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    • 专利摘要:
    method of manufacturing a steel element formed in a hot press. the present invention relates to a method of manufacturing a steel element formed in a hot press that has high strengths, an excellent balance between strength and ductility, and good deformation properties after collision crushing (collision protection). the manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. the method makes a steel element by heating a steel sheet having a specific chemical composition and by subjecting the steel sheet to at least one formation in a hot press. in the method, the heating temperature is equal to or higher than the transformation temperature ac3, and an initial temperature of formation in the hot press is in the range of the heating temperature for the initial temperature of formation of martensite (ms). the cooling from [(ms temperature) - 150 ° c] to 80 ° c is carried out so that a tempering parameter (à) specified by expression (1) is in the range from 7100 to 8030.
    公开号:BR112015016863B1
    申请号:R112015016863-9
    申请日:2014-01-17
    公开日:2020-09-01
    发明作者:Naoki Mizuta；Tatsuya Asai；Takayuki Yamano；Tetsuji Hoshika
    申请人:Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)；
    IPC主号:
    专利说明:

    TECHNICAL FIELD
    [001] The present invention relates to methods of fabricating steel elements formed in a hot press. In specific terms, the present invention relates to a method of manufacturing a steel element formed in a hot press that has high strengths and an excellent balance between strength and ductility (balance strength - ductility). BACKGROUND OF THE INVENTION
    [002] Specific steel automotive parts are designed in order to present greater resistance in order to offer both collision protection and a reduction in weight. Regardless, the parts after manufacture require good functionality of the steel sheets in question. It is assumed that such high-strength steel sheets, in particular steel sheets with a tensile strength of 980 MPa or more, are subjected to cold work (for example, cold press forming). As a disadvantage, however, this requires a higher press forming load or causes a remarkably low dimensional accuracy.
    [003] As a possible solution to the disadvantage, there is a hot press training technique. Hot press formation is also called hot pressing or hot stamping. In the hot press formation, a steel plate in question is subjected to a press formation, at the same time being heated in order to carry out the formation and simultaneously obtain greater strengths. In this technique, the steel sheet at a high temperature (for example, in the single-phase austenitic region) is formed or molded with tools (drill and cutter), maintained and cooled in the lower dead center (formation) so extracting the heat from the steel plate to the tools to quickly cool the steel plate. In this way, the steel sheet in question is tempered. The forming technique, when performed, can result in a product formed with good dimensional accuracy and high resistance. The technique can be performed with a lower forming load compared to the cold forming of parts that have resistances at the same similar level.
    [004] However, the technique requires a retention in the lower dead center for ten to several seconds in order to extract heat for the tools; and during this period, the formation of another piece becomes impossible. Unfortunately, a steel element, therefore, occupies the pressing machine for a long time, and this results in low productivity.
    [005] In addition, hot press formation requires cooling at a cooling rate of 30 ° C / seconds or more. Therefore, training, as well as transfer, has to be carried out within a short time of ten to several seconds; press training can be performed approximately only once; and there is a limit to the form capable of forming by press training. To the disadvantage, the technique, therefore, cannot manufacture parts with complicated shapes. Even more disadvantageously, the resulting steel element obtained after working has high strength and low ductility, it is not expected to have high shock absorption after collision, and is limited to the uses in which the steel element is applicable.
    [006] In order to solve these disadvantages, studies have been carried out to improve productivity and / or increase the degree of training freedom in hot press training techniques.
    [007] Typically, Patent Literature (PTL) 1 describes that productivity can be increased by holding a workpiece in the lower dead center for a short time, separating the workpiece from the tools at a higher temperature, and submit the workpiece to a subsequent step. This technique, however, requires rapid cooling (at a rate of 150 ° C / s in the working examples) after training, and therefore requires a special installation design, and is probably not very versatile, although the technique provides shorter tool holding times. In addition, the manufacturing method specified in the PTL 1 document performs a formation for a short time of ten to several seconds as in conventional techniques, it is difficult to perform a multi-stage press formation, and cannot process the workpiece. work for a complicated shape.
    [008] The document PTL 2 describes a method of forming in a hot press. In this method, cooling water is injected from the tools during press formation in order to reduce the retention time in the lower dead center and obtain high strengths and satisfactory productivity. The method, however, requires complicated production facilities in order to inject cooling water from tools and is not versatile.
    [009] PTL documents 3 to 5 propose hot press training methods. In the method presented in the PTL 3 document, a steel plate heated to 1000 ° C or less is subjected to a multi-stage press formation from two to five stages (steps) in a temperature range of 600 ° C or greater and, then cooled at a cooling rate of 10 ° C / second or more. In multi-stage press formation, the stage press formation is carried out within 3 seconds, and a subsequent press formation is carried out within 4 seconds. In the method described in document PTL 4, a steel sheet is heated to a temperature range from the temperature Acs to the melting point, is partially supported by a drill provided in the tools, is subjected to formation at an initial temperature greater than a temperature in which all deferrite, pearlite, bainite, and martensite transformations occur, and is quickly cooled after formation. In the method described in PTL 5, press forming is performed so that a formed product is released from the press forming tools within 5 seconds after the press forming tools reach the bottom dead center, and cooled at a rate cooling temperature of 30 ° C / s or more to allow the formed part to have an HV hardness of 400 or more.
    [010] However, other improvements, such as the adjustment of chemical compositions, are probably required in order to certainly provide an excellent resistance-ductility balance. List of Citations Patent Literature PTL Document 1: Japanese Patent Application Publication Not Examined (JP-A) N. 2011-218436 PTL Document 2: JP-A N. 2002-282951 PTL Document 3: JP-A N. 2005-152969 Document PTL 4: JP-A N. 2009-82992 Document PTL 5: JP-A N. 2005-288528 SUMMARY OF THE INVENTION Technique Problem
    [011] The present invention was made according to these circumstances, and an object of the present invention is to establish and provide a technique for the economical and efficient manufacture of a steel element by forming in a hot press with a high degree of freedom in the form to be formed, with the steel element having high strengths and excellent resistance - ductility balance and offering good deformation properties (collision protection) after collision deformation.
    [012] As used herein, the term "high strength" in the steel element according to the present invention refers to having a yield strength of 800 MPa or more (preferably 850 MPa or more, and more preferable) - roughly 900 MPa or more) and a tensile strength of 980 MPa or more (preferably 1270 MPa or more, and more preferably 1470 MPa or more). The yield strength and the breaking stress in this document are determined using the methods described in the experimental examples mentioned below. Also as used in this document, the term “excellent resistance-ductility balance” in the steel element according to the present invention refers to the fact that the steel element has a product (TS x EL) of tensile strength TS and EL elongation of 13550 MPa% or more (preferably 13600 MPa% or more, more preferably 13700 MPa% or more, even more preferably 13800 MPa% or more, and particularly preferably 14000 MPa% or more), the product TS x EL being determined by a method described in the experimental examples. Solution to the Problem
    [013] The present invention targets the object and provides a method of fabricating a steel element formed in a hot press. The method includes heating a steel sheet and subjecting the steel sheet to at least one hot press formation. The steel sheet contains, as a chemical composition in percentage by mass (hereinafter the same for the chemical composition), - C in a content of 0.15% to 0.4%; - Si in a content of more than 1.0% to 1.65%; - Al in a content of 0.5% or less (excluding 0%); - Mn in a content of 1% to 3.5%; - Ti in a content of 0.10% or less (excluding 0%); and - B with a content of 0.005% or less (excluding 0%),
    [014] - with the rest being iron and unavoidable impurities. In the method, a temperature (heating temperature) for heating is equal to or greater than the transformation temperature Acs. An initial formation temperature in the hot press is a range from the heating temperature to the initial formation temperature of martensite (Ms). The cooling of [(Ms temperature) - 150 ° C] to 80 ° C is performed so that a tempering parameter (À) as specified by Expression (1) is in the range of 7100 to 8030. The Expression (1 ) is represented as follows: Expression 1:

    [015] - where À represents the tempering parameter; [Si] represents the content (in percentage by mass) of Si in the steel; and tn 'is a value as specified by Expression (2): Expression 2:

    [016] - where tn represents an umpteenth time (in seconds) when a total cooling time from [(temperature Ms) - 150 ° C] to 80 ° C is divided into 5000 equal parts; Tn represents a temperature (° C) in the nth time tn, where to is 0 seconds, and To is a [(temperature Ms) - 150 ° C]; and 10A {} represents the {} - th power of 10.
    [017] Cooling from [(Ms temperature) - 150 ° C] to 80 ° C can be performed at an average cooling rate of 5 ° C / s to 20 ° C / s.
    [018] The final temperature of the final hot press formation at least once hot press formation can be equal to or less than the temperature Ms.
    [019] The step of heating the steel sheet to the heating temperature is preferably carried out at an average temperature rise rate of 5 ° C / s or more in the temperature range of 100 ° C to the heating temperature.
    [020] After forming in a hot press, tempering can be carried out at a temperature of 100 ° C to less than 600 ° C.
    [021] The steel plate for use in the method of manufacturing a steel element formed in a hot press can also contain at least one element selected from the group consisting of:
    [022] Cr in a content of 5% or less (excluding 0%),
    [023] at least one element selected from the group consisting of Ni and Cu in a total content of 0.5% or less (excluding 0%),
    [024] Mo at a content of 1% or less (excluding 0%), θ
    [025] Nb in a content of 0.1% or less (excluding 0%).
    [026] The present invention also includes a steel element formed in a hot press obtained by means of the manufacturing method. The present invention also includes an automotive steel part obtained by working the steel element formed in a hot press. Advantageous Effects of the Invention
    [027] The manufacturing method according to the present invention can provide a steel element by forming in a hot press, the steel element having high strengths and excellent resistance - ductility balance, offering good deformation properties ( collision protection) after collision crushing, and is useful as or for high-strength steel parts for automobiles. Unlike conventional hot press forming techniques, the manufacturing method does not require long-term retention in the lower dead center, it can efficiently manufacture steel elements, it can perform hot press forming multiple times, and it has a high degree of freedom in the form to be formed. BRIEF DESCRIPTION OF THE DRAWINGS
    [028] Figures 1 (a), 1 (b), and 1 (c) are explanatory drawings illustrating the moment before, intermediate, and in the formation of the lower dead center of the hot press formation in the experimental examples, respectively;
    [029] Figure 2 is a schematic explanatory drawing illustrating a multi-stage formation process;
    [030] Figure 3 is an explanatory drawing illustrating the processes of formation of multiple stages according to modalities;
    [031] Figure 4 is a cross-sectional view of a steel element including a reinforcer;
    [032] Figure 5 is a schematic explanatory drawing illustrating an outburst in a multi-stage formation process according to a modality;
    [033] Figure 6 is a schematic explanatory drawing illustrating a flange in a multi-stage formation process according to a modality;
    [034] Figure 7 is a schematic explanatory drawing illustrating drilling and trimming (peripheral) in multi-stage formation processes according to modalities;
    [035] Figure 8 is an explanatory drawing illustrating how to derive the Expression (1) as specified in the present invention;
    [036] Figure 9 is a process graph illustrating a hot press formation in the experimental examples;
    [037] Figure 10 is a schematic perspective view illustrating the shape of a steel element obtained in the experimental examples;
    [038] Figure 11 is a schematic perspective view illustrating a position in which a thermocouple is embedded in order to measure the temperature of a steel sheet in the experimental examples; and
    [039] Figure 12 is a schematic perspective view illustrating a position in which a test sample for a tensile test is sampled with respect to a steel element in the experimental examples. DESCRIPTION OF THE MODALITIES OF THE INVENTION
    [040] The present inventors have made intensive studies in order to provide a steel element with the properties. As a result, electrodes have discovered a special method of fabricating a steel element using a steel sheet having a specific chemical composition by heating the steel sheet and subjecting the heated steel sheet to a hot press formation at least one turn. The method is carried out at the same time meeting conditions (i) to (iii). In specific terms, (i) the heating temperature is controlled so that it is equal to or higher than the transformation temperature Aça. In addition, (ii) the hot press formation is initiated at a temperature in the range of the heating temperature to below the temperature Ms. In addition, (iii) the cooling of [(Ms temperature) - 150 ° C] to 80 ° C is performed so that a quenching parameter (À) as specified by Expression (1) is from 7100 to 8030. The present invention was made based on these findings.
    [041] The reasons why manufacturing conditions including conditions (i) to (iii) are specified in the present invention will be described in detail below. Manufacturing Conditions
    [042] Heating is carried out at a temperature (heating temperature) equal to or greater than the transformation temperature Aça
    [043] Heating to a temperature equal to or greater than the transformation temperature Acs allows the steel element to have a single martenite phase as its microstructure and to have predetermined resistances. The “transformation temperature Acs” refers to an austenitic transformation temperature and is henceforth also simply referred to as the “temperature Acs”. Heating, when carried out at a temperature below the transformation temperature Acs, causes the ferrite and other phases to remain and act as nuclei during a hot formation and thus allow the ferrite to grow easily . This probably makes it difficult for the steel element to have high strengths even when the cooling rate after heating is controlled.
    [044] The heating temperature is preferably equal to or greater than [(temperature Acs) + 10 ° C]. Heating, when carried out at an excessively high temperature, can cause the microstructure that constitutes the steel element to become thicker, and can cause the steel element to show a small resistance - ductility balance. To avoid this, the heating temperature can be controlled at an upper limit preferably equal to or less than [(temperature Acs) + 180 ° C], and more preferably equal to or less close to [(temperature Acs) + 150 ° C].
    [045] Retention at heating temperature can be carried out for a time (heating retention time) preferably 15 minutes or less, and more preferably 5 minutes or less. This is typically preferred in the sense of restricting the growth of austenitic grain. The retention does not have to be carried out, that is, the heating retention time can be zero, as long as the heating temperature falls within the specific range.
    [046] Heating can be carried out in an atmosphere as selected from oxidizing atmospheres, reducing atmospheres, and non-oxidizing atmospheres. In specific terms, the atmosphere is exemplified by the air atmosphere, the flue gas atmosphere, and the nitrogen gas atmosphere.
    [047] In the step of heating the steel sheet to the heating temperature, the heating is preferably carried out at an average temperature rise rate of 5 ° C / second or more in the temperature range of 100 ° C to the temperature of heating. Heating at such a high rate of temperature rise may refine the (reduce the grain size of the) phase (anterior austenite) in order to contribute to an even better resistance - ductility balance. The average rate of temperature rise is more preferably 50 ° C / s or more, and even more preferably 100 ° C / s or more. The average rate of temperature rise is not a critical issue with regard to the upper limit for a better resistance-ductility balance, but is preferably around 500 ° C / s or less, typically taking into account the size of the heating equipment and the size part to be manufactured.
    [048] Hot press formation is initiated at a temperature in the range of heating temperature to below the temperature of Ms.
    [049] Hot press formation, when started at a temperature in the heating temperature range below Ms temperature, allows easy work and sufficiently reduces the press formation formation load. Hot press formation, when started at a temperature below the Ms temperature, must be carried out on a high-strength martensitic steel. To a disadvantage, this causes overloading of the press forming, since a pressing machine for hot press forming is generally not as powerful, or causes a greater risk of delayed fracture due to the high residual stress. To avoid this, the initial temperature of the hot press formation is controlled so that it is equal to or greater than the temperature Ms. The initial temperature of the hot press formation is preferably equal to or greater than [(temperature Ms) + 30 ° C], and more preferably equal to or greater than [(temperature Ms) + 50 ° C],
    [050] In the present invention, the term "initial" of hot press forming refers to a time in which a part of the steel sheet (blank) first comes into contact with at least part of the tools in the first formation . The term “final” of the hot press formation refers to a time in which the entire part of the product formed is separated from the tools in the final formation.
    [051] In the present invention, an initial temperature (in other words, the blank temperature when a part of the blank first comes into contact with at least a part of the tools in the first formation) of hot press formation is specified, although there is no limitation on a final temperature (in other words, the temperature of the blank (steel element) at the time the entire formed product (the steel element) is separated from the tools in the final formation) from the hot press formation. A preferred final temperature of the hot press formation will be described in detail below.
    [052] The Aça temperature and the Ms temperature are respectively calculated according to Expressions (a) and (b) described in the document “The Physical Me-tallurgy of Steels”, by William C. Leslie (Maruzen Co., Ltd. , May 31, 1985, p. 273). In the expressions, an element indicated in the dashes represents the content (in percentage by mass) of the element, and the calculation can be carried out, at the same time defining the content of an element not contained in the steel plate as 0 percent by mass. Expressions (a) and (b) are represented as follows:
    [053] Temperature Ac3 (° C) = 910 - 203 x ([C] 05) - 15.2 x [Ni] + 44.7 x [Si] + 31.5 x [Mo] - 30 x [Mn] -11 x [Cr] - 20 x [Cu] + 700 x [P] + 400 x [Al] + 400 x [Ti] (a)
    [054] Ms Temperature (° C) = 561 - 474 x [C] - 33 x [Mn] -17 x [Ni] -17 x [Cr] -21 x [Mo] (b) Number of Press Formation a Training and Training in Multiple Stages
    [055] Hot press training can be performed once or multiple times. Hot press formation, when performed multiple times, can result in a complicated shaped element and can provide better dimensional accuracy. A mechanism for providing better dimensional accuracy is as follows.
    [056] During the press forming process, different portions of the blank are in contact with the tools for different durations, and this can cause a temperature difference (non-uniformity) in the formed product. Typically, it is assumed that the hot press formation is carried out once as a curvature illustrated in Figure 1. In this case, portion A of the blank comes into contact with the tools for a long time and undergoes a great deal of temperature reduction (great heat extraction for the tools), and, in contrast, the B portions of the blank are in contact with the tools for a short time and undergo a small temperature reduction, each according illustrated in Figure 1. Thus, the product formed may have a difference in magnitude of temperature reduction, and thus have a difference in magnitude of thermal shrinkage, undergo a thermal deformation (plastic deformation), and present a lower dimensional accuracy.
    [057] In one embodiment, the hot press formation is performed as a multi-stage formation, in specific terms, the press formation is performed several times at a temperature or temperatures equal to or greater than the Ms. temperature. , even when a deterioration of dimensional accuracy occurs in a previous formation, a subsequent formation is still carried out at a high temperature and can easily correct the deterioration of dimensional accuracy. In addition, the repeated forming processes also eliminate or mitigate non-uniform temperature and, thus, will more readily reduce the deterioration of dimensional accuracy due to the non-uniform temperature.
    [058] In addition, the method, when carrying out the hot press formation in multiple stages, as above, may also include a straightening process by shape restriction and, with advantage, provides a better dimensional accuracy , and dimensional accuracy is a concern in multi-stage hot press forming technology. Deterioration of dimensional accuracy is a concern in a multi-stage hot forming process geared to productivity. The deterioration of dimensional accuracy, however, can be significantly improved by separating the product formed from the tools at a temperature equal to or less than the temperature Ms in the final hot press formation (including in the case where the hot press formation is performed once), as described below. In specific terms, the final hot press formation is preferably carried out at a final temperature equal to or less than the Ms. temperature. In a more preferred embodiment, the product formed is kept in contact with the tools (tool restriction) at [( temperature Ms) - 150 ° C] before separation. This may offer better and even more stable dimensional accuracy. In particular, this will be useful when the element is made of a thin blank having a thickness of typically 1.4 mm or less, since such a thin blank may undergo a great deal of deterioration in dimensional accuracy after forming in multiples stages.
    [059] Hot press training, when performed multiple times, can be performed on the same tools, or on two or more different combinations of tools in different ways, in specific terms, on tools that have shapes that differ from a process training for another.
    [060] Hot press forming, when performed as a multi-stage forming, requires a smaller amount of work per process than the final required amount of work and allows the blank to form in an element that has a shape more complicated.
    [061] For example, the rear side elements and other parts are curved three-dimensionally and have different shapes in cross section (width and height) in the longitudinal direction. These parts are, in general, difficult to be formed into final forms in a single step. However, the process of forming multiple stages (which include multiple stages), as illustrated in Figure 2, allows the formation of parts with these complicated shapes. In specific terms, the formation process can be designed as including the formation (drag and / or curvature) in a coarse form, as in Figure 2 (a) in the first stage, and additional work (reorientation and / or relaunch) to a final shape, as indicated by the solid line in Figure 2 (b) in the second step.
    [062] In addition, the forms to be worked on in the first stage and in the second stage of the multi-stage formation process can be appropriately designed. Typically, a portion of excess metal can be arranged in an appropriate form, and the order of work operations can be properly defined. This may provide elements formed with significantly complicated shapes, as illustrated in Figures 3 (a) and 3 (b). The formed elements, when they have these complicated shapes, can contribute to greater functions (for example, greater rigidity and / or greater collision protection) and reduced wall thicknesses of the resulting parts.
    [063] Current automobile body structures often employ a structure that includes a part (A), and a reinforcer (C) within the part (A), as illustrated in Figure 4 (seen in cross section) . These structures or parts are exemplified by the central pillars and balancers. A part, when it has such a shape or structure, can withstand deformation in the sectional form and can present better protection against collision after an impact on the part (A). However, part (A), when it has a complicated shape, as described above, may present better protection to the collision by itself. This may omit or reduce the thickness of the reinforcer (C) and contribute to a lighter weight and lower cost.
    [064] In the multi-stage training modalities, a bulging and / or a flanging can be performed in the second or later stage, as described below. Typically, the bulging can be carried out in the second stage or later stage of the multi-stage formation process, as illustrated in Figure 5. This formation allows the steel element to have an additional curved shape in order to present, thus, a greater function, such as greater rigidity and / or greater collision protection. Typically still, a flanging can be performed in the second or later stage of the multi-stage forming process, as illustrated in Figures 6 (a) and 6 (b). The flanging is exemplified by an upward flange, a downward flange, a flange stretch, deburring, and flange shrinkage. This formation may also allow the steel element to have a greater function, such as greater rigidity and / or greater collision protection.
    [065] In another form of multi-stage formation, a perforation or similar process can be carried out in a state in which the material in the second or subsequent stage becomes soft at a relatively high temperature. Typically, perforation (punching) and / or peripheral trimming (shearing) can be performed in the second or later stage, as illustrated in Figures 7 (a), 7 (b), and 7 (c). This allows drilling and / or trimming to be carried out as a process in press formation and allows cost reduction, with such drilling and / or trimming being typically performed by laser work as an extra step in conventional forming processes, including only one retention step in the lower dead center. Hot peripheral trimming and / or hot drilling (drilling) can also be performed before forming, as illustrated in Figure 7 (d).
    [066] Hot Press Formation Termination Temperature (Final Tool Release Temperture)
    [067] The expression “final temperature of the hot press formation” refers to the final temperature of the final hot press formation and, when the hot press formation is carried out only once, it simply refers to the “temperature -final break of the formation in hot press ”. The final temperature of the hot press formation is not a critical issue and can be equal to or greater than the temperature Ms, or equal to or less than the temperature Ms.
    [068] For better dimensional accuracy, the final temperature of the final hot press formation is preferably equal to or less than the Ms. temperature. In this case, the final temperature is more preferably equal to or greater than [(Ms temperature) - 150 ° C]. In specific terms, the hot press formation is preferably finished in a temperature range equal to or less than the Ms temperature (that is, at a time when the martensitic transformation occurs), and the hot press formation, in the This document refers to the final hot press formation in the case of multi-stage hot press formation. This can provide significantly better dimensional accuracy, both in single-stage and multi-stage formation.
    [069] Initial Temperatures and Final Temperatures in the respective Hot Press Formation Modes
    [070] Modalities of the hot press formation are exemplified as follows.
    [071] Training in a Single Stage Hot Press (a training operation)
    [072] (l-1) Hot press formation is carried out at an initial temperature of the heating temperature falling to the temperature Ms and at a final temperature equal to or greater than the temperature Ms.
    [073] (l-2) Hot press formation is carried out at an initial temperature of the heating temperature falling to the temperature Ms and at a final temperature equal to or less than the temperature Ms.
    [074] (ll) Multiple Stage Hot Press Training (multiple training operations)
    [075] (ll-1) The first formation in the hot press is carried out at an initial temperature of the heating temperature descending to the temperature Ms, and the final formation in the hot press is carried out at a final temperature equal to or greater that Ms. temperature
    [076] (ll-2) The first formation in the hot press is carried out at an initial temperature of the heating temperature descending to the temperature Ms, and the final formation in the hot press is carried out at a final temperature equal to or less at Ms. temperature
    [077] Hot press formation, when performed multiple times, can also include a reheating process and / or a temperature retention process between two press formation processes. However, hot press formation is preferably carried out without reheating and temperature retention so that all hot press forming processes are carried out at initial temperatures equal to or greater than Ms. temperature. This is preferred from the from the point of view of productivity, installation cost, and energy cost.
    [078] The rate of cooling of the heating temperature to [(Ms temperature) - 150 ° C] is not a critical issue. Typically, cooling can be performed from the heating temperature down to [(Ms temperature) - 150 ° C] at an average cooling rate of 2 ° C / s or more (more preferably 5 ° C / s or more). Cooling at a cooling rate within such a range contributes to the formation of martensite in a temperature range downstream of or below the MS temperature, while approximately preventing the formation of other phases, such as ferrite and bainite. This can easily result in a high strength element. The cooling rate is not a critical issue when it comes to the upper limit and can be around 500 ° C / s or less, and preferably 200 ° C / s or less in consideration of the operation in question. For example, the average cooling rate can be controlled from 2 ° C / s to 10 ° C / s.
    [079] The cooling rate can be controlled by any combination within the conditions typified as follows:
    [080] - time of removal of the blank from the heating furnace to start the press formation (rate after cooling typically by transfer);
    [081] - total contact time with the press training tools in hot press training, with the total contact time given by multiplying the contact time by one process by the number of training processes;
    [082] - cooling conditions (for example, natural cooling or forced wind cooling) between two adjacent forming processes when the press formation is carried out several times; and
    [083] - cooling conditions (for example, natural cooling or forced wind cooling) after finishing the press formation (after the tool is released). In particular, it is effective to determine a longer contact time with the press forming tools in order to increase the cooling rate to a temperature equal to or greater than [(temperature Ms) - 150 ° C]. These cooling conditions can be previously estimated typically by simulation.
    [084] The cooling of [(Ms temperature) - 150 ° C] to 80 ° C is carried out so that a tempering parameter (À) as specified by Expression (1) is from 7100 to 8030, the Expression being (1) is represented as follows: Expression 3:

    [085] - where À represents the quenching parameter; [Si] represents the content (in percentage by mass) of Si in the steel; and tn 'represents a value as specified by Expression (2), hereinafter the symbols are defined as above, Expression 4:

    [086] - where tn represents an umpteenth time (in seconds) when a total cooling time from [(Ms temperature) - 150 ° C] to 80 ° C is divided into 5000 equal parts; Tn represents a temperature (° C) in the nth time tn, where to is 0 seconds, and To is a [(temperature Ms) - 150 ° C]; and 10A {} represents {} - th power of 10, henceforth the symbols are defined as above.
    [087] In accordance with the present invention, the cooling of [(Ms temperature) - 150 ° C] to 80 ° C is controlled so that a tempering parameter (À) as specified by Expression (1) is 7100 to 8030. The tempering parameter (À) is hereinafter also simply referred to as “parameter À”.
    [088] It will initially be described how to derive parameter À.
    [089] Expression (1) is derived based on Expression (3). Expression (3) is typically described in “Tekko Zairyo” (edited by the Japan Institute of Metals and Materials) and is generally used for the specification of the quenching parameter (Àg). Based on Expression (3), the hardness of a martensitic steel after isothermal retention at temperature T [K] for a time t [h] can be assumed. Materials having an identical Ág as specified by Expression (3) have an identical hardness as a result of heat treatment at any temperature for any time. In the case of steel materials, the constant C can be set to 20. Expression (3) is represented as follows:

    [090] Expression (3) is represented on the assumption that the workpiece is isothermally contained, as mentioned above. In contrast, a part (formed product) after forming in a hot press is, in general, necessarily cooled with a coolant, such as tools, air, or water, and its cooling is traced as a continuous cooling , as illustrated in Figure 8 (a). Expression (3) defined in the assumption of an isothermal retention is not applicable to this case without modification.
    [091] Expression (3) is, therefore, modified as follows to be applicable to a continuous cooling process of the product formed after forming in a hot press.
    [092] The modification (improvement) is carried out based on the following considerations. Initially, the cooling curve is divided into micro-times at identical intervals and is approximated to a set of isothermal retention heat treatments in micro-times, as shown in Figure 8 (a). Isothermal retentions at temperatures T1, T2, and T3 for a micro time period Δt, as exemplified in Figure 8 (a) are respectively converted to the time periods tT, t2 ', and t3' at a given base temperatureTb, as shown in Figure 8 (b). The converted time periods are added together to produce a total converted time, and the base expression is applied to the isothermal retention at the base temperature Tb for the total converted time [tT + t2 ’+ t3’].
    [093] Next, how, in specific terms, to derive the modified expression will be described with reference to Figures 8 (a) and 8 (b) as a modality.
    [094] As shown in Figure 8 (a), the cooling curve is approximated to three isothermal retention processes. In specific terms, the isothermal retention is performed in T1 (K) in a period of time Δt from t0 to t1; is performed in T2 (K) over a period of time Δt from t1 to t2; and is performed in T3 (K) over a period of time Δt from t2 to t3. The isothermal retention processes are respectively converted into heat treatments at the base temperature Tb (K) for certain periods of time tT, t2 ', and t3'. In specific terms, an isothermal retention process at a temperature higher than the base temperature Tb is converted into a heat treatment for a longer period of time; while an isothermal retention process at a temperature below the baseTb temperature is converted into a heat treatment for a shorter period of time.
    [095] Typically, when the isothermal retention is performed in T1 (K) in the time period Δt from tO to t1, this isothermal retention process can be converted into a heat treatment at the base temperature Tb (K) for a period of time tT, as specified by and derived from Expressions (4) to (6):
    Expression 5:
    Expression 6:

    [096] The time periods t2 'and t3' are determined in a similar way, and the converted time periods tT, t2 ', and t3' are added, and the total sum is replaced in Expression (3) of to result in Expression (7):

    [097] Expression (7) and Expression (6) are generalized in order to respectively result in Expression (8) and Expression (9):

    [098] - where tn 'is a value as specified by Expression (9):

    [099] In this respect, the measuring temperature and the measuring time are respectively “degree in Celsius scale (° C)” and “second (sec.)”. In contrast, the temperature and time in Expression (9), as derived from the base expression, are respectively indicated in “Kelvin scale (K)” and in “hour (h)”. The unit systems in Expression (9) are converted respectively to the degree in Celsius scale (° C) and to the second (s, sec.). The base temperature Tb can be any temperature, but this document is defined as 20 ° C. The constant C is defined as 20, a value that is generally used in steel materials. The unit systems are converted, and the base temperature Tb of 20 ° C and the constant C of 20 are replaced in Expression (9) in order to result in Expression (10): Expression 10:

    [0100] In addition, the calculation according to Expression (8) is performed by dividing the entire cooling process into 5000 parts. The number “n”, therefore, takes a value from 1 to 5000. The temperature specified by [(temperature Ms) - 150 ° C] is defined as a starting point. Therefore, to is 0 (sec.), To is [(temperature Ms-) 150 ° C], and Δt is [tn-tn-i]. These conditions (parameters) are replaced in Expression (8) and Expression (10) in order to respectively result in Expression (11) and Expression (2): Expression 11:

    [0101] - where tn 'is as specified by Expression (2): Expression 2:

    [0102] Then, Expression (11) is modified into an expression with respect to resistance to temper softening due to the addition of Si. The parameter of temper (Àg) in the base expression, Expression (3), is not affected by variation in chemical composition. In contrast, silicon (Si) effectively increases the resistance to quenching softening, and the quenching parameter becomes apparently smaller with a higher Si content. In specific terms, Si affects the quenching parameter. The quenching parameter used in this document is calculated, while defining the specific temperature [(temperature Ms) - 150 ° C] as the starting point (To), the specific temperature being determined by the chemical compositions of the steel. The tempering parameter is therefore adaptable to variations in chemical compositions within specific ranges. However, the expression for temperature Ms does not have the Si content term, and the Si content term is added. The quenching parameter apparently decreases with a higher Si content, as described above, and a minus (negative) term with respect to the Si content is added to Expression (11) in order to result in Expression (1). The Si content in Expression (1) has a coefficient of 430, the coefficient of which is experimentally determined. Expression (1) is represented as follows: Expression 1:

    [0103] - where tn 'is as specified by Expression (2): Expression 2:

    [0104] The quenching parameter can also be determined by determining the micro-quenching parameters one by one, and by adding the determined quenching parameters. However, Ag, as specified by the base expression, Expression (3), is defined in order to determine a parameter in a single heat treatment process. When the parameters obtained in two or more processes are added to produce a value, the resulting value may become abnormal (extremely high) and will not meet the parameter in the operation in question.
    [0105] Q cooling, in this case, is performed so that the parameter À, as specified by the expression defined above, is 7100 or more. The À parameter of 7100 corresponds to cooling in the [(Ms temperature) - 150 ° C] range of 80 ° C at a cooling rate of about 20 ° C / s (holding time on tools of 10 seconds). Cooling, when performed on an A parameter less than 7100, may require a long retention time on tools and cause poor productivity, which differs little from conventional methods. The À parameter is preferably 7300 or more, and more preferably 7500 or more. In contrast, cooling, when performed on an A parameter greater than 8030, may fail to allow the steel element to have a desired resistance - ductility balance. To avoid this, the cooling of [(Ms temperature) - 150 ° C] to 80 ° C can be carried out so that the parameter À is 8030 or less. The parameter À is preferably 7900 or less, and more preferably 7800 or less.
    [0106] Q cooling, in this case, from [(Ms temperature) - 150 ° C] to 80 ° C has only to be carried out so that the parameter À falls within the specific range, and can be carried out by means of any specific, unrestricted procedure. The cooling procedure is exemplified by forced wind cooling and natural cooling. The cooling process from [(Ms temperature) - 150 ° C] to 80 ° C can be performed at an identical (constant) cooling rate or at a variable cooling rate according to the temperature ranges, as long as the parameter (À) falls within the range. The manufacturing method can also include an isothermal retention process and / or a reheat process in the middle of cooling the [(Ms temperature) - 150 ° C] to 80 ° C, as long as the parameter (À) falls within the track. When the method includes the isothermal retention process, the workpiece can, for example, be kept in a waiting oven and then cooled by the cooling procedure.
    [0107] In a cooling mode performed so that the parameter A falls within the range, the workpiece can be cooled from [(temperature Ms) - 150 ° C] to 80 ° C at an average cooling rate from 5 ° C / s to 20 ° C / s.
    [0108] Q cooling, when performed at an average cooling rate above 20 ° C / s, may require a long retention time on tools and cause poor productivity, thus differing little from conventional methods. The average cooling rate is more preferably 15 ° C / s or less. In contrast, cooling, when performed at an excessively low average cooling rate, may fail to allow the steel element to have a desired resistance - ductility balance. To avoid this, the average cooling rate is preferably 5 ° C / s or more, and more preferably 10 ° C / s or more.
    [0109] Cooling to 80 ° C at room temperature can be performed at any non-critical cooling rate. Typically, cooling can be accomplished by natural cooling. Quenching after Hot Press Formation
    [0110] The method can also include, after forming in a hot press, the tempering at a temperature of 100 ° C to less than 600 ° C. The blank (also the resulting steel element) has a specific chemical composition system as mentioned below and will therefore be able to undergo a resistance adjustment by tempering without impairing the resistance - ductility balance (balance TS x EL ). For sufficient quenching purposes, quenching is preferably carried out at a temperature of 100 ° C or higher, and more preferably 200 ° C or higher. In contrast, tempering, when carried out at a temperature of 600 ° C or higher, may fail to allow the steel element to have a high yield stress (YS). To avoid this, tempering, when employed, is preferably carried out at a temperature below 600 ° C, and more preferably at 300 ° C or less. Tempering can be carried out for a time (holding time at tempering temperature) of 60 minutes or less in order to control costs. Blank (Steel Sheet) for Use in Hot Press Formation
    [0111] Next, the blank (steel sheet) for use in hot press forming will be illustrated. Initially, the blank for use in the manufacturing method has a chemical composition as follows. Chemical Composition of the Carbon Crude Part (C): 0.15% to 0.4%
    [0112] To allow the steel element to have a tensile strength of 980 MPa or more, the carbon content can be 0.15% or more, preferably 0.17% or more, and more preferably 0.20% or more. In contrast, with respect to the weldability of the resulting element, the carbon content may be 0.4% or less, preferably 0.30% or less, and more preferably 0.26% or less in terms of the upper limit. Silicon (Si): greater than 1.0% to 1.65%
    [0113] Silicon (Si) essentially increases the resistance to temper softening and guarantees high strengths (in order to guarantee an excellent resistance - ductility balance). Si also effectively offers better ductility of weld beads after the element is subjected to welding. Si is also effective in ensuring excellent resistance to delayed fracture. In order to show such effects sufficiently, the Si content can be controlled to more than 1.0%, preferably 1.1% or more, and more preferably 1.2% or more. On the other hand, Si, when contained in excess, can cause a significant occurrence of internal oxidation (grain boundary oxidation) in the hot ironing process, but it can also cause the subsequent acid washing process to proceed at a significantly higher rate. low, resulting in lower productivity. To avoid this, the Si content can be controlled to 1.65% or less, preferably 1.45% or less, and more preferably 1.35% or less. Aluminum (Al): 0.5% or less (excluding 0%)
    [0114] Aluminum (Al) is used for deoxidation, and for this purpose, the Al content is preferably 0.01% or more. On the other hand, Al, when contained in an excessively high content, may cause the temperature of Acs to rise, and this may require a higher heating temperature when forming in a hot press, resulting in less production efficiency. To avoid this, the Al content can be controlled to 0.5% or less, preferably 0.20% or less, more preferably 0.10% or less, and even more preferably 0.050% or less. Manganese (Mn): 1% to 3.5%
    [0115] Manganese (Mn) is necessary to allow the steel plate to have a better hardening capacity in order to thus produce an element with high strengths. From this point of view, the Mn content can be controlled to 1% or more, preferably 1.5% or more, more preferably 1.8% or more, and even more preferably 2.0% or more. However, Mn, when contained in a content of more than 3.5%, may exhibit saturated effects and cause an increase in cost. To avoid this, the Mn content is controlled, in this case, to 3.5% or less, preferably 3.0% or less, and more preferably 2.8% or less. Titanium (Ti): 0.10% or less (excluding 0%)
    [0116] Titanium (Ti) fixes nitrogen (N) like TiN, allows boron (B) as a solute, and, thus, effectively guarantees the hardening capacity. From this point of view, the Ti content is preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, Ti, when contained in an excessively high content, can cause the steel sheet in question (blank) to have greater resistance than necessary and produce cutting tools - drilling with shorter lives , resulting in increased costs. To avoid this, the Ti content can be controlled to 0.10% or less, preferably 0.06% or less, and more preferably 0.04% or less. Boron (B): 0.005% or less (excluding 0%)
    [0117] Boron (B) allows the steel to have a better hardening capacity and to present high strengths even when subjected to slow cooling, and it is necessary. To have these effects, boron can preferably be contained in a content of 0.0003% or more, more preferably 0.0015% or more, and even more preferably 0.0020% or more. In contrast, boron, when contained in excess, may form excess boron nitride (BN) and cause a deterioration in hardness. To avoid this, the boron content can be controlled to 0.005% or less, preferably 0.0040% or less, and more preferably 0.0035% or less.
    [0118] Steel (the blank, also the steel element), in this case, has the chemical composition as above, with the rest including iron and unavoidable impurities, such as P, S, N, O, As , Sb, and Sn. Among such unavoidable impurities, the levels of phosphorus (P) and sulfur (S) are preferably controlled to 0.02% or less. Nitrogen (N), when contained in an excessively high content, can cause a deterioration in hardness after hot formation and / or deterioration in weldability. To avoid this, the nitrogen content is preferably controlled to 0.01% or less. Oxygen (O) causes surface failures. To avoid this, the oxygen content is preferably controlled to 0.001% or less.
    [0119] Steel may also contain one or more additional elements, as follows, within ranges that will not adversely affect the advantageous effects of the present invention. Chromium (Cr): 5% or less (excluding 0%)
    [0120] Chromium (Cr) effectively allows the steel plate to have a better hardening capacity and undoubtedly to have an excellent resistance to oxidation. In specific terms, the steel sheet becomes resistant to the generation of slag after heating before forming in a press. To have these effects, Cr can preferably be contained in a content of 0.01% or more, and more preferably 0.1% or more. However, Cr, when contained in excess, may have saturated effects and cause an increase in cost. To avoid this, the Cr content can be controlled, preferably, to an upper limit of 5%, more preferably 3.5% or less, and even more preferably 2.5% or less.
    [0121] At least one element selected from the group consisting of nickel (Ni) and copper (Cu): total content of 0.5% or less (excluding 0%)
    [0122] Nickel (Ni) and copper (Cu) effectively allow the steel plate to have a better hardening capacity and allow the formed product to have better resistance to delayed fracture and better resistance to oxidation. To have these effects, at least one of the elements may preferably be contained in a total content of 0.01% or more, and more preferably 0.1% or more. However, the elements, when contained in excess, may cause surface failures after the manufacture of the steel sheet. This may result in lower acid wash properties and lower productivity. To avoid this, the total content of these elements is preferably 0.5% or less, and more preferably 0.3% or less. Molybdenum (Mo): 1% or less (excluding 0%)
    [0123] Molybdenum (Mo) effectively allows the steel plate to have a better hardening capacity. The steel sheet, when containing this element, will probably allow the formed product to have a reduced variation in hardness (which is less non-uniform in hardness). To have these effects, the element may preferably be contained in a content of 0.01% or more, and more preferably 0.1% or more. However, Mo, when contained in excess, can have saturated effects and cause increased costs. To avoid this, the Mo content is preferably controlled at an upper limit to 1%, more preferably 0.8% or less, and even more preferably 0.5% or less. Niobium (Nb): 0.1% or less (excluding 0%)
    [0124) Niobium (Nb) effectively refines the microstructure and contributes to greater resistance. Nb, when contained, may preferably be contained in a content of 0.005% or more, and more preferably 0.01% or more. In contrast, Nb, when contained in excess, may cause the steel sheet in question (blank) to have excessively high strengths, and this may shorten the lives of tools used in a stamping process and cause increased costs , the stamping process being the process of typically cutting the blank into a predetermined shape before forming a hot press. To avoid this, the Nb content may preferably be controlled to 0.1% or less, and more preferably 0.05% or less. Raw Part Manufacturing Method
    [0125] Such a blank having a chemical composition that meets the conditions can be manufactured by any non-limited method. The blank can, for example, be manufactured by casting, heating, hot rolling, another acid washing, cold inaction, and, as necessary, annealing, each according to a common procedure . The resulting hot-rolled steel sheet or resulting cold-rolled steel sheet may still be subjected to coating (for example, coating containing zinc). In specific terms, coated steel sheets (for example, galvanized steel sheets) can be used; and hot dip galvanized and annealed steel sheets obtained by still connecting the coated steel sheets. Hot Press Formed Steel Element
    [0126] The steel element formed in a hot press obtained by the method according to the present invention has a chemical composition identical to that of the blank (steel plate) used. The steel element is not limited to microstructure. The microstructure of the steel element is exemplified by one that includes a single phase of martensite; and one that includes martensite as a matrix and that still includes retained austenite (y retained) in a content of 2 percent by volume or more, preferably 3 percent by volume or more, and more preferably 5 percent by volume or more , regarding the microstructure as a whole. The steel element containing y retained at a content of 2 percent by volume or more may be excellent in tensile elongation (ductility), collision protection, and delayed fracture resistance, as described above.
    [0127] The steel element may include, as a steel microstructure, the retained y with the remainder including low temperature transformation phases, such as approximately phases of martensite, tempered martensite, bainite, and bainitic ferrite. The term "approximately" refers to the fact that the steel element can include ferrite and other transformation phases that are formed at a temperature equal to or greater than the temperature Ms, such as the phases that are inevitably formed in the manufacturing process.
    [0128] The resulting steel element can be subjected to cutting (machining), such as trimming and / or drilling in order to produce, for example, a self-motivating steel part.
    [0129] The steel element can be used as an intact or working (processing) steel automotive part as mentioned above. The self-motivating steel part is exemplified by impact bars, bumpers, reinforcements, and central pillars.
    [0130] The present application claims priority to the Patent Application for Ja-Pão N. 2013-007808, filed on January 18, 2013, the contents of which are incorporated in their entirety into this document for reference. Examples
    [0131] The present invention will be illustrated in more detail below with reference to several examples (experimental examples). It should be noted, however, that the examples are in no way intended to limit the scope of the present invention; that various changes and modifications can of course be made to them without departing from the spirit and scope of the present invention as described herein; and all such changes and modifications must be considered to be within the scope of the present invention. Experimental Example 1
    [0132] Steel sheets having the chemical compositions shown in Table 1 (with the remainder including iron and unavoidable impurities) were prepared as blanks. The steel sheets were dimensionally 1.4 mm thick, 190.5 mm wide, and 400 mm long. The steel sheets were subjected to a hot press formation, such as the press formation according to the procedure illustrated in Figure 9. The hot press formation is henceforth also simply referred to as “press formation” or “ pressing ”. Table 1 also indicates Aça temperatures and Ms temperatures, as calculated by the expressions. The calculations according to the ex pressures for the Aça temperatures and for the Ms temperatures were carried out, at the same time defining the content of an element not contained in zero.
    [0133] Each sample in Experimental Example 1 was manufactured according to conditions as follows. In specific terms, the blank was heated to 900 ° C (heating temperature) for 6 minutes and subjected to hot press formation at an initial temperature of 800 ° C to 700 ° C. During the process of heating the steel sheet (blank) to the heating temperature, the steel sheet was heated in the range of 100 ° C to the heating temperature at an average temperature rise rate of about 10 ° C / s. The steel plate was cooled from the heating temperature down to [(Ms temperature) [(Ms temperature) - 150 ° C] at an average cooling rate of 10 to 30 ° C / s.
    [0134] Hot press formation was performed as press formation (in curvature (shape) using a main pad) as illustrated in Figure 1 using a press machine (400 ton mechanical press) to produce a steel element with hat-shaped channel illustrated in Figure 10. The main pad used, as a pressure source, a spring with a force of about 1 ton.
    [0135] Figure 1 illustrates the training process. Figure 1 illustrates a drill 1, a cutter 2, a main pad 3, a steel plate (blank) 4, and a pin (floating pin with integrated spring) 5.
    [0136] As shown in Figure 1 (a), the pins with integrated spring 5 are placed on the tools (cutter 2 and main pad 3), and the blank 4 removed from the heating oven is, in turn, placed on the pins 5. This is done in order to minimize the contact between the blank 4 and the tools (cutter 2 and main pad 3) before the start of press formation.
    [0137] Figure 1 (b) illustrates a state during formation, that is, a state during the pressing of drill 1. Figure 1 (c) illustrates a state in which drill 1 is pressed down and reaches the center lower dead (lower limit position).
    [0138] As in Table 2 below, Test N. 16 has been press-trained three times, and Tests Ns. 17 and 18 underwent press training four times.
    [0139] Tests Ns. 1 to 7 and 10 to 51 of Tables 2 and 3 were performed on the tools for a time (tool contact time, lower dead center retention time) from about 0.8 to about 7 seconds, as in Figure 9. Among these, the Ns Tests. 3, 4, and 19 in Table 2 were performed on the tools for a time of about 7 seconds.
    [0140] After forming in a hot press, cooling to room temperature was performed as illustrated in Figure 9. In specific terms, after forming in a press, Tests Ns. 1, 2, 11 to 14, 17, 18, and 20 to 51 of Tables 2 and 3 were cooled by forced wind cooling; while the Ns Tests. 10, 15, and 16 in Table 2 were cooled by natural cooling. After the hot press, Tests Ns. 5 to 7 of Table 2 were carried out in a waiting oven for 6 minutes and then naturally cooled. In contrast, Tests Ns. 3, 4, and 19 in Table 2 were naturally cooled after press formation. The Ns Tests. 8 and 9 of Table 2 were also naturally cooled after press formation.
    [0141] As illustrated in Figure 11, thermocouples were embedded in the positions corresponding to the central parts of the top and the vertical wall of the resulting steel element in order to measure the temperature history of the steel plate after the fabrication element was manufactured. steel. The temperatures measured at the two positions were approximately identical.
    [0142] A cooling time from [(Ms temperature) - 150 ° C] to 80 ° C was read outside the measured temperature history in order to calculate an average cooling rate and a quenching parameter (À) as indicated in Tables 2 and 3. In specific terms, the quenching parameter was calculated, at the same time defining a to 0 (second) and a To of [Ms-150 ° C] as starting points, and adjusting the measured cooling history from [(Ms temperature) - 150 ° C] to 80 ° C and the Si content. A final tool release temperature shown in Tables 2 and 3 was determined based on the temperatures indicated by the thermocouples and the tool positions at that time . In this Experimental Example, the final tool release temperature corresponds to the final temperature of the final hot press formation.
    [0143] The steel elements (formed elements) thus obtained were each subjected to a tensile test and productivity assessment using the methods as follows. The prepared steel elements included a single phase of martensite as a microstructure. Traction test
    [0144] From a portion of the formed part (steel element), a JIS No. 5 test sample was cut as a tensile test sample, as shown in Figure 12. The test sample was subjected to the test tensile strength according to the method prescribed in the JIS Z 2241 Standard using an AG-IS 250kN Autographic Traction Analyzer (Shimadzu Corporation) to measure yield strength (YS), tensile strength (TS), and elongation (EL ). The test was performed at a strain rate of 10 mm / min. Based on the data, the product (TS x EL; MPa *%) of the TS tensile strength and EL elongation were determined. Productivity Assessment
    [0145] Productivity was evaluated by the length of the tool holding time, and the tool holding is a rate determination step in the hot press formation process. A sample subjected to tool retention for 10 seconds or more was evaluated as being equivalent to conventional techniques and showing low productivity (x); while a sample submitted to tool retention for less than 10 seconds was evaluated as having good productivity (o).
    * Cada teste foi realizado a uma temperatura de aquecimento de 900°C e a uma temperatura inicial de formação de 800°C a 700°C. * Cada teste foi realizado em uma taxa média de elevação de temperatura de cerca de 10°C/s. na faixa de 100°C até a temperatura de aquecimento. Tabela 3 * Cada teste foi realizado a uma temperatura de aquecimento de 900°C e a uma temperatura inicial de formação de 800°C a 700°C. * Cada teste foi realizado em uma taxa média de elevação de temperatura de cerca de 10°C/s. na faixa de 100°C até a temperatura de aquecimento.[0146] The above results are shown in Tables 1.2 and 3. Table 1 * Each test was performed at a heating temperature of 900 ° C and an initial formation temperature of 800 ° C to 700 ° C. * Each test was performed at an average rate of temperature rise of around 10 ° C / s. in the range of 100 ° C to the heating temperature. Table 3 * Each test was performed at a heating temperature of 900 ° C and an initial formation temperature of 800 ° C to 700 ° C. * Each test was performed at an average rate of temperature rise of around 10 ° C / s. in the range of 100 ° C to the heating temperature. Tables 1 to 3 demonstrate as follows.
    [0147] Tests Ns. 1 to 4 have an insufficient Si content and have failed to obtain an excellent resistance-ductility balance. Test data Ns. 2 and 3 demonstrate that the samples, when having an insufficient Si content, fail to present a sufficiently high resistance - ductility balance, even at different average cooling rates in the range of [(Ms temperature) - 150 ° C] to 80 ° Ç. Test N. 4 presented an A parameter greater than the specific range and presented a remarkably weak resistance - ductility balance.
    [0148] Tests Ns. 5 to 7 subjected the cooling of [(Ms temperature) - 150 ° C] to 80 ° C performed so that the À parameter becomes larger than the specific range and presented a weak resistance - ductility balance.
    [0149] Tests Ns. 8 and 9 submitted the cooling of [(Ms temperature) - 150 ° C] to 80 ° C performed so that the parameter À becomes smaller than the specific range and produced a steel element with low productivity.
    [0150] The Tests Ns. 41,42, 47, and 48 submitted the cooling of [(Ms temperature) - 150 ° C] to 80 ° C performed so that the À parameter becomes greater than the specific range and presented a weak resistance - ductility balance.
    [0151] In contrast, The Tests Ns. 10 to 40, 43 to 46, and 49 to 51 employed steel sheets having the chemical compositions as specified in the present invention, subjected the method of fabricating steel elements as specified in the present invention, and produced steel elements with high resistance and excellent balance resistance - ductility with good productivity.
    [0152] Among them, Tests Ns. 10 to 14 used an identical blank, but submitted the cooling of [(Ms temperature) - 150 ° C] to 80 ° C in different parameters À. The resulting steel elements showed good resistance - ductility balance.
    [0153] Tests Ns. 15 to 18 used an identical blank, but subjected to press formation in different numbers of operations at different final tool release temperatures. A comparison between the data of these tests shows that the resulting steel element can present good levels of productivity and resistance - ductility balance when performing a formation in multiple stages at a low final tool release temperature. Experimental Example 2
    [0154] In Experimental Example 2, the sample steel plates were heated up to the heating temperature, having been examined how the average rate of temperature rise from 100 ° C to the heating temperature affects the properties of the resulting steel elements .
    [0155] In Experimental Example 2, each sample was prepared as a small flat plate having a thickness of 1.4 mm, a width of 180 mm, and a length of 50 mm, subjected to a heat treatment using an electric heating equipment , and evaluated.
    [0156] The steel plate of the blank “A” in Table 1 was heated to a heating temperature of up to 900 ° C. In this process, the steel sheet was heated from 100 ° C to heating temperature at different average rates of temperature rise, as given in Table 4 below. At the point of time when the sample temperature reached the heating temperature of 900 ° C, the power supply was interrupted, the sample was naturally cooled to 800 ° C, further cooled so that the average cooling rate of the tempera -heating time (900 ° C) to [(Ms temperature) - 150 ° C] was about 10 ° C / s, even cooler from [(Ms temperature) - 150 ° C] to 80 ° C so that the quenching parameter (À) becomes a value given in Table 4, and, in this way, produce a heat-treated sample. Conditions other than those mentioned above occurred according to Test No. 15 in Table 2.
    [0157] A JIS No. 5 test sample was sampled from the resulting hot-treated sample and subjected to a tensile test in order to determine YS, TS, EL, and TS x EL. The results of this process are shown in Table 4. Table 4
    [0158] Each test used the blank "A".
    [0159] Each test was carried out at a heating temperature of 900 ° C and an initial cooling temperature (corresponding to the initial formation temperature) of 800 ° C.
    [0160] Each test was performed at an average cooling rate of around 10 ° C in the heating temperature range below [(MS temperature) - 150 ° C],
    [0161] Table 4 demonstrates as follows. The steel sheet can have a greater TS x EL balance when heating the steel sheet to the heating temperature at a higher average rate of temperature rise in the temperature range of 100 ° C to the heating temperature. In particular, heating is preferably carried out at an average temperature rise rate of 50 ° C / s or more in order to allow the steel sheet to have a good TS x EL balance of 14000 MPa *% or more. Experimental Example 3
    [0162] In Experimental Example 3, the steel sheets were prepared by forming in a hot press and subjected to tempering. In this process, it was investigated how the tempering temperature affects the mechanical properties.
    [0163] The steel elements were prepared using the procedure of Test N. 16 (final tool release temperature: 380 ° C) in Table 2 of Experimental Example 1, except that the steel elements prepared by press formation hot were further quenched, as described below. Tempering was carried out so that the steel elements after forming in a hot press were heated to a tempering temperature given in Table 5 at an average temperature rise rate of 40 ° C / s, and cooled to water without being retained at quenching temperature.
    [0164] The resulting steel elements were subjected to tensile tests using the procedure of Experimental Example 1 in order to determine YS, TS, EL, and TS x EL. Their results are shown in Table 5. Each test sample had a tool retention time of less than 10 seconds in the hot press formation process and showed good productivity (o). Table 5
    [0165] Each test used the blank "A".
    [0166] Each test was performed at an average temperature rise rate of about 10 ° C / s in the range of 100 ° C above the heating temperature.
    [0167] Each test was performed at a heating temperature of 900 ° C and an initial formation temperature of 800 ° C to 700 ° C.
    [0168] Each test was performed at an average cooling rate of around 10 ° C / s in the heating temperature range down to [(MS temperature) - 150 ° C],
    [0169] Table 5 demonstrates as follows. A comparison of Test No. 58 with Test Nos. 59 to 62 shows that the samples, even when still subjected to tempering, can be controlled in resistance without presenting a lower TS x EL balance. The data from Test N. 63 shows that the quench is preferably carried out at a quench temperature less than 600 ° C in terms of the upper limit in order to provide a desired flow resistance (800 MPa or more). List of Reference Signs 1 - drill 2 - cutter 3 - main pad 4 - steel plate (blank) 5 - pin
    权利要求:
    Claims (6)
    [0001]
    1. Method of manufacturing a steel element formed in a hot press, the method CHARACTERIZED by the fact that it comprises the heating of a steel sheet and the submission of the steel sheet to hot press formation at least once, to steel sheet consisting of, as a chemical composition in percentage by mass (hereinafter the same for the chemical composition), C in a content of 0.15% to 0.4%; Si in a content of more than 1.0% to 1.65%; Al at a content of 0.5% or less (excluding 0%); Mn in a content of 1% to 3.5%; Ti in a content of 0.10% or less (excluding 0%); and B in a content of 0.005% or less (excluding 0%), optionally, at least one element selected from the group consisting of: (a) Cr in a content of 5% or less (excluding 0%); (b) at least one element selected from the group consisting of Ni and Cu in a total content of 0.5% or less (excluding 0%); (c) Mo in a content of 1% or less (excluding 0%); (d) Nb in a content of 0.1% or less (excluding 0%); (e) P in a content of 0.02% or less; (f) S with a content of 0.02% or less; and (g) N in a content of 0.01% or less, with the remainder being iron and unavoidable impurities, where: a temperature (heating temperature) for heating is equal to or greater than the transformation temperature Acs, a initial hot press formation temperature is in a range from the heating temperature to the initial martensite formation temperature (Ms), and the cooling from [(Ms temperature - 150 ° C] to 80 ° C is performed at an average cooling rate of 20 ° C / s or less so that a tempering parameter (À) as specified by Expression (1) is in the range of 7100 to 8030, Expression (1) being represented as follows: Expression 1:
    [0002]
    2. Manufacturing method, according to claim 1, CHARACTERIZED by the fact that the cooling from [(Ms temperature) - 150 ° C] to 80 ° C is carried out at an average cooling rate of 5 ° C / at 20 ° C / s.
    [0003]
    3. Manufacturing method according to claim 1, CHARACTERIZED by the fact that a final temperature of the final hot press formation at least once of the hot press formation is equal to or less than the temperature Ms.
    [0004]
    4. Manufacturing method, according to claim 1, CHARACTERIZED by the fact that the heating of the steel sheet to the heating temperature is carried out at an average temperature rise rate of 5 ° C / s or more in a range from 100 ° C to heating temperature.
    [0005]
    5. Manufacturing method, according to claim 1, CHARACTERIZED by the fact that it comprises quenching at a temperature of 100 ° C to less than 600 ° C after forming in a hot press.
    [0006]
    6. Method of manufacturing an automotive steel part, CHARACTERIZED by the fact that it comprises: manufacturing a steel element formed in a hot press, as defined in any one of claims 1 to 5, and working the steel element formed in a press the hot.
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    同族专利:
    公开号 | 公开日
    JP5595609B2|2014-09-24|
    EP2946848A1|2015-11-25|
    ES2683149T3|2018-09-25|
    CN104936716B|2016-09-07|
    US20150354035A1|2015-12-10|
    BR112015016863A2|2017-07-11|
    WO2014112594A1|2014-07-24|
    MX368319B|2019-09-27|
    EP2946848A4|2016-08-31|
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    KR20150095921A|2015-08-21|
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    CN104936716A|2015-09-23|
    JP2014156653A|2014-08-28|
    BR112015016863A8|2019-10-29|
    KR101640358B1|2016-07-15|
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    法律状态:
    2018-11-13| B06F| Objections, documents and/or translations needed after an examination request according art. 34 industrial property law|
    2019-12-03| B06U| Preliminary requirement: requests with searches performed by other patent offices: suspension of the patent application procedure|
    2020-07-07| B09A| Decision: intention to grant|
    2020-09-01| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 17/01/2014, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2013007808|2013-01-18|
    JP2013-007808|2013-01-18|
    PCT/JP2014/050833|WO2014112594A1|2013-01-18|2014-01-17|Manufacturing method for hot press formed steel member|
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