HOT-LAMINATED STEEL SHEET AND ITS PRODUCTION METHOD

专利摘要:
abstract patent of invention: "hot rolled steel plate and its production method". a steel plate in a composition containing, in mass%: c: 0.01 to 0.2 ?; si: 2.5? or less; mn: 4.0? or less; p: 0.10? or less; s: 0.03? or less; al: 0.001 to 2.0 ?; n: 0.01? or less; and o: 0.01? or less, and one element or a total of two elements between ti and nb in 0.01 to 0.30 ?. the average effective diameter of the crystal grain and 1/4 of the plate thickness is 10 µm or less, and the average effective diameter of the crystal grain in a part in the range of 50 µm from the surface is 6 µm or less . the structure of the steel sheet is tempered martensite or lower bainite, and its low volume is 90% or more in total.
公开号:BR112015021149B1
申请号:R112015021149-6
申请日:2014-05-16
公开日:2020-03-10
发明作者:Hiroshi Shuto；Masafumi Azuma；Akifumi SAKAKIBARA；Yuuki Kanzawa；Ken Kimura
申请人:Nippon Steel Corporation；
IPC主号:

专利说明:

Descriptive Report of the Invention Patent for "HOT LAMINATED STEEL SHEET AND ITS PRODUCTION METHOD".
Technical field [0001] The present invention relates to a hot-rolled sheet having a maximum tensile strength of 980 MPa or more, excellent in fatigue strength and low temperature toughness, and a method for its production. In particular, the present invention relates to a hot-rolled steel sheet that supports use in an element where stresses are repeatedly loaded, and includes fatigue strength and low temperature toughness allowing use in a cryogenic region, and to a method of its production.
Background of the technique [0002] Reducing the weight of a vehicle chassis has advanced with the use of a high-strength steel plate to suppress the emission of carbon dioxide gas from a vehicle. In addition, many high-strength steel plates with a maximum tensile strength of 980 MPa or more have been used for the vehicle chassis in addition to the mild steel plate to ensure passenger safety. In addition, it is necessary to increase the level of resistance to the use of high-strength steel plate more than before in order to advance the weight reduction of the vehicle chassis. The high reinforcement of the steel sheet is generally accompanied by the deterioration of the material properties such as low temperature toughness and, therefore, it is important in the development of the high strength steel sheet as to allow high reinforcement without deteriorating the material properties.
[0003] The fatigue strength of the steel sheet is a necessary property for important safety-related parts centered on the steel sheets for parts under the vehicle chassis. The reduction of the weight of the parts cannot be allowed if the fatigue resistance is not improved together with the high reinforcement of the steel plate. Consequently, fatigue properties are defined not only by resistance, but also by a fatigue limit in which the fatigue fracture does not occur due to less stress than that, and a fatigue resistance in the useful life being the fatigue life when a certain stress or tension is loaded.
[0004] In addition, there is a requirement for the steel plate used for the element as stated above that the element is difficult to fracture even if it is impacted by a collision or similar after the steel plate is conformed as the element and be attached to the vehicle. In addition, there is also a need to improve toughness at low temperature to ensure resistance to impact in a cold place. This tenacity at low temperature is defined by vTrs (transition temperature of the appearance of the Charpy fracture) or similar. Therefore, it is necessary to consider the impact resistance on the steel sheet itself. In addition, the plastic deformation of the steel sheet becomes difficult due to the high reinforcement of the steel sheet and therefore the anxiety for fracture becomes greater. Consequently, tenacity is necessary as an important property.
[0005] It is effective to refine a structure to improve fatigue properties. For example, In Patent Literature 1 and Patent Literature 2, a hot rolled steel sheet is described in which the average diameter of ferrite grain is adjusted to be 2 pm or less, and the balance between strength and strength ductility and the fatigue limit ratio (fatigue strength / TS) are good. However, these steel sheets have a structure whose main phase is ferrite, and it is difficult to guarantee resistance of 980 MPa or more.
[0006] In addition, fatigue fractures occur from the vicinity of a surface and therefore it is particularly important to refine the structure in the vicinity of the surface. For example, in Patent Literature 3, a hot-rolled steel sheet is described in which its main phase is polygonal ferrite, the average diameter of the crystal grain of the polygonal ferrite gradually becomes smaller from the center of the sheet thickness towards the surface layer to be an inclined crystal grain structure. This hot-rolled steel sheet is one in which the polygonal ferrite fraction is gradually refined from the central part of the sheet thickness towards the part of the surface layer of the sheet thickness by the application of folding after hot rolling.
[0007] In addition, in Patent Literature 4, a hot-rolled steel sheet is described in which polygonal ferrite is the main phase, and the diameter of the crystal grain in the vicinity of the surface layer is adjusted to be 20% or less the grain diameter in the central part of the plate thickness. This hot-rolled steel sheet is one in which the lamination is carried out in a ferrite region, the surface layer is transformed inversely by a process of heat generation at the time of the hot rolling to thus refine the structure in the surface layer.
[0008] These hot-rolled steel sheets are excellent in fatigue properties, but their production method is complicated, the preferred rolling conditions are narrow and, therefore, the decrease in productivity and yield is worrying. In addition, the main phase is ferrite, and therefore it is difficult to guarantee strength of 980 MPa or more.
[0009] On the other hand, a martensite structure is extremely hard, and therefore on a steel plate having a resistance grade of 980 MPa or more it is often the case that the martensite structure is used as the main phase or as a second phase for reinforcement. In Patent Literature 5, the improvement of fatigue properties by refining the grain in a structure in which the martensite structure is the main phase is described. Note that this is a technique in the field of steel tubes, and it is one in which the diameter is reduced after tube production, and the average diameter of the martensite block is adjusted to be 3 gm or less by rapid cooling after the heating. It is necessary to install heating and cooling equipment after finishing laminating to apply the similar method to hot rolled steel plate to improve fatigue strength, and there is the problem that a large investment is required.
[00010] On the other hand, for example, in Patent Literature 6, a production method is described in which the martensite phase, whose aspect ratio is adjusted, is adjusted to be the main phase of a steel sheet structure , as a method to improve the toughness of the steel sheet.
[00011] In general, it is known that the aspect ratio of martensite depends on the aspect ratio of an austenite grain before processing. Here, the martensite whose aspect ratio is large means the martensite transformed from non-recrystallized austenite (austenite that is extended by lamination). In addition, martensite whose aspect ratio is small means the transformed martensite from recrystallized austenite.
[00012] Therefore, it is necessary that the steel sheet according to Patent Literature 6 recrystallize austenite to reduce the aspect ratio. In addition, it is necessary to increase the temperature of the finishing laminate to allow recrystallization of the austenite. Consequently, there is a tendency in which the grain diameter of austenite, in addition to the grain diameter of martensite, becomes large. In general, grain refining is known to be effective in improving toughness. Therefore, when the aspect ratio is decreased, it is possible to reduce a deterioration factor of the toughness resulting from the shape, but it is accompanied by the deterioration of toughness by the hardening of the crystal grain, and therefore there is a limit to the improvement of toughness. In addition, it is not mentioned as fatigue resistance, and it is difficult to say that sufficient fatigue resistance is guaranteed.
List of citations Patent Literature [00013] Patent Literature 1: Japanese Patent Publication Open for Public Inspection No. H11-92859 [00014] Patent Patent Literature 2: Japanese Patent Publication Open for Public Inspection No. H11-152544 [ 00015] Patent Literature 3: Japanese Patent Publication Open for Public Inspection No. 2004-211199 [00016] Patent Literature 4: Japanese Patent Publication Open for Public Inspection No. 2007-162076 [00017] Patent Literature 5: Publication Japanese Patent Application Open for Public Inspection No. 2010-70789 [00018] Patent Literature 6: Japanese Patent Publication Open for Public Inspection No. 2011-52321 [00019] Patent Literature 7: Japanese Patent Publication Open for Public Inspection n ° 2011-17044 [00020] Patent Literature 8: Japanese Patent Publication open to public inspection n ° 2012-62561 [00021] Patent Literature 9: Japanese Patent Publication nesa open to public inspection n ° 2009-52106 [00022] Patent Literature 10: Japanese Patent Publication open to public inspection n ° 2008-285748 [00023] Patent Literature 11: Japanese Patent Publication open to public inspection n ° 2008 -255484 [00024] Patent Literature 12: Japanese Patent Publication Open to Public Inspection No. 2005-200673 Summary of the invention Technical problem [00025] The present invention is made in consideration of the problems shown above, and its purpose is to provide a plate hot-rolled steel having a maximum tensile strength of 980 MPa or more, excellent fatigue strength and low temperature toughness, and its production method.
Solution to the problem [00026] The present inventors were successful in producing a hot-rolled steel sheet that was excellent in fatigue strength and low temperature toughness and had a maximum tensile strength of 980 MPa or more by optimizing a composition of the hot-rolled steel sheet and its production conditions, and by controlling the structure of the hot-rolled steel sheet. Their summaries are as follows. (1) A hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more, excellent in fatigue strength and low temperature toughness, includes a composition containing, in mass% C: 0.01 to 0 ,2%;
Si: 2.5% or less (excluding “0” (zero));
Mn: 4.0% or less (excluding “0” (zero)); P: 0.10% or less; S: 0.03% or less;
Al: 0.001 to 2.0%; N: 0.01% or less (excluding “0” (zero)); O: 0.01% or less (excluding “0” (zero)); and or one or both between Ti and Nb for a total of 0.01 to 0.30%; and the balance being made of iron and impurities, and a structure in which the volume fraction of any or the total tempering martensite and lower bainite is 90% or more, where the average diameter of the effective crystal grain in a part of a range of 1/4 the thickness of the plate from the surface is 10 gm or less, and the average diameter of the effective crystal grain in a part of a range of 50 gm from the surface is 6 gm or less. (2) The hot-rolled steel plate according to item (1), where iron-based carbides that exist in one or both of the tempered martensite and the lower bainite are 1 χ 106 (pieces / mm2) or more, and the average aspect ratio of the effective crystal grains of one or both temperature martensite and lower bainite is two or less. (3) The high-strength hot-rolled steel plate according to item (1) or (2), also contains, in mass%, one element or two or more elements selected from a group made of: Cu: 0, 01 to 2.0%; Ni: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.3%; and Cr: 0.01 to 2.0%. (4) The hot-rolled steel plate according to any of the items (1) to (3), also contains, in mass%, one element or two or more elements selected from the group made of: Mg: 0.0005 a 0.01%; Ca: 0.0005 to 0.01%; and REM: 0.0005 to 0.1%. (5) The hot-rolled steel sheet according to any of items (1) to (4), also contains, in mass%: B: 0.0002 to 0.01%. (6) The hot-rolled steel sheet according to any of items (1) to (5), where a galvanized layer or a bonded galvanized layer is included on a surface of the steel sheet. (7) The method of producing a hot-rolled steel sheet includes: melting a composition containing, in mass%, C: 0.01 to 0.2%;
Si: 2.5% or less (excluding “0” (zero));
Mn: 4.0% or less (excluding “0” (zero)); P: 0.10% or less; S: 0.03% or less;
Al: 0.001 to 2.0%; N: 0.01% or less (excluding “0” (zero)); O: 0.01% or less (excluding “0” (zero)); and either or both between Ti and Nb from 0.01 to 0.30%, and the balance made of iron and impurities and, subsequently, heat a cast plate to 1200 ° C or more directly or after it has been cooled once, completing the hot lamination under the condition that satisfies R / (100 - r)> 8, and at 900 ° C or more when the diameter of the lamination cylinder is set to be R / mm, and the reduction ratio is adjusted to be r%, in the final stage of the finishing lamination; cool to an average cooling rate of 60 ° C / s or more from the finishing laminate temperature to 700 ° C, to an average cooling rate of 50 ° C / s or more than 700 ° C to 400 ° C, and at an average cooling rate of less than 50 ° C from 400 ° C to room temperature, and winding at a temperature of less than 400 ° C. (8) The method of production of the hot-rolled steel sheet according to item (7), where the cast plate also contains, in mass%, one element or two or more elements selected from a group made of: Cu: 0 , 01 to 2.0%;
Ni: 0.01 to 2.0%;
Mo: 0.01 to 1.0%; V: 0.01 to 0.3%; and Cr: 0.01 to 2.0%. (9) The method of producing the hot-rolled steel sheet according to item (7) or (8), where the cast plate also contains, in mass%, one element or two or more elements selected from the group consisting of: Mg: 0.0005 to 0.01%;
Ca: 0.0005 to 0.01%; and REM: 0.0005 to 0.1%. (10) The method of producing the hot-rolled steel sheet according to any of items (7) to (9), where the cast plate also contains, in mass%, B: 0.0002 to 0.01%. (11) The method of producing the hot-rolled steel sheet according to any of items (7) to (10) also includes: performing a galvanizing treatment or a bonded galvanizing treatment after winding.
Advantageous effects of the invention [00027] According to the present invention it is possible to provide a hot-rolled steel sheet which is excellent in fatigue strength and low temperature toughness, has a maximum tensile strength of 980 MPa or more, and its production method.
Description of configurations [00028] Hereinafter, the content of the present invention will be described in detail.
[00029] As a result of arduous studies by the present inventors or the like, it has been found that a high strength of 980 MPa or more, a high fatigue strength and low temperature toughness can be guaranteed by adjusting the structure of the rolled steel sheet to hot to be a structure in which the average diameter of the effective crystal grain in a part of a range of 1/4 of the plate thickness from the surface is 10 gm or less. The average diameter of the effective crystal grain in a part in the range of 50 gm from the surface is 6 gm or less, and one or both of tempered martensite and lower bainite are contained in total in 90% or more in fraction of volume, it is also preferably set to be one in which the average aspect ratio of tempered martensite and lower bainite is adjusted to be two or less, and the iron-based carbides in tempered martensite and lower bainite are contained to 1 χ 106 (pieces / mm2).
[00030] Here, the effective crystal grain diameter is a region surrounded by the grain edge of a disorientation of 15 ° or more, and is capable of being measured using an EBSD or similar. Its details are described later.
Microstructure of the hot-rolled steel sheet [00031] Initially, the microstructure of a hot-rolled steel sheet of the present invention is described.
[00032] In the hot rolled steel sheet of the present invention, the main phase is adjusted to be either one or both of tempered martensite and lower bainite, and the total volume fraction is adjusted to be 90% or more, and so maximum tensile strength of 980 MPa or more and high fatigue strength are guaranteed. Consequently, it is necessary for the main phase to be adjusted to be one or both of tempered martensite and lower bainite.
[00033] The tempered martensite in the present invention is the most important microstructure for hot-rolled steel sheet to have strength, fatigue strength, and low temperature toughness. Tempered martensite is an aggregation of crystal grains in the form of a blade, and contains the iron-based carbide within which the largest axis is 5 nm or more. In addition, iron-based carbide is one that belongs to a plurality of variants, i.e., a plurality of groups of iron-based carbides that extend in different directions.
[00034] The structure of tempered martensite is obtainable when the cooling rate at a cooling time of one point Ms (temperature of initiation of the transformation of martensite) or less is decreased, and when it is tempered at 100 ° C at 600 ° C after it was once made a martensite structure. In the present invention, precipitation is controlled by a cooling control between 400 ° C and room temperature.
[00035] The lower bainite is also the aggregation of crystal grains in the form of a blade, and contains within itself the iron-based carbide whose major axis is 5 nm or more. Furthermore, the iron-based carbide is one that belongs to a unique variant, that is, to a group of iron-based carbides that extends in the same direction. It can be easily distinguished whether it is tempered martensite or lower bainite by observing the direction of extension of the iron-based carbide. Here, the group of carbides based on iron that extends in the same direction means that whose difference in the direction of extension of the group of iron carbides is up to 5 °.
[00036] The lower bainite is generated from 400 ° C up to around the Ms point, and there is a case where it is generated at the Ms point or less while it is contradictory to the martensite. When the cooling rate at 400 ° C or less is relatively large, martensite is formed, and when it is small, lower bainite is formed.
[00037] When the volume fraction of either or the total of both tempered martensite and lower bainite is less than 90%, the maximum tensile strength of 980 MPa or more, which is an essential point of the present invention, does not can be guaranteed, and the high fatigue strength, which is the effect of the present invention, cannot be achieved. Consequently, its lower limit is 90%. On the other hand, when the volume fraction is adjusted to be 100%, strength, high fatigue strength, and excellent low temperature toughness, which are the effects of the present invention, are exerted. Note that the bottom bainite is desirably 5% or more, and also desirably 9%. The volume fraction of the lower bainite increases, so there is a tendency for the effective average diameter of the crystal grain in a part 1/4 of the thickness of the plate to become small. [00038] In the hot-rolled steel plate structure, one or two or more types can be contained by 10% or less in the fraction of total volume of ferrite, new martensite, upper bainite, perlite, retained austenite, like other structures .
[00039] Here, new martensite is defined as martensite that does not contain carbide. New martensite is highly resistant, but deteriorates in toughness at low temperature and, therefore, it is necessary to limit the volume fraction to be 10% or less.
[00040] The retained austenite becomes the new martensite when the steel material is plastically deformed at a press forming time or when the vehicle element is plastically deformed at the time of a collision, and therefore there is a similar negative effect as new martensite mentioned above. Therefore, the volume fraction needs to be limited to 10% or less. [00041] The upper bainite is an aggregation of blade-shaped crystal grains, and is an aggregation of carbides containing blades between the blades. The carbides contained between the blades become the starting point for fractures, and therefore the low temperature toughness is reduced. In addition, the upper bainite is formed at a high temperature compared to the lower bainite, and therefore it has low strength. Consequently, when the upper bainite is formed excessively, it is difficult to guarantee the maximum tensile strength of 980 MPa or more, and the fatigue strength is decreased. This trend becomes noticeable when the volume fraction of the upper bainite exceeds 10%, and therefore it is necessary to limit the volume fraction to be 10% or less.
[00042] Ferrite has a solid crystal grain, and means a structure in which a substructure such as a blade is not contained within. Ferrite is a softer structure, and it decreases tensile strength and fatigue strength, and therefore it is necessary to limit it to 10% or less to ensure maximum tensile strength of 980 MPa or more, and high fatigue resistance. In addition, ferrite is extremely soft compared to either or both between tempered martensite and lower bainite which are the main phases, and therefore the deformation is concentrated at the interfaces of both structures, and they can easily be the points starting point of fractures. As stated above, ferrite decreases toughness at low temperature. This trend becomes noticeable when the volume fraction exceeds 10% and, therefore, it is necessary to limit the volume fraction to be 10% or less.
[00043] Perlite also causes a decrease in tensile strength and fatigue strength, and deteriorates tenacity at low temperature in the same way as ferrite, and therefore it is necessary to limit the volume fraction to be 10% or less .
[00044] Tempered martensite, new martensite, upper bainite, lower bainite, ferrite, perlite, austenite and the remaining structure that constitute the steel sheet of the present invention defined above are liable to be performed to identify these structures, check existing positions, and measure area ratios according to the methods described below. That is, the cross section in a rolling direction or the cross section in a direction orthogonal to the rolling direction of a steel sheet is corroded using a nital reagent and a reagent described in Patent Publication open to public inspection No. S59- 219473 to be observed with a scanning microscope and electronic transmission at a magnification of 1000 times to 100000 times, and thus it is possible to identify these structures, check the existing positions, and measure the area ratios.
[00045] In addition, it is also possible to distinguish the structure from a crystal orientation analysis using a crystal orientation analysis method using a FESEM-EBSP method [backscattered electron diffraction (EBSD) linked to field emission scanning electron microscope (FE-SEM)], and a hardness measurement of a minimal region such as the micro Vickers hardness measurement. For example, culm defined above, tempered martensite, upper bainite, and lower bainite have different formation sites and crystal orientation relationships (extension directions) for carbide, and therefore, iron-based carbide in the grain. blade-shaped crystal is observed using a FE-SEM, its direction of extension is examined, and thus it is possible to easily distinguish the upper bainite, the lower bainite, and the tempered martensite. [00046] In the present invention, each of the volume fractions of ferrite, perlite, upper bainite, lower bainite, tempered martensite, and new martensite is obtained by accumulating a sample while using a sheet thickness cross section that is parallel the rolling direction of the hot-rolled steel sheet as an observation surface, and etching with nital is carried out. A range of 1/8 of the thickness to 3/8 of the thickness centering in a position at a depth of 1/4 from the surface of a sheet thickness is observed by an FE-SEM to measure the area fraction, and the result is used as the volume fraction. In addition, 10 visual fields are each measured at a magnification of 5000 times, and the average value is adjusted to be the area ratio.
[00047] New martensite and retained austenite are not corroded sufficiently by caustication with nital, and therefore it is possible to clearly distinguish from the structures defined above (ferrite, bainite ferrite, upper bainite, lower bainite, and tempered martensite) in observation by FE-SEM. Consequently, the volume fraction of the new martensite is capable of being discovered as the difference between the non-corroded region observed by FE-SEM and the fraction of area of the austenite retained measured by X-ray.
[00048] Subsequently, the diameter of the crystal grain of the hot-rolled steel sheet is described.
[00049] It is necessary that the effective average diameter of the crystal grain in a part in the range of 1/4 of the thickness of the sheet from the surface of a hot-rolled steel sheet (hereinafter, that part is referred to as “ the 1/4 part of the plate thickness ”) is adjusted to be 10 pm or less, and the effective average diameter of the crystal grain in a part of a 50 pm range from the surface is 6 pm or less to allow the improvement in fatigue resistance. This is to suppress the occurrence of fatigue fractures from the surface layer. The effective average diameter of the crystal grain in the surface layer is specially refined. The present inventors have examined the relationship between fatigue strength and the effective average diameter of the crystal grain in the part 1/4 of the thickness of the plate and in the part in the range of 50 pm from the surface, so it becomes obvious that an excellent Fatigue strength can be obtained when they are 10 pm or less in the part at 1/4 of the thickness of the sheet, and 6 pm or less in the part of the 50 pm range from the surface. The average effective grain diameter is desirably 5 gm or less in the part of the 50 gm strip from the surface, and also desirably 4 gm or less in the part of the 50 gm strip from the surface.
[00050] Here is described a method of identifying the effective average diameter of the crystal grain. In the present invention, the effective average diameter of the crystal grain is defined using the EBSP-OIM method (backscattered electron diffraction pattern - image orientation microscopy). In the EBSO-OIM method, equipment and software are constituted in which an electron beam is irradiated in a sample that tilts highly in the scanning electron microscope (SEM), the Kikuchi pattern formed by backscattering is photographed by a highly sensitive camera , your image is processed by a computer to measure the crystal orientation at an irradiation point in a short period of time. According to the EBSP-OIM method, it is possible to quantitatively analyze a microstructure and the crystal orientation on a surface of a crude sample. In addition, the area of analysis of the EBSP-OIM method is an area that can be observed by the SEM, and it is possible to analyze at a resolution of at least 20 nm according to the EBSP-OIM method although it depends on the SEM resolution. In the present invention, the disorientation of the crystal grains is defined to be 15 ° with the minimum value of a high-angle grain border that is generally recognized as the crystal grain border, a grain is visualized from the mapped image, and the effective average diameter of the crystal grain is discovered.
[00051] When the effective average diameters of the crystal grains are measured in the part a1 / 4 of the plate thickness and in the part in the range of 50 gm from the surface, 10 visual fields are measured centering in each position at a magnification of 1200 times, and the average effective diameter of the crystal grain is adjusted to be the average effective diameter of the crystal grain.
[00052] In addition, it is preferable that the iron-based carbide is contained in the structure to 1 χ 106 (pieces / mm2) or more in each between the tempered martensite and the lower bainite.
[00053] The reason for containing iron-based carbide to 1 χ 106 (pieces / mm2) or more is to increase the low temperature toughness of a main phase, and to achieve a balance between excellent strength and low temperature toughness. . That is, tempered martensite is excellent in strength, but fails in toughness, and its improvement is necessary. Consequently, the iron-based carbide is precipitated by a certain number or more in order to improve the toughness of the main phase.
[00054] The present inventors have examined the relationship between low temperature toughness and the numerical density of iron-based carbide, and then it became clear that it is possible to guarantee excellent low temperature toughness by adjusting the numerical density of carbide-based iron in tempered martensite and lower bainite to 1 χ 106 (pieces / mm2) or more. Therefore, the numerical density of the iron-based carbide is desirably adjusted to be 1 χ 106 (parts / mm2) or more. It is more desirably 5 χ 106 (pieces / mm2) or more, and still desirably 1 χ 107 (pieces / mm2) or more.
[00055] In addition, the size of the iron-based carbide precipitated by the process of the present invention is small on the order of 300 nm or less, and most of them are precipitated on the blades of martensite and bainite, and therefore are estimated ensure that the low temperature toughness is not deteriorated.
[00056] As a method to measure the numerical density of iron-based carbide, initially a sample is accumulated while using the cross section of the plate thickness that is parallel to the rolling direction of the hot-rolled steel plate as the observation surface . The observation surface of the sample is polished, etched with nital, and the range from 1/8 of the thickness to 3/8 of the thickness centered in the position at a depth of 1/4 of the thickness from the surface of the steel plate is observed by the FE-SEM to measure the numerical density of the iron-based carbide. At that time, 10 visual fields are observed individually at a magnification of 5000 times to measure the numerical density of the iron-based carbide.
[00057] In addition, it is desirable that the average effective aspect ratio of the tempered martensite and lower bainite crystal grains (here, means the region surrounded by the grain edge of 15 ° or more) is adjusted to be two or less . The crystal grain that is flat in a specific direction has a large anisotropy, and there is often the case that fractures propagate along the grain edge at the time of the Charpy test to decrease the toughness value. Consequently, it is effective for effective crystal grains to be made to be as equiaxial grains as possible. In the present invention, the cross section in the rolling direction of the hot-rolled steel sheet is observed, the ratio between the length (L) in the rolling direction and the length (T) in the direction of the sheet thickness (= L / T ) is defined as the aspect ratio, and its average value (that is, the average aspect ratio) is adjusted to be two or more.
Chemical composition of hot-rolled steel sheet [00058] The following are the reasons for the limitations of the chemical composition of hot-rolled steel sheets of the present invention. Note that "%" of a grade is a mass%.
[00059] C is an element that contributes to increase the strength and improve the fatigue strength of a base material, but it is also the element that generates iron-based carbides such as cementite (Fe3C) to be starting points for fractures at the time of bore expansion, and deteriorates toughness at low temperature. It is impossible to obtain an effect of improving the resistance due to the reinforcement of the structure by a phase of generation of transformation at low temperature when the C content is less than 0.01%. On the other hand, when the C content is above 0.2%, the ductility of the steel sheet decreases, the iron-based carbides such as cementite (Fe3C) increase to be the starting points for fractures when applied. impact, and low temperature toughness deteriorates. Consequently, the C content is adjusted to be within the range of 0.01 to 0.2%. (Si: 2.5% or less (excluding “0” (zero)) [00060] Si is an element that contributes to increase the strength of the base material, and can be used as a deoxidizing material for molten steel and, therefore, it is preferably contained within a range of 0.001% or more as needed, however, when the content exceeds 2.5%, the contribution to saturation resistance, and in addition the formation of ferrite is accelerated due to Si. As a result, it becomes difficult to adjust the volume fraction of either or the total of both between tempered martensite and lower bainite to be 90% or more, and the resistance and tenacity at low temperature deteriorate. Si is adjusted to be 2.5% or less. (Mn: 4% or less (excluding “0” (zero)) [00061] Mn is contained to adjust one or both of the tempered martensite and the lower bainite as the main phase in the steel plate structure by reinforcing the cooling in addition to the reinforcement of the solid solution.When the Mn content is above 4%, it becomes difficult to adjust the density of the iron-based carbides of either or both between the tempered martensite and the lower bainite to be 1 χ 106 (pieces / mm2) or more. In addition, in effect of the strengthening of the cooling it also saturates. Consequently, the Mn content is adjusted to be 4% or less. On the other hand, when the Mn content is less than 1%, it is difficult to exert the effect of suppressing the transformation of ferrite and the transformation of bainite during cooling. Consequently, the Mn content is desirably 1% or more. (Ti, Nb: 0.01 to 0.30% each or the total of both) [00062] Either or both between Ti and Nb are the most important elements to allow both excellent low temperature toughness and high strength 980 MPa or more. These carbonitrides, or either or both of the solid solutions of Ti and Nb, delay the growth of the grain in hot rolling, and thus it is possible to refine the grain diameter of the hot rolled steel sheet, and contribute to the improvement of toughness at low temperature. Among them, Ti is especially important because it contributes to the improvement of low temperature toughness by refining the diameter of the crystal grain at the time of heating the plate by the existence as TiN in addition to the grain growth property due to the solid solution of N It is necessary that one or both between Ti and Nb must be contained by 0.01% or more to make the average effective diameter of the crystal grain of the hot rolled steel sheet to be 10 pm or less. In addition, when the content of one or the total of both between Ti and Nb exceeds 0.30%, austenite is difficult to be recrystallized at normal lamination temperature, and thus the toughness is deteriorated. Consequently, the range of the content of one or the total of both between Ti and Nb is adjusted to be 0.01 to 0.30%. A more desirable range is 0.02 to 0.25%. (P: 0.10% or less) [00063] P is an element contained in cast iron, and it is the element that secretes at the edges of the grains, and decreases the toughness at low temperature according to the increase in the content. Consequently, it is desirable that the P content is lower, and is adjusted to be 0.10% or less because when it is contained above 0.10%, it adversely affects the processing capacity and the welding capacity. In particular, the P content is desirably 0.03% or less when the weldability is considered. (S: 0.03% or less) [00064] S is an element contained in cast iron, and it is an element that generates an inclusion such as MnS in which not only fractures occur when hot rolling but also the toughness to low temperature is deteriorated when its content is very high. Consequently, the S content should be reduced as much as possible, but 0.03% or less is an acceptable range, and therefore it is adjusted to 0.03% or less. Note that the S content, when the hole expansion property by some extent is required, is desirably 0.01% or less, more desirably 0.005% or less. (Al: 0.001 to 2.0%) [00065] Al suppresses the formation of crude cementite, and improves toughness at low temperature. In addition, it can also be used as a deoxidizing material. However, when it is excessively contained, the formation of ferrite is accelerated, and therefore it is difficult to adjust the volume fraction of each or the total of both between tempered martensite and lower bainite to be 90% or more. Therefore. the Al content is adjusted to be 2.0% or less. The content of Al is desirably 1.5% or less. It is difficult to adjust the Al content to be less than 0.001%, and therefore this value is adjusted to be the lower limit (N: 0.01% or less (excluding “0” (zero)) [00066] N improves the resistance, however, when it is excessively contained, the fatigue resistance is decreased accompanied by an increase in nitrogen-based inclusion such as TiN, AlN, etc., to be starting points for fatigue fractures. the N content to be 0.01% or less. On the other hand, it is not economically desirable to adjust the N content to be less than 0.0005%. Therefore, it is desirable to adjust the N content to be 0.0005% or more. (O: 0.01% or less (excluding “0” (zero)) [00067] O generates oxide, and deteriorates the forming capacity, and therefore it is necessary to suppress its content. In particular, when the content O exceeds 0.01%, this trend becomes noticeable. Consequently, it is necessary to adjust the O content to be 0.01% or less. On the other hand, it is not economically desirable to adjust the O content to be less than 0.001%, and therefore it is desirable to adjust the O content to be 0.001% or more.
[00068] Here above is a basic chemical composition of the hot rolled steel sheet of the present invention, but it can also contain compositions as shown below.
[00069] (One element or two or more elements selected from a group made of Cu, Ni, Mo, V, Cr) (One element or two or more elements selected from a group made of Cu, Ni, Mo, V, Cr ) [00070] Cu, Ni, Mo, V, Cr suppress the transformation of ferrite at the time of cooling, and an element or two or more elements selected from this group may be contained because the main phase of the structure of the hot-rolled steel sheet is adjusted to be one or both of tempered martensite and lower bainite. On the other hand, they are elements that have an effect on improving the strength of the hot-rolled steel sheet by reinforcing precipitation, and therefore an element or two or more elements selected from this group may be contained. However, when the content of each of Cu, Ni, Mo, V, Cu is less than 0.01%, the effect cannot be achieved sufficiently. Furthermore, when the Cu content is above 2.0%, the Ni content is above 2.0%, the Mo content is above 1.0%, the V content is above 0.3%, the Cr content is above 2.0%, the effect shown above saturates and the economic efficiency is reduced. Consequently, when Cu, Ni, Mo, V, Cr are contained as needed, it is desirable that the Cu content is 0.01 to 2.0%, the Ni content is 0.01 to 2.0%, the content of Mo is 0.01 to 1.0%, the V content is 0.01 to 0.3%, and the Cr content is 0.01 to 2.0%. (One element or two or more elements selected from the group made of Mg, Ca, and REM) [00071] Mg, Ca, and REM (rare earth element) are elements that control modes of non-metallic inclusions to be starting points of fractures, and a cause of deterioration in processing capacity, and improvement in processing capacity. Consequently, one element or two or more elements selected from that group can be contained. Mg, Ca and REM contents are each adjusted to be 0.0005% or more because the effect becomes noticeable when it is 0.0005% or more. In addition, the effects mentioned above saturate and economic efficiency is decreased when the Mg content is above 0.01%, the Ca content is above 0.01%, and the REM content is above 0.1 %. Consequently, it is desirable for the Mg content to be adjusted to be 0.0005 to 0.01%, the Ca content to be adjusted to be 0.0005 to 0.01%, and the REM content to be adjusted to be 0, 0005 at 0.1%. (B: 0.0002 to 0.01%) [00072] B contributes to adjust the main phase of the steel sheet structure to be one or both of tempered martensite and lower bainite due to the delay in the transformation of ferrite. In addition, B secretes at the grain edges as does C, and low temperature toughness is improved by increasing the resistance at the grain edges. Therefore, B may be contained in the hot rolled steel sheet. However, this effect becomes noticeable when the B content is 0.0002% or more, and therefore the lower limit of the B content is adjusted to be 0.0002% or more. On the other hand, when the B content is above 0.01%, not only the saturation effect, but also the economic efficiency deteriorates. Consequently, the B content is desirably 0.0002% to 0.01%. It is most desirably 0.0005 to 0.005%, and even more desirably 0.0007 to 0.0030%.
[00073] Note that it is found that the effects of the present invention are not damaged if Zr, Sn, Co, Zn, W are contained by 1% or less as a total as for other elements. Among these elements there is a possibility in Sn that a failure occurs at the time of hot rolling, and therefore the Sn content is desirably 0.05% or less.
[00074] The balance is composed of iron and impurities. As impurities are exemplified those contained in the raw material such as ore and scrap, and those contained during the production process. [00075] The hot-rolled steel sheet of the present invention which has the structure and chemical composition as set out above is capable of improving corrosion resistance by providing a hot-dip galvanized layer by performing a galvanizing treatment by hot dip, also a galvanized layer bonded by performing a bonding treatment after coating on a hot-rolled steel sheet surface described above. In addition, a coating layer is not limited to pure zinc, and may contain elements such as Si, Mg, Zn, Al, Fe, Mn, Ca, Zr to also improve corrosion resistance. The coating layer as set out above does not damage the excellent fatigue strength and low temperature toughness of the present invention.
[00076] Furthermore, the effects of the present invention can be obtained if any of the surface layers treated by an organic coating formation, lamination of a film, a treatment with organic salts / inorganic salts, a treatment without chromium can be provided , etc. Hot-rolled steel sheet production method [00077] The following describes a method of producing the hot-rolled steel sheet of the present invention.
[00078] The volume fraction of either or the total of both between tempered martensite and lower bainite is adjusted to be 90% or more, the effective average diameter of the crystal grain in the 1/4 part of the plate thickness is adjusted to be 10 gm or less, and the effective average diameter of the crystal grain in the part in the range of 50 gm from the surface is adjusted to be 6 gm or less to allow for excellent fatigue strength and low temperature toughness. It is desirable that either or both of the tempered martensite and the lower bainite contain the iron-based carbide to 1 χ 106 (pieces / mm2), and the effective aspect ratio of the crystal grains of either or both of them the tempered martensite and the bottom bainite are adjusted to be two or less. Details of the production method are described below to simultaneously satisfy the above.
[00079] The production method preceding hot rolling is not particularly limited. That is, the composition is adjusted to be the composition mentioned above by performing the melting in a vat oven, an electric oven, etc., and several subsequent secondary fusions, and then it can be cast by a method such as casting normal continuous, a casting by the conventional method (ingots), and casting of thin plates.
[00080] In the case of continuous casting, hot rolling can be performed after it is cooled once to a low temperature, and then heated again, or the ingot can be hot rolled without cooling to room temperature. In addition, the cast plate can be hot rolled continuously. Scrap can be used as a raw material as long as the composition can be controlled within the range of the present invention.
[00081] The hot rolled steel sheet excellent in fatigue strength and low temperature toughness of the present invention can be obtained when the following essential conditions can be achieved.
[00082] When the hot rolled steel sheet is produced, after being cast to have a predetermined steel sheet composition, the cast plate is cooled directly or once, and then heated to 1200 ° C or more. In the final stage of the finishing lamination, when the diameter of the lamination cylinder is adjusted to be R / mm, and the reduction ratio is adjusted to be r%, the hot lamination is completed under a condition that satisfies R / (100 - r)> 8, and at 900 ° C or more. It is cooled at an average cooling rate of 60 ° C / s or more from the finishing laminate temperature to 700 ° C, cooled at an average cooling rate of 50 ° C / s or more from 700 ° C to 400 ° C, and cooled to an average cooling rate of less than 50 ° C / s from 400 ° C to room temperature. It is thus possible to produce hot-rolled steel sheet that is excellent in fatigue strength and low temperature toughness and has a tensile strength of 980 MPa or more.
[00083] The heating temperature of the hot rolling plate needs to be adjusted to 1200 ° C or more. In the hot-rolled steel sheet of the present invention, the suppression of the austenite grains' hardening is performed using one or both between the solid solutions of Ti and Nb, and therefore it is necessary to re-melt one or both between NbC and TiC which are precipitated at the time of casting. When the heating temperature of the plate is less than 1200 ° C, carbides of one or both of them between Nb and Ti require a long time to be melted, and therefore the subsequent refinement of the crystal grain diameter and the enhancement effect low temperature toughness does not occur. Consequently, it is necessary to adjust the heating temperature of the plate to 1200 ° C or more. In addition, the upper limit of the heating temperature of the plate is not particularly limited, and the effect of the present invention is exercised, but it is not economically preferable to adjust an excessively high heating temperature. Therefore, the upper limit of the plate heating temperature is desirably set to be less than 1300 ° C.
[00084] In the final stage of the finishing lamination, when the diameter of the lamination cylinder is adjusted to be R / mm, and the reduction ratio is adjusted to be r%, the condition of R / (100-r)> 8 needs to be satisfied. The inventors carried out intensive studies, and found that when the condition of R / (100 - r)> 8 is satisfied, it is possible to produce a hot-rolled steel sheet that has high fatigue strength because the effective average grain diameter crystal in the part in the range of 50 pm from the surface is 6 pm or less. Its causes are not certain, but it can be estimated that the value of R / (100 - r) is adjusted to be large, the shear stress applied to the surface layer of the hot rolled steel sheet in the final step of the finishing lamination is increased, and thus a portion of concentrated stress becomes a ferrite nucleation site at the time of transformation after lamination, and the structure of the surface layer is refined. When the condition is set to be R / (100 - r)> 9.2, the effective average diameter of the crystal grain the part in the range of 50 gm from the surface is 5 gm or less, also when the condition is set to be R / (100 - r)> 10.3, the structure whose effective average diameter of the crystal grain in the part of the range of 50 gm from the surface is 4 gm or less can be obtained.
[00085] It is necessary to adjust the temperature of the finishing laminate to 900 ° C or more. The hot-rolled steel sheet of the present invention contains too much of one or both between Ti and Nb to refine the diameter of the austenite grain. As a result, austenite is difficult to be recrystallized, it becomes a grain extended in the lamination direction to incur the deterioration of the toughness resulting from the finishing lamination in a temperature region of less than 900 ° C. Note that the upper limit of the finishing laminating temperature is desirably 1040 ° C.
[00086] It is necessary to cool at an average cooling rate of 60 ° C / s or more from the temperature of the finishing laminate to 700 ° C. When the average cooling rate is less than 60 ° C / s, recrystallization proceeds excessively in the surface layer after lamination, and the effective average diameter of the crystal grain in the part of the 50 gm range from the surface exceeds 6 gm . This is because when the value of R / (100 - r) is large, the large shear stress is applied to the surface layer, and stress-induced recrystallization occurs.
[00087] As mentioned above, in the final stage of the finishing lamination, the hot foot lamination is completed under the condition in which R / (100 - r)> 8 is satisfied and at 900 ° C or more when the diameter of the rolling cylinder is adjusted to be R / mm, and the reduction ratio is adjusted to be r%. It is therefore possible to control so that the effective average diameter of the crystal grain becomes 10 pm or less in the part 1/4 of the thickness of the plate, and 6 pm or less in the part in the range of 50 pm from the surface . Fatigue strength and low temperature toughness are therefore improved. Note that the volume fraction of one or the total of both between tempered martensite and lower bainite is 90% or more, and therefore most of the effective average diameter of the crystal grain is made from the crystals of these structures.
[00088] It is cooled at an average rate of 50 ° C / s or more than 700 ° C to 400 ° C.
[00089] When the cooling rate from the finishing temperature to 400 ° C is less than 50 ° C / s, ferrite is generated during cooling, and it is difficult to adjust the volume fraction of one or the total of both between martensites tempering and lower bainite which is the main phase to be 90% or more. The average cooling rate is therefore adjusted to be 50 ° C / s or more than 700 ° C to 400 ° C. Note that if no ferrite is formed during the cooling process, air cooling can be performed in a temperature region along this path.
[00090] Note that the cooling rate from point Bs to the lower bainite formation temperature is preferably adjusted to be 50 ° C / s or more. This is to prevent the generation of superior bainite. When the cooling rate from point Bs to the lower bainite formation temperature is less than 50 ° C / s, the upper bainite is formed, and there is the case that new martensite (martensite whose displacement density is high) is generated between the blades of bainite, or there is retained austenite (it becomes the martensite whose displacement density is high at the time of processing). Consequently, the curing ability in cooking and low temperature toughness deteriorate. Note that point Bs is the starting temperature of the upper bainite generation, and is determined by the composition, but here it is conveniently adjusted to be 550 ° C. In addition, the lower bainite generation temperature is also determined by the composition, but here it is conveniently adjusted to be 400 ° C. The cooling rate is adjusted to be 50 ° C / s or more from the temperature of the finishing laminate to 400 ° C, especially from 550 ° C to 400 ° C. The average cooling rate is adjusted to be 50 ° C / s or more from the rolling end temperature up to 400 ° C.
[00091] Note that cooling at an average cooling rate of 50 ° C / s or more from the finishing laminate temperature to 400 ° C (60 ° C / s or more at 700 ° C or more) means that adjustment of the condition of the average cooling rate from the temperature of the finishing laminate to 550 ° C to less than 50 ° C / s is included. However, there is a case where the upper bainite is easy to generate under this condition, and there is the case where an upper bainite of 10% or more in the volume fraction can be partially generated. Therefore, it is desirable that the cooling rate from 550 ° C to 400 ° C be adjusted to be 50 ° C / s or more.
[00092] It is necessary to adjust the average cooling rate from 400 ° C to room temperature by less than 50 ° C / s. This is to allow the structure in either or both of tempered martensite and lower bainite to be adjusted to be the main phase, and the numerical density of the iron-based carbide is within the range stipulated above. When the average cooling rate is 50 ° C / s or more, iron-based carbide cannot be adjusted to be in the range set out above, and high fatigue and toughness resistance cannot be achieved. Consequently, it is necessary to adjust the average cooling rate to less than 50 ° C / s.
[00093] Here, "cooling at a cooling average rate of less than 50 ° C / s from 400 ° C to room temperature" does not only mean cooling, but also an isothermal retention, that is, winding less than 400 ° C is included. In addition, the control of the cooling rate in this temperature region aims to control the displacement density and the numerical density of the iron-based carbide in the structure of the hot-rolled steel sheet, and therefore it is possible to obtain the maximum strength tensile strength of 980 MPa or more, the high resistance to fatigue and the high tenacity that are the effects of the present invention, even when it is cooled once to the temperature at which martensite begins to transform (point Ms) or less, and later the temperature is increased by reheating.
[00094] The winding temperature must be less than 400 ° C. When the winding temperature is 400 ° C or more, one or both between the ferrite and the upper bainite are formed after winding, and it is difficult to adjust the volume fraction of one or the total of both between the tempered martensite and the lower bainite, which are the main phase, by 90% or more. Consequently, the winding temperature is adjusted to be less than 400 ° C.
[00095] In general, it is necessary to suppress the transformation of ferrite to obtain martensite, and cooling to 50 ° C / s or more is necessary. In addition, a temperature region is transient from a temperature region that is difficult to cool, whose heat transfer coefficient called the film boiling region is relatively low at low temperature to a temperature region which is easy to cool. be cooled whose coefficient of heat transfer called the boiling temperature region of the nucleate is large. Consequently, when the temperature region below 400 ° C is set to be the temperature of the cooling stop, the winding temperature is easy to be varied, and the quality of the material varies according to the variation of the winding temperature. Consequently, it is often the case that the normal winding temperature is set to either more than 400 ° C or the winding at room temperature.
[00096] As a result, it is estimated that it is conventionally difficult to discover that the maximum tensile strength of 980 MPa or more, and the excellent fatigue strength and low temperature toughness can be guaranteed simultaneously by winding from 400 ° C to room temperature and by decreasing the cooling rate as the present invention [00097] It is noted that it is desirable to perform a skin pass lamination at a reduction rate of 0.1 to 2% after all the processing is completed in order to allow the correction of the shape hot-rolled steel plate and improved ductility by introducing mobile displacement. In addition, acid cleaning can be performed on the hot-rolled steel sheet as needed after all processes have been completed to remove the scales attached to the surface of the obtained hot-rolled steel sheet. In addition, a skin pass or cold rolling can be performed online or offline at a reduction rate of 10% or less for the hot rolled steel sheet obtained after acid cleaning.
[00098] The hot-rolled steel sheet of the present invention is produced by passing through continuous casting, roughing rolling, finishing rolling, or acid cleaning being the normal hot rolling process. Note that it is possible to guarantee the maximum tensile strength of 980 MPa or more and excellent fatigue strength and low temperature toughness which are the effects of the present invention even if it is produced while excluding part of the process.
[00099] Furthermore, it is possible to guarantee the maximum tensile strength of 980 MPa or more and the high fatigue strength and low temperature toughness that are the effects of the present invention even if the heat treatment is performed online or offline in a range of temperatures from 100 to 600 ° C after the hot rolled steel sheet is produced once aiming for the precipitation of carbides.
[000100] The steel plate with maximum tensile strength of 980 MPa or more according to the present invention indicates a steel plate whose maximum tensile stress by a tensile test performed based on JIS Z 2241 by the use of a JIS n specimen 5 ° cut in a vertical direction in relation to the rolling direction of the hot rolling is 980 MPa or more.
[000101] The steel sheet which is excellent in fatigue strength according to the present invention indicates a steel sheet whose fatigue strength at ten million cycles obtained by the flat bending fatigue test carried out based on JIS Z 2275 is 600 MPa or more.
[000102] The steel sheet which is excellent in low temperature toughness according to the present invention indicates a steel sheet whose fracture appearance transition temperature (vTrs) of the Charpy test performed based on JIS Z 2242 is -40 ° C or less.
[000103] In the present invention, the desired hot-rolled steel sheet is used mainly in vehicles, and, therefore, it is often the case that the thickness of the sheet becomes about 3 mm. Therefore, the surface of the hot-rolled steel sheet is polished, and the hot-rolled steel sheet is processed to be a 2.5 mm specimen to perform the test.
Examples [000104] Examples of the present invention are cited to describe the technical content of the present invention. Note that the condition in the current examples is a conditional example that is applied to verify the possibility of configurations and effects of the present invention, and the present invention is not limited to that conditional example. The invention can apply several conditions as long as the objective of the present invention is achieved without leaving its spirit or its essential characteristics.
[000105] The results of studies are described using steels having the compositions illustrated in Table 1. The steels A to P illustrated in Table 1 are examples that satisfy the conditions of the composition of the present invention, and the aaj steels are examples. that do not satisfy the conditions of the composition of the present invention. Note that specifically La and Ce are used as REM.
[000106] After these steels were cast, they were heated in the state, or reheated after being cooled once to room temperature to be in the temperature range from 1170 ° C to 12950 ° C, and later hot rolling was performed under each condition in Tables 2-1 and 2-2, they were finished laminated at 889 ° C to 1095 ° C, cooling and winding were performed under each of the conditions illustrated in Tables 2-1 and 2-2 to be hot-rolled steel sheets each with a sheet thickness of 3.4 mm. After that, acid cleaning was performed, and then the 0.5% skin pass lamination was performed.
[000107] In Tables 2-1 and 2-2, “R” means the diameter of the lamination cylinder (mm) in the final stage of the finishing lamination, and “r” indicates the reduction ratio (%) in the final stage of the finishing lamination. The underline means to be outside the range of the present invention. Note that the steels in Tables 2-1, 2-2 are represented by providing serial numbers for the steels used in Table 1. [000108] Several specimens were cut from the obtained hot-rolled steel sheet, and a material tests, a structural observation, etc., were performed. As a tensile test, JIS No. 5 specimen was cut in a vertical direction to the rolling direction, and the test was performed based on JIS Z 2242.
[000109] Fatigue strength was assessed using the 10 million cycle fatigue strength discovered by the flat bending fatigue test performed based on JIS Z 2275.
[000110] The Charpy test was performed based on JIS Z 2242, and the transition temperature of fracture appearance was measured. The thickness of the hot-rolled steel sheet of the present invention was less than 10 mm, and therefore the front and rear faces of the obtained hot-rolled steel sheet were polished to adjust the thickness to be 2.5 mm, and subsequently the Charpy test was performed.
[000111] A portion of the hot-rolled steel sheets was heated to 660 to 720 ° C, a hot dip galvanizing treatment or a bonding treatment was carried out at 540 to 580 ° C after the coating treatment to make it a hot-dip galvanized steel sheet (GI) or a hot-dip galvanized steel sheet (GA), and subsequently the material test was performed.
[000112] The microstructure observation is performed according to the method shown above, and then the volume fraction of each structure, the numerical density of the iron-based carbide, the effective average diameter of the crystal grain and the ratio were measured medium in appearance.
[000113] The results are shown in tables 3-1 to 3-3. In Tables 3-1 to 3-3 the underline means to be outside the range of the present invention. In addition, “HR” means hot-rolled steel sheet, “GI” means hot-dip galvanized steel sheet, “GA” means a coated steel sheet on which bonded hot-dip galvanizing is performed on hot rolled steel sheet.
[000114] Only those that satisfy the condition of the present invention had the maximum tensile strength of 980 MPa or more, high fatigue strength and low temperature toughness.
[000115] On the other hand, in each of the B-10, G-3, K-4 steels, the heating temperature of the plate was less than 1200 ° C, and the iron-based carbides of each or both between Ti and Nb they precipitated at the time of casting were difficult to be dissolved. Consequently, the effective diameter of the crystal grain cannot be adjusted within the range of the present invention although the other lamination conditions were adjusted within the range of the present invention. And the low temperature toughness has deteriorated. [000116] In each of the steels A-8, C-4, F-3, K-5, O-2, when the diameter of the rolling cylinder was adjusted to R / mm, and the reduction ratio was adjusted to being r%, the expression R (100 - r) became less than eight in the final stage of the finishing lamination, a sufficient tension was not applied to the surface layer of the hot rolled steel sheet, and therefore the effective average diameter of the crystal grain in the part of the 50 pm range from the surface was stiffened, and the fatigue resistance was deteriorated.
[000117] In each of the A-9, B-11, G-4, K-6 steels, the temperature of the finishing lamination was too low to be the lamination in the non-recrystallized austenite region, and therefore the grain was not extended in the lamination direction. Consequently, the average aspect ratio was large, and the low temperature toughness was deteriorated.
[000118] In each of the steels A-10, D-3, H-3, K-7, N-2, O-3, the cooling rate from the temperature of the finishing laminate to 700 ° C was less than 60 ° C / s, the crystal grains in the vicinity of the surface layer were recrystallized during cooling, the effective average diameter of the crystal grain became large, and therefore the fatigue resistance was deteriorated.
[000119] In each of the steels A-11, A-13, D-4, H-4, K-8, K-10, the cooling rate of 700 to 400 ° C was less than 50 ° C / s , and ferrite and upper bainite were formed during cooling. Therefore, tensile strength and fatigue strength were difficult to guarantee, and low temperature toughness was deteriorated because the boundaries between ferrite and martensite became the starting points for fractures.
[000120] In each of the A-12, B-12, I-2, K-9 steels, the cooling rate from 400 ° C to room temperature was 50 ° C / s or more, the amount of precipitation of carbides was insufficient, and therefore low temperature toughness was deteriorated.
[000121] In each of the A-13, D-4, K-10 steels, the winding temperature was 400 ° C or more, and the amount of upper ferrite or bainite structure becomes large in the steel plate structure . As a result, it was difficult to guarantee tensile strength and fatigue strength. In addition, the crude iron-based carbides precipitated between the blades that exist in the upper bainite structure have become the starting points for fractures, and therefore, the low temperature toughness has deteriorated.
[000122] Furthermore, as can be seen in each of the steels A-4, B-2, 9, C-2, 3, K-3, L-7, M-1, the quality of the material of the present invention can be guaranteed even if hot dip galvanizing treatment or bonded hot dip galvanizing treatment has been carried out.
[000123] On the other hand, in each of the steels al to j1 whose steel plate composition did not satisfy the range of the present invention, the tensile strength of 980 MPa or more, the excellent fatigue strength and the low temperature toughness defined in the present invention could not be obtained.
[000124] Note that when the sample was attempted to be produced under the same conditions as A1 steel except that the O content exceeds 0.01% by mass, there was a problem with processing capacity, and it was found that it could not be treated as a product.
Table 1 Table 2-1 _____________, ___________, _______________, __________________, ________________________, _____________________, Table 2-2 Table 3-1 Table 3-2 _________ _______________________.___________________.___________ _______________________ Industrial applicability [000125] The present invention is an effective technique for a hot rolled steel sheet having a maximum tensile strength of 980 MPa or more and excellent fatigue strength and low temperature toughness, and its production method. According to the present invention, it is possible to provide hot rolled steel sheet which is excellent in fatigue strength and low temperature toughness, and has a maximum tensile strength of 980 MPa or more. This hot-rolled steel sheet is easy to process, capable of being used in very cold places and, therefore, its industrial applicability is extremely remarkable.

权利要求:
Claims (11)
[1]
1. Hot-rolled steel sheet, characterized by the fact that it comprises: a composition containing, in mass%, C: 0.01 to 0.2%; Si: 2.5% or less (excluding “0” (zero)); Mn: 4.0% or less (excluding “0” (zero)); P: 0.10% or less; S: 0.03% or less; Al: 0.001 to 2.0%; N: 0.01% or less (excluding “0” (zero)); O: 0.01% or less (excluding “0” (zero)); Cu: "0" (zero) at 2.0%; Ni: “0” (zero) at 2.0%; Mo: "0" (zero) at 1.0%; V: “0” (zero) at 0.3%; Cr: "0" (zero) at 2.0%; Mg: "0" (zero) at 0.01%; Ca: "0" (zero) at 0.01%; REM: "0" (zero) at 0.1%; B: "0" (zero) at 0.01%; and one or both in a total of Ti and Nb of 0.01 to 0.30%, and the balance being made of iron and impurities, and a structure in which the total volume fractions of tempered martensite and lower bainite is 90 % or more, where the average effective diameter of the crystal grain in a part 1/4 of the thickness of the steel sheet from the surface is 10 pm or less, and the average effective diameter of the crystal grain in, a part in 50 pm range from the surface is 6 pm or less.
[2]
2. Hot-rolled steel sheet according to claim 1, characterized by the fact that iron-based carbides that exist in tempered martensite and lower bainite are 1 x 106 (pieces / mm2) or more, and the average ratio Aspect ratio of the crystal grains of tempered martensite and lower bainite is two or less.
[3]
3. Hot-rolled steel sheet according to claim 1 or 2, characterized by the fact that it also comprises, in mass%, one element or two or more elements selected from a group made of: Cu: 0.01 to 2.0%; Ni: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.3%; and Cr: 0.01 to 2.0%.
[4]
4. Hot-rolled steel sheet according to any one of claims 1 to 3, characterized in that it also comprises, in mass%, one element or two or more elements selected from a group made of: Mg: 0, 0005 at 0.01%; Ca: 0.0005 to 0.01%; and REM: 0.0005 to 0.1%.
[5]
Hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that it also comprises, in mass%, B: 0.0002 to 0.01%.
[6]
6. Hot-rolled steel sheet according to any one of claims 1 to 5, characterized in that a galvanized layer or a bonded galvanized layer is included on the surface of the steel sheet.
[7]
7. Method of producing a hot-rolled steel sheet, characterized by the fact that it comprises: melting a composition containing, in mass%, C: 0.01 to 0.2%; Si: 2.5% or less (excluding “0” (zero)); Mn: 4.0% or less (excluding “0” (zero)); P: 0.10% or less; S: 0.03% or less; Al: 0.001 to 2.0%; N: 0.01% or less (excluding “0” (zero)); O: 0.01% or less (excluding “0” (zero)); Cu: "0" (zero) at 2.0%; Ni: “0” (zero) at 2.0%; Mo: "0" (zero) at 1.0%; V: “0” (zero) at 0.3%; Cr: "0" (zero) at 2.0%; Mg: "0" (zero) at 0.01%; Ca: "0" (zero) at 0.01%; REM: "0" (zero) at 0.1%; B: "0" (zero) at 0.01%; and either or the total of both between Ti and Nb from 0.01 to 0.30%, and the balance being iron and impurities, and subsequently heat a cast plate to 1200 ° C or more directly or after it has been cooled one turn; perform hot rolling under a condition that satisfies R / (100 - r)> 8, and set the hot rolling finish temperature to 900 ° C or more when the diameter of the laminating cylinder is set to be R / mm and the reduction ratio is adjusted to be r%, in the final stage of a finishing laminate, and to cool at an average cooling rate of 60 ° C / s or more from the temperature of the finishing laminate to 700 ° C, an average cooling rate of 50 ° C / s or more from 700 ° C to 400 ° C, and an average cooling rate of 50 ° C / s or less than 400 ° C to room temperature, and wind to a less than 400 ° C.
[8]
8. Method of production of the hot-rolled steel sheet according to claim 7, characterized by the fact that the cast plate also contains, in mass%, one element or two or more elements selected from the group made of: Cu: 0.01 to 2.0%; Ni: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.3%; and Cr: 0.01 to 2.0%.
[9]
9. Method of production of hot-rolled steel sheet according to claim 7 or 8, characterized by the fact that the cast plate also contains, in mass%, one element or two or more elements selected from a group made of : Mg: 0.0005 to 0.01%; Ca: 0.0005 to 0.01%; and REM: 0.0005 to 0.1%.
[10]
10. Method of production of the hot-rolled steel sheet according to any of claims 7 to 9, characterized in that the cast plate also contains, in mass%, B: 0.0002 to 0.01%.
[11]
11. Method of producing the hot-rolled steel sheet according to any one of claims 7 to 10, characterized in that it also comprises: carrying out a galvanizing treatment or a bonded galvanizing treatment after winding.

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同族专利:

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公开号 | 公开日

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JP6048580B2|2016-12-21|

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WO2014188966A1|2014-11-27|

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US20150376730A1|2015-12-31|

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CN105143488A|2015-12-09|

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TW201502287A|2015-01-16|

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MX2015011027A|2015-10-22|

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KR20150114540A|2015-10-12|

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EP3000905A1|2016-03-30|

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ES2759051T3|2020-05-07|

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JPWO2014188966A1|2017-02-23|

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EP3000905A4|2016-11-02|

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US10023929B2|2018-07-17|

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EP3000905B1|2019-10-30|

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KR101766567B1|2017-08-08|

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CN105143488B|2017-05-17|

---

BR112015021149A2|2017-07-18|

---

PL3000905T3|2020-04-30|

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TWI510649B|2015-12-01|

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法律状态:
2018-11-06| B06F| Objections, documents and/or translations needed after an examination request according art. 34 industrial property law|

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2019-06-11| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application according art. 36 industrial patent law|

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2019-08-20| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

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2020-01-14| B09A| Decision: intention to grant|

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2020-03-10| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 16/05/2014, OBSERVADAS AS CONDICOES LEGAIS. |

优先权:

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申请号 | 申请日 | 专利标题

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JP2013-107324|2013-05-21|

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JP2013107324|2013-05-21|

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PCT/JP2014/063026|WO2014188966A1|2013-05-21|2014-05-16|Hot-rolled steel sheet and method for manufacturing same|

---