steel sheet and method for making a steel sheet

专利摘要:
STEEL SHEET AND METHOD FOR MAKING A STEEL SHEET. The invention relates to a cold-rolled and tempered steel sheet, whose chemical composition comprises, in which the contents are expressed by weight percentage: 0.10 (less than equal) C (less than equal) 0.13%, 2, 4 (less than equal) Mn (less than equal) 2.8%, 0.30 (less than equal) Si (less than equal) 0.55%, 0.30 (less than equal) Cr (less than equal) 0.56%, 0.020 (less than equal) Ti (less than equal) 0.050%, 0.0020 (less than equal) B (less than equal) 0.0040%, 0.005 (less than equal) Al (less than equal) 0.050%, Mo (less than equal) 0.010%, Nb (less than equal) 040%, 0.002 N (less than equal) 0.008%, S (less than equal) 0.005%, P (less than equal) 0.020%, where the remainder consists of iron and unavoidable impurities resulting from fusion, where steel sheet has a microstructure that consists of, in proportion of surface, martensite and / or lower bainite, in which said martensite comprises fresh martensite and / or self-tempered martensite, the sum of the surface proportions of martensite and lower bainite being between 60 to 95%, 4 to 35% of bainite that contains a low carbide content, 0 to 5% ferrite and less than 5% austenite retained in the shape of an island.
公开号:BR112016013130B1
申请号:R112016013130-4
申请日:2014-12-05
公开日:2021-03-09
发明作者:Josée DRILLET；Véronique HEBERT
申请人:Arcelormittal；
IPC主号:

专利说明:

FIELD OF THE INVENTION
[001] The present invention relates to a cold-rolled and tempered steel sheet that has a very high tensile strength and a deformability to manufacture forming parts, particularly in the automotive industry, for the manufacture of vehicle structural elements motorized and for the manufacture of such steel sheet. BACKGROUND OF THE INVENTION
[002] Steels have been developed which have a very favorable elastic limit / tensile strength ratio during forming operations.
[003] Their reinforcement capacity is very high, which allows a good distribution of deformations in the event of a collision and allows to obtain a significantly higher elastic limit in parts after forming. It is then possible to produce parts that are as complex as with traditional steels, but with higher mechanical properties, which allows for a decrease in thickness to maintain identical functional specifications. In this way, these steels have an effective response to the lightness and safety requirements of vehicles.
[004] In particular, steels of which the structure comprises martensite, optionally bainite, within a ferritic matrix, have experienced a great development, since they combine high strength with significant deformation possibilities.
[005] Recent requirements for lightness and reduced energy consumption have led to a high demand for very high strength steels, in which the tensile strength TS of which is greater than 1,180 MPa.
[006] Despite this level of resistance, these steels must have good ductility, good weldability and good coatability, in particular good suitability for continuous galvanizing by hardening.
[007] These steels must also have a high elastic limit and elongation at break, as well as good formability.
[008] In fact, certain automotive parts are manufactured through forming operations that combine different ways of deformation. Certain microstructural resources of steel may prove to be well suited for one deformation mode, but unfavorable in relation to another mode. Certain portions of the parts must have a high tensile strength and / or good flexibility and / or good edge cut formability.
[009] This edge cut formability is evaluated by determining an orifice expansion ratio, indicated Ac%. This orifice expansion ratio measures the suitability of the steel to undergo expansion during cold pressing and therefore provides an assessment of the formability of the steel in this deformation mode.
[010] The orifice expansion ratio can be assessed as follows: after producing an orifice by cutting a sheet of steel, a frustoconical tool is used in order to expand the edges of such an orifice. During this operation, it is possible to observe early damage close to the orifice edges during the expansion, in which this damage starts in second phase particles or at the interfaces between the different microstructural components in the steel.
[011] Described in ISO standard 16630: 2009, the orifice expansion method consists of measuring the initial diameter Di of the orifice before pressing, then the final diameter Df of the orifice after pressing, determined when cracks are observed through the thickness of the orifice. steel sheet at the edges of the hole. The suitability for Ac% orifice expansion is then determined using the following formula:

[012] Ac% therefore makes it possible to quantify the suitability of a sheet of steel to withstand pressing in a cut-out orifice. According to this method, the initial diameter is 10 millimeters.
[013] According to documents noUS 2012/0312433 A1 and US 2012/132327 A1, steels are known to have a TS tensile strength greater than 1,180 MPa. However, this rupture stress is obtained to the detriment of formability and weldability.
[014] In addition, according to documents noUS 2013/0209833 A1, US 2011/0048589 A1, US 2011/01683000 A1 and WO 2013/144376 A1, steels are known to have a high tensile strength that can exceed 1,000 MPa , but without having satisfactory formability and weldability at the same time. BRIEF DESCRIPTION OF THE INVENTION
[015] Under these conditions, a focus of the invention is to provide a sheet of steel that has a high tensile strength, in particular that is between 1,180 and 1,320 MPa, along with a high elastic limit, in particular that is between 800 and 970 MPa, where this value is determined before any hardening lamination operation on the steel sheet, and a good formability, in particular an Ac% orifice expansion ratio greater than or equal to 30%, an angle of flexion, for a sheet of steel with a thickness that is between 0.7 mm and 1.5 mm, greater than or equal to 55 ° and an elongation at break greater than 5%.
[016] For this purpose, the invention refers to a cold-rolled and tempered steel sheet, which has a chemical composition that comprises the contents expressed by weight percentage: 0.10 <C <0.13% 2,4 <Mn <2.8% 0.30 <Si <0.55% 0.30 <Cr <0.56% 0.020 <Ti <0.050% 0.0020 <B <0.0040% 0.005 <Al <0.050% Mo <0.010% Nb <0.040% 0.002 <N <0.008% S <0.005% P <0.020%, where the remainder consists of iron and unavoidable impurities resulting from melting, where the steel sheet has a microstructure that consists of, in proportion of surface, martensite and / or lower bainite, in which said martensite comprises initial martensite and / or self-prevented martensite, with the sum of the surface proportions of martensite and lower bainite being between 60 to 95%, 4 to 35% of bainite that contains low carbide content, 0 to 5% of ferrite and less than 5% of austenite retained in the shape of an island.
[017] In some embodiments, the steel sheet additionally includes, according to the invention, one or more of the following resources: - said microstructure comprises, in surface proportion, 4% to 20% of initial martensite, preferably 4% at 15%; - said microstructure comprises, in surface proportion, 40 to 95% of self-prevented martensite and lower bainite; - said self-prevented martensite and said lower bainite contain rod-shaped carbides oriented in the directions <111> of the slats of bainite and martensite; - the so-called low carbide bainite contains less than 100 carbides per unit area of 100 square micrometers; - said microstructure comprises, in proportion to the surface, 4 to 5% of ferrite; - the smallest size of the retained austenite islands is less than 50 nanometers; - the fraction of old austenite grains created by the annealing of which the size is less than one micrometer represents less than 10% of the total population of said old austenite grains; - said steel sheet has a tensile strength that is between 1,180 MPa and 1,320 MPa, and an Ac% orifice expansion ratio greater than or equal to 40%; - said steel sheet has a thickness that is between 0.7 mm and 1.5 mm, and said steel sheet has a bending angle greater than or equal to 55 °; 2.5 <Mn <2.8%; 0.30 <Si <0.5%; 0.005 <Al <0.030%; - said steel sheet comprises a coating of zinc or zinc alloy obtained through hot dip coating; - said zinc or zinc alloy coating is a galvanized coating, wherein said zinc or zinc alloy coating comprises from 7 to 12% iron; - said steel sheet comprises a coating of zinc or zinc alloy obtained by vacuum deposition.
[018] The invention also relates to a method for manufacturing a cold-rolled and tempered steel sheet according to the invention, which comprises the following successive steps: - providing a semi-finished steel that has a chemical composition that comprises the levels expressed by weight percentage: 0.10 <C <0.13% 2.4 <Mn <2.8% 0.30 <Si <0.55% 0.30 <Cr <0.56% 0.020 <Ti <0.050% 0.0020 <B <0.0040% 0.005 <Al <0.050%. Mo <0.010% Nb <0.040% 0.002 <N <0.008% S <0.005% P <0.020% in which the remainder consists of iron and unavoidable impurities resulting from melting, then - heat said semi-finished steel to a temperature above or above at 1,250 ° C, then - hot-rolling said semi-finished steel, where the end of the rolling temperature is greater than the Ar3 temperature of the start of the austenite transformation by cooling, to obtain a hot-rolled steel sheet, then - cool said hot rolled steel sheet at a rate sufficient to prevent the formation of ferrite and pearlite, then - cool said hot rolled steel sheet to a temperature below 580 ° C, then - cold laminate said sheet hot-rolled steel sheet to obtain a cold-rolled steel sheet, then - reheat said cold-rolled steel sheet between 600 ° C and Ac1, where Ac1 designates the start of the austenitic transformation temperature upon heating, with a rate heating V R that is between 1 and 20 ° C / s, then - reheat said cold-rolled steel sheet to a temperature Tm that is between Ac3'-10 ° C and Ac3 '+ 30 ° C and keep said sheet cold-rolled steel at said temperature Tm for a time Dm that is between 50 and 150 seconds, with Ac3 '= Min {Ac3 + 1,200 / Dm; 1,000 ° C}, where Ac3 and Ac3 'are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates the end of the austenitic transformation temperature upon heating as determined regardless of the residence time at such temperature Ac3, then - cool the steel sheet at a rate that is between 10 and 150 ° C / s for a temperature Te that is between 460 ° C and 490 ° C, then - keep said steel sheet at temperature Te for a time that is it takes between 5 and 150 seconds, then - coat the steel sheet by continuous immersion in a zinc or zinc alloy bath at a temperature TZn that is between 450 ° C and 480 ° C, in which the said temperatures Te and TZn are such that 0 <(Te-TZn) <10 ° C, then - optionally heat the coated steel sheet to a temperature between 490 ° C and 550 ° C for a time tG which is between 10 s 40 s.
[019] The invention also relates to a method for manufacturing a cold-rolled and tempered steel sheet according to the invention, which comprises the following successive steps: - providing a semi-finished steel that has a chemical composition that comprises the expressed levels by weight percentage: 0.10 <C <0.13% 2.4 <Mn <2.8% 0.30 <Si <0.55% 0.30 <Cr <0.56% 0.020 <Ti <0.050 % 0.0020 <B <0.0040% 0.005 <Al <0.050%. Mo <0.010% Nb <0.040% 0.002 <N <0.008% S <0.005% P <0.020% in which the remainder consists of iron and unavoidable impurities resulting from melting, then - heat said semi-finished steel to a temperature above or above at 1,250 ° C, then - hot-laminate said semi-finished steel, where the end of the rolling temperature is greater than Ar3, to obtain a hot-rolled steel sheet, then - cool said hot-rolled steel sheet in a rate sufficient to prevent the formation of ferrite and perlite, then - cool said hot-rolled steel sheet to a temperature below 580 ° C, then - cold-laminate said hot-rolled steel sheet to obtain a sheet of cold-rolled steel, then - reheat said cold-rolled steel sheet between 600 ° C and Ac1, where Ac1 designates the start of the austenitic transformation temperature by heating, at a heating rate VR, which is between 1 and 20 ° C / s, then - reheat said steel sheet cold rolled at a temperature Tm which is between Ac3-10 ° C and Ac3 + 30 ° C and keep said cold-rolled steel sheet at temperature Tm for a time Dm which is between 50 and 150 seconds, with, where Ac3 and Ac3 'are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates the end of the austenitic transformation temperature upon heating as determined regardless of the residence time at such temperature Ac3, then - cool the steel sheet in a rate that is between 10 and 100 ° C / s for a temperature Te that is between 460 ° C and 490 ° C, then - keep said steel sheet at temperature Te for a time that is between 5 and 150 seconds, then - cool said steel sheet to room temperature.
[020] In embodiments, the latter method additionally includes one or more of the following features: - a coating of zinc or zinc alloy is carried out by vacuum deposition after said cooling step to room temperature; - said vacuum deposition is carried out by physical vapor deposition (PVD); - said vacuum deposition is carried out by jet vapor deposition (JVD). BRIEF DESCRIPTION OF THE DRAWINGS
[021] The features and advantages of the invention will be shown by reading the description below, provided as an example and carried out in reference to the attached Figures, in which: - Figure 1 shows the microstructure of a steel sheet according to the invention , shown by a first type of thinning; and - Figure 2 shows the microstructure of the steel sheet of Figure 1, shown by a second type of thinning. DESCRIPTION OF REALIZATIONS OF THE INVENTION
[022] Throughout the order, Ar3 will designate the start of the transformation temperature of austenite by cooling.
[023] In addition, Ac1 will designate the start of the allotropic transformation temperature by heating the steel.
[024] In addition, Ac3 will designate the end of the austenitic transformation temperature by heating as calculated by the Thermo-Calc® program, known therein. This calculation does not involve the residence time at the Ac3 temperature.
[025] However, the end of the austenitic transformation temperature upon heating depends on the residence time on the plateau, indicated Dm. The reference Ac3 'will then refer to the minimum corrected end of the austenitic transformation temperature upon heating, determined using the formula: Ac3' = Min {Ac3 + 1,200 / Dm; 1,000 ° C}, where Ac3 and Ac3 'are expressed in degrees Celsius and Dm in seconds. Min {Ac3 + 1,200 / Dm; 1,000 ° C} designates the lowest value here between the two quantities: (Ac3 + 1,200 / Dm) and 1,000 ° C. Thus, if Ac3 + 1,200 / Dm is less than or equal to 1,000 ° C, Ac3 ’= Ac3 + 1,200 / Dm. However, if Ac3 + 1,200 / Dm is greater than 1,000 ° C, Ac3 '= 1,000 ° C. It is considered, then, that even when the residence time on the plateau is very short, a temperature of 1,000 ° C makes it possible to obtain an austenitic structure.
[026] This temperature Ac3 'is such that when the steel is kept at the temperature Ac3' for a residence time equal to Dm, the steel sheet is entirely in the austenitic phase.
[027] The martensite results from the transformation without diffusion of the austenite Y below the start of the martensitic transformation temperature Ms by cooling.
[028] The martensite takes the form of thin slats elongated in one direction, and oriented inside each initial austenite grain. The term martensite includes both initial martensite and self-prevented martensite.
[029] A distinction will be made below between self-alerting martensite and initial martensite (ie, not tempered and not self-alerted).
[030] In particular, self-alerting martensite is present in the form of thin slats that comprise iron carbides dispersed in these slats, in the form of poles oriented in the directions <111> of the slat mesh α. This self-alerting martensite is formed in the case of a rapid cooling cycle below the martensitic transformation temperature Ms. The iron carbides dispersed in the slats are formed by precipitation below the martensitic transformation temperature Ms when the cooling is not slow enough to produce martensite initial. Otherwise, the initial martensite does not comprise carbides.
[031] The bainite, formed during the cooling of the austenitic band, above the start of the martensitic transformation temperature Ms, takes the form of an aggregate of ferrite slats and cementite particles. The formation of the same involves a short distance diffusion.
[032] A distinction will be made below between lower bainite and bainite that contains low carbide content.
[033] The lower bainite is formed, during cooling, in a temperature range immediately above the initial martensitic transformation temperature Ms. It takes the form of thin slats and comprises carbides dispersed in those slats.
[034] In addition, bainite containing low carbide content will refer to bainite containing less than 100 carbides per unit surface area of 100 square micrometers. Bainite containing a low carbide content is formed during cooling between 550 ° C and 450 ° C.
[035] Unlike low carbide bainite, lower bainite always contains more than 100 carbides per unit area of 100 square micrometers.
[036] In the chemical composition of steel, carbon plays a role in the formation of the microstructure and mechanical properties.
[037] The carbon weight content is between 0.10% and 0.13%. This range of carbon content makes it possible to simultaneously achieve a tensile strength greater than 1,180 MPa, an elongation at break greater than 5% and a satisfactory orifice expansion ratio Ac%, greater than 30%, or even greater than 40 %. In particular, a carbon content level below 0.10% does not make it possible to achieve a sufficient breakdown voltage. For a higher carbon content, greater than 0.13%, weldability tends to decrease. In addition, the temperature Ms falls, such that the initial martensite fraction, that is, not tempered and not self-correcting, in the microstructure tends to increase and, thus, deteriorate the orifice expansion ratio.
[038] The weight content of manganese is between 2.4% and 2.8%, preferably between 2.5% and 2.8%. Manganese is a gamma-like element, which reduces the temperature Ac3 and reduces the temperature Ms for the beginning of martensite formation. The low carbon content of steel can lead to a high Ac3 temperature, above 850 ° C. A manganese content greater than 2.4% makes it possible, by reducing the value of the Ac3 temperature, to achieve complete austenization of the steel between 840 ° C and 850 ° C, after maintaining it at that temperature for a time of at least 50 s . Manganese also allows the formation of self-prevented martensite and, therefore, contributes to obtaining an Ac% orifice expansion ratio greater than or equal to 40%. The manganese content level is limited to 2.8%, in order to limit the formation of band structures.
[039] Silicon is an element that participates in hardening in solid solution, the level of which, by weight in steel, is between 0.30% and 0.55%, preferably between 0.30% and 0, 5%. A content level of at least 0.30% makes it possible to obtain sufficient hardening of the ferrite and / or bainite. The content by weight of silicon is limited to 0.55% to guarantee an Ac% orifice expansion ratio greater than or equal to 40%, while limiting the formation of upper bainite. In addition, an increase in the level of silicon content would deteriorate the coatability of the steel by favoring the formation of oxides that adhere to the surface of the steel sheet.
[040] In addition, silicon reduces weldability. Weldability can, in particular, be estimated using carbon-equivalent Ceq, for example, calculated using the formula published by Nishi, T et al., In “Evaluation of high-strength steels for automobile use”, report Nippon Steel technician, n ° 20, pages 37 to 44, 1982, in which the content levels of the elements are expressed as a percentage by weight:

[041] A level of silicon content below 0.55% contributes, in particular, to guarantee both very good weldability, in particular a carbon equivalent calculated using the Nishi formula of less than or equal to 0.30% , as well as good coatability.
[042] Silicon is also alfagenic and, therefore, contributes to increasing the Ac3 temperature and favoring the formation of bainite that contains low carbide content. A level of silicon content below 0.55% therefore helps to prevent the formation of an excessive amount of bainite that contains a low carbide content.
[043] The composition of the steel sheet additionally includes chromium in a content greater than or equal to 0.30%, in order to improve the hardening capacity of the steel, and to increase its hardness as well as the tensile strength. The level of chromium content must be less than or equal to 0.56%, in order to retain a satisfactory elongation at break and limit costs.
[044] Titanium is present in steel in a content that is between 0.020% and 0.050%. At a content between 0.020% and 0.050%, titanium essentially combines with nitrogen and carbon to precipitate as nitrides and / or carbonitrides. Below 0.020%, the tensile strength of 1,180 MPa is not achieved. Titanium also has a positive influence on the weldability of steel.
[045] In addition to a titanium content level of 0.050%, there is a risk of precipitating thick liquid titanium nitrides, which tend to reduce ductility, and lead to early damage during orifice expansion. In fact, when nitrides with a size greater than 6 microns are present, it is observed that most of them are at the origin of cleavage with the matrix during the cutting and pressing stages. Titanium also makes it possible to ensure that nitrogen is fully combined in the form of nitrides or carbonitrides, in such a way that boron is in free form and can play an effective role in the hardening capacity.
[046] The boron weight content is between 0.0020% and 0.0040%. By limiting carbon activity, boron actually makes it possible to control and limit diffusive phase transformations (ferritic or pearlitic transformation during cooling) and to form hardening phases (bainite or martensite) necessary to obtain high voltage characteristics of break. The addition of boron further makes it possible to limit the addition of hardening elements such as Mn, Mo, Cr and to reduce the analytical cost of the steel grade. According to the invention, the minimum level of boron content to guarantee an effective hardening capacity is 0.0020%. In addition to 0.0040%, the effect on hardening capacity is saturated and a harmful effect on coating and ductility is observed.
[047] The composition of the steel sheet includes additional and optionally molybdenum, in a content below 0.010%. Molybdenum, like chromium, plays an effective role in hardening capacity. However, a content level above 0.010% increases the cost of additions excessively.
[048] The chemical composition of the hot-rolled steel sheet optionally includes niobium, with a weight content level below 0.040%. In addition to a weight content level of 0.040%, the recrystallization of austenite is delayed. The structure then contains a significant fraction of elongated grains, which no longer makes it possible to achieve the target Ac% orifice expansion ratio.
[049] In addition, the level of nitrogen weight content is guaranteed to be between 0.002% and 0.008%. In order to form a sufficient amount of nitrides and carbonitrides, the level of nitrogen content must be above 0.002%. The level of nitrogen content must be below 0.008% in order to avoid a precipitation of boron nitrides, which would decrease the amount of free boron.
[050] An aluminum weight content level between 0.005% and 0.050% makes it possible to guarantee the deoxidation of steel during its manufacture. An aluminum content level should be below 0.050%, or below 0.030% to avoid an increase in the Ac3 temperature and to prevent the formation of ferrite during cooling.
[051] The composition optionally comprises sulfur and phosphorus.
[052] The level of sulfur content should be below 0.005%. In addition to a sulfur content level of 0.005%, ductility is reduced due to the excessive presence of sulfides such as MnS which decrease the deformability, in particular the Ac% orifice expansion ratio.
[053] The level of phosphorus content should be below 0.020%. In fact, phosphorus is an element that gives solid solution hardening, but that reduces spot weldability and heat ductility, particularly due to its suitability for segregation at the grain boundaries or cosegregation with manganese.
[054] The microstructure of the steel sheet comprises, according to the invention, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite that contains low carbide content, 0 to 5% of ferrite and less than 5% of austenite retained in the shape of an island.
[055] In the context of the invention, the sum of the surface proportions of martensite and lower bainite is considered, where this fraction of total surface is between 60 and 95%.
[056] As previously indicated, a distinction is made between self-alerting martensite and initial martensite, that is, not tempered and not self-prevented.
[057] According to one embodiment, the martensite is formed, in particular, by self-alerting martensite, in which the surface proportion of the sum of the self-alerting martensite and the lower bainite is at least 40% of the entire microstructure and up to 95% .
[058] Self-prevented martensite and lower bainite are present in the form of thin slats and comprise carbides dispersed in those slats.
[059] In particular, the self-prevented martensite and the lower bainite comprise Fe2C and Fe3C iron carbides in the form of rods oriented in the <111> directions of the α mesh of the bainite and martensite slats.
[060] The proportions of self-prevented martensite and lower bainite are specified together, since the self-prevented martensite and lower bainite have substantially the same function in relation to the steel's use properties. In addition, these two components, present in the form of thin slats, cannot be identified individually from each other through observations by scanning electron microscopy.
[061] A percentage of the surface of self-prevented martensite and lower bainite that is between 40% and 95% makes it possible to favor the formability of the steel, in particular the flexibility and the formability of its edge cut. A percentage of at least 40% of self-prevented martensite and lower bainite makes it possible, therefore, to obtain a good bending angle, in particular a bending angle for steel sheets with a thickness between 0.7 mm and 1, 5 mm, at least 55 °, and a good expansion ratio for Ac% orifice, in particular greater than or equal to 40%.
[062] The percentage of self-prevented martensite and lower bainite in the microstructure is preferably less than 95%, in order to retain a sufficient percentage of lower carbide containing bainite, which makes it possible to obtain an elongation at break of at least 5%.
[063] The martensite may additionally comprise partially initial martensite, in a surface ratio that is between 4 and 20% of the entire microstructure, preferably between 4% and 15%.
[064] The initial martensite does not comprise carbides.
[065] The percentage of initial martensite surface should be below 20%, preferably below 15%, in particular to avoid deteriorating the brittleness of the steel and to guarantee a good orifice expansion ratio.
[066] In particular, the surface percentage of the initial band-shaped martensite should be minimized. A martensite band refers to a long martensite island that has an elongated morphology. In particular, such a strip has a greater length corresponding to the rolling direction of the steel sheet, to about 10 degrees. This elongated morphology is characterized on the one hand by the ratio between the longest length Lmax and the shortest length Lmin on the island, and on the other hand by the value of the maximum size Lmax on the island. A given island is considered to have an elongated morphology that, in this way, forms a band, when its Lmax / Lmin ratio is greater than or equal to 15 and when its longest Lmax length is greater than 30.
[067] The formation of these bands is favored by high levels of manganese and carbon content.
[068] The minimization of the percentage of initial martensite surface in band form, in particular below 10% of the entire microstructure, in particular makes it possible to avoid any deterioration of the flexion angle.
[069] The microstructure additionally comprises from 4 to 35% of bainite which contains low carbide content, that is, which comprises less than 100 carbides per unit area of 100 square micrometers.
[070] Bainite that contains low carbide content is formed during cooling between 550 ° C and 450 ° C. The formation of the same is, in particular, favored by the addition of silicon, which tends to delay the precipitation of the carbides, together with a small amount of hardening elements such as carbon or manganese.
[071] Bainite, which contains low carbide content, makes it possible to increase the elongation at break. In particular, a proportion of bainite surface that contains a low carbide content of at least 4% makes it possible to obtain an elongation at break of at least 5%. The proportion of bainite surface that contains a low carbide content should be limited to 35% in order to guarantee an orifice expansion ratio greater than or equal to 40% and a tensile strength greater than or equal to 1,180 MPa.
[072] The microstructure additionally comprises 0 to 5% ferrite, preferably 4 to 5%. The proportion of ferrite surface should be no more than 5% in order to guarantee good flexibility, as well as a strength greater than 1,180 MPa.
[073] The microstructure may contain austenite retained in the shape of an island, which in particular forms small plates between the slats of self-prevented martensite and lower bainite. The proportion of retained austenite surface is below 5%.
[074] Preferably, the smallest size of these retained austenite islands is less than 50 nanometers.
[075] In addition, the inventors also showed the importance of controlling the size of the austenite grains created during the annealing of the cold-rolled steel sheet, that is, existing at high temperatures at the end of the annealing period, prior to the subsequent cooling . These austenite grains are classified as “old austenite grains”, since these grains are replaced by other components during subsequent allotropic transformations by cooling. As will be explained, the size of these old austenite grains can, however, be shown through different methods, in the final product. According to the invention, the fraction of old austenite grains of which the size is less than one micrometer represents less than 10% of the total population of these old austenite grains.
[076] The fraction of old austenite grains of which the size is smaller than a micrometer is, for example, determined using an appropriate reagent, the thinning speed on which depends on certain local segregations in the old borders, for example , the Béchet-Beaujard reagent. For this purpose, a steel sample in the final state, that is, at the end of the manufacturing method according to the invention, is thinned out with the use of an appropriate reagent, in particular a reagent composed of an aqueous solution saturated with picric acid with at least 0.5% sodium alkyl sulfonate added, for a time between several minutes and an hour.
[077] At the end of this thinning, a micrographic examination of the sample makes it possible to visualize the boundaries of the old austenite grains and produce a histogram of the size of those old austenite grains, in particular to determine the fraction of the old austenite grains of which the size is less than a micrometer.
[078] Alternatively, the size of the old austenite grains can be determined with the use of sudden cooling interrupted during cooling after annealing, by adopting initial cooling conditions in order to cause intergranular ferritic germination, then to interrupt the last one by sudden cooling .
[079] The inventors have shown that the size of these ancient austenite grains conditions the kinetics of phase transformation during cooling following annealing. In particular, the small grains of austenite, smaller than one micrometer, contribute to reduce the temperature value Ms and, thus, increase the formation of initial martensite.
[080] Conversely, the presence of large grains of austenite reduces the formation of bainite that contains low carbide content.
[081] A fraction of old austenite grains of which the size is smaller than a micrometer, which makes up less than 10% of the total population of austenite grains, therefore contributes in particular to obtain an Ac% expansion ratio of orifice greater than or equal to 40% and a bending angle, for steel sheets with a thickness between 0.7 mm and 1.5 mm, of at least 55 °.
[082] These microstructural characteristics are, for example, determined by observing the microstructure by scanning electron microscopy using a field effect drum (“SEM-FEB” technique) with a magnification greater than 1,200x, coupled with an EBSD detector (“Backscatter Electron Diffraction”). The morphologies of the slats and grains are then determined by image analysis using programs known in them, for example, the Aphelion® program.
[083] The cold-rolled and tempered steel sheet according to the invention can be produced exposed, without coating, but it can also be coated. For example, such a coating can be formed by zinc or a zinc alloy, in particular a galvanized coating comprising 7 to 12% iron.
[084] In particular, such a steel sheet is well suited to the deposition of the metallic coating, in particular by immersion according to common methods.
[085] In particular, the composition and tensile characteristics of steel are compatible with the restrictions and heat cycles of zinc coating methods with continuous dip coating.
[086] The coating method used depends on the target application. In particular, the coating can be obtained by immersion, using a vacuum deposition method such as JVD (jet vapor deposition), or by cationic electrodeposition.
[087] The inventors have shown that a sheet of steel, according to the invention, has a tensile strength that is between 1,180 and 1,320 MPa, along with an elastic limit that is between 800 and 970 MPa, before any operation hardening lamination, an elongation at break of at least 5%, in particular greater than 8%, and an Ac% of orifice expansion ratio greater than or equal to 30%, in particular greater than or equal to 40%.
[088] In particular, an elastic limit that is between 800 and 970 MPa is obtained while retaining a tensile strength below 1,320 MPa. In addition, such a steel sheet has a high bending angle. In particular, when the steel sheet has a thickness that is between 0.7 mm and 1.5 mm, the bending angle is at least 55 °.
[089] The implementation of the manufacturing method for a laminated sheet, according to the invention, includes the following successive steps.
[090] A steel with a composition according to the invention is provided, and a semi-finished steel is molded from such steel. This molding can be carried out in ingots or continuously in the form of sheets with a thickness of about 200 mm.
[091] Molded semifinished steels are first brought to a temperature TR greater than 1,250 ° C, in order to homogenize the steel and completely dissolve the precipitations.
[092] Then, the semi-finished steel is hot rolled in a temperature range in which the steel structure is completely austenitic, that is, at a TFL temperature higher than the initial transformation temperature Ar3 of the austenite by cooling. If the TFL temperature is lower than the Ar3 temperature, the ferrite grains are hardened by rolling deformation and the ductility is reduced. Preferably, an end lamination temperature greater than 875 ° C will be chosen.
[093] The hot-rolled steel sheet is cooled at a rate sufficient to prevent formation of ferrite and pearlite, in particular greater than 30 ° C / s, so the hot-rolled steel sheet is wound in a spiral at a TBob temperature which is between 500 ° C and 580 ° C. The winding temperature must be below 580 ° C to prevent oxidation during the winding. An excessively low winding temperature, that is, below 500 ° C, leads to an increase in the hardness of the steel, which increases the forces required during subsequent cold rolling. The winding temperature range also makes it possible to prevent perlite formation.
[094] Cold rolling is then carried out, with a reduction rate that is, for example, between 40% and 70% in order to introduce an amount of deformation that allows later recrystallization.
[095] The cold-rolled steel sheet is then heated, preferably within a continuous annealing facility, with an average heating rate VC that is between 1 ° C / s and 20 ° C / s between 600 ° and the temperature Ac1 (start of the allotropic transformation temperature for austenite by heating).
[096] The temperature Ac1 can be measured by dilatometry, or evaluated using the formula published below in “Darstellung der Umwandlungen für technische Anwendungen und Moglichkeiten ihrer Beeinflussung”, HP Hougardy, Werkstoffkunde Stahl Band 1, 198-231, Verlag Stahleis , Düsseldorf, Germany, 1984:

[097] In this formula, the temperature Ac1 is expressed in degrees Celsius, and C, Mn, Si Cr, Mo and Ni designate the weight percentages of C, Mn, Si, Cr, Mo and Ni respectively in the composition.
[098] During the heating of the steel between 600 ° C and Ac1, a start of recrystallization occurs and TiNbCN precipitations form in the steel, which makes it possible to control the size distribution of the austenite grains formed from Ac1.
[099] Surprisingly, the inventors have shown that controlling the average VC heating rate between 600 ° C and Ac1 and thus the heating time between 600 ° C and Ac1, which corresponds to the time between the start of recrystallization and the start of the phase transformation, is decisive for the kinetics of the last phase transformations, in particular during the subsequent residence phase at the annealing temperature TM. The inventors then unexpectedly showed that choosing an average heating rate VC between 600 ° C and Ac1 that is between 1 ° C / s and 20 ° C / s makes it possible, at the end of the manufacturing method, to obtain a steel of which the microstructure consists of, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite that contains a low carbide content, 0 to 5% of ferrite and less than 5% of austenite retained in the shape of an island.
[0100] In particular, an average VC heating rate below 1 ° C / s would lead to an excessively long heating time between 600 ° C and Ac1 and, therefore, excessive ferrite formation and very low mechanical strength.
[0101] Otherwise, an average VC heating rate greater than 20 ° C / s would lead to an excessively short heating time between 600 ° C and Ac1, and insufficient growth of ferrite grains during heating between 600 ° C and Ac1 .
[0102] Furthermore, the inventors have shown that the size of the ferrite grains obtained at the end of heating between 600 ° C and Ac1 has an influence on the size of the austenite grains at the end of austenization. Insufficient growth of ferrite grains does in fact cause an excessively small fraction of small austenite grains to form, thus insufficient self-prevented martensite formation at the end of annealing, ie less than 40%, due to the decrease in the MS temperature value. .
[0103] The cold-rolled steel sheet is then heated from the temperature Ac1 to an annealing temperature TM that is between Ac3'-10 ° C and Ac3 '+ 30 ° C, and the steel sheet laminated to cold is kept at a temperature TM for a time Dm that is between 50 and 150 seconds.
[0104] As indicated above, the temperature Ac3 ’depends on the time spent on the plateau. The Dm time is chosen in such a way that the fraction of austenite grains of which the size is less than one micrometer represents less than 10% of the total population of the austenite grains. In particular, the residence time Dm must be long enough to form sufficiently large austenite grains. Preferably, an average grain size will be chosen greater than 3 microns, which is ideally between 5 and 10 microns.
[0105] In addition, a residence time Dm shorter than 50 s would lead to the formation of an excessively large proportion of ferrite at the end of the method.
[0106] The size of the austenite grains conditions the kinetics of phase transformation during the cooling that follows the annealing. In particular, the small grains of austenite, smaller than one micrometer, contribute to reduce the temperature value Ms and, thus, decrease the formation of self-prevented martensite.
[0107] The heating of the cold-rolled sheet at an average heating rate VC which is between 1 ° C / s and 20 ° C / s between 600 ° C and the temperature Ac1, followed by the heating of the cold-rolled steel sheet between Ac1 and TM and the permanence of the cold-rolled steel sheet at temperature TM during the time Dm, which is between 50 and 100 seconds, makes it possible, in this way, to control the size of the formed austenite grains and, more particularly, to control the fraction of those grains of which the size is smaller than a micrometer.
[0108] These heating parameters make it possible to obtain the microstructure according to the invention at the end of annealing and thus contribute to obtaining the desired mechanical characteristics.
[0109] The steel sheet is then cooled at a rate VR that is between 10 and 100 ° C / s to a temperature Te that is between 460 ° C and 490 ° C. The cooling rate VR should be greater than 10 ° C / s in order to form less than 5% of ferrite and not to form much bainite that contains low carbide content.
[0110] This cooling can be carried out from the TM temperature in one or more steps and may, in the latter case, involve different cooling modes, such as gas jets, water jets or boiling or cold water bath.
[0111] The steel sheet is then kept at the temperature Te for a time of between 5 and 150 seconds.
[0112] A partial transformation from austenite to bainite occurs at this stage. The stay in Te must be shorter than 150 s in order to limit the proportion of bainite surface and, in this way, obtain a sufficient proportion of martensite.
[0113] The steps to follow in the method differ depending on whether an individual intends to manufacture a sheet of continuously galvanized steel, in particular galvanized, or uncoated.
[0114] According to a first embodiment, corresponding to the manufacture of a continuously galvanized steel sheet, the steel sheet is coated by continuous passage immersed in a bath of zinc or zinc alloy at a temperature TZn which is between 450 ° C and 480 ° C, for several seconds. Te and TZn temperatures are such that 0 <(Te-TZn) <10 ° C.
[0115] The galvanized product is then cooled to room temperature by transforming a large fraction of the remaining austenite into initial martensite and / or lower bainite. In this way, a cold-rolled, tempered and galvanized steel sheet is obtained that contains, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite that contains low carbide content, 0 to 5 % of ferrite and less than 5% of austenite retained in the shape of an island.
[0116] If an individual wishes to manufacture a cold-rolled, tempered and “galvanized” (galvanized bonded) steel sheet, the galvanized product is heated immediately by leaving the zinc or zinc alloy bath at a TG temperature that is between 490 and 550 ° C for a time tG that is between 10 and 40 s. An individual then causes the iron to diffuse and the thin layer of zinc or zinc alloy deposited during immersion, which makes it possible to obtain a galvanized steel sheet.
[0117] The galvanized steel sheet is then cooled to room temperature, while transforming a large fraction of the remaining austenite into initial martensite and / or lower bainite. In this way, a cold-rolled, tempered and galvanized steel sheet is obtained that contains, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite that contains low carbide content, 0 to 5 % of ferrite and less than 5% of austenite retained in the shape of an island.
[0118] According to a second embodiment, corresponding to the manufacture of an uncoated steel sheet, the cooling of the steel sheet is carried out from temperature Te to room temperature in order to obtain a cold-rolled and tempered uncoated steel sheet which contains, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite which contains low carbide content, 0 to 5% of ferrite and less than 5% of austenite retained in the shape of an island.
[0119] According to a third embodiment, corresponding to the manufacture of a vacuum-coated steel sheet, an individual proceeds as in the second realization, from temperature Te, with cooling of the steel sheet to room temperature, then perform vacuum deposition of the zinc coating or zinc alloy, for example, by physical vapor deposition (PVD) or a jet vapor deposition (JVD) method. A cold-rolled and tempered coated steel sheet is obtained, which contains, in surface proportion, 60 to 95% of martensite and lower bainite, 4 to 35% of bainite which contains low carbide content, 0 to 5% of ferrite and less than 5% of austenite retained in the shape of an island.
[0120] The inventors have shown that implementing this method makes it possible to obtain a sheet of steel that has a tensile strength that is between 1,180 and 1,320 MPa, together with an elastic limit that is between 800 and 970 MPa (before any operation hardening lamination), an elongation at break of at least 5%, or even 8%, of which the Ac% orifice expansion ratio is greater than or equal to 30%, and even greater than or equal to 40%.
[0121] Furthermore, the implementation of this method makes it possible to grant the steel sheet an angle of flexion of at least 55 ° when the steel sheet has a thickness that is between 0.7 mm and 1.5 mm.
[0122] In addition, the steel sheet obtained has good weldability using common assembly methods, such as spot resistance welding.
[0123] As a non-limiting example, the following results show the advantageous features conferred by the invention.
[0124] Semi-finished steels were provided of which the compositions, expressed in weight (%), are given in Table 1 below.
[0125] In addition to the steels I1 to I4 used to manufacture steel sheets according to the invention, the composition of steels R1 to R9 used for the manufacture of the reference steel sheets are indicated as a comparison.
TABLE 1
[0126] The underlined values are not in accordance with the invention.
[0127] The Ac3 temperature was calculated using the Thermo-Calc® program, in which the start of the martensitic transformation temperature Ms and the carbon equivalent Ceq correspond to each of these compositions. These values are provided in Table 2 below.
[0128] In particular, the Ms temperature was determined from the following formula published by KW Andrews, in “Empirical Formula for Calculating Some Transformation Temperatures”, Journal of the Iron and Steel Institute, 203, Part 7, 1965, in that the content levels of the elements are expressed as a percentage by weight:

[0129] The carbon equivalent Ceq was determined from the Nishi formula indicated above.
[0130] The underlined values are not in accordance with the invention.
TABLE 2
[0131] The molded semi-finished steels corresponding to the above compositions have been reheated to a reheat temperature greater than 1,250 ° C, then hot rolled, where the end of the rolling temperature is equal to 850 ° C, therefore greater than Ar3 for all these steels.
[0132] Then, the hot-rolled steel sheets were cooled, while the formation of ferrite and perlite was avoided, then wound in a spiral at a temperature of 545 ° C.
[0133] The steel sheets were then cold rolled to a thickness of 1.4 mm.
[0134] The steel sheets were then reheated between 600 ° C and Ac1, where Ac1 designates the start of the austenitic transformation temperature by heating, with a reheating rate VC, then reheated to a temperature Tm and kept at temperature Tm for a time Dm.
[0135] According to a first set of tests, the steel sheets were cooled at a rate VR to a temperature Te, then kept at the temperature Te for a time De.
[0136] These tests were performed according to five different treatment conditions (a to e), shown in Table 3.
[0137] According to a sixth test (f in Table 3), the steel sheets were cooled from temperature Tm to room temperature, at a cooling rate VR, without maintaining at an intermediate temperature between Tm and room temperature. In Table 3, NA means not applicable. In fact, according to treatment f, no residence at a temperature Te is carried out and the residence time De is therefore irrelevant.
[0138] The manufactured steel sheets are then uncoated steel sheets.
TABLE 3
[0139] The underlined values are not in accordance with the invention.
[0140] With the use of tensile strength tests, the elastic limit Ys, the tensile strength TS and the total elongation A were determined for the steel sheets obtained by the different manufacturing modes. The flexibility of these steel sheets was also determined by determining the maximum angle before rupture.
[0141] The maximum angle before the rupture of the steel sheet is determined by applying a hole in the steel sheets in order to flex the said sheet. The force to be applied to perform the flexion increases until the steel sheet breaks. The measurement of the force applied during the bending makes it possible, in this way, to detect the beginning of the rupture of the steel sheet, and to measure the angle of bending when such rupture occurs.
[0142] The Ac% orifice expansion ratio was also determined in each steel sheet by producing an orifice by cutting the steel sheet using a frusto-conical tool in order to produce an expansion at the edges of said orifice. As described in ISO standard 16630: 2009, the initial diameter Di of the orifice was measured before pressing, then the final diameter Df of the orifice after pressing, when cracks are observed following through the thickness of the steel sheet at the edges of the orifice. The expansion capacity of the Ac% orifice was determined according to the following formula:

[0143] The microstructure of the steels has also been determined. The surface proportions of martensite (including self-prevented martensite and initial martensite) and lower bainite (together), self-prevented martensite and lower bainite (together), and bainite containing low carbide content were quantified after roughing with sodium bisulfite. The proportion of initial martensite surface was quantified after thinning by a NAOH-NaNO3 reagent.
[0144] The proportion of ferrite surface was also determined by observations by scanning electron microscope and optics, in which the ferritic phase was identified.
[0145] The microstructures of the steel sheets were provided in Table 4 below.

TABLE 4
[0146] The underlined values are not in accordance with the invention.
[0147] The mechanical properties of the steel sheets are given in Table 5 below.

TABLE 5
[0148] In this table, n.d. means that the property values have not been determined.
[0149] The analysis of these results shows the relationships between the steel compositions, the microstructure and their mechanical properties.
[0150] The steel sheets I1-b, I2-b, I3-b, I4-b and I5-b have a composition and a microstructure according to the invention. Subsequently, these steel sheets have a tensile strength, an elastic limit, an elongation, a bending angle and an orifice expansion ratio that satisfy the target values.
[0151] Figures 1 and 2 illustrate the microstructure of the I4-b steel sheet. Figure 1 results from the thinning of the steel sheet by sodium bisulfite, while Figure 2 results from the thinning of the steel sheet by NAOH-NaNO3. Figure 1 shows the self-prevented martensite and the lower bainite (M + Bl), as well as the bainite that contains low carbide content (BFC). Figure 2 shows the initial martensite (MF) in the form of darker areas.
[0152] The steel sheet, according to example R1-b, shows an excessively high Cr content level and an excessively high B content level, such that the breaking strength TS of the same is very high . Thus, although a satisfactory elastic limit YS is obtained, this elastic limit is obtained together with an excessively high tensile strength TS.
[0153] Steel sheets, according to R2-b and R3-b, have an excessively low C level and, therefore, have no satisfactory breaking stress.
[0154] In particular, the steel sheet, according to example R2-b, comprises a high lower carbide containing a fraction of bainite, due to the low level of C content of the steel that makes up the steel sheet, which takes to obtain a relatively low breaking voltage.
[0155] In addition, a low level of C content leads to an increase in the Ac3 temperature and, therefore, the Ac3 'temperature.
[0156] For example, R3, according to manufacturing condition c, an excessively high number of small grains of austenite therefore forms, which leads to the formation at the end of the annealing of an excessively low proportion of the self-prevented and bainite martensite surface. lower, an excessively high proportion of initial martensite and an excessively high proportion of bainite surface that contains low carbide content.
[0157] This leads to insufficient TS resistance and YS elastic limit, and an orifice expansion ratio less than the desired minimum ratio.
[0158] The steel sheet, according to example R4-b, has excessively high C and Mn levels, excessively low Cr and Si levels, an excessively high Nb level and a level of excessively low B content, which leads to a very small bending angle.
[0159] In particular, the high levels of Mn and C content in example R4-b lead to the excessive formation of initial band-shaped martensite, which causes a deterioration of the flexion angle. In addition, due to the high levels of carbon and manganese content, the weldability of steel is deteriorated. In particular, the carbon equivalent Ceq determined from the aforementioned Nishi formula is equal to 0.34%, therefore greater than the maximum desired value of 0.30%.
[0160] Example R5 shows an excessively low level of Mn content, which, combined with a high level of Si content (0.507%), leads to an excessive formation of bainite containing low carbide content according to the treatments be c.
[0161] Thus, examples R5-b and R5-c have an insufficient tensile strength and elastic limit.
[0162] In addition, the excessively low Mn content level leads to a high temperature Ac3 = 820 ° C. According to manufacturing condition c, Ac3 = 840 ° C therefore has a high value, such that the temperature Tm = 820 ° C is less than Ac3'-10 ° C. As a result, the optimization is not complete, an excessively high proportion of the ferrite surface remains in the steel and very little self-prevented martensite and lower bainite.
[0163] Consequently, the tensile strength TS and the elastic limit YS are insufficient, as well as the hole expansion ratio Ac%.
[0164] Similarly, example R6 shows an excessively low level of Mn content, which, combined with a high level of Si content (0.511%), leads to the excessive formation of bainite containing low carbide content.
[0165] In addition, the excessively low Mn content level, which leads to a high temperature Ac3 = 820 ° C, such that according to route c, the temperature Tm = 820 ° C is less than Ac3 ' -10 ° C = 830 ° C.
[0166] As a result, the optimization is not complete, and an excessively high proportion of ferrite surface remains in the steel, as well as very little self-prevented martensite and lower bainite. The elastic limit and the orifice expansion ratio are consequently deteriorated. However, the high level of Mo content makes it possible to maintain a high breakdown voltage TS.
[0167] Example R7 has an excessively high Mo level. Due to the low level of Nb content in example R7, this high level of Mo content results in a decrease in the size of the old austenitic grains and therefore leads, due to the decrease in the MS temperature value, to the insufficient formation of martensite and bainite lower, in particular self-prevented martensite and lower bainite at the end of annealing and excessive formation of initial martensite.
[0168] For example, R7, this results in an insufficient elastic limit. However, the high level of Mo content makes it possible to maintain a high breakdown voltage TS.
[0169] The composition of R8 steel also has an excessively high Mo content level.
[0170] Example R9 shows an excessively high Si content level, which leads to an excessive formation of bainite containing a low carbide content and an insufficient formation of martensite and lower bainite, which leads to obtaining an insufficient elastic limit. For example, I1- d, the heating rate Vc and dwell time Dm are very low. Thus, the temperature Tm is below Ac3'-10. As a result, the optimization is not complete, and an excessive growth of ferrite grains is observed. A proportion of excessive ferrite surface, self-prevented martensite and insufficient lower bainite and bainite that contains an insufficient low carbide content thus remain in the steel. The tensile strength and the elastic limit are therefore insufficient.
[0171] For example, I1-e, the heating rate Vc is very low. In this way, an excessive growth of ferrite grain is observed. A proportion of excessive ferrite surface, self-prevented martensite and insufficient lower bainite and bainite that contains an insufficient low carbide content thus remain in the steel. The tensile strength and the elastic limit are therefore insufficient.
[0172] For example, I4-f, the VR cooling rate is very high. In this way, a lot of martensite and inferior bainite and insufficient bainite are formed, which contains a low content of carbide and initial martensite.
[0173] The tensile strength and elastic limit are, therefore, well beyond those that are aimed at.

权利要求:
Claims (19)
[0001]
1. STEEL SHEET, cold rolled and tempered, whose chemical composition consists of, and the contents are expressed by weight percentage: 0.10 <C <0.13% 2.4 <Mn <2.8% 0, 30 <Si <0.55% 0.30 <Cr <0.56% 0.020 <Ti <0.050% 0.0020 <B <0.0040% 0.005 <Al <0.050% Mo <0.010% Nb <0.040% 0.002 < N <0.008% S <0.005% P <0.020%, characterized in that the remainder consists of iron and unavoidable impurities resulting from melting, in which the steel sheet has a microstructure that consists of, in proportion to the surface: - martensite and / or lower bainite, in which the sum of the surface proportions of martensite and lower bainite is between 60 and 95%, the martensite consisting of initial martensite and / or self-prevented martensite, the surface proportion of self-alerted martensite and lower bainite being 40 to 95 %; - 4 to 35% bainite, which contains low carbide content, containing less than 100 carbides per unit area of 100 square micrometers; - 0 to 5% ferrite; and - less than 5% of austenite retained in the shape of an island.
[0002]
2. STEEL SHEET, according to claim 1, characterized in that the microstructure comprises, in surface proportion, 4% to 20% of initial martensite, preferably 4% to 15%.
[0003]
3. STEEL SHEET, according to claim 1, characterized in that the self-correcting martensite and the lower bainite contain rod-shaped carbides oriented in the <111> directions of the bainite and martensitic slats.
[0004]
STEEL SHEET, according to any one of claims 1 to 3, characterized in that the microstructure comprises, in surface proportion, 4 to 5% of ferrite.
[0005]
5. STEEL SHEET, according to any one of claims 1 to 4, characterized in that the smallest size of the retained austenite islands is less than 50 nanometers.
[0006]
6. STEEL SHEET, according to any one of claims 1 to 5, characterized in that the fraction of old austenite grains created by the annealing of which the size is less than one micrometer represents less than 10% of the total population of the old grains of austenite.
[0007]
7. STEEL SHEET, according to any one of claims 1 to 6, characterized in that the steel sheet has a tensile strength that is between 1,180 MPa and 1,320 MPa, and an Ac% orifice expansion ratio greater or equal to 40%.
[0008]
STEEL SHEET according to any one of claims 1 to 7, characterized in that the steel sheet has a thickness that is between 0.7 mm and 1.5 mm, and in which the steel sheet has an angle flexion greater than or equal to 55 °.
[0009]
9. STEEL SHEET, according to any one of claims 1 to 8, characterized in that the chemical composition comprises the content expressed by weight percentage: 2.5 <Mn <2.8%.
[0010]
10. STEEL SHEET, according to any one of claims 1 to 9, characterized in that the chemical composition comprises the content expressed by weight percentage: 0.30 <Si <0.5%.
[0011]
11. STEEL SHEET, according to any one of claims 1 to 10, characterized in that the chemical composition comprises the content expressed by weight percentage: 0.005 <Al <0.030%.
[0012]
12. STEEL SHEET according to any one of claims 1 to 11, characterized in that the steel sheet comprises a coating of zinc or zinc alloy obtained through continuous dip coating.
[0013]
13. STEEL SHEET, according to claim 12, characterized in that the zinc or zinc alloy coating is a galvanized coating, wherein the zinc or zinc alloy coating comprises from 7 to 12% iron.
[0014]
STEEL SHEET according to any one of claims 1 to 11, characterized in that the steel sheet comprises a coating of zinc or zinc alloy obtained by vacuum deposition.
[0015]
15. METHOD FOR MANUFACTURING A STEEL SHEET cold-rolled and tempered, as defined in any one of claims 1 to 13, characterized by comprising the following successive steps: - providing a semi-finished steel that has a chemical composition consisting of, the contents expressed as a percentage by weight: 0.10 <C <0.13% 2.4 <Mn <2.8% 0.30 <Si <0.55% 0.30 <Cr <0.56% 0.020 <Ti < 0.050% 0.0020 <B <0.0040% 0.005 <Al <0.050% Mo <0.010% Nb <0.040% 0.002 <N <0.008% S <0.005% P <0.020% where the remainder consists of iron and unavoidable impurities resulting from the melting, then - heat the semi-finished steel to a preheat temperature greater than or equal to 1,250 ° C, then - hot-laminate the semi-finished steel, where the end-rolling temperature is higher than the Ar3 temperature at the beginning of the transformation of the austenite by cooling, to obtain a hot-rolled steel sheet, then - cool the hot-rolled steel sheet at a rate sufficient to ev facilitate the formation of ferrite and perlite, then - cool the hot-rolled steel sheet to a temperature below 580 ° C, then - cold-laminate the hot-rolled steel sheet to obtain a cold-rolled steel sheet, then - reheat the cold-rolled steel sheet between 600 ° C and Ac1, where Ac1 designates the start of the austenitic transformation temperature by heating, at a heating rate VR that is between 1 and 20 ° C / s, then - reheat the cold-rolled steel sheet to a temperature Tm between Ac3'-10 ° C and Ac3 '+ 30 ° C and keep the cold-rolled steel sheet at temperature Tm for a time Dm which is between 50 and 150 seconds, with Ac3 '= Min {Ac3 + 1,200 / Dm; 1,000 ° C}, where Ac3 and Ac3 'are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates the end of the austenitic transformation temperature upon heating as determined regardless of the residence time at such temperature Ac3, then - cool the steel sheet at a rate that is between 10 and 150 ° C / s for a temperature Te that is between 460 ° C and 490 ° C, so - keep the steel sheet at temperature Te for a while between 5 and 150 seconds, then - coat the steel sheet by continuous immersion in a bath of zinc or zinc alloy at a temperature TZn that is between 450 ° C and 480 ° C, in which the said temperatures Te and TZn so that 0 <(Te-TZn) <10 ° C, then - optionally heat the coated steel sheet to a temperature between 490 ° C and 550 ° C for a time tG which is between 10 s and 40 s.
[0016]
16. METHOD FOR MANUFACTURING A STEEL SHEET cold-rolled and tempered, as defined in any one of claims 1 to 11 and 14, characterized by comprising the following successive steps: - providing a semi-finished steel that has a chemical composition consisting of, the contents expressed by weight percentage: 0.10 <C <0.13% 2.4 <Mn <2.8% 0.30 <Si <0.55% 0.30 <Cr <0.56% 0.020 < Ti <0.050% 0.0020 <B <0.0040% 0.005 <Al <0.050% Mo <0.010% Nb <0.040% 0.002 <N <0.008% S <0.005% P <0.020% where the remainder consists of iron and unavoidable impurities resulting from melting, then - heat the semi-finished steel to a temperature of Treaquecer greater than or equal to 1,250 ° C, then - hot-laminate the semi-finished steel, where the end of the rolling temperature is greater than Ar3, to obtain a sheet hot-rolled steel, then - cool the hot-rolled steel sheet at a rate sufficient to prevent the formation of ferrite and perlite, then - cool the laminated steel sheet hot at a temperature below 580 ° C, then - cold-roll the hot-rolled steel sheet to obtain a cold-rolled steel sheet, then - reheat the cold-rolled steel sheet between 600 ° C and Ac1 , where Ac1 designates the start of the austenitic transformation temperature by heating at a heating rate VR that is between 1 and 20 ° C / s, then - reheat the cold-rolled steel sheet to a temperature Tm that is between Ac3-10 ° C and Ac3 + 30 ° C and keep the cold-rolled steel sheet at temperature Tm for a time Dm that is between 50 and 150 seconds, with Ac3 '= Min {Ac3 + 1200 / Dm; 1000 ° C}, where Ac3 and Ac3 'are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates the end of the austenitic transformation temperature by heating as determined regardless of the residence time at such temperature Ac3, then - cool the steel sheet at a rate that is between 10 and 100 ° C / s for a temperature Te that is between 460 ° C and 490 ° C, so - keep the steel sheet at temperature Te for a while between 5 and 150 seconds, then - cool the steel sheet to room temperature.
[0017]
17. METHOD FOR MAKING A STEEL SHEET cold rolled, tempered and coated, as defined in claim 16, characterized in that a coating of zinc or zinc alloy is carried out by vacuum deposition after the cooling step to room temperature.
[0018]
18. METHOD FOR MANUFACTURING A STEEL SHEET, according to claim 17, characterized in that the vacuum deposition is carried out by physical vapor deposition (PVD).
[0019]
19. METHOD FOR MANUFACTURING A STEEL SHEET, according to claim 17, characterized in that the vacuum deposition is carried out by jet vapor deposition (JVD).

类似技术:

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公开号 | 公开日 | 专利标题

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BR112016013130B1|2021-03-09|steel sheet and method for making a steel sheet

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法律状态:
2019-07-30| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|

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2020-09-29| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application [chapter 6.1 patent gazette]|

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2021-01-05| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|

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2021-03-09| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 05/12/2014, 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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USPCT/US2013/074482|2013-12-11|

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PCT/US2013/074482|WO2015088523A1|2013-12-11|2013-12-11|Cold rolled and annealed steel sheet|

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