Martensitic steel sheet and method for producing same

Abstract

A martensitic steel sheet containing the following elements: 0.08% < = C < = 0.14%; 1.95% < = Mn < = 2.6%; 0.1% < = Si < = 0.8%; 0.01% < = Al < = 0.1%; 0.001% < = Ti < = 0.1%; 0.0001% < = B < = 0.05%; 0% < = S < = 0.09%; 0% < = P < = 0.09%; 0% < = N < = 0.09%; 0.1% < = Cr < = 1%; 0% < = Ni < = 1%; 0% < = Cu < = 1%; 0% < = Mo < = 0.4%; 0% < = Nb < = 0.1%; 0% < = V < = 0.1%; 0% < = Sn < = 0.1%; 0% < = Pb < = 0.1%; 0% < = Sb < = 0.1%; 0.001% < = Ca < = 0.01%; the remainder consists of iron and unavoidable impurities, the microstructure of the steel comprising, in area percent, at least 92% martensite, 1% to 8% of a cumulative amount of ferrite and bainite, and 0% to 2% of an optional amount of retained austenite.

Description

Technical Field

[0001] The present invention relates to a method for producing martensitic steel suitable for the automotive industry, engineering machinery parts (yellow goods) and prototype parts (green goods). More particularly, it relates to a martensitic steel having a tensile strength of 1180 MPa or more. Background Art

[0002] Auto parts need to meet two inconsistent requirements, that is, easy formability and strength, but in recent years, in view of global environmental issues, a third requirement of improving fuel consumption has been imposed on automobiles. Therefore, now auto parts must be made of materials with high formability to meet the standard of easy assembly of complex automobile components, and at the same time, strength must be improved for vehicle crash resistance and durability while reducing the weight of the vehicle to improve fuel efficiency.

[0003] Therefore, a great deal of research and development efforts have been invested to reduce the amount of material utilized in automobiles by increasing the strength of the material. Conversely, an increase in the strength of a steel sheet reduces formability, and therefore a material having both high strength and high formability must be developed.

[0004] Early research and development in the field of high strength and high formability steel sheets has resulted in several methods for producing high strength and high formability steel sheets, some of which are listed herein for a clear understanding of the present invention:

[0005] The steel sheet of WO2017 / 065371 is manufactured by the following steps: rapidly heating a material steel sheet for 3 seconds to 60 seconds to the Ac3 transformation point or higher and holding the material steel sheet, the material steel sheet comprising 0.08 wt% to 0.30 wt% C, 0.01 wt% to 2.0 wt% Si, 0.30 wt% to 3.0 wt% Mn, 0.05 wt% or less P, and 0.05 wt% or less S, and the remainder being Fe and other inevitable impurities; rapidly cooling the heated steel sheet with water or oil at 100°C / second or higher; and rapidly tempering to 500°C to the A1 transformation point for 3 seconds to 60 seconds, including heating and holding time. However, the steel of WO2017 / 065371 does not mention a tensile strength of 1180 MPa or more in both the rolling direction and the transverse direction. Furthermore, even after having a tempered martensite single-phase structure, the hole expansion ratio of the steel of WO2017 / 065371 is no more than 40%.

[0006] WO2010 / 036028 relates to hot-dip galvanized steel sheet and its manufacturing method. The hot-dip galvanized steel sheet comprises a steel sheet comprising a martensitic structure as a matrix and a hot-dip galvanized layer formed on the steel sheet. The steel sheet comprises 0.05 wt % to 0.30 wt % C, 0.5 wt % to 3.5 wt % Mn, 0.1 wt % to 0.8 wt % Si, 0.01 wt % to 1.5 wt % Al, 0.01 wt % to 1.5 wt % Cr, 0.01 wt % to 1.5 wt % Mo, 0.001 wt % to 0.10 wt % Ti, 5 ppm to 120 ppm N, 3 ppm to 80 ppm B, impurities, and the remainder Fe. However, the steel of WO2010 / 036028 does not mention the hole expansion rate. Summary of the invention

[0007] The object of the present invention is to solve these problems by making available a cold rolled martensitic steel sheet having simultaneously:

[0008] - An ultimate tensile strength of 1180 MPa or more in both the transverse direction and the rolling direction, and preferably greater than 1220 MPa in both the transverse direction and the rolling direction.

[0009] - a yield strength of 900 to 1180 MPa in both the transverse direction and the rolling direction, and preferably 950 to 1180 MPa in both the transverse direction and the rolling direction.

[0010] - Total elongation greater than 3% both in transverse direction and in rolling direction.

[0011] - A hole expansion ratio greater than 40%, and preferably greater than 50%.

[0012] Preferably, such a steel may also have good formability (for rolling) as well as good weldability and paintability.

[0013] Another object of the invention is also to make available a method for manufacturing these panels that is compatible with conventional industrial applications and at the same time robust to variations in manufacturing parameters. DETAILED DESCRIPTION

[0014] The above objects and other advantages of the present invention will become more apparent through detailed description of preferred embodiments of the present invention.

[0015] The chemical composition of cold rolled martensitic steel contains the following elements:

[0016] The carbon present in the steel of the present invention is 0.08% to 0.14%. Carbon is an element necessary to improve the strength of the steel of the present invention by producing low temperature transformation phases such as martensite, and therefore, carbon plays two key roles, one of which is to improve strength. However, a carbon content of less than 0.08% will not impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.14%, the steel exhibits poor spot weldability, which limits its application for automotive parts. The preferred content for the present invention can be maintained at 0.09% to 0.13%, and more preferably 0.1% to 0.12%.

[0017] The manganese content of the steel of the present invention is 1.95% to 2.6%. This element is a gamma-phase generating element (gammagenous). Manganese provides solid solution strengthening and suppresses the ferrite transformation temperature and reduces the ferrite transformation rate, thus contributing to the formation of martensite. An amount of at least 1.95% is required to impart strength and contribute to the formation of martensite. However, when the manganese content is greater than 2.6%, it has an adverse effect, for example, it hinders the transformation of austenite to martensite during cooling after annealing. A manganese content higher than 2.6% may be excessively segregated in the steel during solidification and the uniformity of the material inside is impaired, which may lead to surface cracks during the hot working process. The preferred limit of manganese presence is 2% to 2.5%, and more preferably 2% to 2.4%, even more preferably 2% to 2.3%.

[0018] The silicon content of the steel of the present invention is 0.1% to 0.8%. Silicon is an element that contributes to the improvement of strength by solid solution strengthening. Silicon is a component that can hinder the precipitation of carbides during cooling after annealing, and therefore, silicon promotes the formation of martensite. But silicon is also a ferrite former and also increases the Ac3 transformation point, which pushes the annealing temperature to a higher temperature range, which is why the silicon content is kept at a maximum of 0.8%. A silicon content higher than 0.8% may also cause temper embrittlement and in addition silicon also impairs coatability. The preferred limit of silicon presence is 0.15% to 0.7%, and more preferably 0.2% to 0.6%.

[0019] The content of aluminum is 0.01% to 0.1%. In the present invention, aluminum removes oxygen present in molten steel to prevent oxygen from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, thereby reducing the grain size. Higher aluminum content (above 0.1%) increases the Ac3 point to a high temperature, thereby reducing productivity. The preferred limit of aluminum presence is 0.01% to 0.05%.

[0020] Titanium is added to the steel of the present invention at 0.001% to 0.1%. It forms titanium nitrides which appear during solidification of the cast product. The amount of titanium is therefore limited to 0.1% to avoid the formation of coarse titanium nitrides which are detrimental to formability. In the case of titanium contents below 0.001%, no effect is imparted to the steel of the present invention.

[0021] Boron is an important optional element of the steel of the present invention and may be present in an amount of 0.0001% to 0.05%. Boron forms boron nitrides when added in an amount of at least 0.0001% and imparts additional strength to the steel of the present invention.

[0022] Sulfur is not an essential element but may be contained in the steel as an impurity, and from the viewpoint of the present invention, the sulfur content is preferably as low as possible, but from the viewpoint of manufacturing cost, the sulfur content is 0.09% or less. In addition, if higher sulfur is present in the steel, it combines with manganese to form sulfides and reduces the beneficial effects of manganese on the present invention.

[0023] The phosphorus content of the steel of the present invention is 0% to 0.09%. Phosphorus reduces spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.09%, and preferably below 0.06%.

[0024] Nitrogen is limited to 0.09% to avoid material aging and to minimize precipitation of aluminum nitride during solidification which is detrimental to the mechanical properties of the steel.

[0025] Chromium is an optional element that may constitute 0% to 1% of the steel. Chromium provides strength to the steel by solid solution strengthening and a minimum of 0.1% is required to impart strength, but when used above 1%, the surface finish of the steel is compromised. The preferred limit of chromium presence is 0.1% to 0.5%.

[0026] Molybdenum is an optional element constituting 0% to 0.4% of the steel of the present invention; Molybdenum plays an effective role in improving hardenability and hardness, especially when added in an amount of at least 0.001% or even at least 0.002%, delaying the appearance of bainite and thus promoting the formation of martensite. However, excessive addition of Mo increases the addition cost of the alloying element, so its content is limited to 0.4% for economic reasons.

[0027] Niobium is present in the steel of the invention at 0% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the invention by precipitation hardening. Niobium will also influence the size of the microstructural components by its precipitation as carbonitrides and by hindering recrystallization during the heating process. Thus, the finer microstructure formed at the end of the holding temperature and therefore after full annealing will cause hardening of the product. However, niobium contents above 0.1% are not economically interesting because a saturation effect of its influence is observed (which means that an additional amount of niobium does not cause any strength improvement of the product).

[0028] Vanadium effectively improves the strength of steel by forming carbides or carbonitrides, and from an economical viewpoint, the upper limit is 0.1%.

[0029] Nickel can be added as an optional element in an amount of 0% to 1% to increase the strength of the steel of the present invention and improve its toughness. A minimum of 0.01% is preferred to obtain such effects. However, when its content is higher than 1%, nickel causes deterioration of ductility.

[0030] Copper can be added as an optional element in an amount of 0% to 1% to increase the strength of the steel of the present invention and improve its corrosion resistance. A minimum of 0.01% is preferred to obtain such effects. However, when its content is higher than 1%, it may deteriorate the surface appearance.

[0031] Calcium can be added to the steel of the present invention in an amount of 0.001% to 0.01%. Calcium is added to the steel of the present invention as an optional element, especially during inclusion treatment. Calcium aids in the refining of steel by binding harmful sulfur contents in the form of globules, thereby hindering the harmful effects of sulfur.

[0032] Other elements such as Sn, Pb or Sb may be added alone or in combination in the following proportions: Sn≤0.1%, Pb≤0.1% and Sb≤0.1%. Up to the indicated maximum content levels, these elements allow grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0033] The microstructure of the martensitic steel plate will now be described in detail, with all percentages being in terms of area fraction.

[0034] Martensite constitutes at least 92% of the microstructure by area fraction. The martensite of the present invention may contain both fresh martensite and tempered martensite. However, fresh martensite is an optional microscopic component, which is preferably limited in the steel to an amount of 0% to 4%, preferably 0% to 2%, or even better equal to 0%. Fresh martensite may be formed during cooling after tempering. Tempered martensite is formed by martensite formed during the second cooling step after annealing and in particular below the Ms temperature, and more particularly after Ms-10°C to 20°C. Such martensite is then tempered during the tempering temperature T tempering maintained at 150°C to 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is 93% to 99%, and more preferably 94% to 98%.

[0035] The cumulative amount of ferrite and bainite is 1% to 8% of the microstructure. The cumulative presence of bainite and ferrite does not adversely affect the present invention before 8%, but above 8%, the mechanical properties may be adversely affected. Therefore, the preferred limit of the cumulative presence of ferrite and bainite is maintained at 1% to 7%, and more preferably 1% to 6%.

[0036] Bainite is formed during reheating before tempering. In a preferred embodiment, the steel of the present invention contains 1% to 3% bainite. Bainite can impart formability to the steel, but when present in too large a quantity it can adversely affect the tensile strength of the steel.

[0037] Ferrite may form during the first cooling step after annealing, but is not required as a microstructural component. Ferrite formation must be kept as low as possible and is preferably less than 2% or even less than 1%.

[0038] Retained austenite is an optional microstructure which may be present in the steel at 0% to 2%.

[0039] The microstructure of the cold-rolled martensitic steel sheet does not contain microstructure components such as pearlite and cementite in addition to the above-mentioned microstructure.

[0040] The steel according to the invention can be manufactured by any suitable method. However, as a non-limiting example, it is preferred to use the method according to the invention which will be described in detail.

[0041] Such a preferred method comprises providing a semi-finished steel casting having the chemical composition of the high-quality steel according to the invention. The casting can be made into an ingot or continuously into the form of a thin slab or thin strip, i.e., a thickness ranging from about 220 mm for a slab to tens of millimeters for a thin strip.

[0042] For example, a slab having a chemical composition according to the present invention is produced by continuous casting, wherein the slab is optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and ensure that the ratio of local carbon to nominal carbon remains below 1.10. The slab provided by the continuous casting process can be used directly at high temperature after continuous casting, or can be first cooled to room temperature and then reheated for hot rolling.

[0043] The temperature of the slab subjected to hot rolling must be at least 1000°C and must be lower than 1280°C. In the case where the temperature of the slab is lower than 1280°C, an excessive load is imposed on the rolling mill, and furthermore, the temperature of the steel may be lowered to the ferrite transformation temperature during the finish rolling, whereby the steel will be rolled in a state where the structure contains transformation ferrite. Therefore, the temperature of the slab must be high enough so that the hot rolling should be completed in the temperature range of Ac3 to Ac3+100°C. Reheating at a temperature higher than 1280°C must be avoided because it is industrially expensive.

[0044] The plate obtained in this way is then cooled at a cooling rate of at least 20° C. / second to a coiling temperature which must be below 650° C. Preferably, the cooling rate will be less than or equal to 200° C. / second.

[0045] The hot rolled steel sheet is then coiled at a coiling temperature below 650° C. to avoid ovalization, and preferably 475° C. to 625° C. to avoid scale formation, wherein a more preferred range of such coiling temperature is 500° C. to 625° C. The coiled hot rolled steel sheet is then cooled to room temperature and then subjected to an optional hot strip annealing.

[0046] The hot rolled steel sheet may be subjected to an optional descaling step to remove the scale formed during hot rolling prior to the optional tropical annealing. The hot rolled sheet may then be subjected to an optional tropical annealing. In a preferred embodiment, such tropical annealing is carried out at a temperature of 400° C. to 750° C., preferably for at least 12 hours and not more than 96 hours, the temperature preferably being kept below 750° C. to avoid partially transforming the hot rolled microstructure and, therefore, possibly losing microstructural homogeneity. Thereafter, the optional descaling step of the hot rolled steel sheet may be carried out, for example, by pickling of such sheet.

[0047] The thickness of the hot rolled steel sheet thus obtained is preferably 0.5 mm to 10 mm, and more preferably 1 mm to 8 mm, even more preferably 1 mm to 6 mm. The hot rolled steel sheet can optionally be directly conveyed for heat treatment to produce an optional hot rolled heat treated martensitic steel suitable for engineering machinery parts and prototype parts. The heat treatment process for both the hot rolled steel sheet and the cold rolled steel sheet is the same.

[0048] The hot-rolled steel sheet is then subjected to cold rolling at a thickness reduction rate of 35% to 90% to obtain a cold-rolled steel sheet.

[0049] The cold rolled steel sheet is then heated in a two-step heating process, wherein the first step of heating starts from room temperature and heats the cold rolled steel sheet to a temperature HT1 in the range of 410° C. to 750° C. at a heating rate HR1 of at least 10° C. / sec. In a preferred embodiment, the heating rate HR1 of such a first step of heating is at least 12° C. / sec., and more preferably at least 13° C. / sec. The preferred HT1 temperature of such a first step is 425° C. to 725° C., and more preferably 435° C. to 690° C.

[0050] In the second heating step, the cold rolled steel sheet is heated from HT1 to A at a heating rate HR2 of 0.5°C / sec to 50°C / sec. c 3 to A c 3+100℃, preferably A c 3+10℃ to A c In a preferred embodiment, the heating rate HR2 of the second step heating is 0.7°C / sec to 25°C / sec, and more preferably 0.8°C / sec to 20°C / sec, wherein the A of the steel sheet isc 3 Calculated by using the following formula:

[0051] Ac3=910-203[C]^(1 / 2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11[Cr]-20[Cu]+700[P]+400[AE]+120[As]+400[Ti]

[0052] The element contents are expressed as weight percentages of the cold-rolled steel sheets.

[0053] The cold rolled steel sheet is held at Tsoak for a period of 10 to 500 seconds to ensure complete recrystallization and complete transformation of the heavily work-hardened initial structure into austenite.

[0054] The cold rolled steel sheet is then cooled in a two-step cooling process, wherein the first step of cooling starts from Tsoak and cools the cold rolled steel sheet to a temperature T1 in the range of 620°C to 750°C at a cooling rate CR1 of 15°C / sec to 150°C / sec. In a preferred embodiment, the cooling rate CR1 of such first step of cooling is 20°C / sec to 120°C / sec. The preferred T1 temperature of such a first step is 630°C to 725°C.

[0055] In the second cooling step, the cold rolled steel sheet is cooled from T1 to a temperature T2 of Ms-10°C to 20°C at a cooling rate CR2 of at least 50°C / sec. In a preferred embodiment, the cooling rate CR2 of the second cooling step is at least 100°C / sec, and more preferably at least 150°C / sec. The preferred T2 temperature of such a second step is Ms-10°C to 20°C. s -50℃ to 20℃.

[0056] The Ms of the steel plate is calculated by using the following formula:

[0057] Ms=545-601.2*(1-EXP(-0.868[C]))-34.4[Mn]-13.7[Si]-9.2[Cr]-17.3[Ni] -15.4[Mo]+10.8[V]+4.7[Co]-1.4[Al]-16.3[Cu]-361[Nb]-2.44[Ti]-3448[B]

[0058] Thereafter, the cold rolled steel sheet is reheated to a tempering temperature Ttemper of 150° C. to 300° C. for a time of 100 seconds and 650 seconds at a heating rate of at least 1° C. / second, and preferably at least 2° C. / second and greater, at least 10° C. / second. The preferred temperature range for tempering is 200° C. to 300° C., and the preferred duration of holding at Ttemper is 200 seconds to 600 seconds.

[0059] Then, the cold rolled steel sheet is cooled to room temperature to obtain cold rolled martensitic steel.

[0060] The cold rolled martensitic steel sheet of the present invention may be optionally coated with zinc or a zinc alloy, or with aluminum or an aluminum alloy to improve its corrosion resistance.

[0061] Example

[0062] The following tests, examples, graphical illustrations and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only and will demonstrate the advantageous features of the present invention.

[0063] Table 1 summarizes the steel plates made of steels with different compositions, where each element is indicated by its presence in percentage by weight, and the remainder is iron and other process impurities, where the steel plates were respectively produced according to the process parameters as noted in Table 2. Thereafter, Table 3 summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the results of the evaluation of the properties obtained.

[0064]

[0065] Table 2

[0066] Table 2 summarizes the hot rolling process parameters for imparting the desired mechanical properties to the steels in Table 1 to form martensitic steels and the annealing process parameters for the cold-rolled steel sheets.

[0067]

[0068] Table 3 illustrates the results of tests carried out according to standards on different microscopes, such as scanning electron microscopes, for determining the microstructure of both the invention steel and the reference steel, expressed as area fraction. The results are noted here:

[0069] Table 3:

[0070]

[0071] I = according to the invention; R = reference; underlined values: not according to the invention.

[0072] Table 4

[0073] The results of various mechanical tests performed according to the standards are summarized. Ultimate tensile strength, yield strength and total elongation are tested according to ISO-6892. To evaluate the hole expansion, a test called hole expansion is applied, in which the sample is subjected to a punch (10 mm hole) and deformed, after which we measure the hole diameter and calculate HER% = 100*(Df-Di) / Di

[0074]

[0075] I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

1. A martensitic steel plate, comprising the following elements expressed in weight percentage: 0.08％≤C≤0.14％； 1.95%≤Mn≤2.6%; 0.1%≤Si≤0.8%; 0.01％≤Al≤0.1％； 0.001%≤Ti≤0.1%; 0.0001％≤B≤0.05％； 0％≤S≤0.09％； 0％≤P≤0.09％； 0％≤N≤0.09％； and can include one or more of the following optional elements: 0.1％≤Cr≤1％； 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Mo≤0.4%; 0%≤Nb≤0.1%; 0%≤V≤v.1%; 0%≤Sn≤0.1%; 0%≤Pb≤0.1%; 0%≤Sb≤0.1%; 0.001%≤Ca≤0.01%; The remainder of the composition is composed of iron and unavoidable impurities caused by processing, and the microstructure of the steel comprises, by area percentage, at least 92% of martensite, 1% to 8% cumulative amounts of ferrite and bainite, and 0% to 2% optional amount of retained austenite.

2. The martensitic steel sheet according to claim 1, wherein the composition contains 0.15% to 0.7% silicon.

3. The martensitic steel sheet according to claim 1 or 2, wherein the composition contains 0.09% to 0.13% carbon.

4. The martensitic steel sheet according to any one of claims 1 to 3, wherein the composition contains 2% to 2.5% manganese.

5. The martensitic steel sheet according to any one of claims 1 to 4, wherein The amount of martensite is 93% to 99%.

6. The martensitic steel plate according to any one of claims 1 to 5, wherein the hole expansion ratio of the plate is greater than 40%.

7. A method for producing a martensitic steel sheet, comprising the following steps in sequence: - providing a steel composition according to any one of claims 1 to 4; - reheating the semi-finished product to a temperature of 1000° C. to 1280° C.; - rolling the semi-finished product in the austenite range to obtain a hot-rolled steel plate, wherein the hot rolling final rolling temperature is Ac3 to Ac3+100°C; - cooling the plate to a coiling temperature below 650°C at a cooling rate of at least 20°C / sec; and coiling the hot rolled plate; - cooling the hot rolled sheet to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling process; - optionally the hot rolled steel sheet can be annealed; - optionally subjecting the hot rolled steel sheet to a descaling process; - cold rolling the hot rolled steel sheet at a reduction ratio CR of 35% to 90% to obtain a cold rolled steel sheet; - The cold rolled steel sheet is then heated in two steps, wherein: o A first step of heating, heating the cold-rolled steel sheet from room temperature to a temperature HT1 of 410° C. to 750° C. at a heating rate HR1 of at least 10° C. / sec; ○ The second step of heating is to heat from HT1 to Ac3 to Ac3+100°C at a heating rate HR2 of 0.5°C / s to 50°C / s, wherein the cold-rolled steel sheet is kept for 10 seconds to 500 seconds, - The cold rolled steel sheet is then cooled in two steps, wherein: ○ The first step of cooling is to cool the cold-rolled steel sheet from T soaking to a temperature T1 of 620° C. to 750° C. at a cooling rate CR1 of 15° C. / s to 150° C. / s; ○ The second step of cooling is cooling from T1 to a temperature T2 of Ms-10°C to 20°C at a cooling rate CR2 of at least 50°C / sec, - then reheating the cold rolled steel sheet to a tempering temperature T tempering of 150°C to 300°C at a rate of at least 1°C / second, wherein the cold rolled steel sheet is maintained for a time of 100 seconds to 650 seconds; - Then cooled to room temperature at a cooling rate of at least 1°C / sec to obtain a martensitic steel sheet.

8. The method of claim 7, wherein the coiling temperature is 475°C to 625°C.

9. The method according to claim 7 or 8, wherein Tsoak is Ac3+10°C to Ac3+100°C.

10. The method according to any one of claims 7 to 9, wherein CR1 is 20°C / sec to 120°C / sec.

11. The method according to any one of claims 7 to 10, wherein T1 is 630°C to 725°C.

12. The method according to any one of claims 7 to 11, wherein CR2 is greater than 100°C / sec.

13. The method according to any one of claims 7 to 13, wherein T2 is Ms-50°C and 20°C.

14. The method according to any one of claims 7 to 14, wherein T temper is 200°C to 300°C.

15. Use of a steel sheet obtainable according to any one of claims 1 to 6 or manufactured according to the method of any one of claims 7 to 14 for manufacturing structural parts of a vehicle.

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