Martensitic steel and method for the same

Abstract

The following elements, namely, 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%, with the balance of the composition consisting of iron and unavoidable impurities, and the microstructure of the steel containing, in area percent, at least 92% martensite, 1% - 8% cumulative amount of ferrite and bainite, and 0% - 2% optional amount of retained austenite, a martensitic steel sheet.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing martensitic steel suitable for the automotive industry, durable consumer goods, and environmentally friendly products. More specifically, the present invention relates to martensitic steel having a tensile strength of 1180 MPa or more.

Background Art

[0002] Automotive parts are required to meet two conflicting needs, namely, ease of forming and strength. In recent years, from the perspective of the global environment, a third requirement of improving fuel efficiency has been imposed on automobiles. Therefore, nowadays, automotive parts need to be made of materials with high formability to meet the criterion of ease of adaptation to complex automotive assemblies. At the same time, it is necessary to improve the strength for the collision resistance and durability of the vehicle and to reduce the weight of the vehicle for improving fuel efficiency. For this reason, in order to reduce the amount of material used in vehicles by increasing the strength of the material, active research and development are being carried out. Conversely, when the strength of the steel sheet is increased, the formability decreases. Therefore, it is necessary to develop materials having both high strength and high formability.

[0003] Previous research and development in the field of high-strength and high-formability steel sheets have led to several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed herein for the ultimate understanding of the present invention.

[0004]

[0005] ​The steel sheet of WO2017 / 065371 is manufactured through the following steps: namely, rapidly heating a base steel sheet containing 0.08 to 0.30 wt% C, 0.01 to 2.0 wt% Si, 0.30 to 3.0 wt% Mn, with P at 0.05 wt% or less and S at 0.05 wt% or less, and the balance being Fe and other inevitable impurities, to above the Ac3 transformation point and maintaining it for 3 to 60 seconds; rapidly quenching the heated steel sheet using water or oil at 100 °C / second or more; and rapidly tempering it from 500 °C to the A1 transformation point for 3 to 60 seconds including the heating and maintaining time. However, the steel of WO2017 / 065371 does not mention a tensile strength of 1180 MPa or more in both the rolling and transverse directions. Further, even after having a tempered martensite single-phase structure, the hole expansion rate of the steel of WO2017 / 065371 is 40% or less.

[0006] WO2010 / 036028 relates to a hot-dip galvanized steel sheet and a method for manufacturing the same. The hot-dip galvanized steel sheet includes a steel sheet having a martensite structure as a base material and a hot-dip galvanized layer that has an adverse effect on the steel sheet. The steel sheet contains 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, and impurities, with the balance being Fe. However, the steel of WO2010 / 036028 does not mention the hole expansion rate.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] An object of the present invention is to solve these problems by making it possible to obtain a cold-rolled martensitic steel sheet having the following simultaneously. - A maximum tensile strength of 1180 MPa or more in both the transverse direction and the rolling direction, preferably exceeding 1220 MPa in both the transverse direction and the rolling direction, - A yield strength of 900 MPa to 1180 MPa in both the transverse direction and the rolling direction, preferably 950 MPa to 1180 MPa in both the transverse direction and the rolling direction, - An elongation of more than 3% in both the transverse direction and the rolling direction, - An expansion ratio of more than 40%, preferably more than 50%.

[0009] Preferably, such a steel can also have good weldability and coatability, as well as good compatibility with forming and rolling.

[0010] Another object of the present invention is also to make available a method for manufacturing these plates that is stable against shifts in manufacturing parameters while conforming to conventional industrial applications.

Means for Solving the Problems

[0011] The above objects and other advantages of the present invention will become more apparent by describing the preferred embodiments of the present invention in detail.

[0012] The chemical composition of the cold-rolled martensitic steel contains the following elements.

[0013] The carbon present in the steel of the present invention is 0.08% to 0.14%. Carbon is an element necessary to increase the strength of the steel of the present invention by generating a low-temperature transformation phase such as martensite. Therefore, carbon plays two important roles, one of which is to increase strength. However, a carbon content of less than 0.08% cannot impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.14%, the steel exhibits insufficient spot weldability and its application to automotive parts is restricted. The preferred content for the present invention is 0.09% to 0.13, more preferably 0.1% to 0.12%.

[0014] The manganese content of the steel of the present invention is 1.95% to 2.6%. This element is gammagenous. Manganese brings about solid-solution strengthening, suppresses the ferrite transformation temperature, reduces the ferrite transformation rate, and thus aids in the formation of martensite. An amount of at least 1.95% is required to impart strength and aid in the formation of martensite. However, when the manganese content exceeds 2.6%, adverse effects such as delaying the transformation from austenite to martensite during cooling after annealing occur. A manganese content exceeding 2.6% may excessively segregate in the steel during solidification, impairing the homogeneity inside the material and causing surface cracks during the hot working process. The preferred limit of the presence of manganese is 2% to 2.5%, more preferably 2% to 2.4%, and even more preferably 2% to 2.3%.

[0015] 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 delay the precipitation of carbides during cooling after annealing, and thus silicon promotes the formation of martensite. However, silicon is also a ferrite former and raises the Ac3 transformation point, pushing the annealing temperature into a higher temperature range, so the silicon content is maintained at a maximum of 0.8%. A silicon content exceeding 0.8% also causes temper embrittlement, and in addition, silicon also impairs the coating property. The preferred limit of the presence of silicon is 0.15% to 0.7%, more preferably 0.2% to 0.6%.

[0016] The aluminum content is 0.01 - 0.1%. In the present invention, aluminum removes the oxygen present in the molten steel in order to prevent oxygen from forming a gas phase during the solidification process. Aluminum also fixes the nitrogen in the steel to form aluminum nitride and reduces the grain size. If the aluminum content exceeds 0.1%, the Ac3 point rises to a high temperature, reducing productivity. The preferred limit of the presence of aluminum is 0.01% - 0.05%.

[0017] Titanium is added to the steel of the present invention in an amount of 0.001% - 0.1%. This forms titanium nitride that appears during the solidification of the cast product. The amount of titanium is limited to 0.1% in order to avoid the formation of coarse titanium nitride that is harmful to formability. A titanium content of less than 0.001% has no effect on the steel of the present invention.

[0018] Boron is an optional element essential for the steel of the present invention and may be present in an amount of 0.0001% - 0.05%. When boron is added in an amount of at least 0.0001%, it forms boron nitride and imparts additional strength to the steel of the present invention.

[0019] Sulfur is not an essential element, but may be contained as an impurity in the steel. From the perspective of the present invention, it is preferable that the sulfur content is as low as possible, but from the perspective of manufacturing cost, it is 0.09% or less. Furthermore, when the amount of sulfur present in the steel is large, sulfur particularly combines with manganese to form sulfides, reducing its beneficial effect on the present invention.

[0020] The phosphorus component of the steel of the present invention is 0% - 0.09%. Phosphorus reduces the spot weldability and hot workability due to its tendency to segregate particularly at the grain boundaries or to co-segregate with manganese. For these reasons, its content is limited to 0.09%, preferably less than 0.06%.

[0021] Nitrogen is limited to 0.09% to avoid the aging deterioration of the material and minimize the precipitation of aluminum nitride during solidification, which is harmful to the mechanical properties of steel.

[0022] Chromium is an optional element that can constitute 0% - 1% of the steel. Chromium provides strength to the steel through solid-solution strengthening, and a minimum of 0.1% is required to impart strength. However, if it exceeds 1%, it will damage the surface finish of the steel. The preferred limit of the chromium content is 0.1% - 0.5%.

[0023] Molybdenum is an optional element that constitutes 0% - 0.4% of the steel of the present invention. Molybdenum plays an effective role in improving hardenability and hardness, delays the appearance of bainite, and thus promotes the formation of martensite, especially when added in an amount of at least 0.001% or even at least 0.002%. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.4%.

[0024] Niobium is present in the steel of the present invention in an amount of 0% - 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also affects the size of the microstructure components by its precipitation as carbonitrides and by delaying recrystallization during the heating process. Thus, at the end of the holding temperature, the finer microstructure formed results in the hardening of the product after complete annealing. However, a niobium content exceeding 0.1% shows a saturation effect of its influence, which means that an additional amount of niobium does not bring about an improvement in the strength of the product and is thus not economically interesting.

[0025] Vanadium is effective in improving the strength of steel by forming carbides or carbonitrides, and its upper limit is 0.1% from the perspective of economy.

[0026] Nickel can be added as an optional element in an amount of 0 to 1% in order to increase the strength of the steel of the present invention and improve its toughness. In order to obtain such an effect, a minimum of 0.01% is preferred. However, if its content exceeds 1%, nickel will cause a decrease in ductility.

[0027] Copper can be added as an optional element in an amount of 0 to 1% in order to increase the strength of the steel of the present invention and improve its corrosion resistance. In order to obtain such an effect, a minimum of 0.01% is preferred. However, if its content exceeds 1%, copper may deteriorate the surface properties.

[0028] Calcium can be added to the steel of the present invention in the range of 0.001% to 0.01%. Calcium is added as an optional element to the steel of the present invention, particularly during inclusion treatment. Calcium delays the harmful effects of sulfur by binding the harmful sulfur content in a spherical form and contributes to the refinement of the steel.

[0029] Other elements such as Sn, Pb or Sb can be added individually or in combination at the following ratios, i.e., Sn ≤ 0.1%, Pb ≤ 0.1% and Sb ≤ 0.1%. Up to the maximum content level shown, these elements enable the refinement of crystal grains during solidification. The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

[0030] The microstructure of the martensitic steel sheet will be described in detail here. All percentages are expressed as area fractions.

[0031] Martensite constitutes at least 92% of the microstructure in terms of area fraction. The martensite of the present invention can include both fresh martensite and tempered martensite. However, fresh martensite is an optional microconstituent that is preferably limited to an amount equal to 0% to 4%, preferably 0% to 2%, and even more preferably 0% in the steel. Fresh martensite can be formed during cooling after tempering. Tempered martensite is formed from martensite that occurs during the second stage of cooling after annealing, particularly after below the Ms temperature, more specifically from Ms - 10°C to 20°C. Such martensite is tempered during holding at a tempering temperature Ttemper of 150°C to 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the martensite content is 93% to 99%, more preferably 94% to 98%.

[0032] The cumulative amount of ferrite and bainite corresponds to 1% to 8% of the microstructure. The cumulative presence of bainite and ferrite does not adversely affect the present invention up to 8%, but may adversely affect the mechanical properties if it exceeds 8%. Therefore, the preferred limit of the cumulative presence of ferrite and bainite is maintained at 1% to 7%, more preferably 1% to 6%.

[0033] 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 if the amount is too large, it may adversely affect the tensile strength of the steel.

[0034] Ferrite can occur during the first stage of cooling after annealing, but is not required as a constituent of the microstructure. Ferrite formation should be kept as low as possible, preferably less than 2% or even less than 1%.

[0035] Retained austenite is an optional microstructure that can be present in the steel in an amount of 0% to 2%.

[0036] In addition to the above-mentioned microstructure, the microstructure of the cold-rolled martensitic steel sheet does not contain microstructure components such as pearlite and cementite.

[0037] The steel according to the present invention can be produced by any suitable method. However, as a non-limiting example, it is preferable to use the method according to the present invention described in detail below.

[0038] Such a preferred method is to provide a semi-finished casting of steel having the chemical composition of the prime steel according to the present invention. The casting can be in the form of an ingot or continuously in the form of a thin slab or thin strip, i.e., having a thickness ranging from about 220 mm for a slab to several tens of millimeters for a thin strip.

[0039] For example, a slab having the chemical composition according to the present invention is produced by continuous casting, and the slab is optionally directly subjected to soft reduction during the continuous casting process to avoid center segregation and keep the ratio of local carbon to nominal carbon below 1.10. The slab provided by the continuous casting process can be used directly at a high temperature after continuous casting or can be first cooled to room temperature and then reheated for hot rolling.

[0040] The temperature of the slab to be subjected to hot rolling must be at least 1000 °C and must be less than 1280 °C. If the temperature of the slab is less than 1280 °C, an excessive load is applied to the rolling mill, and furthermore, the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, and the steel will be rolled in a state where transformed ferrite is contained in the structure. Therefore, the temperature of the slab must be high enough so that hot rolling should be completed in the temperature range of Ac3 to Ac3 + 100 °C. Reheating at a temperature exceeding 1280 °C must be avoided because it is industrially expensive.

[0041] Subsequently, the plate thus obtained is cooled to a coiling temperature that must be less than 650°C at a cooling rate of at least 20°C / second. Preferably, the cooling rate is 200°C / second or less.

[0042] Subsequently, the hot-rolled steel plate is coiled at a coiling temperature of less than 650°C, preferably 475°C to 625°C to avoid ovalization and preferably to avoid scale formation. A more preferred range of such coiling temperature is 500°C to 625°C. Subsequently, after the coiled hot-rolled steel plate is cooled to room temperature, it is subjected to optional hot band annealing.

[0043] The hot-rolled steel plate may be subjected to an optional descaling process to remove the scale formed during hot rolling before the optional hot band annealing. Subsequently, the hot-rolled plate may be subjected to optional hot band annealing. In a preferred embodiment, such hot band annealing is carried out at a temperature of 400°C to 750°C, preferably for at least 12 hours and 96 hours or less, to avoid partial deformation of the hot-rolled microstructure and thus, in some cases, to avoid loss of microstructure homogeneity. Therefore, the temperature preferably remains below 750°C. Thereafter, an optional descaling process of this hot-rolled steel plate may be carried out, such as by pickling such a plate.

[0044] The hot-rolled steel plate thus obtained preferably has a thickness of 0.5 mm to 10 mm, more preferably 1 mm to 8 mm, and even more preferably 1 mm to 6 mm. This hot-rolled steel plate can optionally be directly sent to a heat treatment for manufacturing an optionally hot-rolled heat-treated martensitic steel suitable for durable consumer goods and environmentally friendly products. The heat treatment processes for hot-rolled steel plates and cold-rolled steel plates are the same.

[0045] Subsequently, this hot-rolled steel plate is cold-rolled to obtain a cold-rolled steel plate with a thickness reduction rate of 35 to 90%.

[0046] Next, the cold-rolled steel sheet is heated in a two-step heating process. The first step of heating starts from room temperature, and the cold-rolled steel sheet is heated to a temperature HT1 in the range of 410°C to 750°C at a heating rate HR1 of at least 10°C / second. In a preferred embodiment, the heating rate HR1 in such a first step of heating is at least 12°C / second, more preferably at least 13°C / second. A preferred HT1 temperature for such a first step is 425°C to 725°C, more preferably 435°C to 690°C.

[0047] In the second step of heating, the cold-rolled steel sheet is heated from HT1 to an annealing temperature Tsoak which is Ac3 to Ac3 + 100°C, preferably Ac3 + 10°C to Ac3 + 100°C, at a heating rate HR2 which is 0.5°C / second to 50°C / second. In a preferred embodiment, the heating rate HR2 in the second step of heating is 0.7°C / second to 25°C / second, more preferably 0.8°C / second to 20°C / second, and the Ac3 of the steel sheet is calculated using the following formula. 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[Al] + 120[As] + 400[Ti] In the formula, the element content is expressed in weight percent of the cold-rolled steel sheet.

[0048] The cold-rolled steel sheet is held at Tsoak for 10 seconds to 500 seconds to ensure complete recrystallization and complete transformation of the strongly work-hardened initial structure into austenite.

[0049] Next, the cold-rolled steel sheet is cooled in a two-step cooling process where the first step of cooling starts from Tsoak, and the cold-rolled steel sheet is cooled to a temperature T1 in the range of 620°C to 750°C at a cooling rate CR1 of 15°C / second to 150°C / second. In a preferred embodiment, the cooling rate CR1 in such a first step of cooling is 20°C / second to 120°C / second. A preferred T1 temperature for such a first step is 630°C to 725°C.

[0050] In the second cooling step, the cold-rolled steel sheet is cooled from T1 to a temperature T2 which is Ms - 10°C to 20°C at a cooling rate CR2 of at least 50°C / second. In a preferred embodiment, the cooling rate CR2 in the second cooling step is at least 100°C / second, more preferably at least 150°C / second. A preferred T2 temperature for such a second step is Ms - 50°C to 20°C.

[0051] The Ms of the steel sheet is calculated using the following formula. 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]

[0052] Thereafter, the cold-rolled steel sheet is reheated at a heating rate of at least 1°C / second, preferably at least 2°C / second, more preferably at least 10°C / second to a tempering temperature Ttemper of 150°C to 300°C for 100 seconds to 650 seconds. A preferred temperature range for tempering is 200°C to 300°C, and a preferred duration for holding at Ttemper is 200 seconds to 600 seconds.

[0053] Next, the cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled martensitic steel.

[0054] The cold-rolled martensitic steel sheet of the present invention may be coated with zinc or a zinc alloy, or aluminum or an aluminum alloy in order to improve its corrosion resistance.

Examples

[0055] The following tests, examples, illustrative examples, and tables presented in this specification are essentially non-limiting and are to be considered for illustrative purposes only and show advantageous features of the present invention.

[0056] Table 1 summarizes steel plates made of steel with different compositions, showing the presence of each element in weight percent, with the balance being iron and other process impurities. The steel plates are manufactured according to the process parameters specified in Table 2, respectively. Subsequently, Table 3 summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the results of evaluating the obtained properties.

[0057]

Table 1

[0058] Table 2 summarizes the hot rolling and annealing process parameters carried out on the cold-rolled steel plates to impart the mechanical properties necessary for the steel in Table 1 to become martensitic steel.

[0059] Table 2 is as follows.

[0060]

Table 2

[0061] Table 3 exemplifies the results of tests carried out according to standards on different microscopes such as scanning electron microscopes to determine the microstructures of both the steel of the present invention and the reference steel with respect to the area fraction. The results are specified in this specification.

[0062]

Table 3

[0063] Summarize the results of various mechanical tests carried out according to the standards. The maximum tensile strength, yield strength and total elongation are tested according to ISO-6892. To estimate the hole expansion, a test called hole expansion is applied. In this test, a 10 mm hole is drilled in the sample, deformed, and the diameter of the hole is measured after deformation, and HER% = 100*(Df - Di) / Di is calculated.

[0064]

Table 4

Claims

1. A martensitic steel sheet comprising the following elements expressed in weight percent, namely, 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 capable of containing one or more of the following optional elements, namely, 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% wherein the remaining composition is composed of iron and inevitable impurities generated during processing, and the microstructure of the steel contains, in area percent, at least 92% martensite, 1% - 8% cumulative amount of ferrite and bainite, and 0% - 2% optional amount of retained austenite, a martensitic steel sheet.

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

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

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

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

6. The martensitic steel sheet according to any one of claims 1 - 5, having a hole expansion rate exceeding 40%.

7. A method for manufacturing a martensitic steel sheet comprising the following continuous steps. - A step of providing a steel composition according to any one of claims 1 - 4, - A step of reheating the semi-finished product to a temperature of 1000°C - 1280°C, - A step of rolling the semi-finished product in the austenite range where the hot rolling finishing temperature is Ac3 - Ac3 + 100°C to obtain a hot rolled steel sheet, - A step of cooling the sheet at a cooling rate of at least 20°C / second to a coiling temperature below 650°C and coiling the hot rolled sheet, - A step of cooling the hot rolled sheet to room temperature, - Optionally, a step of performing a scale removal process on the hot rolled steel sheet, - Optionally, a step where annealing may be performed on the hot rolled steel sheet. - Optionally, a process of performing a descaling process on the hot-rolled steel sheet; - A process of cold-rolling the hot-rolled steel sheet at a reduction ratio CR of 35 to 90% to obtain a cold-rolled steel sheet - Next, a process of heating the cold-rolled steel sheet by heating in two steps, · The first step of heating the cold-rolled steel sheet starts from room temperature and reaches a temperature HT1 of 410°C to 750°C at a heating rate HR1 of at least 10°C / second, · The second step of heating starts from HT1 and reaches a temperature Tsoak of Ac3 to Ac3 + 100°C at a heating rate HR2 of 0.5°C / second to 50°C / second, and holds it for 10 to 500 seconds; - Next, a process of cooling the cold-rolled steel sheet by cooling in two steps, · The first step of cooling the cold-rolled steel sheet starts from Tsoak and reaches a temperature T1 of 620°C to 750°C at a cooling rate CR1 of 15°C / second to 150°C / second, · The second step of cooling starts from T1 and reaches a temperature T2 of Ms - 10°C to 20°C at a cooling rate CR2 of at least 50°C / second; - Then, the cold-rolled steel sheet is reheated to a tempering temperature Ttemper of 150°C to 300°C at a rate of at least 1°C / second and held for 100 to 650 seconds; - Then, it is cooled to room temperature at a cooling rate of at least 1°C / second to obtain a martensitic steel sheet.

8. The method according to 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 / second to 120°C / second.

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 / second.

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 Ttemper is 200°C to 300°C.

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

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