CN113811624B

CN113811624B - Cold rolled martensitic steel and method for martensitic steel thereof

Info

Publication number: CN113811624B
Application number: CN202080033667.5A
Authority: CN
Inventors: 马蒂厄·西本特里特; 文森特·洛伊斯特; 奥雷莉·埃斯诺
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2019-06-12
Filing date: 2020-06-05
Publication date: 2023-10-20
Anticipated expiration: 2040-06-05
Also published as: WO2020250098A1; JP2022537932A; MA56172A; EP3983568A1; CN113811624A; MX2021015171A; US20220298614A1; ZA202108295B; KR20220005572A; WO2020250009A1; CA3139633A1

Abstract

A cold rolled martensitic steel sheet comprising the following elements: c is more than or equal to 0.3% and less than or equal to 0.4%; mn is more than or equal to 0.5% and less than or equal to 1%; si is more than or equal to 0.2% and less than or equal to 0.6%; cr is more than or equal to 0.1% and less than or equal to 1%; al is more than or equal to 0.01% and less than or equal to 1%; mo is more than or equal to 0.01% and less than or equal to 0.5%; ti is more than or equal to 0.001% and less than or equal to 0.1%; s is more than or equal to 0% and less than or equal to 0.09%; p is more than or equal to 0% and less than or equal to 0.09%; n is more than or equal to 0% and less than or equal to 0.09%; nb is more than or equal to 0% and less than or equal to 0.1%; v is more than or equal to 0% and less than or equal to 0.1%; ni is more than or equal to 0% and less than or equal to 1%; cu is more than or equal to 0% and less than or equal to 1%; b is more than or equal to 0% and less than or equal to 0.05%; ca is more than or equal to 0.001% and less than or equal to 0.01%; sn is more than or equal to 0% and less than or equal to 0.1%; pb is more than or equal to 0% and less than or equal to 0.1%; sb is more than or equal to 0% and less than or equal to 0.1%; the remainder consists of iron and unavoidable impurities resulting from the working, the microstructure of the steel being in area percent: at least 95% martensite, a cumulative amount of ferrite and bainite of 1% to 5%, and an optional amount of residual austenite of 0% to 2%.

Description

Cold rolled martensitic steel and method for martensitic steel thereof

The present invention relates to a method for manufacturing cold rolled martensitic steel suitable for the automotive industry, in particular martensitic steel having a tensile strength of 1700MPa or more.

Automotive parts are required to meet two inconsistent demands, i.e., ease of forming and strength, but in recent years, a third demand for improvement of fuel consumption by automobiles has been given in view of global environmental problems. Therefore, the automobile parts must now be made of a material having high formability to meet the standards for easy assembly of complex automobile components, and at the same time, the strength must be improved for the crashworthiness and durability of the vehicle while reducing the weight of the vehicle to improve fuel efficiency.

Accordingly, a great deal of research and development effort has been made to reduce the amount of material used in automobiles by increasing the strength of the material. Conversely, an increase in the strength of the steel sheet decreases formability, and thus it is necessary to develop a material having both high strength and high formability.

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:

the steel sheet of WO2017/065371 is manufactured by the following steps: rapidly heating and holding a material steel sheet containing 0.08 to 0.30 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P, and 0.05 wt% or less of S, the remainder being Fe and other unavoidable impurities, to an Ac3 transformation point or more over 3 to 60 seconds; rapidly cooling the heated steel sheet with water or oil at 100 ℃/sec or more; and flash tempered to 500 ℃ to A1 transition point for 3 seconds to 60 seconds, including heating and holding times. But the steel of WO2017/065371 is not able to provide a 22% hole expansion ratio while having a tensile strength of 1700 MPa.

The object of the present invention is to solve these problems by manufacturing a cold rolled martensitic steel sheet simultaneously having:

an ultimate tensile strength of 1700MPa or more, and preferably of 1750MPa or more,

-a yield strength of greater than or equal to 1500MPa, and preferably greater than 1550 MPa.

-a hole expansion ratio of at least 22% and preferably greater than 25%.

Preferably, such a steel may also have good formability (for rolling) as well as good weldability and coatability.

It is also an object of the invention to make available a method for manufacturing these panels compatible with conventional industrial applications while being robust to variations in manufacturing parameters.

The above objects and other advantages of the present invention will become more apparent from the detailed description of the preferred embodiments of the present invention.

The chemical composition of the cold rolled martensitic steel comprises the following elements:

the carbon present in the steel of the invention is 0.3% to 0.4%. Carbon is an element necessary to improve the strength of the steel of the present invention by generating a low temperature transformation phase such as martensite, and thus, carbon plays two key roles, one of which is to improve the strength. However, a carbon content of less than 0.3% will not impart tensile strength to the steel of the present invention. On the other hand, at carbon contents exceeding 0.4%, the steel exhibits poor spot weldability, which limits its use for automotive parts. The preferable content for the present invention may be maintained at 0.3% to 0.38%, and more preferably 0.3% to 0.36%.

The manganese content of the steel according to the invention is 0.5% to 1%. The element is a gamma phase generating element (gamma phase). Manganese provides solid solution strengthening and suppresses ferrite transformation temperature and reduces ferrite transformation rate, thus contributing to the formation of martensite. An amount of at least 0.5% is required to impart strength and to aid in the formation of martensite. However, when the manganese content is more than 1%, it has an adverse effect such as it hinders the transformation of austenite to martensite during cooling after annealing. Manganese contents of greater than 1% may excessively segregate in the steel during solidification and the uniformity of the interior of the material is impaired, which may lead to surface cracking during hot working. The preferable limit of the presence of manganese is 0.5% to 0.9%, and more preferably 0.6% to 0.8%.

The silicon content of the steel according to the invention is 0.2% to 0.6%. Silicon is an element that contributes to the improvement of strength by solid solution strengthening. Silicon is a component that can hinder carbide precipitation during cooling after annealing, and therefore, silicon promotes the formation of martensite. But silicon is also a ferrite forming element 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 0.6%. Silicon contents above 0.6% may also temper embrittlement and furthermore silicon also impair coatability. The preferable limit of the presence of silicon is 0.2% to 0.5%, and more preferably 0.25% to 0.45%.

The chromium content of the combined coil of the steel according to the invention is 0.1% to 1%. Chromium is an essential element for providing strength to steel by solid solution strengthening, and requires a minimum of 0.1% to impart strength, but impairs the surface finish of steel when more than 1% is used. The preferred limit of the presence of chromium is 0.3% to 0.9% and more preferably 0.4% to 0.8%.

The content of aluminum is 0.01% to 1%. In the present invention, aluminum removes oxygen present in molten steel to prevent oxygen from forming a gas phase during a solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, thereby reducing grain size. Higher aluminum content (higher than 1%) increases the Ac3 point to high temperature, thereby decreasing productivity. The preferred limit of the presence of aluminum is 0.01% to 0.5%.

0.001% to 0.1% titanium is added to the steel of the present invention. Which forms titanium nitride that occurs during solidification of the casting. The amount of titanium is thus limited to 0.1% to avoid formation of coarse titanium nitride which is detrimental to formability. At a titanium content of less than 0.001%, no effect is exerted on the steel of the present invention.

Molybdenum is an essential element, which constitutes 0.01% to 0.5% of the steel of the invention; molybdenum plays an effective role in improving hardenability and hardness, delaying the occurrence of bainite to promote the formation of martensite, especially when added in an amount of at least 0.01%. Molybdenum also promotes the formation of ferrite and pearlite microstructure during cooling after hot rolling, which ferrite and pearlite microstructure promotes cold rolling. However, excessive addition of molybdenum increases the addition cost of the alloying element, and thus the content thereof is limited to 0.5% for economic reasons. The preferred limit of molybdenum is 0.1% to 0.3%.

Sulfur is not an essential element, but may be contained as an impurity in steel, and the sulfur content is preferably as low as possible from the viewpoint of the present invention, but from the viewpoint of manufacturing cost, the sulfur content is 0.09% or less. Furthermore, if higher sulphur is present in the steel, it especially combines with manganese to form sulphides and reduces the beneficial effect of manganese on the invention.

The phosphorus content of the steel of the invention is 0% to 0.09%. Phosphorus reduces the spot weldability and hot ductility, especially because it tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, the content thereof is limited to 0.09%, and preferably less than 0.06%.

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

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 affect the size of the microstructure components by its precipitation as carbonitride and by impeding recrystallization during the heating process. Thus, a finer microstructure formed at the end of the holding temperature and thus after the complete annealing will cause hardening of the product. However, niobium contents above 0.1% are economically unattractive because of the saturation effect it affects (which means that additional amounts of niobium do not cause any strength improvement in the product).

Vanadium effectively increases the strength of steel by forming carbide or carbonitride, and the upper limit is 0.1% from an economical point of view.

Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength and improve the toughness of the steel of the present invention. A minimum value of 0.01% is preferred to obtain such an effect. However, when the content thereof is more than 1%, nickel causes deterioration in ductility.

Copper may be added as an optional element in an amount of 0% to 1% to thereby increase the strength and improve the corrosion resistance of the steel of the present invention. A minimum value of 0.01% is preferred to obtain such an effect. However, when the content thereof is more than 1%, it may deteriorate the surface appearance.

Boron is an optional element of the steel of the present invention and may be present at 0% to 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.

Calcium may be added to the steel of the present invention in an amount of 0.001% to 0.01%. Calcium is added as an optional element to the steel of the invention, especially during inclusion treatment. Calcium aids in the refining of steel by binding the detrimental sulfur content in the form of spheres, thereby impeding the detrimental effects of sulfur.

Other elements such as Sn, pb, or Sb may be added alone or in combination in the following proportions: sn is less than or equal to 0.1%, pb is less than or equal to 0.1%, and Sb is less than or equal to 0.1%. Up to the maximum content level shown, these elements make it possible to refine the grains during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from the processing.

The microstructure of the martensitic steel sheet will now be described in detail, with all percentages being area fractions.

Martensite constitutes at least 95% of the microstructure in area fraction. The martensite of the present invention may comprise both fresh martensite and tempered martensite. However, fresh martensite is an optional microscopic component, preferably it is limited in the steel by an amount of 0% to 4%, preferably 0% to 2%, and even better equal to 0%. Fresh martensite may be formed during cooling after tempering. Tempered martensite is formed from martensite formed during the second step of cooling after annealing and in particular after a temperature below Ms, and more in particular Ms-10 ℃ to 20 ℃. Such martensite is then tempered during a tempering temperature ttempering maintained at 150 ℃ to 300 ℃. The martensite of the present invention imparts ductility and strength to such steels. Preferably, the content of martensite is 96% to 99%, and more preferably 97% to 99%.

The cumulative amount of ferrite and bainite is 1% to 5% of the microstructure. The cumulative presence of bainite and ferrite does not adversely affect the present invention until 5%, but mechanical properties exceeding 5% may be adversely affected. Therefore, the preferable limit of the cumulative presence of ferrite and bainite is maintained to be 1% to 4%, and more preferably 1% to 3%.

Bainite is formed during reheating prior to tempering. In a preferred embodiment, the steel of the invention comprises 1% to 3% bainite. Bainite may impart formability to steel, but when present in too large amounts, it may adversely affect the tensile strength of the steel.

Ferrite may be formed during the first cooling step after annealing, but is not required as a microstructure component. The ferrite formation must be kept as low as possible and preferably less than 2% or even less than 1%.

Retained austenite is an optional microstructure that may be present in the steel at 0% to 2%.

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

The steel according to the invention may be manufactured by any suitable method. However, as a non-limiting example, the method according to the invention, which will be described in detail, is preferably used.

Such a preferred method consists in providing a semifinished casting of steel having the chemical composition of the initial steel according to the invention. The castings may be formed into ingots or continuously into thin slabs or strips, i.e. thickness ranging from about 220mm for slabs to several tens of millimeters for thin strips.

For example, slabs having the chemical composition according to the invention are manufactured by continuous casting, wherein the slab is optionally subjected to a direct gentle pressure during the continuous casting process to avoid centre segregation and to ensure that the ratio of local carbon to nominal carbon remains below 1.10. The slab provided through the continuous casting process may be directly used at a high temperature after continuous casting, or may be first cooled to room temperature and then heated for hot rolling.

The temperature of the slab subjected to hot rolling must be at least 1000 ℃ and must be lower than 1280 ℃. In the case where the temperature of the slab is lower than 1280 c, an excessive load is applied to the rolling mill, and furthermore, the temperature of the steel may be lowered to the ferrite transformation temperature during finish rolling, so that the steel may be rolled in a state where the structure includes the transformation ferrite. Therefore, the temperature of the slab must be high enough that hot rolling should be completed in the temperature range of Ac3 to Ac3+100 ℃. Reheat at temperatures above 1280 ℃ must be avoided, as this is industrially expensive.

The plate obtained in this way is then cooled at a cooling rate of at least 20 ℃/sec to a coiling temperature that must be lower than 650 ℃. Preferably, the cooling rate is less than or equal to 200 ℃/sec.

The hot rolled steel sheet is then coiled at a coiling temperature below 650 ℃ to avoid ovalization, and preferably 475 ℃ to 625 ℃ to avoid scale formation, wherein even preferred ranges of such coiling temperatures are 500 ℃ to 625 ℃. The coiled hot rolled steel sheet is then cooled to room temperature and then subjected to an optional tropical anneal.

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

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

The cold-rolled steel sheet is thereafter subjected to a heat treatment, which imparts the required mechanical properties and microstructure to the steel according to the invention.

The cold-rolled steel sheet is then heated in a two-step heating process, wherein the first heating step starts from room temperature and the cold-rolled steel sheet is heated to a temperature HT1 in the range of 550 ℃ to 750 ℃ at a heating rate HR1 of at least 10 ℃/sec. In a preferred embodiment, the heating rate HR1 for such first step heating is at least 15 ℃/sec and more preferably at least 18 ℃/sec. The preferred HT1 temperature for such first step is from 575 ℃ to 725 ℃.

In the second heating step, the cold rolled steel sheet is heated from HT1 to A at a heating rate HR2 of 1 to 50 ℃/sec _c 3 to A _c 3+100 ℃, preferably A _c 3+10 ℃ to A _c Annealing temperature T at 3+100℃. In a preferred embodiment, the heating rate HR2 of the second step heating is from 1 ℃/sec to 25 ℃/sec and more preferably from 1 ℃/sec to 20 ℃/sec, wherein Ac3 of the steel sheet is calculated by 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]

wherein the element content is expressed as weight percent of the cold rolled steel sheet.

The cold rolled steel sheet is maintained under T soaking for a period of 10 seconds to 500 seconds to ensure complete recrystallization and complete transformation of the strong work hardened initial structure to austenite.

The cold-rolled steel sheet is then cooled in a two-step cooling process, wherein the first cooling step starts from T soaking and the cold-rolled steel sheet is cooled to a temperature T1 in the range of 630 to 750 ℃ at a cooling rate CR1 of 30 to 150 ℃/sec. In a preferred embodiment, such first step cooling has a cooling rate CR1 of from 30 ℃/sec to 120 ℃/sec. The preferred T1 temperature for such first step is 640℃to 725 ℃.

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

M of steel sheet _s Calculated by 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[At]-16.3[cu]-361[Nb]-2.44[Ti]

-3448[B]

thereafter, the cold rolled steel sheet is re-heated to a tempering temperature T tempering of 150 to 300 ℃ for a time of 100 and 600 seconds at a heating rate of at least 1 ℃/sec, and preferably at least 2 ℃/sec and more, at least 5 ℃/sec. The preferred temperature range for tempering is 200 ℃ to 300 ℃ and the preferred duration of holding under T-tempering is 200 seconds to 500 seconds.

Then, the cold-rolled steel sheet is cooled to room temperature to obtain cold-rolled martensitic steel.

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

Examples

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

In table 1, steel sheets made of steels having different compositions are summarized, wherein the steel sheets were produced according to the process parameters as noted in table 2, respectively. Thereafter, table 3 summarizes the microstructure of the steel sheet obtained during the test, and table 4 summarizes the evaluation results of the obtained characteristics.

TABLE 2

Table 2 summarizes the hot rolling process parameters and the annealing process parameters performed on the cold rolled steel sheet to impart the mechanical properties required for the steel of table 1 to become a cold rolled martensitic steel.

Table 3 illustrates the results of tests performed according to standards on different microscopes, e.g. scanning electron microscopes, for determining the microstructure of both the steel of the present invention and the reference steel, expressed in area fractions. The results are noted herein:

table 3:

test	Martensitic phase	Ferrite + bainite	Retained austenite
				I1	98	2％	-
l2	98	2％	-
				l3	98	2％	-
R1	89％	11％	-
				R2	93.5％	6.5％	-
R3	94％	6％	-

I = according to the invention, R = reference; underlined values: not according to the invention.

TABLE 4 Table 4

The results of various mechanical tests performed according to the standards are summarized. To test the ultimate tensile strength and yield strength, the ultimate tensile strength and yield strength were tested in accordance with JIS-Z2241. To evaluate reaming, a test called reaming was applied, in which the sample was punched (10 mm hole) and deformed, after which we measured the pore size and calculated HER% = 100 x (Df-Di)/Di

Claims

1. A cold rolled martensitic steel sheet comprising, in weight percent:

0.3％≤C≤0.4％；

0.5％≤Mn≤1％；

0.2％≤Si≤0.6％；

0.1％≤Cr≤1％；

0.01％≤Al≤1％；

0.01％≤Mo≤0.5％；

0.001％≤Ti≤0.1％；

0％≤S≤0.09％；

0％≤P≤0.09％；

0％≤N≤0.09％；

and optionally one or more of the following optional elements:

0％≤Nb≤0.1％；

0％≤V≤0.1％；

0％≤Ni≤1％；

0％≤Cu≤1％；

0％≤B≤0.05％；

0.001％≤Ca≤0.01％；

0％≤Sn≤0.1％；

0％≤Pb≤0.1％；

0％≤Sb≤0.1％；

the remainder of the composition consists of iron and unavoidable impurities resulting from the working, the microstructure of the steel comprising, in area percent: at least 95% martensite, a cumulative amount of ferrite and bainite of 1% to 5%, and an optional amount of residual austenite of 0% to 2%;

the cold rolled martensitic steel sheet has an ultimate tensile strength of 1700MPa or greater, a yield strength of 1500MPa or greater, and a hole expansibility of at least 22%.

2. The cold rolled martensitic steel sheet according to claim 1, wherein the composition comprises 0.3% to 0.36% carbon.

3. The cold rolled martensitic steel sheet according to claim 1 or 2, wherein the composition comprises 0.3% to 0.38% carbon.

4. The cold rolled martensitic steel sheet according to claim 1 or 2, wherein the composition comprises 0.01% to 0.5% aluminium.

5. The cold rolled martensitic steel sheet according to claim 1 or 2, wherein the composition comprises 0.5% to 0.9% manganese.

6. Cold rolled martensitic steel sheet according to claim 1 or 2, wherein the composition comprises 0.3 to 0.9% chromium.

7. The cold rolled martensitic steel sheet according to claim 1 or 2, wherein the amount of martensite is 96 to 99%.

8. The cold rolled martensitic steel sheet according to claim 1 or 2, wherein said cumulative amount of ferrite and bainite is 1 to 4%.

9. A method of producing a cold rolled martensitic steel sheet, said method comprising the following sequential steps:

-providing a semi-finished product having a steel composition according to any one of claims 1 to 6;

-reheating the semifinished product to a temperature of 1000 ℃ to 1280 ℃;

-rolling the semifinished product in the austenitic range to obtain a hot rolled steel sheet, wherein the hot rolling finishing temperature is Ac3 to Ac3+100 ℃;

-cooling the plate to a coiling temperature below 650 ℃ at a cooling rate of at least 20 ℃/sec; and coiling the hot rolled sheet;

-cooling the hot rolled sheet to room temperature;

-optionally subjecting the hot rolled steel sheet to a scale removal process;

-optionally annealing the hot rolled steel sheet;

-optionally subjecting the hot rolled steel sheet to a scale removal process;

-cold rolling said hot rolled steel sheet at a reduction of 35-90% to obtain a cold rolled steel sheet;

-then heating the cold rolled steel sheet in a two-step heating, wherein:

o a first step of heating, heating the cold rolled steel sheet from room temperature to a temperature HT1 of 550 ℃ to 750 ℃ at a cooling rate HR1 of at least 10 ℃/sec;

o a second heating step of soaking at a heating rate HR2 of 1 to 50 ℃/sec from HT1 to a temperature T of AC3 to AC3+100 ℃, holding the cold rolled steel sheet at the temperature T soaking for a time of 10 to 500 seconds,

-then cooling the cold rolled steel sheet in a two-step cooling, wherein:

a first step of cooling, wherein the cold-rolled steel sheet is cooled from T-average temperature to a temperature T1 of 630 ℃ to 750 ℃ at a cooling rate CR1 of 30 ℃ to 150 ℃ per second;

a second cooling step, starting from T1 and cooling at a cooling rate CR2 of at least 50 ℃/sec to a temperature T2 of Ms-10 ℃ to 20 ℃,

-then reheating the cold-rolled steel sheet to a tempering temperature twempering of 150 ℃ to 300 ℃ at a rate of at least 1 ℃/sec, at which tempering temperature the cold-rolled steel sheet is held for a time of 100 seconds to 600 seconds;

-then cooling to room temperature at a cooling rate of at least 1 ℃/sec to obtain a cold rolled martensitic steel sheet.

10. The method of claim 9, wherein the take-up temperature is 475 ℃ to 625 ℃.

11. The method of claim 9 or 10, wherein tsoaking is from ac3+10 ℃ to ac3+100 ℃.

12. The method of claim 9 or 10, wherein CR1 is 30 ℃/sec to 120 ℃/sec.

13. The method of claim 9 or 10, wherein T1 is 640 ℃ to 725 ℃.

14. The method of claim 9 or 10, wherein CR2 is at least 100 ℃/sec.

15. The method of claim 9 or 10, wherein T2 is Ms-50 ℃ to 20 ℃.

16. The method of claim 9 or 10, wherein T-tempering is 200 ℃ to 300 ℃.

17. Use of a steel sheet obtained according to any one of claims 1 to 8 or manufactured according to the method of any one of claims 9 to 16 for manufacturing structural parts of a vehicle.