CN112689684B

CN112689684B - Cold rolled and coated steel sheet and method for manufacturing the same

Info

Publication number: CN112689684B
Application number: CN201980059157.2A
Authority: CN
Inventors: 萨马内哈·阿利贝吉
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2018-09-20
Filing date: 2019-09-17
Publication date: 2022-12-09
Anticipated expiration: 2039-09-17
Also published as: HUE062231T2; JP2022501504A; KR102647462B1; EP3853387B1; KR20210061382A; JP7422143B2; WO2020058829A1; US20220033925A1; ZA202101225B; UA126725C2; WO2020058748A1; CN112689684A; MX2021003290A; FI3853387T3; CA3110629C; ES2946086T3; EP3853387A1; CA3110629A1; BR112021003583A2; MA53640B1

Abstract

A cold rolled and heat treated steel sheet having a composition comprising the following elements: 0.13% to less than or equal to 0.18% carbon, 1.1% to less than or equal to 1.8% manganese, 0.5% to less than or equal to 0.9% silicon, 0.6% to less than or equal to 1% aluminum, 0.002% to less than or equal to 0.02% phosphorus, 0% to less than or equal to 0.003% sulfur, 0% to less than or equal to 0.007% nitrogen, and may include one or more of the following optional elements: chromium is more than or equal to 0.05 percent and less than or equal to 1 percent, molybdenum is more than or equal to 0.5 percent and less than or equal to 0.001 percent and less than or equal to 0.1 percent, titanium is more than or equal to 0.001 percent and less than or equal to 0.1 percent, copper is more than or equal to 0.01 percent and less than or equal to 2 percent, nickel is more than or equal to 0.01 percent and less than or equal to 3 percent, calcium is more than or equal to 0.0001 percent and less than or equal to 0.005 percent, vanadium is more than or equal to 0 percent and less than or equal to 0.1 percent, boron is more than or equal to 0.003 percent and less than or equal to 0.1 percent, magnesium is more than or equal to 0 percent and less than or equal to 0.010 percent, zirconium is more than or equal to 0.010 percent and the rest is composed of iron and inevitable impurities generated due to processing, and the microstructure of the steel plate comprises the following components in terms of area fraction: 60% to 75% of ferrite, 20% to 30% of bainite, 10% to 15% of residual austenite and 0% to 5% of martensite, wherein the cumulative amount of residual austenite and ferrite is 70% to 80%.

Description

Cold rolled and coated steel sheet and method for manufacturing the same

The present invention relates to a cold-rolled and coated steel sheet suitable for use as a steel sheet for automobiles.

Automobile parts are required to satisfy two inconsistent requirements, i.e., easy forming and strength, but in recent years, in view of global environmental issues, automobiles are also given a third requirement to improve fuel consumption. Therefore, automobile parts must now be made of materials having high formability to meet the standards for easy assembly of complex automobile components, and at the same time, must improve strength for vehicle crashworthiness and durability while reducing the weight of the vehicle to improve fuel efficiency.

Accordingly, a great deal of research and development efforts have been made to reduce the amount of materials used in automobiles by increasing the strength of the materials. 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 enumerated herein for a clear understanding of the present invention:

US20140234657 is a patent application claiming a hot-dip galvanized steel sheet having the following microstructure: one or both of martensite and bainite in total is equal to or more than 20% and equal to or less than 99% by volume fraction, and the residual structure contains less than 8% by volume fraction of one or both of residual austenite, ferrite, and equal to or less than 10% by volume fraction of pearlite. Furthermore, US20140234657 achieves a tensile strength of 980MPa but does not achieve an elongation of 25%.

US8657969 claims a high strength galvanized steel sheet having tensile strength of 590MPa or more and excellent workability. The composition of the components comprises the following components in percentage by mass: c:0.05 to 0.3%, si:0.7 to 2.7%, mn:0.5% to 2.8%, P:0.1% or less, S:0.01% or less, al:0.1% or less, N:0.008% or less, and the balance: fe or unavoidable impurities. The microstructure comprises, by area ratio: ferrite phase: 30% to 90%, bainite phase: 3% to 30%, martensite phase: 5% to 40%, wherein, in the martensite phase, the martensite phase having an aspect ratio of 3 or more is present in a proportion of 30% or more.

The object of the present invention is to solve these problems by making available a cold-rolled and coated steel sheet having at the same time the following characteristics:

an ultimate tensile strength greater than or equal to 600MPa and preferably greater than 620MPa,

-a total elongation greater than or equal to 31% and preferably greater than 33%.

In a preferred embodiment, the steel sheet according to the present invention may also exhibit a yield strength of 320MPa or more.

In a preferred embodiment, the steel sheet according to the present invention may also exhibit a yield strength to tensile strength ratio of 0.6 or more.

Preferably, such a steel may also have good suitability for forming (in particular for rolling) and good weldability and coatability.

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

The cold rolled and heat treated steel sheet of the present invention may optionally be coated with zinc or a zinc alloy, or with aluminum or an aluminum alloy to improve its corrosion resistance.

Carbon is present in the steel at 0.13% to 0.18%. Carbon is an element necessary for increasing the strength of a steel sheet by generating a low-temperature transformation phase such as bainite, and in addition, carbon also plays a key role in austenite stabilization and is therefore an essential element for ensuring residual austenite. Thus, carbon plays two key roles, one is to increase strength and the other is to retain austenite to impart ductility. However, a carbon content of less than 0.13% will not stabilize austenite in a sufficient amount required for the steel of the present invention. On the other hand, at carbon contents exceeding 0.18%, the steel exhibits poor spot weldability, which limits its application to automotive parts.

The manganese content of the steel of the invention is 1.1% to 1.8%. This element is a gamma-phase-generating element (gamma-genius). The purpose of adding manganese is essentially to obtain a structure comprising austenite and to give strength to the steel. Manganese in an amount of at least 1.1 wt.% was found to provide strength and hardenability of the steel sheet and to stabilize austenite. However, when the manganese content is more than 1.8%, it exerts adverse effects, such as hindering the transformation of austenite into bainite during the overaging holding for the bainite transformation. Further, a manganese content of more than 1.8% also reduces the ductility of the inventive steel and also deteriorates weldability, so that the elongation target may not be achieved. The preferred content for use in the present invention may be maintained at 1.2% to 1.8%, further more preferably 1.3% to 1.7%.

The silicon content of the steel of the invention is 0.5% to 0.9%. Silicon is a component that can hinder the precipitation of carbides during overaging, and therefore, carbon-rich austenite is stable at room temperature due to the presence of silicon. Furthermore, because of the poor solubility of silicon in carbides, it effectively inhibits or hinders the formation of carbides and therefore also promotes the formation of a bainite structure, which is sought according to the invention in order to impart to the steel its essential characteristics. However, the disproportionate silicon content does not produce the effects mentioned and leads to problems such as temper embrittlement. Therefore, the concentration is controlled within the upper limit of 0.9%. The preferred content for use in the present invention may be maintained at 0.6% to 0.8%.

Aluminum is an essential element and is present in the steel at 0.6% to 1%. Aluminum is an alpha phase generating element (alphagene element) and imparts a total elongation to the steel of the present invention. A minimum of 0.6% aluminum is required to have a minimum of ferrite to impart elongation to the steel of the present invention. Aluminum also serves to remove oxygen from the molten state of the steel to clean the steel of the present invention, and aluminum also prevents oxygen from forming a gas phase. But as long as the aluminum is greater than 1%, it forms AlN, which is detrimental to the steel of the present invention, and therefore, the preferred range of presence of aluminum is 0.6% to 0.8%.

The phosphorus content of the steel of the invention is 0.002% to 0.02%. Phosphorus reduces the spot weldability and hot ductility, in particular due to its tendency to segregate at grain boundaries or to co-segregate with manganese. For these reasons, the content thereof is limited to 0.02% and preferably below 0.014%.

Sulfur is not an essential element but may be contained as an impurity in the steel, and the sulfur content is preferably as low as possible from the viewpoint of the present invention, but is 0.003% or less from the viewpoint of the production cost. Furthermore, if higher sulphur is present in the steel, it combines especially with manganese to form sulphides and reduces its beneficial effect on the steel of the invention.

Nitrogen is limited to 0.007% to avoid aging of the material and to minimize precipitation of nitrides during solidification, which is detrimental to the mechanical properties of the steel.

Chromium is an optional element for the present invention. The chromium content which may be present in the steel according to the invention is between 0.05% and 1%. Chromium is an essential element to provide strength and hardening to the steel, but when used at greater than 1%, it impairs the surface finish of the steel. In addition, a chromium content of less than 1% coarsens the dispersion pattern of carbides in the bainite structure, and therefore, the density of carbides in bainite is kept low.

Molybdenum is an optional element constituting 0.001% to 0.5% of the steel of the invention; molybdenum plays an effective role in determining hardenability and hardness, delaying the appearance of bainite and avoiding carbide precipitation in bainite. However, the addition of molybdenum excessively increases the addition cost of the alloying element, so that the content thereof is limited to 0.5% for economic reasons.

Niobium is an optional element for the present invention. The niobium content may be present in the steel of the present invention at 0.001% to 0.1% and added to the steel of the present invention to form carbonitride by precipitation hardening to impart strength to the steel of the present invention. Niobium also affects the size of the microstructure components by its precipitation as carbonitrides and by hindering recrystallization during the heating process. The microstructure formed at the end of the holding temperature and therefore after the completion of the annealing is therefore finer, which leads to hardening of the steel according to the invention. However, a niobium content of more than 0.1% is economically unattractive, since its influencing saturation effect is observed (which means that the additional amount of niobium does not cause any strength improvement of the product).

Titanium is an optional element and may be added to the steel of the present invention at 0.001% to 0.1%. Like niobium, it participates in carbonitride formation and thus plays a role in hardening of the steel of the present invention. In addition, titanium also forms titanium nitrides that occur 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 where the titanium content is less than 0.001%, it does not impart any effect to the steel of the present invention.

Copper may be added as an optional element in an amount of 0.01% to 2% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% copper is required to achieve this effect. However, when the content thereof is more than 2%, it may deteriorate the surface appearance.

Nickel may be added as an optional element in an amount of 0.01% to 3% to increase the strength and improve the toughness of the steel. A minimum of 0.01% is required to produce such an effect. However, when the content thereof is more than 3%, nickel causes deterioration of ductility.

The calcium content in the steel of the present invention is 0.0001% to 0.005%. Calcium is added as an optional element to the steel of the invention, especially during inclusion treatment. Calcium aids in the refining of the steel by preventing the detrimental sulfur content of the globular form, thereby hindering the detrimental effects of sulfur.

Vanadium effectively increases the strength of steel by forming carbides or carbonitrides, and the upper limit is 0.1% for economic reasons. Other elements such as cerium, boron, magnesium or zirconium may be added singly or in combination in the following weight ratios: less than or equal to 0.1 percent of cerium, less than or equal to 0.003 percent of boron, less than or equal to 0.010 percent of magnesium and less than or equal to 0.010 percent of zirconium. These elements make it possible to refine the grains during solidification, up to the maximum content level indicated. The remainder of the composition of the steel consists of iron and unavoidable impurities resulting from the working.

The microstructure of the steel sheet comprises:

for the steel of the invention, ferrite constitutes 60% to 75% of the microstructure in area fraction. Ferrite as a matrix constitutes the primary phase of the steel. In the present invention, the ferrite cumulatively includes polygonal ferrite and acicular ferrite. Ferrite imparts high strength and elongation to the steel of the present invention. In order to secure an elongation of 31% and preferably 33% or more, it is necessary to have 60% ferrite. Ferrite is formed in the steel of the present invention during cooling after annealing. However, as long as the ferrite content is present in the steel of the present invention at more than 75%, strength cannot be achieved.

For the steel according to the invention, the bainite constitutes 20 to 30% of the microstructure in area fraction. In the present invention, bainite cumulatively consists of lath bainite and granular bainite, with 20% bainite being required in order to ensure a tensile strength of 620MPa and preferably 630MPa or more. Bainite is formed during the overaging holding.

The retained austenite constitutes 10 to 15% by area fraction of the steel. Residual austenite is known to have a higher solubility for carbon than bainite and therefore acts as an effective carbon trap, hindering the formation of carbides in bainite. The percentage of carbon in the retained austenite according to the invention is preferably higher than 0.9% and preferably lower than 1.1%. The retained austenite of the steel according to the invention imparts increased ductility.

Martensite is an optional ingredient, and may be present in an area fraction of 0% to 5% of the microstructure and found in trace amounts. Martensite used in the present invention includes both fresh martensite and tempered martensite. Due to the cooling after annealing, the invention forms martensite and gets tempered during overaged holding. Fresh martensite is also formed during cooling after coating of the cold-rolled steel sheet. When the martensite is less than 5%, the martensite imparts ductility and strength to the steel of the present invention. When the martensite exceeds 5%, it imparts excessive strength but reduces the elongation beyond acceptable limits. The preferable limit of martensite is 0% to 3%.

The total amount of ferrite and retained austenite must always be 70% to 80% to have a total elongation of 31%, and a minimum of 70% is required to ensure a total elongation of more than 31% while having a tensile strength of 600 MPa. Ferrite and retained austenite are soft phases compared to martensite and bainite, thus imparting elongation and ductility, but as long as more than 80% is cumulatively present, the strength drops beyond acceptable limits.

The microstructure of the cold-rolled and heat-treated steel sheet does not contain microstructure components such as pearlite and cementite in addition to the above-described microstructure without impairing the mechanical properties of the steel sheet.

The steel sheet according to the invention may be produced by any suitable method. A preferred method comprises providing a semi-finished casting of steel having a chemical composition according to the invention. The casting may be made in the form of an ingot or continuously in the form of thin slabs or thin strips, i.e. with a thickness ranging from about 220mm for slabs to tens of millimeters for thin strips.

For example, a slab having the above chemical composition is manufactured by continuous casting, wherein the slab is optionally subjected to direct light reduction during the continuous casting process to avoid center segregation and ensure that the local carbon to nominal carbon ratio remains below 1.10. The slab provided through the continuous casting process may be used directly at a high temperature after continuous casting, or may be first cooled to room temperature and then reheated to be hot rolled.

The temperature of the slab subjected to hot rolling is at least 1150 ℃ and must be lower than 1280 ℃. In the case where the temperature of the slab is lower than 1150 ℃, an excessive load is applied to the rolling mill. Therefore, preferably, the temperature of the slab is sufficiently high so that hot rolling can be completed in a temperature range of Ac1+50 ℃ to Ac1+250 ℃, preferably Ac1+50 ℃ to Ac1+200 ℃, while the finish rolling temperature is always maintained above Ac1+50 ℃. Reheating at temperatures above 1280 ℃ must be avoided as this is industrially expensive.

The finishing rolling temperature range of Ac1+50 deg.C to Ac1+250 deg.C is preferred to have a structure favorable for recrystallization and rolling. It is necessary to carry out the final pass at a temperature higher than Ac1+50 c because below this temperature the steel sheet shows a significant reduction in the rollability. The sheet obtained in this way is then cooled at a cooling rate of more than 30 ℃/s to a coiling temperature which must be below 625 ℃. 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 625 ℃ to avoid ovalization (ovalization) and preferably below 600 ℃ to avoid scale formation. The preferable range of such a take-up temperature is 350 ℃ to 600 ℃. The coiled hot rolled steel sheet may be cooled to room temperature and then subjected to optional hot band annealing (hot band annealing).

The hot rolled steel sheet may be subjected to an optional scale removal step to remove scale formed during hot rolling prior to optional hot strip annealing. The hot rolled sheet may then be subjected to an optional hot strip annealing at a temperature of 400 ℃ to 750 ℃ for at least 12 hours and not more than 96 hours, the temperature being kept below 750 ℃ to avoid partially transforming the hot rolled microstructure and, therefore, 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. Subjecting the hot-rolled steel sheet to cold rolling at a gauge reduction of 35% to 90% to obtain a cold-rolled steel sheet. The cold rolled steel sheet obtained from the cold rolling process is then subjected to annealing to impart microstructure and mechanical properties to the steel of the invention.

In annealing, the cold rolled steel sheet is subjected to two-step heating to reach a soaking temperature of Ac1+30 ℃ to Ac3, wherein Ac1 and Ac3 are calculated for the inventive steel by using the following formula:

Ac1＝723-10，7[Mn]-16[Ni]+29，1[Si]+16，9[Cr]+6，38[W]+290[As]

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 in weight percent.

In a first step, the cold rolled steel sheet is heated to a temperature range of 550 ℃ to 650 ℃ at a heating rate of 10 ℃/sec to 40 ℃/sec. Thereafter, in the subsequent second heating step, the cold rolled steel sheet is heated to the soaking temperature of annealing at a heating rate of 1 to 5 ℃/sec.

The cold rolled steel sheet is then preferably held at the soaking temperature for a time of 10 to 500 seconds to ensure at least 30% of the strong work hardening initial structure is transformed into an austenitic microstructure. The cold rolled steel sheet is then cooled in two steps to an overaging holding temperature. In the first cooling step, the cold rolled steel sheet is cooled to a temperature in the range of 600 ℃ to 720 ℃, preferably 625 ℃ to 720 ℃ at a cooling rate of less than 5 ℃/sec, preferably less than 3 ℃/sec. During this first cooling step, the ferritic matrix of the invention is formed. Thereafter, in the subsequent second cooling step, the cold rolled steel sheet is cooled to an overaging temperature range of 250 ℃ to 470 ℃ at a cooling rate of 10 ℃/sec to 100 ℃/sec. The cold rolled steel sheet is then held in the overaging temperature range for a period of 5 seconds to 500 seconds. The cold rolled steel sheet is then brought to a temperature in the coating bath temperature range of 400 ℃ to 480 ℃ to facilitate coating of the cold rolled steel sheet. Then, the cold rolled steel sheet is coated by any known industrial process such as electro-galvanizing, JVD, PVD, hot dip coating (GI), etc.

Examples

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

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

TABLE 1

Underlined values: not in accordance with the invention

TABLE 2

Table 2 summarizes the annealing process parameters performed on the steels of table 1. Steel compositions a and B were used to manufacture the plate according to the invention. The table also illustrates the reference steels designated C and D in the table. Table 2 also shows a list of Ac1 and Ac 3. For the inventive and reference steels, these Ac1 and Ac3 are defined as follows:

Ac1＝723-10，7[Mn]-16[Ni]+29，1[Si]+16，9[Cr]+6，38[W]+290[As]

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 in weight percent.

All the plates were cooled after hot rolling at a cooling rate of 34 ℃/sec and finally reached a temperature of 460 ℃ before coating. All sheets had a cold rolling reduction of 65%.

Table 2 is as follows:

TABLE 3

Table 3 illustrates the results of tests performed according to the standard on different microscopes, such as scanning electron microscopes, used to determine the microstructure of both the inventive steel and the reference steel.

The results are noted herein:

i = according to the invention; r = reference; underlined values: not in accordance with the invention

TABLE 4

Table 4 illustrates the mechanical properties of both the inventive steel and the reference steel. In order to determine the tensile strength, yield strength and total elongation, a tensile test was performed according to JIS Z2241 standard.

The results of each mechanical test performed according to the standard are summarized.

TABLE 4

Claims

1. Cold rolled steel sheet, the composition of which comprises, in weight percent:

carbon is between 0.13 and 0.18 percent

Manganese is more than or equal to 1.1 percent and less than or equal to 1.8 percent

Silicon is more than or equal to 0.5 percent and less than or equal to 0.9 percent

0.6 percent to 1 percent of aluminum

Phosphorus is more than or equal to 0.002% and less than or equal to 0.02%

Sulfur is more than or equal to 0 percent and less than or equal to 0.003 percent

Nitrogen is more than or equal to 0 percent and less than or equal to 0.007 percent

And optionally one or more of the following elements:

chromium is between 0.05 and 1 percent

Molybdenum is more than or equal to 0.001 percent and less than or equal to 0.5 percent

Niobium is more than or equal to 0.001 percent and less than or equal to 0.1 percent

Titanium is more than or equal to 0.001 percent and less than or equal to 0.1 percent

Copper is more than or equal to 0.01 percent and less than or equal to 2 percent

Nickel is more than or equal to 0.01 percent and less than or equal to 3 percent

Calcium is more than or equal to 0.0001 percent and less than or equal to 0.005 percent

Vanadium is more than or equal to 0 percent and less than or equal to 0.1 percent

Boron is between 0 and 0.003 percent

Cerium is more than or equal to 0 percent and less than or equal to 0.1 percent

Magnesium is between 0 and 0.010 percent

Zirconium is between 0 and 0.010 percent

The remainder of the composition is composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel sheet comprises, in area fraction: 60% to 75% of ferrite, 20% to 30% of bainite, 10% to 15% of residual austenite and 0% to 5% of martensite, wherein the cumulative amount of residual austenite and ferrite is 70% to 80%.

2. The cold rolled steel sheet of claim 1, wherein the composition comprises 0.6% to 0.8% silicon.

3. The cold rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.14 to 0.18% carbon.

4. The cold rolled steel sheet according to claim 3, wherein the composition comprises 0.6 to 0.8% of aluminum.

5. The cold rolled steel sheet according to claim 1 or 2, wherein the composition comprises 1.2 to 1.8% manganese.

6. The cold rolled steel sheet according to claim 5, wherein the composition comprises 1.3 to 1.7% manganese.

7. The cold rolled steel sheet as claimed in claim 1 or 2, wherein the cumulative amount of ferrite and residual austenite is 73 to 80% and the percentage of residual austenite is less than 13%.

8. Cold rolled steel sheet according to claim 1 or 2, wherein the amount of martensite is between 0% and 3%.

9. The cold rolled steel sheet according to claim 1 or 2, wherein the carbon content of the retained austenite is 0.9 to 1.1%.

10. The cold rolled steel sheet according to claim 1 or 2, wherein the steel sheet has an ultimate tensile strength of 600MPa or more and a total elongation of 31% or more.

11. The cold rolled steel sheet according to claim 10, wherein the steel sheet has a yield strength of 320MPa or more and a total elongation of 33% or more.

12. The cold rolled steel sheet according to claim 1 or 2, wherein the steel sheet is coated.

13. A method of producing a cold rolled steel sheet according to any one of claims 1 to 6, comprising the following sequential steps:

-providing a semi-finished product having the elemental composition of the cold rolled steel sheet;

-reheating the semi-finished product to a temperature of 1150 ℃ to 1280 ℃;

-rolling the semi-finished product in the austenite range to obtain a hot rolled steel sheet, wherein the hot rolling end temperature should be Ac1+50 ℃ to Ac1+250 ℃;

-cooling the sheet to a coiling temperature below 625 ℃ at a cooling rate of more than 30 ℃/s; and coiling the hot rolled sheet;

-cooling the hot rolled sheet to room temperature;

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

-optionally annealing the hot rolled steel sheet at a temperature of 400 ℃ to 750 ℃;

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

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

-then annealing at a soaking temperature of Ac1+30 ℃ to Ac3 for a duration of 10 seconds to 500 seconds by heating the cold rolled steel sheet via two-step heating, wherein:

heating the cold rolled steel sheet to a temperature range of 550 ℃ to 650 ℃ at a heating rate of 10 ℃/sec to 40 ℃/sec in the first heating step;

then in a second step, the cold rolled steel sheet is heated at a heating rate of 1 to 5 ℃/sec from the temperature range of 550 to 650 ℃ and to the annealing soaking temperature at which it is maintained,

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

cooling the cold rolled steel sheet in a first step cooling to a temperature range of 600 ℃ to 720 ℃ at a cooling rate of less than 5 ℃/sec

O in a second step, the plate is then cooled from a temperature range of 600 ℃ to 720 ℃ to an overaging temperature at a cooling rate of 10 ℃/s to 100 ℃/s

-then subjecting the cold rolled steel sheet to overaging at a temperature range of 250 ℃ to 470 ℃ for a time of 5 seconds to 500 seconds, and

-then cooling to room temperature to obtain a cold rolled steel sheet.

14. The method of claim 13, wherein the coiling temperature is less than 600 ℃.

15. The method of claim 13 or 14, wherein the hot rolling end temperature is Ac1+50 ℃ to Ac1+200 ℃.

16. The method of claim 13, wherein the cooling rate after annealing in the temperature range of 625 ℃ to 720 ℃ is less than 3 ℃/sec.

17. A method according to claim 13, wherein the cold rolled steel sheet is annealed at Ac1+30 ℃ to Ac3 and the annealing temperature is selected to ensure that at least 30% austenite is present at the end of the soaking.

18. The method according to claim 13, wherein the cold rolled steel sheet is coated at a temperature in the range of 400 ℃ to 480 ℃.

19. Use of a steel sheet according to any one of claims 1 to 12 or produced according to the method of claims 13 to 18 for the manufacture of a structural or safety part of a vehicle.

20. A vehicle comprising a component obtained according to claim 19.