CN115698365A

CN115698365A - Heat-treated cold-rolled steel sheet and method for producing same

Info

Publication number: CN115698365A
Application number: CN202080101746.5A
Authority: CN
Inventors: 文森特·洛伊斯特; 韦罗妮克·埃贝尔; 马蒂厄·西本特里特
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2020-07-20
Filing date: 2020-07-20
Publication date: 2023-02-03
Anticipated expiration: 2040-07-20
Also published as: US20230287531A1; JP2023534180A; EP4182488A1; KR20230016218A; WO2022018481A1; CN115698365B; MX2023000811A; CA3183159A1; ZA202212175B; BR112022023896A2

Abstract

A heat-treated cold-rolled steel sheet comprising the following elements: 0.1% to 0.2% of C; 1.2% to 2.2% of Mn; 0.05% Si 0.6%;0.001% to 0.1% of Al; 0.01% and 0.5% of Cr; 0% and S0.09%; 0% and P0.09%; 0% to 0.09% of N; mo is between 0% and 0.5%;0% and less than or equal to 0.1% of Ti; nb is between 0% and 0.1%;0% to 0.1% of V; ni is less than or equal to 0% and less than or equal to 1%;0% and Cu 1%;0% to 0.005% of Ca; b is less than or equal to 0% and less than or equal to 0.05%; the remainder consisting of iron and unavoidable impurities resulting from working, the microstructure of the steel comprising, in area percentages: 60% to 85% tempered martensite, 15% to 38% ferrite and bainite in cumulative amounts, 0% to 5% residual austenite in optional amounts, and 0% to 5% fresh martensite in optional amounts.

Description

Heat-treated cold-rolled steel sheet and method for producing same

The present invention relates to a cold rolled steel sheet having high strength and high formability.

Automotive parts need to satisfy two inconsistent requirements, i.e., easy formability and strength, but in recent years, in consideration 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 improving the strength of the materials. Conversely, an increase in the strength of the steel sheet reduces 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:

EP3187608 provides a high-strength hot-dip galvanized steel sheet having a Tensile Strength (TS) of 1,300mpa or more and excellent in ductility and in-plane uniformity of material characteristics, and also provides a method for manufacturing the steel sheet. The high-strength hot-dip galvanized steel sheet has a specific composition containing C, si, mn, and the like. In the chemical composition, the content of Ti [ Ti ] and the content of N [ N ] satisfy [ Ti ] >4[N ]. The high-strength hot-dip galvanized steel sheet has a microstructure including: 60% or more and 90% or less of martensite in an area fraction, more than 5% and 40% or less of polygonal ferrite in an area fraction, and less than 3% (including 0%) of retained austenite in an area fraction. In terms of vickers hardness, the martensite has an average hardness of 450 or more and 600 or less, and the martensite has an average grain diameter of 10 μm or less. The standard deviation of the grain diameter of martensite is 4.0 μm or less. EP3187608 is capable of providing tensile strength of greater than 980MPa but does not have an elongation of 8% or greater.

EP3473741 is a steel plate having tensile strength of 950MPa or more and good toughness and a method for manufacturing the same. The steel sheet has a specific composition and a metallographic structure comprising: ferrite having an area fraction of 30% or less (including 0%), tempered martensite having an area fraction of 70% or more (including 100%), and retained austenite having an area fraction of 4.5% or less (including 0%), wherein the average aspect ratio of iron-based carbides precipitated in grains of the tempered martensite, the grain size of which is first 10% large, is 3.5 or more. The steel of EP3473741, however, does not provide an ultimate tensile strength of 950 or more in both the rolling direction and the transverse direction.

The known prior art relating to the manufacture of high strength and high formability steel sheets has one or another drawback: there is therefore a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same.

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

-an ultimate tensile strength of 980 to 1150MPa in both the transverse direction and the rolling direction, and preferably an ultimate tensile strength of 980 to 1150MPa in both the transverse direction and the rolling direction.

In a preferred embodiment, the total elongation of the steel sheet is greater than or equal to 8%,

in a preferred embodiment, the yield strength in both the transverse direction and the rolling direction is 700MPa to 850MPa, and the yield strength in both the transverse direction and the rolling direction is preferably 720MPa to 850MPa.

Preferably, such steels may also have good suitability for forming, in particular for rolling, and good weldability and coatability.

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

Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

Carbon is present in the steel at 0.1% to 0.2%. Carbon is an element necessary for improving the strength of the steel sheet by generating a low-temperature transformation phase such as martensite. A content of less than 0.1% will not allow martensite to be formed there by tempered martensite, thereby reducing strength and ductility. On the other hand, at a carbon content exceeding 0.2%, the welded zone and the heat-affected zone are significantly hardened, and thus the mechanical characteristics of the welded zone are impaired. The preferable limit of carbon is 0.12% to 0.19%, and more preferably 0.13% to 0.17%.

The manganese content of the steel of the invention is 1.2% to 2.2%. Manganese is an element imparting strength. In order to provide strength and hardenability to the steel sheet, it has been found that the amount of manganese is at least about 1.2 wt%. Therefore, higher percentages of manganese, e.g., 1.3% to 2.1%, are preferred. But when manganese is more than 2.2%, this produces adverse effects such as slowing the transformation of austenite to ferrite during slow cooling after annealing, resulting in a reduction in ductility. Furthermore, a manganese content higher than 2.2% will also reduce the weldability of the steel according to the invention. Therefore, the preferable limit of the steel of the present invention is 1.3% to 2.1%, and more preferably 1.6% to 2.0%.

Silicon is an essential element of the steel of the present invention, and is present in the range of 0.05% to 0.6%. Silicon is added to the steel of the present invention to impart strength by solid solution strengthening. Silicon plays a role in the formation of the microstructure by preventing the precipitation of carbides and by promoting the formation of martensite. But the surface characteristics and weldability of the steel deteriorate whenever the silicon content is more than 0.6%, so the silicon content is preferably 0.1% to 0.5%, and more preferably 0.1% to 0.4%.

The aluminum content of the invention is 0.001% to 0.1%. Aluminum is added to deoxidize the steel of the present invention. Aluminum is an alpha phase generating element (alphageneous element). This may improve the formability and ductility of the steel. In order to obtain such an effect, the aluminum content needs to be 0.001% or more. However, when the aluminum content exceeds 0.1%, the Ac3 point rises beyond the acceptable, and the austenite single phase is very difficult to achieve industrially, so that hot rolling in the complete austenite region cannot be performed. Therefore, the aluminum content should not be more than 0.1%. The preferred limit of the presence of aluminum is from 0.001% to 0.09%, and more preferably from 0.001% to 0.06%.

The chromium content of the steel of the invention is between 0.01% and 0.5%. Chromium is an essential element to provide strength and hardening to the steel, but above 0.5% in use impairs the surface finish of the steel. The preferable limit of chromium is 0.1% to 0.4%, and more preferably 0.1% to 0.3%.

The phosphorus content of the steel of the invention is limited to 0.09%. Phosphorus is an element that hardens in solid solution and also interferes with the formation of carbides. Thus, a small amount of phosphorus of at least 0.002% may be advantageous, but phosphorus also has adverse effects, such as a reduction in 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 phosphorus content is preferably limited to a maximum of 0.09%.

Sulfur is not an essential element, but may be contained as an impurity in the steel at most 0.09%. The sulfur content is preferably as low as possible, but from the viewpoint of manufacturing cost, 0.001% to 0.03% is preferable. Furthermore, if higher sulfur is present in the steel, the sulfur combines especially with Mn and Ti to form sulfides and reduces their beneficial effect on the invention.

To avoid material degradation, nitrogen was limited to 0.09%. Nitrogen can form nitrides with vanadium and niobium or carbonitrides together with carbon by precipitation strengthening, which can impart strength to the steel of the present invention, but when nitrogen is present at more than 0.09%, it can form a large amount of aluminum nitrides disadvantageous to the present invention, and thus the preferable limit of nitrogen is 0.001% to 0.01%.

Molybdenum is an optional element constituting 0% to 0.5% of the steel of the invention; molybdenum increases the hardenability of the steel of the invention and influences the transformation of austenite to ferrite and bainite during cooling after annealing. However, excessive addition of molybdenum increases the cost of adding alloying elements, and therefore the molybdenum content is limited to 0.5% for economic reasons.

Niobium is an optional element that may be added to the steel at up to 0.1%, preferably 0.0010% to 0.1%. Which is adapted to form carbonitrides by precipitation hardening to impart strength to the steel according to the invention. Due to the recrystallization during the delayed heating of niobium, the microstructure formed at the end of the holding temperature and therefore after the full annealing is finer, which leads to hardening of the product. However, when the niobium content is more than 0.1%, the amount of carbonitride is disadvantageous for the present invention because a large amount of carbonitride tends to decrease the ductility of steel.

Titanium is an optional element that may be added to the steel of the invention in up to 0.1%, preferably 0.001% to 0.1%. Like niobium, it participates in carbonitrides and thus plays a role in hardening. But titanium also participates in the formation of TiN which occurs during solidification of the cast product. The amount of Ti is therefore limited to 0.1% to avoid coarse TiN which is detrimental to the hole expansion. In the case where the titanium content is less than 0.001%, titanium does not exert any influence on the steel of the present invention.

Vanadium is an optional element that may be added to the steel of the invention in up to 0.1%, preferably 0.001% to 0.01%. Like niobium, it participates in carbonitrides and thus plays a role in hardening. But vanadium also participates in the formation of VN which occurs during the solidification of the cast product. The amount of V is therefore limited to 0.1% to avoid a coarse VN which is detrimental to reaming. In case the vanadium content is below 0.001%, vanadium does not have any influence on the steel of the present invention.

Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel and improve its toughness. A minimum of 0.01% is required to produce such an effect. However, when the content thereof is more than 1%, nickel causes ductility to deteriorate.

Copper may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% is required to produce such an effect. However, when the content thereof is more than 1%, copper causes deterioration of hot ductility during hot rolling.

Calcium is an optional element that may be added to the steel of the invention at up to 0.005%, preferably 0.001% to 0.005%. Calcium is added as an optional element to the steel of the invention, especially during inclusion processing. Calcium contributes to the refining of steel by capturing the harmful sulphur content in the spheroidization of steel.

Other elements such as cerium, boron, magnesium or zirconium may be added alone or in combination in the following proportions: ce is less than or equal to 0.1 percent, B is less than or equal to 0.05 percent, mg is less than or equal to 0.05 percent, and Zr is less than or equal to 0.05 percent. 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 according to the invention comprises in area fraction 60% to 85% tempered martensite, which constitutes the matrix phase of the steel according to the invention, 0% to 5% of retained austenite, 0% to 5% of fresh martensite and 15% to 38% of ferrite and bainite in cumulative amounts.

The tempered martensite constitutes 60% to 85% of the microstructure in terms of area fraction. The tempered martensite is formed by martensite, which is formed during the second step of cooling after annealing, in particular when the temperature is reduced below the Ms temperature and more particularly from Ms-10 ℃ to 15 ℃. Such martensite is then tempered during holding at a tempering temperature trempering of 150 ℃ to 300 ℃. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the martensite content is 62% to 80%, and more preferably 62% to 75%.

Fresh martensite is an optional microscopic component, which is limited in the steel to an amount of 0% to 5%, preferably 0 to 2%, even better equal to 0%. Fresh martensite may be formed during final cooling after tempering.

The ferrite and bainite account for 15% to 38% of the microstructure in cumulative amount. An accumulated amount of bainite and ferrite greater than 15% is mandatory to ensure a balance between strength and elongation, wherein the presence of bainite imparts a tensile strength of 980MPa, and ferrite ensures elongation. Bainite is formed during reheating prior to tempering. Bainite can impart strength to the steel, but when present in too large an amount, it can adversely affect the yield strength of the steel. Ferrite imparts elongation as well as formability to the steel of the present invention. In order to ensure an elongation of 8% and preferably 9% or more, it is preferable to have 10% ferrite. Ferrite is formed during the first step of cooling after annealing. However, when bainite and ferrite, which are cumulatively present, are present at more than 38%, mechanical properties, particularly tensile strength and yield strength in the transverse direction, may be adversely affected. Therefore, the preferred limit of ferrite and bainite accumulated remains from 20% to 37%, and more preferably from 25% to 36%.

Retained austenite is an optional microstructure that may be present in the steel from 0% to 5%. The presence of up to 5% of retained austenite does not adversely affect the mechanical properties. Up to 5% of the retained austenite imparts ductility and elongation to the steel. The retained austenite is preferably 0% to 3%, and more preferably 0% to 2%.

In addition to the above-described microstructure, the microstructure of the cold rolled steel sheet does not contain a microstructure component 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 semi-finished casting of steel having the chemical composition of the high-grade steel 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 several tens of mm for thin strips.

For example, a slab having a chemical composition according to the invention is manufactured 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 local carbon to nominal carbon ratio remains below 1.10. The slab provided by 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 for hot rolling.

The temperature of the slab subjected to hot rolling must be at least 1000 ℃ and must be below 1280 ℃. In the case where the temperature of the slab is less than 1000 ℃, an excessive load is applied on the rolling mill, and further, the temperature of the steel may be lowered to the ferrite transformation temperature during finish rolling, so that the steel will be rolled in a state where the transformed ferrite is included in the structure. 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 ℃. Reheating at temperatures above 1280 ℃ must be avoided because they are industrially expensive.

The steel sheet obtained in this way is then cooled at a cooling rate of at least 20 ℃/s to a coiling temperature which must be lower than 650 ℃. Preferably, the cooling rate will be 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 at 475 ℃ to 625 ℃ to avoid scale formation, with even preferred ranges for such coiling temperatures being 500 ℃ to 625 ℃. The coiled hot rolled steel sheet is then cooled down to room temperature and then subjected to an optional hot strip 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 steel sheet may then be subjected to an optional hot strip annealing. In a preferred embodiment, such a hot band annealing 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 partial transformation of the hot rolled microstructure and thus possible loss of microstructure homogeneity. Thereafter, the optional scale removal step of the hot rolled steel sheet may be performed by, for example, pickling of such a steel sheet.

And then subjecting the hot rolled steel sheet to cold rolling at a thickness reduction of 35 to 90% to obtain a cold rolled steel sheet.

Thereafter, the cold rolled steel sheet is subjected to a heat treatment, which will impart the mechanical properties and microstructure required for the steel of the present invention.

The cold rolled steel sheet is then heated in a two-step heating process, wherein the first step of heating starts from room temperature, 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 of the first step of such heating is at least 12 ℃/sec, and more preferably at least 15 ℃/sec. The preferred HT1 temperature for such a first step is from 575 ℃ to 725 ℃, and more preferably from 575 ℃ to 700 ℃.

In the second step of heating, the cold rolled steel sheet is heated from HT1 to an annealing temperature tset of Ac3 to Ac3+100 ℃, preferably Ac3+10 ℃ to Ac3+100 ℃, at a heating rate HR2 of 1 ℃/sec to 15 ℃/sec. In a preferred embodiment, the heating rate HR2 of the second step of heating is from 1 ℃/sec to 8 ℃/sec, and more preferably from 1 ℃/sec to 4 ℃/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 in weight percentage of the cold rolled steel sheet.

The cold rolled steel sheet is held at T-soak for a time of 10 seconds to 500 seconds to ensure complete recrystallization and complete transformation to austenite of the strongly work hardened initial structure.

The cold rolled steel sheet is then cooled in a two-step cooling process, wherein the first step of cooling starts from tsoaking, the cold rolled steel sheet is cooled down to a temperature T1 in the range of 630 ℃ to 685 ℃ at a cooling rate CR1 of 1 ℃/sec to 15 ℃/sec. In a preferred embodiment, the cooling rate CR1 of the first step of such cooling is comprised between 1 ℃/s and 10 ℃/s, and more preferably between 1 ℃/s and 4 ℃/s. The preferred T1 temperature for such a first step is from 640 ℃ to 685 ℃ and more preferably from 650 ℃ to 685 ℃.

In the second step of cooling, the cold rolled steel sheet is cooled down from T1 to a temperature T2 of Ms-10 ℃ to 15 ℃ at a cooling rate CR2 of at least 100 ℃/sec. In a preferred embodiment, the cooling rate CR2 of the second step of cooling is at least 200 ℃/sec, and more preferably at least 300 ℃/sec. The preferred T2 temperature for such a second step is Ms-20 ℃ to 20 ℃ and more preferably Ms-50 ℃ to 20 ℃.

Ms of the steel sheet is 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[Al]-16.3[Cu]

-361[Nb]-2.44[Ti]-3448[B]

thereafter, the cold rolled steel sheet is reheated to a tempering temperature Ttempering of 150 ℃ to 300 ℃ at a heating rate of at least 5 ℃/sec, and preferably at least 10 ℃/sec, and more preferably 12 ℃/sec or more for a time of 100 seconds to 600 seconds. The preferred temperature range for tempering is 175 ℃ to 280 ℃ and the preferred duration of holding in T temper is 200 seconds to 500 seconds.

According to the invention, the tempering temperature is selected such that the difference between T1 and T tempering is between 415 ℃ and 455 ℃. Δ T is determined as follows:

delta T = (T1-T temper)

When Δ T is less than 415 ℃, the cumulative amount of bainite and ferrite exceeds 38%, which is disadvantageous in mechanical characteristics, particularly tensile strength in the transverse direction. When Δ T is greater than 455 ℃, the amount of tempered martensite is too high, whereby the steel of the present invention exceeds 1150MPa in the rolling direction. Preferred Δ T is from 420 ℃ to 440 ℃.

Then, the cold-rolled steel sheet is cooled down to room temperature to obtain a heat-treated cold-rolled steel sheet.

The heat-treated cold rolled 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.

The heat treated cold rolled steel sheet may also be coated by any known industrial process such as electro-galvanizing, JVD, PVD, etc.

An optional post batch annealing (post batch annealing) may then be performed at a temperature of 150 ℃ to 300 ℃ for a time period of 30 minutes to 120 hours.

Examples

The following tests and examples provided herein are non-limiting in nature and must be considered for illustrative purposes only, and will show the advantageous features of the invention and illustrate the importance of the parameters chosen by the inventors after a number of experiments and further establish the properties that can be achieved by the steel according to the invention.

Steel sheet samples according to the invention and steel sheet samples according to some comparative grades were prepared with the compositions summarized in table 1 and the process parameters are summarized in table 2. The respective microstructures of those steel sheets are summarized in table 3, and the properties are summarized in table 4.

Table 1 depicts steels having compositions expressed in weight percent.

Table 1: composition of the test

Steel	C	Mn	Si	Al	Cr	P	S	N	Mo	Ti	Nb	V	Ni	B
															1	0.15	1.90	0.19	0.04	0.18	0.0031	0.001	0.0034	0.002	0.02	0..001	0.06	0.015	0.001
2	0.15	1.90	0.18	0.04	0.18	0.0031	0.001	0.004	0.0021	0.02	0.001	0.06	0.015	0.001
															3	0.15	1.89	0.21	0.03	0.19	0.0025	0.0013	0.005	0.0031	0.025	0.001	0.03	0.02	0.001
4	0.15	1.88	0.20	0.03	0.18	0.0018	0.0012	0.0045	0.0022	0.025	0.001	0.02	0.015	0.001
															5	0.15	1.91	0.20	0.03	0.19	0.0016	0.0025	0.004	0.0035	0.025	0.001	0.02	0.017	0.001
6	0.15	1.87	0.21	0.027	0.18	0.0018	0.0023	0.004	0.0043	0.025	0..001	0.02	0.015	0.001
															7	0.14	1.87	0.21	0.027	0.18	0.0018	0.0023	0.004	0.0043	0.025	0..001	0.02	0.015	0.001
8	0.15	1.88	0.20	0.025	0.20	0.0023	0.0021	0.004	0.0043	0.025	0..001	0.03	0.015	0.001

Table 2 summarizes the annealing process parameters performed on the steel of table 1.

Table 2: process parameters of the experiment

All examples and comparative examples were reheated to a temperature of 1200 c and then hot rolled with a hot final rolling temperature of 890 c, after which the hot rolled steel strip was cooled at a rate of 80 c/sec and coiled at 530 c, and the cold rolling reduction of all examples and comparative examples was 50%.

TABLE 2a

Underlined values: not in accordance with the present invention.

TABLE 2b

Underlined values: not in accordance with the present invention.

Table 3 summarizes the results of tests performed on different microscopes, such as scanning electron microscopes, according to standards to determine the microstructure composition of both the inventive steel test and the reference test.

Table 3: microscopic Structure of the test

Underlined values: not in accordance with the present invention.

Table 4 summarizes the mechanical properties of both the inventive and reference steels. Tensile strength, yield strength and total elongation tests were carried out according to the NF EN ISO 6892 standard,

table 4: mechanical Properties tested

Underlined values: not in accordance with the present invention.

The examples show that the steel sheet according to the invention is the only steel sheet showing all the targeted properties due to its specific composition and microstructure.

Claims

1. A heat-treated cold rolled steel sheet comprising the following elements, expressed in weight percent:

0.1％≤C≤0.2％；

1.2％≤Mn≤2.2％；

0.05％≤Si≤0.6％；

0.001％≤Al≤0.1％；

0.01％≤Cr≤0.5％；

0％≤S≤0.09％；

0％≤P≤0.09％；

0％≤N≤0.09％；

and can include one or more of the following optional elements:

0％≤Mo≤0.5％；

0％≤Ti≤0.1％；

0％≤Nb≤0.1％；

0％≤V≤0.1％；

0％≤Ni≤1％；

0％≤Cu≤1％；

0％≤Ca≤0.005％；

0％≤B≤0.05％；

the remainder consisting of iron and unavoidable impurities resulting from processing, the microstructure of the steel comprising, in area percent: 60% to 85% tempered martensite, 15% to 38% ferrite and bainite in cumulative amounts, 0% to 5% residual austenite in optional amounts, and 0% to 5% fresh martensite in optional amounts.

2. The heat-treated cold rolled steel sheet of claim 1, wherein composition comprises 0.12% to 0.19% carbon.

3. The heat-treated cold rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.1 to 0.5% of silicon.

4. The heat-treated cold rolled steel sheet of any one of claims 1 to 3, wherein composition comprises 0.001 to 0.09% aluminum.

5. Heat-treated cold rolled steel sheet according to anyone of claims 1 to 4, wherein the composition comprises from 1.3% to 2.1% of manganese.

6. Heat-treated cold rolled steel sheet according to anyone of claims 1 to 5, wherein the composition comprises from 0.1 to 0.4% of chromium.

7. Heat-treated cold rolled steel sheet according to anyone of claims 1 to 6, wherein the amount of tempered martensite is comprised between 62% and 80%.

8. The heat-treated cold rolled steel sheet as claimed in any one of claims 1 to 7, wherein the cumulative amount of ferrite and bainite is 20 to 37%.

9. The heat-treated cold rolled steel sheet according to any one of claims 1 to 8, wherein said steel sheet has an ultimate tensile strength in the transverse direction of 980 to 1150MPa and an ultimate tensile strength in the rolling direction of 980 to 1150MPa.

10. A method of producing a heat-treated cold rolled steel sheet comprising the sequential steps of:

-providing a steel composition according to any one of claims 1 to 6;

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

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

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

-cooling the hot rolled steel sheet to room temperature;

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

-optionally annealing the hot rolled steel sheet;

-optionally subjecting said hot rolled steel sheet to an descaling process;

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

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

-a first step of heating the cold-rolled steel sheet starting from room temperature to a temperature HT1 of 550 ℃ to 750 ℃ at a heating rate HR1 of at least 10 ℃/s;

the second step of heating is preceded by a temperature tsoaking from HT1 to Ac3+100 ℃ at a heating rate HR2 of from 1 ℃/s to 15 ℃/s, holding it at said temperature tsoaking for a time of from 10 seconds to 500 seconds,

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

the first step of cooling the cold-rolled steel sheet is lowered from tset to a temperature T1 of 630 ℃ to 685 ℃ at a cooling rate CR1 of 1 ℃/s to 15 ℃/s;

the second step of cooling down starts from T1 with a cooling rate CR2 of at least 100 ℃/s down to a temperature T2 of Ms-10 ℃ to 15 ℃,

-then reheating the cold rolled steel sheet at a rate of at least 5 ℃/s to a tempering temperature trempering of 150 ℃ to 300 ℃, holding it at the tempering temperature trempering for a time of 100 seconds to 600 seconds, wherein Δ T = (T1-trempering) must be 415 ℃ to 455 ℃;

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

11. The method of claim 10, wherein the coiling temperature is 475 ℃ to 625 ℃.

12. The method of claim 10 or 11, wherein tsak is Ac3+10 ℃ to Ac3+100 ℃.

13. The method of any one of claims 10 to 12, wherein CR1 is from 1 ℃/sec to 10 ℃/sec.

14. The method of any one of claims 10 to 13, wherein T1 is from 640 ℃ to 685 ℃.

15. The method of any one of claims 10 to 14, wherein CR2 is at least 200 ℃/sec.

16. The method of any one of claims 10 to 15, wherein T2 is Ms-20 ℃ to 20 ℃.

17. The method of any one of claims 10 to 16, wherein T-temper is 175 ℃ to 280 ℃.

18. Use of a steel sheet obtainable according to any one of claims 1 to 9 or manufactured according to the method of any one of claims 10 to 17 for manufacturing a structural part of a vehicle.