Cold rolled and heat treated steel sheet and a method of manufacturing thereof

Abstract

The invention relates to a cold rolled and heat treated steel sheet having a composition comprising in percentage by weight: C : 0.02 - 0.12 %, Mn : 1.2 - 2.3%, Al : 0.01 - 0.1%, Nb : 0.01% - 0.1%, Ti : 0.01 - 0.12%, P ≤ 0.09 %, S ≤ 0.09 %, N ≤ 0.009% and can contain Si ≤ 2 %, Cr ≤ 0.5 %, Ni ≤ 3%, Ca ≤ 0.005%, Cu ≤ 2%, Mo ≤ 0. 5%, V ≤ 0.1%, B ≤ 0.003%, Ce ≤ 0.1%, Mg ≤ 0.010%, Zr ≤ 0.010%, the remainder being iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 2% to 10% Cementite, 30 to 60% of Recrystallized ferrite, 38 to 68% of non- recrystallized ferrite, wherein the cumulated amount of Recrystallized ferrite and Non-recrystallized ferrite is from 85% to 96% and includes 0.5% to 2% of Carbides of Niobium, the optional cumulated amount of residual austenite and martensite being from 0% to 5%.

Description

[0001] COLD ROLLED AND HEAT TREATED STEEL SHEET AND A METHOD OF MANUFACTURING THEREOF

[0002]

[0001] The present invention relates to cold rolled and heat treated steel sheets suitable for use as steel sheet for industry, construction steel and the steel for automobiles.

[0003]

[0002] Structural steels are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in terms of CO2 consumption impact is also bestowed upon these structural steels, that are intended to be used for building solar frames, racking, silos, roofing, cladding and other similar purposes, in view of global environment concerns. Thus, now structural steel must be made of material having high strength in order that to fit in the criteria of durability and longevity.

[0004]

[0003] Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in solar panel frames, silos, cladding and roofing structures car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.

[0005]

[0004] Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:

[0006]

[0005] US10920293 is a steel sheet of a composition comprising, in mass %, C: 0.07 to 0.19%, Si: 0.09% or less, Mn: 0.50 to 1.60%, P: 0.05% or less, S: 0.01 % or less, Al: 0.01 to 0.10%, N: 0.010% or less, and the balance Fe and unavoidable impurities, and of a micro structure that contains ferrite as a primary phase, and 2 to 12% of perlite, and 3% or less of martensite by volume, and in which the remainder is a low-temperature occurring phase, the ferrite having an average crystal grain diameter of 25pm or less, the perlite having an average crystal grain diameter of 5pm or less, the martensite having an average crystal grain diameter of 1.5 pm or less, and the perlite having a mean free path of 5.5 pm or more. However the steel of US10920293 is not able to achieve the tensile strength of 600MPa or more.

[0007]

[0006] The purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:

[0008] - an ultimate tensile strength of 700MPa or more and preferably more than 740MPa and

[0009] - a total elongation of more than 12.0%

[0010]

[0007] In a preferred embodiment, the steel according to the invention has a yield strength from 630 MPa or more and preferably more than 650 MPa. In another preferred embodiment, the steel has a TS / YS ratio can be set at a value greater than or equal to 1 .05 when measured according to the EN1993- 1 -12 Eurocodes standards and preferably TS / YS ratio greater than or equal to 1 .07 when measured according to the EN1993-1 -12 Eurocodes standards.

[0011]

[0008] Preferably, such steel can also have a good suitability for forming, for rolling with good weldability, bendability and coatability.

[0012]

[0009] Another object of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.

[0013]

[0010] The cold rolled and heat treated steel sheet of the present invention is be coated with zinc or zinc alloys, or with aluminum or aluminum alloys or Magnesium or Magnesium alloys to improve its corrosion resistance.

[0014]

[0011] Carbon is present in the steel from 0.02% to 0.12%. Carbon is an element necessary for increasing the strength of the steel sheet by interstitial strengthening as well as via forming microalloyed precipitates. If C is lower than 0.02 wt%, it is difficult to achieve the required yield strength 630 MPa or more and total elongation of more than 12% simultaneously. Whenever the carbon content is higher than 0.12% it degrades coatability and exhibits poor adhesion at the steel-coating interface. Carbon content higher than 0.12% decreases the Ac1 temperature due to which second phases like pearlite, bainite, martensite can form at relatively low soaking temperatures which decreases the yield strength as well as increases work-hardening during bending which is not recommended. Additionally, high carbon content of more than 0.1 % is not recommended due to the deterioration in the weldability of the steel. Hence, the preferred range for carbon for the steel of present invention is therefore 0.05% to 0.09% and more preferably 0.055% to 0.085%.

[0015]

[0012] Manganese content of the steel of present invention is from 1.2% ro 2.3%. The purpose of adding Manganese is essentially to impart strength to the steel by solid solution strengthening. If Mn is lower than 1 .2%, it is difficult to achieve the required yield strength 630 MPa or more and the total elongation higher than 12% simultaneously. When Mn content is added more than 2.3% the transformation from austenite to pearlite is suppressed and martensite and / or, bainite is formed, resulting into poor weldability in terms of increased hardness in the heat-affected zone (HAZ) and surface cracking becomes likely to occur during welding. A preferable content for the present invention may be kept from 1.2% to 2.1 %, furthermore preferably 1.3% to 2.0% to ensure good bendability of the steel of present invention.

[0016]

[0013] Aluminum is an essential element and is present in the steel of present invention from 0.01 % to 0.1 %. Aluminum promotes ferrite formation and increases the Ms temperature which allows the present invention to have Ferrite in adequate amount as required by the. steel of present invention to impart steel of present invention with ductility as well as strength. However, when the presence of Aluminum is more than 0.1 % increases the Ac3 temperature which makes the annealing and hot rolling finishing temperature in complete Austenitic region which will increase the amount of recrystallized ferrite beyond the limits of the present invention and it is also economically unreasonable. Aluminum content is preferably limited from 0.01 % to 0.09% and more preferably 0.01 % to 0.05%.

[0017]

[0014] Niobium is an essential element for the Steel of present invention from 0.01 % to 0.1 % and suitable for forming carbides and Carbonitrides to impart strength of the steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbides and by retarding the recrystallization during heating process. Thus, finer microstructure formed in the final product as a consequence the steel of present invention is able to reach the targeted strength. However, Niobium content above 0.1 % is not economically interesting as well as forms coarser precipitates which are detrimental for the properties like hole expansion ratio, elongation of the steel and also when the content of niobium is 0.1 % or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling. The preferred limit for niobium content is from 0.01 % to 0.09% and more preferably from 0.01 % to 0.07%

[0018]

[0015] Titanium is an essential element and is added to the Steel of present invention from 0.01 % to 0.12%. As Niobium, it is involved in carbo-nitrides formation so plays a role in hardening of the Steel of present invention. In addition, Titanium also forms Titanium-nitrides which appear during solidification of the cast product. The amount of Titanium is so limited to 0.12% to avoid formation of coarse Titanium-nitrides detrimental for formability. The preferred limit for titanium content is from 0.02% to 0.1 % and more preferably from 0.05% to 0.09%

[0019]

[0016] Phosphorus is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the phosphorus content is preferably as low as possible, and below 0.09%. Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with manganese. For these reasons, its content is limited to less than 0.09%, preferably less than 0.03 % and more preferably less than 0.014%.

[0020]

[0017] Sulfur is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the Sulfur content is preferably as low as possible. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the steel of present invention. For these reasons, its content is limited to less than 0.09%, preferably less than 0.03 %.

[0021]

[0018] Nitrogen is limited to 0.009% to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel.

[0019] Silicon is optional element and the content of the steel of present invention is up to 2%. Silicon adds strength to ferrite through solid solution strengthening because of this effect the hole expansion rate tends to increase and also ensures good ductility. However, when contained in an amount more than 2%, silicon concentrates at the steel sheet surface in the form of an oxide during annealing, and the coatability deteriorates and causes embrittlement. An excess silicon content of more than 2% also impairs toughness at high temperature, and often causes surface cracking at the time of welding. For this reason, the Silicon content is restricted to 2% or less. The Silicon content is preferably from 0.01 % to 1 % and more preferably from 0.01 % to 0.09%.

[0022]

[0020] Chromium is an optional element for the present invention. Chromium content may be present in the steel of present invention up to 0.5%. Chromium provides strength and hardening to the steel but when used above 0.5% it impairs surface finish of steel. The preferred limit for Chromium for the present invention is from 0.1 % to 0.4%.

[0023]

[0021] Nickel may be added as an optional element in an amount up to 3% to increase the strength of the steel and to improve its toughness. A minimum of 0.01 % is preferred to produce such effects. However, when its content is above 3%, Nickel causes ductility deterioration.

[0024]

[0022] Calcium content in the steel of present invention is up to 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment with a preferred minimum amount of 0.0001 %. Calcium contributes towards the refining of Steel by arresting the detrimental Sulfur content in globular form, thereby, retarding the harmful effects of Sulfur.

[0025]

[0023] Copper may be added as an optional element in an amount up to 2% to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01 % of Copper is preferred to get such effect. However, when its content is above 2%, it can degrade the surface aspects.

[0026]

[0024] Molybdenum is an optional element that constitutes up to 0.5% of the Steel of present invention; Molybdenum plays an effective role in determining hardenability and hardness, delays the appearance of Bainite and avoids carbides precipitation in Bainite. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%.

[0027]

[0025] Vanadium is effective in enhancing the strength of steel by forming carbides or carbo-nitrides and the upper limit is 0.1 % due to the economic reasons. Other elements such as Cerium, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Cerium ^0.1 %, Boron 0.003%, Magnesium 0.010% and Zirconium 0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

[0028]

[0026] The microstructure of the Steel sheet will now be described. Nonrecrystallized ferrite constitutes from 38% to 68% of microstructure by area fraction of the steel of present invention. Non-recrystallized ferrite grains are defined as dislocation containing elongated ferrite grains that formed during cold-rolling and did not recrystallize during heating and soaking temperature during annealing after cold-rolling. Non-recrystallized ferrite contributes to the high strength in the steels of present invention and to ensure yield strength of 630MPa or, more, it is necessary to have at least 38% non-recrystallized ferrite. But when the non-recrystallized ferrite content is present above 68% in the matrix of the steel of present invention, it is not possible to achieve the total elongation of at least 12%. The preferred limit for presence of the non- recrystallized ferrite for the present invention is therefore from 42% to 65% by area fraction and more preferably from 45% to 60%.

[0029]

[0027] Recrystallized ferrite constitutes from 30% to 60% of microstructure by area fraction of the steel of present invention and it is advantageous to have an average grain size of less than 4 microns and preferably the average grain size is from 0.5 micron to 3.5 microns and more preferably from 1 micron to 3 microns. This recrystallized ferrite imparts the steel of present invention with a total elongation of at least 12%. However, when the recrystallized ferrite content is present above 60% in the matrix of the steel of present invention, it is not possible to achieve the yield strength of 630MPa. Recrystallized ferrite grains are defined as dislocation-free equiaxed grains that nucleate and grow during heating and soaking below the Ac1 temperature during annealing after cold-rolling. The preferred limit for the presence of recrystallized ferrite in the matrix for the present invention is therefore from 33% to 58% by area fraction and more preferably from 35% to 55%.

[0030]

[0028] The cumulative presence of non-recrystallized ferrite and recrystallized ferrite can be from 85% to 96% and preferably from 89% to 95% and includes 0.5 to 2% of niobium carbides. Etching with Dino’s etchant (140ml of distilled water, 100ml of H2O2, 4g of oxalic acid, 2ml of H2SO4 and 1.5ml of HF) is used to clearly visualize the grain boundaries and thereafter differentiate between recrystallized and non-recrystallized ferrite microconstituents is done from an optical micrograph base on the shape of the grain. The area fraction for each constituent is measured as per the ASTM E562.

[0031]

[0029] Cementite is an essential microstructure of the steel of present invention and present from 2% to 10%. Cementite imparts strength and to the steel. Whenever the Cementite is present more than 10% the steel of the present invention is detrimental for the elongation of the steel of present invention. The preferred limit for presence of the cementite for the present invention is therefore from 2% to 9% by area fraction.

[0032]

[0030] Titanium and Niobium Carbides are present in the steel of present invention. It is advantageous according to the present invention that the size of the carbide precipitates is from 2nm to 200nm and more preferably 2nm to 20nm. The carbides of the present invention include both intragranular niobium carbides (i.e. precipitate inside the ferrite grains so called intragranular carbides) and intergranular carbides (i.e. precipitate on the ferrite grain boundaries so called intergranular carbides). The homogenous and coherent precipitation of the carbide increases the strength of the steel. The limit for the presence of the niobium carbide from 0.5% to 2% by area fraction and more preferably from 0.5% to 1.5% by area fraction, such amount being included in the ferrites total amount.

[0031] Residual Austenite and Martensite are optional constituent and may be present from 0% to 5 % of microstructure by area fraction and found in traces. Further The preferable limit for Residual Austenite and Martensite is from 0% to 3%.

[0033]

[0032] In addition to the above-mentioned microstructure, the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite, bainite without impairing the mechanical properties of the steel sheets.

[0034]

[0033] A steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip.

[0035]

[0034] For example, a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.

[0036]

[0035] The temperature of the slab, which is subjected to hot rolling, is at least 1000° C and must be below 1280°C. In case the temperature of the slab is lower than 1000° C the dissolution of Titanium and Niobium does not takes places completely and consequently Titanium and Niobium will not form adequate carbides during annealing and additionally there may be excessive load is imposed on a rolling mill if temperature if less than 1000° C and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the temperature range of 800°C to1000°C and final rolling temperature must remain above 800°C. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive.

[0037]

[0036] A final rolling temperature range above 800°C is necessary to have a structure that is favorable to recrystallization and rolling. It is preferred that the final rolling pass to be performed at a temperature greater than 850°C, because below this temperature the steel sheet exhibits a significant drop in rollability. The hot rolled steel obtained in this manner is then cooled at a cooling rate above 20°C / s to the coiling temperature which must be from 450°C to 650°C. The objective of keeping the coiling temperature from 450°C to 650°C is to keep the microalloying elements such as Niobium and / or Titanium in solid solution in the hot band to maximize precipitation during annealing after cold rolling. Preferably, the cooling rate after finishing hot rolling will be less than or equal to 200° C / s.

[0038]

[0037] The hot rolled steel is then coiled at a coiling temperature from 450°C to 650°C to avoid ovalization and preferably from 450°C to 625°C to avoid scale formation. A more preferred range for such coiling temperature is from 460°C to 625°C. The coiled hot rolled steel is cooled down to room temperature before subjecting it to optional hot band annealing.

[0039]

[0038] The hot rolled steel may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then subjected to an optional Hot Band Annealing at, for example, temperatures from 400°C to 750°C for at least 12 hours and not more than 96 hours, the temperature remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel may performed through, for example, pickling of such sheet. This hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction from 35 to 90%. The cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.

[0039] The annealing of the cold rolled steel sheet is performed in two steps heating wherein the first step starts from heating the steel sheet from room temperature to a temperature T1 which is from 550°C to 680°C, with a heating rate HR1 of at least 2°C / s. It is advantageous to keep T1 temperature below the recrystallisation initiation temperature which is calculated by differential scanning calorimetry experiments as per paper published as “Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery, recrystallisation and ferrite to austenite phase transformation” on Pages 765-773 in Taylor and Francis on 06 Dec 2013. Thereafter the second step starts from heating further the steel sheet from T1 to a soaking temperature T2 from 720°C to 850°C, with a heating rate HR2 of at least 0.5°C / s, HR2 being lower than HR1 , then perform annealing at T2 during 1 to 500 seconds. In a preferred embodiment, the heating rate for the second step the heating rate is less than 10°C / s and more preferably less than 5°C / s. The preferred temperature T1 is from 575°C to 670°C and more preferably T1 temperature is from 600°C to 660°C. The preferred temperature T2 for soaking is from 720°C to 840°C and more preferably T2 temperature for soaking is from 750°C to 830°C.

[0040]

[0040] Then the cold rolled steel is cooled from T2 to temperature range T3 from 400°C to 500°C, preferably from 420°C to 490°C, at an average cooling rate of at least 5°C / s and preferably at least 8°C / s, wherein the cooling step may include an optional slow cooling sub-step within the T3 temperature range with a cooling rate of 2°C / s or less and preferably of 1 °C / s or less. The cold rolled steel sheet is held within the temperature range T3 during 10 to 500 seconds.

[0041]

[0041] Then the cold rolled steel sheet can then be brought to the temperature of the coating bath from 420°C to 480°C, depending on the nature of the coating, to facilitate hot dip coating of the cold rolled steel sheet.

[0042]

[0042] The cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc, which may not require bringing it to the above-mentioned temperature range before coating.

[0043] Then an optional post batch annealing may be done at a temperature from 150°C to 300°C during 30 minutes to 120 hours.

[0043]

[0044] Thereafter, optional skin pass rolling may be performed on the cold rolled steel sheet.

[0044]

[0045] EXAMPLES

[0045]

[0046] The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention.

[0046]

[0047] Steel sheets made of steels with different compositions in weight percentage are gathered in Table 1 , where the steel sheets are produced according to process parameters as stipulated in Table 2 and 2A, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties.

[0047]

[0048] Table 1

[0048]

[0049] I = according to the invention; R = reference; underlined values: not according to the invention.

[0049]

[0050] Table 2 and 2A

[0050]

[0051] Table 2 gathers the processing parameters till cold rolling and thereafter the Table2A shows annealing process parameters implemented on steels of Table 1. The Steel compositions I1to I4 and R1 to R3 serve for the manufacture of sheets according to the invention.

[0052] Following processing parameters are the same for all the steels of Table 1 . All steels of table 1 were cooled at a rate above 20°C / s before coiling and all were finally brought at a temperature of 460°C before zinc hot dip coating.

[0051]

[0053] The table 2 is as follows:

[0052]

[0054] Table 2A : Annealing Process parameters are as follows :

[0053]

[0055] I = according to the invention; R = reference; underlined values: not according to the invention.

[0054]

[0056] Table 3

[0055]

[0057] Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels.

[0058] The microstructure is measured using Etching with Dino’s etchant (140ml of distilled water, 100ml of H2O2, 4g of oxalic acid, 2ml of H2SO4 and 1 ,5ml of HF) to clearly visualize the grain boundaries and thereafter differentiate between recrystallized and non-recrystallized ferrite microconstituents is done from an optical micrograph base on the shape of the grain. The area fraction for each constituent is measured as per the ASTM E562 through an optical microscope at 100X objective magnification. All samples include 0.5 to 2% of niobium carbide dispersed in the ferrites.

[0056]

[0059] The results are stipulated herein:

[0057]

[0060] I = according to the invention; R = reference; underlined values: not according to the invention.

[0058]

[0061] Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance with NBN EN ISO 6892-1 , method B on an A80 specimens.

[0059]

[0062] The results of the various mechanical tests conducted in accordance with the standards are gathered

[0060]

[0063] Table 4

[0061] = according to the invention; R = reference; underlined values: not according to the invention.

Claims

CLAIMS1 . A cold rolled and heat-treated steel sheet having a composition comprising of the following elements, expressed in percentage by weight:0.02 % < Carbon < 0.12 %1 .2% < Manganese < 2.3%0.01 % < Aluminum < 0.1 %0.01 % < Niobium < 0.1 %0.01 % < Titanium < 0.12%Phosphorus < 0.09 % Sulfur < 0.09 %.Nitrogen < 0.009% and can contain one or more of the following optional elements 0 % < Silicon < 2 %0 % < Chromium < 0.5 %0 % < Nickel < 3%0 % < Calcium < 0.005%0 % < Copper < 2%0 % < Molybdenum < 0. 5%0 % < Vanadium < 0.1 %0 % < Boron < 0.003%0 % < Cerium < 0.1 %0 % < Magnesium0 % < Zirconium 0.010% the remainder being iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 2% to 10% Cementite, 30 to 60% of Recrystallized ferrite, 38 to 68% of non-recrystallized ferrite, wherein the cumulated amount of Recrystallized ferrite and Nonrecrystallized ferrite is from 85% to 96% and includes 0.5% to 2% of Carbides of Niobium, the optional cumulated amount of residual austenite and martensite being from 0% to 5%.

2. Cold rolled and heat-treated steel sheet according to claim 1 , wherein the composition includes 0.01 % to 1 % of Silicon.

3. Cold rolled and heat-treated steel sheet according to claim 1 or 2, wherein the composition includes 0.05% to 0.09% of Carbon.

4. Cold rolled and heat-treated steel sheet according to anyone of claims 1 to 3, wherein the composition includes 0.01 % to 0.09% of Aluminum.

5. Cold rolled and heat-treated steel sheet according to anyone of claims 1 to 4, wherein, the cumulated amounts of Recrystallized ferrite and nonrecrystallized ferrite is from 89% to 95%.

6. Cold rolled and heat-treated steel sheet according to anyone of claims 1 to 5, wherein, the amount of Recrystallized ferrite is from 35% to 55%.

7. Cold rolled and heat-treated steel sheet according to anyone of claims 1 to 6, wherein the non-recrystallized ferrite is from 42% to 65%.

8. Cold rolled and heat treated steel sheet according to anyone of claims 1 to 8, wherein said steel sheet has an ultimate tensile strength of 700 MPa or more, and a total elongation of 12.0% or more and a yield strength to tensile strength ratio equal to or greater than 1 .05.

9. Cold rolled and heat-treated steel sheet according to anyone of claims 1 to 9, wherein said steel sheet has a yield strength of 630 MPa or more.

10. A method of production of a cold rolled and heat-treated steel sheet comprising the following successive steps:- providing a steel composition according to anyone of claims 1 to 4;- reheating said semi-finished product to a temperature from 1000°C to 1280°C;- rolling the said semi-finished product in the temperature range from 800°C to 1000°C wherein the hot rolling finishing temperature shall be above 800°C to obtain a hot rolled steel;- cooling the hot rolled steel at a cooling rate above 20°C / s to a coiling temperature which is from 450°C to 650°C; and coiling the said hot rolled steel;- cooling the said hot rolled steel to room temperature;- optionally performing scale removal process on said hot rolled steel sheet;- optionally annealing is performed on hot rolled steel sheet;- optionally performing scale removal process on said hot rolled steel sheet;- cold rolling the said hot rolled steel sheet with a reduction rate from 35 to 90% to obtain a cold rolled steel sheet;- annealing the said cold rolled steel sheet in two steps heating wherein: o the first step starts from heating the steel sheet from room temperature to a temperature T1 from 550°C to 680°C, with a heating rate HR1 of at least 2°C / s, o the second step starts from heating further the steel sheet from T1 to a soaking temperature T2 from 720°C to 850°C, with a heating rate HR2 of 0.5°C / s or more, HR2 being lower than HR1 , then perform annealing at T2 during 1 to 500 seconds, then cooling the cold rolled steel sheet from T2 to a holding temperature T3 from 400°C to 500°C at an average cooling rate of at least 5°C / s,- then the said cold rolled steel sheet is held at T3 during 10 to 500 seconds to obtain a cold rolled and heat-treated steel sheet.

11. A method according to claim 10, wherein the coiling temperature is from 450°C to 625°C.

12. A method according to claim 10 or 11 , wherein the finishing rolling temperature is more than 850°C.

13. Use of a steel sheet according to anyone of claims 1 to 9 or of a steel sheet produced according to the method of claims 10 to 12, for the manufacture of structural steel.

14. Steel Structures comprising a part obtained according to claim 13.

Images