COLD-ROLLED AND HEAT-TREATED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

The invention relates to a cold-rolled, heat-treated steel sheet having a composition comprising, by weight percent: C: 0.12% to 0.25%, Mn: 3.0% to 8.0%, Si: 0.70% to 1.50%, Al: 0.3% to 1.2%, B: 0.0002% to 0.004%, S: 0.010%, P: 0.020%, N: 0.008%, the remainder of the composition being iron and unavoidable impurities resulting from smelting, and having a microstructure consisting, by surface fraction: between 5% and 45% ferrite, between 25% and 85% partitioned martensite, this partitioned martensite having a carbide density strictly less than 2 x 10⁶ / mm², between 10% and 30% retained austenite, less than 8% fresh martensite, a portion of this fresh martensite being combined with retained austenite in the form of martensite-austenite islands in a total surface fraction less than 10%.

Description

COLD-ROLLED AND HEAT-TREATED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME The present invention relates to a high-strength steel sheet having high ductility and formability and to a method for obtaining such a steel sheet. To manufacture different items such as structural body member parts and body panels for automotive vehicles, it is known to use sheets made of dual phase steels (DP) or transformation induced plasticity steels (TRIP). One of the main challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency in light of global environmental conservation, without compromising safety requirements. To meet these requirements, the steel industry is continuously developing new high-strength steels to produce sheets with improved performance and tensile strength, as well as good ductility and formability. WO2019123245 describes a method for obtaining a high-strength, high-formability cold-rolled steel sheet with a yield strength YS between 1000 MPa and 1300 MPa, a tensile strength TS between 1200 MPa and 1600 MPa, a uniform elongation UE of at least 10%, and a hole expansion ratio HER of at least 20%, due to a quenching and parting process. The microstructure of the cold-rolled steel sheet consists of, in surface fraction: between 10% and 45% ferrite, which has an average grain size of maximum 1.3 pm, the product of the ferrite surface fraction by the average ferrite grain size is a maximum of 35% pm, between 8% and 30% retained austenite, said retained austenite having a Mn content greater than 1.1*Mn%, Mn% designating the Mn content of the steel, maximum 8% fresh martensite, maximum 2.5% cementite and the remainder is partitioned martensite. A surface fraction of at least 8% retained austenite, with a Mn content greater than 1.1%Mn, allows for a combination of high ductility and high strength. During the annealing of hot-rolled steel sheet, the austenite is enriched with manganese. Annealing after cold rolling according to the invention homogenizes the microstructure with finer fresh martensite and MA islands, and consequently does not provide the characteristics of WO2019123245. Publication WO2018220430 refers to steel sheets that are hot-formed to produce parts. The steel parts are then cooled before being reheated and held at a post-treatment temperature and cooled to room temperature. This hot-forming process induced intense local deformations of the steel part, due to the geometry of the part and the forming tools, leading to local modifications of the microstructure. Therefore, the purpose of the invention is to solve the aforementioned problem and provide a steel sheet that has a yield strength greater than 950 MPa, a resistance to RCfr / nn / zznz / E / YiAi stress greater than 1180 MPa, a uniform elongation greater than 10% and a HER hole expansion ratio greater than 25%. The objective of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also comprise features according to any of claims 2 to 10. Another objective is achieved by providing the method according to claim 11. The invention will now be described in detail and illustrated by examples without including limitations. Hereafter, Ae1 designates the equilibrium transformation temperature below which austenite is completely unstable, Ae3 designates the equilibrium transformation temperature above which austenite is completely stable, and Ms designates the martensite onset temperature, i.e., the temperature at which austenite begins to transform into martensite after cooling. These temperatures can be calculated using the following formula: Ae1 =670 + 15*%Si - 13*%Mn + 18*%AI Ae3 = 890 - 20 * / %C + 20 * %Si - 30 * %Mn + 130* %AI Ms= 560 - (30*%Mn+13*%Si-15*%AI+12*%Mo)-600*(1-exp(-0.96*C)) The composition of the steel according to the invention comprises, in percentage by weight: According to the invention, the carbon content ranges from 0.12% to 0.25%. Above 0.25%, the weldability of the steel sheet may be reduced. If the carbon content is less than 0.12%, the retained austenite fraction is not sufficiently stabilized to achieve adequate elongation and tensile strength. In a preferred embodiment, the carbon content ranges from 0.15% to 0.25%. According to the invention, the manganese content ranges from 3.0% to 8.0% to achieve sufficient elongation with austenite stabilization. Above 8.0% addition, the risk of central segregation increases, negatively impacting the yield strength and tensile strength. Below 3.0%, the final structure comprises an insufficient fraction of retained austenite, thus failing to achieve the desired combination of ductility and strength. In a preferred embodiment, the manganese content ranges from 3.0% to 5.0%. According to the invention, the silicon content ranges from 0.70% to 1.50%. A silicon addition of at least 0.70% helps to stabilize a sufficient amount of retained austenite. Above 1.50%, silicon oxides form on the surface, which impairs the steel's coating capacity. In a preferred embodiment, the silicon content ranges from 0.80% to 1.30%. The aluminum content ranges from 0.3% to 1.2%, as aluminum is highly effective at deoxidizing steel in the liquid phase during processing. The aluminum content does not exceed 1.2% to prevent inclusions and avoid oxidation problems. In a preferred embodiment, the aluminum content ranges from 0.3% to 0.8%. The boron content ranges from 0.0002% to 0.004% to increase the hardenability of the steel and improve the weldability of the steel sheet. Optionally, some elements are RCfr / nn / zznz / E / YiAi can be added to the composition of the steel according to the invention. Niobium may be optionally added up to 0.06% to refine the austenite grains during hot rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%. Above 0.06%, the yield strength and elongation are not guaranteed at the desired level. Molybdenum can be added up to 0.5%. Molybdenum stabilizes the retained austenite, thereby reducing austenite decomposition during partitioning. Above 0.5%, the addition of Mo is costly and ineffective given the required properties. Vanadium can be optionally added up to 0.2% to provide precipitation strengthening. Titanium can be added up to 0.05% to provide precipitation strengthening. If the titanium level is above or equal to 0.05%, the yield strength and elongation at the desired level are not guaranteed. Preferably, a minimum of 0.01% titanium is added in addition to boron to protect the boron against BN formation. The remainder of steel's composition is iron and impurities resulting from the smelting process. In this respect, phosphorus (P), sulfur (S), and nitrogen (N) are considered residual elements that are unavoidable impurities. Their content is less than 0.010% for sulfur, less than 0.020% for phosphorus, and less than 0.008% for nitrogen. The microstructure of the cold-rolled, heat-treated steel sheet according to the invention is described below. The cold-rolled, heat-treated steel sheet has a microstructure consisting of, by surface fraction, between 5% and 45% ferrite, between 25% and 85% partitioned martensite (the partitioned martensite having a carbide density strictly less than 2 x 10⁶ / mm²), between 10% and 30% retained austenite, and less than 8% fresh martensite. A portion of the fresh martensite combines with retained austenite to form martensite-austenite (MA) islands, in a total surface fraction of less than 10%. In a preferred embodiment, these MA islands have a shape factor less than or equal to 2. Ferrite forms during annealing at a temperature between (Ae1+Ae3) / 2 and Ae3. If the ferrite fraction is less than 5%, the uniform elongation does not reach 10%. If the ferrite fraction is greater than 45%, the tensile strength of 1180 MPa and the yield strength of 950 MPa are not achieved. The microstructure of cold-rolled, heat-treated steel sheet comprises between 25% and 85% partitioned martensite to ensure high ductility. This partitioned martensite has a carbide density strictly less than 2 x 10⁶ / mm². Partitioned martensite is the martensite formed after cooling following annealing and then partitioned during the parting step. Preferably, the microstructure comprises between 40% and 80% partitioned martensite. The microstructure of the cold-rolled, heat-treated steel sheet comprises between 10% and 30% retained austenite to ensure high ductility and less than 8% fresh martensite. Preferably, the microstructure comprises a maximum of 6% fresh martensite. Fresh RCfr / nn / zznz / E / YiAi forms during the cooling to room temperature of the cold-rolled, heat-treated steel sheet. The size of the fresh martensite and martensite-austenite islands is less than 0.7 pm. The steel sheet according to the invention can be produced by any suitable manufacturing method, and a person skilled in the art can define one. However, the method according to the invention, comprising the following steps, is preferred: A semi-finished product capable of further hot rolling is provided with the steel composition described above. The semi-finished product is heated to a preheating temperature (Treheating) between 1150 °C and 1300 °C to facilitate hot rolling, with a final rolling temperature (FRT) between 800 °C and 950 °C, to obtain a hot-rolled steel sheet. The maximum FRT value is chosen to avoid oiling of the austenitic grains. Preferably, the FRT is between 800 °C and 910 °C. Hot-rolled steel is cooled and coiled at a cooling temperature between 200 °C and 700 °C. Preferably, the coiling temperature is between (Ms-100 °C) and 550 °C. After rolling, the sheet can be pickled to remove oxidation. The hot-rolled steel sheet is then annealed to a first annealing temperature TA1 between 550 °C and 700 °C, and held at this annealing temperature for a holding time tA1 between 30 s and 50 h, in order to improve the cold rolling capability and hardness of the hot-rolled steel sheet. The hot-rolled and annealed steel sheet is then cold-rolled to obtain a cold-rolled steel sheet with a thickness that can be, for example, between 0.7 mm and 3 mm, or even better, in the range of 0.8 mm to 2 mm. The cold-rolling reduction ratio is preferably between 20% and 80%. Below 20%, recrystallization is not favored during subsequent heat treatment, which can impair the ductility of the heat-treated, cold-rolled steel sheet. Above 80%, there is a risk of edge cracking during cold rolling. The cold-rolled steel sheet is then reheated to a second annealing temperature TA2 above Ae3-10 °C, and held at this TA2 temperature for a holding time tA2 between 1 s and 1000 s, in order to obtain, from annealing, a microstructure comprising martensite and bainite, the sum of which is greater than 80%, strictly less than 20% ferrite and strictly less than 20% of the sum of carbides and martensite-austenite (MA) islands. The martensite in the martensite-austenite islands is fresh martensite. Martensite included in the sum of martensite and bainite greater than 80% is self-hardening martensite. The type of martensite can be determined and quantified using a field emission scanning electron microscope (FEG-SEM). The cold-rolled steel sheet then undergoes a quenching and parting (Q&P) process. The quenching and parting process comprises the following steps: RCfr / nn / zznz / E / YiAi - reheat the cold-rolled steel sheet to a temperature TA3 strictly lower than Ae3 and higher than (Ae1+Ae3) / 2 and hold at this annealing temperature TA3 for a holding time tA3 between 3 s and 1000 s, in order to obtain an austenitic and ferritic structure. - Temper the cold-rolled steel sheet to a tempering temperature (TQ) lower than (Ms-50 °C) to obtain a tempered steel sheet. During this tempering step, the austenite partially transforms into martensite. If the tempering temperature is higher than (Ms-50 °C), the fraction of tempered martensite in the final structure is too low, leading to a fresh martensite fraction above 8%, which is detrimental to the overall elongation of the steel. - reheat the tempered steel to a partition temperature TP between 350 °C and 550 °C and hold at this partition temperature for a partition time between 1 s and 1000 s before cooling to room temperature. The cold-rolled steel sheet, heat-treated according to the invention, has a yield strength YS greater than 950 MPa, a tensile strength TS greater than 1180 MPa, a uniform elongation UE greater than 10%, and a hole expansion ratio HER greater than 25%. Preferably, the cold-rolled, heat-treated steel sheet according to the invention has YS and TS expressed in MPa, UE, total elongation TE and HER expressed in %, and silicon content %Si expressed as a weight percent, satisfying the following equation: (YS*UE +TS*TE+TS*HER) / %Si > 65000 This equation shows the level of mechanical properties for a given silicon content. Preferably, the total elongation (TE) is greater than 14%. YS, TS, UE and TE are measured according to ISO 6892-1. HER is measured according to ISO 16630. The invention will now be illustrated by the following examples, which are by no means limiting. Examples of grades, whose compositions are compiled in Table 1, were cast into semi-finished products and processed into steel sheets following the process parameters compiled in Table 2. Table 1 - Compositions RCfr / nn / zznz / E / YiAi The tested compositions are listed in the following table where the contents of the elements are expressed as a percentage by weight: Steel C Mn Si Al BSPN Mo V Nb Ti Ae1 (°C) Ae3 (°C) Ms (°C) A 0.19 3.8 0.98 0.50 0.0005 0.002 0.013 0.003 0.3 0.15 - - 644 852 337 B 0.19 3.9 1.17 0.39 0.0021 0.001 0.011 0.003 0.2 - 0.02 0.029 644 838 331 C 0.19 3.8 0.98 0.51 0.0005 0.002 0.013 0.002 0.3 - - - 644 853 337 AC steels are in accordance with the invention. Table 2 - Process parameters The steel semi-finished products, as melted, were reheated to 1200 °C, hot-rolled to a finish rolling temperature (FRT), coiled, first heat-treated to a temperature TA1, and held at this temperature TA1 for a specified holding time before cold rolling. A second annealing was performed at a temperature TA2, and the cold-rolled steel was held at this temperature TA2 for a holding time ta2 before the quenching and parting (Q&P) process, followed by cooling to room temperature. The following specific conditions were applied: RCfr / nn / zznz / E / YiAi ω ω μ γο σι ο σι ο οι ο σι FRT Steel Test (°C) Tensile (°C) First Annealing Cold Rolled Proportion (%) Second Annealing Q&P TA1 (°C) tA1 (min) TA2 (°C) tA2 (s) TA3 (°C) tA3 (s) TQ (°C) TP (°C) t (s) r A 900 450 620 420 50 850 120 750 230 60 400 250 2* A 900 450 620 420 50 850 120 800 230 120 400 250 3* A 900 450 620 420 50 900 220 800 230 150 400 250 4* B 850 450 630 900 50 850 150 800 230 100 400 250 5 A 900 450 620 420 50 850 120 710 230 30 400 250 6 C 900 450 600 420 50 800 220 170 430 250 7 A 900 450 600 420 50 - - 800 220 170 430 250 *: tests according to the invention. Underlined values: do not correspond to the invention. BSMOO / ZW / XIAI The annealed sheets were then analyzed and the corresponding microstructure elements before Q&P, after Q&P and mechanical properties after Q&P were gathered respectively in Table 3, 4 and 5. Table 3 - Microstructure of the steel sheet before the Q&P process The microstructure of the tested samples was determined and compiled in the following table: RCfr / nn / zznz / E / YiAi Assay 0 Microstructure before Q&P F(%) B + M(%) MA + carbides (%) 1* 2 98 0 2* 2 98 0 3* 0 100 0 4* 2 98 0 5 2 98 0 6 97 0 3 7 97 0 3 *: trials according to the invention / Underlined values: do not correspond to the invention B: represents the surface fraction of bainite F: represents the ferrite surface fraction M: represents the surface fraction of martensite MA: represents the surface fraction of martensite-austenite islands The surface fractions are determined by the following method: a specimen is cut from the cold-rolled, heat-treated steel sheet, polished, and etched with a known reagent to reveal the microstructure. The section is then examined using an optical or scanning electron microscope, for example, a field emission scanning electron microscope (FEG-SEM) at a magnification greater than 5000x, coupled to a backscattered electron diffraction (EBSD) device. The determination of the surface fraction of each constituent is carried out using image analysis with a known method. The retained austenite fraction is determined, for example, by X-ray diffraction (XRD). For tests 6 and 7, which were not annealed at a temperature TA2 during tA2, the microstructure before Q&P is the microstructure of the cold-rolled steel sheet. For tests 1 to 5, the microstructure determined before Q&P is the microstructure obtained after the second annealing. Table 4 - Microstructure of the steel sheet after the Q&P process ACfr / nn / zznz / E / YiAi The microstructure of the tested samples was determined and compiled in the following table: Assay 0 Microstructure after Q&P Assays F(%) PM (%) Y (%) FM (%) Carbide density in PM (x106 / mm2) MA islands (%) FM and MA size (pm) 1* 38 47 15 0 1 1 0.4 2* 15 66 16 3 1 6 0.5 3* 15 63 17 5 1 8 0.5 4* 20 64 16 0 1 1 0.4 5 52 28 20 0 1 1 0.4 6 15 57 16 12 2 20 1 7 15 48 17 20 1 15 1.2 *: tests according to the invention / Underlined values: do not correspond to the invention and: represents the residual austenite surface fraction PM: represents the fraction of partitioned martensite surface FM: represents the surface fraction of fresh martensite F: represents the ferrite surface fraction MA: represents the surface fraction of martensite-austenite islands Due to the second annealing, there is a more homogeneous microstructure with fine fresh martensite and MA islands, with a size smaller than 0.7 pm. In contrast, tests 6 and 7, in which there is no second annealing, and consequently, no more significant Mn enrichment in austenite, form more than 10% larger fresh martensite and MA islands with a more heterogeneous size distribution. Table 5 - Mechanical properties of the cold-rolled and heat-treated steel sheet after the Q&P process The mechanical properties of the tested samples were determined and compiled in the following table: Test YS (MPa) TS (MPa) UE (%) HER (%) TE (%) (YS*UE +TS*TE+TS*HER) / %S¡ 1* 1065 1276 13 28 16 71417 2* 1173 1328 12 32 16 79408 3* 1092 1322 10 32 14 73196 4* 1221 1355 12 43 15 79694 5 762 1254 14 ne 18 33918 6 1155 1323 9 19 12 52457 7 1132 1351 10 7 13 39133 *: tests according to the invention. Underlined values: do not match mechanical properties, ne: value not evaluated RCfr / nn / zznz / E / YiAi The examples show that the steel sheets according to the invention, i.e., examples 1 to 4, are the only ones that exhibit all the target properties thanks to their specific composition and microstructures. In test 5, steel A is hot-rolled, coiled, annealed once, and cold-rolled before being annealed a second time according to the invention. During the quenching and parting step, the steel is heated to a low temperature TA3, which limits austenite and consequently favors ferrite during cooling. The yield strength of the final steel sheet is then less than 950 MPa, and the equation (YS*LJE +TS*TE+TS*HER) / %Si did not reach 65000. In tests 6 and 7, steels C and A, respectively, were not reheated before the quenching and parting process. The microstructure before quenching and parting was 97% ferritic, leading to a high fresh martensite content after quenching and parting. This high fraction of large-sized fresh martensite resulted in a hole expansion ratio of less than 25%, and consequently, a (YS*UE+TS*TE+TS*HER) / %S ratio of less than 65000.

Claims

1. A cold-rolled, heat-treated steel sheet made of steel having a composition comprising, in weight percent: C: 0.12% to 0.25% Mn: 3.0% to 8.0% Si: 0.70% to 1.50% Al: 0.3% to 1.2% B: 0.0002% to 0.004% S < 0.010% P < 0.020% N < 0.008% and optionally comprising one or more of the following elements, in weight percent: Mo < 0.5% V < 0.2% Nb < 0.06% Ti < 0.0.5% the remainder of the composition is iron and unavoidable impurities resulting from smelting, said steel sheet having a microstructure consisting of, in surface fraction: - between 5% and 45% ferrite, - between 25% and 85% partitioned martensite, this partitioned martensite having a carbide density strictly less than 2x106 / mm2, - between 10% and 30% retained austenite, - less than 8% fresh martensite, - a portion of this fresh martensite which combines with retained austenite in the form of martensite-austenite (MA) islands in a total surface fraction less than 10%.

2. A cold-rolled, heat-treated steel sheet according to claim 1, wherein the manganese content is between 3.0% and 5.0%.

3. A cold-rolled steel sheet heat-treated according to any of claims 1 to 2, wherein the silicon content is between 0.80% and 1.30%.

4. A cold-rolled steel sheet heat-treated according to any of claims 1 to 3, wherein the size of fresh martensite and martensite-austenite islands is less than 0.7 pm.

5. A cold-rolled steel sheet heat-treated according to any of claims 1 to 4, wherein said microstructure includes a maximum of 6% fresh martensite.

6. A cold-rolled steel sheet heat-treated according to any of claims 1 to 5, wherein the yield strength YS is greater than 950 MPa.

7. A cold-rolled, heat-treated steel sheet according to any of claims 1 to 6, wherein the tensile strength TS is greater than 1180 MPa.

8. A cold-rolled steel sheet heat-treated according to any of claims 1 to 7, wherein the uniform elongation is greater than 10%.

9. A cold-rolled steel sheet heat-treated according to any of claims 1 to 8, wherein the hole expansion ratio is greater than 25%.

10. A cold-rolled, heat-treated steel sheet according to any of claims 1 to 9, wherein the yield strength YS expressed in MPa, the tensile strength TS expressed in MPa, the uniform elongation UE expressed in %, the total elongation TE expressed in %, the hole expansion ratio HER expressed in %, and the silicon content expressed as a weight percent, satisfy the following equation: (YS*UE +TS*TE+TS*HER) / %S¡ > 65000 11. A method for manufacturing a heat-treated, cold-rolled steel sheet, comprising the following successive steps: - melting a steel to obtain a semi-product, this semi-product having a composition according to claim 1, - reheating the slab to a heating temperature between 1150 °C and 1300 °C, - hot-rolling the reheated slab to a final rolling temperature between 800 °C and 950 °C to obtain a hot-rolled steel sheet, - coiling the hot-rolled steel sheet at a coiling temperature between 200 °C and 700 °C. - annealing the hot-rolled steel sheet to a first annealing temperature TA1 between 550 °C and 700 °C, and holding the steel sheet at this temperature TA1 for a holding time tA1 between 30 s and 50 h, - cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet,- reheat the cold-rolled steel sheet to a second annealing temperature TA2 above Ae3-10 °C, and hold the steel sheet at this temperature TA2 for a holding time tA2 between 1 s and 1000 s, so as to obtain, from annealing, a microstructure comprising martensite and bainite, the sum of which is greater than 80%, strictly less than 20% ferrite, and strictly less than 20% of the sum of carbides and martensite-austenite islands (MA), Ae3 calculated from the formula: Ae3 = 890 - 20 * %C + 20 * %Si - 30 * %Mn + 130 * %Al - reheat the cold-rolled steel sheet to a temperature TA3 strictly less than Ae3 and greater than (Ae1+Ae3) / 2, and hold the steel sheet at this temperature of TA3 annealing for a retention time tA3 between 3 s and 1000 s,Ae1, calculated from the formula: ACfr / nn / zznz / E / YiAi Ae1 = 670 + 15*%Si - 13*%Mn + 18*%Al - temper the cold-rolled steel sheet to a tempering temperature TQ lower than (Ms-50 °C), to obtain a tempered steel sheet, Ms, calculated from the formula: Ms = 560 - (30*%Mn + 13*%Si - 15*%Al + 12*%Mo) - 600*(1 - exp(-0.96*%C)) - reheat the tempered steel sheet to a parting temperature TP between 350 °C and 550 °C, and hold the tempered steel sheet at this parting temperature for a parting time between 1 s and 1000 s, - cool the steel sheet to room temperature, to obtain a steel sheet cold-rolled and heat-treated.