Cold rolled and heat treated steel sheet and its manufacturing method

Abstract

The invention deals with a cold rolled and heat-treated steel sheet having a composition comprising, by weight percent C: 0.12 - 0.25 % Mn: 3.0 - 8.0 %, Si: 0.70 – 1.50 %, Al: 0.3 – 1.2 %, B: 0.0002 – 0.004 %, S ≤ 0.010 %, P ≤ 0.020 %\nN ≤ 0.008 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: between 5% and 45% of ferrite, between 25% and 85% of partitioned martensite, said partitioned martensite having a carbides density strictly less than 2x106 / mm2, between 10% and 30% of retained austenite, less than 8% of fresh martensite, a part of said fresh martensite being combined with retained austenite in the shape of martensite-austenite islands in total surface fraction less than 10%.

Description

Cold rolled and heat treated steel sheet and its manufacturing method The present invention relates to a cold-rolled high-strength steel sheet having high ductility and formability and a method for obtaining such a steel sheet. For the manufacture of various items such as structural body parts and body panels of automotive vehicles, it is known to use sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels. One of the major challenges in the automotive industry is to reduce the weight of vehicles in order to improve fuel efficiency from a global environmental perspective, without neglecting safety requirements. To meet these requirements, new high-strength steels are continuously developed by the steelmaking industry to produce sheets with improved yield and tensile strengths and good ductility and formability. Publication WO2019123245 describes a method for obtaining a high-strength, high-ductility 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%, a hole expansion ratio HER of at least 20%, thanks to a quenching and debonding process. The microstructure of cold rolled steel sheet includes between 10 and 45% surface fraction of ferrite, having an average grain size of up to 1.3 μm, the product of the surface fraction of ferrite and the average grain size of up to 35 μm, between 8 and 30% retained austenite, retained austenite having a manganese content higher than 1.1*Mn%, where Mn% represents the manganese content of the steel, up to 8% fresh martensite, up to 2.5% cementite and the rest is segregated martensite. A surface fraction of at least 8% of retained austenite, having a manganese content higher than 1.1*Mn%, allows for a combination of high ductility and high strength. During 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 islands and MA and therefore does not provide the features of publication WO2019123245. Publication WO2018220430 relates to steel sheets that have been hot-formed to produce parts. The steel parts are then cooled before being reheated and held to the post-treatment temperature and cooled to room temperature. This hot-forming process, due to the geometry of the part and the forming tool, causes severe local deformations in the steel part, which leads to local changes in the microstructure. Therefore, the purpose of the invention is to solve the above problem and provide a steel sheet with a yield strength higher than 950 MPa, a tensile strength higher than 1180 MPa, a uniform elongation higher than 10 percent, and a HER hole expansion ratio higher than 25 percent. The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also include the features of any one of claims 2 to 10. Another object is achieved by providing a method according to claim 11. The invention will now be described in detail and illustrated by non-limiting examples. Hereafter, Ae1 determines the equilibrium transformation temperature below which austenite is completely unstable, Ae3 determines the equilibrium transformation temperature above which austenite is completely stable, Ms determines the martensite onset temperature, i.e. the temperature at which austenite begins to transform into martensite after cooling. These temperatures can be calculated from a formula: Ae1=670 + 15*%Si – 13*%Mn + 18*%Al Ae3 = 890 – 20 * %C + 20 * %Si – 30 * %Mn + 130 * %Al Ms= 560 - (30*%Mn+13*%Si-15*%Al+12*%Mo)-600*(1-exp(-0,96*C)) The composition of the steel according to the invention, in terms of weight percentage, includes the following: According to the invention, the carbon content is between 0.12% and 0.25%. By adding more than 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 obtain sufficient elongation and tensile strength. In a preferred embodiment, the carbon content is between 0.15% and 0.25%. According to the invention, the manganese content is between 3.0 and 8.0% to obtain sufficient elongation by stabilizing the austenite. With an addition of more than 8.0%, the risk of central segregation increases to the detriment of yield strength and tensile strength. Below 3.0%, the final structure contains an insufficient fraction of retained austenite, so that the desired combination of ductility and strength is not achieved. In a preferred embodiment, the manganese content is between 3.0% and 5.0%. According to the invention, the silicon content is between 0.70% and 1.50%. Adding silicon of at least 0.70% helps to stabilize a sufficient amount of retained austenite. At more than 1.50%, silicon oxides form on the surface, which impairs the coatability of the steel. In a preferred embodiment, the silicon content is between 0.80% and 1.30%. The aluminum content is between 0.3% and 1.2%, since aluminum is a very effective element for deoxidizing steel in the liquid phase during operation. To prevent the formation of inclusions and to avoid oxidation problems, the aluminum content is not more than 1.2%. In a preferred embodiment, the aluminum content is between 0.3% and 0.8%. The boron content is between 0.0002% and 0.004% to increase the quenchability of the steel and improve the weldability of the steel sheet. If desired, some elements can be added to the steel composition according to the invention. Niobium can be optionally added up to 0.06% to refine the austenite grains during hot rolling and to provide precipitation hardening. Preferably, the minimum amount of niobium added is 0.0010%. More than 0.06% will not achieve the desired yield strength and elongation at the surface. Molybdenum can be added up to 0.5%. Molybdenum stabilizes the retained austenite and therefore reduces the decomposition of austenite during separation. Adding more than 0.5% molybdenum is costly and ineffective given the required properties. Vanadium can be optionally added up to 0.2% to create a hard deposit. Titanium can be added up to 0.05% to provide precipitation hardening. If the titanium level is greater than or equal to 0.05%, the desired yield strength and elongation at break will not be achieved. Preferably, at least 0.01% titanium is added in addition to boron to protect boron from BN formation. The remainder of the steel is a combination of iron and impurities from the smelting process. In this regard, P, S and N are considered as residual elements, which are unavoidable impurities. Their content is less than 0.010% for S, less than 0.020% for P and less than 0.008% for N. The microstructure of the cold-rolled and heat-treated steel sheet according to the invention will now be described. The cold-rolled and heat-treated steel sheet has a microstructure comprising between 5% and 45% of the surface fraction of ferrite, between 25% and 85% of the precipitated martensite, the precipitated martensite having a carbide density necessarily less than 2x106 / mm2, between 10 and 30% of the retained austenite, less than 8% of the fresh martensite. A portion of the fresh martensite combines with the retained austenite to form martensite-austenite (MA) islands, the total surface fraction of which is less than 10%. In a preferred embodiment, these MA islands have a shape factor less than or equal to 2. Ferrite is formed during annealing at a temperature between (Ae1+Ae3) / 2 and Ae3. If the ferrite fraction is less than 5%, uniform elongation of 10% will not be achieved. If the ferrite fraction is higher than 45%, tensile strength of 1180 MPa and yield strength of 950 MPa will not be achieved. The microstructure of cold-rolled and heat-treated steel sheet includes between 25 and 85% of precipitated martensite to ensure high ductility of the steel, and the precipitated martensite has a carbide density necessarily less than 2x106 / mm2. The precipitated martensite is martensite that is formed after cooling after annealing and then precipitated in the precipitation step. Preferably, the microstructure includes between 40 and 80% of precipitated martensite. The microstructure of cold-rolled and heat-treated steel sheet contains between 10 and 30% retained austenite to ensure high ductility of the steel and less than 8% fresh martensite. Preferably, the microstructure contains a maximum of 6% fresh martensite. Fresh martensite is formed during cooling at room temperature of cold-rolled and heat-treated steel sheet. The size of fresh martensite and martensite-austenite islands is less than 0.7 μm. The steel sheet according to the invention can be produced by any suitable manufacturing method and can be defined by a person skilled in the art. However, it is preferred to use a method according to the invention which comprises the following steps: A semi-product that can be further hot rolled is provided with the steel composition described above. This semi-product is heated to a Treheat temperature of between 1150°C and 1300°C, for ease of hot rolling, with a final hot rolling temperature FRT of between 800°C and 950°C, to obtain a hot rolled steel sheet. The maximum value of FRT is selected in order to prevent austenitic grain coarsening. Preferably, FRT is between 800°C and 910°C. The hot rolled steel is then cooled and coiled at a temperature Tcoil between 200°C and 700°C. Preferably, the coiling temperature is from (Ms-100°C) to 550°C. After coiling, the sheet can be acid-washed to remove oxidation. The hot-rolled steel sheet is then annealed at a first annealing temperature TA1 between 550°C and 700°C and held at the said annealing temperature for a holding time tA1 between 30 and 50 hours to improve the cold rolling ability and toughness 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 which can be, for example, between 0.7 mm and 3 mm or even better in the range of 0.8 mm and 2 mm. The reduction ratio of cold rolling is preferably between 20 and 80%. Below 20%, recrystallization during subsequent heat treatment is not desirable, which may impair the formability of the cold rolled and heat treated steel sheet. Above 80% there is a risk of edge cracking in cold rolling. The cold rolled steel sheet is then reheated to the second annealing temperature TA2 higher than Ae3-10°C and held at the mentioned TA2 temperature for a holding time tA2 between 1 second and 1000 seconds, so that after annealing, a microstructure consisting of martensite and bainite is obtained, the total of which is more than 80%, precisely less than 20% ferrite and precisely less than 20% of the total of martensite-austenite (MA) islands and carbides. Martensite of martensite-austenite islands is fresh martensite. The martensite present in the sum of martensite and bainite is more than 80% a self-tempered martensite. The determination of the type of martensite can be carried out and quantified thanks to field emission scanning electron microscopy ("FEG-SEM"). The cold rolled steel sheet is then subjected to a quenching and tempering (Q&P) process. The quenching and tempering process includes the following steps: - Reheating the cold rolled steel sheet to a temperature TA3 necessarily lower than Ae3 and higher than (Ae1+Ae3) / 2 and holding at the annealing temperature TA3 for a holding time tA3 between 3 seconds and 1000 seconds, in order to obtain an austenitic and ferritic structure. - Quenching the cold rolled steel sheet to a quenching temperature TQ lower than (Ms-50°C), to obtain a quenched steel sheet. During this quenching step, austenite is partially transformed into martensite. If the quenching temperature is higher than (Ms-50°C), the fraction of tempered martensite in the final structure is too low, resulting in a fresh martensite fraction above 8%, which is detrimental to the overall elongation of the steel. - Reheating the quenched steel to a separation temperature between 350°C and 550°C and maintaining it at said separation temperature for a separation time between 1 second and 1000 seconds before cooling to room temperature. The cold-rolled and heat-treated steel sheet according to the invention has a yield strength YS of higher than 950 MPa, a tensile strength TS of higher than 1180 MPa, a uniform elongation UE of higher than 10%, and a hole expansion ratio HER of higher than 25%. Preferably, the cold rolled and heat treated steel sheet according to the invention has YS and TS expressed in MPa, UE, total elongation TE and HER expressed in percent and silicon content Si percentage expressed in percent by weight which satisfies 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 standard ISO 6892-1. HER is measured according to ISO standard ISO 16630. The invention is now illustrated by the following examples, which are in no way limiting. Examples The 3 grades, whose compositions are summarized in Table 1, were cast into semi-finished products and converted into steel sheets according to the process parameters summarized in Table 2. Table 1 – Compositions The tested compounds are summarized in the table below, where the element content is expressed in weight percent. 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 according to the invention. Table 2 - Process parameters The steel semi-products, after casting, were reheated at 1200°C, hot rolled to the final rolling temperature FRT, coiled, first heat treated at TA1 and held at the said TA1 temperature for a holding time ta1, before cold rolling. The second annealing is performed at TA2 and the cold rolled steel is held at the said TA2 temperature for a holding time ta2, before the quenching and tempering (Q&P) process and subsequent cooling to room temperature. The following specific conditions were applied: Steel Test FRT (°C) TCoil (°C) First Annealing Cold Rolling Rate (%) Second Annealing Q&P TA1 (°C) tA1 (min) TA2 (°C) tA2 (s) TA3 (°C) tA3(s) TQ (°C) TP (°C) t(s) 1* 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 *: Experiments according to the invention. Underlined values: not in accordance with the invention Then the annealed sheets were analyzed and the corresponding microstructural elements before Q&P, after Q&P and mechanical properties after Q&P were collected in Tables 3, 4 and 5, respectively. Table 3 - Microstructure of steel sheet before Q&P process The microstructure of the tested samples was determined and collected in the following table: Microstructure test 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 *: Tests based on the invention / Underlined values: Not in accordance with the invention B: stands for bainite surface fraction F: stands for ferrite surface fraction M: stands for martensite surface fraction MA: stands for Martensite-Austenite Island Surface Fraction Surface fractions are determined by the following procedure: A sample of cold-rolled and heat-treated steel sheet is cut, polished, and etched with a known reagent to reveal the microstructure. The section is then examined by light or scanning electron microscopy, for example with a field emission gun scanning electron microscope ("FEG-SEM") at a magnification greater than 5000 times, coupled with a BSE (back-scattered electron) instrument. The determination of the surface fraction of each component is carried out by image analysis using 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 TA2 during tA2, the microstructure before Q&P is that of the cold rolled steel sheet. For tests 1 to 5, the microstructure given before Q&P is the microstructure obtained after the second annealing. Table 4 - Microstructure of steel sheet after Q&P process The microstructure of the tested samples was determined and collected in the following table: Microstructure test after Q&P tests F(%) PM (%) γ (%) FM (%) Density of carbides in terms of PM (x106 / mm2) MA islands (%) Size of FM and mA (µm) 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 based on the invention / Underlined values: Not in accordance with the invention γ: stands for surface fraction of retained austenite PM: abbreviation for surface fraction of precipitated martensite FM: stands for fresh martensite surface fraction F: stands for ferrite surface fraction MA: stands for Martensite-Austenite Island Surface Fraction Thanks to the second annealing, there is a more homogeneous microstructure with fresh martensite and MA islands with a size of less than 0.7 μm. In contrast, experiments 6 and 7 where there is no second annealing and therefore a significant enrichment of manganese in the austenite constitute more than 10% of the fresh martensite and larger MA islands with a more heterogeneous size distribution. Table 5 - Mechanical properties of cold rolled and heat treated steel sheet after Q&P process The mechanical properties of the tested samples were determined and collected in the following table: Test YS (MPa) TS (MPa) UE (%) HER (%) TE (%) (YS*UE +TS*TE+TS*HER) / %Si 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 na 18 33918 6 1155 1323 9 19 12 52457 7 1132 1351 10 7 13 39133 *: Experiments based on the invention Underlined values: do not correspond to mechanical properties. na: unevaluated value The examples show that the steel sheets according to the invention, namely Examples 1 to 4, are the only sheets which, thanks to their specific composition and microstructure, exhibit all the target properties. In test 5, steel A according to the invention is hot rolled, coiled, annealed for the first time and then cold rolled before being annealed for the second time. During the quenching and separation step, the steel is heated to a low temperature of TA3, which confines the austenite and thus promotes ferrite during cooling. Then the yield strength of the final steel sheet is below 950 MPa and the equation (YS*UE +TS*TE+TS*HER) / %Si does not reach 65000. In experiments 6 and 7, steels C and A are not reheated before quenching and annealing, respectively. The microstructure before Q&P is 97% ferrite, which results in a high content of fresh martensite after Q&P. This high fraction of fresh martensite with large size results in a hole expansion ratio of less than 25% and therefore the equation (YS*UE +TS*TE+TS*HER) / %Si of less than 65,000.

Claims

Claims 1. A cold rolled and heat treated steel sheet, made from a steel having a composition comprising the following by weight percentage: C: 0.12 - 0.25 % Mn: 3.0 - 8.0 % Si: 0.70 - 1.50 % Al: 0.3 - 1.2 % B: 0.0002 - 0.004 % S ≤ 0.010 % P ≤ 0.020 % N ≤ 0.008 % and optionally comprising one or more of the following elements by weight percentage: Mo ≤ 0.5% V ≤ 0.2 % Nb ≤ 0.06 % Ti ≤ 0.05 % The remainder is a combination of iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising the following by surface fraction: - between 5 and 45% ferrite, - between 25 and 85% Precipitated martensite Precipitated martensite has a carbide density necessarily less than 2x106 / mm2, - between 10 and 30% retained austenite, - less than 8% fresh martensite, - a portion of said fresh martensite combined with the retained austenite in the form of martensite-austenite (MA) islands in a total surface fraction of less than 10%. - in this case, the size of the fresh martensite and martensite-austenite islands is less than 0.7 µm, and has a yield strength YS expressed in MPa, tensile strength TS expressed in MPa, uniform elongation UE expressed in %, total elongation TE expressed in %, pore expansion ratio HER expressed in % and silicon content expressed in % by weight which satisfies the following equation: (YS*UE +TS*TE+TS*HER) / %Si > 65000.

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

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

4. A cold rolled and heat treated steel sheet according to any one of claims 1 to 3, wherein said microstructure comprises at most 6% fresh martensite.

5. A cold rolled and heat treated steel sheet according to any one of claims 1 to 4, wherein the yield strength YS is higher than 950 MPa.

6. A cold rolled and heat treated steel sheet according to any one of claims 1 to 5, wherein the tensile strength TS is higher than 1180 MPa.

7. A cold rolled and heat treated steel sheet according to any one of claims 1 to 6, wherein the uniform elongation is higher than 10%.

8. A cold rolled and heat treated steel sheet according to any one of claims 1 to 7, wherein the hole expansion ratio is higher than 25%.

9. A method for manufacturing cold-rolled and heat-treated steel sheet comprising the following sequential steps: - casting a steel to obtain a semi-finished product, said semi-finished product having a composition according to claim 1, - reheating the semi-finished product at a temperature Treheat between 1150 and 1300 ° C, - hot rolling the heated semi-finished product with a final rolling temperature FRT between 800 ° C and 950 ° C to obtain a hot rolled steel sheet, - coiling the hot rolled steel sheet at a coil temperature Tcoil between 200 ° C and 700 ° C, - annealing the hot rolled steel sheet at a first annealing temperature TA1 between 550 ° C and 700 ° C and holding the steel sheet at said temperature TA1 for a holding time tA1 between 30 and 50 hours, - cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet, reheating the cold rolled steel sheet to a temperature Second annealing TA2 above Ae3-10°C and holding the steel sheet at the mentioned TA2 temperature for a holding time tA2 between 1 second and 1000 seconds until after annealing, a microstructure consisting of martensite and bainite is obtained, the total of which is more than 80%, precisely less than 20% ferrite and precisely less than 20% of the sum of martensite-austenite (MA) islands and carbides, where Ae3 is calculated from the following formula: Ae3 = 890 – 20 * %C + 20 * %Si – 30 * %Mn + 130 * %Al - Reheating the cold rolled steel sheet to a temperature TA3 necessarily less than Ae3 and higher than (Ae1+Ae3) / 2 and holding the steel sheet at the annealing temperature TA3 for a holding time tA3 between 3 seconds and 1000 seconds, where Ae1 is calculated from the following formula: Ae1=670 + 15*%Si – 13*%Mn + 18*%Al - Quenching the cold rolled steel sheet to a quenching temperature TQ less than (Ms-50°C), to obtain a quenched steel sheet, where Ms is calculated from the following formula becomes: Ms= 560 - (30*%Mn+13*%Si-15*%Al+12*%Mo)-600*(1-exp(-0.96*%C)) - reheating the quenched steel sheet to a separation temperature between 350°C and 550°C and maintaining the quenched steel sheet at the specified separation temperature for a separation time between 1 second and 1000 seconds, - cooling the steel sheet to room temperature, to obtain a cold rolled and heat treated steel sheet