HOT ROLLED STEEL SHEET, HIGH TENACITY AND METHOD OF MANUFACTURING THE SAME.

Abstract

The invention relates to a hot-rolled steel sheet having a composition comprising, in weight percent: C: 0.10% to 0.25%, Mn: 3.5% to 5.0%, Si: 0.80% to 1.60%, B: 0.0003% 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 of, in surface fraction: between 50% and 80% lath bainite, less than 30% granular bainite, the remainder being martensite, martensite-austenite islands and austenite films, and having less than 20% martensite and the MA islands having the multiplication of the maximum length Lmax of the grain by the maximum width Wmax. of the grain larger than 1 µm2.

Description

HOT-ROLLED STEEL SHEET, HIGH TENACITY AND ITS MANUFACTURING METHOD The present invention relates to a high strength steel sheet having high toughness and good malleability and a method for obtaining such a steel sheet. To manufacture various items such as structural body member parts and body panels for automotive vehicles, it is known to use sheets made of dual-phase (DP) steels or transformation-induced plasticity (TRIP) steels. One of the main challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency for global environmental protection, while also addressing safety requirements. To meet these requirements, the steel industry is continually developing new high-strength steels to produce blades with greater performance and tensile strength, as well as good ductility and formability. One development to improve mechanical properties is to increase the manganese content in steels. The presence of manganese helps increase the ductility of steels by stabilizing austenite. However, these medium-manganese steels have brittleness weaknesses. WO2007101921 describes a method for producing hot-rolled sheets of multiphase steels, in particular, with a manganese content between 1% and 3%. The microstructure consists of at least 75% bainite, residual austenite greater than or equal to 5%, and martensite greater than or equal to 2%. To achieve a Charpy V notch fracture energy greater than 28 J (corresponding to 0.52 J / mm2) and the target microstructure, the cooling of the hot-rolled steel sheet must be controlled. Two cooling stages are actually necessary to obtain the desired properties, which complicates the manufacturing process. Therefore, the purpose of the invention is to solve the aforementioned problem and provide a steel sheet that has high hardness with a Charpy impact energy at 20SC greater than 0.50 J / mm2, a tensile strength TS greater than or equal to 1450 MPa, a high uniform elongation greater than or equal to 5%, and is easily processable by the conventional process route. Another purpose of the invention is to provide a steel sheet with good weldability. 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 6. Another objective is achieved by providing the method according to claim 7. Another objective of the invention is achieved by providing a steel sheet comprising the features according to claim 8. The invention will now be described in detail and illustrated by examples without including limitations. Hereafter, Ms designates the martensite initiation temperature, i.e., the temperature at which austenite begins to transform into martensite upon cooling. This temperature can be calculated from a formula based on the weight percentage of the corresponding elements: 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 will now be described, where the content is expressed as a percentage by weight. The carbon content is between 0.10% and 0.25%. If the carbon content is too high, the weldability of the steel sheet is insufficient. If the carbon content is less than 0.10%, the austenite fraction is not sufficiently stabilized to obtain target properties. In a preferred embodiment of the invention, the carbon content is between 0.15% and 0.20%, respectively. The manganese content is between 3.5% and 5.0% manganese. Greater than 5.0% of addition, the risk of central segregation increases, to the detriment of hardness. Less than 3.5% manganese, the final structure comprises an insufficient fraction of retained austenite to obtain the desired properties. In a preferred embodiment of the invention, the manganese content is between 3.5% and 4.5% manganese. According to the invention, the silicon content is between 0.80 and 1.60. A silicon addition of at least 0.80 helps stabilize a sufficient amount of retained austenite. Above 1.60, silicon is detrimental to toughness. In addition, silicon oxides form on the surface, which impairs the coating ability of the steel. In a preferred embodiment of the invention, the silicon content is between 1.00 and 1.60. According to the invention, the boron content is between 0.0003 7% and 0.004 7% . The presence of boron delays the bainitic transformation to a lower temperature, and the bainite formed at low temperatures has a lath-like morphology that increases hardness. In addition, boron improves the weldability of the steel sheet. Greater than 0.004 7%, the formation of boron-carbides at the anterior austenite grain boundaries is promoted, making the steel more brittle. Less than 0.0003 7%, there is not a sufficient concentration of free B to segregate at the anterior austenite grain boundaries to increase the hardness of the steel. In a preferred embodiment of the invention, the boron content is between 0.001 7% and 0.003 7% . Optionally, some elements may be added to the composition of the steel according to the invention: Titanium may optionally be added up to 0.04 7% to provide precipitation enhancement. Preferably, a minimum of 0.01 7% of titanium is added in addition to boron to protect the boron from BN formation. Niobium may be added up to 0.05 7o to refine austenite grains during hot rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010 7o. Optionally, molybdenum can be added, up to a maximum limit of 0.37o. Molybdenum stabilizes austenite and increases the hardness of the steel. Furthermore, molybdenum improves the weldability of the steel sheet. Above 0.37o, the addition of Mo is costly and ineffective, given the required properties. Optionally, aluminum can be added up to 0.90%, since it is a very effective element in deoxidizing the steel in the liquid phase during processing. In addition, aluminum improves the weldability of the steel sheet. The aluminum content is less than 0.90% to prevent the occurrence of inclusions and avoid oxidation problems. Preferably, the aluminum content is between 0.10% and 0.90%. More preferably, the aluminum content is between 0.20% and 0.90%. Even more preferably, the aluminum content is between 0.30% and 0.90%, even more preferably between 0.40% and 0.90%. According to the invention, a maximum of 0.80% chromium is permitted. Above this level, a saturation effect is observed, and adding chromium is both unnecessary and costly. The remaining composition of steel is iron and impurities resulting from smelting. In this sense, P, S, and N are at least considered residual elements that 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. In particular, phosphorus is segregated at the grain boundary and for a phosphorus content above 0.020%, the hardness of the steel is reduced. The microstructure of the hot-rolled steel sheet according to the invention will now be described. Hereinafter, the aspect ratio is the ratio of the maximum length Lmax of a grain to the maximum width Wmax of the grain measured at 90° to said maximum length. Hot-rolled steel sheet has a microstructure consisting of, in surface fraction, 50–80% lath bainite, less than 30% granular bainite, and the remainder martensite, martensite-austenite (MA) islands, and austenite films, the sum of which comprises 15–35%. In addition, less than 20% of the martensite and MA islands have a ratio of maximum grain length Lmax by maximum grain width Wmax greater than 1 pm2. The morphology of lath bainite is obtained thanks to the presence of boron, which delays the bainitic transformation, and thanks to low-temperature coiling. According to the present invention, lath bainite will be a bainite with an aspect ratio greater than or equal to 3. The presence of between 50% and 80% lath bainite is beneficial for the hardness of hot-rolled steel. Granular bainite has an aspect ratio less than 3. The remaining microstructure comprises martensite, MA islands, and austenite films, the sum of which ranges from 15% to 35%, to ensure a uniform elongation greater than 5%. Above 35% of the sum of martensite, MA islands, and austenite films, the austenite in the MA islands and austenite films becomes unstable and transforms into martensite, leading to a degradation of elongation. Less than 20% of the martensite and MA island fraction has a multiplication of Lmax by Wmax greater than 1 pm2. Greater than 20%, the MA islands transform into fresh martensite, resulting in elongation degradation. Martensite-austenite (MA) islands have an aspect ratio less than or equal to 2. These MA islands develop during rolling. A portion of the austenite transforms into lath bainite as described above. Part of the austenite transforms into martensite generating MA islands during rolling. A final portion of the austenite remains in the final microstructure. Austenite ηαΐτ / ηη / ζζηζ / Ε / γίΛA films are austenite between bainite laths with an aspect ratio greater than or equal to 2. Both MA islands and austenite films are beneficial to the toughness of hot-rolled steel sheet. The hot-rolled steel sheet according to the invention has a Charpy impact energy at 20 °C strictly greater than 0.50 J / mm2 measured in accordance with ISO 148-1:2006(F) and ISO 148-1:2017(F). The hot-rolled steel sheet according to the invention has a tensile strength TS greater than or equal to 1450 MPa, and a uniform elongation UE greater than or equal to 5 %. Preferably, the hot-rolled steel sheet according to the invention has a total elongation TE strictly greater than 7 %. TS, UE and TE are measured in accordance with ISO 6892-1. The steel sheet according to the invention can be produced by any suitable manufacturing method, and one can be defined by the person skilled in the art. However, it is preferred to use the method according to the invention, which comprises the following steps: A semi-finished product capable of being hot-rolled thereafter is provided with the steel composition described above. The semi-finished product is heated to a temperature between 1150 °C and 1300 °C, so as to facilitate hot rolling, with a final hot-rolling temperature (FRT) of between 750 °C and 900 °C. Preferably, the FRT is between 800 °C and 900 °C. When the FRT is higher than 900 °C, the bainite transformation kinetics are significantly slowed down during winding, leading to the formation of a high fraction of martensite, MA islands and austenite in the final microstructure. Furthermore, the presence of a large fraction of martensite and MA islands having Lmax * Wmax greater than 1 pm2 leads to a degradation in elongation. The hot rolled steel is then cooled and rolled into a coil at a temperature between (Ms-100 °C) and 550 °C. The hot-rolled steel sheet is cooled to room temperature. After winding, the sheet can be pickled to remove oxidation. Another purpose of the invention is to provide a steel sheet with good weldability. The welded assembly is manufactured by producing two sheets of hot-rolled steel, and resistance spot welding the two steel parts. Spot welding under ISO 18278-2 standard condition has been performed on the hot-rolled steel sheets. In the test used, the samples consist of two steel sheets formed into a cross-weld equivalent. A force is applied to break the weld. This force, known as cross-tensile strength (CTS), is expressed in daN. It depends on the weld spot diameter and the metal thickness, i.e., the thickness of the steel and the metal coating. It allows the calculation of the coefficient a, which is the ratio of the CTS value to the product of the weld spot diameter multiplied by the substrate thickness. This coefficient is expressed in daN / mm2. The plug ratio is equal to the plug diameter divided by the cast zone diameter. nofr / nn / zznz / E / YiAi Strength spot welds joining the first sheet to the second sheet are characterized by high strength in the cross-tension test defined by an a value of at least 50 daN / mm2, and a plug ratio of at least 80%. The invention will now be illustrated by the following examples, which are in no way limiting. Example 1 grades, whose compositions are listed in Table 1, were melted into semi-finished products and processed into ηοίτ / ηη / ζζηζ / Ε / γίΛΐ steel sheets Table 1 - Compositions 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 B s PN Ti Nb Mo Al Cr Ms (°C) A 0.17 3.7 1.03 0.0019 0.001 0.014 0.004 0.025 0 0.21 0.81 0.5 355 B 0.19 3.9 1.27 0.0021 0.001 0.011 0.004 0.029 0.02 0.20 0.39 0 330 C 0.18 3.5 0.97 0 0.001 0.013 0.004 0 0.03 0.20 0 0 345 D 0.17 3.6 1.01 0 0.001 0.016 0.004 0 0 0 0 0 349 Steels A and B are according to the invention, C and D outside the invention. Table 2 - Process parameters The steel semi-finished products, as molded, were reheated to 1200°C, hot-rolled, and coiled. The following specific conditions were applied: FRT Steel Test (°C) Tenoned (°C) 1 A 900 450 2 B 830 450 3 B 845 500 4 B 91Q 500 5 C 900 450 6 D 900 450 Underlined values: not in accordance with the invention. The hot-rolled sheets were analyzed and the corresponding microstructure elements, mechanical properties and weldability properties were collected respectively in Tables 3, 4 and 5. Table 3 - Microstructure of hot-rolled steel sheet The phase percentages of the microstructures of the hot-rolled steel sheet obtained were determined: ηοίτ / ηη / ζζηζ / Ε / γίΛΐ Assays Ribbon bainite (%) Martensite + MA + austenite (%) Granular bainite (%) Fraction of martensite and MA islands having Lmax*Wmax > 1 pm2 (%) 1 75 25 - 14 2 77 23 - 12 3 75 25 - 13 4 60 40 - 25 5 Ξ 50 50 ne 6 - 60 40 ne Underlined values: not in accordance with the invention. ne: unevaluated value The surface phase fractions in the microstructure are determined by the following method: a specimen is cut from the hot laminate, polished, and etched with a known reagent to reveal the microstructure. The section is subsequently examined using a scanning electron microscope, e.g., a field emission scanning electron microscope (FEG-SEM) at magnification greater than 5000x, in secondary electron mode. The determination of the surface fraction of austenite films and MA islands is carried out thanks to SEM observations after etching with Nital or Picral / Nital reagent. According to the present invention, the lath bainite will be a bainite having an aspect ratio greater than or equal to 3. According to the invention, the MA islands have an aspect ratio less than or equal to 2. Table 4 - Mechanical properties of hot-rolled steel sheet The mechanical properties of the tested samples were determined and compiled in the following table: Test Charpy impact energy (J / mm2) TS (MPa) EU (%) TE(%) 1 0.89 nenene 2 0.81 1492 6.6 11 3 0.76 1522 7.4 11 4 0.82 1485 4.1- 7 5 0.31 nenene 6 0.16 nenene Underlined values: target values ​​do not match. ne: unevaluated value Table 5 - Weldability properties of hot-rolled steel sheet The weldability properties of some samples were determined and compiled in the following table: ηοίτ / ηη / ζζηζ / Ε / γίΛΐ Test at (daN / mm2) Shutter ratio (%) 1 66 84 5 45 77 6 47 70 Underlined values: target values ​​do not match. The examples show that the steel sheets according to the invention, i.e. examples 1 to 3, are the only ones that exhibit all the target properties thanks to their specific composition and microstructures. In Test 4, the steel sheet was hot-rolled at a temperature of 910°C, resulting in a high proportion of martensite and MA islands. This results in a uniform elongation of less than 5%. The absence of boron in steels C and D leads to a low level of Charpy impact energy in tests 5 and 6, with the formation of more than 30% granular bainite, decreasing the steel's fracture toughness. Regarding weldability parameters, the absence of boron, molybdenum, and aluminum is detrimental to the plug ratio.

Claims

1. Hot-rolled steel sheet made of a steel having a composition comprising, in weight percent: C: 0.10% to 0.25% Mn: 3.5% to 5.0% Si: 0.80% to 1.60% B: 0.0003% to 0.005% S < 0.010% P < 0.020% N < 0.008% and optionally comprising one or more of the following elements, in weight percent: Ti < 0.04% Nb < 0.05% Mo < 0.3% Al < 0.90% Cr < 0.80% the remainder of the composition is iron and unavoidable impurities resulting from smelting, said steel sheet having a microstructure comprising, in surface fraction: - from 50% to 80% of lath bainite with a dimensional ratio greater than or equal to 3, - less than 30% of granular bainite with a dimensional ratio less than 3, - the remainder being martensite, MA islands of martensite-austenite having a dimensional ratio less than or equal to 2, and austenite films, the sum of which is from 15% to 35%, - and less than 20% of the islands of said martensite and said MA having the multiplication of the maximum grain length Lmax by the maximum grain width Wmax greater than 1 pm2.

2. A hot-rolled steel sheet according to claim 1, wherein the manganese content is between 3.5% and 4.5%.

3. A hot-rolled steel sheet according to any of claims 1 and 2, wherein the silicon content is between 1.00% and 1.60%.

4. A hot-rolled steel sheet according to any of claims 1 to 3, wherein the hot-rolled steel sheet has a Charpy impact energy at 20 °C strictly greater than 0.50 J / mm2.

5. A hot-rolled steel sheet according to any of claims 1 to 4, wherein the hot-rolled steel sheet has a tensile strength TS greater than or equal to 1450 MPa.

6. A hot-rolled steel sheet according to any of claims 1 to 5, wherein the hot-rolled steel sheet has a uniform elongation UE greater than or equal to 5%.

7. A method for manufacturing a hot-rolled steel sheet, comprising the following successive steps: - melting a steel to obtain a semi-product, said semi-product having a composition according to claim 1, - reheating the semi-product to a reheating temperature T between 1,150 °C and 1,300 °C, - hot-rolling the semi-product to a final hot-rolling temperature between 750 °C and 900 °C to obtain a hot-rolled steel sheet, - cooling the hot-rolled steel sheet, - winding the hot-rolled steel sheet at a winding temperature T between (MS-100 °C) and 550 °C to obtain a wound steel sheet 8. A resistance spot weld of two steel parts of hot-rolled steel sheet according to any of claims 1 to 6 or obtained by the method according to claim 7, wherein said resistance spot weld has a value a of at least 50 daN / mm2 and a plug ratio of at least 80%.