HOT-ROLLED AND ANNEALED HIGH-TENACITY STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

The invention relates to a hot-rolled and annealed steel sheet with a composition comprising, in weight percent: C: 0.1-0.25%, Mn: 3.00-5.00%, Si: 0.80-1.60%, B: 0.0003-0.004%, 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: more than 20% recrystallized ferrite, the remainder being non-recrystallized ferrite, more than 15% of the recrystallized ferrite having a grain size greater than 5 µm and a carbide density at the grain boundary of recrystallized ferrite less than 5 carbides per 10 µm of grain boundary length.

Description

HOT-ROLLED AND ANNEALED HIGH-TENACITY STEEL SHEET AND METHOD OF MANUFACTURING THE SAME Field of invention The present invention relates to a high-strength steel sheet having high toughness and low hardness and to a method for obtaining such a steel sheet. Background of the invention To manufacture various items, such as parts of structural body members and body panels for automotive vehicles, it is known to use sheets made from DP (dual phase) steels or TRIP (transformation induced plasticity) steels. One of the main challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency while considering global environmental protection, without neglecting safety requirements. To meet these requirements, the steel industry is continuously developing new high-strength steels to obtain sheets with improved performance, tensile strength, ductility, and formability. One of the developments aimed at improving mechanical properties is increasing the manganese content in steels. The presence of manganese helps increase the ductility of steels by stabilizing austenite. However, these steels exhibit weaknesses related to brittleness. To overcome this problem, elements such as boron are added. These boron additive chemical compounds are very tough during hot rolling, but the hot strip is too difficult to process further. The most efficient way to soften the hot strip is through batch annealing, but this leads to a loss of hardness. For example, publication US20050199322 discloses a hot-rolled high-carbon steel sheet that has excellent ductility and elastic flange formability; the hot-rolled steel sheet is annealed to reduce the hardness of the steel sheet. Brief description of the invention Therefore, there is an unsolved problem in the state of the art to obtain a hot-rolled steel sheet with high toughness and low hardness, compatible with a downstream process. Therefore, the purpose of the invention is to solve the aforementioned problem and provide a steel sheet having a combination of a hardness level below 300 HV and high toughness with a Charpy impact energy at 20°C greater than 0.40J / mm2. The objective of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also include features of any of claims 2 to 7. The objective of the invention is also to provide a steel according to claim 8. Detailed description of the invention The invention will now be described in detail and illustrated with examples without setting limitations. Hereafter, Ms indicates the martensite onset temperature, that is, the temperature at which austenite begins to transform into martensite upon cooling. These temperatures can be calculated using the formula: 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. According to the invention, the carbon content is between 0.1 and 0.25%. Above 0.25% carbon, the weldability of the steel sheet may be reduced. If the carbon content is less than 0.1%, the austenite fraction is not sufficiently stabilized to obtain the target microstructure after annealing. In a preferred embodiment of the invention, the carbon content is between 0.15 and 0.20%. The manganese content ranges from 3.00 to 5.00%. Above 5.00% addition, the risk of core segregation increases, negatively impacting hardness. The minimum content is defined to stabilize the austenite and obtain the target microstructure after annealing. Preferably, the manganese content is between 3.50 and 5.00%. In a preferred embodiment of the invention, the manganese content is between 3.50 and 4.50%. According to the invention, the silicon content ranges from 0.80 to 1.60%. Above 1.60%, silicon is detrimental to hardness. Furthermore, silicon oxides form on the surface, which impairs the steel's coatability. A silicon addition of at least 0.80% helps stabilize a sufficient amount of austenite to obtain, after annealing, the microstructure according to the invention. In a preferred embodiment of the invention, the silicon content ranges from 1.00 to 1.60%. According to the invention, the boron content ranges from 0.0003% to 0.004%. The presence of boron delays the bainitic transformation to a lower temperature, and the bainite formed at this lower temperature has a lattice morphology that increases toughness. Above 0.004%, the formation of borocarbides at the former austenite grain boundaries is promoted, making the steel more brittle. Below 0.0003%, there is not a sufficient concentration of free boron to segregate at the former austenite grain boundaries to increase the steel's toughness. In a preferred embodiment of the invention, the boron content ranges from 0.001% to 0.003%. Optionally, some elements can be added to the composition of the steel according to the invention. Titanium can be added at up to 0.04% to provide precipitation enhancement. Preferably, a minimum of 0.01% titanium is added in addition to boron to protect the boron from BN formation. Niobium may be optionally added in amounts up to 0.05% to refine the austenite grains during hot rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%. Molybdenum can be optionally added at up to 0.3% to decrease phosphorus segregation. Above 0.3%, adding molybdenum is costly and ineffective considering the required properties. Aluminum is a very effective element for deoxidizing steel in the liquid phase during manufacturing. The aluminum content can be added up to a maximum of 0.90% to prevent inclusions and avoid oxidation problems. A maximum of 0.80% chromium is allowed, above a saturation effect, and adding chromium is useless and expensive. The remainder of steel's composition consists of 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. Specifically, phosphorus segregates at the grain boundaries, and for phosphorus content above 0.020%, the steel's toughness is reduced. The microstructure of the hot-rolled and annealed steel sheet according to the invention will now be described. The hot-rolled and annealed steel sheet has a microstructure consisting, in the surface fraction, of 20% or more of recrystallized ferrite, the remainder being non-recrystallized ferrite (including 0%), 15% or more of the recrystallized ferrite having a grain size greater than 5 pm, and a carbide density at the grain boundary of recrystallized ferrite less than or equal to 5 carbides per 10 pm of grain boundary length. Recrystallized ferrite consists of ferrite grains that recrystallized during hot strip annealing. During hot rolling, the austenite grains elongate and take on a pancake shape. Hot rolling generates dislocations, which store energy. During annealing, this stored energy drives the formation of ferrite grains with a very low dislocation density within the grain. As recrystallization progresses, the hardness of the steel decreases. The target properties are not achieved below 20% recrystallized ferrite. In a preferred embodiment of the invention, the recrystallized ferrite content is between 40% and 60%. In another preferred embodiment, the recrystallized ferrite content is between 80% and 100%. 15% or more of recrystallized ferrite has a grain size greater than 5 pm, in order to achieve a low level of hardness. Recrystallized ferrite can be distinguished from non-recrystallized ferrite by its equiaxed morphology. Recrystallized ferrite observed in backscattered electron microscopy (BSE) exhibits homogeneous contrast due to its low dislocation density. The remainder of the microstructure is unrecrystallized ferrite, comprising between 0% (inclusive) and 80%. The portion of bainite and martensite that cannot recrystallize during hot band annealing is the unrecrystallized ferrite. The density of carbides at the grain boundary of recrystallized ferrite is less than or equal to 5 carbides per 10 pm of grain boundary length to improve the toughness of the steel. The hot-rolled and annealed steel sheet according to the invention has an energy IVIA / a / ZUZZ / UU 134» Charpy E impact at 20°C greater than 0.40J / mm2measured according to ISO 148-1:2006 (F) and ISO 1481:2017(F). The hot-rolled and annealed steel sheet according to the invention has a Vickers hardness level of less than 300 HV. 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, which comprises the following steps, is preferred: A semi-finished product suitable for hot rolling is provided with the steel composition described above. The semi-finished product is heated to a temperature between 1,150 and 1,300°C, thus facilitating hot rolling, with the final rolling temperature (FRT) depending on the steel's chemical composition. To obtain specific properties, a skilled technician must select the FRT (Finish Roll Temperature) that promotes matrix recrystallization after hot strip annealing. Beyond a certain FRT value, which depends directly on the steel's chemical composition, the stored energy is no longer sufficient to recrystallize the ferrite after hot strip annealing. Preferably, the FRT is between 750 and 1000°C. More preferably, the FRT is between 800 and 950°C. Hot-rolled steel is cooled and coiled at a coil temperature between 20 and 550°C. Preferably, the coil temperature ranges from (Ms-100°C) to 550°C. After winding, the sheet can be pickled to remove oxidation. The coiled steel sheet is annealed at an annealing temperature Ta below Ac1. The steel sheet is held at Ta for a holding time of between 0.1 and 100 h to reduce hardness while maintaining toughness above 0.4 J / mm² of the hot-rolled steel sheet. To obtain specific properties, the technician must select Ta to favor ferrite recrystallization. Annealing at too low a temperature limits ferrite recrystallization and promotes carbides at grain boundaries, decreasing the toughness of the steel sheet. Preferably, Ta is between 500°C and Ac1. After hot strip annealing, the carbide density at the grain boundaries is less than 5 carbides per 10 pm of grain boundary length, which improves the steel's toughness. The hot-rolled and annealed steel sheet is then cooled to room temperature. Hot-rolled and annealed steel sheet has good toughness and hardness properties, which allows for further processing. For example, hot-rolled and annealed steel sheet can be cold-rolled to obtain cold-rolled steel sheet with a thickness of, for example, between 0.7 and 3 mm, or even better, in the range of 0.8 to 2 mm. The cold-rolling reduction ratio is preferably between 20 and 80%. The invention will now be illustrated by the following examples, which are by no means limiting. Example 1 degrees, whose compositions are compiled in Table 1, were strained into semi-finished products and ML / a / ZUZZ / UU 1043 were processed into steel sheets following the process parameters compiled in Table 2. Table 1 - Compositions The tested compositions are compiled in the following table where the content of the elements is expressed as a percentage by weight. IVIA / a / ¿U¿¿ / UU 134» Steel C Mn Si BSPN Ti Mo Al Ac1 (°C) Ms (°C) A 0.18 3.94 1.29 0.0022 0.002 0.01 0.002 0.023 0.19 0.03 640 328 B 0.18 3.85 1.27 0.0024 0.002 0.01 0.003 0.026 0.21 0.6 655 339 C 0.18 3.96 1.48 0.0022 0.002 0.01 0.002 0.024 0.19 0.03 640 325 AC steels are in accordance with the invention. The temperature of Ac1 has been determined by dilatometry tests and metallographic analysis. Table 2 - Process parameters The steel semi-finished products, as cast, were reheated to 1,200°C for 1,800 seconds, hot-rolled, and then coiled before hot-strip annealing. The following specific conditions were applied: Test Steel FRT (°C) Tcoil (°C) Annealing Ta (°C) ta (h) 1 A 950 450 620 23 2 A 900 450 620 23 3 A 850 450 620 23 4 A 800 450 620 23 5 B 950 450 620 23 6 B 900 450 620 23 7 B 850 450 620 23 8 B 800 450 620 23 9 C 950 450 620 23 10 C 900 450 620 23 11 C 850 450 620 23 12 C 800 450 620 23 Next, hot-rolled and annealed steel sheets were analyzed and the corresponding microstructure elements and mechanical properties were compiled in Tables 3 and 4. Table 3 - Microstructure of hot-rolled and annealed steel sheet The phase percentages of the microstructures of the hot-rolled and annealed steel sheet obtained were determined: MA / a / ZUZZ / UU f 34» Assay Recrystallized Ferrite (%) Non-recrystallized Ferrite (%) Carbide density at grain boundary (number / 10 pm) Recrystallized ferrite with size >5 pm (%) 1 20 80 5 15 2 40 60 4 30 3 95 5 2 65 4 98 2 1.5 75 5 5 95 6 0 6 10 90 5 1 7 98 2 2 80 8 100 0 2 80 9 5 95 6 0 10 5 95 5 1 11 25 75 5 7 12 80 20 4 40 Underlined values: not corresponding to the invention The surface fractions are determined using the following method: a sample is cut from the hot-rolled and annealed steel sheet, polished, and etched with a known reagent to reveal the microstructure. The section is then examined using a scanning electron microscope, for example, a field emission electron microscope (FEG-SEM) at magnifications greater than 5000x, in both secondary electron and backscattered electron modes. Table 4 - Mechanical properties of hot-rolled and annealed steel sheet The mechanical properties of the analyzed samples were determined and compiled in the following table: Charpy Impact Energy Test (J / mm2) Hardness (HV) 1 0.40 278 2 0.49 263 3 0.69 211 4 0.70 204 5 0.34 285 6 0.30 293 7 0.69 210 8 0.66 214 9 0.31 296 10 0.28 290 11 0.30 269 12 0.44 221 Underlined values: do not match target values To obtain specific properties, the technician in the field must select the FRT finishing rolling temperature to favor the recrystallization of the matrix after annealing. To obtain a final hot-rolled and annealed steel sheet with more than 20% recrystallized ferrite, the remainder being non-recrystallized ferrite, FRT tests were carried out at 800, 850, 900 and 950°C, before annealing at a temperature Ta of 620°C for a time ta of 23 h. In tests 1-4, steel A is hot-rolled at FRTs of 950, 900, 850, and 800°C, respectively. These examples demonstrate all the specific properties resulting from their unique composition and microstructure. In tests 5-8, steel B is hot rolled with FRT of 800, 850, 900 and 950°C. The high FRT (Fertile Rendering Temperature) of tests 5 and 6, at 950 and 900°C respectively, leads to a level of ferrite recrystallized after annealing of 5% and 10%, lower than the desired level. In tests 7-8, more than 98% of the ferrite recrystallizes due to the low FRT of 850 and 800°C. In tests 9-12, C steel is hot rolled with FRT of 800, 850, 900 and 950°C. In this case, a FRT above 900°C implies a microstructure outside the scope of the invention. For tests 9-11, the carbide density at the grain boundary is higher than the desired level, leading to low steel toughness. Example 2 grade, whose composition is compiled in table 6, was cast into semi-finished products and processed into steel sheets following the process parameters compiled in table 7. Table 6 - Chemical Composition Steel C Mn Si BSPN Ti Nb Mo Al Ac1 (°C) Ms (°C) D 0.19 3.86 1.27 0.0021 0.001 0.01 0.003 0.029 0.02 0.20 0.39 650 331 Steel D is in accordance with the invention. Table 7 - Process parameters The steel semi-finished products, as cast, were reheated to 1,200°C for 1,800 s, hot-rolled, and then coiled before hot-band annealing. The following specific conditions were applied: Test Steel FRT (°C) TCoil (°C) Annealing Ta (°C) ta (h) 13 D 845 300 594 23 14 D 845 300 605 7 15 D 845 300 619 7 16 D 845 300 633 7 17 D 845 300 648 7 Next, hot-rolled and annealed steel sheets were analyzed, and the corresponding microstructure elements and mechanical properties were compiled respectively in Tables 8 and 9. Table 8 - Microstructure of hot-rolled and annealed steel sheet The phase percentages of the microstructures of the hot-rolled and annealed steel sheet obtained were determined: Tests Recrystallized Ferrite (%) Non-recrystallized Ferrite (%) Density of carbides at the grain boundary (number / 10 pm) Recrystallized Ferrite with size >5 pm (%) 13 5 95 10 0 14 30 70 6 7 15 45 55 3 40 16 55 45 2 48 17 60 40 1.5 50 Underlined values: not related to the invention. The surface fractions are determined by the following method: a sample is cut from the hot-rolled and annealed steel sheet, polished, and etched with a known reagent to reveal the microstructure. The section is then examined using a scanning electron microscope, for example, a field emission electron microscope (FEG-SEM) at a magnification greater than 5000x, in both secondary electron and backscattered electron modes. Table 9 - Mechanical properties of hot-rolled and annealed steel sheet The mechanical properties of the analyzed samples were determined and compiled in the following table: Charpy Impact Energy Test (J / mm2) Hardness (HV) 13 0.20 324 14 0.26 300 15 0.41 271 16 0.53 235 17 0.56 223 Underlined values: not corresponding to the invention Tests 13-17 were performed with an FRT of 845°C and by varying the annealing temperature Ta, in order to obtain a final annealed steel sheet with more than 20% recrystallized ferrite, the remainder being non-recrystallized ferrite, and limiting carbides at the grain boundaries. If the hardness (ta) is too low, as in tests 13 and 14, the ferrite is not sufficiently recrystallized and the steel is too hard. The high amount of carbides formed at the grain boundaries reduces the toughness of the steel.

Claims

1. The hot-rolled and annealed steel sheet, manufactured from a steel having a composition comprising, in weight percent: C: 0.1-0.25% Mn: 3.00-5.00% Si: 0.80-1.60% B: 0.0003-0.004% 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 being iron and unavoidable impurities resulting from smelting, the steel sheet having a microstructure comprising, in the surface fraction, 20% or more of recrystallized ferrite the remainder being non-recrystallized ferrite, 15% or more of the recrystallized ferrite with a grain size greater than 5 pm and a carbide density at the recrystallized ferrite grain boundary less than or equal to 5 carbides per 10 pm of grain boundary length.

2. A hot-rolled and annealed steel sheet according to claim 1, wherein the recrystallized ferrite is between 40 and 60%.

3. A hot-rolled and annealed steel sheet according to claim 1, wherein the recrystallized ferrite is between 80 and 100%.

4. A hot-rolled and annealed steel sheet according to any of claims 1 to 3, wherein the manganese content is between 3.50 and 4.50%.

5. A hot-rolled and annealed steel sheet according to any of claims 1 to 4, wherein the silicon content is between 1.00 and 1.60%.

6. A hot-rolled and annealed steel sheet according to any of claims 1 to 5, wherein the hot-rolled and annealed steel sheet has a Charpy impact energy at 20°C greater than 0.40J / mm2, measured according to ISO 148-1:2006 (F) and ISO 148-1:2017 (F).

7. A hot-rolled and annealed steel sheet according to any of claims 1 to 6 of IVIA / a / ZUZZ / UU 104» wherein the hot-rolled and annealed steel sheet has a hardness level of less than 300HV.

8. A cold-rolled steel sheet obtained from the cold rolling of hot-rolled and annealed steel sheet according to any of claims 1 to 7.