Low carbon low alloy Q&P steel or hot dip galvanized Q&P steel with tensile strength ≥ 1180 MPa and its manufacturing method

Abstract

A low carbon low alloy Q&P steel or hot dip galvanized Q&P steel with a tensile strength of ≥1180MPa and a manufacturing method thereof. Its chemical composition, by mass percentage, is C: 0.16-0.23%, Si: 1.1-2.0%, Mn: 1.6-3.0%, P≦0.015%, S≦0.005%, Al: 0.02-0.05%, and may further contain one or two of Cr, Mo, Ti, Nb, and V, and Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and unavoidable impurities. Its manufacturing method includes smelting, casting, hot rolling, cold rolling, and rapid heat treatment or rapid heat treatment hot dip plating steps. In the present invention, by controlling the rapid heating, short-time holding temperature and rapid cooling processes in the rapid heat treatment process, the recovery, recrystallization and austenite transformation processes of the deformed structure are changed and the recrystallization of ferrite is suppressed, thereby obtaining a microstructure with equiaxed fine crystals and a multi-phase structure. Finally, the metallographic structure of the obtained steel is a multi-phase structure of martensite, retained austenite and ferrite, with a grain size of 1 to 3 μm, optimizing the mechanical properties and expanding the range of the material performance zone.

Description

[Technical Field]

[0001] The present invention belongs to the technical field of rapid thermal treatment of materials, and particularly relates to a low-carbon, low-alloy Q&P steel or a low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength of ≥ 1180 MPa, and a manufacturing method thereof. [Background technology]

[0002] As people's awareness of energy conservation and material service safety gradually increases, the use of high-strength steel, especially advanced high-strength steel, is increasing day by day, and as a result, steel companies and scientific research institutes are increasingly focusing on the development of advanced high-strength steel.In order to further improve the strength and elongation properties of steel products, more and more emphasis is being placed on the development of third-generation advanced high-strength steel, represented by Q&P (Quenching and Partitioning) steel.

[0003] Q&P heat treatment technology is a new continuous heat treatment process technology proposed by Speer et al. in the early 21st century. This process mainly consists of four steps: First, the strip steel is heated to the austenitizing temperature and kept at that temperature; Second, the sample is s ~M f Rapid cooling to a temperature between 0.1 and 1.0 results in a two-phase structure consisting primarily of martensite and retained austenite; Third, the strip steel is s Heating to the following temperature and keeping it at this temperature will cause carbon element to diffuse and distribute from supersaturated martensite into austenite, reducing the carbon content and hardness of martensite and improving its plasticity, while increasing the carbon content of austenite and increasing its stability; Fourth, if the stability of the retained austenite is insufficient during the cooling process to room temperature, some of the austenite will transform into martensite, reducing the amount of retained austenite available at room temperature.

[0004] Q&P steel is essentially a martensitic steel, but unlike conventional tempered martensitic steel, the plasticity of Q&P steel is significantly improved at the same strength as tempered martensitic steel. This is because the structure of Q&P steel contains retained austenite, which transforms into martensite during deformation, resulting in the so-called TRIP effect, which significantly improves the plasticity of the steel.

[0005] Currently, there are two development approaches for the Q&P process. One is to add alloying elements to enhance the ability of the alloying elements in the steel to inhibit carbide precipitation; the other is to optimize the process by finding the optimal temperature and time, and adjusting the temperature and time of the quenching and distribution processes in the Q&P process to change the structural performance of the Q&P steel.

[0006] US patent application US2003 / 027825 provides a rough outline of the production process for Q&P steel, but limits the austenitizing process to high temperatures, which requires the entire material structure to be austenitized. In actual production, this temperature is too high (850-950°C) and takes too long (usually, the austenitizing process for steel plate requires 2-5 minutes of heat retention), which requires high equipment requirements and leads to high production costs.

[0007] Chinese Patent CN1081931138B discloses "980MPa-class cold-rolled high-strength Q&P steel for automobiles and its production method," and the chemical composition of the steel, in mass percentage, is C: 0.18-0.24%, Si: 0.6-1.3%, Mn: 1.6-2.4%, P: 0.02-0.04%, S≦0.005%, Nb: 0.04-0.07%, N≦0.006%, Als: 0.05-1.0%, and the balance is Fe and other unavoidable impurity elements. The hot rolling step has a rolling finish temperature of 870-910°C, a coiling temperature of 660-710°C, a cold rolling reduction of ≥ 45% in the cold rolling step, a soaking segment temperature of 770-840°C, an overaging segment temperature of 300-440°C, a soaking segment time of 60-225 s, an overaging segment time of 300-1225 s, and a flattening elongation of 0.3-0.9% in the flattening step. The resulting steel sheet has a yield strength of greater than 550 MPa, a tensile strength of greater than 980 MPa, and a fracture elongation of greater than 18%.

[0008] The main feature of the steel of this invention is that it achieves a good synergy between strength and plasticity through the conventional Q&P process. However, due to the use of conventional heat treatment methods, its soaking time and distribution time are both long, and at the same time, its alloy content is relatively high, which also increases production costs and reduces production flexibility.

[0009] Chinese patent application CN109136779A discloses a "Method for Producing Martensitic Matrix 1100 MPa Grade Rare Earth Q&P Steel." The chemical composition of this steel, by mass percentage, is: 0.15-0.22% C, 0.6-1.7% Si, 1.1-2.4% Mn, 0.1-0.5% Mo, 0.1-0.5% Al, 0.05-0.11% V, 0.01-0.05% Y, 0.02-0.04% P, ≤0.005% S, 0.04-0.07% Nb, ≤0.006% N, 0.001-0.006% B, with the balance being Fe and other unavoidable impurities. The resulting steel sheet had a tensile strength of approximately 1100 MPa and an elongation at break of approximately 20%.

[0010] The main feature of this steel is that it is made by simultaneously adding rare earth elements such as Y and Mo, V, and Nb to refine the grains and reduce the Mn content, improving weldability. The manufacturing process requires two casting steps: Smelting: After blending the raw materials according to the composition of the invention, they are smelted in a converter, then refining in a vacuum furnace, and finally casting to obtain a billet; Melting of trace elements: Adding trace alloy element powders (Mo, Al, V, Y, Nb, N, B, etc.) to an arc melting furnace to obtain a secondary billet. Hot rolling process: The billet is heated to 1100-1150°C in a heating furnace and kept at this temperature for 1-3 hours. After that, it is hot rolled to a rolling end temperature of 820-880°C and a coiling temperature of 550-650°C. The resulting steel plate has a thickness of 1.5-3.0mm, and then cooled to room temperature by water quenching. Cold rolling process: After pickling, it is cold rolled through multiple passes to obtain a steel plate with a thickness of 1.2-1.5mm. The entire annealing process is divided into three stages: 1st: Manganese distribution process in the two-phase region: The material was heated at 10-30℃ / s, and C3 and A C1 (two-phase region), and then hold for 3-15 minutes, then cool to room temperature by water quenching; 2nd: 1st carbon distribution process: material, M S and M f The material is kept at a temperature T0 between 10 and 300 seconds, and then cooled to room temperature by water quenching; 3rd: Second carbon distribution process: Material is S and M f The material is then cooled to room temperature by water quenching.

[0011] The manufacturing process of this invention is complicated, requires high energy consumption, and involves high alloy content and multiple water quenching processes to remove the oxide layer on the material surface, which brings about many problems in terms of environment and energy consumption, and leads to increased manufacturing costs and reduced manufacturing flexibility.

[0012] Chinese patent application CN10843148A discloses a "method for producing high-strength steel plate with improved ductility and formability, and the steel plate obtained therefrom," and the chemical composition of the steel of this invention is, by mass percentage, 0.15-0.23% C, 2.0-2.8% Mn, 1.0-2.1% Si, 0.02-1.0% Al, 1.0-2.1% Al + Si, 0-0.035% Nb, ≦0.3% Mo, ≦0.04% Cr, with the balance being Fe and other unavoidable impurities. The steel plate is annealed at an annealing temperature TA to obtain a structure containing at least 65% austenite and at most 35% ferrite. The steel sheet was quenched from a temperature of at least 600°C to a quenching temperature QT of Ms-170°C to Ms-80°C at a cooling rate of at least 20°C / s, the steel sheet was heated to a distribution temperature Pt of 350°C to 450°C, the steel sheet was held for a distribution time Pt ​​of 80 to 440 seconds, and the steel sheet was immediately cooled to room temperature. The resulting steel sheet had a tensile strength of greater than 1180 MPa and an elongation at break of greater than 12%.

[0013] The main feature of the steel of this invention is that it uses high Mn, high Si and high Al components, and through the traditional Q&P process, controls the proportion of each phase in the final structure to achieve a good synergy between strength and plasticity. Because it uses traditional heat treatment methods, the soaking time and distribution time are both long, which can increase production costs and reduce production flexibility.

[0014] Chinese patent application CN10912923A discloses a "Heat treatment method for low-carbon microalloyed high-strength and elongation cold-rolled TRIP980 steel," and the chemical composition of the steel of this invention is, in mass percentages, C: 0.18-0.23%, Si: 1.6-1.8%, Mn: 1.5-2.0%, Nb: 0.025-0.045%, Ti: 0.08-0.15%, P≦0.015%, S≦0.005%, with the balance being Fe and other unavoidable impurities. The main manufacturing steps of the steel of this invention are as follows: 1) A billet with a certain chemical composition is forged into a forged billet, which is then reheated and hot-rolled, water-cooled, and coiled to obtain hot-rolled strip steel. 2) The hot-rolled steel strip is pickled and then cold-rolled to obtain a cold-rolled steel strip; 3) After the cold-rolled steel strip is fully austenitized, it is kept at the temperature for a certain period of time and then water-cooled to room temperature to produce a pre-quenched steel strip with a fully martensite structure; 4) The surface of the pre-quenched steel strip is descaled, thereby removing the oxide scale layer and decarbonized layer, and then heated and annealed again, kept at that temperature for a certain period of time, and then cooled to a certain temperature in a salt bath, kept at that temperature for a certain period of time, and then water-cooled to room temperature to produce the final steel strip product.

[0015] The reheating temperature range of the forged billet in step 1) is 1100-1200°C, and the soaking time is 3-5 hours. The starting temperature of hot rolling is 1050-1150°C, and the finishing temperature is 850-900°C. Hot rolling is performed using a four-roll reversing mill, with 7 passes of reciprocating rolling, with the reduction rate of the first 2 passes being 30-50% and the reduction rate of the last 5 passes being 20-30%. After that, it is water-cooled to 650-750°C, and then asbestos is added and soaked for 8-10 hours, thereby simulating the coiling process. The thickness of the hot-rolled strip is 4-5.5mm.

[0016] The cold rolling in step 2) is performed by unidirectional rolling using a four-roll mill, with 10-15 rolling passes, including 3-5 flattening passes, and the thickness of the final cold-rolled strip steel is 1.0-1.5 mm.

[0017] The austenitizing temperature of the cold rolled steel strip described in step 3) is 870 to 920°C, and the austenitizing temperature holding time is 5 to 15 minutes.

[0018] In step 4), the thickness of the oxide scale and decarbonized layer removed is 50-100 μm on each of the top and bottom surfaces, and then the pre-hardened steel strip is reheated to an annealing temperature of 780-830°C with an annealing temperature holding time of 3-8 minutes. This is followed by salt bath cooling at a salt bath cooling rate of 100-200°C / s, a salt bath temperature holding time of 320-400°C, and an annealing time of 5-10 minutes.

[0019] The main feature of this steel is that it uses large amounts of the microalloy elements Nb and Ti to refine the grains, resulting in high elongation (A%≧24%) and high strength (≧980 MPa). Compared to the conventional TRIP steel production process, this invention employs a two-stage heat treatment method for cold-rolled strip steel: after pickling, the cold-rolled strip steel is first annealed once to fully austenitize, then quenched to a fully martensite structure, and then the surface is descaled and decarbonized, and then annealed once again to obtain the final finished strip steel. This method has problems such as the high amount of microalloy elements added, increased manufacturing costs due to the two annealing stages, and increased manufacturing process complexity.

[0020] Chinese Patent CN105543674B discloses a "Manufacturing Method for Cold-Rolled Ultra-High Strength Dual-Phase Steel with High Local Forming Performance." The chemical composition of the high-strength dual-phase steel of this invention is, by weight percentage, C: 0.08-0.12%, Si: 0.1-0.5%, Mn: 1.5-2.5%, Al: 0.015-0.05%, and the remainder is Fe and other unavoidable impurities. The raw materials are selected and blended according to the chemical composition and melted into a billet. The billet is heated at 1150-1250°C for 1.5-2 hours, and then hot-rolled. The starting temperature of the hot-rolling is 1080-1150°C, and the finishing temperature is 880-930°C. After rolling, the steel is cooled to 450-620°C at a cooling rate of 50-200°C / s, and then coiled. The hot-rolled steel has a bainite-based structure. The hot-rolled steel sheets were cold-rolled, heated to 740-820°C at a rate of 50-300°C / s, annealed, and then cooled to 620-680°C at a rate of 2-6°C / s with a holding time of 30 s-3 min. The cooling rate was then 30-100°C / s to 250-350°C, and overaged for 3-5 min to obtain ultra-high-strength dual-phase steel with a ferrite-martensite dual-phase structure. This ultra-high-strength dual-phase steel has a yield strength of 650-680 MPa, a tensile strength of 1023-1100 MPa, and an elongation of 12.3%. It can be bent 180° along the rolling direction without cracking.

[0021] The main feature of this patent is that it combines the control of the cooling conditions after hot rolling with rapid heating in the continuous annealing process, that is, by controlling the cooling process after hot rolling, it eliminates band-like structures and achieves uniform structure; rapid heating is used during the subsequent continuous annealing process to ensure uniform structure and achieve fine structure. The premise of this patent technology is to obtain hot-rolled raw material with bainite as the main structure after hot rolling, and its purpose is mainly to ensure uniform structure and avoid local deformation caused by band-like structures.

[0022] The main drawbacks of the patent are: First, it is necessary to obtain hot-rolled raw materials with bainite structure. These hot-rolled raw materials have high strength and high deformation resistance, which brings great difficulties to the subsequent pickling and cold rolling processes. Second, the understanding of rapid heating is limited to shortening the heating time and refining the crystal grains. The heating rate is not differentiated according to the changes in the material structure at different temperature stages, but is instead all heated at a rate of 50-300℃ / s, which leads to high rapid heating production costs; Third, the soaking time is 30s-3min; increasing the soaking time will inevitably weaken the grain refinement effect caused by rapid heating, which is unfavorable to improving the material strength and toughness; Fourth, the patent requires overaging for 3 to 5 minutes, which is too long for rapid heat treatment of DP steel and is therefore unnecessary. Furthermore, the increase in soaking time and overaging time is not conducive to energy savings, reduction in equipment investment, and reduction in the equipment footprint. Furthermore, it is also not conducive to rapid and stable operation of the steel strip in the furnace. It is clear that this is not a rapid heat treatment process in the strict sense.

[0023] Chinese Patent Application No. 201711385126.5 discloses a "780 MPa-grade low-carbon low-alloy TRIP steel" with a chemical composition, in mass percentages, of 0.16-0.22% C, 1.2-1.6% Si, 1.6-2.2% Mn, with the remainder being Fe and unavoidable impurities, obtained by the following rapid heat treatment process: the steel strip was rapidly heated from room temperature to the austenite-ferrite dual-phase region of 790-830°C at a heating rate of 40-300°C / s; the residence time in the dual-phase region heating target temperature range was 60-100 seconds; the steel strip was rapidly cooled from the dual-phase region temperature to 410-430°C at a cooling rate of 40-100°C / s, with a residence time in this temperature range of 200-300 seconds; and the steel strip was rapidly cooled from 410-430°C to room temperature. Its characteristics are that the metal structure of the TRIP steel is a three-phase structure consisting of bainite, ferrite, and austenite; the average grain size of the TRIP steel is significantly refined; the tensile strength is 950-1050 MPa; the elongation is 21-24%; and the strength-strain product can reach up to 24 GPa%.

[0024] The main drawbacks of the patent are: First, the patent discloses a 780MPa-class low-carbon, low-alloy TRIP steel product and its processing technology. However, the tensile strength of the TRIP steel product is 950-1050MPa, which is too high for a 780MPa-class product and cannot provide a satisfactory user experience. However, it is too low for a 980MPa-class product and cannot fully meet the user's strength requirements. Second, the patent uses a single-stage rapid heating method, and the same rapid heating rate is used throughout the entire heating temperature range. The material structure changes at different temperature stages are not differentiated, but are all rapidly heated at a rate of 40-300°C / s, which inevitably leads to high production costs during the rapid heating process; Third, the soaking time of this patent is set to 60-100 seconds, which is close to the soaking time of conventional continuous annealing. Increasing the soaking time will inevitably weaken the grain refinement effect caused by rapid heating, which is very unfavorable to improving the material strength and toughness. Fourth, the patent requires a bainite isothermal treatment time of 200-300 seconds, which is too long for the actual rapid heat treatment products, and is therefore unnecessary and unable to fulfill its intended purpose. Furthermore, the increase in the soaking time and isothermal treatment time is not conducive to energy savings, reduction in equipment investment and equipment footprint, and is also detrimental to the rapid and stable operation of the steel strip in the furnace. It is clear that this is not a rapid heat treatment process in the strict sense.

[0025] Chinese patent application CN107794357B and US patent application US2019 / 0153558A1 are entitled "Method for producing ultra-high strength martensitic cold rolled steel sheet by ultra-rapid heating process." The chemical composition of the high strength dual phase steel is, in weight percentage, C: 0.10-0.30%, Mn: 0.5-2.5%, Si: 0.05-0.3%, Mo: 0.05-0.3%, Ti: 0.01-0.04%, Cr: 0.10-0.3%, B: 0.001-0.004%, P≦0.02%, S≦0.02%, and the balance is Fe and other unavoidable impurities. Mechanical properties of the dual phase steel: Yield strength R p0.2 is over 1100MPa, tensile strength R m =1800-2300 MPa, elongation up to 12.3%, uniform elongation 5.5-6%. This invention provides an ultra-rapid heating production process for ultra-high strength martensitic cold-rolled steel sheet, characterized in that the cold-rolled steel sheet is first heated to 300-500°C at 1-10°C / s, and then reheated to the single-phase austenite region of 850-950°C at a heating rate of 100-500°C / s; the steel sheet is then water-cooled to room temperature after a holding time of no more than 5 seconds, yielding an ultra-high strength cold-rolled steel sheet.

[0026] Drawbacks of the process described in the patent include: First, the annealing temperature of the steel in this invention is in the ultra-high temperature range of the austenite single phase region, and it contains a large amount of alloying elements, so that the yield strength and tensile strength are both above 1000 MPa, which brings great difficulties to the heat treatment process itself, the pre-heat treatment process, manufacturing, and subsequent user use; Second, the ultra-rapid heating annealing method of this invention adopts a heating time of no more than 5 seconds, which not only makes it difficult to control the heating temperature, but also leads to uneven distribution of alloying elements in the final product, and uneven and unstable product structure performance; Thirdly, the final rapid cooling was performed by water quenching to room temperature, without the necessary tempering treatment, so the resulting final product microstructure and alloying element distribution profile in the final microstructure did not provide the optimal toughness for the product, resulting in excessive strength in the final product and insufficient plasticity and toughness; Fourth, the cooling rate of the water quenching method of the invention is too high, which causes problems such as poor steel plate shape and surface oxidation, so the patented technology has little or no practical value.

[0027] Chinese Patent CN1081931138B discloses "980MPa-class cold-rolled high-strength Q&P steel for automobiles and its production method," and the chemical composition of the steel, in mass percentage, is C: 0.18-0.24%, Si: 0.6-1.3%, Mn: 1.6-2.4%, P: 0.02-0.04%, S≦0.005%, Nb: 0.04-0.07%, N≦0.006%, Als: 0.05-1.0%, and the balance is Fe and other unavoidable impurity elements. The hot rolling step has a rolling finish temperature of 870-910°C, a coiling temperature of 660-710°C, a cold rolling reduction of ≥ 45% in the cold rolling step, a soaking segment temperature of 770-840°C, an overaging segment temperature of 300-440°C, a soaking segment time of 60-225 s, an overaging segment time of 300-1225 s, and a flattening elongation of 0.3-0.9% in the flattening step. The resulting steel sheet has a yield strength of greater than 550 MPa, a tensile strength of greater than 980 MPa, and a fracture elongation of greater than 18%.

[0028] The main feature of the steel of this invention is that it achieves a good synergy between strength and plasticity through the conventional Q&P process. However, due to the use of conventional heat treatment methods, its soaking time and distribution time are both long, and at the same time, its alloy content is relatively high, which also increases production costs and reduces production flexibility.

[0029] Chinese patent application CN109136779A discloses a "Method for Producing Martensitic Matrix 1100 MPa Grade Rare Earth Q&P Steel." The chemical composition of this steel, by mass percentage, is: 0.15-0.22% C, 0.6-1.7% Si, 1.1-2.4% Mn, 0.1-0.5% Mo, 0.1-0.5% Al, 0.05-0.11% V, 0.01-0.05% Y, 0.02-0.04% P, ≤0.005% S, 0.04-0.07% Nb, ≤0.006% N, 0.001-0.006% B, with the balance being Fe and other unavoidable impurities. The resulting steel sheet had a tensile strength of approximately 1100 MPa and an elongation at break of approximately 20%.

[0030] The main feature of this steel is that it is made by simultaneously adding rare earth elements such as Y and Mo, V, and Nb to refine the grains and reduce the Mn content, improving weldability. The manufacturing process requires two casting steps: Smelting: After blending the raw materials according to the composition of the invention, they are smelted in a converter, then refining in a vacuum furnace, and finally casting to obtain a billet; Melting of trace elements: Adding trace alloy element powders (Mo, Al, V, Y, Nb, N, B, etc.) to an arc melting furnace to obtain a secondary billet. Hot rolling process: The billet is heated to 1100-1150°C in a heating furnace and kept at this temperature for 1-3 hours. After that, it is hot rolled to a rolling end temperature of 820-880°C and a coiling temperature of 550-650°C. The resulting steel plate has a thickness of 1.5-3.0mm, and then cooled to room temperature by water quenching. Cold rolling process: After pickling, it is cold rolled through multiple passes to obtain a steel plate with a thickness of 1.2-1.5mm. The entire annealing process is divided into three stages: 1st: Manganese distribution process in the two-phase region: The material was heated at 10-30℃ / s, and C3 and A C1 (two-phase region), and then hold for 3-15 minutes, then cool to room temperature by water quenching; 2nd: 1st carbon distribution process: material, M S and M f The material is kept at a temperature T0 between 10 and 300 seconds, and then cooled to room temperature by water quenching; 3rd: Second carbon distribution process: Material is S and M f The material is then cooled to room temperature by water quenching.

[0031] The manufacturing process of this invention is complicated, requires high energy consumption, and involves high alloy content and multiple water quenching processes to remove the oxide layer on the material surface, which brings about many problems in terms of environment and energy consumption, and leads to increased manufacturing costs and reduced manufacturing flexibility.

[0032] Chinese patent application CN10843148A discloses a "method for producing high-strength steel plate with improved ductility and formability, and the steel plate obtained therefrom," and the chemical composition of the steel of this invention is, by mass percentage, 0.15-0.23% C, 2.0-2.8% Mn, 1.0-2.1% Si, 0.02-1.0% Al, 1.0-2.1% Al + Si, 0-0.035% Nb, ≦0.3% Mo, ≦0.04% Cr, with the balance being Fe and other unavoidable impurities. The steel plate is annealed at an annealing temperature TA to obtain a structure containing at least 65% austenite and at most 35% ferrite. The steel sheet was quenched from a temperature of at least 600°C to a quenching temperature QT of Ms-170°C to Ms-80°C at a cooling rate of at least 20°C / s, the steel sheet was heated to a distribution temperature Pt of 350°C to 450°C, the steel sheet was held for a distribution time Pt ​​of 80 to 440 seconds, and the steel sheet was immediately cooled to room temperature. The resulting steel sheet had a tensile strength of greater than 1180 MPa and an elongation at break of greater than 12%.

[0033] The main feature of the steel of this invention is that it uses high Mn, high Si and high Al components, and through the traditional Q&P process, controls the proportion of each phase in the final structure to achieve a good synergy between strength and plasticity. Because it uses traditional heat treatment methods, the soaking time and distribution time are both long, which can increase production costs and reduce production flexibility.

[0034] Chinese patent application CN10912923A discloses a "Heat treatment method for low-carbon microalloyed high-strength and elongation cold-rolled TRIP980 steel," and the chemical composition of the steel of this invention is, in mass percentages, C: 0.18-0.23%, Si: 1.6-1.8%, Mn: 1.5-2.0%, Nb: 0.025-0.045%, Ti: 0.08-0.15%, P≦0.015%, S≦0.005%, with the balance being Fe and other unavoidable impurities. The main manufacturing steps of the steel of this invention are as follows: 1) A billet with a certain chemical composition is forged into a forged billet, which is then reheated and hot-rolled, water-cooled, and coiled to obtain hot-rolled strip steel. 2) The hot-rolled steel strip is pickled and then cold-rolled to obtain a cold-rolled steel strip; 3) After the cold-rolled steel strip is fully austenitized, it is kept at a certain temperature for a while, and then water-cooled to room temperature to produce a pre-quenched steel strip with a fully martensite structure; 4) The surface of the pre-quenched steel strip is descaled, thereby removing the oxide scale layer and decarbonized layer, and then heated and annealed again, kept at that temperature for a certain period of time, and then cooled to a certain temperature in a salt bath, kept at that temperature for a certain period of time, and then water-cooled to room temperature to produce the final steel strip product.

[0035] The reheating temperature range of the forged billet in step 1) is 1100-1200°C, and the soaking time is 3-5 hours. The starting temperature of hot rolling is 1050-1150°C, and the finishing temperature is 850-900°C. Hot rolling is performed using a four-roll reversing mill, with 7 passes of reciprocating rolling, with the reduction rate of the first 2 passes being 30-50% and the reduction rate of the last 5 passes being 20-30%. After that, it is water-cooled to 650-750°C, and then asbestos is added and soaked for 8-10 hours, thereby simulating the coiling process. The thickness of the hot-rolled strip is 4-5.5mm.

[0036] The cold rolling in step 2) is performed by unidirectional rolling using a four-roller mill, with 10-15 rolling passes, including 3-5 flattening passes, and the thickness of the final cold-rolled strip steel is 1.0-1.5 mm.

[0037] The austenitizing temperature of the cold rolled steel strip described in step 3) is 870 to 920°C, and the austenitizing temperature holding time is 5 to 15 minutes.

[0038] In step 4), the thickness of the oxide scale and decarbonized layer removed is 50-100 μm on each of the top and bottom surfaces, and then the pre-hardened steel strip is reheated to an annealing temperature of 780-830°C with an annealing temperature holding time of 3-8 minutes. This is followed by salt bath cooling at a salt bath cooling rate of 100-200°C / s, a salt bath temperature holding time of 320-400°C, and an annealing time of 5-10 minutes.

[0039] The main feature of this steel is that it uses large amounts of the microalloy elements Nb and Ti to refine the grains, resulting in high elongation (A%≧24%) and high strength (≧980 MPa). Compared to the conventional TRIP steel production process, this invention employs a two-stage heat treatment method for cold-rolled strip steel: after pickling, the cold-rolled strip steel is first annealed once to fully austenitize, then quenched to a fully martensite structure, and then the surface is descaled and decarbonized, and then annealed once again to obtain the final finished strip steel. This method has problems such as the high amount of microalloy elements added, increased manufacturing costs due to the two annealing stages, and increased manufacturing process complexity.

[0040] Currently, limited by the equipment capacity of conventional continuous annealing furnace production lines, research on cold-rolled Q&P steel products and annealing processes is based on the heating rate (5-20°C / s) of existing industrial equipment, which heats the strip slowly, sequentially completing recrystallization and austenitization transformation. This results in relatively long heating and soaking times and high energy consumption. At the same time, conventional continuous annealing production lines have a large number of strip rollers in the high-temperature furnace segment. Based on the product outline and production capacity requirements, conventional continuous annealing units generally require a soaking time of 1-3 minutes. For conventional production lines with a unit speed of around 180 meters / minute, the number of rollers in the high-temperature furnace segment is generally 20-40, which increases the difficulty of controlling the quality of the strip surface. Summary of the Invention [Problem to be solved by the invention]

[0041] The present invention provides a low-carbon, low-alloy Q&P steel with a tensile strength of ≥ 1180 MPa, a low-carbon, low-alloy hot-dip galvanized Q&P steel with a tensile strength of ≥ 1180 MPa, and a method for manufacturing the same through rapid heat treatment. The present invention uses rapid heat treatment to change the recovery, recrystallization, and austenite transformation processes of the deformation structure, increasing the nucleation rate (including the recrystallization nucleation rate and the austenite transformation nucleation rate), shortening the grain growth time, refining the grains, and increasing the retained austenite content, thereby further improving the strength and plasticity of the material. The matrix structure of the low-carbon, low-alloy Q&P steel of the present invention is uniform, with obvious sheet-like tempered martensite appearing, with a grain size of 1-3 μm. The retained austenite and ferrite phases are uniformly distributed around the martensite-strengthening phase grains, with the sheet-like volume fractions being 75-90% martensite, 10-25% retained austenite, and 3-10% ferrite. The low-carbon, low-alloy Q&P steel of the present invention has a yield strength of ≥ 660 MPa, a tensile strength of ≥ 1180 MPa, an elongation of ≥ 18%, and a strength-strain product of ≥ 24 GPa, with good synergy between strength and toughness, and user-friendly performance in forming, welding, etc. The present invention uses a rapid heat treatment process to improve production efficiency and reduce the alloy content in the same level of steel, thereby reducing production costs and manufacturing difficulties in the pre-heat treatment process, significantly reducing the number of furnace rollers, and improving material surface quality. [Means for solving the problem]

[0042] To achieve the above objectives, the technical solution of the present invention is: A low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or more, the chemical composition of which is, by mass percentage, C: 0.16 to 0.23%, Si: 1.1 to 2.0%, Mn: 1.6 to 3.0%, P≦0.015%, S≦0.005%, Al: 0.02 to 0.05%, and may further contain one or two of Cr, Mo, Ti, Nb, and V, where Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and unavoidable impurities. Preferably, the low-carbon, low-alloy Q&P steel with a tensile strength of 1180 MPa or greater has a multiphase structure of 75-90% martensite, 10-25% retained austenite, and 3-10% ferrite, with a uniform matrix distribution, clear sheet-like tempered martensite, grain sizes of 1-3 μm, and ferrite uniformly distributed around the martensite-strengthened phase grains, with the martensite-strengthened phase grains predominantly having a sheet-like structure. Preferably, the austenite in the Q&P steel's metal structure has good thermal stability, with a -50°C austenite transformation rate of less than 8% and a -190°C austenite transformation rate of less than 30%. Preferably, the Q&P steel has a yield strength of 668-1112 MPa, a tensile strength of 1181-1350 MPa, an elongation of 18.9-24.2%, and a strength-strain product of 24.1-28.97 GPa%.

[0043] Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the C content range can be selected from 0.17-0.23%, 0.19-0.21%, and 0.18-0.21%. Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the Si content range can be selected from 1.1-1.7%, 1.3-1.5%, 1.4-2.0%, and 1.6-1.8%. Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the Mn content range can be selected from 1.6-2.2%, 1.8-2.0%, 2.4-3.0%, and 2.6-2.8%.

[0044] Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or more, the Cr content is ≦0.35%, for example ≦0.25%; the Mo content is ≦0.25%; the Nb content is ≦0.06%, for example ≦0.04%; the Ti content is ≦0.065%, for example ≦0.04%, for example 0.006 to 0.016%; and the V content is ≦0.055%, for example ≦0.035%.

[0045] Preferably, the low carbon low alloy Q&P steel with a tensile strength of ≥ 1180 MPa according to the present invention is obtained by the following process: 1) Smelting and Casting Smelting and casting slabs according to the above chemical composition; 2) Hot rolling and coiling Coiling temperature 550-680℃; 3) Cold rolling The reduction ratio of cold rolling is 40-85%; 4) Rapid heat treatment The cold-rolled steel sheet is rapidly heated to 770-845°C, and the rapid heating can be performed in one or two stages; when one-stage rapid heating is performed, the heating rate is 50-500°C / s; when two-stage rapid heating is performed, the first stage is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and the second stage is heated from 550-625°C to 770-845°C at a heating rate of 50-500°C / s; then, soaking is performed, with the soaking temperature being 770-845°C and the soaking time being 10-60 seconds; After the soaking is complete, the material is slowly cooled to 700-770°C at a cooling rate of 5-15°C / s, then rapidly cooled to 230-280°C at a cooling rate of 50-200°C / s, and kept at this temperature for 2-10 seconds. After that, the material is heated to 300-470°C at a heating rate of 10-30°C / s and tempered for 10-60 seconds. After tempering, the material is cooled to room temperature at a cooling rate of 30-100°C / s.

[0046] Preferably, in step 2), the rolling finish temperature of the hot rolling is ≧A r3 . Preferably, in step 2), the coiling temperature is 580 to 650°C.

[0047] Preferably, in step 3), the cold rolling reduction is 60 to 80%. Preferably, the total process time of the rapid thermal processing described in step 4) is 71 to 186 seconds.

[0048] Preferably, in step 4), when single-stage heating is employed for the rapid heating, the heating rate is 50 to 300° C. / s.

[0049] Preferably, in step 4), the rapid heating is performed in two stages: in the first stage, the heating is performed from room temperature to 550-625°C at a heating rate of 15-500°C / s; in the second stage, the heating is performed from 550-625°C to 770-845°C at a heating rate of 50-300°C / s.

[0050] Preferably, in step 4), the rapid heating is performed in two stages: in the first stage, the heating is performed from room temperature to 550-625°C at a heating rate of 30-500°C / s; in the second stage, the heating is performed from 550-625°C to 770-845°C at a heating rate of 80-300°C / s.

[0051] Preferably, in step 4), the quenching rate of the steel sheet is 50 to 150°C / s.

[0052] In some embodiments, the chemical composition of the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or greater is, by mass, 0.17-0.23% C, 1.1-1.7% Si, 1.6-2.2% Mn, 0.015% P, 0.005% S, and 0.02-0.05% Al. The steel may further contain one or two of Cr, Mo, Ti, Nb, and V, where Cr + Mo + Ti + Nb + V ≤ 0.5%, with the balance being Fe and unavoidable impurities. Preferably, the low-carbon, low-alloy Q&P steel has a C content of 0.19-0.21%. Preferably, the low-carbon, low-alloy Q&P steel has a Si content of 1.3-1.5%. Preferably, the low-carbon, low-alloy Q&P steel has a Mn content of 1.8-2.0%. Preferably, the metallographic structure of this Q&P steel is a multiphase structure consisting of 75-85% martensite, 10-25% retained austenite, and 3-10% ferrite, with a uniform matrix distribution, clear sheet-like tempered martensite, grain size of 1-3 μm, uniformly distributed ferrite phase around the martensite-strengthened phase grains, and the martensite-strengthened phase grains mainly have a sheet-like structure. Preferably, the austenite in the metallographic structure of this Q&P steel has good thermal stability, with a -50°C austenite transformation rate of less than 8% and a -190°C austenite transformation rate of less than 30%. Preferably, this Q&P steel has a yield strength of 668-1002 MPa, a tensile strength of 1181-1296 MPa, an elongation of 18.9-24.2%, and a strength-strain product of 24.1-28.6 GPa%.

[0053] In some embodiments, the chemical composition of the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or greater is, by mass, 0.16-0.23% C, 1.4-2.0% Si, 2.4-3.0% Mn, 0.006-0.016% Ti, P≦0.015%, S≦0.002%, and 0.02-0.05% Al. The steel may further contain one or two of Cr, Mo, Nb, and V, where Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and unavoidable impurities. Preferably, the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or greater is a low-carbon, low-alloy Q&P steel having a tensile strength of 1280 MPa or greater. Preferably, the low-carbon, low-alloy Q&P steel has a C content of 0.18-0.21%. Preferably, in this low-carbon, low-alloy Q&P steel, the Si content is 1.6 to 1.8%. Preferably, in this low-carbon, low-alloy Q&P steel, the Mn content is 2.6 to 2.8%. The metallographic structure of this Q&P steel is a multiphase structure of 80 to 90% martensite, 10 to 20% retained austenite, and 3 to 5% ferrite, with a uniform matrix structure distribution, clear sheet-like tempered martensite, grain size 1 to 3 μm, and ferrite phase uniformly distributed around the martensite strengthening phase grains, with the martensite strengthening phase grains mainly having a sheet-like structure. Preferably, the austenite in the metallographic structure of this Q&P steel has good thermal stability, with an austenite transformation rate at −50°C of less than 8% and a −190°C austenite transformation rate of less than 30%. Preferably, the Q&P steel has a yield strength of 754 to 1112 MPa, a tensile strength of 1281 to 1350 MPa, an elongation percentage of 19 to 22.2%, and a strength-strain product of 24.1 to 28.97 GPa%.

[0054] Another aspect of the present invention provides a low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength of ≥ 1180 MPa, whose chemical composition, in mass percentages, is C: 0.16-0.23%, Si: 1.1-2.0%, Mn: 1.6-3.0%, P≦0.015%, S≦0.005%, preferably ≦0.002%, and Al: 0.02-0.05%, and may further contain one or two of Cr, Mo, Ti, Nb, and V, where Cr + Mo + Ti + Nb + V ≦ 0.5%, with the balance being Fe and unavoidable impurities. Preferably, the hot-dip galvanized Q&P steel has a metallographic structure of a three-phase structure of martensite, ferrite, and austenite, with a uniform matrix distribution, sheet-like tempered martensite, a grain size of 1 to 3 μm, and ferrite phase uniformly distributed around the martensite-reinforced phase grains, with the martensite-reinforced phase grains mainly having a sheet-like structure. Preferably, the hot-dip galvanized Q&P steel has a metallographic structure of a three-phase structure with volume fractions of 45 to 75% martensite, 15 to 30% ferrite, and 10 to 25% austenite. Preferably, the hot-dip galvanized Q&P steel has a yield strength of ≥ 720 MPa, a tensile strength of ≥ 1180 MPa, an elongation of ≥ 19%, and a strength-strain product of ≥ 23.0 GPa%. Preferably, the hot-dip galvanized Q&P steel has a yield strength of 721 to 956 MPa, a tensile strength of 1184 to 1352 MPa, an elongation of 19 to 22.5%, and a strength-strain product of 23.6 to 28.9 GPa%. Preferably, the austenite in the metal structure of the hot-dip galvanized Q&P steel has good thermal stability, with a -50°C austenite transformation rate of less than 8% and a -190°C austenite transformation rate of less than 30%.

[0055] Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the C content range can be selected from 0.17-0.23%, 0.19-0.21%, and 0.18-0.21%. Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the Si content range can be selected from 1.1-1.7%, 1.3-1.5%, 1.4-2.0%, and 1.6-1.8%. Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of 1180 MPa or higher, the Mn content range can be selected from 1.6-2.2%, 1.8-2.0%, 2.4-3.0%, and 2.6-2.8%.

[0056] Preferably, in the low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa, the Cr content is ≤ 0.35%, for example ≤ 0.25%; the Mo content is ≤ 0.25%; the Nb content is ≤ 0.06%, for example ≤ 0.04%; the Ti content is ≤ 0.065%, for example ≤ 0.04%, for example 0.006-0.016%; and the V content is ≤ 0.055%, for example ≤ 0.035%.

[0057] In some embodiments, the low carbon, low alloy, hot dip galvanized Q&P steel having a tensile strength of ≥ 1180 MPa is obtained by the following method: 1) Smelting and Casting Smelting and casting slabs according to the above chemical composition; 2) Hot rolling and coiling Hot rolling finish temperature ≧ A r3 , then cooled to 550-680℃ and coiled; 3) Cold rolling The reduction ratio of cold rolling is 40-80%; 4) Rapid heat treatment, hot dip galvanizing The cold-rolled steel sheet is rapidly heated to 770-845°C, and the rapid heating can be performed in one or two stages; when one-stage rapid heating is performed, the heating rate is 50-500°C / s; when two-stage rapid heating is performed, the first stage is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and the second stage is heated from 550-625°C to 770-845°C at a heating rate of 30-500°C / s (e.g., 50-500°C / s); then, soaking is performed, with the soaking temperature being 770-845°C and the soaking time being 10-60 seconds; After the soaking is completed, the material is cooled slowly to 700-770°C at a cooling rate of 5-15°C / s, then rapidly cooled to 230-280°C at a cooling rate of 50-200°C / s, and kept at this temperature for 2-10 seconds. Then, the material is heated to 460-470°C at a heating rate of 10-30°C / s, and then subjected to a distribution treatment for 10-60 seconds; and then immersed in a zinc pot for hot-dip galvanizing. After hot dip galvanizing, the product is rapidly cooled to room temperature at a cooling rate of 30-150°C / s to obtain a hot dip pure galvanized GI product; alternatively, after hot dip galvanizing, the product is heated to 480-550°C at a heating rate of 10-300°C / s to perform alloying treatment, with the alloying treatment time being 5-20 seconds; after alloying treatment, the product is rapidly cooled to room temperature at a cooling rate of 30-250°C / s to obtain an alloyed hot dip galvanized GA product.

[0058] Preferably, in step 2), the coiling temperature is 580 to 650°C. Preferably, in step 3), the cold rolling reduction is 60 to 80%.

[0059] Preferably, the total process time for the rapid heat treatment and hot dip galvanizing in step 4) is 43 to 186 seconds.

[0060] Preferably, in step 4), when single-stage heating is employed for the rapid heating, the heating rate is 50 to 300° C. / s.

[0061] Preferably, in step 4), the rapid heating is performed in two stages: in the first stage, the material is heated from room temperature to 550-625°C at a heating rate of 15-300°C / s, and in the second stage, the material is heated from 550-625°C to 770-845°C at a heating rate of 50-300°C / s.

[0062] Preferably, in step 4), the rapid heating is performed in two stages: in the first stage, the material is heated from room temperature to 550-625°C at a heating rate of 30-300°C / s, and in the second stage, the material is heated from 550-625°C to 770-845°C at a heating rate of 80-300°C / s.

[0063] Preferably, in step 4), the cooling rate in the rapid cooling step of the steel strip or steel plate is 50 to 150° C. / s.

[0064] In some embodiments, the chemical composition of the low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength of 1180 MPa or greater is, by mass, 0.17-0.23% C, 1.1-1.7% Si, 1.6-2.2% Mn, 0.015% P, 0.005% S, and 0.02-0.05% Al. The steel may further contain one or two of Cr, Mo, Ti, Nb, and V, where Cr + Mo + Ti + Nb + V ≤ 0.5%, with the balance being Fe and unavoidable impurities. Preferably, the hot-dip galvanized Q&P steel has a C content of 0.19-0.21%. Preferably, the hot-dip galvanized Q&P steel has a Si content of 1.3-1.5%. Preferably, the hot-dip galvanized Q&P steel has a Mn content of 1.8-2.0%. Preferably, the hot-dip galvanized Q&P steel has a metal structure consisting of a three-phase structure with volume fractions of 45-75% martensite, 15-30% ferrite, and 10-25% austenite, a uniform matrix structure distribution, sheet-like tempered martensite, grain size of 1-3 μm, ferrite phase uniformly distributed around the martensite strengthening phase grains, and the martensite strengthening phase grains mainly have a sheet-like structure. Preferably, the hot-dip galvanized Q&P steel has a yield strength of 721-805 MPa, a tensile strength of 1184-1297 MPa, an elongation of 19.1-22.4%, and a strength-strain product of 23.6-28 GPa%. Preferably, the austenite in the hot dip galvanized Q&P steel metallography has good thermal stability, with a -50°C austenite transformation rate being less than 8% and a -190°C austenite transformation rate being less than 30%.

[0065] In some embodiments, the chemical composition of the low-carbon, low-alloy galvanized Q&P steel having a tensile strength of 1180 MPa or greater is, by mass percentage, 0.16-0.23% C, 1.4-2.0% Si, 2.4-3.0% Mn, 0.006-0.016% Ti, P≦0.015%, S≦0.002%, and Al: 0.02-0.05%. It may further contain one or two of Cr, Mo, Nb, and V, where Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and unavoidable impurities. Preferably, the low-carbon, low-alloy galvanized Q&P steel having a tensile strength of 1180 MPa or greater is a low-carbon, low-alloy galvanized Q&P steel having a tensile strength of 1280 MPa or greater. Preferably, the C content of the hot-dip galvanized Q&P steel is 0.18-0.21%. Preferably, the hot-dip galvanized Q&P steel has a Si content of 1.6 to 1.8%. Preferably, the hot-dip galvanized Q&P steel has a Mn content of 2.6 to 2.8%. Preferably, the metal structure of the hot-dip galvanized Q&P steel is a three-phase structure of martensite, ferrite, and austenite (the martensite structure accounts for 75 to 90%, the retained austenite structure accounts for 10 to 25%, and the ferrite structure accounts for 3 to 10%), the matrix structure distribution is uniform, clear sheet-like tempered martensite appears, the grain size is 1 to 3 μm, the ferrite phase is present uniformly distributed around the martensite strengthening phase grains, and the martensite strengthening phase grains mainly have a sheet-like structure. Preferably, the hot-dip galvanized Q&P steel has a yield strength of 802 to 956 MPa, a tensile strength of 1280 to 1352 MPa, an elongation of 19 to 22.5%, and a strength-strain product of 25.2 to 28.9 GPa%. Preferably, the austenite in the metal structure of the hot-dip galvanized Q&P steel has good thermal stability, with a -50°C austenite transformation rate of less than 8% and a -190°C austenite transformation rate of less than 30%.

[0066] In the design of the steel composition and process of the present invention: Carbon: Carbon is the most common strengthening element in steel. It increases the strength of steel and reduces its plasticity. However, because forming steel requires low yield strength, high uniform elongation, and high total elongation, the carbon content should not be too high. Steel contains two carbon phases: ferrite and cementite. The carbon content has a significant effect on the mechanical properties of steel. As the carbon content increases, the number of strengthening phases, such as martensite and pearlite, increases, significantly improving the steel's strength and hardness, but significantly reducing its plasticity and toughness. If the carbon content is too high, a clear network of carbides will appear in the steel. The presence of network carbides significantly reduces the strength, plasticity, and toughness, significantly weakening the strengthening effect of increased carbon content. Furthermore, because it reduces the steel's processing capabilities, the carbon content should be kept as low as possible to ensure strength.

[0067] For Q&P steels, carbon is one of the most effective strengthening elements in the martensite matrix. It dissolves in austenite, expands the austenite phase region, significantly increases austenite stability, shifts the pearlite-bainite transformation C curve to the right, delays the pearlite-bainite transformation, and lowers the Ms temperature. A too low carbon content reduces the stability of retained austenite, while a too high carbon content leads to twinning in martensite, reducing the steel's plasticity, toughness, and weldability. Considering these factors, the carbon content is limited to a range of 0.16 to 0.23%. In some embodiments, the carbon content is 0.18 to 0.21%. In other embodiments, the carbon content is 0.19 to 0.21%.

[0068] Mn: Manganese can form a solid solution with iron and further increase the strength and hardness of ferrite and austenite in carbon steel. It can also produce relatively fine, high-strength pearlite during the cooling process after hot rolling. The pearlite content also increases with increasing Mn content. Manganese is also a carbide-forming element, and manganese carbide can dissolve in cementite, thereby indirectly increasing the strength of strengthening phases such as martensite and pearlite. Manganese also significantly enhances the hardenability of steel, further increasing its strength. In some embodiments, the Mn content is 1.8-2.0%. In other embodiments, the C content is 2.6-2.8%.

[0069] In Q&P steel, manganese lowers the martensitic transformation temperature Ms, increases the amount of retained austenite, and improves the stability of the retained austenite. Manganese does not significantly affect the toughness of the steel. However, a high manganese content tends to coarsen the grains in the steel, increasing the steel's susceptibility to overheating. Insufficient cooling after melt casting and hot rolling can easily cause white spots in carbon steel. The present invention limits the manganese content to a range of 1.6 to 3.0%.

[0070] Si: Silicon forms a solid solution in ferrite or austenite, thereby strengthening the yield strength and tensile strength of steel. Furthermore, silicon can increase the cold-work deformation hardening rate of steel, making it a beneficial element in alloy steels. Silicon also exhibits significant uneven distribution along the crystal cross-section of silicon-manganese steel. The uneven distribution of silicon at grain boundaries alleviates the distribution of carbon and phosphorus along the grain boundaries, further improving the embrittlement state of the grain boundaries. Silicon can increase the strength, hardness, and wear resistance of steel, without significantly reducing its plasticity within a certain range. Silicon has strong deoxidizing properties and is commonly used as a deoxidizer in steelmaking. Silicon also increases the fluidity of molten steel, so silicon is commonly included in steels. However, if the silicon content in steel is too high, its plasticity and toughness will be significantly reduced. Regarding Q&P steel: Firstly, silicon element is a non-carbide forming element, and its solubility in carbides is extremely low, which can inhibit the formation of Fe3C in the isothermal process of QP steel, making the untransformed austenite rich in carbon, thereby greatly enhancing the stability of austenite and allowing it to be maintained at room temperature; Secondly, silicon element is a ferrite-forming element, which can enhance the stability of retained austenite and play the role of solid solution strengthening, thereby increasing the strength of steel; Thirdly, silicon has the effect of reducing the austenite phase region and increasing the activity of carbon in ferrite.

[0071] A high silicon content is advantageous for obtaining a large amount of retained austenite, but too high a silicon content can cause a hard oxide layer and poor surface performance on the steel, reducing the wettability and surface quality of the hot-rolled steel sheet. Silicon does not significantly affect the growth rate of austenite, but it does have a significant effect on the morphology and distribution of austenite. An increase in silicon content increases the manufacturing difficulty of the pre-heat treatment process. The present invention limits the silicon content to a range of 1.1 to 2.0%. In some embodiments, the Si content is 1.3 to 1.5%. In other embodiments, the Si content is 1.6 to 1.8%.

[0072] Cr: The main function of chromium in steel is to improve its hardenability, resulting in relatively good overall mechanical properties after quenching and tempering. Chromium and iron form a continuous solid solution, reducing the austenite phase region. Chromium and carbon form various carbides, with a stronger affinity for carbon than iron and manganese. Chromium and iron can form the intermetallic compound σ phase (FeCr). Chromium reduces the carbon concentration in pearlite and its limiting solubility in austenite. Chromium slows the decomposition rate of austenite, significantly improving the hardenability of steel. However, it also increases the steel's tendency toward temper embrittlement. When added with other alloying elements, chromium significantly increases the strength and hardness of steel. Cr increases the hardenability of steel during air cooling, adversely affecting the weldability of steel. However, if the chromium content is less than 0.3%, the adverse effect on weldability is negligible; if the content exceeds this limit, defects such as cracks and slag entrapment are more likely to occur during welding. When Cr is present simultaneously with other alloying elements (e.g., coexisting with V), the adverse effect of Cr on weldability is significantly reduced. For example, when elements such as Cr, Mo, and V are present simultaneously in a steel, even if the Cr content reaches 1.7%, there is no significant adverse effect on the weldability of the steel. In the present invention, chromium is a beneficial and unnecessary element, and its addition in excessive amounts is undesirable due to factors such as increased cost. In some embodiments, the Cr content is ≦0.35%, e.g., ≦0.25%.

[0073] Mo: Molybdenum can suppress the self-diffusion of iron and the diffusion rate of other elements. The atomic radius of Mo is larger than that of α-Fe. When Mo dissolves in α-ferrite solid solution, it generates strong lattice distortion in the solid solution. At the same time, Mo increases the lattice atomic bond attraction, raising the recrystallization temperature of α-ferrite. The strengthening effect of Mo is evident in pearlitic, ferritic, and martensitic steels, and even in high-alloy austenitic steels. The beneficial effect of Mo in steel depends on its interaction with other alloying elements in the steel. The solid-solution strengthening effect of Mo is more pronounced when strong carbide-forming elements such as V, Nb, and Ti are added to steel. This is because the strong carbide-forming elements combine with C to form stable carbides, promoting the more effective dissolution of Mo into solid solution, which is beneficial for improving the hot strength of steel. Adding Mo can also improve the hardenability of steel, but the effect is less pronounced than that of C and Cr. Mo inhibits transformation in the pearlite region and accelerates transformation in the intermediate temperature region. Therefore, Mo-containing steels can form a certain amount of bainite and eliminate ferrite formation even at high cooling rates. This is one of the reasons why Mo has a beneficial effect on the thermal strength of low-alloy heat-resistant steels. Mo can also significantly reduce the tendency of steels to become hot brittle and reduce the rate of pearlite spheroidization. A Mo content of 0.15% or less does not adversely affect the weldability of the steel. In the present invention, molybdenum is a beneficial but unnecessary element, and adding too much is undesirable due to factors such as increased cost. In some embodiments, the Mo content is ≦0.25%.

[0074] Nb: Nb is a carbide and nitride former and can meet these requirements at relatively low concentrations. At room temperature, most of its form in steel is carbide, nitride, and carbonitride, with a small portion dissolved in ferrite. Adding Nb inhibits austenite grain growth and raises the grain coarsening temperature of steel. Nb and carbon form highly stable NbC, and adding trace amounts of Nb to steel can enhance the matrix strength through its precipitation strengthening effect. Nb has a significant inhibitory effect on the growth of ferrite recrystallization and austenite grain growth, refining grains and improving the strength and toughness of steel. Nb also influences grain boundary mobility, transformation behavior, and carbide formation. Nb increases the carbon content in retained austenite, inhibits the formation of bainite, promotes martensite nucleation, and results in a dispersed martensite structure. It also enhances the stability of retained austenite. Adding Nb increases the strength of dual-phase steels, allowing for consistent strength even with lower martensite and lower C content, thereby enhancing the toughness of the dual-phase steel. Another benefit of adding Nb is that it enhances the strength of the steel over a wide annealing temperature range. In the present invention, Nb is a beneficial but unnecessary element, and excessive addition is undesirable due to factors such as increased cost. In some embodiments, the Nb content is ≤0.06%, e.g., ≤0.04%.

[0075] Ti: Ti is a microalloying element and a ferrite-forming element that closes the gamma region. It can increase the critical point of steel. The combination of Ti and C in steel can form highly stable TiC, which is highly soluble within the austenitizing temperature range of common heat treatments. TiC particles refine austenite grains, increasing the opportunity for nucleation of new phases during austenite decomposition and transformation, both of which accelerate the austenite transformation. Ti also forms precipitates (TiC and TiN), which are more stable than the carbides and nitrides of C, N, Nb, and V. This significantly reduces the diffusion rate of C in austenite, significantly slowing the rate of austenite formation. The formed carbides and nitrides precipitate in the matrix, pinning the austenite grain boundaries and inhibiting austenite grain growth. During the cooling process, the precipitated TiC has a precipitation strengthening effect; during the tempering process, Ti mitigates the diffusion of C in the α phase, mitigates the precipitation and growth of carbides such as Fe and Mn, increases tempering stability, and performs secondary hardening by precipitating TiC. Microalloying with Ti can increase the high-temperature strength of steel. Adding trace amounts of Ti to steel can, on the one hand, reduce the carbon equivalent content and simultaneously improve the strength and weldability of the steel, and, on the other hand, fix impurities such as oxygen, nitrogen, and sulfur, improving the weldability of the steel. Second, the effect of Ti microparticles, such as the insolubility of TiN at high temperatures, can prevent grain coarsening in the heat-affected zone, increasing the toughness of the heat-affected zone and thereby improving the weldability of the steel. In some embodiments, the Ti content is ≦0.065%, e.g., ≦0.04%. In some embodiments, when Ti is added, the amount added may be in the range of 0.006-0.016%.

[0076] Microalloy element V: V is a ferrite stabilizer and a strong carbide former. It has a strong grain refining effect and can densify the steel structure. Adding V to steel simultaneously improves the strength, plasticity, and toughness of the steel. Vanadium can also increase the high-temperature strength of structural steel. Vanadium cannot improve hardenability. When trace amounts of the microalloy element V are added to steel, if the steel has a low carbon equivalent, the dispersed precipitation of carbide / nitride particles (less than 5 nm in size) and the solid solution of V refines the grains, significantly improving the strength and toughness (especially low-temperature toughness) of the steel and providing it with good usability, such as good weldability. Adding trace amounts of V to steel can reduce the carbon equivalent content while simultaneously increasing the strength and weldability of the steel, and can also fix impurities such as oxygen, nitrogen, and sulfur, improving the weldability of the steel. Second, the effect of V(CN), such as its microparticle insolubility at high temperatures, can prevent grain coarsening in the heat-affected zone and increase the toughness of the heat-affected zone, thereby improving the weldability of the steel. In the present invention, trace alloy elements are beneficial but unnecessary elements, and adding excessive amounts is undesirable due to factors such as increased cost. In some embodiments, the V content is ≦0.055%, e.g., ≦0.035%.

[0077] Adding trace amounts of Nb, V, and Ti to steels with low carbon equivalents can refine grains through the dispersed precipitation of carbide / nitride particles (less than 5 nm in size) and the solid solution of Nb, V, and Ti. This significantly improves the strength and toughness (especially low-temperature toughness) of steels, thereby providing good weldability and service performance. Nb, V, and Ti are carbide and nitride formers, and these elements can meet these requirements at relatively low concentrations. Nb, V, and Ti are strong carbide formers, and at room temperature, they exist mostly in the form of carbides, nitrides, and carbonitrides in steels, with a small portion solid-solubilizing in ferrite. For Q&P steels, the addition of these microalloying elements strengthens the ferrite matrix through grain refinement and precipitation. The formation of ferrite enriches the residual austenite with carbon, delaying the transformation of austenite to bainite. Finely dispersed carbides and nitrides inhibit the nucleation of bainite and slow down the kinetics of bainite formation. The addition of Nb, V, and Ti can inhibit the growth of austenite grains and increase the coarsening temperature of steel; this is because the small dispersed particles of these carbides and nitrides can pin austenite grain boundaries, inhibit their migration, increase the austenite recrystallization temperature, and expand the unrecrystallized region, thereby inhibiting the growth of austenite grains.

[0078] The present invention uses a rapid heat treatment method (including rapid heating, short-term heat retention, and rapid cooling processes) to precisely control the recovery, recrystallization, and transformation processes of the deformed structure during the heat treatment of rolled hard strip steel, ultimately achieving a fine, uniform, dispersedly distributed structure and a good synergy between strength and plasticity.

[0079] The specific principle is to use different heating rates at different temperature stages during the heating process. The low-temperature segment primarily involves the recovery of deformation structures, so a relatively low heating rate can reduce energy consumption. The high-temperature segment primarily involves recrystallization and grain growth of different phase structures. Therefore, a relatively high heating rate must be used to shorten the dwell time of the structure in the high-temperature section to ensure grain refinement. Controlling the heating rate during the heating process inhibits the recovery of deformation structures and the ferrite recrystallization process, allowing the recrystallization and austenite transformation processes to overlap, increasing the nucleation points for recrystallized and austenite grains, and ultimately refines the grains. Short-term heating and rapid cooling shortens the grain growth time during the soaking process, ensuring a refined and uniformly distributed grain structure.

[0080] The heat treatment processes disclosed in Chinese Patent CN107794357B and U.S. Patent US2019 / 0153558A1 involve a step-by-step heating process: first, heating to 300-500°C at a heating rate of 1-10°C / s, then heating to the single-phase austenite range of 850-950°C at a heating rate of 100-500°C / s, and then cooling to room temperature by water quenching after a temperature hold of no more than 5 seconds. This treatment method requires heating the steel plate to the high-temperature range of single-phase austenite, which increases the high-temperature resistance requirements of the equipment and increases manufacturing difficulty. The use of water quenching requires a very high cooling rate, which significantly reduces the grain structure growth time in the high-temperature range, but it also inevitably results in uneven distribution of alloying elements in the final product, resulting in uneven and unstable product microstructural performance. Furthermore, a water quench cooling rate that is too high can cause a series of problems, such as poor plate shape and surface oxidation.

[0081] By comprehensively controlling the entire heat treatment process (including rapid heating (controlling the heating rate in stages), short-time soaking, and rapid cooling processes), it is possible to obtain precisely controlled, optimal crystal grain size, uniform distribution of alloying elements and each phase structure, and ultimately obtain a product with the optimal synergy of strength and toughness.

[0082] The main phase structure of the Q&P steel obtained by the rapid heat treatment method of the present invention is martensite (75-90% by volume) and retained austenite (10-25% by volume), and also contains a very small amount of ferrite (3-10% by volume). Therefore, strictly speaking, the phase structure is a multiphase structure, the matrix structure is uniformly distributed, and obvious sheet-like tempered martensite appears, with a grain size of 1-3 μm. The martensite strengthening phase grains are surrounded by uniformly distributed ferrite phase, and the martensite strengthening phase grains mainly have a sheet-like structure.

[0083] The method for producing a low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to the present invention includes the following steps: 1) Smelting and Casting Smelting and casting slabs according to the above chemical composition; 2) Hot rolling and coiling Coiling temperature 550-680℃; 3) Cold rolling Cold rolling reduction ratio of 40-85% to obtain rolled hard steel strip or steel plate; 4) Rapid heat treatment a) Rapid heating The cold-rolled steel strip or steel plate is rapidly heated to 770-845°C, and the rapid heating can be performed in one or two stages; when one-stage rapid heating is performed, the heating rate is 50-500°C / s; when two-stage rapid heating is performed, the first stage is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and the second stage is heated from 550-625°C to 770-845°C at a heating rate of 50-500°C / s; b) Soaking The target temperature for the austenite-ferrite two-phase region is 770-845°C, and the soaking time is 10-60 seconds. c) cooling After the soaking of the steel strip or steel plate is completed, it is slowly cooled to 700-770°C (for example, 720-770°C) at a cooling rate of 5-15°C / s, and then rapidly cooled to 230-280°C at a cooling rate of 50-200°C / s (for example, 50-150°C / s), and kept at this temperature range for 2-10 seconds; d) Tempering After the temperature is maintained, the steel strip or steel plate is heated to 300-470°C at a heating rate of 10-30°C / s and tempered for 10-60 seconds. e) After tempering, the strip steel or steel plate is cooled to room temperature at a cooling rate of 30 to 100°C / s.

[0084] In the method for producing the low-carbon low-alloy Q&P steel, preferably, in step 2), the rolling finish temperature of the hot rolling is ≧A r3 Preferably, in step 2), the coiling temperature is 580 to 650°C. Preferably, in step 3), the cold rolling reduction is 60 to 80%. Preferably, in step 4), the total process time of the rapid heat treatment is 71 to 186 s. Preferably, in step 4), when single-stage heating is used for the rapid heating, the heating rate is 50 to 300°C / s. Preferably, in step 4), two-stage heating is used for the rapid heating: in the first stage, heating is performed from room temperature to 550 to 625°C at a heating rate of 15 to 300°C / s; and in the second stage, heating is performed from 550 to 625°C to 770 to 845°C at a heating rate of 50 to 300°C / s. Preferably, in step 4), the rapid heating employs two-stage heating: in the first stage, heating is performed from room temperature to 550-625°C at a heating rate of 30-300°C / s; and in the second stage, heating is performed from 550-625°C to 770-845°C at a heating rate of 80-300°C / s. Preferably, in step 4), the final temperature of the rapid heating is 790-845°C. Preferably, in step 4), the rapid cooling rate of the steel strip or steel sheet is 50-150°C / s. Preferably, in the soaking process described in step 4), the steel strip or steel sheet is heated to the target temperature in the austenite-ferrite two-phase region, and then the temperature is maintained for soaking. Preferably, in the soaking process described in step 4), the temperature of the steel strip or steel sheet is increased or decreased by a small amount during the soaking period, so that the temperature after the increase does not exceed 845°C and the temperature after the decrease does not fall below 770°C. Preferably, the soaking time is 10 to 40 seconds.

[0085] The rapid heat treatment, hot-dip galvanizing manufacturing method for low-carbon, low-alloy galvanized Q&P steel with a tensile strength of ≥ 1180 MPa according to the present invention includes the following steps: 1) Smelting and Casting Smelting and casting slabs according to the above chemical composition; 2) Hot rolling and coiling Hot rolling finish temperature ≧ A r3 , then cooled to 550-680℃ and coiled; 3) Cold rolling Cold rolling reduction ratio is 40-80%, and after cold rolling, hard rolled steel strip or steel plate is obtained; 4) Rapid heat treatment, hot dip galvanizing a) Rapid heating The cold-rolled steel strip or steel plate is rapidly heated from room temperature to a target temperature of 770-845°C in the austenite-ferrite two-phase region, and the rapid heating is performed in one or two stages; When using single-stage rapid heating, the heating rate is 50~500℃ / s; When two-stage rapid heating is adopted, in the first stage, the material is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and in the second stage, the material is heated from 550-625°C to 770-845°C at a heating rate of 30-500°C / s (e.g., 50-500°C / s); b) Soaking The target temperature for the austenite-ferrite two-phase region is 770-845°C, and the soaking time is 10-60 seconds. c) cooling After the soaking of the steel strip or steel plate is completed, it is slowly cooled to 720-770°C at a cooling rate of 5-15°C / s; then it is rapidly cooled to 230-280°C at a cooling rate of 50-200°C / s (e.g., 50-150°C / s), and kept at this temperature range for 2-10 seconds (e.g., 2-8 seconds); d) distribution After the heat retention is completed, the strip steel or steel plate is heated to 460-470°C at a heating rate of 10-30°C / s, and then subjected to distribution treatment, with a distribution time of 10-60s; e) Hot-dip galvanizing After the distribution is completed, the steel strip or steel plate is immersed in a zinc pot for hot dip galvanizing; f) After hot-dip galvanizing the steel strip or steel sheet, it is rapidly cooled to room temperature at a cooling rate of 30 to 150°C / s to obtain a hot-dip pure galvanized GI product; or after hot-dip galvanizing the steel strip or steel sheet, it is heated to 480 to 550°C at a heating rate of 10 to 300°C / s to perform an alloying treatment, and the alloying treatment time is 5 to 20 seconds; after the alloying treatment, it is rapidly cooled to room temperature at a cooling rate of 30 to 250°C / s to obtain an alloyed hot-dip galvanized GA product.

[0086] In the method for rapid heat treatment and hot-dip galvanizing of low-carbon, low-alloy galvanized Q&P steel, preferably, the total process time for the rapid heat treatment and hot-dip galvanizing is 43 to 186 seconds. Preferably, in step 2), the coiling temperature is 580 to 650°C. Preferably, in step 3), the cold rolling reduction is 60 to 80%. Preferably, in step 4), when single-stage heating is used for the rapid heating, the heating rate is 50 to 300°C / s. Preferably, in step 4), two-stage heating is used for the rapid heating, and in the first stage, heating is performed from room temperature to 550 to 625°C at a heating rate of 15 to 300°C / s, and in the second stage, heating is performed from 550 to 625°C to 770 to 845°C at a heating rate of 50 to 300°C / s. Preferably, in step 4), the rapid heating employs two-stage heating, in which in the first stage, heating is performed from room temperature to 550-625°C at a heating rate of 30-300°C / s, and in the second stage, heating is performed from 550-625°C to 770-845°C at a heating rate of 80-300°C / s. Preferably, in step 4), the final temperature of the rapid heating is 790-845°C. Preferably, in step 4), the cooling rate in the rapid cooling stage is 50-150°C / s. Preferably, in the soaking process of step 4), the steel strip or steel plate is heated to the target temperature in the austenite-ferrite two-phase region, and then soaked at that temperature. Preferably, in the soaking process of step 4), the steel strip or steel sheet is heated or cooled in small increments during the soaking period, so that the temperature after heating does not exceed 845°C and the temperature after cooling does not fall below 770°C. Preferably, the soaking time is 10 to 40 seconds. Preferably, in step 4), after hot-dip galvanizing the steel strip or steel sheet, an alloying treatment is performed by heating to 480 to 550°C at a heating rate of 30 to 200°C / s, and the alloying treatment time is 5 to 20 seconds; after the alloying treatment, the product is quenched to room temperature at a cooling rate of 30 to 200°C / s to obtain an alloyed hot-dip galvanized GA product.

[0087] In the method for producing a low-carbon, low-alloy Q&P steel and a low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength of 1180 MPa or more according to the present invention, 1. Heating rate control The recrystallization kinetics during continuous heating can be quantitatively described by a relationship that is affected by the heating rate. The functional relationship between the ferrite recrystallization volume fraction and temperature T during continuous heating is:

[0088]

number

[0089] where X(t) is the ferrite recrystallization volume fraction; n is the Avrami exponent, which is related to the transformation mechanism and generally ranges from 1 to 4 depending on the decay period of the recrystallization nucleation rate; T is the heat treatment temperature; T star is the recrystallization onset temperature; β is the heating rate; b(T) is given by:

[0090]

number

[0091] From the above equation and related experimental data, the recrystallization start temperature (T star ) and end temperature (T fin ) both increase; when the heating rate is 50°C / s or more, the austenite transformation and recrystallization processes overlap, and the recrystallization temperature rises to the two-phase region temperature; it is known that the faster the heating rate, the higher the ferrite recrystallization temperature.

[0092] Under conventional slow heating conditions, the deformed matrix undergoes complete recovery, recrystallization, and grain growth, followed by the transformation of ferrite to austenite, and the transformation nucleation occurs mainly at the boundaries of the grown ferrite grains, with a relatively slow nucleation rate, resulting in a relatively coarse grain structure.

[0093] Under rapid heating conditions, the deformed matrix begins to recrystallize before it has fully recovered. The ferrite-to-austenite transformation begins before recrystallization is complete or before grain growth begins. The fine grains and large grain boundary area at the beginning of recrystallization significantly increase the nucleation rate and significantly refine the grains. In particular, after the ferrite recrystallization and austenite transformation processes overlap, a large number of crystal defects, such as dislocations, remain within the ferrite crystals, providing numerous nucleation sites for austenite, resulting in explosive nucleation and further refinement of the austenite grains. At the same time, the high density of dislocation defects remaining also serves as a pathway for rapid diffusion of carbon atoms, allowing each austenite grain to rapidly form and grow, resulting in an increase in the austenite volume fraction.

[0094] Precise control of the microstructural changes and distribution of alloying elements and phases during rapid heating lays a good foundation for the growth of austenite during the subsequent soaking process, and for the distribution of alloying elements and the transformation of austenite to martensite during the rapid cooling process. Ultimately, a final product structure with refined grains and a reasonable distribution of elements and phases can be obtained. Taking into consideration factors such as the effects of rapid heating and refined grains, manufacturing cost, and manufacturability, the present invention specifies a heating rate of 50-500°C / s for single-stage rapid heating and a heating rate of 15-500°C / s for two-stage rapid heating.

[0095] Rapid heating has different effects on structural transformation processes such as material recovery, recrystallization, and grain growth within different temperature ranges. To achieve optimal structural control, the preferred heating rates for different heating temperature ranges also vary: from 20°C to 500-625°C, the heating rate has the greatest effect on the recovery process, so the heating rate should be controlled at 5-300°C / s, more preferably 50-300°C / s; from the heating temperature of 500-625°C to the austenitizing temperature of 770-845°C, the heating rate has the greatest effect on the recrystallization nucleation rate, transformation nucleation rate, and grain growth process, so the heating rate should be controlled at 50-300°C / s, more preferably 80-300°C / s.

[0096] 2. Controlling the heat-soaking speed The selection of the soaking temperature must be combined with the control of the material structure change process at each temperature stage of the heating process, and at the same time, the change and control of the structure in the subsequent quenching process must be taken into consideration, so that a desirable structure and distribution can be finally obtained.

[0097] The soaking temperature depends on the C content. In conventional technology, the soaking temperature is generally set to A C3 The present invention utilizes rapid heating technology to form a large amount of dislocations in ferrite, providing nucleation energy for austenite transformation, so the temperature is set to A C1 From A C3 It is sufficient to heat the steel between 0.16 and 0.23%. C1 and A C3 The soaking temperatures are approximately 730°C and 870°C, respectively. Q&P steels contain a large amount of undissolved, fine, and uniformly distributed carbides, which act as a mechanical inhibitor on austenite grain growth during soaking, contributing to the refinement of the alloy steel grain size. However, excessively high soaking temperatures significantly reduce the number of undissolved carbides, weakening this inhibitory effect and strengthening the tendency for grain growth, further reducing the strength of the steel. Too many undissolved carbides can cause agglomeration, resulting in uneven distribution of chemical elements locally. High carbon content in these agglomerations can lead to local overheating. Therefore, ideally, alloy steels should contain a small amount of uniformly distributed, fine-grained undissolved carbides, which can prevent abnormal austenite grain growth and, accordingly, increase the content of each alloying element in the matrix, thereby improving the alloy steel's mechanical properties, such as strength and toughness.

[0098] The soaking temperature must be selected with the aim of obtaining fine and uniform austenite grains, so that a higher volume fraction and a uniform and fine martensite structure can be obtained after cooling. A too high soaking temperature will cause the workpiece to crack easily during quenching, and the resulting martensite structure will become coarse after quenching, reducing the mechanical performance of the steel; it will also reduce the amount of retained austenite, reducing the hardness and wear resistance of the workpiece. A too low soaking temperature will result in an insufficient content of carbon and alloying elements dissolved in austenite, resulting in an uneven distribution of alloying element concentrations in austenite, significantly reducing the hardenability of the steel and adversely affecting the mechanical performance of the alloy steel. The soaking temperature for hypoeutectoid steels is A. c3 The soaking temperature should be +30 to +50°C. In ultra-high strength steels, the presence of carbide-forming elements inhibits the transformation of carbides, so the soaking temperature can be appropriately increased. Taking all of these factors into consideration, the present invention selects a soaking temperature of 770 to 845°C, hoping to achieve a rational hardening process and ideal structural performance.

[0099] 3. Controlling the soaking time The present invention employs rapid heating, which contains a large number of dislocations in the two-phase region material, providing a large number of nucleation points for austenite formation and rapid diffusion paths for carbon atoms, allowing austenite to form extremely quickly. The shorter the soaking time, the shorter the carbon diffusion distance, resulting in a larger carbon concentration gradient within the austenite and ultimately a higher carbon content in the remaining retained austenite. However, if the soaking time is too short, the distribution of alloying elements in the steel will be uneven, resulting in insufficient austenitization. If the soaking time is too long, the austenite grains will likely become coarse. The length of the soaking time also depends on the carbon and alloying element contents in the steel. High carbon and alloying element contents in the steel not only reduce the thermal conductivity of the steel, but also significantly delay the structural transformation of the steel because the alloying elements diffuse more slowly than carbon. In this case, the soaking time must be appropriately extended. Therefore, the soaking time must be controlled by strictly combining the soaking temperature, quenching, and rapid heating processes and taking them into consideration comprehensively, otherwise an ideal structure and element distribution cannot be obtained. For these reasons, in the present invention, the soaking time is set to 10 to 60 seconds.

[0100] 4. Control of quenching speed To obtain martensite, the cooling rate during quenching must be greater than the critical cooling rate. The critical cooling rate depends primarily on the material's composition. In this study, the Si content was 1.1-2.0% and the Mn content was 1.6-3.0%. Because these contents are relatively high, the addition of Si and Mn significantly enhanced the hardenability of Q&P steel, lowering the critical cooling rate. The cooling rate must also take into account the microstructural changes and alloy diffusion distribution during the heating and soaking processes to achieve a reasonable distribution of phases and alloying elements. If the cooling rate is too low, the martensite structure will not be obtained, resulting in reduced strength and insufficient mechanical performance. On the other hand, if the cooling rate is too high, large quenching stresses (i.e., microstructural and thermal stresses) will occur, causing severe defects in the sheet shape. Uneven cooling can lead to particularly severe defects in the sheet shape and even severe deformation and cracking of the sample. Therefore, in the present invention, the quenching rate is set to 50 to 200° C. / s.

[0101] 5. Control of tempering temperature Typically, when alloy steels are tempered below 150°C, the temperature is too low to allow diffusion of alloying elements, with only carbon remaining capable of diffusing. Therefore, low-temperature tempered steels have relatively high hardness, but are too brittle and have poor toughness, failing to meet the performance requirements of their intended use. When tempered at temperatures above 200°C, large amounts of carbon and other alloying elements in martensite begin to precipitate, reducing residual stress until it disappears. The hardness of the tempered steel gradually decreases with increasing tempering temperature, but its toughness increases. However, when the tempering temperature reaches around 500°C, martensite decomposition is complete, cementite gradually aggregates and grows, and the α-phase begins to recover. As the temperature continues to increase, the α-phase begins to recrystallize, forming polygonal ferrite, resulting in a significant decrease in strength. The higher the tempering temperature, the coarser the α phase and cementite phase become, and the lower the hardness of the tempered steel becomes. Since the ultimate purpose of the present invention is to obtain better strength and plasticity at the same time, the present invention sets the tempering temperature to 300 to 470°C.

[0102] 6. Controlling the tempering time During the tempering process of steel, the tempering time plays three roles: (1) ensuring sufficient transformation of the structure; (2) reducing or eliminating internal stress; and (3) achieving the required performance of the workpiece in accordance with the tempering temperature. In the steel of the present invention, rapid heating technology is used to refine the austenite grains, thereby shortening the spacing between the retained austenite and martensite formed after the primary quench. This improves the efficiency of carbon diffusion from the supersaturated martensite to the retained austenite, significantly reducing the time required for the tempering process. However, if the tempering time is too short, it is difficult to relieve internal stress and the brittleness of the workpiece cannot be reduced. Considering all aspects, the present invention sets the tempering time to 10 to 60 seconds.

[0103] 7. Control of distribution temperature Typically, when high-alloy Q&P steels are tempered below 150°C, the alloying elements cannot diffuse because the temperature is too low. Only carbon has a certain degree of diffusion capacity. Therefore, low-temperature tempered steels have relatively high hardness, but are too brittle and have poor toughness, failing to meet the performance requirements of their intended use. When tempered at temperatures above 200°C, the carbon and other alloying elements in the martensite begin to precipitate in large quantities, reducing residual stress until it disappears. The hardness of the tempered steel also gradually decreases with increasing tempering temperature. When the tempering temperature reaches around 500°C, martensite decomposition is complete, cementite gradually aggregates and grows, and the α phase begins to recover. As the temperature continues to increase, the α phase begins to recrystallize, forming polygonal ferrite. The higher the partitioning temperature, the coarser the α phase and cementite phase become and the lower the hardness of the partitioned steel. The main purpose of the partitioning process of the present invention is to diffuse the carbon in the already obtained martensite into the retained austenite that has not yet undergone martensitic transformation, reducing the carbon in the martensite and increasing its plasticity, while at the same time increasing the carbon concentration in the retained austenite with the diffused carbon and increasing the stability of the retained austenite, thereby simultaneously providing the final product with good strength and plasticity, i.e., a good synergy between strength and plasticity. Therefore, taking into account the hot-dip galvanizing temperature, the partitioning temperature is set to 460 to 470°C.

[0104] 8. Controlling distribution time During the steel partitioning process, the partitioning time plays three roles: (1) ensuring sufficient transformation of the structure; (2) reducing or eliminating internal stress; and (3) achieving the required performance of the workpiece in accordance with the partitioning temperature. In the steel of the present invention, rapid heating technology is used to refine the austenite grains, thereby shortening the spacing between the retained austenite and martensite formed after the primary quench. This improves the efficiency of carbon diffusion partitioning from supersaturated martensite to the retained austenite, significantly reducing the time required for the partitioning process. However, if the partitioning time is too short, it is difficult to relieve internal stress and the brittleness of the workpiece cannot be reduced. Considering all aspects, the partitioning time in the present invention is set to 10 to 60 seconds.

[0105] 10. Hot-dip galvanizing and alloying control For high-strength hot-dip galvanized products, the rapid heat treatment process reduces the residence time of the steel strip in the high-temperature furnace, which significantly reduces the concentration of alloying elements on the surface of the high-strength steel strip during the heat treatment process, thereby improving the galvanizability of the high-strength hot-dip galvanized products, reducing surface coating non-adhesion defects, improving corrosion resistance, and increasing yield.

[0106] The method of the present invention can reduce the alloy content in the same level of steel, refine the crystal grains, and achieve a good soft and hard phase structure and a good synergy between strength and toughness; at the same time, by modifying the conventional continuous hot-dip galvanizing unit into a rapid heating and quenching process and realizing a rapid heat treatment process, the length of the heating segment and soaking segment of the conventional continuous hot-dip galvanizing unit can be greatly shortened (by at least one-third compared with the conventional continuous annealing furnace), which improves the production efficiency of the conventional continuous hot-dip galvanizing unit, reduces production costs and energy consumption, and reduces the number of furnace rollers in the continuous annealing furnace, especially the number of furnace rollers in the high-temperature furnace segment, thereby reducing energy consumption and capital investment.

[0107] At the same time, by building a new continuous annealing and hot-dip galvanizing unit with rapid heat treatment and hot-dip galvanizing process technology, it can achieve the goals of simplifying the unit, flexible material transfer, and high controllability; for the material, it can refine the grain of the strip steel, further increase the material strength, reduce the alloy cost and manufacturing difficulty of the heat treatment pre-process, and improve the user's performance in material forming and welding.

[0108] By modifying the conventional continuous annealing unit with a rapid heating and cooling process and implementing a rapid heat treatment process, the length of the heating and soaking segments of the conventional continuous annealing furnace can be significantly shortened (by at least one-third compared to conventional continuous annealing furnaces), improving the production efficiency of the conventional continuous annealing unit, reducing production costs and energy consumption, and reducing the number of furnace rollers in the continuous annealing furnace, especially the number of furnace rollers in the high-temperature furnace segment, thereby improving the surface quality control of the steel strip and achieving high-surface quality steel strip products. At the same time, the construction of a new continuous annealing unit using rapid heat treatment process technology can achieve the goals of unit simplification, flexible material transfer, and high controllability; for the material, it can refine the grain of the steel strip, further increasing the material's strength and plasticity, reducing alloy costs and the manufacturing difficulties of pre-heat treatment processes, and improving the user's performance in material forming, welding, etc. [Effects of the Invention]

[0109] The present invention has the following advantages over the prior art: (1) The present invention uses rapid heat treatment to suppress the recovery of deformed structures and the ferrite recrystallization process during the heat treatment process, overlap the recrystallization process with the austenite transformation process, increase the nucleation points of recrystallized grains and austenite grains, shorten the grain growth time, and refine the grains. The resulting Q&P steel has a multiphase structure containing 75-90% martensite, 10-25% retained austenite, and 3-10% ferrite. The resulting hot-dip galvanized Q&P steel has a three-phase structure of refined martensite, ferrite, and austenite, preferably with a volume fraction of 45-75% martensite, 10-25% retained austenite, and 15-30% ferrite. The resulting Q&P steel and hot-dip galvanized Q&P steel have a uniform matrix structure, with obvious sheet-like tempered martensite and refined grains of 1-3 μm. Ferrite is uniformly distributed around the martensite-strengthening phase grains, and the martensite-strengthening phase grains are mainly in a sheet-like structure. The austenite in the structure has various forms, such as block, strip, and granular, and has good thermal stability. The austenite transformation rate at -50°C is less than 8% and at -190°C is less than 30%. The TRIP effect can be sustained under different strain conditions, resulting in excellent product mechanical properties and user usability.

[0110] (2) Compared with Q&P steel obtained by conventional heat treatment methods, the alloying elements of the Q&P steel obtained by the present invention are significantly reduced, the grain size is reduced by 40-80%, and the performance is excellent: the yield strength is 668-1112 MPa, the tensile strength is 1181-1350 MPa, the elongation is 18.9-24.2%, and the strength-strain product is 24.1-28.97 GPa%. Compared with hot-dip galvanized Q&P steel obtained by conventional continuous annealing hot-dip galvanizing, provided that the manufacturing conditions of the previous process remain unchanged, the Q&P steel obtained by rapid heat treatment of the present invention has an average grain size of 1-3 μm, a reduction of the average grain size by 10-40%, and good grain refinement effects can be achieved; its yield strength is ≥ 720 MPa, tensile strength ≥ 1180 MPa, elongation ≥ 19%, and strength-strain product ≥ 23.0 GPa%; preferably, the hot-dip galvanized Q&P steel has a yield strength of 721-956 MPa, a tensile strength of 1184-1352 MPa, an elongation of 19-22.5%, and a strength-strain product of 23.6-28.9 GPa%.

[0111] (3) The rapid heat treatment process for low-carbon, low-alloy Q&P steel with a tensile strength of ≥ 1180 MPa and the rapid heat treatment process for low-carbon, low-alloy hot-dip galvanized Q&P steel described in the present invention can shorten the entire heat treatment time to 71-186 s and 43-186 s, respectively, which can significantly reduce the entire heat treatment process time (traditional continuous annealing process time is usually 5-8 min), thereby improving production efficiency, reducing energy consumption, and lowering production costs.

[0112] (4) Compared with conventional Q&P steel and its heat treatment process, the rapid heat treatment method of the present invention shortens the heating and soaking segment times by 60-80%, shortening the overall heat treatment process time to 71-186 seconds. Compared with conventional hot-dip galvanized Q&P steel and its heat treatment process, the rapid heat treatment method of the present invention shortens the length and time of the heating and soaking segments of the continuous hot-dip galvanizing annealing furnace (the heating and soaking segments are shortened by 60-80% compared with conventional continuous hot-dip galvanizing annealing furnaces) and the overall heat treatment process time. Therefore, the present invention significantly reduces energy consumption, emissions, and consumption, significantly reduces one-time investment in furnace equipment, and significantly reduces production, operating, and equipment maintenance costs. Furthermore, when rapid heat treatment is used to produce products with the same strength, the alloy content can be reduced, reducing production costs for heat treatment and pre-treatment processes and the manufacturing difficulty of each process before heat treatment.

[0113] (5) Compared with conventional process-produced Q&P steel and hot-dip galvanized Q&P steel and their heat treatment processes, the use of rapid heat treatment technology can reduce the time for the heating and soaking processes, shorten the length of the furnace, and reduce the number of furnace rollers, thereby reducing the probability of surface defects occurring in the furnace and on the product surface, and significantly improving the surface quality of the product. In addition, due to the refinement of the product's crystal grain and the reduction of the material alloy content, the use performance of the Q&P steel obtained using the technology of this invention, such as forming properties such as hole expansion properties and bending properties, and welding properties, has also been improved.

[0114] For high-strength hot-dip galvanized products, the rapid heat treatment process reduces the residence time of the steel strip in the high-temperature furnace, which significantly reduces the concentration of alloying elements on the surface of the high-strength steel strip during the heat treatment process, thereby improving the galvanizability of the high-strength hot-dip galvanized products, reducing surface coating non-adhesion defects, improving corrosion resistance, and increasing yield.

[0115] The low-carbon, low-alloy Q&P steel with a tensile strength of ≥ 1180 MPa obtained by the present invention is of great value to the development of next-generation lightweight automobiles, trains, ships, aircraft and other means of transportation, as well as the healthy development of corresponding industries and advanced manufacturing. [Brief explanation of the drawings]

[0116] [Figure 1] FIG. 1 is a microstructure image of the Q&P steel produced from the test steel A according to Example 1 of the present invention. [Figure 2] FIG. 2 is a microstructure image of the Q&P steel produced from the test steel A according to the conventional process 1 in Example 1 of the present invention. [Figure 3] FIG. 3 shows microstructure images of the Q&P steels produced according to Example 7 from Test Steel K according to Example 1 of the present invention. [Figure 4] FIG. 4 is a microstructure image of the Q&P steel produced according to Example 8 from Test Steel R according to Example 1 of the present invention. [Figure 5] FIG. 5 shows microstructure images of the Q&P steels produced according to Example 22 from Test Steel P according to Example 1 of the present invention. [Figure 6] FIG. 6 shows microstructure images of the Q&P steels produced according to Example 23 from Test Steel S according to Example 1 of the present invention. [Figure 7] FIG. 7 is a microstructure image of the Q&P steel produced according to Example 1 from Test Steel A according to Example 2 of the present invention. [Figure 8] FIG. 8 is a microstructure image of the Q&P steel produced from the test steel A according to the conventional process 1 in Example 2 of the present invention. [Figure 9] FIG. 9 shows microstructure images of the Q&P steels produced according to Example 7 from Test Steel K according to Example 2 of the present invention. [Figure 10] FIG. 10 is a microstructure image of the Q&P steel produced according to Example 8 from Test Steel R according to Example 2 of the present invention. [Figure 11]FIG. 11 shows microstructure images of the Q&P steels produced according to Example 22 from Test Steel P according to Example 2 of the present invention. [Figure 12] FIG. 12 shows microstructure images of the Q&P steels produced according to Example 23 from Test Steel S according to Example 2 of the present invention. [Figure 13] FIG. 13 is a microstructure image of hot-dip pure galvanized Q&P steel (GI) produced according to Example 1 from test steel A according to Example 3 of the present invention. [Figure 14] FIG. 14 is a microstructure image of hot-dip pure galvanized Q&P steel (GI) produced from test steel A according to conventional process 1 in Example 3 of the present invention. [Figure 15] FIG. 15 shows microstructure images of galvannealed dual-phase steels (GA) produced according to Test Steel I to Example 17 of Example 3 of the present invention. [Figure 16] FIG. 16 shows microstructure images of hot-dip pure galvanized dual-phase steels (GI) produced according to Example 22 from Test Steel D according to Example 3 of the present invention. [Figure 17] FIG. 17 shows microstructure images of galvannealed dual-phase steels (GA) produced according to Test Steel I to Example 34 of the present invention. [Figure 18] FIG. 18 is a microstructure image of hot-dip pure galvanized Q&P steel (GI) produced according to Example 1 from test steel A according to Example 4 of the present invention. [Figure 19] FIG. 19 is a microstructure image of hot-dip pure galvanized Q&P steel (GI) produced from test steel A according to conventional process 1 in Example 4 of the present invention. [Figure 20] FIG. 20 shows microstructure images of the galvannealed dual-phase steels (GA) produced according to Example 17 to Example 4 of the present invention. [Figure 21] FIG. 21 is a microstructure image of the hot-dip pure galvanized dual-phase steel (GI) produced according to Example 22 from Test Steel D according to Example 4 of the present invention. [Figure 22]FIG. 22 shows microstructure images of the galvannealed dual-phase steels (GA) produced according to Example 4 of the present invention, from Test Steel I to Example 34. DETAILED DESCRIPTION OF THE INVENTION

[0117] The present invention will be further described below in conjunction with examples and drawings. The examples are implemented based on the technical solutions of the present invention, and provide detailed embodiments and specific operation procedures. However, the scope of protection of the present invention is not limited to the following examples.

[0118] In the examples, the yield strength, tensile strength and elongation were measured in accordance with GB / T 228.1-2010 Metallic Materials Tensile Test Part 1: Room Temperature Test Method, using P7 specimens in the transverse direction.

[0119] Example 1 See Table 1 for the composition of the test steel in this example, Tables 2 and 3 for specific parameters of this example and the conventional process, and Tables 4 and 5 show the main performance of steels manufactured from the composition of the test steel in this example by the example and the conventional process.

[0120] Tables 1 to 5 show that the method of the present invention can reduce the alloy content in the same level steel, refine the crystal grains, and obtain a synergistic effect on the material structure and strength and toughness. The Q&P steel obtained by the method of the present invention has a yield strength of 668 to 1002 MPa, a tensile strength of 1181 to 1296 MPa, an elongation of 18.9 to 24.2%, and a strength-strain product of 24.1 to 28.6 GPa%.

[0121] Figure 1 shows a microstructure image of Steel A with typical composition obtained in Example 1, and Figure 2 shows a microstructure image of Steel A with typical composition obtained in Conventional Process Example 1. The images reveal significant differences between the microstructures processed using different heat treatment methods. The microstructure of Steel A processed in the present invention (Figure 1) consists of a fine, uniform austenite structure dispersed mainly in a martensite matrix, along with a small amount of carbides. The austenite, martensite grain structure, and carbides are all very fine and uniformly distributed throughout the matrix, which is highly beneficial for improving the strength and plasticity of the material. In contrast, the microstructure of Steel A processed using a conventional process (Figure 2) has a relatively non-uniform distribution, with a small amount of bulk white ferrite present at the grain boundaries of black martensite and austenite. The microstructure of the conventional process is characterized by relatively coarse grains and a certain non-uniform microstructure distribution.

[0122] Figure 3 shows a microstructure image obtained in Example 7 from K steel with typical composition, and Figure 4 shows a microstructure image obtained in Example 8 from R steel with typical composition. Figure 5 shows a microstructure image obtained in Example 22 from P steel with typical composition, and Figure 6 shows a microstructure image obtained in Example 23 from S steel with typical composition. Examples 7, 8, 22, and 23 all used a short overall heat treatment cycle. As can be seen from the images, the method of the present invention allows for a more uniform, fine, and dispersed phase structure to be obtained through a short rapid annealing treatment. Therefore, the manufacturing method of the present invention can refine the crystal grains, uniformly distribute the phase structures of the material throughout the matrix, improve the material structure, and enhance material performance.

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[0131] Example 2 See Table 6 for the composition of the test steel of the present invention, Tables 7 and 8 for specific parameters of the examples of the present invention and the conventional process, and Tables 9 and 10 show the main performance of steels manufactured from the test steel composition of the present invention by the examples and the conventional process.

[0132] Tables 6 to 9 show that the method of the present invention can reduce the alloy content in the same level steel, refine the crystal grains, and obtain a good synergy between the material structure, strength, and toughness. The Q&P steel obtained by the method of the present invention has a yield strength of 754 to 1112 MPa, a tensile strength of 1281 to 1350 MPa, an elongation of 19 to 22.2%, and a strength-strain product of 24.8 to 28.97 GPa%.

[0133] Figure 7 shows a microstructure image obtained from Steel A with typical composition in Example 1, and Figure 8 shows a microstructure image obtained from Steel A with typical composition in Conventional Process Example 1. The images reveal significant differences between the microstructures processed using different heat treatment methods. The microstructure of the steel processed in the present invention example is primarily composed of fine, uniform martensite and a small amount of carbides dispersed in the ferrite matrix. Both the martensite grain structure and the small amount of carbides are very fine and uniformly distributed in the ferrite matrix, which is highly beneficial for improving the strength and plasticity of the material. The microstructure of steel processed using traditional processes has relatively uneven distribution, relatively coarse martensite grains, and a small amount of retained austenite and carbide grains distributed at the martensite grain boundaries, resulting in uneven distribution. The microstructure of steel processed using traditional processes is characterized by relatively coarse grains and a certain uneven microstructure distribution.

[0134] Figure 9 shows a microstructure image obtained from Example 7 using K steel with typical chemical compositions, and Figure 10 shows a microstructure image obtained from Example 8 using R steel with typical chemical compositions. Figure 11 shows a microstructure image obtained from Example 22 using P steel with typical chemical compositions, and Figure 12 shows a microstructure image obtained from Example 23 using S steel with typical chemical compositions. Examples 7, 8, 22, and 23 all used short heat treatment cycles. As can be seen from the images, the method of the present invention allows for a more uniform, fine, and dispersed phase structure to be obtained through a short rapid annealing process. Therefore, the manufacturing method of the present invention can refine the crystal grains, uniformly distribute the phase structures of the material throughout the matrix, improve the material structure, and enhance material performance.

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[0143] According to the results of Examples 1 and 2, by using rapid heating and rapid cooling technology, a traditional continuous annealing unit can be modified to realize a rapid heat treatment process, which can significantly shorten the length of the heating and soaking segments of the traditional continuous annealing furnace, improve the production efficiency of the traditional continuous annealing unit, reduce production costs and energy consumption, reduce the number of furnace rollers in the continuous annealing furnace, improve the controllability of the surface quality of the steel strip, and obtain a steel strip product with high surface quality; at the same time, by constructing a new continuous annealing unit using rapid heat treatment process technology, it has the advantages of simplifying the continuous heat treatment unit, flexible material transfer, and high controllability; for the material, it can refine the grain of the steel strip, further increase the material strength, reduce the alloy cost and the manufacturing cost and manufacturing difficulty of the pre-heat treatment process, and improve the user's performance such as the welding performance of the material.

[0144] As described above, the rapid heat treatment process of the present invention has a significant effect on the advancement of continuous annealing technology for cold-rolled steel strips, and it is expected that the cold-rolled steel strip will complete the process from room temperature to final austenitization within tens of seconds, tens of seconds, or even just a few seconds. This significantly shortens the length of the heating section of the continuous annealing furnace, improves the speed and production efficiency of the continuous annealing unit, and significantly reduces the number of rollers in the furnace of the continuous annealing unit. For a rapid heat treatment line with a unit speed of about 180 meters per minute, the number of rollers in the high-temperature furnace segment will not exceed 10, significantly improving the quality of the steel strip surface. At the same time, the rapid heat treatment process method for recrystallization and austenitization completed in an extremely short time also provides a more flexible and flexible method for designing the microstructure of high-strength steel, improving the material microstructure and improving material performance without changing the upstream process conditions such as alloy composition or rolling process.

[0145] Advanced high-strength steels, represented by Q&P steel, have a broad future, and rapid heat treatment technology also has great value for development and application. The combination of the two will provide greater space for the development and production of Q&P steel.

[0146] Example 3 See Table 11 for the composition of the test steel in this example, and Table 12 (single-stage heating) and Table 13 (two-stage heating) for the specific parameters of the examples of the present invention and the conventional process; Tables 14 and 15 show the main performance of GI and GA hot-dip galvanized QP steels manufactured from the test steel compositions of the present invention using the examples in Tables 12 and 13 and the conventional process.

[0147] Tables 11 to 15 show that the method of the present invention can reduce the alloy content in the same level steel, refine the crystal grains, and obtain a synergistic effect on the structure of the material and the strength and toughness. The Q&P steel obtained by the method of the present invention has a yield strength of 721 to 805 MPa, a tensile strength of 1184 to 1297 MPa, an elongation of 19.1 to 22.4%, and a strength-strain product of 23.6 to 28 GPa%.

[0148] Figures 13 and 14 are images of the structure of Steel A with typical chemical compositions after Example 1 and Conventional Process Example 1. The images reveal significant differences in the structure after hot-dip galvanizing. The structure of Steel A after the rapid heat treatment of the present invention (Figure 13): The matrix structure distribution is uniform, and sheet-like tempered martensite is evident in the structure, with grain sizes of 1 to 3 μm. Ferrite phase is present, uniformly distributed around the grains of the martensite-strengthening phase. Because the stability of martensite, formed by the growth of some prior austenite, is reduced, a small amount of tempered martensite appears in the structure after heat treatment. The retained martensite-strengthening phase still mainly has a sheet-like shape. The ferrite, martensite grain structure, and carbides are all very fine and uniformly distributed in the matrix, which is highly advantageous for improving the strength and plasticity of the material.

[0149] The microstructure of conventionally processed steel (Figure 14) is a typical Q&P steel microstructure, with coarse lath martensite grains, austenite and carbides distributed along the martensite grain boundaries, and an uneven distribution of the multiphase structure.

[0150] Figure 15 shows a microstructure image obtained from Example 17 (GA) of typical-component I steel, and Figure 16 shows a microstructure image obtained from Example 22 (GI) of typical-component D steel. Figure 17 shows a microstructure image obtained from Example 34 (GA) of typical-component I steel. Examples 17, 22, and 34 all used short heat treatment cycles; as can be seen from the images, the method of the present invention can produce a highly uniform, fine, and dispersedly distributed microstructure of each phase. Therefore, the manufacturing method of hot-dip galvanized Q&P steel of the present invention refines crystal grains, uniformly distributes each phase in the matrix, and improves the material's microstructure and performance.

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[0159] Example 4 See Table 16 for the composition of the test steel in this example, and Table 17 (single-stage heating) and Table 18 (two-stage heating) for the specific parameters of this example and the conventional process; Tables 19 and 20 show the main performance of the hot-dip pure galvanized GI products manufactured from the test steel compositions of the present invention using the examples in Tables 17 and 18 and the conventional process, and Table 19 shows the main performance of the GI and GA hot-dip galvanized QP steel products manufactured from the test steel compositions of the present invention using the examples in Tables 17 and 18 and the conventional process.

[0160] Tables 16 to 20 show that the method of the present invention can reduce the alloy content in the same level steel, refine the crystal grains, and obtain a synergistic effect on the structure of the material and the strength and toughness. The Q&P steel obtained by the method of the present invention has a yield strength of 802 to 956 MPa, a tensile strength of 1280 to 1352 MPa, an elongation of 19 to 22.5%, and a strength-strain product of 25.2 to 28.9 GPa%.

[0161] Figures 18 and 19 are images of the structure of Steel A with typical chemical compositions after Example 1 and Conventional Process Example 1. The images reveal significant differences in the structure after hot-dip galvanizing. The structure of Steel A after rapid heat treatment according to the present invention (Figure 18) consists of martensite, austenite, and a small amount of ferrite and carbides. The matrix structure is uniformly distributed, and sheet-like tempered martensite appears in the structure, with grain sizes of 1 to 3 μm. Most of the strengthening phase grains are surrounded by ferrite. Because the stability of martensite, formed by the growth of some prior austenite, is reduced, a small amount of tempered martensite appears in the structure after heat treatment. The remaining strengthening phase still mainly has a blocky shape. The ferrite, martensite grain structure, and carbides are all very fine and uniformly distributed in the matrix, which is highly advantageous for improving the strength and plasticity of the material.

[0162] The microstructure of conventionally processed steel (Figure 19) is a typical Q&P steel microstructure, with coarse lath martensite grains, austenite and carbides distributed along the martensite grain boundaries, and an uneven distribution of the multiphase structure.

[0163] Figure 20 shows a microstructure image obtained from Example 17 (GA) of typical-component I steel, and Figure 21 shows a microstructure image obtained from Example 22 (GI) of typical-component D steel. Figure 22 shows a microstructure image obtained from Example 34 (GA) of typical-component I steel. Examples 17, 22, and 34 all used short heat treatment cycles; as can be seen from the images, the method of the present invention can produce a highly uniform, fine, and dispersedly distributed microstructure of each phase. Therefore, the manufacturing method of hot-dip galvanized Q&P steel of the present invention refines grains, uniformly distributes each phase in the matrix, and improves the material's microstructure and performance.

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[0172] The present invention uses rapid heating and quenching technology to modify a traditional continuous annealing and hot-dip galvanizing unit, allowing it to achieve rapid heat treatment and hot-dip galvanizing processes. This significantly shortens the length of the heating segment and soaking segment of the traditional continuous annealing and hot-dip galvanizing furnace, improving the production efficiency of the traditional continuous annealing and hot-dip galvanizing unit, reducing production costs and energy consumption, and reducing the number of furnace rollers in the continuous annealing and hot-dip galvanizing furnace, significantly reducing surface defects such as roller marks, pits, and scratches, improving the controllability of the surface quality of the steel strip, and achieving high-surface-quality steel strip products. At the same time, the construction of a new continuous annealing unit using rapid heat treatment and hot-dip galvanizing process technology has the advantages of simplifying the hot-dip galvanizing unit, allowing for flexible material transfer and high controllability. For the material, it can refine the grain of the steel strip, further increasing the material strength, reducing alloy costs and the manufacturing difficulties of the pre-heat treatment process, and improving the user's performance in material forming, welding, etc.

[0173] As described above, the use of rapid heat treatment and hot-dip galvanizing processes in this invention has had a significant impact on the advancement of continuous annealing and hot-dip galvanizing technology for cold-rolled steel strip. It is expected that cold-rolled steel strip will complete the process from room temperature to final austenitization in just a few seconds, significantly shortening the length of the heating section of the continuous annealing and hot-dip galvanizing furnace, improving the speed and production efficiency of the continuous annealing and hot-dip galvanizing unit, and significantly reducing the number of rollers in the furnace. For a rapid heat treatment and hot-dip galvanizing line with a unit speed of approximately 180 meters per minute, the number of rollers in the high-temperature furnace segment will not exceed 10, significantly improving the quality of the steel strip surface. At the same time, the rapid heat treatment and hot-dip galvanizing process, which completes the recrystallization and austenitization processes in an extremely short time, also provides a more flexible and flexible method for designing the microstructure of high-strength steel, improving the material microstructure and performance without changing the alloy composition or rolling process.

[0174] Advanced high-strength steels, represented by hot-dip galvanized Q&P steel, have a broad future prospect. Rapid heat treatment and hot-dip galvanizing technologies also have great value for development and application. The combination of these two technologies will provide greater scope for the development and production of hot-dip galvanized Q&P steel.

Claims

1. A low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa, The chemical components are, in mass percentage, C: 0.16 to 0.23%, Si: 1.1 to 2.0%, Mn: 1.6 to 3.0%, P≦0.015%, S≦0.005%, Al: 0.02 to 0.05%, and may further contain one or two of Cr, Mo, Ti, Nb, and V, and the sum of Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and inevitable impurities; The metal structure of the low-carbon, low-alloy Q&P steel is a multiphase structure consisting of 75 to 90% martensite, 10 to 25% retained austenite, and 3 to 10% ferrite, and the matrix structure distribution is uniform, with sheet-like tempered martensite having a grain size of 1 to 3 μm appearing, and ferrite phases being uniformly distributed around the grains of the martensite strengthening phase, A low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa, characterized in that the low-carbon, low-alloy Q&P steel has a yield strength of ≥ 660 MPa, a tensile strength of ≥ 1180 MPa, an elongation of ≥ 18%, and a strength-elongation product of ≥ 24 GPa%.

2. In the low-carbon low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa, the C content range is 0.17-0.23%; The content range of Si is 1.4 to 2.0%; The content range of Mn is 2.4 to 3.0%; The Cr content is ≦0.35%; The content of Mo is ≦0.25%; The content of Nb is ≦0.06%; The content of Ti is ≦0.065%; The content of V is ≦0.055% 2. A low carbon, low alloy Q&P steel according to claim 1, characterized in that it has a tensile strength of ≥ 1180 MPa.

3. A low-carbon, low-alloy Q&P steel having a tensile strength of ≧1180 MPa as described in claim 1, wherein the Ti content is 0.006 to 0.016%.

4. The low-carbon, low-alloy Q&P steel has a −50°C austenite transformation rate of less than 8% and a −190°C austenite transformation rate of less than 30%; and / or The yield strength is 668 to 1112 MPa, the tensile strength is 1181 to 1350 MPa, the elongation is 18.9 to 24.2%, and the strength-strain product is 24.1 to 28.97 GPa%.

2. A low carbon, low alloy Q&P steel according to claim 1, characterized in that it has a tensile strength of ≥ 1180 MPa.

5. 2. A low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to claim 1, characterized in that the chemical compositions of the low-carbon, low-alloy Q&P steel are, in mass percentages, C: 0.17 to 0.23%, Si: 1.1 to 1.7%, Mn: 1.6 to 2.2%, P≦0.015%, S≦0.005%, Al: 0.02 to 0.05%, and may further include one or two of Cr, Mo, Ti, Nb, and V, wherein Cr+Mo+Ti+Nb+V≦0.50%, and the balance being Fe and unavoidable impurities.

6. A low-carbon, low-alloy Q&P steel having a tensile strength ≧ 1180 MPa as described in claim 5, wherein the C content is 0.19 to 0.21%, and / or the Si content is 1.3 to 1.5%, and / or the Mn content is 1.8 to 2.0%.

7. The metal structure of the low-carbon low-alloy Q&P steel is a multiphase structure consisting of 75 to 85% martensite, 10 to 25% retained austenite, and 3 to 10% ferrite, The low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to claim 5, wherein the low-carbon, low-alloy Q&P steel has a yield strength of 668 to 1002 MPa, a tensile strength of 1181 to 1296 MPa, an elongation of 18.9 to 24.2%, and a strength-elongation product of 24.1 to 28.6 GPa%.

8. 2. A low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to claim 1, characterized in that the chemical compositions of the low-carbon, low-alloy Q&P steel are, in mass percentages, C: 0.16 to 0.23%, Si: 1.4 to 2.0%, Mn: 2.4 to 3.0%, Ti: 0.006 to 0.016%, P≦0.015%, S≦0.002%, Al: 0.02 to 0.05%, and may further contain one or two of Cr, Mo, Nb, and V, where Cr+Mo+Ti+Nb+V≦0.5%, and the balance being Fe and unavoidable impurities.

9. The tensile strength of the low carbon low alloy Q&P steel is ≥ 1280 MPa; and / or The low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to claim 8, wherein the metal structure of the low-carbon, low-alloy Q&P steel is a multiphase structure of 80 to 90% martensite, 10 to 20% retained austenite, and 3 to 5% ferrite.

10. A low-carbon, low-alloy Q&P steel having a tensile strength ≧ 1180 MPa as described in claim 8, wherein the C content is 0.18 to 0.21%, and / or the Si content is 1.6 to 1.8%, and / or the Mn content is 2.6 to 2.8%.

11. A low-carbon, low-alloy Q&P steel having a tensile strength ≧ 1180 MPa as set forth in claim 8, wherein the low-carbon, low-alloy Q&P steel has a yield strength of 754 to 1112 MPa, a tensile strength of 1281 to 1350 MPa, an elongation of 19 to 22.2%, and a strength-strain product of 24.8 to 28.97 GPa%.

12. A low-carbon, low-alloy, hot-dip galvanized Q&P steel having a tensile strength of ≥ 1180 MPa, The chemical components are, in mass percentage, C: 0.16 to 0.23%, Si: 1.1 to 2.0%, Mn: 1.6 to 3.0%, P≦0.015%, S≦0.005%, Al: 0.02 to 0.05%, and may further contain one or two of Cr, Mo, Ti, Nb, and V, and the sum of Cr+Mo+Ti+Nb+V≦0.5%, with the balance being Fe and inevitable impurities; The metal structure of the hot-dip galvanized Q&P steel is a three-phase structure of martensite, ferrite, and austenite, and the matrix structure distribution is uniform, with sheet-like tempered martensite having a grain size of 1 to 3 μm appearing, and ferrite phases uniformly distributed around the grains of the martensite strengthening phase; the metal structure of the hot-dip galvanized Q&P steel is a three-phase structure with volume fractions of 45 to 75% martensite, 10 to 25% austenite, and 15 to 30% ferrite; and The hot-dip galvanized Q&P steel has a yield strength of 720 MPa or more, a tensile strength of 1180 MPa or more, an elongation percentage of 19%, and a strength-strain product of 23.0 GPa% or more. A low-carbon, low-alloy, hot-dip galvanized Q&P steel having a tensile strength of ≥ 1180 MPa.

13. The hot-dip galvanized Q&P steel has a yield strength of 721 to 956 MPa, a tensile strength of 1184 to 1352 MPa, an elongation of 19 to 22.5%, and a strength-strain product of 23.6 to 28.9 GPa%; and / or 13. The low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength ≥ 1180 MPa according to claim 12, wherein the −50°C austenite transformation rate of the hot-dip galvanized Q&P steel metallography is less than 8%, and the −190°C austenite transformation rate is less than 30%.

14. 13. A low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength ≥ 1180 MPa according to claim 12, characterized in that the chemical compositions of the low-carbon, low-alloy hot-dip galvanized Q&P steel are, in mass percentages, C: 0.17 to 0.23%, Si: 1.1 to 1.7%, Mn: 1.6 to 2.2%, P≦0.015%, S≦0.005%, Al: 0.02 to 0.05%, and may further include one or two of Cr, Mo, Ti, Nb, and V, where Cr + Mo + Ti + Nb + V≦0.5%, with the balance being Fe and unavoidable impurities.

15. A low-carbon, low-alloy, hot-dip galvanized Q&P steel having a tensile strength ≧1180 MPa as set forth in claim 14, wherein the C content is 0.19 to 0.21%, and / or the Si content is 1.3 to 1.5%, and / or the Mn content is 1.8 to 2.0%.

16. The metal structure of the hot-dip galvanized Q&P steel is a three-phase structure consisting of, by volume fraction, 45-75% martensite, 15-30% ferrite, and 10-25% austenite; and / or The hot-dip galvanized Q&P steel according to claim 14, wherein the yield strength is 721 to 805 MPa, the tensile strength is 1184 to 1297 MPa, the elongation is 19.1 to 22.4%, and the strength-elongation product is 23.6 to 28 GPa%.

17. 13. A low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength ≥ 1180 MPa according to claim 12, characterized in that the chemical composition of the low-carbon, low-alloy hot-dip galvanized Q&P steel is, in mass percentages, C: 0.16 to 0.23%, Si: 1.4 to 2.0%, Mn: 2.4 to 3.0%, Ti: 0.006 to 0.016%, P≦0.015%, S≦0.002%, Al: 0.02 to 0.05%, and may further contain one or two of Cr, Mo, Nb, and V, where Cr + Mo + Ti + Nb + V≦0.5%, with the balance being Fe and unavoidable impurities.

18. A low carbon, low alloy, hot-dip galvanized Q&P steel having a tensile strength ≧1180 MPa as described in claim 17, wherein the tensile strength of said low carbon, low alloy, hot-dip galvanized Q&P steel is ≧1280 MPa.

19. In the hot-dip galvanized Q&P steel, the C content is 0.18 to 0.21%, and / or the Si content is 1.6 to 1.8%, and / or the Mn content is 2.6 to 2.8%; and / or The low-carbon, low-alloy hot-dip galvanized Q&P steel having a tensile strength ≧ 1180 MPa according to claim 17, wherein the hot-dip galvanized Q&P steel has a yield strength of 802 to 956 MPa, a tensile strength of 1280 to 1352 MPa, an elongation of 19 to 22.5%, and a strength-elongation product of 25.2 to 28.9 GPa%.

20. A method for producing a low carbon, low alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to any one of claims 1 to 11, said method comprising the following steps: 1) Smelting and Casting smelting and casting slabs according to said chemical composition; 2) Hot rolling and coiling Hot rolling end temperature ≧ A ｒ３ , then cooled to 550-680°C and coiled; 3) Cold rolling Cold rolling is performed at a reduction ratio of 40 to 85%, and after cold rolling, a rolled hard steel strip or steel plate is obtained; 4) Rapid thermal processing a) Rapid heating The cold-rolled steel strip or steel plate is rapidly heated to 770-845°C, and the rapid heating can be performed in one or two stages; when one-stage rapid heating is performed, the heating rate is 50-500°C / s; when two-stage rapid heating is performed, the first stage is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and the second stage is heated from 550-625°C to 770-845°C at a heating rate of 50-500°C / s; b) Soaking The target temperature for the austenite and ferrite two-phase region is 770 to 845°C, and the soaking time is 10 to 60 seconds; c) cooling After the soaking of the steel strip or steel plate is completed, it is slowly cooled to 700 to 770°C at a cooling rate of 5 to 15°C / s, and then rapidly cooled to 230 to 280°C at a cooling rate of 50 to 200°C / s, and kept at this temperature range for 2 to 10 seconds; d) Tempering After the temperature is maintained, the steel strip or steel plate is heated to 300 to 470°C at a heating rate of 10 to 30°C / s, and then tempered for 10 to 60 seconds; e) After tempering, the strip steel or steel plate is cooled to room temperature at a cooling rate of 30 to 100 ° C. / s. A method for producing a low-carbon, low-alloy Q&P steel having a tensile strength of ≥ 1180 MPa according to any one of claims 1 to 11.

21. The method according to claim 20, wherein the coiling temperature is 580 to 650°C.

22. The method according to claim 20, wherein the cold rolling reduction is 60 to 80%.

23. The total process time of the rapid heat treatment of the low carbon low alloy Q&P steel is 71 to 186 seconds; and / or When the rapid heating is performed in one stage, the heating rate is 50 to 300°C / s; and / or The rapid heating is performed in two stages, with the first stage heating from room temperature to 550-625°C at a heating rate of 15-300°C / s, and the second stage heating from 550-625°C to 770-845°C at a heating rate of 50-300°C / s; In the rapid heating step, the final temperature of said rapid heating is 790-845°C; and / or In the cooling step, the quenching rate of the strip or sheet is 50 to 150 ° C / s; and / or In the soaking process, the steel strip or steel plate is heated to the target temperature in the two-phase region of austenite and ferrite, and then the temperature is maintained to soak the steel strip or steel plate; and / or During the soaking process, the temperature of the strip or steel plate is increased or decreased slightly during the soaking period, and the temperature after the increase does not exceed 845°C, and the temperature after the decrease does not fall below 770°C; and / or The method of claim 20, wherein the soaking time is 10 to 40 seconds.

24. The method described in claim 20, wherein the steel plate is heated from room temperature to 550 to 625°C at a heating rate of 30 to 300°C / s in a first stage, and from 550 to 625°C to 770 to 845°C at a heating rate of 80 to 300°C / s in a second stage.

25. A method for producing a low-carbon, low-alloy, hot-dip galvanized Q&P steel according to any one of claims 12 to 19, comprising the following steps: 1) Smelting and Casting Smelting and casting slabs according to the above chemical composition; 2) Hot rolling and coiling The hot rolling finish temperature is ≧A r3 , and then the steel sheet is cooled to 550 to 680°C and coiled; 3) Cold rolling Cold rolling is performed at a reduction ratio of 40-80%, and after cold rolling, a rolled hard steel strip or steel plate is obtained; 4) Rapid heat treatment, hot-dip galvanizing a) Rapid heating The cold-rolled steel strip or steel plate is rapidly heated to a target temperature of 770 to 845°C, which is the austenite-ferrite two-phase region, and the rapid heating is performed in one or two stages; When one-stage rapid heating is adopted, the heating rate is 50-500°C / s; When two-stage rapid heating is adopted, in the first stage, the temperature is heated from room temperature to 550-625°C at a heating rate of 15-500°C / s, and in the second stage, the temperature is heated from 550-625°C to 770-845°C at a heating rate of 30-500°C / s; b) Soaking The target temperature for the austenite and ferrite two-phase region is 770 to 845°C, and the soaking time is 10 to 60 seconds; c) cooling After the soaking of the steel strip or steel plate is completed, it is slowly cooled to 720-770°C at a cooling rate of 5-15°C / s; then it is rapidly cooled to 230-280°C at a cooling rate of 50-200°C / s and kept at this temperature range for 2-10 seconds; d) distribution After the temperature is maintained, the steel strip or steel plate is heated to 460-470°C at a heating rate of 10-30°C / s, and then subjected to a distribution treatment, with the distribution time being 10-60s; e) Hot-dip galvanizing After the distribution is completed, the steel strip or steel plate is immersed in a zinc pot for hot dip galvanizing; f) After hot-dip galvanizing the steel strip or steel sheet, it is quenched to room temperature at a cooling rate of 30 to 150 ° C / s to obtain a hot-dip pure galvanized GI product; or after hot-dip galvanizing the steel strip or steel sheet, it is heated to 480 to 550 ° C at a heating rate of 10 to 300 ° C / s to perform alloying treatment, and the alloying treatment time is 5 to 20 seconds; after alloying treatment, it is quenched to room temperature at a cooling rate of 30 to 250 ° C / s to obtain an alloyed hot-dip galvanized GA product. A method for producing a low-carbon, low-alloy, hot-dip galvanized Q&P steel having a tensile strength of ≧1180 MPa according to any one of claims 12 to 19.

26. 26. The method of claim 25, wherein the coiling temperature is 580 to 650°C.

27. 26. The method of claim 25, wherein the cold rolling reduction is 60 to 80%.

28. The total process time of the rapid heat treatment of the low carbon low alloy Q&P steel is 43 to 186 seconds; and / or When the rapid heating is performed in one stage, the heating rate is 50 to 300°C / s; and / or The rapid heating is performed in two stages, with the first stage heating from room temperature to 550-625°C at a heating rate of 15-300°C / s, and the second stage heating from 550-625°C to 770-845°C at a heating rate of 50-300°C / s. In the rapid heating step, the final temperature of said rapid heating is 790-845°C; and / or In the cooling step, the quenching rate of the strip or sheet is 50 to 150 ° C / s; and / or In the soaking process, the steel strip or steel plate is heated to the target temperature in the two-phase region of austenite and ferrite, and then the temperature is maintained to soak the steel strip or steel plate; and / or During the soaking process, the temperature of the strip or steel plate is increased or decreased slightly during the soaking period, and the temperature after the increase does not exceed 845°C, and the temperature after the decrease does not fall below 770°C; and / or The soaking time is 10 to 40 seconds.

26. The method of claim 25.

29. The method described in claim 25, wherein the steel plate is heated from room temperature to 550 to 625°C at a heating rate of 30 to 300°C / s in a first stage, and from 550 to 625°C to 770 to 845°C at a heating rate of 80 to 300°C / s in a second stage.

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