1470MPa-grade transformation induced plasticity steel and preparation method thereof

Abstract

The invention provides 1470MPa-grade transformation induced plasticity steel and a preparation method thereof, and belongs to the field of steel preparation. The method comprises the following steps: obtaining a plate blank with set chemical components; carrying out heating, two-stage rolling, cooling and coiling on the plate blank to obtain a hot-rolled coil; cold rolling and continuous annealing are conducted on the hot-rolled coil, and the 1470 MPa-grade transformation induced plasticity steel is obtained; wherein the continuous annealing comprises a preheating section, a first heating section, a soaking section, a first cooling section, a second heating section and a second cooling section, and the final temperature of the first heating section is controlled to be 900-950 DEG C. A continuous annealing process mode of high-temperature heating, low-temperature rapid cooling and high-temperature aging is adopted, and the high-strength steel of a mixed structure mainly comprising a lath bainite ferrite hard matrix and film retained austenite is obtained. In the structure, the hard phase matrix provides strength, the retained austenite TRIP effect provides ductility of the steel, and the fine and uniform structure composition provides high bendability, so that the steel has high bendability and ductility.

Description

Technical Field

[0001] This application relates to the technical field of steel preparation, and particularly to a 1470 MPa grade transformation-induced plasticity steel and a preparation method thereof. Background Art

[0002] In recent years, due to the continuous increase in carbon dioxide emissions, resulting in the global greenhouse effect and the gradual deterioration of the human living environment, many countries have restricted the emissions of greenhouse gases such as CO2 to control the increasingly serious environmental problems. In response to the current environmental protection issues, the improvement of automobile fuel consumption is strongly required. The lightweight of the vehicle body is an effective way to improve fuel consumption, but it is also necessary to ensure the safety of passengers. Therefore, while reducing the vehicle body weight, it is also necessary to ensure collision safety. In order to ensure the lightweight and collision safety of the vehicle body, high-strength materials need to be used. In recent years, the strength of automotive steel used on vehicle bodies has reached the 1180 MPa level. With the annual increase in automotive safety requirements and lightweight standards, automotive steel is developing towards higher strength levels. Currently, 1470 MPa high-strength steel plates are being considered for use in manufacturing the bumper reinforcement components at the front of the vehicle to further achieve the lightweight of the vehicle body. At the same time, with the continuous expansion of the application of ultra-high-strength steel, the performance requirements are getting higher and higher. It is required to have not only high strength but also good formability, such as high elongation, high yield, bendability, hole expansion property, etc.

[0003] The microstructure of traditional transformation-induced plasticity steel (TRIP steel) is that it contains 5% - 15% retained austenite in the ferrite and bainite matrix. However, the flange stretching performance of TRIP steel is not good, so it cannot be used as a suspension part. In high-strength steel, the TRIP microstructure contains high-plasticity ferrite and hard second phases, such as bainite or martensite, which is beneficial to the improvement of total elongation. The presence of retained austenite further improves the elongation performance. However, with the increase in total elongation rate, the performance determined by local elongation, such as flange flanging performance and bending performance, will be damaged. Starting from improving the hole expansion performance and bending performance, it is hoped that the microstructure of high-strength steel is a uniform single-phase structure. Otherwise, local strain concentration will occur in the soft phase near the hard phase, which is not conducive to flange flanging performance and bending performance. Although flattening pre-yielding or introducing precipitates can reduce the hardness difference between the hard phase and the soft phase and improve the flange flanging performance and bending performance to a certain extent, its essential structure remains unchanged, and it cannot completely solve the defect of transformation-induced plasticity steel in hole expansion, and at the same time, a certain amount of elongation rate will be lost. Therefore, how to improve the bendability on the basis of ensuring the high elongation rate of transformation-induced plasticity steel is a technical problem that urgently needs to be solved at present. Summary of the Invention

[0004] This application provides a 1470 MPa grade transformation-induced plasticity steel and a preparation method thereof to solve the technical problem in the prior art that it is difficult to improve the bendability on the basis of ensuring the high elongation rate of transformation-induced plasticity steel.

[0005] In the first aspect, the present application provides a method for preparing a 1470 MPa transformation-induced plasticity steel, the method comprising:

[0006] Obtaining a slab with a set chemical composition;

[0007] Heating, two-stage rolling, cooling, and coiling the slab to obtain a hot-rolled coil;

[0008] Cold rolling and continuous annealing the hot-rolled coil to obtain a 1470 MPa transformation-induced plasticity steel; wherein, the continuous annealing includes a preheating section, a first heating section, a soaking section, a first cooling section, a second heating section, and a second cooling section, and controlling the end temperature of the first heating section to be 900 °C to 950 °C.

[0009] Optionally, the set chemical composition includes: C, Si, Mn, Cr, Mo, Cu, Nb, P, S, N, and Fe; wherein, by mass fraction,

[0010] The content of C is 0.18% to 0.20%, the content of Si is 0.5% to 1%, the content of Mn is 1.8% to 2.5%, the content of Cr is 0.3% to 0.8%, the content of Mo is 0.2% to 0.5%, the content of Cu is 0.1% to 0.2%, the content of Nb is 0.015% to 0.03%, the content of P ≤ 0.01%, the content of S ≤ 0.01%, and the content of N ≤ 0.004%.

[0011] Optionally, the end temperature of the preheating section is 210 °C to 230 °C, and the heating rate of the preheating section is 8 °C / s to 12 °C / s; the heating rate of the first heating section is 1 °C / s to 3 °C / s; the end temperature of the soaking section is 900 °C to 950 °C, and the soaking time of the soaking section is 80 s to 120 s.

[0012] Optionally, the first cooling section is air-blowing rapid cooling, and the end temperature of the air-blowing rapid cooling is 250 °C to 300 °C; the end temperature of the second heating section is 320 °C to 380 °C, and the soaking time of the second heating section is 120 s to 180 s; the end temperature of the second cooling section is 150 °C to 170 °C, and the cooling rate of the second cooling section is 2 °C / s to 4 °C / s.

[0013] Optionally, the method further includes:

[0014] The molten steel is smelted and continuously cast. During the smelting process, the converter end temperature is 1650°C to 1670°C, and Al-Fe is used for deoxidation. The addition amount of Al-Fe is 3.5 kg / t of molten steel to 4.5 kg / t of molten steel; the end temperature of the continuous casting is 1640°C to 1660°C.

[0015] Optionally, the heating temperature is 1220°C to 1280°C; the coiling temperature is 550°C to 620°C.

[0016] Optionally, the two-stage rolling includes rough rolling and finish rolling, and the finish rolling end temperature is 870°C to 920°C.

[0017] Optionally, the total reduction ratio of the cold rolling is 50% to 60%.

[0018] In a second aspect, the present application provides a 1470 MPa grade transformation-induced plasticity steel prepared by the method described in any one of the embodiments of the first aspect. The microstructure of the transformation-induced plasticity steel includes a hard lath bainitic ferrite phase matrix and thin film retained austenite; wherein, the volume fraction of the hard lath bainitic ferrite phase matrix is 90% to 95%, and the volume fraction of the thin film retained austenite is 5% to 10%.

[0019] Optionally, the transformation-induced plasticity steel satisfies at least one of the following properties: yield strength Rp0.2 > 1000 MPa, tensile strength Rm > 1470 MPa, elongation A50 > 10%, R / T ≤ 1.5.

[0020] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0021] The present application provides a preparation method for a 1470 MPa grade transformation-induced plasticity steel. By adopting a continuous annealing process mode of high-temperature heating - low-temperature rapid cooling - high-temperature aging, a high-strength steel with a mixed structure of lath bainitic ferrite hard matrix and thin film retained austenite is obtained. This tissue type is different from the soft ferrite phase as the matrix, mainly composed of lath or film-like fine tissues, with very few massive tissues, and the overall tissue is relatively uniform and refined, which will not cause local strain concentration. The hard phase matrix in this tissue provides strength, and the TRIP effect of retained austenite provides the ductility of the steel, while the fine and uniform tissue structure provides high bendability, thus having high bendability and elongation. The tensile strength of the transformation-induced plasticity steel obtained in the present application > 1470 MPa, the yield strength > 1000 MPa, and the A50 elongation > 10%, meeting the special requirements of automotive parts for processing performance. Description of the Drawings

[0022] The accompanying drawings here are incorporated into and form a part of this specification, showing embodiments consistent with this application, and are used together with the specification to explain the principles of this application.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] Figure 1 It is a schematic flowchart of a preparation method for a 1470 MPa grade transformation-induced plasticity steel provided in an embodiment of this application;

[0025] Figure 2 It is a microstructural photograph of a 1470 MPa grade transformation-induced plasticity steel provided in Embodiment 1 of this application. Detailed implementation manners

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are some but not all of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of this application without creative efforts belong to the scope protected by this application.

[0027] The various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of this application; therefore, it should be considered that the described range description has specifically disclosed all possible sub-ranges and individual values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and individual numbers within that range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0028] In addition, in the description of the specification of this application, terms such as "including" and "comprising" mean "including but not limited to". In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this text, "and / or" describes the association relationship of associated objects and indicates that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this text, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following items", or similar expressions refer to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0029] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in this application can be obtained through market purchases or can be prepared by existing methods.

[0030] In a first aspect, this application provides a method for preparing a 1470 MPa grade transformation-induced plasticity steel. Please refer to Figure 1 , the method includes:

[0031] S1. Obtain a slab with a set chemical composition;

[0032] In some embodiments, the set chemical composition includes: C, Si, Mn, Cr, Mo, Cu, Nb, P, S, N, and Fe; wherein, by mass fraction,

[0033] the content of C is 0.18% - 0.20%, the content of Si is 0.5% - 1%, the content of Mn is 1.8% - 2.5%, the content of Cr is 0.3% - 0.8%, the content of Mo is 0.2% - 0.5%, the content of Cu is 0.1% - 0.2%, the content of Nb is 0.015% - 0.03%, the content of P ≤ 0.01%, the content of S ≤ 0.01%, and the content of N ≤ 0.004%.

[0034] Positive effects of controlling the C content to be 0.18% - 0.20%: C is the most effective solid-solution strengthening element and the most important element to ensure the content of hard phases in steel. If the C content is too small, the content of hard phases cannot be guaranteed and it is difficult to achieve the required strength. If the C content is too large, the weldability will deteriorate. Exemplarily, the content of this C can be 0.18%, 0.19%, 0.195%, 0.20%, etc.

[0035] Positive effects of controlling the Si content to be 0.5% - 1%: Si is an important element that inhibits the precipitation of cementite. If the Si content is too small, it is difficult to inhibit the precipitation of cementite, resulting in a small amount of retained austenite and affecting the ductility of the steel. If the Si content is too large, a large amount of scale will be generated during the hot rolling process, resulting in surface defects. Exemplarily, the content of this Si can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, etc.

[0036] Positive effects of controlling the Mn content to be 1.8% - 2.5%: Mn is a solid-solution strengthening element and an important element that stabilizes austenite. If the Mn content is too small, it is difficult to guarantee the hard phases of the steel and it is difficult to achieve high strength. If the Mn content is too large, the workability and weldability will deteriorate. Exemplarily, the content of this Mn can be 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, etc.

[0037] Positive effects of controlling the Cr content to be 0.3% - 0.8%: Cr is a solid-solution strengthening element, which can improve the hardenability and the strength of the steel plate. If the Cr content is too small, it is difficult to guarantee the hard phases of the steel and it is difficult to achieve high strength. If the Cr content is too large, the workability will deteriorate. Exemplarily, the content of this Cr can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0038] Positive effects of controlling the Mo content to be 0.2% - 0.5%: Mo is also a solid-solution strengthening element, which can improve the hardenability and the strength of the steel plate. If the Mo content is too small, it is difficult to guarantee the high strength of the steel. If the Mo content is too large, the cost will be too high. Exemplarily, the content of this Mo can be 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0039] Positive effects of controlling the Cu content to be 0.1% - 0.2%: Adding Cu that forms a thin film layer prohibits the entry of hydrogen, thereby improving the hydrogen-induced cracking resistance of the steel. Exemplarily, the content of this Cu can be 0.1%, 0.12%, 0.14%, 0.18%, 0.2%, etc.

[0040] Positive effects of controlling the Nb content to be 0.015% - 0.03%: Nb can effectively refine grains and improve the microstructure uniformity of steel. If the Nb content is too small, it cannot play the role of grain refinement. If the Nb content is too large, the ductility will deteriorate. Exemplarily, the content of Nb can be 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0041] Positive effects of controlling the P content ≤ 0.01%: P easily causes a significant decrease in the plasticity and toughness of steel. Therefore, the content requirement is as low as possible. Exemplarily, the content of P can be 0.005%, 0.007%, 0.009%, 0.01%, etc.

[0042] Positive effects of controlling the S content ≤ 0.01%: S is a harmful impurity element in steel, which causes hot brittleness of steel, reduces the ductility and toughness of steel, and causes cracks during forging and rolling. Therefore, the content requirement is as low as possible. Exemplarily, the content of S can be 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.01%, etc.

[0043] Positive effects of controlling the N content ≤ 0.004%: Like carbon, nitrogen is also a solid solution element. As the N content in steel increases, it will lead to deterioration of its stamping processability. At the same time, solid solution N is the main cause of the aging of galvanized sheet products. Especially for the strain aging effect after tempering, the influence of nitrogen is particularly large. Therefore, N is required to be as low as possible. Exemplarily, the content of N can be 0.002%, 0.003%, 0.004%, etc.

[0044] In some embodiments, the method further includes:

[0045] Smelting and continuous casting of molten steel. During the smelting process, the converter end temperature is 1650°C - 1670°C, and Al-Fe is used for deoxidation. The addition amount of Al-Fe is 3.5 - 4.5 kg / t of molten steel; the end temperature of the continuous casting is 1640°C - 1660°C.

[0046] Exemplarily, the converter end temperature can be 1650°C, 1655°C, 1660°C, 1665°C, 1670°C, etc.; the addition amount of Al-Fe can be 3.5 kg / t of molten steel, 3.7 kg / t of molten steel, 3.9 kg / t of molten steel, 4 kg / t of molten steel, 4.2 kg / t of molten steel, 4.5 kg / t of molten steel; the end temperature of the continuous casting can be 1640°C, 1645°C, 1650°C, 1655°C, 1660°C, etc.

[0047] In some embodiments, during the converter tapping process of the smelting process, slag materials are added, the tapping slag amount ≤ 80 mm, and the tapping time ≥ 4 min.

[0048] Exemplarily, the amount of slag carried over during tapping can be 40mm, 45mm, 50mm, 55mm, 60mm, 65mm, 70mm, 75mm, 80mm, etc.; the tapping time can be 4min, 5min, 6min, 7min, 8min, 9min, 10min, etc.

[0049] In some embodiments, during the tapping process of the converter in the smelting process, 200 kg - 800 kg of lime as slag material, 0 kg - 1000 kg of pre - molten slag, and 100 kg - 400 kg of fluorite are added. The slag materials start to be added with the steel flow in the early stage of tapping.

[0050] Exemplarily, the addition amount of lime as slag material can be 200 kg, 300 kg, 400 kg, 500 kg, 600 kg, 700 kg, 800 kg, etc.; the addition amount of pre - molten slag can be 0 kg, 100 kg, 200 kg, 300 kg, 400 kg, 500 kg, 600 kg, 700 kg, 800 kg, 900 kg, 1000 kg, etc.; the addition amount of fluorite can be 100 kg, 150 kg, 200 kg, 250 kg, 300 kg, 350 kg, 400 kg, etc.

[0051] In some embodiments, during the tapping process of the converter in the smelting process, all slag materials are added before the tapping amount reaches 1 / 5, including 800 kg of small - sized lime and 200 kg of fluorite per furnace.

[0052] In some embodiments, during the tapping process of the smelting, Mn - Fe is used to adjust Mn, Al - Fe is used to adjust Al, and Si - Fe is used to adjust Si.

[0053] S2. Heat, two - stage rolling, cooling, and coiling are performed on the slab to obtain a hot - rolled coil;

[0054] In some embodiments, the heating temperature is 1220°C - 1280°C;

[0055] The positive effect of controlling the heating temperature at 1220°C - 1280°C: If the slab heating temperature is lower than 1220°C, the nitrogen carbides cannot dissolve completely, affecting the required strength and elongation. On the contrary, if the heating temperature is higher than 1280°C, the hot - working plasticity deteriorates. Exemplarily, the heating temperature can be 1220°C, 1230°C, 1240°C, 1250°C, 1260°C, 1270°C, 1280°C, etc.

[0056] In some embodiments, the coiling temperature is 550°C - 620°C.

[0057] Positive effects of controlling the coiling temperature at 550°C to 620°C: Adopting low-temperature coiling enables the head, tail, and the edges and middle in the width direction of the hot-rolled sheet to have uniform tissue properties, avoiding performance fluctuations in the subsequent cold-rolled sheet. In addition, adopting low-temperature coiling avoids grain boundary oxidation and enables the acquisition of hot-rolled raw materials with good surfaces. Exemplarily, the coiling temperature can be 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, etc.

[0058] In some embodiments, the two-stage rolling includes rough rolling and finish rolling, and the finish rolling final rolling temperature is 870°C to 920°C.

[0059] Positive effects of controlling the finish rolling final rolling temperature at 870°C to 920°C: If the finish rolling end temperature is lower than 870°C, large and coarse ferrite generated during hot rolling affects the subsequent elongation. On the contrary, if the finish rolling end temperature is higher than 920°C, large austenite generated during hot rolling affects the subsequent strength. The final rolling temperature can be 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, etc.

[0060] S3. Cold-roll and continuously anneal the hot-rolled coil to obtain a 1470 MPa grade transformation-induced plasticity steel; wherein, the continuous annealing includes a preheating section, a first heating section, a soaking section, a first cooling section, a second heating section, and a second cooling section, and the end temperature of the first heating section is controlled at 900°C to 950°C.

[0061] In some embodiments, the total reduction ratio of the cold rolling is 50% to 60%.

[0062] Positive effects of controlling the total reduction ratio of the cold rolling at 50% to 60%: The total reduction ratio of the cold rolling cannot be too large, otherwise the cold rolling process is difficult to carry out; if the total reduction ratio of the cold rolling is too small, the target thickness cannot be achieved. The total reduction ratio can be 50%, 52%, 55%, 57%, 60%, etc.

[0063] In some embodiments, the end temperature of the preheating section is 210°C to 230°C, and the heating rate of the preheating section is 8°C / s to 12°C / s; the heating rate of the first heating section is 1°C / s to 3°C / s; the end temperature of the soaking section is 900°C to 950°C, and the soaking time of the soaking section is 80 s to 120 s.

[0064] In some embodiments, the first cooling section is a blowing rapid cooling, and the end temperature of the blowing rapid cooling is 250°C to 300°C; the end temperature of the second heating section is 320°C to 380°C, and the soaking time of the second heating section is 120 s to 180 s; the end temperature of the second cooling section is 150°C to 170°C, and the cooling rate of the second cooling section is 2°C / s to 4°C / s.

[0065] Controlling the end temperature of the preheating section to be 210°C to 230°C and the heating rate of the preheating section to be 8°C / s to 12°C / s has the following positive effects: The cold-rolled coil is first preheated, and during this process, the cold-deformed ferrite undergoes recovery. Exemplarily, the end temperature of this preheating section can be 210°C, 215°C, 220°C, 225°C, 230°C, etc.; the heating rate of this preheating section can be 8°C / s, 9°C / s, 10°C / s, 11°C / s, 12°C / s, etc.

[0066] Controlling the end temperature of the first heating section to be 900°C to 950°C and the heating rate of the first heating section to be 1°C / s to 3°C / s has the following positive effects: The preheated strip steel is further heated, and during this process, the recrystallization of the cold-rolled ferrite structure is achieved, and the pearlite first transforms into austenite and grows into ferrite. Exemplarily, the end temperature of this first heating section is 900°C, 910°C, 920°C, 940°C, 950°C, etc.; the heating rate of this first heating section can be 1°C / s, 1.5°C / s, 2.5°C / s, 3°C / s, etc.

[0067] The end temperature of the soaking section is 900°C to 950°C, and the holding time of the soaking section is 80s to 120s has the following positive effects: This process achieves full or partial austenitization and obtains a larger amount of austenite. At the same time, the austenite grains are effectively controlled, and the hole expansion property is effectively improved. Exemplarily, the end temperature of this soaking section can be 900°C, 910°C, 920°C, 930°C, 940°C, 950°C, etc.; the holding time of this soaking section can be 80s, 90s, 100s, 110s, 120s, etc.

[0068] Controlling the first cooling section to be fast cooling by blowing air, and the end temperature of the fast cooling by blowing air is 250°C to 300°C has the following positive effects: The strip steel is fast cooled to the aging temperature by blowing air, and during this process, the formation of ferrite is avoided, and a certain proportion of untransformed austenite and martensite are obtained. The martensite formed during this process provides effective nucleation sites and stresses for the bainite transformation, thereby promoting the bainite transformation. Exemplarily, the end temperature of the fast cooling by blowing air can be 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, etc.

[0069] Control the end temperature of the second heating section to be 320°C to 380°C, and the holding time of the second heating section to be 120 s to 180 s. Positive effects: After rapid cooling, induction heating is carried out to heat to a temperature of 320°C to 380°C, and overaging treatment is carried out by holding for 120 s to 180 s. During this process, part of the austenite transforms into bainite, further enriching carbon in the retained austenite and increasing the stability of austenite. In the present invention, a low-temperature cooling-aging heating process is adopted, so that the martensite phase formed at low temperature promotes the full occurrence of bainite transformation, so that more carbon is enriched in the retained austenite, avoiding a large amount of unstable austenite in the final rapid cooling section from transforming into blocky martensite-austenite structure, which affects the yield strength and bendability of the steel. At the same time, the hard martensite is tempered and softened to obtain a uniform structure. Exemplarily, the end temperature of the second heating section can be 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, etc.; the holding time of the second heating section is 120 s, 130 s, 140 s, 150 s, 160 s, 170 s, 180 s, etc.

[0070] Control the end temperature of the second cooling section to be 150°C to 170°C, and the cooling rate of the second cooling section to be 2°C / s to 4°C / s. Positive effects: The strip after overaging treatment is finally cooled. During this process, a small amount of unstable austenite may transform into martensite, which is beneficial to improving the strength of the strip. The strip is coiled into a finished product after temper rolling. Exemplarily, the end temperature of the second cooling section can be 150°C, 155°C, 160°C, 165°C, 170°C, etc.; the cooling rate of the second cooling section can be 2°C / s, 2.5°C / s, 3°C / s, 3.5°C / s, 4°C / s, etc.

[0071] In a second aspect, the present application provides a 1470 MPa grade transformation-induced plasticity steel prepared by the method according to any one of the embodiments in the first aspect. The microstructure of the transformation-induced plasticity steel includes a hard lath bainite ferrite phase matrix and thin-film retained austenite; wherein, the volume fraction of the hard lath bainite ferrite phase matrix is 90% to 95%, and the volume fraction of the thin-film retained austenite is 5% to 10%.

[0072] Adopt a heat treatment process mode of high-temperature heating - low-temperature rapid cooling - aging heating to obtain a matrix of more than 90% hard bainitic ferrite phase and a thin-film-like retained austenite of more than 5% hard phase. There is also a small amount of tempered martensite in this structure. The refined and uniform hard phase matrix in this structure provides strength and improves bendability at the same time. The film-like retained austenite enriched in high carbon provides the ductility of the steel. Exemplarily, the volume fraction of the hard lath bainitic ferrite phase matrix can be 90%, 91%, 92%, 93%, 94%, 95%, etc., and the volume fraction of the thin-film retained austenite can be 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0073] In some embodiments, the transformation-induced plasticity steel satisfies at least one of the following properties: yield strength Rp0.2 > 1000 MPa, tensile strength Rm > 1470 MPa, elongation A50 > 10%, R / T ≤ 1.5.

[0074] The transformation-induced plasticity steel obtained by the method of the present application has higher bendability and elongation compared with the traditional transformation-induced plasticity steel. Exemplarily, the yield strength Rp0.2 can be 1010 MPa, 1030 MPa, 1050 MPa, 1075 MPa, 1100 MPa, 1120 MPa, 1140 MPa, 1150 MPa, etc.; the tensile strength Rm can be 1480 MPa, 1490 MPa, 1495 MPa, 1500 MPa, 1505 MPa, 1510 MPa, etc.; the elongation A50 can be 11%, 11.5%, 12%, 12.5%, etc.; the R / T can be 0.75, 1.0, 1.25, 1.5, etc.

[0075] The preparation method of the transformation-induced plasticity steel is realized based on the chemical composition of the above transformation-induced plasticity steel. The chemical composition of the transformation-induced plasticity steel can be specifically referred to the above embodiments. Since the preparation method of the transformation-induced plasticity steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be elaborated one by one here.

[0076] The following further elaborates the present application in combination with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods without specific conditions in the following embodiments are usually determined according to industry standards. If there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0077] Prepare the molten steel of Preparation Examples 1 to 4 and Comparative Example 1 and cast it into slabs. The chemical compositions of the slabs are shown in Table 1.

[0078] Table 1 Chemical composition of the slab in mass percentage (wt%), the balance being Fe and unavoidable impurities

[0079] Group C Si Mn Cr Mo Cu Nb P S N Example 1 0.18 1 2.3 0.6 0.4 0.2 0.015 0.006 0.005 0.003 Example 2 0.20 0.9 2.5 0.3 0.3 0.1 0.02 0.007 0.007 0.003 Example 3 0.19 0.7 2.0 0.5 0.5 0.15 0.015 0.008 0.009 0.004 Example 4 0.20 0.5 1.8 0.8 0.4 0.2 0.03 0.009 0.003 0.004 Comparative Example 1 0.25 0.2 2.6 0.3 0.2 -- 0.03 0.006 0.004 0.004

[0080] Based on the chemical composition of the above transformation-induced plasticity steel, embodiments and comparative examples of the present application provide a method for preparing a transformation-induced plasticity steel, the method comprising the following steps:

[0081] Obtain a slab by smelting molten steel followed by continuous casting; during the smelting process, the target temperature at the end of the converter is 1660°C. During the tapping process, slag materials are added, specifically 800 kg of lime and 200 kg of fluorite are added. The slag materials are added along with the steel flow at the early stage of tapping, and all the slag materials are added before the tapping amount reaches 1 / 5. The tapping slag amount is ≤80 mm, and the tapping time is 6 minutes. After continuous casting, a slab with a set chemical composition is obtained;

[0082] Heat the slab, then obtain a hot-rolled sheet through rough rolling and finish rolling, and then perform laminar cooling on the hot-rolled sheet, and coil it into a hot-rolled coil after cooling; the heating temperature of the slab is 1250°C; the finish rolling temperature of the finish rolling is 900°C; the coiling temperature of the hot-rolled sheet is 600°C;

[0083] Perform cold rolling, continuous annealing, tempering, and coiling on the hot-rolled coil; wherein, the cold rolling reduction rate is 55%; the continuous annealing includes a preheating section, a first heating section, a soaking section, a first cooling section, a second heating section, and a second cooling section; the continuous annealing process parameters of Examples 1-4 and Comparative Example 1 are shown in Table 2; the continuous annealing process is specifically: preheating section: heating from room temperature to 210°C - 230°C at a rate of 8°C / s - 12°C / s; first heating section: heating from 210°C - 230°C to 900°C - 950°C at a rate of 1°C / s - 3°C / s; soaking section: maintaining at 900°C - 950°C and holding for 80 s - 120 s; first cooling section: cooling from 900°C - 950°C to 250°C - 300°C at a rate of 20°C / s - 40°C / s; the second heating section includes: heating from 250°C - 300°C to 320°C - 380°C at a rate of 1°C / s - 3°C / s and holding for 120 s - 180 s; the second cooling section includes: cooling from 320°C - 380°C to 150°C - 170°C at a rate of 2°C / s - 4°C / s; to obtain a 1470 MPa grade transformation-induced plasticity steel.

[0084] Table 2 Continuous annealing process parameters of transformation-induced plasticity steel

[0085]

[0086] The microstructure of the 1470MPa transformation-induced plasticity steel obtained in the embodiments of the present invention is a mixed microstructure of fine lath bainitic ferrite hard matrix, thin-film retained austenite, and tempered martensite. The mechanical properties of the continuous annealing high-strength steel obtained in the present invention are shown in Table 3.

[0087] Table 3 Mechanical Properties of Transformation-Induced Plasticity Steel

[0088] Group Rp0.2, MPa Rm, MPa A50，％ R / T Example 1 1050 1480 12 0.75 Example 2 1100 1500 11.5 1.25 Example 3 1090 1509 12.5 1.5 Example 4 1140 1493 12 0.75 Comparative Example 1 740 1289 11 2.3

[0089] As can be seen from Table 3, for the production method of a 1470MPa bainitic matrix transformation-induced plasticity steel with high bendability provided in the embodiments of the present invention, the obtained 1470MPa bainitic matrix transformation-induced plasticity steel has higher bendability and elongation compared to the conventional transformation-induced plasticity steel of the comparative example. The tensile strength is greater than 1470MPa, the yield strength is greater than 1000MPa, and the A50 elongation is greater than 10%.

[0090] From the atta Figure 2 ched drawings, it can be known that the microstructure of the 1470MPa transformation-induced plasticity steel obtained in Example 1 is a mixed microstructure of fine lath bainitic ferrite, tempered martensite hard matrix, and about 10% thin-film retained austenite.

[0091] In summary, for the production method of a 1470MPa bainitic matrix transformation-induced plasticity steel with high bendability provided in the present application, a high-temperature heating-low-temperature rapid cooling-high-temperature aging process mode is adopted to obtain a high-strength steel with a mixed microstructure mainly composed of lath bainitic ferrite hard matrix and thin-film retained austenite. This microstructure type is different from that with a ferrite soft phase as the matrix, mainly being fine lath or film-like structures, with very few blocky structures, and the overall microstructure is relatively uniform and refined, which will not cause local strain concentration. The hard-phase matrix in this microstructure provides strength, the TRIP effect of retained austenite provides the ductility of the steel, and the fine and uniform microstructure composition provides high bendability. According to the present invention, a 1470MPa continuous annealing high-strength steel with high bendability can be obtained, meeting the special requirements of automotive parts for processing performance.

[0092] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A preparation method of a 1470 MPa grade transformation-induced plasticity steel, characterized in that, The method includes: obtaining a slab with a set chemical composition; heating, two-stage rolling, cooling, and coiling the slab to obtain a hot-rolled coil; performing cold rolling and continuous annealing on the hot-rolled coil to obtain a transformation-induced plasticity steel of 1470 MPa grade; wherein, the continuous annealing includes a preheating section, a first heating section, a soaking section, a first cooling section, a second heating section, and a second cooling section, and the end temperature of the first heating section is controlled to be 900°C - 950°C.

2. The method according to claim 1, characterized in that, The set chemical composition includes: C, Si, Mn, Cr, Mo, Cu, Nb, P, S, N, and Fe; wherein, by mass fraction, the content of C is 0.18% - 0.20%, the content of Si is 0.5% - 1%, the content of Mn is 1.8% - 2.5%, the content of Cr is 0.3% - 0.8%, the content of Mo is 0.2% - 0.5%, the content of Cu is 0.1% - 0.2%, the content of Nb is 0.015% - 0.03%, the content of P ≤ 0.01%, the content of S ≤ 0.01%, and the content of N ≤ 0.004%.

3. The method according to claim 1, wherein The end temperature of the preheating section is 210°C - 230°C, and the heating rate of the preheating section is 8°C / s - 12°C / s; the heating rate of the first heating section is 1°C / s - 3°C / s; the end temperature of the soaking section is 900°C - 950°C, and the soaking time of the soaking section is 80 s - 120 s.

4. The method according to claim 1, characterized in that The first cooling section is fast cooling by blowing air, and the end temperature of the fast cooling by blowing air is 250°C - 300°C; the end temperature of the second heating section is 320°C - 380°C, and the soaking time of the second heating section is 120 s - 180 s; the end temperature of the second cooling section is 150°C - 170°C, and the cooling rate of the second cooling section is 2°C / s - 4°C / s.

5. The method according to claim 1, characterized in that The method further includes: smelting and continuous casting the molten steel. During the smelting process, the end temperature of the converter is 1650°C - 1670°C, Al-Fe is used for deoxidation, and the addition amount of Al-Fe is 3.5 kg / t·molten steel - 4.5 kg / t·molten steel; the end temperature of the continuous casting is 1640°C - 1660°C.

6. The method according to claim 1, wherein The temperature of the heating is 1220°C - 1280°C; the temperature of the coiling is 550°C - 620°C.

7. The method according to claim 1, characterized in that, The two-stage rolling includes rough rolling and finish rolling, and the finish rolling temperature of the finish rolling is 870°C - 920°C.

8. The method according to claim 1, wherein The total reduction ratio of the cold rolling is 50% - 60%.

9. A 1470 MPa transformation-induced plasticity steel prepared by the method according to any one of claims 1 to 8, characterized in that, The microstructure of the transformation-induced plasticity steel includes a hard lath bainitic ferrite phase matrix and a thin film of retained austenite; wherein, the volume fraction of the hard lath bainitic ferrite phase matrix is 90% - 95%, and the volume fraction of the thin film of retained austenite is 5% - 10%.

10. The transformation-induced plasticity steel according to claim 9, wherein, The transformation-induced plasticity steel meets at least one of the following properties: yield strength Rp0.2 > 1000 MPa, tensile strength Rm > 1470 MPa, elongation A50 > 10%, R / T ≤ 1.5.

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