600MPa-grade high-strength steel and preparation method thereof

Abstract

The invention relates to 600MPa-grade high-strength steel and a preparation method thereof, and belongs to the technical field of steel preparation. The method comprises the steps that a plate blank with set chemical components is heated in a furnace, and the set chemical components comprise C, Si, Mn, P, S, Alt and Fe; the plate blank heated in the furnace is sequentially subjected to rough rolling, finish rolling and cooling, and a hot-rolled plate is obtained; the hot-rolled plate is coiled, the coiling temperature is controlled, so that the hot-rolled plate has uniform and fine ferrite grains, and a hot-rolled coil is obtained; the hot-rolled coil is sequentially subjected to acid pickling and cold rolling, the reduction rate of cold rolling is controlled, and a chilled plate is obtained; and the chilled plate is subjected to continuous annealing, the continuous annealing temperature is controlled so as to regulate and control the dislocation density of the chilled plate, and the 600MPa-grade high-strength steel is obtained. According to the embodiment of the invention, chemical components are reasonably designed, and proper coiling, cold rolling and annealing processes are matched, so that low cost and mechanical properties of the 600MPa-grade high-strength steel are considered.

Description

Technical Field

[0001] This application relates to the technical field of steel preparation, and in particular, to a 600 MPa grade high-strength steel and a preparation method thereof. Background Art

[0002] The 600 MPa grade high-strength steel mainly consists of microalloyed steel and dual-phase steel, and its microstructure is of types such as ferrite + pearlite, ferrite + bainite, etc. It has good formability and weldability, and thus has been widely applied in the fields of automobiles, construction machinery, etc. At present, with the development of automotive lightweighting, the demand for 600 MPa grade products is increasing, especially for those with a thickness within 1.5 mm, and the thinnest can reach 0.6 mm.

[0003] At present, the production processes of 600 MPa grade high-strength steel mainly include three categories: hot continuous rolling process, thin slab continuous casting and rolling process, and cold rolling and annealing process. However, there are still the following problems: (1) For the 600 MPa grade hot-rolled products of domestic advanced steel mills, the thinnest thickness specification produced by the hot continuous rolling process is 1.5 mm, and thinner specifications are not producible; (2) For the 600 MPa grade hot-rolled products produced by the thin slab continuous casting and rolling line, the thinnest thickness specification that can be produced is 1.2 mm, and thickness specifications below 1.2 mm are not producible; (3) For thickness specifications below 1.2 mm, they are generally produced by the cold rolling and annealing process. The annealing heating temperature is generally above 800 °C, and higher microalloying elements need to be added to ensure that the strength reaches 600 MPa after annealing, resulting in higher costs. Summary of the Invention

[0004] This application provides a 600 MPa grade high-strength steel and a preparation method thereof to solve the following technical problems: how to balance the low cost and mechanical properties of 600 MPa grade high-strength steel.

[0005] In a first aspect, an embodiment of this application provides a preparation method of a 600 MPa grade high-strength steel, and the method includes:

[0006] Heating a slab with a set chemical composition in a furnace, where the set chemical composition includes C, Si, Mn, P, S, Alt, and Fe;

[0007] Successively subjecting the slab after in-furnace heating to rough rolling, finish rolling, and cooling to obtain a hot-rolled sheet;

[0008] Coiling the hot-rolled sheet and controlling the coiling temperature so that the hot-rolled sheet has uniformly fine ferrite grains to obtain a hot-rolled coil;

[0009] Successively subjecting the hot-rolled coil to pickling and cold rolling, and controlling the reduction rate of the cold rolling to obtain a cold hard sheet;

[0010] The cold hard plate is subjected to continuous annealing, and the temperature of the continuous annealing is controlled to regulate the dislocation density of the cold hard plate, thereby obtaining a 600 MPa grade high-strength steel.

[0011] Optionally, the coiling temperature is 600°C to 650°C.

[0012] Optionally, the reduction rate of the cold rolling is 50% to 60%.

[0013] Optionally, the temperature of the continuous annealing is 540°C to 590°C.

[0014] Optionally, by mass fraction, the content of C is 0.060% to 0.10%, the content of Si is ≤0.25%, the content of Mn is 0.85% to 1.25%, the content of P is ≤0.015%; the content of S is ≤0.005%, and the content of Alt is 0.02% to 0.05%.

[0015] Optionally, the process parameters of the in-furnace heating include: the soaking section temperature is 1220°C to 1250°C, the soaking section time is ≥35 min, and the total in-furnace heating time is 180 min to 220 min; and / or,

[0016] Optionally, the process parameters of the rough rolling include: the total reduction rate of the rough rolling is ≥85%, the reduction rate of the fifth pass of R2 is ≥30%, the rough rolling exit temperature is ≥1150°C, and the final thickness of the rough rolling is 28 mm to 34 mm; and / or,

[0017] The process parameters of the finish rolling include: the finish rolling exit temperature is 880°C to 920°C, and the reduction rate of the last pass of the finish rolling is ≤10%.

[0018] Optionally, the thickness of the 600 MPa grade high-strength steel is 0.6 mm to 2.0 mm.

[0019] Optionally, after the cold hard plate is subjected to continuous annealing, and the temperature of the continuous annealing is controlled to regulate the dislocation density of the cold hard plate to obtain a 600 MPa grade high-strength steel, the following steps are further included:

[0020] The 600 MPa grade high-strength steel is leveled, and the elongation of the leveling is 0.3% to 0.6%.

[0021] Optionally, the 600 MPa grade high-strength steel meets the following mechanical properties: the yield strength is ≥500 MPa, the tensile strength is ≥600 MPa, the elongation is ≥10%, and it meets the requirement of no cracking in the 180° d = 1a bending.

[0022] In a second aspect, an embodiment of the present application provides a 600 MPa grade high-strength steel prepared by the method according to any one of the embodiments of the first aspect.

[0023] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0024] The method for preparing the 600 MPa grade high-strength steel provided by the embodiment of the present application includes: heating a slab with a set chemical composition in a furnace, where the set chemical composition includes C, Si, Mn, P, S, Alt, and Fe; successively performing rough rolling, finish rolling, and cooling on the slab after heating in the furnace to obtain a hot-rolled sheet; coiling the hot-rolled sheet and controlling the coiling temperature to make the hot-rolled sheet have uniform and fine ferrite grains to obtain a hot-rolled coil; successively performing pickling and cold rolling on the hot-rolled coil and controlling the reduction ratio of the cold rolling to obtain a cold-rolled hard sheet; continuously annealing the cold-rolled hard sheet and controlling the temperature of the continuous annealing to regulate the dislocation density of the cold-rolled hard sheet to obtain a 600 MPa grade high-strength steel. The set chemical composition includes C, Si, Mn, P, S, Alt, and Fe, which reduces the addition amount of low alloy from the source to achieve cost reduction of the alloy; heating the slab with a set chemical composition in a furnace can improve the mechanical property uniformity of the slab, so that the internal structure of the slab reaches a uniform austenitized state and ensures the stability of subsequent rolling; rough rolling can break the coarse as-cast grains of the slab and improve the internal structure; finish rolling can further reduce the thickness of the slab and control the shape, and ensure the dimensional accuracy and surface quality of the hot-rolled sheet; cooling the hot-rolled sheet after finish rolling at a high temperature can make the structure of the hot-rolled sheet fine and improve the mechanical properties of the steel; coiling the hot-rolled sheet and controlling the coiling temperature to make the hot-rolled sheet have uniform and fine ferrite grains to ensure the mechanical properties of the hot-rolled sheet; successively pickling the hot-rolled coil can remove the scale on the surface of the hot-rolled coil and make the hot-rolled coil have good surface quality; cold rolling the pickled hot-rolled coil and controlling the reduction ratio of the cold rolling can balance the strength and plasticity of the pickled hot-rolled coil; continuously annealing the cold-rolled hard sheet and controlling the temperature of the continuous annealing to regulate the dislocation density of the cold-rolled hard sheet to ensure that the flattened ferrite does not recrystallize, thereby ensuring the high strength of the steel, and the energy consumption and cost of continuous annealing are low. Therefore, this method takes into account the low cost and mechanical properties of the 600 MPa grade high-strength steel. Description of the Drawings

[0025] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0026] To more clearly illustrate the technical solutions in the embodiments of the present 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.

[0027] Figure 1 Schematic flow chart of a preparation method of a 600 MPa grade high-strength steel provided by an embodiment of the present application;

[0028] Figure 2 Metallographic structure diagram of a 600 MPa grade high-strength steel provided by an embodiment of the present application. Detailed implementation manners

[0029] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0030] The various embodiments of the present 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 the present application; therefore, it should be considered that the described range description has specifically disclosed all possible sub-ranges and single 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 single 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.

[0031] In this application, unless otherwise specified, the directional terms such as "upper" and "lower" specifically refer to the drawing directions in the attached drawings. Additionally, in the description of the specification of this application, terms such as "include" and "comprise" mean "include 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 represent: 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 "multiple" 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 represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0032] Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments, equipment, etc. used in this application can be obtained through market purchase or can be prepared by existing methods.

[0033] Currently, for 600MPa - grade cold - rolled high - strength steel, it is generally precipitation - strengthened low - alloy high - strength steel or dual - phase steel; the former obtains high strength through solid - solution strengthening and precipitation strengthening, and needs to add relatively high micro - alloying elements to increase the contribution of precipitation strengthening and ensure good strength and plasticity; the latter obtains medium - and low - temperature structures such as bainite or martensite through structure strengthening to increase strength, and also needs to add certain alloying elements to improve hardenability to ensure the acquisition of medium - and low - temperature structures. Both of the above - mentioned products need to be realized through smelting - continuous casting - hot rolling - cold rolling - continuous annealing (full annealing), and the annealing temperature is generally above 800°C.

[0034] Therefore, to balance the low cost and mechanical properties of 600 MPa grade high-strength steel. On the one hand, the embodiments of the present application provide a preparation method for 600 MPa grade high-strength steel. To obtain high strength, it no longer depends on precipitation strengthening and microstructure strengthening methods, that is, by reducing the addition amount of alloy to achieve cost reduction of the alloy. The high strength is mainly obtained through strain hardening. By controlling the coiling temperature, the hot-rolled sheet has uniform and fine ferrite grains, thereby ensuring the mechanical properties of the hot-rolled sheet; through cold rolling, the microstructure is deformed, the dislocation content is increased to improve the strength, but the elongation will be significantly reduced after cold rolling. Therefore, the reduction ratio of cold rolling is controlled to obtain appropriate deformation ability; in order to improve the elongation of the finished product, continuous annealing is carried out. The purpose of continuous annealing is to regulate the dislocation density of the steel, ensure that the flattened ferrite does not recrystallize, and balance the strength and plasticity of the steel. And continuous annealing is a low-temperature and short-time treatment, with extremely high efficiency and cost savings. Therefore, the comprehensive cost reduction is mainly achieved by reducing the alloy addition amount and regulating the continuous annealing process. Figure 1 It is a schematic flow chart of a preparation method for 600 MPa grade high-strength steel provided by the embodiments of the present application; please refer to Figure 1 , the method includes:

[0035] S1. Heating the slab with a set chemical composition in the furnace. The set chemical composition includes C, Si, Mn, P, S, Alt, and Fe;

[0036] The set chemical composition includes C, Si, Mn, P, S, Alt, and Fe, which realizes cost reduction of the alloy by reducing the addition amount of low alloy from the source; heating the slab with a set chemical composition in the furnace can improve the uniformity of the mechanical properties of the slab, so that the internal structure of the slab reaches a uniform austenitized state, and ensure the stability of subsequent rolling.

[0037] In some embodiments, in terms of mass fraction, the content of C is 0.060% - 0.10%, the content of Si is ≤0.25%, the content of Mn is 0.85% - 1.25%, the content of P is ≤0.015%; the content of S is ≤0.005%, and the content of Alt is 0.02% - 0.05%.

[0038] In the embodiments of the present application, C: C is one of the most economical strengthening elements in steel, mainly playing a role in solid solution strengthening. If the C content is too low, it is easy to cause insufficient strength after cold rolling; but when the C content is too high, the welding performance and forming performance of the steel will deteriorate, making it difficult to meet the requirements of welding performance and cold forming performance. Therefore, the C content can be 0.060% - 0.10%. Exemplarily, the C content can be 0.060%, 0.070%, 0.080%, 0.090%, 0.10%, etc.

[0039] Si: Si is a solid solution strengthening element, which is beneficial to improve the solid solution strengthening effect. However, a high Si content is not conducive to the surface quality of the plate and is also not conducive to welding performance. Therefore, considering the strength, weldability and surface quality of the material, the Si content can be ≤0.25%. For example, the Si content can be 0.25%, 0.24%, 0.23%, 0.22%, etc.

[0040] Mn: Mn is a solid solution strengthening element. In order to achieve the requirements of the present invention of yield strength ≥ 500MPa and tensile strength ≥ 600MPa, Mn cannot be too low, and a higher Mn content increases the cost. Therefore, the Mn content can be 0.85% to 1.25%. Exemplarily, the Mn content can be 0.85%, 0.87%, 0.89%, 0.91%, 0.95%, 1.00%, 1.10%, 1.20%, 1.25%, etc.

[0041] P and S: P and S are impurity elements in steel. P element is easy to cause central segregation of steel, deteriorating the weldability and plastic toughness of steel; S element is easy to form MnS inclusions with Mn element, reducing toughness. Therefore, considering the weldability and plastic toughness of the material, the P content can be ≤0.015%, and the S content can be ≤0.005%. For example, the P content can be 0.015%, 0.014%, 0.013%, 0.012%, etc.

[0042] Alt: Alt is added to steel mainly for deoxidation. Incomplete deoxidation will lead to a decrease in the cold forming performance of the material. In order to meet the requirements of steel plate forming performance, the Alt content should be ≥ 0.02%. However, too high an Alt content will lead to excessive AlN inclusions in the steel, reducing the elongation of the material. Therefore, the Alt content can be 0.02% to 0.05%. For example, the Alt content can be 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0043] In some embodiments, the process parameters of heating in the furnace include: the soaking section temperature is 1220°C to 1250°C, the soaking section time is ≥35 minutes, and the total heating time in the furnace is 180 minutes to 220 minutes.

[0044] In the embodiments of the present application, the soaking section temperature can be 1220°C to 1250°C, ensuring that the slab is fully austenitized during soaking, providing a guarantee for uniform slab quality for the uniformity of mechanical properties. The soaking section time can be ≥35 min, ensuring sufficient diffusion time for the slab during soaking to achieve the uniformity of slab quality, ensuring the stability of rolling and the consistency of mechanical properties. The total in-furnace heating time can be 180 min to 220 min, ensuring sufficient diffusion time for the slab during heating to achieve the uniformity of slab quality, ensuring the stability of rolling and the consistency of mechanical properties. Exemplarily, the soaking section temperature can be 1220°C, 1230°C, 1240°C, 1250°C, etc.; the soaking section time can be 35 min, 38 min, 40 min, 42 min, 43 min, 45 min, etc.; the total in-furnace heating time can be 180 min, 190 min, 200 min, 210 min, 220 min, etc.

[0045] S2. The slab after in-furnace heating is successively subjected to rough rolling, finish rolling, and cooling to obtain a hot-rolled sheet.

[0046] Rough rolling can break the large as-cast grains of the slab and improve the internal structure; finish rolling can further reduce the thickness of the slab and control the shape, and ensure the dimensional accuracy and surface quality of the hot-rolled sheet; cooling the hot-rolled sheet after finish rolling, which has a relatively high temperature, can make the structure of the hot-rolled sheet fine and improve the mechanical properties of the steel.

[0047] In some embodiments, the process parameters of the rough rolling include: the total reduction ratio of rough rolling is ≥85%, the reduction ratio of the fifth pass of R2 is ≥30%, the rough rolling exit temperature is ≥1150°C, and the final thickness of rough rolling is 28 mm to 34 mm; and / or,

[0048] The process parameters of the finish rolling include: the finish rolling exit temperature is 880°C to 920°C, and the reduction ratio of the last pass of finish rolling is ≤10%.

[0049] In the embodiments of the present application, the total rough rolling reduction ratio can be ≥85%. The rough rolling is in the high-temperature zone, and large reduction is beneficial to the occurrence of recrystallization, obtaining a uniform and fine structure. The reduction ratio of the fifth pass of R2 can be ≥30%. In the fifth pass, recrystallization is not likely to occur due to the decrease in rolling temperature. To ensure the occurrence of recrystallization, the rolling reduction ratio is increased to ensure recrystallization and obtain a uniform and fine structure. The rough rolling exit temperature can be ≥1150 °C to ensure that the intermediate billet is fully recrystallized during hot rolling and obtain a uniform and fine structure. The final thickness of the rough rolling can be 28 mm to 34 mm, which is determined according to the requirements of the subsequent finish rolling process and the specifications of the finished sheet. An appropriate final thickness of the rough rolling can not only ensure sufficient reduction in the finish rolling to accurately control the flatness and dimensional accuracy, but also ensure the stability of the subsequent finish rolling and the product quality. Exemplarily, the total rough rolling reduction ratio can be 85%, 86%, 87%, 88%, etc.; the reduction ratio of the fifth pass of R2 can be 30%, 31%, 32%, 33%, 34%, 35%, etc.; the rough rolling exit temperature can be 1150 °C, 1152 °C, 1155 °C, 1160 °C, 1170 °C, etc.; the final thickness of the rough rolling (intermediate billet thickness) can be 28 mm, 29 mm, 30 mm, 31 mm, 32 mm, 33 mm, 34 mm, etc. In addition, the rough rolling adopts a 1+5 mode.

[0050] The finish rolling exit temperature can be 880 °C to 920 °C. Since the thickness of the hot rolled coil is relatively thin, a higher finishing temperature is beneficial to reducing the deformation resistance and ensuring the stability of thin gauge rolling. The reduction ratio of the last pass of the finish rolling can be ≤10%. Rolling with a small reduction ratio in the last pass can avoid poor flatness caused by excessive deformation, such as edge waves and center waves. At the same time, a smaller reduction ratio also helps to reduce the damage of the rolling roll to the surface of the sheet and improve the surface finish. Exemplarily, the finish rolling exit temperature can be 880 °C, 890 °C, 900 °C, 910 °C, 920 °C, etc.; the reduction ratio of the last pass of the finish rolling can be 10%, 9%, 8%, 7%, etc. In addition, the finish rolling rolls the intermediate billet through 7 passes, and the above cooling is laminar cooling to cool the strip steel to the coiling temperature, and the cooling adopts an upper 2 and lower 2 mode.

[0051] S3. Coil the hot rolled sheet and control the coiling temperature so that the hot rolled sheet has uniform and fine ferrite grains to obtain a hot rolled coil;

[0052] The hot rolled sheet is coiled, and the coiling temperature is controlled so that the hot rolled sheet has uniform and fine ferrite grains, thereby ensuring the mechanical properties of the hot rolled sheet.

[0053] In some embodiments, the coiling temperature is 600 °C to 650 °C.

[0054] In the embodiments of the present application, the coiling temperature can be 600°C to 650°C. High-temperature coiling is beneficial to obtaining uniform and moderately sized ferrite grains. If the coiling temperature is higher than 650°C, it may cause the strength of the hot-rolled coil to be too low, affecting the strength of the finished product and resulting in insufficient strength of the finished product. If the coiling temperature is lower than 600°C, it may cause medium- and low-temperature structures to appear in the hot-rolled coil, which is not conducive to the elongation of the finished product. Exemplarily, the coiling temperature can be 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, etc.

[0055] S4. Subject the hot-rolled coil to pickling and cold rolling in sequence, and control the reduction ratio of the cold rolling to obtain a cold hard sheet.

[0056] Subjecting the hot-rolled coil to pickling in sequence can remove the scale on the surface of the hot-rolled coil, making the hot-rolled coil have good surface quality. After pickling, the hot-rolled coil is cold-rolled, and by controlling the reduction ratio of this cold rolling, the strength and plasticity of the pickled hot-rolled coil can be balanced.

[0057] In some embodiments, the reduction ratio of the cold rolling is 50% to 60%.

[0058] In the embodiments of the present application, the reduction ratio of the cold rolling can be 50% to 60%, which can fully balance the strength and plasticity of the pickled hot-rolled coil. If the reduction ratio of the cold rolling is lower than 50%, it may cause insufficient strength of the cold hard sheet. If the reduction ratio of the cold rolling is higher than 60%, it may cause a significant increase in the strength of the cold hard sheet, a significant decrease in plasticity, and a significant decrease in cold forming ability, making it difficult to meet the requirements of subsequent roll forming. Exemplarily, the reduction ratio of the cold rolling can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, etc.

[0059] In some embodiments, the thickness of the 600 MPa grade high-strength steel is 0.6 mm to 2.0 mm.

[0060] In the embodiments of the present application, the thickness of the 600 MPa grade high-strength steel can be 0.6 mm to 2.0 mm, which is a thin-gauge high-strength steel. Exemplarily, the thickness of the 600 MPa grade high-strength steel can be 0.6 mm, 0.7 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2.0 mm, etc.

[0061] S5. Subject the cold hard sheet to continuous annealing and control the temperature of the continuous annealing to regulate the dislocation density of the cold hard sheet to obtain 600 MPa grade high-strength steel.

[0062] The cold hard plate is subjected to continuous annealing, and the temperature of the continuous annealing is controlled to regulate the dislocation density of the cold hard plate, ensuring that the flattened ferrite does not recrystallize, thereby ensuring the high strength of the steel. Moreover, compared with batch annealing, the heating and holding time of the batch annealing process needs to reach more than 10 hours, resulting in high costs. While continuous annealing has a short time and a low temperature, saving energy and reducing costs. Also, the annealing time is short (the strip running speed is 30m / min - 120m / min), with extremely high efficiency and cost savings. In the embodiments of the present application, the continuous annealing is a semi-annealing process, mainly including a heating section, a soaking section, and a cooling section. The temperature of the continuous annealing refers to the temperature of the soaking section.

[0063] In some embodiments, the temperature of the continuous annealing is 540°C - 590°C.

[0064] In the embodiments of the present application, the temperature of the continuous annealing can be 540°C - 590°C, fully controlling the dislocation density of the cold hard plate, and the flattened ferrite does not recrystallize, thereby ensuring good strength-plasticity matching of the finished product. If the temperature of the continuous annealing is higher than 590°C, it may cause the flattened ferrite structure to recrystallize and the ferrite to become an equiaxed structure, resulting in a significant decrease in the strength of the finished product. If the temperature of the continuous annealing is lower than 540°C, it may cause insufficient recovery of the flattened ferrite and the plasticity cannot be guaranteed. Exemplarily, the temperature of the continuous annealing can be 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, etc.

[0065] In some embodiments, after subjecting the cold hard plate to continuous annealing and controlling the temperature of the continuous annealing to regulate the dislocation density of the cold hard plate to obtain a 600MPa grade high-strength steel, the following steps are further included:

[0066] The 600MPa grade high-strength steel is leveled, and the elongation of the leveling is 0.3% - 0.6%.

[0067] In the embodiments of the present application, the 600MPa grade high-strength steel can also be leveled, and the elongation of the leveling can be 0.3% - 0.6%, which can ensure that the shape of the product is flat and free of warping. Exemplarily, the elongation of the leveling can be 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0068] In some embodiments, the 600MPa grade high-strength steel meets the following mechanical properties: the yield strength is ≥500MPa, the tensile strength is ≥600MPa, the elongation is ≥10%, and it meets the requirement that the 180°d = 1a bend does not crack.

[0069] In the embodiments of the present application, the 600MPa grade high-strength steel has excellent mechanical properties, and the aspect ratios of ferrite and pearlite do not exceed 5. Figure 2The metallographic structure diagram of a 600 MPa grade high-strength steel provided by an embodiment of the present application; please refer to Figure 2 , which are flattened and elongated ferrite and pearlite structures. The gray is the ferrite structure and the black is the pearlite structure. Both of these structures show a flattened and elongated morphology.

[0070] The embodiment of the present application provides a preparation method for a 600 MPa grade high-strength steel, which has the following advantages:

[0071] 1. The set chemical composition only includes C, Si, Mn, P, S, Alt and Fe, which reduces the alloy cost from the source. While ensuring that the strength of the steel reaches the 600 MPa level, the high-cost problem caused by the use of expensive alloying elements is avoided;

[0072] 2. Adopting a low C - low Mn series ultra-low-cost design, combined with appropriate hot rolling process, acid rolling process and annealing process, high-strength steel with a thickness of 0.6 mm to 2.0 mm and excellent cold forming performance can be produced. The yield strength reaches more than 500 MPa, the tensile strength reaches more than 600, the elongation rate reaches more than 10%, and it has excellent sheet shape quality and surface quality, meeting the requirement of no cracking for 180° d=a bending. Compared with the existing processes such as hot rolling, continuous casting and rolling, cold rolling with bell annealing, and cold rolling with continuous annealing at the same level, under the performance of meeting the tensile strength of 600 MPa grade and elongation rate ≥10%, its production cost is the lowest and the cost performance is the best.

[0073] In the second aspect, the embodiment of the present application provides a 600 MPa grade high-strength steel prepared by the method described in any one of the embodiments in the first aspect.

[0074] This 600 MPa grade high-strength steel is realized based on the preparation method of the above-mentioned 600 MPa grade high-strength steel. The specific steps of the preparation method of this 600 MPa grade high-strength steel can refer to the above embodiments. Since this 600 MPa grade high-strength 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.

[0075] The following combines specific embodiments to further elaborate the present application. 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 noted in the following embodiments are usually measured according to national standards. If there is no corresponding national standard, they are carried out according to general international standards, conventional conditions or the conditions recommended by the manufacturer.

[0076] The embodiment of the present application provides a preparation method for 600MPa grade high-strength steel. For the chemical composition of the 600MPa grade high-strength steel, please refer to Table 1. For the process parameters of in-furnace heating, rough rolling and finish rolling of the 600MPa grade high-strength steel, please refer to Table 2. For the process parameters of coiling, cold rolling, continuous annealing and temper rolling of the 600MPa grade high-strength steel, please refer to Table 3. For the mechanical properties and microstructures of the 600MPa grade high-strength steel, please refer to Table 4.

[0077] Table 1 Chemical composition (wt%) of 600MPa grade high-strength steel, the rest is Fe and inevitable impurities

[0078] Serial number C Si Mn P S Alt Example 1 0.065 0.15 1.20 0.0010 0.0025 0.030 Example 2 0.080 0.14 1.10 0.0010 0.0030 0.035 Example 3 0.090 0.08 0.90 0.0012 0.0026 0.035 Example 4 0.10 0.04 0.85 0.0011 0.0018 0.025 Example 5 0.090 0.2 1.25 0.0010 0.0030 0.05 Comparative example 1 0.065 0.15 1.20 0.0010 0.0025 0.030 Comparative example 2 0.065 0.15 1.20 0.0010 0.0025 0.030

[0079] Table 2 Process parameters of in-furnace heating, rough rolling and finish rolling of 600MPa grade high-strength steel

[0080]

[0081]

[0082] Table 3 Process parameters of coiling, cold rolling, continuous annealing and temper rolling of 600MPa grade high-strength steel

[0083]

[0084] Table 4 Mechanical properties and microstructures of 600MPa grade high-strength steel

[0085]

[0086]

[0087] In summary, as can be seen from Table 4, the 600MPa grade high-strength steel prepared in the embodiment of the present application meets the following mechanical properties: the yield strength is ≥500MPa, the tensile strength is ≥600MPa, the elongation is ≥10%, it meets the requirement that no crack occurs in the 180°d = 1a bending, and the volume fraction of ferrite is 90% - 94%. In Comparative Example 1, the cold rolling rate is too high and the annealing temperature is too high, which will cause the recrystallization of flattened ferrite, resulting in insufficient strength; in Comparative Example 2, the coiling temperature is too low and the annealing temperature is too low, which will cause a significant increase in strength and a low elongation.

[0088] One or more technical solutions in the embodiment of the present application at least further have the following technical effects or advantages:

[0089] (1) The 600MPa grade high-strength steel prepared in the embodiment of the present application meets the following mechanical properties: the yield strength is

[0090] ≥500 MPa, the tensile strength is ≥600 MPa, the elongation is ≥10%, it meets the requirement that there is no cracking in the 180° bending with d = 1a, and it has excellent sheet shape and surface quality;

[0091] (2) By adopting a very low-cost design of the low C - low Mn system and combining with appropriate hot rolling process, pickling and cold rolling process, and annealing process, high-strength steel with a thickness of 0.6 - 2.0 mm and excellent cold forming performance can be produced. This method takes into account both the low cost and mechanical properties of 600 MPa grade high-strength steel.

[0092] The above are only the specific implementation manners of the present application, which enable 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 600 MPa grade high-strength steel, the method comprising: Heating a slab with a set chemical composition in a furnace, the set chemical composition including C, Si, Mn, P, S, Alt, and Fe; Successively subjecting the slab after in-furnace heating to rough rolling, finish rolling, and cooling to obtain a hot-rolled sheet; Coiling the hot-rolled sheet and controlling the coiling temperature so that the hot-rolled sheet has uniformly fine ferrite grains to obtain a hot-rolled coil; Successively pickling and cold rolling the hot-rolled coil and controlling the reduction rate of the cold rolling to obtain a cold-rolled hard sheet; Continuously annealing the cold-rolled hard sheet and controlling the continuous annealing temperature to regulate the dislocation density of the cold-rolled hard sheet to obtain a 600 MPa grade high-strength steel.

2. The method according to claim 1, wherein The coiling temperature is 600 °C to 650 °C.

3. The method according to claim 1, wherein The reduction rate of the cold rolling is 50% to 60%.

4. The method according to claim 1, characterized in that The continuous annealing temperature is 540 °C to 590 °C.

5. The method according to claim 1, wherein By mass fraction, the content of C is 0.060% to 0.10%, the content of Si is ≤0.25%, the content of Mn is 0.85% to 1.25%, the content of P is ≤0.015%; the content of S is ≤0.005%, and the content of Alt is 0.02% to 0.05%.

6. The method according to claim 1, wherein The process parameters of the in-furnace heating include: the soaking section temperature is 1220 °C to 1250 °C, the soaking section time is ≥35 min, and the total in-furnace heating time is 180 min to 220 min; and / or, The process parameters of the rough rolling include: the total rough rolling reduction rate is ≥85%, the reduction rate of the fifth pass of R2 is ≥30%, the rough rolling exit temperature is ≥1150 °C, and the final thickness of the rough rolling is 28 mm to 34 mm; and / or, The process parameters of the finish rolling include: the finish rolling exit temperature is 880 °C to 920 °C, and the reduction rate of the last pass of the finish rolling is ≤10%.

7. The method according to claim 1, wherein The thickness of the 600 MPa grade high-strength steel is 0.6 mm to 2.0 mm.

8. The method according to claim 1, characterized in that, After continuously annealing the cold-rolled hard sheet and controlling the continuous annealing temperature to regulate the dislocation density of the cold-rolled hard sheet to obtain a 600 MPa grade high-strength steel, it further includes: Leveling the 600 MPa grade high-strength steel, and the elongation of the leveling is 0.3% to 0.6%.

9. The method according to claim 1, wherein The 600 MPa grade high-strength steel meets the following mechanical properties: the yield strength is ≥500 MPa, the tensile strength is ≥600 MPa, the elongation is ≥10%, and it meets the requirement that the 180°d = 1a bending does not crack.

10. A 600 MPa grade high-strength steel prepared by the method according to any one of claims 1 to 9.

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