Multipurpose high-strength steel for automobiles and method for manufacturing the same

Abstract

The application belongs to cold-rolled high-strength steel for automobiles, and particularly relates to a multipurpose high-strength steel for automobiles and a preparation method thereof. The chemical components of the steel include, in mass percentage, C: 0.12-0.16%, Mn: 1.80-2.30%, Si: 0.60-1.30%, Al: 0.015-0.5%, P: 0.007-0.012%, S: 0.001-0.004%, and the balance of Fe and inevitable impurities. The method comprises the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous annealing and galvanizing. The application proposes to realize the coverage of various categories and strength steel grades by using the same or similar component steel plates through different heat treatment processes, so as to reduce the casting mixed pouring section scrap steel, improve the rolling production efficiency, and optimize the management process on the production end; and to reduce the welding difficulty and broaden the application range of the material on the application end.

Description

Technical Field

[0001] This invention belongs to the technical field of cold-rolled high-strength steel for automobiles, specifically relating to a multi-purpose high-strength steel for automobiles and its preparation method. Background Technology

[0002] Major CO2-emitting industries include cement, steel, and automobiles, with the automotive industry accounting for a significant portion. Considering global CO2 emissions, both the steel and automotive industries are high-carbon emitters, with the automotive industry's main carbon emission pathway concentrated in the steel production stage. Therefore, achieving "dual carbon" in automotive steel requires effective improvement measures. Lightweighting is currently a primary focus for the automotive industry and automotive steel manufacturers. Clearly, reducing material usage through high-strength steel products in automobiles ensures safety at the production end while effectively reducing steel output, and achieves weight reduction and emissions reduction at the application end. However, from the current development perspective of high-strength steel, the market demand for automotive high-strength steel remains limited, and it needs to cater to personalized customization, resulting in a wide variety of products. Taking QP980 steel as an example, products cover high-ductility QP980 products, high-hole-expanding QP980 products, high-ductility galvanized QP980 products, and high-hole-expanding galvanized QP980 products, etc. A single type of steel may involve several, or even a dozen, grades. Producing different grades of products requires corresponding component designs, significantly increasing production costs and carbon emissions during the production process. Therefore, addressing the issue of the diverse types of high-strength steel produced for automobiles is crucial for reducing costs for steel companies and promoting carbon reduction.

[0003] Chinese patent CN11979488A discloses a 780MPa grade alloyed hot-dip galvanized DH steel and its preparation method. The weight percentages of the steel plate components are as follows: C: 0.11-0.17%, Mn: 1.4-2.4%, Si: 0.15-0.60%, Al: 0.02-1.0%, Mo: 0.20-0.70%, P≤0.03%, S≤0.03%, B≤0.005%, V≤0.05%, Ti≤0.05%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, and continuous annealing galvanizing, resulting in DH galvanized steel plates with a tensile strength of 780-880MPa and an elongation of 20-25%. The key idea behind this steel is to introduce retained austenite through an alloy design with a high proportion of C and Mn within the 780MPa range, thereby improving the plasticity of the steel plate.

[0004] Chinese patent CN110983198A discloses an alloyed hot-dip galvanized duplex steel and its preparation method. The alloy composition of the steel is C: 0.10-0.15%, Si: 0.2-0.5%, Mn: 2.5-2.9%, Al: 0.02-0.05%, Ti: 0.015-0.03%, Nb: 0.015-0.03%, Cr: 0.4-0.6%, Mo: 0.1-0.3%, P≤0.01%, S≤0.01%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, and continuous annealing galvanizing processes to obtain a steel plate with a tensile strength of over 1180 MPa.

[0005] Chinese patent CN111118397A discloses a 980MPa quenched steel and its preparation method. The alloy composition of this steel is C: 0.20-0.25%, Si: 1.4-1.8%, Mn: 1.8-2.2%, V: 0-0.10%, Nb: 0-0.050%, Ti: 0-0.050%, P≤0.010%, S≤0.012%, Al≤0.060%, with the balance being Fe and other unavoidable impurities. High-strength cold-rolled annealed steel sheets with an elongation of 20-25% and a strength of 980-1150MPa are produced.

[0006] The aforementioned DH steel, DP steel, and QP steel have different alloy composition design systems, exhibiting different strength-ductility performance combinations. The design of multi-purpose high-strength steel that achieves multiple mechanical properties while meeting various application performance indicators, thus fulfilling the requirement of a single steel for multiple uses, remains a gap in the market. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a multi-purpose high-strength steel for automobiles and its preparation method. Using the same alloy composition system, multi-purpose high-strength steel can be prepared by only changing the subsequent process.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] This invention provides a multi-purpose high-strength steel for automobiles, wherein the chemical composition of the steel, by mass percentage, comprises:

[0010] C: 0.12–0.16%, Mn: 1.80–2.30%, Si: 0.60–1.30%, Al: 0.015–0.5%, P: 0.007–0.012%, S: 0.001–0.004%, with the balance being Fe and unavoidable impurities.

[0011] In the above technical solution, the chemical composition of the steel further includes one or more of Ni, Cr, Mo, Nb, and Ti, wherein, by mass percentage, Ni: 0.10–0.30%, Cr: 0.10–0.30%, Mo: 0.05–0.30% and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.015–0.025%, and Ti: 0.01–0.025%.

[0012] The selection principles and content design rationale for the various chemical components of the steel of this invention are as follows:

[0013] C: Carbon (C) is a common strengthening element in steel. As an interstitial solid solution atom, C in the matrix increases the strength of the solid solution by causing lattice distortion. In the multi-purpose automotive high-strength steel of this invention, C's role in DH and QP steels is mainly to ensure the stability of retained austenite, while in DP steel it mainly improves hardenability in the critical zone and ensures the martensite transformation content. However, in this invention, the overall C content is in the low C range, taking into account resistance spot welding performance, hot-rolled edge cracking, and cold-rolled edge cracking. Therefore, the C content in this invention is controlled at 0.12%–0.16%.

[0014] Mn: Mn is a low-cost element, and Mn atoms strengthen solid solutions by inducing lattice distortion through substitutional solid solution. In this invention, the addition of Mn is the main element for obtaining 780–980 MPa. However, the added Mn content should not exceed the range of this invention, considering the C / Mn segregation problem caused by excessive Mn content, and the problem that excessive Mn in DH steel leads to the inhibition of bainite formation and improved hardenability. Therefore, this invention comprehensively considers the composite addition content of Mn / Cr / Mo / Ni. In this invention, the Mn content is controlled at 1.80–2.30%.

[0015] Si: Si is one of the key elements in this invention. Sufficient Si addition ensures the matrix strength of the ferrite. Simultaneously, Si addition increases the AC3 point of the steel plate, effectively adjusting the annealing process window during continuous annealing and ensuring an appropriate ratio of ferrite and austenite in the critical region at industrial continuous annealing temperatures. Furthermore, sufficient Si content can suppress the formation of carbides during the over-aging stage, preventing performance degradation due to carbide precipitation. In this invention, Si is within the moderate range for high-strength steel. Excessive Si content leads to surface quality issues such as "incomplete galvanizing" on the galvanized surface, and increases the difficulty of pickling (due to the internal oxide layer and grain boundary oxide layer). The Si content cannot be too low, as this will fail to suppress carbide precipitation, which is the main application of QP and DH steels. In this invention, the Si content is controlled between 0.60% and 1.30%.

[0016] Al: In this invention, Al is used only as a deoxidizer. This invention aims to solve the problem of "multiple uses for one type of steel," therefore, adding Al would significantly increase costs and make continuous casting more difficult. In this invention, the Al content is controlled at 0.015–0.5%.

[0017] P: P is an impurity element in steel, which readily agglomerates at grain boundaries. When the P content in steel is high, Fe2P particles are easily formed, reducing the steel's plasticity and toughness. Therefore, the lower the P content, the better. In this invention, the P content is controlled at 0.007–0.012%.

[0018] S: S is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which worsens the plasticity of the steel plate. Therefore, the lower its content, the better. In this invention, the S content is controlled at 0.001-0.004%.

[0019] You can also add elements:

[0020] Ni: Ni itself is a solid solution strengthening element, similar to C and Mn, improving the stability of austenite. At the same time, Ni also improves the corrosion resistance of steel plates to a certain extent. It can be added in appropriate amounts to the optional components of this invention to enhance corrosion resistance. In this invention, the Ni content is controlled at 0.10–0.30%.

[0021] Cr and Mo: Cr and Mo are solid solution strengthening elements, which strengthen the steel plate. In this invention, Cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite during the cooling stage, and promote the formation of martensite. Simultaneously, Cr and Mo can change the type of iron oxide scale during the coiling process, limit the oxidation within the steel plate, and improve the surface quality of the steel plate. In this invention, Cr and Mo are added after Mn addition, balancing the problems of edge cracking in hot rolling and edge cracking in cold rolling. In this invention, the Cr content is controlled at 0.10–0.50%, and the Mo content is controlled at 0.05–0.30%.

[0022] As mentioned earlier, alloying elements such as Ni, Cr, and Mo are all substitutes for Mn, and their main role in this invention is to improve the stability of austenite. However, considering factors such as cost, casting difficulty, hot rolling difficulty, and cold rolling difficulty, the overall addition should meet the integrated goals of low cost, ease of production, and high yield. Mn+Ni+Cr+Mo ≤ 2.6%.

[0023] Nb: Nb is a microalloying strengthening element. It is added to Ti in the production of DP and DH series products to refine the grain size, increase the yield strength ratio, and thus improve the subsequent flanging performance of the steel sheet. In this invention, the Nb content is controlled at 0.015–0.025%.

[0024] Ti: In this invention, Ti is appropriately added as a strength supplement. In the "one steel for multiple uses" process, some planned compositions cannot meet the strength requirements. The precipitation of Ti helps to refine the original austenite grains, strengthen the grains, and supplement the strength through precipitation strengthening. In this invention, the Ti content is controlled at 0.015-0.025%.

[0025] Another aspect of the present invention provides a method for preparing the above-mentioned multi-purpose high-strength steel for automobiles, the method comprising the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous annealing galvanizing; the specific steps of the method are as follows:

[0026] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel;

[0027] (2) Hot rolling: The billet is heated and then subjected to rough rolling, fine rolling and coiling into hot rolled coil;

[0028] (3) Pickling and cold rolling: Cold rolling is carried out after pickling;

[0029] (4) Continuous annealing galvanizing: The cold-rolled steel plate is heated to 820-950℃, isothermaled for 35-120s, then slowly cooled to 700-780℃ at a cooling rate of 1.2-3.6℃ / s, then cooled to 380-470℃ at a rate of 15-25℃ / s, isothermaled for 15-25s, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 480-560℃, and the alloying time is 15-25s. In the above technical solution, further, in step (1), the casting temperature is 1580-1620℃, and the billet thickness is 220-280mm.

[0030] In the above technical solution, further, in step (2), the heating temperature is 1230~1280℃, the furnace time is 180~240min, the rough rolling temperature is 1150~1200℃, the intermediate billet thickness is 50~80mm, the finishing rolling is divided into two stages, the recrystallization rolling temperature is 1070~1130℃, the final rolling temperature is above 920℃, the coiling temperature is between 450~520℃, and the hot-rolled coil thickness is 2.8~3.5mm.

[0031] The heating temperature is controlled at 1230–1280℃, and the furnace time is 180–240 min. The purpose is to promote full solid solution of the alloy and control the banded structure caused by segregation. The two-stage rolling in the finishing rolling stage aims to promote the recrystallization of the original austenite grains and inhibit the coarsening of unrecrystallized austenite grains. The coiling temperature is controlled at 450–520℃ to prevent the formation of Si-rich oxides on the surface of the steel plate after adding Si, which would lead to the formation of internal oxide layers and grain boundary oxide layers.

[0032] In the above technical solution, further, in step (3), the thickness of the cold-rolled plate is 1.4 / 1.6 / 1.8mm, the 1.4mm plate thickness corresponds to the 2.8mm hot-rolled steel plate, the 1.6mm and 1.8mm plate thicknesses correspond to the 3.0~3.5mm hot-rolled steel plates, and the cold rolling reduction rate is 46.7~48.6%.

[0033] Too low a rolling reduction rate cannot guarantee sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization during the continuous annealing stage; too high a rolling reduction rate significantly increases the load on the cold rolling mill, which cannot guarantee the achievement of the target thickness.

[0034] In the above technical solution, further, in step (4), the continuous annealing galvanizing process is as follows: the cold-rolled steel plate is heated to 820-850℃, isothermaled for 60-105s, then slowly cooled to 700-740℃ at a cooling rate of 1.2-3.6℃ / s, then cooled to 380-460℃ at a rate of 15-25℃ / s, isothermaled for 15-25s, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 495-525℃, and the alloying time is 15-25s. The resulting steel product meets the requirements of DH780 standard.

[0035] A high proportion of ferrite microstructure is obtained through isothermal treatment in the critical region. Simultaneously, the carbon concentration in the austenite in the critical region increases during ferrite formation. After slow cooling, a relatively obvious concentration gradient appears in the undercooled austenite. In the 380–460℃ stage, bainite formation is promoted, and carbon enrichment continues to extend to the surrounding undercooled austenite along the concentration gradient. After alloying, the C-rich and smaller-grained undercooled austenite is retained at room temperature to participate in the austenite form, while the relatively C-poor and larger-grained undercooled austenite undergoes martensitic transformation.

[0036] In the above technical solution, further, in step (4), the continuous annealing galvanizing process is as follows: the cold-rolled steel plate is heated to 860-900℃, isothermal for 35-45s, then slowly cooled to 750-780℃ at a cooling rate of 1.2-3.6℃ / s, then cooled to 380-460℃ at a rate of 15-25℃ / s, isothermal for 15-25s, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 495-505℃, and the alloying time is 15-25s. The resulting steel product meets the requirements of the DH980 standard.

[0037] A high proportion of austenitic microstructure is obtained through isothermal treatment in the critical region. Excessive epitaxial ferrite formation is suppressed by slow cooling to ensure yield strength. Bainite formation is promoted in the 380–460℃ range, while carbon enrichment continuously extends into the surrounding supercooled austenite along the concentration gradient. After alloying, the carbon-rich, smaller-grained supercooled austenite remains at room temperature to participate in austenite formation, while the carbon-depleted, larger-grained supercooled austenite undergoes martensitic transformation. Adjusting the alloying temperature prevents excessive carbide formation from affecting the steel plate's strength.

[0038] In the above technical solution, further, in step (4), when the product steel meets the requirements of the DP980 standard, the continuous annealing galvanizing process is as follows: the cold-rolled steel plate is heated to 860-900℃, isothermaled for 100-120s, then slowly cooled to 750-780℃ at a cooling rate of 1.2-3.6℃ / s, then cooled to 380-460℃ at a rate of 15-25℃ / s, isothermaled for 15-25s, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 530-560℃, and the alloying time is 15-25s. The resulting product steel meets the requirements of the DP980 standard.

[0039] A high proportion of austenitic microstructure is obtained through isothermal treatment in the critical region, and increasing the isothermal time promotes coarsening and equalizes the C concentration. Slow cooling is used to suppress the formation of excessive epitaxial ferrite to ensure yield strength, and controlling the alloying temperature promotes the formation of secondary martensite.

[0040] In the above technical solution, further, in step (4), the continuous annealing galvanizing process is as follows: the cold-rolled steel plate is heated to 920-950℃, isothermal for 60-100s, then slowly cooled to 750-780℃ at a cooling rate of 1.2-3.6℃ / s, then cooled to 340-400℃ at a rate of 15-25℃ / s, then heated to 450-470℃, isothermal for 15-25s, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 480-495℃, and the alloying time is 15-25s. The resulting steel product meets the requirements of QP980 standard.

[0041] A fully austenitic microstructure is obtained through isothermal treatment in the critical region. Excessive epitaxial ferrite formation is suppressed by slow cooling to ensure yield strength. Martensitic transformation is promoted in the 340–400℃ range while retaining a corresponding amount of residual austenite. Isothermal treatment is then carried out by raising the temperature, and the martensite undergoes tempering to form tempered martensite. The residual austenite is rich in carbon, which improves stability. Alloying continues, and excessively high alloying temperatures are prevented from causing the precipitation of tempered martensite carbides.

[0042] The beneficial effects of this invention are as follows:

[0043] This invention proposes to use steel plates with the same or similar composition and different heat treatment processes to cover multiple types and strengths of steel. On the production side, this will reduce scrap steel in the casting mixing section, improve rolling production efficiency, and optimize management processes. On the application side, it can reduce welding difficulty and broaden the application range of materials.

[0044] The application of this invention can achieve energy conservation and emission reduction in the production and application processes, which is in line with the national "dual carbon" policy. Attached Figure Description

[0045] Figure 1The microstructure diagrams are of the four types of steel obtained from composition-1 according to the present invention. a is DH780-1, b is DH980-1, c is DP980-1, and d is QP980-1. Detailed Implementation

[0046] The present invention will be described in more detail through embodiments. These embodiments are merely descriptions of the best mode of the invention and do not limit the scope of the invention in any way.

[0047] Table 1 lists the chemical composition of the steels used in the examples.

[0048] Table 1 Chemical composition of the steel in the examples, wt%.

[0049] Example C Mn Si Al Ni Cr Mo Mn+Ni+Cr+Mo Ti Nb P S Ingredients-1 0.148 2.23 0.95 0.015 - - - 2.23 0.025 - 0.009 0.002 Ingredients-2 0.126 2.28 0.75 0.42 0.15 - 0.05 2.48 0.018 0.022 0.011 0.001 Ingredients-3 0.158 2.08 1.02 0.48 0.1 0.12 0.15 2.45 0.022 - 0.012 0.003 Ingredients-4 0.135 2.15 0.96 0.015 0.13 0.18 2.46 0.015 0.019 0.007 0.004 Ingredients-5 0.145 2.31 1.23 0.24 0.15 2.46 0.018 0.018 0.008 0.002

[0050] A multi-purpose high-strength steel for automobiles, the preparation method includes the following steps:

[0051] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, the casting temperature is 1580~1620℃, and the billet thickness is 220~280mm;

[0052] (2) Hot rolling: The heating temperature is between 1230 and 1280℃, the furnace time is 180 to 240 min, the rough rolling temperature is 1150 to 1200℃, the intermediate billet thickness is 50 to 80 mm, the finishing rolling is divided into two stages, the recrystallization rolling temperature is between 1070 and 1130℃, the final rolling temperature is above 920℃, the coiling temperature is between 450 and 520℃, and the hot-rolled coil thickness is between 2.8 and 3.5 mm;

[0053] (3) Pickling and cold rolling: After pickling, cold rolling is carried out. The thickness of the cold rolled plate is 1.4 / 1.6 / 1.8mm. The 1.4mm plate thickness corresponds to the 2.8mm hot rolled steel plate, and the 1.6 and 1.8mm plate thicknesses correspond to the 3.0~3.5mm hot rolled steel plates. The cold rolling reduction is controlled at 46.7~48.6%.

[0054] (4) Continuous annealing galvanizing: The cold-rolled steel plate is heated to 820-850℃ and isothermaled for 60-105s. Then it is slowly cooled to 700-740℃ at a cooling rate of 1.2-3.6℃ / s. Then it is cooled to 380-460℃ at a rate of 15-25℃ / s and isothermaled for 15-25s. Then it is put into the zinc pot and finally into the alloying furnace. The alloying temperature is 495-525℃ and the alloying time is 15-25s. The resulting steel product meets the requirements of DH780 standard.

[0055] The cold-rolled steel sheet is heated to 860–900℃ and held at a constant temperature for 35–45 seconds. Then, it is slowly cooled to 750–780℃ at a cooling rate of 1.2–3.6℃ / s, followed by cooling to 380–460℃ at a rate of 15–25℃ / s and held at a constant temperature for 15–25 seconds. The sheet is then placed in a zinc pot and finally in an alloying furnace. The alloying temperature is 495–505℃ and the alloying time is 15–25 seconds. The resulting steel product meets the requirements of the DH980 standard.

[0056] The cold-rolled steel sheet is heated to 860–900℃ and held at a constant temperature for 100–120 seconds. Then, it is slowly cooled to 750–780℃ at a cooling rate of 1.2–3.6℃ / s, followed by cooling to 380–460℃ at a rate of 15–25℃ / s and held at a constant temperature for 15–25 seconds. The sheet is then placed in a zinc pot and finally in an alloying furnace. The alloying temperature is 530–560℃ and the alloying time is 15–25 seconds. The resulting steel product meets the requirements of the DP980 standard.

[0057] The cold-rolled steel sheet is heated to 920–950℃ and held at a constant temperature for 60–100 seconds. Then, it is slowly cooled to 750–780℃ at a cooling rate of 1.2–3.6℃ / s, followed by cooling to 340–400℃ at a rate of 15–25℃ / s. The temperature is then raised to 450–470℃ and held at a constant temperature for 15–25 seconds. The sheet is then placed in a zinc pot and finally in an alloying furnace. The alloying temperature is 480–495℃ and the alloying time is 15–25 seconds. The resulting steel product meets the requirements of the QP980 standard.

[0058] Following the above preparation method, DH780-1, DH980-1, DP980-1, and QP980-1 were prepared using components-1 in Table 1, and so on.

[0059] Table 2 lists the continuous casting and hot rolling process parameters for the steel in the examples, and Table 3 lists the cold rolling and continuous annealing galvanizing process parameters for the steel in the examples.

[0060] Table 2. Continuous casting and hot rolling process parameters for the steel in the examples.

[0061]

[0062]

[0063] Table 3. Process parameters for cold rolling and continuous annealing galvanizing of steel in the examples.

[0064]

[0065] Table 4 shows the mechanical properties of the steels in the examples.

[0066] Table 4 Mechanical properties of the steel in the examples

[0067] Example F / ％ M / ％ TM / % RA / % B / ％ Rp0.2 / MPa Rm / MPa A80 / ％ λ / % DH780-1 58.6 20.3 - 6.5 6.8 454 834 24.6 56.3 DH780-2 54.2 22.5 - 7.2 8.9 462 836 25.4 48.6 DH780-3 53.8 21.4 - 5.8 8.4 458 846 23.8 49.2 DH780-4 52.7 15.8 - 5.4 9.6 443 827 26.2 52.1 DH780-5 58.2 18.6 - 6.3 9.2 459 852 25.1 53.4 DH980-1 39.4 26.5 - 8.2 6.7 705 1035 18.7 31.6 DH980-2 42.8 32.4 - 6.5 7.3 722 1046 19.2 34.5 DH980-3 44.2 34.2 - 7.8 5.9 716 1028 18.4 32.5 DH980-4 38.6 33.5 - 8.6 7.2 725 1034 20.1 33.7 DH980-5 38.5 29.7 - 9.2 7.8 706 1029 19.8 32.1 DP980-1 52.6 35.2 - - - 648 1062 16.5 24.5 DP980-2 57.4 38.4 - - - 652 1053 17.2 22.8 DP980-3 62.3 35.6 - - - 562 1048 16.8 21.6 DP980-4 54.1 34.7 - - - 598 1058 17.2 22.5 DP980-5 55.8 42.2 - - - 604 1066 16.3 25.6 QP980-1 12.4 - 68.4 11.8 6.8 798 1012 14.2 68.5 QP980-2 6.8 - 72.5 12.2 7.2 823 1036 14.8 72.4 QP980-3 9.6 - 71.8 10.9 7.8 812 1042 14.1 66.4 QP980-4 8.2 - 65.6 10.5 9.5 822 1018 15.8 68.2 QP980-5 11.8 - 74.2 11.6 9.2 805 1026 15.2 67.4

[0068] As shown in Table 4, the microstructure of DH780 consists of ferrite, retained austenite, bainite, and martensite, with ferrite content of 50-60%, martensite content of 10-25%, bainite content of 5-10%, and retained austenite content of 5-7.5%. The steel has a yield strength of over 440 MPa, a tensile strength of over 780 MPa, an elongation of over 23%, and a porosity of over 45%.

[0069] In DH980, the microstructure consists of ferrite + retained austenite + bainite + martensite, with ferrite content of 35-45%, martensite content of 25-35%, bainite content of 5-10%, and retained austenite content of 5-7.5%. The steel has a yield strength of over 700 MPa, a tensile strength of over 980 MPa, an elongation of over 16%, and a porosity of over 30%.

[0070] In DP980, the microstructure is ferrite + martensite, with ferrite content of 45% to 65% and martensite content of 35% to 55%; the steel has a yield strength of 550 MPa or more, a tensile strength of 980 MPa or more, an elongation of 16% or more, and a porosity of 20% or more.

[0071] In QPHE980, the microstructure consists of ferrite + retained austenite + bainite + tempered martensite, with ferrite content of 5-15%, tempered martensite content of 65-75%, bainite content of 5-10%, and retained austenite content of 10-12.5%. The steel has a yield strength of ≥780MPa, a tensile strength of ≥980MPa, an elongation of ≥14%, and a porosity of ≥65%.

[0072] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A multi-purpose high-strength steel for automobiles, characterized in that, The chemical composition of the steel, by mass percentage, includes: C: 0.12~0.145%, Mn: 1.80~2.30%, Si: 0.95~1.30%, Al: 0.015~0.5%, P: 0.007~0.012%, S: 0.001~0.004%, balance being Fe and unavoidable impurities; The chemical composition of the steel also includes one or more of Ni, Cr, Mo, Nb, and Ti, wherein, by mass percentage, Ni: 0.10~0.30%, Cr: 0.10~0.30%, Mo: 0.05~0.30% and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.015~0.025%, and Ti: 0.01~0.025%. The preparation method of the steel includes the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous annealing galvanizing. The specific steps of the method are as follows: (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel; (2) Hot rolling: The billet is heated and then subjected to rough rolling, finish rolling and coiling into hot rolled coil; (3) Pickling and cold rolling: Cold rolling is carried out after pickling; (4) Continuous annealing galvanizing: The cold-rolled steel plate is heated to 820~850℃, isothermal for 60~105s, then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, then cooled to 380~460℃ at a rate of 15~25℃ / s, isothermal for 15~25s, then put into the zinc pot, and finally into the alloying furnace. The alloying temperature is 495~525℃ and the alloying time is 15~25s. Alternatively, the cold-rolled steel sheet can be heated to 860~900℃, held at a constant temperature for 35~45s, then slowly cooled to 750~780℃ at a cooling rate of 1.2~3.6℃ / s, then cooled to 380~460℃ at a rate of 15~25℃ / s, held at a constant temperature for 15~25s, then placed in a zinc pot, and finally placed in an alloying furnace at an alloying temperature of 495~505℃ for 15~25s. Alternatively, the cold-rolled steel sheet can be heated to 860~900℃, held at a constant temperature for 100~120s, then slowly cooled to 750~780℃ at a cooling rate of 1.2~3.6℃ / s, then cooled to 380~460℃ at a rate of 15~25℃ / s, held at a constant temperature for 15~25s, then placed in a zinc pot, and finally placed in an alloying furnace at an alloying temperature of 530~560℃ for 15~25s. Alternatively, the cold-rolled steel sheet can be heated to 920~950℃, held at a constant temperature for 60~100s, then slowly cooled to 750~780℃ at a cooling rate of 1.2~3.6℃ / s, then cooled to 340~400℃ at a rate of 15~25℃ / s, then heated to 450~470℃, held at a constant temperature for 15~25s, then placed in a zinc pot, and finally placed in an alloying furnace at an alloying temperature of 480~495℃ for 15~25s.

2. The multi-purpose high-strength steel for automobiles according to claim 1, characterized in that, In step (1), the casting temperature is 1580~1620℃ and the billet thickness is 220~280mm.

3. The multi-purpose high-strength steel for automobiles according to claim 1, characterized in that, In step (2), the heating temperature is 1230~1280℃, the furnace time is 180~240min, the rough rolling temperature is 1150~1200℃, the intermediate billet thickness is 50~80mm, the finishing rolling is divided into two stages, the recrystallization rolling temperature is 1070~1130℃, the final rolling temperature is above 920℃, the coiling temperature is 450~520℃, and the hot-rolled coil thickness is 2.8~3.5mm.

4. The multi-purpose high-strength steel for automobiles according to claim 1, characterized in that, In step (3), the thickness of the cold-rolled sheet is 1.4 / 1.6 / 1.8mm. The 1.4mm sheet thickness corresponds to the 2.8mm hot-rolled steel sheet, and the 1.6mm and 1.8mm sheet thicknesses correspond to the 3.0~3.5mm hot-rolled steel sheets. The cold rolling reduction rate is 46.7~48.6%.

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