Super bainite steel and preparation method thereof

Abstract

The invention belongs to the field of steel, and particularly relates to super bainite steel and a preparation method thereof. The super bainite steel provided by the invention comprises the following chemical components in percentage by mass: 0.15 to 0.22 percent of C, 0.80 to 1.35 percent of Si, 1.8 to 2.3 percent of Mn, less than or equal to 0.01 percent of P, less than or equal to 0.005 percent of S, 0.01 to 0.12 percent of Als, 0.1 to 0.5 percent of Cr, 0.02 to 0.06 percent of Nb, 0.01 to 0.04 percent of Ti, less than or equal to 0.0045 percent of N and the balance of Fe and inevitable impurities, and meanwhile, Ceq is equal to C + Si / 30 + Mn / 20 + 2P + 4S < = 0.41. The super bainite steel provided by the invention is excellent in performance and can be widely applied to the field of automobile steel.

Description

Technical Field

[0001] The invention belongs to the field of steel materials, and in particular relates to super bainite steel and a preparation method thereof. Background Art

[0002] As people pay more attention to automobile lightweighting and collision safety, and with the increasing prominence of energy shortages and environmental pollution, high-strength automobile steel is an important way to achieve automobile lightweighting and improve safety performance. However, high-strength automobile steel is prone to cracking during cold forming, especially during local forming of complex parts (such as flanging, bending, etc.), which seriously restricts its promotion and application. Bainitic steel with bainite and retained austenite as the matrix structure and second phase respectively has attracted much attention. It avoids the problem of soft-hard phase hardness difference existing in dual-phase steel, and can improve the ductility and local forming performance of steel.

[0003] Compared with Q&P steel, the representative steel grade of the third generation of advanced high-strength steel of 1180MPa grade, the organizational characteristics of super bainite steel are composed of fine and regular carbide-free bainite lath bundles and residual austenite distributed between the lath bundles on the bainite matrix, and there is also a very small amount of tempered martensite structure. By designing the unique organizational structure of super bainite steel, it can ensure excellent strength and plasticity while having excellent local forming performance (such as flanging and hole expansion). Therefore, super bainite steel with TRIP effect has been developed in recent years to meet the automotive industry's demand for new steel plates with high strength, high plasticity and high local forming performance. Through the query of relevant patents, the patents that are similar to 1180MPa super bainite steel plates are as follows:

[0004] CN 110724877 A discloses a 1180MPa grade high plasticity bainite complex phase steel plate for automobiles and a preparation method thereof. The chemical composition of the steel plate by weight is as follows: C: 0.03-0.20, Si: 0.05-1.50%, Mn: 0.5-2.5%, P≤0.05%, S≤0.05%, Nb: 0.01-0.04%, Ti: 0.01-0.04%, V: 0.01-0.15%, Al: 0.015-1.5%, and replaceable components, by weight percentage, contain: Cr: 0-2%, Mo: 0-0.5%, Cu: 0.05-0.5%, and the balance is Fe and unavoidable impurities. The rolling process is: the temperature of the hot rolling ingot is between 400 and 750℃, the heating temperature is between 1150 and 1250℃, the rolling temperature is between 1150 and 1250℃, the final rolling temperature is above 900℃, and the coiling temperature is between 600 and 750℃; the annealing process is: the annealing temperature is between 780 and 880℃, the annealing time is between 1 and 5min, the cooling rate is greater than 30℃ / s, and the aging temperature is between 300 and 500℃, the aging time is between 300 and 600s, and then cooled to room temperature. The Si and Mn content added in this patent has a wide range, and the low Si and low Mn state is not enough to achieve the strength grade of the product. In addition, the hot rolling process only provides heating temperature, finishing rolling temperature and coiling temperature, and does not specify the cooling path and intermediate temperature control, which is not conducive to the stable control of product structure performance and surface quality.

[0005] CN 109778062 A discloses a cold-rolled complex phase steel with a tensile strength of 1200 MPa and a preparation method thereof. The chemical composition by weight percentage is: C: 0.1-0.15%, Si: 0.1-0.5%, Mn: 1.5-2.6%, Cr: 0.4-0.7%, Mo: 0.2-0.5%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, P≤0.02%, S≤0.015%, and the remainder is iron and other inevitable impurity elements; and the following conditions are satisfied at the same time: C+(Si+Mn) / 6+(Cr+Mo+V) / 5≤0.8. This patent uses Nb-Ti microalloying to prepare a cold-rolled complex phase steel with a tensile strength of 1200MPa. The product has a high Mn content (1.5-2.6%), which increases the thermal sensitivity of the plate during continuous casting and rolling annealing, which is not conducive to mass production; and the plasticity index needs to be improved. Summary of the invention

[0006] In view of this, an object of the present invention is to provide a super bainite steel and a preparation method thereof. The super bainite steel provided by the present invention has excellent performance and can be widely used in the field of automotive steel.

[0007] The invention provides a super bainite steel. The chemical composition of the super bainite steel is as follows, by mass percentage: C: 0.15-0.22%, Si: 0.80-1.35%, Mn: 1.8-2.3%, P≤0.01%, S≤0.005%, Als: 0.01-0.12%, Cr: 0.1-0.5%, Nb: 0.02-0.06%, Ti: 0.01-0.04%, N≤0.0045%, and the remaining elements are Fe and inevitable impurities, and at the same time, Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 is satisfied.

[0008] Preferably, the microstructure of the super bainite steel consists of 80-90 vol% bainite, 3-11 vol% martensite and 5-9 vol% retained austenite.

[0009] Preferably, the yield strength of the super bainite steel is 870-950 MPa, the tensile strength is 1180-1250 MPa, and the elongation A is 50 The value is 15-20%, and the hole expansion rate is 30-40%.

[0010] The present invention provides a method for preparing the super bainite steel described in the above technical solution, comprising the following steps:

[0011] (a) Smelting process: smelting molten steel according to the chemical composition of super bainite steel;

[0012] (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs;

[0013] (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, dephosphorizing, rough rolling, finish rolling, laminar cooling and coiling to obtain a hot rolled coil;

[0014] (d) Pickling and cold rolling process: the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil;

[0015] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is subjected to continuous annealing to obtain super bainite steel.

[0016] Preferably, in step (b), the continuous casting temperature is 1460-1550° C.; and the thickness of the slab is 120-240 mm.

[0017] Preferably, in step (c), the heating temperature is 1210-1270°C; the water pressure of the dephosphorization is ≥22MPa, and the temperature is ≥1173°C; the starting temperature of the finishing rolling is 1000-1100°C, the finishing passes are 7 passes, the final rolling temperature is 890-950°C, and the reduction rate of the last pass is ≤15%; the laminar cooling method is sparse cooling; the thickness of the hot-rolled coil is 2.3-4mm.

[0018] Preferably, in step (c), the coiling adopts a U-type coiling mode; when coiling, the coiling temperature of the strip in the first 20m is 560-650°C, the coiling temperature of the strip in the last 70m is 560-640°C, and the coiling temperature of the middle section is 580°C.

[0019] Preferably, in step (d), the cold rolling reduction rate is 40-70%; and the thickness of the cold-rolled thin strip steel is 1-2 mm.

[0020] Preferably, in step (e), the continuous annealing includes a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a rapid cooling section and an over-aging section in sequence; wherein the temperature of the soaking section is 850-870°C, the terminal temperature of the slow cooling section is 740-770°C, the terminal temperature of the rapid cooling section is 350-380°C, and the temperature of the over-aging section is 360-380°C.

[0021] Preferably, in step (e), the specific process conditions of each section of the continuous annealing are as follows:

[0022] The preheating section is heated from room temperature to 200-240°C at a rate of 1-5°C / s;

[0023] The oxidation stage: heating from 200-240°C to 620-660°C at a rate of 0.8-1.2°C / s;

[0024] The heating section is heated from 620-660°C to 850-870°C at a rate of 0.2-0.4°C / s;

[0025] The soaking section: maintain 850-870°C and keep warm for 215-280s;

[0026] The slow cooling section: cooling from 850-870°C to 740-770°C at a rate of 0.6°C / s-1.0°C / s;

[0027] The fast cooling section: cooling from 740-770°C to 350-380°C at a rate of 16°C / s-24°C / s;

[0028] The over-aging stage: maintain 360-380°C and keep warm for 1000-1300s.

[0029] Compared with the prior art, the present invention provides a super bainite steel and a preparation method thereof as described above, which has at least the following beneficial effects:

[0030] (1) The chemical composition of the super bainite steel of the present invention mainly includes C, Si and Mn as main elements, and the Si content and the Mn content are controlled to be less than 1.35% and 2.30% respectively, which further reduces the surface defects and element segregation problems of the ultra-high strength steel plate in the production process, and at the same time plays the role of inhibiting the precipitation of cementite and stabilizing austenite. Compared with other ultra-high strength steel products for automobiles with a strength grade of ≥1180MPa, the design of alloy composition and the coordinated optimization of process form a cold-rolled steel plate with a higher strength grade and takes into account both plasticity and formability.

[0031] (2) In the preparation method of the present invention, the hot rolling adopts a U-shaped coiling mode to compensate for the temperature loss at the head and tail of the strip, thereby improving the uniformity of the structure; the continuous annealing process adopts a heating process system of full austenite temperature, and the over-aging stage in the continuous annealing does not require the use of quenching and re-partitioning technical means, which can meet the needs of the traditional acid rolling-continuous annealing production line on a large scale, and has the advantages of low production cost and stable process.

[0032] (3) The synergistic control of the C, Si, Mn, Cr, Nb and Ti contents of the super bainite steel prepared by the present invention and the hot rolling, cold rolling and continuous annealing processes ensures that the steel plate achieves fine grain strengthening, solid solution precipitation strengthening and phase transformation strengthening. The final structure is composed of bainite + a small amount of martensite and residual austenite. The high proportion of bainite can improve the existing steel plate strength level to 1180MPa while taking into account the high yield, high elongation and high forming characteristics. The residual austenite improves the ductility of the steel plate, so that the prepared 1180MPa super bainite steel plate can meet the requirements of high strength plasticity and hole expansion and flanging performance for automotive structural parts (subframe, B-pillar, front longitudinal beam). BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0034] Figure 1 Metallographic photograph of the super bainite steel plate provided in Example 1 of the present invention;

[0035] Figure 2 This is a scanning electron microscope photo of the super bainite steel plate provided in Example 1 of the present invention;

[0036] Figure 3EBSD phase diagram of the super bainite steel plate provided in Example 1 of the present invention;

[0037] Figure 4 This is the XRD spectrum of the super bainite steel plate provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The invention provides a super bainite steel. The chemical composition of the super bainite steel is as follows, by mass percentage: C: 0.15-0.22%, Si: 0.80-1.35%, Mn: 1.8-2.3%, P≤0.01%, S≤0.005%, Als (acid soluble aluminum): 0.01-0.12%, Cr: 0.1-0.5%, Nb: 0.02-0.06%, Ti: 0.01-0.04%, N≤0.0045%, and the remaining elements are Fe and inevitable impurities, and at the same time, Ceq (carbon equivalent)=C+Si / 30+Mn / 20+2P+4S≤0.41 is satisfied.

[0040] In the super bainite steel provided by the present invention, the C content may specifically be 0.15wt%, 0.155wt%, 0.16wt%, 0.165wt%, 0.17wt%, 0.172wt%, 0.175wt%, 0.18wt%, 0.185wt%, 0.19wt%, 0.195wt%, 0.2wt%, 0.202wt%, 0.205wt%, 0.21wt%, 0.215wt% or 0.22wt%.

[0041] In the super bainite steel provided by the present invention, the content of Si may specifically be 0.8wt%, 0.82wt%, 0.85wt%, 0.87wt%, 0.9wt%, 0.92wt%, 0.95wt%, 0.97wt%, 1wt%, 1.02wt%, 1.05wt%, 1.07wt%, 1.1wt%, 1.12wt%, 1.15wt%, 1.17wt%, 1.2wt%, 1.23wt%, 1.25wt%, 1.27wt%, 1.3wt%, 1.32wt% or 1.35wt%.

[0042] In the super bainite steel provided by the present invention, the Mn content may specifically be 1.8wt%, 1.82wt%, 1.85wt%, 1.87wt%, 1.9wt%, 1.92wt%, 1.95wt%, 1.97wt%, 2wt%, 2.03wt%, 2.05wt%, 2.07wt%, 2.1wt%, 2.13wt%, 2.15wt%, 2.17wt%, 2.2wt%, 2.23wt%, 2.25wt%, 2.27wt% or 2.3wt%.

[0043] In the super bainite steel provided by the present invention, the content of P may specifically be 0.001wt%, 0.0015wt%, 0.002wt%, 0.0025wt%, 0.003wt%, 0.0035wt%, 0.004wt%, 0.0045wt%, 0.005wt%, 0.0055wt%, 0.006wt%, 0.0065wt%, 0.007wt%, 0.0075wt%, 0.008wt%, 0.0085wt%, 0.009wt%, 0.0095wt% or 0.01wt%.

[0044] In the super bainite steel provided by the present invention, the S content may specifically be 0.001wt%, 0.0012wt%, 0.0015wt%, 0.0017wt%, 0.002wt%, 0.0023wt%, 0.0025wt%, 0.0027wt%, 0.003wt%, 0.0032wt%, 0.0035wt%, 0.0037wt%, 0.004wt%, 0.0042wt%, 0.0045wt%, 0.0047wt% or 0.005wt%.

[0045] In the super bainite steel provided by the present invention, the content of Als may specifically be 0.01wt%, 0.015wt%, 0.02wt%, 0.025wt%, 0.03wt%, 0.035wt%, 0.04wt%, 0.045wt%, 0.05wt%, 0.055wt%, 0.06wt%, 0.065wt%, 0.07wt%, 0.075wt%, 0.08wt%, 0.085wt%, 0.09wt%, 0.095wt%, 0.1wt%, 0.105wt%, 0.11wt%, 0.115wt% or 0.12wt%.

[0046] In the super bainite steel provided by the present invention, the Cr content may specifically be 0.1wt%, 0.12wt%, 0.15wt%, 0.17wt%, 0.2wt%, 0.22wt%, 0.25wt%, 0.27wt%, 0.3wt%, 0.32wt%, 0.35wt%, 0.37wt%, 0.4wt%, 0.42wt%, 0.45wt%, 0.47wt% or 0.5wt%.

[0047] In the super bainite steel provided by the present invention, the Nb content may specifically be 0.02wt%, 0.023wt%, 0.025wt%, 0.027wt%, 0.03wt%, 0.032wt%, 0.035wt%, 0.037wt%, 0.04wt%, 0.042wt%, 0.045wt%, 0.047wt%, 0.05wt%, 0.052wt%, 0.055wt%, 0.057wt% or 0.06wt%.

[0048] In the super bainite steel provided by the present invention, the content of Ti may specifically be 0.01wt%, 0.012wt%, 0.015wt%, 0.016wt%, 0.018wt%, 0.02wt%, 0.022wt%, 0.025wt%, 0.027wt%, 0.03wt%, 0.032wt%, 0.035wt%, 0.036wt%, 0.038wt% or 0.04wt%.

[0049] In the super bainite steel provided by the present invention, the N content may specifically be 0.001wt%, 0.0012wt%, 0.0015wt%, 0.0017wt%, 0.002wt%, 0.0023wt%, 0.0025wt%, 0.0027wt%, 0.003wt%, 0.0031wt%, 0.0032wt%, 0.0035wt%, 0.0037wt%, 0.004wt%, 0.0042wt% or 0.0045wt%.

[0050] In the super bainite steel provided by the present invention, the microstructure of the super bainite steel is preferably composed of 80-90 vol% bainite, 3-11 vol% martensite and 5-9 vol% retained austenite.

[0051] In the super bainite steel provided by the present invention, the yield strength of the super bainite steel is preferably 870-950 MPa, specifically 870 MPa, 875 MPa, 880 MPa, 885 MPa, 890 MPa, 895 MPa, 897 MPa, 900 MPa, 901 MPa, 905 MPa, 910 MPa, 913 MPa, 915 MPa, 920 MPa, 925 MPa, 930 MPa, 935 MPa, 940 MPa, 945 MPa or 950 MPa.

[0052] In the super bainite steel provided by the present invention, the tensile strength of the super bainite steel is preferably 1180-1250MPa, specifically 1180MPa, 1182MPa, 1185MPa, 1187MPa, 1190MPa, 1192MPa, 1195MPa, 1197MPa, 1200MPa, 1202MPa, 1205MPa, 1207MPa, 1210MPa, 1212MPa, 1215MPa, 1217MPa, 1220MPa, 1222MPa, 1225MPa, 1227MPa, 1230MPa, 1232MPa, 1235MPa, 1237MPa, 1240MPa, 1242MPa, 1245MPa, 1247MPa or 1250MPa.

[0053] In the super bainite steel provided by the present invention, the elongation A of the super bainite steel is 50 The value is preferably 15-20%, specifically 15%, 15.2%, 15.5%, 15.7%, 16%, 16.2%, 16.5%, 16.7%, 17%, 17.2%, 17.5%, 17.7%, 18%, 18.2%, 18.5%, 18.7%, 19%, 19.2%, 19.5%, 19.7% or 20%.

[0054] In the super bainite steel provided by the present invention, the hole expansion rate of the super bainite steel is preferably 30-40%, specifically 30%, 30.5%, 31%, 31.5%, 32%, 32.5%, 33%, 33.5%, 34%, 34.5%, 35%, 35.5%, 36%, 36.5%, 37%, 37.5%, 38%, 38.5%, 39%, 39.5% or 40%.

[0055] The present invention also provides a method for preparing the super bainite steel described in the above technical solution, comprising the following steps:

[0056] (a) Smelting process: smelting molten steel according to the chemical composition of super bainite steel;

[0057] (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs;

[0058] (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, dephosphorizing, rough rolling, finish rolling, laminar cooling and coiling to obtain a hot rolled coil;

[0059] (d) Pickling and cold rolling process: the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil;

[0060] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is subjected to continuous annealing to obtain super bainite steel.

[0061] In the preparation method provided by the present invention, in step (b), the continuous casting temperature is preferably 1460-1550°C, specifically 1460°C, 1470°C, 1480°C, 1490°C, 1500°C, 1510°C, 1520°C, 1530°C, 1540°C or 1550°C.

[0062] In the preparation method provided by the present invention, in step (b), the thickness of the slab is preferably 120-240 mm, specifically 120 mm, 130 mm, 140 mm, 150 mm, 160 mm, 170 mm, 180 mm, 190 mm, 200 mm, 210 mm, 220 mm, 230 mm or 240 mm.

[0063] In the preparation method provided by the present invention, in step (c), the heating temperature is preferably 1210-1270°C, specifically 1210°C, 1215°C, 1218°C, 1220°C, 1225°C, 1230°C, 1231°C, 1235°C, 1240°C, 1245°C, 1246°C, 1250°C, 1255°C, 1260°C, 1265°C or 1270°C.

[0064] In the preparation method provided by the present invention, in step (c), the water pressure of the dephosphorization is preferably ≥22MPa; the temperature of the dephosphorization is preferably ≥1173°C to ensure the removal of the low-melting-point FeO·Fe2SiO4 eutectic compound.

[0065] In the preparation method provided by the present invention, in step (c), the start rolling temperature of the finish rolling is preferably 1000-1100°C, specifically 1000°C, 1010°C, 1020°C, 1026°C, 1030°C, 1040°C, 1048°C, 1050°C, 1060°C, 1063°C, 1070°C, 1080°C, 1090°C or 1100°C; to avoid the formation of Fe2O3, the finish rolling pass is preferably 7 passes. The final rolling temperature of the finishing rolling is preferably 890-950°C, specifically 890°C, 895°C, 900°C, 905°C, 908°C, 910°C, 912°C, 915°C, 920°C, 925°C, 930°C, 935°C, 940°C, 941°C, 945°C or 950°C; the final reduction rate of the finishing rolling is preferably ≤15%, and lubrication rolling is adopted to avoid the III-level oxide scale formed in the finishing rolling process from being broken in the final pass.

[0066] In the preparation method provided by the present invention, in step (c), in order to avoid excessive stress in the oxide layer and the formation of excessive crack defects during cooling, thereby affecting the forming performance of the oxide layer, the temperature of the laminar cooling is preferably 720-770°C, specifically 720°C, 725°C, 730°C, 731°C, 735°C, 740°C, 745°C, 750°C, 755°C, 760°C, 762°C, 765°C or 770°C, and the laminar cooling method preferably adopts a sparse cooling method.

[0067] In the preparation method provided by the present invention, in step (c), the thickness of the hot-rolled coil is preferably 2.3-4 mm, specifically 2.3 mm, 2.4 mm, 2.5 mm, 2.6 mm, 2.7 mm, 2.8 mm, 2.9 mm, 3 mm, 3.1 mm, 3.2 mm, 3.3 mm, 3.4 mm, 3.5 mm, 3.6 mm, 3.7 mm, 3.8 mm, 3.9 mm or 4 mm.

[0068] In the preparation method provided by the present invention, in step (c), the coiling preferably adopts a U-shaped coiling mode; the hot coil box is used to adjust the strip head and tail before the strip enters the finishing mill, preferably controlling the coiling temperature of the strip head 20m to 560-650℃, the coiling temperature of the strip tail 70m to 560-640℃, and the coiling temperature of the middle section to 580℃, so as to ensure the stability of the longitudinal structure and performance of the strip. The appropriate coiling temperature keeps the Fe3O4 generated before coiling stable, and avoids the adverse effects of the grain boundary oxide layer and carbide on the surface and plasticity.

[0069] In the preparation method provided by the present invention, in step (d), the cold rolling reduction rate is preferably 40-70%, specifically 40%, 42%, 45%, 46.7%, 47%, 48.5%, 50%, 52%, 53%, 55%, 57%, 60%, 62%, 65%, 67% or 70%.

[0070] In the preparation method provided by the present invention, in step (d), the thickness of the cold-rolled thin strip steel is preferably 1 to 2 mm, specifically 1 mm, 1.05 mm, 1.1 mm, 1.15 mm, 1.2 mm, 1.25 mm, 1.3 mm, 1.35 mm, 1.4 mm, 1.45 mm, 1.5 mm, 1.55 mm, 1.6 mm, 1.65 mm, 1.7 mm, 1.75 mm, 1.8 mm, 1.85 mm, 1.9 mm, 1.95 mm or 2 mm.

[0071] In the preparation method provided by the present invention, in step (e), the continuous annealing includes a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a fast cooling section and an over-aging section in sequence. In the present invention, the temperature of the soaking section is preferably 850-870°C to ensure that the steel plate is heated at a fully austenitizing temperature and obtains a suitable soaking section structure requirement; in a specific embodiment of the present invention, the temperature of the soaking section can be specifically 850°C, 851°C, 852°C, 853°C, 854°C, 855°C, 856°C, 857°C, 858°C, 859°C, 860°C, 861°C, 862°C, 863°C, 864°C, 865°C, 866°C, 867°C, 868°C, 869°C or 870°C. In the present invention, the terminal temperature of the slow cooling section is preferably 740-770°C, the terminal temperature of the fast cooling section is preferably 350-380°C, and the temperature of the over-aging section is preferably 360-380°C, so as to ensure the formation of bainite at this stage, and then control the phase ratio of bainite, martensite and residual austenite in the final structure, so as to ensure that both strong plasticity and formability are satisfied; in a specific embodiment of the present invention, the terminal temperature of the slow cooling section is preferably 740-770°C, the terminal temperature of the fast cooling section is preferably 350-380°C, and the temperature of the over-aging section is preferably 360-380°C. The specific temperature may be 740°C, 741°C, 742°C, 743°C, 744°C, 745°C, 746°C, 747°C, 748°C, 749°C, 750°C, 751°C, 752°C, 753°C, 754°C, 755°C, 756°C, 757°C, 758°C, 759°C, 760°C, 761°C, 762°C, 763°C, 764°C, 765°C, 766°C, 767°C, 768°C , 769°C or 770°C, and the terminal temperature of the fast cooling section can be specifically 350°C, 351°C, 352°C, 353°C, 354°C, 355°C, 356°C, 357°C, 358°C, 359°C, 360°C, 361°C, 362°C, 363°C, 364°C, 365°C, 366°C, 367°C, 368°C, 369°C, 370°C, 371°C, 372°C, 373°C, 374°C , 375℃, 376℃, 377℃, 378℃, 379℃ or 380℃, and the temperature of the over-aging section may be 360℃, 361℃, 362℃, 363℃, 364℃, 365℃, 366℃, 367℃, 368℃, 369℃, 370℃, 371℃, 372℃, 373℃, 374℃, 375℃, 376℃, 377℃, 378℃, 379℃ or 380℃.

[0072] In the preparation method provided by the present invention, in step (e), in a specific embodiment of the present invention, the specific process conditions of each section of the continuous annealing are preferably as follows:

[0073] The preheating section is heated from room temperature to 200-240°C at a rate of 1-5°C / s;

[0074] The oxidation stage: heating from 200-240°C to 620-660°C at a rate of 0.8-1.2°C / s;

[0075] The heating section is heated from 620-660°C to 850-870°C at a rate of 0.2-0.4°C / s;

[0076] The soaking section: maintain 850-870°C and keep warm for 215-280s;

[0077] The slow cooling section: cooling from 850-870°C to 740-770°C at a rate of 0.6°C / s-1.0°C / s;

[0078] The fast cooling section: cooling from 740-770°C to 350-380°C at a rate of 16°C / s-24°C / s;

[0079] The over-aging stage: maintain 360-380°C and keep warm for 1000-1300s.

[0080] In the preparation method provided by the present invention, in step (e), the unit speed of the equipment used for continuous annealing is preferably 40-60 m / min, specifically 40 m / min, 42 m / min, 45 m / min, 47 m / min, 50 m / min, 52 m / min, 55 m / min, 57 m / min or 60 m / min. In the present invention, the unit speed of the cold-rolled thin strip steel is preferably reduced by 5 m / min for every 0.2 mm increase in speed, so as to ensure the formation of ferrite and tempered martensite during the heating and heat preservation process due to the reverse temperature during the heating process, and to avoid the transitional growth of the grains of the steel plate after cold rolling.

[0081] In the preparation method provided by the present invention, in step (e), the flattening elongation of the cold-rolled thin steel strip is preferably 0.3-0.5%, specifically 0.3%, 0.32%, 0.35%, 0.37%, 0.4%, 0.42%, 0.45%, 0.47% or 0.5%. In the present invention, the flattening elongation is preferably reduced by 0.05% for every 0.1 mm increase in the thickness of the cold-rolled thin steel strip.

[0082] For the purpose of greater clarity, the invention is described in detail through the following embodiments.

[0083] Examples 1 to 3

[0084] Three sets of 1180MPa grade super bainite steel plates for automobiles are provided, and their chemical compositions are shown in Table 1:

[0085] Table 1 Chemical composition of 1180MPa grade super bainite steel for automobile (wt.%)

[0086] serial number C Si Mn P S Als Cr Ti Nb N Example 1 0.190 1.35 2.27 0.008 0.002 0.045 0.22 0.036 0.040 0.0032 Example 2 0.202 1.15 2.30 0.008 0.003 0.030 0.32 0.016 0.035 0.0030 Example 3 0.172 1.25 2.15 0.008 0.002 0.045 0.42 0.022 0.055 0.0031

[0087] The specific process of the preparation method of the above-mentioned 1180MPa grade super bainite steel plate for automobiles is as follows:

[0088] (a) Smelting process: smelting molten steel according to the chemical composition of the complex phase steel plate shown in Table 1;

[0089] (b) Continuous casting process: the casting temperature is 1520°C, and the thickness of the slab obtained by continuous casting is 230 mm;

[0090] (c) Hot rolling process: The slab obtained by continuous casting is subjected to heating, dephosphorization, rough rolling, finishing rolling, laminar cooling and coiling in sequence to obtain a hot rolled coil; wherein, the dephosphorization water pressure is ≥22MPa, the dephosphorization temperature is ≥1173°C, the finishing rolling passes are 7, the last pass reduction is ≤15% and lubrication rolling is adopted, and other key hot rolling process parameters are detailed in Table 2:

[0091] Table 2 Main process parameters of hot rolling of 1180MPa grade super bainite steel for automobile

[0092]

[0093] (d) Pickling and cold rolling process: the hot rolled coil is pickled to remove the iron oxide scale on the surface, and then cold rolled to form cold rolled thin strip steel, wherein the thickness of the strip steel in Example 1, Example 2, and Example 3 are 1.6 mm, 1.8 mm, and 1.0 mm, respectively, and the cold rolling reduction ratios are 46.7%, 48.5%, and 50.0%, respectively;

[0094] (e) Continuous annealing process: The cold-rolled thin strip is processed into the desired product by a continuous annealing process. The strip is first slowly heated to the soaking temperature (complete austenitization temperature) and kept for a period of time to form a complete austenite. Then, it is slowly cooled to the slow cooling end temperature to decompose part of the supercooled austenite, forming a small amount of oriented epitaxial ferrite and "carbon-enriching" the remaining austenite. Then, the strip is rapidly cooled to transform the austenite into martensite and bainite, and the austenite transforms into bainite in the over-aging stage. Finally, it is cooled to room temperature and the strip shape is adjusted by a skin pass mill to increase the yield strength.

[0095] The continuous annealing specifically includes a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a fast cooling section and an over-aging section in sequence; wherein, the heating rates of the preheating section of Example 1, Example 2 and Example 3 are 3.8°C / s, 4.1°C / s and 4.5°C / s, respectively, and the terminal temperatures are 220°C, 205°C and 240°C, respectively; the heating rates of the oxidation section are 0.80°C / s, 0.97°C / s and 0.81°C / s, respectively, and the terminal temperatures are 632℃, 621℃, 658℃, the heating rates in the heating section are 0.30℃ / s, 0.34℃ / s, 0.23℃ / s, respectively, the terminal temperatures are 862℃, 865℃, 853℃, respectively, the holding time in the soaking section is 262.8s, 219.0s, 245.5s, respectively, the holding time in the over-aging section is 1237.9s, 1031.6s, 1143.3s, respectively. For other key continuous annealing process parameters, see Table 3:

[0096] Table 3 Main process parameters of continuous annealing of 1180MPa grade super bainite steel for automobiles

[0097]

[0098] The microstructure of the 1180MPa grade super bainite steel for automobiles prepared by the above process in Example 1 is as follows: Figure 1 As shown in the scanning electron microscope photos Figure 2 As shown. From the steel structure of Example 1 shown in the figure and combined Figure 3 (EBSD phase diagram) and Figure 4 (XRD spectrum) Phase volume fraction measurement results show that the microstructure of the 1180 MPa grade super bainite steel of Example 1 of the present invention is composed of 80% to 90% lath bainite and 5 to 9% retained austenite and residual tempered martensite.

[0099] The performance of the above-mentioned 1180MPa grade super bainite steel for automobiles was tested in accordance with GB / T228-2010 "Room Temperature Tensile Test Method for Metallic Materials" and GB / T24524-2021 "Hole Expansion Test Method for Metallic Materials Sheet and Strip", and its mechanical properties are shown in Table 4 below:

[0100] Table 4 Mechanical properties of 1180MPa grade super bainite steel for automobiles

[0101] serial number Yield strength / MPa Tensile strength / MPa Elongation % Yield-to-strength ratio Hole expansion rate / % Example 1 901 1212 16.0 0.733 35 Example 2 897 1192 17.5 0.753 33 Example 3 913 1205 16.5 0.740 36 CN110724877A 677 1192 17.2 0.568 33 CN109778062A 973 1234 7.5 0.788 /

[0102] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A super bainite steel, characterized in that: The chemical composition of the super bainite steel is, by mass percentage, C: 0.15-0.22%, Si: 0.80-1.35%, Mn: 1.8-2.3%, P≤0.01%, S≤0.005%, Als: 0.01-0.12%, Cr: 0.1-0.5%, Nb: 0.02-0.06%, Ti: 0.01-0.04%, N≤0.0045%, and the remaining elements are Fe and unavoidable impurities, and at the same time satisfy Ceq=C+Si / 30+Mn / 20+2P+4S≤0.

41.

2. The super bainite steel according to claim 1, characterized in that: The microstructure of the super bainite steel consists of 80-90 vol% of bainite, 3-11 vol% of martensite and 5-9 vol% of retained austenite.

3. The super bainite steel according to claim 1, characterized in that: The yield strength of the super bainite steel is 870-950MPa, the tensile strength is 1180-1250MPa, and the elongation A 50 The value is 15-20%, and the hole expansion rate is 30-40%.

4. A method for preparing super bainite steel according to any one of claims 1 to 3, characterized in that: The following steps are involved: (a) Smelting process: smelting molten steel according to the chemical composition of super bainite steel; (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs; (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, dephosphorizing, rough rolling, finish rolling, laminar cooling and coiling to obtain a hot rolled coil; (d) Pickling and cold rolling process: the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil; (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is subjected to continuous annealing to obtain super bainite steel.

5. The preparation method according to claim 4, characterized in that: In step (b), the continuous casting temperature is 1460-1550° C.; the thickness of the slab is 120-240 mm.

6. The preparation method according to claim 4, characterized in that: In step (c), the heating temperature is 1210-1270°C; the water pressure of the dephosphorization is ≥22MPa, and the temperature is ≥1173°C; the starting temperature of the finishing rolling is 1000-1100°C, the finishing rolling passes are 7 passes, the final rolling temperature is 890-950°C, and the reduction rate of the last pass is ≤15%; the laminar cooling method is sparse cooling; the thickness of the hot-rolled coil is 2.3-4mm.

7. The preparation method according to claim 4, characterized in that: In step (c), the coiling adopts a U-shaped coiling mode; when coiling, the coiling temperature of the strip in the first 20m is 560-650°C, the coiling temperature of the strip in the last 70m is 560-640°C, and the coiling temperature of the middle section is 580°C.

8. The preparation method according to claim 4, characterized in that: In step (d), the cold rolling reduction rate is 40-70%; the thickness of the cold rolled thin strip steel is 1-2 mm.

9. The preparation method according to claim 4, characterized in that: In step (e), the continuous annealing includes a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a rapid cooling section and an over-aging section in sequence; wherein the temperature of the soaking section is 850-870°C, the terminal temperature of the slow cooling section is 740-770°C, the terminal temperature of the rapid cooling section is 350-380°C, and the temperature of the over-aging section is 360-380°C.

10. The preparation method according to claim 9, characterized in that: In step (e), the specific process conditions of each section of the continuous annealing are as follows: The preheating section is heated from room temperature to 200-240°C at a rate of 1-5°C / s; The oxidation stage: heating from 200-240°C to 620-660°C at a rate of 0.8-1.2°C / s; The heating section is heated from 620-660°C to 850-870°C at a rate of 0.2-0.4°C / s; The soaking section: maintain 850-870°C and keep warm for 215-280s; The slow cooling section: cooling from 850-870°C to 740-770°C at a rate of 0.6°C / s-1.0°C / s; The fast cooling section: cooling from 740-770°C to 350-380°C at a rate of 16°C / s-24°C / s; The over-aging stage: maintain 360-380°C and keep warm for 1000-1300s.

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