Super bainite steel and preparation method therefor

Abstract

The present invention belongs to the field of steel, and particularly relates to super bainite steel and a preparation method therefor. The super bainite steel provided by the present invention comprises the following chemical components in percentages by mass: 0.15-0.22% of C, 0.80-1.35% of Si, 1.8-2.3% of Mn, less than or equal to 0.01% of P, less than or equal to 0.005% of S, 0.01-0.12% of Als, 0.1-0.5% of Cr, 0.02-0.06% of Nb, 0.01-0.04% of Ti, and less than or equal to 0.0045% of N, with the balance being Fe and inevitable impurities. Moreover, the steel satisfies: Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41. The super bainite steel provided by the present invention has good performance and can be widely used in the field of automotive steel.

Description

A super bainitic steel and its preparation method

[0001] This application claims priority to Chinese Patent Application No. 202411437339.8, filed on October 15, 2024, entitled "A Super Bainitic Steel and a Method for Preparing the Same", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention belongs to the field of steel, and particularly relates to a super bainitic steel and its preparation method. Background Technology

[0003] With increasing considerations for lightweighting and crash safety in automobiles, coupled with growing concerns about energy shortages and environmental pollution, high-strength automotive steel is a crucial pathway to achieving lightweighting and improved safety performance. However, high-strength automotive steel is prone to cracking during cold forming, particularly in the localized forming of complex parts (such as flanging and bending), which severely restricts its widespread application. Bainitic steel, with bainite as the matrix and retained austenite as the second phase, has attracted considerable attention. It avoids the hardness difference between the soft and hard phases present in dual-phase steels and can improve the ductility and localized forming properties of the steel.

[0004] Compared to Q&P steel, a representative of third-generation advanced high-strength steel with a strength of 1180MPa, super bainitic steel is characterized by a microstructure composed of finely ordered, carbide-free bainitic lath bundles and retained austenite distributed between the lath bundles in a bainitic matrix, while also containing a very small amount of tempered martensite. By designing the unique microstructure of super bainitic steel, it can ensure both excellent strength and plasticity while exhibiting superior local forming properties (such as flanging and hole enlargement). Therefore, super bainitic steel with the TRIP effect has been developed in recent years to meet the automotive industry's demand for new steel sheets with high strength, high plasticity, and high local forming performance. A search of relevant patents reveals the following patents that are similar to 1180MPa-grade super bainitic steel sheets:

[0005] CN 110724877 A discloses a 1180MPa grade high-plasticity bainitic multiphase steel sheet for automobiles and its preparation method. The chemical composition by weight percentage is: 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%. The substituted components, by weight percentage, are: Cr: 0-2%, Mo: 0-0.5%, Cu: 0.05-0.5%, with the balance being Fe and unavoidable impurities. The rolling process is as follows: the hot-rolled billet's furnace entry temperature is between 400 and 750℃, the heating temperature is between 1150 and 1250℃, the initial 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 as follows: the annealing temperature is between 780 and 880℃, the annealing time is between 1 and 5 minutes, the cooling rate is greater than 30℃ / s, cooling to the aging temperature is between 300 and 500℃, the aging time is between 300 and 600 seconds, and then cooling to room temperature. This patent adds a wide range of Si and Mn contents. Low Si and low Mn conditions are insufficient to achieve the desired strength grade in the product. Furthermore, the hot rolling process only provides the heating temperature, finishing rolling temperature, and coiling temperature, without specifying the cooling path and intermediate temperature control, which is detrimental to the stable control of the product's microstructure, properties, and surface quality.

[0006] CN 109778062 A discloses a cold-rolled multiphase steel with a tensile strength of 1200MPa and its preparation method. 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%, with the balance being iron and other unavoidable impurity elements; and simultaneously satisfying the following condition: C+(Si+Mn) / 6+(Cr+Mo+V) / 5≤0.8. This patent uses Nb-Ti microalloying to prepare a cold-rolled multiphase 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.

[0007] Summary of the Invention

[0008] In view of this, the purpose of this invention is to provide a super bainitic steel and its preparation method. The super bainitic steel provided by this invention has excellent properties and can be widely used in the field of automotive steel.

[0009] This invention provides a super bainitic steel, the chemical composition of which, by mass percentage, is: 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%, with the remaining elements being Fe and unavoidable impurities, and simultaneously satisfying Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41.

[0010] Preferably, the microstructure of the super bainitic steel consists of 80-90 vol% bainite, 3-11 vol% martensite, and 5-9 vol% retained austenite.

[0011] Preferably, the super bainitic steel has a yield strength of 870–950 MPa, a tensile strength of 1180–1250 MPa, and an elongation A. 50 The value is 15-20%, and the porosity is 30-40%.

[0012] This invention provides a method for preparing the super bainitic steel described above, comprising the following steps:

[0013] (a) Smelting process: Smelting molten steel according to the chemical composition of super bainitic steel;

[0014] (b) Continuous casting process: The molten steel processed in step (a) is continuously cast into slabs;

[0015] (c) Hot rolling process: The slab obtained in step (b) is heated by hot or cold charging, and then descaled, rough rolled, finish rolled, laminar flow cooled and coiled to obtain a hot rolled coil;

[0016] (d) Pickling and cold rolling process: After the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, it is cold-rolled into a cold-rolled thin strip steel coil.

[0017] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed to obtain super bainitic steel.

[0018] Preferably, in step (b), the continuous casting temperature is 1460–1550°C; and the slab thickness is 120–240 mm.

[0019] Preferably, in step (c), the heating temperature is 1210–1270°C; the dephosphorization water pressure is ≥22 MPa and the temperature is ≥1173°C; the initial rolling temperature of the finishing rolling is 1000–1100°C, the finishing rolling passes are 7, the final rolling temperature is 890–950°C, and the reduction rate of the last pass is ≤15%; the laminar flow cooling method is sparse cooling; and the thickness of the hot-rolled coil is 2.3–4 mm.

[0020] Preferably, in step (c), the winding adopts a U-shaped winding mode; when winding, the winding temperature of the first 20m of the strip is 560-650℃, the winding temperature of the last 70m of the strip is 560-640℃, and the winding temperature of the middle section is 580℃.

[0021] Preferably, in step (d), the cold rolling reduction rate is 40-70%; and the thickness of the cold-rolled thin strip is 1-2 mm.

[0022] Preferably, in step (e), the continuous annealing sequentially 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; wherein the temperature of the soaking section is 850–870°C, the final temperature of the slow cooling section is 740–770°C, the final temperature of the rapid cooling section is 350–380°C, and the temperature of the over-aging section is 360–380°C.

[0023] Preferably, in step (e), the specific process conditions for each stage of the continuous annealing are as follows:

[0024] The preheating section: heats from room temperature to 200-240°C at a rate of 1-5°C / s;

[0025] The oxidation section is heated from 200–240°C to 620–660°C at a rate of 0.8–1.2°C / s.

[0026] The heating section is heated from 620-660℃ to 850-870℃ at a rate of 0.2-0.4℃ / s.

[0027] The heat spreader section is maintained at 850–870°C for 215–280 seconds.

[0028] The slow cooling section cools the temperature from 850–870°C to 740–770°C at a rate of 0.6°C / s–1.0°C / s.

[0029] The rapid cooling section cools the temperature from 740-770℃ to 350-380℃ at a rate of 16℃ / s-24℃ / s.

[0030] The over-aging period: maintain 360-380℃ and keep warm for 1000-1300s.

[0031] Compared with the prior art, the present invention provides the super bainitic steel described above and its preparation method, which has at least the following beneficial effects:

[0032] (1) The chemical composition of the super bainitic steel of the present invention is mainly composed of C, Si and Mn as the main elements. The Si content and 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 ultra-high strength steel plates during production. At the same time, it plays the role of inhibiting cementite precipitation and stabilizing austenite. Compared with other ultra-high strength steel products for automobiles with strength grades ≥1180MPa, the design of alloy composition and the synergistic optimization of process have formed a cold-rolled steel plate with higher strength grade and takes into account plasticity and formability.

[0033] (2) In the preparation method of the present invention, the hot rolling adopts the U-shaped coiling mode to supplement the temperature loss at the head and tail of the strip and improve the uniformity of the structure; the continuous annealing process adopts the heating process system of the complete austenitic temperature, and the over-aging stage in the continuous annealing does not require the technical means of quenching and re-distribution, which can meet the requirements of the traditional pickling-rolling-continuous annealing production line on a large scale, and has the advantages of low production cost and stable process.

[0034] (3) The synergistic control of the C, Si, Mn, Cr, Nb and Ti contents of the super bainitic steel prepared by the present invention with 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 microstructure is composed of bainite + a small amount of martensite and retained austenite. The high proportion of bainite can improve the existing strength level of the steel plate to 1180MPa while taking into account the characteristics of high yield, high elongation and high formability. The retained austenite improves the ductility of the steel plate, so that the prepared 1180MPa super bainitic steel plate can meet the requirements of high strength plasticity and hole expansion and flanging performance for use in automotive structural parts (subframe, B-pillar, front longitudinal beam). Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0036] Figure 1 is a metallographic photograph of the super bainitic steel plate provided in Embodiment 1 of the present invention;

[0037] Figure 2 is a scanning electron microscope image of the super bainitic steel plate provided in Embodiment 1 of the present invention;

[0038] Figure 3 is the EBSD phase diagram of the super bainitic steel plate provided in Embodiment 1 of the present invention;

[0039] Figure 4 shows the XRD pattern of the super bainitic steel plate provided in Embodiment 1 of the present invention. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] This invention provides a super bainitic steel, the chemical composition of which, by mass percentage, is: 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%, with the remaining elements being Fe and unavoidable impurities, and simultaneously satisfying Ceq (carbon equivalent) = C + Si / 30 + Mn / 20 + 2P + 4S ≤ 0.41.

[0042] In the super bainitic steel provided by this invention, the content of C can 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%.

[0043] In the super bainitic steel provided by this invention, the Si content can 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%.

[0044] In the super bainitic steel provided by this invention, the Mn content can 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%.

[0045] In the super bainitic steel provided by this invention, the content of P can specifically be 0.001 wt%, 0.0015 wt%, 0.002 wt%, 0.0025 wt%, 0.003 wt%, 0.0035 wt%, 0.004 wt%, 0.0045 wt%, 0.005 wt%, 0.0055 wt%, 0.006 wt%, 0.0065 wt%, 0.007 wt%, 0.0075 wt%, 0.008 wt%, 0.0085 wt%, 0.009 wt%, 0.0095 wt%, or 0.01 wt%.

[0046] In the super bainitic steel provided by this invention, the content of S can 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%.

[0047] In the super bainitic steel provided by this invention, the content of Als can 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%.

[0048] In the super bainitic steel provided by this invention, the Cr content can 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%.

[0049] In the super bainitic steel provided by this invention, the Nb content can 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%.

[0050] In the super bainitic steel provided by this invention, the Ti content can 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%.

[0051] In the super bainitic steel provided by this invention, the N content can 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%.

[0052] In the super bainitic steel provided by the present invention, the microstructure of the super bainitic steel preferably consists of 80-90 vol% bainite, 3-11 vol% martensite and 5-9 vol% retained austenite.

[0053] In the super bainitic steel provided by the present invention, the yield strength of the super bainitic 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.

[0054] In the super bainitic steel provided by the present invention, the tensile strength of the super bainitic steel is preferably 1180-1250 MPa, specifically 1180 MPa, 1182 MPa, 1185 MPa, 1187 MPa, 1190 MPa, 1192 MPa, 1195 MPa, 1197 MPa, 1200 MPa, 1202 MPa, 1205 MPa, 1207 MPa, 1210 MPa, 1212 MPa, 1215 MPa, 1217 MPa, 1220 MPa, 1222 MPa, 1225 MPa, 1227 MPa, 1230 MPa, 1232 MPa, 1235 MPa, 1237 MPa, 1240 MPa, 1242 MPa, 1245 MPa, 1247 MPa, or 1250 MPa.

[0055] In the super bainitic steel provided by this invention, the elongation A of the super bainitic steel is... 50 The preferred value is 15% to 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%.

[0056] In the super bainitic steel provided by the present invention, the porosity of the super bainitic 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%.

[0057] This invention also provides a method for preparing the super bainitic steel described above, comprising the following steps:

[0058] (a) Smelting process: Smelting molten steel according to the chemical composition of super bainitic steel;

[0059] (b) Continuous casting process: The molten steel processed in step (a) is continuously cast into slabs;

[0060] (c) Hot rolling process: The slab obtained in step (b) is heated by hot or cold charging, and then descaled, rough rolled, finish rolled, laminar flow cooled and coiled to obtain a hot rolled coil;

[0061] (d) Pickling and cold rolling process: After the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, it is cold-rolled into a cold-rolled thin strip steel coil.

[0062] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed to obtain super bainitic steel.

[0063] In the preparation method provided by the present invention, in step (b), the continuous casting temperature is preferably 1460-1550℃, specifically 1460℃, 1470℃, 1480℃, 1490℃, 1500℃, 1510℃, 1520℃, 1530℃, 1540℃ or 1550℃.

[0064] 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.

[0065] In the preparation method provided by the present invention, in step (c), the heating temperature is preferably 1210-1270℃, specifically 1210℃, 1215℃, 1218℃, 1220℃, 1225℃, 1230℃, 1231℃, 1235℃, 1240℃, 1245℃, 1246℃, 1250℃, 1255℃, 1260℃, 1265℃ or 1270℃.

[0066] In the preparation method provided by the present invention, in step (c), the water pressure for phosphorus removal is preferably ≥22MPa; the temperature for phosphorus removal is preferably ≥1173℃, so as to ensure the removal of low-melting-point FeO·Fe2SiO4 eutectic compounds.

[0067] In the preparation method provided by this invention, in step (c), the initial rolling temperature of the finishing rolling is preferably 1000–1100℃, specifically 1000℃, 1010℃, 1020℃, 1026℃, 1030℃, 1040℃, 1048℃, 1050℃, 1060℃, 1063℃, 1070℃, 1080℃, 1090℃, or 1100℃; to avoid the formation of Fe2O3, the finishing rolling passes are preferably 7 passes. The final rolling temperature of the finishing mill is preferably 890-950℃, specifically 890℃, 895℃, 900℃, 905℃, 908℃, 910℃, 912℃, 915℃, 920℃, 925℃, 930℃, 935℃, 940℃, 941℃, 945℃ or 950℃; the reduction rate of the final pass of the finishing mill is preferably ≤15%, and lubricated rolling is used to avoid the breakage of the tertiary iron oxide scale formed during the finishing mill process in the final pass.

[0068] In the preparation method provided by the present invention, in step (c), in order to avoid excessive internal stress in the oxide layer causing excessive crack defects during cooling and affecting the forming performance of the oxide layer, the laminar cooling temperature is preferably 720-770℃, specifically 720℃, 725℃, 730℃, 731℃, 735℃, 740℃, 745℃, 750℃, 755℃, 760℃, 762℃, 765℃ or 770℃, and the laminar cooling method is preferably a sparse cooling method.

[0069] In the preparation method provided by the present invention, in step (c), the thickness of the hot-rolled coil is preferably 2.3 to 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.

[0070] In the preparation method provided by this invention, in step (c), the coiling preferably adopts a U-shaped coiling mode; before the strip enters the finishing mill, the strip head and tail are adjusted using a hot coiling box. Preferably, the coiling temperature is controlled at 560–650°C for the first 20m of the strip head, 560–640°C for the last 70m of the strip tail, and 580°C for the middle section, to ensure stable longitudinal microstructure of the strip. A suitable coiling temperature keeps the Fe3O4 generated before coiling stable, while avoiding the adverse effects of grain boundary oxide layers and carbides on surface properties and plasticity.

[0071] In the preparation method provided by the present invention, in step (d), the reduction rate of the cold rolling is preferably 40% to 70%, specifically 40%, 42%, 45%, 46.7%, 47%, 48.5%, 50%, 52%, 53%, 55%, 57%, 60%, 62%, 65%, 67%, or 70%.

[0072] 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.

[0073] In the preparation method provided by this invention, step (e) of the continuous annealing sequentially 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 this 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 to obtain the appropriate microstructure requirements of the soaking section. In specific embodiments of this invention, the temperature of the soaking section can specifically be 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 this invention, the preferred endpoint temperature of the slow cooling section is 740–770°C, the preferred endpoint temperature of the rapid cooling section is 350–380°C, and the preferred temperature of the over-aging section is 360–380°C. This ensures the formation of bainite during this stage, thereby controlling the phase ratio of bainite, martensite, and retained austenite in the final microstructure to ensure a balance between strength, plasticity, and formability. In a specific embodiment of this invention, the endpoint temperature of the slow cooling section can 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, or 764°C. The temperatures can be 765℃, 766℃, 767℃, 768℃, 769℃, or 770℃, and the final temperature of the rapid cooling section can specifically be 350℃, 351℃, 352℃, 353℃, 354℃, 355℃, 356℃, 357℃, 358℃, 359℃, 360℃, 361℃, 362℃, 363℃, 364℃, 365℃, 366℃, 367℃, or 368℃. The temperatures of the over-aging period can be 360℃, 361℃, 362℃, 373℃, 374℃, 375℃, 376℃, 377℃, 378℃, 379℃, or 380℃, and the specific temperatures of the over-aging period can be 360℃, 361℃, 362℃, 363℃, 364℃, 365℃, 366℃, 367℃, 368℃, 369℃, 370℃, 371℃, 372℃, 373℃, 374℃, 375℃, 376℃, 377℃, 378℃, 379℃, or 380℃.

[0074] In the preparation method provided by the present invention, in step (e), in a specific embodiment of the present invention, the preferred process conditions for each stage of the continuous annealing are as follows:

[0075] The preheating section: heats from room temperature to 200-240°C at a rate of 1-5°C / s;

[0076] The oxidation section is heated from 200–240°C to 620–660°C at a rate of 0.8–1.2°C / s.

[0077] The heating section is heated from 620-660℃ to 850-870℃ at a rate of 0.2-0.4℃ / s.

[0078] The heat spreader section is maintained at 850–870°C for 215–280 seconds.

[0079] The slow cooling section cools the temperature from 850–870°C to 740–770°C at a rate of 0.6°C / s–1.0°C / s.

[0080] The rapid cooling section cools the temperature from 740-770℃ to 350-380℃ at a rate of 16℃ / s-24℃ / s.

[0081] The over-aging period: maintain 360-380℃ and keep warm for 1000-1300s.

[0082] In the preparation method provided by this 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 this invention, the unit speed is preferably reduced by 5 m / min for every 0.2 mm increase in cold-rolled thin strip steel to ensure the formation of ferrite and tempered martensite due to temperature reversal during heating and holding, while avoiding excessive grain growth in the steel sheet after cold rolling.

[0083] In the preparation method provided by this invention, in step (e), the leveling elongation of the cold-rolled thin strip steel is preferably 0.3% to 0.5%, specifically 0.3%, 0.32%, 0.35%, 0.37%, 0.4%, 0.42%, 0.45%, 0.47%, or 0.5%. In this invention, for every 0.1 mm increase in the thickness of the cold-rolled thin strip steel, the leveling elongation is preferably reduced by 0.05%.

[0084] For clarity, the following examples will be used to provide a detailed description.

[0085] Examples 1-3

[0086] Three sets of 1180MPa grade super bainitic steel sheets for automotive use are provided, and their chemical compositions are shown in Table 1:

[0087] Table 1. Chemical composition (wt.%) of 1180MPa grade super bainitic steel for automotive applications.

[0088] The specific process for preparing the aforementioned 1180MPa grade super bainitic steel sheet for automobiles is as follows:

[0089] (a) Smelting process: Smelt the molten steel according to the chemical composition of the multiphase steel plate shown in Table 1;

[0090] (b) Continuous casting process: The casting temperature is 1520℃, and the thickness of the slab obtained by continuous casting is 230mm;

[0091] (c) Hot rolling process: The slab obtained from continuous casting is sequentially heated, descaled, rough rolled, finish rolled, laminar flow cooled, and coiled to obtain a hot-rolled coil; wherein, the descaling water pressure is ≥22MPa, the descaling temperature is ≥1173℃, the finish rolling passes are 7, the reduction rate of the last pass is ≤15%, and lubricated rolling is used. Other key hot rolling process parameters are detailed in Table 2:

[0092] Table 2 Main process parameters for hot rolling of 1180MPa grade super bainitic steel for automobiles

[0093] (d) Pickling and cold rolling process: After the hot-rolled coil is pickled to remove the iron oxide scale on the surface, it is cold-rolled into cold-rolled thin strip steel. The thicknesses of the strip steel in Examples 1, 2 and 3 are 1.6 mm, 1.8 mm and 1.0 mm, respectively, and their cold rolling reduction rates are 46.7%, 48.5% and 50.0%, respectively.

[0094] (e) Continuous annealing process: After the cold-rolled thin strip steel is processed by continuous annealing process to make the desired product, the strip steel is first slowly heated to the homogenization temperature (complete austenitization temperature) and held at the temperature for a period of time to achieve complete austenitization. Then, it is slowly cooled to the slow cooling endpoint temperature to decompose some of the supercooled austenite, forming a small amount of oriented epiphytic ferrite and "enriching" the remaining austenite with carbon. After that, the strip steel is rapidly cooled to transform the austenite into martensite and bainite. In the over-aging section, the austenite to bainite transformation occurs. Finally, it is cooled to room temperature and the strip shape is adjusted by the finishing machine to increase the yield strength.

[0095] The continuous annealing process specifically includes, in sequence, a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a rapid cooling section, and an aging section; wherein, the heating rates of the preheating section in Examples 1, 2, and 3 are 3.8℃ / s, 4.1℃ / s, and 4.5℃ / s, respectively, and the final temperatures are 220℃, 205℃, and 240℃, respectively; the heating rates of the oxidation section are 0.80℃ / s, 0.97℃ / s, and 0.81℃ / s, respectively, and the final temperatures are respectively... The heating rates in the heating section were 0.30℃ / s, 0.34℃ / s, and 0.23℃ / s, respectively, with final temperatures of 862℃, 865℃, and 853℃. The soaking time was 262.8s, 219.0s, and 245.5s, respectively, and the aging time was 1237.9s, 1031.6s, and 1143.3s, respectively. Other key continuous annealing process parameters are detailed in Table 3.

[0096] Table 3. Main process parameters for continuous annealing of 1180MPa grade super bainitic steel for automotive applications.

[0097] The microstructure of the 1180MPa grade super bainitic steel for automobiles prepared by the above process is shown in Figure 1, and the scanning electron microscope image is shown in Figure 2. Based on the microstructure of the steel in Example 1 shown in the figures, and combined with the phase volume fraction determination results in Figure 3 (EBSD phase diagram) and Figure 4 (XRD pattern), the microstructure of the 1180MPa grade super bainitic steel of Example 1 of this invention consists of 80%–90% lath bainite and 5–9% retained austenite and residual tempered martensite.

[0098] The properties of the above-mentioned 1180MPa grade super bainitic steel for automobiles were tested according to GB / T228-2010 "Metallic materials - Tensile testing at room temperature" and GB / T24524-2021 "Metallic materials - Test method for hole expansion in thin plates and strips". The mechanical properties are shown in Table 4 below:

[0099] Table 4 Mechanical Properties of 1180MPa Grade Super Bainitic Steel for Automotive Use

[0100] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A super bainite steel, characterized in that, The chemical composition of the super bainitic steel, by mass percentage, is as follows: 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%, with the remaining elements being Fe and unavoidable impurities, and simultaneously satisfying 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 bainitic steel consists of 80–90 vol% bainite, 3–11 vol% martensite, and 5–9 vol% retained austenite.

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

4. A method of producing the super bainite steel according to any one of claims 1 to 3, characterized in that, Includes the following steps: (a) Smelting process: Smelting molten steel according to the chemical composition of super bainitic steel; (b) Continuous casting process: The molten steel processed in step (a) is continuously cast into slabs; (c) Hot rolling process: The slab obtained in step (b) is heated by hot or cold charging, and then descaled, rough rolled, finish rolled, laminar flow cooled and coiled to obtain a hot rolled coil; (d) Pickling and cold rolling process: After the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, it is cold-rolled into a cold-rolled thin strip steel coil. (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed to obtain super bainitic 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℃; the dephosphorization water pressure is ≥22MPa and the temperature is ≥1173℃; the initial rolling temperature of the finishing rolling is 1000–1100℃, the finishing rolling passes are 7, the final rolling temperature is 890–950℃, and the reduction rate of the last pass is ≤15%; the laminar flow cooling method is sparse cooling; and 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 winding adopts a U-shaped winding mode; when winding, the winding temperature of the first 20m of the strip is 560-650℃, the winding temperature of the last 70m of the strip is 560-640℃, and the winding temperature of the middle section is 580℃.

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 is 1-2 mm.

9. The preparation method according to claim 4, characterized in that, In step (e), the continuous annealing sequentially 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; wherein, the temperature of the soaking section is 850-870°C, the final temperature of the slow cooling section is 740-770°C, the final 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 for each stage of the continuous annealing are as follows: The preheating section: heats from room temperature to 200-240°C at a rate of 1-5°C / s; The oxidation section is heated 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℃ to 850-870℃ at a rate of 0.2-0.4℃ / s. The heat spreader section is maintained at 850–870°C for 215–280 seconds. The slow cooling section cools the temperature from 850–870°C to 740–770°C at a rate of 0.6°C / s–1.0°C / s. The rapid cooling section cools the temperature from 740-770℃ to 350-380℃ at a rate of 16℃ / s-24℃ / s. The over-aging period: maintain 360-380℃ and keep warm for 1000-1300s.

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