1000MPa cold-rolled continuous annealing CH steel with ultrahigh reaming performance and preparation method of cold-rolled continuous annealing CH steel

Abstract

The invention belongs to the field of metal materials, and provides 1000 MPa cold-rolled continuous annealing CH steel with the ultrahigh broaching performance and a preparation method of the 1000 MPa cold-rolled continuous annealing CH steel with the ultrahigh broaching performance. 0.80% to 1.40% of Si; the content of Mn is 2.00%-2.20%, and the content of Ti is 0.025%-0.040%; p is less than or equal to 0.01%; s is less than or equal to 0.008%, and the balance is Fe and inevitable impurities. The preparation method comprises the steps of smelting, continuous casting, hot rolling, acid pickling, cold rolling, continuous annealing and flattening. The yield strength of the steel plate produced by the method is 850-950 MPa, the ductility is greater than 15%, and the hole expansion rate is greater than 80%; good fatigue performance is achieved, and the high-cycle fatigue strength limit is larger than or equal to 485 MPa.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a cold-rolled continuous annealed CH steel with ultra-high hole expansion performance of 1000MPa and its preparation method. Background Technology

[0002] Studies have shown that for every 100 kg reduction in vehicle weight, carbon emissions can be reduced by approximately 8.5 g / 100 km. Reducing vehicle weight through material selection to achieve emission reduction is a major method of lightweighting for automakers. Among advanced high-strength steels are quenched and distributed steel (QP), twinned-inducible plasticity (TWIP), transformation-induced plasticity (TRIP), dual-phase steel (DP), plastic-reinforced dual-phase steel (DH), and complex-phase steel (CP). Complex-phase steel has attracted much attention from automakers, suppliers, and manufacturers, mainly because: (1) Due to its multiphase structure, complex-phase steel can effectively improve the hole-expanding performance of materials; (2) Compared with products of the same tensile strength, complex-phase steel has higher yield strength and better local forming performance, making it particularly suitable for manufacturing safety parts with high flanging performance. Plastic-reinforced complex-phase steel (CH) introduces a certain amount of residual austenite into complex-phase steel and uses transformation-induced plasticity to increase the elongation of the complex-phase steel. During the period of rapid development of new energy vehicles, the design of automotive parts has become more diversified, and the requirements for the comprehensive performance of steel plates have become increasingly higher. Under the circumstances of intensified competition in the automotive market, how to produce cold-rolled continuous annealing plastic-reinforced multiphase steel (CH steel) with high hole expansion performance to meet market demand is an urgent problem for upstream steel companies.

[0003] The patent document "A Formed Reinforced Multiphase Steel and Its Preparation Method" (Publication No.: CN112251668A) discloses a formed reinforced multiphase steel with the following chemical composition: C: 0.17%~0.21%, Si: 0.2%~0.7%, Mn: 1.8%~2.5%, P: ≤0.01%, S: ≤0.005%, Al: 0.4%~0.8%, Cr: 0.1%~0.39%, Nb: 0.02%~0.04%; and containing at least one of the following elements: Ti: 0.01%~0.04%, Cu: 0.03%~0.1%, with the balance being Fe; the steel plate has a yield strength of 780MPa~920MPa, a tensile strength of 980MPa~1150MPa, a uniform elongation of not less than 8%, an elongation after fracture of not less than 10%, and a porosity greater than 40%. The addition of precious metal elements such as Nb and Cr to its cold-rolled multiphase steel increases production costs; moreover, the hole expansion rate is not high, which cannot meet the high flanging requirements of new models.

[0004] The patent document "A Cold-Rolled Multiphase Steel with Good Hole-Expanding Performance and Its Preparation Method" (Publication No.: CN108913991A) discloses a multiphase steel with the following chemical composition: C: 0.15%–0.20%, Si: 0.30%–0.80%, Mn: 1.7%–2.3%, P≤0.015%; S≤0.008%, Als≤0.08%, Nb: 0.02%–0.08%, Cr: 0.4%–0.8%, Ti: 0.02%–0.06%, with the balance being Fe and unavoidable impurities. The multiphase steel exhibits a yield strength exceeding 700 MPa, a tensile strength exceeding 980 MPa, an elongation exceeding 8%, and a hole-expanding rate exceeding 15%. Adding precious metal elements such as Nb and Cr to cold-rolled multiphase steel increases production costs, and a hole expansion rate of over 15% can no longer meet the high flanging performance requirements of car manufacturers.

[0005] To address the aforementioned problems, this invention, through the selection of alloying elements and ingenious process design, develops a 1000MPa ultra-high hole-expansion performance cold-rolled continuous annealed CH steel and its preparation method, thereby improving the steel sheet performance to meet the automotive industry's requirements for high flanging performance and high elongation. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel with excellent fatigue performance and its preparation method.

[0007] The objective of this invention is achieved as follows:

[0008] A cold-rolled continuous annealed CH steel with ultra-high hole expansion performance of 1000MPa, the composition of the CH steel by weight percentage is as follows: C: 0.18%~0.22%; Si: 0.80%~1.40%; Mn: 2.00%~2.20%; Ti: 0.025%~0.040%; P≤0.01%; S≤0.008%, with the balance being Fe and unavoidable impurities.

[0009] Furthermore, the CH steel also includes one or more of Cu, Al, and RE; wherein Cu: 0.1%~0.2%; Al: 0.04%~0.08%; RE: 0.01%~0.05%.

[0010] Furthermore, in this CH steel, 0.95%≤Si+Al+RE≤1.42%.

[0011] Furthermore, the microstructure of the CH steel includes epitaxial ferrite, bainite, tempered martensite, secondary martensite, and retained austenite; the area percentages of each microstructure are as follows: 8%≤epitaxy ferrite≤12%, 50%≤bainite≤62%, 10%≤tempered martensite≤15%, 8%≤secondary martensite≤12%, and the remainder is retained austenite; preferably, the width of the bainite laths in the CH steel is 5.2μm~8μm, the width of the secondary martensite laths is 3.4μm~4.7μm, the average size of the carbides precipitated in the tempered martensite is ≤80nm, and the retained austenite in the steel is distributed in a thin film form in the bainite and secondary martensite; preferably, the hardness ratios of each microstructure are: 1.8≤bainite / epitaxy ferrite≤2.5; 1.5≤tempered martensite / bainite≤2.0; 1.8≤secondary martensite / tempered martensite≤2.2.

[0012] Furthermore, the CH steel has a tensile strength ≥1000MPa, a yield strength between 850MPa and 950MPa, an elongation >15%, and a hole expansion rate >80%. It also exhibits good fatigue performance, with a high-cycle fatigue strength limit ≥485MPa.

[0013] The rationale for the design of the components in this invention is as follows:

[0014] C: Carbon, as an interstitial atom in steel, usually exists in solid solution form, enhancing the strength of steel plates through solid solution strengthening. Carbon dissolved in austenite increases the stability of supercooled austenite, allowing retained austenite to remain at room temperature, thus improving the plasticity of the steel plate by retaining a certain amount of retained austenite. However, excessively high carbon content affects the weldability and fatigue strength of the steel plate; therefore, the C content in this invention is set at 0.18%~0.22%.

[0015] Si dissolves in ferrite, increasing the strength of epitaxial ferrite. Si does not form carbides, effectively preventing carbide precipitation. However, excessive Si content can increase the carbon content in supercooled austenite, leading to the formation of high-strength martensite during forming, which is detrimental to the steel sheet's properties. Therefore, the Si content in this invention is set at 0.8%~1.4%.

[0016] Mn, dissolved in ferrite and austenite, expands the austenite phase region. It can lower the martensitic transformation temperature and critical cooling rate in steel, improve hardenability, and enhance the stability of retained austenite. Too low a manganese content leads to insufficient steel plate strength and affects the stability of retained austenite; too high a manganese content causes Mn segregation, severely affecting the formability of the steel plate. Therefore, the Mn content in this invention is set at 2.0%~2.2%.

[0017] Ti can form fine, dispersed precipitates with C and N elements in steel, refining the grain size and significantly improving the strength of the steel. However, excessive titanide precipitation can reduce the formability of the steel; therefore, the Ti content in this invention is set at 0.025%~0.04%.

[0018] P: Dissolves in ferrite and can improve the strength and hardness of steel. However, P tends to segregate at grain boundaries in steel, causing grain boundary embrittlement. P is a harmful element and should be strictly controlled. Therefore, the P content in this invention is less than or equal to 0.01%.

[0019] S is a harmful element in steel. It can form MnS inclusions with Mn, which are prone to crack initiation and reduce the flanging performance of steel plates. Therefore, the S content in this invention is less than or equal to 0.008%.

[0020] CH steel also includes one or more of Cu, Al, and RE.

[0021] Cu: Dissolves in austenite to increase the strength of the steel plate. During annealing, elemental Cu precipitates from the austenite, acting as precipitation strengthening. Adding Cu also increases resistance to atmospheric corrosion, extending the service life of the steel plate. Therefore, the Cu content in this invention is set at 0.1%~0.2%.

[0022] Al has a strong affinity for both oxygen and nitrogen, and is a deoxidizer used in steelmaking. Al can capture nitrogen in steel to form AlN, pinning grain boundaries, hindering grain migration, preventing grain coarsening at high temperatures, and reducing the strength of the steel plate. However, excessive Al content will increase inclusions in the steel and worsen the hole-expanding performance of the steel plate. Therefore, the Al content in this invention is set at 0.1%~0.2%.

[0023] RE (reactive oil) can transform oxides and sulfides in steel into fine, dispersed inclusions, thus reducing the harmful effects of inclusions such as MnS in steel. Simultaneously, RE improves the fluidity of steel, reduces non-metallic inclusions, and makes the steel microstructure denser and purer, significantly improving the fatigue performance of the steel. When used in combination with elements such as Si and Al in steel, it significantly enhances the strength of the steel through inclusion modification. Therefore, the RE content in this invention is set at 0.001%~0.005%.

[0024] This invention requires controlling the content of Si + Al + RE to 0.95% ≤ Si + Al + RE ≤ 1.42%. ① Si is a strong deoxidizer, combining with O to form chain-like or cluster-like SiO2 inclusions, which easily become crack initiations during forming. RE can react with SiO2 to form rare earth silicates, which transform into spherical shapes, reducing stress concentration and improving the fatigue performance of the steel plate. Al reacts with N in the steel to form AlN. AlN is prone to coarsening, and RE adsorbs onto its surface, inhibiting its growth and improving the hot strength of the steel plate. ② Excessive Si exacerbates the hot brittleness of the steel plate and reduces its surface quality; excessive Al will clog the gate, making tapping difficult. Therefore, the addition amounts of Si, Al, and RE need to be controlled to maximize their effects, enabling the steel plate to achieve high hole expansion performance and high fatigue performance.

[0025] The second technical solution of the present invention is to provide a method for preparing 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel, including smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and leveling;

[0026] Continuous casting:

[0027] The casting speed should be between 1.00 and 1.30 m / min. Excessive speed can lead to steel leakage, while insufficient speed will affect production efficiency. The billet thickness should be between 260 and 280 mm.

[0028] Hot-rolled:

[0029] The slab heating temperature is 1225℃~1250℃, requiring a holding time of 60~90 minutes within this range; the initial rolling temperature is 1060℃~1120℃, and the final rolling temperature is 900℃~930℃; the rolled steel plate is then laminar cooled to 620~670℃ at a laminar flow rate of 25~30℃ / s; the final thickness of the hot-rolled plate is 2.2~4.0mm. The coiling temperature is 620~670℃. The microstructure of the hot-rolled and coiled steel plate is ferrite + bainite + martensite, with the following area percentages: 25% ≤ ferrite ≤ 40%; 35% ≤ bainite ≤ 45%; the remainder is martensite.

[0030] The slab is heated to 1225℃~1250℃ and held for 60~90 minutes within this temperature range to ensure the dissolution of Ti compounds and maintain Ti supersaturation for subsequent precipitation. The fine, dispersed Ti compounds pin the original austenite, hindering austenite migration and refining the grain size. A reasonable holding time homogenizes the alloying elements and reduces compositional segregation. The initial rolling temperature is 1060℃~1120℃, and the final rolling temperature is 900℃~930℃. This final rolling temperature falls within the austenitic region, ensuring austenite recrystallization. After rolling, the steel plate undergoes laminar flow cooling to 620~670℃ at a rate of 25~30℃ / s. The final thickness of the hot-rolled plate is 2.2~4.0mm. The coiling temperature is 620~670℃, followed by slow cooling in a slow cooling pit. The microstructure of the coiled steel plate is ferrite + bainite + martensite. Excessive coiling temperature will result in poor surface quality and coil collapse; excessively low coiling temperature will lead to excessively high strength in the hot-rolled plate, making cold rolling difficult or even impossible. At this coiling temperature, the steel plate microstructure obtains 35%~45% bainite, which is beneficial for grain refinement during continuous annealing.

[0031] Pickling: Remove the iron oxide scale generated on the surface of the strip steel. The pickling temperature is set to 75℃~90℃ and the pickling speed is set to 150~170m / min.

[0032] Cold rolling: The cold rolling reduction rate is 50%~65%. The cold rolling deformation allows the steel sheet sufficient deformation energy storage, which is beneficial for annealing recrystallization and reduces A... C1 and A C3 Temperature control, while saving energy consumption and reducing production costs.

[0033] ⑥ Continuous annealing: This includes heating, initial heat preservation, slow cooling, rapid cooling, aging, temperature increase, secondary heat preservation, and cooling.

[0034] The strip steel is heated to 870-900℃ at a heating rate of 5-8℃ / s, held for 80-120s, slowly cooled to 730-770℃, cooled to 370-390℃ at a cooling rate of 20-25℃ / s, held for 350-420s, and then over-aged at this stage. The temperature is then raised to 440-460℃, held for 10-20s, and then cooled at a cooling rate of 12-17℃ / s before entering the cooling guide rollers. The temperature range is 200-220℃. After that, it is air-cooled and then leveled.

[0035] The strip steel is heated to 870-900℃ at a heating rate of 5-8℃ / s. If the heating rate is too low, the strip steel will remain in the furnace for too long, resulting in severe surface oxidation and affecting the final product quality. If the heating rate is too high, dislocation recovery and recrystallization cannot proceed sufficiently, the cold-rolled inherited banded structure cannot be eliminated, and the elongation of the steel plate will be reduced. This heating temperature is AC3+30℃~50℃, the purpose of which is to obtain a fully austenitic structure, eliminate critical ferrite, and improve the yield strength of the steel plate. The initial holding time is 80-120 seconds, followed by slow cooling to 730℃-770℃. Insufficient holding time leads to uneven alloy element homogenization; excessive holding time causes grain growth, resulting in a coarse final microstructure and affecting steel plate properties. This yields 8%-12% oriented epiphytic ferrite. Oriented epiphytic ferrite has a higher alloy content and higher strength than critical zone ferrite, providing good ductility and formability. It also reduces the hardness difference between bainite and tempered martensite, harmonizing microstructure deformation and resulting in good hole-expanding properties. Cooling is then performed at a rate of 20-25℃ / s to 370℃-390℃, followed by holding for 350-420 seconds. Over-aging is carried out during this stage to prevent the transformation of overcooled austenite into pearlite during cooling, which would reduce steel plate strength. A certain amount of martensite is generated during rapid cooling. Cooling to 370℃-390℃ completes the over-aging stage; this temperature range corresponds to the bainitic phase transformation region. If the overaging temperature is too low, the bainitic transformation rate decreases, thus reducing the bainite content. If the overaging temperature is too high, excessive bainite is generated at this stage, preventing the tensile strength of the steel plate from reaching 1000 MPa. The overaging stage yields 50%–62% bainite, which helps improve the hole-expanding performance of the steel plate. Carbon atoms in the bainite diffuse into the supercooled austenite, increasing the stability of the retained austenite. After subsequent cooling, a certain amount of retained austenite is obtained, ensuring the formability of the steel plate. Raising the temperature to 420–440℃ and holding it for 10–20 seconds allows the martensite generated during the initial rapid cooling to temper, forming tempered martensite. This reduces the strength of the martensite and decreases the hardness difference with the bainite phase, effectively improving the hole-expanding performance of the steel plate. Cooling to the cooling steering roller chamber at 12~17℃ / s, within the temperature range of 200~220℃, is mainly to obtain a small amount of secondary martensite to ensure the strength of the steel plate and a certain amount of residual austenite to improve the forming performance of the steel plate and meet the requirements of new vehicle parts.

[0036] Leveling: Leveling elongation is 0.2%~0.4%, ensuring the shape of the steel plate and improving the yield strength of the strip. Leveling can produce a surface roughness of 0.8μm~1.5μm on the strip, which is beneficial to subsequent steel plate coating processes.

[0037] The beneficial effects of this invention are as follows:

[0038] 1. The 1000MPa grade ultra-high hole-expanding performance cold-rolled continuous annealed CH steel of this invention uses C and Mn as the main alloying elements, without precious alloying elements such as Cr, Mo, Nb, and Ni, and has the advantage of lower raw material cost; the precipitation strengthening of Ti and Cu ensures the tensile strength of the steel plate; the combination of Al, Si, and RE modifies the inclusions in the steel, refines the grains, and significantly improves the hole-expanding performance and fatigue performance of the steel plate, making it an innovative industrial product.

[0039] 2. The microstructure obtained by this invention mainly consists of epitaxial ferrite, bainite, tempered martensite, combined with secondary martensite and retained austenite. Tempered martensite and retained austenite are introduced into the traditional CH ferrite + bainite + martensite microstructure. The tempered martensite reduces the hardness difference between the bainite and secondary martensite microstructures, improving the hole-expanding performance of the steel plate; the retained austenite exhibits a TRIP effect, enhancing the plasticity of the steel plate. This results in the product of this invention having excellent comprehensive performance and strong market competitiveness. The formation of this microstructure can be achieved using an innovative full austenitization combined with a quenching and temperature-raising process.

[0040] 3. The CH steel described in this invention has a tensile strength ≥1000MPa, a yield strength of 850MPa~950MPa, an elongation >15%, a hole expansion rate >80%, and a fatigue strength limit ≥485MPa, which meets the needs of upgrading and iterating automotive steel products. Attached Figure Description

[0041] Figure 1 This is a metallographic image of the microstructure of Embodiment 1 of the present invention. Detailed Implementation

[0042] The present invention will be further illustrated below through examples.

[0043] According to the technical solution's composition (the composition of the CH steel by weight percentage is as follows: C: 0.18%~0.22%; Si: 0.80%~1.40%; Mn: 2.00%~2.20%; Ti: 0.025%~0.040%; P≤0.01%; S≤0.008%, with the balance being Fe and unavoidable impurities), the process involves smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing, and leveling.

[0044] Continuous casting:

[0045] The billet pulling speed is 1.00~1.30m / min, and the billet thickness is 260~280mm;

[0046] Hot-rolled:

[0047] The slab heating temperature is 1225℃~1250℃, and the holding time is 60~90min; the initial rolling temperature is 1060℃~1120℃, and the final rolling temperature is 900℃~930℃; the rolled steel plate is cooled by laminar flow to 620~670℃; the coiling temperature is 620~670℃.

[0048] Pickling: Removes the iron oxide scale generated on the surface of the strip steel. The pickling temperature is 75℃~90℃ and the pickling speed is 150~170m / min.

[0049] Cold rolling: The cold rolling reduction rate is 50%~65%;

[0050] Continuous annealing: The strip steel is heated to 870~900℃ at a heating rate of 5~8℃ / s, held for 80~120s, and then slowly cooled to 730℃~770℃ at a slow cooling rate of 2.73℃ / s~3.11℃ / s. It is then cooled to 370℃~390℃ at a cooling rate of 20~25℃ / s, held for 350s~420s, and over-aging is performed during this stage. After that, the temperature is raised to 420℃~440℃, held for 10~20s, and then cooled at a cooling rate of 12~17℃ / s. It then enters the cooling guide roller chamber, with a temperature range of 200~220℃. Finally, it is air-cooled to room temperature.

[0051] Smoothness: Smoothness elongation rate is 0.2%~0.4%.

[0052] Furthermore, the microstructure of the hot-rolled coiled steel plate is ferrite + bainite + martensite, of which, by area percentage: 25% ≤ ferrite ≤ 40%; 35% ≤ bainite ≤ 45%; and the remainder is martensite.

[0053] Furthermore, the laminar cooling rate of the hot-rolled steel plate is 25~30℃ / s.

[0054] The composition of the steel in this embodiment is shown in Table 1. The main process parameters for continuous casting and rolling of the steel in this embodiment are shown in Table 2. The main process parameters for continuous annealing of the steel in this embodiment are shown in Table 3. The properties of the steel in this embodiment are shown in Table 4. The microstructure of the steel in this embodiment is shown in Table 5.

[0055] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0056] Example C Mn Si Ti P S Cu Al RE Si+Al+RE 1 0.186 2.14 0.95 0.038 0.008 0.004 0.17 - 0.03 0.98 2 0.194 2.15 1.24 0.032 0.009 0.005 - 0.04 0.04 1.32 3 0.217 2.07 0.88 0.031 0.01 0.006 0.12 0.06 0.01 0.95 4 0.206 2.13 1.36 0.026 0.006 0.008 0.13 - 0.03 1.39 5 0.192 2.03 1.40 0.035 0.007 0.007 - - 0.05 1.45 6 0.211 2.11 0.99 0.025 0.008 0.008 - 0.05 0.01 1.05 7 0.180 2.20 1.28 0.040 0.009 0.007 0.20 - 0.02 1.3 8 0.220 2.09 0.95 0.034 0.01 0.005 - - - 0.95

[0057] Table 2 Main process parameters for continuous casting and rolling of steel in the embodiments of the present invention.

[0058] Example Casting speed (m / min) Billet thickness (mm) Heating temperature ℃ Insulation time (min) Rolling temperature (°C) Final rolling temperature ℃ Laminar cooling rate (°C / s) Laminar cooling temperature ℃ Hot-rolled coil thickness (mm) Ferrite content / % Bainite content / % Martensite content / % 1 1.13 277 1227 67 1069 913 28.6 633 2.2 30.3 40.3 29.4 2 1.21 267 1245 84 1084 900 27.5 620 2.8 25.9 43.7 30.4 3 1.30 260 1242 70 1110 924 25.6 662 2.9 38.7 36.3 25.0 4 1.18 270 1240 77 1105 930 28.4 620 3.2 28.4 43.1 28.5 5 1.26 263 1236 69 1096 908 26.3 637 4.0 32.5 38.7 28.8 6 1.00 280 1228 85 1060 919 29.4 645 3.7 35.1 38.4 26.5 7 1.17 274 1247 90 1120 922 26.7 670 3.6 39.2 35.0 25.8 8 1.22 265 1250 88 1118 911 25.1 656 2.9 36.5 35.8 27.7

[0059] Table 3 Main process parameters of continuous annealing in the embodiments of the present invention

[0060] Example Cold rolling reduction rate / % Pickling temperature ℃ Pickling rate (m / min) Heating rate ℃ / s Heating temperature ℃ Isothermal time s Slow cooling rate (℃ / s) Slow cooling temperature ℃ Rapid cooling rate (℃ / s) Over-aging temperature ℃ Insulation time (s) Temperature rise ℃ Secondary heat preservation time (s) Cooling rate ℃ / s Cooling guide roller chamber / ℃ Smooth elongation % 1 56 81 151 6.7 874 92 3.02 738 21.9 386 377 430 13 13.4 200 0.28 2 61 76 163 7.1 870 85 3.11 730 23.2 378 381 432 15 15.1 215 0.32 3 52 83 159 7.8 896 80 3.04 759 20 390 405 436 10 17.0 203 0.26 4 59 89 169 5.0 879 96 2.97 745 22.5 388 420 440 16 14.3 212 0.24 5 63 77 160 5.7 885 103 3.04 748 23.1 370 364 423 20 15.5 209 0.33 6 54 82 166 8.0 893 107 2.73 770 22.7 382 350 420 15 12.0 205 0.29 7 58 86 157 7.5 888 120 2.75 764 24.5 373 379 428 18 16.2 220 0.35 8 55 79 153 5.2 900 114 2.95 767 25.0 384 393 435 19 14.4 211 0.28

[0061] Table 4 Properties of steel in embodiments of the present invention

[0062] Example Yield strength MPa Tensile strength (MPa) elongation % Hole expansion rate % Fatigue strength limit MPa 1 859 1024 15.5 80.6 498 2 882 1067 15.2 81.9 501 3 943 1042 15.3 81.3 489 4 950 1058 16.5 83.8 497 5 887 1044 15.7 82.4 504 6 863 1000 16.1 81.5 492 7 850 1012 15.7 80.4 487 8 877 1033 16.6 82.2 485

[0063] Table 5. Ratio of steel microstructure content to hardness of each phase in embodiments of the present invention.

[0064] Example Epitaxial ferrite % Bainite% Tempered martensite% Secondary martensite% Residual austenite % Bainite / epitaxial ferrite Tempered martensite / bainite Secondary martensite / tempered martensite A B C 1 11.2 57.5 10.5 10.3 10.5 2.03 1.75 1.93 5.5 3.6 75 2 10.9 56.7 14.3 8.2 9.9 1.92 1.67 2.04 7.6 4.7 64 3 10.5 57.8 12.2 9.5 10.0 2.46 1.95 2.20 6.8 3.9 68 4 10.7 51.9 13.5 11.4 12.5 1.80 1.58 2.15 5.9 4.0 76 5 12.0 56.3 15.0 9.4 7.3 2.14 1.74 1.83 7.8 3.4 55 6 8.0 56.3 13.7 8.0 14.0 2.31 1.92 1.89 5.9 3.8 74 7 8.6 62.0 11.9 9.5 8.0 2.18 1.77 1.92 7.4 4.2 67 8 11.3 50.0 15.0 9.7 14.0 2.05 1.69 2.01 6.3 4.3 79

[0065] Note: A is the width of the bainite lath, μm; B is the width of the secondary martensite lath, μm; C is the average size of the carbides precipitated in the tempered martensite, nm.

[0066] The microstructure of CH steel produced using this invention comprises epitaxial ferrite, bainite, tempered martensite, secondary martensite, and retained austenite; its area percentage content is as follows: 8% ≤ epitaxial ferrite ≤ 12%, 50% ≤ bainite ≤ 62%, 10% ≤ tempered martensite ≤ 15%, 8% ≤ secondary martensite ≤ 12%, and the remainder is retained austenite; the bainite lath width is 5.2μm~8μm, the martensite lath width is 3.4μm~4.7μm, the average size of carbides precipitated in tempered martensite is ≤80nm, and the retained austenite in the steel is distributed in a thin film form in the bainite and secondary martensite. The CH steel plate has a tensile strength ≥1000MPa, a yield strength of 850MPa~950MPa, an elongation >15%, and a porosity >80%. It exhibits good fatigue performance, with a fatigue strength limit ≥485MPa.

[0067] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A cold-rolled continuously annealed CH steel with ultra-high hole expansion performance of 1000MPa, characterized in that, The composition of this CH steel by weight percentage is as follows: C: 0.18%~0.22%; Si: 0.80%~1.40%; Mn: 2.00%~2.20%; Ti: 0.025%~0.040%; P≤0.01%; S≤0.008%, with the balance being Fe and unavoidable impurities.

2. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, The CH steel also includes one or more of Cu, Al, and RE; wherein Cu: 0.1%~0.2%; Al: 0.04%~0.08%; RE: 0.01%~0.05%.

3. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, In this CH steel, 0.95% ≤ Si + Al + RE ≤ 1.42%.

4. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, The microstructure of the CH steel includes epitaxial ferrite, bainite, tempered martensite, secondary martensite, and retained austenite; the microstructures are as follows by area percentage: 8%≤epitudinal ferrite≤12%, 50%≤bainite≤62%, 10%≤tempered martensite≤15%, 8%≤secondary martensite≤12%, and the remainder is retained austenite.

5. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 4, characterized in that, The width of the bainite laths is 5.2μm~8μm, the width of the secondary martensite laths is 3.4μm~4.7μm, the average size of the carbides precipitated in the tempered martensite is ≤80nm, and the retained austenite in the steel is distributed in the form of a thin film in the bainite and secondary martensite.

6. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 4, characterized in that, The hardness ratios of various microstructures are as follows: 1.8≤bainite / elongated ferrite≤2.5; 1.5≤tempered martensite / bainite≤2.0; 1.8≤secondary martensite / tempered martensite≤2.

2.

7. The 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, The CH steel has a tensile strength ≥1000MPa, a yield strength of 850MPa~950MPa, an elongation >15%, a hole expansion rate >80%, and a high cycle fatigue strength limit ≥485MPa.

8. A method for preparing a 1000MPa ultra-high hole-expanding performance cold-rolled continuously annealed CH steel according to any one of claims 1-7, comprising smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing, and leveling; characterized in that: Continuous casting: The billet pulling speed is 1.00~1.30m / min, and the billet thickness is 260~280mm; Hot-rolled: The slab heating temperature is 1225℃~1250℃, and the holding time is 60~90min; the initial rolling temperature is 1060℃~1120℃, and the final rolling temperature is 900℃~930℃; the rolled steel plate is cooled by laminar flow to 620~670℃; the coiling temperature is 620~670℃. Pickling: Removes the iron oxide scale generated on the surface of the strip steel. The pickling temperature is 75℃~90℃ and the pickling speed is 150~170m / min. Cold rolling: The cold rolling reduction rate is 50%~65%; Continuous annealing: The strip steel is heated to 870-900℃ at a heating rate of 5-8℃ / s, held for 80-120s, and then slowly cooled to 730-770℃ at a slow cooling rate of 2.73-3.11℃ / s. It is then cooled to 370-390℃ at a cooling rate of 20-25℃ / s, held for 350-420s, and over-aging is performed during this stage. After that, the temperature is raised to 420-440℃, held for 10-20s, and then cooled at a cooling rate of 12-17℃ / s. It then enters the cooling guide roller chamber at a temperature range of 200-220℃. Finally, it is air-cooled to room temperature. Smoothness: Smoothness elongation rate is 0.2%~0.4%.

9. The method for preparing 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 8, characterized in that: The microstructure of the hot-rolled steel plate is ferrite + bainite + martensite, with the following percentages by area: 25% ≤ ferrite ≤ 40%; 35% ≤ bainite ≤ 45%; and the remainder is martensite.

10. The method for preparing 1000MPa ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 8, characterized in that: The laminar cooling rate of the hot-rolled steel plate is 25~30℃ / s.

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