800mpa grade ultra-high hole expansion performance cold-rolled continuous annealing ch steel and preparation method thereof

Abstract

This invention belongs to the field of metallic materials and provides a cold-rolled, continuously annealed CH steel with ultra-high hole-expanding performance of 800MPa and its preparation method. The composition of the CH steel, by weight percentage, is as follows: C: 0.13%~0.16%, Mn: 2.0%~2.3%, Si: 0.4%~0.7%, Ti: 0.015%~0.03%, P≤0.01%, S≤0.008%, with the balance being Fe and unavoidable impurities. The preparation method includes smelting, continuous casting, hot rolling, pickling, cold rolling, and continuous annealing. The CH steel produced by this invention has a tensile strength of over 800MPa, a yield strength of 750~770MPa, an elongation of >16%, a hole-expanding rate of >90%, and good fatigue performance, with a high-cycle fatigue strength limit ≥500MPa and no cracking after 180° bending.

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 800MPa and its preparation method. Background Technology

[0002] The automotive industry is transitioning from the era of gasoline-powered vehicles to new energy vehicles. In recent years, consumers have prioritized economic value, safety, and environmental friendliness in their vehicle demands. Automakers are using material selection to achieve lightweighting and reduce fuel consumption. Advanced high-strength steel can replace traditional high-strength steel in this regard. New generations include DP steel, CP steel, TWIP steel, TRIP steel, hot-formed steel, and DH steel. DH steel, also known as plasticity-reinforced dual-phase steel, is an upgraded version of DP steel. Compared to CP steel, CH steel further enhances plasticity and hole-expanding properties through microstructure control while maintaining the high flanging (i.e., high hole-expanding ability) of CP steel, making it an upgraded replacement for CP steel. Currently, developing CH steel that balances hole-expanding performance and plasticity is a hot research topic in composite CH steel research.

[0003] The patent document "A Cold-Rolled Multiphase Steel and Its Annealing Method" (Publication No. CN115572899A) discloses the following chemical composition of the multiphase steel: 0.05%≤C≤0.14%, 0.2%≤Si+Al≤0.6%, 1.5%≤Mn+Cr≤2.8%, 0.02%≤Nb+Ti≤0.08%, Mo≤0.3%, V≤0.15%, B≤0.003%, with the remainder being Fe and unavoidable impurities. When its tensile strength is 780~880MPa, its elongation after fracture (A80) ≥20% and its hole expansion rate ≥40%. This steel plate has high elongation and low hole expansion performance, which is more conducive to stamping. However, it cannot meet the requirements for roll forming, which requires high hole expansion performance. Secondly, the addition of expensive alloying elements such as Cr, Nb, V, and B increases production costs.

[0004] The patent document "A Cold-Rolled Multiphase Steel with Excellent Formability of 800MPa and Its Preparation Method" (Publication No. CN116043121B) discloses the following chemical composition of the steel plate: C: 0.06%~0.075%, Si: 0.1%~0.4%, Mn: 1.8%~2.5%, Cr: 0.1%~0.4%, 2.2%≤Mn+Cr≤3.0%, Mo: 0.05%~0.3%, 1 / 3≤Mo / Cr≤2 / 3, P≤0.02%, S≤0.005%, Ti: 0.01%~0.03%, Nb: 0.015%~0.03%, with the balance being Fe and unavoidable impurities; the finished steel plate has a tensile strength of over 800MPa, a yield strength of 620~750MPa, an elongation ≥14%, and a hole expansion rate ≥70%. The performance of this steel plate is improved compared to traditional multiphase steel, but it cannot meet the forming requirements of complex parts; secondly, the large number of expensive alloying elements such as Cr, Nb, V, B, etc. greatly increases the production cost and cannot meet the low-cost requirements of automobile manufacturers. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide an 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel with excellent fatigue performance and its preparation method.

[0006] The objective of this invention is achieved as follows:

[0007] A cold-rolled continuously annealed CH steel with ultra-high hole expansion performance of 800MPa grade, the composition of the steel plate by weight percentage is as follows: C: 0.13%~0.16%, Mn: 2.0%~2.3%, Si: 0.4%~0.7%, Ti: 0.015%~0.03%, P≤0.01%, S≤0.008%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the CH steel also includes one or more of Cu, Al, and RE, wherein Cu: 0.05%~0.1%; Al: 0.01%~0.04%; RE: 0.005%~0.012%. Preferably, in the CH steel, 0.484%≤Si+Al+RE≤0.711%.

[0009] Further, the microstructure of the CH steel comprises epitaxial ferrite, bainite, tempered martensite, and secondary martensite. By area percentage: 10% ≤ epitaxial ferrite ≤ 15%, 50% ≤ bainite ≤ 55%, 6% ≤ tempered martensite ≤ 11%, 5% ≤ secondary martensite ≤ 10%, with the remainder being retained austenite. Preferably, the bainite lath width is 4.5~6μm, the secondary martensite lath width is 3.7~4.8μm, and the carbide precipitation size in the tempered martensite is <80nm. The hardness ratios of the various microstructures are: 1.5 ≤ bainite / epitaxyferrite ≤ 1.9; 1.3 ≤ tempered martensite / bainite ≤ 1.7; 1.6 ≤ secondary martensite / tempered martensite ≤ 2.1.

[0010] Furthermore, the CH steel has a tensile strength of over 800 MPa, a yield strength of 750~770 MPa, an elongation of >16%, a hole expansion rate of >90%, does not crack when bent at 180°, has good fatigue performance, and a high-cycle fatigue strength limit of ≥500 MPa.

[0011] The rationale for the design of the components in this invention is as follows:

[0012] C: Carbon is a relatively economical solid solution strengthening element in steel materials and is an essential element, ensuring the tensile strength of steel plates. Carbon content affects the austenitization process; the carbon content in supercooled austenite influences the carbon content and phase ratio in the final bainite and tempered martensite, directly affecting hole-expanding performance. On the other hand, it affects the amount of TiC precipitation; fine, dispersed TiC precipitation is beneficial for strengthening ferrite, reducing the hardness difference between ferrite, bainite, and secondary martensite, and thus improving hole-expanding performance. Excessive carbon content directly increases the carbon equivalent, deteriorating the weldability of the steel plate.

[0013] Manganese (Mn) is an economical strengthening element in steel. It can expand the austenite phase region, thereby lowering the A1 point temperature. As an austenite stabilizing element, it can shift the critical transformation curve of supercooled austenite to the right, thus delaying the pearlite transformation and ensuring the strength of the steel plate. However, excessive addition of Mn can lead to carbon-manganese segregation with carbon, which deteriorates the mechanical properties of the steel plate.

[0014] Silicon (Si): As a ferrite strengthening element, silicon strengthens ferrite and increases its yield strength, tensile strength, and hardness. In this invention, the addition of silicon inhibits carbide precipitation during the aging stage, reduces the amount of carbide precipitation, and improves the formability of the steel sheet.

[0015] Ti: As a microalloying element, carbon can "capture" free nitrogen in steel. During hot rolling and homogenization, it can hinder the migration of the original austenitic grain boundaries and refine the microstructure. During annealing, Ti carbides or carbonitrides precipitate to strengthen the matrix and ensure the tensile strength of the steel.

[0016] P: Phosphorus is a harmful element in steel, easily segregating at grain boundaries and reducing grain boundary strength; its content should be as low as possible. Considering cost, the P content in this invention is controlled at P≤0.01%.

[0017] Sulfur (S) is a harmful element in steel, easily forming MnS inclusions with manganese (Mn), which reduces the fatigue performance of the steel plate. Therefore, the lower the sulfur content, the better. Considering cost, the sulfur content in this invention is controlled at S ≤ 0.008%.

[0018] This CH steel also includes one or more of Cu, Al, and RE.

[0019] Cu: Dissolves in austenite to increase the strength of steel plates. During annealing, elemental Cu precipitates from austenite, providing 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 0.05~0.1%.

[0020] 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 0.01%~0.04%.

[0021] 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 0.005%~0.012%.

[0022] This invention requires controlling the addition amounts of Si + Al + RE to be 0.484 ≤ Si + Al + RE ≤ 0.711. 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, transforming their morphology 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, which is prone to coarsening. 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.

[0023] The second technical solution of the present invention is to provide a method for preparing 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel, including smelting, continuous casting, hot rolling, pickling, cold rolling, and continuous annealing;

[0024] Continuous casting: Industrial continuous casting is carried out according to the above composition, with a casting temperature of 1550℃-1600℃, a billet pulling speed of 0.9~1.2m / min, and a billet thickness of 220~280mm.

[0025] Hot rolling: Heating temperature between 1230~1250℃, furnace time between 210~250min, roughing temperature between 1120~1150℃, finishing temperature above 900℃. Coiling temperature between 630~650℃, hot-rolled coil thickness between 3.5~4.0mm.

[0026] The heating temperature is between 1230 and 1250℃, and the furnace time is between 210 and 250 minutes to ensure homogenization of alloying elements and minimize component segregation. The rough rolling temperature is between 1120 and 1150℃, and the finish rolling temperature is above 900℃ to ensure that the original recrystallization behavior is promoted during hot rolling and to inhibit the coarsening of non-recrystallized grains. The coiling temperature is between 630 and 650℃. Preferably, the coiled microstructure is ferrite + bainite + martensite, wherein, by area percentage: ferrite content: 40% to 50%; bainite content: 27% to 35%, and the remainder is martensite.

[0027] Pickling: Pickling temperature is 75℃~90℃, and pickling speed is 150m / min~170m / min.

[0028] Cold rolling: The cold rolling reduction rate is 50%~65%. The cold rolling stage stores a certain amount of deformation energy, reduces the AC1 and AC3 points of the steel plate, and is beneficial to recovery and recrystallization in the continuous annealing stage.

[0029] Continuous annealing: The cold-rolled slab is heated to 880℃~930℃, isothermaled for 100~130s, then slowly cooled to 760℃~790℃ at a cooling rate of 1.3~2.5m / s, and then rapidly cooled to 330~380℃ at a cooling rate of 20-30℃. It is then aged at 330~380℃ and held for 300~420s. The temperature is then increased to 430~450℃ at a rate of 8℃ / s~12℃ / s and held for 10s~20s. Finally, it is cooled at a cooling rate of 12~20℃ / s until the temperature at the top of the cooling tower is ≤200℃, and then it enters the finishing process.

[0030] The cold-rolled slab is heated to 880℃~930℃, which is the full austenitization temperature, with the aim of obtaining a fully austenitized microstructure. It is isothermally cooled for 100~130s to ensure uniform diffusion of alloying elements during austenitization. It is then slowly cooled to 760℃~790℃. This slow cooling temperature is designed to obtain a certain amount of epitaxial ferrite, and its content is controlled to be 10%≤epitudinal ferrite≤15% to prevent excessive ferrite formation from reducing the strength of the steel plate. On the other hand, it also promotes the diffusion of excess carbon into the cold austenite, thereby affecting the phase transformation variables of the subsequent transformation from supercooled austenite to bainite / martensite. Subsequently, the temperature is rapidly cooled to 330-380℃ at a rate of 20-30℃ / s to ensure the transformation of supercooled austenite into bainite / martensite. Aging is then performed at 330-380℃ for 300-420 seconds to promote bainite transformation, yielding 50% ≤ bainite ≤ 55%. The temperature is then increased to 430-450℃ at a rate of 8-12℃ / s to ensure the martensite undergoes a tempering reaction, transforming into tempered martensite. Compared to martensite, tempered martensite exhibits a higher carbon content, originating from the solid solution. Carbide precipitation reduces the degree of martensite lattice distortion, thereby reducing its hardness and mitigating the hardness difference with ferrite / bainite. During forming, it reduces stress concentration at the ferrite / bainite interface, improving formability. Then, it is cooled at a cooling rate of 12~20℃ / s until the temperature at the top of the cooling tower is ≤200℃. At this point, some of the untransformed supercooled austenite in the steel will transform into secondary martensite to compensate for the lack of strength. The untransformed austenite is retained as residual austenite, which improves the plasticity of the steel plate.

[0031] The beneficial effects of this invention are as follows:

[0032] (1) The 800MPa grade ultra-high hole-expanding performance cold-rolled continuous annealed CH steel involved in this invention is mainly composed of C and Mn elements, breaking the traditional CH steel manufacturing process that adds a large number of precious alloying elements such as Cr, Mo, Nb, and B, greatly reducing the production cost of steel plates and creating greater profit margins for enterprises. Through the precipitation strengthening of Ti and Cu, the tensile strength of the steel plate is guaranteed; by using Al, Si, and RE in combination, the inclusions in the steel are further modified, the grains are refined, and the hole-expanding performance and fatigue performance of the steel plate are significantly improved. It is an innovative industrial product.

[0033] (2) The microstructure of this invention is mainly epitaxial ferrite, bainite, tempered martensite, combined with secondary martensite and retained austenite. It makes full use of bainite phase transformation, martensite phase transformation, martensite tempering softening and other microstructure transformations, effectively controlling the proportion of each phase and hardness difference in the final microstructure. The final product takes into account high porosity, high plasticity and good bending properties. The microstructure can be achieved through a brand-new full austenitization combined with quenching and heating process.

[0034] (3) The CH steel described in this invention has a tensile strength of over 800 MPa, a yield strength of 750~770 MPa, an elongation of >16%, and a hole expansion rate of >90%, which has good hole expansion performance and fatigue performance, thus broadening the application scenarios of CH steel and making it more suitable for the manufacturing needs of new energy vehicles. Attached Figure Description

[0035] Figure 1 This is a metallographic image of the microstructure of Embodiment 1 of the present invention. Detailed Implementation

[0036] The present invention will be further illustrated below through embodiments. The embodiments of the present invention, based on the component ratio of the technical solution, involve smelting, continuous casting, hot rolling, pickling, cold rolling, and continuous annealing.

[0037] Continuous casting:

[0038] The casting temperature is 1550℃-1600℃, the billet pulling speed is 0.9~1.2m / min, and the billet thickness is 220~280mm.

[0039] Hot rolling: heating temperature 1230~1250℃, furnace time 210~250min, rough rolling temperature 1120~1150℃, finishing rolling temperature above 900℃; coiling temperature 630~650℃, hot rolled coil thickness 3.5~4.0mm.

[0040] Pickling: Pickling temperature is 75℃~90℃; pickling speed is 150~170m / min.

[0041] Cold rolling: The cold rolling reduction rate is 50%~65%;

[0042] Continuous annealing: The cold-rolled slab is heated to 880~930℃, isothermaled for 100~130s, then slowly cooled to 760~790℃ at a cooling rate of 1.3~2.5m / s, and then rapidly cooled to 330~380℃ at a cooling rate of 20~30℃ / s. It is then aged at 330~380℃ and held for 300~420s. The temperature is then increased to 430~450℃ at a cooling rate of 8℃ / s~12℃ / s, and then cooled at a cooling rate of 12~20℃ / s before entering the finishing process.

[0043] Finishing: A constant elongation control method is adopted, and the finishing elongation is 0.6%~0.8%.

[0044] Furthermore, the microstructure of the hot-rolled steel plate is ferrite + bainite + martensite, with the following percentages by area: 40% ≤ ferrite ≤ 50%, 27% ≤ bainite ≤ 35%, and the remainder being martensite.

[0045] The composition of the steel in this embodiment of the invention is shown in Table 1. The microstructure of the steel in this embodiment of the invention is shown in Table 2. The main process parameters for continuous casting and rolling of the steel in this embodiment of the invention are shown in Table 3. The main process parameters for continuous annealing of the steel in this embodiment of the invention are shown in Table 4. The properties of the steel in this embodiment of the invention are shown in Table 5.

[0046] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0047] Example C Mn Si Ti P S Cu Al RE Si+Al+RE 1 0.134 2.30 0.68 0.027 0.01 0.008 0.054 0.023 0.008 0.711 2 0.160 2.02 0.44 0.022 0.005 0.007 0.073 0.032 0.012 0.484 3 0.152 2.12 0.56 0.016 0.008 0.008 - 0.032 0.009 0.601 4 0.144 2.26 0.53 0.019 0.006 0.008 0.082 0.014 0.005 0.549 5 0.140 2.21 0.60 0.024 0.007 0.007 - 0.038 0.007 0.645 6 0.155 2.06 0.59 0.015 0.009 0.005 - 0.029 0.009 0.628 7 0.130 2.30 0.66 0.030 0.007 0.006 0.085 - 0.010 0.670 8 0.137 2.28 0.64 0.008 0.005 0.091 - 0.005 0.645

[0048] Table 2. Ratio of steel microstructure content to hardness of each phase in embodiments of the present invention.

[0049] Example Epitaxial ferrite content / % Bainite content / % Tempered martensite content / % Secondary martensite / % Residual austenite / % Bainite / epitaxial ferrite Tempered martensite / bainite Secondary martensite / tempered martensite A B C 1 14.9 61.2 11.0 7.7 12.9 1.66 1.37 1.68 4.8 3.9 77 2 12.6 60.3 9.7 5.6 11.8 1.83 1.59 2.05 5.7 4.6 73 3 13.3 60.1 8.4 5.0 13.2 1.65 1.70 2.10 6.0 4.3 70 4 15.0 53.0 9.3 7.1 15.6 1.78 1.61 1.95 4.9 4.8 75 5 14.2 58.8 6.2 6.4 14.4 1.69 1.55 1.74 5.3 4.0 71 6 13.7 56.0 7.7 7.3 14.3 1.90 1.42 1.91 5.5 3.7 76 7 11.8 57.7 10.1 6.0 14.4 1.56 1.57 1.68 4.5 3.9 72 8 10 60.4 10.5 10.0 11.1 1.50 1.80 1.92 5.1 4.5 74

[0050] 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 carbide precipitates in the martensite, nm.

[0051] Table 3. Main process parameters and microstructure content of steel continuous casting and rolling in the embodiments of the present invention.

[0052] Example Casting temperature / ℃ Casting speed / m / min Slab thickness / mm Heating temperature / ℃ Furnace time / min Rough rolling temperature / ℃ Finishing rolling temperature / ℃ Winding temperature / ℃ Hot-rolled coil thickness / mm Cold rolling reduction rate / % Ferrite content / % Bainite content / % Martensite content / % 1 1567 0.98 228 1246 218 1134 923 632 3.67 56 43.2 34.5 22.3 2 1582 1.13 245 1232 246 1123 914 643 3.52 58 47.5 33.4 19.1 3 1593 1.05 276 1250 210 1130 931 630 3.71 53 42.8 32.1 25.1 4 1574 1.09 280 1237 225 1141 910 639 3.54 57 46.1 30.6 23.3 5 1580 1.20 258 1242 241 1150 916 650 3.91 55 50 27.4 22.6 6 1550 0.95 247 1250 213 1147 925 644 4.0 60 48.7 29.2 22.1 7 1586 0.91 239 1235 239 1121 916 649 3.50 62 49.2 28.3 22.5 8 1579 0.14 224 1230 250 1139 920 635 3.79 64 45.6 29.8 24.6

[0053] Table 4 Main heat treatment process parameters for cold rolling and annealing of steel in embodiments of the present invention.

[0054] Example Heating temperature / ℃ Isothermal time / s Slow cooling rate / ℃ / s Slow cooling temperature / ℃ Rapid cooling rate / ℃ / s Over-aging temperature / ℃ Insulation time / s Heating rate / ℃ / s Final temperature rise / °C Rapid cooling rate / ℃ / s Temperature at the top of the cooling tower / °C Elongation / % 1 889 126 1.7 778 25.4 337 319 8.5 448 20 148 0.75 2 915 107 1.3 782 28.1 349 365 10.3 441 16.8 189 0.80 3 930 100 1.9 769 23.2 380 342 9.4 437 15.3 182 0.73 4 895 113 2.0 760 21.3 354 420 12.0 447 18.7 167 0.64 5 880 130 1.4 765 26.4 362 386 11.5 430 12.0 199 0.60 6 902 106 2.2 773 29.1 373 412 10.1 433 14.6 193 0.72 7 884 119 2.5 786 22.6 332 411 9.7 444 17.1 174 0.69 8 923 123 1.6 790 27.2 343 354 8.3 446 17.9 165 0.62

[0055] Table 5 Mechanical properties of steel in the embodiments of the present invention

[0056] Example Yield strength / MPa Tensile strength / MPa Elongation / % Hole expansion rate / % High-cycle fatigue strength limit / MPa 180° bend 1 758 820 16.9 107 505 No cracking 2 763 823 17.5 103 522 No cracking 3 752 800 17.3 99 517 No cracking 4 750 802 17.9 104 509 No cracking 5 754 813 16.4 100 502 No cracking 6 769 817 16.8 102 514 No cracking 7 765 811 16.9 97 506 No cracking 8 770 834 16.3 95 512 No cracking

[0057] The microstructure of CH steel produced using this invention comprises epitaxial ferrite, bainite, tempered martensite, secondary martensite, and retained austenite; by area percentage: 10% ≤ epitaxial ferrite ≤ 15%, 50% ≤ bainite ≤ 55%, 6% ≤ tempered martensite ≤ 11%, 5% ≤ secondary martensite ≤ 10%, with the remainder being retained austenite. The bainite lath width is 4.5~6 μm, the secondary martensite lath width is 3.7~4.8 μm, and the average size of carbide precipitates in the tempered martensite is < 80 nm. The hardness ratios of the various microstructures are: 1.5 ≤ bainite / epitaxyferrite ≤ 1.9; 1.3 ≤ tempered martensite / bainite ≤ 1.7; 1.6 ≤ secondary martensite / tempered martensite ≤ 2.1. With tensile strength above 800MPa, yield strength between 750 and 770MPa, elongation >16%, and hole expansion rate >90%, CH steel has good hole expansion and fatigue performance, which broadens the application scenarios of CH steel and makes it more suitable for the manufacturing needs of new energy vehicles.

[0058] 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 800MPa, characterized in that, The composition of CH steel by weight percentage is as follows: C: 0.13%~0.16%, Mn: 2.0%~2.3%, Si: 0.4%~0.7%, Ti: 0.015%~0.03%, P≤0.01%, S≤0.008%, with the balance being Fe and unavoidable impurities. The microstructure of CH steel includes epitaxial ferrite, bainite, tempered martensite, secondary martensite, and retained austenite. By area percentage: 10%≤epitudinal ferrite≤15%, 50%≤bainite≤55%, 6%≤tempered martensite≤11%, 5%≤secondary martensite≤10%, with the remainder being retained austenite. CH steel has a tensile strength of over 800MPa, a yield strength of 750~770MPa, an elongation of >16%, a porosity of >90%, and a high-cycle fatigue strength limit of ≥500MPa. The method for preparing 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel includes the following steps: Continuous casting: The casting temperature is 1550℃~1600℃, the billet pulling speed is 0.9~1.2m / min, and the billet thickness is 220~280mm; Hot rolling: heating temperature 1230~1250℃, furnace time 210~250min, rough rolling temperature 1120~1150℃, finishing rolling temperature above 900℃; coiling temperature 630~650℃, hot rolled coil thickness 3.5~4.0mm; Pickling: Pickling temperature is 75℃~90℃; pickling speed is 150~170m / min; Cold rolling: The cold rolling reduction rate is 50%~65%; Continuous annealing: The cold-rolled slab is heated to 880~930℃, isothermaled for 100~130s, then slowly cooled to 760~790℃ at a cooling rate of 1.3~2.5℃ / s, and then rapidly cooled to 330~380℃ at a cooling rate of 20~30℃ / s. It is then aged at 330~380℃ and held for 300~420s. The temperature is then increased to 430~450℃ at a cooling rate of 8℃ / s~12℃ / s, and then cooled at a cooling rate of 12~20℃ / s before entering the finishing process. Finishing: A constant elongation control method is adopted, and the finishing elongation is 0.6%~0.8%.

2. The 800MPa grade 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: 0.05~0.1%; Al: 0.01%~0.04%; RE: 0.005%~0.012%.

3. The 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 2, characterized in that, In this CH steel, 0.484% ≤ Si + Al + RE ≤ 0.711%.

4. The 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, The width of the bainite laths is 4.5~6μm, the width of the secondary martensite laths is 3.7~4.8μm, and the average size of the carbide precipitates in the tempered martensite is <80nm.

5. The 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that, The hardness ratios of various microstructures are as follows: 1.5≤bainite / epipolar ferrite≤1.9; 1.3≤tempered martensite / bainite≤1.7; 1.6≤secondary martensite / tempered martensite≤2.

1.

6. The 800MPa grade ultra-high hole expansion performance cold-rolled continuous annealed CH steel according to claim 1, characterized in that: The microstructure of the hot-rolled and coiled steel plate is ferrite + bainite + martensite, of which, by area percentage: 40% ≤ ferrite ≤ 50%, 27% ≤ bainite ≤ 35%, and the remainder is martensite.

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