1000mpa ultra-high hole expansion performance cold-rolled continuous annealing ch steel and preparation method thereof

Abstract

The application belongs to the technical field of metal materials, and provides a 1000MPa super-high hole expansion performance cold-rolled continuous annealing CH steel and a preparation method thereof.The composition of the CH steel is as follows in terms of percentage by weight: C: 0.18% to 0.22%; Si: 0.80% to 1.40%; Mn: 2.00% to 2.20%, Ti: 0.025% to 0.040%; P: ≤0.01%; S: ≤0.008%, and the balance is Fe and inevitable impurities.The preparation method comprises smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and skin passing.The yield strength of the steel plate produced by the application is 850MPa to 950MPa, the elongation is >15%, the hole expansion rate is >80%, the steel plate has good fatigue performance, and the high-cycle fatigue strength limit is ≥485MPa.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials, and particularly relates to a 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel and a preparation method thereof. BACKGROUND

[0002] Studies have shown that for every 100 kg reduction in vehicle weight, carbon emissions can be reduced by about 8.5 g / 100 km. Reducing vehicle weight through material selection is the main way for automobile manufacturers to achieve emission reduction. Among them, advanced high-strength quenching and partitioning steel (QP), twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP), dual-phase steel (DP), plastic-enhanced dual-phase steel (DH), and complex phase steel (CP) are the main ways. The complex phase steel is of great concern to automobile manufacturers, suppliers, and manufacturers, mainly because: (1) the complex phase steel can effectively improve the hole expansion performance due to its multi-phase structure; (2) the complex phase steel has higher yield strength compared to products of the same tensile strength, and has better local forming performance, especially for high-flanging performance safety parts. Complex phase steel with high ductility (CH) is a type of complex phase steel that introduces a certain amount of residual austenite to increase the elongation of complex phase steel through transformation-induced plasticity. In the period of rapid development of new energy vehicles, the design of automobile parts is becoming more and more diversified, and the comprehensive performance requirements of steel plates are becoming higher and higher. In the form of increasing competition in the automobile market, how to produce cold-rolled continuous annealing complex phase steel with high hole expansion performance (CH steel) to meet market demand is a problem that upstream steel enterprises need to solve urgently.

[0003] The patent document "A shaped enhanced complex phase steel and a preparation method thereof" (publication number: CN112251668A) discloses a shaped enhanced complex phase 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 at least one of the following elements: Ti: 0.01%~0.04%, Cu: 0.03%~0.1%, the balance being Fe; the yield strength of the steel plate is 780MPa~920MPa, the tensile strength is 980MPa~1150MPa, the uniform elongation is not less than 8%, the elongation after fracture is not less than 10%, and the hole expansion rate is greater than 40%. The cold-rolled complex phase steel adds noble metal elements such as Nb and Cr, increasing the production cost; and the hole expansion rate is not high, which cannot meet the demand for high flanging performance of new vehicle models.

[0004] The patent document "980MPa grade cold-rolled multiphase steel with good hole expansion performance and preparation method thereof" (publication number: CN108913991A) discloses a multiphase steel with the following chemical composition of the steel plate: 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%, and the balance of Fe and inevitable impurities. The yield strength of the multiphase steel reaches 700MPa or more, the tensile strength reaches 980MPa or more, the elongation is 8% or more, and the hole expansion rate is 15% or more. The cold-rolled multiphase steel adds noble metal elements such as Nb and Cr, which increases the production cost, and the hole expansion rate of 15% or more cannot meet the demand of automobile manufacturers for high flanging performance.

[0005] To overcome the above problems, the present application develops a 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel and its preparation method by selecting alloy elements and ingenious process design, which improves the performance of the steel plate to meet the demand of automobile manufacturers for high flanging performance and high elongation. SUMMARY

[0006] The present application aims to overcome the above problems and deficiencies and provide a 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel with excellent fatigue performance and its preparation method.

[0007] The purpose of the present application is achieved as follows:

[0008] A 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel, the composition of the CH steel is as follows in terms of weight percentage: 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%, and the balance of Fe and inevitable impurities.

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

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

[0011] Further, the microstructure of the CH steel includes epitaxial ferrite, bainite, tempered martensite, secondary martensite and residual austenite; the area percentage of each microstructure is as follows: 8%≤epitaxial ferrite≤12%, 50%≤bainite≤62%, 10%≤tempered martensite≤15%, 8%≤secondary martensite≤12%, and the rest is residual austenite; preferably, the bainite lath width in the CH steel is 5.2μm~8μm, the secondary martensite lath width is 3.4μm~4.7μm, the average size of precipitated carbides in the tempered martensite is ≤80nm, and the residual austenite in the steel is distributed in the bainite and secondary martensite in the form of a film; preferably, the hardness ratio of each microstructure is as follows: 1.8≤bainite / epitaxial ferrite≤2.5; 1.5≤tempered martensite / bainite≤2.0; 1.8≤secondary martensite / tempered martensite≤2.2.

[0012] Further, the tensile strength of the CH steel is ≥1000MPa, the yield strength is between 850MPa~950MPa, the elongation is >15%, and the hole expansion ratio is >80%. The steel has good fatigue performance, and the high-cycle fatigue strength limit is ≥485MPa.

[0013] The component design reasons of the present application are as follows:

[0014] C: Carbon is an interstitial atom in steel, usually exists in the form of solid solution, and can improve the strength of the steel plate through solid solution strengthening. Carbon dissolved in austenite can increase the stability of undercooled austenite, so that the residual austenite is retained at room temperature, and a certain content of residual austenite is retained in the steel plate to improve the plasticity of the steel plate. However, too high carbon content affects the welding performance and fatigue strength of the steel plate, so the C content in the present application is set to 0.18%~0.22%;

[0015] Si: dissolved in ferrite, improves the strength of epitaxial ferrite. Si does not form carbides, which can effectively prevent the precipitation of carbides. However, excessive Si content can increase the carbon content in the undercooled austenite, generating high-strength martensite during the forming process, which is not conducive to the performance of the steel plate. Therefore, the Si content in the present application is set to 0.8%~1.4%;

[0016] Mn: dissolved in ferrite and austenite, can expand the austenite phase region. It can reduce the martensite transformation temperature and critical cooling rate of the steel, improve the hardenability of the steel, and improve the stability of the residual austenite. Too low manganese content leads to insufficient strength of the steel plate and affects the stability of the residual austenite; too high manganese content will cause Mn segregation, which seriously affects the forming performance of the steel plate, therefore, the Mn content in the present application is set to 2.0%~2.2%.

[0017] Ti: can form fine and dispersed precipitates with C, N elements in steel, refine grains, and can significantly improve the strength of the steel. Excessive titanium precipitates will reduce the forming performance of the steel, therefore the Ti content of the present application is set to 0.025%~0.04%.

[0018] P: dissolved in ferrite, can improve the strength and hardness of the steel. But P in the steel will be segregated in the grain boundary, making the grain boundary brittle. P is a harmful element and should be strictly controlled. Therefore, the P content of the present application is less than or equal to 0.01%.

[0019] S: in steel is a harmful element, can form MnS inclusions with Mn, easy to be the starting point of crack initiation, reduce the flanging performance of the steel plate, therefore the S content in the present application is less than or equal to 0.008%.

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

[0021] Cu: solid solution in austenite to improve the strength of the steel plate. In the annealing process, elemental Cu will precipitate from the austenite, play a precipitation strengthening effect. Adding Cu can also increase the atmospheric corrosion resistance, improve the service life of the steel plate. Therefore, the Cu content in the present application is set to 0.1%~0.2%.

[0022] Al: has strong affinity with oxygen and nitrogen, is a deoxidizer in the steelmaking process. Al can capture N in steel, form AlN, pin the grain boundary, hinder grain boundary migration, prevent grain coarsening at high temperature, and deteriorate the strength of the steel plate. But too high Al content will increase the inclusions in the steel, deteriorate the hole expansion performance of the steel plate. Therefore, the Al content in the present application is set to 0.1%~0.2%.

[0023] RE: can make the oxides and sulfides in the steel into fine and dispersed inclusions, therefore can reduce the harm of MnS and other inclusions in the steel; at the same time, RE can improve the fluidity of the steel, reduce non-metallic inclusions, make the steel structure become dense and pure, greatly improve the fatigue performance of the steel. Combined with Si, Al and other elements in the steel, through inclusion modification, the strength of the steel is significantly improved. Therefore, the RE content in the present application is set to 0.001%~0.005%.

[0024] The present application needs to control 0.95%≤Si+Al+RE≤1.42%, ①Si is a strong deoxidizer, combined with O to form chain or cluster SiO2 inclusions, easy to become a crack source in the forming process, RE can react with SiO2 to form rare earth silicate, its form changes into spherical, reduces stress concentration, and improves the fatigue performance of the steel plate. Al forms AlN with N in the steel, AlN is easy to coarsen, RE adsorbs its surface to inhibit its growth, and improves the hot strength of the steel plate. ②Excessive Si aggravates the hot brittleness of the steel plate and reduces the surface quality of the steel plate; excessive Al will block the water gap, and it is difficult to tap the steel. Therefore, the addition amount of Si, Al and RE needs to be controlled to maximize the effect of the three, so that the steel plate realizes high hole expansion performance and high fatigue performance.

[0025] The second technical scheme of the present application is to provide a preparation method of 1000MPa ultra-high hole expansion performance cold rolled continuous annealing CH steel, comprising smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and flattening.

[0026] Continuous casting:

[0027] The blank drawing speed is 1.00~1.30m / min, and the drawing speed is too large to easily leak steel, and the drawing speed is too small to affect the production efficiency. The thickness of the casting blank is 260~280mm.

[0028] Hot rolling:

[0029] The slab heating temperature is 1225℃~1250℃, and needs to be kept for 60~90min in this temperature range; the rolling temperature is 1060℃~1120℃, and the final rolling temperature is 900℃~930℃; after rolling, the steel plate is cooled to 620~670℃ by laminar flow, and the laminar flow cooling speed is 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 steel plate after hot rolling and coiling is ferrite + bainite + martensite, wherein the area percentage is: 25%≤ferrite≤40%; 35%≤bainite≤45%; the rest is martensite.

[0030] The slab heating temperature is 1225-1250℃, and the temperature needs to be kept for 60-90min in this temperature range to ensure the dissolution of Ti compounds and the supersaturation of Ti to facilitate subsequent precipitation. The fine and dispersed Ti compounds can pin the original austenite, hinder the migration of austenite, and play a role in refining the grains. Reasonable holding time can homogenize the alloying elements and reduce composition segregation. The opening rolling temperature is 1060-1120℃, and the final rolling temperature is 900-930℃. This final rolling temperature is in the austenite region to ensure austenite recrystallization; the hot-rolled plate is cooled to 620-670℃ by laminar cooling at a speed of 25-30℃ / s; the final thickness of the hot-rolled plate is 2.2-4.0mm. The coiling temperature is 620-670℃, and then the steel plate is slowly cooled in the slow cooling pit. The microstructure of the steel plate after coiling is ferrite + bainite + martensite. Too high coiling temperature will cause poor surface quality and coil collapse; too low coiling temperature will result in too high strength of the hot-rolled plate, which will cause difficulty in cold rolling and even failure to complete cold rolling. At this coiling temperature, the steel plate microstructure obtains 35%-45% bainite structure, which is beneficial to grain refinement in continuous annealing.

[0031] Pickling: remove the iron oxide scale formed on the surface of the steel strip, 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 is 50%-65%. The cold rolling deformation amount makes the steel plate have enough deformation energy, which is beneficial to annealing recrystallization, reduces the A C1 and A C3 temperature, saves energy consumption, and reduces production cost.

[0033] ⑥Continuous annealing: including heating, primary holding, slow cooling, rapid cooling, overaging, temperature raising, secondary holding, and cooling.

[0034] The steel strip is heated to 870-900℃ at a heating speed of 5-8℃ / s, primary holding for 80-120s, slow cooling to 730-770℃, cooling to 370-390℃ at a cooling speed of 20-25℃ / s, holding for 350-420s, overaging in this stage, raising the temperature to 440-460℃, secondary holding for 10-20s, and then cooling at a cooling speed of 12-17℃ / s to enter the cooling reversing roller, the temperature range is 200-220℃, and then air cooling for flattening process.

[0035] The strip steel is heated to 870-900 DEG C at a heating rate of 5-8 DEG C / s; if the heating rate is too small, the strip steel stays in the heating furnace for too long, the surface is seriously oxidized, and the final quality of the product is affected; if the heating rate is too large, dislocation recovery and recrystallization cannot be fully carried out, and the cold-rolled genetic banded structure cannot be eliminated, and the elongation of the steel plate is reduced. The heating temperature is AC3+30 DEG C-50 DEG C, the purpose is to obtain full austenite structure, eliminate critical zone ferrite, and improve the yield strength of the steel plate. Once the temperature is kept for 80-120s, and slowly cooled to 730 DEG C-770 DEG C; if the holding time is too short, the alloy elements cannot be homogenized; if the holding time is too long, the grains grow, the final original structure is coarse, the performance of the steel plate is affected, 8%-12% of oriented accretion ferrite is obtained, the oriented accretion ferrite has higher alloy content than the critical zone ferrite, has higher strength, can make the steel plate have good ductility, and is helpful to the formability of the steel plate; and the hardness difference between the bainite and the tempered martensite is reduced, the organization deformation is coordinated, and the steel plate has good hole expansion performance. The cooling rate is 20-25 DEG C / s, the temperature is cooled to 370 DEG C-390 DEG C, and the temperature is kept for 350s-420s, and overaging is carried out in this stage; the overaging is avoided in the cooling process, the pearlite is converted into the overaging, and the strength of the steel plate is reduced. A certain amount of martensite is generated in the fast cooling process. The temperature is cooled to 370 DEG C-390 DEG C, which is the overaging stage, and the temperature interval is the bainite phase transition zone. If the overaging temperature is too low, the bainite phase transition rate is reduced, and then the bainite content is reduced. If the overaging temperature is too high, more bainite is generated in this stage, and the tensile strength of the steel plate cannot reach 1000 MPa. In the overaging stage, 50%-62% of bainite is obtained, and the bainite is helpful to improve the hole expansion performance of the steel plate. The carbon atoms in the bainite diffuse to the supercooled austenite, increase the stability of the residual austenite, obtain a certain amount of residual austenite after subsequent cooling, and ensure the formability of the steel plate. The temperature is raised to 420 DEG C-440 DEG C, and the second holding is kept for 10-20s; in this stage, the martensite generated in the first fast cooling is tempered to form tempered martensite, the strength of the martensite is reduced, the hardness difference with the bainite is reduced, and the hole expansion performance of the steel plate is effectively improved. The cooling rate is 12-17 DEG C / s, and the temperature is cooled to a cooling transition roller chamber, and the temperature interval is 200 DEG C-220 DEG C; mainly for obtaining a small amount of secondary martensite to ensure the strength of the steel plate and a certain amount of residual austenite to improve the formability of the steel plate, and meet the requirements of new vehicle parts.

[0036] Skin pass: the skin pass elongation is 0.2%-0.4%, the plate shape of the steel plate is ensured, and the yield strength of the strip steel is improved. The skin pass can make the strip steel surface have 0.8-1.5 mu m roughness, and be helpful to subsequent steel plate coating process.

[0037] The beneficial effects of the application are as follows:

[0038] 1. The 1000MPa grade ultra-high hole expansion performance cold-rolled continuous annealing CH steel according to the present application is prepared by taking C and Mn as main alloying elements and without precious alloying elements such as Cr, Mo, Nb and Ni, and has the advantage of low raw material cost; the tensile strength of the steel plate is ensured by the precipitation strengthening of Ti and Cu; the inclusions in the steel are modified by the combination of Al, Si and RE, the grains are refined, and the hole expansion performance and fatigue performance of the steel plate are significantly improved, so that the present application is an innovative industrialized product.

[0039] 2. The obtained structure is mainly epitaxial ferrite, bainite, tempered martensite, secondary martensite and residual austenite, and the tempered martensite and residual austenite are introduced into the traditional CH steel ferrite + bainite + martensite structure. The tempered martensite can reduce the hardness difference between the bainite and secondary martensite structure and improve the hole expansion performance of the steel plate; the residual austenite has TRIP effect and improves the plasticity of the steel plate, so that the product of the present application has excellent comprehensive performance and good market competitiveness, and the formation of the structure can adopt the innovative full austenitization and quenching temperature raising process.

[0040] 3. The CH steel according to the present application has a tensile strength of ≥1000MPa, a yield strength of 850MPa~950MPa, an elongation of >15%, a hole expansion rate of >80%, and a fatigue strength limit of ≥485MPa, and can meet the needs of upgrading and iteration of automobile steel products. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 Figure 1 is a microstructure metallographic phase diagram of Example 1 of the present application. DETAILED DESCRIPTION

[0042] The present application will be further described below by means of examples.

[0043] Example 1 of the present application is prepared according to the component of the technical scheme (the composition of the CH steel is as follows in terms of weight percentage: 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%, and the balance is Fe and inevitable impurities.), and is subjected to smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and flattening.

[0044] Continuous casting:

[0045] The breakout speed is 1.00~1.30m / min, and the thickness of the cast blank is 260~280mm.

[0046] Hot rolling:

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

[0048] Pickling: removing the iron oxide scale generated on the surface of the strip, pickling temperature 75-90℃, pickling speed 150-170m / min;

[0049] Cold rolling: the cold rolling reduction is 50-65%;

[0050] Continuous annealing: the strip is heated to 870-900℃ at a heating speed of 5-8℃ / s, the first holding time is 80-120s, the slow cooling is to 730-770℃ at a slow cooling speed of 2.73-3.11℃ / s, the cooling is to 370-390℃ at a cooling speed of 20-25℃ / s, the holding time is 350-420s, the overaging is carried out in this stage, then the temperature is raised to 420-440℃, the second holding time is 10-20s, the cooling is carried out at a cooling speed of 12-17℃ / s, and the temperature interval is 200-220℃ in the cooling reversing roller chamber, and finally the air cooling is to room temperature.

[0051] Skin passing: the skin passing elongation is 0.2-0.4%.

[0052] Further, the microstructure of the steel plate after hot rolling and coiling is ferrite + bainite + martensite, wherein the area percentage is 25%≤ferrite≤40%, 35%≤bainite≤45%, and the rest is martensite.

[0053] Further, the laminar flow cooling speed of the steel plate after hot rolling is 25-30℃ / s.

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

[0055] Table 1 Composition of the steel in the embodiment of the present application (wt%)

[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 of continuous casting and rolling of the steel in the embodiment of the present application

[0058] Example Casting speed m / min Casting thickness mm Heating temperature °C Soaking time min Opening temperature °C Finishing temperature °C Layer cooling speed °C / s Layer cooling temperature °C 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 of the steel in the embodiment of the present application

[0060] Example Cold rolling reduction / % Pickling temperature °C Pickling speed m / min Heating rate °C / s Heating temperature °C Primary isothermal time s Tempering cooling speed °C / s Tempering temperature °C Fast cooling speed °C / s Overaging temperature °C Soaking time s Temperature raising temperature °C Secondary soaking time s Cooling rate °C / s Cooling turning roll chamber / °C Temper extension % 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 the steel of the embodiment of the application

[0062] Example Yield strength MPa Tensile strength MPa Elongation % Hole expansion ratio % 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 Microstructure content and hardness ratio of each phase of the steel of the embodiment of the application

[0064] Example Epitaxial ferrite % Bainite % Tempered martensite % Secondary martensite % Retained 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 bainite lath, μm; B is the width of secondary martensite lath, μm; C is the average size of precipitated carbide in tempered martensite, nm.

[0066] The microstructure of the CH steel produced by the application comprises epitaxial ferrite, bainite, tempered martensite, secondary martensite and residual austenite; the area percentage content is as follows: 8%≤epitaxial ferrite≤12%, 50%≤bainite≤62%, 10%≤tempered martensite≤15%, 8%≤secondary martensite≤12%, and the rest is residual austenite; the width of bainite lath is 5.2 μm~8 μm, the width of martensite lath is 3.4 μm~4.7 μm, the average size of precipitated carbide in tempered martensite is ≤80 nm, and the residual austenite in the steel is distributed in the bainite and secondary martensite in the form of film. The tensile strength of the CH steel plate is ≥1000 MPa, the yield strength is 850 MPa~950 MPa, the elongation is >15%, and the hole expansion ratio is >80%. The CH steel has good fatigue performance, and the fatigue strength limit is ≥485 MPa.

[0067] In order to describe the application, the application is adequately and sufficiently described by the above embodiments, and the above embodiments are only used to illustrate the application, but not limit the application. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art without departing from the spirit and scope of the application shall be included in the protection scope of the application, and the patent protection scope of the application shall be defined by the claims.

Claims

1. A 1000 MPa ultra-hole expandable cold rolled continuous annealed CH steel characterized in that, The CH steel has the following components in percentage by weight: 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%, the balance being Fe and unavoidable impurities; the CH steel further comprises one or more of Cu, Al, RE; wherein, Cu: 0.1%~0.2%; Al: 0.04%~0.08%, RE: 0.01%~0.05%; the CH steel has 0.95%≤Si+Al+RE≤1.42%; the microstructure of the CH steel comprises epitaxial ferrite, bainite, tempered martensite, secondary martensite and residual austenite; each microstructure has the following percentage by area: 8%≤epitaxial ferrite≤12%, 50%≤bainite≤62%, 10%≤tempered martensite≤15%, 8%≤secondary martensite≤12%, the rest being residual austenite; the bainite lath width is 5.2μm~8μm, the secondary martensite lath width is 3.4μm~4.7μm, the average size of precipitated carbide in the tempered martensite is ≤80nm, the residual austenite in the steel is distributed in the bainite and secondary martensite in the form of film; the hardness ratio of each microstructure is: 1.8≤bainite / epitaxial ferrite≤2.5; 1.5≤tempered martensite / bainite≤2.0; 1.8≤secondary martensite / tempered martensite≤2.2; the tensile strength of the CH steel is ≥1000MPa, the yield strength is in the range of 850MPa~950MPa, the elongation is >15%, the hole expansion rate is >80%; the high cycle fatigue strength limit is ≥485MPa; The preparation method of the 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel comprises smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and flattening; The casting speed is in the range of 1.00~1.30m / min, and the thickness of the cast blank is in the range of 260~280mm; The slab heating temperature is in the range of 1225℃~1250℃, and the holding time is 60~90min; the starting rolling temperature is in the range of 1060℃~1120℃, and the final rolling temperature is in the range of 900℃~930℃; after rolling, the steel plate is cooled to 620~670℃ by laminar flow; the coiling temperature is in the range of 620~670℃; the microstructure of the steel plate after hot rolling and coiling is ferrite + bainite + martensite, wherein the percentage by area is: 25%≤ferrite≤40%; 35%≤bainite≤45%; the rest is martensite; Pickling: the scale formed on the surface of the strip steel is removed, the pickling temperature is in the range of 75℃~90℃, and the pickling speed is in the range of 150~170m / min; Cold rolling: the cold rolling reduction is in the range of 50%~65%. ​ ​ Continuous annealing: the strip steel is heated to 870-900℃ at a heating rate of 5-8℃ / s, the first holding time is 80-120s, slow cooling to 730-770℃ at a slow cooling rate of 2.73-3.11℃ / s, cooling to 370-390℃ at a cooling rate of 20-25℃ / s, holding time of 350-420s, overaging is carried out in this stage, then the temperature is raised to 420-440℃, the second holding time is 10-20s, then cooling at a cooling rate of 12-17℃ / s, entering the cooling reversing roller chamber, the temperature interval is 200-220℃; finally air cooling to room temperature; Tempering: the tempering elongation is 0.2%-0.4%.

2. A preparation method of the 1000MPa ultra-high hole expansion performance cold-rolled continuous annealing CH steel of claim 1, comprising smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and tempering; characterized in that: Continuous casting: The casting speed is 1.00-1.30m / min, and the casting thickness is 260-280mm; Hot rolling: The slab heating temperature is 1225-1250℃, the holding time is 60-90min; the starting rolling temperature is 1060-1120℃, the final rolling temperature is 900-930℃; the steel plate is cooled to 620-670℃ by laminar cooling after rolling; the coiling temperature is 620-670℃; Pickling: the iron oxide scale generated on the surface of the strip steel is removed, the pickling temperature is 75-90℃, and the pickling speed is 150-170m / min; Cold rolling: the cold rolling reduction is 50%-65%; Continuous annealing: the strip steel is heated to 870-900℃ at a heating rate of 5-8℃ / s, the first holding time is 80-120s, slow cooling to 730-770℃ at a slow cooling rate of 2.73-3.11℃ / s, cooling to 370-390℃ at a cooling rate of 20-25℃ / s, holding time of 350-420s, overaging is carried out in this stage, then the temperature is raised to 420-440℃, the second holding time is 10-20s, then cooling at a cooling rate of 12-17℃ / s, entering the cooling reversing roller chamber, the temperature interval is 200-220℃; finally air cooling to room temperature; Tempering: the tempering elongation is 0.2%-0.4%.

3. A process for the production of 1000 MPa ultra hole expanding performance cold rolled continuous annealed CH steel as claimed in claim 2, wherein: The laminar cooling rate of the steel plate after hot rolling is 25-30℃ / s.

Images