Preparation method of 1180 MPa grade automobile steel produced through continuous annealing

Abstract

The invention relates to a preparation method of 1180MPa-grade automobile steel produced by continuous annealing, which comprises the following steps: S1, smelting process: preparing molten steel from 0.17%-0.20% of C, 1.0%-1.5% of Si, 1.80%-2.30% of Mn, less than or equal to 0.008% of P, less than or equal to 0.005% of S, 0.01%-0.07% of Als, 0.15%-0.30% of Cr, 0.02%-0.06% of Nb, 0.01%-0.04% of Ti, less than or equal to 0.0045% of N and the balance of Fe and inevitable impurities, and Ceq = C + Si / 30 + Mn / 20 + 2P + 4S < = 0.41; s2, a slab is manufactured through continuous casting; s3, hot rolling; s4, acid pickling and cold rolling; and S5, continuous annealing. U-shaped coiling is adopted for hot rolling, so that the structure uniformity is improved; and the continuous annealing process is adjusted, and a product with ultrahigh strength and good plasticity and hole expanding performance is obtained.

Description

Technical Field

[0001] The invention relates to a method for preparing 1180MPa grade automobile steel produced by continuous annealing, belonging to the technical field of metallurgy. Background Art

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

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

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

[0005] The domestic patent CN 109778062 A discloses a cold-rolled complex phase steel with a tensile strength of 1200MPa and its preparation method. The chemical composition by weight percentage is: C: 0.1-0.15%, Si: 0.1-0.5%, Mn: 1.5-2.6%, Cr: 0.4-0.7%, Mo: 0.2-0.5%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, P≤0.02%, S≤0.015%, the remainder is iron and other unavoidable impurity elements; and at the same time, the following conditions are met: C+(Si+Mn) / 6+(Cr+Mo+V) / 5≤0.8. In this scheme, Nb-Ti microalloying is used to prepare a cold-rolled complex phase steel with a tensile strength of 1200MPa, with an elongation of about 7%, and the hole expansion and flanging performance is not tested, which is not conducive to the forming of complex parts and the application scenarios are severely limited. Summary of the invention

[0006] The technical problem to be solved by the present invention is that the automobile steel prepared by the existing method has poor plasticity, hole expansion performance and formability, which is not conducive to the forming of complex parts.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a method for preparing 1180MPa grade automobile steel produced by continuous annealing, comprising the following steps:

[0008] S1. Smelting process, according to the chemical composition mass percentage: C 0.17%-0.20%, Si 1.0%-1.5%, Mn 1.80%-2.30%, P≤0.008%, S≤0.005%, Als 0.01%-0.07%, Cr 0.15%-0.30%, Nb 0.02%-0.06%, Ti 0.01%-0.04%, N≤0.0045%, the remaining elements are Fe and unavoidable impurities, and Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 to obtain molten steel;

[0009] S2, a continuous casting step, continuously casting the obtained molten steel into a slab;

[0010] S3, hot rolling process, heating the obtained slab by hot charging or cold charging, descaling, rough rolling, finish rolling and laminar cooling to obtain hot rolled coil;

[0011] S4, a pickling and cold rolling process, wherein the hot-rolled coil is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil;

[0012] S5, a continuous annealing process, wherein the obtained cold-rolled thin strip steel coil is subjected to continuous annealing to obtain a product.

[0013] Among them, the heating temperature in step S3 of the above method is 1220-1270°C, the total furnace time is 230-400min, soft water blowing is added in the rough rolling to pre-cool the iron sheet, the entire length of the rolling line is descaled, the descaling water pressure is ≥22MPa, and the descaling temperature of the steel billet is ≥1173°C; the finishing rolling start temperature is 1040-1130°C, the finishing rolling passes are 7 passes, the final rolling temperature is 870-920°C, the last pass reduction rate is ≤15% and lubrication rolling is adopted; the intermediate temperature is 720-770°C, the coiling temperature is 600-650°C, the laminar cooling adopts the front-stage centralized cooling method, and the thickness of the hot-rolled coil is 2.3-4.0mm.

[0014] In the above method, in step S3, the coiling adopts a U-type coiling mode, and the head and tail of the strip are adjusted by a hot coil box before the strip enters the finishing mill, and the coiling temperature of the strip 30m before the head is controlled to be 630-680°C, the coiling temperature of the strip 70m after the tail is controlled to be 630-680°C, and the coiling temperature of the middle section is controlled to be 610°C to 630°C.

[0015] In the method, in step S4, the hot-rolled strip is pickled and cold-rolled after being uncoiled, straightened and scaled, and the strip shape is adjusted; the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest is uncoiled at a speed of 120-180 m / min; a six-roll straightening machine is used to pull the strip head out of the uncoiler, and the strip head and tail are straightened to make the strip easy to thread, the six-roll straightening machine is maintained at a speed of 120-180 m / min for scale breaking, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons; the pickling adopts immersion acid tank pickling, three-stage pickling and four-stage rinsing, wherein the acid temperature of the three-stage pickling process is 75-93°C, the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, 9-15%, respectively, and the Fe 2+ The four-stage rinsing temperatures are 30-50℃, 40-60℃, 50-70℃, 60-80℃, and the pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5, respectively. -1 The concentration is ≤4g / l; drying and cold rolling, the thickness of the cold rolled thin strip is 1.0-2.0mm, after 5 rolling passes, the cold rolling reduction rate is 40-70%.

[0016] Furthermore, in the above method, the cold rolling reduction ratio is 45 to 65%.

[0017] Furthermore, in step S5 of the method, the steel is heated to 300°C and 870-900°C at a rate of 5-10°C / s and 0.2-2.0°C / s respectively, the heat-averaging and heat-keeping time is 200-280s, and then cooled to the slow cooling endpoint temperature of 740-760°C at a rate of 1-5°C / s, and then cooled to the fast cooling endpoint temperature of 340-360°C at a rate of 16-20°C / s, the over-aging temperature is 340-360°C, and the over-aging heat preservation time is 1000-1300s.

[0018] Furthermore, in the above method, the unit speed is 40-60 m / min, and the flattening elongation is 0.3-0.5%.

[0019] Furthermore, in the above method, the unit speed is reduced by 5m / min for every 0.2mm increase in the thickness of the cold-rolled thin strip steel, and the flat elongation is reduced by 0.05% for every 0.1mm increase in the thickness of the cold-rolled thin strip steel.

[0020] Furthermore, in the above method, the pickled steel strip is dried in a dryer, and the air supply temperature of the dryer is 80-140°C.

[0021] The microstructure of the product obtained in step S5 of the above method is composed of 80% to 90% bainite, 1% to 3% martensite, 8% to 15% retained austenite and 1% to 5% tempered martensite.

[0022] The beneficial effects of the present invention are:

[0023] 1. The chemical composition of the steel plate produced by this scheme mainly contains C, Si and Mn as the main elements, and the Si content and Mn content are controlled below 1.5% and 2.30% respectively, which further reduces the surface defects and element segregation problems of ultra-high strength steel plates in the production process, and at the same time plays the role of inhibiting the precipitation of cementite and stabilizing austenite. Compared with other ultra-high strength steel products for automobiles with a strength grade of ≥1180MPa, the coordinated optimization of the design of alloy composition and process has formed a higher strength grade cold-rolled steel plate and taken into account both plasticity and formability.

[0024] 2. In this preparation method, the hot rolling adopts a U-type coiling mode to supplement the temperature loss at the head and tail of the strip and improve the uniformity of the structure; the continuous annealing process adopts a heating process system of full austenite temperature, and the over-aging stage in the continuous annealing does not require the use of quenching and re-partitioning technical means, which can meet the traditional acid rolling-continuous annealing production line on a large scale and has the advantages of low production cost and stable process.

[0025] 3. The synergistic control of the C, Si, Mn, Cr, Nb and Ti contents of the steel plate prepared by this method and the hot rolling, cold rolling and continuous annealing processes ensures that the steel plate achieves fine grain strengthening, solid solution precipitation strengthening and phase transformation strengthening. The final structure is composed of bainitic ferrite + residual austenite + trace martensite. The high proportion of bainite can improve the existing steel plate strength level to 1180MPa while taking into account the high yield, high elongation and high forming characteristics. The residual austenite improves the ductility of the steel plate, so that the prepared 1180MPa super bainitic steel plate can meet the requirements of high strength plasticity and hole expansion and flanging performance for automotive structural parts (subframes, B-pillars, front longitudinal beams). BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a metallographic schematic diagram of the steel plate of the present invention;

[0027] Figure 2 This is a schematic diagram of a scanning electron microscope of a steel plate of the present invention;

[0028] Figure 3 This is a schematic diagram of the structure of the first 20 m of the hot rolled coil obtained by U-shaped curling in the present invention;

[0029] Figure 4 This is a schematic diagram of the structure of the hot-rolled coil obtained by U-shaped curling in the present invention;

[0030] Figure 5This is a schematic diagram of the structure of the hot rolled coil 70 m after the tail end obtained by U-shaped curling of the present invention;

[0031] Figure 6 This is a schematic diagram of the structure of the first 20m of the hot rolled coil without U-shaped curling;

[0032] Figure 7 Schematic diagram of the structure of the hot rolled strip obtained without U-shaped curling;

[0033] Figure 8 This is a schematic diagram of the structure of the hot-rolled coil 70m after the tail without U-shaped curling. DETAILED DESCRIPTION

[0034] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0035] like Figures 1 to 8 As shown, a method for preparing 1180MPa grade automobile steel produced by continuous annealing of the present invention comprises the following steps:

[0036] S1. Smelting process, according to the chemical composition mass percentage: C 0.17%-0.20%, Si 1.0%-1.5%, Mn 1.80%-2.30%, P≤0.008%, S≤0.005%, Als 0.01%-0.07%, Cr 0.15%-0.30%, Nb 0.02%-0.06%, Ti 0.01%-0.04%, N≤0.0045%, the remaining elements are Fe and unavoidable impurities, and Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 to obtain molten steel;

[0037] S2, a continuous casting step, continuously casting the obtained molten steel into a slab;

[0038] S3, hot rolling process, heating the obtained slab by hot charging or cold charging, descaling, rough rolling, finish rolling and laminar cooling to obtain hot rolled coil;

[0039] S4, a pickling and cold rolling process, wherein the hot-rolled coil is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil;

[0040] S5, continuous annealing process, the obtained cold-rolled thin strip steel coil is subjected to continuous annealing to obtain the product. It can be understood by those skilled in the art that, under the premise of ensuring the economy of the steel plate through alloy composition and specific process, the method simultaneously maintains high plasticity and high hole expansion performance under the premise of strength level greater than 1180MPa, and the production process is stable, which is conducive to the mass production of products, thereby improving the competitiveness of products under the same strength level conditions and broadening the application scope of products in the field of automotive steel. Specifically, in S1, by appropriately adding microalloying elements and reducing the necessary Si and Mn element content at the same time, the surface defects and element segregation problems of ultra-high strength steel plates in the production process are reduced, and at the same time, the effects of inhibiting cementite precipitation and stabilizing austenite are played. In step S3, it is preferred that hot rolling adopts "U-shaped" coiling to supplement the temperature loss of the head and tail of the strip steel to improve the uniformity of the organization. At the same time, in step S5, by adjusting the continuous annealing process, a 1180MPa-level ultra-high strength steel product composed of bainitic ferrite + residual austenite is obtained, and the plasticity and hole expansion performance are good.

[0041] Preferably, the heating temperature in step S3 of the above method is 1220-1270°C, the total furnace time is 230-400min, soft water blowing is added in the rough rolling to pre-cool the iron sheet, the entire length of the rolling line is descaled, the descaling water pressure is ≥22MPa, and the descaling temperature of the steel billet is ≥1173°C; the finishing rolling start temperature is 1040-1130°C, the finishing rolling passes are 7 passes, the final rolling temperature is 870-920°C, the last pass reduction rate is ≤15% and lubrication rolling is adopted; the intermediate temperature is 720-770°C, the coiling temperature is 600-650°C, the laminar cooling adopts the front-stage centralized cooling method, and the thickness of the hot-rolled coil is 2.3-4.0mm. It can be understood by those skilled in the art that in step S3, the heating temperature is further preferably 1220-1270°C, the total furnace time is 230-400min, soft water is blown in the rough rolling to pre-cool the iron sheet, the whole length of the rolling line is descaled, and the descaling water pressure is ≥22MPa; the descaling temperature of the steel billet is ≥1173°C to ensure the removal of the low-melting-point FeO·Fe2SiO4 eutectic compound. The finishing rolling start temperature is 1040-1130°C to avoid the formation of Fe2O3, the finishing rolling passes are 7 passes, the final rolling temperature is 870-920°C, the last pass reduction is ≤15%, and lubrication rolling is used to avoid the III-level iron oxide scale formed during the finishing rolling process from breaking in the last pass. In order to avoid excessive stress in the oxide layer, excessive crack defects are formed during cooling, affecting the forming performance of the oxide layer. The intermediate temperature is 720-770°C. The intermediate temperature here is due to the laminar cooling of 13 groups of water pipes. The intermediate point temperature is measured after the 6th group of water pipes. The coiling temperature is 600-650°C. The laminar cooling adopts the front-stage centralized cooling method. The thickness of the hot rolled coil is 2.3-4.0mm.

[0042] Preferably, in the above method, in step S3, the coiling adopts a U-type coiling mode, and the hot coil box is used to adjust the strip head and tail before the strip enters the finishing mill, and the coiling temperature of the strip head 30m before the strip head is controlled to be 630-680°C, the coiling temperature of the strip tail 70m after the strip tail is controlled to be 630-680°C, and the coiling temperature of the middle section is 610°C to 630°C. It can be understood by those skilled in the art that in step S3, the preferred coiling adopts a U-type coiling mode, and the hot coil box is used to adjust the strip head and tail before the strip enters the finishing mill, and the coiling temperature of the strip head 30m before the strip head is controlled to be 630-680°C, the coiling temperature of the strip tail 70m after the strip tail is controlled to be 630-680°C, and the coiling temperature of the middle section is 610°C to 630°C. The actual preferred coiling temperature of the middle section is 620°C, so as to ensure the stability of the longitudinal structure and performance of the strip, obtain ferrite + pearlite structure in the hot rolled state, and improve the cold rolling performance.

[0043] Preferably, in step S4 of the method, the hot-rolled strip is pickled and cold-rolled after being uncoiled, straightened and scaled, and the plate shape is adjusted; wherein the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest is uncoiled at a speed of 120-180 m / min; a six-roll straightening machine is used to pull the strip head out of the uncoiler, and the strip head and tail are straightened to make the strip easy to thread, the six-roll straightening machine is maintained at a speed of 120-180 m / min for scale breaking, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons; the pickling adopts immersion acid tank pickling, three-stage pickling and four-stage rinsing, wherein the three-stage pickling process The acid solution temperature is 75-93°C, the free HCl concentrations of the three-stage pickling are 2.5-5.0%, 6-12%, and 9-15%, respectively, the Fe2+ of the three-stage pickling is 110-130g / l, 90-120g / l, and 50-70g / l, the temperatures of the four-stage rinsing are 30-50°C, 40-60°C, 50-70°C, and 60-80°C, respectively, the pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, and 5.0-6.5, respectively, and the Cl-1 concentration is ≤4g / l; drying and cold rolling, the thickness of the cold-rolled thin strip is 1.0-2.0mm, and after 5 rolling passes, the cold rolling reduction rate is 40-70%. It is understood by those skilled in the art that in step S4, the hot-rolled strip is pickled and cold-rolled after unwinding, straightening, descaling and adjusting the plate shape, and preferably the pickling is performed by i-BOX (shallow turbulent pickling). The unwinding machine has a strip threading speed of 50-70 m / min, a tail swinging speed of 100-140 m / min, and the rest is unwinding at a speed of 120-180 m / min. The six-roll straightening machine is used to pull the strip head out of the unwinder and straighten the strip head and tail to make the strip easy to thread. The six-roll straightening machine is maintained at a descaling speed of 120-180 m / min, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons. The pickled plate is obtained by removing the oxide scale on the surface of the hot-rolled coil using an acid solution. The pickling adopts a low-acid consumption, high-efficiency immersion acid tank (i-Box) pickling, three-stage pickling, and four-stage rinsing. The acid solution temperature of the three-stage pickling process is 75-93° C., and the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, and 9-15%, respectively. The Fe 2+ The four-stage rinsing temperatures are 30-50℃, 40-60℃, 50-70℃, 60-80℃, and the pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5, respectively. -1 The concentration is required to be ≤ 4. The thickness of the cold-rolled thin strip steel is 1.0-2.0 mm, and after 5 passes of rolling, the cold rolling reduction rate is 40-70%.

[0044] Preferably, the cold rolling reduction ratio in the above method is 45-65%. Those skilled in the art will appreciate that, in order to further ensure the crystal grain size of the cold-rolled thin strip, the cold rolling reduction ratio in this method is preferably 45-65%.

[0045] Preferably, in step S5 of the method, the steel is heated to 300°C and 870-900°C at a rate of 5-10°C / s and 0.2-2.0°C / s respectively, the heat-averaging and heat-keeping time is 200-280s, and then cooled to the slow cooling endpoint temperature of 740-760°C at a rate of 1-5°C / s, and then cooled to the fast cooling endpoint temperature of 340-360°C at a rate of 16-20°C / s, the over-aging temperature is 340-360°C, the over-aging heat preservation time is 1000-1300s, the unit speed is 40-60m / min, and the flattening elongation is 0.3-0.5%. It can be understood by those skilled in the art that in step S5, the steel plate is heated to 300°C and 870-900°C at a rate of 5-10°C / s and 0.2-2.0°C / s, respectively, and the heat-maintaining time is 200-280s to ensure that the steel plate is heated at a complete austenitizing temperature to obtain a uniform austenite structure, cooled to a slow cooling end temperature of 740-760°C at a rate of 1-5°C / s to avoid the formation of proeutectoid ferrite, cooled to a fast cooling end temperature of 340-360°C at a rate of 16-20°C / s, the over-aging temperature is 340-360°C, and the over-aging heat preservation is 1000-1300s to ensure that the bainite transformation is complete at this stage, thereby controlling the phase ratio of bainite, residual austenite and martensite in the final structure to ensure that both strong plasticity and formability are met.

[0046] Preferably, in the above method, the unit speed is 40-60 m / min, and the flattening elongation is 0.3-0.5%. It can be understood by those skilled in the art that the method further limits the unit speed during annealing to achieve grain refinement and avoid the transitional growth of the grains of the steel plate after cold rolling. Specifically, the preferred unit speed is 40-60 m / min, and the flattening elongation is 0.3-0.5%.

[0047] Preferably, in the above method, the speed of the cold-rolled thin strip steel unit is reduced by 5 m / min for every 0.2 mm increase in thickness, and the flat elongation of the cold-rolled thin strip steel is reduced by 0.05% for every 0.1 mm increase in thickness. It can be understood by those skilled in the art that, in order to further avoid the transitional growth of the grains of the steel plate after cold rolling, the method further preferably reduces the speed of the cold-rolled thin strip steel unit by 5 m / min for every 0.2 mm increase in thickness, and reduces the flat elongation by 0.05% for every 0.1 mm increase in thickness of the cold-rolled thin strip steel.

[0048] Preferably, in the above method, the pickled steel strip is dried in a dryer, and the air supply temperature of the dryer is 80-140° C. Those skilled in the art will appreciate that, in order to ensure that the grain boundary oxide layer of the rinsed steel strip is pickled clean, the present method preferably uses a dryer for drying, and further limits the air supply temperature of the dryer to 80-140° C., and improves the output efficiency.

[0049] Preferably, the product microstructure obtained in step S5 of the above method is composed of 80% to 90% bainite, 1% to 3% martensite, 8% to 15% retained austenite and 1% to 5% tempered martensite. It can be understood by those skilled in the art that in order to ensure that the final product has high tensile strength and meets the requirements of high strength plasticity and hole expansion and flanging performance, the present method further limits the product microstructure, specifically bainitic ferrite + retained austenite + trace martensite, preferably the product microstructure is composed of 80% to 90% bainite, 1% to 3% martensite, 8% to 15% retained austenite and 1% to 5% tempered martensite, a high proportion of bainite can improve the existing steel plate strength level to 1180MPa and take into account the characteristics of high yield, high elongation and high forming; the retained austenite improves the ductility of the steel plate.

[0050] This scheme provides three groups of 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing, and its chemical composition is shown in Table 1:

[0051] serial number C Si Mn P S Als Cr Ti Nb N Example 1 0.184 1.45 2.27 0.008 0.002 0.045 0.22 0.036 0.040 0.0032 Example 2 0.198 1.34 2.30 0.008 0.003 0.032 0.20 0.026 0.035 0.0030 Example 3 0.175 1.25 2.15 0.008 0.002 0.041 0.30 0.022 0.045 0.0031

[0052] Table 1 Chemical composition (wt.%) of 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing

[0053] The method for preparing the above-mentioned 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing is specifically as follows:

[0054] S1, a smelting process, smelting molten steel according to the chemical composition of the complex phase steel plate shown in Table 1;

[0055] S2, continuous casting process, the casting temperature is 1460 ~ 1550 ℃, the thickness of the slab obtained by continuous casting is 120 ~ 240mm;

[0056] S3, hot rolling process, the slab obtained by continuous casting is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling in sequence to obtain a hot rolled coil;

[0057]

[0058] Table 2 Main process parameters of hot rolling of 1180MPa grade super bainite steel for automobile

[0059] S4, pickling and cold rolling process, after pickling the hot rolled coil to remove the surface iron oxide scale, cold rolling into cold rolled thin strip steel, wherein the thickness of the strip steel numbered Example 1, Example 2, and Example 3 are 1.6 mm, 1.8 mm, and 1.2 mm, respectively, and the cold rolling reduction ratios are 46.7%, 48.5%, and 50.0%, respectively;

[0060] S5, continuous annealing process, the cold-rolled thin strip is processed into the desired product by continuous annealing process, firstly, the strip is slowly heated to the soaking temperature (complete austenitization temperature), kept for a period of time to be completely austenitic, and then slowly cooled to the slow cooling end temperature, then the strip is rapidly cooled, and the austenite is transformed into lath bainite in the over-aging section, and finally cooled to room temperature, and the strip shape is adjusted by the skin pass mill; the specific continuous annealing process parameters are shown in Table 3:

[0061]

[0062] Table 3 Main process parameters of continuous annealing of 1180MPa grade super bainite steel for automobiles

[0063] The microstructure of the 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing through the above process is as follows: Figure 1 As shown in the scanning electron microscope photos Figure 2 shown.

[0064] According to GB / T228-2010 "Room Temperature Tensile Test Method for Metallic Materials" and GB / T24524-2021 "Hole Expansion Test Method for Metallic Materials Sheet and Strip" the performance of the above-mentioned 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing and the steels produced by comparative documents 1 and 2 are tested, and their mechanical properties are shown in Table 4 below:

[0065]

[0066] Table 4 Mechanical properties of 1180MPa grade bainitic ferrite based ultra-high strength steel for automobiles produced by continuous annealing

[0067] In summary, the yield strength of the 1180MPa grade bainitic ferrite-based ultra-high strength steel for automobiles produced by continuous annealing in Examples 1, 2 and 3 is 870-950MPa, the tensile strength is 1180-1250MPa, the elongation A50 value is 14%-18%, and the hole expansion rate is 30%-50%. The yield strength is significantly higher than that of the comparative document 1, and the elongation is significantly higher than that of the comparative document 2, so the overall comprehensive performance is better than that of the comparative documents 1 and 2.

[0068] Comparison of hot rolled U-shaped coiling and non-U-shaped coiling structure and performance:

[0069]

[0070]

[0071] In summary, the performance of the hot-rolled U-type coiling and the non-U-type coiling tissues is compared. After U-type coiling, the tissue types of the head, middle and tail are consistent, all F+P, and the strength fluctuation is small; without U-type coiling, the tissue types of the head, middle and tail are quite different, and B and M tissues appear at the head and tail, resulting in high strength at the head and tail, which need to be cut off before cold rolling, greatly affecting the yield rate.

Claims

1. A method for preparing 1180MPa grade automobile steel produced by continuous annealing, characterized in that The steps include: S1. Smelting process, according to the chemical composition mass percentage: C 0.17%-0.20%, Si 1.0%-1.5%, Mn 1.80%-2.30%, P≤0.008%, S≤0.005%, Als 0.01%-0.07%, Cr 0.15%-0.30%, Nb 0.02%-0.06%, Ti 0.01%-0.04%, N≤0.0045%, the remaining elements are Fe and unavoidable impurities, and Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 to obtain molten steel; S2, a continuous casting step, continuously casting the obtained molten steel into a slab; S3, hot rolling process, heating the obtained slab by hot charging or cold charging, descaling, rough rolling, finish rolling and laminar cooling to obtain hot rolled coil; S4, a pickling and cold rolling process, wherein the hot-rolled coil is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil; S5, a continuous annealing process, wherein the obtained cold-rolled thin strip steel coil is subjected to continuous annealing to obtain a product.

2. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 1, characterized in that: The heating temperature in step S3 is 1220-1270°C, the total furnace time is 230-400min, soft water blowing is added in the rough rolling to pre-cool the iron sheet, the entire length of the rolling line is descaled, the descaling water pressure is ≥22MPa, and the descaling temperature of the steel billet is ≥1173°C; the start rolling temperature of the finishing rolling is 1040-1130°C, the finishing rolling passes are 7 passes, the final rolling temperature is 870-920°C, the last pass reduction rate is ≤15% and lubrication rolling is adopted; the intermediate temperature is 720-770°C, the coiling temperature is 600-650°C, the laminar cooling adopts the front-stage centralized cooling method, and the thickness of the hot-rolled coil is 2.3-4.0mm.

3. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 1, characterized in that: In step S3, the coiling adopts the U-type coiling mode. Before the strip enters the finishing mill, the hot coil box is used to adjust the strip head and tail. The coiling temperature of the strip 30m before the head is controlled to be 630-680℃, the coiling temperature of the strip 70m after the tail is controlled to be 630-680℃, and the coiling temperature of the middle section is controlled to be 610℃ to 630℃.

4. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 1, characterized in that: In step S4, the hot-rolled strip is pickled and cold-rolled after being uncoiled, straightened and scaled, and the strip shape is adjusted; wherein the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest is uncoiled at a speed of 120-180 m / min; a six-roll straightening machine is used to pull the strip head out of the uncoiler, and the strip head and tail are straightened to make the strip easy to thread, the six-roll straightening machine is maintained at a speed of 120-180 m / min for scale breaking, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons; pickling adopts immersion acid tank pickling, three-stage pickling and four-stage rinsing, wherein the acid temperature of the three-stage pickling process is 75-93°C, the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, 9-15%, respectively, and the Fe 2+ The four-stage rinsing temperatures are 30-50℃, 40-60℃, 50-70℃, 60-80℃, and the pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5, respectively. -1 The concentration is ≤4g / l; drying and cold rolling, the thickness of the cold rolled thin strip is 1.0-2.0mm, after 5 rolling passes, the cold rolling reduction rate is 40-70%.

5. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 4, characterized in that: The cold rolling reduction ratio is 45-65%.

6. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 1, characterized in that: In the step S5, the steel is heated to 300°C and 870-900°C at a rate of 5-10°C / s and 0.2-2.0°C / s respectively, the heat-averaging and heat-keeping time is 200-280s, and then cooled to the slow cooling end point temperature of 740-760°C at a rate of 1-5°C / s, and then cooled to the fast cooling end point temperature of 340-360°C at a rate of 16-20°C / s. The over-aging temperature is 340-360°C, and the over-aging heat preservation time is 1000-1300s.

7. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 6, characterized in that: The unit speed is 40-60m / min, and the flattening elongation is 0.3-0.5%.

8. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 7, characterized in that: The speed of the unit is reduced by 5 m / min for every 0.2 mm increase in the thickness of the cold-rolled thin strip steel, and the flattening elongation is reduced by 0.05% for every 0.1 mm increase in the thickness of the cold-rolled thin strip steel.

9. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 4, characterized in that: The pickled strip is dried in a dryer with an air supply temperature of 80 to 140°C.

10. The method for preparing 1180MPa grade automobile steel produced by continuous annealing according to claim 1, characterized in that: The microstructure of the product obtained in step S5 is composed of 80% to 90% bainite, 1% to 3% martensite, 8% to 15% residual austenite and 1% to 5% tempered martensite.

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