1180MPa-grade super bainite steel for automobile and preparation method thereof

Abstract

The invention discloses 1180MPa-grade super bainite steel for an automobile and a preparation method of the 1180MPa-grade super bainite steel, and belongs to the technical field of cold-rolled strip production. The super bainite steel comprises the following chemical components in percentage by mass: 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, 0.04%-0.08% of V, 0.04%-0.08% of Ti, less than or equal to 0.0045% of N and the balance of Fe and impurities, and Ceq = C + Si / 30 + Mn / 20 + 2P + 4S < = 0.41. The preparation method comprises the steps of smelting, continuous casting, hot rolling, pickling cold rolling and continuous annealing. Through alloy component design and process collaborative optimization, the prepared super bainite steel has high strength, high plasticity and high reaming performance.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled strip production, and specifically relates to a 1180MPa-grade super bainite steel for automobiles and a preparation method thereof. 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 grade of the third generation of advanced high-strength steel of 1180MPa grade, the organizational characteristics of super bainite steel are 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 organizational structure 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. Through the query of relevant patents, the patents that are similar to 1180MPa super bainite steel plates are as follows:

[0004] CN 110724877 A discloses a 1180MPa grade high plasticity bainite complex phase steel plate for automobiles and a preparation method thereof. The chemical composition of the steel plate by weight is as follows: 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 replaceable components, by weight percentage, contain: Cr: 0-2%, Mo: 0-0.5%, Cu: 0.05-0.5%, and the balance is Fe and unavoidable impurities. The rolling process is: the temperature of the hot rolling billet entering the furnace is between 400 and 750°C, the heating temperature is between 1150 and 1250°C, the rolling temperature is between 1150 and 1250°C, the final rolling temperature is above 900°C, and the coiling temperature is between 600 and 750°C; the annealing process is: the annealing temperature is between 780 and 880°C, the annealing time is between 1 and 5 minutes, the cooling rate is greater than 30°C / s, and the aging temperature is between 300 and 500°C, the aging time is between 300 and 600s, and then cooled to room temperature. The content range of C, Si and Mn in this patent is very wide. The low C, low Si and low Mn state is not enough to achieve the strength grade of the product, and 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] CN 109778062 A discloses a cold-rolled complex phase steel with a tensile strength of 1200MPa and a preparation method thereof. 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%, and the remainder is iron and other unavoidable impurity elements; and the following conditions are met at the same time: C+(Si+Mn) / 6+(Cr+Mo+V) / 5≤0.8. The patent adopts Nb-Ti microalloying 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 purpose of the present invention is to provide a 1180MPa grade super bainite steel for automobiles and a preparation method thereof. By means of alloy composition and specific process, the economy of the steel plate is ensured, and high plasticity and high hole expansion performance are simultaneously maintained under the premise that the strength level is greater than 1180MPa. The production process is stable, which is conducive to the mass production of the product, thereby improving the competitiveness of the product under the same strength level conditions and broadening the application scope of the product in the field of automobile steel.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] In a first aspect, the present invention provides a 1180MPa grade super bainite steel for automobiles, wherein the chemical composition is as follows by 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%, V 0.04%-0.08%, Ti 0.04%-0.08%, N≤0.0045%, and the remaining elements are Fe and unavoidable impurities, and at the same time satisfies Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41.

[0009] The microstructure of the above-mentioned 1180MPa grade super bainite steel for automobiles is composed of 80% to 90% bainite, 1% to 3% martensite and 8% to 15% retained austenite.

[0010] The yield strength of the above 1180MPa grade super bainite steel for automobiles is 870-970MPa, the tensile strength is 1180-1250MPa, and the elongation A 50 The value is 14% to 18%, and the hole expansion rate is 30% to 50%.

[0011] In a second aspect, the present invention provides a method for preparing the above-mentioned 1180MPa grade super bainite steel for automobiles, comprising the following steps:

[0012] (a) Smelting process: smelting molten steel according to the chemical composition of 1180MPa grade super bainite steel for automobiles;

[0013] (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs;

[0014] (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, dephosphorizing, rough rolling, finish rolling and laminar cooling to obtain a hot-rolled coil;

[0015] Among them, the heating temperature is 1220-1270°C; the finishing rolling temperature is 1040-1130°C, the final rolling temperature is 870-920°C; the intermediate temperature is 720-770°C; the thickness of the hot rolled coil is 2.3-4.0mm;

[0016] (d) Pickling and cold rolling process: the hot rolled coil obtained in step (c) is uncoiled, stretched and straightened to break scale, the plate shape is adjusted, and the surface oxide layer is removed by pickling, and then cold rolled into a cold rolled thin strip steel coil;

[0017] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed, that is, after soaking, slow cooling, rapid cooling, over-aging treatment and smoothing, a 1180MPa grade super bainite steel sheet for automobiles is obtained;

[0018] Among them, the soaking temperature is 910-950°C; the slow cooling endpoint temperature is 750-780°C, the fast cooling endpoint temperature is 340-360°C; the over-aging temperature is 340-360°C; the unit speed is 40-60m / min; and the flattening elongation is 0.3-0.5%.

[0019] In the above preparation method, in step (b), the casting temperature is 1460-1550° C., and the slab thickness is 120-240 mm.

[0020] In the above preparation method, in step (c), the total heating time in the furnace is 230 to 400 minutes.

[0021] In the above preparation method, in step (c), soft water blowing is added during rough rolling to pre-cool the iron sheet.

[0022] In the above preparation method, in step (c), the entire length of the rolling line is descaled, the descaling water pressure is ≥22Mpa, and the descaling temperature is ≥1173°C.

[0023] In the above preparation method, in step (c), the finishing rolling passes are 7 passes, the reduction rate of the last pass is ≤15% and lubrication rolling is adopted.

[0024] In the above preparation method, in step (c), the laminar cooling adopts a front-stage centralized cooling method.

[0025] Furthermore, in the above preparation method, in step (c), 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 600-650°C.

[0026] In the above preparation method, in step (d), the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the remaining unwinding speed is maintained at 120-180 m / min.

[0027] In the above preparation method, in step (d), a six-roll straightening machine is used for tension straightening and descaling, the speed is 120-180 m / min, the elongation is 1.2-2.5%, and the tension of the tension straightening machine is 40-50 tons.

[0028] In the above preparation method, in step (d), the cold rolling is performed through 5 passes, the cold rolling reduction rate is 40-70%, and the thickness of the cold rolled thin strip is 1.0-2.0 mm.

[0029] Furthermore, the cold rolling reduction ratio is 45 to 65%.

[0030] In the above preparation method, in step (d), pickling is carried out by immersion acid tank pickling, specifically:

[0031] 1) Three-stage pickling: The acid temperature of the pickling process is 75-93°C; the free HCl concentration is 2.5-5.0%, 6-12%, and 9-15% respectively; Fe 2+ The concentrations are 110-130 g / l, 90-120 g / l, and 50-70 g / l respectively;

[0032] 2) Four-stage rinsing: rinsing temperatures are 30-50°C, 40-60°C, 50-70°C, 60-80°C; pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5; Cl -1 The concentration is ≤4g / l.

[0033] Furthermore, the rinsed strip is dried in a dryer, and the air supply temperature of the dryer is 80-140°C.

[0034] In the above preparation method, in step (e), heating is performed to 300°C at a rate of 5 to 10°C / s, and then heating is performed to the absorptive temperature at a rate of 0.2 to 2.0°C / s, and the absorptive holding time is 200 to 280s.

[0035] In the above preparation method, in step (e), cooling is performed at a rate of 1 to 5°C / s to a slow cooling endpoint temperature, and then cooling is performed at a rate of 16 to 20°C / s to a fast cooling endpoint temperature.

[0036] In the above preparation method, in step (e), the aging treatment is kept warm for 1000 to 1300 seconds.

[0037] In the above preparation method, in step (e), the unit speed is reduced by 5m / min for every 0.2mm increase in the cold-rolled thin strip steel.

[0038] In the above preparation method, in step (e), the flattening elongation decreases by 0.05% for every 0.1 mm increase in the thickness of the cold-rolled thin strip steel.

[0039] The beneficial effects of the present invention are:

[0040] 1. The chemical composition of the 1180MPa grade super bainite steel for automobiles of the present invention mainly includes C, Si and Mn as main elements, and the Si content and the Mn content are controlled at less than 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. A small amount of vanadium and titanium elements are added to significantly refine the original austenite grains, and obtain uniform and fine lath bainite blocks to obtain good strength-plasticity matching. Compared with other ultra-high strength steel products for automobiles with strength grades ≥1180MPa, the design of alloy components and the coordinated optimization of processes form a cold-rolled steel plate with a higher strength grade and take into account both plasticity and formability.

[0041] 2. In the preparation method of the present invention, the hot rolling adopts a U-shaped coiling mode to supplement the temperature loss at the head and tail of the strip and improve the uniformity of the structure; the continuous annealing process adopts 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 meets the traditional acid rolling-continuous annealing production line on a large scale and has the advantages of low production cost and stable process.

[0042] 3. The synergistic control of the C, Si, Mn, Cr, V and Ti contents of the 1180MPa grade super bainite steel for automobiles prepared by the present invention 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 bainite + 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 grade super bainite steel plate can be used for automotive structural parts (subframes, B-pillars, front longitudinal beams) that meet the requirements of high strength plasticity and hole expansion and flanging performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a metallographic photo of 1180MPa grade super bainite steel for automobiles;

[0044] Figure 2 This is a scanning electron microscope photo of 1180MPa grade super bainite steel for automobiles;

[0045] Figure 3 This is the EBSD photo of 1180MPa grade super bainite steel for automobiles;

[0046] Figure 4 This is the XRD pattern of 1180MPa grade super bainite steel for automobiles. DETAILED DESCRIPTION

[0047] The technical solution of the present invention can be specifically implemented in the following manner.

[0048] 1180MPa grade super bainite steel for automobiles, whose chemical composition by mass percentage is: C 0.17%~0.20%, Si1.0%~1.5%, Mn 1.80%~2.30%, P≤0.008%, S≤0.005%, Als 0.01%~0.07%, Cr 0.15%~0.30%, V 0.04%~0.08%, Ti 0.04%~0.08%, N≤0.0045%, and the remaining elements are Fe and unavoidable impurities, and at the same time satisfy Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41.

[0049] The role of alloying elements in the present invention:

[0050] C is an important alloying element in advanced high-strength automotive steels. All phase transformations and the final microstructure are affected by C. C is an element that stabilizes austenite and affects the stability of retained austenite by affecting the C content in the retained austenite. Generally, the Ms point of a material decreases with the increase of the C content in the steel, and the carbon content in the retained austenite at room temperature also increases, thereby improving the stability of the retained austenite. In Fe-C alloys, each increase of 1wt.% in carbon content can reduce the Ms point by about 320°C, and has a decisive impact on the microstructure at room temperature. Too low a C content cannot ensure sufficient C partitioning during the continuous annealing process of the experimental steel, which directly affects the stability of the austenite phase at room temperature. A high C content will make the overly stable retained austenite unable to perform the TRIP effect well during the subsequent deformation process, and the work hardening behavior will be damaged, reducing the strength of the experimental steel and deteriorating the welding performance of the steel plate.

[0051] The Si element can inhibit the precipitation of carbides in the third generation of automotive steel, provide conditions for the diffusion of C atoms in the partitioning process, and improve the partitioning effect of C. At the same time, the Si element can increase the C content in austenite, which is easy to retain at room temperature to form residual austenite, and improve the stability of residual austenite. Too high Si content will reduce the hot rolling performance of the steel plate, form stable oxides on the surface, and reduce the surface quality of the steel plate. Si has a strong effect on improving the strength of the solid solution in the steel and the cold working deformation hardening rate, but it also reduces the toughness and plasticity of the steel to a certain extent.

[0052] As an austenite-forming element, Mn plays a role in solid solution strengthening and reducing the Ms point in steel, reducing the critical quenching temperature of the experimental steel, delaying the transformation of austenite to pearlite, stabilizing austenite and ensuring the stability of retained austenite. The research results of Seung et al. on 0.15C-1Si-(2.0~2.4)Mn TRIP steel showed that excessive Mn content would reduce the stability of retained austenite. When the mass fraction of Mn in the steel increased from 2.1% to 2.4%, the Ms point rose by about 100℃. This is because the addition of Mn slowed down the ferrite transformation rate and reduced the carbon content of ferrite diffusing into austenite. When the carbon content in the steel is constant, the structure of the steel gradually changes from pearlite to martensite and then to austenite with the increase of Mn content. Excessive Mn content is prone to Mn segregation, causing hot cracks in the continuous casting billet, and also increases the C equivalent of the steel plate, affecting the welding performance.

[0053] Cr is an austenite stabilizing element in steel, which expands the austenite phase region and improves the hardenability of the steel plate. Considering that too high Mn addition will cause excessive formation of C / Mn segregation, a small amount of Cr can be added.

[0054] Ti is a strong carbide-forming element, and has a strong bonding force with C to form stable TiC, which can strongly slow down the diffusion rate of C in steel and slow down the nucleation process of austenite. TiN solidified during the heating stage can pin the original austenite grain boundary, significantly refine the original austenite grains, and the precipitation strengthening of TiC can improve the yield strength of steel. Too much Ti combined with N will form liquid TiN, which will act as a crack source and affect the hole expansion and flanging performance; the precipitation of Ti will also occupy the C atoms required for the retention of residual austenite, affecting the stability of the residual austenite.

[0055] V is a strong carbide-forming element, which is mainly precipitated in the hot rolling coiling stage and the continuous annealing stage. It can promote the bainite phase transformation in the continuous annealing stage, but excessive VC precipitation will form C-rich clusters around VC, occupying the C required for austenite formation in the critical zone.

[0056] In a second aspect, the present invention provides a method for preparing the above-mentioned 1180MPa grade super bainite steel for automobiles, comprising the following steps:

[0057] (a) Smelting process: smelting molten steel according to the chemical composition of 1180MPa grade super bainite steel for automobiles;

[0058] (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs;

[0059] (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, and then performing dephosphorization, rough rolling, finish rolling and laminar cooling;

[0060] (d) Pickling and cold rolling process: the hot-rolled coil obtained in step (c) is pickled to remove the oxide layer on the surface, and then cold-rolled to form a cold-rolled thin strip steel coil;

[0061] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is subjected to continuous annealing to produce a 1180 MPa grade super bainite steel sheet for automobiles.

[0062] In the above technical solution, preferably, in the step (b), the casting temperature is 1460-1550° C., and the slab thickness is 120-240 mm.

[0063] In the above technical solution, preferably, in the step (c), the heating temperature is 1220-1270°C, the total furnace time is 230-400 min, soft water blowing is added during rough rolling to precool the iron sheet, the entire length of the rolling line is descaled, and the descaling water pressure is ≥22 MPa; the descaling temperature of the steel billet is ≥1173°C to ensure that the low melting point FeO·Fe 2 SiO 4 The finishing rolling temperature is 1040~1130℃ to avoid Fe 2 O 3 The formation and finishing passes are 7 passes, the final rolling temperature is 870-920℃, the final reduction rate is ≤15%, and lubrication rolling is adopted to avoid the Ⅲ-level oxide scale formed in the finishing rolling process from breaking in the final pass. In order to avoid excessive stress in the oxide layer, excessive crack defects are formed during cooling, which affects the forming performance of the oxide layer. The intermediate temperature is 720-770℃, and the laminar cooling adopts the front-stage centralized cooling method. The thickness of the hot rolled coil is 2.3-4.0mm.

[0064] In the above technical scheme, preferably, 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 600-650°C, so as to ensure the stability of the longitudinal organizational performance of the strip, obtain ferrite + pearlite organization in the hot rolled state, and improve the cold rolling ability.

[0065] In the above technical solution, preferably, in the step (d), the hot-rolled strip is uncoiled, straightened, scaled and adjusted in shape, and then i-BOX (shallow turbulent pickling) is used. The uncoiler has a strip threading speed of 50-70 m / min, a tail swinging speed of 100-140 m / min, and the rest is uncoiled at a speed of 120-180 m / min. The six-roll straightening machine is used to pull the strip head out of the uncoiler and straighten the strip head and tail to make the strip easy to thread. The six-roll straightening machine is maintained at a scale breaking 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°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. -1 The concentration is required to be ≤4. The strip steel after rinsing is dried in a dryer (the air supply temperature of the dryer is 80-140°C) to ensure that the grain boundary oxide layer is pickled clean. 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%, preferably 45-65%.

[0066] In the above technical solution, preferably, in the step (e), the steel plate is heated to 300°C and 910-950°C at a rate of 5-10°C / s and 0.2-2.0°C / s, respectively, and the soaking time is 200-280s to ensure that the steel plate is heated at a fully austenitizing temperature to obtain a uniform austenite structure, cooled to a slow cooling end temperature of 750-780°C at a rate of 1-5°C / s to avoid the formation of proeutectoid ferrite, and cooled to a fast cooling end temperature of 340-360°C at a rate of 16-20°C / s, and the over-aging temperature is The temperature is 340-360℃, and the overaging insulation is 1000-1300s to ensure that the bainite transformation is complete at this stage, and then the phase ratio of bainite, residual austenite and martensite in the final organization is controlled to ensure that both strong plasticity and formability are taken into account. The unit speed is 40-60m / min, and the unit speed is reduced by 5m / min for every 0.2mm increase in cold-rolled thin strip steel to avoid the transitional growth of steel plate grains after cold rolling. The flat elongation is 0.3-0.5%, and the flat elongation is reduced by 0.05% for every 0.1mm increase in thickness of cold-rolled thin strip steel.

[0067] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0068] Example

[0069] The present invention provides three groups of Examples 1 to 3 in which 1180 MPa grade super bainite steel for automobiles is prepared by the preparation method of the present invention, and the chemical compositions thereof are shown in Table 1.

[0070] Table 1 Chemical composition of 1180MPa grade super bainite steel for automobile (wt.%)

[0071] serial number C Si Mn P S Als Cr Ti V N Example 1 0.191 1.45 2.27 0.008 0.002 0.045 0.22 0.056 0.044 0.0032 Example 2 0.196 1.34 2.30 0.008 0.003 0.032 0.20 0.061 0.051 0.0030 Example 3 0.180 1.25 2.15 0.008 0.002 0.041 0.30 0.066 0.055 0.0031

[0072] The specific process of the preparation method of the above-mentioned 1180MPa grade super bainite steel plate for automobiles is as follows:

[0073] (a) Smelting process: smelting molten steel according to the chemical composition of the super bainite steel plate shown in Table 1;

[0074] (b) Continuous casting process: the casting temperature is 1460-1550°C, and the thickness of the slab obtained by continuous casting is 120-240 mm;

[0075] (c) Hot rolling process: The slab obtained by continuous casting is subjected to heating, dephosphorization, rough rolling, finish rolling and laminar cooling in sequence to obtain a hot-rolled coil. The main hot rolling process parameters are shown in Table 2;

[0076] Table 2 Main process parameters of hot rolling of 1180MPa grade super bainite steel for automobile

[0077]

[0078] (d) Pickling and cold rolling process: the hot rolled coil is pickled to remove the iron oxide scale on the surface, and then cold rolled to form cold rolled thin strip steel, wherein the thickness of the strip steels in Examples 1 to 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;

[0079] (e) Continuous annealing process: The cold-rolled thin strip is processed into the desired product by continuous annealing process. The strip is first slowly heated to the soaking temperature (complete austenitization temperature) and kept for a period of time to become completely austenitic. Then, it is slowly cooled to the slow cooling end temperature. Then, the strip is rapidly cooled. In the over-aging stage, austenite transforms into lath bainite. Finally, it is cooled to room temperature and the strip shape is adjusted by a skin pass mill. The specific continuous annealing process parameters are shown in Table 3:

[0080] Table 3 Main process parameters of continuous annealing of 1180MPa grade super bainite steel for automobiles

[0081]

[0082] The microstructure of the 1180MPa grade super bainite steel for automobiles prepared by the above process is as follows: Figure 1 As shown in the scanning electron microscope photos Figure 2 As shown in the figure, the structure of the experimental steel No. 1 is combined with Figure 3 (EBSD phase diagram) and Figure 4 (XRD spectrum) Phase volume fraction measurement results show that the microstructure of the 1180MPa super bainite steel of the present invention is composed of 80% to 90% lath bainite, 8 to 15% retained austenite and a small amount of martensite.

[0083] The performance of the above-mentioned 1180MPa grade super bainite steel for automobiles was tested in accordance with 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", and its mechanical properties are shown in Table 4 below:

[0084] Table 4 Mechanical properties of 1180MPa grade super bainite steel for automobiles

[0085]

Claims

1. 1180MPa grade super bainite steel for automobiles, characterized in that: Its chemical composition by mass percentage is: 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%, V 0.04% ~ 0.08%, Ti 0.04% ~ 0.08%, N ≤ 0.0045%, and the remaining elements are Fe and unavoidable impurities, and at the same time satisfy Ceq = C + Si / 30 + Mn / 20 + 2P + 4S ≤ 0.

41.

2. The 1180MPa grade super bainite steel for automobiles according to claim 1, characterized in that: Its microstructure is composed of 80% to 90% bainite, 1% to 3% martensite and 8% to 15% retained austenite.

3. The 1180MPa grade super bainite steel for automobiles according to claim 1, characterized in that: Its yield strength is 870-950MPa, tensile strength is 1180-1250MPa, elongation A 50 The value is 14% to 18%, and the hole expansion rate is 30% to 50%.

4. The method for preparing 1180MPa grade super bainite steel for automobiles according to any one of claims 1 to 3, characterized in that: The steps include: (a) Smelting process: smelting molten steel according to the chemical composition of 1180MPa grade super bainite steel for automobiles; (b) continuous casting process: continuously casting the molten steel treated in step (a) into slabs; (c) hot rolling process: heating the slab obtained in step (b) by hot charging or cold charging, dephosphorizing, rough rolling, finish rolling and laminar cooling to obtain a hot-rolled coil; Among them, the heating temperature is 1220-1270°C; the finishing rolling temperature is 1040-1130°C, the final rolling temperature is 870-920°C; the intermediate temperature is 720-770°C, the coiling temperature is 600-650°C; the thickness of the hot rolled coil is 2.3-4.0mm; (d) Pickling and cold rolling process: the hot rolled coil obtained in step (c) is uncoiled, stretched and straightened to break scale, the plate shape is adjusted, and the surface oxide layer is removed by pickling, and then cold rolled into a cold rolled thin strip steel coil; (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed, that is, after soaking, slow cooling, rapid cooling, over-aging treatment and smoothing, a 1180MPa grade super bainite steel sheet for automobiles is obtained; Among them, the soaking temperature is 910-950°C; the slow cooling endpoint temperature is 750-780°C, the fast cooling endpoint temperature is 340-360°C; the over-aging temperature is 340-360°C; the unit speed is 40-60m / min; and the flattening elongation is 0.3-0.5%.

5. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 4, characterized in that: In step (b), the casting temperature is 1460-1550° C. and the slab thickness is 120-240 mm.

6. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 4, characterized in that: In step (c), at least one of the following is satisfied: The total heating time in the furnace is 230 to 400 minutes; Soft water blowing is used in the rough rolling process to pre-cool the iron sheet; The entire length of the rolling line is fully descaled, the descaling water pressure is ≥22Mpa, and the descaling temperature is ≥1173℃; The finishing rolling passes are 7, the last pass reduction is ≤15% and lubrication rolling is adopted; Laminar cooling adopts front-stage centralized cooling method.

7. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 6, characterized in that: The coiling adopts U-type coiling mode. Before the strip enters the finishing mill, the head and tail of the strip are adjusted using a hot coil box. The coiling temperature of the first 30m of the strip is controlled at 630-680℃, the coiling temperature of the 70m after the tail is controlled at 630-680℃, and the coiling temperature of the middle section is controlled at 620℃.

8. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 4, characterized in that: In step (d), at least one of the following is satisfied: The unwinding machine has a threading speed of 50-70m / min, a tail-swinging speed of 100-140m / min, and the rest of the unwinding speed is 120-180m / min. A six-roller straightening machine is used for tension and descaling, with a speed of 120-180 m / min, an elongation of 1.2-2.5%, and a tension of 40-50 tons; The cold rolling is performed through 5 passes, the cold rolling reduction rate is 40-70%, and the thickness of the cold rolled thin strip is 1.0-2.0 mm.

9. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 4, characterized in that: In step (d), pickling is carried out by immersion pickling in an acid tank, specifically: 1) Three-stage pickling: The acid temperature of the pickling process is 75-93°C; the free HCl concentration is 2.5-5.0%, 6-12%, and 9-15% respectively; Fe 2+ The concentrations are 110-130 g / l, 90-120 g / l, and 50-70 g / l respectively; 2) Four-stage rinsing: rinsing temperatures are 30-50°C, 40-60°C, 50-70°C, 60-80°C; pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5; Cl -1 The concentration is ≤4g / l.

10. The method for preparing 1180MPa grade super bainite steel for automobile according to claim 4, characterized in that: In step (e), at least one of the following is satisfied: Heat to 300°C at a rate of 5-10°C / s, and then heat to the absorptive temperature at a rate of 0.2-2.0°C / s, with a absorptive holding time of 200-280s; Cool at a rate of 1-5°C / s to the slow cooling end temperature, and then cool at a rate of 16-20°C / s to the fast cooling end temperature; The insulation time of over-aging treatment is 1000~1300s; The speed of the cold-rolled thin strip steel is reduced by 5m / min for every 0.2mm increase; For every 0.1mm increase in thickness of cold-rolled thin strip, the flat elongation decreases by 0.05%.

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