High-strength hot-rolled steel plate for passenger car framework and production method

Abstract

The steel comprises the following chemical components in percentage by weight: 0.098% to 0.156% of C, 0.60% to 1.50% of Si, 1.75% to 2.63% of Mn, 0.035% to 0.045% of Al, 0.145% to 0.203% of V, 0.082% to 0.110% of Ti, 0.20% to 0.44% of Cr, 0.20% to 0.40% of Mo, 0.020% to 0.035% of W, 0.021% to 0.072% of Bi, 0.050% to 0.148% of Sb, 0.010% to 0.018% of Y, 0.0031% to 0.0042% of Ca, 0.1153% to 0.1173% of Mg, 0.70% to 1.40% of Al + Si, 1.0% to 1.8% of C / Ti, 0.1190% to 0.1211% of Ca + Mg, less than or According to the steel plate, the yield strength is larger than or equal to 740 MPa, the tensile strength is larger than or equal to 840 MPa, the transverse elongation A is larger than or equal to 23%, the hole expansion rate ranges from 55% to 65%, transverse cold bending is conducted by 180 degrees, D = a, and the steel plate has good high forming performance on the premise that high-strength mechanical performance is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and particularly relates to a high-strength hot-rolled steel plate for bus frames and a production method thereof. The steel plate of the present invention is mainly applicable to the manufacture of bus frames Background Art

[0002] High formability hot-rolled sheets are widely used in fields such as automobiles, household appliances, and construction. Especially in automobile manufacturing, the elongation and hole expansion rate of steel plates are important indicators for measuring the formability of materials. The hole expansion rate reflects the anti-rupture ability of materials under local deformation conditions, and is particularly important for components that require high formability (such as automobile chassis, body structure parts, etc.).

[0003] The hole expansion rate is a key indicator for evaluating the local formability of materials, especially in applications that require high local deformation ability (such as punching and flanging processes of automobile parts). A high hole expansion rate means that the material is not easily ruptured during local deformation and can withstand greater plastic deformation. The elongation directly reflects the amount of plastic deformation that the material can withstand before fracture in the application of high formability hot-rolled sheets. High hole expansion rate and high elongation are of great significance for applications in fields such as automobiles. By optimizing the composition design, refining the grains, controlling the second-phase particles, and improving the production process, the hole expansion rate of hot-rolled sheets can be significantly increased. In the future, with the continuous emergence of new materials and new processes, high formability hot-rolled sheets will play an important role in more fields and meet the growing market demand.

[0004] The Chinese patent application with the publication number CN 104561791 A discloses a steel for automobile boxes at the 800 MPa level and its production method. Under the existing smelting and rolling production equipment conditions, this invention utilizes the effects of microalloying elements vanadium and titanium in the steel, that is: the microalloying elements V and Ti combine with C and N to form carbonitrides, which play a precipitation strengthening role at low temperatures; the combined strengthening of vanadium and titanium can more fully exert the effect of precipitation strengthening and improve the overall performance of the steel plate. The elongation of the steel plate produced by this method is at most 27% and at least 22.5%, and no evaluation is made on the hole expansion rate. Its formability is poor and does not meet the requirements of high-strength forming automotive parts. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-strength hot-rolled steel plate for bus frames and a production method thereof. The yield strength of the steel plate is ≥740 MPa, the tensile strength is ≥840 MPa, the transverse elongation A is ≥23%, the hole expansion rate is between 55% and 65%, and the transverse cold bending 180° D = a is qualified. On the premise of ensuring high-strength mechanical properties, the steel plate has good high formability.

[0006] In order to achieve the above purpose, the present invention is realized by adopting the following technical solutions:

[0007] A high-strength hot-rolled steel plate for bus frame, the chemical components in the steel are as follows by weight percentage: C: 0.098% - 0.156%, Si: 0.60% - 1.50%, Mn: 1.75% - 2.63%, Al: 0.035% - 0.045%, V: 0.145% - 0.203%, Ti: 0.082% - 0.110%, Cr: 0.20% - 0.44%, Mo: 0.20% - 0.40%, W: 0.020% - 0.035%, Bi: 0.021% - 0.072%, Sb: 0.050% - 0.148%, Y: 0.010% - 0.018%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al + Si: 0.70% - 1.40%, C / Ti: 1.0 - 1.8, Ca + Mg: 0.1190% - 0.1211%, and P≤0.010%, S≤0.005%, N≤0.006% are limited, and the balance is Fe and inevitable impurities.

[0008] The structure in the steel is as follows: the volume percentage of ferrite is 20% - 30%, the volume percentage of martensite is 35% - 45%, the volume percentage of retained austenite is 12% - 15%, and the volume percentage of bainite is 10% - 20%.

[0009] The yield strength of the steel plate is ≥740 MPa, the tensile strength is ≥840 MPa, the transverse elongation A≥23%, the hole expansion rate is 55% - 65%, and the transverse cold bending 180° D = a is qualified.

[0010] A production method of a high-strength hot-rolled steel plate for bus frame, including smelting, heating, rolling and cooling, specifically as follows:

[0011] 1) Smelting process: The RH + LF process is adopted, the contents of H and O are strictly controlled, H≤0.0002%, O≤0.0015%, calcium treatment is carried out in the refining process, electromagnetic stirring and soft reduction technology are put into use during continuous casting, the casting speed of the billet is ≤1.0 m / min, the reduction amount of soft reduction is 2.0 - 5.0 mm, reducing the center segregation of the continuous casting billet is beneficial to reducing the banded structure of the subsequent rolled hot-rolled steel plate finished product and improving the hole expansion performance.

[0012] 2) Heating process: The continuous casting slab with a thickness of (110 - 210) mm and a width of (1050 - 2010) mm is directly hot-charged and hot-delivered into a walking beam heating furnace for heating, the heating temperature is 1115 - 1210 °C, and the holding time is 162 - 193 min. The chemical composition of the present invention contains Sb, the melting point of Sb is low, and it is easy to enrich at the grain boundary to generate cracks, so the heating temperature should not be too high. And the appropriate holding time makes the alloy elements in the slab completely dissolve, the slab composition is uniform, and plays a role in controlling the original austenite grain size, etc.

[0013] 3) Rolling and cooling process: The rough rolling adopts a 3+3 rolling process (R1 has 3 passes of rolling, R2 has 3 passes of rolling), with a total of 6 passes of rolling. The rough rolling exit temperature is 1055 - 1105 °C, the intermediate slab thickness is 32 - 44 mm, and the width is 1050 - 2010 mm. Before entering the hot rolling finishing mill, the intermediate slab is insulated by a heat preservation cover to reduce the temperature drop of the intermediate slab on the delay roller table and the temperature difference at the head, tail, and plate width direction. The finishing rolling is 7-stand continuous rolling. High-pressure water descaling is carried out before finishing rolling. The finishing rolling entry temperature is not higher than 1055 °C, and the finishing rolling temperature is 800 - 915 °C. After finishing rolling, a cooling mode of rapid cooling + air cooling + ultra-rapid cooling is adopted. The rapid cooling rate is about 50 - 60 °C / s. After cooling to 645 - 690 °C, air cooling is carried out for 6 - 12 s, and then ultra-rapid cooling is carried out. The ultra-rapid cooling rate ≥122 °C / s. The steel plate is cooled to 235 - 285 °C and then coiled. The hot-rolled plate after coiling immediately enters a slow cooling pit with heating, and the heat preservation cover is covered. The heating temperature of the slow cooling pit is 435 - 470 °C, and the heat preservation time is 15 - 20 min. Then the steel coil is taken out and air-cooled to room temperature. The purpose of rapid cooling to 645 - 690 °C + air cooling for 6 - 12 s is to rapidly precipitate ferrite. While inhibiting grain growth, it also ensures the content of ferrite, thus making the ferrite grains refined. The purpose of ultra-rapid cooling to 235 - 285 °C is to cool to the martensite region for coiling at a cooling rate greater than the critical cooling rate of pearlite transformation, avoiding the pearlite formation region. While inhibiting grain growth, it also ensures the content of martensite, thus making the martensite grains refined. Immediately after coiling, it enters a slow cooling pit with heating, and the heat preservation cover is covered. The heating temperature of the slow cooling pit is 435 - 470 °C, and the heat preservation time is 15 - 20 min. The purpose is to retain a large amount of V precipitation phase in the structure as a hydrogen trap, reducing the risk of delayed crack occurrence in the Sb-containing steel plate of the present invention during use; and through the combined addition of V and Mo, combined with the slow cooling process design, a large amount of V, Mo composite carbides are retained in the Sb-containing steel plate of the present invention, and these are used as hydrogen traps, greatly improving the hydrogen-induced crack resistance of the Sb-containing steel plate of the present invention during service, and obtaining a hot-rolled steel plate for bus skeletons with excellent mechanical properties, hole expansion properties, and hydrogen-induced crack resistance. The convexity control accuracy of the steel plate is ±25 μm, the flatness is controlled within 15 I, the thickness control accuracy is ±25 μm, and the finished product thickness is 2.0 - 5.0 mm.

[0014] The main functions of the composition of a high-strength hot-rolled steel plate for bus skeletons in the present invention are:

[0015] C: Carbon is a common strengthening element in steel. As an interstitial solid solution atom, carbon is dissolved in the matrix to increase the solid solution strength by causing lattice distortion. The role of carbon in the present invention can also ensure the stability of residual austenite, thereby improving the formability and hole expansion performance of the steel plate. If the carbon content is too low, the mechanical properties of the steel plate in the present invention cannot be obtained. If the content is too high, the steel plate will become brittle, and there will be risks of delayed fracture and hot-rolled edge cracking, which is also detrimental to the welding performance, plasticity and toughness of the steel plate. In the present invention, the carbon requirement is generally in the low carbon range, which is beneficial to reduce the risk of delayed fracture and hot-rolled edge cracking, and is also beneficial to the welding performance of the steel plate. Therefore, the optimal range of carbon in the present invention is 0.098% to 0.156%.

[0016] Si: Silicon is one of the important elements of the present invention. Sufficient silicon addition to ferrite can ensure the strength of the ferrite matrix. At the same time, the role of silicon addition is that sufficient silicon addition can also reduce inclusions in steel, inhibit the decomposition of residual austenite and the formation of carbides, and avoid the steel plate from reducing the mechanical properties and hole expansion performance due to the decomposition of residual austenite and the formation of carbides. However, if the silicon content is too low, it will not ensure the strength of the ferrite matrix and inhibit the decomposition of residual austenite and the formation of carbides. If the content is too high, it will affect the hot rolling surface quality, and a large amount of iron oxide scale and welding performance will appear. Therefore, the silicon content in the present invention is 0.60% to 1.50%.

[0017] Mn: Manganese strengthens the solid solution in steel by substitutional solid solution inducing lattice distortion, and is also an austenite stabilizing element in steel, expanding the austenite zone, reducing the critical quenching rate of steel, and delaying the transformation of austenite to pearlite. However, if the manganese content is too low, the supercooled austenite will be unstable, reducing the plasticity, toughness and hole expansion performance of the steel plate, and the added content of manganese should not exceed the scope of the present invention. The main consideration is the C or Mn segregation problem caused by excessive manganese content, which deteriorates the uniformity of the steel plate structure during hot rolling, easily causes serious banded structure defects in the structure, and is not conducive to hole expansion performance. In addition, excessive manganese content will lead to poor welding performance of the steel plate. Therefore, based on comprehensive considerations, the present invention selects a manganese content of 1.75% to 2.63%.

[0018] P: Phosphorus is an impurity element in steel and is easily concentrated at grain boundaries. When the phosphorus content in steel is high, Fe2P particles are easily formed, which reduces the plasticity, toughness and hole expansion performance of the steel. Therefore, the lower the content, the better. In order to obtain a higher elongation, the upper limit is set at 0.010%.

[0019] S: Sulfur is an impurity element in steel, which is easily combined with Mn to form MnS inclusions, becoming the starting point of cracks and deteriorating the processing performance, seriously affecting the plasticity, formability and hole expansion performance of the steel plate. Therefore, the lower the content, the better, and its upper limit is set at 0.006%.

[0020] Al: In traditional processes, Al is a deoxidizer in the steelmaking process. At the same time, Al can also combine with N in the steel to form AlN and refine the grains. However, in the present invention, the main purpose of adding a relatively large amount of Al is to accelerate the transformation kinetics of austenite to ferrite during the cooling process. At the same time, together with Si, it inhibits the precipitation of cementite, and at the same time raises the austenitizing temperature, facilitating a better selection of the process window. Too little Al content has a limited effect on the austenitizing temperature and slows down the precipitation rate of ferrite during cooling; while too high Al content will cause clogging of the nozzle during continuous casting, affecting production efficiency. Therefore, in the present invention, the Al content is limited to 0.035% - 0.045%, and it satisfies Al + Si: 0.70% - 1.40%.

[0021] V: Vanadium has significant precipitation strengthening and grain refinement strengthening effects. The effects of vanadium are mainly achieved by forming precipitates with carbon and nitrogen. In particular, the precipitation of VN formed with nitrogen can greatly improve the strength of the steel plate. In addition, the addition of V can also combine with H to improve the anti-delayed fracture ability of the steel plate, and a large amount of V precipitation phase is retained inside the steel plate structure as a hydrogen trap, reducing the risk of delayed cracks in the Sb-containing steel plate of the present invention during use; and through the combined addition of V and Mo and the design of slow cooling process, a large amount of V and Mo complex carbides are retained in the Sb-containing steel plate of the present invention, and these are used as hydrogen traps, greatly improving the anti-hydrogen-induced cracking ability of the Sb-containing steel plate during service, and obtaining a hot-rolled steel plate with excellent mechanical properties, hole expansion properties and anti-hydrogen-induced cracking ability. When the V content is relatively high, the low-temperature toughness of the steel plate is significantly deteriorated, and the toughness of the heat-affected zone of the weld is also poor. Therefore, the optimal range of the V content in the present invention is between 0.145% and 0.203%.

[0022] Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, raise the austenite recrystallization temperature, refine the grains, and at the same time improve the strength and toughness of the steel. Moreover, Ti is a strong carbide and nitride forming element, which can combine with carbon and nitrogen to form stable and fine carbides and nitrides, playing a significant role in grain refinement strengthening and precipitation strengthening, and can also strengthen ferrite and bainite, which is beneficial to improving the hole expansion performance. Therefore, the optimal range of the Ti content in the present invention is between 0.082% and 0.110%. And it satisfies C / Ti: 1.0 - 1.8.

[0023] Cr: It is a carbide forming element, which can delay the pearlite transformation, improve the hardenability of the steel, thus being beneficial to the formation of martensite structure, refine the structure, and play a strengthening effect. It can also stabilize the retained austenite, which is beneficial to the improvement of the hole expansion performance. Too low chromium content will affect the hardenability of the steel, and too high chromium content will increase the production cost and also make the processing and formability of the material worse. The principle for the selection of the chromium content is to promote the formation of martensite. Therefore, the chromium content in the present invention is selected to be 0.20% - 0.44%.

[0024] Mo: Molybdenum is a carbide-forming element that can improve the strength and toughness of steel plates. Mo can significantly enhance the stability of austenite, increase the hardenability of steel, facilitate the formation of martensite structure, and ensure the martensite content obtained during the rapid cooling stage. However, excessive hardenability limits the formation of retained austenite, which is not conducive to obtaining high plasticity and hole-expanding performance. Therefore, the Mo content in this invention is selected to be in the range of 0.20% - 0.40%.

[0025] W: Tungsten is a metal with the highest melting point. The formed WC has high hardness, and its function in steel is similar to that of molybdenum. Its wear resistance enhancement effect is better than that of molybdenum. Tungsten can also improve the hardenability of steel and effectively inhibit grain growth. When its content is less than 0.020%, the effect is slight. When it exceeds 0.035%, the brittleness increases. Therefore, the optimal range of W content in this invention is between 0.020% - 0.035%.

[0026] Bi: Bismuth element is mainly distributed at grain boundaries and inside grains in steel, which can improve the strength of steel plates, reduce the diffusion rate of elements such as carbon and oxygen at grain boundaries, reduce decarburization and oxidation phenomena, and improve the surface and mechanical properties of steel plates. Therefore, the Bi content in this invention is limited to 0.021% - 0.072%.

[0027] Sb: Antimony can make the corrosion products dense and inhibit the diffusion of H2O, O2, Cl, and SO4 2- etc. into the steel matrix. It can be enriched near the steel matrix in an acidic environment, prompting the formation of a uniform and dense oxide film (rich in elements such as Sb) on the surface of the steel plate matrix to resist further erosion of the steel matrix. However, Sb is a low-melting-point element that easily accumulates at grain boundaries, causing grain boundary cracks. With the addition of Sb content, the crack risk of the steel plate increases rapidly. Therefore, the Sb content in this invention is limited to 0.050% - 0.148%.

[0028] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, facilitate the improvement of hole-expanding performance, and can also improve the welding performance and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the Y content in this invention is limited to 0.010% - 0.018%.

[0029] Ca: Calcium can change the morphology of sulfides (MnS) in steel, prevent the formation of strip-shaped MnS inclusions, and improve the plasticity, toughness, and hole-expanding performance of steel plates. It can also improve the quality of continuous casting billets. Therefore, the Ca content in this invention is controlled at 0.0031% - 0.0042%, and Ca + Mg: 0.1190% - 0.1211%.

[0030] Mg: Magnesium is a good deoxidizer, desulfurizer and spheroidizer in steel. Magnesium can reduce the number of inclusions in steel, make their size smaller, distribution uniform and morphology improved. Trace amounts of magnesium can improve the size and distribution of carbides in steel, promote the carbide particles to be fine and uniform, which is beneficial to the improvement of the reaming rate. Therefore, the Mg content in this invention is controlled within 0.1153% - 0.1174%.

[0031] N: Regarding the N content in steel, the lower the N content, the better, but too low will lead to production difficulties and increased costs. However, in this invention, VN formed with V needs to precipitate for precipitation strengthening and grain refinement strengthening to improve the strength and reaming performance of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0032] Compared with the prior art, the beneficial effects of this invention are as follows:

[0033] 1) The addition of Mo can improve the strength and toughness of the steel plate. Mo can significantly improve the stability of austenite, increase the hardenability of the steel, be beneficial to the formation of martensite structure, ensure obtaining martensite in the rapid cooling stage, and is beneficial to obtaining higher strength.

[0034] 2) WC formed by W has high hardness, enhances the wear resistance, improves the hardenability of the steel, and effectively inhibits grain growth.

[0035] 3) The Bi bismuth element is mainly distributed at grain boundaries and inside grains in steel, playing a role in improving the strength of the steel plate, reducing the diffusion rate of elements such as carbon and oxygen at grain boundaries, reducing decarburization and oxidation phenomena, and improving the surface and mechanical properties of the steel plate.

[0036] 4) The addition of Sb can make the corrosion products dense, inhibit the diffusion of H2O, O2, Cl and SO4 2- etc. to the steel matrix, and can be enriched near the steel matrix in an acidic environment, promoting the formation of a uniform and dense oxide film (rich in elements such as Sb) on the surface of the steel plate matrix to resist further erosion of the steel matrix.

[0037] 5) The addition of Y can refine the grains in steel, enhance the strength and plasticity of grain boundaries, be beneficial to the improvement of reaming performance, and can also improve the welding performance and oxidation resistance of the steel, thereby increasing the service life of the steel at high temperatures.

[0038] 6) The addition of Ca can change the morphology of sulfides in the steel grade, improve the plasticity, toughness and reaming performance of the steel plate.

[0039] 7) The addition of Mg can reduce the number of inclusions in steel, make their size smaller, distribution uniform and morphology improved. Trace amounts of magnesium can improve the size and distribution of carbides in steel, promote the carbide particles to be fine and uniform, which is beneficial to the improvement of reaming performance.

[0040] 8) After rolling, a cooling mode of rapid cooling + air cooling + ultra-rapid cooling is adopted, and various phase structures at different cooling stages can be obtained.

[0041] 9) After coiling, it enters a slow cooling pit with heating. The purpose is to retain a large amount of V precipitation phases inside the structure as hydrogen traps, reduce the risk of delayed cracking during the use of the Sb-containing steel plate of the present invention, significantly improve the hydrogen-induced cracking resistance of the Sb-containing steel plate of the present invention during service, and obtain excellent mechanical properties, hole expansion properties and hydrogen-induced cracking resistance.

[0042] 10) The structure in the steel of the present invention is ferrite, martensite, retained austenite and bainite, which can significantly improve the hole expansion performance of the steel plate during the forming process.

[0043] 11) The present invention has excellent mechanical properties, with a yield strength ≥ 740 MPa, a tensile strength ≥ 840 MPa, a transverse elongation A ≥ 23%, a hole expansion rate between 55% and 65%, and a qualified transverse cold bend of 180° D = a. Specific Embodiments

[0044] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further illustrates the specific embodiments of the present invention in conjunction with embodiments. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0045] The specific components, hot rolling process systems, properties and tissue volume percentages of 6 embodiments of the present invention are shown in Tables 1-4.

[0046] Table 1 Chemical Compositions (wt, %) of Embodiments of the Present Invention

[0047]

[0048]

[0049] Table 2 Hot Rolling Process Systems of Embodiments of the Present Invention

[0050]

[0051] Table 3 Mechanical Property Parameters of Embodiments of the Present Invention

[0052]

[0053] Table 4 Tissue Volume Percentages in Embodiments of the Present Invention

[0054] Number Ferrite Martensite Retained austenite Bainite Example 1 21.0％ 45.0％ 15.0％ 19.0％ Example 2 28.8％ 38.7％ 14.5％ 18.0％ Example 3 24.0％ 41.6％ 15.0％ 19.4％ Example 4 27.0％ 42.0％ 14.0％ 17.0％ Example 5 29.5％ 37.5％ 13.0％ 20.0％ Example 6 30.0％ 44.0％ 12.3％ 13.7％

[0055] In order to describe the present invention, the present invention has been appropriately and sufficiently described by way of examples above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. should be included within the protection scope of the present invention. The patent protection scope of the present invention shall be defined by the claims.

Claims

1. A high-strength hot-rolled steel sheet for bus frame, characterized in that, The chemical components in the steel by weight percentage are as follows: C: 0.098% - 0.156%, Si: 0.60% - 1.50%, Mn: 1.75% - 2.63%, Al: 0.035% - 0.045%, V: 0.145% - 0.203%, Ti: 0.082% - 0.110%, Cr: 0.20% - 0.44%, Mo: 0.20% - 0.40%, W: 0.020% - 0.035%, Bi: 0.021% - 0.072%, Sb: 0.050% - 0.148%, Y: 0.010% - 0.018%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al + Si: 0.70% - 1.40%, C / Ti: 1.0 - 1.8, Ca + Mg: 0.1190% - 0.1211%. And P ≤ 0.010%, S ≤ 0.005%, N ≤ 0.006% are restricted, and the balance is Fe and inevitable impurities.

2. The high-strength hot-rolled steel plate for a bus skeleton according to claim 1, wherein The microstructure in the steel is as follows: the volume percentage of ferrite is 20% - 30%, the volume percentage of martensite is 35% - 45%, the volume percentage of retained austenite is 12% - 15%, and the volume percentage of bainite is 10% - 20%.

3. The high-strength hot-rolled steel sheet for bus skeleton according to claim 1, characterized in that, The yield strength of the steel plate ≥ 740 MPa, the tensile strength ≥ 840 MPa, the transverse elongation A ≥ 23%, the hole expansion rate is 55% - 65%, and the transverse cold bending 180° D = a is qualified.

4. The high-strength hot-rolled steel sheet for a bus frame according to claim 1, characterized in that, The finished thickness of the steel plate is 2.0 - 5.0 mm.

5. The high-strength hot-rolled steel sheet for a bus skeleton according to claim 1, characterized in that, The convexity control accuracy of the steel plate is ±25 μm, the flatness is controlled within 15 I, and the thickness control accuracy is ±25 μm.

6. A production method of high-strength hot-rolled steel plate for bus frame according to any one of claims 1-5, including smelting, heating, rolling and cooling, characterized in that, In the smelting process: the casting speed ≤ 1.0 m / min, and the reduction amount of soft reduction is 2.0 - 5.0 mm; In the heating process: the heating temperature is 1115 - 1210 °C, and the holding time is 162 - 193 min; In the cooling process: the hot-rolled plate after coiling immediately enters the slow cooling pit with heating, covers the heat preservation cover, the heating temperature of the slow cooling pit is 435 - 470 °C, holds for 15 - 20 min, takes out the steel coil, and air cools to room temperature.

7. The preparation method of a high-strength hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the rolling process, the rough rolling exit temperature is 1055 - 1105 °C.

8. The preparation method of a high-strength hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that In the rolling process, the finishing rolling entry temperature is not higher than 1055 °C, and the finishing rolling temperature is 800 - 915 °C.

9. A preparation method of a high-strength hot-rolled steel plate for a bus skeleton according to any one of claims 6-8, characterized in that In the rolling process, the thickness of the intermediate billet before finishing rolling and after rough rolling is 32 - 44 mm, the width is 1050 - 2010 mm, and the intermediate billet is insulated by a heat preservation cover before entering the hot rolling finishing mill.

10. The preparation method of a high-strength hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the cooling process, after finishing rolling, a cooling mode of rapid cooling + air cooling + ultra-rapid cooling is adopted. The rapid cooling rate is 50 - 60 °C / s, after cooling to 645 - 690 °C, air cooling is carried out for 6 - 12 s, and then ultra-rapid cooling is carried out. The ultra-rapid cooling rate ≥ 122 °C / s, and the steel plate is cooled to 235 - 285 °C and then coiled.

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