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

Abstract

The invention relates to a high-formability ultrahigh-strength hot-rolled steel plate for a passenger car framework and a production method, the steel comprises the following chemical components: C, Si, Mn, Al, Nb, V, Ti, Cr, Mo, W, Bi, Sb, Y, Ca and Mg, and P is limited to be less than or equal to 0.010%, S is limited to be less than or equal to 0.005%, N is limited to be less than or equal to 0.006%, and the balance is Fe and inevitable impurities. According to the steel plate, the yield strength is larger than or equal to 810 MPa, the tensile strength is larger than or equal to 935 MPa, the transverse elongation A is larger than or equal to 21%, the hole expansion rate ranges from 45% to 55%, transverse cold bending is conducted by 180 degrees, D = a, the steel plate is qualified, and good high forming performance is achieved.

Description

Technical Field

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

[0002] Hot-rolled sheets with high formability are widely used in fields such as automobiles, household appliances, and construction. Especially in automobile manufacturing, the elongation and hole expansion ratio of steel sheets are important indicators for measuring the formability of materials. The hole expansion ratio 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 structural parts, etc.).

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

[0004] The Chinese patent application with the publication number CN 105369134 B discloses a 400MPa grade pickling-free automotive structural hot-rolled steel sheet and its production method, which is designed with a common C-Mn composition system. The hot-rolled steel sheet is produced by solid-solution strengthening with carbon and manganese elements to increase the strength. However, the yield strength of this steel sheet is at most 311MPa, the elongation is ≥34%, and the hole expansion ratio is not evaluated. Its strength level is relatively low, and the requirements for elongation and hole expansion ratio do not meet the requirements of high-strength forming automotive parts.

[0005] The Chinese patent application with the application number 201811563499.1 discloses a 1300 MPa grade ultra-high strength cold-rolled steel sheet for automobiles and its production method. In the composition design of the present invention, C, Mn, and Mo elements are used to improve the hardenability of the steel and ensure the strength of the strip steel. At the same time, Nb, V, and Ti are used for microalloying, and the yield strength and toughness are improved by the effect of fine grain strengthening. While achieving ultra-high strength, good elongation is maintained, and the decrease in elongation caused by excessive Nb elements is avoided, with excellent comprehensive performance and low cost. The obtained ultra-high strength cold-rolled steel sheet for automobiles has a yield strength ≥ 850 MPa and a tensile strength ≥ 1300 MPa. However, on the premise of high strength, its elongation is only ≥ 7%, and the hole expansion rate is not evaluated, and its formability does not meet the requirements of high-strength formed automobile parts. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-formability ultra-high strength hot-rolled steel sheet for bus frames and its production method, with a yield strength ≥ 810 MPa, a tensile strength ≥ 935 MPa, a transverse elongation rate A ≥ 21%, a hole expansion rate of 45% - 55%, and a qualified transverse cold bend of 180° D = a, having good high-formability.

[0007] In order to achieve the above purpose, the present invention is implemented by adopting the following technical solutions:

[0008] A high-formability ultra-high strength hot-rolled steel sheet for bus frames, the chemical components in the steel are calculated by weight percentage as follows: C: 0.079% - 0.123%, Si: 0.60% - 1.50%, Mn: 1.82% - 2.50%, Al: 0.20% - 0.34%, Nb: 0.053% - 0.072%, V: 0.135% - 0.182%, Ti: 0.092% - 0.122%, Cr: 0.70% - 0.85%, Mo: 0.25% - 0.45%, W: 0.021% - 0.035%, Bi: 0.022% - 0.070%, Sb: 0.050% - 0.151%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al + Si: 0.90% - 1.70%, Mo + Mn: 2.3% - 2.9%, Ca + Mg: 0.1190% - 0.1211%, and P ≤ 0.010%, S ≤ 0.005%, N ≤ 0.006% are restricted, and the balance is Fe and unavoidable impurities.

[0009] The structure in the steel is: ferrite volume percentage 20% - 30%, martensite volume percentage 40% - 50%, retained austenite volume percentage 9% - 13%, bainite volume percentage 10% - 20%.

[0010] The yield strength of the steel plate is ≥810 MPa, the tensile strength is ≥935 MPa, the transverse elongation A is ≥21%, the hole expansion rate is 45% - 55%, and the transverse cold bending of 180° with D = a is qualified.

[0011] A production method of a high-formability ultra-high-strength hot-rolled steel plate for a bus skeleton, including smelting, heating, rolling and cooling, is as follows:

[0012] 1) Smelting process: The RH+LF process is adopted to strictly control the contents of H and O, with H ≤ 0.0002% and O ≤ 0.0015%. Calcium treatment is carried out in the refining process, and electromagnetic stirring and soft reduction technologies are introduced during continuous casting. The casting speed of the billet is ≤1.0 m / min, and 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 hot-rolled steel plate finished product and improving the hole expansion performance.

[0013] 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-loaded into a walking beam reheating 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. Since the melting point of Sb is low and it is easy to enrich at the grain boundaries to generate cracks, the heating temperature should not be too high. And the appropriate holding time enables the alloying elements in the slab to be completely dissolved, the slab composition to be uniform, and plays a role in controlling the original austenite grain size, etc.

[0014] 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 1040°C to 1105°C, the thickness of the intermediate slab is 32 - 45 mm, 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 inlet temperature is not higher than 1040°C, the finishing rolling temperature is 780 - 912°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 655 - 695°C, air cooling is carried out for 6 - 12 s, and then ultra-rapid cooling is carried out. The ultra-rapid cooling rate ≥132°C / s. The steel plate is cooled to 255 - 305°C and then coiled. 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 455 - 505°C, and it is insulated for 15 - 20 min. Then the steel coil is taken out and air cooled to room temperature. The purpose of rapid cooling to 655 - 695°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 255 - 305°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 455 - 505°C, and it is insulated for 15 - 20 min. The purpose is to retain a large amount of V precipitation phases in the tissue as hydrogen traps, reducing the risk of delayed cracking of 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 cracking resistance of the Sb-containing steel plate of the present invention during service, and obtaining excellent mechanical properties, hole expansion properties and hydrogen-induced cracking-resistant hot-rolled steel plates for bus skeletons. The convexity control accuracy of the steel plate is ±20μm, the flatness is controlled within 15I, the thickness control accuracy is ±20μm, and the finished product thickness is 1.8 - 5.5 mm.

[0015] The main functions of the composition of a high-formability ultra-high-strength hot-rolled steel plate for bus skeletons in the present invention are:

[0016] C: Carbon is a common strengthening element in steel. As an interstitial solid solution atom, carbon dissolves in the matrix to increase the strength of the solid solution by causing lattice distortion. The role of carbon in the present invention can also ensure the stability of retained austenite, thereby improving the formability and hole-expanding performance of the steel plate. If the carbon element 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, with risks of delayed fracture and hot-rolled edge cracking, and it is also unfavorable for the welding performance, plasticity, and toughness of the steel plate. In the present invention, carbon is required to be in the low-carbon range as a whole, which is beneficial to reducing the risks 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 0.079% - 0.123%.

[0017] Si: Silicon is one of the important elements in the present invention. Adding sufficient silicon to ferrite can ensure the strength of the ferrite matrix. At the same time, the role of silicon addition is also that adding sufficient content of silicon can reduce inclusions in the steel, inhibit the decomposition of retained austenite and the formation of carbides, and avoid the reduction of the mechanical properties and hole-expanding performance of the steel plate due to the decomposition of retained austenite and the formation of carbides. However, if the silicon content is too low, it cannot play the role of ensuring the strength of the ferrite matrix and inhibiting the decomposition of retained austenite and the formation of carbides. If the content is too high, it will affect the hot-rolled surface quality, resulting in a large amount of scale and welding performance. Therefore, the content of silicon in the present invention is 0.60% - 1.50%.

[0018] Mn: Manganese strengthens the solid solution in the way of substitutional solid solution causing lattice distortion in steel, and it is also an austenite stabilizing element in steel, expanding the austenite region, reducing the critical quenching speed of steel, and delaying the transformation of austenite to pearlite. However, if the manganese content is too low, the supercooled austenite is unstable, reducing the plasticity, toughness, and hole-expanding performance of the steel plate. And the addition content of manganese element should not exceed the scope of the present invention. The main consideration is the problem of C or Mn segregation caused by too high manganese content, which deteriorates the tissue uniformity of the steel plate during the hot-rolling process, is easy to cause serious banded tissue defects in the tissue, and is also unfavorable for the hole-expanding performance. Moreover, too high manganese content will lead to poor welding performance of the steel plate. Therefore, the selected manganese content is 1.82% - 2.50%.

[0019] P: Phosphorus is an impurity element in steel and is extremely easy to segregate at grain boundaries. When the phosphorus content in steel is relatively high, it is easy to form Fe2P particles, reducing the plasticity, toughness, and hole-expanding performance of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation rate, its upper limit is set to 0.010%.

[0020] S: Sulfur is an impurity element in steel and is easy to combine with Mn to form MnS inclusions, becoming the starting point of cracks and deteriorating the processing performance, seriously affecting the plasticity, formability, and hole-expanding performance of the steel plate. Therefore, the less the content, the better, and its upper limit is set to 0.005%.

[0021] Al: In traditional processes, Al is a deoxidizer in the steelmaking process. Meanwhile, Al can also combine with N in the steel to form AlN and refine the grains. However, in this 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, and at the same time, together with Si, inhibit the precipitation of cementite, and increase the austenitizing temperature to facilitate a better selection of the process window. Too little Al content has limited influence on the austenitizing temperature and slows down the precipitation rate of ferrite during cooling; while too high Al content will cause clogging of the tundish during continuous casting, affecting production efficiency. Therefore, in this invention, the Al content is limited to 0.20% - 0.34%, and it satisfies Al + Si: 0.90% - 1.70%.

[0022] Nb: Niobium mainly plays roles such as fine grain strengthening and precipitation strengthening in steel, can also expand the rolling process window, improve the uniformity of the through-coil performance of the steel coil, and can also refine the microstructure and improve the cold forming performance of the steel plate. At high temperatures, niobium exists in a solid solution state in austenite, which can inhibit the growth of austenite grains and static and dynamic recrystallization during hot deformation, and increase the recrystallization termination temperature, enabling the finishing rolling temperature to be increased; at the same time, the precipitation of niobium carbonitrides can also delay recrystallization and prevent the growth of austenite grains, with obvious fine grain strengthening and precipitation strengthening effects, and can effectively reduce the grade of the steel plate's banded structure and improve the hole expansion performance. Therefore, the optimal range of the Nb content in this invention is between 0.053% and 0.072%.

[0023] V: Vanadium has significant precipitation strengthening and fine grain 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 steel plate's resistance to delayed fracture, and a large number of V precipitation phases are retained inside the steel plate microstructure as hydrogen traps, reducing the risk of delayed cracks occurring during the use of the Sb-containing steel plate in this invention; and through the combined addition of V and Mo, combined with the slow cooling process design, a large number of V and Mo composite carbides are retained in the Sb-containing steel plate in this invention and used as hydrogen traps, greatly improving the hydrogen-induced crack resistance of the Sb-containing steel plate during service, and obtaining a hot-rolled steel plate with excellent mechanical properties, hole expansion performance, and hydrogen-induced crack resistance. When the V content is relatively high, the low-temperature toughness of the steel plate deteriorates significantly, and the toughness of the heat-affected zone of welding also becomes worse. Therefore, the optimal range of the V content in this invention is between 0.135% and 0.182%.

[0024] Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase 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 fine grain 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 Ti content in the present invention is between 0.092% and 0.122%.

[0025] 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. If the chromium content is too low, it will affect the hardenability of the steel. If the chromium content is too high, the production cost will increase, and the processing and formability of the material will also become worse. The selection principle of the chromium content is to promote the formation of martensite. Therefore, the chromium content selected in the present invention is 0.70% - 0.85%.

[0026] Mo: Molybdenum is a carbide forming element, which can improve the strength and toughness of the steel plate. Mo can significantly improve the stability of austenite, increase the hardenability of the steel, and is beneficial to the formation of martensite structure, ensuring the martensite content obtained in the rapid cooling stage. However, too high hardenability limits the formation of retained austenite, which is not conducive to obtaining high plasticity and hole expansion performance. Therefore, the Mo content selected in the present invention is 0.25% - 0.45%, and it satisfies Mo + Mn: 2.3% - 2.9%.

[0027] W: Tungsten is the metal with the highest melting point. The formed WC has high hardness. Its function in steel is similar to that of molybdenum, and its wear resistance enhancement effect is better than that of molybdenum. Tungsten can also improve the hardenability of the 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 the present invention is between 0.021% and 0.035%.

[0028] Bi: Bismuth element is mainly distributed at grain boundaries and inside grains in the steel, playing a role in increasing 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. Therefore, the Bi content in the present invention is limited to 0.022% - 0.070%.

[0029] Sb: Antimony can make the corrosion products dense and inhibit H2O, O2, Cl and SO4 2-Diffusing into the isotropic steel matrix, it 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. However, Sb is a low-melting-point element, which is prone to enrichment at grain boundaries, causing grain boundary cracks. With the addition of Sb content, the crack risk of the steel plate increases rapidly. Therefore, in the present invention, the Sb content is limited to 0.050% - 0.151%.

[0030] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, is beneficial to 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, in the present invention, the Y content is limited to 0.012% - 0.020%.

[0031] 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 the steel plate. It can also improve the quality of continuous casting billets. Therefore, in the present invention, the Ca content is controlled at 0.0031% - 0.0042%, and Ca + Mg: 0.1190% - 0.1211%.

[0032] Mg: Magnesium is a good deoxidizer, desulfurizer, and nodulizer 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 carbides to be fine and uniform, which is beneficial to the improvement of the hole-expanding rate. Therefore, in the present invention, the Mg content is controlled at 0.1153% - 0.1174%.

[0033] N: Regarding the N content in steel, the lower the N content, the better. However, too low N content will lead to production difficulties and increased costs. But in the present invention, VN formed with V needs to precipitate for precipitation strengthening and grain refinement strengthening to improve the strength and hole-expanding performance of the steel plate. Therefore, in the present invention, the N content ≤ 0.006%.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 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 steel, is beneficial to the formation of martensite structure, ensure the acquisition of martensite in the rapid cooling stage, and is beneficial to obtaining higher strength.

[0036] 2) WC formed by W has high hardness, enhances the wear resistance, improves the hardenability of steel, and effectively inhibits grain growth.

[0037] 3) The bismuth element Bi in steel is mainly distributed at grain boundaries and inside grains, 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.

[0038] 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.

[0039] 5) The addition of Y can refine the grains in the steel, enhance the strength and plasticity of the grain boundaries, be beneficial to the improvement of the hole-expanding 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.

[0040] 6) The addition of Ca can change the morphology of sulfides in the steel type, and improve the plasticity, toughness, and hole-expanding performance of the steel plate.

[0041] 7) The addition of Mg can reduce the number of inclusions in the steel, make their size smaller, with uniform distribution and improved morphology. Trace amounts of magnesium can improve the size and distribution of carbides in the steel, promote the carbide particles to be fine and uniform, which is beneficial to the improvement of the hole-expanding performance.

[0042] 8) Adopting a cooling mode of rapid cooling + air cooling + ultra-rapid cooling after rolling can obtain various phase structures at different cooling stages.

[0043] 9) After coiling, it enters a slow-cooling pit with heating. The purpose is to retain a large number of V precipitation phases in the structure as hydrogen traps, reduce the risk of delayed cracks occurring during the use of the Sb-containing steel plate of the present invention; significantly improve the hydrogen-induced crack resistance of the Sb-containing steel plate of the present invention during service, and obtain excellent mechanical properties, hole-expanding performance, and hydrogen-induced crack resistance.

[0044] 10) The structure in the steel of the present invention is ferrite, martensite, retained austenite, and bainite, which can significantly improve the hole-expanding performance of the steel plate during the forming process.

[0045] 11) The present invention has excellent mechanical properties, with a yield strength ≥ 810 MPa, a tensile strength ≥ 935 MPa, a transverse elongation A ≥ 21%, a hole-expanding rate of 45% - 55%, and a qualified transverse cold bend of 180° D = a. Specific embodiments

[0046] 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 combination with examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0047] The specific compositions, hot rolling process systems, performances, and tissue volume percentages of 6 examples of the present invention are shown in Tables 1 - 4.

[0048] Table 1 Chemical composition of the embodiments of the present invention (wt, %)

[0049]

[0050] Table 2 Hot rolling process system of the embodiments of the present invention

[0051]

[0052] Table 3 Mechanical property parameters of the embodiments of the present invention

[0053]

[0054] Table 4 Volume percentage of the structure in the embodiments of the present invention

[0055] Number Ferrite Martensite Retained austenite Bainite Example 1 21.0％ 50.0％ 9.0％ 20.0％ Example 2 28.8％ 48.7％ 10.5％ 12.0％ Example 3 25.4％ 42.6％ 12.5％ 19.5％ Example 4 27.0％ 49.0％ 9.5％ 14.5％ Example 5 30.0％ 47.0％ 13.0％ 10.0％ Example 6 26.0％ 44.0％ 10.3％ 19.7％

[0056] In order to describe the present invention, the present invention has been properly and fully 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 make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent replacements, improvements, etc. shall 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 formability ultra-high strength hot-rolled steel sheet for bus frame, characterized in that The chemical components in the steel are by weight percentage: C: 0.079% - 0.123%, Si: 0.60% - 1.50%, Mn: 1.82% - 2.50%, Al: 0.20% - 0.34%, Nb: 0.053% - 0.072%, V: 0.135% - 0.182%, Ti: 0.092% - 0.122%, Cr: 0.70% - 0.85%, Mo: 0.25% - 0.45%, W: 0.021% - 0.035%, Bi: 0.022% - 0.070%, Sb: 0.050% - 0.151%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al + Si: 0.90% - 1.70%, Mo + Mn: 2.3% - 2.9%, Ca + Mg: 0.1190% - 0.1211%. And it is restricted that P ≤ 0.010%, S ≤ 0.005%, N ≤ 0.006%, and the balance is Fe and unavoidable impurities.

2. The high formability ultra-high strength hot-rolled steel sheet for bus frame according to claim 1, characterized in that, The microstructure in the steel is: ferrite volume percentage 20% - 30%, martensite volume percentage 40% - 50%, retained austenite volume percentage 9% - 13%, bainite volume percentage 10% - 20%.

3. The high formability ultra-high strength hot-rolled steel sheet for bus frame according to claim 1, wherein, The yield strength of the steel plate ≥ 810 MPa, the tensile strength ≥ 935 MPa, the transverse elongation A ≥ 21%, the hole expansion rate is 45% - 55%, and the transverse cold bend 180° D = a is qualified.

4. The high formability ultra-high strength hot-rolled steel sheet for a bus skeleton according to claim 1, characterized in that, The finished thickness of the steel plate is 1.8 - 5.5 mm.

5. The high formability ultra-high strength hot-rolled steel sheet for bus frame according to claim 1, characterized in that, The convexity control precision of the steel plate is ±20 μm, the flatness is controlled within 15 I, and the thickness control precision is ±20 μm.

6. A production method of a high formability ultra-high strength hot-rolled steel sheet for a bus skeleton 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: after the hot rolled coil is taken up, it immediately enters the slow cooling pit with heating, the heat preservation cover is covered, the heating temperature of the slow cooling pit is 455 - 505 °C, the heat preservation is 15 - 20 min, the steel coil is taken out and air cooled to room temperature.

7. The preparation method of a high formability ultra-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 1040 - 1105 °C.

8. The preparation method of a high formability ultra-high strength hot rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the rolling process, the finish rolling entry temperature is not higher than 1040 °C, and the finish rolling temperature is 780 - 912 °C.

9. A preparation method of a high formability ultra-high strength hot-rolled steel sheet 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 finish rolling and after rough rolling is 32 - 45 mm, the width is 1050 - 2010 mm, and the intermediate billet is heat preserved by a heat preservation cover before entering the hot rolling finish rolling mill.

10. The preparation method of a high formability ultra-high strength hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the cooling process, after finish rolling, the 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 655 - 695 °C, air cooling is carried out for 6 - 12 s, and then ultra - rapid cooling is carried out. The ultra - rapid cooling rate ≥ 132 °C / s, and the steel plate is cooled to 255 - 305 °C and then coiled.

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