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

Abstract

The steel comprises the following chemical components in percentage by weight: 0.025 percent to 0.042 percent of C, 0.10 percent to 0.21 percent of Si, 1.35 percent to 1.98 percent of Mn, 0.18 percent to 0.34 percent of Al, 0.007 percent to 0.017 percent of Nb, 0.010 percent to 0.019 percent of V, 0.25 percent to 0.45 percent of Mo, 0.022 percent to 0.034 percent of W, 0.021 percent to 0.063 percent of Bi, 0.050 percent to 0.102 percent of Sb, 0.012 percent to 0.020 percent of Y, 0.0031 percent to 0.0042 percent of Ca, 0.1153 percent to 0.1173 percent of Mg, 0.9 percent to 3.0 percent of Al / Si, 1.75 percent to 2.30 percent of Mo + Mn, 0.1190 percent to 0.1211 percent of Ca + Mg, less than or equal to 0.010 percent of P, less than or equal to 0.005 percent The yield strength of the steel plate ranges from 345 MPa to 370 MPa, the tensile strength ranges from 445 MPa to 512 MPa, the transverse elongation A is larger than or equal to 36%, the hole expansion rate ranges from 105% to 115%, transverse cold bending is conducted by 180 degrees, D = a, and the steel plate has good high forming performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and particularly relates to a high-elongation 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 plates 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 structural 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 capabilities (such as punching and flanging processes for automobile parts). A high hole-expansion rate means that the material is not easily broken 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 plates. High hole-expansion rate and high elongation are of great significance for applications in fields such as automobiles. By optimizing composition design, refining grains, controlling second-phase particles, and improving production processes, the hole-expansion rate of hot-rolled plates can be significantly increased. In the future, with the continuous emergence of new materials and new processes, high-formability hot-rolled plates will play an important role in more fields and meet the growing market demand.

[0004] Chinese patent application with publication number CN 105369134 B discloses a 400MPa grade pickling-free hot-rolled steel plate for automobile structures and its production method, which is designed with a common C-Mn composition system and produces a hot-rolled steel plate by solid-solution strengthening with carbon and manganese elements. However, the yield strength of this steel plate is at most 311MPa, the elongation is ≥34%, and no evaluation is made on the hole-expansion rate. Its strength level is low, and the requirements for elongation and hole-expansion rate do not meet the requirements of high-strength forming automobile parts. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-elongation hot-rolled steel plate for bus frames and a production method thereof. The yield strength of the steel plate is 345-370MPa, the tensile strength is 445-512MPa, the transverse elongation A≥36%, the hole-expansion rate is 105%-115%, and the transverse cold bending of 180° D=a is qualified. The steel plate of the present invention has good high formability.

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

[0007] A high-elongation hot-rolled steel plate for bus frames, the chemical components in the steel by weight percentage are: C: 0.025% - 0.042%, Si: 0.10% - 0.21%, Mn: 1.35% - 1.98%, Al: 0.18% - 0.34%, Nb: 0.007% - 0.017%, V: 0.010% - 0.019%, Mo: 0.25% - 0.45%, W: 0.022% - 0.034%, Bi: 0.021% - 0.063%, Sb: 0.050% - 0.102%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al / Si: 0.9% - 3.0%, Mo + Mn: 1.75% - 2.30%, 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.

[0008] The microstructure in the steel is: the volume percentage of ferrite is 60% - 80%, the volume percentage of bainite is 10% - 30%, and the volume percentage of retained austenite is 4% - 10%.

[0009] The yield strength of the steel plate is 345 - 370 MPa, the tensile strength is 445 - 512 MPa, the transverse elongation rate A ≥ 36%, the hole expansion rate is 105% - 115%, and the transverse cold bending 180° D = a is qualified.

[0010] A production method of a high-elongation hot-rolled steel plate for bus frames, including smelting, heating, rolling and cooling. The specific method is as follows:

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

[0012] 2) Heating process: Directly hot charge and heat the continuous casting slab with a thickness of (110 - 210) mm and a width of (1050 - 2010) mm 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, and the melting point of Sb is low, which is easy to enrich at the grain boundaries and 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, and R2 has 3 passes of rolling), with a total of 6 passes of rolling. The rough rolling exit temperature is 1070 - 1105°C, the intermediate slab thickness is 33 - 45 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 between the head and tail and in the 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 1070°C, and the finishing rolling temperature is 860 - 908°C. After finishing rolling, a laminar cooling + ultra-fast cooling mode is adopted. The laminar cooling rate is about 12 - 17°C / s. After cooling to 750 - 798°C, ultra-fast cooling is carried out, and the cooling rate ≥100°C / s. The steel plate is cooled to 400 - 550°C and then coiled. After coiling, the hot rolled plate immediately enters a slow cooling pit with heating, and the heat preservation cover is covered. The heating temperature of the slow cooling pit is 500 - 600°C, and it is kept warm for 10 - 15 minutes. Then the steel coil is taken out and air-cooled to room temperature. The purpose of laminar cooling to 750 - 798°C is to rapidly precipitate ferrite. While inhibiting grain growth, it also ensures the content of ferrite, thereby refining the ferrite grains. The purpose of ultra-fast cooling to 400 - 550°C is to rapidly precipitate a large amount of bainite. While inhibiting grain growth, it also ensures the content of bainite, thereby refining the bainite grains. 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 500 - 600°C, and the purpose of keeping warm for 10 - 15 minutes is to retain a large amount of V precipitation phases as hydrogen traps inside the structure, reducing the risk of delayed cracks occurring 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 and 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 a high-elongation hot rolled steel plate for bus frames with excellent mechanical properties, hole expansion properties, and hydrogen-induced cracking resistance. The flatness control accuracy of the steel plate is ±40μm, the flatness is controlled within 15I, the thickness control accuracy is ±40μm, and the finished product thickness is 2.0 - 7.0 mm.

[0014] The main functions of the components of a high-elongation hot rolled steel plate for bus frames in the present invention are:

[0015] C: Carbon is a common strengthening element in steel. As an interstitial solid solution atom, carbon dissolves in the matrix and increases 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 forming performance and hole expansion 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 be embrittled, posing risks of delayed fracture and hot rolling 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 rolling 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 0.025% - 0.042%.

[0016] Si: Silicon is one of the important elements of the present invention. Sufficient addition of silicon to ferrite can ensure the strength of the ferrite matrix. At the same time, the role of silicon addition is that sufficient content of silicon addition can also 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 expansion 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 guarantee the strength of the ferrite matrix and inhibit the decomposition of retained austenite and the formation of carbides. If the content is too high, it will affect the hot rolling surface quality, resulting in a large amount of scale and welding performance. Therefore, the content of silicon in the present invention is 0.10% - 0.21%.

[0017] 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 undercooled austenite is unstable, reducing the plasticity, toughness, and hole expansion 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 that too high manganese content leads to C or Mn segregation problems, deteriorating the tissue uniformity of the steel plate during hot rolling, and easily causing serious banded tissue defects in the tissue. Also, too high manganese in the steel involved in the present invention leads to an increase in hardenability and the inhibition of bainite formation, which is also unfavorable for the hole expansion performance. Moreover, too high manganese content will cause the welding performance of the steel plate to deteriorate. Therefore, considering comprehensively in the present invention, the manganese content is selected to be 1.35% - 1.98%.

[0018] P: Phosphorus is an impurity element in steel and is extremely prone to segregation 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 expansion 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%.

[0019] 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 expansion performance of the steel plate. Therefore, the less the content, the better, and its upper limit is set to 0.005%.

[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. Meanwhile, together with Si, it inhibits the precipitation of cementite and increases the austenitizing temperature, facilitating a better selection of the process window. Too little Al content has a limited impact 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 0.18% - 0.34%, and Al / Si satisfies 0.9% - 3.0%.

[0021] Nb: Niobium mainly plays roles such as fine grain strengthening and precipitation strengthening in steel. It can also expand the rolling process window, improve the uniformity of the through-roll performance of the steel coil, refine the structure, 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 an increase in the finish rolling temperature. 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, which can effectively reduce the grade of the banded structure of the steel plate and improve the hole expansion performance. Therefore, the optimal range of Nb content in the present invention is between 0.007% and 0.017%.

[0022] 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 VN precipitate 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 resistance of the steel plate to delayed fracture, and a large amount of V precipitate phase is retained inside the steel plate structure as a hydrogen trap, reducing the risk of delayed cracks occurring during the use of the Sb-containing steel plate in the present invention; and through the combined addition of V and Mo and the design of the slow cooling process, a large amount of V and Mo composite carbides are retained in the Sb-containing steel plate in the present invention, and these are 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, it significantly deteriorates the low-temperature toughness of the steel plate, and the toughness of the heat-affected zone of welding also becomes poor. Therefore, the addition amount of V in the present invention is 0.010% - 0.019%.

[0023] 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, and is beneficial to the formation of bainite structure. Both Mn and Mo are elements that increase hardenability, ensuring the bainite content obtained in 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 the present invention is selected to be between 0.25% and 0.45%, and it satisfies Mo + Mn: 1.75% - 2.30%.

[0024] 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, which can 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 0.034%, the brittleness increases. Therefore, the optimal range of W content in the present invention is between 0.022% and 0.034%.

[0025] Bi: Bismuth elements are 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 the present invention is limited to 0.021% - 0.063%.

[0026] Sb: Antimony can make the corrosion products dense, inhibit the diffusion of H2O, O2, Cl, and SO4 2- etc. into 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. 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, the Sb content in the present invention is limited to 0.050% - 0.102%.

[0027] Y: Yttrium can refine 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 antioxidant performance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the Y content in the present invention is limited to 0.012% - 0.020%.

[0028] Ca: Calcium can change the morphology of sulfides (MnS) in steel, prevent the formation of long - strip MnS inclusions, improve the plasticity, toughness, and hole - expanding performance of steel plates, and can also improve the quality of continuous casting billets. Therefore, the Ca content in the present invention is controlled at 0.0031% - 0.0042%, and Ca + Mg: 0.1190% - 0.1211%.

[0029] 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%.

[0030] N: Regarding the N content in steel, the lower the N content, the better. However, too low N content will cause production difficulties and increase costs. But 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%.

[0031] Compared with the prior art, the beneficial effects of this invention are as follows:

[0032] 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 bainite structure, ensure obtaining bainite in the rapid cooling stage, and be beneficial to obtaining higher plasticity and reaming performance.

[0033] 2) WC formed by W has high hardness, can enhance the wear resistance, improve the hardenability of the steel, and effectively inhibit grain growth.

[0034] 3) The bismuth element Bi in steel is mainly distributed at grain boundaries and inside grains, which can improve the strength of the steel plate, 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 the steel plate.

[0035] 4) The addition of Sb can make the corrosion products dense, inhibit the diffusion of H2O, O2, Cl and SO42 etc. to the steel matrix, can be enriched near the steel matrix in an acidic environment, and promote 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.

[0036] 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.

[0037] 6) The addition of Ca can change the morphology of sulfides in the steel grade, and improve the plasticity, toughness and reaming performance of the steel plate.

[0038] 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.

[0039] 8) After rolling, a laminar cooling + ultra-fast cooling mode can be adopted to obtain various phase structures at different cooling stages.

[0040] 9) The structure in the steel of the present invention is ferrite, bainite and retained austenite, which can significantly improve the hole-expanding performance of the steel plate during the forming process.

[0041] 10) After coiling, it enters a slow-cooling pit with heating. The purpose is to retain a large number of V precipitation phases inside the structure as hydrogen traps, reduce the risk of delayed cracking in the Sb-containing steel plate of the present invention during use; 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.

[0042] 11) It has excellent mechanical properties, with a yield strength of 345 - 370 MPa, a tensile strength of 445 - 512 MPa, a transverse elongation rate A ≥ 36%, a hole-expanding rate of 105% - 115%, and a qualified transverse cold bend of 180° D = a, having good high-elongation and hole-expanding performance. Specific Embodiments

[0043] In order to make the purpose, technical solution 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.

[0044] The specific components, hot rolling process systems, performances and tissue volume percentages of 6 examples of the present invention are shown in Tables 1 - 4.

[0045] Table 1 Chemical Compositions (wt, %) of Examples of the Present Invention

[0046]

[0047]

[0048] Table 2 Hot Rolling Process Systems of Examples of the Present Invention

[0049]

[0050] Table 3 Mechanical Property Parameters of Examples of the Present Invention

[0051]

[0052] Table 4 Tissue Volume Percentages in Examples of the Present Invention

[0053] Number Ferrite Bainite Retained austenite Example 1 65.0％ 30.0％ 5.0％ Example 2 69.6％ 21.2％ 9.2％ Example 3 78.4％ 15.1％ 6.5％ Example 4 75.0％ 15.5％ 9.5％ Example 5 80.0％ 12.0％ 8.0％ Example 6 60.7％ 29.3％ 10.0％

[0054] 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 still 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-elongation hot-rolled steel plate for bus frame, characterized in that, The chemical components in the steel are by weight percentage: C: 0.025% - 0.042%, Si: 0.10% - 0.21%, Mn: 1.35% - 1.98%, Al: 0.18% - 0.34%, Nb: 0.007% - 0.017%, V: 0.010% - 0.019%, Mo: 0.25% - 0.45%, W: 0.022% - 0.034%, Bi: 0.021% - 0.063%, Sb: 0.050% - 0.102%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al / Si: 0.9% - 3.0%, Mo + Mn: 1.75% - 2.30%, Ca + Mg: 0.1190% - 0.1211%. And it is limited that P ≤ 0.010%, S ≤ 0.005%, N ≤ 0.006%, and the balance is Fe and unavoidable impurities.

2. The high-elongation hot-rolled steel sheet for a bus skeleton according to claim 1, characterized in that The microstructure in the steel is: the volume percentage of ferrite is 60% - 80%, the volume percentage of bainite is 10% - 30%, and the volume percentage of retained austenite is 4% - 10%.

3. A high-elongation hot-rolled steel sheet for a bus skeleton according to claim 1, characterized in that, The yield strength of the steel plate is 345 - 370 MPa, the tensile strength is 445 - 512 MPa, the transverse elongation A ≥ 36%, the hole expansion rate is 105% - 115%, and the transverse cold bending 180° D = a is qualified.

4. A high-elongation hot-rolled steel plate for bus frame according to claim 1, characterized in that, The finished thickness of the steel plate is 2.0 - 7.0 mm.

5. The high-elongation hot-rolled steel sheet for a bus skeleton according to claim 1, characterized in that, The convexity control accuracy of the steel plate is ±40 μm, the flatness is controlled within 15 I, and the thickness control accuracy is ±40 μm.

6. A production method of a high-elongation hot-rolled steel plate for a bus skeleton according to any one of claims 1-5, comprising 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 coiling, the hot rolled plate immediately enters the slow cooling pit with heating, covers the heat preservation cover, the heating temperature of the slow cooling pit is 500 - 600 °C, holds for 10 - 15 min, takes out the steel coil, and air cools to room temperature.

7. The preparation method of a high-elongation hot-rolled steel plate for a bus skeleton according to claim 6, characterized in that, In the rolling process, the rough rolling exit temperature is 1070 - 1105 °C.

8. The preparation method of a high-elongation hot-rolled steel plate for a bus skeleton according to claim 6, characterized in that, In the rolling process, the finish rolling entry temperature is not higher than 1070 °C, and the finish rolling temperature is 860 - 908 °C.

9. The preparation method of a high-elongation 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 finish rolling and after rough rolling is 33 - 45 mm, the width is 1050 - 2010 mm, and the intermediate billet is heat-insulated by a heat preservation cover before entering the hot rolling finish rolling mill.

10. The preparation method of a high-elongation hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the cooling process, after finish rolling, the laminar flow cooling + ultra-fast cooling cooling mode is adopted. The laminar flow cooling rate is 12 - 17 °C / s. After cooling to 750 - 798 °C, ultra-fast cooling is carried out, and the cooling rate ≥ 100 °C / s. After cooling the steel plate to 400 - 550 °C, it is coiled.

Images