556 MPa grade hot-rolled steel plate for passenger car framework and production method of 556 MPa grade hot-rolled steel plate

Abstract

The steel comprises the following chemical components in percentage by weight: 0.062 to 0.092 percent of C, 0.10 to 0.35 percent of Si, 1.53 to 1.87 percent of Mn, 0.18 to 0.34 percent of Al, 0.035 to 0.055 percent of V, 0.062 to 0.076 percent of Ti, 0.25 to 0.45 percent of Mo, 0.021 to 0.035 percent of W, 0.022 to 0.068 percent of Bi, 0.050 to 0.110 percent of Sb, 0.012 to 0.020 percent of Y, 0.0031 to 0.0042 percent of Ca, 0.1153 to 0.1174 percent of Mg, 0.30 to 0.60 percent of Al + Si, 1.2 to 2.0 percent of C / V, 0.1190 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 of S, less than or equal to 0.006 percent of N and the The yield strength of the steel plate ranges from 556 MPa to 618 MPa, the tensile strength ranges from 660 MPa to 735 MPa, the transverse elongation A is larger than or equal to 26%, the hole expansion rate ranges from 70% to 85%, transverse cold bending is conducted by 180 degrees, D = a, and the steel plate is qualified. The steel plate has excellent mechanical performance and high forming performance.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and particularly relates to a 556 MPa grade hot-rolled steel plate for bus frames and a production method thereof. Background Art

[0002] Hot-rolled plates with high formability 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 to measure 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 hot-rolled plates with high formability. 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 plates can be significantly improved. In the future, with the continuous emergence of new materials and new processes, hot-rolled plates with high formability will play an important role in more fields and meet the growing market demand.

[0004] Chinese Patent Application with Publication No. CN 104342598 A discloses a production method of a 600 MPa grade hot-rolled steel strip for automobile beams. The design principle of this invention is to systematically analyze the influence of elements such as C, Mn and micro-alloying elements Nb, Ti in steel on the properties such as strength and toughness of steel theoretically, and at the same time take into account the production cost and process, design the chemical composition of this new type of 600 MPa grade high-strength automobile beam steel, and through the production technology of controlled rolling and controlled cooling, a production method of a 600 MPa grade automobile beam steel produced by Nb-Ti composite micro-alloying process can be obtained. The maximum elongation after fracture of the steel plate produced by this method is 26%, and the hole expansion rate is not evaluated, which does not meet the use requirements of high-formability automobile parts.

[0005] Chinese Patent Application with Publication No. CN 114807780 B discloses a 600 MPa grade steel for hot stamping of automobile axle housings and a production method thereof. This invention provides a production method of a steel plate with a yield strength ≥ 600 MPa, a tensile strength ≥ 780 MPa, and a transverse elongation A50 ≥ 28%, but this invention does not elaborate and evaluate the hole expansion rate, and is limited in the use requirements of high-formability automobile parts. Summary of the Invention

[0006] The object of the present invention is to provide a 556 MPa grade hot-rolled steel plate for bus frame and its production method. The yield strength of the steel plate is 556 - 618 MPa, the tensile strength is 660 - 735 MPa, the transverse elongation rate A≥26%, the hole expansion rate is 70% - 85%, and the transverse cold bending 180° D=a is qualified. The steel plate of the present invention has excellent mechanical properties and high formability.

[0007] To achieve the above object, the present invention is realized by adopting the following technical solutions:

[0008] A 556 MPa grade hot-rolled steel plate for bus frame, the chemical components in the steel are by weight percentage: C: 0.062% - 0.092%, Si: 0.10% - 0.35%, Mn: 1.53% - 1.87%, Al: 0.18% - 0.34%, V: 0.035% - 0.055%, Ti: 0.062% - 0.076%, Mo: 0.25% - 0.45%, W: 0.021% - 0.035%, Bi: 0.022% - 0.068%, Sb: 0.050% - 0.110%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al+Si: 0.30% - 0.60%, C / V: 1.2 - 2.0, 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: the volume percentage of ferrite is 55% - 75%, the volume percentage of martensite is 10% - 30%, and the volume percentage of retained austenite is 5% - 15%.

[0010] The yield strength of the steel plate is 556 - 618 MPa, the tensile strength is 660 - 735 MPa, the transverse elongation rate A≥26%, the hole expansion rate is 70% - 85%, and the transverse cold bending 180° D=a is qualified.

[0011] A production method of a 556 MPa grade hot-rolled steel plate for bus frame, including smelting, heating, rolling and cooling. The specific method is as follows:

[0012] 1) The 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 casting 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 rolled hot-rolled steel plate finished product and improving the hole expansion performance.

[0013] 2) The 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 rolling process of 3 + 3 mode (R1 adopts 3 passes of rolling, R2 adopts 3 passes of rolling), with a total of 6 passes of rolling. The rough rolling exit temperature is 1050 °C - 1105 °C, the thickness of the intermediate billet is 35 - 44 mm, and the width is 1050 - 2010 mm. Before the intermediate billet enters the hot rolling finishing mill, it is insulated with a heat preservation cover to reduce the temperature drop of the intermediate billet 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 1050 °C, and the finishing rolling temperature is 775 - 905 °C. After finishing rolling, a cooling mode of rapid cooling + air cooling + ultra-fast cooling is adopted. The rapid cooling rate is about 50 - 60 °C / s. After cooling to 695 - 755 °C, air cooling is carried out for 6 - 12 s, and then ultra-fast cooling is carried out. The ultra-fast cooling rate ≥ 122 °C / s. The steel plate is cooled to 235 - 285 °C and then coiled. After the hot-rolled steel plate is coiled, it 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 - 468 °C, and it is held for 15 - 20 min. Then the steel coil is taken out and air-cooled to room temperature. The purpose of rapid cooling to 695 - 755 °C + air cooling for 6 - 12 s is to rapidly precipitate ferrite. While inhibiting grain growth, it also ensures the content of ferrite, thereby making the ferrite grains refined. The purpose of ultra-fast 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, thereby 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 - 468 °C, and the purpose of holding for 15 - 20 min is to retain a large amount of V precipitation phase as a hydrogen trap inside the structure, 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, 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 frames with excellent mechanical properties, hole expansion performance, and hydrogen-induced crack resistance. The convexity control accuracy of the steel plate is ±30 μm, the flatness is controlled within 15 I, the thickness control accuracy is ±30 μm, and the finished product thickness is 2.0 - 7.0 mm.

[0015] In the present invention, the main functions of the chemical components of the 556 MPa grade hot-rolled steel plate for bus frames are as follows:

[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 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 become brittle, posing risks of delayed fracture and hot-rolled edge cracking, and also being unfavorable to 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 0.062% - 0.092%.

[0017] Si: Silicon is one of the important elements in 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 also lies in that sufficient content of silicon addition 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 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 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 0.10% - 0.35%.

[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 rate 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 will lead to C or Mn segregation problems, deteriorating the tissue uniformity of the steel plate during hot rolling, making it easy to appear serious banded tissue defects in the tissue, and also being unfavorable to the hole expansion performance. Additionally, 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 as 1.53% - 1.87%.

[0019] P: Phosphorus is an impurity element in steel and is extremely likely 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 expansion performance of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation, its upper limit is set to 0.010%.

[0020] S: Sulfur is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which become the starting points of cracks and deteriorate 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 at 0.005%.

[0021] Al: In traditional processes, Al is a deoxidizer during steelmaking. 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 more Al is to accelerate the transformation kinetics process of austenite to ferrite during the cooling process, and at the same time, together with Si, inhibit the precipitation of cementite, and also increase the austenitizing temperature to facilitate 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 tundish 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 0.30% - 0.60%.

[0022] 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 number of V precipitation phases are retained inside the steel plate structure as hydrogen traps, 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, combined with the slow cooling process design, a large number 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 to greatly improve the hydrogen-induced cracking resistance of the Sb-containing steel plate during service, obtaining a hot-rolled steel plate with excellent mechanical properties, hole expansion performance, and hydrogen-induced cracking 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 worse. Therefore, in the present invention, the optimal range of the V content is between 0.035% and 0.055%, and C / V satisfies 1.2 - 2.0.

[0023] 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 grain refinement strengthening and precipitation strengthening, and can also strengthen ferrite and martensite. Therefore, in the present invention, the optimal range of the Ti content is between 0.062% and 0.076%.

[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 the present invention is selected to be between 0.25% and 0.45%.

[0025] W: Tungsten is a metal with the highest melting point. The formed WC has high hardness. Its role 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 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 the W content in the present invention is between 0.021% and 0.035%.

[0026] 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 0.022% - 0.068%.

[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, 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 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 the present invention is limited to 0.050% - 0.110%.

[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 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%.

[0029] Ca: Calcium can change the morphology of sulfides (MnS) in steel, prevent the formation of long-strip 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 the present 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 hole expansion rate. Therefore, the Mg content in this invention is controlled at 0.1153% - 0.1174%.

[0031] 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. In this invention, VN formed with V needs to precipitate for precipitation strengthening and grain refinement strengthening to improve the strength and hole expansion 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 martensite is obtained in the rapid cooling stage, and is beneficial to obtaining higher strength.

[0034] 2) WC formed by W has high hardness, can enhance the wear resistance, improve the hardenability of the steel, and effectively inhibit grain growth.

[0035] 3) Bi element 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 C and O at grain boundaries, reduce decarburization and oxidation phenomena, and improve 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, 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.

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

[0042] 10) 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 cracks in the Sb-containing steel plate of the present invention during use; significantly improve the hydrogen-induced cracking resistance of the Sb-containing steel plate of the present invention during service, and excellent mechanical properties, hole-expanding performance, and hydrogen-induced cracking resistance are obtained.

[0043] 11) The present invention has excellent mechanical properties, with a yield strength of 556 - 618 MPa, a tensile strength of 660 - 735 MPa, a transverse elongation rate A ≥ 26%, a hole-expanding rate of 70% - 85%, and a qualified transverse cold bend of 180° D = a. Detailed Embodiments

[0044] In order to make the purpose, technical solutions, and advantages of the present invention clearer, the following further explains the detailed embodiments of the present invention in conjunction 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.

[0045] The specific compositions, hot rolling process systems, performances, and volume percentages of the structures of 6 embodiments of the invention are shown in Tables 1 - 4.

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

[0047]

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

[0049]

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

[0051]

[0052]

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

[0054] Number Ferrite Martensite Retained austenite Example 1 75.0％ 10.5％ 14.5％ Example 2 63.8％ 27.0％ 9.2％ Example 3 65.4％ 26.7％ 7.9％ Example 4 72.0％ 23.0％ 5.0％ Example 5 65.5％ 27.5％ 7.0％ Example 6 55.0％ 30.0％ 15.0％

[0055] To describe the present invention, the present invention has been described appropriately and sufficiently through embodiments above. The above embodiments are only used to illustrate the present invention and are not a limitation to 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 replacements, 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 hot-rolled steel plate with a yield strength of 556 MPa for bus frames, characterized in that, The chemical components in the steel are by weight percentage: C: 0.062% - 0.092%, Si: 0.10% - 0.35%, Mn: 1.53% - 1.87%, Al: 0.18% - 0.34%, V: 0.035% - 0.055%, Ti: 0.062% - 0.076%, Mo: 0.25% - 0.45%, W: 0.021% - 0.035%, Bi: 0.022% - 0.068%, Sb: 0.050% - 0.110%, Y: 0.012% - 0.020%, Ca: 0.0031% - 0.0042%, Mg: 0.1153% - 0.1174%, and Al + Si: 0.30% - 0.60%, C / V: 1.2 - 2.0, 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.

2. The 556 MPa grade hot-rolled steel plate for bus frame according to claim 1, wherein, The microstructure in the steel is: ferrite volume percentage 55% - 75%, martensite volume percentage 10% - 30%, and retained austenite volume percentage 5% - 15%.

3. A hot-rolled steel sheet with a yield strength of 556 MPa for bus frames according to claim 1, characterized in that, The yield strength of the steel plate is 556 - 618 MPa, the tensile strength is 660 - 735 MPa, the transverse elongation rate A ≥ 26%, the hole expansion rate is 70% - 85%, and the transverse cold bend 180° D = a is qualified.

4. A 556 MPa grade 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. A hot-rolled steel sheet with a yield strength of 556 MPa for bus frames according to claim 1, characterized in that, The convexity control precision of the steel plate is ±30 μm, the flatness is controlled within 15 I, and the thickness control precision is ±30 μm.

6. A production method of 556 MPa grade hot-rolled steel plate for 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 steel plate is coiled, it immediately enters a slow cooling pit with heating, the heat preservation cover is covered, the heating temperature of the slow cooling pit is 435 - 468 °C, the holding time is 15 - 20 min, the steel coil is taken out, and air-cooled to room temperature.

7. The preparation method of a 556 MPa grade 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 1050 °C - 1105 °C.

8. The preparation method of a 556 MPa grade 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 1050 °C, and the finish rolling temperature is 775 - 905 °C.

9. A preparation method of a 556 MPa grade 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 35 - 44 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 556 MPa grade hot-rolled steel sheet for a bus skeleton according to claim 6, characterized in that, In the cooling process, after finish 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 695 - 755 °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. After the steel plate is cooled to 235 - 285 °C, it is coiled.

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