High formability hot-rolled steel sheet for bus frame and production method

Abstract

The present application relates to a kind of high formability hot-rolled steel plate for bus frame and production method, the chemical composition in steel is as follows by weight percentage: C:0.050%~0.081%, Si:0.10%~0.35%, Mn:1.05%~1.75%, Al:0.18%~0.34%, Nb:0.035%~0.055%, Ti:0.035%~0.055%, Mo:0.10%~0.35%, W:0.023%~0.035%, Bi:0.021%~0.063%, Sb:0.050%~0.070%, Y:0.010%~0.018%, Ca:0.0031%~0.0042%, Mg:0.1153%~0.1174%, and Al+Si:0.30%~0.60%, Mo+Mn:1.20%~2.00%, Ca+Mg:0.1190%~0.1211%, and limit P≤0.010%, S≤0.005%, N≤0.003%, the balance is Fe and inevitable impurities.The yield strength of steel plate is in 405~430MPa, tensile strength is in 515~555MPa, transverse elongation A≥28%, hole expansion ratio is in 90%~100%, and transverse cold bending 180 °D=a is qualified, the steel plate of the present application has excellent mechanical properties and high forming performance.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a high formability hot-rolled steel plate for bus frames and its production method. Background Technology

[0002] High-formability hot-rolled steel sheets are widely used in the automotive, home appliance, and construction industries. Especially in automobile manufacturing, the elongation and expansion rate of steel sheets are important indicators for measuring the formability of materials. The expansion rate reflects the material's resistance to fracture under local deformation conditions, which is particularly important for components requiring high formability (such as automobile chassis and body structural parts).

[0003] Hole expansion ratio is a key indicator for evaluating the local formability of materials, especially in applications requiring high local deformation capacity (such as punching and flanging processes in automotive parts). A high hole expansion ratio means the material is less prone to fracture during local deformation and can withstand greater plastic deformation. Elongation, in the application of high-formability hot-rolled sheets, directly reflects the amount of plastic deformation the material can withstand before fracture. High hole expansion ratio and high elongation are of great significance for applications in the automotive and other fields. The hole expansion ratio of hot-rolled sheets can be significantly improved by optimizing composition design, refining grain size, controlling second-phase particles, and improving production processes. In the future, with the continuous emergence of new materials and processes, high-formability hot-rolled sheets will play an important role in more fields, meeting the growing market demand.

[0004] Chinese patent application CN 107641760 A discloses a hot-rolled automotive structural steel sheet with good fatigue performance at 460MPa and a manufacturing method thereof. The hot-rolled steel sheet is produced by adding a certain amount of Nb and Ti to a standard C-Mn composition system. The trace amounts of Ti are used to fix the S and N in the steel, fully utilizing the grain-refining effect of Nb and Ti. The steel has a yield strength of 460–560MPa, a tensile strength of 500–640MPa, and an elongation A80 ≥ 15%. However, the hole expansion rate was not evaluated, which does not meet the requirements for high-formability automotive parts. Summary of the Invention

[0005] The purpose of this invention is to provide a high-formability hot-rolled steel plate for bus frames and a production method thereof. The steel plate has a yield strength of 405-430 MPa, a tensile strength of 515-555 MPa, a transverse elongation A≥28%, a hole expansion rate of 90%-100%, and a qualified transverse cold bending of 180° D=a. The steel plate of this invention has excellent mechanical properties and high formability.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A high-formability hot-rolled steel plate for a bus frame, wherein the chemical composition of the steel, by weight percentage, is: C: 0.050%–0.081%, Si: 0.10%–0.35%, Mn: 1.05%–1.75%, Al: 0.18%–0.34%, Nb: 0.035%–0.055%, Ti: 0.035%–0.055%, Mo: 0.10%–0.35%, W: 0.023%–0.035%, Bi: 0.021%–0.063%. Sb: 0.050%–0.070%, Y: 0.010%–0.018%, Ca: 0.0031%–0.0042%, Mg: 0.1153%–0.1174%, and Al+Si: 0.30%–0.60%, Mo+Mn: 1.20%–2.00%, Ca+Mg: 0.1190%–0.1211%, with P ≤ 0.010%, S ≤ 0.005%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities.

[0008] The microstructure of the steel is as follows: ferrite volume percentage 45%–60%, bainite volume percentage 30%–45%, and retained austenite volume percentage 10%–15%.

[0009] The steel plate has a yield strength of 405-430 MPa, a tensile strength of 515-555 MPa, a transverse elongation A≥28%, a hole expansion rate of 90%-100%, and is a qualified hot-rolled plate with a transverse cold bending of 180° D=a.

[0010] A method for producing a high-formability hot-rolled steel sheet for a bus frame includes smelting, heating, rolling, and cooling, as detailed below:

[0011] 1) The smelting process described above: The RH+LF process is adopted, and the H and O contents are strictly controlled, with H≤0.0002% and O≤0.0015%. Calcium treatment is carried out in the refining process. Electromagnetic stirring and light reduction technology are applied in the continuous casting process. The billet casting speed is ≤1.0m / min, and the light reduction is 2.0~5.0mm. This reduces the center segregation of the continuous casting billet, which is beneficial to reducing the banded structure and improving the expansion performance of the finished hot-rolled steel plate.

[0012] 2) Heating process: The continuously cast slab, (110-210) mm thick × (1050-2010) mm wide, is directly hot-charged into a walking beam furnace for heating at a temperature of 1115-1210℃ for a holding time of 162-193 min. The chemical composition of this invention contains Sb, which has a low melting point and easily accumulates at grain boundaries, leading to cracks. Therefore, the heating temperature should not be too high. Furthermore, a suitable holding time ensures complete solid solution of the alloying elements in the slab, uniform slab composition, and controls the original austenite grain size.

[0013] 3) Rolling and Cooling Process: The roughing process adopts a 3+3 rolling mode (R1 is rolled in 3 passes, and R2 is rolled in 3 passes), for a total of 6 passes. The exit temperature of the roughing mill is 1060℃~1105℃. The thickness of the intermediate billet is 33~44mm and the width is 1050~2010mm. The intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference in the head, tail and width directions. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing milling. The entry temperature of the finishing mill is not higher than 1060℃. The final rolling temperature is 860~918℃. After the final rolling, a laminar flow cooling + ultra-fast cooling mode is adopted. The laminar flow cooling rate is about 12~17℃ / s. After cooling to 680~730℃, ultra-fast cooling is performed with a cooling rate ≥100℃ / s. The steel plate is cooled to 370~430℃ before being coiled. Immediately after coiling, the coil is placed in a heated slow-cooling pit, covered with an insulating cover, and heated to 470–530°C for 10–15 minutes. The coil is then removed and air-cooled to room temperature. Laminar flow cooling to 680–730°C aims to rapidly precipitate ferrite, inhibiting grain growth while ensuring sufficient ferrite content, thus refining the ferrite grains. Ultra-rapid cooling to 370–430°C aims to rapidly precipitate a large amount of bainite, inhibiting grain growth while ensuring sufficient bainite content, thus refining the bainite grains. Immediately after winding, the steel is placed in a heated slow-cooling pit, covered with an insulating cover. The slow-cooling pit is heated to 470–530°C and held for 10–15 minutes. The purpose is to retain a large amount of Ti precipitates within the microstructure as hydrogen traps, reducing the risk of delayed cracking in the Sb-containing steel plate during use. Through the addition of Ti and Mo composites, combined with the slow-cooling process design, the Sb-containing steel plate retains a large amount of Ti and Mo composite carbides, which act as hydrogen traps, significantly improving its resistance to hydrogen-induced cracking during service. This results in a high-elongation hot-rolled steel plate for bus frames with excellent mechanical properties, hole-expanding properties, and resistance to hydrogen-induced cracking. The steel plate convexity is controlled with an accuracy of ±30μm, flatness within 15I, and thickness with an accuracy of ±30μm, resulting in a finished thickness of 2.0–5.0 mm.

[0014] The main function of the high formability hot-rolled steel plate composition for bus frame in this invention is as follows:

[0015] C: Carbon is a common strengthening element in steel. As an interstitial solid solution atom, carbon dissolved in the matrix increases the strength of the solid solution by causing lattice distortion. In this invention, carbon also ensures the stability of retained austenite, thereby improving the formability and hole-expanding properties of the steel plate. Too low a carbon content will not yield the mechanical properties of the steel plate described in this invention, while too high a content will cause the steel plate to become brittle, posing a risk of delayed fracture and hot-rolling edge cracking, and also negatively impacting the weldability, plasticity, and toughness of the steel plate. In this invention, the overall carbon content is required to be within a low range, which helps reduce the risk of delayed fracture and hot-rolling edge cracking, and is also beneficial to the weldability of the steel plate. Therefore, the optimal range for carbon in this invention is 0.050% to 0.081%.

[0016] Silicon (Si): Silicon is one of the key elements in this invention. Sufficient silicon addition ensures the strength of the ferrite matrix. Furthermore, adequate silicon content reduces inclusions in the steel, inhibits the decomposition of retained austenite and the formation of carbides, and prevents the steel plate's mechanical properties and hole-expanding performance from being reduced due to these factors. However, too low a silicon content fails to guarantee the strength of the ferrite matrix and inhibit the decomposition of retained austenite and the formation of carbides, while too high a content affects the surface quality of hot-rolled steel, resulting in a large amount of iron oxide scale and impaired weldability. Therefore, the silicon content in this invention is 0.10%–0.35%.

[0017] Mn: Manganese strengthens the solid solution in steel by inducing lattice distortion through substitution solid solution. 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 supercooled austenite is unstable, reducing the plasticity, toughness, and hole-expanding performance of the steel plate. Furthermore, the added manganese content should not exceed the scope of this invention, mainly considering the problem of C or Mn segregation caused by excessive manganese content, which deteriorates the uniformity of the steel plate structure during hot rolling and easily leads to severe banded structural defects. In addition, excessive manganese in the steel involved in this invention increases hardenability, inhibits bainite formation, and is also detrimental to hole-expanding performance. Moreover, excessive manganese content will lead to poor weldability of the steel plate. Therefore, considering all factors, this invention selects a manganese content of 1.05% to 1.75%.

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

[0019] S: Sulfur is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which become the starting point of cracks and deteriorate the processing performance. It seriously affects the plasticity, formability and hole expansion performance of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0020] Al: In traditional steelmaking processes, Al is a deoxidizer. It can also combine with nitrogen (N) in steel to form AlN, refining the grain size. However, in this invention, the main purpose of adding a significant amount of Al is to accelerate the austenite-ferrite transformation kinetics during cooling, while simultaneously inhibiting cementite precipitation along with Si, and raising the austenitizing temperature for better selection of the process window. Too little Al content has limited impact on the austenitizing temperature and slows down ferrite precipitation during cooling; while too much Al content will cause nozzle blockage during continuous casting, affecting production efficiency. Therefore, in this invention, the Al content is limited to 0.18%–0.34%, satisfying the Al+Si ratio of 0.30%–0.60%.

[0021] Niobium (Nb) in steel primarily functions as a grain refiner and precipitation strengthener. It also expands the rolling process window, improves the uniformity of coil performance, refines the microstructure, and enhances the cold forming properties of steel sheets. At high temperatures, niobium exists in austenite in a solid solution state, inhibiting austenite grain growth and static and dynamic recrystallization during hot deformation, and increasing the recrystallization termination temperature, thus raising the final rolling temperature. Simultaneously, the precipitation of niobium carbonitrides delays recrystallization, preventing austenite grain growth and exhibiting significant grain refinement and precipitation strengthening effects. This effectively reduces the banded microstructure grade of the steel sheet and improves its expansion properties. Therefore, the optimal range of Nb content in this invention is between 0.035% and 0.055%.

[0022] Ti: Titanium effectively delays the recrystallization of deformed austenite, inhibits austenite grain growth, increases the austenite recrystallization temperature, refines grains, and improves the strength and toughness of steel. Furthermore, Ti is a strong carbide and nitride forming element, capable of combining with carbon and nitrogen to form stable and fine carbides and nitrides, resulting in significant grain refinement and precipitation strengthening. It can also strengthen ferrite and bainite. Moreover, it retains a large amount of Ti precipitates within the steel plate microstructure as hydrogen traps, reducing the risk of delayed cracking in the Sb-containing steel plate during use. Through the composite addition of Ti and Mo, combined with a slow cooling process design, the Sb-containing steel plate of this invention retains a large amount of Ti-Mo composite carbides, which act as hydrogen traps, significantly improving the Sb-containing steel plate's resistance to hydrogen-induced cracking during service. This results in hot-rolled steel plates with excellent mechanical properties, hole-expanding properties, and resistance to hydrogen-induced cracking. Higher Ti content also reduces the toughness of the weld heat-affected zone; therefore, the optimal range of Ti content in this invention is between 0.035% and 0.055%.

[0023] Mo: Molybdenum is a carbide-forming element that can improve the strength and toughness of steel plates. Mo can significantly improve the stability of austenite, increase the hardenability of steel, and is conducive to the formation of bainite. Both Mn and Mo are elements that improve hardenability, ensuring the bainite content obtained in the rapid cooling stage. However, excessively high hardenability restricts the formation of retained austenite, which is not conducive to obtaining high plasticity and pore-expanding properties. Therefore, the Mo content in this invention is selected from 0.10% to 0.35%, and satisfies the condition Mo + Mn: 1.20% to 2.00%.

[0024] W: Tungsten is the metal with the highest melting point. The WC it forms has high hardness. Its effect in steel is similar to that of molybdenum, but it is superior to molybdenum in enhancing wear resistance. 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%, brittleness increases. Therefore, the optimal range of W content in this invention is between 0.023% and 0.035%.

[0025] Bismuth (Bi) is mainly distributed in steel at grain boundaries and within grains, improving the strength of the steel plate, reducing the diffusion rate of elements such as carbon and oxygen at grain boundaries, minimizing decarburization and oxidation, and improving the surface and mechanical properties of the steel plate. Therefore, this invention limits the Bi content to 0.021%–0.063%.

[0026] Sb: Antimony can densify corrosion products and inhibit the production of H2O, O2, Cl, and SO4. 2- It diffuses into the steel matrix and can accumulate 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, resisting further corrosion of the steel matrix. However, Sb is a low-melting-point element, which easily accumulates at grain boundaries, causing grain boundary cracks. As the Sb content increases, the risk of steel plate cracking increases rapidly. Therefore, this invention limits the Sb content to 0.050% to 0.070%.

[0027] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the Y content is limited to 0.010% to 0.018% in this invention.

[0028] Ca: Calcium can alter the morphology of sulfides (MnS) in steel, preventing the formation of elongated MnS inclusions and improving the plasticity, toughness, and porosity of steel plates. It can also improve the quality of cast billets. Therefore, this invention controls the Ca content to 0.0031%–0.0042%, and the Ca+Mg ratio to be 0.1190%–0.1211%.

[0029] Mg: Magnesium is a good deoxidizer, desulfurizer, and spheroidizing agent in steel. Magnesium can reduce the number of inclusions in steel, make them smaller in size, and improve their uniform distribution and morphology. Trace amounts of magnesium can improve the size and distribution of carbides in steel, promote fine and uniform carbide particles, and improve the porosity. Therefore, the Mg content in this invention is controlled at 0.1153% to 0.1174%.

[0030] N: For the N content in steel, the lower the N content, the better, but too low a content will lead to production difficulties and increased costs. Therefore, the N content in this invention is ≤0.003%.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1) The addition of Mo can improve the strength and toughness of steel plates. Mo can significantly improve the stability of austenite, increase the hardenability of steel, and facilitate the formation of bainite structure. It can ensure that bainite is obtained in the rapid cooling stage, which is conducive to obtaining higher plasticity and hole expansion performance.

[0033] 2) The WC formed by W has high hardness, which enhances wear resistance, improves the hardenability of steel, and effectively inhibits grain growth.

[0034] 3) Bismuth is mainly distributed in the grain boundaries and inside the grains of steel. It can improve the strength of steel plates, reduce the diffusion rate of elements such as carbon and oxygen at the grain boundaries, reduce decarburization and oxidation, and improve the surface and mechanical properties of steel plates.

[0035] 4) The addition of Sb can densify the corrosion products and inhibit the production of H2O, O2, Cl and SO4. 2- Isotropic diffusion into the steel substrate allows it to accumulate near the steel substrate 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 substrate, resisting further corrosion of the steel substrate.

[0036] 5) The addition of Y can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures.

[0037] 6) The addition of Ca can change the morphology of sulfides in steel, and improve the plasticity, toughness and hole expansion performance of steel plates.

[0038] 7) The addition of Mg can reduce the number of inclusions in steel, make them smaller in size, and improve their distribution and morphology. Trace amounts of magnesium can improve the size and distribution of carbides in steel, promote fine and uniform carbide particles, and improve the hole-expanding performance.

[0039] 8) The post-rolling cooling mode of laminar flow cooling + ultra-fast cooling can obtain the microstructure of each phase under different cooling stages.

[0040] 9) The microstructure of the steel of this invention consists of ferrite, bainite and retained austenite, which significantly improves the hole expansion performance of the steel plate during the forming process.

[0041] 10) After winding, the steel plate is immediately placed in a heated slow cooling pit. The purpose is to retain a large amount of Ti precipitate phase inside the structure as a hydrogen trap, thereby reducing the risk of delayed cracking in the Sb-containing steel plate of the present invention during use. It also significantly improves the resistance to hydrogen-induced cracking in the service process of the Sb-containing steel plate of the present invention, and obtains excellent mechanical properties, hole expansion properties and resistance to hydrogen-induced cracking.

[0042] 11) The present invention has excellent mechanical properties, with a yield strength of 405-430 MPa, a tensile strength of 515-555 MPa, a transverse elongation A≥28%, a hole expansion rate of 90%-100%, and a qualified transverse cold bending of 180° D=a. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0044] The main process of this invention is as follows:

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

[0046] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0047]

[0048]

[0049] Table 2 Hot rolling process of the present invention embodiment

[0050]

[0051] Table 3 Mechanical performance parameters of embodiments of the present invention

[0052]

[0053] Table 4. Percentage of tissue volume in embodiments of the present invention

[0054]

[0055]

[0056] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can 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 should be defined by the claims.

Claims

1. A high-formability hot-rolled steel plate for a bus frame, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.050%–0.081%, Si: 0.10%–0.35%, Mn: 1.05%–1.75%, Al: 0.18%–0.34%, Nb: 0.035%–0.055%, Ti: 0.035%–0.055%, Mo: 0.10%–0.35%, W: 0.023%–0.035%, Bi: 0.021%–0.063%, Sb: 0.050%. %~0.070%, Y: 0.010%~0.018%, Ca: 0.0031%~0.0042%, Mg: 0.1153%~0.1174%, and Al+Si: 0.30%~0.60%, Mo+Mn: 1.20%~2.00%, Ca+Mg: 0.1190%~0.1211%, with P≤0.010%, S≤0.005%, N≤0.003%, and the balance being Fe and unavoidable impurities.

2. The high formability hot-rolled steel plate for a passenger car frame according to claim 1, characterized in that, The microstructure of the steel is as follows: ferrite volume percentage 45%–60%, bainite volume percentage 30%–45%, and retained austenite volume percentage 10%–15%.

3. The high formability hot-rolled steel plate for a passenger car frame according to claim 1, characterized in that, The steel plate has a yield strength of 405-430MPa, a tensile strength of 515-555MPa, a transverse elongation A≥28%, a hole expansion rate of 90%-100%, and is a qualified hot-rolled plate with a transverse cold bending of 180° D=a.

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

5. A method for producing a high-formability hot-rolled steel plate for a bus frame as described in any one of claims 1-4, comprising smelting, heating, rolling, and cooling, characterized in that, In the aforementioned smelting process, the billet casting speed is ≤1.0m / min, and the light pressing reduction is 2.0~5.0mm; The heating process involves a heating temperature of 1115–1210℃ and a holding time of 162–193 min. In the cooling process, the hot-rolled steel plate is immediately placed into a heated slow cooling pit after being coiled, covered with an insulation cover, and heated to 470-530°C for 10-15 minutes. The steel plate is then removed and air-cooled to room temperature. In the cooling process, after final rolling, a laminar flow cooling + ultra-fast cooling mode is adopted. The laminar flow cooling rate is 12-17℃ / s. After cooling to 680-730℃, ultra-fast cooling is carried out with a cooling rate ≥100℃ / s. The steel plate is cooled to 370-430℃ and then coiled.

6. The method for preparing a high-formability hot-rolled steel plate for a bus frame according to claim 5, characterized in that, In the rolling process, the roughing mill exit temperature is 1060℃~1105℃.

7. The method for preparing a high-formability hot-rolled steel plate for a bus frame according to claim 5, characterized in that, In the rolling process, the entry temperature of the finishing mill is not higher than 1060℃, and the final rolling temperature is 860~918℃.

8. A method for preparing a high-formability hot-rolled steel plate for a bus frame according to claim 6 or 7, characterized in that, In the rolling process, the thickness of the intermediate billet before finishing rolling and after rough rolling is 33-44 mm, and the width is 1050-2010 mm. The intermediate billet is insulated with a heat insulation cover before entering the hot rolling finishing mill.

Images