780 MPa grade wheel steel and manufacturing method thereof

Abstract

The invention relates to 780 MPa grade wheel steel and a manufacturing method thereof. The technological process comprises the steps of KR molten iron desulphurization, converter smelting, argon blowing, LF refining, RH vacuum treatment, continuous casting blank forming, plate blank heating, rolling, cooling and coiling. According to the method, the economical titanium alloy is adopted, precipitation strengthening of Ti is not used for improving the strength of base metal, and solution strengthening and structure strengthening are used for enabling the strength performance of the material to meet the requirement; meanwhile, the process that the front section is rapidly cooled to the medium-low temperature to be coiled is adopted, precipitation of Ti is inhibited, precipitation of TiC is inhibited, growth of columnar crystals is inhibited, the Ti element in the steel in a solid solution mode can be reserved, most of Ti in the material is inhibited by the high cooling speed and the low cooling stopping temperature, and the Ti is still in a solid solution mode and not precipitated in base metal; the Ti element is re-precipitated in the welding stage to the user side, and the strength and hardness of the welding heat affected zone and the welding seam are improved by re-precipitation strengthening of the Ti element in the process, so that the softening phenomenon of the welding heat affected zone and the welding seam can be avoided.

Description

Technical Field

[0001] The invention relates to the technical field of steel smelting, and in particular to a 780MPa grade wheel steel and a manufacturing method thereof. Background Art

[0002] The lightweighting of commercial vehicles was mainly focused on beams and upper parts in the early stage. In the past two years, the lightweighting of wheels has gradually accelerated, and high-strength steel has also begun to be widely used. The processing method of commercial vehicle rims is basically the same as that of passenger vehicles. The main requirements for the forming performance of the material after welding are high. In order to achieve the ultra-high strength requirement of 780MPa, it is necessary to add a strengthening mechanism on the basis of the pure Nb microalloying component system in the early stage. At the same time, it is also necessary to ensure that the material has good welding performance at the user end, the hardness of the heat-affected zone and weld of the welding specimen cannot show obvious softening, and the material has good flash butt welding performance.

[0003] At present, the production of high-strength wheel steel in the same industry mainly has the following problems: 1. When the thickness of the steel is relatively thin, it cannot achieve the high yield strength and tensile strength described in the present invention; 2. Even if the parent material reaches the high yield strength and tensile strength described in the present invention, during welding at the user end, the hardness of the welding heat affected zone and the weld will be significantly softened, resulting in significantly insufficient strength of the welding heat affected zone and the weld, even 100MPa lower than the strength of the parent material, ultimately resulting in the scrapping of the entire wheel steel; some are too costly, and expensive special alloys are added, which is costly.

[0004] The reason for the above problems is that other peers add expensive alloys to improve the strength of steel, or use Ti precipitation strengthening. However, premature Ti precipitation strengthening only guarantees the strength of the parent material. The strength of the heat affected zone and the weld cannot be guaranteed during subsequent welding by users, such as: patents CN200710158965.3, CN201010183475.0, CN201010622948.2, CN201110046426.7, CN201110263972.6, CN201310310003.0, etc.

[0005] How to ensure very high yield strength and tensile strength of wheel steel in a low-cost mode without adding expensive alloys while keeping the steel thin, and most importantly, ensuring that the strength and hardness of the heat affected zone and weld are close to those of the parent material when welding at the user end, so as to meet the user's requirements, has become a technical challenge in the industry.

[0006] The present invention is to solve this technical problem. Summary of the invention

[0007] In order to solve the above problems, the present invention provides a 780MPa grade wheel steel and a manufacturing method thereof, which can not only improve the strength of the base material without adding expensive alloy materials, but also avoid softening of the welding heat affected zone and the weld.

[0008] The technical solution adopted by the present invention is: a 780MPa grade wheel steel, characterized in that: its chemical composition and weight percentage content are: C: 0.055-0.075%, Si: 0.05-0.12%, Mn: 1.7-1.8%, P: ≤0.015%, S: ≤0.002%, Alt: 0.020-0.050%, Cr: 0.18-0.30%, Nb: 0.035-0.050%, Ti: 0.11-0.14%, N≤0.0050%, O≤0.0025%, H≤0.00025%, and the balance is Fe and inevitable inclusions.

[0009] Preferably, the yield strength is 650-800 MPa, the tensile strength is 750-900 MPa, the strength of the welding heat affected zone is equivalent to the strength of the base material, the elongation A is ≥15%, the hole expansion rate is ≥45%, and the thickness is 4-4.5 mm.

[0010] Preferably, Cr: 0.20-0.25%, Nb: 0.038-0.045%, and Ti: 0.115-0.13%.

[0011] A method for manufacturing the 780MPa grade wheel steel as described above, characterized in that the process flow is: KR hot metal desulfurization → converter smelting → argon blowing → LF refining → RH vacuum treatment → continuous casting → slab heating → rolling → cooling → coiling; specifically comprising the following steps:

[0012] S1, after molten iron pretreatment, converter, argon station, ladle furnace and vacuum furnace treatment, continuous casting into billets;

[0013] S11. The molten iron is desulfurized by KR. During this period, the slag is removed twice or more, i.e., the slag before and after removal, and the residence time between the two removals is required to be no less than 7 minutes; the exposed surface is not less than 93%; S in the molten iron after desulfurization is ≤ 0.001%;

[0014] S12, after smelting in a converter, the converter terminal temperature is controlled to be 1596°C to 1622°C, and the terminal oxygen is 0.040 to 0.078%;

[0015] S13, aluminum iron is added to the converter to completely deoxidize the steel, and Als in the steel is controlled to 0.020% to 0.050%;

[0016] S14, argon is blown from the bottom of the molten steel through the argon blowing station, and the Als in the molten steel is controlled to be 0.035% to 0.065% at the end of the argon blowing station;

[0017] S15, the molten steel is treated by deep desulfurization in the ladle furnace, and the S content in the molten steel is controlled to be ≤ 0.002%, and the Als content in the molten steel is controlled to be 0.025% to 0.050%;

[0018] S16, the molten steel is degassed and deslaged in a vacuum furnace, and the alloying is fine-tuned on the composition and temperature. The vacuum cycle time is 30 to 40 minutes, the vacuum degree is controlled at ≤10Pa, and the vacuum end temperature is controlled at 1560 to 1570℃;

[0019] S17, mold protection slag during continuous casting, basicity is 1.17-1.27; viscosity is 0.05-0.15 Pa·S at 1300°C; protective slag for medium carbon steel with a melting point of 1040-1100°C; mold taper is set to 1.18-1.23%, cooling water flow rate of the wide surface of the mold is 3300-3450 L / min, and cooling water flow rate of the narrow surface of the mold is 620-640 L / min;

[0020] S2. Slab heating: the slab heating temperature is 1260~1300℃;

[0021] S3, rolling: the rough rolling temperature is 1080-1120°C, and the finishing temperature of the finishing rolling is 870-910°C;

[0022] S4, cooling: cooling rate is 40-55℃ / s, and final cooling temperature is designed to be 460-500℃;

[0023] S5. Coiling: The coiling temperature is 400-460°C.

[0024] Preferably, in step S15, the content ratio range of the ladle furnace refined slag is controlled as follows: CaO:SiO2:Al2O3=60-65%:4-6%:30-35%.

[0025] Preferably, in step S17, the superheat of the continuous casting process is controlled at 10-20°C, and the tundish temperature is controlled at 1530-1540°C.

[0026] Preferably, in step S17, the billet drawing speed during the continuous casting process is controlled at 0.9-1.1 m / min; and the fluctuation range of the steel liquid level in the crystallizer is controlled within ±3 mm during the continuous casting process.

[0027] Preferably, in step S3, water is sprayed on the surface for cooling during the rough rolling process to adjust the cooling gradient from the surface to the core of the ingot and reduce the easily segregated structure and coarse structure in the core; and the finishing rolling is carried out in the austenite non-recrystallization zone, with a reduction rate of more than 12% for each pass and a cumulative reduction rate of 70% to 73% for finishing rolling.

[0028] Preferably, in step S3, one pass of rough rolling is performed with large deformation, and the reduction ratio of each pass is 16-20%. The second stage of rough rolling adopts seven passes of large deformation rolling, and the reduction ratio of each pass is greater than 13%, and the cumulative reduction ratio of rough rolling is 65%-68%.

[0029] The beneficial effects achieved by the present invention are as follows: the present invention does not add expensive alloys such as Cu, Ni, Mo, V, etc., but uses economical titanium alloys. However, the addition of Ti elements does not utilize precipitation strengthening of Ti to improve the strength of the base material like other counterparts, but utilizes solid solution strengthening and organization strengthening to make the material strength performance meet the requirements; at the same time, the most critical thing is to use the front-end rapid cooling to medium and low temperature coiling process to inhibit the precipitation of Ti, while inhibiting the precipitation of TiC and the growth of columnar crystals, so that the Ti element dissolved in the steel can be retained, and most of the Ti in the material is suppressed by the higher cooling rate and the lower cooling stop temperature, and is still dissolved in the base material without precipitation; in the user-end welding stage, the Ti element is re-precipitated, and this process utilizes the re-precipitation strengthening of the Ti element to improve the strength and hardness of the welding heat affected zone and the weld, and can avoid the softening phenomenon of the welding heat affected zone and the weld. DETAILED DESCRIPTION

[0030] The following specific embodiments are used to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The invention discloses a 780MPa grade wheel steel, whose chemical composition and weight percentage content are as follows: C: 0.05-0.075%, Si: 0.05-0.12%, Mn: 1.7-1.8%, P: ≤0.015%, S: ≤0.002%, Alt: 0.020-0.050%, Cr: 0.18-0.30%, Nb: 0.035-0.050%, Ti: 0.11-0.14%, N≤0.0050%, O≤0.0025%, H≤0.00025%, and the balance is Fe and inevitable inclusions. The yield strength is 650-800MPa, the tensile strength is 750-900MPa, the strength of the welding heat affected zone is equivalent to the strength of the base material, the elongation A is ≥15%, the hole expansion rate is ≥45%, and the thickness is 4-4.5mm.

[0032] In this embodiment, Cr: 0.20-0.25%, Nb: 0.038-0.045%, Ti: 0.115-0.13%.

[0033] The mechanism of action of various elements in the present invention is as follows:

[0034] Carbon (C): A conventional strengthening element that can increase yield strength and tensile strength and improve the hardness of steel, but too much carbon will reduce plasticity and impact toughness. The most suitable carbon addition amount is 0.055-0.075%.

[0035] Manganese (Mn): Mn can be used as a good deoxidizer and desulfurizer in the steelmaking process, which can reduce harmful inclusions in steel and improve the purity of molten steel. Mn can improve the strength of steel through solid solution strengthening and can refine grains. Mn can improve the weldability of steel. However, too high Mn will reduce the plasticity and toughness of steel. The most suitable manganese addition is 1.7-1.8%.

[0036] Silicon (Si): Silicon has a strong chemical reaction with oxygen in steel, which can remove oxygen from molten steel and improve its purity. It can make the grains in steel fine and evenly distributed, and can improve the strength of steel. However, the silicon content should not be too high, which will reduce plasticity and welding performance. The most suitable silicon addition amount is 0.05-0.12%.

[0037] Chromium (Cr): Chromium easily forms a continuous solid solution and reduces the austenite phase area. Chromium forms a variety of carbides with carbon, which are not easy to decarburize during heat treatment and can improve the strength of steel in the rolled state. However, when the chromium content in steel is too high, the toughness of the steel plate will deteriorate and the weldability of the steel will be reduced. The most suitable Cr addition amount is 0.18-0.30%.

[0038] Niobium (Nb): It can optimize the grain boundary structure, promote precipitation hardening of steel, adjust the morphology of non-metallic inclusions in steel and improve the boundary morphology, make the grain boundary thinner and reduce its number, improve the strength and hardness of steel, and improve the machinability and plasticity of steel. However, too much niobium will reduce the toughness of steel and lead to uneven carburized layer. The most suitable niobium addition amount is 0.035-0.050%, preferably 0.038-0.045%.

[0039] Titanium (Ti): Titanium is the most important element for solid solution strengthening, with good strengthening effect and low price. Ti reduces the austenite phase area. Titanium has a strong affinity with nitrogen, oxygen and carbon. It is a good deoxidizing and degassing agent and an effective element for fixing nitrogen and carbon. TiC formed by titanium and carbon has high stability. TiC particles can prevent the growth and coarsening of steel grains and have the effect of refining grains. In the present invention, the Ti element is first retained in the steel as a solid solution to play a role in solid solution strengthening. At the user-end welding stage, the Ti element re-precipitates. This process utilizes the re-precipitation and strengthening of the Ti element to improve the strength and hardness of the welding heat affected zone and the weld, and can avoid the softening of the welding heat affected zone and the weld. However, when the titanium content is too high, the toughness of the steel will be reduced. The more suitable titanium addition amount is 0.11-0.14%, preferably 0.115-0.130%.

[0040] Aluminum (Al): Aluminum is the main deoxidizing element in steel, which can significantly reduce the oxygen content in steel. At the same time, the combination of aluminum and nitrogen forms AlN, which can effectively refine the grains. However, when the aluminum content in steel exceeds 0.05%, it is easy to cause a significant increase in aluminum oxide inclusions, reduce the cleanliness of the steel, and be detrimental to the toughness of the steel. The more suitable aluminum addition amount is 0.020-0.050%.

[0041] Phosphorus and sulfur (P, S): Phosphorus can easily cause cold brittleness of steel; sulfur can easily cause hot brittleness; therefore, the content of phosphorus and sulfur in steel should be reduced as much as possible.

[0042] Nitrogen (N): Nitrogen, oxygen and hydrogen are important indicators for measuring the purity of molten steel. Nitrogen, oxygen and hydrogen are easy to form inclusions, pores, etc. To improve the purity of molten steel, the content of nitrogen, oxygen and hydrogen in the steel must be reduced as much as possible.

[0043] The manufacturing method of the 780MPa grade wheel steel of the present invention has the following process flow: KR molten iron desulfurization→converter smelting→argon blowing→LF refining→RH vacuum treatment (SiCa treatment)→continuous casting into billets→slab heating→rolling→cooling→coiling.

[0044] The production method of steel plate comprises the following steps: continuous casting into billets after molten iron pretreatment, converter, argon station, ladle furnace and vacuum furnace treatment;

[0045] The molten iron is desulfurized by KR, during which: more than two slag removal treatments are carried out, namely, front slag removal and back slag removal, and the residence time between each two back slag removals is required to be no less than 7 minutes; the exposed surface is not less than 93%; S in the molten iron after desulfurization is ≤ 0.001%; ​​desulfurization and clean slag removal of molten iron is to reduce the sulfur content of the molten iron entering the converter and prevent the subsequent slag from returning to sulfur due to incomplete slag removal.

[0046] After converter smelting, the converter terminal temperature is controlled at 1596℃~1622℃, and the terminal oxygen is 0.040~0.078%;

[0047] The addition of aluminum and iron during the converter deoxidizes the steel completely, and the Als content in the steel is controlled at 0.020% to 0.050%. The deoxidation of the steel during the converter and the control of Als are to produce reducing slag in advance for the subsequent desulfurization, and the subsequent desulfurization must require the reducing properties of the slag.

[0048] The molten steel is blown with argon at the bottom of the argon blowing station, and the Als in the molten steel is controlled to be 0.035%~0.065% at the end of the argon blowing station. The argon station controls Als in order to make reducing slag in advance for subsequent desulfurization, and the subsequent desulfurization must require the reducing properties of the slag.

[0049] The molten steel is treated by deep desulfurization in the ladle furnace: S in the molten steel is controlled to be ≤ 0.002%; Als in the molten steel is controlled to be 0.025%-0.050%;

[0050] Controlling the content ratio range of the ladle furnace refining slag: CaO: SiO2: Al2O3 = 60-65%: 4-6%: 30-35%, is the key parameter. Because the LF furnace performs deep desulfurization on low-sulfur steel, in addition to requiring a higher slag basicity, the slag basicity R = CaO / SiO2, the Al2O3 content in the slag should be maintained at 30-35%, reducing the oxygen potential of the molten steel and slag, so the Als in the molten steel is controlled at 0.025% to 0.050%, so as to maintain a very low oxygen potential between steel and slag. The lower the oxygen potential, the higher the desulfurization efficiency and the better the desulfurization effect.

[0051] The molten steel is degassed and deslaged by vacuum furnace circulation, and the alloying is fine-tuned on the composition and temperature. The vacuum cycle time is 30 to 40 minutes, the vacuum degree is controlled at ≤10Pa, and the vacuum end temperature is controlled at 1560 to 1570°C. The vacuum cycle process strictly controls the cycle time and vacuum degree parameters, which can further reduce the nitrogen content in the steel, further remove harmful impurities and gases, and improve the purity of the molten steel. In the case of thin steel, in order to ensure the high strength and high hole expansion rate of the wheel steel of the present invention, the purity of the molten steel must be improved.

[0052] The parameters of the mold protection slag are a difficult point. During the continuous casting process, the mold protection slag uses high alkalinity, low viscosity, and low melting point protection slag. The alkalinity is 1.17-1.27, the viscosity is 0.05-0.15 Pa·S (at 1300°C), and the melting point is 1040-1100°C for medium carbon steel protection slag. The crystallizer taper is set to 1.18-1.23%, the cooling water flow rate on the wide side of the crystallizer is 3300-3450L / min, and the cooling water flow rate on the narrow side of the crystallizer is 620-640L / min.

[0053] In order to avoid defects such as cracks and segregation in the casting process, the continuous casting process controls the superheat at a low level, the superheat is controlled at 10-20°C, and the tundish temperature is controlled at 1530-1540°C. Because the lower the superheat, the better the central segregation can be controlled during the casting process, but the lower the superheat, the closer the tundish temperature is to the liquidus temperature of the molten steel, resulting in the interruption of the solidification of the molten steel during the casting process.

[0054] The casting speed is controlled at 0.9-1.1 m / min during continuous casting. The fluctuation range of the steel liquid level in the crystallizer is controlled within ±3 mm during continuous casting.

[0055] The various parameters of the continuous casting steady-state casting of the present invention are technical difficulties. The control of the basicity, viscosity and melting point of the crystallizer protective slag, and the matching of low superheat, low pulling speed and crystallizer cooling rate are the key. The use of the above-mentioned control method which is different from traditional casting can overcome this technical difficulty and avoid the above-mentioned defects in the cast billet.

[0056] Rolling process:

[0057] 1) Slab heating: The slab heating temperature is 1260~1300℃;

[0058] 2) Rolling: The rough rolling temperature is 1080-1120°C, the rough rolling is performed in one pass with large deformation, and the pass reduction rate is 16-20%. The second stage of rough rolling adopts 7 passes with large deformation, and the reduction rate of each pass is greater than 13%, and the cumulative reduction rate of rough rolling is 65%-68%; during the rough rolling process, the surface is sprayed with water for cooling, and the cooling gradient from the surface to the core of the ingot is adjusted to reduce the easily segregated structure and coarse structure in the core; the finishing rolling is performed in the austenite non-recrystallization zone, and the reduction rate of each pass is greater than 12%, the cumulative reduction rate of finishing rolling is 70%-73%, and the finishing temperature is 870-910°C.

[0059] 3) Cooling; adopt a higher cooling rate method: the cooling rate is 40-55℃ / s, and the final cooling temperature is designed to be 460-500℃. On the one hand, in order to inhibit the precipitation of Ti, the precipitation of TiC, the growth of columnar grains, and retain the Ti element dissolved in the steel, on the other hand, the final cooling temperature is reduced, ultra-fast cooling is used to increase the cooling rate, increase the fluctuation of chemical potential, and thus increase the driving force of phase transformation, which is conducive to enhancing the phase transformation strengthening effect, avoiding excessive coarsening of substructures, and making the grains and substructures finer and more uniform, which is conducive to organizational strengthening.

[0060] 4) Coiling: The coiling temperature is 400-460°C. The present invention adopts the method of lowering the coiling temperature to further inhibit the precipitation of Ti, while inhibiting the precipitation of TiC, inhibiting the growth of columnar crystals, retaining the Ti element dissolved in the steel, and re-precipitating the Ti element in the user-end welding stage. This process utilizes the re-precipitation strengthening of the Ti element to improve the strength and hardness of the welding heat affected zone and the weld, and can avoid the softening of the welding heat affected zone and the weld. Other peers generally have a higher coiling temperature for this type of steel, generally 600-660°C, and promote the precipitation of Ti through high-temperature coiling, and ensure the strength of the base material through the precipitation strengthening of Ti, but the strength of the welding heat affected zone and the weld does not meet the requirements.

[0061] The composition, process and performance of the embodiments of the present invention are shown in Tables 1 to 6 respectively. It can be seen from Tables 1 to 6 that the 780MPa grade high-strength and high-hole expansion rate wheel steel and the manufacturing method thereof of the present invention meet the performance requirements, specifically the yield strength of 650-800MPa, the tensile strength of 750-900MPa, the strength of the welding heat affected zone is equivalent to the strength of the parent material, the elongation A ≥15%, the hole expansion rate ≥45%, and the thickness 4-4.5mm.

[0062] element 1 2 3 4 5 6 7 8 9 10 Comparative Example 1 Comparative Example 2 C 0.055 0.057 0.06 0.062 0.065 0.067 0.068 0.07 0.072 0.075 0.045 0.085 Si 0.05 0.06 0.07 0.08 0.09 0.1 0.085 0.11 0.095 0.12 0.03 0.15 Mn 1.7 1.71 1.72 1.73 1.74 1.75 1.76 1.77 1.79 1.8 1.5 2.5 P 0.012 0.011 0.013 0.015 0.01 0.009 0.014 0.015 0.011 0.01 0.02 0.028 S 0.002 0.0015 0.0018 0.0017 0.002 0.0016 0.0013 0.0011 0.0014 0.0013 0.005 0.007 Alt 0.025 0.024 0.022 0.032 0.046 0.035 0.041 0.046 0.033 0.05 0.008 0.011 Cr 0.18 0.19 0.2 0.21 0.22 0.25 0.24 0.25 0.27 0.3 0.1 0.4 Nb 0.035 0.036 0.038 0.04 0.043 0.045 0.048 0.042 0.046 0.05 0.01 0.07 Ti 0.11 0.115 0.12 0.118 0.122 0.125 0.13 0.132 0.135 0.14 0.05 0.25 N 0.004 0.0045 0.0042 0.0048 0.0035 0.0032 0.0037 0.003 0.0039 0.0028 0.007 0.0065 O 0.0023 0.0025 0.002 0.0024 0.0019 0.0018 0.0022 0.0021 0.0018 0.0016 0.005 0.0046 H 0.00018 0.00023 0.00021 0.00018 0.00015 0.00012 0.00015 0.00011 0.00013 0.00009 0.00037 0.00048

[0063] Table 1 List of composition values ​​of various embodiments and comparative examples of the present invention (wt, %)

[0064]

[0065] Table 2 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0066]

[0067] Table 3 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0068]

[0069] Table 4 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0070]

[0071] Table 5 List of main rolling process parameters of various embodiments of the present invention and comparative examples

[0072]

[0073] Table 6 Main performance test statistics of various embodiments of the present invention and comparative examples

[0074] It should be noted that the description of the above technical solutions is exemplary, and this specification can be embodied in different forms and should not be interpreted as being limited to the technical solutions set forth herein. On the contrary, providing these descriptions will make the disclosure of the present invention thorough and complete, and will fully convey the scope disclosed in this specification to those skilled in the art. In addition, the technical solutions of the present invention are limited only by the scope of the claims.

[0075] Finally, it should be pointed out that the above embodiments are only representative examples of the present invention. Obviously, the present invention is not limited to the above embodiments, and there are many variations. Any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention should be considered to belong to the protection scope of the present invention.

Claims

1. A 780MPa grade wheel steel, characterized in that: Its chemical composition and weight percentage content are: C: 0.055-0.075%, Si: 0.05-0.12%, Mn: 1.7-1.8%, P: ≤0.015%, S: ≤0.002%, Alt: 0.020-0.050%, Cr: 0.18-0.30%, Nb: 0.035-0.050%, Ti: 0.11-0.14%, N≤0.0050%, O≤0.0025%, H≤0.00025%, and the balance is Fe and unavoidable inclusions.

2. The 780MPa grade wheel steel according to claim 1, characterized in that: The yield strength is 650-800MPa, the tensile strength is 750-900MPa, the strength of the welding heat affected zone is equivalent to that of the parent material, the elongation A≥15%, the hole expansion rate≥45%, and the thickness is 4-4.5mm.

3. The 780MPa grade wheel steel according to claim 1, characterized in that: Cr: 0.20~0.25%, Nb: 0.038~0.045%, Ti: 0.115~0.13%.

4. A method for manufacturing a 780 MPa grade wheel steel according to any one of claims 1 to 3, characterized in that: The process flow is: KR hot metal desulfurization → converter smelting → argon blowing → LF refining → RH vacuum treatment → continuous casting → slab heating → rolling → cooling → coiling; specifically includes the following steps: S1, after molten iron pretreatment, converter, argon station, ladle furnace and vacuum furnace treatment, continuous casting into billets; S11. The molten iron is desulfurized by KR. During this period, the slag is removed twice or more, i.e., the slag before and after removal, and the residence time between the two removals is required to be no less than 7 minutes; the exposed surface is not less than 93%; S in the molten iron after desulfurization is ≤ 0.001%; S12, after smelting in a converter, the converter terminal temperature is controlled to be 1596°C to 1622°C, and the terminal oxygen is 0.040 to 0.078%; S13, aluminum iron is added to the converter to completely deoxidize the steel, and Als in the steel is controlled to 0.020% to 0.050%; S14, argon is blown from the bottom of the molten steel through the argon blowing station, and the Als in the molten steel is controlled to be 0.035% to 0.065% at the end of the argon blowing station; S15, the molten steel is treated by deep desulfurization in the ladle furnace, and the S content in the molten steel is controlled to be ≤ 0.002%, and the Als content in the molten steel is controlled to be 0.025% to 0.050%; S16, the molten steel is degassed and deslaged in a vacuum furnace, and the alloying is fine-tuned on the composition and temperature. The vacuum cycle time is 30 to 40 minutes, the vacuum degree is controlled at ≤10Pa, and the vacuum end temperature is controlled at 1560 to 1570℃; S17, mold protection slag during continuous casting, basicity is 1.17-1.27; viscosity is 0.05-0.15 Pa·S at 1300°C; protective slag for medium carbon steel with a melting point of 1040-1100°C; mold taper is set to 1.18-1.23%, cooling water flow rate of the wide surface of the mold is 3300-3450 L / min, and cooling water flow rate of the narrow surface of the mold is 620-640 L / min; S2. Slab heating: the slab heating temperature is 1260~1300℃; S3, rolling: the rough rolling temperature is 1080-1120°C, and the finishing temperature of the finishing rolling is 870-910°C; S4, cooling: cooling rate is 40-55℃ / s, and final cooling temperature is designed to be 460-500℃; S5. Coiling: The coiling temperature is 400-460°C.

5. The 780 MPa grade wheel steel and the method for manufacturing the same according to claim 1, characterized in that: In step S15, the content ratio range of the ladle furnace refined slag is controlled as follows: CaO: SiO2: Al2O3 = 60-65%: 4-6%: 30-35%.

6. The 780 MPa grade wheel steel and the method for manufacturing the same according to claim 1, characterized in that: In step S17, the superheat degree of the continuous casting process is controlled at 10-20°C, and the tundish temperature is controlled at 1530-1540°C.

7. The 780 MPa grade wheel steel and the method for manufacturing the same according to claim 1, characterized in that: In step S17, the billet drawing speed during the continuous casting process is controlled at 0.9-1.1 m / min; and the fluctuation range of the steel liquid level in the crystallizer is controlled within ±3 mm during the continuous casting process.

8. The 780 MPa grade wheel steel and the method for manufacturing the same according to claim 4, characterized in that: In step S3, during the rough rolling process, the surface is cooled by spraying water to adjust the cooling gradient from the surface to the core of the ingot, and the easily segregated structure and coarse structure in the core are reduced; the finishing rolling is carried out in the austenite non-recrystallization zone, the reduction rate of each pass is greater than 12%, and the cumulative reduction rate of finishing rolling is 70% to 73%.

9. The 780 MPa grade wheel steel and the method for manufacturing the same according to claim 4, characterized in that: In step S3, one pass of rough rolling is performed with large deformation, and the reduction rate of each pass is 16-20%. The second stage of rough rolling adopts 7 passes of large deformation rolling, and the reduction rate of each pass is greater than 13%. The cumulative reduction rate of rough rolling is 65%-68%.

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