800MPa-grade high-strength automobile beam steel and manufacturing method thereof

Abstract

The 800MPa-grade high-strength automobile beam steel comprises the following chemical components in percentage by weight: 0.055 to 0.075 percent of C, less than or equal to 0.050 percent of Si, 1.6 to 1.8 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.002 percent of S, 0.020 to 0.050 percent of Alt, 0.30 to 0.40 percent of Cr, 0.040 to 0.060 percent of Nb, 0.12 to 0.15 percent of Ti, less than or equal to 0.0040 percent of N and the balance of Fe and inevitable impurities. According to the invention, expensive alloys such as Cu, Ni, Mo, V and the like are not added, the economical titanium alloy is adopted, and an LF furnace deep desulfurization technology under the condition of low Si content of molten steel is adopted, so that the difficulty of deep desulfurization under the condition of low silicon is overcome, meanwhile, the component requirements of low silicon and ultralow sulfur are met, high yield strength and tensile strength are achieved under the condition that the steel is thin, the yield strength is 750-850MPa, and the tensile strength is 750MPa-850MPa. The tensile strength is 800-950 MPa, the elongation A is larger than or equal to 15%, the thickness is 3-5 mm, and the requirements for high strength and light weight are met.

Description

Technical Field

[0001] The invention belongs to the field of automobile beam steel manufacturing, and specifically relates to 800MPa-grade high-strength automobile beam steel and a manufacturing method thereof. Background Art

[0002] Based on the social needs of green environmental protection and carbon reduction, there is a strong demand for lightweight commercial vehicles. Automobile beam steel also needs to take into account the requirements of high strength and lightweight. The lightweight of automobiles means reducing the thickness of automobile steel as much as possible while ensuring the strength and safety of the automobile, including automobile beam steel. While automobile beam steel is required to achieve ultra-high strength of 800MPa, it is also necessary to ensure that the material has good roll forming performance at the user end. Since the user's processing is cold deformation, the comprehensive performance requirements for automobile beam steel are relatively high.

[0003] At present, the main problems in the industry for the production of automobile beam steel are: 1. When the steel thickness is thin, the yield strength and tensile strength are not enough. Generally, the yield strength is lower than 580MPa and the tensile strength is lower than 700MPa. 2. For the smelting of automobile beam steel, due to the inability to overcome the difficulty of deep desulfurization under low silicon conditions, it is impossible to simultaneously meet the low silicon and ultra-low sulfur components. When the silicon content is high, iron oxide scale will be produced on the rolling surface, and the yield rate is low. When the sulfur content is high, the hot brittleness of the steel will be increased, and the safety will be reduced. 3. Expensive special alloys are added, and the cost is high. For example, CN200810036415.9, CN201010242965.3, CN201010235928.X, CN201210511575.0, CN201510241118.8, CN201511020842.4, etc., all have one or more of the above problems.

[0004] How to ensure very high yield strength and tensile strength of automobile beam steel and its comprehensive performance under the condition of thin steel thickness and without adding expensive alloys in a low-cost mode; at the same time, overcome the difficulties of deep desulfurization under low-silicon conditions and meet the requirements of low silicon and ultra-low sulfur composition, which is a technical problem in this field at present. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide an 800MPa grade high-strength automobile beam steel and a manufacturing method thereof in view of the deficiencies in the above-mentioned prior art, so that the steel can achieve high yield strength and tensile strength when the thickness is relatively thin. The thickness is controlled at 3 to 5mm, and the yield strength reaches 750 to 850MPa, the tensile strength reaches 800 to 950MPa and the elongation A ≥ 15%, which fully meets the requirements of high strength and lightweight.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is:

[0007] The invention discloses an 800MPa grade high-strength automobile beam steel, wherein the chemical composition and weight percentage content are as follows: C 0.055%-0.075%, Si≤0.050%, Mn 1.6%-1.8%, P≤0.015%, S≤0.002%, Alt 0.020%-0.050%, Cr0.30%-0.40%, Nb 0.040%-0.060%, Ti 0.12%-0.15%, N≤0.0040%, and the balance is Fe and unavoidable inclusions.

[0008] The 800MPa-grade high-strength automobile beam steel has a yield strength of 750-850MPa, a tensile strength of 800-950MPa, an elongation A≥15%, and a thickness of 3-5mm.

[0009] The present invention also provides a method for manufacturing 800MPa grade high-strength automobile beam steel, the process flow comprising: KR molten iron desulfurization → converter smelting → argon blowing station → LF refining → RH vacuum treatment → continuous casting into billets → slab heating → rolling → (after rolling) cooling → coiling.

[0010] In the above scheme, the KR hot metal desulfurization process is performed twice or more, i.e., the front slag and the rear slag, and the residence time between the two rear slags is required to be no less than 7 minutes; the exposed surface is no less than 93%; the S in the hot metal after desulfurization is ≤ 0.001%. Hot metal desulfurization and clean slag removal are to reduce the sulfur content of the hot metal entering the converter and prevent the subsequent slag from returning to sulfur due to incomplete slag removal.

[0011] In the above scheme, the converter smelting controls the converter terminal temperature to 1600℃~1620℃, the terminal oxygen to 0.045%~0.075%, controls the converter slag volume to ≤40mm, and blows argon from the bottom of the ladle throughout the whole process; in the converter tapping process, aluminum iron is added for deoxidation, the aluminum iron addition amount is 2.0-4.0kg / ts, and ferromanganese is added for preliminary alloying, the ferromanganese addition amount is 21.0~24.0kg / ts, and active lime 4.0~7.0kg / ts and refining agent 1.0~3.0kg / ts are added at the same time, the TFe content in the slag is reduced from 15%~25% to below 4%, and the Als in the molten steel is controlled to 0.030%~0.060%. The slag mixing dynamics principle in the tapping process is used to achieve rapid slag formation and pre-reduction of the slag, creating better conditions for LF refining to make white slag and shorten the refining time.

[0012] In the above scheme, aluminum wire is fed into the molten steel during 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. The argon station controls Als to 0.035% to 0.065% in order to produce reducing slag in advance for subsequent desulfurization, and the subsequent desulfurization must require the reducing properties of the slag.

[0013] In the above scheme, the molten steel is subjected to deep desulfurization treatment after LF refining, and the S in the molten steel is controlled to be ≤0.002% at the end of LF refining, and the Als content in the molten steel is controlled to be 0.030%~0.060%; the LF furnace refining slag system is controlled to be CaO+Al2O3, with a content range of CaO45~60%, Al2O3 20~30%, and CaO:Al2O3=1.7~2.0, and FeO+MnO in the slag is ≤1.5%.

[0014] In the above scheme, the vacuum cycle time of the RH vacuum treatment is 20 to 30 minutes, the vacuum degree is controlled at ≤15Pa, and the vacuum end temperature is controlled at 1555 to 1565°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 of the automobile beam steel of the present invention, the purity of the molten steel must be improved.

[0015] In the above scheme, the crystallizer protection slag in the process of continuous casting into billets adopts high basicity, low viscosity, low melting point protection slag, the basicity is 1.06-1.16, the viscosity is 0.03-0.12 Pa·S (at the temperature of 1300°C), and the melting point is 1000-1050°C for medium carbon steel protection slag; the crystallizer taper is set to 1.05%-1.15%, the cooling water flow rate of the wide surface of the crystallizer is 3250-3400L / min, and the cooling water flow rate of the narrow surface of the crystallizer is 610-630L / min.

[0016] In the above scheme, in the process of continuous casting, in order to avoid defects such as cracks and segregation in the cast billet during the casting process, the superheat is controlled at a low level during the continuous casting process, 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.

[0017] In the above scheme, during the continuous casting process, the billet drawing speed is controlled at 0.9-1.2 m / min, and the fluctuation range of the crystallizer steel liquid level is within ±3 mm.

[0018] In the above scheme, the slab is heated to a temperature of 1270-1310°C.

[0019] In the above scheme, the rolling includes rough rolling and finish rolling, the rough rolling temperature is 1070-1110°C, the first stage of rough rolling adopts one pass of large deformation rolling, the pass reduction rate is 12%-16%, and the second stage of rough rolling adopts multiple passes of large deformation rolling, the reduction rate of each pass is greater than 12%, and the cumulative reduction rate of rough rolling is 63%-65%; during the rough rolling process, water is sprayed on the surface for cooling, the cooling gradient from the surface to the core of the ingot is adjusted, the surface temperature of the ingot is controlled at 1150-1200°C, the core temperature of the ingot is controlled at 1200-1250°C, and the segregation-prone structure and coarse structure in the core are reduced; the finish rolling is carried out in the austenite non-recrystallization zone, the reduction rate of each pass is greater than 11%, the cumulative reduction rate of finish rolling is 67%-70%, and the finishing temperature of finish rolling is 870-910°C.

[0020] In the above scheme, the post-rolling cooling process adopts a higher cooling rate method: the cooling rate is 30-45℃ / s, and the final cooling temperature is designed to be 580-620℃. On the one hand, post-rolling cooling is to inhibit the precipitation of Ti, while inhibiting the precipitation of TiC and the growth of columnar grains, retaining the Ti element dissolved in the steel; on the other hand, the final cooling temperature is reduced, and 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.

[0021] In the above scheme, the coiling temperature is 540-580° 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, and retaining the Ti element dissolved in the steel.

[0022] The components and their content ranges and key processes of the present invention are mainly based on the following principles:

[0023] Carbon (C): Carbon is 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 amount of carbon added in the present invention is 0.055% to 0.075%.

[0024] Manganese (Mn): Manganese significantly improves the strength of steel through solid solution strengthening and precipitation strengthening mechanisms; Manganese refines grains and improves microstructure, balancing the relationship between high strength and toughness, so that steel has good energy absorption capacity in collisions and improves safety; Manganese helps reduce the tendency of thermal cracking during welding and improves the toughness and strength of welds. However, too high a Mn content will reduce the plasticity and toughness of steel. The more suitable manganese addition amount of the present invention is 1.6% to 1.8%.

[0025] Silicon (Si): The present invention reduces the silicon content as much as possible and no longer relies on the Si element to improve the strength of steel, because silicon will reduce the plasticity of steel. When the silicon content is high, iron oxide scale will be produced on the rolling surface, and the yield rate will be low. However, during the steelmaking process, Si in the molten iron is oxidized into the slag. During the LF furnace refining process, part of the Si in the slag will be reduced into the molten steel. The Si in the molten steel cannot be completely eliminated, so the present invention requires that the maximum Si content is 0.050%.

[0026] 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 and improve thermal stability. Chromium alloys are not only cheap, but can also replace expensive Cu, Mo, and Ni to improve the strength, oxidation resistance and hardenability of steel. However, when the chromium content in steel is too high, the toughness of the steel plate will deteriorate. The most suitable Cr addition amount is 0.30% to 0.40%.

[0027] Niobium (Nb): Niobium 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 finer and reduce its number, improve the strength and hardness of steel, and improve the machinability and plasticity of steel. Nb can form Nb (C, N) precipitates, which also have the effect of grain refinement and precipitation strengthening; Nb can improve the toughness and fracture resistance of the material, so Nb can replace expensive alloys such as V and Mo that also play the role of grain refinement. However, too much niobium will reduce the toughness of steel and lead to uneven carburized layer. The most suitable niobium addition is 0.040% to 0.060%.

[0028] Titanium (Ti): As a strong deoxidizer and grain-refining element, titanium can effectively reduce inclusions in steel and promote grain refinement. The refined grains improve the strength and toughness of the material. Titanium combines with carbon and nitrogen to form fine precipitates (such as TiC or TiN). These precipitates are distributed in the ferrite matrix, hindering dislocation movement, thereby significantly improving the strength and hardness of the steel. Titanium can improve the toughness and fracture toughness of the material, reduce the brittle transition temperature, help improve the corrosion resistance of steel, and extend the service life of automobile beam steel in complex environments. Titanium alloys are not only cheap, but can also replace expensive Cu, Mo, and Ni to improve the strength, toughness, and corrosion resistance of steel. The most suitable titanium addition is 0.12% to 0.15%.

[0029] 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% to 0.050%.

[0030] Phosphorus, sulfur, and nitrogen (P, S, N): Phosphorus can easily cause cold brittleness of steel; sulfur can easily cause hot brittleness, so the phosphorus and sulfur content in steel should be reduced as much as possible; nitrogen is an important indicator for measuring the purity of molten steel. Nitrogen can easily form inclusions, pores, etc. To improve the purity of molten steel, the nitrogen content in steel must be reduced as much as possible.

[0031] In the present invention, when the molten steel is subjected to LF refining deep desulfurization treatment, S≤0.002% and Als0.030%~0.060% in the molten steel are controlled. Since the upper limit of Si content of the steel grade of the present invention is 0.050%, it belongs to aluminum-killed steel in a strict sense, and the activity of slag oxygen is relatively high and the oxidizing property is relatively strong. However, deep desulfurization requires the slag to have a relatively high reducibility, so deep desulfurization of aluminum-killed steel has always been a technical problem. The present invention develops a LF furnace deep desulfurization technology using low Si content in molten steel, which can increase the sulfur capacity of the slag, the activity coefficient of sulfur, and the temperature of the molten steel, reduce the activity of oxygen, and is conducive to increasing the distribution ratio of sulfur, and improving the mass transfer of sulfur on both sides of the slag and steel, thereby improving the desulfurization capacity of the slag.

[0032] In the present invention, the LF furnace refining slag is controlled to be CaO+Al2O3, and the content range is respectively CaO 45%-60%, Al2O3 20%-30%, and CaO:Al2O3=1.7-2.0, and FeO+MnO≤1.5% in the slag is a key parameter. The LF furnace performs deep desulfurization treatment on ultra-low sulfur steel. In addition to requiring a higher slag basicity, the content of Al2O3 in the slag should not be less than 20%, and Als in the molten steel should not be less than 0.030%. In this way, a very low oxygen potential between steel and slag can be maintained, the activity of slag oxygen can be effectively reduced, and the reducibility of slag can be improved. The higher the desulfurization efficiency, the better the desulfurization effect. However, in order to prevent the SiO2 in the slag from being reduced to the molten steel and causing an increase in Si, the content of Al2O3 in the slag should not be higher than 30%, and Als in the molten steel should not be higher than 0.060%. FeO+MnO≤1.5% in the LF furnace refined slag, which further reflects that the oxygen activity of the slag is very low and the reducibility of the slag is very high.

[0033] In the present invention, various parameters of continuous casting steady-state casting are technical difficulties. The control of the alkalinity, viscosity and melting point of the mold protection slag, and the matching of low superheat, low pulling speed and the cooling speed of the crystallizer are the key. In the continuous casting process, the mold protection slag adopts a medium carbon steel protection slag with high alkalinity, low viscosity and low melting point. The lower superheat makes the molten steel easier to solidify after entering the crystallizer. The crystallizer protection slag must be able to effectively cover the surface of the molten steel under low superheat conditions to provide lubrication and insulation effects, otherwise the molten steel will solidify too quickly and interrupt the casting. The appropriate cooling rate can ensure that the solidification process is stable and avoid local overheating or uneven cooling. At the same time, the lower pulling speed allows the protection slag to play a role in the crystallizer for a longer time, promotes a uniform solidification process, and promotes the formation of a uniform billet shell. However, too low a pulling speed will further reduce the superheat and increase the risk of cold steel blocking the casting in the later stage of casting, so it is necessary to comprehensively consider and select an appropriate pulling speed. The above-mentioned method control of the present invention, which is different from traditional casting, can overcome this technical problem and avoid the above-mentioned defects of the cast billet.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] First, the automobile beam steel developed by the present invention can control the thickness to 3-5 mm, and achieve a yield strength of 750-850 MPa, a tensile strength of 800-950 MPa and an elongation A≥15%, which fully meets the requirements of high strength and lightweight.

[0036] Second, the present invention does not add expensive alloys such as Cu, Ni, Mo, V, etc., and uses the LF furnace deep desulfurization technology under low Si content conditions in the economical titanium alloy, which overcomes the difficulty of deep desulfurization under low silicon conditions and meets the low silicon and ultra-low sulfur composition requirements, so that the steel can achieve high yield strength and tensile strength with a relatively thin thickness. DETAILED DESCRIPTION

[0037] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0038] In the following embodiments, the aluminum content in the aluminum iron used is 40-43wt%, and the Mn content in the manganese iron is 75-78wt%. The main components of the refining agent include Al2O3 38-55wt%, CaO 28-40wt%, SiO2 3-12wt%, MgO 3-12wt%, and Fe1-2.5wt%. The refining agent can quickly combine with CaO to increase the Al2O3 content in the slag, which plays a role in promoting the pre-reduction of the top slag of the converter. The above raw materials are all commercially available raw materials.

[0039] Examples 1 to 10 and Comparative Examples 1 to 2

[0040] The chemical composition and weight percentage of the steel in each embodiment and comparative example are shown in Table 1.

[0041] Table 1 List of composition values ​​of various embodiments and comparative examples of the present invention (wt, %)

[0042]

[0043]

[0044] The process flow of manufacturing 800MPa grade high-strength automobile beam steel in each embodiment includes: KR hot metal desulfurization → converter smelting → argon blowing station → LF refining → RH vacuum treatment → continuous casting → slab heating → rolling → post-rolling cooling → coiling, which is mainly carried out in the following steps:

[0045] (1) During the KR desulfurization of molten iron, the following procedures are carried out: slag removal is performed twice or more, i.e., pre-slag removal and post-slag removal, and the residence time between the two post-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%;

[0046] (2) Converter smelting, controlling the converter endpoint temperature to 1600°C to 1620°C, the endpoint oxygen to 0.045% to 0.075%, and controlling the converter slag volume to ≤40mm, with argon blowing from the bottom of the ladle throughout the entire process; adding aluminum iron for deoxidation during the converter steelmaking process, with the aluminum iron added in an amount of 2.0-4.0kg / ts, and adding ferromanganese for preliminary alloying, with the ferromanganese added in an amount of 21.0-24.0kg / ts, while adding active lime 4.0-7.0kg / ts and refining agent 1.0-3.0kg / ts, reducing the TFe content in the slag from 15% to 25% to below 4%, and controlling the Als content in the molten steel to 0.030% to 0.060%;

[0047] (3) 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% to 0.065% at the end of the argon blowing station;

[0048] (4) The molten steel is subjected to deep desulfurization treatment after IF refining, 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 0.030% to 0.060%; at the same time, the LF furnace refining slag system is controlled to be CaO+Al2O3, with the content range of CaO 45% to 60%, Al2O3 20% to 30%, and CaO:Al2O3=1.7 to 2.0, and FeO+MnO in the slag is ≤1.5%;

[0049] (5) The molten steel is subjected to RH vacuum cycle degassing and slag removal treatment, and the alloying is fine-tuned to adjust the composition and temperature so that the final composition content of the molten steel meets Table 1. The vacuum cycle time is 20 to 30 minutes, the vacuum degree is controlled at ≤15Pa, and the vacuum end temperature is controlled at 1555 to 1565°C;

[0050] (6) Continuous casting: During the continuous casting process, the mold protection slag uses a medium carbon steel protection slag with a basicity of 1.06-1.16, a viscosity of 0.03-0.12 Pa·S (at a temperature of 1300°C), and a melting point of 1000-1050°C. The mold taper is set to 1.05%-1.15%, the cooling water flow rate of the wide surface of the mold is 3250-3400 L / min, and the cooling water flow rate of the narrow surface of the mold is 610-630 L / min; the superheat during the continuous casting process is controlled at 10-20°C, and the tundish temperature is controlled at 1530-1540°C; the billet drawing speed during the continuous casting process is controlled at 0.9-1.2 m / min, and the fluctuation range of the mold steel liquid level is within ±3 mm;

[0051] (7) Slab heating: The slab heating temperature is 1270-1310°C;

[0052] (8) Rolling: The rough rolling temperature is 1070-1110°C, and the rough rolling is performed with one large deformation rolling pass, with a pass reduction rate of 12%-16%. The second stage of the rough rolling adopts 7 large deformation rolling passes, with a reduction rate of more than 12% for each pass, and a cumulative reduction rate of 63%-65% for the rough rolling. During the rough rolling process, the surface temperature of the ingot is controlled to be 1150-1200°C, and the core temperature of the ingot is controlled to be 1200-1250°C. The finishing rolling is performed in the austenite non-recrystallization zone, with a reduction rate of more than 11% for each pass, and a cumulative reduction rate of 67%-70% for the finishing rolling. The finishing rolling end temperature is 870-910°C.

[0053] (9) Cooling: adopt a method with a higher cooling rate: the cooling rate is 30-45°C / s, and the final cooling temperature is 580-620°C;

[0054] (10) Coiling: The coiling temperature is 540-580°C.

[0055] The manufacturing process of each comparative example steel is the same as that of the embodiment, except for the control of parameters. The main process parameters of each embodiment and comparative example are shown in Tables 2 to 5.

[0056] Table 2 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0057]

[0058] Table 3 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0059]

[0060] Table 4 List of main steelmaking process parameters of various embodiments of the present invention and comparative examples

[0061]

[0062] Table 5 List of main rolling process parameters of various embodiments of the present invention and comparative examples

[0063]

[0064]

[0065] According to the national standard GB / T 228.1 “Room Temperature Tensile Test Method for Metallic Materials”, the performance parameters of the steels of each embodiment and comparative example were tested, and the test results are shown in Table 6.

[0066] Table 6 Main performance test statistics of various embodiments of the present invention and comparative examples

[0067]

[0068] It can be seen from Table 6 that the embodiments of the present invention can obtain 800MPa grade high-strength automobile beam steel, specifically, a yield strength of 750-850MPa, a tensile strength of 800-950MPa, an elongation A≥15%, and a thickness of 3-5mm, so that the steel can achieve high yield strength and tensile strength when the thickness is relatively thin, which not only meets the requirements of high strength and lightweight, but also overcomes the difficulty of deep desulfurization under low silicon conditions, and meets the requirements of low silicon and ultra-low sulfur components; moreover, the cost is low, and no expensive alloys such as Cu, Ni, Mo, V, etc. are added.

[0069] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and changes without departing from the creative concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. An 800MPa grade high strength automobile beam steel, characterized in that: The chemical composition and weight percentage content are: C 0.055% ~ 0.075%, Si ≤ 0.050%, Mn 1.6% ~ 1.8%, P ≤ 0.015%, S ≤ 0.002%, Alt 0.020% ~ 0.050%, Cr 0.30% ~ 0.40%, Nb 0.040% ~ 0.060%, Ti 0.12% ~ 0.15%, N ≤ 0.0040%, and the balance is Fe and unavoidable inclusions.

2. The 800MPa high-strength automobile beam steel according to claim 1, characterized in that: The yield strength is 750-850MPa, the tensile strength is 800-950MPa, the elongation A≥15%, and the thickness is 3-5mm.

3. A method for manufacturing 800MPa grade high-strength automobile beam steel according to claim 1 or 2, the process flow comprising: KR molten iron desulfurization → converter smelting → argon blowing station → LF refining → RH vacuum treatment → continuous casting into billets → slab heating → rolling → cooling after rolling → coiling, characterized in that the molten steel is subjected to deep desulfurization treatment through the LF refining, and the S in the molten steel is controlled to be ≤0.002% and Als 0.030% to 0.060% at the end of LF refining, and the LF furnace refining slag system is controlled to be a CaO-Al2O3 slag system; wherein the LF furnace refining slag includes CaO 45% to 60% and Al2O3 20% to 30% by mass percentage, and the mass ratio of CaO to Al2O3 is (1.7 to 2.0): 1, and the sum of the mass percentages of FeO and MnO in the LF furnace refining slag is less than 1.5%.

4. The method for manufacturing 800MPa grade high-strength automobile beam steel according to claim 3, characterized in that: The rolling includes rough rolling and finish rolling. The rough rolling temperature is 1070-1110°C. The first stage of rough rolling adopts one large deformation rolling, and the pass reduction rate is 12%-16%. The second stage of rough rolling adopts multiple large deformation rolling, and the reduction rate of each pass is greater than 12%. The cumulative reduction rate of rough rolling is 63%-65%. During the rough rolling process, water is sprayed on the surface for cooling, and the surface temperature of the casting is controlled to be 1150-1200°C, and the core temperature of the casting is controlled to be 1200-1250°C. The finish rolling is carried out in the austenite non-recrystallization zone, and the reduction rate of each pass is greater than 11%. The cumulative reduction rate of finish rolling is 67%-70%, and the finishing temperature of finish rolling is 870-910°C.

5. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: During the KR molten iron desulfurization, slag removal is performed more than twice, namely, pre-slag removal and post-slag removal, and the residence time between each two post-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%.

6. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: The converter smelting controls the converter terminal temperature to 1600°C-1620°C, the terminal oxygen to 0.045%-0.075%, controls the converter slag volume to be ≤40mm, and blows argon from the bottom of the ladle throughout the whole process; during the converter steel tapping process, aluminum iron is added for deoxidation, the aluminum iron added amount is 2.0-4.0kg / ts, and manganese iron is added for preliminary alloying, the manganese iron added amount is 21.0-24.0kg / ts, and at the same time, active lime 4.0-7.0kg / ts and refining agent 1.0-3.0kg / ts are added to reduce the TFe content in the slag from 15%-25% to below 4%, and control the Als content in the molten steel to be 0.030%-0.060%.

7. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: During the argon blowing station, aluminum wire is fed into the molten steel, and the argon blowing station is controlled to end with 0.035% to 0.065% Als in the molten steel; The vacuum cycle time of the RH vacuum treatment is 20 to 30 minutes, the vacuum degree is controlled at ≤15Pa, and the vacuum end temperature is controlled at 1555 to 1565°C.

8. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: During the continuous casting process, the mold protection slag is a medium carbon steel protection slag with high basicity, low viscosity and low melting point, with a basicity of 1.06-1.16, a viscosity of 0.03-0.12 Pa·S at a temperature of 1300° C., and a melting point of 1000-1050° C. The mold taper is set to 1.05%-1.15%, the cooling water flow rate of the wide surface of the crystallizer is 3250-3400 L / min, and the cooling water flow rate of the narrow surface of the crystallizer is 610-630 L / min; During the continuous casting process, the superheat is controlled at 10-20°C, the tundish temperature is controlled at 1530-1540°C, the billet drawing speed is controlled at 0.9-1.2 m / min, and the fluctuation range of the crystallizer steel liquid level is within ±3 mm.

9. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: The temperature of the slab heating is 1270-1310°C; The cooling rate of the post-rolling cooling is 30-45°C / s, and the final cooling temperature is designed to be 580-620°C; The coiling temperature is 540-580°C.

10. The method for manufacturing 800MPa grade high strength automobile beam steel according to claim 3, characterized in that: The steps include: (1) KR hot metal desulfurization: During the process, slag removal is carried out twice or more, i.e., pre-slag removal and post-slag removal, and the residence time between the two post-slag removals is required to be no less than 7 minutes; the exposed surface is not less than 93%; S in the hot metal after desulfurization is ≤ 0.001%; (2) Converter smelting: Control the converter endpoint temperature to 1600°C to 1620°C, the endpoint oxygen to 0.045% to 0.075%, and control the converter slag volume to ≤40mm, and blow argon from the bottom of the ladle throughout the process; add aluminum iron for deoxidation during the converter steelmaking process, the amount of aluminum iron added is 2.0 to 4.0 kg / ts, and add ferromanganese for preliminary alloying, the amount of ferromanganese added is 21.0 to 24.0 kg / ts, and at the same time add 4.0 to 7.0 kg / ts of active lime and 1.0 to 3.0 kg / ts of refining agent, reduce the TFe content in the slag from 15% to 25% to below 4%, and control the Als content in the molten steel to 0.030% to 0.060%; (3) Argon blowing station: Argon is blown into the molten steel through the bottom of the argon blowing station, and the Als content in the molten steel is controlled to be 0.035% to 0.065% at the end of the argon blowing station; (4) LF refining: The molten steel is subjected to deep desulfurization treatment after IF refining, 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 0.030% to 0.060%; the LF furnace refining slag is controlled to be CaO+Al2O3, with the content range of CaO 45% to 60%, Al2O3 20% to 30%, and CaO:Al2O3=1.7 to 2.0, and FeO+MnO in the slag is ≤1.5%; (5) RH vacuum treatment: The molten steel is degassed and deslaged in a vacuum furnace, and the alloying is performed to fine-tune the composition of the molten steel to meet the target composition of the beam steel. The vacuum cycle time is 20 to 30 minutes, the vacuum degree is controlled at ≤15Pa, and the vacuum end temperature is controlled at 1555 to 1565°C; (6) Continuous casting: During the continuous casting process, the mold protection slag is a medium carbon steel protection slag with high basicity, low viscosity and low melting point, with a basicity of 1.06-1.16, a viscosity of 0.03-0.12 Pa·S at a temperature of 1300°C, and a melting point of 1000-1050°C; the mold taper is set to 1.05%-1.15%, the cooling water flow rate of the wide surface of the mold is 3250-3400 L / min, and the cooling water flow rate of the narrow surface of the mold is 610-630 L / min; the superheat during the continuous casting process is controlled at 10-20°C, and the tundish temperature is controlled at 1530-1540°C; the billet drawing speed during the continuous casting process is controlled at 0.9-1.2 m / min, and the fluctuation range of the mold steel liquid level is within ±3 mm; (7) Slab heating: The slab heating temperature is 1270-1310°C; (8) Rolling: The rough rolling temperature is 1070-1110°C, and the rough rolling is performed with one large deformation rolling pass, with a pass reduction rate of 12%-16%. The second stage of the rough rolling adopts 7 large deformation rolling passes, with a reduction rate of more than 12% for each pass, and a cumulative reduction rate of 63%-65% for the rough rolling. During the rough rolling process, the surface temperature of the ingot is controlled to be 1150-1200°C, and the core temperature of the ingot is controlled to be 1200-1250°C. The finishing rolling is performed in the austenite non-recrystallization zone, with a reduction rate of more than 11% for each pass, and a cumulative reduction rate of 67%-70% for the finishing rolling. The finishing rolling end temperature is 870-910°C. (9) Cooling: The cooling rate is 30-45°C / s, and the final cooling temperature is 580-620°C; (10) Coiling: The coiling temperature is 540-580° C. to obtain the 800 MPa grade high-strength automobile beam steel.

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