Long-service-life steel for automobile water pump shaft connecting bearing and production method of long-service-life steel

Abstract

The invention relates to steel for a long-service-life automobile water pump shaft connecting bearing and a production method of the steel, and belongs to the field of iron-based alloys, the steel comprises the following element components in percentage by weight: 0.80-0.90% of C, 0.65-0.85% of Si, 0.20-0.40% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 1.60-1.70% of Cr, 0.40-0.50% of Ni, 0.30-0.40% of Mo, less than or equal to 0.050% of Al, 0.0010-0.0020% of rare earth La, less than or equal to 0.0020% of Ti, less than or equal to 0.0010% of Ca, less than or equal to 0.0010% of O and the balance of Fe and inevitable impurities. The production process of the steel comprises the steps of electric furnace or converter primary smelting, external refining, VD or RH vacuum degassing, continuous casting, continuous rolling, normalizing, finishing and piece making and warehousing. The metallographic structure of the steel surface layer is a ferrite and pearlite structure, and the metallographic structure of the core part is cementite and pearlite. The rolling contact fatigue rated life of the steel material under the contact stress of 4.5 GPa is more than 5 * 107 times.

Description

Technical Field

[0001] The invention relates to the technical field of automobile steel and its smelting technology, and in particular to a steel for automobile water pump shaft bearing and a production method thereof. Background Art

[0002] The cooling system of the automobile engine is an important part of the automobile. The stability, reliability and compactness of the system not only directly affect the mechanical properties of the automobile, but also affect the overall appearance of the automobile. For this reason, the cooling system of automobile engines produced by industrially developed countries generally adopts cooling water pump shaft bearings. The function of the automobile water pump is to circulate the cooling water between the engine water jacket and the radiator to ensure the normal operation of the engine within a certain temperature range. Automobile water pump shaft bearings have been widely used in domestic and imported automobiles. The water pump is composed of a water pump housing, shaft bearings, water seals, impellers, fans and transmission parts. The shaft bearing is an important part of the water pump.

[0003] The steel used for conventional automobile water pump shaft bearings is high carbon chromium bearing steel (GCr15), which is smelted through vacuum deoxidation and electromagnetic stirring, with a low oxide content. It can withstand large contact compressive stress while also withstanding large high tensile stress. After heat treatment, the hardness is above 60HRC.

[0004] Since the speed of the water pump is relatively high, the speed of a general water pump is up to 6000r / min, and the speed of a high-speed water pump is up to 9000r / min, the shaft bearing is required to have a higher load-bearing capacity, so as to meet the life requirements of the water pump at a certain speed. At the same time, due to the limited spatial position of the water pump, the small heat dissipation area, and the harsh working conditions, the shaft bearing should be able to meet the requirements of maintaining normal operation at high temperatures without reducing the bearing's load-bearing capacity. At present, the high-carbon chromium bearing steel GCr15 produced by conventional processes is prone to failure under high speed and high temperature conditions, which ultimately leads to the early failure of the water pump shaft bearing. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention proposes a new type of steel for automobile water pump shaft bearing and a production method thereof, based on conventional bearing steel GCr15, by reasonably optimizing the elemental composition of the steel and improving the steel production process. The produced water pump shaft bearing has the characteristics of high temperature resistance, high stability and long life.

[0006] The main technical indicators of the steel material of the present invention are as follows: the metallographic structure of the surface of the steel material (round steel) after hot rolling is bainite structure, and the core structure is cementite + pearlite structure; after normalizing treatment, the surface of the steel material is ferrite + pearlite structure, and the core is cementite + pearlite structure. After normalizing treatment, the decarburization layer depth of the steel material is ≤0.6%D (D is the diameter of the steel material).

[0007] The macro defects of the finished steel after normalizing are inspected by water immersion high frequency flaw detection using the SEP 1927 method. The total ultrasonic defect index does not exceed 5mm / dm 3 , the maximum length of a single ultrasonic defect does not exceed 5mm; the macrostructure requirements are GB / T1979 for macrostructure rating of steel, central porosity ≤1.0, general porosity ≤1.0, ingot segregation ≤1.0, central segregation ≤1.0, and no shrinkage, cracks or subcutaneous bubbles; steel is tested for carbide network, banding, liquid precipitation and micropores according to GB / T 18254-2016, carbide network ≤1.5, carbide banding ≤2.0, and no carbide liquid precipitation and micropores. Non-metallic inclusions in steel are tested according to GB / T 10561A method, among which brittle non-deformable inclusions that affect the service life of bearings are B fine ≤1.0, B coarse ≤1.0, D fine ≤1.0, D coarse ≤0.5, and Ds ≤1.0. The rolling contact fatigue rating life of steel under a contact stress of 4.5 GPa is 5×10 7 More than times.

[0008] The weight percentage of the elemental components of the steel of the present application is C: 0.80-0.90%, Si: 0.65-0.85%, Mn: 0.20-0.40%, P: ≤0.020%, S: ≤0.010%, Cr: 1.60-1.70%, Ni: 0.40-0.50%, Mo: 0.30-0.40%, Al: ≤0.050%, rare earth La: 0.0010%-0.0020%, Ti: ≤0.0020%, Ca≤0.0010%, O: ≤0.0010%, and the balance is Fe and unavoidable impurities.

[0009] The elemental composition of steel in this application is set based on:

[0010] 1) Determination of C content

[0011] C is an element necessary to ensure the wear resistance of steel. Increasing the carbon content in steel will increase its martensitic transformation ability, thereby increasing its hardness and strength, and further improving wear resistance. However, too high a C content is not good for the toughness of steel. In addition, too high a C content will also lead to severe central C segregation, thereby affecting the core toughness of the steel. The present invention controls its content to 0.80-0.90%. The steel of the present invention belongs to the category of high carbon steel.

[0012] 2) Determination of Si content

[0013] Adding Si to steel can strengthen ferrite, improve strength, elastic limit and hardenability, but Si increases the overheating sensitivity, cracking and decarburization tendency in the steel. The range of Si content in the present invention is determined to be 0.65-0.85%.

[0014] 3) Determination of Mn content

[0015] Mn, as a deoxidizing element in the steelmaking process, is an effective element for strengthening steel and plays a role in solid solution strengthening. Moreover, Mn can improve the hardenability of steel and improve the hot working performance of steel. Mn can eliminate the influence of S (sulfur): Mn can form high melting point MnS with S in steel smelting, thereby weakening and eliminating the adverse effects of S. However, a high Mn content will reduce the toughness of steel. The Mn content of the present invention is controlled at 0.20-0.40%.

[0016] 4) Determination of Al content

[0017] Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride inclusions to refine the grains. However, when the Al content is too high, large particles of Al2O3 and other brittle inclusions are easily formed during the molten steel smelting process, which reduces the purity of the molten steel and affects the service life of the finished product. The range of the Al content in the present invention is determined to be ≤0.05%.

[0018] 5) Determination of Cr content

[0019] Cr is a carbide-forming element that can improve the hardenability, wear resistance and corrosion resistance of steel. However, if the Cr content is too high, it will combine with the carbon in the steel to easily form large carbides. Such insoluble carbides reduce the toughness of the steel and shorten the life of the bearing. The Cr content of the present invention is designed to be in the range of 1.60-1.70%.

[0020] 6) Determination of Ni content

[0021] Nickel in steel can reduce the ability of the surface to absorb carbon atoms, accelerate the diffusion of carbon atoms in austenite, and reduce the concentration of carbon in the carburized layer, so nickel can slow down the carburization rate; at the same time, the addition of nickel can improve the toughness of steel. The content of Ni in the present invention is determined to be in the range of 0.40% to 0.50%.

[0022] 7) Determination of Mo content

[0023] Molybdenum can refine the grains of steel, improve hardenability and heat resistance, and maintain sufficient strength and creep resistance at high temperatures. However, molybdenum is a ferrite-forming element. When the molybdenum content is high, ferrite delta phase or other brittle phases are likely to appear, which reduces toughness. The Mo content of the present invention is determined to be in the range of 0.30-0.40%.

[0024] 8) Determination of rare earth La content

[0025] Adding rare earth elements to steel can refine the metamorphic inclusions, purify the molten steel, and improve the plasticity, heat resistance and wear resistance of the steel. However, improper addition of rare earth elements will cause blockage of the continuous casting nozzle and poor performance. Therefore, the content of rare earth La in the present invention is determined to be within the range of 0.0010-0.0020%.

[0026] 9) Determination of Ca content

[0027] The Ca content will increase the number and size of the point oxides in the steel. At the same time, due to the high hardness and poor plasticity of the point oxides, they do not deform when the steel is deformed, and are prone to form gaps at the interface, which deteriorates the performance of the steel. The range of the Ca content in the present invention is determined to be ≤0.001%.

[0028] 10) Determination of O content

[0029] The oxygen content represents the total amount of oxide inclusions. Oxide brittle inclusions limit the service life of the finished product. A large number of tests have shown that reducing the oxygen content is significantly beneficial to improving the purity of steel, especially reducing the content of oxide brittle inclusions in steel grades. The range of the oxygen content in the present invention is determined to be ≤0.0010%.

[0030] 11) Determination of P and S content

[0031] Phosphorus causes serious segregation during solidification in steel. Phosphorus dissolves in ferrite to cause grain distortion and coarsening, and increases cold brittleness. The content of Phosphorus in the present invention is determined to be ≤0.020%. Phosphorus causes hot brittleness in steel, reduces the ductility and toughness of steel, and the content of Phosphorus in the present invention is determined to be ≤0.010%.

[0032] The production process of steel for automobile water pump shaft and bearing: primary refining in electric furnace or converter - refining outside the furnace - VD or RH vacuum degassing - continuous casting - continuous rolling - normalizing - finishing - punching and warehousing.

[0033] The main production steps are as follows:

[0034] Converter or electric furnace primary refining: High-quality molten iron and scrap steel are added to the converter or electric furnace for primary refining, and 4000-5000m 3 Oxygen and 1000~2000m 3 Argon gas is used and active limestone (CaCO3) is added to react with harmful elements in steel to remove harmful elements such as phosphorus (P≤0.020%) and titanium (Ti≤0.002%). The final carbon content when tapping from the primary smelting furnace is 0.15% to 0.20%, and the tapping temperature is ≥1650℃. Slag blocking is used when tapping, and some alloys (initial adjustment of composition) are added during tapping. Slag removal is carried out immediately after tapping is completed. After slag removal, it is quickly lifted to the refining LF furnace for smelting.

[0035] Refining furnace: The whole refining process adopts argon protective atmosphere smelting. During the process, SiC and CaO-SiO2-MgO high-performance composite slag-making agent are used for deoxidation and removal of harmful non-metallic inclusions. SiC is added to the molten steel for diffusion and precipitation deoxidation. The CaO-SiO2-MgO high-performance composite slag-making agent further diffuses and deoxidizes on the surface of the molten steel and adsorbs and removes harmful non-metallic inclusions. During the smelting process, the power is cut off every 15 minutes, SiC (30-50Kg / 100t molten steel) and CaO-SiO2-MgO high-performance composite slag-making agent (100-150Kg / 100t molten steel) are added to the molten steel, and the molten steel is temperature measured and sampled for analysis. Alloy elements are added according to the target requirements. The number of temperature measurement and sampling of the refining furnace is controlled at 2-3 times until the element composition meets the design requirements. The refining time is controlled at more than 45 minutes, and the soft blowing time of the molten steel is 15-20 minutes.

[0036] Vacuum degassing: During RH or VD vacuum degassing, the maximum vacuum degree in the vacuum furnace is ≤1.33mbar, and the vacuum cycle treatment time of the molten steel is maintained at ≥30min to ensure that the harmful gases in the steel are effectively removed. After the vacuum treatment is completed, argon is blown into the bottom of the ladle to stir the molten steel. The argon flow rate is controlled so that the molten steel is not exposed to the air. The soft blowing time of the molten steel is 15-20min. After the vacuum treatment is completed and the air is broken, La-Fe alloy is added to the molten steel for rare earth treatment. Since the density of La-Fe is lower than that of molten steel, it cannot sink into the molten steel if it is directly added. If a large amount of argon is used to stir the alloy into the molten steel after addition, it is bound to pollute the molten steel and affect the purity. Specific addition method: 10-15kg / 100t of La-Fe in molten steel is loaded into a special sleeve (iron-based alloy) and covered with soft cotton cloth. The above sleeve is tied with a slag rod and deeply inserted into the molten steel for 1 minute. After the sleeve is melted, the rare earth alloy enters the molten steel. The purpose of adding rare earths is to modify the inclusions in the steel into fine granular inclusions. In addition, it can purify the molten steel and improve the plasticity, heat resistance and wear resistance of the material.

[0037] Step 4 Continuous casting: During the continuous casting process, the ladle to the tundish is protected by argon blowing from a long nozzle, and the argon flow rate is ≤250L / min; and the ladle slag detection technology is used to prevent the residual metallurgical slag from the ladle from rushing into the tundish to pollute the pure molten steel. The molten steel in the tundish is protected by a double layer of tundish covering agent and carbonized rice husk to isolate the air, prevent the secondary oxidation of the molten steel, and reduce the temperature drop of the tundish, and control the superheat of the molten steel at 10-30℃. A slag retaining wall is provided in the tundish to filter and block some harmful impurities. The opening degree of the stopper rod is accurately controlled by the computer system, and the molten steel flows into the submerged water nozzle through the stopper rod and is injected into the crystallizer. The fluctuation of the crystallizer liquid level is controlled to fluctuate within the range of -5mm to +5mm. Special crystallizer protection slag is added to the crystallizer, and the molten steel quickly forms a shell under the cooling condition of the crystallizer cooling water; the molten steel in the crystallizer is electromagnetically stirred by an external crystallizer electromagnetic stirring device. The billet drawing machine draws billets at a speed of ≤0.8m / min; the secondary cooling zone cools the continuously cast billets in sections according to the cooling capacity required at different times during the solidification process of the billets; an electromagnetic stirrer is equipped at the cooling end to stir the liquid core of the continuously cast billets and break up coarse columnar crystals to form more equiaxed crystals and reduce positive segregation at 1 / 2R; the billets are cut into fixed-length billets in the vertical direction by a flame cutting machine synchronized with the billet drawing speed, transferred to the horizontal roller via a conveyor roller, and the continuously cast billets are hot transported to a heating furnace for heating.

[0038] Step 5: Steel rolling heating and high temperature diffusion: The continuous casting billet is heated in a furnace at a temperature of 1200-1250°C, and kept at this temperature for more than 6 hours. The air-to-coal ratio of the gas is controlled at 1.01-1.05 to reduce the residual oxygen content and prevent the steel from decarburizing beyond the standard;

[0039] Step 6 rolling: After high-pressure water descaling, the rolling temperature is controlled at 1100-1150°C, so that the steel starts rolling in the austenite single-phase region, and then it is rolled alternately through multiple horizontal and vertical rolling mills. Because the product has a high alloy content, water penetration is not allowed during the rolling process to prevent the entire steel from generating excessive organizational stress due to organizational transformation, resulting in cracking of the steel. The final rolling temperature of the steel is controlled at 950°C-1050°C, so that the entire rolling process is carried out in the austenite single-phase region. The steel is then cooled normally on the cooling bed and then taken off the line. Due to rolling in a high-temperature single phase, the steel grains are coarse. At this time, after the hot-rolled steel is taken off the cooling bed, the metallographic structure of the surface of the steel is bainite structure, and the core structure is cementite + pearlite structure. At the same time, the steel does not use controlled rolling and controlled cooling technology during the rolling process. The carbide network is serious in the hot-rolled state, and the carbide network is above level 3.0. In order to ensure the performance of the steel, normalizing heat treatment is required in the future.

[0040] Step 7 Normalizing: First, keep the rolled steel at 880℃±(0-10℃) to austenitize it, and keep it at this temperature for more than 3 hours, then air-cool the steel, and take it out of the furnace after cooling it to room temperature. After normalizing, the bainite structure of the original steel surface is transformed into ferrite + pearlite structure. At the same time, after normalizing, the carbide network of the steel is significantly improved, and the carbide network is ≤1.5 level.

[0041] Compared with the prior art, the advantages of the present invention are:

[0042] 1) The present invention adds alloys such as Ni and Mo when designing the element composition, and also adds a trace amount of rare earth La, which significantly improves the contact fatigue life of the product and can meet the use requirements of long-life automobile water pump shaft bearings.

[0043] 2) The present invention uses special steelmaking, rolling and normalizing processes to ensure that the steel reaches the required metallographic structure and performance and meets the use requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a typical metallographic structure diagram of the water pump shaft bearing steel after hot rolling according to an embodiment of the present invention. The metallographic structure of the surface of the steel after hot rolling is bainite structure, and the core structure is cementite + pearlite structure.

[0045] Figure 2 This is a typical metallographic structure diagram of the comparative bearing steel after hot rolling using controlled rolling and controlled cooling. The metallographic structure of the surface of the steel after hot rolling is troostite structure, and the core structure is cementite + pearlite structure.

[0046] Figure 3 This is a typical metallographic structure diagram of the water pump shaft bearing steel after normalizing according to an embodiment of the present invention. After normalizing, the surface of the steel is ferrite + pearlite structure, and the core is cementite + pearlite structure. DETAILED DESCRIPTION

[0047] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0048] The chemical compositions (wt%) of the steel for automobile water pump shaft connecting bearings corresponding to Examples 1-3 of the present invention and the comparative example GCr15 steel are shown in Tables 1 and 2, and the detection results of non-metallic inclusions are shown in Table 3.

[0049] Table 1

[0050] Example C Si Mn P S Cr Cu Ni Al The present invention 1 0.86 0.65 0.23 0.018 0.002 1.62 0.01 0.42 0.015 The present invention 2 0.81 0.72 0.32 0.016 0.002 1.65 0.01 0.46 0.023 The present invention 3 0.88 0.75 0.38 0.015 0.001 1.67 0.02 0.49 0.020 Comparative Example GCr15 0.95 0.23 0.36 0.018 0.003 1.45 0.03 0.03 0.025

[0051] Table 2

[0052] Example Mo La Ca Ti O The present invention 1 0.32 0.0012 0.0002 0.0009 0.00048 The present invention 2 0.35 0.0015 0.0002 0.0011 0.00053 The present invention 3 0.38 0.0019 0.0001 0.0008 0.00052 Comparative Example GCr15 0.02 ＜0.0002 0.0005 0.0012 0.00058

[0053] Table 3 Non-metallic inclusion detection of various examples and comparative steels

[0054]

[0055] The typical metallographic structure after hot rolling and normalizing of the embodiment of the present invention is shown in Figure 1 and Figure 3 , the metallographic structure after hot rolling of the comparative example is shown in Figure 2 .

[0056] The carbide bands, networks and liquid precipitation of the steel according to the embodiment of the present invention and the rolling contact fatigue rating life results under a contact stress of 4.5 GPa are shown in Table 4.

[0057] Table 4 Carbide and contact fatigue life test results of various embodiments and comparative steels

[0058] Example Carbide ribbon Carbide Network Carbide liquid precipitation Contact fatigue life The present invention 1 1.5 1.0 0 <![CDATA[6.3×10 7 ]]> The present invention 2 1.0 1.0 0 <![CDATA[5.7×10 7 ]]> The present invention 3 1.5 1.0 0 <![CDATA[7.8×10 7 ]]> Contrast steel 2.0 2.5 0 <![CDATA[2.8×10 7 ]]>

[0059] The manufacturing process of the long-life automobile water pump shaft bearing steel of Examples 1-3 is electric furnace or converter - refining outside the furnace - VD or RH vacuum degassing - continuous casting - continuous rolling - normalizing - finishing - punching and warehousing.

[0060] The tapping end point C of the three embodiments is controlled at 0.15-0.25% respectively, and the end point P is required to be ≤0.020%. The electric furnace / converter, refining furnace, vacuum degassing and continuous casting processes are performed according to the requirements of the aforementioned smelting production steps, and the continuous casting superheat is controlled within 10-30°C, and the pulling speed is 0.40-0.70m / min. The specific rolling process of the continuous casting billet in each embodiment is: the continuous casting billet is heated in a walking beam heating furnace, the heating temperature is 1200-1250°C, and the temperature is kept at this temperature for more than 6 hours, and the air-to-coal ratio of the coal gas is controlled at 1.01-1.05 to reduce the residual oxygen content and prevent the steel from decarburizing beyond the standard; the starting rolling temperature is set to 1100-1150°C, and the final rolling temperature is set to 950-1050°C, so that the entire rolling process is carried out in the austenite single-phase region, and then the steel is cooled normally on the cooling bed and then off the line. After rolling off the production line, the metallographic structure of the steel surface is bainite structure, and the core structure is cementite + pearlite structure.

[0061] The steel is then normalized, specifically kept at 880℃±(0-10℃) to austenitize it, and kept at this temperature for 4 hours, and then air-cooled. The steel is taken out of the furnace after cooling to room temperature. After normalizing, the bainite structure on the surface of the steel is transformed into ferrite + pearlite structure, and the core is cementite + pearlite structure. At the same time, after normalizing, the carbide network of the steel is significantly improved, and the carbide network is ≤1.5 level.

[0062] It can be seen from the data in Tables 1 to 3 that the automobile water pump shaft bearing steel in the above embodiments is compared with the traditional GCr15. In terms of composition design, the performance of the material is improved by adding alloy elements such as Ni and Mo, and the contact fatigue life is more than twice that of the traditional bearing steel GCr15. In addition, after normalizing treatment, the carbide network of the steel of the present invention is significantly improved, and the network is ≤1.5 level. At the same time, the metallographic structure of the surface layer of the hot-rolled steel is transformed from the original bainite structure to ferrite + pearlite structure, which avoids the internal stress caused by different structures during subsequent use and causes internal cracking. In summary, the steel of the present invention is significantly superior to the comparative steel in terms of performance and has better competitiveness.

Claims

1. A round steel, characterized in that: The weight percentage of the elemental components of the steel is C: 0.80-0.90%, Si: 0.65-0.85%, Mn: 0.20-0.40%, P: ≤0.020%, S: ≤0.010%, Cr: 1.60-1.70%, Ni: 0.40-0.50%, Mo: 0.30-0.40%, Al: ≤0.050%, rare earth La: 0.0010%-0.0020%, Ti: ≤0.0020%, Ca ≤0.0010%, O: ≤0.0010%, and the balance is Fe and unavoidable impurities.

2. The steel material according to claim 1, characterized in that: The metallographic structure of the steel surface is ferrite + pearlite, and the metallographic structure of the core is cementite + pearlite.

3. The steel material according to claim 1, characterized in that: Macro defects of steel are inspected by water immersion high frequency flaw detection using SEP 1927 method, and the total ultrasonic defect index does not exceed 5mm / dm 3 The maximum length of a single ultrasonic defect does not exceed 5mm; the macrostructure adopts GB / T 1979 to grade the macrostructure of steel: central porosity ≤1.0 level, general porosity ≤1.0 level, ingot segregation ≤1.0 level, central segregation ≤1.0 level, no shrinkage holes, cracks and subcutaneous bubbles; the carbide network, banding, liquid precipitation and micropores are inspected according to GB / T18254-2016: carbide network ≤1.5 level, carbide banding ≤2.0 level, no carbide liquid precipitation and micropores.

4. The steel material according to claim 1, characterized in that: The non-metallic inclusions in steel are tested according to GB / T 10561A method. The brittle non-deformable inclusions that affect the service life of the bearing are B fine ≤ 1.0, B coarse ≤ 1.0, D fine ≤ 1.0, D coarse ≤ 0.5, Ds ≤ 1.0; the rolling contact fatigue rating life of the steel under 4.5GPa contact stress is 5×10 7 More than times.

5. A method for manufacturing the steel material according to claim 1, characterized in that: It involves molten steel smelting, continuous casting, heating, continuous rolling and normalizing processes, as follows: The molten steel smelting involves primary refining, refining and vacuum degassing. The molten steel is dephosphorized and detitanized in the primary refining stage. The final carbon content of the primary refining steel is 0.15% to 0.20%, and the steel tapping temperature is ≥1650°C. Part of the alloy is added during steel tapping to adjust the composition. The whole refining process is smelted in an argon protective atmosphere. During the process, SiC and CaO-SiO2-MgO high-performance composite slag-making agents are used for deoxidation and removal of harmful non-metallic inclusions. SiC is added to the molten steel for diffusion and precipitation deoxidation. The CaO-SiO2-MgO high-performance composite slag-making agent further diffuses and deoxidizes on the surface of the molten steel and removes harmful non-metallic inclusions by adsorption. Vacuum degassing is used to remove gas components in the molten steel. After vacuum degassing, argon is blown into the bottom of the ladle. The argon flow rate is controlled so that the molten steel is not exposed to the air. The soft blowing time is 15 to 20 minutes. After the vacuum treatment is completed and the air is broken, La-Fe alloy is added to the molten steel for rare earth treatment. The continuous casting: the continuous casting billet is cut into fixed-length billets and then hot-fed into a furnace for heating; Heating: The continuous casting billet is heated in the furnace to achieve high temperature diffusion, the heating temperature is 1200-1250°C, and the temperature is kept at this temperature for 6 hours, and the air-to-coal ratio of the gas is controlled at 1.01-1.05; The continuous rolling: the continuous casting billet is descaled by high-pressure water and then continuously rolled, the starting rolling temperature is 1100-1150°C, the rolling starts in the austenite single phase region, and then the rolling is alternately performed through multiple horizontal and vertical rolling mills, the final rolling temperature is 950°C-1050°C, the entire rolling process is carried out in the austenite single phase region, the rolled steel is cooled on a cooling bed and rolled off the line, after the hot-rolled steel is rolled off the cooling bed, the metallographic structure of the surface of the steel is bainite structure, the core structure is cementite + pearlite structure, and the carbide network in the hot-rolled state is above level 3.0; Normalizing: The steel is kept at 880°C ± (0-10°C) to austenitize the structure, and kept at this temperature for more than 3 hours, and then the steel is air-cooled. The steel is cooled to room temperature and then taken out of the furnace, and the bainite structure on the surface is transformed into ferrite + pearlite structure, and the carbide network is ≤1.5 level.

6. The method according to claim 5, characterized in that: In the initial smelting stage of molten steel, high-quality molten iron and scrap steel are added to the converter or electric furnace for primary smelting, and 4000-5000m 3 Oxygen and 1000~2000m 3 Argon gas is added, and active limestone is added to react with harmful elements in steel to achieve dephosphorization and detitanium, to the extent that phosphorus P ≤ 0.020% and titanium Ti ≤ 0.002%. Slag blocking is used for initial steelmaking, and slag removal is carried out immediately after steelmaking is completed.

7. The method according to claim 5, characterized in that: During the refining stage of molten steel smelting, power is cut off every 15 minutes, 30-50 kg of SiC and 100-150 kg of CaO-SiO2-MgO high-performance composite slag-making agent are added to the molten steel, and the molten steel is temperature measured and sampled for analysis. Alloy elements are added according to target requirements, and the number of temperature measurements and sampling of the refining furnace is controlled at 2-3 times until the component content reaches the designed range; the refining time is controlled at more than 45 minutes, and the soft blowing time of the molten steel is 15-20 minutes.

8. The method according to claim 5, characterized in that: During the vacuum degassing stage of molten steel smelting, the maximum vacuum degree in the vacuum furnace is ≤1.33mbar, and the vacuum circulation treatment time of the molten steel is maintained at ≥30min; 10-15kg of La-Fe alloy is loaded into the iron-based alloy sleeve and covered with soft cotton cloth, the sleeve is tied with a slag rod and deeply inserted into the molten steel for 1 minute, and then the sleeve is melted and the rare earth alloy is melted into the molten steel.

9. The method according to claim 5, characterized in that: During the continuous casting process, argon blowing is adopted from the ladle to the tundish with a long nozzle, and the argon flow rate is ≤250L / min. The ladle slag detection technology is used to control the slag rolling. The molten steel in the tundish is protected by a double layer of tundish covering agent and carbonized rice husk to isolate the air and control the superheat of the molten steel at 10-30℃. A slag retaining wall is provided in the tundish to filter and block some harmful impurities. The opening degree of the stopper rod is accurately controlled by the computer system. The molten steel flows into the submerged nozzle through the stopper rod and is injected into the crystallizer. The fluctuation of the crystallizer liquid level is controlled within the range of -5mm to +5mm. Crystallizer protection slag is added to the crystallizer, and the molten steel is cooled in the crystallizer. The billet shell is formed quickly under water cooling conditions; the molten steel in the crystallizer is electromagnetically stirred by an external crystallizer electromagnetic stirring device; the billet drawing machine draws the billet at a drawing speed of ≤0.8m / min; the secondary cooling zone cools the continuous casting billet in sections according to the solidification process of the billet; an electromagnetic stirrer at the cooling end is equipped to stir the liquid center of the continuous casting billet, break up the coarse columnar crystals to form more equiaxed crystals and reduce the positive segregation at 1 / 2R; the billet is cut into fixed-length billets in the vertical direction by a flame cutting machine synchronized with the billet drawing speed, transferred to the horizontal roller via a conveyor roller, and the continuous casting billet is hot sent to the heating furnace for heating.

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