Long-service-life bearing steel for new energy automobile electric drive system and production method of long-service-life bearing steel

Abstract

The invention relates to long-life bearing steel, and belongs to the field of iron-based alloys. The microcosmic brittle inclusions of the bearing steel meet the conditions that the B fine system is less than or equal to 1.5 grade, the B coarse system is less than or equal to 1.0 grade, the D fine system is less than or equal to 1.0 grade, the D coarse system is less than or equal to 1.0 grade, and the DS system is less than or equal to 1.0 grade; the total ultrasonic defect index does not exceed 5mm / dm < 3 >, and the maximum length of a single ultrasonic defect does not exceed 2mm; the macrostructure meets the conditions that the center porosity is smaller than or equal to 1.0 level, the general porosity is smaller than or equal to 1.0 level, the ingot segregation is smaller than or equal to 1.0 level, and the center segregation is smaller than or equal to 1.0 level; a metallographic method is adopted to detect that no microscopic gap exists; a metallographic method is adopted to detect that carbide liquation is smaller than or equal to 0 grade, carbide banding is smaller than or equal to 2.0 grade, and carbide netlike is smaller than or equal to 2.5 grade. The end quenching J1.5 is greater than or equal to 62HRC, and the core impact performance AKU is greater than or equal to 80J. The steel is formed by rolling a blank produced by a continuous casting process, an electroslag remelting production process can be replaced, and the production cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgy and relates to an iron-based alloy, and in particular to a bearing steel and a production method thereof. Background Art

[0002] New energy vehicles (NEVs) use unconventional fuels as their power source, integrating advanced technologies in vehicle power control and propulsion to create advanced technical principles, new technologies, and new structures. Driven by energy and environmental concerns, NEVs are poised to become the future of automotive development. Their rapid development promises significant savings in oil and other energy sources. Considering China's energy resource situation and international automotive technology trends, it is projected that by 2025, conventional gasoline vehicles will account for only approximately 50% of China's passenger car fleet, while advanced diesel, gas, and biofuel vehicles, among other new energy vehicles, will experience rapid growth.

[0003] Unlike traditional vehicles, the most critical core system in new energy vehicles has shifted from the internal combustion engine to the electric drive system. The electric drive system is a core component of new energy vehicles, and high-performance electric drive systems will become a core competitive advantage for major automakers. Bearings, as the most important component of drive motors, have been designated as a key core technology for breakthroughs. The domestic bearing industry for motor bearings used in general industrial motors has undergone continuous development. With advancements in bearing design theory and bearing steel material technology, domestically produced bearings have replaced imports and occupied a major portion of the domestic market. However, the wide speed regulation range, high starting torque, high power density, and high efficiency of automotive drive motors place higher demands on bearings for high speed, high and low temperature performance, durability, stability, and reliability.

[0004] Bearing quality is influenced not only by structural design and manufacturing precision, but also by the steel used. This crucial factor impacts the quality, service life, and reliability of bearing products. Domestically, high-speed, long-life bearing steel is commonly produced using high-cost, specialized smelting methods, such as a dual-process involving an electric furnace and electroslag remelting, or a dual-process involving a vacuum induction furnace and vacuum consumables. Continuously cast or die-cast steel produced using highly efficient, off-furnace refining methods cannot be used due to high total oxygen content or large, brittle inclusions, resulting in poor purity. Summary of the Invention

[0005] In order to overcome the shortcomings of bearing production by continuous casting steel, the present invention proposes a bearing steel suitable for continuous casting production using an off-furnace refining method, which can meet the use requirements of bearing steel for electric drive systems of new energy vehicles.

[0006] The bearing steel of the present invention meets the following technical indicators: Non-metallic inclusions in steel destroy the continuity and uniformity of the metal. Depending on the operating conditions of the bearing, under the action of alternating stress, inclusions can easily cause stress concentration, become a source of fatigue cracks, and reduce the fatigue life of the bearing. In order to increase the service life of the final product, the purity of the steel is very important, and the size and number of non-metallic inclusions in the steel, especially the non-deformable hard and brittle inclusions, must be reduced as much as possible. The level of non-metallic inclusions in the present invention is shown in Table 1 below. Macroscopic inclusions significantly reduce the wear resistance of steel, cause serious stress concentration, and easily cause early failure of bearings during use. The macroscopic defects of the present invention are tested by water immersion high-frequency flaw detection using the SE9 1927 method to meet the requirement that the total ultrasonic defect index does not exceed 5mm / dm 3 , the maximum length of a single ultrasonic defect does not exceed 2mm.

[0007] Table 1

[0008] The uniformity and density of the macrostructure of steel have an impact on the life of bearings. The macrostructure adopts GB / T 1979 for the grading of macrostructure of steel, and meets the following requirements: central porosity ≤1.0 level, general porosity ≤1.0 level, ingot segregation ≤1.0 level, and central segregation ≤1.0 level.

[0009] The carbide banding in bearing steel is the crystal segregation formed during the solidification process of molten steel, forming segregation bands with different carbon concentrations. After rolling and stretching, a large amount of excess secondary carbides precipitate in the high-concentration area during the cooling process, thus forming a black and white (high and low carbon) alternating carbide strip-like structure; the carbide network in bearing steel is caused by the reduction of the solubility of carbon in austenite during the cooling process after rolling, and the excess carbon precipitates as secondary carbides at the austenite grain boundaries; when the liquid phase of bearing steel transforms from liquid to solid, the carbon and alloy elements in the last solidified part are enriched to produce metastable eutectic ledeburite. The liquid phase has high hardness and brittleness. After hot rolling, it breaks into small pieces and distributes along the rolling direction, which significantly reduces the wear resistance and fatigue strength of bearing parts and is prone to quenching cracks. Micropores are one of the internal structural defects of bearing steel, which are irregular cracks or pores formed intermittently along the grain boundaries. All of these defects will reduce the service life of bearing parts. Therefore, the present invention adopts metallographic detection to disallow the existence of micropores; metallographic detection is adopted to determine carbide liquid separation ≤ level 0, carbide banding ≤ level 2.0, and carbide network ≤ level 2.5.

[0010] As bearings for electric drive systems in new energy vehicles, the steel surface must have good wear resistance and hardness after heat treatment, and the core needs to withstand large impact loads. The mechanical properties of the steel are: end quenching J1.5 ≥ 62HRC, and the impact performance AKU requirement of the steel core at room temperature is ≥ 80J.

[0011] The technical solution of the present invention is: a long-life new energy vehicle electric drive system bearing steel, the chemical composition of which is C: 0.93-1.05%, Si: 0.15-0.35%, Mn: 0.60-0.80%, Cr: 1.70-1.90%, S≤0.015%, P≤0.020%, Ni: ≤0.25%, Cu≤0.30%, Mo: 0.40-0.50%, Al: ≤0.05%, Ca≤0.0010%, Ti≤0.0015%, O≤0.0008%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, rare earth Ce: 0.005-0.008%, and the balance is Fe and unavoidable impurities. The chemical composition design basis of the new energy vehicle electric drive system bearing steel of the present invention is as follows: 1) Determination of C content Carbon is an essential element for ensuring the wear resistance of steel. Increasing the carbon content in steel increases its martensitic transformation ability, thereby improving its hardness and strength, and thus its wear resistance. However, excessive carbon content is detrimental to the toughness of the steel. The present invention controls the carbon content to 0.93-1.05%. This invention belongs to the category of high-carbon bearing steel.

[0012] 2) Determination of Si content Si is a key element in the present invention. Si dissolves in the ferrite phase, exerting a strong solid-solution strengthening effect, significantly increasing ferrite strength while simultaneously reducing ferrite plasticity and toughness. The Si content of the invention steel is set within a range of 0.15-0.35%.

[0013] 3) Determination of Mn content Manganese, as a deoxidizing element in the steelmaking process, is an effective element for strengthening steel, exerting a solid solution strengthening effect. Furthermore, manganese increases the hardenability of steel and improves its hot workability. Manganese can also counteract the effects of sulfur (S): during steelmaking, Mn reacts with S to form high-melting-point MnS, thereby weakening and eliminating the adverse effects of S. However, high Mn contents can reduce the toughness of steel. In the present invention, the Mn content is controlled to be between 0.60% and 0.80%.

[0014] 4) Determination of Cr content Cr is a carbide-forming element that improves the hardenability, wear resistance, and corrosion resistance of steel. However, if the Cr content is too high, the steel will be too hard, which is not conducive to customer processing and use. The Cr content in the present invention is determined to be within the range of 1.70-1.90%.

[0015] 5) Determination of Al content Al is added to steel as a deoxidizing element. Besides reducing dissolved oxygen in molten steel, Al and N form dispersed, fine aluminum nitride inclusions, which refine grain size. However, excessive Al content can easily form large, brittle inclusions such as Al₂O₃ during smelting, reducing the purity of the molten steel and shortening the service life of the finished product. Therefore, the present invention requires Al content to be ≤ 0.05%.

[0016] 6) Determination of Mo content Molybdenum can refine the grain size 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 too 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 within the range of 0.40-0.50%. 7) Determination of Ti content Ti forms titanium carbonitride inclusions in steel. These inclusions are hard and angular, severely impacting the material's fatigue life. As automotive safety components, the raw materials must minimize the presence of these inclusions. Therefore, the present invention has very strict requirements for Ti content, with a Ti content range of ≤0.0015%.

[0017] 8) Determination of Ca content Ca content increases the number and size of point-like oxides in steel. Furthermore, because these oxides are hard and have poor plasticity, they do not deform when the steel is deformed, and tend to form voids at the interface, degrading the steel's performance. The range of Ca content in the present invention is determined to be ≤0.0010.

[0018] 9) Determination of O content Oxygen content represents the total amount of oxide inclusions. Oxide brittle inclusions limit the service life of finished products. Extensive testing has shown that reducing oxygen content significantly improves steel purity, particularly reducing the content of oxide brittle inclusions within a given steel grade. The oxygen content in this invention is specified to be ≤0.0008%.

[0019] 10) Rare earth Ce content Rare earth element Ce transforms hard inclusions like Al2O3 in steel into fine, easily deformable, and dispersed CeAlO3 and Ce2O3, even eliminating sulfides. This purifies the molten steel and exhibits strong synergistic deformation with the steel matrix, significantly improving and enhancing the steel's plasticity, toughness, and fatigue properties. However, excessive Ce content can easily lead to nodules in the continuous casting nozzle, affecting the castability of the molten steel. The Ce content in this invention is specified to be within the range of 0.005-0.008%.

[0020] 11) Determination of P and S content Phosphorus (P) causes segregation during solidification in steel. It dissolves in ferrite, causing grain distortion and coarsening, and increasing cold brittleness. Therefore, the P content in this invention is set to ≤ 0.020%. Sulphur (S) causes hot brittleness in steel, so the S content in this invention is set to ≤ 0.015%.

[0021] The entire production process is as follows: molten iron pretreatment - electric furnace or converter (oxygen blowing for primary refining) - refining outside the furnace - (VD or RH) vacuum degassing - continuous casting - rolling - finishing - punching and storage. The main production steps are as follows: Molten steelmaking involves sequentially undergoing hot metal pretreatment (KR), electric furnace or converter smelting, LF refining, and RH or VD vacuum degassing to produce high-purity molten steel with specified chemical composition. The smelting process ensures that the molten steel meets the chemical composition requirements of the steel material. High-quality hot metal, scrap steel, and raw and auxiliary materials, along with high-quality refractory materials, are used. During the electric furnace or converter smelting process, the tapping endpoint C and P are controlled: C is 0.10% to 0.40%, and P is controlled at ≤0.018%. The tapping temperature is 1620°C to 1700°C, and slag blocking is used to prevent slag from the electric furnace or converter. Al is added during the tapping process for deoxidation, and Al is added all at once. Al adjustment is not required during the refining process to reduce Al2O3 inclusions. During the LF refining process, slag formation and deoxidation must be strengthened. The LF refining furnace uses SiC for surface diffusion deoxidation, and bottom-blown argon is used to stir the molten steel, forcing inclusions to float to the surface slag. The refining furnace temperature is controlled between 1520°C and 1650°C, and the entire refining process is kept at ≥50 minutes. Vacuum degassing is performed with argon stirring throughout, with a degassing time of 15 to 35 minutes under high vacuum (≤133 Pa). After vacuum degassing is completed and the air is broken, a low-oxygen Ce ferroalloy with a 30% Ce content is added to the molten steel. Due to its greater density than the specific gravity of the molten steel, it sinks directly into the molten steel and melts. High argon stirring is used during this process to accelerate the melting of the Ce ferroalloy and modify the molten steel. Based on the composition and alloy yield, approximately 25-25 kg of Ce-iron alloy is added to a heat of steel (approximately 100 tons of molten steel) to ensure the Ce content reaches the target composition of 0.005-0.008%. After stirring with high argon for approximately 3 minutes, a covering agent is added to the molten steel surface to ensure that the molten steel surface is completely covered by the covering agent, leaving no exposed steel surface to prevent secondary oxidation caused by contact with air. Subsequently, a low argon stirring (soft argon blowing) is used for a total of more than 30 minutes to ensure that the modified inclusions can fully float to the slag.

[0022] Continuous casting involves pouring molten steel into rectangular ingots measuring 300×340mm or larger. Anti-oxidation protection is employed throughout the casting process to prevent secondary oxidation of the molten steel, and tundish slag is used to effectively absorb inclusions. An appropriate steel flow to water ratio is employed, and electromagnetic stirring is used in the mold to disrupt bridging of columnar crystals during solidification. Continuous casting utilizes low superheat, with a required superheat of 10-35°C. Advanced induction heating in the tundish, soft reduction at the end of solidification (controlled at 14-16mm), and electromagnetic stirring are employed to control segregation.

[0023] Heating before rolling: The temperature of the preheating section is controlled at 800-950℃, the temperature of the heating section is controlled at 1100-1250℃, and the temperature of the soaking section is controlled at 1150-1250℃ to ensure that the billet is fully and evenly heated, thereby improving the carbide segregation of the steel. At the same time, in order to prevent the generation of micropores due to excessive heating time, the total heating time is controlled at 10-15 hours.

[0024] The traditional rolling process of bearing steel usually adopts isothermal rolling, but due to the uneven temperature distribution, it is easy to cause problems such as stress concentration inside the material and coarse grains, which affect the final performance of the bearing steel. For this reason, the present invention adopts differential temperature rolling technology, which can effectively improve the microstructure and mechanical properties of the material by controlling the temperature gradient during the rolling process. At the same time, the traditional bearing steel rolling technology usually adopts unidirectional rolling. Although this method is simple, it is easy to produce stress concentration during the rolling process, resulting in uneven internal structure of the material, which affects the performance of the bearing steel. In addition, unidirectional rolling may also lead to poor surface quality of the material, increasing the difficulty of subsequent processing. The present invention adopts a bearing steel U-turn rolling technology, which improves the internal structure and surface quality of the bearing steel by changing the rolling direction, thereby improving its comprehensive performance.

[0025] Specifically, the heated billet is rapidly fed into a differential temperature rolling device. A high-pressure water descaling system sprays high-pressure water on the billet surface, rapidly cooling the billet surface and forming a temperature gradient between the surface and the core. The billet surface temperature is controlled between 1050°C and 1100°C, and the core temperature is controlled between 1180°C and 1200°C. The billet is then fed into a reversing rolling mill for the first preliminary rolling, with the rolling direction being forward (preferably, a reduction of 15% and a rolling speed of 1.0 m / s). After the forward rolling is completed, the billet is turned around and fed into the rolling mill for reverse rolling (preferably, a reduction of 15% and a rolling speed of 1.0 m / s). After the first preliminary rolling is completed, the billet is subjected to another high-pressure water descaling step, and the billet surface is rapidly cooled again. At this point, the billet's surface temperature is controlled at 850-950°C, and its core temperature is approximately 1050-1100°C. The billet is then fed back into a reversing mill for a second preliminary rolling cycle in the forward direction (preferably, with a reduction of 10-20% and a rolling speed of 0.5 m / s). After the forward rolling cycle is complete, the billet is reversed and fed back into the mill for reverse rolling (preferably, with a reduction of 10-20% and a rolling speed of 0.5 m / s). After the second preliminary rolling cycle, intermediate and finish rolling continue until the target round bar is achieved, with the final rolling temperature controlled at 700-750°C. After finish rolling, to further control the bearing steel's network carbides and quickly escape the precipitation zone of large amounts of secondary cementite, the round bar is rapidly cooled by water cooling (specifically, the bar is passed through a water tank that sprays water on the bar surface). However, to prevent excessive cooling rates, which can lead to the formation of brittle bainite and martensite on the steel surface, the water tank's water spray intensity should be kept low. After the steel is water-penetrated, the core temperature of the steel is higher than the surface, causing heat transfer from the core to the surface, causing the surface to return to a red state. By controlling the water tank's water spray intensity, the red-return temperature of the steel surface is controlled between 500°C and 600°C. This series of rolling controls ensures a uniform core structure that meets the required technical specifications. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0027] The chemical compositions (wt%) of the electroslag remelted GCr15 used in the present invention and (for comparison) the GCr15 currently on the market are shown in Tables 2 and 3.

[0028] Table 2

[0029] Table 3

[0030] The comparison of inclusions in the steel materials of each embodiment is shown in Table 4.

[0031] Table 4

[0032] The comparison of carbide inhomogeneity and micropores of the steel materials of each embodiment is shown in Table 5.

[0033] Table 5

[0034] The macroscopic data of the steel materials in each example are shown in Table 6.

[0035] Table 6

[0036] Table 7 shows the room temperature impact energy of the round steel core and the end quenching J1.5 and the comparative steel data.

[0037] Table 7

[0038] The manufacturing process of the bearing steel for the long-life new energy vehicle electric drive system in each embodiment is to produce steel by adopting the forming process of molten iron pretreatment + top and bottom blown converter BOF (high-power electric arc furnace EAF) - ladle refining furnace LF - vacuum circulation degassing furnace RH (VD furnace) - large-section continuous casting CCM large continuous casting billet - continuous rolling - finishing.

[0039] During the specific smelting process, high-quality molten iron, scrap steel, and raw and auxiliary materials are selected, as well as high-quality deoxidizers and refractory materials. During the electric furnace / converter production process, the three embodiments achieve a tapping endpoint C of 0.10%-0.40%, an endpoint P of ≤0.018%, and a continuous casting superheat of 10-35°C. The resulting continuous casting ingots are then heated in a walking beam furnace and rolled into the target bar. The heating, rolling, and cooling processes are shown in Table 8 below. The bars are then straightened and inspected to produce the target finished bar.

[0040] Table 8

[0041] As can be seen from Tables 2, 3, 4, 5, and 6, the control levels of harmful elements such as oxygen, titanium, and non-metallic inclusions in each embodiment are significantly better than those in the comparative example, meaning that the purity of the steel is significantly superior to that of products produced using electroslag remelting technology. Low-magnification inspection results show that the low-magnification quality of the present invention is not inferior to that of the comparative steel, and the carbide heterogeneity is also substantially close to that of the comparative steel, reflecting that the uniformity and density of the present invention are comparable to those of steel produced using the electroslag remelting process. Based on the above analysis, the bearing steel for new energy vehicle electric drive systems produced by the present invention using a vacuum degassing and continuous casting production process can replace the original electroslag remelting process, significantly improving production efficiency, reducing production costs, and significantly enhancing product competitiveness.

[0042] In summary, the bearing steel for the long-life new energy vehicle electric drive system of the present invention adopts the overall idea of improving the purity of steel by adopting rare earth microalloying, etc., adopts a high-efficiency, high-capacity, low-cost process route of vacuum degassing, continuous casting, and rolling, and optimizes and controls the key processes, so that the steel obtains high purity, high structural uniformity and high density, and can replace the original electroslag remelting process.

Claims

1. A long-life bearing steel, characterized by: The mass percentage of chemical composition is C: 0.93~1.05%, Si: 0.15~0.35%, Mn: 0.60~0.80%, Cr: 1.70~1.90%, S≤0.015%, P≤0.020%, Ni: ≤0.25%, Cu≤0.30%, Mo: 0.40~0.50%, Al: ≤0.05%, Ca≤0.0010%, Ti≤0.0015%, O≤0.0008%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, rare earth Ce: 0.005~0.008%, and the balance is Fe and unavoidable impurities.

2. The long-life bearing steel according to claim 1, characterized in that: The metallographic method is used to detect microscopic brittle inclusions that meet the following requirements: B fine series ≤1.5 level, B coarse series ≤1.0 level, D fine series ≤1.0 level, D coarse series ≤1.0 level, DS series ≤1.0 level; Use SEP 1927 water immersion high-frequency flaw detection to detect macro defects, and the total ultrasonic defect index does not exceed 5mm / dm 3 , the maximum length of a single ultrasonic defect does not exceed 2mm; GB / T 1979 is used to grade the macrostructure of steel, meeting the requirements of central porosity ≤ level 1.0, general porosity ≤ level 1.0, ingot segregation ≤ level 1.0, and central segregation ≤ level 1.0; metallographic detection is used to determine the absence of microscopic voids; metallographic detection is used to determine carbide liquid separation ≤ level 0, carbide banding ≤ level 2.0, and carbide network ≤ level 2.

5.

3. A method for producing the long-life bearing steel according to claim 1, characterized in that: include: Step 1: Molten steel smelting: smelting molten steel according to chemical composition; Step 2, continuous casting: casting the molten steel from step 1 into continuous casting billets; Step 3: Heating the continuous casting billet: reheat the billet in the furnace, with the soaking zone temperature at 1150-1250°C, to austenitize the structure and solidify the elements; Step 4, rolling: adopt differential temperature rolling, use descaling high-pressure water to spray water cooling on the surface of the billet to form a temperature difference between the billet surface and the core, control the billet surface temperature 1050-1100 ℃, and the core temperature 1180 ℃-1200 ℃; the first initial rolling adopts a reversible rolling mill, the rolling direction is forward, after the forward rolling is completed, the billet is turned around and sent to the rolling mill for reverse rolling; after the first rolling is completed, the second initial rolling is carried out, before the second initial rolling, the billet is descaled by high-pressure water, the billet surface is cooled again, the billet surface temperature is controlled to 850-950 ℃, and the core temperature is 1050-1100 ℃, the second initial rolling adopts a reversible rolling mill, the rolling direction is forward, after the forward rolling is completed, the billet is turned around and sent to the rolling mill for reverse rolling, after the second initial rolling is completed, intermediate rolling and finishing rolling are carried out to roll the billet into round steel, and the final rolling temperature is controlled at 700-750 ℃; Step 5. Cooling after rolling: The round steel is cooled by water after rolling. The round steel passes through the water tank, and the water tank sprays water on the surface of the round steel to cool it. The water spray intensity is adjusted to control the red temperature of the round steel surface after water spray cooling at 500℃-600℃.

4. The method according to claim 3, wherein: Step 1: The smelting raw materials are sequentially subjected to molten iron pretreatment, oxygen blowing primary refining, furnace refining, and vacuum degassing. The oxygen blowing primary refining is controlled to have an endpoint C of 0.10% to 0.40% and an endpoint P of ≤ 0.018%. The tapping temperature is 1620°C to 1700°C and slag blocking is used for tapping. Al is added during the tapping process for deoxidation, and the Al content is adjusted to the target range. No Al is added during the refining process. Refining adopts SiC surface diffusion deoxidation, and bottom argon blowing is used to stir the molten steel to make inclusions float to the surface slag for adsorption. The refining temperature is: 1520~1650℃, and the entire refining process time is controlled at ≥50min. Argon stirring is implemented during the entire vacuum degassing process. After the vacuum degassing is completed and the air is broken, Ce iron alloy is added to the molten steel. The process adopts large argon stirring. After large argon stirring for 3-5 minutes, a covering agent is added to the molten steel surface. The molten steel liquid surface is fully covered by the covering agent, and then soft argon blowing is switched. The soft argon blowing treatment is carried out for more than 30 minutes to make the inclusions after the modification treatment fully float to the slag.

5. The method according to claim 3, wherein: Step 2: Continuous casting is to pour the molten steel into rectangular continuous casting billets with a specification of 300mm×340mm or more, with an overheating temperature of 10-35℃. Anti-oxidation protection is used throughout the continuous casting process. The light reduction at the end of continuous casting solidification is 14-16mm, and the remaining steel in the ladle is ≥5 tons.

6. The method according to claim 3, wherein: Step 3: Send the continuous casting billet to the walking beam heating furnace for heating. The temperature of the preheating section is controlled at 800-950℃, the temperature of the heating section is controlled at 1100-1250℃, and the total heating time is controlled at 10-15 hours.

7. The method according to claim 3, wherein: Step 4: The reduction of the first initial rolling in the forward direction is 15%, the rolling speed is 1.0 m / s, and the reduction of the reverse rolling is 15%, and the rolling speed is 1.0 m / s; the reduction of the second initial rolling in the forward direction is 10-20%, the rolling speed is 0.5 m / s, and the reduction of the reverse rolling is 10-20%, and the rolling speed is 0.5 m / s.

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