A high wear resistant steel for passenger car outer wheel and a manufacturing method thereof

Abstract

The present application relates to a kind of high wear-resistant passenger car outer star wheel steel and its manufacturing method, chemical composition contains C:0.50~0.56%, Si:0.20%~0.40%, Mn:0.75~0.85%, P:≤0.010%, S:0.010~0.030%, Cr:0.30~0.50%, Ni:0.20~0.30%, Cu:0.20~0.40%, Mo:≤0.05%, Al:0.010%~0.050%, N:0.015%~0.025%, the balance is Fe and inevitable impurity element by mass%;The delivery state of steel is hot-rolled state, and the metallographic organization is ferrite and pearlite.Hardness≤255HBW;Hot-rolled state Rp0.2≥520MPa, Rm≥800MPa, elongation A5≥20%, reduction of area Z≥50%.After intermediate frequency induction quenching, the contact fatigue performance, characteristic life can reach more than 900000 times.

Description

Technical Field

[0001] This invention belongs to the field of iron-based alloy technology, and in particular relates to a high wear-resistant steel for passenger car outer wheels and its manufacturing method. Background Technology

[0002] The automotive transmission system is a safety component, and the universal joint is a crucial part of it. Its quality directly impacts the transmission performance and is a vital safety element, placing high demands on both the component and its manufacturing process. With the continuous advancement of the automotive industry and the maturing of my country's automotive market, the requirements for vehicle safety and durability are constantly increasing. The universal joint is part of the constant velocity joint assembly in the transmission system, connecting one end of the fixed joint to the vehicle's tire and wheel hub. Its internal ball joint and spherical rolling elements are in constant contact, resulting in rolling friction between them.

[0003] The outer wheel is generally made of medium carbon structural steel. After heating and forging, the ball track is machined into the inner wall of the bowl-shaped area formed by extrusion. The surface of the ball track is induction hardened to achieve a hardening effect, ensuring that the surface remains intact during the rolling contact and friction with the spherical rolling body, so as to realize the function of constant velocity universal joint to transmit power.

[0004] However, due to the insufficient performance of the steel, the outer planetary wheels made of existing medium carbon structural steel often experience premature rolling contact fatigue failure. The failure manifests as cracking and material spalling on the surface of the outer planetary wheel, which causes the constant velocity universal joint to be unable to continue to transmit power, thus affecting the normal operation of the vehicle. Summary of the Invention

[0005] The purpose of this invention is to obtain a medium carbon steel with high wear resistance by designing chemical composition and combining it with production methods, ensuring that its rolling contact fatigue performance is significantly higher than that of existing steels after induction hardening.

[0006] The technical solution adopted in this invention is as follows: a high wear-resistant steel for outer wheels of passenger vehicles, wherein the chemical composition of the steel, by mass %, contains C: 0.50-0.56%, Si: 0.20%-0.40%, Mn: 0.75-0.85%, P: ≤0.010%, S: 0.010-0.030%, Cr: 0.30-0.50%, Ni: 0.20-0.30%, Cu: 0.20-0.40%, Mo: ≤0.05%, Al: 0.010%-0.050%, N: 0.015%-0.025%, with the balance being Fe and unavoidable impurity elements; the microstructure of the steel is ferrite and pearlite, and the steel is delivered in a hot-rolled state, preferably hot-rolled black steel.

[0007] The main working principle and content settings of the alloy components of this invention are explained in detail below:

[0008] C content requirements

[0009] Carbon (C) is the most economical and fundamental strengthening element in steel, significantly improving its strength. As the carbon content in steel increases, yield strength and tensile strength rise, while plasticity and impact resistance decrease. In this invention, the C content is defined as 0.50–0.56%. This invention relates to medium-carbon steel.

[0010] Si content requirements

[0011] Si does not form carbides in steel, but can dissolve in ferrite to improve the matrix strength of the material. Si has a strong effect on increasing the cold work hardening rate of ferrite and makes a significant contribution to the yield strength and tensile strength of the material. The Si content range of this invention is determined to be 0.20-0.40%.

[0012] Mn content requirements

[0013] Mn can improve the tensile strength and hardenability of steel through solid solution strengthening. As a deoxidizing element in the steelmaking process, Mn can also fix the form of sulfur in steel and form MnS and (Fe,Mn)S, which are less harmful to the steel's properties, thus reducing or inhibiting the formation of FeS. However, Mn is an element that is prone to segregation, and excessive content will cause severe segregation in the center of the steel. The range of Mn content in this invention is determined to be 0.75% to 0.85%.

[0014] P content requirements

[0015] Phosphorus (P) severely causes segregation during solidification in steel. P dissolves in ferrite, causing grain distortion and coarsening, and increasing cold brittleness. In this invention, the P content is defined as ≤0.010%.

[0016] S content requirements

[0017] S can combine with Mn to form hard and brittle MnS, which improves the machinability of steel and promotes the formation of intragranular ferrite. However, excessive S content can cause hot brittleness in steel, reduce its ductility and toughness, and increase segregation at the center of the steel. The S content range of this invention is determined to be 0.01 to 0.03%.

[0018] Al content requirements

[0019] Al is added as a deoxidizing element in steel. Besides reducing dissolved oxygen in molten steel, Al combines with N to form dispersed, fine AlN particles, which can refine the grain structure. However, excessive Al content can easily lead to the formation of brittle inclusions such as Al₂O₃ during steel smelting, reducing the purity of the molten steel. Therefore, this invention controls the Al content to be between 0.010% and 0.050%.

[0020] N content requirements

[0021] In steel, nitrogen (N) combines with al (Al) to form nitrides, refining the grain size. Fine grains improve plasticity, toughness, and strength, thus enhancing the overall performance of the steel. However, excessively high N content is detrimental to continuous casting production, easily leading to surface cracks and poor surface quality. Therefore, this invention controls the N content to be between 0.015% and 0.025%. To achieve grain refinement, the Al / N ratio should be controlled to ≥3. A lower Al / N ratio will not have a significant grain-refining effect and will not improve plasticity and toughness.

[0022] In nitrogen-containing ferritic steel, precipitation hardening can occur due to the precipitation of ultramicro nitrides during tempering after rapid cooling or prolonged residence at room temperature. Nitrogen also causes strain aging in low-carbon steel, improving the creep strength and high-temperature endurance strength of the steel. It can form extremely stable nitrides with elements such as chromium, aluminum, vanadium, and titanium, becoming surface hardening and strengthening elements.

[0023] Cu content requirements

[0024] Cu plays a role in solid solution strengthening in steel, improving the overall strength level and significantly enhancing the secondary strengthening effect. It also stabilizes austenite to increase the effectiveness of hot deformation. Cu precipitation in steel lowers the austenite transformation temperature, inhibits the formation of high-temperature transformation products, and affects the transformation and refinement of the microstructure. This results in a significant strengthening effect, effectively improving the strength, corrosion resistance, weldability, and fatigue resistance of the steel.

[0025] Cr content requirements

[0026] Cr is a carbide-forming element. When heated, Cr dissolves into austenite, improving hardenability. In steel, some Cr dissolves into ferrite, resulting in solid solution strengthening and increasing the strength and hardness of the ferrite. Cr also reduces the tendency of steel to overheat and the rate of surface decarburization. However, excessively high Cr content can combine with carbon in the steel, easily forming large carbides. These insoluble carbides reduce the steel's ductility and toughness. Furthermore, excessively high Cr content results in excessively hard steel, which is detrimental to customer processing and use. Considering all these factors, the Cr content range in this invention is determined to be 0.30–0.50%.

[0027] Ni content requirements

[0028] Ni is a non-carbide-forming element. Ni exists in steel in solid solution form, providing solid solution strengthening, but this effect is relatively weak. When Ni is used in combination with Cr, it can significantly improve hardenability. Higher Ni content makes it easier for bainite to form in steel. In this invention, the Ni content is determined to be in the range of 0.2% to 0.3%.

[0029] Mo content requirements

[0030] Mo is a non-specially added element. It can exist as a residual element or as some non-essential added elements. Mo will inevitably exist in the raw materials during the steelmaking process. When the Mo content is high, it will increase the strength but deteriorate the plasticity and toughness of the steel. Therefore, this application limits the upper limit of its content, requiring Mo: ≤0.05%.

[0031] Another objective of this invention is to provide a method for manufacturing high wear-resistant steel for passenger vehicle outer wheels, mainly including step one, steel smelting; step two, casting steel billets; step three, hot forming; and step four, sawing and cooling. The specific requirements and implementation methods of the main processes are as follows:

[0032] Step one of the steelmaking process involves: electric arc furnace primary refining, refining, and vacuum degassing, among which...

[0033] The electric arc furnace primary smelting process preferably controls the proportion of pretreated molten iron in the raw materials fed into the furnace to 50%–60%, with the remainder being scrap steel. The scrap steel is baked before entering the furnace, and scrap steel with low Cr, Mo, Ni, and Cu content is selected. Power is supplied to the electric arc furnace to form a molten pool, melting the scrap steel and raising the temperature. The carbon content at the end of the smelting process is controlled to be below 0.04%, and the tapping temperature is 1580–1620℃. Slag is blocked during the tapping process to prevent the formation of oxidizing slag. Furthermore, due to the low carbon content at tapping, there is a kinetic relationship between the C and O contents in the molten steel; when the C content is low, the probability of a high O content is extremely high. Therefore, correspondingly, Al blocks and / or Al particles are used during the tapping process for strong deoxidation of the molten steel and partial alloying with Mn and Al elements to prevent over-oxidation of the molten steel.

[0034] The refining process involves argon stirring, with the intensity of the chemical argon stirring adjusted to prevent the molten steel from being exposed. One or more of aluminum powder, aluminum granules, and carbon powder are used for diffusion deoxidation, and Al wire is used for precipitation deoxidation. Through precipitation and diffusion deoxidation, the non-metallic inclusion content in the molten steel is significantly reduced. By using baked alloys, the content of Cu, Ni, and other components is adjusted to meet the specified requirements. The N content in the molten steel is sampled and analyzed. The Al / N ratio in the molten steel is controlled to be ≥3 by supplementing with nitrogen and manganese wire to achieve the grain-refining effect of AlN.

[0035] The vacuum degassing process involves the ladle entering a vacuum degassing tank, being evacuated to the vacuum level specified in the process, and maintaining this high vacuum for a sufficient time to reduce the content of harmful gases in the molten steel. After the vacuum is broken, argon is gently blown for more than 30 minutes to ensure that inclusions are fully floated to the surface and guarantee the purity of the steel.

[0036] Step two, casting the steel billet, involves either producing a continuously cast billet from molten steel using continuous casting or by first casting into an ingot and then hot-working the ingot into a billet. Considering improved production efficiency and cost savings, continuous casting is more advantageous than ingot casting and is the preferred casting method. The continuous casting process is crucial to the entire smelting process. Given the high [Al] content and [S] element in the steel used in this application, the risk of molten steel nodule formation is high. Improper control of the continuous casting process parameters can lead to surface cracking or dead nozzle formation on the billet, resulting in scrap. Specific measures for optimizing the continuous casting process include: sealing the molten steel from the ladle to the tundish and then to the crystallizer to isolate it from air and control secondary oxidation; using low superheat casting (10℃~20℃) and a suitable casting speed of 0.8~1.3m / min to improve internal quality; and requiring slow cooling in a pit at a temperature above 400℃ for more than 12 hours to prevent cracking.

[0037] Step 3 Hot forming: Preferably, the steel billet is heated to 1050-1080℃ and held for 3-4 hours. After the steel billet is taken out of the furnace, it is descaled by high pressure water and rolled by a continuous rolling mill to hot roll into the required specifications. The initial rolling temperature is 950-1020℃ and the final rolling temperature is 780-820℃. The rolling is controlled in the two-phase region, with an average pass compression ratio of 1.1-1.2 and 10-15 passes. After rolling, it is slowly cooled to obtain round steel with ferrite and pearlite structure.

[0038] This application describes a high wear-resistant automotive outer wheel steel. Through a reasonable alloy composition ratio and excellent steel purity, during the rolling contact fatigue test according to YB / T 5345-2014 "Metallic Materials Rolling Contact Fatigue Test Method", after medium-frequency induction hardening of the sample, the surface layer of the sample has a relatively fine martensitic structure. According to the microstructure classification of standard JB / T9204 "Metallographic Inspection of Induction Hardened Steel Parts", it is grade 5, the hardness of the hardened layer reaches above 60 HRC, and the depth of the hardened layer reaches above 2 mm. Under the conditions of 800 rpm, slip rate of 20%, lubrication with 32# engineering machinery lubricating oil, and 5250 MPa, the characteristic life Vs can reach more than 900,000 cycles.

[0039] Compared with the prior art, the advantages or features of the present invention are as follows:

[0040] (1) Through chemical composition design, the plasticity, toughness and machinability of the steel are guaranteed to fully meet the design requirements of steel for outer wheels of passenger car transmission system: hardness ≤255HBW; hot rolled state Rp0.2≥520MPa, Rm≥800MPa, elongation A5≥20%, reduction of area Z≥50%.

[0041] (2) The elements that improve strength in the chemical composition are restricted. The grains are refined by controlling Al and N and the Al / N ratio ≥3. The grain size is controlled at level 5 or above to achieve fine grain strengthening.

[0042] (3) During steel refining, diffusion deoxidation and long-term high-vacuum degassing processes are adopted to reduce the content of non-metallic inclusions and reduce fatigue sources in steel in the later stage. The inclusion level reaches Class A ≤ 2.0, Class B ≤ 1.0, Class C ≤ 0.5, and Class D ≤ 1.0.

[0043] (4) By adding Cu, the N content is increased, which improves the contact fatigue performance of steel after medium frequency induction hardening, and the characteristic life can reach more than 900,000 cycles. Attached Figure Description

[0044] Figure 1 This is a part drawing of the high wear-resistant automotive outer wheel of this application;

[0045] Figure 2 This is a typical metallographic diagram of the steel used for high wear-resistant automotive outer wheels in this application;

[0046] Figure 3 Rolling contact fatigue test specimens were used for the high wear-resistant automotive outer wheel steel of this application.

[0047] Figure 4 The PN curves are obtained from rolling contact fatigue tests on the high wear-resistant automotive outer wheel steel of this application after induction hardening. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0049] Manufacturing method of high wear-resistant steel bars for automotive outer wheels: The production process for steel bars with specifications of Φ40mm-Φ70mm is as follows: electric furnace primary refining—refining—vacuum degassing—continuous casting—continuous casting billet cooling—continuous casting billet heating—rolling—cooling—finishing—inspection—finished product warehousing. Details are as follows:

[0050] The steel specification for Example 1 is Φ40mm, with a cumulative rolling ratio of 81.2.

[0051] The electric arc furnace has a loading capacity of 100 tons, including 50 tons of molten iron and 50 tons of scrap steel. During the electric arc furnace smelting process, oxygen is blown through the furnace, the tapping temperature is 1599℃, and the final carbon content is 0.03%. Ferromanganese, ferroaluminum, lime, and refining slag are added during tapping; the ladle is then hoisted into the LF furnace.

[0052] The LF furnace is powered on. After the slag is oxidized, aluminum powder, aluminum granules, and carbon powder are added in batches for diffusion deoxidation. Al wire is fed in for precipitation deoxidation. At the same time, the chemical composition of Cu, Ni and other components is adjusted to meet the specifications. In particular, the Al / N ratio is precisely controlled by feeding nitrogen and manganese wire to ensure that the white slag time is 15 minutes and the temperature at the LF furnace outlet is 1575℃.

[0053] After the ladle is placed in position, the vacuum tank cover is closed, and the vacuum is evacuated to 0.3 mbar and maintained for 5 minutes to remove harmful gaseous elements from the steel. After venting, soft argon blowing is performed for 35 minutes, and the ladle temperature is 1569℃.

[0054] The continuous casting process is carried out under protective conditions to isolate the molten steel from the air. The tundish temperature is 1555℃, the superheat is 12-22℃, and the casting speed is set at 1.5m / min. The resulting continuously cast billet is then slowly cooled in the pit at 600℃ for 24 hours.

[0055] The continuously cast billet is heated to 1040℃ and held for 2 hours. The initial rolling temperature is 1010~1050℃, and the final rolling temperature is 810~840℃. The continuously cast billet is rolled into round steel in the two-phase region. The round steel is then slowly cooled.

[0056] Example 2 involves steel with a specification of Φ70mm and a rolling ratio of 26.5.

[0057] The electric arc furnace has a loading capacity of 100 tons, including 50 tons of molten iron and 50 tons of scrap steel. During the electric arc furnace smelting process, oxygen is blown through the furnace. The tapping temperature is 1597℃, and the final carbon content is 0.032%. Ferromanganese, ferroaluminum, lime, and refining slag are added during tapping. The ladle is then hoisted into the LF furnace.

[0058] The LF furnace is powered on. After the slag is oxidized, aluminum powder, aluminum granules, and carbon powder are added in batches for diffusion deoxidation. Al wire is fed in for precipitation deoxidation. At the same time, the chemical composition of Cu, Ni and other components is adjusted to meet the specifications. In particular, the Al / N ratio is precisely controlled by feeding nitrogen and manganese wire to ensure that the white slag time is 15 minutes and the temperature at the LF furnace outlet is 1577℃.

[0059] After the steel ladle is placed in position, the vacuum tank cover is closed, and the vacuum is evacuated to 0.3 mbar and maintained for 5 minutes to remove harmful gaseous elements from the steel. After venting, soft argon blowing is performed for 33 minutes, and the ladle temperature is 1570℃.

[0060] The continuous casting process is protected during pouring, isolating the molten steel from the air. The tundish temperature is 1559℃, the superheat is 11-22℃, and the casting speed is set at 1.5m / min. The resulting continuous casting billet is then slowly cooled in the pit at 600℃ for 24 hours.

[0061] The continuously cast billet is heated to 1040℃ and held for 2 hours. The initial rolling temperature is 1015~1050℃ and the final rolling temperature is 810~840℃. The continuously cast billet is rolled into round steel in the two-phase region. The round steel is then slowly cooled.

[0062] The chemical composition of the steel involved in the above embodiments is shown in Table 1.

[0063] Table 1 Chemical composition (%)

[0064] 1 0.53 0.27 0.79 0.002 0.018 0.25 0.26 0.39 0.01 0.022 0.021 2 0.54 0.28 0.79 0.003 0.021 0.26 0.25 0.38 0.01 0.024 0.020

[0065] The non-metallic inclusions in the above embodiments were tested according to ISO 4967, as shown in Table 2 below.

[0066] Table 2 Non-metallic inclusion levels

[0067] 1 1.5 1.0 0.5 0.0 0.0 0.0 0.5 0.5 2 1.5 0.5 0.5 0.0 0.0 0.0 0.5 0.5

[0068] The tensile strength and hardness of the above embodiments were directly tested. The tensile strength was tested according to EN ISO 6892-1 standard, and the hardness was tested according to EN ISO 6507-1 standard, as shown in Table 3.

[0069] Table 3 Tensile strength, hardness, and microstructure

[0070] 1 Hot-rolled 562 875 27 54 236 2 Hot-rolled 570 889 27 55 241

[0071] The rolling contact fatigue tests were conducted on the samples that had undergone medium-frequency induction hardening in the above examples according to YB / T 5345-2014 "Metallic Materials Rolling Contact Fatigue Test Method". The results are shown in Table 4.

[0072] Table 4 Rolling Contact Fatigue Life

[0073] 1 934347 2 952667

[0074] The product in the example has a metallographic structure of ferrite and pearlite spheres, and exhibits good strength, hardness, and plasticity.

[0075] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A type of high wear-resistant steel for outer gears of passenger vehicles, characterized in that: The chemical composition of the steel, by mass%, contains: C: 0.50–0.56%, Si: 0.20–0.40%, Mn: 0.75–0.85%, P: ≤0.010%, S: 0.010–0.030%, Cr: 0.30–0.50%, Ni: 0.20–0.30%, Cu: 0.20–0.40%, Mo: ≤0.05%, Al: 0.010–0.050%, N: 0.015–0.025%, with the balance being Fe and unavoidable impurity elements; The microstructure of the steel is ferrite and pearlite; The method for manufacturing the steel includes the following steps: Step 1: Steel smelting Steelmaking includes primary refining, refining, and vacuum degassing. The carbon content at the end of primary refining is controlled to be below 0.04%, and the tapping temperature is 1580-1620℃. Slag is blocked during tapping to prevent the addition of oxidizing slag. In addition, Al blocks and / or Al particles are used to strongly deoxidize the steel and partially alloy it with Mn and Al elements during tapping. Argon stirring is used in the refining process. The intensity of argon stirring should be such that the molten steel is not exposed. One or more of aluminum powder, aluminum granules, and carbon powder are used for diffusion deoxidation, and Al wire is used for precipitation deoxidation. The Cu and Ni alloy composition is adjusted to meet the specified content requirements. The N content in the molten steel is sampled and analyzed. The Al / N ratio in the molten steel is controlled to be ≥3 by supplementing with nitrogen and manganese wire to give full play to the grain refining effect of AlN. Vacuum degassing removes light elements from molten steel; after breaking the vacuum, argon is gently blown in for more than 30 minutes. Step 2: Casting steel billets The molten steel obtained in step one is cast into steel billets; Step 3: Thermoforming The billet is heated to fully austenitize the microstructure. After exiting the furnace, it is descaled by high-pressure water and rolled on a continuous rolling mill to achieve the design specifications. The initial rolling temperature is 950-1020℃, and the final rolling temperature is 780-820℃. The rolling is controlled in the two-phase region with an average pass compression ratio of 1.1-1.2 and 10-15 passes. After rolling, it is slowly cooled to obtain round steel with ferrite and pearlite microstructure. Step 4: Sawing and Cooling After rolling, the round steel bars are subjected to slow cooling in a pile. The steel is produced in bars with a diameter of Φ40mm-Φ70mm and a hardness of ≤255HBW; in the hot-rolled state, Rp0.2≥520MPa, Rm≥800MPa, elongation A5≥20%, and reduction of area Z≥50%. During the rolling contact fatigue test of the steel according to the rolling contact fatigue test method of metallic materials in YB / T 5345-2014, after medium-frequency induction hardening of the sample, the surface layer of the sample is martensitic. According to the microstructure classification of metallographic inspection of induction hardened steel parts in standard JB / T9204, it is grade 5. The hardness of the hardened layer reaches more than 60 HRC and the depth of the hardened layer reaches more than 2 mm. Under the conditions of 800 rpm, slip rate of 20%, lubrication with 32# engineering machinery lubricating oil, and 5250 MPa, the characteristic life Vs reaches more than 900,000 cycles.

2. The high wear-resistant steel for passenger vehicle outer wheels according to claim 1, characterized in that: The steel is delivered in a hot-rolled state, specifically as hot-rolled black steel.

3. A method for manufacturing the high wear-resistant steel for passenger vehicle outer wheels as described in claim 1, characterized in that: include: Step 1: Steel smelting Steelmaking includes primary refining, refining, and vacuum degassing. The carbon content at the end of primary refining is controlled to be below 0.04%, and the tapping temperature is 1580-1620℃. Slag is blocked during tapping to prevent the addition of oxidized slag. In addition, Al blocks and / or Al particles are used to strongly deoxidize the steel and partially alloy it with Mn and Al elements during tapping. Argon stirring is used in the refining process. The intensity of argon stirring should be such that the molten steel is not exposed. One or more of aluminum powder, aluminum granules, and carbon powder are used for diffusion deoxidation, and Al wire is used for precipitation deoxidation. The Cu and Ni alloy composition is adjusted to meet the specified content requirements. The N content in the molten steel is sampled and analyzed. The Al / N ratio in the molten steel is controlled to be ≥3 by supplementing with nitrogen and manganese wire to give full play to the grain refining effect of AlN. Vacuum degassing removes light elements from molten steel; after breaking the vacuum, argon is gently blown in for more than 30 minutes. Step 2: Casting steel billets The molten steel obtained in step one is cast into steel billets; Step 3: Thermoforming The billet is heated to fully austenitize the microstructure. After exiting the furnace, it is descaled by high-pressure water and rolled on a continuous rolling mill to achieve the design specifications. The initial rolling temperature is 950-1020℃, and the final rolling temperature is 780-820℃. The rolling is controlled in the two-phase region with an average pass compression ratio of 1.1-1.2 and 10-15 passes. After rolling, it is slowly cooled to obtain round steel with ferrite and pearlite microstructure. Step 4: Sawing and Cooling After rolling, the round steel is subjected to slow cooling in a pile.

4. The method according to claim 3, characterized in that: In step one, 50% to 60% of the raw materials fed into the furnace are pretreated molten iron, and the remainder is scrap steel. The scrap steel is baked before being fed into the furnace, and scrap steel with the lowest possible Cr, Mo, Ni and Cu content is selected.

5. The method according to claim 3, characterized in that: In step two, molten steel is cast into billets using a continuous casting process. The molten steel is poured through a sealed system from the ladle to the tundish and then to the crystallizer to isolate it from the air. The continuous casting process uses low superheating: the superheat is 10℃~20℃, and a suitable casting speed is provided: 0.8~1.3m / min. The obtained continuous casting billets are slowly cooled in the pit at a temperature above 400℃ for more than 12 hours.

6. The method according to claim 3, characterized in that: In step three, before rolling, the billet is heated to 1050-1080°C and held for 3-4 hours to austenitize the microstructure.

Images