A carburized bearing steel for heavy rail rollers and a method for producing the same

Abstract

The present application relates to a kind of carburizing bearing steel for heavy load railway roller, the chemical composition of the steel material is as follows: C:0.17-0.23%, Si:0.15-0.40%, Mn:0.60-0.90%, P:≤0.020%, S:≤0.025%, Cr:0.35-0.85%, Al:0.020-0.060%, Cu≤0.30%, Mo≤0.10-0.30%, Ni:0.40-0.80%, As+Sn+Sb≤0.065%, Ti≤0.0010%, Ca≤0.0005%, O≤0.0008%, N:0.008-0.012%, the balance is Fe and inevitable impurity elements.The present application also provides a kind of carburizing bearing steel manufacturing method for heavy load railway roller: using continuous casting CCM large continuous casting billet-slow cooling-heating rolling into intermediate billet-slow cooling-surface treatment-heating rolling wire rod-slow cooling-finishing forming production method.The organization of the steel material of the present application is more uniform than electroslag steel, grain size is smaller, harmful elements are less, DI value fluctuation is more stable, heart hardenability is better, production cost is lower, significantly improve the quality level and market competitiveness of the steel material.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a carburized bearing steel for heavy-duty railway rollers and its production method. Background Technology

[0002] In recent years, with the continuous changes in my country's railway transportation pattern, the pace of upgrading and replacement of railway freight cars has accelerated, and the maintenance cycle has been extended. Under these circumstances, higher requirements have been placed on the components used in the vehicles. As an important component of railway freight cars, the performance of rolling bearings directly affects the safe operation of the entire railway vehicle. This inevitably places higher demands on the quality control of the rollers, the core component of railway bearings.

[0003] Railway rollers are generally large in size and operate under high-load contact stress, frequently enduring impact and wear. Therefore, in addition to the requirements for general bearings, the rolling elements of these large bearings also require sufficient toughness, high compressive strength, and hardness in the core. Carburized bearing steel is a suitable material for this purpose. Carburized steel, after carburizing, quenching, and low-temperature tempering, exhibits high surface hardness, wear resistance, and contact fatigue strength, while the core has a low carbon content and good toughness, allowing it to withstand significant impact loads. This makes it highly suitable for manufacturing rolling elements in large bearings.

[0004] Currently, the national standard GB / T 3203 for carburized bearing steel clearly stipulates that high-grade carburized steel must be produced using the electroslag remelting process. Steel produced by electroslag remelting has many quality advantages, such as fine and uniformly distributed non-metallic inclusions, high microstructure uniformity, and high density, resulting in consistently good quality stability. However, the electroslag remelting process also has significant disadvantages, including very low production efficiency and capacity, and very high energy consumption and production costs. Therefore, electroslag remelted steel has very low market competitiveness.

[0005] Patent CN 114774771 A discloses a carburized bearing steel for heavy-load rolling mill bearings and its production method. It adjusts the content of trace alloying elements such as Nb and V, and the Ni content is as high as 3.25-3.75%. The addition of trace elements increases the difficulty of smelting and the intensity of labor. At the same time, the high Ni content will significantly increase the cost. In this invention, the grain size and properties are adjusted only by Al and N. While meeting the working conditions required for railway rollers, the production cost is greatly reduced.

[0006] Patent CN 112030065 B discloses a carburized bearing steel and its preparation method. The smelting methods described in this invention include electroslag refining, double vacuum smelting, ladle refining, or vacuum induction furnace smelting. However, this invention employs a high-efficiency, high-capacity, and low-cost process route involving vacuum degassing, continuous casting, and rolling. By optimizing and controlling key processes, the steel achieves higher purity, smaller grain size, more uniform microstructure, and a more stable DI value, meeting the requirements for heavy-duty railway bearings. This significantly improves production efficiency and capacity while substantially reducing energy consumption and production costs, thus giving it a greater competitive advantage.

[0007] This invention relates to a high-quality steel, G20CrNiMo, which is based on the technical requirements of the existing national standard GB / T 3203. Through optimized composition design and a high-efficiency, high-capacity, low-cost process route involving vacuum degassing, continuous casting, and rolling, along with optimized control of key processes, the steel achieves higher purity, smaller grain size, more uniform microstructure, and a more stable DI value. This improves the contact fatigue performance of carburized bearing steel, meeting the requirements of heavy-duty railway bearings. Furthermore, it replaces the current electroslag remelting production process, significantly increasing production efficiency and capacity while substantially reducing energy consumption and production costs, thus giving it a greater competitive advantage. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a carburized bearing steel for heavy-duty railway roller bearings and a method for manufacturing the same, so that the steel meets the quality requirements for carburized bearing steel for heavy-duty railway roller bearings and has strong market competitiveness.

[0009] The technical solution adopted by the present invention to solve the above problems is as follows: a carburized bearing steel for heavy-duty railway rollers, wherein the chemical composition and mass percentage of the steel plate are: C: 0.17-0.23%, Si: 0.15-0.40%, Mn: 0.60-0.90%, P: ≤0.020%, S: ≤0.025%, Cr: 0.35-0.85%, Al: 0.020-0.060%, Cu≤0.30%, Mo≤0.10-0.30%, Ni: 0.40-0.80%, As+Sn+Sb≤0.065%, Ti≤0.0010%, Ca≤0.0005%, O≤0.0008%, N: 0.008-0.012%, with the balance being Fe and unavoidable impurity elements.

[0010] This steel is produced using a continuous casting process, overcoming the significant disadvantages of existing electroslag remelting processes, such as low production efficiency, low capacity, high energy consumption, and high production costs. Through optimized composition, smelting, and continuous casting processes, along with appropriate heating and rolling processes, the resulting wire rod exhibits excellent quality, meeting the following requirements: inclusions: low magnification should not exceed S1, R1, C1; non-metallic inclusions: A coarse ≤ 1.5, A fine ≤ 2.5, B coarse ≤ 0.5, B fine ≤ 1.5, D coarse ≤ 0.5, D fine ≤ 1.0, DS ≤ 1.0; DI value 2.2~2.6; decarburization ≤ 0.10mm; grain size ≤ grade 8.

[0011] The following details the function and dosage selection of the components contained in this invention:

[0012] C: Carbon is a crucial element affecting the tensile strength and yield strength of steel. It also significantly influences the toughness of steel. Carbon's role in steel is a double-edged sword; as the carbon content increases, the yield point and tensile strength of the steel rise, but its ductility and impact resistance decrease significantly. Even slight fluctuations in carbon content can lead to noticeable changes in the steel's strength and toughness. The magnitude of these fluctuations across the steel's cross-section determines the stability of its strength and toughness. Therefore, the carbon content range selected in this invention is C: 0.17–0.23%.

[0013] Si: Silicon can form alloys with iron in steel, altering its atomic structure. This alloying effect makes the steel stronger and more durable. The interaction between silicon and iron atoms increases the lattice strength of the steel, thereby improving its overall strength. Furthermore, adding ferrosilicon in the early stages of refining can also assist in deoxidation. In this invention, the Si content is controlled at 0.15–0.40%.

[0014] Mn: Adding an appropriate amount of ferromanganese during the steelmaking process can play a good deoxidation role under the strong argon gas stirring in the early stage of refining. The manganese element in the steel can effectively improve the strength of the steel. However, a high manganese content will cause the steel of this invention to produce more obvious temper brittleness. The control range is 0.60 to 0.90%.

[0015] P: Phosphorus in this invention is a harmful element. At room temperature, phosphorus can make steel brittle, leading to a significant decrease in its strength and toughness. Therefore, the phosphorus content in the steel of this invention is selected within the range of P: ≤0.020%.

[0016] S: Sulfur in the steel of this invention easily combines with manganese to form manganese sulfide inclusions, which leads to a decrease in the tensile strength and impact performance of the steel. Therefore, the sulfur content in the steel must be strictly controlled. The selection range of sulfur in the steel is S: ≤0.025%.

[0017] Cr and Mo: Both Cr and Mo are elements that enhance hardenability. For 5.5Ni steel plates with low C and low Ni content, additional Cr and Mo are needed to compensate for the insufficient C content and improve hardenability. Simultaneously, they facilitate the diffusion of alloying elements into austenite, stabilizing the austenite microstructure. However, it is undeniable that Cr and Mo have a negative impact on low-temperature impact toughness. Therefore, this invention controls their content at Cr: 0.35–0.85%; Mo ≤ 0.10–0.30%.

[0018] Nitrogen (N): Nitrogen reacts with iron in steel to form hard and brittle compounds, and also combines with other elements to form corrosion-resistant compounds, improving the corrosion resistance of steel. Furthermore, it readily induces the formation of a tough and wear-resistant martensitic structure in steel, enhancing its hardness and strength. Simultaneously, at high temperatures, nitrogen can prevent excessive grain boundary growth, preventing excessive microstructure refinement during quenching, thus reducing quenching embrittlement and improving the steel's toughness, extending its service life. In this invention, the nitrogen content in the steel is set in the range of 0.008–0.012%, more preferably 0.009–0.010%.

[0019] Al: Besides acting as a deoxidizer to reduce the oxygen content in steel, aluminum can also form aluminum nitride with nitrogen, refining grains and providing strong solid solution strengthening, thus improving tempering stability. To control the residual austenite ratio in the final product and obtain better hardness and wear resistance, the Al content should be designed to be greater than 0.020%. However, since the deoxidation product Al2O3 is brittle, it breaks during forging and rolling, forming chain-like bands along the deformation direction, affecting fatigue performance. Furthermore, steel with high aluminum content has poorer surface quality in the cast billet. To ensure comprehensive performance, the Al content range of the steel in this invention is set to 0.020–0.060%, more preferably 0.035–0.045%.

[0020] Ni (Ni): It can improve the strength of steel while maintaining good plasticity and toughness. It is insoluble in carbides and completely enters austenite, forming an infinite solid solution, thus fully utilizing its effect of improving hardenability. It refines ferrite grains, improving the plasticity and toughness of steel, especially low-temperature toughness, under the same strength conditions. The Ni content in this invention is determined to be in the range of 0.40%–0.80%.

[0021] Another objective of this invention is to provide a method for producing carburized bearing steel for heavy-duty railway rollers, which uses continuous casting instead of electroslag remelting to smelt the billet. The main steps are as follows:

[0022] (1) Smelting: This steel is smelted entirely with molten iron. Refining uses a combination of precipitation and diffusion for deoxidation. Nitrogen is used throughout the refining stirring and RH boosting process. The alloy composition is not adjusted after refining, and the purity of the molten steel is strictly controlled.

[0023] (2) Continuous casting: Advanced equipment and processes such as first and last electromagnetic stirring, weak cooling and light pressure are used to match the casting process. The superheat of the casting is controlled at 10-20℃ to obtain 390mm*510mm continuous casting billets with excellent segregation quality.

[0024] (3) Slow cooling of continuous casting billet: Slow cooling of continuous casting billet involves slowly cooling the continuous casting billet in a pit, wherein the temperature in the pit is greater than 650℃, the slow cooling time in the pit is greater than 48 hours, and the temperature when it comes out of the pit is less than 200℃.

[0025] (4) Rolling into intermediate billets: In a furnace with a neutral or weakly oxidizing atmosphere, the heating rate is 150-250℃ / h, and the temperature is raised to 1020-1220℃ and held for 4.5-7.5h; the billet is rolled using a high compression ratio rolling process, with an initial rolling temperature ≥1050℃, an initial rolling compression ratio ≥2.5, a continuous rolling temperature ≥980℃, a final rolling temperature ≥800℃, and a line exit temperature ≥500℃. The total compression ratio of the billet rolling is ≥4.5, and the billet is rolled to 120-240mm. 2 Square central blank.

[0026] (5) Rolling wire rod: Heat the middle square billet at a heating rate of 600~800℃ / h; heat to 1050±30℃ and hold for 0.5~1h; control the rolling of wire rod by setting the starting rolling temperature ≥900℃, the initial rolling compression ratio ≥1.5, the intermediate rolling temperature ≥900℃, the finishing rolling temperature 930±30℃, the wire drawing temperature 900±30℃, and the total compression ratio ≥40.

[0027] (6) Cooling control: After spinning, the wire rod is air-cooled to a temperature of 700-800℃, and then slowly cooled to ≤450℃ in an insulation cover before being coiled and unwound.

[0028] (7) Inspection: The DI value, grain size, inclusions and decarburization of the offline wire rod are inspected. If all are qualified, the hot-rolled wire rod of carburized bearing steel for heavy-duty railway rollers is obtained.

[0029] The metallographic structure of the hot-rolled wire rod obtained by this invention is a pearlite + bainite + ferrite structure with a lamellar spacing of 0.10-0.25 μm. This structure is easy to spheroidize during subsequent heat treatment. After quenching and tempering, the structure is fine acicular martensite + nano austenite + fine granular carbides. About 15% of the retained austenite in the structure is mainly in the form of ultrafine blocky morphology. The ultrafine blocky retained austenite is relatively stable and helps to improve the fatigue life of the rolling elements.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] 1) In the initial stage of refining, the present invention adopts a composite deoxidation method, and no composition adjustment is made in the final stage of refining. RH vacuum and soft blowing treatment fully float Al2O3 and large particle composite inclusions to the surface, thereby improving the cleanliness of the molten steel.

[0032] 2) The present invention adopts a converter + refining LF + vacuum degassing RH + continuous casting process, which can significantly reduce harmful elements O, Ti and Ca compared with the electroslag remelting process. Therefore, O can be controlled at ≤0.0008%, Ti at ≤0.0010%, Ca at ≤0.0005%, and As+Sn+Sb at ≤0.065%.

[0033] 3) During the continuous casting process, low superheat (10-20℃) casting is adopted, and heavy pressure is applied at the end to reduce the thickness by more than 12mm. Combined with high current electromagnetic stirring (>300A) at the end, the core segregation is reduced and the segregation of the billet is effectively improved.

[0034] 4) In the initial rolling process, high reduction rolling is adopted. By combining large cross-section billet casting with non-uniform temperature rolling in the production process, and combining the production technology of high reduction in initial rolling, the matrix structure in the steel can be greatly refined and homogenized.

[0035] 5) Slow cooling treatment is applied to the continuously cast billets and intermediate billets in the pit to avoid surface cracking caused by changes in surface and internal thermal stress and structural stress, thus ensuring the surface quality of the wire rod.

[0036] 6) This invention performs controlled rolling and cooling treatment on wire rod, with high-temperature initial rolling and low-temperature final rolling, to obtain a hot-rolled pearlite + bainite + ferrite structure. The structure after quenching and tempering can significantly improve fatigue life.

[0037] 7) The heavy-duty railway roller carburized bearing steel wire rod produced by this invention has high purity, shallow decarburization, fine grains, good material homogeneity, low production cost, and high production efficiency. Compared with the electroslag process, it greatly improves the yield, improves the environment, meets customers' low-carbon needs, and can achieve ultra-long service life, increasing customer loyalty. Attached Figure Description

[0038] Figure 1 This is a 100X magnified metallographic photograph of the material structure in an embodiment of the present invention.

[0039] Figure 2 This is a 500X magnified metallographic photograph of the material structure in an embodiment of the present invention.

[0040] Figure 3 This is a metallographic photograph with a grain size magnified 200X in an embodiment of the present invention. Detailed Implementation

[0041] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0042] Implementation Cases 1-2

[0043] The carburized bearing steel for heavy-duty railway rollers in this invention is manufactured according to the following procedures:

[0044] 1) Smelting: After smelting in a 100-ton steelmaking furnace, the steel is refined outside the furnace, and then the molten steel is degassed under vacuum. The mass percentage of each chemical element is strictly controlled according to requirements.

[0045] 2) Continuous casting: The billets are continuously cast into 390mm × 510mm square billets, with the superheat in the tundish controlled at 10–20℃. To control material segregation, advanced end-stage electromagnetic stirring is used during continuous casting, with a current >300A and a total light reduction of 10mm. The percentage of chemical composition of the resulting continuously cast billets is shown in Table 1 below.

[0046] Table 1. (wt.%), balance is Fe and other unavoidable impurity elements.

[0047]

[0048]

[0049] 3) The continuous casting billet was placed in the pit at a temperature of 680℃, and the slow cooling time in the pit was 52 hours. The temperature when it came out of the pit was 85℃.

[0050] 4) Billet Heating + Opening: The billet is cold-charged into a walking beam furnace and rapidly heated at 200℃ / h to 1100~1200℃ for homogenization and holding for ≥4.5 hours. The initial rolling temperature is ≥1050℃, and the final rolling temperature is ≥800℃. The billet is rolled to a thickness of 150mm. 2 The intermediate blank.

[0051] The main specific process parameters in the above embodiments are shown in Table 2 below:

[0052] Table 2. (Main specific process parameters for billet heating and hot rolling)

[0053]

[0054] 5) Intermediate billet heating + rolled wire rod + controlled cooling of wire rod: After cleaning the surface of the intermediate billet to a depth of 2.0 mm, it is cold-loaded into the furnace and rapidly heated to 800℃ over 1 hour. It is then heated again over 0.5 hours to the soaking zone temperature of 1050±30℃ and held at that temperature for 0.5–1 hour. The initial rolling temperature is set to ≥900℃, the intermediate rolling temperature to ≥900℃, the finishing rolling temperature to 930±30℃, and the wire drawing temperature to 900±30℃. After wire drawing, the wire rod is air-cooled to 700–800℃, then slowly cooled to ≤450℃ in an insulation hood before being coiled and removed from the production line.

[0055] The main specific process parameters in the above embodiments are shown in Table 3 below:

[0056] Table 3. (Main specific process parameters for intermediate billet heating and hot rolling)

[0057]

[0058] The finished wire rods of carburized bearing steel for heavy-duty railway rollers in Examples 1 and 2 were subjected to relevant tests. The purity, grain size, carbides, and core segregation were measured and are shown in Table 4 below.

[0059] Table 4 (Purity, Grain Size, DI Value and Segregation Results of Finished Wire Rods)

[0060]

[0061] As can be seen from Table 4, the inclusions, grain size, DI value, and central segregation in the examples meet the design requirements, indicating that the material has very high cleanliness, fine grains, and excellent homogeneity. Furthermore, the use of a new composition optimization design has greatly reduced costs, resulting in low-cost carburized railway rollers with high fatigue life.

[0062] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing carburized bearing steel for heavy-duty railway rollers, characterized in that: The chemical composition and mass percentage of the steel are as follows: C: 0.17–0.23%, Si: 0.15–0.40%, Mn: 0.60–0.90%, P: ≤0.020%, S: ≤0.025%, Cr: 0.35–0.85%, Al: 0.020–0.060%, Cu ≤0.30%, Mo: 0.10–0.30%, Ni: 0.40–0.80%, As+Sn+Sb ≤0.065%, Ti ≤0.0010%, Ca ≤0.0005%, O ≤0.0008%, N: 0.008–0.012%, with the balance being Fe and unavoidable impurities. The bearing steel's low magnification is not greater than S1, R1, and C1. Non-metallic inclusions: A coarse ≤1.5, A fine ≤2.5, B coarse ≤0.5, B... Fineness ≤ 1.5, Coarseness (D) ≤ 0.5, Fineness (D) ≤ 1.0, Dimer (DS) ≤ 1.0; Di (DI) value 2.2~2.6; Decarburization ≤ 0.10mm; Grain size ≤ Grade 8; The metallographic structure of the bearing steel is pearlite + bainite + ferrite with a lamellar spacing of 0.10~0.25μm. This structure is easy to spheroidize during subsequent heat treatment. After quenching and tempering, this structure is fine acicular martensite + nano-austenite + fine-grained carbides. 15% of the retained austenite in the structure is mainly in the form of ultrafine blocky morphology. The method includes the following steps: (1) Smelting: All iron smelting, refining uses precipitation and diffusion combined deoxidation, refining stirring and RH boosting gas use nitrogen throughout the process, the alloy composition is not adjusted after refining, and the purity of molten steel is strictly controlled; (2) Continuous casting: The process of first and last electromagnetic stirring, weak cooling and light pressure reduction is matched, and the superheat of casting is controlled at 10-20℃ to obtain 390mm*510mm continuous casting billet with excellent segregation quality. (3) Slow cooling of continuous casting billet: The continuous casting billet is slowly cooled in the pit, and the temperature when it comes out of the pit is less than 200℃; (4) Rolling into intermediate billets: The billet is rolled using a high compression ratio rolling process, with an initial rolling temperature ≥1050℃, an initial rolling compression ratio ≥2.5, a continuous rolling temperature ≥980℃, a final rolling temperature ≥800℃, and a final rolling temperature ≥500℃. The total compression ratio of the billet rolling process is ≥4.5, and the billet is rolled to a thickness of 120~240mm. 2 Square central blank; (5) Rolled wire rod: The wire rod controlled rolling setting is ≥900℃ for the initial rolling temperature, ≥1.5 for the initial rolling compression ratio, ≥900℃ for the intermediate rolling temperature, 930±30℃ for the finishing rolling temperature, 900±30℃ for the wire drawing temperature, and ≥40 for the total compression ratio. (6) Cooling control: After spinning, the wire rod is air-cooled to 700~800℃, and then slowly cooled to ≤450℃ in an insulation cover before being coiled and unwound. (7) Inspection: The DI value, grain size, inclusions and decarburization of the offline wire rod are inspected. If all are qualified, the hot-rolled wire rod of carburized bearing steel for heavy-duty railway rollers is obtained.

2. The method for producing carburized bearing steel for heavy-duty railway rollers according to claim 1, characterized in that: In step (3), the temperature in the pit is greater than 650℃ and the slow cooling time in the pit is greater than 48 hours.

3. The method for producing carburized bearing steel for heavy-duty railway rollers according to claim 1, characterized in that: In step (4), before rolling, the temperature is raised at a rate of 150~250℃ / h in a heating furnace with a neutral or weak oxidizing atmosphere, and then held at 1020~1220℃ for 4.5~7.5h.

4. The method for producing carburized bearing steel for heavy-duty railway rollers according to claim 1, characterized in that: In step (5), the middle square billet is heated before the wire rod is rolled, with a heating rate of 600~800℃ / h; the temperature is raised to 1050±30℃ and held for 0.5~1h.

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