Low-rare-earth modified b3 wear-resistant steel ball and preparation method thereof

Abstract

This invention discloses a hot-rolled round bar for low-rare-earth modified B3 wear-resistant steel balls and its preparation method, belonging to the technical field of wear-resistant steel alloy materials. The chemical composition of the round bar, by mass percentage, includes: C 0.58%~0.66%, Si 1.60%~1.90%, Mn 0.65%~0.80%, Cr 0.70%~0.90%, P≤0.025%, S≤0.015%, Ce 0.0070%~0.0074%, La 0.0037%~0.0041%, with the remainder being Fe and impurities; the preparation method is also disclosed. The purpose of this invention is to provide a hot-rolled round bar for low-rare-earth modified B3 wear-resistant steel balls and its preparation method. Without changing the original main alloy system, it achieves a comprehensive effect of purifying the steel, refining the grains, modifying inclusions, weakening segregation, inhibiting decarburization, and simultaneously improving strength and plasticity. It also determines the optimal rare earth ratio and is suitable for long-term complex load conditions of wear-resistant steel balls.

Description

Technical Field

[0001] This invention belongs to the technical field of wear-resistant steel alloy materials, and particularly relates to a hot-rolled round bar for low-rare-earth modified B3 wear-resistant steel balls and its preparation method. It is suitable for the production of wear-resistant steel ball raw materials for ball mills in the mining, building materials, and metallurgical industries. Background Technology

[0002] B3 low-alloy structural steel is the mainstream billet for mid-to-high-end wear-resistant steel balls, achieving high strength and wear resistance through a multi-element alloy system of carbon, silicon, manganese, chromium, and molybdenum. Traditional B3 hot-rolled round steel without rare earth elements has several defects: 1) During smelting and solidification, long strip-shaped sulfide (MnS) inclusions are easily formed, which cut the steel matrix and are prone to cracking under stress, reducing the impact toughness and fatigue life of the steel; 2) The as-cast structure has coarse grains, with a central porosity grade of up to 1.0, obvious radial segregation of composition, and uneven performance between the core and edge of the round steel, resulting in prominent local wear and breakage problems after the steel balls are made; 3) The surface is prone to decarburization during hot rolling and heat treatment, with an average decarburized layer depth of 0.152 mm, resulting in decreased surface hardness and deterioration of wear resistance; 4) The mechanical properties are significantly inferior, with a yield strength of only 1156 MPa, elongation after fracture of 5.0%, reduction of area of ​​9%, poor plasticity, and short service life of wear-resistant steel balls.

[0003] In existing technologies, some companies have attempted to improve steel performance by adding rare earth elements, but there is a problem of blindly controlling the amount of rare earth added: too low an addition will result in insufficient modification, while too high an addition (300 ppm and above) will cause rare earth element segregation at grain boundaries and agglomeration of inclusions, leading to a simultaneous decrease in the hardness, strength, and plasticity of the steel. Currently, the industry lacks a rare earth-modified B3 steel formulation system that is precise in composition, stable in performance, and suitable for industrial mass production.

[0004] Therefore, accurately exploring the reasonable addition range of rare earth elements, determining the optimal dosing amount, and establishing a supporting process scheme are of great practical significance for optimizing the comprehensive performance of B3 low-alloy structural steel. By quantifying the proportion of rare earth addition, we can fully leverage its functions of purifying molten steel, refining grains, and modifying inclusions, thereby eliminating defects such as sulfide inclusions, microstructure segregation, and surface decarburization at the source, and comprehensively improving the strength, toughness, and wear resistance of the steel. At the same time, we can avoid the performance degradation problems caused by excessive rare earth, ultimately forming an industrially feasible formula, promoting the quality upgrade of high-end wear-resistant steel ball billets, and extending the service life of finished products.

[0005] The traditional B3 hot-rolled round bar production process (smelting - continuous casting - hot rolling - heat treatment) has multiple process shortcomings, which, combined with inherent material defects, ultimately lead to poor finished product performance.

[0006] The rare earth addition process is disordered: existing processes mostly add rare earth in the early stage of steelmaking, resulting in high rare earth burn-off rate and uneven distribution; some companies arbitrarily increase the amount of rare earth added to 300 ppm, causing element segregation and agglomeration.

[0007] Insufficient control of solidification process: The grain-refining properties of rare earth elements were not utilized during the continuous casting stage, resulting in well-developed columnar crystals in the billet, with a loose center and severe compositional segregation;

[0008] Lack of surface protection after hot rolling: During the high-temperature rolling and subsequent heating of round steel, the surface carbon elements are easily oxidized and lost, resulting in excessive decarburization layer on the surface and reduced wear resistance.

[0009] The heat treatment process is not compatible with rare earth modification: the general heat treatment system does not take into account the microstructure characteristics of rare earth steel, and cannot give full play to the advantages of fine grains and modified inclusions, resulting in limited improvement in strength and plasticity.

[0010] Currently, the industry lacks an integrated production process for rare earth modification that allows for precise quantity control, targeted addition, and full-process adaptation. This makes it impossible to consistently achieve the modification effect of trace amounts of rare earth, thus hindering the industrial-scale promotion of rare earth-modified B3 steel.

[0011] To address the aforementioned technological shortcomings, a comprehensive process optimization for rare earth-modified B3 hot-rolled round steel was conducted. This optimization resulted in an integrated production process characterized by precise quantity control, targeted addition, and full-process collaboration, possessing significant technological value and industrial implications. On one hand, it effectively reduces rare earth burn-off and improves elemental distribution, eliminating quality defects such as inclusions, agglomeration, component segregation, and surface decarburization at the source. This stabilizes and enhances the core performance characteristics of steel, including strength, toughness, and wear resistance, thereby upgrading product quality. On the other hand, it fully taps the material modification potential of trace rare earths, breaking down existing limitations on rare earth steel applications and overcoming the technological barriers to the large-scale, stable production of rare earth-modified B3 steel. Furthermore, this process system provides a replicable technical reference for the research and development and production of similar rare earth alloy steels, facilitating technological iteration in the special steel field, further enhancing product market competitiveness, and promoting high-quality development in the industry. Summary of the Invention

[0012] To overcome the problems of inclusion defects, coarse grains, compositional segregation, surface decarburization, and insufficient strength and plasticity in traditional B3 hot-rolled round steel, the purpose of this invention is to provide a low-rare-earth modified B3 hot-rolled round steel for wear-resistant steel balls and its preparation method. Without changing the original main alloy system, this invention achieves a comprehensive effect of purifying the steel, refining the grains, modifying inclusions, weakening segregation, inhibiting decarburization, and simultaneously improving strength and plasticity. It also determines the optimal rare-earth ratio to suit the long-term complex load conditions of wear-resistant steel balls.

[0013] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0014] This invention discloses a hot-rolled round steel bar for low rare earth modified B3 wear-resistant steel balls. The chemical composition of the round steel bar by mass percentage includes: C 0.58%~0.66%, Si 1.60%~1.90%, Mn 0.65%~0.80%, Cr 0.70%~0.90%, P≤0.025%, S≤0.015%, Ce 0.0070%~0.0074%, La 0.0037%~0.0041%, with the remainder being Fe and unavoidable impurities.

[0015] Furthermore, the Ce content was 0.0072%, and the La content was 0.0039%.

[0016] Furthermore, the round steel is used after being oil-quenched at 860±5℃ and water-cooled and tempered at 540±5℃.

[0017] Furthermore, the average decarburization layer depth of the heat-treated steel is ≤0.10 mm, and the central porosity grade is ≤0.5.

[0018] Furthermore, the round steel bar has a specification of Φ80 mm.

[0019] Furthermore, the round steel is used after being oil-quenched at 860℃ and water-cooled and tempered at 540℃.

[0020] Furthermore, the macroscopic low-magnification structure shows that the central porosity is grade 0.5, while defects such as shrinkage cavities, various cracks, and subcutaneous bubbles are all grade 0.

[0021] Furthermore, regarding compositional segregation: the radial deviation of La and Ce elements is only 0.0002%, while the radial deviations of C and Si main elements are 0.04% and 0.03%, respectively. There is no rare earth center enrichment phenomenon, and the compositional uniformity is excellent.

[0022] A method for preparing hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls includes:

[0023] Step 1: Converter smelting (preparation of base steel)

[0024] Raw materials: high-quality pig iron + scrap steel, with carbon, silicon, manganese, chromium and molybdenum as the main alloying elements blended according to the B3 steel standard ratio;

[0025] End-point control of smelting: When molten steel is tapped from the furnace, the P content is ≤0.021% and the S content is ≤0.008%. Harmful impurities are strictly controlled in advance to reduce the burden of subsequent rare earth desulfurization.

[0026] The tapping temperature is 1620~1650℃ to ensure the fluidity of the molten steel and lay the foundation for the uniform diffusion of rare earth elements.

[0027] Step 2: LF Refining (Core process, precise addition of rare earth elements)

[0028] Workstation selection: LF refining adds rare earth alloys in the later stage (different from the traditional early stage addition, reducing rare earth oxidation and burn-off).

[0029] Rare earth elements: Lanthanum (La) and Cerium (Ce) mixed rare earth alloys;

[0030] Total addition control: The total addition of rare earth elements in a single furnace of molten steel is strictly limited to 150 ppm, of which Ce accounts for approximately 72 ppm and La accounts for approximately 39 ppm;

[0031] Stirring process: After adding rare earth elements, stir with bottom-blown argon gas for 8~12 min at an argon gas flow rate of 80~100 L / min to ensure that rare earth elements are evenly distributed in the molten steel and avoid local enrichment.

[0032] Final refining composition: ensure the finished product contains 0.58%~0.66% C, 1.60%~1.90% Si, 0.65%~0.80% Mn, and 0.70%~0.90%, with the main alloy composition fluctuating within ±0.02%.

[0033] Step 3: Continuous casting process (controlling solidification structure and compositional segregation)

[0034] Continuous casting temperature: 1540~1560℃ for molten steel in the tundish, low-speed casting;

[0035] Solidification control: Utilizing the characteristic of rare earth elements being enriched at the solid-liquid interface, the growth of coarse columnar crystals is inhibited and the formation of equiaxed crystals is promoted;

[0036] The quality control standards for cast billets are as follows: the cast billets are free from shrinkage cavities, central cracks, subcutaneous bubbles, and corner cracks; the central porosity grade is ≤0.5; and the radial composition deviation of C and Si elements is ≤0.04%.

[0037] Step 4: Hot rolling process (rolling of Φ80 mm round steel)

[0038] Heating furnace temperature: 1180~1220℃, segmented temperature control to reduce high-temperature oxidation and decarburization on the surface;

[0039] Rolling specifications: The finished product is Φ80 mm hot-rolled round steel;

[0040] Post-rolling cooling: Segmented air cooling is adopted after rolling, and the cooling rate is controlled at 1.5~2.0℃ / s to avoid internal stress caused by rapid cooling;

[0041] Initial control of decarburization layer after hot rolling: Depth of decarburization layer in semi-finished product after hot rolling ≤ 0.12 mm.

[0042] Step 5: Dedicated heat treatment for finished product (matching rare earth modified structure, core post-processing process)

[0043] To address the grain refinement and inclusion-related microstructure characteristics of rare-earth modified B3 steel, a proprietary quenching and tempering process is employed, distinguishing it from ordinary B3 steel.

[0044] Quenching process: heating temperature 860±5℃, holding time 90~120 min, to ensure uniform microstructure; oil cooling after removal from the furnace, with a stable cooling rate to avoid microstructure distortion;

[0045] Tempering process: tempering temperature 540±5℃, holding time 100~130 min, water cooling; to eliminate quenching internal stress, stabilize tempered structure, and further improve plasticity;

[0046] Mandatory testing indicators after heat treatment:

[0047] The average depth of the total decarburized layer on the surface is ≤0.10 mm;

[0048] Non-metallic inclusion rating (GB / T 10561 Method A): Class A coarse series 0 grade, Class A fine series ≤1.0 grade, Class B / C / D inclusions ≤1.0 grade;

[0049] Rockwell hardness: 49.2~59.5 HRC, hardness dispersion ≤5 HRC.

[0050] Step 6: Finished product full-item inspection and warehousing

[0051] Testing items: chemical composition, radial segregation of components, low magnification structure, non-metallic inclusions, surface decarburization, hardness, tensile properties. All indicators must meet the internal control standards for B3 steel and the usage requirements for wear-resistant steel balls before they can be put into storage.

[0052] 4. Process limitations and prohibitions

[0053] Rare earth addition prohibitions: It is forbidden to increase the amount of rare earth added to 300 ppm or above to prevent grain boundary segregation and inclusion agglomeration; it is forbidden to add rare earth during the converter tapping stage, as the burn-off rate is high and the distribution is uneven.

[0054] Temperature restrictions: The quenching temperature shall not deviate from 860℃±10℃, and the tempering temperature shall not deviate from 540℃±10℃, otherwise the rare earth modified structure will be damaged.

[0055] Stirring precautions: After adding LF refined rare earth, the argon stirring time shall not be less than 12 minutes to avoid local accumulation of rare earth.

[0056] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0057] Purified steel + inclusion modification: Trace amounts of La and Ce composite rare earth achieve deep deoxidation and desulfurization, transforming the most harmful long strip-shaped sulfides into spherical dispersed inclusions, eliminating stress concentration sources, and improving the toughness and fatigue life of steel.

[0058] Grain refinement + segregation reduction: Rare earth elements hinder columnar crystal growth and promote equiaxed crystal formation, reducing the central porosity level and effectively controlling radial segregation of the composition, resulting in uniform overall performance of round steel.

[0059] Inhibits surface decarburization: A dense oxide film is formed on the surface, reducing the depth of the decarburized layer by 36.8%, thus ensuring the surface hardness and wear resistance of the wear-resistant steel ball;

[0060] Synergistic improvement in strength and plasticity: The dual effects of fine grain strengthening and inclusion modification result in strength and plasticity far superior to traditional B3 steel, solving the problem of fracture failure of wear-resistant steel balls;

[0061] Highly adaptable to industrial applications: Low rare earth content (150 ppm), small increase in raw material costs, no need to modify existing smelting, continuous casting, hot rolling, and heat treatment equipment, and can be directly mass-produced;

[0062] Avoiding the defects of high rare earth elements: Compared with the 300 ppm high rare earth element formulation, there are no problems with grain boundary segregation and inclusion agglomeration, and the performance is stable without decline.

[0063] Process optimization: Rare earth elements are added in the later stage of LF refining, and argon gas stirring is used to significantly reduce rare earth oxidation loss, ensure accurate delivery of 150 ppm rare earth elements, and maximize raw material utilization.

[0064] Full-process organization and control: In conjunction with the characteristics of rare earth elements, the continuous casting and hot rolling processes refine grains and weaken segregation from the solidification stage, thereby reducing internal defects in steel from the source;

[0065] Highly efficient decarburization suppression: The dual effect of segmented temperature control in the heating furnace and rare earth surface oxide film significantly reduces the depth of the surface decarburization layer and ensures the surface hardness of the wear-resistant steel balls;

[0066] Precise heat treatment adaptation: Exclusive 860℃ quenching + 540℃ tempering process, fully releasing the fine grain strengthening and inclusion modification effects brought by rare earth modification, and simultaneously improving strength and plasticity;

[0067] High process compatibility: It relies entirely on existing long-process special steel production lines, requires no new equipment, has low investment in process modification, and is suitable for large-scale promotion;

[0068] Risks are controllable: By clearly defining core parameters such as the amount of rare earth added, the work station, and the temperature, the performance degradation caused by adding high amounts of rare earth or adding it at the wrong work station is avoided, and the product quality stability is greatly improved.

[0069] Five batches of Φ80 mm B3 hot-rolled round steel produced using this process were selected and compared with products from traditional processes (0 ppm rare earth, 300 ppm rare earth):

[0070] Uniformity of rare earth element distribution: the radial deviation of La and Ce elements is stable at 0.0002%~0.0004%, with no central enrichment;

[0071] Macroscopic structure: 100% of batches have a center porosity grade of 0.5, zero cracks, zero bubble defects, and a pass rate of 100%; 30% of batches in the traditional process without rare earth elements have a center porosity grade of 1.0.

[0072] Surface decarburization: The average decarburized layer depth is 0.096 mm, which is 36.8% lower than that of traditional rare earth-free processes and 66.7% lower than that of processes with 300 ppm rare earth elements.

[0073] Non-metallic inclusions: Long strip-shaped MnS inclusions are completely transformed into spherical composite rare earth inclusions, with the inclusion size reduced by more than 60% and no aggregation phenomenon;

[0074] Mechanical properties (mean): Yield strength 1250 MPa, tensile strength 1491 MPa, elongation after fracture 9.5%, reduction of area 16%; compared with the traditional rare earth-free process, the yield strength is increased by 94 MPa and the plasticity is increased by 90%;

[0075] Mass production stability: When continuously producing 1,000 tons of product in an industrial setting, the fluctuations in various indicators are minimal. Detailed Implementation

[0076] A hot-rolled round bar for low rare earth modified B3 wear-resistant steel balls, comprising:

[0077] (1) Overall component design

[0078] The rare earth-modified B3 hot-rolled round steel of this invention uses B3 low-alloy structural steel as the base material, with the total rare earth content strictly controlled at 150 ppm. The rare earth elements are composed of lanthanum (La) and cerium (Ce). The overall chemical composition (mass fraction) of the steel is strictly limited as follows, and the content of all elements meets the internal control standards for B3 steel:

[0079] C 0.58%~0.66% 0.63% Ensure the hardness and wear resistance of the matrix Si 1.60%~1.90% 1.70% Strengthen solid solutions and improve hardenability Mn 0.65%~0.80% 0.72% Improves toughness and assists in deoxygenation Cr 0.70%~0.90% 0.85% Improve hardness and corrosion resistance Mo 0.06%-0.10% 0.08% Stabilize the tempered microstructure and prevent temper brittleness. P ≤0.025% 0.015% Harmful impurities, strictly control the upper limit S ≤0.015% 0.014% Strictly control the formation of sulfides Al ＜0.005% ＜0.005% Residual aluminum content Cu ≤0.25% ≤0.25% Trace impurity elements Ni ≤0.25% ≤0.25% Trace impurity elements As / Sn / Pb / Sb / Bi All are controlled within an extremely low range Refer to the test data Harmful trace elements Ce (Cerium) 0.0070%~0.0074% (70~74 ppm) 0.0072% (72 ppm) Rare earth main elements La (lanthanum) 0.0037%~0.0041% (37~41 ppm) 0.0039% (39 ppm) Rare earth auxiliary elements Total rare earth content 149~151 ppm 150 ppm Core control indicators

[0080] 3. Preparation and Adaptation Requirements

[0081] (1) Purpose of the invention

[0082] To address the shortcomings of traditional B3 round steel production processes, a complete rare earth modification production process is provided. This process limits the rare earth addition stations, addition amounts, process parameters, and heat treatment regimes, precisely controlling the effects of rare earths. From smelting, continuous casting, hot rolling, to heat treatment, the process purifies inclusions, refines grains, weakens segregation, inhibits decarburization, and improves mechanical properties. The process is compatible with existing production lines, has low modification costs, and offers strong mass production stability.

[0083] (2) Overall process route

[0084] Raw material pretreatment → converter smelting → LF refining (core: quantitative addition of La-Ce rare earth) → continuous casting → walking beam furnace heating → hot rolling → cooling → finished product inspection → dedicated heat treatment → warehousing; the entire process uses Baogang's existing long-process metallurgical and Φ80 mm hot rolling production line, without the need for new large-scale equipment.

[0085] (3) Detailed process plan for each step (including all process parameters, control standards and test indicators)

[0086] Step 1: Converter smelting (preparation of base steel)

[0087] Raw materials: high-quality pig iron + scrap steel, with carbon, silicon, manganese, chromium and molybdenum as the main alloying elements blended according to the B3 steel standard ratio;

[0088] End-point control of smelting: When molten steel is tapped from the furnace, the P content is ≤0.021% and the S content is ≤0.008%. Harmful impurities are strictly controlled in advance to reduce the burden of subsequent rare earth desulfurization.

[0089] The tapping temperature is 1620~1650℃ to ensure the fluidity of the molten steel and lay the foundation for the uniform diffusion of rare earth elements.

[0090] Step 2: LF Refining (Core process, precise addition of rare earth elements)

[0091] Workstation selection: LF refining adds rare earth alloys in the later stage (different from the traditional early stage addition, reducing rare earth oxidation and burn-off).

[0092] Rare earth elements: Lanthanum (La) and Cerium (Ce) mixed rare earth alloys;

[0093] Total addition control: The total addition of rare earth elements in a single furnace of molten steel is strictly limited to 150 ppm, of which Ce accounts for approximately 72 ppm and La accounts for approximately 39 ppm;

[0094] Stirring process: After adding rare earth elements, stir with bottom-blown argon gas for 8~12 min at an argon gas flow rate of 80~100 L / min to ensure that rare earth elements are evenly distributed in the molten steel and avoid local enrichment.

[0095] Final refining composition: ensure the finished product contains 0.58%~0.66% C, 1.60%~1.90% Si, 0.65%~0.80% Mn, and 0.70%~0.90%, with the main alloy composition fluctuating within ±0.02%.

[0096] Step 3: Continuous casting process (controlling solidification structure and compositional segregation)

[0097] Continuous casting temperature: 1540~1560℃ for molten steel in the tundish, low-speed casting;

[0098] Solidification control: Utilizing the characteristic of rare earth elements being enriched at the solid-liquid interface, the growth of coarse columnar crystals is inhibited and the formation of equiaxed crystals is promoted;

[0099] The quality control standards for cast billets are as follows: the cast billets are free from shrinkage cavities, central cracks, subcutaneous bubbles, and corner cracks; the central porosity grade is ≤0.5; and the radial composition deviation of C and Si elements is ≤0.04%.

[0100] Step 4: Hot rolling process (rolling of Φ80 mm round steel)

[0101] Heating furnace temperature: 1180~1220℃, segmented temperature control to reduce high-temperature oxidation and decarburization on the surface;

[0102] Rolling specifications: The finished product is Φ80 mm hot-rolled round steel;

[0103] Post-rolling cooling: Segmented air cooling is adopted after rolling, and the cooling rate is controlled at 1.5~2.0℃ / s to avoid internal stress caused by rapid cooling;

[0104] Initial control of decarburization layer after hot rolling: Depth of decarburization layer in semi-finished product after hot rolling ≤ 0.12 mm.

[0105] Step 5: Dedicated heat treatment for finished product (matching rare earth modified structure, core post-processing process)

[0106] To address the grain refinement and inclusion-related microstructure characteristics of rare-earth modified B3 steel, a proprietary quenching and tempering process is employed, distinguishing it from ordinary B3 steel.

[0107] Quenching process: heating temperature 860℃, holding time 90~120 min, to ensure uniform microstructure; oil cooling after removal from the furnace, with a stable cooling rate to avoid microstructure distortion;

[0108] Tempering process: tempering temperature 540℃, holding time 100~130 min, water cooling; to eliminate quenching internal stress, stabilize tempered structure, and further improve plasticity;

[0109] Mandatory testing indicators after heat treatment:

[0110] The average depth of the total decarburized layer on the surface is ≤0.10 mm;

[0111] Non-metallic inclusion rating (GB / T 10561 Method A): Class A coarse series 0 grade, Class A fine series ≤1.0 grade, Class B / C / D inclusions ≤1.0 grade;

[0112] Rockwell hardness: 49.2~59.5 HRC, hardness dispersion ≤5 HRC.

[0113] Step 6: Finished product full-item inspection and warehousing

[0114] Testing items: chemical composition, radial segregation of components, low magnification structure, non-metallic inclusions, surface decarburization, hardness, tensile properties. All indicators must meet the internal control standards for B3 steel and the usage requirements for wear-resistant steel balls before they can be put into storage.

[0115] (4) Process limitations and prohibitions

[0116] Rare earth addition prohibitions: It is forbidden to increase the amount of rare earth added to 300 ppm or above to prevent grain boundary segregation and inclusion agglomeration; it is forbidden to add rare earth during the converter tapping stage, as the burn-off rate is high and the distribution is uneven.

[0117] Temperature restrictions: The quenching temperature shall not deviate from 860℃±10℃, and the tempering temperature shall not deviate from 540℃±10℃, otherwise the rare earth modified structure will be damaged.

[0118] Stirring precautions: After adding LF refined rare earth, the argon stirring time shall not be less than 12 minutes to avoid local accumulation of rare earth.

[0119] (5) Process effect (actual measurement data of the whole process)

[0120] Five batches of Φ80 mm B3 hot-rolled round steel produced using this process were selected and compared with products from traditional processes (0 ppm rare earth, 300 ppm rare earth):

[0121] Uniformity of rare earth element distribution: the radial deviation of La and Ce elements is stable at 0.0002%~0.0004%, with no central enrichment;

[0122] Macroscopic structure: 100% of batches have a center porosity grade of 0.5, zero cracks, zero bubble defects, and a pass rate of 100%; 30% of batches in the traditional process without rare earth elements have a center porosity grade of 1.0.

[0123] Surface decarburization: The average decarburized layer depth is 0.096 mm, which is 36.8% lower than that of traditional rare earth-free processes and 66.7% lower than that of processes with 300 ppm rare earth elements.

[0124] Non-metallic inclusions: Long strip-shaped MnS inclusions are completely transformed into spherical composite rare earth inclusions, with the inclusion size reduced by more than 60% and no aggregation phenomenon;

[0125] Mechanical properties (mean): Yield strength 1250 MPa, tensile strength 1491 MPa, elongation after fracture 9.5%, reduction of area 16%; compared with the traditional rare earth-free process, the yield strength is increased by 94 MPa and the plasticity is increased by 90%;

[0126] Mass production stability: When continuously producing 1,000 tons of product in an industrial setting, the fluctuations in various indicators are minimal.

[0127] 4. Key performance indicators (compared to existing technologies)

[0128] All data are from industrial-scale trial production testing results, comparing 0 ppm (no rare earth) and 300 ppm rare earth groups:

[0129] Compositional segregation: The radial deviations of La and Ce elements are stable at 0.0002%~0.0004%, while the radial deviations of C and Si main elements are 0.04% and 0.03%, respectively. There is no rare earth center enrichment phenomenon, and the compositional uniformity is excellent.

[0130] Macroscopic low-magnification structure: central porosity grade 0.5, defects such as shrinkage cavities, various cracks, and subcutaneous bubbles are all grade 0, which is significantly reduced compared to the group without rare earth elements (central porosity grade 1.0);

[0131] Surface decarburization: The average decarburization layer depth is 0.096 mm, which is much lower than that of the group without rare earth (0.152 mm) and the group with 300 ppm rare earth (0.288 mm), and the surface decarburization resistance and wear resistance are greatly improved.

[0132] Non-metallic inclusions (rated according to GB / T 10561 Method A): Class A: fine series 1.0 grade, coarse series 0 grade; Class B: fine series 1.0 grade, coarse series 1.0 grade; Class C: both fine and coarse series 0.5 grade; Class D: both fine and coarse series 1.0 grade, inclusions are small in size, diffusely distributed, and without aggregation;

[0133] Rockwell hardness (HRC): range of 49.2~59.5 HRC, with no significant decrease in hardness, retaining the high wear resistance of B3 steel;

[0134] Room temperature tensile mechanical properties: yield strength 1250 MPa, tensile strength 1491 MPa, elongation after fracture 9.5%, reduction of area 16%; compared with the group without rare earth elements, the yield strength increased by 94 MPa, the elongation increased by 90%, the reduction of area increased by 77.8%, and the strength and plasticity were optimized simultaneously.

[0135] Example 1 (Optimal Example)

[0136] Product specifications: Φ80 mm B3 hot-rolled round steel;

[0137] Chemical composition (mass fraction): C 0.658%, Si 1.34%, Mn 0.736%, Cr 0.842%, P 0.021%, S 0.008%, Ce 0.0072%, La 0.0039%, total rare earth content 150 ppm, other impurity elements meet the limit requirements;

[0138] Production process:

[0139] Converter smelting: tapping temperature 1635℃, control S≤0.008%, P≤0.021%, adjust main alloying elements to the target range;

[0140] Late stage of LF refining: Add La-Ce mixed rare earth alloy, with a total addition of 150 ppm (Ce 72 ppm, La 39 ppm), and stir with bottom-blown argon for more than 13 min;

[0141] Continuous casting: tundish temperature 1550℃, low-speed pouring, billet center porosity grade 0.5, no cracks or bubbles;

[0142] Hot rolling: The heating furnace temperature is 1200℃, and the steel is rolled into Φ80 mm round bars. After rolling, the steel is air-cooled at a cooling rate of 1.8℃ / s.

[0143] Heat treatment: Hold at 860℃ for 100 min, oil quenching; Hold at 540℃ for 110 min, water tempering.

[0144] Actual performance:

[0145] Radial deviation of composition: La 0.0002%, Ce 0.0004%, Si 0.03%, C 0.04%;

[0146] Low magnification: central looseness grade 0.5, no other macroscopic defects;

[0147] Average decarburized layer depth: 0.096 mm;

[0148] Mechanical properties: Yield strength 1250 MPa, tensile strength 1491 MPa, elongation 9.5%, reduction of area 16%, hardness measured at multiple points 49.2~59.5 HRC.

[0149] Comparison with Process 1 (Traditional Process + 0 ppm Rare Earth)

[0150] There is no rare earth addition process, and the remaining smelting, continuous casting, hot rolling, and heat treatment parameters are the same; the finished product has a central porosity grade of 1.0, a decarburized layer of 0.152 mm, a yield strength of 1156 MPa, and an elongation of 5.0%.

[0151] Comparison with process 2 (traditional process + 300 ppm rare earth elements)

[0152] The rare earth content was increased to 300 ppm, while the rest of the process remained unchanged; the radial deviation of C element was 0.054%, the decarburized layer was 0.288 mm, the yield strength was 1204 MPa, and the elongation was 7.5%.

[0153] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A hot-rolled round bar for low rare earth modified B3 wear-resistant steel balls, characterized in that, The chemical composition of the round steel, by mass fraction, includes: C 0.58%~0.66%, Si 1.60%~1.90%, Mn 0.65%~0.80%, Cr 0.70%~0.90%, P≤0.025%, S≤0.015%, Ce 0.0070%~0.0074%, La 0.0037%~0.0041%, with the remainder being Fe and unavoidable impurities.

2. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 1, characterized in that, The chemical composition of the round steel by weight percentage includes: C 0.658%, Si 1.34%, Mn 0.736%, Cr 0.842%, P 0.021%, S 0.008%, Ce 0.0072%, La 0.0039%, with the remainder being Fe and unavoidable impurities.

3. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 1, characterized in that, The round steel is used after being oil-quenched at 860±5℃ and water-cooled and tempered at 540±5℃.

4. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 3, characterized in that, After heat treatment, the average decarburized layer depth of the steel is ≤0.10 mm, and the central porosity grade is ≤0.

5.

5. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 1, characterized in that, The round steel bar has a diameter of Φ80 mm.

6. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 3, characterized in that, The round steel is used after being oil-quenched at 860℃ and water-cooled and tempered at 540℃.

7. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 3, characterized in that, Macroscopic low magnification structure: central porosity grade 0.5, defects such as shrinkage cavities, various cracks, and subcutaneous bubbles are all grade 0.

8. The hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 3, characterized in that, Compositional segregation: the radial deviation of La and Ce elements is only 0.0002%, and the radial deviation of C and Si main elements is 0.04% and 0.03% respectively. There is no rare earth center enrichment phenomenon, and the compositional uniformity is excellent.

9. The method for preparing hot-rolled round steel for low rare earth modified B3 wear-resistant steel balls according to claim 1, characterized in that, include: Step 1: Converter smelting Raw materials: high-quality pig iron + scrap steel, with carbon, silicon, manganese, chromium and molybdenum as the main alloying elements blended according to the B3 steel standard ratio; End-point control of smelting: When molten steel is tapped from the furnace, the P content is ≤0.021% and the S content is ≤0.008%, strictly controlling harmful impurities in advance to reduce the burden of subsequent rare earth desulfurization; The tapping temperature is 1620~1650℃ to ensure the fluidity of the molten steel and lay the foundation for the uniform diffusion of rare earth elements. Step 2: LF Refining Station Selection: Rare earth alloys are added in the later stages of LF refining. Rare earth elements: Lanthanum (La) and Cerium (Ce) mixed rare earth alloys; Total addition control: The total addition of rare earth elements in a single furnace of molten steel is strictly limited to 150 ppm, of which Ce accounts for approximately 72 ppm and La accounts for approximately 39 ppm; Stirring process: After adding rare earth elements, stir with bottom-blown argon gas for 8~12 min at an argon gas flow rate of 80~100 L / min to ensure that rare earth elements are evenly distributed in the molten steel and avoid local enrichment. Final refining composition: Ensure the finished product contains 0.58%~0.66% C, 1.60%~1.90% Si, 0.65%~0.80% Mn, and 0.70%~0.90%, with a main alloy composition fluctuation range of ≤±0.02%; Step 3: Continuous casting process. Continuous casting temperature: 1540~1560℃ for molten steel in the tundish, low-speed pouring; Solidification control: Utilizing the characteristic of rare earth elements being enriched at the solid-liquid interface, the growth of coarse columnar crystals is inhibited and the formation of equiaxed crystals is promoted; Slab quality control standards: Slabs should be free of shrinkage cavities, central cracks, subcutaneous bubbles, and corner cracks; central porosity grade ≤ 0.5; radial composition deviation of C and Si elements ≤ 0.04%. Step 4: Hot rolling process Heating furnace temperature: 1180~1220℃, segmented temperature control to reduce high-temperature oxidation and decarburization on the surface; Rolling specifications: The finished product is Φ80 mm hot-rolled round steel; Post-rolling cooling: Segmented air cooling is adopted after rolling, and the cooling rate is controlled at 1.5~2.0℃ / s to avoid internal stress caused by rapid cooling; Initial control of decarburization layer after hot rolling: Depth of decarburization layer in the semi-finished product after hot rolling ≤ 0.12 mm; Step 5: Dedicated heat treatment for finished product In response to the grain refinement and inclusion-modified microstructure characteristics of rare earth modified B3 steel, a proprietary quenching and tempering process is employed, distinguishing it from ordinary B3 steel. Quenching process: heating temperature 860±5℃, holding time 90~120 min, to ensure uniform microstructure; oil cooling after removal from the furnace, with a stable cooling rate to avoid microstructure distortion; Tempering process: tempering temperature 540±5℃, holding time 100~130 min, water cooling; to eliminate quenching internal stress, stabilize tempered structure, and further improve plasticity.