Ultrafine lower bainite steel and method for producing the same

Abstract

The application belongs to the technical field of steel material preparation, and particularly relates to an ultra-fine lower bainite steel and a preparation method thereof, which comprises the following steps: S1, alloy casting preparation of a steel ingot, the steel ingot comprising Cu elements and Ti elements, the content of the Cu elements being 0.1wt%-0.3wt%, and the content of the Ti elements being 0.01wt%-0.15wt%; S2, homogenization annealing of the steel ingot, and multi-directional forging of the steel ingot; S3, cooling of the steel ingot to a first temperature and normalizing treatment of the steel ingot; S4, cooling of the steel ingot to a second temperature, austenitizing of the steel ingot, water quenching of the steel ingot and tempering, and cooling to room temperature to obtain the ultra-fine lower bainite steel; the application significantly optimizes the preparation efficiency and surface performance of the lower bainite structure through the synergistic effect of micro-alloying and a multi-stage hot working process.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, specifically an ultrafine lower bainitic steel and its preparation method. Background Technology

[0002] With increasingly stringent requirements for the synergistic effect of surface finish and toughness in precision molds, optical device molds, and automotive lighting molds, the limitations of traditional high-carbon, high-alloy tool steels are becoming increasingly apparent. Due to the coarsening and uneven distribution of carbides, these materials exhibit high surface roughness after polishing (Ra > 0.15 μm) and insufficient core toughness (impact energy < 50 J), making them unsuitable for high-precision manufacturing. In recent years, lower bainite microstructures have become a research hotspot due to their high strength and toughness, uniform microstructure, and potential polishing performance. However, conventional lower bainite preparation techniques still face significant challenges: isothermal quenching processes require prolonged salt bath holding (up to 4 hours), resulting in high production costs and low efficiency; the high retained austenite content (greater than 10%) easily leads to dimensional instability after polishing. To achieve ultra-fine bainite, existing technologies often rely on high-alloying designs, such as Ni and Mo contents greater than 3%, and complex multi-stage heat treatments, further increasing material costs and process complexity.

[0003] To address the aforementioned issues, this invention proposes a short-process preparation technology based on the synergistic effect of Cu-Ti microalloying and gradient controlled cooling. By optimizing the composite addition of Cu (0.1-0.3 wt%) and Ti (0.05-0.15 wt%), the synergistic effect of Cu promoting the precipitation of nano-ε-Cu phase to strengthen the matrix and Ti forming nano-TiC particles to refine grains and inhibit austenite growth is utilized. Combined with an innovative process chain of electric arc furnace smelting-multidirectional forging-dynamic normalizing-water quenching and tempering, ultra-fine lower bainite microstructure is achieved, with lamellar widths less than 0.5 μm and uniform distribution. This technology abandons traditional salt bath quenching and uses water cooling medium to achieve ultra-fast cooling of 100-200℃ / s. While shortening the process cycle, it reduces the polishing roughness to 0.05-0.10 μm and improves the impact toughness to over 85 J / cm², providing a material solution for high-precision manufacturing that combines low cost, high efficiency, and excellent comprehensive performance. Summary of the Invention

[0004] This invention provides a method for preparing ultrafine lower bainitic steel, comprising the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots include Cu and Ti elements, with the Cu element content being 0.1wt%-0.3wt% and the Ti element content being 0.01wt%-0.15wt%; S2. The steel ingot is subjected to homogenization annealing, and after the homogenization annealing is completed, the steel ingot is subjected to multi-directional forging. S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot; S4. Cool the steel ingot to a second temperature, austenitize the steel ingot, after austenitization, quench the steel ingot in water and temper it, and cool it to room temperature to obtain ultrafine lower bainitic steel.

[0005] In step S4, the cooling method for cooling the steel ingot to the second temperature is air cooling, the second temperature is 150-200℃, the austenitizing temperature is 850-900℃, the austenitizing holding time is 1-3h, the tempering temperature is 500-560℃, and the tempering holding time is 3-6h. Specifically, the second temperature is any one or any two of 150℃, 160℃, 170℃, 180℃, 190℃, and 200℃; the austenitizing temperature is any one or any two of 850℃, 860℃, 870℃, 880℃, 890℃, and 900℃; the austenitizing holding time is any one or any two of 1h, 2h, and 3h; the tempering temperature is any one or any two of 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, and 560℃; the tempering holding time is any one or any two of 3h, 4h, 5h, and 6h; and the austenitizing temperature heating rate is less than 80℃ / h.

[0006] In step S2, the homogenization annealing temperature is 1260-1280℃, the holding time for homogenization annealing is 6-12h, the starting temperature for multi-directional forging is 1150-1200℃, the final forging temperature for multi-directional forging is 950℃, and the cumulative deformation of multi-directional forging is 70%-90%.

[0007] Specifically, the homogenization annealing temperature can be any one of 1260℃, 1270℃, 1280℃ or any two of them; the multi-directional forging starting temperature can be any one of 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃ or any two of them.

[0008] In step S3, the first temperature is 800-900℃, and the normalizing treatment is to keep the temperature at the first temperature for 1-3 hours.

[0009] Specifically, the first temperature is any one or any two of 800℃, 820℃, 840℃, 860℃, 880℃, and 900℃, and the holding time of the normalizing treatment is any one or any two of 1h, 2h, and 3h.

[0010] In step S4, the water quenching uses a water-cooling medium, the cooling rate of the water quenching is 100-200℃ / s, and the tempering is cooled by air cooling. Specifically, the cooling rate of the water quenching is any one of 100℃ / s, 120℃ / s, 140℃ / s, 160℃ / s, 180℃ / s, 200℃ / s, or a range between any two of them.

[0011] In step S4, the surface hardness of the water-quenched steel ingot is 40-47 HRC, the core hardness is 37-39 HRC, the overall hardness of the tempered steel ingot is 38-42 HRC, the volume fraction of lower bainite in the core structure of the ultrafine lower bainitic steel is ≥85%, and the volume fraction of retained austenite is <5%.

[0012] The present invention also provides an ultrafine lower bainitic steel, which is prepared by the above-mentioned method for preparing ultrafine lower bainitic steel of a low alloy steel.

[0013] The chemical composition of the ultrafine lower bainitic steel, by mass percentage, is as follows: C: 0.25wt%-0.30wt%, Si: 0.2wt%-0.4wt%, Mn: 1.4wt%-1.6wt%, Cr: 1.55wt%-1.70wt%, Ni: 1.00wt%-1.11wt%, Mo: 0.5wt%-0.6wt%, V: 0.15wt%-0.20wt%, Cu: 0.1wt%-0.3wt%, Ti: 0.05wt%-0.15wt%, P≤0.02wt%, S≤0.03wt%, with the balance being Fe and unavoidable impurities.

[0014] The width of the lower bainite laths in the ultrafine lower bainitic steel is ≤0.5μm, and the surface roughness Ra of the ultrafine lower bainitic steel after polishing is 0.05-0.10μm.

[0015] The ultrafine bainitic steel has an overall hardness of 38-42 HRC and an impact toughness of ≥85 J / cm. 2 The difference between the surface hardness and the core hardness of the ultrafine bainitic steel is ≤2.5 HRC.

[0016] This invention significantly optimizes the preparation efficiency and surface properties of lower bainite structure through the synergistic effect of microalloying and multi-stage hot working process; Cu element precipitates nanoscale ε-Cu phase during aging, effectively filling grain boundary micropores and uniformly distributing stress; Ti element inhibits austenite grain coarsening, and the two work together to refine the lower bainite lamellars to a width ≤0.5μm, so that the surface roughness Ra after polishing can be stably reached 0.05-0.10μm. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0018] Figure 1 This is a microstructure diagram of the ultrafine lower bainitic steel prepared in Example 1 of the present invention.

[0019] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0020] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a method for preparing ultrafine lower bainitic steel, comprising the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots include Cu and Ti elements, with the Cu element content being 0.1wt%-0.3wt% and the Ti element content being 0.01wt%-0.15wt%; In all embodiments of this invention, 30MnCrNiMo series steel is used, with a carbon content (0.25%-0.30%) in the medium carbon range. This ensures sufficient dissolved carbon to drive the bainitic transformation while avoiding carbide coarsening caused by high carbon content. Mn (1.4%-1.6%) and Cr (1.55%-1.70%) form a synergistic hardenability system, significantly reducing the critical cooling rate for bainitic transformation and providing feasibility for water quenching. Ni (1.00%-1.11%)... The combined addition of Cu (0.5%-0.6%) and Mo (0.5%-0.6%) not only improves the matrix strength, but also widens the bainite formation window by inhibiting pearlite transformation. Trace amounts of V (0.15%-0.20%) and Ti (0.05%-0.15%) form nanoscale carbonitrides that pin austenite grain boundaries, controlling the original austenite grain size to below 20μm, laying the foundation for subsequent ultrafine bainite laths (≤0.5μm). The addition of Cu promotes the formation of a finer lath structure. S2. The steel ingot is subjected to homogenization annealing, and after the homogenization annealing is completed, the steel ingot is subjected to multi-directional forging. In the embodiments of the present invention, high-temperature homogenization annealing (1260-1280℃) is used to completely eliminate dendritic segregation of elements such as Cu and Ti by holding for 6-12 hours, thereby reducing the risk of subsequent forging cracking. Multi-directional forging (70%-90% deformation) refines the grains and breaks up the coarse carbides in the casting state.

[0022] S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot; In the embodiments of the present invention, temperature-controlled normalizing (holding at 800-900℃) is used to allow proeutectoid ferrite to fully precipitate and form a uniform austenite-ferrite dual-phase structure, providing a uniform phase transformation starting point for quenching and tempering.

[0023] S4. Cool the steel ingot to a second temperature, austenitize the steel ingot, after austenitization, quench the steel ingot in water and temper it, and cool it to room temperature to obtain ultrafine lower bainitic steel.

[0024] In the embodiments of this invention, after austenitization at 850℃-900℃, immediate water cooling is performed, overcoming the limitations of traditional oil quenching cooling rates, resulting in a bainite transformation of over 85% and retained austenite <5%. This achieves a surface hardness to core hardness difference of ≤2.5 HRC for the ultrafine low-bainitic steel. Tempering (500-560℃) promotes the dispersed precipitation of ε-carbides within the bainite laths, eliminating quenching stress while maintaining a hardness within the ideal polishing range of 38-42 HRC.

[0025] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0026] Example 1 A method for preparing ultrafine low bainitic steel includes the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots contain Cu and Ti elements, with the Cu element content being 0.2 wt% and the Ti element content being 0.1 wt%. S2. The steel ingot is subjected to homogenization annealing. After homogenization annealing, the steel ingot is subjected to multi-directional forging. The homogenization annealing temperature is 1270℃ and the holding time is 6 hours. The starting temperature of multi-directional forging is 1150℃, the final forging temperature of multi-directional forging is 950℃, and the cumulative deformation of multi-directional forging is 70%. S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot, wherein the first temperature is 880°C and the holding time of the normalizing treatment is 3 hours. S4. Cool the steel ingot to a second temperature and austenitize the steel ingot. After austenitization, quench the steel ingot in water and temper it. Cool it to room temperature to obtain ultrafine lower bainitic steel. The second temperature is 180℃, the austenitization temperature is 880℃, the holding time is 2h, the water quenching cooling rate is 160℃ / s, the tempering temperature is 540℃, and the holding time is 4h.

[0027] Please see Figure 1 , Figure 1 The diagram shows the microstructure of the ultrafine lower bainitic steel prepared in Example 1 of this invention. The lower bainitic structure is black needle-like, with these "needles" or "bamboo leaves" interspersed at a certain angle. Fine, diffusely distributed ε-carbide structures are clearly visible.

[0028] The composition of the ultrafine lower bainitic steel prepared in Example 1 is shown in Table 1; Table 1 (mass percentage, wt%) Example 2 A method for preparing ultrafine low bainitic steel includes the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots contain Cu and Ti elements, with the Cu element content being 0.25 wt% and the Ti element content being 0.08 wt%. S2. The steel ingot is subjected to homogenization annealing. After homogenization annealing, the steel ingot is subjected to multi-directional forging. The homogenization annealing temperature is 1280℃ and the holding time is 6 hours. The starting temperature of multi-directional forging is 1150℃, the final forging temperature of multi-directional forging is 950℃, and the cumulative deformation of multi-directional forging is 70%. S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot, wherein the first temperature is 850°C and the holding time of the normalizing treatment is 3 hours. S4. Cool the steel ingot to a second temperature and austenitize the steel ingot. After austenitization, quench the steel ingot in water and temper it. Cool it to room temperature to obtain ultrafine lower bainitic steel. The second temperature is 160℃, the austenitization temperature is 860℃, the holding time is 2h, the water quenching cooling rate is 180℃ / s, the tempering temperature is 540℃, and the holding time is 4h.

[0029] The composition of the ultrafine lower bainitic steel prepared in Example 2 is shown in Table 2; Table 2 (mass percentage, wt%) Example 3 A method for preparing ultrafine low bainitic steel includes the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots contain Cu and Ti elements, with the Cu element content being 0.1 wt% and the Ti element content being 0.01 wt%. S2. The steel ingot is subjected to homogenization annealing. After homogenization annealing, the steel ingot is subjected to multi-directional forging. The homogenization annealing temperature is 1260℃ and the holding time is 6 hours. The starting temperature of multi-directional forging is 1200℃, the final forging temperature of multi-directional forging is 950℃, and the cumulative deformation of multi-directional forging is 90%. S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot, wherein the first temperature is 800°C and the holding time of the normalizing treatment is 3 hours. S4. Cool the steel ingot to a second temperature and austenitize the steel ingot. After austenitization, quench the steel ingot in water and temper it. Cool it to room temperature to obtain ultrafine lower bainitic steel. The second temperature is 160℃, the austenitization temperature is 860℃, the holding time is 2h, the water quenching cooling rate is 100℃ / s, the tempering temperature is 500℃, and the holding time is 6h.

[0030] The composition of the ultrafine lower bainitic steel prepared in Example 2 is shown in Table 3; Table 3 (mass percentage, wt%) Comparative Example 1 The difference from Example 1 is that the Cu content is 0.25 wt% and Ti is not present.

[0031] Comparative Example 2 The difference from Example 1 is that the Ti element content is 0.15wt% and Cu is not present.

[0032] Comparative Example 3 The difference from Example 1 is that the homogenization annealing temperature in step S2 is different; the homogenization annealing temperature is 1250°C.

[0033] Comparative Example 4 The difference from Example 1 is that the normalizing temperature in step S3 is different; the normalizing temperature is 700°C.

[0034] Comparative Example 5 The difference from Example 1 is that the austenitizing temperature in step S4 is different; the austenitizing temperature is 1000°C.

[0035] Comparative Example 6 The difference from Example 1 is that the water quenching cooling rate in step S4 is different, and the water quenching cooling rate is different, which is 50℃ / h.

[0036] Comparative Example 7 The difference from Example 1 is that the cooling to the second temperature after normalizing in step S4 is different; the second temperature is room temperature.

[0037] Comparative Example 8 The difference from Example 1 is that the tempering temperature in step S4 is different; the tempering temperature is 400°C.

[0038] Please refer to Table 3, which compares the performance of the steel ingots prepared in Examples 1-3 and Comparative Examples 1-8.

[0039] As shown in Table 4, Examples 1, 2, and 3 exhibit excellent overall performance. The difference between surface hardness and core hardness is ≤2.5 HRC, indicating outstanding uniformity of the microstructure gradient. Impact toughness exceeds 85 J / cm², superior to the comparative examples. Surface roughness Ra is 0.05-0.10 μm, significantly lower than the 0.15-0.39 μm of Comparative Examples 1-8. This performance advantage mainly stems from the synergistic effect of the ultrafine lower bainitic microstructure (lamellae width ≤0.5 μm) and the nano-TiC / ε-Cu precipitates (size ≤50 nm). The dense lower bainitic structure effectively suppresses micro-deformation during polishing, while the nano-precipitates strengthen the matrix while maintaining toughness. Furthermore, the microstructure characteristics of a core lower bainitic volume fraction ≥85% and retained austenite <5% ensure the stability of the hardness gradient and high impact toughness, thus meeting the stringent requirements of high-precision tools for integrated strength and toughness and mirror polishing.

[0040] Table 4 This invention significantly optimizes the preparation efficiency and surface properties of lower bainite structure through the synergistic effect of microalloying and multi-stage hot working process; Cu element precipitates nanoscale ε-Cu phase during aging, effectively filling grain boundary micropores and uniformly distributing stress; Ti element inhibits austenite grain coarsening, and the two work together to refine the lower bainite lamellars to a width ≤0.5μm, so that the surface roughness Ra after polishing can be stably reached 0.05-0.10μm.

[0041] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing ultrafine low bainitic steel, characterized in that, Includes the following steps: S1. Alloy casting is used to prepare steel ingots, wherein the steel ingots include Cu and Ti elements, with the Cu element content being 0.1wt%-0.3wt% and the Ti element content being 0.01wt%-0.15wt%; S2. The steel ingot is subjected to homogenization annealing at a temperature of 1260-1280℃ and a holding time of 6-12h. After homogenization annealing, the steel ingot is subjected to multi-directional forging. S3. Cool the steel ingot to a first temperature and perform normalizing treatment on the steel ingot; the first temperature is 800-900℃, and the normalizing treatment is to hold the steel ingot at the first temperature for 1-3 hours; S4. Cool the steel ingot to a second temperature, austenitize the steel ingot, and after austenitization, quench the steel ingot in water and temper it, then cool it to room temperature to obtain ultrafine lower bainitic steel; the second temperature is 150-200℃, the austenitization temperature is 850-900℃, the austenitization holding time is 1-3h, the tempering temperature is 500-560℃, the tempering holding time is 3-6h, and the water quenching cooling rate is 100-200℃ / s; The width of the lower bainitic lamellae in the ultrafine lower bainitic steel is ≤0.5μm, and the surface roughness Ra of the ultrafine lower bainitic steel after polishing is 0.05-0.10μm; the overall hardness of the ultrafine lower bainitic steel is 38-42HRC, and the impact toughness is ≥85J / cm. 2 The difference between the surface hardness and the core hardness of the ultrafine lower bainitic steel is ≤2.5HRC.

2. The method for preparing ultrafine lower bainitic steel according to claim 1, characterized in that, In step S4, the cooling method for cooling the steel ingot to the second temperature is air cooling.

3. The method for preparing ultrafine lower bainitic steel according to claim 1, characterized in that, In step S2, the starting temperature of the multi-directional forging is 1150-1200℃, the final forging temperature of the multi-directional forging is 950℃, and the cumulative deformation of the multi-directional forging is 70%-90%.

4. The method for preparing ultrafine lower bainitic steel according to claim 1, characterized in that, In step S4, the water quenching uses a water-cooling medium, and the tempering is cooled by air cooling.

5. The method for preparing ultrafine lower bainitic steel according to claim 1, characterized in that, In step S4, the surface hardness of the water-quenched steel ingot is 40-47 HRC, the core hardness is 37-39 HRC, the overall hardness of the tempered steel ingot is 38-42 HRC, the volume fraction of lower bainite in the core structure of the ultrafine lower bainitic steel is ≥85%, and the volume fraction of retained austenite is <5%.

6. An ultrafine lower bainitic steel, characterized in that, The ultrafine lower bainitic steel is prepared by the preparation method of ultrafine lower bainitic steel as described in any one of claims 1-5.

7. The ultrafine lower bainitic steel according to claim 6, characterized in that, The chemical composition of the ultrafine lower bainitic steel, by mass percentage, is: C: 0.25wt%-0.30wt%, Si: 0.2wt%-0.4wt%, Mn: 1.4wt%-1.6wt%, Cr: 1.55wt%-1.70wt%, Ni: 1.00wt%-1.11wt%, Mo: 0.5wt%-0.6wt%, V: 0.15wt%-0.20wt%, Cu: 0.1wt%-0.3wt%, Ti: 0.05wt%-0.15wt%, P≤0.02wt%, S≤0.03wt%, with the balance being Fe and unavoidable impurities.

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