Economical high-strength and high-toughness low-carbon steel with bimodal structure and preparation process thereof

Abstract

This invention discloses an economical bimodal high-strength, high-toughness low-carbon steel and its preparation process, belonging to the field of steel material heat treatment technology. The process includes: heat-treating hot-rolled low-carbon steel at ±10°C from temperature Ac1, followed by air cooling to room temperature; heating the air-cooled low-carbon steel to the Ac1-Ac3 temperature range for two-phase heat treatment, followed by quenching with water as the quenching medium; heating the quenched low-carbon steel at a heating rate of 50°C / s to a temperature between +20°C and -20°C from Ac1 to Ac3 for two-phase heat treatment, followed by quenching with different cooling media, namely quenching oil or water at 30°C-50°C, ultimately obtaining a bimodal microstructure. The resulting low-carbon steel has a tensile strength greater than 910 MPa and a total elongation greater than 22%. The bimodal structure preparation process of this invention is simple, and the prepared bimodal low-carbon steel exhibits excellent high strength and high toughness properties.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for steel materials, and particularly relates to an economical bimodal high-strength, high-toughness, low-carbon steel and its preparation process. Background Technology

[0002] With the rapid development of science and technology, modern manufacturing demands increasingly higher performance from structural materials. Steel, as the most important structural material, plays a crucial supporting role in the national economy and aerospace fields. For the past few decades, microalloying and alloying combined with controlled rolling and cooling techniques to regulate the phases and microstructure of steel and improve its overall performance have been the mainstream processes. However, adding expensive alloying elements increases the cost of steel, slows the improvement in performance, and creates difficulties in recycling and reuse. Against the backdrop of resource depletion, excessive energy consumption, and environmental degradation, optimizing the overall performance of steel based on simplified alloy composition is a major pathway to achieving sustainable development in the steel industry. Therefore, how to simplify the chemical composition of steel and optimize its microstructure through improved processing techniques to ultimately enhance its overall performance and meet the application needs of different fields has become a key focus of current steel material research. Summary of the Invention

[0003] The purpose of this invention is to provide an economical bimodal high-strength, high-toughness, low-carbon steel and its preparation process, in order to solve the technical problem of the inverse relationship between the strength and plasticity of materials.

[0004] To achieve the above objectives, the specific technical solution of the present invention for an economical bimodal high-strength, high-toughness, low-carbon steel and its preparation process is as follows:

[0005] For a long time, fine-grained strengthening combined with two-phase and multi-phase microstructures has been the main strengthening method for improving the strength of steel materials without deteriorating toughness. However, when the grain size is refined to the submicron level, although the strength increase is significant, the plasticity is severely reduced. The inverse relationship between strength and plasticity of materials has always been a key problem that needs to be solved in the field of materials science. Research over the years has found that constructing heterogeneous structures can effectively improve both the strength and plasticity of materials simultaneously. Currently, for steel materials, the main processes for constructing heterogeneous structures are large plastic deformation processes, such as cumulative stacking, multi-layer composite rolling, incomplete recrystallization after large plastic deformation, ultrasonic surface rolling, and surface mechanical rolling / impact / friction processes. Among these, the cumulative stacking process is cumbersome, multi-layer composite and incomplete recrystallization after large plastic deformation exhibit obvious anisotropy in mechanical properties, and ultrasonic rolling and surface mechanical rolling / impact / friction have little strengthening effect on materials with large dimensions in all three dimensions, which limits the universality of the above processes. Heat treatment, as the most universal way to effectively improve the mechanical properties of metal structural materials, is the most important back-end process in the entire processing of metal structural materials. The performance of metallic structural materials can be improved by constructing heterogeneous structures through phase transformation during heat treatment. Compared with large plastic deformation processes, it is easier to realize the industrial production of heterogeneous structural materials.

[0006] An economical process for preparing high-strength, high-toughness, and low-carbon steel involves using ordinary low-carbon steel as raw material and obtaining high-strength, high-toughness, and low-carbon steel through three heat treatment steps.

[0007] The chemical composition of the raw steel used is as follows (mass percentage): C content is 0.17%-0.24%; Si content is 0.2%-0.5%; Mn content is 0.5%-1.4%, and the balance is Fe and other unavoidable impurity elements.

[0008] Includes the following steps:

[0009] Step S1: The hot-rolled raw steel is placed in a heat treatment furnace and heat-treated at a temperature of ±10℃ at Ac1, held at that temperature, and then air-cooled to room temperature. Step S1 achieves spheroidization of cementite in low-carbon steel and controls the distribution of cementite on the ferrite grain boundaries as much as possible.

[0010] Step S2: The raw steel treated in Step S1 is heated to the temperature range Ac1 (680℃-720℃)-Ac3 (795℃-855℃) for two-phase heat treatment, held at that temperature, and then quenched. Step S2 enables rapid austenite growth along the ferrite grain boundaries, forming austenite surrounding ferrite. After quenching, the microstructure exhibits martensite surrounding ferrite. The two-phase heat treatment temperature must ensure that a martensite volume fraction of over 60% is obtained.

[0011] Step S3: The raw steel processed in step S2 is heated at a certain rate, specifically at 50℃ / s or higher, to a temperature of +20℃ above Ac1 and below -20℃ below Ac3 for two-phase heat treatment, held at that temperature, and then quenched. Step S3 enables rapid nucleation of austenite in martensite and inhibits austenite growth by holding the temperature for a short time. After quenching in quenching oil or water at 30℃-50℃, the steel exhibits a microstructure characterized by ultrafine-grained quasi-polygonal ferrite grains encapsulating fine-grained polygonal ferrite grains.

[0012] Furthermore, the quenching medium in step S2 is water.

[0013] Furthermore, in step S3, the quenching medium is quenching oil or water at 30℃-50℃.

[0014] Furthermore, in step S1, the heat preservation time is 2-20 hours.

[0015] Furthermore, in step S2, the heat preservation time is 0.5-20 hours.

[0016] Furthermore, in step S3, the heat preservation time is 3-10 minutes.

[0017] The present invention also provides an economical method for preparing a high-strength and high-toughness low-carbon steel with a bimodal structure. The microstructure of the low-carbon steel consists of fine-grained polygonal ferrite grains and ultrafine-grained quasi-polygonal ferrite grains interlocking and coexisting. The ultrafine grains are uniformly distributed around the fine grains, and the grain size has a bimodal scale distribution.

[0018] Furthermore, the grain size distribution of the low-carbon steel is as follows: the equivalent diameter of the fine grain size is 3-8 μm, and the equivalent diameter of the ultrafine grain size is 0.5-2 μm.

[0019] Furthermore, the area ratio of ultrafine ferrite with a diameter of less than 2 μm in the low carbon steel is 45%-75% in the field of view.

[0020] Furthermore, the mechanical properties of the low-carbon steel are as follows: tensile strength ≥910MPa, yield strength ≥400MPa, total elongation ≥22%, strength-ductility product ≥20GPa%, work hardening index ≥0.44, and yield strength ratio <0.5.

[0021] The present invention provides an economical bimodal high-strength, high-toughness, low-carbon steel and its preparation process, which has the following advantages:

[0022] This process is simple and low-cost. Moreover, the method can produce a bimodal structure through simple heat treatment. The hot-rolled low-carbon steel is heat-treated at ±10℃ from the Ac1 temperature point, held at that temperature for a certain time, and then air-cooled. It is then heated to the Ac1-Ac3 temperature range for two-phase heat treatment, with water as the quenching medium. The steel is then heated at a heating rate of 50℃ / s to the Ac1 temperature point +20℃ to the Ac3 temperature point -20℃ for two-phase heat treatment, followed by quenching. The quenching medium is oil or water at 30℃-50℃. This process produces a high-strength and high-toughness low-carbon steel with a bimodal structure.

[0023] The low-carbon steel obtained by the method of this invention has the characteristics of high strength and high toughness. Its tensile strength can reach more than 910 MPa, yield strength ratio is less than 0.5, total elongation is greater than 22%, and strength-ductility product is greater than 20 GPa%. It is suitable for use as a material for key components of automotive structural parts and shock dampers that require high strength and high toughness. Attached Figure Description

[0024] Figure 1 The image shows the metallographic structure of the sample from Example 1.

[0025] Figure 2 The image shows the metallographic structure of the sample from Example 2.

[0026] Figure 3 This is a metallographic image of the sample from Example 3.

[0027] Figure 4 This is a metallographic image of the sample from Example 4.

[0028] Figure 5 This is a metallographic image of the sample from Example 5.

[0029] Figure 6 This is a metallographic image of the sample from Example 6.

[0030] Figure 7 This is a metallographic image of the sample from Example 7.

[0031] Figure 8 This is a metallographic image of the sample from Example 8.

[0032] Figure 9 This is a metallographic image of the sample from Example 9.

[0033] Figure 10 The graph shows the mechanical properties (tensile strength) of the samples obtained in Examples 8 and 9.

[0034] Figure 11 This is a process flow diagram for an example. Detailed Implementation

[0035] To better understand the purpose, structure, and function of this invention, the following detailed description, in conjunction with the accompanying drawings, provides an economical bimodal high-strength, high-toughness, low-carbon steel and its preparation process.

[0036] Example 1:

[0037] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0038] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 800℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 780℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 5 minutes, and then quenched using oil quenching. Figure 1 As shown.

[0039] Depend on Figure 1 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 74.3% of the area in the field of view.

[0040] Example 2:

[0041] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0042] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 760℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 760℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 6 minutes, and then quenched using 30℃ water as the quenching medium. Figure 2 As shown.

[0043] Depend on Figure 2 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 60.3% of the area in the field of view.

[0044] Example 3:

[0045] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0046] According to the process flow chart, the low-carbon steel was placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel was then heated to 760℃ for two-phase region heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel was then heated to 740℃ at a heating rate of 80℃ / s for two-phase region heat treatment, held for 10 minutes, and then quenched using 50℃ water as the quenching medium. This resulted in a microstructure characterized by ultrafine-grained ferrite mixed with fine-grained martensite encapsulating coarse-grained ferrite, as shown below. Figure 3 As shown.

[0047] Depend on Figure 3 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 45.7% of the area in the field of view.

[0048] Example 4:

[0049] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0050] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 780℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 740℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 8 minutes, and then quenched using oil quenching. Figure 4 As shown.

[0051] Depend on Figure 4 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 55.3% of the area in the field of view.

[0052] Example 5:

[0053] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0054] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 760℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 760℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 5 minutes, and then quenched using 50℃ water as the quenching medium. Figure 5 As shown.

[0055] Depend on Figure 5 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 58.7% of the area in the field of view.

[0056] Example 6:

[0057] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0058] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 780℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 760℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 7 minutes, and then quenched using 50℃ water as the quenching medium. Figure 6 As shown.

[0059] Depend on Figure 6 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 62.7% of the area in the field of view.

[0060] Example 7:

[0061] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0062] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 790℃ for 2 hours, then air-cooled to room temperature. The air-cooled low-carbon steel is then heated to 780℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 780℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 3 minutes, and then quenched using 30℃ water as the quenching medium. Figure 7 As shown.

[0063] Depend on Figure 7 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 69.9% of the area in the field of view.

[0064] Example 8:

[0065] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0066] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 780℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 760℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 4 minutes, and then quenched using quenching oil as the quenching medium. Figure 8 As shown.

[0067] Depend on Figure 8 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 66.8% of the area in the field of view.

[0068] Example 9:

[0069] The selected raw material steel has the following chemical composition by mass percentage: C: 0.21%, Si: 0.46%, Mn: 1.4%, with the balance being Fe and other unavoidable impurity elements and microalloying elements. The Ac1 line of this low-carbon steel is 695℃, and the Ac3 line is 823℃.

[0070] According to the process flow chart, the low-carbon steel is placed in a heat treatment furnace and heat-treated at 700℃ for 2 hours, followed by air cooling to room temperature. The air-cooled low-carbon steel is then heated to 780℃ for two-phase heat treatment, held for 1 hour, and then quenched using water as the quenching medium. The quenched low-carbon steel is then heated to 760℃ at a heating rate of 80℃ / s for two-phase heat treatment, held for 4 minutes, and then quenched using 40℃ water as the quenching medium. Figure 9 As shown.

[0071] Depend on Figure 9 It can be seen that the microstructure of this high-strength, high-toughness, low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocked, with ultra-fine grains smaller than 2μm accounting for approximately 63.7% of the area in the field of view.

[0072] The high-strength, high-toughness, low-carbon steel samples obtained in Examples 8 and 9 were tested according to relevant national standards. The test results are shown in Table 1 and 2. Figure 10 .

[0073] Table 1. Summary of Sample Performance

[0074]

[0075] In conclusion:

[0076] (1) This invention uses C, Si, and Mn as alloying elements for low-carbon steel, wherein the C content is between 0.17 and 0.24%, the Si content is between 0.2 and 0.5%, and the Mn content is between 0.5 and 1.4%. The alloying elements are inexpensive, and the addition of these elements is simple and easy to recycle and reuse. By improving the comprehensive performance of low-carbon steel through simple alloying, this invention embodies the concept of sustainable development of steel materials.

[0077] (2) This invention proposes that low carbon steel with a bimodal structure can be prepared by heat treatment alone, and the comprehensive mechanical properties can be significantly improved. The process method is simple, universal, and applicable to various profiles and parts.

[0078] (3) Based on a simple alloy composition, the present invention constructs a low carbon steel with a bimodal structure through heat treatment, which has the characteristics of high strength and high toughness. Its tensile strength can reach more than 910 MPa, the yield strength ratio is less than 0.5, the total elongation is greater than 22%, and the strength-ductility product is greater than 20 GPa%. It is suitable for key components of automotive structural parts and shock dampers with high strength and high toughness requirements.

[0079] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. An economical process for preparing high-strength, high-toughness, low-carbon steel, characterized in that, High-strength and high-toughness low-carbon steel is produced by heat treatment using ordinary low-carbon steel as raw material. The chemical composition of the raw steel used is as follows (mass percentage): C content is 0.17%-0.24%; Si content is 0.2%-0.5%; Mn content is 0.5%-1.4%, with the balance being Fe and other unavoidable impurity elements. Includes the following steps: Step S1: Place the hot-rolled raw steel in a heat treatment furnace and heat treat it at a temperature of ±10℃ at point Ac1, hold it at that temperature, and then air cool it to room temperature. Step S2: The raw steel processed in Step S1 is heated to the temperature range Ac1-Ac3 for two-phase heat treatment, held at that temperature, and then quenched; the two-phase heat treatment temperature achieves a martensite volume fraction of more than 60%; the value range of Ac1 is 680℃-720℃; the value range of Ac3 is 795℃-855℃. Step S3: The raw steel processed in step S2 is heated at a certain heating rate to a temperature point Ac1 above +20°C to a temperature point Ac3 below -20°C for two-phase heat treatment, held at the temperature, and then quenched. In step S2, the quenching medium is water; in step S3, the quenching medium is quenching oil or water at 30℃-50℃; in step S1, the holding time is 2-20h; in step S2, the holding time is 0.5-20h; and in step S3, the holding time is 3-10min. The mechanical properties of the low-carbon steel are as follows: tensile strength ≥910MPa, yield strength ≥400MPa, total elongation ≥22%, strength-ductility product ≥20GPa%, work hardening index ≥0.44, and yield strength ratio <0.

5.

2. An economical, high-strength, high-toughness low-carbon steel with a bimodal structure, prepared by the process described in claim 1, characterized in that... The microstructure of the low-carbon steel consists of fine-grained polygonal ferrite grains and ultra-fine-grained quasi-polygonal ferrite grains interlocking and coexisting, with the ultra-fine grains uniformly distributed around the fine grains, and the grain size exhibiting a bimodal scale distribution.

3. The economical bimodal high-strength and high-toughness low-carbon steel according to claim 2, characterized in that, The grain size distribution of the low-carbon steel is as follows: the equivalent diameter of fine grains is 3-8 μm, and the equivalent diameter of ultrafine grains is 0.5-2 μm.

4. The economical bimodal high-strength and high-toughness low-carbon steel according to claim 2, characterized in that, The area ratio of ultrafine ferrite smaller than 2μm in the field of view of the low carbon steel is 45%-75%.

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