A method for improving the strength and toughness of low-carbon microalloyed steel

Abstract

This invention relates to the field of material microstructure control and heat treatment, and particularly to a method for improving the strength and toughness of low-carbon microalloyed steel. The specific process of this method is as follows: (1) adding a small amount of V element to the standard composition of low-carbon microalloyed steel; (2) using a high-temperature diffusion annealing process to reduce or eliminate segregation; (3) obtaining a certain proportion of martensitic structure through multiple critical zone holding and variable-speed quenching, while refining the grain size; (4) controlling the martensite decomposition products through a tempering process to obtain the final microstructure. This invention can effectively improve the final microstructure of low-carbon microalloyed steel and significantly enhance the mechanical properties of the low-carbon microalloyed steel material.

Description

Technical Field

[0001] This invention relates to the field of material microstructure control and heat treatment, and in particular to a method for improving the strength and toughness of low-carbon microalloyed steel. Background Technology

[0002] Low-carbon microalloyed steel possesses excellent comprehensive mechanical properties and good weldability, making it suitable for manufacturing structural components subjected to dynamic loads, such as pressure vessels, oil tanks, vehicles, mining machinery, and bridges. my country has a large production volume of low-carbon microalloyed steel, but it is mainly concentrated in low-to-mid-range products. The performance ceiling of domestically produced low-carbon microalloyed steel lags significantly behind that of foreign products with the same composition. This necessitates the use of higher-grade materials for high-end products with stringent performance requirements, which substantially increases production costs. Therefore, further improving the performance of domestically produced low-carbon microalloyed steel is of great significance. Summary of the Invention

[0003] The purpose of this invention is to provide a method for improving the strength and toughness of low-carbon microalloyed steel, thereby further improving the mechanical properties of low-carbon microalloyed steel materials while keeping the material composition essentially unchanged.

[0004] The technical solution of this invention is:

[0005] A method for improving the strength and toughness of low-carbon microalloyed steel, the specific process of which is as follows:

[0006] (1) Microalloying adjustment is made on the basis of the standard composition of low carbon microalloyed steel, and 0.05~0.15wt% of V element is added;

[0007] (2) High-temperature diffusion annealing process is used to reduce or eliminate segregation;

[0008] (3) Martensitic structure is obtained by holding the critical zone and quenching at different speeds twice or more, while refining the grain size.

[0009] (4) The martensite decomposition products are controlled by tempering process to obtain the final microstructure.

[0010] The method for improving the strength and toughness of low carbon microalloyed steel, in step (2), the high temperature diffusion annealing process is as follows: the low carbon microalloyed steel is heated to 1200~1250℃ and held for a holding time of 0.5~2h / 25mm thickness, and then furnace cooled to room temperature after the holding time is completed.

[0011] In the method for improving the strength and toughness of low-carbon microalloyed steel, in step (3), the critical zone heat preservation and variable speed quenching process has the following process parameters: the first process parameter is: directly heat to Ac1~Ac3 temperature, heat preservation time is 1~3h, then air cool to 550~650℃, and then water cool to room temperature; the second process parameter is: heat to 500~550℃, heat preservation time is 1~3h, then heat to Ac1~Ac3 temperature, heat preservation time is 1~3h, then air cool to 550~650℃, and then water cool to room temperature; the subsequent critical zone heat preservation and variable speed quenching process is consistent with the second process parameter.

[0012] In the method for improving the strength and toughness of low-carbon microalloyed steel, the critical zone quenching times in step (3) are usually 3 to 5 times.

[0013] In the method for improving the strength and toughness of low-carbon microalloyed steel, in step (3), after the critical zone heat preservation and variable speed quenching process, the volume ratio of martensite is 15~30% and the grain size is 20μm~40μm.

[0014] In the method for improving the strength and toughness of low-carbon microalloyed steel, the tempering process parameters in step (4) are: heating to 500~600℃ for tempering, holding time is 1~3h, and after holding, the steel is removed from the furnace and air-cooled to room temperature.

[0015] The mechanism by which this invention improves the strength and toughness of low-carbon microalloyed steel is:

[0016] This invention utilizes a multi-critical-zone holding and variable-speed quenching process to introduce a certain proportion of martensite into the quenched microstructure of low-carbon microalloyed steel. Compared to ferrite and bainite, tempered martensite exhibits higher dislocation and sub-interface density, thereby improving material strength. After appropriate tempering, the fine and dispersed spherical carbides precipitated in the martensite are less likely to become crack initiation sources and do not reduce the crack initiation energy of the material. Furthermore, because the martensite is distributed in an island-like pattern on the ferrite matrix, its introduction brings a large number of phase interfaces. Combined with the inherent abundance of interfaces in the martensite itself, these interfaces deflect the crack propagation direction, increasing the crack propagation path and effectively improving the crack propagation energy of the material. In addition, the addition of a small amount of vanadium can introduce fine MC carbides, thereby refining the austenite grains, which also contributes to the simultaneous improvement of material strength and toughness.

[0017] The advantages and beneficial effects of this invention are:

[0018] 1. This invention achieves a significant improvement in the performance of low-carbon microalloyed steel while keeping the material composition basically unchanged. Its mechanical properties can be achieved as follows: yield strength (Re) ≥ 450MPa, tensile strength (Rm) ≥ 520MPa, elongation (A) ≥ 20%, and impact energy (AKv) at -46℃ ≥ 70J.

[0019] 2. This invention is simple to operate, and the resulting sample has a uniform and stable structure with excellent performance, making it very suitable for mass production.

[0020] 3. This invention can be widely applied to various low-carbon microalloyed steels. Attached Figure Description

[0021] Figure 1 The final microstructure of the improved low-carbon microalloyed steel in Example 1 is shown.

[0022] Figure 2 The final microstructure of the improved low-carbon microalloyed steel in Example 2 is shown.

[0023] Figure 3 The final microstructure of the improved low-carbon microalloyed steel in Example 3 is shown. Detailed Implementation

[0024] In the specific implementation process, the specific process of the method of the present invention is as follows: (1) Add a small amount of V element on the basis of the standard composition of low carbon microalloy steel; (2) Use high temperature diffusion annealing process to reduce or eliminate segregation; (3) Obtain a certain proportion of martensite structure by multiple critical zone heat preservation and variable speed quenching, and at the same time refine the grain size; (4) Control the martensite decomposition products by tempering process and obtain the final structure.

[0025] The existing standard composition range requirements for low-carbon microalloyed steel are as follows:

[0026] element Content (wt%) C 0.08~0.12 Si 0.20~0.35 Mn 0.90~1.40 P ≤0.01 S ≤0.006 Ni 0.60~0.70 Cr 0.20~0.30 Mo 0.20~0.30 Fe margin

[0027] The present invention will now be further described in detail with reference to embodiments and accompanying drawings.

[0028] Example 1

[0029] In this embodiment, based on the existing composition range of low-carbon microalloyed steel, 0.12 wt% V is added. The final material composition is: 0.11C-0.30Si-0.95Mn-0.01P-0.005S-0.61Ni-0.20Cr-0.22Mo-0.12V (weight percentage, Fe balance), and its preparation method includes smelting, forging, and heat treatment. Wherein:

[0030] Melting: The low-carbon microalloyed steel is prepared according to its chemical composition and composition, and then added to a vacuum induction melting furnace. It is melted at a temperature of 1650℃ and cast into an ingot.

[0031] Forging: After holding the ingot at 1150℃ for 6 hours, it is subjected to three forging and three drawing processes. After forging, it is air-cooled to room temperature.

[0032] The heat treatment process involved the following steps: ① After forging, the sample was heated to 1250℃ and held for 2 hours, then furnace cooled to room temperature. ② The sample was heated to 820℃ and held for 2 hours, then air-cooled to 600℃, and finally water-cooled to room temperature. ③ The sample was heated to 540℃ and held for 2 hours, then heated to 820℃ and held for 2 hours, then air-cooled to 600℃, and finally water-cooled to room temperature. ④ The sample was heated to 540℃ and held for 2 hours, then heated to 820℃ and held for 2 hours, then air-cooled to 600℃, and finally water-cooled to room temperature. In this embodiment, after critical zone holding and variable-speed quenching, the volume ratio of martensite was approximately 20%, and the average grain size was approximately 25 μm. ⑤ The sample was heated to 540℃ for tempering, held for 2 hours, and then air-cooled to room temperature.

[0033] The above heat treatment can significantly improve the final microstructure of low-carbon microalloyed steel. The microstructure of the material is shown in the figure. Figure 1 Its performance is as follows:

[0034] The yield strength (Re) is 566 MPa, the tensile strength (Rm) is 481 MPa, the elongation (A) is 21%, and the impact energy (AKv) at -46℃ is 113 J.

[0035] Example 2

[0036] In this embodiment, based on the existing composition range of low-carbon microalloyed steel, 0.06 wt% V is added. The final material composition is: 0.09C-0.32Si-1.07Mn-0.008P-0.004S-0.63Ni-0.26Cr-0.25Mo-0.06V (weight percentage, Fe balance), and its preparation method includes smelting, forging, and heat treatment. Wherein:

[0037] Melting: The low-carbon microalloyed steel is prepared according to its chemical composition and composition, and then added to a vacuum induction melting furnace. It is melted at a temperature of 1650℃ and cast into an ingot.

[0038] Forging: After holding the ingot at 1150℃ for 6 hours, it is subjected to three forging and three drawing processes. After forging, it is air-cooled to room temperature.

[0039] The heat treatment process involved the following steps: ① After forging, the sample was heated to 1216℃ and held for 1.5 hours, then furnace cooled to room temperature. ② The sample was heated to 810℃ and held for 2.5 hours, then air-cooled to 580℃, and finally water-cooled to room temperature. ③ The sample was heated to 520℃ and held for 2.5 hours, then heated to 810℃ and held for 2.5 hours, then air-cooled to 580℃, and finally water-cooled to room temperature. ④ The sample was heated to 520℃ and held for 2.5 hours, then heated to 810℃ and held for 2.5 hours, then air-cooled to 580℃, and finally water-cooled to room temperature. In this embodiment, after critical zone holding and variable-speed quenching, the volume ratio of martensite was 17%, and the average grain size was approximately 22 μm. ⑤ The sample was heated to 520℃ for tempering, held for 2.5 hours, and then air-cooled to room temperature.

[0040] The above heat treatment can significantly improve the final microstructure of low-carbon microalloyed steel. The microstructure of the material is shown in the figure. Figure 2 Its performance is as follows:

[0041] The yield strength (Re) is 557 MPa, the tensile strength (Rm) is 476 MPa, the elongation (A) is 21.5%, and the impact energy (AKv) at -46℃ is 119 J.

[0042] Example 3

[0043] In this embodiment, based on the existing composition range of low-carbon microalloyed steel, 0.10 wt% V is added. The final material composition is: 0.10C-0.26Si-1.31Mn-0.005P-0.003S-0.67Ni-0.22Cr-0.28Mo-0.10V (weight percentage, Fe balance), and its preparation method includes smelting, forging, and heat treatment. Wherein:

[0044] Melting: The low-carbon microalloyed steel is prepared according to its chemical composition and composition, and then added to a vacuum induction melting furnace. It is melted at a temperature of 1650℃ and cast into an ingot.

[0045] Forging: After holding the ingot at 1150℃ for 6 hours, it is subjected to three forging and three drawing processes. After forging, it is air-cooled to room temperature.

[0046] The heat treatment process involved the following steps: ① After forging, the sample was heated to 1235℃ and held for 1 hour, then furnace cooled to room temperature. ② The sample was heated to 830℃ and held for 1.5 hours, then air-cooled to 615℃, and finally water-cooled to room temperature. ③ The sample was heated to 530℃ and held for 1.5 hours, then heated to 830℃ and held for 1.5 hours, then air-cooled to 615℃, and finally water-cooled to room temperature. ④ The sample was heated to 530℃ and held for 1.5 hours, then heated to 830℃ and held for 1.5 hours, then air-cooled to 615℃, and finally water-cooled to room temperature. In this embodiment, after critical zone holding and variable-speed quenching, the volume ratio of martensite was 15%, and the average grain size was approximately 20 μm. ⑤ The sample was heated to 560℃ for tempering, held for 1.5 hours, and then air-cooled to room temperature.

[0047] The above heat treatment can significantly improve the final microstructure of low-carbon microalloyed steel. The microstructure of the material is shown in the figure. Figure 3 Its performance is as follows:

[0048] The yield strength (Re) is 572 MPa, the tensile strength (Rm) is 493 MPa, the elongation (A) is 19.5%, and the impact energy (AKv) at -46℃ is 106 J.

[0049] The results of the examples show that the present invention, through heat treatment process control, introduces island-like martensite structure, thereby achieving a significant improvement in the performance of low-carbon microalloyed steel materials.

[0050] For those skilled in the art, various other corresponding changes can be made based on the above technical solutions and concepts, and all such changes should fall within the protection scope of the claims of this invention.

Claims

1. A method for improving the strength and toughness of low-carbon microalloyed steel, characterized in that, The specific process is as follows: (1) Microalloying adjustment is made on the basis of the standard composition of low carbon microalloyed steel, and 0.05~0.12wt% of V element is added; the standard composition of the low carbon microalloyed steel by weight percentage is: C 0.08~0.12%, Si 0.20~0.35%, Mn 0.90~1.40%, P≤0.01%, S≤0.006%, Ni 0.60~0.70%, Cr 0.20~0.30%, Mo 0.20~0.30%, with the balance being Fe; (2) High-temperature diffusion annealing process is adopted to reduce or eliminate segregation. The high-temperature diffusion annealing process is as follows: the low carbon microalloy steel is heated to 1200~1250℃ and held for 0.5~2h / 25mm thickness. After the holding time is completed, the furnace is cooled to room temperature. (3) Martensitic structure is obtained by more than two critical zone heat preservation and variable speed quenching processes, and the grain size is refined at the same time. In the critical zone heat preservation and variable speed quenching process, the first process parameters are: directly heat to Ac1~Ac3 temperature, heat preservation time is 1~3h, then air cool to 550~650℃, and then water cool to room temperature; the second process parameters are: heat to 500~550℃, heat preservation time is 1~3h, then heat to Ac1~Ac3 temperature, heat preservation time is 1~3h, then air cool to 550~650℃, and then water cool to room temperature; the subsequent critical zone heat preservation and variable speed quenching processes are consistent with the second process parameters. After critical zone heat preservation and variable speed quenching process, a certain proportion of martensite is introduced into the quenched structure of low carbon microalloy steel. The martensite is distributed in islands on the ferrite matrix, which increases the crack propagation path and effectively improves the crack propagation energy of the material. The volume ratio of martensite ranges from 15% to 30%, and the grain size ranges from 20μm to 40μm. (4) The martensite decomposition products are controlled by tempering process and the final structure is obtained. The tempering process parameters are: heat to 500~600℃ for tempering, hold for 1~3h, and air cool to room temperature after holding.

2. The method for improving the strength and toughness of low-carbon microalloyed steel according to claim 1, characterized in that, In step (3), the critical zone is quenched 3 to 5 times.

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