Heat treatment method for reducing yield ratio of low alloy steel and low alloy steel with low yield ratio

Abstract

The invention provides a heat treatment method for reducing the yield ratio of low alloy steel and the low alloy steel with the low yield ratio, and belongs to the technical field of heat treatment of the low alloy steel. According to the method, a small amount of dislocation is introduced through cold rolling deformation to provide nucleation sites for an austenite phase in a two-phase region, so that the austenite phase in the two-phase region grows along a martensite lath to form a new martensite phase, and Ni element enrichment is established at a new martensite grain boundary; and then, by utilizing the short-distance diffusion path characteristic of the grain boundary and aging heat treatment, the Ni element is further enriched at the new martensite grain boundary to obtain the film-shaped inverted austenite. The result of the embodiment shows that film-shaped inverted austenite is separated out in the low alloy steel obtained through the treatment process, and meanwhile, the tensile strength of the low alloy steel is larger than 990MPa, the yield strength of the low alloy steel is larger than 880MPa, the yield ratio of the low alloy steel is smaller than 0.9, and the ductility of the low alloy steel is larger than 26%.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment of low alloy steel, in particular to a heat treatment method for reducing the yield ratio of low alloy steel and low alloy steel with low yield ratio. Background Art

[0002] High-strength low-alloy steel, with its exceptional strength, good plasticity, and relatively simple welding process, has become the material of choice in many industrial fields, and is widely used in the automotive industry, shipbuilding, bridge construction, and oil pipeline construction. However, with the continuous improvement of industrial development, the performance requirements of steel materials continue to increase. To meet the long-term service requirements of steel materials, it is necessary to achieve high ductility and a low yield-to-strength ratio while maintaining the yield strength of steel, thereby improving the service performance and application potential of the material.

[0003] Film-like austenite can improve the shaping and work hardening capabilities under high strain conditions through the TRIP (Transformation Induced Plasticity) effect, allowing the performance of low-alloy steel to meet higher mechanical properties. In alloy steels with high alloy content such as medium manganese steel and nickel steel, researchers use the quenching-critical quenching-tempering (QLT) process to introduce the reverse transformed austenite phase. However, for low-alloy steels, due to the low content of alloy elements and the lack of austenite stabilizing elements, it is difficult to achieve the enrichment of austenite stabilizing elements to maintain reverse austenite. Therefore, the precipitation of film-like reverse austenite in low-alloy steels has received increasing attention.

[0004] Patent document CN114058793A discloses a heat treatment technology for reducing the yield ratio of ultra-high-strength marine steel EH890. This technology achieves this reduction by adding at least 0.13% carbon to the material and using the QLT process to precipitate soft-phase critical ferrite during air cooling. However, practical verification shows that the yield ratio of the treated material still exceeds 0.94. Furthermore, a high carbon content adversely affects welding processes, increasing welding difficulty and potential risks, and presenting certain obstacles to practical engineering applications. Patent document CN117626126A discloses a method for producing ultra-low carbon steel reaching a grade of 980 MPa. This method utilizes the addition of nickel (Ni) exceeding 4.6% and the QLT process to precipitate reversed austenite, leveraging the TRIP effect to increase steel strength. However, the yield ratio remains above 0.9, and the high Ni content makes production relatively expensive.

[0005] Therefore, how to improve the ductility of low alloy steel and reduce the yield strength ratio while stably maintaining the yield strength of the steel has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0006] The object of the present invention is to provide a heat treatment method for reducing the yield ratio of low alloy steel and low alloy steel with a low yield ratio. The method provided by the present invention achieves the optimization goals of improving ductility and reducing the yield ratio on the basis of stably maintaining the yield strength of the steel.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a heat treatment method for reducing the yield ratio of low alloy steel, comprising the following steps:

[0009] (1) heating the low alloy steel to above Ac3 and performing solid solution treatment to obtain solid solution low alloy steel;

[0010] (2) pre-deforming the solid solution low alloy steel obtained in step (1), and then heating it to a temperature between Ac1 and Ac3 for critical quenching to obtain critical quenched low alloy steel; the total deformation amount of the pre-deformation treatment is 0.5 to 4%;

[0011] (3) The critically quenched low alloy steel obtained in step (2) is subjected to aging treatment below Ac1 to obtain a low alloy steel with a low yield ratio.

[0012] Preferably, the chemical composition of the low alloy steel in step (1) is, by mass percentage, C: 0.03-0.08%, Si: 0.2-0.3%, Mn: 0.8-1.2%, Cu: 1-1.5%, Ni: 2.5-3.5%, Mo: 0.3-0.6%, Ti: 0.05-0.1% and the balance iron.

[0013] Preferably, the holding temperature of the solution treatment in step (1) is 870-930° C., and the holding time of the solution treatment is 0.5-4 h.

[0014] Preferably, the holding temperature of the solution treatment in step (2) is 900° C., and the holding time of the solution treatment is 1 hour.

[0015] Preferably, the total deformation amount of the pre-deformation treatment in step (2) is 1 to 2%.

[0016] Preferably, the holding temperature of the critical quenching treatment in step (2) is 720-780° C., and the holding time of the critical quenching treatment is 0.5-4 h.

[0017] Preferably, the holding temperature of the critical quenching treatment is 750° C., and the holding time of the critical quenching treatment is 1 hour.

[0018] Preferably, the holding temperature of the aging treatment in step (3) is 500-600° C., and the holding time of the aging treatment is 0.5-25 h.

[0019] Preferably, the holding temperature of the aging treatment is 550° C., and the holding time of the aging treatment is 1 hour.

[0020] The present invention provides low-alloy steel with a low yield ratio prepared by the heat treatment method described in the above technical solution.

[0021] The present invention provides a heat treatment method for reducing the yield ratio of low alloy steel, comprising the following steps: (1) heating the low alloy steel to above Ac3 and performing a solid solution treatment to obtain a solid solution low alloy steel; (2) pre-deforming the solid solution low alloy steel obtained in step (1), and then heating it to between Ac1 and Ac3 and performing a critical quenching treatment to obtain a critical quenching low alloy steel; the total deformation amount of the pre-deformation treatment is 0.5-4%; (3) performing an aging treatment on the critical quenching low alloy steel obtained in step (2) at a temperature below Ac1 to obtain a low alloy steel with a low yield ratio. The present invention utilizes cold rolling deformation to introduce a small amount of dislocations to provide nucleation sites for the austenite phase in the two-phase region, so that the austenite phase in the two-phase region grows along the martensite lath to form a new martensite phase and establishes Ni enrichment at the new martensite grain boundary; then, utilizing the short-range diffusion path characteristics of the grain boundary and aging heat treatment, the Ni element is further enriched at the new martensite grain boundary to obtain a thin film-shaped reverse austenite. The heat treatment process provided by the present invention precipitates stable film-like reverse austenite in low-alloy steel, achieving the optimization goal of improving ductility and reducing yield strength ratio on the basis of stably maintaining the yield strength of the steel. This result successfully achieves a good balance with high toughness, greatly meeting the stringent requirements of modern industry for the comprehensive performance of low-alloy steel, and providing reliable performance guarantee for it in the application environment, thereby significantly improving the service performance and application potential of low-alloy steel materials. The treatment process provided by the present invention only needs to adjust a small amount of cold rolling pre-deformation to achieve the precipitation of film-like reverse austenite in low-alloy steel. It is simple, convenient, low-cost, and suitable for widespread promotion and application in industrial production. The results of the embodiment show that the low-alloy steel obtained by the heat treatment method provided by the present invention precipitates film-like reverse austenite, and at the same time, the tensile strength of the low-alloy steel is greater than 990MPa, the yield strength is greater than 880MPa, the yield strength ratio is less than 0.9, and the elongation is greater than 26%. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of a process flow for precipitating thin-film reversed austenite in low alloy steel provided in Example 2 of the present invention;

[0023] Figure 2 This is a microstructure diagram of the low alloy steel with low yield ratio prepared in Example 1 of the present invention;

[0024] Figure 3 This is a microstructure diagram of the low alloy steel with low yield ratio prepared in Example 2 of the present invention;

[0025] Figure 4 This is a microstructure diagram of the low alloy steel with low yield ratio prepared in Example 3 of the present invention;

[0026] Figure 5 This is a TEM bright field image of the low alloy steel with low yield ratio prepared in Example 1;

[0027] Figure 6 for Figure 5 Electron diffraction patterns of the corresponding selected areas;

[0028] Figure 7 Based on Figure 6 Dark field image obtained from the electron diffraction pattern in;

[0029] Figure 8 These are the stress-strain curves of the low alloy steels prepared in Examples 1 to 3 and Comparative Examples 1 to 2. DETAILED DESCRIPTION

[0030] The present invention provides a heat treatment method for reducing the yield ratio of low alloy steel, comprising the following steps:

[0031] (1) heating the low alloy steel to above Ac3 and performing solid solution treatment to obtain solid solution low alloy steel;

[0032] (2) pre-deforming the solid solution low alloy steel obtained in step (1), and then heating it to a temperature between Ac1 and Ac3 for critical quenching to obtain critical quenched low alloy steel; the total deformation amount of the pre-deformation treatment is 0.5 to 4%;

[0033] (3) The critically quenched low alloy steel obtained in step (2) is subjected to aging treatment below Ac1 to obtain a low alloy steel with a low yield ratio.

[0034] The present invention heats low alloy steel to above Ac3 (Ac3 refers to the complete transformation temperature of austenite) and performs solid solution treatment to obtain solid solution low alloy steel.

[0035] The present invention has no special limitation on the specific type and source of the low alloy steel, and low alloy steels well known to those skilled in the art can be used. As an embodiment of the present invention, the chemical composition of the low alloy steel can be, by mass percentage, C: 0.03-0.08%, Si: 0.2-0.3%, Mn: 0.8-1.2%, Cu: 1-1.5%, Ni: 2.5-3.5%, Mo: 0.3-0.6%, Ti: 0.05-0.1% and the balance iron, or C: 0.04-0.07%, Si: 0.22-0.2 The chemical composition of the low-alloy steel can be controlled to achieve both high ductility and a low yield strength ratio while maintaining yield strength.

[0036] In the present invention, the holding temperature of the solution treatment is preferably 870-930°C; the holding time of the solution treatment is preferably 0.5-4h; the cooling method after the solution treatment is preferably air cooling, furnace cooling, water cooling or oil cooling. As an embodiment of the present invention, the holding temperature of the solution treatment can be 880°C, 890°C, 900°C, 910°C or 920°C; the holding time of the solution treatment can be 1h, 1.5h, 2h, 2.5h, 3h or 3.5h. The present invention can obtain lath martensite structure in low alloy steel by heating low alloy steel to above Ac3 and performing solution treatment.

[0037] After obtaining the solid solution low alloy steel, the present invention performs a pre-deformation treatment on the solid solution low alloy steel, and then heats it to Ac1-Ac3 (Ac1 refers to the austenite start transformation temperature) for critical quenching treatment to obtain critical quenched low alloy steel.

[0038] In the present invention, the total deformation amount of the pre-deformation treatment is 0.5-4%; the pre-deformation treatment is preferably cold rolling. The present invention has no special limitation on the deformation amount of a single pass and the number of rolling passes of the pre-deformation treatment, which can be determined according to the technical common sense of those skilled in the art. As an embodiment of the present invention, the total deformation amount of the pre-deformation treatment can be 1%, 1.5%, 2%, 2.5%, 3% or 3.5%; the number of rolling passes of the pre-deformation treatment can be 1-15 passes, or can be 2 passes, 3 passes, 4 passes, 5 passes, 6 passes, 7 passes, 8 passes, 9 passes, 10 passes, 11 passes, 12 passes, 13 passes or 14 passes. The present invention utilizes cold rolling deformation added after solution treatment to introduce a small amount of dislocations to provide nucleation sites for the austenite phase in the two-phase region, so that the austenite phase in the two-phase region grows along the martensite laths to form a new martensite phase, and establishes Ni element enrichment at the new martensite grain boundaries. In the subsequent aging treatment process, this nickel enrichment promotes the distribution of secondary elements, and precipitates stable thin-film inversion austenite at the new martensite crystal planes.

[0039] In the present invention, the holding temperature of the critical quenching treatment is preferably 720-780°C; the holding time of the critical quenching treatment is preferably 0.5-4h; and the cooling method after the critical quenching treatment is preferably air cooling, furnace cooling, water cooling or oil cooling. As an embodiment of the present invention, the holding temperature of the critical quenching treatment can be 730°C, 740°C, 750°C, 760°C or 770°C; the holding time of the critical quenching treatment can be 1h, 1.5h, 2h, 2.5h, 3h or 3.5h. The present invention can obtain lath martensite structure + fresh martensite structure through critical quenching treatment.

[0040] After obtaining the critical quenching low alloy steel, the present invention performs aging treatment on the critical quenching low alloy steel at a temperature below Ac1 to obtain low alloy steel with a low yield ratio.

[0041] In the present invention, the holding temperature of the aging treatment is preferably 500-600°C; the holding time of the aging treatment is preferably 0.5-25h; the cooling method after the aging treatment is preferably air cooling, furnace cooling, water cooling or oil cooling. As an embodiment of the present invention, the holding temperature of the aging treatment can be 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C or 590°C; the holding time of the aging treatment can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h or 24h. The present invention utilizes the short-range diffusion path characteristics of grain boundaries and aging heat treatment to further enrich the Ni element at the new martensite grain boundaries to obtain thin-film reverse austenite, and can precipitate thin-film reverse austenite in low-carbon low-alloy steel.

[0042] The present invention utilizes cold rolling deformation to introduce a small amount of dislocations to provide nucleation sites for the austenite phase in the two-phase region, so that the austenite phase in the two-phase region grows along the martensite lath to form a new martensite phase and establishes Ni enrichment at the new martensite grain boundary; then, utilizing the short-range diffusion path characteristics of the grain boundary and aging heat treatment, the Ni element is further enriched at the new martensite grain boundary to obtain a thin film of reversed austenite. The heat treatment process provided by the present invention precipitates stable thin film of reversed austenite in low alloy steel, achieving the optimization goal of improving ductility and reducing yield strength ratio while stably maintaining the yield strength of the steel. This result successfully achieves a good balance with high toughness, greatly meeting the stringent requirements of modern industry for the comprehensive performance of low alloy steel, and providing reliable performance guarantee for it in the application environment, thereby significantly improving the service performance and application potential of low alloy steel materials.

[0043] The treatment process provided by the present invention only requires adjusting a small amount of cold rolling pre-deformation to achieve the precipitation of thin-film inverted austenite in low-alloy steel. It is simple, convenient, low-cost, and suitable for wide promotion and application in industrial production.

[0044] The present invention provides low-alloy steel with a low yield ratio prepared by the heat treatment method described in the above technical solution.

[0045] In the present invention, the chemical composition of the low alloy steel with low yield ratio is preferably: C: 0.03-0.08%, Si: 0.2-0.3%, Mn: 0.8-1.2%, Cu: 1-1.5%, Ni: 2.5-3.5%, Mo: 0.3-0.6%, Ti: 0.05-0.1% and the balance of iron, more preferably: C: 0.04-0.07%, Si: 0.22-0.28 %, Mn: 0.9~1.1%, Cu: 1.1~1.4%, Ni: 2.8~3.2%, Mo: 0.4~0.5%, Ti: 0.06~0.09% and the balance iron, further preferably: C: 0.05~0.06%, Si: 0.25%, Mn: 1%, Cu: 1.2~1.3%, Ni: 3%, Mo: 0.4~0.5%, Ti: 0.07~0.08% and the balance iron.

[0046] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] Example 1

[0048] A heat treatment method for reducing the yield ratio of low alloy steel comprises the following steps:

[0049] (1) low alloy steel is heated to above Ac3 for solution treatment, and then water-cooled to room temperature to obtain a solid solution low alloy steel with a lath martensite structure; the chemical composition of the low alloy steel is, by mass percentage, C: 0.03%, Si: 0.2%, Mn: 0.8%, Cu: 1%, Ni: 2.5%, Mo: 0.3%, Ti: 0.05% and the balance is iron; the holding temperature of the solid solution treatment is 930°C, and the holding time of the solid solution treatment is 0.5h;

[0050] (2) subjecting the solid solution low alloy steel obtained in step (1) to one pre-deformation treatment, then heating it to a temperature between Ac1 and Ac3 for critical quenching treatment, and water cooling it to room temperature to obtain a critical quenched low alloy steel having a lath martensite structure + a newly formed martensite structure; the total deformation amount of the pre-deformation treatment is 0.5%, and the pre-deformation treatment is cold rolling; the holding temperature of the critical quenching treatment is 720° C., and the holding time of the critical quenching treatment is 0.5 h;

[0051] (3) The critically quenched low alloy steel obtained in step (2) is subjected to aging treatment below Ac1, and water-cooled to room temperature to obtain low alloy steel with a low yield ratio; the holding temperature of the aging treatment is 500°C, and the holding time of the aging treatment is 0.5h.

[0052] Example 2

[0053] A heat treatment method for reducing the yield ratio of low alloy steel comprises the following steps:

[0054] (1) low alloy steel is heated to above Ac3 for solution treatment, and then water-cooled to room temperature to obtain a solid solution low alloy steel with a lath martensite structure; the chemical composition of the low alloy steel is, by mass percentage, C: 0.05%, Si: 0.25%, Mn: 0.9%, Cu: 1.2%, Ni: 3.2%, Mo: 0.5%, Ti: 0.05% and the balance is iron; the holding temperature of the solid solution treatment is 900°C, and the holding time of the solid solution treatment is 1 hour;

[0055] (2) The solid solution low alloy steel obtained in step (1) is subjected to three pre-deformation treatments, and then heated to a temperature between Ac1 and Ac3 for critical quenching treatment, and water-cooled to room temperature to obtain a critical quenched low alloy steel with lath martensite structure + new martensite structure; the total deformation amount of the pre-deformation treatment is 2%, and the pre-deformation treatment is cold rolling; the holding temperature of the critical quenching treatment is 750° C., and the holding time of the critical quenching treatment is 1 hour;

[0056] (3) The critically quenched low alloy steel obtained in step (2) is subjected to aging treatment at a temperature below Ac1, and water-cooled to room temperature to obtain a low alloy steel with a low yield ratio; the holding temperature of the aging treatment is 550° C., and the holding time of the aging treatment is 1 hour.

[0057] The process flow diagram of precipitating thin film reverse austenite in low alloy steel provided in Example 2 of the present invention is as follows: Figure 1 As shown. Figure 1 It can be seen that the method provided by the present invention first heats the low alloy steel to above Ac3 for solution treatment, water cools it to room temperature and then performs three cold rolling pre-deformation treatments, then heats it to between Ac1 and Ac3 for critical quenching treatment, and water cools it to room temperature and then performs aging treatment below Ac1 to obtain a low alloy steel with a low yield ratio.

[0058] Example 3

[0059] A heat treatment method for reducing the yield ratio of low alloy steel comprises the following steps:

[0060] (1) low alloy steel is heated to above Ac3 for solution treatment, and then water-cooled to room temperature to obtain a solid solution low alloy steel with a lath martensite structure; the chemical composition of the low alloy steel is, by mass percentage, C: 0.08%, Si: 0.3%, Mn: 1.2%, Cu: 1.5%, Ni: 3.5%, Mo: 0.6%, Ti: 0.1% and the balance is iron; the holding temperature of the solid solution treatment is 870°C, and the holding time of the solid solution treatment is 4 hours;

[0061] (2) The solid solution low alloy steel obtained in step (1) is subjected to 10 pre-deformation treatments, and then heated to a temperature between Ac1 and Ac3 for critical quenching treatment, and water-cooled to room temperature to obtain a critical quenched low alloy steel having a lath martensite structure + a newly formed martensite structure; the total deformation amount of the pre-deformation treatment is 4%, and the pre-deformation treatment is cold rolling; the holding temperature of the critical quenching treatment is 780° C., and the holding time of the critical quenching treatment is 4 hours;

[0062] (3) The critical quenched low alloy steel obtained in step (2) is subjected to aging treatment below Ac1 and water-cooled to room temperature to obtain low alloy steel with a low yield ratio; the holding temperature of the aging treatment is 600°C, and the holding time of the aging treatment is 25 hours.

[0063] Comparative Example 1

[0064] A method for processing low alloy steel comprises the following steps:

[0065] (1) low alloy steel is heated to above Ac3 for solution treatment, and then water-cooled to room temperature to obtain a solid solution low alloy steel with a lath martensite structure; the chemical composition of the low alloy steel is, by mass percentage, C: 0.05%, Si: 0.25%, Mn: 0.9%, Cu: 1.2%, Ni: 3.2%, Mo: 0.5%, Ti: 0.05% and the balance is iron; the holding temperature of the solid solution treatment is 900°C, and the holding time of the solid solution treatment is 1 hour;

[0066] (2) heating the solid solution low alloy steel obtained in step (1) to a temperature between Ac1 and Ac3 for intercritical quenching treatment, and water cooling to room temperature to obtain intercritical quenched low alloy steel; the holding temperature of the intercritical quenching treatment is 750° C., and the holding time of the intercritical quenching treatment is 1 hour;

[0067] (3) The critical quenched low alloy steel obtained in step (2) is subjected to aging treatment at a temperature below Ac1, and water-cooled to room temperature to obtain low alloy steel; the holding temperature of the aging treatment is 550° C., and the holding time of the aging treatment is 1 hour.

[0068] Comparative Example 2

[0069] A method for processing low alloy steel comprises the following steps:

[0070] (1) low alloy steel is heated to above Ac3 for solution treatment, and then water-cooled to room temperature to obtain a solid solution low alloy steel with a lath martensite structure; the chemical composition of the low alloy steel is, by mass percentage, C: 0.05%, Si: 0.25%, Mn: 0.9%, Cu: 1.2%, Ni: 3.2%, Mo: 0.5%, Ti: 0.05% and the balance is iron; the holding temperature of the solid solution treatment is 900°C, and the holding time of the solid solution treatment is 1 hour;

[0071] (2) The solid solution low alloy steel obtained in step (1) is subjected to 10 pre-deformation treatments, and then heated to a temperature between Ac1 and Ac3 for critical quenching treatment, and water-cooled to room temperature to obtain critical quenched low alloy steel; the total deformation amount of the pre-deformation treatment is 5%, and the pre-deformation treatment is cold rolling; the holding temperature of the critical quenching treatment is 750° C., and the holding time of the critical quenching treatment is 1 hour;

[0072] (3) The critical quenched low alloy steel obtained in step (2) is subjected to aging treatment at a temperature below Ac1, and water-cooled to room temperature to obtain low alloy steel; the holding temperature of the aging treatment is 550° C., and the holding time of the aging treatment is 1 hour.

[0073] The microstructure of the low alloy steel with low yield ratio prepared in Examples 1 to 3 of the present invention was observed, and the results were as follows: Figure 2 、 Figure 3 and Figure 4 .Depend on Figures 2-4 It can be seen that after the low alloy steel is treated by the heat treatment method provided by the present invention, the microstructure of the low alloy steel prepared is uniform.

[0074] Figure 5 This is a TEM bright field image of the low alloy steel with low yield ratio prepared in Example 1. Figure 5 Selected area electron diffraction is performed on the corresponding selected area (circled area) in the figure, and the results are as follows Figure 6 As shown; using Figure 6 The diffraction spot is used as the central dark field, and the results are as follows Figure 7 As shown. Figure 5 It can be seen that there are other phases at the grain boundaries of the newly formed martensite, and then combined with Figure 6 and Figure 7 It can be seen that Figure 5 The other phases in the film are actually austenite phase (i.e., reverse austenite) and appear in a film-like form.

[0075] Figure 7 This is a TEM dark field image of the low alloy steel with low yield ratio prepared in Example 1. Figure 7It can be seen that the white part is reversed austenite, which indicates that the heat treatment method provided by the present invention obtains reversed austenite.

[0076] The mechanical properties of the low alloy steels prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were tested, and the obtained results were as follows: Figure 8 As shown in Table 1, Figure 8 The stress-strain curves of the low alloy steels prepared in Examples 1 to 3 and Comparative Examples 1 to 2 are as follows:

[0077] Table 1 Mechanical properties of low alloy steels prepared in Examples 1 to 3 and Comparative Examples 1 to 2

[0078] Group Yield strength / MPa Tensile strength / MPa Yield-to-strength ratio Elongation / % Example 1 886±11 993±19 0.89 27.1±1.9 Example 2 922±15 1042±24 0.88 26.7±1.1 Example 3 907±7 1018±14 0.89 28.7±1.9 Comparative Example 1 912±8 940±9 0.97 20.3±0.9 Comparative Example 2 913±14 958±17 0.95 21.1±1.3

[0079] The tensile test was conducted in accordance with the national standard "Metallic Materials Tensile Test Methods" (GB / T 228-2002). Plate-shaped tensile specimens with a gauge length of 12.5 mm were machined using a CNC machine tool, and the length of the specimens was in the rolling direction (RD). The surfaces of the tensile specimens were polished before testing to avoid errors caused by surface defects. Tensile tests were conducted using an INSTRON-5500R universal testing machine at room temperature and under axial loading conditions. The tensile strain rate was approximately 1.3 × 10 -3 s -1 , and the strain was calibrated using an extensometer; each group of experiments was repeated 3 times, and the average value was taken. The error was the standard deviation of the 3 repeated tests. After the tensile test, the tensile strength (MPa), yield strength (MPa) and elongation (%) of the material were calculated according to the stress-strain curve.

[0080] Depend on Figure 8As can be seen from Table 1, the yield strength ratio of the low alloy steel prepared in Examples 1 to 3 of the present invention is less than 0.9, while the yield strength ratio of the low alloy steel prepared in Comparative Examples 1 to 2 is ≥0.95, indicating that the method provided by the present invention can significantly reduce the yield strength ratio of the low alloy steel, and the low alloy steel prepared in the present invention has a higher elongation than that in Comparative Examples 1 to 2; by comparing Example 2 with Comparative Example 1, it can be seen that, under the same other conditions, the present invention can not only improve the tensile strength and yield strength of the low alloy steel by introducing a pre-deformation treatment, but also reduce the yield strength ratio to below 0.9, and the elongation is significantly improved; by comparing Comparative Examples 1 and 2 It can be seen from the comparison that, under the condition that other conditions are the same, the pre-deformation treatment is introduced and the total deformation of the pre-deformation treatment is controlled at 5%, the tensile strength, yield strength and elongation of the low alloy steel are slightly improved, and the yield strength ratio is reduced from 0.97 to 0.95; through the comparison of Example 2 and Comparative Example 2, it can be seen that when the total deformation of the pre-deformation treatment is increased from 2% to 5%, the tensile strength, yield strength and elongation of the low alloy steel are significantly reduced, and the yield strength ratio is increased from 0.88 to 0.95, indicating that the present invention controls the total deformation of the pre-deformation treatment to 0.5-4%, which can significantly improve the mechanical properties of the low alloy steel and reduce the yield strength ratio.

[0081] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A heat treatment method for reducing the yield ratio of low alloy steel, characterized in that: The following steps are involved: (1) heating the low alloy steel to above Ac3 and performing solid solution treatment to obtain solid solution low alloy steel; (2) pre-deforming the solid solution low alloy steel obtained in step (1), and then heating it to a temperature between Ac1 and Ac3 for critical quenching to obtain critical quenched low alloy steel; the total deformation amount of the pre-deformation treatment is 0.5 to 4%; (3) The critically quenched low alloy steel obtained in step (2) is subjected to aging treatment below Ac1 to obtain a low alloy steel with a low yield ratio.

2. The heat treatment method according to claim 1, characterized in that Calculated by mass percentage, the chemical composition of the low alloy steel in step (1) is: C: 0.03-0.08%, Si: 0.2-0.3%, Mn: 0.8-1.2%, Cu: 1-1.5%, Ni: 2.5-3.5%, Mo: 0.3-0.6%, Ti: 0.05-0.1% and the balance iron.

3. The heat treatment method according to claim 1, characterized in that The holding temperature of the solution treatment in step (1) is 870-930° C., and the holding time of the solution treatment is 0.5-4 h.

4. The heat treatment method according to claim 3, characterized in that The holding temperature of the solution treatment in step (2) is 900° C., and the holding time of the solution treatment is 1 hour.

5. The heat treatment method according to claim 1, characterized in that The total deformation amount of the pre-deformation treatment in step (2) is 1 to 2%.

6. The heat treatment method according to claim 1, characterized in that The holding temperature of the critical quenching treatment in step (2) is 720-780° C., and the holding time of the critical quenching treatment is 0.5-4 hours.

7. The heat treatment method according to claim 6, characterized in that The holding temperature of the critical quenching treatment is 750° C., and the holding time of the critical quenching treatment is 1 hour.

8. The heat treatment method according to claim 1, wherein The holding temperature of the aging treatment in step (3) is 500-600° C., and the holding time of the aging treatment is 0.5-25 h.

9. The heat treatment method according to claim 8, characterized in that The holding temperature of the aging treatment is 550° C., and the holding time of the aging treatment is 1 hour.

10. Low alloy steel with low yield ratio prepared by the heat treatment method according to any one of claims 1 to 9.

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