A low-temperature resistant steel and its heat treatment process

Abstract

The present invention relates to the field of steel materials, and provides a low-temperature resistant steel and its heat treatment process. The heat treatment process includes multiple steps such as melting and homogenization treatment, rolling treatment, quenching treatment, redistribution treatment and aging treatment. By precisely controlling the process parameters and alloy component ratios, the microstructure is optimized. The microstructure of this steel includes large-angle grain boundaries and small-angle grain boundaries in optimized proportions, nanoscale retained austenite, copper-rich precipitate phases and NbC precipitate phases, ultrafine needle-shaped bainite and thin-film retained austenite. The synergistic effect of each microstructure significantly improves the strength, toughness and balance performance of the steel. In this process, the size and volume fraction of retained austenite, the morphology and distribution of precipitate phases, and the thickness of retained austenite are reasonably designed to ensure the comprehensive performance of the material in extremely low-temperature environments, solve the problem of material failure in low-temperature environments, and have broad application value.

Description

Technical Field

[0001] The present invention relates to the field of steel materials, and particularly to a low-temperature resistant steel and its heat treatment process. Background Art

[0002] With the development of cryogenic engineering technology, the demand for low-temperature resistant steel is increasing in fields such as liquefied natural gas storage and transportation, polar architecture, deep-sea drilling, and aerospace. In these scenarios, materials are in an extreme low-temperature environment for a long time. For example, in a liquefied natural gas storage tank, the performance needs to be maintained at -196°C. Traditional steel is prone to brittle fracture at low temperatures. Therefore, low-temperature resistant steel must have high tensile strength, good low-temperature impact toughness, excellent fracture toughness, and fatigue resistance, and at the same time, it must have long-term service stability and the ability to resist low-temperature embrittlement. The research and development of high-performance low-temperature resistant steel is of great significance for promoting industrial technological progress and coping with environmental challenges. Although certain progress has been made in the research and development of low-temperature resistant steel, there are still obvious deficiencies in meeting low-temperature strength and toughness in the existing technology. For example, Chinese Patent No. CN105886957A discloses a low-temperature ring steel, which improves strength and toughness by adding specific alloying elements and optimizing the heat treatment process, but its fracture toughness in an extremely low-temperature environment is still difficult to meet higher performance requirements, especially prone to brittle fracture under complex stress conditions. The main reason is that the existing technology has insufficient control ability over the microstructure, unable to achieve the coordinated optimization of multiple strengthening mechanisms, resulting in problems such as grain coarsening and uneven distribution of precipitation phases. At the same time, another Chinese Patent No. CN105886958A proposes a new type of low-temperature steel, emphasizing improving low-temperature performance through measures such as grain refinement, but the single mechanism limits the plastic deformation ability of the material under low-temperature impact. In addition, although some studies have proposed strategies for grain refinement and strengthening precipitation phases, there is a lack of in-depth research on the formation and coordination of retained austenite and ultra-fine acicular bainite, resulting in the performance of the material in an extreme low-temperature environment not meeting the engineering application requirements.

[0003] Therefore, the deficiencies in low-temperature strength and toughness of the existing technology mainly stem from the poor coordination among microstructure design, alloy composition optimization, and heat treatment process. These problems urgently need to be solved through innovative technical solutions to comprehensively improve the performance of low-temperature resistant steel. Summary of the Invention

[0004] (1) Technical Problems to be Solved

[0005] The purpose of the present invention is to provide a low-temperature resistant steel and its heat treatment process to solve the problem of insufficient low-temperature performance of current low-temperature resistant steel.

[0006] (2) Technical Solutions

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A heat treatment process for low-temperature resistant steel, comprising the following steps:

[0009] S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are sequentially subjected to melting and homogenization treatment to obtain a square billet;

[0010] S2: Rolling treatment: The square billet obtained in step S1 is sequentially subjected to primary rolling and finish rolling treatments to obtain a rolled steel plate;

[0011] S3: Quenching treatment: The rolled steel plate obtained in step S2 is quenched to obtain martensitic steel;

[0012] S4: Redistribution treatment: The martensitic steel obtained in step S3 is sequentially subjected to redistribution treatment and re-quenching treatment to obtain quenching-partitioning steel;

[0013] S5: Aging treatment: The quenching-partitioning steel obtained in step S4 is subjected to aging treatment and third quenching to obtain low-temperature resistant steel.

[0014] Furthermore, the components of the low-temperature resistant steel in step S1 include the following by weight: 0.03 - 0.07% C, 0.01 - 0.03% Ce, 0.0005 - 0.003% B, 0.20 - 0.30% Si, 2.20 - 2.60% Mn, 4.5 - 4.9% Ni, 0.85 - 1.00% Al, 1.35 - 1.55% Cu, 0.50 - 0.65% Cr, 0.50 - 0.65% Mo, 0.01 - 0.03% Ti, 0.04 - 0.08% Nb, with the balance being Fe and unavoidable impurities;

[0015] In view of the performance requirements for strengthening steel under low-temperature conditions, the present invention significantly improves the nano-scale precipitation strengthening effect and enhances the tolerance performance of dislocations to the low-temperature environment by adding a large proportion of Ni, Al, and Cu; by adding appropriate amounts of Cr, Mo, Nb, and Ti, the hardenability of the steel is improved, the martensite transformation during cooling is promoted, and the grain structure is refined, thereby further enhancing the low-temperature toughness and comprehensive mechanical properties. The synergistic effect of the above alloying elements enables the low-temperature resistant steel of the present invention to exhibit excellent strength and toughness in extremely low-temperature environments, providing a reliable material guarantee for the practical application of low-temperature engineering.

[0016] Furthermore, the process method for melting and homogenization in step S1 is: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating at a heating rate of 9.0 - 11.0 °C / min to 1600 - 1650 °C, holding for 20 - 30 min, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot at a heating rate of 9.0 - 11.0 °C / min to 1280 - 1290 °C, holding for 30 - 35 min to complete the homogenization treatment;

[0017] Further, the process method of the rolling treatment in step S2 is as follows: The steel that has completed homogenization treatment in step S1 is cooled to 1145 - 1160°C at an air cooling rate of 2.5 - 3.5°C / s for primary rolling, and the rolling deformation amount is 20 - 30%. Then, the steel after primary rolling is cooled to 980 - 1000°C at the same air cooling rate for continuous rolling, and the continuous rolling deformation amount is 50 - 60%. Next, the steel after continuous rolling is continuously cooled to 850 - 860°C at the same rate for finish rolling, and the finish rolling deformation amount is 70 - 80%. Finally, the rolled steel plate is obtained.

[0018] Further, the process method of the quenching treatment in step S3 is as follows: The rolled steel plate in step S2 is heated to 895 - 905°C at a heating rate of 9.0 - 11.0°C / min, held for 30 - 35 min, and then water quenched to obtain martensitic steel after quenching treatment.

[0019] Further, the process method of the re-distribution treatment and re-quenching treatment in step S4 is as follows: The martensitic steel obtained in step S3 is heated to 660 - 680°C at a heating rate of 9.0 - 11.0°C / min, held for 30 - 35 min to complete the re-distribution treatment, and then re-quenched. The quenching medium is water, and quenching - partitioned steel is obtained after re-quenching is completed.

[0020] Further, the process method of the aging treatment and the third quenching in step S5 is as follows: The quenching - partitioned steel obtained in step S4 is heated to 550 - 560°C at a heating rate of 9.0 - 11.0°C / min, held for 50 - 60 min, and then subjected to the third quenching. The quenching medium is water. After the third quenching is completed and dried, low-temperature resistant steel is finally obtained.

[0021] The present invention provides a heat treatment process for low-temperature resistant steel, which achieves excellent comprehensive performance of the material in extremely low-temperature environments through precise multi-step processing. First, through melting and homogenization treatment (S1), the alloy components are evenly distributed, and the macrosegregation in the ingot is eliminated, providing a billet with uniform chemical composition and pure structure for subsequent processing. Subsequently, in the rolling treatment (S2) stage, the billet is subjected to multi-pass rolling, including primary rolling, continuous rolling, and finish rolling. By controlling the cooling rate and deformation amount, the grain structure is refined and the tissue uniformity of the material is improved. Then, in the quenching treatment (S3) stage, through rapid heating and water quenching processes, the rolled steel plate is transformed into a martensite-based structure, effectively improving the strength and hardness of the material, and providing a high density of dislocations and a stable martensite matrix for subsequent processes. In the partitioning treatment (S4), by heating and holding to promote the redistribution of carbon and alloying elements in the structure, stable retained austenite and nano-precipitates are formed. At the same time, re-quenching further optimizes the tissue performance to obtain quenching-partitioning steel with high strength and high toughness. Finally, in the aging treatment (S5) stage, by long-term holding to promote the full precipitation of precipitates, and finally through the third quenching to refine the structure and stabilize the retained austenite, the low-temperature toughness and balanced strength of the steel are significantly improved. Through reasonable heat treatment design and precise composition optimization, this process enables the low-temperature resistant steel to have excellent strength, toughness, and balanced performance, meeting the application requirements of extremely low-temperature engineering and providing a reliable guarantee for material selection in low-temperature environments.

[0022] The present invention also discloses a low-temperature resistant steel, which is prepared by the above preparation method;

[0023] Furthermore, the ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is (60 - 70):(30 - 40).

[0024] Furthermore, the low-temperature resistant steel contains retained austenite with an average diameter of 200 - 500 nm and a volume fraction of 12 - 16%.

[0025] Furthermore, the low-temperature resistant steel contains copper-rich precipitate phases with an average diameter of 2.0 - 7.0 nm and a density of (1.5 - 6.2)×10 22 / m 3 。

[0026] Furthermore, the low-temperature resistant steel contains NbC precipitate phases, and the NbC precipitate phases and α-Fe in the low-temperature steel form a semi-coherent interface.

[0027] Furthermore, the average diameter of the NbC precipitate phases is 3 - 10 nm and the density is (1.0 - 3.0)×10 22 / m.

[0028] Furthermore, the low-temperature resistant steel contains ultra-fine acicular bainite with an average width of 50 - 200 nm.

[0029] Furthermore, the low-temperature resistant steel contains thin-film-like retained austenite with an average thickness of 20 - 40 nm.

[0030] The present invention adopts an optimized microstructure design mainly to enhance the strength and toughness properties of the low-temperature resistant steel under extreme low-temperature conditions. Through precise control of the heat treatment process and composition design, the microstructure of the low-temperature resistant steel has achieved a variety of mutually synergistic micro-characteristics, including the optimized ratio of large-angle grain boundaries to small-angle grain boundaries, the stable distribution of transformation-induced austenite, the dispersed precipitation of copper-rich precipitates, the precise control of NbC precipitate phases, the formation of ultra-fine acicular bainite, and the regulation of thin-film-like retained austenite. In the specific parameter range of the microstructure, the ratio of large-angle grain boundaries to small-angle grain boundaries is (60 - 70):(30 - 40). When the ratio of large-angle grain boundaries is less than 60%, the hindering effect on the crack propagation path is insufficient, resulting in a decrease in the anti-fracture performance of the material; while exceeding 70% will reduce the plasticity of the material and increase the risk of brittle failure.

[0031] In the present invention, the average diameter of the transformation-induced austenite is 200 - 500 nm, and the volume fraction is 12 - 16%. When its diameter is less than 200 nm or the volume fraction is lower than 12%, the transformation-induced plasticity effect in the material is insufficient, and the crack propagation energy cannot be fully absorbed, thus reducing the low-temperature toughness; while when the diameter exceeds 500 nm or the volume fraction is higher than 16%, the stability of the transformation-induced austenite decreases, and uncontrolled martensitic transformation is likely to occur, resulting in a decrease in the ductility of the material.

[0032] In the present invention, the average diameter of the copper-rich precipitate phase is 2.0 - 7.0 nm, and the density is (1.5 - 6.2)×10 22 / m 3 , if the diameter is less than 2.0 nm or the density is lower than 1.5×10 22 / m 3 , then the precipitation strengthening effect is insufficient, and the strength of the material does not meet the standard; while when the diameter exceeds 7.0 nm or the density is higher than 6.2×10 22 / m 3 it will lead to the coarsening of the precipitate phase, affecting the uniformity of the matrix and reducing the overall performance.

[0033] In the present invention, the diameter of the NbC precipitate phase is 3 - 10 nm, and the density is (1.0 - 3.0)×10 22 / m 3 , if the diameter is less than 3 nm or the density is lower than 1.0×10 22 / m 3, the ability to pin dislocations is insufficient, resulting in a decrease in the strength and toughness of the material; while when the diameter exceeds 10 nm or the density is higher than 3.0×10 22 / m 3 it will cause the precipitation phase to aggregate, weakening the fine grain strengthening effect of the material.

[0034] In the present invention, the width of the ultra-fine acicular bainite is 50 - 200 nm. When the width is less than 50 nm, the formation of the bainite structure is incomplete and cannot effectively prevent crack initiation and propagation; while when the width exceeds 200 nm, it will cause grain coarsening and reduce the low-temperature anti-brittleness of the material.

[0035] In the present invention, the thickness of the film-like retained austenite is 20 - 40 nm. When its thickness is less than 20 nm, the stability of the retained austenite is insufficient and it is prone to rapidly transform into martensite at low temperatures, resulting in a decrease in the plasticity of the material; while when the thickness exceeds 40 nm, it will affect the balance of the strength and toughness of the material and reduce the comprehensive performance in a low-temperature environment. The synergy of these microstructural characteristics ensures excellent mechanical properties and reliability of the low-temperature resistant steel in extremely low-temperature environments by optimizing the size, morphology and distribution of each component. However, once the parameter range is exceeded, the synergistic effect of the microstructure will be damaged, and the strength-toughness balance of the material will decrease significantly, thus affecting its actual application effect in cryogenic engineering.

[0036] (3) Beneficial technical effects

[0037] 1. The present invention realizes excellent strength, toughness and balance performance of the low-temperature resistant steel in extremely low-temperature environments by precisely controlling the melting, rolling, quenching, redistribution and aging treatment processes. Compared with the prior art, it significantly improves the low-temperature anti-brittleness and fracture toughness of the material, solves the problems of brittle fracture and performance failure of the material in low-temperature environments, and is widely applicable to fields such as liquefied natural gas storage and transportation, polar architecture and deep-sea engineering. The synergistic effect among the components is the core advantage of the present invention. Niobium, titanium and carbon enhance the dislocation pinning effect by forming fine NbC precipitation phases, copper and nickel promote the stability of the copper-rich precipitation phase and the retained austenite, and the alloying elements and multi-stage heat treatment jointly refine the grains and optimize the microstructure to ensure the balance of the strengthening and toughening mechanisms. The reasonable process parameters and component ratios improve the stability and synergy of the microstructure, endowing the present invention with unique performance advantages in low-temperature environments and promoting the technological progress and engineering application in the field of low-temperature steel.

[0038] 2. The present invention realizes excellent strength and toughness and balanced properties of cryogenic-resistant steel under extreme low-temperature conditions by optimizing the microstructure design and precisely controlling the heat treatment process. Compared with the prior art, the fracture resistance and low-temperature toughness of the material are significantly improved, solving the problem of easy brittle fracture of traditional steel in low-temperature environments. It is widely applicable to fields such as liquefied natural gas storage and transportation, polar engineering, and deep-sea drilling. The core innovation of the present invention lies in the synergistic effect among components: the optimized ratio of large-angle and small-angle grain boundaries effectively hinders the crack propagation path, the retained austenite absorbs the crack propagation energy through the transformation-induced plasticity effect, the precise control of copper-rich precipitate phase and NbC precipitate phase enhances the precipitation strengthening effect, and the composite structure of ultra-fine needle-like bainite and thin-film-like retained austenite achieves the balance between strength and toughness. The reasonable component ratio and key parameters ensure that the microstructure synergy is superior to the prior art, thus promoting the technological progress and engineering applications in the field of low-temperature steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is the electron backscatter diffraction technology pattern of the cryogenic-resistant steel prepared in Example 1 of the present invention.

[0040] Figure 2 It is the transmission electron microscope photograph of the semi-coherent interface formed by the NbC precipitate phase in the cryogenic-resistant steel prepared in Example 1 of the present invention and α-Fe in the low-temperature steel.

[0041] Figure 3 It is the transmission electron microscope photograph of the ultra-fine needle-like bainite morphology of the cryogenic-resistant steel prepared in Example 1 of the present invention.

[0042] Figure 4 It is the transmission electron microscope photograph of the thin-film-like retained austenite morphology of the cryogenic-resistant steel prepared in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0044] Example 1

[0045] A heat treatment process for cryogenic-resistant steel includes the following steps:

[0046] S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are successively subjected to melting and homogenization treatment to obtain a square billet; the components of the low-temperature resistant steel by weight include the following composition: 0.03% C, 0.01% Ce, 0.0005% B, 0.20% Si, 2.20% Mn, 4.5% Ni, 0.85% Al, 1.35% Cu, 0.50% Cr, 0.50% Mo, 0.01% Ti, 0.04% Nb, and the balance is Fe and inevitable impurities; the process method of melting and homogenization is: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating to 1600 °C at a heating rate of 9.0 °C / min, holding for 20 min, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot to 1280 °C at a heating rate of 9.0 °C / min and holding for 30 min to complete the homogenization treatment.

[0047] S2: Rolling treatment: The square billet obtained in step S1 is successively subjected to primary rolling and finish rolling to obtain a rolled steel plate; the process method of rolling treatment is: cooling the steel after completing the homogenization treatment in step S1 to 1145 °C at an air cooling rate of 2.5 °C / s for primary rolling, and the rolling deformation is 20%; cooling the steel after primary rolling to 980 °C at the same air cooling rate for continuous rolling, and the continuous rolling deformation is 50%; continuing to cool the steel after continuous rolling to 850 °C at the same rate for finish rolling, and the finish rolling deformation is 70%, and finally obtaining a rolled steel plate.

[0048] S3: Quenching treatment: The rolled steel plate obtained in step S2 is subjected to quenching treatment to obtain martensitic steel; the process method of quenching treatment is: heating the rolled steel plate in step S2 to 895 °C at a heating rate of 9.0 °C / min, holding for 30 min and then performing water quenching, and martensitic steel is obtained after quenching treatment.

[0049] S4: Redistribution treatment: The martensitic steel obtained in step S3 is successively subjected to redistribution treatment and re-quenching treatment to obtain quenching-partitioning steel; the process method of redistribution treatment and re-quenching treatment is: heating the martensitic steel obtained in step S3 to 660 °C at a heating rate of 9.0 °C / min, holding for 30 min to complete the redistribution treatment, and then performing re-quenching, and the quenching medium is water, and quenching-partitioning steel is obtained after re-quenching is completed.

[0050] S5: Aging treatment: The quenching-partitioning steel obtained in step S4 is subjected to aging treatment and third quenching to obtain low-temperature resistant steel. The process method of aging treatment and third quenching is: heating the quenching-partitioning steel obtained in step S4 to 550 °C at a heating rate of 9.0 °C / min, holding for 50 min, and then performing third quenching, and the quenching medium is water, and low-temperature resistant steel is finally obtained after the third quenching is completed and dried.

[0051] A low-temperature resistant steel prepared in this embodiment is obtained by using the above preparation method; the ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is 60:40. The low-temperature resistant steel contains rotational austenite with an average diameter of 500 nm and a volume fraction of 12%. The low-temperature resistant steel contains copper-rich precipitate phases with an average diameter of 7 nm and a density of 1.5×1022 / m 3 . The low-temperature resistant steel contains NbC precipitate phases, and the NbC precipitate phases and α-Fe in the low-temperature steel form a semi-coherent interface. The average diameter of the NbC precipitate phases is 10 nm and the density is 1×10 2 2 / m 3 . The low-temperature resistant steel contains ultrafine needle-shaped bainite with an average width of 200 nm. The low-temperature resistant steel contains thin-film retained austenite with an average thickness of 40 nm.

[0052] It can be seen from Figure 1 that the low-temperature resistant steel prepared in Example 1 of the present invention shows a ratio of large-angle grain boundaries to small-angle grain boundaries of 60:40 in the backscattered electron diffraction technology pattern, indicating that the grain boundary structure plays a significant role in optimizing the material properties. Figure 2 The transmission electron microscope photo showing the semi-coherent interface formed by the NbC precipitate phases and α-Fe in the low-temperature steel is presented, further proving the existence of the semi-coherent interface characteristics in the material, and this interface helps to improve the strength and toughness of the material. Figure 3 The transmission electron microscope photo is of the morphology of ultrafine needle-shaped bainite, proving the existence of this tissue morphology and effectively improving the strength and toughness of the low-temperature steel. Figure 4 The transmission electron microscope photo shows the thin-film retained austenite, proving the distribution of the thin-film retained austenite. This organizational structure can significantly improve the low-temperature impact performance and enhance the comprehensive performance of the material. In summary, these microstructures and interface characteristics jointly verify the rationality and superiority of the optimized process of the present invention.

[0053] Example 2

[0054] A heat treatment process for a low-temperature resistant steel, comprising the following steps:

[0055] S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are successively subjected to melting and homogenization treatment to obtain a square billet; the components of the low-temperature resistant steel by weight include the following composition: 0.04% C, 0.02% Ce, 0.001% B, 0.23% Si, 2.32% Mn, 4.6% Ni, 0.89% Al, 1.41% Cu, 0.54% Cr, 0.54% Mo, 0.01% Ti, 0.05% Nb, and the balance is Fe and unavoidable impurities; the process method of melting and homogenization is: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating to 1615 °C at a heating rate of 9.6 °C / min, holding for 23 min, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot to 1283 °C at a heating rate of 9.6 °C / min and holding for 32 min to complete the homogenization treatment.

[0056] S2: Rolling treatment: The square billet obtained in step S1 is successively subjected to rough rolling and finish rolling to obtain a rolled steel plate; the process method of rolling treatment is: cooling the steel after completing the homogenization treatment in step S1 to 1149 °C at an air cooling rate of 2.8 °C / s for rough rolling, with a rolling deformation of 23%, cooling the steel after rough rolling to 986 °C at the same air cooling rate for continuous rolling, with a continuous rolling deformation of 53%; cooling the steel after continuous rolling to 853 °C at the same rate for finish rolling, with a finish rolling deformation of 73%, and finally obtaining a rolled steel plate.

[0057] S3: Quenching treatment: The rolled steel plate obtained in step S2 is subjected to quenching treatment to obtain martensitic steel; the process method of quenching treatment is: heating the rolled steel plate in step S2 to 898 °C at a heating rate of 9.6 °C / min, holding for 32 min and then performing water quenching, and martensitic steel is obtained after quenching treatment.

[0058] S4: Redistribution treatment: The martensitic steel obtained in step S3 is successively subjected to redistribution treatment and re-quenching to obtain quenching-partitioning steel; the process method of redistribution treatment and re-quenching is: heating the martensitic steel obtained in step S3 to 666 °C at a heating rate of 9.6 °C / min, holding for 32 min to complete the redistribution treatment, and then performing re-quenching, with the quenching medium being water, and quenching-partitioning steel is obtained after re-quenching is completed.

[0059] S5: Aging treatment: The quenching-partitioning steel obtained in step S4 is subjected to aging treatment and third quenching to obtain low-temperature resistant steel. The process method of aging treatment and third quenching is: heating the quenching-partitioning steel obtained in step S4 to 553 °C at a heating rate of 9.6 °C / min, holding for 53 min, and then performing third quenching, with the quenching medium being water, and finally obtaining low-temperature resistant steel after the third quenching is completed and dried.

[0060] A low-temperature resistant steel prepared in this embodiment is prepared by the above preparation method; the ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is (64:36). The low-temperature resistant steel contains retained austenite with an average diameter of 400 nm and a volume fraction of 15%. The low-temperature resistant steel contains copper-rich precipitate phases with an average diameter of 4.8 nm and a density of 3.5×10 2 2 / m 3 . The low-temperature resistant steel contains NbC precipitate phases, and the NbC precipitate phases and α-Fe in the low-temperature steel form a semi-coherent interface. The average diameter of the NbC precipitate phases is 8.0 nm and the density is 1.8×10 2 2 / m 3 . The low-temperature resistant steel contains ultra-fine acicular bainite with an average width of 150 nm. The low-temperature resistant steel contains thin-film retained austenite with an average thickness of 30 nm.

[0061] Example 3

[0062] A heat treatment process for a low-temperature resistant steel includes the following steps:

[0063] S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are sequentially subjected to melting and homogenization treatment to obtain a bloom; the components of the low-temperature resistant steel by weight include the following composition: 0.05% C, 0.02% Ce, 0.002% B, 0.26% Si, 2.44% Mn, 4.8% Ni, 0.94% Al, 1.48% Cu, 0.59% Cr, 0.59% Mo, 0.02% Ti, 0.07% Nb, and the balance is Fe and unavoidable impurities; the process method of melting and homogenization is: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating to 1630 °C at a heating rate of 10.2 °C / min, holding for 26 min, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot to 1287 °C at a heating rate of 10.2 °C / min and holding for 33 min to complete the homogenization treatment.

[0064] S2: Rolling treatment: The bloom obtained in step S1 is sequentially subjected to primary rolling and finish rolling to obtain a rolled steel plate; the process method of rolling treatment is: cooling the steel after completing the homogenization treatment in step S1 to 1153 °C at an air cooling rate of 3.1 °C / s for primary rolling, and the rolling deformation is 26%, cooling the steel after primary rolling to 993 °C at the same air cooling rate for continuous rolling, and the continuous rolling deformation is 56%; continuing to cool the steel after continuous rolling to 857 °C at the same rate for finish rolling, and the finish rolling deformation is 76%, and finally obtaining a rolled steel plate.

[0065] S3: Quenching treatment: The rolled steel plate obtained in step S2 is subjected to quenching treatment to obtain martensitic steel; the process method of the quenching treatment is: heating the rolled steel plate in step S2 to 902 °C at a heating rate of 10.2 °C / min, holding for 33 min and then performing water quenching, and martensitic steel is obtained after the quenching treatment.

[0066] S4: Redistribution treatment: The martensitic steel obtained in step S3 is subjected to redistribution treatment and re-quenching treatment in sequence to obtain quenching-partitioning steel; the process methods of the redistribution treatment and the re-quenching treatment are: heating the martensitic steel obtained in step S3 to 673 °C at a heating rate of 10.2 °C / min, holding for 33 min to complete the redistribution treatment, and then performing re-quenching, the quenching medium is water, and quenching-partitioning steel is obtained after the re-quenching is completed.

[0067] S5: Aging treatment: The quenching-partitioning steel obtained in step S4 is subjected to aging treatment and the third quenching to obtain low-temperature resistant steel. The process methods of the aging treatment and the third quenching are: heating the quenching-partitioning steel obtained in step S4 to 558 °C at a heating rate of 10.2 °C / min, holding for 56 min, and then performing the third quenching, the quenching medium is water, and low-temperature resistant steel is finally obtained after the third quenching and drying.

[0068] A kind of low-temperature resistant steel prepared in this embodiment is prepared by the above preparation method; the ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is 67:33. The low-temperature resistant steel contains retained austenite with an average diameter of 350 nm and a volume fraction of 15.5%. The low-temperature resistant steel contains copper-rich precipitate phases with an average diameter of 3.8 nm and a density of 4.5×10 2 2 / m 3 . The low-temperature resistant steel contains NbC precipitate phases, and the NbC precipitate phases and α-Fe in the low-temperature steel form a semi-coherent interface. The average diameter of the NbC precipitate phases is 6.5 nm and the density is 2.3×10 2 2 / m 3 . The low-temperature resistant steel contains ultra-fine acicular bainite with an average width of 120 nm. The low-temperature resistant steel contains thin-film retained austenite with an average thickness of 25 nm.

[0069] Example 4

[0070] A heat treatment process for a low-temperature resistant steel, comprising the following steps:

[0071] S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are successively subjected to melting and homogenization treatment to obtain a square billet; the components of the low-temperature resistant steel by weight include the following composition: 0.07% C, 0.03% Ce, 0.003% B, 0.30% Si, 2.60% Mn, 4.9% Ni, 1.00% Al, 1.55% Cu, 0.65% Cr, 0.65% Mo, 0.03% Ti, 0.08% Nb, and the balance is Fe and unavoidable impurities; the process method of melting and homogenization is: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating to 1650 °C at a heating rate of 11.0 °C / min, holding for 30 min, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot to 1290 °C at a heating rate of 11.0 °C / min and holding for 35 min to complete the homogenization treatment.

[0072] S2: Rolling treatment: The square billet obtained in step S1 is successively subjected to rough rolling and finish rolling to obtain a rolled steel plate; the process method of rolling treatment is: cooling the steel after completing the homogenization treatment in step S1 to 1160 °C at an air cooling rate of 3.5 °C / s for rough rolling, and the rolling reduction is 30%; cooling the steel after rough rolling to 1000 °C at the same air cooling rate for continuous rolling, and the continuous rolling reduction is 60%; continuing to cool the steel after continuous rolling to 860 °C at the same rate for finish rolling, and the finish rolling reduction is 80%, and finally obtaining a rolled steel plate.

[0073] S3: Quenching treatment: The rolled steel plate obtained in step S2 is subjected to quenching treatment to obtain martensitic steel; the process method of quenching treatment is: heating the rolled steel plate in step S2 to 905 °C at a heating rate of 11.0 °C / min, holding for 35 min and then performing water quenching, and martensitic steel is obtained after quenching treatment.

[0074] S4: Redistribution treatment: The martensitic steel obtained in step S3 is successively subjected to redistribution treatment and re-quenching treatment to obtain quenching-partitioning steel; the process method of redistribution treatment and re-quenching treatment is: heating the martensitic steel obtained in step S3 to 680 °C at a heating rate of 11.0 °C / min, holding for 35 min to complete the redistribution treatment, and then performing re-quenching, and the quenching medium is water, and quenching-partitioning steel is obtained after re-quenching is completed.

[0075] S5: Aging treatment: The quenching-partitioning steel obtained in step S4 is subjected to aging treatment and third quenching to obtain low-temperature resistant steel. The process method of aging treatment and third quenching is: heating the quenching-partitioning steel obtained in step S4 to 560 °C at a heating rate of 11.0 °C / min, holding for 60 min, and then performing third quenching, and the quenching medium is water. After the third quenching is completed and dried, low-temperature resistant steel is finally obtained.

[0076] A low-temperature resistant steel prepared in this embodiment is prepared by the above preparation method; the ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is 70:30. The low-temperature resistant steel contains retained austenite with an average diameter of 200 nm and a volume fraction of 16%. The low-temperature resistant steel contains copper-rich precipitate phases with an average diameter of 2 nm and a density of 6.2×10 2 2 / m 3 . The low-temperature resistant steel contains NbC precipitate phases, and the NbC precipitate phases and α-Fe in the low-temperature steel form a semi-coherent interface. The average diameter of the NbC precipitate phases is 3 nm and the density is 3×10 2 2 / m 3 . The low-temperature resistant steel contains ultra-fine needle-shaped bainite with an average width of 50 nm. The low-temperature resistant steel contains thin-film retained austenite with an average thickness of 20 nm.

[0077] Comparative Example 1

[0078] It is basically the same as Example 1, except that the Ce element is removed.

[0079] Comparative Example 2

[0080] It is basically the same as Example 1, except that the aging temperature is increased to 580 °C (higher than the upper limit).

[0081] Comparative Example 3

[0082] It is basically the same as Example 1, except that the first quenching temperature is reduced to 880 °C (lower than the lower limit).

[0083] Comparative Example 4

[0084] It is basically the same as Example 1, except that the redistribution temperature is reduced to 640 °C (lower than the lower limit).

[0085] Comparative Example 5

[0086] It is basically the same as Example 1, except that the redistribution treatment is cancelled.

[0087] Comparative Example 6

[0088] It is basically the same as Example 1, except that the homogenization temperature is reduced to 1260 °C (lower than the lower limit).

[0089] Comparative Example 7

[0090] It is basically the same as Example 1, except that the finish rolling deformation is reduced to 60% (lower than the lower limit).

[0091] Comparative Example 8

[0092] It is basically the same as Example 1, except that the heating rate is reduced to 7.0 °C / min (lower than the lower limit).

[0093] Comparative Example 9

[0094] It is basically the same as Example 1, except that the aging treatment is cancelled.

[0095] Performance test:

[0096] Low-temperature impact test: For the impact performance test, a JBDW-300D ultra-low temperature impact testing machine is used, which is carried out in accordance with the latest national standard. The impact rate is set at 5.24 m / s, and the maximum impact energy is 450 J. The tests are carried out at room temperature and in the liquid nitrogen temperature range respectively. At the same time, a NI750C oscilloscope impact testing machine equipped with a high-resolution rotary encoder is used to record the displacement-load curve and displacement-energy curve data at different temperatures.

[0097] Low-temperature tensile test: The low-temperature tensile performance test is completed on a GNT1000Y electro-hydraulic servo universal testing machine, using the microcomputer control mode. The test methods and procedures strictly follow the national standard specifications, and the test environment is -196 °C (liquid nitrogen temperature).

[0098] The performance data of the steels in Examples 1-4 and Comparative Examples 1-9 are summarized in Table 2.

[0099] Table 2 Summary of the performance data of the steels in Examples 1-4 and Comparative Examples 1-9

[0100]

[0101] The main difference between Comparative Example 1 and Example 1 is the removal of the Ce element. It can be seen from Table 2 that the low-temperature impact performance, tensile strength and elongation of Example 1 are significantly better than those of Comparative Example 1. The presence of the Ce element can significantly improve the low-temperature impact performance and elongation because Ce can refine the grains and reduce the size and quantity of inclusions, thereby optimizing the grain boundary distribution and improving the toughness of the material. In Comparative Example 1, due to the absence of the Ce element, the grains are coarsened and the grain boundary stability decreases, resulting in a significant reduction in the low-temperature impact performance and elongation.

[0102] The main difference between Comparative Example 2 and Example 1 is that the aging temperature is increased to 580 °C (higher than the upper limit). It can be seen from Table 2 that the low-temperature impact performance, tensile strength and elongation of Comparative Example 2 are all lower than those of Example 1. This is because too high an aging temperature will cause significant coarsening of the copper-rich precipitate phase and NbC precipitate phase, reducing their strengthening effect, thereby resulting in a decrease in the strength and toughness of the material. In addition, the coarsened precipitate phase cannot effectively pin the dislocations, further reducing the elongation and impact performance.

[0103] The main difference between Comparative Example 3 and Example 1 is that the first quenching temperature is reduced to 880 °C (lower than the lower limit). As can be seen from Table 2, both the low-temperature impact property and the tensile strength of Comparative Example 3 are lower than those of Example 1. Reducing the first quenching temperature will result in incomplete martensite transformation, forming more retained ferrite or pearlite structures, thereby reducing the strength and toughness of the matrix. The incompletely quenched structure will also weaken the effect of subsequent redistribution treatment, leading to a decline in overall performance.

[0104] The main difference between Comparative Example 4 and Example 1 is that the redistribution temperature is reduced to 640 °C (lower than the lower limit). As can be seen from Table 2, both the low-temperature impact property and the elongation of Comparative Example 4 are lower than those of Example 1. Too low a redistribution temperature will result in insufficient stability of reverse austenite, a decrease in its volume fraction, thereby weakening the energy absorption capacity and toughness of the material. In addition, the precipitation rate of the precipitated phase at low temperature slows down, resulting in a weakened strengthening effect, ultimately causing the low-temperature impact property and elongation to decline.

[0105] The main difference between Comparative Example 5 and Example 1 is that the redistribution treatment is cancelled. As can be seen from Table 2, the low-temperature impact property, tensile strength, and elongation of Comparative Example 5 are significantly lower than those of Example 1. The redistribution treatment is an important step in forming reverse austenite. Cancelling this treatment will cause a significant reduction in the content of reverse austenite in the material, thereby weakening its low-temperature toughness. In addition, the lack of redistribution treatment will affect the distribution and size of the precipitated phase, reducing the overall performance of the material.

[0106] The main difference between Comparative Example 6 and Example 1 is that the homogenization temperature is reduced to 1260 °C (lower than the lower limit). As can be seen from Table 2, both the low-temperature impact property and the elongation of Comparative Example 6 are lower than those of Example 1. Too low a homogenization temperature will result in uneven distribution of alloying elements, forming composition segregation, thereby making the microstructure of the material uneven and reducing the low-temperature toughness and plasticity. In addition, composition segregation will also cause the presence of hard and brittle phases in local areas, further weakening the elongation and impact performance.

[0107] The main difference between Comparative Example 7 and Example 1 is that the finish rolling deformation is reduced to 60% (lower than the lower limit). As can be seen from Table 2, both the low-temperature impact property and the elongation of Comparative Example 7 are slightly lower than those of Example 1. Reducing the finish rolling deformation will reduce the grain refinement effect, resulting in an increase in grain size, thereby weakening the toughness of the material. In addition, insufficient deformation will also affect the distribution and morphology of the precipitated phase, weakening its strengthening effect, ultimately causing the low-temperature impact property and elongation to decline.

[0108] The main difference between Comparative Example 8 and Example 1 is that the heating rate is reduced to 7.0 °C / min (lower than the lower limit). As can be seen from Table 2, both the low-temperature impact performance and tensile strength of Comparative Example 8 are lower than those of Example 1. An excessively low heating rate will lead to grain growth, affecting the tissue uniformity and refinement degree of the material, and thus reducing its strength and toughness. In addition, slow heating will weaken the precipitation driving force of the precipitated phase, increase its size and decrease its density, further weakening the performance of the material.

[0109] The main difference between Comparative Example 9 and Example 1 is that the aging treatment is cancelled. As can be seen from Table 2, the low-temperature impact performance, tensile strength and elongation of Comparative Example 9 are significantly lower than those of Example 1. Aging treatment can promote the formation and refinement of copper-rich precipitate phases and NbC precipitate phases, thus effectively enhancing the strength and toughness of the material. Cancelling the aging treatment will lead to insufficient precipitate phases, significantly reducing the strengthening effect. At the same time, since the residual stress in the structure is not fully released, the elongation and toughness of the material also decrease.

[0110] In summary, through one-by-one comparison, it can be found that the optimized process of Example 1 shows significant advantages in grain refinement, precipitate phase control and tissue stability, significantly improving the low-temperature impact performance, tensile strength and elongation. Compared with Example 1, deviating from the key process parameters or omitting important treatment steps in the comparative examples will lead to a significant decrease in the material performance, further verifying the necessity and effectiveness of process optimization.

[0111] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the above examples, those of ordinary skill in the art should understand that any equivalent structural transformation made under the concept of the present invention using the content of the specification and drawings of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A heat treatment process for low temperature resistant steel, characterized in that: The following steps are involved: S1: Melting and homogenization treatment: The components of the low-temperature resistant steel are sequentially melted and homogenized to obtain a square billet; The components of low temperature resistant steel include the following by weight: 0.03~0.07% C, 0.01~0.03% Ce, 0.0005~0.003% B, 0.20~0.30% Si, 2.20~2.60% Mn, 4.5~4.9% Ni, 0.85~1.00% Al, 1.35~1.55% Cu, 0.50~0.65% Cr, 0.50~0.65% Mo, 0.01~0.03% Ti, 0.04~0 .08%Nb, the balance is Fe and inevitable impurities; the smelting and homogenization process is as follows: add the components of low-temperature resistant steel into a vacuum melting furnace, heat to 1600~1650℃ at a heating rate of 9.0~11.0℃ / min, keep warm for 20~30min, and then naturally cool to room temperature to obtain an ingot; then heat the ingot to 1280~1290℃ at a heating rate of 9.0~11.0℃ / min, keep warm for 30~35min, and complete the homogenization treatment; S2: rolling treatment: cooling the steel homogenized in step S1 to 1145-1160°C at an air cooling rate of 2.5-3.5°C / s for initial rolling, with a deformation of 20-30%, cooling the steel after initial rolling to 980-1000°C at the same air cooling rate for continuous rolling, with a deformation of 50-60%; cooling the steel after continuous rolling to 850-860°C at the same rate for finish rolling, with a deformation of 70-80%, and finally obtaining a rolled steel plate; S3: quenching treatment: heating the rolled steel plate in step S2 to 895-905°C at a heating rate of 9.0-11.0°C / min, keeping the temperature for 30-35min, and then water quenching to obtain martensitic steel after quenching treatment; S4: redistribution treatment: the martensitic steel obtained in step S3 is heated to 660-680°C at a heating rate of 9.0-11.0°C / min, and kept at this temperature for 30-35 minutes to complete the redistribution treatment, and then quenched again, the quenching medium is water, and after the quenching is completed again, the quenched-distributed steel is obtained; S5: Aging treatment: The quenched-distributed steel obtained in step S4 is heated to 550-560°C at a heating rate of 9.0-11.0°C / min, kept at this temperature for 50-60 minutes, and then subjected to a third quenching, wherein the quenching medium is water. After the third quenching is completed, the low-temperature resistant steel is finally obtained after drying.

2. A low temperature resistant steel, characterized in that: Prepared by the heat treatment process as claimed in claim 1; The ratio of large-angle grain boundaries to small-angle grain boundaries in the low-temperature resistant steel is (60-70):(30-40).

3. A low temperature resistant steel as claimed in claim 2, characterized in that: The low temperature resistant steel contains rotational austenite, the average diameter of which is 200-500 nm and the volume fraction of which is 12-16%; The low temperature resistant steel contains a copper-rich precipitate phase with an average diameter of 2.0-7.0 nm and a density of (1.5-6.2)×10 22 / m 3 ; The low temperature resistant steel contains NbC precipitate phase, and the NbC precipitate phase forms a semi-coherent interface with α-Fe in the low temperature steel; The average diameter of the NbC precipitate phase is 3-10 nm, and the density is (1.0-3.0)×10 22 / m.

4. A low temperature resistant steel as claimed in claim 2, characterized in that: The low temperature resistant steel contains ultrafine needle-shaped bainite with an average width of 50-200 nm; The low-temperature resistant steel contains film-like retained austenite, the average thickness of which is 20-40 nm.

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