Low-temperature-resistant steel and heat treatment process thereof

Abstract

The invention relates to the field of steel and iron materials, and provides low-temperature-resistant steel and a heat treatment process thereof, the heat treatment process comprises multiple steps of smelting and homogenizing treatment, rolling treatment, quenching treatment, redistribution treatment and aging treatment, and a microstructure is optimized by accurately controlling process parameters and alloy component proportion. The microstructure of the steel comprises large-angle grain boundaries, small-angle grain boundaries, nanoscale rotary austenite, copper-rich precipitated phases, NbC precipitated phases, superfine needle-shaped bainite and film-shaped retained austenite in an optimized proportion, and the strength, toughness and balance performance of the steel are remarkably improved through the synergistic effect of all the structures. In the process, the size and volume fraction of the rotary austenite, the morphology and distribution of the precipitated phase and the thickness of the retained austenite are reasonably designed, the comprehensive performance of the material in the extremely low-temperature environment is ensured, the problem of material failure in the low-temperature environment is solved, and the method has wide application value.

Description

Technical Field

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

[0002] With the development of cryogenic engineering technology, the demand for low-temperature resistant steel in the fields of liquefied natural gas storage and transportation, polar construction, deep-sea drilling, and aerospace is growing. In these scenarios, the material is in an extremely low temperature environment for a long time, such as the need to maintain performance at -196°C in a liquefied natural gas storage tank. Traditional steel is prone to brittle fracture at low temperatures, so low-temperature resistant steel must have high tensile strength, good low-temperature impact toughness, excellent fracture toughness and fatigue resistance, and must have long-term service stability and resistance to low-temperature embrittlement. The research and development of high-performance low-temperature resistant steel is of great significance to promoting industrial technological progress and responding to environmental challenges. Although the research and development of low-temperature resistant steel has made certain progress, the existing technology still has obvious deficiencies in meeting low-temperature strength and toughness. For example, the Chinese patent with publication number CN105886957A discloses a low-temperature ring steel that improves strength and toughness by adding specific alloy 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 under complex stress conditions. Brittle fracture is prone to occur. The main reason is that the existing technology has insufficient control over the microstructure and cannot achieve the coordinated optimization of multiple strengthening mechanisms, resulting in grain coarsening and uneven distribution of precipitation phases. At the same time, another Chinese patent with publication number CN105886958A proposed a new type of low-temperature steel, emphasizing the improvement of low-temperature performance through measures such as grain refinement, but the single mechanism makes the material's plastic deformation ability limited under low-temperature impact. In addition, although some studies have proposed strategies for refining grains and strengthening precipitation phases, there is a lack of in-depth research on the formation and synergy of rotated austenite and ultrafine needle-shaped bainite, resulting in the performance of the material in extreme low-temperature environments failing to meet engineering application requirements.

[0003] Therefore, the shortcomings of existing technologies in low-temperature strength and toughness are mainly due to the poor coordination between 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 issues solved

[0005] The purpose of the present invention is to provide a low temperature resistant steel and a heat treatment process thereof, so as to solve the problem that the current low temperature resistant steel has insufficient low temperature performance.

[0006] (2) Technical solution

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

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

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

[0010] S2: rolling process: the billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate;

[0011] S3: quenching treatment: quenching the rolled steel plate obtained in step S2 to obtain martensitic steel;

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

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

[0014] Further, the components of the low temperature resistant steel in step S1 include the following compositions 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, and the balance is Fe and unavoidable impurities;

[0015] The present invention aims at the performance requirements of strengthening steel under low temperature conditions. By adding a large proportion of Ni, Al and Cu, the nano-scale precipitation strengthening effect is significantly improved, and the tolerance of dislocations to low temperature environments is enhanced; by adding appropriate amounts of Cr, Mo, Nb and Ti, the hardenability of steel is improved, the martensitic transformation during the cooling process is promoted, and the grain structure is refined, thereby further improving 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, which can provide reliable material guarantee for the practical application of cryogenic engineering.

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

[0017] Furthermore, the process method of the rolling treatment in step S2 is: cooling the steel that has completed the homogenization treatment in step S1 to 1145-1160°C at an air cooling rate of 2.5-3.5°C / s for initial rolling, and the deformation of the rolling is 20-30%; cooling the steel after the initial rolling to 980-1000°C at the same air cooling rate for continuous rolling, and the deformation of the continuous rolling is 50-60%; cooling the steel after the continuous rolling to 850-860°C at the same rate for finish rolling, and the deformation of the finish rolling is 70-80%, and finally obtaining a rolled steel plate.

[0018] Further, the process method of the quenching treatment in step S3 is: 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 30-35 minutes, and then water quenching to obtain martensitic steel after quenching;

[0019] Furthermore, the process method of the redistribution treatment and re-quenching treatment of step S4 is: heating the martensitic steel obtained in step S3 to 660-680°C at a heating rate of 9.0-11.0°C / min, keeping it warm for 30-35 minutes to complete the redistribution treatment, and then quenching it again, the quenching medium is water, and after the re-quenching is completed, the quenched-distributed steel is obtained.

[0020] Furthermore, the process method of the aging treatment and the third quenching in step S5 is: heating the quenched-distributed steel obtained in step S4 to 550-560°C at a heating rate of 9.0-11.0°C / min, keeping it warm for 50-60 minutes, and then performing a third quenching, the quenching medium is water, and after the third quenching is completed, the low-temperature resistant steel is finally obtained after drying.

[0021] The present invention provides a heat treatment process for low-temperature resistant steel, which realizes the excellent comprehensive performance of the material in an extremely low temperature environment through precise multi-step processing. First, through smelting and homogenization treatment (S1), the alloy components are evenly distributed, the macro segregation in the ingot is eliminated, and a square billet with uniform chemical composition and pure organization is provided for subsequent processing. Subsequently, in the rolling treatment (S2) stage, the square billet is subjected to multiple rolling, including initial rolling, continuous rolling and finishing rolling, and the grain structure is refined and the organizational uniformity of the material is improved by controlling the cooling rate and deformation amount. Then, in the quenching treatment (S3) stage, the rolled steel plate is transformed into a martensite-based organization through rapid heating and water quenching process, which effectively improves the strength and hardness of the material, and provides high-density dislocation and stable martensite matrix for subsequent processes. In the redistribution treatment (S4), heating and heat preservation are used to promote the redistribution of carbon and alloy elements in the organization, forming stable residual austenite and nano-precipitation phases, and quenching again to further optimize the organizational properties, and obtain high-strength and high-toughness quenching-distribution steel. Finally, in the aging treatment (S5) stage, long-term heat preservation is used to promote the full precipitation of the precipitated phase, and finally the third quenching is used to refine the structure and stabilize the residual austenite, significantly improving the low-temperature toughness and balanced strength of the steel. This process uses reasonable heat treatment design combined with precise component optimization to make the low-temperature resistant steel have excellent strength, toughness and balanced performance, meet the application requirements of extreme low-temperature engineering, and provide 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-mentioned 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 rotational austenite, the average diameter of which is 200 to 500 nm and the volume fraction of which is 12 to 16%.

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

[0026] Furthermore, the low-temperature resistant steel contains NbC precipitation phase, and the NbC precipitation phase forms a semi-coherent interface with α-Fe in the low-temperature steel.

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

[0028] Furthermore, the low temperature resistant steel contains ultrafine needle-shaped bainite with an average width of 50 to 200 nm.

[0029] Furthermore, the low temperature resistant steel contains film-like retained austenite, the average thickness of which is 20 to 40 nm.

[0030] The present invention adopts an optimized microstructure design mainly to enhance the strength and toughness of low-temperature resistant steel under extreme low temperature conditions. By precisely controlling the heat treatment process and component design, the organizational structure of the low-temperature resistant steel achieves a variety of mutually synergistic microscopic characteristics, including the optimized ratio of large-angle grain boundaries to small-angle grain boundaries, the stable distribution of rotational austenite, the dispersed precipitation of copper-rich precipitation phases, the precise control of NbC precipitation phases, the formation of ultrafine needle-shaped bainite, and the regulation of film-like residual 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 blocking effect of the crack propagation path is insufficient, resulting in a decrease in the material's fracture resistance; and exceeding 70% will reduce the material's plasticity and increase the risk of brittle failure.

[0031] In the present invention, the average diameter of the rotated austenite is 200-500nm, and the volume fraction is 12-16%. When its diameter is less than 200nm 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, thereby reducing the low-temperature toughness; and when the diameter exceeds 500nm or the volume fraction is higher than 16%, the stability of the rotated austenite decreases, and uncontrolled martensitic phase transformation is prone to occur, resulting in reduced ductility of the material.

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

[0033] The diameter of the NbC precipitate phase in the present invention is 3-10 nm, and the density is (1.0-3.0)×10 22 / m 3 , if the diameter is less than 3nm or the density is less 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; and when the diameter exceeds 10nm or the density is higher than 3.0×10 22 / m 3 This will cause the precipitated phase to aggregate and weaken the fine grain strengthening effect of the material.

[0034] The width of the ultrafine needle-shaped bainite in the present invention is 50 to 200 nm. When the width is less than 50 nm, the formation of the bainite structure is incomplete and crack initiation and expansion cannot be effectively prevented; and when the width exceeds 200 nm, it will lead to grain coarsening and reduce the low-temperature brittleness resistance of the material.

[0035] In the present invention, the thickness of the film-like retained austenite is 20 to 40 nm. When the thickness is less than 20 nm, the residual austenite is not stable enough and is easy to quickly transform into martensite at low temperatures, resulting in reduced plasticity of the material; and when the thickness exceeds 40 nm, it will affect the balance between the strength and toughness of the material and reduce the overall performance in low temperature environments. The synergy of these microstructural characteristics ensures that the low-temperature resistant steel exhibits excellent mechanical properties and reliability in extreme low temperature environments by optimizing the size, morphology and distribution of each component. However, once the parameter range is exceeded, the synergy of the microstructure will be destroyed, and the strength and toughness balance of the material will be significantly reduced, thereby affecting its actual application effect in cryogenic engineering.

[0036] (3) Beneficial technical effects

[0037] 1. The present invention achieves excellent strength, toughness and balanced performance of low-temperature resistant steel in extreme low-temperature environments by precisely controlling the smelting, rolling, quenching, redistribution and aging treatment processes. Compared with the prior art, the low-temperature brittleness resistance and fracture toughness of the material are significantly improved, and the problem of brittle fracture and performance failure of the material in low-temperature environments is solved. It is widely used in the fields of liquefied natural gas storage and transportation, polar construction and deep-sea engineering. The synergistic effect between 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 copper-rich precipitation phases and residual austenite, and alloy elements and multi-stage heat treatment jointly refine the grains and optimize the organizational structure to ensure the balance of strengthening and toughening mechanisms. Reasonable process parameters and component ratios improve the stability and synergy of the microstructure, so that the present invention has unique performance advantages in low-temperature environments and promotes technological progress and engineering applications in the field of low-temperature steel.

[0038] 2. The present invention achieves excellent strength, toughness and balanced performance of low-temperature resistant steel under extreme low temperature conditions by optimizing the microstructure design and accurately controlling the heat treatment process. Compared with the prior art, it significantly improves the material's fracture resistance and low-temperature toughness, solves the problem of brittle fracture of traditional steel in low-temperature environments, and is widely used in the fields of liquefied natural gas storage and transportation, polar engineering and deep-sea drilling. The core innovation of the present invention lies in the synergistic effect between the components: the optimization of the ratio of large-angle and small-angle grain boundaries effectively hinders the crack propagation path, the rotational austenite absorbs the crack propagation energy through the transformation-induced plasticity effect, the precise control of the copper-rich precipitation phase and the NbC precipitation phase enhances the precipitation strengthening effect, and the composite structure of ultrafine needle-shaped bainite and film-like residual austenite achieves a balance between strength and toughness. Reasonable component ratios and key parameters ensure that the microstructure synergy is superior to the prior art, thereby promoting technological progress and engineering applications in the field of low-temperature steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the backscattered electron diffraction technology spectrum of the low-temperature resistant steel prepared in Example 1 of the present invention.

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

[0041] Figure 3 This is a transmission electron microscope photograph of the ultrafine needle-shaped Bainite morphology of the low-temperature resistant steel prepared in Example 1 of the present invention.

[0042] Figure 4 This is a transmission electron microscope photograph of the film-like retained austenite morphology of the low-temperature resistant steel prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0043] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0044] Example 1

[0045] A heat treatment process for low temperature resistant steel comprises the following steps:

[0046] 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 the low-temperature resistant steel include the following components by weight: 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 melting and homogenization process method is: add the components of the low-temperature resistant steel into a vacuum melting furnace, heat to 1600°C at a heating rate of 9.0°C / min, keep warm for 20 minutes, and then naturally cool to room temperature to obtain an ingot; then heat the ingot to 1280°C at a heating rate of 9.0°C / min, keep warm for 30 minutes, and complete the homogenization treatment.

[0047] S2: rolling treatment: the square billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate; the process method of the rolling treatment is: the steel that has completed the homogenization treatment in step S1 is cooled to 1145°C at a wind cooling rate of 2.5°C / s for initial rolling, and the deformation of the rolling is 20%; the steel after initial rolling is cooled to 980°C at the same wind cooling rate for continuous rolling, and the deformation of the continuous rolling is 50%; the steel after continuous rolling is further cooled to 850°C at the same rate for finish rolling, and the deformation of the finish rolling is 70%, and finally a rolled steel plate is obtained.

[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 the quenching treatment is: the rolled steel plate in step S2 is heated to 895°C at a heating rate of 9.0°C / min, and then water quenched after keeping the temperature for 30 minutes, and martensitic steel is obtained after quenching treatment.

[0049] S4: Redistribution treatment: The martensitic steel obtained in step S3 is subjected to redistribution treatment and re-quenching treatment in sequence to obtain quenched-distributed steel; the process method of the redistribution treatment and re-quenching treatment is: the martensitic steel obtained in step S3 is heated to 660°C at a heating rate of 9.0°C / min, and the redistribution treatment is completed by keeping the temperature for 30 minutes, followed by re-quenching, the quenching medium is water, and the quenched-distributed steel is obtained after the re-quenching is completed.

[0050] S5: Aging treatment: The quenched-distributed 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: the quenched-distributed steel obtained in step S4 is heated to 550°C at a heating rate of 9.0°C / min, kept warm for 50 minutes, and then subjected to third quenching, the quenching medium is water, the third quenching is completed, and finally the low-temperature resistant steel is obtained after drying.

[0051] A low-temperature resistant steel prepared in this embodiment is prepared by the above-mentioned preparation method; the ratio of large-angle grain boundary to small-angle grain boundary in the low-temperature resistant steel is 60:40. The low-temperature resistant steel contains rotational austenite with an average diameter of 500nm and a volume fraction of 12%. The low-temperature resistant steel contains a copper-rich precipitate phase with an average diameter of 7nm and a density of 1.5×1022 / m 3 The low temperature resistant steel contains NbC precipitates, which form a semi-coherent interface with α-Fe in the low temperature steel. The average diameter of the NbC precipitates is 10nm 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 200nm. The low temperature resistant steel contains film-like retained austenite with an average thickness of 40nm.

[0052] pass Figure 1 It can be seen 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 spectrum, which indicates that the grain boundary structure plays a significant role in optimizing material properties. Figure 2 The transmission electron microscope photograph of the semi-coherent interface formed by the NbC precipitation phase and α-Fe in the low-temperature steel is shown, which further proves the existence of semi-coherent interface characteristics in the material, and this interface helps to improve the strength and toughness of the material. Figure 3 This is a transmission electron microscope photograph of the ultrafine needle-shaped bainite morphology, which proves the existence of this organizational morphology and effectively improves the strength and toughness of low-temperature steel. Figure 4 The transmission electron microscope photo of the film-like retained austenite is shown, which proves the distribution of the film-like 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 optimization process of the present invention.

[0053] Example 2

[0054] A heat treatment process for low temperature resistant steel comprises the following steps:

[0055] 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 the low-temperature resistant steel include the following components by weight: 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 inevitable impurities; the melting and homogenization process method is: add the components of the low-temperature resistant steel into a vacuum melting furnace, heat to 1615°C at a heating rate of 9.6°C / min, keep warm for 23 minutes, and then naturally cool to room temperature to obtain an ingot; then heat the ingot to 1283°C at a heating rate of 9.6°C / min, keep warm for 32 minutes, and complete the homogenization treatment.

[0056] S2: rolling treatment: the square billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate; the process method of the rolling treatment is as follows: the steel that has completed the homogenization treatment in step S1 is cooled to 1149°C at a wind cooling rate of 2.8°C / s for initial rolling, and the deformation of the rolling is 23%; the steel after initial rolling is cooled to 986°C at the same wind cooling rate for continuous rolling, and the deformation of the continuous rolling is 53%; the steel after continuous rolling is further cooled to 853°C at the same rate for finish rolling, and the deformation of the finish rolling is 73%, and finally a rolled steel plate is obtained.

[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 the quenching treatment is: the rolled steel plate in step S2 is heated to 898°C at a heating rate of 9.6°C / min, kept warm for 32 minutes, and then water quenched to obtain martensitic steel after quenching treatment.

[0058] S4: Redistribution treatment: The martensitic steel obtained in step S3 is subjected to redistribution treatment and re-quenching treatment in sequence to obtain quenched-distributed steel; the process method of the redistribution treatment and re-quenching treatment is: the martensitic steel obtained in step S3 is heated to 666°C at a heating rate of 9.6°C / min, and the redistribution treatment is completed by keeping the temperature for 32 minutes, followed by re-quenching, the quenching medium is water, and the quenched-distributed steel is obtained after the re-quenching is completed.

[0059] S5: Aging treatment: The quenched-distributed 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: the quenched-distributed steel obtained in step S4 is heated to 553°C at a heating rate of 9.6°C / min, kept warm for 53 minutes, and then subjected to third quenching, the quenching medium is water, the third quenching is completed, and finally the low-temperature resistant steel is obtained after drying.

[0060] A low-temperature resistant steel prepared in this embodiment is prepared by the above-mentioned 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 rotational austenite with an average diameter of 400nm and a volume fraction of 15%. The low-temperature resistant steel contains a copper-rich precipitate phase with an average diameter of 4.8nm and a density of 3.5×10 2 2 / m 3 The low temperature steel contains NbC precipitates, which form a semi-coherent interface with α-Fe in the low temperature steel. The average diameter of the NbC precipitates is 8.0 nm and the density is 1.8×10 2 2 / m 3 The low temperature resistant steel contains ultrafine needle-shaped bainite with an average width of 150nm. The low temperature resistant steel contains film-like retained austenite with an average thickness of 30nm.

[0061] Example 3

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

[0063] 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 the low-temperature resistant steel include the following components by weight: 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 inevitable impurities; the melting and homogenization process method is: add the components of the low-temperature resistant steel into a vacuum melting furnace, heat to 1630°C at a heating rate of 10.2°C / min, keep warm for 26 minutes, and then naturally cool to room temperature to obtain an ingot; then heat the ingot to 1287°C at a heating rate of 10.2°C / min, keep warm for 33 minutes, and complete the homogenization treatment.

[0064] S2: rolling treatment: the square billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate; the process method of the rolling treatment is as follows: the steel that has completed the homogenization treatment in step S1 is cooled to 1153°C at a wind cooling rate of 3.1°C / s for initial rolling, and the deformation of the rolling is 26%; the steel after initial rolling is cooled to 993°C at the same wind cooling rate for continuous rolling, and the deformation of the continuous rolling is 56%; the steel after continuous rolling is further cooled to 857°C at the same rate for finish rolling, and the deformation of the finish rolling is 76%, and finally a rolled steel plate is obtained.

[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: the rolled steel plate in step S2 is heated to 902°C at a heating rate of 10.2°C / min, kept warm for 33 minutes, and then water quenched to obtain martensitic steel after 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 quenched-distributed steel; the process method of the redistribution treatment and re-quenching treatment is: the martensitic steel obtained in step S3 is heated to 673°C at a heating rate of 10.2°C / min, and the redistribution treatment is completed by keeping the temperature for 33 minutes, followed by re-quenching, the quenching medium is water, and the quenched-distributed steel is obtained after the re-quenching is completed.

[0067] S5: Aging treatment: The quenched-distributed steel obtained in step S4 is subjected to aging treatment and a third quenching to obtain low-temperature resistant steel. The process method of aging treatment and the third quenching is: the quenched-distributed steel obtained in step S4 is heated to 558°C at a heating rate of 10.2°C / min, kept warm for 56 minutes, and then subjected to a third quenching, the quenching medium is water, the third quenching is completed, and the low-temperature resistant steel is finally obtained after drying.

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

[0069] Example 4

[0070] A heat treatment process for low temperature resistant steel comprises the following steps:

[0071] 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 the low-temperature resistant steel include the following components by weight: 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 inevitable impurities; the melting and homogenization process method is: add the components of the low-temperature resistant steel into a vacuum melting furnace, heat to 1650°C at a heating rate of 11.0°C / min, keep warm for 30 minutes, and then naturally cool to room temperature to obtain an ingot; then heat the ingot to 1290°C at a heating rate of 11.0°C / min, keep warm for 35 minutes, and complete the homogenization treatment.

[0072] S2: rolling treatment: the square billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate; the process method of the rolling treatment is: the steel that has completed the homogenization treatment in step S1 is cooled to 1160°C at a wind cooling rate of 3.5°C / s for initial rolling, and the deformation of the rolling is 30%; the steel after initial rolling is cooled to 1000°C at the same wind cooling rate for continuous rolling, and the deformation of the continuous rolling is 60%; the steel after continuous rolling is further cooled to 860°C at the same rate for finish rolling, and the deformation of the finish rolling is 80%, and finally a rolled steel plate is obtained.

[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 the quenching treatment is: the rolled steel plate in step S2 is heated to 905°C at a heating rate of 11.0°C / min, kept warm for 35 minutes, and then water quenched to obtain martensitic steel after quenching treatment.

[0074] S4: Redistribution treatment: The martensitic steel obtained in step S3 is subjected to redistribution treatment and re-quenching treatment in sequence to obtain quenched-distributed steel; the process method of the redistribution treatment and re-quenching treatment is: the martensitic steel obtained in step S3 is heated to 680°C at a heating rate of 11.0°C / min, and the redistribution treatment is completed by keeping the temperature for 35 minutes, followed by re-quenching, the quenching medium is water, and the quenched-distributed steel is obtained after the re-quenching is completed.

[0075] S5: Aging treatment: The quenched-distributed 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: the quenched-distributed steel obtained in step S4 is heated to 560°C at a heating rate of 11.0°C / min, kept warm for 60 minutes, and then subjected to third quenching, the quenching medium is water, the third quenching is completed, and finally the low-temperature resistant steel is obtained after drying.

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

[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. (below the lower limit).

[0083] Comparative Example 4

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

[0085] Comparative Example 5

[0086] It is basically the same as the first embodiment, except that the reallocation process is cancelled.

[0087] Comparative Example 6

[0088] The method is substantially the same as Example 1, except that the homogenization temperature is reduced to 1260° C. (below the lower limit).

[0089] Comparative Example 7

[0090] It is basically the same as Example 1, except that the finishing deformation is reduced to 60% (below 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 (below 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: The impact performance test adopts JBDW-300D ultra-low temperature impact tester, which is implemented in accordance with the latest national standards. The impact rate is set to 5.24m / s, the maximum impact energy is 450J, and the test is carried out at room temperature and liquid nitrogen temperature. At the same time, the displacement-load curve and displacement-energy curve data at different temperatures are recorded using NI750C oscilloscope impact tester equipped with high-resolution rotary encoder.

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

[0098] The properties of the steel materials of Examples 1 to 4 and Comparative Examples 1 to 9 are summarized in Table 2.

[0099] Table 2 Performance summary of steel materials of Examples 1 to 4 and Comparative Examples 1 to 9

[0100]

[0101] The main difference between Comparative Example 1 and Example 1 is that the Ce element is removed. As can be seen from Table 2, 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 number of inclusions, thereby optimizing the grain boundary distribution and improving the toughness of the material. In Comparative Example 1, due to the lack of the Ce element, the grains are coarsened and the grain boundary stability is reduced, resulting in a significant decrease 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). As can be seen from Table 2, the low-temperature impact performance, tensile strength and elongation of Comparative Example 2 are lower than those of Example 1. This is because too high an aging temperature will cause the copper-rich precipitate phase and the NbC precipitate phase to coarsen significantly, reducing their strengthening effect, thereby reducing the strength and toughness of the material. In addition, the coarsened precipitate phase cannot effectively pin 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, the low-temperature impact properties and tensile strength of Comparative Example 3 are lower than those of Example 1. Lowering the first quenching temperature will lead to incomplete martensite transformation, forming more residual ferrite or pearlite structure, thereby reducing the strength and toughness of the matrix. Incompletely quenched structure will also weaken the effect of subsequent redistribution treatment, resulting in a decrease 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, the low-temperature impact performance and elongation of Comparative Example 4 are lower than those of Example 1. Too low a redistribution temperature will lead to insufficient stability of the rotational austenite, and its volume fraction will be reduced, 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, and ultimately causing a decrease in low-temperature impact performance and elongation.

[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 performance, tensile strength and elongation of Comparative Example 5 are significantly lower than those of Example 1. The redistribution treatment is an important step in the formation of rotational austenite. The cancellation of this treatment will lead to a significant reduction in the content of rotational 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, the low-temperature impact performance and elongation of Comparative Example 6 are lower than those of Example 1. Too low a homogenization temperature will lead to uneven distribution of alloy elements and form component segregation, which will make the microstructure of the material uneven and reduce the low-temperature toughness and plasticity. In addition, component segregation will also lead to 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 finishing deformation is reduced to 60% (below the lower limit). As can be seen from Table 2, the low-temperature impact performance and elongation of Comparative Example 7 are slightly lower than those of Example 1. The reduction in finishing 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, and ultimately leading to a decrease in low-temperature impact performance and elongation.

[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, the low-temperature impact performance and tensile strength of Comparative Example 8 are lower than those of Example 1. Too low a heating rate will lead to grain growth, affect the uniformity and refinement of the material's structure, and thus reduce its strength and toughness. In addition, slow heating will weaken the precipitation driving force of the precipitated phase, increase its size, reduce its density, and further weaken 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 precipitation phase and NbC precipitation phase, thereby effectively enhancing the strength and toughness of the material. Canceling the aging treatment will lead to insufficient precipitation phase and significantly reduce the strengthening effect. At the same time, since the residual stress in the organization is not fully released, the elongation and toughness of the material are also reduced.

[0110] In summary, by comparing them one by one, it can be found that the optimized process of Example 1 shows significant advantages in grain refinement, precipitation phase control and organizational stability, and significantly improves low-temperature impact performance, tensile strength and elongation. Compared with Example 1, deviation from key process parameters or omission of important processing steps in the comparative example will lead to a significant decrease in material properties, further verifying the necessity and effectiveness of process optimization.

[0111] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that all equivalent structural changes made under the concept of the present invention and using the contents of the present invention specification and drawings should be covered within the scope of protection 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; S2: rolling process: the billet obtained in step S1 is subjected to initial rolling and finish rolling in sequence to obtain a rolled steel plate; S3: quenching treatment: quenching the rolled steel plate obtained in step S2 to obtain martensitic steel; S4: Redistribution treatment: The martensitic steel obtained in step S3 is subjected to redistribution treatment and re-quenching treatment in sequence to obtain quenched-distribution steel; S5: Aging treatment: The quenched-distributed steel obtained in step S4 is subjected to aging treatment and a third quenching to obtain low-temperature resistant steel.

2. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The components of the low-temperature resistant steel in step S1 include the following compositions 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, and the remainder is Fe and unavoidable impurities.

3. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The smelting and homogenization process of step S1 is as follows: adding the components of the low-temperature resistant steel into a vacuum melting furnace, heating to 1600-1650°C at a heating rate of 9.0-11.0°C / min, keeping warm for 20-30 minutes, and then naturally cooling to room temperature to obtain an ingot; then heating the ingot to 1280-1290°C at a heating rate of 9.0-11.0°C / min, keeping warm for 30-35 minutes, and completing the homogenization treatment.

4. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The process method of the rolling treatment in step S2 is: cooling the steel that has completed the homogenization treatment in step S1 to 1145-1160°C at an air cooling rate of 2.5-3.5°C / s for initial rolling, and the deformation of the rolling is 20-30%; cooling the steel after initial rolling to 980-1000°C at the same air cooling rate for continuous rolling, and the deformation of the continuous rolling is 50-60%; cooling the steel after continuous rolling to 850-860°C at the same rate for finish rolling, and the deformation of the finish rolling is 70-80%, and finally obtaining a rolled steel plate.

5. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The process method of the quenching treatment in step S3 is: 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 30-35 minutes, and then water quenching to obtain martensitic steel after quenching.

6. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The process method of the redistribution treatment and re-quenching treatment in step S4 is: heating the martensitic steel obtained in step S3 to 660-680°C at a heating rate of 9.0-11.0°C / min, keeping the temperature for 30-35 minutes to complete the redistribution treatment, and then quenching again, the quenching medium is water, and after the re-quenching is completed, the quenched-distributed steel is obtained.

7. A heat treatment process for low temperature resistant steel according to claim 1, characterized in that: The process method of the aging treatment and the third quenching in step S5 is as follows: 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 warm for 50-60 minutes, and then subjected to the third quenching, the quenching medium being water, and after the third quenching is completed, the low-temperature resistant steel is finally obtained after drying.

8. A low temperature resistant steel, characterized in that: Prepared by the preparation method according to any one of claims 1 to 7; The ratio of the large-angle grain boundary to the small-angle grain boundary in the low-temperature resistant steel is (60-70):(30-40).

9. The low temperature resistant steel according to claim 8, characterized in that: The low temperature resistant steel contains rotation austenite, the average diameter of which is 200-500nm 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 to 7.0 nm and a density of (1.5 to 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.

10. The low temperature resistant steel according to claim 8, characterized in that: The low temperature resistant steel contains ultrafine needle-shaped bainite, the average width of which is 50 to 200 nm; The low temperature resistant steel contains film-like retained austenite, the average thickness of which is 20-40nm.

Images