A method for improving the strength and impact toughness of 3.5Ni steel plate in low temperature environment

Abstract

The application belongs to the field of metallurgy, and particularly relates to a method for improving the strength and impact toughness of 3.5Ni steel plates under low-temperature environment. The technical scheme of the application is as follows: a method for improving the strength and impact toughness of 3.5Ni steel plates under low-temperature environment, by precisely controlling the adding amount of Mo in the 3.5Ni steel plate and the heat treatment process, a nano coherent B2 precipitated phase with specific crystallographic characteristics is induced to form, so that the impact energy of the 3.5Ni steel plate under-196 DEG C reaches 100J or more. The method for improving the strength and impact toughness of 3.5Ni steel plates under low-temperature environment provided by the application makes the impact energy of the 3.5Ni steel under-196 DEG C reach 100J or more, provides low-cost and high-performance low-temperature structural materials for the fields of liquefied natural gas storage and transportation, cryogenic engineering and the like, and can expand the application boundary of low-Ni steel under extreme working conditions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metallurgy, and particularly relates to a method for improving the strength and impact toughness of 3.5Ni steel plate in a low-temperature environment. BACKGROUND

[0002] With the increasing demand of petroleum and chemical industries, low-temperature steel is increasingly widely used in liquefied petroleum gas (LPG) and liquefied ethylene (LNG) storage and transportation equipment. As an important representative of low-temperature steel, Ni-based low-temperature steel is widely used in various low-temperature equipment and containers worldwide. Ni element can comprehensively improve the mechanical properties of low-temperature steel, however, the demand for 9Ni steel is increasing year by year, and the high price of Ni element and the limited Ni resource reserves in China have gradually become prominent.

[0003] In order to meet the needs of national economic development, the development of Ni-saving low-temperature steel helps China to achieve the strategic goal of sustainable development of resources. However, reducing the Ni content in steel will significantly deteriorate the strength and toughness of the steel plate. Therefore, it is necessary to explore the change rule of the organization and strength and toughness of Ni-saving low-temperature steel, develop the industrialization key manufacturing technology of Ni-saving low-temperature steel, and finally replace the traditional 9%Ni steel in the construction of ultra-low-temperature storage and transportation facilities, so that China's Ni-saving ultra-low-temperature steel can reach the international advanced level from the current blank state.

[0004] Micro-alloying of nickel-based low-temperature steel can improve the mechanical properties of nickel-based low-temperature steel. Among them, the addition of alloying elements to improve the mechanical properties of alloys mainly has two forms: one is that the alloying elements are segregated during heat treatment, forming a microstructure with favorable shape, size and reasonable volume fraction for mechanical properties; the other is the precipitation of alloy carbides.

[0005] Nanometer coherent precipitation is considered to be one of the most effective methods for strengthening bcc structure steel, but these precipitates mostly present approximate strict stoichiometric composition, and the higher alloy concentration leads to higher brittleness, which improves the strength while significantly deteriorates the toughness and plasticity.

[0006] At present, the low-temperature steel with reduced Ni content has made significant progress in research, and there are numerous related patent technologies. For example, some patents (such as patent application number 201910429287.2) adopt a controlled rolling, online quenching, two-phase zone quenching and tempering process, and obtain 7Ni steel thick plates for ultra-low-temperature containers with excellent low-temperature toughness and high strength. Another patent (patent application number 202310562946.6) produces continuous casting slabs with a thickness of more than 150 mm with low center segregation and porosity through a continuous casting process, adopts a multiple dehydrogenation process, uses a large reduction rolling mill, and uses a QLT heat treatment process to manufacture 5-30 mm thick 5.5Ni steel plates that guarantee mechanical properties and process performance. Some patents (such as patent application number 202311085017.7) reduce the density of 5Ni steel through chemical composition and content design, and further ensure that the steel plate has good comprehensive performance through the manufacturing method. At present, the mainstream 3.5%Ni steel adopts normalizing + tempering (NT) or quenching + tempering (QT) process, although it can achieve -101℃ impact energy >80J (such as 08Ni3DR steel plate), but the performance below -120℃ decreases significantly, which is difficult to meet the demand of deep cold equipment. And with the decrease of Ni content, the strength of the steel plate decreases, and the ductile-brittle transition temperature increases; this limits the application of 3.5%Ni steel plate in lower environmental temperature. SUMMARY

[0007] The present application provides a method for improving the strength and impact toughness of 3.5Ni steel plate in low temperature environment, which makes the impact energy of 3.5Ni steel at -196℃ reach more than 100J, providing low-cost and high-performance low-temperature structural materials for liquefied natural gas storage and transportation, deep cold engineering and other fields, and expanding the application boundary of low-Ni steel in extreme working conditions.

[0008] The technical solution of the present application is as follows:

[0009] A method for improving the strength and impact toughness of 3.5Ni steel plate in low temperature environment, by precisely controlling the addition amount of Mo in 3.5Ni steel plate and heat treatment process, inducing the formation of nano-coherent B2 precipitates with specific crystallographic characteristics, so that the impact energy of 3.5Ni steel plate at -196℃ reaches more than 100J.

[0010] Further, the method for improving the strength and impact toughness of 3.5Ni steel plate in low temperature environment has the following specific process steps:

[0011] (1) Vacuum induction furnace smelting and casting: the vacuum induction furnace is used for smelting to obtain an alloy melt, and the alloy melt is cast into an ingot; the chemical composition of the ingot is as follows in percentage by mass: carbon content 0.04-0.05%, silicon content ≤0.1%, manganese content ≤0.1%, nickel content 3.35-3.7%, phosphorus content ≤0.005%, sulfur content ≤0.004%, molybdenum content ≤0.05%, 0.08-0.12%, 0.17-0.23% or 0.33-0.37%, and the rest is iron and other inevitable impurities;

[0012] (2) Heating: the ingot is heated to a temperature range of 1150-1250℃, and is kept for 1-2 hours to ensure uniform heating of the ingot and stability of the structure;

[0013] (3) Two-stage rolling is performed:

[0014] Coarse rolling stage: 3-5 passes of coarse rolling are performed in the austenite recrystallization zone, the rolling-in temperature is set to 1100-1150℃, and the cumulative reduction rate of a single pass is 53-65%;

[0015] Finish rolling stage: 3-5 passes of finish rolling are performed in the austenite unrecrystallization zone, the rolling-in temperature is 860-890℃, and the cumulative reduction rate of a single pass is 72.5-82.5%; the finish rolling temperature is set to 790-810℃, and finally a hot-rolled plate with a thickness of 12-18mm is obtained;

[0016] (4) Quenching: the hot-rolled plate is heated to 830-870℃ and kept for 1h, and then is quenched to reduce the temperature of the hot-rolled plate to below 200℃, so as to ensure that the steel plate can maintain a stable metallographic structure at a low temperature;

[0017] (5) Two-phase zone quenching treatment: the quenched steel plate is heated to a two-phase zone temperature of 700-730℃ and kept for 30-70 minutes; after the structure is fully transformed, quenching is performed to reduce the temperature to below 200℃, so as to obtain a subcritical quenched steel plate;

[0018] (6) Tempering treatment: the subcritical quenched steel plate is heated to 630-650℃ for tempering, and the tempering time is 45-80 minutes; after tempering, water cooling to room temperature is performed to ensure the stability of the structure and the toughness of the steel plate during tempering, and finally a 3.5Ni steel plate with a thickness of 12-18mm is obtained.

[0019] The technical key points of the application are as follows:

[0020] 1. Introduce precise amount of Mo in 3.5wt.% Ni steel, induce the formation of new Mo-rich nano B2 precipitates through process optimization, the precipitates are body-centered cubic ordered structure, completely coherent with bcc matrix, average size about 1-2nm, through Mo element regulation to realize the efficient precipitation of B2 phase, break through the "high Ni dependent" solid solution strengthening path.

[0021] 2. Through "quenching + critical zone quenching + tempering" process, use the diffusion kinetics characteristics of Mo to control B2 phase nucleation and growth, inhibit the coarsening of precipitates, and ensure the structural stability at low temperature.

[0022] 3. Mo-rich nano B2 precipitates through "coherent strengthening + dislocation shear" dual mechanism, significantly improve the yield strength, tensile strength of steel at 77K (liquid nitrogen temperature), and the impact energy increases to 280J, which is equivalent to the performance of traditional 9Ni steel, breaking the "strength-toughness" trade-off effect at low temperature.

[0023] The beneficial effects of the present application are:

[0024] 1. The present application successfully generates nano coherent B2 precipitates by precisely regulating the Mo addition amount and heat treatment process in 3.5Ni steel plate, the Mo-rich B2 nano precipitates in the steel are coherent with the matrix, effectively hindering dislocation movement, and at the same time, the coherent interface coordinates deformation to avoid crack initiation caused by stress concentration; the formation of nano coherent B2 precipitates not only improves the static strength of the material, but also significantly improves the dynamic mechanical properties of the material, so that the yield strength of the material is increased from 450MPa of traditional 3.5Ni steel to 5000MPa, and the tensile strength is increased from 550MPa to 630MPa, while the elongation is ≥27%, so that the material can absorb more energy when subjected to impact load, and improve the fatigue resistance and service life.

[0025] 2. At -196℃ low temperature environment, the impact energy of 3.5Ni steel is significantly improved from 50J of traditional steel to ≥200J, reaching or even exceeding the performance level of some 9Ni steel, effectively solving the problem of insufficient material toughness in low temperature environment.

[0026] 3. Compared with traditional 9Ni steel, the present application reduces the Ni content to 3.5% and adds a small amount of Mo, while maintaining or even improving the performance, the material cost can be reduced by about 30-40%. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The dark field phase of TEM in Example 1 shows nano B2 phase when Mo element is 0.35%.

[0028] Figure 2 TEM dark field phase of nanoscale B2 phase when Mo element is 0.1% in Example 3;

[0029] Figure 3 TEM high-resolution characterization of the matrix and nanoscale B2 phase when Mo element is 0.35% in Example 1;

[0030] Figure 4 Mechanical property comparison chart when Mo element is 0.01%, 0.1%, 0.2% and 0.35% respectively;

[0031] Figure 5 SEM microstructure and tensile fracture when Mo element is 0.1% and 0.35% respectively;

[0032] Figure 6 77K oscilloscope impact curve and impact macro and micro fracture when Mo element is 0.35% in Example 1. DETAILED DESCRIPTION Example 1

[0033] A method for improving the strength and impact toughness of 3.5Ni steel plate in a low temperature environment, the specific process steps are as follows:

[0034] (1) Vacuum induction furnace melting and casting: adopt vacuum induction furnace to melt and get alloy melt and cast it into ingot; the chemical composition of the ingot is as follows: carbon content 0.05%, silicon content ≤0.1%, manganese content ≤0.1%, nickel content 3.5%, phosphorus content ≤0.005%, sulfur content ≤0.004%, molybdenum content 0.35%, and the rest is iron and other unavoidable impurities;

[0035] (2) Heating: heat the ingot to 1200℃ and keep for 1.5 hours to ensure uniform heating of the ingot and stability of the structure;

[0036] (3) Two-stage rolling:

[0037] Coarse rolling stage: 3-5 passes of coarse rolling in the austenite recrystallization zone, the rolling temperature is set to 1100-1150℃, and the cumulative reduction rate of single pass is 53~65%;

[0038] Finish rolling stage: enter the austenite unrecrystallization zone and carry out 3 to 5 passes of finish rolling, the rolling temperature is 860-890℃, and the cumulative reduction rate of single pass is 72.5-82.5%; the final rolling temperature is set to 790-810℃, and finally get hot-rolled plate with thickness of 12-18mm;

[0039] (4) Quenching: The hot-rolled plate is heated to 850°C, held for 1 h, and then quenched to reduce the temperature of the hot-rolled plate to below 200°C, so as to ensure that the steel plate can maintain a stable microstructure at a lower temperature;

[0040] (5) Two-phase zone quenching treatment: The quenched steel plate is heated to a two-phase zone temperature of 720°C and held for 50 minutes; after the microstructure is fully transformed, quenching is performed to rapidly reduce the temperature to below 200°C, thereby obtaining a subcritical quenched steel plate;

[0041] (6) Tempering treatment: The subcritical quenched steel plate is heated to 640°C for tempering, and the tempering time is 60 minutes; after tempering, water cooling to room temperature is performed to ensure the stability of the microstructure during tempering and the improvement of the toughness of the steel plate, and finally a 3.5Ni steel plate with a thickness of 12 to 18 mm is obtained. Example 2

[0042] The difference from Example 1 is that the molybdenum content is 0.2%. Example 3

[0043] The difference from Example 1 is that the molybdenum content is 0.1%. Example 4

[0044] The difference from Example 1 is that the molybdenum content is 0.01%.

[0045] As shown in Figure 1 , the molybdenum element content is adjusted to 0.35%, and the nanoscale B2 phase under this content is displayed by transmission means. As shown in Figure 2 , the molybdenum element is adjusted to 0.1% in the working condition, and the microstructure of the nanoscale B2 phase is directly presented by transmission dark field image, which provides a basic microstructure reference for subsequent research. Figure 2 When the molybdenum content is 0.1Mo, the number of B2 phases is small, sparse, and unevenly distributed, Figure 1 When the molybdenum content is 0.35Mo, the number of B2 phases is large, more dense, and dispersed. The Mo-rich B2 nanoscale precipitates are mostly near-spherical, and are cut into elongated shapes by dislocations after being deformed by impact load, losing their original equiaxed spherical morphology; and their average size is extremely small (about 1-2 nm), uniformly distributed and with extremely high number density, without obvious segregation in the matrix. It can be found that with the increase of Mo introduction, the number of Mo-rich B2 nanoscale precipitates increases.

[0046] As shown in Figure 3As shown, the advanced technology of transmission high resolution microscope was used to observe the sample with 0.35% molybdenum content. Under the high resolution imaging, the lattice structure of the body phase and B2 phase precipitates can be clearly distinguished, and there is obvious difference between them. This solid evidence strongly verifies the real existence of B2 phase, and provides key microstructure basis for further exploring the influence of molybdenum element on material performance. And we found that B2 phase is coherent with the matrix, combined with Figure 1 , 2 Comparing the above, it is found that the number, density and dispersed distribution state of nanoscale B2 phase of 0.35% Mo are beneficial to uniform dispersion of stress, hinder the movement of dislocation, and can improve the tensile and yield strength of the material. When impacted, it can also absorb energy through its own deformation to improve impact performance.

[0047] As shown in Figure 4 , the results of tensile property test of samples with different molybdenum content (0.01%, 0.1%, 0.2% and 0.35%) are shown. From the figure, it can be clearly seen that with the gradual increase of molybdenum content, the yield strength and tensile strength of the material show a steady upward trend, and the tensile strength increases from about 550 MPa to more than 600 MPa.

[0048] As shown in Figure 5 , the scanning structure diagram and tensile fracture morphology of samples with molybdenum content of 0.1% and 0.35% are compared. Through careful observation, it can be found that the fracture mainly presents typical dimple characteristics, and there are a large number of size dimple composite cases. Among them, large and deep dimples and equiaxed dimples are interwoven with each other. This unique microstructure is a strong evidence of the good plastic deformation and coordinated deformation ability of the material. This structure can effectively disperse stress and avoid local stress concentration when the material is subjected to external force, so as to ensure that the material does not break rapidly when it is deformed greatly.

[0049] As shown in Figure 6As shown, the impact curve of the sample with 0.35% Mo content at 77K is shown. From the curve data, the impact absorption energy of the material is up to about 290J, the crack initiation energy is about 35J, and the propagation energy is about 250J. These data comprehensively show that the material is ductile fracture at 77K low temperature environment. Further analysis of the macroscopic fracture also shows typical ductile fracture characteristics. In the expansion area of the fracture, very fine and dense dimple structure can be observed. Notably, these dimples gradually transition from elongated to transitional and finally equiaxed, which fully embodies the material's excellent coordinated deformation ability and outstanding impact absorption capacity when subjected to impact load. This ordered evolution of microstructure enables the material to effectively dissipate energy through dimple deformation and expansion when facing complex loads, thereby significantly improving the material's impact resistance and toughness.

[0050] According to the results of normal temperature tensile test and 77K charpy impact test, it can be found that with the increase of Mo introduction, the yield strength and tensile strength of the material increase by 13%, and the elongation is still ≥ 28%. The 3.5Ni produced by other inventions is mostly brittle fracture at 77K or the impact energy is 60-80J. The 3.5Ni Mo steel plate prepared by the industrial production test of the method has a ductile-brittle transition temperature below 77K, and the impact absorption energy is ≥ 280J. In its tensile fracture, compared to the low Mo example, a large number of uniformly distributed large and deep equiaxed dimples appear; and several deep secondary dimples appear inside a large dimple, and inclusions are also observed at the bottom, which is one of the manifestations of its higher strength. In its impact fracture: the fiber-radiation transition zone of the fracture, the continuous evolution of the dimple morphology is observed: on the fiber side, the dimples are significantly elongated and distributed in the direction of the fracture, accompanied by dense tear ridges, embodying the ductile fracture characteristics of 'plastic deformation dominated, high energy dissipation' during stable crack propagation; towards the radiation zone, the dimples gradually equiaxed, the size decreases, and the tear ridge density decreases, reflecting the accelerated crack propagation and limited plastic deformation; on the radiation side, equiaxed dimples and cleavage characteristics coexist, which confirms the brittle mechanism of 'insufficient plastic energy dissipation, rapid brittle fracture' during unstable crack propagation. The transition fully presents the 'ductile to brittle' fracture mode transition, providing micro-morphology basis for analyzing the ductile-brittle transition behavior of the material under impact.

Claims

1. A method for improving the strength and impact toughness of 3.5Ni steel plate in a low temperature environment, characterized by, By precisely controlling the Mo addition amount and heat treatment process of 3.5Ni steel plate, a nano coherent B2 precipitate phase with specific crystallographic characteristics is induced to form, so that the impact energy of the 3.5Ni steel plate at-196℃ reaches more than 100J; The specific process steps are as follows: (1) Vacuum induction furnace melting and casting: the vacuum induction furnace is used for melting to obtain an alloy melt and casting the alloy melt into an ingot; the chemical composition of the ingot is as follows in mass percentage: carbon content 0.04-0.05%, silicon content ≤0.1%, manganese content ≤0.1%, nickel content 3.35-3.7%, phosphorus content ≤0.005%, sulfur content ≤0.004%, molybdenum content 0.08-0.12%, 0.17-0.23% or 0.33-0.37%, and the rest is iron and other unavoidable impurities; (2) Heating: the ingot is heated to a temperature range of 1150-1250℃ and is kept for 1-2 hours to ensure uniform heating of the ingot and stability of the structure; (3) Two-stage rolling: Coarse rolling stage: 3-5 passes of coarse rolling are carried out in the austenite recrystallization zone, the rolling temperature is set to 1100-1150℃, and the cumulative reduction of the coarse rolling stage is 53-65%; Fine rolling stage: 3-5 passes of fine rolling are carried out in the austenite unrecrystallization zone, the rolling temperature is 860-890℃, and the cumulative reduction of the fine rolling stage is 72.5-82.5%; the final rolling temperature is set to 790-810℃, and finally a hot-rolled plate with a thickness of 12-18mm is obtained; (4) Quenching: the hot-rolled plate is heated to 830-870℃ and kept for 1h, and then is quenched to reduce the temperature of the hot-rolled plate to below 200℃, so as to ensure that the steel plate can maintain a stable metallographic structure at a lower temperature; (5) Two-phase zone quenching treatment: the quenched steel plate is heated to a two-phase zone temperature of 700-730℃ and kept for 30-70 minutes; after the structure is fully transformed, quenching is carried out to rapidly reduce the temperature to below 200℃, and a subcritical quenched steel plate is obtained; (6) Tempering treatment: the subcritical quenched steel plate is heated to 630-650℃ for tempering, and the tempering time is 45-80 minutes; after tempering, water cooling to room temperature is carried out to ensure the stability of the structure and the toughness of the steel plate during tempering, and finally a 3.5Ni steel plate with a thickness of 12 to 18mm is obtained.

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