High strength and toughness low temperature marine steel regulated by v-n microalloying and multi-stage heat treatment and method of manufacturing the same

Abstract

The application discloses a V-N micro-alloying and multi-stage heat treatment regulated high-strength and high-toughness low-temperature marine engineering steel and a manufacturing method thereof, and belongs to the technical field of low-temperature alloy steel smelting.The V-N micro-alloying is combined with multi-stage heat treatment, the content of Ni is reduced, V and N elements are added, and a multi-stage heat treatment process of hot rolling-direct quenching-high temperature quenching-intercritical quenching-tempering is combined, so that the strength and low-temperature toughness of the material are improved while resources are saved, better strength and toughness matching is achieved, and a low-temperature marine engineering steel with good comprehensive performance is obtained, thereby providing guidance for alloy design and process optimization of high-performance marine engineering steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of low-temperature alloy steel smelting, and particularly relates to a V-N micro-alloyed and multi-stage heat treatment regulated high-strength and high-toughness low-temperature marine steel and a manufacturing method thereof. BACKGROUND

[0002] With the development of China's industry and the improvement of the national living standard, clean energy such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG) is increasingly favored by people. However, due to the lack of domestic land oil (the main material for producing liquefied petroleum gas) and natural gas resources, the production cannot meet the growing demand. The ocean contains abundant oil and gas resources. At present, more than 80% of the global marine oil and gas reserves are distributed in deep sea areas with a water depth of 500 m or more, of which the oil and gas reserves in the Arctic region account for 13% and 30% of the global unproven and unexploited reserves, respectively. Therefore, it is inevitable for China's oil and gas resource exploitation and transportation to move from land to deep sea and polar regions.

[0003] However, the development and transportation of deep-sea and polar resources cannot be achieved without deep diving-transportation technology, which is a comprehensive system composed of submersibles, surface support ships and land bases. Deep-sea exploration and development of deep-sea resources require the use of submersibles. In order to cope with complex marine environments, steel materials used in submersibles should have high strength, excellent low-temperature toughness, good ductility and other characteristics to adapt to the static pressure and cyclic load generated by the working depth in deep-sea operations. Icebreakers are key engineering ships for polar exploration, exploration and development, which need to continuously perform icebreaking operations in the Arctic environment (air temperature below -50℃) and cooperate with submersibles for exploration work. Due to the action of sea ice, ships will be subjected to additional ice load during navigation in the Arctic, and this long-term cyclic ice load not only easily causes damage to ship structure components, but also puts high requirements on the low-temperature toughness of materials. In addition, during oil exploitation, the offshore LPG production system separates crude oil from LPG, and qualified LPG is transported to the storage tank for storage. The natural gas obtained by exploitation is liquefied into LNG, which is smaller in volume and more convenient for storage and transportation. However, due to the characteristics of LPG and LNG such as ultra-low temperature and flammability, the transportation and storage tanks thereof need to have high strength, ultra-low temperature toughness and low-temperature crack propagation resistance.

[0004] Currently, the steels used in ocean engineering are mostly low-alloy high-strength steels, mainly having a martensitic, bainitic or tempered martensitic structure, and a low austenite content, which leads to a high yield strength ratio and poor formability, and thus causes difficulties in forming and low service safety. Moreover, the thermal and mechanical stability of the austenite in the structure is poor, and martensitic phase transformation easily occurs under low-temperature isothermal or low stress-strain conditions, which converts the steel into brittle martensite, resulting in poor cryogenic toughness of the steel and failing to meet the ultra-low service temperature conditions. In addition, to obtain a high-strength martensitic / bainitic matrix, the traditional marine steel usually uses a high carbon content, which leads to a large amount of cementite precipitation, seriously deteriorating the low-temperature impact toughness and crack resistance, and reducing the weldability of the steel plate, which poses a great challenge to the manufacture of ship hulls and high-end marine equipment and their use safety under harsh conditions. Therefore, how to ensure high strength while retaining more stable austenite in the steel, reduce the yield strength ratio of the steel, obtain good formability, improve the low-temperature toughness, and have excellent service safety at ultra-low temperature is a current research hotspot.

[0005] Ni element has the effect of improving the stability of austenite, and the stable austenite can improve the impact toughness of the steel at low temperature through local phase transformation induced plasticity, crack tip blunting and matrix purification, etc. Therefore, the Ni-based low-temperature marine steel can obtain good comprehensive mechanical properties, and has a broad research prospect in the fields of deep submersibles, icebreakers and low-temperature storage tanks, etc. Among them, 9Ni steel is the most widely used due to its good low-temperature toughness at ultra-low temperature, and can be used as a manufacturing material for devices serving in-196℃ environment, such as the manufacture of LNG storage tanks. According to the EN 10028-4 standard, the performance requirements of traditional 9Ni steel are as follows: yield strength: 585 MPa; tensile strength: 680-820 MPa; elongation: 18%; -196℃ impact energy: 80 J.

[0006] In order to ensure high low-temperature toughness, a large amount of Ni alloying elements are added in traditional 9Ni steel to obtain a certain amount of metastable austenite. However, with the rapid development of science and technology, the demand for Ni alloy in China is increasing, and a large amount of import is needed, which greatly increases the cost of steel smelting. At the same time, too high Ni content will lead to too large viscosity of molten steel, which is prone to cause continuous casting billet drawing failure after smelting, and even cause the accident of molten steel leakage. At the same time, although a high content of Ni element has been added in 9Ni steel, there is still a problem of insufficient strength, the main reasons are as follows: on the one hand, in order to ensure low-temperature toughness, methods such as adding ultra-low carbon (≤0.05%) and removing impurity N are usually used to reduce the content of interstitial atoms (such as C, N, etc.) to prevent brittle fracture. On the other hand, 9Ni steel often ignores the addition of micro-alloying elements such as Nb, V and Ti. Related research on micro-alloyed steel shows that the addition of 0.1% micro-alloying elements to steel can obtain a strength increment of hundreds of megapascals through the precipitation of nanoscale micro-alloyed carbides and nitrides and the grain refinement caused by the pinning of grain boundaries. Therefore, due to the lack of C, N and micro-alloying elements in 9Ni steel, the effects of precipitation strengthening and fine-grain strengthening cannot be fully utilized, and there is usually a problem of insufficient strength.

[0007] In addition to the influence of alloy composition on the mechanical properties of Ni-based low-temperature marine steel, heat treatment process is also a necessary way to control the mechanical properties of low-temperature marine steel. The commonly used heat treatment process after hot rolling and air cooling of Ni-based low-temperature marine steel includes three kinds: normalizing + tempering process, double normalizing + tempering process, and high temperature quenching + tempering process. Among them, normalizing + tempering and double normalizing + tempering processes have low strength and low-temperature toughness of the steel plate, and high energy consumption, so they are rarely used in actual production. High temperature quenching + tempering is the method commonly used in industry. However, the metastable austenite content obtained by high temperature quenching + tempering process is less. Studies have shown that metastable austenite often occurs at grain boundaries, which can effectively prevent the generation and propagation of cracks and improve low-temperature toughness. Therefore, the lack of metastable austenite content will lead to poor crack propagation resistance of the steel, and the low-temperature toughness of the steel cannot be guaranteed. At the same time, impurity atoms may severely segregate at grain boundaries, leading to tempering martensite embrittlement and further reducing the impact toughness of the steel. In addition, due to the poor enrichment effect of austenite stabilizing elements during high temperature quenching + tempering process, the stability of austenite is insufficient, and unstable metastable austenite will be converted to brittle martensite under low temperature isothermal stage or low stress and strain conditions, the content of residual metastable austenite is further reduced, the effect of inhibiting crack propagation is weakened, and the low-temperature toughness is poor.

[0008] Therefore, it is urgent to develop a high-quality high-toughness low-temperature marine engineering steel, to optimize the chemical composition of the steel, and to develop a reasonable multi-stage heat treatment process, to develop a nickel-saving type low-temperature marine steel that can reach the performance standard of 9Ni steel, so as to improve the mechanical properties of the Ni-based low-temperature marine steel, realize the production and development of the nickel-saving type low-temperature marine steel with low cost, low yield ratio, high strength and excellent low-temperature toughness, and further promote the development of the ship and marine engineering industry in China. SUMMARY

[0009] The purpose of the present application is to provide a V-N micro-alloyed and multi-stage heat treated high-toughness low-temperature marine steel and its manufacturing method, which combines V-N micro-alloying with multi-stage heat treatment to obtain a low-temperature marine engineering steel with good comprehensive performance, and provides guidance for alloy design and process optimization of high-performance marine engineering steel.

[0010] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0011] A V-N micro-alloyed and multi-stage heat treated high-toughness low-temperature marine steel, the chemical composition of which is as follows: C 0.08%~0.15%, Ni 5.5%~7.5%, Mn 0.6%~1.5%, Mo 0.2%~0.84%, Si 0.2%~0.5%, V 0.05%~0.3%, N 0.005%~0.014%, Al 0.02%~0.15%, S 0.002%~0.01%, P 0.002%~0.01%, and the balance being Fe and inevitable impurities.

[0012] The microstructure of the low-temperature marine steel after heat treatment includes tempered martensite, metastable austenite, fresh martensite and V(C,N) precipitates.

[0013] The thickness of the low-temperature marine steel is 12mm~50mm, the yield strength is 764MPa~925MPa, the tensile strength is 840MPa~1040MPa, the yield ratio is 0.88~0.91, the elongation is 20.2%~25.6%, and the-196℃ impact energy is 107J~152J.

[0014] The manufacturing method of the V-N micro-alloyed and multi-stage heat treated low-temperature high-toughness marine steel simulates the steps of industrial production, including smelting, forging, hot rolling quenching, high temperature quenching, intercritical quenching and tempering, and the specific steps are as follows:

[0015] Step 1. Smelting and forging:

[0016] The alloy ingot with the above components is smelted in a vacuum induction furnace, the alloy ingot is heated to an austenitizing temperature range of 1140-1220°C in a heating furnace, and is kept for 1-3.5 hours, and then is forged into a billet at a temperature range of 980-1220°C, and the thickness of the billet is 60-150 mm.

[0017] Step 2. Hot rolling and quenching treatment:

[0018] The forged billet is heated to 1140-1220°C, and is kept for 1-3.5 hours, and then is subjected to homogenization treatment. Subsequently, the hot rolling treatment is performed, and the billet is rolled into a hot-rolled steel plate with a thickness of 12-50 mm through 5-7 rolling passes, and finally the hot-rolled steel plate is directly water quenched to room temperature.

[0019] Step 3. High-temperature quenching treatment:

[0020] The hot-rolled steel plate after water quenching is heated to 800-830°C, and is kept for 30-60 minutes after temperature stabilization, and then is water quenched to room temperature.

[0021] Step 4. Inter-critical quenching treatment:

[0022] The steel plate after high-temperature quenching treatment is heated to a two-phase region temperature of 650-690°C, and is kept for 40-70 minutes after temperature stabilization, and then is water quenched to room temperature.

[0023] Step 5. Tempering treatment:

[0024] The steel plate after inter-critical quenching treatment is heated to 520-580°C, and is kept for 60-90 minutes after temperature stabilization, and then is air cooled to room temperature, and thus a V-N micro-alloyed and multi-stage heat treatment regulated high-strength and high-toughness low-temperature marine steel is obtained.

[0025] The marine steel provided by the application is based on the traditional 9Ni low-temperature steel component process, reduces the addition of Ni element, adds V and N elements, and combines a multi-stage heat treatment process of hot rolling direct quenching-high temperature quenching-intercritical quenching-tempering, thereby saving resources, improving the strength and low-temperature toughness of the material, and achieving better matching of strength and toughness. On the one hand, V(C, N) precipitates are formed in the multi-stage heat treatment process, and the nanoscale V(C, N) precipitates can pin the grain boundary, thereby obtaining high strength with fine-grain strengthening effect. On the other hand, compared with the traditional heat treatment method, the cooling method after hot rolling is adjusted to direct quenching and an intercritical quenching process is added, which can better optimize the structure and performance of the steel. The direct water quenching method after hot rolling can greatly improve the cooling speed, increase the supercooling degree, improve the nucleation rate and increase the phase change driving force, thereby achieving the goal of refining the grains and helping to produce more stable austenite. The intercritical quenching is performed in the two-phase region of austenite formation, and this process can form a large amount of metastable austenite, and can make the austenite stabilizing elements such as Ni, Mn, C and N in the alloy composition rich in the metastable austenite, thereby improving the stability of the metastable austenite, making it still maintain a high content at ultra-low temperature to hinder the expansion of cracks, and making the material have excellent low-temperature toughness. Under the condition of low-cost Ni saving, the application combines V-N micro-alloying with a multi-stage heat treatment process, avoids the problem of insufficient strength caused by weak precipitation strengthening and fine-grain strengthening ability in the original alloy system, and overcomes the problem of low metastable austenite content and poor stability in a single heat treatment process, so that the difficult problem of low-temperature toughness is solved. This innovative method helps to reduce production costs and improve the performance and competitiveness of products.

[0026] The application adopts low-carbon medium-nickel steel and adds a small amount of micro-alloy elements. C is an austenite stabilizing element, which can improve the strength of the steel, but when the content of C exceeds 0.2%, the low-temperature toughness of the steel will be significantly reduced and the welding performance will be deteriorated, therefore the mass percentage of C is 0.08% to 0.15%. Ni is a key austenite stabilizing element, which can significantly reduce the martensite transformation temperature, so that the metastable austenite can maintain high stability in a low-temperature environment, which is very beneficial to the improvement of the low-temperature toughness of the steel. However, the price of Ni element is expensive, and under the premise of ensuring that the low-temperature mechanical properties of the steel meet the use requirements, the amount of Ni element in the steel should be considered to be saved, therefore compared with the traditional 9Ni low-temperature marine steel, the mass percentage of Ni in the application is 5.5% to 7.5%. Mn element is also an important austenite stabilizing element, which is beneficial to the formation of reversed austenite in the steel, and can also reduce the martensite transformation temperature of the steel, which is very beneficial to the improvement of the toughness of the steel. Mn element is also a matrix strengthening element, which has a certain solid solution strengthening effect on the matrix and can improve the hardenability of the steel. If the content of Mn in the steel is too low, the strength of the steel will be reduced and cannot meet the use requirements, and if the content of Mn is too high, the center segregation of the steel billet will be serious and large-size MnS inclusions will be generated, which will damage the toughness of the steel, therefore the mass percentage of Mn in the application is 0.6% to 1.5%. Mo can stabilize austenite, avoid martensite temper brittleness and improve low-temperature impact performance, but Mo is relatively expensive, therefore the mass percentage of Mo in the application is 0.2% to 0.84%. Si can be dissolved in austenite to produce solid solution strengthening effect, improve the hardness and strength of the experimental steel, but if the content of Si exceeds a certain range, the plasticity and toughness of the steel will be significantly reduced, therefore the mass percentage of Si in the application is 0.2% to 0.5%. The V mineral reserves in China are abundant, and the use of V solves the problem that other micro-alloy elements such as Nb are long-term imported at high prices due to foreign monopoly, however, too much V will deteriorate the material processing performance and cause waste of resources. Therefore the mass percentage of V in the application is 0.05% to 0.3%. N is an excellent austenite stabilizing element, and its ability to stabilize austenite is about 30 times that of Ni. In addition, N can combine with V to form V-N micro-alloying, promote the precipitation of V(C, N) and improve the strength, but too much N will exist in the steel as an interstitial atom, which will significantly increase the brittleness of the steel and damage the welding capacity and cold bending performance of the steel. Therefore the mass percentage of N in the application is 0.005% to 0.014%. Al is mainly used for deoxidization and grain refinement to improve the toughness of the steel at low temperature, but excessive Al will make Al combine with N to form AlN, which will cause the steel to be hot brittle and prone to cracks. Therefore the mass percentage of Al in the application is 0.02% to 0.15%. S and P are impurity elements in the experimental steel, which should be controlled within a certain range.

[0027] The application has the advantages that:

[0028] (1) Optimize the alloy composition and introduce the concept of micro-alloying. The present application proposes to introduce the concept of V-N micro-alloying on the basis of ensuring a certain C content. On the one hand, V and N elements have a strong affinity, which can control N not to exist in the form of interstitial atoms, and realize the precipitation of V(C, N) phase. This strengthens the precipitation strengthening effect while pinning the original austenite and lath martensite grain boundaries, plays a role in refining the grain and hindering the movement of dislocations, and can effectively improve the strength of the material. And the fine grains also effectively hinder the crack propagation, which is also helpful to improve the toughness. On the other hand, a small amount of N will exist in the metastable austenite, which can improve the stability of the metastable austenite and contribute to the improvement of low-temperature toughness.

[0029] (2) Control the structure and performance through multi-stage heat treatment. On the basis of the traditional heat treatment process, direct water quenching instead of air cooling after hot rolling greatly improves the cooling speed, the nucleation rate is improved, the grain is refined, which is beneficial to obtain fine lath martensite structure. Combined with the subsequent high temperature quenching (Q) process, a cyclic quenching is formed, which further strengthens the fine grain strengthening effect and contributes to the improvement of strength. And these fine grains can also effectively prevent crack propagation, better improve the low-temperature toughness of the steel. In addition, the intercritical quenching (L) process is introduced, some martensite in the matrix which is rich in C, Mn, Ni, N and other austenite stabilizing elements will undergo reverse transformation to form a high content and uniform distribution of metastable austenite, which purifies the matrix and improves the stability of the metastable austenite due to the enrichment of alloying elements. Finally, excellent impact toughness at -196℃ can be achieved, which provides performance support for practical application.

[0030] (3) Multiple strengthening methods work together to improve strength. Compared with the traditional single optimization method, the combination of composition optimization and heat treatment process optimization used in the present application can make the steel strength far exceed the strength standard of 9Ni steel through multiple strengthening methods such as solid solution strengthening of metal elements, martensite lath strengthening, precipitation strengthening, fine grain strengthening and work hardening, which plays a role in structure weight reduction, is beneficial to reduce energy consumption and environmental pollution, and is of great significance to the upgrading of steel enterprises' marine products, energy saving and consumption reduction.

[0031] (4) saving cost. The V mineral reserves are rich in China, so the use of V solves the problem of long-term reliance on high-priced import due to foreign monopoly of other metals, and reduces the cost. N is a cheap element, widely exists in nature, and has a cost much lower than that of Ni and a capability of stabilizing austenite much higher than that of Ni. Moreover, N and V have a high affinity, can promote V(C, N) precipitation, and play a more excellent precipitation strengthening and fine-grain strengthening effect. Grain refinement is helpful to the stabilization of austenite. In recent years, researches have shown that the cost of Ni-based low-temperature steel can be reduced by about 5% for each 1% reduction of Ni. Therefore, the N element is added to the steel in the present application, which has the effect of replacing Ni with N, can reduce the use of expensive Ni alloy, and greatly reduces the smelting cost of the steel. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is a schematic diagram of the heat treatment process of the present application.

[0033] Figure 2 It is an SEM diagram of the microstructure of Example 1.

[0034] Figure 3 It is an XRD diagram of Example 2.

[0035] Figure 4 It is a raw austenite grain distribution diagram of Example 3.

[0036] Figure 5 It is a load / impact work-displacement curve of Example 4.

[0037] Figure 6 It is an impact fracture morphology of Example 5.

[0038] Figure 7 It is an SEM diagram of the microstructure of Example 6.

[0039] Figure 8 It is a raw austenite grain distribution diagram of the comparative example. DETAILED DESCRIPTION

[0040] The heating furnace used in the heat treatment process of the present application is an atmosphere furnace produced by Shiyang Precision Equipment (Shanghai) Co., Ltd.

[0041] The Japanese field emission scanning electron microscope JSM-7800F is used to observe the microstructure in the examples of the present application.

[0042] The Japanese Rigaku Smart Lab 9kW is used for XRD testing in the examples of the present application.

[0043] The Japanese Shimadzu tensile testing machine is used for tensile property testing in the examples of the present application.

[0044] The impact performance test in the embodiment of the present application adopts a PSW 750 Zwick pendulum impact testing machine.

[0045] The heat treatment process in the embodiment of the present application is as shown in Figure 1

[0046] The present application will be further described below in combination with the drawings and specific embodiments.

[0047] Embodiment 1

[0048] Step 1. Smelting and forging: smelting an alloy ingot in a vacuum induction furnace, the composition of the alloy ingot is as follows in terms of mass percentage: C: 0.1%, Ni: 6.0%, Mn: 0.9%, Mo: 0.4%, Si: 0.25%, V: 0.2%, N: 0.007%, Al: 0.15%, S: 0.005%, P: 0.005%, and the rest is Fe and inevitable impurities. The alloy ingot is heated to an austenitizing temperature in a heating furnace, the heating temperature is controlled at 1220℃, and is kept for 3h, and then is forged into a billet with a thickness of 130mm at a temperature range of 1100℃-1220℃.

[0049] Step 2. Hot rolling and quenching treatment: the forged billet is heated to 1220℃ and kept for 3h for homogenization treatment. Subsequently, hot rolling treatment is carried out, and the billet is rolled into a hot-rolled steel plate with a thickness of 40mm through a 5-pass rolling process, and finally the hot-rolled steel plate is directly water quenched to room temperature.

[0050] Step 3. High-temperature quenching treatment: the hot-rolled steel plate after water quenching is heated to 810℃, and after temperature stabilization, is kept for 50min, and then is water quenched to room temperature.

[0051] Step 4. Subcritical quenching treatment: the steel plate after high-temperature quenching treatment is heated to a two-phase region temperature of 650℃, and after temperature stabilization, is kept for 60min, and then is water quenched to room temperature.

[0052] Step 5. Tempering treatment: the steel plate after subcritical quenching treatment is heated to 580℃, and after temperature stabilization, is kept for 90min, and then is air cooled to room temperature, thereby obtaining a V-N micro-alloyed and multi-stage heat treatment regulated high-strength and high-toughness low-temperature marine steel.

[0053] The marine steel sample blank prepared is ground into a mechanical property sample, performance test is carried out, and morphology analysis test is carried out, and the SEM graph of the final state structure is as shown in Figure 2 ​As shown in the figure, the microstructure is tempered martensite + metastable austenite + fresh martensite + V(C, N) precipitates. The metastable austenite is dispersedly distributed on the grain boundary, in the grain and the martensite lath boundary. The V(C, N) precipitates are spherical and uniformly dispersed in the matrix. The mechanical properties of the prepared offshore steel are detected, and the yield strength is 838 MPa, the tensile strength is 930 MPa, the yield strength ratio is 0.90, the elongation is 21.5%, and the impact energy at-196 ℃ is 115 J.

[0054] Example 2

[0055] Step 1. Smelting and forging: smelting an alloy ingot in a vacuum induction furnace, the composition of the alloy ingot is as follows in mass percentage: C: 0.08%, Ni: 7.1%, Mn: 0.6%, Mo: 0.2%, Si: 0.5%, V: 0.05%, N: 0.005%, Al: 0.12%, S: 0.002%, P: 0.004%, and the rest is Fe and unavoidable impurities. The alloy ingot is heated to the austenitizing temperature in a heating furnace, the heating temperature is controlled at 1220℃, and the temperature is kept for 3.5h, and then forged into a billet with a thickness of 150mm at a temperature of 1120℃-1220℃.

[0056] Step 2. Hot rolling and quenching treatment: the forged billet is heated to 1220℃ and kept for 3.5h for homogenization treatment. Then, hot rolling treatment is carried out, and the billet is rolled into a hot-rolled steel plate with a thickness of 50mm through 5 rolling passes, and finally the hot-rolled steel plate is directly water quenched to room temperature.

[0057] Step 3. High temperature quenching treatment: the hot-rolled steel plate after water quenching is heated to 830℃, and after the temperature is stabilized, it is kept for 60min, and then water quenched to room temperature.

[0058] Step 4. Intercritical quenching treatment: the steel plate after high temperature quenching treatment is heated to the two-phase region temperature of 690℃, and after the temperature is stabilized, it is kept for 70min, and then water quenched to room temperature.

[0059] Step 5. Tempering treatment: the steel plate after intercritical quenching treatment is heated to 540℃, and after the temperature is stabilized, it is kept for 90min, and then air cooled to room temperature, to obtain a V-N microalloyed and multi-stage heat treatment regulated high strength and toughness low temperature offshore steel.

[0060] The offshore steel prepared in this example is sampled for XRD test to calculate the content of metastable austenite, and the distribution of each substance peak is as shown in the figure. Figure 3The content of metastable austenite in the offshore steel prepared in this example was calculated to be 14.2% according to the integral intensity of the austenite and martensite peaks. The offshore steel sample blank was ground into a mechanical property sample and tested, and the mechanical properties were measured to be: yield strength of 764 MPa, tensile strength of 840 MPa, yield strength ratio of 0.91, elongation of 20.2%, and impact energy at -196 ℃ of 107 J.

[0061] Example 3

[0062] Step 1. Smelting and forging: An alloy ingot was prepared by smelting in a vacuum induction furnace, and the composition of the alloy ingot was as follows in mass percent: C: 0.15%, Ni: 6.2%, Mn: 1.2%, Mo: 0.8%, Si: 0.2%, V: 0.3%, N: 0.014%, Al: 0.02%, S: 0.01%, P: 0.005%, and the rest was Fe and unavoidable impurities. The alloy ingot was heated to an austenitizing temperature in a heating furnace, and the heating temperature was controlled at 1140 ℃, and then the alloy ingot was forged into a billet with a thickness of 60 mm at a temperature of 980 ℃ to 1140 ℃.

[0063] Step 2. Hot rolling and quenching treatment: the forged billet was heated to 1140 ℃ and held for 1 h for homogenization treatment. Then, the hot rolling treatment was carried out, and the billet was rolled into a hot-rolled steel plate with a thickness of 12 mm through 7 rolling passes. Finally, the hot-rolled steel plate was directly water quenched to room temperature.

[0064] Step 3. High-temperature quenching treatment: the hot-rolled steel plate after water quenching was heated to 800 ℃, and after temperature stabilization, it was held for 30 min, and then water quenched to room temperature.

[0065] Step 4. Intercritical quenching treatment: the steel plate after high-temperature quenching treatment was heated to a two-phase region temperature of 660 ℃, and after temperature stabilization, it was held for 40 min, and then water quenched to room temperature.

[0066] Step 5. Tempering treatment: the steel plate after intercritical quenching treatment was heated to 550 ℃, and after temperature stabilization, it was held for 60 min, and then air cooled to room temperature, thereby obtaining a V-N microalloyed and multi-stage heat treatment regulated high-strength and high-toughness low-temperature offshore steel.

[0067] The offshore steel prepared in this example was subjected to morphology testing, and the original austenite grain distribution was as shown in Figure 4The original austenite grain size was counted, and the grain size was fine, with an average size of 15.41 μm, and the grain size distribution was uniform. Such fine and uniform grains help to improve the mechanical properties of the material. The mechanical properties of the marine steel sample blank prepared by grinding into a mechanical property sample were tested, and the mechanical properties were: yield strength 925 MPa, tensile strength 1040 MPa, yield strength ratio 0.89, elongation 22.4%, and -196 ℃ impact energy 132 J.

[0068] Example 4:

[0069] Step 1. Smelting and forging: smelting an alloy ingot in a vacuum induction furnace, the composition of the alloy ingot is as follows in mass percent: C: 0.12%, Ni: 5.5%, Mn: 1.0%, Mo: 0.62%, Si: 0.32%, V: 0.2%, N: 0.012%, Al: 0.04%, S: 0.006%, P: 0.01%, and the rest is Fe and unavoidable impurities. The alloy ingot is heated to the austenitizing temperature in a heating furnace, the heating temperature is controlled at 1200 ℃, and the temperature is kept for 2.5 h, and then forged into a billet with a thickness of 120 mm at a temperature of 1060 ℃ to 1200 ℃.

[0070] Step 2. Hot rolling and quenching treatment: the forged billet is heated to 1180 ℃ and kept for 3 h for homogenization treatment, and then subjected to hot rolling treatment, and after 6 passes of rolling process, the billet is rolled into a hot-rolled steel plate with a thickness of 30 mm, and finally the hot-rolled steel plate is directly water quenched to room temperature.

[0071] Step 3. High temperature quenching treatment: the hot-rolled steel plate after water quenching is heated to 810 ℃, and after temperature stabilization, it is kept for 45 min, and then water quenched to room temperature.

[0072] Step 4. Subcritical quenching treatment: the steel plate after high temperature quenching treatment is heated to the two-phase region temperature of 660 ℃, and after temperature stabilization, it is kept for 60 min, and then water quenched to room temperature.

[0073] Step 5. Tempering treatment: the steel plate after subcritical quenching treatment is heated to 560 ℃, and after temperature stabilization, it is kept for 80 min, and then air cooled to room temperature, to obtain a V-N microalloyed and multi-stage heat treatment regulated high strength and toughness low temperature marine steel.

[0074] The mechanical properties of the marine steel sample blank prepared in this example were tested after grinding into a mechanical property sample, and the mechanical properties were: yield strength 865 MPa, tensile strength 960 MPa, yield strength ratio 0.90, elongation 21.8%, and -196 ℃ impact energy 119 J. The load / impact energy-displacement curve obtained during the -196 ℃ low temperature impact process is as follows: Figure 5As shown, according to the load / impact work-displacement curve, the energy corresponding to the crack formation from the beginning of the impact to the maximum impact load is 41 J, and the energy corresponding to the crack propagation process from the maximum load to the fracture is 78 J, and the sum of the two is the total impact work 119 J.

[0075] Example 5

[0076] Step 1. Smelting and forging: smelting an alloy ingot in a vacuum induction furnace, the composition of the alloy ingot is as follows in mass percent: C: 0.1%, Ni: 7.5%, Mn: 0.9%, Mo: 0.84%, Si: 0.43%, V: 0.12%, N: 0.009%, Al: 0.09%, S: 0.008%, P: 0.007%, and the rest is Fe and unavoidable impurities. The alloy ingot is heated to the austenitizing temperature in a heating furnace, the heating temperature is controlled at 1150℃, and the temperature is kept for 2h, and then forged into a billet with a thickness of 90mm at a temperature range of 1000℃-1150℃.

[0077] Step 2. Hot rolling and quenching treatment: the forged billet is heated to 1150℃ and kept for 2h for homogenization treatment. Then, hot rolling treatment is carried out, and the billet is rolled into a hot-rolled steel plate with a thickness of 17mm through 7 rolling passes, and finally the hot-rolled steel plate is directly water quenched to room temperature.

[0078] Step 3. High temperature quenching treatment: the hot-rolled steel plate after water quenching is heated to 810℃, and after the temperature is stabilized, it is kept for 35min, and then water quenched to room temperature.

[0079] Step 4. Intercritical quenching treatment: the steel plate after high temperature quenching treatment is heated to the two-phase region temperature of 690℃, and after the temperature is stabilized, it is kept for 45min, and then water quenched to room temperature.

[0080] Step 5. Tempering treatment: the steel plate after intercritical quenching treatment is heated to 570℃, and after the temperature is stabilized, it is kept for 65min, and then air cooled to room temperature, thereby obtaining a V-N microalloyed and multi-stage heat treatment regulated high strength and toughness low temperature marine steel.

[0081] The marine steel sample blank prepared in this example is ground into a mechanical property sample and tested, and the mechanical properties are measured as follows: yield strength is 841MPa, tensile strength is 955MPa, yield strength ratio is 0.88, elongation is 25.6%, and impact energy at-196℃ is 152J. After observing the microfracture morphology of the sample after impact at-196℃ (as shown in Figure 6 the figure), it is found that the fracture of the impact sample produces significant plastic deformation, which is composed of large dimples and surrounding small equiaxed dimples, showing ductile fracture characteristics, indicating good deep cryogenic impact toughness.

[0082] Example 6:

[0083] Step 1. Smelting and Forging: Alloy ingots are smelted in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.13%, Ni: 6.7%, Mn: 1.5%, Mo: 0.7%, Si: 0.35%, V: 0.3%, N: 0.01%, Al: 0.06%, S: 0.004%, P: 0.002%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 100 mm within a temperature range of 1050℃ to 1200℃.

[0084] Step 2. Hot rolling and quenching treatment: The forged steel billet is heated to 1180℃ and held for 2 hours for homogenization. Then, it is hot rolled through 6 rolling passes to form a hot-rolled steel plate with a thickness of 25mm. Finally, the hot-rolled steel plate is directly water quenched to room temperature.

[0085] Step 3. High-temperature quenching treatment: Heat the hot-rolled steel plate after water quenching to 800℃, hold it at that temperature for 40 minutes after the temperature stabilizes, and then water quench it to room temperature.

[0086] Step 4. Sub-temperature quenching treatment: Heat the steel plate after high-temperature quenching to the two-phase region temperature of 670℃, hold it at the temperature for 50 minutes after the temperature stabilizes, and then quench it in water to room temperature.

[0087] Step 5. Tempering treatment: Heat the steel plate after sub-temperature quenching to 520℃, hold it at that temperature for 70 minutes after the temperature stabilizes, and then air cool it to room temperature to obtain high strength and toughness low temperature marine steel with VN microalloying and multi-stage heat treatment control.

[0088] Morphological analysis was performed on the marine steel microstructure obtained in this embodiment, and the final microstructure SEM image is shown below. Figure 7 As shown, the microstructure is the same as in other embodiments: the matrix is ​​tempered martensite, and the bright lining region on the matrix mainly consists of metastable austenite and fresh martensite generated during the cooling process, while V(C,N) precipitates are also formed. The metastable austenite is in the form of fine, discontinuous dots, dispersed at grain boundaries, within grains, and on martensite lath boundaries; while the fresh martensite is more scattered and coarse. The V(C,N) precipitates are spherical, small in size, and uniformly dispersed in the matrix. The marine steel sample blank prepared in this embodiment was ground into a mechanical property specimen, and its mechanical properties were tested. The mechanical properties are: yield strength of 890 MPa, tensile strength of 1000 MPa, yield-to-tensile ratio of 0.89, elongation of 23.5%, and impact energy at -196℃ of 137 J.

[0089] Comparative example:

[0090] Step 1. Smelting and forging: An alloy ingot was prepared by smelting in a vacuum induction furnace, and the composition of the alloy ingot was as follows in mass percent: C: 0.08%, Ni: 7.2%, Mn: 0.72%, Mo: 0.35%, Si: 0.23%, Al: 0.07%, S: 0.003%, P: 0.003%, and the balance being Fe and unavoidable impurities. The alloy ingot was heated to an austenitizing temperature in a heating furnace, the heating temperature was controlled at 1220℃, and the alloy ingot was kept for 3.5h, and then the alloy ingot was forged into a billet with a thickness of 150mm at a temperature in the range of 1120℃ to 1220℃.

[0091] Step 2. Hot rolling and quenching treatment: the forged billet was heated to 1220℃ and kept for 3.5h for homogenization treatment. Subsequently, the hot rolling treatment was performed, and the billet was rolled into a hot-rolled steel plate with a thickness of 50mm through a 5-pass rolling process, and finally the hot-rolled steel plate was directly water quenched to room temperature.

[0092] Step 3. High-temperature quenching treatment: the hot-rolled steel plate after water quenching was heated to 820℃, and after the temperature was stabilized, the steel plate was kept for 60min, and then water quenched to room temperature.

[0093] Step 4. Inter-critical quenching treatment: the steel plate after high-temperature quenching treatment was heated to a two-phase region temperature of 680℃, and after the temperature was stabilized, the steel plate was kept for 70min, and then water quenched to room temperature.

[0094] Step 5. Tempering treatment: the steel plate after inter-critical quenching treatment was heated to 580℃, and after the temperature was stabilized, the steel plate was kept for 90min, and then air cooled to room temperature, thereby obtaining the offshore steel product.

[0095] After the comparative sample was corroded with picric acid, the original austenite grain distribution diagram obtained is shown in FIG. 2. Figure 8 The original austenite grain size was found to be relatively coarse, with an average size of 22.14μm. The grain size distribution was uneven, and the difference between the largest grain size and the smallest grain size was about 13μm, which was relatively large. Meanwhile, the offshore steel sample blank prepared in the present comparative example was ground into a mechanical property sample, and the corresponding performance test was performed, and the mechanical properties were measured as follows: the yield strength was 748MPa, the tensile strength was 797MPa, the yield strength ratio was 0.94, the elongation was 16.4%, and the impact energy at -196℃ was 75J.

[0096] In summary, by comparing the examples with the comparative examples, it is found that the concept of introducing V-N micro-alloying in the steel and combining with multi-stage heat treatment can make more nanoscale V(C, N) precipitates precipitate in the low-temperature marine steel, the structure becomes finer, the pinning effect of dislocation is more obvious, and the strength is significantly improved. And a certain amount of stable metastable austenite is obtained during heat treatment, which still has better low temperature impact toughness at-196℃ compared with the comparative examples, which is beneficial to form excellent strength and toughness matching, and makes a certain contribution to the development of low-temperature marine steel.

Claims

1. A high-strength, high-toughness, low-temperature marine steel with VN microalloying and multi-stage heat treatment control, characterized in that, The chemical composition of the marine steel, by mass percentage, is as follows: C 0.12%–0.15%, Ni 5.5%–6.7%, Mn 1.0%–1.5%, Mo 0.62%–0.8%, Si 0.2%–0.35%, V 0.2%–0.3%, N 0.01%–0.014%, Al 0.02%–0.06%, S 0.004%–0.01%, P 0.002%–0.01%, with the balance being Fe and unavoidable impurities. The microstructure of the low-temperature marine steel after heat treatment includes: tempered martensite, metastable austenite, fresh martensite and V(C,N) precipitates; The low-temperature marine steel has a thickness of 12mm to 30mm, a yield strength of 865MPa to 925MPa, a tensile strength of 960MPa to 1040MPa, a yield-to-tensile ratio of 0.89 to 0.90, an elongation of 21.8% to 23.5%, and an impact energy of 119J to 137J at -196℃. The method for preparing high-strength, high-toughness, low-temperature marine steel using VN microalloying and multi-stage heat treatment control simulates the steps of industrial production: smelting, forging, hot rolling and quenching, high-temperature quenching, sub-temperature quenching, and tempering. The specific steps are as follows: Step 1. Smelting and forging: The alloy ingot of the marine steel composition is smelted in a vacuum induction furnace, and after heating and holding the alloy ingot at a temperature range of 980℃~1220℃, it is forged into a steel billet. Step 2. Hot rolling and quenching treatment: The forged steel billet is heated and held at a certain temperature, then homogenized and hot-rolled to obtain a hot-rolled steel plate. The hot-rolled steel plate is then directly water-quenched to room temperature. The hot rolling process consists of 5 to 7 passes. Step 3. High-temperature quenching treatment: The hot-rolled steel plate after water quenching is heated to 800℃~830℃, held at the temperature after it stabilizes, and then water quenched to room temperature. Step 4. Sub-temperature quenching treatment: The steel plate after high-temperature quenching is heated to the two-phase region temperature of 650℃~690℃, held at the temperature after it stabilizes, and then water-quenched to room temperature. Step 5. Tempering treatment: The steel plate after sub-temperature quenching is heated to 520℃~580℃, held at the temperature after it stabilizes, and then air-cooled to room temperature to obtain high strength and toughness low temperature marine steel with VN micro-alloying and multi-stage heat treatment control. In step 1, the alloy ingot is heated to 1140℃~1220℃ and held for 1h~3.5h. The thickness of the steel billet is 60mm to 150mm; In step 2, the heating temperature of the steel billet is 1140℃~1220℃, and the holding time is 1h~3.5h; The thickness of the hot-rolled steel plate is 12mm to 30mm.

2. The high-strength, high-toughness, low-temperature marine steel with VN microalloying and multi-stage heat treatment control according to claim 1, characterized in that, In step 3, the heat preservation time is 30 min to 60 min.

3. The high-strength, high-toughness, low-temperature marine steel with VN microalloying and multi-stage heat treatment control according to claim 1, characterized in that, In step 4, the heat preservation time is 40 min to 70 min.

4. The high-strength, high-toughness, low-temperature marine steel with VN microalloying and multi-stage heat treatment control according to claim 1, characterized in that, In step 5, the heat preservation time is 60 min to 90 min.

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