Medium-thickness high-toughness low-yield-ratio marine steel and preparation method thereof

Abstract

The invention relates to the technical field of high-strength steel preparation, and particularly discloses medium-thickness type high-toughness low-yield-ratio marine steel and a preparation method thereof.The marine steel plate comprises, by weight, 0.075%-0.115% of C, 0.85%-1.15% of Mn, 0.08%-0.24% of Si, 3.30%-3.60% of Ni, 0.35%-0.55% of Cr, 0.55%-0.75% of Mo, 0.95%-1.25% of Cu, 0.040%-0.070% of V, 0.010%-0.030% of Nb, 0.010%-0.030% of Ti and the balance Fe and inevitable impurities. The process comprises the steps of smelting, casting, rolling, cooling and heat treatment. A critical annealing process is introduced on the basis of a hardening and tempering process, so that the microstructure of the steel plate is adjusted into a composite structure of tempered martensite, critical ferrite, retained austenite, secondary martensite and nanoscale precipitates. The thickness of the prepared steel plate is 20 mm, the yield strength of the core part is larger than or equal to 800 MPa, the tensile strength is about 1000 MPa, the ductility is larger than or equal to 19%, the yield ratio is smaller than or equal to 0.83, the transverse impact energy at the temperature of minus 40 DEG C is larger than or equal to 120 J, and the steel plate is excellent in performance, high in plasticity and toughness, low in yield ratio and wide in application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength steel preparation, and specifically relates to a medium-thick marine engineering steel with high strength and toughness and low yield ratio and a preparation method thereof. Background Art

[0002] The sustained and rapid development of economic pillar industries such as the offshore oil and gas industry, modern marine fisheries, marine tourism industry and marine transportation industry has promoted the construction of ultra-large marine engineering equipment such as submarine oil and gas exploration equipment, offshore platforms and large ships, and also provided broad prospects for the application of marine engineering steel (referred to as "marine steel"). The complex service environment such as wind and waves, tides, ice collisions, earthquakes, etc. determines the particularity of marine steel. The product must not only meet the high strength requirements, but also have many properties such as low-temperature toughness, low yield strength ratio, fatigue resistance, layer tearing resistance, weldability, corrosion resistance and cold processing to ensure the safety and reliability of marine equipment during service. At the same time, the development direction of large-scale, polar and deep-sea marine equipment requires marine steel to have both thick wall and large plate width. In recent years, although the research and development and application of ultra-high strength marine steel have continued to increase, the increase in strength level has caused a decrease in impact toughness and an increase in yield strength ratio, which directly affects its development in high-end manufacturing. Impact toughness reflects its ability to resist external impact loads, while high yield strength ratio will result in a small allowable margin for deformation and fracture of structural parts, increasing the safety risk during service. Therefore, how to coordinate the strength, yield strength ratio and low-temperature toughness of marine engineering steel has become its main development direction.

[0003] At present, steel enterprises are constantly launching new products to meet the needs of marine engineering, with yield strength levels ranging from 460MPa to 960MPa. 460MPa-level high-strength and low-yield ratio marine engineering steel has been popularized in various industries. The yield ratio of 550MPa-level marine engineering steel is as high as 0.85, and it is gradually used in various structural application scenarios such as marine engineering, high-rise buildings, and bridges. The yield ratio of 800MPa-level medium and thick steel plates reaches 0.95, which exceeds the requirements of some classification societies (yield ratio ≤ 0.94), and it is difficult to meet the requirements of large steel structure seismic specifications. In addition, in order to further reduce the deadweight of engineering structures and improve the variable load and self-sustaining capacity of platforms, the demand for 960MPa-level high-strength marine engineering steel with superior comprehensive performance and high added value has increased significantly. At present, controlled rolling and controlled cooling (TMCP) and quenching and tempering (quenching + high temperature tempering) processes are often used to achieve the preparation of this level of marine engineering steel. The TMCP process refines the quenched martensite structure by controlling the deformation of austenite to improve the strength and toughness of the steel plate. However, due to the small amount of reduction of thick steel plates, the grain refinement is limited, resulting in uneven structure in the thickness direction of the plate, and the mechanical properties in the thickness direction are significantly different. In contrast, quenching and tempering treatment has certain advantages in improving the stability and uniformity of production performance, but high-temperature tempering will lead to an increase in the yield strength ratio (> 0.95), which cannot meet the engineering applications that require a low yield strength ratio. Therefore, EH690 is the highest grade of steel currently used in batches.

[0004] The patent "Low Yield Ratio Easy Welding High Strength Steel Plate and Its Preparation Process" with publication number CN102433507B adopts the TMCP process to produce a finished steel plate with a thickness of ≥15mm, a structure of ferrite + bainite, a yield strength of 460-560MPa, a tensile strength of 700-790MPa, an elongation after fracture of ≥14%, and a yield ratio of <0.7. The patent "A Low Yield Ratio Marine Steel Plate with Excellent Low-Temperature Toughness and Its Manufacturing Method" with publication number CN112877601B also adopts the TMCP process to develop a marine steel with a yield ratio of <0.8, but its strength level is only 420MPa. It can be seen that the TMCP process can meet the requirements of low yield strength, but the strength level is low, and there are deficiencies in weldability, cold and hot processing stability, etc.

[0005] The patent with publication number CN102851622B, "A Ultra-High Strength and High Toughness Steel Plate for Marine Engineering and Its Production Method", has developed a marine steel with a yield strength of 710-800MPa, a tensile strength of 770-840MPa, and a transverse impact energy of ≥90J at -40℃ through a quenching and tempering process, achieving a good balance between strength and low-temperature toughness, but due to the main single-phase structure, its yield ratio is greater than 0.90. The patent with publication number CN114892072A, "A High Strength and High Toughness Hydrogen Embrittlement Resistant Steel Plate and Its Composition Optimization and Preparation Method", has even produced GPa-grade marine steel through a quenching and tempering process, but its yield ratio is as high as 0.93-0.98, which makes it difficult to ensure that marine facilities absorb more energy during plastic deformation and ensure the safety of the facilities.

[0006] In order to achieve a good balance between strength, toughness and yield strength ratio of offshore steel, various steel enterprises and research institutions have conducted in-depth research on the processing technology of offshore steel. The patent "Heat Treatment Method of 890MPa Grade Ultra-High Strength and Low Yield Ratio Offshore Steel" with publication number CN114231714A adopts a heat treatment method of post-rolling online quenching plus sub-temperature quenching plus tempering water cooling to prepare 890MPa grade offshore steel, showing excellent mechanical properties of yield strength ≥ 890MPa, tensile strength 940~1100MPa, and average impact energy ≥ 120J at -40℃, but the yield strength ratio is also maintained at 0.90~0.93. The publication number CN114000056A "A medium-thick steel plate with a yield strength of 960MPa and a low yield ratio for marine engineering and its preparation method" is prepared through controlled rolling and controlled cooling process and (α+γ) two-phase annealing + complete austenitizing quenching + medium and low temperature tempering treatment process. It shows a yield strength of ≥960MPa, a tensile strength of 1100MPa and a -40℃ impact toughness of ≥69J, but the yield ratio also increases by 0.95.

[0007] In addition, the patent "A low yield ratio marine engineering steel and its preparation method" with publication number CN111057965A adds quenching and tempering treatment times on the basis of quenching and tempering treatment, and develops marine engineering steel with yield strength of ~732MPa, tensile strength of ~832MPa, and yield ratio of ~0.9, but there are many production processes, high energy consumption and cost. China Baoshan Iron and Steel Co., Ltd. has successively developed a series of 800MPa quenched and tempered steel plates with high toughness and excellent weldability (such as patents CN105603322B, CN106319388B, etc.) using offline quenching and tempering heat treatment technology (i.e. RQ+T). The mechanical properties of the steel plates (especially low temperature toughness, crack resistance and crack arrest characteristics) and weldability are excellent, and the physical quality of the products has reached the international leading level; however, the yield ratio of the steel plates is still as high as 0.92, and the manufacturing cost is relatively high.

[0008] In summary, current technologies still face challenges in solving the problem of mismatch in strength, toughness and yield strength ratio of medium and thick marine steels.

[0009] In response to this situation, the present invention is based on the "multi-phase, metastable, multi-scale" M3 microstructure control principle, and utilizes a critical annealing process to obtain a complex phase structure composed of ferrite, martensite, residual austenite and nanoscale precipitates. A medium-thick, high-strength, low-yield ratio marine steel with a yield strength ≥800MPa, a tensile strength of ~1000MPa, a yield ratio ≤0.83, and a -40°C lateral impact energy ≥120J has been developed to meet the safety requirements of special large-scale marine engineering structure design. Summary of the invention

[0010] The object of the present invention is to provide a medium-thick marine engineering steel with high strength and toughness and low yield ratio and a preparation method thereof, so as to solve the problems raised in the above-mentioned background technology.

[0011] To achieve the above object, the present invention provides the following technical solutions:

[0012] A medium-thick high-strength and low-yield ratio marine engineering steel and a preparation method thereof, comprising the following steps:

[0013] The components of the marine steel plate are calculated by weight percentage as follows: C: 0.075-0.115%, Mn: 0.85-1.15%, Si: 0.08-0.24%, Ni: 3.30-3.60%, Cr: 0.35-0.55%, Mo: 0.55-0.75%, Cu: 0.95-1.25%, V: 0.040-0.070%, Nb: 0.010-0.030%, Ti: 0.010-0.030%, and the rest are Fe and unavoidable impurities.

[0014] The preparation method of the steel plate adopts a quenching+critical annealing+tempering heat treatment process.

[0015] Quenching: heat the hot-rolled steel plate to 850-870°C, keep it warm for 26-30 minutes, and then water quench (t=1.3-1.5min / mm×h, h: plate thickness);

[0016] Critical annealing: heat the quenched steel plate to 690-710°C, keep it warm for 46-50 minutes, and then air cool it (t=2.3-2.5min / mm×h, h: plate thickness);

[0017] Tempering: Heat the critical annealed steel plate to 560-580°C, keep it at this temperature for 46-50 minutes, and then air cool it (t=2.3-2.5min / mm×h, h: plate thickness).

[0018] Preferably, the yield strength of the steel plate is ≥800MPa, the tensile strength is ≤1000MPa, the yield strength ratio is ≤0.83, and the transverse impact energy at -40°C is ≥120J. Its microstructure consists of tempered martensite, critical ferrite, retained austenite, secondary martensite and nano-scale precipitation phase. The finished steel plate has a thickness range of 20±2mm.

[0019] The composition design reasons of the steel plate of the present invention are as follows:

[0020] C: It is an essential element to ensure strength. It can increase the strength and hardness of the hard phase in the multiphase structure through solid solution strengthening, increase the difference in mechanical properties between the soft and hard phases, and help reduce the yield strength ratio. At the same time, it can form fine precipitations with V, Nb, and Ti to achieve precipitation strengthening. However, an increase in carbon content is not good for plasticity, toughness, and weldability. For this reason, the present invention sets the C content range to 0.075-0.115%.

[0021] Mn: It is an essential element to ensure the strength and toughness of steel. When the content is above 0.8%, it can improve the hardenability of steel and play a role in solid solution strengthening and fine grain strengthening, but it is easy to lead to an increase in the yield strength ratio. Mn can also increase the content of metastable austenite and form a complex phase structure, but it is not conducive to the formation of soft phase ferrite. When the Mn content exceeds 1.6%, it will aggravate the segregation of the continuous casting billet and deteriorate the plasticity, low temperature toughness and welding performance of the steel. For this reason, the present invention sets the Mn content range to 0.85-1.15%.

[0022] Si: It is an important reducing agent and deoxidizer. It can be dissolved in ferrite and austenite to improve the hardness and strength of steel, which is beneficial to the regulation of yield strength ratio. At the same time, the addition of Si inhibits the precipitation and coarsening of cementite and improves toughness. However, when the Si content exceeds 0.30%, it is easy to produce red iron scale and deteriorate the toughness of martensitic high-strength steel. For this reason, the present invention sets the Si content range to 0.08-0.24%.

[0023] Ni: It has the function of improving the strength, elongation and low-temperature toughness of the steel plate at the same time; as the main alloying element for stabilizing austenite, it can delay the pearlite transformation, which is beneficial to microstructure control and grain refinement, and improves low-temperature toughness; at the same time, its addition will affect the lateral slip of dislocations and reduce the toughness-brittle transition temperature of steel, but too high Ni content will affect weldability and greatly improve the yield strength ratio of the steel plate; at the same time, Ni is a precious metal, and excessive addition is not conducive to price control. For this reason, the Ni content of the present invention is controlled between 3.30% and 3.60%.

[0024] Cr: 0.2% or more Cr can increase the hardenability of steel, strengthen the hard phase structure, and is conducive to the control of strain and yield strength ratio; in addition, it has the effect of secondary hardening, which can improve the hardness and wear resistance of steel. However, when its content exceeds 0.70%, large sparks will appear during welding, affecting the welding quality. The present invention selects the Cr content range of 0.35-0.55%.

[0025] Mo: can improve hardenability and enhance the strength of steel plates; can effectively improve the stability of toughness in the thickness direction of steel plates; can also improve tempering stability, so that steel parts can be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and improving plasticity. However, while Mo promotes the formation of martensite / bainite, it increases the size of martensite / bainite crystals and the orientation difference between the formed martensite / bainite laths is very small. Therefore, while Mo greatly improves the strength of quenched and tempered steel plates and reduces the yield ratio of steel plates, it reduces the low-temperature toughness and elongation of online quenched and tempered steel plates. In addition, Mo is a precious metal and will increase costs. The present invention selects a Mo content range of 0.55 to 0.75%.

[0026] Cu: It is also an austenite stabilizing element, which can reduce the phase transition temperature, improve the hardenability and corrosiveness of the steel plate, refine the microstructure, and improve the low-temperature toughness of the steel plate; when the content reaches a certain value, it can form a nano-strengthening phase through the tempering heat treatment process, so that the steel still has ultra-high strength at a low carbon content, improves the yield strength ratio, but has no adverse effect on welding performance. When the content is higher than 0.45%, it is easy to cause copper brittleness during hot deformation processing, so it is often necessary to add it together with the Ni element. Considering the strength level, the present invention selects the Cu content range of 0.95-1.25%.

[0027] V, Nb, Ti: As strong carbide-forming elements, they can combine with carbon to form fine precipitation to achieve precipitation strengthening. Compared with Nb and Ti, VC has a relatively low precipitation temperature, which is easy to promote non-uniform nucleation and refine phase transformation structure during rolling; precipitation during welding can increase the hardness of the welded joint and reduce the softening rate. However, too high V content will affect toughness. For this reason, the present invention partially replaces V with Nb and Ti to play a corresponding role, and controls the V content to 0.04-0.07%. Nb and Ti both have strong solid solution and fine grain effects, which can improve the strength of steel; however, Nb elements need to increase the heating temperature of the continuous casting billet to ensure the solid solution effect, while Ti elements are easy to combine with N to form square TiN, which affects the plastic toughness of steel. In addition, too high Nb and Ti contents will inhibit the formation of V-containing precipitation phases. For this reason, the contents of Nb and Ti in the present invention are controlled at 0.01-0.03%, and the total amount of microalloys is controlled at 0.06-0.13% to ensure comprehensive performance.

[0028] A method for preparing medium-thick high-strength and low-yield ratio marine engineering steel comprises the following steps:

[0029] Step 1: Smelt in a vacuum induction furnace according to the designed chemical composition, fill with argon gas for protection during the smelting process, and then cast. The ingot weighs 80Kg and is 100-120mm thick;

[0030] Step 2: The ingot is heated to 1150-1200℃, kept at this temperature for 2 hours, and then hot rolled. The initial rolling temperature is controlled at 1050-1150℃, and the final rolling temperature is controlled at 850-1000℃. After hot rolling to 20±2mm, it is air-cooled to room temperature. During the rolling process, the average single-pass reduction rate is above 15%, in order to fully recrystallize the austenite and achieve the purpose of refining the austenite grains.

[0031] Step 3: The steel plate is treated by a three-step heat treatment process. The hot-rolled plate described in step 2 is heated to 850-870°C and kept at this temperature for 26-30 minutes, and then water quenched to obtain a uniform and fine martensitic structure, followed by critical annealing at 690-710°C for 46-50 minutes and air cooling, and finally tempering at 560-580°C for 46-50 minutes and air cooling to obtain a medium-thick marine engineering steel with high strength and toughness and low yield ratio.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention adopts low carbon, Si-Cu-Mn-Ni-Cr-Mo composite alloying, and Nb+V+Ti composite microalloying combined design, which can give full play to the beneficial effects of inhibiting recrystallization, solid solution strengthening, precipitation strengthening, grain refinement, and improving the strain strengthening ability of steel plates, laying the foundation for obtaining the final organizational structure. The production processes such as smelting, heating, rolling and cooling that match the alloy composition solve the problem of matching the comprehensive technical characteristics of medium and thick marine steel plates such as high strength, high toughness, low yield ratio, excellent weldability and thick specifications. The designed medium and thick high-toughness low yield ratio marine steel has a thickness of 20mm, has good strength-plasticity-toughness matching and low yield ratio, that is, yield strength ≥800MPa, tensile strength ~1000MPa, yield ratio YR≤0.83, -40℃ transverse impact energy ≥120J, meets the safety requirements of special large-scale marine engineering structure design, can be used in harsh use environments and stress conditions, and is of great significance for engineering construction.

[0034] 2. The present invention adopts the heat treatment process of quenching + critical annealing + tempering to obtain a mixed structure mainly composed of critical ferrite (soft phase), tempered martensite (hard phase), residual austenite and secondary martensite, and is accompanied by a large amount of carbides and Cu-rich phase precipitation. Uniform and fine martensitic structure can be obtained by controlling the quenching process parameters. As a hard phase, the formation of martensitic structure can significantly improve the tensile strength of steel, while grain refinement and grain boundary strengthening and solid solution strengthening produced by supersaturated solid solution of carbon atoms are conducive to improving yield strength. At the same time, grain refinement increases the density of grain boundaries in steel, provides nucleation sites for the formation of reverse austenite in the subsequent critical annealing process, and promotes the generation of reverse austenite. In the critical annealing process, with the diffusion of austenite stabilizing elements such as C, Ni, Mn, Cu from martensite to reverse austenite, the matrix gradually changes to a mixed structure of tempered martensite and critical ferrite. Wherein, C, as an interstitial atom, will rapidly gather in reverse austenite due to its high diffusion rate. However, as the critical annealing time goes by, its concentration in the reverse transformed austenite gradually decreases, reducing the thermal stability of the reverse transformed austenite. Some low-stability reverse transformed austenite transforms into secondary martensite during the subsequent cooling process. The formation of secondary martensite will increase the tensile strength of the steel. In the subsequent tempering process, a large number of second phases begin to precipitate and are dispersed. It is worth noting that the diffusion of Cu elements during the critical annealing process weakens the precipitation ability of the Cu-rich phase on the matrix, but promotes its precipitation in the secondary martensite, compensating for the yield strength loss caused by the critical annealing. It can be seen that by controlling the quenching, critical annealing and tempering process parameters, the ideal control of the proportion of each phase, grain size and precipitation phase can be achieved, thereby optimizing the mechanical properties of the steel, that is, tempered martensite and secondary martensite as hard phases ensure the tensile strength of the steel, and grain refinement and dispersed precipitation of Cu-rich phases enhance the yield strength. The synergistic deformation of critical ferrite and retained austenite improves the low-temperature toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The microstructure diagrams observed by scanning electron microscope and transmission electron microscope of the present invention are shown in FIG. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] Embodiment 1:

[0038] See also Figure 1 As shown:

[0039] According to the composition range of the steel grade of the present invention, 6 kinds of experimental steels were prepared. The high-purity raw materials were configured into 80Kg raw materials according to the mass percentage, smelted in a 100Kg vacuum induction furnace, and then molded to a thickness of 100-120mm, followed by the following process treatments:

[0040] S1: heating the ingot to 1150-1200°C for 2 hours and performing homogenization treatment;

[0041] S2: hot rolling the steel ingot obtained in S1 into a steel plate with a thickness of 20±2 mm, with an initial rolling temperature of 1050°C≤≤1150°C and a final rolling temperature of 850°C≤≤1000°C;

[0042] S3: quenching the steel plate obtained in S2, keeping it at 850-870°C for 26-30 minutes and then quenching it with water to room temperature;

[0043] S4: subjecting the steel plate obtained in S3 to intercritical annealing treatment, keeping the temperature at 690-710°C for 46-50 minutes and then air cooling to room temperature;

[0044] S5: The steel plate obtained in S4 is tempered, kept at 560-580°C for 46-50 minutes, and then air-cooled to room temperature.

[0045] The process control principles involved in the medium-thick high-strength and low-yield ratio marine engineering steel and its preparation method of the present invention are as follows:

[0046] 1. The process principle of the quenching heat treatment involved in the present invention is: the designed alloy is smelted and hot-rolled into a plate, and then water-quenched at 850-870°C for 26-30 minutes to obtain a uniform and fine martensite structure. The supersaturated solid solution of carbon causes the lattice to distort, resulting in a very high shear strain inside the martensite, and the strain energy is reduced by generating substructures such as phase transformation dislocations, phase transformation twins, and stacking faults, resulting in a higher strength. Compared with bainite, polygonal ferrite or martensite-austenite mixed structures with the same strength level, the fine lath martensite structure is easier to achieve a good balance of strength and toughness. The quenching temperature affects the final strength of the steel plate and the strain strengthening ability of the steel plate; low temperature cannot ensure that the precipitated phase of the alloy element is fully dissolved back into the austenite, while high temperature will cause the austenite grains to be too coarse, which is not conducive to the control of the toughness of the steel plate. The quenching temperature of the present invention is determined based on the austenitizing temperature, and the austenitizing temperature is determined to be ~800°C by thermal expansion experiments.

[0047] 2. The process principle of the critical annealing heat treatment involved in the present invention is: the hot-rolled plate after quenching is heated to the (α+γ) two-phase region for 46 to 50 minutes to promote the formation of reverse austenite, and then air-cooled to room temperature to obtain a mixed structure mainly composed of critical ferrite (soft phase), tempered martensite (hard phase), residual austenite and secondary martensite. By adjusting the holding temperature of the two-phase region, the volume fraction and stability of the reverse austenite can be controlled to achieve the purpose of regulating the ratio of soft and hard phases. The low critical annealing temperature is not conducive to the nucleation and growth of reverse austenite, and although the high temperature can increase the content of reverse austenite, its thermal stability is reduced due to the full distribution of alloy elements. In the subsequent cooling process, the reverse austenite with low thermal stability may be transformed into secondary martensite (hard phase), reducing the content of residual austenite in the steel. The soft phase can dominate the coordinated deformation, obtain a low yield ratio and high plasticity, and the hard phase can ensure the strength of the steel plate.

[0048] The process principle of the tempering heat treatment involved in the present invention is: the annealed plate is reheated to 560-580°C and tempered for 46-50 minutes to obtain critical ferrite and tempered martensite structures with precipitated nanoparticles. After the steel plate is tempered, the matrix further recovers and recrystallizes, promoting the transformation of martensite to critical ferrite, and at the same time, carbides and nano Cu-rich phases are precipitated in the matrix. The precipitation of a large amount of nanophases will significantly increase the yield strength of the steel plate; while the transformation of martensite to critical ferrite causes the matrix to soften, which is conducive to the reduction of the yield ratio. Therefore, the type, size and volume fraction of the precipitates and the degree of recovery / recrystallization in the steel plate after the two-phase region annealing treatment are controlled by regulating the tempering and holding temperature to control the yield ratio. When tempered at 560-580°C, the yield ratio of the steel plate can be guaranteed to be no higher than 0.83. In addition, when tempered in this temperature range, temper brittleness can be avoided, the impact toughness is improved, and the steel plate has a good strength-toughness match and a lower yield ratio.

[0049] The components of the prepared steel plate were measured, and the actual chemical composition of the steel plate in the embodiment is shown in Table 1. Three groups of samples were cut from the prepared steel plate, and different heat treatment temperatures were selected within the heat treatment temperature range for treatment. The heat treatment process parameters of all samples are shown in Table 2. Standard round bar tensile specimens and V-notch Charpy impact specimens were processed from the core of the heat-treated steel plate according to the national standard, and then room temperature tensile tests and -40°C Charpy impact tests were carried out. The mechanical properties of the steel plate in the embodiment are shown in Table 3.

[0050] Table 1 Chemical composition of steel plates (wt%).

[0051] Example C Mn Si Ni Cr Mo Cu V Nb Ti 1 0.096 1.03 0.11 3.37 0.52 0.72 1.13 0.041 0.028 0.020 2 0.108 0.94 0.19 3.59 0.37 0.58 1.01 0.056 0.017 0.018 3 0.075 1.08 0.24 3.30 0.35 0.55 1.25 0.069 0.011 0.026 4 0.082 1.13 0.08 3.48 0.44 0.67 1.06 0.062 0.025 0.016 5 0.115 0.85 0.22 3.51 0.49 0.75 0.96 0.047 0.015 0.029 6 0.103 0.97 0.16 3.45 0.55 0.64 1.21 0.050 0.023 0.012

[0052] Table 2 Heat treatment process parameters of steel plates

[0053]

[0054]

[0055] Note: Q : Quenching temperature; T L : critical annealing temperature; T T : Tempering temperature; t Q : Quenching time; t L : critical annealing time; t T : Tempering time.

[0056] Table 3 Mechanical properties of steel plates

[0057]

[0058]

[0059] Note: Rp0.2: yield strength; Rm: tensile strength; A: elongation; Y / T: yield strength ratio; Akv: -40℃ transverse impact energy.

[0060] From the data in Table 1, Table 2 and Table 3, it can be seen that the steel plate prepared by the alloy composition and production process of the present invention has a yield strength of ≥800MPa, a tensile strength of ~1000MPa, an elongation of ≥19%, a yield strength ratio of ≤0.83, and a transverse impact energy of ≥120J at -40℃, with excellent strength-toughness matching and small fluctuations within the full composition range and process window. Compared with marine steel of the same strength level, the introduction of the critical annealing process significantly reduces the yield strength ratio. Compared with marine steel with a similar yield strength ratio, the strengthening level is about 460-550MPa, and the marine steel prepared by the present invention has a strength level increased by 45-74%.

[0061] Through scanning electron microscopy and transmission electron microscopy analysis, Figure 1 As shown in the figure, the critical annealing process is introduced on the basis of the traditional quenching and tempering process to transform the steel matrix structure from a single tempered martensite structure to a composite structure consisting of tempered martensite, retained austenite, critical ferrite and a small amount of secondary martensite, and nano-scale Cu-rich phase, MC-type carbides, M 23 Various second phases such as C6 type carbide.

[0062] in Figure 1 Shown: TM: tempered martensite; RA: retained austenite; IF: intercritical ferrite; FM: secondary martensite; Rich-Cu: Cu-rich phase

[0063] During intercritical annealing at 690-710℃, the matrix structure will undergo strong recovery / recrystallization, resulting in the transformation of the matrix into tempered martensite or even intercritical ferrite, while also promoting the formation of reversed austenite. The higher the intercritical annealing temperature, the more intercritical ferrite and reversed austenite will be formed. However, a higher intercritical annealing temperature will reduce the carbon content in the reversed austenite, weaken its thermal stability, and cause it to transform into secondary martensite again during the subsequent cooling process. Secondary martensite, as a hard phase, can improve the tensile strength of the experimental steel, while intercritical ferrite improves the plastic deformation ability of the steel. In addition, the retained austenite with appropriate stability during the strain process will undergo the TRIP effect, further improving the strength and plasticity of the experimental steel. The synergistic effect of intercritical ferrite and retained austenite enables the experimental steel to exhibit a transverse low-temperature impact energy of up to 120J. However, the presence of too much soft phases such as intercritical ferrite and retained austenite will weaken the yield strength of the experimental steel. Therefore, tempering treatment is needed to promote the precipitation of nano-Cu-rich phase and compensate for the loss of yield strength through its precipitation strengthening effect.

[0064] The precipitation behavior of nano Cu-rich phase during tempering is also affected by the critical annealing temperature. For the experimental steel after annealing, there are two precipitation positions of nano Cu-rich phase during tempering, one is precipitation from the matrix and the other is precipitation from the secondary martensite. When the critical annealing temperature is too low, the content of reverse transformation austenite is low and the stability is high, resulting in almost no secondary martensite in the steel. At this time, the Cu-rich phase is precipitated from the matrix. However, due to the diffusion of Cu element between the matrix and reverse transformation austenite during the critical annealing process, the solid solution of Cu in the matrix is ​​reduced, resulting in a decrease in precipitation strengthening ability. When the critical annealing temperature is too high, the content of reverse transformation austenite increases, but the stability is low, resulting in a large amount of secondary martensite in the steel. At this time, part of the Cu-rich phase precipitates from the matrix and part from the secondary martensite. The enrichment of Cu element in the reverse transformed austenite causes the Cu content in the secondary martensite to be much higher than that in the matrix. Therefore, the re-precipitation of Cu-rich phase in the secondary martensite becomes the key to improving the yield strength of the experimental steel, which will again lead to an increase in the yield strength ratio.

[0065] It will be appreciated that in the development of any actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made. Such a development effort may be complex and time-consuming, but will be a routine task of design, fabrication, and production for those of ordinary skill having the benefit of this disclosure without undue experimentation.

[0066] 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 preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A medium-thick high-strength and low-yield ratio marine steel, characterized in that: The components of the marine steel plate are calculated by weight percentage as follows: C: 0.075-0.115%, Mn: 0.85-1.15%, Si: 0.08-0.24%, Ni: 3.30-3.60%, Cr: 0.35-0.55%, Mo: 0.55-0.75%, Cu: 0.95-1.25%, V: 0.040-0.070%, Nb: 0.010-0.030%, Ti: 0.010-0.030%, and the rest are Fe and unavoidable impurities.

2. The method for preparing a medium-thick high-strength and low-yield ratio marine engineering steel according to claim 1, characterized in that: The production process includes the following steps: smelting, casting, slab heating, rolling and heat treatment process; in: S1, the smelting and casting: prepare 80Kg of raw materials, smelt them in a 100Kg vacuum induction furnace, and then cast them into a mold with a thickness of 100-120mm; S2. Slab heating: heat the ingot to 1150-1200°C for 2 hours and perform homogenization treatment; S3, rolling: hot rolling the steel ingot to a thickness of 20±2mm with an average single-pass reduction rate of more than 15%, and air-cooling to room temperature, 1050℃≤initial rolling temperature≤1150℃, 850℃≤final rolling temperature≤1000℃; S4, quenching the hot-rolled steel plate, keeping it at 850-870°C for 26-30 minutes, and then quenching it with water to room temperature (t=1.3-1.5min / mm×h, h: plate thickness); S5. Perform critical annealing on the quenched steel plate, keep it at 690-710°C for 46-50 minutes, and then air-cool it to room temperature (t=2.3-2.5min / mm×h, h: plate thickness); S6. Temper the critical annealed steel plate by keeping it at 560-580°C for 46-50 min and then air-cooling it to room temperature (t=2.3-2.5 min / mm×h, h: plate thickness).

3. The medium-thick high-strength and low-yield ratio marine steel according to claim 1, characterized in that: The microstructure of the marine steel plate consists of tempered martensite, critical ferrite, retained austenite, secondary martensite and nanoscale precipitation phases. The retained austenite is evenly distributed in the tempered martensite / critical ferrite matrix, the precipitation phase is evenly distributed in the entire microstructure, and the nano copper-rich phase is distributed in both the matrix and the secondary martensite.

4. The medium-thick high-strength and low-yield ratio marine engineering steel according to claim 1, characterized in that: The yield strength of the marine engineering steel plate is ≥800MPa, the tensile strength is ∼1000MPa, the elongation is ≥19%, the yield strength ratio is ≤0.83, and the transverse impact energy at -40°C is ≥120J.

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