Low-temperature steel for marine engineering, and production method therefor

Abstract

A low-temperature steel for marine engineering, and a production method therefor. The steel comprises the following chemical components: 0.07-0.13% of carbon, 0.1-0.4% of silicon, 0.6-1.5% of manganese, 5-9% of nickel, 0.6-0.8% of chromium, 0.6-0.9% of molybdenum, 0.1-0.16% of vanadium, 0.4-0.8% of copper, with the balance being iron and impurities. A process route of smelting → refining → continuous casting → heating → rolling → cooling → three-stage heat treatment is used. In the first-stage heat treatment, the temperature is 800-850ºC, the heat preservation time coefficient is 1-1.5 min / mm, and water cooling is performed; in the second-stage heat treatment, the temperature is 700-720ºC, the heat preservation time coefficient is 1-1.5 min / mm, and water cooling is performed; and in the third-stage heat treatment, the temperature is 600-620ºC, the heat preservation time coefficient is 2-2.5 min / mm, and air cooling is performed. The steel has good comprehensive performance in an environment at -80ºC or lower and satisfies the requirements of the marine engineering field.

Description

Low-temperature steel for marine engineering and production method thereof Technical Field

[0001] The present invention belongs to the technical field of steel material preparation, and relates to low-temperature steel for marine engineering and a production method of low-temperature steel for marine engineering. Background Art

[0002] In recent years, the rapid development of marine resources in low-temperature regions, such as the polar regions, has created an urgent need for cold-resistant marine engineering equipment. Marine engineering steels must not only possess high strength, good elongation, and weldability in low-temperature environments, but also possess excellent ultra-low-temperature toughness to ensure reliability during safe service. Furthermore, the increasing difficulty of extracting deep-sea oil and gas resources, such as those at depths exceeding 3,000 meters, necessitates the use of a large number of ultra-high-strength structural materials. Therefore, high-strength, high-ductility, easily weldable, and excellent low-temperature toughness structural steels hold significant application prospects in marine engineering fields, such as those in extreme cold and deep seas.

[0003] Existing marine engineering steels generally adopt a single martensitic structure with a high dislocation density in order to obtain high-strength properties with a yield strength of 600MPa or above. However, such steel plates have the following defects: 1) Insufficient low-temperature toughness. For example, they cannot meet the requirements of low-temperature marine engineering projects below -80°C; 2) Insufficient plasticity and a high yield-to-strength ratio. As the yield strength increases, the elongation of the steel plate decreases, and the plastic deformation redundancy from yield to fracture is reduced, which increases the risk of brittle fracture and seriously affects the service safety of the structure; 3) Insufficient strength. For example, such steel plates cannot reach a high strength of 1GPa.

[0004] Summary of the Invention

[0005] The purpose of the present invention is to provide a low-temperature steel for marine engineering and a production method for low-temperature steel for marine engineering. The steel can meet the comprehensive requirements of marine engineering fields such as extreme cold and deep sea for ultra-high strength, high plasticity, easy welding and excellent low-temperature toughness of steel.

[0006] To achieve the above-mentioned purpose of the invention, one embodiment provides a low-temperature steel for marine engineering, whose chemical composition, in percentage by mass, is as follows: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P≤0.003%, S≤0.002%, and the balance is iron and unavoidable impurities.

[0007] Furthermore, the microstructure of the low-temperature steel is martensite+austenite; the volume percentage of martensite is greater than 75% and less than 85%, and the volume percentage of austenite is greater than 15% and less than 25%.

[0008] Furthermore, the low-temperature steel has a nano-precipitated phase of (MoV)C particles and Cu particles, and the volume percentage of the precipitated phase is less than 0.1%.

[0009] Furthermore, under low temperature conditions of -196°C, the yield strength of the low-temperature steel is greater than 1200 MPa, the elongation after fracture is greater than 30%, the yield strength ratio is less than 0.84, and the impact energy is greater than 150 J.

[0010] Furthermore, under low temperature conditions of -80°C, the yield strength of the low-temperature steel is above 1000 MPa, the elongation after fracture is above 25%, the yield strength ratio is below 0.94, and the impact energy is above 160 J.

[0011] To achieve the above-mentioned object of the invention, one embodiment provides a method for producing low-temperature steel for marine engineering, which comprises the following steps:

[0012] Smelting, refining and continuous casting are performed in sequence to produce a continuous casting billet; the chemical composition of the continuous casting billet is as follows, in percentage by mass: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P≤0.003%, S≤0.002%, and the balance is iron and unavoidable impurities;

[0013] Heat the continuous casting billet in a heating furnace to 1100-1150°C and keep it warm for 2-3 hours;

[0014] The continuous casting slabs coming out of the heating furnace are rolled into steel plates at a rolling temperature of 950-1050°C;

[0015] The obtained steel plate is laminar-cooled to below 200°C at a cooling rate greater than 20°C / s, and then subjected to three-stage heat treatment in sequence to obtain the low-temperature steel product; wherein, the temperature of the first stage heat treatment is 800-850°C, the holding time coefficient is 1.0-1.5 min / mm, and the cooling method is water cooling; the temperature of the second stage heat treatment is 700-720°C, the holding time coefficient is 1.0-1.5 min / mm, and the cooling method is water cooling; the temperature of the third stage heat treatment is 600-620°C, the holding time coefficient is 2.0-2.5 min / mm, and the cooling method is air cooling.

[0016] Preferably, in the step of "rolling the continuously cast slab exiting the heating furnace into a steel plate", the average single-pass reduction rate is above 15%, and the thickness of the obtained steel plate is 10 to 100 mm.

[0017] Preferably, in the step of "smelting, refining and continuous casting in sequence to obtain a continuously cast billet", the superheat of the molten steel in the tundish during continuous casting is ≤20°C, and full-process protection casting is adopted, and the inclusion levels of Class A, Class B, Class C and Class D of the obtained continuously cast billet are all ≤0.5.

[0018] Preferably, the superheat degree of the molten steel in the tundish is 10-20°C.

[0019] Preferably, in the step of "cooling the obtained steel plate to below 200°C by laminar flow at a cooling rate greater than 20°C / s", laminar cooling is performed at a cooling rate of below 30°C / s until the temperature drops to 30-200°C.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) Based on low carbon, a high nickel, high vanadium, low manganese + molybdenum, chromium and copper design is adopted. The low carbon content ensures the weldability and toughness of the steel, and the low carbon and low manganese content avoids the formation of a large amount of segregated banded structure, ensuring the uniformity of structure and performance. The high Ni content promotes the formation of retained austenite and improves the stability of austenite together with C. Mo and V can form nano-sized complex phase carbide (MoV)C particles to improve strength. The Cu element increases the stability of retained austenite while improving strength.

[0022] 2) Based on the chemical composition, through high-temperature rolling + rapid cooling + three-stage heat treatment, especially two heat treatments with different high and low temperatures in the two-phase region, precise control of the steel structure is achieved. In particular, the content, size and morphology of high-stability austenite in the finished steel plate can be precisely controlled, thereby obtaining the comprehensive properties of ultra-high strength, high plasticity, low yield ratio, excellent toughness and excellent welding performance; moreover, based on low carbon + high nickel as the components of stable austenite, combined with the thickness of the steel plate, a multi-scale complex phase micro-precision control heat treatment process technology is adopted to simultaneously achieve the stable control of the comprehensive performance quality such as ultra-high strength, good plasticity and low-temperature toughness, which is conducive to improving the yield rate of large-scale industrial production;

[0023] 3) Overall, it achieves the comprehensive performance advantages of ultra-high strength, high plasticity, low yield ratio, excellent toughness and excellent welding performance in ultra-low temperature environment. For example, the steel plate has an ultra-high yield strength of more than 1000MPa, elongation after fracture>20%, yield ratio<0.93, and impact energy≥150J in ultra-low temperature environment of -80~-196℃, thus meeting the application requirements of marine engineering fields such as extreme cold and deep sea. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 is a scanning electron microscope microstructure diagram of martensite and retained austenite of a finished steel plate of Example 5 of the present invention at a microscopic scale of 5 μm;

[0025] FIG2 is a transmission electron micrograph of a nano-precipitated phase of a finished steel plate of Example 5 of the present invention at a microscopic scale of 200 nm;

[0026] FIG3 is an energy spectrum analysis diagram of Cu element distribution at a microscopic scale of 200 nm of the finished steel plate of Example 5 of the present invention;

[0027] FIG4 is an X-ray diffraction pattern of austenite γ phase and martensite α phase of the finished steel plate of Example 5 of the present invention at 25° C., −80° C., and −196° C. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further introduced below in conjunction with specific implementation methods. The technical content described below is only an exemplary implementation method of the present invention and is not intended to limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the attached claims.

[0029] This embodiment provides a low-temperature steel, particularly a steel plate suitable for use in low-temperature environments in marine engineering. The chemical composition of the steel, in percentage by mass, is as follows: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P ≤ 0.003%, S ≤ 0.002%, with the remainder being iron and unavoidable impurities.

[0030] The functions of the various chemical elements of the wire rod described in this embodiment are described in detail below.

[0031] C: A strengthening element. An increase in its content helps to improve the strength of the steel, but has a negative impact on the plasticity and toughness of the steel, especially its weldability. In this embodiment, the C content is 0.07 to 0.13% by mass.

[0032] Si: A strengthening element that can suppress bainite formation, cementite precipitation and coarsening, and improve toughness. In this embodiment, the Si content is 0.10 to 0.40% by mass.

[0033] Mn: mainly used to increase the strength and toughness of steel, and at the same time can increase the content of metastable austenite to form a duplex structure; in this embodiment, the Mn content is 0.60-1.50% by mass.

[0034] Ni: A major alloying element with a solid solution strengthening effect, stabilizing austenite. This lowers the Ar3 point, shifts the CCT curve to the right, promotes the formation of metastable austenite and increases its stability. It can also form a fine martensitic structure, thereby improving the strength and toughness of the steel. Ni also affects the lateral slip of dislocations, lowering the ductile-brittle transition temperature of the steel. In this embodiment, the Ni content is 5.00 to 9.00% by mass.

[0035] Cr: shifts the continuous cooling transformation curve of steel to the right, improving the hardenability of steel, i.e., martensite-forming ability. Cr is dissolved in untransformed austenite, increasing the stability of supercooled austenite. In this embodiment, the Cr content is 0.60-0.80% by mass.

[0036] Mo: An element that improves hardenability, expands the austenite phase region, plays an important role in controlling the phase transformation structure, and can effectively improve the strength of the material; it reduces the phase transformation temperature and the critical cooling rate of bainite transformation, which can effectively improve the stability of the strength and toughness properties in the thickness direction of the steel plate; Mo is also an element with a high affinity for C and can form nano-sized complex phase carbide (MoV)C particles together with V, which has a precipitation strengthening effect; in this embodiment, the Mo content is calculated as a percentage by mass in the range of 0.60 to 0.90%.

[0037] V: effectively refines the grain size of the steel and simultaneously forms nano-sized complex carbide (MoV)C particles in the steel together with Mo. This strongly hinders dislocation movement during deformation, thereby significantly improving the strength. In this embodiment, the V content is 0.1-0.16% by mass.

[0038] Cu: forms nano-sized Cu particles in the matrix structure and precipitates, which plays a precipitation strengthening role. In addition, Cu can stabilize the retained austenite and increase the content and stability of the retained austenite. Its content is controlled at 0.40-0.80%.

[0039] As mentioned above, the chemical composition of this embodiment is based on low carbon and adopts a design scheme of high nickel, high vanadium, low manganese + molybdenum, chromium and copper. The low C content ensures the weldability and toughness of the steel, and the low carbon and low manganese content avoid the large-scale formation of segregated banded structures, ensuring the uniformity of the organization and performance. The high Ni content promotes the formation of retained austenite and improves the stability of austenite together with C. Mo and V can form nano-sized complex phase carbide (MoV) C particles to improve the strength, and the Cu element increases the stability of the retained austenite while improving the strength. As a result, the overall performance advantages of ultra-high strength (for example, above 1 GPa level), high plasticity, low yield ratio, excellent toughness and excellent welding performance are achieved, thereby meeting the application requirements of marine engineering fields such as extreme cold and deep sea.

[0040] Specifically, the microstructure of the steel plate of this embodiment is martensite + austenite, wherein the volume percentage of martensite is greater than 75% and less than 85%, and the volume percentage of austenite is greater than 15% and less than 25%. This microstructure can further ensure high strength, excellent low-temperature toughness, and good elongation. In the present invention, the volume percentages of martensite and austenite are obtained by conventional techniques known in the art. For example, in accordance with the international standard "ASTM E3-11 (2017) Standard Guide for Preparation of Metallographic Specimens", a metallographic specimen is sampled at 1 / 4 of the thickness of the steel plate to prepare the metallographic specimen. The obtained metallographic specimen is observed and measured under optical and scanning electron microscopes at 500x and 1000x magnifications, respectively. The martensite and austenite are distinguished by the difference in contrast in the micrographs, and the volume percentages of martensite and austenite are calculated.

[0041] In addition, the steel plate of this embodiment further comprises a trace amount of nano-precipitated phases of (MoV)C particles and Cu particles, with the volume percentage of the precipitated phases being less than 0.1%. The precipitation phases significantly improve the strength of the steel. In the present invention, the observation method for the precipitated phases is conventionally known in the art. For example, a sample of (MoV)C particles and Cu particles in the steel is sampled from 1 / 4 of the width and 1 / 4 of the thickness of the steel plate, and a 30-50 μm thick thin film sample is prepared. A thin area is prepared using an electrolytic double-spray method. Observation, measurement, and statistical analysis are performed using a transmission electron microscope with a field of view of 20,000 to 100,000 times magnification. EDS energy spectrum analysis confirms that the precipitated phases are nano-precipitated phases of (MoV)C particles and Cu particles with a diameter of 5 nm or greater, and the volume fraction of the precipitated phases is obtained.

[0042] The steel plate of this embodiment can have a thickness of 10 to 100 mm and exhibits excellent performance and strong stability under low-temperature conditions. Specifically, at -196°C, the steel plate exhibits a yield strength of 1200 MPa or greater, an elongation of 30% or greater, a yield strength ratio of 0.84 or less, and an impact energy KV2 of 150 J or greater. At -80°C, the steel plate exhibits a yield strength of 1000 MPa or greater, an elongation of 25% or greater, a yield strength ratio of 0.94 or less, and an impact energy KV2 of 160 J or greater. In the present invention, the performance test of the steel plate under low temperature conditions is a conventional technical means known in the art. For example, the obtained steel plate is sampled according to the international standard "ASTM A673 / A673M-17 Standard Specification for Sampling Procedures for Impact Tests of Structural Steel", and the sample is subjected to low temperature performance testing according to the international standard "ASTM E2298-18 Standard Test Method for Instrumented Impact Testing of Metallic Materials" to obtain the yield strength, elongation after fracture, yield strength ratio and impact energy KV2 under low temperature conditions of -196°C and -80°C.

[0043] Furthermore, this embodiment also provides a method for producing the aforementioned steel, which employs the following process: smelting → refining → continuous casting → heating → rolling → cooling → heat treatment. The manufacturing method is described in detail below in sequential order.

[0044] Step 1) Smelting → Refining → Continuous Casting

[0045] According to the chemical composition described above, smelting, refining and continuous casting are carried out in sequence to produce a continuous casting billet; the chemical composition of the continuous casting billet is consistent with the chemical composition described above, that is, in percentage by mass: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P ≤ 0.003%, S ≤ 0.002%, and the balance is iron and unavoidable impurities;

[0046] Step 2) Heating

[0047] The continuous casting billet is heated to 1100-1150°C in a heating furnace and kept at this temperature for 2-3 hours; for example, the soaking zone of the heating furnace is set to any temperature value between 1100-1150°C, and the residence time of the continuous casting billet in the soaking zone of the heating furnace is controlled to be 2-3 hours;

[0048] Step 3) Rolling

[0049] The continuous casting slabs exiting the heating furnace are rolled into steel plates by high-temperature hot rolling at a rolling temperature (start rolling temperature) of 950-1050°C. This allows for sufficient recrystallization of the austenite and refinement of the austenite grain size. High-temperature rolling can also effectively improve rolling efficiency and reduce mill load.

[0050] Step 4) Cooling → Heat treatment

[0051] The steel plate rolled in the above step 3 is laminar-cooled to below 200°C at a cooling rate greater than 20°C / s. In this way, the steel plate is rapidly cooled online using a laminar cooling system after rolling, and the final cooling temperature is controlled below 200°C, so that the phase transformation structure can be controlled to be martensite.

[0052] After cooling, the low-temperature steel product is obtained by sequentially performing three-stage heat treatment;

[0053] Among them, the temperature of the first stage heat treatment is 800-850°C, the holding time coefficient is 1.0-1.5min / mm, and the cooling method for cooling after holding is water cooling. The heat treatment in this stage can refine the martensite and precipitate residual austenite, that is, the obtained structure is martensite + austenite, of which the volume of austenite accounts for about 5-10%; the temperature of the second stage heat treatment is controlled in the two-phase region (between Ac1 and Ac3), specifically 700-720°C, the holding time coefficient is 1.0-1.5min / mm, and the cooling method for cooling after holding is water cooling. Residual austenite is further precipitated in the second stage heat treatment. The residual austenite, that is, in the obtained composite structure of martensite + austenite, the volume proportion of austenite increases to more than 15% and less than 25%; the temperature of the third stage heat treatment is controlled in the two-phase region (between Ac1' and Ac3', due to the change of the organizational phase, Ac1' is less than Ac1, and Ac3' is less than Ac3), specifically the temperature is 600-620°C, the holding time coefficient is 2.0-2.5min / mm, and the cooling method during cooling after holding is air cooling. A precipitate phase can be precipitated in the third stage heat treatment, and the precipitate phase is a trace amount (less than 0.1% by volume) of nano-scale (MoV)C particles and Cu particles.

[0054] In this way, the production method of this embodiment, based on the chemical composition, realizes precise control of the steel structure through high-temperature rolling + rapid cooling + three-stage heat treatment, especially two heat treatments with different high and low temperatures in the two-phase region, and especially can accurately regulate the high-stability austenite content, size and morphology in the finished steel plate, thereby obtaining the comprehensive performance of ultra-high strength, high plasticity, low yield ratio, excellent toughness and excellent welding performance; moreover, the product performance is stable, which is conducive to improving the yield rate of large-scale industrial production.

[0055] The smelting and refining in step 1 can be carried out using conventionally available technologies, such as converter smelting or electric furnace smelting, or a LF furnace + RH furnace refining technique. Preferably, when using the LF furnace + RH furnace refining technique, the duration of both the LF furnace refining and the RH furnace refining is controlled to be 20-30 minutes.

[0056] Preferably, in step 1, the superheat of the molten steel in the tundish during continuous casting is ≤ 20°C, specifically 10-20°C, and full-protected casting is used. The resulting continuous casting ingot has non-metallic inclusion levels of Class A (sulfides), Class B (alumina), Class C (silicates), and Class D (spherical oxides) of ≤ 0.5. This allows for precise control of steel cleanliness, thereby improving the performance of the resulting finished steel product.

[0057] Preferably, in step 3, the average single-pass reduction is above 15%, and the resulting steel plate has a thickness of 10 to 100 mm. On the one hand, the high reduction ratio combined with high-temperature rolling can fully achieve austenite recrystallization and refine the initial austenite grain size of the steel plate. Furthermore, the present invention is suitable for the production of medium and thick plates, meeting the demand for steel used in marine engineering applications.

[0058] More preferably, in step 4, when laminar cooling is performed on the steel plate, the cooling rate is controlled to be below 30°C / s, that is, greater than 20°C / s and not more than 30°C / s, and the final cooling temperature is between 30°C and 200°C. In this way, the metallographic structure can be precisely controlled.

[0059] In summary, the beneficial effects of the present invention are:

[0060] 1) Based on low carbon, a high nickel, high vanadium, low manganese + molybdenum, chromium and copper design is adopted. The low carbon content ensures the weldability and toughness of the steel, and the low carbon and low manganese content avoids the formation of large amounts of segregated banded structure, ensuring uniformity of structure and performance. The high nickel content promotes the formation of retained austenite and, together with carbon, improves the stability of austenite. Mo and vanadium can form nano-sized complex phase carbide (MoV)C particles to improve strength. The Cu element increases the stability of retained austenite while improving strength.

[0061] 2) Based on the chemical composition, through high-temperature rolling + rapid cooling + three-stage heat treatment, especially two heat treatments with different high and low temperatures in the two-phase region, precise control of the steel structure is achieved. In particular, the content, size and morphology of high-stability austenite in the finished steel plate can be precisely controlled, thereby obtaining the comprehensive properties of ultra-high strength, high plasticity, low yield ratio, excellent toughness and excellent welding performance; moreover, based on low carbon + high nickel as the components of stable austenite, combined with the thickness of the steel plate, a multi-scale complex phase micro-precision control heat treatment process technology is adopted to simultaneously achieve the stable control of the comprehensive performance quality such as ultra-high strength, good plasticity and low-temperature toughness, which is conducive to improving the yield rate of large-scale industrial production;

[0062] 3) Overall, it achieves the comprehensive performance advantages of ultra-high strength, high plasticity, low yield ratio, excellent toughness and excellent welding performance in ultra-low temperature environment. For example, the steel plate has an ultra-high yield strength of more than 1000MPa, elongation after fracture>20%, yield ratio<0.93, and impact energy≥150J in ultra-low temperature environment of -80~-196℃, thus meeting the application requirements of marine engineering fields such as extreme cold and deep sea.

[0063] To make the technical solutions and advantages of an embodiment of the present invention more clear, the embodiment will be further described below in conjunction with Examples 1 to 6 according to an embodiment of the present invention. Obviously, Examples 1 to 6 described are only part of the embodiments of the present invention, not all of them.

[0064] Specifically, Examples 1 to 6 each provide a steel material, and the manufacturing method of each is as follows:

[0065] (1) Smelting → Refining → Continuous Casting

[0066] According to the chemical composition described above, smelting, LF refining, RH refining, and continuous casting were carried out in sequence to produce a continuously cast ingot. The treatment time of LF refining, the treatment time of RH refining, the superheat degree of the molten steel in the tundish during continuous casting, and the test results of the four types of inclusions A, B, C, and D in the obtained continuously cast ingot are all shown in Table 1.

[0067] [Table 1]

[0068] The chemical composition of the continuous casting slab is shown in Table 2 in terms of mass percentage.

[0069] [Table 2]

[0070] (2) Heating → Rolling → Cooling

[0071] The continuous casting slab is heated in a heating furnace and kept warm for 2 to 3 hours. After exiting the heating furnace, it is rolled into a steel plate, which is then cooled on a laminar cooling system. The soaking temperature, rolling temperature, average single-pass reduction ratio, cooling rate, and final cooling temperature in the heating furnace of each embodiment are shown in Table 3.

[0072] [Table 3]

[0073] (3) Three-stage heat treatment

[0074] The cooled steel plate is subjected to three-stage heat treatment in sequence according to the method shown in Table 4 below to obtain the finished steel plate.

[0075] [Table 4]

[0076] The microstructure and mechanical properties of the finished steel plates of each embodiment were tested respectively:

[0077] 1) The microstructure of the finished steel plate of each embodiment is martensite + austenite, wherein the volume percentage of martensite is greater than 75% and less than 85%, and the volume percentage of austenite is greater than 15% and less than 25%, and the austenite has excellent low-temperature stability; and further comprises a nano-precipitated phase of (MoV)C particles and Cu particles with a total volume percentage of less than 0.1%;

[0078] 1 to 4 , Example 5 is used as an example to illustrate the microstructure of the finished steel plate of the present invention. FIG1 shows that the matrix structure of Example 5 is tempered lath martensite, and the white color is high-stability retained austenite with a high volume fraction. FIG2 and FIG3 respectively show the nanoscale (MoV)C particles and Cu precipitates of Example 5. FIG4 shows the X-ray diffraction patterns of the finished steel plate of Example 5 at 25°C, -80°C, and -196°C, indicating the volume fractions of the austenite γ phase and the martensite α phase, with the volume fraction of the γ phase being stable at 15.5%, 15.3%, and 15.3%.

[0079] 2) The mechanical properties of the finished steel plates of each embodiment are shown in Table 5. It can be seen that the steel plates of each embodiment have an ultra-high yield strength of more than 1000 MPa, an elongation after fracture of >20%, a yield strength ratio of <0.93, and an impact energy KV2 ≥150 J under an ultra-low temperature environment of -80 to -196°C. They have excellent comprehensive ultra-low temperature performance and can be used as low-temperature steel in marine engineering fields such as extreme cold and deep sea.

[0080] [Table 5]

Claims

1. A low-temperature steel for marine engineering, characterized in that: Its chemical composition is calculated in percentage by mass: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P≤0.003%, S≤0.002%, and the balance is iron and unavoidable impurities.

2. The low-temperature steel for marine engineering according to claim 1, characterized in that: Its microstructure is martensite + austenite.

3. The low-temperature steel for marine engineering according to claim 2, characterized in that: The volume percentage of martensite is 75% or more and less than 85%, and the volume percentage of austenite is 15% or more and less than 25%.

4. The low-temperature steel for marine engineering according to claim 1, characterized in that: The composite material has a nano-precipitated phase of (MoV)C particles and Cu particles, and the volume percentage of the precipitated phase is less than 0.1%.

5. The low-temperature steel for marine engineering according to claim 1, characterized in that: Under the low temperature condition of -196°C, the yield strength of the low temperature steel is above 1200 MPa, the elongation after fracture is above 30%, the yield strength ratio is below 0.84, and the impact energy is above 150 J.

6. The low-temperature steel for marine engineering according to claim 1, characterized in that: Under low temperature conditions of -80°C, the low temperature steel has a yield strength of more than 1000 MPa, an elongation after fracture of more than 25%, a yield strength ratio of less than 0.94, and an impact energy of more than 160 J.

7. A method for producing low-temperature steel for marine engineering, characterized in that: The following steps are involved: Smelting, refining and continuous casting are performed in sequence to produce a continuous casting billet; the chemical composition of the continuous casting billet is as follows, in percentage by mass: C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60-1.50%, Ni: 5.00-9.0%, Cr: 0.60-0.80%, Mo: 0.60-0.90%, V: 0.10-0.16%, Cu: 0.40-0.80%, P≤0.003%, S≤0.002%, and the balance is iron and unavoidable impurities; Heat the continuous casting billet to 1100-1150℃ in a heating furnace and keep it warm for 2-3h; The continuous casting slabs coming out of the heating furnace are rolled into steel plates at a rolling temperature of 950-1050°C; The obtained steel plate is laminar-cooled to below 200°C at a cooling rate greater than 20°C / s, and then subjected to three-stage heat treatment in sequence to obtain the low-temperature steel product; wherein, the temperature of the first stage heat treatment is 800-850°C, the holding time coefficient is 1.0-1.5 min / mm, and the cooling method is water cooling; the temperature of the second stage heat treatment is 700-720°C, the holding time coefficient is 1.0-1.5 min / mm, and the cooling method is water cooling; the temperature of the third stage heat treatment is 600-620°C, the holding time coefficient is 2.0-2.5 min / mm, and the cooling method is air cooling.

8. The method for producing low-temperature steel for marine engineering according to claim 7, characterized in that: In the step of "rolling the continuous casting slab out of the heating furnace into a steel plate", the average single-pass reduction rate is above 15%, and the thickness of the obtained steel plate is 10 to 100 mm.

9. The method for producing low-temperature steel for marine engineering according to claim 7, characterized in that: In the step of "smelting, refining and continuous casting in sequence to obtain a continuously cast billet", the superheat of the molten steel in the tundish during continuous casting is ≤20°C, and full-process protection casting is adopted. The inclusion levels of Class A, Class B, Class C and Class D of the obtained continuously cast billet are all ≤0.

5.

10. The method for producing low-temperature steel for marine engineering according to claim 9, characterized in that: The superheat of molten steel in the tundish is 10-20℃.

11. The method for producing low-temperature steel for marine engineering according to claim 7, characterized in that: In the step of "cooling the obtained steel plate to below 200° C. by laminar flow at a cooling rate greater than 20° C. / s", laminar cooling is performed at a cooling rate less than 30° C. / s until the temperature drops to 30 to 200° C.

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