Low temperature steel for marine engineering and its manufacturing method

Abstract

The present invention discloses a low-temperature steel for marine engineering and a method for its production. The steel's chemical composition includes 0.07-0.13% carbon, 0.1-0.4% silicon, 0.6-1.5% manganese, 5-9% nickel, 0.6-0.8% chromium, 0.6-0.9% molybdenum, 0.1-0.16% vanadium, and 0.4-0.8% copper, with the remainder being iron and impurities. The steel is produced using a process line consisting of smelting, refining, continuous casting, heating, rolling, cooling, and a three-stage heat treatment. The first heat treatment involves water cooling at a temperature of 800-850°C with a holding time of 1-1.5 min / mm. The second heat treatment involves water cooling at a temperature of 700-720°C with a holding time of 1-1.5 min / mm. The third heat treatment involves air cooling at a temperature of 600-620°C with a holding time of 2-2.5 min / mm. This steel has excellent overall performance in environments below -80°C, meeting the needs of the marine engineering field.

Description

[Technical Field]

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

[0002] In recent years, the rapid development of marine resource development in polar regions and other low-temperature regions has created an urgent need for cold-resistant marine engineering equipment. In low-temperature environments, marine engineering steels must not only have high strength, good elongation, and weldability, but also excellent ultra-low temperature toughness to ensure reliability throughout their service life. Furthermore, as the extraction of deep-sea oil and gas resources at depths of 3,000 meters or more becomes increasingly difficult, the use of large quantities of ultra-high strength structural materials is also becoming necessary. Therefore, structural steels with high strength, high ductility, weldability, and excellent low-temperature toughness have important application prospects in marine engineering fields such as polar regions and the deep sea.

[0003] Current marine engineering steels generally use a single martensite structure with a high dislocation density to achieve high strength performance with a yield strength of 600 MPa or higher. However, this type of steel plate has the following drawbacks: 1) It lacks low-temperature toughness and cannot meet the demands of low-temperature marine engineering projects, for example, at temperatures below -80°C. 2) It lacks ductility and has a high yield ratio, which means that as the yield strength increases, the steel plate's fracture elongation decreases, reducing the plastic deformation margin from yield to fracture, increasing the risk of brittle fracture and seriously affecting the in-service safety of structures. 3) Furthermore, it lacks strength, meaning this type of steel plate cannot reach a high strength of 1 GPa. Summary of the Invention

[0004] The object of the present invention is to provide a low-temperature steel for marine engineering and a manufacturing method thereof, which can meet the overall requirements for steel in the field of marine engineering, such as in arctic regions and deep seas, including ultra-high strength, high ductility, excellent weldability, and excellent low-temperature toughness.

[0005] To achieve the above object, in one embodiment, there is provided a low temperature steel for marine engineering, the chemical composition of which, in mass percent, is: C: 0.07~0.13%, Si: 0.10 to 0.40% Mn: 0.60 to 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%, The remainder is iron and unavoidable impurities.

[0006] Furthermore, the microstructure of the low temperature steel is martensite + austenite, and 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%.

[0007] Furthermore, the low temperature steel has a nano-precipitated precipitate phase of (MoV)C particles and Cu particles, and the volume percentage of the precipitate phase is less than 0.1%.

[0008] Furthermore, under low temperature conditions of -196°C, the low temperature steel has a yield strength of 1200 MPa or more, a fracture elongation of 30% or more, a yield ratio of 0.84 or less, and an impact value of 150 J or more.

[0009] Furthermore, under low temperature conditions of -80°C, the low temperature steel has a yield strength of 1000 MPa or more, a fracture elongation of 25% or more, a yield ratio of 0.94 or less, and an impact value of 160 J or more.

[0010] To achieve the above object, in one embodiment, there is provided a method for producing a low temperature steel for marine engineering, the method comprising the steps of: a step of sequentially carrying out smelting, refining, and continuous casting to obtain a continuously cast slab, the chemical composition of which is, in mass percent, 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 being iron and unavoidable impurities; A process of heating the continuously cast slab to 1100-1150°C in a heating furnace and keeping the temperature for 2-3 hours; A process in which the continuous cast slab discharged from the heating furnace is rolled into a steel plate at a rolling temperature of 950 to 1050°C; The steel sheet thus obtained is subjected to laminar cooling at a cooling rate exceeding 20°C / s to a temperature of 200°C or less, and then three stages of heat treatment are sequentially carried out to obtain the low-temperature steel product, wherein the first stage of heat treatment has a temperature of 800-850°C, a heat-holding time coefficient of 1.0-1.5 min / mm, and a cooling method of water cooling, the second stage of heat treatment has a temperature of 700-720°C, a heat-holding time coefficient of 1.0-1.5 min / mm, and a cooling method of water cooling, and the third stage of heat treatment has a temperature of 600-620°C, a heat-holding time coefficient of 2.0-2.5 min / mm, and a cooling method of air cooling.

[0011] Preferably, in the step of "rolling the continuously cast slab discharged from the heating furnace to form a steel plate", the average reduction rate per pass is 15% or more, and the thickness of the resulting steel plate is 10 to 100 mm.

[0012] Preferably, in the process of "sequentially performing smelting, refining, and continuous casting to obtain a continuously cast slab," the superheat of molten steel in the tundish during continuous casting is ≦20°C, protective casting is adopted throughout the entire process, and the resulting continuously cast slab has inclusion grades of Class A, Class B, Class C, and Class D of ≦0.5.

[0013] Preferably, the degree of superheat of the molten steel in the tundish is 10 to 20°C.

[0014] Preferably, in the step of "laminar cooling the obtained steel sheet at a cooling rate exceeding 20°C / s to 200°C or less," laminar cooling is continued at a cooling rate of 30°C / s or less until the temperature falls to 30 to 200°C. [Effects of the Invention]

[0015] Compared with the prior art, the beneficial effects of the present invention include:

[0016] 1) Based on low carbon, it adopts a design scheme of high nickel, high vanadium, low manganese + molybdenum chromium copper, and the low carbon content ensures the weldability and toughness of the steel, while the low carbon and low manganese content prevents the formation of large amounts of segregation bands, ensuring uniform 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 form nano-sized complex carbide (MoV)C particles to increase strength, and the copper element increases the stability of retained austenite while also increasing strength.

[0017] 2) Based on the chemical composition, high-temperature rolling + rapid cooling + three-stage heat treatment, especially two heat treatments combining high and low temperatures in the two-phase region, allows for precise control of the steel structure, particularly the content, size, and morphology of highly stable austenite in the product steel plate, resulting in the comprehensive properties of ultra-high strength, high ductility, low yield ratio, excellent toughness, and excellent weldability. Furthermore, by using low carbon + high nickel as the stable austenite composition basis and combining it with the steel plate thickness, multi-scale multi-phase micro-precise control heat treatment process technology can simultaneously achieve stable control of the comprehensive properties such as ultra-high strength, good ductility, and low-temperature toughness, which is beneficial for improving the yield of large-scale industrial production.

[0018] 3) Overall, it achieves comprehensive property advantages, including ultra-high strength, high ductility, low yield ratio, excellent toughness, and excellent weldability in cryogenic environments. For example, the steel plate has an ultra-high yield strength of over 1000 MPa, fracture elongation of over 20%, yield ratio of less than 0.93, and impact strength of ≥ 150J in cryogenic environments of -80 to -196°C, meeting the application needs of marine engineering in extremely cold regions and deep seas. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a scanning electron microscope micrograph of martensite and retained austenite at a microscopic scale of 5 μm in the steel plate product of Example 5 of the present invention. [Figure 2] FIG. 2 is a transmission electron microscope image of nano-precipitates at a microscopic scale of 200 nm in the steel sheet product of Example 5 of the present invention. [Figure 3] FIG. 3 is an energy dispersive X-ray analysis diagram of Cu element distribution at a microscopic scale of 200 nm in the steel sheet product of Example 5 of the present invention. [Figure 4] FIG. 4 shows X-ray diffraction spectra of the austenite γ phase and the martensite α phase at 25° C., −80° C., and −196° C. of the steel plate product of Example 5 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The technical solutions of the present invention will be further described below in combination with specific embodiments, but the technical contents described below are merely exemplary embodiments of the present invention and do not limit the protection scope of the present invention, which is determined by the appended claims.

[0021] This embodiment provides a low-temperature steel, particularly a steel plate suitable for use in low-temperature environments such as marine engineering, having a chemical composition, in mass percent, of 0.07-0.13% C, 0.10-0.40% Si, 0.60-1.50% Mn, 5.00-9.0% Ni, 0.60-0.80% Cr, 0.60-0.90% Mo, 0.10-0.16% V, 0.40-0.80% Cu, P≦0.003%, S≦0.002%, and the balance being iron and unavoidable impurities.

[0022] The role of each chemical element in this embodiment will be described in detail below.

[0023] C: A strengthening element, an increase in the C content helps to improve the strength of the steel material, but has a negative effect on the ductility and toughness of the steel, particularly on weldability. In this embodiment, the C content is 0.07 to 0.13% by mass.

[0024] Si: A strengthening element that suppresses the formation of bainite, the precipitation and coarsening of carburized bodies, and improves toughness. In this embodiment, the Si content is 0.10 to 0.40% by mass.

[0025] Mn: Mainly used to increase the strength and toughness of steel, and at the same time, to increase the content of metastable austenite and form a multi-phase structure. In this embodiment, the Mn content is 0.60 to 1.50% by mass.

[0026] Ni: A major alloying element that has a solid solution strengthening effect and stabilizes austenite, lowering the Ar3 point, shifting the CCT curve to the right, promoting the formation of metastable austenite and increasing its stability, and forming a small martensite structure, thereby improving the toughness of the steel. Ni also affects dislocation glide and lowers the tough-brittle transition temperature of the steel. In this embodiment, the Ni content is 5.00 to 9.00% by mass.

[0027] Cr: Shifts the continuous cooling transformation curve of the steel to the right, improving the hardenability of the steel, i.e., the martensite forming ability. At the same time, Cr dissolves in untransformed austenite and increases the stability of supercooled austenite. In this embodiment, the Cr content is 0.60 to 0.80% by mass.

[0028] Mo: An element that improves hardenability, expands the austenite phase region, plays an important role in controlling the transformation structure, and effectively improves material strength. It lowers the transformation temperature and the critical cooling rate for bainite transformation, effectively improving the stability of toughness performance in the steel plate thickness direction. At the same time, Mo is an element with high affinity for C, and can form nano-sized multi-phase carbide (MoV)C particles together with V, resulting in a precipitation strengthening effect. In this embodiment, the Mo content is 0.60 to 0.90% by mass.

[0029] V: effectively refines the grain size of steel and simultaneously forms nano-sized multi-phase carbide (MoV)C particles together with Mo in the steel, which strongly hinders dislocation movement during deformation, thereby significantly improving strength. In this embodiment, the V content is 0.1 to 0.16% by mass.

[0030] Cu: Forms nano-sized Cu particle precipitation phase in the matrix structure, which plays a role in precipitation strengthening. Furthermore, Cu stabilizes retained austenite, improving the content and stability of retained austenite. Its content is controlled to 0.40-0.80%.

[0031] As described above, the chemical composition of this embodiment is based on a low-carbon, high-nickel, high-vanadium, low-manganese, molybdenum-chromium-copper design. The low carbon content ensures the steel's weldability and toughness, while the low carbon and manganese content prevents the formation of large amounts of segregation bands, ensuring uniform structure and performance. The high nickel content promotes the formation of retained austenite and, together with carbon, enhances austenite stability. Mo and vanadium form nano-sized multi-phase carbide (MoV)C particles, enhancing strength. Cu enhances strength while also increasing the stability of retained austenite. This results in comprehensive property advantages, including ultra-high strength (e.g., above 1 GPa), high ductility, low yield ratio, excellent toughness, and excellent weldability, which can meet the application needs of marine engineering in extreme cold regions and deep seas.

[0032] Specifically, the microstructure of the steel sheet of this embodiment is martensite plus austenite, with the volume percentage of martensite being 75% or more but less than 85% and the volume percentage of austenite being 15% or more but 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 determined by conventional techniques known in the art. For example, a metallographic specimen is prepared by taking a sample from the quarter-thickness position of the steel sheet in accordance with the international standard "ASTM E3-11 (2017) Standard Guide for Metallographic Specimen Preparation." The prepared metallographic specimen is then subjected to structural observation and measurement under an optical microscope and a scanning electron microscope at 500x and 1000x magnifications. The martensite and austenite are distinguished from each other based on the contrast difference in the micrographs, and the respective volume percentages of martensite and austenite are calculated.

[0033] The steel sheet of this embodiment also contains trace amounts of nano-precipitated precipitates of (MoV)C particles and Cu particles. The volume percentage of the precipitates is less than 0.1%, and the precipitates significantly improve the strength of the steel. In the present invention, the method for observing the precipitates is conventional, as known in the art. For example, a sample for observing the (MoV)C and Cu particles in the steel is taken from a position corresponding to 1 / 4 of the width and 1 / 4 of the thickness of the steel sheet, and a thin film specimen with a thickness of 30-50 μm is prepared. The thin area is obtained using a twin-jet electrolytic method. Observation, measurement, and analysis statistics are performed using a transmission electron microscope at a magnification of 20,000 to 100,000 times. Energy-dispersive X-ray analysis (EDS) is used to confirm that the precipitates are nano-precipitated precipitates of (MoV)C particles and Cu particles with a diameter of 5 nm or more, and the volume fraction of the precipitates is obtained.

[0034] The steel plate of this embodiment can have a thickness of 10 to 100 mm, and has excellent performance under low-temperature conditions and high performance stability. Specifically, at a low temperature of -196°C, the steel plate has a yield strength of 1200 MPa or more, a fracture elongation of 30% or more, a yield ratio of 0.84 or less, and an impact value KV2 of 150 J or more. At a low temperature of -80°C, the steel plate has a yield strength of 1000 MPa or more, a fracture elongation of 25% or more, a yield ratio of 0.94 or less, and an impact value KV2 of 160 J or more. 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 in accordance with the international standard "ASTM A673 / A673M-17 Standard Specification for Sampling Procedures for Impact Testing of Structural Steels," and the sample is subjected to low temperature performance testing in accordance with the international standard "ASTM E2298-18 Standard Test Method for Instrumented Impact Testing of Metallic Materials," to obtain the yield strength, fracture elongation, yield ratio, and impact value KV2 under low temperature conditions of -196°C and -80°C.

[0035] Furthermore, this embodiment also provides a method for manufacturing the aforementioned steel material. This manufacturing method employs the following process route: smelting → refining → continuous casting → heating → rolling → cooling → heat treatment. Hereinafter, the manufacturing method will be described in detail in accordance with the process order.

[0036] Process 1) Smelting → Refining → Continuous casting According to the aforementioned chemical composition, smelting, refining, and continuous casting are carried out in sequence to obtain a continuously cast slab, whose chemical composition is the same as the aforementioned chemical composition, i.e., in mass percent, 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 being iron and unavoidable impurities.

[0037] Step 2) Heating The continuously cast slab is heated to 1100 to 1150°C in a heating furnace and kept at that temperature for 2 to 3 hours. For example, the soaking zone of the heating furnace is set to a temperature value between 1100 and 1150°C, and the residence time of the continuously cast slab in the soaking zone of the heating furnace is controlled to 2 to 3 hours.

[0038] Process 3) Rolling The continuously cast slabs discharged from the heating furnace are rolled into steel sheets using a high-temperature hot rolling method, with the rolling temperature (rolling start temperature) being 950-1050°C. This method allows for sufficient recrystallization of austenite and refines the austenite grain size. High-temperature rolling also effectively improves rolling efficiency and reduces the load on the rolling mill.

[0039] Process 4) Cooling → Heat treatment The steel sheet rolled in step 3 is laminar-cooled at a cooling rate exceeding 20°C / s to 200°C or less. In this way, by performing online rapid cooling using a laminar-cooling system after rolling the steel sheet and controlling the final cooling temperature to 200°C or less, the transformed structure can be controlled to martensite.

[0040] After cooling, three stages of heat treatment are carried out in sequence to obtain the low temperature steel product.

[0041] The first heat treatment is performed at a temperature of 800-850°C, with a holding time of 1.0-1.5 min / mm and water cooling after the heat treatment. This heat treatment refines martensite and precipitates retained austenite, resulting in a martensite-austenite structure with a volume fraction of approximately 5-10% austenite. The second heat treatment is performed at a temperature in the two-phase region (between Ac1 and Ac3), specifically at 700-720°C, with a holding time of 1.0-1.5 min / mm and water cooling after the heat treatment. This second heat treatment further precipitates retained austenite, resulting in a martensite-austenite composite structure with a volume fraction of austenite of 15% or more but less than 25%. The temperature of the third stage heat treatment is controlled in the two-phase region (between Ac1' and Ac3', where Ac1' is smaller than Ac1 and Ac3' is smaller than Ac3 due to the change in the structural phase), specifically 600-620°C, the heat-holding time coefficient is 2.0-2.5 min / mm, and the cooling method after heat-holding is air-cooling. This third stage heat treatment allows the precipitation of a precipitate phase, which is a trace amount (less than 0.1% by volume) of nano-level (MoV)C particles and Cu particles.

[0042] As described above, the manufacturing method of this embodiment, based on the chemical composition, achieves precise control of the steel structure by high-temperature rolling + rapid cooling + three-stage heat treatment, especially by combining two heat treatments with different high and low temperatures in the two-phase region, and in particular, can precisely adjust the content, size and morphology of highly stable austenite in the product steel plate, and further obtain the comprehensive properties of ultra-high strength, high ductility, low yield ratio, excellent toughness and excellent weldability. In addition, the product performance is stable, which is advantageous for improving the yield of large-scale industrial production.

[0043] Here, the smelting and refining in step 1 can be performed using existing available technologies, for example, converter smelting or electric furnace smelting, and LF furnace + RH furnace refining technology is used. Preferably, when LF furnace + RH furnace refining technology is used, the LF furnace refining and RH furnace refining times are both controlled to 20 to 30 minutes.

[0044] Preferably, in step 1, the superheat of the molten steel in the tundish during continuous casting is ≦20°C, specifically 10-20°C, protective casting is employed throughout the entire process, and the non-metallic inclusion grades of Class A (i.e., sulfides), Class B (i.e., alumina), Class C (i.e., silicates), and Class D (spherical oxides) in the resulting continuously cast slab are all ≦0.5. In this way, precise control of the cleanliness of the steel can be achieved, improving the performance of the final steel product.

[0045] Preferably, in step 3, the average single-pass rolling reduction is 15% or more, and the thickness of the resulting steel plate is 10 to 100 mm. On the one hand, the combination of a large rolling reduction method and high-temperature rolling can fully realize austenite recrystallization and refine the primary austenite grain size of the steel plate. On the other hand, the present invention can be applied to the production of medium-thickness plates, which can meet the demand for steel used in marine engineering.

[0046] More preferably, in step 4, during laminar cooling of the steel sheet, the cooling rate is controlled to 30° C. / s or less, that is, more than 20° C. / s but not more than 30° C. / s, and the final cooling temperature is set to 30 to 200° C. In this way, the metallographic structure can be precisely adjusted.

[0047] In summary, the beneficial effects of the present invention are as follows:

[0048] 1) Based on low carbon, it adopts a design scheme of high nickel, high vanadium, low manganese + molybdenum chromium copper, with a low carbon content to ensure the weldability and toughness of the steel, and low carbon and manganese to avoid the formation of large amounts of segregation banding, ensuring uniform 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 form nano-sized complex carbide (MoV)C particles to increase strength, and copper increases the stability of retained austenite while also increasing strength.

[0049] 2) Based on the chemical composition, high-temperature rolling + rapid cooling + three-stage heat treatment, especially two heat treatments combining high and low temperatures in the two-phase region, allows for precise control of the steel structure, particularly the content, size, and morphology of highly stable austenite in the product steel plate, resulting in the comprehensive properties of ultra-high strength, high ductility, low yield ratio, excellent toughness, and excellent weldability. Furthermore, by using low carbon + high nickel as the stable austenite composition basis and combining it with the steel plate thickness, multi-scale multi-phase micro-precise control heat treatment process technology can simultaneously achieve stable control of the comprehensive properties such as ultra-high strength, good ductility, and low-temperature toughness, which is beneficial for improving the yield of large-scale industrial production.

[0050] 3) Overall, it achieves comprehensive property advantages such as ultra-high strength, high ductility, low yield ratio, excellent toughness, and excellent welding performance in ultra-low temperature environments. For example, the steel plate has an ultra-high yield strength of 1000 MPa or more, fracture elongation > 20%, yield ratio < 0.93, and impact value ≥ 150J in ultra-low temperature environments of -80 to -196°C, which can meet the application needs of marine engineering in extremely cold regions and deep seas.

[0051] In order to clarify the technical solution and advantages of one embodiment of the present invention, the present embodiment will be further described below in combination with Example 16 based on one embodiment of the present invention. Obviously, Example 16 described is only a part of the present invention, but not all of the examples.

[0052] Specifically, each of Examples 1 to 6 provides a steel material, and the manufacturing method for each is as follows.

[0053] (1) Smelting → Refining → Continuous Casting According to the above-mentioned chemical composition, smelting, LF refining, RH refining, and continuous casting were carried out in this order to obtain a continuously cast slab. The processing times of LF refining, RH refining, the superheat of the molten steel in the tundish during continuous casting, and the inspection results of the four types of inclusions A, B, C, and D in the obtained continuously cast slab are all shown in Table 1.

[0054] [Table 1]

[0055] The chemical composition of the continuously cast slab is shown in Table 2 in mass percent.

[0056] [Table 2]

[0057] (2) Heating → Rolling → Cooling The continuously cast slab was heated in a heating furnace and kept at that temperature for 2 to 3 hours, then removed from the heating furnace and rolled into a steel plate, which was then cooled in a laminar flow cooling system. The soaking temperature in the heating furnace, rolling temperature, average single-pass reduction, cooling rate, and final cooling temperature for each example are shown in Table 3.

[0058] [Table 3]

[0059] (3) Three-stage heat treatment The cooled steel sheet is subjected to three heat treatment stages in sequence according to the method in Table 4 below, thereby obtaining the above steel sheet product.

[0060] [Table 4]

[0061] The structure and mechanical properties of the steel plate products of each example were examined.

[0062] 1) The microstructure of each steel sheet product is martensite + austenite, with the martensite volume fraction being 75% or more but less than 85% and the austenite volume fraction being 15% or more but less than 25%. The austenite has excellent low-temperature stability. Furthermore, the steel sheet products also contain nano-precipitates of (MoV)C particles and Cu particles with a total volume fraction of less than 0.1%.

[0063] Figures 1 to 4 show the microscopic structure of a steel sheet product of the present invention, with Example 5 as a representative example. In Figure 1, the matrix structure of Example 5 is tempered lath martensite, and the white color indicates a high volume fraction of highly stable retained austenite. Figures 2 and 3 show nano-level (MoV)C particles and Cu precipitates, respectively, in Example 5. Figure 4 shows the X-ray diffraction patterns of the steel sheet product of Example 5 at 25°C, -80°C, and -196°C, showing the volume fractions of the austenite γ phase and the martensite α phase. The volume fractions of the γ phase are stable at 15.5%, 15.3%, and 15.3%, respectively.

[0064] 2) The mechanical properties of the steel plate products of each Example are as shown in Table 5. The steel plates of each Example all have an ultra-high yield strength of 1000 MPa or more, a fracture elongation of more than 20%, a yield ratio of less than 0.93, and an impact value KV2 ≥ 150J in an ultra-low temperature environment of -80 to -196°C, and have excellent overall performance at ultra-low temperatures, making them suitable for use as low-temperature steel in marine engineering fields such as extreme cold and deep sea environments.

[0065] [Table 5]

Claims

1. A low temperature steel for marine engineering, The low temperature steel has a chemical composition, in mass%, C: 0.07-0.13%, Si: 0.10-0.40%, Mn: 0.60 to 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 consisting of Fe and impurities.

2. 2. A low temperature steel for marine engineering according to claim 1, wherein said microstructure is martensite + austenite.

3. 3. A low temperature steel for marine engineering according to claim 2, in which the volume percentage of martensite is greater than or equal to 75% and less than 85%, and the volume percentage of austenite is greater than or equal to 15% and less than 25%.

4. 2. A low temperature steel for marine engineering according to claim 1, having a nano-precipitated precipitate phase of (MoV)C particles and Cu particles, the volume percentage of said precipitate phase being less than 0.1%.

5. 2. The low-temperature steel for marine engineering according to claim 1, wherein, under a low-temperature condition of -196 ° C, the low-temperature steel has a yield strength of 1200 MPa or more, a fracture elongation of 30% or more, a yield ratio of 0.84 or less, and an impact value of 150 J or more.

6. 2. The low-temperature steel for marine engineering according to claim 1, wherein, under a low-temperature condition of -80 ° C, the low-temperature steel has a yield strength of 1000 MPa or more, a fracture elongation of 25% or more, a yield ratio of 0.94 or less, and an impact value of 160 J or more.

7. A method for producing low temperature steel for marine engineering, comprising: a step of sequentially carrying out smelting, refining, and continuous casting to obtain a continuously cast slab, the chemical composition of which is, in mass percent, 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 balance being iron and unavoidable impurities; a step of heating the continuously cast slab to 1100 to 1150°C in a heating furnace and maintaining the temperature for 2 to 3 hours; a step of rolling the continuously cast slab discharged from the heating furnace into a steel plate at a rolling temperature of 950 to 1050°C; and laminar cooling the obtained steel plate at a cooling rate exceeding 20°C / s to 200°C or less, and then sequentially performing three-stage heat treatment to obtain the low-temperature steel product, wherein the first-stage heat treatment has a temperature of 800 to 850°C, a heat-holding time coefficient of 1.0 to 1.5 min / mm, and a cooling method of water cooling, the second-stage heat treatment has a temperature of 700 to 720°C, a heat-holding time coefficient of 1.0 to 1.5 min / mm, and a cooling method of water cooling, and the third-stage heat treatment has a temperature of 600 to 620°C, a heat-holding time coefficient of 2.0 to 2.5 min / mm, and a cooling method of air cooling.

8. 8. The method for producing a low-temperature steel for marine engineering according to claim 7, wherein in the step of "rolling the continuously cast slab discharged from the heating furnace to produce a steel plate", the average single-pass reduction rate is 15% or more, and the thickness of the obtained steel plate is 10 to 100 mm.

9. 8. A method for producing a low-temperature steel for marine engineering according to claim 7, wherein in the step of "sequentially carrying out smelting, refining and continuous casting to obtain a continuously cast slab", the superheat of the molten steel in the tundish during continuous casting is ≦20°C, protective casting is adopted throughout the entire process, and the inclusion grades of Class A, Class B, Class C and Class D in the obtained continuously cast slab are all ≦0.

5.

10. 10. The method for producing low temperature steel for marine engineering according to claim 9, wherein the degree of superheat of the molten steel in the tundish is 10 to 20°C.

11. 8. The method for producing a low-temperature steel for marine engineering according to claim 7, wherein in the step of "laminar cooling the obtained steel plate at a cooling rate exceeding 20°C / s to 200°C or less," laminar cooling is continued at a cooling rate of 30°C / s or less until the temperature drops to 30 to 200°C.

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