785MPa marine steel plate with good -80℃ impact toughness and its manufacturing method

Abstract

The invention provides a 785MPa marine steel plate with good impact toughness at 80°C and a manufacturing method thereof. The steel plate components are calculated by weight percentage: C: 0.06%-0.08%, Si: 0.26%-0.4%, Mn: 0.8%-1.3%, P: ≤0.02%, S: ≤0.01%, Als: 0.005%-0.02%, Ni: 5.3%-6.8%, Cr: 0.35%-0.5%, Mo: 0.2%-0.4%, Cu: 0.9%-1.5%, Nb: 0.02%-0.06%, V: 0.08%-0.15%, Ti: 0.01%-0.02%, N: 0.008%-0.012%, and the rest are Fe and unavoidable impurities. The production method of the steel plate includes smelting, low temperature heating, high-efficiency hot rolling, and normalizing processes. The yield strength of the above-mentioned marine engineering steel plate shall be ≥785MPa, the tensile strength shall be 865-980MPa, the elongation shall be ≥16%, the Charpy impact energy at -80℃ shall be ≥90J, and the thickness shall be ≤60mm.

Description

[Technical field]

[0001] The invention belongs to the field of steel material preparation, and particularly relates to a 785MPa marine engineering steel plate with good -80°C impact toughness and a manufacturing method thereof. [Background technology]

[0002] The 21st century is the century of the ocean. With the development of science and technology and the improvement of people's living standards, countries around the world are focusing on the huge resources contained in the ocean. In recent years, the sustained and rapid development of the offshore equipment industry has promoted the large demand for offshore steel and the upgrading of products. The market urgently needs high-strength, extra-thick, ultra-low temperature tough offshore steel plates with good comprehensive performance.

[0003] As conventional exploitable oil and gas resources are gradually depleted, the rich energy reserves in the Arctic region have received more and more attention. A survey completed by the United States Geological Survey shows that the reserves of oil, natural gas and liquefied natural gas in the Arctic Circle have reached 90 billion barrels, 47 trillion cubic meters and 44 billion barrels respectively, accounting for about 22% of the world's total undiscovered energy. At the same time, the global greenhouse effect has intensified the warming of the Arctic, and the sea ice coverage area has continued to decrease rapidly, which is increasingly conducive to resource development and ship navigation. In recent years, Russia and Nordic countries have significantly accelerated the exploration and development of oil and gas in the Arctic. The potential of energy and trade routes in the Arctic region has received more and more attention, which has promoted the demand and development of large-scale high-tech polar marine equipment, and put forward higher requirements for ultra-low temperature steel for polar ships and marine engineering that meet polar service conditions. High-strength polar low-temperature marine steel with excellent low-temperature toughness and easy weldability is the development trend.

[0004] The service environment of offshore engineering platforms is harsh. In addition to conventional forces, they must also consider the influence of multiple factors such as strong winds, surges, tides, ice collisions, earthquakes, etc. This determines the particularity of steel used in offshore platforms. The material selection for platform construction must be able to adapt to various sea conditions. At the same time, steel plates are in a humid and high-salinity marine environment for a long time, and are affected by humid air, seawater, and marine organisms, causing paint film shedding, steel plate surface corrosion, corrosion fatigue and other problems, which reduce the mechanical properties of the steel plates, shorten the service life, and seriously affect the normal use of offshore engineering platforms. In addition, offshore platforms are far away from the coast and cannot be regularly docked for repair and maintenance like ships. In order to enable offshore engineering platforms to be used safely in extremely cold and complex environments such as polar regions, it is urgent to develop a high-quality ultra-high-strength steel for offshore engineering with excellent comprehensive performance. This high-strength steel plate for offshore engineering must have the advantages of high strength, ultra-low temperature toughness, fatigue resistance, easy welding, resistance to marine environmental corrosion, and resistance to marine organism attachment.

[0005] At present, marine engineering steel can meet most of the market demand in the marine engineering field, but special steel with excellent comprehensive performance and lower ductile-brittle transition temperature is still the development goal of countries around the world. High-strength steel plates with high service safety have high scientific research problems, strict production processes, high equipment requirements, and great development difficulties. The patent "Ni-based low-temperature steel and manufacturing method, liquefied natural gas storage tank and transport ship hull" with publication number CN102586683B proposes a Ni-based low-temperature steel plate. The invention adopts a high-Ni and high-Cu component system, and the impact toughness has been improved. Only thin-gauge steel plates can be produced. The patent "A new type of steel plate with impact toughness higher than P690QL2 steel plate for LPG ship storage tanks and its production method" with publication number CN111621708A proposes an ultra-high-strength low-temperature steel, which adopts Nb+V micro-alloy component design and is in a quenched and tempered state. The patent does not use V, Ti, and N elements to increase the strength of the steel plate, and the strength of the steel plate cannot reach 785MPa. The patent "785MPa-grade extra-thick quenched and tempered rack steel plate for offshore platforms and its preparation method" with publication number CN111621711A proposed a rack steel plate for offshore platforms with a yield strength of 785MPa. The Ni and Cu alloy content in its chemical composition is low, and the steel plate cannot guarantee the low-temperature impact toughness of -80℃ with conventional rolling quenching and tempering treatment. The patent "Yield 785MPa-grade thick steel plate for offshore engineering and its production method" with publication number CN110616369A proposed a quenched and tempered 785MPa thick ultra-high strength offshore steel plate, and normalizing treatment production cannot obtain qualified comprehensive performance. [Summary of the invention]

[0006] In view of the shortcomings of the prior art, the purpose of the present invention is to propose an ultra-high strength steel plate with large thickness and good low-temperature impact toughness suitable for the field of marine engineering, whose mechanical properties and high service safety performance can meet the service conditions of -80℃ marine engineering equipment, and the -80℃ low-temperature impact energy ≥90J.

[0007] The present invention proposes a 785MPa marine steel plate with good -80°C impact toughness and a manufacturing method thereof, and the specific scheme is as follows:

[0008] A 785MPa marine steel plate with good -80℃ impact toughness comprises the following components, measured by weight percentage: C: 0.06%-0.08%, Si: 0.26%-0.4%, Mn: 0.8%-1.3%, P: ≤0.02%, S: ≤0.01%, Als: 0.005%-0.02%, Ni: 5.3%-6.8%, Cr: 0.35%-0.5%, Mo: 0.2%-0.4%, Cu: 0.9%-1.5%, Nb: 0.02%-0.06%, V: 0.08%-0.15%, Ti: 0.01%-0.02%, N: 0.008%-0.012%, and the rest is Fe and unavoidable impurities.

[0009] Further, the size of TiN in the steel plate is 10-20 nm;

[0010] Furthermore, in the steel plate, (Nb+V+Ti) / N accounts for 12%-24% by weight.

[0011] Furthermore, the size of Nb(CN) in the steel plate is 10-20 nm.

[0012] Furthermore, after normalizing treatment, the steel plate has a yield strength of ≥785 MPa, a tensile strength of 865-980 MPa, an elongation of ≥16%, and a Charpy impact energy of ≥90 J at -80°C.

[0013] Furthermore, the thickness of the steel plate is ≤60 mm.

[0014] The reasons for the composition design of the present invention are as follows:

[0015] C is a basic strengthening element in steel. It is the main element to ensure strength and hardness in the present invention. If the C content is too low, the C solid solution content and carbide content will be reduced, and the strength of the steel plate cannot be guaranteed. If the C content is too high, a large amount of hardened structure will be produced, which will reduce the elongation and low-temperature impact toughness of the steel plate. Therefore, the C content in the steel should be accurately controlled. The C content is 0.06%-0.08%.

[0016] Si is a deoxidizing element in the steelmaking process and also has a solid solution strengthening effect. In the present invention, a large amount of V and N elements are used, so Al element deoxidation cannot be used. Therefore, in order to fully deoxidize and improve the cleanliness of steel, the Si content needs to be appropriately increased. The Si content of the present invention is 0.26%-0.4%.

[0017] The atomic radius of Mn is similar to that of Fe, and it can be dissolved in Fe matrix in large quantities to improve the strength of steel plate. When the Mn content is less than 0.8%, it contributes little to the strength of steel plate. At the same time, Mn is an element that expands the austenite phase region and improves the stability of austenite. When the mass percentage of Mn element is greater than 1.3%, the segregation of Mn element will make the low-temperature toughness of the core of the thick plate poor. The Mn content is 0.8%-1.3%.

[0018] P and S elements have no benefit on the mechanical properties of steel plates, especially elongation, and P should be controlled to be ≤ 0.02% and S ≤ 0.01%.

[0019] Al is the main deoxidizing element in steel. However, when the steel contains a large amount of N, excessive Al content will form large inclusions, affecting the low-temperature impact toughness of the steel plate. The Al content should be reduced as much as possible, and the Als content is 0.005%-0.02%.

[0020] The role of Ni is to improve the toughness and hot workability of the steel plate. A large amount of Ni addition can obtain a lower tough-to-brittle transition temperature and improve the toughness and plasticity of the steel plate. At the same time, the addition of Ni can improve the hot cracking tendency of Cu in steel. Ni has a certain corrosion resistance effect. The Ni content is 5.3%-6.8%.

[0021] The Cr element in steel can effectively improve the strength of the steel plate; the C content in the steel is low, and the Cr content can be appropriately increased, but too high a Cr content will reduce the impact toughness of the steel plate. The Cr content is 0.35%-0.5%.

[0022] Mo element can improve the hardenability of steel plate. At the same time, Mo element can form fine carbides in steel, which can effectively improve the strength of steel plate. Adding appropriate amount of Mo element to heat-treated steel plate can improve the toughness of steel plate. The Mo content is 0.2%-0.4%.

[0023] Cu is an important element added in the present invention. It can improve the strength of the steel plate. Cu can work together with Ni in the steel to reduce the toughness-brittleness transition temperature of the steel plate. Adding excessive Cu alone will cause the low-temperature toughness of the steel plate to decrease and cause hot brittleness. However, a large amount of Ni is added in the present invention. Combined with the heating rolling process in the present invention, the Cu brittleness problem can be effectively avoided and the role of Cu can be played. The Cu content is 0.9%-1.5%.

[0024] Nb is a strong carbonitride forming element. When heated, the C and N-oxide particles of Nb that are not dissolved are distributed on the austenite grain boundaries, which can hinder the growth of austenite grains when the steel is heated; Nb can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness at the same time; when Nb in the steel is cooled after normalizing the steel plate, a large amount of Nb (CN) is precipitated, with a size of 10-20nm, which further promotes dislocation entanglement and refines the grains. The Nb content is 0.02%-0.06%.

[0025] Adding V to steel can refine the grain structure and improve the strength and toughness of the steel plate. When the addition amount is less than 0.08%, it cannot produce a significant strengthening effect with the N element; when it is greater than 0.15%, the toughness and weldability of the steel are reduced, and the surface quality of the steel plate is reduced. When an appropriate amount of N is added to the steel, V can be fully precipitated, so that the particle size and spacing in the steel are significantly reduced, precipitation strengthening occurs, and the strength and low-temperature toughness of the steel plate are improved. In the present invention, the V content is 0.08%-0.15%. .

[0026] Ti can improve the toughness of steel, refine grains, and play a role in the form of 10-20nm TiN. However, when it exceeds 0.02%, it is easy to form large particles of TiN. The Ti content is 0.01%-0.02%.

[0027] The N element can be combined with Nb, Ti, and V elements to form a fine dispersed N-de precipitate phase, which can effectively promote the nucleation of intracrystalline ferrite and effectively control the growth of original austenite grains. The weight percentage of (Nb+V+Ti) / N is 12%-24%. The increase in N content can increase the TiN in the steel. When the N content is increased, there is still a large amount of undissolved TiN in the high temperature stage. Increasing the nitrogen content can increase the amount of TiN in the steel at high welding temperature and improve the material's ability to inhibit the tendency of austenite grain coarsening during welding. VN can play a role in the normalizing process to improve the strength and toughness of the steel plate. However, when the solid solution N content is too large, the thermoplasticity of the steel decreases and the toughness of the steel plate decreases. Therefore, the N content is 0.008% to 0.012%.

[0028] The second technical solution of the present invention is to provide a method for manufacturing a 785MPa marine engineering steel plate with good -80°C impact toughness, including smelting, low temperature heating, high-efficiency hot rolling, and normalizing processes;

[0029] Smelting:

[0030] The molten steel is refined through a converter, LF furnace, RH or VD furnace to further reduce the contents of P, S and non-metallic inclusions.

[0031] Low temperature heating:

[0032] The ingot is placed in a heating furnace at a furnace temperature of 500-600℃ and kept warm for 1-2 hours. The purpose is to keep the temperature of the billet consistent in the thickness direction at the low temperature stage, so as to prepare for uniform organization in the high temperature stage. The heating rate of the ingot is controlled at 1.5-3℃ / min in the subsequent heating process to avoid the billet being heated too quickly and causing uneven heating inside the billet. The absorptive temperature is 1150-1250℃ and kept warm for 1-2.5 hours. The purpose of short-term absorptive heat preservation is to ensure that the C / N compounds are fully dissolved while avoiding the liquefaction of the Cu element on the surface of the high Cu content billet at high temperature for a long time, resulting in thermal cracking and preventing edge cracking during rolling.

[0033] Efficient hot rolling:

[0034] The starting rolling temperature is 1120-1200℃, the average pass reduction rate is 5%-12%, the average reduction rate of the last three passes is 10%-15%, and the final rolling temperature is 1000-1050℃. The purpose of high-temperature hot rolling is to ensure that the steel plate does not become Cu brittle while quickly rolling the steel plate to the target thickness at the stage of low deformation resistance. The purpose of emphasizing the average pass reduction rate is to avoid surface cracks and lamination defects on the surface of the steel plate due to the liquefaction of Cu at the grain boundary. The purpose of the last three rolling process design is to take advantage of the large drop in surface temperature, increase the deformation of the core of the steel plate, improve the grain size of the core of the steel plate, and promote the flattening and refinement of austenite grains. The single pass reduction rate of the last three passes near the final rolling temperature is most important for grain refinement. After the last three rolling processes, the austenite grains are refined to a grain size of more than 5 levels.

[0035] Normalizing process

[0036] The normalizing temperature is 780-880℃, and the normalizing holding time is 0.8-1.4min / mm. Normalizing can significantly improve the low-temperature impact toughness of the steel plate, while achieving the consistency of performance at different positions of the steel plate. The reason is that the normalizing treatment causes a large amount of dispersed precipitated phase VN to precipitate in the matrix to produce precipitation strengthening.

[0037] The offshore steel plates produced according to the above scheme have the following beneficial effects:

[0038] 1. The innovative alloy composition system of the present invention can ensure that the yield strength of the steel plate after normalizing treatment is ≥785MPa, the tensile strength is 865-980MPa, the elongation is ≥16%, and the -80℃ Charpy impact energy is ≥90J.

[0039] 2. By utilizing the key production technology of large-thickness marine engineering steel plates that combine the elements of C, Mn, Ni, Mo, V, Ti, Nb, and N, ultra-high-strength 785MPa marine engineering steel with a maximum thickness of 60mm can be produced. The efficient hot rolling + normalizing process adopted can greatly improve production efficiency.

Brief Description of the Drawings

[0040] Figure 1 This is the microstructure diagram of Example 1 of the present invention. [Specific implementation method]

[0041] In view of the composition and performance requirements of ultra-high strength marine steel in an extremely cold marine environment of -80°C, the present invention is further described below through examples.

[0042] The embodiment of the present invention performs smelting, low-temperature heating, high-efficiency hot rolling and normalizing according to the component ratio of the technical solution.

[0043] Smelting:

[0044] The molten steel is refined through a converter, LF furnace, RH or VD furnace to further reduce the contents of P, S and non-metallic inclusions.

[0045] Low temperature heating:

[0046] The ingot is placed in a heating furnace at a temperature of 500-600°C and kept warm for 1-2 hours. During the heating process, the heating rate is controlled at 1.5-3°C / min, and the aquifer temperature is kept at 1150-1250°C for 1-2.5 hours.

[0047] Efficient hot rolling:

[0048] The starting rolling temperature is 1120-1200°C, the average pass reduction rate is 5%-12%, the average reduction rate of the last three passes is 10%-15%, and the final rolling temperature is 1000-1050°C.

[0049] Normalizing process:

[0050] The normalizing temperature is 780-880℃, and the normalizing holding time is 0.8-1.4min / mm.

[0051] Furthermore, in the manufacturing method of the steel plate, the austenite grains are refined to a grain size of 5 or above after the last three rolling processes.

[0052] The chemical composition of the steel of the present invention is shown in Table 1, the production method is shown in Table 2, and the tensile and impact properties are shown in Table 3.

[0053] Table 1 Chemical composition of steel according to the present invention (wt%)

[0054]

[0055] Table 2 Steel preparation method according to the present invention

[0056]

[0057] Table 3 Mechanical properties of steel plates in the embodiments of the present invention

[0058]

[0059] As can be seen from the above, the yield strength of the steel plate of the present invention is ≥785MPa, the tensile strength is 865-980MPa, the elongation is ≥16%, and the -80℃ Charpy impact energy is ≥90J. By utilizing the key production technology of large-thickness marine steel plates matched with C, Mn, Ni, Mo, V, Ti, Nb, and N elements, ultra-high-strength 785MPa marine steel with a maximum thickness of 60mm can be produced, and the efficient hot rolling + normalizing process adopted can greatly improve production efficiency.

[0060] In order to describe the present invention, the present invention is appropriately and fully illustrated by the embodiments above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A 785MPa marine steel plate with good -80℃ impact toughness, characterized in that: The steel plate comprises the following components by weight percentage: C: 0.06%-0.069%, Si: 0.26%-0.4%, Mn: 0.8%-1.3%, P: ≤0.02%, S: ≤0.01%, Als: 0.005%-0.02%, Ni: 5.3%-6.8%, Cr: 0.35%-0.5%, Mo 0.2%-0.29%, Cu: 0.9%-1.5%, Nb: 0.02%-0.06%, V: 0.102%-0.15%, Ti: 0.01%-0.02%, N: 0.008%-0.012%, and the rest is Fe and unavoidable impurities; The size of TiN in the steel plate is 10-20 nm; The weight percentage of (Nb+V+Ti) / N in the steel plate is 12%-24%; The size of Nb (CN) in the steel plate is 10-20 nm; The yield strength of the steel plate is ≥785MPa, the tensile strength is 865-980MPa, the elongation is ≥16%, the -80℃ Charpy impact energy is ≥90J, and the thickness of the steel plate is ≤60mm; The manufacturing method of 785MPa marine steel plate with good -80℃ impact toughness includes smelting, low temperature heating, high-efficiency hot rolling and normalizing process. Low temperature heating The temperature of the ingot entering the furnace is 500-600℃, and it is kept warm for 1-2 hours. During the heating process, the heating rate is controlled at 1.5-3℃ / min, and the soaking temperature is 1195-1235℃ and kept warm for 1-2.5 hours; Efficient hot rolling The starting rolling temperature is 1150-1200℃, the average pass reduction rate is 5%-12%, the average reduction rate of the last three passes is 10%-15%, and the final rolling temperature is 1023-1050℃; Normalizing process The normalizing temperature is 780-880℃, and the normalizing holding time is 0.8-1.4min / mm; The manufacturing method of the steel plate refines the austenite grains to a grain size of 5 or above after the last three rolling processes.

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