Steel Plate for Extremely Cold Marine Environment with Yield Strength of 900 MPa and Manufacturing Method

Abstract

The present invention provides a high-strength and high-toughness low-temperature offshore steel plate and a production method thereof. The chemical composition of the steel plate by weight percentage is as follows: C: 0.06% - 0.12%, Si: 0.22% - 0.4%, Mn: 1.2% - 1.5%, P: ≤0.02%, S: ≤0.01%, Als: 0.01% - 0.03%, Ni: 7% - 10%, Cr: 0.55% - 0.9%, Mo: 0.5% - 0.8%, Cu: 0.4% - 0.8%, Nb: 0.02% - 0.06%, V: 0.1% - 0.3%, Ti: 0.016% - 0.04%, N: 0.002% - 0.006%, and the balance is Fe and unavoidable impurities. The production method of the steel plate includes smelting, casting, electroslag remelting, heating, rolling, and quenching and tempering. The above-mentioned offshore steel plate has a yield strength ≥900 MPa, a tensile strength of 940 - 1100 MPa, an elongation ≥13%, a Charpy impact energy at -80°C ≥70 J, a ductile-brittle transition temperature ≤ -95°C, and a thickness ≤60 mm.

Description

Technical field

[0001] The present invention belongs to the field of steel material preparation, in particular to a steel plate for use in extremely cold marine environments with a yield strength of 900 MPa and a manufacturing method thereof. [Background Technology]

[0002] The 21st century is the century of the ocean. With the advancement of science and technology and the improvement of people's living standards, countries around the world are focusing their attention on the vast resources contained in the ocean. The sustained and rapid development of the offshore equipment industry in recent years has driven a large demand for offshore steel and promoted product upgrades. The market urgently needs high-strength, extra-thick, ultra-low-temperature tough offshore steel plates with excellent comprehensive performance and capable of operating in ultra-low-temperature environments of -80°C to -60°C.

[0003] As conventional oil and gas resources gradually deplete, the Arctic's vast energy reserves are attracting increasing attention. A survey conducted by the United States Geological Survey indicates that the Arctic Circle holds 90 billion barrels of oil, 47 trillion cubic meters of natural gas, and 44 billion barrels of liquefied natural gas reserves, respectively, representing approximately 22% of the world's total undiscovered energy resources. Furthermore, global warming is exacerbating Arctic warming, and the sea ice cover continues to decline rapidly, making it increasingly favorable for resource development and navigation. In recent years, Russia and Nordic countries have significantly accelerated Arctic oil and gas exploration and development. The increasing attention paid to the Arctic's potential for energy and trade routes is driving demand for and the development of large, high-tech polar offshore equipment. This is placing higher demands on ultra-low-temperature steels for polar ships, offshore structures, bridges, and vessels, capable of operating in polar temperatures ranging from -80°C to -60°C.

[0004] Offshore platforms operate in harsh environments. Besides conventional stresses, they must also consider a variety of factors, including strong winds, surges, tides, ice impacts, and earthquakes. This dictates the unique characteristics of offshore platform steel, requiring the materials used in platform construction to be adaptable to various sea conditions, including low temperatures, surges, and ice impacts. Steel plates are exposed to humid, high-salinity marine environments for extended periods, where they are exposed to humid air, seawater, and the adhesion of marine organisms, causing paint loss, surface corrosion, and corrosion fatigue. This degrades the plates' mechanical properties and shortens their service life, severely impacting the normal operation of offshore platforms. Furthermore, offshore platforms are located far from the coast and cannot undergo regular dry docking for repairs and maintenance like ships. To ensure the safe operation of offshore platforms in extremely cold and complex environments, such as the polar regions, there is an urgent need to develop high-quality, ultra-high-strength steel for offshore engineering applications with excellent comprehensive performance. Such high-strength steel plates must possess high strength, ultra-low temperature toughness, fatigue resistance, ease of welding, and resistance to marine corrosion and marine organism adhesion.

[0005] At present, the steel for offshore engineering can already meet most of the market demands in the offshore field. However, special steel with excellent comprehensive properties and a lower ductile-brittle transition temperature is still the development goal of various countries in the world. High-strength steel plates with high service safety have high research difficulties, strict production processes, high requirements for equipment, and great development difficulties. The patent "Corrosion-resistant offshore engineering steel with yield strength ≥ 890 MPa and its production method" with the publication number CN110331334A proposes a super-high-strength steel plate resistant to marine corrosion. This invention adopts a composition system with high C and high Mn, and a large amount of Cr, Re, and Sn elements are added to the steel to improve the corrosion resistance of the steel plate. However, the addition of these elements will significantly reduce the low-temperature impact toughness of the steel plate and cannot meet the toughness requirements of the steel plate in the extremely cold marine environment. The patent "An 890 MPa grade high-strength steel, steel pipe and its manufacturing method" with the publication number CN105586529B proposes a super-high-strength steel pipe. In the alloy composition design, high C and low Ni are adopted, and the strength is improved, but the low-temperature toughness is low, and the chemical composition and process proposed in this patent are only suitable for producing steel pipes. The patent "A steel plate with yield strength of 890 MPa grade and low welding crack sensitivity and its manufacturing method" with the publication number CN103898406B proposes a steel plate with a yield strength of 890 MPa grade and a low crack sensitivity index. In order to optimize the welding performance of the steel plate, the alloying elements are low, and the low-temperature impact toughness of the steel plate rolled by the conventional TMCP process is low.

Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the purpose of the present invention is to propose a steel plate for extremely cold marine environment with a yield strength of 900 MPa grade and a manufacturing method thereof. To achieve the purpose of the present invention, the inventor uses the composition design of Ni, Cr, Mo and micro-alloying elements in combination and the key production technology of steel plates for extremely cold and low-temperature offshore engineering. The yield strength of the steel plate is ≥ 900 MPa, and its mechanical properties and high service safety performance can meet the service conditions of offshore engineering equipment in extremely cold environments.

[0007] The purpose of the present invention is achieved as follows:

[0008] A steel plate for extremely cold marine environment with a yield strength of 900 MPa grade, by weight percentage, includes the following components: C: 0.06% - 0.12%, Si: 0.22% - 0.4%, Mn: 1.2% - 1.5%, P: ≤ 0.02%, S: ≤ 0.01%, Als: 0.01% - 0.03%, Ni: 7% - 10%, Cr: 0.55% - 0.9%, Mo: 0.5% - 0.8%, Cu: 0.4% - 0.8%, Nb: 0.02% - 0.06%, V: 0.1% - 0.3%, Ti: 0.016% - 0.04%, N: 0.002% - 0.006%, and the rest is Fe and unavoidable impurities.

[0009] Furthermore, the yield strength of the steel plate is ≥900 MPa, the tensile strength is 940 - 1100 MPa, the elongation is ≥13%, the Charpy impact energy at -80°C is ≥70 J, and the ductile-brittle transition temperature is ≤ -95°C.

[0010] Furthermore, the thickness of the steel plate is ≤60 mm.

[0011] Furthermore, the volume percentage of martensite in the microstructure of the steel plate is 90% - 95%.

[0012] C, as a basic strengthening element in steel, is the main element to ensure strength and hardness in the solution of the present invention. If the C content is too low, the solid solution content of C and the carbide content will decrease, and the grain refinement effect will be low, resulting in insufficient strength of the steel plate. If the C content is too high, the crack tendency will be large, reducing the low-temperature toughness of the steel plate. Therefore, the content of C element in the steel should be precisely controlled. The C content in the present invention is 0.06% - 0.12%.

[0013] Si can improve the strength of the steel plate. At the same time, as a deoxidizer, Si can reduce the O content. When the Si content is less than 0.22%, the deoxidation effect is not obvious. When the Si content is greater than 0.4%, it will cause the coarsening of the structure. The Si content in the present invention is 0.22% - 0.4%.

[0014] The atomic radius of Mn element is similar to that of Fe atom, and it can be dissolved in the Fe matrix in large amounts to improve the strength of the steel plate. When the Mn content is less than 1.2%, the contribution to the strength of the steel plate is small. At the same time, Mn is an element that expands the austenite phase region, improves the stability of austenite, and also strongly increases the hardenability of the steel plate. In the present invention, the billet is prepared by the electroslag remelting method, and the Mn element can be appropriately increased to further play the role of the Mn element. However, when the mass percentage content of the Mn element is greater than 1.5%, the martensite transformation tendency of the steel plate weakens during quenching, which is not conducive to quenching and tempering treatment. Moreover, the segregation of the Mn element will make the low-temperature toughness of the core of the thick plate poor. The Mn content is 1.2% - 1.5%.

[0015] The P and S elements are not beneficial to the mechanical properties of the steel plate, especially the elongation. It should be controlled that P ≤ 0.02% and S ≤ 0.01%.

[0016] Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is not good, and microalloying elements such as Ti cannot play the role of refining grains due to being oxidized. Ultra-high strength steel plates with larger thicknesses need to appropriately increase the Als content in the steel; on the contrary, if the Al element is too high, large inclusions will be formed. The Als content is 0.01% - 0.03%.

[0017] The role of Ni is to improve the toughness and hot workability of steel plates. Adding a large amount of Ni can enhance the fatigue resistance of the steel, reduce the notch sensitivity, and achieve a lower ductile-brittle transition temperature. Meanwhile, the addition of Ni can improve the hot cracking tendency of Cu in the steel. Ni has a certain effect on resisting atmospheric and salt spray corrosion, and the Ni content is 7%-10%.

[0018] Cr in the steel can effectively increase the strength of the steel plate. For ultra-high strength quenched and tempered steel with a relatively large thickness, it can improve the hardenability of the steel plate. However, if the Cr content is too high, it will reduce the impact toughness of the steel plate. The Cr content is 0.55%-0.9%.

[0019] Mo can improve the hardenability and tempering stability of the steel plate. Adding a certain amount of Mo to quenched and tempered steel can improve the hardenability of thick steel plates and appropriately increase the tempering temperature of the steel plate. Meanwhile, Mo can form fine carbides in the steel, effectively increasing the strength of the steel plate. Mo can also cooperate with Ni to play a certain corrosion resistance role. The Mo content is 0.5%-0.8%.

[0020] The addition of Cu can increase the strength and toughness of the steel plate. The effect of adding a small amount is similar to that of Ni, and the combined action with Ni can improve the low-temperature toughness of the steel plate. However, excessive addition will cause Cu embrittlement in the steel plate. The Cu content is 0.4%-0.8%.

[0021] Nb plays a role in solution strengthening in the steel. Dissolving in austenite significantly improves the hardenability of the steel. When Nb exists in the form of carbonitrides, it can refine the grains and improve the strength and low-temperature toughness of the steel plate. The Nb content is 0.02%-0.06%.

[0022] V can form V(C,N) particles in the matrix, which can play a role in refining and strengthening the grains. When dissolved in the solid solution at high temperature, it increases the hardenability; conversely, when it exists in the form of carbides, it reduces the hardenability. V increases the tempering stability of quenched steel and produces a secondary hardening effect. The V content is 0.1%-0.3%.

[0023] Ti has a very strong affinity with N, O, and C. It is a good deoxidizer and degassing agent and an effective element for fixing nitrogen and carbon. Although Ti is a strong carbide-forming element, it does not form complex compounds in combination with other elements. The Ti C, N compounds have strong binding forces and are not easily decomposed. In steel, they can only slowly dissolve into the solid solution when heated above 1000 °C. Before dissolution, titanium carbide particles have the effect of preventing grain growth. Ti is also one of the strong ferrite-forming elements, which significantly increases the A1 and A3 temperatures of steel. In low-alloy steel, Ti can improve plasticity and toughness. Since Ti fixes N, S and forms TiC, the strength of the steel is increased. After quenching and tempering, grain refinement and the precipitation of carbides can significantly improve the plasticity and impact toughness of the steel. The Ti content is 0.016% - 0.04%.

[0024] Part of N can be used in the matrix, which has the effects of solid solution strengthening and improving hardenability. In steel, it combines with microalloying elements such as Ti and V to play a precipitation strengthening role. The N content is 0.002% - 0.006%.

[0025] The second technical solution of the present invention is to provide a manufacturing method for a steel plate with a yield strength of 900 MPa for extremely cold marine environments, including smelting, casting, electroslag remelting, heating, rolling, and quenching and tempering;

[0026] Smelting:

[0027] 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.

[0028] Casting:

[0029] Full protection casting is carried out, and the superheat of the tundish molten steel is 20 - 30 °C.

[0030] Electroslag remelting:

[0031] The ingot-cast or continuous-cast billet is used as the electroslag electrode to prepare the electroslag remelted billet, and the steel billet is slowly cooled for ≥72 h after being taken off the production line.

[0032] Heating:

[0033] The cast billet is charged into the heating furnace at a furnace temperature of 400 - 700 °C and kept warm for 1 - 2 hours. The purpose is to keep the temperature consistent in the thickness direction of the steel billet at the low temperature stage and prepare for the uniform structure in the high temperature stage. During the subsequent heating process of the cast billet, the heating rate is controlled at 2 - 7 °C / min to avoid uneven heating inside the steel billet caused by too fast heating of the steel billet. The soaking temperature is 1300 - 1350 °C and kept warm for 1 - 3 hours. The purpose of high temperature heating and heat preservation is to fully soak the high alloy composition steel plate while avoiding abnormal growth of the electroslag remelting structure.

[0034] Rolling:

[0035] The starting rolling temperature is 1200 - 1330 °C, the average reduction per pass is 6 - 12%, the finishing rolling temperature is 900 - 1150 °C, and the rolling reduction ratio ≥ 5. The purpose of hot rolling at high temperature is to increase the reduction per pass in a single pass as much as possible in the stage with lower deformation resistance and improve the structure of the electroslag ingot. The purpose of the rolling process design to ensure the average reduction per pass is to utilize the relatively large decrease in surface temperature, increase the deformation amount of the steel plate core, improve the grain size of the steel plate core, promote the flattening and refinement of austenite grains, and the austenite grain size of the steel plate core is 6 - 8 grades. Since ferrite nucleates at the austenite grain boundaries, increasing the austenite grain boundary area can achieve the effect of grain refinement.

[0036] Quenching and tempering:

[0037] The quenching and tempering process is the key to affecting the low-temperature toughness of the steel plate. The process of two quenching + high-temperature tempering is adopted. The quenching temperature is 750 - 900 °C, the holding time is 0.8 - 2.5 min / mm, and the cooling rate at 1 / 4 of the steel plate thickness is ensured to be 5 - 30 °C / s during quenching and cooled to room temperature; the subcritical quenching temperature is 600 - 800 °C, the holding time is 0.8 - 2.5 min / mm, the tempering temperature is 400 - 700 °C, and the holding time is 2 - 4.5 min / mm. The purpose of high-temperature quenching is to fully austenitize the steel plate, ensure that the austenite of the steel plate is neither overheated nor underheated, improve the cooling capacity, accelerate the phase transformation under the premise of ensuring no cracking and deformation, make a large amount of the structure transform into martensite, and increase the hardened structure. High-temperature tempering can adjust the quality of the precipitated second phase while adjusting the matrix structure; the purpose of subcritical quenching is to form retained austenite and improve the low-temperature toughness of the steel plate; sufficient tempering in terms of temperature and time can make the tempering transformation products transform as much as possible, reduce the strength and hardness of the steel plate while improving the toughness and plasticity.

[0038] The marine atmospheric corrosion-resistant steel plate produced according to the above scheme has the following beneficial effects:

[0039] 1. Combining the composition design of alloying elements such as C, Mn, Ni, Cr, Cu, Mo, Nb, V, etc. and the key production technology of large-thickness arctic offshore steel plates, a 900 MPa grade offshore steel plate with a maximum thickness of 60 mm is produced through electroslag remelting + multi-stage heating + hot rolling at high temperature + two quenching + high-temperature tempering.

[0040] 2. The innovative alloy composition system of the present invention can ensure that the yield strength of the steel plate after quenching and tempering treatment is ≥ 900 MPa, the tensile strength is 940 - 1100 MPa, the elongation is ≥ 13%, the Charpy impact energy at -80 °C is ≥ 70 J, and the ductile-brittle transition temperature is ≤ -95 °C.

Description of the Drawings

[0041] Figure 1 It is the metallographic diagram of the microstructure of Example 1 of the present invention.

Detailed Embodiment

[0042] The following specific embodiments further illustrate the present invention in detail.

[0043] In the embodiments of the present invention, smelting, casting, electroslag remelting, heating, rolling, and quenching and tempering are carried out according to the component ratios of the technical solutions.

[0044] Heating: The cast billet is charged into a heating furnace at a furnace temperature of 400 - 700 °C and kept warm for 1 - 2 hours. During the subsequent heating process, the heating rate is controlled at 2 - 7 °C / min, and the soaking temperature is 1300 - 1350 °C and kept warm for 1 - 3 hours.

[0045] Rolling: The starting rolling temperature is 1200 - 1330 °C, the average reduction per pass is 6% - 12%, the finishing rolling temperature is 900 - 1150 °C, and the rolling reduction ratio ≥ 5.

[0046] Quenching and tempering: A two - stage quenching + high - temperature tempering process is adopted. The quenching temperature is 750 - 900 °C, the holding time is 0.8 - 2.5 min / mm, and the cooling rate at 1 / 4 of the steel plate thickness during quenching is 5 - 30 °C / s until it cools to room temperature; the sub - critical quenching temperature is 600 - 800 °C, the holding time is 0.8 - 2.5 min / mm, the tempering temperature is 400 - 700 °C, and the holding time is 2 - 4.5 min / mm.

[0047] Furthermore, the whole casting process is carried out under protective casting, and the superheat of the tundish molten steel is 20 - 30 °C.

[0048] Furthermore, in the electroslag remelting, the electroslag electrode is a die - cast billet or a continuous - cast billet.

[0049] Furthermore, during the electroslag remelting process, the slow - cooling time of the electroslag billet offline is ≥ 72 h.

[0050] Furthermore, the austenite grain size of the core of the steel plate after rolling is 6 - 8 grades.

[0051] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1. The process parameters of the steel in the embodiments of the present invention are shown in Table 2. The properties of the steel in the embodiments of the present invention are shown in Table 3.

[0052] Table 1 Chemical composition of the steel in the embodiments of the present invention

[0053] Component C Si Mn P S Als Ni Cr Mo Cu Nb V Ti N 1 0.073 0.34 1.25 0.011 0.01 0.015 8.22 0.75 0.53 0.56 0.043 0.211 0.023 0.0037 2 0.062 0.29 1.43 0.02 0.004 0.029 9.73 0.61 0.57 0.62 0.038 0.152 0.031 0.0023 3 0.096 0.22 1.33 0.013 0.002 0.022 8.81 0.82 0.61 0.44 0.058 0.118 0.017 0.0025 4 0.084 0.31 1.48 0.018 0.009 0.021 7.89 0.58 0.51 0.76 0.024 0.233 0.019 0.0054 5 0.067 0.27 1.39 0.011 0.007 0.021 9.2 0.69 0.79 0.41 0.033 0.187 0.026 0.0042 6 0.061 0.39 1.44 0.012 0.004 0.013 8.96 0.87 0.59 0.52 0.037 0.247 0.029 0.0051 7 0.078 0.24 1.29 0.014 0.009 0.028 7.09 0.71 0.62 0.47 0.021 0.3 0.033 0.0044 8 0.081 0.28 1.42 0.015 0.01 0.021 8.54 0.55 0.78 0.59 0.027 0.171 0.022 0.0021 9 0.092 0.4 1.21 0.011 0.005 0.013 9.49 0.83 0.67 0.63 0.059 0.218 0.038 0.0042 10 0.112 0.37 1.29 0.012 0.008 0.026 9.98 0.74 0.56 0.66 0.053 0.138 0.019 0.0058 11 0.107 0.25 1.49 0.017 0.007 0.023 7.01 0.56 0.71 0.78 0.055 0.292 0.026 0.0033 12 0.088 0.36 1.35 0.01 0.009 0.026 7.78 0.64 0.54 0.72 0.024 0.263 0.039 0.0049 13 0.094 0.27 1.47 0.02 0.004 0.022 7.18 0.9 0.74 0.69 0.035 0.111 0.016 0.0022 14 0.099 0.26 1.36 0.018 0.003 0.012 8.22 0.85 0.57 0.53 0.044 0.169 0.028 0.0059 15 0.102 0.34 1.45 0.019 0.008 0.016 9.31 0.79 0.64 0.64 0.047 0.227 0.018 0.0047 16 0.109 0.38 1.22 0.016 0.006 0.024 9.81 0.72 0.69 0.79 0.025 0.179 0.027 0.0035 17 0.119 0.27 1.40 0.011 0.01 0.027 9.69 0.66 0.76 0.74 0.034 0.212 0.017 0.0039 18 0.116 0.31 1.48 0.013 0.005 0.006 7.62 0.61 0.8 0.57 0.054 0.203 0.024 0.0043 19 0.104 0.37 1.23 0.014 0.009 0.023 7.93 0.57 0.58 0.43 0.051 0.171 0.037 0.0053 20 0.115 0.39 1.26 0.011 0.01 0.012 8.66 0.78 0.66 0.48 0.022 0.125 0.032 0.0024

[0054]

[0055] Table 3 Mechanical properties of the steel plate in the embodiments of the present invention

[0056]

[0057] As can be seen from the above, the steel plate obtained by the present invention has a yield strength ≥ 900 MPa, a tensile strength of 940 - 1100 MPa, an elongation ≥ 13%, a Charpy impact energy at -80 °C ≥ 70 J, a ductile-brittle transition temperature ≤ -95 °C, and a maximum thickness of 60 mm.

[0058] In order to describe the present invention, the present invention has been appropriately and fully described by way of examples above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill 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 replacements, improvements, etc. should be included within the protection scope of the present invention. The patent protection scope of the present invention shall be defined by the claims.

Claims

1. A steel plate for extremely cold marine environments with a yield strength of 900 MPa, characterized in that, The chemical composition of the steel plate is as follows by weight percentage: C: 0.06% - 0.12%, Si: 0.22% - 0.4%, Mn: 1.2% - 1.5%, P: ≤0.02%, S: ≤0.01%, Als: 0.01% - 0.03%, Ni: 7.0% - 9.31%, Cr: 0.79% - 0.9%, Mo: 0.5% - 0.8%, Cu: 0.53% - 0.8%, Nb: 0.044% - 0.06%, V: 0.169% - 0.3%, Ti: 0.016% - 0.04%, N: 0.002% - 0.006%, and the rest is Fe and inevitable impurities; The yield strength of the steel plate is ≥900 MPa, the tensile strength is 940 - 1100 MPa, the elongation is ≥13%, the Charpy impact energy at -80°C is ≥70 J, and the ductile-brittle transition temperature is ≤ -95°C; the thickness of the steel plate is 45 - 60 mm; the volume percentage of martensite in the microstructure of the steel plate is 92% - 95%; A manufacturing method of a steel plate with a yield strength of 900 MPa for extremely cold marine environments, including smelting, casting, electroslag remelting, heating, rolling, and quenching and tempering; Heating: The cast billet is charged into a heating furnace at a furnace temperature of 400 - 700°C and kept warm for 1 - 2 hours. The heating rate of the cast billet during the subsequent temperature rise process is controlled at 2 - 7°C / min, and the soaking temperature is 1300 - 1350°C and kept warm for 1 - 3 hours; Rolling: The rolling start temperature is 1200 - 1330°C, the average pass reduction rate is 6% - 12%, the final rolling temperature is 900 - 1150°C, and the rolling reduction ratio is ≥5; Quenching and tempering: A two-stage quenching + high-temperature tempering process is adopted. The quenching temperature is 750 - 900°C, the holding time is 0.8 - 2.5 min / mm, and the cooling rate at 1 / 4 of the steel plate thickness during quenching is 5 - 30°C / s and cooled to room temperature; the subcritical quenching temperature is 600 - 800°C, the holding time is 0.8 - 2.5 min / mm, the tempering temperature is 400 - 700°C, and the holding time is 2 - 4.5 min / mm; The casting is full-protection casting, and the superheat of the tundish molten steel is 20 - 30°C; In the electroslag remelting, the electroslag electrode is a die-cast billet or a continuous-cast billet; During the electroslag remelting process, the slow cooling time of the electroslag billet offline is ≥72 h; The austenite grain size of the core of the steel plate after rolling is 6 - 8 grades.

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