Steel for polar marine engineering and method for producing the same

Abstract

The application discloses a kind of steel for polar ocean engineering and its production method, the chemical composition of steel is: C 0.02~0.07%, Si 0.10~0.25%, Mn 1.75~2.50%, P 0.022~0.026%, S≤0.008%, Nb 0.025~0.030%, V 0.02~0.03%, Ti 0.02~0.03%, Cr 0.15~0.35%, Ni 0.15~0.35%, Co 0.02~0.05%, Als 0.035~0.045%, N 0.010~0.012%, Cu 0.15~0.22%, Sn 0.03~0.05%, Sb 0.03~0.05%, RE 0.03~0.05%.After smelting, slab continuous casting, casting blank heating, rolling and cooling, the Rp0.2 of steel is 420~480MPa, A≥26%, -80 ℃ impact energy≥210J, corrosion resistance rate is less than 0.080mm / a, fatigue life under peak stress 200MPa load is greater than 2 million times, maximum thickness can reach 150mm, Z direction performance is greater than or equal to 45%.The steel plate provided by the application has higher strength and excellent low temperature toughness, and also has good corrosion resistance and fatigue resistance.

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation technology, and specifically relates to a steel for engineering in extremely cold marine environments and its production method. Background Technology

[0002] Steel used in marine equipment manufacturing faces complex marine environments with varying waves, temperatures, humidity, and salinity, placing higher demands on its performance, particularly its corrosion and fatigue resistance. Steels require higher strength, greater thickness, low-temperature impact toughness, good Z-axis properties, and simultaneously good corrosion resistance and fatigue resistance. Existing steels struggle to meet all these performance requirements simultaneously.

[0003] Patent application number 201510314696.X discloses a material with a yield strength ≥420MPa, tensile strength: 640~850MPa, elongation A ≥25%, yield strength ratio ≤0.64, and corrosion resistance index I not less than 6.44.

[0004] A low yield strength ratio weathering steel for wind turbine towers with KV2≥200J at -50℃ and KV2≥185J at -60℃, and its production method. However, it failed to provide data on the production of thick steel plates, and lacked evaluations of corrosion resistance and fatigue resistance, making it unsuitable for the needs of steel plates used in marine engineering equipment.

[0005] Patent application number 202010557893.5 discloses a 550MPa grade weathering steel plate with excellent weldability and its manufacturing method. It employs an optimized TMCP process, achieving a yield strength ≥460MPa, tensile strength ≥570MPa, Charpy impact energy (single value) ≥120J at -40℃, impact toughness KV2 ≥100J at -40℃, and weld heat-affected zone (HAZ) Akv ≥100J at -40℃. However, its low-temperature toughness is only at -40℃, and the issues of corrosion resistance and fatigue resistance remain unresolved.

[0006] Patent application CN201410036368.3 discloses a corrosion-resistant steel plate for use in the South China Sea marine environment and its production process. The production process includes converter smelting, LF refining, vacuum degassing, continuous casting, controlled rolling and controlled cooling, etc. Theoretically, the microstructure of this steel plate is a single-phase polygonal ferrite fine structure (average grain size 10.17μm). In actual industrial production, it inevitably contains a very small amount of pearlite. Compared with conventional ship hull structural steel EH36, its corrosion resistance in marine environments (marine atmosphere, tidal range, full immersion, etc.) is improved by more than 50%, and it has good strength-toughness matching and weldability. However, its low-temperature toughness is insufficient, and its fatigue resistance has not been evaluated.

[0007] Patent application number 201910712227.1 discloses a high-fatigue structural steel with a yield strength of 345MPa and its manufacturing method. The chemical composition of the steel is: C 0.13%~0.16%, Mn 1.30%~1.60%, Nb 0.020%~0.050%, Alt 0.020%~0.030%, Ti≤0.010%, Si≤0.12%, P≤0.010%, S≤0.005%, with the balance being iron and unavoidable impurities. By employing a high-pressure reduction + controlled cooling process, the resulting steel plate exhibits good comprehensive mechanical properties and a relatively good surface quality. However, its corrosion resistance was not evaluated; only the impact toughness at -20℃ was assessed, which is far from meeting the requirements for use in different marine environments.

[0008] Patent application number 202110068169.0 discloses an engineering corrosion-resistant fatigue steel and its preparation method. Based on the main elements of E690 steel (C 0.04%~0.07%, Si 0.20%~0.26%, Mn 1.45%~1.60%, P≤0.01%, S≤0.015%, Cr 0.44%~0.50%), element regulation and characteristic element addition are carried out, Cu 0.28%~0.66%, Ni 0.76%~1.55%, Sb 0.03%~0.12%, with the remainder being Fe and unavoidable impurities. Its corrosion fatigue strength can be increased by up to 52%, but the low-temperature toughness of the steel is not evaluated. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a steel for engineering in extremely cold marine environments and its production method. This steel is characterized by low carbon and low alloying, and incorporates multi-element alloying strengthening elements such as Cr, Ni, and Co, as well as precipitation strengthening elements such as V and N. It also features the addition of P, Al, Sb, Sn, and RE, thereby improving the steel plate's strength and low-temperature toughness, while also exhibiting good corrosion resistance and fatigue resistance. This invention solves the problems of insufficient corrosion resistance and fatigue resistance in existing marine engineering steel plates, particularly addressing the issue of poor impact toughness in ultra-low temperature environments.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] This invention is based on a steel plate with excellent low-temperature toughness, high strength, corrosion resistance, and fatigue resistance. The chemical composition of the steel plate in this invention is as follows: C: 0.02%–0.07%, Si: 0.10%–0.25%, Mn: 1.75%–2.50%, P: 0.022%–0.026%, S: ≤0.008%, Nb: 0.025%–0.030%, V: 0.02%–0.03%, Ti: 0.02%–0.03%, Cr: 0.15%–0.35%, Ni: 0.15%–0.35%, Co: 0.02%–0.05%, Als: 0.035%–0.045%, N: 0.0 10%–0.012%, Cu: 0.15%–0.22%, Sn: 0.03%–0.05%, Sb: 0.03%–0.05%, RE: 0.03%–0.05%, of which 0.05≥(Nb+V+Ti) / Mn≥0.03, 8≥Si / Al≥2, (Cu+P+Co) / Cr≥0.65, 20≥Cr / Co≥3.5, Ni / Cr≥0.6, (Nb+V+Ti+Als) / N≥9, Ni / Cu≥0.8, 36Nb / Mn≥0.40, (Nb+V+Ti) / RE≥1.4, RE / P≥1.2, the remainder being Fe and unavoidable impurities.

[0012] The above-mentioned alloying elements and their contents were selected in this invention because of their respective roles in improving the strength, toughness, corrosion resistance, and fatigue resistance of marine engineering steel plates.

[0013] C: C and Cr can form alloy cementite (Fe·Cr)3C, and can also form carbides, such as Cr7C3, Cr 23 C6 and other carbides have higher melting points, hardness, wear resistance, and stability than Fe3C, thus improving steel strength. C, along with strong carbide-forming elements such as V, Nb, and Ti, preferentially forms VC, NbC, and TiC carbides, which have the highest stability, melting point, hardness, and wear resistance. Therefore, C is the most effective element for improving steel plate strength; however, its content below 0.02% will significantly reduce the strength of the steel plate. But C greatly affects the low-temperature toughness, elongation, and weldability of steel. From the perspective of improving the toughness, corrosion resistance, fatigue resistance, and weldability of steel, the C content in steel should be controlled to be appropriately low. Therefore, the C content in this invention is selected to be between 0.02% and 0.07%.

[0014] Si: Si is an essential element for deoxidation in steelmaking and has a certain solid solution strengthening effect. Although Si can increase the strength of steel plates, it reduces the critical cooling rate of martensitic transformation, severely impairing the low-temperature toughness, elongation, and weldability of high-strength steel plates. Si not only promotes the formation of martensitic islands, but also results in larger and unevenly distributed martensitic islands, severely damaging the toughness of the weld heat-affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. A certain Si content can effectively improve the steel's resistance to marine corrosion, and the combined addition of Si and Al can improve corrosion resistance and high-temperature oxidation resistance. In this invention, the Si content is controlled at 0.10%–0.25%, with 8 ≥ Si / Al ≥ 2.

[0015] Mn: Mn is a key element for improving strength and toughness, significantly enhancing the hardenability of steel, and is very inexpensive, making it a major additive element in steel. When the carbon content is low, a higher Mn content can effectively improve the hardenability of steel, and improve the strength of the steel plate by refining the microstructure and promoting bainite transformation, while also exhibiting excellent low-temperature toughness. Mn expands the austenite region and promotes grain enlargement, requiring the combined addition of grain-refining elements Nb, V, and Ti to further refine the grains and improve the fatigue resistance of the steel. However, Mn is prone to segregation during the solidification process of molten steel, exacerbating segregation and porosity in the center of the billet, leading to low low-temperature toughness of high-strength steel plates and cracks in welded joints. This needs to be improved by optimizing the continuous casting process and heating process. In this invention, the Mn content is selected to be 1.75%–2.50%, and 0.05≥(Nb+V+Ti) / Mn≥0.03.

[0016] Polymer (P): Polymer has a strong solid solution strengthening effect in steel. When added as an alloying element to low-alloy structural steel, it can improve its strength and atmospheric corrosion resistance. A P content of 0.02% or higher can significantly improve corrosion resistance. However, excessive P content can negatively impact the low-temperature toughness of the base metal and the toughness of the weld heat-affected zone. Therefore, its content should be controlled within a reasonable range. In this invention, the P content is controlled between 0.022% and 0.026%.

[0017] Sulfur (S): Severe segregation of sulfur in steel deteriorates its quality. S is an inclusion-forming element, forming inclusions such as FeS and MnS, which reduces the ductility of the steel. Furthermore, the vicinity of these inclusions becomes a corrosion initiation site, negatively impacting the corrosion resistance of the steel plate. FeS, due to its low melting point, easily melts at grain boundaries, weakening the bonding force between grains and leading to hot brittleness in the steel. Therefore, a certain amount of manganese (Mn) should be added to form MnS, which has a higher melting point and deformability. Thus, in this invention, its content is controlled to be less than or equal to 0.008%.

[0018] Co (Co) primarily functions as a solid solution strengthener in steel, improving its oxidation resistance and corrosion resistance. Co increases the interaction between Fe atoms, lowers the critical concentration for Cr atom cluster formation, and thus enhances the stability of Cr atom clusters. When Co and Cr atoms act simultaneously in steel, a smooth passivation film forms on its surface, exhibiting high structural stability and effectively protecting the matrix, resulting in excellent corrosion resistance. Co promotes precipitation hardening by fostering more nucleation sites for (Ti, Al)Ni3 precipitates, thereby improving strength and toughness. The addition of Co has little effect on the Ni / Ti precipitation ratio, but it can reduce the size of the precipitated phases. Co increases the nucleation rate of Ni3Ti and simultaneously increases the number density of precipitates, meaning that Co can make the distribution of precipitates more dispersed, improving the fatigue resistance of steel and enhancing the precipitation strengthening effect of the precipitates. Therefore, in this invention, its content is controlled at 0.02%–0.05%, with 20 ≥ Cr / Co ≥ 3.5.

[0019] Cr: Cr can improve the strength and hardness of steel. Cr is a ferrite-forming element, which helps increase the ferrite content in steel, thereby improving its low-temperature toughness. Cr is an element that improves the corrosion resistance of steel; however, adding Cr alone can sometimes reduce corrosion resistance, even making it worse than ordinary carbon steel. It needs to be used in combination with other corrosion-resistant alloying elements, such as Cu, P, and Co, to significantly improve corrosion resistance. In this invention, its content is controlled at 0.15%–0.35%, and (Cu+P+Co) / Cr ≥ 0.65.

[0020] Ni: Ni strengthens ferrite and refines pearlite in steel, resulting in increased strength overall, with little effect on plasticity. Ni can improve the fatigue resistance of steel and reduce its notch sensitivity, thus improving the fatigue performance of steel. Ni does not form carbides; instead, it strengthens ferrite by forming simple substitutional solid solutions, lowering the ductile-brittle transition temperature and improving the low-temperature toughness of steel. A certain Ni content ensures sufficient hardenability and uniform properties in the thickness direction of the steel plate, while also ensuring a balance between strength and toughness and low-temperature toughness. Adding Ni to steel can also reduce copper embrittlement in Cu-containing steel, mitigate intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of the steel plate. The combination of Ni and Cr can significantly improve the corrosion resistance of steel. In this invention, the Ni content is controlled at 0.15%–0.35%, Ni / Cr ≥ 0.6, and Ni / Cu ≥ 0.8.

[0021] Cu: Cu is the most important and widely used alloying element in corrosion-resistant steel. Cu can activate the cathode, promote anodic passivation, and slow down corrosion. During corrosion, a copper-rich phase forms on the surface of the steel. Between the corrosion layer and the copper-rich layer, there is a dense and strongly adherent intermediate layer, which further alleviates corrosion. In particular, when used in combination with P, it can significantly improve the resistance to marine atmospheric corrosion and seawater corrosion. In addition to reducing copper embrittlement in copper-containing steel and mitigating intergranular cracking during hot rolling, the composite addition of Cu and Ni is also important because both Cu and Ni are austenite stabilizing elements. The composite addition of Cu and Ni can significantly reduce Ar3, increasing the driving force for the austenite-to-ferrite phase transformation. At the same time, Cu can accelerate the high-temperature strain-induced precipitation of niobium carbonitride, raise the recrystallization stopping temperature, and facilitate controlled rolling in the non-recrystallization zone to refine phase transformation products and improve the fatigue resistance of the material. In this invention, its content is controlled at 0.15% to 0.22%.

[0022] Nb: Nb is an important element in controlled-rolled and controlled-cooled steel. As a strong carbide-forming element, Nb forms NbC and NbN two-phase particles with C and N, which are crucial elements in controlled-rolled and controlled-cooled steel. This effectively refines the grain structure, thereby simultaneously improving strength and low-temperature impact toughness. The combined addition of Nb and Mn effectively inhibits austenite recovery and recrystallization during rolling. On the one hand, it increases the austenite recrystallization temperature, thus increasing the rolling temperature and reducing the load on the rolling mill; on the other hand, it effectively refines the phase transformation structure of the steel plate, thereby simultaneously improving strength and low-temperature impact toughness; it also prevents intergranular corrosion of the steel by oxidizing media. Nb can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of the steel. In this invention, its content is controlled at 0.025%–0.030%, with 36Nb / Mn ≥ 0.40.

[0023] Vanadium (VC) has a strong affinity for both oxygen (O) and nitrogen (N), making it a strong carbide-forming element. VC generally has high dispersion and is extremely stable, thus facilitating deoxidation and degassing to achieve a dense, fine-grained structure, improving plasticity, toughness, and strength. Its impact performance and fatigue strength are higher than vanadium-free steel, exhibiting high strength and toughness at both high and low temperatures (<0°C). Due to the high dispersion of VC, it prevents coarse grain growth in the weld, thus improving the weldability of the steel. However, heating to the VC melting temperature will cause strong grain growth in the steel. When dissolved in a solid solution at high temperatures, it increases hardenability; conversely, if present in carbide form, it decreases hardenability. VC increases the tempering stability of quenched steel and produces a secondary hardening effect. VC can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of the steel. In this invention, its content is controlled at 0.020%–0.030%.

[0024] Ti: Ti has a strong affinity for N, O, and C, and its affinity for S is stronger than that for iron. Therefore, it is an excellent deoxidizer and degassing agent, and an effective element for fixing N and C. Ti is a strong carbonitride forming element; trace amounts of Ti can combine with N in steel to form TiN, preventing the growth of austenite grains during homogenization and also preventing austenite grain growth in the weld heat-affected zone, thereby improving weldability. TiC and TiN have strong, stable, and non-decomposing bonding forces. In steel, they only slowly dissolve into the solid solution when heated to above 1000℃, thus significantly controlling grain growth in the weld heat-affected zone and improving the weldability of the material. Because Ti fixes N and S and forms TiN, the plasticity and impact toughness of steel can be significantly improved. Ti can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of steel. However, Ti has a strong affinity for N and O and readily forms TiN and TiO2, which at lower temperatures leads to more non-metallic inclusions and subcutaneous porosity defects. The content of this invention is controlled at 0.02% to 0.03%.

[0025] Nitrogen (N): Like carbon (C), nitrogen (N) can dissolve in fe to form interstitial solid solutions. N expands the austenite phase region of steel and is a strong austenite-forming element. Within certain limits, it can replace some nickel. N that penetrates the steel surface can combine with elements such as Nb, Al, V, and Ti to form extremely stable nitrides, improving the corrosion resistance of the steel. However, excessive residual nitrogen in steel can lead to a loose macrostructure or porosity. Therefore, a certain amount of Al needs to be added to nitrogen-containing steel to form stable AlN, preventing nitrogen from escaping during solidification and forming defects such as porosity. Therefore, in this invention, the N content is controlled at 0.010%–0.012%, and (Nb+V+Ti+Als) / N ≥ 9.

[0026] Al: Al is mainly used for deoxidation and grain refinement. Al reacts with N or O to form effective fine dispersions, inhibiting grain growth during steel heating. During steel cooling, it promotes austenite decomposition, improving hardenability. It also acts as a nucleation point for recrystallization, promoting ferrite nucleation and refining grains, thus improving fatigue resistance. AlN itself has high stability during heating, thereby improving the thermal stability of steel, reducing overheating tendency, and improving oxidation resistance. Al forms an effective surface hardening layer through the lower-temperature diffusion (nitriding) of N, improving oxidation and corrosion resistance. Adding a certain amount of Si during Al deoxidation can significantly improve the deoxidation effect of Al; however, excessive Al content can lead to abnormal structures and promote graphitization. Therefore, the Al content in this invention is controlled at 0.035%–0.045%.

[0027] Sn and Sb: At austenitic temperatures, antimony (Sb) in steel precipitates at MnS inclusions and along the original austenite grain boundaries, thus inhibiting the enrichment and precipitation of MnS inclusions at the grain boundaries. Sb can also refine the grain size of secondary recrystallization, refining the steel's microstructure and improving its toughness, thereby enhancing the steel's fatigue resistance and corrosion resistance. Sn and Sb help improve the corrosion resistance of materials; both adding Sn alone and adding Sn and Sb in combination significantly improve the corrosion resistance of the material. Sn and Sb form a corrosion-resistant oxide film of SnO2 and Sb2O5 on the steel surface, which effectively prevents the interaction between the matrix and the corrosive medium, inhibiting the corrosion of the steel in the corrosive medium. Sn and Sb, after hydration, yield Sn... 2+ Sb 3+ The precipitates formed in the anodic micro-regions fill corrosion cracks or cavities, enhancing the resistance to corrosion. - The ability to penetrate; Sn and Sb can also be absorbed by Cl - Hydration under environmental media inhibits Fe 3+ Hydrolysis produces H + The process improves the pH value of the corrosion micro-zone and alleviates the anodic dissolution process. Sn and Sb, acting as corrosion inhibitors, alter the anode and cathode reaction processes, significantly improving the steel's resistance to marine corrosion. In this invention, their contents are controlled at 0.03%–0.05%.

[0028] Rare earth elements (REs) are highly reactive and have strong binding properties. Adding REs to steel can improve solidification structure, alter solid-state phase transformation structure, form harmless low-melting-point inclusions, strengthen interfaces through segregation, and passivate surface rust layers. REs can increase the self-corrosion potential and polarization resistance of weathering steel, thereby inhibiting anodic reactions, increasing the resistance to the entire electrochemical reaction, and significantly reducing the corrosion rate of the steel. REs are enriched at grain boundaries through diffusion mechanisms, inhibiting the segregation of inclusions at grain boundaries, and improving the low-temperature performance and corrosion resistance of steel. Adding REs to phosphorus-containing steel can reduce macroscopic segregation, reducing the segregation of phosphorus at grain boundaries and ferrite interfaces, making the distribution of phosphorus in the steel more reasonable, thereby significantly improving the toughness, corrosion resistance, and fatigue resistance of the steel. However, REs are a scarce resource, and their addition must be controlled. In this invention, the RE content is controlled at 0.03% to 0.05%, (Nb+V+Ti) / RE ≥ 1.4, and RE / P ≥ 1.2.

[0029] The above describes the content range and function of various added elements. The manufacturing method of this invention for producing steel for extreme cold marine environment engineering includes:

[0030] 1. The steel is smelted according to the above composition, and the process includes:

[0031] 1) During converter smelting, the content of elements such as P and S is adjusted to be within the range of this invention. During the smelting process, the molten iron is first desulfurized and pretreated. After desulfurization, the S content in the molten iron is ≤0.0025%. A combined top and bottom blowing process is adopted, and the converter tapping temperature is 1630~1650℃. Then, Si-Ca wire feeding treatment is carried out, and the Ca content is controlled between 0.0015% and 0.0025%.

[0032] 2) Refine the molten steel. 7-9 minutes before the end of vacuum treatment in the LF furnace, add 1.95-3.25 kg / ton of 20% rare earth alloy to the LF furnace and then purge with argon for 5-10 minutes. Then perform RH treatment for 30-35 minutes. Nitrogen is purged throughout the RH treatment process. Control the [H] in the steel to be ≤1.0 ppm and [O] to be ≤20 ppm. The net circulation time before removal is 6-10 minutes.

[0033] 3) Add Sn and Sb elements before the RH treatment is completed, ensuring that the amount added is 1.2 to 1.3 times the target control amount, so as to ensure that the endpoint content can be controlled within the target range.

[0034] 2. Cast the molten steel obtained in step 1 into the required continuous casting billets. To control the content of isoaxial crystals in the continuous casting billets, the superheating temperature of the tundish is 25-35℃. A lower superheating temperature can reduce the solidification time of the molten steel and reduce the segregation of elements such as carbon and manganese in the center of the billet, thereby reducing defects such as porosity and shrinkage cavities in the billet and ensuring the Z-axis properties and fatigue resistance of the steel plate. Full-process protective casting is adopted, and the billet pulling speed is controlled at 1.1-1.3 m / min, with a secondary cooling water ratio of 0.80-0.90 m³ / min. 3 / t, so that the equiaxed crystal ratio of the continuously cast billet is >25.0%, electromagnetic stirring is used at the end of solidification to make the molten steel uniform and achieve high strength and high density solidification as soon as possible, and heavy reduction is used in the continuous casting process, with a reduction of 15.0~20.0mm.

[0035] 3. In order to control the grain size of the continuously cast billet to be no more than 500μm, the continuously cast billet is rapidly cooled. The initial cooling temperature is 950~980℃, the cooling rate is 8.0~9.0℃ / s, and after cooling to 680~720℃, it is put into a slow cooling pit for slow cooling, and then cooled to below 150℃ at a cooling rate of 3.0~20.0℃ / h.

[0036] 4. The continuously cast billet obtained in step 3 is fed into a heating furnace for heating. A segmented heating process is adopted. The billet is fed into the furnace at a temperature of 650–750℃ and held for 1.0–2.0 hours to release internal stress. Below 950℃, a slow heating process is used to further release internal stress caused by cooling and heavy pressure, and to prevent excessively rapid heating from causing new temperature stress. The heating rate is controlled at 6–8℃ / min, and the billet is held at 950℃ for 15–30 minutes. Above 950℃, a rapid heating process with appropriately extended holding time is adopted to prevent austenite grain coarsening while allowing alloying elements in the billet to fully diffuse and dissolve, further reducing element segregation. The heating rate is controlled at 10–12℃ / min, and the billet is heated to 1180–1200℃ for homogenization, with a holding time of 3.0–4.0 hours.

[0037] 5. The billet is rolled into finished steel plate in two stages. In the first stage, in order to fully break the columnar crystals of the continuously cast billet, a high-temperature slow rolling process with a large reduction is adopted. After descaling, the billet is directly rolled at a rolling speed of 1.0-1.5 m / s. The reduction in each of the first three passes is >40 mm. During the rolling process, the billet is cooled with mill cooling water between each pass for 5-7 seconds. The final rolling temperature is 980-1040℃. The thickness of the billet waiting to be heated is 1.5-2.2 times the thickness of the finished product. In order to suppress the growth of grains in the intermediate billet, the billet waiting to be heated is cooled by water spray at a cooling rate of 6.0-8.0℃ / s, cooling to 10-25℃ above the starting rolling temperature of the second stage. The starting rolling temperature of the second stage is 920-950℃, the rolling speed is 1.6-2.0 m / s, and the final rolling temperature is 820-850℃.

[0038] 6. The rolled steel plate adopts the rapid cooling (ACC) process with a cooling rate of 5.0~15.0℃ / s and a steel plate reddening temperature of 550~620℃, which can maintain fine grains after rolling and prevent grain growth.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] This invention employs a low-carbon and high-manganese content design, controlling the range of composite alloying elements as follows: 0.05≥(Nb+V+Ti) / Mn≥0.03, 8≥Si / Al≥2, (Cu+P+Co) / Cr≥0.65, 20≥Cr / Co≥3.5, Ni / Cr≥0.6, (Nb+V+Ti+Als) / N≥9, Ni / Cu≥0.8, 36Nb / Mn≥0.40, (Nb+V+Ti) / RE≥1.4, RE / P≥1.2. This improves the strength of the steel while ensuring that the steel plate has excellent low-temperature toughness, corrosion resistance, and fatigue resistance. By optimizing the continuous casting process to control the impact of elemental segregation on the fatigue and low-temperature performance of steel plates, a segmented heating process with slow speed and long duration is adopted for heating. A two-stage process of high-temperature, slow-speed, high-reduction rolling and recrystallization zone rolling is adopted, combined with the subsequent ACC controlled cooling process. The strength of the steel plate is guaranteed by grain refinement, dislocation strengthening, solid solution strengthening and second-phase strengthening; the low-temperature toughness of the steel plate is guaranteed by grain refinement; the oxides formed by elements such as Cr, Ni, and Co are guaranteed by the good corrosion resistance of the steel plate; the fatigue resistance of the steel plate is improved by controlling the fine and dispersed two-phase particles and elemental segregation; and the corrosion resistance of the steel plate is improved by adding a small amount of Sn, Sb and RE and P elements to reduce the segregation of P element.

[0041] 1. A composite addition of Co, Ni, and Cr elements is proposed, which results in a steel plate with good comprehensive mechanical properties, yield strength of 420-480 MPa, elongation ≥26%, and impact energy at -80℃ greater than or equal to 210 J.

[0042] 2. A composite method is proposed to add small amounts of Sn, Sb and RE and P elements to improve the corrosion resistance of steel plates, with a marine atmospheric corrosion rate of less than 0.080 mm / a;

[0043] 3. The steel plate has good fatigue resistance, with a fatigue life of more than 2 million cycles under a peak stress of 200MPa.

[0044] 4. It can produce a wide range of thicknesses, with a maximum thickness of 150mm, and the Z-axis performance is greater than or equal to 45%. Detailed Implementation

[0045] The present invention will be described in more detail below through examples. According to the above chemical composition and production process, the actual smelting composition of the present invention is shown in Table 1, the actual process parameters of the present invention are shown in Tables 2 to 6, and the actual properties are shown in Table 7.

[0046] Table 1 Smelting composition, Wt%

[0047]

[0048] Table 2 Steelmaking Process Parameters

[0049] Serial Number Argon blowing time / min RH processing time / min Net cycle time / min 1 5 30 6 2 5 30 6 3 10 35 6 4 10 35 6 5 8 30 8 6 8 30 8 7 8 35 8 8 6 35 9 9 6 32 10 10 7 32 10

[0050] Table 3 Continuous Casting Process Parameters

[0051]

[0052]

[0053] Table 4 Heating process parameters

[0054]

[0055] Table 5. First-stage rolling process parameters

[0056]

[0057] Table 6. Process parameters for two-stage rolling and post-rolling cooling

[0058]

[0059]

[0060] Table 7 Physical Performance

[0061]

[0062] As shown in Table 7, the yield strength of the steel in the embodiments of the present invention ranges from 425 to 475 MPa, all exceeding the design strength of 420 MPa. The elongation is greater than 26%, the Z-axis performance is greater than 40%, and the impact energy at -80℃ is greater than 200 J. This indicates that the strength and toughness of the steel in each embodiment not only meet the design requirements but also have a certain margin. Fatigue performance tests were conducted using an Instron 8802 fatigue testing machine under normal conditions. The loading method was tension-compression fatigue, the stress ratio Rs = -1, and the test frequency was 20 Hz. The fatigue life at a peak stress of 200 MPa exceeded 2 million cycles. Following the test methods specified in GB / T 19746-2005, a 168-hour cyclic immersion rapid corrosion evaluation test was conducted on the steel. The corrosion resistance performance of the steel was evaluated using the weight loss method, and the annual average corrosion rate was calculated.

[0063] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A type of steel for engineering in extremely cold marine environments, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.02%–0.07%, Si: 0.10%–0.25%, Mn: 1.75%–2.50%, P: 0.022%–0.026%, S: ≤0.008%, Nb: 0.025%–0.030%, V: 0.02%–0.03%, Ti: 0.02%–0.03%, Cr: 0.15%–0.35%, Ni: 0.15%–0.35%, Co: 0.02%–0.05%, Als: 0.035%–0.045%, N: 0.010%–0.012%, Cu: 0.15%–0.22%, Sn: 0.03%–0.05%, Sb: 0.03%–0.05%, RE: 0.03%–0.05%, of which 0.05 ≥ (Nb+V+Ti) / Mn ≥ 0.03, 8 ≥ Si / Al ≥ 2, (Cu+P+Co) / Cr ≥ 0.65, 20 ≥ Cr / Co ≥ 3.5, Ni / Cr ≥ 0.6, (Nb+V+Ti+Als) / N ≥ 9, Ni / Cu ≥ 0.8, 36 Nb / Mn ≥ 0.40, (Nb+V+Ti) / RE ≥ 1.4, RE / P ≥ 1.2, with the remainder being Fe and unavoidable impurities; the production method includes smelting, slab continuous casting, slab heating, rolling and cooling. The slab is rolled into finished steel plates in two stages. The first-stage slab is descaled after exiting the furnace and then directly rolled. The rolling speed is... The rolling speed is 1.0–1.5 m / s, with a reduction of >40 mm per pass in the first three passes. During the rolling process, the billet is cooled with mill cooling water between each pass for 5–7 seconds. The final rolling temperature is 980–1040℃. The thickness of the billet waiting to be heated is 1.5–2.2 times the thickness of the finished product. The billet waiting to be heated is cooled by spraying water at a rate of 6.0–8.0℃ / s, cooling to 10–25℃ above the second stage starting rolling temperature. The second stage starting rolling temperature is 920–950℃, the rolling speed is 1.6–2.0 m / s, and the final rolling temperature is 820–850℃. The rolled steel plate is subjected to a rapid cooling process at a rate of 5.0–15.0℃ / s, and the steel plate red-hot temperature is 550–620℃.

2. The steel for engineering in extremely cold marine environments according to claim 1, characterized in that, The steel has a yield strength of 420–480 MPa, an elongation of ≥26%, and an impact energy of ≥210 J at -80℃.

3. The steel for engineering in extremely cold marine environments according to claim 1, characterized in that, The corrosion rate of steel against marine atmosphere is <0.080 mm / a.

4. The steel for engineering in extremely cold marine environments according to claim 1, characterized in that, Fatigue life > 2 million cycles under peak stress of 200 MPa.

5. The steel for engineering in extremely cold marine environments according to claim 1, characterized in that, The maximum thickness of the steel plate can reach 150mm, and the Z-axis performance is ≥45%.

6. A method for producing steel for extreme cold marine environment engineering as described in any one of claims 1 to 5, comprising smelting, slab continuous casting, slab heating, rolling and cooling, characterized in that, During the continuous casting process, the continuously cast billet is rapidly cooled at an initial cooling temperature of 950–980℃ at a cooling rate of 8.0–9.0℃ / s. After cooling to 680–720℃, it is transferred to a slow cooling pit for further slow cooling, and then cooled to below 150℃ at a rate of 3.0–20.0℃ / h. The billet heating process employs a segmented heating method: it is introduced into the furnace at a temperature of 650–750℃ and held for 1.0–2.0h; below 950℃, the heating rate is controlled at 6–8℃ / min, and it is held at 950℃ for 15–30min; above 950℃, the heating rate is controlled at 10–12℃ / min, and it is heated to 1180–1200℃ for homogenization and held for 3.0–4.0h. The billet is rolled into finished steel plates in two stages. The first-stage billet exits… After descaling in the furnace, the billet is directly rolled at a speed of 1.0–1.5 m / s. The reduction in each of the first three passes is greater than 40 mm. During the rolling process, the billet is cooled with mill cooling water between each pass for 5–7 seconds. The final rolling temperature is 980–1040℃. The thickness of the billet waiting to be heated is 1.5–2.2 times the thickness of the finished product. The billet waiting to be heated is cooled by water spray at a rate of 6.0–8.0℃ / s, cooling to 10–25℃ above the second stage rolling temperature. The second stage rolling temperature is 920–950℃, the rolling speed is 1.6–2.0 m / s, and the final rolling temperature is 820–850℃. The rolled steel plate is subjected to a rapid cooling process at a rate of 5.0–15.0℃ / s, and the steel plate reddening temperature is 550–620℃.

7. A method for producing steel for extreme cold marine environment engineering according to claim 6, characterized in that, During the smelting process, the molten iron is first desulfurized and pretreated. After desulfurization, the S content in the molten iron is ≤0.0025%. A combined top and bottom blowing process is adopted, and the converter tapping temperature is 1630-1650℃. Then, Si-Ca wire feeding treatment is carried out, and the Ca content is controlled at 0.0015%-0.0025%. 7-9 minutes before the end of vacuum treatment in the LF furnace, 1.95-3.25 kg / ton of steel with 20% rare earth alloy is added to the LF furnace, followed by argon blowing for 5-10 minutes. Then, RH treatment is carried out for 30-35 minutes. Nitrogen is blown throughout the RH treatment, and the net circulation time before unloading is 6-10 minutes.

8. A method for producing steel for extreme cold marine environment engineering according to claim 6, characterized in that, During the continuous casting of slabs, the tundish is superheated to 25-35℃, and full-process protective casting is adopted. The billet pulling speed is controlled at 1.1-1.3m / min, the secondary cooling water ratio is 0.80-0.90m3 / t, and heavy reduction is adopted during the continuous casting process, with a reduction of 15.0-20.0mm.

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