Steel for extremely cold marine environment engineering and production method thereof

Abstract

The invention discloses steel for extremely cold marine environment engineering and a production method of the steel. The steel comprises the following chemical components: 0.02 to 0.07 percent of C, 0.10 to 0.25 percent of Si, 1.75 to 2.50 percent of Mn, 0.022 to 0.026 percent of P, less than or equal to 0.008 percent of S, 0.025 to 0.030 percent of Nb, 0.02 to 0.03 percent of V, 0.02 to 0.03 percent of Ti, 0.15 to 0.35 percent of Cr, 0.15 to 0.35 percent of Ni, 0.02 to 0.05 percent of Co, 0.035 to 0.045 percent of Als, 0.010 to 0.012 percent of N, 0.15 to 0.22 percent of Cu, 0.03 to 0.05 percent of Sn, 0.03 to 0.05 percent of Sb and 0.03 to 0.05 percent of RE. Through smelting, slab continuous casting, casting blank heating, rolling and cooling, the Rp0.2 of the steel is 420-480 MPa, A is larger than or equal to 26%, the impact energy at the temperature of-80 DEG C is larger than or equal to 210 J, the corrosion resistance rate is smaller than 0.080 mm / a, the fatigue life under the peak stress load of 200 MPa is longer than 2 million times, the maximum thickness can reach 150 mm, and the Z-direction performance is larger than or equal to 45%. The steel plate provided by the invention has relatively high strength and excellent low-temperature toughness, and also has relatively good corrosion resistance and fatigue resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material preparation, and in particular relates to steel for extremely cold marine environment engineering and a production method thereof. Background Art

[0002] Steel used in marine equipment manufacturing faces complex environments such as complex ocean waves, temperature, humidity, and salinity, which places higher demands on the performance of steel, especially corrosion and fatigue performance, such as higher strength, greater thickness, low-temperature impact toughness, good Z-direction performance, as well as better corrosion resistance and fatigue resistance, etc. It is difficult for existing steel to meet the above performance requirements at the same time.

[0003] The patent application number 201510314696.X discloses 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] Low yield strength ratio weather-resistant steel for wind turbine towers with -50℃ KV2≥200J, -60℃ KV2≥185J and production method. However, it fails to provide data on the production of thick steel plates, has no corrosion and fatigue performance evaluation, and is not suitable for the demand for steel plates for marine engineering equipment.

[0005] Patent application number 202010557893.5 discloses a 550MPa grade weathering steel plate with excellent weldability and its manufacturing method, using optimized TMCP process, yield strength ≥460MPa, tensile strength ≥570MPa, -40℃ Charpy impact energy (single value) ≥120J, -40℃ impact toughness KV2 ≥100J, welding heat affected zone (HAZ) -40℃ Akv ≥100J. However, its low temperature toughness is only -40℃, and the corrosion and fatigue resistance problems are not solved.

[0006] The patent with application number CN201410036368.3 discloses a corrosion-resistant steel plate for the South China Sea marine environment and its production process. The production process includes converter smelting process, LF refining process, vacuum degassing process, continuous casting process, controlled rolling and controlled cooling process, etc. The organizational type of the steel plate is theoretically 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 structure. Compared with conventional hull structural steel EH36, its corrosion resistance in marine environment (marine atmosphere, tidal range, full immersion, etc.) is improved by more than 50%, and it has good strength and toughness matching and welding performance. However, its low-temperature toughness is insufficient, and its fatigue resistance has not been evaluated.

[0007] The patent with 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%, Nb0.020%-0.050%, Alt 0.020%-0.030%, Ti≤0.010%, Si≤0.12%, P≤0.010%, S≤0.005%, and the remainder is iron and unavoidable impurities. By adopting a large reduction + controlled cooling process, the obtained steel plate has good comprehensive mechanical properties and good surface quality. However, its corrosion performance was not evaluated, and only the impact toughness at -20℃ was evaluated, which is far from meeting the use requirements of different marine environments.

[0008] The patent with application number 202110068169.0 discloses an engineering corrosion-resistant fatigue steel and a preparation method thereof. The steel is 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%), and element regulation and characteristic element addition are performed, Cu0.28%~0.66%, Ni 0.76%~1.55%, Sb 0.03%~0.12%, and the rest are Fe and unavoidable impurities. The corrosion fatigue strength of the steel can be increased by up to 52%, but the low-temperature toughness of the steel is not evaluated. Summary of the invention

[0009] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a steel for extremely cold marine environment engineering and a production method thereof, which has low carbon and low alloying as basic characteristics, composite addition of multi-element alloy strengthening elements such as Cr, Ni, Co and precipitation strengthening elements such as V and N, and addition of P, Al and Sb, Sn, RE as basic characteristics, so as to improve the strength and low-temperature toughness of the steel plate, and at the same time have good corrosion resistance and fatigue resistance; solve the problem of insufficient corrosion resistance and fatigue resistance of existing marine engineering steel plates, especially solves the problem of poor impact toughness in ultra-low temperature environment.

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

[0011] The present invention is characterized by a steel plate having excellent low temperature toughness, high strength, corrosion resistance and fatigue resistance. In the present invention, the chemical composition of the steel plate is as follows: C: 0.02% to 0.07%, Si: 0.10% to 0.25%, Mn: 1.75% to 2.50%, P: 0.022% to 0.026%, S: ≤ 0.008%, Nb: 0.025% to 0.030%, V: 0.02% to 0.03%, Ti: 0.02% to 0.03%, Cr: 0.15% to 0.35%, Ni: 0.15% to 0.35%, Co: 0.02% to 0.05%, Als: 0.035% to 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%, among 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, and the rest are Fe and unavoidable impurities.

[0012] The present invention selects the above alloying element types and their contents because of the role of each element in improving the strength, toughness, corrosion resistance and fatigue resistance of the steel in 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, etc., the melting point, hardness, wear resistance and stability of these carbides are higher than Fe3C, which can improve the strength of steel; C and strong carbide forming elements such as V, Nb, Ti, etc. will preferentially form carbides such as VC, NbC, TiC, etc., which have the highest stability, melting point, hardness and wear resistance; therefore, C is the most effective element to improve the strength of steel plates, and when its content is lower than 0.02%, the strength of the steel plate will be greatly reduced. However, C has a great influence on 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 lower. Therefore, the C content of the present invention is selected to be 0.02% to 0.07%.

[0014] Si: Si is a necessary element for steelmaking deoxidation and has a certain solid solution strengthening effect. Although Si can improve the strength of steel plates, Si reduces the critical cooling rate of martensitic phase transformation, seriously damaging the low-temperature toughness, elongation and weldability of high-strength steel plates. Si not only promotes the formation of MA islands, but also the size of the formed MA islands is relatively large and unevenly distributed, which seriously damages the toughness of the welding 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 marine corrosion resistance of steel, and the composite addition of Si and Al can improve the corrosion resistance and high-temperature oxidation resistance. The Si content of the present invention is controlled at 0.10% to 0.25%, 8≥Si / Al≥2.

[0015] Mn: Mn is the main element for improving strength and toughness, and can significantly improve the hardenability of steel. It is very cheap and is the main additive element in steel. When the C 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 structure and promoting bainite transformation, while having excellent low-temperature toughness; Mn expands the austenite region and promotes grain enlargement. It needs to be combined with the grain-refining elements Nb, V, and Ti to promote grain refinement and improve the fatigue resistance of steel; but Mn is prone to segregation during the solidification of molten steel, aggravating the segregation and looseness of the center of the ingot, resulting in low low-temperature toughness of high-strength steel plates and cracks in welded joints, which need to be improved by optimizing the continuous casting process and heating process. The Mn content of the present invention is selected to be 1.75% to 2.50%, and 0.05≥(Nb+V+Ti) / Mn≥0.03.

[0016] P: P has a strong solid solution strengthening effect in steel. When added to low alloy structural steel as an alloying element, it can improve its strength and atmospheric corrosion resistance. When its content is greater than or equal to 0.02%, it can significantly improve corrosion resistance. When the P content is too high, it will have an adverse effect on the low temperature toughness of the parent material and the toughness of the welding heat affected zone, so its content should be controlled within a reasonable range as much as possible. In the present invention, its content is controlled within 0.022% to 0.026%.

[0017] S: S segregates severely in steel, deteriorating the quality of steel. S is an inclusion-forming element, forming inclusions such as FeS and MnS, thereby reducing the ductility of steel, and the area near the inclusions will become the source of corrosion, which is not conducive to the corrosion resistance of the steel plate. Moreover, FeS is easy to melt at the grain boundary due to its low melting point, weakening the bonding force between the grains and causing the hot brittleness of the steel. Therefore, a certain amount of Mn element should be added to form MnS with a higher melting point and deformation ability. Therefore, the content of S in the present invention is controlled to be less than or equal to 0.008%.

[0018] Co: Co plays a major role in solid solution strengthening in steel, improving the oxidation resistance and corrosion resistance of steel. Co can increase the interaction between Fe atoms, reduce the critical concentration of Cr atoms forming clusters, and thus improve the stability of Cr atom clusters. When Co and Cr atoms act on steel at the same time, a smooth passivation film is formed on its surface, which has high structural stability and can well protect the matrix, thus having excellent corrosion resistance. Co promotes precipitation hardening by promoting more (Ti, Al) Ni3 precipitation nucleation sites, improving strength and toughness. The addition of Co has little effect on the precipitation ratio of Ni and Ti, but can reduce the size of the precipitated phase. Co increases the nucleation rate of Ni3Ti and increases the number density of the precipitated phase, that is, Co can make the distribution of the precipitated phase more dispersed, improve the fatigue resistance of steel, and enhance the precipitation strengthening effect of the precipitated phase. Therefore, the content of Co in the present invention is controlled at 0.02% to 0.05%, 20≥Cr / Co≥3.5.

[0019] Cr: Cr can improve the strength and hardness of steel. Cr is a ferrite-forming element, which helps to increase the ferrite content in steel and thus improve the low-temperature toughness of steel. Cr is an element that improves the corrosion resistance of steel, but adding Cr alone sometimes reduces the corrosion resistance, and even has a lower corrosion resistance than ordinary carbon steel. It needs to be used in combination with other corrosion-resistant alloying elements, such as Cu, P, Co, etc., to significantly improve its corrosion resistance. The content of Cr in the present invention is controlled at 0.15% to 0.35%, (Cu+P+Co) / Cr≥0.65.

[0020] Ni: Ni strengthens ferrite and refines pearlite in steel. The overall effect is to improve strength, and the effect on plasticity is not significant. Ni can improve the fatigue resistance of steel and reduce the sensitivity of steel to notches, so Ni can improve the fatigue performance of steel. Ni does not form carbides, but plays a role in strengthening ferrite by forming a simple substitutional solid solution, reducing the toughness-brittleness transition temperature of steel, and improving the low-temperature toughness of steel; a certain Ni content can ensure that the steel plate has sufficient hardenability and uniform performance in the thickness direction, while ensuring the strength and toughness matching and low-temperature toughness of the steel plate. Adding Ni to steel can also reduce the copper brittleness of Cu-containing steel, reduce 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 the present invention, its content is controlled at 0.15% to 0.35%, Ni / Cr≥0.6, Ni / Cu≥0.8.

[0021] Cu: Cu is the most important and most commonly used alloying element in corrosion-resistant steel. Cu can activate the cathode, promote the passivation of the steel anode and slow down the corrosion. During the corrosion process, a copper-rich phase will form on the surface of the steel. There is a dense and highly adherent intermediate layer between the surface corrosion layer and the copper-rich layer of the steel, which further alleviates the corrosion of the steel. In particular, when used in combination with P, it can significantly improve the performance of resisting marine atmospheric corrosion and seawater corrosion. In addition to reducing the copper brittleness of copper-containing steel and reducing the intergranular cracking during the hot rolling process, the composite addition of Cu and Ni is more importantly austenite stabilizing element. The composite addition of Cu and Ni can significantly reduce Ar3 and increase the driving force of the phase transformation from austenite to ferrite. At the same time, Cu can accelerate the high-temperature strain-induced precipitation of niobium carbonitride and increase the recrystallization stop temperature, which is conducive to controlled rolling in the non-recrystallization zone to refine the phase transformation products and improve the fatigue resistance of the material. The content of Cu in the present invention is controlled at 0.15% to 0.22%.

[0022] Nb: Nb is an important element in controlled rolling and controlled cooling steel. Nb is a strong carbide-forming element. The two-phase particles NbC and NbN formed with C and N are important elements in controlled rolling and controlled cooling steel. They can effectively refine the grains, thereby improving the strength and low-temperature impact toughness at the same time. The composite addition of Nb and Mn can effectively inhibit the recovery and recrystallization of austenite during the rolling process. On the one hand, it can increase the austenite recrystallization temperature, thereby increasing the rolling temperature and reducing the load of the rolling unit; on the other hand, it can effectively refine the phase transformation structure of the steel plate, thereby improving the strength and low-temperature impact toughness at the same time; it can prevent the intergranular corrosion of the steel by the oxidizing medium. Nb can increase the solid solution amount of rare earth in steel, thereby improving the corrosion resistance of steel. In the present invention, its content is controlled at 0.025% to 0.030%, 36Nb / Mn≥0.40.

[0023] V: V has a great affinity with O and N and is a strong carbide-forming element; generally, VC has a high degree of dispersion and is extremely stable, so it is beneficial for deoxidation and degassing to obtain a dense fine-grained structure, improve plasticity, toughness and high strength. Its impact performance and fatigue strength are higher than those of vanadium-free steel, and it has high strength and toughness at both high and low temperatures (<0°C). Since the high dispersion of VC prevents the weld grains from being coarse, it can improve the weldability of the steel, but heating to the VC dissolution temperature will cause the steel crystals to grow strongly. When dissolved into a solid solution at high temperature, the hardenability is increased; conversely, when it exists in the form of carbides, the hardenability is reduced. V increases the tempering stability of quenched steel and produces a secondary hardening effect. V can increase the solid solution amount of rare earth in steel, thereby improving the corrosion resistance of steel. In the present invention, its content is controlled at 0.020% to 0.030%.

[0024] Ti: Ti has a strong affinity with N, O, and C, and its affinity with S is stronger than that of iron. Therefore, it is a good deoxidizing and degassing agent and an effective element for fixing N and C. Ti is a strong carbonitride-forming element. A trace amount of Ti can combine with N in steel to form TiN, which prevents the growth of austenite grains during soaking and can also prevent the growth of austenite grains in the heat-affected zone of welding, thereby improving weldability. TiC and TiN have strong binding force, stability, and are not easy to decompose. They can only be slowly dissolved into the solid solution in steel when heated to above 1000°C. Therefore, they can significantly control the growth of grains in the heat-affected zone of welding and improve the welding performance of the material. Since 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 amount of rare earth in steel, thereby improving the corrosion resistance of steel. However, Ti has a great affinity with N and O and is very easy to form TiN and TiO2. At lower temperatures, more non-metallic inclusions and subcutaneous porosity defects are formed. The content of the present invention is controlled at 0.02% to 0.03%.

[0025] N: Like C, N can be dissolved in Fe to form interstitial solid solutions. N expands the austenite phase of steel and is a strong austenite-forming element. It can replace a portion of nickel within a certain limit. N that penetrates into the surface of steel can be combined with Nb, Al, V, Ti and other elements to form extremely stable nitrides, improving the corrosion resistance of steel. However, excessive residual nitrogen in steel can lead to loose macroscopic structure or pores, so a certain amount of Al needs to be added to nitrogen-containing steel to form stable AlN to avoid defects such as pores formed by nitrogen escaping during solidification. Therefore, the N content in the present invention is controlled at 0.010% to 0.012%, (Nb+V+Ti+Als) / N≥9.

[0026] Al: Al is mainly used for deoxidation and grain refinement. Al and N or O form effective fine dispersions to inhibit grain growth when steel is heated. When steel is cooled, it promotes the decomposition of austenite to improve the hardenability of steel. It also becomes a nucleation point for recrystallization, promotes ferrite nucleation and grain refinement, and improves the fatigue resistance of steel. AlN itself has high stability when heated, so it can improve the thermal stability of steel, help reduce the overheating tendency of steel, and improve the oxidation resistance of steel; Al forms an effective surface hardening layer through the low-temperature diffusion (nitridation) of N, improves the oxidation resistance and corrosion resistance of steel, and adding a certain amount of Si when deoxidizing with Al can significantly improve the deoxidation of Al; but if the amount of Al is too much, it will cause abnormal structure of steel and promote the graphitization tendency of steel. Therefore, the Als content of the present invention is controlled at 0.035% to 0.045%.

[0027] Sn, Sb: Antimony (Sb) in steel at austenite temperature precipitates at MnS inclusions and along the original austenite grain boundaries, thereby inhibiting the enrichment and precipitation of MnS inclusions on the grain boundaries. Sb can also refine the size of secondary recrystallized grains, refine the structure of steel and improve toughness, thereby improving the fatigue resistance and corrosion resistance of steel; Sn and Sb help to improve the corrosion resistance of materials. Adding Sn alone or adding Sn and Sb elements in combination to steel significantly improves the corrosion resistance of materials; Sn and Sb form SnO2, Sb2O5 corrosion-resistant oxide films on the surface of steel, which effectively prevent the interaction between the matrix and the corrosive medium and inhibit the corrosion of steel in the corrosive medium; Sn and Sb are obtained after hydration. 2+ , Sb 3+ The precipitates formed in the anode micro-area fill the corrosion cracks or cavities, improving the barrier to corrosion. - Penetration ability; Sn and Sb can also pass through Cl - Hydration in ambient media inhibits Fe 3+ Hydrolysis produces H + The process improves the pH value of the corrosion micro-area and alleviates the anodic dissolution process. Sn and Sb, as corrosion inhibitors, change the cathode and anode reaction process and can greatly improve the marine environment corrosion resistance of steel. The content of the present invention is controlled at 0.03% to 0.05%.

[0028] RE: Rare earth (RE) atoms are active and have strong binding force. Adding RE to steel can improve solidification structure, change solid phase change structure, form harmless low melting point inclusions, strengthen interface through segregation, passivate surface rust layer, etc.; RE can improve the self-corrosion potential and polarization resistance of weathering steel, thereby inhibiting the anodic reaction, increasing the resistance of the entire electrochemical reaction, and significantly reducing the corrosion rate of steel; RE is enriched at the grain boundary through the diffusion mechanism, inhibiting the segregation of inclusions at the grain boundary, and improving the low temperature performance and corrosion resistance of steel; RE added to P-containing steel can reduce macrosegregation, reduce the segregation of P at the grain boundary and ferrite interface, and make the distribution of P in steel more reasonable, thereby significantly improving the toughness, corrosion resistance, fatigue resistance, etc. of steel. However, RE is a scarce resource, and its addition amount must be controlled. The RE content of the present invention is controlled at 0.03% to 0.05%, (Nb+V+Ti) / RE≥1.4, RE / P≥1.2.

[0029] The above are the content ranges and functions of various elements added. The manufacturing method of the present invention for producing steel for extremely 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 contents of elements such as P and S are adjusted to be within the scope of the present invention. During the smelting process, the molten iron is first subjected to a desulfurization pretreatment, and the S content in the molten iron after desulfurization is ≤0.0025%. A top and bottom composite blowing process is adopted, and the converter tapping temperature is 1630-1650°C. After that, Si-Ca wire feeding treatment is performed, and the Ca content is controlled between 0.0015% and 0.0025%.

[0032] 2) Refining the molten steel, adding 20% ​​rare earth alloy 1.95-3.25 kg / ton steel into the LF furnace 7-9 minutes before the end of vacuum treatment in the LF furnace, and then blowing argon for 5-10 minutes; then RH treatment, the RH treatment time is 30-35 minutes, nitrogen is blown throughout the RH treatment, controlling [H] ≤ 1.0 ppm, [O] ≤ 20 ppm in the steel, and the net circulation time before moving out is 6-10 minutes.

[0033] 3) Add Sn and Sb elements before the end of RH treatment, and ensure that the added amount is 1.2 to 1.3 times the target control amount 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 desired continuous casting billet. In order to control the content of equiaxed crystals in the continuous casting billet, the tundish is superheated at 25-35°C. A lower superheat can reduce the solidification time of the molten steel, reduce the segregation of carbon, manganese and other elements in the center of the billet, reduce defects such as looseness and shrinkage of the billet, and ensure the Z-direction performance and fatigue resistance of the steel plate; adopt full-process protection casting, the continuous casting billet drawing speed is controlled at 1.1-1.3m / min, and the secondary cooling water ratio is 0.80-0.90m 3 / t, so that the proportion of equiaxed crystals in the continuous casting billet is greater than 25.0%, and electromagnetic stirring is used at the end of solidification to make the molten steel evenly stirred and achieve high-strength and high-density solidification as soon as possible. Heavy pressure is used in the continuous casting process, and the pressure reduction is 15.0-20.0mm.

[0035] 3. In order to control the grain size of the continuous casting billet to no more than 500μm, the continuous casting billet is cooled rapidly, with the start cooling temperature of 950-980℃ and the cooling rate of 8.0-9.0℃ / s. After cooling to 680-720℃, it enters the slow cooling pit for slow cooling, and then cools to below 150℃ at a cooling rate of 3.0-20.0℃ / h.

[0036] 4. Send the continuous casting billet obtained in step 3 into the heating furnace for heating. The heating adopts a segmented heating process, enters the furnace at a furnace temperature of 650-750°C, and is kept warm for 1.0-2.0 hours to release the internal stress of the billet; a slow heating process is adopted below 950°C to further release the internal stress caused by the continuous casting billet cooling and heavy pressure, and to prevent new temperature stress caused by excessive heating. The heating rate is controlled at 6-8°C / min, and heated to 950°C and kept warm for 15-30 minutes; above 950°C, a process of rapid heating and appropriately extending the holding time is adopted to prevent the austenite grains from coarsening while allowing the alloy elements in the billet to fully diffuse and dissolve, further reducing element segregation, and the heating rate is controlled at 10-12°C / min, and heated to 1180-1200°C for uniform temperature, and the holding time is 3.0-4.0 hours.

[0037] 5. The ingot is rolled into a finished steel plate in two stages. In the first stage, in order to fully break the columnar crystals of the continuous casting ingot, a high-temperature slow rolling and large reduction process is adopted. The ingot is rolled directly after being descaled. The rolling speed is 1.0-1.5 m / s. The reduction of each of the first three passes is greater than 40 mm. During the rolling process, the billet is cooled by the mill cooling water between each pass. The cooling time is 5-7 s. The final rolling temperature is 980-1040°C. The thickness of the warm billet is 1.5-2.2 times the thickness of the finished product. In order to suppress the growth of the intermediate billet grains, the warm billet is cooled by water spraying at a cooling rate of 6.0-8.0°C / s until it is cooled to 10-25°C above the second stage rolling temperature. The second stage rolling temperature is 920-950°C, the rolling speed is 1.6-2.0 m / s, and the final rolling temperature is 820-850°C.

[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 red-return temperature of 550-620℃, which can maintain fine grains after rolling and prevent grain growth.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The present invention adopts a low carbon and relatively high manganese content design, controls the composite addition of alloy elements in the range of 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, improves the strength of the steel and ensures that the steel plate has excellent low-temperature toughness, corrosion resistance and fatigue resistance. By optimizing the continuous casting process to control the influence of element segregation on the fatigue performance and low-temperature performance of the steel plate, a slow and long-time segmented heating process is adopted for heating, and a two-stage process of high-temperature, slow, large reduction and recrystallization zone rolling is adopted for rolling, combined with the subsequent ACC controlled cooling process, the strength of the steel plate is ensured by relying on fine grain strengthening, dislocation strengthening, solid solution strengthening and second phase strengthening; the good low-temperature toughness of the steel plate is ensured by relying on grain refinement; the oxides formed by elements such as Cr, Ni, and Co are used to ensure that the steel plate has good corrosion resistance; the fatigue resistance of the steel plate is improved by controlling fine dispersed two-phase particles and element segregation; and the segregation of the P element is reduced and the corrosion resistance of the steel plate is improved by adding a small amount of Sn, Sb, RE, and P elements in combination.

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

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

[0043] 3. The steel plate has good fatigue resistance, and the fatigue life is more than 2 million times under the peak stress of 200MPa;

[0044] 4. The thickness range that can be produced is wide, the maximum thickness can reach 150mm, and the Z-direction performance is greater than or equal to 45%. DETAILED DESCRIPTION

[0045] The present invention is 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 performance is shown in Table 7.

[0046] Table 1 Melting composition, Wt%

[0047]

[0048] Table 2 Steelmaking process parameters

[0049] Serial number Argon blowing time / min RH treatment 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 of two-stage rolling and post-rolling cooling

[0058]

[0059]

[0060] Table 7 Physical properties

[0061]

[0062] As can be seen from Table 7, the yield strength of the embodiment steel of the present invention is 425-475MPa, all reaching the design strength of 420MPa or above, the elongation is greater than 26%, the Z-direction performance is greater than 40%, and the -80℃ impact energy is greater than 200J, indicating that the strength and toughness of each embodiment steel not only meet the design requirements, but also have a certain margin. The fatigue performance test was carried out under a conventional environment using an Instron 8802 fatigue testing machine, the loading mode was tension-compression fatigue, the stress ratio Rs=-1, the test frequency was 20Hz, and the fatigue life number under the peak stress of 200MPa load was greater than 2 million times. According to the test method specified in GBT 19746-2005, the invention steel was subjected to a 168-hour periodic immersion rapid corrosion evaluation test, and the corrosion resistance of the invention steel was evaluated using the weight loss method, and converted into an annual average corrosion rate.

[0063] It is to be 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 replacement or modification that does not deviate from the essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A steel for extremely cold marine environment engineering, characterized in that: The chemical composition of steel is as follows by weight: 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%, among 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, and the rest are Fe and unavoidable impurities.

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

3. The steel for extremely cold marine environment engineering according to claim 1, characterized in that: The marine atmospheric corrosion resistance rate of steel is less than 0.080mm / a.

4. The steel for extremely cold marine environment engineering according to claim 1, characterized in that: The fatigue life is greater than 2 million times under peak stress of 200MPa.

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

6. A method for producing steel for extremely cold marine environment engineering according to 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 of the slab, the continuous casting billet is cooled rapidly, with the cooling temperature at 950-980℃ and the cooling rate at 8.0-9.0℃ / s. After cooling to 680-720℃, it enters the slow cooling pit for slow cooling, and then cools to below 150℃ at a cooling rate of 3.0-20.0℃ / h. The billet is heated by a segmented heating process, entering the furnace at a furnace temperature of 650-750℃ and keeping it warm for 1.0-2.0h; the heating rate below 950℃ is controlled at 6-8℃ / min, and heating to 950℃ is kept warm for 15-30min; the heating rate above 950℃ is controlled at 10-12℃ / min, heating to 1180-1200℃ for uniform temperature, and keeping it warm for 3.0-4.0h; the billet is rolled into a finished steel plate in two stages, and the first stage billet is rolled out The furnace is directly rolled after descaling, with a rolling speed of 1.0-1.5 m / s, and the reduction of each of the first three passes is greater than 40 mm. During the rolling process, the billet is cooled by mill cooling water between each pass, with a cooling time of 5-7 s, a final rolling temperature of 980-1040 ° C, and a thickness of the warm billet of 1.5-2.2 times the thickness of the finished product. The warm billet is cooled by water spraying at a cooling rate of 6.0-8.0 ° C / s, and cooled to 10-25 ° C above the second stage rolling temperature; the second stage rolling temperature is 920-950 ° C, the rolling speed is 1.6-2.0 m / s, and the final rolling temperature is 820-850 ° C; the steel plate after rolling adopts a rapid cooling process, with a cooling rate of 5.0-15.0 ° C / s, and the red-returning temperature of the steel plate is 550-620 ° C.

7. The method for producing steel for extremely cold marine environment engineering according to claim 6, characterized in that: During the smelting process, the molten iron is first subjected to a desulfurization pretreatment, and the S in the molten iron after desulfurization is ≤0.0025%. A furnace top and bottom composite blowing process is adopted, and the converter tapping temperature is 1630-1650°C. Thereafter, Si-Ca wire feeding treatment is carried out, and the Ca content is controlled at 0.0015%-0.0025%; 20% rare earth alloy 1.95-3.25 kg / ton of steel is added to the LF furnace 7-9 minutes before the end of the vacuum treatment in the LF furnace, and then argon blowing is carried out for 5-10 minutes; then RH treatment is carried out, the RH treatment time is 30-35 minutes, nitrogen blowing is carried out throughout the RH treatment, and the net circulation time before moving out is 6-10 minutes.

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

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