Ultra-high strength offshore steel with high ductility resistant to ice load erosion and its manufacturing method

Abstract

The present invention provides a high-ductility anti-ice-load erosion-resistant ultra-high-strength marine steel and a manufacturing method thereof. The composition of the marine steel is as follows by weight percentage: C: 0.045 - 0.065%, Si: 0.25 - 0.35%, Mn: 1.00 - 1.30%, Cr: 1.30 - 1.60%, Ni: 1.10 - 1.50%, Cu: 0.45 - 0.65%, Mo: 0.40 - 0.80%, Sn: 0.05 - 0.12%, Sb: 0.05 - 0.12%, Nb: 0.03 - 0.05%, V: 0.03 - 0.04%, Ti: 0.005 - 0.025%, Ca: 0.006% - 0.009%, P: ≤0.01%, S: ≤0.001%, Als: 0.02% - 0.05%, and the balance is Fe and inevitable impurities. The manufacturing method includes smelting, continuous casting, heating, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling, secondary cooling, and heat treatment. The elongation after fracture of the marine steel plate produced by applying the present invention is ≥29%, the yield strength is ≥550 MPa, the tensile strength is 670 - 830 MPa, the Charpy impact energy at -60°C is ≥180 J, NDTT ≤ -70°C, and the surface hardness of the steel plate is ≥325 HBW.

Description

Technical Field

[0001] The present invention belongs to the field of metals, and particularly relates to an anti-ice-load erosion-resistant ultra-high-strength marine steel with high ductility and a manufacturing method thereof. Background Art

[0002] Due to the particularity of the marine engineering application environment, high requirements have always been placed on the service safety of marine equipment and structural materials. In recent years, with the gradual transformation and upgrading of the nearshore industry to the marine industry, the service environment of marine equipment has become increasingly complex and harsh. For marine equipment and structures that serve in ice-covered waters or waters with seasonal sea ice for a long time, ice loads are external loads that need to be considered key in the process of marine engineering design and use. Ice load refers to the force generated by the interaction between sea ice and marine engineering equipment and structures in ice-covered areas. Its main acting modes include: the impact force or frictional force generated by drifting ice under the push of wind and ocean currents; the extrusion force generated by large-area ice blocks under the action of wind and ocean currents; the vertical force of the ice layer on open-type buildings caused by sea level rise and fall; the vertical static load caused by the accumulation of sea ice on buildings; and the extrusion force generated by the expansion of water due to ice formation.

[0003] The above-mentioned acting modes of ice loads correspond to three forms of failure threats that marine engineering structures and equipment in ice-covered areas may encounter during service, namely extrusion deformation, collision impact, and erosion wear. These three forms of failure threats also correspond to three performance requirements for the safe service of materials applied under ice loads: ductility, low-temperature impact toughness, and erosion wear resistance. Currently, the American Bureau of Shipping (ABS) has incorporated the ductility performance index into the relevant specification "Material requirements for higher-ductility hull structural steel plates and sections". The purpose is to enable the steel plate to absorb the energy of the collision when a ship, marine engineering equipment, or structure collides, making it less likely to experience structural instability and thus improving the service safety of the equipment. Nippon Steel and Sumitomo Metal in Japan have developed such marine engineering steel plates with high ductility and anti-collision functions (the "NSafe-Hull" series), and they are currently in the application stage. In China, there is currently no systematic research and large-scale application of such steel plates. On the other hand, regarding erosion wear resistance, most current domestic and foreign research focuses on the erosion wear process and performance of steel at room temperature, and there is less research on the low-temperature erosion wear of steel under sea ice loads. In fact, the key parts near the sea surface of marine engineering structures and equipment in ice-covered areas are constantly subjected to repeated impact and friction of sea ice and superimposed seawater corrosion, and their service states are more complex and harsh. Therefore, it is necessary to develop a marine engineering steel with high ductility, good low-temperature impact toughness, and erosion wear resistance to ensure the engineering operation safety of marine engineering structures and equipment in ice-covered waters under ice loads.

[0004] Public reports of related patents:

[0005] The invention "A high-ductility EH36 grade marine engineering steel and its manufacturing method" (publication number CN114807761A) discloses the chemical composition of the marine engineering steel by weight percentage: C: 0.05%~0.08%, Si: 0.10%~0.30%, Mn: 1.00%~1.50%, Nb: 0.01%~0.04%, V: 0.02%~0.05%, Ti: 0.005%~0.02%, P: ≤0.01%, S: ≤0.01%, Als: 0.01%~0.05%, and the rest is Fe and unavoidable impurities. The chemical composition of the invented steel does not add precious alloy elements such as Ni; by controlling the sulfur and phosphorus content, the microstructure and grain size of the steel plate are regulated by induction heating of the continuous casting billet, two-stage controlled rolling, relaxation and controlled cooling processes, and a steel plate with a high yield rate, stable strength and toughness, and especially high ductility is obtained; its disadvantage is that the strength level of the produced steel plate is not high, and the erosion and wear performance evaluation has not been carried out.

[0006] The invention "A high-ductility FH500 grade ship plate steel and its production method" (publication number CN108517463A) discloses that the ship plate steel composition adopts Nb and V micro-alloy strengthening composition design, and cooperates with controlled rolling and controlled cooling technology to obtain soft ferrite and hard bainite structure. The controlled cooling stage adopts water cooling-air cooling-water cooling three-stage cooling, and the first stage of cooling requires ultra-fast cooling of 100-150℃ / s. Its shortcomings are that there are also problems such as complex process control, high requirements for controlled cooling equipment capacity and uniformity of steel plate structure. The steel plate was not evaluated for corrosion performance.

[0007] The invention "A 440MPa grade polar hull steel and its preparation method" (publication number CN112831720A) discloses that the components for the hull are, by weight percentage, C: 0.07-0.12%, Si: 0.30-0.40%, Mn: 1.30-1.60%, Cu: 0.25-0.40%, Ni: 1.0-1.5%, V: 0.06-0.10%, Ti: 0.008-0.012%, Als: 0.05-0.07%, N≤0.004%, P≤0.005%; S≤0.002%, and the rest are Fe and unavoidable impurity elements. In terms of process, a composite structure of ferrite and pearlite is obtained by two-stage controlled rolling and controlled cooling. The steel plate has a -60°C impact energy ≥200J, an elongation ≥26%, and has good welding performance. However, the disadvantage is that the erosion and wear performance of the steel is not evaluated.

[0008] The chemical composition of the wear-resistant steel plate disclosed in the invention "A Wear-Resistant Steel Plate and Its Preparation Method" (publication number: CN110656291A) is as follows: C: 0.05 - 0.12%, Si: 0.10 - 0.50%, Mn: 1.00 - 1.50%, P ≤ 0.020%, S ≤ 0.005%, Nb: 0.015 - 0.025%, Cr: 0.50 - 0.90%, Ni: 0.08 - 0.25%, Ti: 0.008 - 0.035%, Als: 0.015 - 0.040%, As ≤ 0.04%, Sn ≤ 0.03%, N ≤ 0.005%, O ≤ 0.003%, H ≤ 0.0002%, and the balance is Fe and unavoidable impurities. The preparation method adopts the TMCP method, and through two-stage controlled rolling plus controlled cooling, a composite structure of bainite, ferrite and pearlite is obtained. Combining the effects of alloying elements, the steel plate has certain corrosion resistance and wear resistance under the condition of sufficient toughness. Its disadvantage is that this technology improves the bainite ratio (>70%) by high-speed cooling plus low-temperature reverse redness to ensure the hardness and wear resistance of the steel plate, and the cooling will inevitably have a negative impact on the plasticity of the steel plate.

[0009] The steel plate composition design disclosed in the invention "An Economical Corrosion-Resistant and Wear-Resistant Steel and Its Preparation Method" (publication number: CN112195405A) uses low-cost C-Mn-B as the main wear resistance strengthening element, gives full play to the role of element B in enhancing hardenability, and realizes a substantial increase in the strength and wear resistance of the material by solid solution elements such as C and Mn. Combining the synergistic effects of elements such as P, Sb, and Ce improves the corrosion resistance of the material; the preparation method adopts the method of TMCP plus quenching and tempering. After two-stage controlled rolling plus controlled cooling, high (medium) frequency induction quenching and tempering with a high heating rate are used to refine the structure, and a tempered martensite / bainite structure is obtained, making the steel have good wear resistance and corrosion resistance. Its disadvantage is that this technology uses induction heating to perform quenching and tempering treatment on the whole plate, resulting in high power consumption and being limited by the skin effect of induced eddy currents on the thickness of the heated body, making it difficult to achieve uniform heating of thick steel plates.

[0010] The production method disclosed in the invention "A Method for Improving the Production Efficiency of Heavy Plates in a Single-Strand Mill" (publication number: CN111014292A) mainly directly cools the intermediate billet after the first heating rolling and then reheats and rolls it for the second time. After reheating to the target temperature and tapping, the finished product thickness is directly rolled. By adopting the method of directly cooling the intermediate billet and then reheating and rolling it again, the cooling time of the intermediate billet is saved, which can significantly improve the mill efficiency and thus greatly increase the hourly output of the mill. Its disadvantage is that since this method increases the heating and cooling processes in the process route, it increases the consumption of energy and industrial water, and the extent of the increase in production efficiency remains to be discussed. Summary of the Invention

[0011] The purpose of the present invention is to overcome the above problems and shortcomings and provide an ultra-high strength marine engineering steel with high ductility and resistance to ice load corrosion and a manufacturing method thereof, which has comprehensive performance and can meet the long-term service conditions of marine engineering equipment.

[0012] The object of the invention is achieved in this way:

[0013] The present invention adopts the composition design of low C, Mn + "Cr-Cu-Ni-Mo-Sn-Sb" alloying + Nb, V, Ti microalloying, and combines the secondary heating rolling, controlled cooling and induction quenching and tempering process to control the microstructure of the steel plate, so as to obtain a 550Mpa grade steel plate with high yield rate, stable strength and toughness, especially resistance to ice load erosion and wear and high ductility. Compared with the above-mentioned public patents, this patent prepares a steel plate with resistance to ice load erosion and wear, high ductility and low temperature toughness suitable for the field of marine engineering. Its comprehensive performance can meet the long-term service conditions of marine engineering equipment, with tensile elongation after fracture ≥29%, -60℃ impact energy ≥180J, NDTT temperature ≤-70℃, and surface hardness ≥325HBW.

[0014] The invention discloses an ultra-high strength marine engineering steel with high ductility and resistance to ice load corrosion. The components of the steel plate are as follows by weight percentage: C: 0.045-0.065%, Si: 0.25-0.35%, Mn: 1.00-1.30%, Cr: 1.30-1.60%, Ni: 1.10-1.50%, Cu: 0.45-0.65%, Mo: 0.40-0.80%, Sn: 0.05-0.12%, Sb: 0.05-0.12%, Nb: 0.03-0.05%, V: 0.03-0.04%, Ti: 0.005-0.025%, Ca: 0.006-0.009%, P: ≤0.01%, S: ≤0.001%, Als: 0.02%-0.05%, and the rest are Fe and unavoidable impurities.

[0015] The steel surface microstructure is tempered martensite, and the steel plate matrix microstructure is ferrite and bainite.

[0016] The tempered martensite hardened layer on the surface of the steel plate has a thickness of 2 to 3 mm; the C content in the 3 mm area from the surface of the steel plate to the inside of the matrix is ​​0.12% to 0.15%.

[0017] The ferrite phase is acicular ferrite + equiaxed ferrite, and the average equiaxed grain size is less than 20μm; the proportion of the bainite phase to the total area ranges from 20% to 30%; the average length of the tempered martensite lath is <20μm; the ferrite phase is acicular ferrite plus equiaxed ferrite, and the average equiaxed grain size is less than 20μm, and the interior of the original austenite grains is fine acicular ferrite, which accounts for 70% to 80% of the ferrite phase area fraction.

[0018] The elongation after fracture of the steel plate is ≥29%, the yield strength is ≥550 MPa, the tensile strength is 670 - 830 MPa, the Charpy impact energy at -60°C is ≥180 J, the nil-ductility transition temperature (NDTT) is ≤ -70°C, the surface hardness of the steel plate in HBW is ≥325, and the finished thickness range of the steel plate is 40 - 70 mm.

[0019] The reasons for the composition design of the present invention are as follows:

[0020] C: C is an essential element in steel materials. The steel plate of the present invention can be divided into a matrix and a surface hardened layer according to its tissue characteristics. The role of C in the matrix part of the steel plate of the present invention is mainly reflected in two points: one is to contribute to the strength of the steel plate matrix in the form of solid solution strengthening; the other is to contribute to the strength of the steel plate matrix in the form of precipitation strengthening by co-acting with microalloying elements. However, on the premise of meeting the strength design requirements of the present invention, from the perspective of the high ductility of the steel of the present invention, it is necessary to control the C content in the matrix as much as possible. This is mainly because the matrix of the steel plate of the present invention is a ferrite / bainite duplex structure, and it is necessary to control the C content to ensure a high ferrite phase ratio in the final structure, so as to release the stress generated by the plastic deformation of bainite during the overall coordinated deformation process and avoid necking due to premature stress concentration. On the other hand, the surface hardenability of the steel does not require increasing the C content in the matrix, but is solved by the surface carburizing process. Therefore, the C content in the steel plate matrix is preferably C: 0.045% - 0.065%. The role of C in the surface hardened layer part of the steel plate of the present invention is mainly reflected in two points: one is to ensure the martensite formation ability of the steel plate during surface quenching, so that the steel plate can form a surface hardened layer with an obvious hardness difference from the matrix through surface quenching; the other is to improve the wear resistance of the steel in the form of dispersed carbonitrides with Nb, V, and Ti in the surface hardened layer, both of which require a relatively high C content. Therefore, considering the comprehensive performance of the product's erosion wear resistance, the C content on the surface layer of the steel plate is increased by surface carburizing to achieve C: 0.12% - 0.15% from the surface to 3 mm inward.

[0021] Si: It is the main deoxidizing component in the steelmaking process. Si existing in solid solution form will increase the ductile-brittle transition temperature, elastic limit, and fatigue strength while increasing the strength. In order to obtain a sufficient deoxidizing effect, it must contain more than 0.10%. However, if the upper limit is too high, it will reduce the toughness of the base metal and the welded part, and have an adverse effect on the surface quality of the steel. Therefore, the Si content is preferably controlled at 0.25% - 0.35%.

[0022] Mn: Mn is an essential element to ensure the strength and toughness of steel. It also has the effect of expanding the austenite phase region, reducing the temperatures of Ac1, Ac3, Ar1, and Ar3 points, and refining the ferrite grains. In this invention, the role of Mn is mainly reflected in two aspects: one is to contribute to the matrix strength of the steel plate through solid solution strengthening; the other is that Mn can greatly reduce the martensite transformation temperature of the steel, improve the hardenability of the steel, and ensure the martensite formation ability during surface quenching of the steel plate. However, Mn element is prone to segregation in steel, and it is easy to segregate at 1 / 4 and 1 / 2 thicknesses of medium and heavy steel plates, resulting in a significant reduction in low-temperature toughness and even the phenomenon of impact fracture delamination; more importantly, with the increase of Mn content, the corrosion resistance of the steel plate will decrease sharply; therefore, considering the material mechanics and corrosion resistance comprehensively, the preferred range of Mn content is controlled at 1.00% - 1.30%.

[0023] Cr: Cr is an effective element to improve the corrosion resistance of steel. The addition of Cr in steel has an obvious impact on the electrochemical process of steel. Cr element can inhibit the anodic dissolution process in the electrochemical process, and the corrosion potential of the steel matrix shifts positively. The higher the addition amount of Cr, the more obvious the inhibitory effect on the anodic process. On the one hand, Cr can form Cr2O3 and Cr(OH)3 products with better stability and corrosion resistance in the rust layer; on the other hand, Cr can promote the formation of α-FeOOH in the rust layer, and a part of it will also be doped in α-FeOOH and Fe3O4 to form α-(Fe,Cr)OOH and FeCr2O4 with more stable thermodynamics. Under the combined action of the two, the stability and protection of the rust layer of Cr-containing steel are greatly improved. The Cr-containing products in the rust layer can improve the originally loose and porous network structure of the rust layer, provide more nucleation sites, and the rust particles gradually become spherical and refined, which is conducive to the subsequent close aggregation and fusion between particles, and the defects in the rust layer are reduced. As time goes by, the structure of the rust layer gradually becomes denser, which can effectively hinder the penetration of erosive Cl- ions in seawater. In addition, Cr is also an effective element to improve the strength of the steel plate. However, excessive Cr can cause a decrease in welding performance. Therefore, in this invention, the Cr content is controlled at 1.30% - 1.60%.

[0024] Ni: Ni has a solid solution strengthening effect, can improve the strength and toughness of steel, and has the characteristic of reducing the Ar3 point with the minimum increase in carbon equivalent or cold crack sensitivity coefficient Pcm. Ni is also one of the key alloying elements to improve the corrosion resistance characteristics of steel plates. Adding Ni element in steel can promote the formation of a stable, dense, and anti-chloride-ion erosion and peeling amorphous layer on the surface of the steel plate, inhibit the permeability of chloride ions and oxygen atoms, and improve the corrosion resistance characteristics of the steel plate. However, the cost of adding Ni element is relatively high. Therefore, in this invention, the Ni content range is controlled at 1.10% - 1.50%.

[0025] Cu: Cu is an essential element for the corrosion resistance of steel in marine environments. During the corrosion process of steel, it will be enriched in the inner rust layer in the form of CuO, which can effectively isolate corrosive media, reduce the promoting effect of chloride ions on corrosion, increase the polarization resistance of steel, enhance the protection of the rust layer, and thus improve the corrosion resistance of steel. In terms of mechanical properties, Cu can increase the strength of the steel plate with little impact on the low-temperature toughness of the steel plate. However, when excessive Cu is added, during hot rolling and normalizing processes, fine and dispersed ε-Cu precipitates will occur (the solubility of Cu in ferrite is about 0.45% or so), which will damage the low-temperature toughness of the steel plate and may also cause copper brittleness, billet cracks, liquid segregation and other defects. Therefore, considering both the mechanical and corrosion resistance properties of the material, the preferred range of Cu content is controlled at 0.45% - 0.65%.

[0026] Mo: Mo is a recognized element for resistance to local corrosion. Whether it exists in the rust layer in metallic or oxidized form, Mo can increase the resistivity of the rust layer and the densification degree of the rust layer, thereby enhancing the protection of the rust layer. Mo and Cr also have a certain synergistic effect, which can promote the transformation of alloying elements in the rust layer into corrosion-resistant products. In addition, Mo can also improve the hardenability of steel. The range of Mo controlled in this invention is 0.40% - 0.80%.

[0027] Sn: Sn can increase the corrosion potential of steel itself, reduce the self-corrosion current density, and slow down the corrosion rate and improve its corrosion resistance by increasing the double-layer resistance and inhibiting ion exchange. After the formation of the rust layer, the presence of SnO2 in the rust layer can inhibit the further dissolution of steel and reduce the corrosion rate. There is also a synergistic effect between Sn and Cu, forming a continuous film layer containing both tin oxide and copper oxide on the steel surface, improving the corrosion resistance of the steel. However, too high Sn content will damage the toughness of the steel and increase the risk of local acidification pitting, so its content is controlled at 0.05% - 0.25%.

[0028] Sb: When compounded with Sn in steel, it can improve the corrosion resistance and wear resistance of steel, resulting in the enrichment of Sn and the uniform distribution of Sb in the rust layer, and forming a SnO2 - Sb2O5 corrosion-resistant oxide film on the steel surface, which can enhance the ability to block the penetration of Cl- ions. However, Sb has an adverse effect on the mechanical properties of steel, reducing the strength and increasing the brittleness of the steel. The range of Sb controlled in this invention is 0.10 - 0.25%.

[0029] Nb: Nb has a very strong affinity with C, N, and O, forming corresponding extremely stable compounds with them. Nb can refine the grains of steel, reduce the overheating sensitivity of steel, and under certain existing conditions, can improve the strength and toughness of steel. Especially under the condition of induction heating with a high heating rate, fine Nb carbonitrides precipitate to refine austenite grains. Moreover, according to the two-dimensional misfit theory, during the phase transformation from austenite to ferrite, the misfit degree between Nb carbonitrides and ferrite is higher than that of V carbonitrides. Theoretically, its effectiveness in promoting the formation of acicular ferrite is moderately effective, but the precipitation efficiency of Nb carbonitrides during this phase transformation is 3 times that of V. Therefore, it actually has a better promoting effect on inducing acicular ferrite. The Nb content of this invention is controlled at 0.03% - 0.05%.

[0030] V: V has a strong affinity with C and N. The fine V(C,N) particles dispersed in ferrite are the main way of vanadium microalloying strengthening. The V(C,N) precipitates in ferrite conform to the B-N orientation relationship with the ferrite matrix and maintain a coherent relationship with the ferrite matrix at the initial stage of uniform precipitation in ferrite. Compared with other microalloying elements such as Nb and Ti, the V carbides and nitrides have the smallest misfit degree with the ferrite matrix. The coherent relationship between the V carbides and nitrides and the ferrite matrix can be maintained to a larger particle size. Therefore, it is beneficial to improve the resistance to fatigue crack initiation and propagation of steel, and also improve the yield ratio, low-temperature toughness, and welding performance of steel. And the smaller the misfit degree value, the easier the heterogeneous nucleation occurs. Therefore, the addition of V plays an important role in the formation of acicular ferrite. However, the V content should not be too high, as too high a content will reduce the toughness of steel and is not conducive to the creep performance of steel. The V content of this invention is controlled at 0.03% - 0.04%.

[0031] Ti: Ti can not only improve the strength of steel, refine grains, reduce aging sensitivity and cold brittleness, but also a small amount of titanium can improve welding performance. Ti exists in the form of TiN and plays a role. In this invention, the property that TiN has a good coherent relationship with acicular ferrite is mainly utilized, and it can become the core for the formation of acicular ferrite. This heterogeneous nucleus with a good coherent relationship with acicular ferrite can effectively promote the formation of matrix acicular ferrite. Therefore, the preferred Ti content is 0.005% - 0.025%.

[0032] Ca: Ca treatment is carried out on the molten steel to purify the molten steel. The Ca element can spheroidize the MnS inclusions, disperse their sizes, prevent the MnS inclusions from forming long strip plastic shapes during the rolling process, and form dispersed spherical shapes. In addition, it can refine the size of sulfide inclusions, inhibit the hot brittleness of S, improve the impact toughness and Z-direction performance of the steel plate, and improve the anisotropy of the impact toughness of the steel plate. In addition, the calcium oxide inclusions generated by Ca treatment in the metallurgical process are dispersed while carbon monoxide bubbles are generated. The dispersed calcium oxide inclusions serve as heterogeneous nucleation cores and can induce the formation of intragranular acicular ferrite during the subsequent controlled cooling process. The content range of Ca element in the present invention is 0.006 - 0.009%.

[0033] P: P can segregate in the center part of the slab and aggregate at the grain boundaries, which is an element that has an adverse effect on the low-temperature toughness, ductility and weldability of the steel plate. Although P has the effect of improving the corrosion resistance of the steel plate, it has a negative effect on the fatigue performance of the steel plate. Considering the requirements of the corrosion fatigue resistance of the steel plate, P cannot be used as a characteristic element. The present invention requires that the P content be controlled not to exceed 0.01%.

[0034] S: S can combine with Mn in the steel to form MnS inclusions. During hot rolling, MnS extends along the rolling direction to form a MnS inclusion band along the rolling direction, which is likely to become a crack source under long-term complex loads, seriously damaging the anti-fatigue performance, low-temperature impact toughness and Z-directionality of the steel plate. Therefore, the present invention requires that the S content be controlled not to exceed 0.001%.

[0035] Als: As a deoxidizing and grain-refining element that must be added in the present invention, the added content is above 0.01%, but when it exceeds 0.08%, it is easy to generate hot cracks in the continuous casting billet, and at the same time, the toughness of the steel decreases. The Als content is controlled at 0.02% - 0.05%.

[0036] The composition design idea of the present invention is that the steel plate matrix is designed with a low C content to ensure a high ferrite phase ratio in the final structure to achieve high ductility of the steel plate, and the surface layer of the steel plate is carburized to increase the surface C content to ensure the martensite formation ability to achieve the wear resistance of the steel plate. Adding Cr, Cu, Ni, Mo, Sn, Sb can improve the corrosion resistance of the steel, and microalloying with Nb, V, Ti can improve the matrix strength of the steel and promote the formation of acicular ferrite, and improve the wear resistance of the steel in the form of dispersed carbonitrides in the surface hardened layer. Through the synergistic effect between various elements and process control, the steel plate has excellent anti-ice load erosion wear resistance, elongation performance and low-temperature impact toughness.

[0037] The second technical solution of the present invention is to provide a manufacturing method of a high-ductility anti-ice load erosion-resistant ultra-high-strength offshore steel, including smelting, continuous casting, heating, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling, secondary cooling, heat treatment;

[0038] (1) Smelting process:

[0039] a) During converter smelting, adjust the contents of elements such as C, Si, Mn, P, and S to bring their contents within the scope of the present invention, and add other alloying components for smelting according to requirements.

[0040] b) Refine the molten steel and adjust the contents of other alloying elements to within the scope of the present invention.

[0041] c) Conduct RH treatment on the refined molten steel. The RH treatment time is ≥ 30 min. Before continuous casting and drawing, add calcium carbide to the mold for final deoxidation, and control [H] ≤ 2.0 ppm and [O] ≤ 18 ppm in the steel. Calcium carbide reacts violently with the molten steel at high temperature, and at the same time, CaO inclusions are generated. Since the time from the generation of CaO inclusions to the solidification of the molten steel is short and the movement of the molten steel is relatively gentle, the possibility of the just-generated CaO inclusions aggregating and growing is relatively small, and they will be distributed in a dispersed state when CO bubbles are generated. A large number of dispersed small CaO inclusions can promote the formation of acicular ferrite as heterogeneous nucleation cores during the subsequent controlled cooling process. Finally, the number of Ca-containing composite inclusions with a size of 0.3 - 2 μm in the steel plate matrix reaches 2000 - 2400 per mm 2 .

[0042] (2) Obtain the required casting blank from the molten steel obtained in step (1) through continuous casting. To improve the central segregation of the casting blank, control the superheat of the tundish ≤ 30 °C; conduct full protection casting, and apply electromagnetic stirring and soft reduction. Electromagnetic stirring: I ≥ 450 A, and the current frequency is 200 - 250 kHz.

[0043] (3) Primary heating process: Heat the casting blank obtained in step (2) to a temperature of 1160 - 1200 °C. Since the steel alloy in the present invention has a high content and poor thermal conductivity, in order to control the heating quality of the continuous casting blank, preferably, conduct segmented heating; during the first heating, adopt a segmented heating process. Below 850 °C, use slow heating to relieve the thermal stress caused by the thermal conductivity, and the heating rate is 9 - 13 °C / min. Above 850 °C, use a fast heating process to prevent abnormal growth of austenite grains, and the heating rate is 17 - 22 °C / min. After reaching the target temperature, hold for 1 - 2.5 h.

[0044] (4) Primary rolling process: The rolling temperature is in the austenite recrystallization temperature range of 1050 - 1100 °C, and roll the casting blank to 1.5 - 2.0 times the finished product thickness. The reduction in the first pass ≥ 50 mm, and the average single-pass reduction rate is above 15%. The purpose is to fully break the austenite grains of the casting blank and make the austenite dynamic recrystallization process proceed fully to refine the austenite grain size;

[0045] (5) Primary cooling process: After the first rolling, a rapid laminar cooling system with an average cooling rate of 8 - 15 °C / s is used to quickly cool the intermediate billet to room temperature, completing the austenitization pretreatment of the intermediate billet and obtaining a fine martensite / bainite structure.

[0046] (6) Secondary heating process: The intermediate billet obtained in step (5) is sent into a carburizing furnace for secondary heating. The intermediate billet is heated to 920 - 960 °C for re-austenitization at a heating rate of 17 - 22 °C / min. Subsequently, drip-feed gas surface carburization is carried out using methanol and kerosene as carburizing agents, and the holding time is 4 - 6 h. The pre-treatment structure after the first rolling and cooling is a fine martensite / bainite structure. This non-steady-state structure has a high internal energy storage. A large number of crystal defects such as dislocations and interfaces can provide a large number of recrystallization nucleation sites during the secondary heating austenitization process, and a refined austenite structure (average equivalent diameter of the original austenite grains ≤ 20 μm) can be obtained. On the other hand, the secondary heating is equivalent to the effect of normalizing treatment, which is beneficial to the uniform distribution of corrosion-resistant solid solution elements in the steel. Surface carburization can increase the carbon content on the surface layer of the steel plate (carbon content within 3 mm from the surface > 0.12%) and form a certain carbon concentration gradient to enhance surface hardenability without increasing the overall carbon content of the steel plate, and cooperate with subsequent induction surface quenching to achieve surface hardening.

[0047] (7) Secondary rolling process: The secondary rolling starting temperature is 820 - 850 °C, the average single-pass reduction rate reaches 15% - 20%, and the finishing rolling temperature is 770 - 810 °C. The purpose is to fully deform the austenite grains, provide energy storage and positions for phase transformation nucleation, increase the phase transformation nucleation rate, and further reduce the ferrite grains during the two-phase region deformation, ultimately achieving the purpose of grain refinement.

[0048] (8) Straightening process: The rolled steel plate needs to be straightened to improve the flatness of the steel plate and prevent problems such as warping of the steel plate during the cooling process.

[0049] (9) Secondary cooling process: After the steel plate is straightened and relaxed to a temperature of 730 - 760 °C, it enters a rapid laminar cooling system with an average cooling rate of 10 - 15 °C / s, and the return red temperature is controlled at 500 - 550 °C. The purpose of selecting high cooling rate and low return red in this invention is to control the phase transformation structure, mainly reflected in two points: one is to ensure a certain proportion of bainite to meet the design strength requirements of the steel plate; the other is that under this cooling condition, the thermodynamic conditions for the formation of a large amount of acicular ferrite can be achieved. This interlocking structure has higher strength than equiaxed ferrite, enabling the steel plate to be effectively strengthened without reducing its plasticity and toughness, and can also well prevent the propagation of cracks, which is beneficial to improving the fatigue resistance of the steel plate. Subsequently, the steel plate is air-cooled to room temperature to obtain a ferrite + bainite dual-phase matrix structure. Among them, the ferrite phase is composed of acicular ferrite and equiaxed ferrite, the average grain size of equiaxed ferrite is less than 20 μm, and the interior of the original austenite grains is fine acicular ferrite, accounting for 70 - 80% of the ferrite phase area; the bainite phase accounts for 20 - 30% of the total area.

[0050] (10) High-frequency induction surface hardening: Adopting the method of longitudinal magnetic flux induction heating, the surface of the cooled steel plate is heated by high-frequency induction to 960 - 1000 °C at a current frequency above 200 kHz and then quenched in water to room temperature. Subsequently, the surface-hardened steel plate is heated to 150 - 200 °C for low-temperature tempering for 0.5 - 1.0 h. The purpose is to utilize the skin effect of high-frequency induced eddy current to rapidly austenitize the surface of the steel plate and then quench it to form a surface hardened layer of 2 - 3 mm. After low-temperature tempering, a high-hardness tempered structure of fine-grained martensite (average length of martensite laths < 20 μm) plus dispersed carbide and nitride is formed on the surface of the steel plate, which can effectively improve the wear resistance of the steel plate without affecting the overall performance of the steel plate.

[0051] The design reason of this invention lies in:

[0052] (1) In this invention, the purity of the steel is improved by controlling the sulfur and phosphorus content in the pure steel smelting, and the steel quality is improved by spheroidizing inclusions through Ca treatment. At the same time, the dispersed calcium oxide inclusions formed by the reaction with the molten steel serve as heterogeneous nucleation cores, which can induce the formation of intragranular acicular ferrite during the subsequent controlled cooling process. The chemical composition of the steel plate is reasonably designed. The steel plate matrix is designed with a low C content to ensure a high ferrite phase ratio in the final structure to achieve high ductility of the steel plate. The surface C content of the steel plate is increased through carburizing technology to ensure the martensite formation ability to achieve the wear resistance of the steel plate. Cr, Cu, Ni, Mo, Sn, and Sb are added to improve the corrosion resistance of the steel, and Nb, V, and Ti are microalloyed to improve the matrix strength of the steel and promote the formation of acicular ferrite, and improve the wear resistance of the steel in the form of dispersed carbide and nitride in the surface hardened layer. Through the synergistic effect of various elements combined with process control, the steel plate has excellent anti-ice load erosion wear resistance, elongation performance, and low-temperature impact toughness.

[0053] (2) The methods of secondary heating rolling, surface carburizing, controlled cooling, and high-frequency induction surface hardening are innovatively adopted. The intermediate billet after rapid cooling is reheated. The metastable structure with high energy storage provides released energy and a large number of nucleation sites during the re-austenitization process, refining the austenite grain size of the intermediate billet. Meanwhile, it is beneficial to the good structure and alloy element uniformity of the steel plate in the second-stage rolling, and is conducive to improving the plasticity and toughness of the product. The high cooling rate and low reverse redness of secondary cooling ensure a certain proportion of bainite to meet the design strength requirements of the steel plate, and at the same time realize the thermodynamic conditions for the formation of a large amount of acicular ferrite. The introduction of acicular ferrite effectively strengthens the steel plate without reducing its plasticity and toughness. Its interlocking structure can well prevent the propagation of cracks, which is beneficial to improving the fatigue resistance of the steel plate. Surface carburizing can increase the carbon content on the surface layer of the steel plate and form a certain carbon concentration gradient to enhance the surface hardenability without increasing the overall carbon content of the steel plate. Subsequently, it is combined with high-frequency induction heating for rapid austenitization on the surface and then quenching to form a surface hardened layer. After low-temperature tempering, a high-hardness tempered structure of fine-grained martensite plus dispersed carbonitrides is formed on the surface of the steel plate, which can effectively improve the wear resistance of the steel plate without affecting the overall performance of the steel plate, providing guarantee for the safety of the steel plate in polar service in ocean engineering.

[0054] (3) The steel plate prepared by the present invention has anti-ice load erosion and wear resistance (more than 2 times that of the same-level steel plate), ultra-high strength (yield strength ≥ 550 MPa, tensile strength 670 - 830 MPa), high ductility (elongation after fracture ≥ 29%), and low-temperature impact toughness (-60 °C Charpy impact energy ≥ 180 J, nil ductility transition temperature (NDTT) ≤ -70 °C), surface hardness HBW ≥ 325, and the product thickness range is 40 - 70 mm.

[0055] (4) The tissue characteristics of the steel plate of the present invention are as follows: The matrix of the steel plate is a duplex structure of ferrite plus bainite, among which the ferrite phase is acicular ferrite plus equiaxed ferrite, the average equiaxed grain size is less than 20 μm, and the interior of the prior austenite grains is fine acicular ferrite, accounting for 70 - 80% of the area fraction of the ferrite phase; the bainite phase accounts for 20 - 30% of the total area. The surface is a tempered martensite hardened layer with a depth of 2 - 3 mm, and the average length of martensite laths < 20 μm. Description of the Drawings

[0056] Figure 1 It is the microstructural diagram of the surface hardened layer of Example 1 of the present invention.

[0057] Figure 2 It is the microstructural diagram of the matrix of Example 1 of the present invention. Detailed Embodiments

[0058] The present invention will be further described below through examples.

[0059] In the embodiments of the present invention, smelting, continuous casting, heating, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling, secondary cooling, and heat treatment are carried out according to the component ratios of the technical solutions.

[0060] Primary heating: The heating temperature of the slab is 1160 - 1200 °C, and the holding time is 1 - 2.5 h.

[0061] Primary rolling: The rolling temperature is 1050 - 1100 °C, the reduction in the first pass is ≥50 mm, the average reduction per pass is above 15%, and the slab is rolled to 1.5 - 2.0 times the finished product thickness.

[0062] Primary cooling: After the primary rolling is completed, a rapid laminar cooling system with an average cooling rate of 8 - 15 °C / s is used to quickly cool the intermediate billet to room temperature.

[0063] Secondary heating: The intermediate billet is sent into a carburizing furnace for secondary heating. The intermediate billet is heated to 920 - 960 °C for re-austenitization at a heating rate of 17 - 22 °C / min, and then the surface of the steel plate is carburized and held for 4 - 6 h; preferably, drip-feed gas surface carburizing is carried out using methanol and kerosene as carburizing agents.

[0064] Secondary rolling: The rolling start temperature is 820 - 850 °C, the average reduction per pass reaches 15 - 20%, and the final rolling temperature is 770 - 810 °C.

[0065] Secondary cooling: The steel plate is relaxed and held at 730 - 760 °C and then enters a rapid laminar cooling system with an average cooling rate of 10 - 15 °C / s. The return red temperature is controlled at 500 - 550 °C; then the steel plate is air-cooled to room temperature.

[0066] Heat treatment: High-frequency induction surface hardening is adopted. In the way of longitudinal magnetic flux induction heating, the surface of the cooled steel plate is heated by high-frequency induction to 960 - 1000 °C at a current frequency above 200 kHz and then water-quenched to room temperature. Subsequently, the surface-hardened steel plate is heated to 150 - 200 °C for low-temperature tempering for 0.5 - 1.0 h.

[0067] Furthermore, for smelting:

[0068] a) During converter smelting, the contents of elements such as C, Si, Mn, P, and S are adjusted to be within the scope of the present invention, and other alloying components are added for melting according to requirements.

[0069] b) The molten steel is refined, and the contents of other alloying elements are adjusted to be within the scope of the present invention.

[0070] c) Subject the refined molten steel to RH treatment with a RH treatment time ≥ 30 min. Add calcium carbide to the mold before continuous casting to conduct final deoxidation, and control [H] ≤ 2.0 ppm and [O] ≤ 18 ppm in the steel. The number of Ca-containing complex inclusions with a size of 0.3 - 2 μm in the final steel plate matrix reaches 2000 - 2400 per mm 2 .

[0071] Furthermore, for continuous casting: control the superheat in the tundish ≤ 30°C; conduct casting under full protection, and apply electromagnetic stirring and soft reduction. For electromagnetic stirring: I ≥ 450 A.

[0072] Furthermore; the primary heating adopts a segmented heating process. Below 850°C for the slab, use slow heating with a heating rate of 9 - 13°C / min; above 850°C for the slab, use a rapid heating process with a heating rate of 17 - 22°C / min.

[0073] Furthermore, the microstructure of the intermediate billet after primary cooling is fine martensite / bainite structure.

[0074] Furthermore, the microstructure of the steel plate after secondary heating is a refineable austenite structure, and the average equivalent diameter of the original austenite grains ≤ 20 μm.

[0075] The compositions of the steel in the examples and comparative examples of the present invention are shown in Table 1. The main process parameters for steelmaking, heating, and surface carburizing of the steel in the examples of the present invention are shown in Table 2. The main process parameters for rolling and cooling of the steel in the examples of the present invention are shown in Table 3. The main process parameters for heat treatment of the steel in the examples of the present invention are shown in Table 4. The microstructures of the steel in the examples of the present invention are shown in Table 5. The properties and NDTT temperatures of the steel in the examples of the present invention are shown in Table 6. The anti-ice load erosion wear weight loss (g) of the examples and comparative examples of the present invention are shown in Table 7.

[0076] Table 1 Compositions of the steel in the examples and comparative examples of the present invention (wt%)

[0077]

[0078] Note: P: ≤ 0.01%, S: ≤ 0.001%

[0079] Table 2 Main process parameters for steelmaking, heating, and surface carburizing of the steel in the examples of the present invention

[0080]

[0081] Table 3 Main process parameters for rolling and cooling of the steel in the examples of the present invention

[0082]

[0083] Table 4 Main process parameters for heat treatment of the steel in the examples of the present invention

[0084]

[0085] Table 5 Microstructure of the steel in the embodiments of the present invention

[0086]

[0087] Table 6 Properties of the steel in the embodiments of the present invention and NDTT temperature

[0088]

[0089] Table 7 Erosion wear weight loss (g) of ice load erosion wear of the embodiments and comparative examples of the present invention

[0090] Embodiment 1.0 m / s 2.0 m / s 1 0.0014 0.0026 2 0.0018 0.0029 3 0.0017 0.0032 4 0.0016 0.0025 5 0.0019 0.0032 6 0.0017 0.0028 7 0.0019 0.0034 8 0.0021 0.0033 Comparative Example 0.0055 0.0074

[0091] Remarks: Detection of ice load erosion wear performance: Introduce a 50-mm-thick FH550 steel plate as a comparative example. The sample is processed into a steel sample of 10 mm × 10 mm × 4 mm using a wire cutting machine, and the outer surface is polished step by step with 180#, 400#, 800#, and 1200# sandpapers. Then, it is ultrasonically cleaned, dried, weighed, and the results are recorded and sealed for standby. Prepare a 3.5% NaCl solution according to the ASTM D1141-98 standard to simulate the seawater environment, which is used as the test medium for this erosion wear test. The ice load erosion wear test is completed on a low-temperature sea ice erosion wear testing machine. Before the erosion wear test, first use a mold to prepare 10 mm × 10 mm × 10 mm cube ice blocks from 3.5% NaCl aqueous solution in a cryogenic environmental chamber at -60°C. The ice-making time is 24 h. Then, add the ice blocks to the 3.5% NaCl aqueous solution pre-insulated at 0°C at a ratio of 1:1 for the ice load erosion wear test. The environmental temperature is controlled at -5 ± 1°C throughout the test. The erosion test adopts the rotary test method and is carried out at rotational speeds of 1.0 m / s and 2.0 m / s. The test time is 30 h. After the erosion wear test, place the specimen in a rust-removing solution (3.5 g hexamethylenetetramine + 500 ml hydrochloric acid + 500 ml deionized water) for 30 s of ultrasonic treatment, then rinse it successively with alcohol and deionized water, dry it, and weigh it. The erosion wear weight loss of the specimen is calculated from the mass difference of the specimen before and after the test.

[0092] The elongation after fracture of the offshore steel plate produced by applying the technical solution of the present invention is ≥29%, the yield strength is ≥550 MPa, the tensile strength is 670 - 830 MPa, the Charpy impact energy at -60°C is ≥180 J, NDTT ≤ -70°C, the surface hardness of the steel plate is ≥325 HBW, and the finished thickness range of the steel plate is 40 - 70 mm; the ice load erosion wear performance is more than twice that of the steel plates of the same grade.

[0093] In order to describe the present invention, the present invention has been properly 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 also 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. An ultra-high strength offshore steel with high ductility and resistance to ice load erosion, characterized in that, The composition of the offshore steel is as follows by weight percentage: C: 0.045% - 0.065%, Si: 0.25% - 0.35%, Mn: 1.00% - 1.30%, Cr: 1.30% - 1.60%, Ni: 1.10% - 1.50%, Cu: 0.45% - 0.65%, Mo: 0.40% - 0.80%, Sn: 0.05% - 0.12%, Sb: 0.05% - 0.12%, Nb: 0.03% - 0.05%, V: 0.03% - 0.04%, Ti: 0.005% - 0.025%, Ca 0.006% - 0.009%, P: ≤0.01%, S: ≤0.001%, Als: 0.02% - 0.05%, and the balance is Fe and unavoidable impurities; the elongation after fracture of the offshore steel plate is ≥29%, the yield strength is ≥550 MPa, the tensile strength is 670 - 830 MPa, the Charpy impact energy at -60°C is ≥180 J, NDTT ≤ -70°C, the surface hardness of the steel plate is ≥325 HBW, and the finished thickness range of the steel plate is 40 - 70 mm; the manufacturing method of a high-ductility anti-ice load erosion-resistant ultra-high-strength offshore steel includes smelting, continuous casting, heating, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling, secondary cooling, and heat treatment; Primary heating: The casting blank is heated to a temperature of 1160 - 1200°C and held for 1 - 2.5 h; Primary rolling: The rolling temperature is 1050 - 1100°C, the reduction in the first pass is ≥50 mm, the average single-pass reduction rate is above 15%, and the casting blank is rolled to 1.5 - 2.0 times the finished thickness; Primary cooling: After the primary rolling is completed, a rapid laminar cooling system with an average cooling rate of 8 - 15°C / s is used to quickly cool the intermediate billet to room temperature; Secondary heating: The intermediate billet is sent to a carburizing furnace for secondary heating, and the intermediate billet is heated to 920 - 960°C for re-austenitization at a heating rate of 17 - 22°C / min, and then the steel plate is surface carburized and held for 4 - 6 h; Secondary rolling: The rolling start temperature is 820 - 850°C, the average single-pass reduction rate reaches 15% - 20%, and the finishing rolling temperature is 770 - 810°C; Secondary cooling: The steel plate is relaxed and held at 730 - 760°C and enters a rapid laminar cooling system with an average cooling rate of 10 - 15°C / s, and the return red temperature is controlled at 500 - 550°C; then the steel plate is air-cooled to room temperature; Heat treatment: High-frequency induction surface quenching is adopted. In the way of longitudinal magnetic flux induction heating, the surface of the cooled steel plate is high-frequency induction heated to 960 - 1000°C at a current frequency above 200 kHz and then water-quenched to room temperature, and then the surface-quenched steel plate is heated to 150 - 200°C for low-temperature tempering for 0.5 - 1.0 h.

2. The ultra-high strength offshore steel with high ductility and resistance to ice load erosion loss according to claim 1, wherein The surface microstructure of the offshore steel is tempered martensite, and the matrix microstructure of the steel plate is ferrite and bainite.

3. The ultra-high strength marine steel with high ductility and resistant to ice load erosion according to claim 2, characterized in that, The thickness of the hardened layer of the tempered martensite on the surface of the offshore steel plate is 2 - 3 mm; the C content in the area 3 mm from the surface of the steel plate to the matrix is 0.12% - 0.15%.

4. An ultra-high strength offshore steel with high ductility and resistance to ice load erosion according to claim 2, characterized in that, The ferritic phase of the offshore steel is acicular ferrite + equiaxed ferrite, and the average grain size of the equiaxed ferrite is less than 20 μm; the proportion of the bainite phase in the total area ranges from 20% to 30%; the average length of the tempered martensite laths < 20 μm; among them, the ferritic phase is acicular ferrite plus equiaxed ferrite, the average grain size of the equiaxed ferrite is less than 20 μm, and the interior of the original austenite grains is fine acicular ferrite, accounting for 70-80% of the area fraction of the ferritic phase.

5. The ultra-high strength offshore steel with high ductility and resistant to ice load erosion according to claim 1, characterized in that: Smelting: a) Adjust the contents of elements such as C, Si, Mn, P, S, etc. during converter smelting to make their contents within the scope of the present invention, and add other alloying components for smelting according to requirements; b) Refine the molten steel and adjust the contents of other alloying elements to within the scope of the present invention; c) The refined molten steel is subjected to RH treatment with a RH treatment time ≥ 30 min. Calcium carbide is added to the mold before continuous casting withdrawal for final deoxidation, controlling [H] ≤ 2.0 ppm and [O] ≤ 18 ppm in the steel. The number of Ca-containing complex inclusions with a size of 0.3 - 2 μm in the final steel plate matrix reaches 2000 - 2400 pieces / mm 2 .

6. The ultra-high strength offshore steel with high ductility and resistant to ice load erosion according to claim 1, characterized in that: Continuous casting: Control the superheat of the tundish ≤ 30 °C; protect the whole process of pouring, and input electromagnetic stirring and soft reduction, electromagnetic stirring: I ≥ 450 A.

7. The ultra-high strength offshore steel with high ductility and resistance to ice load erosion according to claim 1, characterized in that: The primary heating adopts a segmented heating process. When the temperature of the slab is below 850 °C, slow heating is adopted, and the heating rate is 9-13 °C / min; when the temperature of the slab is greater than 850 °C, a rapid heating process is adopted, and the heating rate is 17-22 °C / min.

8. An ultra-high strength offshore steel with high ductility and resistance to ice load erosion according to claim 1, characterized in that: The microstructure of the intermediate slab after the primary cooling is fine martensite / bainite microstructure.

9. An ultra-high strength offshore steel with high ductility and resistance to ice load erosion according to claim 1, characterized in that: The microstructure of the steel plate after the secondary heating is austenite microstructure that can be refined, and the average equivalent diameter of the original austenite grains ≤ 20 μm.

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