High-strength marine steel with high ductility and resistance to ice load corrosion and its manufacturing method

Abstract

The present invention provides a high-strength marine engineering steel with high ductility and resistance to ice load corrosion and a manufacturing method thereof. The components of the marine engineering steel are as follows by weight percentage: C: 0.065% to 0.085%, Si: 0.25% to 0.35%, Mn: 1.30% to 1.50%, Cr: 0.80% to 1.20%, Ni: 0.35% to 0.55%, Cu: 0.45% to 0.65%, Mo: 0.20% to 0.50%, Sn: 0.05% to 0.12%, Sb: 0.05% to 0.12%, Nb: 0.02% to 0.04%, V: 0.02% to 0.04%, Ti: 0.005%~0.025%, Ca:0.002%~0.005%, P:≤0.01%, S:≤0.002%, Als:0.02%~0.05%, and the rest are Fe and inevitable impurities; the manufacturing method comprises smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary rolling, secondary cooling and heat treatment; the marine engineering steel plate produced by the present invention has a tensile elongation after fracture of ≥31%, a yield strength of ≥420MPa, a tensile strength of 530~680MPa, a Charpy impact energy at ‑60℃ of ≥220J, a zero plastic transition temperature NDTT≤‑70℃, and a surface hardness of ≥285HBW.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and in particular relates to high-ductility ice-load-resistant high-strength marine engineering steel and a manufacturing method thereof. Background Art

[0002] Due to the particularity of the application environment of marine engineering, the service safety requirements of marine engineering equipment and structural materials have always been high. In recent years, as the offshore industry is gradually transforming and upgrading to the marine industry, the service environment of marine equipment has become increasingly complex and harsh. For marine engineering equipment and structures that serve for a long time in ice-covered waters or seas with seasonal sea ice, ice load is an external load that needs to be considered during the design and use of marine engineering. Ice load refers to the force generated by the interaction between sea ice and marine engineering equipment and structures in ice areas. Its main modes of action include: the impact force or friction force generated by drifting ice driven by wind and current; the extrusion force generated by large areas of ice under the action of wind and current; the vertical force of ice on open-type buildings caused by the rise and fall of the sea level; the vertical static load caused by the accumulation of sea ice on buildings; the extrusion force caused by the expansion of water bodies due to freezing. The above-mentioned ice load action mode reflects the three failure threat forms that ice-covered marine engineering structures and equipment may encounter during service, namely, extrusion deformation, collision impact and erosion wear. These three failure threat forms also reflect the three performance requirements for safe service of application materials under ice loads: ductility, low-temperature impact toughness and erosion wear resistance. At present, the American ABS classification society has included the ductility performance index into the relevant specification "Material requirements for higher-ductility hull structural steel plates and sections". Its purpose is to allow the steel plate to absorb the energy of the collision when the ship, marine engineering equipment and structure collide, so that it is not easy to cause structural instability, thereby improving the service safety of the equipment. Japan's Nippon Steel and Sumitomo Metal have developed such marine engineering steel plates with high ductility and collision resistance ("NSafe-Hull" series), which are currently in the application stage. At present, my country has not formed a systematic study and large-scale application of such steel plates. On the other hand, regarding the erosion wear resistance, most of the current domestic and foreign research focuses on the erosion wear process and performance of steel at room temperature, while there are few studies on the low-temperature erosion wear of steel under sea ice loads. In fact, the key parts of offshore structures and equipment in ice-covered areas are constantly subjected to repeated impact and friction from sea ice and corrosion from seawater, making their service conditions more complex and severe. Therefore, it is necessary to develop a marine engineering steel with high ductility, good low-temperature impact toughness and erosion and wear resistance to ensure the safety of engineering operations of marine engineering structures and equipment in ice-covered areas under ice loads.

[0003] Public reports of related patents:

[0004] The invention "A high-ductility EH36 grade marine engineering steel and its manufacturing method" (publication number CN114807761A) discloses marine engineering steel whose chemical composition is calculated 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. The disadvantage is that the strength level of the steel plate produced by the invented manufacturing method is not high, and the erosion and wear performance evaluation has not been carried out.

[0005] The invention "A 440MPa grade polar hull steel and its preparation method" (publication number CN112831720A) discloses a shipbuilding steel whose chemical composition, in weight percentage, is 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. 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. The disadvantage is that the erosion and wear performance of the steel is not evaluated.

[0006] The invention "A wear-resistant steel plate and its preparation method" (publication number CN110656291A) discloses the following components in terms of weight percentage: 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: 0.008-0.0 35%, Als: 0.015-0.040%, As≤0.04%, Sn≤0.03%, N≤0.005%, O≤0.003%, H≤0.0002%, the balance is Fe and inevitable impurities, the preparation method adopts the TMCP method, and the bainite, ferrite and pearlite composite structure is obtained by two-stage controlled rolling and controlled cooling. Combined with the effect of alloy components, the steel plate has sufficient toughness and certain corrosion resistance and wear resistance. The disadvantage is that this technology uses high-speed cooling and low-temperature anti-red to increase the proportion of bainite (>70%) to ensure the hardness and wear resistance of the steel plate, and cooling will inevitably have a negative impact on the plasticity of the steel plate.

[0007] The invention "An Economical Corrosion-Resistant Wear-Resistant Steel and Its Preparation Method" (publication number CN112195405A) discloses a corrosion-resistant steel with low-cost C-Mn-B as the main wear-resistance strengthening element, which plays the role of B element in enhancing hardenability, realizes a substantial improvement in the strength and wear resistance of the material by solid solution elements such as C and Mn, and improves the corrosion resistance of the material by combining the synergistic effect of elements such as P, Sb, and Ce. The preparation method adopts the TMCP plus quenching and tempering method, and achieves microstructure refinement by high (medium) frequency induction quenching and tempering with a high heating rate after two-stage controlled rolling and controlled cooling, and obtains tempered martensite / bainite structure, so that the steel has both good wear resistance and corrosion resistance; its disadvantage is that the technology uses induction heating to temper the entire plate, which consumes a lot of electricity and is difficult to achieve uniform heating of thick steel plates due to the skin effect of the induced eddy current, which limits the thickness of the heated body.

[0008] The invention "A method for improving the production efficiency of thick plates in a single-stand rolling mill" (publication number CN111014292A) discloses a thick plate production method in which the intermediate billet is directly cooled after the first heating and rolling, and then heated and rolled for the second time, and then rolled to the finished product thickness after the second heating to the target temperature. The method of directly cooling the intermediate billet and then heating and rolling it again saves the cooling time of the intermediate billet, which can significantly improve the efficiency of the rolling mill and thus greatly increase the mill hourly output. The disadvantage is that this method increases the consumption of energy and industrial water due to the increase in the heating and cooling process in the process path, and the extent of the improvement in production efficiency remains to be discussed. Summary of the invention

[0009] The object of the present invention is to overcome the above-mentioned problems and shortcomings and to provide a high-strength marine steel having a tensile elongation after fracture of ≥31%, a yield strength of ≥420 MPa, a tensile strength of 530-680 MPa, a Charpy impact energy of -60°C of ≥220 J, a zero plastic transition temperature (NDTT) of ≤-70°C, high ductility and ice load corrosion resistance, and a method for manufacturing the same.

[0010] The object of the invention is achieved in this way:

[0011] 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 420Mpa 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 ≥31%, -60℃ impact energy ≥220J, NDTT temperature ≤-70℃, and surface hardness HBW ≥285.

[0012] A high-strength marine steel with high ductility and ice load corrosion resistance, the steel plate has the following components by weight percentage: C: 0.065% to 0.085%, Si: 0.25% to 0.35%, Mn: 1.30% to 1.50%, Cr: 0.80% to 1.20%, Ni: 0.35% to 0.55%, Cu: 0.45% to 0.65%, Mo: 0.20% to 0.50%, Sn: 0.05%~0.12%, Sb: 0.05%~0.12%, Nb: 0.02%~0.04%, V: 0.02%~0.04%, Ti: 0.005%~0.025%, Ca: 0.002%~0.005%, P: ≤0.01%, S: ≤0.002%, Als: 0.02%~0.05%, and the rest are Fe and unavoidable impurities.

[0013] The steel surface microstructure is tempered martensite, and the steel plate matrix microstructure is ferrite and bainite.

[0014] The tempered martensite hardened layer thickness of the steel plate surface is 2-3 mm, and the average length of the martensite lath is less than 15 μm; the volume percentage of the ferrite is 80%-90%, and the average grain size of the ferrite is 5.0-10.0 μm.

[0015] The offshore steel plate has a tensile elongation of ≥31%, a yield strength of ≥420 MPa, a tensile strength of 530-680 MPa, a -60°C Charpy impact energy of ≥220 J, a zero plastic transition temperature (NDTT) ≤-70°C, a surface hardness HBW ≥285, and a steel plate thickness of 50-80 mm.

[0016] The reasons for the composition design of the present invention are as follows:

[0017] The overall composition design idea of ​​the present invention is embodied in two parts: one is to select appropriate C and Mn contents to ensure the martensite forming ability of the steel plate under the condition of ensuring strength-plastic matching, so as to ensure the performance difference between the surface hardened layer and the matrix; the second is to add Cr, Cu, Ni, Mo, Sn, Sb corrosion-resistant characteristic elements and cooperate with Nb, V, Ti micro-alloying to improve the steel's resistance to ice load erosion, wear and fatigue performance, among which Cr, Cu, Ni, Mo, Sn, Sb improve the steel's corrosion resistance, and Nb, V, Ti micro-alloying improves the steel's strength and improves the steel's wear resistance in the form of dispersed carbonitrides in the surface hardened layer.

[0018] C: is a necessary element to ensure strength. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. In the present invention, the role of C is mainly reflected in four points: first, it contributes to the strength of the steel plate matrix through solid solution strengthening; second, it contributes to the strength of the steel plate matrix through precipitation strengthening in conjunction with microalloying elements; third, it ensures the martensite forming ability of the steel plate during surface quenching, so that the steel plate can form a surface hardening layer with a significant hardness difference from the matrix through surface quenching; fourth, it improves the wear resistance of the steel in the form of dispersed carbonitrides in the surface hardening layer with Nb, V, and Ti. However, under the premise of meeting the strength design requirements of the present invention, it is necessary to reduce the C content as much as possible from the perspective of high ductility and erosion wear performance of the steel of the present invention. Therefore, from the perspective of product performance, it is preferred that the C content be controlled at C: 0.065-0.085%.

[0019] Si: It is the main deoxidizing component in the steelmaking process. Si in the form of solid solution will increase the strength while also increasing the ductile-brittle transition temperature, elastic limit and fatigue strength. In order to obtain a sufficient deoxidation effect, it must contain more than 0.10%. However, too high an upper limit will reduce the toughness of the base material and the weld, and have an adverse effect on the surface quality of the steel. Therefore, it is preferred that the Si content be controlled at 0.25-0.35%.

[0020] Mn: Mn is an essential element to ensure the strength and toughness of steel. In the present invention, the role of Mn is mainly reflected in two points: first, it contributes to the strength of the steel plate matrix through solid solution strengthening; second, Mn can greatly reduce the martensitic transformation temperature of steel, improve the hardenability of steel, and ensure the martensite formation ability of the steel plate during surface quenching. However, the Mn element is easy to segregate in steel, and it is easy to segregate at 1 / 4 and 1 / 2 thickness of medium and thick steel plates, thereby causing a strong reduction in low-temperature toughness and even the phenomenon of impact fracture stratification; more importantly, with the increase of Mn content, the corrosion resistance of the steel plate will drop sharply; therefore, considering the material mechanics and corrosion resistance, it is preferred that the Mn content range is controlled within 1.30-1.50%.

[0021] Cr: Cr is an effective element to improve the corrosion resistance of steel. The addition of Cr in steel has a significant effect on the electrochemical process of steel. Cr can inhibit the anodic dissolution process in the electrochemical process, and the corrosion potential of the steel matrix shifts positively. The higher the amount of Cr added, 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 be doped in α-FeOOH and Fe3O4 to form thermodynamically more stable α-(Fe,Cr)OOH and FeCr2O4. Under the combined effect 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 original loose and porous network structure of the rust layer, provide more nucleation sites, and the rust particles gradually spheroidize and refine, which is conducive to the subsequent close agglomeration and fusion between particles, and the defects in the rust layer are reduced. With the extension of time, the structure of the rust layer gradually densifies, which can effectively hinder the penetration of corrosive Cl- ions in seawater. In addition, Cr is also an effective element for improving the strength of steel plates, but excessive Cr may cause a decrease in welding performance, so the present invention controls the Cr content to 0.80-1.20%.

[0022] Ni: Ni has a solid solution strengthening effect, which can improve the strength and toughness of steel, and has the characteristics of reducing the Ar3 point and minimizing the increase in carbon equivalent or cold crack sensitivity coefficient Pcm. Ni is also one of the key alloying elements for improving the corrosion resistance of steel plates. Adding Ni elements to steel can promote the formation of a stable, dense, chloride ion-resistant amorphous layer on the surface of the steel plate, inhibit the permeability of chloride ions and oxygen atoms, and improve the corrosion resistance of the steel plate. However, the cost of adding Ni elements is relatively high, so the present invention controls the Ni content range to 0.35-0.55%.

[0023] Cu: Cu is an essential element for steel to resist corrosion 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 well isolate corrosive media, reduce the promoting effect of chloride ions on corrosion, and increase the polarization resistance of steel, resulting in enhanced protection of the rust layer, thereby improving the corrosion resistance of steel. In terms of mechanical properties, Cu can improve the strength of steel plates and has little effect on the low-temperature toughness of steel plates; but when too much Cu is added, fine and dispersed ε-Cu precipitation will occur during hot rolling and normalizing treatment (the solid solubility of Cu in ferrite is about 0.45%), which will damage the low-temperature toughness of the steel plate, and may also cause copper embrittlement, ingot cracks, liquid precipitation and other defects. Therefore, considering the material mechanics and corrosion resistance, it is preferred that the Cu content range be controlled within 0.45-0.65%.

[0024] Mo: Mo is a recognized local corrosion resistant element. Whether in the metallic state or in the oxidized state, Mo in the rust layer can increase the resistivity of the rust layer, the densification degree of the rust layer, and thus improve the protectiveness of the rust layer. Mo and Cr also have a certain synergistic effect, which can promote the transformation process of alloy elements in the rust layer into corrosion-resistant products. In addition, Mo can also improve the hardenability of steel. The present invention controls the range of Mo to be 0.20-0.50%.

[0025] Sn: Sn can increase the corrosion potential of steel itself, reduce the self-corrosion current density, increase the double layer resistance, inhibit ion exchange, and slow down the corrosion rate, thereby improving its corrosion resistance. After the rust layer is formed, 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 tin oxide and copper oxide on the surface of steel, which improves the corrosion resistance of steel. However, too high a Sn content will damage the toughness of the steel and increase the risk of local acid pitting, so its content is controlled at 0.10-0.25%.

[0026] Sb: When added in combination with Sn in steel, it can improve the corrosion resistance and wear resistance of the steel, make Sn enriched and Sb evenly distributed in the rust layer, form a SnO2-Sb2O5 corrosion-resistant oxide film on the surface of the steel, and enhance the ability to block the penetration of Cl- ions. However, Sb has an adverse effect on the mechanical properties of steel, which will reduce the strength of the steel and increase its brittleness. The present invention controls the range of Sb to be 0.10-0.25%.

[0027] Nb: Nb has a strong affinity with C, N, and O, and forms corresponding extremely stable compounds therewith. Nb can refine the grains of steel, reduce the overheating sensitivity of steel, and under certain conditions, can improve the strength and toughness of steel, especially under the conditions of induction heating at a high heating rate, precipitating fine Nb carbonitrides to refine austenite grains. The Nb content of the present invention is controlled at 0.02-0.04%.

[0028] V: V has a strong affinity with both C and N. The fine V (C, N) particles dispersed in the ferrite are the main way of strengthening by vanadium microalloying. The V (C, N) precipitates in the ferrite conform to the BN orientation relationship with the ferrite matrix, and maintain a coherent relationship with the ferrite matrix in the initial stage of uniform precipitation in the ferrite. Compared with other microalloying elements such as Nb and Ti, the carbides and nitrides of V have the smallest mismatch with the ferrite matrix, and the coherent relationship between the carbides and nitrides of V and the ferrite matrix can be maintained to a larger particle size, which is beneficial to improve the resistance of steel to fatigue crack initiation and expansion, and also improve the yield strength ratio, low temperature toughness and welding performance of steel. However, the V content should not be too high, otherwise it will reduce the toughness of the steel and be detrimental to the creep performance of the steel. The V content of the present invention is controlled at 0.02-0.04%.

[0029] Ti: Ti can not only improve the strength of steel, refine grains, reduce aging sensitivity and cold brittleness, but also improve welding performance in small amounts. Ti works in the form of TiN. When it is less than 0.005%, the effect is small. When it exceeds 0.04%, it is easy to form large particles of TiN and lose the effect. Therefore, the preferred Ti content is 0.005-0.025%.

[0030] Ca: Ca treatment is performed on molten steel to purify the molten steel. The Ca element can spheroidize the MnS inclusions and disperse their size, preventing the MnS inclusions from forming long strip plastic shapes during the rolling process, and forming dispersed spherical shapes, and refining the size of sulfide inclusions, inhibiting the hot brittleness of S, improving the impact toughness and Z-direction performance of the steel plate, and improving the anisotropy of the impact toughness of the steel plate. The amount of Ca added depends on the S content in the steel. If the Ca addition is too low, the treatment effect is not great; if the Ca addition is too high, the size of Ca(O,S) is too large, the brittleness is also increased, and it can become the starting point of the fracture crack, reducing the low-temperature toughness of the steel, while also reducing the purity of the steel and polluting the molten steel. The present invention controls the Ca element content to 0.002-0.005%.

[0031] P: P can segregate in the center of the slab and gather at the grain boundaries. It 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 for the corrosion fatigue performance of the steel plate, P cannot be used as a characteristic element. The present invention requires that the P content be controlled to be no more than 0.01%.

[0032] S: S can combine with Mn in steel to form MnS inclusions. During hot rolling, MnS extends along the rolling direction to form a MnS inclusion belt along the rolling direction, which can easily become a crack source under long-term complex loads, seriously damaging the fatigue resistance, low-temperature impact toughness and Z-direction properties of the steel plate. Therefore, the present invention requires that the S content be controlled to be no higher than 0.002%.

[0033] Als: As a deoxidation and grain refinement element that must be added in the present invention, the added content is above 0.01%, but when it exceeds 0.08%, hot cracks of the casting are likely to occur, and the toughness of the steel is reduced. The Als content is controlled at 0.01% to 0.05%.

[0034] The second technical solution of the present invention is to provide a method for manufacturing high-strength marine engineering steel with high ductility and ice load corrosion resistance, including smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary rolling, secondary cooling, and heat treatment;

[0035] (1) Smelting process: Smelting is carried out according to the composition range of the present invention, and a continuous casting billet is obtained after smelting and continuous casting. The LF and RH refining furnace treatments each require 20 to 40 minutes, the superheat of the molten steel in the tundish is ≤25°C, and the casting is protected throughout the process; the A, B, C, and D inclusions in the steel meet the requirements of A≤0.5, B≤0.5, C≤0.5, and D≤0.5.

[0036] (2) One-pass rolling process: the first heating temperature is 1150-1200°C, the rolling temperature is in the austenite recrystallization temperature zone of 1000-1060°C, and the average single-pass reduction rate is above 10%, in order to fully carry out the dynamic recrystallization process of austenite and refine the austenite grain size;

[0037] (3) Primary cooling process: After the primary rolling is completed, a rapid laminar cooling system with an average cooling rate of 10-20°C / s is used to quickly cool the intermediate billet to room temperature, completing the austenitization pretreatment of the intermediate billet to obtain a fine martensite / bainite structure;

[0038] (4) Secondary heating process: The temperature of the intermediate billet during secondary heating is raised to 930-980℃ to re-austenize the intermediate billet. The pre-treated structure after one rolling and cooling is a fine martensite / bainite structure. This non-stable 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 points during the secondary heating austenitization process, and a finer austenite structure can be obtained. The average equivalent diameter of the original austenite grains is ≤15μm. The subsequent controlled cooling structure morphology will also be constrained due to the inheritance of the structure. On the other hand, secondary heating is equivalent to normalizing treatment, which is conducive to the uniform distribution of alloy elements in the steel.

[0039] (5) Secondary rolling process: The secondary rolling temperature is 790-830℃ in the austenite / proeutectoid ferrite two-phase region, the average single-pass reduction rate reaches 15-20%, and the final rolling temperature is 780-810℃. The purpose is to fully deform the austenite grains, provide energy storage and position for the phase deformation nuclei, increase the phase deformation nucleus rate, and further reduce the ferrite grains in the two-phase region, ultimately achieving the purpose of grain refinement.

[0040] (6) Straightening process: The steel plate must be straightened after rolling to improve the flatness of the steel plate and prevent problems such as warping of the steel plate during the cooling process.

[0041] (7) Secondary cooling process: After straightening, the steel plate is cooled to 760-790°C and enters a rapid laminar cooling system with an average cooling rate of 5-10°C / s. The return temperature is controlled at 550-600°C to control the phase transformation structure and size. The steel plate is then air-cooled to room temperature to obtain a ferrite plus bainite dual-phase structure, in which the average ferrite grain size is 5.0-10.0 μm and the ferrite phase ratio is in the range of 80%-90%.

[0042] (8) Heat treatment: High-frequency induction surface quenching is adopted. The surface of the cooled steel plate is high-frequency induction heated to 950-1000°C by longitudinal magnetic flux induction heating at a current frequency of more than 200kHz, and then water quenched to room temperature. The surface-quenched steel plate is then heated to 130-170°C for low-temperature tempering for 0.5-1.0h. 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 hardening layer of 2-3mm. After low-temperature tempering, a layer of refined martensite (average length of martensite lath <15μm) plus dispersed carbonitrides with high hardness tempering structure 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.

[0043] The beneficial effects of the present invention are:

[0044] (1) The present invention improves the purity of steel by controlling the sulfur and phosphorus content in pure steel smelting, and improves the steel quality by treating spheroidized inclusions with Ca. The chemical composition of the steel plate is rationally designed, and the appropriate C and Mn contents are selected to ensure the martensite forming ability of the steel plate under the condition of ensuring the matching of strength and plasticity, so as to ensure the performance difference between the surface hardened layer and the matrix, and the corrosion-resistant characteristic elements of Cr, Cu, Ni, Mo, Sn, and Sb are added to cooperate with Nb, V, and Ti microalloying to improve the steel's resistance to ice load erosion and wear fatigue. Among them, Cr, Cu, Ni, Mo, Sn, and Sb improve the corrosion resistance of steel, and Nb, V, and Ti microalloying improves the strength of steel and improves the wear resistance of steel in the form of dispersed carbonitrides in the surface hardened layer. Through the synergistic effect between the elements and the process control, the steel plate has excellent resistance to ice load erosion and wear, elongation and low-temperature impact toughness.

[0045] (2) The innovative method of secondary heating rolling, controlled cooling and high-frequency induction surface quenching is adopted. The intermediate billet is heated again after rapid cooling. The non-steady-state structure with high energy storage provides energy release and a large number of nucleation positions during the re-austenitization process, which refines the austenite grain size of the intermediate billet. At the same time, it is conducive to the good organization and uniformity of alloy elements of the second-stage rolled steel plate, which is conducive to improving the plasticity and toughness of the product. After the high-frequency induction quenching surface is rapidly austenitized, it is quenched to form a surface hardened layer. After low-temperature tempering, the surface of the steel plate forms a high-hardness tempered structure of refined martensite and dispersed carbonitride, which can effectively improve the wear resistance of the steel plate without affecting the overall performance of the steel plate, and provide a guarantee for the safety of polar service of marine engineering.

[0046] (3) The steel plate prepared by the present invention has ice load erosion and wear resistance (more than twice that of the same grade of steel plate), high strength (yield strength ≥420MPa, tensile strength 580-660MPa), high ductility (elongation after fracture ≥31%) and low-temperature impact toughness (-60℃ Charpy impact energy ≥220J, zero plastic transition temperature (NDTT) ≤-70℃), surface hardness HBW ≥285, and product thickness range 50-80mm.

[0047] (4) The matrix microstructure of the inventive steel plate is ferrite plus bainite (wherein the average ferrite grain size is 5.0-10.0 μm, and the ferrite phase ratio is in the range of 80-90%), and the surface is a 2-3 mm deep tempered martensite (the average length of the martensite lath is less than 15 μm) hardened layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is the metallographic microstructure of the surface hardened layer of Example 1.

[0049] Figure 2 This is the metallographic microstructure of the matrix of Example 1. DETAILED DESCRIPTION

[0050] The present invention will be further described below by way of examples.

[0051] The embodiment of the present invention performs smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary rolling, secondary cooling and heat treatment according to the component ratio of the technical solution.

[0052] Primary heating: heating temperature is 1150~1200℃;

[0053] One-pass rolling: rolling temperature is 1000-1060℃, and the average single-pass reduction rate is above 10%;

[0054] Primary cooling: After the primary rolling is completed, a rapid laminar cooling system with an average cooling rate of 10-20℃ / s is used to quickly cool the intermediate billet to room temperature to complete the austenitization pretreatment of the intermediate billet.

[0055] Secondary heating: The secondary heating intermediate billet is heated to 930-980℃ to re-austenitize the intermediate billet;

[0056] Secondary rolling: the secondary rolling temperature is 790-830℃, the average single-pass reduction rate reaches 15-20%, and the final rolling temperature is 780-810℃;

[0057] Secondary cooling: After straightening, the steel plate is cooled to 760-790℃ and enters a rapid laminar cooling system with an average cooling rate of 5-10℃ / s. The return temperature is controlled at 550-600℃, and then the steel plate is air-cooled to room temperature.

[0058] Heat treatment: high-frequency induction surface quenching is adopted. The surface of the cooled steel plate is high-frequency induction heated to 950-1000°C by longitudinal magnetic flux induction heating at a current frequency of more than 200kHz, and then water quenched to room temperature. Subsequently, the surface-quenched steel plate is heated to 130-170°C for low-temperature tempering for 0.5-1.0h.

[0059] Further, the smelting: smelting according to the composition range of the present invention, smelting, continuous casting to obtain continuous casting billets, LF and RH refining furnace treatment each requires 20 to 40min,

[0060] The continuous casting: the superheat of the molten steel in the tundish is ≤25°C, and the casting is protected throughout the whole process; the A, B, C, and D inclusions in the steel meet the requirements of A≤0.5, B≤0.5, C≤0.5, and D≤0.5.

[0061] Furthermore, after the primary cooling, the microstructure of the steel plate is a fine martensite / bainite structure.

[0062] Furthermore, the microstructure of the steel plate after secondary heating is that the average equivalent diameter of the original austenite grains is ≤15μm.

[0063] The composition of the steel of the embodiment of the present invention and the comparative example is shown in Table 1. The main process parameters for smelting the steel of the embodiment of the present invention are shown in Table 2. The main process parameters for controlled rolling and controlled cooling of the steel of the embodiment of the present invention are shown in Table 3. The main process parameters for heat treatment of the steel of the embodiment of the present invention are shown in Table 4. The microstructure of the steel of the embodiment of the present invention is shown in Table 5. The mechanical properties and NDTT temperature of the steel of the embodiment of the present invention are shown in Table 6. The weight loss (g) of ice load erosion and wear resistance of the embodiment of the present invention and the comparative example is shown in Table 7.

[0064] Table 1 Composition (wt%) of steel of the present invention and comparative example

[0065]

[0066] Note: P:≤0.01%, S:≤0.002%

[0067] Table 2 Main process parameters for steel smelting in the embodiment of the present invention

[0068]

[0069] Table 3 Main process parameters of controlled rolling and controlled cooling of steel in the embodiment of the present invention

[0070]

[0071] Table 4 Main process parameters of heat treatment of steel in the embodiment of the present invention

[0072]

[0073] Table 5 Microstructure of steel according to the present invention

[0074]

[0075] Table 6 Mechanical properties and NDTT temperature of steel according to the present invention

[0076]

[0077] Table 7 Ice load erosion and wear weight loss of the embodiments of the present invention and the comparative examples (g)

[0078] Example 0.5m / s 1.0m / s 1 0.0017 0.0027 2 0.0013 0.0023 3 0.0015 0.0029 4 0.0013 0.0021 5 0.0022 0.0032 6 0.0014 0.0022 7 0.0018 0.0030 8 0.0014 0.0026 Comparative Example 0.0053 0.0066

[0079] Note: Ice load erosion and wear resistance test: A 50mm thick FH420 steel plate was introduced as a comparative example. The sample was processed into a 10mm×10mm×4mm steel sample using a wire cutting machine. The outer surface was polished step by step with 180#, 400#, 800#, and 1200# sandpapers. After ultrasonic cleaning and drying, the sample was weighed and the results were recorded and sealed for later use. According to ASTM D1141-98 standard, a 3.5% NaCl solution was configured to simulate the seawater environment as the test medium for this erosion and wear test. The ice load erosion and wear test was completed on a low-temperature sea ice erosion and wear tester. Before the erosion wear test, a 3.5% NaCl aqueous solution was first prepared into a 10mm×10mm×10mm cube ice block in an ultra-low temperature environment box at -60℃ using a mold. The ice making time was 24h. Then the ice block was added to the 3.5% NaCl aqueous solution pre-warmed at 0℃ in a ratio of 1:1 for ice load erosion wear test. The ambient temperature was controlled at -5±1℃ throughout the test. The erosion test was carried out using a rotation test method at a speed of 0.5m / s and 1.0m / s for 30h. After the erosion wear test, the sample was placed in a rust removal solution (3.5g hexamethylenetetramine + 500ml hydrochloric acid + 500ml deionized water) for ultrasonic treatment for 30s, then rinsed with alcohol and deionized water in turn, and weighed after drying. The erosion wear weight loss of the sample was calculated from the mass difference of the sample before and after the test.

[0080] The marine engineering steel plate produced by the present invention has a tensile elongation of ≥31%, a yield strength of ≥420MPa, a tensile strength of 530-680MPa, a Charpy impact energy of ≥220J at -60°C, a zero plastic transition temperature NDTT≤-70°C, a surface hardness of ≥285HBW, a steel plate thickness of 50-80mm, and an ice load erosion and wear resistance performance that is more than twice that of the same level of steel plates.

[0081] In order to describe the present invention, the present invention is appropriately and fully illustrated by the examples in the above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A high-strength marine steel with high ductility and resistance to ice load corrosion, characterized in that: The composition of the marine engineering steel is as follows by weight percentage: C: 0.065%-0.085%, Si: 0.25%-0.35%, Mn: 1.30%-1.50%, Cr: 0.80%-1.20%, Ni: 0.35%-0.55%, Cu: 0.45%-0.65%, Mo: 0.20%-0.50%, Sn: 0.05%-0.12%, Sb: 0.05%-0.12%, Nb: 0.02%-0.04%, V: 0.02%-0.04%, Ti: 0.005%-0.025%, Ca: 0.002%-0.005%, P: ≤0.01%, S: ≤0.002%, Als: 0.02%-0.05%, and the rest is Fe and unavoidable impurities; The surface microstructure of the offshore steel plate is tempered martensite, and the steel plate matrix microstructure is ferrite and bainite; the tempered martensite hardened layer thickness of the offshore steel plate surface is 2-3 mm, and the average length of the martensite lath is less than 15 μm; the volume percentage of ferrite is 80%-90%, and the average ferrite grain size is 5.0-10.0 μm; The offshore steel plate has a tensile elongation of ≥31%, a yield strength of ≥420 MPa, a tensile strength of 530-680 MPa, a -60°C Charpy impact energy of ≥220 J, a zero plastic transition temperature NDTT ≤-70°C, a surface hardness of ≥285 HBW, and a steel plate thickness of 50-80 mm; and an ice load erosion and wear resistance of more than twice that of the same grade of steel plates; The manufacturing method of the ice load erosion resistant high-strength marine engineering steel with high ductility comprises smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary rolling, secondary cooling, and heat treatment; Primary heating: heating temperature is 1150~1200℃; One-pass rolling: rolling temperature is 1000-1060℃, and the average single-pass reduction rate is above 10%; Primary cooling: After the primary rolling is completed, a rapid laminar cooling system with an average cooling rate of 10-20°C / s is used to quickly cool the intermediate billet to room temperature to complete the austenitization pretreatment of the intermediate billet; Secondary heating: The secondary heating intermediate billet is heated to 930-980℃ to re-austenitize the intermediate billet; Secondary rolling: the secondary rolling temperature is 790-830℃, the average single-pass reduction rate reaches 15%-20%, and the final rolling temperature is 781-810℃; Secondary cooling: After straightening, the steel plate is cooled to 760-790℃ and enters a rapid laminar cooling system with an average cooling rate of 5-10℃ / s. The return temperature is controlled at 550-600℃, and then the steel plate is air-cooled to room temperature. Heat treatment: high-frequency induction surface quenching is adopted. The surface of the cooled steel plate is high-frequency induction heated to 950-1000°C at a current frequency of more than 200kHz, and then water quenched to room temperature. Subsequently, the surface-quenched steel plate is heated to 130-170°C for low-temperature tempering for 0.5-1.0h.

2. The high-strength marine engineering steel with high ductility and ice load corrosion resistance according to claim 1, characterized in that: The smelting is carried out according to the composition range of the present invention, and a continuous casting billet is obtained after smelting and continuous casting, and the LF and RH refining furnace treatments each require 20 to 40 minutes; The continuous casting: the superheat of the molten steel in the tundish is ≤25°C, and the casting is protected throughout the whole process; the A, B, C, and D inclusions in the steel meet the requirements of A≤0.5, B≤0.5, C≤0.5, and D≤0.

5.

3. The high-strength marine engineering steel with high ductility and ice load corrosion resistance according to claim 1, characterized in that: After the primary cooling, the microstructure of the steel plate is a fine martensite / bainite structure.

4. The high-strength marine engineering steel with high ductility and ice load corrosion resistance according to claim 1, characterized in that: The microstructure of the steel plate after secondary heating is that the average equivalent diameter of the original austenite grains is ≤15μm.

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