Ultra-high strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion and its manufacturing method

Abstract

The invention provides an ultra-high strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion and a manufacturing method thereof. The steel has the following components by weight percentage: C: 0.045-0.075%, Si: 0.15-0.25%, Mn: 0.90-1.20%, Cr: 1.20-1.50%, Ni: 0.90-1.30%, Cu: 0.40-0.70%, Mo: 0.40-0.60%, Sn: 0.05-0.15%, Nb: 0.03-0.05%, V: 0.025-0.035%, Ti: 0.005-0.025%, Ca: 0.003-0.006%, P: ≤0.01%, S: ≤0.002%, Als: 0.02%-0.05%, and the rest is Fe and unavoidable impurities. The manufacturing method includes smelting, continuous casting, controlled rolling and controlled cooling; the steel plate produced by the present invention has a yield strength of ≥550MPa, a tensile strength of 670-830MPa, a tensile elongation after fracture of ≥29%, a low-temperature impact energy of ‑60°C of ≥180J, a NDTT temperature of ≤‑70°C, and a steel plate thickness of 20-50mm.

Description

Technical Field

[0001] The present invention belongs to the field of steel materials, and in particular relates to an ultra-high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion, and a manufacturing method thereof. Background Art

[0002] The particularity of the application environment of marine engineering has always placed high requirements on the service safety of marine equipment and structural materials. 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. Not only does it require marine engineering steel to have good strength, but it also specifically requires steel plates to have high ductility and ultra-low temperature toughness to cope with sudden accidents and operational safety in severe sea conditions that ships and marine engineering equipment may encounter. The American ABS classification society has currently incorporated high ductility performance indicators into the relevant specifications "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 anti-collision functions ("NSafe-Hull" series), which are currently in the application stage. At present, my country has not formed a systematic research and large-scale application of such steel plates. On the other hand, since the marine atmospheric environment is characterized by high humidity, high salt fog, and high solar radiation intensity compared to the inland atmospheric environment, conventional steel grades are extremely susceptible to corrosion damage in this environment, seriously affecting the long-term service stability of marine engineering equipment and structures.

[0003] For a long time, the design and manufacturing of marine engineering steel have focused more on strength and toughness indicators, and less on the ductility and corrosion properties of the materials. However, with the deepening of human exploration of marine resources, marine engineering applications are developing towards the deep sea and offshore. The long-term service safety and stability brought by high ductility and resistance to marine atmospheric corrosion will become an inevitable trend in the future demand for marine engineering steel.

[0004] Public reports of related patents:

[0005] The invention with publication number CN110846554A discloses a method for manufacturing a high-ductility EH32 grade marine engineering steel and steel plate. The patent adopts a composition design scheme with alloy reduction, does not add high-priced metals and micro-alloying elements, and achieves high ductility and low-temperature toughness of the steel plate through a three-stage controlled rolling + controlled cooling TMCP process. However, since the three-stage rolling process requires two intermediate billets to be heated, the process is relatively cumbersome and the production efficiency is low. The corrosion performance of the steel plate was not evaluated.

[0006] The invention with publication number CN108517462A proposes a high-ductility EH40 grade ship plate steel and its preparation method. The patent is designed with low carbon and micro-alloy strengthening components, and adopts two-stage rolling plus water cooling-air cooling-water cooling three-stage cooling. The production process of this technology is complex, the control capability of the cooling equipment is high, and the process window is narrow. The corrosion performance of the steel plate has not been evaluated.

[0007] The invention with publication number CN110714171A discloses a high-ductility EH420 grade ship plate steel and its production method. The patent uses the TMCP method of low carbon plus Nb and Ti microalloy strengthening combined with two-stage rolling and two-stage cooling, and uses ultra-fast cooling technology with a cooling rate of 60℃ / s to obtain a composite structure of soft phase ferrite and hard bainite. However, this technology is difficult and it is difficult to achieve the required cooling rate conditions in actual industrial production, and there are limitations on the thickness of the steel plate. The corrosion performance of the steel plate was not evaluated.

[0008] The invention with publication number CN108517463A discloses a high-ductility FH500 grade ship plate steel and its production method, which is designed with Nb and V microalloy strengthening components, and cooperates with controlled rolling and controlled cooling technology to obtain soft ferrite and hard bainite structures. 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. 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.

[0009] The invention with publication number CN104046898A discloses a high-performance marine climate resistant steel plate and a manufacturing method thereof, which adopts a low-alloy weathering steel composition design of ultra-low C-low Mn-high Als-low N-(Cu+high Ni) alloying-Nb micro-alloying-ultra-micro Ti treatment, and is combined with a two-stage controlled rolling + controlled cooling process in the austenite single-phase region to obtain a ferrite+pearlite (controlled rolling) or ferrite+bainite (TMCP) structure, to obtain a 355Mpa grade steel plate with resistance to marine atmospheric corrosion and a yield strength of less than 0.8, and the tensile elongation of the steel plate after fracture ranges from 30% to 35%.

[0010] The invention with publication number CN109136752A discloses a 420MPa grade low yield ratio marine atmospheric corrosion resistant bridge steel and a production method thereof. The composition design adopts the addition of Cu and Ni corrosion resistant elements combined with Nb and Ti micro-alloying, and combines the process route of TMCP+low temperature tempering heat treatment to obtain a steel plate with marine atmospheric corrosion resistance and a yield strength lower than 0.85, and the steel plate tensile elongation after fracture ranges from 28.5% to 33.5%.

[0011] The invention with publication number CN108004488A discloses a high-toughness bridge steel plate resistant to marine climate and a production method thereof. Ca is used to control the number and shape of inclusions in the steel, and Sb, Cu, Ni, Cr, and Mo are added to improve the corrosion resistance of the steel. A 690Mpa grade steel plate with resistance to marine atmospheric corrosion and a yield strength of less than 0.86 is obtained through a TMCP post-tempering process, and the tensile elongation of the steel plate ranges from 25% to 28%.

[0012] The invention with publication number CN111014292A discloses a method for improving the production efficiency of thick plates of a single-stand rolling mill, by directly cooling the intermediate billet after the first heating and rolling, and then heating and rolling for the second time, and then directly rolling 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 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. However, this method increases the consumption of energy and industrial water due to the increase of 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

[0013] The object of the present invention is to overcome the above-mentioned problems and shortcomings and to provide a kind of marine engineering equipment such as drilling platform, pile shoe and cantilever track suitable for offshore platform, whose comprehensive performance can reach the long-term service conditions of offshore engineering, tensile elongation after fracture ≥29%, yield strength ≥550MPa, tensile strength 670~830MPa, -60℃ Charpy impact energy ≥180J, zero plastic transition temperature (NDTT) ≤-70℃, high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion and its manufacturing method.

[0014] The object of the invention is achieved in this way:

[0015] The present invention adopts a composition design of low C + "Cr-Ni-Cu-Mo-Sn" alloying + Nb, V, Ti micro-alloying, and combines secondary heating controlled rolling and controlled cooling processes to regulate the microstructure and grain size of the steel plate, thereby obtaining an ultra-high strength marine engineering steel plate with high yield rate, stable strength and toughness, and especially resistance to marine atmospheric corrosion and high ductility.

[0016] The invention discloses an ultra-high strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion. The steel has the following components by weight percentage: C: 0.045%-0.075%, Si: 0.15%-0.25%, Mn: 0.90%-1.20%, Cr: 1.20%-1.50%, Ni: 0.90%-1.30%, Cu: 0.40%-0.70%, Mo: 0.40%-0.60%, Sn: 0.05%-0.15%, Nb: 0.03%-0.05%, V: 0.025%-0.035%, Ti: 0.005%-0.025%, Ca: 0.003%-0.006%, P: ≤0.01%, S: ≤0.002%, Als: 0.02%-0.05%, and the rest is Fe and unavoidable impurities.

[0017] The offshore steel plate has a yield strength of ≥550MPa, a tensile strength of 670-830MPa, a tensile elongation of ≥29%, a low-temperature impact energy of -60°C of ≥180J, a NDTT temperature of ≤-70°C, and a steel plate thickness of 20-50mm.

[0018] The microstructure of the marine steel plate is ferrite+bainite, wherein the ferrite is equiaxed ferrite and acicular ferrite, the volume percentage of the bainite is 20% to 30%, and the average grain size of the equiaxed ferrite is 11 to 18 μm.

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

[0020] 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. However, too high C content has a negative impact on the ductility, low temperature toughness and fracture toughness, weldability and corrosion resistance of steel. Too low C content will lead to high austenite crystal mobility, affect the uniform refinement of the TMCP process, and easily form mixed crystal structure. Therefore, from the perspective of product performance, it is preferred that the C content be controlled at C: 0.045-0.075%.

[0021] Si: It is the main deoxidizing component in the steelmaking process. 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. Si in the form of solid solution will increase the strength while also reducing plasticity and increasing the ductile-brittle transition temperature. Therefore, the preferred Si content is 0.15% to 0.25%.

[0022] Mn: Mn is an essential element to ensure the strength and toughness of steel. With the increase of Mn content, the strength of steel increases significantly. 1% Mn can increase the tensile strength by about 100MPa. In addition to improving strength and toughness, it also has the effect of expanding the austenite phase, reducing the temperature of Ac1, Ac3, Ar1, Ar3 points, and refining ferrite grains. However, Mn elements are easy to segregate in steel, and are easy to segregate at 1 / 4 and 1 / 2 thickness of medium and thick steel plates, resulting in a strong decrease in low-temperature toughness and even impact fracture delamination. 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 be controlled in the range of 0.90% to 1.20%.

[0023] Cr: Cr is an effective element for improving the corrosion resistance of steel. It can form a dense passivation layer containing Cr that is closely bonded to the substrate. This relatively stable structure can improve the corrosion resistance of steel. In addition, Cr is also an effective element for improving the strength of steel plates. However, excessive Cr can cause a decrease in welding performance. Therefore, the present invention controls the Cr content to 1.20% to 1.50%.

[0024] Ni: Ni has a solid solution strengthening effect, which can improve the strength and toughness of steel. It 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 to improve the marine atmospheric corrosion resistance of steel plates. Adding Ni 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 marine atmospheric corrosion resistance of the steel plate; secondly, the amorphous oxide amorphous layer with a high pH value inhibits electrochemical reactions and improves the marine atmospheric corrosion resistance of the steel plate, but the cost of adding Ni is relatively high, so the Ni content is controlled in the range of 0.90% to 1.30%.

[0025] Cu: Cu is an essential element for resisting marine atmospheric corrosion. 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 marine atmospheric 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 is controlled within 0.40% to 0.70%.

[0026] 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 0.40% to 0.60%.

[0027] 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 Sn content will damage the toughness of steel and increase the risk of local acid pitting, so its content is controlled at 0.05% to 0.15%.

[0028] 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 0.03-0.05%.

[0029] V: V is a ferrite refining element, which improves the strength, yield strength ratio and low temperature toughness of steel, improves the welding performance of steel, and can also increase the thermal strength and creep resistance of steel. However, the V content should not be too high, otherwise it will reduce the toughness of steel and be detrimental to the creep performance of steel. The V content of the present invention is controlled at 0.025% to 0.035%.

[0030] 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, and 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% to 0.025%.

[0031] 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 properties of the steel plate, and improving the anisotropy of the impact toughness of the steel plate. In addition, the inclusion of Ca oxides also helps to promote the formation of acicular ferrite during the controlled cooling of the steel plate. The present invention controls the Ca element content to 0.003-0.006%.

[0032] P: P can segregate in the center of the slab and gather at the grain boundaries, and 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 marine atmospheric corrosion resistance of the steel plate, considering the damage of P to the mechanical properties of the steel plate, P cannot be used to improve the marine atmospheric corrosion resistance of the steel plate. The present invention requires that the P content be controlled to be no higher than 0.01%.

[0033] 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 band along the rolling direction, which becomes a crack source and seriously damages the transverse low-temperature impact toughness and Z-direction properties of the steel plate. The content of S in the material of the present invention is controlled to be no higher than 0.001%.

[0034] 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 ingot are easily generated, and the toughness of the steel is reduced. The Als content is controlled at 0.02% to 0.05%.

[0035] The second technical solution of the present invention is to provide a method for manufacturing ultra-high strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion, including smelting, continuous casting, controlled rolling and controlled cooling;

[0036] (1) Smelting process:

[0037] 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 alloy components are added as required for smelting.

[0038] b) refining the molten steel and adjusting the contents of other alloying elements within the scope of the present invention.

[0039] c) The refined molten steel is subjected to RH treatment for a period of ≥30 min, and [H] in the steel is controlled to be ≤2.0ppm and [O] ≤18ppm.

[0040] (2) Continuous casting: The molten steel obtained in step (1) is continuously cast to obtain the desired ingot. In order to improve the center segregation of the ingot, the superheat of the tundish is controlled to be ≤30°C. The pouring is protected throughout, and electromagnetic stirring and light pressure are applied. The electromagnetic stirring: I≥450A.

[0041] (3) Controlled rolling and controlled cooling: including primary heating + primary rolling + primary cooling + secondary heating + secondary rolling + secondary cooling;

[0042] One-time heating process: The heating temperature of the billet is 1160-1200℃. The steel of the present invention has high alloy content and poor thermal conductivity. In order to control the heating quality of the continuous casting billet, a segmented heating process is used for the first heating. When the heating temperature is below 850℃, slow heating is used to relieve the thermal stress caused by thermal conductivity. The heating rate is 9-13℃ / min. When the heating temperature is greater than 850℃, a rapid heating process is used to prevent abnormal growth of austenite grains. The heating rate is 17-22℃ / min; the holding time is 1-2.5h.

[0043] One-pass rolling process: The rolling temperature is in the austenite recrystallization temperature zone of 1050-1100℃, and the ingot is rolled to 1.5-2.0 times the finished product thickness. The first pass reduction is ≥50mm, and the average single pass reduction rate is above 15%. The purpose is to fully break the austenite grains of the ingot, so that the dynamic recrystallization process of austenite can be fully carried out to refine the austenite grain size;

[0044] Primary cooling process: 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, complete the austenitization pretreatment of the intermediate billet, and obtain fine martensite / bainite structure;

[0045] Secondary heating process: The secondary heating intermediate billet is heated to 900-950℃ at a heating rate of 17-22℃ / min 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 dislocations, interfaces and other crystal defects can provide a large number of recrystallization nucleation points during the secondary heating austenitization process, and a finer austenite structure can be obtained. On the other hand, the secondary heating is equivalent to the normalizing treatment, which is beneficial to the uniform distribution of corrosion-resistant solid solution elements in the steel.

[0046] Secondary rolling process: The secondary rolling temperature is 820-850℃, the average single-pass reduction rate is 15%-20%, and the final rolling temperature is 770-810℃. 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 by deformation in the two-phase region, ultimately achieving the purpose of grain refinement.

[0047] Straightening process: The rolled steel plate needs to be straightened to improve the straightness of the steel plate and prevent problems such as warping of the steel plate during the cooling process.

[0048] Secondary cooling process: After straightening, the steel plate is relaxed and heated to 730-760℃ and then enters a rapid laminar cooling system with an average cooling rate of 7-12℃ / s. The return temperature is controlled at 500-550℃ to control the phase change structure and size. The steel plate is then slowly cooled to room temperature for ≥24h.

[0049] The beneficial effects of the present invention are:

[0050] (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 a low C content is used and Cr, Cu, Ni, Mo, and Sn are added to improve the marine atmospheric corrosion resistance of the steel. The addition of a small amount of Sn plays a synergistic role with other corrosion-resistant elements, and reduces the amount of high-valent alloy elements such as Cu, Cr, Mo, and Ni to a certain extent. The combination of other elements such as Si and Mn and microalloying of Nb and V ensures the toughness of the steel. Through the interaction between the elements, the steel plate has ultra-high strength, excellent marine atmospheric corrosion resistance, elongation, and low-temperature impact performance.

[0051] (2) The method of secondary heating rolling and controlled cooling is adopted. The intermediate billet is heated again after rapid cooling. The non-steady-state structure with high energy storage releases energy and provides a large number of nucleation sites during the re-austenitization process, which refines the austenite grain size of the intermediate billet. At the same time, it is beneficial to the good organization and composition uniformity of the steel plate in the second stage of rolling, which is beneficial to the improvement of the plastic toughness of the product and provides a guarantee for the service safety of marine engineering.

[0052] (3) The steel plate prepared by the present invention has resistance to marine atmospheric corrosion, ultra-high strength (yield strength ≥550MPa, tensile strength 670-830MPa), high ductility (tensile elongation after fracture ≥29%) and low-temperature impact toughness (-60℃ impact energy ≥180J), good fracture toughness (brittle transition temperature NDTT ≤-70℃), and the product thickness ranges from 20 to 50mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is the SEM microstructure of Example 1 of the present invention.

[0054] Figure 2 This is the SEM morphology of the impact fracture of Example 1 of the present invention. DETAILED DESCRIPTION

[0055] The present invention will be further described below by way of examples.

[0056] The embodiment of the present invention performs smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling and secondary cooling according to the component ratio of the technical solution.

[0057] One-time heating process: billet heating temperature 1160~1200℃, holding time 1~2.5h;

[0058] One-pass rolling process: rolling temperature is 1050-1100℃, first pass reduction is ≥50mm, average single pass reduction is above 15%; rolling the ingot to 1.5-2.0 times the finished product thickness;

[0059] Primary cooling process: After the primary rolling is completed, the intermediate billet is quickly cooled to room temperature, with an average cooling rate of 8-15℃ / s;

[0060] Secondary heating process: secondary heating temperature is 900-950°C, and the heating rate is 17-22°C / min;

[0061] Secondary rolling process: Secondary rolling temperature is 820-850℃, average single-pass reduction rate reaches 15-20%, and final rolling temperature is 770-810℃;

[0062] Secondary cooling process: After straightening, the steel plate is relaxed and cooled to 730-760℃, with a cooling rate of 7-12℃ / s and a red-return temperature of 500-550℃. The steel plate is then slowly cooled to room temperature, with a cooling time of ≥24h.

[0063] Furthermore, during the first heating, a segmented heating process is adopted, the heating temperature is below 850°C, slow heating is adopted, and the heating rate is 9 to 13°C / min; when the heating temperature is greater than 850°C, a rapid heating process is adopted, and the heating rate is 17 to 22°C / min.

[0064] Further; the smelting process:

[0065] 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 alloy components are added as required for smelting.

[0066] b) refining the molten steel and adjusting the contents of other alloying elements within the scope of the present invention.

[0067] c) The refined molten steel is subjected to RH treatment for a period of ≥30 min, and [H] in the steel is controlled to be ≤2.0ppm and [O] ≤18ppm.

[0068] Further, the continuous casting: the molten steel obtained in step (1) is continuously cast to obtain the desired ingot, in order to improve the center segregation of the ingot, the superheat of the tundish is controlled to be ≤30°C; the whole casting process is protected, and electromagnetic stirring and light pressure are put into use, and the electromagnetic stirring: I≥450A.

[0069] The composition of the steel of the present invention is shown in Table 1. The main process parameters of smelting and heating of the steel of the present invention are shown in Table 2. The main process parameters of controlled rolling and controlled cooling of the steel of the present invention are shown in Table 3. The mechanical properties and NDTT temperature of the steel of the present invention are shown in Table 4. The marine atmospheric corrosion resistance rate (g / m 2 h) See Table 5. The microstructure of the steel of the embodiment of the present invention is shown in Table 6.

[0070] Table 1 Composition of steel according to the present invention (wt%)

[0071]

[0072] Remarks: P: ≤0.01%, S: ≤0.002%.

[0073] Table 2 Main process parameters for steel smelting and heating in the embodiment of the present invention

[0074]

[0075] Table 3 Main process parameters of controlled rolling and controlled cooling of steel in the embodiment of the present invention

[0076]

[0077] Table 4 Mechanical properties and NDTT temperature of steel according to the present invention

[0078]

[0079] Table 5 Marine atmospheric corrosion resistance rate of the steel of the present invention and the comparative steel (g / m 2 h)

[0080] Example 168h 360h 1 0.5663 0.5214 2 0.5565 0.5135 3 0.5459 0.5086 4 0.5381 0.4947 5 0.5532 0.5013 6 0.5254 0.4827 7 0.5178 0.4633 8 0.5216 0.4716 Comparative Example 0.7741 0.7298

[0081] Note: The corrosion resistance test experiment adopts the laboratory salt spray accelerated corrosion test to simulate the marine atmospheric environment, and the accelerated corrosion test is carried out on the embodiment of the present invention, and a 50mm thick FH550 steel plate is introduced as a comparative example. The test solution is 3.5% NaCl solution, the temperature in the salt spray chamber is controlled at 35±2℃, and the salt spray settling speed is 80cm / s. 2 The area settles 1.5 to 2 ml per hour. The corrosion cycle nodes are set at 168h and 360h. After accelerated corrosion, the weight loss method is used to measure and calculate the average corrosion rate (g / m 2 h) Evaluate the corrosion resistance of the steel plate.

[0082] Table 6 Microstructure of steel in the embodiment of the present invention

[0083] Example Bainite / % Average grain size of equiaxed ferrite / μm 1 27.2 12.5 2 26.4 13.6 3 25.9 12.3 4 24.4 14.1 5 23.1 14.7 6 22.9 15.1 7 24.6 16.2 8 21.7 16.3

[0084] 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 engineering steel with high ductility and resistance to marine atmospheric corrosion, characterized in that: The chemical composition of the marine steel is as follows by weight percentage: C: 0.045%-0.075%, Si: 0.15%-0.25%, Mn: 0.90%-1.20%, Cr: 1.20%-1.50%, Ni: 0.90%-1.30%, Cu: 0.46%-0.70%, Mo: 0.40%-0.60%, Sn: 0.05%-0.15%, Nb: 0.03%-0.05%, V: 0.025%-0.035%, Ti: 0.005%-0.025%, Ca: 0.003%-0.006%, P: ≤0.01%, S: ≤0.002%, Als: 0.02%-0.05%, and the rest is Fe and unavoidable impurities; the tensile elongation after break is ≥29%; The method for manufacturing a high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion comprises smelting, continuous casting, primary heating, primary rolling, primary cooling, secondary heating, secondary rolling, and secondary cooling; One-time heating process: billet heating temperature 1160~1200℃, holding time 1~2.5h; One-pass rolling process: rolling temperature is 1050~1100℃, first pass reduction is ≥50mm, average single pass reduction is above 15%; the ingot is rolled to 1.5~2.0 times the finished product thickness; Primary cooling process: After the primary rolling is completed, the intermediate billet is quickly cooled to room temperature, with an average cooling rate of 8~15 ℃ / s; Secondary heating process: secondary heating temperature is 900~950℃, heating rate is 17~22℃ / min; Secondary rolling process: Secondary rolling temperature is 821~850℃, average single-pass reduction rate reaches 15%~20%, and final rolling temperature is 770~810℃; Secondary cooling process: After straightening, the steel plate is relaxed and cooled to 730~760 ℃ and then rapidly cooled at a rate of 7~12 ℃ / s and a red-return temperature of 500~550 ℃. The steel plate is then slowly cooled to room temperature for ≥24 h.

2. The high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion according to claim 1, characterized in that: The offshore steel plate has a yield strength of ≥550 MPa, a tensile strength of 670-830 MPa, a low-temperature impact energy of -60°C of ≥180 J, a NDTT temperature of ≤-70°C, and a steel plate thickness of 20-50 mm.

3. The high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion according to claim 1, characterized in that: The microstructure of the marine steel plate is ferrite+bainite, wherein the ferrite is equiaxed ferrite and acicular ferrite, the volume percentage of the bainite is 20%-30%, and the average grain size of the equiaxed ferrite is 11-18 μm.

4. The high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion according to claim 1, characterized in that: During the first heating, a segmented heating process is adopted. When the heating temperature is below 850°C, slow heating is adopted with a heating rate of 9 to 13°C / min. When the heating temperature is greater than 850°C, a rapid heating process is adopted with a heating rate of 17 to 22°C / min.

5. The high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion according to claim 1, characterized in that: The smelting process: a) During converter smelting, the contents of elements such as C, Si, Mn, P, and S are adjusted to be within the range of the present invention, and other alloy components are added as required for smelting; b) refining the molten steel and adjusting the contents of other alloy elements within the range of the present invention; c) The refined molten steel is subjected to RH treatment for a period of ≥30 min, and the [H] in the steel is controlled to be ≤2.0ppm and [O] ≤18ppm.

6. The high-strength marine engineering steel with high ductility and resistance to marine atmospheric corrosion according to claim 1, characterized in that: The continuous casting comprises: continuously casting the molten steel obtained in step (1) to obtain the desired ingot; in order to improve the center segregation of the ingot, controlling the superheat of the tundish to be ≤30°C; protecting the pouring throughout the process, and putting into electromagnetic stirring and light pressure, and the electromagnetic stirring: I≥450A.

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