550MPa-grade ultrahigh-strength ship plate steel with excellent low-temperature impact performance and preparation method of 550MPa-grade ultrahigh-strength ship plate steel

Abstract

The invention discloses 550MPa-grade ultrahigh-strength ship plate steel with excellent low-temperature impact performance and a preparation method of the 550MPa-grade ultrahigh-strength ship plate steel, and belongs to the technical field of ship steel. The steel plate comprises the following chemical components in percentage by weight: 0.07 to 0.09 percent of C, 0.20 to 0.30 percent of Si, 1.35 to 1.40 percent of Mn, 0.8 to 0.9 percent of Ni, 0.45 to 0.50 percent of Cr and 0.35 to 0.40 percent of Mo, microalloy elements of Nb, Ti and B are added, and the carbon equivalent Ceq is 0.52 to 0.56 percent. The steel is prepared through the processes of molten iron pretreatment, converter smelting, duplex refining, continuous casting and slow cooling, two-stage controlled rolling and quenching and tempering heat treatment, the initial rolling temperature in the rough rolling stage is controlled to be larger than or equal to 1080 DEG C, the finish rolling temperature in the finish rolling stage is controlled to be 840 + / -20 DEG C, and the processes of quenching (930 + / -10 DEG C) and tempering (630 + / -5 DEG C) are combined. The total cross section of the obtained 50-80mm steel plate is of a tempered martensite structure, the yield strength is larger than or equal to 550MPa, the tensile strength is larger than or equal to 650MPa, the ductility is larger than or equal to 16%, the impact energy of 1 / 4 and 1 / 2 thickness parts at the temperature of minus 60 DEG C is larger than or equal to 120J and 100J respectively, the steel plate has high strength, high toughness and excellent low-temperature performance, the technical problem that the strength and the toughness of the plate are difficult to consider is solved, and the steel plate is suitable for key force bearing components of ships and ocean engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-strength ship plate materials, and particularly relates to a super-high-strength ship plate steel with excellent low-temperature impact performance at the 550 MPa level and a preparation method thereof. Background Art

[0002] With the rapid development of the global shipbuilding industry and offshore engineering, the performance requirements for steels used in ships and offshore platforms are increasing day by day. Especially in structures such as large ships and deep-sea drilling platforms, extremely strict standards are put forward for the mechanical properties and safety of steel plates. Ship plate steels with high strength and excellent low-temperature impact performance have become an important direction for material research and development due to their reliability and stability in extreme environments.

[0003] Traditional 550 MPa-level super-high-strength ship plates usually adopt quenching and tempering treatment, and the matrix structure is mainly tempered martensite. Although it has good strength and toughness, there are difficulties in balancing large thickness, high strength and extremely low-temperature toughness. In the prior art, although some patents have reduced costs through processes such as controlled rolling and controlled cooling, there are still deficiencies in the comprehensive performance of steel plate thickness, strength and low-temperature impact performance. For example, the finished product thickness is limited to thin steel plates, or some strength indicators do not meet the standards, or there are problems such as crack risks in the production of steel plates.

[0004] Under this background, the research and development of a super-high-strength ship plate that can meet the strength requirements of the 550 MPa level, has excellent low-temperature impact performance, and is suitable for the 50-80 mm specification, as well as its production method, has important practical significance and broad application prospects for improving the safety and reliability of ship and offshore engineering structures and promoting the technological progress of related industries. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a super-high-strength ship plate steel with excellent low-temperature impact performance at the 550 MPa level and a preparation method thereof. The steel components and their weight percentage contents are as follows: C 0.07 - 0.09%, Si 0.20 - 0.30%, Mn 1.35 - 1.40%, P ≤0.006%, S ≤0.002%, Alt 0.030 - 0.040%, Nb 0.035 - 0.040%, Ti 0.010 - 0.015%, Ni 0.8 - 0.9%, Cr 0.45 - 0.50%, Mo 0.35 - 0.40%, B 0.0009 - 0.0012%, and the rest are Fe and unavoidable impurities.

[0006] According to the above technical features, C represents carbon, Si represents silicon, Mn represents manganese, P represents phosphorus, S represents sulfur, Alt represents aluminum, Nb represents niobium, Ti represents titanium, Ni represents nickel, Cr represents chromium, Mo represents molybdenum, and B represents boron.

[0007] According to the above technical features, the further preferred steel components and their weight percentage contents are as follows:

[0008] C 0.083wt%, Si 0.3wt%, Mn 1.38wt%, P 0.005wt%, S 0.002wt%, Alt 0.032wt%, Nb 0.037wt%, Ti 0.012wt%, Ni 0.82wt%, Cr 0.457wt%, Mo 0.352wt%, B 0.0009wt%, and C eq is 0.533;

[0009] Or, C 0.071wt%, Si 0.28wt%, Mn 1.40wt%, P 0.006wt%, S 0.002wt%, Alt 0.04wt%, Nb 0.035wt%, Ti 0.015wt%, Ni 0.80wt%, Cr 0.451wt%, Mo 0.40wt%, B 0.0012wt%, and C eq is 0.531;

[0010] Or, C 0.09wt%, Si 0.21wt%, Mn 1.35wt%, P 0.005wt%, S 0.001wt%, Alt 0.031wt%, Nb 0.04wt%, Ti 0.011wt%, Ni 0.90wt%, Cr 0.50wt%, Mo 0.35wt%, B 0.0011wt%, and C eq is 0.548.

[0011] According to the above technical features, the thickness of the steel plate is 50 - 80mm, the yield strength ReH ≥ 550MPa, the tensile strength Rm ≥ 650MPa, the elongation A ≥ 16%, the impact energy KV2 at -60°C at the 1 / 4 thickness part of the steel plate ≥ 120J, the impact energy KV2 at -60°C at the 1 / 2 thickness part ≥ 100J, and the microstructure of each part of the steel plate is tempered martensite (tempered M).

[0012] The carbon equivalent C of the steel in the present invention eq is in the range of 0.52 - 0.56%, and the carbon equivalent Ceq is calculated through the chemical composition. Its calculation formula is C eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (%).

[0013] The production process route for producing the steel of the present invention according to the weight percentages of chemical components given within the above ranges is as follows: hot metal pretreatment → converter smelting → LF furnace refining → RH furnace vacuum treatment → slab casting → continuous casting billet slow cooling → continuous casting billet heating → controlled rolling → laminar flow cooling → air cooling → quenching → tempering → air cooling. The steps of the preparation method are as follows:

[0014] 1) Conduct hot metal pretreatment and control the S content in hot metal ≤ 0.002%;

[0015] 2) Carry out top and bottom combined blowing smelting in the converter, control the end-point composition: C ≤ 0.04%, P ≤ 0.005%. The stirring time before and after tapping is ≥ 1 min. Tapping with slag blocking, and after deoxidation alloying, enter the argon station for bottom blowing argon. The composition in the argon station: P ≤ 0.006%, S ≤ 0.011%, and the alloy components such as Mn, Cr, Mo, Ni, Nb reach the target requirements;

[0016] 3) Add deoxidizers such as aluminum, fluorite, and calcium carbide for refining in the LF furnace, add lime for desulfurization, S ≤ 0.002%. The RH ultimate vacuum degree (<67 Pa) has a circulation times ≥ 6 times. After adding alloys such as Ti and B, the circulation times is 3 times, and add a covering agent when leaving the station;

[0017] 4) During continuous casting, adopt dynamic soft reduction and electromagnetic stirring. The electromagnetic stirring current is 460 A, the frequency is 6 Hz, control the superheat degree at 10 - 25 °C, pour with a constant casting speed, the slab thickness ≥ 300 mm, and the slow cooling time of the continuous casting billet ≥ 72 hours;

[0018] 5) When heating the continuous casting billet, control the heating temperature in the range of 1200 - 1285 °C, the heating rate is 9 - 13 min / cm, and the tapping temperature is in the range of 1200 ± 20 °C;

[0019] 6) The required reduction ratio for slab rolling is ≥ 3.6:1. Adopt two-stage controlled rolling. Specifically: After the slab is taken out of the furnace, remove the scale with high-pressure water descaling, and then cool it to ≥ 1080 °C for the first-stage rolling. The reduction per pass in the longitudinal rolling is controlled at 20 - 35 mm, and roll to (finished product thickness + 50) mm for temperature waiting; The starting rolling temperature in the second stage ≤ 900 °C, the cumulative reduction rate in the last three passes is controlled at ≥ 30%, and the finishing rolling temperature is controlled at 840 ± 20 °C; After rolling, water-cool to 670 ± 30 °C;

[0020] 7) Quenching heat treatment: Control the temperature of the heat treatment furnace in the range of 930 ± 10 °C. After the steel plate reaches this temperature range, the holding time is: product thickness × 3 min / mm. After the steel plate is taken out of the furnace, immediately water-cool it to room temperature with a quenching machine;

[0021] 8) Tempering heat treatment: Control the temperature of the heat treatment furnace at 630 ± 5 °C for tempering. After the steel plate reaches this temperature range, the holding time is: product thickness × 3 - 4 min / mm. After tempering is completed, take it out of the furnace and air-cool it to room temperature.

[0022] According to the above technical features, the reasons for limiting the main chemical components of the steel of the present invention are as follows:

[0023] C is a strong solid solution strengthening element, which can significantly improve the strength of the steel plate. However, as the carbon content increases, the degree of structural segregation of the steel increases, the toughness and plasticity of the steel decrease, and the welding performance of the steel plate deteriorates. In order to make the steel plate have good welding performance and good low-temperature impact toughness, the carbon content in the steel must be reduced. Therefore, the carbon content of the steel of the present invention is controlled to be 0.07-0.09%.

[0024] The Si element can improve the hardness and strength of the solid solution in the steel, not only increasing the hardenability of the steel, but also increasing the tempering resistance of the quenched steel, so that the steel can be tempered at a higher temperature, thereby improving the toughness and delayed fracture resistance of the steel. However, when the Si content increases, it will promote the formation of island martensite, which is harmful to the toughness of the welding heat affected zone and is prone to cracking or crack defects. The Si content of the steel of the present invention is controlled to be 0.20~0.30%.

[0025] Mn is an effective element for improving strength and toughness. It can deoxidize and eliminate the influence of S during the smelting process. It is an effective element for expanding the austenite phase area, refining the grains, ensuring comprehensive performance and improving hardenability during the rolling process. It does not deteriorate the deformation ability of steel. At the same time, it can slow down the speed of structural decomposition and transformation during the tempering process after quenching, and improve the stability of the tempered structure. When used as welding structural steel, the larger the Mn / C ratio (reaching more than 2.5), the better the low-temperature toughness of the steel. The Mn content of the present invention is controlled within the range of 1.35-1.40%.

[0026] P and S are impurity elements in steel. P has strong solid solution strengthening and cold work hardening effects in steel. As an alloying element, it can improve the strength and atmospheric corrosion resistance of steel. However, P is prone to segregation in steel, causing serious segregation in parts of the steel, reducing the plasticity and toughness of the steel, and is extremely harmful to low-temperature toughness. S is often released in the form of strip-like sulfides along the rolling direction, destroying the continuity of the steel and significantly reducing the ductility and toughness of the steel. In terms of metallurgical quality, the sulfur and phosphorus content levels of the steel of the present invention are strictly controlled, that is, P≤0.006% and S≤0.002%.

[0027] Al is the main deoxidizing element in steel. Al in steel can form AlN with N, which can hinder the growth of high-temperature austenite and play a role in grain refinement. However, when the Al content is too high, it is easy to cause more inclusions in the steel, which is not good for the toughness of the steel and reduces the hardenability and toughness of the steel. The Alt content of the steel of the present invention is controlled to be 0.030-0.040%.

[0028] Nb and Ti are two strong carbide and nitride forming elements, which have extremely strong affinity with nitrogen and carbon and can form extremely stable carbonitrides with them. Nb has the strongest effect on increasing the recrystallization temperature, can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, and refine the grains. Ti can produce a strong precipitation strengthening effect, increase the strength of the steel, and can also prevent austenite recrystallization and greatly improve the low-temperature toughness of the heat-affected zone of welding. However, when the Ti content is relatively high, large particles of harmful inclusions that affect low-temperature toughness will be formed. Therefore, through the grain refinement strengthening, precipitation strengthening and solid solution strengthening effects of Nb and Ti microalloying elements, excellent strength and toughness can be obtained for the steel plate. The steel of the present invention has an Nb content of 0.035 - 0.040% and a Ti content of 0.010 - 0.015%.

[0029] Ni is an effective element for expanding the γ phase, refining grains, spheroidizing carbides, ensuring comprehensive properties and increasing hardenability. It can refine ferrite grains to improve the low-temperature toughness of the steel and significantly reduce the low-temperature ductile-brittle transition temperature of the steel plate and welded joints. However, the Ni content should not be too high, as it will not only increase the steelmaking cost but also cause the scale to be difficult to fall off. The content of Ni in the present invention is designed to be 0.8 - 0.9%.

[0030] Cr is beneficial to improving the strength, hardenability, corrosion resistance and high-temperature oxidation resistance of the steel. A stable structure can be obtained after heat treatment. The enrichment of Cr can improve the stability of the corrosion film, delay the deterioration of corrosion and improve the corrosion resistance of ship plate steel. At the same time, the composite strengthening of Cr and Mo can greatly improve the anti-hydrogen sulfide corrosion performance of the steel plate. However, too high a Cr content is not conducive to welding. Considering the alloy cost and service requirements, the Cr content is controlled at 0.45 - 0.50%.

[0031] Mo can increase hardenability. When it coexists with Cr, Mn, etc., it can reduce or eliminate the temper brittleness caused by other alloying elements, which is greatly beneficial to the toughness of the steel, improve the temper stability and effectively eliminate or reduce the residual stress in the steel. However, too high a Mo content is extremely likely to obtain coarse martensite during rapid cooling and welding cooling processes, reducing the low-temperature toughness of the base material and deteriorating the welding performance. Therefore, the Mo content in the present invention is controlled at 0.35 - 0.40%.

[0032] B is an element that strongly increases hardenability. The addition of B can effectively inhibit the nucleation and growth of proeutectoid ferrite. Due to the non-equilibrium segregation of B at the austenite grain boundary, it strongly inhibits the γ-α phase transformation, promoting the formation of fine low-carbon martensite during quenching of austenite, thereby increasing the yield strength and tensile strength of the steel. However, too much B content will reduce the low-temperature toughness of the steel. The B content in the present invention is selected to be 0.0009 - 0.0012%.

[0033] Based on the above technical features, the reasons for setting the main production process parameters in the present invention are as follows:

[0034] During the top and bottom combined blowing smelting process in the converter, control the end-point composition: C≤0.04%, P≤0.005%. The stirring time before and after tapping is ≥1 minute, and slag blocking tapping is carried out, which is beneficial to alloying and controlling the P content in the molten steel at a relatively low level. During the hot metal pretreatment, control the S in the hot metal ≤0.002%; during the refining in the LF furnace, add aluminum, fluorite, calcium carbide, etc., and add lime, with S≤0.002%, which is beneficial to deoxidation, desulfurization, and reducing inclusions. The RH ultimate vacuum degree (<67 Pa) has a cycle number ≥6 times, and the cycle number is 3 times after adding alloys such as Ti and B, which helps to reduce the harmful gas elements N, H, and O in the steel. Reducing the content of harmful elements in the steel can improve the low-temperature toughness of the steel.

[0035] During the continuous casting process, dynamic soft reduction and electromagnetic stirring are adopted. The electromagnetic stirring current is 460 A, the frequency is 6 Hz, and the superheat is controlled at 10~25°C, and continuous casting is carried out at a constant drawing speed, which is beneficial to preventing slag entrainment, improving the central density of the slab and improving the central segregation of the slab, thereby enhancing the properties of the center of the steel plate.

[0036] Increasing the compression ratio can improve the comprehensive mechanical properties of the steel plate. The maximum thickness of the steel plate in the present invention is 80 mm, and the compression ratio ≥3.6:1. Therefore, the slab thickness needs to reach ≥300 mm. To improve the plasticity of the steel, reduce the deformation resistance, and improve the internal structure of the slab, the slab needs to be heated to the temperature range of austenite single-phase solid solution structure, so that it has enough time to homogenize the structure and dissolve carbides at a relatively high temperature. The silicon-manganese steel containing Nb is heated to 1200°C and soaked for 2 hours, so that more than 90% of Nb can be dissolved into the austenite matrix. When heated to 1260°C and held for 30 minutes, it can be completely dissolved. The solubility of Ti in the silicon-manganese steel is similar to that of Nb. To fully dissolve the two micro-alloying elements Nb and Ti, the steel heating process in the present invention is controlled: the heating temperature is in the range of 1200~1285°C, the heating rate is 9~13 min / cm, and the tapping temperature is in the range of 1200±20°C.

[0037] The present invention adopts two-stage controlled rolling, which can give full play to the effects of fine grain strengthening, precipitation strengthening, solid solution strengthening, and phase transformation strengthening of microalloying elements such as Nb and Ti, as well as alloying elements such as Ni, Cr, Mo, and B during rolling and cooling processes, thereby improving the properties of the steel plate. To promote the deformation of the core, after the slab is taken out of the furnace, it is cooled to ≥1080°C for the first-stage rolling. The reduction per longitudinal rolling pass is controlled at 20 - 35 mm. Through high-temperature large reduction rolling, grain refinement can be achieved through recovery and recrystallization. The second-stage rolling is carried out in the austenite non-recrystallization zone. To obtain uniformly sized deformation bands and fine and uniform ferrite grains, the total reduction ratio in the non-recrystallization zone needs to be greater than a certain value, generally greater than 45%. Therefore, the intermediate waiting temperature thickness is (finished product thickness + 50) mm. According to the precipitation characteristics of Nb and Ti microalloys in the steel of the present invention, the starting rolling temperature of the second stage is ≤900°C, the cumulative reduction ratio of the last three passes is controlled at ≥30%, the finishing rolling temperature is controlled at 840 ± 20°C, and after rolling, it is water-cooled to 670 ± 30°C.

[0038] According to the formula Ac3 = 937.2 - 436.5×C + 56×Si - 19.7×Mn - 16.3×Cu - 26.6×Ni - 4.9×Cr + 38.1×Mo + 124.8×V + 136.3×Ti - 19.1×Nb + 198.4×Al + 3315×B, the Ac3 temperature of the steel of the present invention is 890°C. Therefore, the heat treatment furnace temperature during quenching is controlled in the range of 930 ± 10°C. After the steel plate reaches this temperature range, the holding time is: product thickness × 3 min / mm. Through tempering tests, the optimal tempering process for the steel of the present invention is determined as: tempering temperature 630 ± 5°C. After the steel plate reaches this temperature range, the holding time is: product thickness × 3 - 4 min / mm.

[0039] Technical advantages:

[0040] 1. The steel plate of the present invention has a thickness of 50 - 80 mm, a yield strength ReH ≥ 550 MPa, a tensile strength Rm ≥ 650 MPa, an elongation A ≥ 16%, the impact energy KV2 at -60°C at the 1 / 4 thickness part of the steel plate ≥ 120 J, the impact energy KV2 at -60°C at the 1 / 2 thickness part ≥ 100 J, and the microstructure of each part of the steel plate is tempered martensite (tempered M).

[0041] 2. The present invention is a super high-strength ship plate with excellent low-temperature impact performance at 550 MPa level, which can be used for supports and components with high requirements for steel plate strength in ships, construction machinery, ocean engineering, etc.

[0042] 3. The present invention has the advantages of simple manufacturing processes and can be implemented in each metallurgical enterprise. Description of the drawings

[0043] Figure 1Flow chart of a super high strength ship plate steel with excellent low temperature impact performance at 550 MPa level and its preparation method;

[0044] Figure 2 For the surface layer of the metallographic structure (tempered M) at each thickness part of the steel plate in Example 1;

[0045] Figure 3 For 1 / 4 plate thickness of the metallographic structure (tempered M) at each thickness part of the steel plate in Example 1;

[0046] Figure 4 For 1 / 2 plate thickness of the metallographic structure (tempered M) at each thickness part of the steel plate in Example 1. Detailed implementation mode

[0047] The technical solutions of the present invention will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work conditions belong to the scope of protection of the present invention.

[0048] The present invention prepares a steel plate with high strength, high toughness and good cross-section impact stability by optimizing the chemical composition design of the steel plate and the process parameters of heating, rolling and heat treatment. The present invention will be described in detail below in combination with specific embodiments, but the content of the present invention is not limited to the following embodiments.

[0049] The thickness of the steel plate is 50mm, 51mm, 52mm, 53mm, 54mm, 55mm, 56mm, 57mm, 58mm, 59mm, 60mm, 61mm, 62mm, 63mm, 64mm, 65mm, 66mm, 67mm, 68mm, 69mm, 70mm, 71mm, 72mm, 73mm, 74mm, 75mm, 76mm, 77mm, 78mm, 79mm, 80mm.

[0050] C eq The preferred range is: 0.52%, 0.53%, 0.54%, 0.55%, 0.56%.

[0051] Example 1

[0052] Chemical composition design of ultra-high strength ship plate steel with excellent low-temperature impact performance at 550 MPa level. The specific composition is: C 0.083 wt%, Si 0.3 wt%, Mn 1.38 wt%, P 0.005 wt%, S 0.002 wt%, Alt 0.032 wt%, Nb 0.037 wt%, Ti 0.012 wt%, Ni 0.82 wt%, Cr 0.457 wt%, Mo 0.352 wt%, B 0.0009 wt%, and C eq is 0.533.

[0053] Hot rolling process: Heat the continuous casting billet to 1189 °C at a rate of 11 min / cm, and then roll after discharging from the furnace. The finished thickness is 80 mm, the billet discharging temperature is 1189 °C, the rough rolling starting temperature is 1080 °C, the reduction per pass in longitudinal rolling is 20.4 - 31.2 mm, the thickness during holding is 130 mm, the finishing rolling starting temperature is 846 °C, the finishing rolling final temperature is 840 °C, and the recrystallization temperature is 668 °C.

[0054] Heat treatment process: Quench the rolled steel plate at a quenching heating temperature of 930 °C for 240 min, and then perform tempering treatment at a tempering temperature of 632 °C for 240 min.

[0055] Example 2

[0056] Chemical composition design of ultra-high strength ship plate steel with excellent low-temperature impact performance at 550 MPa level. The specific composition is: C 0.071 wt%, Si 0.28 wt%, Mn 1.40 wt%, P 0.006 wt%, S 0.002 wt%, Alt 0.04 wt%, Nb 0.035 wt%, Ti 0.015 wt%, Ni 0.80 wt%, Cr 0.451 wt%, Mo 0.40 wt%, B 0.0012 wt%, and C eq is 0.531.

[0057] Hot rolling process: Heat the continuous casting billet to 1202 °C at a rate of 12.8 min / cm, and then roll after discharging from the furnace. The finished thickness is 70 mm, the billet discharging temperature is 1202 °C, the rough rolling starting temperature is 1087 °C, the reduction per pass in longitudinal rolling is 21.5 - 32.8 mm, the thickness during holding is 120 mm, the finishing rolling starting temperature is 855 °C, the finishing rolling final temperature is 843 °C, and the recrystallization temperature is 676 °C.

[0058] Heat treatment process: Quench the rolled steel plate. The quenching heating temperature is 932 °C, and the holding time is 210 min. Then, perform tempering treatment. The tempering temperature is 630 °C, and the holding time is 210 min.

[0059] Example 3

[0060] Chemical composition design of a super high strength ship plate steel with excellent low temperature impact performance at 550 MPa level. The specific composition is: C 0.09 wt%, Si 0.21 wt%, Mn 1.35 wt%, P 0.005 wt%, S 0.001 wt%, Alt 0.031 wt%, Nb 0.04 wt%, Ti 0.011 wt%, Ni 0.90 wt%, Cr 0.50 wt%, Mo 0.35 wt%, B 0.0011 wt%, and C eq is 0.548.

[0061] Hot rolling process: Heat the slab to 1185 °C, heat it at a rate of 9.2 min / cm, and perform rolling after discharging from the furnace. The finished thickness is 60 mm, the slab discharging temperature is 1185 °C, the rough rolling starting temperature is 1084 °C, the reduction per pass in longitudinal rolling is 22 - 31.1 mm, the thickness after holding temperature is 110 mm, the finishing rolling starting temperature is 859 °C, the finishing rolling final temperature is 835 °C, and the recrystallization temperature is 672 °C.

[0062] Heat treatment process: Quench the rolled steel plate. The quenching heating temperature is 939 °C, and the holding time is 180 min. Then, perform tempering treatment. The tempering temperature is 630 °C, and the holding time is 200 min.

[0063] Comparative Example 1

[0064] Chemical composition design of a steel. The specific composition is: C 0.14 wt%, Si 0.25 wt%, Mn 1.27 wt%, P 0.016 wt%, S 0.006 wt%, Alt 0.027 wt%, Nb 0.022 wt%, Ti 0.011 wt%, Ni 0.251 wt%, Cr 0.72 wt%, Mo 0.35 wt%, B 0.0015 wt%, and C eq is 0.586.

[0065] Hot rolling process: The billet is heated to 1184°C at a rate of 9.3 min / cm, and then rolled after being taken out of the furnace. The thickness of the finished product is 80 mm, the billet temperature when taken out of the furnace is 1184°C, the rough rolling starting temperature is 1126°C, the reduction per pass in longitudinal rolling is 14 - 32.5 mm, the thickness during temperature holding is 110 mm, the finish rolling starting temperature is 852°C, the finish rolling ending temperature is 840°C, and the recrystallization temperature is 651°C.

[0066] Heat treatment process: The rolled steel plate is quenched. The quenching heating temperature is 915°C and the holding time is 160 min. Then it is tempered. The tempering temperature is 660°C and the holding time is 130 min.

[0067] Comparative Example 2

[0068] Chemical composition design of a kind of steel, the specific components are: C 0.134 wt%, Si 0.23 wt%, Mn 1.51 wt%, P 0.015 wt%, S 0.005 wt%, Alt 0.029 wt%, Nb 0.052 wt%, Ti 0.013 wt%, Ni 0.11 wt%, Cr 0.5 wt%, Mo 0.37 wt%, B 0.0013 wt%, and C eq is 0.57.

[0069] Hot rolling process: The billet is heated to 1187°C at a rate of 10.2 min / cm, and then rolled after being taken out of the furnace. The thickness of the finished product is 70 mm, the billet temperature when taken out of the furnace is 1187°C, the rough rolling starting temperature is 1182°C, the reduction per pass in longitudinal rolling is 12.8 - 25.5 mm, the thickness during temperature holding is 120 mm, the finish rolling starting temperature is 874°C, the finish rolling ending temperature is 866°C, and the recrystallization temperature is 734°C.

[0070] Heat treatment process: The rolled steel plate is quenched. The quenching heating temperature is 910°C and the holding time is 142 min. Then it is tempered. The tempering temperature is 590°C and the holding time is 161 min.

[0071] Performance data, see Table 1.

[0072] Table 1.

[0073]

[0074] According to the comparison of performance data, Examples 1-3, while maintaining a high strength level of 550 MPa (ReH 693-713 MPa, Rm 741-750 MPa), exhibit excellent low-temperature toughness (impact energy at -60°C of 233-277 J at 1 / 4 thickness and 113-223 J at 1 / 2 thickness) and plasticity (elongation of 20.5-21.5%), which are significantly superior to those of the comparative examples (impact energy generally lower than 120 J and elongation of 13.5-15%). In particular, although Comparative Example 2 has a higher strength (ReH 772 MPa), its low-temperature toughness deteriorates severely (impact energy at 1 / 2 thickness is only 16-31 J). The examples achieve a synergistic improvement in strength and toughness through alloy design with low C (0.071-0.09%) and low impurities (P≤0.006%, S≤0.002%), combined with controlled rolling and controlled cooling (finishing rolling temperature of 835-843°C) and quenching and tempering treatment (quenching at 930°C + tempering at 630°C), forming a uniform tempered martensite structure.

[0075] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A super high strength ship plate steel with excellent low temperature impact performance at 550 MPa level, characterized in that, The components and their weight percentages of the ultra-high strength steel for ship plates are as follows: C 0.07 - 0.09%, Si 0.20 - 0.30%, Mn 1.35 - 1.40%, P ≤0.006%, S ≤0.002%, Alt 0.030 - 0.040%, Nb 0.035 - 0.040%, Ti 0.010 - 0.015%, Ni 0.8 - 0.9%, Cr 0.45 - 0.50%, Mo 0.35 - 0.40%, B 0.0009 - 0.0012%, and the rest are Fe and inevitable impurities; The whole cross-section of the structure of the ultra-high strength steel for ship plates is tempered martensite.

2. The ultra-high strength ship plate steel with excellent low-temperature impact performance at 550 MPa level according to claim 1, characterized in that, The components and their weight percentages of the ultra-high strength steel for ship plates are: C 0.083wt%, Si 0.3wt%, Mn 1.38wt%, P 0.005wt%, S 0.002wt%, Alt 0.032wt%, Nb 0.037wt%, Ti 0.012wt%, Ni 0.82wt%, Cr 0.457wt%, Mo 0.352wt%, B 0.0009wt%; Or, C 0.071wt%, Si 0.28wt%, Mn 1.40wt%, P 0.006wt%, S 0.002wt%, Alt 0.04wt%, Nb 0.035wt%, Ti 0.015wt%, Ni 0.80wt%, Cr 0.451wt%, Mo 0.40wt%, B 0.0012wt%; Or, C 0.09wt%, Si 0.21wt%, Mn 1.35wt%, P 0.005wt%, S 0.001wt%, Alt 0.031wt%, Nb 0.04wt%, Ti 0.011wt%, Ni 0.90wt%, Cr 0.50wt%, Mo 0.35wt%, B 0.0011wt%.

3. An ultra-high strength ship plate steel with excellent low-temperature impact performance at 550 MPa level according to claim 1, characterized in that, The yield strength ReH of the ultra-high strength steel for ship plates ≥550MPa, the tensile strength Rm ≥650MPa, and the elongation A ≥16%.

4. An ultra-high strength ship plate steel with excellent low-temperature impact properties at 550 MPa level according to claim 1, characterized in that, The impact energy KV2 at -60°C at the 1 / 4 thickness part of the steel ≥120J, and the impact energy KV2 at -60°C at the 1 / 2 thickness part ≥100J.

5. The ultra-high strength ship plate steel with excellent low-temperature impact performance at 550 MPa level according to claim 1, characterized in that, The carbon equivalent C of the steel eq is between 0.52% and 0.56%.

6. The production method of a super high strength ship plate steel with excellent low temperature impact performance at 550 MPa level according to any one of claims 1-5, characterized in that, The production method includes: Carrying out hot metal pretreatment; Converting top and bottom combined blowing smelting, controlling the end point composition: C ≤0.04%, P ≤0.005%, the stirring time before and after tapping ≥1min, slagging off during tapping, and blowing argon after deoxidation alloying; Refining by adding aluminum, fluorite, and calcium carbide for deoxidation, adding lime for desulfurization, S ≤0.002%, the RH ultimate vacuum <67Pa, the number of cycles ≥6 times, the number of cycles is 3 times after adding Ti and B-containing alloys, and adding a covering agent when leaving the station; During continuous casting, dynamic soft reduction and electromagnetic stirring are adopted, controlling the superheat degree at 10 - 25°C, casting at a constant drawing speed, the slab thickness ≥300mm, and the slow cooling time of the casting billet ≥72 hours; When heating the continuous casting billet, control the heating temperature at 1200 - 1285 °C, the heating rate at 9 - 13 min / cm, and the tapping temperature at 1200 ± 20 °C; The reduction ratio requirement for slab rolling is ≥ 3.6, and two-stage controlled rolling is adopted; Quenching heat treatment: Control the heat treatment furnace temperature in the range of 930 ± 10 °C, and the holding time is: product thickness × 3 min / mm. After the steel plate is taken out of the furnace, it is water-cooled to room temperature; Tempering heat treatment: Control the heat treatment furnace temperature at 630 ± 5 °C for tempering. After the steel plate reaches this temperature range, the holding time is: product thickness × 3 - 4 min / mm. After tempering is completed, it is taken out of the furnace and air-cooled to room temperature.

7. The production method of a super high strength ship plate steel with excellent low temperature impact performance at 550 MPa level according to claim 6, characterized in that, When performing hot metal pretreatment, control the S in the hot metal ≤ 0.002%.

8. The production method of an ultra-high strength ship plate steel with excellent low temperature impact property at 550 MPa level according to claim 6, characterized in that, The composition of the argon station is: P ≤ 0.006%, S ≤ 0.011%.

9. The production method of an ultra-high strength ship plate steel with excellent low temperature impact performance at 550 MPa level according to claim 6, characterized in that, The specific method of the two-stage controlled rolling is as follows: After the slab is taken out of the furnace, scale is removed by high-pressure water descaling, and then it is cooled to ≥ 1080 °C for the first-stage rolling. The reduction per pass in longitudinal rolling is controlled at 20 - 35 mm, and when rolling to the finished thickness + 50 mm, temperature waiting is carried out; The starting rolling temperature in the second stage is ≤ 900 °C, and the cumulative reduction ratio in the last three passes is controlled at ≥ 30%. The finishing rolling temperature is controlled at 840 ± 20 °C; After rolling, it is water-cooled to 670 ± 30 °C.

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