Low-temperature high-strength container steel with tensile strength larger than or equal to 600 MPa and production method thereof

Abstract

The invention discloses low-temperature high-strength container steel with the tensile strength larger than or equal to 600 MPa. The low-temperature high-strength container steel comprises C, Si, Mn, P, S, Ni, Mo, Alt, Cr, V, Fe and residual elements. The main structure of the steel plate obtained through controlled rolling and controlled cooling and normalizing and rapid cooling is 60-75% of bainite, 25-35% of ferrite and a small amount of pearlite, the yield strength of the heat treatment delivery state steel plate is 533-620 MPa, the tensile strength is 640-700 MPa, the elongation is larger than or equal to 19%, the impact energy at the temperature of 20 DEG C below zero is 200-310 J, and after maximum die welding is conducted for 10-12 h at the temperature of 580-620 DEG C, the yield strength is 530-600 MPa, the tensile strength is 630-680 MPa, the elongation is larger than or equal to 20%, and the impact energy at the temperature of 20 DEG C below zero is 170-250 J. According to the scheme, the alloy cost is low, the production process is short, the obtained steel plate is stable in performance, and after maximum die welding, the tensile strength and impact energy of the steel plate still meet the use requirements of low-temperature container steel.

Description

Technical Field

[0001] The invention is applicable to the field of medium and thick plate production, and specifically relates to a low-temperature high-strength container steel with a tensile strength of ≥600MPa and a production method thereof. Background Art

[0002] Steel plates for low-temperature and high-strength containers with a tensile strength of ≥600MPa play an indispensable role in many important industries due to their excellent physical and chemical properties. Especially in the fields of petroleum, chemical, electric power and energy, their high strength, high toughness and good welding performance make them the preferred material for manufacturing pressure vessels, boilers, storage tanks and pipelines. However, after simulated post-weld heat treatment, their tensile strength and impact energy will decrease to a certain extent. The decrease in tensile strength is usually between 30-50MPa, while the decrease in impact energy may reach 30-60%. What is more serious is that the instability of the impact value increases, and even single-digit impact values ​​appear. These changes undoubtedly have an adverse effect on the performance of the steel plate and also bring greater challenges to equipment manufacturing and quality control.

[0003] The patent with publication number CN110088339A discloses "Pressure vessel steel plate with excellent PWHT resistance and its manufacturing method", and its composition weight percentage is: C: 0.10% ~ 0.20%, Si: 0.15% ~ 0.40%, Mn: 1.15% ~ 1.50%, Mo: 0.45% ~ 0.60%, Cu: 0.03% ~ 0.30%, P ≤ 0.025%, S ≤ 0.025%, sol.Al: 0.005% ~ 0.06%, Cr: 0.03% ~0.30, Nb: 0.002%~0.025%, Zr: 0.002%~0.025%, the rest is Fe and inevitable impurities; the hot rolled steel plate is subjected to normalizing heat treatment at 820~950℃, and then the cooled steel plate is subjected to tempering heat treatment at 550~680℃; the tensile strength after the maximum 60 hours of post-weld heat treatment at 600~660℃ (all 630℃ in the embodiments) is 550MPa or higher, and the Charpy impact energy value at -10℃ is 100J or higher. However, the steel contains a relatively large amount of precious element Mo (0.45%~0.60%), and in addition to the normalizing heat treatment, a tempering heat treatment process is added, which leads to a high production cost.

[0004] The patent with publication number CN108431272A discloses "a steel plate for low-temperature pressure vessels with excellent resistance to PWHT and a method for manufacturing the same", wherein the steel plate maintains a tensile strength of 600 MPa or more even after a PWHT of up to 20 hours at 580-640°C (630°C in the embodiments), and a Charpy impact energy value of 200 J or more at -110°C. However, the steel contains a large amount of precious element Ni (3.0%-4.0%) and a small amount of precious element W (0.03%-0.25%). In addition, the hot-rolled steel plate is subjected to a heat treatment process of quenching + high-temperature tempering, and its production cost is significantly high. Summary of the invention

[0005] In order to solve the above technical defects, the purpose of the present invention is to provide a low-temperature high-strength container steel with a tensile strength ≥600MPa. The steel plate has low alloy cost, short production process, stable performance, and a tensile strength of ≥600MPa.

[0006] Another object of the present invention is to provide a method for producing low-temperature high-strength container steel with a tensile strength ≥600MPa.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is as follows: a low-temperature high-strength container steel with a tensile strength of ≥600MPa, the steel plate thickness is 36mm~100mm, and the chemical composition is as follows by mass percentage: C: 0.06~0.09, Si: 0.15~0.25, Mn: 1.35~1.48, P≤0.008, S≤0.003, Ni: 0.30~0.40, Mo: 0.16~0.30, Alt: 0.018~0.030, Cr: 0.15~0.20, V: 0.025~0.040, and the rest are Fe and residual elements.

[0008] Carbon: As the C content increases, the tensile strength increases, but the impact energy decreases. To ensure the impact energy, the C content needs to be appropriately reduced, so the C content is limited to 0.06-0.09wt%.

[0009] Silicon: In the subsequent heat treatment process, there is normalizing and rapid cooling, and the Si content needs to be increased to increase the steel's ability to obtain martensite. However, high Si will cause cracks in the steel plate after rolling, so the Si content is limited to 0.15% to 0.25wt%.

[0010] Manganese: Increasing the Mn content can increase the number of Mn substitution atoms in the matrix and improve the strength of the steel plate, but too high a Mn content makes the banded structure more obvious and reduces the low-temperature toughness of the steel plate. In order to ensure strength while improving impact toughness, Mn needs to be set according to the lower and middle limits, and the Mn content is limited to 1.35-1.48wt%.

[0011] Nickel: Ni is an important component to ensure low-temperature impact toughness at 1 / 2 thickness. Increasing the Ni content will improve the stability of austenite and enhance the inhibitory effect on ferrite transformation and pearlite transformation. However, this type of steel does not require impact toughness at too low a temperature, and excessive Ni will increase the alloy cost. Therefore, the Ni content is limited to 0.30-0.40wt%.

[0012] Molybdenum: Adding a small amount of Mo element can promote low-temperature bainite phase transformation, and finally generate more lath bainite in the structure. Adding Mo can also promote the bainite phase transformation dynamics under low temperature conditions and effectively shorten the phase transformation time. Therefore, the Mo content is limited to 0.16-0.30wt%.

[0013] Vanadium: V can refine the structure and grain of steel, thereby improving the strength and toughness of steel; when vanadium is dissolved into solid solution at high temperature, it can increase the hardenability of steel. It can improve the anti-die welding ability of steel plates for low-temperature and high-strength containers, so the V content is limited to 0.25-0.40wt%.

[0014] The production method of the above-mentioned low-temperature high-strength container steel with a tensile strength of ≥600MPa includes continuous casting, billet heating, controlled rolling and controlled cooling, and normalizing and rapid cooling, as follows:

[0015] ①Continuous casting: Use slabs ≥350mm thick, and the slab low multiple reaches Class C 1.5 or above;

[0016] ② Billet heating: Billet temperature at furnace loading is 250-600℃, heating temperature of heating furnace 1 is 860-1000℃, heating temperature of heating furnace 2 is 1130-1150℃, heating temperature of soaking section is 1220-1240℃, total heating time is 15-20min / cm; this steel is crack-sensitive steel, and excessive heating temperature will cause thermal cracks;

[0017] ③ Controlled rolling and controlled cooling: two-stage rolling is adopted, the rough rolling start temperature is 1000-1020℃, the pass reduction is controlled at 25-35mm, the pass deformation rate is>14%, and the deformation coefficient is controlled to be>0.55 for more than 3 passes. The cumulative reduction in one stage is 150-180mm, the finishing rolling start temperature is 800-830℃, the pass deformation is 25-30mm, and the last pass deformation is controlled according to 1-3mm to flatten the steel plate, and the final rolling temperature is 750-790℃; after rolling, the steel plate enters ACC cooling, the upper and lower water ratio is controlled to be 1:1.2-1.4, and the red-return temperature is 630-650℃;

[0018] It should be noted that the recrystallization temperature of the steel is 845°C, and the entire rough rolling is carried out in a range above the austenite recrystallization temperature, which promotes dynamic recrystallization, refines austenite grains, and forms recrystallized austenite. During the rough rolling and the drying process after the rough rolling, the upper surface is in contact with the air and the lower surface is in contact with the roller, resulting in a temperature difference in the thickness direction of the slab. The upper surface cools slowly and no proeutectoid ferrite transformation occurs, but compared with the core, its cooling rate is relatively fast, so the deformed austenite grains will not be fully recrystallized in time, and a part of the coarse austenite grains will be retained. The coarse deformed austenite grains will be retained in the subsequent finishing rolling process. In order to avoid the formation of coarse bainite structure during the ACC water cooling process, the grain size distribution on the upper surface is uneven, and the upper and lower water ratio is optimized to 1:1.2~1.4.

[0019] ④ Normalizing and rapid cooling: Normalizing temperature 865~885℃, holding time 1.5~2.0mm / min, after holding, the steel plate enters the quenching machine for rapid cooling, the water inlet temperature of the steel plate is controlled at 840~860℃, the cooling rate is 65~75℃ / S, the red-return temperature is 635~645℃, and then air-cooled to room temperature.

[0020] The alloy cost of this scheme is low and the production process is short. The main organization of the steel plate obtained by this scheme is 60-75% bainite, 25-35% ferrite, and a small amount of pearlite. The yield strength of the heat-treated delivery steel plate is 533-620MPa, the tensile strength is 640-700MPa, the elongation is ≥19%, and the impact energy at -20℃ is 200-310J. After the maximum die welding at 580-620℃ and 10-12h, the yield strength is 530-600MPa, the tensile strength is 630-680MPa, the elongation is ≥20%, and the impact energy at -20℃ is 170-250J. After the maximum die welding, the tensile strength and impact energy of the steel plate still meet the use requirements of low-temperature container steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0022] Figure 1 This is a 500X metallographic structure diagram of a 60 mm thick steel plate at 1 / 4 of the thickness of the present invention.

[0023] Figure 2 It is a 500X metallographic structure diagram of a 60 mm thick steel plate at 1 / 2 thickness of the present invention. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0025] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0026] Several low-temperature high-strength container steels with a thickness ranging from 36 mm to 100 mm and a tensile strength of ≥ 600 MPa are produced using the chemical composition shown in Table 1 below (unit: wt%):

[0027] Table 1: Chemical composition of 36mm to 100mm thick low-temperature high-strength container steel (Wt,%)

[0028] Examples Thickness(mm) C Si Mn P S Als Ni Mo Cr V 1 36 0.08 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.22 0.035 2 60 0.09 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.24 0.035 3 100 0.09 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.25 0.035

[0029] The production process of the steel mainly includes converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, controlled rolling and controlled cooling, and heat treatment, as follows:

[0030] (1) Converter smelting: steel tapping P ≤ 0.007%, steel tapping C ≤ 0.10%; converter tapping must be slag-proof, slag layer thickness ≤ 65 mm; argon must be blown throughout the tapping process.

[0031] (2) LF refining: During the refining process, the white slag is kept for ≥20 min, fully desulfurized, and the S content of the finished product is controlled to be ≤0.003 wt%; the C content of the finished product is controlled in the range of 0.08-0.10 wt%.

[0032] (3) VD vacuum refining: The holding time is controlled at 15-25 minutes, and the hydrogen content is required to be ≤1.6ppm; the VD leaving station temperature ensures that the superheat of the molten steel in the tundish is 15-25℃.

[0033] (4) Continuous casting: Ensure that the thickness of the ingot is at least 350mm, the pouring superheat is controlled at 15-25℃, and the casting speed is 0.85-0.88m / min. The low multiple must be in Class C 1.0-1.5 level or above.

[0034] (5) Billet heating: billet charging temperature 250-600°C, heating temperature of heating furnace 1 860-1000°C, heating temperature of heating furnace 2 1130-1150°C, heating temperature of soaking section 1220-1240°C, total heating time 15-20 min / cm;

[0035] (6) Controlled rolling and controlled cooling: two-stage rolling is adopted, the starting temperature of rough rolling is 1000-1020℃, the pass reduction is controlled at 25-35mm, the pass deformation rate is greater than 14%, and the deformation coefficient is controlled to be greater than 0.55 for more than 3 passes. The cumulative reduction in one stage is 150-180mm, the starting temperature of finishing rolling is 800-830℃, the pass deformation is 25-30mm, and the last pass deformation is controlled according to 1-3mm to flatten the steel plate, and the final rolling temperature is 750-790℃; after rolling, the steel plate enters ACC cooling, the upper and lower water ratio is controlled to be 1:1.2-1.4, and the red-return temperature is 630-650℃;

[0036] (7) Normalizing and rapid cooling: Normalizing temperature is 865-885℃, holding time is 1.5-2.0mm / min. After the holding period, the steel plate enters the quenching machine for rapid cooling. The water inlet temperature of the steel plate is controlled at 840-860℃, the cooling rate is 65-75℃ / S, the red-return temperature is 635-645℃, and then air-cooled to room temperature.

[0037] The steel plate obtained in the example was tested for performance. Among them, the maximum mold welding insulation temperature is 580-620℃, the insulation time is 10-12h, the minimum mold welding insulation temperature is 580-620℃, the insulation time is 2-3h, the furnace loading temperature is ≤400℃, the heating and cooling rate is 50-80℃ / h, and the furnace is air-cooled. The performance indicators are shown in Table 2:

[0038] Table 2: Mechanical properties of steel plates with a thickness of 36mm to 100mm and a tensile strength of ≥600MPa for low-temperature high-strength containers

[0039]

[0040]

[0041] This trial production of 36mm, 60mm, 100mm thick steel plates with tensile strength ≥600MPa for low temperature high strength containers, 10 batches each, through reasonable chemical composition design and production process control, the organization of the steel plate is 60-75% bainite + 25-35% ferrite + a small amount of pearlite. The yield strength of the heat-treated delivery steel plate is 533-620MPa, the tensile strength is 640-700MPa, the elongation is ≥19%, and the impact energy at -20℃ is 200-310J. After the maximum die welding at 580-620℃ and 10-12h, its yield strength is 530-600MPa, the tensile strength is 630-680MPa, the elongation is ≥20%, and the impact energy at -20℃ is 170-250J. After the maximum die welding, the tensile strength and impact energy of the steel plate still meet the use requirements of low temperature container steel.

Claims

1. A low-temperature high-strength container steel with a tensile strength of ≥600MPa, characterized in that: The steel plate has a thickness of 36 mm to 100 mm, and contains the following chemical compositions in mass percentage (unit, wt%): C: 0.06 to 0.09, Si: 0.15 to 0.25, Mn: 1.35 to 1.48, P≤0.008, S≤0.003, Ni: 0.30 to 0.40, Mo: 0.16 to 0.30, Alt: 0.018 to 0.030, Cr: 0.15 to 0.20, V: 0.025 to 0.040, and the rest are Fe and residual elements; The main structure of the steel plate is 60-75% bainite, 25-35% ferrite, and a small amount of pearlite. The yield strength of the heat-treated delivered steel plate is 533-620MPa, the tensile strength is 640-700MPa, the elongation is ≥19%, and the impact energy at -20°C is 200-310J. After maximum mold welding at 580-620°C for 10-12h, the yield strength is 530-600MPa, the tensile strength is 630-680MPa, the elongation is ≥20%, and the impact energy at -20°C is 170-250J.

2. The method for producing low-temperature high-strength container steel with a tensile strength of ≥600 MPa according to claim 1, characterized in that It includes continuous casting, billet heating, controlled rolling and controlled cooling, and normalizing and rapid cooling, as follows: ①Continuous casting: Use slabs ≥350mm thick, and the slab low multiple reaches Class C 1.5 or above; ② Billet heating: Billet charging temperature is 250-600℃, heating temperature of heating furnace 1 is 860-1000℃, heating temperature of heating furnace 2 is 1130-1150℃, heating temperature of soaking section is 1220-1240℃, total heating time is 15-20min / cm; ③ Controlled rolling and controlled cooling: two-stage rolling is adopted, the rough rolling start temperature is 1000-1020℃, the pass reduction is controlled at 25-35mm, the pass deformation rate is>14%, and the deformation coefficient is controlled to be>0.55 for more than 3 passes. The cumulative reduction in one stage is 150-180mm, the finishing rolling start temperature is 800-830℃, the pass deformation is 25-30mm, and the last pass deformation is controlled according to 1-3mm to flatten the steel plate, and the final rolling temperature is 750-790℃; after rolling, the steel plate enters ACC cooling, the upper and lower water ratio is controlled to be 1:1.2-1.4, and the red-return temperature is 630-650℃; ④ Normalizing and rapid cooling: Normalizing temperature 865~885℃, holding time 1.5~2.0mm / min, after holding, the steel plate enters the quenching machine for rapid cooling, the water inlet temperature of the steel plate is controlled at 840~860℃, the cooling rate is 65~75℃ / S, the red-return temperature is 635~645℃, and then air-cooled to room temperature.

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