High-strength and high-toughness 20MnNiMo steel plate with thickness of 250-300 mm and production method thereof

Abstract

The invention discloses a high-strength and high-toughness 20MnNiMo steel plate with the thickness of 250-300 mm. The high-strength and high-toughness 20MnNiMo steel plate comprises the following chemical components in percentage by weight (wt%): 0.20-0.22% of C, 0.30-0.40% of Si, 1.50-1.60% of Mn, less than or equal to 0.010% of P, less than or equal to 0.002% of S, 1.20-1.30% of Ni, 0.60-0.65% of Mo and the balance of Fe and residual elements, and after thermal refining, the structure of the steel plate is lower bainite, undissolved ferrite and a small amount of tempered sorbite; the yield strength is 570-650 MPa, the tensile strength is 670-760 MPa, the fracture elongation is larger than or equal to 20%, the percentage reduction of area is larger than or equal to 50%, the impact energy at the temperature of-40 DEG C is larger than or equal to 150 J, and the shear section rate is 100%. The thickness surface structure grain size of the steel plate is 9.0-10.0 grades, the thickness 1 / 4 position structure grain size is 8.5-9.5 grades, the thickness 1 / 2 position structure grain size is 8.0-9.0 grades, and the grain size grade difference of the whole thickness section is smaller than or equal to 2.0 grades. A normal probe is adopted for ultrasonic detection, under the sensitivity of phi 5 + 32 db, the primary bottom wave attenuation is smaller than or equal to 20%, and defects such as looseness, white spots and inclusions do not exist inside. And the comprehensive performance is good, and the quality requirements of the 20MnNiMo steel plate with large thickness, high strength and high toughness are completely met.

Description

Technical Field

[0001] The invention relates to the field of extra-thick plate production, and in particular to a 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate and a production method thereof. Background Art

[0002] 250-300mm thick 20MnNiMo steel plates are mainly used in large die forging presses and nuclear power leading materials. They require yield strength ≥570MPa, tensile strength ≥670MPa and impact energy ≥150J at low temperature of -40℃. Such a large thickness and high performance are required, which becomes a difficulty.

[0003] Publication No. CN 102260835 A discloses 18MnNiMo steel for nuclear power and its preparation method. The proposal only introduces the composition and smelting method of 18MnNiMo steel for nuclear power, and cannot prove that it can obtain the corresponding steel plate properties, nor can it explain that it can produce thick steel plates.

[0004] Publication No. CN101845593A provides a 20 Cr controlled nuclear power steel and a production method thereof. The scheme adopts 220-250 mm thick slab rolling, and the yield strength and tensile strength of the obtained steel plate are low, and the low temperature impact performance is also low and cannot meet the requirements.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] In order to improve the above technical defects, the purpose of the present invention is to provide a 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate. The obtained steel plate has strong yield strength and tensile strength, and high low-temperature impact performance, which can meet the requirements for improving technical defects.

[0007] Another object of the present invention is to provide a method for producing a 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate.

[0008] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate, comprising the following chemical components in mass percentage: C: 0.20-0.22, Si: 0.30-0.40, Mn: 1.50-1.60, P≤0.010, S≤0.002, Ni: 1.20-1.30, Mo: 0.60-0.65, and the rest are Fe and residual elements, and its carbon equivalent CEV=0.65-0.70.

[0009] The structure of the steel plate is lower bainite + undissolved ferrite + a small amount of tempered ferrite, the grain size difference of the entire thickness section is ≤2.0, and the strength difference is within 50MPa; the surface structure grain size is 9.0-10.0, the yield strength is 600-650MPa, the tensile strength is 700-760MPa, the elongation at fracture is ≥20%, the cross-sectional shrinkage is ≥50%, and the impact energy at -40℃ is ≥200J; the grain size of the structure at 1 / 4 of the thickness is 8. Grade 5-9.5, yield strength 580-620MPa, tensile strength 680-750MPa, elongation at fracture ≥20%, section shrinkage ≥50%, impact energy at -40℃ ≥200J; grain size at 1 / 2 thickness position is grade 8.0-9.0, yield strength 570-630MPa, tensile strength 670-730MPa, elongation at fracture ≥20%, section shrinkage ≥50%, impact energy at -40℃ ≥150J.

[0010] The obtained steel plate was ultrasonically tested with a straight probe. At a sensitivity of φ5+32db, the attenuation of the primary bottom wave was ≤20%, and there were no looseness, white spots or inclusion defects inside.

[0011] The production method of the above-mentioned 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate is as follows: the steel plate uses a 900-1000 mm thick steel ingot cast by a water-cooled ingot mold as a blank, and is rolled. After rolling, it enters ACC controlled cooling, and then is transferred to a trolley furnace with temperature for cycle tempering, and finally obtains a finished steel plate after heat treatment, as follows:

[0012] 1) Rolling: The starting rolling temperature is ≥1000℃. When the thickness is greater than 600mm, the pass is uniformly reduced by 50mm. When the steel plate is rolled to 600mm, the upper and lower surfaces are sprayed with water for cooling before rolling. The reduction is adjusted according to the temperature difference between the surface and the core. If the temperature difference is less than 50℃, the rolling reduction is 60mm. If the temperature difference is 50-100℃, the rolling reduction is 55mm. If the temperature difference is greater than 100℃, the rolling reduction is 50mm.

[0013] It should be particularly emphasized that: this scheme adopts differential temperature rolling. When the temperature difference between the surface and the core is small, the differential temperature rolling effect is poor and the reduction amount needs to be increased to ensure the permeability of the core. When the temperature difference between the surface and the core is large, reducing the reduction amount can not only ensure that the penetration effect reaches the core of the steel plate, but also ensure the safety of the rolling equipment, thereby achieving the effect of rolling instead of forging.

[0014] 2) ACC controlled cooling: The steel plate water entry temperature is > 800°C. After each cooling, it returns to red for 40 to 60 seconds and then enters the water for cooling. Repeat cooling 6 to 8 times, and the final cooling temperature is 550 to 600°C.

[0015] 3) Cycle tempering: The steel plate controlled by ACC is hoisted into the trolley furnace at 500-600℃ for tempering. The temperature is raised to 600-650℃ and kept for 24h, then cooled to 150-200℃ at a rate of ≤100℃ / h. The temperature is raised to 600-650℃ at a rate of ≤100℃ / h again and kept for 24h, then cooled to 150-200℃ at a rate of ≤100℃ / h. After 3 cycles, the steel plate is hoisted out. The total holding time is 72h.

[0016] It should be noted that during smelting and casting, H element will inevitably remain in the molten steel, and the solubility of H in ferrite is the lowest. After rolling, the steel plate is cyclically tempered in the temperature range of 600-650℃, which can diffuse and remove the residual H in the steel plate to the greatest extent.

[0017] 4) Heat treatment: adopt two quenching + tempering process, the first quenching holding temperature is 880~900℃, the holding time is 2.0~2.2min / mm, and the quenching is carried out to room temperature after leaving the furnace. The quenching water temperature is ≤20℃ and the quenching water flow rate is ≥400m 3 / h, total time in water ≥20min; second quenching holding temperature 800~820℃, holding time 2.3~2.5min / mm, quench to room temperature after leaving the furnace, quenching water temperature ≤15℃, quenching water flow ≥500m 3 / h, total time in water ≥20min; tempering holding temperature is 630-660℃, holding time is 3.5-3.8min / mm, air cooling + spray cooling is adopted after out of the furnace, cooling speed is controlled at 0.05-0.3℃ / S, and natural cooling is performed after cooling to below 300℃.

[0018] It should be noted that during the first quenching, the steel plate can be fully austenitized and become a fine bainite + martensite structure after quenching; when the steel plate is subjected to secondary quenching, the holding temperature is 10 to 20°C below the AC3 temperature, so that a portion of fine undissolved ferrite is retained in the structure, and the structure in the entire thickness direction is consistent after quenching, and the grain size difference is small; after tempering, the steel plate is air-cooled + spray-cooled to below 300°C, which can avoid the tempering brittleness range of the steel plate and improve the low-temperature impact toughness after tempering.

[0019] The beneficial effects of the present invention are: through the reasonable design of the steel plate composition, the precise control of the production process and the heat treatment process, the structure of the obtained steel plate is lower bainite + undissolved ferrite + a small amount of tempered troostite; its yield strength is 570-650MPa, the tensile strength is 670-760MPa, the fracture elongation is ≥20%, the cross-sectional shrinkage is ≥50%, the -40℃ impact energy is ≥150J, and the shear cross-sectional rate is 100%. The grain size of the surface layer of the steel plate thickness is 9.0-10.0 level, the grain size of the structure at the 1 / 4 position of the thickness is 8.5-9.5 level, the grain size of the structure at the 1 / 2 position of the thickness is 8.0-9.0 level, and the grain size difference of the entire thickness section is ≤2.0 level. The steel plate is ultrasonically tested with a straight probe. At a sensitivity of φ5+32db, the attenuation of the primary bottom wave is ≤20%, and there are no defects such as looseness, white spots, and inclusions inside. The comprehensive performance is good and fully meets the quality requirements of 20MnNiMo steel plates with large thickness, high strength and high toughness. DETAILED DESCRIPTION

[0020] In order to better understand the present invention, the content of the present invention is further clearly set forth in conjunction with the embodiments below, but the protection content of the present invention is not limited to the following embodiments. In the following description, a large number of specific details are provided in order to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details.

[0021] Embodiment 1:

[0022] a. Melting composition: C: 0.20%, Si: 0.33%, Mn: 1.60%, P: 0.008%, S: 0.001%, Ni: 1.23%, Mo: 0.65%, the rest are Fe and residual elements, carbon equivalent CEV = 0.68%.

[0023] b. Electric furnace smelting is adopted, and the scrap steel ratio is 25% low alloy plate edge + 75% Ni / Mo series scrap steel, of which Ni content is 1.70% and Mo content is 0.64%. The molten steel is refined outside the furnace in LF, the chemical composition is adjusted, and the white slag process is adopted to absorb the non-metallic inclusions in the molten steel. The molten steel is subjected to VD vacuum degassing, and the pressure is maintained at a vacuum degree of 50Pa for 24 minutes. The H content in the molten steel after vacuuming is 0.85PPm.

[0024] c. Use a water-cooled ingot mold with a thickness of 950mm for casting, a compression ratio of 3.1 to 3.8, a casting superheat of 38°C, a main body casting time of 20min, a riser casting time of 7min, and demolding after 12h after casting.

[0025] d. The steel ingot is braised at 450℃ for 6h, the heating rate is 74℃ / h, and it is kept at 1220~1240℃ for 14h. After keeping it for 7h, the steel is turned over;

[0026] e. The steel ingot is rolled in one heat, with the starting rolling temperature of 1042℃. When the thickness is greater than 600mm, the single pass reduction is uniformly reduced by 50mm. When the steel plate is rolled to 600mm, the upper and lower surfaces of the steel plate are sprayed with water for cooling before rolling. The temperature difference between the surface and the core is 120℃, and the rolling reduction is 50mm.

[0027] f. After rolling, enter ACC cooling, the water temperature is 814℃, cooling 7 times, after each cooling, return to red for 40-60S and then enter water cooling again, the final cooling temperature is 567℃;

[0028] g. After rolling, the steel plate is hoisted into a trolley furnace at 560℃ for tempering. After keeping at 630℃ for 24h, the temperature is lowered to 180℃ at a rate of 60℃ / h, and then the temperature is raised to 630℃ at a rate of 80℃ / h and kept in the range for a long time. After three cycles, the steel plate is hoisted out. The total holding time is 72h.

[0029] h. First quenching: steel plate insulation temperature 900℃, insulation time 2.2min / mm, quench to room temperature after leaving the furnace, quenching water temperature 18℃, quenching water flow 450m 3 / h, total water time 28min; second quenching: steel plate insulation temperature 800℃, insulation time 2.5min / mm, quenching to room temperature after leaving the furnace, quenching water temperature 13℃, quenching water flow 550m 3 / h, total time in water 26min; Tempering: holding temperature 645℃, holding time 3.7min / mm, air cooling + spray cooling after out of the furnace, cooling rate 0.15℃ / S, cool to below 300℃ and then cool naturally.

[0030] The steel plate obtained in the example was tested for performance, and its mechanical properties are shown in Table 1:

[0031] Table 1 Mechanical properties

[0032]

[0033] Embodiment 2:

[0034] a. Melting composition: C: 0.21%, Si: 0.35%, Mn: 1.55%, P: 0.007%, S: 0.001%, Ni: 1.26%, Mo: 0.62%, the rest are Fe and residual elements, carbon equivalent CEV = 0.68%.

[0035] b. Electric furnace smelting is adopted, and the scrap steel ratio is 30% low alloy plate edge + 70% Ni / Mo series scrap steel, of which Ni content is 1.65% and Mo content is 0.72%. The molten steel is refined outside the furnace in LF, the chemical composition is adjusted, and the white slag process is adopted to absorb the non-metallic inclusions in the molten steel. The molten steel is subjected to VD vacuum degassing, and the pressure is maintained at a vacuum degree of 48Pa for 25 minutes. The H content in the molten steel after vacuuming is 0.82PPm;

[0036] c. Use a water-cooled ingot mold with a thickness of 950mm for casting, a compression ratio of 3.1-3.8, a casting superheat of 36°C, a main body casting time of 20min, a riser casting time of 7min, and demoulding after 12h after casting;

[0037] d. The steel ingot is braised at 460℃ for 6h, the heating rate is 63℃ / h, and it is kept at 1220~1240℃ for 13h. After keeping it for 7h, the steel is turned over;

[0038] e. The steel ingot is rolled in one heat, with the starting rolling temperature of 1038℃. When the thickness is greater than 600mm, the single-pass reduction is uniformly reduced by 50mm. When the steel plate is rolled to 600mm, the upper and lower surfaces of the steel plate are sprayed with water for cooling before rolling. The temperature difference between the surface and the core is 80℃, and the rolling reduction is 55mm;

[0039] f. After rolling, enter ACC cooling, water temperature is 820℃, cooling is 7 times, after each cooling, it returns to red for 40-60S and then enters water cooling again, the final cooling temperature is 582℃;

[0040] g. After rolling, the steel plate is hoisted into a trolley furnace at 580℃ for tempering. After keeping at 630℃ for 24h, the temperature is lowered to 180℃ at a rate of 75℃ / h, and then the temperature is raised to 630℃ at a rate of 82℃ / h and kept in the range for a long time. After three cycles, the steel plate is hoisted out. The total holding time is 72h.

[0041] h. First quenching: steel plate insulation temperature 890℃, insulation time 2.2min / mm, quench to room temperature after leaving the furnace, quenching water temperature 17℃, quenching water flow 450m 3 / h, total water time 30min; second quenching: steel plate insulation temperature 800℃, insulation time 2.5min / mm, quenching to room temperature after leaving the furnace, quenching water temperature 14℃, quenching water flow 550m 3 / h, total time in water 28min; tempering: holding temperature 640℃, holding time 3.8min / mm, air cooling + spray cooling after out of the furnace, cooling rate 0.18℃ / S, cool to below 300℃ and then cool naturally.

[0042] The steel plate obtained in the example was tested for performance, and its mechanical properties are shown in Table 2:

[0043] Table 2 Mechanical properties

[0044]

[0045]

[0046] The metallographic structure of the steel plates obtained in Examples 1 and 2 was tested. The grain size of the surface layer of the steel plate was 9.0-10.0, the grain size of the 1 / 4 position of the thickness was 8.5-9.5, the grain size of the 1 / 2 position of the thickness was 8.0-9.0, and the grain size difference of the entire thickness section was ≤2.0. The steel plate was ultrasonically tested with a straight probe. At a sensitivity of φ5+32db, the attenuation of the primary bottom wave was ≤20%, and there were no defects such as looseness, white spots, and inclusions inside.

[0047] In summary, the production method of the present invention is effective in producing 250-300 mm thick high-strength, high-toughness 20MnNiMo steel plates. The various steps cooperate with each other and synergistically enhance the efficiency, thereby obtaining 250-300 mm thick high-strength, high-toughness steel plates with excellent comprehensive performance.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate, characterized in that: The steel plate comprises the following chemical components in mass percentage: C: 0.20-0.22, Si: 0.30-0.40, Mn: 1.50-1.60, P≤0.010, S≤0.002, Ni: 1.20-1.30, Mo: 0.60-0.65, and the rest are Fe and residual elements, and its carbon equivalent CEV=0.65-0.70; The structure of the steel plate is lower bainite + undissolved ferrite + a small amount of tempered ferrite, the grain size difference of the entire thickness section is ≤2.0, and the strength difference is within 50MPa; the surface structure grain size is 9.0-10.0, the yield strength is 600-650MPa, the tensile strength is 700-760MPa, the elongation at fracture is ≥20%, the cross-sectional shrinkage is ≥50%, and the impact energy at -40℃ is ≥200J; the grain size of the structure at 1 / 4 of the thickness is 8. Grade 5-9.5, yield strength 580-620MPa, tensile strength 680-750MPa, elongation at fracture ≥20%, section shrinkage ≥50%, -40℃ impact energy ≥200J; grain size at 1 / 2 thickness is grade 8.0-9.0, yield strength 570-630MPa, tensile strength 670-730MPa, elongation at fracture ≥20%, section shrinkage ≥50%, -40℃ impact energy ≥150J; The obtained steel plate was ultrasonically tested with a straight probe. At a sensitivity of φ5+32db, the attenuation of the primary bottom wave was ≤20%, and there were no looseness, white spots or inclusion defects inside.

2. The method for producing a 250-300 mm thick high-strength and high-toughness 20MnNiMo steel plate according to claim 1, characterized in that: The steel plate adopts a 900-1000 mm thick steel ingot cast by a water-cooled ingot mold as a blank, and is rolled. After rolling, it enters ACC controlled cooling, and then is transferred to a trolley furnace for cycle tempering with temperature, and finally a finished steel plate is obtained after heat treatment, as follows: 1) Rolling: The rolling temperature is ≥1000℃. When the thickness is greater than 600mm, the pass is uniformly pressed down by 50mm. When the steel plate is rolled to 600mm, the upper and lower surfaces are sprayed with water for cooling before rolling. The reduction is adjusted according to the temperature difference between the surface and the core. If the temperature difference is less than 50℃, the rolling reduction is 60mm. If the temperature difference is 50-100℃, the rolling reduction is 55mm. If the temperature difference is greater than 100℃, the rolling reduction is 50mm. 2) ACC controlled cooling: The steel plate water entry temperature is > 800°C, and after each cooling, it returns to red for 40-60S and then enters the water for cooling. Repeat cooling 6-8 times, and the final cooling temperature is 550-600°C; 3) Cycle tempering: The steel plate controlled by ACC is hoisted into the trolley furnace with a temperature of 500-600℃ for tempering. The temperature is raised to 600-650℃ and kept for 24h, then cooled to 150-200℃ at a rate of ≤100℃ / h, and then raised to 600-650℃ at a rate of ≤100℃ / h and kept for 24h, then cooled to 150-200℃ at a rate of ≤100℃ / h. After 3 cycles, the steel plate is hoisted out. The total holding time is 72h. 4) Heat treatment: adopt two quenching + tempering process, the first quenching holding temperature is 880~900℃, the holding time is 2.0~2.2min / mm, and the quenching is carried out to room temperature after leaving the furnace. The quenching water temperature is ≤20℃ and the quenching water flow rate is ≥400m 3 / h, total time in water ≥20min; second quenching holding temperature 800~820℃, holding time 2.3~2.5min / mm, quench to room temperature after leaving the furnace, quenching water temperature ≤15℃, quenching water flow ≥500m 3 / h, total time in water ≥20min; tempering holding temperature is 630-660℃, holding time is 3.5-3.8min / mm, air cooling + spray cooling is adopted after out of the furnace, cooling speed is controlled at 0.05-0.3℃ / S, and natural cooling is performed after cooling to below 300℃.

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