A 250-300mm thick high-strength, high-toughness 20MnNiMo steel plate and its production method

Abstract

This invention discloses a 250-300mm thick high-strength, high-toughness 20MnNiMo steel plate, comprising the following chemical composition (wt%): 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, with the remainder being Fe and residual elements. After quenching and tempering, its microstructure consists of lower bainite + undissolved ferrite + a small amount of tempered sorbite; its yield strength is 570-650MPa, tensile strength is 670-760MPa, elongation at break is ≥20%, reduction of area is ≥50%, impact energy at -40℃ is ≥150J, and shear area is 100%. The grain size of the surface layer of the steel plate is grade 9.0–10.0, the grain size at 1 / 4 of the thickness is grade 8.5–9.5, and the grain size at 1 / 2 of the thickness is grade 8.0–9.0. The grain size difference across the entire thickness section is ≤2.0. Ultrasonic testing was performed using a straight probe. At a sensitivity of φ5+32dB, the first-order backwave attenuation was ≤20%, and there were no internal defects such as porosity, white spots, or inclusions. The overall performance is excellent, fully meeting the quality requirements for thick, high-strength, and high-toughness 20MnNiMo steel plates.

Description

Technical Field

[0001] This invention relates to the field of extra-thick plate production, specifically to a 250-300mm thick high-strength, high-toughness 20MnNiMo steel plate and its production method. Background Technology

[0002] 250-300mm thick 20MnNiMo steel plates are mainly used in large die forging presses and as the main material for nuclear power. The requirements of yield strength ≥570MPa, tensile strength ≥670MPa and impact energy ≥150J at -40℃ are challenging due to the large thickness and high performance.

[0003] Publication number CN 102260835 A discloses 18MnNiMo steel for nuclear power and its preparation method. This scheme only introduces the composition and smelting method of 18MnNiMo steel for nuclear power, and cannot prove that it can obtain the corresponding steel plate performance, nor can it explain that it can produce thick steel plates.

[0004] The publication number CN101845593A discloses a 20-Cr nuclear power steel and its production method. The method uses 220-250mm thick slabs for rolling, and the yield strength and tensile strength of the obtained steel plates are relatively low, and the low-temperature impact performance is also low, which cannot meet the requirements.

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

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

[0007] Another objective of this invention is to provide a method for producing 250-300mm thick high-strength, high-toughness 20MnNiMo steel plates.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is: a 250-300mm thick high-strength and high-toughness 20MnNiMo steel plate, comprising the following chemical composition by 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, with the remainder being Fe and residual elements, and its carbon equivalent CEV = 0.65-0.70.

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

[0010] The obtained steel plate was ultrasonically tested using a straight probe. At a sensitivity of φ5+32db, the attenuation of the first wave was ≤20%, and there were no defects such as porosity, white spots, or inclusions inside.

[0011] The production method of the above-mentioned 250-300mm thick high-strength and high-toughness 20MnNiMo steel plate involves using a 900-1000mm thick steel ingot cast in a water-cooled ingot mold as the billet, rolling it, then subjecting it to ACC controlled cooling, followed by heated transfer to a bogie furnace for circulating tempering, and finally heat treatment to obtain the finished steel plate, as detailed below:

[0012] 1) Rolling: When the initial rolling temperature is ≥1000℃ and the thickness is >600mm, the rolling reduction is 50mm per pass. When the thickness reaches 600mm, the upper and lower surfaces of the steel plate are sprayed with water to cool it before rolling. The reduction is adjusted according to the temperature difference between the surface and the core. If the temperature difference is <50℃, the rolling reduction is 60mm. If the temperature difference is 50~100℃, the rolling reduction is 55mm. If the temperature difference is >100℃, the rolling reduction is 50mm.

[0013] It is particularly important to emphasize that this scheme uses 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 core penetration rate. 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 and achieve the effect of replacing forging with rolling.

[0014] 2) ACC controlled cooling: The steel plate is immersed in water at a temperature >800℃. After each cooling, it is allowed to glow red for 40-60 seconds before being immersed in water for further cooling. This cooling process is repeated 6-8 times, with a final cooling temperature of 550-600℃.

[0015] 3) Cyclic tempering: The steel plate, cooled by ACC control, is hoisted into the bogie furnace at a temperature of 500-600℃ for tempering. The temperature is then raised to 600-650℃ and held for 24 hours. After that, the temperature is lowered to 150-200℃ at a rate of ≤100℃ / h. The temperature is then raised again to 600-650℃ at a rate of ≤100℃ / h and held for another 24 hours. After that, the temperature is lowered to 150-200℃ at a rate of ≤100℃ / h. This cycle is repeated 3 times before the steel plate is hoisted out. The total holding time is 72 hours.

[0016] It should be noted that during smelting and casting, hydrogen (H) will inevitably remain in the molten steel. Since H has the lowest solubility in ferrite, the rolled steel plate is subjected to cyclic tempering 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: A two-stage quenching and tempering process is adopted. The first quenching temperature is 880-900℃, and the holding time is 2.0-2.2 min / mm. After removing from the furnace, the water is quenched to room temperature. The quenching water temperature is ≤20℃, and the quenching water flow rate is ≥400 m³ / min. 3 / h, total water time ≥20min; second quenching holding temperature 800~820℃, holding time 2.3~2.5min / mm, after furnace removal, quench to room temperature, quenching water temperature ≤15℃, quenching water flow rate ≥500m³ / h. 3 / h, total water time ≥20min; tempering holding temperature 630~660℃, holding time 3.5~3.8min / mm, after exiting the furnace, use air cooling + spray cooling, control the cooling rate 0.05~0.3℃ / S, cool to below 300℃ and then let it cool naturally.

[0018] It should be noted that during the first quenching of the steel plate, the microstructure can be fully austenitized, and after quenching, it becomes a fine bainite + martensite microstructure. When the steel plate is quenched for the second time, the holding temperature is 10-20℃ below AC3 temperature, so that some fine undissolved ferrite is retained in the microstructure. After quenching, the microstructure is consistent throughout the thickness direction and the grain size difference is small. After tempering, the steel plate is air-cooled and spray-cooled to below 300℃, which can avoid the temper brittle range of the steel plate and improve the low-temperature impact toughness after tempering.

[0019] The beneficial effects of this invention are as follows: Through the rational design of the steel plate composition and the precise control of the production process and heat treatment process, the resulting steel plate has a microstructure of lower bainite + undissolved ferrite + a small amount of tempered sorbite; its yield strength is 570-650 MPa, tensile strength is 670-760 MPa, elongation at break is ≥20%, reduction of area is ≥50%, impact energy at -40℃ is ≥150 J, and shear area is 100%. The grain size of the surface microstructure of the steel plate is 9.0-10.0 grade, the grain size at 1 / 4 of the thickness is 8.5-9.5 grade, the grain size at 1 / 2 of the thickness is 8.0-9.0 grade, and the grain size difference across the entire thickness section is ≤2.0 grade. The steel plate is ultrasonically tested using a straight probe, and at a sensitivity of φ5+32db, the first-wave attenuation is ≤20%, and there are no defects such as porosity, white spots, or inclusions inside. It has excellent overall performance and fully meets the quality requirements of thick, high-strength, and high-toughness 20MnNiMo steel plates. Detailed Implementation

[0020] To better understand the present invention, the following embodiments further illustrate the content of the invention, but the scope of protection of the present invention is not limited to the following embodiments. Numerous specific details are set forth in the following description to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the present invention can be practiced without one or more of these details.

[0021] Example 1:

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

[0023] b. Electric furnace smelting is employed, with a scrap ratio of 25% low-alloy plate edges and 75% Ni / Mo series scrap steel, wherein the Ni content is 1.70% and the Mo content is 0.64%. The molten steel undergoes ladle refining in the LF furnace, with chemical composition adjusted and a white slag-making process used to adsorb non-metallic inclusions. The molten steel is then subjected to VD vacuum degassing, held at a vacuum of 50 Pa for 24 minutes. After vacuuming, the H content in the molten steel is 0.85 ppm.

[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℃, a casting time of 20 minutes for the main body, a casting time of 7 minutes for the riser, and demold 12 hours after casting.

[0025] d. The steel ingot is simmered at 450℃ for 6 hours with a heating rate of 74℃ / h, held at 1220~1240℃ for 14 hours, and then turned over after holding at 1220~1240℃ for 7 hours.

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

[0027] f. After rolling, the roll enters the ACC cooling system with a water temperature of 814℃. It is cooled 7 times, and after each cooling, it is reddened for 40-60 seconds before being immersed in water for cooling again. The final cooling temperature is 567℃.

[0028] g. After rolling, the steel plate is hoisted into the bogie furnace at 560°C for tempering. After holding at 630°C for 24 hours, the temperature is reduced to 180°C at a rate of 60°C / h. Then, the temperature is increased to 630°C at a rate of 80°C / h and held for another 24 hours. After repeating this cycle 3 times, the steel plate is hoisted out. The total holding time is 72 hours.

[0029] h. First quenching: The steel plate is held at 900℃ for 2.2 min / mm, then quenched to room temperature with quenching water at 18℃ and a flow rate of 450 m³ / min. 3 / h, total water time 28min; Second quenching: steel plate holding temperature 800℃, holding time 2.5min / mm, after taking it out of the furnace, quench to room temperature, quenching water temperature 13℃, quenching water flow rate 550m³ / h 3 / h, total water time 26min; tempering: holding temperature 645℃, holding time 3.7min / mm, after exiting the furnace, air cooling + spray cooling is used, cooling rate 0.15℃ / S, and after cooling to below 300℃, it is naturally cooled.

[0030] The mechanical properties of the steel plates obtained in the example were tested and are shown in Table 1.

[0031] Table 1 Mechanical Properties

[0032]

[0033] Example 2:

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

[0035] b. Electric furnace smelting is employed, with a scrap ratio of 30% low-alloy plate edges + 70% Ni / Mo series scrap steel, wherein the Ni content is 1.65% and the Mo content is 0.72%. The molten steel undergoes ladle refining in the LF furnace, with chemical composition adjusted and a white slag-making process used to adsorb non-metallic inclusions. The molten steel is then subjected to VD vacuum degassing, held at a vacuum of 48 Pa for 25 minutes. After vacuuming, the H content in the molten steel is 0.82 ppm.

[0036] 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 36℃, a casting time of 20 minutes for the main body, a casting time of 7 minutes for the riser, and demold 12 hours after casting.

[0037] d. The steel ingot is simmered at 460℃ for 6 hours with a heating rate of 63℃ / h, held at 1220~1240℃ for 13 hours, and then turned over after holding at 1220~1240℃ for 7 hours.

[0038] e. The steel ingot is rolled in one heat. The initial rolling temperature is 1038℃. When the thickness is >600mm, the single-pass reduction is 50mm. When the thickness reaches 600mm, the upper and lower surfaces of the steel plate are sprayed with water to cool it before rolling. The temperature difference between the surface and the core is 80℃. The rolling reduction is 55mm.

[0039] f. After rolling, the roll enters the ACC cooling system with a water temperature of 820℃. It is cooled 7 times, and after each cooling, it is reddened for 40-60 seconds before being immersed in water for cooling again. The final cooling temperature is 582℃.

[0040] g. After rolling, the steel plate is hoisted into the bogie furnace at 580°C for tempering. After holding at 630°C for 24 hours, the temperature is reduced to 180°C at a rate of 75°C / h. Then, the temperature is increased to 630°C at a rate of 82°C / h and held for another 24 hours. After repeating this cycle 3 times, the steel plate is hoisted out. The total holding time is 72 hours.

[0041] h. First quenching: The steel plate is held at 890℃ for 2.2 min / mm, then quenched to room temperature with quenching water at 17℃ and a flow rate of 450 m³ / min. 3 / h, total water time 30min; Second quenching: steel plate holding temperature 800℃, holding time 2.5min / mm, after taking it out of the furnace, quench to room temperature, quenching water temperature 14℃, quenching water flow rate 550m³ / h 3 / h, total water time 28min; tempering: holding temperature 640℃, holding time 3.8min / mm, after exiting the furnace, air cooling + spray cooling is used, cooling rate 0.18℃ / S, and after cooling to below 300℃, natural cooling is performed.

[0042] The mechanical properties of the steel plates obtained in the example were tested and are shown in Table 2.

[0043] Table 2 Mechanical Properties

[0044]

[0045]

[0046] Metallographic analysis was performed on the steel plates obtained in Examples 1 and 2. The grain size of the surface layer of the steel plates was 9.0–10.0 grade, the grain size at 1 / 4 of the thickness was 8.5–9.5 grade, and the grain size at 1 / 2 of the thickness was 8.0–9.0 grade. The grain size difference across the entire thickness section was ≤2.0 grade. Ultrasonic testing was performed on the steel plates using a straight probe. At a sensitivity of φ5+32dB, the first-order backwave attenuation was ≤20%, and no defects such as porosity, white spots, or inclusions were found inside.

[0047] In summary, the production method of this invention is highly effective in producing 250-300mm thick high-strength and high-toughness 20MnNiMo steel plates. The various steps work together synergistically to achieve excellent overall performance, resulting in 250-300mm thick high-strength and high-toughness steel plates.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing a 250-300mm thick high-strength, high-toughness 20MnNiMo steel plate, characterized in that, The steel plate contains the following chemical composition by 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, with the remainder being Fe and residual elements, and its carbon equivalent CEV = 0.65-0.70; The production method of the above-mentioned 250-300mm thick high-strength and high-toughness 20MnNiMo steel plate involves using a 900-1000mm thick steel ingot cast in a water-cooled ingot mold as the billet, rolling it, then subjecting it to ACC controlled cooling, followed by heated transfer to a bogie furnace for circulating tempering, and finally heat treatment to obtain the finished steel plate, as detailed below: 1) Rolling: When the initial rolling temperature is ≥1000℃ and the thickness is >600mm, the rolling reduction should be uniformly 50mm per pass. When the thickness reaches 600mm, the upper and lower surfaces of the steel plate should be sprayed with water to cool it before rolling. The reduction should be adjusted according to the temperature difference between the surface and the core. If the temperature difference is <50℃, the rolling reduction is 60mm. If the temperature difference is 50~100℃, the rolling reduction is 55mm. If the temperature difference is >100℃, the rolling reduction is 50mm. 2) ACC controlled cooling: The steel plate is immersed in water at a temperature >800℃. After each cooling, it is allowed to glow red for 40-60 seconds before being immersed in water for further cooling. This cooling process is repeated 6-8 times, with a final cooling temperature of 550-600℃. 3) Cyclic tempering: The steel plate cooled by ACC is hoisted into the bogie furnace at a temperature of 500-600℃ for tempering. The temperature is raised to 600-650℃ and held for 24 hours. Then, the temperature is lowered to 150-200℃ at a rate of ≤100℃ / h. The temperature is then raised to 600-650℃ again at a rate of ≤100℃ / h and held for another 24 hours. Then, the temperature is lowered to 150-200℃ at a rate of ≤100℃ / h. After this cycle is repeated 3 times, the steel plate is hoisted out. The total holding time is 72 hours. 4) Heat treatment: A two-stage quenching and tempering process is adopted. The first quenching temperature is 880-900℃, and the holding time is 2.0-2.2 min / mm. After removing from the furnace, the water is quenched to room temperature. The quenching water temperature is ≤20℃, and the quenching water flow rate is ≥400 m³ / min. 3 / h, total water time ≥20min; second quenching holding temperature 800~820℃, holding time 2.3~2.5min / mm, after furnace removal, quench to room temperature, quenching water temperature ≤15℃, quenching water flow rate ≥500m³ / h. 3 / h, total water time ≥20min; tempering holding temperature 630~660℃, holding time 3.5~3.8min / mm, after exiting the furnace, use air cooling + spray cooling, control the cooling rate 0.05~0.3℃ / S, cool to below 300℃ and then let it cool naturally.

2. The method for producing 250-300mm thick high-strength, high-toughness 20MnNiMo steel plates according to claim 1, characterized in that, The microstructure of the steel plate consists of lower bainite + undissolved ferrite + a small amount of tempered ferrite. The grain size difference across the entire thickness section is ≤2.0 grade, and the strength difference is within 50 MPa. The surface microstructure has a grain size of 9.0–10.0 grade, a yield strength of 600–650 MPa, a tensile strength of 700–760 MPa, an elongation at break ≥20%, a reduction of area ≥50%, and an impact energy at -40℃ ≥200 J. At the 1 / 4 thickness position, the grain size is 8.5–9.5 grade, the yield strength is 580–620 MPa, and the tensile strength is 68 MPa. The steel plate has a strength of 0-750 MPa, an elongation at break of ≥20%, a reduction of area of ​​≥50%, and an impact energy of ≥200 J at -40℃. The grain size at the 1 / 2 thickness position is grade 8.0-9.0, the yield strength is 570-630 MPa, the tensile strength is 670-730 MPa, the elongation at break of ≥20%, the reduction of area of ​​≥50%, and the impact energy of ≥150 J at -40℃. The obtained steel plate is ultrasonically tested using a straight probe. At a sensitivity of φ5+32 dB, the attenuation of the first wave is ≤20%, and there are no defects such as porosity, white spots, or inclusions inside.

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