A production method of high-strength high-low-temperature toughness 3.5Ni steel plate

Abstract

The application discloses a production method of high-strength high-low-temperature toughness 3.5Ni steel medium plate, and the production process comprises the following steps: molten iron pretreatment, converter, LF and RH / VD refining, continuous casting, heating, rolling, MULPIC cooling, finishing and heat treatment; the chemical components of the steel are as follows in percentage by weight: C=0.03%-0.10%, Mn=0.02%-0.12%, Si=0.05%-0.15%, Ni=3.2%-3.8%, P<=0.010%, S<=0.005%, Mo=0.20%-0.50%, and the rest is Fe and inevitable impurity elements. The application refines the grain through micro-alloying and controlled rolling+on-line quenching+intercritical quenching+tempering heat treatment process, optimizes the proportion of critical ferrite and reverse transformation austenite, obtains the ferrite and martensite structure between soft and hard phases, improves the strength and low-temperature toughness, and the yield strength is greater than or equal to 420MPa, the tensile strength is greater than or equal to 520MPa, the total elongation is greater than or equal to 18%, and the impact energy at-140 DEG C is greater than or equal to 100J.

Description

Technical Field

[0001] This invention belongs to the field of steel material manufacturing technology and relates to a production method of high-strength, high-low temperature toughness 3.5Ni steel medium-thick plate. Background Technology

[0002] With the increasing demand for clean energy, the import volume of liquid ammonia has been increasing year by year. According to IGC specifications, steel grades with a Ni content exceeding 5% cannot be used in liquid ammonia marine transport tanks or fuel tanks. Currently, only 5Ni steel meets the requirements for both liquid ammonia and LEG transport tanks. Under technical agreements, the Ni content of 5Ni steel is controlled within the range of 4.75% to 5.00%. Furthermore, my country is Ni-poor and Ni prices remain high. With the emergence of Ni-saving low-temperature steels, achieving the excellent low-temperature strength and toughness of high-Ni steel using low-Ni steel remains a challenge.

[0003] Currently, most mainstream 3.5Ni steels are processed using normalizing + tempering (NT) or quenching + tempering (QT) processes. Although they can achieve an impact energy of >80J at -101℃ (such as 08Ni3DR steel plates), their strength does not meet the requirements of liquid ammonia storage tanks. In addition, the impact performance at -140℃ decreases significantly, making it difficult to meet the needs of tank equipment at even lower temperatures, such as LEG. Summary of the Invention

[0004] The present invention aims to provide a production method for high-strength, high-low temperature toughness 3.5Ni steel medium-thick plates, which obtains high strength and high-low temperature toughness mechanical properties through micro-alloying, optimized rolling and heat treatment processes.

[0005] The technical solution of the present invention: A method for producing high-strength, high- and low-temperature toughness 3.5Ni steel medium-thick plates, the production process includes hot metal pretreatment, converter, LF and RH / VD refining, continuous casting, heating, rolling, MULPIC cooling, finishing, and heat treatment; the chemical composition of the steel, by weight percentage, is C=0.03%~0.10%, Mn=0.02%~0.12%, Si=0.05%~0.15%, Ni=3.2%~3.8%, P≤0.010%, S≤0.005%, Mo=0.20%~0.50%, with the remainder being Fe and unavoidable impurity elements; key process steps include: (1) Before entering the furnace, spray an anti-oxidation coating on the upper and lower surfaces of the continuously cast billet. The heating temperature range is 1100~1200℃, and the holding time is 200~300 minutes. (2) The rolling method is two-stage rolling. The rough rolling start temperature is ≥1050℃, the finish rolling start temperature is ≤930℃, and the final rolling temperature is 800~850℃. After rolling, rapid quenching and cooling are carried out. The water immersion temperature is ≥780℃ and the cooling rate is ≥10℃ / s. (3) After online quenching, offline quenching and tempering are performed. The quenching temperature is 690~750℃ and the tempering temperature is 610~670℃. (4) After rolling and heat treatment, a 3.5Ni steel plate is obtained. The microstructure of the steel plate is tempered martensite + critical ferrite as the matrix, with a small amount of reversed austenite distributed in the matrix. The yield strength of the steel plate is ≥420MPa, the tensile strength is ≥520MPa, the total elongation is ≥18%, and the impact energy at -140℃ is ≥100J.

[0006] Compared with the prior art, the beneficial effects of the present invention are as follows: the hardenability of the steel plate is improved and the grain size is refined by microalloying, and the grains are further refined by controlled rolling and online quenching to obtain a refined hard phase structure; the sub-temperature quenching + tempering heat treatment process optimizes the ratio of critical ferrite and reverse transformation austenite to obtain a soft and hard phase ferrite and martensite structure, which improves strength and low temperature toughness and meets the requirements of low temperature ship tank manufacturing. Attached Figure Description

[0007] Figure 1 This is a microstructure diagram of the 3.5Ni steel in Example 1 of the present invention.

[0008] Figure 2 This is a microstructure diagram of the impact fracture surface of 3.5Ni steel in Example 1 of the present invention. Detailed Implementation

[0009] The present invention will be further described below with reference to the embodiments. Example 1

[0010] A method for producing high-strength, high- and low-temperature toughness 3.5Ni steel medium-thick plates, with a chemical composition by mass percentage of C=0.05%, Si=0.08%, Mn=0.07%, Ni=3.61%, P=0.004%, S=0.001%, Mo=0.30%, and the remainder being Fe and unavoidable impurities; including the following process steps: (1) The smelting process is converter decarburization, ladle furnace dephosphorization and alloying, vacuum furnace dephosphorization and dephosphorization, and continuous casting. The thickness of the continuous casting billet is 220mm.

[0011] (2) The surface of the continuous casting billet is coated with an anti-oxidation coating. The furnace temperature is controlled at 1150~1200℃ and the furnace time is 240 minutes. The roughing rolling temperature is 1187℃. The first three reductions are 40, 38 and 34 mm respectively. The intermediate billet thickness is 80 mm. The finishing rolling temperature is 952℃ and the final rolling temperature is 836℃. The rolling thickness is 30 mm. After rolling, the billet is quickly cooled in water at a water temperature of 796℃. MULPIC uses the DQ water flow mode and quenches to below 100℃.

[0012] (3) Heat the quenched steel plate to the two-phase region temperature of 730℃, hold for 50 minutes and then quench to below 200℃ to obtain the sub-temperature quenched steel plate; heat the sub-temperature quenched steel plate to 630℃ for tempering, the tempering time is 60 minutes, and after taking it out of the furnace, air cool to room temperature.

[0013] (4) Rolling and heat treatment to obtain the microstructure of 3.5Ni steel plate with tempered martensite + critical ferrite as the matrix and a small amount of reversed austenite distributed in the matrix; tensile strength is 550MPa, yield strength is 443MPa, elongation is 27%, and impact energy at -140℃ is 291J. Example 2

[0014] A method for producing high-strength, high- and low-temperature toughness 3.5Ni steel medium-thick plates, with a chemical composition by mass percentage of C=0.08%, Si=0.05%, Mn=0.10%, Ni=3.4%, P=0.005%, S=0.002%, Mo=0.36%, and the remainder being Fe and unavoidable impurities; including the following process steps: (1) The smelting process is as follows: converter decarburization, ladle furnace dephosphorization and alloying, vacuum furnace dephosphorization and N removal, and continuous casting. The thickness of the continuous casting billet is 220 mm.

[0015] (2) The surface of the continuous casting billet is coated with an anti-oxidation coating. The furnace temperature is controlled at 1140~1180℃ and the furnace time is 268 minutes. The roughing rolling temperature is 1164℃. The first three reductions are 38, 37 and 32 mm respectively. The intermediate billet thickness is 70 mm. The finishing rolling temperature is 935℃ and the final rolling temperature is 816℃. The rolling thickness is 20 mm. After rolling, the billet is quickly cooled in water at a water temperature of 785℃. MULPIC uses the DQ water flow mode and quenches to below 100℃.

[0016] (3) Heat the quenched steel plate to the two-phase region temperature of 700℃, hold for 40 minutes and then quench to below 200℃ to obtain the sub-temperature quenched steel plate; heat the sub-temperature quenched steel plate to 650℃ for tempering, the tempering time is 40 minutes, and after taking it out of the furnace, air cool to room temperature.

[0017] (4) Rolling and heat treatment to obtain the microstructure of 3.5Ni steel plate with tempered martensite + critical ferrite as the matrix and a small amount of reversed austenite distributed in the matrix; tensile strength is 547MPa, yield strength is 439MPa, elongation is 26%, and impact energy at -140℃ is 255J.

Claims

1. A method for producing high-strength, high-low temperature toughness 3.5Ni steel medium-thick plates, the production process including hot metal pretreatment - converter - LF and RH / VD refining - continuous casting - heating - rolling - MULPIC cooling - finishing - heat treatment, characterized in that: The chemical composition of the steel, by weight percentage, is C=0.03%~0.10%, Mn=0.02%~0.12%, Si=0.05%~0.15%, Ni=3.2%~3.8%, P≤0.010%, S≤0.005%, Mo=0.20%~0.50%, with the remainder being Fe and unavoidable impurity elements; Key process steps include: (1) Before entering the furnace, spray an anti-oxidation coating on the upper and lower surfaces of the continuously cast billet. The heating temperature range is 1100~1200℃, and the holding time is 200~300 minutes. (2) The rolling method is two-stage rolling. The rough rolling start temperature is ≥1050℃, the finish rolling start temperature is ≤930℃, and the final rolling temperature is 800~850℃. After rolling, rapid quenching and cooling are carried out. The water immersion temperature is ≥780℃ and the cooling rate is ≥10℃ / s. (3) After online quenching, offline quenching and tempering are performed. The quenching temperature is 690~750℃ and the tempering temperature is 610~670℃. (4) After rolling and heat treatment, a 3.5Ni steel plate is obtained. The microstructure of the steel plate is tempered martensite + critical ferrite as the matrix, with a small amount of reversed austenite distributed in the matrix. The yield strength of the steel plate is ≥420MPa, the tensile strength is ≥520MPa, the total elongation is ≥18%, and the impact energy at -140℃ is ≥100J.

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