Production method of 960MPa-grade extra-thick rack steel plate

Abstract

The invention relates to a production method of a 960MPa-grade extra-thick rack steel plate, the production process route comprises smelting, die casting, rolling, rapid cooling, finishing, quenching and tempering, the thickness of the produced steel plate is 150-180mm, and the steel comprises the following chemical components in percentage by mass: 0.14-0.18% of C, 0.15-0.40% of Si, 1.0-1.2% of Mn, less than or equal to 0.008% of P, less than or equal to 0.002% of S, 0.8-1.8% of Cr, 0.05-0.08% of V, 1.6-3.0% of Ni, 0.8-1.0% of Mo, 0.07-0.012% of Al, 0.0015-0.0025% of B and the balance of Fe and inevitable impurities. Low-carbon alloying component design is adopted, the 1 / 2 thickness yield strength of the produced steel plate is larger than or equal to 960 MPa, the tensile strength is larger than or equal to 1000 MPa, the ductility is larger than or equal to 16%, the yield ratio is smaller than or equal to 0.9, the 1 / 2 thickness transverse and longitudinal impact absorption energy at minus 40 DEG C is larger than or equal to 100 J, and production of the 960 MPa-level extra-thick rack steel is achieved through the low-temperature quenching, high-temperature quenching and tempering processes.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and relates to a method for producing a low-alloy high-strength steel plate with a strength level of 960MPa, high strength in the core and high toughness at -40°C. Background Art

[0002] With the development of wind power and marine engineering, the demand for rack steel is increasing, and the thickness and strength requirements of steel are getting higher and higher. However, rack steel has the characteristics of high nickel and molybdenum. As the thickness increases, the difficulty of flame cutting and welding gradually increases. At present, the maximum thickness of 690MPa grade rack steel has reached 254mm. Considering that extra-thick rack steel has strict requirements on flame cutting and welding processes, the production and processing of thicker rack steel is more difficult and no longer has practical performance.

[0003] With further demand, rack steel with higher strength levels has become the object of competitive research and development by various steel mills. Unlike the 960MPa grade steel plates used in ordinary engineering machinery, rack steel often requires thicker steel plates and better weldability. In addition, due to its special use scenario, the strength and toughness of the core of the steel plate are extremely high. Therefore, a special production process is required to ensure this.

[0004] Chinese patent CN201910537633.9 discloses "a thick layered tearing resistant yield strength 960MPa grade high-strength steel plate and its production method", which uses a quenching and tempering process to produce 50-100mm 960MPa grade high-strength steel. It produces 100mm thick quenched and tempered ultra-high strength plates based on the continuous casting billet production process. Due to the insufficient dissolution of the solute elements in the first quenching, the impact toughness value of the steel plate is low, and it does not have the production capacity of rack steel of this strength level.

[0005] Chinese patent CN202311262274.3 "A 960MPa-grade structural steel for bridges with excellent low internal stress fracture toughness and its preparation method" uses sub-temperature quenching and annealing processes to produce ultra-low internal stress 960MPa-grade high-strength steel. However, due to the huge difference in cooling rate within the thickness of the extra-thick steel plate, sub-temperature quenching easily leads to huge differences in the structure within the thickness of the steel plate, so it is not suitable for extra-thick steel plates.

[0006] Chinese patent CN202111253774.1 "A 960MPa-grade low-yield ratio marine engineering steel plate and its preparation method" uses a two-phase annealing + tempering process to produce 100mm 960MPa-grade low-yield ratio ultra-high strength steel, which has the characteristics of low yield ratio and high toughness. However, the micro-component segregation produced during the annealing process causes the steel plate to have a relatively complex organizational structure and poor micro-uniformity, which is not conducive to the welding of the steel plate. Summary of the invention

[0007] The purpose of the present invention is to provide a production method of a 960MPa grade extra-thick rack steel plate, by which various impurities and inclusions in molten steel are reduced through pure steel smelting, and the solid solution of solute elements and uniformity of structure of the steel plate are improved through double quenching. The yield strength of 1 / 2 thickness of the steel plate is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is ≥16%, the yield strength ratio is ≤0.9, and the impact absorption energy of 1 / 2 thickness in the transverse and longitudinal directions at -40°C is ≥100J.

[0008] The present invention is achieved through the following technical solutions: A method for producing a 960MPa grade extra-thick rack steel plate, the production process route is smelting-die casting-rolling-finishing-low temperature quenching-high temperature quenching-tempering, and the thickness of the produced steel plate is 150-180mm. The chemical composition mass percentage of the steel is C=0.14-0.18, Si=0.15-0.40, Mn=1.0-1.2, P≤0.008, S≤0.002, Cr=0.8-1.8, V=0.05-0.08, Ni=1.6-3.0, Mo=0.8-1.0, Al=0.07-0.012, B=0.0015-0.0025, and the remainder is Fe and unavoidable impurities; the method comprises the following process steps: (1) Smelting: The BOF-LF-VD production process is adopted. The P content of the converter is controlled at ≤0.01%. After steelmaking, the secondary slag forming method is used to reduce the P content of the molten steel to below 0.006%. The refining LF furnace adopts a deep desulfurization process to control the sulfur content of the finished product S ≤0.002%. The VD furnace production adopts a deep degassing process, the total vacuum treatment time is ≥15min, and the nitrogen content of the molten steel N is controlled to be ≤0.004%. 300~500kg of silicon calcium barium composite deoxidizer is added in the later stage of vacuum treatment. The soft blowing of VD is ≥20 minutes after breaking the air. (2) Mold casting: The superheat is controlled at 40-45°C. During casting, a full-cover protective cover is used to cover the center pouring pipe to prevent air inhalation during casting. The N content in the finished steel is controlled to be ≤0.006%. Then, an integral insulation cover is used to insulate the entire ingot mold. (3) Rolling: Two-stage rolling is adopted. The initial rolling adopts a pit-type heating furnace with a heating temperature of 1230~1250℃ and a billet thickness of 400mm. The secondary heating adopts a roller-bottom heating furnace with a heating temperature ≥1190℃. The rolling is carried out in two stages. The first stage adopts a low-speed and high-pressure rolling mode, and the second stage adopts high-temperature rolling to ensure the shape of the steel plate. After rolling, the steel plate is air-cooled to room temperature. (4) Quenching and tempering: The rolled steel plate is subjected to secondary quenching and tempering heat treatment in a roller hearth furnace. The primary quenching temperature is 890-900°C, the secondary quenching temperature is 930-940°C, and the holding time is 150-180 minutes. The steel plate is quenched in a roller quenching machine, the tempering temperature is 600-630°C, and the holding time is 200-300 minutes. After being taken out of the tempering furnace, the steel plate is cut to length at about 200°C, and the surface and the quality of the cut surface are inspected before storage.

[0009] The invention is suitable for the production of 150-180 mm 960 MPa grade rack steel plates, and the produced steel has good core strength and toughness and excellent welding performance.

[0010] The reasons for limiting the main chemical components in the steel of the present invention are as follows: The composition design of the steel of the present invention is mainly designed by ensuring the hardenability of the steel plate, while ensuring that the steel plate has a certain weldability.

[0011] The C element is the guarantee of steel strength. However, too high a C content affects the weldability of the steel plate. Taking into account the strength requirements of the steel plate, the C content of this design is selected to be relatively low at 0.14-0.18, which not only guarantees the strength of the steel plate, but also ensures a certain weldability.

[0012] Si is generally used as a reducing agent and deoxidizer in the steelmaking process. Silicon can significantly improve the fatigue strength and fatigue ratio of 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. However, it is generally believed that the Si content below 0.40% has little effect on impact toughness; considering comprehensively, the Si content of the steel of the present invention is controlled within the range of 0.15% to 0.40%.

[0013] Mn is an important element of strength and toughness in steel, but it is easy to aggregate and segregate when the molten steel solidifies, and it is easy to produce banded structure in the rolled steel plate. In order to avoid excessive segregation of the Mn element, the Mn content of the steel of the present invention is designed to be 1.00-1.20%.

[0014] P is one of the harmful elements in steel. When the phosphorus content increases, the strength of the steel increases, the plasticity and toughness decrease, and the processing performance decreases. For extra-thick plates, the P content tends to concentrate in the center and affect the impact toughness of the steel plate. Therefore, the P content of the steel of the present invention is designed to be P≤0.008%.

[0015] S is an impurity element in steel. S is easy to segregate and enrich in steel, forming long strips of MnS inclusions in steel, reducing the impact toughness of steel plates, and S is easy to make steel hot brittle. The sulfur content level of the steel of the present invention is strictly controlled, that is, S≤0.002%.

[0016] Cr is an effective element for improving the strength of steel plates. At the same time, Cr slows down the decomposition rate of austenite and significantly improves the hardenability of steel. The effect is more significant when added in combination with Mo and Ni. However, when the Cr content is high, large particles of alloy carbide inclusions are easily generated, which has a greater impact on impact toughness and welding performance. Taking all factors into consideration, the Cr content of the present invention is controlled to be 0.8% to 1.8%.

[0017] The Ni element can improve the hardenability of ultra-high strength steel and is beneficial to improving the low-temperature toughness of the steel plate. However, too high Ni content has a certain influence on the cutting and welding performance of the steel plate. Taking comprehensive considerations, the Ni content of the steel of the present invention is controlled within the range of 1.6 to 3.00%.

[0018] Mo plays a very important role in steel. It can improve the strength and hardness of steel, improve the ductility and toughness of steel, and improve the hardenability of steel, which is slightly inferior to Cr. Mo can improve the creep ability of ferrite, effectively inhibit the aggregation of cementite, and improve thermal stability. However, too much Mo content can easily lead to the formation of coarse martensite in the steel plate, affecting the toughness of the steel plate. The Mo content of the steel of the present invention is controlled at: 0.80% to 1.00%.

[0019] Ti is a strong carbide and nitride forming element, which can fix the N element in the molten steel and refine the grain size of the steel plate. However, in die casting production, excessive enrichment of the Ti element will lead to a large number of large-particle TiN inclusions in the core of the steel plate, resulting in reduced plastic toughness in the core of the steel plate.

[0020] V is a strong carbide and nitride forming element, which can improve the hardenability of the steel plate, and can precipitate during the tempering process to improve the strength of the steel plate. However, too high a V content will lead to excessive V solid solution, reducing the brittleness of the steel plate. In the present invention, the V content is controlled to 0.05% to 0.08%; Al deoxidation significantly reduces the oxygen content in the steel. At the same time, it can refine the grains, fix the N element, and improve the low-temperature impact toughness of the base material and the welding heat-affected zone. In order to ensure that the B element in the molten steel can play a role, the Als content of the steel of the present invention is controlled to be: 0.07% to 0.12%.

[0021] The B element can be enriched at the austenite grain boundary, slowing down the generation of bainite, thereby significantly improving the hardenability of the steel plate. However, the solid solution B element in the steel plate can improve the toughness of the steel plate when it is above 0.0007%, and can no longer continue to improve the toughness of the steel plate when it is above 0.002%. The B content of the present invention is controlled to be 0.0015% to 0.002%.

[0022] Reasons for setting the steel production process of the present invention: By using silicon-calcium-barium composite deoxidation in the vacuum process and soft blowing time ≥ 20 minutes after vacuum, the influence of inclusions in molten steel on performance can be reduced; theory and practice have proved that inclusions larger than 20μm in molten steel will affect the impact toughness of steel plates. In order to reduce large particle inclusions in molten steel, composite deoxidation can quickly generate low-melting point composite inclusions, which has better modification effect than calcium feeding wire. At the same time, in order to fully float large particle inclusions, 20 minutes of soft blowing time is conducive to the floating of inclusions and fusion with slag for removal; The vacuum treatment is maintained for ≥15 minutes. During die casting, a closed sleeve is used to cover the pouring nozzle to reduce the amount of air inhaled during the casting process, and the N content of the finished product is controlled to be ≤0.006%. The impact of N content on extra-thick ultra-high strength steel plates is mainly that free N will affect the impact toughness and welding toughness of the steel plates. Under the condition that Ti cannot be used to fix N, excessive Al content and excessive N content will cause the AlN particles in the steel plate to be too large, affecting the toughness of the steel plate. Reducing the N content can effectively avoid the generation of large AlN particles.

[0023] In the two-time quenching process, the first quenching can achieve complete austenitization and homogenization of the steel plate. However, due to the slow cooling rate in the core of the extra-thick steel plate, the core is prone to produce incompletely hardened structures. During the second quenching, due to the genetic effect of the structure, the steel plate can be quickly austenitized and further homogenized on the basis of the first quenching. The secondary heating can promote the moderate growth of the austenite grain size and the improvement of the hardenability of the steel plate, thereby ensuring that the core of the steel plate is hardened and has higher strength and toughness. Compared with the long-term high-temperature single heating, it is not easy to produce mixed crystals and overheated structures, and the structure uniformity is better.

[0024] The outstanding features of the present invention are reflected in: reducing the residual elements in the steel plate through a pure steel smelting method to avoid affecting the impact toughness of the steel plate, promoting the full solid solution and homogenization of the alloy elements in the steel plate through a double quenching process, improving the hardenability of the steel plate, ensuring the core hardening, and producing 960MPa grade rack steel that can ensure the core strength and impact toughness of the steel plate, the yield strength of 1 / 2 thickness of the steel plate is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is ≥16%, the yield strength ratio is ≤0.9, and the 1 / 2 thickness transverse and longitudinal -40°C transverse and longitudinal impact absorption energy is ≥100J. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the metallographic structure diagram of the 180mm960MPa rack steel plate produced in Example 4. DETAILED DESCRIPTION

[0026] The following is a further description of the main contents of the present invention such as the component control range and the best mode of implementation in conjunction with the embodiments: Example 1

[0027] The P content of molten steel is controlled at 0.009% in converter smelting. After steelmaking, the secondary slag forming method is used to reduce the P content of molten steel to 0.004%. The refining LF furnace adopts a deep desulfurization process, and the sulfur content of the finished product is ≤0.0015%; the VD furnace production adopts a deep degassing process, and the total vacuum treatment time is 18 minutes; after VD is broken, calcium treatment and soft blowing are carried out for 25 minutes; the mold casting superheat is controlled at 42-44°C, and then an integral insulation cover is used to keep the ingot mold warm for 72 hours; the rolling adopts two-fire billet rolling, and the finished product thickness is rolled to 152mm and then air-cooled to room temperature; the rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 895℃, the holding time is 160 minutes, the second quenching temperature is 935℃, the holding time is 154 minutes, both times use roller quenching machine to quench for 40 minutes and cool to room temperature, the tempering temperature is 630℃, the holding time is 210 minutes, and the steel is cut to size at 200℃ after being taken out of the tempering furnace, and the surface and fire-cut surface quality are checked before storage.

[0028] Embodiment 2: The P content of molten steel is controlled at 0.010% in converter smelting. After steelmaking, the secondary slag forming method is used to reduce the P content of molten steel to 0.005%. The refining LF furnace adopts a deep desulfurization process, and the sulfur content of the finished product is ≤0.0013%; the VD furnace production adopts a deep degassing process, and the total vacuum treatment time is 18 minutes; after VD is broken, calcium treatment and soft blowing are carried out for 26 minutes; the mold casting superheat is controlled at 42-45°C, and then an integral insulation cover is used to keep the ingot mold warm for 72 hours; the rolling adopts two-fire billet rolling, and the finished product thickness is rolled to 160mm and then air-cooled to room temperature; the rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 896℃, the holding time is 162 minutes, the second quenching temperature is 937℃, the holding time is 164 minutes, both times use roller quenching machine to quench for 45 minutes and cool to room temperature, the tempering temperature is 620℃, the holding time is 240 minutes, and the steel is cut to size at 220℃ after being taken out of the tempering furnace, and the surface and fire-cut surface quality are checked before storage.

[0029] Smelting Example 3: The P content of the molten steel is controlled at 0.008% in the converter smelting. After steelmaking, the secondary slag forming method is used to reduce the P content of the molten steel to 0.006%. The refining LF furnace adopts a deep desulfurization process, and the sulfur content of the finished product is ≤0.0010%; the VD furnace production adopts a deep degassing process, and the total vacuum treatment time is 19 minutes; after the VD is broken, calcium treatment and soft blowing are carried out for 29 minutes; the mold casting superheat is controlled at 43~44℃, and then an integral insulation cover is used to keep the ingot mold warm for 72 hours; the rolling adopts two-fire billet rolling, and the finished product thickness is rolled to 170mm and then air-cooled to room temperature; the rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 896℃, the holding time is 173 minutes, the second quenching temperature is 938℃, the holding time is 176 minutes, both times use roller quenching machine to quench for 40 minutes and cool to room temperature, the tempering temperature is 610℃, the holding time is 270 minutes, and the steel is cut to size at 230℃ after being taken out of the tempering furnace, and the surface and fire-cut surface quality are checked before storage.

[0030] Embodiment 4: The P content of the molten steel is controlled at 0.009% in converter smelting. After steelmaking, the secondary slag forming method is used to reduce the P content of the molten steel to 0.003%. The refining LF furnace adopts a deep desulfurization process, and the sulfur content of the finished product is ≤0.0011%; the VD furnace production adopts a deep degassing process, and the total vacuum treatment time is 18 minutes; after VD is broken, calcium treatment and soft blowing are carried out for 26 minutes; the mold casting superheat is controlled at 42-43°C, and then an integral insulation cover is used to keep the ingot mold warm for 72 hours; the rolling adopts two-fire billet rolling, and the finished product thickness is rolled to 180mm and then air-cooled to room temperature; the rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 899℃, the holding time is 179 minutes, the second quenching temperature is 940℃, the holding time is 180 minutes, both times use roller quenching machine to quench for 40 minutes and cool to room temperature, the tempering temperature is 600℃, the holding time is 300 minutes, and the steel is cut to size at 230℃ after being taken out of the tempering furnace, and the surface and fire-cut surface quality are checked before storage.

[0031] The chemical composition control of each smelting example is shown in Table 1, and the detection performance of the steel is shown in Table 2.

[0032] Table 1 Chemical composition of the steel used in the example (wt%) .

[0033] Table 2 Various test properties of example steel .

[0034] As can be seen from Table 2, the present invention can produce 150-180 mm thick 960 MPa grade rack steel, with stable performance of steel plates of various thicknesses, dense core, and steel plates that fully utilize two quenchings, significantly improved toughness, uniform structure, and convenient for welding of steel plates.

[0035] from Figure 1 It can be seen that after quenching and tempering, the structure of the steel plate is a tempered rope solid + a small amount of bainite structure, the structure is fine and uniform, and meets the requirements of rack steel for core strength and toughness.

Claims

1. A method for producing 960MPa grade extra-thick rack steel plate, the production process route is smelting - die casting - rolling - finishing - low temperature quenching - high temperature quenching - tempering, characterized in that: The thickness of the steel plate is 150~180mm, and the chemical composition of the steel is C=0.14~0.18, Si=0.15~0.40, Mn=1.0~1.2, P≤0.008, S≤0.002, Cr=0.8~1.8, V=0.05~0.08, Ni=1.6~3.0, Mo=0.8~1.0, Al=0.07~0.012, B=0.0015~0.0025, and the balance is Fe and unavoidable impurities; The yield strength of the steel plate 1 / 2 thickness is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is ≥16%, the yield strength ratio is ≤0.9, and the impact absorption energy of the 1 / 2 thickness in the transverse and longitudinal directions at -40℃ is ≥100J, including the following key process steps: (1) Smelting: The BOF-LF-VD production process is adopted. The P content of the converter is controlled at ≤0.01%. After steelmaking, the secondary slag forming method is used to reduce the P content of the molten steel to below 0.006%. The refining LF furnace adopts a deep desulfurization process to control the sulfur content of the finished product S ≤0.002%. The VD furnace production adopts a deep degassing process, the total vacuum treatment time is ≥15min, and the nitrogen content of the molten steel N is controlled to be ≤0.004%. 300~500kg of silicon calcium barium composite deoxidizer is added in the later stage of vacuum treatment. The soft blowing of VD is ≥20 minutes after breaking the air. (2) Mold casting: The superheat is controlled at 40-45°C. During casting, a full-cover protective cover is used to cover the center pouring pipe to prevent air inhalation during casting. The N content in the finished steel is controlled to be ≤0.006%. Then, an integral insulation cover is used to insulate the entire ingot mold. (3) Rolling: Two-stage rolling is adopted. The initial rolling adopts a pit-type heating furnace with a heating temperature of 1230~1250℃ and a billet thickness of 400mm. The secondary heating adopts a roller-bottom heating furnace with a heating temperature ≥1190℃. The rolling is carried out in two stages. The first stage adopts a low-speed and high-pressure rolling mode, and the second stage adopts high-temperature rolling to ensure the shape of the steel plate. After rolling, the steel plate is air-cooled to room temperature. (4) Quenching and tempering: The rolled steel plate is subjected to secondary quenching and tempering heat treatment in a roller hearth furnace. The primary quenching temperature is 890~900℃, the secondary quenching temperature is 930~940℃, and the holding time is 150~180 minutes. The steel plate is quenched by a roller quenching machine, the tempering temperature is 600~630℃, and the holding time is 200~300 minutes. After being taken out of the tempering furnace, it is cut to length at about 200℃, and the surface and fire-cut surface quality are inspected before storage.

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