A production method of 960mpa grade super-thick rack steel plate

Abstract

A method for producing 960MPa grade extra-thick rack steel plates, the production process route is smelting—in-mold casting—rolling—rapid cooling—finishing—quenching—tempering, producing steel plates with a thickness of 150~180mm, and the chemical composition of the steel by mass percentage 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, balance is Fe and unavoidable impurities; the present invention adopts a low-carbon alloying composition design, and the produced steel plate has a yield strength ≥960MPa, tensile strength ≥1000MPa, elongation ≥16%, yield strength ratio ≤0.9, and the transverse and longitudinal impact absorption energy ≥100J at -40℃ for 1 / 2 thickness. The production of 960MPa level extra-thick rack steel is achieved through low temperature quenching + high temperature quenching + tempering process.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a method for producing a low-alloy high-strength steel plate with a strength level of 960MPa, high core strength, and high toughness at -40℃. Background Technology

[0002] With the development of wind power and marine engineering, the demand for rack steel is increasing, and the requirements for steel thickness and strength are becoming more stringent. However, rack steel has high nickel and molybdenum content, and as the thickness increases, the difficulty of fire cutting and welding also gradually increases. Currently, the maximum thickness of 690MPa grade rack steel used has reached 254mm. Considering the stringent requirements for fire cutting and welding processes for extra-thick rack steel, even thicker rack steel presents greater production and processing difficulties, rendering it impractical.

[0003] Driven by further demand, higher-strength rack steel has become the focus of research and development for various steel mills. Unlike the 960MPa grade steel plates used in ordinary construction machinery, rack steel often requires thicker plates with better weldability, and due to its special application scenarios, it demands extremely high strength and toughness in the core of the steel plate. Therefore, special production processes are required to ensure its quality.

[0004] Chinese patent CN201910537633.9 discloses "A thick high-strength steel plate with a yield strength of 960MPa against lamellar tearing and its production method," which uses a quenching and tempering process to produce 50-100mm thick 960MPa grade high-strength steel. However, its production of a 100mm thick quenched and tempered ultra-high-strength plate, based on a continuous casting billet production process, suffers from insufficient dissolution of solute elements during the first quenching process, resulting in a lower impact toughness value for the steel plate and making it unsuitable for producing rack steel of this strength level.

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

[0006] Chinese patent CN202111253774.1, "A low yield strength ratio marine steel plate with a yield strength of 960MPa and its preparation method," describes the production of 100mm thick 960MPa grade low yield strength ratio ultra-high strength steel using a two-phase annealing + quenching and tempering process. This steel has the characteristics of low yield strength ratio and high toughness. However, the micro-component segregation generated during the annealing process leads to a relatively complex microstructure and poor micro-uniformity in the steel plate, which is not conducive to welding. Summary of the Invention

[0007] The purpose of this invention is to provide a production method for 960MPa grade extra-thick rack steel plates. This method reduces various impurities and inclusions in molten steel through pure steel smelting, and improves the solid solution of solute elements and the uniformity of the microstructure through two quenching processes. The steel plate has a yield strength ≥960MPa, tensile strength ≥1000MPa, elongation ≥16%, yield strength ratio ≤0.9, and transverse and longitudinal impact absorption energy ≥100J at -40℃ for half the thickness.

[0008] This invention is achieved through the following technical solution:

[0009] A method for producing 960MPa grade extra-thick rack steel plates, the production process route is smelting—in-mold casting—rolling—finishing—low-temperature quenching—high-temperature quenching—tempering, producing steel plates with a thickness of 150~180mm. The chemical composition of the steel (mass percentage) 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, with the balance being Fe and unavoidable impurities; including the following process steps:

[0010] (1) Smelting: The BOF-LF-VD production process is adopted. The converter smelting controls P≤0.01%. After tapping, the secondary slag formation method is used to reduce the P content of the molten steel to below 0.006%. The refining LF furnace adopts the deep desulfurization process to control the sulfur content of the finished product S≤0.002%. The VD furnace production adopts the deep degassing process. The total vacuum treatment time is ≥15min, and the nitrogen content of the molten steel N≤0.004%. 300~500kg of silicon-calcium-barium composite deoxidizer is added in the later stage of vacuum treatment. After the VD furnace breaks the vacuum, soft blowing is carried out for ≥20 minutes.

[0011] (2) Ingot casting: The superheat is controlled at 40~45℃. During casting, a full-coverage protective cover is used to cover the middle injection pipe to prevent air intake during the casting process. The N in the finished steel is controlled to be ≤0.006%. Then, an overall heat preservation cover is used to keep the steel ingot mold warm.

[0012] (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 hearth heating furnace with a heating temperature of ≥1190℃. Two-stage rolling is adopted. The first stage adopts a low-speed, high-reduction rolling mode, and the second stage adopts high-temperature rolling to ensure the shape of the steel plate. After rolling, the plate is air-cooled to room temperature.

[0013] (4) Quenching and tempering: After rolling, the 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 using a roller quenching machine. The tempering temperature is 600-630℃ and the holding time is 200~300 minutes. After the steel plate is taken out of the tempering furnace, it is fire-cut to length at about 200℃. After checking the surface and fire-cut surface quality, it is put into storage.

[0014] This invention is applicable to the production of 150-180mm 960MPa grade rack steel plates, and the produced steel has good core strength and toughness and excellent weldability.

[0015] The reasons for limiting the main chemical composition of the steel in this invention are as follows: The steel composition design of this invention is mainly to ensure the hardenability of the steel plate, while ensuring that the steel plate has a certain degree of weldability.

[0016] Carbon (C) is essential for the strength of steel. However, excessively high C content can negatively impact the weldability of the steel plate. Considering the strength requirements of the steel plate, this design selects a relatively low C content of 0.14–0.18%, ensuring both the strength of the steel plate and a certain level of weldability.

[0017] In steelmaking, silicon (Si) is generally used as a reducing agent and deoxidizer. Silicon can significantly improve the fatigue strength and fatigue ratio of steel; however, when the Si content increases, it promotes the formation of island martensite, which is detrimental to the toughness of the weld heat-affected zone. It is generally believed that Si content below 0.40% has little effect on impact toughness. Taking all factors into consideration, the Si content of the steel in this invention is controlled within the range of 0.15% to 0.40%.

[0018] Mn is an important element for strength and toughness in steel, but it tends to agglomerate and segregate during the solidification of molten steel, and easily forms banded structures in rolled steel plates. To avoid excessive Mn segregation, the Mn content of the steel in this invention is designed to be 1.00–1.20%.

[0019] Phosphorus (P) is one of the harmful elements in steel. Increased phosphorus content increases the strength of steel but decreases its plasticity and toughness, reducing its processing performance. For extra-thick plates, P content tends to accumulate in the core, affecting the impact toughness of the steel. Therefore, the P content of the steel in this invention is designed to be P≤0.008%.

[0020] Sulfur (S) is an impurity element in steel. S readily segregates and accumulates in steel, forming elongated MnS inclusions that reduce the impact toughness of the steel plate and cause hot brittleness. The steel of this invention strictly controls the sulfur content level, i.e., S ≤ 0.002%.

[0021] Cr is an effective element for improving the strength of steel plates. At the same time, Cr slows down the decomposition rate of austenite, significantly improving the hardenability of steel. The effect is even more pronounced when added in combination with Mo and Ni. However, high Cr content easily leads to the formation of large alloy carbide inclusions, which significantly affect impact toughness and weldability. Taking all factors into consideration, the Cr content in this invention is controlled at 0.8%–1.8%.

[0022] Ni can improve the hardenability of ultra-high strength steel and is beneficial to the improvement of the low-temperature toughness of steel plates. However, excessively high Ni content has a certain impact on the cutting and welding performance of steel plates. Taking all factors into consideration, the Ni content of the steel in this invention is controlled at 1.6 to 3.00%.

[0023] Mo plays a crucial role in steel. It can improve the strength and hardness of steel, enhance its ductility and toughness, and improve its hardenability, though slightly less than chromium. Mo can improve the creep resistance of ferrite, effectively inhibit cementite aggregation, and improve thermal stability. However, excessive Mo content can lead to the formation of coarse martensite in the steel plate, affecting its toughness. The Mo content of the steel in this invention is controlled at 0.80% to 1.00%.

[0024] Ti is a strong carbide and nitride forming element, which can fix the N element in molten steel and refine the grain size of steel plate. However, in die casting production, excessive enrichment of Ti element will lead to a large number of large TiN particles in the core of the steel plate, resulting in a decrease in the plasticity and toughness of the steel plate core.

[0025] V is a strong carbide and nitride forming element, which can improve the hardenability of steel plates and precipitate during tempering, thereby increasing the strength of the steel plates. However, excessive V content will lead to excessive V solid solution, reducing the brittleness of the steel plates. In this invention, the V content is controlled at 0.05% to 0.08%.

[0026] Al deoxidation significantly reduces the oxygen content in steel. It also refines grains, fixes nitrogen (N) element, and improves the low-temperature impact toughness of the base metal and the weld heat-affected zone. To ensure that boron (B) in the molten steel can play its role, the ALs (alcohol content) of the steel in this invention are controlled at 0.07%–0.12%.

[0027] Boron can accumulate at austenite grain boundaries, slowing down the formation of bainite and thus significantly improving the hardenability of steel plates. However, the solid solution of boron in steel plates can improve the toughness of steel plates when it is above 0.0007%, but it can no longer improve the toughness of steel plates when it is above 0.002%. The boron content in this invention is controlled at 0.0015% to 0.002%.

[0028] The rationale for setting up the steel production process in this invention:

[0029] By employing a silicon-calcium-barium composite deoxidation process during vacuum deoxidation, coupled with a soft blowing time of ≥20 minutes after vacuum deoxidation, the impact of inclusions in molten steel on performance is reduced. Theory and practice have proven that inclusions larger than 20μm in molten steel affect the impact toughness of steel plates. To reduce large particle inclusions in molten steel, composite deoxidation can rapidly generate low-melting-point composite inclusions, showing better modification effects compared to calcium wire feeding. Simultaneously, the 20-minute soft blowing time facilitates the full flotation of large particle inclusions, allowing them to fuse with the steel slag and be removed.

[0030] Vacuum treatment holding time ≥15 minutes; during ingot casting, a closed sleeve is used to cover the sprue to reduce air intake during casting and control the N content of the finished product to ≤0.006%. The main impact of N content on ultra-thick, high-strength steel plates is that free N affects the impact toughness and weld toughness of the steel plate. When Ti cannot be used to fix N, excessively high Al and N contents will lead to excessively large AlN particles in the steel plate, affecting its toughness. Reducing the N content can effectively avoid the formation of large AlN particles.

[0031] In a two-stage quenching process, the first quenching achieves complete austenitization and homogenization of the steel plate. However, for extra-thick steel plates, the core cooling rate is too slow, leading to incompletely hardened microstructures in the core. During the second quenching, due to the hereditary effect on microstructure, the steel plate can rapidly austenitize and further homogenize based on the first quenching. The secondary heating promotes appropriate austenite grain growth, improves the hardenability of the steel plate, and ensures core hardening, resulting in higher strength and toughness. Compared to a single heating process with prolonged high temperature, it is less prone to mixed grains and overheating, and the microstructure is more uniform.

[0032] The key features of this invention are: reducing 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 solidification and homogenization of alloying elements in the steel plate through a two-stage quenching process, improving the hardenability of the steel plate, ensuring core hardening, and producing 960MPa grade rack steel that can guarantee the core strength and impact toughness of the steel plate. The yield strength of the steel plate at 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 in the transverse and longitudinal directions at -40℃ at 1 / 2 thickness is ≥100J. Attached Figure Description

[0033] Figure 1 The metallographic structure of the 180mm 960MPa rack steel plate produced in Example 4 is shown in the figure. Detailed Implementation

[0034] The following examples further illustrate the main aspects of the present invention, including the range of component control and the best implementation method: Example 1

[0035] The converter smelting process controls P=0.009%. After tapping, a secondary slag-forming method is used to reduce the P content of the 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, with a total vacuum treatment time of 18 minutes. After VD degassing, calcium treatment and soft blowing are performed for 25 minutes. The superheat of the ingot casting is controlled at 42-44℃, and then the ingot mold is kept warm for 72 hours using an overall insulation cover. The rolling process adopts two-fire billet rolling, and after rolling to a finished thickness of 152mm, it is air-cooled to room temperature. The rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 895℃, and the holding time is 160 minutes. The second quenching temperature is 935℃, and the holding time is 154 minutes. Both quenchings are performed using a roller quencher for 40 minutes and then cooled to room temperature. The tempering temperature is 630℃, and the holding time is 210 minutes. After exiting the tempering furnace, the material is fire-cut to length at 200℃. The surface and fire-cut surface quality are then checked before the material is put into storage.

[0036] Example 2:

[0037] The converter smelting process controls P=0.010%. After tapping, a secondary slag-forming method is used to reduce the P content of the 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, with a total vacuum treatment time of 18 minutes. After VD degassing, calcium treatment and soft blowing are performed for 26 minutes. The superheat of the ingot casting is controlled at 42-45℃, and then the ingot mold is kept warm for 72 hours using an overall heat preservation cover. The rolling process adopts two-fire billet rolling, and after rolling to a finished thickness of 160mm, it is 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 quenchings are done with a roller quencher for 45 minutes and then cooled to room temperature. The tempering temperature is 620℃, the holding time is 240 minutes, and after exiting the tempering furnace, the material is fire-cut to length at 220℃. The surface and fire-cut surface quality are then checked before the material is put into storage.

[0038] Smelting Example 3:

[0039] The converter smelting process controls the P content to 0.008%. After tapping, a 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, with a total vacuum treatment time of 19 minutes. After VD degassing, calcium treatment and soft blowing are performed for 29 minutes. The superheat of the ingot casting is controlled at 43~44℃, and then the ingot mold is kept warm for 72 hours using an overall insulation cover. The rolling process adopts two-fire billet rolling, and after rolling to a finished thickness of 170mm, it is 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 quenchings are done with a roller quencher for 40 minutes and then cooled to room temperature. The tempering temperature is 610℃, the holding time is 270 minutes, and after exiting the tempering furnace, the material is fire-cut to length at 230℃. The surface and fire-cut surface quality are then checked before the material is put into storage.

[0040] Example 4:

[0041] The converter smelting process controls the P content to 0.009%. After tapping, a 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, with a total vacuum treatment time of 18 minutes. After VD degassing, calcium treatment and soft blowing are performed for 26 minutes. The superheat of the ingot casting is controlled at 42-43℃, and then the ingot mold is kept warm for 72 hours using an overall insulation cover. The rolling process adopts a two-fire billet rolling, and after rolling to a finished thickness of 180mm, it is air-cooled to room temperature. The rolled steel plate is quenched and tempered in a roller hearth furnace. The first quenching temperature is 899℃, and the holding time is 179 minutes. The second quenching temperature is 940℃, and the holding time is 180 minutes. Both quenchings are performed using a roller quencher for 40 minutes and then cooled to room temperature. The tempering temperature is 600℃, and the holding time is 300 minutes. After exiting the tempering furnace, the material is fire-cut to length at 230℃. The surface and fire-cut surface quality are then checked before the material is put into storage.

[0042] The chemical composition control for each smelting example is shown in Table 1, and the test performance of the steel is shown in Table 2.

[0043] Table 1 Chemical composition (wt%) of the controlled steel in the examples

[0044] .

[0045] Table 2. Test properties of the steel in the examples

[0046] .

[0047] As can be seen from Table 2, the present invention can produce 960MPa grade rack steel with a thickness of 150-180mm. The steel plates of each thickness have stable performance, dense core, and make full use of two quenching processes, which significantly improves toughness and uniform structure, and is also conducive to welding of the steel plates.

[0048] from Figure 1 It can be seen that after the steel plate is quenched and tempered, the microstructure is a tempered solid structure with a small amount of bainite. The microstructure is fine and uniform, which meets the requirements of rack steel for core strength and toughness.

Claims

1. A production method of 960MPa grade super-thick rack steel plate, the production process route is smelting-mold casting-rolling-finishing-low temperature quenching-high temperature quenching-tempering, characterized in that: The steel plates produced have a thickness of 150~180mm, and the chemical composition of the steel by mass percentage 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, with the balance being Fe and unavoidable impurities; The steel plate, with a 1 / 2 thickness, has a yield strength ≥960MPa, tensile strength ≥1000MPa, elongation ≥16%, yield strength ratio ≤0.9, and a transverse and longitudinal impact absorption energy ≥100J at -40℃. This includes the following key process steps: (1) Smelting: The BOF-LF-VD production process is adopted. The P content in the converter smelting is controlled to be less than 0.01%. After tapping, the P content in the molten steel is reduced to less than 0.006% by the secondary slag forming method. The deep desulfurization process is adopted in the refining LF furnace to control the sulfur content of the finished product to be less than 0.002%. The deep degassing process is adopted in the VD furnace production. The total vacuum treatment time is ≥15min, and the nitrogen content of the molten steel is controlled to be less than 0.004%. 300~500kg of silicon-calcium-barium composite deoxidizer is added in the later stage of vacuum treatment. After the VD furnace breaks the vacuum, soft blowing is carried out for ≥20 minutes. (2) Ingot casting: The superheat is controlled at 40~45℃. During casting, a full-coverage protective sleeve is used to cover the middle injection pipe to prevent air from being drawn in during the casting process. The N in the finished steel is controlled to be ≤0.006%. Then, an overall heat preservation cover is used to keep the steel ingot mold warm. (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 hearth heating furnace with a heating temperature of ≥1190℃. Two-stage rolling is adopted. The first stage adopts a low-speed, high-reduction rolling mode. The second stage adopts high-temperature rolling to ensure the shape of the steel plate. After rolling, the plate is air-cooled to room temperature. (4) Quenching and tempering: After rolling, the 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 using a roller quenching machine. The tempering temperature is 600~630℃, and the holding time is 200~300 minutes. After the steel plate is taken out of the tempering furnace, it is fire-cut to length at 200℃. After checking the surface and fire-cut surface quality, it is put into storage.

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