Heat treatment method for high-strength hydroelectric steel 980CF with thickness of 120mm

Abstract

The invention provides a heat treatment method for high-strength hydroelectric steel 980CF with the thickness of 120 mm. The heat treatment method comprises two times of quenching and step tempering. The steel plate has good hardenability and high strength and toughness through two times of high-pressure quenching, so that the hardenability is reduced, the adding amount of alloy elements is increased, the carbon equivalent of the steel plate is also properly reduced, and the alloy cost of the steel plate is reduced to a certain extent; and the problems of poor surface impact performance and low core tensile strength of the high-strength super-thick plate are solved through stepped tempering. According to the 120mm 980CF steel plate produced by the process, the unevenness is less than or equal to 2mm / m, the yield strength Rp0.2 is greater than or equal to 870MPa, the tensile strength Rm is 920-1120MPa, the percentage elongation after fracture (A) is greater than or equal to 14%, the impact at the full-thickness position at the temperature of 60 DEG C below zero is greater than or equal to 80J, the impact at the 5% surface strain aging at the temperature of 40 DEG C below zero is greater than or equal to 90J, and the plate shape, the high-strength tensile property and the excellent low-temperature impact toughness completely meet the use requirements of large-scale hydroelectric steel.

Description

Technical Field

[0001] The invention belongs to the field of heat treatment of steel plates, and in particular relates to a heat treatment method for 980CF high-strength hydropower steel with a thickness of 120 mm. Background Art

[0002] Based on the demand for high-head large-scale hydropower steel, hydropower steel is also developing in the direction of high strengthening and large-scale. In order to solve the construction difficulties of high HD value pumped storage power stations, it also puts forward new requirements for the wall thickness of 1000MPa hydropower steel. At the same time, hydropower steel has strict requirements on the chemical composition yield strength, tensile strength, low-temperature toughness, weldability and other performance indicators of steel plates. A customer requires the yield strength Rp0.2 of 120mm980CF to be ≥870MPa, the tensile strength Rm to be between 920MPa and 1120MPa, the elongation after fracture (A) to be ≥14%, the impact at -60℃ to be ≥80J, the impact at -40℃ to be ≥60J at 5% surface strain aging, the carbon content to be no more than 0.61%, and the welding crack sensitivity index to be no more than 0.31%.

[0003] Nippon Steel uses direct quenching to provide 950MPa high-strength steel plates with a thickness of less than 50mm. At present, there is no mature method for the production of thick, especially 120mm thick hydropower high-strength steel at home and abroad. How to ensure the performance of thick steel plates in the full thickness direction from the surface to the core is a difficult problem. The existing methods are to control the C content to achieve the carbon equivalent and crack sensitivity index required by the standard, and to add a large amount of alloy elements to change the hardenability and strength of the steel. However, these two methods have little room for optimization in composition for low welding crack sensitive steels. At the same time, if a large amount of alloy elements are added, it is not conducive to industrial production.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] In order to solve the above-mentioned technical defects, the purpose of the present invention is to provide a heat treatment method for 120mm thick high-strength hydropower steel 980CF, which can fully meet the requirements of high strength and toughness and low welding sensitivity index of thick hydropower steel and realize mass production.

[0006] To achieve the above object, the present invention is implemented in the following manner: A heat treatment method of 120mm thick high-strength hydropower steel 980CF, including two quenching and step tempering, is as follows:

[0007] a. First quenching: The steel plate is required to be heated to 900℃~930℃ in a roller hearth heating furnace with a holding time of 1.9~2.3min / mm. After the holding time is completed, the steel plate is sequentially quenched in a roller quenching machine with a high-pressure zone and a low-pressure zone. The roller speed is controlled at 0.6~1.2m / min and the quenching water temperature is 15~20℃. Among them, the ratio of lower water to upper water in the high-pressure zone is 1.1~1.3, and the ratio of lower water to upper water in the low-pressure zone is 1.3~1.5. The cooling rate of the core is controlled at 3~5℃ / S, and quenched to room temperature. It should be noted that the first quenching ensures a large cooling rate to fully penetrate into the core and ensure that a small amount of pearlite + bainite is formed at the 1 / 2 position to maintain a high strength.

[0008] b. Secondary quenching: The steel plate is heated to 860℃~890℃ in a roller hearth heating furnace, and the holding time is 2.0~2.5min / mm. After the holding time is over, the steel plate enters the roller quenching machine with high-pressure zone and low-pressure zone for quenching in turn, and the roller speed is controlled to be 0.8~1.2m / min, and the quenching water temperature is 15~20℃. Among them, the ratio of lower water to upper water in the high-pressure zone is 1.1~1.3, and the ratio of lower water to upper water in the low-pressure zone is 1.3~1.5. The core cooling rate is controlled to be 2~4℃ / S, and quenched to room temperature; It should be noted that the secondary quenching uses critical quenching. If a higher heating temperature is selected, firstly, it will cause the austenite grains to coarsen, and secondly, a large amount of carbides will dissolve, which will increase the carbon content in austenite (or martensite), resulting in a decrease in toughness. It is more sensitive to microcracks generated under strong loads and is not conducive to the increase of strength.

[0009] c. Step tempering: When the steel plate is loaded into the furnace, the furnace temperature is ≤200℃, and the temperature is raised to 615℃~640℃ in the normalizing furnace. The heating time is controlled at 1~1.3min / mm, and then the furnace temperature is controlled to be kept at 620~640℃ for 130~160min. After the insulation is completed, the temperature of the steel plate is raised to 655℃~670℃ within 10~20min, and kept at this temperature for 30~40min.

[0010] It should be noted that after quenching the thick steel plate, the amount and grain size of martensite formed in the core and 1 / 4 of the steel plate are less than those on the surface of the steel plate, and their hardness and stress are also different. In order to make the structure and performance of the steel plate surface consistent with those of the core and 1 / 4, the temperature is increased in a short time so that the surface temperature of the steel plate is greater than the temperature of the core and 1 / 4. The increase in the surface tempering temperature allows the martensite on the surface of the thick plate to be fully merged. At the same time, the first stage of tempering mainly controls the consistency of carbide precipitation in the core and 1 / 4; step tempering is to make the structure and performance of the internal and external steel plate consistent, ensuring the overall strength and toughness of the steel plate.

[0011] The beneficial effects of the present invention include: using a combination of two high-pressure quenching and step tempering processes to ensure that thick high-strength hydropower steel has good hardenability, high strength, and good low-temperature impact toughness. After being kept at the austenitizing temperature, it is quickly cooled under high pressure to generate different volume fractions of martensite and residual austenite structures at the full thickness position to ensure good plastic elongation. The selection of sub-temperature quenching for the secondary quenching is conducive to reducing the grain size and better balancing the relationship between tensile strength and impact toughness. The application of the step tempering process greatly improves the problem of poor surface impact performance and low core tensile strength of high-strength and ultra-thick plates.

[0012] The 120mm980CF steel plate produced by this process has an unevenness of ≤2mm / m, a yield strength Rp0.2≥870MPa, a tensile strength Rm between 920MPa and 1120MPa, an elongation after fracture (A)≥14%, an impact resistance of -60℃ at full thickness position≥80J, and an impact resistance of -40℃ after 5% surface strain aging≥90J. Its plate shape, high-strength tensile properties and excellent low-temperature impact toughness fully meet the use requirements of large hydropower steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The technical features of the present invention are further described below in conjunction with the accompanying drawings and embodiments.

[0014] Figure 1 Schematic diagram of the metallographic structure at the upper surface, 1 / 2 and 1 / 4 of the steel plate obtained in an example of the present invention. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0016] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0017] The production of a 120mm thick, high-strength, low-temperature impact-resistant, low-crack-sensitive hydropower steel 980CF includes the following process points:

[0018] (1) Smelting: The hot metal after KR desulfurization and high-quality scrap steel are smelted in a converter-LF furnace-VD vacuum furnace in sequence. Its spectral melting composition is as follows (unit: wt%): C (0.10), Si (0.20), Mn (0.9), P ≤ 0.007, S ≤ 0.003, Cu (0.20), Nb (0.3), V (0.04), Cr (0.53), Mo (0.5), Ni (0.28), Alt (0.030), [H] ≤ 1.0 ppm, [0] ≤ 15 ppm.

[0019] (2) Casting: The molten steel obtained in step (1) is cast into a 780 mm thick steel ingot using a water-cooled mold.

[0020] (3) Ingot heating: The steel ingot obtained in step (2) is coated with high temperature paint and then fully cleaned. It is heated to 1200° C. in a soaking pit at a heating time of 10 min / cm and kept at this temperature for 10 h.

[0021] (4) slab rolling: the steel ingot heated in step (3) is descaled under high pressure and then rolled. The starting rolling temperature is 965° C. and the intermediate slab thickness is 600 mm.

[0022] (5) Heating the intermediate billet: The intermediate billet obtained in step (4) is stacked in a slow cooling pit for 24 hours, and then placed in a soaking pit for 10 hours at a temperature of 1220° C. The heated intermediate billet is then descaled with high-pressure water, and the first rolling temperature is 980° C. and the final rolling temperature is 950° C.

[0023] (6) First quenching: The steel plate is heated to 900°C in a roller hearth heating furnace, and the holding time is 2.2 min / mm. After the holding time is completed, the steel plate enters the high-pressure zone and the low-pressure zone for quenching in turn, maintaining the roller speed at 1 m / min and controlling the quenching water temperature at 15°C. The water pressure in the high-pressure zone is 0.8±0.05Mpa, the lower water ratio is 1.1, and the water pressure in the low-pressure zone is 0.4±0.05Mpa, the lower water ratio is 1.3.

[0024] (7) Secondary quenching: The steel plate is heated to 870°C in a roller hearth heating furnace, and the holding time is 2.0 min / mm. After the holding time is completed, the steel plate enters the high-pressure zone and the low-pressure zone for quenching in turn, and the roller speed is maintained at 0.8 / min. The quenching water temperature is controlled at 16°C. The water pressure in the high-pressure zone is 0.8±0.05Mpa, and the ratio of the lower water to the upper water is 1.1. The water pressure in the low-pressure zone is 0.4±0.05Mpa, and the ratio of the lower water to the upper water is 1.3.

[0025] (8) Step tempering: The steel plate is heated to 615℃~640℃ in a normalizing furnace. The furnace temperature is ≤200℃ when loading the furnace, and the heating time is controlled within 120~130min. Then the steel plate is kept at 630℃ for 145min, and the temperature deviation is ±10℃. After the insulation, the temperature of the steel plate is raised to 655℃ within 15min and kept at this temperature for 30~40min. After the insulation, the temperature is lowered with the furnace. When the furnace temperature drops below 200℃, the steel plate is taken out of the furnace and air-cooled to room temperature.

[0026] The steel plate obtained in the example was inspected for appearance, and the unevenness of the whole plate was ≤2mm / m; the steel plate was sampled and tested for mechanical properties, and the test results are shown in the following table.

[0027]

[0028] The steel plate obtained by this scheme has a yield strength Rp0.2 ≥ 870MPa, a tensile strength Rm between 920MPa and 1120MPa, an elongation after fracture (A) ≥ 14%, a -60℃ impact at the full thickness position ≥ 80J, and a 5% surface strain aging -40℃ impact ≥ 90J. Its plate shape, high-strength tensile properties and excellent low-temperature impact toughness fully meet the use requirements of large hydropower steel. The metallographic structure of the steel plate is tested and the metallographic structure is as follows Figure 1 shown.

Claims

1. A heat treatment method for 120mm thick high-strength hydropower steel 980CF, characterized in that It includes two quenching and step tempering, as follows: 1) First quenching: The steel plate is required to be heated to 900℃~930℃ in a roller hearth heating furnace, and the holding time is 1.9~2.3min / mm. After the holding time is completed, the steel plate enters the roller quenching machine with high pressure zone and low pressure zone for quenching in turn, and the roller speed is controlled to be 0.6~1.2m / min, and the quenching water temperature is 15~20℃. Among them, the ratio of water in the high pressure zone to water is 1.1~1.3, and the ratio of water in the low pressure zone to water is 1.3~1.

5. The core cooling speed is controlled to be 3~5℃ / S, and quenched to room temperature; 2) Secondary quenching: The steel plate is heated to 860℃~890℃ in a roller hearth heating furnace, and the holding time is 2.0~2.5min / mm. After the holding time is over, the steel plate enters the roller quenching machine with high pressure zone and low pressure zone for quenching in turn, and the roller speed is controlled at 0.8~1.2m / min and the quenching water temperature is 15~20℃. Among them, the ratio of lower water to upper water in high pressure zone is 1.1~1.3, and the ratio of lower water to upper water in low pressure zone is 1.3~1.

5. The core cooling speed is controlled at 2~4℃ / S, and quenched to room temperature; 3) Step tempering: When the steel plate is loaded into the furnace, the furnace temperature is ≤200℃, and the temperature is raised to 615℃~640℃ in the normalizing furnace, and the heating time is controlled at 1~1.3min / mm; then the furnace temperature is controlled to be kept at 620~640℃, and the holding time is 130~160min; after the holding is completed, the temperature of the steel plate is raised to 655℃~670℃ within 10~20min, and kept at this temperature for 30~40min. After the holding is completed, the temperature is lowered with the furnace, and the furnace temperature drops to below 200℃ and then the steel plate is taken out of the furnace and air-cooled to room temperature.

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