Steel plate with high shear section rate in 590 Mpa low-temperature environment and manufacturing method of steel plate

Abstract

The invention discloses a steel plate with a high shear section rate in a 590 Mpa low-temperature environment and a manufacturing method of the steel plate. The steel comprises the following chemical components in percentage by mass: 0.08-0.012% of C, 0.10-0.35% of Si, 0.35-0.70% of Mn, less than or equal to 0.010% of P, less than or equal to 0.0005% of S, 0.70-1.10% of Cu, 2.10-2.60% of Ni, 1.10-1.75% of Cr, 0.15-0.30% of Mo, 0.020-0.060% of Alt, less than or equal to 0.0050% of N, less than or equal to 0.0030% of O and the balance of Fe and inevitable impurities. The method comprises the key process steps of converter smelting, external refining, slab continuous casting, slab heating, steel plate controlled rolling, quenching and tempering. The thickness of the manufactured steel plate is 60-70 mm, the yield strength is not lower than 590 Mpa, and the ductility is not lower than 20%; at the test temperature of 40 DEG C, the impact energy is not lower than 200 J, and the shear section rate reaches 100%; at the test temperature of 80 DEG C, the impact energy is not lower than 180 J, and the shear section rate is not lower than 70%; at the test temperature of 100 DEG C, the impact energy is not lower than 150 J, and the shear section rate is not lower than 45%.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a high shear section ratio steel plate for low temperature environments of 590 MPa and its manufacturing method. Background Technology

[0002] High shear ratio steel plates are commonly used in shipbuilding and marine structural engineering, such as ships on the Arctic and Antarctic routes or icebreakers. In low-temperature collision service environments, conventional steel plates are prone to brittleness and fracture, while high shear ratio steel plates with a low temperature of 590 MPa can better ensure the safety of the hull structure, while making the hull structure lighter, ensuring strength, and improving the ship's load-bearing capacity and fuel efficiency. Summary of the Invention

[0003] This invention aims to provide a method for manufacturing 590MPa grade steel plates with high shear ratio and high toughness under low-temperature conditions. The produced steel plates have a thickness of 60~70mm, a yield strength of 590~650MPa, a tensile strength of 650~720MPa, and an elongation of not less than 20%. Under a test temperature of −40℃, the impact energy is not less than 200 J and the shear ratio reaches 100%. Under a test temperature of −80℃, the impact energy is not less than 180 J and the shear ratio is not less than 70%. Under a test temperature of −100℃, the impact energy is not less than 150 J and the shear ratio is not less than 45%.

[0004] This invention is achieved through the following technical solution: A high shear ratio steel plate for low-temperature environments with a strength of 590 MPa is disclosed. The steel's chemical composition (mass percentage) is: C = 0.08–0.012, Si = 0.10–0.35, Mn = 0.35–0.70, P ≤ 0.010, S ≤ 0.0005, Cu = 0.70–1.10, Ni = 2.10–2.60, Cr = 1.10–1.75, Mo = 0.15–0.30, Alt = 0.020–0.060, N ≤ 0.0050, O ≤ 0.0030, with the balance being Fe and unavoidable impurities. The steel plate has a thickness of 60–70 mm, a yield strength of not less than 590 MPa, and an elongation of not less than 20%. At a test temperature of −40℃, the impact energy is not less than 200 J, and the shear ratio reaches 100%. At a test temperature of −80℃, the impact energy is not less than 180 J. J, shear cross-sectional area ratio not less than 70%; at a test temperature of −100℃, impact energy not less than 150 J, shear cross-sectional area ratio not less than 45%.

[0005] A method for manufacturing a high shear ratio steel plate under low-temperature conditions of 590 MPa includes the following process steps: (1) Converter smelting: After the molten iron produced by the blast furnace is pre-desulfurized, it is poured into a 120~300t top and bottom blowing converter for treatment. The molten iron is oxidized and decarburized to C=0.04%~0.06%, dephosphorized, desulfurized and added with calculated weight of Mn, Cu and Ni raw materials. Oxygen is continued to adjust the temperature of the molten steel to 1600~1650°C. Then, according to the detected [O] content in the molten steel, aluminum ingots are added for calming and deoxidation. Bottom blowing argon is carried out before tapping the steel. (2) Ladle refining: The molten steel with adjusted temperature and preliminary chemical composition is poured into the ladle and transported to the LF refining furnace. The molten steel is stirred by bottom blowing argon gas, and high basicity slag is used for stirring and desulfurization to achieve the target sulfur content in the molten steel and allow solid inclusions to float to the surface. Before the LF treatment is completed, 200 meters of pure calcium wire is fed in. The ladle is then sent to the RH vacuum refining furnace for vacuum degassing treatment for ≥12 minutes to remove the gas content of the molten steel to H≤2 ppm, O≤20 ppm, and N≤45 ppm. (3) Slab continuous casting: After the steel is refined twice, the temperature reaches the casting superheat of 25~50°C and is then transported to the casting machine. It is continuously cast into slabs with a thickness of 220~450mm and a width of 1650~2800mm through the tundish under the protective atmosphere and protective slag. (4) Slab heating: After the continuous casting high temperature slab is flame-cut, it is stacked in the billet yard for slow cooling for 72~96 hours to prevent slab cracks. Then it is transported to the heating furnace and heated to a temperature of 1180~1280°C for 6~10 hours. The austenite grain size is above grade 6. (5) Controlled rolling of steel plate: After removing the iron oxide scale from the surface of the billet with high pressure water, it is rolled in 5 to 7 passes along the width direction at a rolling temperature of 1100 to 1150℃. After rolling to the required width, the steel is rotated 90° and then rolled at 900 to 1050℃ to the required steel plate thickness, keeping the austenite grains in a recrystallized equiaxed shape with a grain size of grade 7 or above. (6) Quenching: The quenching temperature is controlled at 860~870℃, the heating coefficient is controlled at 1.2~2.0min / mm based on the thickness of the finished steel plate, the holding time is controlled at 20~60min, and the quenching cooling rate is 2~3℃ / s; (7) Tempering: Tempering temperature is 660~670℃, heating coefficient is controlled at 2.5~4.0min / mm, holding time is controlled at 100~150min based on the thickness of the finished steel plate, and air cooling is performed after exiting the furnace.

[0006] The alloy composition and content settings of this invention are based on the following mechanism of action: Carbon (C) is a fundamental strengthening element in ultra-high strength steel, effectively improving the strength of steel plates. However, excessively high C content is detrimental to weldability, low-temperature toughness, and shear ratio. To ensure a balance between strength, weldability, low-temperature toughness, and shear ratio in the steel of this invention, the C content is controlled between 0.08% and 0.12%.

[0007] Si is one of the main deoxidizing elements in the steelmaking process and is also a solid solution strengthening element. However, if the Si content is too high, ferrous silicate Fe2SiO4 will be formed during the heating process, which is difficult to remove. This makes it difficult to peel off the iron oxide scale on the surface of the steel plate and affects the surface quality of the steel plate. Therefore, the Si content is controlled at 0.10%~0.35%.

[0008] Mn is the most important solid solution strengthening element and a strong austenite stabilizing element, playing a role in grain refinement and strengthening. Too low an Mn content leads to insufficient strength, while too high an Mn content easily leads to segregation and the formation of banded structures. Therefore, the Mn content in this invention is controlled at 0.35%~0.70%.

[0009] Mo in quenched and tempered steel can significantly improve hardenability and tempering stability, allowing the steel plate to be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and improving ductility and toughness. Taking into account the thickness, strength, and toughness of the steel plate, the content is controlled between 0.15% and 0.30%.

[0010] Al is a strong deoxidizing and grain-refining element in the steelmaking process. It effectively removes oxygen from molten steel and forms a stable AlN precipitate with nitrogen, which pins the austenite grain boundaries and effectively inhibits austenite grain growth. In this invention, the Al content is controlled at 0.020%~0.060%.

[0011] The prominent role of Cu in steel is to improve its resistance to atmospheric corrosion. When steel plates have a low carbon content and need to maintain high strength, increasing the Cu content can improve the strength without adversely affecting the weldability. Taking all factors into consideration, the Cu content should be controlled between 0.70% and 1.10%.

[0012] Ni is a solid solution strengthening agent in steel and a good hardenability additive. It can significantly improve low-temperature performance, especially low-temperature toughness and shear ratio. Ni alloys are relatively expensive, so the content is controlled at 2.10% to 2.60% after comprehensive consideration.

[0013] Cr enhances hardenability in quenched and tempered steel. Through tempering after quenching, it exhibits excellent comprehensive mechanical properties. The content is controlled at 1.10% to 1.75%.

[0014] Both phosphorus (P) and sulfur (S) are harmful elements and should be removed as much as possible during the steelmaking process. P readily segregates at the original austenite grain boundaries, forming a high-P enrichment zone at the solidification end of the billet core, thus reducing toughness and shear ratio. During steelmaking, sulfur (S) reacts with manganese (Mn) to form MnS inclusions, especially in the Mn segregation region of the billet core, where numerous coarse MnS inclusions form and deform into strips during rolling. Therefore, this invention aims to control P ≤ 0.010% and S ≤ 0.005% at low levels.

[0015] The beneficial effects of this invention are as follows: Using the method of this invention, 60-70mm thick steel plates with a 590MPa grade and high shear ratio under low-temperature conditions can be produced. The yield strength is not less than 590MPa, and the elongation is not less than 20%. At a test temperature of −40℃, the impact energy is not less than 200 J, and the shear ratio reaches 100%. At a test temperature of −80℃, the impact energy is not less than 180 J, and the shear ratio is not less than 70%. At a test temperature of −100℃, the impact energy is not less than 150 J, and the shear ratio is not less than 45%. Simultaneously, the steel plates exhibit good corrosion resistance and easy weldability. Attached Figure Description

[0016] Figure 1 Metallographic image of the 70mm thick steel plate manufactured in Example 2. Detailed Implementation

[0017] The present invention will be further described below with reference to the embodiments. Example 1

[0018] A method for producing a 60mm thick steel plate with high shear ratio under low-temperature conditions (590 MPa). The process steps are as follows: Converter smelting: molten steel was smelted according to the set composition and cast into 300 mm thick slabs. The chemical composition by weight percentage is shown in Table 1, with the balance being Fe and unavoidable impurities. The converter smelting was successful on the first attempt, with the endpoint control C=0.05% and the endpoint oxygen content O=412ppm. High-basicity slag desulfurization was used in the LF furnace, and the vacuum treatment time in the RH furnace was 15 min, with the residual H measured to be 1.2ppm. 2) Ladle refining: When the molten steel reaches a casting superheat of 35°C, it is transported to the casting machine for continuous casting. A 300mm cross section is used, the casting speed is 0.75m / min, the crystallizer surface fluctuation is ±2mm, and the actual control range of superheat during molten steel casting is 10~15°C. 3) Slab continuous casting: After being flame-cut, the high-temperature slabs are stacked in the billet yard for slow cooling for 85 hours, and then transported to the heating furnace for heating to 1207℃ for 8.2 hours, with an austenite grain size of 6.5.

[0019] 4) Slab heating: After the slab heated to 1207℃ is held at the temperature for 3 hours, it is taken out of the heating furnace and the iron oxide scale on the upper and lower surfaces of the slab is removed with 20MPa high-pressure water. 5) Controlled rolling of steel plates: Two-stage controlled rolling is performed: The first stage begins at a rolling temperature of 1133℃ with a cumulative reduction of 80%, down to an intermediate billet thickness of 130mm, ensuring sufficient austenite recrystallization and refinement; the second stage begins at a rolling temperature of 920℃ with a reduction of 80%, finally hot-rolling the steel plate to 60mm at 815℃ with a grain size of 7.5; after rolling, the steel plate is stacked and cooled for 24 hours, followed by quenching and tempering treatment. 6) Quenching: Quenching temperature 862~869℃, heating coefficient 1.5min / mm, holding time 56min, quenching cooling rate 2.4℃ / s; 7) Tempering: Tempering temperature 663~665℃, heating coefficient 3.2min / mm, holding time 125min, air cooling after taking out of the furnace.

[0020] The properties of the steel plates prepared by the above method are shown in Tables 2 and 3. Example 2

[0021] A method for producing a 70mm thick steel plate with high shear ratio under low-temperature conditions (590 MPa). The process steps are as follows: 1) Converter smelting: molten steel was smelted according to the set composition and cast into 300 mm thick slabs. The chemical composition by weight percentage is shown in Table 1, with the balance being Fe and unavoidable impurities. The converter smelting was successful on the first attempt, with the endpoint control C=0.05% and the endpoint oxygen content O=412ppm. High-basicity slag desulfurization was used in the LF furnace, and the vacuum treatment time in the RH furnace was 15 min, with the residual H measured to be 1.2ppm. 2) Ladle refining: When the molten steel reaches a casting superheat of 35°C, it is transported to the casting machine for continuous casting. A 300mm cross section is used, the casting speed is 0.75m / min, the crystallizer surface fluctuation is ±2mm, and the actual control range of superheat during molten steel casting is 10~15°C. 3) Continuous casting of slabs: After being flame-cut, the high-temperature slabs are stacked in the billet yard for slow cooling for 86 hours, and then transported to the heating furnace for heating to 1206℃ for 8.3 hours, with an austenite grain size of 6.5. 4) Slab heating: After the slab is heated to 1206℃ and held for 3 hours, it is taken out of the heating furnace and the iron oxide scale on the upper and lower surfaces of the slab is removed with 20MPa high-pressure water. 5) Controlled rolling of steel plates: Two-stage controlled rolling is performed: The first stage begins at a rolling temperature of 1130℃ with a cumulative reduction of 76%, down to an intermediate billet thickness of 140mm, ensuring sufficient recrystallization and refinement of austenite; the second stage begins at a rolling temperature of 923℃ with a reduction of 75%, and finally hot-rolls the steel plate to 70mm at 819℃, with a grain size of grade 7; after rolling, the steel plate is stacked and cooled for 24 hours, followed by quenching and tempering treatment. 6) Quenching: Quenching temperature 863~868℃, heating coefficient 1.5min / mm, holding time 54min, quenching cooling rate 2.3℃ / s; 7) Tempering: Tempering temperature 662~667℃, heating coefficient 3.3min / mm, holding time 122min, air cooling after taking out of the furnace.

[0022] The properties of the steel plates prepared by the above method are shown in Tables 2 and 3.

[0023] As shown in Table 2, the steel plate produced by this invention has a yield strength of over 590 MPa, a tensile strength of 650-720 MPa, and an elongation of not less than 20%. Table 3 shows that at a test temperature of −40℃, the impact energy is not less than 200 J, and the shear area ratio reaches 100%; at a test temperature of −80℃, the impact energy is not less than 180 J, and the shear area ratio is not less than 70%; at a test temperature of −100℃, the impact energy is not less than 150 J, and the shear area ratio is not less than 45%. The steel plate exhibits good strength and toughness matching, a high shear area ratio under low-temperature test conditions, and excellent comprehensive mechanical properties.

[0024] Table 1 Chemical composition (wt%) of the steel in the examples .

[0025] Table 2 Tensile properties of steels in the examples .

[0026] Table 3. Series of temperature shock properties of the steels in the examples. .

Claims

1. A high shear ratio steel plate for low-temperature environments at 590 MPa, characterized in that: The steel's chemical composition (mass percentage) is: C=0.08~0.012, Si=0.10~0.35, Mn=0.35~0.70, P≤0.010, S≤0.0005, Cu=0.70~1.10, Ni=2.10~2.60, Cr=1.10~1.75, Mo=0.15~0.30, Alt=0.020~0.060, N≤0.0050, O≤0.0030, with the balance being Fe and unavoidable impurities; the steel plate thickness is 60~70mm, yield strength is not less than 590MPa, elongation is not less than 20%; impact energy at −40℃ test temperature is not less than 200 J, shear area ratio reaches 100%; impact energy at −80℃ test temperature is not less than 180 J, shear area ratio is not less than 70%; impact energy at −100℃ test temperature is not less than 150 J. J, the shear cross-sectional area ratio is not less than 45%.

2. A method for manufacturing a high shear ratio steel plate under low-temperature conditions of 590 MPa, characterized in that... The key process steps include the following: (1) Converter smelting: After the molten iron produced by the blast furnace is pre-desulfurized, it is poured into a 120~300t top and bottom blowing converter for treatment. The molten iron is oxidized and decarburized to C=0.04%~0.06%, dephosphorized, desulfurized and added with calculated weight of Mn, Cu and Ni raw materials. Oxygen is continued to adjust the temperature of the molten steel to 1600~1650°C. Then, according to the O content in the molten steel, aluminum ingots are added for calming and deoxidation. Bottom blowing argon is carried out before tapping the steel. (2) Ladle refining: The molten steel with adjusted temperature and preliminary chemical composition is poured into the ladle and transported to the LF refining furnace. The molten steel is stirred by bottom blowing argon gas, and high basicity slag is used for stirring and desulfurization. Before the LF treatment is completed, 200 meters of calcium wire is fed in. The ladle is then sent to the RH vacuum refining furnace for vacuum degassing treatment for ≥12 minutes, and the gas content of the molten steel is reduced to H≤2 ppm, O≤20 ppm, and N≤45 ppm. (3) Slab continuous casting: After the steel is refined twice, the temperature reaches the casting superheat of 25~50°C and is then transported to the casting machine. It is continuously cast into slabs with a thickness of 220~450mm and a width of 1650~2800mm through the tundish under the protective atmosphere and protective slag. (4) Slab heating: After the continuous casting high temperature slab is flame-cut, it is stacked in the billet yard for slow cooling for 72~96h, and then transported to the heating furnace for heating to a temperature of 1180~1280°C for 6~10h, with austenite grain size of grade 6 or above. (5) Controlled rolling of steel plate: After removing the iron oxide scale from the surface of the billet with high pressure water, it is rolled in 5 to 7 passes along the width direction at a rolling temperature of 1100 to 1150℃. After rolling to the required width, the steel is rotated 90° and then rolled at 900 to 1050℃ to the required steel plate thickness, keeping the austenite grains in a recrystallized equiaxed shape with a grain size of grade 7 or above. (6) Quenching: The quenching temperature is controlled at 860~870℃, the heating coefficient is controlled at 1.2~2.0min / mm according to the thickness of the finished steel plate, the holding time is controlled at 20~60min, and the quenching cooling rate is 2~3℃ / s; (7) Tempering: Tempering temperature is 660~670℃, heating coefficient is controlled at 2.5~4.0min / mm, holding time is controlled at 100~150min according to the thickness of the finished steel plate, and air cooling is performed after taking it out of the furnace.

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