Preparation method of 400 Mpa grade low-temperature steel for polar region

Abstract

The invention discloses a preparation method of 400 Mpa low-temperature steel for a polar region, and belongs to the technical field of metal materials. The steel comprises the following chemical components in percentage by mass: 0.04%-0.06% of C, 0.15%-0.50% of Si, 1.45%-1.55% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.012%-0.02% of Ti, more than or equal to 0.015% of Al, 1.15%-1.50% of Ni, 0.20%-0.30% of Mo, 0.60%-1.00% of Cr and the balance of Fe and inevitable impurities. The low-carbon high-manganese component design is adopted, Ni-Mo-Cr alloy elements are added in a compounded mode, the F40-ARC polar region steel plate with the maximum thickness being 60 mm is produced through the DQ + LT technology, and high strength, high toughness and high crack arrest performance are integrated. The low-temperature toughness and crack-arrest toughness of the thick steel plate produced by the heat treatment process are improved through a component and structure regulation and control technology, the process is simple, the production cost is low, the production efficiency is high, energy is saved, consumption is reduced, and environment friendliness is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy and relates to a preparation method of 400Mpa low-temperature steel for polar regions. Background Art

[0002] With the development of polar resource exploitation, transportation, and scientific research in polar seas, the demand for developing low-temperature steel for extremely cold polar environments is increasing day by day. The steel used for the hull structures of polar icebreakers, scientific research vessels, and transport vessels usually has to withstand repeated impacts from ice layers. The special steel used must have good comprehensive properties such as low-temperature toughness, crack arrest performance, and corrosion resistance on the premise of ensuring strength. Currently, for F-class ship plate steel traditionally manufactured by low-carbon microalloying and thermo-mechanical control process (TMCP), the ductile-brittle transition temperature of the steel plate is usually around -60°C. However, for polar steel, the ductile-brittle transition temperature usually needs to be below -100°C, and at the same time, it is necessary to ensure that there is no cracking under impact in the -60°C polar environment. Therefore, the ship plates produced by traditional processes can no longer meet the usage requirements of polar environments. Summary of the Invention

[0003] The purpose of the present invention is to provide a 400Mpa low-temperature steel for polar regions and its preparation method. The representative steel grade is F40-ARC, with a thickness of 20 - 60mm. In particular, it can meet the requirements of low-temperature impact performance at -100°C and crack arrest toughness at -60°C.

[0004] The technical solution of the present invention is as follows: A preparation method of 400Mpa low-temperature steel for polar regions, the chemical composition of the steel by mass percentage is C = 0.04% - 0.06%, Si = 0.15% - 0.50%, Mn = 1.45% - 1.55%, P ≤ 0.008%, S ≤ 0.003%, Ti = 0.012% - 0.02%, Al ≥ 0.015%, Ni = 1.15% - 1.50%, Mo = 0.20% - 0.30%, Cr = 0.60% - 1.00%, and the rest is Fe and inevitable impurities; it includes the following technological steps: (1) Smelting: After subjecting the smelting raw materials to KR hot metal pretreatment, converter smelting and LF furnace refining are carried out. After the refining is completed, VD vacuum treatment is carried out, and then it is cast into a continuous casting billet with a thickness of ≥260mm; (2) Heating: The continuous casting billet is reheated to 1100 - 1150°C, and the soaking time is 20 - 30min; (3) Rolling: After discharging from the furnace, two-stage rolling is carried out. The finishing rolling temperature in the first stage of rough rolling is ≥980°C, and the finishing rolling temperature in the second stage of finish rolling is controlled at 760 - 820°C, and then it is rapidly water-cooled to below 200°C; (4) Heat treatment: Reheat the rolled steel plate to 780 - 810°C, hold for 20 - 30 min, then water-cool to room temperature. Finally, heat the steel plate to 600 - 650°C, hold for 30 - 60 min, and then air-cool to room temperature.

[0005] Process technical principle of the present invention: In the present invention, the rolled steel plate is reheated to A C1 ~A C3 temperature, causing partial reverse transformation of the on-line quenched martensite obtained during the rolling process. After rapid cooling, it transforms into a duplex structure of martensite + critical ferrite + a small amount of reverse transformed austenite, improving the low-temperature toughness and crack arrest performance of the steel plate through the composite action of the hard and soft phase structures.

[0006] Beneficial effects of the present invention: 1) The present invention uses continuous casting billets to produce high-toughness steel plates with a low-temperature impact requirement of -100°C, and the maximum thickness can reach 60 mm. While ensuring product quality, it can meet the usage requirements of polar environments. 2) The process of the present invention is simple, with low production costs, energy-saving and consumption-reducing, providing a metallurgical idea for the production of high-strength, tough and high crack arrest thick steel plates by heat treatment processes. 3) The steel produced by the method of the present invention is a new variety of energy-saving, low-carbon and environmentally friendly steel, and can also be widely used in the manufacture of low-temperature steels in multiple fields such as polar environment offshore engineering, wind power, bridges, and buildings. Description of the drawings

[0007] Figure 1 Metallographic structure photo of the steel plate in Example 1 of the present invention.

[0008] Figure 2 Metallographic structure photo of the steel plate in Comparative Example 1 of the present invention. Specific embodiments

[0009] The present invention will be further described below through examples and comparative examples.

[0010] A preparation method of a 400 Mpa low-temperature steel for polar regions. After subjecting the smelting raw materials to KR hot metal pretreatment, carry out 120 t converter smelting, LF furnace refining, VD furnace vacuum treatment, and pour into a continuous casting billet with a thickness of 260 mm. Then roll it into the finished thickness on a 5000 mm double-stand medium and heavy plate production line, and finally carry out heat treatment to adjust the structure and properties. The chemical compositions of the steel plates in the examples and comparative examples are shown in Table 1, the key process parameters of production are shown in Table 2, and the physical properties are shown in Table 3.

[0011] Table 1 Chemical compositions of the steel plates in the examples and comparative examples (weight, %) 。

[0012] Table 2 Key process parameters for the preparation of the steel plates in the examples and comparative examples 。

[0013] Table 3 Comprehensive mechanical property test results of steel plates in examples and comparative examples 。

[0014] Among them, in both the examples and the comparative examples, a double tensile testing machine was used for the crack arrest test, and an isothermal test at -60°C was carried out under the set main tensile stress. The specimen size was the original plate thickness × 500 mm × 1100 mm.

[0015] As can be seen from Table 3, the low-temperature toughness and crack arrest performance of the steel plates in Example 1, Example 2, and Example 3 of the present invention are good. The metallographic structure of the steel plates in the examples was observed, and the microstructure was mainly martensite + critical ferrite + a small amount of reverse transformed austenite, as Figure 1 shown. The low-temperature impact toughness and crack arrest performance of the steel plate in Comparative Example 1 were poor compared with those in the examples. Comparative Example 1 was a common F40 hull structural steel, a 60-mm steel plate produced by the traditional TMCP process, and its impact performance at -100°C and crack arrest performance at -60°C could not meet the requirements. The metallographic structure was mainly bainite, as Figure 2 shown. Therefore, it is very difficult to stably produce arctic steel plates that meet the requirements of both -100°C low-temperature toughness and -60°C crack arrest performance in industrial production by using the traditional process, which fully demonstrates the ingenuity, consideration, and uniqueness of the present invention in terms of composition and process design.

Claims

1. A method for preparing 400Mpa grade low-temperature steel for polar use, characterized in that: The chemical composition of steel is C=0.04%~0.06% by mass, Si=0.15%~0.50%, Mn=1.45%~1.55%, P≤0.008%, S≤0.003%, Ti=0.012%~0.02%, Al≥0.015%, Ni=1.15%~1.50%, Mo=0.20%~0.30%, Cr=0.60%~1.00%, and the rest is Fe and unavoidable impurities; The process steps include: (1) Smelting: The raw materials are pre-treated with KR molten iron and then smelted in a converter and refined in an LF furnace. After refining, they are vacuum treated with VD and then cast into continuous casting billets with a thickness of ≥260 mm; (2) Heating: reheat the continuous casting billet to 1100~1150℃, soaking time 20~30min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out. The first stage of rough rolling has a final rolling temperature of ≥980°C, and the second stage of finishing rolling has a final rolling temperature controlled at 760~820°C, and then rapidly water-cooled to below 200°C; (4) Heat treatment: The rolled steel plate is reheated to 780~810℃, kept at this temperature for 20~30min, and then water-cooled to room temperature. Finally, the steel plate is heated to 600~650℃, kept at this temperature for 30~60min, and then air-cooled to room temperature.

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