A. Hot metal pretreatment.
To ensure the low-sulfur content of pipeline steel, hot metal pretreatment is required before steelmaking. This is a convenient and economical method to obtain low-sulfur hot metal. Usually, spraying, stirring and other hot metal desulfurization methods are used to spray calcium carbide, magnesium, or lime and other desulfurizing agents into torpedo tanks of molten iron tanks to reduce the sulfur in the molten iron from 300×10 -6 to below 50×10 -6.
B. The converter top and bottom composite blowing.
In addition to desulfurization, converter smelting also has dephosphorization tasks. Through top-bottom composite blowing, it can not only reduce phosphorus and sulfur, reduce the oxidizing atmosphere, but also reduce splashing and strengthen smelting.
C. Refining outside the furnace.
Pipeline steel is often refined outside the furnace, such as RH vacuum degassing, LF refining, ladle powder spraying, silicon calcium feeding, and rare earth wire, etc. Especially RH and calcium treatment has become indispensable technological measures for the production of high-grade pipeline steel. The application of vacuum degassing makes it possible to produce pure steel. Also, the accurate control of alloying elements in the vacuum treatment process can control the chemical composition within a narrow range, which can minimize the performance fluctuation of the steel pipe. For X80 and higher strength pipeline steel, performance fluctuation control is a very important issue. Vacuum treatment can reduce the hydrogen content, and the oxygen content of high-strength pipeline steel must be controlled at a lower level than traditional steel grades because high-strength steel is more sensitive to cracks.
D. Continuous casting.
In the production of pipeline steel continuous casting, how to prevent large-particle inclusion, component segregation, surface, and internal cracks are the main issues to improve the quality of the material. The secondary oxidation of molten steel from the ladle to the tundish and from the tundish to the mold should be prevented, and argon protection is generally used. Besides, when the continuous casting billet is above 1300°C, rapid water spray cooling should be avoided to avoid surface cracks. At the same time, continuous casting adopts electromagnetic stirring, light reduction technology, and preventing the flow of the mother liquor enriched in the liquid cavity, which has a great effect on reducing the segregation of alloy elements.
E. Controlled cooling by drawing and rolling.
To make full use of the effect of grain refinement and achieve the matching of strength and toughness, most pipeline steel is produced by controlled rolling. It recrystallizes through deformation in the high-temperature austenite zone and the low-temperature austenite un-recrystallized zone. The deformation of (r+a) enriched nuclei, and the deformation of the (r+a) two-phase region to achieve the best refinement effect. Steel microalloying, rolling process, and grain refinement are closely related. It is not enough to use the general post-rolling cooling for pipeline steel above X70. It is necessary to use controlled cooling to adjust the ferrite crystal Grain size, bainite type, and proportion, number of carbonitride precipitates, and particle size distribution.