Recently, Baosteel's steelmaking team has continuously conquered many technical difficulties, such as precise logistics organization and scheduling, efficient converter dephosphorization, precise control of refining components, billet surface quality control, and so on. Through logistics optimization of production organization, smelting and continuous casting process improvement, 7 heats of continuous casting of 9Ni steel in a single tundish have been successfully completed for the first time, with a single production volume of more than 2000 tons, setting a new domestic record for continuous casting of 9Ni steel.
Extremely difficult 9Ni steel
9Ni steel for large LNG storage tanks is recognized as a high-tech and high value-added product in the industry. there are only a handful of domestic steel mills qualified for the production of LNG storage tanks and marine 9Ni steel. This kind of high alloy steel requires high precision of composition control, strict technical standard restrictions on harmful elements and gases such as phosphorus, sulfur, oxygen and nitrogen, as well as special requirements for billet surface quality and flatness. there are many technical bottlenecks in smelting, out-of-furnace refining and continuous casting. The number of continuous casting furnaces of 9Ni steel in steelmaking production is generally low, which is one of the main reasons why the yield of 9Ni steel is much lower than that of other steels. It is generally believed that for steelmaking plants, the number of continuous casting furnaces of 9 nickel steel not only determines the yield, but also shows the comprehensive level of production, technology and management of a steel plant. At present, the number of continuous casting furnaces in tundish of domestic 9Ni steel manufacturers is almost less than 5 heats, and the casting capacity of single continuous casting is no more than 1200 tons.
Innovative technology
Accurate logistics organization and scheduling is the premise and guarantee to ensure the efficient smelting of 9Ni steel. In the production of 9Ni steel, the steelmaking team closely implements the production organization around the train timetable, and does a solid job in iron ladle baking, alloy preparation, converter, refining and continuous casting process status optimization and preparation; actively confirm smelting control difficulties with technicians, and reasonably arrange pre-ladle washing and trough washing to ensure the best production condition. By pre-arranging the production plan one day in advance, precise control and reasonable and efficient optimization of production arrangement are realized, and the accuracy of smelting start time error is successfully controlled within 0.5 hours. As one of the bottleneck processes of multi-furnace continuous casting of 9Ni steel, the converter process plays a decisive role in the stable production of the product. The first-line steelmaking team studied the temperature control theory of dephosphorization furnace and decarbonization furnace, and realized the precise control of temperature. through a series of technological innovations, such as the foaming control technology of dephosphorization slag and the one-time hit control technology of nickel content in the decarbonization furnace, the efficient dephosphorization of converter and the precise control of nickel content are realized.
The technical standard of 9Ni steel composition control is strict, which brings great pressure on LF desulphurization and nitrogen control, seriously restricts the production rhythm and affects the number of continuous casting furnaces in tundish. In order to overcome this "bottleneck", the key team continuously fought the front line, reviewing the process after each production. Through tackling key problems continuously for nearly a year, the LF furnace treatment cycle of 9Ni steel has been stably controlled within the prescribed range, which can meet the requirements of multi-furnace continuous casting in the later process. Due to the influence of solidification microstructure characteristics of 9 nickel steel, the control of subcutaneous microcracks in 9 nickel steel billet has always been a recognized technical problem in the industry. The front-line research team focused on improving the heat transfer uniformity in the mold and increasing the surface temperature of the billet. Through process optimization and improvement for a year, the subcutaneous micro-cracks of the 9-nickel steel continuous casting slab are stably controlled within 3 mm, which greatly reduces the surface grinding amount of the slab, which not only improves the production efficiency but also reduces the quality loss.
Baosteel has successfully achieved its annual target of 20, 000 tons of 9Ni steel production in 2021 and is expected to reach 30, 000 tons in 2022. On the basis of the current realization of seven heats of continuous casting, the steelmaking plant will continue to overcome difficulties, enhance manufacturing capacity, and give full play to the contribution and wisdom of steelmakers for the annual goal.
Author: Zhang Xie Wang Jia
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