Steelmaking Production Practice of 9Ni Steel for Ulta-low Temperature Pressure Vessel

Abstract

This article presents the smelting practice of 9Ni steel for ultra-low-temperature pressure vessel in a certain steel plant by the "BOF → LF → scraping slag/fetching slag → LF → VD → CC" process. The decarburization and desulfurization in the BOF + LF process require that the steel produced by the converter has a low phosphorus （w［P］） ≤ 0.009%） and carbon content （w［C］） ≤ 0.05%）， with an average desulfurization rate of 74.3% and an average decarburization rate of 51.2%. The control of phosphorus content is more difficult than carbon content in the production process. In the scraping slag process， the average phosphorus reversion rate in the molten steel is 16.89%， while in the fetching slag process， it is 22.61%. Although the scraping slag process results in a lower phosphorus reversion rate in the molten steel， the processing time is increased by 15 to 20 minutes， and the average molten steel loss is increased by 3.1 t per heat. After adjusting the addition of nickel plates from the converter to the ladle furnace， the average nickel recovery rate increases by 3.74%. Through optimizing the production process， the molten steel in the tundish meets internal control requirements for phosphorus（ P）， sulfur （S）， nitrogen N， total oxygen （T.O）， and carbon （C）， ensuring high purity of the molten steel， the macrostructure center segregation of the cast slab reaches 1.0～1.5 grade in class C， the internal and surface quality of the cast slab are good. The rolled steel plate exhibits excellent performance at -196 ℃.

Keywords

• 9ni steel， dephosphorization； decarbonization； scraping slag； fetching slag