Control of TiN precipitation behavior in titanium-containing micro-alloyed steel

Abstract

As a kind of high-strength steel, low-carbon titanium-containing micro-alloyed steel has been widely used in the industry recently. Fine TiN precipitates can refine the austenite grains, improve the toughness of the steel, strengthen the good formability and weldability of the steel, and reduce the production costs of the process. However, due to the improper technical operations or steps, a large amount of TiN will precipitate in the billet, which will affect the crack sensitivity of the steel. According to the thermodynamic calculation results, when the nitrogen content is reduced from 50 ppm to 20 ppm, the precipitation temperature of TiN can be reduced to 1400 °C, whilst the amount of TiN precipitate quantity will be significantly reduced. This article conducted the Oxygen and Nitrogen Analysis towards the samples throughout the entire steelmaking process. The results indicated an abnormal increase in nitrogen content during the Ladle Furnace (LF) process. In addition to the reaction between air and molten steel, the raw and auxiliary materials for steelmaking may also be the reasons which caused the increase of nitrogen content in the molten steel, as they contain nitrogen as well. However, there are only a small number of studies that had explored the impact of raw and auxiliary materials on steel quality. This article investigated the cleanliness of ferrotitanium alloys. The results showed that the cleanliness (inclusions and nitrogen content) of TiFe-30 is much higher than that of TiFe-70. When TiFe-70 was replaced by TiFe-30 for steelmaking, the content of nitrogen in the molten steel as well as the content of TiN were reduced, whilst the size of TiN was correspondingly decreased. In this experiment, not only was the performance of steel improved, the cost of steelmaking was also reduced. At last, this work could also awaken the judgment standards for steelmaking alloys.