Study On Inclusion Control Technology Of High Strength Bolt Steel

China’s automobile industry presents good development, but the automobile advanced bolt material has a large gap compared with foreign countries and such material depends on import all the time. Inclusion in bolt steel is one of the main reasons for cracks of cold heading and early fatigue, so to develope inclusion control technology has a vital significance for high strength bolt steel produced domestics.In this paper, based on a steel corporation, the formation of inclusions was predicted and the deoxidation capacity of refining slag was analyzed by thermodynamics, and then inclusions in high strength bolt steel were controlled by the optimization of steelmaking and refining process in the steel works. The main conclusions were obtained as follows:(1) Low dissolved oxygen content can be obtained by aluminum deoxidation. When the acid-soluble aluminum content is0.02%-0.05%, the dissolved oxygen content of steel is about (4～6)×10-4%. Spinel inclusions can be formed with little amounts of Magnesium. Meanwhile, alumina inclusions are not stable with trace amounts of calcium, and would transform into calcium aluminate inclusions.(2) During the refining process, TiN and TiC inclusions can not be formed in liquid steel, and with the solidification of steel, they would precipitate in solidification process.(3) With the increase of basicity, the deoxidation capability of slag and the formation possibility of globular inclusion increase as well. When the basicity is above4, the refining slag almost reaches the largest deoxidation capability. For low oxygen high strength bolt steel which requires globular inclusion controlled, refining slag with basicity about4is suitable, and the basicity can be controlled about3.5-4.5in practice. The experimental results show that Slag4-3with basicity of3.4-4.0almost gets the best deoxidation result, and gets the inclusion shape partly controlled, so this type of slag can be chosen to refine high strength bolt steel.(4) According to the optimization methods, such as using high quality raw materials, promoting carbon content when tapping, changing tapping deoxidation method, enhancing refining time, controlling soft blowing gas amount and canceling calcium treatment, new process gets average total oxygen content of7.10×10-4%, average nitrogen content of28.38×10-4%, average sulfur content of34×10-4%and titanium content below20×10-4%. When the new process is applied, the main size of inclusions is about2～4μm; D-type inclusions are better controlled with the grade no more than1.0, and the grade of B-type inclusions increases little with the grade no more than0.5, and all the inclusion grades satisfy the request.