Refinement of Solidification Structure of H13 Steel by Rare Earth Sulfide

To improve the solidification structure of H13 steel, rare earth sulfide (Ce-S) is used as the nucleation core of delta-ferrite or gamma-austenite to refine the original austenite grain. The relationship between rare earth inclusions and grain refinement is discussed in terms of the type, distribution, size, number, and formation characteristic of rare earth inclusions. The results show that to form a large amount of Ce-S (> 35 mm(-2)) and inhibit the formation of rare earth oxide (Ce-O), rare earth oxysulfide (Ce-O-S), and MnS, the content of O, S, and Ce must be strictly controlled. Most Ce-S in the solidification structure is located in the original austenite grain interior, indicating that Ce-S can act as the nucleation core of delta-ferrite or gamma-austenite. The most effective size of Ce-S as nucleation core is 1-2 mu m, followed by 2-3 mu m, and the Ce-S with the size larger than 3 mu m has the least effect. The original austenite grain size is related to the number density of Ce-S, and the higher the number density of Ce-S, the smaller the grain size. The precipitation size of Ce-S during solidification is mostly 1-3 mu m, and these Ce-S can act as the nucleation core more effectively.