Effect And Controlling Of Precipitation Behavior Of Secondary Phases On The Evolution Of Slab Surface For Microalloyed Steel Casting

As one of the important products in the steel industry, microalloyed steel is sensitive to cracking. Surface defects are the major problem encountered during the continuous casting of microalloyed steel, especially the transverse cracking on the slab surface. A lot of studies show that micro cracks is related to the surface structure of slab. The coarse austenite, film-like ferrite and secondary phases precipitated on the austenite grain boundary of surface structure lead to microscopic cracks during bending or straightening. Moreover, secondary phases affect the microstructure of surface layer, such as refining austenite by pinning the grain boundary and promoting the nucleation of ferrite. Therefore, studying the relationship between secondary phases and microstructure is of great significance to solve the problem of surface transverse cracking of microalloyed steel from controlling the structure.In this paper, the study included many steels which have different microalloying element content. The effect of cooling rates, reheating condition on the precipitation-dissolution behavior of secondary phases during double phase transformation is analyzed. The effect of precipitation behavior of secondary phases on austenite grain growth, proeutectoid ferrite distribution and refinement of austenite during reheating is also analyzed. It’s hopeful to obtain the fine and uniform austenite grains by controlling the precipitation behavior of secondary phases and provide the theoretical and experimental basis for the choice of technical parameters of double phase transformation. The results of this study can be briefly summarized as follows:① Thermodynamics and kinetics calculation of secondary phases’ precipitation in microalloyed steel shows that: secondary phases’ precipitation sequence is Ti CN, Nb CN, and VCN. They precipitate on the austenite grain boundary firstly, then homogeneously in the austenite grain of slab.② Solidification of microalloyed steel was simulated by confocal laser scanning microscopy. Small reliefs on the samples’ surface are related to the precipitation of secondary phases. The precipitation of secondary phases can be indirectly characterized by in-situ observing surface reliefs. This is a new method of secondary phases’ characterization.③ While the theoretical precipitation of Ti reaches 0.008 %, grain boundary pinning is effective, which can limit the austenite grain for growth. While it’s over 0.008 %, the prior austenite grain is not further refined, because the more precipitation part put little strength to pinning.④ Nb CN can promote the nucleation of proeutectoid ferrite. Secondary phases are dispersed in the austenite grain, as well as proeutectoid ferrite by the cooling rate of 5.0 ℃?s-1, which is helpful to refine the austenite during reheating later.⑤ VCN can expand the incubation time of transformation of ferrite to austenite by inhibition of austenite nucleation during reheating. The austenite is uniform and fine when reheating time is 1000 ℃ and reheating rate is 3.0 ℃?s-1.