Morphology and Formation Mechanism of Ti and Nb Liquid Precipitation Phase in Medium Carbon CrMo Steel

Abstract

The morphology of the liquid precipitation phase of micro alloys Ti， Nb， and V in 42CrMo steel continuous casting billets was observed， as well as the changes of the liquid precipitation phase after heating and rolling， then the formation mechanism was derived by using thermodynamic calculations. The results showed that Ti and Nb precipitated in the form of composite carbonitrides can be observed in 42CrMo steel 410 mm ×530 mm continuous casting billets with Ti 0. 012%，Nb 0. 030%， V 0. 030%， N 0. 003 5%， which was formed a micrometer scale liquid precipitation phase. Those with higher Ti content（70%-83%）were mostly polygonal blocks， while others with higher Nb content（70%-85%）were mostly thin films or irregular blocks， and no significant V segregation in the liquid phase； The carbonitrides precipitation phases of Ti and Nb can dissolve and grow together， the liquid precipitation with higher Nb content grew into blocks with the liquid precipitation with higher Ti content as the core， while some phases nucleate and grew with sulfides as the core or basal planes； There were many carbonitrides liquid precipitation phase in the center of continuous casting billets， and their sizes were relatively large （ up to 35 μm）， but its quantity and size under the surface were relatively small （below 10μm）； After heating and rolling the continuous casting billet at 1 200 ℃， liquid precipitation phase can be partially dissolved， resulting in a decrease in quantity and size， and it will break during the rolling process.

Keywords

• ti； nb； crmo； micro alloy； medium carbon steel； liquid precipitation phase