Effect Of RE On The Microstructure And Impact Toughness Of B450NbRE Steel

RE weathering steel is a type of steel with Chinese source characteristics. To improve impact properties of RE weathering steel, the effects and mechanisms of RE should be further studied. In this paper, the high strength weathering B450NbRE steel with high strength and good weathing preformance produced by BaoGang Group was chosen as the research subject. The effects of RE on the microstructure and impact toughness were studied.In this paper, the effect of RE in B450NbRE steel on the rolled microstructure, precipitated phase and impact toughness were further studied by metallographic analysis and impact test. The reason of the unstable impact toughness of B450NbRE steel was analyzed. The following results were obtained:1. The microstructure of B450NbRE steel is ferrite and small quantities of bainite, perlite and tertiary cementite along the ferrite grain boundary after being rolled and air cooled. The predominant microstructure is bainite and small amounts of martensite by wind cooling.2. Tertiary cementite along the grain boundary of ferrite is the main reason for the unstable impact toughness of high strength weathering steel B450NbRE produced by BaoGang Group. There are two factors of tertiary cementite forming in B450NbRE steel. First, RE could promote the dissolution of alloy elements in austenite and, with deformation induced action oversaturated alloy elements precipitated as cementite increased in rolling process. Second, during the cooling process after rolling, the cooling rate is much slower at the temperature range near Ac1.3. With increasing RE content, the proportion of ferrite first increased and then decreased. The max proportion of ferrite is 94.4% as RE content is 0.0047%.4. Trace RE could improve the impact toughness obviously. With increasing RE content, the impact toughness first increased and then decreased. The max impact toughness of B450NbRE is 45.1J(20℃) and 13.9J(-40℃) as RE content is 0.0047%.5. With increasing RE content, the content of Nb solution decreased and the content of Nb compounds increased.It would have an important significance for the application of RE in clean weathing steel.