The Research of Phase Transformation Rules at Different Al Content on Continuous Cooling Condition of δ-TRIP Steel

摘要

The continuous cooling phase transformation rules of under-cooled austenite in δ-TRIP steel were researched in this paper.It was measured by DIL805 quenching deformation dilatometer.This equipment was used to determine the continuous cooling transformation (CCT) diagram.The microstructure of the steels was studied by the metallographic method.The effect of Al content (2.25％ and 3.08％) on continuous cooling transformation of δ-TRIP steel was analyzed.The results indicated that ferrite (or pearlite) formed in super-cooled austenite of δ-TRIP steel during higher temperature interval.Bainite transformation and martensite transformation occured during intermedium temperature and low temperature zones respectively.The Ac1 temperature of No.1 steel(2.25％Al)was 736.9°C and the Ac1 temperature of No.2 steel(3.08％Al) was 763.4°C,It indicated that Ac1 temperature raised in the higher Al content steels; So that it accelerated the ferrite transformation and enlarged the area of ferrite phase transformation,on the other hand it delayed the pearlite transformation and reduced the area of pearlite phase transformation.It also improved bainite transformation starting temperature in δ-TRIP steels; With an increasing cooling rate,the starting temperature of ferrite phase transformation gradually decreased; In order to avoid forming pearlite structure,the cooling rate should not be less than 5°C/s in No.1 steel (2.25％Al).and the cooling rate should not be less than 3°C/s No.2 steel (3.08％Al).