Effect Of Niobium On The Microstructure And Mechanical Properties Of High Strength Bainite Steels

In the present work,the effect of Nb on microstructure and properties of Mn-Si-Cr series bainitic steel was investigated.Based on the Nb microalloyed technology and phase transformation theory for Mn-series bainitic steel,phase transformation kinetics,heat treatment process optimization,strength-toughness mechanism and wear resistance properties of Mn-Si-Cr series bainite steel with Nb was studied.Attained results show that the characteristic properties of experimental steels with Nb are benificial for the application of bainitic steels.The main results are as following:Micro-alloying Nb addition can enhance the phase transformation point of bainitic steel(Ac1,Ac3 and Ms).In continuous cooling transformation curve(CCT),temperature interval and cooling rate range for ferrite and pearlite transformation are extended.For bainite transformation,the temperature interval is enlarged,while critical cooling rate is decreased.The quanties of solid-soluted Nb and Nb(C,N)precipitation show obvious effect on the phase transformation process and final produce.The higher the austenitizing temperature,the more Nb solution can be detected.Higher Nb solution can improve the stability of austenite and delay the transformation at high temperature stage.Austenitizing temperature has a certain effect on the final microsturcture.Under the employed experimental condition,small NbC precipitates were detect and their size is around 10 nm.The austenite grain size decreased from 29 ?m to 18 ?m with the addition of Nb.For as-treated steels,high cooling rate following austenitation is good to mechanical properties and the corresponding wear resistance can be greatly improved compared with traditional treatment.This may be due to precipitation strengthening and grain refinement of the Nb carbides.After tempered at about 320 ? for oil-quenced treated steels,optimum combinations of strength and toughness can be obtained.