Tailoring Microstructures And The Resulting Abrasive Wear Of High Strength And Ductility Q&P Steels

In this thesis,an advanced Q&P（Quenching&Partitioning）steel was chosen for the current study.On the basis of the heat treatment regime,a series of microstructural features was obtained by designing the various heat treatments.Systematic abrasive wear experiments were conducted.Combining 3D-profile tester,SEM,XRD,EPMA,FIB+TEM,et al.,the microstructural features were observed and the abrasion resistance and the resulting abrasion mechanism of microstructures were studied.During the abrasive wear process,the change of retained austenite and the microstructure of plastic deformation layer were unraveled.The relationship of heat treatment regime,microstructure and the resulting abrasive wear resistance was discussed,which consequently provides the knowledge to guide the optimization of heat treatment and the design of microstructures for abrasion resistant Q&P steel with high strength and ductility.The main conclusions are as follows:1.Based on the inter-critical annealing（IA）with different temperatures,Dual Phase（DP）steel with different volume fraction of martensite were obtained.With the increase of IA temperature,the volume fraction of martensite increases and its morphology evolves from block to lath.The increase in volume fraction of martensite leads to the increase of tensile strength and hardness but the decrease in uniform elongation.The wear test results demonstrate that the wear loss first decreases and then starts to rise for all loading conditions as the volume fraction of martensite increases.Compared to the full martensite with high hardness,90%M-DP steel with a low hardness has an equivalent and/or even better abrasion resistance.2.On the basis of the Q&P heat treatment with respect to the different quenching temperature,it aims to introduce the retained austenite in dual phase microstructure to form ferrite+martensite+retained austenite.With increasing the quenching temperature,the content of retained austenite first increase and then decrease,having a maximum of（8%）at275℃.The results show that the quenching temperature has a little influence on the mechanical properties and abrasion resistance.XRD results show that compared to the original sample,the content of retained austenite after wear test decrease,which verify the occurrence of phase transformation during abrasive wear process.3.Based on the Q&P heat treatment with respect to the different partitioning time,the content of retained austenite in Q&P steel gradually increases with the partitioning time increases,while the martensite evolves from blocky morphology to lath morphology.The results show the tensile strength and hardness of Q&P steel gradually decreases but its elongation increases with the partitioning time.It was found that the Q&P steel with 100s of partitioning time has a lowest wear loss,which could be the result of strongest mechanical stability（high carbon content）and its highest degree of martensite transformation.Furthermore,the characterization of FIB+TEM on the cross section shows that the retained austenite exists with film shape in the deformation layer,and there are lots of dislocations located on the edge of M-A island.4.Compared with the DP steel and commercial abrasion resistant steels,a Q&P steel obtained via an optimal heat treatment gives a best performance resisting to abrasion with low hardness(Hv_0.2 380)and high strength/ductility.