Research On Microstructures And Properties Of Quenching-partitioning-tempering Medium Carbon Steels With The Addition Of Nb And Cu

The industry of iron and steel is the economic foundation of a nation, and at the same timeas an economic development engine. The steel industry is also one of the main CO2emissionproducers. Till2010, the steel yield all over the world has reached up to about1.3billion tons.In order to conserve energy and reduce environmental pollution, it is urgent to develophigh-strength steels to achieve the aim of transportation, device and equipment lightweight.The quenching-partitioning-tempering (Q-P-T) process proposed by Xu Zuyao (T.Y. Hsu) is anovel process to obtain steels with excellent combination of strength and elongation.In this paper, in order to evaluate the effect of chemical composition,partitioning-tempering time on microstructures and hardness of the steels treated byquenching-partitioning-tempering process, Nb+Cu-free, Nb-bearing and Nb+Cu-bearingmedium carbon steels with different partitioning-tempering time were investigated. In order toevaluate the effect of quenching temperature on microstructures and hardness of the steelstreated by quenching-partitioning-tempering process, Nb+Cu-bearing medium carbon steelswith quenched at100°C and210°C were investigated. The microstructural observations wereanalyzed by means of optical microscope, scanning electron microscopy, transmission electronmicroscope, and X-ray diffraction. The hardness was applied by Vickers hardness tester. Theresults obtained are as follows:(1) The ultra fine-grained microstructures of the three kinds of steel byquenching-partitioning-tempering process were mainly made up of martensite and retainedaustenite. Very fine carbides were precipitated in the martensite matrix duringpartitioning-tempering period. With the increase of partitioning-tempering time, the martensitelath was coarsened, the volume fraction and carbon content of retained austenite was alsochanged. Compared with the Nb+Cu-free steel, the addition of Nb and Cu contributed to theincrease in the volume fraction of retained austenite.(2) The addition of Nb and Cu was responsible for not only the more volume fraction ofretained austenite but also the precipitation strengthening. With the addition ofcarbide-forming element of Nb, the precipitation strengthening effect was obvious for thequenching-partitioning-tempering treated steel. The fine precipitates were identified to beNb(C, N) particles. When the tempering temperature was450°C, the peak value of hardness(450HV1) occurred at the partitioning-tempering time of1800s for Nb-bearing steel. Theaddition of Cu promoted the precipitation of Nb, and thereby it played an important role in theprecipitation strengthening. When the tempering temperature was450°C, the peak value ofhardness (461HV1) occurred at the partitioning-tempering time of1800s for Nb+Cu-bearingsteel. The peak value of hardness for Nb+Cu-bearing steel was higher than that of theNb-bearing steel. (3) For Nb+Cu-bearing steel, when the quenching temperature was100°C and210°C, theultra fine-grained microstructures were mainly made up of martensite, retained austenite andcarbides. With the increase of quenching temperature, the volume fraction of retainedaustenite was increased and the hardness of the steel was accordingly decreased.