Regulation Of Retained Austenite And Study Of Properties In Medium-carbon Nanostructured Complex-phase Steels

As a metastable phase in advanced high-strength steels,the morphology,size and distribution of retained austenite have important effects on the properties of the material.In order to gain more ideal properties,it was necessary to regulate the retained austenite in the nanostructured complex-phase steel,and this article started with the addition of alloy elements as well as the design and optimization of the heat treatment process.The regulation of retained austenite with the addition of alloy element of Si,Al partial replacement of Si,and intercritical heat treatment as well as several new multi-step heat treatment process had been studied by means of optical microscope,scanning electron microscopy,electron back scattering diffraction,electron probe microanalysis,transmission electron microscopy and X-ray diffraction.At the same time,the mechanical and wear-resisting properties were studied by means of microhardness,tensile and impact testing,and impact abrasive wear machine.The main contents of this dissertation are given as follows:In order to explore the effect of Si on the regulation of retained austenite and the influence of properties,two kinds of medium carbon steel with different content of Si(0.30 wt % and 1.5 wt %)transformation at 250? for 6-24 h was studied.The results showed that Si can inhibit the precipitation of cementite.With the same heat treatment,compared with 0.3Si sample,the volume fraction and carbon content of retained austenite and the hardness were higher,and the V-notch impact performance was better.With the constant total content of Si and Al,the effect of partial replacement of Si by Al on the regulation of retained austenite and the influence of properties were studied.Results showed that with partial replacement of Si by Al,there was a small amount of ferrite,and the volume fraction of retained austenite was lower,the carbon content in retained austenite was higher,and the morphology of retained austenite was mainly film.With partial replacement of Si by Al,although the hardness and tensile strength were relatively lower,the retained austenite surrounded by ferrite will show more obvious TRIP effect,which resulted in higher elongation after fracture and higher product of strength and elogation.In order to explore the intercritical heating on the regulation of retained austenite and the influence of properties,Q&P and I&QP treatment were carried out on the steel of 1.54 Si and 1.26 Al.The results showed that in addition to martensite and retained austenite,there was some ferrite in the samples treated with I&QP process.Compared with Q&P samples,the volume fraction of retained austenite in I&QP samples was lower.With the presence of soft ferrite,the hardness and tensile strength of I&QP samples were lower but the elongation was higher.In the process of tensile testing,retained austenite showed more obvious TRIP effect due to the presence of soft ferrite,which will improve the elongation and the working hardening index of the samples.Therefore,compared with Q&P samples,I&QP samples had slightly lower hardness and tensile strength,but higher elongation,higher product of strength and elogation and better comprehensive performance.In order to explore multi-step treatment on the regulation of retained austenite and the influence of properties,the experimental steels with Si content of 1.5 wt% treated with one-step bainite treatment(300? for 6 h),two-step bainite treatment(300? for 3 h+250? for 20 h),low temperature bainite + quenching-partitioning(B&QP),quenching-partitioning + low temperature bainite(QP&B),bainite transformation + tempering(BT)and quenching-bainite transformation + tempering(Q&BT)treatment were studied.In the two-step bainite sample,the volume fraction of retained austenite was relatively lower,but the carbon content in retained austenite was relatively higher.The morphology of retained austenite was mainly film,and the size of retained austenite with film and block morphology were smaller than that of one-step bainite sample.The mechanical properties and wear resistance were better;Compare to the B&QP sample,for the QP&B sample,the volume fraction of retained austenite was lower but the carbon content in retained austenite was higher.The hardness and impact toughness were slightly higher,and the wear resistance was better;Due to the pre-martensite released carbon into the nearby retained austenite during the tempering process,which improved the stability of retained austenite and delayed the coarsening of bainite ferrite,thus improving the tempering stability of the test steel,the tempering hardness of bainite steel with pre-martensite was stable at the temperature range from 200? to 500?.While,the tempering hardness of bainite steel without pre-martensite was stable at the temperature range from 200? to 400?,and then dropped rapidly.