Research And Application On Low-cost High Strength And Toughness Of Bainite/Martensite Multiphase Steels

Due to superior cost performance, the Mn-series bainite/martensite multiphase steels have been paid more attentions and widely used in many aspects of construction industry, high-rise buildings, bridge beams, automobile manufacturing etc. In order to satisfy the requirement of high performance, reduce the alloying cost, simple production process and the guarantee of welding performance, the present work aims to develop a new type low-cost Mn-series bainite/martensite (B/M) multiphase steels. The B/M multiphase micro structure with fine sub-structure was achieved to improve the mechanical properties, especially the excellent combination of strength and toughness.The present work designed the new types of 0.2wt%C-Mn-Si-Cr series bainite/ martensite multiphase steels with reduced alloying cost. The CCT curves were calculated and measured, and the micro structure evolution during the continuous cooling process show that the bainite/martersite multiphase can be abtained with a wide cooling rate range. With the increase of Si content, the high temperature phase transition point of the steel was significantly increased, and the Mo element significantly increased the hardenability of this serious steels.In order to improve the strength and toughness, the present work was mainly through three ideas to control the refinement of microstructure (austenite grain), i.e., micro-alloying, deformation and phase transformation refinement. The volume fraction and morphologies of bainite is depended on the cooling rate. The Gleeble-1500D thermal simulation testing machine was employed to study the effect of cooling rate on the microstructrue and mechanical properties. The results show that the mechanical properties can reach the following levels:ultimate tensile strength about 1300MPa, impact toughness at room temperature 110J when cooling rate of approximately 0.75℃ /s, followed low temperature tempering. The mechanical properties can reach the following levels:ultimate tensile strength about 1300MPa, impact toughness at room temperature 110J. The best match for the strength and toughness after TMCP process: Rm≈1321MPa, Akv≈185J (20℃). The quenching and partitioning process combined with bainitic transformation (BQ&P) process was employed to refine the multiphase microstructure. The bainite formed during initial cooling step can divide the prior austenite grains and refine the martensite block. The mechanical properties of bainite/martensite multiphase steel by BQ&P-T process can reach the following levels: Rp≈1061MPa,Rm≈1414MPa,A≈16%,Akv≈114J(20)℃、47J(-40℃)、43J(-60℃), when QT=200℃, PT=280℃,TT=250℃.The mechanism of microstructure evolution and refinement was discussed based on the various microstructures which were observed by OM, SEM, TEM and EBSD technique. The relationship between microstructure and mechanical properties was studied. The results showed that the B/M multiphase steel with excellent combination of strength and impact toughness was based on the retained austenite and the refinement of lath bainite/martensite in the deformation process to absorb a lot of impact energy.A heavy drill rod without Ni and with advanced technology, low cost, the tensile strength greater than 1300Mpa has been developed successfully by adjusting the process parameters. The industrial production has been realized. The strength and toughness of the steel without carburization:Rp≈1176MPa, Rm≈1488MPa, A≈15%, Akv≈83J(20℃); the strength and toughness of the steel with carburization:Rp≈1170MPa, Rm≈1484MPa, A≈15%,Akv≈73J(20℃).