Study Of Ultra Fast Cooling Process And Strength-toughening Mechanism Of Ti Microalloyed Steels

In this thesis,the project "Control of Nano Precipitates in Low Carbon Steels under Ultra Fast Cooling(UFC)and Comprehensive Strengthening Mechanisms",supported by the National Natural Science Foundation of China(Key Program),and the actual project of"Development of Ultra Fast Cooling after Rolling System and Process in 3450 mm Production Line" are taken as the research background.Aiming at common C-Mn steels with Ti adding,the effects of Ti content and UFC processing parameters on the microstructure and the mechanical properties of Ti microalloyed steels and the strength-toughening mechanism are investigated.The main works and research results of this thesis are as follows:(1)By means of the continuous cooling and the isothermal quenching thermal simulation experiments,the transformation behaviors of supercooled austenite and microstructure evolution rules of the experimental steels and the effects of Ti content and UFC processing parameters on phase transformation and precipitation have been investigated.The continuous cooling transformation(CCT)curves,the tip nose temperature of "C" curves for ferrite isothermal transformation and precipitation completion time were determined.(2)By means of laboratory hot rolling experiments,the effects of Ti content on the microstructure and the mechanical properties of the experimental steels and the precipitation behaviors of carbide inside of ferrite have been investigated.(3)By means of laboratory hot rolling experiments,the effects of UFC process parameters on the microstructure and the mechanical properties of the experimental steels have been investigated.(4)Based on the calculation of the contributions of strengthening mechanisms in the experimental steels with different Ti content and UFC process parameters,the strengthening and toughening mechanisms are analyzed.The results illustrate that,for all the experimental steels with different Ti content and different UFC process parameters under UFC process,grain refining strengthening(??GB)is the major strengthening mechanism,while precipitation strengthening(??Orowan)is the secondary;With Ti content increasing,both??Orowan and ?GB increases,however,the increase of ??Orowan is greater;With Final cooling temperature increasing,??Orowan increases and ??GB decreases;With cooling rates increasing,??Orowan and ??GB increases Simultaneously.