Relationship Research between Microstructure Characteristics and Mechanical Property of Advanced High Strength Steels

Author:Zhao Hong Shan

Supervisor:xu zu yao jin xue jun


Degree Year:2016





With the prosperity of the automotive industry,the high performance advanced high strength steels(AHSSs)are developed rapidly.The AHSSs are developed from the first generation of steels,including dual-phase steels and transformation-induced plasticity steels to the twinning-induced plasticity second generation of steels,and to the current third generation of steels,including quenching and partitioning treated steels and medium-manganese steels.The generational change of AHSSs indicates that the development of new generation AHSS is increasingly focused on the multi-phase interaction and the coordination deformation ability.This means that the relationship between microstructures and the mechanical properties grows increasingly intricate,so the relevant research is more and more important.Generally speaking,the mechanical properties of individual phase,the phase interactions and the role of retained austenite will affect the deformation mechanism and the mechanical properties of meta-stable retained austenite-containing multi-phase AHSSs.A comprehensive model or concept which can systematically study these factors is needed.The multi-phase,multi-scale,meta-stable and morphology,namely M4 model not only embody the complexity of the deformation mechanism,but also can be used to guide the study of the relationship between microstructures and the mechanical properties.Therefore,in this paper,guided by the M4 model,the Q&P treated steels and corresponding counterpart samples are used to study the following questions:1.The quantification method of differently distributed retained austenite study and the effect of distribution on the stability of retained austenite.2.The study of the effect of martensite morphology and distribution on the strain/stress distribution of matrix phases,the stability of retained austenite and the mechanical properties of meta-stable retained austenite-containing multi-phase AHSSs.The exploration and application of digitized microstructure reconstruction method for meta-stable retained austenite-containing multi-phase AHSSs.3.The analysis and modeling of the relationship between microstructures and mechanical properties of meta-stable retained austenite-containing multi-phase AHSSs,including the role of individual phases,phase interactions and transformation induced plasticity effect.The experiment methods and main conclusions are as follows:The complex microstructures of Q&P steel have been investigated using two-step color tint-etching method and further verified by X-ray diffraction,electron backscattering diffraction,magnetization measurements and Mossbauer(5)(5)spectroscopy.A method based on color tint-etching micrographs and the concept of categorized linear interception has been proposed to study the amount of retained austenite residing in different phases in Q&P treated multi-phase steel.A multistage transformation model is proposed and proved to be more accurate than a single stability model.This finding facilitates a quantitative evaluation of the contribution of surrounding phases to the retained austenite stability.Two types of either blocky or fine martensite-containing multi-phase steels,obtained by step-quenching and partitioning(S-QP)or intercritical-quenching and partitioning(I-QP),have been used to analyze the effect of martensite morphology on the phase interaction and the retained austenite stability.The strength and elongation of finely dispersed martensite-containing I-QP specimen are optimized,due to that the martensite morphology will affect the strain/stress distribution of matrix phases and the retained austenite transformation rate and uniformity.A digitized microstructure reconstruction method is used to the microstructure feature description and microstructure reconstruction.The mean free path of martensite with the volume fraction increase of martensite is studied by the analyzing of the reconstructed images.The relationship between nucleation number and the volume fraction of martensite is discussed.These results and discussion prove that martensite morphology optimization is an effective method to improve the strength and ductility of Q&P steels.The digitized microstructure reconstruction method is an effective method to improve the efficiency.Finally,in order to study the interactions among different phases and the effect of the stability of retained austenite on mechanical properties,Q&P steels and corresponding quenching or quenching and tempering treated dual phase steels are studied.The role of retained austenite on the deformation mechanism is analyzed by comparing the Q&P and DP samples,and the effect of retained austenite located inside ferrite is studied quantitatively.By analyzing the change of martensite carbon content before and after partitioning treatment and the value of microhardness of partitioned martensite,it is proved that the mechanical property and deformation mechanism of partitioned martensite are similar to the tempered martensite.By a coordinate conversion method applied to the microhardness test,the load partitioning between ferrite and partitioned martensite is proved to comply with the linear distribution.A mechanical model based on the Mecking-Kocks theory,Bouquerel’s spherical assumption and Gladman-type mixture law is used to model the behavior of meta-stable retained austenite-containing multi-phase Q&P steel.Through reasonable experiment design,the quantitative study of the stability of differently distributed retained austenite,the analysis of deformation mechanism of multi-phase steels and the study of mechanical modeling are proved to be valid.Above all,the study of multi-phase,multi-scale,meta-stable and morphology on the effect of mechanical properties will be helpful to understand the relationship between microstructures and mechanical properties,the relationship between microstructures and heat treatments,and the optimization direction of microstructure and mechanical properties,which is the theoretical basis of new generation AHSSs design.