On the Correlation between Microstructure Homogeneity and Formability in Two Multi Phase Steels Treated with Different Cooling Strategies

Marion Bechtold,Marco Witte,Sascha Kluge

doi:10.4028/www.scientific.net/msf.854.22

Abstract

In the scope of the optimization of multi phase steels, e.g. for the automotive industry, control of the microstructure is essential to tailor the mechanical properties. In this study, two cold rolled steels varying in carbon content were annealed and cooled under different laboratory conditions. The microstructure is investigated using optical and electron microscopy and EBSD. The results are correlated to the mechanical properties obtained from tensile, hole expansion and bending test. It is found that tensile strength and elongation are mainly dependent on martensite volume fraction, while yield strength is less affected by chemical composition or annealing treatment. In contrast, hole expansion capacity and maximum bending angle are significantly improved by the homogenization of the microstructure which is independent of strength and elongation. The microstructure homogeneity is expressed by analyzing the Lorenz curves derived from the kernel average misorientation from EBSD measurements.

Highlights

Cold rolled multi phase steels such as dual phase steels and complex phase steels are the predominant materials in structural and crash-relevant components of many car bodies [1]
The HC-intermediate quench (IQ) is the only specimen that contains a significant amount of bainite and tempered martensite
A small fraction of bainite forms and the martensite volume fraction is reduced in comparison to direct quenching

Summary

Introduction

Cold rolled multi phase steels such as dual phase steels and complex phase steels are the predominant materials in structural and crash-relevant components of many car bodies [1]. High strength levels are desired to reduce sheet thickness and vehicle weight, while high formability is needed to realize complex forming operations and high energy absorption in case of crash. The material’s strength is mainly determined by the volume fraction of hard phases, number and size of precipitates as well as grain size. Considering cold rolled and annealed steels, the cooling conditions after annealing play a critical role for the final microstructure. The mechanical properties are compared by tensile tests, hole expansion tests and bending tests.

Experimental Procedures

Results and Discussion

Summary