Characterization of the Stretch Flangeabitity of High-Strength Bainitic Steel: The Significance of Variant Pairs.

Zhiquan Wang,Yajun Hui,Chengjia Shang,Zifeng Guo,Bin Chen

doi:10.3390/ma15010276

Zhiquan Wang, Yajun Hui + Show 3 more

Open Access

https://doi.org/10.3390/ma15010276

Copy DOI

Abstract

Variant pairs have an indispensable function on mechanical properties such as low impact toughness. Therefore, it was assumed that they would also affect the HER (Hole Expansion Ratio, an indicator to evaluate stretch flanging performance). To clarify this, a comprehensive analysis of the common influential factors in an 800 MPa grade low carbon micro-alloyed steel, i.e., the retained austenite, the M/A (Martensite/Austenite) island, the titanium precipitations, the grain diameter, the density of high angle grain boundaries and the textures, was first conducted. It was found that they did not match well with the HER, suggesting that they were not the governing factor for HER in this steel. However, the dominating crystallography groups and the variant pairing results indicated that they fitted well with the HER. In the samples with high HERs, the CP (Close Packed) groups dominated the transformation, wherein one individual CP group consisted of two or more Bain groups, whereas it evolved into the domination of joint CP groups and Bain groups for the low HER sample. Further analysis on the variant pairing features indicated that a correlation occurred between the HER and the high angle variant pairs. In the steels with high HERs, high-angle variant pairs of V1/V2, V1/V3 that transformed from the same CP group, particularly of V1/V2 pair, were mostly generated. They turned to V1/V9, V1/V10, V1/V12, V1/V15, V1/V17, and V1/V18 pairs from differential CP groups, especially the V1/V12 and V1/V15 pair for low-HER steel. This result showed that V1/V2, V1/V12, and V1/V15 might have accounted most for the HER in this steel. The underlying reason was that the V1/V2 pair was specialized in supplying a slip passage for dislocation transmission across a grain boundary with little resistance, whereas the dislocation transmission ability for V1/V12 and V1/V15 pair was particularly poor. Thus, to efficiently enhance the HER, one should regulate the variant pairs by augmenting the V1/V2 fraction and suppressing the formation of the V1/V12 and V1/V15 pair.

Highlights

The hole-expansion ratio (HER) is a measure of stretch flangeability performance for automotive structural steels [1]
With the advanced-high strength steels nowadays becoming extensively employed in fabricating automobile structural parts to improve fuel efficiency and to eliminate gas emissions [2,3,4,5,6,7,8], the cracking caused by dislocation pile-ups or strength difference between differential microstructures during stretch flangeability operation of AHSS has been the major restriction in their use [2,3,4]
Though the variants belonging to differential Bain groups formed some CP regions concurrently in specimen C, the variants belonging to differential CP groups had a higher chance of forming as a Bain region in specimen C than specimens A and B. This led to a decrease in the density of high-angle grain boundaries. These results proved that the HER was closely related to the dominating crystallography groups

Summary

Introduction

The hole-expansion ratio (HER) is a measure of stretch flangeability performance for automotive structural steels [1]. Most steel researchers focus on the material itself, and a considerable number of experimental and simulation works [4,9,11] have been done on the effect of the microstructure to the HER, respectively, for the aspects of the phases constituting the microstructure [5,10,12,14] the anisotropy [15,16,17,18,19,20,21], the austenite stability [22], strain hardening exponent (n) [17], stress state [23], etc All these studies will assist developers in developing a high

Methods

Results

Conclusion