Interpretation of dynamic tensile behavior by austenite stability in ferrite-austenite duplex lightweight steels

Jaeyeong Park,Hyoung Seop Kim,Jai-Hyun Kwak,Min Cheol Jo,Hyeok Jae Jeong,Sunghak Lee,Seok Su Sohn

doi:10.1038/s41598-017-15991-5

Abstract

Phenomena occurring in duplex lightweight steels under dynamic loading are hardly investigated, although its understanding is essentially needed in applications of automotive steels. In this study, quasi-static and dynamic tensile properties of duplex lightweight steels were investigated by focusing on how TRIP and TWIP mechanisms were varied under the quasi-static and dynamic loading conditions. As the annealing temperature increased, the grain size and volume fraction of austenite increased, thereby gradually decreasing austenite stability. The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was closely related with deformation mechanisms. Under the dynamic loading, the temperature rise due to adiabatic heating raised the austenite stability, which resulted in the reduction in the TRIP amount. Though the 950 °C-annealed specimen having the lowest austenite stability showed the very low ductility and strength under the quasi-static loading, it exhibited the tensile elongation up to 54% as well as high strain-hardening rate and tensile strength (1038 MPa) due to appropriate austenite stability under dynamic loading. Since dynamic properties of the present duplex lightweight steels show the excellent strength-ductility combination as well as continuously high strain hardening, they can be sufficiently applied to automotive steel sheets demanded for stronger vehicle bodies and safety enhancement.

Highlights

Since automotive steels generally require excellent strength for sustaining structures and improving resistances to shock or impact during car crashes[1,2,3], highly-deformable TWinning Induced Plasticity (TWIP) and TRansformation Induced Plasticity (TRIP) steels have been used in automotive industries
Quasi-static and dynamic tensile properties of the (α + γ) duplex lightweight Fe-0.3C-8.8Mn-5.1Al steel specimens were investigated in relation with TRIP and TWIP mechanisms
The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was attributed to deformation mechanisms of TRIP + TWIP or TRIP

Summary

Introduction

Since automotive steels generally require excellent strength for sustaining structures and improving resistances to shock or impact during car crashes[1,2,3], highly-deformable TWinning Induced Plasticity (TWIP) and TRansformation Induced Plasticity (TRIP) steels have been used in automotive industries. The addition of 5~6 wt.% of Al as well as 8~9 wt.% of Mn provides excellent tensile properties including tensile strength above 700 MPa and elongation above 70%10, while it leads to about 9% of lightweight effect compared to conventional steels[11] This excellent strength-ductility combination is mainly achieved at an optimal austenite stability obtained when both TWIP and TRIP mechanisms are actively working[10]. To effectively use duplex lightweight steels for automotive steel sheet applications, for collision-absorption components, the detailed information of dynamic deformation should be obtained This is because automotive steel sheets require high resistance to impact energy upon vehicle collision as well as high strength and fracture toughness for sustaining sufficient structural stability. Detailed deformation mechanisms were examined by analyzing how TWIP and TRIP mechanisms were varied under the quasi-static and dynamic loadings, and the correlation with microstructural evolution processes was verified

Methods

Results

Discussion

Conclusion