Assessment of Heat-Affected Zone Softening of Hot-Press-Formed Steel over 2.0 GPa Tensile Strength with Bead-On-Plate Laser Welding

Kwangsoo Kim,Minjung Kang,Namhyun Kang,Cheolhee Kim

doi:10.3390/app11135774

Abstract

High-strength hot-press-formed (HPF) steels with a fully martensitic microstructure are being widely used in the fabrication of automotive body structure, and 2.0 GPa-strength HPF steel has recently been commercially launched. However, heat-affected zone (HAZ) softening is unavoidable in welding martensitic steel. In this study, the HAZ softening characteristic of 2.0 GPa HPF steel was investigated by applying a high-brightness laser welding process, wherein the heat input was controlled by varying the welding speed. Microstructural evaluation and hardness test results showed that the base metal with a fully martensitic microstructure was changed to the same type of fully martensitic microstructure in the weld metal, while relatively soft microstructures of tempered martensite and ferrite phase were partially formed in the intercritical HAZ (ICHAZ) and subcritical HAZ (SCHAZ) areas. In the tensile test, the joint strength was 10–20% lower than that of the base metal, and the fracture initiation was estimated at the ICHAZ/SCHAZ boundary, where the lowest hardness was confirmed by the nanoindentation technique.

Highlights

The strength of steel utilized in the automobile body structures has been increasing rapidly, and the proportion of martensite-based ultra-high-strength steel (UHSS) has been expanding drastically in the last two decades [1,2,3,4]
Among modern martensite-based UHSS, fully martensitic steel and hot-press-formed (HPF) steel have higher strength owing to the higher martensitic phase fraction than that in martensite-ferrite dual-phase (DP) steel and transformation-induced plasticity steel, which have a partial martensite fraction [4,5,6]
During the HPF process, an HPF-steel sheet is heated for complete austenization and quenched in a water-cooled die; this process enables the achievement of high dimensional accuracy owing to the high temperature forming process and a fully martensitic microstructure formation owing to the rapid cooling

Summary

Introduction

The strength of steel utilized in the automobile body structures has been increasing rapidly, and the proportion of martensite-based ultra-high-strength steel (UHSS) has been expanding drastically in the last two decades [1,2,3,4]. HPF steel, a boron-alloy steel, has a ferritic-pearlitic microstructure with a tensile strength of approximately 600 MPa before the HPF process. During the HPF process, an HPF-steel sheet is heated for complete austenization and quenched in a water-cooled die; this process enables the achievement of high dimensional accuracy owing to the high temperature forming process and a fully martensitic microstructure formation owing to the rapid cooling. Since 2010, HPF steel has been widely commercialized in the automotive industry, and 22MnB5 steel with a tensile strength of 1.5 GPa has been most commonly applied [7].

Results

Conclusion