Strain Amount and Strain Path Effects on Instrumented Charpy Toughness of Baked Third Generation Advanced High Strength Steels

Third generation advanced high strength steels (AHSS) that rely on the transformation of austenite to martensite have gained growing interest for implementation into vehicle architectures. Previous studies have identified a dependency of the rate of austenite decomposition on the amount of strain and the associated strain path imposed on the sheet. The rate and amount of austenite transformation can impact the work hardening behavior and tensile properties. However, a deeper understanding of the impact on toughness, and thus crash performance, is not fully developed. In this study, the strain path and strain amounts were systematically controlled to understand the associated correlation to impact toughness in the end application condition (strained and baked). Impact toughness was evaluated using an instrumented Charpy machine with a single sheet v-notch sample configuration. The instrumented striker provides a load - displacement curve as well as a total impact energy measurement, which is the integration of the load - displacement curve. Using these measurements, this study intends to understand any potential correlation between amount of retained austenite transformation, through the varying of strain paths and amount of strain, and resulting impact toughness after paint bake.