Austenite reconstruction via EBSD measurements: a tool to understand low Carbon martensite steel properties

Lieven Bracke,Laura Moli Sanchez,Nicolas Bernier,N Schryvers,J Van Humbeeck

doi:10.1051/matecconf/20153307005

Abstract

The basic characterization of the austenite grain size and shape prior to quenching to martensite was already used in the past to optimize the mechanical properties and impact toughness of low Carbon martensitic steel. This basic characterization can typically be done by optical microscopy. To better understand the mechanisms that generate the different properties, however, a more detailed analysis is required. An algorithm has been developed to calculate the austenite orientation starting from the martensite orientation measured by EBSD. The method has been applied to explain the different properties of martensite after austenitization and quenching and of direct quenched martensite with different levels of accumulated strain. It is shown that a small austenite size is needed to improve the impact toughness, regardless of the process route. The strength is strongly depending on the dislocation density in the austenite.

Highlights

The strength and the properties of hot rolled martensitic low carbon steels are known to depend strongly on the austenite condition prior to quenching [1]
The strength increases with a lower finish rolling temperature for the direct quenching (DQ) materials
For the EBSD result, the prior austenite grain size (PAGS) smaller than 3.5μm have been discarded because the reconstruction routine creates a large number of artificial small grains

Summary

Introduction

The strength and the properties of hot rolled martensitic low carbon steels are known to depend strongly on the austenite condition prior to quenching [1]. Two main processes can be used to produce martensitic steel plates [2]. The first one is through re-austenitizing and quenching (RAQ) In this case, the austenite is fully recrystallized and equiaxed before quenching. In case of a finish rolling temperature well above temperature of non-recrystallization (Tnr), the quenching occurs from coarse, recrystallized austenite. In case the finish rolling temperature is at or just above the Tnr, the quenching starts from a finegrained, equiaxed austenite. This process is called “recrystallization controlled rolling” (RCR). For a finish rolling temperature below Tnr, the quenching is done from deformed austenite and the process is called “controlled rolling” (CR).

Results

Discussion

Conclusion