Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

Javier Hidalgo,Maria Jesus Santofimia

doi:10.1007/s11661-016-3525-4

Javier Hidalgo, Maria Jesus Santofimia

Open Access

https://doi.org/10.1007/s11661-016-3525-4

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Abstract

Abstract Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

Highlights

LATH martensite is a key constituent of many advanced high strength steels such as martensitic steels, dual-phase (DP) steels, and other multi-phase steels like low-alloyed transformation-induced plasticity and quenching and partitioning steels
It can be presumed that misorientations in the gap between these are due to boundaries corresponding to former austenite grains
Boundaries presumably corresponding to prior austenite grain (PAG) are emphasized with white lines

Summary

Introduction

LATH martensite is a key constituent of many advanced high strength steels such as martensitic steels, dual-phase (DP) steels, and other multi-phase steels like low-alloyed transformation-induced plasticity and quenching and partitioning steels. Many efforts are devoted to understand these microstructures and their effects on the mechanical properties. Prior austenite grain size (PAGS), steel compositions and processing routes play an important role in the formation and characteristics of lath martensite. There is not a clear explanation yet of how the PAGS affects martensite formation and the resulting microstructures. To understand the effect of the reduction of PAGS on the martensitic microstructure, it is important to have a clear picture of how martensitic microstructures are sub-divided. The Kurdjumov–Sachs (K–S) orientation relation between parent austenite and lath martensite predicts that 24 unique crystallographic lath variants may develop from a single parent austenite grain.

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