Strong Interactions between Austenite and the Matrix of Medium-Mn Steel during Intercritical Annealing.

Cunyu Wang,Chang Wang,Tianpeng Zhou,Zejun Chen,Wenquan Cao

doi:10.3390/ma13153366

Abstract

The effects of heat treatment on the microstructure evolution was studied in regards to austenite nucleation and grain growth. It was found that the austenite nucleation and matrix recrystallization kinetics of samples annealed at 675 °C for different times were revealed, implying a strong interaction between the ferrite matrix and austenite. The recrystallization of the matrix during annealing provided favorable conditions for austenite nucleation and growth, and the formation of austenite during this process reduced the matrix recrystallization kinetics, thus delaying the recrystallization process of the matrix around the austenite grains. The statistical results for the austenite grain size under different annealing temperatures indicated that the average grain size of the austenite slightly increases with increasing of the annealing temperature, but the austenite with the largest grain size grows faster at the same temperature. This difference is attributed to the strict Kurdjumov Sachs (KS) orientation relationship (OR) between the austenite grains and the matrix, because the growth of austenite with a strict KS OR with the matrix is often inhibited during annealing. In contrast, the austenite maintains a non-strict KS OR with the matrix and can grow preferentially with increasing annealing temperature and time.

Highlights

The development of advanced ultrahigh strength steels is promoted by the need for lightweight bodies, and medium-Mn steels with 3–10% Mn are considered one of the most promising materials for automobile mass reduction [1,2,3]
From a statistical point of view, it is interesting that a large number of austenite grains were distributed at the low-angle grain boundaries, they were small in number and size
Distributed at the high-angle grain boundaries in the matrix, but there were still austenite grains we believe that different grain boundary types were not one of the main factors that impacted the distributed at the low-angle grain boundaries, they were small in number and size

Summary

Introduction

The development of advanced ultrahigh strength steels is promoted by the need for lightweight bodies, and medium-Mn steels with 3–10% Mn are considered one of the most promising materials for automobile mass reduction [1,2,3]. A typical heat treatment process during the production of cold-rolled medium-Mn steels produces a mixed structure of ferrite (F) and retained austenite (A) by an intercritical annealing process [6,7,8,9,10]. The microstructure evolution of cold-rolled medium-Mn steels during intercritical annealing is a complex process that involves the recovery and recrystallization of the matrix and the formation and growth of austenite. There are reports about a potential relationship between matrix recrystallization and phase transformation processes during the intercritical annealing process of cold-rolled steels. The primary objective is to study the recrystallization behavior of the matrix and austenite nucleation and growth in medium-Mn steel under the synergistic influences of the annealing temperature and time to reveal their intrinsic relationship

Experimental Procedures

Microstructure

Microstructure of the Samples

Microstructure of the Samples Annealed at Different Temperatures

Ferrite Recrystallization

Factors that Influenced the Austenite Grain Size

Conclusions