Microstructure and Mechanical Properties of Cast and Hot-Rolled Medium-Carbon Steels under Isothermal Heat-Treatment Conditions

Byungsue Shin,Soongkeun Hyun,Sanggyu Choi,Sung Yi,Kwangyuk Kim

doi:10.3390/met11121950

Abstract

In this study, the changes in the microstructure and mechanical properties during isothermal heat treatment of cast steel before and after hot deformation were investigated using medium-carbon steel with low alloy content. The microstructural characteristics of the cast and hot-rolled medium-carbon steel under isothermal heat-treatment conditions were examined using optical microscopy and scanning electron microscopy in conjunction with electron backscatter diffraction. The variation in the mechanical properties was evaluated using Rockwell hardness and tensile tests. After maintaining an austenitizing condition at 1200 °C for 30 min, an isothermal heat treatment was performed in the range 350–500 °C, followed by rapid cooling with water. Both the cast steel and hot-rolled steel did not completely transform into bainitic ferrite during isothermal heat treatment. The partially untransformed microstructure was a mixture of martensite and acicular ferrite. At 500 °C, the prior austenite phase changed to Widmanstätten ferrite and pearlite. At 450 °C, bainitic ferrite and cementite were coarsened by the coalescence of ferrite and subsequent diffusive growth. The mechanical properties increased as the isothermal heat-treatment temperature decreased, and the hardness of the cast steel was generally higher than that of the hot-rolled steel. Hardness and strength showed similar trends, and overall mechanical properties tend to decrease as the isothermal heat-treatment temperature increases, but there are slight differences depending on complex factors such as various phase fractions and grain size.

Highlights

Steel needs to be of high strength with a light weight for automobiles and with durability for civil engineering applications, machinery, and building structures
Bainitic steels are typically used in applications that require high strength and wear resistance combined with good toughness
The current study evaluated the mechanical properties of medium-carbon low-alloy steel with respect to microstructure under low deformation and cast steels

Summary

Introduction

Steel needs to be of high strength with a light weight for automobiles and with durability for civil engineering applications, machinery, and building structures. In the era of carbon neutrality, the features of materials have become more critical to their applications. Bainitic steels are typically used in applications that require high strength and wear resistance combined with good toughness. When choosing the optimal material for a particular component, geometric constraints and desired properties are the determining factors. Alloys should have features such as the desired hardenability, economic feasibility, and resistance to unwanted phase formation, which can lead to poor mechanical properties or thermal cracking [1]. The entire scope and mechanisms of bainite formation and the structures controlling its mechanical properties are not fully understood. Several studies have been conducted on the contribution of various bainite microstructures to the mechanical properties [2,3,4,5,6,7]

Methods

Results

Conclusion