The Kinetics of Phase Transition of Austenite to Ferrite in Medium-Carbon Microalloy Steel

Liushun Wu,Yun Zhou,Kunlong Liu

doi:10.3390/met11121986

Abstract

To reduce energy and resource consumption, high-strength hot-rolled rebars with yield strengths of ≥400 MPa (HRB500) and ≥500 MPa (HRB600) have been designed and produced in recent years. Optimizing the microstructure in the steel to improve strength necessitates determining the kinetics of the phase transition of austenite to polygonal ferrite. Therefore, in the study, the effect of temperature and holding time on the volume fraction of ferrite is investigated in HRB500 and HRB600 steels. Experimental results show that the ferrite percentage initially increases with an increase in temperature and then decreases as the temperature increases from 600 to 730 °C. The optimum temperature range is 680–700 °C for HRB500 steel and 650–680 °C for HRB600 steel. Based on the Johnson–Mehl–Avrami equation, phase transition kinetic models are established. Model predictions are consistent with the validation data. Thus, this study establishes a reference for studying ferrite formation during cooling.

Highlights

In recent years, to reduce energy and resource consumption, high-strength hot-rolled rebars, namely HRB500 and HRB600, have been developed [1,2] that will substitute for low-grade rebars, such as HRB335
The nucleation of ferrite in Fe-X-Y systems (X represents substitutional elements such as Mn, Si, Ni, Cr, Mo, Co, V, Ti, and Nb, and Y represents interstitial elements, such as C and N) has been studied extensively [5], and the investigations have demonstrated that interface character [6,7], carbides [8], austenite grain size [9,10], and compressive stress [11,12] influence ferrite formation
In this study, taking hot-rolled ribbed steel (HRB500 and HRB600 grade steels) as an example, we investigate the kinetics of ferrite formation in medium-carbon microalloy steel by controlling the cooling process and developing kinetics equations based on the Johnson–Mehl–Avrami equation

Summary

Introduction

To reduce energy and resource consumption, high-strength hot-rolled rebars, namely HRB500 (yield strength of ≥400 MP) and HRB600 (yield strength of ≥500 MPa), have been developed [1,2] that will substitute for low-grade rebars, such as HRB335. The transformation from austenite to ferrite in steel is very important because the morphology and volume fraction of ferrite in steels significantly impact its mechanical properties [3,4]. It is necessary to clearly understand the mechanism of transformation from austenite to ferrite during cooling, i.e., nucleation, thermodynamics, and kinetics. The nucleation of ferrite in Fe-X-Y systems (X represents substitutional elements such as Mn, Si, Ni, Cr, Mo, Co, V, Ti, and Nb, and Y represents interstitial elements, such as C and N) has been studied extensively [5], and the investigations have demonstrated that interface character [6,7], carbides [8], austenite grain size [9,10], and compressive stress [11,12] influence ferrite formation

Methods

Results

Discussion

Conclusion