The Effect of Direct Strip Casting on the Kinetics of Phase Transformation of a Dual Phase Steel

Nam Mai,Christiane Schulz,Nikki Stanford

doi:10.3390/met12020170

Abstract

A dual phase steel has been produced directly from the liquid under conditions that simulate direct strip casting and thin slab casting. The kinetics of polygonal ferrite formation during the inter-critical anneal were quantified using the JMAK approach, and this revealed significantly retarded transformation kinetics in the strip cast samples compared to the commercial steel that was processed through the conventional hot rolling approach. The transformation rate in the strip cast samples were as much as three orders of magnitude slower compared to the commercial steel. It was found that the kinetics of the ferrite formation were retarded principally by the large prior austenite grain size in the strip cast samples, and this hypothesis was tested experimentally by both coarsening of the prior austenite grain size, and by refinement of the prior austenite grain size. However, even after grain size normalization, small differences in transformation kinetics between the direct strip cast and commercial steel specimens were observed. These differences were explained by investigation of MnS precipitation in the steels. It was found that the transformation rate is high when the solutes are in solid solution, and that the rate of transformation slows significantly when precipitation of nano-precipitates occurs.

Highlights

Dual phase steels (DP steels) have a microstructure containing a mixture of soft ferrite grains and hard martensite islands
Two different microstructures were observed. For those samples that were isothermally held at temperatures above 550 ◦ C, polygonal ferrite formed at the prior austenite grain boundaries during the inter-critical annealing, and upon water quenching the remaining austenite transformed to martensite
The microstructure and mechanical properties of dual phase steel produced by simulated thin slab casting (TSC) and direct strip casting (DSC) have been compared with those from a commercially sourced steel of the same composition

Summary

Introduction

Dual phase steels (DP steels) have a microstructure containing a mixture of soft ferrite grains and hard martensite islands. In the case of dual phase steels, the microstructures that will develop during rapid cooling are not known, and their behaviour during secondary processing has only been studied briefly [7]. For the case of strip cast material, the most energy efficient method to make a DP steel is to perform the inter-critical annealing step immediately after casting. It is experimentally challenging to simulate this process: the steel must be rapidly solidified, liberated from its casting, and placed into the inter-critical annealing furnace before the specimen cools to the ferrite transformation temperature. In the present work, direct strip casting and thin slab casting of a dual phase steel is simulated, and the specimens placed into the inter-critical annealing furnace before the beginning of ferrite transformation

Methods

Results

Discussion

Conclusion