Processing–Microstructure Relation of Deformed and Partitioned (D&amp;P) Steels

Li Liu,Mingxin Huang,Binbin He

doi:10.3390/met9060695

Abstract

An ultrastrong and ductile deformed and partitioned (D&P) steel developed by dislocation engineering has been reported recently. However, the microstructure evolution during the D&P processes has not yet been fully understood. The present paper aims to elucidate the process–microstructure relation in D&P process. Specifically, the evolution of phase fraction and microstructure during the corresponding D&P process are captured by means of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Subsequently, the effect of partitioning temperature on dislocation density and mechanical properties of D&P steel is investigated with the assistance of uniaxial tensile tests and synchrotron X-ray diffraction. It is found that a heterogeneous microstructure is firstly realized by hot rolling. The warm rolling is crucial in introducing dislocations, while deformation-induced martensite is mainly formed during cold rolling. The dislocation density of the D&P steel gradually decreases with the increase of partitioning temperature, while the high yield strength is maintained owing to the bake hardening. The ductility is firstly enhanced while then deteriorated by increasing partitioning temperature due to the strong interaction between dislocation and interstitial atoms at higher partitioning temperatures.

Highlights

Strong and ductile metallic materials are ideal to develop high-performance yet energy-efficient structural components for many applications [1,2]
The volume fraction of austenite was slightly decreased after intercritical annealing, indicating that ferrite transformation is negligible during intercritical annealed (IA)
Around 18% of austenite was retained after cold rolling and it basically remained unchanged during subsequent partitioning retained after cold rolling and it basically remained unchanged during subsequent partitioning process, process, confirming that almost no phase transformations took place during low-temperature confirming that almost no phase transformations took place during low-temperature partitioning partitioning process (Figure 2b)

Summary

Introduction

Strong and ductile metallic materials are ideal to develop high-performance yet energy-efficient structural components for many applications [1,2]. Strength and ductility are, in general, mutually exclusive in metallic materials [3,4]. Alloying by the addition of cobalt and titanium is an effective way to simultaneously improve the strength and ductility. This strategy is not cost-efficient for the industrial application and not sustainable for the limited resources on earth [5]. The retained austenite can provide transformation-induced plasticity (TRIP) effect to improve strain hardening rate, delaying the onset of necking and leading to the high strength and good ductility [9]. Ductility, which requires the glide of Metals 2019, 9, 695; doi:10.3390/met9060695 www.mdpi.com/journal/metals

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Discussion

Conclusion