Structure-property relation in a quenched-partitioned low alloy steel involving bainite transformation

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Abstract

The impact of bainite transformation during initial quenching and partitioning steps on the microstructural evolution was studied in a Fe-0.4C-2.0Mn-1.7Si-0.4Cr (wt%) steel. By optimizing quenching cooling rate and partitioning time, the final microstructure comprised of initial-quenched bainite, carbon-depleted martensite, bainite formed during partitioning, and final-quenched martensite, together with retained austenite. High volume fraction of retained austenite with desired carbon-content was obtained by prolonging the partitioning time to 2700 s The initial-quenched bainite, bainite formed during partitioning, and martensite provided carbon atoms to austenite, leading to the formation of retained austenite with different degree of stability. Consequently, a good combination of strength and elongation (ultimate tensile strength: 1688 MPa, total elongation: 25.2%) was obtained.

Introduction

There is currently a strong demand for high strength steels with good ductility to reduce the weight of steel components for automotive and railway applications. Advanced high strength steels (AHSS) with adequate toughness and formability, such as dual-phase (DP), transformation induced plasticity (TRIP) and multiphase steels are being developed to meet the demands of the automotive industry [1], [2]. Quenching and partitioning (Q&P) process is one of the most promising methods to obtain a good combination of strength and ductility [3], [4], [5], [6]. Generally, the Q&P process consists of two steps: the first step involves quenching the steel to a temperature (Tq) between martensite-start (Ms) temperature and martensite-finish (Mf) temperature after partial or completed austenitization such that carbon-supersaturated martensite and untransformed austenite are obtained. Subsequently, the steel is held at Tq or slightly above Tq to promote carbon diffusion from martensite to austenite and render retained austenite stable. Based on the Q&P theory, a modified quenching, partitioning and tempering (Q-P-T) processes was proposed to combine the TRIP effect with precipitation strengthening, which further improves the mechanical properties [7], [8]. The earlier studies of Q&P or Q-P-T process suggested that carbon-depleted martensite and retained austenite was obtained in the absence of bainite transformation during the partitioning process [1], [2], [3], [4], [5], [6], [7], [8]. It was also suggested that the formation of bainite during the Q&P process was favorable for further improvement of strength, ductility and toughness through the introduction of additional carbon-enriched retained austenite with high stability [9], [10], [11], [12].

The Q&P or Q-P-T process involving the formation of carbide-free bainite during quenching step (abbreviated as bainite-based Q&P, i.e., BQ&P or BQ-P-T process) can significantly improve the mechanical properties of steels [9], [12]. The multi-phase microstructure of BQ&P steels comprised of bainite, martensite and retained austenite, resulting in high product of tensile strength and elongation (PSE) of ~42 GPa%. However, the effect of carbide-free bainite on microstructural evolution continues to be unclear and requires further studies.

Numerous studies have reported the presence of bainite transformation during Q&P treatment [10], [11], [12], [13], [14], but there still exists some uncertainty on the role of bainite transformation. One view point is that the bainite transformation has a competing effect with respect to carbon partitioning from martensite to austenite. Consequently, the carbon enrichment is delayed and the volume fraction of retained austenite is reduced [13], [14]. On the other hand, the formation of carbide-free bainite plays the same role as in TRIP treatment, contributing to the stabilization of austenite through additional carbon diffusion during partitioning [9], [10], [11], [12]. Thus, it is important to understand bainite transformation and how it influences microstructure during the partitioning step in Q&P steels.

In this study, a Fe-0.4C-2.0Mn-1.7Si-0.4Cr steel was designed to obtain the best combination of mechanical properties via Q&P process involving bainite transformation. The objective is to explore the role of bainite transformation on microstructural evolution, especially the formation of retained austenite in the attempt to acquire a better understanding of the Q&P process.

Section snippets

Experimental procedure

The nominal chemical composition of experimental steel in wt% was Fe-0.4 C-2.0Mn-1.7Si-0.4Cr. The composition was designed on the basis of Mn-containing bainitic steel [15], [16]. Si was added to prevent the formation of carbides and promote the formation of carbide-free bainite during quenching and partitioning steps. A 50 kg ingot of the experimental steel was cast, reheated at 1200 °C and forged to 30 mm×80 mm×500 mm dimension with a finish-forging temperature of 950 °C. The forged plate was

Microstructure prior to partitioning step

The phase volume fraction and morphology prior to the partitioning step has a significant influence on the final microstructure. In this regard, the fraction of bainite and martensite measured by dilatometer as a function of quenching temperature (Tq) is presented in Fig. 2. The dilatometric curves indicated that a fully martensitic structure was obtained when the cooling rate was 100 °C/s (Ms=252 °C), whereas bainite transformation occurred when the cooling rate was 1 °C/s. The volume fraction of

Conclusions

In the present study, quenching and partitioning process involving the formation of bainite (i. e. BQ&P process) was applied to a Fe-0.4C-2.0Mn-1.7Si-0.4Cr (wt%) steel. Multiphase microstructure was developed by optimizing quenching cooling rate and partitioning time: bainite formed during initial quenching, carbon-depleted martensite, bainite formed during partitioning, ‘fresh’ martensite formed during final quenching after partitioning and different types of retained austenite. This

Acknowledgements

This research is supported by National Natural Science Foundation of China (No. 51301012). One of authors (RDKM) gratefully acknowledges support from the University of Texas at El Paso, USA.

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