Effect of a quenching–long partitioning treatment on the microstructure and mechanical properties of a 0.2C% bainitic steel

https://doi.org/10.1016/j.jmatprotec.2015.03.010Get rights and content

Highlights

  • A Q–LP treatment is proposed for large-scale steel components.

  • The Q–LP treatment can improve the impact toughness notably.

  • The Q–LP treated steel has a triplex-phase microstructure including bainite.

  • The retained austenite exhibits either a inter-lath plates or a film morphology.

Abstract

A quenching–long partitioning (Q–LP) heat treatment has been proposed for large-scale industrial production of steel components. Compared to the conventional quenching–tempering (Q–T) and quenching–partitioning (Q–P) process, the steel treated by air-quenching from austenitizing temperature and 60 min-partitioning at 200 °C exhibits the best combination of strength and toughness. With a nearly unchanged product of strength and elongation of around 18 GPa%, the room-temperature impact toughness has been significantly improved from 84 J/cm2 for the air Q–T treatment to 104 J/cm2 for the Q–LP treatment, which is attributed to the more retained austenite and the effective relief of the microstresses. The multi-phase microstructure of the Q–LP treated steel contains martensite, bainite and retained austenite exhibiting two different morphologies, i.e., one is the inter-lath plates, and the other is the austenite films within the bainite plates.

Introduction

One of the most important heat-treatment processes to improve the product of strength and elongation (PSE) of steel components is the quenching and partitioning (Q–P) treatment developed by Edmonds et al. (2006), though the idea of carbon partitioning to austenite has been proposed earlier by Speer et al. (2003). During the Q–P treatment, Edmonds et al. (2006) suggested that the specimens should be quenched from austenite-field temperature to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by a ‘partitioning’ treatment at or above the quenching temperature through which carbon can diffuse from the supersaturated martensite to the untransformed austenite, resulting in an increased amount of retained austenite phase stabilized to room temperature. According to Edmonds et al. (2011), if the competing reactions with respect to the carbon partitioning to the retained austenite, e.g., cementite precipitation, can be suppressed by appropriate alloying with elements such as Si or Al, microstructures with a certain amount of austenite can be retained through the Q–P process, and then a martensite/austenite multiphase structures with high strength and good plasticity combination can be obtained for the third generation advanced high strength sheet steels (AHSS). The potential of improving ductility by Q–P treatment has been recognized and explored widely by the pioneers in the AHSS field.

In order to emphasize the addition of carbide-forming elements such as Nb to explore the effect of grain refining and precipitation strengthening on the strength of Q–P steel, Hsu (2007) further proposed the quenching–partitioning–tempering (Q–P–T) process. Zhong et al. (2009) have conducted a Q–P–T treatment to a Fe–0.2C–1.5Mn–1.5Si–0.05Nb–0.13Mo steel and a good combination of strength (1500 MPa) and elongation (15%) was obtained, showing the beneficial effect of the Q–P–T process. Wang et al. (2011) have examined the strengthening and toughening mechanism of the Q–P–T process by examining the microstructure. They found that the optimized microstructure with multiphase and multiscale, i.e., the lath martensite (submicron-scale in thickness), the retained austenite (nano-scale in thickness for interlath film-like type and submicron in diameter for island-like type) and the carbide precipitates (nano-scale in diameter) obtained by the Q–P–T process are the origin of the good mechanical properties. However, most of the above works focus on the improved strength and plasticity by different processes and little attention has been paid on the toughness, which is important for the large components in the large-scale industrial use.

Considering the temperature of the Q–P process is not easy to control in industrial use, Yi et al. (2013) has proposed a process called quenching and tempering-associated partitioning (Q–T & P) treatment. By proper alloy design, the quenching temperature in the Q–T & P process can be designed at ambient temperature and the components can be lighter. Other process with the purpose of improving the combined properties also explore the use of multiphase structure of ferrite, martensite and austenite, such as the recently proposed dual stabilization heat treatment (DSHT) by Qu et al. (2013). However, the processing steps of the DSHT treatment are complex for applications in industry.

According to Edmonds et al. (2006), though Si or Al can suppress the precipitation of cementite, they are ineffective in suppressing the precipitation of the transitional epsilon carbide. As a result, during the partitioning step, the holding time is usually restricted to be less than 1 min to retard the transitional epsilon carbides precipitation, and therefore a maximum amount of retained austenite can be obtained. However, such a short partitioning time is hard to produce a uniform temperature distribution in large-scale steel components, thus it is still difficult for application in large-scale industrial production. Moreover, the considerable residual stress that exists in the quenched steel-components may not be relived effectively by the short partitioning process. As a result, the steel components may still have a poor toughness for applications in various fields.

On the other hand, besides martensite and the retained austenite, bainite would form during a longer partitioning time during the conventional Q–P or Q–P–T process as pointed by Bhadeshia and Honeycombe (2011). Though the bainite may have a positive effect on the combined mechanical properties, much less attention has been paid on that. As a result, the potential effect of bainite on the mechanical properties of steels has not been fully dug out in the above-mentioned processes. Recently, Gao et al. (2013) have proposed a two-step Q–P–T process for a Mn–Si–Cr alloyed steel containing 0.4%C, and a good combined property (with a PSE of 31.4 GP%) has been obtained without using expensive alloying elements. The excellent properties were attributed to the particular triplex microstructure containing carbide-free bainite, martensite and various types of retained austenite. Moreover, the best strength/plasticity combination was achieved after tens of minutes of partitioning treatment, which is easier for application to a large-scale industrial production. However, compared to the one-step treatment, the two-step method may still be difficult to control.

In the present study, in order to seek a more simple Q–P(–T) process for large steel components in industrial use, a one-step Q–P treatment with a longer partitioning time (denoted Q–LP hereafter) has been proposed to a Mn–Si–Cr–Mo bainitic steel containing 0.2%C. In order to obtain a certain amount of bainite at the air-quenching process, which is easy to be realized in the large-industry use, the composition of the steel has been designed in a fixed range. The effects of the partitioning temperature and partitioning time on the strength, plasticity and the impact toughness of the steel have also been investigated systematically.

Section snippets

Experimental procedure

The nominal chemical composition of the steel under investigation is Fe–0.2C–2.2Mn–1.3Si–1.1Cr–0.25Mo–0.25Ni in weight percent. Mn and Cr were added to lower the bainite transformation temperature, while Ni and Mo were added to improve the toughness of the alloy. The Ms temperature was estimated by the equation Ms = 539–423C–30.4Mn–17.7Ni–12.1Cr–7.5Mo according to Andrews (1965), i.e., it is about 368 °C for the investigated steel. The amount of martensite at different quenching temperatures can

Mechanical properties of the steel after different treatment

The mechanical properties of the steel after different heat-treatments are shown in Table 1. It can be seen that the samples after conventional water Q–T process has a maximum tensile strength of 1400 MPa, and an elongation of 12.1%. If the austenitized sample is quenched by air, the strength will decrease a little with a slight increase in elongation. Compared with the water-quenched samples, the air-quenched ones have a better combination of strength and elongation, as can be seen from the PSE

Conclusions

A one-step Q–P treatment with prolonged partitioning time (Q–LP) has been proposed for large-scale steel components production in industrial use. The effect of partitioning temperature and time on the mechanical properties and microstructure of a 0.2C–2.2Mn–1.3Si–1.1Cr–0.25Ni–0.25Mo steel has been investigated. Several conclusions have been drawn as follows:

  • (1)

    Compared to the conventional Q–T process, the Q–LP treatment with a partitioning time of 60 min does not change the PSE value much, but the

Acknowledgement

The present study is financially supported from the Scientific Research Foundation for Young Teachers of Sichuan University (No. 2014SCU11019), Science and Technology Department of Sichuan Province, China (No. 2014GZ0087), Science &Technology and Intellectual Property Department of Panzhihua, China (Grant No. 2014CY-C-3) and National Natural Science Foundation of China (No. 51401135). The authors would express their great gratitude to the reviewers for their constructive suggestions.

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