Influence of bainite/martensite-content on the tensile properties of low carbon dual-phase steels

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Abstract

This investigation aims to examine structure–property relations of ferrite–bainite dual-phase (FBDP) steels and to compare these against that of ferrite–martensite dual-phase (FMDP) steels. For this purpose, a series of FBDP and FMDP steels containing wide variation (20–90%) of the harder constituents have been prepared from a low carbon Nb-micro-alloyed base material by suitable heat treatments. Hardness and tensile properties of the developed steels have been examined against the volume fraction of bainite or martensite. The nature of variation of the estimated mechanical properties such as hardness, yield and tensile strength, percentage elongation and strain-hardening exponent with the amount of the harder constituents of the FBDP and FMDP steels exhibits subtle to significant differences. These differences have been explained using the influence of the nature of the microstructural constituents and their mutual interactions. Low carbon FBDP steel with 60–70% bainite appears to possess excellent potential for structural applications.

Introduction

Dual-phase steels developed over the past few decades offer impressive mechanical properties, such as continuous yielding behaviour and superior strength–ductility combination, in addition to the advantage of reduced cost, better formability, and excellent surface finish over other high-strength low-alloy (HSLA) steels of similar chemistry [1]. As a consequence, these steels find wide applications in automobile components, such as car body panels, wheels, bumpers [2], [3], etc. in which the current drive is to reduce weight and to achieve higher crash resistance [4]. Dual-phase steels usually consist of some specific volume fraction of high-strength phase, such as martensite or bainite, contained within a softer matrix, ferrite [5]. The chemical composition, specifically the micro-alloying elements, plays an important role and affects the structure and properties of these steels. The influence of the different types of micro-alloying elements on the mechanical properties of ferrite–martensite [6], [7], [8] and ferrite–bainite [9], [10], [11] steels has been discussed in several earlier reports and has been the major issue for a number of conferences (e.g. Refs. [12], [13], [14]). These steels are produced by either intercritical annealing or controlled rolling [4], [15]. Imposition of slow cooling rate during controlled rolling process is favourable for the bainite transformation [16]. However, investigations on ferrite–martensite steels are extensive compared with that on the structure–property correlations of ferrite–bainite dual-phase (FBDP) steels. In addition, reports related to structure–property relations of ferrite–bainite dual-phase steels of low carbon (usually <0.1 wt% C) grades are limited. The present report is aimed to understand the structure–property (tensile) relations of ferrite–bainite steels vis-à-vis ferrite–martensite steels of low carbon variety containing higher volume fractions of the harder phase.

Only a few investigators have discussed the salient aspects of the structure–property relations of low carbon ferrite–bainite (LCFB) steels. Sudo et al. [17], [18] have examined a few LCFB steels, and have reported that increase in bainite content in these steels generally increases their yield ratio (ratio of yield strength to tensile strength), reduction of area and fatigue endurance limit. The effect of bainite content on ferrite–bainite–martensite steels, on the other hand, has been examined by Sudo et al. [17], [18], Kim et al. [19] and Choi et al. [20]. Sudo and Iwai [17] have indicated that decrease in bainite content in ferrite–bainite–martensite steels, causes lowering of strength and yield ratio, but leads to improved percentage elongation and strain-hardening exponent. Kim et al. [19] have suggested that small amounts of bainite in ferrite–martensite dual-phase (FMDP) steels leads to improvement in yield strength and ductility but deterioration in tensile strength. These investigators have also reported the occurrence of discontinuous yielding behaviour in these multiphase steels. Discontinuous yielding in three phase steels at slow strain rates has also been observed by Choi et al. [20]. In addition, attempts have also been made to study the effect of bainite on TRIP aided dual-phase steels [21], [11]. However, systematic studies on LCFB dual-phase steels with wide variation of bainite are lacking.

This investigation presents the characteristics of a series of LCFB dual-phase microstructures containing 50–90% bainite, prepared by varying the heat treatment of a Nb-bearing steel. The effect of the amount of bainite on the tensile behaviour of these steels has also been ascertained and the results have been examined in the light of the available literature. The structure–property relations of the LCFB steels has been compared with that of a series of ferrite–martensite steels of identical chemistry, but prepared by different heat treatment schedule.

Section snippets

Material and heat treatments

A commercial low carbon niobium-micro-alloyed steel has been used in this investigation. The steel was available in the form of 30 mm thick plates cut from a hot rolled transfer bar. The chemical composition of the steel is shown in Table 1, whereas its microstructure in the as-received state is shown in Fig. 1.

Cylindrical bars of 10 mm diameter and 120 mm length were cut from the steel plates and were first subjected to heat treatment for achieving ferrite–bainite structures. This heat treatment

Heat treatment and microstructures

The time–temperature-transformation (TTT) diagram of the steel under investigation has been computed using the formalism developed by Bhadeshia [25]. The TTT diagram of the steel, shown in Fig. 3, was used to design the heat treatment schedules (Fig. 2) for obtaining the various dual-phase microstructures. A schematic cooling curve is superimposed on the TTT diagram for illustration. Some typical microstructures of the ferrite–bainite and ferrite–martensite dual-phase steels are shown in Fig. 4

On the nature of microstructural constituents

Ferrite–bainite dual-phase steels have been produced by isothermal treatment in the bainitic transformation range. Isothermal transformation temperature was selected based on the calculated bainitic (Bs) and martensitic start temperatures (Ms) shown in Fig. 3. The Bs and the Ms temperatures for the selected steel were found to be 590 and 475 °C, respectively. However, after carbon enrichment of austenite due to ferrite formation during air-cooling, the Bs and the Ms temperatures are expected to

Conclusions

The structure–property relations of a series of ferrite–bainite and ferrite–martensite dual-phase steels made from low carbon Nb-bearing base material have been examined. The results of the investigation lead to the following conclusions:

  • (1)

    Macro-hardness of the developed FBDP steels remain almost constant for bainite content up to 50%, beyond which it increases with increasing bainite content. Further micro-hardness of bainite region decreases while that of ferrite phase increases with increasing

Acknowledgements

The authors gratefully acknowledge the help of M/s Tata Steel for the supply of the steel for this investigation. One of the authors (SBS) gratefully acknowledges the financial support received from Department of Science and Technology, Ministry of Science and Technology, Govt. of India, under scheme no. SR/FTP/ETA-279/01.

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