Microstructures and mechanical properties of dual phase steel produced by laboratory simulated strip casting
Graphical abstract
Introduction
Dual phase (DP) steels are widely used in automotive industry due to a good combination of continuous yielding behaviour, high strength, high strain hardening rate, low yield stress-to-tensile strength ratio and good formability [1], [2], [3]. DP steels have high tensile strength in the range of 500–1200 MPa and total elongation in the range of 12–34%, which depend on fractions of ferrite, martensite and bainite [1], [4]. Traditional microstructures of DP steels consist of polygonal ferrite and martensite. To satisfy custom requirements, ferrite–bainite–martensite and ferrite–bainite steels were produced in order to modify mechanical properties: bainite instead of martensite were shown to improve formability with a little decrease of strength [5], [6], [7]. The effect of martensite fraction, distribution and martensite region size, and the effect of ferrite fraction and grain size on mechanical behaviour of DP steels have been intensively studied. With increasing the martensite fraction, the yield strength and ultimate tensile strength increase while uniform and total elongations decrease [8], [9]. The distribution of martensite also affects the mechanical behaviour [10], [11], [12]. Martensite regions existing as isolated areas within ferrite matrix result in a better combination of strength and ductility than martensite regions forming a chain-like network structure surrounding ferrite [10]. Refinement of ferrite or/and martensite regions simultaneously enhances strength and ductility [13], [14], [15], [16]. Ultrafine-grained DP steels with the average ferrite grain size of ~ 1.2 μm exhibit a high ultimate tensile strength up to 1000 MPa [13].
In industry, the DP steels are successfully produced using hot rolling and cold rolling & annealing [1]. As alternative, the strip casting could be suggested as a more economic and environmentally friendly way for DP steels production. This technology is already used for carbon steels, silicon steels and stainless steels in industry [17], [18]. It allows obtaining several millimetres thick strips directly from molten metals [17], [18]. Due to this and elimination of many subsequent hot rolling stages required by conventional continuous casting and thin slab casting, the strip casting process has many advantages, such as energy saving and emission reduction, lower capital and operating costs, a smaller and more flexible operating regime, a higher tolerance to a high residual scrap and easy adjustment for different steel grades [17], [18], [19], [20].
In this study, the feasibility of producing DP steel using strip casting was investigated in the laboratory. Heat treatment schedules were designed to obtain microstructures with 40%–90% ferrite. Mechanical properties were tested and compared to hot rolled DP steels. The correlation between microstructure and mechanical properties was analysed.
Section snippets
Materials and experimental techniques
The chemical composition of studied DP steel is shown in Table 1. Square specimens of 36 × 36 mm2 and 1.2 mm thickness and cylindrical specimens of 20 mm diameter and 6 mm length were produced via dip tester at Deakin University in order to simulate rapid cooling during solidification in strip casting process [21]. The dip tester is used to immerse a copper substrate into molten steel for a short and controlled period of time to simulate a rapid solidification. As-cast microstructure consisted
Prior austenite microstructure simulation
Fig. 4 shows typical prior austenite grain structures in as-cast and heat treated samples. PAGS distributions after different austenitisation schedules are shown in Fig. 5. PAGS increased with an increase in holding time or holding temperature: after holding at 1250 °C for 120 and 180 s, the average grain size was 48 ± 19 and 83 ± 37 μm respectively; while after holding at 1300 °C for 120 and 180 s, the average grain size was 72 ± 30 and 117 ± 44 μm respectively. The average grain size after holding at 1300
The effect of ferrite fraction on mechanical behaviour
In the studied steel YS increased with a decrease in ferrite fraction (an increase in martensite fraction) due to dislocation density in ferrite increasing with volume change accommodation of martensite formation [29]. There is non-uniform distribution of dislocations within ferrite grains: low density within the grains (Fig. 17a); medium density in the regions adjacent to ferrite–ferrite boundaries (Fig. 17b); high density in the regions adjacent to martensite due to accommodation of volume
Conclusion
The characterisation of microstructure and mechanical properties of DP steel produced by the laboratory simulated strip casting leads to the following conclusions:
- (1)
The developed processing route for simulation of the strip casting in laboratory, resulted in DP steel microstructures containing 40–90% polygonal ferrite, martensite and small amounts of bainite and Widmänstatten ferrite.
- (2)
For DP steel with nominal composition of 0.08C–0.81Si–1.47Mn–0.03Al wt.%, experimental CCT and TTT diagrams were
Acknowledgements
This project was supported by the Australian Research Council (DP130101887). The JEOL JSM-7001F FEG-SEM was funded by the Australian Research Council (LE0882613). The authors thank Dr. A.A. Gazder, UOW for modification of tensile stage.
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