Dependency of Natural Aging on the Ferritfoe Grain Size in Dual-Phase Steel
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grain size on the enhancement of mechanical properties of metallic materials is well known. Severe plastic deformation (SPD), thermomechanical processing, and thermal treatments are used for grain refinement.[1–3] Regarding low-carbon steels, remarkable grain refinement can be achieved by cold deformation and annealing of martensite.[4,5] The effect of intercritical annealing on the enhancement of tensile properties of low-carbon steels is also well known, which results in the formation of ferritic–martensitic dualphase (DP) microstructure.[6–9] Similarly, the intercritical annealing of the cold-rolled martensite is known as one of the most effective methods for grain refinement of DP steels compared to those obtained by intercritical annealing of undeformed microstructures, which results in the enhancement of work-hardening behavior and tensile properties.[10–12] Another important approach for altering the mechanical properties is the bake hardening (BH) effect of DP steel at temperatures above ambient temperature after prestraining.[13–18] Nevertheless, is has been shown that the yield stress of DP steel can be significantly increased
MEHRAN ZAMANI, HAMED MIRZADEH, and HAMID M. GHASEMI are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted March 25, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
via aging at ambient temperature due to the formation of fine precipitates in ferrite through quench aging, where this effect is pronounced for low-silicon steels.[19,20] However, the influence of ferrite grain size of DP steel on the aging behavior and their synergistic effects on the mechanical properties need to be investigated, which is the aim of the present study. A low-carbon ST-12 steel sheet (0.035C-0.005N0.035Si-0.268Mn, wt pct) was received in the normalized condition. Different processing routes were followed as shown in Figure 1 with complete details for obtaining DP steels with fine (DPf), medium (DPm), and coarse (DPc) ferrite grain sizes. As demonstrated and fully characterized in the previous study on the same steel,[20] the highest level of natural aging was obtained at ~ 216 hours for all samples, and hence, its details are not discussed in the present study. Accordingly, the aged samples were represented as DPfa, DPma, and DPca. Optical microscopy after etching by the LePera and Nital solutions was used for revealing the microstructure of the samples. The JIS Z 2201 tensile samples with gauge length of 10.4 mm were used, and tensile testing at room temperature with the constant cross-head speed of 1 mm/min was carried out, where for the as-quenched samples, the elapsed time from quenching to tensile testing was less than 30 minutes. Figure 2 shows the microstructure of different samples. The average ferrite grain sizes of DPf, DPm, and DPc were determined as 11.8, 32.8, and 63.6 lm, respectively. The average sizes of martensite islands (with volume fraction of ~ 0.13) are
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