Deformation characteristics of retained ferrite and transformed ferrite in a dual-phase steel
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I.
INTRODUCTION
STEELS whose structure consists of a mixture of ferrite and martensite are commonly referred to as dual-phase steels, l The deformation characteristics of dual-phase steels are known to be different from those of normal high strength low alloy (HSLA) steels in that at equivalent tensile strength levels, dual-phase steels exhibit a much higher ductility. 2'3'4 Two types of ferrite are present in a dual-phase steel structure. Retained ferrite is the ferrite phase which existed during the intercritical hold, and that formed during cooling is called transformed ferrite.5-8 Matlock et al. 6 indicated that the transformed ferrite might grow epitaxially on the retained ferrite into the austenite on cooling after intercritical annealing. Recently, Jeong and Kim 7 reported that the growth behavior of the transformed ferrite could vary with the cooling rate and that this variation in the growth behavior of the transformed ferrite resulted in different microstructures. It is then realistic to expect that such changes in the microstructure caused by different growth behaviors of transformed ferrite also influence the deformation behaviors of dual-phase steels. The aim of the present study is to gain an understanding of the deformation behavior of dual-phase steels by observing the difference in deformation characteristics between retained ferrite and transformed ferrite.
II.
2 hours, followed by furnace-cooling. Then they were intercritically annealed at 830 °C for 30 minutes in a flowing argon atmosphere in an electric tube furnace and cooled at a rate of 0.1 °C/sec or 5.6 °C/sec to intermediate temperatures, and water-quenched. The average grain size of the retained ferrite produced by this heat treatment was about 55/xm. This coarse-grained microstructure facilitated accurate observation of the deformation behaviors of the two different types of ferrite. The gage length portions of the intercritically heat-treated tensile specimens were polished to metallographic quality, etched in 2 pct nital, and stained with a boiling (about 118 °C) alkaline chromate solution (8 g CrO3 and 40 g NaOH in 72 ml distilled water). With this treatment, martensite phase appears black, retained ferrite appears gray, and transformed ferrite, white. 6 Optical micrographs were taken of the polished areas of the dualphase steel specimens before straining and after applying various levels of tensile strain. The tensile strains were applied at a cross-head speed of 2 mm/min. Point counting methods were used on photomicrographs to determine the volume fractions of the different phases.
III.
RESULTS AND DISCUSSION
The starting microstructure which consisted of a coarsegrained ferrite-pearlite aggregate is shown in Figure 1. The volume fraction and grain size of ferrite was about 0.84 and
EXPERIMENTAL P R O C E D U R E
The steel used in this study was prepared as a 22.6 kg, air induction melted heat. The chemical composition of the steel in weight percent was 0.13 pct C, 0.63 pct Si, 1.42 pct Mn, 0.16 pct Mo, and the balance, Fe.
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