On Factors Affecting the Phase Transformation and Mechanical Properties of Cold-Rolled Transformation-Induced-Plasticity
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TION
THE remarkable strength-ductility balance in transformation-induced-plasticity (TRIP) steel results from the occurrence of the TRIP phenomenon during deformation.[1] The coexistence of austenite with a certain microstructural stability is of vital importance in order for this phenomenon to occur and, hence, to achieve the desired properties. The austenite retention is usually obtained by combining the effects of chemical composition and typical heat treatment. In this respect, adding large amounts of silicon to TRIP steel ensures that cementite precipitation is unlikely to occur in the microstructure during bainite formation.[2] The absence of cementite ensures that the carbon will enrich the austenite rather than form cementite plates. Therefore, after the bainite transformation finishes by further cooling to room temperature (RT), the austenite is stabilized. Jeong et al. concluded that austenite retention in these low-alloyed steels is almost impossible with silicon concentrations much below 1 wt pct.[3] However, these high required silicon levels are outside standard industrial practice for producing flat products because of the following.
MOHAMED SOLIMAN, Doctor, and HEINZ PALKOWSKI, Professor of Metal Forming, are with the Institute of Metallurgy, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted February 24, 2008. Article published online July 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
(a) Steel with more than 1 pct Si has a poor Zn coating quality after continuous galvanizing, due to the presence of Si-Mn oxides on the strip surface.[4] (b) The high Si content of these steels causes red scales to form in bands. After pickling, the oxides are completely removed, but the band remains visible on the surface of the pickled steel.[5] Consequently, studies have been performed with other elements that can substitute for the role of silicon. Presently, aluminum seems to be the most promising candidate. However, in addition to the fact that high aluminum content in steel causes serious casting problems, a full substitution of silicon by an equivalent amount of aluminum leads to a marked deterioration of the strength-ductility balance.[6,7] De-Meyer et al.[8] proposed that silicon can best be partially replaced by aluminum with an increase in the carbon content. Bleck also suggested that combining silicon, aluminum, and phosphorus is a reasonable compromise and could be the most important alloying concept for low-alloyed TRIP steels.[9] Little or no information can be found in the literature about the sole effect of aluminum content on the phase transformation and mechanical properties of the TRIPaided steels. De-Meyer et al.[8] and E. Emadoddin et al.[10] presented some results about Si-Al-alloyed TRIP steels. However, their comparison of the number of phases and the mechanical properties is misleading, because the carbon content was different in their alloys. Carbon is the main alloying element by which both all tr
