Galvanizing and Galvannealing Behavior of CMnSiCr Dual-Phase Steels
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WITH the increasing demand in lightweight automobiles with improved fuel efficiency, advance highstrength steel (AHSS) has gained ever-increasing attention for car bodies. The high strength of dual-phase (DP) steel and transformation-induced plasticity (TRIP) steel allows a thinner strip and guarantees the passengers’ safety.[1] To enhance the corrosion protection properties, hot-dip galvanizing (GI) or galvannealing (GA) treatment is essential for steels. The GI steels are usually produced by dipping a steel sheet into a molten Zn bath with approximately 0.1 to 0.3 wt pct Al at a temperature of around 723 K (450 °C). It is generally recognized that the addition of Al can produce a ductile Zn coating via the formation of an Fe-Al inhibition layer, which suppresses the formation of brittle Fe-Zn phases[2,3] The relevant reaction for the formation of the Fe-Al inhibition layer is briefly shown below:[4] 2Fe þ 5½Albath ! Fe2 Al5
½1
KO-CHUN LIN, Ph.D. Candidate, PENG-WEI CHU, Undergraduate Student, and CHAO-SUNG LIN, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan (ROC). Contact e-mail: csclin@ntu. edu.tw HON-BOR CHEN, formerly Senior Engineer with the China Steel Corporation, Kaohsiung 812, Taiwan, is now retired. Manuscript submitted November 4, 2012. Article published online January 24, 2013 2690—VOLUME 44A, JUNE 2013
The DP steels are generally strengthened by adding alloying elements such as Mn, Si, and Cr. However, these elements can be troublesome for the GI and GA treatments, especially the Si.[5,6] Most of the oxides can be reduced by aluminothermic reduction during the hot dipping process;[7] however, Si and/or Mn oxides cannot be completely reduced.[8,9] Remnant oxides tend to slow down the GA reactions.[3,4] The Si often causes oxide scales that are difficult to remove during the continuous GI process. For example, Mn2SiO4 has been found to block the interaction between the steel substrate and Zn coating, resulting in bare spots. External oxides generally have a harmful effect on the wettability between the steels and the molten Zn.[5–7,10,11] The Si also accounts for the well-known Sandelin effect, specifically the poor coating adhesion, non-uniform coating thickness, and the absence of a uniform intermetallic compounds’ (IMCs) layer.[12–15] Researchers have employed the electron microprobe measurement and thermodynamic calculation to analyze the solubility of Si in the Fe-Zn IMCs. It was found that the Si is almost insoluble in the f phase and C phase, but can be dissolved up to about 1 at.pct and 0.3 at.pct, respectively, in the d phase and C1 phase at 723 K (450°C).[14–16] Accordingly, the distinct morphology and crystallization mechanism of the various Fe-Zn IMCs in GI and GA steels can be seen in the literature.[17–19] In this study, hot-dip galvanized and galvannealed coatings were made on two kinds of Si-containing DP steel sheets and a traditional interstitial-free (IF) steel to METALLURGICAL AND MATERIALS TRANSACTIONS A
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