On the Processing of Martensitic Steels in Continuous Galvanizing Lines: Part II
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INTRODUCTION
CURRENT automotive body-in-white designs place a great emphasis on passenger safety aspects. Whereas dual-phase (DP) and transformation-induced plasticity (TRIP) steels are ideally suited for energy absorption parts for frontal impact, ultra-high-strength martensitic grades must be used for anti-intrusion barriers to protect passengers in the case of side impact or rollover. In spite of the great demand, however, the use of ultra-highstrength steel (UHSS) in the automotive industry has been limited due to the difficulties in press forming. Press hardening or hot stamping, therefore, was widely used to produce UHSS for automotive bodies. Recent developments in roll forming technology have expanded the application of UHSS in automotive industries, making it possible to produce ultra-high-strength automotive components with high productivity and low production cost. Combined with a high strength level, the corrosion resistance must also be ensured for materials used as parts of automotive components to guarantee long-term vehicle durability. In this regard, continuous hot-dip galvanizing (GI) or galvannealing (GA) compatible martensitic steel is a promising material, as it provides not only ultrahigh strength, but also a cost-effective approach for obtaining corrosion resistance. Two thermal cycle concepts were proposed to obtain a zinc-coated martensitic steel. In the first concept, the TAEJIN SONG, Graduate Student, and B.C. DE COOMAN, Professor, are with the Graduate Institute of Ferrous Technology, Materials Design Laboratory, POSTECH, Pohang 790-784, South Korea. JAIHYUN KWAK, Principal Researcher, is with the Technical Research Laboratory, POSCO, Gwangyang 545-711, South Korea. Contact e-mail: [email protected] Manuscript submitted March 15, 2011. Article published online August 12, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
coating process was carried out on the pretransformed martensitic steel, as described in the companion article. The results showed that the tempering phenomena taking place during GI and GA significantly degraded the mechanical properties of martensite formed prior to these processes. The yield strength increased, and the steel deformed by the propagation of localized strain. The strain hardening rate decreased considerably, leading to the loss of tensile strength. In the second concept, which is discussed in the present article, the coating process is carried out on the untransformed austenitic steel and the martensitic transformation occurs during the final cooling from the coating process temperature, as described in the schematic shown in Figure 1(a). In this concept, the austenite decomposition is effectively suppressed during the coating processes. Thus, an alloy design suppressing the austenite decomposition during cooling, isothermal holding, hot dipping, and Fe-Zn alloying is essential. The hardenability concept used in this study is schematically illustrated in Figure 1(b).[1] As a strong austenite stabilizing element, the addition of carbon suppresses all the au
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