Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel
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TRODUCTION
IMPROVED safety standards and a trend toward automotive body-in-white weight containment have led to a strong interest in advanced high strength steels (AHSS) such as transformation-induced plasticity (TRIP) steel. The Zn coating of TRIP steel by means of hot dip galvanizing (HDG) and galvannealing is a key factor which determines whether TRIP steel can be used in automotive body-in-white construction, as thin TRIP steel sheets used for lighter car bodies should be resistant to perforation corrosion. The HDG process is used to produce pure Zn (g)coated steel by the hot dipping of recrystallizationannealed strips into an Al-containing Zn bath. The Al content of the Zn bath for galvanized sheet production is typically 0.2 mass pct Al. Production of galvannealed (GA) sheet steel consists of two processes: the galvanizing itself and the subsequent short diffusion annealing process lasting ~20 s in the range from 763 K to 823 K LAWRENCE CHO, Graduate Student, and BRUNO C. DE COOMAN, Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, 790-784, South Korea. Contact e-mail: [email protected] MYUNG SOO KIM and YOUNG HA KIM, Researchers, are with the POSCO Technical Research Laboratories, Gwangyang, 545-090, South Korea. Manuscript submitted April 10, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(490 C to 550 C). The latter process step transforms the Zn layer into an Fe-Zn alloy layer by interdiffusion and reaction of Fe and Zn. This results in the formation of Fe-Zn intermetallic compounds. The reaction between Fe in the solid state and Zn in the liquid state during the galvannealing results in the formation of the coating layer which may contain the following Fe-Zn intermetallic compounds: zeta (f)FeZn13, delta (d)-FeZn10, gamma1 (U1)-Fe11Zn40, and gamma (U)-Fe3Zn10 phases. The Fe content and structure of the Fe-Zn intermetallic compounds in GA coating should be properly controlled to minimize powdering. Powdering is the delamination of the coating during forming operations such as deep drawing. Coatings with a good powdering resistance have been reported to consist of a layer of U phase 2), more particle-type crystalline c-xMnOÆSiO2 (x > 1) oxides are formed on the surface than film-type amorphous a-SiO2 or a-xMnOÆSiO2 (x < 1) oxides. In general, oxides with a particle-type morphology are known to be less harmful to the wettability than filmtype oxides. The Mn/Si mass pct ratio of the CMnSi TRIP steel was 1.57 and bare spots were not present in the Zn coating surface after the galvanizing simulation.[15] The cold-rolled TRIP steel had a thickness of 1.2 mm and panels with dimensions of 220 9 120 mm were cut from the sheet. The panels were annealed and
GA in an IWATANI SURTEC HDG simulator (IWATANI Inc., Germany). This HDG simulator consists of a hot dipping simulation unit, a gas mixing station, a hybrid humidification system, and a control system. In the heating section of the main unit, an infrared furnace was used for the continuous a
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