Influence of Gas Atmosphere Dew Point on the Selective Oxidation and the Reactive Wetting During Hot Dip Galvanizing of
- PDF / 1,126,210 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 41 Downloads / 209 Views
CTION
IMPROVED safety standards and reduced automotive body-in-white weight have led to a strong interest in advanced high-strength steels (AHSS) such as transformation-induced plasticity (TRIP) steel. The galvanizability of TRIP steels by means of hot dip galvanizing (HDG) 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 gas atmosphere in annealing furnaces of continuous HDG lines reduces the iron oxides formed during cold rolling to elemental Fe. The alloying elements, Mn, Si, and Al are subject to selective oxidation as they have a high affinity for oxygen. This selective oxidation occurs at the steel surface in the low dew point (DP) gas atmosphere LAWRENCE CHO, Ph.D. Student, SEOK JAE LEE, Professor (Assistant), and BRUNO C. DE COOMAN, Professor (Supervisor), are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact email: decooman@postech. ac.kr MYUNG SOO KIM, Senior Principal Researcher, and YOUNG HA KIM, Senior Researcher, are with the POSCO Technical Research Laboratories, Gwangyang, 545-090, South Korea. Manuscript submitted December 15, 2011. Article published online September 12, 2012 362—VOLUME 44A, JANUARY 2013
of HDG lines operated in standard conditions. The presence of film-forming surface oxides, in particular, the amorphous a-xMnOÆSiO2 (x < 0.9) and a-SiO2 oxides, leads to a deterioration of the wettability of the intercritically annealed strip by the molten Zn and prevents the formation of the Fe2Al5 xZnx inhibition layer at the steel surface.[1–3] This selective oxidation phenomenon occurring at the surfaces and in the subsurfaces of most industrial steel grades has been investigated for interstitial free steel, dual phase steel, and TRIP steel during the last decade.[4–10] Three methods which have been proposed to improve the quality of the hot dip Zn coatings are shown in Figure 1. The first method consists of annealing the strip in a high DP atmosphere, which results in internal oxidation rather than external oxidation.[7–9] The second method involves the deposition of a thin layer of pure Fe, Ni, or Cu before the annealing stage in HDG lines.[11–13] The third method is a two-step procedure consisting of an oxidation followed by a reduction.[14–17] In the oxidation stage, the surface is fully oxidized to a Fe oxide layer. The Mn, Si, and Al, which diffuse to the interface between the ferrite and the Fe oxide layer, form compound oxides. In the reduction stage, the Fe oxide is fully reduced to a pure Fe layer with embedded selective oxides. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Schematic of the three methods proposed to prevent formation of Zn-coating defects resulting from the selective oxidation during HDG of AHSS steel.
The first method, i.e., the control of the DP, was suggested as one of the promising methods for g
Data Loading...
