Analyses of the Selective Oxides and Fe-Zn Interface of Annealed and Galvanized TRIP Steels Having Three Si/Mn Ratios

摘要

This study aims at clarifying the effect of Si/Mn ratio on selective oxidation in annealing and on the formation of inhibition layer of transformation-induced plasticity (TRIP) steels in hot-dip galvanizing, using X-ray photoelectron spectroscopy and transmission electron microscopy as complementary tools. Three TRIP steels with Si/Mn ratio of 0.5 and 1.0 and 1.3, respectively, were annealed at 800 °C in a 10% H_2-N_2 atmosphere with a dew point of -70, -30 or 0 °C, and galvanized in a zinc bath of 0.14-0.20 wt% Al for 4 s. The oxides formed on the surface were mainly SiO_2 and xMnO-SiO_2 at dp=-70 °C and were xMnO-SiO_2 and MnSiO_3 at dp=0 °C. At dp = -30 °C, the oxides were a mixture of SiO_2, xMnO-SiO_2 and Mn_2SiO_4. The relative quantity of SiO_2 increased with increasing the Si/Mn ratio of the steel. More than 70 % of the surface could be covered by the Fe_2Al_5 crystals after galvanizing for steels whose surface was free from SiO_2 after annealing. Increasing the Al content in the range of 0.14 to 0.20 wt% effectively increased the coverage ratio of the inhibition layer and improved the coatability for steels having an oxide layer of mixed SiO_2, xMnO-SiO_2 and Mn_2SiO_4/MnSiO_3.