PSI - Issue 33

Saki Hayashi et al. / Procedia Structural Integrity 33 (2021) 1162–1172 Hayashi et al / Structural Integrity Procedia 00 (2019) 000 – 000

1164

3

total content of impurity element i ∆ McLean segregation energy , constants for normal distribution function

1. Introduction Blast furnace method, which has supported human life since the Industrial Revolution as the capital steel making process, is facing a demand for the drastic change due to the global issue of carbon dioxide emission. Itaya et al. (2010) found the steel industry emitted more carbon dioxide than any other industry. On the other hand, electric furnace method has been developed in this one hundred year. The electric furnace method is a promising approach in terms of carbon emissions, because all processes are all powered by electricity. The Japan Iron and Steel Federation found the share of electric furnace steels in crude steel production has not grown, especially in Japan. It is shown that electric furnace steels are used for common use, but not for high value added products e.g., large buildings by Ministry of Economy, Trade and Industry of Japan. The reasons for this are thought to be as follows. Since electric furnace steels are made from steel scraps, they can contain impurity elements that are seldom contained in blast furnace steels, and previous studies have shown that some of the elements called tramp elements, such as Sn and Cu, which are difficult to remove by refining, cause different problems. For example, Nagasaki et al. (1997) found Cu and Sn can cause cracking during manufacturing, and Inushima et al. (2019) found Sn can decrease toughness after welding. In order to increase the share of electric arc furnace steels in crude steel production, it is necessary to solve these problems. In the previous studies, cracking during steelmaking due to Cu has been well investigated so far, but the toughness reduction after welding due to Sn is still not well understood. Therefore, in this study, fracture tests were carried out on specimens with various amounts of Sn and heat treatments which simulate welding, and the mechanism of the effect of Sn on the toughness of the heat-affected zone of welding and the allowable upper limit of Sn were discussed. When welding steels, a phenomenon like tempering can occur in the heat-affected zone. Tempering is a heat treatment in which steels are reheated to a temperature below the transformation temperature after quenching, in which steels are heated to a temperature above the transformation temperature. During welding, the heat-affected zone is subjected to repeated heating and cooling. The same area can be heated to a high temperature, as in quenching, when it is close to the welded area and heated to a lower temperature than the transformation point, as in tempering, when it is far away from the welded area. In this way, a phenomenon like tempering can occur in the heat-affected zone. When steels containing Sn are tempered in a certain temperature range, the toughness of the steel may decrease, contrary to the original purpose of tempering. This phenomenon is referred to as temper embrittlement. Mimura (1974) claim that temper embrittlement happens because Sn segregates at grain boundaries in a certain temperature range, making intergranular brittle fracture more likely to occur. Since tempering is not intentionally performed in welding, temper embrittlement may occur in some parts of the weld heat affected zone. This is the reason why Sn is considered to affect the toughness after welding badly. 2.2. Material The chemical composition was prepared as Table 1, with three different amounts of Sn: 0.01 mass%, 0.1 mass%, and 0.2 mass%, with and without boron (B) for each. In this study, we focused on boron because it was shown boron 2. Experiment 2.1. The reason why Sn affects the toughness after welding.