Abstract
One promising direction in steelmaking to respond to global climate change issue is to increase the use of electric furnace manufacturing methods using zero-carbon electricity, which have received more and more attention recently. Considering the replacement from blast furnace steel, the temper embrittlement problem induced by some tramp elements, which is unique behavior for electric furnace methods, must be resolved.Sn is considered as a tramp element that cannot be removed in steel refining process, and it has been reported that the toughness of the base metal and the welded heat affected zone can be significantly degraded. In the case of electric furnace steels with degraded toughness, segregation of Sn is observed at grain boundaries and the resulting fracture mode shifts to intergranular fracture mode. In this study, in order to quantify the allowable amount of Sn added to thick steel plates that is used for large-heat input welding, experiments were conducted on specimens with different amounts of Sn and heat-treated for different holding times at 400℃, which is a typical embrittlement temperature and investigated the effect of the amount of Sn on the change in low-temperature fracture toughness. As a result of fracture toughness tests, clear evidence of temper embrittlement was observed. Apart from previous famous study by McMahon, there was no intergranular fracture in fracture surface but transgranular and cleavage fracture was observed. Therefore, the mechanism by which Sn tends to cause cleavage fracture in the weld heat affected zone was proposed. Based on our newly proposed mechanism, a model was developed to estimate the transition temperature change for various conditions of tempering time, tempering temperature and Sn content.Finally, the authors propose allowable maximum content of Sn based on the model for the establishment of the rational ore sorting system from scraps in near future society.
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