Suppression of Surface Hot Shortness due to Cu in Recycled Steels

Abstract

The most serious problem in the recycling of steel is the occurrence of surface hot shortness during hot deformation due to the mixing of Cu from scrap into steels. Tin accelerates the effect of Cu. The surface hot shortness is caused by liquid embrittlement, that is, formation of the liquid Cu-enriched phase through preferential oxidation of Fe atoms at the steel/scale interface during heating for hot deformation and penetration of this Cu-enriched phase into the grain boundaries. Decrease in the amount of the liquid Cu-enriched phase penetrating into grain boundaries can suppress the surface hot shortness. The amount of the liquid Cu-enriched phase penetrating into the grain boundaries can be reduced by the suppression of oxidation, occlusion of the Cu-enriched phase into the scale, back-diffusion of Cu into the steel matrix and suppression of penetration of the liquid Cu-enriched phase. Therefore, the effects of various elements and conditions of heating and deformation on the surface hot shortness, oxidation, amount of the Cu-enriched phase at the interface and the penetration were examined by tensile tests at high temperatures, thermogravimetry and optical microscopy. The conclusion can be summarized as follows. Silicon, Mn, S (+Mn) and B reduce the susceptibility to the surface hot shortness through decreasing the amount of Cu-enriched phase at the steel/scale interface. The effect of Si is significant. Carbon reduces the oxidation rate in LNG combustion gas. Phosphorus, Si, B and C reduce the susceptibility to the surface hot shortness through restraining the penetration of the Cu-enriched phase into grain boundaries. Heating at higher temperatures reduces the susceptibility mainly through a reduction in the amount of the Cu-enriched phase at the steel/scale interface, although the loss of steels by oxidation increases. A large grain size accelerates the surface hot shortness. A small amount of H 2 O in air significantly accelerates the surface hot shortness. Effects of H 2 O in heating atmosphere depend on the steel composition and more detailed research on this is desired. Very slow deformation does not cause liquid embrittlement through dynamical re-crystallization, while at a fast deformation rate the embrittlement is suppressed by an increase in the critical stress for the liquid embrittlement. Multiple methods using physical metallurgy suggested by the present research for suppressing the surface hot shortness should be applied together with other methods through separation, smelting and design of fabrication in order to promote the recycling of steels.

Effect of Alloying Elements on Mechanical Properties of High P Low Alloy Steels

“…The reduction in production costs has been also reported by effectively using low-quality iron ores and by omitting or shortening the dephosphorization process. Furthermore, Shibata et al [14] have found that susceptibility to surface hot shortness decreases on increasing P content up to 0.1 % in Fe-0.1 %C-0.5 %Mn-0.5 %Cu steel. In this case, the possibilities can be explored to replace or reduce judiciously the costlier element Ni with P to even more than 0.1 %.…”

Effect of Oxygen Concentration on Surface Hot Shortness of Steel Induced by Copper

“…However, the addition of certain elements, such as nickel, silicon, boron, carbon, and phosphorus, is reported to be effective in controlling this problem. [1][2][3][4][5][6] Furthermore, controlling the reheating conditions, such as temperature and the atmospheric water vapor concentration in the atmosphere, also contribute to resolving this problem. 7,8) Therefore, it is important to understand how Cu is distributed when steel is oxidized.…”

Effect of Atmospheric Conditions on Copper Behaviour during High Temperature Oxidation of a Steel Containing Copper

“…At conditions of higher or lower temperatures, the sensitivity of surface cracks is reduced. 5,8) In addition, the oxidizing atmosphere also has an impact; atmospheric water vapour increases the number of crack on the steel surface.…”