Effects of Cu and Other Tramp Elements on Steel Properties. Evaluation of Susceptibility to Surface Hot Shortness in Cu-containing Steels by Tensile Test.

Abstract

The feasibility of a new method for evaluating the susceptibility of steels to surface hot shortness due to Cu and Sn was examined. Steels used are IF steels containing 0.1-0.2%Cu and 0.1%C-0.5%Cu steels. Chemical compositions of the IF steels were controlled with the purpose of being as similar as possible to those of steels of which the susceptibility was previously examined by a visual method in a practical rolling process at Nippon Steel Corporation. Tensile tests were carried out at 1 100°C after heating a specimen in air or Ar gas. The total elongation and maximum load were measured from a load-elongation curve. By dividing the difference between the total elongations or maximum loads obtained in air and Ar gas by the total elongation or maximum load in Ar gas, the parameters Ee or Ep were calculated. By considering the reduction in specimen diameter due to oxidation, the Ep corrected, Ep', was calculated. The Ee, Ep and Ep' of the IF steels corresponded to the degree of surface cracks observed in the practical rolling process. Penetration of Cu-enriched phase into grain boundaries tends to occur under thermal and external stresses. Surface cracks stop growing along the depth direction at the earlier stage of deformation when the amount of Cu-enriched phase is the smaller at steel/scale interface. Tensile tests using specimens having artificial round cracks clarified that the Ee and Ep' increased with an increase in the crack depth and that, in contrast, the Ep' increased slightly and the Ee decreased with an increase in the number of cracks. The parameter Ee and Ep' showed a maximum value at a strain rate of around 3 x 10 -2 s -1 , and the difference in their values among the steels is largest around this strain rate. Therefore it is possible to know sensitively the effects of any factors like alloying or impurity elements on surface hot shortness at around this strain rate although this strain rate is much slower than those of hot workings.