Sj. Seo et al., EFFECTS OF 0.4-PERCENT SI AND 0.02-PERCENT P ADDITIONS ON SURFACE HOTSHORTNESS IN O.1-PERCENT-C-0.5-PERCENT-MN STEELS CONTAINING 0.5-PERCENT CU, ISIJ international, 37(3), 1997, pp. 240-249
The objective of this paper is to examine the effects of Si and P in l
ow carbon steels on surface hot shortness due to Cu. Susceptibility of
the steels to surface hot shortness was evaluated by a new method usi
ng tensile tests which is proposed by the present authors. Tensile tes
ts were carried out after heating specimens at 1 000, 1 100 and 1 200
degrees C in air and in Ar gas. Tensile tests using specimens implanti
ng a Cu rod, observation of oxidation rate by thermogravimetry, optica
l microscopy and EPMA of steel/scale interface region etc. were also p
erformed. At 1 100 degrees C, single additions of 0.4% Si and 0.02% P
were effective to decrease susceptibility to surface hot shortness, al
though these increased the oxidation rate. Duplex addition of 0.4% Si
and 0.02% P decreased the oxidation rate and exhibited a substantial e
ffect on a decrease in the susceptibility. Addition of Si decreased th
e amount of Cu-enriched phase at steel/scale interface. This is contri
butable to the reduction of the susceptibility to surface hot shortnes
s. Internal oxidation of Si is thought to decrease the amount of the C
u-enriched phase. Single addition of 0.02% P seems to increase slightl
y the amount of the Cu-enriched phase. A critical stress exists to fra
cture the specimens by Cu-enriched liquid phase. The additions of Si a
nd P increase this critical stress. Silicon also contributes to a decr
ease in the growth rate of the crack created by the penetration. At 1
200 degrees C, the susceptibility to surface hat shortness in all stee
ls decreased compared with that at 1 100 degrees C, but trends of effe
cts of single and duplex additions of 0.4% Si and 0.02% P on the susce
ptibility were similar to those at 1 100 degrees C. The oxidation rate
for all steels was much higher than at 1 100 degrees C, but the amoun
t of Cu-enriched phase at steel/scale interface was reduced compared w
ith at 1 100 degrees C. The amount of the Cu-enriched phase in the ste
els containing 0.4% Si is smaller than that in other steels. Liquid ph
ase which appears in scale at temperatures higher than 1 177 degrees C
(eutectic temperature of FeO-2FeO . SiO2) is a contributing factor in
both increases of the oxidation rate and occlusion of Cu into the sca
le at 1 200 degrees C.