of the equations 2.303-C%Y=-C2×0.006+3×(-16.3)门，〔%Y)=0.018 2 2.303c%0>-C2×(-90.58)-3×(-0,20);〔%01<0.0072 3 As a result of calculation the oxygen content in steel can be decreased at most to a level~0.0072%by deoxidation with yttrium in the condition of equilibrium. More content of yttrium even promates to pick up some oxygen in the steel. However the other elements -deoxidizers in the steel and their interaction can decrease the oxygen content in more extent.This is indicated in the chemical analysis of experimented steel. 2.3 Composition of Nonmetallic Inclusions The metallographic pictures of nonmetallic inclusions in experimented heats are shown in Fig.3.The nonmetallic inclusions in comparative steel,which was not modified with yttrium,is mainly consisted of Al2Os and (Mn,Fe)S.The biggest inclusion is not beyond 400um.The finished steel with yttrium contains nonme- tallic inclusions with complex composition;besides,the inclusions morphology is b 50林m 2写4知 25科m Fig.3 Nonmetallic inclusions in steel a.without Y,b.~0.014%Y, c.0.053%Y,d.-0.190%Y 2002222 a8200223 Fig.4 Distribution of Y(b)and S(c)Inside Incluslons (a)In steel with Y (0.190%Y) 542， 瓦万万厄五灭万石 一 〔 。 一 〕 〔 〕 二 。 、 气万，万万丁一丁 万二尸二二二 产洲 一 乙。 吕 吕 仁为 〔 一 。 一 一 。 〕 〔 〕 ， 了 。 。 一 五 · ， ， ， 拼 ， 竺一欢豪 一 少 一 〔了 警， …彝妙 忱砒 了 臀 段反醉 ， · ， ， 蒸 馨 ‘ 洲户 一 矛 一卜 ， 侧