Influences Of Al-Ti Complex Deoxidation On Inclusions And The Microstructure Of Non-quenched And Tempered Steel

Non-quenched and tempered steel is acquired by adding microalloy elements in mediumcarbon steel and processed by temperature controlled rolling and cooling. It is a kind of steelwhose mechanical property is equivalent to that of quenched and tempered carbon structuralsteel or structural alloy steel. Non-quenched and tempered steel is named as “green steel” due toits energy saving, high efficiency, and environmental friendly by omitting of thermal treatmentprocess. But the toughness of non-quenched and tempered steel is poor because of theprecipitation of proeutectoid ferrite along with austenite grain boundary, and the thickmicrostructure of ferrite-pearlite.O_xide metallurgy is a method which can change the morphology, size distribution ofinclusion, refine grains of steel, and improve the toughness of steel by choosing appropriatedeoxidant and deoxidization operation to form the oxides which has high-melting-point, smallsize, and uniformly dispersed in the steel. Those oxides can become the nucleation core of thesecond phase and promote the nucleation of acicular ferrite.Based on the consideration that oxide metallurgy can change the morphology, sizedistribution of inclusions, refine grains of steel, and improve the toughness of steel, throughthermodynamic calculation and high temperature experiment, the effects of Al-Ti deoxidation onthe morphology, size distribution of inclusions in non-quenched and tempered steel, the effectsof different Ti/Al ratio on the morphology, size distribution of MnS, and the effects of carboncontent on the inclusions and the nucleation of acicular ferrite in the Al-Ti deoxidationnon-quenched and tempered steel were investigated in this paper. Specific research results are asfollows:Thermodynamic calculation determined the thermodynamic stable region in the Al-Tideoxidation non-quenched and tempered steel for different deoxidizing products. In order toavoid the formation of Al₂O₃observed in steel, when the titanium content is in the range of0.01%～0.2%in the steel, the aluminum content should be less than0.01%.Adding titanium into non-quenched and tempered steel, the complex inclusions of Al-Ti-Oand MnS can be observed which are irregular in shape, large in size when the aluminum contentis more than0.04%; Adding0.02%Ti to steel can form the complex inclusions ofMgO-Al₂O₃-TiO_x-SiO₂-MnS which are fine, dispersed and spherical when the aluminum contentin the steel is low. The complex inclusions of MgO-Al₂O₃-TiO_x-SiO₂-MnS are beneficial to thegeneration of acicular ferrite in the non-quenched and tempered steel.When the Ti/Al ratio is more than0.9, the morphology of MnS is spherical in thenon-quenched and tempered steel; When the Ti/Al ratio is0.37and0, the morphology of MnS isirregular and its size is lager; With the increase of the Ti/Al ratio, the proportion of MnS thosediameter is less than1μm and the MnS precipitation ratio on the oxides increase. Contrary, theproportion of MnS those diameter is larger than1μm would be decrease.Carbon content affects the inclusions and the generation of acicular ferrite in Ti-deoxidized non-quenched and tempered steel. The complex inclusions of MgO-Al₂O₃-TiO_x-SiO₂-MnSwhich is fine, dispersed and spherical would be easy to form, when the carbon content in thesteel is low. The ration of acicular ferrite would decrease in Ti-deoxidized steel with highercarbon content.