| Ti can react with N in the steel to form TiN which can improve the toughness and weldingof steel. Ti also can react with C in steel to form TiC which can make grain refinement andprecipitation strengthening. Adding proper amount of Ti to steel can increase the strength of steeland improve the thermoplastic and toughness. Nb in the steel can increase the austeniterecrystallization temperature so as to achieve the purpose of austenite grain refinement. There issiginifancant importance to develop niobium and titium compound micro-alloying steel toimprove the strength, performance, save raw materials and decrease of the production cost.Using Si-Ti-Fe alloy in niobium-alloying structure steel for micro-alloying has great practicalsignificance in improving the performance of steel, promoting the recycling of metallurgicalproducts and learn the interaction mechnism of titium in the steel.The experiments were conducted in a high frequency vacuum induction furnace forsmelting Nb-microalloying steel with different titanium alloy addtion at1550℃in Aratmosphere. The composition, microstructure and phase structure, morphology, size of theinclusions in Nb-microalloying steel with different titanium alloy addtion were analysised. Thenmetallographic specimen of Nb-microalloying steel with different titanium alloy addtion wereprepared, solid phase tranformation was observe online using high temperature confocal lasermicroscope at1200℃for understanding the exhalation behavior of titanium and niobiuminclusion and grasping the influence of titanium on the microstructure of niobium micro alloyingsteel. The microstructure, precipitation of the inclusions and marcography mechnics inNb-microalloying steel with different titanium alloy addtion were analysised.The results were showed as follows.（1） The oxygen content in steel decreased from0.028%to below0.003%after using Al to deoxygenate and oxygen content in steel decreased to about0.002%after micro-alloying with titanium and niobium which increased the utilization ratio ofthe alloy. The content of Ti increasedwith the amount of the Si-Ti-Fe alloy. The utilization ratioof alloy elements Ti exceeded80%.（2） Inclusions in steel were mainly Al2O3, SiO2, TiOxbasedcomplex compound with spherical or near spherical shape.The were tipical （Ti, Nb）N complexcompounds with cube shape in the steel. And NbC, TiC precipitated around spherical shapeoxide inclusions. There were MnS, FeS inclusions in the steel. Furthermore, part of inclusions inthe samples became from spherical to rectangle, which appeared as typical Ti （C, N） compoundinclusion with increasing titanium addition.（3） There were a lot of fine （Ti, Nb）（C, N）precipitations after the steel heat treated by using confocal scanning laser microscope （CSLM） atdifferent heating rate. After heat treated, with the increase of the content of titanium steel,quantities of inclusions with the size less than1μm increased dramatically, and the totalquantities of inclusions also had certain degree increase, while the quantities of inclusions withthe size larger than1μm had the decrease trend.（4） The metallographical microstructure of theas-cast steels changed from martensite to bainite with the increase of titanium addtioin. Thecontent of the titanium in the steel increased gradually with the increase of titanium alloyaddition and the microstructure was obviously refining in steel, bainite structure could be obtained more easily under the same cooling conditions. Microstructure of the steel became fromferrite to martensite when the cooling speed increased which led to high strength and badtoughness.（5） The hardness of the steel increased with the increase of cooling rate. The hardnessof steel had a marked change comparision no titanium addtion with that titanium addition, andthe hardness of the steel also changed with different titanium content in the steel. |
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