Effect Of Ti/V/Nb Microalloying Based On Oxide Metallurgy On The Formation Of Acicular Ferrite In Seamless Steel Tubes

Hot rolled seamless steel tubes are widely used in shipbuilding,oil and gas transportation and other industries,and are known as industrial blood vessels.Due to its special piercing and rolling characteristics at high temperature,the resulting structure is relatively coarse,which cannot effectively improve the mechanical properties of seamless steel tubes.At present,most of the domestic and foreign researches focus on the development of on-line heat treatment of steel pipes and the development of controlled cooling technologies through thermal simulation tests.Both of these methods have their limitations and it is difficult to simulate the shear deformation and cooling rate in seamless steel pipes.Therefore,the method of adding oxide metallurgy+microalloying elements is proposed in this study.The method is to induce the ferrite phase transition in the crystal by adding the second phase and microalloying elements that are stable at high temperature.The method of obtaining acicular ferrite structure is explored.In this paper,six kinds of Q345 steels with the same basic composition are used as research objects,and Ti-Mg-Ca-Ce and other composite deoxidation methods are used.On this basis,Ti,V,and Nb microalloying elements are added.Optical microscopy(OM)and field emission scanning are used.Electron microscopy(SEM),field emission electron probe(EPMA),high temperature confocal microscopy(LCSM),etc.have studied the behavior of inclusions in steel to induce intragranular ferrite transformation.The experimental results show that:(1)In K1,K2,K3,and K4 test steels with single element addition of Nb,V,or Ti,the inclusions are mainly circular composite oxides and carbonitride precipitation phases,and their morphology is related to the composition of the inclusions.Inclusions that induce acicular ferrite nucleation are complex oxides such as Ti-Mg-Ca-Ce,which can induce one or more acicular ferrites to nucleate on the inclusions.The MnS and Ti(C,N)inclusions in the test steel induced elliptic or equiaxed intracrystalline ferrite structure.(2)In the K5 and K6 alloy steels in which Ti-Nb and Ti-V are added in addition,in addition to the composite oxide,there are also carbonitride precipitates adhering to the oxide with a size of about 1 μm.Phase,forming complex inclusions with the oxide.The inclusions have single ferrite laths with a size width of about 2 μm induced by the composite oxide precipitated by niobium carbonitride.In both K5 and K6 test steels,MnS induced intranuclear ferrite nucleation.Among them,acicular ferrite was formed in K5 test steel alone,and in K6 test steel,Single acicular and multiple equiaxed ferrites.(3)Observation results under a high temperature confocal microscope show that K4 and K6 test steels preferentially grow grain boundary ferrite(GBF)structures on the austenite grain boundaries(AGBs)at about 610℃ and 622℃.With the decrease of temperature at 570℃ and 590℃ the intragranular ferrite starts to nucleate and grow on the inclusions,and at the same time,induced nucleation occurs between the slat bundles,forming more needle-like shapes.The ferrite structure and the slat bundles collide with each other to form a cross-interlocking structure.(4)The characterization methods of EDS and EPMA were used to analyze the inclusions of induced ferrite and non-induced nucleation in K4 and K6 component steels,and the composite oxide inclusions(Ti-Mg-The formation of MDZ on Ca)is beneficial to promote acicular ferrite nucleation,which belongs to the mechanism of inducing nucleation in the manganese-depleted region.The existence of Mn and S-free inclusions(including Ti,Mg,Ce)in K6 test steel induced nucleation and MnS induced nucleation may be related to the mechanism of minimum mismatch.