Formation And Control Of Acicular Ferrite In Low Carbon Steel On Thin Strip Casting

Thin strip casting is an important advanced technology in the materials field in twenty-first Century.It has the characteristics of short process,low-energy,fine solidification structure and low composition segregation.However,the austenite structure of low carbon steel strip in strip casting is larger than that of traditional hot rolled strip.On the other hand,thin strip casting technology has the characteristics of near final forming,and the metal liquid is continuously cast into 1?5mm thick strip,and the subsequent processing deformation is small.Therefore,it is difficult to achieve grain refinement by thermomechanical treatment,so it is urgent to explore a new way to achieve grain refinement.Through simulation experiments,in-depth study of the influence of cooling rate and other parameters on the microstructure,clarify the acicular ferrite microstructure characteristics and inclusions inducing acicular ferrite mechanism,and the effects of acicular ferrite content and polygonal ferrite size on mechanical properties were preliminarily clarified.(1)There are a large number of non-metallic inclusions in low carbon steel strip,mainly compound oxides such as Ti,Al,Mn and Si.The size of inclusions is 0.2?2.0?m,and the size of most inclusions is concentrated in the range of 0.2?1.0?m.According to the observation of the distribution of Si,Mn,Ti and Al in composite oxides,the inclusion can be divided into three parts:core,shell and surface precipitation.The core is composed of oxides of Al2O3 and Ti(Ti3O5,etc.),and the shell is probably composed of SiO2·MnO and MnO·TiO2,while surface precipitation is some sulfides.The composite oxides can induce acicular ferrite,and the nucleation rate of acicular ferrite can be improved by appropriately increasing the size of inclusion.(2)The results show that oxide induced acicular ferrite is mainly caused by solute elements depleted mechanism.In the experiment,the carbon depleted zone can be observed in the matrix around the oxide.Because the thermal expansion coefficient of oxide is different from that of matrix,a large number of dislocations are produced in the matrix around the oxide during the cooling process.Carbon atoms are readily diffused along the dislocation from the surrounding matrix to the oxide surface,resulting in a carbon depleted zone.The formation of the carbon depleted zone reduces the stability of austenite around the inclusion,providing additional chemical driving force for ???,and promoting the nucleation of acicular ferrite.The acicular ferrite has a K-S orientation relationship with the original austenite.On the same inclusions surface nucleation and growth of in the opposite direction of acicular ferrite nucleate on the same habit plane.(3)The influence of cooling rate on the content and size of acicular ferrite in two temperature ranges of 1200??650?,and 700 ??20 ? was investigated.In the range of 1200??650?,the acicular ferrite content increases with the increase of cooling rate.When the cooling rate is over 20?/s,the acicular ferrite content decreases with the increase of cooling rate.In the range of 700?20?,the acicular ferrite content increases with the increase of cooling rate,but the increase extent decreases gradually.When the cooling rate is over 10?/s,the acicular ferrite content is almost no longer changed.When the cooling rate is in the range of 0.2?30?s,the acicular ferrite size decreases continuously with the increase of cooling rate.(4)The effects of three cooling temperatures at 600?,650 ? and 700?on acicular ferrite content and acicular ferrite size were investigated.When the cooling temperature is 700?,a large amount of undercooled austenite is transformed into pre-eutectoid ferrite,and the acicular ferrite content and acicular ferrite size are the smallest.When the cooling temperature is 650?,the acicular ferrite content and acicular ferrite size are the largest.(5)Controlling the heating and holding time mainly controls the size of austenite grain and increases the size of austenite grain,which will reduce the resistance to acicular ferrite transformation.However,when the heating time is more than 10min,the austenite grain almost ceases to grow,and the increase of holding time will reduce the content of acicular ferrite.(6)Under simulated conditions,the increase of acicular ferrite content in low carbon steel strip can greatly increase the tensile strength,but the elongation will decrease,the increase of acicular ferrite content in low carbon steel strip can greatly increase the tensile strength,but the elongation will decrease.The tensile strength of low carbon steel containing 1%acicular ferrite is 387MPa,and elongation is 51.65%.When the acicular ferrite content is increased to 56%,the elongation is 27.42%,and the tensile strength can be increased to 513MPa.Besides the content of acicular ferrite,many factors such as acicular ferrite size,polygonal ferrite size,acicular ferrite region size and dispersion conditions will have a great impact on the formation of thin strip mechanics,so the follow-up work of this experiment still has great space.