The Research On The Influence Of Inclusions To The Acicular Ferrite Formation In The High Strength Steel

The production of high strength steel in China comprehensively adopted micro alloying technology which faces the problem of metal resources and production cost.It has been a new research emphasis to adopt a new technology to reduce or even replace micro alloying elements to produce high strength steel in low cost.The basic idea of oxide metallurgy technology is using non-metallic inclusions to induce intracrystalline acicular ferrite nucleating.The formation of fine acicular ferrite has great facilitation in improving strength and restraining the extension of crack.Researching the infulence of inclusions to acicular ferrite nucleating has great significance on the development of new steel grade and reducing production cost.In this paper,the 20MnSiV was chosen as experimental subject.The continuous cooling phase transformation curve was tested by thermo-simulated test,and in order to get the continuous cooling transformation curve(CCT),the methods of thermo-simulated,metallograph analysis and leveraged calculation were used.Through the analysis of the CCT curve and metallograph,the change of metallurgical structure at different cooling rate in the cooling process were researched.In the process of experiments,the steel in crystallizer and ladle were deoxidized respectively by calcium carbide and Ca-Si wire,and the heat treatment experiment was carried out on the samples that after forging,at last large number of samples with acicular ferrite were got,and the microstructure were determined by the methods of microstructure observation and microhardness testing.The result showed that:(1)the acicular ferrite could formed in the 20MnSiV in cooling rate at a range of 1?to 20?/s,and the intragranular acicular ferrite could formed in cooling rate at a range of 5?to 20?;(2)the vickers hardness of acicular ferrite structure was between 210 and 240 HV.In this paper,in order to research the influence of the size and composition of non-metallic inclusion to the nucleating and growth of intragranular acicular ferrite,the size and composition of inclusions of the sample with intragranular acicular ferrite.The size and composition of inclusions that induced intracrystalline acicular ferrite nucleating and the microstructure of acicular ferrite were analysed.The result showed that:the phase interface of large-size inclusions can play the role of grain boundary to promote intracrystalline polygonal ferrite nucleating and growing;(2)the inclusions that could induced intracrystalline acicular ferrite nucleating were mainly Mn S,followed by Mn O·Si O₂ and Mn S·Si O₂,and and the size of three kinds of inclusions mainly less than 3?m;(3)the nucleating of intracrystalline acicular ferrite induced by Mn S was determined by stress-strain energy and the inert interfacial energy;(4)the composite inclusions and the inclusions in inlay condition could provide larger suitable nucleation area,and promote more acicular ferrite nucleating and growing at the same time.