| Hot rolled seamless pipe is an irreplaceable steel material.which is widely used in electric power,petroleum,chemical industry,shipbuilding and other fields.In the traditional production process of hot-rolled seamless pipes,the pipes need to go through heating,high-temperature piercing,high-temperature rolling,sizing and other processes.Restricted by geometry,the austenite grains of hot-rolled seamless pipes can't be refined by controlled rolling like other types of steel,resulting a coarse microstructure after cooling which decreases the mechanical properties of seamless pipes.At present,only rapid cooling and on-line normalization are available for the microstructure refinement,but the improvement of product performance is limited.Therefore,the purpose of this project is to induce acicular ferrite transformation and refine grains by combining oxide addition with controlling cooling process,so as to refine grains and improve the comprehensive mechanical properties of steel pipes.In this study,traditional aluminum deoxidized steels and oxide metallurgical steels are used for comparison.Oxide metallurgical steels are divided into Ti-Mg-Ca-Ce composite deoxidization and Ti-Zr composite deoxidization.The effects of cooling rate,austenitizing temperature,deformation temperature and deformation amount on microstructure of experimental steel were studied by OM,SEM and TEM.At the same time,EPMA was used to study the behavior of inclusions induced acicular ferrite transformation in different deoxidization systems.The experimental results are as follows:In the Al deoxidized steel,the type of inclusions are mainly Si-Mn-O with large size,the inclusions which are larger than 1 ?m accounting for 79%.In the Ti-Mg-Ca-Ce deoxidized steel,the inclusions are mainly Ti-Mg-Ca-Ce-Al-O with smaller size,the inclusions which are larger than 1 ?m accounting for 47%;In the Ti-Zr deoxidized steel,the inclusions are mainly Ti-Zr-Al-O+MnS.Due to the addition of Zr element,the inclusions are further refined,the inclusions which are larger than 1 ?m only accounting for 36%.In the Al deoxidized steel,only when the cooling rate is 1 ?/s?5 ?/s acicular ferrite can form.The content of acicular ferrite is low and the lath is short and thick.The cooling rate range of acicular ferrite formed in Ti-Mg-Ca-Ce deoxidized steel and Ti-Zr deoxidized steel is 1 ?/s?10 ?/s,and the range with the high content of acicular ferrite is 3 ?/s?5 ?/s.The content of acicular ferrite in Ti-Zr deoxidized steel is higher than that of Ti-Mg-Ca-Ce deoxidized steel,and the lath is relatively slender,and the content of acicular ferrite reaches the maximum value when the cooling rate is 3 ?/s.The austenitizing temperature affects the grain size of the original austenite.Combined with the microstructure of three experimental steels at different austenitizing temperatures,it could be found that the acicular ferrite formed when the average austenite grain size was larger than 43 ?m.The acicular ferrite formed in Al deoxidized steel and Ti-Mg-Ca-Ce deoxidized steel when the temperature was 1050 ??1150 ?.However,because the fine size of oxides in Ti-Zr deoxidized steel significantly refine the austenite grain size,acicular ferrite can not form below 1150 ?,a large amount of acicular ferrite formed at 1250?.When the deformation of Ti-Mg-Ca-Ce deoxidized steel and Ti-Zr deoxidized steel is less than 50%,with the increase of deformation amount,the grains are refined and the of acicular ferrite slightly increases.When the deformation amount reaches 70%,the acicular ferrite decreases obviously and the content of granular bainite increases.Ti-Zr-Al-O+MnS inclusions in Ti-Zr deoxidized steel can be the nucleation core of acicular ferrite. |
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