Effect Of Nitrogen On Microstructure And Mechanical Properties In TMCP Processed Low Carbon Mo-V-Ti Steels

High strength low alloy steel was considered as an important structural steel,which widely used in construction,oil pipelines,bridges,ships or other industries.Adding nitrogen?N?in microalloy steel can significantly refine ferritic grain and promote the formation of ferrite grain.In addition,increased nitrogen content can increase the content of martensite-austenite?M-A?constituent in the steel and improve the strength of the steel.It is valuable and significative for engineering to explore the mechanism of increased nitrogen and controlled rolling and cooling of unrecrystallization zone to improve the microstructure and properties of low carbon Mo-V-Ti steels.Chemical component of 0.06C-0.20Si-1.50Mn-0.27Mo-0.06V-0.01Ti and series nitrogen content?0.0030-0.0140 wt.%?of steels was designed and prepared in this paper.The continuous cooling transformation of deformation austenitic of low carbon Mo-V-Ti steel with different nitrogen contents was studied.The microstructure and tensile properties of Mo-V-Ti-N steel through controlled rolling and cooling process were studied.By means of optical microscopy?OM?,scanning electron microscopy?SEM?,electron backscatter diffraction?EBSD?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?,the microstructure and tensile properties of the steels were characterized.The results show that:The dynamic recrystallization was easy to be carried out under the condition of higher deformation temperature and lower strain rate.The dynamic recrystallization activation energy of 30N and 140N steel were 229 kJ/mol and 251 kJ/mol,respectively.The static recrystallization volume fraction increased with the increase of the deformation temperature or the increase of the interval time between passes.The strain-induced precipitation inhibited the progress of static recrystallization.The activation energy of static recrystallization in 30N and 140N steel were 259 kJ/mol and 288 kJ/mol,respectively.With the increasing of nitrogen content,the average size of austenite grain decreases gradually,Ar₃ increases.At the cooling rate of 1?/s,with the increasing of nitrogen content,Ar₃ increases,polygonal ferrite and pearlite grain size increases gradually.At the cooling rate of 10³0?/s,30N steel consisted of acicular ferrite,granular bainite and lath bainite.However,the microstructure in 80N and 120N steels is a mixture of polygonal ferrite,acicular ferrite and granular bainite.With the increasing of nitrogen content,?Ti,V??C,N?precipitation increases,polygonal ferrite,acicular ferrite and M-A constituent increases.With the increasing of cooling rate,acicular ferrite increases while M-A constituent decreases.Low carbon Mo-V-Ti steels with two-stage controlled rolling and the cooling rate of10?/s,with the nitrogen content increased from 0.0030 wt.%to 0.0120 wt.%,the yield ratio reduced from 0.74 to 0.69,the yield strength reduced,tensile strength decreased,the strain hardening increment??increased from 171 MPa to 188 MPa,impact energy increased significantly from 65 J to 145 J.At the nitrogen content of 0.0120 wt.%,the steel plate with excellent comprehensive mechanical properties was obtained.The microstructure of low-carbon Mo-V-Ti-N steel steel are granular bainite and polygonal ferrite in the finish rolling temperature of 800⁹00?,start cooling temperature of 780⁸40?,cooling rate of 15²0?/s and finish cooling temperature450⁴80?.The comprehensive performance of industrial trial steel which in the finish rolling temperature of 850?,start cooling temperature of 800?,cooling rate of 20?/s and finish cooling temperature 450?is excellent,and the enhanced-nitrogen steel shows that the yield ratio is less than 0.8.The strain hardening index of 30N and 140N steel in the first stage of tensile deformation is 1.01 and 0.89 respectively,and the strain hardening index of the second stage is 0.27 and 0.32 respectively.In addition,with the increasing of tensile deformation,dislocation density and internal stress increases,the strain hardening capacity decreases.