| With the rapid development of large-scale engineering construction technology,the large-scale and high performance of structural steels are urgently required.As an important support material for large offshore oil platforms,all-weather trains and large Bridges,thick H-beam has attracted more and more attention.Large thick hot rolled H beams need to be refined to meet the performance requirements of high strength,toughness and good weldability.Although controlled rolling and controlled cooling(TMCP)is the most widely used grain refinement technology in industry.However,because of the high rolling temperature and small compression ratio,the traditional TMCP technology with the core of "low temperature and large reduction" is difficult to be applied to the production of large thick H-beam.It is necessary to control austenite size,adjust the critical strain value of dynamic recrystallization and control the deformation recrystallization behavior of austenite by microalloying design and second phase particles during rolling process.To reveal the precipitation behavior of microalloyed second phase particles in steel is the basis of controlling austenite size and the austenitic deformation recrystallization behavior during rolling process.In this paper,Nb microalloyed and Nb-Ti microalloyed steel is taken as the research object to systematically analyze the precipitation behavior of Nb C,Ti N,(Nb,Ti)(C,N)composite precipitates.To be more specific,the precipitation behavior of different types of second phase particles and its influence on original austenite and the deformation recrystallization behavior of austenite were studied in different roughing and finishing rolling stages.The research reveals the role of microalloyed elements in hot rolled H-beams,which provides a foundation for the microalloyed design of large thick hot rolled H-beams.In this work,the results show that the original austenite grain size of Nb microalloyed experimental steel increases with the increase of roughing temperature and with the increase of holding time.According to the phase diagram thermodynamic comprehensive analysis:the volume fraction of Nb C in Nb microalloyed experimental steel decreases with the increase of roughing temperature,and the total solution temperature is about 1070 ? and the second phase particles in steel hardly precipitate at 1150 ?.The grain growth rates of the experimental steel are 0.061 ?m/? and 0.699 ?m/? respectively when the experimental steel is heated from 950 ? to 1050 ? and 1150 ? for 30 s.It is mainly due to the precipitation behavior of the second phase particles of Nb C varies with the change of roughing temperature.The Nb C second phase particles have higher volume fraction(0.027%)and lower solid solubility at 950 ?.The volume fraction and particle size of Nb C decrease with the increase of temperature,while the solid solubility increases with the increase of temperature.The size of Nb C particles at 1050 ? is smaller than that at 950 ?,but the number and distribution of Nb C particles at 950 ? are more and more dispersed than that at 1050 ?.Nb C particles are difficult to be found at 1150 ?,so the migration and growth of grain boundary cannot be effectively hindered at this temperature.At 950 ?,the size of Nb C particles grows with the increase of holding time,but the growth rate of Nb C particles increases rapidly when the holding time is more than 40 s.Therefore,it is obvious that the austenite grain growth can not be inhibited by the precipitation of Nb particles only by the microalloyed design of Nb.The results of OM,SEM and TEM on the microstructure and the second phase particles of Nb-Ti microalloyed experimental steel show that: Compared with Nb microalloy experimental steel,the precipitation temperature of Ti N particles in Nb-Ti microalloy experimental steel is significantly higher than that of Nb microalloyed experimental steel.It can be seen that the precipitation of Ti N particles at high temperature can effectively nail the austenite grain boundary in experimental microalloy steels containing Ti.At 950 ? and1150 ?,the austenite grain size is significantly smaller than that of Nb microalloy experimental steel due to the action of Ti N particles.The results show that the mixed precipitation of Ti N and Nb C greatly increases the volume fraction of the second phase particles in steel.At 950 ? and 1150 ?,the size of Ti N and(Nb,Ti)(C,N)particles is fine,the distribution is uniform,and the shape is square or rectangular.All these shows strong hindrance to the original austenite grain boundary.Ti has a significant effect on the precipitation behavior of Nb C second phase particles in the experimental Nb-Ti microalloyed steel,which greatly increases the number and density of the second phase particles and reduces the size of the second phase particles in the steel.During the simulated finishing rolling stage(950 ?),the precipitation behavior of Nb C epitaxial growth promoted by Ti N also has a significant effect on the static recrystallization of steel.Compared without a large number of second phase particles in the steel,the volume fraction of the second phase particles in the forged Nb-Ti microalloyed experimental steel with Pre-Ti N is higher,the particle size is larger,and there are more(Nb,Ti)(C,N)composite precipitates.These second phase particles can effectively hinder grain boundary migration of austenite and inhibit static recrystallization of austenite between passes during finishing rolling.Inhibition of static recrystallization is beneficial to the accumulation of strain,which is helpful to reduce the critical strain value of dynamic recrystallization and promote the occurrence of dynamic recrystallization of austenite.The results show that the microalloyed of Nb has almost no inhibitory effect on the growth of austenite grains during rough rolling.Therefore,it is necessary to realize this process through Ti N second phase particles formed by microalloyed element Ti.It is also difficult to realize strain accumulation and inhibit the recrystallization of austenite between passes in the finishing rolling process only by Ti microalloyed.Therefore,the method of forming Nb C second phase particles by Nb microalloyed is needed.The existence of Ti N can provide a preferential nucleation location for the effective precipitation of Nb particles,that is,Nb grows epitaxial on Ti N particles,which is not only conducive to the precipitation of a large number of small second phase particles,but also can give full play to the role of Nb and reduce the amount of Nb addition,thus saving the cost.A large number of fine second phase particles in Nb-Ti microalloy experimental steels can effectively inhibit the static recrystallization of austenite between passes and provide the basis for microstructure refinement. |
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