| High strength low alloy(HSLA)steel is generally added by the multi-microalloying to obtain higher grain refinement strengthening and precipitation strengthening to meet the property requirements of steel by taking advantage of each element in steel.Controlling stain-induced precipitation behavior of fine carbide and/or nitride during hot rolling process can control the recrystallization kinetics of HSLA steel,and obtaining nanometer-sized precipitates during coiling or tempering after rolling and restraining their coarsening are directly related to the mechanical properties of HSLA steel.High Nb(<0.1 wt.%,which is relative to the Nb content of 0.5 wt.%)low-carbon steel containing Mo is widely used.Microalloy element Mo has a certain effect on the precipitation and coarsening behavior of Nb carbonitride in high Nb steel.However,the mechanism of this effect is not very clear or even controversial.In the present paper,the high Nb low-carbon steel containing Mo(-0.2 wt.%)(Nb-Mo steel)was prepared to investigate the effect of Mo on the precipitation and coarsening kinetics of nanometer-sized carbides both in austenite and ferrite,as well as the effect of Mo on the static recrystallization of deformed austenite of Nb-Mo steel.Meanwhile,the precipitation kinetics of complex precipitate formed in ferrite of Nb-Mo steel was calculated by existing theory.And the mechanism of the effect of Mo on the coarsening kinetics of complex precipitate both in austenite and ferrite of Nb-Mo steel was also explained,which provides theoretical guidance and experimental basis for the development of high strength steel mainly strengthened by nanometer-sized precipitates.And an "intelligent" fire resistant steel with low cost and saving Mo was developed.The main contents and innovations of this paper are as follows:1)The strain-induced carbide precipitation in austenite of 0.1wt.%Nb steel and 0.1 wt.%Nb-0.19wt.%Mo steel were investigated by stress relaxation test and transmission electron microscopy(TEM).The results show that the precipitation-time-temperature(PTT)diagram of the two tested steels exhibits a typical " C " shape curve.Mo addition can slightly move the "C" curve to the top left,implying the Mo promoted the precipitation kinetics of NbC in austenite,this is consistent with the result of TEM observation.2)In addition,TEM observation shows that the precipitate formed in austenite of Nb-Mo bearing steel is identified as(Nb,Mo)C with the NaCl-type crystal structure containing a small amount of Mo.And the Mo content in(Nb,Mo)C depends on its particle size,thus those kind of particles can be called as "metastable" precipitates which exhibit a higher coarsening resistance as compared with NbC particles in Nb bearing steel,which is attributed to the decrease of the interfical energy of MC/austenite and the concentration of Nb in austenite due to Mo addition.3)Static recrystallization kinetics of 0.1wt.%Nb steel and 0.1wt.%Nb-0.19 wt.%Mo steels were investigated by stress relaxation test.The static recrystallization kinetics equations of two experimental steels were established via studying the static recrystallization behaviors at different strains,deformation temperatures,strain rates,etc.The results show that Mo addition can significantly improve the static recrystallization activation energy of Mo-Nb bearing steel.Mo addition delayed the recrystallization kinetics of deformed austenite both at higher temperature(?1020?)and lower temperature(?1020?)due to the solute drag effect of Mo in solid solution and the finer precipitates of(Nb,Mo)C.4)The effect of Mo on the precipitation behavior of Nb carbide in ferrite of quenched Nb-Mo bearing steel was investigated by hardness test and theoretical calculation.The results show that the carbide precipitated in ferrite is complex(Nb,Mo)C particle with higher Mo content and the size less than 10 nm,the sublattice fraction of Mo in(Nb,Mo)C reaches 30%.And with the increase of sublattice fractioin of Mo in(Nb,Mo)C,the PTT curve of Nb carbide in ferrite moved to the top left,indicating Mo addition can promote the precipitation kinetics of NbC in the ferrite.This is consistent with the result of physical-chemical phase analysis.Analysis showed that the lattice misfit between(Nb,Mo)C and ferrite was decreased by Mo incorporation into NbC lattice,thus promoting the precipitation of Nb carbide.What's more,those particles,(Nb,Mo)C,precipitated in ferrite Nb-Mo-bearing steel during that the carbonitride didn't precipitate during TMCP process but precipitate when catching tempering exhibit superior coarsening resistance compared to that of NbC particles in ferrite of Nb-bearing steel.This is mainly attributed to the smaller misfit between(Nb,Mo)C and ferrite and the low concentration of Nb in ferrite due to rapid precipitation in early stage in Nb-Mo bearing steel.5)A "intelligent" fire resistant steel with low cost and saving Mo was developed via redesigning the composition of steel with high M(M=Nb,V,Ti,?O.1wt.%)and low Mo(<0.2wt.%),which is a steel fire so as to generate a certain amount of precipitation hardening at elevated temperature.The microstructure of this as-rolled steel consists of granular bainite and polygonal ferrite.The constant load(280MPa)tensile test shows that the 0.2 wt.%Mo addition in Fe-C-Nb/V steel increase the failure temperature of steels by 40 ?,which is attributed,on the one hand,to the more low-angle grain boundary which can provide the favorable nucleation sites for MC-type carbides formed by Mo addition,on the other hand,to the larger amount of Nb carbide due to the role of promoting precipitation by Mo addition.Based on the above basic research,a high Nb low Mo "intelligent" fire resistant steel with yield strength of 521 MPa at room temperature and 360 MPa at 600 ?,and the impact energy of 249 J at-40? was developed. |
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