Study On High Temperature Plasticization Control For Surface Microstructure Of Continuous Casting Slab Of Microalloyed Steel

Hot delivery and hot charging technology of continuous casting slab has the characteristics of reducing energy consumption,shortening process flow and improving yield.It has brought many economic benefits for iron and steel enterprises.However,when Nb and other microalloyed steel slab are hot-charged,the surface crack rate of the rolled steel often increases greatly.Surface quenching technology can change the thermal history of slab surface and effectively solve the hot transport crack problem of microalloyed steel slab containing Nb.However,some specific technological parameters of surface quenching technology for continuous casting slab,such as quenching start temperature,quenching speed,and the change of slab temperature during quenching,still need further study.The medium and heavy slabs of Nb-containing microalloyed steel produced by a domestic steel plant were studied.Firstly,the static CCT curves of Nb-containing microalloyed steels were measured.The thermodynamics and kinetics of carbonitride precipitation in Microalloyed element were calculated.It was found that with the increase of cooling rate,the phase transformation temperature decreases.When the cooling rate is 0.2?/s,Ar3 is about 782?,Ar1 is about 695?;when cooling rate is 5?/s,Ar3 is about 665? and Ar1 is about 610?.The precipitation temperature of Ti is higher,and the precipitation of Ti is basically completed when the slab is cast out of the continuous caster.The relative precipitation temperature of Nb is lower.The "nose point" temperatures of Nb(C,N)under grain boundary,dislocation and homogeneous nucleation mechanism are 928,820 and 836?.Secondly,the quenching process of medium and heavy slab was simulated by using MSC.Marc finite element simulation software.It was found that the central temperature of wide surface of slab decreased from 955? to 403? during the quenching process,and then after the latent heat of the core of slab,the surface temperature could rise to more than 780?.The quenching cooling temperature of 10mm below the center of the wide surface of the slab decreased from 980? to 550? after quenching,which met the requirement of the structure transformation temperature of the fast cooling.Thirdly,the effect of different quenching start temperature and quenching speed on the microstructure evolution and precipitation behavior of carbonitride in slab was studied by Gleeble thermal simulation experiment.It was found that no preeutectoid ferrite was formed at grain boundary at 950?.The precipitates were square,and mainly containing Ti,and the size of precipitates was fine about 10 nm.When quenching speed is 5?/s,there is no widmanstatten structure,ferrite is fine,precipitate size is about 20 nm,and the distribution is dispersive.Finally,Combining with the actual production situation,a surface quenching device for slab was designed and field production test was carried out.Under the surface quenching process,the precipitates in the slab were dispersed and the size was less than 20 nm;there was no the net preeutectoid ferrite and widmanstatten structure,and equiaxial or acicular ferrite could be formed on the surface of the slab with a thickness of more than 20 mm.The quenched slab is directly heated to the heating furnace and rolled into a 35 mm thick plate.All the first and second inspections are qualified,and all the flaw detection are qualified,which can realize the qualified supply.