Formation Mechanism And Control Technology Of Crack Defect In Peritectic Steel Slab

Peritectic steel is apt to cause crack defect during continuous casting process,which severely affect the quality of slab shell and the production efficiency.The optimization of the slab continuous casting process parameters of peritectic steel is a key but common problem to be solved very urgently.In peritectic steel typical crack defects are divided into two types:surface crack(longitudinal face crack and transverse corner crack)and internal crack(central crack and centreline crack).Research result shows that longitudinal face crack is in connection with large amplitude periodic mold level fluctuation,mold copper plate cooling and steel grade composition;transverse corner crack with secondary cooling,steel grade composition and large amplitude periodic mold level fluctuation;while central crack and centreline crack with secondary cooling,roll gap accuracy and roll gap shrinkage.In order to reduce the crack defects in peritectic steel slab shell,in this paper the continuous casting process of peritectic steel in a slab casting machine is researched.An on-line electromagnetic sensor is used to measure the mold level fluctuation wave under different steel grades carbon contents,casting speeds,slab sizes and physical parameters of mold slag.And the wave data is subjected to frequency and amplitude analysis using fast Fourier transform spectrum analysis for investigating the mechanism for the large amplitude periodic mold level fluctuation,for reducing the slab longitudinal face cracks.A 3D heat-transfer finite element model is established for the cooling slot in mold copper plate and the temperature distributions in transvers direction of the wide copper plate is numerically calculated under the newly proposed cooling slot structure for decreasing slab longitudinal face cracks.A 3D thermal/stress coupled finite element model of the slab shell is also established for analyzing the temperature distribution in slab during its solidification process and the stress-strain change in slab shell during the reduction process for reducing internal cracks.On the basis of the actual production data,the effects of steel grade composition on the slab crack defects are studied.A new location method by relating slab quality to process parameters of continuous casting is proposed,with which the real-time production parameters in the process can be precisely positioning to specific slice of slab according to the modified casting length for realizing the precision management and treatment of the crack defects in peritectic steel slab.Base on the theoretical analysis of data,the simulation analysis and the practical production results,the following conclusions are obtained:(1)In the slab continuous casting process of steels,the main frequency of the large amplitude periodic mold level fluctuations changes with carbon content from ultra-low carbon steel to medium carbon steel,the mold size and the mold flux type.By analyzing the value change principle of the main frequency of the large amplitude periodic mold level fluctuation,it can be deduced that the mold level fluctuation is caused by the dynamic compression and expanding of the liquid core of the slab shell when the unstable bulging passing through the roller,and the large amplitude periodic mold level fluctuation not induced by the oscillation of the mold but the resonance of the steel in the mold when the mold level fluctuation frequency approaches to the natural frequency of the steel in the mold.A new 2-D resonance frequency controlling equation containing the slab cross section size,the mold flux and steel surface tension coefficients and densities is summed up in different steel carban content range as#12 The natural frequency calculated by the above model is well according with the measured main frequency of the large amplitude periodic mold level fluctuation in practical production.The error is within 4.3%.By adopting the following measures:?change the mold level control system model(compensation for the mold level)to decrease the effect of the unstable bulging on the amplitude of the mold level fluctuation;?change the type of the mold slag to change the surface tension coefficient so as to staggered the unstable bulging frequency from the natural frequency of the liquid steel in mold;?improve the cooling intensity in the second cooling zone to reduce the occurrence of unsteady bulging;as a result,in the continuous casting of peritectic steel the large amplitude mold level fluctuation occurrence probability decreases by 12.35%,and in the case of low carbon steel,the probability decreases by 3.45%.After adopting the non-periodic arrangement of the guiding rolls and the compound structure rolls,in the continuous casting of peritectic steel and microalloyed steel,the large amplitude periodic mold level fluctuation occurrence probability decreases by 8.3%.(2)A three dimensional heat transfer model for mold copper plate is established and the principle of the effect of the cooling slot structure on the heat transfer is obtained.According to the simulation analysis result,the copper plate water slot structure in the bolt region is changed from straight to curve.Consequently,the temperature distribution becomes more uniform in the wide face of the copper plate and the temperature at the meniscus on the hot side of the copper plate is decreased by about 10? while the temperature fluctuation range at the bolt region reduced by 6 ? than before and the slab surface longitudinal crack formation probability decreased by 84%.(3)For reduction process of the peritectic steel slab,a three dimensional thermo-mechanical coupling model is established.The change principles of the temperature and the stress/strain distribution are obtained.The internal crack is effectively controlled by increasing the casting speed by 0.1m/min,holding the cooling water intensity before the 4th sector of segment,decreasing the cooling water intensity by 10%between the 5th and 8th sector of segment,shifting the reduction zone from formerly 8th and 9th sector of segment to currently 9th and 10th.(4)A design philosophy of the key composition of easy-growth cracked peritectic steel grade is acquired according to the effect of the steel composition on the crack defect obtained in this research.In the boronated peritectic steel and in the Nb containing steel,trace titanium treatment and reasonable low nitrogen composition control is recommended.The transverse corner cracks in both cases are decreased remarkably and the corresponding edge seams defect indexes of hot rolled coil are decreased from 4.2 to almost nothing and from 5.5 to 2.0,respectively.(5)A new location model by relating slab quality(including crack defect)to process parameters of continuous casting is obtained.The process parameters can be precisely positioning to specific slice of slab for precision management and treatment of the crack defects in peritectic steel slab.The positioning accuracy can reach the level of 50mm.