Study On The Formation Mechanism And Control Technology For Transverse Corner Crack Of Continuous Casting Slab Of High Strength Microalloyed Steel

The transverse corner crack of continuous casting slab is one of the main factors that affect the products quality and the producing process of high strength microalloyed steel.In the present work,the high strength microalloyed steels which were produced by Meishan Steel were taken as the research object.The causes and control directions of the transverse corner crack of continuous casting slab were analyzed,and the high temperature thermoplastic properties of the test steels under different cooling conditions were studied.The numerical simulation techniques were used to investigate the thermal and mechanical behaviors of the shell in mold and secondary cooling zones during different process conditions;Furthermore,the optimization study of the taper of a new type of curved surface mold,and the developing research on the process and equipment of grain refinement control in the secondary cooling zone were carried out.As well as the industrial tests of the new type mold and secondary cooling optimization based on the transverse crack control were achieved.The main contents and results show as below:(1)At first,the precipitates and microstructure of the slab corner samples of high strength microalloyed steel produced by Meishan Steel were surveyed by TEM and optical microscope.The results show that the main factors of inducing lateral corner cracks is the existence of chaining high titanium and low niobium carbonitride,and the pro-eutectoid ferrite films forms at the original austenite grain boundaries.The main size of the precipitates is from tens to hundred of nanometers.(2)Through analyzing the difference of high temperature thermoplastic performance behavior of test steel under different cooling strength conditions,it can be seen that when the temperature is lower than 1150? and increasing the cooling rate more than 5?/s can realize the refinement and scattering distribution of micro-alloying carbonitrides both on the grain boundaries and in the grain.This also can eliminate the pro-eutectoid ferrite film at the original austenite grain boundaries which can be found at traditional continuous casting condition.All these are conducive to improving the crack resistance of the steel as a whole.(3)Aimed at the No.3 slab casting machine at Meishan Steel(max section size 1650mm X 230mm),a coupled thermo-mechanical finite element model of the shell-mold system with dynamic flux and air gap distribution was established.Through the study of the distribution characteristics of the flux film and air gap,the temperature distribution and evolution of the shell,the stress distribution and so on,the changing patterns of the thermal/mechanical behavior of the shell during the solidification in the mold were comprehensively analyzed.The results show that during the process of producing high strength microalloyed steel at Meishan slab caster,the air gap is generated in the middle and lower part of the mold,and it mainly concentrates in the range of 0?20mm from the wide/narrow face corner of the shell;the average thickness of the flux film in the narrow face corner region is thicker than that of the wide face.The thickness of the mold flux increases at first and then decreases along both the center of the wide face and the narrow face,concentrating in the range of 0?40mm from the corner of the slab.The shell surface temperature decrease smoothly in the upper part of mold,but the temperature of the shell corner falls most rapidly in this height range.The temperature of the shell corner decrease slowly in the lower part of the mold,which is unfavorable for the dispersion precipitation control of microalloy carbonitride of Nb,Ti,and etc.(4)In order to simulate the temperature and stress fields of slab during continuous casting process more precisely,taking the secondary cooling zones of slab caster in Meishan Steel as the research object,Marc finite element analysis software is used to establish the three-dimensional temperature and the stress-strain field numerical simulation model for the slab solidification process of Meishan Steel.The variation of slab temperature,shell thickness,thermal shrinkage and thermal stress at different casting speeds are studied.It is show that all the corner temperature of slab passing through the straightening zone is in the third brittleness temperature region of high strength microalloyed steel grades.In the whole continuous casting process,the thermal shrinkage of the slab increases continuously,and the increasing rate shows a trend pattern of speed up-slowing down-speed up.The equivalent thermal stress of the slab shows a tendency of two stages.In the first stage,the equivalent thermal stress of the slab shows a trend of slow and fluctuated increasing;in the second stage,the equivalent thermal stress of the slab increases rapidly when the temperature of the corresponding position is below 900?.(5)Based on the above research,the development of a new type mold with inner convex curved taper that can effectively inhibit the precipitation of carbonitride in the grain boundary of slab corner was realized.And.a new equipment.and technology of the secondary cooling to control twice phase-transition that can refine the grain size of microstructure was achived.After the relevant industrial tests had been carried out,the results show that the integrated technology can significantly control the occurrence of slab transverse corner cracks,and the upwarping defects rate of hot rolled coils decreased from 1.31%to 0.018%.