Study On Microalloyed Steel Slab Transverse Corner Cracks Control By Grain Refinement

Microalloyed steel has advantage of high strength,high toughness and excellent weldability,for which it was widely applied to energy chemical industry,transportation,offshore engineering and many other areas.Nowadays microalloyed steel has become one of the major products in steel enterprises at home and abroad.However,addition of microalloy element in steel increases crack sensitivity at the corner of slab and transverse corner cracks occur frequently which impose enormous economic losses on almost every Iron&Steel enterprise.Now transverse corner cracks on slab has developed into a general technical problem that restricts microalloyed steel's highefficient and quality production.Based on the intrinsic factor of corner crack,this paper observed the slab corner morphology,analyzed the growth and evolution of the corner micro organization during the continuous casting process and illustrated the mechanism of corner crack on micro alloyed steel slab by using metallographic examination and other methods.Then a solidification heat-transfer FEM model(SHFM)of microalloyed steel slab in continuous casting was built on the original vertical banding type slab caster in some manufactory,the solidification and thermal behavior of slab during continuous casting were revealed,the effect of casting speed and cooling intensity on the thermal history of slab corner were inspected.Finally,an equipment modification was put forward and correspondingly a new cooling model was created whose target was to realize the grain refinement and microstructure reorganization by the mechanism of steel phase transformation.At this point,thermal behavior of microalloyed steel slab under new cooling mode mentioned above was studied by simulation,then a new controlled cooling system called double phase transformation was unveiled.By application of new controlled cooling system in some manufactory,it was verified new process can eliminate transverse slab corner cracks efficiently.Through metallographic examination,the change of microstructure was to be see,there were fine-grained ferrite covered the slab corner which improved the plasticity and prevent the slab from transverse crack.The main conclusions are obtained as follows:(1)Under conventional cooling mode,cooling intensity of the microalloyed steel slab surface is uneven,corner of slab cools down more intensely as a result of two-dimension heat transferring,when slab reaches straightening segment,the corner temperature within the trough regions of the third brittle temperature,plasticity of slab corner turns deteriorated and strength turns fragile.The transvers cracks would occur at the corner of slab,once the straightening force exceeds the ultimate tensile strength of slab corner.(2)The slab corner cools off gradually under current continuous casting condition,during which slab corner should be exposed to high temperature above Ac3 for a long time,and interfacial surface energy promote austenite grains growth,coarse austenite structure weakens coordinating ability between grains,impairs plasticity of slab corner.When passing through straightening segment,inner arc of slab bears enormous tensile stress,while,coarse microstructure makes it difficult for austenite grains glide and distortion as well as accommodate to straightening force,which account for the existence of the third brittle temperature range.(3)Under microscope,the corner crack that attach to film-like or mesh-like ferrite was seen.It is known that proeutectoid ferrite is often a result of coarse austenite,as the strength of ferrite is inferior to that of austenite,film-like ferrite deteriorates the bonding force between austenite,makes corner crack easier to generate alone austenite grain boundary.(4)The calculated result of SHFM shows that surface temperature of microalloyed steel slab is not uniform,corner temperature is lower than the other place because of two-dimension heat transferring.In addition,casting speed play an important role in the temperature history of slab surface as well as shell thickness and sump depth,for every 0.1 m/min of increased speed there are about 25? of elevated corner temperature at the mold exit and about 45? at the entrance of straightening segment.What's more,the sump depth would dramatic increase 3m,for every 0.1 m/min of increased speed,so it should be notice whether the cooling strength corresponding to the casting speed.(5)By modification of narrow side roller segment on slab caster,cooling capacity of narrow side on slab can be enhanced.Based on the equipment modification,it is known that slab corner can be dramatic cooled down to the temperature under Ar3 and rebound to the temperature above Ac3 by enhancing the cooling intensity from the mold exit to the middle of bending segment and reducing the water quantity that come from underpart of bending segment.The corner temperature reaches its peak and turns down again before entering fan-shaped sections.At the straighten point the temperature of slab surface drop a little.(6)After practice in manufactory,it is well known that transverse corner cracks of microalloyed steel slab can be eliminated by applicating double phase transformation controlled cooling technology in continuous casting.Through metallurgical microscope,fine grains that derived from double phase change can be see,the plasticity of slab corner improved significantly,the crack sensitivity reduced correspondingly.