Study On Transverse Crack Of Q345 Steel Slab

With the development of bridge construction and highway transportation,the application of Q345 steel is more and more extensive.This requires Q345 steel could maintain good performance even in harsh working environment,but slab is often accompanied by surface transverse crack,although the transverse cracks can be eliminated by heat blow sweep method,the heat blowing is a time-consuming and laborious process,which could reduce the productivity seriously.At present,a domestic steel production of 260mm×2070mm Q345 steel billet exists serious surface transverse crack,for the Q345 steel surface transverse crack problems.In order to control of slab surface crack fundamentally,this paper starts from the behavior of solidification,analysis the influence factors and control measures of slab surface transverse cracks in-depth by adjusting the parameters of continuous casting technology.To solve the above problems,the typical billet specimens which have crack on were analyzed by metallographic microscope and scanning electron microscope,the surface cracks were mainly distributed in the corner of steel slab,and extend into the along the proeutectoid ferrite film which at the grain boundaries,the thickness of proeutectoid ferrite film was too large that analysis under the microscope;And the second phase particles which precipitated in austenite grain boundaries were distributed like a chain,that would embrittle the grain boundary,easy to form a hole when billet was under stress,it is not conducive to reduce the crack,the composition analysis of internal crack showed that there was no mold powder in the crack,so the crack should be formed under stress at the bending or straightening process.It is found through macroscopic examination that the equiaxed grain ratio is low and the columnar crystal is big,the oscillation mark of the slab surface is deep,which may lead to the nonuniform heat transfer in the narrow surface of the slab,that is easy to produce cracks.Through metallographic microscope casting microstructure showed that the surface was a certain thickness of the chilled layer,inside was widmanstatten,it is found in the statistical analysis of slab surface chilling layer thickness,chilled layer thickness in the narrow face slab is very thin or nonexistent chilling layer,the inside widmanstatten structure under the action of force will easily lead to cracking;proeutectoid ferrite in the slab is narrow membranous distribution phenomenon is more obvious,the continuous distribution of the soft phase is also conducive to the expansion of crack;at the same time in the billet outside the arc corner discovered sorbite region,which may caused by excessive mold taper,that would increase the friction,which would cause transverse cracks appeared in the casting process.Three different process schemes of mould taper were made based on the above results and current taper mould parameters(1.1%-1.2%),which were 0.9%,1.0% and 1.1%,study on the quality of billet under different process parameters could found that,when the mould taper was 1.0%,the thickness of chilled layer was the largest and the micro structure is more uniform;At this time,the hardness data is concentrated relatively while measured with vickers hardness tester;for Q345 billet,it is better able to compensate the air gap caused by shell solidification shrinkage when the mould taper is 1.0%,which could guarantee the heat transfer in billet surface is good.The final results show that the mould taper parameter in the casting process is too large,resulting in the slab surface transverse crack,the billet quality is obviously improved and the crack is under control when the mould taper parameter is set to 1.0%.