Research On The Formation Mechanism Of Corner Cracks In Q345 Bridge Plate Steel In A Steel Plan

At present,the Q345 qD cast billet produced by a domestic steel plant has a high incidence of corner cracks and surface transverse cracks,which seriously affects its continuous casting production and product quality.This paper uses Gleeble 3500 thermal simulation test,Factsage7.0,scanning electron microscope,infrared thermal imager,ANSYS finite element software and other means to analyze the causes of crack formation,high temperature thermoplasticity,temperature field,and thermal stress of the Q345 qD cast slab produced by a steel plant.The main conclusions obtained are as follows:(1)Through the analysis of the crack morphology and inclusions of the Q345 qD cast slab,the cracking morphology of the transverse cracks and corner cracks on the surface of the cast slab is mainly serrated,which is a typical brittle crack.Large inclusions are not found at the cracks.It shows that the crack formation is basically not related to inclusions and may be related to excessive deformation in the brittleness-sensitive temperature range.(2)The high temperature thermoplasticity test shows that as the tensile deformation temperature decreases,the shrinkage of area of Q345 qD steel shows a trend of first increasing and then decreasing.The first brittle range of Q345 qD is melting point ～ 1250 ℃,the high temperature plastic range is 1200 ℃ ～ 900 ℃ and the third brittle range is 850 ℃ ～ 800 ℃(or lower);The formation of corner cracks of Q345 qD steel billet may be related to continuous casting.The temperature of the inner and outer arc corners of the second cooling section 8 and 9 are in the brittle temperature range and the corner cracks are related to static reduction.(3)Through the Q345 qD slab three-dimensional temperature field simulation,it is concluded that the drawing speed has a greater impact on the slab temperature field,the solidification end point and the central solid phase ratio,followed by the secondary cooling water ratio,while the slab width and superheat are relatively small..Increasing the drawing speed will cause the slab solidification end point to move back and the liquid phase depth to elongate.When the drawing speed is increased by 0.1 m/min,the solidification end point moves back by about 2.7 m,and the slab is at the end of the second cooling stage 7 and 8 The center temperature of the wide face of the cast slab at the end of the section and the 9th section increased by23 ℃,28.3 ℃ and 61.3 ℃ respectively,and the temperature of the center of the narrow face of the slab increased by 62.3 ℃,63.6 ℃ and 65.2 ℃ respectively.Therefore,it is possible to avoid the formation of cracks caused by the straightening of the Q345 qD billet in the hot crack sensitive zone by optimizing the drawing speed or the specific water volume of the secondary cooling,or to prevent the temperature of the inner and outer arc corners of the billet from being brittle when the billet is statically reduced.The temperature range leads to the generation of surface transverse cracks and corner cracks.(4)Through the Q345 qD slab three-dimensional thermal stress field simulation,the thermal stress field distribution of the full-flow full-size slab is very uneven and the maximum thermal stress is concentrated in the corner of the slab transition from the bending section to the straightening section.It is easy to cause cracks in the corners of the cast slab in this area.(5)By increasing the compressed air in the secondary cooling straightening section of Q345 qD continuous casting(adding a tube to blow air,the pressure is 4 bar)to blow off the surface water of the casting slab and reduce the specific water volume of the secondary cooling(the specific water volume is reduced from 0.4 L/kg to 0.35L/kg),significantly reduced the crack occurrence rate of its cast slab.The crack occurrence rate in 2020 is reduced by about 54.81% compared with 2019,and the incidence of corner crack on the north side in 2020 is reduced by about 73.03%compared with 2019.