Study On Surface Quality Control Of Medium Carbon Microalloyed Steel Continuous Casting Slab

In order to improve the mechanical properties and processing performance of the steel,the carbon content of the microalloyed steel is increased to the range of medium carbon steel,and the straight arc continuous casting machine combined with the argon blowing process is used for continuous casting slab production,eliminate the inclusion aggregation defect at 1/4 of the inner arc side of continuous casting slab produced by arc continuous caster,to realize the production of defect-free medium carbon microalloyed steel continuous casting slab.However,in practice,the outer arc corner crack defects and subcutaneous bubble defects occur frequently in the production process of carbon microalloyed steel continuous casting slab,which has a serious impact on the quality of continuous casting slab and subsequent processing,and has become a common technical problem limiting the high-quality and efficient production of steel.Therefore,the mechanism of corner crack and subcutaneous bubble defect of continuous casting slab is systematically studied,and a reasonable process system for surface quality control of continuous casting slab is formulated,so as to realize the defect free production of medium carbon microalloyed steel continuous casting slab in straight arc caster.Through field process parameters and continuous casting slab surface defect investigation,the formation reasons and key influencing factors of typical surface defects of medium carbon microalloyed steel continuous casting slab were determined.The reason for the formation of cracks at the corner of the continuous casting slab of medium carbon microalloyed steel is that the corner of the continuous casting slab contains large Al2O3 inclusions and the temperature of the corner of the continuous casting slab is in the high temperature brittleness range during the bending/straightening process.At the same time,the superheat of molten steel and the large fluctuation range of the liquid level of the mold also have a significant impact on the corner cracks.The reason for the formation of subcutaneous bubbles is that the plug,upper nozzle and argon sealing combined argon blowing process are unreasonable.A large amount of gas enters the mold to form small bubbles,and the bubbles collide with the micro inclusions in the liquid steel during the movement of liquid steel flow and are captured by the solidification front.By analyzing the composition,morphology and size of typical large inclusions and microscopic inclusions in the continuous casting slab,the composition and source of typical inclusions were clarified.The microscopic inclusions in the continuous casting slab are mainly composed of spherical deoxidation product inclusions with a size of 2～3 μm,which are captured by small floating bubbles during the casting process to form a "bubble+micro inclusion"subcutaneous bubble defect with Ca,Al and O micro inclusions.The large-scale inclusions in the corner defects of the continuous casting slab are mainly largescale inclusions formed by the Al2O3 precipitate at the nozzle during the casting process.At the same time,the slag entrained on the liquid surface of the mold will also form large-scale inclusions containing Na and K mold slag components.The formation mechanism of corner crack was revealed by high temperature tensile test,microstructure transformation and typical precipitate detection.The brittleness range of 30CrMo medium carbon microalloyed steel is 765～710℃.The cooling process of continuous casting slab did not reach the austenite phase transformation temperature,and the main precipitate(Cr,Fe)XCY had little pinning effect on the austenite grain boundary.It was clear that the formation mechanism of corner crack of continuous casting slab was that a large number of grain boundary ferrite induced by austenite deformation in the temperature range of 825～725 ℃ and bending stress caused the plastic deterioration of continuous casting slab,and the corner crack occurred after the corner structure on the outer arc side reached the stress limit.Based on the traditional two-dimensional heat transfer model,the solidification heat transfer model of 30CrMo steel continuous casting slab in the continuous casting process was established,and combined with the corner crack mechanism,the secondary cooling water distribution process of the continuous casting slab was optimized.Under the condition of raw water,the corner temperature of the continuous casting slab is in the brittle temperature range,which is easy to produce corner cracks.Based on the mechanism of corner crack of continuous casting slab,the control strategy of secondary cooling water quantity in the cooling area and weak cooling in the reheating area of continuous casting bending section under the condition of strong cooling is put forward.Field industrial test results show that the incidence of corner cracks in the continuous casting slab is reduced from 2.16%before improvement to 0.26%after improvement.Through mold physical model,the characteristics of bubble movement in the mold were studied,and the total argon blowing amount of stopper and upper nozzle was determined.Combined with the field industrial test,the compound argon blowing process of "stopper-upper nozzle-argon sealing" was determined.In actual industrial production,the amount of argon blowing at the stopper rod and nozzle is controlled to be 7 L/min and 3 L/min respectively,the amount of argon blowing at the nozzle argon seal is 10 L/min,and the immersion depth is 150～170 mm,which can achieve bubbles and inclusions.Quickly float to remove and reduce mold slag.The field industrial test results show that after using the optimized secondary cooling water volume combined with the "stopper-upper nozzle-argon sealing" composite argon blowing process,the cracks at the corners are controlled,and the incidence of subcutaneous bubble defects is reduced from 8.46%before improvement to after improvement.of 1.11%.