Mechanism And Control Of Surface Crack Initiation And Propagation Of Continuous Casting Beam Blank

With the development of beam blank continuous casting technology,the problem of surface crack has been the key factor limiting the improvement of casting blank quality.The casting blanks with surface cracks need to be finished in light cases,while serious ones lead to steel leakage and waste products,which seriously affect the production schedule and increase the production cost,and it is difficult to meet the development requirements of modern continuous casting technology.Therefore,many metallurgical scholars have carried out a series of theoretical research and practical investigation on the mechanism and influencing factors of casting blank cracks.However,due to the limitation of production site and equipment,it is almost impossible to measure the stress causing crack propagation,and it is even more difficult to monitor the dynamic propagation of cracks,which makes the behavior of crack initiation and further propagation unclear.Therefore,it is impossible to deeply study the propagation process of beam blank surface cracks in production.In order to further understand the initiation and propagation mode of surface cracks on beam blank in continuous casting,truly show the dynamic process of crack propagation,and realize quantitative analysis of crack propagatio.This paper aims at the surface cracks on beam blank in continuous casting of a steel plant,based on the traditional macro-qualitative research methods,on the basis of other slab crack mechanism,combined with the special cross-section shape of beam blank,using finite element software to simulate the solidification process of beam blank in the mold.And on this basis,determines the crack initiation position.Combined with macro and micro simulation methods,a series of basic research work on surface crack initiation and dynamic propagation has been carried out.The mechanism of surface crack initiation and propagation is deeply explored,which provides theoretical basis for further improving the surface quality of beam blank.The specific research contents and results are as follows:(1)The microstructure analysis of surface cracks and inclusions on beam blank was carried out.Starting from the surface cracks in the production of beam blank continuous casting,the macroscopic analysis of surface cracks was carried out,and the occurrence rate,location,morphology,size and law of cracks were counted.By means of metallographic microscope,scanning electron microscope and energy dispersive spectrometer,the morphology of the microcracks,the microstructures around the cracks,the types of inclusions and the micromorphology of the microcracks were observed.The main position of the surface cracks on the beam blank were put forward,and the causes were analyzed.(2)Based on the characteristics of heat transfer and stress and genetic behavior of temperature and stress in continuous casting of beam blank,a thermo-mechanical coupling analysis model of beam blank was established.Based on the test results of mechanical properties of as-cast steel at high temperature,the temperature and stress distribution of beam blank were studied,and the germination positions of cracks in beam blank in mold were determined to be the center of web 180 mm away from meniscus and the R angle position 200 mm away from meniscus.Combined with the temperature and stress parameters at the crack germination position,the dynamic propagation process of macro cracks at the easy-to-germinate crack position of beam blank in mold was simulated by using the extended finite element method and sub-model technology.It is found that the longitudinal surface cracks propagate more easily,and the final propagating direction is basically perpendicular to the main tensile stress direction.The crack propagation at the web is greater than that in the same stress direction at R angle,and the crack propagation length gradually increases with the increase of the inclination angle in the stress direction.It is not easy to propagate the transverse cracks in the web or at the R angle.(3)A polycrystalline model of meso-crack initiation in as-cast steel was established.By using the extended finite element method,combined with the properties and distribution of inclusions and pores in the slab,the influence law of crack initiation on the properties,size,ratio,position and shape of inclusions and pores was systematically and quantitatively studied from the microscopic point of view.It is found that soft inclusions,slender inclusions and polygonal inclusions are more likely to induce cracks,while round(spherical)inclusions and fine inclusions are difficult to germinate cracks.Cracks can be induced when the size of soft inclusions is larger than 2.4 μm and that of hard inclusions is larger than 5μm;The bigger the difference of elastic modulus between inclusion and matrix,the easier it is to germinate cracks.The enlargement of pores,the increase of porosity and the slender shape of pores all aggravate the germination of cracks.Inclusions and pores located on the surface of slab are easy to induce cracks and further propagate.(4)A polycrystalline model of meso-crack propagation in strand was established.The influence of inclusions,pores and the coupling effect of inclusions and pores on the dynamic propagation of surface cracks in different positions and different stress directions of beam blank was systematically studied from the microscopic point of view.It is found that inclusions and pores located at a certain distance below the crack will aggravate the crack propagation,whereas hard inclusions and pores located at a certain distance above the crack will inhibit the crack propagation.The coupling effect of pore and inclusions will further aggravate the crack propagation.(5)A crack arrest polycrystalline model for meso-cracks of slab was established.Based on the polycrystalline model of porosity crack and inclusion crack in as-cast steel,the influence of porosity,inclusion property,size and position on crack propagation is explored.It is found that pores and hard inclusions can inhibit crack growth to a certain extent.When the distance between the crack tip and the center of the crack-stop hole is equal,the size of the crack-stop hole with crack-stop effect increases with the crack growth.When the distance between crack tip and crack stop hole is 1 μm,the crack stop hole has the best crack stop effect.When the distance between hard inclusion and crack tip is close to 0,the crack arrest effect of hard inclusion is the best.(6)The macroscopic measures for controlling surface cracks on beam blank were put forward.Through the combination of macroscopic and microscopic analysis method,it is found that the increase of drawing speed and superheat will promote the crack initiation and propagation,and the increase of superheat has greater influence on crack initiation and propagation than that of drawing speed.Therefore,the casting process parameters are optimized,and it is suggested that the casting speed should be controlled at 1.0～1.1 m/min and the superheat should be controlled at 20℃～25℃.In order to reduce the stress of beam blank,the optimization of single taper and multi-taper of beam blank mold was carried out,and it was suggested to use the optimized multi-taper mold.Based on the research on crack arrest of inclusions and pores,the measures to control the cleanness of beam blank in production are put forward.The combination of micro-research and macro-research can provide basic theoretical and technical support for reducing the incidence of surface defects and improving the quality of beam blank,which has high academic research value and production practice significance.