Introduction
Interrupted continuous cast slab and its surface cracks are an important quality dysfunction of modern steel production. Surface cracks of steel slabs that appear between guides are called pullout defects. The defects not only cause serious economic losses to the steel production enterprises, but also bring potential safety risks to the subsequent processing and end use of slabs. The surface crack mainly exists in the eyes, up and down horizontal seams, web cracks and vertical cracks. The formation of surface network crack before guide, during and after guide is an important problem of ingot quality control in modern steel production.
Through overall standardization of continuous casting process, technological parameters of tundish and set sand, guide and steel system optimization, rational layout of billet guide, reasonable operation and repair of equipment, etc., the surface of slab can be stabled to a great extent. However, due to the unbalanced nature of the steelmaking and continuous casting process, the quality of surface crack defects is still a difficult problem to solve. Understanding the principle and mechanism of surface crack formation is of great importance to the control of surface crack quality of finished slabs.
1 Historical research on the cause of surface cracks
In the 1970s and 1980s, based on actual observations, European and American steel companies and experts from institutions such as American Iron and Steel Institute (AISI) linked surface cracks to guide plates, tundish and set sand, temperature and chemical composition, and formed a common conclusion about the cause of surface cracks.
The earliest is the theory of guide plate pressing. It believes that the stress and strain caused by pressing lead to the low plasticity of the billet area near the guide plate, thus forming cracks. Then the interference friction theory was proposed. The steel liquid containing fine sand particles is strongly sheared in the slot due to the interference of the guide rails, so that an additional frictional force is generated and the micro cracks are induced at the heated surface.
Later, the mechanism of fine sand introduction and blockage was proposed. The particles of set sand enter and stop the eye of heat when they flow, thus forming high volume stress and causing surface micro cracks. The thermal fatigue theory believes that the surface of the billet experiences rapid heating and cooling due to interference friction and induction self-excitation. Surface cracks are formed due to the difference in thermal expansion coefficient between steel and sand particles.
2 Recent research on the cause of surface cracks
In recent years, with the development of research on mathematical model and numerical simulation, the main mechanism of surface crack has been further detected. First, numerical simulations of continuous casting process are conducted by using simulation experiments. The frictional force at the guide plate, the interfacial method of fine sand in the steel liquid and the thermal fatigue at the surface of the slab are investigated. Then the thermal history at the surface of the slab is analyzed by the 3-D shape recognition method, and the gradient of thermal strain in the billet is found out by the 2-D thermomechanical stapling method.
The simulation result show that the decrease of heat input and the increase of temperature difference between center of slab and surface will accelerate the formation of surface crack. The calculation results also show that the lower the casting speed is, the larger the gradient between the center of slab and surface is. Based on this analysis, the surface temperature transient history and stress distribution of continuous casting slabs can be obtained, so as to further discuss the cause of surface crack.
3 Current prevention and control of surface crack
In the current production process, relevant personnel pay more attention to the corresponding preventive and control measures on the cause of surface crack.
(1) Interficial friction of guide plate. At present, it is one of the important preventive measures to reduce the stress and strain of the nozzle near the guide plate, which can be realized by setting or sizing the chamfered guide plate slot, number and surface of the guide plate roller, lubrication and lubrication system of the guide plate, etc.
(2) Reduction of temperature difference. The influence of the temperature difference on the stress of slab surface and the formation of surface crack can be reduced by the rational distribution of set sand in the steel liquid, the reasonable regulation of fine sand ratio, the comprehensive control of heat input, temperature and flow rate of steel liquid, as well as the adoption of inserts with insulation and heat preservation effect in the nozzle.
(3) Prevention of thermal fatigue. In order to prevent thermal fatigue, different cooling patterns at different parts of the slab surface can be used. Generally, intensive cooling can be used in the clearance of the rollers, and slower cooling operation can be adopted in the middle and slender side of the slab, so as to greatly reduce the thermal fatigue of the surface. In addition, during the design process of continuous casting tundish, reasonable water channel structure can be adopted, so as to reduce the thermal fatigue between different parts of the slab surface.
Conclusion
In general, the main cause of surface network cracks in continuous cast billets is that the operating parameters such as flow rate, temperature, pressure, casting speed, chemical composition and set sand are not balanced, resulting in abnormal heat input into the surface of the billet. Therefore, it is of great significance for the stable production of continuous cast billets to reduce the surface temperature field and the thermal strain of the billet at the same time by optimizing the continuous casting process and other parameters. In view of this, it is suggested that the specific measures to reduce surface network crack should be combined with the specific measures taken according to the actual situation of the workshop, so as to achieve the expected results of improving the quality of slabs and protecting the safety of work.
