Study On The Control Of Continuous Casting Slab Quality Under Unsteady Casting Conditions

It is becoming the key point for controlling the continuous casting slab quality during the period of unsteady casting such as casting start, casting speed variation and casting end that the cleaness of casting molten steel is deteriorated. In this thesis, the problems of unsteady casting slab quality with the high casting speed caster were selected and studied monographically. Furthermore, the reasons giving rise to the quality problems were analyzed. The ways to solve the problems were explored by mathematical modeling and water modeling for continuous casting tundish and mold. The strand samples before and after the process optimization were evaluated with macroscopic examination, quantitative metallography and so on. The feasibility and effectiveness of the schemes for solving the quality problems were analyzed and verified by on-site experiments. The technique schemes suitable for actual situation have come into being at last. The on-site experiments show that the technologies for improving slab quality under the steady and unsteady casting conditions are apparently effective, which are beneficial to the quality of final products. The main researches and results are as follows.1. On-site investigation and present situation analyzing of slab quality(1) The strands under 15 operating conditions were sampled and analyzed, and the subcutaneous T.[O] was inferior to 27ppm. The statistical result of metallographic specimen shows that inclusion is spherical Al2O3 or silconaluminate with the size less than 50μm mainly, and the castability and cleanliness of molten steel need improvement.(2) Argon bubbling primarily leads to the pinhole of strand scattering mainly on the narrow face during steady casting periods. Defect is aggravated with the increase of argon flowrate. Immersing depth of nozzle has an effect on medium-small inclusion, and gas and inclusion increase with immersing depth of the nozzle.(3) Strand quality is greatly influenced by casting starting, casting ending, casting speed variating and so on under unsteady casting periods, and the amount of inclusion is as 2-6 times as that at the normal casting periods. Ladle change, nozzle change and nozzle blocking, however, have less impact on strand quality.2. Physical modelling and control of flow field in tundish(1) Casting starting and state of fluid flow affect the inhalation, and the amount of inhalation at turbulent flow is more than that at laminar flow apparently.(2) During the changing of ladle, the higher the liquid level is, the more advantageous it is to remove the inclusions. It is of use for removing inclusions to increase pouring speed, moreover, the higher the casting speed is, the more disadvantageous it is to remove the inclusions.(3) With the increase of casting speed, the variation law of critical height for slag entrapment due to vortex is from high to low and then to high, and the vortex critical height is the minimum at 1.4m/min casting speed. Proper slag depth should be selected, 40mm is regarded as "critical depth". Critical height increases at first and decreases afterwards with the viscosity of covering flux increasing. Stopper controlling flow makes a contribution to controlling vortex slag entrapment.(4) During continuous casting grade transition process, grades with similar and intercross composition should be selected to sequentially cast. During the sequentially casting of low carbon grades to high carbon grades, the transition time is short.3. Physical modeling and control of flow field in continuous casting mold(1) While casting small-section strands, factors affecting slag entrapment in mold are casting speed mostly, blowing argon flowrate secondly, and immersion depth of nozzle least. Factors affecting level fluctuations and narrow face bubbles are casting speed mostly, immersion depth of nozzle secondly, and blowing argon flowrate least. While casting large-section strands, factors affecting slag entrapping, liquid flux distribution and narrow face bubbles are casting speed mostly, blowing argon flowrate secondly, and immersion depth of nozzle least. Generally, casting speed has much more effect on mould flow field than other factors.(2) The frequency of slag entrapping and the thickness of liquid flux increase with the casting speed, decrease with immersion depth of nozzle increasing, and increase with blowing argon flowrate. Narrow face bubbles increases with the casting speed, and increases slightly with blowing argon flowrate.(3) Imperfect alignment of mold nozzle leads to slag entrapment and uneven distribution of liquid flux, which become worse with the drift of mold nozzle. While nozzle clogged, the possibility of slag entrapment is diminished at the side of outlet clogged, and the possibility of slag entrapment caused by the shearing flow becomes higher at the other side. When the area of nozzle clogged at one side reaches 40% of the opening, mixed types of slag entrapment appear at the other side, and narrow face is exposed largely.(4) While nozzle clogs doposited, and casting speed is between 1.6 m/min and 2.0 m/min, the general probability of different volume nozzle clogs captured by solidified shell reaches 20.4%.The probability of nozzle clogs captured increases with the size reducing and casting speed increasing.(5) During shifting the casting speed, the frequency of slag entrapment and the uneven distribution of liquid flux increase with the acceleration increasing. As viscosity of liquid flux increases, the possibility of slag entrapment decreases, and the degree of uneven distribution of liquid flux decreases.(6) Parameters of structure and dimension for nozzle are optimized. While the section is 21 O×1300mm2, and casting speed exceeds 1.8 m/min, the phenomena slag entrapment in the liquid surface and mold exposure are obvious. The optimal nozzle is suitable to cast the 210×1300mm2 strand section with various molten steel flowrate that the correspondent casting speed is between 1.2m/min and 2.0m/min.4. Numerical simulation of mould flow field(1)While casting large-section strands and the casting speed reaching 2.0m/min, it can be calculated that the maximum molten steel flowrate at centerline of meniscus level reaches 0.553m/s, which surpasses the molten steel flowrate at the meniscus level, therefore it should be controlled within 0.3m/s.(2) Comparing the distribution of bubbles of 300μm and 2.5mm in the mold,300μm bubbles reach the narrow face and are captured by shell more easily, thus the probability of subsurface porosity is high.(3)Comparing the mold flow fields with/without electromagnetic braking, electromagnetic braking suppresses greatly outflowing speed of molten steel, which lessens the shake of molten steel on solidified shell. If reducing the upper circumfluence region and controlling the disturbance of meniscus level, the molten steel flowrate at the meniscus level can be decreased greatly. Under no electromagnetic braking, while casting speed surpasses 1.8m/min, exposure appears at the boundary of liquid flux layer, and the probability of breakout increases.(4) Imperfect alignment of mold nozzle causes the unsymmetrical degree of mold flow field increasing, the molten steel flowrate at the meniscus level ununiforming and the meniscus level vortex, which makes the mold flux and liquid flux entrapped into the molten steel more easily.5. On-the-spot test and application effect(1) The porosity at the edge of slab. Subsurface porosity at the edge of slab is caused by excessive argon stirring at the mold. From on-the-spot test, after argon stirring improved, pinhole porosity decreased by 61.8% than the former process, the slab quality is improved apparently.(2) Controlling the tundish flow field and optimizing the submerging nozzle. Suitable parameters of structure and dimension for nozzle are introduced. Stopper makes a contribution to control slag entrapping at casting beginning and casting finishing. Different grades should be sequentially cast according to carbon content from low to high.(3) Optimum slab cutting during unsteady casting. Many on-the-spot tests are carried and samples are analyzed. Accurate affected area longitudinally is put forward for quality of unsteady casting strand.(4) Establishing process system and formulating standard. Suitable argon stirring processes are introduced for different casting speeds and sections. The critical value of argon flowrate is defined. Relating regulations are applied on the spot, which have been brought into operation controlling standards and regulations.