Inner Quality Control Of Continuous Casting Billet With High Carbon Content

Continuous casting high carbon billets and blooms serve as basic material to manufacture steel products for special purposes such as bearings, springs and cords. Accordingly, their quality, especially inner quality, directly affects the yield and quality of final products. Therefore, this paper chooses the inner quality control of continuous casting high carbon billet as main contents. During the continuous casting process of high carbon billet with a cross section of 160 mm×160 mm in a steel plant, the muti-strand mode makes the residul time of molten steel in tundish of each strand differ greatly causing large difference of super heat and inclusion level among them. Moreover, because of intrinsic solidification characteristics, the high carbon steel billet suffers from central segregation much. It is known to all that those potential quality problems do much harm to high inner quality of high carbon steel billet. Therefore, the present work focus on the flow characteristics of molten steel in tundish and central segregation of billet, in order to develop an appropriate casting process of high quality billet with high carbon. Firstly, Flow characterization of tundish in different control device and operating parameter was studied by physical simulation and numerical simulation. Based on the simulated results, the reasonable control device and operating parameter was proposed. Secondly, this paper developed a heat transfer model for continuously cast billet, together with billet surface temperature tests by ThermaCAMTM, nail shooting tests and corrosion tests of solidification dendrites, to investigate variation pattern of billet surface temperature and shell thickness and the distribution of cooling intensity and to predict the thickness of liquid core under different casting conditions at the location of final electromagnetic stirring (F-EMS), to perfect the casting speed and secondary cooling intensity as well. Then, based on the results of secondary cooling intensity a mathematical model of F-EMS was developed to investigate the variation pattern of flow and electromagnetic field under different F-EMS parameters and perfect them. The metallurgical results were investigated through industial tests. According to the research above, this paper developed a produing process including low cating speed, intensive secondary cooling and F-EMS to improve the central segregation of high carbon billet. The main conclusions are summarized as follows.(1) The results of physical and numerical simulations on the flow field of the tundish with different control devices show that the volume fraction of plug region was small with large dead region and short actual residence time of melt steel, indicating the bad removal result of inclusions. Therefore the control device of tundish needs to be further improved. According to the results from the previous references and the research achievements of the present author’s laboratory, combined with real practice four different figurate turbulence inhibitors and two different shaped porous baffle walls were designed, and the flow under different control flow schemes was simulated by numerical and physical modeling. Considering the volume fraction ratio between plug region and dead region and the consistency of each strand, the tundish flow of Scheme 10 (Fig.1) with 3# turbulence inhibitor and 2# porous baffle wall was rather suitable than other schemes. The optimal operating parameters for the ten-strand tundish are the submergence depth of 250 mm, the bath depth of 800 mm and the casting speed of 2.0 m/min. After optimization, the average temperature difference among each strand decreased from 8℃ to 4℃, and the number of inclusions is significantly reduced from 2.44 per square millimetre to 1.42 per square millimetre, especially, the inclusions larger than 21 μm were also removed from 15.2×10-3 per square millimetre to 6.2×10-3 per square millimetre.(2) According to billet surface temperature tests by ThermaCAMTM, shooting tests and dendrite corrosion tests, together with mathematical model of solidification heat transfer during billet continuous casting, this paper investigated the uniformity of secondary cooling and the effect of secondary cooling pattern on the heat transfer and liquid core thickness at F-EMS. Under intensive cooling conditions (1.03-1.06 L/kg), at the casting speed of 1.90 m/min,1.99 m/min and 2.05 m/min, the measured shell thickness of 70 steel at shooting place (11.786m from the meniscus) is 67.0 mm,65.0 mm and 62.0 mm, and liquid core thickness at inlet and outlet of F-EMS are 72.2 mm and 64.4 mm,74.8 mm and 67.6 mm and 75.0 mm and 68.0 mm, repectively. The corresponding values of SWRH82B steel at 11.476 m from the meniscus under the casting speed of 1.90 m/min,1.99 m/min and 2.05 m/min are 64.0 mm, 61.0 mm and 55.0 mm,73.0 mm and 65.2 mm,75.4 mm and 68.2 mm and 81.8 mm and 75.2 mm, repectively. Under casting speed of 1.80 m/min, reasonable liquid core thickness of 70 steel billet at F-EMS is 63.9-55.0 mm, according to industrial tests. Meanwhile, as the current and frequency are 340 A and 6 Hz, the casting speed and specific water flow are 1.81 m/min and 1.03 L/kg, respectively, columnar dendrites in SWRH82B steel billet with some angles from billet surface. From billet surface to center, the cooling rate in mushy zone decreases from 50.2 ℃/s to 0.7 ℃/s, promoting a tiny equixed dendrite zone with thickness of 2.0 mm and a columnar zone with thickness of 28.0 mm, meanehile the primary dendrite arm spacing increses rapid from 136.3 μm to 415.9 μm, however the secondary dendrite arm spacing keeps within 180.0 μm and 280.0 μm. Under weak cooling conditions (0.44-0.46 L/kg), suface temperature recovery in the air cooling zone of 60 steel and 70 steel are 105.1 ℃ and 109.1 ℃, respectively, which increases the probability of inner cracks,(3) Based on the validated measured magnetic flux density, the mathematical model coupled electromagmetic field and fluid flow was estiblished. The distribution of electromagnetic field and flow field at the solidification end was studied and the variations of flow velocity at different process parameters were numerically investigated. The magnetic flux density in the billet center raise with the increase of current intensity. With the effect of electromagnetic stirring, the molten steel in the two-phase region swirl intensely and the stirring intensity reach its maximum in the lower part of the agitator. And the stirring velocity increase with the rise of current intensity and frequency. As the final electromagnetic stirring current is fixed in 6Hz, the maximum tangential velocity raises from 11.2 cm/s to 18.2 cm/s with the increase of current intensity from 300 A to 400 A. Similarly, the maximum tangential velocity raises 1.7 cm/s with increasing frequency of 1Hz eachly as the current intensity is fixed in 320 A. With the casting speed rising from 1.8 m/min to 2.0 m/min, the increase of maximum tangential velocity is 7.16 cm/s. While the maximum stirring velocity increase only 2.64 cm/s due to the small variation of temperature field with the rise of superheat 20 ℃. As the 70 final electromagnetic stirring current is fixed in 6 Hz, the maximum stirring velocity raises from 13.4 cm/s to 17.7 cm/s with the increase of current intensity from 360 A to 420 A. And the maximum stirring velocity raised 2.23cm/s with increasing frequency of 1 Hz. Similarly, As the SWRH82B final electromagnetic stirring current is set for 6 Hz, the maximum stirring velocity raises from 12.4 cm/s to 17.6 cm/s with the increase of current intensity from 360 A to 440 A. And the maximum stirring tangential velocity raises 2.23cm/s with increasing frequency of 1 Hz, when the current density is 420 A. The optimal final electromagnetic stirring parameters for 60,70 and SWRH82B are 380 A/6 Hz,400 A/6 Hz and 420 A/6 Hz, respectively.(4) The results from industrial tests show that the operational route with intensive secondary cooling (1.0L/kg), properly low casting speed and reasonable F-EMS parameters is beneficial to reduce central segregation. Under casting speed of 1.9 m/min and parameters of F-EMS of 380 A/6 Hz, the central segregation index of 60 steel billet is almost lower than 1.05. Under casting speed of 1.8 m/min and parameters of F-EMS of 400 A/6 Hz, the central segregation index of 70 steel billet is almost lower than 1.08. Moreover, under casting speed of 1.8 m/min and parameters of F-EMS of 420 A/6 Hz, the central segregation index of SWRH82B steel billet is almost with in 1.07-1.15. Meawhile, casting speed doses a more powerful effect on central segregation compared with super heat.