Quality improvements by use of FC Mold on hot rolled coils

Transcription

1 Quality improvements by use of FC Mold on hot rolled coils Hongliang Yang*, Jingxin Song**, Nils Jacobson*, Olof Sjöden*, Jan-Erik Eriksson*, Helmut Hackl* *ABB AB/Metallurgy Västerås, Sweden JFE Steel was the first company to publish the results of static magnetic field on as-cast slabs. Reduction of the penetration depth by static magnetic field was Without FC Mold Disturbed meniscus Thin powder film Vortices With FC Mold Calmer and hotter meniscus Location of iron cores Less penetration depth Upper field Lower field ** Meishan Steelworks, Baosteel group, China Abstract: The effects of FC Mold in a slab caster mold are studied by inspecting the surface quality of hot rolled coils. A defect index is proposed to represent the casting-related defects of hot rolled coils. FC Mold improves the coils surface quality by up to 60% depending on the casting parameters and FC Mold operations. A dimensionless mold index ( Mo ) is proposed to unite the casting parameters such as casting speed, throughput, slab width and thickness, and it is found that the mold index can be the controlling parameter in determining the FC Mold effects. It is also found that the magnetic interaction number (IA) can be used to optimize the operation of FC Mold under different casting conditions. Both numerical and water model simulations are carried out to study the influence of the FC Mold on the flow field in the mold. Both types of simulations show that the jet flow from the SEN turns into a horizontal flow when it comes into contact with the DC magnetic field. The jet flow momentum, DC magnetic field strength and length of the horizontal flow will determine the braking effect of DC magnetic field in total. The findings from the simulations also justify the use of mold index and magnetic interaction number. Key words: FC Mold, Mold index, Magnetic interaction number 1. Introduction FC Mold, an electromagnetic flow control technology in the slab caster mold, has been widely installed worldwide since the 1990s. The FC Mold uses two static magnetic fields, an upper field at the slab meniscus level to control the meniscus metal flow velocity and a second independently controlled lower field in the lower part of the mold to minimize the penetration depth of the steel jets from the SEN, see Fig.1. EMBR Ruler is an earlier generation of FC Mold with only one level static magnetic field. Inclusions Figure 1: FC Mold principles confirmed by checking the inclined angle of the dendrites at the narrow face [1]. The increase of the effective mixing in the mold by the static magnetic field is shown by the addition of iron sulfur at the outlets of SEN [1]. Due to the increase of meniscus temperature and stabilization of meniscus fluctuations by the static magnetic field, the variation of the pitch and depth of oscillation marks on middle carbon steel are reduced, as well as the frequency of longitudinal cracks.[] The quantity of inclusions in the slab subsurface area is effectively reduced because of the reduced penetration depth.[3] D. van der Plas et. al [4] have made plant trials investigating the influence of EMBR Ruler on slab cleanliness and final product quality. It was found that EMBR Ruler can reduce the rejection ratio on critical steel grades up to 50% after cold rolling, and that EMBR performance is affected by casting parameters as mold width, casting speed and argon flow rate. H. Yamamura et. al [5] have studied the influence of magnetic flux density on the flow patterns by analyzing solid structures and numerical simulations. The magnetic interaction number was proposed to correlate the casting conditions and the quantity of inclusions in the as-cast slabs. Y. Miki and S. Takeuchi [6] have made slab analyses and LES simulations and found that bubbles and inclusions can penetrate deeply into the slabs due to asymmetric unbalanced flow in the mold, and FC Mold can reduce the time dependent flow

2 and thereby prevent the internal defects caused by bubbles and inclusions. In this paper, as a continuation of the previous paper [7], steel plant measurements on the influence of FC Mold on the hot rolled coils quality were presented. Two dimensionless numbers, mold index ( Mo ) and magnetic interaction number (IA ) were employed to study the influence of the casting parameters and FC Mold operations.. Plant data This work was carried out on caster CC in Meishan Steelworks, Baosteel Group in China. The production route of Meishan Steelworks includes three 150 ton BOFs, two LF stations and one RH station. Originally, there were in total one double-strand caster (CC1) and one single strand caster (CC). FC Mold was installed on the single strand of CC. In 008, another strand equipped with FC Mold was added into CC. Consequently, the highest casting speed of CC was lowered from.4 m/min to 1.8 m/min and the maximum throughput was lowered to.8 ton/min per strand due to the limited liquid steel supply, see Table 1. Machine type Casting speed Throughout Steel grades Slab thickness Slab width Tundish capacity Vertical bending 0.8~1.8 m/min <.8 ton/min per strand LCAK, MC, ULC 10 mm 800~1300 mm 55 ton Table 1: Caster conditions for the coil surface investigation Coil surface quality is a consequence of the slab internal and surface quality, and also the rolling process quality. By counting the specific defects which are considered to have originated from the casting process, the effects of FC Mold can be studied. During the test, FC Mold is only turned-on on one strand and turned-off on another strand of CC in order to make comparisons under the same casting conditions. An automatic surface detection system is installed in the finishing mill. The thickness of the coils vary from 1 mm to 8 mm. Results of the surface detection system were manually checked to identify the defects related to the casting process and the number of defects were recorded for comparison. The casting-related coil surface defects include those originating from bubbles, endogenous inclusions, slag inclusions, cracks, etc in the slab. Fig. shows some characteristic coil surface defects encountered in this study. The defect number was counted for each coil and divided by the weight of the coil in order to remove the factor of the different slab length. A coil defect index based on the defect number per ton of coil was used in this paper. In total about 3000 coils were counted in this work. 3. Results and discussions The improvement ratio with FC Mold is defined as D1 D α = 100, (1) D1 where α is the improvement ratio of average coil defect index, %, D1 is the average coil defect index without FC Mold, and D is the average coil defect index with FC Mold. 3.1 Influence of casting speeds and slab width Fig. 3 shows the improvement ratio of the average coil defect index with FC Mold under different casting speeds. It can be seen that, in general, the FC Mold effects increase with higher casting speed.

3 (a) Endogeneous inclusions (b) Slag inclusions (c) Bubble (d) Bubble attached with inclusions Figure 1: Morphology and originations of some characteristic slab-related coil surface defects Improvement ratio with FC Mold (%) slab width because the working length of DC magnetic field on the jet flow increases with the slab width. Fig.4 shows almost the same tendency with the exception of the slab width close to 1300mm. An investigation was made to look at the relationship between the slab width and the average casting speed. As shown in Fig.5, the average casting speed for the slab width close to 1300mm is very low. 10 0,8 1 1, 1,4 1,6 1,8 Average casting speeed (m/min) Figure 3: Influence of average casting speed on the FC Mold effects Improvement ratio with FC Mold (%) Slab width ( mm) Figure 4: Influence of slab width on the FC Mold effects Besides the casting speed, a previous study [4] showed that the FC Mold effects increase with larger Average casting speed (m/min) 1,6 1,5 1,4 1,3 1, 1, Slab width (mm) Figure 5: Relationship between slab width and average casting speed 3. Mold index A mold index is proposed to combine the casting parameters, namely casting speed, slab width and thickness.

4 VcQ Mo = ρgd 3 c W 3 V 3 10 = 10, () gd where Mo is the mold index, Vc is the casting speed, m/s, Q is the throughput, kg s, ρ is the steel density, 3 m kg, g is the gravitational acceleration, m,w is the slab width, m, and D is the slab thickness, m. The mold index characterizes the flow intensity in the mold which is a combined effect of both flow speed and turbulence intensity. A higher mold index represents higher flow intensity in the mold. FC Mold is more effective when the flow intensity is high in the mold because the braking force is proportional to both the flow speed and the turbulence intensity. Casting speed (m/min) Casting speed ( m/min) Fig.6 shows the mapping among the mold index, casting speed and the throughput. The mold indices of Meishan caster lie in the range between 0.6 and.0, while the mold indices of most casters lie in the range between 0.6 and 3.5. Improvement ratio (%) ,50 0,75 1,00 1,5 1,50 Mold Index 1,75 Figure 7: Improvement ratio with FC Mold at different Mold Indices s 5 4, Operation window of Meishan 3,5 3 Operation window of most casters, , ,5 1,50,50 3,50 4,50 5,50 Throughput ( ton/min) Mo=0,5 Mo=1 Mo=1,5 Mo= Mo=3 Mo=4 Figure 6: Mold index with casting speed and throughput (slab thickness: 10mm),00 Fig.7 shows the improvement ratio with FC Mold at different mold indices. In general, the improvement ratio increases with mold index. The improvement ratio reaches around 60% when the mold index is close to.0. This result confirms that mold index may be an important parameter in studying the FC Mold effects. There are many casters producing with higher casting speeds and throughputs, where the mold index will be higher than this work. A higher mold index means the stronger flow momentum and turbulence level in the mold. The improvement ratio with FC Mold can be even higher than 60% with higher mold index. 3.3 Magnetic interaction number The magnetic interaction number is employed to study the influence of magnetic flux density under different casting conditions. B W IA = (3) ρu σ Q U = (4) Aρ where IA is the magnetic interaction number, σ is the 6 electrical conductivity of liquid steel, S m, B is the magnetic flux density, T, U is the average speed from SEN outlet, m s, A is the area of the SEN outlet, m, and Q is the throughput, kg sec,w is the slab width, m. Only the effects of the lower magnetic field are presented in this paper. The magnetic interaction number represents the ratio of the magnetic braking force to the inertia force of the jet flow from the SEN outlet. One difference between Eq. (3) and the one proposed by the authors in Ref. [5] is that slab width instead of slab thickness is employed in the formula. Both water modeling and numerical simulations show that the slab width is directly correlated to the effective working length of the magnetic field on the jet flow. Fig.8a shows the relationship between the defect index and the magnetic interaction number. It can be seen that defect indices decreases with higher magnetic interaction number. In production, it is the upper band of the defect indices which is essential for the probable rejection, therefore a modified defect index is defined as D m = D + 3σ, (5)

5 (a) Figure 8: Influence of Magnetic Interaction Number on FC Mold effects (a) Defect Index versus magnetic interaction number. (b) Modified Defect Index versus magnetic interaction number (b) Where D m is the modified Defect Index, D is the average Defect index, σ is the standard deviation of the defect index. Fig.8b shows the relationship between the modified defect index and the magnetic interaction number. It shows clearly there is an optimal value of magnetic interaction number around 5. Previous study [8] showed that in the mold, the jet flow from the SEN turns into a horizontal flow when it comes into contact with the DC magnetic field. The jet flow momentum, DC magnetic field strength and length of the horizontal flow (slab width) will determine the braking effect of DC magnetic field in total. When the magnetic interaction number is small, the braking force is not high enough comparing to the jet flow momentum and turbulence. When the magnetic interaction number is too high, the jet flow and the turbulence are over braked, this means that the meniscus will have a risk of freezing. 4. Conclusions The effects of FC Mold in a slab caster mold are studied by inspecting the surface quality of hot rolled coils. FC Mold improves the coils surface quality by up to 60% depending on the casting parameters and FC Mold operations in this work where the maximum mold index is. The improvement ratio with FC Mold can be even higher for the casters with higher mold index than. FC Mold operation can be optimized based on magnetic interaction number. References [1] S. Kenichiro, et. al, Flow field control of liquid steel in the mold of a continuous slab caster by an electromagnetic brake, Tetsu-to-Hagane,, Vol. 69, No.1,1983, pp. 91. [] H. Okuda, et. al, Effects of electro-magnetic brake on surface quality of medium carbon strand cast slabs, Tetsu-to-Hagane, Vol. 7, No.4,1986, pp. 196 [3] Nagai et. al, Effect of electromagnetic brake on inclusions decrease, Tetsu-to-Hagane, Vol. 69, No.1,1983, pp [4] D. van der Plas, J-M Galpin, et. al, Combined investigations of the EMBR performance at continuous casters of Sollac Dunkerque and Hoogovens Ijmuiden, Proceedings nd European conference on continuous casting, 1994, pp

6 [5] H. Yamamura, T. Toh, et. al, Optimum magnetic flux density in quality control of casts with level DC magnetic field in continuous casting mold, ISIJ International, Vol.41, No. 10, 001, pp [6] Y. Miki, S. Takeuchi, Internal defects of continuous casting slabs caused by asymmetric unbalanced steel flow in mold, ISIJ International, Vol. 43, No. 10, 003, pp [7] J. Song, H. Yang. et. al, Effects of FC Mold on slab and final product qualities at low throughputs, AISTech 010 proceedings, Vol., Pittsburg, pp [8] H. Yang, et.al, water modeling of stirring and braking processes in a slab caster mold, AISTech 010 proceedings, Vol., Pittsburg, pp