METAL 2008 13. –15. 5. 2008, Hradec nad Moravicí ___________________________________________________________________________ 1 MAGNETIC PROPERTY IMPROVEMENT OF A NON – GRAIN ORIENTED ELECTRICAL STEEL VIA CONTINUOUS ANNEALING SIMULATIONS J. Huňady, A. Balunová, E. Hilinski, M. Predmerský, M. Černík U. S. Steel Košice, s.r.o., Research and Technology Center, Vstupný areál U. S. Steel, 044 54 Košice, Slovak Republic [email protected], [email protected], [email protected], [email protected], [email protected]Abstract In May of 2006, United States Steel Košice (USSK) at its Research and Development Center commissioned the latest version of a Hot-Dip Process Simulator (HDPS), of Generation IV, produced by Iwatani-Surtec-Rhesca. This unique unit provides a realistic simulation of continuous annealing processes on a laboratory scale and was constructed to support product development and process optimization, as well as to allow USSK to better serve customers. The capabilities of the HDPS and how it is used to evaluate product and improve material processing in support of developing non-oriented electrical steels will be illustrated in studies to determine chemistry – microstructure – property relationships in high silicon non-oriented steel alloyed with and without antimony. The HDPS provides a platform to accurately simulate the commercial annealing process so that optimal decarburization and grain growth temperatures are determined. Final magnetic properties are measured on a Single Sheet Tester (SST) and correlated to Epstein values. 1. INTRODUCTION When U. S. Steel acquired the former VSŽ steel plant in the Slovak Republic in 2001, it was reintroduced to fully finished non-oriented electrical steel, a product not made at U. S. Steel since the divestiture of the Vandergrift Plant in 1983. VSZ, now U. S. Steel— Košice (USSK), was producing about 150,000 metric tons of non-oriented electrical steel per year in grades ranging from M800 to M470 in sheet thicknesses of 0.50 and 0.65 mm on two Dynamo Lines. In 2004, a third Dynamo Line was commissioned with the capability of processing lower core loss grades such as M400, M350, and M300 which could not be reliably produced on the existing lines. The new Dynamo Line provides USSK the opportunity to produce the more value added grades non-oriented electrical steel grades in sheet thicknesses down to 0.35 mm. In order to support the development of these grades USSK required laboratory scale equipment that could accurately simulate the Dynamo Line annealing process. USSE Research therefore purchased a Generation IV Hot Dip Process Simulator (HDPS) from Iwatani-Rhesca to model all continuous annealing processes. An illustration of how the HDPS is used to support new grade development is provided in an example of simulating commercial Dynamo Line annealing to determine optimum processing parameters for an antimony alloy high silicon M400-50 non-oriented electrical steel. 2. DESCRIPTION OF HDPS The USSK HDPS was designed to meet or exceed the capabilities of any of the USSK Continuous Annealing Lines (CAL) currently in operation. It is capable of producing a steel sample with a thermal history that matches any CAL design. While annealing, all important functions of the production process such as thermal treatment, soaking, and cooling can be simulated in a real annealing atmosphere on a laboratory scale with the help of a command
Transcript
METAL 2008 13. –15. 5. 2008, Hradec nad Moravicí ___________________________________________________________________________
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MAGNETIC PROPERTY IMPROVEMENT OF A NON – GRAIN
ORIENTED ELECTRICAL STEEL VIA CONTINUOUS ANNEALING
SIMULATIONS
J. Huňady, A. Balunová, E. Hilinski, M. Predmerský, M. Černík
U. S. Steel Košice, s.r.o., Research and Technology Center, Vstupný areál U. S. Steel, 044 54
In May of 2006, United States Steel Košice (USSK) at its Research and Development Center commissioned the latest version of a Hot-Dip Process Simulator (HDPS), of Generation IV, produced by Iwatani-Surtec-Rhesca. This unique unit provides a realistic simulation of continuous annealing processes on a laboratory scale and was constructed to support product development and process optimization, as well as to allow USSK to better serve customers. The capabilities of the HDPS and how it is used to evaluate product and improve material processing in support of developing non-oriented electrical steels will be illustrated in studies to determine chemistry – microstructure – property relationships in high silicon non-oriented steel alloyed with and without antimony. The HDPS provides a platform to accurately simulate the commercial annealing process so that optimal decarburization and grain growth temperatures are determined. Final magnetic properties are measured on a Single Sheet Tester (SST) and correlated to Epstein values.
1. INTRODUCTION
When U. S. Steel acquired the former VSŽ steel plant in the Slovak Republic in 2001, it was reintroduced to fully finished non-oriented electrical steel, a product not made at U. S. Steel since the divestiture of the Vandergrift Plant in 1983. VSZ, now U. S. Steel— Košice (USSK), was producing about 150,000 metric tons of non-oriented electrical steel per year in grades ranging from M800 to M470 in sheet thicknesses of 0.50 and 0.65 mm on two Dynamo Lines. In 2004, a third Dynamo Line was commissioned with the capability of processing lower core loss grades such as M400, M350, and M300 which could not be reliably produced on the existing lines. The new Dynamo Line provides USSK the opportunity to produce the more value added grades non-oriented electrical steel grades in sheet thicknesses down to 0.35 mm. In order to support the development of these grades USSK required laboratory scale equipment that could accurately simulate the Dynamo Line annealing process. USSE Research therefore purchased a Generation IV Hot Dip Process Simulator (HDPS) from Iwatani-Rhesca to model all continuous annealing processes. An illustration of how the HDPS is used to support new grade development is provided in an example of simulating commercial Dynamo Line annealing to determine optimum processing parameters for an antimony alloy high silicon M400-50 non-oriented electrical steel. 2. DESCRIPTION OF HDPS
The USSK HDPS was designed to meet or exceed the capabilities of any of the USSK Continuous Annealing Lines (CAL) currently in operation. It is capable of producing a steel sample with a thermal history that matches any CAL design. While annealing, all important functions of the production process such as thermal treatment, soaking, and cooling can be simulated in a real annealing atmosphere on a laboratory scale with the help of a command
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based program structure. The consumption of operating materials is extremely small compared with mini or commercial lines. Also, a wide choice of innovative thermal treatments can be performed on the HDPS that cannot be performed on commercial lines. Figure 1 shows a schematic diagram and picture of the main sections of the simulator. The HDPS is a vertical system that consists of the driving mechanism at the top of the simulator, upper chamber in the middle section, and lower chamber at the base. The steel sample is hung on the drive rod through the sample entry/cooling chamber and transferred up and down through the processing sections. Below this chamber is an Infrared (IR) furnace, of 54 kW capacity, with 9 quartz lamps on each side. They are separated vertically into 3 heating zones that are individually controlled and heat the steel sample through a transparent quartz tube. At the base of the upper chamber is an induction furnace. The upper chamber is separated from the lower chamber by an atmosphere-sealing gate valve which allows evacuation of the upper chamber at the beginning of each test cycle. The lower chamber consists of a zinc pot that allows zinc coating capability. Currently, the USSK HDPS does not have the zinc coating option, however the HDPS is designed for easy to this galvanizing capability in the future. An in-situ dual-color pyrometer is for sample temperature control.
Fig. 1. Schematic diagram and photograph of the USSK Generation IV HDPS. Integral to the system is a computer control cabinet and a cooling system. The control system has several functions, such as preparing the simulation by adjusting all necessary parameters, performing the simulation process, monitoring the process, online visualization, displaying and editing the results and archiving files. An easy-to-use editor enables the simulations to be programmed using short command words. The total working volume of the controlled atmosphere in the simulator is approximately 35 liters. The HDPS monitors many parameters such as the temperatures, gas flows, power settings, positioning sensors, dew points and pressures. The data collection interval of these parameters can be as low as 20 milliseconds. Some of the relevant performance capabilities of the USSK HDPS are shown in Tab. 1. A picture of an HDPS sample, showing uniform heating area for acquiring subsize tensile samples or Single Sheet Tester (SST) coupons for magnetics testing is provided in Fig. 2, [6].
Drive mechanism
Entry/Cooling chamber N2, H2, He
IR Furnace
Induction Furnace
Zinc pot
Gate Valve
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Figure 3. Sample prepared for the simulation (1) with thermocouples (2) and sample holder (3).
2
1
3
5 cm
Table 1. Capabilities of the USSK Research HDPS.
3. EXPERIMENTAL PREPARATION
Each experimental project on the HDPS involves certain procedures prior to simulation. Typically, extra steel samples are prepared for thermal cycle tuning to ensure that accurate thermal profiles matching the temperature, and heating and cooling rate targets for the subject CAL are achieved. This requires calculations based on the commercial line parameters. During tuning three thermocouples are used, Fig. 3, in order to precisely measure the temperature in the uniform area and to help balance the three zones of the IR furnace. The tuning also involves managing temperature over-shoots and offsets via optimization of the annealing cycle. This is basically accomplished via the self learning (autotuning) infrared heater control optimization software which allows convenient and accurate heater power adaptation to cope with the changing masses and reflectivities of samples. The end result is an optimized temperature distribution with minimized temperature gradients (maximum temperature differential being +/- 7°C). Manual tuning of the process controller can be performed as well, by adjusting the proportional and integral settings within the program. Once the experimental cycles are tuned, simulation commences. The sample is attached to the drive rod through the entry /cooling chamber which is then closed and evacuated to a pressure of 0.14 torr. Then, the chamber is pressurized with nitrogen and the annealing cycle begins with the intended nitrogen/hydrogen concentrations [6].
4. EXPERIMENTAL INTRODUCTION
The magnetic behavior of non-oriented electrical steels is controlled by the alloy content and several microstructural parameters, such as texture, grain size, and impurities. Silicon, aluminum, manganese and other microalloying elements such as antimony and tin are the
Sample size 120 mm x 200 mm, max 130 mm x 220mm
Gauge range 0.18 mm - 3.0 mm
IR furnace max 1100 °C
Uniform area 90 x 90 mm
Heating rate max 50 °C/s
Cooling rate max 200 °C/s
Dew point control -60 °C → +50 °C
Withdraw speed 0,1 – 1300 mm/s
Acceleration 20 m/s2
2
3
1
Fig. 2. Sample (1) for simulation with marked uniform area (2) from which SST coupons and tensile specimens (3) are obtained.
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A, B
830, 900
without, 900, 920, 940, 960, 980, 1000,
1020, 1040, 1060
Cooling
steel Tdecarb. [˚C] Tgrain growth. [˚C] l
Dew point +35 ˚C Soaking time 85 s
Dew point -40 ˚C Soaking time 23 s
chief elements used to obtain the desired properties. Texture or preferred crystallographic orientation, which is extremely important for obtaining high polarization levels, is produced as a consequence of deformation and subsequent grain boundary migration that occurs during recovery, recrystallization and grain growth. The (100) and (111) components are very important factors that affect the magnetic flux density of silicon steels. High intensities of (100) components improve magnetic properties whereas high intensities of (111) components have a detrimental effect [1]. Of the major alloying elements used for electrical steels, antimony and tin are particularly noted for their effects on texture [2, 3]. They are surface active elements that segregate on grain boundaries and selectively affects the deformation accumulation within grains and their subsequent recrystallization texture, impairing growth of (111) oriented grains and favoring recrystallization textures rich in (100) and (110)<uvw> orientations [4]. Also, antimony additions have been noted to lead to increased grain size which improves core loss [4, 5].
5. EXPERIMENTAL DETAILS
The aim of the simulations was to improve the magnetic properties in high silicon grades of electrical steel produced at USSK, M400-50A and better grades in particular. Two types of commercially produced, high-silicon, non-grain oriented steels were used. The chemical composition differed only in the antimony content, as shown in Tab. 2. Two decarburization temperatures, 830 and 900˚C, were used for each steel to explore its affect on properties. Several grain growth temperatures ranging from 900 to 1060˚C, in 20˚C increments, were subsequently applied following each decarburization temperature. Fig. 4 is a block diagram showing the experimental plan for the annealing simulations. The decarburization and grain growth atmospheres were composed of 25 % H2 and 75 % N2 with dew points of + 35˚C for decarburization process and -40˚C for grain growth. The decarburization and grain growth soaking times of 85 and 23 seconds respectively, were derived from the commercial annealing line furnace lengths and line speeds. After the annealing cycle was completed, the magnetic properties of the samples were measured on a Brockhaus Single Sheet Tester (SST) which requires sample dimensions of 70 by 70 mm. Aging was conducted at 225˚C for 24 hours followed by retesting to determine the aging index or percent change in core loss. The SST determined electromagnetic properties of J5000 magnetic polarization and P1.5 core loss were correlated to Epstein values. Correlations, for longitudinal and transverse properties, shown in Fig. 5 and 6, were obtained by regressing SST to Epstein measurements on all grades and gages of non-grain oriented silicon steels produced by USSK.
Figure 4. Block diagram showing the experimental plan used for simulating the annealing cycles.
Table 2. Base Chemical Compositions
steel
[wt. %] C Mn Si Al Sb
A 0,007 0,25 2,4 0,4
without B 0,06
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6. RESULTS 6.1 Magnetic properties
The Epstein correlated and SST measured magnetic properties are compared in Fig. 7 and 8 for P1.5 core loss and in Fig. 9 and 10 for J5000 magnetic polarization. The SST results tend to be optimistic with regard to the Epstein results. More specifically, it increases values of J5000 magnetic polarization by about 0,06 – 0,07 T and about 0,2 - 0,3 W/kg for the P1.5 core loss. The influence of the annealing cycle and antimony content on core loss and polarization is seen in Fig. 11 and 12. A strong influence of grain growth temperature was found where the higher grain growth temperatures yielded better values for core loss. The best value of core loss was achieved in the antimony added steel subjected to a decarburization and grain growth temperatures of 830 and 1060˚C. The P1.5 core loss of 3,32 W/kg actually meets the specifications for M350-50A, the next grade better than the M400-50A being studied. The best value of J5000 magnetic polarization was of 1,652 T, also in the antimony alloyed steel. In this case the 900˚C decarburization and 1040˚C grain growth temperatures yielded this result. The results show that antimony has a very important role in relation to the magnetic properties. Fig. 11 and 12 confirm the beneficial effect of antimony on the magnetic properties. The antimony alloyed steel shows lower core loss and higher values of magnetic polarization for both decarburization temperatures.
Figure 11. Relation between P1.5 core loss, decarburization temperature, and Sb content.
Figure 12. Relation between J5000 polarization, decarburization temperature, and Sb content.
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Grain growth temperature without grain growth 900 ˚C 920 ˚C 980 ˚C
1060 ˚C
6.2 Grain size and microstructure
Decarburization temperature 830 ˚C, material A
J5000 = 1,607 T
4,432 W/kg 4,170 W/kg
4,280 W/kg 3,841 W/kg
3,514 W/kg
24 µm
37 µm 42 µm
85 µm
66 µm
Decarburization temperature 900 ˚C, material A
4,051 W/kg 4,127 W/kg 3,990 W/kg
3,842 W/kg
3,543 W/kg
107 µm
60 µm
39 µm
49 µm 49 µm
J5000 = 1,617 T
Decarburization temperature 830 ˚C, material B
4,512 W/kg
4,209 W/kg 4,080 W/kg
3,731 W/kg
3,315 W/kg
120 µm
66 µm
38 µm 40 µm
22 µm J5000 = 1,625 T
Decarburization temperature 900 ˚C, material B
4,156 W/kg
3,928 W/kg
3,950 W/kg
3,702 W/kg0
3,378 W/kg
J5000 = 1,611 T
46 µm 56 µm
48 µm
69 µm
86 µm
Figure 13. Grain size in relation to the core loss P1.5.
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Grain size measurements were performed on scanning electron microscope by EBSD method. TSL OIM program was used for the texture measurements evaluation which provides several methods for the grain size calculation and to point, diameter, interception and ASTM. For the grain size calculation, the best method appeared was the point method which result was distribution allocation of the grain size. Fig. 14 presents the example of the grain size distribution of the material B, antimony alloyed, which was decarburized at 830°C and non-grain growth annealed. For the given sample, the average grain size value was 22 µm, Fig. 13. Ferrite microstructure with different grain size was obtained after the annealing cycles. Influence of grain size and antimony content on magnetic properties is seen in the Fig. 13. There are two parameters, grain growth temperature and antimony content, which affect the magnetic properties significantly. The lowest core loss of 3,315 W/kg was achieved after decarburization annealing at 830 ˚C and grain growth annealing at 1060 ˚C. It was antimony alloyed material B where 120 µm grain size was observed, fig. 13. Regarding the grain size, the grain growth temperature of 1060 ˚C appears to be the most convenient temperature in order to achieve low core loss values. Grain growth increase and decrease of core losses was observed at this grain growth temperature for both materials. Antimony alloyed steel B, decarburized at 830 ˚C and grain growth annealed at 1060 ˚C showed the best magnetic polarization J5000 value of 1,625 T.
6.3 Grain size and microstructure Texture measurements were performed on scanning electron microscope by EBSD
method. TSL OIM program was used for the texture measurements evaluation. Each sample was measured twelve times on the area of 500 by 500 micrometers. Measured data files were summarized which produced the total number of approximately 160 000 measured points. By this procedure, the sufficient number of grains was ensured in order to calculate the grain size and texture. Orientation distribution function – ODF was calculated by the Bunge method by the TSL OIM program. Regarding the achievement of the lowest core loss in relation to the highest magnetic polarization values, the samples with and without antimony, annealed at both decarburization temperature and grain growth temperature of 1060 °C, were taken for the texture analysis. Figures 15 and 16 present the fibres of the selected samples. Textures of the investigated samples are described by the eta and gamma fibres. Material without antimony shows lower density values of the orientation f(x) of eta fiber and, at the same time, higher values of gamma fiber. Material with antimony addition shows, on the other hand, higher values of eta fiber and lower values of gamma fiber. Ratio of (100)/(111) texture components is more favourable for the samples with antimony in comparison to the material without antimony addition which is also confirmed by the achieved values of magnetic polarization J5000 and core losses P1.5.
Figure 14. Grain size distribution, sample B, decarburization temperature 830 ˚C, without grain growth
[µm]
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0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
0 10 20 30 40 φ φ φ φ
f(g)900/1060-Sb
900/1060
830/1060-Sb
830/1060
ϕϕϕϕ2222 = 45o
φ φ φ φ = 0o
(001)
<010>
(001)
<120>
(001)
<1-10>
0
1
2
3
4
5
6
7
8
9
10
11
60 70 80 90ϕϕϕϕ1111
f(g)900/1060-Sb
900/1060
830/1060-Sb
830/1060
ϕϕϕϕ2222 = 45o
φφφφ = 55o
(111)
<0-11>
(111)
<-1-12>
60
7. CONCLUSSIONS (1) the positive effect of the antimony addition on the magnetic polarization and core loss
was confirmed after simulations of various continuous annealing processes. The antimony alloyed material possessed higher magnetic polarization values and lower core loss values than the material without antimony,
(2) grain size also significantly influences the core loss of both antimony alloyed and unalloyed, materials. The largest grain size and the lowest core loss value was achieved with the 1060˚C grain growth temperature,
(3) the texture parameters, described by the eta and gamma fibers, showed antimony to have a positive effect on magnetic properties. Higher values of magnetic polarization and lower values of core losses were achieved in the antimony alloyed material. The fraction of (111) oriented grains was decreased concomitantly with an increase in the fraction of (100) oriented grains,
(4) from the point of view of final thermal treatment, the decarburization temperature of 830°C followed by a grain growth temperature of 1060°C, applied to antimony containing material produced the best combination of magnetic properties (highest magnetic polarizations coupled with the lowest core loss values).
REFFERENCES [1] Lyudkovsky, G. Effect of Antimony on Recrystalization Behaviour and Magnetic Properties of a Nonoriented Silicon Steel,Matallurgical and Materials Transaction A, Vol. 15, No.2, February 1984 [2] Jenko, M. et al. Orientation – dependent antimony segregation on FeSi alloy surfaces Fizika A 4, pp. 91-98, 1995 [3] Chang, S.K. Texture Effect on Magnetic Properties by Alloying Specific Elements in Non – grain Oriented Silicon Steels. ISIJ International, Vol. 45, No. 6, pp. 918-922, 2005 [4] De Wulf M. Development of non grain oriented electrical steels at the ARCELOR group. Advances on experimental techniques, new material and modelling tools. 2nd International workshop magnetism and metallurgy, Freiberg, Germany, June 2006 [5] Shimanaka H. Non-grain oriented Si steels useful for energy efficient electrical apparatus, Research laboratories, Kawasaki Steel Corporation, Chiba, Japan [6] A. Balunova. Modeling, Simulations and Optimalization of Continuous Annealing Processes on Hot Dip Process Simulator, Acta Metalurgica Slovaca,13/2007, s. 26-31
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