87_03.pdf
- 1. NKK TECHNICAL REVIEW No.87 (2002) –12– Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE Yoshihiko Oda*, Yasushi Tanaka**, Nobuo Yamagami***, Atsushi Chino**** and Katsumi Yamada***** * Senior Research Engineer, Sheet Products Research Dept., Materials and Processing Research Center ** Group Manager, Dr,. Planning & Coordination Dept. *** Group Manager, Product Design and Quality Control, Strip and Coated Products Fukuyama Works **** Team Manager, Dr,. Materials Characterization Research Dept., Applied Technology Research Center ***** Senior Research Engineer, Materials Characterization Research Dept., Applied Technology Research Center A new type of non-oriented electrical steel sheet, NKB-CORE, has been developed by NKK through the application of a unique ultra-low sulfur technology. This type of steel sheet shows: (1) low hysteresis loss; (2) high magnetic flux density; (3) excellent punching property, and (4) low production cost. The low core loss was obtained by reduc- ing the sulfur content in steel and suppressing surface nitriding. The high magnetic flux density was obtained by re- ducing silicon and aluminum content. This paper describes the material design of NKB-CORE, and presents the ap- plication of this material to motors for electric power steering systems for automobiles. 1. Introduction An electric motor is a device which converts electrical energy to mechanical energy, and is used extensively in daily life for driving air-conditioners, refrigerators, vacuum cleaners, CD-ROM drives, just to mention a few of an al- most infinite number of applications. The efficiency in elec- tric power consumption by these appliances is almost uni- laterally determined by the electric motor’s performance. Recently, in the automotive field, high-efficiency motors have been extensively employed for driving HEV (Hybrid Electric Vehicles) and electric power steering systems1),2) . Non-oriented electrical steel sheets are used as the core material for such high-efficiency motors. The non-oriented electrical steel sheet is a key material that transmits mag- netic energy thus affecting the efficiency of electric motors, and is highly desirable in terms of achieving both low core loss and high magnetic flux density. The traditional method of achieving low core loss was to add Si and Al to the steel and reduce the eddy current loss in the electrical steel sheets that form the motor core. However, there are important drawbacks to this method. First, the magnetic flux density declines3) . Second, the hardness increases and consequently the punching property suffers. In contrast, lowering hysteresis loss by high purification technology does not lower magnetic flux density nor affects the hard- ness. This is the preferred method for achieving low core loss. Conventionally, efforts have been made to reduce the contents of elements such as oxygen, nitrogen, and sulfur at the stage of steelmaking in order to produce purified steel4),5) . With regard to the sulfur content, recent remark- able advances in steelmaking technologies have made it easier to produce ultra-low sulfur steel that contains sulfur at less than 10 ppm, a level hitherto difficult to achieve. Against this background, detailed investigations were made on the magnetic properties of ultra-low sulfur steel (S<10 ppm). Based on the findings obtained through these investigations, a new type of ultra-low sulfur electrical steel sheet was developed for the manufacture of highly energy efficient motors. 2. Effect of sulfur content on magnetic properties of steel 2.1 Experimental method In order to study the effect of extremely low sulfur con- tent on the improvement of magnetic properties, steel samples with specific chemical compositions, shown in Table 1, were prepared by vacuum melting, and cast into ingots. These ingots were soaked at 1200℃ for one hour and hot-rolled into 2.3 mm thick sheets. After pickling, these hot-rolled sheets were annealed at 830℃ for three hours in a pure hydrogen atmosphere. Thereafter, these sheets were cold-rolled into 0.50 mm thick sheets, which were then subjected to final annealing at temperatures of 850-1050℃ for 2 min in an atmosphere of 10%H2 - 90%
- 2. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE –13– NKK TECHNICAL REVIEW No.87 (2002) N2. The magnetic properties of steel sheets processed in this manner were measured by a single-sheet tester at 50 Hz and 60 Hz. Loss separation was performed by applying the two-frequency method. The cross-sectional microstructures of the steel sheets were observed under an optical microscope. The portions near the surfaces of these steel sheets were closely exam- ined by a SEM (Scanning Electron Microscope). To de- termine the amount of nitrides contained in the surface portion, the content of N that was present as AlN was de- termined by electrolytic extraction at depth intervals of 30μm from the sheet surface. Auger electron spectrome- try was applied to investigate the segregation of elements in the surface portion. Table 1 Chemical composition of sample steels 2.2 Results of the experiment and discussion Fig.1 shows the relationship between the sulfur content and ferrite grain size after final annealing. The grain size was measured based on the JIS G 0552 method by polish- ing the central portion of the thickness of the sheets. It is noted that the grain growth is markedly improved over that of conventional materials (S content 30 to 50 ppm) by lowering the sulfur content to 4 ppm. This is because the MnS content, which inhibits grain growth, is lowered by reducing the sulfur content. Fig.2 shows the total core loss at 50 Hz. In the final an- nealing temperature below 900℃, the core loss decreases with increasing final annealing temperature for both high-sulfur steel (S content of 32 or 54 ppm) and ultra-low sulfur steel (S content of 4 ppm). However, in terms of absolute values, the core loss of the ultra-low sulfur steel is much lower than that of high-sulfur steel. In the final annealing temperature above 900℃, the core loss of high-sulfur steel decreases with increasing final annealing temperature, but in contrast, the core loss of the ultra-low sulfur steel increases with increasing final annealing tem- perature. In the final annealing temperature zone above 1050℃, the core loss of the ultra-low sulfur steel becomes larger than that of high-sulfur steel. Fig.2 Effects of sulfur content on total core loss To find the cause of such phenomena, loss separation was performed by the two-frequency method. The results are shown in Fig.3. Eddy current loss increases as the final annealing temperature increases for both high-sulfur and ultra-low sulfur steels. These behaviors are considered to be attributable to the fact that the grain diameter increases as the final annealing temperature increases. This increases the width of the magnetic domain, thereby increasing the eddy current loss. Eddy current loss is larger for ultra-low sulfur steel than for high-sulfur steel. This result is con- ceivably attributable to the same reason. The grain size is larger, hence the magnetic domain is wider for ultra-low sulfur steel than for high-sulfur steel. On the other hand, hysteresis loss steadily decreases as the final annealing temperature increases for high-sulfur steel. Hysteresis loss also decreases for ultra-low sulfur steel as the final annealing temperature increases to 900℃, but once the final annealing temperature goes beyond 900℃, hysteresis loss increases as the temperature in- creases. (Unit: mass %) No. Si Mn Al S N Fe 1 2.63 0.18 0.27 0.0004 0.0020 bal. 2 2.75 0.22 0.30 0.0032 0.0019 bal. 3 2.73 0.22 0.30 0.0054 0.0019 bal. 0 50 100 150 200 800 850 900 950 1000 1050 1100 S=4ppm S=32ppm S=54ppm Grain diameter, μm Final annealing temperature, ℃ Fig.1 Relationship between sulfur content and ferrite grain size after final annealing 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 800 850 900 950 1000 1050 1100 S=4ppm S=32ppm S=54ppm W15/50, W/kg Final annealing temperature, ℃ W 15/50 , W/kg
- 3. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE NKK TECHNICAL REVIEW No.87 (2002) –14– As above, the final annealing temperature dependence of hysteresis loss is markedly different for ultra-low sulfur steel and high-sulfur steel. This is considered to be the major factor that increases core loss for ultra-low sulfur steel when the final annealing temperature is high. In order to study why hysteresis loss of ultra-low sulfur steel increases when it is annealed at high temperatures, cross-sectional microstructures of high-sulfur steel and ul- tra-low sulfur steel were compared under optical micro- scope (Photo 1). The ferrite grain size of the ultra-low sul- fur steel near the center of the thickness is about 1.5 times as large as that of high-sulfur steel. Nevertheless, a small ferrite grain layer is noticeable near the surface of ul- tra-low sulfur steel. The cross-sectional microstructures at the depth of 10μm from the surface which is part of the small ferrite grain layer, was observed using SEM. Fine AlN particles measuring 0.1 to 0.5μm across were no- ticed, see Photo 2. This observation suggests that the small ferrite grain surface layer was formed due to the pinning effect on grain boundaries caused by surface nitriding. In order to quantify the amount of AlN in the surface layer, electrolyte extraction was conducted at intervals, 30μm from the surface. The measured results obtained for the nitride that is present as AlN are shown in Figs.4 and 5. The ultra-low sulfur steel exhibits pronounced ni- triding to the depth of about 60μm when annealed at 1050℃, see Fig.4. The degree of nitriding on the surface increases as the final annealing temperature rises for both types of steel, see Fig.5. However, the degree of nitriding with ultra-low sulfur steel (S = 4 ppm) is 9 times as high as that with high-sulfur steel (S = 54 ppm) when annealed at 1050℃. Fig.4 The amount of AlN close to the surface 100μｍ (a) S = 54 ppm (b) S = 4 ppm 1μm AlN Depth from the surface , μｍ 0 200 400 600 800 1000 1200 1050℃, 2min S=4ppm S=54ppm 0～30 30～60 60～90 N as AlN, ppm 0 200 400 600 800 1000 1200 800 850 900 950 1000 1050 1100 S=4ppm S=54ppm N as AlN, ppm Temperature , ℃ 0～30μｍ Fig.3 Effect of sulfur content on hysteresis loss and eddy current loss (Bm = 1.5T, f = 50Hz) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 800 850 900 950 1000 1050 1100 Final annealing temperature , ℃ Hysteresis loss Eddy current loss S=4ppm S=54ppm Bm=1.5T f=50Hz Core loss , W/kg Photo 1 Optical micrograph of the steel (final annealing temperature: 1050℃) Photo 2 SEM micrograph at the depth of 10μm from the surface (S = 4 ppm, annealed at 975℃ for 2 min) Fig.5 Relationship between final annealing temperature and the amount of AlN
- 4. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE –15– NKK TECHNICAL REVIEW No.87 (2002) These results suggest that the increase in hysteresis loss in ultra-low sulfur steel noticeable in Fig.3 is attributable to the magnetic domain walls being prevented from mov- ing by the small ferrite grain layer and AlN that are pre- sent near the surface. Auger electron spectrometry was applied to the steel sheet surfaces to clarify the causes of increased surface ni- triding of the ultra-low sulfur steel. First, in order to clean the surfaces of cold-rolled steel sheet specimens, their surfaces were subjected to Ar ion spattering in an Auger chamber. The specimens were then kept at the elevated temperature for 30 minutes, then cooled to room tempera- ture, and an elemental analysis was performed on the sur- faces of the specimens. The temperature inside the cham- ber was raised to only 850℃, since it was difficult to keep the specimens in the chamber at higher temperatures. A prominent sulfur peak was noticed for high-sulfur steel as shown in Fig.6. This specimen was subjected to Ar ion spattering for 30 seconds to remove the surface layer, then analyzed again. The sulfur peak had disappeared. This in- dicates that segregation of sulfur occurs near the surface of high-sulfur steel during the annealing. In contrast, the sul- fur peak is very weak in ultra-low sulfur steel, and the segregation of sulfur near the surface is hardly detectable. Driscoll has pointed out that surface segregation of sulfur affects the ability of the steel sheet surface to absorb at- mospheric oxygen6) . From this, it is also conceivable that the segregated sulfur near the surface may interfere with the process of nitrogen absorption. In other words, sulfur in high-sulfur steel conceivably segregates near the surface during the annealing process after hot-rolling and also during the initial period of the final annealing. The sulfur segregated near the surface inhibits nitrogen absorption on the surface during the high-temperature annealing. In the case of ultra-low sulfur steel, sulfur is virtually absent near Fig.6 Auger spectra of steel surface following annealing the surface. Therefore, nitrogen in the atmosphere is easily absorbed though the steel surface during the final anneal- ing, and absorbed nitrogen further diffuses into the interior of the steel. It then combines with Al to form AlN, which in turn precipitates near the steel surface and increases the core loss. 3. Development of an electrical steel sheet for energy efficient motor 3.1 Material design concept The results of these investigations indicate the following possibilities. Ultra-low sulfur steel is apt to suffer from prominent surface nitriding, but has excellent property in terms of interior grain growth. If the surface nitriding of ultra-low sulfur steel is effectively prevented, core loss will possibly be substantially reduced. Two approaches appeared possible to achieve this objective: (1) to increase hydrogen partial pressure in the annealing atmosphere, and (2) to add elements such as P, Sb, Sn which, like S, tend to cause surface segregation but do not form precipitates that inhibit grain growth. Taking the second option, an element that causes surface segregation was added to ultra-low sulfur steel to prevent surface nitriding and obtain large uniform grains throughout the steel sheet thickness. Fig.7 shows the degree of nitriding at the depth of 30μm from the surface when 40 ppm of Sb was added to ultra-low sulfur steel. Addition of Sb substantially inhibits surface nitriding of ultra-low sulfur steel. A similar effect of in- hibiting surface segregation was also confirmed with the addition of Sn and P. A new type of electrical steel sheet was developed by adding a surface-nitriding inhibitor to ultra-low sulfur steel. Fig.8 compares eddy current loss and hysteresis loss of 0.35 mm thick conventional 35A300 grade steel, purified steel (ultra-low sulfur steel) and newly developed steel. Hysteresis loss, hence the total core loss as well, was sig- nificantly reduced by adding a surface-nitriding inhibitor to ultra-low sulfur steel. Fe S P 0 200 400 600 800 1000 Electron Energy, eV Fe S=4ppm,850℃,30min S P S=54ppm,850℃,30min 0 100 200 300 400 500 600 700 Sb free Sb=40ppm N as AlN , ppm Fig.7 Effects of Sb on surface nitriding of ultra-low sulfur steel (final annealing temperature: 975℃)
- 5. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE NKK TECHNICAL REVIEW No.87 (2002) –16– Fig.9 shows the features and key technologies of the newly developed steel. The newly developed steel achieves low core loss by distributing large grains uni- formly in the thickness direction. With a lower Si and Al content than in conventional materials, the newly devel- oped steel achieves higher magnetic flux density than that of conventional materials. The lower Si and Al content soften the material, thus making punching operations eas- ier and extending the life of punching dies. Furthermore the lower Si and Al content make the material easier to cold-roll, this in turn has led to higher production yields and cost reductions. Fig.9 Characteristics of newly developed materials 3.2 Features of the newly developed materials Figs.10 and 11 show the magnetic properties and Vickers hardness of the 0.50 mm thick newly developed steel sheets. These newly developed steel sheets show higher magnetic flux density than conventional steel sheets that have nearly the same level of core loss. The newly devel- oped steel sheets exhibit 20 to 30 points lower Vickers hardness than the JIS grade materials with comparable core loss. These improvements make cold-rolling easier, thus improving steel maker productivity, and also making the punching operation easier, thus reducing the wear on the customer’s dies. Fig.10 Magnetic properties of newly developed materials Fig.11 Vickers hardness of newly developed materials (Load : 500g) Fig.12 shows a magnetization in high strength magnetic fields of the newly developed steel sheet and a conven- tional material (35A230) that has comparable core loss. The newly developed material shows a saturation mag- netization of 2.00T as compared with 1.95T for the con- ventional material, an improvement of 0.05T. The newly developed technology makes it possible to reduce the need to add non-magnetic elements such as Si and Al. Energy efficient motors for automobiles are sometimes designed in such a way as to make the magnetic flux density reach 2.0T in order to increase their torque. In such applications newly developed steel sheet with high levels of magnetic saturation can reduce flux leakage. Developed material Ultra low S steel 35A300 Core loss , W/kg 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Bm=1.5T , f=50Hz Hysteresis loss Eddy current loss Low core loss Low cost High punching property Uniform grain size Reduced hardness ●High purity ●Ultra low sulfur ●Suppressed nitriding in surface layer Reduced Si,Al content Developed materials Reduced production cost 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 W15/50 , W/kg 50A290 50A310 50A350 50A400 50A470 50A600 50A700 50A800 50A1000 50A1300 JIS grade materials Developed materials Thickness ：0.50mm 1.80 1.78 1.76 1.74 1.72 1.70 1.68 1.66 1.64 B 50 , T 50A600 50A800 100 120 140 160 180 200 220 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 50A290 50A310 50A350 50A400 50A470 50A700 50A1000 50A1300 JIS grade materials Developed materials Thickness ：0.50mm W15/50 , W/kg HV Fig.8 Comparison of core loss of 35A300, ultra-low sulfur steel, and newly developed material
- 6. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE –17– NKK TECHNICAL REVIEW No.87 (2002) Fig.12 Magnetic properties in high strength magnetic fields 4. Applying newly developed steel sheet to electric power steering EPS (Electric Power Steering) systems are being in- creasingly used as a means of achieving fuel economy. Applying the newly developed steel sheet to the EPS sys- tem is worth mentioning. The EPS system is said to reduce fuel consumption by 3 to 5% when compared with the conventional hydraulic power steering system. This reduc- tion is due to the fact that the hydraulic pump in the con- ventional hydraulic power steering system operates con- tinuously even when the automobile is driving in a straight line and the steering wheel is not being turned. This means that conventional hydraulic power steering systems waste energy when the automobile is driving fast and in a straight line on the high way. In contrast, the motor in the EPS system is put into operation only when the driver turns the steering wheel, thus does not consume energy when the automobile is driving in a straight line. As a re- sult of this advantage in terms of fuel consumption, about 30% of passenger cars are expected to be equipped with EPS systems by 2006. However, there is a drawback to the EPS system. Its steering response is inferior to the hydraulic power steer- ing system. This is due to loss torque generated when the motor is idling. Loss torque is attributable to the bearings, brushes, and other mechanical parts, as well as hysteresis loss originating in the motor core material. As a result it is necessary that the motor core material for the EPS system should exhibit low hysteresis loss. The newly developed material provides lower hysteresis loss than the JIS grade materials of comparable core loss, and is considered the best kind for use as EPS motor core material. Brush DC motors were fabricated experimentally using electrical steel sheets of various grades, and their loss torque was analyzed. The result is shown in Fig.13. The loss torque is expressed as a ratio with that of the conven- tional 50A1000 grade material being unity. The newly de- veloped material exhibits a loss torque of about 60% that of the conventional material. Lowering the loss torque im- proves the steering response of the EPS system. For this reason, several new model automobiles have already adopted this material in their EPS motors. Furthermore, the newly developed material has al- ready been used in hybrid electric vehicle motors, high-efficiency induction motors, and other motors that need to be highly energy efficient. It is expected that its applications in various motors will further expand in future. 5. Conclusion NKK carried out extensive investigations on the effect of sulfur on the magnetic properties of electrical steel sheets, and obtained the following findings. The hysteresis loss of the ultra-low sulfur steel sheet containing sulfur at less than 10 ppm increases with increased temperatures in high-temperature annealing. This is because AlN precipi- tates in the surface layer, prevents the move of the mag- netic domain wall. Ultra-low sulfur steel exhibits pro- nounced surface nitriding due to the decreased levels of surface segregation of sulfur. On the basis of the above findings, NKK developed a new type of electrical steel sheet for energy efficient mo- tors. This was achieved by reducing the sulfur content to an extremely low level while inhibiting surface nitriding. The newly developed material has lower hysteresis loss, higher magnetic flux density, and better punching property when compared with conventional materials. The newly developed material is also easy to produce, and very promising as a core material for highly energy efficient motors. 6 5 4 3 2 1 1.75 1.80 1.85 1.90 1.95 2.00 2.05 H , ×104 A/m 35A230 Developed material J, T EPS type : column Motor type : blush DC 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Developed material （0.5mm） 50A700 50A1000 Loss torque（50A1000=1） Hysteresis loss Blush loss Bearing loss Fig.13 Effect of hysteresis loss on loss torque of the EPS motor
- 7. Ultra-low Sulfur Non-oriented Electrical Steel Sheets for Highly Efficient Motors: NKB-CORE NKK TECHNICAL REVIEW No.87 (2002) –18– References 1) Nishiyama, T. “Development of Small-size High-efficiency IPM Mo- tors for EV”. Prospect of Metal-related Materials for Electric Vehicles. p.53(1997). 2) Tsuruta, K. “Hybrid Electric Cars of Toyota”. Nikkei Mechanical. No.563, p.12(2001). 3) Bozorth, R. M. Ferromagnetism (1951) 77. 4) Obara, T. “Trends in Development of Highly Functional Non-oriented Silicon-bearing Steel Sheet”. The 155th and 156th Nishiyama Memo- rial Technical Symposium, p.151(1995). 5) Lyudkovsky, G. et al. “Non-oriented Electrical Steels”. Journal of Metals. January, p.18(1986). 6) Driscoll, T. J. “The Initial Oxidation of Iron at 200℃ and 300℃ and the Effect of Surface Sulfur”. Oxidation of Metals. Vol.16, Nos.1/2, p.107(1981). <Please refer to> Yoshihiko Oda Sheet Products Research Dept., Materials and Processing Research Cen- ter Tel : (81)-84-945-3614 e-mail : [email protected]