Development of Thickness Measurement System for Hot Steel with Laser-Ultrasonic Wave Technology

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1 Materials Transactions, Vol. 55, No. 7 (2014) pp to 1016 Special Issue on Laser Ultrasonics and Advanced Sensing The Japanese Society for Non-Destructive Inspection Development of Thickness Measurement System for Hot Steel with Laser-Ultrasonic Wave Technology N. Fuse +, K. Kaneshige and H. Watanabe Daido Corporate R&D Center, Daido Steel Co., Ltd., Nagoya , Japan Laser-ultrasonic is one of the non-contact and non-destructive inspection methods for hot materials, complicated shapes and minute materials. It has possibilities to measure dimensions and inspect metallographic structures and defects. It is difficult to measure or inspect them by traditional methods such as an ultrasonic diagnostics. Though laser-ultrasonic can be applicable to many industrial fields, we have studied the practical measures to apply the technology to a thickness measurement system for hot materials, such as robustness of optical systems and safety measures for high-power laser. The thickness measuring techniques for cold and hot bulk steels using laser-ultrasonic were tested in our laboratory. The measuring accuracy was within 2% for cold bulk steels and 4% for hot bulk steels (compared with measured value by gauges or X-ray system). Then an experimental system has been installed in a hot rolling process line and verified in terms of the measuring accuracy, safety counter-measures and durability for high temperature or dust. As the results of this experiment, the measuring accuracy is within 4% compared with X-ray system, dust on optical devices was free due to air curtains and an air jet nozzle for more than 5 months and laser reflection are shielded completely with fireproof metallic fiber curtains. [doi: /matertrans.i-m ] (Received July 11, 2013; Accepted March 28, 2014; Published May 30, 2014) Keywords: laser-ultrasonic, ablation, non-contact, non-destructive, thickness measuring, hot rolled steel, cold rolled steel, speckle interferometer, X-ray 1. Introduction Laser-ultrasonic 1) is the new technology for non-contact and non-destructive inspection. It is specially expected to be applied to measuring thickness and material temperature, inspecting metallographic structure or defects of hot bulk steels, and so on. As it can be applicable to many industrial fields, 2 10) we have studied measurements for robustness of optical systems and safety for high-power laser in the practical use of laser-ultrasonic technology. The thickness measuring techniques for cold and hot bulk steels using laser-ultrasonic were tested in our laboratory. The bulk steels were cut down to specific dimensions from rolled steels in the rolling process line. The measuring accuracy was within 2% for cold bulk steels and 4% for hot bulk steels (compared with measured value by gauges or X-ray system). Later the experimental system has been installed in the hot rolling process line and verified in terms of the measuring accuracy, safety counter-measures and durability for high temperature or dust. Though X-ray has been used to measure thickness of hot rolled steel in the hot rolling process line, laser-ultrasonic technique has possibilities to inspect metallographic structures and defects unlike X-ray. 2. Principle of Measurement 2.1 Ultrasonic generation using laser The principle of ultrasonic generation by laser has two modes, such as thermo-elastic effect and ablation. In the case of thermo-elastic effect, the ultrasonic wave is generated by the stress with thermal expansion when the laser is irradiated on objects. On the other hand, ablation is the phenomenon that momentary evaporation of the objects occurs as soon as high power density pulsed laser is irradiated on them. In + Corresponding author, n-fuse@ac.daido.co.jp Fig. 1 Laser-ultrasonic propagation in hot steel. this time, plasma shock wave is also generated and propagates into the objects as ultrasonic wave. We use ablation in this experiment because of its stronger ultrasonic wave (Fig. 1). 2.2 Ultrasonic detection by laser The ultrasonic wave is finally attenuated after reflecting from bottom to surface of the objects repeatedly. The wave causes the surface of the objects to vibrate. The vibration can be detected with a laser interferometer. Speckle interference (Fig. 2) 11 13) is adopted in this experiment. Speckle pattern appears as reflection when laser beam is irradiated on the objects. The pattern is different before and after vibration (Fig. 3). 14) As the pattern is less susceptible to surface texture of the objects than fringes used in general interferometer such as Fabry-Perot, it is easier to detect vibration on even hot rolled steel surface. 2.3 Calculation of thickness The 1st wave is defined as a wave that is generated with laser and returns to the surface from bottom of objects for the first time. Similarly all other echo waves are defined as 2nd, 3rd, n th (n >= 2) repeatedly (Fig. 4). As the time lag between the n-1 echo wave and the n echo wave is equal to

2 1012 N. Fuse, K. Kaneshige and H. Watanabe Fig. 2 Principle of laser interferometer. (c) Fig. 5 Location of installed system. Fig. 3 Speckle pattern; before vibration, after vibration, (c) difference between and. for ultrasonic detection, a loader/un-loader for them and its incidental equipment (Fig. 7). Optical equipment has cooling mechanisms for the laser system with air and water, and dustproofing mechanisms, like air curtains. 3.3 Laser equipment In Tables 1 and 2, main specifications of the generation laser and the detection laser are described and their appearances are shown in Figs. 8 and 9.15) Fig. 4 Measuring time of flight. the turnaround time from the bottom of objects to the surface (Tof: Time of flight), the thickness (t) is calculated by the eq. (1) with the Tof and the propagation velocity (Vp) that is specified by the steels and its temperature. t¼ 3. A Summary and Experimental System 1 Tof Vp 2 Safety ð1þ Measures of the 3.1 Outline of the experimental system Figure 5 shows the location where the experimental system is installed. We installed the system 20 meter away from a steckel mill in order to measure thickness just after hot rolling (Fig. 6). The temperature of the steel at this location is Celsius (slightly low compared with that of the steel in a coiler furnace). 3.2 Structure of experimental system The experimental system consists of a 1064 nm-yag laser for ultrasonic generation, a 532 nm-yag laser interferometer 3.4 Environmental-resistance Since the interferometer is located just above the hot rolled process line, counter-measures against heat radiation and dust should be taken. As we supposed that the experimental period was from February to June, measures against environmental temperature were taken during the same term Speckle laser interferometer (Ultrasonic detection laser) The laser interferometer is stored in a case filled with heat insulator. The body of the interferometer is ventilated for cooling and dust-proof. The bottom panel and a nozzle are cooled with running water because the distance from hot rolled steel to them is 200 mm. High pressure air is injected from the laser nozzle to prevent the invasion of dust. It also blows heat-resistant and heat absorbing glasses inside the nozzle against dust. Four heat absorbing glasses are provided on a rotary plate in order to replace them easily when they are dirty or damaged (Fig. 10) Ultrasonic generation laser The generation laser is stored in a case filled with heat insulator. The body of the laser is cooled with running water and ventilated for dust-proof (Fig. 11). 3.5 Safety measures As it is important for experimenters and third-party to protect themselves from high power laser, we took countermeasures conformed to the Japanese Industrial Standard C6802 (the same as IEC 60825). Laser reflection has been shielded completely by covering the area of laser exposure with fireproof metallic fiber curtains (NASLONμ, Nippon Seisen Co., Ltd.) (Fig. 12).16)

3 Development of Thickness Measurement System for Hot Steel with Laser-Ultrasonic Wave Technology 1013 Fig. 6 Detailed arrangement. Fig. 7 Table 1 Fig. 8 Ultrasonic generation laser. Fig. 9 Ultrasonic detection laser. Construction of system. Specification of ultrasonic generation laser. Item Specification Type of oscillation Nd:YAG Oscillation wavelength Oscillation power 1064 nm 450 mj Pulse width 10 ns Pulse period 10 Hz Table 2 Specification of ultrasonic detection laser Item Specification Interferometry Oscillation wavelength Speckle 532 nm Oscillation power 180 mw Experimental Result Setting up the database for material temperature vs. ultrasonic propagation velocity The ultrasonic propagation velocity is calculated using the thickness value by X-ray at the specified temperature of the Fig. 10 Environmental measures for laser interferometer. steel surface measured with a radiation thermometer. So we have prepared the correlative database about the steel temperature vs. the ultrasonic propagation velocity for various steels. Examples of detected ultrasonic signals for 2 kinds of steel are shown in Fig. 13 and ultrasonic propagation velocity dependence upon temperature is shown in Fig. 14. As the correlation between steel temperature and ultrasonic propagation velocity is high, linear approximation was performed in the specific temperature region.17)

4 1014 N. Fuse, K. Kaneshige and H. Watanabe Fig. 11 Environmental measures for generation laser. Fig. 12 Safety measures for shielding laser. Fig. 13 Detected ultrasonic wave signals; DF42N (alloy, t3.5 mm, 670 celsius), D38NR (high alloy, t3.5 mm, 690 celsius). Fig. 14 Correlation between ultrasonic propagation velocity vs. temperature; Steel: DF42N, Steel: D38NR. Fig. 15 Comparison of measured thickness with laser and X-ray; DF42N, D38NR. 4.2 Thickness measurement using correlation databases Thickness is calculated by using the eq. (1) in section 2.3. The calculated thickness and the thickness by X-ray system are compared in Fig. 15. The thickness measurement errors for several steels are also shown in Fig. 16. The thickness is measured at the center in width direction of the hot rolled steel. The thickness profile about part of backward 240 and 180 m are shown for DF42N and D38NR in Fig Discussion 5.1 Factors that may affect accuracy of measuring thickness We have verified the major factors that may affect accuracy

5 Development of Thickness Measurement System for Hot Steel with Laser-Ultrasonic Wave Technology Fig. 16 Error span of measured thickness vs. several hot rolled steels (alloy & high alloy) Fig. 18 Measurement error vs. surface conditions. (c) Fig. 17 Surface texture; Lathe s finish, rough grinding finish, (c) mirror finish. of measuring thickness. These are caused by specific character of laser interferometer. As the premise it is important for laser interferometer to receive laser reflection from object sufficiently, some of major factors are surface condition and inclination or waviness of rolled steel. Verification results about these effects are described. We have evaluated the dependence of the thickness measurement accuracy upon these factors, including other factors, such as temperature measurement accuracy, elasticity at high temperature and steel type Effect of surface condition We measured thickness of the three blocks of cold steel, which are the same material (SC50) and the same dimension (t15 mm), but have different surface textures. These surface textures are lathe s finish, rough grinding finish and mirror finish respectively (Fig. 17). The results are shown in Fig. 18. In these results, as it is impossible to detect displacement from mirror finish for speckle interferometer in principle, the measuring accuracy is low because of less S/N. Also because of large diffused reflection from lathe s finish, it leads large loss of laser power and low level S/N. As the surface condition of rolled steels depends on that of the reduction roll, it affects measurement accuracy Effect of angle The measurement accuracy of thickness was investigated when the tilting angle of detection laser to the cold steel surface was changed (Fig. 19). The tilt angle was controlled to either one of 0, 3 and 5. The result is shown in Fig. 20. It is presumed that the ultrasonic S/N decreases because input power of laser reflection to optical detectors is getting smaller as the tilt angle is larger. Actually as the top and end of hot rolled steel have waviness and inclination, the measurement accuracy gets worse Repeatability of measurement The measuring repeatability of ultrasonic propagating velocity of cold and hot bulk steels (seven kinds of steel, thickness: mm, surface finish: rough grinding) was investigated. In case of hot bulk steels, they were measured at Fig. 19 Experimentation. Fig. 20 Measurement error vs. angle of laser beam Celsius after heated until 1100 Celsius in an electric furnace. The actual thickness of hot bulk steels was calculated taking account of coefficient of thermal expansion against measured value of cold steels. The measurement accuracy (3 ) of hot bulk steel is within 4%, and that of cold bulk steel is within 2% (Fig. 21) Error factors The causes have been surveyed that the measuring accuracy of hot rolled steels is lower than that of cold bulk steels. The deterioration of measuring accuracy is caused by variety of steel type, elasticity at high temperature, measurement accuracy of temperature, in addition to the surface condition and inclination or flap of rolled steel. As the ultrasonic attenuation in hot bulk steels is larger than that in cold bulk steel, due to its elasticity, the work

6 1016 N. Fuse, K. Kaneshige and H. Watanabe compared with X-ray system. The error factors are concerned in surface condition, measurement accuracy of material temperature, elasticity at high temperature, and steel type. (2) Dust on optical devices was free due to air curtain and air jet nozzle for more than 5 months. (3) Laser reflection were shielded completely with fireproof metallic fiber curtain. The evaluation continues for more studying about measurement errors. Fig. 21 Average and 3 of ultrasonic propagation velocity at cold and hot bulk steels (N = 5 each). temperature affects the S/N of ultrasonic signals. Temperature was measured only at the surface of steels. However, strictly speaking, the distribution of the temperature has to be measured over the thickness direction. For example if temperature changes 20 Celsius, the measurement value of thickness has about 0.5% difference. This value may become errors that is not ignored. On the other hand, grain size affects ultrasonic signal level and propagation velocity because there are considerable difference in size between grains of cold rolled steels and those of hot rolled steels. 18) These influence are being investigated individually now. The temperature of the steel is measured by a infrared thermometer and the thermal emissivity is specified each steel. Though we calibrate the thermal emissivity, it may have errors owing to surface condition of rolled steels. Collectively, it is supposed there are 2% special errors concerned in hot rolled process line. 6. Conclusion The results are as follows; (1) Measuring accuracy of hot rolled steel is within 4% REFERENCES 1) C. B. Scruby and L. E. Drain: Laser Ultrasonics Techniques and Applications, (Taylor & Francis, 1990). 2) J. Takatsubo: Butsuri-Tansa 54 (2001) 388 (in Japanese). 3) J. Takatsubo: Inspect. Eng. 15 (2010) 24 (in Japanese). 4) J. Takatsubo: INSS JOURNAL 19 (2012) ) K. Yamanaka: J. JSNDI 49 (2000) ) M. Ochiai: Toshiba Rev. 61 (2006) 44. 7) M. Ochiai: J. JSNDI 57 (2008) 19. 8) M. Ochiai: J. JSTP 51 (2010) ) H. Yamada, A. Kosugi and I. Ihara: Japan J. Appl. Phys. 50 (2011) 07HC06. 10) Y. Matsuda: Japan J. Appl. Phys. Part 2-Letters 39 (2000) L59. 11) B. Pouet, B. Sebastian and P. Clémenceauet: QNDE (2005) ) B. Pouet, B. Sebastian and P. Clémenceauet: QNDE (2006) ) B. Pouet, B. Sebastian and P. Clémenceauet: QNDE (2007) ) M. Uchino, T. Harada, M. Nagai and M. Koganemaru: Construction of Measurement System of Out-of-Plane Deformation Using Speckle Pattern, (Fukuoka, Japan, Fukuoka Industrial Technology Center, 2002). 15) Thales Japan K.K: 2012 edition laser equipment catalog, (Tokyo, Japan). 16) Nippon Seisen Co., Ltd.: 2012 edition product catalog, (Osaka, Japan). 17) T. Fukuchi, T. Okuyama and T. Okuyama: Application of laser ultrasound to noncontact temperature measurement of composites, Electric Power Engineering Research Laboratory Rep. No. H06012, (2007, Tokyo, Japan). 18) Y. Nagata: J. JSNDI 49 (2000) 369.