# Laser-Ultrasound-Based Grain Size Gauge for the Hot Strip Mill
## 3 Results and Discussion
### Online Measurements  
The grain size gauge consists of one measurement head and four smaller cabinets containing:  
1. the climate control for the head and driver for the generation laser,
2. the interferometer and the control computers,
3. the detection laser, and
4. the electronic wiring and control unit for the motors. The control computer communicates with the mill and moves the head into measurement position and turns on the lasers when a strip is present. The control computer collects the ultrasonic response and the necessary mill data and stores it in a database that is then accessed by the analyzing computer. This second computer extracts the relevant information from the A-scan and calculates the grain size, which is then written back to the database and/or is sent to the mill control system. The algorithm finds the longitudinal echoes and decides which part of the spectrum should be utilized in the attenuation calculation. Below, in Figure 7, some bandpass-filtered A-scans from the grain size gauge are displayed. The ultrasonic generation occurs at time \(\sim\)0.7 \(\upmu\)s, and at \(\sim\)2 \(\upmu\)s, the first back wall echo is present. The consecutive echoes have a separation of \(\sim\)1.2 \(\upmu\)s, and their amplitude is clearly attenuated, which is expected from both the grain scattering and geometric effects, as well as the shedding of energy into other waves, such as shear waves. In between the main longitudinal echoes, much smaller negative peaks are visible, which are the mode-converted PS-waves. The signal-to-noise ratio of this system is above 400, but it is expected to deteriorate slightly with time together with the output of the lasers. The grain size gauge has some overcapacity in terms of the generation laser, which is currently operated at around 50% of the maximum output power. Therefore, up to 50% degradation of the generation laser system can be fully compensated by the control computer by allowing more laser light through the internal attenuator. The calculated mean linear intercept grain size including twin boundaries from two online LUS measurements are displayed below in Figure 8.  
Figure 6: (**a**) Graph of the measured grain size as a function of time at the soaking temperature for six of the stainless steel samples. (**b–g**), EBSD inverse pole figure (IPF) maps of the samples post annealing at 900–1150 \(\lx@math@degree\)C, respectively. The micrographs are, for pedagogic reasons, cropped and resized to share the same size scale. The strips had a length of 1000 m and 650 m and had average grain sizes of just over 5 and around 6 \(\upmu\)m in (a) and (b), respectively. The head and tail end of the strip are not measured to provide extra-large safety margins, and, hence, the first and last part of the strip are not displayed. In principle, the lasers could start as soon as the system detects hot material above the gauge by using the detection laser as a material detector [31], which could potentially allow the LUS grain size gauge to make the first measurement just a few millimeters into the strip head end. Figure 8: Calculated grain size along the length of a 1000 m (**a**) and 650 m (**b**) long strip as measured by the online LUS grain size gauge. The head and tail end were not measured due to extra safety margins employed during the first few LUS measurements performed with the new grain size gauge. Figure 7: A few typical bandpass–filtered A–scans from the online LUS-based grain-size gauge installed in the Borlänge hot strip mill. Both measurements display a slightly increasing grain size along the length of the strip, which could be the result of a variation in rolling speed. Changing the zoom during rolling implies that the grain-size measurement will be performed at a different time from the last stand comparing the head and the tail end of the strip. Since the material usually experiences a time dependent recrystallization the head and tail end of the strip will be at slightly different stages of the recrystallization kinetics if the rolling speed varies.