The real-time grain size measurement has recently been realized by laser ultrasonics in the hot-rolling process [1]. This enables quality control, the possibility of direct feedback to the process control system, as well as feedback to the set-up calculation which is performed before each transfer bar is sent through the finishing mill to be rolled. The gauge provides novel insights into how the material behaves during production. This is especially useful for low-alloyed steels that phase transform at cooling to room temperature making it difficult to use traditional metallographic methods to estimate prior austenite grain structure. However, the grain size gauge is currently only installed at one position in the mill, thus, providing measurements at a single point in the process. To better understand how material behaves during the whole process the GLUS® testbed at Swerim, which is the combination of the thermo-mechanical simulator GLEEBLE and laser ultrasonics can be used. The method provides a unique possibility to explore and validate alloying concepts on a smaller scale to increase the understanding of how material properties evolve during for example annealing or hot-rolling processes. In this work, this is demonstrated for an austenitic stainless steel making it possible to confirm LUS measurements with room temperature observations. Thermomechanical simulations are made corresponding to a 6-stand finishing mill, with different deformation strategies reaching the same total deformation. Grain structure is monitored with laser ultrasonics on 316L. In addition, we will present the results from grain size measurement during the deformation showing the capability of GLUS to capture the microstructure evolution such as dynamic recrystallization.

References

[1] Malmström M, Jansson A, Hutchinson B, Lönnqvist J, Gillgren L, Bäcke L, et al. Laser-Ultrasound-Based Grain Size Gauge for the Hot Strip Mill. Applied Sciences 2022;12:10048. https://doi.org/10.3390/app121910048

Residual stresses in metallic materials occur to a varying degree depending on the process stages and different process parameters. The residual stresses may become problematic not only when the material is further processed down-stream in the value chain, but also affects the performance of the final product negatively due to impaired fatigue properties. Today there is no measuring technique that is fast, reliable, and robust enough to measure the residual stresses, which impede process developments due to that no feedback to the operators is available. The overall purpose of the project is to develop a new measuring technique for detection of varying levels of residual stress in the material, which would lead to a drastically increased knowledge on how different processes and parameters affects the final stress state. By using a non-destructive method suited for on-line monitoring, the dynamic changes can be evaluated and enable real-time feedback to the operators. This will be achieved by developing a model that correlates the birefringence in ultrasonic data to the stress level and applied as an electro-magnetic acoustic transducer (EMAT) based on-line measurements demonstrator, in the steel works environment. The non-contact EMAT probe will be installed close to the plate leveling to measure the ultrasonics response before and after leveling. The results from this on-line measurement campaign will be presented. 

The aim of this work is to acquire material data at elevated temperatures for different chemical compositions and phases as basis for numerical simulations incorporating the effect of crystalline texture. Several samples; AISI 304, interstitially free, low carbon, martensite, and pearlite, were measured in a Gleeble® Thermal-Mechanical Simulator 3800 in conjunction with LUS (GLUS®) during annealing from room temperature up to a maximum temperature in the range between 675 and 1000 °C.

Weight savings in mobility and transport are mandatory in order to reduce CO2 emissions and energy consumption. The steel industry offers weight saving solutions by a growing portfolio of Advanced High Strength Steel (AHSS) products. AHSS owe their strength to their largely refined and complex microstructures, containing multiple metallurgical phases. Optimal control of the thermo-mechanical processing of AHSS requires inline sensors for real-time monitoring of evolution and consistency of microstructure and material properties.

In this study, we demonstrate the significance of austenite annealing twin boundaries when calibrating laser ultrasonic measurements for gauging austenite grain size in situ during the thermomechanical processing of high-strength low-alloy steels. Simple calculations show how differences in twinning density can lead to errors in grain size measurements if twins are disregarded during calibration and the method is used for a broad range of steels. Conversely, when calibration is performed using alloys with a metastable austenite microstructure at room temperature, the same calibration is suitable for a broad range of HSLA steels, provided that annealing twins are taken into account. Since light optical microscopy does not allow the characterization of annealing twins in low-alloy steel, the verification of the laser ultrasonic results was conducted using the novel approach of comparing the twinned grain sizes obtained using the ultrasonic method in low-alloy steels with the austenite grain maps reconstructed from martensite orientation maps measured using electron backscatter diffraction. Finally, we show how differences in twinning density occur even for alloys with a roughly similar stacking fault energy, further highlighting the importance of annealing twins in the calibration of laser ultrasonic measurements for industrial use.

Various approaches are reviewed for determining elastic anisotropy and its coupling to crystallographic texture, with special reference to ultrasonic measurements. Two new methods are described for measuring the anisotropy of P-wave velocity using laser-ultrasonics. Making measurements across the diameter of a cylindrical specimen as it is rotated makes it possible to maintain a very constant known path length. This permits extremely accurate measurements with a precision of better than 0.01%. Results on 316 stainless steel in different conditions are compared with calculated values obtained from EBSD textures together with measured densities and crystalline coefficients from the literature. Excellent agreement is obtained when applying the Hill geometrical average procedure. A similar approach is adopted to measure the variation of wave velocity in a martensitic steel, after tempering at a range of temperatures. Changes in the anisotropy associated with thermal softening are discussed. The second method uses Galvano mirrors to steer the generating laser to different positions over a sheet surface, allowing wave velocities to be determined along different directions in the anisotropic material.

Anisotropy of elastic wave propagation in metals is controlled by texture together with crystalline anisotropy, so laser-ultrasonic measurements can provide valuable information about a material’s underlying elastic phenomena. Evidently, anisotropy cannot be deduced from a single measurement and various approaches have been used to detect and quantify this which are reviewed briefly in the introduction. These include:

• Measurements of velocity by rotating the material with respect to the instrument. This is seldom feasible in an industrial environment but we demonstrate how high precision can be achieved this way in laboratory experiments.

• Changing the wave path using a masked axicon lens or by deflecting the generating laser using galvano-mirror optics. This latter approach is well suited to industrial application such as in steel processing. Examples of this method will be presented. • Combining different wave types having the same direction of propagation such as S0 and SH0 or S0, SH0 and P waves.

• Using P-waves arrivals measured after different numbers of reflections through the thickness of the plate. Although the same fixed positions are used for generation and detection, the successive pulses pass along different directions in the material.

The largest uncertainty in LUS generally comes from the measurement of distance between the two laser points. By machining the material into a cylinder using a lathe, the diameter is extremely constant as the specimen is rotated. This has allowed velocities to be measured with a precision of better than 1 part in 10,000. Results on stainless steels show excellent agreement between measured wave velocities and values calculated from the texture. Another application to quenched and tempered martensite is shown below. Tempering between 20°C and 650°C causes reduction in hardness and leads to increases in stiffness and wave velocity but the anisotropy is almost unchanged. The Galvano mirror technique is demonstrated in application to hot rolled steels where many path directions in the material can be rapidly scanned. Finally, we discuss some limitations of texture measurements from ultrasonic measurements.

The paper summarizes the creation of a robust online grain size gauge for a hot strip mill. A method and algorithm for calculating the grain size from the measured ultrasonic attenuation is presented. This new method is self-calibrating, does not rely on a geometrical reference sample and can cope with the effects of diffraction on the attenuation. The model is based on 52 quenched samples measured with more than 23,000 laser ultrasonics shots and has a correlation coefficient R2 of 0.8. Typical online laser ultrasonic measurements from the hot strip mill and the calculated grain size versus length are presented for a couple of steel strips.