## 1 Introduction  
The steel industry is a crucial component of the global economy, providing essential raw materials for a wide range of other industries. Research in this area is critical to the development of new processes, materials, and technologies that improve the efficiency, quality, and sustainability of steel production. Considering the market size of the steel industry, the environmental, social, and economic impact of these types of improvements cannot be overstated. High-strength low-alloy (HSLA) steels typically undergo a processing schedule in which the steel spends some time in a high-temperature, fully austenitic state. During subsequent processing, the steel eventually cools down, resulting in one or several phase transformations from the face-centered cubic austenite phase into the body-centered cubic (or near-equivalent) phases, along with any precipitates that may have formed during the processing steps. The role of the austenite phase is to serve as a parent microstructure, affecting the morphology and crystallography of the transformation products, as well as the kinetics and thermodynamics of the transformations. Information regarding the parent austenite structure is therefore important both from a scientific as well as an industrial processing perspective. Real-time measurement of the parent austenite microstructure would allow improved control of production processes toward improved product quality and economy, as well as tailored properties in the final product. An important aspect of the parent austenite structure is the tendency to form annealing twins during grain growth. In this context, twinning implies that the arrangement of atoms in one grain can be generated in the other via a reflection across a common plane, mostly parallel to the grain boundary interface. For steels, a major part of the twinning interface is parallel to one of the [111] planes, with a 60\({}^{\circ}\) rotation around the plane normal required to shift the atomic structure across the boundary to that of its twin. A special feature of this arrangement is that it results in a coherent, low-energy boundary between the twin and the matrix. The twinning boundaries play an important role in the recrystallization and grain growth of the parent austenite microstructure. However, their quantification from the final product imposes some special difficulties. Grain size measurements using laser ultrasonics (LUS) have a long history. The grain size in cubic phases can be inferred from the attenuation of the ultrasonic waves in polycrystalline materials with high confidence [1; 2; 3; 4; 5; 6]. Additionally, it has been shown that the attenuation is the same regardless of the coherency of the boundary [7; 8; 9]. This means that LUS has the capability to detect annealing twins as well as other boundaries. While LUS has been applied to study low-alloy high-temperature austenite structure evolution in the past [1; 3; 5], the metallographic data for verification have been obtained using selective etching followed by light optical microscopy, which imposes some limitations to identifying annealing twin boundaries. The direct quantitative measurement of the parent austenite structure via conventional means is difficult in any case due to the high temperatures (600-1000 \({}^{\circ}\)C) in which the phase is naturally present. Instead, the parent structure is inferred by studying room-temperature specimens that have undergone a solid-state phase transformation. Lath martensite is a convenient structure for these kinds of studies. The martensite transformation occurs at temperatures below 500 \({}^{\circ}\)C and requires rapid cooling for most alloying compositions. The rapid cooling ensures that most of the alloying and impurity elements are left approximately in the same place as at the beginning of cooling. This has the important consequence that the parent grain structure can be determined for martensitic steels by optical metallography of polished and selectively etched cross-section surfaces.