## 1 Introduction
In the high-cycle regime, fatigue cracks occurred from internal defects or nonmetallic inclusions such as TiN, Al\({}_{2}\)O\({}_{3}\), MnS, and Al\({}_{2}\)O\({}_{3}\). The effect of nonmetallic inclusions on the fatigue strength of high-strength steels was studied by [PERSON] and [PERSON] [17] and by [PERSON] et al. [18]. They expressed fatigue strength as a function of Vickers hardness, stress ratio, and the root of the projected area of inclusion. Most studies have focused on high-strength steels or alloys with hardening heat treatment. However, the railway axle material in this study is normalized and has a medium strength level (tensile strength 590 MPa), and it does not contain Cr, Mo, Ni, Cu, etc. It is composed of five elements (C, Mn, Si, S, P) and Fe. Whether this kind of manganese steel has a fatigue limit or not is interesting. [PERSON] et al. [19] investigated the giga-cycle fatigue behavior of normalized carbon steel (Ck15, tensile strength 418 MPa) using an ultrasonic fatigue tester. The chemical composition of the material is 0.15 wt % C, 0.45 Mn, 0.20 Si, P, S, Fe balance. All fatigue cracks initiated at surfaces. The fatigue strength decreased until \(10^{7}\) cycles; after that, it was nearly constant. The fatigue strength at \(10^{9}\) cycles was 2.5% lower than at \(10^{7}\) cycles. The highest number of cycles to failure was \(1.3\times 10^{7}\) cycles. [PERSON] et al. [20] carried out fatigue tests in a giga-cycle regime using specimens produced from a real axle with chemical composition C, Si, Mn, Cr, Ni, Cu, Al, and Fe. A duplex S-N curve (inclined-horizontal-inclined-horizontal curve) was obtained. All cracks initiated at surfaces. Damaged spots around subsurface inclusions that occurred during production were observed, but did not propagate beyond a certain size. [PERSON] et al. [21] compared the fatigue strengths of structural steel 40 Cr (0.4% C, 1% Cr) in air, fresh water, and 3.5% NaCl aqueous solution. Specimens were tested to \(2\times 10^{9}\) cycles with a rotary bending machine. The S-N curves in air and in water showed inclined-horizontal-inclined shapes, but the S-N curve in NaCl presented a continuously decreasing shape. According to the experimental results for 25 Cr2 Ni2 MoV welded specimens by [PERSON] et al. [22], the S-N curve at room temperature presented duplex slopes. However, at 370 \({}^{\circ}\)C, the fatigue strength decreased continuously. The crack initiation mechanism was similar at both temperatures. Fatigue studies at high temperatures are numerous, but comparative studies on fatigue properties at room temperature and low temperatures for manganese steels are rarely found in the literature. The purpose of this study is to investigate the fatigue and impact characteristics at low and high temperatures in consideration of the actual operating environment of railway vehicles. Ultrasonic fatigue testing equipment capable of fatigue tests to \(-\)160 \({}^{\circ}\)C was developed, and the fatigue and Charpy impact characteristics of the axle material were examined over a temperature range of \(-\)60 to +60 \({}^{\circ}\)C. Crack initiation and fracture surfaces were investigated.