## 2 Materials and Methods  
A hot-rolled steel sheet with 10 mm thickness was obtained from the strip mill production line from SSAB, Raahe, Finland. The large sheet was cut to smaller test plates with dimensions of 300 mm \(\times\) 250 mm. The plates were then reheated and held at 980 \({}^{\circ}\)C for 40 min before submerging into a water tank for a rapid quenching (cooling rate \(>\) 80 \({}^{\circ}\)C/s). The quenched samples were then tempered for 2 h at selected temperatures (150 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 400 \({}^{\circ}\)C). A hot-rolled sample and water quenched sample without any tempering were also included for comparison (abbreviated HR and WQ in the article, respectively). A Gleeble 3800 thermomechanical simulator was used in order to create a continuous cooling transformation (CCT) diagram for the steel. Samples were cut from the received hot-rolled material in the longitudinal direction. A total of six cooling rates in the range 3-96 \({}^{\circ}\)C/s were applied. Samples were machined to a cylindrical shape with dimensions of 6 mm \(\times\) 9 mm (cooling rates from 3 to 48 \({}^{\circ}\)C/s) or 5 mm \(\times\) 7.5 mm (cooling rate 96 \({}^{\circ}\)C/s). The heating rate was 10 \({}^{\circ}\)C/s and austenitization temperature was 920 \({}^{\circ}\)C. The holding time was 10 min for each sample. No strain was applied during the Gleeble testing. Phase transformation temperatures were identified from the Gleeble dilatation data and phase fractions were calculated from the images taken with an optical microscope. In order to simulate the industrial reheating and quenching process, the samples were not homogenized prior to the Gleeble testing. The chemical composition for the steel is presented in Table 1. The composition has been designed to give full hardening for a plate thickness of approximately 12 mm. Carbon content was 0.4 wt.%, which should provide hardness exceeding 650 HV10 (Vickers hardness number) or 58 HRC (Rockwell C hardness). The avoidance of expensive alloying elements, such as nickel, was a major consideration in the alloy design. Instead of nickel and chromium, molybdenum has been selected as the main hardenability element along with the medium carbon content. Earlier trials with laboratory casts showed that molybdenum had a strong effect on the hardenability of the steel and could be utilized to achieve the desired hardness levels. Manganese has been kept relatively low to avoid the possible deterioration of impact toughness inflicted by segregation. Microalloying with Nb and Ti has been used to prevent excessive grain growth at higher austenitization temperatures. The studied steel has a lower manganese, chromium, and nickel content in comparison to the well-known commercial steel grades, such as the AISI 4340, 300M, and 51 CrV4. Furthermore, commercial boron steels used in similar applications often have notably higher manganese levels. The test plates were water jet cut and machined to suitable dimensions for mechanical testing and microstructural inspection. Three longitudinal flat tensile test specimens of each material variant were tested using an MTS 810 servohydraulic universal testing machine. The dimensions for the reduced section of the tensile test specimens were 10 mm \(\times\) 20 mm \(\times\) 120 mm. Charpy-V impact toughness testing was conducted at room temperature and at \(-\)40 \({}^{\circ}\)C with three specimens for each material condition at both temperatures. The transverse impact tests were made according to ISO 148-1 standard. The specimen size was 7.5 mm \(\times\) 10 mm \(\times\) 55 mm for the impact tests for all samples. A Struers Duramin A-300 tester was used to measure hardness with a minimum of seven indentations (HV10) through the thickness of the material. Microstructural characterization was done with a field emission scanning electron microscope (FESEM, Zeiss Sigma) and with a laser scanning confocal microscope (Keyence VK-X200). Samples were polished and etched with Nital (2%) prior to the inspection.