*2.1. Sample Preparation*

A hundred metallographic samples were prepared by selecting rolls of 55SiCr, 65Mn, 72A, and SWRH82B hot rolled wire rods with a diameter of 6–12 mm, 25 in each. After cutting, grinding, and polishing, a 4% nitric acid alcohol solution was used for corrosion for 25–35 s (the higher the carbon content is, the longer the corrosion time is). When a clear microstructure was displayed, the ZEISS Axio Observer optical metallographic microscope (Manufactured by Carl Zeiss, Munich, Germany) was used for observation. Twenty-five metallographic photographs were taken randomly from the edge to the center of each metallographic specimen, totaling 2500.
