When sourcing steel billets, you must prioritize chemical composition and mechanical properties. Ensure the supplier provides a Mill Test Certificate (MTC) according to EN 10204 3.1 or 3.2. Key elements like Carbon (C), Manganese (Mn), Silicon (Si), Phosphorus (P), and Sulfur (S) must fall within the ranges specified by international standards such as ASTM A615, DIN 17100, or JIS G3112. Additionally, inspect for physical defects like surface cracks, blowholes, or slag inclusions, as these will compromise the quality of the final rolled product.