How does A36 mild steel and SS400 difference?

Both of them are designed for structural purposes, in common application cases their mechanical property can achieve the same purpose, if you have special steel requirements, you need to look up at the chemical composition and mechanical property table to make sure, here are ASTM A36 Specifications:ASTM A36 Carbon Structural Steel Plate, you can compare it with SS400 and other similar standard.
These two steel products are basically the same thing. A36 steel, or as it should properly be know as, "ASTM A36" falls under the jurisdiction of the American Society for Testing and Materials (ASTM). The product SS400 steel, or as it should properly be known as, "JIS G3101 SS400" falls under the jursidiction of Japanese Industrial Standard (JIS). The product form for both standards is plate, bar & shapes, basically structural steel material. The ASTM Standard and the JIS Standard chemical composition are also much the same.
BBN steel offer carbon steel  A36 and SS400 steel stock in China,If you have questions regarding the scope of carbon steel  A36 and SS400 steel,please feel free to reach out to BBN steel, or visit our main steel plate products page for more BBN steel.