1. A105N is the American ASTM standard number, with A indicating plain carbon structural steel.

2. A105 is a material code for a special steel, known as a cold-drawn steel. It is a low-carbon steel forging similar to 20 steel. There are two standards for it: the American standard "ASTMA105/A105M Carbon Steel Forgings for Pipe Fittings," and the Chinese standard "GB/T12228-2006 Technical Conditions for Carbon Steel Forgings for General Valves."

3. In pipeline systems, A105 can generally replace Grade 20, but Grade 20 cannot fully replace A105, as there is still a significant difference in strength. However, A105 has slightly poorer resistance to stress corrosion cracking (SCC) compared to Grade 20.

4. Carbon structural steel contains approximately 0.05% to 0.70% carbon, with some grades reaching up to 0.90%.

5. Available in two categories: ordinary carbon structural steel and carbon structural steel. They have wide applications and are extensively used, primarily in railway, bridge, and various construction projects. They are also used for manufacturing metal components that bear static loads, as well as for mechanical parts that are not heat-treated and general welding parts.