"硬纸板（用于写布告）" can be translated as "cardboard (for writing notices)" or more specifically, when referring to the material typically used for posting announcements, you could say "stiff cardboard or poster board (for notices)".
In a concise phrase for your specific context, "cardboard for notices" or "notice cardboard" would also be understandable, though "poster board (for notices)" might be more commonly used in some English-speaking regions to describe the type of sturdy cardboard often used for making posters or announcements.
For the most straightforward and commonly understood translation, "cardboard for writing notices" or simply "notice cardboard" would suffice.