Here are multiple models and variations od screws, bolts and nuts with different heads and threads. They have a realistic procedural metal material.
BLEND (Blender, filesize: 1.56 GB), BLEND (Blender, filesize: 1.56 GB), OBJ (OBJ, filesize: 799 MB), FBX (Autodesk FBX, filesize: 1.56 GB), STL (Stereolithography, filesize: 1.56 GB)