Hexagon socket head bolts are divided into ordinary and high-strength ones according to their strength levels. Ordinary hexagon socket bolts refer to grade 4.8, and high-strength hexagon socket bolts refer to grade 8.8 or above, including grade 10.9 and 12.9.Among them, the inner hexagon sockets include DIN 912, DIN 7991, and ISO 7380.

1.DIN 912
Cylindrical head hexagon socket head screws, also referred to as hexagon socket head bolts, cup head screws, and hexagon socket head screws, have different names, but they mean the same thing. Commonly used hexagon socket head screws include grade 4.8, grade 8.8, grade 10.9, and grade 12.9. Also called hexagon socket screws, also called hexagon socket bolts. The head is either a hexagonal head or a cylindrical head. Grade 8.8-12.9 hexagon socket head screws are called high-strength and high-grade hexagon socket head bolts.
Grade 8.8-12.9 hexagon socket head screws are called high-strength and high-grade hexagon socket head bolts.

2.DIN 7991
IN7991 countersunk head hexagon socket head screw has a conical head and a concave hexagon in the middle. After installation, the screw head can be embedded in the installation surface, and the surface is smooth and beautiful.
The head of the countersunk head hexagonal socket screw can be embedded in the component, can exert larger torque, and has higher connection strength, so it can replace the hexagonal bolt. It is often used in joints that require compact structure and smooth appearance.
DIN7991 hexagon socket countersunk head screws refer to the German standard DIN7991-1986. The standard stipulates the ISO metric thread specifications from M3 to M24 and the product grade of hexagon socket countersunk head screws of grade A.

3.ISO 7380
ISO7380 hexagon socket head screw, a type of hexagon socket head screw, has a flat round head and a concave hexagon in the middle, which is called a hexagon socket head screw.
ISO7380 hexagon socket head screws adopt the international standard ISO7380:1997. The standard stipulates the thread specifications from M3 to M16, performance levels of 8.8, 10.9 and 12.9, and product grade A-level hexagon socket head screws. According to the material, hexagon socket head screws are divided into carbon steel hexagon socket head screws and stainless steel hexagon socket head screws.
Carbon steel flat round head hexagon socket head screws are divided into grade 8.8, grade 10.9 and grade 12.9 according to their performance levels. They are high-strength and high-grade hexagon socket head screws. Generally, low carbon steel, medium carbon steel, high carbon steel and alloy steel are distinguished by the carbon content in carbon steel.