According to the thread type, it is divided into two types: coarse thread and fine thread. The coarse thread type is not displayed in the bolt mark. Bolts are divided into eight grades of 3.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9 according to their performance grades. Among them, bolts of grade 8.8 and above (including grade 8.8) are made of low-carbon alloy steel or medium-carbon steel and are heat-treated (quenched). + Tempering), generally referred to as high-strength bolts, generally referred to as ordinary bolts below grade 8.8 (excluding grade 8.8).