① pearlescent shape transformation (high temperature ~, diffusion type ~, A 1 ~ 550℃): A 1 ~ 650℃, austenite A→ pearlite P650 ~ 600℃, austenite A→S (sorbite, flake P) 600 ~ 550℃, austenite a. 550 ~ 350℃, A→ upper bainite B (feathery B: parallel F+ discontinuous CM) 350 ~ 230℃, A → lower bainite B (fine flake or granular carbide distributed in needle-like B: F) B has better strength and toughness than B, which is the desired structure for heat treatment. ③ Martensite transformation (low temperature ~, non-diffusion type ~, MS ~ MF): A is cooled by V >; When VK is cooled below 100 ms, the transition of 100 ms will occur. The supersaturated solid solution of martensite M- carbon in α-Fe. High hardness, high strength and low toughness