Production flowsheet of steel 420M (/% ： 0.18 〜0.23 C

≤1.00Si

0.25 ~ 1.00Mn

≤0.020P

≤ 0.005S

12.0 ~ 14.0Cr

≤ 0.50Mo

≤ 0.50Ni) is 20 t EAF-AOD-LF-3 t ingot-Φ190 mm tube shell-Φ193 mm x29 mm shell-rolled by ASSEL mill to Φ158.75 mm x25.4 mm tube. Because of poor hot plasticity of steel 420M the piercing point consumption is large in piercing process

the inner wall of shell easily produces overlap and the tube surface cracks easily form after rolled by ASSEL mill. With optimized process including decreasing tube shell heating temperature from original 1 240 ~ 1 280 °C to 1 150 ~ 1 250 °C，using bearing molybdenum piercing point to replace medium carbon CrMo steel piercing point

improving tube rolling parameters and slow cooling or annealing after rolling the surface quality of tube improved markedly and the qualified rate of first nondestructive test increases from original less than 50% to more than 95% .