The mechanism of fine crack at narrow faces of slab of HSLA steel (/% ：0.16 ~ 0.18C

0.20 ~0.40Si

1.42 ~1.55Mn

≤ 0.025P

≤0.012S

≤ 0.015 ~ 0.025Nb

0.100 ~0.115V

0.0100 ~ 0.0150N) was analyzed and researched by the means of metallographic analysis. The results show that the mechanism of fine crack is unreasonable microstructure of surface of slab narrow faces

such as coarse austenite grain

as well as formation of pre-eutetoid ferrite film and segregation of the second phase particle at the austenite grain boundaries. By optimization of cooling process i. e. increasing narrow face cooling water rate by 35%

the austenite grains are fine

the formation of ferrite film is suppressed and the participation of second phase particle is changed as well as the surface structure of slab is improved

the defect of fine crack at narrow faces of slab is eliminated.