Based on analysis on MnS in 016 mm rolled products and in 150 mm x 150 mm casting billet of sulfur-bearing gear steel QT20CrMo (/%：0. 20C

0. 020S

0.020Al

0. 95Cr

0. 20Mo) to get the factors to occur non-qualified rating of A-type inclusions (Anne >3 rating and Aeuaret. >2. 5 rating) in steel

it is obtained that the large size MnS inclusions occurring in central zone of casting billet are source of non-qualified rating A-type inclusions in rolled products and the serious center segregation is main reason of formed large size MnS inclusions in central zone of casting billet. With increasing secondary cooling water rate from 0. 37 L/kg to 0. 59 L/kg

the billet central carbon segregation index decreases from 1. 12 ~ 1. 44 to 0. 93 ~ 1. 23

the sulfur segregation index decreases from 1. 28 ~ 1. 70 to 1. 01 ~ 1. 31 and the size of max network MnS decreases from 2 000μm to 1 000μm. As a result the qualified rate of rating of A-type inclusions in steel QT20CrMo rolled products is up to 100%

and the ratio of ≤2. 0 rating of A-type thin inclusions increases from 25. 0% to 97. 0%.