The effect of 0. 75% ~ 1. 54% Mn on structure and properties of 4)16 mm hot-rolled coil of medium-carbon cold-heading steel (/% : 0.45C

0. 22Si

0. 75 ~ 1. 54Mn

0. 007 ~ 0. 008P

0. 003 ~ 0.004S) has been analyzed and studied. The end-rolling temperature of hot-rolling coil is 880 X.

slow-cooling. Results show that with increasing manganese in steel from 0. 75% to 1. 54%

the tensile strength of coil increases from 661 MPa to 821 MPa

i. e. that increases by 160 MPa； the percentage of ferrite in structure of steel decreases from 49% ~53% to 9% 〜13%

the pearlite content in structure increases obviously and the pearlite lamellar spacing reduces from 0.5 μm to 0.25μm ； and the hardenability of steel improves significantly

that is the hardenability coefficient o£ steel increases from 3. 19 mm to 8. 81 mm.