The forming cause and fracture mechanism of pen-tip shaped fracture of Φ12.5 mm SWRH82B steel (/%: 0. 80C

0. 74Mn

0. 22Si

0. 013P

0. 008S) wire rod is analyzed by optical microscope

scanning electron microscope and energy dispersive spectrometer etc. to get the main causes of formation of pen-tip shaped fracture are center carbon segregation and poor network cementite. With using optimum process measures including controlling 120 t converter end [C]30. 20%

controlling [C]0. 79% ~0. 81% in LF refining process

in 150 mm x 150 mm billet casting controlling liquid over heating extent ≤25 °C

casting speed 1.5 m/min

weak secondary cooling water ratio 0. 30 〜0. 45 L/kg

mold end electromagnetic stirring 3.5 Hz and 300 A

controlling and rolling temperature 900 〜930 ℃

wire rod exiting temperature 〜750 °C with cooling speed 〜5°C/min

the average carbon segregation index of casting billet decreases form original 1. 15 to 1.08

equiaxed ratio increases from 20% to 35%

sorbite ratio increases from 85% to 90%

rating of network cementite decreases to less than 2 and the percentage of pen-tip fracture decreases from original 20% to 3%.