The production flowsheet for cold heading steel 10B21 (/% ：0.18-0.23C

≤0.10Si

0.60-0.90Mn

≤0.030P

≤0.035S

0.0008-0.0030B

≥0.02Al) is 50t top and bottom combined blowing converter-LF-280 mm x 325 mm casting bloom-continuous rolling to Φ12 mm rod products. It is obtained by examination that as nitrogen content in steel is 0.0046%

the HRC hardness value of quenched steel is 41-43

while as the nitrogen content in steel is 0.0081%

the HRC value of quenched steel is 21-27 that is less than the specification requirement HRC value 38. And it is obtained by test that as Ti/N ≤8

with increasing Ti/N the yield of boron in steel increases obviously

for instance on condition that the nitrogen content in steel is 0.0046% with Ti/N being 7. 4

the yield of boron is up to 90%

and on this condition the nitrogen in steel is 0.0081% with Ti/N being 4.3

the yield of boron in steel is less than 70% led to low boron content in steel and lower HRC value of quenched steel. By improving the BOF and LF operation to control the nitrogen in steel less than 0.005% (0.0039%-0.0041% ) with Ti/N being 7.5-8.1

the quenched HRC value of core of rod product is 40-42 to meet the requirement of user.