Prevention of halfway cracks in continuous cast A36 steel slabs containing boron

The formation of halfway cracks in continuous cast A36 steel slabs containing 17 ppm boron was studied by investigating the zero ductility temperature(ZDT), zero strength temperature(ZST), solidification characteristics, and strain distribution in the steels. Results show that the formation of halfway cracks in A36-B slab is attributed to the increase in both the internal crack susceptibility(ZDT and ZST) of the steel and the external tensile strain at the solidification front. The ZST of both steels with(A36-B) and without(A36) boron addition is nearly the same, but the ZDT of A36-B steel is found 50 oC lower due to a considerable increase of boron content at the final stage of solidification. The decrease of ZDT enlarges the solidification cracking susceptibility zone and results in the A36-B steel being more prone to cracking. In addition, during the unbending segments, a large tensile strain, resulting from the unbending process and the misalignment deviation of supporting rolls, occurs in the upper part of the solidified shell, which is another reason causing halfway cracks in A36-B slab. By decreasing the P and S content to be less than 50 ppm and 150 ppm, respectively, controlling B content in the range of 10 to 15 ppm, increasing the secondary cooling specific water ratio from 0.76 to 0.85 L·kg-1, and restricting roll alignment deviation to less than plus or minus 0.3 mm, halfway cracks in the boron containing slab are almost eliminated.