PURPOSE: To improve weldability, hot-dip galvanization resistance and grooving characteristic, in the construction steel which is used for outer steel structures, etc., by specifying the chemical composition of the steel and the heating temp. and the rolling conditions of the billet and applying tempering.
CONSTITUTION: A billet is used which is in wt.% C: 0.04-0.15, Si: 0.15-0.25, Mn: 0.50-1.50, Mo: 0.10-0.40, Nb: 0.005-0.060, V: 0.005-0.060, Ti: 0.005-0.030, and the balance Fe, and which has the composition of hot-dip galvanization crack sensitivity equivalent as expressed by formula (LMECeq)≤0.28. The billet is heated to 1050°C or above; after rolling completed at 850-950°C, acceleratedly cooled from the temp. at Ar3 transformation point or above to 400-650°C at the cooling velocity of 3-20°C/sec; and after that, tempered at 500-650°C. Incidentally, the above steel may contain one kind or more from Cu: 0.05-0.40, Ni: 0.05-0.50, Cr: 0.10-0.40, and Ca: 0.0005-0.0050.
IWAI KIYOSHI
NAKATANI YOSHIYUKI
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