METHOD FOR MANUFACTURING ROUND BILLET OF Ni-CONTAINING HIGH ALLOY HAVING IMPROVED INTERNAL QUALITY

A method for manufacturing a round billet of NI-containing high alloy, the method comprising the steps of:

• continuously casting a molten steel by means of a casting mold, thereby obtaining a rectangular billet;
• and
• shaping the rectangular billet into a round billet by forging or rolling,
• wherein the casting mold has a shape such that a w/h ratio between long side length (w) and short side length (h) of a cross section of the rectangular billet is 1.0 to 2.0, and the cross section is perpendicular to the casting direction of the rectangular billet;
• the chemical composition of the molten steel is:
• C:
• 0.005 to 0.250% by mass,
• Si:
• 0.05 to 2.00% by mass,
• Mn:
• 0.05 to 3.00% by mass,
• P:
• no more than 0.04% by mass,
• S:
• no more than 0.004% by mass,
• Cu:
• 0.01 to 3.00% by mass,
• Cr:
• 10 to 35% by mass,
• Ni:
• 10 to 80% by mass,
• Mo:
• 1.5 to 10.0% by mass,
• Al:
• 0.001 to 0.300% by mass,
• N:
• 0.001 to 0.300% by mass,
• W:
• 0.00 to 6.00% by mass,
• Ti:
• 0.00 to 2.00% by mass, and
• the balance:
• Fe and impurities;
• a Mo segregation degree of the round billet is no more than 0.20;
• and
• the Mo segregation degree is a ratio ((cMo′−cMo)/cMo) of a difference (cMo′−cMo) between Mo concentration at the axial center of the round billet (cMo′) and Mo concentration in the molten steel (cMo) to Mo concentration in the molten steel (cMo).