PURPOSE: To obtain a billet having extremely few internal defects by adjusting the content of C in the components of a molten steel to the prescribed range of the critical strain quantity at which the internal crack of the billet arises, increasing the content of Mn and/or content of Si in said components to make up the required strength and casting continuously the billet by using such molten steel.
CONSTITUTION: The [C]% in the components of a molten steel is adjested to 0.10W0.14%, more preferably 0.10W0.13% in this invention in excluding the conventional concept of assuring the strength of a steel material simply by adjusting the [C]% in the components of the molten steel. Satisfactory continuous castability is assured by increasing the critical strain at which the internal crack of the steel material arises and the strength which is not enough with the [C]% adjusted in the above-mentioned way is made up by increasing and adjusting the [Mn]% and/or [Si]% in the components of the molten steel. The increased [Mn]% and/or [Si]% for the purpose of making up the deficient strength is also effective in improving additionally toughness when the [Mn] and/or [Si] is used as a positive component.
NOMURA FUMIO
OZAWA KOUSAKU
