To provide a method for determining an occurrence condition of a surface flaw that is a surface defect of a steel material which occurs between continuous casting and rolling processes by focusing on the relationship between temperature and the average particle radius (rave) of a granular oxide.

The method for determining an occurrence condition of a surface flaw of a steel material, which is a harmful surface defect when using the steel material, estimates a temperature range when a surface flaw that is a surface defect of a steel material occurs from the average particle radius (rave) of a granular oxide composing a subscale layer (Fig. 1) generated near the flaw. In this case, the granular oxide is formed of two kinds of phases, a first phase oxide and a second phase oxide, and is generated near a surface flaw of a structural steel such that oxide particles are generated by precipitation of the first phase oxide, the oxide particles of the first phase oxide serves as a precipitation nucleus and causes precipitation of the second phase oxide, and thus the coarsening progresses. The temperature region when a surface flaw being an object occurs is estimated from the granular oxide.

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