| 摘要 |
In cutting soft steel or low-carbon steel with a plasma, the concentration of oxygen in an assist gas is controlled so as to diminish dross adhesion around the hole during piercing. As the plasma gas may be used oxygen, air, a gas mixture of oxygen and nitrogen, etc. As the assist gas may be used nitrogen, oxygen, air, a gas mixture of oxygen and air, etc. The oxygen concentration in the assist gas in the piercing step is regulated so as to be higher than in the cutting step. The oxygen concentration in the assist gas in the piercing step is 20 mol% or higher, preferably 100 mol% or a high concentration close to it, while that in the cutting step is 20% or higher and lower than the concentration in burning, e.g., about 40-80 mol%.
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