Method for producing seamless metal tube

A seamless metal tube production method for producing a first seamless metal tube with a first wall thickness and a second seamless metal tube with a second wall thickness, which is different from the first wall thickness, by using an inclined rolling mill, wherein:

• the inclined rolling mill comprises:
• a plug located on a pass line;
• and
• three inclined rolls arranged equiangularly around the pass line, each of which has an entrance-side lateral surface and an exit-side lateral surface, the distance between the pass line and the entrance-side lateral surface decreasing gradually with increasing distance from an entrance of the inclined rolls and decreasing distance from an exit of the inclined rolls along the pass line, and the distance between the pass line and the exit-side lateral surface increasing gradually with increasing distance from the entrance of the inclined rolls and decreasing distance from the exit of the inclined rolls along the pass line;
• and
• the production method comprises:
• a first inclination rolling step of rolling a first heated workpiece by the inclined rolling mill;
• a setting changing step of changing a setup condition of the inclined rolling mill in the following manner (a) or (b):
• (a) when the second wall thickness is smaller than the first wall thickness, positioning the three inclined rolls closer to the pass line than during the first inclination rolling step and making a cross angle of each of the inclined rolls greater than a cross angle of each of the inclined rolls set for the first inclination rolling step;
• or
• (b) when the second wall thickness is larger than the first wall thickness, positioning the three inclined rolls farther from the pass line than during the first inclination rolling step and making the cross angle of each of the inclined rolls smaller than the cross angle of each of the inclined rolls set for the first inclination rolling step;
• and
• a second inclination rolling step of rolling a second heated workpiece by the inclined rolling mill under the changed condition.