In manufacturing of a seamless steel pipe by the Mannesmann-mandrel mill process, a round billet heated in a rotary hearth furnace is pierced by a piercer to form a hollow shell, that is, a stock pipe. A mandrel bar with a lubricant applied tothe surface is inserted into the shell in a skewering manner, and the shell is rolled into a predetermine dimension by a mandrel mill consisting of 5 to 9 stands at one pass. This is called elongation rolling.

After elongation rolling, the pipe is drawn out with the mandrel bar, cut off a portion with a poor pipe end shape with a hot saw, and reheated in a reheating furnace, and then the outer surface thereof is descaled with high-pressure water. Theresulting pipe’s outer diameter is successively reduced and the wall thickness is slightly reduced by a stretch reducer, and made into a predetermined product dimension. Thereafter, the pipe is cooled in a cooling bed, cut to a required length by a coldsaw, and sent to a shaping line.

In FIG. 1, the actual line shows a case where the thickness of scales becomes 5 μm, and the dotted line shows a case where the thickness of scale became 20 μm. Namely, when the heating temperature of the test piece was 1150° C.,the thickness of scale becomes 5 μm when exposed to air for 5 seconds, and 20 μm when exposed to air for 65 seconds.

Since the above-mentioned test was a so-called laboratory test, it was slightly different from an actual production line of the seamless steel pipe. However, it can be said, permitting for this difference, that the scale of thickness 5 to 20μm is generated by taking an exposure time of 5 to 60 seconds and setting the exposure temperature on the inner surface of pipe at not lower than 1150° C. The temperature of inner surface of the stock pipe after piercing is about 1250° C. maximum. Since the temperature of inner surface of stock pipe is reduced with the lapse of time, the scale thickness never exceeds 30 μm after the lapse of 60 seconds even at 1250° C.

According to the process of the present invention, in manufacturing a seamless steel pipe by Mannesmann-mandrel mill process, the friction coefficient in the mandrel mill rolling can be reduced and then the seamless steel pipe can be manufacturedwithout causing flaws on the inner surface.