Method for producing hot-rolled seamless pipes having thickened ends

A method for producing hot-rolled, seamless pipes having at least one wall thickening which can be arranged at any positions over the length of the pipe, comprising:

• rolling a hollow shell on a mandrel bar as an inner tool with rolls of a multiple-stand mandrel bar rolling mill having at least three rolling stands and at least two rolls per stand to a required nominal wall thickness;
• producing at specified positions over the length of the pipe a required wall thickening on the outer side of the pipe in comparison with the nominal wall thickness by opening the rolls in the rolling stands, wherein the thickened wall is produced and finish-rolled only by two rolling stands of the at least three rolling stands, which are consecutive as seen in a rolling direction, at the specified positions, in which deviations of the finished contour of the thickening from an ideal circular cross-section, which are produced by the roll contours when the rolls are being opened, are minimised, wherein the rolling stands of the at least three rolling stands located upstream thereof as seen in the rolling direction are also opened for a required wall thickness stepping of the rolling stands and all of the subsequent rolling stands of the at least three rolling stands are opened at least to such an extent as to avoid any contact of the rolls of these rolling stands with the previously produced thickening and thus any subsequent reduction of the produced wall thickening;
• and
• establishing the two rolling stands which roll the wall thickening by a calculated evaluation parameter BWV,i, wherein the rolling stand having the smallest geometric deviation from an ideal circular cross-section in numerical terms and thus the smallest evaluation parameter BWV,i is selected as the first stand for finish-rolling of the wall thickening and the rolling stand arranged upstream thereof as seen in the rolling direction is selected as the second stand, wherein the evaluation parameter BWV,i is defined as follows:
• BWV,i=|ev,i−1+ev,i| for i=2 to i-max
• where i=sequential number of the stand, starting with 1 and ascending in the rolling direction i-max=sequential number of the last stand
• wherein
• ev,i=centre point displacement of the groove base radius R1,i in comparison with the rolling axis (+=above rolling centre, −=below rolling centre during rolling of the thickened wall s-V)
• where
• ev,i=eR,i+A,i,
• wherein
• eR,i=the centre point displacement of the groove base radius R1,i in comparison with the rolling axis (+=above rolling centre, −=below rolling centre) during rolling of the nominal pipe wall thickness s-R
• and
• A,i=theoretical opening dimension of stand i, in order to roll the thickened wall s-V in the groove base,
• where
• A,i=s-V−s,i,
• wherein
• s,i=wall thickness in the groove base of stand i.