| 摘要 |
PURPOSE:To manufacture good quality seamless tubes having no linear flaw on the inner surface by using stands provided with koliber for which a gap between a roll groove and a mandrel at an intermediate position between the groove bottom and groove edges narrowest and the gap at other positions is larger as the position is farther from the intermediate position. CONSTITUTION:Nine, for example, stands provided with a couple of grooved roll 21 having a koliber forming groove 21b on the periphery are installed so that each draft direction of the stands is arranged to be sequentially different to each other by 90 deg. around the pass line and a mandrel bar 22 is inserted into a hollow tube stock 10, which is sequentially rolled to be formed into a seamless tube. In that case, from a 1st stand to a 4th stand, the grooved roll 21 are eccentrically formed so that a gap between the groove 21b and the mandrel bar 22 at positions 45 deg. each apart from the groove bottom around the pass line is narrowest and the gap is larger as a position is farther from the above 45 deg. positions. From a 5th stand to a 8th stand, the die is the same as that of a conventional stand and a 9th stand has dies of a constant dia. Generation of inner surface flaws is prevented and smooth seamless tubes are manufactured by sequential rolling of the stock 10 by the stands having the above structure. |
