HIGH CARBON STEEL PIPE HAVING EXCELLENT COLD WORKABILITY, MACHINABILITY, AND QUENCHING PROPERTIES, AND METHOD FOR MANUFACTURING SAME

摘要

Provided are a high carbon resistance-welded steel pipe having excellent cold workability machinability, and quenching properties; and a method for manufacturing the same. Specifically a high carbon steel pipe having a composition containing- by mass 0.25 to 0.60% of C 0.01 to 2.0% of Si, 0.2 to 3.0% of Mn, 0.001 to 0.1% of Al, 0.001 to 0.05% of P, 0.02% or less of S 0.0010 to 0.0100% of N, 0.0003 to 0.0050% of B and 0.0001 to 0.0050% of Ca, with the remainder made up by residual Fe and unavoidable impurities, is used as a material steel pipe. Heating and soaking is performed to Ac transformation point or above, and diameter reducing rolling is subsequently performed at a rolling finishing temperature of 900°C or above (Ac transformation point) and a cumulative diameter reduction ratio of 30 to 70% in a temperature range no greater than 900°C. This makes it possible to obtain, without performing spheroidizing annealing, a structure in which cementite particles having an average particle diameter (d) from 0.1 µm to less than 0.5 µm and in which the average distance (L) between the surfaces of adjacent cementite particles is 0.5 to 10 µm are dispersed in a ferrite phase. In particular machinability is significantly improved.