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

摘要

Provided are a high-carbon electric-resistance-welded steel tube that combines superior cold workability, machinability, and hardenability and a method for manufacturing such a high-carbon electric-resistance-welded steel tube. Specifically, a high-carbon steel tube is provided as a raw steel tube. The raw steel tube has a composition containing, by mass, 0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0% manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus, 0.02% or less sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to 0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being iron and incidental impurities. The raw steel tube is heated to and soaked at an Ac 3 transformation point or higher and is then subjected to stretch-reducing rolling at a finishing temperature of rolling of 900°C to (Ac 1 transformation point) with a cumulative reduction in diameter of 30% to 70% within the temperature range of 900°C or lower. This allows the formation, without spheroidizing annealing, of a microstructure containing a ferrite phase and cementite grains dispersed therein that have an average grain size d of 0.1 to less than 0.5 µm and an average distance L between surfaces of adjacent cementite grains of 0.5 to 10 µm. In particular, the steel tube has significantly improved machinability.