Free-cutting steel

Steel with elements added that effectively improve machinability (ease of cutting). This steel is essential for continuous high-speed cutting with automatic machine tools, and various structural steels, tool steels, stainless steels, etc. with added S, Pb, Se, Te, etc. are in practical use. Steel with S increased to 0.10-0.25 mass%, several times higher than normal, and Mn added at the same time at about 1 mass% is called sulfur free-cutting steel. This steel is soft and brittle, but the crystal grains are finely distributed in the direction of hot rolling, so the cutting chips are easy to break, and it has excellent machinability, and has been used widely for a long time. Lead free-cutting steel with added 0.1-0.2 mass% Pb does not dissolve in the steel, but is distributed in fine grains of 2-3 μm, and its lubricating effect mainly prevents tool wear and improves machinability. It can be used for machine parts where strength is important without deteriorating mechanical properties, but its use other than high-grade steel has decreased due to lead pollution issues. There are also steels that combine Pb and S, or even Te, and steels that have been deoxidized with Ca from molten steel to improve machinability are called Ca free-cutting steels or deoxidized adjusted steels.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

被削性(削られやすさ)を有効に高める元素を添加した鋼．自動工作機械で連続高速切削を行うのに不可欠の鋼材で，各種の構造用鋼，工具鋼，ステンレス鋼などにS，Pb，Se，Teなどを添加した鋼種が実用されている．Sを通常の数倍の0.10～0.25質量％ に高め，同時にMnを約1質量％ 添加した鋼は硫黄快削鋼とよばれる．これは柔らかくてもろいが，結晶粒が熱間圧延の方向に伸びて微細に分布し，削りくずが破砕しやすく，すぐれた被削性を示し，もっとも古くから広く利用されている．Pbを0.1～0.2質量％ 添加した鉛快削鋼はPbが鋼中に固溶せず，2～3 μm の微細な粒状で分布し，主として潤滑効果で工具の摩耗を防いで快削性を改善する．機械的性質を劣化せず強度を重視する機械部品にも利用できるが，鉛公害が問題となって高級鋼以外の利用は減少している．PbとSを組み合わせ，あるいはさらにTeを組み合わせた鋼もあり，また溶鋼にCa脱酸を行って被削性を高めたものはCa快削鋼または脱酸調整鋼とよばれている．

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