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5Cr21Mn9Ni4N（简称21─4N）钢,是一种被国内、外所广泛采用的内燃机排气阀钢,此钢种对提高排气门寿命效果显著。国内重庆特殊钢厂、上钢五厂、大冶钢厂、长城钢厂等单位已可大批量生产。农机系统各气门制造厂也开始试制或批量生产21─4N排气门。为使大家及时了解21─4N钢的性能及方便订货,特转登1980年4月“5Cr21Mn9Ni4N气阀钢鉴定转产会”通过的“5Cr21Mn9Ni4N气阀钢技术条件”（报批稿）供参考。待批准后,应以正式公布的技术条件为准。）
5Cr21Mn9Ni4N (referred to as 21 ─ 4N) steel, is a domestic and foreign widely used internal combustion engine exhaust valve steel, the steel to improve the life of the exhaust valve effect is significant. Special steel plant in Chongqing, the steel Wuchang, Daye Steel, Great Wall steel mill and other units have been mass-produced. Agricultural machinery system valve manufacturers also began trial production or mass production of 21 ─ 4N exhaust valve. In order to make everyone keep abreast of 21 - 4N steel performance and convenient ordering, special turn Deng April, “5Cr21Mn9Ni4N valve steel steel identification of the conversion will be” passed the “5Cr21Mn9Ni4N valve steel technical conditions” (for draft) for reference. Pending approval, the official published technical conditions shall prevail. )