Development of Bainitic Coral Graphite Cast Iron Dies for Plastics Injection

本课题旨在开发适用于塑料注塑模具的贝氏体珊瑚石墨铸铁。为了弄清细小过冷石墨即珊瑚石墨的形成条件，首先测量了含钛铸铁或真空处理铸铁单向凝固时奥氏体-石墨共晶凝固前沿的温度。研究了氧和氮对石墨结构的影响。其次，对含镍和铜的珊瑚石墨铸铁进行等温回火，并对其力学性能进行了测试。所得结果总结如下：1)提高生长速率使石墨形貌由片状向细过冷型转变。在熔体中加入钛或真空处理后，发生变化的生长速率降低。2)珊瑚石墨的形成并不一定需要大的过冷。3)细珊瑚石墨结构样品的溶质氧和溶质氮含量低于片状石墨结构样品。由此可以得出结论：Ti添加或真空处理对石墨微细结构的促进是由熔体中氧和氮的还原引起的。4)对含镍、含铜珊瑚石墨铸铁进行等温回火处理，得到贝氏体组织。该贝氏体珊瑚石墨铸铁的力学性能适合于注塑模具。

The aim of this project is to develop bainitic coral graphite cast iron which is suitable for the plastics injection dies. In order to make clear the condition forming fine undercooled graphite, i.e. coral graphite, firstly the temperature of the solidification front of austenite-graphite eutectic was measured in Ti-bearing cast iron or vacuum-treated cast iron solidified unidirectionally. And the effect of oxygen and nitrogen on the graphite structure was investigated.Secondary, coral graphite cast irons containing nickel and copper were austempered and the mechani-cal properties of the irons were examined. The results obtained are summarized as follows.1) Increasing the growth rate changed the graphite morphology from flaky to fine undercooled type. The growth rate where the change took place decreased by the addition of titanium or vacuum treatment of melts.2) The formation of the coral graphite did not necessarily need large undercooling.3) The content of solute oxygen and solute nitrogen of the specimen with fine coral graphite structure was lower than those of the specimen with flaky graphite structure. Accordingly, it was concluded that the promotion of fine graphite structure by means of Ti addition or vacuum treatment was caused by the reduction of oxygen and nitrogen from the melt.4) Bainitic structure were obtained by austempering treatement of the coral graphite cast iron contain-ing nickel and copper. The mechanical properties of this bainitic coral graphite cast iron was judged to be suitable for the plastics injection dies.