PDF《直接铸造变韧铁系球墨铸铁之研究》

Journal of China Institute of Technology Vol.

29-2003.12 直接铸造变韧铁系球墨铸铁之研究 Study on As-Cast Nodular Cast lron with Bainitic Matrix 洪学舜*.金重**.陈兴时*** Shyue-Shuenn Horng. Tsung-Shune Chin.Hsing-Shih Chen * 中华技术学院机械系讲师.**清华大学材研所教授.***荣刚材料科技公司董事长 摘要本研究设计一种利用二段冷却速率控制法,进行直接铸造变韧铁基地之球墨 铸铁的可行性研究.在添加 Ni , Mo , Cu 三种主要合金元素,不同的合金组成情 况下配合冷却速率,探讨所得变韧铁基地球墨铸铁性质之差异,及合金元素间 Cu 取代 Ni 的问题. 实验结果显示: (1)本研究成功地以直接铸造、并控制冷却速率之法,得到高 於90的变韧铁基地的球墨铸铁,其成份围为C:3.47-3.62wt%;

Si: 2.3-2.40wt%;

Ni:0.0-3.1wt%;

Mo:0.16-0.35wt%;

Cu:0-0.84wt%,其抗拉强度:

100 ~

115 kgf / mm2 ,降伏强度:

90 ~

100 kgf / mm2 ,延伸率 1.5 ~ 3.0. (2) 石墨周围旁生成下变韧铁、远离石墨区域生成上变韧铁,碳的作用为其主要因素 之一. (3)Cu 可取代 Ni ,添加

1 Cu 相当於添加 2Ni所致之S曲线右移变 韧铁的变态效果.(4)添加合金元素固然有助於变韧铁的变态生成右移,但要适当 地控制热平衡点温度及瞬间切线斜率,才能得到理想的变韧铁基地. 关键词:变韧铁基地,球墨铸铁,直接铸造,两段冷却过程 Abstract The purpose of this study was to explore the possibility of obtaining as-cast bainitic nodular cast iron by a designed of two-stage-cooling after castings being removed from the mold. In order to enhance bainitic transformation, Ni-Mo combinations with varying amount were added, and the feasibility of replacing Ni by cheaper Cu was also persuaded. Optical and electron metallography, X-ray diffractometry, and mechanical testings were applied to study the cast matrix with nodularity better than 80. From the experimental results, conclusions can be drawn that, (1) nodular cast -

119 - 直接铸造变韧铁系球墨铸铁之研究 irons with more than

90 bainitic matrix are obtainable by the two-stage-cooling process with C : 3.47 ~ 3.62wt% , Si: 2.30 ~ 2.40wt%, Ni:

0 ~ 3.10wt%, Mo: 0.16 ~ 0.35wt%, Cu:

0 ~ 0.84wt%, having tensile strengths

100 ~

115 kgf / mm2 , yield strengths

90 ~

100 kgf / mm2 , and elongation 1.5 ~ 3.0 ;

(2) lower bainite was found to appear nearby the graphite , while upper bainite away from it, the difference in carbon distribution is thought to be one of the main reasons;

(3)Cu was found to be approximately twice as effective as Ni in the enhancement of bainite formation;

(4) although alloy addition is important, it was found that the way of controlling cooling processes is even more vital in obtaining bainitic matrix. Keywords:Bainitic Matrix, Nodular Cast Iron ,Direct Casting, Two- Stage-Cooling Process.

一、前言 球墨的形成藉铁水成份,球化剂及接种剂予以控制.接种剂的作用,使得 铁水熔液充满藉以均质性孕育球墨的核,使其在共晶前或共晶时稳定出现,并持 续等方向性地成长;

其中球化剂强烈除硫并提供铁水中的 Mg 含量.当Mg 含量 在某一适当围内,可促使石墨球化,如果低於某个临界值,则会造成球墨退化. 如果接种剂存在铁水时间太长,会造成接种退化,易促使碳化物析出,因此如何 适当的选择球化剂、接种剂的种类及量,和处理的过程,皆影响著石墨的品质, 而石墨的品质和机械性质又息息相关. 铸铁中的石墨型态常分成三种.一为片状石墨层互相连结而彼此纠缠,热传性 优良,吸震性极佳,同时质软易於加工.二为球状石墨,由於为球状,造成应力 集中较小,机械强度较高,但球墨为一单独之个体,因此热传性较差,同时吸震 性也不好.第三类即为缩状 ( vermicular ) 石墨,其兼具片状石墨和球状石墨的特 点,大致上头部呈圆形,也属彼此相通特点,其生长的机构也并包片状和球状的 生长型态,在文献中[1]论述中甚详,其性质的表现介於片状石墨和球状石墨之间. 变韧铁的形成机构,大致上和波来铁变态机构有所不同,主要分别在於变韧铁 是由沃斯田铁中先析出肥粒铁,再在沃斯田铁和肥粒铁界面处析出碳化物,然后 又造成新的肥粒铁区域,如此成长而形成变韧铁[2].变韧铁随著材料的化学组成, 处理方式而展现不同形态.最常被提及的则是碳钢中的羽毛状的上变韧铁,针状 的下变韧铁,而其性质也回然而异.大抵上上变韧铁在降伏强度、抗拉强度、硬 度值皆比下变韧铁差[3].常常下变韧铁伴随著较多的残留沃斯田铁.进一步的解 释则在於处理时恒温温度的偏低,导致变韧铁变态量和时间相对的关系,温度较 -