【作者】 任怀喜；

【导师】 杨华；

【作者基本信息】 山东大学 ， 材料加工工程， 2008， 硕士

【摘要】 压铸是最先进的金属成型方法之一,应用很广,发展很快。但压铸模的服役条件非常苛刻。由于铜合金的浇注温度较高,因此铜合金压铸模的使用寿命非常低,仅相当于铝合金压铸模寿命的1/2-1/3。压铸模具钢中合金元素的含量较高,形成的碳化物种类也较多。这些碳化物沿着初生奥氏体晶界偏析,从而造成组织中有沿晶界连续分布的网状碳化物,导致模具钢的使用性能严重下降。此外,钢中的磷易偏析,也严重降低钢的性能。因此,减轻磷的偏析、破碎沿晶界连续分布的网状碳化物是提高压铸模综合性能的有效途径。对于压铸模具钢来说,合理选材、采用合适的变质处理及热处理工艺,可以提高模具钢的质量,降低压铸生产成本。本文在实验室条件下,重点研究了化学成分选择、复合变质及热处理对铜合金压铸模具钢的组织和力学性能的影响。利用金相显微镜及高倍视频显微镜(HSVM)、电子探针(EPMA)、DSC等手段,首先研究了锌对磷在钢中偏聚的影响,发现:锌能减弱磷在钢中的偏聚,提高钢的力学性能。此外,还研究了K/Na、RE复合变质对钢组织和性能的影响。结果显示:压铸模具钢经过复合变质处理后,晶粒得到了细化,碳化物的形貌也发生了显著的变化,网状碳化物断裂。K/Na具有很强的脱氧、脱硫能力,使得低熔点的硫、氧化物不能在晶界聚集,晶界大大净化,有效减少杂质的数量,使得结晶核心周围过冷度均匀。稀土在钢中主要偏聚在晶界上,具有减轻碳及其他合金元素向晶界的偏析,净化晶界,减少夹杂物的数量,改善夹杂物的形貌和分布的作用;在凝固过程中,强烈吸附在碳化物的某些生长表面上,阻碍碳化的生长,有利于碳化物的断网和孤立化。结果表明:当加入0.5%的K/Na和0.5%的RE复合变质处理时,新型铜合金压铸模具钢的组织及力学性能最好。研究了淬火温度、回火温度和回火保温时间等工艺条件对压铸模具钢组织和性能的影响。发现模具钢随着淬火温度的升高,晶粒逐渐变大,在1100℃以下时,晶粒长大并不是很明显,但是1150℃淬火后,晶粒特别粗大,故淬火温度不宜超过1100℃。模具钢随着淬火温度的升高,硬度增加,线膨胀系数减小,抗热疲劳性能增强,故较高的淬火温度有利于提高模具钢的性能。模具钢550℃回火后硬度达到峰值。随着保温时间延长,硬度先增加后变小,在5小时左右时,硬度最大。最优热处理工艺为:1150-1100℃×50min(三次),油冷;550℃×3-5h(二次),空冷。更多还原

【Abstract】 Die-casting is one of the most advanced metal forming technologies, and it is used widely and developing fast. It is usually used in severe rugged environment. But the point of pouring of copper alloy is very high, so the operational life span is very short. The die steel contains high carbon and a large amount of alloy elements, which results in different kinds of carbides. The carbides segregate due to density variation, so the properties of the steel would become unhomogeneous radially. Meanwhile, some carbides distribute along the boundary of austenite grains while they are precipitating from the liquid phase, which leads to the emergence of continuous network of carbides at room temperature, and this would cause a severe decline of die cast dies’ properties. Besides, phosphorus is usually segregated, and this can reduce the properties of steel. Thus it is an efficient approach to improve die cast dies’ properties by preventing segregation of phosphorus and breaking the network of carbides distributing along the grain boundaries. And reasonable selection of materials, modification and heat treatment could save a large number of die cast dies materials, reduce production cost of die cast dies and improve quality and output of die cast dies.In this thesis, we investigated the effects of chemical composition, modification treatment and heat treatment of die cast dies on the microstructure and mechanical. The effect of zinc on the segregation of phosphorus was investigated utilizing optical microscope、Hi-scope video microscope (HSVM) and electroc probe microanalysis(EPMA). We find that, zinc can reduce the segregation of phosphorus, so the mechanical properties of steel can be improved. We also investigated the effect of K/Na and RE compound modification on the microstructure of die cast dies. The results showed that the grain size was refined, and the morphology of carbides was changed greatly after compound modification. The network of carbides was broken. The result show that the die cast dies modified by 0.5%K/Na+0.5%RE has the best structure and mechanical properties.In this thesis, the effects of quenching temperature, temper temperature and temper thermal retardation time have also been researched. The result show that, with the development of quenching temperature, the crystal grains are bigger. The crystal grains largen slowly under 1100℃,but the grain largens extremely above 1150℃. So the quenching temperature should be under 1100℃. With the development of quenching temperature, the hardness increases , and the thermal fatigue property of the die steel increased. The hardness after 550℃temper reach the maximizing. With the lengthen of temper thermal retardation time, the hardness first largens and then dimimishes. The hardness after 5 hours temper thermal retardation reach the maximizing.It was found that the optimum heat treatment was oil quenching thrice from 1050～1100℃for 50 minutes and tempering twice at 550℃for 3 to 5 hours, then air-cooling.更多还原