【作者】 齐凯；

【导师】 李廷举；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 球墨铸铁作为一种廉价的结构材料,因其良好的耐磨性、减震性、低缺口敏感性以及优良的切削加工性能和铸造性能,已广泛用于汽车、农业机械、机床、冶金机械等多个领域,具有重要的应用价值和广阔的应用前景。目前,几乎所有的球墨铸铁件都是直接铸造成型或经机加工后使用,为了达到强度的特殊要求,常规的处理方法是在球墨铸铁冶炼过程中添加一些合金元素或者增加后续热处理工艺。但是,前者大大增加了球墨铸铁的生产成本,后者耗时、耗能,且对环境污染严重。而塑性加工由于具有较高的生产效率和材料利用率,并使得铸坯结构致密、粗晶破碎细化和均匀,从而显著提高机械性能。因此,通过塑性加工使球墨铸铁获得近（净）终成形是提高球墨铸铁性能,扩展其应用领域的有效途径。本文以工业中最常用的QT450-10球墨铸铁为实验材料,利用物理模拟实验系统地研究了球墨铸铁的塑性变形行为,分析了高温压缩过程中金属基体以及第二相石墨颗粒的变化规律,进而探讨不同变形参数下微观组织的演变机制,利用优化的最佳变形温度确定了QT450-10高温锻造和高温轧制工艺,并尝试了锻造余热淬火在球墨铸铁磨球上的应用。主要研究结果如下:利用物理模拟实验系统地研究了不同变形参数下QT450-10的高温塑性变形行为。通过真应力-应变曲线分析可知,球墨铸铁热变形过程中出现明显的加工硬化和加工软化现象;曲线上峰值应力在不同应变速率下均随温度的增加而下降,而峰值应变随温度的增加呈先增加后下降的趋势。QT450-10在650-950℃温度区间,应变速率为0.01-10s^-1时的热压缩变形激活能Q=391.52 kJ/mol,流变应力σ与Z参数表述的流变应力方程为其中Z参数可表述为:发生动态再结晶的临界应变为对球墨铸铁变形试样的基体组织观察分析表明:球墨铸铁在变形过程中存在形变诱导相变现象。球墨铸铁中形变诱导铁素体转变温度比常规先共析铁素体转变温度提高150℃,且诱导相主要在变形的石墨颗粒周围以碎布块状形态存在;形变诱导珠光体转变开始温度较未变形试样提高43℃,经奥氏体形变诱导珠光体转变后,珠光体发生球化,即碳化物相为均匀分布于铁素体基底的极细的球状颗粒沉淀,其平均尺寸仅为338nm,而铁素体基底分割为许多等轴的亚晶粒,其平均直径仅为1.16μm。研究了变形参数影响石墨颗粒演变的规律,结果表明:随温度的升高,石墨体积百分含量和石墨颗粒形状因子β（石墨短轴/长轴）呈明显下降趋势,而相邻石墨间距则逐渐增加;当应变速率增加时,石墨间距略有下降,而石墨体积百分含量和石墨颗粒形状因子β均无明显变化;随着应变量的增加,相邻石墨间距和石墨颗粒形状因子β呈明显下降趋势,而石墨体积百分含量基本不变。计算表明,石墨由球形演变为椭球状后,当外界拉应力方向与椭球长轴平行时,石墨形状改变可缓和石墨对基体的应力集中效应。球铁热变形过程中,石墨发生塑性变形是因受到基体的摩擦力,导致石墨沿主流变应力方向伸长,即最终的石墨破碎是拉伸断裂而非脆性断裂,当变形量大于石墨颗粒破断的最小变形量(ε>ε_min)时石墨随即发生破裂。采用逐步逼近法得出QT450-10最佳压力加工温度区间。在该温度区间设计了球墨铸铁的高温锻造和高温轧制工艺。锻造过程中,球墨铸铁发生奥氏体变形后的相变,得到了间距小于0.319μm的细珠光体,较常规正火珠光体片层间距（0.46μm）小30.6%。尽管锻件较铸态试样的伸长率降低了14%,但是抗拉强度却由原来的560 MPa增加到905 MPa,提高了61.6%,抗弯强度增加了61.2%。当球墨铸铁轧坯的压缩比达到8.83（压缩量89%）时仍保持良好板型,并未出现破碎现象,说明球墨铸铁具有良好的高温可塑性。轧坯中心部位珠光体含量低于边缘部分,石墨由球形演变为片层状。优化轧制参数后,轧坯抗拉强度较铸态球铁提高了26.5%,尤其在平行轧制方向取样的冲击功达到34 J。轧板出现各向异性,板坯平行轧制方向（纵向）的抗拉强度比垂直轧制方向（横向）高18.9%,延伸率高13.3%。最后,本文进行了球墨铸铁磨球的锻造余热淬火实验,与常规淬火球墨铸铁磨球以及含铬铸铁磨球相比,锻造余热淬火球墨铸铁磨球的马氏体组织由于继承了形变奥氏体中的位错结构而得到充分细化,磨球边缘硬度值比常规淬火磨球的高5.3%。并且锻造余热淬火磨球冲击韧性是常规淬火磨球和高铬铸铁磨球的3倍。锻造余热淬火的磨球磨损率比常规淬火球墨铸铁磨球的磨损率低53.1%,比含铬铸铁磨球的磨损率低51.1%。摩擦系数比常规淬火球墨铸铁磨球的低7.4%,比含铬铸铁磨球的低9.6%。更多还原

【Abstract】 Nodular cast iron is widely used as a structural material in various fields, such as automotive industry, agricultural machinery, machine tools and metallurgy and so on, because of its several advantages over steel including wear resistance, shock absorption, machining, casting abilities, especially from the economical point of view. At present, almost all of the nodular cast iron is used directly as cast or after simply machining. Although its mechanical properties can be improved via alloying and heat treatments usully, alloying increases the opration costs and heat treatments leads to inefficient production, energy dissipation and environmental pollution. Compared with these two former processes, plastic deformaiton method is a more efficient processing technology which leads to a product with optimal properties, not only for dimensional precision and appearance, but also with respect to the mechanical and physical properties. So the applications of nodular cast iron will be extended significantly when the plastic deformation method is employed.In the present paper, the hot deformation behaviors of nodular cast iron were studied systematically by physical simulation. According to the analysis of the graphite particles morphology evolution and phase transition during the plastic deformation process, it can be seen that the final microstructure had a close relationship with deformation parameters, such as deformation temperature, strain rate and strain. In addition, the die forging and hot rolling processes parameters were optimized. A new quenching process for nodular cast iron （QT450-10） named forge-quenching heat treatment was developed, and preliminary process specification was proposed. Main research details and results are as follows:In order to study the hot deformation behaviors of nodular cast iron （QT450-10）, isothermal compression tests were carried out with a Gleeble 1500 over a range of temperature from 650 to 950℃and constant strain rates from 10^-2 to 10 s^-1. The flow curves are marked by a peak stress and softening which decline as temperature rises. The peak stress diminishes with the increase of temperature and the decrease of strain rate while the peak strain presents a maximum with rise in temperature. The activation energy is 391.52 kJ/mol for nodular cast iron in peak stress. The flow stress （σ） of nodular cast iron was described well by the Zener-Hollomon parameter （Z） Where, Z =εexp（?）The relationship of critical strain for dynamic recrystallization （DRX） initiation and peak strain was described byε_c=0.61ε_pMicrostructure analysis showed that the phase transition occurs during the compression process of nodular cast iron. The transformation start temperature of ferrite and pearlite are raised 150℃and 43℃, respectively. In addition, the polygonal type ferrite emerges, and the pearlite spheroidizes, that is to say, the microstructure consists of ferrite and partially spheroidized cementite. The average size of ferrite grain is about 1.16μm and the granular cementite distributed at the ferrite grain boundaries is only 338 nm.The graphite particles morphology evolution is investigated. The results show that both of the volume fraction of graphite and shape ratio （major axis/minor axis） decrease while the space between two graphite particles increase as the temperature increases; when the strain rate increases the space between two adjacent graphite particles becomes smaller slightly, and the volume fraction of graphite and shape ratio are invariant; the space and the volume fraction of graphite decreases significantly while the volume fraction of graphite is constant with the increase of the strain.The results of mechanical calculation show that the stress concentration of nodular cast iron caused by graphite particles can be weakened when the direction of applied stress is parallel to the major axis of elliptic graphite compared with the nodular graphite particles. Because of the friction between the matrix and the graphite, the graphite becomes elongated on the direction of stress until the graphite occurs fracture when the strain is higher than the minimum strain (ε>ε_min).The optimized hot working temperatures of QT450-10 are determined by successive approximation method. In comparison with the interlamellar spacing （0.46μm） of pearlite in nodular cast iron after common normalizing, the finer pearlite whose interlamellar spacing is < 0.319μm is obtained during the die forging process. Although the elongation of forged nodular cast iron decreases 14%, the tensile strength and the bending strength are increased 61.6% and 61.2%, respectively. Nodular cast iron also showed good workability since the rolled nodular cast iron still kept plate without any shatter with the reduction of 89% on the compression direction. Besides, the nodular cast iron paltes show anisotropic mechanical properties. For example, the tensile strength and the elongation of samples parallel to the rolling direction are higher 18.9% and 13.3% than these values of samples perpendicular to the rolling direction, respectively, as compared with the samples perpendicular to the rolling direction.At last, the forge-quenching treatment test of nodular cast iron was designed. The results show that an existence of finer acicular martensitic structure and prolate ellipsoids graphite particles. The hardness and impact tests results show that the forge-quenched nodular cast iron displays superior mechanical properties to conventional quenched nodular cast iron. The wear rates of forge-quenched nodular cast iron are lower 53.1% and 51.1% than conventional quenched nodular cast iron and white cast iron, respectively. Meanwhile, the friction coefficients are also lower 7.4% and 9.6% than conventional quenched nodular cast iron and white cast iron, respectively.更多还原