【作者】 张洋；

【导师】 马永庆； 齐育红；

【作者基本信息】 大连海事大学 ， 载运工具运用工程， 2007， 博士

【摘要】 钢中碳化物类型、数量、尺寸及分布对钢的工艺、组织及性能有极为重要的影响。碳化物的细化对提高钢的强韧性、抗疲劳断裂等性能均有重要作用。目前应用于工模具材料的高碳合金钢分为两种,一种为高碳低合金工模具钢,另一种为高碳高合金工模具钢。对于低合金钢,当为了提高其耐磨性而增加合金含量使之成为高碳中合金钢时,碳化物细化却成为难题。而且,由于高碳中合金钢的淬火残余奥氏体量比高碳低合金钢高得多,势必要求更高的回火温度,由此却可能导致硬度下降。因此,国内外工模具钢历来缺少高碳中合金钢品种。现有的大多数高碳高合金钢中由于碳化物数量多、分布密度大容易聚集长大,通过常规的锻轧加工和热处理工艺很难达到细化的目的,而通过复杂的锻轧加工或热处理工艺又会增加工艺上的困难,并消耗较多的能源,往往也很难达到理想的细化程度。因此,进一步研究超细碳化物高碳中,高合金钢的合金设计方法,对于开发研制性能优越的新型工模具钢材料具有重要的理论意义和实际应用价值。本文通过对高碳合金钢的相平衡热力学计算和实验结果分析,对超细碳化物高碳中、高合金钢的合金设计进行了研究,提出了完整的合金设计方法。针对实际的工模具应用开发出具有超细碳化物的DM7S高合金钢和DM8B中合金钢。由高合金钢的理论计算和实验结果分析得出,Cr,W,Mo元素含量设计合理时,高合金钢中的M₆C的数量增加,在一定程度上抑制M₂₃C₆和M₇C₃碳化物以使退火碳化物均匀细化。由于MC碳化物是淬火未溶碳化物细化的关键,为了避免共晶MC碳化物的出现,应控制V含量。通过亚稳定平衡计算得到高合金钢在回火过程中存在的稳定碳化物相为M₂₃C₆,MC和M₆C,亚稳定碳化物相为M₇C₃,M₃C渗碳体和M₂C（HCP）。亚稳定平衡计算结果与实验结果基本一致。中合金钢实际热处理过程偏离平衡态,需对相平衡热力学计算结果进行修正。中合金钢退火碳化物的细化较之高合金钢更为重要。钢中Cr/（W+2Mo）值越小,M₆C碳化物数量越大,M₆C,MC和M₂₃C₆碳化物数量搭配合适才有利于碳化物细化。因此可以通过相平衡热力学计算推测M₆C碳化物的数量,进而对合金成分进行调整控制,以得到碳化物细化的中合金钢。建立了高Cr,低W、Mo的高合金钢的平衡碳计算公式,提出Cp=0.016W+0.063Mo+0.099Cr+0.19V,二次硬化的回火硬度的计算公式为H_c=a（1+b）/（0.0127a+0.00267）,其中a为基体碳饱和度,b为碳化物沉淀量的修正因子。提出淬火硬度计算公式HRc=α（1+β）/0.00915α+0.00586,α为基体中含C量的平方根,β为基体中合金元素固溶强化对马氏体硬度影响的修正系数,此公式隐含着洛氏硬度测定和马氏体强化机理的对应关系,对其它钢种也适用。研制开发出具有超细碳化物的DM7S高合金钢。DM7S高合金钢退火碳化物类型为M₂₃C₆,M₆C和MC,退火碳化物颗粒的平均尺寸为0.42μm,淬火未溶碳化物类型为M₆C和MC,未溶碳化物颗粒的平均尺寸为0.485μm。二次硬化硬度HRC62.8,同时具有优良的综合机械性能。其组织结构特征和性能与合金设计计算预测的结果相符。研制开发出具有超细碳化物的DM8B中合金钢。DM8B中合金钢退火碳化物由M₂₃C₆,M₆C,MC三种类型组成。退火碳化物尺寸主要分布在0.2～0.5μm范围内,淬火未溶碳化物类型为M₂₃C₆,M₆C和MC,平均尺寸为0.33μm,淬火未溶碳化物主要以超细碳化物为主。890℃淬火硬度为HRC65.8,240℃附近回火时,出现低温二次硬化效应,回火硬度为HRC62.4～63.8,具有较高的抗回火性。其组织结构特征和性能与合金设计计算预测的结果相符。通过合理的成分设计,研究开发的中、高合金钢热处理后碳化物达到预期的细化程度,机械性能得到全面提高,对于超细碳化物高碳合金钢的开发和应用具有指导意义。为实现高碳合金钢的合金设计完整计算提供了新思路。更多还原

【Abstract】 The type,amount and size distribution of carbides have important effect on the process,microstructure and properties of the steel.Fine carbides in the steel are very important to improve the performance of strength-toughness and fatigue crack and so on. This kind of tool and die steel is divided two classes,one is high carbon high alloy steel, and another is high carbon low alloy steel.In order to improve the performance of wear-resistance,increasing the content of alloy elements in the low alloy steel,which makes the low alloy steel become medium alloy steel.Because of the amount of retained austenite after quenching of medium alloy steel is larger than that of low alloy steel,medium alloy steel should be tempered at higher temperature,which may make the tempering hardness decrease.At the same time,due to the large amount and high distribution density of carbides that easily gather in high carbon alloy steel,it is difficult to make carbides refine by normal forging and heat treatment.But for complicated forging and heat treatment,it increases the difficulty on the process and consume more energy source.Therefore,as far as development of new tool and die steel is concerned, it is theoretical and practical significance for researching alloy design method for high carbon medium and high alloy steel with fine carbides.In this paper,the new complete method of the alloy design for high carbon medium and high alloy steel with ultra-fine carbides is put forward through the phase equilibrium thermodynamic calculation and experiment results.For practical application, new medium alloy steel DM8B and high alloy steel DM7S are developed.The results of calculation and experiment of high alloy steel show,when the content of the Cr,W and Mo elements in the steel are rational,the amount of M₆C carbides increase and the M₂₃C₆ and M₇C₃ carbides are restricted,which makes the annealing carbides refine.The V content in the steel should be controlled in order to avoid appearing eutectic MC carbides.Through the metastable equilibrium calculation, the tempering carbide phases contain M₂₃C₆,M₆C and MC stable carbides and M₇C₃, M₃C and M₂C metastable carbide,which is agreement with experimental results. The phase equilibrium thermodynamic calculation should be corrected for the medium alloy steel deviating equilibrium states in the practical annealing treatment.The Cr/（W+2Mo）ratio lower,the amount of M₆C carbides larger.The rational partition of M₆C,M₂₃C₆ and MC is advantaged to carbides refinement.So according to the amount of M₆C carbides calculated by thermodynamics,the medium alloy steel with fine carbides can be obtained.The balanced carbon formula Cp=0.016W+0.063Mo+0.099Cr+0.19V of high alloy steel is put forward.The tempering hardness value can be obtained from HRC=a（1+b）/（0.0127a+0.00267）,in which a is the saturation level of carbon in the matrix and b is correction factor.The quenching hardness value of the medium alloy steel can be calculated from HRC=α（1+β）/（0.00915α+0.00586）,in whichαis the square root of carbon content in the matrix andβis correction coefficient of solid solution strengthening.This formula implies the relationship of hardness measurement and martin site strengthening.High carbon high alloy steel DM7S with ultra-fine carbides is developed.This steel’s annealing carbides contain M₆C,M₂₃C₆ and MC.The annealing carbides average size is 0.42μm,undissolving carbides are primarily M₆C and MC and about 0.485μm in diameter.This steel has the value of hardness HRC62.8 and other excellent mechanical properties.High carbon medium alloy steel DM8B with ultra-fine carbides is developed.This steel has M₆C,M₂₃C₆ and MC three types of annealing carbides which size is primarily in the range of 0.2～0.5μm.Undissolving carbides is M₆C,M₂₃C₆ and MC and the average size is 0.33μm.The steel has the hardness of HRC65.8 after quenching and tempering hardness is HRC62.4～63.8 after tempering at 240℃.The microstructure and properties are agreement with calculation results.The high carbon medium and high alloy steel with ultra-fine carbides and excellent mechanical properties are obtained by rational alloy design which is significant for the development of the high carbon alloy steel with ultra-fine carbides.The new complete research method is provided by the alloy design for high carbon alloy steel.更多还原