【作者】 李绍宏；

【导师】 吴晓春；

【作者基本信息】 上海大学 ， 材料学， 2011， 博士

【摘要】 本文针对Cr12MoV、D2等常用冷作模具钢因碳化物偏析严重,凝固时易形成带状莱氏体碳化物导致模具因韧性不足而崩刃、断裂或塌陷的实际情况,采用热力学原理并结合计算机辅助优化设计,以减少莱氏体碳化物、增加二次析出碳化物数量来提高韧性的合金化思路,在Cr12MoV成分的基础上适当降低C和Cr含量以减少莱氏体碳化物偏析,提高Mo、V含量以细化晶粒、改善韧性。开发出兼备高强韧性和高耐磨性的SDC99冷作模具钢,以适应当前冷冲压成形材料强度不断提高的发展趋势。同时采用深冷处理技术对组织进行控制,进一步改善使用性能。通过力学性能测试、光学显微镜(OM)、扫描电镜(SEM)、高分辨透射电镜(HRTEM)、X射线衍射仪(XRD)、DIL805A膨胀仪、电阻仪、内耗仪等测试手段展开研究,揭示了高强韧冷作模具钢的合金化规律和强韧化机制,澄清了深冷处理过程中的微观组织转变及相变机理,获得以下主要结论:本研究设计的高强韧冷作模具钢SDC99克服了传统Cr12MoV易崩刃或开裂的弱点,高温抗回火稳定性优于Cr12MoV。在硬度为62HRC条件下,韧性达80J以上,比Cr12MoV提高一倍,耐磨性能与Cr12MoV相当。其性能与进口同类型高品质冷作模具钢相当,可替代进口DC53,SLD-Magic及ASSAB88等高强韧冷作模具钢。通过电解萃取碳化物的激光粒度分析表明,SDC99低温回火后钢中碳化物的中位径为1.23μm,而Cr12MoV低温回火后钢中碳化物的中位径为11.37μm。采用统计方法得出SDC99钢中有45.5%的碳化物尺寸在0.25~1μm之间,而Cr12MoV钢中91%的碳化物尺寸均大于5μm。通过改善钢中碳化物的形态、数量及尺寸分布,使钢的力学性能得到了提高。采用深冷处理技术实现了对高强韧冷作模具钢热处理后组织的有效控制,进一步提高了耐磨性能。经不同工艺深冷处理后,硬度都有1~3HRC的提高,但冲击韧性均下降到常规淬回火处理的一半左右。深冷处理工艺顺序对性能影响显著:试样经淬火+回火+深冷处理(HTC24)以及淬火+一次回火+长时间深冷+一次回火(HTC24T),硬度均无明显增加,但冲击韧性较淬火+深冷处理+回火的冲击韧性更高。长时间深冷处理后残余奥氏体仍不能完全转变为马氏体,经深冷处理后钢中的残余奥氏体以纳米级薄膜状存在于马氏体板条间。淬火+回火+深冷处理的试样中剩余奥氏体量高于淬火+深冷处理+回火试样的残余奥氏体量,其原因可能是回火过程中部分碳原子从马氏体中扩散进入残余奥氏体,从而使残余奥氏体发生了部分稳定化。经深冷处理+回火后马氏体基体碳含量比淬火+回火处理马氏体的碳含量低,表明深冷处理的试样在回火过程中有更多的碳化物析出。深冷处理后试样的Snoek峰明显降低,即深冷处理后试样中可提供应力感生有序产生Snoek弛豫的碳原子减少。SKK峰增高表明深冷处理过程中的应力作用可导致碳原子发生迁移,迁移的碳原子偏聚于位错周围对位错产生强烈的钉扎作用。延长深冷处理保持时间使SKK峰高度进一步增加的结果表明:间隙原子在应力作用下发生了迁移,并且与时间具有一定的关系,即间隙碳原子在低温下应力诱导扩散需要较长时间,深冷处理保持时间对SKK峰的影响可用耦合模型进行解释。回火后SKK峰弛豫强度随回火温度升高而降低。经过深冷处理与未深冷处理的试样在相同温度回火相同时间后发现,经过深冷处理试样的SKK弛豫强度明显低于未经深冷处理试样的弛豫强度,且回火温度升高后,深冷处理前后峰高的差距变小。由此表明:深冷处理使碳化物析出的动力增强,与未深冷处理的试样相比,经深冷处理的试样在较低温度回火就能析出更多的碳化物。使用电阻测试淬火态和深冷处理后的试样得出了相同的结论。深冷处理过程中的应力诱导作用导致碳原子发生迁移。迁移的碳原子偏聚于位错周围形成偏聚区或原子团簇,在回火过程中偏聚于位错周围的碳原子团簇无需进行长程扩散而形成碳化物。因此,在相同温度回火相同时间条件下,深冷处理后的试样比未经深冷处理的试样能析出更多的碳化物。这是除残余奥氏体转变为马氏体外,深冷处理提高耐磨性能的第二诱因。采用计算机模拟技术对深冷处理过程的温度场、应力场和应变场进行计算表明:深冷处理有利于降低淬火过程中产生的内应力。冷却温度越低,消除淬火应力的效果越明显。深冷处理过程由于收缩作用将产生很大的内应力,应在深冷处理前进行回火处理。冷却速度、深冷次数和深冷时间的对应力和应变的影响甚微。更多还原

【Abstract】 It is directed at the ledeburite carbides segregated seriously in solidification of Cr12 die steels (e.g. Cr12MoV or D2 etc.). The morphology characterization of ledeburite carbide is distributed in banding and net with fishbone shapes. Thus the moulds are usually failure with tipping, fracture and collapse in services due to low toughness. In order to improve this situation, a new high strength and high toughness cold work die steel was developed to adapt for the requirement of high strength materials forming. The thermodynamic theory and equilibrium principle of carbon as well as computer aided optimizing design are used in. The alloying ideas are decreasing ledeburite to improve toughness and increasing second precipitation carbides to strength matrix also improve wear resistance. The composition design is to reduce the content of carbon and chromium in order to cut down the segregation of ledeburite carbide; to increase content of molybdenum and vanadium in order to refining grain as well as improve toughness based on Cr12MoV. At the same time, deep cryogenic treatment (DCT) was carried out in order to give full play to proficiency and improve properties.By means of Mechanics Performance Testing, optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometer, DIL805A dilatometer, electric resistance instrument and internal friction system and so on used in research. The alloying regularity and mechanism of toughness have been discovered. And the variation of microstructure as well as mechanism of phase transformation in the process of DCT has been clarified in this dissertation. The main conclusions are summarized as follows:The new developed high strength and high toughness cold work die steel SDC99 have overcome the weakness of Cr12MoV which failed with tipping, fracture and collapse in services due to low toughness. The temper resistance is better than Cr12MoV. The toughness achieves to 80J which is twice as Cr12MoV while the hardness is 62HRC. In addition, the wear-resisting property is as same as Cr12MoV. It can instead of the imported high strength high toughness steel such as DC53, SLD-Magic and ASSAB88 etc. The carbides were obtained by electroextraction in quenched and tempered steel. The distribution of carbides size was analyzed by Laser Particle Size Analyzer (LPSA). The average size of carbide in SDC99 and Cr12MoV are 1.23μm and 11.37μm respectively after tempering. In addition, the statistical method was used to study the carbides distribution in the tempered two steals. There are 45.5% of carbides distributed between 0.25~1μm in tempered SDC99 steel, while 91%of carbides size lager than 5μm in tempered Cr12MoV. The SDC99 steel was made the carbides size distribution better than Cr12MoV and a large number of fine carbides precipitated in the tempering process. It is the reason that higher toughness obtained in SDC99 while the wear resistance is as well as Cr12MoV.Deep cryogenic treatment was carried out in order to improve the wear resistance and dimensional stability. The hardness is increased 1~3HRC after DCT, however, the toughness reduced. The volume of retained austenite is reduced after DCT. The volume of retained austenite in tempering then DCT treated sample is higher than DCT treated after quenching. The reason is that carbon atoms diffuse into retained austenite in the process of tempering to make the retained austenite stabilized. Besides, the carbon content in martensite is lower than tempered sample after DCT. It can be concluding that more carbide was precipitated in the tempering after DCT.It is shown the Snoek peak reduced after DCT treating by means of internal friction. The Snoek relaxation which is associated with the reorientation of interstitial solute atoms(C, N) disappeared almost in the body-centered cubic metals under the application of oscillatory stress. The decreasing of the height of Snoek peak indicates the solute C amount is reduced after DCT treating. However, the Snoek-Kê-K?ster (SKK) peak increased after DCT means the carbon atoms segregate to nearby dislocations and produced strong interactions including the interstitial carbon atom themselves and between the interstitial carbon atoms with time-dependent strain field of dislocations. The interactions between the interstitial carbon atoms in the Cottrell atmosphere are stronger than the long range interactions between interstitial carbon atoms in the perfect bcc lattice. These effects are enhanced with the soaking time in temperature of cryogenic treatments due to more transformation of austenite to martensite and increase the lattice distortion with prolonging soaking time. Effect of deep cryogenic treatment on internal friction behaviors of cold work die steel can be explained by coupling model.The SKK peak decreased with increasing tempering temperature. It is shown that the height of SKK peak in DCT treated sample is lower than non DCT treated sample which were tempered at same temperature with 2 h. This indicates that there is more carbide precipitated from matrix after DCT treated during tempering. The same result is obtained from experiment of electric resistance. DCT promotes the carbon atoms segregated nearby dislocations and the segregated carbon atoms forming clusters as well as the carbides precipitation in the process of tempering. This is an important reason in improvement of wear resistance besides the transformation of retained austenite into martensite after DCT.Moreover, the computer modeling and simulating technology was used to calculate the temperature field, stress and strain field in the process of DCT. It is shown that DCT is helpful for reducing internal stress which produced in quenching process. It is obviously that the lower temperature, the lower internal stress after DCT treating. It is necessary to carry out tempering before DCT because large internal stress produced in the process of DCT. However, cooling speed, cryogenic times and cryogenic time have very little effect of stress and strain from simulations.更多还原