【作者】 石淑琴；

【导师】 陈光；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2006， 博士

【摘要】 本文以Fe-1.5C-1.5Cr-xAl超高碳钢为研究对象，采用理论分析与实验相结合的方法，系统研究了铝元素与热处理工艺对超高碳钢组织转变及力学性能的影响，提出了铝元素抑制先共析碳化物析出的机理，确定了Fe-1.5C-1.5Cr-xAl超高碳钢铝元素的合理添加量，并提出了Fe-1.5C-1.5Cr-2.0Al超高碳钢不经形变获得组织超细化、综合性能良好的热处理工艺。主要结论如下：(1)采用Thermo-Calc热力学计算软件系统，对Fe-1.5C-1.5Cr-xAl超高碳钢通过热力学相图计算发现：铝的添加使该超高碳钢单相奥氏体区域变窄并左移，进入液、固两相区的温度降低，同时共析转变温度提高。Fe-1.5C-1.5Cr-2.0Al超高碳钢共析转变温度区间约为40℃，共析转变开始温度约为790℃。(2)铝元素对超高碳钢组织中的先共析碳化物(网状碳化物和魏氏组织碳化物)具有显著的抑制作用。当含铝量增加到1.0％以上时，魏氏组织碳化物被抑制；接近2.0％时，网状碳化物开始断网。(3)铝元素抑制先共析碳化物析出机理为：奥氏体化后未溶碳化物增多降低了奥氏体碳含量，冷却过程中先共析碳化物减少；同时未溶碳化物阻碍奥氏体晶粒长大，奥氏体晶粒细化使晶界上分布的碳化物减少；并且铝在晶界附近的偏聚对晶界碳化物的形核及长大有抑制作用。(4)随含铝量增加，珠光体片层间距减小，相变组织完全球化温度下降；含铝2.0％时，经850℃×1h等温球化后可获得完全的球化组织；含铝大于2.54％时，出现石墨化；Fe-1.5C-1.5Cr-xAl超高碳钢铝的合理添加量应取2.0％。(5)铝元素提高了超高碳钢的回火稳定性，推迟了ε-碳化物向渗碳体的转变。400℃回火组织无塑性；450℃回火组织具有超高强度、高硬度、低塑性；650℃回火组织具有高强度、高塑性、低硬度。单纯采用淬火-回火工艺难以实现Fe-1.5C-1.5Cr-2.0Al超高碳钢综合性能的提高。(6)测定了Fe-1.5C-1.5Cr-2.0Al超高碳钢等温转变开始点和马氏体转变开始点Ms，测定的300℃孕育期最短为30分钟，Ms为150℃。(7)提出了Fe-1.5C-1.5Cr-2.0Al超高碳钢不经形变获得组织超细化、综合性能良好的等温淬火热处理工艺。300℃×10h等温淬火处理，获得了含条宽约100nm下贝氏体铁素体+宽度小于100nm残余奥氏体+直径约250nm球状渗碳体的超细复相组织，其屈服强度1568MPa，抗拉强度1744MPa，总延伸率6.9％，硬度HV532。更多还原

【Abstract】 In this paper, a system research on the effects of Al element addition and heattreatment on the phase transformation and mechanical properties of Fe-1.5C-1.5Cr-xAlUltra High Carbon Steel (UHCS) was done based on theory analysis combined withexperimental tests. The control mechanism of Al element on the separation of theproeutectoid carbide was also discussed. Based on optimizing proper Al element addition,a comprehensive heat treatment technics without deformation to gain excellent combinedproperties with ultrafine grained microstructure in Fe-1.5C-1.5Cr-2.0Al UHCS wasproposed. The main research conclusions were presented as follows:(1) Thermodynamics phase graphs of Fe-1.5C-1.5Cr-xAl UHCS were calculated under"Thermo-Calc" thermodynamics calculating system. The addition of Al element wasfound to make the austenitic mono-phase region of UHCS become narrower andshift towards left. The temperature to reach liquid and solid binal phases decreases.With increasing the Al content, the onset temperature as well as phase transformationinterval for the eutectoid transformation increases. The eutectoid transformationtemperature interval of Fe-1.5C-1.5Cr-2.0Al UHCS is in about 40℃, and the starttemperature of eutectoid transformation is about 790℃.(2) Remarkable restrain effect of Al element on proeutectoid carbide (netlike carbide andcarbide in wildmanstatten tissue) was found in UHCS. When the Al content reaches1.0%, wildmanstatten tissue is suppressed. As Al content increases further close to2.0%, netlike carbide begins to be broken.(3) The mechanism for the Al element suppressing the formation of proeutectoid carbideis proposed: The increase of undissolved carbide after austenite treatment decreasesthe carbon content in austenite which results in a decrease in the amount ofproeutectoid carbide during cooling. The unresolved carbide suppresses the growthof austenite grains. Refinement in austenite grains reduces the number of carbidedistributed in grain boundary. Aggregation of Al in grain boundary greatlysuppresses the nucleation and growth of carbide in the boundary.(4) Space between pearlite flakes decreases and the temperature for pearlite tospheroidization totally decreases with increasing Al content. When Al contentreaches 2.0%, a totally glomeration tissue can be gained after 850℃×1h isothermalspheroidization treatment. However, when Al content exceeds 2.54%, graphitizationis found. The proper addition quantity for Al in Fe-1.5C-1.5Cr-xAl UHCS is 2.0%。 (5) Al element can increase thermal stability of UHCS against tempering and retardscementite formation fromε-carbide, which attribute to the low plastic after mediumtemperature tempering. Based on this research, we bring forward that it is verydifficult to increase the combined properties of Fe-1.5C-1.5Cr-2.0Al UHCS underquenching-tempering treatment.(6) Isothermal transformation points and Ms temperature of Fe-1.5C-1.5Cr-2.0Al UHCSwere measured. The shortest gestation time is about 30minutes at 300℃, while itsMs is 150℃.(7) A comprehensive isothermal quenching heat treatment technology withoutdeformation to gain excellent combined properties was proposed forFe-1.5C-1.5Cr-2.0Al UHCS. After isothermal quenching of 300℃×10h, a kind ofultrafine complex phase structure composed of low-bainite ferrite with item width of100nm、residual austenitic less than 100 nm in width and spherical cemenite withdiameter of about 250nm was gained. The yield strength reaches 1568MPa, fracturestrength 1744 MPa, the total elongation 6.9%, and microhardness HV532.更多还原