【作者】 吴涛；

【导师】 王明智；

【作者基本信息】 燕山大学 ， 材料学， 2013， 博士

【摘要】 组织细化是开发新一代钢铁材料的核心技术，现行组织细化工艺可将铁素体晶粒细化至微米级。为进一步细化钢的组织达到超细晶的目标，需研究新的理论，开发新的技术。本文研究目的是通过淬火碳钢在低温低应力下变形制备超细晶钢，并研究其温变形流变行为和变形过程中组织演变特征。利用Gleeble-3500热模拟试验机对淬火10、20、T8、T10A和T12钢在Ac1温度以下的温变形流变行为进行了研究，并建立了各碳钢温变形本构关系方程；利用光学显微镜、扫描电镜和透射电镜研究了不同工艺条件下碳钢温变形试样的组织特征；基于加工图理论，建立碳钢温变形能量耗散图和加工图，为温加工工艺制定和优化提供理论依据。实验结果表明，随着变形温度的提高和应变速率的降低，淬火碳钢温变形流变应力下降。在较低温度和较高应变速率(600℃，0.1-1s^-1)下，流变应力随碳含量的增加而增大；在较高温度和较低应变速率下(700℃，0.01-0.001s^-1)，流变应力在高碳范围内呈下降趋势。碳含量低于0.78wt.%，淬火碳钢温变形激活能随含碳量增加而降低，碳含量高于0.78wt.%时，淬火碳钢温变形激活能随含碳量增加而增加。在相同变形条件下，能量耗散效率最大值随着碳含量的增加呈增大趋势。淬火碳钢温变形可制备出由亚微米级等轴铁素体晶粒和纳米级碳化物颗粒组成的超细晶组织。温变形过程中不仅发生了动态再结晶，同时发生了形变诱导碳化物析出。T12钢马氏体组织温变加工软化率大于退火组织温变形的软化率；马氏体组织软化机制主要为动态再结晶，退火组织温变形软化机制主要是动态回复。加工图表明，T12钢淬火组织失稳区域是能量耗散效率η低于19%的区域，安全区是能量耗散效率η为19％-33％的区域；球化退火组织失稳区域是能量耗散效率η低于21%的区域，安全区是能量耗散效率η为21％-32％的区域。温变形过程促进了短片状碳化物的溶断和颗粒碳化物的细化；碳化物在温变形过程中的细化主要为短片状碳化物的溶断和碳原子的扩散共同作用的结果；短片状碳化物的溶断是由亚晶界处凹槽的扩展导致。淬火T8钢不同变形量温变形后硬度随着变形量的增大呈先降低后增大的趋势，在变形量为50%时，碳化物完全细化，硬度值最低。更多还原

【Abstract】 Refinement of microstructure is the key technology of new steel, current techniquefor microstructure refinement can just obtain micron-size ferrite grains. In order toproduce ultrafine grain steels，it need new theory and new technology.The aim for this dissertation is to obtain ultrafine grain steels by warm deformationof initially quenched steels at low temperature and low flow stress, and investgate thewarm deformation behavior of quenched carbon steels and the microstructure evolutionduring the warm deformation.The warm deformation behavior of initially quenched steels, such as10,20, T8,T10A and T12, at temperatures below Ac1was studied by Gleeble-3500thermal simulationmachine, and constitutive equations of the warm deformation were set up. Microstructurecharacteristics during the warm deformation with different processes were examined byoptical microcopy, scanning electron microscopy （SEM） and transmission electronmicroscopy （TEM）. Based on the theory of processing map, the power dissipationefficiency and processing maps were established to determine the region of plasticinstability.Experiment results show that the flow stress of the warm deformation of the initiallyquenched steels decreases with the deformation temperature elevating and the strain ratereducing. The flow stress increases with the increase of carbon content at low deformationtemperature and high strain rate(600℃，0.1-1s^-1); which decrease at high deformationtemperature and low strain rate(700℃，0.01-0.001s^-1) with high carbon content. Thewarm deformation activation energy decrease with increase of carbon content（<0.78wt%）and increase with increase of carbon content（>0.78wt%）. The maximum of powerdissipation efficiency of warm deformation at strain of0.5increases with increase ofcarbon content at same deformation conditions. Ultrafine microstructures with equiaxialultrafine/submicron-grained ferrite and nano cementite particles can be obtained by warmdeformation of initially quenched steels. Dynamic recrystallization of ferrites andcementites precipitation can be seen during warm deformation of initially quenched steels.The main softening mechanism during warm deformation of initially quenched T12steel is dynamic recrystallization, with higher work-softening effect than that of initiallyspheroidized T12steel, and the power dissipation efficiency of initially quenched steel ishigher tan that of initially spheroidized steel. The warm compression promotes thefragmentation and the spheroidization of lamellar cementites in the initially quenchedsteels. The fragmentation of lamellar cementites was due to the extension ofsub-grainboundary in the cementite, and the spheroidization of cementites depended onthe diffusion of carbon atoms at the tip of bended and breakup cementites. Themicrohardness of initially quenched T8steel after warm deformation shows a trend fromdescent to ascent with the increase of strain, and microhardness reaches minimum at strainof0.5, with fully globularize cementites.更多还原