马氏体温变形超细化过共析钢

ULTRAFINED MICROSTRUCTURE OF HYPEREUTECTOID STEEL BY WARM DEFORMATION OF MARTENSITE

利用Gleeble 1500热模拟试验机进行单轴热压缩实验,研究了含Al和不含Al两种过共析钢马氏体温变形和等温回火过程中的组织超细球化演变及超细球化组织的力学性能。结果表明:与马氏体等温回火相比,马氏体温变形加快马氏体的分解动力学,在较短的时间内即获得超细化(α+θ)复相组织。温变形过程中的组织超细化演变主要经历渗碳体粒子的析出与粗化及铁素体基体的动态回复和动态再结晶;而在等温回火过程中,铁素体主要发生静态回复和晶粒长大,并没有再结晶现象发生。合金元素Al的加入在等温回火和温变形过程中均抑制马氏体的分解,阻碍渗碳体粒子的粗化和铁素体晶粒的长大,导致复相组织的细化。同时,Al的加入使马氏体温变形和等温回火后所得超细化(α+θ)复相组织在不降低总延伸率的前提下,强度得以明显提高。

The continuous network of brittle proeutectoid carbide will formed along the grain boundaries when cooled slowly from austenite in hypereutectoid steels. Steels with such high carbon content have been neglected in industry because of they are inherently brittle. By properly processed, such as hot and warm working （HWW）, isothermal warm working （IWW）, divorced eutectoid transformation （DET） and divorced eutectoid transformation with associated deformation （DETWAD）, the steels will exhibit ultrafine microduplex structure with fine spheroidized cementite （θ） particles dispersed in fine-grained and equiaxed ferrite （α） matrix （grain size is less than 1 μm）. This microduplex structure shows superplasticity at elevated temperature and exhibits better mechanical properties at room temperature. However, these processes are relatively complicated and should break the proeutectoid cementiets firstly. By warm deformation of martensite, a simple process and the ultrafined microstructure can be obtained easily. In the present work, the effects of Al on the microstructural ultrarefinement and mechanical properties of hypereutectoid steel during warm deformation of martensite as well as tempering of martensite were investigated by uniaxial hot compression simulation experiment. The results indicate that the warm deformation accelerates the martensite decomposition compared to tempering, leading to the formation of ultrafine （α＋θ） microduplex structures. The microstructure evolution of martensite during warm deformation involves the precipitation and coarsen of cementite particles, and the dynamic recovery and dynamic recrystallization of ferrite, while tempering of martensite, the precipitation and coarsen of cementite particles, static recovery and grain growth of ferrite occm＇red, but no recrystallization of ferrite occurred. With the addition of Al, the decomposition of martensite is impeded during warm deformation and tempering, the microduplex structure is refined, and its strength is improved, while the elongation is not decreased.