【作者】 李红娟；

【导师】 王宝奇；

【作者基本信息】 河北工业大学 ， 材料学， 2005， 硕士

【摘要】 利用真空熔炼技术,制备了UHCS-OAl,UHCS-1.OAl,UHCS-1.5Al,UHCS-2.0Al,UHCS-2.3Al,UHCS-3.5Al六种铝含量递增的超高碳钢和UHCS-0.9Si,UHCS-0.6Al-0.6Si,UHCS-1.6Al-0.6Si三炉铝硅复合添加的超高碳钢,并锻造成棒材作为试验用料。利用FactSage软件对不同成分的铝合金化超高碳钢进行了热力学计算;结合淬火-硬度试验,确定了各种钢的相变特征温度。利用金相显微镜、扫描电镜、透射电镜对锻态组织及各种工艺处理后的组织进行了观察,对各种热处理后组织的力学性能进行了测定。 热力学计算与试验均表明:超高碳钢中添加铝使先共析碳化物数量降低是锻造空冷后网状碳化物形成受到有效抑制的根本原因。同时,共析转变温度及转变温度的范围随着铝含量的增加而提高,变宽;先共析碳化物析出的温度范围减小,超高碳钢易于形成细化的伪共析珠光体组织。在碳含量低于1.7wt%时,添加1.5～2.3wt%的铝,锻造空冷后网状碳化物的析出受到有效的抑制。 组织演变的分析表明,对于铝合金化的超高碳钢,利用加热控制,在成分不均匀奥氏体的冷却过程中,先共析碳化物将以弥散的形式在基体上以粒状析出,没有显示晶界优先形核的现象,无网状碳化物的形成。 通过向超高碳钢添加铝,可以采用较为简便的热处理工艺使碳化物得到球化。铝含量高于1.5wt%时,采用离异共析退火工艺或淬火+高温回火工艺处理后,均可获得细小的碳化物分布于铁素体基体的均匀球化组织。 超高碳钢中高的铝含量将降低球化处理过程中碳化物的长大倾向,减小基体晶粒的长大趋势。但过高的铝含量(>3wt%),使超高碳钢的室温塑性降低。含碳量为1.5wt%—1.8wt%时,铝含量控制在1.5—2.5wt%的范围内,并加入1.5wt%左右的Cr,可取得良好的室温综合力学性能。 铝合金化超高碳钢淬火后的显微组织为超细马氏体基体及分布于其上的细小碳化物的复相组织,马氏体的亚结构是板条马氏体和孪晶马氏体并存的隐晶马氏体,回火过程中碳化物主要沿板条界和板条内析出,回火稳定性提高,回火各阶段微观组织变化均推到更高的温度。更多还原

【Abstract】 Using the vacuum induction melting furnace, 6 kinds of UHCS with increasing content of A1(UHCS-OA1, UHCS-1.0A1, UHCS-1.5A1, UHCS-2.0A1, UHCS-2.3A1, UHCS-3.5A1) and 3 kinds of UHCS with different content of Al and Si (UHCS-0.9Si, UHCS-0.6Al-0.6Si, UHCS-1.6Al-0.6Si) were prepared. The ingots were hot-forged and worked into experimental samples. The FactSage software has been applied to calculate the thermodynamics data of UHCS with different addition of Al. With the help of quenching-hardness test, the phase transformation temperatures were determined. The microstructures processed by different techniques were analyzed by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM), and the mechanical properties were tested.Both the thermodynamic calculation and experiments showed that the main reason for inhibition of network carbide of forged UHCS was that the quantity of proeutectoid carbide decreased with the addition of Al. Meantime, the phase transformation temperatures were improved. The areas of eutectoid transformations were widened and that of proeutectoid carbide precipitation were shrunk with the increasing of Al content so that it was easy to form fine pseudo-eutectoid structure. With the C content under 1.7wt%, the addition of 1.5-2.3wt%Al restrained the carbide precipitation of forged UHCS effectively.The analyses of microstructure evolution showed that by heating the austenite with inhomogeneous carbon concentration, the proeutectoid carbide particles of UHCS with addition of Al precipitated dispersedly in the matrix during cooling, no tendency to nuclear at the grain boundary and no formation of network carbide.Compared with the UHCS without Al, the spheroidization techniques of UHCS withaddition of Al were simplified. UHCS with the addition of Al above 1.5wt% can be processed by the divorced eutectoid annealing technique or quenching-and-high-tempering technique, which both can obtain the desired microstructure with the fine spheroidal carbides dispersed on the ferrite matrix.With high content of Al, the carbide growth tendency of UHCS was decreased during spheroidization, which avail to restrain the growth of matrix. But too much Al (>3wt%) may decrease the ductility of UHCS. The optimal composition with 1.5-1.8wt% C content was the addition of 1.5-2.3wt%Al and 1.5wt%Cr, which exhibited good ambient mechanical properties after spheroidization.The UHCS with addition of Al after quenching can obtain the microstructure with the fine spheroidal carbides dispersed on the martensite matrix. The sub-structure of martensite was the mixture of lath and twin-crystal martensite. The carbides mainly precipitated along and in the lath boundary during tempering. With addition of Al, the tempering stability of UHCS was improved and the changes of microstructure at each stage were deferred to higher temperatures.更多还原