【作者】 孔德群；

【导师】 刘庆锁；

【作者基本信息】 天津理工大学 ， 材料学， 2012， 硕士

【摘要】 本文采用中频感应真空炉熔炼高碳低合金含硅钢Fe-0.88C-1.35Si-1.03Cr-0.43Mn，经880-1000℃不同的奥氏体化后在200-300℃等温转变，形成了束状贝氏体组织，采用XRD、金相显微镜、透射电镜等研究了奥氏体化温度对束状贝氏体的转变点与显微组织的影响，探究了束状贝氏体的生长行为。实验结果表明，随着奥氏体温度的不断升高，实验钢的中温等温冷却转变曲线逐渐向左下方移动：在中温区的高温段，贝氏体铁素体的初始形成时间逐渐延长；在中温度的低温段，贝氏体铁素体的初始形成时间则逐渐减少；而在马氏体转变点附近，贝氏体铁素体的初始形成时间没有太大变化。随着奥氏体化温度的升高，奥氏体晶粒尺寸逐渐增大，最终得到的束状贝氏体铁素体尺寸也逐渐增大，主要体现在长度变化上。950℃与980℃奥氏体化的试验钢在低温转变时，短时等温（≤30min）即可获得大量的束状贝氏体，转变终了时贝氏体铁素体的体积分数达到80%，具有不完全转变的现象。与常规上、下贝氏体不同的是，本实验钢在中温区高温段与低温段等温时得到的贝氏体组织形态及其演变规律是一致的，均是长大初期表现为单独的针状，后期形成平行的束状贝氏体。束状贝氏体组织是由多条细长的同位向的铁素体和残留奥氏体薄膜平行并列组成的，宽度在三十到五十个纳米单位，长度在十几到几十个微米单位。束状贝氏体具有非常硬的性能特点，洛氏硬度达到62-64HRC，显微硬度也高达800HV左右，同时其精细结构还可以提高冲击韧性。精细结构观察发现，金相显微镜下束状贝氏体内部的每一根铁素体，其实都是由多条平行的铁素体条并列组成的；每一个铁素体条是由很多的亚片条连接而成的，交错着延续下去；亚片条内部存在高密度的位错结构。束状贝氏体铁素体在原奥氏体母相中形核后以切变的方式而伸长或者展宽，在原奥氏体的晶粒内部生长，没有穿越晶界的迹象。贝氏体铁素体条的展宽是通过切变机制新的同位向铁素体微条不断地在已长大铁素体条的两侧形核与长大的结果，在邻近铁素体条侧界面的奥氏体基体内存在碳原子过量的偏析区域，因而铁素体条进一步展宽困难。贝氏体铁素体条的伸长是通过切变机制在其生长前沿重复形成亚片条而实现的，生长前端的界面前沿周围的母相中存在强烈的应力-应变场，形成应变-诱发相变，导致亚单元的不断积累，从而使铁素体条逐渐增长。更多还原

【Abstract】 This paper has investigated the effect of austenitizing temperature on the transition startpoint, microstructure, and growth behavior of sheave-like bainite ferrite in the new designedhigh-carbon low-alloy silicon-containing steel isothermally transformed at200-300℃forshort time after austenitization at880-1000℃, by means of X-Ray Diffraction, metallurgicalmicroscopy, and transmission electron microscopy.The results show that the fast bainite is generated in the form of sheaves duringlow-temperature isothermal transformation of the high-carbon steel，which can be finishedonly in about30min with a bainitic ferrite volume fraction of80%, and anincomplete-reaction phenomenon. The microstructure evolutions of upper and lower bainitein present study are similarly consistent, known from conventional bainite. The typicalmicrostructure of sheave-like bainite gives a most elegant fine scale of an intimate mixtureof austenite films and slender ferrite plates with a width of just30-50nm and a length in μmscale. These ferrite plates are carbon-supersaturated and lattice-distorted, indicating thecharacteristics of diffusionless transformation. The isothermal cooling curve of theexperiment steel gradually moves to the lower left, with increasing the austenitizingtemperature, which leads to a gradual increase of austenite grain size and is responsible forthe larger length of bainite ferrite. The bainitic ferrite in the sample corresponding toaustenitization at980℃has the best bundle phenomenon with high density, hardness of62-64HRC, or in excess of800HV, and excellent impact toughness.Bainitic ferrite nuclears in different forms at the austenite grain boundaries or defects inaustenitic grain. The sheave-like bainite can only grow inside the austenite grains, and noevidence has been found that bainite plates traverse the austenite grain boundaries. Finestructure observation shows that every ferrite plate in the bainite sheave is composed ofseveral paralleling ferrite platelets with the same orientation relationship, and each platelet iscomposed of several conterminous sub-platelets with high-density of dislocation. Thickeningof the bainite ferrite plate is a consequence of new platelets nucleation and growth by shearmechanism at sides of the already formed platelets. Due to the yield strength of austenite,thicken of platelets has a limited side, and total number of which is also limited to more than adozen. Lengthening of the bainite ferrite plate can be regarded as a consequence of newplatelets nucleation and growth by shear mechanism at leading tip of the already formedplatelets.更多还原