利用热压缩实验, 研究了中碳钢回火马氏体在700 ℃/0.01 s-1条件下变形时的组织演变规律, 分析了渗碳体粒子状态的影响. 实验结果表明: 中碳钢回火马氏体热变形过程中, 发生了渗碳体粒子粗化和铁素体动态再结晶, 形成由微米级的等轴铁素体晶粒与均匀分布的渗碳体粒子组成的超细化(α+θ)复相组织. 与静态回火相比, 形变促进Fe原子和C原子的扩散, 使渗碳体粒子粗化动力学提高2-3个数量级. 渗碳体粒子的粗化主要来自铁素体晶界上粒子尺寸的增加, 铁素体晶粒内部的细小粒子尺寸无明显变化但数量减少, 前者有助于以多粒子协同方式实现粒子激发形核, 后者减小了晶界迁移的阻力, 两者均有利于铁素体动态再结晶的发生. 随着初始组织中渗碳体粒子尺寸的减小, 发生动态再结晶所需应变量增大, 但所得复相组织更加均匀、细化.
For plain carbon steels, the microduplex (α+θ) structure consisting of ultrafine
ferrite matrix and dispersed cementite particles demonstrates a good balance between strength
and ductility as compared with a normal microstructure, e.g. ferrite plus pearlite in hypoeutectoid
steels or pearlite in eutectoid steels etc.. Various thermo-mechanical treatments have been developed
to obtain such microduplex (α+θ) structure. It is commonly considered that the formation
of fine equiaxed ferrite grains during thermo-mechanical treatments involving hot (warm) deformation
could be attributed to dynamic recrystallization (DRX) of ferrite. However, the mechanism of DRX of
ferrite during the formation of the microduplex (α+θ) structure as well as the effect of
cementite particles on DRX of ferrite are still not well understood. In the present work, DRX of ferrite in a
medium-carbon steel with different tempered martensites was investigated by hot uniaxial compression
tests at 0.01 s-1 and 700 ℃, and the effect of cementite particles with different initial states
was analyzed. The results indicate that during hot deformation the coarsening of cementite particles
and DRX of ferrite took place, leading to the formation of the microduplex (α+θ) structure.
In comparison with static tempering, the diffusions of Fe atoms and C atoms were enhanced by hot
deformation and thus the coarsening kinetics of cementite particles was accelerated by 2-3 orders of magnitude. During hot deformation the sizes of cementite particles at the boundaries
of ferrite grains increased and the amount of cementite particles in ferrite grains decreased. The
former is beneficial to the particle stimulated nucleation under coordinated effects of several
particles, and the later can reduce the drag effect of cementite particles on the migration of grain
boundaries, both of them are advantageous to DRX of ferrite. With the decrease in the size of
cementite particles in the initial microstructures, the critical strain for DRX of ferrite
increased, but the resultant microduplex ($\alpha+\theta$) structure is fine, and more uniform.
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