烘烤硬化钢应变时效中cottrell气团形成与碳原子偏聚的研究

The experimental studies on the formation of the Cottrell atmosphere and carbon redistributions during strain aging will be carried out in order to explore the mechanism of bake harden, make the appropriate production process, and so that get bake hardening properties as expect. (1) Study on the formation of Cottrell atmosphere and the effect on BH value during strain aging. BH value and Cottrell atmosphere density, pre-strain dislocation density and internal friction curves during different strain aging were measured. The relationship of BH value and solution Carbon content was researched about different dislocation density and the largest density of the corresponding Cottrell atmosphere. (2)Study on the segregation of carbon atom to grain boundaries and the effect on BH value during strain aging. Internal friction curves at grain boundaries in different strain aging condition were discussed. The effect of grain size and pre-strain on carbon segregation towards grain boundaries and BH value were studied by adjusting annealing grain size and pre-strain, changing grain boundary area and structure. (3) The experiments were carried out with the specimens of different carbon content to study the effect of carbon content and pre-strain on the kinetics of the carbide precipitation and the effect of carbide precipitation on BH value by changing pre-strain with different aging time in certain temperature.

针对烘烤硬化钢应变时效过程中Cottrell气团形成与碳原子偏聚进行试验研究，探究烘烤硬化机理，为合理制定生产工艺获得预期的烘烤硬化性能提供理论指导。（1）应变时效过程中Cottrell气团的形成及对BH值影响的研究。测量预变形时的位错密度和不同应变时效工艺下的内耗谱线（Q-1～T），研究BH值与固溶碳含量的关系，不同位错密度所对应的最大Cottrell气团密度，BH值与Cottrell气团密度的关系。（2）应变时效过程中碳原子向晶界偏聚及对BH值影响的研究。通过调整退火晶粒尺寸和预变形量，改变晶界面积和结构，分析不同应变时效工艺下所对应的晶界内耗谱，研究晶粒尺寸和预变形对碳原子向晶界偏聚及其对BH值的影响规律。（3）选用含碳量不同的试验材料，改变预变形量，进行一定温度下不同时间的时效，研究含碳量和预变形量对应变时效碳化物沉淀动力学的影响，和碳化物沉淀对BH值的影响。

针对低碳钢和超低碳钢的应变时效工艺，分别进行了Cottrell气团形成、碳原子偏聚及碳化物析出对烘烤硬化性能影响的研究。结果表明：（1）在时效过程中，随着时效时间延长，碳原子快速由间隙位置向位错处扩散形成Cottrell气团，BH值快速增加；随后Cottrell气团中的碳原子浓度增速减慢，BH值缓慢增大；当烘烤时间达到一定值后，Cottrell气团中的碳原子浓度趋于稳定，BH值保持不变；不同温度下的应变时效过程基本相同，Cottrell气团形成速率与时效温度成正比，时效温度越高，形成Cottrell气团所需要的时间越短。（2）采用BH值、屈服点延伸率（YPE）和SKK峰高的变化均可以较好地描述Cottrell气团形成动力学，以BH值、屈服点延伸率（YPE）和SKK峰高拟合的Harper时效动力学方程中的时间指数n值分别为0.53，0.59和0.59，分析了BH值与屈服点延伸率（YPE）间的关系，建立了BH值与SKK峰高之间的关系式。（3）在应变时效过程中间隙碳原子向晶界处偏聚，偏聚量与晶粒尺寸、预变形和时效工艺有关。（4）钢的烘烤硬化性能与原始碳含量、显微组织、预变形和时效工艺等有关，其强化机制是间隙原子的固溶强化、Cottrell气团强化和沉淀强化的共同作用。该研究将为烘烤硬化钢板的合理选用和应变时效工艺制定提供理论指导。