硼在晶界的偏聚行为对弹簧钢淬透性的影响机理

Addition of a tiny amount of B remarkably increases the hardenability of high-strength spring steel. However, it is difficult to quantitatively detect the grain boundary segregation of boron in steel with conventional measurement technique because of the trace amount of boron, so that the segregation mechanism of solute boron in austenite grain boundaries and its influence on the hardenability of spring steel have not been fully understood up to now. In this project, 3DAP technique, SIMS combined with TEM as well as SEM method are intentionally used for such following purposes as to observe and explain the segregation mechanism of B in austenite grain boundaries, clarify the influencing factors and establish kinetic model of B segregation in austenite grain boundaries considering B precipitation, build up phase field model for kinetics of γ→α phase transformation including nucleation and grain growth in cooling process, reveal the stagnation mechanism for nucleation of proeutectoid ferrite using in-situ observation method with high temperature confocal microscope and continuous cooling transformation experiment. The results of this research work contribute to illustrate the effect of B on hardenability of spring steel and would provide theoretical and practical significance for research and development for new generation of high-strength, economical spring steel.

微量硼元素能够显著地提高弹簧钢的淬透性。然而由于硼在钢中微量存在，其存在形式和状态难以用常规手段进行直观、定量地检测，对于硼原子向晶界偏聚的机制及其对弹簧钢淬透性的影响机理，长期以来并未完全弄清。本项目拟采用三维原子探针技术、二次离子质谱分析结合透射电镜、扫描电镜等实验手段观察弹簧钢中硼在奥氏体晶界的偏聚和析出状态，阐明硼在晶界的偏聚机制；弄清硼在奥氏体晶界偏聚的影响因素，考虑硼在晶界的沉淀析出，建立硼的非平衡晶界偏聚动力学模型，得出硼在晶界的最大偏聚浓度和临界时间等参数；构建冷却过程中γ→α相变形核和晶粒长大的动力学相场模型，并结合连续冷却转变实验和高温共聚焦原位观察，揭示硼的晶界偏聚和析出行为对奥氏体晶界上先共析铁素体形核的延迟机理。本项目研究工作有助于揭示硼元素对弹簧钢淬透性的影响机理，对开发资源节约型、高使用寿命的新型高强度弹簧钢有非常重要的理论和实际意义。

本项目研究了硼在奥氏体晶界的偏聚状态，从硼在晶界非平衡偏聚的动力学的角度，进一步研究奥氏体化温度、冷却速率以及钢种成分等因素对硼的晶界偏聚行为的影响；从硼的晶界偏聚对铁素体相变的延迟机理角度，建立γ→α相变非均匀形核的动力学模型。为合理制定优质弹簧钢热处理工艺提供新思路，为现场生产高强度优质弹簧钢提供理论依据。.通过三维原子探针和飞行时间-二次离子质谱仪观察了在不同奥氏体化温度和冷却速率时冷却过程的合金弹簧钢中硼元素在晶界的偏聚状态。硼原子能够明显取代C原子并与Fe、C形成较为稳定的渗碳体相Fe3(C, B)，随着B含量的增加，γ-Fe区明显减小。结合空位-原子复合体扩散模型和临界冷却速率模型，建立硼的晶界非平衡偏聚动力学模型，对硼元素在奥氏体晶界的偏聚状态进行模拟计算。结果表明由晶界到晶内自由硼原子浓度先逐渐下降，到最低点后逐渐增加，存在贫硼区（宽度约12μm），表明了硼原子在晶界附近会向晶内反向扩散。通过不同硼含量的弹簧钢连续冷却转变实验，测得含硼29ppm 60Si2Mn弹簧钢的Ac1=758℃，Ac3=799℃，Ar1=537℃，Ar3=714℃，Ms=267℃。含硼与不含硼弹簧钢马氏体完全转变临界冷却速率分别为10℃/s和13℃/s, 硼元素的加入，使临界冷却速率降低，冷却曲线局部右移，提高了过冷奥氏体的稳定性，阻碍了奥氏体的共析分解转变。并通过高温共聚焦显微镜进行原位观察γ→α相转变分析，发现硼元素的加入，使相同冷却速率下先共析铁素体含量明显降低，硼元素阻碍了奥氏体的共析分解转变，影响效果随奥氏体化温度的升高而降低，与Gleeble实验结果互为验证。