成分和微结构调控的抗氢脆高强塑中锰钢基础研究

Usually, ultrahigh strength steel exhibits very high hydrogen embrittlement (HE) susceptibility due to the microstructure mainly consisting of martensite but little of austensite phase. At present, medium Mn steel, as a new generation of automotive steel, is developing. It provides a possibility for improving the HE susceptibility by controlling the microstructure due to exactly it contains a certain volume fraction of austenite phase in medium Mn steel. Under this situation, based on the concept of enhancing resistance to hydrogen-induced intergranular cracking (HIGC), the proposal was put forward. The main means include: (1) By decorating the prior austenite grain boundary (GB) and rising the special GB fraction, such as low coincidence site lattice boundary, which will reduce hydrogen concentration enriched on GB. (2) By optimizing the rolling process and intercritical annealing parameter in order to precipitate nano-film-like austenite phase in the interface of martensite lath, and ensuring a certain degree of wide size dispersion of austenite grain within the martensite block, at the same time, the austenite grain size keeps small. The aim is to ensure more hydrogen keeps being stay in austenite phase to reduce the probability of HIGC. (3) By controlling the partitioning process to adjust the distribution of alloying element in austenite phase, and adding the alloying element of Al, to enhance stacking fault energy, which could promote multiple deformation behavior to play a role to improve resistance of HE. Simultaneously, the law of hydrogen-induced delayed cracking was constructed. Based on the above whole research process, from the macroscopic experiment to the microstructure evolution, the HE mechanism of medium Mn steel was discussed. As an ultimate goal, the objective of the study will provide theory foundation for the developing of medium Mn steel.

通常超高强钢的微观组织以马氏体为主，奥氏体相非常少，因而钢的氢脆性能很差。目前正在研发中的新一代汽车用中锰钢，由于微观组织中含有一定比例的奥氏体相，为通过调控微观结构提高钢的氢脆性能提供了可能。本申请正是在此前提下，以提高氢脆性能为理念，通过：(1)改善原始奥氏体晶界形态、提高低重合点阵晶界比例，降低晶界氢富集来提高氢致沿晶开裂性能；(2)优化加工和处理工艺，调控微结构，促使马氏体板条间的奥氏体以薄膜状析出，马氏体晶粒内的奥氏体保持弥散细小而有一定的分布区间；通过变形过程中的逐渐转变降低氢在板条界面间的富集，从而降低氢致开裂几率；(3)配分工艺调整相中的合金元素含量以及适当添加Al等合金元素，增加奥氏体相层错能，促使多种形变模式开动，从而改善氢脆性能。与此同时，找出中锰钢的氢致开裂规律。通过上述从宏观实验到微观结构的演化，探讨中锰钢的氢脆机理，为中锰钢的发展及应用提供有价值的理论依据。

为了达到通过Mn和C的配分实现良好的力学性能抗氢性能，结合文献资料，选定了6Mn和11Mn两种成分体系，制备了Fe-0.2C-6Mn-3Al-(0/0.6)Si和Fe-0.2C-11Mn-(0/2/4)Al两大类中锰钢样品。同时通过调控热处理工艺，包括淬火配分、临界退火和回火等，结合不同的轧制工艺（热轧、温轧和冷轧），前后共制备了十个批次共40余种试样。筛选出力学性能较好的样品，再进行微观组织、TRIP效应、断裂行为以及氢脆敏感性等方面的系统研究。获得主要结论有：.针对6Mn体系，对比研究了0Si和0.6Si中锰钢。发现0.6Si试样的微观组织中存在较多的高温铁素体相，经较长时间临界退火后，0.6Si样品的强塑积和抗氢脆性能均大幅度提高，归因于高温铁素体相与基体的界面微裂纹扩展被抑制。另外，随应变速率增加，两种材料的力学性能均下降，而0.6Si的延伸率损失更快。不同轧制工艺的对比研究发现，温轧导致中锰钢的微结构为纤维织构，纵向裂纹和断口分层可以释放微裂纹尖端的三轴应力，使得微裂纹形核和扩展方式不同于QP和IA样品，而明显改善中锰钢的抗氢脆性能。 .针对11Mn体系，通过添加0、2、4 wt%Al，配合不同的轧制和热处理工艺，首先发现热轧中锰钢的原奥氏体晶界和马氏体为易开裂点，温/冷轧+退火工艺可获得等轴的铁素体晶粒均匀镶嵌在奥氏体基体中，可减少氢致裂纹萌生位点。其次，对热轧后的样品进行温、冷轧，再分别进行回火和退火，发现回火态2Al的微观组织为铁素体和少量的马氏体，抗氢脆能力明显改善；而退火态2Al样品，由于具有更均匀细小的微观组织、更稳定的奥氏体而具有更低的氢脆敏感性。在这一成分体系中，我们得到了强塑积超过70GPa%、同时抗氢性能优良的样品。.第一原理计算表明，层错上锰的覆盖度在浓度为25%附近有极大值。层错能随着锰浓度单调降低。锰与氢复合，可使合金的层错能进一步降低。因此复合效应对层错能的影响非常值得关注。.综上，温轧是一种有效地提高中锰钢机械性能和抗氢脆性能的工艺手段。添加合金元素Si的样品宜采用较长时间退火，而不含Si试样宜采用短时退火。薄膜状奥氏体和含有退火孪晶的块状奥氏体稳定性高，能够有效阻碍中锰钢的氢致开裂。