汽车用高强高塑性中锰钢延迟开裂的机理及规律研究

The delayed cracking phenomena for high strength steels are normally due to hydrogen embrittlement. The sensitivity to hydrogen embrittlement increases with the increase of the strength. For medium manganese steels, their fracture strength can be over 1100MPa. Their total elongation can be more than 40%. Normally materials with such good ductility should show low sensitivity to hydrogen embrittlement. However, their product of strength-ductility can reduce from 56 GPa•% free of hydrogen to 10 GPa•% with hydrogen. Their high strength and high ductility are from the TRIP effect and ultra-fine grains. Therefore, we want to find the effect of different hydrogen concentrations to KIH and da/dt. This study will focus on how hydrogen distributes in ultra-fine grains and its effect on the grain boundaries. We also want to investigate the effect of hydrogen to residual austenite and martensite from TRIP. In addition, how cold working can affect the hydrogen embrittlement will be studied. The synergistic effect of various factors (chemical composition, microstructure, TRIP and the concentration of hydrogen) will be emphasized. Eventually, we want to adjust the chemical composition and the heat treatment in order to improve the anti-delayed cracking properties of medium manganese steels.

强度-塑性/抗延迟开裂（耐氢脆）的这种“倒置关系”已经成为制约高强钢发展和应用的瓶颈。中锰钢通过调制成分和热处理工艺，虽然其抗拉强度可达1100MPa以上（处于对氢致延迟开裂较为敏感的范围），但其超高延伸率（可以超过40%）和超细晶粒的特点是否具有高的抗氢脆性能呢？这是值得研究的。我们前期对不同强度和Mn含量的中锰钢进行试验发现，其强塑积可由无氢时的56 GPa•%急剧降低至含氢时的10 GPa•%。本申请首先要建立氢含量与氢致开裂门槛值KIH和裂纹扩展速率da/dt的定量关系；阐明氢在超细晶粒中的分布和作用规律，以及超细晶晶界对氢的陷阱作用；研究TRIP效应中奥氏体和新生马氏体对延迟开裂的影响机理；获得冷加工对氢的吸附和扩散的影响规律。寻找成分、微观组织结构、TRIP效应和氢含量等因素的协同作用机制对中锰钢延迟开裂性能的影响规律。通过优化组织结构和时效工艺，获得抗氢脆延迟开裂的中锰钢。

锰含量为5-10%的中锰钢的性能、成本和易加工性等方面较为平衡，从而成为第三代汽车用高强钢体系中的重要钢种。中锰钢通过调制成分和热处理工艺，虽然其抗拉强度可达1100MPa以上，但是其强度级别处于对氢致延迟开裂较为敏感的范围。我们对不同强度和Mn含量的中锰钢进行试验发现，其强塑积在含氢的条件下可下降80%。研究发现几种中锰钢在60%相对湿度值及以上的环境下，存在较为稳定的氢含量（约为0.8ppm）。在外载荷的作用下，出现应力诱导氢扩散情况的发生，更多的氢可以进入试样。随着氢浓度的增加，材料的抗拉强度和延伸率都逐渐下降，建立了氢含量与氢致开裂门槛值的定量关系。研究还发现中锰钢经过塑性变形时，奥氏体中溶解氢在相变过程中扩散到界面处，因而降低晶粒间作用力，同时在裂纹处存在应力集中，出现沿晶开裂；另外部分裂纹向铁素体内部扩展，氢通过促进位错发射，导致铁素体处局部变形，裂纹尖端在铁素体处存在应力集中。利用原位拉伸TEM等手段，获得了中锰钢在含氢条件下裂纹萌生和扩展的规律。研究成果为今后改善中锰钢组织和加工工艺，获得较低延迟开裂敏感性的中锰钢提供了理论依据和数据基础。