微合金化对超高强度低合金钢中氢行为与氢脆的影响

Mooring chain steel is one of the most important materials in marine engineering equipment construction. To meet the demand of building deep ocean oil drill platform, the R5 steel is developed for higher strength. However, the higher strength is partly attributed to the dislocation strengthening. It is well known that dislocation can trap hydrogen reversibly, and hence the potential of hydrogen-induced embrittlement may bury in this steel. Therefore, it is necessary for us to find methods inhibiting the sensitivity of hydrogen embrittlement in this higher strength mooring chain steel, and at the same time, considering how to increase its strength. This project aims at developing an ultrahigh strength steel by microalloying Nb, V and Ti elements in R5-based low-alloyed steel and studying the following aspects in these microalloyed steels with homogeneous fine sorbite microstructure after technology and heat treatment control. First, microstructure changings in carbide, dislocation and hydrogen distribution caused by microalloying Nb, V and Ti elements are observed from micro and/or nanoscale level. In addition, the strengthening and toughing is also systematically researched. Hence, the microstructure is understood better and the mechanism of strengthening brought by these elements is clarified. Second, the hydrogen diffusion behavior without stress and under different stress is also studied and the effect of these microalloying elements on diffusible hydrogen, trapping hydrogen, hydrogen diffusivity and activation energy is figured out. Thus, a relationship among hydrogen diffusion, precipitates and dislocation is established in theory. Third, the hydrogen-induced embrittlement is thoroughly investigated by slow strain rate tensile test, fatigue and stress relaxation. Then the critical factor which affects the hydrogen-induced embrittlement is found and therefore the mechanism to inhibit the sensitivity of hydrogen-induced embrittlement can be easily illustrated. In a word, the final goal is to provide a theory law for developing ultrahigh strength steels used securely in ocean engineering.

系泊链钢是海洋工程装备的关键材料，针对深海石油钻井平台要求而开发的R5系泊链钢强度大幅提高，很大程度上靠的是位错强化，但存在潜在的氢脆敏感性，如何在提高R5系泊链钢强度的同时降低氢脆敏感性，是亟需解决的材料科学问题。本项目运用Nb、V、Ti微合金化，使得以R5系泊链钢为基的低合金钢达到超高强度，经过工艺和热处理控制，获得均匀细小的索氏体组织，使其具有超高强度和高韧性。以此为研究对象，研究Nb、V、Ti微合金化的强化作用机理；研究在不同应力作用下的氢扩散行为；研究氢对慢拉伸、疲劳及应力松弛性能的影响。揭示微合金化元素对组织的影响，对不同应力状态下的氢扩散行为以及氢与位错交互作用的影响，对不同受力情况下的氢脆敏感性的影响。建立纳米析出相、氢扩散、位错、氢脆之间的关系；阐明微合金元素对氢脆敏感性影响的机理。最终为开发具有自主知识产权的超高强度低合金钢、提高海洋结构部件的安全性提供必要的技术支持。

系泊链钢是海洋工程装备的关键材料，随着钻井平台应用在深海，要求系泊链钢强度大幅提高，为此就潜在氢脆敏感性。我们提出以R5系泊链钢为基础，通过Nb、V、Ti微合金化，研究微合金化对超高强度低合金钢中氢行为与氢脆的影响。.通过添加不同Nb、V、Ti微合金化熔炼低合金钢，进行工艺和热处理控制，得到900-1500MPa的强度。.通过TEM、X衍射、EBSD、APT等手段研究了碳化物、氢陷阱、位错等微观组织，认为Nb能明显细化原奥氏体晶粒；而V不能明显细化原奥氏体晶粒。相同的添加量，Nb的碳化物细小，而V的碳化物粗大。(Cr,V)的碳化物不是氢的不可逆陷阱，(Nb,V)的碳化物是氢的不可逆陷阱。并随回火温度的升高，小角度晶界减少，大角度晶界增加；微合金元素V能阻止组织回复。因此，通过拉伸得到微合金化低合金钢的力学性能表明，Nb不能提高钢的强度，对钢的塑性影响较小；V能明显增加钢的强度，但对其塑性影响也较小；Nb与V含量增加都降低加工硬化指数。通过工艺控制达到1500MPa强度的钢，强度与微合金元素几乎无关，但V降低塑性，Nb与V复合添加，明显提高塑性。.通过氢扩散行为的研究，认为Nb与V微合金元素的添加，都能降低氢扩散系数，主要与微合金元素阻止组织回复相关，而与碳化物析出无关。应力对扩散行为的影响与组织、所加应力的大小及形式有关，可以从本身的位错、形变过程形成的位错得到较好的解释，从而通过充氢的饱和氢浓度预测可逆氢陷阱密度、氢扩散系数以及氢脆严重程度。.从DIC技术在位研究动态充氢条件下的应变场变化、动态充氢下的疲劳及预充氢后的应力松弛，得出氢能促进应力的松弛，从而促进局部变形，降低裂纹形成所需应力。通过疲劳裂纹的扩展研究，认为氢能增加较小应力场强度范围的裂纹扩展速率，结合预充氢、动态充氢的氢脆敏感性研究，从宏观试样上得出微观变形机制及氢脆机理。为提高海洋结构部件的安全性提供必要的技术支持。