軋延製程及合金元素添加對鋼材及其銲件在含氫環境下機械性質之影響

The Influence of Rolling Process and alloy Addition on Mechanical Properties of Steel and Weldments in Hydrogen Environment

本研究針對相同成分不同軋延製程鋼材（傳統軋延製程SM490C與熱機軋延加速冷卻（TMCP）製程EH36）及銲件，探索微觀組織型態改變對鋼材在氫環境下的機械性能影響。另外對以相同TMCP製程而有無微合金添加之鋼材（TMCP EH36與TMCP API 5L X65）及銲件，探索微合金對鋼材在氫環境下機械性能之作用。結果發現控制軋延加速冷卻製程的細化等軸晶粒，除提高材料延性外，在不同充氫濃度下，API 5L X65平行軋延方向除外，三種材料母材的強度不受充氫所影響，而且強度大小依次為API 5L X65最優、EH36次之、SM490C為後。二種材料銲件其強度亦依次為API 5L X65最優、EH36次之、SM490C為後。氫環境對延性的影響極為明顯，母材及銲件均隨著充氫電流密度增加，伸長量和斷面縮率都相對降低。在未充氫前，EH36與SM490C母材及銲件的延性優於API 5L X65，但充氫後API 5L X65母材及銲件反而表現出較優異的抗氫脆能力，EH36次之，SM490C較差。在相同成分不同軋延製程下，微觀組織型態不同，導致帶狀組織之傳統控制軋延SM490C鋼沿厚度方向的氫擴散速率小於隨機均勻分佈微觀組織之TMCP製程EH36鋼。然而在相同TMCP製程，有無添加微合金的條件下，含有Nb、V、Cu、Al、Ni等微合金之API 5L X65鋼，其氫擴散速率與滲透速率都比未添加合金之EH36鋼小。其原因可能是API 5L X65中含有Cu合金，易在鋼鐵表面生成氧化銅，有遲緩氫滲透的效果。

The purpose of this study focuses on two aspects of hydrogen embrittlement of steels: (1) the effect of microstructural mophology on the hydrogen embrittlement susceptibility of steels produced by different processes; (2) the effect of microalloying addition on the tensile property of steels in the hydrogen environment under the same thermo-mechanical control rolling processing (TMCP) with accelerated cooling. The results show that the ultimate tensile strength and yeild strength of the base metals and weldments of all three steels (TMCP API 5L X65, TMCP EH36, and SM490C) are independent of increasing hydrogen-charging current density. The superior strength and ductility of TMCP API 5L X65 after hydrogen charging can be attributed to the solid solution strengthening and fine precipitates superimposed on the equiaxed refined grains. The relative hard band structures in SM490C steel and weldment are the major cause that renders the lower ultimate tensile strength and elongation inevitable. Under the same composition, the hydrogen diffusivity along the through-surface direction in a banded structure of SM490C steel shows a lower value than that in the equiaxed grain EH36 steel. Whereas, the hydrogen diffusivity and hydrogen permeability of microallyed (Nb, V, Cu, Al, Ni) API 5L X65 steel are lower than that of EH36 steel with no alloying. The reason could be that the API 5L X65 steel formed a protective film enriched Cu arrests hydrogen diffusion.