不同抗氧化塗層之高強度熱衝壓鋼材延遲破裂敏感性評估研究

Hydrogen-induced Delayed Fracture Susceptibility of Hot-stamping High Strength Steels with Different Anti-Oxidation Coatings

為了減少車輛燃料消耗，降低溫室氣體排放量，車體輕量化近幾年來逐漸成為車廠發展之主流趨勢，部分車廠於車體上使用高強度熱衝壓鋼材取代部分傳統鋼材來降低車重。高強度熱衝壓鋼材乃是將Mn-B鋼加熱至沃斯田鐵化溫度之上，再以模具快速衝壓成形並急速冷卻，在完成部件所需形狀的同時，使鋼材變態至麻田散鐵相而獲得極高之強度。然鋼材於加熱時會發生高溫氧化導致大量銹皮脫落，而導致於後續衝壓成形及塗裝上的問題，故鋼材表面多使用抗氧化塗層以提供高溫氧化保護。由於鋼材經衝壓成形後之強度可達1300 MPa以上，若鋼材製程、後續加工組裝，或產品使用中存在氫滲入的環境時，可能導致氫誘發延遲破裂而造成材料無預警的破裂失效問題，進而影響車輛使用安全。故本研究主要針對新開發之AO抗氧化塗層材與目前廣泛使用之Al-Si鍍層材，探討於熱衝壓製程後、於不同殘留應力下之氫誘發延遲破裂敏感性差異，並建立有效且快速的評估技術。試驗結果顯示：當外加應力≦0.5YS時，兩種塗層材及不塗裝之裸材均不會發生延遲破裂。而當外加應力≧0.75YS時，部分鋼材發生延遲破裂現象，其中，不塗裝裸材即使於外加應力為0.9YS時，亦無延遲破裂現象發生；而Al-Si鍍層材之延遲破裂敏感度高於AO塗層材，其原因可能於酸性腐蝕性環境中，含Al量較高之Al-Si材與酸反應生成較大量之氫氣，導致較多氫原子滲入鋼材內部所致。

High strength hot-stamping steel is used as the candidate of traditional steel substitute for lowering the auto weight in order to reduce fuel for vehicle use and greenhouse gas effect. Hot-stamping steels own the extremely high stress and get the specific form by employing heating the Mn-B steel plate to the temperature above austenitizing temperature and then stamped and cooled down with rapid speed by the mold with coolant flowing inside to make the phase transformation from the martensite to austenite. A mount of scales, formed and separated from the surface during the high temperature oxidation for the bare steels, which cause some damages to the mould in the stamping and make some disadvantages in the following painting process. Therefore, anti-oxide layer was applied on the plate surface to avoid high temperature oxidation. Due to the hot-stamping steel owning the extremely high stress (over 1300 MPa), there might result in the hydrogen-induced delayed fracture in the hydrogen-containing environment. It might cause the unexpected failure of materials or components during their operation. In this study, the susceptibility of the hydrogen-induced delayed fracture of different residual stress for the developed AO steel and traditional Al-Si coated steels were evaluated. Besides, the efficient and rapid evaluation technique was also then established. Experimental results showed that AO, Al-Si coated and bare 15B22 steels will not induce the delayed fracture while the applied stress less than 0.5 yield stress. The different results were revealed while increasing the applying stress. Some kinds of steels induce the delayed fracture while applied 0.75 yield stress. For bared steels, they are immunized from delayed fracture, even applying 0.9 yield stress. However, Al-Si coated steels owned the higher delayed fracture sensitivity than the AO and bared steels, which is resulted from that more content of hydrogen is generated in acid test solution and own higher possibility to permeate into the substrate in the former due to its higher aluminum content.