冷滾軋對於465麻時效不鏽鋼之顯微結構與析出行為影響

Abstract

465麻時效不鏽鋼在工業中，常會在時效處理前，先施行冷加工至所需形狀，以降低加工成本。然而，過去卻僅有少數文獻，具系統性的探討此材料施行冷加工後的析出行為。因此，本研究引入40%冷滾軋，並透過熱膨脹儀、OM、XRD、EBSD、TEM等儀器，研究本材料在熱處理各階段的相變特性、顯微結構，以及時效處理各階段的析出行為。 首先，本研究利用熱膨脹儀，首次於此材料中，發現特殊的恆溫麻田散鐵相變現象，並觀察、量化麻田散鐵基地之層次結構、次結構的變化。接著對於沃斯田鐵在時效過程中的成長趨勢及析出特性進行研究。最後，利用晶體結構學的知識，配合TEM在[211]α晶帶軸所拍攝η-Ni3Ti析出物之中心暗場影像，精準量化析出物的尺寸分布，發現引入40%冷滾軋所導致的差排密度上升，將因為管道擴散效應，使η-Ni3Ti析出物迅速成長，並且抑制逆沃斯田鐵生成，證明了η-Ni3Ti析出物與逆沃斯田鐵之間的析出競爭關係。 本研究亦透過高解析穿透式電子顯微鏡，在[111]α晶帶軸發現η-Ni3Ti析出物之V5、V6變體傾向依附生長，並在析出物中形成以(11̅02̅)η為雙晶面的雙晶結構。

465 maraging stainless steel is often formed to the desired shape before conducting aging treatment in order to reduce processing cost. However, there are only few literatures have systematically investigated the precipitation behavior of 465 maraging stainless steel after cold working. Therefore, this study introduced 40% cold rolling then investigated the phase transformation, microstructure and precipitation behavior of 465 maraging stainless steel provided by Gloria Material Technology Corp. at each heat treatment stage by dilatometer, OM, XRD, EBSD, and TEM. First of all, this study derived the isothermal martensite transformation in this material for the first time by dilatometer then observed and quantified the changes of the hierarchical structure and substructure of martensite. After that, we studied the precipitating characteristics of reversed austenite during the aging process. Finally, by using the knowledge of crystallography, this study precisely quantify the size distribution of precipitation. It was found that the increase in dislocation density caused by introducing 40% cold rolling will promote the growth rate of η-Ni3Ti due to pipe diffusion and inhibit the forming of reversed austenite, therefore, proof the competition relationship between η-Ni3Ti and reversed austenite. In this work, the V5 and V6 variants of η-Ni3Ti were also found to grow in pair by using HRTEM at [111]α zone axis, and formed a twinning structure with (11̅02̅)η as twinning plane in the precipitation.