JIS SKD61改良鋼種之淬火前顯微組織對熱處理後之衝擊韌性的影響

Abstract

本研究以 SKD61 改良鋼種為實驗材料，由四塊鋼胚的表面與中心部位取下鋼材製作成實驗試片，並分析各鋼材的巨觀成分偏析、偏析帶、碳化物形貌、非金屬夾雜物等淬火前組織。為獲得更多種淬火前組織，將原材的實驗試片施以兩種退火處理。接著將試片進行淬火，淬火冷速模擬現場淬火作業兩種不同厚度（100mm 與 300mm）之模具中心實際的溫度變化歷程，經過淬火的試片接著回火至目標硬度，並比較衝擊值，最後探討淬火前顯微組織對淬火回火後的硬度、衝擊值與金相組織的影響。為瞭解鋼料原有組織對淬火時沃斯田鐵晶粒大小的影響，將試片依淬火條件升溫及保溫後，冷卻至適當溫度長時間持溫，使碳化物在沃斯田鐵晶界上析出、凝聚，以方便觀察淬火時沃斯田鐵晶粒的大小。 研究結果顯示，各鋼胚的中心材由於較表面材有嚴重之帶狀偏析導致其韌性較差，退火後也不會使其韌性優於表面材。若鋼材淬火前組織為擬似波來鐵與針狀肥粒鐵，淬火回火後的組織會較球化組織淬火回火後粗大。比對各鋼材之金相組織與衝擊值後發現，帶狀偏析對衝擊韌性造成的影響比淬火時沃斯田鐵的晶粒尺寸更為顯著。比較原材與退火材之衝擊值後發現，原材若有粗大的晶界網狀碳化物，透過適當的退火處理後可以改善此不良組織並提昇韌性。鋼材之韌性也明顯受到非金屬夾雜物影響，若鋼材含有巨觀夾雜物，淬火回火後的韌性值會顯著偏低；而電渣重熔後的鋼材若出現大量含鋁夾雜物，將導致其韌性無法獲得改善。

The effect of four kinds of prior-microstructure on the impact toughness of the modified JIS SKD61 hot work tool steel was investigated in this study. These four kinds of prior-microstructure are the macro-segregation which causes the alloy content deviation in the different ingot position, the micro-segregation which causes the segregation band, the carbide morphology and the non-metallic inclusion. The specimens were extracted from commercial ingots and were divided into two groups, the surface and center material. In order to evaluate more kinds of the prior-microstructure, the specimens were annealed by two kinds of processes. In order to compare the toughness, the specimens were heated slowly and quenched at two kinds of the specific rate to simulate the quenching thermal history of the center of the 100mm and 300mm thick molds, then were tempered to the same hardness. For understanding the effect of the prior-microstructure on the austenite grain growing during quenching, the observation specimens were prepared to reveal the grain boundary clearly. After observing the prior-microstructure and comparing the hardness, quenched-tempered microstructure, the effect of the prior-microstructure on the impact toughness was verified. The results are as follow: 1. The severe band segregation of the center material of the ingots induces the lower toughness than the surface one in the untreated and annealed states. 2. The specific prior-microstructure, the degenerate pearlite and acicular ferrite, induces its quenched-tempered microstructure much coarser than the one with the well-spheroidized prior-microstructure. 3. The toughness significantly affected by the band segregation and the size of the non-metallic inclusion, not the austenite grain size. 4. The toughness could be improved by breaking the carbide-networks through a proper annealing process. 5. The relatively high concentration of the micro non-metallic inclusion in the ESR steel may reduce the advantage of less sulfur on the toughness.