22MnB5板條狀麻田散鐵的晶體方位關係與顯微組織研究

Structures of Lath Martensite in 22MnB5 Low Carbon Steel

程銘奕(Ming-Yi Cheng)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202101784

摘要

本研究使用CBMM公司所提供之含碳量為0.22wt%的低碳鋼22MnB5，來進行板條狀麻田散鐵的微結構分析。首先將材料放進高溫爐內，以1200°C的溫度均質化三天，再將其切割成直徑5mm，長度10mm的圓柱形式片進行熱處理。熱處理流程為將試片分別升溫至不同的沃斯田鐵化溫度(1000°C及1200°C)持溫五分鐘，再以極快的冷卻速度(50°C/s 及100°C/s)冷卻至室溫，以確保沃斯田鐵得以完全形成麻田散鐵，並透過熱膨脹儀所得之溫度對試片升長量的曲線圖，知道材料的Ms溫度約落在350°C左右。熱處理完成後，利用OM、SEM、EBSD、TEM對板條狀麻田散鐵進行觀察，再輔以各項軟體進行後續的顯微結構及方位關係的分析。

由OM所拍攝的照片可以清楚的看到原沃斯田鐵的晶界，利用截線法可得原沃斯田鐵的晶粒大小，無論冷卻速度為50°C/s 還是100°C/s，沃斯田鐵化溫度1000°C持溫五分鐘的試片其平均晶粒大小在7到10μm之間，而沃斯田鐵化溫度1200°C持溫五分鐘的試片其平均晶粒大小大約在60μm左右。由SEM的實驗結果搭配pole figure，可以分析出一個原沃斯田鐵晶粒裡所有block及sub-block的晶體方位關係。本實驗利用EBSD技術找到了三種特殊晶帶軸(zone axis) 下的原沃斯田鐵晶粒，分別為〈111〉γ、〈110〉γ、〈100〉γ，並在這三軸下分別選了一個較大(～150μm)及一個較小(～25μm)的原沃斯田鐵晶粒來進行麻田散鐵的晶體結構分析及block的厚度統計。晶體結構分析上吻合晶體幾何學所得之結果；而block厚度統計實驗結果發現，無論是哪一種大小的原沃斯田鐵晶粒，在〈110〉γ、〈100〉γ下其block的厚度都差不多，然而在〈111〉γ下觀察到的block特別厚，因此推測其形貌更接近為長條形的盤狀(plate)。由OM、SEM、EBSD所得之照片，搭配KAM，可清楚看到板條狀麻田散鐵的粗化行為，搭配錯位角度(misorientation angle)分析，可知粗化發生在鄰近的兩個相同變體(variant)間。由TEM可觀察到粗化的板條狀麻田散鐵裡有雪明碳鐵的析出，亦可利用明、暗場找到相互穿透雙晶(Inter-penetrating twin)的結構，由照片分析推測此雙晶結構發生在鄰近的兩個互為雙晶結構的變體間。另外，藉由TEM所拍攝之照片配合軟體Digital micrograph，可以對未粗化的板條狀麻田散鐵進行厚度統計。統計結果顯示無論在哪個參數條件下，其厚度都十分接近，配合硬度測試，也發現結果都相當接近。

關鍵字

板條狀麻田散鐵 ； 方位關係 ； 相互穿透雙晶 ； 背向散射電子繞射技術

並列摘要

The material used for this research is 22MnB5, a low-carbon steel that contains carbon content of 0.22wt%, provided by CBMM company. To begin with, we put the material into the furnace, homogenizing it at 1200°C for three days, and then cut it into several cylindrical –like specimen. The heat treatment process can be done as follow: raising the specimen’s temperature up to different austenitizing temperature (1000°C and 1200°C), and then quench the specimen to room temperature in a very fast rate (50°C/s and 100°C/s) to ensure that austenite phase be completely transformed to martensitic phase. By using dilatometer, we can obtain the temperature verses specimen elongation line graph and get the Ms temperature which is around 350°C. After the heat treatment, further investigation for martensite microstructure and crystallography can be done by using OM, SEM, EBSD, TEM, and various software.

Through OM image, we can clearly define the prior austenite grain boundary, and get the average grain size which are approximately 7μm and 60μm when the austenitizing temperature are 1000°C and 1200°C respectively by using the intercept method.

Through SEM image and pole figure, we can analyze every block and sub-block’s orientation relationship in one prior austenite grain. By using EBSD, we are able to find prior austenite grain under three special zone axis, which are 〈111〉γ、〈110〉γ、〈100〉γ. For each zone axis, we analyze the orientation relationship of different variants, and calculate the block size inside one large(～150μm) and small (～25μm) prior austenite grain. The experimental result matches the theory of crystallography. Beside, we found out that no matter the size of prior austenite grain is small or large, the thickness of different blocks is nearly the same under 〈110〉γ、〈100〉γ zone axis. However, the average block size in packet 1 under 〈111〉γ zone axis is much larger than those which are under other zone axis. Therefore, we suppose that the 3D structure of lath martensite is an elongated-plate structure.

From KAM image and misorientation angle analysis, we can see coalescence effect happens when two or more nearby laths which have the same orientation (variant) combine with each other. This effect can also be observed under TEM, when cementite precipitated in laths. From TEM bright field and dark field images, we can also see inter-penetrating twin structure. We suppose that this structure formed within two adjacent variants which are twin relation. In addition, by using the software Digital micrograph, we can calculate the thickness of laths. The result shows that the average lath thickness is nearly the same under each condition, and this result matches the hardness test result.

並列關鍵字

lath martensite ； orientation relationship ； inter-penetrating twin ； EBSD

參考文獻

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