Effect of Mg on carbides in 21-4N valve steel
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摘要: 为了探究不同镁含量对21-4N气阀钢中液析碳化物的影响规律，采用实验室高温电阻炉冶炼了不同镁添加量的21-4N气阀钢，借助蔡司金相显微镜、扫描电子显微镜、电解腐蚀等方法开展研究。结果表明，随21-4N钢中Mg含量的增加，碳化物平均最大尺寸及面积占比均先减小后增大。添加微量的镁有助于改善钢中富Cr型液析碳化物的尺寸和分布，镁的质量分数为1.6×10−5时，液析碳化物平均最大尺寸为60.55 μm，面积占比10.43%，在钢中多呈颗粒状分布，连续性被破坏，其原因是镁元素在晶界处的偏聚，能够抑制C、Cr等合金元素的扩散，阻断碳化物连续生长，从而达到细化碳化物尺寸的效果。Abstract: In order to explore the influence of different magnesium content on primary carbides in 21-4N valve steel, 21-4N valve steel with different magnesium content was smelted in a laboratory high temperature resistance furnace, and the research was carried out by use of Zeiss metallographic microscope, scanning electron microscope and electrolytic corrosion. The results show that with the increase of Mg content in 21-4N steel, both the average maximum size and area ratio of carbides firstly decrease and then increase. The addition of trace magnesium is helpful to improve the size and distribution of Cr-rich precipitated carbides in steel. When the mass fraction of magnesium is 1.6×10−5, the average maximum size of precipitated carbides is 60.55 μm and its area ratio is 10.43%, and most of them are granular in steel, and the continuity is destroyed. The reason is that the segregation of magnesium at the grain boundary can inhibit the diffusion of alloy elements such as C and Cr, and then block the continuous growth of carbide, thus achieving the effect of refining the carbide size.
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Key words:
- 21-4N valve steel /
- Mg modification /
- primary carbide
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图 2 21-4N气阀钢的平衡凝固相转变示意
Figure 2. Equilibrium solidification phase diagram of 21-4N valve steel
图 3 不同镁含量下碳化物典型形貌与分布
（1: 0 ; 2: 1.2×10−5; 3: 1.6×10−5; 4: 3.4×10−5; 5: 5.8×10−5; 6: 6.0×10−5）（（a）：典型腐蚀金相照片；（b）：典型液析碳化物形貌）
Figure 3. Typical morphology and distribution of carbide with different magnesium content
图 4 Mg含量对钢中碳化物尺寸及分布的影响
Figure 4. Effect of Mg content on carbide size and its distribution
表 1 21-4N气阀钢和Ni-Mg合金主要化学成分
Table 1. Main chemical compositions of raw materials and Ni-Mg alloy
% C Si Mn P S Cr Ni Al N Mg 21-4N气阀钢 0.52 0.22 9.00 0.03 0.03 20.38 3.37 0.02 0.42 Ni-Mg合金 0.56 0.11 74.19 24.19 
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表 2 镁含量检测结果及收得率
Table 2. Test results magnesium content and its yield
组别 目标镁含量×106 镁检测值×106 镁收得率/% 1 0 2 5 12 7.14 3 10 16 4.86 4 20 34 4.97 5 40 58 3.37 6 60 60 2.29
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