Modification of inclusions in Ni-Al maraging steels by rare earths
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摘要: 钢中的非金属夹杂物对超高强度钢的塑韧性具有重要的影响。在Fe-Ni-Al系马氏体时效钢中加入稀土La、Ce元素，通过扫描电子显微镜及能谱仪（SEM-EDS）与电子探针（EPMA）分析结合FactSage热力学计算，研究了不同稀土元素对钢中非金属夹杂物的变质机理。结果表明，马氏体时效钢中加入稀土元素后形成了含稀土的RE-O-S和RE-Al-O，抑制了Al2S3的形成，对AlN夹杂物的形成没有明显影响，夹杂物经稀土改性后形状由条状和不规则的几何形状转变为接近球状。热力学计算结果表明，添加稀土元素后，钢液中可能形成的夹杂物热力学稳定性大致为REAlO3→Al2O3→稀土硫化物→AlN。加入稀土后，在高温熔融态下钢液中RE2O3和Al2O3就已稳定存在，并且随着冷却温度的降低，钢中的夹杂物按REAlO3→RE2O2S→RES的顺序逐渐析出，且稀土La和Ce的夹杂物演化路径基本相同。Abstract: Non-metallic inclusions in steel have an important influence on the ductility and toughness of ultra-high strength steels. In this paper, rare earth La and Ce elements were added to Fe-Ni-Al system martensitic aging steel, and the metamorphic mechanism of non-metallic inclusions in steel with different rare earth elements was investigated by scanning electron microscopy, energy spectrometry (SEM-EDS) and electron probe (EPMA) analyses combined with FactSage thermodynamic calculations. The results showed that the addition of rare earth elements to martensitic ageing steel resulted in the formation of rare earth-containing RE-O-S and RE-Al-O, which inhibited the formation of Al2S3 and had no significant effect on the formation of AlN inclusions, and the shape of the inclusions was transformed from a strip-like and irregular geometry to a near-spherical shape after the modification of the inclusions by rare earths. Thermodynamic calculations showed that after the addition of rare earth elements, the thermodynamic stability of inclusions in the steel might be formed roughly as REAlO3 → Al2O3 → rare earth sulfides → AlN. After the addition of rare earths, the RE2O3 and Al2O3 had been stable in the high-temperature molten state of the liquid steel, and with the lowering of the cooling temperature inclusions formed in the order of REAlO3 → RE2O2S → RES, and the inclusions of rare-earth La and Ce showed basically the same evolutionary path.
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图 1 1#试验钢中夹杂物的EPMA图
（a）一类夹杂；（b）二类夹杂；（c）三类夹杂
Figure 1. EPMA plot of inclusions in 1# test steel
图 2 2#试验钢中夹杂物的EPMA图
（a）一类夹杂；（b）A为二类夹杂；B为三类夹杂
Figure 2. EPMA plot of inclusions in 2# test steel
图 3 3#试验钢中夹杂物的EPMA图
（a）一类夹杂；（b）二类夹杂
Figure 3. EPMA plot of inclusions in 3# test steel
图 4 夹杂物的OTS统计结果
（a）试验钢中的夹杂物数量；（b）试验钢中夹杂物的尺寸分布
Figure 4. OTS statistical results of inclusions
图 6 夹杂物析出量随温度的变化
Figure 6. Variation of the amount of mixture precipitation with ttemperature
(a) 1#；(b) 2#； (c) 3#
图 5 钢液中生成物的∆G与温度的关系
(a) 加Ce; (b) 加La
Figure 5. Temperature dependence of ∆G of the product in the steel liquid
表 1 试验钢化学成分
Table 1. Chemical compositions of experimental steels
% 样本 C Ni Al Mo Nb B O N S La Ce Fe 1 # 0.039 19.157 2.342 3.886 0.657 0.017 0.0018 0.011 0.0033 Bal 2 # 0.049 19.735 2.338 3.133 0.646 0.016 0.0011 0.012 0.0021 0.019 Bal 3 # 0.046 19.109 2.641 3.262 0.675 0.018 0.0010 0.010 0.0020 0.018 Bal 
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表 2
1600 ℃时钢中各元素活度的相互作用系数Table 2. Interaction coefficients of the elements activities in steel at
1600 ℃元素 作用系数 C Al O S N B Ni Ce La O −0.450 −3.900 −0.200 −0.133 −2.600 −2.600 0.006 −0.570 −0.570 S 0.110 0.035 −0.270 −0.028 0.010 0.130 0.000 −0.231 −0.231 Al 0.091 0.045 −6.600 0.030 −0.058 −0.043 −0.043 N 0.130 −0.028 0.050 0.007 0.000 0.094 Ce −0.077 −2.250 −5.030 −39.8 −6.599 La −4.980
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Table 3. Standardized Gibbs free energies of inclusions in steel liquids
化学方程式 ∆Gθ / (J·mol−1) [Al] + [N] = AlN(s) − 245900 + 107. 59T2[Al] + 3[O] = Al2O3(s) − 1203623 + 386.7T[Ce]+2[O]=CeO2(s) −852720 +249. 96T2[Ce]+3[O]= 2Ce2O3(s) − 714380 +179. 74T2[Ce]+2[O]+[S]=Ce2O2S(s) − 675700 +165. 5T2[Ce]+3[S]= Ce2S3(s) − 536420 +163. 86T3[Ce]+4[S]= Ce3S4(s) − 497670 +146. 3T[Ce]+[S]=CeS(s) − 422100 +120. 38T[Ce]+3[O]+[Al]=CeAlO3(s) − 1366460 +364. 3T2[La] + 3[O] = La2O3(s) − 1511520 + 379.2T2[La] + 2[O] + [S] = La2O2S(s) − 1425820 + 351.0T[La] + [S] = LaS(s) − 490000 + 171.0T[Al] +[La] + 3[O] = LaAlO3(s) − 1188616 + 310.6T
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