中碳<strong>Nb</strong>合金化钢液析碳化物的析出行为

doi:10.11900/0412.1961.2023.00053

金属学报

2025, Vol. 61

Issue (4): 653-664 DOI: 10.11900/0412.1961.2023.00053

研究论文

本期目录 | 过刊浏览

中碳Nb合金化钢液析碳化物的析出行为

梁炫¹^,², 侯廷平¹^,²(

), 张东¹^,², 谭昕暘¹^,², 吴开明¹^,²(

)

1 武汉科技大学省部共建耐火材料与冶金国家重点实验室武汉 430081
2 武汉科技大学高性能钢铁材料及其应用省部共建协同创新中心武汉 430081

Precipitation Behavior of Primary Carbides in Medium Carbon Nb-Alloyed Steel

LIANG Xuan¹^,², HOU Tingping¹^,²(

), ZHANG Dong¹^,², TAN Xinyang¹^,², WU Kaiming¹^,²(

)

1 State Key Laboratory of Refractories and Metallury, Wuhan University of Science and Technology, Wuhan 430081, China
2 Hubei Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, China

引用本文:

梁炫, 侯廷平, 张东, 谭昕暘, 吴开明. 中碳Nb合金化钢液析碳化物的析出行为[J]. 金属学报, 2025, 61(4): 653-664.
Xuan LIANG, Tingping HOU, Dong ZHANG, Xinyang TAN, Kaiming WU. Precipitation Behavior of Primary Carbides in Medium Carbon Nb-Alloyed Steel[J]. Acta Metall Sin, 2025, 61(4): 653-664.

全文: PDF(3223 KB) HTML

摘要:

液析碳化物严重影响钢的力学性能。本工作利用SEM、TEM等手段表征和分析了液析碳化物的形貌及元素分布，研究Nb含量对中碳Nb合金化钢液析碳化物的影响。基于密度泛函理论的第一性原理与准谐Debye模型相结合的方法，分析了不同温度下NbC与晶格振动、电子相关的热力学参数演变规律。建立了耦合凝固相变的微观偏析模型，定量分析了凝固相变和Nb含量对溶质元素偏析的影响。结果表明，随着Nb含量的增加，液析碳化物的形貌由球形向多面体转变，带状分布趋势加强。不同温度下NbC中电子导致的自由能改变量增加被晶格振动导致的自由能改变量减小所补偿，总Gibbs自由能改变量为负，证实了NbC的热力学稳定性。凝固过程中L + δ向L + γ转变，降低了溶质元素C的偏析程度，增加了Nb元素的偏析程度。Nb含量的增加促使NbC在更低的固相分数时开始析出。

关键词 ：铌合金钢, 液析碳化物, 热力学机制, 第一性原理, 微观偏析, 凝固相变

Abstract：

High-performance steels containing niobium have widespread use in high-end manufacturing sectors such as aerospace, automotive, energy, and construction engineering. Due to its capacity as a strong carbide-forming element, Nb favors the formation of NbC. These carbides, dispersed within the matrix, significantly contribute to precipitation strengthening, precipitation hardening, and grain refinement. In addition, Nb serves as a positive segregation element. When steel solidifies from its molten state, Nb segregates in the liquid phase and forms primary NbC carbides. These carbides significantly affect the mechanical properties of steel. Herein, to investigate the effect of Nb content on primary carbides in medium-carbon Nb-alloyed steel, the microstructure including the morphology, size, and distribution of niobium and carbon was characterized through SEM and TEM. To further understand the role of lattice vibrations and electrons at different temperatures, the first principle calculations with quasi-harmonic Debye model were combined to study the evolution of thermodynamic parameters. Primary carbides form because of solute segregation at the solid-liquid interface. For a more detailed investigation, a solute microsegregation model coupled with solidification phase transition was developed. This model was adopted to quantitatively analyze the effects of solidification phase transition and Nb content on solute microsegregation. The experiments yielded the following results: with increasing Nb content, the morphology of primary carbides shifted from spherical to polyhedral geometry. Furthermore, a distinct zonal distribution of primary carbides was observed. The laws of thermodynamics indicate that the increase in free energy change due to electrons at different temperatures is compensated by the decrease in free energy change arising from lattice vibrations, indicating the key role of lattice vibrations in maintaining the stability of NbC. The Gibbs free energy change at different temperatures was negative, indicating the thermostatic stability of NbC. Furthermore, the absence of imaginary frequency in the phonon spectrum indicates the dynamic stability of NbC. From a microsegregation viewpoint, the mass fraction of solute carbon decreases while that of the solute Nb increases at the solidification front during the phase transition from L + δ to L + γ. As the Nb content increases, the solid fraction of the solidification phase transition increases. Increased Nb content promotes the precipitation of primary carbides at low solid fractions.

Key words： Nb-alloyed steel primary carbide thermodynamic mechanism first principle calculation microsegregation solidification phase transition

收稿日期: 2023-02-13

ZTFLH:

O614.51

基金资助:国家自然科学基金项目(12174296, U1532268, U20A20279);湖北省重点研发计划项目(2021BAA057);湖北省高等学校优秀中青年科技创新团队项目(T201903);浙江省领军型创新创业团队项目(2021R01020)

通讯作者: 侯廷平，houtingping@wust.edu.cn，主要从事金属材料相变研究；
吴开明，wukaiming@wust.edu.cn，主要从事高性能钢铁材料相变及应用性能的研究

Corresponding author: HOU Tingping, professor, Tel: 18140522212, E-mail: houtingping@wust.edu.cn;
WU Kaiming, professor, Tel: 13100610041, E-mail: wukaiming@wust.edu.cn

作者简介: 梁炫，女，1978年生，博士

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表1 实验用钢的化学成分 (mass fraction / %)

图1 NbC、Nb和C的晶体结构

Element	$D 0 δ$	$Q δ$	$D 0 γ$	$Q γ$	$k δ / L$	$k γ / L$
C	0.0127	81370.80	0.0761	134557.44	0.19	0.340
Nb	50.2000	251960.48	0.8300	266478.96	0.40	0.220
Mn	0.7600	224429.76	0.0550	249366.40	0.76	0.780

表2 溶质元素的扩散常数、扩散激活能和平衡分配系数[33]

图2 不同Nb含量钢的SEM像

图3 碳化物的TEM明场像及Nb、C元素分布图

表3 NbC、Nb和C在0 K、0 Pa结构优化后的晶格常数、晶胞角度和形成能

图4 NbC的声子谱及声子态密度

图5 NbC的电子总态密度和分态密度，及Nb、C的总态密度

图6 NbC的振动自由能和电子自由能随温度的变化

图7 晶格振动和电子引起的自由能改变量及NbC总Gibbs自由能改变量随温度的变化

图8 凝固过程Thermo-Calc热力学计算Nb含量对相变的影响

表4 不同Nb含量中碳钢的三相共存温度区间(ΔTL + δ + γ )和碳化物析出温度(Tstart) (K)

图9 1.6Nb钢的溶质平衡分配系数随温度的变化

图10 L + δ→L + γ相变对0.1Nb钢溶质偏析的影响

图11 Nb含量对溶质元素偏析比的影响

图12 4种实验用钢凝固前沿实际浓度积和平衡浓度积

$f s$	$w (C) L, f s$				$w (N b) L, f s$
	0.1Nb	0.6Nb	1.1Nb	1.6Nb	0.1Nb	0.6Nb	1.1Nb	1.6Nb
0.380	0.289	0.289	0.289	0.289	0.130	0.780	1.432	2.084
0.516	0.344	0.344	0.344	0.344	0.146	0.879	1.612	-
0.689	0.369	0.369	0.453	0.453	0.240	1.440	-	-
0.948	0.547	0.546	0.548	0.549	0.684	-	-	-

表5 当fs=fsNbC时溶质元素C和Nb在凝固前沿液相中的质量分数 (%)

图13 NbC析出对0.1Nb钢溶质浓度的影响

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