Effects of transformation temperature and austenitization temperature on the transformation kinetics and microstructure of a Nb microalloyed high-carbon steel
-
摘要: 为了优化一种含Nb高碳钢的热处理工艺，采用热膨胀法、OM、SEM、EBSD、硬度仪等研究了加热温度和相变温度对珠光体相变动力学和组织性能的影响。结果表明：升高加热温度将减慢珠光体相变动力学。原因：①奥氏体晶粒增大导致珠光体形核点数量降低；②奥氏体中固溶Nb含量增多，其对C扩散系数的减慢程度增强，从而降低珠光体长大速率。相变温度由625 ℃降低至575 ℃时，珠光体相变动力学同样减慢。此外，相变温度由625 ℃降低至575 ℃时，珠光体形核方式发生了变化，由主要在晶隅和晶棱处形核转变为主要在晶面和晶棱处形核。降低相变温度可以细化珠光体片层间距，提高珠光体钢的硬度，珠光体团簇尺寸同样随相变温度和加热温度的降低而显著细化。在试验所研究的三个工艺中，900 ℃加热+575 ℃相变工艺具有最高的硬度和最细的团簇尺寸。因此，为了获得最佳的强韧性匹配，建议降低加热温度和相变温度。Abstract: In order to optimize the heat treatment process of a Nb microalloyed high-carbon steel, the effects of reheating temperature and transformation temperature on pearlite transformation kinetics and microstructure and properties were studied by dilatometry, optical microscope, scanning electron microscope, electron backscattering diffraction and hardness tests. The results show that the pearlite transformation kinetics is reduced by increasing the reheating temperature. One reason is that the number of pearlite nucleation sites decreases with the increase of prior austenite grain size. On the other hand, the solubility of Nb increases with the reheating temperature, which decreases the carbon diffusion coefficient and thus decreases the pearlite growth rate. The pearlite transformation kinetics decreases as the transformation temperature decreases from 625 ℃ to 575 ℃. In addition, when the transformation temperature decreases from 625 ℃ to 575 ℃, the pearlite nucleation mechanism changes from nucleation mainly at the corners and edges of prior austenite to nucleation mainly at the surfaces and edges. Moreover, the decrease in transformation temperature can refine the pearlite lamellar spacing and improve the hardness of the pearlitic steel. The pearlite nodule size is also significantly refined with the decrease of transformation temperature and reheating temperature. Among the three studied processes, 900 ℃ reheating with 575 ℃ transformation technology provides the highest hardness and the smallest nodule size. Therefore, in order to obtain the best combination of strength and toughness, it is recommended to reduce the reheating temperature and transformation temperature.
-
Key words:
- high-carbon steel /
- pearlite /
- Nb /
- reheat temperature /
- transformation kinetics /
- microstructure
-
图 1 加热和冷却过程膨胀量随温度变化规律
Figure 1. Dilatation versus temperature during heating and cooling processes
图 2 (a) 珠光体相变动力学; (b) 珠光体相变速率
Figure 2. (a) Pearlite transformation kinetics; (b) Pearlite transformation rate
图 3 (a) Nb在奥氏体中的固溶量; (b) NbC含量随温度变化规律
Figure 3. (a) solubility of Nb in austenite; (b) the amount NbC versus temperature
图 4 珠光体相变动力学拟合
Figure 4. Fitting results of pearlite transformation kinetics
(a) 900-625; (c) 1080-625; (d) 900-575
图 7 圆形截线法测量珠光体片层间距示意
Figure 7. Schematic diagram of measuring pearlite lamellar spacing by the circular intercept method
图 8 EBSD珠光体团簇晶粒形貌
Figure 8. Pearlite grain morphology characterized by EBSD
(a) 900-625; (c) 1180-625; (d) 900-575
-
[1] Zhou Qingyue, Zhang Yinhua, Chen Zhaoyang, et al. Research and selection of rail steel in China[J]. Chinese Railways, 2011,(11):47-51. (周清跃, 张银花, 陈朝阳, 等. 我国铁路钢轨钢的研究及选用[J]. 中国铁路, 2011,(11):47-51.Zhou Qingyue, Zhang Yinhua, Chen Zhaoyang, et al. Research and Selection of Rail Steel in China[J]. Chinese Railways, 2011, (11): 47-51. [2] Calvo J, Jung I H, Elwazri A M, et al. Influence of the chemical composition on transformation behavior of low carbon microalloyed steels[J]. Materials Science & Engineering A, 2009,520:90-96. [3] Xie K Y, Zheng T, Cairney J M, et al. Strengthening from Nb-rich clusters in a Nb-microalloyed steel[J]. Scripta Materialia, 2012,66:710-713. [4] Deardo A J. Niobium in modern steels[J]. International Materials Reviews, 2013,48:371-402. [5] Fu Liming, Wang Huanrong, Wang Wei, et al. Effect of solute drag and precipitate pinning on austenite grain growth in Ti-Nb microalloyed steels[J]. Journal of Iron and Steel Research, International, 2011,18(s1):383-387. [6] Ray A. Niobium microalloying in rail steels[J]. Materials Science and Technology, 2017,33:1584-1600. [7] Zhou Jiehu, Wu Feng, Feng Kaiming, et al. Ultrafine pearlitic transformation behavior of 2000 MPa ultra-high-strength steel wire under Nb microalloying[J]. Steel Research International, 2021,92:2000516. [8] Dey I, Chandra S, Saha R, et al. Effect of Nb micro-alloying on microstructure and properties of thermo-mechanically processed high carbon pearlitic steel[J]. Materials Characterization, 2018,140:45-54. [9] Jansto S G. MicroNiobium alloy approach in medium and high carbon steel bar, plate and sheet products[J]. Metallurgical & Materials Transactions B, 2014,45:438-444. [10] Liu Chengjun, Huang Yahe, Liu Hongliang, et al. Effects and mechanisms of niobium on the fracture toughness of heavy rail steel[J]. Advanced Materials Research, 2011,163-167:110-116. [11] Tian Junyi, Wang Houxin, Zhu Min, et al. Improving mechanical properties in high-carbon pearlitic steels by replacing partial V with Nb[J]. Materials Science & Engineering A, 2022,834:142622. [12] Liu Feng, Xu Gang, Zhang Yulong, et al. In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel[J]. International Journal of Minerals, Metallurgy, and Materials, 2013,20:1060-1066. [13] Zener C. Kinetics of the decomposition of austenite[j]. Transactions AIME, 1945, 167: 550-595. [14] Offerman S E, Wilderen L J G W, Dijk N H, et al. In-situ study of pearlite nucleation and growth during isothermal austenite decomposition in nearly eutectoid steel[J]. Acta Materialia, 2003,51:3927-3938. [15] Yong Qilong, Zhang Zhengyan, Sun Xinjun, et al. Effect of dissolved niobium on eutectoid transformation behavior[J]. Journal of Iron and Steel Research, International, 2017, 24 973-978. [16] Lee K J, Lee J K. Modelling of γ/α transformation in niobium-containing microalloyed steels[J]. Scripta Materialia, 1999, 40: 831-836. [17] Yeong T P. Measurement and modelling of diffusional transformation of austenite in C-Mn steels[D]. Taibei: National Sun Yat-Sen University, 2001. -
下载:
计量
- 文章访问数: 189
- HTML全文浏览量: 110
- PDF下载量: 13
- 被引次数: 0
