Effect of Mn content on microstructure and mechanical properties of medium carbon low Si bainitic steel
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摘要: 设计并熔炼了四种锰含量（质量分数分别为0.76%、1.29%、1.53%、1.85%）的中碳低硅贝氏体钢，利用OM、SEM、XRD仪器和拉伸、冲击试验机等研究了锰含量的添加对试验钢显微组织和力学性能的影响。结果表明，锰元素抑制了铁素体与珠光体相变；锰元素的添加降低了贝氏体的形核驱动力，减少了贝氏体最大转变量，使得贝氏体转变停滞后剩余的过冷奥氏体尺寸增大，含量增加；这些奥氏体在后续等温过程中大部分分解为粗大碳化物和铁素体混合组织，并围绕在贝氏体板条周围，形成了局部粗化组织，少部分保存至室温形成残余奥氏体；随着锰含量的增加，试验钢的屈服强度和冲击韧性逐步下降，抗拉强度由于合金化和组织粗化两因素的相互竞争先升高后降低；综合考虑贝氏体钢热处理难度与力学性能，锰含量为1.29% 时最适合作为弹簧用钢。Abstract: Four medium-carbon low-Si bainitic steels with different Mn contents(0.76%, 1.29%, 1.53%, 1.85%) were designed and smelted. The effect of manganese content on the microstructure and mechanical properties of the test steel was studied by OM, SEM, XRD and tensile and impact tests. The results show that the phase transformation between ferrite and pearlite is inhibited by manganese. Manganese addition decreases the nucleation driving force of bainite, causing the declining maximum transformation amount of bainite, and increases the size and content of the remaining supercooled austenite after bainite transformation stagnation. Most of the remaining austenite decomposes into coarse carbide and ferrite mixed structure in the subsequent isothermal process, and forms local coarse structure around the bainite lath. A small amount of the austenite stabilize to room temperature and then form retained austenite. With the increase of manganese content, the yield strength and impact toughness of the test steel gradually decrease, and the tensile strength firstly increase and then decrease due to the competition between alloying and microstructure coarsening. Considering the complexity of heat treatment and balanced mechanical properties of bainitic steel, 1.29% manganese content is proposed for spring steel.
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Key words:
- low silicon bainite steel /
- manganese content /
- carbide /
- retained austenite
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图 1 锰含量对正火状态试验钢显微组织的影响
Figure 1. Effect of manganese content on microstructure of normalizing test steel
(a) 0.76%Mn; (b) 1.29%Mn; (c) 1.53%Mn; (d) 1.85%Mn
图 2 锰含量对试验钢空冷+等温后显微组织的影响
Figure 2. Effect of Mn content on microstructure of test steel after isotherm
(a) 0.76%Mn; (b) 1.29%Mn; (c) 1.53%Mn; (d) 1.85%Mn
图 4 粗大碳化物的SEM形貌及分布
(a) 线型分布碳化物； (b) 粗大碳化物与铁素体混合组织
Figure 4. SEM morphology and distribution of coarse carbides (a) Linear distribution of carbides (b) Coarse carbide and ferrite mixed structure
图 5 等温热处理过程中低硅贝氏体钢中碳化物析出示意
Figure 5. Schematic diagram of carbide precipitation in low silicon bainite steel during isothermal heat treatment
图 6 不同锰含量试验钢贝氏体形核及长大驱动力与温度的关系
Figure 6. The relationship between temperature and driving force of Bainite core and growth of steel with different Mn content
图 7 不同锰含量试验钢等温过程中残余奥氏体中碳质量分数变化与${\rm{T}}'_0 $曲线拟合
Figure 7. The variation of C mass fraction in residual austenite during isothermal process is fitted with ${\rm{T}}'_0 $ lines
图 8 0.79%Mn试验钢的相变热力学曲线${\rm{T}}_0、{\rm{T}}'_0$、${\rm{A}}'_{{\rm{e}}3}、{\rm{A}}'_{{\rm{e}}3}$
Figure 8. Thermodynamic curve of phase transformation of 0.79%Mn test steel ${\rm{T}}_0,{\rm{T}}'_0 $, ${\rm{A}}_{{\rm{e}}3},{\rm{A}}'_{{\rm{e}}3}$
图 9 不同锰含量试验钢的冲击断口形貌
Figure 9. Impact fracture morphology of steel tested with different manganese content
(a) 0.76%Mn; (b) 1.29%Mn; (c) 1.53%Mn; (d) 1.85%Mn
表 1 试验钢的化学成分
Table 1. Chemical compositions of experimental steels
% No. C Mn Cr+Mo+V Si P S Fe M1 0.54 0.76 1.45 0.31 0.013 0.003 Bal. M2 0.55 1.29 1.44 0.24 0.013 0.005 Bal. M3 0.54 1.53 1.46 0.42 0.013 0.003 Bal. M4 0.56 1.85 1.53 0.37 0.017 0.004 Bal. 
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表 2 不同锰含量试验钢的力学性能
Table 2. Mechanical properties of steel tested with different manganese content
No. 抗拉强度/MPa 屈服强度/MPa 伸长率/% 强塑积/(GPa·%) 冲击功/J M1 1203±3 903±6 11.5±0.4 13.56 27.9±2 M2 1592±2 1290±23 8.8±0.3 13.99 26.7±5 M3 1629±7 1239±17 9.5±0.2 15.54 24.1±6 M4 1565±18 1136±31 9.4±0.4 14.54 21.7±3
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