Research on cracking mechanism of girth and weld straightening of grade 800 MPa automobile torsion beam steel
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摘要: 利用光镜、扫描电镜、透射电镜、EBSD、Thermo-Calc热力学软件、JMatPro软件研究了Nb-Ti复合微合金化800 MPa级扭力梁用钢热轧板在汽车扭力梁围管和焊缝矫直过程中的开裂原因。对三种不同成分的试验钢的显微组织和纳米析出相的研究结果表明，围管和焊缝矫直开裂的主要原因为：3#钢中添加了Cr、Mo、V元素，使钢的TTT曲线右移，提高了奥氏体的稳定性，使轧后冷却过程中生成了大量的含有高密度位错的板条马氏体和板条铁素体等硬相组织，而等轴铁素体的数量较少，而且晶粒大小不均匀，在力学性能上表现为抗拉强度偏高。由于板条铁素体和板条马氏体含有高密度位错，降低了钢的塑韧性，使其在围管和焊缝矫直过程中容易开裂。Abstract: The cracking mechanism of grade 800 MPa steel for automobile torsion beam during girth and weld straightening were studied by OM, SEM, TEM and EBSD. Thermos-Calc thermodynamic software and J Mat Pro software were used to calculate equilibrium precipitated phase in steel and TTT curves. The microstructure and nano precipitates of experimental steels with different compositions were observed. The results show that the main reasons for cracking of experimental steel during girth and weld straightening are as follows: the addition of Cr, Mo and V elements in the 3# steel makes the TTT curve of the steel shift to the right, improving the stability of austenite and generating a large number of lath martensite and lath ferrite with high density dislocation during cooling after rolling. However, the amount of equiaxed ferrite is less and the grain size is uneven, so the tensile strength is higher. High density dislocation in lath martensite and lath ferrite results in the decrease of plasticity and toughness, and then causing crack in girth and weld during straightening.
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图 1 光镜组织形貌：(a)1#钢；(b)2#钢；(c)3#钢
Figure 1. Optical microscope morphology: (a)1# steel; (b)2# steel; (c)3# steel
图 2 扫描电镜组织形貌：(a)1#钢；(b)2#钢；(c)3#钢
Figure 2. SEM microscope morphology: (a)1# steel; (b)2# steel; (c)3# steel
图 3 铁素体晶粒及位错的TEM形貌: (a) 1#钢；(b) 2#钢；(c)、(d)3#钢
Figure 3. TEM photos showing ferrite grains and dislocation: (a)1# steel; (b)2# steel; (c)3# steel
图 4 EBSD组织形貌：(a)、(b)、(c)铁素体晶粒；(d)、(e)、(f)晶界取向
Figure 4. EBSD microscope morphology: (a)、(b)、(c)ferrite grains；(d)、(e)、(f)grain boundary orientation
图 5 热力学计算结果：(a)1#钢；(b)2#钢；(c)3#钢
Figure 5. Calculation results by Thermo-Calc: (a)1# steel; (b)2# steel; (c)3# steel
图 6 钢中析出相形貌及成分: (a)、(b) 1#钢; (c)、(d) 2#钢; (e)、(f) 3#钢
Figure 6. TEM photos showing and compositions of precipitates: (a) (b) 1# steel, (c) (d) 2# steel, (e)(f) 3# steel
表 1 化学成分
Table 1. Chemical compositions of used steels
% 编号 C Mn Si Als N Nb Ti Mo V Cr 1# 0.069 1.75 0.09 0.046 0.004 0.05 0.11 0 0 0 2# 0.045 1.50 0.05 0.032 0.004 0.03 0.09 0.17 0 0 3# 0.060 1.56 0.10 0.045 0.003 0.05 0.07 0.03 0.23 0.13 
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表 2 力学性能
Table 2. Mechanical properties of used steels
编号 抗拉强度/MPa 屈服强度/MPa 断后伸长率A50/% 开裂情况 1 828 767 22 未开裂 2 804 741 23 未开裂 3 838 753 27 开裂
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表 3 相变温度计算结果
Table 3. Calculated phase transition temperatures
试样 Fs/℃ Ps/℃ Bs/℃ Ms/℃ $M_{{\rm{f}}} $/℃ 1# 838.4 670.6 618.5 451.2 344.2 2# 854.1 676.2 630.7 463.0 356.9 3# 847.4 688.3 619.1 451.7 344.8
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表 4 晶界长度
Table 4. Grain boundary length of three steels
钢种 晶界长度/μm 2～5° 5～15° >15° 1# 410.78 465.98 2010 2# 454.95 586.41 2270 3# 124.65 202.82 1450
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