Study on the hot ductility of titanium microalloyed high-strength beam steels
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摘要: 通过高温拉伸试验研究了600～1300 ℃温度范围内不同Ti含量的钛微合金化高强大梁钢热塑性规律，基于Thermo-calc热力学计算软件对不同试验钢中主要析出相的析出区间进行计算，利用光学显微镜（OM）、扫描电镜（SEM）和透射电镜（TEM）对热拉伸断口形貌、断口析出物、显微组织特征进行观察和讨论。结果表明，随着四种试验钢中Ti含量的增加，低塑性区逐渐向下移动，低塑性区间宽度增加，含Ti量最高的950L钢热塑性显著下降。主要原因是沿晶界析出的网膜状铁素体弱化了晶界强度，为裂纹的萌生和扩展提供了条件。此外，在应力的作用下，钢中存在的微米级TiN颗粒及其析出物易于与基体间形成产生微孔，微孔聚集形成裂纹，从而降低试验钢的热塑性。因此，提出在保证矫直温度的前提下适当提高钢水冷却速率，抑制薄膜状先共析铁素体的析出及第二相析出，可有效改善试验钢在第III脆性区间的高温热塑性。Abstract: High-temperature tensile tests have been conducted to examine the thermoplasticity of titanium microalloyed high-strength beam steels in the temperature range of 600 ℃ to 1300 ℃. Thermo-calc, the thermodynamic calculation software, was used to calculate the main precipitation intervals in various experimental steels. Optical microscopy (OM), field-emission type scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to observe and analyze the microstructural characteristics and the thermal tensile fracture morphology. The findings demonstrate that thermoplasticity declines dramatically with increasing Ti content, and the hot brittle range is gradually lowered and widened in four experimental steels. The 950L steel with the highest Ti content shows the lowest thermoplasticity. The main reason is that the formed proeutectoid ferrite weakens the grain boundary strength and provides conditions for the initiation and propagation of cracks. In addition, microvoids are easily formed between the micron TiN particles and precipitates. Then the microvoids aggregate to form cracks under stress, thus reducing the thermoplasticity of the experimental steel. Therefore, it is suggested that increasing the cooling rate on the premise that the straightening temperature is assured and preventing the precipitation of proeutectoid ferrite and the second phase can effectively improve the thermoplasticity in the third brittle zone.
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图 2 不同温度下试验钢的抗拉强度及断面收缩率
Figure 2. Tensile strength and area reduction of experimental steel at different temperatures
（a）610L；（b）700L；（c）800L；（d）950L
图 3 800 ℃下不同试验钢断口SEM形貌
Figure 3. SEM morphology of fracture surface of different experimental steels at 800 ℃
（a)、(b）610L；（c)、(d）700L；（e)、(f）800L；（g)、(h）950L
图 4 800 ℃下不同试验钢断口附近组织形貌
Figure 4. Microstructure near the fracture area of different experimental steels at 800 ℃;
（a)、(b）610L，（c)、(d）700L，（e)、(f）800L，（g)、(h）950L
图 5 950L钢900 ℃（a)、(b）及1150 ℃（c)～(f）微观组织TEM观察
Figure 5. TEM microstructures of the 950L steel observed steel at 900 ℃ (a), (b) and 1150 ℃ (c)～(f)
图 6 不同试验钢Thermo-cala相图计算结果
Figure 6. Calculation results of Thermo-calc phase diagrams of different experimental steels
（a）610L;（b）700L;（c）800L;（d）950L
图 7 不同试验钢铸态金相组织
Figure 7. As-cast metallograph of different experimental steels
（a）610L;（b）700L;（c）800L;（d）950L
图 8 不同类型析出物数密度
（a）610L;（b）700L;（c）800L;（d）950L
Figure 8. The number density of different types of precipitates
图 9 950L试验钢析出物扫描图片及面扫结果
Figure 9. Scanning pictures and surface scanning results of 950L experimental steel precipitates
表 1 试验钢主要化学成分
Table 1. Main chemical compositions of the tested steel
% 钢种 C Si Mn N Nb Ti Mo V Al Ca 610L 0.06 0.07 1.16 0.0028 0.025 0.038 0.047 0.0002 700L 0.06 0.06 1.49 0.003 0.037 0.075 0.034 0.0018 800L 0.05 0.09 1.55 0.003 0.038 0.089 0.17 0.038 0.0008 950L 0.087 0.08 1.90 0.004 0.034 0.106 0.21 0.09 0.029 0.0012 
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