Effect of Ca-Mg compound modifier on microstructure and plasicity in cold-rolled high strength steel
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摘要: 利用光学显微镜(OM)、透射电镜(TEM)和电子背散射衍射(EBSD)等设备研究了Ca-Mg复合变质剂对冷轧超高强钢铸坯中TiN粒子析出的影响机制，并对显微组织在各工序的遗传性及塑性改善进行了系统对比。结果发现：添加变质剂以后，铸坯中TiN第二相粒子尺寸明显变小，数量增多，分布更弥散；添加改质剂后钢坯、热轧钢板中沿晶界铁素体减少，冷轧退火后显微组织变细；与未添加变质剂试验钢比，屈服强度和屈强比提高，材料折弯、扩孔率得到改善。变质剂的添加改变了钢液中TiN形核机制。弥散的TiN可以细化铸态组织，并通过遗传效应对热轧、冷轧连退组织产生影响，改善力学性能和成型性能。Abstract: The influence mechanism of Ca-Mg compound modifier on TiN precipitation in ingot of producing ultra-high cold-rolled strength steel were studied by means of optical microscope (OM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD), etc. Andmicrostructure hereditary effect and plastic improvement at various process steps were compared systematically. The result showed that the size of TiN reduced with the addition of modifier, whereas its density increased with a relatively uniform distribution. Addition of modifier decreased the volume fraction of ferrite along grain boundary for both ingot and hot rolled strip, and resulted in fine microstructure in the as-cold rolled sheet. Comparing modifier-free sample, the yield strength, yield ratio dramatically increased and the bending properties and hole expanding ratio improved for the modifier-bearing sheet. The nucleation mechanism of TiN was changed by the addition of Ca-Mg compound modifier. The dispersed TiN particles could effectively refine the casting microstructure, and the microstructure for hot-rolled or cold-rolled were all refined effectively by hereditary effect which could significantly improve the mechanical properties and formability of steel.
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Key words:
- high strength steel /
- TiN /
- modifier /
- microstructure /
- mechanical property /
- formability
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图 1 试验钢冷轧后的退火工艺
Figure 1. Heat treatment process for as cold-rolled experimental steels
图 2 铸坯中第二相粒子形貌和化学成分
Figure 2. Morphology and chemical compositions of secondary particle in casting ingot
图 4 冷板晶粒取向成像及有效晶粒尺寸
Figure 4. Crystal orientation mapping (COM) and effective grain size of cold rolled sheet
表 1 试验钢的主要化学成分
Table 1. Main chemical compositions of the experimental steels
% 编号 C Si Mn S P Nb+Ti+Cr Alt N 改质剂 1#钢 ≤0.35 ≤1.50 ≤2.50 0.015 ≤0.030 ≤0.200 ≥0.020 ≤0.007 未添加 2#钢 ≤0.35 ≤1.50 ≤2.50 0.015 ≤0.030 ≤0.200 ≥0.020 ≤0.007 添加 
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表 2 变质剂对试验钢力学性能和成型性能的影响
Table 2. Effect of modifier on the mechanical properties and forming performance
钢号 力学性能 成型性能 抗拉强度/MPa 屈服强度/MPa 屈强比 伸长率/% 扩孔率/% 180°折弯 d=0a 1#热轧 723 490 0.68 13 25 不开裂 2#热轧 795 564 0.71 16 31 不开裂 1#冷轧 984 683 0.69 20 35 开裂 2#冷轧 1000 783 0.78 25.5 55 不开裂
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