目的 针对高扩孔钢扩孔性能富余量不足的问题,以4种不同卷取工艺制备的工业试制铁素体基高扩孔钢为研究对象,研究卷取温度对高扩孔钢拉伸和扩孔性能的影响,进而总结出扩孔性能提升存在的问题。方法 采用万能试验机、显微硬度计（HV-1002）、场发射扫描电镜（ZEISS GeminiSEM 300）、电子探针微区分析仪（JXA-8530F Plus）和俄歇电子（AES）-电子背散射衍射仪（EBSD,PHI 710）进行力学性能及显微组织研究。结果 在630 ℃卷曲时,高扩孔钢的屈服强度和抗拉强度达到最大值,分别为722 MPa和798 MPa;然而,此时热轧板的厚度中心由于C/Mn偏析而析出粗大珠光体,在形变过程中,由于中心和近表面位置变形不均匀而易在中心分层开裂,从而导致扩孔率降低至34.1%。降低卷取温度至500~550 ℃和提升卷曲温度至700 ℃均能抑制热轧板厚度中心处珠光体的形成,提升扩孔率;但是由于卷取温度偏离碳化钒的鼻尖析出温度,因此强度降低,屈服强度和抗拉强度分别为530~640 MPa和620~730 MPa。结论 当卷取温度为630 ℃和700 ℃时,厚度中心C元素的偏析会导致生成珠光体/渗碳体组织,进而导致中心与边缘硬度差异增大,从而发生开裂。除此之外,当卷取温度提升至630 ℃时,屈服强度和抗拉强度达到最大值,分别为722 MPa和798 MPa。为了在高强度下提升铁素体扩孔钢的扩孔率,需改善铸坯中心C/Mn的偏析程度,使其在VC析出鼻尖温度附近卷取时能有效抑制厚度中心珠光体的形成。
Abstract
To address the issue of insufficient performance margin in the hole expansion capacity of high hole expansion steel, the work aims to systematically investigate the effect of coiling temperatures on the tensile and hole expansion properties of four industrially trial-produced ferrite-based high hole expansion steels subject to different coiling processes, thus identifying the prevailing challenges in enhancing the hole expansion performance. The mechanical properties and microstructure were characterized with a universal testing machine, a microhardness tester (HV-1002), a field emission scanning electron microscope (ZEISS GeminiSEM 300), an electron probe microanalyzer (JXA-8530F Plus), and an Auger electron spectrometer (AES)-electron backscatter diffraction (EBSD) system (PHI 710). It was found that high hole expansion steel, when coiled at 630 ℃, exhibited peak yield and tensile strengths of 722 MPa and 798 MPa, respectively. However, at this stage, coarse pearlite precipitated at the thickness center of the hot-rolled sheet due to C/Mn segregation. During deformation, the non-uniform strain distribution between the center and near-surface regions made the material prone to centerline delamination cracking, thereby reducing the hole expansion ratio to 34.1%. Lowering the coiling temperature to 500-550 ℃ or raising it to 700 ℃ could suppress the formation of pearlite at the center of the hot-rolled sheet and improve the hole expansion ratio. However, because the coiling temperature deviated from the nose precipitation temperature of vanadium carbide (VC), the strength decreased and the yield and tensile strengths fell within the ranges of 530-640 MPa and 620-730 MPa, respectively. In conclusion, when the coiling temperature is 630 ℃/700 ℃, the segregation of C at the sheet center thickness promotes the formation of pearlite/cementite microstructure, resulting in increased hardness difference between the center and edge regions, which leads to cracking. Additionally, when the coiling temperature increases to 630 ℃, the yield strength and tensile strength reach their maximum values of 722 MPa and 798 MPa, respectively. To enhance the hole expansion ratio of high-strength ferritic hole expansion steel, it is necessary to mitigate the C/Mn segregation at the center of the slab. This improvement ensures that when coiling is performed near the nose temperature for vanadium carbide (VC) precipitation, the formation of pearlite at the sheet center can be effectively suppressed.
关键词
铁素体高扩孔钢 /
强度 /
中心开裂分层 /
偏析 /
珠光体
Key words
ferritic high hole expansion steel /
strength /
central delamination cracking /
segregation /
pearlite
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基金
北京市自然科学基金（2242048）; 云南省重点研发计划（202403AA080013）
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