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<p>The non-metallic inclusions (NMIs), microstructures, fracture characteristics, and low-temperature impact toughness (LTIT) of coarse-grained heat-affected zone (CGHAZ) for Ca-Ti-deoxidized high-strength low-alloy steel with different Cu contents after high-heat input welding (HHIW) of 400&#xa0;kJ/cm are studied. As the Cu content is increased from 0.069 to 0.22&#xa0;pct, the average size of prior austenite grains decreases, while the area fraction of intragranular acicular ferrites (IAFs) and polygonal ferrites (PFs) increases from 53.11 to 77.80&#xa0;pct, leading to that the frequency of high-angle grain boundaries (HAGBs) increases from 56.4 to 75.7&#xa0;pct and the average effective grain size decreases from 8.11 to 7.07&#xa0;μm. The effective NMIs inducing IAFs nucleation are complex oxides of Al<sub>2</sub>O<sub>3</sub>-Ti<sub>2</sub>O<sub>3</sub>, CaO-Al<sub>2</sub>O<sub>3</sub>-Ti<sub>2</sub>O<sub>3</sub>, and CaO-Ti<sub>2</sub>O<sub>3</sub> embedded with MnS and (Mn,Cu)S, with the number density increasing from 27 to 46&#xa0;mm<sup>−2</sup>. The number density of the effective NMIs containing (Mn,Cu)S is obviously lower in 69Cu steel compared to 220Cu steel, with the Cu content in these NMIs increasing from 0.50 to 5.34&#xa0;pct. Thermodynamic and first-principles calculation results show that the increase of Cu content reduces the precipitation temperature of MnS while increase that of CuS. The ferrite is prone to expel Cu atom, while the expelled Cu atom dopes in MnS, lowering the energy of MnS and favoring the formation of (Mn,Cu)S. The ability of effective NMIs containing (Mn,Cu)S inducing IAFs nucleation is stronger than that of effective NMIs containing MnS. Specifically, the effective NMIs containing (Mn,Cu)S can induce approximately 5 ferrite laths, while those containing MnS can only induce approximately 3 ferrite laths, due to the lower disregistry between CuS and ferrite and the significant overall energy reduction when the IAFs forms at (Mn,Cu)S. Additionally, the area fraction of fibrous zone in impact fracture surface increases from 0 to 49.01&#xa0;pct. The propagation path of primary crack becomes more curved and that of secondary crack becomes wider and shorter in 220Cu steel, due to cracks being deflected and arrested by finer IAFs and PFs with HAGBs. Therefore, the LTIT of CGHAZ at –&#xa0;20&#xa0;°C increases from 14 to 199&#xa0;J after HHIW of 400&#xa0;kJ/cm.</p>

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Effect of Cu on CGHAZ Low-Temperature Impact Toughness in Ca-Ti-Deoxidized HSLA Steels After HHIW: Insights from Inclusions, Microstructures, and Fracture Analysis

  • 期刊论文

  • 发表日期：

    2025年6月27日

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  • Liang Wang,
  • Zhong Ding,
  • Jian Yang,
  • Yinhui Zhang,
  • Yuqi Zhang,
  • Yanli Chen,
  • Fengqi Zhou,
  • Zhipeng Dai

摘要

The non-metallic inclusions (NMIs), microstructures, fracture characteristics, and low-temperature impact toughness (LTIT) of coarse-grained heat-affected zone (CGHAZ) for Ca-Ti-deoxidized high-strength low-alloy steel with different Cu contents after high-heat input welding (HHIW) of 400 kJ/cm are studied. As the Cu content is increased from 0.069 to 0.22 pct, the average size of prior austenite grains decreases, while the area fraction of intragranular acicular ferrites (IAFs) and polygonal ferrites (PFs) increases from 53.11 to 77.80 pct, leading to that the frequency of high-angle grain boundaries (HAGBs) increases from 56.4 to 75.7 pct and the average effective grain size decreases from 8.11 to 7.07 μm. The effective NMIs inducing IAFs nucleation are complex oxides of Al2O3-Ti2O3, CaO-Al2O3-Ti2O3, and CaO-Ti2O3 embedded with MnS and (Mn,Cu)S, with the number density increasing from 27 to 46 mm−2. The number density of the effective NMIs containing (Mn,Cu)S is obviously lower in 69Cu steel compared to 220Cu steel, with the Cu content in these NMIs increasing from 0.50 to 5.34 pct. Thermodynamic and first-principles calculation results show that the increase of Cu content reduces the precipitation temperature of MnS while increase that of CuS. The ferrite is prone to expel Cu atom, while the expelled Cu atom dopes in MnS, lowering the energy of MnS and favoring the formation of (Mn,Cu)S. The ability of effective NMIs containing (Mn,Cu)S inducing IAFs nucleation is stronger than that of effective NMIs containing MnS. Specifically, the effective NMIs containing (Mn,Cu)S can induce approximately 5 ferrite laths, while those containing MnS can only induce approximately 3 ferrite laths, due to the lower disregistry between CuS and ferrite and the significant overall energy reduction when the IAFs forms at (Mn,Cu)S. Additionally, the area fraction of fibrous zone in impact fracture surface increases from 0 to 49.01 pct. The propagation path of primary crack becomes more curved and that of secondary crack becomes wider and shorter in 220Cu steel, due to cracks being deflected and arrested by finer IAFs and PFs with HAGBs. Therefore, the LTIT of CGHAZ at – 20 °C increases from 14 to 199 J after HHIW of 400 kJ/cm.