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<p>Interstitial-Free (IF) steel exhibits excellent formability and is one of the typical representative steel grades for automotive sheet steel. Currently, the development of IF steel manufacturing technology through the Electric Arc Furnace (EAF) short process has become the focus of industry attention. IF steel typically requires extremely low content of carbon (C) and nitrogen (N) in the steel—less than 30 ppm. However, the molten steel produced <i>via</i> the EAF short process exhibits elevated N content (60 to 70 ppm), significantly exceeding the required N content requirement for IF steel. Excessively high N content can lead to the precipitation of excessive nitrides in IF steel during the continuous casting process. This subsequently deteriorates the hot ductility of the steel and increases the risk of surface cracks in slabs. Cooling rate is a key factor in determining nitride precipitation behavior. Current research primarily focuses on the effect of cooling rate on nitride precipitation. However, studies on how low cooling rates (1.0&#xa0;°C/s) and low-ductility conditions promote microcrack formation through nitride precipitation, thereby deteriorating hot ductility, are insufficient. This study investigated the hot ductility of EAF-produced IF steel under different cooling rates within the 700&#xa0;°C to 1050&#xa0;°C range using a Gleeble-3800 thermal simulation tester. The mechanism promoting nitride precipitation and its effect on microcrack formation under low cooling rates was analyzed through scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS). Quantitative relationships were established between the cooling rate and hot ductility and the proportion of nitrides precipitated along grain boundaries, as well as their number density and average size. The results indicate that within the cooling rate range of 1.0 to 3.0&#xa0;°C/s, an increase in the cooling rate improves the hot ductility of IF steel. At low-ductility temperature points, the precipitation of micron-sized nitrides at grain boundaries and within the matrix promotes the formation of microvoids under stress. These microvoids then propagate into microcracks, deteriorating the hot ductility of the steel. As the cooling rate increases, in terms of nitride precipitation behavior, the proportion of nitrides precipitated along grain boundaries decreases from 42.5 to 17.5 pct, the number density decreases from 633 to 247 mm<sup>−2</sup>, and the average size decreases from 1.76 to 1.46 <i>μ</i>m. The reduction in all three dimensions can enhance grain-boundary cohesion, promote the dispersed distribution of nitrides, and reduce the degree of stress concentration, thereby reducing the occurrence of microcracks. Therefore, to reduce the crack incidence rate, the cooling rate of the slab prior to entering the bending segment should not be lower than 1.898&#xa0;°C/s.</p>

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Study on the Influence Mechanism of Cooling Rate on Hot Ductility and Microcrack Formation in IF Steel Slabs with EAF-Typical Nitrogen Levels

  • 期刊论文

  • 发表日期：

    2026年5月18日

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  • Jiawei He,
  • Xuewen Xiao,
  • Dongfeng He,
  • Zhiwei Han

摘要

Interstitial-Free (IF) steel exhibits excellent formability and is one of the typical representative steel grades for automotive sheet steel. Currently, the development of IF steel manufacturing technology through the Electric Arc Furnace (EAF) short process has become the focus of industry attention. IF steel typically requires extremely low content of carbon (C) and nitrogen (N) in the steel—less than 30 ppm. However, the molten steel produced via the EAF short process exhibits elevated N content (60 to 70 ppm), significantly exceeding the required N content requirement for IF steel. Excessively high N content can lead to the precipitation of excessive nitrides in IF steel during the continuous casting process. This subsequently deteriorates the hot ductility of the steel and increases the risk of surface cracks in slabs. Cooling rate is a key factor in determining nitride precipitation behavior. Current research primarily focuses on the effect of cooling rate on nitride precipitation. However, studies on how low cooling rates (1.0 °C/s) and low-ductility conditions promote microcrack formation through nitride precipitation, thereby deteriorating hot ductility, are insufficient. This study investigated the hot ductility of EAF-produced IF steel under different cooling rates within the 700 °C to 1050 °C range using a Gleeble-3800 thermal simulation tester. The mechanism promoting nitride precipitation and its effect on microcrack formation under low cooling rates was analyzed through scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS). Quantitative relationships were established between the cooling rate and hot ductility and the proportion of nitrides precipitated along grain boundaries, as well as their number density and average size. The results indicate that within the cooling rate range of 1.0 to 3.0 °C/s, an increase in the cooling rate improves the hot ductility of IF steel. At low-ductility temperature points, the precipitation of micron-sized nitrides at grain boundaries and within the matrix promotes the formation of microvoids under stress. These microvoids then propagate into microcracks, deteriorating the hot ductility of the steel. As the cooling rate increases, in terms of nitride precipitation behavior, the proportion of nitrides precipitated along grain boundaries decreases from 42.5 to 17.5 pct, the number density decreases from 633 to 247 mm−2, and the average size decreases from 1.76 to 1.46 μm. The reduction in all three dimensions can enhance grain-boundary cohesion, promote the dispersed distribution of nitrides, and reduce the degree of stress concentration, thereby reducing the occurrence of microcracks. Therefore, to reduce the crack incidence rate, the cooling rate of the slab prior to entering the bending segment should not be lower than 1.898 °C/s.