采用DIL805A型相变仪，测定含硼P20B钢在850～1 000 ℃的奥氏体化条件下的过冷奥氏体在640～720 ℃等温时的C曲线；结合850～1 000 ℃奥氏体化条件下晶粒度的测定和第二相分析，研究硼对P20B钢淬透性的影响行为.结果表明：在850～920 ℃的温度范围内，随着奥氏体化温度的升高，晶粒略有长大，固溶硼含量显著增加，使高温C曲线明显右移，即淬透性明显提高；而在920～1 000 ℃的温度范围内，随着奥氏体化温度的升高，晶粒显著长大，固溶硼含量不再明显增加，此时淬透性不再明显提高.

The C curve of supercooled austenite of plastic mould steel P20B was investigated with the DIL805A dilatometric instrument. The austenitizing temperature was between 850 ℃ and 1 000 ℃ and the isothermal temperature between 640 ℃ and 720 ℃. We compared austenite grain size at different austenitizing temperatures and analyzed the second phase which obtained from the electrolytic extraction procedure. The results show that at 850~920 ℃ in the temperature range when the austenitizing temperature is increased, the grains grow slightly, the content of B increased significantly to make the C curve strongly shift right. At 920~1 000 ℃ in the temperature range when the austenitizing temperature is increased, the grains grow significantly, and the content of B no longer increases so hardenability of P20B steel does not increased. 