本文研究了微量元素Al和Ti对800 MPa 低碳低合金钢模拟焊接HAZ的组织和韧性的影响。微量元素 Al 和 Ti的加入，有效地提高了焊接HAZ的低温韧性：加入Al后，在5kJ/mm的线能量作用下，-20℃ 和 -40℃ 的冲击韧性分别为 103.5J 和19.7J，与不加时相比，分别提高了 4.9倍和1.6倍；再加微量 Ti 元素同时降低碳含量，则使5 kJ/mm线能量下-40℃ 的冲击韧性高达 186.0 J，提高了 14.9 倍。微量元素 Al 提高HAZ低温韧性是通过降低M－A组元的平均弦长和HAZ固溶N量，以及提高相变转变温度来实现；微量元素 Ti 有细化晶粒、降低固溶N量和促进块型Bg形成的作用；降低钢中的碳含量，致使M－A组元中的碳含量降低，而低碳M－A组元本身韧性较好，因此，它对HAZ韧性损害较少。本文首次探讨了焊接冷却过程中，第二相质点TiN 和 AlN的析出行为，即 TiN 和 AlN 在冷却过程中的析出是由于加热过程中未完全溶解的微细质点的长大所致。
其他摘要
The effects of micro-alloying elements Al and Ti on microstructures and toughness of the synthetic weld HAZ of low carbon 800 MPa HSLA steels have been studied. The low temperature toughness of synthetic weld HAZ is increased remarkably with the addition of Al and Ti. The Akv-20℃ and Akv-40℃ of HAZ bearing Al at 5 kJ/mm heat input are 103.5J and 19.7J respectively, which is 4.9 and 1.6 times of that in the steel without any micro-alloying elements. With the addition of both Al and Ti as well as reducing the C content, the Akv-40℃ of HAZ at 5 kJ/mm heat input is 186.0 J, which is 14.9 times of that in the steel without any micro-alloying elements. The mechanism of Al to improve HAZ toughness at large heat input is reducing the mean string length of M-A constituents and the soluble N content, increasing the phase transformation temperature. Ti can refine austenite grains, reduce the soluble N content and increase the ratio of rectangular bainite in whole microstructures. The combined addition of Al and Ti further increases the low temperature toughness. Decreasing the C content in base metal can decrease the C content in M-A constituents, thus reduce their harmful effect on toughness. It is the first time to study the reprecipitation behavior of second phase particles (TiN and AlN) during cooling period of the weld thermal cycle. The reason for the reprecipitation of TiN and AlN during cooling period is the growth of the very fire remained particles not dissolved completely during heating period.
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