Al对热挤压模具钢SDAH13连续冷却转变规律的影响

Effect of Al on the continuous cooling transformation characteristic of hot extrusion die steel SDAH13

采用热膨胀仪测定Al质量分数分别为0．77％和1．43％以及无Al的热挤压模具钢SDAHl3的连续冷却转变曲线，并结合光学显微镜、扫描电镜及显微硬度仪分析Al元素对SDAH13钢相变点、连续转变规律、组织以及硬度的影响．结果表明：Al元素显著提高SDAH13钢的Ac1、Ac3和Ms点，降低淬火残留奥氏体含量，同时扩大铁素体及奥氏体两相区．在1060℃奥氏体化温度下，Al元素对SDAH13钢贝氏体相变的临界冷速（0．30℃·s-1）无明显影响，但使贝氏体相区变宽，Al质量分数分别为0．77％和1．43％的SDAH13钢的珠光体相变的临界冷速（0．05℃·s-1和0．3℃·S-1）均高于无Al的SDAHl3钢的临界冷速（0．02℃·S-1），且Al质量分数为1．43％的SDAH13钢在0．02—0．08℃·s-1冷速下出现先共析铁素体组织．Al的加入还使SDAH13钢淬火硬度有所降低．

The continuous cooling transformation （CCT） diagrams of SDAH13 hot extrusion die steels with different mass fractions of A1 （0. 77% and 1.43% ） and without A1 were measured by dilatometry. The effects of A1 on the CCT diagrams, microstructure and hardness of SDAH13 steels were studied by optical microscopy, scanning electron microscopy （SEM） and Vickers hardness test. The results show that the phase transformation temperature points （Acl, Acz and Ms） of SDAH13 steels significantly increase, tile contents of retained austenite in the quenched steels decrease, and the a-y multi-phase region expands with the increase in mass fraction of A1. However, when austenited at 1060 ℃, A1 has no obvious influence on the critical cooling rate （0. 30 ℃· s-l） of bainite transforma- tion, but bainite phase region broadens. Meanwhile, the critical cooling rates of pearlite transformation of SDAH13 steels with the A1 content of 0. 77% and 1.43 % are 0. 05 ℃ · s - 1 and 0. 3 ℃ · s- 1, respectively, which are higher than the critical cooling rate （0, 02 ℃·s-1 ） of pearlite transformation of SDAH13 steel without A1. Pro-eutectoid ferrite appears at the cooling rate from 0. 02 ℃ ·s-1 to 0. 08℃ ·s-1 when the mass fraction of AI reaches 1.43%. Nevertheless, the quenched hardness decreases with A1 addition.