The microstructure evolution and tensile properties of laser directed energy deposition (LDED) 300 M steel with isothermal heat treatment at 320 ℃ for different holding time were investigated in this paper. The results showed that the microstructure of the as-deposited LDED 300 M steel consisted of martensite and bainite. After heat treatment, the microstructure of the LDED 300 M steel is still the mixture of martensite, bainite and retained austenite, while the size of the lower bainite significantly decreased. As the increasing holding time, the content of martensite gradually decreased, while the content of bainite and austenite gradually increased. When the holding time of LDED 300 M steel increased from 20 min to 40 min, the number and size of austenite increased slightly. However, when the holding time was 60 min, the number and size significantly increased. As holding time continuously increase, it contributes little to the number and size of austenite. With regard to the hardness of the LDED 300 M steel, it was 464 HV when the holding time was 20 min. With the increment of holding time, the hardness gradually decreased until 428 HV when the holding time was 80 min. While holding time had less effect on the tensile strength and yield strength. To be specific, the tensile strength decreased from 1348 MPa to 1332 MPa, and yield strength decreased from 853 MPa to 759 MPa. The elongation gradually increased, and it increases to 33 % when the holding time was 80 min, which was higher than that of the forged 300 M steel standard (15 %). Effect of various factors, like microstructure, dislocation density and ε-carbides precipitated in the bainite on the ductility of the LDED 300 M steel with isothermal heat treatment was also studied.

研究了在320℃下等温热处理不同保温时间的激光定向能量沉积（LDED）300 M钢的组织演变和拉伸性能。结果表明，沉积后的LDED 300 M钢的显微组织由马氏体和贝氏体组成。热处理后，LDED 300 M钢的显微组织仍是马氏体，贝氏体和残余奥氏体的混合物，而下部贝氏体的尺寸则明显减小。随着保温时间的增加，马氏体含量逐渐减少，贝氏体和奥氏体含量逐渐增加。当LDED 300 M钢的保温时间从20分钟增加到40分钟时，奥氏体的数量和大小略有增加。但是，当保持时间为60分钟时，数量和大小显着增加。随着保温时间的不断增加，它对奥氏体数量和尺寸的贡献很小。关于LDED 300 M钢的硬度，保持时间为20分钟时为464 HV。随着保持时间的增加，当保持时间为80分钟时，硬度逐渐降低至428 HV。保持时间对拉伸强度和屈服强度的影响较小。具体而言，抗拉强度从1348MPa降低至1332MPa，屈服强度从853MPa降低至759MPa。伸长率逐渐增加，当保持时间为80分钟时，伸长率增加至33％，高于锻造的300 M钢标准品的伸长率（15％）。各种因素的影响，例如微观结构，