基于仿真的M54超高强度钢真空自耗重熔工艺优化

采用Meltflow-VAR软件,选取稳态熔速为3.6、3.9、4.2、4.5、4.8、5.1 kg/min六种工艺曲线,对M54超高强度钢Φ660 mm大锭型真空自耗重熔工艺进行仿真。计算结果表明,随熔速增加,熔池深度和体积增加,熔速4.8 kg/min及以上熔池与结晶器完全接触,冷却效果得到改善。一次枝晶间距随熔速增大单调上升,局部凝固时间和二次枝晶间距在3.6~4.8 kg/min范围随熔速增加下降、在4.8~5.1 kg/min无明显变化。选取4.2 kg/min熔速开展工业化试制验证,熔池形状与计算结果吻合良好。钢锭宏观偏析程度较低,但微观偏析(枝晶偏析)的程度可达30%以上。微观偏析在有二次枝晶结构存在的位置与二次枝晶间距正相关,在无二次枝晶结构的位置与一次枝晶间距正相关。综合考虑熔速对熔池形状、枝晶间距、元素偏析的影响,建议稳态熔速优化为4.8 kg/min。

时效温度对二次硬化超高强度钢M54力学性能和组织的影响

试验用M54钢(/%:0.28~0.32C,≤0.10Si,≤0.10Mn,≤0.005S,≤0.008P,9.5~10.5Ni,6.6~7.4Co,1.8~2.2Mo,1.1~1.5W,0.7~1.3Cr,0.04~0.16V)由5.8 t真空感应炉+2.2t真空自耗炉熔炼并锻成Ф170mm棒材。锻材1/2R切取的Ф5mm试样经1075℃90min空冷和1060℃75min油冷固溶处理+在-73℃120min深冷处理+400~600℃300min空冷时效处理。采用光学和扫描电子显微镜等分析研究了时效温度对二次硬化超高强度钢M54的力学性能及组织的影响。结果表明,试验M54钢在520℃300min时效后具有优异的综合力学性能,抗拉强度2040MPa、冲击功62J、断裂韧性110MPa·m^(1/2);在560℃时效时,出现了沿晶断裂形貌,该钢冲击功K_(U2)和断裂韧性K_(IC)分别降至26J和80.6 MPa·m^(1/2),随着时效温度升高至600℃时试验钢的冲击功K_(U2)和断裂韧性K_(IC)分别增加到56 J和131 MPa·m^(1/2)。

Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-flee bainite-martensite （CFB/M） ultra-high strength steel was established by applying controlled cooling of small samples. In addition, the influence of thermomechanical treatment tem- perature on the structure and properties was discussed. The experimental results showed that when deformed at 860℃ and below, ferrite transformation occurred due to strain. With the decrease of ausforming temperature, the quantity of ferrite increased and strength and toughness were deteriorated. Therefore, certain information was provided for optimizing technical parameter of ausforming process., firstly, the thermomechanical treatment temperature should not be lower than 860 ℃ in order to avoid ferrite formation induced by deformation; secondly, rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.

Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting

300 M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles,owing to its high strength,excellent transverse plasticity,fracture toughness and fatigue resistance.Herein,low and high power selective laser melting(SLM)of 300 M steel and their microstructural evolution and mechanical properties have been reported.The results show that the optimal energy density range with the highest relative density for SLMed 300 M steel is between 60 and160 J/mm^3.Furthermore,molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300~600 W but a keyhole mode with"U"shape at a higher laser power of 800~1900 W.The heterogeneous microstructure of as-built samples is cha racterized by a skin-core structure which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone.The skin-core structure of SLMed 300 M steel has the characteristics of hard inside and soft outside.The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900 W.Interestingly,ultimate tensile strength(1156-1193 MPa)and yield tensile strength(1085-1145 MPa)of dense samples fabricated at diffe rent laser powers vary marginally.But,the elongation(6.8-9.1%)of SLMed 300 M steel is greatly affected by the laser power.

基于JMatPro软件的超高强度钢强度设计

以M54合金成分为基础,利用JMatPro软件计算了碳、钴、镍元素含量对钴镍系超高强度钢强度性能的影响,以及M2C析出相体积分数随钼、铬、钨元素含量的变化规律,根据计算结果设计了一种新的超高强度钢。结果表明:合金中碳含量在0.25%~0.45%范围,钴含量在5%~15%范围,镍含量在5%~10%范围时,随碳、钴、镍元素含量增加,合金强度提高。钼元素含量为2%~3%,铬元素含量为1.5%~2.5%时,析出相体积分数为4%,具有较好的沉淀强化效果。设计合金成分为0.35C-10Co-10Ni-2.5Mo-2Cr-1.3W-0.5V。热处理后组织为回火马氏体组织以及弥散分布的纳米级棒状M2C析出相,抗拉强度最高可达2380MPa,与M54钢相比提高了360MPa。