新型超高强度钢的合金优化及其组织性能研究

【作者】 路妍；

【导师】 谢刚； 苏杰；

【作者基本信息】 昆明理工大学 ， 钢铁冶金， 2011， 博士

【摘要】 本文在对现有低合金超高强度钢强韧化机理分析的基础上,设计出了30Cr3Si2Mn2NiWMoVNb和30Cr3Si2Mn2NiMoVNb两种新型超高强度钢。采用力学性能测试、光学显微镜(OM)、冷场发射扫描电子显微镜(SEM)、能谱衍射分析(EDAX)、X射线衍射分析(XRD)、透射电子显微镜(TEM)以及物理化学相分析等方法系统地研究了新型超高强度钢及现有的Eglin钢的力学性能以及微观组织,分析了未溶碳化物与热处理温度及组织性能的关系。本文主要的研究内容及获得的成果如下：以Eglin钢为原型钢,借用在G50钢中加入较高Si以及加入Nb以改善韧性的设计思路,同时加入约2%的Mn以进一步提高钢的韧性,为保证足够的强度,适当提高了Mo的含量。采用Thermo-Calc热力学方法优化了平衡态高温相组织,设计出了新型低成本超高强度钢30Cr3Si2Mn2NiWMoVNb钢。结果表明,30Cr3Si2Mn2NiWMoVNb钢的最佳热处理制度为950℃×1h,油淬+260℃×2h,空冷。其力学性能为：抗拉强度1870MPa,屈服强度1460MPa,冲击功为66J,延伸率为12%。与Eglin钢相比,在韧性相当的前提下,该钢的抗拉强度提高了约200MPa。30Cr3Si2Mn2NiWMoVNb钢的研制成功,对开发及拓展新的低成本超高强度钢提供了有益的借鉴。以G50钢为原型钢,降低Ni元素含量以节约成本,在此基础上,提高1%的Mn以提高钢的韧性,同时提高2%的Cr以改善淬透性,为了适当提高强度并细化晶粒,加入了少量的V元素,同时采用Thermo-Calc热力学方法优化了平衡态高温相组织,设计出了新型低成本超高强度钢30Cr3Si2Mn2NiMoVNb钢。试验结果指出,30Cr3Si2Mn2NiMoVNb (?)冈的最佳热处理制度为950℃×1h,油淬+260℃×2h,空冷,其力学性能为：抗拉强度1750MPa,屈服强度1330MPa,冲击功可达74J,延伸率为13%。相比于Eglin钢,该钢的抗拉强度提高了100MPa,韧性和塑性与之相当,而且其强度对淬火温度不太敏感。与G50钢相比,该钢的力学性能与之相当,成本大幅度降低。通过OM、SEM、EDAX、XRD以及TEM等分析手段研究发现：新型超高强度钢的基体均为高位错密度的板条马氏体组织,板条间分布有薄膜状的残余奥氏体及-些未溶碳化物。30Cr3Si2Mn2NiWMoVNb钢中的未溶碳化物为富W、Mo的M6C相以及富Nb、v的MC相,其未溶碳化物颗粒尺寸细小,数量显著增多,具在再加精细的板条组织,这也是30Cr3Si2Mn2NiWMoVNb钢具有更高抗拉强度及屈服强度,而韧性、塑性却与Eglin钢相当的重要原因。30Cr3Si2Mn2NiMoVNb钢未溶碳化物为富Mo的M6C相以及富Nb、V的MC相。相对于Eglin钢,该钢微观组织中的未溶碳化物颗粒尺寸、数量与之相差不显著,但该钢中由于增加了可保持到高温的富Nb、V的MC相,有效细化了晶粒,使得其强度随着奥氏体化温度的升高没有显著变化。采用Thermo-Calc热力学软件计算了新型超高强度钢加热时高温相的变化规律,其变化规律与试验结果基本一致。计算结果表明,30Cr3Si2Mn2NiWMoVNb钢的高温相主要为富Nb、V的MC相及富W、Mo的M6C相,其中富W、Mo的M6C相于约970℃时完全溶解；而30Cr3Si2Mn2NiMoVNb钢的高温相主要为富Nb、V的MC相及富Mo的M6C相,其中富Mo的M6C相在约920℃时完全溶解。试验结果也表明,30Cr3Si2Mn2NiWMoVNb钢中的M6C相的完全溶解温度在950℃～980℃之间；30Cr3Si2Mn2NiMoVNb钢中的M6C相的完全溶解温度在890℃～920℃。更多还原

【Abstract】 Based on the analysis of strengthening and toughening mechanism of low-alloy ultra-high strength steels,30Cr3Si2Mn2NiWMoVNb steel and 30Cr3Si2Mn2NiMoVNb steel are designed. The machaical properties and microstructure of newly designed steels and Eglin steel are systematic investigated by means of mechanics performance testing, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, the relations between undissolved carbides and austenited temperature are also studied. The main research and conclusions are obtained as follows:The toughness of G50 steel can be improved by adding Si and Nb in G50 steel. Based on the method, the toughness of Eglin steel can be improved by adding 2% Mn in Eglin steel. However the strength must be reduced. So the content of Mo can be increased to improve the strength of Eglin steel. So based on Eglin steel,30Cr3Si2Mn2NiWMoVNb steel, a new low cost ultra-high strength steel, can be by thermodynamic calculation by application of Thermo-Calc software. The results show that austenizing at 950℃for 1 hour, oil quenched and then tempered at 260℃for 2 hours followed by air cooling is the optimal heat treatment for 30Cr3Si2Mn2NiWMoVNb steel. Under the conditions, tensile strength of 187OMPa, yield strength of 1460MPa, Charpy U-notch impact toughness of 66J and ductility of 12% were obtained. The tensile strength of new steel is increased about 200Mpa with almost the same toughness against Eglin steel. The successful development of 30Cr3Si2Mn2NiWMoVNb steel can offer a method to develop a low cost ultrahigh strength steels.In order to reduce the cost, based on the G50 steel, the content of Mn increases by 1% to improve the toughness, the content Cr increases by 2% to improve hardenability and V is added to improve the strength.30Cr3Si2Mn2NiMoVNb steel, a new low cost ultra-high strength steel has been designed by thermodynamic calculation by assistant of Thermo-Calc software. The results show that austenizing at 950℃for 1 hour, oil quenched and then tempered at 260℃for 2 hours followed by air c(?)oling is the optimal heat treatment of 30Cr3Si2Mn2NiMoVNb steel. Under the conditions, tensile strength of 1750MPa, yield strength of 133OMPa, Charpy U-notch impact toughness of 74J and ductility of 13% were obtained. Tensile strength of 30Cr3Si2Mn2NiMoVNb steel is increased about 100MPa with almost the same toughness against Eglin steel and insensitive to austenitizing temperature. 30Cr3Si2Mn2NiMoVNb steel has almost the same mechanical properties as G50 steel, but is much cheaper.The microstructure of newly designed steels are characterized through OM, SEM, EDAX, XRD and TEM. The results show that matrix of new designed steels is lath-martensite with high dislocation density, interlath retained austenite and some undissolved carbides.30Cr3Si2Mn2NiWMoVNb steel has finer and more undissolved carbides of W, Mo-rich M6C and Nb, V-rich MC compared with Eglin steel. The undissolved carbides are lead to a finer microstructure, which ensured toughness and increased strength significantly. The undissolved carbides of 30Cr3Si2Mn2NiMoVNb steel are Mo-rich M6C and Nb, V-rich MC. Size and quantity of the undissolved carbides in 30Cr3Si2Mn2NiMoVNb steel are the same as Eglin steel. However, finer grain size according to Nb, V-rich MC lead to insensitive strength of 30Cr3Si2Mn2NiMoVNb steel.Variation of high temperature phases during heating in newly designed steels are calculated by Thermo-Calc software and verification test is carried out. Calculation results show that the W, Mo-rich M6C phases of 30Cr3Si2Mn2NiWMoVNb steel are dissolved completely at about 970℃. And the Mo-rich M6C phases of 30Cr3Si2Mn2NiMoVNb steel are dissolved completely at about 920℃. Experimental results showed that undissolved W, Mo-rich M6C carbides of 30Cr3Si2Mn2NiWMoVNb steel are dissolved completely in the range of 950℃-980℃; and undissolved Mo-rich M6C carbides of 30Cr3Si2Mn2NiMoVNb steel are dissolved completely in the range of 890℃-920℃. The calculation results were in accordance with test results.更多还原