【作者】 赵杰；

【导师】 蒲健；

【作者基本信息】 华中科技大学 ， 材料学， 2012， 硕士

【摘要】 细晶强化、位错强化、第二相强化和固溶强化是马氏体钢的主要强化机制。对于碳钢而言，位错密度、晶粒尺寸和残余奥氏体含量等组织参数均和碳含量密切相关。因而碳含量是控制马氏体钢强度的重要因素。碳在淬火马氏体钢中偏聚于位错和晶界等缺陷处，对其在淬火马氏体钢中的强化作用形式至今没有定论。马氏体钢组织结构层次复杂，按照晶体学位向关系分为板条束、板条块和板条。对马氏体钢的强度结构控制单元也得出不一致的结论，认为屈服强度结构控制单元是原奥氏体、板条束和板条块的观点都是存在的。而且文献大都研究了屈服强度的结构控制单元，并未研究抗拉强度的结构控制单元。中碳马氏体钢淬火后内应力较大，加上孪晶的出现使其脆性极大。如何提高其塑韧性，充分利用其高强度的力学性能特点一直是马氏体钢研究的内容之一。本文采用多种实验手段定量表征了淬火中锰马氏体钢的组织结构，在此基础上探讨了淬火马氏体钢中碳的作用形式和强度结构控制单元；中碳中锰钢淬火后脆性极大，分析认为和组织中较高含量的不稳定奥氏体相关（体积分数14%），通过传统回火工艺，控制奥氏体的含量和稳定性，有效提高了中碳马氏体钢的塑性；低碳中锰马氏体钢淬火态强度较高，但是塑韧性较低，为了提高综合力学性能，进行了回火处理，并研究了回火温度对其组织和力学性能的影响。对淬火中锰马氏体钢的研究表明，增加碳含量提高了位错密度，细化了马氏体板条束和板条块结构。晶粒细化和位错密度增加使屈服强度和抗拉强度提高，同时塑性逐渐降低。计算证明，碳在淬火马氏体钢中并不是起到类似间隙固溶强化的作用。碳的的作用可以理解为增强了位错强化能力和细晶强化能力。屈服强度的结构控制单元是原奥氏体结构；抗拉强度的结构控制单元是板条结构。对中碳中锰钢（Fe-0.4C-5Mn）的研究表明，低中温回火处理得到抗拉强度在1600MPa以上，延伸率在12%~15%间，具有良好塑性的超高强度钢。马氏体基体强化是其保持超高强度的原因；较高含量和高稳定性的奥氏体，是其同时拥有良好塑性的原因。对低碳中锰钢的（Fe-0.2C-5Mn）的研究表明，回火处理能够提高马氏体钢的塑韧性。200℃低温回火处理得到最佳的综合力学性能（抗拉强度约1600MPa，屈服强度约1250MPa，延伸率15%，V型冲击功28J）。300℃左右的回火马氏体脆是板条间残余奥氏体分解的缘故。更多还原

【Abstract】 Grain refinement strengthening, dislocation strengthening, second phase strengtheningand solid solution strengthening are the main strengthening mechanisms in martensiticsteels. To carbon steels, microstructure parameters, such as dislocation densities, grain size,and retained austenite content, all closely relates with the carbon content. Thus carboncontent is the most important factor controlling the martensite strength. Carbon segregatesat the defects of dislocations and grain boundaries in quenched martensite. There is nofinal conclusion about the strengthening form of the carbon atoms in quenched martensite.The martensite structure is complex, and is divided into packet, block and lath by thecrystallography orientations. Conclusions about the microstructural unit controlling themartensite strength are inconsistent. Viewpoints regarding prior austenite, packet andblock as the microstructural unit controlling the yield strength all exits. However, mostarticles considered microstructural unit controlling the yield strength, and didn’t study themicrostructural unit controlling the ultimate tensile strength. The medium carbonquenched martensite steel is brittle due to the large internal stress and the appearance oftwins. It is one of the martensite study content at all times that how to improve theductility and toughness to fully utilize the high strength of the medium carbon martensitesteel.This paper quantitatively characterized the microstructure of the quenchedmedium-Mn martensitic steels using various experimental means, on the base of whichdiscussed the carbon strengthening form in quenched martensite and the microstructuralunit controlling the strength. The quenched medium carbon medium-Mn steel is verybrittle. It was deemed that the brittleness is related to relatively high content of unstableretained austenite (14%, volume fraction) in the microstructure. The ductility of thequenched medium carbon medium-Mn martensite steel was effectively improved throughcontrolling the the content and stability of the retained austenite by conventional tempering process. The quenched martensite strength is very high, but the ductility andtoughness is low. To improve the comprehensive mechanical properties, the low carbonmedium-Mn martenise steel was tempered, and the tempering temperature effect on themicrostructure and mechanical properties was studied.The study on the quenched medium-Mn martensite steels indicated that, increasingcarbon content increase the dislocation density and refine the packet and block size. Grainrefinement and dislocation density increase made the increase of the yield strength andultimate tensile strength, while the ductility lowered gradually. Calculations proved that,carbon in quenched martensite doesn’t act solid solution like strengthening effect. Thecarbon effect can be understood as enhancing the ability of dislocation strengthening andgrain refinement strengthening. The microstructural unit controlling the yield strength andultimate tensile strength is prior austenite and lath, respectively.The study on medium carbon medium-Mn steel (Fe-0.4C-5Mn) indicated that,ultrahigh strength steel with good ductility was obtained by low temperature tempering,with ultimate tensile strength of above1600MPa and ductility of12%~15%. The reason ofits ultrahigh strength is martensite matrix strengthening; Meanwhile, the high volumefraction of retained austenite with high stability favors the good ductility.The study on the low carbon medium-Mn steel (Fe-0.2C-5Mn) indicated that,tempering heat treatment improve the ductility and toughness of the martensite steel. Lowtemperature tempering when tempered at200℃obtained the best comprehensivemechanical properties with the ultimate tensile strength of1600MPa, yield strength ofabout1250MPa, elongation of15%and room temperature Charpy-V impact energy of28J.The reason of the tempered martensite embrittlement when tempered at about300℃is thedecomposition of the inter-lath retained austenite.更多还原