中碳珠光体型高速车轮钢的韧化机理
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摘要
韧性是影响高速车轮运行安全的关键性能指标。为了阐明中碳珠光体型高速车轮钢的韧化机理,对夹杂物改性和组织韧化两方面进行了深入研究。研究结果表明,硫化物包裹氧化物的夹杂物改性提高了车轮钢的韧性。车轮钢中的氧化物夹杂容易在夹杂物与基体界面处产生裂纹,并向周围基体扩展;当氧化物被硫化物包裹后,裂纹仅在夹杂物本身产生,保护了周围基体。在w(Mn)=0.75%的成分体系下,当硫的质量分数提高到0.006%及以上时,硫化物在固相线温度以上析出,可以实现对氧化物的较好包裹,改善车轮钢的韧性;硫化物在车轮热加工过程中会发生回溶与再析出,破坏复合夹杂物的包裹效果,提高硫质量分数或降低热加工温度,可以提高复合夹杂物的热稳定性。奥氏体晶粒尺寸和先共析铁素体体积分数是车轮钢组织韧化的关键控制因素。细化奥氏体晶粒尺寸、提高铁素体体积分数,断口中解理面尺寸减小,韧性撕裂区增多。
Toughness is the key performance index affecting the safety of high speed wheel. In order to clarify the toughening mechanism of medium carbon pearlitic steel for high speed wheel,toughening through inclusion modification and microstructural control was intensively investigated. It was found that the toughness of wheel steel was improved by formation of oxide-sulfide duplex inclusion(sulfide-encapsulated oxide). The oxide inclusions easily induced crack initiation at the inclusion/matrix interface and the crack could propagate into the surrounding matrix,while the crack initiation of duplex inclusions was only on the inclusion itself so that the crack propagation to the matrix was hindered. When the mass percent of sulfur was increased to more than 0.006%(for a given manganese mass percent of 0.75%),the oxide inclusions could be well encapsulated due to the precipitation of sulfides beyond the solidus temperature. During hot working of wheel steel,the sulfides could be dissolved and re-precipitated in austenite,leading to a change for the duplex inclusions. However,the thermal stability of duplex inclusions could be improved by increasing the mass percent of sulfur or decreasing the hot working temperature. The austenite grain size and ferrite volume fraction were the key reasons for the toughening of wheel steel microstructure. The cleavage facet size was decreased and the ductile tearing area was increased with refinement of austenite grain size and enhancement of ferrite volume fraction.
Toughness is the key performance index affecting the safety of high speed wheel. In order to clarify the toughening mechanism of medium carbon pearlitic steel for high speed wheel,toughening through inclusion modification and microstructural control was intensively investigated. It was found that the toughness of wheel steel was improved by formation of oxide-sulfide duplex inclusion(sulfide-encapsulated oxide). The oxide inclusions easily induced crack initiation at the inclusion/matrix interface and the crack could propagate into the surrounding matrix,while the crack initiation of duplex inclusions was only on the inclusion itself so that the crack propagation to the matrix was hindered. When the mass percent of sulfur was increased to more than 0.006%(for a given manganese mass percent of 0.75%),the oxide inclusions could be well encapsulated due to the precipitation of sulfides beyond the solidus temperature. During hot working of wheel steel,the sulfides could be dissolved and re-precipitated in austenite,leading to a change for the duplex inclusions. However,the thermal stability of duplex inclusions could be improved by increasing the mass percent of sulfur or decreasing the hot working temperature. The austenite grain size and ferrite volume fraction were the key reasons for the toughening of wheel steel microstructure. The cleavage facet size was decreased and the ductile tearing area was increased with refinement of austenite grain size and enhancement of ferrite volume fraction.
引文
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[8] ZHOU S,ZUO Y,LI Z,et al. Microstructural analysis on cleavage fracture in pearlitic steels[J]. Materials Characterization,2016,119:110.
[9] Choudhary S,Ghosh A. Thermodynamic evaluation of formation of oxide-sulfide duplex inclusions in steel[J]. ISIJ International,2008,48(11):1552.
[10] Sakamoto H,Toyama K,Hirakawa K. Fracture toughness of medium-high carbon steel for railroad wheel[J]. Materials Science and Engineering:A,2000,285(1):288.
[11] Furuhara T,Kikumoto K,Saito H,et al. Phase transformation from fine-grained austenite[J]. ISIJ International,2008,48(8):1038.
[12] Park Y,Bernstein I. The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel[J]. Metallurgical Transactions A,1979,10(11):1653.
[13]吴斯,李秀程,张娟,等.铌微合金化对高铁车轮钢奥氏体形核和长大的影响[J].钢铁,2015,50(7):100.(WU Si,LI Xiucheng,ZHANG Juan,et al. Influence of Nb micro-alloying on austenite nucleation and growth in high speed railway wheels steel[J]. Iron and Steel,2015,50(7):100.)
[14] WU S,LI X,ZHANG J,et al. Microstructural refinement and mechanical properties of high-speed niobium-microalloyed railway wheel steel[J]. Steel Research International,2015,86(7):775.
[15] LI Z,YONG Q,ZHANG Z,et al. Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels[J]. Metals and Materials International,2014,20(5):801.
[2] Okagata Y. Design technologies for railway wheels and future prospects[J]. Nippon Steel and Sumitomo Metal Technical Report,2013,105:26.
[3]李吉东,韩培德,王烽,等. LZ50车轴钢边部增碳低倍缺陷的检验和分析[J].中国冶金,2017,27(2):53.(LI Ji-dong,HAN Pei-de,WANG Feng,et al. Inspection and analysis on edge carburization macroscopic defect of LZ50 axle steel[J]. China Metallurgy,2017,27(2):53.)
[4]余音宏,潘涛,尹建成,等. S含量对高速车轮钢性能和夹杂物的影响[J].钢铁,2013,48(10):57.(YU Yin-hong,PAN Tao,YIN Jian-cheng,et al. Effect of sulfur contents on properties and inclusions in high speed wheel steels[J]. Iron and Steel,2013,48(10):57.)
[5]马跃,潘涛,江波,等. S含量对高速车轮钢断裂韧性影响的研究[J].金属学报,2011,47(8):978.(MA Yue,PAN Tao,JIANG Bo,et al. Study of the effect of sulfur contents on fracture toughness of railway wheel steels for high speed train[J]. Acta Metallurgica Sinica,2011,47(8):978.)
[6] Brooksbank D,Andrews K. Stresses associated with duplex oxide-sulphide inclusions in steel[J]. J Iron Steel Inst,1970,208(6):582.
[7]左越,周世同,李昭东,等. V和Si对珠光体车轮钢显微组织和力学性能的影响规律[J].材料研究学报,2016,30(6):401.(ZUO Yue,ZHOU Shi-tong,LI Zhao-dong,et al. Effect of V and Si on microstructure and mechanical properties of mediumcarbon pearlitic steels for wheel[J]. Chinese Journal of Materials Research,2016,30(6):401.)
[8] ZHOU S,ZUO Y,LI Z,et al. Microstructural analysis on cleavage fracture in pearlitic steels[J]. Materials Characterization,2016,119:110.
[9] Choudhary S,Ghosh A. Thermodynamic evaluation of formation of oxide-sulfide duplex inclusions in steel[J]. ISIJ International,2008,48(11):1552.
[10] Sakamoto H,Toyama K,Hirakawa K. Fracture toughness of medium-high carbon steel for railroad wheel[J]. Materials Science and Engineering:A,2000,285(1):288.
[11] Furuhara T,Kikumoto K,Saito H,et al. Phase transformation from fine-grained austenite[J]. ISIJ International,2008,48(8):1038.
[12] Park Y,Bernstein I. The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel[J]. Metallurgical Transactions A,1979,10(11):1653.
[13]吴斯,李秀程,张娟,等.铌微合金化对高铁车轮钢奥氏体形核和长大的影响[J].钢铁,2015,50(7):100.(WU Si,LI Xiucheng,ZHANG Juan,et al. Influence of Nb micro-alloying on austenite nucleation and growth in high speed railway wheels steel[J]. Iron and Steel,2015,50(7):100.)
[14] WU S,LI X,ZHANG J,et al. Microstructural refinement and mechanical properties of high-speed niobium-microalloyed railway wheel steel[J]. Steel Research International,2015,86(7):775.
[15] LI Z,YONG Q,ZHANG Z,et al. Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels[J]. Metals and Materials International,2014,20(5):801.