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Development of Case Hardening Steels for Automotive Transmission

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Development of Case Hardening Steels for Automotive Transmission NIPPON STEEL TECHNICAL REPORT No. 122 NOVEMBER 2019 Technical Report UDC 629 . 11 : 621 . 833 : 621 . 785 . 53 Development of Case Hardening Steels for Automotive Transmission Masayuki HORIMOTO* Kei MIYANISHI Shoji TODO Akira SHIGA Hideki IMATAKA Abstract The reduction of carbon dioxide emission has become an important issue in the preven- tion of global warming in the world. For automotive transmission, reduction of the size and weight of parts improves the fuel efficiency of automobiles. Therefore, it is important to strengthen gears and CVT pulleys which are the main parts of automotive transmission. Furthermore, in case hardening heat treatment, vacuum carburizing and gas nitriding in addition to conventional gas carburizing are starting to be adopted. We have developed case hardening steels for high strength transmission parts through a combination of the steel material component and the heat treatment process. 1. Introduction their performance after the completion of the customers’ manufac- In recent years, from the viewpoint of global environmental pro- turing processes. tection, there has been a strong demand for improvement of fuel ef- This report introduces Nippon Steel’s developed steels: high ficiency of vehicles and reduction of carbon dioxide emissions. For strength gear steel, high impact fatigue resistant strength gear steel, automotive transmission, reduction of the size and weight of parts is wear resistant CVT (Continuously Variable Transmission) steel, becoming important and complex part shapes are being adopted to mild carburizing steel and high strength nitriding gear steel. reduce the size of the unit. Then in addition to the conventional de- mands for weight reduction and further strength improvement, the 2. High Strength Gear Steel gear steel is also required to have the low heat treatment distortion Conventionally, for the manufacturing of automotive gears, JIS- property in order to realize quietness. Furthermore, in order to re- SCr420 steel and SCM420 steel, to which the gas carburizing hard- duce the carbon dioxide emission in the gear manufacturing process, ening treatment has been applied, are used and the incompletely a shift from the conventionally employed gas carburizing treatment hardened microstructure generated in the carburized layer remains a to the gas nitriding treatment and the vacuum carburizing treatment problem. In the gas carburizing hardening, since the steel is heated is promoted. Thus automobile companies employ various surface in the carburizing gas atmosphere containing oxidizing gases such hardening heat treatment processes, and provide necessary functions as CO2, H2O and O2, the oxides of Mn, Cr and Si are produced at the to their respective transmission parts. In this regard, Nippon Steel grain boundaries of the prior-austenite (intergranular oxidation) dur- Corporation has developed case hardening steels to respond to the ing heating. As a result, the alloying elements such as Mn, Cr and Si requirements of various surface hardening heat treatment processes decrease in the matrix around the intergranular oxidation, and the and the transmission parts. The steels have been developed to pro- hardenability is decreased. 1, 2) Since the diffusion velocity of oxygen vide a superior performance to those of alloy steels for machine in steel is lower than that of carbon, the depth of the intergranular structural use of the domestically used JIS-SCr and SCM systems. oxidation layer is smaller than the carburized depth. Consequently, The developed steel is compositionally designed so that the hard- in the gas-carburized hardened steel, despite the high carbon con- ness and the toughness of the carburized layer under the tooth sur- tent, the hardenability of the subsurface layer is locally decreased face and the hardness of the gear core portion are optimized so as to and the incompletely hardened microstructure with low hardness satisfy the requirements of the subject units, and designed to exert (fine pearlite microstructure) is formed in the subsurface layer. The * Chief Researcher, Bar & Wire Rod Research Lab., Steel Research Laboratories 20-1 Shintomi, Futtsu City, Chiba Pref. 293-8511 - 99 - NIPPON STEEL TECHNICAL REPORT No. 122 NOVEMBER 2019 incompletely hardened microstructure with low hardness becomes the shot-peening is more effectively applied to the developed the origin of the fatigue fracture, and decreases the bending fatigue CM201 steel wherein the incompletely hardened microstructure is strength and the pitting fatigue strength, a type of rolling contact fa- reduced. tigue strength of the gear teeth. It is also important to suppress the pitting failure, the fatigue To reduce the incompletely hardened microstructure, a reduction failure initiated at the gear tooth surface layer, as well as to suppress of the formation of intergranular oxidation products and an increase the gear bending fatigue failure. For instance, one method consid- in the matrix hardenability are considered. To suppress the inter- ered to improve the fatigue strength is, instead of employing the granular oxidation, the device on the part of the processor’s side to high strength gear steel, to grind off the gear tooth subsurface layer reduce the oxidizing gas components of the carburizing gas and that contains the incompletely hardened microstructure. However, composition design on the steelmaker’s side to reduce the elements even in the case without the incompletely hardened microstructure, that form intergranular oxidation are considered. For example, as depending on the loading conditions such as contact stress, slipping Fig. 1 shows, Si is most influential on the depth of the intergranular speed and the like, the tooth surface temperature increases due to oxidation among the intergranular oxide producing elements. There- the friction-induced heat caused by the repetition of the meshing of fore, by reducing the Si content, and by increasing the content of the gear teeth, and the tooth surface is tempered thereby. Therefore, Mo, an element that does not produce intergranular oxide products, the hardness decreases during its operation. When the tooth surface to compensate for the deficiency of the hardenability produced hardness decreases, pitting failure is generated more readily, and so thereby, a microstructure free of incompletely hardened microstruc- it is important to maintain the tooth surface hardness even during its ture is developed. 3) operation. Table 1 shows the chemical compositions of the developed steel It is reported that a good correlation is established between the CM201, and Fig. 2 shows the microstructure of the subsurface layer. pitting fatigue strength and the hardness of the carburized layer ob- As compared with JIS-SCM420 steel, in the high strength gear steel tained with tempering at 300°C, 4) and the pitting fatigue strength is wherein the incompletely hardened microstructure is reduced, the considered to be enhanced by providing the chemical compositions rotating bending fatigue strength is improved by 13%, and the bend- that impart high tempering hardness to the matrix when tempered at ing fatigue strength of the gear is improved. 300°C. The hardness obtained with tempering at 300°C is deter- Furthermore, in order to further improve the bending fatigue mined by the hardened carburized layer hardness governed by the strength, the compressive residual stress is imparted to the subsur- carbon content of the carburized layer and by the resistance to tem- face layer. The higher the matrix strength is, the larger the compres- per softening governed by the alloying element contents. As Fig. 3 sive residual stress developed by shot-peening becomes. Therefore, shows, the element Si has the highest resistance to temper softening in the temperature region around 300°C. The process of tempering the carbon steel martensite is consid- ered to go through the following three stages: in the first stage, the fine ε carbide (Fe2–3C) as a transition carbide is precipitated, in the second stage, the retained austenite is decomposed to form cement- Fig. 2 Micrographs of carburizing layer (nital etching), a) CM201, b) Fig. 1 Intergranular oxidation depths varying alloy contents SCM420 Table 1 Chemical compositions of developed steels (mass%) Grade C Si Mn Cr Mo V B Developed steel CM201 0.20 0.10 0.75 1.00 0.40 – – XG5 0.20 0.50 0.35 1.45 0.35 – – NT-B 0.20 0.25 0.40 1.65 – – add Wear resistance 0.17 0.25 0.80 1.85 – – – MSB20 0.20 0.80 0.80 0.10 – – 0.0020 Nitriding 0.10 0.15 0.55 1.25 – 0.17 – Conventional steel JIS-SCM420 0.20 0.25 0.80 1.10 0.20 – – JIS-S35C 0.35 0.20 0.75 – – – – - 100 - NIPPON STEEL TECHNICAL REPORT No. 122 NOVEMBER 2019 ite θ and ferrite α, and in the third stage, the ε carbide transforms to gram and the dark field image formed at the spot indicated as DF in the cementite θ. 5) The third stage of the Si-added steel is considered the diagram are shown. In the dark field image, a number of fine ε to move to the higher temperature side, and the transformation of carbides several nm in size and aligned in the same orientation in the ε carbide to the cementite θ is suppressed; therefore, the decrease what is considered to be the martensite lath are confirmed. of the matrix hardness during the low temperature tempering is con- The Si content is increased in the developed steel XG5 as shown sidered to be suppressed. In Fig. 4, the bright field image of the in the chemical compositions in Table 1, and as Fig. 5 shows, the transmission electron microscopy (TEM) image of 0.6C-1Si steel pitting resistant strength obtained in a roller pitting fatigue test is tempered at 400°C, the selected area diffraction pattern, the key dia- improved by 21% from that of JIS-SCM420. Furthermore, by ad- justing the contents of Mn and Cr appropriately, the hardness after the spheroidizing annealing treatment is reduced, and as Fig.
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