Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-19
2003-12-09
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Ferrous
C148S654000, C420S120000, C420S127000, C420S128000
Reexamination Certificate
active
06660105
ABSTRACT:
TECHNICAL FIELD
This invention relates to a case hardening steel having good grain coarsening properties during carburization, to a method for producing the steel, and to a blank material for carburized parts.
BACKGROUND ART
Gear-wheels, bearing parts, rolling parts, shafts. and constant velocity joint parts are normally manufactured by a process using medium-carbon steel alloy for mechanical structures prescribed by, for example, JIS G 4052, JIS G 4104, JIS G 4105 and JIS G 4106 that is cold forged (including form rolling), machined to a specified shape and carburization hardened. Because cold forging produces a good product surface layer and dimensional precision, and results in a better yield, with a lower manufacturing cost, than hot forging, there is an increasing trend for parts that were conventionally produced by hot forging to be produced by cold forging which, in recent years, has produced a pronounced increase in the focus on carburized parts manufactured by the cold forging—carburizing process. A major problem with carburized parts is reducing heat treatment strain. This is because a shaft that warps as a result of strain from heat treatment can no longer function as a shaft, or in the case of gear-wheels or constant-velocity joint parts, high strain from heat treatment can cause noise and vibration. The major factor in such heat-treatment induced strain is grain coarsening produced during the carburizing. In the prior art, grain coarsening has been suppressed by annealing after cold forging and before carburization hardening. With respect to this, in recent years there is a strong trend toward omitting the annealing as a way of reducing costs. Therefore, there has been a strong need for steel in which grain coarsening does not occur even if the annealing is omitted.
Bearing and rolling parts that have to take a high contact stress are subjected to deep carburization. As deep carburization requires an extended period of time ranging from ten-plus hours to several tens of hours, it gives rise to another important issue, that of reducing the carburization time for the purpose of saving energy. One effective way of reducing the carburization time is to use a higher carburizing temperature. Carburization is normally performed at around 930° C. The problem with performing carburization at a higher temperature, in the range of 990 to 1090° C., is that it results in grain coarsening and a lack of the necessary material qualities, such as rolling fatigue characteristics and the like. Thus, there is a demand for case hardening steel that is suitable for high-temperature carburizing, that is, the grains of which are not coarsened by high-temperature carburizing. Many of the bearing and rolling parts that have to take a high contact stress are large parts that are normally manufactured by the steps of hot forging bar steel, heat treatment such as normalizing or the like, if required, machining, carburization hardening, and, if required, polishing. To suppress grain coarsening during carburizing, following the hot forging step, that is, when the parts are still blanks, it is necessary to optimize a material for suppressing the grain coarsening.
For this, JP-A-56-75551 discloses steel for carburizing comprising steel containing specific amounts of Al and N that is heated to not less than 1200° C. and then hot worked, whereby even after it has been carburized at 980° C. for six hours it is able to maintain fine grains, with the core austenite grains being fine grains having a grain size number of not less than six. However, the grain coarsening suppression ability of the steel is not stable and, depending on the process used to produce the steel, the steel may be unable to prevent grain coarsening during carburizing.
JP-A-61-261427 discloses a method of manufacturing steel for carburizing in which steel is used that contains specific amounts of Al and N, wherein after the steel has been heated to a temperature corresponding to the amounts of Al and N, then hot rolled at a finishing. temperature of not more than 950° C., the precipitation amount of AlN is not more than 40 ppm and the ferrite grain size number is from 11 to 9. Again, however, the grain coarsening suppression ability of the steel is not stable and, depending on the process used to produce the steel, the steel may be unable to prevent grain coarsening during carburizing.
JP-A-58-45354 discloses a case hardening steel containing specified amounts of Al, Nb and N. Again, however, the ability of the steel to suppress grain coarsening is not stable, so that in some cases grain coarsening is suppressed, and in other cases it is not. Moreover, in the examples the steel is described as having a nitrogen content of not less than 0.021%. If anything, that would have the effect of worsening the grain coarsening properties, making the steel susceptible to cracking and blemishes during the production process, in addition to which, because of the hardness, the material would have poor cold workability.
Thus, the above methods are not able to stably. suppress grain coarsening during carburization hardening, and therefore are not able to prevent strain and warping. With respect also to bearing and rolling parts that are subjected to high contact stresses, there are no examples in which such parts that have been subjected to deep carburizing by carburizing at a high temperature exhibit adequate strength properties. That is, there are no prior examples of blank materials for carburized parts or case hardening steel suitable for high-temperature carburization.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide case hardening steel with low heat-treatment strain having good grain coarsening prevention properties during carburization, a method of producing the steel, and, with respect to the production of carburized parts produced in the hot forging process, blank material for carburized parts that are able to prevent grain coarsening even during high-temperature carburizing and have adequate strength properties.
To attain the above object, the present inventors investigated what the dominant factors in grain coarsening were, and clarified the following points.
1. Even though steels may have the same chemical composition, in some cases they may be able to suppress grain coarsening and in other cases they may not be able to: grain coarsening cannot be prevented just by limiting the chemical composition. An important factor, apart from the chemical composition, is the state of precipitation of carbonitrides after the steel has been hot rolled or hot forged.
2. A key to preventing grain coarsening during carburization is, during carburization heating, to effect dispersion of a large amount of fine AlN and Nb(CN) as pinning particles.
3. To ensure a stable manifestation of the pinning effect of the Nb(CN) during carburization heating, the hot rolled or hot forged steel needs a prior fine precipitation of at least a given amount of Nb(CN). Moreover, if coarse AlN is precipitated or TiN or Al
2
O
3
is present in the steel after the steel has been hot rolled or hot forged, it will form coarse Nb(CN) precipitation nuclei, impeding the fine precipitation of the Nb(CN). This being the case, it is necessary to keep the Ti content and O content as low as possible.
4. To ensure a stable manifestation of the pinning effect of the AlN during carburization heating, in contrast to Nb(CN), it is necessary to minimize the AlN precipitation amount in the steel in the hot rolled or hot forged condition. This is an essential requirement for achieving fine precipitation of the Nb(CN). Moreover, any TiN or Al
2
O
3
that is present in the steel after the steel has been hot rolled or hot forged will form AlN precipitation nuclei, increasing the amount of AlN precipitation, so in this case, too, the Ti and O contents have to be minimized.
5. Even if carbonitrides are controlled as described, any admixture of bainitic structure in the steel after hot rolling will promote grain coarsening during carburization heating.
6. Moreover, grai
Kubota Manabu
Ochi Tatsuro
Combs Morillo Janelle
Nippon Steel Corporation
Wyszomierski George
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