JPH0567698B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an alloy steel for mechanical structures, particularly a carburized alloy, which is suitable for use in drive system transmission parts such as gears in mechanical devices such as automobiles and industrial machines. Regarding steel. [Prior art] There are two disadvantages of the conventional gas carburizing method: (i) abnormal carburization layer occurs, and (ii) carburization takes a long time.
There are several points. In order to solve these shortcomings,
Vacuum carburizing method appeared. Since vacuum carburizing is carried out at a high carburizing temperature as shown in FIG. 2, for example, carburizing can be carried out in an extremely short time. However, if the steel used for vacuum carburizing is a low-alloy case-hardened steel (JIS G 4052) with the composition shown in Table 1, which is conventionally known as carburizing steel, crystal grains will be removed during vacuum carburizing. Since the grains become coarse, it is common to perform a reburning treatment to refine the grains after the vacuum carburizing treatment. In addition, recently, a low-alloy duplex steel has been reported that does not require the reburning treatment for grain refinement after vacuum carburizing treatment, has excellent cold forgeability, and can omit intermediate annealing treatment. has been done. Regarding such low-alloy duplex steels, see Tetsu to Hagane, Vol. 66 S117,
It is described in Vol. 70 S1288, Vol. 71 S711, Vol. 71 S1333, etc. As an example, Tetsu to Hagane, Vol. 71
The composition of the low alloy duplex steel described in S1333 is shown in Table 2. In addition, the amount of each element in Tables 1 and 2 is weight%.
is shown. Moreover, % used in this specification indicates weight %. [Problems to be Solved by the Invention] When general low-alloy case hardened steel is subjected to vacuum carburizing treatment, the austenite crystal grains become coarse due to the high temperature of the treatment, and reburning treatment is required to refine the grains. is required, and the expected reduction in processing time cannot be achieved. In addition, the conventional low-alloy duplex steel that does not require reburning treatment has a large amount of 2% Si added, so
In normal gas carburizing treatment, internal oxidation becomes noticeable.
To prevent this, vacuum carburizing is essential.
Gas carburizing treatment is not possible. In addition, since low-alloy duplex steel has two phases, ferrite and martensite, in the core, if the strength of the ferrite is low or the area ratio of each phase is not appropriate,
Materials whose fatigue strength is currently used, e.g. JIS
It is inferior to SCr 420 gas carburized material. [Means for Solving the Problems] In order to solve the problems of the above-mentioned prior art, the duplex steel for high-temperature gas carburizing of the present invention contains C, Mo and/or V of more than 0.05% and less than 0.15%, and 1.0 % exceeds 5.0%
Less than Mo, more than 1.0% and less than 2.0, Nb more than 0.01% and less than 0.05%, and more than 0.005% and less than 0.015%
The content of Si is 0.10% or less, the amount of Mn is 0.70% or less, the amount of P is less than 0.010%, and the balance is substantially Fe. When the C content is less than 0.05%, the ferrite area ratio increases and fatigue strength decreases, and when it is more than 0.15%, the two-phase region is narrowed and cold workability deteriorates, both of which are unfavorable. If the Mo amount and V amount are outside the above range, the C amount should be 0.05% to 0.05% as above.
When it is 0.15% and the target carburizing temperature is 1000 to 1100°C, there is a tendency that a two-phase structure is not formed, which is not preferable. When the amount of Nb is outside the above range, the structure of the carburized layer tends to become coarse, and when it is above the above range, the area ratio of ferrite increases, both of which are unfavorable. If the amount of Si and Mn is above the above, there will be a tendency to produce grain boundary oxides during gas carburizing treatment, and if the amount of P is above the above, there will be a tendency of grain boundary embrittlement, both of which are unfavorable. If the amount of S is less than 0.005%, the gear cutting workability will be greatly reduced, and if it is more than 0.015%, the mechanical properties of the cross-grain will deteriorate, both of which are unfavorable. [Function] Conventionally, a large amount of Si was added to create a two-phase structure in the core, which caused internal oxidation during carburization. In the present invention, the amount of Si is kept as low as possible, and Mo and/or V, which are less susceptible to internal oxidation, are added as elements for forming a two-phase structure. During gas carburizing, Mo, V, Nb,
W (Be, As, Sb, P, etc. have similar functions, but
However, considering the cost, Mo and V are preferable. Furthermore, Mo,
V is a ferrite reinforcing element and can prevent deterioration of fatigue strength. Furthermore, the amounts of Si and Mn added are kept to the minimum necessary for deoxidation so that grain boundary oxides are difficult to form even when gas carburizing is performed. Cr improves hardenability, but at the same time it is an element that tends to cause internal oxidation, so it is not added to prevent internal oxidation. A small amount of S is added to improve workability. Coarsening of crystal grains in the core is suppressed by the two-phase structure. However, since the carburized layer has a single phase of martensite, the effect of suppressing coarsening due to the structure cannot be expected. Therefore
Nb is added to suppress coarsening due to the pinning effect. The composition of the duplex steel for high-temperature gas carburizing of the present invention is selected to have the above-mentioned effects, so internal oxidation is difficult to occur even when high-temperature gas carburizing is performed, and it also has excellent fatigue strength. . [Example] Composition shown in Table 3 (the amount of each element is expressed as weight %)
A test piece was cut out of low-alloy steel, subjected to gas carburization treatment, and then subjected to a rotating bending fatigue test. The conditions for gas carburizing treatment are as follows: (i) For Invention Steel 1 and Comparative Steel 3, gas carburize at 1000°C, furnace cooling to 950°C, oil quenching, and further heating to 180°C and air cooling. , (ii) For comparative steel 4, after gas carburizing at 925℃,
Furnace cooled to 850℃, then oil quenched, then further heated to 180℃
There were two types: one was heated to a temperature of Figure 1 shows the invention steel 1, comparative steel 3, and
4 is an S-N curve of a rotary bending fatigue test of a material obtained by carburizing No. 4 as described above. As is clear from FIG. 1, the duplex steel for high temperature gas carburizing of the present invention has excellent fatigue properties. FIG. 3 is a micrograph showing the metallographic structures of the invention steel 1 and comparative steel 3 which were carburized. invention steel 1
It can be seen from this photo that the metal structure of Steel 3 is superior to that of Comparative Steel 3. Invention Steel 2 also has substantially the same fatigue properties and metal structure as Invention Steel 1. [Effect of the invention] The duplex steel for high-temperature gas carburizing of the present invention has a temperature of 1000°C to
Even when gas carburizing is performed at a high temperature of 1100°C, the crystal grains do not become coarse and internal oxidation does not occur. Furthermore, it has a two-phase structure with a stable amount of ferrite, and the ferrite itself is reinforced with Mo and V, so it has high fatigue strength.

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【table】

【table】 [Brief explanation of the drawing]

FIG. 1 shows an S-N curve showing the results of a rotary bending fatigue test of carburized steel, FIG. 2 shows a heat treatment cycle in vacuum carburization, and FIG. 3 shows a microscopic structure of gas carburized steel.

Claims (1)

[Claims] 1. C, Mo and/or V of more than 0.05% and less than 0.15%, Mo of more than 1.0% and less than 5.0%, V of more than 1.0% and less than 2.0, more than 0.01% and less than 0.05% Nb,
and contains more than 0.005% and less than 0.015% S, and
Si content is 0.10% or less, Mn content is 0.70% or less, P content is
A duplex steel for high-temperature gas carburizing, characterized by less than 0.010% Fe, with the remainder being substantially Fe. 2 The above Mo and/or V is 2.5% or more 5.0
The duplex steel for hot gas carburizing according to claim 1, characterized in that the content of Mo is less than %. 3 The above Mo and/or V is 1.5% or more2.0
The duplex steel for hot gas carburizing according to claim 1, characterized in that the V content is less than %.