JPH0465893B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention has almost no carburized abnormal structure on the surface even when carburized and quenched, and can be used in mechanical structural parts to achieve high fatigue strength, rolling fatigue strength, and wear resistance. The present invention relates to case hardening steel that can be case hardened. [Prior Art] Conventionally, mechanical structural parts have been widely subjected to surface hardening treatment, particularly carburizing and quenching, which is particularly effective, in order to improve their fatigue strength, rolling contact fatigue strength, and wear resistance. . However, when conventional case hardening steel (e.g. JIS-SCr420, SCM420) is carburized and quenched, grain boundary oxidation with a depth of 10 to 20 μm occurs on the surface of the carburized part, accompanied by an incompletely quenched structure ( troostite) is formed and the surface hardness decreases. The crystal boundary oxidation on the surface of the carburized portion and the incompletely quenched structure are collectively referred to as the abnormal carburized structure. This abnormal structure is caused by O in the oxidizing gases (CO ₂ , H ₂ O) in the carburizing atmosphere preferentially penetrating and diffusing into the austenite grain boundaries of the steel during carburizing, and is more concentrated than Fe such as Si, Mn, and Cr. In addition to forming oxides by combining with elements that are easily oxidized, these oxides reduce the concentrations of solid solution Si, solid solution Mn, and solid solution Cr near the grain boundaries, and as a result of these factors, hardenability decreases. As a result of the extreme decline,
This is thought to be caused by the formation of an incompletely hardened structure on the surface. Then, when grain boundary oxides are formed in the surface layer,
Since these act as notches, fatigue strength and impact resistance deteriorate, and a soft incompletely quenched layer is generated along with the formation of grain boundary oxides, resulting in fatigue strength, rolling contact fatigue strength, Abrasion resistance also deteriorates. However, in recent years, demands on the durability of mechanical structural parts used in industrial machinery, transportation machinery, construction machinery, etc. have become increasingly sophisticated.In particular, high fatigue strength and high High rolling fatigue strength is desired, and high wear resistance is desired for sliding members. For example, in the case of automobile transmission gears, tooth root fatigue strength and tooth surface rolling fatigue strength have improved as engines become more powerful, and hydraulic pistons and cylinders in plastic molding machines, rock drills, etc. As machines become larger and sliding cycles become faster, there is a strong desire to improve the wear resistance of the surfaces of sliding parts. For this reason, case-hardening steels have been developed for the purpose of preventing the formation of abnormal carburized structures and improving fatigue strength, impact resistance, or wear resistance.
No. 60-21359, JP-A No. 60-21359, JP-A-60-21359
It has been proposed in Publication No. 243252, etc. [Problems to be solved by the invention] However, all of these proposals attempt to solve the problem only by regulating the amount of Si, Mn, and Cr added, which promote grain boundary oxidation. Therefore, it is difficult to completely suppress the carburized abnormal structure. Therefore, it has been difficult to completely meet recent high demands for mechanical structural parts. In addition, introducing compressive residual stress into the surface layer improves fatigue strength and rolling contact fatigue strength, so shot peening has traditionally been performed after carburizing and quenching, but incompletely quenched structures still exist. As a result, the shot peening caused extreme roughness (surface unevenness), which could even reduce fatigue strength, so it could not be said to be an effective countermeasure. In view of this situation, the present invention provides a case hardening steel that exhibits extremely high resistance to the formation of abnormal carburized structures. [Means for Solving the Problems] The present inventors have determined that in order to suppress the formation of abnormal carburized structures, a comprehensive approach should be taken from a wide range of components, not limited to Si, Mn, and Cr, as well as grain boundary oxidation. We believed that it was necessary to take measures from both sides of the incompletely quenched structure, and as a result of extensive experimental research, we came to the following knowledge. It was found that grain boundary oxidation can be greatly suppressed by adding ΓCu. This occurs in the surface layer during carburizing.
This is thought to be because Cu is concentrated and the outermost surface is covered with an extremely thin Cu compound, which prevents O from entering the grain boundaries. Γ However, the effect of suppressing grain boundary oxidation by Cu is limited to elements with lower oxide formation free energy than Fe, in other words, elements that are more easily oxidized than Fe (as shown in Figure 1, for example, Si, Mn, Cr, Al, etc.)
If these elements are added too much, grain boundary oxidation cannot be suppressed. From this point of view, it is best to avoid containing Al, which is extremely easy to oxidize, as much as possible, and even if it is included, it is solvable.
Less than 0.05% for Al, then less than 0.15% for easily oxidized Si, less than 1.5% for Cr, and Mn
It is necessary to limit it to 1.2% or less. Regarding ΓCu, there is also Cu under the Cu compound coating.
Even if the amount of solute Si, solute Mn, and solute Cr decreases due to grain boundary oxidation, an incompletely quenched structure will not form in the surface layer due to the hardenability improvement effect of Cu. It turned out to be difficult. ΓSi, Mn, and Cr are also elements that improve hardenability, so if the amounts of these elements are restricted as described above, not only will the hardenability of case hardening steel not be ensured, but the type of quenching refrigerant will In some cases, a boiling film is formed on the steel surface during quenching, which slows down the cooling rate at the outermost surface, resulting in incompletely quenched structures on the surface layer even if the inside is sufficiently quenched. As a result of further investigation, it became clear that Ni and Mo do not promote grain boundary oxidation and can prevent the formation of incompletely hardened layers. this is,
As shown in FIG. 1, this is thought to be because Ni and Mo have a higher free energy of oxide formation than Fe, that is, they are elements that are more difficult to oxidize than Fe and improve hardenability. When the hardenability of the base material is insufficient, it is also effective to add B. Γ It has also been revealed that Nb and V work effectively against coarsening of austate grains due to regulation of Al content. The present invention has been made based on the above findings, and its gist is a case hardening steel based on the following component composition, with or without additional component systems added thereto as necessary. C: 0.1 to 0.4%, Si: less than 0.15% by weight,
Mn: 1.2% or less, Cr: 1.5% or less, Sol.Al:
Contains less than 0.05%, Cu: 0.02 to less than 0.5%,
Remaining Fe and unavoidable impurities. Weight ratio: Ni: 5.0% or less, Mo: 2.5% or less,
B: One or more of 0.005% or less. One or two types of Nb: 0.15% or less and V: 0.15% or less in weight ratio Next, the reason for limiting the composition range of the steel of the present invention will be described. ΓC: 0.1 to 0.4% C is a basic component necessary to ensure strength as a mechanical structural part, and as a case hardening steel, the hardness of the core (non-carburized part) after carburizing and quenching and tempering is at least H _R C25 is necessary, and for this purpose, the amount of C must be 0.1% or more. However, if it is added in excess of 0.4%, the difference in hardness between the case (carburized part) and the core becomes small, making it difficult to introduce compressive residual stress that is effective in improving fatigue strength and rolling contact fatigue strength. The upper limit was set at 0.4% because it deteriorates the hardness and machinability. ΓSi: less than 0.15% Si is an element that is much more easily oxidized than Fe, and has the effect of significantly promoting the formation of grain boundary oxides in the carburized surface layer. For this reason, less is better, but if it is less than 0.15%, grain boundary oxidation will be negligible, so the upper limit should be set at 0.15%.
(0.15% is not included). The lower limit is not particularly stipulated because the lower the Si content, the more grain boundary oxidation can be suppressed, but it is preferable because it is difficult to reduce the Si content to less than 0.05% in terms of steel manufacturing.
It is 0.05%. ΓMn: 1.2% or less Mn is also an element that is more easily oxidized than Fe, although not as much as Si, and therefore promotes grain boundary oxidation.
For this reason, the smaller the amount of Mn added, the better;
When adding 1.2% or less, grain boundary oxides will hardly be generated. For this reason, Mn
The upper limit was set at 1.2%. The lower limit is not particularly defined because the smaller the amount of Mn, the more grain boundary oxidation can be suppressed, but in consideration of ensuring the hardenability of the core part, 0.3% or more is preferable. ΓCr: 1.5% or less Cr is also an element that tends to generate grain boundary oxides,
Therefore, the smaller the amount of Cr added, the more preferable. However, when adding Cu, if the content is 1.5% or less, grain boundary oxides will hardly be generated. For this reason, the upper limit of Cr was set at 1.5%. The lower limit is not particularly specified because the less Cr is, the more grain boundary oxidation can be suppressed, but if it is less than 0.5%, the carburizability will decrease, so 0.5% or more is preferable. ΓSol.Al: Less than 0.05% Al is much more easily oxidized than Si, Mn, and Cr and can cause grain boundary oxidation, so the smaller the better, but when adding Cu, 0.05%
If it is less than %, grain boundary oxides will hardly be generated. The lower limit is not particularly defined because the lower the Al content, the more effective it is in preventing grain boundary oxidation. ΓCu: 0.02 to less than 0.5% As already mentioned, by adding Cu,
An extremely thin Cu compound layer is formed on the outermost surface, which prevents grain boundary oxidation. In addition, a Cu-enriched layer is formed under the Cu compound layer, and even if grain boundary oxidation occurs and the hardenability of the surface layer decreases, an incompletely hardenable structure is formed due to the hardenability-improving effect of Cu. It becomes difficult to do. At least 0.02% Cu is required to fully express these effects, but
Adding more than 0.5% will increase the thickness of the Cu compound layer, sharply reducing carburizability and forming an incompletely quenched structure, so the lower limit should be set.
0.02%, with an upper limit of less than 0.5%. ΓNi: 5.0% or less In order to prevent grain boundary oxidation, the steel of the present invention
The amount of Si, Mn, and Cr, which are more easily oxidized, is limited. Since these elements are all elements that improve hardenability, the hardenability of the core portion as a case hardening steel cannot be sufficiently ensured. Furthermore, since the hardenability of the case is insufficient, incompletely hardened structures are likely to be formed in the surface layer. Therefore,
Addition of Ni, which does not cause grain boundary oxidation and improves hardenability, is effective in preventing abnormal carburized structures (grain boundary oxidation, incompletely hardened layer). Moreover, this Ni also has the effect of improving the deterioration of hot workability caused by the addition of Cu. However, if Ni is added in an amount exceeding 5.0%, carburizing properties are inhibited and machinability is significantly deteriorated, so when Ni is added, the upper limit is set to 5.0%. Regarding the lower limit, in order to prevent deterioration of hot workability due to the addition of Cu, it is preferable to set the amount to be equal to or higher than the amount of Cu added. ΓMo: 2.5% or less Like Ni, Mo is an element that does not cause grain boundary oxidation and improves hardenability, so it not only ensures hardenability in the core but also improves the incompletely hardened structure in the surface layer. It is useful for preventing. However, when added in excess of 2.5%, the carburized case part
Since Mo carbides precipitate and the hardenability of the surface layer deteriorates, the upper limit when adding Mo is 2.5%. The lower limit is preferably 0.1% or more in consideration of ensuring the hardenability of the core portion. ΓB: 0.005% or less B is an element that significantly improves hardenability when added in a small amount, but the effect becomes weaker as the amount of C increases. Therefore, it is very effective in improving only the hardenability of the core portion. However, B
It is also an element that is more easily oxidized than Fe, and when added in excess of 0.005%, grain boundary oxidation begins to occur in the surface layer. Therefore, if added, the upper limit should be 0.005
%. The lower limit is 0.0003, where the hardenability improvement effect becomes noticeable.
It is preferable that it exceeds %. ΓNb: 0.01-0.1%, V: 0.01-0.1% Usually, Al combines with N to form fine AlN, which has the effect of preventing coarsening of austenite grains during carburization. However, in order to suppress grain boundary oxidation in the steel of the present invention, Al is not added or the amount added is limited to less than 0.05%, so depending on the history before carburizing and carburizing conditions, austenite grains may become coarse. There are cases where In order to prevent this, it is effective to add Nb and V to finely precipitate NbC and VC. In order to fully demonstrate this effect, both Nb and V must be 0.01.
It is necessary to add more than %. However, Nb, V
Both are more easily oxidized than Fe, and when added in excess of 0.01%, grain boundary oxides begin to form. Therefore, the lower limit was set to 0.01% and the upper limit was set to 0.1%. [Example] Next, the present invention will be explained with reference to an example. 42 types of steel having the chemical composition shown in Table 1 are melted in a high-frequency vacuum furnace and hot-forged to a diameter of 40 mm.
A round bar and a square bar of 35 mm x 35 mm were formed, and the bending fatigue test piece shown in Figure 2 was made from the square bar, and the disc-shaped rolling fatigue test piece shown in Figure 3 was made from the round bar. did. These test pieces were first held at 930°C for 2 hours in a carburizing atmosphere with a carbon potential of 0.8%, and then quenched with hot quench oil at 200°C. After carburizing and quenching, the cross section of the bending fatigue test piece was polished,
Thereafter, the grain boundary oxidation depth was measured without corrosion, and the depth of the incompletely hardened layer due to nital corrosion was measured by optical microscopic observation. Measurements were performed at 20 locations for each, and the average value was taken as the measured value. In addition, bending fatigue tests and rolling fatigue tests were conducted. In the bending fatigue test, as shown in Figure 4, compressive stress is repeatedly applied to the center of the specimen up to ¹⁰ times with both ends supported, and the relationship between the compressive stress and the number of repetitions is plotted as an S-N curve. The fatigue limit was determined from the results. As for the rolling fatigue test pieces, as shown in Figure 5, the two-roller type rolling fatigue test (slip rate 0%) was repeated up to 10 ^{to 7} times on the same test pieces, and the relationship between the presser surface pressure and the number of repetitions was calculated. The relationship was expressed as an S-N curve to determine the durability limit. These test results are also listed in Table 1. Steel types No. 1 to No. 72 are steels according to the present invention. On the other hand, steel grades No. 73 to No. 75 are similar to steel grade No. 76 in terms of Si content.
- No. 78 is a comparative steel that is outside the scope of the present invention in terms of Mn content, steel types No. 79 to 81 in terms of Cr content, and steel types No. 82 to No. 85 in terms of Cu content. It is.

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〔Effect of the invention〕

As is clear from the above description, the case-hardened steel of the present invention has extremely little carburized abnormal structure, so it is used in severe applications such as automobile transmissions, plastic injection molding machines, and hydraulic pistons and cylinders of rock drilling machines. It can be used in mechanical structural parts used in various conditions to provide them with high fatigue strength, rolling fatigue strength, and wear resistance, and is highly effective in improving the durability of these mechanical structural parts. It is something.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between temperature and free energy of formation of metal oxides, Figures 2 and 3 are illustrations of the shape and dimensions of the test piece, and Figures 4 and 5 are illustrations of the test method. , and FIGS. 6 to 9 are graphs showing the influence of characteristic elements on the formation of abnormal carburized structures in the steel of the present invention.

Claims (1)

[Claims] 1. C: 0.1 to 0.4%, Si: less than 0.15% by weight,
Mn: 1.2% or less, Cr: 1.5% or less, Sol.Al: 0.05%
A case hardening steel characterized by containing less than 0.02 to less than 0.5% Cu, with the balance consisting of Fe and inevitable impurities. 2 C: 0.1 to 0.4%, Si: less than 0.15% by weight,
Mn: 1.2% or less, Cr: 1.5% or less, Sol.Al: 0.05%
Cu: less than 0.02 to less than 0.5%, and
Contains one or more of Ni: 5.0% or less, Mo: 2.5% or less, B: 0.005% or less, and the balance is Fe.
Case-hardened steel characterized by comprising: and unavoidable impurities. 3 C: 0.1 to 0.4%, Si: less than 0.15% by weight,
Mn: 1.2% or less, Cr: 1.5% or less, Sol.Al: 0.05%
Cu: less than 0.02 to less than 0.5%, and
Nb: 0.15% or less, V: 0.15% or less type 1 or 2
A case hardening steel characterized by containing seeds and the remainder consisting of Fe and inevitable impurities. 4 C: 0.1 to 0.4%, Si: less than 0.15% by weight,
Mn: 1.2% or less, Cr: 1.5% or less, Sol.Al: 0.05%
Cu: less than 0.02 to less than 0.5%, and
Contains one or more of Ni: 5.0% or less, Mo: 2.5% or less, B: 0.005% or less, and further
Nb: 0.15% or less, V: 0.15% or less type 1 or 2
A case hardening steel characterized by containing seeds and the remainder consisting of Fe and inevitable impurities.