JPH0713293B2 - Carbonitriding method for chromium-containing steel members - Google Patents

Description

TECHNICAL FIELD The present invention relates to surface treatment of a chromium-containing steel member, and in particular,
The present invention relates to a method of hardening a surface by carburizing and nitriding a chromium-containing steel member.

(Prior Art) It has been conventionally known that a steel member is carburized and then hardened to harden the surface. However, when carburizing, alloy elements such as Cr, Mn, and Si in the steel members combine with a small amount of oxygen in the atmosphere to form oxides, and the generated oxides have hardenability. A bad abnormal structure is formed near the surface of the steel member, which causes a problem that the surface hardening of the steel member cannot be sufficiently promoted.
Carburizing in a vacuum can solve this problem, but it is not practical in terms of manufacturing cost.

Further, as a method for hardening the surface of a steel member, a nitriding treatment in which nitrogen is infiltrated into the member is known. This nitrogen has the effect of improving the hardenability and forming a stable retained austenite structure in the steel member, and this retained austenite structure gives a compressive stress to the steel surface by shot peening etc. Since it becomes a site, the surface of the member can be hardened by this, and as a result, the fatigue strength of the member can be increased. It is also known that this nitriding treatment is combined with a carburizing treatment, and the nitriding treatment is continuously performed after the carburizing treatment. The nitriding treatment can be effectively used for carbon steel having poor hardenability. However, in alloy steel, an abnormal structure having poor hardenability ( (Torstite) is formed, so that a sufficient fatigue strength increasing effect cannot be obtained, and as a result, the nitriding treatment is not often used for alloy steel at present.

The abnormal structure in the nitriding treatment increases according to the amount of nitride generated in the member. This nitride increases as the concentration of nitrogen in the nitriding treatment, the nitriding treatment temperature, and the treatment time increase, and tends to occur at a deeper position inside the member.

(Problems to be solved) As described above, the carburizing and nitriding treatments can be effectively utilized for the surface hardening of the steel member, but on the other hand, the cause of the abnormal structure with poor hardenability is generated. Also, conventionally, the cause of the occurrence of this abnormal structure has not been sufficiently clarified, and as a result, the surface hardening of the steel member cannot be sufficiently performed by carburizing and nitriding treatments, and therefore, There is a problem that desired fatigue strength cannot be obtained.
Further, in order to make up for such deterioration of the hardenability of the steel member, it is possible to use a quenching agent with good quenching performance, but quenching is performed using a quenching agent with good quenching performance. And the thermal deformation of steel member products is large, which causes a problem in terms of finishing accuracy.

(Means for Solving the Problems) The present invention is configured based on the finding that the occurrence of an abnormal structure having poor hardenability is caused by the nitride formed in the alloy steel member. An object of the present invention is to provide a carbonitriding treatment method capable of imparting a desired fatigue strength to alloy steels, particularly chromium-containing steel members, by suppressing the generation of an abnormal structure in the nitriding treatment as much as possible. The method for carbonitriding a chromium-containing steel member of the present invention is a carburizing treatment for a chromium-containing steel member, and a depth of half or less of a generation depth of chromium oxide generated in the chromium-containing steel member during the carburizing treatment step. Nitrogen content is 0.1% by weight or more in the deep portion, and nitriding treatment is performed so that the nitrogen content is 0.1% by weight or less in the depth portion that is more than the formation depth of chromium oxide, and then quenching treatment. It is characterized by doing. According to the present invention, the chromium-containing steel member is preferably at a temperature of about 900 ° C to about 930 ° C for about 2 hours to about.
Carburizing is performed for about 4 hours. In the carburizing treatment of the present invention, it is not always necessary to keep the temperature constant. Further, as the chromium-containing steel member to which the present invention can be applied,
Examples include SCM420 and SCr420. By the above carburizing treatment, chromium oxide is generated from the surface of the steel member to a depth of about 0.07 mm. The nitriding treatment of the present invention is performed after the above carburizing treatment. In this case, the nitriding temperature may be obtained by reheating after the carburizing treatment, or the nitriding temperature can be secured by utilizing the carburizing temperature. The nitriding treatment temperature thus obtained is about 800 ° C to about 850 ° C.
Is appropriate. The nitriding time in this case is about 10 minutes to about 20 minutes.
Minutes are preferable, and the higher the processing temperature, the shorter the processing time.
Further, nitrogen for nitriding can be added from NH ₃ gas, and RX gas can be used as a carrier gas. RX gas is, for example, CO 24%, H ₂ 3.0%, CO ₂₀ .
It has a composition such as 2%, CH ₄ 0.04%, H ₂ O 0.4% and the balance N ₂ . The proportion of nitrogen in the nitrogen-containing atmosphere gas, that is, the nitrogen potential is preferably about 0.3% to about 0.4%.
The higher the nitrogen potential and the longer the treatment time, the deeper the nitrogen penetration depth, that is, the effective nitriding depth, and therefore the higher the nitrogen potential, the shorter the nitriding treatment time. The nitrogen content in the chromium-containing steel member obtained by the nitriding treatment of the present invention is 0.1% by weight or more in the depth of half or more in the formation depth of chromium oxide generated during the carburizing treatment step. preferable. When the nitrogen content exceeds 0.1% by weight inside the above-mentioned formation depth, more chromium nitride is formed inside the chromium oxide formation region, and as a result, the occurrence of abnormal structures such as torstite becomes remarkable. Because. In addition, 0.1% by weight at half the depth of chromium oxide formation
If it is less than the above range, the nitrogen content is too small, and sufficient curing performance cannot be obtained.

(Effect of the present invention) According to the present invention, the nitriding treatment is carried out so that the desired nitrogen content is within the production range of chromium oxide inevitably generated in the carburizing treatment step. In the chromium oxide generation region, the amount of solid solution chromium is smaller than in other regions, so the amount of chromium nitride generated is small, and therefore formation of abnormal structure can be suppressed. Further, in the region deeper than the chromium oxide generation region, the nitriding is suppressed as much as possible, and thus similarly, the generation of chromium nitride is small and the formation of the abnormal structure can be suppressed. As a result, according to the present invention, it is possible to form a stable retained austenite structure through quenching while effectively suppressing the occurrence of an abnormal structure, and improve the hardenability due to the presence of solute nitrogen. According to the present invention, the carbonitrided chromium-containing steel member thus obtained can secure sufficient fatigue strength through subsequent treatments such as quenching and shot peening.

(Description of Examples) Examples of the present invention will be described below.

Example 1 Chromium-containing steel member used as a secondary shaft gear
SCM420 was carbonitrided under the following conditions. The above SCM4
The composition of 20 is C0.21% by weight, Si0.27% by weight, Mn0.81% by weight, P0.016% by weight, S0.014% by weight, Cr1.02% by weight, Mo0.
It was 15% by weight and the balance Fe.

Treatment conditions Carburizing treatment Carburizing treatment temperature 900 ° C Carburizing treatment time 4 hours Nitriding treatment After the carburizing treatment, the temperature was lowered to perform the carburizing treatment.

Treatment conditions Nitriding temperature 830 ° C Nitriding time 15 minutes Nitrogen-added gas NH ₃ Carrier gas RX gas Nitrogen potential 0.4% Comparative example 1 Carburizing the same secondary gear parts of SCM420 as in Example 1 under the same conditions as in Example 1. I went.

Nitriding treatment Similar to Example 1, after the carburizing treatment, the temperature was lowered to carry out the nitriding treatment.

Treatment conditions Nitriding temperature 830 ° C Nitriding time 3 minutes Nitrogen-added gas NH ₃ Carrier gas RX Nitrogen potential 0.4% Comparative example 2 Secondary gear parts having the same composition as in Example 1 were carburized under the same conditions as in Example 1. It was

Nitriding treatment After the carburizing treatment, the temperature was lowered to perform the nitriding treatment.

Treatment conditions Nitriding temperature 830 ° C Nitriding time 75 minutes Nitrogen added gas NH ₃ Carrier gas RX gas Nitrogen potential 0.4% Example 2 Chromium-containing member SC for manufacturing main drive gear
Carbonitriding treatment was performed on r420. The composition of SCr420 is as follows: C0.22% by weight, Si0.24% by weight, Mn0.82% by weight, P
The content was 0.019% by weight, S0.013% by weight, Cr1.03% by weight, Mo0.03% by weight, and the balance Fe.

Carburizing treatment Conditions Carburizing temperature 930 ° C Carburizing time 2 hours Nitriding After carburizing, the temperature was lowered and nitriding was performed.

Treatment conditions Nitriding temperature 810 ° C Nitriding time 20 minutes Nitrogen-added gas NH ₃ Carrier gas RX gas Nitrogen potential 0.3% Comparative example 3 Carburizing treatment on the same parts as in Example 2 under the same conditions as in Example 2 SCr420. went.

Nitriding treatment After the carburizing treatment, the temperature was lowered to perform the nitriding treatment.

Treatment condition Nitriding temperature 760 ° C Nitriding time 20 minutes Nitrogen-added gas NH ₃ Carrier gas RX gas Nitrogen potential 0.3% by volume Comparative Example 4 Carburizing treatment of the same chromium-containing steel part SCr420 as in Example 2 under the same conditions as in Example 2 I went.

Nitriding treatment After the carburizing treatment, the temperature was lowered to perform the nitriding treatment.

Treatment conditions Nitriding temperature 870 ° C Nitriding time 20 minutes Nitrogen-added gas NH ₃ Carrier gas RX gas Nitrogen potential 0.3% by volume Example 3 Carbide treatment of the same chromium-containing steel part SCr420 as in Example 2 under the same conditions as in Example 2. I went.

Nitriding treatment After the carburizing treatment, the temperature was lowered and the treatment temperature was 800 ° C., the treatment time was 10 minutes, and the other conditions were the same as those in Example 2 for the nitriding treatment.

Comparative Example 5 The same chromium-containing steel part SCr420 as in Example 3 was subjected to the nitriding treatment under the same conditions as in Example 3 except that the nitriding treatment time was 3 minutes.

Comparative Example 6 The same chromium-containing steel part SCr420 as in Example 3 was subjected to nitriding treatment under the same conditions as in Example 3 except that the nitriding time was set to 60 minutes.

Example 4 The same chromium-containing steel part SCr420 as in Example 3 was carbonitrided under the same conditions as in Example 3, except that the nitriding temperature was 850 ° C. and the treatment time was 10 minutes.

Comparative Example 7 Carbonitriding was performed on the same chromium-containing steel part SCr420 as in Example 4 under the same conditions as in Example 4, except that the nitriding time was 2 minutes.

Comparative Example 8 The same chromium-containing steel part SCr420 as in Example 4 was carbonitrided under the same conditions as in Example 4, except that the nitriding time was 40 minutes.

Example 5 The same chromium-containing steel part SCr420 as in Example 4 was carbonitrided under the same conditions as in Example 4, except that the nitriding temperature was 830 ° C. and the nitriding time was 5 minutes.

Comparative Example 9 The same chromium-containing steel part SCr420 as in Example 5 was subjected to carbonitriding under the same conditions as in Example 5, except that the nitriding temperature was 770 ° C.

Comparative Example 10 The same chromium-containing steel part SCr420 as in Example 5 was carbonitrided under the same conditions as in Example 5, except that the nitriding temperature was 890 ° C.

The nitrogen distribution of the chromium-containing steel parts obtained by the carbonitriding treatment of Examples 1 to 5 and Comparative Examples 1 to 10 described above was measured to be 5%.
The cross-sectional structure was observed after performing a weak etching process using nitric acid alcohol. The results are shown in Table 1. In addition, in Example 1, Comparative Example 1, and this, the thickness of the chromium oxide layer on the surface generated through the carburizing treatment is 0.07 mm, and
5, Comparative Examples 3 to 10 have a chromium oxide layer thickness of 0.08 mm
Met.

Further, the distribution state of nitrogen was examined for the parts obtained by the treatments of Example 1 and Comparative Examples 1 and 2. The results are shown in FIG.

Further, FIG. 2 shows a cross-sectional photograph when the parts obtained by the treatments of Example 1, Comparative Example 1 and Comparative Example 2 were weakly etched using the above 5% nitric acid alcohol.

As is clear from Table 1, in Examples 1 to 5, the nitrogen concentration becomes 0.1% by weight in the vicinity of the maximum depth of the chromium oxide generation region of the component. In Comparative Examples 1, 3, 5, 7, and 9, the nitrogen concentration was lowered to 0.1% by weight or less at a relatively shallow position (below the midpoint of the chromium oxide layer) in the chromium oxide generation region. On the contrary, in Comparative Examples 2, 4, 6, 8 and 10, the nitrogen concentration is 0.1% by weight at a depth where the nitrogen concentration exceeds the maximum depth of the chromium oxide layer. The results shown in FIG. 1 show changes in the nitrogen concentration in the component when the nitriding time was changed. Further, in FIG. 1, the change in the amount of acid-soluble chromium is shown in relation to the depth, but it can be considered that the amount of acid-soluble chromium corresponds to the amount of solid-soluble chromium, and chromium oxide is formed. The amount of acid-soluble chromium decreases in the region where the acid-soluble chromium exists. Therefore,
In FIG. 1, the region where the amount of acid-soluble chromium changes is the region where chromium oxide is generated. From this result, the maximum depth of chromium oxide formation is 0.07mm from the surface.
It can be seen that it is. On the other hand, the nitrogen concentration in Example 1 was about 0.1 near the maximum depth of formation of the chromium oxide.
%, But in the case of Comparative Example 1, the amount was much less than 0.1% by weight before reaching the maximum oxide formation depth, and in the case of Comparative Example 2, conversely, chromium oxide was used. There is a nitrogen concentration far exceeding 0.1% by weight in the region beyond the production range of. In Table 1, as a result of observing the cross-sectional structure after the etching treatment, in Comparative Examples 1, 3, 5, 7, and 9, it was observed that a layered abnormal structure was generated near the surface. In addition, Comparative Examples 2, 4, 6, 8 and 10
It was observed that a net-like abnormal tissue occurred in a deep part of the part. On the other hand, Examples 1 to
In the case of No. 5, abnormal tissue was hardly generated, but even if it was generated, it was only slightly observed in an extremely shallow region. This is also clear from the cross-sectional photograph of FIG. 2. In Comparative Example 1, many black streaks are present near the surface, indicating the occurrence of abnormal tissue. In addition, Comparative Example 3
On the other hand, innumerable streaks are formed from the surface to a considerably deep part, and the occurrence of abnormal tissue is observed. On the other hand, in Example 1, the above-mentioned streaks are hardly seen, and it is understood that the abnormal tissue is hardly generated. FIG. 3 shows the above observation results in the relationship between the nitriding treatment time and the nitriding treatment temperature. Third
In the figure, the circles show the cases where the treatment was carried out at a nitrogen potential of 0.3% by volume, and the triangles show the cases where the treatment was carried out at a nitrogen potential of 0.4% by volume. Further, those with all black marks are those in which a net-like abnormal tissue as shown in Comparative Example 2 in FIG. 2 was observed, and those with white marks are the layered forms as shown in Comparative Example 1 in FIG. The abnormal tissue was observed. The ones marked with semi-black color are those treated in Examples 1 to 5 above, and the occurrence of abnormal tissues is hardly seen. From the results shown in FIG. 3, it is clear that it is desirable to select the nitriding treatment time and the nitriding treatment temperature from the shaded area in FIG.

Next, the chromium-containing steel parts obtained by the treatments of Examples 1 to 5 and Comparative Examples 1 to 10 were quenched and tested for fatigue strength of the parts. In this test, gear specifications were manufactured as a module: 2.25, number of teeth: 17, tooth width; 22 mm of main drive gear or gear specifications were manufactured as a module: 2.0, number of teeth: 21, secondary shaft gear with tooth width: 26 mm. The above parts were rotated under a given torque load, and the failure cycle was investigated. The results are shown in Table 2.

From these results, it can be seen that Examples 1 to 5 have excellent fatigue strength in comparison with Comparative Examples 1 to 10.

[Brief description of drawings]

FIG. 1 is a graph showing a distribution state of nitrogen content in a chromium-containing steel member in the case of performing nitriding treatment, and FIG. 2 is a photograph showing a metal structure of a cross section of the chromium-containing steel member at a magnification of 400 times, FIG. 3 is a graph showing the relationship between the nitriding treatment time and the treatment temperature.

Claims (1)

[Claims] 1. A chromium-containing steel member is carburized, and a nitrogen content in a depth portion of half or less of a generation depth of chromium oxide generated in the chromium-containing steel member during the carburizing treatment step is 0.
Chromium which is 1% by weight or more and which is subjected to nitriding treatment so that the nitrogen content is 0.1% by weight or less in a depth portion which is equal to or more than the formation depth of chromium oxide, and is then quenched. Carbonitriding method for steel containing material.