WO2013121794A1

WO2013121794A1 - Soft-nitriding steel and soft-nitrided component using steel as material

Info

Publication number: WO2013121794A1
Application number: PCT/JP2013/000838
Authority: WO
Inventors: 岩本　隆; 佳祐安藤; 冨田　邦和; 大森　靖浩; 清史上井; 三田尾　眞司
Original assignee: Ｊｆｅ条鋼株式会社; Ｊｆｅスチール株式会社
Priority date: 2012-02-15
Filing date: 2013-02-15
Publication date: 2013-08-22
Also published as: EP2816128A4; JP5449626B1; CN104114733A; JPWO2013121794A1; WO2013121794A8; EP2816128B1; EP2816128A1; MY177826A; US20150020926A1; KR20140129081A

Abstract

The present invention can provide soft-nitriding steel which has a predefined component composition and, before soft nitriding, excellent machinability by having a structure in which the bainite area ratio prior to soft nitriding is greater than 50%, and after soft nitriding, strength/toughness equal to that of a carburized material of conventional steel, for example, SCr420 steel and provides more superior fatigue characteristics.

Description

Soft nitriding steel and soft nitriding parts made from this steel

The present invention relates to a soft nitriding steel and a soft nitriding part made of this steel, in particular, a steel for soft nitriding which has excellent fatigue characteristics after soft nitriding and is preferable for automobiles and construction machines, and a soft nitriding part made of this steel It is about.

Machine structural parts such as automobile gears are usually required to have excellent fatigue characteristics and are subjected to surface hardening treatment. Carburizing treatment, induction hardening treatment and nitriding treatment are well known as surface hardening treatments.

The carburizing treatment allows C to penetrate and diffuse in a high temperature austenite region, so that a deep hardening depth can be obtained and effective in improving fatigue strength.

However, since heat treatment distortion occurs, it has been difficult to apply to parts that require strict dimensional accuracy from the viewpoint of quietness.

The induction hardening process is a process in which the surface layer is quenched by induction heating, and is inferior in dimensional accuracy as in the carburizing process.

Nitriding is a treatment that increases the surface hardness by infiltrating and diffusing nitrogen in the temperature range below the Ac ₁ transformation point, but the treatment time is as long as 50 to 100 hours, and the brittle compound layer on the surface layer is removed after the treatment. There was a need to do.

For this reason, a soft nitriding treatment that develops nitriding in a short time at a treatment temperature similar to that of the nitriding treatment has been developed, and in recent years, it has been widely used for machine structural parts and the like. Soft nitriding is a process in which N and C are simultaneously infiltrated and diffused in the temperature range of 500 to 600 ° C. to harden the surface, and the processing time is less than half that of conventional nitriding. It is possible.

However, in the carburizing treatment, the core hardness can be increased by quench hardening, whereas in the nitrocarburizing treatment, the core hardness is not increased because the treatment is performed at a temperature below the transformation point of the steel. The nitrocarburized material is inferior in fatigue strength compared to the carburized material.

In order to increase the fatigue strength of the nitrocarburized material, the core hardness is usually increased by quenching and tempering before nitrocarburizing, but the obtained fatigue strength is hardly sufficient, Manufacturing costs increase and machinability also decreases.

In order to solve such problems, the steel component composition is a component composition containing Ni, Al, Cr, Ti, and the core portion is Ni—Al, Ni—Ti intermetallic compound or Cu during soft nitriding. It has been proposed to age harden with a compound and to precipitate and harden nitrides and carbides such as Cr, Al, and Ti in the nitride layer (Patent Document 1 and Patent Document 2).

In Patent Document 3, a steel containing 0.5 to 2% of Cu is forged by hot forging and then air-cooled to form a ferrite main structure in which Cu is solid-solved. Cu is further precipitated, and further, precipitation hardening of Ti, V, and Nb carbonitrides is used in combination to obtain a steel having excellent bending fatigue characteristics after soft nitriding. Patent Document 4 discloses a steel for soft nitriding in which Ti—Mo carbides and further carbides containing one or more of Nb, V, and W are dispersed.

JP-A-5-59488 JP-A-7-138701 JP 2002-69572 A JP 2010-163671 A

However, the nitrocarburized steels described in Patent Documents 1 and 2 are improved in bending fatigue strength due to precipitation hardening of Cu or the like, but it is difficult to say that the workability is sufficiently secured. Steel requires a relatively large amount of Cu, Ti, V, and Nb, and has a high production cost. The steel for soft nitriding described in Patent Document 4 has a problem of high production cost because it contains a relatively large amount of Ti and Mo.

Accordingly, the present invention provides a soft nitrided part that has a property of low hardness and excellent machinability before soft nitriding, and can increase the core hardness by soft nitriding and has excellent fatigue characteristics. An object of the present invention is to provide a soft nitriding steel that can be manufactured at a relatively low cost and a soft nitriding component made of this steel.

In order to solve the above problems, the inventors diligently studied the influence of the structure and composition on the fatigue properties of steel after soft nitriding. As a result, a steel layer containing a specific amount of V and Nb as a steel composition and having a bainite main structure as a pre-soft nitriding structure is subjected to soft nitriding treatment, and a surface layer that is soft nitrided using the temperature rise at that time It has been found that if the core hardness is increased by aging precipitation of fine precipitates in the core structure other than the part, excellent fatigue characteristics can be obtained after soft nitriding.

The present invention has been made on the basis of the above findings and further studied, and has the following features.
1. By mass%, C: 0.01% or more and less than 0.10%, Si: 1.0% or less, Mn: 0.5 to 3.0%, Cr: 0.30 to 3.0%, Mo: 0 0.005 to 0.4%, V: 0.02 to 0.5%, Nb: 0.003 to 0.15%, Al: 0.005 to 0.2%, S: 0.06% or less, P : Steel for soft nitriding comprising 0.02% or less and B: 0.0003 to 0.01%, balance: Fe and inevitable impurities, and having a structure with a bainite area ratio exceeding 50% before soft nitriding.
2. The steel for soft nitriding according to 1 above, wherein a precipitate containing V and Nb is dispersed and precipitated in the bainite phase after soft nitriding.
3. A nitrocarburized part made of the nitrocarburized steel according to 1 or 2 above.

According to the present invention, before soft nitriding, it has excellent machinability, and after nitrocarburizing, it has the same strength and toughness as a carburized material of conventional steel, for example, SCr420 steel, and further has excellent fatigue characteristics. A nitrocarburizing steel and a nitrocarburized component made of this steel can be obtained, which is extremely useful industrially.

It is the schematic which shows the manufacturing process for manufacturing a soft nitriding component using the steel for soft nitriding of this invention.

The microstructure, component composition and manufacturing conditions of the soft nitriding steel according to the present invention will be described below.

1. Microstructure The microstructure before soft nitriding is a bainite area ratio exceeding 50%, and a structure in which V and Nb precipitates are dispersed and precipitated in the bainite phase after soft nitriding. When the parent phase before soft nitriding is a bainite main structure with a bainite area ratio exceeding 50%, the formation of V and Nb precipitates in the parent phase is remarkably suppressed as compared with a ferrite-pearlite structure. As a result, it is possible to suppress the precipitation of V and Nb precipitates before soft nitriding and increase the hardness of the steel, and the machinability usually performed before soft nitriding is improved. Further, when soft nitriding is applied to this, the surface layer portion is nitrided, and at the same time, V and Nb precipitates are aged in the core bainite structure other than the surface nitrided portion, and the core hardness increases. To do. As a result, the fatigue strength and strength after soft nitriding are significantly improved.

In the present invention, the structure having a bainite area ratio exceeding 50% means that the area ratio of the bainite structure (phase) exceeds 50% in cross-sectional structure observation (observation of a 200-fold optical microscope structure). Preferably, the area ratio of the bainite structure exceeds 60%, and more preferably exceeds 80%. Moreover, it is preferable that fine precipitates having a particle size of less than 10 nm are dispersed as V and Nb precipitates precipitated in the bainite structure. Further, it is preferable that 500 or more V and Nb precipitates having a particle size of less than 10 nm are present per 1 μm ² for sufficient precipitation strengthening.

2. Component Composition The reasons for limiting the component composition in the soft nitriding steel of the present invention will be described. Hereinafter,% of the steel component is mass%.

C: 0.01% or more and less than 0.10% C is added to form a bainite structure and ensure strength. When the amount of C added is less than 0.01%, the amount of bainite produced decreases, and the amounts of V and Nb precipitates decrease, making it difficult to ensure strength. On the other hand, when 0.10% or more of C is added, the hardness of the bainite structure increases and the machinability deteriorates. Therefore, the amount of C added is in the range of 0.01% or more and less than 0.10%. More preferably, it is 0.03% or more and less than 0.10%.

Si: 1.0% or less Si is added because it is effective for deoxidation and bainite structure formation. When the Si addition amount exceeds 1.0%, the machinability and Deteriorates cold workability. Accordingly, the Si addition amount is set to 1.0% or less. More preferably, it is 0.5% or less. More preferably, it is 0.3% or less. In order to effectively contribute Si to deoxidation, the Si addition amount is preferably 0.01% or more.

Mn: 0.5 to 3.0%
Mn is added because it is effective for bainite structure formation and strength improvement. When the amount of Mn added is less than 0.5%, the amount of bainite structure formed decreases and V and Nb precipitates are generated, so the hardness before soft nitriding increases and the V and Nb precipitates after nitrocarburizing treatment. The amount produced will decrease, the hardness will decrease after soft nitriding, and it will be difficult to ensure strength. On the other hand, when the Mn addition amount exceeds 3.0%, the machinability and the cold workability are deteriorated. Therefore, the amount of Mn added is in the range of 0.5 to 3.0%. More preferably, it is 0.5% or more and 2.5% or less. More preferably, it is 0.6% or more and 2.0% or less.

Cr: 0.30 to 3.0%
Cr is added because it is effective in forming a bainite structure. When the amount of Cr added is less than 0.30%, the amount of bainite structure formed decreases and V and Nb precipitates are generated. Therefore, the hardness before soft nitriding increases and the amount of V and Nb precipitates generated after nitrocarburizing treatment. As a result, the hardness decreases after soft nitriding, making it difficult to ensure strength. On the other hand, when the Cr addition amount exceeds 3.0%, the machinability and the cold workability are deteriorated. Therefore, the Cr addition amount is set in the range of 0.30 to 3.0%. More preferably, it is 0.5% or more and 2.0% or less. More preferably, it is 0.5% or more and 1.5% or less.

V: 0.02 to 0.5%
V is an important element that forms fine precipitates together with Nb due to the temperature rise during soft nitriding, increases the core hardness, and improves the strength. When the V addition amount is less than 0.02%, the effect of addition is poor. On the other hand, when the amount of V added exceeds 0.5%, the precipitate becomes coarse. Therefore, the V addition amount is set in the range of 0.02 to 0.5%. More preferably, it is 0.03% or more and 0.3% or less. More preferably, it is 0.03% or more and 0.25% or less.

Nb: 0.003 to 0.15%
Nb is a very effective element that forms fine precipitates together with V due to temperature rise during soft nitriding, increases the core hardness, and improves fatigue strength. When the Nb addition amount is less than 0.003%, the effect of addition is poor. On the other hand, if the amount of Nb added exceeds 0.15%, the precipitate becomes coarse. Therefore, the amount of Nb added is set in the range of 0.003 to 0.15%. More preferably, it is 0.02% or more and 0.12% or less.

Mo: 0.005 to 0.4%
Mo has the effect of finely depositing V and Nb precipitates to improve the strength of the soft nitriding material, and is an important element in the present invention. Mo is also effective for generating a bainite structure. Although 0.005% or more is added for strength improvement, it is an expensive element, and if it exceeds 0.4%, the component cost increases. Therefore, the amount of Mo added is within the range of 0.005 to 0.4%. More preferably, it is 0.01 to 0.3%. More preferably, it is 0.04 to 0.2%.

Al: 0.005 to 0.2%
Al is an element effective for improving the surface hardness and the effective hardened layer depth after soft nitriding, and is positively added. Moreover, since it is an element useful also for refine | miniaturizing a structure | tissue by suppressing the austenite grain growth at the time of hot forging and improving toughness, 0.005% or more is added. On the other hand, even if the content exceeds 0.2%, the effect is saturated, and a disadvantage that causes an increase in the component cost is caused. Therefore, the amount of Al added is within the range of 0.005 to 0.2%. Preferably, it is more than 0.020% and 0.1% or less. More preferably, it is more than 0.020% and 0.040% or less.

S: 0.06% or less S is a useful element that forms MnS in steel and improves the machinability, but if it exceeds 0.06%, it impairs toughness. Therefore, the S addition amount is set to 0.06% or less. Preferably, it is 0.04% or less. In order to express the effect of improving machinability due to S, the amount of S added is preferably 0.002% or more.

P: 0.02% or less P segregates at austenite grain boundaries and lowers the grain boundary strength, thereby lowering strength and toughness. Therefore, it is desirable that the P content is reduced as much as possible, but 0.02% is acceptable. Therefore, the P content is 0.02% or less. In addition, since it will require high cost to make P less than 0.001%, it should just reduce to 0.001% industrially.

B: 0.0003 to 0.01%
B has an effect of promoting the formation of a bainite structure. If the amount of addition of B is less than 0.0003%, the effect of addition is poor. On the other hand, even if B is added in excess of 0.01%, the effect is saturated and the component cost is increased. Therefore, the B addition amount is set in the range of 0.0003 to 0.01%. More preferably, it is 0.0010% or more and 0.01% or less.

In addition, in order to obtain the bainite structure formation promotion effect, it is preferable that B is dissolved in steel. However, when solid solution N exists in steel, B in steel is consumed for formation of BN, and when B exists in steel as BN, it does not contribute to improvement of hardenability. Therefore, when solid solution N is present in the steel, it is preferable to add more B than is consumed for the formation of BN. The amount of B (% B) and the amount of N (% N in the steel) are preferably added. It is preferable that the relationship represented by the following formula (1) holds.

% B ≧% N / 14 × 10.8 + 0.0003 --- (1)

In the steel for soft nitriding of the present invention, when improving the machinability of the nitrocarburized material after forging, it is possible to add one or more selected from Pb ≦ 0.2% and Bi ≦ 0.02%. it can. The effects of the present invention are not impaired by the content of these elements and the presence or absence of addition.

Further, in the steel for soft nitriding of the present invention, the balance other than the above additive elements is Fe and inevitable impurities. In particular, Ti not only adversely affects the precipitation strengthening of V and Nb, but also increases the core hardness. Since it will be reduced, try not to contain as much as possible. Preferably, it is less than 0.010%, more preferably less than 0.005%.

3. Manufacturing Conditions FIG. 1 is a schematic view showing a manufacturing process for manufacturing a soft nitrided part using the soft nitriding steel of the present invention.

In FIG. 1, S1 is a steel bar manufacturing process as a raw material, S2 is a conveying process, and S3 is a product (soft-nitriding part) finishing process.

That is, in the steel bar manufacturing process (S1), the steel ingot is hot-rolled into a steel bar and shipped after quality inspection. After the shipment, the conveyed (S2) steel bar is cut into a predetermined size in the product (soft-nitriding part) finishing step (S3), and hot forging or cold forging is performed. After making the desired shape by cutting such as drilling or turning, soft nitriding is performed to obtain a product.

Moreover, the hot rolled material may be finished as it is by cutting such as turning or drilling, and then subjected to soft nitriding to obtain a product. In the case of hot forging, cold straightening may be performed after hot forging. In addition, the final product may be subjected to a coating treatment such as paint or plating. Hereinafter, desirable manufacturing conditions will be described.

Rolling heating temperature The rolling heating temperature is preferably in the range of 950 to 1250 ° C. The steel for soft nitriding of the present invention heats the remaining carbide from the time of melting so that the forgeability is not impaired by the precipitation of fine precipitates on the rolled material (the bar steel used as the material of the hot forged part). This is because they are dissolved at the time of hot rolling.

That is, when the rolling heating temperature is less than 950 ° C., the remaining carbides are difficult to be dissolved from the time of dissolution. On the other hand, if it exceeds 1250 ° C., the crystal grains become coarse and the forgeability tends to deteriorate. Accordingly, it is desirable that the rolling heating temperature be in the range of 950 ° C. to 1250 ° C.

Rolling finishing temperature The rolling finishing temperature is desirably 800 ° C or higher. When the rolling finish temperature is less than 800 ° C., a ferrite structure is generated. Therefore, as a next step, particularly when soft nitriding is performed after cold forging or cutting, a bainite structure in which the parent phase exceeds 50% in area ratio after soft nitriding. Because it is disadvantageous to get. Further, when the rolling finishing temperature is less than 800 ° C., the rolling load is high and the roundness of the rolled material is deteriorated. Therefore, the rolling finishing temperature is desirably 800 ° C. or higher.

Cooling rate It is desirable to regulate the cooling rate after rolling so that fine precipitates are deposited before forging and the forgeability is not impaired. It is desirable to cool the precipitation temperature range of 700 to 550 ° C. of the fine precipitate at a rate exceeding the critical cooling rate (0.5 ° C./sec) at which the fine precipitate is obtained.

Soft nitriding treatment (precipitation treatment)
The obtained steel bar is used as a raw material, and after forging, it is made into a part shape by cutting or the like. Thereafter, soft nitriding is performed. In the soft nitriding treatment, it is desirable that the soft nitriding temperature is in the range of 550 to 700 ° C. and the treatment time is 10 minutes or more so that fine precipitates containing V and Nb are precipitated. When the temperature is lower than 550 ° C., a sufficient amount of precipitates cannot be obtained. On the other hand, when the temperature is higher than 700 ° C., it becomes an austenite region and soft nitriding becomes difficult. More desirably, the temperature is in the range of 550 to 630 ° C. The reason for setting the treatment time to 10 minutes or more is that a sufficient amount of V and Nb precipitates can be obtained.

In addition, when hot forging is used, in order to make the parent phase a bainite structure with an area ratio exceeding 50% after soft nitriding, and from the viewpoint of cold straightening and machinability after hot forging, fine precipitation It is desirable that the heating temperature during hot forging is in the range of 950 to 1250 ° C., the forging finishing temperature is 800 ° C. or higher, and the cooling rate after forging is higher than 0.5 ° C./sec so that no product is precipitated.

Next, the present invention will be further described with reference to examples.

Steels having the compositions shown in Table 1 (steel Nos. 1 to 17) were melted in a 150 kg vacuum melting furnace, rolled at 1150 ° C. and finished at 970 ° C., and then the cooling rate was 0.9 ° C./sec. After cooling to room temperature, a 50 mmφ steel bar was prepared. No. 17 is a conventional material JIS SCr420. In addition, about all the steels in Table 1, P is not positively added. Therefore, the P content in Table 1 indicates a value mixed as an inevitable impurity. For Ti, steel No. 1 in Table 1 was used. 14 and steel no. No. 15 was added. 1 to 13 and steel no. 16 to 17 are not actively added. Therefore, in Table 1, steel No. 1 to 13 and steel no. The Ti contents of 16 to 17 all show values mixed as inevitable impurities.

These materials are further heated to 1200 ° C. and hot forged at 1100 ° C. to 30 mmφ. Cooling rate: 0.8 ° C./sec and partly, cooling rate: 0.1 ° C. / It cooled to room temperature in sec.

The above materials were examined for structure observation, hardness measurement and machinability. In the structure observation, the cross section was observed with an optical microscope, the core structure was identified, and the area fraction of the bainite phase in the core was determined for those having a bainite phase in the core. The machinability was evaluated by a drill cutting test. Specifically, a hot forged material cut to a thickness of 20 mm was used as a test material, a JIS high-speed tool steel SKH51 6 mmφ straight drill, feed: 0.15 mm / rev, rotation speed: 795 rpm, Five through holes were made per cross section, and the total number of holes until the drill became uncut was evaluated.

Further, the hardness measurement was conducted by examining the hardness of the core using a Vickers hardness tester with a test load of 100 g.

Steel No. Nos. 1 to 16 were obtained by subjecting the hot forged material to gas soft nitriding, In No. 17, the hot forging was subjected to gas carburizing treatment. The gas soft nitriding treatment was performed by heating to 570 to 620 ° C. in an atmosphere of NH ₃ : N ₂ : CO ₂ = 50: 45: 5 and holding for 3.5 hours. The gas carburizing treatment was carried out under the condition that after 930 ° C. × 3 h carburizing, holding at 850 ° C. × 40 min, and then oil cooling, and further tempering at 170 ° C. × 1 h.

These heat-treated materials were subjected to structure observation, hardness measurement, precipitate observation, impact property investigation, and fatigue property investigation.

In the structure observation, the cross-section was observed with an optical microscope to identify the core structure, and the area fraction of the bainite phase was determined for those having a bainite phase in the core.

The hardness of the soft nitriding material and the carburized material was measured for the core hardness and the surface hardness. The surface hardness was measured at a position of 0.02 mm from the surface, and the effective hardened layer depth was defined as the depth from the surface to be HV400. Moreover, a transmission electron microscope observation sample was prepared from the core portion of the soft nitriding material and the carburized material by an electrolytic polishing method using a twin jet method, and the obtained sample was a transmission type with an acceleration voltage of 200 kV. The precipitate was observed using an electron microscope. Furthermore, the composition of the observed precipitate was determined by an energy dispersive X-ray spectrometer (EDX).

The impact characteristics were evaluated by a Charpy impact test, and the impact value (J / cm ² ) was obtained. As the test piece, a notched test piece (R: 10 mm, depth: 2 mm) was used. The notched test piece was sampled from a hot forged material, and subjected to the Charpy impact test after subjecting the sampled test piece to the soft nitriding treatment or carburizing treatment described above.
Fatigue property evaluation was performed by an Ono-type rotary bending fatigue test to determine the fatigue limit. A test piece with a notch (notch R: 1.0 mm, notch diameter: 8 mm, stress concentration factor: 1.8) was used as the test piece. This test piece was taken from the hot forging material and subjected to the soft nitriding treatment or carburizing treatment described above, and then subjected to the test.

Table 2 shows the test results. No. 1 to 6 are invention examples, Nos. 7 to 17 are comparative examples. 18 is a conventional example of JIS SCr420 steel.

As can be seen from Table 2, no. The nitrocarburized materials 1 to 6 have better fatigue strength than those obtained by carburizing and tempering the conventional example (No. 18). No. The drill cutting workability of materials 1 to 6 before soft nitriding (hot forged material) is practically equivalent to or higher than that of conventional materials. Further, as a result of the observation of the precipitate composition by transmission electron microscope observation and EDX, No. In the nitrocarburized materials 1 to 6, it was confirmed that 500 or more fine precipitates having a particle size of less than 10 nm containing V and Nb in the bainite phase were dispersed per 1 μm ² . From this result, it is considered that the nitrocarburized material according to the present invention exhibited high fatigue strength by precipitation strengthening due to the fine precipitates.

In contrast, Comparative Example No. Nos. 7 to 17 are inferior in fatigue strength or drillability because the chemical composition or the obtained microstructure is outside the scope of the present invention.

In particular, No. No. 7 has a lower fatigue strength than the inventive examples because the cooling rate after hot forging is slow. Here, no. In No. 7, as a result of observation with a transmission electron microscope, dispersion of fine precipitates having a particle size of less than 10 nm was not observed, but coarse precipitates having a particle size greatly exceeding 10 nm were observed. From this result, it is considered that the precipitate formed in this way is coarse, which is a cause of low fatigue strength. That is, if the cooling rate after hot forging is slow and the desired bainite structure cannot be obtained, coarse precipitates are generated before soft nitriding, and the amount of fine precipitates generated after soft nitriding is small. Therefore, it is considered that precipitation strengthening is insufficient as a result.

No. In No. 8, since the amount of C is high outside the range of the present invention, the hardness of the bainite structure is increased and the drill workability is reduced.

No. In No. 9, the amount of Si and Mn is high outside the scope of the present invention, so that the hot forged material has high hardness, and the drillability is reduced to about 1/5 of the conventional material.

No. No. 10, the amount of Mn is low outside the range of the present invention, the ferrite-pearlite structure is formed before soft nitriding (after hot forging), and the area ratio of the bainite structure is lowered, and V and Nb precipitates are precipitated in the structure. Therefore, the hardness before soft nitriding is increased and the drill workability is reduced.

No. No. 11, since the Cr content is low outside the scope of the present invention, a ferrite-pearlite structure is formed before soft nitriding (after hot forging), and the area ratio of the bainite structure is reduced, and V and Nb precipitates are present in the structure. Since it precipitated, the hardness before soft nitriding increased and drill workability fell.

No. No. 12, since the amount of Mo is low outside the scope of the present invention, the amount of fine precipitates after soft nitriding is small, and sufficient core hardness cannot be obtained, so the fatigue strength is lower than in the conventional example.

No. In No. 13, V and Nb are low outside the scope of the present invention, so that the amount of precipitates after soft nitriding is small and sufficient core hardness cannot be obtained, so the fatigue strength is lower than that of the conventional material.

No. No. 14 has a low Nb outside the range of the present invention, so the amount of precipitates after the soft nitriding treatment is small, so that a sufficient core hardness cannot be obtained, and the fatigue strength is lower than that of the conventional material.

No. 15 and no. In No. 16, since Ti was added, the amount of precipitates containing V and Nb after soft nitriding was small, so that sufficient core hardness could not be obtained, and the fatigue strength was lower than that of conventional materials. Furthermore, the impact value also shows a low value.

No. Since Al is low outside the range of the present invention, the surface hardness and effective hardened layer depth after soft nitriding were low, so that the fatigue strength is lower than that of the conventional material.

Claims

% By mass
C: 0.01% or more and less than 0.10%,
Si: 1.0% or less,
Mn: 0.5 to 3.0%,
Cr: 0.30 to 3.0%,
Mo: 0.005 to 0.4%,
V: 0.02 to 0.5%,
Nb: 0.003 to 0.15%,
Al: 0.005 to 0.2%,
S: 0.06% or less,
P: 0.02% or less and B: 0.0003-0.01%,
Remaining: Steel for soft nitriding consisting of Fe and inevitable impurities and having a structure with a bainite area ratio exceeding 50% before soft nitriding.
The steel for soft nitriding according to claim 1, wherein precipitates containing V and Nb are dispersed and precipitated in the bainite phase after soft nitriding.
Soft nitriding parts made of the soft nitriding steel according to claim 1 or 2.