JP2018003076A - Steel for soft nitriding and components, and manufacturing method therefor - Google Patents

Abstract

The present invention provides a steel for soft nitriding that is excellent in machinability before soft nitriding treatment and ensures fatigue strength after soft nitriding. SOLUTION: In mass%, C: 0.02 to less than 0.10%, Si: 1.0% or less, Mn: 0.50 to 3.0%, Cr: 0.30 to 3.0%, Mo: 0.005 to 0.4%, Al: more than 0.020 to 0.2% N: 0.0200% or less, P: 0.02% or less, and S: 0.06% or less, with the balance being Fe and inevitable impurities, and the bainite phase satisfying an area ratio of more than 50% with respect to the entire structure Organization. [Selection figure] None

Description

  The present invention relates to a steel for soft nitriding and parts obtained from the steel for soft nitriding, and further to a manufacturing method thereof, and particularly provides a material excellent in fatigue characteristics after soft nitriding and suitable as a part for automobiles and construction machinery. It is something to try.

  Since mechanical structure parts such as automobile gears are required to have excellent fatigue characteristics, surface hardening treatment is usually performed. As the surface hardening treatment, carburizing treatment, induction hardening treatment, nitriding treatment and the like are well known.

  Among these, the carburizing treatment penetrates and diffuses C in the high temperature austenite region, so that a deep hardening depth is obtained and effective in improving fatigue strength. However, since heat treatment distortion occurs due to the carburizing treatment, it has been difficult to apply to parts that require strict dimensional accuracy from the viewpoint of quietness and the like.

  In addition, since the induction hardening process is a process in which the surface layer portion is quenched by induction heating, heat treatment distortion is also generated, and there is a problem in terms of dimensional accuracy as in the carburizing process.

On the other hand, the nitriding treatment is a treatment for increasing the surface hardness by invading and diffusing nitrogen in a relatively low temperature range below the Ac ₁ transformation point, and thus there is no fear that the heat treatment distortion described above occurs. However, there is a problem that the processing time is as long as 50 to 100 hours, and the brittle compound layer on the surface layer needs to be removed after the processing.

  For this reason, a so-called soft nitriding process has been developed in which the processing time is shortened at a processing temperature comparable to that of the nitriding process, and in recent years, it has been widely used for machine structural parts and the like. This soft nitriding treatment hardens the surface by simultaneously intruding and diffusing N and C in the temperature range of 500-600 ° C, and the processing time can be reduced to less than half compared to conventional nitriding treatment. It is.

However, in the soft nitriding treatment, it is difficult to obtain a sufficient depth of the hardened layer formed along with the diffusion of N into the steel due to the problems of the treatment temperature and time. For this reason, in order to obtain sufficient fatigue strength, an increase in the hardness of the core portion where N does not diffuse can be an effective means. However, since soft nitriding is performed at a temperature below the transformation point of steel, it is generally difficult to increase the core hardness during the processing.
Conventionally, 0.1% or more of C is added to the steel material before soft nitriding to ensure the strength of the core, but the increase in C in the steel is the same as that of the steel material before soft nitriding. In order to increase the hardness, cold workability such as cutting and cold forging is significantly hindered, resulting in deterioration of dimensional accuracy of finished parts and severe consumption of tools during cutting and cold forging. This will increase the manufacturing cost.
In order to increase the fatigue strength of the nitrocarburized material, quenching and tempering are performed before nitrocarburizing to increase the core hardness, and the machinability before processing and the fatigue characteristics after nitrocarburizing Although there is a method to achieve both, it is inevitable that the manufacturing cost increases due to giving sufficient strength to the core part by quenching and tempering, and further, it cannot be said that securing the machinability is sufficient.

As a solution to such a problem, Patent Document 1 allows high bending fatigue strength to be obtained after soft nitriding by including Ni, Cu, Al, Cr, Ti, or the like in steel. A steel for soft nitriding has been proposed.
In other words, this steel is age-hardened with Ni-Al, Ni-Ti intermetallic compound or Cu compound at the core by soft nitriding treatment, while Cr, Al, Ti, etc. in the nitrided layer at the surface layer The bending fatigue strength is improved by precipitation hardening of nitrides and carbides.

  In Patent Document 2, steel containing 0.5 to 2% of Cu is forged by hot forging, then air-cooled to obtain a ferrite-based structure in which Cu is solid-solved, and softened at 580 ° C. for 120 minutes. There has been proposed a soft nitriding steel that provides excellent bending fatigue properties after soft nitriding treatment by precipitation hardening of Cu during nitriding treatment and further using precipitation hardening of Ti, V and Nb carbonitrides. .

  Furthermore, Patent Document 3 proposes a steel for soft nitriding in which Ti—Mo carbides and carbides containing one or more of Nb, V, and W are further dispersed.

  Furthermore, in Patent Document 4, in the steel containing V and Nb, the structure before nitriding is a structure mainly composed of bainite, and the precipitation of V and Nb carbonitrides at the stage before nitriding is suppressed. There has been proposed a nitriding steel material that is excellent in fatigue strength with improved core hardness by precipitating carbonitride.

JP-A-5-59488 JP 2002-69572 A JP 2010-163671 JP 2013-166997 A

However, the soft nitrided steel described in Patent Document 1 is not sufficient to ensure workability, although the bending fatigue strength is improved by precipitation hardening of Ni—Al, Ni—Ti intermetallic compounds and Cu. It was.
Moreover, the steel for soft nitriding described in Patent Document 2 has a problem that the production cost is high because it is necessary to add a relatively large amount of Cu, Ti, V, and Nb.
Furthermore, since the steel for soft nitriding described in Patent Document 3 contains a relatively large amount of Ti and Mo, there is still a problem of high cost.

  On the other hand, the steel for nitriding described in Patent Document 4 adds Nb and V to increase the fatigue strength after nitriding by utilizing nitride precipitation including the core of these elements during nitriding. However, the addition of Nb and V raises the cost of alloy addition and causes the deterioration of manufacturability due to the formation of these carbonitrides, resulting in a decrease in product yield and an increase in manufacturing cost. In addition, Nb and V carbonitrides are also present in the steel before soft nitriding, and the machinability is lowered by these hard precipitates.

The present invention advantageously solves the above problems, and provides a nitrocarburizing steel that achieves extremely excellent machinability by advantageously controlling the structure before nitrocarburizing treatment, along with its manufacturing method. For the purpose.
Another object of the present invention is to provide, together with a manufacturing method thereof, a component that maintains high core hardness after machining and after soft nitriding treatment and that ensures fatigue characteristics.

Now, in order to solve the above-mentioned problems, the present inventors have intensively studied the influence of the steel component composition and the structure.
As a result, the core required to achieve extremely excellent machinability and high fatigue strength by making the pre-soft-nitriding structure of steel limited to less than 0.1% by mass a bainite phase of more than 50%. The knowledge that it was possible to balance the part hardness with a high dimension was obtained.
The present invention was completed after further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.02 to less than 0.10%,
Si: 1.0% or less,
Mn: 0.50 to 3.0%
Cr: 0.30 to 3.0%
Mo: 0.005-0.4%,
Al: more than 0.020 to 0.2%,
N: 0.0200% or less,
P: 0.02% or less and S: 0.06% or less are contained within a range satisfying the following formula (1), the balance has a component composition of Fe and inevitable impurities, and the area ratio of the bainite phase is 50% A steel for soft nitriding characterized by having a super-fine structure.
Record
6.3 ≦ [Mn] + 2.5 × [Cr] + 20 × [Mo] ≦ 12 --- (1)
However, [] is the element content in parentheses (mass%)

2. A component comprising: a core portion having the component composition and structure described in 1 above; and a surface layer portion having a component composition having a high nitrogen and carbon content relative to the component composition of the core portion.

3. % By mass
C: 0.02 to less than 0.10%,
Si: 1.0% or less,
Mn: 0.50 to 3.0%
Cr: 0.30 to 3.0%
Mo: 0.005-0.4%,
Al: more than 0.020 to 0.2%,
N: 0.0200% or less,
P: 0.02% or less and S: 0.06% or less are contained within a range satisfying the following formula (1), and the balance is a steel material having a component composition of Fe and inevitable impurities. Heating temperature: 950 to 1250 ° C., finishing temperature : Hot-working under conditions of 800 ° C. or higher, and thereafter cooling at a temperature range of at least 700 to 550 ° C. at a rate of 0.4 ° C./s or higher.
Record
6.3 ≦ [Mn] + 2.5 × [Cr] + 20 × [Mo] ≦ 12 --- (1)
However, [] is the element content in parentheses (mass%)

4). The soft nitriding steel obtained by the production method described in 3 above is finished in a desired shape, and then subjected to soft nitriding under conditions of processing temperature: 500 to 700 ° C. and processing time: 10 minutes or more. A method for producing a featured part.

According to the present invention, it is possible to obtain a nitrocarburizing steel with an inexpensive component system and extremely excellent machinability, ensuring excellent dimensional accuracy after processing and after nitronitriding, and reducing cold processing costs. In addition, after the soft nitriding treatment, it is possible to obtain a part having fatigue characteristics equal to or higher than those of the JIS SCr420 and SCM420 materials similarly subjected to the soft nitriding treatment.
The parts obtained according to the present invention are extremely useful when applied to machine structural parts such as automobiles.

It is a figure which shows the manufacturing process which manufactures a nitrocarburized component.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding the following component composition shall mean "mass%".
C: 0.02% or more and less than 0.10% C is added to form a bainite phase and ensure strength. However, if the content is less than 0.02%, not only a sufficient amount of bainite phase area ratio can not be obtained, but also the hardness of bainite itself is insufficient, and it is difficult to ensure strength, so it is necessary to add 0.02% or more is there. On the other hand, when the content is 0.10% or more, the hardness of the produced bainite phase is increased, and the machinability is remarkably lowered. Therefore, the C content is set in the range of 0.02% or more and less than 0.10%. More preferably, it is 0.05% or more and less than 0.10%.

Si: 1.0% or less
Si is added because it is effective not only for deoxidation but also for the formation of a bainite phase. However, if it exceeds 1.0%, it dissolves in the ferrite and bainite phases, and its solid solution hardening improves the machinability and cold workability. In order to cause deterioration, the Si content is 1.0% or less. Preferably it is 0.5% or less, More preferably, it is 0.3% or less.
In order to effectively contribute Si to deoxidation, the addition amount is preferably set to 0.01% or more.

Mn: 0.50% to 3.0%
Mn has the effect of enhancing the hardenability of the steel and generating a bainite phase stably. When the amount of Mn is less than 0.50%, the above effect is poor, and on the other hand, the amount of MnS produced is not sufficient, so that machinability is lowered. Therefore, the Mn content is 0.50% or more. On the other hand, if it exceeds 3.0%, the machinability and the cold workability deteriorate, so the Mn content should be 3.0% or less. The range is preferably 1.0% or more and 2.5% or less, more preferably 1.5% or more and 2.5% or less.

Cr: 0.30 to 3.0%
Cr is effective for the formation of a bainite phase and is a hard nitride-forming element and is added because it is effective for increasing the surface hardness after soft nitriding. When the content is less than 0.30%, the amount of bainite phase generated is reduced and the core hardness is reduced. Further, Cr nitride formation on the surface layer is insufficient and the surface layer is also reduced in hardness, so the soft nitriding treatment It becomes difficult to secure the strength later. Therefore, the Cr content is 0.30% or more. On the other hand, if it exceeds 3.0%, the machinability and the cold workability deteriorate, so the Cr content is 3.0% or less. Preferably it is 0.5 to 2.0%, more preferably 0.5 to 1.5%.

Mo: 0.005-0.4%
Mo is effective for stably generating the bainite phase. Here, in order to improve strength, 0.005% or more of addition is required. However, since it is an expensive element, if it exceeds 0.4%, the component cost increases. For this reason, the amount of Mo is made 0.005 to 0.4% of range. Preferably it is 0.01 to 0.3%, more preferably 0.04% or more and less than 0.2%.

Al: more than 0.020% and 0.2% or less
Al is an element useful for increasing the surface hardness after soft nitriding and increasing the effective hardened layer depth, and is actively added. Moreover, it is an element useful also for refine | miniaturizing a structure | tissue and improving toughness by suppressing the austenite grain growth at the time of hot forging. From such a viewpoint, Al is contained exceeding 0.020%. On the other hand, even if the content exceeds 0.2%, the effect is saturated, and rather disadvantageous that causes an increase in the component cost occurs, so the Al content is limited to 0.2% or less. Preferably it is in the range of more than 0.020% and 0.1% or less, more preferably in the range of more than 0.020% and 0.04% or less.

N: 0.0200% or less N is an element that forms carbonitrides in steel, improves the strength of soft nitriding materials, and nitrides formed by bonding with Al and the like are useful elements for refining the steel structure. is there. Therefore, it is preferable to contain 0.0020% or more. However, if the content exceeds 0.0200%, the ductility of the steel material is lowered and the surface crack of the slab is generated, so that the slab quality is degraded. For this reason, N is limited to 0.0200% or less.

P: 0.02% or less P is segregated at austenite grain boundaries and lowers the grain boundary strength, thereby lowering strength and toughness. Therefore, it is desirable to suppress the P content as much as possible, but 0.02% is allowed. In addition, since it requires high cost to make P less than 0.001%, it may be industrially reduced to 0.001%.

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%, the toughness is impaired, so it is limited to 0.06% or less. Preferably it is 0.04% or less.
In addition, in order to express the machinability improvement effect by S, it is preferable to make S amount 0.002% or more.

  Furthermore, in the component system of the present invention, when the amount of addition of Mn, Cr and Mo is increased more than necessary, the steel piece surface cracks are likely to occur when casting the steel slab, which is a material before hot rolling, from molten steel. . Therefore, in order to suppress the steel piece surface crack, it is necessary to suppress the addition amount of Mn, Cr and Mo. Specifically, the value calculated by [Mn] + 2.5 × [Cr] + 20 × [Mo] (where [] is the element content (mass%) in parentheses) is 12 or less. It is important. By limiting the amount of Mn, Cr and Mo added in accordance with this equation, the steel slab surface cracking is suppressed.

In order to stably realize the bainite structure exceeding 50% as expected in the present invention with respect to the addition amounts of Mn, Cr and Mo, the above [Mn] + 2.5 × [Cr] + 20 × [Mo The value calculated by the formula is 6.3 or more.
From the above, it was a requirement to satisfy the following formula (1).
6.3 ≦ [Mn] + 2.5 × [Cr] + 20 × [Mo] ≦ 12 --- (1)
Note that [] is the element content in parentheses (% by mass).

  In the steel of the present invention, other than the above-described components are Fe and inevitable impurities. Among unavoidable impurities, Ti, V and Nb are particularly effective in increasing the strength after soft nitriding, but they are also present in the steel before soft nitriding, so that the machinability is significantly reduced. is there. That is, Ti and V are preferably less than 0.010%, and Nb is preferably less than 0.005%.

Next, the reason why the steel structure of the soft nitrogen steel in the present invention is limited to the above range will be described.
Bainitic phase: More than 50% in area ratio with respect to the entire structure In the present invention, it is extremely important that the bainite phase has an area ratio with respect to the entire structure exceeding 50%.
The bainite phase is a structure excellent in machinability that affects the tool life at the time of cutting as compared with a ferrite-pearlite structure having the same hardness. The reason for this is not necessarily clear, but ferrite-pearlite is a mixed structure of soft ferrite and hard pearlite, whereas bainite forms a relatively homogeneous structure and is soft as ferrite. While the structure increases the shear energy required for chip generation, relatively fine carbides are uniformly dispersed in the bainite, which acts as a stress concentration source to reduce the shear energy. Can be considered.
Therefore, the steel structure of the nitrocarburizing steel of the present invention, that is, the steel structure before the nitronitriding treatment is mainly composed of a bainite phase. Specifically, the bainite phase is more than 50% in terms of the area ratio with respect to the entire structure. Preferably it is more than 60%, more preferably more than 80%. It may be 100%.
In addition, as a structure other than the bainite phase, a ferrite phase, a pearlite phase, or the like can be considered, but it goes without saying that the smaller the structure, the better.

  Here, the area ratio of each phase can be determined as follows. That is, a test piece was taken from the obtained nitrocarburizing steel, and the cross section (L cross section) parallel to the rolling direction was corroded with nital after polishing, and was then optical microscope (200 times) or scanning electron microscope (SEM) ) Is used to identify the type of phase by cross-sectional structure observation (200 × optical microscope structure observation), and the area ratio of each phase is obtained.

Next, the manufacturing process of the soft nitriding steel of the present invention and parts using the same will be described.
FIG. 1 shows a typical manufacturing process for manufacturing a part using the soft nitriding steel (for example, steel bar) according to the present invention. Here, S1 is a steel bar manufacturing process as a raw material, S2 is a conveying process, and S3 is a product (soft-nitriding component) finishing process.

First, the steel ingot is hot-rolled into a steel bar in the steel bar manufacturing process (S1), shipped after quality inspection.
Then, after the transfer (S2), in the product (soft-nitriding part) finishing step (S3), the steel bar is cut to a predetermined size, hot forging or cold forging is performed, and drilling or turning is performed as necessary. After cutting into a desired shape (for example, a gear product or a shaft part), soft nitriding is performed to obtain a product (part).
In addition, the hot rolled material may be finished as it is by a cutting process such as turning or drilling, and then subjected to soft nitriding to obtain a product. In the case of hot forging, cold correction may be performed after hot forging. In addition, the final product may be subjected to a coating treatment such as paint or plating.

In the method for producing nitrocarburizing steel of the present invention, in the hot working step immediately before the soft nitriding treatment, the heating temperature at the hot working and the working temperature are set to specific conditions, so that the bainite phase main component as described above is used. Organization.
Here, hot working mainly means hot rolling or hot forging, but hot forging may be further performed after hot rolling. Further, cold forging may be performed after hot rolling.
Here, when the hot working process immediately before the soft nitriding process is a hot rolling process, that is, when hot forging is not performed after hot rolling, it is preferable to satisfy the following conditions in the hot rolling process: .

Rolling heating temperature: 950-1250 ° C
In the hot rolling process, carbides remaining from the time of dissolution are dissolved so that fine precipitates are not deposited on the rolled material (bar steel used as a component material by cold forging and / or cutting) and forgeability is not impaired.
Here, if the rolling heating temperature is less than 950 ° C., the remaining carbides from the time of melting are hardly dissolved. On the other hand, if it exceeds 1250 ° C., the crystal grains become coarse and the forgeability tends to deteriorate. For this reason, it is preferable that rolling heating temperature shall be the range of 950 degreeC-1250 degreeC.

Rolling finish temperature: 800 ° C or more When the rolling finish temperature is less than 800 ° C, a ferrite phase is generated, which is disadvantageous in generating a bainite phase that satisfies more than 50% of the area ratio of the entire structure before soft nitriding. Become. Also, the rolling load is increased. Therefore, the rolling finish temperature is preferably 800 ° C. or higher. From the viewpoint of preventing extreme coarsening of the austenite grains after hot rolling, the upper limit is preferably about 1100 ° C.

Cooling rate in a temperature range of at least 700 to 550 ° C. after rolling: 0.4 ° C./s or more 700 to 550 ° C., which is a temperature range in which at least ferrite-pearlite transformation is active for the purpose of increasing the area ratio of the bainite structure in steel. In the above temperature range, the cooling rate after rolling needs to be 0.4 ° C./s or more, which is a critical cooling rate capable of suppressing ferrite transformation. The upper limit is preferably about 200 ° C./s.

In addition, when the hot working process immediately before nitriding is a hot forging process, that is, when only hot forging is performed or when hot forging is performed after hot rolling, the conditions shown below in the hot forging process are as follows: Satisfy.
In addition, when hot rolling is performed before hot forging, the above-described conditions may not necessarily be satisfied as the hot rolling conditions.

Hot forging conditions In this hot forging, in order to make the bainite phase more than 50% in terms of the area ratio with respect to the entire structure, and from the viewpoint of cold straightening and machinability after hot forging, fine precipitates should not be precipitated. Therefore, it is preferable that the heating temperature during hot forging is 950 to 1250 ° C. and the forging finishing temperature is 800 ° C. or higher.
Furthermore, it is as important as described above that the cooling rate after hot forging is at least 0.4 ° C./s in the temperature range of at least 700 to 550 ° C. The upper limit is preferably about 200 ° C./s.

  Next, the obtained rolled material or forged material is subjected to cutting or the like to obtain a part shape, and then soft nitriding is performed under the following conditions.

Soft nitriding treatment (precipitation treatment) conditions The soft nitriding treatment is performed under conditions of treatment temperature: 500 to 700 ° C. and treatment time: 10 minutes or more so as to precipitate fine precipitates. Here, if the soft nitriding temperature is in the range of 500 to 700 ° C., the diffusion rate of N into the steel cannot be sufficiently obtained unless the temperature is less than 500 ° C., whereas if it exceeds 700 ° C., the austenite region becomes soft. This is because nitriding becomes difficult. More preferably, it is the range of 550-630 degreeC.

In the soft nitriding treatment, N and C are infiltrated and diffused at the same time. Therefore, a mixed atmosphere of a nitriding gas such as NH ₃ or N ₂ and a carburizing gas such as CO ₂ or CO, for example, NH ₃ : N ₂ : Soft nitriding may be performed in an atmosphere of CO ₂ = 50: 45: 5. A salt bath, plasma nitriding, etc. can also be used.

Examples of the present invention will be specifically described below.
Steel with the composition shown in Table 1 is melted in a 150 kg vacuum melting furnace, heated at 1150 ° C, hot-rolled at a rolling finish temperature of 970 ° C, and then cooled to room temperature at a rate of 0.9 ° C / s. 50 mmφ steel bar. Steel grade O is steel corresponding to JIS SCr420.
These materials are further heated to 1200 ° C and then hot forged at a finishing temperature of 1100 ° C to form a 40mmφ steel bar, and then the cooling rate in the range of 700 to 550 ° C is as shown in Table 2. And cooled to room temperature.

The hot forged material thus obtained was evaluated for machinability, particularly drillability, by a drill cutting test. That is, 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, 5 per section The through-hole of the location was opened, and the total number of holes until the drill became uncut was evaluated.
Moreover, about said hot forging material, structure | tissue observation and hardness measurement were performed. In the structure observation, the type of phase was identified and the area ratio of each phase was determined by the method described above.
In the hardness measurement, using a Vickers hardness tester, the hardness of the core part was measured at a test load of 2.94 N (300 gf) in accordance with JIS Z 2244, and the average value was defined as the hardness HV.

Then, after the hot forging described above, soft nitriding was further performed.
The soft nitriding treatment was performed by heating to 570 to 600 ° C. in an atmosphere of NH ₃ : N ₂ : CO ₂ = 50: 45: 5 and maintaining for 3.5 hours.

The heat treated material thus obtained was subjected to structure observation, hardness measurement, and fatigue property evaluation.
Here, in the structure observation, the type of phase was identified and the area ratio of each phase was determined by the method described above, as before soft nitriding.
In the hardness measurement, the surface hardness of the heat-treated material was measured at a position of 0.05 mm from the surface, and the core hardness was measured at the center (core). The surface hardness and the core hardness were both measured using a Vickers hardness tester in accordance with JIS Z 2244, measuring the core hardness with a test load of 2.94 N (300 gf). The average values were the surface hardness HV and the core hardness HV, respectively. Furthermore, the effective hardened layer depth was measured by defining the depth from the surface to be HV400.

Fatigue characteristics were evaluated by the Ono-type rotating bending fatigue test, and the unruptured fatigue strength (fatigue limit) was determined 10 ⁷ times. For the fatigue test, a notched specimen (notch R: 1.0 mm, notch diameter: 8 mm, stress concentration factor: 1.8) was sampled from the hot forged material, and the soft nitriding treatment and carburization described above were performed on this specimen. It performed using the heat processing material which processed.
Table 2 shows the test results. Nos. 1 to 4 are invention examples, Nos. 5 to 15 are comparative examples, and No. 16 is a conventional example obtained by applying nitrocarburizing treatment to JIS SCr420 equivalent steel.

  As apparent from Table 2, Invention Examples No. 1 to No. 4 showed drill workability before nitrocarburizing better than Conventional Example No. 16. Further, all of Invention Examples No. 1 to 4 obtained fatigue strength equal to or higher than that of Conventional Example No. 16.

On the other hand, Comparative Examples Nos. 5 to 15 are inferior in either drill workability or fatigue strength or both because the component composition or the obtained steel structure was outside the scope of the present invention.
That is, No. 5 has a slow cooling rate after hot forging, so that an appropriate amount of bainite phase cannot be obtained, and both drill workability and fatigue strength after soft nitriding are inferior to those of the inventive examples.
In No. 6, since formula (1) deviated from the invention range (upper limit) and surface cracks occurred in continuously cast steel pieces, the subsequent rolling was stopped.
In No. 7, Formula (1) did not satisfy the preferred range (lower limit), and an appropriate amount of bainite phase was not obtained.
In No. 8, C exceeded the proper range, and although surface cracks occurred in the continuously cast steel piece despite the fact that formula (1) was within the preferred range of the present invention, the subsequent rolling was stopped.
No. In C, C exceeds the appropriate range, while equation (1) does not reach the lower limit of the preferred range of the present invention, and as a result, both continuous casting and hot working can proceed without any problem, and the final part can be manufactured. However, the hardness of the hot forged material before the soft nitriding treatment is increased, and the drill workability is reduced.
In No. 10, the amount of C is less than the lower limit of the appropriate range, and the fatigue strength is deteriorated as compared with the conventional example.
In No. 11, since the Si amount and the Mn amount exceed the appropriate ranges, the hardness of the hot forged material before the nitriding treatment is increased, and the drill workability is lower than that of the conventional example No. 16.
Nos. 12 to 14 cannot satisfy the formula (1) because any of Mn, Cr, and Mo is less than the appropriate range, and the steel structure of the hot forged material before soft nitriding is ferrite. It has a mixed structure of a phase-pearlite phase and bainite. For this reason, the bainite fraction in the structure does not satisfy the preferred range of the present invention, and drill workability and fatigue characteristics are reduced.
In No. 15, since the amount of Al is less than the appropriate range, the fatigue strength is reduced.

Claims (4)

% By mass
C: 0.02 to less than 0.10%,
Si: 1.0% or less,
Mn: 0.50 to 3.0%
Cr: 0.30 to 3.0%
Mo: 0.005-0.4%,
Al: more than 0.020 to 0.2%,
N: 0.0200% or less,
P: 0.02% or less and S: 0.06% or less are contained within a range satisfying the following formula (1), the balance has a component composition of Fe and inevitable impurities, and the area ratio of the bainite phase is 50% A steel for soft nitriding characterized by having a super-fine structure.
Record
6.3 ≦ [Mn] + 2.5 × [Cr] + 20 × [Mo] ≦ 12 --- (1)
However, [] is the element content in parentheses (mass%)   A component comprising: a core portion having the component composition and structure according to claim 1; and a surface layer portion having a component composition having a high nitrogen and carbon content relative to the component composition of the core portion. % By mass
C: 0.02 to less than 0.10%,
Si: 1.0% or less,
Mn: 0.50 to 3.0%
Cr: 0.30 to 3.0%
Mo: 0.005-0.4%,
Al: more than 0.020 to 0.2%,
N: 0.0200% or less,
P: 0.02% or less and S: 0.06% or less are contained within a range satisfying the following formula (1), and the balance is a steel material having a component composition of Fe and inevitable impurities. Heating temperature: 950 to 1250 ° C., finishing temperature : Hot-working under conditions of 800 ° C. or higher, and thereafter cooling at a temperature range of at least 700 to 550 ° C. at a rate of 0.4 ° C./s or higher.
Record
6.3 ≦ [Mn] + 2.5 × [Cr] + 20 × [Mo] ≦ 12 --- (1)
However, [] is the element content in parentheses (mass%)   The soft nitriding steel obtained by the manufacturing method according to claim 3 is finished into a desired shape, and then subjected to soft nitriding under conditions of processing temperature: 500 to 700 ° C. and processing time: 10 minutes or more. A method of manufacturing a component characterized by the above.

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