JP2016056450A - Steel and component for soft nitriding and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for soft nitriding having excellent machinability by suppressing hardening before a soft nitriding treatment, exhibiting excellent fatigue characteristic after the soft nitriding treatment and suitable for being applied to a machine construction components such as automobiles.SOLUTION: There is provided a structure having a composition containing, by mass%, C:0.10 to less than 0.20%, Si:1.0% or less, Mn:over 0.5 to 3.0%, Cu:0.10 to 1.0%, Ni:0.05 to 1.0%, Cr:0.3 to 3.0%, Mo:0.005 to 0.4%, V:0.02 to 0.5%, Nb:0.003 to 0.15%, Al:over 0.020 to 0.2%, N:0.0200% or less, P:0.02% or less and S:0.06% or less and the balance Fe with inevitable impurities and a structure having a bainite phase satisfying area ratio of over 50% based on a whole structure.SELECTED DRAWING: 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 allows C to penetrate and diffuse in a 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 infiltrating and diffusing nitrogen in a relatively low temperature range below the Ac ₁ transformation point, and thus there is no possibility 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 it is necessary to remove the brittle compound layer on the surface layer after the processing.

  Therefore, a so-called soft nitriding process has been developed in which the processing time is shortened at the same processing temperature as 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 infiltrating 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. is there.

However, in the carburizing process described above, the core hardness can be increased by quench hardening, whereas the soft nitriding process is performed at a temperature below the transformation point of the steel. The nitrocarburized material has a problem that the fatigue strength is inferior to the carburized material.
Therefore, in order to increase the fatigue strength of the nitrocarburized material, quenching and tempering treatment is usually performed before the nitrocarburizing treatment to increase the core hardness, but the obtained fatigue strength is hardly sufficient. Further, the manufacturing cost has increased, and further, the machinability has been inevitably lowered.

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 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 by precipitating and hardening Cu during nitriding, and also using precipitation hardening of Ti, V, and Nb carbonitrides. .

  Further, 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.

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

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.

The present invention advantageously solves the above-mentioned problems, and provides a relatively inexpensive nitrocarburizing steel, which is ensured in machinability by suppressing hardening before nitrocarburizing, together with its manufacturing method. With the goal.
Another object of the present invention is to provide, together with a manufacturing method thereof, a part having increased core hardness by soft nitriding after machining and improved 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, as a composition of steel, an appropriate amount of Cu is contained together with an appropriate amount of Ni, an appropriate amount of V and Nb is contained, and a steel structure is formed into a bainite phase having an area ratio of more than 50%. Excellent machinability can be obtained without containing relatively expensive elements, and after soft nitriding, fine precipitates containing V and Nb are dispersed and precipitated in addition to Cu precipitation hardening. It was found that excellent fatigue properties can be obtained by increasing the core hardness.
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. Component composition is mass%, C: 0.10 to less than 0.20%, Si: 1.0% or less, Mn: more than 0.5 to 3.0%, Cu: 0.10 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.3 to 3.0% , Mo: 0.005 to 0.4%, V: 0.02 to 0.5%, Nb: 0.003 to 0.15%, Al: more than 0.020 to 0.2%, N: 0.0200% or less, P: 0.02% or less, and S: 0.06% or less The balance is made of Fe and inevitable impurities, and the bainite phase has a structure satisfying an area ratio of more than 50% with respect to the entire structure.

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

3. 3. The soft nitriding component according to 2 above, wherein a precipitate containing V and Nb is dispersed and precipitated in the bainite phase.

4). In mass%, C: 0.10 to less than 0.20%, Si: 1.0% or less, Mn: more than 0.5 to 3.0%, Cu: 0.10 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.3 to 3.0%, Mo: 0.005 -0.4%, V: 0.02-0.5%, Nb: 0.003-0.15%, Al: more than 0.020-0.2%, N: 0.0200% or less, P: 0.02% or less and S: 0.06% or less, the balance being Fe And steel material consisting of inevitable impurities is hot-worked under the conditions of heating temperature: 950-1250 ° C, finishing temperature: 800 ° C or higher, and after processing, at a rate of more than 0.4 ° C / s in the temperature range of at least 700-550 ° C A method for producing a soft nitriding steel, characterized by cooling at a temperature.

5. The soft nitriding steel obtained by the production method described in 4 above is finished into a desired shape, and then subjected to soft nitriding under conditions of processing temperature: 550 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 soft nitriding steel with an inexpensive component system and excellent machinability, and after nitrocarburizing treatment, fatigue characteristics equal to or higher than those of JIS SCr420 material subjected to carburizing treatment are obtained. It is possible to obtain parts having the
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 components.

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. “%” Representing the following component composition means “mass%” unless otherwise specified.
C: 0.10% or more and less than 0.20% C is added to form a bainite phase and ensure strength. However, if the content is less than 0.10%, a sufficient amount of bainite phase may not be obtained, and the amount of V and Nb precipitates is insufficient after nitrocarburizing treatment, and it is difficult to ensure the strength. It is necessary to add. On the other hand, when the content is 0.20% or more, the hardness of the produced bainite phase is increased and the machinability is lowered, so the C content is in the range of 0.10% or more and less than 0.20%. More preferably, it is 0.10% or more and 0.15% or less of range.

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: More than 0.5% and less than 3.0%
Mn is added because it is effective in generating a bainite phase and improving the strength. However, when the amount of Mn is 0.5% or less, the amount of bainite phase generated is reduced, and V and Nb precipitates are generated before the soft nitriding treatment, so that the hardness before soft nitriding increases. In addition, since the absolute amounts of V and Nb precipitates after the soft nitriding process are reduced, the hardness after the soft nitriding process is lowered and it is difficult to ensure the strength. Therefore, Mn is added over 0.5%. 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. Preferably it is more than 0.5% and 2.5% or less, more preferably in the range of 0.6% or more and 2.0% or less.

Cu: 0.10 to 1.0%
Cu is a useful element that improves the strength of the nitrocarburized material by precipitation hardening during the nitrocarburizing treatment, and is also effective for the generation of a bainite phase. However, if the Cu content is less than 0.10%, it is difficult to obtain a desired effect, so the content is made 0.10% or more. On the other hand, if it exceeds 1.0%, the hardenability increases excessively, the amount of martensite generated increases, and the machinability deteriorates. Therefore, the amount of Cu is limited to the range of 0.10 to 1.0%. Preferably it is 0.10 to 0.50% of range.

Ni: 0.05-1.0%
Ni is added to suppress hot brittleness caused by Cu. In order to exert this effect, it is effective to set the ratio of Ni and Cu (Ni / Cu) to 0.5 or more. This is because the addition of Ni increases the solid solubility limit of Cu in γ-Fe, and the melting point of the Cu-concentrated phase increases, so that the appearance of Cu at the oxide scale / base metal interface can be suppressed. . From such a viewpoint, the Ni content is 0.05% or more. On the other hand, if it exceeds 1.0%, the hardness increases, which not only adversely affects the machinability but also is disadvantageous in terms of cost. For this reason, Ni content is limited to 0.05 to 1.0% of range. Preferably it is 0.05 to 0.50% of range.

Cr: 0.3-3.0%
Cr is added because it is effective for forming a bainite phase. However, when the content is less than 0.3%, the amount of bainite phase generated is reduced, and V and Nb precipitates are generated before the soft nitriding treatment, so that the hardness before soft nitriding increases. In addition, since the absolute amounts of V and Nb precipitates after the soft nitriding process are reduced, the hardness after the soft nitriding process is lowered and it is difficult to ensure the strength. Therefore, the Cr content is 0.3% 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 has the effect of precipitating V and Nb precipitates finely and improving the strength of the nitrocarburized material, and is an important element in the present invention. It is also effective for the generation of 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, it is 0.04 to 0.2% of range.

V: 0.02-0.5%
V is an important element that improves the strength by forming fine precipitates together with Nb and increasing the core hardness (hereinafter referred to as core hardness) of the component due to the temperature rise during soft nitriding. However, if the amount of V is less than 0.02%, it is difficult to obtain a desired effect. On the other hand, if it exceeds 0.5%, precipitates are coarsened and a sufficient strength improvement effect cannot be obtained, so the amount of V is in the range of 0.02 to 0.5%. Preferably it is 0.03-0.3%, More preferably, it is 0.03-0.25% of range.

Nb: 0.003-0.15%
Nb is extremely effective in improving fatigue strength because fine precipitates are formed together with V to increase the core hardness due to temperature rise during soft nitriding. However, if the Nb content is less than 0.003%, it is difficult to obtain a desired effect. On the other hand, if it exceeds 0.15%, precipitates are coarsened and a sufficient strength improvement effect cannot be obtained, so the Nb content is in the range of 0.003 to 0.15%. . Preferably it is 0.02 to 0.12% of range.

Al: more than 0.020% and less than 0.2%
Al is an element useful for improving the surface hardness and effective hardened layer depth after the soft nitriding treatment, so it is positively 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 a useful element that forms carbonitrides in steel and improves the strength of the nitrocarburized material. Therefore, it is preferable to contain 0.0050% or more. However, if the content exceeds 0.0200%, the carbonitride to be formed becomes coarse and the toughness of the steel material is lowered. Moreover, the surface crack of a slab arises and slab quality falls. 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.

In the steel of the present invention, other than the above-described components are Fe and inevitable impurities.
Among unavoidable impurities, Ti in particular has an adverse effect on the precipitation strengthening of V and Nb and lowers the core hardness. Therefore, it is necessary to prevent Ti from being contained as much as possible. Preferably it is less than 0.010%, more preferably less than 0.005%.

Next, the reason why the steel structure of the soft nitriding 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%.
In the present invention, V and Nb precipitates are dispersed and deposited on the core portion other than the surface nitriding portion after the soft nitriding treatment, thereby increasing the core hardness and improving the fatigue strength after the soft nitriding treatment. It is.
Here, if V and Nb precipitates exist before the soft nitriding treatment, it is disadvantageous from the viewpoint of machinability at the time of cutting performed before the soft nitriding treatment. In this regard, in the bainite transformation process, V and Nb precipitates are less likely to be generated in the parent phase than in the ferrite-pearlite transformation process.
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 soft nitriding steel, and a vertical cross section (L cross section) parallel to the rolling direction was corroded with nital after polishing, and the cross section was obtained using an optical microscope or a scanning electron microscope (SEM). The type of phase is identified by tissue observation (200 times optical microscope structure observation), and the area ratio of each phase is obtained.

Further, in the component of the present invention, the soft nitriding steel of the present invention is subjected to soft nitriding treatment to disperse precipitates containing V and Nb such as (Nb, V) (C, N) in the bainite phase. .
The reason for this is that the core portion hardness increases and the fatigue strength after the soft nitriding treatment is remarkably improved by dispersing and precipitating the precipitate containing V and Nb in the core structure other than the surface soft nitriding portion. It is.
Here, the particle size of the precipitate containing V and Nb in the bainite phase is preferably less than 10 nm, and it is preferable to disperse and deposit 500 or more per unit area of 1 μm ² in order to contribute to precipitation strengthening after the soft nitriding treatment. The measurement limit of the particle size of the precipitate, that is, the minimum measurable particle size is 1 nm.
In addition, as for the part obtained by performing a soft nitriding process, the surface layer part (parts other than a core part) becomes a component composition with high content of carbon and nitrogen with respect to the component composition of a core part.

Next, the manufacturing process of the steel for soft nitriding and parts of this invention is demonstrated.
FIG. 1 shows a typical manufacturing process for manufacturing a soft nitrided part using the soft nitriding steel (bar steel) 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 into a predetermined dimension, hot forging or cold forging is performed, and drilling or turning is performed as necessary. After cutting into a desired shape (for example, gear product or shaft part), soft nitriding is performed to obtain a product.
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. The structure suppresses the formation of V and Nb precipitates.
Here, hot working mainly means hot rolling and hot forging, but hot forging may be further performed after hot rolling. Needless to say, cold forging may be performed after hot rolling.
Here, when the hot working process immediately before the soft nitriding treatment is a hot rolling process, that is, when hot forging is not performed after hot rolling, the following conditions are satisfied 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, 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 finishing temperature is 800 ° C. or higher. The upper limit is preferably about 1100 ° C.

Cooling rate in a temperature range of at least 700 to 550 ° C. after rolling: More than 0.4 ° C./s Precipitation of fine precipitates so that fine precipitates are deposited before finishing to the desired shape and workability is not impaired. In the temperature range of at least 700 to 550 ° C., which is the temperature range, the cooling rate after rolling is set to a rate exceeding 0.4 ° C./s, which is the critical cooling rate for obtaining fine precipitates. 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, the heating temperature during hot forging is set to 950 to 1250 ° C., the forging finishing temperature is set to 800 ° C. or more, and the cooling rate after forging is set to exceed 0.4 ° C./s in a 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 a treatment temperature of 550 to 700 ° C. and a treatment time of 10 minutes or more so as to precipitate fine precipitates. Here, if the soft nitriding temperature is in the range of 550 to 700 ° C., a sufficient amount of precipitates cannot be obtained unless the temperature is lower than 550 ° C., whereas if it exceeds 700 ° C., it becomes an austenitic region and soft nitriding is difficult. Because it becomes. More preferably, it is the range of 550-630 degreeC.

In the soft nitriding treatment, N and C are simultaneously infiltrated and diffused, so a mixed atmosphere of a nitriding gas such as NH ₃ and N ₂ and a carburizing gas such as CO ₂ and CO, for example, NH ₃ : N ₂ : CO Soft nitriding may be performed in an atmosphere of ₂ = 50: 45: 5.

Examples of the present invention will be specifically described below.
Steels with the composition shown in Table 1 (steel types A to Y) were melted in a 150 kg vacuum melting furnace, heated at 1150 ° C, hot-rolled at a rolling finish temperature of 970 ° C, and then 0.9 ° C / s. The steel was cooled to room temperature at a speed to obtain a 50 mmφ steel bar. Steel type Y is steel corresponding to JIS SCr420.
These materials were further heated to the heating temperatures shown in Table 2 and then hot forged at the finishing temperatures shown in Table 2 to form 30 mmφ bar steel, and then the range of 700 to 550 ° C. as the cooling rate shown in Table 2, Cooled to room temperature. In addition, for a part, it cooled to room temperature by making the range of 700-550 degreeC into the speed of 0.1 degree-C / s for the comparison.

The hot forged material thus obtained was evaluated for machinability, particularly drillability, by a drill cutting test. A hot forged material cut to 20 mm thickness 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 locations per section Through holes were drilled, 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.
For hardness measurement, a Vickers hardness tester was used, and the hardness of the core was 5 points with a test load of 2.94N (300gf) in accordance with JIS Z 2244 (one center position on the steel bar cross section, D / D from the surface in the radial direction). 4 (D is the diameter of the steel bar) was measured at four points at intervals of 90 ° in the circumferential direction, and the average value was defined as the hardness HV.

Next, for steel types A to X, after the hot forging described above, soft nitriding treatment was further performed. On the other hand, the hot forging material of steel type Y was carburized for comparison.
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.
On the other hand, the carburizing treatment was performed by carburizing at 930 ° C. for 3 hours, holding at 850 ° C. for 40 minutes, oil cooling, and tempering at 170 ° C. for 1 hour.

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 and core hardness of the heat-treated material were measured. Here, both the surface hardness and the core hardness were measured with a test load of 2.94 N (300 gf) in accordance with JIS Z 2244 using a Vickers hardness tester. The surface hardness is 0.05 mm from the surface at 6 points at 60 ° intervals in the circumferential direction of the steel bar, and the core hardness is 1 center point on the steel bar cross section and the D / 4 depth position from the surface in the radial direction ( D is the diameter of the steel bar) measured at a total of 5 points, 4 points at 90 ° intervals in the circumferential direction, and the average value of each was defined as the surface hardness HV and the core hardness HV. Furthermore, the effective hardened layer depth was measured by defining the depth from the surface to be HV400.

  In addition, a sample for observation with a transmission electron microscope was prepared from the core of a soft nitrided material and a carburized material by an electrolytic polishing method using a twin jet method, and the obtained sample was subjected to a transmission electron microscope with an acceleration voltage of 200V. Was used to observe the precipitate. Further, the composition of the observed precipitate was determined by an energy dispersive X-ray spectrometer (EDX).

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) is sampled from the hot forged material, and the soft nitriding treatment or carburization described above is performed on the specimen. It performed using the heat processing material which processed.
Table 2 shows the test results. Nos. 1 to 10 and 26 are invention examples, Nos. 11 to 24 are comparative examples, and No. 25 is a conventional example obtained by carburizing JIS SCr420 equivalent steel.

  As is apparent from Table 2, all of Invention Examples No. 1 to 10 and 26 are superior in fatigue strength to Conventional Example No. 25 subjected to carburizing treatment. Moreover, the drill workability before soft nitriding of Nos. 1 to 10 and 26 is practically equivalent to or higher than that of the conventional example No. 25.

Furthermore, as a result of observation of precipitates with a transmission electron microscope and investigation of precipitate compositions with an energy dispersive X-ray spectrometer (EDX), the nitrocarburized materials Nos. 1 to 10 and 26 contained V in the bainite phase. It was confirmed that 500 or more fine precipitates having a particle size of less than 10 nm and containing Nb were dispersed and deposited per 1 μm ² . From this result, it is considered that the nitrocarburized material according to the present invention exhibited high fatigue strength due to precipitation strengthening by the fine precipitate.

On the other hand, Comparative Examples No. 11 to 24 are inferior in fatigue strength or drill workability because the component composition or the obtained steel structure was outside the scope of the present invention.
That is, No. 11 has a slow cooling rate after hot forging, so that an appropriate amount of bainite phase cannot be obtained, and since the amount of fine precipitates produced by soft nitriding is small, precipitation strengthening is insufficient. Fatigue strength is low compared to the examples.
In No. 12, since C exceeds the appropriate range, the hardness of the hot forged material before the soft nitriding treatment is increased and the drill workability is lowered.
In No. 13, since the Si amount exceeds the appropriate range, the hardness of the hot forged material before the nitriding treatment is increased and the drill workability is decreased.
In No. 14, since the Mn content exceeds the appropriate range, the steel structure of the hot forged material before the soft nitriding treatment is mainly composed of the martensite phase. For this reason, the hardness before the nitrocarburizing treatment is increased and the drill workability is reduced.
In No. 15, the amount of Cu is less than the appropriate range, so the age hardening ability is poor and sufficient core hardness is not obtained. For this reason, fatigue strength is low compared with conventional steel No.25.
In No. 16, since the amount of Cu exceeds the appropriate range, the hardness of the hot forged material before the soft nitriding treatment is increased and the drill workability is lowered.
In No. 17, since the Ni content exceeds the appropriate range, the hardness of the hot forged material before the soft nitriding treatment is increased, and the drill workability is reduced.
In No. 18, since the Cr content is less than the proper range, the steel structure of the hot forged material before the soft nitriding treatment is mainly composed of a ferrite phase and a pearlite phase. For this reason, V and Nb precipitates are precipitated in the structure, the hardness before the soft nitriding treatment is increased, and drill workability is lowered.
In No. 19, since the amount of Mo is less than the proper range, the amount of fine precipitates produced after soft nitriding is small, and sufficient core hardness is not obtained. For this reason, fatigue strength is low compared with the conventional example No.25.
In No. 20, the amount of V is less than the appropriate range, so the amount of fine precipitates produced after soft nitriding is small, and sufficient core hardness is not obtained. For this reason, fatigue strength is low compared with conventional steel No.25.
In No. 21, the amount of Nb is less than the appropriate range, so the amount of fine precipitates produced after soft nitriding is small, and sufficient core hardness is not obtained. For this reason, fatigue strength is low compared with conventional steel No.25.
No.22 and No.23 have less Al than the proper range, so sufficient surface hardness and effective hardened layer depth after soft nitriding cannot be obtained, and fatigue strength is lower than conventional steel No.25 .
No. 24 contains a large amount of Ti, which is an impurity component in the present invention, so that the amount of fine precipitates produced after soft nitriding is small and sufficient core hardness is not obtained. For this reason, fatigue strength is low compared with conventional steel No.25.

Claims (5)

  Component composition is mass%, C: 0.10 to less than 0.20%, Si: 1.0% or less, Mn: more than 0.5 to 3.0%, Cu: 0.10 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.3 to 3.0% , Mo: 0.005 to 0.4%, V: 0.02 to 0.5%, Nb: 0.003 to 0.15%, Al: more than 0.020 to 0.2%, N: 0.0200% or less, P: 0.02% or less, and S: 0.06% or less The balance is made of Fe and inevitable impurities, and the bainite phase has a structure satisfying an area ratio of more than 50% with respect to the entire structure.   A component comprising: a core portion comprising the component composition and structure according to claim 1; and a surface layer portion comprising a component composition having a high nitrogen and carbon content relative to the component composition of the core portion.   The component according to claim 2, wherein a precipitate containing V and Nb is dispersed and precipitated in the bainite phase.   In mass%, C: 0.10 to less than 0.20%, Si: 1.0% or less, Mn: more than 0.5 to 3.0%, Cu: 0.10 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.3 to 3.0%, Mo: 0.005 -0.4%, V: 0.02-0.5%, Nb: 0.003-0.15%, Al: more than 0.020-0.2%, N: 0.0200% or less, P: 0.02% or less and S: 0.06% or less, the balance being Fe And steel material consisting of inevitable impurities is hot-worked under the conditions of heating temperature: 950-1250 ° C, finishing temperature: 800 ° C or higher, and after processing, at a rate of more than 0.4 ° C / s in the temperature range of at least 700-550 ° C A method for producing a soft nitriding steel, characterized by cooling at a temperature.   The soft nitriding steel obtained by the production method according to claim 4 is finished into a desired shape, and then subjected to soft nitriding under conditions of processing temperature: 550 to 700 ° C. and processing time: 10 minutes or more. A method of manufacturing a component characterized by the above.

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