JP2005281857A - Raw material for nitrided component having excellent broaching workability and method for manufacturing nitrided component using the raw material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a raw material for nitrided components which permit easy broaching to have intricate hole shapes and have excellent surface hardness and hardening depth after a soft nitriding treatment and a method for manufacturing the nitrided components using the raw material. <P>SOLUTION: The raw material for the nitrided components is composed of a ferrite pearlite structure which contains, by mass %, 0.10 to 0.40% C, ≤0.50% Si, 0.30 to 1.50% Mn, 0.30 to 2.00% Cr, over 0.15 to 0.50% V, and 0.02 to 0.50% Al, further contains Ni, Mo, S, Bi, Se, Ca, Te, Nb, and Ti at need, consists of the balance Fe and impurity elements and has a ferrite hardness of HV ≥190. Also, the raw material for the nitrided components can be obtained by hot forging the hot rolled steel products composed of the above components after heating to 1,000 to 1,300°C and cooling the steel products at a rate of ≥0.25°C/sec until the surface temperature after the forging falls down to 600 to 690°C, then holding the steel products for ≥20 minutes at the above temperature region to complete the ferrite-pearlite transformation and cooling the steel products down to room temperature. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a material suitable for a nitrided part having a complicated hole shape, such as a gear part, and a method for producing a nitrided part using the material, and is particularly excellent in broachability and nitriding characteristics, and by hot forging. The present invention relates to a nitrided part material to be manufactured and a method for manufacturing a nitrided part.

  Among gear parts used for automobiles, construction vehicles, construction equipment, etc., there are many parts having a complicated hole shape in cross section. As a processing method for finishing such a complicated hole shape at a time, a method called broaching has been implemented.

Broaching is a general tool called broach, which is inserted into a pilot hole that has been previously machined into the workpiece, and then pulled downwards. This is a processing method in which a predetermined shape is finished by penetrating the workpiece to the upper finishing blade by the movement of the broach. Although this machining method has an advantage that a complicated hole shape can be machined by a single machining, it is a low-speed but heavy machining, and selection of a material that can sufficiently secure necessary workability is indispensable.

In addition, having a complicated hole shape, as a matter of course, the occurrence of distortion significantly degrades the quality of the component and is not allowed. However, in order to ensure the necessary strength and wear resistance, the occurrence of distortion is unavoidable when induction hardening is applied to gear parts in the past. Studies on soft nitriding, which is a surface hardening treatment method that can be suppressed, are underway.

However, in order to enable soft nitriding, it is necessary to add a component for improving nitriding characteristics, and there is a problem that the addition of this element reduces broachability. In other words, if it is not necessary to secure excellent nitriding properties, carbon steel with a carbon content of about 0.40% (Cr, Mo and other alloys not added) can be used. Processing is possible without problems. However, in such a carbon steel, it is necessary to use a tempered alloy steel such as SCM420 because the required soft nitriding characteristics cannot be obtained, but in that case, broachability is greatly reduced. Since the processing becomes difficult, development of a method for manufacturing a nitrided part having both excellent nitriding characteristics and broachability has been strongly desired.

Although not limited to broaching, it is not characterized by improved machinability, but an invention characterized by improving both machinability and nitriding characteristics has already been patented, for example, patent literature The inventions described in 1 and 2 are disclosed.

JP-A-8-81734 JP 59-190321 A

Among them, in the steel described in Patent Document 1, fine VC is dispersed by cooling the Cr-containing alloy steel containing V at a rate of 5 to 200 ° C./min up to 500 ° C. after hot working. As a ferrite pearlite structure, an attempt is made to obtain a steel excellent in both machinability and nitriding characteristics.

Moreover, the steel of patent document 2 is the speed | rate of 0.15-7.0 degreeC / second from 880 degreeC or more to 580-700 degreeC after hot processing the Cr containing alloy steel containing V similarly to patent document 1. Then, the steel is adjusted and cooled at 580 to 700 ° C. for 15 to 60 minutes to obtain a ferrite pearlite structure, thereby obtaining a steel excellent in both soft nitriding characteristics and machinability.

However, the above-described conventional invention has the following problems.
The steel described in Patent Document 1 is cooled at a speed within a specified range until it reaches 500 ° C. after hot working, and has a good workability structure in which a bainite structure does not occur while ensuring internal hardness. It is characterized by doing. However, when considering actual production, there are variations that cannot be avoided in production, such as variations in chemical composition, processing temperature, and cooling rate, resulting in variations in internal hardness, and stable broaching workability is obtained. There is no problem. In addition, even when the conditions are set on the assumption that the bainite structure is not generated when the initial manufacturing conditions are set, the bainite structure is generated in a part of the manufactured steel due to the influence of the above-described variation. The possibility could not be denied, and there was a problem that it was difficult to ensure stable and excellent workability.

Further, unlike the steel of Patent Document 1, the steel described in Patent Document 2 is characterized in that the transformation is completed by holding it in a temperature range of 580 to 700 ° C. for a certain period of time, so that the ferrite pearlite structure is stabilized. This is an advantageous method in terms of quality stability as compared with Patent Document 1. However, there has been a problem that excellent broachability cannot be stably obtained because no study has been made on the optimum hardness (ferrite hardness or the like) for improving broachability.

The present invention has been made in order to solve the above-described problems, and while ensuring excellent broaching workability capable of easily processing complicated hole shapes, nitrided parts having excellent soft nitriding characteristics are also provided. The purpose is to propose a new manufacturing method.

  The invention of claim 1 is mass%, C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, V: more than 0.15 to 0.50%, Al: 0.02 to A nitrided part material excellent in broachability, characterized by containing 0.50%, the balance being Fe and impurity elements, and a ferrite pearlite structure having a ferrite hardness of HV190 or more.

  The present inventors have investigated in detail about which factors can improve the tool life during broaching, which is characterized by low-speed heavy cutting. In the above-mentioned two patent documents, the structure is ferrite pearlite, and particularly bainite is not preferable because it reduces the machinability. Another structure is a sorbite structure obtained when tempering Cr-containing alloy steel. However, the conventional bad bainite structure means that when the bainite structure occurs, the hardness becomes hard and the workability decreases, and the effect on broaching workability between structures when the hardness is constant. Was not clear.

Therefore, the present inventors have investigated in detail about which structure has the best broachability when the heat treatment conditions are changed variously and the hardness is the same. As a result, the sorbite structure obtained by tempering even with the same hardness has the most deteriorated broachability, and the bainite structure exhibits superior broachability compared to the sorbite structure, but similar to the above-mentioned patent document It has been found that broach workability is most improved when the ferrite pearlite structure is used.

In addition, the nitrided parts obtained from the steel of the present invention have the same internal hardness as the tempered products (quenched and tempered materials) of carbon steel and alloy steel, considering the strength required for the parts. It is desirable to take account of the influence of the hardness of the ferrite portion having a low hardness. Therefore, the relationship between ferrite hardness and broachability was investigated in detail. As a result, if the ferrite hardness is rather soft, the broaching processability is lowered, and in order to obtain excellent broaching processability, the ferrite hardness needs to be HV190 or more. It has been found that it becomes possible by adding more than 0.15%.

The internal hardness described in many past patent documents is almost the average hardness of the entire structure. Moreover, even if the ferrite hardness is less than HV190, it is sufficiently possible to obtain a ferrite pearlite structure having an internal hardness equal to or higher than that of a conventional tempered steel, and the described hardness is Even if it is equal to or higher than the tempered product, the ferrite hardness is not always HV190 or higher. Furthermore, if additionally explained, in the case of an invention in which measures for strengthening ferrite such as addition of V are not taken, even if the hardness is equal to or higher than that of the tempered product, the ferrite hardness is It is natural to think that it is less than HV190. On the other hand, in the present invention, the ferrite hardness is set to HV190 or more, and the internal hardness equal to or higher than that of the tempered product can be secured. As a result, it is possible to obtain a nitrided part that has no problem in terms of strength as a part while ensuring excellent broachability.

Next, the reason for limiting each component range of the steel used in the method for producing a nitrided part excellent in broaching workability according to the present invention will be described.

C: 0.10 ~ 0.40%
C is an element necessary for increasing the internal hardness and ensuring the necessary strength, and needs to be contained in an amount of 0.10% or more. However, if the content exceeds 0.40%, the soft nitriding properties (surface hardness, hardening depth) are reduced, and the internal hardness is too high, so that the machinability and toughness are reduced, so the upper limit is 0.40%. did.

Si: 0.50% or less
Si is an element necessary for deoxidation during the production of steel. When Si is used as a deoxidation element, it is an element that needs to be contained in a small amount. However, Si is an element that deteriorates the nitriding characteristics. If it is contained, the surface hardness and the hardening depth after soft nitriding will decrease, so its content must be the minimum amount for deoxidation treatment. The upper limit was 0.50%.

Mn: 0.30 to 1.50%
Mn is an element necessary for ensuring the necessary hardenability, increasing the internal hardness, and ensuring the required strength. In order to obtain the effect, it is necessary to contain 0.30% or more. However, if it is contained in a large amount, the soft nitriding characteristics are lowered as in C, and a bainite structure is easily generated, the core hardness becomes too high, and the machinability and toughness are lowered. 1.50%.

Cr: 0.30-2.00%
Cr is an extremely important element for the present invention because it has the effect of increasing the surface hardness and softening depth after soft nitriding, improving the hardenability and ensuring the necessary strength. Therefore, it is necessary to contain 0.30% or more. However, if it is contained in a large amount, a bainite structure is likely to be formed, and both toughness and machinability are lowered. Therefore, the upper limit was made 2.00%.

V: Over 0.15 ~ 0.50%
V is the most important element for the present invention, and strengthens the ferrite in the ferrite pearlite structure that is isothermally transformed by holding the temperature after hot forging described later, thereby increasing the ferrite hardness and improving the low-speed machinability. It is an indispensable element for obtaining broachability. Further, since the ferrite hardness is increased by the addition of V, as a result, it is possible to obtain an internal hardness equal to or higher than that of a conventional tempered product. V also has the effect of improving soft nitriding characteristics. In order to obtain the above effects sufficiently, it is necessary to contain V at least more than 0.15%, desirably 0.18% or more. However, even if it is contained in a large amount, the above effect is saturated, and an effect commensurate with the high cost of addition cannot be obtained, so the upper limit was made 0.50%.

Al: 0.02-0.50%,
Al is an element effective for deoxidation, and is an element that increases the surface hardness after nitriding and improves soft nitriding characteristics. In order to obtain a sufficient effect not only for deoxidation but also for improving soft nitriding properties, it is necessary to contain 0.02% or more at least. However, if contained in a large amount, the effect of improving the soft nitriding properties is saturated, and oxide inclusions (alumina) increase to adversely affect fatigue strength, so the upper limit was made 0.50%.

Next, the manufacturing conditions of the method for manufacturing a nitrided part (claim 3) using the nitrided part material according to claim 1 and the reason for limitation will be described below.
Claim 3 is a hot rolled steel material composed of the components defined in claim 1 and is hot forged after heating to 1000 to 1300 ° C., and the surface temperature after forging is 0.25 ° C./second or more until the surface temperature becomes 600 to 690 ° C. After cooling at a speed, the temperature is maintained in the temperature range for 20 minutes or more to complete the ferrite pearlite transformation, and by cooling to room temperature, a ferrite part material having a ferrite pearlite structure having a ferrite hardness of HV190 or more is manufactured, A nitrided part manufacturing method characterized by subjecting this material to predetermined machining such as broaching and then soft nitriding.

  In the method for manufacturing a nitrided part according to claim 3, first, steel having the components specified in claim 1 is melted in an electric furnace or the like, and a base material is manufactured by hot rolling. This base material is cut into predetermined dimensions and heated to perform hot forging.

Here, the lower limit of the heating temperature (surface temperature) at the time of hot forging was set to 1000 ° C., because the V that is precipitated as carbonitride in the rolled steel material at the time of heating at the forging sufficiently This is because the solid is dissolved and precipitated in the ferrite while maintaining the temperature described later, and the ferrite hardness is improved by precipitation hardening. V carbonitride dissolves at low temperatures compared to Al and Nb carbonitrides used for grain refinement in case-hardened steel, but it still exceeds 1000 ° C to achieve sufficient solid solution. It is necessary to heat. When solid solution is insufficient, fine precipitation during temperature holding in the subsequent process becomes insufficient, strengthening by precipitation hardening is reduced, ferrite hardness is lowered, and broach workability is reduced. Become.

In addition, the upper limit of the heating temperature is 1300 ° C. If heated to 1300 ° C, V is already in a completely solid solution state, and any effect can be obtained even if the temperature is increased further. This is because it is not possible and only wastes energy, and the crystal grains become coarse and the toughness is lowered.

  When the forging is completed, it is cooled at a rate of 0.25 ° C./second until the surface temperature reaches 600 to 690 ° C. As this cooling slows down, the ferrite hardness decreases, and the ferrite pearlite structure obtained by maintaining the temperature described later becomes coarse and the toughness decreases, so cooling at a rate of at least 0.25 ° C./second or more did. The designation of the cooling rate is for adjusting the ferrite hardness of the final product, and does not limit the surface temperature of the temperature when charged into the furnace to 600 to 690 ° C. Since the temperature of the material to be heat-treated at the time of charging in the furnace fluctuates due to variations during manufacturing, a slight fluctuation in charging temperature (for example, the charging temperature decreases to the 500 ° C range or slightly exceeds 690 ° C) If there is a temperature), there is no effect on the quality of the final product, and as long as the furnace temperature is accurately controlled, a product with the desired quality can be obtained. This is because if the holding temperature is accurately controlled, even if the charging temperature fluctuates somewhat, the ferrite pearlite transformation proceeds and is completed during the temperature holding, and is dissolved in the ferrite by heating during the forging. This is because V is precipitated finely as carbonitrides and the ferrite hardness is improved, and the intended product can be produced.

Here, the reason why the holding temperature range is set to 600 to 690 ° C. is that the ferrite pearlite transformation takes a long time even if the temperature is lower or higher than this temperature range, so that it cannot be efficiently manufactured. . Furthermore, if the temperature is lower than 600 ° C, there is a risk that bainite nose may occur depending on the components, and bainite may be generated during the temperature holding. If it exceeds 690 ° C, the internal hardness decreases. This is because it is difficult to obtain the required hardness.

In addition, the lower limit of the holding time is set to 20 minutes because if the holding time is shorter than this, it becomes difficult to complete the ferrite pearlite transformation during temperature holding, and there is a concern that bainite may be generated by subsequent cooling. is there. As described above, in order to obtain excellent broachability, the structure needs to be ferrite pearlite, and since the transformation needs to be completed during this temperature holding, the lower limit of the holding time is set to 20 minutes. . If the ferrite pearlite transformation is completed during the temperature holding, the target effect can be obtained, so the upper limit of the holding time is not particularly limited, but if it becomes extremely long, the productivity decreases and the cost increases. Therefore, it is preferable to be within 2 hours at the longest.

When the temperature holding is finished, cool to room temperature. At this time, since the transformation has already been completed, it may be cooled rapidly or slowly, and even if the cooling rate is changed, there is no influence on the tissue. Therefore, the cooling may be performed under conditions that are easy to implement in accordance with the convenience of the actual manufacturing site. A forged part having excellent broachability can be produced by the above process.

The structure after cooling to room temperature is a ferrite pearlite structure, but this does not mean that bainite and sorbite structures are not allowed to be produced. This is because, in actual production, a bainite structure may be generated due to variations in various conditions, and production may be difficult if not allowed at all. Specifically, it is desirable to suppress it to about 3% or less. However, of course, 0% is desirable.

Since the manufactured hot forged part is still in a rough-formed state with respect to the final shape, further predetermined processing is performed. At this time, the broaching described above is also carried out together with other processes, and finished into a predetermined product shape. At this time, since it is adjusted to an appropriate hardness and structure by manufacturing in the above-described process, broaching can be easily performed, and finishing can be completed.

Finally, soft nitriding is performed. This soft nitriding treatment may be carried out under the conditions that are normally carried out for ordinary nitriding steels, but may of course be carried out with a slight optimization of the conditions. Steels with the above component ranges are added with elements that enhance nitriding properties such as Al and V, and have excellent nitriding properties, so that the desired surface hardness and hardening depth can be easily obtained. .

By manufacturing in the process described above, parts with complex hole shapes can be efficiently broached (= long tool life and good production efficiency), and the surface hardness after soft nitriding and the depth of hardening can be achieved. An excellent nitrided part can be manufactured.

By manufacturing the material for nitrided parts comprising the steel component according to claim 1 through the process described above, a part that is easy to broach and has excellent nitriding characteristics can be manufactured. Add one or more elements of Ni, Mo, S, Bi, Se, Ca, Te, Nb, Ti according to the conditions, and further improve soft nitriding properties, strength, toughness, and machinability (Claim 2). Hereinafter, the reason for limitation of each component is demonstrated.

Ni: 2.00% or less, Mo: 0.50% or less
Ni and Mo are effective elements for further improving the strength and toughness of the nitrided part obtained in the present invention, and can be added as necessary. However, Ni and Mo are expensive elements as compared with the alloy elements used in claim 1, and the amount of addition must be appropriately adjusted by comparing the effect obtained and the cost increase due to the addition. However, if added in a large amount, the effect is saturated, and there is no possibility of improving the properties commensurate with the cost increase, so the upper limit was limited to 2.00% for Ni and 0.50% for Mo.

S: 0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Ca: 0.10% or less, Te: 0.30% or less
S, Bi, Se, Ca, and Te are elements well known as elements for improving machinability, and if necessary, in addition to the steel used in claim 1, machinability ( It is an element that can improve broachability. However, the addition of these elements deteriorates hot workability and causes forging formability to deteriorate, and also adversely affects fatigue strength and toughness. Therefore, the addition of these elements must be minimized. Therefore, at most, Bi, Se, and Te are required to be 0.30% or less, S is 0.20% or less, and Ca is 0.10% or less.

Nb: 0.50% or less, Ti: 1.00% or less
Nb and Ti are elements that can increase the surface hardness after nitriding treatment and improve soft nitriding characteristics because they combine with N that penetrates into the steel from the surface by nitriding treatment to form hard nitride. . Therefore, it can be added as required according to the required surface hardness after nitriding. However, the effect is saturated even if it is added in a large amount, and an effect commensurate with the increase in cost due to the addition cannot be obtained. Therefore, the upper limit was set to 0.50% for Nb and 1.00% for Ti.

  In addition, the manufacturing method in the case of manufacturing a nitrided part using the steel which added the arbitrary addition element demonstrated above is completely the same as the method performed when not using an arbitrary addition element mentioned above. Therefore, by similarly performing hot forging and holding the temperature, it is possible to produce a material for nitride parts with excellent broachability, and by performing nitriding after performing mechanical processing such as broaching, A nitrided part excellent in terms of surface hardness and curing depth can be produced.

  Next, the characteristics of the nitrided part material and the method of manufacturing the nitrided part using the material according to the present invention will be described in comparison with comparative examples. Table 1 shows chemical components of the test steels used as examples.

  In Table 1, steels 1 to 14 are steels within the composition range specified in the present invention, among which steels 1 to 5 are test steels corresponding to claims 1 and 3 and steels 6 to 14 are claims 2. This is a test steel corresponding to No.4. In addition, steels 15 to 21 are test steels whose components are outside the range specified in the present invention, and steels 22 and 23 are JIS SCM420 and S45C, which are steels that have been widely used conventionally. (Hereinafter, steels 1 to 14 are referred to as invention steels, steels 15 to 21 are referred to as comparative steels, and steels 22 and 23 are referred to as conventional steels).

In addition, in order to evaluate the steel of many components in a short time for the test steel, the steel of the present invention and the comparative steel were ingots and machined using steel ingots melted in a 30kg vacuum induction melting furnace. Φ60mm × 90mm round bar specimens were prepared, and for conventional steels 22 and 23, a portion of the φ60 round bar that was actually manufactured was sampled to obtain a φ60 × 90mm test. A piece was prepared and the test described below was performed. Among these test pieces, the test pieces of 1 to 21 steel were heated to 1200 ° C and upset to a height of 40 mm, and after processing, cooled to 640 ° C at a rate of 0.4 ° C / second, The temperature was maintained for 30 minutes and then cooled to room temperature. In addition, the conventional steels Nos. 22 and 23 were heated to 1200 ° C., air-cooled as they were after hot forging by the same method as described above, and subjected to quenching (860 ° C.) and tempering treatment. The tempering treatment was carried out by adjusting the core hardness so as to be an average degree (HV230) of the steels 1 to 21 (22 steel was 590 ° C. and 23 steel was 600 ° C.). The test piece thus obtained was cut and machined by a method that does not affect the material properties, and a test piece having a width of 40 mm and a height of 20 mm was prepared. At the same time, a test piece (diameter 20 mm) for soft nitriding property test was prepared, and the test described later was performed.

Next, a method for evaluating broachability will be described. Broach teeth used for actual parts manufacturing are very expensive.In this experiment, broach teeth for evaluation (width 4 mm, rake angle 18 °, second angle 2 °, side clearance angle 13 °, teeth Formula 5) was prepared, and the above-mentioned test piece having a width of 40 mm was milled (cutting speed 7 m / min, cutting allowance 0.05 mm) in the cutting direction as shown in FIG. The cutting resistance inside was measured. In this processing, the better the workability, the smaller the cutting resistance at the time of processing. Therefore, broachability can be evaluated by measuring the cutting resistance during milling. However, as the processing progresses, the broach teeth wear and the influence may affect the cutting resistance, so in the final stage of the above processing (the final stage including when the total cutting length of 40,000 mm is finished) Cutting resistance was measured. Moreover, the ferrite hardness was also measured using a part of the cutting test piece.

The soft nitriding characteristics were obtained by performing the soft nitriding treatment under the condition of 560 ° C x 2.5 hours in the atmosphere gas in which ammonia was mixed with RX gas using the test piece having a diameter of 20 mm as described above. It was measured. Note that the curing depth measured here is a depth at which HV513 (HRC50) is obtained. The results are shown in Table 2.

  As is apparent from Table 2, all the steels 1 to 14 according to the examples of the present invention were upset and forged and maintained at a constant temperature to transform the ferrite pearlite structure having a ferrite hardness of HV190 or more. 10% compared with the tempered product of S45C (23 steel), which is a conventional steel that has no bainite and sorbite structure, resulting in poor soft nitriding properties but no problem with broachability. As described above, cutting resistance was reduced, and excellent broachability was obtained. It was also revealed that not only broachability is excellent, but also the nitriding characteristics are equivalent to or better than the conventional steel SCM420 (22 steel).

  On the other hand, it was clarified that one of the comparative examples 15-21 steel that was upset and forged and kept at a constant temperature and transformed at a constant temperature was inferior in broachability and soft nitriding properties. That is, 15 steel has a high C content, so no specific numerical values are shown in Table 2, but the core hardness increased, broachability decreased, and nitrocarburizing characteristics also decreased. Yes, steel No. 16 has a high Si content and therefore soft nitriding characteristics have been reduced. Steel Nos. 17 and 19 have a high Mn or Cr content, so a bainite structure is formed and the core hardness increases. However, while broaching processability is reduced, 17 steel has a reduced soft nitriding property, and 18 and 20 steels have a low content of Cr or Al, which is an element necessary for improving the soft nitriding property. The soft nitriding characteristics are reduced, the surface hardness after nitriding treatment is low, and the hardening depth is shallow. 21 steel has low V content, so the ferrite hardness decreases and broachability is low. It is a drop. In addition, steels 15, 17, 19, and 21 had broaching workability substantially the same as S45C, although broaching workability decreased compared to the present invention. Of these, steels 19 and 21 have almost the same nitriding properties as the steel of the present invention, and can be used if broaching workability equivalent to S45C is sufficient.

In addition, since 22 steel (tempered product of SCM420), which is a conventional steel, has a sorbite structure due to tempering, broaching workability is maintained even if the core hardness is adjusted to be equivalent to the test piece of the present invention. It is inferior. Note that the 23 steel (S45C tempered product) has not been tested because it is known that the soft nitriding properties are greatly inferior without being tested. The result of the cutting resistance was carried out in order to grasp the reference value that can be broached without any problem, and the comparison with the data of the example of the present invention is as described above.

As a result, the conventional method has only obtained one of broaching workability and soft nitriding characteristics that can satisfy the required level, but by applying the method of the present invention, It became clear that a nitrided part that is easy to process and has excellent surface hardness and depth after soft nitriding can be obtained.

In the said Example, the result of having evaluated forging conditions, the temperature maintenance conditions after forging, etc. was shown. In this example, using one steel shown in Table 1, the forging conditions and the temperature holding conditions are changed as shown in Table 3, and how the broaching workability changes with the above examples. The conditions other than those in Table 3 were evaluated by the same method. The results are shown in Table 4.

In Table 4, it is a case where the manufacturing method of A-C satisfies the conditions of this invention, and the conditions of DH are the cases where any manufacturing conditions do not satisfy the conditions of this invention.
Looking at this result, the test piece obtained under Condition D had a low heating temperature, so the solid solution of V carbonitride in the steel became insufficient, the ferrite hardness decreased, and broachability decreased. In condition E, the cooling rate after upsetting forging was too slow, and as with D, the ferrite hardness decreased and the workability decreased.

In addition, the specimens prepared under the conditions of F and G were not cooled properly because the temperature conditions for isothermal transformation were not appropriate, and the ferrite pearlite transformation during temperature holding did not proceed as expected, and the sample was air-cooled before the transformation was completed. In this condition, bainite was generated by air cooling and broachability decreased, and under condition H, the set temperature was appropriate, but the temperature holding time was too short, 10 minutes, so the ferrite pearlite transformation was completely completed. Before it is air-cooled, bainite is generated and broachability is lowered.
On the other hand, it can be confirmed that all the test pieces produced under the conditions A to C satisfying the conditions specified in the present invention have the same or better broachability than the conventional steel S45C. It was.

  In this way, it is not enough to manage the internal hardness simply by controlling the internal hardness, and the structure should be a ferrite pearlite structure that hardly contains bainite and sorbite, and the ferrite hardness should be HV190 or more. Therefore, it is effective to appropriately adjust the heating temperature at the time of forging, the subsequent cooling rate, and the temperature holding conditions after limiting the components within a specific range.

  The basic evaluation explained above has revealed the possibility of producing a nitrided part that is easy to broach and has excellent soft nitriding characteristics. Steel corresponding to steel No. 1 was melted in an electric furnace for actual production, nitrided parts were produced under the conditions A shown in Table 3, and soft nitriding characteristics and broachability were evaluated. As a result, it was confirmed that substantially the same effect as the basic evaluation result shown in the above example was obtained. In addition, although it did not show about the amount of wear of broach teeth in the above-mentioned example, it has been confirmed that the tool life can be greatly improved because the cutting resistance can be reduced and the wear can be suppressed to a small extent by evaluation with this actual part. .

  As described above, the present invention facilitates broaching by limiting the components to a specific range and forging and isothermal transformation under appropriate conditions to obtain a ferrite pearlite structure having a ferrite hardness of HV190 or more. In addition, a nitrided part having excellent surface hardness and hardening depth after soft nitriding can be produced. Therefore, a nitrided part having a hole shape with a complicated cross-sectional shape can be easily manufactured, and the contribution to the industry is extremely large.

It is a figure explaining the evaluation method of broach workability.

Claims (4)

In mass%, C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, V: more than 0.15 to 0.50%, Al: 0.02 to 0.50%, the balance A material for nitride parts with excellent broachability, characterized in that is made of Fe and an impurity element and has a ferrite pearlite structure with a ferrite hardness of HV190 or higher. In addition to the steel according to claim 1, Ni: 2.00% or less, Mo: 0.50% or less, S: 0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Ca: 0.10% or less, A material for nitride parts having excellent broachability, characterized by containing one or more of Te: 0.30% or less, Nb: 0.50% or less, and Ti: 1.00% or less. In mass%, C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, V: more than 0.15 to 0.50%, Al: 0.02 to 0.50%, the balance Hot-rolled steel consisting of Fe and impurity elements, hot forged after heating to 1000-1300 ° C., after cooling at a rate of 0.25 ° C./second or more until the surface temperature after forging becomes 600-690 ° C., Hold the temperature in the temperature range for 20 minutes or more to complete the ferrite pearlite transformation and cool to room temperature to produce a material for nitride parts consisting of ferrite pearlite structure with a ferrite hardness of HV190 or more. A method for producing a nitrided part, characterized by performing a soft nitriding treatment after a predetermined machining. In addition to the steel used in the production method according to claim 3, Ni: 2.00% or less, Mo: 0.50% or less, S: 0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Applying the production method according to claim 3 to hot rolled steel containing one or more of Ca: 0.10% or less, Te: 0.30% or less, Nb: 0.50% or less, Ti: 1.00% or less. A method for producing a nitrided component.

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