JP2017066477A - Method of manufacturing cold forged age hardened steel component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a component for cold forging, exhibiting high cold forgeability and providing high endurance ratio by work hardening due to cold forging and age hardening after the cold forging.SOLUTION: A cold forged age hardened steel component is obtained using, as a raw material, a hot rolled steel material which contains C:0.02 to 0.13%, Si:0.01 to 0.50%, Mn:0.20 to 0.70%, S:0.005 to 0.020%, Al:0.005 to 0.050%, Cr:0.02 to 1.50%, V:0.02 to 0.50%, N:0.003 to 0.030%, P: limited to 0.020% or less and the balance Fe with impurities and which has mainly ferrite and pearlite and a total area percentage of ferrite and pearlite of 90% or more. The hot rolled steel material is cold rolled in such a manner that equivalent strain of 0.2 or more is added to a region of the hot rolled steel material where fatigue strength is required, the region corresponding to a region of a component being a product, the cold rolled steel material is subjected to age hardening treatment at the temperature range of Ac3 point or less of the raw material, and thus the cold forged age hardened steel component having V deposit amount at the above region of 40% or more is obtained.SELECTED DRAWING: Figure 1

Description

  TECHNICAL FIELD The present invention relates to a method for manufacturing a cold forged part, and more particularly to a method for manufacturing a cold forged age-hardened steel part that is formed by cold forging and then subjected to age hardening to be used as a machine structural part or the like. It is.

Carbon steel for machine structure and alloy steel for machine structure are used as structural steel that is a material for machine structural parts such as automobile parts, industrial machine parts, and construction machine parts.
In order to manufacture parts from these steel materials, conventionally, the "hot forging-cutting" process has mainly been carried out, but in recent years, switching from hot forging to cold forging has been carried out for the purpose of improving productivity. The tendency to adopt the “cold forging-cutting” process is increasing. Thus, if cold forging is applied instead of hot forging, near-net shaping is achieved with the forged material, and productivity can be improved by reducing the amount of cutting after forging.

  However, in general, cold forging has a high degree of processing, and thus causes problems such as high processing load, short mold life, and easy cracking of parts. Therefore, in order to switch to the “cold forging-cutting” process, the cold forgeability (cold forgeability) of the steel material is increased, that is, the load during cold forging is reduced and cracking is suppressed. It has become the most important issue.

  On the other hand, high fatigue strength is required for cold forged parts obtained by cold forging, that is, machine structural parts such as automobile parts, industrial machine parts, and construction machine parts. In order to exhibit high fatigue strength after cold forging, it is effective to increase the hardness after cold forging. For this purpose, it is conceivable to adjust the material components and the like so that the hardness of the material is increased. However, if the hardness of the material before cold forging is increased, the cold forgeability is lowered. That is, it has been extremely difficult to achieve both cold forgeability and fatigue strength in the steel material.

Therefore, in order to solve such a problem, in order to increase the fatigue strength of the cold forged parts, after cold forging, the steel is heated to a temperature of Ac3 or higher and subjected to quenching / tempering or induction quenching heat treatment. Conventionally, the surface is cured.
However, in such a method, since the hardness of the parts after the heat treatment is increased, a reduction in machinability in the final finishing process cannot be avoided, and the merit of improving the productivity by cold forging cannot be effectively exhibited. There was a problem.

  Therefore, a so-called age-hardening steel material has been proposed which does not increase the hardness more than necessary at the stage of cutting, but applies a heat treatment that increases the hardness after cutting, that is, an age hardening treatment.

  For example, in Patent Document 1, the chemical component is mass%, C: 0.01 to 0.15%, Si: 0.05% or less, Mn: 0.10 to 0.90%, P: 0.030. % Or less, S: 0.030% or less, Cr: 0.50 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.10%, N: 0.00080% And O: 0.0030% or less, with the balance being Fe and impurities, 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V ≦ 160 or less, 20 ≦ (669.3 × log C-1959.3 × log N-6983.3) × (0.067 × Mo + 0.147 × V) ≦ 80, 160 ≦ 140 × Cr + 125 × Al + 235 × V, 90 ≦ 511 × C + 33 × Mn + 56 × Cu + 15 × Ni + 36 × Cr + 5 × Mo + 134 × V ≦ 170 However, the structure is a ferrite-pearlite structure, a ferrite-bainite structure, or a ferrite-pearlite-bainite structure, and the area ratio of ferrite is 70% or more, and the V content in the precipitate by the extraction residue analysis is 0.1. Disclosed is a technology related to cold forging and nitriding steel, cold forging and nitriding steel, and cold forging and nitriding parts, wherein the core hardness is not less than 10% and the core hardness is not less than 220, 0, and 20 mm in terms of Vickers hardness ing.

  Patent Document 2 discloses that the chemical component is mass%, C: 0.06 to 0.50%, Si: 0.05% or less, Mn: 0.5 to 1.0% or less, V: 0.10. The total amount of pro-eutectoid ferrite and pearlite is 90% or more in area ratio, and the pro-eutectoid ferrite amount is represented by the formula f = 100-125 [C] +22.5 [V]. There is disclosed a technology relating to a steel for cold heading having an area% equal to or greater than the f value and having excellent cold workability in which VC is precipitated in the pro-eutectoid ferrite.

International Publication No. 2012/053541 JP 2000-273580 A

  The technique disclosed in Patent Document 1 described above provides steel and steel materials having excellent cold forgeability and machinability after cold forging, and parts subjected to cold forging and nitriding treatment In addition, a high core hardness, a high surface hardness, and a deep effective hardened layer depth can be provided. However, Patent Document 1 does not mention fatigue strength at all, and therefore, improvement of the durability ratio (fatigue strength / tensile strength) has not been studied.

  On the other hand, the technique disclosed in Patent Document 2 relates to a steel for cold heading that can be subjected to cold working as it is rolled, and precipitates V carbide (VC) during hot rolling, A steel with improved cold forgeability by reducing the solid solution C is provided. However, the technique of Patent Document 2 does not consider fatigue strength. Further, in order to improve the strength, it is assumed that a tempering treatment is performed, and cutting is necessary in a hardened state after the tempering treatment, and a reduction in machinability cannot be avoided.

  The present invention has been made against the background described above, and shows high cold forgeability in the cold forging process, and at the same time, a high durability ratio is obtained by work hardening by cold forging and age hardening after cold forging. It is an object of the present invention to provide a method for manufacturing a cold forged part.

  As a result of various experiments and examinations by the present inventors in order to solve the above-mentioned problems, the following items (A) to (E) have been clarified.

  (A) In order to obtain excellent cold forgeability, it is necessary to reduce the hardness of the material (steel) used for forging. The forging load can be reduced by reducing the hardness of the material. Moreover, in order to suppress the crack at the time of cold forging, it is effective to reduce the amount of C of steel used as a raw material.

  (B) In order to obtain high fatigue strength while suppressing the hardness after cold forging, it is important to increase the durability ratio (fatigue strength / tensile strength) of the component. In order to increase the durability ratio of parts, it is effective to utilize work hardening by cold forging and precipitation of carbonitrides. In addition, the high durability ratio in this invention points out that it is a durability ratio of 0.55 or more. It is still more desirable if it is 0.60 or more.

  (C) In order to precipitate V carbonitrides while maintaining cold forgeability, it is effective to utilize aging precipitation obtained by raising the temperature to a temperature range of Ac3 point or lower after cold forging. It is.

  (D) It has been clarified that by applying a considerable strain by cold forging, precipitation of V carbonitride is promoted and the durability ratio of the component is improved.

  (E) Even if the amount of C is reduced in order to show high forgeability during cold forging, and the chemical composition of the material is controlled appropriately even if the temperature is increased for aging precipitation after cold forging, it is sufficient As a result, it became clear that the durability ratio of the parts was improved.

  This invention is completed based on the knowledge of the above (A)-(E), The summary is the manufacturing method of the cold forging age-hardening steel components shown to following (1)-(2). is there.

(1) In mass%,
C: 0.02-0.13%,
Si: 0.01 to 0.50%,
Mn: 0.20 to 0.70%,
S: 0.005-0.020%,
Al: 0.005 to 0.050%,
Cr: 0.02 to 1.50%,
V: 0.02 to 0.50%,
N: 0.003-0.030%
And P: limited to 0.020% or less, the balance having a chemical composition consisting of Fe and inevitable impurities,
Moreover, the metal structure is mainly composed of ferrite and pearlite, and a hot rolled steel material having a total area ratio of ferrite and pearlite of 90% or more is used as a material.
For the material, cold forging is performed so that an equivalent strain of 0.2 or more is applied to a part of the product part where at least fatigue strength is required,
Further, the cold forged age hardened steel is obtained by performing age hardening in a temperature range of Ac3 point or less of the raw material to obtain a cold forged age hardened steel part having a V precipitation amount of 40% or more in the part. A manufacturing method for parts.

  (2) The hot-rolled steel material further contains one or more of Cu: 0.20% or less, Ni: 0.20% or less, and Mo: 0.20% or less. The manufacturing method of the cold forging age-hardening steel part of (1).

  (3) It is characterized in that a region where a maximum stress is assumed to be generated at the time of use of a product part is included and at least a region where a stress of 90% or more of the maximum stress is generated is defined as the portion. The manufacturing method of the cold forging age-hardened steel part in any one of said (1) and (2).

  According to the method for producing a cold forged age-hardened steel part of the present invention, the cold forgeability at the time of cold forging is excellent, and a high durability ratio and coverage are achieved by age hardening without performing quenching and tempering or induction hardening. Machinability can be secured. Therefore, instead of the conventional "hot forging-cutting" process, it manufactures machine structural parts such as automobile parts, industrial machine parts, construction machine parts, etc. by the "cold forging-age hardening-cutting" process. It is possible to improve the productivity by forming a near net shape with a forged finished material.

It is a graph which shows the experimental result about the relationship between the equivalent distortion | strain provided in cold forging, and the durability ratio of a product. It is a graph which shows the experimental result about the relationship between the amount of V precipitation in the components of a product, and the endurance ratio of a product.

  Below, the manufacturing method of the cold forging age-hardening steel part of this invention is demonstrated in detail.

  In the manufacturing method of the cold forged age-hardened steel part of the present invention, the subsequent manufacturing process is defined using hot rolled steel as a raw material, but not only the manufacturing process conditions but also the hot rolled steel as a raw material The component composition and the metallographic status are also important. First, the component composition of the material will be described. In the following description, “%” of the content of each element means “% by mass”.

  In the method for producing the cold forged age-hardened steel part of the present invention, the hot-rolled steel used as a raw material includes, as essential components, C: 0.02 to 0.13%, Si: 0.01 to 0.50% , Mn: 0.20 to 0.70%, S: 0.005 to 0.020%, Al: 0.005 to 0.050%, Cr: 0.02 to 1.50%, V: 0.02 It is steel containing -0.50% and N: 0.003-0.030%. First, the reasons for limiting these essential component elements will be described.

<C: 0.02-0.13%>
C is an element necessary for increasing the strength as a machine structural component. However, if the content exceeds 0.13%, cracks occur during cold forging, so the C content is set to 0.13% or less. On the other hand, if the C content is less than 0.02%, a tensile strength of 400 MPa or more and a fatigue strength of 250 MPa or more cannot be ensured after age hardening. Therefore, the C content is set to 0.02% or more. The C content is preferably 0.03% or more and less than 0.10%.

<Si: 0.01 to 0.50%>
Si is an element necessary for deoxidation at the time of melting steel, and in order to obtain this effect, 0.01% or more of Si is contained. However, since Si strengthens the solid solution of ferrite, if the Si content exceeds 0.50%, cold forgeability is lowered. Therefore, the Si content is set to 0.50% or less. Note that the Si content is desirably 0.05% or more and 0.45% or less.

<Mn: 0.20 to 0.70%>
Mn increases the strength of the final part as a solid solution strengthening element. If the Mn content is less than 0.20%, the strength of the final part is insufficient, and if it exceeds 0.70%, the cold forgeability is lowered. Therefore, the Mn content is set to 0.20 to 0.70%. Note that the Mn content is desirably 0.25% or more and 0.65% or less.

<S: 0.005 to 0.020% or less>
S is an element that improves machinability. In order to obtain the effect of improving machinability, it is necessary to contain 0.005% or more of S. On the other hand, if the S content exceeds 0.020%, coarse sulfides are generated in the steel, which causes cracks during cold forging. Therefore, the content of S is set to 0.005 to 0.020%. The S content is desirably 0.018% or less.

<V: 0.02% to 0.50%>
V is effective in increasing the fatigue strength and the durability ratio by forming carbonitride during the age hardening treatment. In order to acquire this effect, 0.02% or more of V is contained. On the other hand, from the viewpoint of alloy cost, the upper limit of V is set to 0.50%. The V content is preferably 0.03% or more.

<Al: 0.005 to 0.050%>
Al is a deoxidizer during steel refining. In order to obtain a deoxidizing effect, 0.005% or more of Al is contained. On the other hand, if the content exceeds 0.050%, coarse Al inclusions are generated in the steel, which causes cracks during cold forging. Therefore, the Al content is set to 0.050% or less. The Al content is preferably 0.045% or less.

<Cr: 0.02 to 1.50%>
Cr has the effect of increasing the fatigue strength after forging as a solid solution strengthening element. If the Cr content is less than 0.02%, this effect cannot be obtained. On the other hand, since Cr is a carbide generating element, if the Cr content exceeds 1.50%, stable Cr carbide is generated in the steel, which may inhibit the precipitation of V carbonitride. Therefore, the Cr content is set to 0.02 to 1.50%. Note that the Cr content is desirably 0.03% or more, and desirably 1.30% or less.

<N: 0.003-0.030%>
N is combined with V in the age hardening treatment after cold forging and precipitates as a carbonitride to improve the durability ratio. In order to acquire this effect, 0.003% or more of N is contained. However, if N is contained excessively, it causes a decrease in cold forgeability, so the content is made 0.030% or less. The N content is preferably 0.025% or less.

  The hot-rolled steel used in the method for producing the cold-forged age-hardened steel part of the present invention basically comprises the above-mentioned elements from C to N as essential components, and the remainder consists of Fe and impurities. However, P in the impurity is limited to 0.05% or less. The reason for the restriction of P will be described next. In addition, an impurity refers to what is mixed from the ore as a raw material, a scrap, or a manufacturing environment, etc. when manufacturing steel materials industrially here.

<P: 0.020% or less>
P is a component inevitably contained in the steel, is easily segregated in the steel, and causes a local decrease in ductility. In particular, when the content exceeds 0.020%, local ductility deterioration becomes remarkable. Therefore, the P content is limited to 0.020% or less. Note that the P content is desirably limited to 0.018% or less.

  Furthermore, the hot-rolled steel used in the method for producing a cold forged age-hardened steel part of the present invention not only restricts P as described above, but also contains elements from C to N as described above. Furthermore, you may contain 1 or more types of elements in Cu, Ni, and Mo. Therefore, these selective additive elements will be described next.

<Cu: 0.20% or less>
Since Cu has an effect of increasing the fatigue strength of steel, it may be contained in an amount of 0.20% or less. On the other hand, if Cu exceeds 0.20%, the cold forgeability is lowered. Therefore, from the viewpoint of ensuring cold forgeability, the amount of Cu in the case of containing Cu is set to 0.20% or less. In addition, when Cu is contained, the amount of Cu is preferably 0.15% or less.

<Ni: 0.20% or less>
Since Ni has the effect of increasing the fatigue strength of steel, it may be contained in an amount of 0.20% or less. On the other hand, if Ni exceeds 0.20%, the cold forgeability is lowered. From the viewpoint of ensuring cold forgeability, the amount of Ni in the case of containing Ni is set to 0.20% or less. When Ni is contained, the amount of Ni is preferably 0.15% or less.

<Mo: 0.20% or less>
Since Mo has an effect of increasing the fatigue strength of steel, it may be contained in an amount of 0.20% or less. On the other hand, if Mo exceeds 0.20%, the cold forgeability is lowered. From the viewpoint of ensuring cold forgeability, the amount of Mo in the case of containing Mo is set to 0.20% or less. In addition, when Mo is contained, the amount of Mo is preferably 0.15% or less.

  The state of the metal structure (micro structure) of the hot rolled steel used as a raw material in the method for producing the cold forged age-hardened steel part of the present invention is also important. The metal structure will be described next.

<About metal structure>
The metal structure (microstructure) of the hot rolled steel used in the present invention is preferably ferrite or a mixed structure of ferrite and pearlite. More specifically, in the microstructure, the preferable total area ratio of ferrite and pearlite is 90% or more. Bainite and martensite are inferior in cold forgeability compared to ferrite and pearlite, and may cause cracks during cold forging. Therefore, in the above microstructure, the total area ratio of bainite and martensite is preferably 5% or less. In addition, from the viewpoint of suppressing cracks during cold forging, the bainite structure and martensite structure may be generated at zero.

<Manufacturing method overview>
In the method for producing a cold forged age-hardened steel part according to the present invention, a hot-rolled steel material having a component composition as described above and a metal structure is used as a raw material, and it corresponds to a part that requires at least fatigue strength in a product part. Cold forging is performed so that an equivalent strain of 0.2 or more is applied, and age hardening is performed in a temperature range of Ac3 point or less of the material, and the amount of V precipitation in the portion is 40%. Obtain cold forged age hardened steel parts. Therefore, first, an example of a method for producing a hot-rolled steel material will be described.

<About the manufacturing method of the material>
First, molten steel satisfying the above-described chemical composition is prepared, and an ingot is manufactured by an ingot forming method, or a slab is manufactured by a continuous casting method. Moreover, you may manufacture a billet by hot-rolling (ingot rolling etc.) an ingot or a slab.
Next, the ingot, slab or billet obtained as described above is hot-rolled to produce a hot-rolled steel material as a material in the method of the present invention. For example, a continuous hot rolling method may be applied to the hot rolling.

  Here, in order to make the V precipitation amount of the final cold forging age-hardened steel part 40% or more, the rolling temperature in the final rolling of the hot rolling (the steel material on the entrance side of the stand where the final rolling is performed) The temperature is preferably 900 ° C. or higher. In order to obtain the above-mentioned microstructure, the steel material after hot rolling is preferably allowed to cool. In this case, martensite and bainite are hardly generated during cooling, and the area ratio of martensite and bainite in the microstructure can be suppressed to 5% or less.

  Next, an example of a process for producing a cold forged age-hardened steel part having a high durability ratio using the hot rolled steel material as described above will be described.

<About the manufacturing method of cold forging age-hardened steel parts>
First, cold forging for obtaining a desired part shape is performed on the hot-rolled steel as described above to produce an intermediate product. Cold forging is performed so that an equivalent strain of 0.2 or more is applied to at least a portion where a fatigue strength is required in a product part.
Next, an age hardening treatment is applied to the intermediate product to obtain a cold forged age hardening steel part having a V precipitation amount of 40% at the above-described site where at least fatigue strength is required.
Specifically, the intermediate product is heated to a temperature not lower than 200 ° C. and not higher than Ac3 point, and is maintained at this temperature, preferably not less than 30 minutes. If the heat treatment temperature is less than 200 ° C., no precipitation of V carbonitride occurs, so that a high durability ratio cannot be obtained. If the heat treatment temperature exceeds the Ac3 point, not only the V precipitates become coarse and a high durability ratio cannot be obtained, but also heat treatment strain occurs due to austenite transformation. Moreover, if the holding time at the said temperature is less than 30 minutes, precipitation of V carbonitride will not occur and there exists a possibility that a high durability ratio may not be obtained. On the other hand, if the holding time at the above temperature is too long, the same effect can be obtained, but the manufacturing cost becomes high. Therefore, a preferable holding time is 180 minutes or less.
The Ac3 point can be calculated by the following equation.
Ac3 (° C.) = − 230.5 × C + 31.6 × Si−20.4 × Mn−39.8 × Cu−18.1 × Ni−14.8 × Cr + 16.8 × Mo + 912

<Equivalent strain in cold forging>
As described above, in cold forging, it is necessary to perform forging so that an equivalent strain of 0.2 or more is applied to a portion where fatigue strength is required at least in a product part. When the equivalent strain applied by cold forging is less than 0.2, the degree of work hardening is small and a high durability ratio cannot be obtained. Here, depending on the part shape, it may be difficult to apply an equivalent strain of 0.2 or more to the entire part. In this case, 0.2 or more is performed by cold forging at a site where fatigue strength is required. The equivalent strain of.

<Parts requiring fatigue strength>
Including a portion where the maximum stress is expected to occur when the product component is used, and at least a region where stress of 90% or more of the maximum stress is generated is a portion where the fatigue strength is required, and at least An equivalent strain of 0.2 or more may be applied to the region by cold forging. Specifically, for example, the position of the maximum stress applied to a product part is specified by finite element method analysis (FEM analysis), and the fatigue strength is required in the region of maximum stress to maximum stress × 0.9. It is preferable to use a site.

<Equivalent strain of 0.2 or more>
In cold forging, the basis for setting the equivalent strain to be applied to a portion requiring fatigue strength to 0.2 or more is based on the following experimental results.
C: 0.05 to 0.12%, Si: 0.05%, Mn: 0.45%, V: 0.09 to 0.25%, the balance being a plurality of diameters of 36 mm mainly composed of Fe For steel materials of the above, drawing to a diameter of 34 mm simulating cold forging with a processing rate of 10%, drawing to a diameter of 32 mm simulating cold forging with a processing rate of 20%, cold forging with a processing rate of 50% A heat treatment (age hardening treatment) is carried out for 30 to 60 minutes below the Ac3 transformation point, with a drawing process simulating 25 mm in diameter, and a 18 mm diameter simulating cold forging with a processing rate of 75%. After the implementation, a tensile test and an Ono-type rotary bending fatigue test were conducted to examine the durability ratio. Here, the equivalent strain of the drawn material with a diameter of 34 mm is 0.1, the equivalent strain of the drawn material with a diameter of 32 mm is 0.2, the equivalent strain of the drawn material with a diameter of 25 mm is 0.7, and the equivalent strain of the drawn material with a diameter of 18 mm. Is 1.4.

  FIG. 1 shows the relationship between these equivalent strain values and the durability ratio. From FIG. 1, it is clear that the endurance ratio can be made 0.55 or more by applying an equivalent strain of 0.2 or more by cold forging to a site where fatigue strength is required and performing age hardening treatment. became. Therefore, the equivalent strain necessary for obtaining a desired durability ratio is set to 0.2 or more.

<V precipitation amount>
The cold-forged age-hardened steel part obtained by the production method of the present invention requires a V precipitation amount of 40% or more at a site where fatigue strength is required as described above. If the amount of V precipitation is less than 40%, the degree of age hardening is small and a high durability ratio cannot be obtained. In addition, V precipitation amount means here the quantity (mass%) of V which precipitated as carbonitride among V contained in cold forging age hardening steel components.

<About the measuring method of V precipitation amount>
V precipitation amount is calculated | required by the following extraction residue analysis method.
That is, a 10 mm × 10 mm × 10 mm sample is cut out from the center position of the steel material and used as an extraction residue analysis sample. The sample is subjected to constant current electrolysis in a 10% AA-based (liquid in which tetramethylammonium chloride, acetylacetone, and methanol are mixed at 1: 10: 100).
More specifically, in order to remove deposits on the surface of the sample, first, preliminary electrolysis is performed on the sample under the conditions of current: 1000 mA, time: 28 minutes, and room temperature. Thereafter, the deposit on the sample surface is ultrasonically cleaned in alcohol to remove it from the sample. The mass of the sample from which deposits have been removed (the weight of the sample before electrolysis) is measured.

Next, the sample is electrolyzed under the conditions of current: 173 mA, time: 142 minutes, and room temperature. The electrolyzed sample is taken out, and the deposit (residue) on the sample surface is ultrasonically washed in alcohol to remove it from the sample. Thereafter, the electrolyzed solution and the solution used for ultrasonic cleaning are suction filtered through a filter having a mesh size of 0.2 μm to collect a residue.
The mass of the sample from which the deposit (residue) has been removed in this way (the mass of the sample after electrolysis) is measured. Then, the “mass of the electrolyzed sample” is obtained from the difference between the measured values of the mass of the sample before and after electrolysis.

On the other hand, the residue collected on the filter is transferred to a petri dish and dried, and the mass is measured. Thereafter, in accordance with JIS G1258, the residue is analyzed by an ICP emission spectrometer (high frequency inductively coupled plasma emission spectrometer) to determine “the mass of V in the residue”.
The obtained “mass of V in the residue” is divided by the above-mentioned “mass of the electrolyzed sample” and the percentage is displayed as “V precipitation amount” (mass%).

<About V precipitation amount 40% or more>
Next, the grounds for defining the V precipitation amount of 40% or more will be described.
C: 0.05 to 0.12%, Si: 0.05%, Mn: 0.45%, V: 0.09 to 0.25%, the balance being a plurality of diameters of 36 mm mainly composed of Fe Of steel, in other words, steel materials with various changes in the V precipitation amount are drawn to a diameter of 34 mm simulating cold forging with a processing rate of 10%, and to a diameter of 32 mm simulating cold forging with a processing rate of 20%. , Drawing to 25 mm in diameter simulating cold forging with a processing rate of 50%, and drawing to 18 mm in diameter simulating cold forging with a processing rate of 75%, and 30 below the Ac3 transformation point. After carrying out heat treatment (aging hardening treatment) for ˜60 minutes, a tensile test and an Ono-type rotary bending fatigue test were conducted to examine the durability ratio. Here, the equivalent strain of the drawn material with a diameter of 34 mm is 0.1, the equivalent strain of the drawn material with a diameter of 32 mm is 0.2, the equivalent strain of the drawn material with a diameter of 25 mm is 0.7, and the equivalent strain of the drawn material with a diameter of 18 mm. Is 1.4.

  FIG. 2 shows the relationship between the measured durability ratio and the V precipitation amount. From FIG. 2, it was revealed that the durability ratio can be 0.55 or more by setting the amount of V precipitation in a portion requiring fatigue strength to 40% or more. Therefore, the V precipitation amount necessary to obtain a desired durability ratio is defined as 40% or more.

  Examples carried out for verifying the operation and effect of the present invention will be described below.

<Production of round bar forging material>
150 kg of steels A to L having chemical components shown in Table 1 were melted by vacuum melting to produce ingots.
As shown in Table 1, the chemical compositions of Steels A to I are within the scope of the present invention, while the chemical compositions of Steels J to L are outside the scope of the present invention.
The manufactured ingot was subjected to hot forging simulating hot rolling to produce a round bar forged material having a diameter of 42 mm. The heating temperature (° C.) and finishing temperature (° C.) in hot forging are as shown in Test Nos. 1 to 12 in Table 2.

<About microstructure observation>
Samples were collected by cutting (crossing) the round bar forgings of test numbers 1 to 12 in a direction perpendicular to the axial direction. After the sample was embedded in the resin, the cut surface (observation surface) was polished. Nittal corrosion was performed on the observation surface after polishing, the microstructure was observed, and phases (ferrite, pearlite, bainite, martensite) in the microstructure were specified. Furthermore, the area ratios of ferrite and pearlite were determined on the observation surface.
“F / P” in “Microstructure” in Table 2 means that the area ratio of ferrite and pearlite was 95% or more in the observed microstructure.

<About cold forgeability evaluation test>
A cylindrical test piece having a diameter of 14 mm and a height of 21 mm was cut out from each of the round bar forging materials of test numbers 1 to 12. Cold forgeability evaluation by a compression test at room temperature was performed using a cylindrical test piece.
Specifically, in the compression test, when the processing rate (= (1−test specimen height / processing specimen height before processing) × 100) is 70% (hereinafter referred to as 70% processing), The specimen was observed for cracks. The observation was performed visually (using the naked eye or a simple magnifier) to observe the presence or absence of fine cracks (length: 0.5 to 1.0 mm).
In each of test numbers 1 to 12, the above compression test was performed five by five. The goal was that no cracks were observed in all five.

<About cold forging>
Peeling processing was carried out on the round bar forgings of test numbers 1 to 12 to produce round bars having a diameter of 36 mm. A round bar having a diameter of 18 mm was manufactured by subjecting the round bar to a drawing process simulating cold forging with a processing rate of 75%.

<About age hardening treatment>
Age hardening treatment was performed on the manufactured round bar having a diameter of 18 mm. The heat treatment temperature in the age hardening treatment was 600 ° C., and the holding time was 60 minutes.

<About V precipitation amount measurement>
An extraction residue test piece of 10 mm × 10 mm × 10 mm was collected from the center of a round bar having a diameter of 18 mm after aging treatment. Using the extraction residue test piece, the amount of V precipitation (% by mass) due to the extraction residue was determined by the method described above. The amount of V precipitation was set to 40% or more.

<About hardness test>
A sample was obtained by cutting (crossing) a 18 mm diameter round bar after age hardening in a direction perpendicular to the axial direction. After the sample was embedded in the resin, the cut surface (observation surface) was polished. Then, the Vickers hardness test based on JIS Z2244 was implemented with respect to the observation surface. The test force was 9.8N. The measurement locations were arbitrary five points near the center of the round bar forging material. The average of the five measured values was taken as the hardness (Hv) of the test number.

<About tensile test and fatigue test>
Tensile strength test (MPa) and fatigue strength (MPa) are performed using a round bar with a diameter of 18 mm after age hardening treatment, and a tensile test according to JIS Z2241 and an Ono-type rotary bending fatigue test according to JIS Z2274. Further, the durability ratio (= fatigue strength / tensile strength) was obtained. The durability ratio was set to 0.55 or more.

<About test results>
Table 2 shows the results of each test. Referring to Table 2, the chemical compositions of test numbers 1 to 9 are within the scope of the present invention, and the microstructure after hot rolling is composed of ferrite and pearlite, and the total area ratio of ferrite and pearlite is 90%. That was all. As a result, no crack was confirmed in 70% processing, and excellent cold forgeability was obtained. Furthermore, the equivalent strain imparted by cold forging to a portion requiring fatigue strength was 0.2 or more, and the amount of V precipitation after aging treatment was 40% or more. As a result, the durability ratio was 0.55 or more, and a high durability ratio was obtained.

On the other hand, in test numbers 10 to 12, the desired cold forgeability or durability ratio was not obtained.
Specifically, in the case of test number 10, the C content exceeded the upper limit of the C content defined in the present invention. Therefore, cracks were observed in 70% processing, and the target cold forgeability was not obtained.
Moreover, in the case of the test number 11, V content was less than the minimum of V content prescribed | regulated by this invention. Therefore, although the target cold forgeability was obtained, the durability ratio was as low as 0.53, and the target durability ratio was not obtained.
Furthermore, in the case of the test number 12, Cr content exceeded the upper limit of Cr content prescribed | regulated by this invention. Therefore, the durability ratio was as low as 0.52, and the target durability ratio was not obtained.

  The embodiment and the example of the present invention have been described above. However, the above-described embodiments and examples are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments and examples, and can be appropriately modified and implemented without departing from the spirit of the present invention.

  As described above, the cold forging age-hardened steel part obtained by the production method of the present invention has high cold forgeability in the cold forging process, and work hardening by cold forging and age hardening after cold forging. Shows a high durability ratio. Therefore, it can be widely applied to the manufacture of machine structural parts such as automobile parts, industrial machine parts, and construction machine parts that have been manufactured in the "hot forging-cutting" process so far, contributing to the near net shape of parts. it can.

Claims (3)

% By mass
C: 0.02-0.13%,
Si: 0.01 to 0.50%,
Mn: 0.20 to 0.70%,
S: 0.005-0.020%,
Al: 0.005 to 0.050%,
Cr: 0.02 to 1.50%,
V: 0.02 to 0.50%,
N: 0.003-0.030%
And P: limited to 0.020% or less, the balance having a chemical composition consisting of Fe and inevitable impurities,
Moreover, the metal structure is mainly composed of ferrite and pearlite, and a hot rolled steel material having a total area ratio of ferrite and pearlite of 90% or more is used as a material.
For the material, cold forging is performed so that an equivalent strain of 0.2 or more is applied to a part of the product part where at least fatigue strength is required,
Further, age-hardening steel is obtained by performing age-hardening treatment in a temperature range of Ac3 point or less of the raw material to obtain a cold-forged age-hardened steel part having a V precipitation amount of 40% or more in the part. A manufacturing method for parts.   The hot-rolled steel material further contains one or more of Cu: 0.20% or less, Ni: 0.20% or less, and Mo: 0.20% or less. The manufacturing method of the cold forging age-hardening steel part of description. 2. A region including a portion where a maximum stress is assumed to occur when a product part is used, and at least a region where a stress of 90% or more of the maximum stress is generated is defined as the portion. The manufacturing method of the cold forging age-hardening steel component of any one of Claim 2 or Claim 2.

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