JP2013185186A - Cold forging and nitriding steel, cold forging and nitriding steel material, and cold forged and nitrided component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold forging and nitriding steel which is excellent in cold forgeability and machinability after cold forging, and is capable of providing a cold forged and nitrided component with high core part hardness, high surface hardness and a deep effective hardened layer depth.SOLUTION: A cold forging and nitriding steel has a chemical composition that contains, by mass, 0.01-0.15% C, less than 0.10% Si, 0.10-0.50% Mn, 0.030% or less P, 0.050% or less S, 0.80-2.0% Cr, 0.03% or more and less than 0.10% V, 0.01-0.10% Al, 0.0080% or less N, and 0.0030% or less O with the balance being Fe and impurities, and satisfies [399×C+26×Si+123×Mn+30×Cr+32×Mo+19×V≤160], [20≤(669.3×logC-1959.6×logN-6983.3)×(0.067×Mo+0.147×V)≤80] and [140×Cr+125×Al+235×V≥160]. Instead of some of Fe, a specific amount of one or more elements among Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb may be contained.

Description

  The present invention relates to a steel for cold forging and nitriding, a steel material for cold forging and nitriding, and a cold forging and nitriding component. Specifically, it has excellent cold forgeability and machinability after cold forging, as well as high core hardness and surface hardness, and deep effective hardened layer depth on parts that have been subjected to cold forging and nitriding. The present invention relates to a steel for cold forging and nitriding suitable for use as a material for a cold forging and nitriding component, and a cold forging and nitriding component using the same.

  The “nitriding” in the present invention includes not only “nitriding” in the strict sense of “intruding and diffusing N” but also “soft nitriding” which is a process of intruding and diffusing N and C. For this reason, in the following description, “nitriding” including “soft nitriding” is simply referred to.

  Further, the above-mentioned “cold forging and nitriding” refers to performing “nitriding” treatment after performing “cold forging”.

  Mechanical structural parts used in automobile transmissions, such as gears and pulleys for belt-type continuously variable transmissions (CVT), are usually surfaced in order to improve bending fatigue strength, pitching strength, and wear resistance. A curing process is performed. Typical surface hardening treatments include carburizing quenching, induction quenching, and nitriding.

Among the above, carburizing and quenching is a process in which low carbon steel is generally used, C is intruded and diffused in a high temperature austenite region of Ac ₃ or higher, and then quenched. Although carburizing and quenching has an advantage that a high surface hardness and a deep effective hardened layer depth can be obtained, there is a problem that heat treatment deformation becomes large because it is a process involving transformation. Therefore, when high part accuracy is required, finishing such as grinding and honing after carburizing and quenching is required. In addition, a so-called “carburized abnormal layer” such as a grain boundary oxide layer or an incompletely hardened layer formed on the surface layer becomes a fracture starting point such as bending fatigue, and there is a problem that the fatigue strength is lowered.

Induction hardening is a process of quenching by rapid heating and cooling to a high temperature austenite region of Ac ₃ or higher. Although there is an advantage that adjustment of the effective hardened layer depth is relatively easy, it is not a surface hardening treatment that penetrates and diffuses C like carburizing, so the required surface hardness, effective hardened layer depth and core hardness In order to achieve this, it is common to use medium carbon steel having a higher C content than carburizing steel. However, since medium carbon steel has a higher material hardness than low carbon steel, there is a problem that machinability is lowered. Moreover, it is necessary to produce a high-frequency heating coil for each part.

On the other hand, nitriding is a process for obtaining high surface hardness and an appropriate effective hardened layer depth by invading and diffusing N at a temperature of 400 to 550 ° C. below the Ac ₁ point. Since the processing temperature is lower than that of carburizing and induction hardening, there is an advantage in that heat treatment deformation is small.

Among the nitriding processes, soft nitriding is a process for obtaining high surface hardness by invading and diffusing N and C at a temperature of about 500 to 650 ° C. below the Ac ₁ point, and the processing time is as short as several hours. Since it is time, it is suitable for mass production.

  Furthermore, with the current trend of reducing greenhouse gases against the background of global warming suppression, there is a demand for reduction of processes that are held at high temperatures such as carburizing and quenching. In addition, by replacing the manufacturing method from hot forging, which also includes a process of holding at high temperature, to cold forging, it is possible not only to reduce greenhouse gases but also to achieve high effects in reducing manufacturing costs. Application to machine structural parts such as For this reason, cold forging and nitriding are methods for manufacturing machine structural parts that are ready for the times.

  However, the conventional nitriding steel has problems as shown in the following <1> to <3>.

  <1> Nitriding is a surface hardening treatment that does not perform quenching treatment from a high temperature austenite region, that is, cannot be strengthened with martensitic transformation. For this reason, in order to secure the desired core hardness in the nitrided part, it is necessary to contain a large amount of alloy element. However, if a large amount of alloy element is contained, the deformation resistance increases, so it is formed by cold forging. It becomes difficult to process.

  <2> As a typical nitriding steel, there is an aluminum chrome molybdenum steel (SACM645) defined in JIS G 4053 (2008). In this steel, Cr, Al, etc. generate nitrides near the surface. Therefore, although high surface hardness can be obtained, high effective bending fatigue strength cannot be ensured because the effective hardened layer is shallow.

  <3> Among nitriding, soft nitriding is held for several hours in a temperature range of about 500 to 650 ° C., so that the core portion of the component is easily tempered and softened. As a result, in a part to which a high surface pressure is applied, plastic deformation is likely to occur at the core, and the contact surface is deformed by being dented.

  Therefore, in order to solve the above-described problems, for example, Patent Literature 1 and Patent Literature 2 disclose techniques relating to nitriding.

  Patent Document 1 discloses that the hardness after rolling is 200 or less in terms of Vickers hardness, and is intended to provide a steel for nitrocarburizing excellent in soft nitriding properties and cold forging properties. An excellent steel for soft nitriding ”is disclosed. Said “steel for soft nitriding” is in mass%, C: 0.05 to 0.25%, Si: 0.50% or less, Mn: 0.55% or less, Cr: 0.50 to 2.00 %, V: 0.02 to 0.35% and Al: 0.005 to 0.050%, and if necessary, Nb: 0.02 to 0.35%, and the balance of Fe And an impurity element.

  Patent Document 2 discloses a “nitriding component manufacturing method” in which the hardened surface layer is hard, the effective hardened layer depth is deep, the required core hardness is obtained, and the amount of machining such as cutting is small. Has been. The above-mentioned “manufacturing method of nitriding parts” is mass%, C: 0.10 to 0.40%, Si: 0.10 to 0.70%, Mn: 0.20 to 1.50%, Cr : 0.50 to 2.50% and V: 0.05 to 0.60%, and further, if necessary, Al, Mo, Ti, Nb, Ta, B, S, Pb, Te, Se, A steel material containing one or more of Ca, Bi, and Sb, with the balance being substantially composed of Fe, was subjected to V precipitation control heat treatment before nitriding, and then cold worked. In addition, it is a technique of further performing nitriding treatment.

JP-A-5-171347 JP-A-7-102343

  The steel disclosed in Patent Document 1 is not necessarily excellent in all of cold forgeability, machinability after cold forging, deformation resistance, bending fatigue strength, and wear resistance. Moreover, the effective hardened layer depth means a depth of 400 or more in terms of Vickers hardness (hereinafter sometimes referred to as “HV”), and does not have a sufficient effective hardened layer depth. It was.

  The steel disclosed in Patent Document 2 contains a large amount of alloying elements. For this reason, if cold forging is performed at a large workability, sufficient cold forgeability cannot always be ensured, which may be a problem.

  The present invention has been made in view of the above situation, and is excellent in cold forgeability and machinability after cold forging, as well as high core hardness and high in parts subjected to cold forging and nitriding. An object of the present invention is to provide a cold forging and nitriding steel and a cold forging and nitriding steel that can be provided with a surface hardness and a deep effective hardened layer depth, and are suitable for use as a material for cold forging and nitriding parts. To do.

  Specifically, the hardness is low and cold forging is easy, and after cold forging is excellent in chip disposal, and the core hardness after cold forging and nitriding is 180 or more in HV, surface Cold forging and nitriding steel and cold forging and nitriding that can obtain hardness characteristics of 650 or more in HV and effective hardened layer depth of 0.15 mm or more and can be used as a material for cold forging and nitriding parts The purpose is to provide steel.

  It is also an object of the present invention to provide a cold forging and nitriding component using the cold forging and nitriding steel and the steel for cold forging and nitriding.

  As described above, nitriding is a surface hardening treatment that does not perform quenching treatment from the austenite region, that is, cannot be strengthened with martensitic transformation. For this reason, in order to ensure the desired core hardness in the nitrided part, it is necessary to contain a large amount of alloy elements, but in this case, it is difficult to form by cold forging.

  Therefore, in order to solve the above-mentioned problems, the present inventors first performed a forming process by cold forging as a method for obtaining a machine structural component without performing high temperature holding such as hot forging and carburizing and quenching. Then, a means for securing the core hardness, surface hardness and effective hardened layer depth necessary as a machine structural component was examined by performing surface hardening treatment by nitriding.

  As a result, the amount of alloying elements can be kept to the minimum necessary to ensure excellent cold forgeability, and high core hardness can be obtained through the combined effect of work hardening by cold forging and age hardening at the nitriding temperature. If possible, we have reached the technical idea that both of the conflicting properties of high core hardness and good cold forgeability can be ensured.

  Therefore, the present inventors have further experimented based on the above technical idea, and obtained the following knowledge (a) to (e).

  (A) When Cr and Al are contained in steel, the surface hardness can be increased by nitriding.

  (B) In order to obtain a higher surface hardness by nitriding and to increase the age hardening amount at the nitriding temperature, it is effective to limit the N content in the steel and to contain V in the steel. If Mo is contained in the above steel, a larger age hardening amount can be obtained.

  (C) On the other hand, when Cr and V are contained, cold forgeability is lowered. In order to ensure cold forgeability without lowering the core hardness, there is a limit to restricting the content of individual component elements. However, if the content of C, Si, Mn, Cr, Mo and V is limited to a specific range after limiting the N content, excellent cold forging is possible even if Cr and V are contained. Sex can be secured. As a result, since cold forging can be performed with a large degree of processing, strengthening by work hardening can be achieved.

  (D) Furthermore, if the contents of C, Mn, Cu, Ni, Cr, Mo and V in steel are limited to a specific range, excellent machinability can be imparted after cold forging.

  (E) High core hardness required as a machine structural component can be ensured by work hardening by cold forging and age hardening at a nitriding temperature.

  The present invention has been completed based on the above findings, and the gist of the present invention is the cold forging and nitriding steel shown in the following (1) to (5), the cold forging and nitriding steel shown in (6) and (7 ).

(1) By mass%, C: 0.01 to 0.15%, Si: less than 0.10%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.050 %: Cr: 0.80-2.0%, V: 0.03% or more and less than 0.10%, Al: 0.01-0.10%, N: 0.0080% or less, and O: 0.0. 0030% or less, and the balance consists of Fe and impurities, Fn1 represented by the following formula (1) is 160 or less, Fn2 represented by the formula (2) is 20 to 80, and ( 3) A steel for cold forging and nitriding characterized by having a chemical composition in which Fn3 represented by the formula is 160 or more.
Fn1 = 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V (1)
Fn2 = (669.3 × log _e C−1959.6 × log _e N−6983.3) × (0.067 × Mo + 0.147 × V) (2)
Fn3 = 140 × Cr + 125 × Al + 235 × V (3)
C, Si, Mn, Cr, Mo, V, N, and Al in the above formulas (1) to (3) mean the content of the element in mass%.

  (2) The steel for cold forging and nitriding as described in (1) above, which contains Mo: 0.50% or less in mass% instead of part of Fe.

  (3) The above (1), characterized in that, instead of a part of Fe, by mass%, at least one selected from Cu: 0.50% or less and Ni: 0.50% or less is contained. Or the steel for cold forge nitriding as described in (2).

  (4) Instead of a part of Fe, it contains at least one selected from Ti: 0.20% or less, Nb: 0.10% or less, and Zr: 0.10% or less in mass%. The steel for cold forging and nitriding according to any one of (1) to (3) above, which is characterized.

  (5) Instead of a part of Fe, in mass%, Pb: 0.50% or less, Ca: 0.010% or less, Bi: 0.30% or less, Te: 0.30% or less, Se: 0 The steel for cold forging and nitriding according to any one of (1) to (4) above, comprising at least one selected from 30% or less and Sb: 0.30% or less.

  (6) The chemical composition according to any one of (1) to (5) above, wherein the V content in the precipitate by extraction residue analysis is 0.05% or less. Steel for forging and nitriding.

  (7) The chemical composition according to any one of (1) to (5) above, having a core hardness of 180 or more in terms of Vickers hardness, a surface hardness of 650 or more in terms of Vickers hardness, and an effective cured layer A cold forged and nitrided part having a depth of 0.15 mm or more.

  The “impurities” in the remaining “Fe and impurities” refer to those mixed from ore as a raw material, scrap, or the manufacturing environment when the steel material is industrially produced.

  The steel for cold forging and nitriding of the present invention is excellent in cold forgeability and machinability after cold forging, and has a high core in a part subjected to cold forging and nitriding treatment. Hardness, high surface hardness and deep effective hardened layer depth can be provided. For this reason, it is suitable for using as a raw material of cold forging nitriding components.

  Further, the cold forged and nitrided parts of the present invention are excellent in deformation resistance, bending fatigue strength, and wear resistance, and thus are suitable as parts for machine structures used in automobile transmissions such as gears and pulleys for CVT. Can be used.

It is a figure which shows the shape of the smooth test piece for a deformation resistance measurement at the time of the cold forging used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the notch test piece for the limit compressibility measurement at the time of the cold forging used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the round bar test piece for measurement, such as the hardness after nitriding used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the rough shape as cut out from the cold drawing material of the Ono type | formula rotation bending fatigue test piece with a notch used in the Example. Except for the part labeled “3.2S”, the unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the block test piece A for abrasion resistance investigation used in the Example. The dimension which does not show the unit in a figure is "mm". It is a figure which shows the shape of the block test piece B for a deformation resistance investigation used in the Example. The dimension which does not show the unit in a figure is "mm". In an Example, it is a figure which shows the heat pattern of the soft nitriding given to the test piece shown in FIGS. It is a figure which shows the finishing shape of the Ono-type rotary bending fatigue test piece with a notch used in the Example. The unit of the dimension in the figure is “mm”. It is a figure explaining the length of the chip | tip produced by the turning process using the NC lathe of an Example. It is a figure explaining the method of the block on ring type abrasion test implemented in the Example. It is a figure which shows the shape of the ring test piece used by the block on-ring-type abrasion test of an Example. The dimension which does not show the unit in a figure is "mm". In an Example, it is a figure which shows the heat pattern of the gas carburizing quenching-tempering performed to the ring test piece and the indentation test jig | tool before finish grinding. It is a figure explaining the measuring method of the wear depth after the block on-ring-type wear test implemented in the Example. It is a figure explaining the method of the indentation test implemented in the Example. It is a figure which shows the shape of the indentation test jig | tool used by the indentation test of the Example. The dimension which does not show the unit in a figure is "mm". It is the figure which arranged the relationship between Fn1 represented by (1) Formula, and the hardness (HV) before cold working in the investigation 1 of an Example. It is the figure which arranged the relationship between Fn1 represented by (1) Formula, and the deformation resistance in the cold forging in the investigation 4 of an Example. It is the figure which arranged the relationship between Fn1 represented by (1) Formula, and the limit compression rate in the cold forging in the investigation 5 of an Example. It is the figure which arranged the relationship between Fn2 represented by (2) Formula, and the core part hardness (HV) after nitriding in the investigation 7 of an Example. It is the figure which arranged the relationship between Fn2 represented by (2) Formula, and the amount of indentation deformation in the investigation 10 of an Example. It is the figure which arranged the relationship between Fn3 represented by (3) Formula, and the surface hardness (HV) after nitriding in the investigation 7 of an Example. It is the figure which arranged the relationship between Fn3 represented by (3) Formula, and the rotation bending fatigue strength in the investigation 8 of an Example. It is the figure which arranged the relationship between Fn3 represented by (3) Formula, and the wear depth in the investigation 9 of an Example.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition:
C: 0.01 to 0.15%
C is an essential element for ensuring the bending fatigue strength and core hardness of the cold forged and nitrided parts, and a content of 0.01% or more is necessary. However, if the C content is too large, the hardness increases and cold forgeability decreases. For this reason, the upper limit was set and the C content was set to 0.01 to 0.15%. The C content is preferably 0.03% or more, and more preferably 0.13% or less.

Si: Less than 0.10% Si is an element contained as an impurity in steel. On the other hand, it is also an element having a deoxidizing action. When there is too much content of Si, hardness will become high and cold forgeability will fall. For this reason, an upper limit was set, and the Si content was less than 0.10%. In order to obtain a deoxidizing action, the Si content is preferably 0.02% or more. The Si content is preferably 0.02% or more, and preferably 0.09% or less.

Mn: 0.10 to 0.50%
Mn has the action of ensuring the bending fatigue strength and core hardness of the cold forged and nitrided parts and the deoxidizing action. In order to obtain these effects, a content of 0.10% or more is necessary. However, when there is too much content of Mn, hardness will become high and cold forgeability will fall. For this reason, an upper limit was set, and the Mn content was set to 0.10 to 0.50%. The Mn content is preferably 0.30% or more, and more preferably 0.46% or less.

P: 0.030% or less P is an impurity contained in steel. When there is too much content of P, P segregated to the grain boundary may make steel embrittle. For this reason, an upper limit is set and the P content is set to 0.030% or less. A more preferable content of P is 0.020% or less.

S: 0.050% or less S is an impurity contained in steel. Further, if S is positively contained, it combines with Mn to form MnS and has an effect of improving machinability. However, if the S content exceeds 0.050%, coarse MnS is formed, and hot workability and bending fatigue strength are reduced. Therefore, the content of S is set to 0.050% or less. The S content is preferably 0.030% or less. In order to obtain the effect of improving the machinability, the S content is preferably 0.005% or more.

Cr: 0.80 to 2.0%
Cr combines with N during nitriding to form nitrides, improves the surface hardness during nitriding, and has the effect of ensuring the bending fatigue strength and wear resistance of cold forged nitriding parts. However, the above effect cannot be obtained if the Cr content is less than 0.80%. On the other hand, if the content of Cr exceeds 2.0%, it becomes hard and the cold forgeability deteriorates. Therefore, the content of Cr is set to 0.80 to 2.0%. The Cr content is preferably 0.90% or more, and more preferably 1.5% or less.

V: 0.03% or more and less than 0.10% V combines with C or / and N during nitriding to form carbide, nitride and carbonitride, and has an effect of improving surface hardness. . In addition, there is an effect of improving the core hardness by age hardening at the nitriding temperature, that is, by forming a carbide. In order to acquire these effects, it is necessary to contain V 0.03% or more. However, if the content of V is large, not only the hardness becomes too high, but also the cold forgeability decreases. Therefore, an upper limit is set, and the V content is set to 0.03% or more and less than 0.10%. The V content is preferably 0.05% or more.

Al: 0.01-0.10%
Al has a deoxidizing action. Moreover, it combines with N at the time of nitriding, forms AlN, and has the effect of improving surface hardness. In order to acquire these effects, it is necessary to contain Al 0.01% or more. However, if there is too much Al content, hard and coarse Al ₂ O ₃ will be formed and the cold forgeability will be lowered, and the effective hardened layer in nitriding will become shallow and the bending fatigue strength and pitting strength will be reduced. Problems arise. Therefore, an upper limit is set, and the Al content is set to 0.01 to 0.10%. The Al content is preferably 0.02% or more, and preferably 0.07% or less.

N: 0.0080% or less N, together with C, combines with elements such as V to form carbonitrides. If carbonitride precipitates during hot rolling, the hardness increases and cold forgeability decreases. Further, the effect of improving the core hardness by age hardening at the nitriding temperature cannot be sufficiently obtained. Therefore, the N content needs to be limited and is set to 0.0080% or less. A preferable N content is 0.0070% or less.

O: 0.0030% or less O forms oxide-based inclusions, causes fatigue failure at the starting point of inclusions, and reduces bending fatigue strength. When the O content exceeds 0.0030%, the bending fatigue strength is significantly reduced. Therefore, the content of O is set to 0.0030% or less. A preferable O content is 0.0020% or less.

  One of the steels for cold forging and nitriding of the present invention contains the above-mentioned elements from C to O, the balance is made of Fe and impurities, and satisfies the conditions for Fn1 to Fn3 described later. Have a chemical composition. As already described, “impurities” in “Fe and impurities” refers to those mixed from ore, scrap, or production environment as raw materials when industrially producing steel materials.

  Another one of the steels for cold forging and nitriding of the present invention is Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te instead of a part of the above-mentioned Fe. , Se and Sb are contained, and the chemical composition satisfies the conditions for Fn1 to Fn3.

  Hereinafter, the effects of the optional elements Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se, and Sb and the reasons for limiting the content will be described.

Mo: 0.50% or less Mo combines with C at the nitriding temperature to form carbides, and has the effect of improving the core hardness by age hardening. Therefore, even if Mo is added to obtain the above effect, Mo Good. However, if it exceeds 0.50% and contains Mo, it will become hard and cold forgeability will fall. Therefore, the amount of Mo in the case of inclusion is set to 0.50% or less. When Mo is contained, the amount of Mo is preferably 0.40% or less.

  On the other hand, in order to stably obtain the effect of Mo described above, the amount of Mo in the case of inclusion is preferably 0.05% or more.

  Both Cu and Ni have the effect of improving the core hardness. For this reason, in order to acquire said effect, you may contain these elements. Hereinafter, the above Cu and Ni will be described.

Cu: 0.50% or less Cu has an action of improving the core hardness, so that Cu may be contained in order to obtain the above effect. However, when the Cu content increases, the cold forgeability decreases, and in addition, Cu melts and becomes liquid at high temperatures such as hot rolling. The liquefied Cu infiltrates between crystal grains, embrittles grain boundaries, and causes surface defects in hot rolling. Therefore, an upper limit is set for the amount of Cu in the case of inclusion, and it is set to 0.50% or less. When Cu is contained, the amount of Cu is preferably 0.40% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the amount of Cu in the case of inclusion is preferably 0.10% or more.

Ni: 0.50% or less Since Ni has an action of improving the core hardness, Ni may be contained in order to obtain the above effect. However, when the Ni content increases, the cold forgeability decreases. Therefore, an upper limit is set for the amount of Ni in the case of inclusion, and it is set to 0.50% or less. When Ni is contained, the amount of Ni is preferably 0.40% or less.

  On the other hand, in order to stably obtain the effect of Ni described above, the amount of Ni in the case of inclusion is preferably 0.10% or more.

  Said Cu and Ni can be contained only in any 1 type or 2 types of composites. The total amount when these elements are contained in combination may be 1.00%, but is preferably 0.80% or less. In addition, when Cu is contained, it is preferable to contain Ni in combination in order to avoid the occurrence of surface flaws in the hot rolling described above.

  Ti, Nb, and Zr all have the effect of refining crystal grains and improving the bending fatigue strength. For this reason, in order to acquire said effect, you may contain these elements. Hereinafter, the Ti, Nb, and Zr will be described.

Ti: 0.20% or less Ti combines with C or / and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains and improve bending fatigue strength. Therefore, Ti may be contained to obtain the above effect. However, when the Ti content is large, coarse TiN is generated, so that the bending fatigue strength decreases. Therefore, an upper limit is set for the amount of Ti in the case of inclusion, and it is set to 0.20% or less. When Ti is contained, the amount of Ti is preferably 0.10% or less.

  On the other hand, in order to stably obtain the effect of Ti described above, the amount of Ti when contained is preferably 0.005% or more.

Nb: 0.10% or less Nb combines with C or / and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains and improve bending fatigue strength. Therefore, Nb may be included to obtain the above effect. However, when there is much content of Nb, hardness will rise and cold forgeability will fall. Therefore, an upper limit is set for the amount of Nb in the case of inclusion, and the content is made 0.10% or less. When Nb is contained, the amount of Nb is preferably 0.07% or less.

  On the other hand, in order to stably obtain the above-described effect of Nb, the amount of Nb when contained is preferably 0.020% or more.

Zr: 0.10% or less Zr also has an action of combining with C or / and N to form fine carbides, nitrides and carbonitrides to refine crystal grains and improve bending fatigue strength. Therefore, Zr may be contained in order to obtain the above effect. However, when there is much content of Zr, hardness will rise and cold forgeability will fall. Therefore, an upper limit is set for the amount of Zr in the case of inclusion, and the content is made 0.10% or less. The amount of Zr when contained is preferably 0.07% or less.

  On the other hand, in order to stably obtain the effect of Zr described above, the amount of Zr when contained is preferably 0.002% or more.

  Said Ti, Nb, and Zr can be contained only in any one of them, or 2 or more types of composites. The total amount when these elements are combined and contained may be 0.40%, but is preferably 0.24% or less.

  Pb, Ca, Bi, Te, Se, and Sb all have an effect of improving machinability. For this reason, in order to acquire said effect, you may contain these elements. Hereinafter, the above Pb, Ca, Bi, Te, Se, and Sb will be described.

Pb: 0.50% or less Pb has an effect of improving machinability. Therefore, Pb may be contained in order to obtain the above effect. However, when the content of Pb is large, the hot workability is lowered, and further, the toughness of the cold forged nitriding part is reduced. Therefore, an upper limit is set for the amount of Pb in the case of inclusion, and it is set to 0.50% or less. When Pb is contained, the amount of Pb is preferably 0.20% or less.

  On the other hand, in order to stably obtain the effect of Pb described above, the amount of Pb when contained is preferably 0.02% or more.

Ca: 0.010% or less Ca has an effect of improving machinability. Therefore, Ca may be contained in order to obtain the above effect. However, when there is much content of Ca, hot workability falls and also the toughness fall of cold forge nitriding components is caused. Therefore, an upper limit is set for the amount of Ca in the case of inclusion, and it is set to 0.010% or less. When Ca is contained, the amount of Ca is preferably 0.005% or less.

  On the other hand, in order to stably obtain the effect of Ca described above, the amount of Ca when contained is preferably 0.0003% or more.

Bi: 0.30% or less Bi also has an effect of improving machinability. Therefore, Bi may be included to obtain the above effect. However, when the Bi content is large, the hot workability is lowered, and further, the toughness of the cold forged and nitrided parts is also reduced. Therefore, an upper limit is set for the amount of Bi in the case of inclusion, and it is set to 0.30% or less. When contained, the amount of Bi is preferably 0.10% or less.

  On the other hand, in order to stably obtain the above-described effects of Bi, the amount of Bi when contained is preferably 0.005% or more.

Te: 0.30% or less Te has an effect of improving machinability. Therefore, Te may be included to obtain the above effect. However, when the content of Te is large, the hot workability is lowered, and further, the toughness of the cold forged and nitrided parts is reduced. Therefore, an upper limit is set for the amount of Te in the case of inclusion, and it is set to 0.30% or less. When Te is contained, the amount of Te is preferably 0.10% or less.

  On the other hand, in order to stably obtain the above-described effect of Te, the amount of Te when contained is preferably 0.003% or more.

Se: 0.30% or less Se also has an effect of improving machinability. Therefore, Se may be contained in order to obtain the above effect. However, when the Se content is large, the hot workability is lowered, and further, the toughness of the cold forged and nitrided parts is also reduced. Therefore, an upper limit is set for the amount of Se in the case of inclusion, and it is set to 0.30% or less. When contained, the amount of Se is preferably 0.10% or less.

  On the other hand, in order to stably obtain the effect of Se described above, the amount of Se when contained is preferably 0.005% or more.

Sb: 0.30% or less Sb has an effect of improving machinability. Therefore, Sb may be contained in order to obtain the above effect. However, when the Sb content is large, the hot workability is lowered, and further, the toughness of the cold forged nitriding component is also lowered. Therefore, an upper limit is set for the amount of Sb in the case of inclusion, and it is set to 0.30% or less. When contained, the amount of Sb is preferably 0.10% or less.

  On the other hand, in order to stably obtain the above-described effect of Sb, the amount of Sb when contained is preferably 0.005% or more.

  Said Pb, Ca, Bi, Te, Se, and Sb can be contained only in one of them, or 2 or more types of composites. The total amount when these elements are contained in combination is preferably 0.50% or less, and more preferably 0.30% or less.

Fn1: 160 or less The steel for cold forge nitriding and the steel for cold forge nitriding of the present invention are:
Fn1 = 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V (1)
Fn1 represented by the following must be 160 or less. However, C, Si, Mn, Cr, Mo and V in the formula (1) mean the content in mass% of the element.

  Said Fn1 is a parameter used as an index of cold forgeability. If Fn1 is 160 or less, the hardness before cold forging becomes low, and good cold forgeability can be ensured. On the other hand, if Fn1 exceeds 160, the hardness before cold forging becomes too high, and cold forgeability deteriorates. Fn1 is preferably 80 or more. When Mo and V are not added and are contained only at the impurity level, Mo and V in the equation (1) are set to “0 (zero)”.

Fn2: 20-80
The steel for cold forging and nitriding and the steel for cold forging and nitriding of the present invention are
Fn2 = (669.3 × log _e C−1959.6 × log _e N−6983.3) × (0.067 × Mo + 0.147 × V) (2)
Fn2 represented by must be 20-80. However, C, N, Mo and V in the formula (2) mean the content in mass% of the element.

  Fn2 is a parameter that serves as an index for the amount of age hardening by nitriding after cold forging, that is, an allowance for improving the core hardness by nitriding. If Fn2 is 20 or more, the amount of age hardening after nitriding is increased, and the core hardness is improved. However, if Fn2 exceeds 80, the above effect is saturated. When Mo and V are not added and are contained only at the impurity level, Mo and V in the equation (2) are set to “0 (zero)”.

Fn3: 160 or more The steel for cold forging and nitriding and the steel for cold forging and nitriding of the present invention are:
Fn3 = 140 × Cr + 125 × Al + 235 × V (3)
Fn3 represented by the following must be 160 or more. However, Cr, Al, and V in the formula (3) mean the content of the element in mass%.

  Said Fn3 is a parameter used as a parameter | index of the surface hardness, bending fatigue strength, and abrasion resistance after nitriding.

  Cr, Al and V can generate nitrides and carbonitrides having high hardness in the vicinity of the surface of the cold forging nitrided part during nitriding, and can improve the surface hardness. By setting Fn3 to 160 or more, the surface hardness is 650 or more in HV, and bending fatigue strength and wear resistance equivalent to those of the carburized and quenched material can be obtained. When Fn3 is smaller than 160, the surface hardness is low, and the bending fatigue strength and wear resistance are inferior to those of the carburized and quenched material. Fn3 is preferably 170 or more, and preferably 300 or less.

(B) V content in precipitates by extraction residue analysis:
In addition to having the chemical composition described in the preceding item (A), the steel for cold forging and nitriding of the present invention defines the V content in the precipitate by extraction residue analysis to be 0.05% or less.

  When a large amount of fine V precipitates, that is, V carbides, nitrides and carbonitrides, are precipitated in the steel for cold forging and nitriding, the ferrite is strengthened and the hardness is increased, and the cold forgeability is lowered. Therefore, in order to ensure cold forgeability, the V content in the precipitates by extraction residue analysis is preferably 0.05% or less. The V content in the precipitate is more preferably 0.03% or less.

  V content in the precipitate by extraction residue analysis is, for example, collecting an appropriate test piece, subjecting it to constant current electrolysis in a 10% AA-based solution, and filtering the extracted solution through a filter having a mesh size of 0.2 μm. Thus, it can be obtained by conducting a general chemical analysis on the filtrate. The 10% AA-based solution is a solution obtained by mixing tetramethylammonium chloride, acetylacetone and methanol at 1: 10: 100.

  As described above, if hot rolling or hot forging is used, V, carbides, nitrides and carbonitrides may precipitate and cold forgeability may not be sufficient. Therefore, in order to obtain a steel for cold forging and nitriding whose V content in the precipitate by extraction residue analysis is 0.05% or less, it is heated to, for example, 850 to 950 ° C. after hot rolling or / and hot forging. Then, it is preferable to “normalize” by forced air cooling and cooling to room temperature.

  After heating in the above temperature range, it is allowed to cool in the atmosphere or gradually cooled and then cooled to room temperature to perform “normalization”, and V, carbide, nitride and carbonitriding are again performed during the cooling process. A thing precipitates and hardness becomes high and cold forgeability may fall. Therefore, after heating, for example, the average cooling rate in the temperature range of 800 to 500 ° C. is 0.5 to 5.0 ° C./second so that carbides, nitrides and carbonitrides of V do not precipitate. It is preferable to cool by forced air cooling.

(C) Cold forged nitriding parts:
In addition to having the chemical composition described in the above item (A), the cold forged nitriding component of the present invention has a core hardness of 180 or more in HV, a surface hardness of 650 or more in HV, and an effective hardened layer depth. Must be at least 0.15 mm.

  When the above conditions are satisfied, the cold forged nitrided parts are excellent in deformation resistance, bending fatigue strength and wear resistance, and are suitable as mechanical structural parts used in automobile transmissions such as gears and pulleys for CVT. Can be used.

  The core hardness is preferably 200 or more in HV and 350 or less. The surface hardness is preferably 670 or more in HV, and preferably 800 or less. The effective hardened layer depth is preferably 0.40 mm or less.

(D) Manufacturing method of cold forged nitriding parts:
For example, when the material has a cylindrical shape, the cold forging and nitriding component of the above (C) is preferably the above-described (C) forging and nitriding steel material having the chemical composition described in the above (A). Cold forging at a compression ratio of 50% or more with respect to the steel for cold forging and nitriding having the chemical composition described in the item A) and the V content in the precipitate by the extraction residue analysis described in the item (B). After performing, it can manufacture by performing nitriding for 1 to 30 hours at 400-650 ° C. And compression ratio, the height of the pre-cold forging material H _0, if the height of the component after cold forging and is H, represented by _{{(H 0 -H) / H} 0} × 100 value It is.

  In order to improve the core hardness of the cold forged nitriding component, it is preferable to increase the degree of work in cold forging, that is, to increase the strain and to utilize the strengthening by work hardening.

  After the cold forging is performed, it is preferable to perform nitriding at 400 to 650 ° C. for 1 to 30 hours in order to utilize strengthening by age hardening in addition to strengthening by work hardening.

  When the nitriding temperature is low and less than 400 ° C., a high surface hardness can be imparted to the cold forged nitriding component, but the effective hardened layer becomes shallow, and it is difficult to achieve core hardness improvement by age hardening. On the other hand, when the nitriding temperature is high and exceeds 650 ° C., the effective hardened layer of the cold forged nitriding component becomes deep, but the surface hardness is lowered and the core hardness is also lowered. The temperature for nitriding is preferably 450 ° C. or higher, and preferably 630 ° C. or lower.

  The nitriding time varies depending on the depth of the effective hardened layer required for the cold forged nitriding component, but when it is less than 1 hour, the effective hardened layer becomes shallow. On the other hand, a long time exceeding 30 hours is not suitable for mass production. The nitriding time is preferably 1 hour or longer, and preferably 20 hours or shorter.

The nitriding method for obtaining the cold forged nitriding component of the present invention is not particularly defined, and gas nitriding, salt bath nitriding, ion nitriding, or the like can be used. In the case of soft-nitriding, for example a combination of RX gas in addition to NH _3, NH ₃ and RX gas 1: processing may be performed in one of an atmosphere.

  The nitriding time varies depending on the processing temperature. For example, when soft nitriding is performed at 590 ° C., the nitriding time is 9 hours, and the surface hardness, core hardness, and effective hardened layer depth described in the above section (C) are set. Can be obtained.

When it is desired to suppress the formation of brittle compounds, or using fluorine gas as a pretreatment for nitridation by NH _3, it is preferable to use a mixed gas of NH ₃ and H ₂ to nitriding.

  Hereinafter, the present invention will be described in more detail by way of examples treated with gas soft nitriding, but the present invention is not limited to these examples.

  Steels 1 to 16 having chemical components shown in Table 1 were melted in a 180 kg vacuum melting furnace and cast into an ingot.

  Steels 1 to 11 in Table 1 are steels according to examples of the present invention whose chemical composition is within the range specified by the present invention, while Steels 12 to 16 are outside the conditions specified by the present invention for chemical composition. It is steel of a comparative example.

  Among the steels of the above comparative examples, steel 12 is steel corresponding to SCr420H defined in JIS G 4052 (2008).

  Each ingot was homogenized by holding at 1250 ° C. for 5 hours, and then hot forging produced a steel bar having a diameter of 35 mm and a length of 1000 mm and a steel bar having a diameter of 45 mm and a length of 1000 mm.

  Among the steel bars, steels 1 to 11 and steels 13 to 16 were kept at 950 ° C. for 1 hour and then subjected to “normalization” by straightening and cooling to room temperature. As a result of measuring by inserting a thermocouple into the steel bar, the average cooling rate in the temperature range of 800 to 500 ° C. with straight air cooling was R / 2 part of the steel bar having a diameter of 35 mm (“R” represents the radius of the steel bar). 1.48 ° C./second, and 0.72 ° C./second in the R / 2 part of a steel bar having a diameter of 45 mm.

  On the other hand, the steel bar of steel 12 containing no V was held at 920 ° C. for 1 hour, then allowed to cool in the atmosphere and cooled to room temperature, and “normalized”.

  About each steel, various test pieces were extract | collected from a part of 35-mm-diameter bar steel normalized as mentioned above.

  Specifically, for each steel, a normalized steel bar having a diameter of 35 mm was cut so-called “crossing”, that is, perpendicular to the axial direction (length direction). Next, after embedding in the resin so that the cut surface becomes the test surface, it is polished so that the cut surface has a mirror finish, and is subjected to normalization, that is, a test piece for measuring Vickers hardness before cold working, did.

  Moreover, about each steel, the sample of 10 mm x 10 mm x 10 mm was cut out from R / 2 part of the normalized steel bar of 35 mm in diameter for extraction residue analysis.

  Further, for each steel, six smooth test pieces shown in FIG. 1 were cut out in parallel to the axial direction from the center part of the normalized steel bar having a diameter of 35 mm for measuring deformation resistance during cold forging. Similarly, five notch test pieces shown in FIG. 2 were cut out in parallel to the axial direction from the center part of the normalized steel bar having a diameter of 35 mm in order to measure the critical compressibility during cold forging.

  For each steel, the remainder of the normalized 35 mm diameter steel bar and the normalized 45 mm diameter steel bar were peeled and strained by cold drawing instead of cold forging, and the properties after drawing The properties after cold forging were evaluated.

  That is, the remainder of the normalized steel bar having a diameter of 35 mm was peeled to a diameter of 28 mm, pickled and lubricated, and then cold-drawn so that the diameter became 15.45 mm.

  The diameters of the dies used for the drawing are 26.5 mm, 23.5 mm, 21.5 mm, 19.95 mm, 18.17 mm and 15.45 mm in this order. In addition, the total area reduction rate when performing a drawing process from a diameter of 28 mm to 15.45 mm is 70%.

  For each steel, a cold drawn material having a diameter of 15.45 mm obtained as described above was traversed. Next, after embedding in the resin so that the cut surface becomes the test surface, the sample is polished so that the cut surface has a mirror finish, and after the drawing process (that is, after the cold working), a test piece for Vickers hardness measurement Was made.

  Furthermore, for each steel, from the center of the cold drawn material having a diameter of 15.45 mm, a 10 mm diameter round bar test piece shown in FIG. Further, a coarse notched Ono type rotating bending fatigue test piece shown in FIG. 4 was cut out.

  Similarly, a block test piece having a length of 15.75 mm, a width of 10.16 mm and a thickness of 6.35 mm shown in FIG. 5 is formed in parallel with the axial direction from the center of the cold drawn material (hereinafter referred to as “block test piece”). A ”) and a block test piece (hereinafter referred to as“ block test piece B ”) having a length of 25 mm, a width of 5 mm, and a thickness of 12.5 mm shown in FIG.

  The dimensions not showing the unit in each of the above-described cutout test pieces shown in FIGS. 1 to 6 are “mm” except for the portion indicated as “3.2S”, and the finish symbol “▽”, “▽▽” and “▽▽▽” are “triangular symbols” indicating the surface roughness described in the explanatory table 1 of JIS B 0601 (1982).

  “3.2S” attached to “▽▽▽” means that the maximum height Rmax is 3.2 μm or less. “Rq: 0.10 to 0.20 μm” attached to “▽” means that the root mean square roughness “Rq” defined in JIS B 0601 (2001) is 0.10 to 0.20 μm. To do.

  On the other hand, the normalized steel bar having a diameter of 45 mm was peeled to a diameter of 34.7 mm, subjected to pickling and lubrication, and then cold-drawn so as to have a diameter of 29 mm.

  The diameters of the dies used for drawing are 32.88 mm, 30.5 mm, and 29 mm in this order. In addition, the total area reduction rate when performing a drawing process from a diameter of 34.7 mm to 29 mm is 30%.

  For each steel, the cold-drawn material with a diameter of 29 mm obtained as described above was cut into a length of 300 mm, and used as a test piece for machinability investigation after drawing (that is, after cold working).

  Among the test pieces produced as described above, a round bar test piece having a diameter of 10 mm, which is used for measuring the hardness after nitriding, an ono type rotary bending fatigue test piece having a rough notch, a block test piece A, and The block specimen B was nitrided. Specifically, “gas soft nitriding” by the heat pattern shown in FIG. 7 was performed. Note that “120 ° C. oil cooling” indicates that the oil was cooled in oil at an oil temperature of 120 ° C.

  The above-notched Ono-type rotating bending fatigue test piece with notches having been subjected to “gas soft nitriding” was finished to produce the Ono-type rotating bending fatigue test piece with notches shown in FIG.

  The unit of dimensions in the notched Ono type rotating bending fatigue test piece shown in FIG. 8 is “mm”, and the finish symbols “▽” and “▽▽▽” in the figure are the same as those in FIGS. These are “triangular symbols” indicating the surface roughness described in Table 1 of JIS B 0601 (1982).

  “˜” is a “waveform symbol”, meaning that it is a dough, that is, it remains a “gas soft-nitrided” surface.

  The following tests were performed using the test pieces prepared as described above.

Investigation 1: Vickers hardness test before cold working Five points of HV, JIS Z 2244, of one center part and four R / 2 parts of a Vickers hardness measurement specimen before mirror working that was mirror finished. In accordance with “Vickers hardness test-test method” described in (2009), the test force was measured with a Vickers hardness tester at 9.8 N, and the arithmetic average value of 5 points was calculated as the hardness before cold working. did.

Investigation 2: Extraction residue analysis A 10 mm × 10 mm × 10 mm sample cut out for extraction residue analysis was subjected to constant current electrolysis in a 10% AA-based solution. That is, in order to remove the deposits on the surface, first, preliminary electrolysis was performed on the sample under the conditions of current: 1000 mA, time: 28 minutes, and then the deposit on the sample surface was ultrasonically washed in alcohol from the sample. The mass of the sample from which the adhering material was removed was measured, and then the mass of the sample before electrolysis to be performed was used.

  Next, the sample was electrolyzed under the conditions of current: 173 mA and time: 142 minutes. The electrolyzed sample was taken out, and the deposit (residue) on the sample surface was ultrasonically washed in alcohol to remove it from the sample. Thereafter, the electrolyzed solution and the solution used for ultrasonic cleaning were suction filtered with a filter having a mesh size of 0.2 μm to collect a residue. About the sample from which the deposit | attachment (residue) was removed, mass was measured and it was set as the mass of the sample after electrolysis. The “mass of the electrolyzed sample” was determined from the difference in the measured values of the mass of the sample before and after electrolysis.

  The residue collected on the filter was transferred to a petri dish, dried, measured for mass, and then subjected to an acid decomposition treatment.

  The acid-decomposed solution was analyzed with an ICP emission spectrometer (high-frequency inductively coupled plasma emission spectrometer) to determine “the mass of V in the residue”.

  Then, for each steel, the “mass of V in the residue” obtained as described above is divided by the “mass of the electrolyzed sample” and expressed as a percentage. V content ”.

Investigation 3: Vickers hardness test after cold working In the same manner as in the case of "Investigation 1", according to JIS Z 2244 (2009), a test piece for measuring Vickers hardness after cold working after mirror finishing was used. HV of 5 points in total, 1 point in the center and 4 points in the R / 2 part, was measured with a Vickers hardness tester with a test force of 9.8 N, and the arithmetic average value of 5 points was the hardness after cold working .

Investigation 4: Measurement of deformation resistance in cold forging Cold compression of the smooth test piece shown in FIG. 1 at 10%, 20%, 30%, 40%, 50% and 60% compression by end face constrained compression The deformation resistance at that time was measured. Each deformation resistance is determined according to the method described in Table 5.2 on page 158 of “Forging” (1997, first edition, second edition, Corona) edited by the Japan Society for Technology of Plasticity. Apparent constraint coefficient) was calculated.

  Next, for each steel, an approximate curve was created from a 6-point plot, with the horizontal axis representing logarithmic strain and the vertical axis representing deformation resistance. From the obtained approximate curve, the deformation resistance when the logarithmic strain was 1.0 was obtained, and when the value was 550 MPa or less, it was set as the target because it was excellent in cold forgeability. The “logarithmic strain” mentioned above refers to the average logarithmic strain ε in Table 5.2 on page 158 of “Forging” edited by the Japan Society for Technology of Plasticity.

Investigation 5: Limit compression ratio measurement in cold forging The notch specimen shown in FIG. 2 was cold-compressed until cracks occurred in the notch with the naked eye, and the compressibility when cracks occurred was obtained. . And about 5 test pieces, the compression rate when a crack generate | occur | produced was calculated | required, and the compression rate of the 3rd test piece from the one where this compression rate is low was made into the limit compression rate. And when the critical compression ratio was 65% or more, this was set as the target because it was excellent in cold forgeability.

Investigation 6: Machinability test The outer periphery of a test piece cut to a length of 300 mm after cold drawing to a diameter of 29 mm was turned using an NC lathe to investigate the machinability.

  Turning uses a carbide tool mainly made of WC without chip breaker, cutting speed: 150 m / min, cutting depth: 0.2 mm, feeding: 0.8 mm / rev, and lubrication with water-soluble lubricant It carried out in the state. The machinability after cold working was evaluated by the chip disposability during turning.

  The chip disposability was evaluated for each steel by selecting a chip having the maximum chip length shown in FIG. 9 from any 10 chips after turning and measuring the length. The chip disposability is defined as “particularly good (」) ”,“ good (◯) ”and“ bad ”(when the chip length is 5 mm or less and over 10 mm or less and over 10 mm, respectively). X) ".

  In the case where the chip disposability was evaluated as good or better (◎ or ○), the machinability was considered excellent, and this was the target.

Survey 7: Measurement of core hardness, surface hardness and effective hardened layer depth after nitriding (gas soft nitriding) Crossing the 10 mm diameter round bar test piece that has been gas soft nitrided, the cut surface becomes the test surface After being embedded in the resin, the surface was polished so as to have a mirror finish, and the core hardness was measured using a Vickers hardness tester. Moreover, the surface hardness and the effective hardened layer depth were investigated using the micro Vickers hardness measuring machine.

  Specifically, in accordance with JIS Z 2244 (2009), a Vickers hardness with a test force of 9.8 N, a total of 5 HVs of 1 center part and 4 parts of R / 2 part of the mirror-finished test piece. Measured with a thickness tester, the arithmetic average value of 5 points was defined as “core hardness”.

  Using the same embedded sample, in accordance with JIS Z 2244 (2009) in the same manner as above, a micro Vickers hardness tester sets the test force to 0.98 N and a depth of 0.01 mm from the surface of the test piece. Arbitrary 10 points of HV at the position were measured, and the values were arithmetically averaged to obtain “surface hardness”.

  Furthermore, using the same embedded sample, HV was measured sequentially from the surface of the test piece mirror-finished with a test force of 1.96 N using a micro Vickers hardness measuring machine in accordance with JIS Z 2244 (2009). A distribution map was created. And the distance from the surface to the position which becomes 550 in HV was made into the "effective hardened layer depth."

Survey 8: Ono type rotating bending fatigue test Using the Ono type rotating bending fatigue test piece finished after nitriding (soft nitriding), the Ono type rotating bending fatigue test was conducted under the following test conditions, and the number of repetitions was 10 ⁷ times. The maximum strength at which fracture does not occur was defined as “rotary bending fatigue strength”. When the rotational bending fatigue strength was 400 MPa or more, the rotational bending fatigue strength was considered to be excellent, and this was targeted.

・ Temperature: Room temperature,
・ Atmosphere: In the air
-Number of rotations: 3000 rpm.

Investigation 9: Investigation of wear resistance The wear resistance was investigated by a block-on-ring wear test. That is, as shown in FIG. 10, a nitrided (soft nitrided) block test piece A having a length of 15.75 mm and a thickness of 6.35 mm (hereinafter referred to as “test surface”) is pressed against the ring test piece. The wear test was carried out by rotating the ring specimen.

  Specifically, after putting 100 ml of a commercially available automatic transmission oil as a lubricating oil in the test chamber and raising the temperature to 90 ° C., the test surface of the block test piece A is pressed against the ring test piece with a test force of 1000 N. The ring specimen was rotated at a sliding speed of 1 m / sec until the total sliding distance reached 8000 m.

  The above-mentioned ring test piece is cut out from a 45 mm diameter steel bar of SCM420 specified in JIS G 4053 (2008), with the steel bar aligned in the axial direction and cut into a shape generally shown in FIG. “Gas carburizing quenching-tempering” was performed according to the heat pattern shown, and then the outer peripheral portion was ground by 100 μm and finished to the shape shown in FIG.

  The dimension which does not show the unit in said ring test piece shown in FIG. 11 is "mm", and the finishing symbol "▽" in the figure is the surface described in the explanatory table 1 of JIS B 0601 (1982). It is a “triangular symbol” indicating roughness. In addition, “Rq: 0.15 to 0.35 μm” attached to “▽” means that the root mean square roughness “Rq” defined in JIS B 0601 (2001) is 0.15 to 0.35 μm. Means.

  “Cp” in FIG. 12 represents a carbon potential. Further, “80 ° C. oil cooling” indicates that the oil is cooled by being put into oil having an oil temperature of 80 ° C.

  After completion of the block-on-ring wear test, the test surface of the block test piece A is measured using a surface roughness meter, as shown by arrows 1, 2, and 3 in FIG. The maximum difference between the non-contact portion and the contact portion in the cross-sectional curve was taken as the wear depth. In addition, it measured for each three places and made the average value the wear depth. If the wear depth at this time was 10.0 μm or less, it was judged that the wear resistance was excellent, and this was the target.

  The above “non-contact part” and “contact part” refer to “non-contact part” and “contact part” with the ring test piece.

Investigation 10: Investigation of deformation resistance Deformation resistance was investigated by an indentation test. That is, as shown in FIG. 14, an indentation test of the shape shown in FIG. 15 is performed on a surface (hereinafter referred to as “test surface”) having a length of 25 mm and a thickness of 12.5 mm of the nitrided (soft-nitrided) block test piece B. The jig was pushed in and the deformation resistance was investigated. The indentation test jig is a 45 mm diameter steel bar of SCM420 specified in JIS G 4053 (2008), as in the case of the ring test piece of the block-on-ring wear test. A test piece was cut into the shape shown in FIG. The “gas carburizing quenching-tempering” by the heat pattern shown in FIG. 12 was performed, and then the outer peripheral portion was ground by 100 μm and finished to the dimensional shape shown in FIG.

  Specifically, the indentation test jig was pushed into the test surface of the block test piece B with a test force of 5000 N using a hydraulic servo tester. After releasing the test force, the amount of indentation deformation on the test surface of the block test piece B was measured with a surface roughness meter at three locations in the same manner as in Investigation 10, and the average value of the three locations was used as the indentation deformation amount. If the indentation deformation amount was 5.0 μm or less, it was judged that the deformation resistance was excellent, and this was the target.

  The dimension which does not show the unit in the above indentation test jig shown in FIG. 15 is “mm”, and the finish symbol “▽” in the figure was described in the explanation table 1 of JIS B 0601 (1982). It is a “triangular symbol” indicating the surface roughness. In addition, “Rq: 0.10 to 0.20 μm” attached to “▽” means that the root mean square roughness “Rq” defined in JIS B 0601 (2001) is 0.10 to 0.20 μm. Means.

  Table 2 summarizes the test results of the surveys 1 to 10. However, in Table 2, the difference between the core hardness (HV) after nitriding (gas soft nitriding) and the hardness after cold working (HV) is shown as age hardening amount (ΔHV) by nitriding. In addition, when the steel 12 which does not contain V was used, V was not recognized in the precipitate by an extraction residue analysis. For this reason, the V content column in the precipitate of the test number 12 was described as “−”.

  Of the above test results, the relationship between Fn1 and hardness before cold working (HV) in Survey 1, the relationship between Fn1 and deformation for cold forging in Survey 4, and the cold forging in Fn1 and Survey 5 The relations of the 4th reduction ratio are shown in FIGS.

  19 and 20 respectively show the relationship between Fn2 and the core hardness (HV) after nitriding in Investigation 7 and the relationship between Fn2 and the amount of indentation deformation in Investigation 10.

  FIGS. 21 to 23 summarize the relationship between Fn3 and the surface hardness (HV) after nitriding in Survey 7, the relationship between Fn3 and rotational bending fatigue strength in Survey 8, and the relationship between Fn3 and wear depth in Survey 9, respectively. Show.

  From Table 2, in the case of test numbers 1 to 11 of the present invention examples where the material before nitriding satisfies the conditions specified in the present invention, it is excellent in cold forgeability and machinability after cold working, and nitriding In order to satisfy all of the core hardness, surface hardness and effective hardened layer depth specified later in the present invention, it has excellent deformation resistance, high rotational bending fatigue strength, and excellent wear resistance.

  Among the examples of the present invention, test number 5 using steel 5 containing Te, test number 6 using steel 6 containing Se and Sb, test number using steel 10 containing Pb and Ca It is clear that all of the test numbers 11 using the steel 11 containing 10 and Bi are particularly excellent in machinability after cold working.

  On the other hand, in the test number 12 of the comparative example, the contents of C, Si, and Mn of the steel 12 used are all more than the range defined in the present invention, and are defined as “160 or less” in the present invention. Fn1 is as high as 219. Therefore, the cold compressibility is inferior because the critical compression ratio is as low as 55% and the deformation resistance is as high as 638 MPa. Moreover, since the steel 12 does not contain V and Fn2 is 0 and is out of the range defined in the present invention, the amount of indentation deformation is 5.1 μm and the deformation resistance is also inferior. Further, the steel 12 has an Fn3 of 140, which deviates from the definition of “160 or more” of the present invention, and in addition, the effective hardened layer depth after nitriding is 0.12 mm, which is shallower than the value specified by the present invention. Since the surface hardness after nitriding is 612, which is HV lower than the value specified in the present invention, the rotational bending fatigue strength is as low as 390 MPa, the wear depth is as large as 13.0 μm, and the wear resistance is also poor.

  Test No. 13 is inferior in cold forgeability because Fn1 of steel 13 used is 169, which is high beyond the range specified in the invention, and the critical compression ratio is as low as 59% and the deformation resistance is as high as 619 MPa.

  Test No. 14 has an Fn2 of 3 for the steel 14 used, which is not within the range of “20 to 80” of the present invention, and the core hardness after nitriding is 168 at HV lower than the value defined by the present invention. Therefore, the amount of indentation deformation is as large as 5.5 μm and the deformation resistance is poor.

  In Test No. 15, the V content of the steel 15 used is 0.15%, which is more than the range specified in the present invention, so the V content in the precipitate in the extraction residue is out of the specification. Therefore, the critical compressibility is as low as 61% and the deformation resistance is as high as 588 MPa, so that the cold forgeability is inferior. In addition, the hardness after cold working (cold drawing) is as high as 251 in HV. Inferior to machinability.

  The test number 16 is that the Fn3 of the steel 16 used deviates from the definition of “160 or more” of the present invention, so the surface hardness after nitriding is as low as 588 in HV, and the effective hardened layer depth is also defined in the present invention. It is 0.12 mm shallower than the value. Therefore, the rotational bending fatigue strength is as low as 370 MPa, the wear depth is as large as 11.0 μm, and the wear resistance is inferior.

  The steel for cold forging and nitriding of the present invention is excellent in cold forgeability and machinability after cold forging, and has a high core in a part subjected to cold forging and nitriding treatment. Hardness, high surface hardness and deep effective hardened layer depth can be provided. For this reason, it is suitable for using as a raw material of cold forging nitriding components.

  Further, the cold forged and nitrided parts of the present invention are excellent in deformation resistance, bending fatigue strength, and wear resistance, and thus are suitable as parts for machine structures used in automobile transmissions such as gears and pulleys for CVT. Can be used.

Claims (7)

In mass%, C: 0.01 to 0.15%, Si: less than 0.10%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.80 to 2.0%, V: 0.03% or more and less than 0.10%, Al: 0.01 to 0.10%, N: 0.0080% or less, and O: 0.0030% or less And the balance is Fe and impurities, Fn1 represented by the following formula (1) is 160 or less, Fn2 represented by the formula (2) is 20 to 80, and the formula (3) A steel for cold forging and nitriding characterized by having a chemical composition in which Fn3 represented by the formula is 160 or more.
Fn1 = 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V (1)
Fn2 = (669.3 × log _e C−1959.6 × log _e N−6983.3) × (0.067 × Mo + 0.147 × V) (2)
Fn3 = 140 × Cr + 125 × Al + 235 × V (3)
C, Si, Mn, Cr, Mo, V, N, and Al in the above formulas (1) to (3) mean the content of the element in mass%.   The steel for cold forging and nitriding according to claim 1, characterized by containing Mo: 0.50% or less in mass% instead of part of Fe.   It replaces with a part of Fe and contains 1 or more types selected from Cu: 0.50% or less and Ni: 0.50% or less by the mass%, The claim 1 or 2 characterized by the above-mentioned. Steel for cold forging and nitriding.   Instead of a part of Fe, it contains one or more selected from Ti: 0.20% or less, Nb: 0.10% or less, and Zr: 0.10% or less in mass%. The steel for cold forging and nitriding according to any one of claims 1 to 3.   Instead of a part of Fe, in mass%, Pb: 0.50% or less, Ca: 0.010% or less, Bi: 0.30% or less, Te: 0.30% or less, Se: 0.30% The steel for cold forging and nitriding according to any one of claims 1 to 4, comprising at least one selected from the following and Sb: 0.30% or less.   A steel material for cold forging and nitriding having the chemical composition according to any one of claims 1 to 5 and having a V content in a precipitate by an extraction residue analysis of 0.05% or less.   It has the chemical composition according to any one of claims 1 to 5, the core hardness is 180 or more in terms of Vickers hardness, the surface hardness is 650 or more in terms of Vickers hardness, and the effective hardened layer depth is 0.15 mm. Cold forged and nitrided parts characterized by the above.

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