JP2013159794A - Rolled steel for cold forging/nitriding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled steel for cold forging/nitriding, capable of being subjected to cold forging without performing heat treatment beforehand, having reduced surface roughness after cold forging, having excellent machinability after cold forging and capable of imparting high core part hardness to a component subjected to cold forging and nitriding.SOLUTION: A rolled steel for cold forging/nitriding has a chemical composition including 0.06 to 0.15% C, 0.02 to 0.35% Si, 0.10 to 0.90% Mn, ≤0.030% S, 0.50 to 2.0% Cr, 0.10 to 0.50% V and 0.010 to 0.090% Al, and the balance Fe with impurities; wherein P, N and O in the impurities satisfy P≤0.030%, N≤0.0080% and O≤0.0030%, and 5.0≤35(C+N)+5(Si+Mn)+Cr+3(Cu+Ni)+4(Mo+V)≤10.0 and 1.8≤{V+(9/20)Mo}/C≤9.0 is satisfied. The rolled steel for cold forging/nitriding has a microstructure of ferrite-pearlite structure, in which the average grain size of the ferrite is ≤50 μmn.

Description

  The present invention relates to a rolled steel material for cold forging nitriding. Specifically, cold forging can be performed in the rolled state without heat treatment in advance, and when cutting is performed after cold forging, cutting resistance is low and machinability is excellent. It is possible to provide a processed core (hereinafter referred to as “cold forging and nitriding part”) with high core hardness and suitable for use as a material for cold forging and nitriding part. The present invention relates to a rolled steel material for nitriding (hereinafter referred to as “rolled steel material for cold forging and nitriding”). The “rolled steel material for cold forging and nitriding” refers to a steel material in an as-rolled state that is used after being subjected to cold forging and nitriding.

  The “nitriding” in the present invention includes not only “nitriding” in the strict sense of “invading 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” may be simply referred to.

  In general, a sliding part of an automobile is manufactured by performing a cutting process after hot forging and then performing a surface hardening heat treatment such as carburizing and quenching. However, with the trend of reducing greenhouse gases against the background of global warming suppression in recent years, cold forging, nitriding, softening, etc. have been carried out from manufacturing methods that require holding at high temperatures such as hot forging and carburizing and quenching. There is a demand for conversion to a manufacturing method using heat treatment at room temperature such as nitriding or at a temperature lower than conventional.

  However, the conventional nitriding steel has the following problems.

  <1> Nitriding is a surface hardening heat 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 ensure the desired core hardness in the nitrided part, it is necessary to contain a large amount of alloying elements. In this case, it is difficult to form by cold forging, such as by hot forging. Molding is required.

  <2> Soft nitriding is a surface hardening heat treatment in which nitrogen and carbon enter and diffuse from the surface of the component while being kept in a temperature range of about 500 to 650 ° C. for several hours. However, the core of the component is tempered by being kept in this temperature range for several hours, and thus is easily softened. For this reason, when a high surface pressure is applied to the soft-nitrided component, the load stress may exceed the yield strength of the component. At this time, the contact surface is dented and deformed.

  In order to solve the above-described problems, for example, Patent Documents 1 to 4 disclose techniques related 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 steel for soft nitriding excellent in cold forgeability and pitting resistance” that has improved cold forgeability before soft nitriding and has sufficient pitting resistance after soft nitriding. . The above “soft nitriding steel” is a low carbon steel containing, by mass%, Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%). If necessary, it further contains at least one of Al, N, Cr, Mo, V, Nb, Ti, Ca, Zr, Te, Bi and Pb.

Patent Document 3 discloses “a method of manufacturing a machine part by cold forging-soft nitriding”. The above-mentioned “manufacturing method of machine parts” is mass%, C: 0.15 to 0.30%, Si: 0.2% or less, Mn: 0.4 to 1.5%, Cr: 0.6 A nitrided steel having an alloy composition containing ~ 1.5%, s-Al: 0.05-0.20% and V: 0.05-0.30%, the balance being substantially made of Fe,
1) Temporary soaking: Heated to 900 ± 15 ° C 2) Intermediate cooling: Cooled at a rate of 10 ° C / sec or less 3) Secondary soaking: Heated to 680 ± 30 ° C, and then cold forged to form the part shape It is a manufacturing method which comprises hardening the surface by gas soft nitriding at 500 to 650 ° C.

  Patent Document 4 discloses “steel for soft nitriding” which can obtain a high surface hardness and a deep effective hardening depth by soft nitriding and is excellent in cold forgeability, fatigue life and pitting resistance. The above “steel for soft nitriding” is in mass%, C: 0.25% or less, Si: 0.30% or less, Mn: 0.30-0.90%, S: 0.020% or less, Cr : 0.50 to 1.50%, Al: 0.030 to 0.500%, V: 0.05 to 0.30%, Ti: 0.10% or less, N: 0.0060% or less, the balance being It consists of Fe and an impurity element, and Ti / N satisfies 4 or more.

JP-A-5-171347 JP-A-10-306343 JP 2006-63378 A JP-A-9-71841

Plasticity and processing, Vol. 22, no. 241 (1981), pp. 139-144 K. Osakada et al. : Anals of the CIRP, vol. 30 (1981) No. 30. 1, p. 135

  As described above, the “cold forging and nitriding component” is subjected to cold forging and cutting as necessary, and then subjected to nitriding treatment. Depending on the part, the free end of cold forging may be finished by light cutting or polishing.

  The steel disclosed in Patent Document 1 described above cannot always reduce the surface roughness when cold forging is performed at a high workability. For this reason, when the surface roughness is large, depending on the part, the unevenness generated at the free end of cold forging may not be removed by light cutting or polishing.

  The steel disclosed in Patent Document 2 contains a large amount of alloy elements such as Cu and Ni. 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 manufacturing method disclosed in Patent Document 3 is a technology in which the steel material after rolling is subjected to the heat treatments 1) to 3), followed by cold forging into a component shape, and then gas soft nitriding to harden the surface. It is. That is, since it is necessary to heat-treat the steel material after rolling before cold forging, the process becomes complicated and the cost increases.

  Although the steel disclosed in Patent Document 4 has been studied for deformation resistance during cold forging, it is not always possible to reduce the surface roughness when cold forging is performed with a large degree of work. For this reason, when the surface roughness is large, depending on the part, the unevenness generated at the free end of cold forging may not be removed by light cutting or polishing.

  In the case of a part formed by cold forging, it is desirable to omit the cutting process in order to reduce costs. However, it is difficult to omit the cutting process for all the parts after cold forging. Therefore, the parts after cold forging are also required to have low cutting resistance and excellent machinability.

  The present invention has been made in view of the above situation, and can be cold forged in a rolled state without heat treatment in advance, has a small surface roughness after cold forging, and is further subjected to cutting after cold forging. In this case, the cutting resistance is low, the machinability is excellent, and the parts subjected to the cold forging and nitriding treatment can have a high core hardness. An object is to provide a rolled steel material for cold forging and nitriding suitable for use as a material.

  In order to solve the above-described problems, the present inventors have made various investigations and studies. As a result, first, the following findings (a) to (h) were obtained.

  (A) In order to provide excellent cold forgeability in which cold forging can be performed in a rolled state without heat treatment in advance, the hardness of the steel material is reduced to Vickers hardness (hereinafter referred to as “HV”) May be 160 or less.

  (B) In order to reduce the hardness of an as-rolled steel material to 160 or less in HV, the microstructure may be a mixed structure composed of ferrite and pearlite (hereinafter also referred to as “ferrite / pearlite structure”).

  (C) The surface roughness after cold forging is affected by the ferrite grain size of the steel material before cold forging. By reducing the ferrite grain size, the irregularities generated at the free end of cold forging can be removed by light cutting or polishing.

  (D) Regardless of the type of cold working such as cold forging or cold drawing, if cold working with a true strain of 0.5 or more is applied, HV by work hardening is performed regardless of the hardness of the rolled steel material. (Hereinafter, the increment of HV is referred to as “ΔHV”) is 50 or more, and the cutting resistance at the time of cutting is reduced as compared with that before cold working. However, if the hardness of the steel material before cold working is excessively low, the hardness is low after cold working, and it is easy for “swelling” to occur. On the other hand, the cutting resistance is even increased, and in this case, the machinability is lowered. Note that “peeling” refers to a phenomenon in which a material to be treated easily adheres to a blade during cutting and is peeled off as it is.

  (E) The HV of the steel material has a correlation with a formula defined by the contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, and N. If the value of the formula is adjusted to a specific range, the steel as-rolled can have excellent cold forgeability, and the occurrence of “peel” when cutting after cold forging. Is suppressed and cutting resistance can be reduced, so that good machinability is ensured.

  (F) The aforementioned sliding parts for automobiles are required to be difficult to plastically deform during sliding, and the difficulty of plastic deformation depends on the core hardness of the parts. And the core hardness can be increased by work hardening at the time of cold forging. As described above, if cold working with a true strain of 0.5 or more is applied, the hardness of the steel as-rolled Regardless, ΔHV due to work hardening is 50 or more.

(G) Since the parts are tempered during holding for several hours at a soft nitriding temperature of 500 to 650 ° C., ΔHV due to the work hardening is reduced. However, if VC or Mo ₂ C is precipitated in the temperature range of 500 to 650 ° C., the hardness of the core can be increased by the precipitation hardening action.

(H) Utilizing the precipitation hardening action at the nitriding temperature, that is, V is deposited as VC and / or Mo is deposited as Mo ₂ C, and ΔHV of the core portion by nitriding treatment is maintained in an appropriate range. In order to increase the core hardness, the value of the formula defined by the contents of C, Mo and V may be adjusted to a specific range.

  This invention is completed based on said knowledge, The summary exists in the rolled steel materials for cold forge nitriding shown to following (1)-(4).

(1) In mass%, C: 0.06-0.15%, Si: 0.02-0.35%, Mn: 0.10-0.90%, S: 0.030% or less, Cr: 0.50 to 2.0%, V: 0.10 to 0.50%, and Al: 0.010 to 0.090%, the balance being Fe and impurities, P, N and O in the impurities Are P: 0.030% or less, N: 0.0080% or less, and O: 0.0030% or less, respectively, and Fn1 represented by the following formula (1) is 5.0 to 10.0. The Fn2 represented by the following formula (2) has a chemical composition of 1.8 to 9.0, the microstructure is a ferrite / pearlite structure, and the average particle diameter of the ferrite is 50 μm or less. Rolled steel for cold forging and nitriding, characterized by
Fn1 = 35 (C + N) +5 (Si + Mn) + Cr + 3 (Cu + Ni) +4 (Mo + V) (1)
Fn2 = {V + (9/20) Mo} / C (2)
However, C, Si, Mn, Cu, Ni, Cr, Mo, V and N in the above formulas (1) and (2) mean the content of the element in mass%.

  (2) The rolled steel material 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) Instead of a part of Fe, by mass%, it contains at least one of Ti: 0.50% or less, Nb: 0.50% or less, and Zr: 0.50% or less. The rolled steel material for cold forging and nitriding as described in (1) or (2) above.

  (4) From the above (1), characterized in that, instead of a part of Fe, by mass%, it contains one or more of Cu: 0.30% or less and Ni: 0.20% or less (3) The rolled steel material for cold forging and nitriding according to any of the above.

  In addition, “impurities” in “Fe and impurities” as the balance refer to those mixed from ore, scrap, or production environment as raw materials when industrially producing steel materials.

  The “ferrite / pearlite structure” refers to a mixed structure composed of ferrite and pearlite.

  The rolled steel material for cold forging and nitriding of the present invention can be cold forged as it is rolled without preheating the steel material, has a small surface roughness after cold forging, and is further subjected to cutting after cold forging In some cases, the cutting resistance is low, the machinability is excellent, and the parts subjected to the cold forging and nitriding treatments can have a high core hardness. For this reason, it is suitable for using as a raw material of cold forging nitriding components.

It is a figure which shows the shape of the smooth test piece used in the Example for measuring the cold forgeability and the surface roughness of the side surface after cold forging. The unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the block-shaped test piece used in measuring the core part hardness after a soft nitriding process in the Example. The unit of the dimension in the drawing is “mm” except for the part described as “Rq: 0.10 to 0.20 μm”. It is a figure which shows the heat pattern of the gas soft nitriding process performed to the block-shaped test piece of FIG. 2 in the Example. It is the figure which arranged the relationship between Fn1 represented by (1) Formula, and the hardness (HV) of the steel materials after rolling in the investigation 1 of an Example. It is the figure which arranged the relationship between Fn1 represented by (1) Formula, and the cutting resistance (main component force) after cold drawing in the investigation 5 of an Example. The relationship between Fn2 expressed by the formula (2) and ΔHV before and after gas soft nitriding in Survey 5 and Survey 6 of the example, that is, “HV after soft nitriding—HV after cold drawing” is arranged. FIG.

  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: 0.06 to 0.15%
C is an essential element for securing the strength of the cold forged nitriding component, and a content of 0.06% or more is necessary. However, if the C content is too large, the hardness becomes too high and the cold forgeability decreases. For this reason, the upper limit was provided and the C content was 0.06 to 0.15%. When the cold forgeability is more important, the C content is preferably 0.13% or less.

Si: 0.02-0.35%
Si has a deoxidizing action. In order to obtain this effect, a Si content of 0.02% or more is required. However, when there is too much content of Si, a ferrite will become hard and cold forgeability will fall. For this reason, the upper limit was provided and Si content was 0.02 to 0.35%. Note that the Si content is preferably 0.15% or less.

Mn: 0.10-0.90%
Mn has an action to ensure the strength of the cold forged and nitrided part and a deoxidizing action. In order to obtain these effects, a Mn content of 0.10% or more is required. However, if the Mn content is too large, the hardness of the ferrite becomes too high and the cold forgeability decreases. For this reason, the upper limit was provided and the content of Mn was set to 0.10 to 0.90%. In addition, when the cold forgeability is regarded as important, the Mn content is preferably 0.70% or less.

S: 0.030% or less S is an element contained as an impurity in steel. Further, when S is contained, it combines with Mn to form MnS and has an effect of improving machinability. However, when the content exceeds 0.030%, coarse MnS is formed, Forgeability is reduced. Therefore, the content of S is set to 0.030%. The S content is preferably 0.010% or more when machinability is more important, and 0.015% or less when cold forgeability is important. Is preferred.

Cr: 0.50 to 2.0%
Cr has the effect of increasing the surface hardness in nitriding and improving the wear resistance of cold forged nitriding parts. If the Cr content is less than 0.50%, the above effect cannot be obtained. 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 0.50 to 2.0%. When the cold forgeability is more important, the Cr content is preferably 1.5% or less.

V: 0.10 to 0.50%
V combines with C in the nitriding temperature range and precipitates as VC, thereby improving the core hardness after nitriding. In order to acquire this effect, it is necessary to contain V 0.10% or more. However, if the content of V is large, it becomes hard and cold forgeability is lowered. Therefore, an upper limit is set and the content of V is set to 0.10 to 0.50%. The V content is preferably 0.40% or less.

Al: 0.010 to 0.090%
Al has the effect of increasing the surface hardness in nitriding and improving the wear resistance of the cold forged nitriding component. In order to acquire this effect, it is necessary to contain Al 0.010% or more. However, when there is too much content of Al, the hardened layer by nitriding will become shallow and the problem that wear resistance falls will arise. Therefore, an upper limit is provided, and the Al content is set to 0.010 to 0.090%. The Al content is preferably 0.020% or more, and is preferably 0.050% or less.

  One of the rolled steel materials for cold forging and nitriding of the present invention contains the above-described elements from C to Al, the balance is composed of Fe and impurities, and has a chemical composition that satisfies the conditions for Fn1 and Fn2 described later. Is. 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.

  However, in the present invention, P, N, and O in impurities must be strictly limited, and the contents thereof are P: 0.030% or less, N: 0.0080% or less, and O: 0.0. It is necessary to make it 0030% or less.

  This will be described below.

P: 0.030% or less P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the P content exceeds 0.030%, the embrittlement occurs. The degree may be noticeable. Therefore, the content of P is set to 0.030% or less. The P content is preferably 0.020% or less.

N: 0.0080% or less N is an impurity contained in the steel and increases the hardness of the ferrite and decreases the cold forgeability by being dissolved in the ferrite. Moreover, since it combines with V to form VN having a high solid solution temperature, the amount of solid solution V is reduced, and it becomes difficult to obtain the effect of precipitation hardening of VC at the nitriding temperature. Therefore, an upper limit is set and the N content is set to 0.0080% or less. The N content is preferably 0.0070% or less.

O: 0.0030% or less O is an impurity contained in steel, forms oxide inclusions, and may cause cracks during cold forging. In particular, when the content of O exceeds 0.0030%, a coarse oxide is generated and cold forgeability is lowered. Therefore, the content of O is set to 0.0030% or less. Note that the O content is preferably 0.0020% or less.

  Another one of the rolled steel materials for cold forge nitriding of the present invention contains one or more elements of Mo, Ti, Nb, Zr, Cu and Ni instead of a part of the above-mentioned Fe, and It has a chemical composition that satisfies the conditions for Fn1 and Fn2.

  Hereinafter, the operational effects of the Mo, Ti, Nb, Zr, Cu, and Ni, which are optional elements, 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 carbide (Mo ₂ C), and has an effect of increasing the core hardness after nitriding. Therefore, you may contain Mo. 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 containing is 0.50% or less. In addition, when the cold forgeability is regarded as important, the amount of Mo in the case of inclusion 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.

  Ti, Nb, and Zr all have the effect of increasing the surface hardness. For this reason, you may contain these elements. Hereinafter, the Ti, Nb, and Zr will be described.

Ti: 0.50% or less Ti is an element effective in increasing the surface hardness by combining with N that penetrates and diffuses from the surface during nitriding to generate nitride. Therefore, Ti may be included. However, even if Ti is contained in an amount exceeding 0.50%, the above effect is saturated and the cost is increased. Therefore, the amount of Ti in the case of containing is 0.50% or less. In addition, when Ti is contained, the amount of Ti is preferably 0.20% 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 0.05% or more.

Nb: 0.50% or less Nb is an element effective in increasing the surface hardness by forming a nitride by combining with N that penetrates and diffuses from the surface during nitriding. Therefore, you may contain Nb. However, even if Nb is contained in an amount exceeding 0.50%, the above effect is saturated and the cost is increased. Therefore, the amount of Nb in the case of containing is 0.50% or less. In addition, it is preferable that the amount of Nb in the case of containing is 0.10% 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 0.02% or more.

Zr: 0.50% or less Zr is an element effective for increasing the surface hardness by forming nitride by combining with N that penetrates and diffuses from the surface during nitriding. Therefore, Zr may be contained. However, even if Zr is contained in an amount exceeding 0.50%, the above effect is saturated and the cost is increased. Therefore, the amount of Zr in the case of containing is 0.50% or less. In addition, it is preferable that the amount of Zr in the case of containing is 0.10% 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 0.02% 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 contained may be 1.50% or less, but is preferably 0.40% or less.

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

Cu: 0.30% or less Cu has an action of increasing the core hardness. Therefore, Cu may be contained. However, when it contains Cu exceeding 0.30%, in addition to becoming hard and cold forgeability falling, it becomes easy to produce hot brittleness. Therefore, the amount of Cu in the case of inclusion is set to 0.30% or less. In addition, when the cold forgeability is regarded as important, the amount of Cu in the case of inclusion is preferably 0.20% 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: 0.20% or less Ni has an action of increasing the core hardness. Ni also has the effect of suppressing the occurrence of hot brittleness due to Cu. Therefore, Ni may be included. However, if Ni exceeds 0.20%, it becomes hard and cold forgeability deteriorates. Therefore, the amount of Ni in the case of inclusion is set to 0.20% or less. In addition, when Ni is contained, the amount of Ni is preferably 0.10% or less.

  On the other hand, in order to stably obtain the above-described effect of Ni, the amount of Ni when contained is preferably 0.05% or more.

  Said Cu and Ni can be contained only in any 1 type or 2 types of composites. The total amount when Cu and Ni are contained in combination may be 0.50% or less, but is preferably 0.30% or less. In addition, when Cu is contained, it is preferable to contain Ni in combination in order to avoid the occurrence of hot brittleness due to Cu.

Fn1: 5.0-10.0
The rolled steel material for cold forging and nitriding of the present invention is
Fn1 = 35 (C + N) +5 (Si + Mn) + Cr + 3 (Cu + Ni) +4 (Mo + V) (1)
Fn1 represented by must be 5.0 to 10.0. However, C, Si, Mn, Cu, Ni, Cr, Mo, V, and N in the formula (1) mean the content in mass% of the element.

  Said Fn1 is a parameter used as the parameter | index of the hardness (HV) of the steel materials after rolling, and HV will also become high when Fn1 is large. When Fn1 becomes large, especially when it exceeds 10.0, the hardness becomes too high and the cold forgeability deteriorates. Furthermore, the cutting resistance after cold forging increases, and the machinability also decreases. However, when Fn1 is 10 or less, the hardness of the steel material after rolling is stably 160 or less in HV, and excellent cold forgeability that can be cold forged without prior heat treatment is obtained. . On the other hand, when Fn1 becomes excessively small, especially below 5.0, the hardness of the steel material after rolling is easily below HV at 100, so that although it is excellent in cold forgeability, it is cut after cold forging. In this case, “peeling” tends to occur, cutting resistance increases, and machinability deteriorates. From these things, Fn1 was set to 5.0-10.0. Fn1 is preferably 6.0 or more, and preferably 9.5 or less.

Fn2: 1.8-9.0
The rolled steel material for cold forging and nitriding of the present invention is
Fn2 = {V + (9/20) Mo} / C (2)
Fn2 represented by the formula must be 1.8 to 9.0. However, C, Mo and V in the formula (2) mean the content in mass% of the element.

  Said Fn2 is a parameter used as the parameter | index of the increment of the hardness of the core part by nitriding after cold forging, ie, (DELTA) HV of the core part by nitriding. If Fn2 is 1.8 to 9.0, moderate precipitation hardening with ΔHV of 20 to 60 occurs at the core during nitriding, so that the amount of plastic deformation during sliding of the cold forged nitriding component can be reduced. it can.

  When Fn2 is less than 1.8, the content of V and Mo is small relative to the C content of the steel material, and sufficient precipitation hardening does not occur. The amount of plastic deformation becomes large. Further, when Fn2 exceeds 9.0, the content of C is small relative to the content of V and Mo in the steel material, and sufficient precipitation hardening does not occur. Therefore, ΔHV is less than 20, and cold forging The amount of plastic deformation when the nitrided part slides increases. Fn2 is preferably 2.0 or more, and preferably 8.0 or less.

(B) Microstructure (B-1) Phase In the rolled steel material for cold forging and nitriding of the present invention, the microstructure (phase) must be a ferrite pearlite structure. Even if it has the chemical composition described in the item (A), the hardness is high when the microstructure includes a low temperature transformation structure, specifically, bainite or / and martensite. This is because cold forging cannot be performed unless heat treatment is performed in advance, and even if cold forging can be performed, the machinability at the time of subsequent cutting is extremely reduced.

  When the area ratio of pearlite in the ferrite pearlite structure increases, especially when it exceeds 50%, the hardness of the steel material after rolling exceeds 160 in HV, and cold forgeability may be reduced. The main phase in the ferrite / pearlite structure (that is, the phase exceeding 50% in terms of area) is preferably ferrite. In the ferrite-pearlite structure, it is very preferable that the area ratio of ferrite is 70% or more.

  When the steel has the chemical composition described in the above (A), for example, according to the manufacturing method described later, the upper limit of the area ratio of ferrite in the ferrite / pearlite structure is about 85%. Become. As described above, the “ferrite / pearlite structure” refers to a mixed structure composed of ferrite and pearlite.

  The above “phase” is obtained by, for example, cutting a sample from a rolled material, mirror-polishing so that an appropriate cut surface becomes a test surface, and then corroding with a nital to reveal a microstructure and using an optical microscope. Can be identified by observation. The area ratio of the ferrite can be obtained by subjecting the photograph taken by observation with the above-mentioned optical microscope to image processing by a usual method.

(B-2) Average particle diameter of ferrite The rolled steel material for cold forging and nitriding of the present invention must have an average particle diameter of ferrite of 50 μm or less in a ferrite / pearlite structure which is a microstructure. Even if the microstructure is a ferrite-pearlite structure, if the average grain size of ferrite exceeds 50 μm, the surface roughness after cold forging increases, so the unevenness that occurs at the free end of cold forging This is because it cannot be removed by light cutting or polishing. The average particle size of the ferrite is preferably 30 μm or less.

  In addition, when steel has the chemical composition as described in said (A) term, according to the manufacturing method mentioned later, the minimum of the average particle diameter of the ferrite in a ferrite pearlite structure | tissue will be about 16 micrometers.

  The above-mentioned “average particle diameter of ferrite” is, for example, a sample cut from a rolled material, mirror-polished so that an appropriate cut surface becomes a test surface, and then corroded with nitral to reveal a microstructure, Using an optical microscope, a photograph taken by observing five visual fields randomly at a magnification of 1000 times is subjected to image processing by a normal method to calculate the area of each ferrite grain, and can be obtained by converting to a circle.

  The rolled steel material for cold forging and nitriding of the present invention can be manufactured, for example, by the method described below.

  After the steel having the chemical composition described in the item (A) is melted by a normal method, continuous casting is performed to form a slab, and then the slab is heated at a temperature of 1250 ° C. or more and rolled into pieces. To produce a steel piece. Since the fuel cost increases, the heating temperature of the partial rolling is preferably 1350 ° C. or less.

  The steel slab thus obtained was hot-rolled in a temperature range of 1000 to 850 ° C., hot-rolled so that the total area reduction was 50% or more, and finish-rolled, and then 800 It cools on the conditions that the average cooling rate in a temperature range of -500 degreeC becomes 0.8-1.5 degreeC / s. In addition, said "total area reduction rate" points out the area reduction rate from a billet area.

  Then, using the rolled steel material for cold forging and nitriding of the present invention as a raw material, by cold forging, for example, the true strain is processed to 0.5 or more, preferably 0.7 or more to form a part, and if necessary, cut Cold forged nitriding parts can be manufactured by performing processing and polishing, and then performing nitriding treatment at 400 to 650 ° C. for 1 to 30 hours by a normal method.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels 1 to 21 having chemical components shown in Table 1 were melted by a 180 kg vacuum melting furnace and cast into an ingot.

  Steels 1 to 11 in Table 1 are steels whose chemical compositions are within the range defined by the present invention, while Steels 12 to 21 are steels of comparative examples whose chemical compositions deviate from the conditions defined by the present invention. It is.

  Each ingot was subjected to a solution treatment that was maintained at 1250 ° C. for 5 hours to homogenize, and then a hot bar forged a round bar having a diameter of 55 mm and a length of 300 mm.

  Next, each round bar having a diameter of 55 mm is held at 1250 ° C. for 30 minutes, and with respect to the steel 5, rolling is started from various temperatures of 1200 to 850 ° C. using a perforated rolling mill. As for the steels, rolling was started from 900 ° C., all were finished into round bars with a diameter of 38 mm, and then air-cooled. The average cooling rate in the 800-500 degreeC temperature range of the round bar surface at this time was 0.8-1.5 degreeC / s. The total area reduction when the 55 mm diameter round bar was hot-rolled into a 38 mm diameter round bar was 52%.

  Various test pieces were sampled from the round bars having a diameter of 38 mm of steels 1 to 21 thus obtained in the rolled state.

  First, a round bar was cut perpendicular to the axial direction, embedded in a resin so that the cut surface became the test surface, and then mirror-polished, and this sample was subjected to hardness measurement and structure observation after rolling. .

  In addition, in order to measure the cold forgeability and the surface roughness of the side surface after cold forging, a smooth test piece having the shape shown in FIG. 1 is cut out from the axial center portion with the axial direction of the round bar and the test piece aligned. It was. In addition, the unit of the dimension in FIG. 1 is “mm”, and the finish symbols “▽▽” and “▽▽▽” are the surface roughness described in the explanatory table 1 of JIS B 0601 (1982). Is a “triangular symbol”.

  Further, each round bar having a diameter of 38 mm of the steels 1 to 21 obtained as described above was peeled to a diameter of 35 mm in a rolled state, subjected to pickling and lubrication, and then the diameter was 24.0 mm. Then, a cold drawing process (hereinafter, the drawing process is simply referred to as “drawing”) was performed. In addition, the total area reduction rate at the time of drawing from diameter 35 mm to 24.0 mm is 53% (average true strain is equivalent to 0.9).

  The round bar after cold drawing was subjected to hardness measurement and cutting resistance measurement by turning. Furthermore, from the round bar after drawing, a block-shaped test piece having the shape shown in FIG. 2 was cut out and subjected to gas soft nitriding treatment with the heat pattern shown in FIG.

  The unit of the dimension in FIG. 2 is “mm” except for a portion described as “Rq: 0.10 to 0.20 μm”. In addition, the finishing symbol “▽” is “Rq: 0.10 to“ Triangle symbol ”indicating the surface roughness described in the explanatory table 1 of JIS B 0601 (1982), and“ Rq: 0.10 ”added to“ ▽ ”. “0.20 μm” means that the root mean square roughness “Rq” defined in JIS B 0601 (2001) is 0.10 to 0.20 μm.

  Further, “120 ° C. oil cooling” in FIG. 3 indicates that the oil is cooled by being put into oil having an oil temperature of 120 ° C.

  The details of the survey will be described below.

Investigation 1: Hardness measurement after rolling One round center of a round bar of steel 1 to 21 having a diameter of 38 mm after rolling and embedded in the resin and mirror-polished and R / 2 part ("R" is a round bar) The Vickers hardness is measured with a test force of 9.8 N according to “Vickers hardness test-test method” described in JIS Z 2244 (2009) for a total of 4 points. The arithmetic average value of 5 points was defined as HV after rolling. In addition, the hardness after rolling aimed at 100-160 by HV.

Investigation 2 Microscopic observation after rolling The cross-section of the round bars of steel 1 to 21 after rolling embedded in the resin and mirror-polished is corroded with nital, and the magnification is set to 1000 times to 5 in the radial direction with an optical microscope. The phase was identified by visual field observation.

  In addition, the obtained microstructure photograph is image-processed by a usual method to determine the area ratio of ferrite in each field of view, and furthermore, by calculating the area of each ferrite grain and converting it into a circle, The average ferrite particle diameter was calculated. And the average ferrite particle diameter of said 5 visual fields was arithmetically averaged, and the average particle diameter of the ferrite was calculated | required.

Investigation 3: Deformation resistance measurement after rolling The smooth test piece having the shape shown in FIG. 1 taken from a round bar of diameters of 38 mm of steel 1 to 21 after rolling was cold-compressed (cold forging), and the deformation resistance was investigated. did. At this time, in order to constrain the sliding of the compression surface, a compression plate with concentric grooves was used with reference to Non-Patent Document 1, which is a provisional standard for the cold upsetting test method of the Japan Society for Technology of Plasticity. The sample was cold-compressed at a compression rate of 10 to 70% in 10% increments (average true strain was equivalent to 0.13 to 1.40), and the load at that time was measured with a load cell attached to the press. The load was converted to stress according to Non-Patent Document 2 proposed by Kosakada et al. In addition, the stress when the value of the equivalent plastic strain at the center of the test piece was 1 was defined as the deformation resistance. The target deformation resistance after rolling was 550 MPa or less.

Investigation 4: Measurement of surface roughness of side surface in the case of cold forging 80% of a smooth test piece having a shape shown in FIG. 1 taken from a round bar having a diameter of 38 mm of rolled steels 1 to 21 to a height of 4.2 mm Cold compression (cold forging) was performed at a compression ratio (average true strain is equivalent to 2.23). At this time, a smooth plate was used as the compression plate, and molybdenum disulfide was applied to the compression plate in advance. The test piece cold-compressed to a height of 4.2 mm was subjected to ultrasonic cleaning, and then the surface roughness of the side portion was measured in the axial direction. Note that a roughness curve was collected with a radius of the tip of the stylus of 2 μm, a measurement length of 1.0 mm, a measurement speed of 0.3 mm / s, an inclination correction of least square curve correction. Among the obtained roughness parameters, the maximum height roughness (“R _Z ”) described in JIS B 0601 (2001) was the surface roughness of the side surface after cold forging. In addition, the surface roughness of the side surface in the case of cold forging was set so that R _Z was 10 μm or less.

Investigation 5: Hardness measurement after cold drawing After cutting a cold drawn round bar with a diameter of 24 mm perpendicular to the axial direction and embedding it in resin so that the cut surface becomes the test surface, mirror polishing In addition, for a total of 5 points including 1 point at the center of the cross section and 4 points at the R / 2 part, the test force is 9.8 N in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2009). The Vickers hardness was measured, and the arithmetic average value of 5 points was defined as HV after cold drawing. In addition, the hardness after cold drawing aimed at 200 or more by HV.

Investigation 6: Measurement of cutting resistance after cold drawing and investigation of occurrence of “peeling” A cylindrical test piece having a length of 400 mm was cut out from a round bar having a diameter of 24 mm and subjected to turning to obtain a cutting resistance. Was measured and the occurrence of "Mushi" was investigated. Cutting conditions were wet machining (feed amount: 20 L / min) with water-soluble emulsion diluted 20 times using a P20 carbide tool, cutting speed was 150 m / min, feed was 0.20 mm / rev, The cut amount was 0.8 mm. In addition, the value of the main component force which is a circumferential direction reaction force among the reaction forces applied to the carbide tool during the turning process was measured with a cutting dynamometer, and this was defined as a cutting resistance. The cutting resistance after cold drawing was targeted at 600 N or less. In addition, the goal was to prevent the occurrence of “smile”.

Investigation 7: Measurement of core hardness after gas soft nitriding After sampling from a cold-drawn round bar with a diameter of 24 mm, a block-shaped test piece having the shape shown in FIG. 2 was subjected to gas soft nitriding with the heat pattern shown in FIG. Was crossed at the center, embedded in resin and mirror polished. Vickers hardness was measured at an arbitrary 5 points 3 mm from the nitrocarburized surface at a test force of 2.94 N according to “Vickers hardness test-test method” described in JIS Z 2244 (2009). The arithmetic average value of 5 points was defined as the HV of the core after soft nitriding. The core hardness after soft nitriding was set to 250 or more in HV.

  Table 2 summarizes the above test results. Table 2 also shows the rolling start temperature when a round bar having a diameter of 55 mm was held at 1250 ° C. for 30 minutes and finished into a round bar having a diameter of 38 mm using a hole-type rolling mill.

  FIG. 4 shows the relationship between Fn1 expressed by the equation (1) and the hardness (HV) after rolling in Investigation 1, and FIG. 5 shows cutting after cold drawing in Fn1 and Investigation 5. The relationship with resistance (main component) is organized and shown. Further, FIG. 6 shows Fn2 expressed by the equation (2) and an increment (ΔHV) of HV before and after the soft nitriding treatment in the investigations 5 and 6, that is, “HV after soft nitriding treatment−HV after cold drawing”. ".

  From Table 2, in the case of “examples of the present invention” of test symbols C to P satisfying the conditions defined in the present invention, characteristics after rolling (hardness (HV), deformation resistance), side surface when cold forged It is clear that surface roughness, properties after cold drawing (hardness (HV), cutting resistance and “peel”), and core hardness (HV) after soft nitriding have all achieved the targets It is.

  On the other hand, test symbols A, B, and Q to Z of “Comparative Examples” that deviate from the conditions specified in the present invention are intended properties after rolling (hardness (HV), deformation resistance), cold Any of the surface roughness of the side surface when forged, characteristics after cold drawing (hardness (HV), cutting resistance and “peel”), and core hardness (HV) after soft nitriding can be achieved. Not.

  That is, test symbols A and B were rolled using steel 5 having a chemical composition within the range defined by the present invention, but the average ferrite grain sizes after rolling were 97.3 μm and 81.5 μm, respectively. The surface roughness of the side surface in the case of cold forging is large because it is out of the conditions defined in the present invention.

  The test symbol Q is 0.02% of the steel 12 used and Fn1 is 4.8, both of which are lower than the range defined in the present invention, so the hardness after rolling is as low as 98 in HV. . For this reason, the hardness after cold drawing is as low as 195 in HV, causing “peeling” during turning, and the cutting resistance is as high as 744 N. Moreover, since Fn2 of the steel 12 is 11.0, which is larger than the range specified in the present invention, the core hardness after soft nitriding is as low as 213 in HV.

  Test symbol R has a Cu content of 0.50%, Ni content of 0.30%, and Fn1 of 11.1 in steel 13 used, all of which are higher than the range defined in the present invention. For this reason, the hardness after rolling is as high as 183 in HV, and the deformation resistance is as high as 560 MPa. Further, the cutting resistance is as high as 618N.

  In the test symbol S, the N content of the steel 14 used is 0.0120%, which is higher than the range specified in the present invention, and Fn2 is 1.3, which is lower than the range specified in the present invention. For this reason, the core hardness after soft nitriding is as low as 211 in HV.

  In the test symbol T, the steel 15 used has a V content of 0.05% and Fn2 of 0.4, both of which are lower than the range defined in the present invention. As a result, the core hardness after soft nitriding is as low as 189 in HV.

  In the test symbol U, the steel 16 used has a C content of 0.17%, a V content of 0.60%, and Fn1 of 12.7, both of which are higher than the range defined in the present invention. For this reason, the hardness after rolling is as high as 175 in HV, and the deformation resistance is as high as 559 MPa. Further, the cutting resistance is as high as 607N.

  In the test symbol V, the steel 17 used has a C content of 0.17%, an Mn content of 1.00%, and Fn1 of 14.2, all of which are higher than the ranges specified in the present invention. For this reason, the hardness after rolling is as high as 178 in HV, and the deformation resistance is as high as 591 MPa. Further, the cutting resistance is as high as 613N.

  In the test symbol W, the Fn1 of the steel 18 used is 12.4, which is higher than the range defined in the present invention. For this reason, the hardness after rolling is as high as 178 in HV, and the deformation resistance is as high as 577 MPa. Further, the cutting resistance is as high as 620N.

  In the test symbol X, the Fn2 of the steel 19 used is 1.3, which is lower than the range defined in the present invention. For this reason, the precipitation hardening amount at the time of soft nitriding becomes small, and the core hardness after soft nitriding is as low as 230 in HV.

  In the test symbol Y, the Fn2 of the steel 20 used is 10.4, which is higher than the range specified in the present invention. For this reason, the precipitation hardening amount at the time of soft nitriding becomes small, and the core hardness after soft nitriding is as low as 230 in HV.

  In test symbol Z, Fn1 of steel 21 used is 4.9, which is lower than the range defined in the present invention, and thus the hardness after rolling is as low as 98 in HV. For this reason, the hardness after cold drawing is as low as 196 in HV, and “peeling” occurs during turning, and the cutting resistance is as high as 660 N.

  The rolled steel material for cold forging and nitriding according to the present invention can be cold forged in a rolled state without preheating the steel material, has a small surface roughness after cold forging, and is further subjected to cutting after cold forging. In this case, the cutting resistance is low, the machinability is excellent, and the parts subjected to the cold forging and nitriding treatments can have a high core hardness. For this reason, it is suitable for using as a raw material of cold forging nitriding components.

Claims (4)

In mass%, C: 0.06-0.15%, Si: 0.02-0.35%, Mn: 0.10-0.90%, S: 0.030% or less, Cr: 0.50 -2.0%, V: 0.10-0.50% and Al: 0.010-0.090%, the balance consists of Fe and impurities, P, N and O in the impurities are respectively P: 0.030% or less, N: 0.0080% or less, and O: 0.0030% or less, and Fn1 represented by the following formula (1) is 5.0 to 10.0, (2) Fn2 represented by the formula has a chemical composition of 1.8 to 9.0, the microstructure is a ferrite / pearlite structure, and the average particle diameter of the ferrite is 50 μm or less. Rolled steel for cold forging and nitriding.
Fn1 = 35 (C + N) +5 (Si + Mn) + Cr + 3 (Cu + Ni) +4 (Mo + V) (1)
Fn2 = {V + (9/20) Mo} / C (2)
However, C, Si, Mn, Cu, Ni, Cr, Mo, V and N in the above formulas (1) and (2) mean the content of the element in mass%.   The rolled steel material for cold forging and nitriding according to claim 1, characterized by containing Mo: 0.50% or less in mass% instead of part of Fe.   Instead of a part of Fe, it is characterized by containing at least one of Ti: 0.50% or less, Nb: 0.50% or less, and Zr: 0.50% or less in mass%. The rolled steel material for cold forging and nitriding according to claim 1 or 2.   Any one of claims 1 to 3, characterized by containing at least one of Cu: 0.30% or less and Ni: 0.20% or less in mass% instead of a part of Fe Rolled steel for cold forging and nitriding according to any one of the above.

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