JP2014129607A - Non-refining type nitrided crankshaft - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-refining type nitrided crankshaft having high flexure fatigue strength with 700 MPa or more and efficient flexure correction property, and excellent in peeling resistance of a compound layer.SOLUTION: There is provided a non-refining type nitrided crankshaft containing C:0.25 to 0.60%, Si:0.10 to 1.0%, Mn:0.60 to 2.0%, P≤0.08%, S≤0.10%, Cr:0.20 to 1.0% and N:0.0030 to 0.0250%, one or more kind of Cu, Ni, Mo, V, Ti and Ca and the balance Fe with impurities and satisfying [40-C+2Mn+5.5Cr≥43.0], and having HV hardness of 0.05 mm position of depth from a surface of 380 to 600, and depth of the compound layer of at least a pin fillet part, a journal fillet part and a pin part of 5 μm or less.

Description

  The present invention relates to a crankshaft subjected to non-tempering nitriding treatment. More specifically, a crankshaft used for automobiles, industrial machines, construction machines, etc., which is manufactured by nitriding without forging and tempering after forging and machining into a required shape. About.

  Hereinafter, the above-mentioned “nitriding crankshaft” is referred to as “nitriding crankshaft”. A nitrided crankshaft that is not subjected to a quenching-tempering tempering treatment and whose dough structure is a mixed structure of ferrite and pearlite is referred to as a “non-tempered nitrided crankshaft”.

  A crankshaft requiring high wear resistance and high fatigue strength (in particular, high bending fatigue strength) is subjected to surface hardening treatment such as induction hardening and nitriding after forging and machining.

The nitriding treatment is a diffusion permeation treatment of nitrogen or nitrogen and carbon at a temperature of A ₁ point or less, and the heat treatment temperature is low. For this reason, in the nitriding treatment, the heat treatment strain is reduced as compared with the induction hardening treatment or the like, and in recent years, the number of crankshafts to which the nitriding treatment is applied has increased. On the surface of the crankshaft that has been subjected to nitriding treatment, a compound layer (mainly a layer in which a nitride such as Fe ₃ N is formed) having a depth of several μm to 40 μm, which is observed white due to nitral corrosion, further includes a compound layer and A diffusion layer having a depth of several hundreds of μm is formed between the dough and is cured by permeated nitrogen or nitrogen and carbon.

  As described above, the heat treatment strain in the nitriding treatment is smaller than that in the induction hardening treatment, but it is not completely absent. Therefore, in a crankshaft that is a rotating shaft component, a decrease in dimensional accuracy due to slight heat treatment strain may be a problem. Therefore, it is necessary to increase the dimensional accuracy by performing bending correction after the nitriding treatment.

  However, when bending correction is performed on a nitrided crankshaft, cracks may occur from the surface layer. Such cracks reduce the bending fatigue strength because stress concentrates. Therefore, the nitrided crankshaft is required to have no crack even when it is subjected to bending correction, that is, to be excellent in bending correction.

  Furthermore, in recent years, consideration for the environment has been demanded, and the nitrided crankshaft is not an exception, and light weight and downsizing are desired. For example, a high bending fatigue strength of 700 MPa or more is required. In order to obtain such a high bending fatigue strength, the hardness of the outermost layer portion of the diffusion layer (hereinafter also referred to as “surface hardness”) is Vickers hardness (hereinafter referred to as “HV hardness”). And at least 380 or more. The “outermost layer portion of the diffusion layer” specifically means a position having a depth of 0.05 mm from the surface after nitriding.

  However, if the surface hardness is 380 or higher in HV hardness, cracks are likely to occur when bending correction is performed on the nitride crankshaft.

In addition, in recent years, it is also required to simplify the heat treatment during production in order to reduce costs, save resources, and reduce exhaust CO ₂ . Therefore, non-tempered steel having a mixed structure of ferrite and pearlite (hereinafter referred to as “ferrite / pearlite structure”) that is not subjected to tempering treatment of quenching and tempering is used as the material for the nitride crankshaft. ing.

  However, in general, the ferrite-pearlite structure is coarser than the quenched-tempered structure. For this reason, non-tempered steel is inferior in bending fatigue strength and bending straightness compared with tempered steel.

  Under such circumstances, there has been a great demand for a non-tempered nitrided crankshaft having a high bending fatigue strength of 700 MPa or more and sufficient bend straightening properties. For example, the following techniques are disclosed.

That is, in Patent Document 1,
In mass%, C: 0.2-0.6%, Si: 0.05-1.0%, Mn: 0.25-1.0%, S: 0.03-0.2%, Cr: 0.2% or less, s-Al: 0.045% or less, Ti: 0.002 to 0.010%, N: 0.005 to 0.025%, and O: 0.001 to 0.005% If necessary, Pb: 0.01 to 0.40%, Ca: 0.0005 to 0.0050%, and Bi: 0.005 to 0.40% And satisfying the conditions of 0.12 × Ti% <O% <2.5 × Ti% and 0.04 × N% <O% <0.7 × N%, with the balance being Fe and inevitable impurities Become
The structure after hot forging is a mixed structure of ferrite and pearlite,
“Non-tempered steel for soft nitriding” is disclosed.

In Patent Document 2,
A crankshaft having a pin portion and a journal portion, made of steel having a surface subjected to nitriding treatment or soft nitriding treatment,
The steel is an alloy component, C: 0.07 mass% or more and 0.12 mass% or less, Si: 0.05 mass% or more and 0.25 mass% or less, Mn: 0.1 mass% or more and 0.5 mass% or less. Hereinafter, Cu: 0.8% by mass to 1.5% by mass, Ni: 2.4% by mass to 4.5% by mass, Al: 0.8% by mass to 1.5% by mass, Ti: 0 0.5% by mass or more and 1.5% by mass or less, and if necessary, S: 0.01% by mass to 0.10% by mass, Ca: 0.0010% by mass to 0.0050% by mass 1 type or 2 types, and the balance consists of Fe and inevitable impurities,
In addition, after a steel sample taken from the center not affected by the nitriding treatment is melted at 1200 ° C. for 1 hour, the temperature range from 900 ° C. to 300 ° C. is 0.3 ° C./second or more. By cooling at an appropriate cooling rate set to 5 ° C./second or less, the ratio of bainite in the steel structure can be 80% or more, and the HV hardness can be 200 or more and 300 or less,
The pin portion and the journal portion subjected to the nitriding treatment or soft nitriding treatment have an HV hardness of 350 to 500 in HV hardness, and an HV hardness of 650 to 950 at a position 0.05 mm from the surface. That
A "crankshaft" featuring the above is disclosed.

JP 2002-226939 A JP 2007-177309 A

  By the technique disclosed in Patent Document 1, a non-tempered nitrided crankshaft can be obtained. However, since the obtained nitride crankshaft has a low surface hardness, the bending fatigue strength is low and does not reach 700 MPa.

  With the technique disclosed in Patent Document 2, a nitrided crankshaft having high fatigue strength can be obtained. However, the nitrided crankshaft obtained has a surface hardness that is too high. Therefore, if bending correction is performed in order to eliminate the heat treatment strain generated during nitriding, it is difficult to avoid the occurrence of cracks from the surface layer.

  Further, the compound layer of the nitrided part is a hard layer having an HV hardness of 500 or more, and conventionally, it has been said that there is an effect of improving the wear resistance of the sliding surface of the nitrided part. However, the compound layer has excellent wear resistance when the surface pressure of the sliding surface is small, and the compound layer peels off in an environment where the surface pressure is high such that a bending fatigue strength of 700 MPa or more is required. Thus, the powdery compound layer acts more like an abrasive on the sliding surface of the nitrided part, and it has become a problem that the wear resistance of the nitrided part is significantly reduced.

  In Patent Document 1 and Patent Document 2, no consideration is given to peeling of a compound layer that occurs in a nitrided part used under such a high surface pressure.

  The present invention has been made in view of the above-described situation, and provides a non-tempered nitrided crankshaft having a high bending fatigue strength of 700 MPa or more, sufficient bending straightening properties, and excellent peeling resistance of a compound layer. The purpose is to provide.

  As a result of conducting various studies to solve the above-described problems, the present inventors have obtained the following findings (A) to (K).

  (A) When a thin plate test piece was collected from the surface layer of a nitrided steel material and subjected to a tensile test, the test piece from which the compound layer was removed showed a significant increase in tensile test compared to the test piece from which the compound layer was not removed. To improve.

  (B) As a result of observing the fracture surface of the thin plate test piece after the tensile test, in the test piece from which the compound layer was not removed, the compound layer was brittlely fractured and became the starting point of cracking, whereas the compound layer In the test piece from which is removed, it is a ductile fracture surface.

  (C) By removing the compound layer on the surface of the nitrided steel material, the fracture mode during bending correction changes from brittle fracture starting from the compound layer to ductile fracture. Can be improved.

  (D) By removing the compound layer on the surface of the nitrided part, practically sufficient bend straightening can be obtained even if the surface hardness after nitriding is 380 or higher in terms of HV hardness.

  (E) However, if the surface hardness of the nitrided crankshaft is excessively high at 600 or more in HV hardness, even if the compound layer is removed, practically sufficient bending straightening can be obtained. Can not.

  (F) The bending fatigue strength hardly changes before and after the removal of the compound layer, and a high bending fatigue strength of 700 MPa or more can be obtained if the surface layer hardness is 380 or more in terms of HV hardness.

  (G) However, even if an attempt is made to remove the compound layer, it is difficult to completely remove only the compound layer without removing the diffusion layer imparted with a surface hardness of 380 or higher in HV hardness.

  (H) And, if the compound layer remains without being removed, the compound layer peels and becomes powdery as described above in an environment with high surface pressure that requires a bending fatigue strength of 700 MPa or more. It acts like an abrasive and significantly reduces the wear resistance of the nitrided crankshaft.

  (I) Although the compound layer breaks brittlely as in the above (B), surprisingly, the higher the hardness of the compound layer, the better the peel resistance of the compound layer.

  (J) And if the contents of C, Cr, Mn, and Mo in the steel material are appropriately controlled, the hardness of the compound layer increases.

  (K) Nitriding excellent in peeling resistance of the compound layer by appropriately controlling the contents of C, Cr, Mn and Mo in the steel material and controlling the depth of the compound layer to 5 μm or less. A crankshaft can be obtained.

The present invention has been completed based on the above findings, and the gist thereof resides in the non-heat treated nitrided crankshaft shown in the following (1 ) .

(1) Steel material of dough is mass%, C: 0.25 to 0.60%, Si: 0.10 to 0.80 %, Mn: 0.60 to 2.0%, P: 0.08 %: S: 0.10% or less, Al: 0.05% or less, Cr: 0.20 to 1.0% and N: 0.0030 to 0.0250%, and the following <1> To <3>, the balance is a non-tempered nitrided crankshaft comprising at least one element selected from the elements listed in the following, with the balance being Fe and impurities, and satisfying the following formula (1 '): Non-refining type characterized in that the HV hardness at a position of 0.05 mm from the surface is 380 to 600, and the compound layer depth of at least the pin fillet portion, journal fillet portion and pin portion is 5 μm or less Nitride crankshaft.
<1> Cu: 1.0% or less, Ni: 0.5% or less, Mo: 0.5% or less, and V: 0.3% or less <2> Ti: 0.050% or less <3> Ca: 0 .010% or less 40−C + 2Mn + 5.5Cr + 26Mo ≧ 44.4 (1 ′)
The element symbol in the above formula (1 ′) means the content of the element in mass%.

  The “non-tempered nitrided crankshaft” refers to a nitrided crankshaft that is not subjected to a tempering treatment of quenching and tempering and whose fabric structure is a mixed structure of ferrite and pearlite.

  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.

  It is sufficient that the compound layer depth is 5 μm or less at the pin fillet portion, journal fillet portion and pin portion of the crankshaft. Moreover, as long as HV hardness of the depth 0.05mm position from the surface has ensured 380-600, it does not need to have a compound layer.

  The non-tempered nitrided crankshaft of the present invention does not require a tempering treatment by quenching and tempering, has a bending fatigue strength of 700 MPa or more and sufficient bending straightening properties, and is excellent in the peel resistance of the compound layer. For this reason, the non-refined nitrided crankshaft of the present invention can be used as a crankshaft for automobiles, industrial machines, construction machines, etc., and can meet the demand for lighter and smaller crankshafts.

It is a figure which shows the relationship between the HV hardness in the center position of the depth direction of a compound layer, and " 40-C + 2Mn + 5.5Cr + 26Mo". In the figure , “ 40−C + 2Mn + 5.5Cr + 26Mo” is represented as “ fn1”. It is a schematic diagram which shows the principal part of a crankshaft. It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece with a groove | channel used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the 4-point bending test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the shape of the scratch test piece used in the Example. The unit of the dimension in the figure is “mm”. The surface of the scratch specimen in FIG. 5 was scratched once in a circle having a diameter of 20 mm with a Vickers indenter with a load of 70 N, and the formed groove was observed with a scanning electron microscope to investigate the peeling state of the compound layer. It is a figure which shows typically an example when peeling arises in a compound layer among results. The surface of the scratch specimen in FIG. 5 was scratched once in a circle having a diameter of 20 mm with a Vickers indenter with a load of 70 N, and the formed groove was observed with a scanning electron microscope to investigate the peeling state of the compound layer. It is a figure which shows typically an example when peeling does not arise in a compound layer among results.

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

<Chemical composition of steel material of dough>
C: 0.25 to 0.60%
C has the effect of increasing the bending fatigue strength by increasing the surface layer hardness. In order to acquire this effect, it is necessary to contain 0.25% or more of C. However, if the C content exceeds 0.60%, the surface layer hardness becomes too large and the bend straightening property is lowered, and the hardness of the compound layer on the surface becomes low, and the compound layer is resistant to peeling. The nature is also reduced. Therefore, the content of C is set to 0.25 to 0.60%. The C content is preferably 0.27% or more and 0.50% or less.

Si: 0.10 to 1.0%
Si is an element necessary for deoxidation at the time of melting, and has an effect of increasing bending fatigue strength. In order to obtain such an effect, the content must be at least 0.10%. However, if the Si content exceeds 1.0%, the surface hardness becomes too high, and the bending straightness decreases. Therefore, the content of Si is set to 0.10 to 1.0%. The Si content is preferably 0.15% or more and 0.80% or less.

Mn: 0.60 to 2.0%
Mn has the effect of increasing the bending fatigue strength by increasing the surface hardness when nitriding. Mn increases the hardness of the compound layer on the surface and also has an effect of increasing the peel resistance of the compound layer. In order to obtain such an effect, it is necessary to contain at least 0.60% of Mn. On the other hand, if the Mn content exceeds 2.0%, the surface hardness becomes too high, and the bending straightness is lowered. Therefore, the content of Mn is set to 0.60 to 2.0%. The Mn content is desirably 0.65% or more and 1.8% or less.

P: 0.08% or less P is an element contained as an impurity, and lowers the bending fatigue strength. In particular, when the content exceeds 0.08%, the bending fatigue strength is significantly reduced. Therefore, the content of P is set to 0.08% or less. Note that the P content is preferably 0.04% or less.

S: 0.10% or less S is an element contained as an impurity. Moreover, S has the effect | action which improves machinability, when it contains positively. When the content is too large and exceeds 0.10%, the bending fatigue strength and the bending straightness are lowered. Therefore, the content of S is set to 0.10% or less. The S content is preferably 0.08% or less. When it is desired to obtain a machinability improving effect, it is desirable to positively contain 0.02% or more of S.

Al: 0.05% or less Al is an element contained as an impurity. Al is an element that may be added for deoxidation. If the content is too large and exceeds 0.05%, the bend straightening property decreases. Therefore, the Al content is set to 0.05% or less. The Al content is preferably 0.04% or less. In order to obtain a deoxidizing effect, it is desirable to positively contain 0.02% or more of Al.

Cr: 0.20% to 1.0%
Cr has the effect of increasing the bending fatigue strength by increasing the surface layer hardness. Cr also has the effect of increasing the hardness of the compound layer on the surface and increasing the peel resistance of the compound layer. In order to acquire such an effect, it is necessary to contain 0.20% or more of Cr. However, if the Cr content exceeds 1.0%, the surface layer hardness becomes unnecessarily high, and the bending straightness decreases. For this reason, the Cr content is set to 0.20 to 1.0%. The Cr content is preferably 0.25% or more and 0.9% or less.

N: 0.0030% to 0.0250%
N is an element that improves the bending fatigue strength and the bending straightness. In order to obtain such an effect, it is necessary to contain 0.0030% or more of N. However, even if N is contained in excess of 0.0250%, the effect is saturated and the manufacturing cost is unnecessarily high. Therefore, the content of N is set to 0.0030 to 0.0250%. The N content is preferably 0.0040% or more and 0.0220% or less.

Microalloyed type nitride crankshaft bets invention, one in which the fabric of the steel, along with containing an element from the above C to N, is selected from the elements listed in up to <3> from the <1> It contains the above, the remainder consists of Fe and impurities, and satisfies the above formula (1 ′).

  Hereafter, the effect of the element which belongs to each of <1>-<3> and the reason for limitation of content are demonstrated.

<1> Cu: 1.0% or less, Ni: 0.5% or less, Mo: 0.5% or less and V: 0.3% or less Cu, Ni, Mo and V which are elements belonging to <1> are , Both have the effect of increasing the bending fatigue strength. Therefore, when it is desired to obtain the above effects, these elements may be contained within the range described below.

Cu: 1.0% or less Cu has an effect of improving bending fatigue strength. For this reason, you may contain Cu as needed. However, when the Cu content exceeds 1.0%, the hot workability is lowered. Therefore, the amount of Cu in the case of inclusion is set to 1.0% or less. When Cu is contained, the amount of Cu is preferably 0.4% or less, and more preferably 0.3% or less. On the other hand, in order to stably obtain the effects of Cu described above, the amount of Cu in the case of inclusion is preferably 0.05% or more.

Ni: 0.5% or less Ni has the effect of improving the bending fatigue strength. For this reason, you may contain Ni as needed. However, even if Ni of 0.5% or more is contained, the above effect is saturated, and thus economical efficiency is impaired. Therefore, the amount of Ni in the case of inclusion is set to 0.5% or less. When Ni is contained, the amount of Ni is preferably 0.3% or less, and more preferably 0.2% 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 0.05% or more.

Mo: 0.5% or less Mo has an effect of improving bending fatigue strength. Mo also has the effect | action which raises the hardness of the compound layer of a surface and improves the peeling resistance of a compound layer. For this reason, you may contain Mo as needed. However, even if 0.5% or more of Mo is contained, the above effect is saturated, and thus the economic efficiency is impaired. Therefore, the amount of Mo in the case of inclusion is set to 0.5% or less. When Mo is included, the amount of Mo is desirably 0.4% 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 desirably 0.05% or more, and more desirably 0.10% or more.

V: 0.3% or less V has an effect of improving bending fatigue strength. For this reason, you may contain V as needed. However, when the content of V exceeds 0.3%, the bending straightness deteriorates. Therefore, when V is included, the amount of V is set to 0.3% or less. When V is contained, the amount of V is preferably 0.25% or less. On the other hand, in order to stably obtain the effect of V described above, the amount of V when contained is preferably 0.05% or more, and more preferably 0.1% or more.

  Said Cu, Ni, Mo, and V can be contained only in any 1 type in them, or 2 or more types of composites. When it is included, the total content of the above elements may be 2.3% when Cu is 1.0%, Ni and Mo are 0.5% and V is 0.3%, respectively. , 0.8% or less is desirable.

<2> Ti: 0.050% or less Ti has the action of forming nitrides, refining crystal grains, and improving the bending straightness. For this reason, you may contain Ti as needed. However, if the Ti content exceeds 0.050%, the nitride becomes coarse and the bending fatigue strength decreases. Therefore, when Ti is included, the amount of Ti is set to 0.050% or less. When Ti is contained, the amount of Ti is preferably 0.035% or less. On the other hand, in order to stably obtain the effect of Ti described above, the amount of Ti in the case of inclusion is preferably 0.005% or more.

<3> Ca: 0.010% or less Ca has an effect of improving machinability. For this reason, you may contain Ca as needed. However, if the Ca content exceeds 0.010%, the formation of coarse inclusions cannot be avoided, so the bending fatigue strength decreases. Therefore, the Ca content in the case of inclusion 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, and more preferably 0.0005% or more.

[ 40-C + 2Mn + 5.5Cr + 26Mo]: 43.0 or more In the non-tempered nitrided crankshaft of the present invention , [ 40-C + 2Mn + 5.5Cr + 26Mo] is 43.0 or more,
40−C + 2Mn + 5.5Cr + 26Mo ≧ 43.0 (1 ′)
In represented by (1 ') is fully plus requires expression. C, Mn, Cr and Mo in the above formulas mean the content of the element in mass%.

  When nitriding is performed, the higher the hardness of the compound layer, the higher the peel resistance of the compound layer.

When nitriding a base steel material in which each element from C to Ca is in the above-described range, if the left side of the formula ( 1 ′) is expressed as “ fn1”, as shown in FIG. Hardness has a positive correlation with fn1. In addition, FIG. 1 is a figure which shows the relationship between HV hardness and fn1 in the center position of the depth direction of a compound layer.

When [ 40-C + 2Mn + 5.5Cr + 26Mo] is less than 43.0, the peel resistance of the compound layer is lowered even when the compound layer depth described below is satisfied. Therefore , [ 40-C + 2Mn + 5.5Cr + 26Mo] needs to satisfy 43.0 or more . [ 40-C + 2Mn + 5.5Cr + 26Mo] is preferably 43.2 or more. On the other hand , the upper limit of [ 40-C + 2Mn + 5.5Cr + 26Mo] is a value obtained from the lower limit value of C and the upper limit values of Mn, Cr and Mo.

<Surface hardness>
The non-tempered nitrided crankshaft in which the steel material is made of the aforementioned elements must have a surface hardness (that is, a hardness at a depth of 0.05 mm from the surface) of HV hardness of 380 to 600.
If the surface hardness is 380 or more in terms of HV hardness, a high bending fatigue strength of 700 MPa or more can be obtained. However, when the surface layer hardness exceeds 600 in terms of HV hardness, practically sufficient bending straightness cannot be obtained. The surface hardness is preferably 400 or more in terms of HV hardness, and more preferably 550 or less.

<Depth of compound layer>
The non-tempered nitrided crankshaft in which the steel material is made of the above-described elements should further have a compound layer depth of at least 5 μm or less at the stress concentration portion and the sliding surface.

  The stress concentration portion is a pin fillet portion and a journal fillet portion of the crankshaft, and by making the compound layer depth of the stress concentration portion 5 μm or less, the bending straightness is improved without reducing the bending fatigue strength. be able to. The pin fillet portion and the journal fillet portion refer to portions 1 and 2, respectively, in the schematic diagram of the main part of the crankshaft illustrated in FIG.

  A sliding surface is a pin portion of a crankshaft that slides with a connecting rod via a sliding bearing and lubricating oil. If the compound layer depth is 5 μm or less, the sliding surface also has a high bending of 700 MPa or more. Even in a situation where fatigue load is applied, the compound layer does not peel off. For this reason, the wear resistance does not deteriorate even under the severe environment as described above. A pin part means the part 3 of the schematic diagram of the crankshaft illustrated in FIG.

  The compound layer depth of the pin fillet portion, journal fillet portion and pin portion of the crankshaft is preferably 4 μm or less, and it is desirable that the compound layer depth is as small as possible. Moreover, as long as HV hardness of the depth 0.05mm position from the surface has ensured 380-600, it does not need to have a compound layer.

  The desired surface hardness and compound layer depth can be obtained, for example, by the following treatments (i) to (iv).

  (I) A steel material having a ferrite / pearlite structure is produced by hot forging a steel satisfying the chemical composition defined in the present invention, or after hot forging and further performing a normalizing treatment.

  (Ii) Next, the steel material having the ferrite-pearlite structure is machined into a crankshaft having a predetermined shape.

  (Iii) The crankshaft obtained by the above machining was subjected to soft nitriding by mixing RX gas and ammonia gas in a 1: 1 ratio and maintaining the temperature in an atmosphere of 600 ° C. for 2 hours, and then at 90 ° C. Cool in oil.

  (Iv) Thereafter, the pin fillet portion, journal fillet portion and pin portion are polished by machining such as lapping.

  The above “RX gas” is a kind of metamorphic gas and is a trade name of gas.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels A to M having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace, and then hot forged and processed into a steel bar having a diameter of 90 mm. After hot forging, it was allowed to cool in the atmosphere and cooled to room temperature.

Steels B to F and H in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels I to M are steels whose chemical compositions deviate from the conditions defined in the present invention. Steels A and G are reference steels.

In Table 1, “40−C + 2Mn + 5.5Cr + 26Mo” which is the left side of the formula ( 1 ′) is also shown as “fn1”. Hereinafter , the left side of the expression ( 1 ′) is referred to as “ fn1”.

  The steel bar having a diameter of 90 mm of each steel thus obtained was heated to 1200 ° C. and hot forged at a finishing temperature of 1050 to 1000 ° C. to produce a steel bar having a diameter of 50 mm. After hot forging, it was allowed to cool in the atmosphere and cooled to room temperature.

  Regarding Steel G and Steel H, the above-mentioned steel bar having a diameter of 50 mm was further heated to 900 ° C. and held for 1 hour, and then allowed to cool in the atmosphere to cool to room temperature.

  Hot-forged 50 mm diameter steel bars of steels A to F and steels I to M, and R / 2 part of steel steels G and H having a diameter of 50 mm heat treated at 900 ° C. (“R” represents the radius of the steel bar) 3), a grooved Ono type rotary bending fatigue test piece having the shape shown in FIG. 3, a four-point bending test piece having the shape shown in FIG. 4, and a scratch test piece having the shape shown in FIG. 5 were cut out in parallel with the forging axis. In the test piece of FIG. 3, the 3R groove bottom is the stress concentration part. Similarly, in the test piece of FIG. 4, the 3R notch bottom is a stress concentration portion. In addition, the unit of the dimension in each above-mentioned test piece shown in FIGS. 3-5 is "mm".

  The three types of test pieces obtained as described above were subjected to soft nitriding by mixing RX gas and ammonia gas in a ratio of 1: 1 and holding in an atmosphere at a temperature of 600 ° C. for 2 hours, and then an oil at 90 ° C. Cooled in. As described above, the above “RX gas” is a kind of metamorphic gas and is a trade name of gas.

  After the above soft nitriding treatment and oil cooling (hereinafter simply referred to as “after soft nitriding”), the hardness of the compound layer was examined using a scratch test piece.

  Specifically, after embedding in a resin so that the longitudinal section of the scratch specimen becomes the test surface and mirror polishing, using a dynamic ultra-micro hardness meter manufactured by SHIMADZU, the center of the compound layer in the depth direction The dynamic hardness of the position was measured. Measurement was carried out at 15 points, and the arithmetic average value thereof was converted to Vickers hardness using a calibration curve prepared with a standard hardness sample, and this was regarded as the hardness of the compound layer.

  Table 2 shows the hardness of the compound layer determined as described above.

  Electrolytic polishing was used to make the compound layer 5 μm or less of the groove bottom of the grooved Ono-type rotating bending fatigue test piece, the notch bottom of the 4-point bending test piece, and the scratch surface of the scratch test piece. Since the electrolytic polishing method differs in polishing time depending on the surface area of the polishing surface, it is necessary to carry out the polishing time suitable for the polishing area of each test piece in order to obtain a desired polishing amount. Therefore, electropolishing was performed while changing the polishing time for each test piece under the following conditions, and an electropolishing time sufficient for the compound layer depth to be 5 μm or less was obtained in advance.

Electrolyte: perchloric acid (HClO ₄ ): acetic acid (CH ₃ COOH) = 1: 9
・ Current value: 0.14A,
-Polishing area: Ono type rotating bending fatigue test piece: 160 mm ²
For 4-point bend specimen: 96 mm ²
For scratch specimens: 552 mm ² .

  As a result, in the case of Ono type rotating bending fatigue test piece, 970 seconds, in the case of 4-point bending test piece, 590 seconds, in the case of scratch test piece, if electrolytic polishing is performed with a polishing time of 3350 seconds, This steel was found to be sufficient for the compound layer depth to be 5 μm or less.

Therefore, the electrolytic polishing of the grooved Ono type rotary bending fatigue test piece of each steel after soft nitriding was performed for the above polishing time according to the above conditions for the notch bottom of the 4-point bending test piece and the scratch surface of the scratch test piece. Carried out
・ Investigation of bending fatigue strength by Ono type rotating bending fatigue test,
・ Investigation of bending straightness by 4-point bending test,
・ Investigation of peel resistance of compound layer by scratch test,
Carried out.

  Further, using a grooved Ono-type rotating bending fatigue test piece, a 4-point bending test piece, and a scratch test piece, the surface layer hardness (that is, the hardness at a depth of 0.05 mm from the surface of the test piece) and the compound layer depth Investigate.

  Regarding the steel A and the steel B, each of the above-mentioned investigations was carried out for those not subjected to electrolytic polishing after soft nitriding.

  The contents of each survey will be described below.

<1> Investigation of bending fatigue strength:
A grooved Ono-type rotating bending fatigue test was conducted under the conditions of swinging at 3000 rpm at room temperature and in the air, and the bending fatigue strength (hereinafter referred to as “σw”) was investigated. The target of σw was 700 MPa or more.

<2> Investigation of bending straightness:
A 2 mm strain gauge was bonded to the notch bottom of the 4-point bending test piece, and bending correction strain was applied until the gauge was disconnected. The reading of the gauge at the time when the gauge was disconnected was evaluated as the bending straightness. The target of the bending straightness was that the gauge reading was 10000 μ (corresponding to a bending straightening strain of 1.0%) or more.

<3> Investigation of peel resistance of compound layer The surface of the scratch specimen was scratched once in a circle with a diameter of 20 mm with a Vickers indenter with a load of 70 N, and the obtained groove was examined in detail with a scanning electron microscope. Observed.

  FIG. 6 schematically shows an example of the case where peeling occurs in the compound layer, and FIG. 7 schematically shows an example where no peeling occurs in the compound layer.

  As shown in FIG. 6, the peeling of the compound layer occurred from the edge of the scratch groove, and the peeling resistance of the compound layer was evaluated based on the presence or absence of the peeling of the compound layer having a maximum width exceeding 50 μm. The target of the peel resistance of the compound layer was that no peeling of the compound layer with a maximum width exceeding 50 μm occurred.

<4> Surface hardness:
After embedding in the resin so that the groove bottom longitudinal section of the grooved Ono-type rotating bending fatigue test specimen, the notch bottom longitudinal section of the four-point bending specimen and the longitudinal section of the scratch specimen are the test surface, the above surface Was polished to a mirror finish, and the surface hardness was examined using a Vickers hardness tester.

  Specifically, in accordance with “Vickers hardness test-test method” described in JIS Z 2244, from the groove bottom surface of the grooved Ono type rotary bending fatigue test piece and the surface of the notch bottom of the four-point bending test piece. The HV hardness at arbitrary 6 points at a position of 0.05 mm in depth was measured with a Vickers hardness tester with a test force of 2.94 N, and the value was arithmetically averaged to obtain the surface hardness.

<5> Compound layer depth:
A sample obtained by mirror polishing the resin-filled test piece used in <4> above was corroded with nital. Next, the grooved Ono-type rotating bending fatigue test piece was photographed using a five-field optical microscope at a magnification of 400 times at the groove bottom, the four-point bending test piece at the notch bottom, and the scratch test piece at an arbitrary surface part. The portion observed white in each field of view was defined as a “compound layer”, the depth of each was measured, and the value was arithmetically averaged to obtain the compound layer depth.

  Table 3 summarizes the results of the above investigations. “◯” and “x” in the “Release Resistance of Compound Layer” column of Table 3 indicate that the compound layer with a maximum width exceeding 50 μm did not peel and that it occurred.

From Table 3, in the case of test numbers 2 to 6 and 8 in which the steel material of the fabric satisfies the chemical composition conditions specified in the present invention and the surface hardness and the compound layer depth satisfy the conditions specified in the present invention, It is clear that it has bending fatigue strength, bending straightening property and peeling resistance of the compound layer.

  On the other hand, in the case of Test No. 9 and Test No. 10, the steel material of the dough satisfies the chemical composition conditions specified in the present invention, but the compound layer depth does not satisfy the conditions specified in the present invention. The peel resistance, or the bending straightness and the peel resistance of the compound layer have not reached the target.

  In the case of test numbers 11 to 15, since the chemical compositions of the steels I to M deviate from the conditions specified in the present invention, they are inferior in any of bending fatigue strength, bending straightening property, or compound layer peeling resistance. .

  That is, in the case of test number 11, the Cr content of steel I, which is the steel material of the fabric, is below the range defined by the present invention, and therefore the surface hardness is also below the specified hardness, and the bending fatigue strength is inferior. Furthermore, the fn1 of the steel I is below the range defined in the present invention, and therefore the hardness of the compound layer is low and the peel resistance of the compound layer is inferior.

  In the case of test number 12, the C content of steel J, which is a steel material, is less than the range defined in the present invention. For this reason, the surface layer hardness is also lower than the specified hardness, and the bending fatigue strength is inferior.

  In the case of test number 13, the Mn content of steel K, which is the steel material of the fabric, is below the range specified in the present invention, and therefore the surface hardness is below the specified hardness and the bending fatigue strength is inferior. Further, the fn1 of the steel K is below the range specified in the present invention, so that the hardness of the compound layer is low and the peel resistance of the compound layer is inferior.

  In the case of test number 14, the Cr content of steel L, which is the steel material of the fabric, exceeds the range specified in the present invention, and thus the surface hardness exceeds the specified hardness and is inferior in bending straightness.

  In the case of test number 15, the fn1 of steel M, which is a steel material of the fabric, is below the range defined in the present invention, and therefore the hardness of the compound layer is low and the peel resistance of the compound layer is inferior.

  The non-tempered nitrided crankshaft of the present invention does not require a tempering treatment by quenching and tempering, has a bending fatigue strength of 700 MPa or more and sufficient bending straightening properties, and is excellent in the peel resistance of the compound layer. For this reason, the non-refined nitrided crankshaft of the present invention can be used as a crankshaft for automobiles, industrial machines, construction machines, etc., and can meet the demand for lighter and smaller crankshafts.

1: Pin fillet part 2: Journal fillet part 3: Pin

Claims (1)

Steel material of dough is mass%, C: 0.25 to 0.60%, Si: 0.10 to 0.80 %, Mn: 0.60 to 2.0%, P: 0.08% or less, S: 0.10% or less, Al: 0.05% or less, Cr: 0.20 to 1.0% and N: 0.0030 to 0.0250%, and the following <1> to <3 The non-tempered nitrided crankshaft containing one or more elements selected from the elements listed up to <1>, the balance being Fe and impurities, and satisfying the following formula (1 '), having a depth from the surface Non-refined nitriding crankshaft having HV hardness of 380 to 600 at 0.05 mm and compound layer depth of at least pin fillet portion, journal fillet portion and pin portion being 5 μm or less .
<1> Cu: 1.0% or less, Ni: 0.5% or less, Mo: 0.5% or less, and V: 0.3% or less <2> Ti: 0.050% or less <3> Ca: 0 .010% or less 40−C + 2Mn + 5.5Cr + 26Mo ≧ 44.4 (1 ′)
The element symbol in the above formula (1 ′) means the content of the element in mass%.

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