JPWO2016098143A1 - Nitriding component manufacturing method and nitriding steel - Google Patents

Abstract

Provided is a method for manufacturing a nitrided part that can increase fatigue strength after nitriding and can suppress deformation after nitriding. The manufacturing method of this embodiment is 0.55 <= Cr + 0.15Si + 0.2 (Mn-1.71S) + 0.1Cr <= 1.10 and 0.55 <= Cr + 0.5Si + 0.35 (Mn-1.71S) -0. .3C ≦ 1.30, a step of preparing a material having a predetermined chemical composition, a step of hot working the material, and a hot-worked material at 600 ° C. or higher and A1 = 723 A step of producing a nitriding steel by carrying out a residual stress release heat treatment after soaking for 20 minutes or more at a temperature defined by 10.7Mn + 29.1Si-16.9Ni + 16.9Cr or less at a temperature of A1 or less, and cooling; The method includes a step of cutting the nitriding steel material after the release heat treatment, and a step of performing a nitriding treatment on the nitriding steel material after the cutting processing.

Description

  The present invention relates to a method for producing a nitrided part using a nitriding steel material, and a nitriding steel material to be nitrided.

  Machine structural parts such as crankshafts and connecting rods are used in automobiles, industrial machines, construction machines, and the like. The mechanical structural parts are required to have high fatigue strength.

  The nitriding treatment is effective for increasing the fatigue strength of the mechanical structural component. For example, a carbon steel material for machine structure or an alloy steel material for machine structure is hot forged to produce an intermediate product having a desired shape. Normalize the intermediate product as necessary. Nitriding is performed on the intermediate product after hot forging or the intermediate product after normalizing to produce a nitrided part that is a machine structural part. By the nitriding treatment, a reinforced layer of nitrogen is formed in the vicinity of the surface layer of the nitrided part. Therefore, the fatigue strength of the nitrided part is increased.

  By the way, the nitriding treatment may cause deformation such as bending in the nitrided part. Cold deformation is usually performed on the deformed nitrided part. If the cold correction process can be omitted and simplified, it is possible to reduce the manufacturing cost and the cycle time. If the deformation of the nitrided part after nitriding can be suppressed, cold straightening can be omitted or simplified.

  Regarding techniques for improving the fatigue strength and bending straightness of nitrided parts, JP-A-9-291339 (Patent Document 1), JP-A-10-46287 (Patent Document 2), JP-A-2010-13729 ( Patent Document 3), Japanese Patent Application Laid-Open No. 2009-270160 (Patent Document 4), and the like.

  The nitrided steel disclosed in Patent Document 1 is, as an essential element, in mass%, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr : 0.05 to 0.50%, as optional elements, Ni, Mo, N, V, Nb, Ti, Zr, Ta, S, Pb, Ca, Bi, Te or one or more Can be contained. Patent Document 1 describes the following matters. By optimizing the C content, Mn content and Cr content in the steel, the ferrite area ratio is increased, and the ferrite and pearlite are further refined. Thereby, excellent fatigue characteristics and bending characteristics can be obtained.

  The steel for nitriding disclosed in Patent Document 2 is, by weight, C: 0.30 to 0.43%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.60%, P : 0.08% or less, S: 0.10% or less, sol. Al: 0.010% or less, Ti: 0.013% or less, Ca: 0.0030% or less, Pb: 0.20% or less and N: 0.010 to 0.030%, with the balance being Fe and Consists of impurities. Further, in this nitriding steel, the Cr content in the impurities is 0.10% or less, and the V content is 0.01% or less. Patent Document 2 describes the following matters. By suppressing the Al content and Ti content in the steel, the fatigue strength is increased. By suppressing the V content, Cr content, and Al content, the surface hardness after nitriding is suppressed, and the bending straightness is improved. By containing P, fatigue strength increases. By setting the nitriding steel to the above-described chemical composition, excellent fatigue strength and bend straightening can be obtained without performing tempering treatment.

  The steel for soft nitriding disclosed in Patent Document 3 is C: more than 0.45% to 0.60% or less, Si: less than 0.50%, Mn: more than 1.30% to 1.70% or less , P: 0.05% or less, S: 0.02 to 0.10%, Cr: 0.30% or less and N: more than 0.007% and 0.030% or less, Al: 0. More than 010% and 0.10% or less, and Ti: more than 0.005% and 0.035% or less, the total content contains 0.015% or more, the balance being Fe and impurities Consists of. The V content in the impurities is 0.010% or less. Further, this soft nitriding steel has fn1 = 1.25C + Mn-0.1Cr of 1.90 or more, and fn2 = N-0.45Al- (1/22) Ti exceeds 0. Patent Document 3 describes the following matters. If fn1 is 1.90 or more, the proportion of ferrite is 10% or less. Furthermore, if fn2 exceeds 0, the pinning effect is obtained by the nitride of Ti and Al, and the crystal grains become finer. Therefore, even if it carries out normalization instead of quenching and tempering, excellent fatigue strength and bending straightness can be obtained.

The manufacturing method of the steel for soft nitriding disclosed in Patent Document 4 is mass%, C: 0.25 to 0.50%, Si: 0.1 to 0.5%, Mn: 0.3 to 1. 5%, P: 0.05% or less, S: 0.1% or less, Ti: 0.005 to 0.05%, Cr: 0.40% or less, Al: 0.05% or less, and N: 0.00. The steel containing 005 to 0.030% with the balance being Fe and impurities is heated to 1100 to 1300 ° C., hot forged at a finishing temperature of 900 ° C. or higher, 570 ° C. or higher, and A ₁ = 723-10.7 (Mn%) + 29.1 (Si%) - 16.9 (Ni%) + 16.9 to annealing at _{a 1} ° C. below the temperature represented by (Cr%). Patent Document 4 describes the following matters. When annealing is performed at A ₁ ° C or less represented by the above formula, cementite for fixing Cr remains in the matrix. For this reason, the Cr concentration in the ferrite is lowered and the bending straightness is improved.

  As described above, in Patent Document 1, crystal grains are refined. However, the fatigue strength may not be improved by itself. In patent document 2, in order to improve bend straightening property, Cr content with high affinity with nitrogen is made low. Therefore, the hardness of the nitrided layer after nitriding tends to be low, and the fatigue strength may be low. In Patent Document 3, the content of an element that strengthens the nitride layer is not sufficient. Therefore, the fatigue strength may not be sufficiently high.

  Furthermore, in Patent Documents 1 to 4, although a technique for improving the bend straightening property of a steel material after deformation such as bending is caused by nitriding treatment, the deformation itself after nitriding treatment is suppressed. There is no disclosure or suggestion.

  An object of the present invention is to provide a method for manufacturing a nitrided part that can increase fatigue strength after nitriding treatment and can suppress deformation after nitriding treatment.

The manufacturing method of the nitride component according to the present embodiment is mass%, C: 0.25 to 0.65%, Si: 0.03 to 0.5%, Mn: 1.3 to 2.7%, P: 0.05% or less, S: 0.005 to 0.10%, Cr: 0.05 to 0.60%, N: 0.003 to 0.025%, Al: 0.05% or less, Ti: 0 ~ 0.05%, Nb: 0 ~ 0.05%, Mo: 0 ~ 0.50%, V: 0 ~ 0.50%, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.50% And Ca: 0 to 0.005%, the balance being made of Fe and impurities, and preparing a material having a chemical composition satisfying the formula (1) and the formula (2), and hot working the material Residual stress release that cools after soaking for 20 minutes or more at a temperature of 600 ° C. or more and A ₁ point or less as defined in (3) Performing heat treatment to manufacture nitriding steel, cutting to nitriding steel after residual stress release heat treatment, and nitriding to nitriding steel after cutting A process.
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
_{A 1 = 723-10.7Mn + 29.1Si-16.9Ni} + 16.9Cr (3)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) to (3).

The steel for nitriding according to the present embodiment is mass%, C: 0.25 to 0.65%, Si: 0.03 to 0.5%, Mn: 1.3 to 2.7%, P: 0.00. 05% or less, S: 0.005 to 0.10%, Cr: 0.05 to 0.60%, N: 0.003 to 0.025%, Al: 0.05% or less, Ti: 0 to 0 0.05%, Nb: 0 to 0.05%, Mo: 0 to 0.50%, V: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and , Ca: 0 to 0.005%, the balance is made of Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2). The area fraction of pearlite in the structure of nitriding steel is higher than 50.0%. In the structure of nitriding steel, cementite having a length of less than 4.0 μm with respect to the total length of cementite in the pearlite colony among pearlite colonies. The ratio of the total area of specific pearlite colonies having a total length ratio of 50% or more to the total area of pearlite colonies is 40% or more.
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

  The steel for nitriding of this embodiment can increase the fatigue strength after nitriding and can suppress the occurrence of bending after nitriding.

FIG. 1 is an example of an SEM image in which a boundary of a pearlite colony is drawn. FIG. 2 is a schematic diagram of a pearlite colony. FIG. 3 is a schematic diagram of another pearlite colony different from FIG. FIG. 4 is a diagram showing a normal pearlite colony and a specific pearlite colony (black portion). FIG. 5 is a side view of an Ono-type rotating bending fatigue test piece. FIG. 6A is a side view of a deformation amount measurement test piece. FIG. 6B is a front view of the deformation amount measuring test piece shown in FIG. 6A. FIG. 7 is a front view of the crankshaft rough profile manufactured in the second embodiment. FIG. 8 is a schematic diagram of a measurement test of the deformation amount of the crankshaft rough profile of FIG.

  The present inventors conducted various studies on deformations such as fatigue strength and bending after nitriding of steel. As a result, the present inventors obtained the following knowledge.

  The reason why deformation such as bending occurs in the steel material after the nitriding treatment is that residual stress introduced into the steel material before nitriding is released during the nitriding treatment.

  Residual stress is generated due to a temperature difference in the steel during cooling after hot forging. As the size of the final steel material (nitrided part) increases or the shape of the steel material becomes more complex, the temperature difference between the locations of the steel material being cooled tends to increase, and the residual stress tends to increase.

If the residual stress of the steel before nitriding can be reduced, deformation of the steel after nitriding can be suppressed. Therefore, in order to reduce the residual stress of the steel before nitriding, “residual stress release heat treatment” is performed before nitriding. Specifically, before nitriding, the steel material is soaked for 20 minutes or more at a ferrite region temperature of 600 ° C. to A ₁ point or less. After soaking, it is cooled to room temperature (25 ° C.) at 2 ° C./second or less.

  If the residual stress release heat treatment is performed, the residual stress generated by hot forging is released before the nitriding treatment. For this reason, it is possible to suppress the occurrence of deformation due to the release of the residual stress during the nitriding process. However, since the residual stress of the steel material is released by the residual stress release heat treatment, the steel material after the residual stress release heat treatment is deformed.

  By the way, in the nitrided part manufactured by normal hot forging, in order to remove the scale formed in the steel surface layer by hot forging, cutting is performed before nitriding. The deformation caused by releasing the residual stress introduced during hot forging is smaller than the cutting allowance removed from the steel material by cutting. Therefore, the deformation due to the residual stress can be removed by cutting.

  Therefore, in the present embodiment, the cutting process is performed after the residual stress releasing heat treatment is performed. Then, nitriding is performed after cutting. In this case, the residual stress introduced during the hot forging is released by the residual stress releasing heat treatment, and deformation occurs. However, this deformation is removed by the next cutting process. The residual stress generated by cutting is much smaller than the residual stress generated by hot forging. Therefore, even if the nitriding treatment is performed, the steel material is hardly deformed.

When the residual stress release heat treatment is performed before the nitriding treatment, the hardness of the entire steel material can be lowered. If the hardness of the whole steel material is low, high fatigue strength cannot be obtained. Then, in order to raise the hardness of the core part and surface layer of steel materials entirely, the chemical composition of steel materials satisfy | fills Formula (1) and Formula (2).
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

  If the chemical composition of the steel material satisfies the formula (1), the hardness of the core portion of the steel material increases. Specifically, the steel material core is solid solution strengthened by Mn, Cr and Si, and the hardness is increased.

  Furthermore, if the chemical composition of the steel material satisfies the formula (2), the hardness of the surface layer of the steel material after the nitriding treatment is increased. Mn, Cr and Si are parent nitrogen elements, which combine with nitrogen to form nitrides and / or Z.O. P. Form a zone. Therefore, these elements harden the nitride layer (surface layer). On the other hand, S and C reduce the hardness of the nitrided layer.

  From the above, if the chemical composition of the steel material satisfies the formula (1) and the formula (2), the hardness of the entire structure (surface layer and core portion) can be increased even in the steel material subjected to the residual stress release heat treatment. it can. Therefore, fatigue strength increases.

  As described above, when the chemical composition of the steel material satisfies the formulas (1) and (2) and the structure of the steel material subjected to the residual stress release heat treatment was observed, the cementite constituting the pearlite was divided, and the pearlite The lamella structure was broken. Therefore, the steel material for nitriding of this embodiment can be specified by the lamellar structure of pearlite in the structure.

  A method for manufacturing a nitrided part and a steel for nitriding completed based on the above knowledge are as follows.

The manufacturing method of the nitride component according to the present embodiment is mass%, C: 0.25 to 0.65%, Si: 0.03 to 0.5%, Mn: 1.3 to 2.7%, P: 0.05% or less, S: 0.005 to 0.10%, Cr: 0.05 to 0.60%, N: 0.003 to 0.025%, Al: 0.05% or less, Ti: 0 ~ 0.05%, Nb: 0 ~ 0.05%, Mo: 0 ~ 0.50%, V: 0 ~ 0.50%, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.50% And Ca: 0 to 0.005%, the balance being made of Fe and impurities, and preparing a material having a chemical composition satisfying the formula (1) and the formula (2), and hot working the material Residual stress release that cools after soaking for 20 minutes or more at a temperature of 600 ° C. or more and A ₁ point or less as defined in (3) Performing heat treatment to manufacture nitriding steel, cutting to nitriding steel after residual stress release heat treatment, and nitriding to nitriding steel after cutting A process.
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
_{A 1 = 723-10.7Mn + 29.1Si-16.9Ni} + 16.9Cr (3)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) to (3).

  The chemical composition of the material may further contain one or two selected from the group consisting of Ti: 0.001 to 0.05% and Nb: 0.003 to 0.05%. Moreover, the chemical composition of the said material is Mo: 0.03-0.50%, V: 0.03-0.50%, Cu: 0.05-0.50%, and Ni: 0.0. You may contain 1 type, or 2 or more types selected from the group which consists of 05-0.50%. Moreover, the chemical composition of the raw material may further contain Ca: 0.0001 to 0.005%.

  The steel for nitriding according to the present embodiment is mass%, C: 0.25 to 0.65%, Si: 0.03 to 0.5%, Mn: 1.3 to 2.7%, P: 0.00. 05% or less, S: 0.005 to 0.10%, Cr: 0.05 to 0.60%, N: 0.003 to 0.025%, Al: 0.05% or less, Ti: 0 to 0 0.05%, Nb: 0 to 0.05%, Mo: 0 to 0.50%, V: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and , Ca: 0 to 0.005%, the balance is made of Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2). The area fraction of pearlite in the structure of nitriding steel is higher than 50.0%. In the structure of nitriding steel, cementite having a length of less than 4.0 μm with respect to the total length of cementite in the pearlite colony among pearlite colonies. The ratio of the total area of specific pearlite colonies having a total length ratio of 50% or more to the total area of pearlite colonies is 40% or more.

  Hereinafter, the manufacturing method of the above-described nitrided part and the steel for nitriding will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the nitriding steel according to the present invention contains the following elements.

C: 0.25 to 0.65%
Carbon (C) increases the strength of the nitrided steel material and increases the fatigue strength. C further increases the wear resistance of the steel material. If the C content is too low, the above effect cannot be obtained. On the other hand, if the C content is too high, the area fraction of cementite in the steel material becomes too high, and the machinability deteriorates. Therefore, the C content is 0.25 to 0.65%. The minimum with preferable C content is higher than 0.25%, More preferably, it is 0.30%, More preferably, it is 0.35%. The upper limit with preferable C content is less than 0.65%, More preferably, it is 0.60%, More preferably, it is 0.58%.

Si: 0.03-0.5%
Silicon (Si) deoxidizes steel. Si further solidifies in ferrite and strengthens the steel (solid solution strengthening). If the Si content is too low, the above effect cannot be obtained. On the other hand, if the Si content is too high, the hardness of the surface layer becomes excessively high during nitriding. If the Si content is too high, the diffusion of nitrogen during the nitriding treatment is further inhibited, the hardened layer depth becomes shallow, and the fatigue strength decreases. Therefore, the Si content is 0.03 to 0.5%. The minimum with preferable Si content is higher than 0.03%, More preferably, it is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Si content is less than 0.5%, More preferably, it is 0.4%, More preferably, it is 0.35%.

Mn: 1.3 to 2.7%
Manganese (Mn) is dissolved in the steel material to strengthen the steel material after the nitriding treatment (solid solution strengthening). Further, Mn combines with N introduced into the steel material by nitriding treatment to form a nitride, thereby increasing the hardness of the surface layer and increasing the fatigue strength. Further, Mn forms MnS in the steel material to enhance the machinability of the steel material. If the Mn content is too low, the above effect cannot be obtained. On the other hand, if the Mn content is too high, the hardenability of the steel material becomes too high. In this case, martensite is formed in the steel material and machinability is lowered. Therefore, the Mn content is 1.3 to 2.7%. The minimum with preferable Mn content is higher than 1.3%, More preferably, it is 1.4%, More preferably, it is 1.5%, More preferably, it is 1.55%. The upper limit with preferable Mn content is less than 2.7%, More preferably, it is 2.4%, More preferably, it is 2.1%.

P: 0.05% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and causes grain boundary embrittlement cracking. Therefore, the P content is preferably as low as possible. The P content is 0.05% or less. A preferable P content is 0.04% or less.

S: 0.005-0.10%
Sulfur (S) combines with Mn in the steel material to form MnS and enhances the machinability of the steel material. If the S content is too low, the above effect cannot be obtained. On the other hand, if S content is too high, coarse MnS will be formed and the fatigue strength of steel materials will fall. Therefore, the S content is 0.005 to 0.10%. The minimum with preferable S content is higher than 0.005%, More preferably, it is 0.01%, More preferably, it is 0.02%. The upper limit with preferable S content is less than 0.10%, More preferably, it is 0.09%, More preferably, it is 0.08%.

Cr: 0.05-0.60%
Chromium (Cr) combines with N introduced into the steel material by nitriding to form CrN in the nitrided layer, strengthening the nitrided layer. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the diffusion of nitrogen during nitriding is inhibited, the hardened layer depth becomes shallow, and the fatigue strength decreases. Therefore, the Cr content is 0.05 to 0.60%. The minimum with preferable Cr content is higher than 0.05%, More preferably, it is 0.10%, More preferably, it is 0.15%. The upper limit with preferable Cr content is less than 0.60%, More preferably, it is 0.50%, More preferably, it is 0.40%.

N: 0.003 to 0.025%
Nitrogen (N) is dissolved in the steel material to increase the strength of the steel material. If the N content is too low, the above effect cannot be obtained. On the other hand, if the N content is too high, bubbles are generated in the steel material. Since bubbles become defects, it is preferable to suppress the generation of bubbles. Therefore, the N content is 0.003 to 0.025%. The minimum with preferable N content is higher than 0.003%, More preferably, it is 0.005%, More preferably, it is 0.007%. The upper limit with preferable N content is less than 0.025%, More preferably, it is 0.022%, More preferably, it is 0.020%.

Al: 0.05% or less Aluminum (Al) is inevitably contained. If the Al content is too high, coarse oxides are formed, and the fatigue strength of the steel material decreases. Therefore, the Al content is 0.05% or less. The upper limit with preferable Al content is less than 0.05%, More preferably, it is 0.045%, More preferably, it is 0.040%.

  On the other hand, Al deoxidizes steel. Therefore, Al may be contained for the purpose of deoxidation. In this case, the preferable lower limit of the Al content is 0.005%, more preferably 0.010%. The Al content in this specification is the content of total Al (Total Al).

  The balance of the nitriding steel according to the present invention consists of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or production environment when industrially producing the steel material, and within a range that does not adversely affect the nitriding steel material of the present invention. It means what is allowed.

[Arbitrary elements]
The steel for nitriding according to the present invention may further contain Ti.

Ti: 0 to 0.05%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti combines with N in the base material to form TiN, and suppresses coarsening of crystal grains during hot forging. However, if the Ti content is too high, TiC is generated and the hardness of the steel material varies greatly. Therefore, the Ti content is 0 to 0.05%. The minimum with preferable Ti content is 0.001%, More preferably, it is 0.003%, More preferably, it is 0.005%. The upper limit with preferable Ti content is less than 0.05%, More preferably, it is 0.040%, More preferably, it is 0.030%.

Nb: 0 to 0.05%
Niobium (Nb) is an optional element and may not be contained. When Nb is contained, Nb combines with N in the base material to form NbN, and suppresses coarsening of crystal grains during hot forging. Nb further delays recrystallization during hot forging and suppresses coarsening of crystal grains. However, if the Nb content is too high, NbC is generated and the hardness of the steel material varies greatly. Therefore, the Nb content is 0 to 0.05%. The minimum with preferable Nb content is 0.003%, More preferably, it is 0.005%. The upper limit with preferable Nb content is less than 0.05%, More preferably, it is 0.040%, More preferably, it is 0.030%.

  The steel for nitriding according to the present invention may further contain one or more selected from the group consisting of Mo, V, Cu and Ni. Mo, V, Cu and Ni all increase the strength of the steel material.

Mo: 0 to 0.50%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel and increases the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. However, if the Mo content is too high, martensite is generated and machinability is lowered. Therefore, the Mo content is 0 to 0.50%. A preferable lower limit of the Mo content is 0.03%. The upper limit with preferable Mo content is less than 0.50%, More preferably, it is 0.40%, More preferably, it is 0.30%.

V: 0 to 0.50%
Vanadium (V) is an optional element and may not be contained. When contained, V forms VC during cooling after hot forging and during nitriding, and increases the strength of the core of the steel material. V further combines with nitrogen to form a nitride and strengthen the nitride layer. Therefore, fatigue strength increases. However, if the V content is too high, the toughness is greatly reduced. Therefore, the V content is 0 to 0.50%. The minimum with preferable V content is 0.03%, More preferably, it is 0.05%. The upper limit with preferable V content is less than 0.50%, More preferably, it is 0.40%, More preferably, it is 0.30%.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When contained, Cu is dissolved in ferrite to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. However, if the Cu content is too high, it segregates at the grain boundaries of steel during hot forging and induces hot cracking. Therefore, the Cu content is 0 to 0.50%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Cu content is less than 0.50%, More preferably, it is 0.40%, More preferably, it is 0.30%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni is dissolved in ferrite to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. Ni further suppresses hot cracking caused by Cu when the steel material contains Cu. However, if the Ni content is too high, the effect is saturated and the manufacturing cost increases. Therefore, the Ni content is 0 to 0.50%. The minimum with preferable Ni content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Ni content is less than 0.50%, More preferably, it is 0.40%, More preferably, it is 0.30%.

  The nitriding steel according to the present invention may further contain Ca.

Ca: 0 to 0.005%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca increases the machinability of the steel material. However, if the Ca content is too high, coarse Ca oxide is generated, and the fatigue strength of the steel material is reduced. Therefore, the Ca content is 0 to 0.005%. The minimum with preferable Ca content for acquiring the said effect stably is 0.0001%, More preferably, it is 0.0003%. The upper limit with preferable Ca content is less than 0.005%, More preferably, it is 0.004%, More preferably, it is 0.003%.

[Regarding Formula (1) and Formula (2)]
The chemical composition further satisfies formulas (1) and (2).
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

[Regarding Formula (1)]
It is defined as F1 = C + 0.15Si + 0.2 (Mn-1.71S) + 0.1Cr. F1 is an index of the hardness of the core of the steel material. C, Si, Mn, and Cr are effective for strengthening the inside of the steel material, and S inhibits strengthening. If F1 is less than 0.55, the hardness inside the steel material is too low, so the fatigue strength is low. On the other hand, if F1 is higher than 1.10, the hardness inside the steel material becomes excessively high, so that machinability is lowered. If F1 is 0.55 to 1.10, the inside of the steel material has appropriate hardness. The minimum with preferable F1 is 0.60, More preferably, it is 0.65. The upper limit with preferable F1 is 1.00, More preferably, it is 0.95.

[Regarding Formula (2)]
It is defined as F2 = Cr + 0.5Si + 0.35 (Mn-1.71S) -0.3C. F2 is an index of the hardness of the surface layer (nitriding layer) of the steel material after the nitriding treatment is performed on the steel material having a pearlite structure in which the lamella is broken. As described above, Cr, Si, and Mn form nitrides and harden the nitride layer. On the other hand, S and C inhibit the hardening of the nitride layer. If F2 is less than 0.55, the hardness of the nitrided layer is too low and the fatigue strength is low. On the other hand, if the value of F2 becomes too large, the hardness only in the vicinity of the surface becomes excessively high, the hardness becomes difficult to increase at a position deeper than the vicinity of the surface, and the toughness further decreases. Therefore, if the value of F2 exceeds 1.30, the fatigue strength decreases. If F2 is 0.55 to 1.30, the nitrided layer has an appropriate hardness, so that excellent fatigue strength can be obtained. The minimum with preferable F2 is 0.60, More preferably, it is 0.65. The upper limit with preferable F2 is 1.10, More preferably, it is 0.95.

[Microstructure of steel]
[Perlite area fraction]
The steel for nitriding according to the present invention has a structure mainly composed of pearlite. More specifically, the area fraction of pearlite in the microstructure of the nitriding steel material exceeds 50.0%. Here, the area fraction of pearlite is defined by the following method.

  A sample for observing the structure is taken from a position that bisects the shortest distance connecting the center and the surface of the nitriding steel. In the case where the steel material is a round bar, the sample is collected at a point where the distance between the center point and the outer circumference of the round bar, that is, the radius R, is divided into two equal parts (hereinafter referred to as R) in a cross section perpendicular to the central axis of the round bar. / 2 position).

  About the collected sample, the cross section perpendicular | vertical to the rolling direction of steel materials is mirror-polished. The mirror-polished surface is used as an observation surface and corroded with a nital etchant. After corrosion, pearlite is identified by observing the microstructure of any four fields of view using a 500 × optical microscope. The identification may be performed by image processing, or may be performed by an operator using a photographic image. Each field of view is 406 μm × 540 μm. The area ratio of pearlite in each visual field (total area of pearlite in each visual field / total area of each visual field × 100 (%)) is obtained. The average value of the pearlite area ratios for the four visual fields obtained is defined as the pearlite area ratio (%).

  Since the steel for nitriding of the present invention has a pearlite-based structure, it has higher strength than a ferrite-based structure. Therefore, the fatigue strength is increased. A preferable pearlite area fraction is 55.0% or more.

[Cementite fragmentation rate]
The steel for nitriding of the present invention has a structure in which cementite in pearlite is divided and a lamellar structure is broken by performing a residual stress releasing heat treatment described later. The pearlite in the tissue includes a plurality of pearlite colonies. Each perlite colony contains a plurality of cementites. Among the plurality of cementites, cementite having a length of less than 4.0 μm in a cross section perpendicular to the rolling direction of the steel material is referred to as “short cementite”.

  Measure the length of each cementite in each perlite colony. When the ratio of the total length (total length) of short cementite to the total length (total length) of all cementite in the pearlite colony (hereinafter referred to as the short cementite ratio) is 50% or more, the pearlite It is judged that the cementite is sufficiently divided in the colony. Such a pearlite colony is referred to as a “specific pearlite colony”. A pearlite colony having a short cementite ratio of less than 50% is referred to as a “normal pearlite colony”.

  The ratio of the total area of the specific pearlite colony to the total area of the pearlite colony in the tissue (that is, the normal pearlite colony and the specific pearlite colony) is defined as “cementite fragmentation rate” (%).

  The cementite fragmentation rate of the nitriding steel according to the present invention is 40% or more. In this case, since the residual stress is sufficiently released in the residual stress releasing heat treatment, deformation of the nitrided part after the nitriding treatment is suppressed. A preferable cementite fragmentation rate is 50% or more, and more preferably 60% or more.

  The cementite fragmentation rate is measured by the following method. A sample for observing the structure is taken from a position that bisects the shortest distance between the center of the steel material and the surface. For example, when the steel material is a round bar, the R / 2 position is the sample collection position in a cross section perpendicular to the central axis of the round bar.

About the collected sample, the cross section perpendicular | vertical to the rolling direction of steel materials is mirror-polished. The mirror-polished surface is used as an observation surface and corroded with a nital etchant. Then, arbitrary 3 visual fields are observed by SEM of 3000 times magnification among observation surfaces, and image data is preserve | saved. The area of each visual field is 1376 μm ² and the total area is 4128 μm ² .

  In the image data of each visual field, the boundary of the pearlite colony is specified by using image processing or by visual observation of the operator. FIG. 1 is an example of an SEM image in which a boundary of a pearlite colony is drawn.

  Subsequently, the length of each cementite contained in each pearlite colony is measured by the following method.

  FIG. 2 is a schematic diagram of the pearlite colony 10. The pearlite colony 10 includes a plurality of cementite 20. A center point P <b> 1 of the end of the cementite 20 and the width of the cementite 20 is specified. In FIG. 2, the end of the cementite 20 is located at the boundary of the pearlite colony 10.

  A straight line is drawn from the point P1 to the opposite end of the cementite 20. The direction of the straight line is the extending direction of cementite 20 (the growth direction of cementite). An intersection point between the straight line and the opposite end of the cementite 20 is defined as a point P2. The length of the line segment LS1 connecting the point P1 and the point P2 is defined as the length of the cementite 20.

  FIG. 3 is a schematic diagram of another pearlite colony 30 different from FIG. The pearlite colony 30 includes a plurality of curved cementites 40.

  The length of the curved cementite 40 is measured by the following method. Of the plurality of cementite 40, attention is focused on cementite 40A. Both ends of the cementite 40A are located at the boundary of the pearlite colony 30. In this case, the center point P3 of the end width is specified by the same method as in the case of the cementite 20 described above. Furthermore, a line segment is drawn from the point P3 toward the opposite end of the cementite 40A. In the cementite 40A, the tip of the line segment crosses the interface between the cementite 40A and the ferrite (hereinafter referred to as the cementite interface) before reaching the opposite end of the cementite 40A. In this case, the line segment LS2 is drawn so that the straight line distance until the line segment extending from the start point P3 crosses the cementite interface of the cementite 40A is maximized. The intersection point between the line segment LS2 and the cementite interface is defined as an intermediate point P4. Subsequently, a line segment is drawn from the intermediate point P4 to the opposite end of the cementite 40A. At this time, the line segment extending from the intermediate point P4 reaches the opposite end of the cementite 40A without crossing the cementite interface and intersects at the point P5. In this case, the line segment LS3 is drawn so that the linear distance between the intermediate point P4 and the point P5 is maximized (without crossing the cementite interface). In this case, the length of the cementite 40A is LS2 length + LS3 length.

  If a line segment is drawn from the intermediate point P4 toward the opposite end of the cementite 40A, and the tip of the line segment crosses the cementite interface again, a new intermediate point is determined by the above method. Starting from the intermediate point, a line segment is drawn toward the opposite end of the cementite 40A. The above operation is repeated until the end of the line segment extending from the midpoint intersects the opposite end of the cementite 40A. And the sum total of the length of each line segment drawn in cementite 40A is defined as the length of cementite 40A.

  Even when the end of the cementite 40 is not in contact with the interface of the pearlite colony 30 (for example, cementite 40B in the figure), the length of the cementite can be measured by the same method as described above.

  The length of each cementite in a pearlite colony is calculated | required by the above method. Then, a short cementite ratio (%) is obtained for each pearlite colony, and whether each pearlite colony is a “normal pearlite colony” or a “specific pearlite colony” is specified. And the area of a normal pearlite colony and the area of a specific pearlite colony are calculated | required. The area can be obtained, for example, by well-known image analysis.

The total area NC of normal pearlite colonies and the total area of specific pearlite colonies DC in all fields of view (total area 4128 μm ² ) are determined. And the cementite parting rate (%) is calculated | required by the following formula.
Cementite breaking rate = DC / (NC + DC) × 100 (%)
Of the pearlite colonies in each field of view, pearlite colonies with an area of less than 10 μm ² are excluded from analysis. That is, such a pearlite colony is not normally recognized as a pearlite colony or a specific pearlite colony. Therefore, it is not included in the total areas DC and NC.

  FIG. 4 is a diagram showing normal pearlite colonies and specific pearlite colonies determined by the above-described specifying method in the SEM image of FIG. 1. In the figure, a black part means a specific pearlite colony. A white part means the pearlite colony or ferrite which became the object of analysis. Parts other than black and white usually mean pearlite colonies.

  Residual stress release heat treatment is performed when nitriding is performed on a steel for nitriding that satisfies the above-mentioned chemical composition, formulas (1) and (2), and has a sufficiently collapsed pearlite lamellar structure with a cementite fragmentation rate of 40% or more. High fatigue strength is obtained despite the fact that

[Production method]
An example of a method for manufacturing a nitriding steel material and a nitrided part according to the present invention will be described.

  The method for manufacturing a nitrided part according to the present invention includes a material preparation process, a hot working process, a residual stress releasing heat treatment process, a cutting process, and a nitriding process. Hereinafter, each process is demonstrated.

[Material preparation process]
A molten steel satisfying the above-described chemical composition and the formulas (1) and (2) is manufactured. The manufactured molten steel is used to make a slab (slab, bloom) by a casting method. Alternatively, the molten steel is used to make an ingot by the ingot-making method. A billet is manufactured by hot working a slab or an ingot. The hot working may be hot rolling or hot forging. The material (steel material) used in the next step may be the above slab or ingot, or may be a billet.

[Hot working process]
The manufactured material is heated. If the heating temperature is too low, an excessive load is applied to the hot working apparatus. On the other hand, if the heating temperature is too high, the scale loss is large. Therefore, preferable heating temperature is 1000-1300 degreeC. A preferable holding time at the heating temperature is 30 to 1000 minutes.

  Hot working is performed on the heated material. Hot working is, for example, hot forging. Hereinafter, the description will be continued assuming that hot working in this step is hot forging.

  The preferable finishing temperature of hot forging is 900 ° C. or higher. This is because if the finishing temperature is too low, the burden on the die of the hot forging device is increased. On the other hand, the preferable upper limit of the finishing temperature is 1250 ° C.

  Cool the material after hot forging. A preferable average cooling rate until the surface temperature of the material reaches 300 ° C. is 5 ° C./second or less. If the average cooling rate exceeds 5 ° C./second, the residual stress generated in the material becomes too large, and the residual stress is not easily released even by the residual stress releasing heat treatment. In this case, deformation (bending) generated in the nitrided part after nitriding tends to be large. Therefore, a preferable average cooling rate is 5 ° C./second or less. A more preferable average cooling rate is 3 ° C./second or less, and further preferably 1 ° C./second or less. After the surface temperature of the material is lowered to 300 ° C., the cooling method to room temperature is not particularly limited. In addition, when a crankshaft is manufactured by hot forging, the average cooling rate is 5 ° C./second or less if the crankshaft is allowed to cool.

[Residual stress release heat treatment]
Residual stress release heat treatment is performed on the material after hot forging. At this time, the heat treatment temperature is 600 ° C. or more and is A ₁ point or less defined by the formula (3).
_{A 1 = 723-10.7Mn + 29.1Si-16.9Ni} + 16.9Cr (3)

  If the heat treatment temperature is less than 600 ° C., the residual stress is not sufficiently released. In this case, the residual stress is released during nitriding, and deformation such as bending is likely to occur.

On the other hand, the heat treatment temperature exceeds the _point A, becomes a two-phase region temperature, a portion of the mixed structure of ferrite and cementite becomes the austenite during the heat treatment. The formed austenite transforms again to ferrite and cementite during cooling. In these transformation processes, the material repeatedly shrinks and expands, resulting in residual stress. Residual stress release heat treatment must be performed at the ferrite region temperature. Therefore, the heat treatment temperature is 600 ° C. to A ₁ point or less.

  The preferable lower limit of the heat treatment temperature of the residual stress release heat treatment is 610 ° C., more preferably 620 ° C. A preferable upper limit of the heat treatment temperature is A.₁Point −5 ° C., more preferably A ₁The point is -10 ° C.

The soaking time at a temperature of 600 ° C. or higher in the residual stress release heat treatment and a temperature of A ₁ or lower defined by the formula (3) is 20 minutes or longer. If the soaking time is less than 20 minutes, the residual stress is not sufficiently released. In this case, the residual stress is released during nitriding, and deformation such as bending is likely to occur. On the other hand, if the soaking time exceeds 24 hours, the effect of the heat treatment is saturated and the heat treatment cost increases. Therefore, the soaking time is within 24 hours.

  After soaking by the heat treatment, the material is cooled to room temperature (25 ° C.). The metal structure does not change depending on the cooling rate after the heat treatment. Therefore, it is not necessary to specify the average cooling rate to room temperature. However, since a new residual stress may occur, it is preferable that the cooling rate is low. The average cooling rate is preferably 2 ° C./second or less, more preferably 1 ° C./second or less.

  The nitriding steel material of the present invention is manufactured through the above steps.

[Cutting]
The above-described nitriding steel material is cut into a predetermined shape. Due to the release of the residual stress in the residual stress releasing heat treatment, deformation such as bending occurs in the nitriding steel material. In the cutting process, the steel material is made into a predetermined shape and the deformation is removed. The type of the cutting process is not particularly limited as long as the nitriding steel material can be formed into a predetermined shape.

[Nitriding treatment]
Nitriding is performed on the steel for nitriding that has been cut. In this embodiment, a well-known nitriding process is employed. The nitriding treatment is, for example, gas nitriding, salt bath nitriding, ion nitriding or the like. The furnace atmosphere during nitriding may be only NH ₃ or a mixture containing NH ₃ and N ₂ and / or H ₂ . In addition, a carburizing gas may be contained in these gases to perform soft nitriding. Therefore, “nitriding” in this specification includes “soft nitriding”.

  When carrying out gas soft nitriding treatment, for example, in an atmosphere in which endothermic metamorphic gas (RX gas) and ammonia gas are mixed 1: 1, the soaking temperature is set to 550 to 620 ° C. and soaking for 1 to 4 hours. do it.

  The nitrided part manufactured by the above manufacturing process has excellent fatigue strength. Further, since the residual stress is released before the nitriding process by the residual stress releasing heat treatment, deformation is hardly caused in the nitriding process. Therefore, the cold correction process after nitriding can be omitted or simplified.

  A 150 kg molten steel of steels A to C having the chemical composition shown in Table 1 was manufactured using a vacuum melting furnace. A 50 kg molten steel of steels D to U was produced using a vacuum melting furnace. An ingot was manufactured using each molten steel.

In the “F1” column in Table 1, the calculated value of F1 described above is described. In the “F2” column, the calculated value of F2 described above is described. In the “A ₁ ” column, A ₁ point (° C.) defined by the formula (3) is described.

  Ingots of each steel type were heated to 1250 ° C. Hot forging was performed on the heated ingot to produce a round bar having a diameter of 55 mm. The finishing temperature in hot forging was 1000 ° C. After hot forging, the round bar was allowed to cool to room temperature. The average cooling rate until the round bar reached 300 ° C. was 0.23 ° C./second in any test number.

  Residual stress release heat treatment was performed on the round bars of each test number under the conditions shown in Table 2.

  The “steel type” column in Table 2 indicates the steel type of the ingot used in the corresponding test number. The “temperature and soaking time” column in the “residual stress releasing heat treatment” column shows the heat treatment temperature (° C.) and soaking time (h) in the residual stress releasing heat treatment. The average cooling rate was 0.17 ° C./sec for all test numbers. The “nitriding conditions” column shows the processing temperature (° C.) and the soaking time (h) in the nitriding treatment.

[Evaluation test]
The following test was implemented using the round bar of each test number.

[Perlite area fraction and cementite fragmentation rate measurement test]
From the R / 2 position of the round bar of each test number, a sample having a cross section parallel to the forging axis direction as a test surface was collected. The pearlite area fraction of each test number was calculated | required by the above-mentioned method using the extract | collected sample. The pearlite area ratio (%) of each test number is shown in the “P fraction” column of Table 2.

  Furthermore, the said sample was observed by 5000 time SEM and the cementite parting rate was calculated | required based on the said method. The cementite fraction (%) of each test number is shown in “θ fraction (%)” in Table 2.

[Preparation of Ono type rotating bending fatigue test piece and deformation measurement test piece]
A plurality of Ono-type rotating bending fatigue test pieces shown in FIG. 5 were collected from the round bars of the respective test numbers. The length L1 in the figure was 106 mm, and the diameter D1 was 15 mm. The radius of curvature R1 of the notch at the center of the test piece was 3 mm, and the diameter of the cross section of the test piece at the notch bottom was 8 mm. The length of the parallel part of the test piece was 18 mm, and the diameter at the parallel part was 10 mm.

  Furthermore, the deformation measurement test pieces shown in FIGS. 6A and 6B were collected from the round bars of the respective test numbers. The diameter D2 of the deformation amount test piece was 50 mm, and the thickness L2 was 50 mm. A notch was provided over the entire length in the thickness direction. The following width W2 was set to a target value of 10.000 mm. The actually measured value of the width W2 of the notch after the notch processing was measured with a micrometer. The measurement location was point M in FIGS. 6A and 6B. The point M was 10 mm, 20 mm, 30 mm, and 40 mm in the width direction from the origin when the left end face shown in FIG. 6B was the origin. The average of the width W2 measured at these four points M was defined as the notch width (mm).

  The collected Ono-type rotating bending fatigue test piece and deformation measurement test piece were subjected to nitriding treatment under the conditions (heat treatment temperature and soaking time) shown in Table 2. Specifically, the test piece was inserted into a heat treatment furnace. After the insertion, ammonia gas and RX gas were introduced into the furnace at a flow rate of 1: 1 while raising the temperature in the furnace to the heat treatment temperature (° C.) in the “nitriding conditions” column of Table 2. Then, nitriding was performed at the heat treatment temperature (° C.) and the holding time (h) shown in the “nitriding conditions” column of Table 2. After the holding time had elapsed, the test piece was taken out of the heat treatment furnace and quenched with 100 ° C. oil.

[Ono type rotating bending fatigue test]
Using the Ono rotary bending fatigue test piece subjected to the above nitriding treatment, an Ono rotary bending fatigue test was performed. A rotating bending fatigue test in accordance with JIS Z2274 (1978) was performed in an air atmosphere at room temperature (25 ° C.). The test was carried out under a double swing condition with a rotation speed of 3400 rpm. Of the test pieces that did not break until the number of repetitions of 1.0 × 10 ⁷ times, the highest stress was defined as the fatigue strength (MPa) of that test number. When the fatigue strength was 510 MPa or more, it was judged that the fatigue strength was excellent.

[Vickers hardness test]
A Vickers hardness test in accordance with JIS Z2244 (2009) was performed on the central axis portion of the Ono type rotating bending fatigue test. The test force was 10 kgf = 98.07N.

[Deformation measurement test]
The notch width W2 of the deformation amount test specimen was measured before nitriding. After the measurement, the above nitriding treatment was performed on the deformation amount test piece. The notch width W2 of the deformation measurement test piece after nitriding was measured again. A value obtained by subtracting the notch width W2 before nitriding from the notch width W2 after nitriding was defined as the deformation (μm). When the absolute value of the deformation amount was 35 μm or less, it was determined that the deformation amount was small.

[Test results]
Table 2 shows the test results. “Fatigue strength” means the fatigue strength (MPa) obtained in the Ono-type rotary bending test, and “deformation amount” means the deformation amount (μm) obtained in the deformation amount measurement test.

Referring to Table 2, in test numbers A1 to A19, the chemical composition was within the scope of the present invention, and the formulas (1) and (2) were also satisfied. Furthermore, the heat treatment temperature in the residual stress release heat treatment was 600 ° C. to A ₁ point (° C.). Therefore, the pearlite area fractions of test numbers A1 to A19 all exceeded 50%, and the cementite fragmentation ratio was 40% or more. Therefore, the fatigue strengths of test numbers A1 to A19 were all 510 MPa or more. Furthermore, the absolute value of the deformation was 35 μm or less.

  In test number A20, the chemical composition was within the scope of the present invention, but the F1 value was less than the lower limit of formula (1). Therefore, the fatigue strength was less than 510 MPa.

  In test number A21, the chemical composition was within the scope of the present invention, but the F1 value exceeded the upper limit of formula (1). For this reason, the core hardness exceeded 270 HV, and there was concern about a decrease in machinability.

  In test number A22, the chemical composition was within the scope of the present invention, but the F2 value was less than the lower limit of formula (2). Therefore, the hardness of the region near the surface of the hardened layer was low, and the fatigue strength was less than 510 MPa.

  In test number A23, the chemical composition was within the scope of the present invention, but the F2 value exceeded the upper limit of formula (2). Therefore, the depth of the hardened layer became shallow and the fatigue strength was less than 510 MPa.

  In test numbers A24 and A25, the amount of Mn was low, and the F2 value was also less than the lower limit of formula (2). Therefore, the fatigue strength was less than 510 MPa.

In Test No. A26, the heat treatment temperature of the residual stress release heat treatment exceeds the _point A. Therefore, the cementite fragmentation rate was less than 40%, and the absolute value of deformation exceeded 35 μm. This is probably because residual stress release heat treatment was performed in the austenite single phase region, and residual stress due to transformation occurred during cooling of the residual stress release heat treatment. In test number A26, the fatigue strength was less than 510 MPa.

  In the test numbers A27 and A30, the residual stress release heat treatment was not performed. Therefore, the cementite fragmentation rate was less than 40%, and the absolute value of deformation exceeded 35 μm.

In test numbers A28 and A29, the heat treatment temperature in the residual stress release heat treatment was less than 600 ° C. Therefore, the cementite fragmentation rate was less than 40%, and the absolute value of deformation exceeded 35 μm.
In test number A31, the heat treatment time in the residual stress release heat treatment was less than 20 minutes. Therefore, the cementite fragmentation rate was less than 40%, and the absolute value of deformation exceeded 35 μm.

  A round billet having the chemical composition of steel type G in Table 1 was prepared. The round billet was heated to 1250 ° C. The heated round billet was hot forged to produce a plurality of in-line four-cylinder crankshaft rough shapes 50 (hereinafter referred to as rough shapes) shown in FIG. The finishing temperature in hot forging was 1100 ° C. The rough shaped material 50 after hot forging was allowed to cool. The cooling rate at this time was 5 ° C./second or less.

  A nitrided part (crankshaft) was manufactured under the test conditions shown in Table 3 for the plurality of manufactured rough members 50.

[Test numbers B1 and B4]
In test numbers B1 and B4, residual stress release heat treatment was performed under the conditions shown in Table 3 (heat treatment temperature and soaking time), and the rough shaped material 50 after soaking was allowed to cool. The average cooling rate during cooling was 2 ° C./second or less.

  Cutting was performed on the rough shape after the residual stress release heat treatment. Specifically, the circumference of the journal portion 51 of the rough profile shown in FIG. 7 was cut. In the cutting process, the outer periphery of all journal parts 51 after the cutting was processed so as to be aligned on the same line. Furthermore, in order to handle easily, the part from the both ends of the journal part 51 was cut | disconnected. The full length of the journal part 51 was 470 mm, and the diameter was 76 mm.

  The rough shaped material 50 after cutting was subjected to nitriding treatment under the same conditions as in Example 1. The deformation amount of the rough shaped material 50 after the nitriding treatment was measured by the following method. As shown in FIG. 8, the two ends of the journal portion 51 of the rough shaped member 50 were supported rotatably. The rough shape member 50 was rotated once around its central axis. At this time, the dial gauge 52 was brought into contact with the central outer peripheral surface of the journal portion 51, and the displacement of the height position was measured. Of the height positions obtained by one rotation, the difference between the maximum value and the minimum value was defined as the deformation amount (μm).

[Test number B2]
In test number B2, the above-mentioned cutting process was implemented with respect to the rough shaped material 50 after hot forging. The above-described nitriding treatment was performed on the rough shaped material 50 after the cutting without performing the residual stress releasing heat treatment. The amount of deformation of the rough shaped material 50 after the nitriding treatment was determined by the same method as in test number B1.

[Test number B3]
In test number B3, the above-described cutting process was performed on the rough shaped material 50 after hot forging. Residual stress release heat treatment was performed on the rough shaped material 50 after cutting under the conditions shown in Table 3. The coarse shaped material 50 after soaking was allowed to cool. The average cooling rate at this time was 2 ° C./second or less. A nitriding treatment was performed on the rough shaped material 50 after the residual stress releasing heat treatment under the same conditions as in the test number B1. The amount of deformation of the rough shaped material 50 after the nitriding treatment was determined by the same method as in test number B1.

[Test results]
The test results are shown in Table 3. Referring to Table 3, the chemical composition of test number B1 was within the scope of the present invention, and the F1 and F2 values were also within the scope of formula (1) and formula (2). Furthermore, the manufacturing conditions were also appropriate. Therefore, the amount of deformation was 35 μm or less, and the occurrence of bending was suppressed.

  On the other hand, in the test number B2, the residual stress release heat treatment was not performed. Therefore, the deformation amount exceeded 35 μm.

  In test number B3, residual stress release heat treatment was performed after cutting. Since the bending occurred in the residual stress release heat treatment and the bending was not removed by the cutting process, the deformation amount exceeded 35 μm.

In Test Number B4, the heat treatment temperature of the residual stress release heat treatment exceeds the _point A. Therefore, the deformation amount exceeded 35 μm. This is probably because residual stress release heat treatment was performed at a two-phase temperature, and residual stress due to transformation occurred during cooling of the residual stress release heat treatment.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

10, 30 Perlite colony 20, 40, 40A, 40B Cementite

Claims (8)

In mass%, C: 0.25 to 0.65%, Si: 0.03 to 0.5%, Mn: 1.3 to 2.7%, P: 0.05% or less, S: 0.005 -0.10%, Cr: 0.05-0.60%, N: 0.003-0.025%, Al: 0.05% or less, Ti: 0-0.05%, Nb: 0-0 0.05%, Mo: 0 to 0.50%, V: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and Ca: 0 to 0.005% And a balance of Fe and impurities, and preparing a material having a chemical composition satisfying the formulas (1) and (2),
Hot working the material;
After the hot-processed material is soaked at a temperature of 600 ° C. or more and at a temperature of A ₁ point or less defined by the formula (3) for 20 minutes or more, a residual stress release heat treatment is performed for cooling. Manufacturing a nitriding steel,
Performing a cutting process on the nitriding steel material after the residual stress releasing heat treatment;
And a step of performing a nitriding process on the steel for nitriding after the cutting process.
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
_{A 1 = 723-10.7Mn + 29.1Si-16.9Ni} + 16.9Cr (3)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) to (3). The manufacturing method according to claim 1,
The chemical composition of the material is
Ti: 0.001 to 0.05%, and
Nb: A method for manufacturing a nitrided part containing one or two selected from the group consisting of 0.003 to 0.05%. It is a manufacturing method of Claim 1 or Claim 2, Comprising:
The chemical composition of the material is
Mo: 0.03-0.50%,
V: 0.03-0.50%,
Cu: 0.05 to 0.50%, and
Ni: A method for producing a nitrided part containing one or more selected from the group consisting of 0.05 to 0.50%. It is a manufacturing method given in any 1 paragraph of Claims 1-3,
The chemical composition of the material is
A method for manufacturing a nitrided part containing Ca: 0.0001 to 0.005%. A nitriding steel,
% By mass
C: 0.25 to 0.65%,
Si: 0.03 to 0.5%,
Mn: 1.3 to 2.7%,
P: 0.05% or less,
S: 0.005-0.10%,
Cr: 0.05-0.60%,
N: 0.003 to 0.025%,
Al: 0.05% or less,
Ti: 0 to 0.05%,
Nb: 0 to 0.05%,
Mo: 0 to 0.50%,
V: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%, and
Ca: 0 to 0.005% is contained, the balance consists of Fe and impurities,
Having a chemical composition satisfying formula (1) and formula (2),
The area fraction of pearlite in the structure of the nitriding steel material is higher than 50.0%,
In the structure of the steel for nitriding, the ratio of the total area of the cementite with a length of less than 4.0 μm to the total length of cementite in the pearlite colony among the pearlite colonies is 50% or more, A steel for nitriding, wherein the ratio of the pearlite colony to the total area is 40% or more.
0.55 ≦ C + 0.15Si + 0.2 (Mn−1.71S) + 0.1Cr ≦ 1.10 (1)
0.55 ≦ Cr + 0.5Si + 0.35 (Mn−1.71S) −0.3C ≦ 1.30 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2). The steel for nitriding according to claim 5,
The chemical composition of the steel for nitriding is
Ti: 0.001 to 0.05%, and
Nb: a steel material for nitriding containing one or two selected from the group consisting of 0.003 to 0.05%. The nitriding steel material according to claim 5 or 6,
The chemical composition of the steel for nitriding is
Mo: 0.03-0.50%,
V: 0.03-0.50%,
Cu: 0.05 to 0.50%, and
Ni: 0.05 to 0.50%
A steel material for nitriding containing one or more selected from the group consisting of: The steel for nitriding according to any one of claims 5 to 7,
The chemical composition of the steel for nitriding is
Ca: Steel material for nitriding containing 0.0001 to 0.005%.