JP2010070831A - Carbonitrided component made of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbonitrided component made of steel which has excellent bending fatigue strength and surface fatigue strength, and can respond to the requirements of reduction in weight, size and higher stress load of a component. <P>SOLUTION: Regarding the carbonitrided component made of steel, in which the base is composed of steel comprising 0.1 to 0.3% C, 0.05 to 1.5% Si, 0.2 to 1.5% Mn, 0.003 to 0.05% S, 0.5 to 2.5% Cr, 0.20 to 0.8% Mo, 0.01 to 0.05% Al and 0.008 to 0.025% N, and the balance Fe with impurities, and, in the region to a depth of 0.1 mm from the surface, the average C concentration Cs is 0.60 to 0.90%, the average N concentration Ns is 0.15 to 0.35%, Cs+Ns is 0.80 to 1.10%, also, in the case of Cr-(Ns×3.7)≥0, [(1+0.7×Si)×(1+3.3×Mn)×[1+2.2×äCr-(Ns×3.7)+(0.048/Ns)}]×(1+3.0×Mo)≥9], and, in the case of Cr-(Ns×3.7)<0, [(1+0.7×Si)×(1+3.3×Mn)×ä1+2.2×(0.048/Ns)}×(1+3.0×Mo)≥9] are satisfied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel part used by carbonitriding, that is, a “carbonitriding part” made of steel. More specifically, with regard to steel carbonitriding parts such as gears and shafts, which have excellent bending fatigue strength and surface fatigue strength, bending that is significantly superior to that produced by carburizing and quenching, which is the most typical surface hardening treatment. The present invention relates to a carbonitrided steel part having fatigue strength and surface fatigue strength.

  Conventionally, steel parts such as gears and shafts of automobiles and industrial machines (hereinafter, “steel parts” are also simply referred to as “parts”) are mechanical structural alloys such as JIS standard SCr420, SCM420 and SNCM420. Carburizing or carbonitriding and quenching using steel as a raw material (hereinafter, carburizing and quenching is referred to as “carburizing and quenching”, and carbonitriding and quenching is referred to as “carbonitriding and quenching”), and then 200 Securing the properties required for each component such as contact fatigue strength, bending fatigue strength, and wear resistance by tempering at a temperature of ℃ or less, and by applying chemical conversion coating treatment and shot peening treatment as necessary. Has been made.

  However, in recent years, parts have become lighter and smaller in size in order to improve the fuel efficiency of automobiles and increase the output of engines, and accordingly, the load on the parts tends to increase. For this reason, there is a great demand from the industry to increase contact fatigue strength and bending fatigue strength among the above characteristics.

  Note that the above "contact fatigue" includes "face fatigue", "line fatigue" and "point fatigue", but in practice there is almost no "line" contact or "point" contact. In addition, “surface fatigue strength” is handled as contact fatigue strength.

  “Pitching” is one of the forms of fracture of surface fatigue, and the form of damage of surface fatigue on the tooth surface of the gear and the shaft is mainly pitching. For this reason, improving the pitching strength corresponds to the improvement of the above-mentioned surface fatigue strength. Therefore, “pitting” as “surface fatigue” will be described below. "

In order to improve the bending fatigue strength and surface fatigue strength of parts,
・ Carburizing and quenching,
-Controlling the C concentration or “C + N” concentration of the surface layer of parts by carburizing or carbonitriding and quenching,
-In the middle of carburizing and quenching or carbonitriding and quenching, the carbide is dispersed by cooling to a temperature of A ₁ point or less.
For example, Patent Documents 1 to 4 propose a technique related to a steel part excellent in bending fatigue strength and surface fatigue strength and a manufacturing method thereof.

  Specifically, the carburizing which prescribes | regulates the chemical composition of steel to patent document 1, and the sum of C and N density | concentration from the surface to the depth of 0.1 mm is 0.70-1.30 weight%. A “high surface pressure mechanical structure member” characterized by being subjected to a nitriding treatment or a carbonitriding treatment is disclosed.

  Patent Document 2 discloses a “high-strength gear” characterized by defining the nitrogen content from the surface to a depth of at least 150 μm and defining the microstructure in that region.

  Patent Document 3 discloses a “carbonitriding component” characterized by defining the chemical composition of steel and defining the amount of C, N, etc. from the surface to 0.1 mm.

  Patent Document 4 defines “Chemical composition” and “Carburizing steel” characterized in that carbide is finely dispersed in the carburized layer on the surface by high-concentration carburizing at a surface carbon concentration of 0.8 to 3.0%. Is disclosed.

JP-A 63-62859 Japanese Patent Laid-Open No. 7-190173 JP 2001-73072 A JP 2002-266053 A

  In the technique proposed in Patent Document 1, the individual concentrations of C and N in the vicinity of the surface are not optimized, and furthermore, the chemical component does not correspond to the N concentration in the vicinity of the surface. For this reason, the surface fatigue strength and the bending fatigue strength are insufficient.

  In the technique proposed in Patent Document 2, the C concentration and “C + N” concentration in the vicinity of the surface are not optimized, and furthermore, the chemical component does not correspond to the N concentration in the vicinity of the surface. For this reason, the surface fatigue strength and the bending fatigue strength are insufficient.

  Since the technique proposed in Patent Document 3 has a high “C + N” concentration in the vicinity of the surface, the amount of retained austenite in the parts subjected to carbonitriding or quenching and tempering after carbonitriding becomes very large. For this reason, it is necessary to perform shot peening as described in the embodiments. However, there are many parts that are difficult to perform shot peening in actual parts, and there is a concern that the bending fatigue strength may decrease.

Technique proposed in Patent Document 4, looking at the examples, after carburizing in order to carbides finely dispersed, once cooled to a temperature below ₁ point A, after heated and held for more than _a point again A, Quenching and heat treatment distortion increases. Further, since it is considered that a carburized abnormal layer is generated, in order to obtain a high bending fatigue strength, a polishing finish is required after quenching and tempering as described in Examples.

  The techniques disclosed in Patent Documents 1 to 4 described above are techniques that can increase the surface fatigue strength and the bending fatigue strength of steel parts, as shown in each example. However, none of these techniques can achieve both the fatigue strength and the cost reduction of parts that can cope with the weight reduction, size reduction, and high stress load of parts recently requested by the industry.

  Therefore, the object of the present invention is to have a bending fatigue strength and a surface fatigue strength that are significantly superior to those produced by carburizing and quenching, which is the most representative surface hardening treatment, without significantly increasing the cost. It is to provide a carbonitriding part made of steel that can meet demands for weight reduction, size reduction, and high stress load.

  In order to solve the above-mentioned problems, the present inventors have conducted research and research focusing on making the vicinity of the surface appropriate hardness, C, N concentration to make a microstructure, and chemical components corresponding thereto. Repeated. As a result, the following findings (a) to (e) were obtained.

  (A) Compared with carburizing and quenching, carbonitriding and quenching are effective in improving surface fatigue strength, but there is an appropriate range for the amount of N, and if it is excessive, a large amount of retained austenite is generated and hardness decreases. Therefore, the bending fatigue strength decreases.

  (B) In the case of carbonitriding and quenching, in order to achieve both surface fatigue strength and bending fatigue strength, it is not sufficient to manage the “C + N” amount of the carbonitrided layer. For example, if the amount of C is low, sufficient hardness cannot be obtained after quenching. The amount of N is as described in (a) above. Therefore, it is necessary to control the C amount, the N amount, and the “C + N” amount.

  (C) When carbonitriding, N is likely to combine with Cr in the dough to produce CrN, and in particular, a large amount of CrN is produced at the prior austenite grain boundaries. Therefore, the solid solution Cr concentration decreases in the vicinity of the prior austenite grain boundary, and a soft layer such as pearlite is formed along the prior austenite grain boundary, so both the surface fatigue strength and the bending fatigue strength are decreased.

  (D) In order to suppress the formation of a soft layer such as pearlite along the prior austenite grain boundary, the amount of CrN to be generated is predicted, and even if CrN is generated, the soft layer is not generated along the prior austenite grain boundary. Thus, what is necessary is just to adjust the quantity of Si, Mn, Cr, and Mo with the big contribution to hardenability on the assumption of the production | generation of CrN.

  (E) In order to enhance the effect of carbonitriding and quenching, it is effective to contain an appropriate amount of Mo.

  The present invention has been completed based on the above findings, and the gist thereof is a carbonitriding component made of steel shown in the following (1) to (3).

  (1) Material | dough is mass%, C: 0.1-0.3%, Si: 0.05-1.5%, Mn: 0.2-1.5%, S: 0.003-0 0.05%, Cr: 0.5-2.5%, Mo: 0.20-0.8%, Al: 0.01-0.05% and N: 0.008-0.025% The balance is a steel material composed of Fe and impurities. In the region from the surface to a depth of 0.1 mm, the average C concentration Cs is 0.60 to 0.90%, and the average N concentration Ns is 0.15 to 0. .35%, Cs + Ns is 0.80 to 1.10%, and the value of X defined by the following formula (A) or (B) is 9.0 or more. Carbonitriding parts.

When Cr- (Ns × 3.7) ≧ 0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × [1 + 2.2 × {Cr− (Ns × 3.7) + (0.048 / Ns)}] × (1 + 3.0 × Mo )···(I)
When Cr- (Ns × 3.7) <0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × {1 + 2.2 × (0.048 / Ns)} × (1 + 3.0 × Mo) (B)
In addition, Cr, Si, Mn, and Mo in each of the above formulas represent the content in mass% of the element in the steel material of the dough.

  (2) The steel material of the material contains one or more of Ti: 0.10% or less, Nb: 0.08% or less, and V: 0.15% or less, instead of part of Fe. The steel carbonitriding component according to (1) above, characterized in that there is.

  (3) The material steel material contains at least one of Ca: 0.003% or less, Mg: 0.003% or less, and rare earth elements: 0.02% or less, instead of part of Fe The carbonitriding part made of steel according to the above (1) or (2), characterized in that:

  The “carbonitriding component” refers to a component that has been subjected to carbonitriding. Further, “concentration” represents the content in mass%.

  The “rare earth element” in the present invention refers to a total of 17 elements of Sc, Y and lanthanoid. The “rare earth element content” refers to the “total content of rare earth elements”. In the following description, the rare earth element is referred to as “REM”.

  Since the steel carbonitrided parts of the present invention have excellent bending fatigue strength and surface fatigue strength, they can be used for gears and shafts of automobiles and industrial machines.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each component element in the steel material of the material and the concentration of the element on the part surface means “mass%”.

(A) About chemical composition of steel material of dough:
C: 0.1 to 0.3%
C is an essential element for ensuring the strength (core strength) of the material of the component when carbonitrided and quenched. However, if the content is less than 0.1%, the above effect is insufficient. On the other hand, if the content of C exceeds 0.3%, the strength after steel bar, wire, and hot forging becomes too high, so that the machinability is greatly reduced. Therefore, the C content in the steel material is set to 0.1 to 0.3%. In addition, the minimum with preferable C content is 0.20%, and a preferable upper limit is 0.25%.

Si: 0.05 to 1.5%
Si has the effect of increasing hardenability and temper softening resistance. However, if the content is less than 0.05%, the above effects are insufficient. On the other hand, if the Si content exceeds 1.5%, the strength after steel bar, wire, and hot forging becomes too high, so that the machinability is greatly reduced. Therefore, the Si content in the steel material is set to 0.05 to 1.5%. Note that if the Si content is 0.4% or more, there is an effect of increasing the surface fatigue strength, so the lower limit of the Si content is preferably 0.4%. In addition, the more preferable minimum of Si content is 0.5%, and a preferable upper limit is 0.8%.

Mn: 0.2 to 1.5%
Mn is an element effective in increasing the bending fatigue strength and the surface fatigue strength because it has the effect of increasing the hardenability. However, if the content is less than 0.2%, the above effect is insufficient. When the Mn content is 0.4% or more, the bending fatigue strength and the surface fatigue strength are significantly improved. On the other hand, if the content of Mn exceeds 1.5%, not only the effect of increasing the bending fatigue strength and the surface fatigue strength is saturated, but also the strength after steel bar, wire rod and hot forging becomes too high. The performance is greatly reduced. Therefore, the Mn content in the steel material is set to 0.2 to 1.5%. The lower limit of the Mn content is preferably 0.4%. In addition, the more preferable minimum of Mn content is 0.8%, and a preferable upper limit is 1.2%.

S: 0.003-0.05%
S combines with Mn to form MnS and improves the machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, when the S content increases, coarse MnS tends to be generated, and the bending fatigue strength and the surface fatigue strength tend to be reduced. In particular, when the content exceeds 0.05%, the bending fatigue strength is increased. In addition, the decrease in surface fatigue strength becomes remarkable. Therefore, the S content in the steel material is set to 0.003 to 0.05%. In addition, the minimum with preferable S content is 0.01%, and a preferable upper limit is 0.03%.

Cr: 0.5 to 2.5%
Cr has an effect of increasing hardenability and temper softening resistance, and is an element effective for increasing bending fatigue strength and surface fatigue strength. However, if the content is less than 0.5%, the above effect is insufficient. When the Cr content is 1.2% or more, the bending fatigue strength and the surface fatigue strength are significantly improved. On the other hand, if the Cr content exceeds 2.5%, not only the effect of increasing bending fatigue strength and surface fatigue strength is saturated, but also the strength after steel bar, wire rod and hot forging becomes too high, so that cutting work Remarkably deteriorates. Therefore, the Cr content in the steel material is set to 0.5 to 2.5%. The lower limit of the Cr content is preferably 1.2%. A more preferable lower limit of the Cr content is 1.4%, and a preferable upper limit is 2.0%.

Mo: 0.20 to 0.8%
Mo has an effect of increasing hardenability and temper softening resistance, and is an effective element for increasing bending fatigue strength and surface fatigue strength. If its content is 0.20% or more, it is the most surface hardening treatment. Bending fatigue strength and surface fatigue strength, which are significantly superior to those produced by typical carburizing and quenching, can be ensured stably and reliably. On the other hand, if the Mo content exceeds 0.8%, not only the effect of increasing the bending fatigue strength and surface fatigue strength is saturated, but the strength after steel bar, wire rod and hot forging becomes too high, so that cutting work The performance is greatly reduced. Therefore, the Mo content in the steel material is set to 0.20 to 0.8%. In addition, the minimum with preferable Mo content is 0.3%, and a preferable upper limit is 0.5%.

Al: 0.01 to 0.05%
Al has a deoxidizing action, and at the same time, easily binds to N to form AlN, is effective for refining crystal grains in the quenched portion, and has an effect of increasing bending fatigue strength. However, this effect is difficult to obtain when the Al content is less than 0.01%. On the other hand, Al tends to form hard oxide inclusions, and if the Al content exceeds 0.05%, the bending fatigue strength is significantly reduced, and the desired bending can be achieved even if other requirements are satisfied. Fatigue strength cannot be obtained. Therefore, the content of Al in the steel material is set to 0.01 to 0.05%. In addition, the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.04%.

N: 0.008 to 0.025%
N easily binds to Al, Ti, Nb, and V to form AlN, TiN, NbN, and VN. Of these, AlN, NbN, and VN are effective in refining crystal grains and have an effect of increasing bending fatigue strength . However, this effect is difficult to obtain when the N content is less than 0.008%. On the other hand, if the N content exceeds 0.025%, coarse TiN is likely to be formed, so that the bending fatigue strength is significantly reduced, and the desired bending fatigue strength can be obtained even if other requirements are satisfied. It becomes impossible. Therefore, the N content in the steel material is set to 0.008 to 0.025%. In addition, the minimum with preferable N content is 0.012%, and a preferable upper limit is 0.020%.

  One of the base steel materials of the carbonitrided component of the present invention has a chemical composition in which the balance is composed of Fe and impurities in addition to the above elements. In addition, it is preferable to restrict | limit the content of P and O (oxygen) as an impurity as follows.

P: 0.025% or less P is an element that easily segregates at the grain boundaries and embrittles the grain boundaries. Therefore, if the content exceeds 0.025%, even if other requirements are satisfied, the frequency is low. Although there is a case where the bending fatigue strength is reduced. Therefore, the P content in the steel material is preferably 0.025% or less. A more preferable upper limit of the P content is 0.018%.

O (oxygen): 0.002% or less O tends to combine with Al to form hard oxide inclusions. In particular, when the O content exceeds 0.002%, other requirements are satisfied. Even if it is, the bending fatigue strength may be reduced although the frequency is low. Therefore, the O content in the steel material is preferably 0.002% or less. Furthermore, it is desirable to reduce the content of O as an impurity as much as possible, but considering the cost of steel making, it is more preferable to make it 0.0010% or less.

  Another chemical composition of the steel material of the carbonitrided component according to the present invention further includes one or more elements selected from Ti, Nb, V, Ca, Mg and REM in addition to the above elements. To do. Below, the effect of these elements and the reason for limitation of content are demonstrated.

  First, Ti, Nb, and V all have the effect of supplementing the grain refinement of the quenched portion with AlN and increasing the bending fatigue strength. For this reason, when ensuring higher bending fatigue strength, you may contain in the following ranges.

Ti: 0.10% or less Ti is easy to form TiC, TiN, Ti (C, N) by combining with C and N, and is effective in supplementing the above-described grain refinement of the quenched portion by AlN. Since it has an action of increasing the bending fatigue strength, Ti may be contained in order to obtain such an effect. However, if the Ti content increases and exceeds 0.10%, coarse TiN is likely to be generated, and the bending fatigue strength is decreased. Therefore, the Ti content in the steel material is set to 0.10% or less. The Ti content is preferably 0.06% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the bending fatigue strength of Ti, the lower limit of the Ti content is preferably 0.01%, and more preferably 0.02%.

Nb: 0.08% or less Nb is easy to form NbC, NbN, Nb (C, N) by combining with C and N, and is effective in supplementing the above-described grain refinement of the quenched portion by AlN. Since it has an action of increasing the bending fatigue strength, Nb may be contained in order to obtain such an effect. However, if the Nb content increases and exceeds 0.08%, coarse Nb (C, N) is likely to be generated, and the bending fatigue strength is decreased. Therefore, the Nb content in the steel material is set to 0.08% or less. The Nb content is preferably 0.05% or less.

  On the other hand, in order to reliably obtain the effect of improving the bending fatigue strength and surface fatigue strength of Nb described above, the lower limit of the Nb content is preferably 0.01%, and more preferably 0.02%.

V: 0.15% or less V is easy to form VN and VC by combining with C and N, and among these, VN is effective in complementing the above-described grain refinement of the quenched portion by AlN. Has the effect of increasing the bending fatigue strength. Moreover, when VN precipitates during carbonitriding, there is an effect of further increasing the bending fatigue strength. For this reason, in order to acquire the effect mentioned above, you may contain V. However, if the content of V increases and exceeds 0.15%, the strength after steel bar, wire rod, and hot forging becomes too high, so that the machinability is greatly reduced. Therefore, the V content in the steel material is set to 0.15% or less. The V content is preferably 0.10% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the bending fatigue strength of V, the lower limit of the V content is preferably 0.02%, and more preferably 0.05%.

  In addition, said Ti, Nb, and V can be contained only in any 1 type in them, or 2 or more types of composites.

  Next, Ca, Mg, and REM all have the effect of increasing the bending fatigue strength. For this reason, when you want to provide higher bending fatigue strength, you may contain in the following ranges.

Ca: 0.003% or less Ca has an effect of increasing bending fatigue strength. Furthermore, Ca also has the effect | action which improves cutting workability. For this reason, in order to acquire the effect mentioned above, you may contain Ca. However, even if Ca is contained in excess of 0.003%, the above effect is saturated and the cost is increased. Therefore, the Ca content in the steel material is set to 0.003% or less.

  On the other hand, in order to reliably obtain the above-described effects of improving the bending fatigue strength and cutting workability of Ca, the lower limit of the Ca content is preferably set to 0.0003%.

Mg: 0.003% or less Mg has an action of increasing bending fatigue strength. Furthermore, Mg also has the effect | action which improves cutting workability. For this reason, in order to acquire the effect mentioned above, you may contain Mg. However, even if Mg is contained in excess of 0.003%, the above effect is saturated and the cost is increased. Therefore, the Mg content in the steel material is set to 0.003% or less.

  On the other hand, in order to surely obtain the effect of improving the bending fatigue strength and cutting workability of Mg described above, the lower limit of the Mg content is preferably set to 0.0003%.

REM: 0.02% or less REM has the effect of increasing the bending fatigue strength. For this reason, in order to acquire the effect mentioned above, you may contain REM. However, even if REM is contained in excess of 0.02%, the above effect is saturated and the cost is increased. Therefore, the REM content in the steel material is set to 0.02% or less. In addition, it is preferable that content of REM shall be 0.01% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the bending fatigue strength of REM, the lower limit of the REM content is preferably 0.0003%, and more preferably 0.001%.

  As described above, “REM” in the present invention refers to a total of 17 elements of Sc, Y, and lanthanoid, and “REM content” refers to “total content of REM”.

  In addition, said Ca, Mg, and REM can be contained only in any 1 type in them, or 2 or more types of composites.

(B) Concentration of C and N in the surface layer portion:
According to the study by the present inventors, the carbonitriding component made of steel according to the present invention is in a region from the surface to a depth of 0.1 mm,
The average C concentration Cs is 0.60 to 0.90%,
The average N concentration Ns is 0.15 to 0.35%,
Cs + Ns is 0.80 to 1.10%,
It became clear that it had to be.

  Hereinafter, the above items will be described in detail.

  It is known that surface fatigue strength and bending fatigue strength are greatly affected by hardness in the vicinity of the surface, temper softening resistance, structure, etc. Conventionally, carbonitriding has been particularly effective in increasing surface fatigue strength by increasing temper softening resistance. It has been said that it is effective in improving

  Certainly, carbonitriding is an effective means for improving the surface fatigue strength, but it is not very effective for other strength properties such as bending fatigue strength, but rather it may be lowered. For this reason, simply performing carbonitriding is not compatible with ensuring the fatigue strength that can meet the demands of the industry, such as weight reduction, downsizing, and high stress loading, and not increasing the cost of parts so much. It is enough.

  Therefore, the present inventors have studied the optimization of the C and N amount of the surface layer by carbonitriding, and the chemical composition of the steel material suitable for it, and the soft structure in the vicinity of the grain boundary, which is likely to be the starting point of fatigue fracture. The following studies were conducted from a different viewpoint from the viewpoint of improving not only the surface fatigue strength but also the bending fatigue strength by increasing the hardness and softening resistance of the surface layer part while suppressing the formation reliably. It was.

  First, steel α and steel β satisfying the chemical composition of the steel material of the dough described in the section (A) shown in Table 1 were melted in a 50 kg vacuum melting furnace, and then cast into an ingot.

  Each ingot after casting was once cooled to room temperature, then heated again at 1250 ° C. for 30 minutes, and hot forged at a finishing temperature of 950 ° C. or higher to obtain a round bar having a diameter of 35 mm.

  Next, each round bar having a diameter of 35 mm was subjected to a treatment that was held at 920 ° C. for 1 hour and allowed to cool to room temperature in the air. Thereafter, a test piece having a diameter of 26 mm and a length of 240 mm was produced by machining from the center of the round bar in parallel with the forging axis.

  Said test piece is carbonitriding and quenching (treatment condition symbols B to M shown in Table 2) or carburizing and quenching (treatment condition symbol A shown in Table 2) under the conditions shown in FIG. 1 and Table 2 using a gas carburizing furnace. Then, after tempering at 170 ° C. for 1.5 hours, the test piece was divided into three equal parts on a plane perpendicular to the axial direction of the test piece. Here, “CP” in FIG. 1 and Table 2 means carbon potential. The reason why tempering was performed at 170 ° C. is that the general tempering temperature after carbonitriding is 160 to 180 ° C.

  Using one of the test pieces divided into three in this manner, chips were collected by lathe processing for a region from the surface to a depth of 0.1 mm (hereinafter also referred to as “surface layer portion”). The contents of C and N were measured by general chemical analysis, and the average C concentration (Cs) and the average N concentration (Ns) in the surface layer portion were obtained.

  Moreover, tempering was further performed at 300 ° C. for 1 hour using another one of the test pieces divided into three. It should be noted that tempering at 300 ° C. may cause the temperature to rise significantly above 200 ° C. near the contact portion of the carbonitrided part, and the hardness of the surface layer portion after tempering at 300 ° C. is the surface fatigue strength. This is because the correlation is large.

  Using the remaining one of the test pieces divided into three equal parts after tempering at 170 ° C. and the test piece further tempered at 300 ° C. described above, the Vickers hardness in the vicinity of the surface was measured according to “JIS Z 2244 (2003)”. Based on “Vickers hardness test—test method”, the measurement was performed by the following method.

  That is, mirror polishing was performed so that the cut surface divided in half by the plane perpendicular to the axial direction of the test piece becomes the test surface, and the test was conducted at the depth of 0.05 mm and 0.10 mm from the outermost surface of the test part. The force was measured at 1.961 N at 5 locations, and the average was calculated as the Vickers hardness of the surface layer.

  Table 3 shows the above-mentioned surface layer analysis results and Vickers hardness (Hv) measurement results. 2 to 7 collectively show the relationship between the average C concentration (Cs), average N concentration (Ns), Cs + Ns, and Vickers hardness of the surface layer portion.

  In addition, in this invention, it aims at exceeding the surface fatigue strength and bending fatigue strength of gas carburized components. For this reason, the surface fatigue strength and the rotational bending fatigue strength are 20% or more of the surface fatigue strength of test number 1 in Table 8 to be described later, which is a gas carburized product of steel satisfying JIS standard SCM420, and The target was 600 MPa, which is 20% or more higher than the rotational bending fatigue strength. In order to achieve this goal, as shown in Table 8, the Vickers hardness (Hv) of the 170 ° C. tempered material is 765 or more, and the Vickers hardness (Hv) of the 300 ° C. tempered material is 710 or more. Since it is necessary, the target Vickers hardness was 765 or higher for the 170 ° C. tempered material and 710 or higher for the 300 ° C. tempered material.

  2-7, there is an example which achieves said target hardness, the average C density | concentration (Cs) in the surface layer part, ie, the area | region from the surface to a depth of 0.1 mm, is 0.60-0. It can be seen that 90%, the average N concentration (Ns) is 0.15 to 0.35%, and Cs + Ns is 0.80 to 1.10%.

  Therefore, looking at the results in Table 3, the average C concentration (Cs) in the surface layer portion is 0.60 to 0.90%, the average N concentration (Ns) is 0.15 to 0.35%, and Cs + Ns is 0. It can be seen that when all of .80 to 1.10% are satisfied, the target hardness is always satisfied.

  From the above, in the region from the surface to a depth of 0.1 mm, the average C concentration Cs is 0.60 to 0.90%, the average N concentration Ns is 0.15 to 0.35%, and Cs + Ns is 0. It was specified to be 80 to 1.10%.

(C) About chemical composition of steel material of dough and N concentration of surface layer part:
According to the study by the present inventors, it is clear that the value of X defined by the following formula (A) or (B) must be 9.0 or more in the steel carbonitriding component according to the present invention. Became.

When Cr- (Ns × 3.7) ≧ 0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × [1 + 2.2 × {Cr− (Ns × 3.7) + (0.048 / Ns)}] × (1 + 3.0 × Mo )···(I)
When Cr- (Ns × 3.7) <0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × {1 + 2.2 × (0.048 / Ns)} × (1 + 3.0 × Mo) (B)
In addition, Cr, Si, Mn, and Mo in each of the above formulas represent the content in mass% of the element in the steel material of the dough.

  Hereinafter, the above items will be described in detail.

  In order to clarify the influence of chemical components on the structure of the surface layer after carbonitriding, the present inventors conducted the following test.

  That is, steels a to k that satisfy the chemical composition of the steel material of the dough described in the section (A) shown in Table 4 were melted in a 50 kg vacuum melting furnace and then cast into an ingot.

  Each ingot after casting was once cooled to room temperature, then heated again at 1250 ° C. for 30 minutes, and hot forged at a finishing temperature of 950 ° C. or higher to obtain a round bar having a diameter of 35 mm.

  Next, each round bar having a diameter of 35 mm was subjected to a treatment that was held at 920 ° C. for 1 hour and allowed to cool to room temperature in the air. Thereafter, eight ono-type rotary bending fatigue test pieces with notches having a shape shown in FIG. 8 were produced by machining from the center of the round bar in parallel to the forging axis. The unit of the dimension in FIG. 8 is “mm”.

  The above test piece was subjected to carbonitriding and quenching (treatment condition symbols F and G shown in Table 2) using the gas carburizing furnace under the conditions shown in FIG. 1 and Table 2, and then at 170 ° C. for 1.5 hours. Tempering was performed.

  For each one of the Ono-type rotating bending fatigue test pieces with notches obtained in this way, chips were collected by lathe processing in the area from the surface of the grip part to a depth of 0.1 mm, that is, the surface layer part. Then, the contents of C and N were measured by a general chemical analysis, and the average C concentration (Cs) and the average N concentration (Ns) in the surface layer portion were obtained.

  Table 5 shows the results of the surface layer analysis.

  As shown in Table 5, the average C concentration (Cs) was 0.60 to 0.90 in the region from the surface to the depth of 0.1 mm as described in the item (B). %, The average N concentration (Ns) was 0.15 to 0.35%, and Cs + Ns was 0.80 to 1.10%.

Therefore, the remaining seven notched Ono-type rotating bending fatigue test pieces of each steel were subjected to the finishing process of the grip part for the purpose of removing heat treatment strain, and then subjected to the Ono-type rotating bending fatigue test at room temperature. did. The test conditions were a rotational speed 3000 rpm, others as a normal method, was the highest stress of that did not break up repetitive number 1.0 × 10 ⁷ times as "rotary bending fatigue strength".

  The rotational bending fatigue strength thus obtained is also shown in Table 5.

  From Table 5, the average C concentration (Cs) is 0.60 to 0.90% and the average N concentration (Ns) is in the region from the surface to the depth of 0.1 mm described in the section (B). Even when the conditions of 0.15 to 0.35% and Cs + Ns satisfy the conditions of 0.80 to 1.10%, it has become clear that the rotational bending fatigue strength may not achieve the target value of 600 MPa described above. .

  Therefore, the mirror surface is such that the cut surface of the notched portion of the Ono-type rotating bending fatigue test piece with notches used for collecting chips at the grip portion is divided into two equal parts by a plane parallel to the axial direction. After polishing and corroding with nital, the vicinity of the surface layer of the test piece was observed at a magnification of 400 times with an optical microscope.

  As a result, in the test piece in which the rotational bending fatigue strength did not reach the target of 600 MPa as described above, a portion exhibiting a darker contrast than the martensite structure was observed along the prior austenite grain boundary.

  Therefore, as a result of further observation using a scanning electron microscope at a magnification of 5000, it was found that the portion exhibiting this dark contrast was a pearlite structure or a bainite structure. In addition, since the above pearlite structure and bainite structure are considered to be softer than the martensite structure, suppressing the formation of a soft structure generated in the vicinity of the prior austenite grain boundary increases the rotational bending fatigue strength. It was found that it is important to improve the hardenability in the vicinity of the prior austenite grain boundaries.

Here, it is known that the hardenability of steel can be estimated by a critical diameter when ideal quenching, that is, D _I , and D _I can be estimated from a chemical component or the like.

  In addition, since CrN is easily generated at the grain boundary of the steel surface layer when carbonitriding, and a Cr-deficient phase is formed in the vicinity of the grain boundary, in order to reliably prevent the generation of a soft phase in the vicinity of the grain boundary, Even if CrN is generated to the maximum extent, it is necessary to ensure a predetermined hardenability.

  From the ratio of the atomic weight of Cr and N, the maximum amount of Cr combined with N having an average nitrogen concentration (Ns) in the surface layer portion is [3.7 × Ns]%. However, at a quenching temperature of general carbonitriding of 830 to 870 ° C., CrN can be dissolved to some extent in austenite as a matrix, and the amount is estimated from the following formula (c) which is a solubility product of CrN. Therefore, it is necessary to subtract this solute Cr amount.

log [Cr] [N] = (6095 / T) +4.11 (C)
In the formula (C), T represents a temperature in absolute temperature (K) units.

  Substituting 850 ° C. (1123 K), which is the quenching temperature at the time of carbonitriding, into the above formula (c), [Cr] [N] = 0.048.

Therefore, the lower limit Cr _g of the Cr amount near the grain boundary is expressed by the following equation,
When Cr- (Ns × 3.7) ≧ 0:
Cr _g = Cr− (Ns × 3.7) + (0.048 / Ns) (d)
When Cr- (Ns × 3.7) <0:
Cr _g = (0.048 / Ns) (e)
It can be obtained from the following formula (d) or formula (e).

A synergistic method to estimate D _I which is generally known from the above, the combination of the lower limit of the Cr content near the grain boundary mentioned above, Formula (I) or formula (B) is obtained.

When Cr- (Ns × 3.7) ≧ 0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × [1 + 2.2 × {Cr− (Ns × 3.7) + (0.048 / Ns)}] × (1 + 3.0 × Mo )···(I),
When Cr- (Ns × 3.7) <0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × {1 + 2.2 × (0.048 / Ns)} × (1 + 3.0 × Mo) (B).

  In Table 5, together with the value of [Cr- (Ns × 3.7)], the value of X obtained from the formula (A) or the formula (B) is also shown.

  FIG. 9 shows the relationship between the rotational bending fatigue strength and the value of X obtained from the formula (A) or the formula (B).

  From FIG. 9, it is clear that when the value of X is less than 9.0, the rotational bending fatigue strength is greatly reduced and the target of 600 MPa cannot be achieved.

  From the above, it is defined that the value of X defined by the above formula (A) or (B) is 9.0 or more.

  The steel carbonitriding component of the present invention may be carbonitrided using a gas carburizing furnace, a vacuum carburizing furnace (vacuum carburizing furnace) or the like.

  In the case of a carbonitriding part having a complicated shape, the average C concentration (Cs) and the average N concentration (Ns) in the surface layer part and the region from the surface to a depth of 0.1 mm are measured by an electron probe microanalysis ( EPMA) may be measured by line analysis.

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

  Steels 1 to 15 having chemical compositions shown in Table 6 were melted in a 50 kg vacuum melting furnace and then cast to obtain an ingot. In addition, steel 1-7 in Table 5 and steel 9-15 are steel which exists in the chemical composition range of the steel material of the dough prescribed | regulated by this invention. On the other hand, the steel 8 is a steel of a comparative example in which the Al content deviates from the chemical composition conditions of the steel material of the fabric defined in the present invention.

  Each ingot after casting was once cooled to room temperature, then heated again at 1250 ° C. for 30 minutes, and hot forged at a finishing temperature of 950 ° C. or higher to obtain a round bar having a diameter of 35 mm.

  Next, each round bar having a diameter of 35 mm was subjected to a treatment that was held at 920 ° C. for 1 hour and allowed to cool to room temperature in the air. Thereafter, a test piece (hereinafter, referred to as “round bar test piece”) having a diameter of 26 mm and a length of 240 mm by machining, parallel to the forging axis from the center of the round bar, and a notch having the shape shown in FIG. An attached Ono type rotating bending fatigue test piece and a roller for roller pitching test having the shape shown in FIG. 10 were prepared. The unit of the dimension in FIG. 10 is “mm”.

  Each of the above test pieces was carbonitrided and quenched (treatment condition symbols C to I, K, and M shown in Table 2) or carburized and quenched (shown in Table 2) under the conditions shown in FIG. 1 and Table 2 using a gas carburizing furnace. Treatment condition symbol A) was performed, followed by tempering at 170 ° C. for 1.5 hours.

  In addition, about the round bar test piece which tempered at said 170 degreeC for 1.5 hours, it divides into 3 equal parts in a surface perpendicular | vertical to an axial direction, and uses one of the test pieces divided into 3 parts. For the surface layer portion, which is a region from the surface to a depth of 0.1 mm, chips are collected by lathe processing, the contents of C and N are measured by general chemical analysis, and the average C concentration in the surface layer portion (Cs) and average N concentration (Ns) were determined.

  Moreover, tempering was further performed at 300 ° C. for 1 hour using another one of the test pieces obtained by dividing the round bar test piece into three equal parts.

  The Vickers hardness in the vicinity of the surface was measured according to JIS Z 2244 by using the remaining one of the test pieces obtained by dividing the round bar test piece into three equal parts after tempering at 170 ° C. and the test piece further tempered at 300 ° C. Based on "Vickers hardness test-test method" in (2003), the measurement was performed by the following method.

  That is, mirror polishing was performed so that the cut surface divided in half by the plane perpendicular to the axial direction of the test piece becomes the test surface, and the test was conducted at the depth of 0.05 mm and 0.10 mm from the outermost surface of the test part. The force was measured at 1.961 N at 5 locations, and the average was calculated as the Vickers hardness of the surface layer.

  The roller pitting test small roller tempered at 170 ° C and the Ono rotary bending fatigue test piece with notch were subjected to the roller pitching test and room temperature after finishing the grip part for the purpose of eliminating heat treatment strain, respectively. It was subjected to an Ono type rotating bending fatigue test.

  The roller pitching test was carried out under the conditions shown in Table 7 with a combination of the above-mentioned small roller pitching test roller and the large roller pitching test roller having the shape shown in FIG. The unit of the dimension in FIG. 11 is “mm”.

  The large roller for the roller pitching test is made of a steel that satisfies the standard of JIS SCM420, which is a general manufacturing process, that is, “normalization → test piece processing → eutectoid carburization by gas carburizing furnace → low temperature tempering → polishing The Vickers hardness Hv at a position of 0.05 mm from the surface, that is, at a depth of 0.05 mm, is 740 to 760, and the depth of the Vickers hardness Hv is 550 or more. Was in the range of 0.8 to 1.0 mm.

For each test number, the number of tests in the roller pitching test was set to 5, an SN graph was created with the surface pressure on the vertical axis and the number of repetitions until the occurrence of pitching on the horizontal axis, and the number of repetitions was 1.0 × 10 ^7. The highest surface pressure from which no pitting occurred was defined as “surface fatigue strength”. In addition, when the area of the largest thing became 1 mm < ² > or more among the places where the surface of the test part of a small roller was damaged, it was set as pitching generation | occurrence | production.

The Ono-type rotating bending fatigue test at room temperature was carried out with the number of tests being 7 each, a rotational speed of 3000 rpm, and others by the usual method, and the highest stress among those that did not break until the number of repetitions of 1.0 × 10 ⁷ times. “Rotating bending fatigue strength”.

  Table 8 summarizes the above test results. In addition, the surface fatigue strength target in the roller pitching test is 20% or more higher than the surface fatigue strength of Test No. 1 carburized and quenched using steel A, and the Ono type rotary bending fatigue strength target is the above The value was 600 MPa, which is 20% or more higher than the rotational bending fatigue strength of Test No. 1. In Table 8, the surface fatigue strength is expressed by assuming the value of test number 1 as “1.00”.

  From Table 8, in the case of test numbers 2 to 8, test number 16 and test number 23 that deviate from the conditions specified in the present invention, the surface fatigue strength in the roller pitching test and the bending fatigue strength in the Ono rotary bending fatigue test It is clear that either or both have not reached their goals.

  In the case of test numbers 9 to 15 and test numbers 17 to 22 that satisfy the conditions specified in the present invention with respect to the above comparative example, the surface fatigue strength in the roller pitching test and the bending fatigue strength in the Ono-type rotary bending fatigue test It is clear that both of them satisfy the target and have significantly better bending fatigue strength and surface fatigue strength than the case of test number 1 manufactured by carburizing and quenching, which is the most typical surface hardening treatment.

  Furthermore, in the test number using steel containing any one or more of Ti, Nb, V, Ca, Mg, and REM, the bending fatigue strength is 630 MPa or more, which is clearly better.

  Since the steel carbonitrided parts of the present invention have excellent bending fatigue strength and surface fatigue strength, they can be used for gears and shafts of automobiles and industrial machines.

It is a figure explaining the heat pattern of carbonitriding quenching or carburizing quenching. The case where the ammonia flow rate G is 0 is carburizing and quenching. It is a figure which shows the relationship between the average C density | concentration (Cs) of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing quenching, and Vickers hardness. It is a figure which shows the relationship between the average C density | concentration (Cs) of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing hardening, and further tempering at 300 degreeC, and Vickers hardness. It is a figure which shows the relationship between the average N density | concentration (Ns) of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing quenching, and Vickers hardness. It is a figure which shows the relationship between the average N density | concentration (Ns) of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing quenching, and further tempering at 300 degreeC, and Vickers hardness. It is a figure which shows the relationship between Cs + Ns of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing quenching, and Vickers hardness. It is a figure which shows the relationship between Cs + Ns of the surface layer part at the time of tempering at 170 degreeC after carbonitriding quenching or carburizing quenching, and further tempering at 300 degreeC, and Vickers hardness. It is a figure which shows the Ono type | formula rotation bending fatigue test piece shape with a notch. The unit of dimension is “mm”. It is a figure which shows the relationship between the rotation bending fatigue strength at the time of tempering at 170 degreeC after carbonitriding and quenching, and the value of X. It is a figure which shows the shape of a roller pitching small roller test piece. The unit of dimension is “mm”. It is a figure which shows the shape of a roller pitching large roller test piece. The unit of dimension is “mm”.

Claims (3)

Dough is mass%, C: 0.1-0.3%, Si: 0.05-1.5%, Mn: 0.2-1.5%, S: 0.003-0.05% , Cr: 0.5-2.5%, Mo: 0.20-0.8%, Al: 0.01-0.05% and N: 0.008-0.025%, the balance being It is a steel material made of Fe and impurities, and in the region from the surface to a depth of 0.1 mm, the average C concentration Cs is 0.60 to 0.90%, and the average N concentration Ns is 0.15 to 0.35%. Cs + Ns is 0.80 to 1.10%, and the value of X defined by the following formula (A) or (B) is 9.0 or more. parts.
When Cr- (Ns × 3.7) ≧ 0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × [1 + 2.2 × {Cr− (Ns × 3.7) + (0.048 / Ns)}] × (1 + 3.0 × Mo )···(I)
When Cr- (Ns × 3.7) <0:
X = (1 + 0.7 × Si) × (1 + 3.3 × Mn) × {1 + 2.2 × (0.048 / Ns)} × (1 + 3.0 × Mo) (B)
In addition, Cr, Si, Mn, and Mo in each of the above formulas represent the content in mass% of the element in the steel material of the dough.   The steel material of the dough contains one or more of Ti: 0.10% or less, Nb: 0.08% or less, and V: 0.15% or less instead of part of Fe. The carbonitriding part made of steel according to claim 1 characterized by the above-mentioned.   The dough steel material contains at least one of Ca: 0.003% or less, Mg: 0.003% or less, and rare earth elements: 0.02% or less, instead of part of Fe. The carbonitriding component made of steel according to claim 1 or 2.

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