JP7151474B2 - Steel for carburized steel parts - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material for carburized steel parts, which is a steel material used for carburized steel parts.

Steel used for machine structural parts generally contains Mn, Cr, Mo, Ni, and the like. Steel materials for carburized steel parts that have chemical compositions containing the above elements and are manufactured through processes such as casting, forging, and rolling are shaped by machining such as forging and cutting, and are further subjected to heat treatment such as carburizing. is applied to obtain a carburized steel part having a carburized layer on the surface and a core portion inside the carburized layer.

Of the costs of manufacturing this carburized steel part, the costs associated with machining are very large. Cutting is not only expensive for cutting tools, but also produces a large amount of chips, which is disadvantageous from the viewpoint of yield. Therefore, attempts have been made to replace cutting with forging. Forging methods can be broadly classified into hot forging, warm forging, and cold forging. Warm forging is characterized by less scale generation and improved dimensional accuracy than hot forging. Cold forging is characterized by no scale generation and dimensional accuracy close to the state after conventional cutting. Therefore, a method of performing rough processing by hot forging and then performing finish processing by cold forging, a method of performing light cutting as a finish after performing warm forging, or a method of performing light cutting as a finish after performing cold forging Methods of carrying out light cutting as a finish, etc. have been investigated. However, when hot forging is replaced with warm forging or cold forging, if the deformation resistance of the steel material for carburized steel parts is high, the contact pressure applied to the die of the forging machine increases, shortening the life of the die. In this case, even if the cutting amount is reduced, the cost advantage is not so large. In addition, when molding into a complicated shape, cracks may occur in portions subjected to extensive processing. Therefore, when carburizing steel parts are manufactured by warm forging or cold forging, it is required to improve the limit workability of steel materials for carburizing steel parts.

International Publication No. 2012/108480 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2012-207244 (Patent Document 2) propose a steel material for carburized steel parts for the purpose of improving cold forgeability (limit workability). .

The carburizing steel described in Patent Document 1 has a chemical composition in mass% of C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001% to 0.80%, S: 0.0001% to 0.100%, Cr: more than 1.30% to 5.00%, B: 0.0005% to 0.0100%, Al: 0.0001% to 1 .0%, Ti: 0.010% to 0.10%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, the balance being Fe and Consists of unavoidable impurities, and the mass % content of each element in the chemical composition satisfies formulas (1) to (3). Here, formulas (1) to (3) are as follows. 0.10<C+0.194×Si+0.065×Mn+0.012×Cr+0.078×Al<0.235 Formula (1), 7.5<(0.7×Si+1)×(5.1×Mn+1)× (2.16×Cr+1)<44 Equation (2), 0.004<Ti−N×(48/14)<0.030 Equation (3). This carburizing steel has the above-mentioned chemical composition, so that the limit workability during cold forging can be increased. Patent Document 1 describes that

The case-hardened steel described in Patent Document 2 has, in mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.1%, Mn: 0.3 to 0.6%, P : 0.03% or less (excluding 0%), S: 0.001-0.02%, Cr: 1.2-2.0%, Al: 0.01-0.1%, Ti: 0 .010 to 0.10%, N: 0.010% or less (not including 0%), B: 0.0005 to 0.005%, the balance being iron and inevitable impurities, equivalent circle diameter 20 nm ^The density of Ti ^- based precipitates less than It is characterized by being 130 HV or less. Patent Document 2 describes that this case-hardened steel is excellent in cold forgeability due to the above-described structure.

WO2012/108480 JP 2012-207244 A

By the way, as described above, cold forging is characterized by dimensional accuracy close to the state after conventional cutting. A processing step is performed. Therefore, for steel materials for carburized steel parts for cold forging applications, it exhibits not only sufficient cold forgeability but also sufficient machinability, So-called chip disposability is required. If the chip disposability is low and long chips are generated, the chips may wind around the steel material or the tool to be processed, reducing the working efficiency of the cutting process. Patent Literatures 1 and 2 do not examine the chip disposability of the steel material after the cold forging process as described above.

An object of the present disclosure is to provide a steel material for carburized steel parts, which has a large critical workability during cold forging and excellent chip disposability during cutting after cold forging.

The steel material for carburized steel parts according to the present disclosure has a chemical composition in mass% of C: 0.11 to 0.15%, Si: 0.17 to 0.35%, Mn: 0.45 to 0.80%. , S: 0.005 to 0.050%, Cr: 1.50 to less than 1.90%, B: 0.0005 to 0.0100%, Al: 0.100 to 0.200%, Ca: 0. 0002 to 0.0030%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, Ti: 0 to less than 0.020%, Nb: 0 to 0.100%, Mo: 0 to 0.500%, Ni: 0 to 0.500%, Cu: 0 to 0.500%, and the balance consists of Fe and impurities, satisfying the formulas (1) to (4), micro In the structure, the total area ratio of ferrite and pearlite is 85.0% or more, and the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to less than 35.0%.
0.100<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.0003<Al×(N-Ti×(14/48))<0.0011 (3)
0.03≦Ca/S≦0.15 (4)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (4).

The steel material for carburized steel parts according to the present disclosure provides a sufficient critical workability during cold forging, and is excellent in chip control during cutting after cold forging.

FIG. 1 is a schematic diagram of the microstructure of the steel material for carburized steel parts according to the present embodiment.

The inventors of the present invention have studied how to improve the critical workability of steel materials for carburized steel parts and improve the machinability (chip disposability) after cold forging. As a result, the inventors obtained the following findings (A) to (G).

(A) The lower the C content, the softer the steel material for carburized steel parts before cold forging can be. However, if the C content is too low, the characteristics (carburized layer depth, core hardness, etc.) of the carburized steel parts after carburizing treatment will be reduced to those of conventional carburized steel parts with a C content of about 0.20%. It becomes difficult to achieve the same level as steel materials (eg, JIS-SCR420). The chemical composition of the steel material for carburized steel parts is C: 0.11 to 0.15%, Si: 0.17 to 0.35%, Mn: 0.45 to 0.80%, S: 0% by mass. .005 to 0.050%, Cr: 1.50 to less than 1.90%, B: 0.0005 to 0.0100%, Al: 0.100 to 0.200%, Ca: 0.0002% to 0 .0030%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, Ti: 0 to less than 0.020%, Nb: 0 to 0.100%, Mo: 0 ∼0.500%, Ni: 0-0.500%, Cu: 0-0.500%, and the balance being Fe and impurities, steel materials for conventional carburized steel parts with a C content Even if it is lower than , it is possible to obtain the core hardness required for carburized steel parts.

(B) In order to obtain as large a carburized layer depth and core hardness as possible with the chemical composition described above, it is preferable to increase the fraction of martensite in the microstructure of the core of the carburized steel part. In order to increase the martensite fraction in the microstructure of the core part of the carburized steel part, the content of alloying elements (hardenability improving elements) that improve the hardenability of steel such as Mn, Cr, Mo and Ni is calculated by the formula It is necessary to contain so as to satisfy (2).
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (2).

(C) However, if the content of the above hardenability improving element increases, the hardenability improving element solid solution strengthens the ferrite. Therefore, the hardness of the steel material for carburized steel parts increases. If the hardness of the steel material for carburized steel parts increases, the cold forgeability decreases and the critical workability decreases. B is an element that enhances the hardenability of steel but does not solid-solution strengthen ferrite. Therefore, as described above, B is included in the chemical composition of the steel material for carburized steel parts, and the content of C and the above-mentioned hardenability improving element is made to satisfy the formula (1). As a result, sufficient core hardness can be obtained in the carburized steel part after carburizing treatment while suppressing a decrease in the critical workability.
0.100<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

(D) In order to stably obtain the effect of improving the hardenability of B, it is necessary to secure a sufficient amount of dissolved B in the steel material during the carburizing treatment. If N exists in the steel, B combines with N to form BN, and solute B is reduced. Therefore, in the present embodiment, Al is included in the steel so that N is combined with Al instead of B. In this case, sufficient solid solution B can be ensured. As a result, a sufficient solid solution B amount can be secured. Furthermore, if AlN is finely dispersed and precipitated, the pinning effect can suppress coarsening of austenite crystal grains during heating for carburizing. Therefore, while satisfying the chemical composition described above, in order to secure the solid solution B amount and obtain the pinning effect of AlN, the Al content, the N content, and the Ti content in the steel material for carburized steel parts should be adjusted according to the formula (3) should be satisfied.
0.0003<Al×(N-Ti×(14/48))<0.0011 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (3).

When the Al content, Ti content, and N content in the chemical composition of the steel material for carburized steel parts satisfy the formula (3), solid solution B combines with N and precipitates as BN, thereby reducing solid solution B. can be suppressed, and sufficient solid solution B can be secured in the steel material. Furthermore, AlN is finely dispersed, and the pinning effect suppresses abnormal grain growth of austenite grains during heating for carburizing. Therefore, sufficient hardness is obtained in the core of the carburized steel part.

(E) B effectively enhances the hardenability of the core of the carburized steel part. However, when gas carburizing is carried out using a metamorphic furnace gas method as the carburizing treatment, the hardenability improvement effect due to the inclusion of B is low in the carburized layer, which is the surface layer portion of the carburized steel part. This is because, during the carburizing treatment, nitrogen enters from the surface of the steel part, bonds with solid solution B and precipitates as BN, and the amount of solid solution B decreases. Therefore, in order to ensure the hardenability of the carburized layer, which is the surface layer of the carburized steel part, the chemical composition of the steel material for the carburized steel part satisfies the formula (2) as described above.

(F) Even if the steel material for carburized steel parts satisfies the above-described chemical composition and satisfies the formulas (1) to (3), if the Ca content is too high relative to the S content, part of Ca is sulfided It does not dissolve in substances and forms oxides. Ca oxide lowers the critical workability of the steel material. If Ca/S in the chemical composition can be set within an appropriate range, it is possible to suppress the formation of oxides while promoting the refinement and spheroidization of sulfides. Forgeability (limit processing rate) can be improved. Specifically, if the chemical composition of the steel material for carburized steel parts satisfies the formula (4), a sufficient critical workability can be obtained.
0.03≦Ca/S≦0.15 (4)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (4).

(G) Among the carburized steel parts, there are parts such as hollow splines that require a lot of cutting work on intermediate members after cold forging. When manufacturing such carburized steel parts that require many cutting processes, the steel material for carburized steel parts is required to have excellent chip disposability as described above. In this specification, the term "chip disposability" refers to the ease with which chips generated during cutting are divided, and is a characteristic that refers to the ease with which chips are removed from the tool and the steel material. As described above, when the C content is kept low, the cold forgeability of the steel increases, but the chip disposability decreases. In the microstructure of the steel material for carburized steel parts having the chemical composition of the above formulas (1) to (4), the total area ratio of ferrite and pearlite is 85.0% or more, and the area is 200 μm ² or more The total area ratio of pearlite grains is 20.0 to less than 35.0%. Coarse pearlite grains having an area of 200 μm ² or more improve the chip disposability of the steel material for carburized steel parts. If the total area ratio of pearlite grains having an area of 200 μm ² or more is less than 20.0 to 35.0%, the steel material for carburizing steel parts can be cut after cold forging while ensuring a sufficient marginal workability. Sufficient chip control is obtained at

The steel material for carburized steel parts according to the present embodiment completed based on the above findings has a chemical composition, in mass%, of C: 0.11 to 0.15%, Si: 0.17 to 0.35%, Mn : 0.45-0.80%, S: 0.005-0.050%, Cr: 1.50-1.90%, B: 0.0005-0.0100%, Al: 0.100- 0.200%, Ca: 0.0002 to 0.0030%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, Ti: 0 to less than 0.020%, Nb: 0 to 0.100%, Mo: 0 to 0.500%, Ni: 0 to 0.500%, Cu: 0 to 0.500%, and the balance consists of Fe and impurities, formula (1) - Satisfying formula (4), in the microstructure, the total area ratio of ferrite and pearlite is 85.0% or more, and the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to 35. less than 0%.
0.100<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.0003<Al×(N-Ti×(14/48))<0.0011 (3)
0.03≦Ca/S≦0.15 (4)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (4).

The chemical composition of the steel material for carburized steel parts is Ti: 0.001 to less than 0.020%, Nb: 0.002 to 0.100%, Mo: 0.005 to 0.500%, Ni: 0.005 0.500%, and Cu: 0.005 to 0.500%.

Details of the steel material for carburized steel parts and the carburized steel parts of the present embodiment will be described below. In this specification, "%" for elements means % by mass unless otherwise specified.

[Chemical composition of steel for carburized steel parts]
The chemical composition of the steel material for carburized steel parts of this embodiment contains the following elements.

C: 0.11% to 0.15%
Carbon (C) increases the hardness of the core of the carburized steel part. If the C content is less than 0.11%, the hardness of the core of the carburized steel part is lowered and the chip disposability is reduced even if the content of other elements is within the range of the present embodiment. descend. On the other hand, the C content of conventional steel materials for carburized steel parts is about 0.20%, but in the steel material for carburized steel parts of the present embodiment, the C content is reduced to 0.15% in order to increase the limit workability. Below. Therefore, the C content is 0.11-0.15%. A preferred lower limit for the C content is 0.12%. A preferable upper limit of the C content is 0.14%.

Si: 0.17% to 0.35%
Silicon (Si) increases the temper softening resistance of carburized steel parts and increases the fatigue strength of carburized steel parts. If the Si content is less than 0.17%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.35%, the hardness of the steel material for carburized steel parts before cold forging becomes excessively high even if the content of other elements is within the range of the present embodiment. The limit processing rate is lowered. Therefore, the Si content is 0.17-0.35%. From the viewpoint of further increasing the fatigue strength, the lower limit of the Si content is preferably 0.18%, more preferably 0.19%. From the viewpoint of further increasing the limit processing rate, the upper limit of the Si content is preferably 0.30%, more preferably 0.25%.

Mn: 0.45% to 0.80%
Manganese (Mn) increases the hardenability of steel and increases the core hardness of carburized steel parts. If the Mn content is less than 0.45%, sufficient hardenability cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.80%, the hardness of the steel material for carburized steel parts before cold forging becomes excessively high even if the content of other elements is within the range of the present embodiment. The limit processing rate is lowered. Therefore, the Mn content is 0.45-0.80%. A preferred lower limit for Mn content is 0.47%, more preferably 0.48%. A preferable upper limit of the Mn content is 0.70%, more preferably 0.60%.

S: 0.005% to 0.050%
Sulfur (S) combines with Mn in steel to form MnS and enhances the chip disposability of steel materials for carburized steel parts. If the S content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the S content exceeds 0.050%, even if the other element contents are within the range of the present embodiment, MnS becomes the starting point of cracking during cold forging, and the critical workability decreases. Therefore, the S content is 0.005-0.050%. A preferable lower limit of the S content is 0.006%, more preferably 0.008%, and still more preferably 0.010%. A preferable upper limit of the S content is 0.040%, more preferably 0.030%, and still more preferably 0.020%.

Cr: 1.50% to less than 1.90% Chromium (Cr) increases the hardenability of steel and increases the core hardness of carburized steel parts. Compared to Mn, Mo, and Ni, which improve hardenability, Cr can improve hardenability while suppressing an increase in hardness of the steel material for carburized steel parts. If the Cr content is less than 1.50%, sufficient hardenability cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content is 1.90% or more, the hardness of the steel material for carburized steel parts before cold forging becomes excessively high even if the content of other elements is within the range of the present embodiment. , the critical processing rate decreases. Therefore, the Cr content is between 1.50 and less than 1.90%. A preferred lower limit for the Cr content is 1.52%, more preferably 1.55%. A preferable upper limit of the Cr content is 1.85%, more preferably 1.80%.

B: 0.0005% to 0.0100%
Boron (B), when dissolved in austenite, greatly enhances the hardenability of steel even in a very small amount. Therefore, the core hardness of the carburized steel part is increased. Furthermore, since B exhibits the above effects even when contained in a very small amount, the hardness of ferrite in the steel material for carburized steel parts is less likely to increase. In other words, the hardenability can be improved while maintaining a high critical workability of the steel material for carburized steel parts. If the B content is less than 0.0005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the B content exceeds 0.0100%, the above effects are saturated. Therefore, the B content is 0.0005-0.0100%. A preferable lower limit of the B content is 0.0007%, more preferably 0.0010%. A preferable upper limit of the B content is 0.0080%, more preferably 0.0050%, and still more preferably 0.0025%.

Al: 0.100% to 0.200%
Aluminum (Al) deoxidizes steel. Al further combines with N to form AlN, and secures the amount of dissolved B in the steel material for carburized steel parts of the present embodiment. Furthermore, finely dispersed AlN suppresses coarsening of austenite crystal grains during heating for carburizing due to the pinning effect. If the Al content is less than 0.100%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.200%, even if the content of other elements is within the range of the present embodiment, coarse oxides are formed in the steel, and the fatigue strength of the carburized steel part is reduced. descend. Therefore, the Al content is 0.100-0.200%. The lower limit of the Al content is preferably 0.101%, more preferably 0.105%, still more preferably 0.110%, still more preferably 0.120%. A preferable upper limit of the Al content is 0.190%, more preferably 0.170%, and still more preferably 0.150%.

Ca: 0.0002% to 0.0030%
Calcium (Ca) dissolves in the sulfides in the steel and makes the sulfides fine and spherical. As a result, the cold forgeability of the steel material for carburized steel parts is improved, and the critical workability is increased. If the Ca content is less than 0.0002%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ca content exceeds 0.0030%, coarse oxides are formed in the steel even if the content of other elements is within the range of this embodiment. In this case, the cold forgeability and limit workability of the steel material for carburized steel parts are rather lowered. Therefore, the Ca content is 0.0002-0.0030%. A preferable lower limit of the Ca content is 0.0005%, more preferably 0.0007%. A preferable upper limit of the Ca content is 0.0025%, more preferably 0.0020%.

N: 0.0080% or less Nitrogen (N) is an unavoidable impurity. That is, the N content is over 0%. N combines with B to form BN and reduces the amount of dissolved B. If the N content exceeds 0.0080%, even if the Al content in the steel material for carburized steel parts is within the above range, Al cannot sufficiently fix N, and BN is excessively generated. do. As a result, the hardenability of the steel material for carburized steel parts is lowered. If the N content exceeds 0.0080%, coarse AlN is further formed, and the coarse AlN serves as a starting point of cracks during cold forging. Therefore, the critical workability of the steel material for carburized steel parts is lowered. Therefore, the N content is 0.0080% or less. A preferable upper limit of the N content is 0.0075%, more preferably 0.0070%, and still more preferably 0.0065%. It is preferable that the N content is as low as possible. However, excessive reduction of the N content increases manufacturing costs. Therefore, in consideration of normal industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.001%. 0030%.

P: 0.050% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is over 0%. P lowers the fatigue strength and hot workability of steel. Therefore, the P content is 0.050% or less. A preferable upper limit of the P content is 0.035%, more preferably 0.020%. The lower the P content is, the better. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the lower limit of the P content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.001%.

O: 0.0030% or less Oxygen (O) is an unavoidable impurity. That is, the O content is over 0%. O forms oxides, lowers the critical workability of the steel material for carburized steel parts, and lowers the fatigue strength of the carburized steel parts. Therefore, the O content is 0.0030% or less. A preferable upper limit of the O content is 0.0028%, more preferably 0.0025%. It is preferable that the O content is as low as possible. However, excessive reduction of O content increases production costs. Therefore, considering normal industrial production, the lower limit of the O content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0007%.

The remainder of the chemical composition of the steel material for carburized steel parts in this embodiment consists of Fe and impurities. The term "impurity" as used herein means an element that is mixed in from the ore, scrap, or the environment during the manufacturing process that is used as a raw material for steel. Impurities other than N, P, and O described above are, for example, Pb, Sn, Cd, Co, and Zn. The contents of these impurities are as follows. Pb: 0.05% or less, Sn: 0.05% or less, Cd: 0.05% or less, Co: 0.05% or less, Zn: 0.05% or less.

[Regarding optional elements]
The chemical composition of the steel material for carburized steel parts of the present embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, Mo, Ni and Cu in place of part of Fe. good. Ti combines with N to further secure the solid solution B amount. Nb forms carbides and carbonitrides to refine grains. Mo, Ni and Cu all enhance the hardenability of steel.

Ti: 0 to less than 0.020% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When included, Ti fixes the N in the steel as TiN. Thereby, the formation of BN is further suppressed, and solid solution B can be further secured. When producing steel materials for carburized steel parts with an electric furnace, it may be difficult to adjust the N content in the steel. In such a case, it is preferable to contain Ti. If even a small amount of Ti is contained, the above effect can be obtained to some extent. On the other hand, if the Ti content is too high, the production costs will be high. Therefore, the Ti content is between 0 and less than 0.020%. A preferable lower limit of the Ti content is more than 0%, more preferably 0.001%, and still more preferably 0.002%. A preferable upper limit of the Ti content is 0.018%, more preferably 0.016%.

Nb: 0-0.100%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and N to form carbides and/or carbonitrides, and suppresses coarsening of austenite grains during heating for carburizing due to the pinning effect. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, coarse carbides and/or carbonitrides are formed, and the critical workability of the steel material for carburized steel parts is lowered. Therefore, the Nb content is 0-0.100%. A preferable lower limit of the Nb content is 0.002%, more preferably 0.004%, and still more preferably 0.010%. A preferable upper limit of the Nb content is 0.080%, more preferably 0.060%, and still more preferably 0.050%.

Mo: 0-0.500%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When included, Mo increases the hardenability of the steel and increases the core hardness of the carburized steel part. Furthermore, Mo does not form oxides and nitrides during the carburizing process when the carburizing process is performed by gas carburizing. Therefore, Mo suppresses the formation of an oxide layer, a nitride layer and an abnormal carburization layer in the carburized layer. These effects can be obtained to some extent if even a small amount of Mo is contained. However, if the Mo content exceeds 0.500%, the hardness of the steel material for carburized steel parts is excessively increased, and the critical workability is lowered. Therefore, the Mo content is 0-0.500%. The lower limit of the Mo content is preferably 0.005%, more preferably 0.010%, still more preferably 0.020%, still more preferably 0.050%. A preferable upper limit of the Mo content is 0.400%, more preferably 0.300%, and still more preferably 0.200%.

Ni: 0-0.500%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When included, Ni increases the hardenability of the steel and increases the core hardness of the carburized steel part. Further, Ni does not form oxides and nitrides during carburizing when carburizing is performed by gas carburizing. Therefore, Ni suppresses the formation of an oxide layer, a nitride layer and an abnormal carburization layer in the carburized layer. If Ni is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.500%, the hardness of the steel material for carburized steel parts is excessively increased, and the critical workability is lowered. Therefore, the Ni content is 0-0.500%. The lower limit of the Ni content is preferably 0.005%, more preferably 0.010%, still more preferably 0.020%, still more preferably 0.050%. A preferable upper limit of the Ni content is 0.400%, more preferably 0.300%, and still more preferably 0.200%.

Cu: 0-0.500%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When included, Cu increases the hardenability of the steel and increases the core hardness of the carburized steel part. Further, Cu does not form oxides and nitrides during the carburizing process when the carburizing process is performed by gas carburizing. Therefore, Cu suppresses the formation of an oxide layer, a nitride layer, and an abnormal carburization layer on the surface of the carburized layer. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.500%, the hardness of the steel material for carburized steel parts is excessively increased, and the critical workability is lowered. Therefore, the Cu content is 0-0.500%. A preferable lower limit of the Cu content is 0.005%, more preferably 0.010%, still more preferably 0.020%, still more preferably 0.050%. A preferable upper limit of the Cu content is 0.400%, more preferably 0.300%. When Cu is contained, the hot workability of the steel material for carburized steel parts is further enhanced by setting the Ni content to 1/2 or more of the Cu content.

[Regarding formulas (1) to (4)]
The chemical composition of the steel material for carburized steel parts of the present embodiment further satisfies formulas (1) to (4).
0.100<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.0003<Al×(N-Ti×(14/48))<0.0011 (3)
0.03≦Ca/S≦0.15 (4)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (4).
Each formula will be described below.

[Regarding formula (1)]
Define F1=C+0.194*Si+0.065*Mn+0.012*Cr+0.033*Mo+0.067*Ni+0.097*Cu+0.078*Al. F1 is an index of the hardness of the steel material for carburized steel parts (and carburized steel parts manufactured using the steel material for carburized steel parts).

When the C content is low, the structure of the steel material for carburized steel parts before cold forging has a significantly higher ferrite fraction than the conventional steel material for carburized steel parts (with a C content of about 0.20%). It has increased. In this case, the hardness of the steel material for carburized steel parts is greatly affected not only by the C content (pearlite fraction) but also by the hardness of ferrite. F1 shows the contribution of each alloying element to the solid solution strengthening of ferrite in the steel material for carburized steel parts.

If F1 is 0.235 or more, the hardness of the steel material for carburized steel parts before cold forging is too high. In this case, the critical workability of the steel material for carburized steel parts is lowered. On the other hand, if F1 is 0.100 or less, the hardness as a carburized steel part is insufficient. Therefore, F1 is greater than 0.100 and less than 0.235. F1 is preferably as low as possible within a range that satisfies the hardenability index (F2) described later. A preferable upper limit of F1 is less than 0.230, more preferably 0.220, and still more preferably 0.210. Note that the F1 value is a value obtained by rounding off the calculated value to the fourth decimal place.

[Regarding formula (2)]
Define F2=(0.70*Si+1)*(5.1*Mn+1)*(2.2*Cr+1)*(3.0*Mo+1)*(0.36*Ni+1). F2 is an index relating to the hardenability of the steel material for carburized steel parts.

As described above, B is effective in increasing the hardenability of the steel material for carburized steel parts and increasing the hardness of the core of the carburized steel parts. On the other hand, when gas carburizing (transformation furnace gas method) is performed as the carburizing treatment, the hardenability improvement effect due to the addition of B is low in the carburized layer, which is the surface layer of the carburized steel part. This is because N in the atmosphere gas in the furnace penetrates into the surface layer of the carburized steel part during the carburizing process, and solid solution B precipitates as BN, and the amount of solid solution B that contributes to the improvement of hardenability is insufficient. . Therefore, when gas carburizing treatment is performed, B can increase the hardness of the core of the carburized steel part, but does not contribute to the improvement of the hardness of the carburized layer of the carburized steel part. Therefore, in order to secure the hardenability of the carburized layer, which is the surface layer of the carburized steel part, it is necessary to use a hardenability improving element other than B.

When F2 is 13.0 or less, the carburized layer depth (Vickers The depth at which the hardness becomes HV550 or more) cannot be sufficiently obtained. On the other hand, if F2 is 45.0 or more, the hardness of the steel material for carburized steel parts before cold forging increases, and the critical workability decreases. Therefore, F2 is greater than 13.0 and less than 45.0. F2 is preferably as large as possible within a range that satisfies the hardness index F1. A preferred lower limit for F2 is 13.2, more preferably 15.0. Note that the F2 value is a value obtained by rounding off the calculated value to the second decimal place.

[Regarding formula (3)]
Define F3=Al×(N−Ti×(14/48)). F3 is an index related to the AlN precipitation amount. When Ti is stoichiometrically excessive with respect to N, all N is fixed as TiN. That is, (N−Ti×(14/48)) in F3 indicates the amount of N in the steel in a form other than TiN. That is, (N−Ti×(14/48)) indicates the amount of N that is not combined with Ti in the steel. In addition, "14" in F3 represents the atomic weight of N, and "48" represents the atomic weight of Ti.

If F3 is 0.0003 or less, the amount of Al that binds to N is insufficient. In this case, dispersion of fine AlN is insufficient. Therefore, the pinning effect does not work effectively, and coarse grains are generated in the austenite grains during heating for carburizing. On the other hand, if F3 is 0.0011 or more, the AlN precipitates are not finely dispersed and are coarsened, so that coarsening of the austenite crystal grains during heating in the carburizing treatment cannot be suppressed. Therefore, F3 is greater than 0.0003 and less than 0.0011. A preferred lower limit for F3 is 0.0004, more preferably 0.0005. A preferred upper limit for F3 is 0.0010, more preferably 0.0009. Note that the F3 value is a value obtained by rounding off the calculated value to the fifth decimal place.

[Regarding formula (4)]
Define F4=Ca/S. F4 is an index for sulfide refinement and spheroidization. As described above, Ca forms a solid solution in sulfides to make the sulfides finer and further spheroidize the sulfides. However, even if the content of each element containing Ca in the chemical composition of the steel material for carburized steel parts is within the above range, if the Ca content relative to the S content is too high, part of Ca will form a solid solution in sulfides. and form oxides. Ca oxide lowers the critical workability of the steel material. If F4 (=Ca/S) can be set in an appropriate range, it is possible to suppress the formation of oxides while promoting the refinement and spheroidization of sulfides. Forgeability and limit workability can be improved. As a result, the steel for carburized steel parts can be formed into complex carburized steel parts.

If F4 is less than 0.03, even if the content of each element in the chemical composition is within the above range and F1 to F3 satisfy formulas (1) to (3), Since the Ca content relative to the S content is too low, the sulfides are insufficiently refined and spheroidized. In this case, the critical working rate of the steel material for carburized steel parts becomes low. On the other hand, if F4 is higher than 0.15, the content of each element in the chemical composition is within the above range, and even if F1 to F3 satisfy the formulas (1) to (3), the steel Since the Ca content relative to the S content in the steel is too high, oxides are produced in excess. In this case, the critical working rate of the steel material for carburized steel parts becomes low. If the content of each element in the chemical composition is within the above range, F1 to F3 satisfy formulas (1) to (3), and F4 is 0.03 to 0.15, The sulfides can be sufficiently refined and spheroidized, and the excessive production of oxides can also be suppressed. Therefore, the steel material for carburized steel parts has a lower deformation resistance during cold forging than the conventional steel, and the limit workability is increased. Furthermore, when the steel material for carburized steel parts is carburized, it is possible to produce carburized steel parts having carburized layer hardness and core hardness equivalent to those of conventional steel. A preferred lower limit for F4 is 0.05, more preferably 0.06. A preferred upper limit for F4 is 0.14, more preferably 0.13. Note that the F4 value is a value obtained by rounding off the calculated value to the third decimal place.

[Microstructure of steel for carburized steel parts]
In the microstructure of the steel material for carburized steel parts, the portion excluding inclusions and precipitates is defined as the matrix (parent phase). The matrix of steel for carburized steel parts consists mainly of ferrite and pearlite. Here, "mainly composed of ferrite and pearlite" means that the total area ratio of ferrite and pearlite in the microstructure is 85.0 to 100.0%. In the matrix, phases other than ferrite and pearlite include, for example, bainite, martensite, and cementite. In the microstructure of the steel material for carburized steel parts of this embodiment, the total area ratio of bainite, martensite and cementite is 0 to 15.0%. In short, in the steel material for carburized steel parts of the present embodiment, the total area ratio of ferrite and pearlite in the microstructure is 85.0 to 100.0%, and the total area ratio of bainite, martensite and cementite in the microstructure is 0. ~15.0%. In the microstructure of the steel material for carburized steel parts of the present embodiment, when the total area ratio of ferrite and pearlite is less than 100.0%, the balance is one selected from the group consisting of bainite, martensite and cementite. Or two or more. Ferrite, pearlite, martensite, bainite, and cementite are included in the calculation of the area ratio of the microstructure. On the other hand, the calculation of the area ratio does not include precipitates other than cementite, inclusions, and retained austenite.

Furthermore, in the microstructure of the steel material for carburized steel parts of the present embodiment, the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to less than 35.0%. As described above, in the microstructure of the steel material having the chemical composition satisfying the formulas (1) to (4), even if the total area ratio of ferrite and pearlite is 85.0% or more, pearlite grains in the microstructure If it is fine, chips are less likely to be broken during cutting. In the microstructure of the steel material having the chemical composition satisfying the formulas (1) to (4), the total area ratio of ferrite and pearlite is 85.0% or more, and the pearlite grains having an area of 200 μm ² or more (hereinafter referred to as , coarse pearlite grains) is an appropriate amount, the chip disposability after cold forging is improved.

Here, perlite grains in the present specification are defined as follows. FIG. 1 is a schematic diagram of a microstructure observation field of steel. Referring to FIG. 1, in the microstructure observation field, a region of pearlite in which ferrite has the same crystal orientation is defined as a pearlite block. A region having the same lamellar orientation in the barlite block is defined as a pearlite colony. In FIG. 1, there are a perlite block 21 containing pearlite colonies 21A and 21B, a pearlite block 22 containing pearlite colonies 22A and 22B, a perlite block 23, and a perlite block 24. In FIG. In the observation field of view, the pearlite blocks 21 and 22 are in contact (adjacent). In this specification, a single perlite block or a plurality of contacting perlite blocks are defined as perlite grains. That is, the perlite blocks 21 and 22 are defined as one perlite grain 2 in FIG. On the other hand, a pearlite block 23 existing alone (that is, not in contact with other pearlite blocks) is defined as one pearlite grain 23 . Similarly, a single perlite block 24 is defined as one perlite grain 24 . Therefore, when the area of the pearlite grains 2 is 200 μm ² or more, the area of the pearlite grains 23 is 200 μm ² or more, and the area of the pearlite grains 24 is 200 μm ² or more, the pearlite grains with an area of 200 μm ² or more in this observation field of view The total area ratio RP (%) of grains is defined by the following formula.
RP = total area of perlite grains 2, 23 and 24/area of observation field x 100

If the total area ratio of pearlite grains having an area of 200 μm ² or more is less than 20.0%, chips are less likely to be broken during cutting of the steel material for carburized steel parts, resulting in reduced chip disposability. On the other hand, if the total area ratio of pearlite grains having an area of 200 μm ² or more is 35.0% or more, the proportion of coarse pearlite in the steel material for carburized steel parts is too high. In this case, the hardness of the steel material for carburized steel parts becomes excessively high, and the critical workability decreases.

Pearlite grains having a total area ratio of 85.0% or more of ferrite and pearlite in the microstructure of a steel material for carburized steel parts having a chemical composition satisfying formulas (1) to (4) and having an area of 200 μm ² or more If the total area ratio of is less than 20.0 to 35.0%, the chip disposability can be improved while maintaining a high marginal workability of the steel material for carburized steel parts. A preferable lower limit of the total area ratio of pearlite grains of 200 μm ² or more is 22.0%, more preferably 25.0%. A preferable upper limit of the total area ratio of pearlite grains of 200 μm ² or more is 33.0%, more preferably 30.0%. The preferable lower limit of the total area ratio of ferrite and pearlite in the microstructure is 90.0%, more preferably 95.0%, still more preferably 97.0%.

[Method for measuring total area ratio in microstructure]
The total area ratio (%) of ferrite and pearlite in the microstructure of the steel material for carburized steel parts of the present embodiment and the total area ratio (%) of pearlite grains having an area of 200 μm ² or more are measured by the following methods. be.

When the steel material for carburized steel parts is a steel bar or wire rod, the center of the radius R connecting the surface and the central axis in the cross section perpendicular to the longitudinal direction (axial direction) of the steel material for carburized steel parts (hereinafter referred to as the cross section) A sample is taken from the position (R/2 position). Among the surfaces of the sampled sample, the surface corresponding to the cross section is used as the observation surface. After the observation surface is mirror-polished, the observation surface is etched using 2% nitric acid alcohol (nital etchant). The etched viewing surface is viewed using a 500x optical microscope to generate photographic images of any 20 fields. The size of each field of view is 500 μm×500 μm.

In each field of view, each phase such as ferrite and pearlite has a different contrast. Therefore, each phase is identified based on the contrast. Among the specified phases, the total area (μm ² ) of ferrite and the total area (μm ² ) of pearlite in each field of view are obtained. The ratio (%) of the sum of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view is defined as the total area ratio (%) of ferrite and pearlite.

Furthermore, in all fields of view, the perlite grains defined above are identified, and the area of each pearlite grain is determined. Then, the total area of pearlite grains of 200 μm ² or more is determined. The ratio (%) of the total area of pearlite grains of 200 μm ² or more to the total area of all visual fields is defined as the total area ratio (%) of pearlite grains of 200 μm ² or more.

The calculation of the area ratio of the microstructure includes ferrite, pearlite, martensite (including tempered martensite), bainite (including tempered bainite), and cementite (including spheroidized cementite). On the other hand, the calculation of the area ratio does not include precipitates other than cementite, inclusions, and retained austenite.

The steel material for carburized steel parts of the present embodiment has each element in the chemical composition within the above range and satisfies the formulas (1) to (4). Furthermore, the total area ratio of ferrite and pearlite in the microstructure is 85.0% or more, and the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to less than 35.0%. Therefore, it is possible to increase the limit processing rate during cold forging, and excellent chip disposability is obtained in cutting after cold forging. Furthermore, in the carburized steel part after carburizing, the hardness of the core can be sufficiently increased, and a carburized layer with a sufficient depth can be obtained. A method of manufacturing the steel material for carburized steel parts will be described later.

[About carburized steel parts]
The carburized steel part of the present embodiment is manufactured using the above steel material for carburized steel part of the present embodiment. Specifically, the carburized steel part is manufactured by carburizing the steel material for the carburized steel part after cold forging. A method of manufacturing the carburized steel component will be described later.

A carburized steel component comprises a carburized layer and a core. The carburized layer has a depth of less than 0.4-2.0 mm from the surface of the carburized steel part. That is, the carburized layer is formed on the surface layer of the carburized steel component. In this embodiment, the carburized layer means a region having a Vickers hardness of 550 HV or higher according to JIS Z 2244:2009. The core portion corresponds to a region inside the carburized layer of the carburized steel component. The chemical composition of the core is the same as the chemical composition of the carburized steel parts described above. In other words, the content of elements in the chemical composition of the core is within the above numerical range and satisfies the formulas (1) to (4).

In the carburized steel part, the carburized layer corresponds to a depth of 50 μm from the surface of the carburized steel part. The Vickers hardness according to JIS Z 2244:2009 at a depth of 50 μm from the surface of the carburized steel part is 650 to 1000 HV. That is, the Vickers hardness of the carburized layer at the above position is 650 to 1000 HV. The carburized layer is formed by carburizing treatment, and the Vickers hardness of the carburized layer is higher than that of the carburized steel component steel material.

In the carburized steel part having the above structure, the core portion is located at a depth of 2.0 mm from the surface of the carburized steel part. The Vickers hardness according to JIS Z 2244:2009 at a depth of 2.0 mm from the surface of the carburized steel part is 250 to 500 HV. That is, the Vickers hardness of the core portion at the above position is 250 to 500 HV.

Vickers hardness of carburized steel parts is measured by the following method. The cross-section perpendicular to any surface of the carburized steel part shall be the measurement plane. On the measurement surface, the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface are measured using a micro Vickers hardness tester according to JIS Z 2244: 2009 Vickers hardness test. Ask. The load during the test shall be 0.49N. The Vickers hardness HV is measured at 10 positions at a depth of 50 μm, and the average value is defined as the Vickers hardness HV at the position at a depth of 50 μm. Also, the Vickers hardness HV at 10 positions at a depth of 0.4 mm is measured, and the average value is taken as the Vickers hardness HV at the position at a depth of 0.4 mm. If the Vickers hardness at the 0.4 mm depth position is 550 HV or more, it is determined that the carburized layer depth is at least 0.4 mm or more. Also, on the measurement surface, the Vickers hardness at a depth of 2.0 mm from the surface is determined by a Vickers hardness test based on JIS Z 2244:2009 using a micro Vickers hardness tester. The load during the test shall be 0.49N. The Vickers hardness HV is measured at 10 positions at a depth of 2.0 mm, and the average value is defined as the Vickers hardness HV at the position at a depth of 2.0 mm.

Carburized steel parts are applied as machine structural parts used in mining machines, construction machines, automobiles, and the like. Mechanical structural parts are, for example, gears, shafts, pulleys and the like.

The steel material for carburized steel parts having the above structure exhibits an excellent marginal workability during cold forging, and an excellent chip disposability during cutting after cold forging. Furthermore, a sufficiently deep carburized layer and a sufficiently hard core can be obtained in the carburized steel part after carburizing.

[Manufacturing method of steel material for carburized steel parts]
An example of the method for manufacturing the steel material for carburized steel parts of the present embodiment will be described. The steel material for carburized steel parts of the present embodiment is not limited to the manufacturing method described below, and the manufacturing method is not limited as long as it has the above configuration. However, the manufacturing method described below is a suitable example of manufacturing the steel material for carburized steel parts of the present embodiment.

An example of the method for manufacturing a steel material for carburized steel parts according to the present embodiment includes a material preparation step and a hot working step. Each step will be described below.

[Material preparation process]
In the material preparation step, a material having a chemical composition that satisfies the above formulas (1) to (4) is prepared. A material is manufactured by the following method, for example. Molten steel having a chemical composition that satisfies the above formulas (1) to (4) is produced. Using the molten steel, a raw material (slab or ingot) is produced by a casting method. For example, the above molten steel is used to produce a slab (bloom) by a well-known continuous casting method. Alternatively, the molten steel is used to produce an ingot by a well-known ingot casting method.

[Hot working process]
In the hot working step, the raw material (bloom or ingot) prepared in the raw material preparation step is hot worked to manufacture a steel material for carburized steel parts. Although the shape of the steel material for carburized steel parts is not particularly limited, it is, for example, a steel bar or a wire rod. In the following description, as an example, a case where the steel material for carburized steel parts is a steel bar will be described. However, even if the steel material for carburized steel parts has a shape other than the steel bar, it can be manufactured by the same hot working process.

Hot working includes a rough rolling process and a finish rolling process. In the rough rolling process, the raw material is hot worked to produce a billet. The rough rolling process uses, for example, a blooming mill. The material is bloomed by a blooming mill to produce a billet. When a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous rolling mill to produce a smaller billet. may In a continuous rolling mill, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are alternately arranged in a row. Through the above steps, the raw material is manufactured into a billet in the rough rolling step.

In the finish rolling step, the billet is heated using a heating furnace or a soaking furnace. The billet after heating is subjected to hot rolling using a continuous rolling mill to produce a steel bar as a steel material for carburized steel parts.

In the finish rolling step, the heating temperature in the heating furnace or soaking furnace is defined as T1 (°C), and the holding time in the heating furnace and soaking furnace is defined as t1 (hour). Here, the heating temperature T1 is the furnace temperature, and the holding time t1 is the stay time (time) of the billet in the furnace. At this time, the preferred heating temperature T1 is 1200 to 1250° C., and the preferred holding time t1 is 1.5 to 15.0 hours. Even if the heating temperature T1 and the holding time t1 are out of the above ranges, the steel material for carburized steel parts having the above configuration can be manufactured as long as the finishing temperature T2 and the average cooling rate CR, which will be described later, are within appropriate ranges.

In finish rolling using a continuous rolling mill, the material temperature at the delivery side of the stand where the final reduction is performed is defined as finish temperature T2. The finishing temperature is the surface temperature (° C.) of the material measured by a thermometer installed on the delivery side of the stand where the final reduction was performed. The thermometer is, for example, a radiation thermometer. In the finish rolling step, the finish temperature T2 and the cooling rate after finish rolling are set in the following ranges.

Finishing temperature T2: 1000-1100°C
If the finishing temperature T2 is less than 1000° C., the total area ratio of pearlite grains having an area of 200 μm ² or more is less than 20.0% even when the cooling described later is performed, and the chip disposability is reduced. do. On the other hand, if the finish rolling temperature T2 exceeds 1100° C., the total area ratio of pearlite grains having an area of 200 μm ² or more becomes 35.0% or more, and the critical working ratio decreases. Therefore, the finishing temperature T2 is 1000-1100.degree. A preferable lower limit of the finishing temperature T2 is 1020°C, more preferably 1030°C. A preferable upper limit of the finishing temperature T2 is 1090°C, more preferably 1080°C.

Average cooling rate CR from finish temperature T2 to steel temperature 600°C: 2.0°C/sec or less

Pearlite transformation is almost completed by the time the steel material temperature reaches 600°C. If the average cooling rate CR from the finish temperature T2 until the steel material temperature reaches 600° C. is 2.0° C./sec or less, the average cooling rate CR will be a cooling rate equal to or less than air cooling. In this case, in the microstructure of the steel material for carburized steel parts, the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0% or more, and sufficient chip control is obtained. On the other hand, if the average cooling rate CR exceeds 2.0°C/sec, the cooling rate is too fast. In this case, the pearlite grains defined above become finer, and the total area ratio of pearlite grains having an area of 200 μm ² or more is less than 20.0%. As a result, the chip disposability of the steel material for carburized steel parts deteriorates. If the average cooling rate CR is further increased, bainite and/or martensite are generated in the microstructure of the steel material for carburized steel parts. In this case, the total area ratio of ferrite and pearlite in the microstructure is less than 85.0%, and the critical workability is lowered.

Incidentally, the average cooling rate CR is measured by the following method. The steel material after finish rolling is conveyed downstream on a conveying line. A plurality of thermometers are arranged along the transfer line, and the temperature of the steel material at each position of the transfer line can be measured. Based on a plurality of thermometers arranged along the transfer line, the time required for the temperature of the steel material to reach 600° C. from the finish temperature T2 is obtained, and the average cooling rate CR (° C./sec) is obtained.

For example, the average cooling rate CR can be adjusted by arranging a plurality of slow cooling covers on the transfer line at intervals. In addition, the method of cooling the steel after the temperature of the steel reaches 600° C. or lower is not particularly limited.

Through the above manufacturing process, the steel material for carburized steel parts of the present embodiment having the above configuration can be manufactured.

[Manufacturing method of carburized steel parts]
Next, an example of a method for manufacturing a carburized steel part according to this embodiment will be described. This manufacturing method includes a cold forging step of cold forging the above steel material for carburized steel parts to produce an intermediate member, a cutting step of cutting the intermediate member, and carburizing the intermediate member. It includes a carburizing step in which a treatment or carbonitriding treatment is performed, and a final heat treatment step in which the intermediate member after the carburizing step is quenched and tempered.

[Cold forging process]
In the cold forging step, the steel material for carburized steel parts produced by the above-described production method is subjected to cold forging to impart a shape to produce an intermediate member. Cold forging conditions such as working ratio and strain rate in this cold forging step are not particularly limited, and suitable conditions may be selected as appropriate.

[Cutting process]
After the cold forging process and before the later-described carburizing process, the intermediate member is subjected to cutting to give a shape. By performing cutting, it is possible to impart a precise shape to the carburized steel part, which is difficult to do with the cold forging process alone. When the steel material for carburized steel parts of the present embodiment is used, chip disposability in the cutting process is excellent.

[Carburizing process]
Carburizing treatment or carbonitriding treatment is performed as a carburizing step on the intermediate member after the cutting step. In order to obtain the carburized steel parts having the Vickers hardness described above, the conditions for carburizing or carbonitriding are as follows: temperature of 830° C. to 1100° C., carbon potential of 0.5% to 1.2%, carburizing time of 1 It is preferable to set the period to an hour or more.

[Finishing heat treatment process]
After the carburizing step, a finishing heat treatment step is performed as necessary. In the final heat treatment step, a quenching treatment or a quenching and tempering treatment is performed to produce a carburized steel component. In order to manufacture the carburized steel parts having the above-described Vickers hardness, it is preferable that the temperature of the quenching medium is room temperature to 250° C. as a condition for the quenching treatment or the quenching and tempering treatment. Moreover, you may implement a sub-zero process after hardening as needed.

[Other processes]
If necessary, the carburized steel part after the finishing heat treatment step may be further subjected to grinding or shot peening. A precision shape can be imparted to the carburized steel component by performing a grinding process. By performing the shot peening process, compressive residual stress is introduced into the surface layer of the carburized steel part. Compressive residual stress suppresses initiation and propagation of fatigue cracks. Therefore, it increases the fatigue strength of the carburized steel part. For example, if the carburized steel part is a gear, the fatigue strength of the tooth root and tooth surface of the carburized steel part can be improved. Shot peening treatment may be performed by a well-known method. The shot peening treatment is preferably carried out, for example, using shot grains having a diameter of 0.7 mm or less and an arc height of 0.4 mm or more.

The effect of one embodiment of the present invention will be described more specifically with reference to examples. The conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the steel material for carburized steel parts and the carburized steel parts of the present embodiment. Therefore, the present invention is not limited to this one conditional example. Various conditions can be adopted for the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Molten steel having the chemical composition shown in Table 1 was prepared. At this time, the molten steel was cast by continuous casting to obtain a slab.

A blank column in Table 1 means that the corresponding elemental content was below the detection limit. After heating the cast slab, blooming, which is a rough rolling step, and subsequent rolling by a continuous rolling mill were carried out to produce a billet with a cross section (perpendicular to the longitudinal direction) of 162 mm×162 mm. The heating temperature in blooming was 1200 to 1250°C.

Using the manufactured billet, a finish rolling process was performed to manufacture a steel bar (steel material for carburized steel parts) having a diameter of 30 mm. Table 2 shows the heating temperature T1 for each test number in the finish rolling process. The retention time t1 was 1.5 to 3.0 hours in any test number. Table 2 shows the finishing temperature T2 of each test number and the average cooling rate CR from the finishing temperature T2 to the temperature of the steel material reaching 600°C.

[Evaluation test]
[Microstructure Observation Test]
A sample for microstructure observation was taken from the R/2 position of the steel bar of each test number. Among the surfaces of the sample, the surface corresponding to the cross section perpendicular to the longitudinal direction of the steel bar was used as the observation surface. After the observation surface was mirror-polished, the observation surface was etched using 2% nitric acid alcohol (nital etchant). The etched viewing surface was observed using a 500x optical microscope to generate photographic images of any 20 fields. The size of each field of view was 500 μm×500 μm. Each phase such as ferrite and pearlite has a different contrast for each phase. Therefore, each phase was identified based on contrast. Among the specified phases, the total area (μm ² ) of ferrite and the total area (μm ² ) of pearlite in each field of view were obtained. The ratio (%) of the sum of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view was defined as the total area ratio (%) of ferrite and pearlite. (Corresponds to "ferrite + pearlite total area ratio" in Table 2.)

Furthermore, in all fields of view, the pearlite grains defined above were identified, and the area of each pearlite grain was determined. Then, the total area of pearlite grains of 200 μm ² or more was obtained. The ratio (%) of the total area of pearlite grains of 200 μm ² or more to the total area of all fields of view was defined as the total area ratio (%) of pearlite grains of 200 μm ² or more ("Total area of coarse perlite grains in Table 2 Equivalent to "rate".). The "remaining structure" column in Table 2 shows the observed phases other than ferrite and pearlite in each test number. A "-" in the column indicates that no phases other than ferrite and pearlite were observed in the matrix in the microstructure. "M" in the column indicates that martensite was observed in addition to ferrite and pearlite in the matrix in the microstructure. "B" in the column indicates that bainite was observed in addition to ferrite and pearlite in the matrix in the microstructure. "C" in the column indicates that cementite was observed in addition to ferrite and pearlite in the matrix in the microstructure.

In addition, ferrite, pearlite, martensite (including tempered martensite), bainite (including tempered bainite), and cementite (including spheroidized cementite) were included in the above calculation of the area ratio. On the other hand, precipitates other than cementite, inclusions, and retained austenite were not included in the calculation of the area ratio.

Table 2 shows the total area ratio of ferrite and pearlite for each test number and the total area ratio (%) of coarse pearlite grains having an area of 200 μm ² or more for each test number.

[Limited compressibility measurement test]
A plurality of critical compressibility test specimens were taken from each test number steel bar. The diameter of the limit compressibility measurement test piece was 6 mm, and the length was 9 mm. The longitudinal direction of the critical compressibility test specimen was parallel to the longitudinal direction of the steel bar. Also, the central axis of the limit compressibility measurement test piece corresponded to the R/2 position of the steel bar of each test number. A notch was formed in the circumferential direction at the central position in the longitudinal direction of the limit compressibility measurement test piece. The notch angle was 30 degrees, the notch depth was 0.8 mm, and the radius of curvature of the notch tip was 0.15 mm.

A 500-ton hydraulic press was used for the limit compressibility measurement test. A limit compressibility measurement test was performed on the produced limit compressibility measurement test piece by the following method. A cold compression test was performed on each specimen using a constraining die at a speed of 10 mm/min. Compression was stopped when microcracks of 0.5 mm or more were generated in the vicinity of the notch, and the compression rate (%) at that time was determined. This measurement was performed a total of 10 times, and the compression ratio (%) at which the cumulative failure probability was 50% was obtained, and the compression ratio was defined as the limit compression ratio (%). Table 2 shows the limit compression rate (%) for each test number. Since the limit compressibility of conventional steel materials for carburized steel parts is 65%, it was determined that the limit workability is excellent when the limit compression ratio is 68% or more, which can be regarded as a value clearly higher than this value. For test numbers with a critical compressibility of less than 68%, the carburized steel parts were not evaluated.

[Machinability test]
Cold drawing simulating cold forging was performed on steel bars with a diameter of 30 mm of each test number. Specifically, a steel bar with a diameter of 30 mm was cold drawn at a rate of area reduction of 30.6% to obtain a steel bar with a diameter of 25 mm. A steel bar after cold drawing was cut into a length of 50 mm to obtain a test piece for turning.

Using an NC lathe, the outer periphery of the test piece for turning was turned. The details of the tools used in the turning test and the turning conditions were as follows.

[tool]
Base material: Carbide P20 grade Coating: None [Turn processing conditions]
Peripheral speed: 150m/min Feed rate: 0.2mm/rev
Notch: 0.4mm
Lubrication: Use water-soluble cutting oil

Chip disposability was evaluated by the following method. Chips ejected during any 10 seconds during the machinability test were collected. The length of the collected chips was examined, and 10 chips were selected in descending order of length. The total weight of 10 selected chips was defined as "chip weight". If the total number of collected chips is less than 10 as a result of long connected chips, the total weight of collected chips is measured and the value converted to 10 is defined as "chip weight". did. For example, when the total number of chips is 7 and the total weight is 12 g, the chip weight is calculated as 12 g×10/7.

If the chip weight for each test number was 15 g or less, it was determined that the chip disposability was high. When the chip weight exceeded 15 g, it was evaluated that the chip disposability was low. Table 2 shows the evaluation results.

[Evaluation test for carburized steel parts]
A test piece with a diameter of 20 mm and a length of 30 mm was taken from the steel bar of each test number. The center of the test piece almost coincided with the center of the steel bar of each test number. Carburizing treatment (gas carburizing treatment) by a metamorphic furnace gas method was performed on the sampled test pieces. In the gas carburizing treatment, the carbon potential was set to 0.8% and the temperature was maintained at 950° C. for 5 hours. Subsequently, it was held at 850° C. for 0.5 hours. After the above steps, the test piece was immersed in an oil bath heated to 130° C. to perform oil quenching. The quenched test pieces were tempered at 150° C. for 90 minutes to produce carburized steel parts.

The following measurements were performed on the carburized layer and the core of the carburized steel parts of each test number. Specifically, on the cut surface perpendicular to the longitudinal direction of the carburized steel part of each test number, the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface were calculated as micro Vickers hardness. It was obtained by a Vickers hardness test based on JIS Z 2244:2009 using a meter. The load during the test was 0.49N. The Vickers hardness HV was measured at 10 positions at a depth of 50 μm, and the average value was defined as the Vickers hardness HV at the position at a depth of 50 μm. Also, the Vickers hardness HV at 10 positions at a depth of 0.4 mm was measured, and the average value was taken as the Vickers hardness HV at the position at a depth of 0.4 mm.

If the hardness at a depth of 0.4 mm from the surface was 550 HV or more, it was determined that the carburized layer was present at least 0.4 mm from the surface. Also, when the Vickers hardness at a depth of 50 μm from the surface is 650 to 1000 HV, it was determined that the hardness of the carburized layer of the carburized steel part is sufficient. Table 2 shows the measurement results.

The Vickers hardness and chemical composition of the core of the carburized steel parts were measured by the following methods. Vickers hardness at a depth of 2.0 mm from the surface of the carburized steel part perpendicular to the longitudinal direction was determined by a Vickers hardness test in accordance with JIS Z 2244:2009 using a micro Vickers hardness tester. . The load during the test was 0.49N. Ten measurements were performed at a depth of 2.0 mm, and the average value was taken as the Vickers hardness (HV) at a depth of 2.0 mm from the surface. Table 2 shows the Vickers hardness obtained. When the Vickers hardness at the 0.2 mm depth position was 250 to 500 HV, the hardness of the core was sufficient and it was judged as acceptable.

In addition, the chemical composition at a depth of 2.0 mm from the surface was quantitatively analyzed for elements with an atomic number of 5 or higher using EPMA (Electron Probe MicroAnalyser). When the chemical composition was the same as the chemical composition of the steel material for carburized steel parts, it was determined that the chemical composition was equivalent. Table 2 shows the determination results.

[Presence or absence of coarse grains in carburized steel parts]
Prior austenite grains were observed at a depth of 2.0 mm from the surface of the steel part of the carburized steel part. Specifically, a cut surface perpendicular to the longitudinal direction of the carburized steel part was used as the observation surface. After the observation surface was mirror-polished, it was etched with a picric acid saturated aqueous solution. A field of view (300 μm×300 μm) including a position 2.0 mm deep from the surface of the etched observation surface was observed with an optical microscope (400×) to identify prior austenite grains. For the identified prior austenite grains, the grain size number of each prior-stenite grain was obtained in accordance with JIS G 0551:2013. No. in grain size number. When even one crystal grain of 4 or less was present, it was determined that "coarse grains were generated".

[Test results]
With reference to Tables 1 and 2, the chemical compositions of the steel materials of Test Nos. 1 to 11 were within the range of the chemical compositions of this embodiment, and satisfied Formulas (1) to (4). Furthermore, the manufacturing conditions were also appropriate. Therefore, in the microstructure of the steel material for carburized steel parts, the total area ratio of ferrite and pearlite is 85.0% or more, and the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to 35.0%. was less than 0%. As a result, the critical compressibility of these test numbers was 68% or more, indicating a sufficient critical working rate. Furthermore, all test numbers were excellent in chip control. Furthermore, in the steel material for carburized steel parts, the carburized layer had a depth of at least 0.4 mm. The Vickers hardness of the carburized layer at a depth of 50 μm is 650 to 1000 HV, and the Vickers hardness of the core at a depth of 2.0 mm is 250 to 500 HV. had a high hardness.

On the other hand, in test number 12, F1 exceeded the upper limit of formula (1). Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 13, the C content was too low. Therefore, the total area ratio of pearlite grains having an area of 200 μm ² or more was less than 20.0%. Therefore, the chip weight exceeded 15 g, and the chip disposability was low.

In test number 14, the C content was too high and F1 exceeded the upper limit of formula (1). Therefore, the total area ratio of pearlite grains having an area of 200 μm ² or more exceeded 35.0%, and the critical workability of the steel material for carburized steel parts was low.

In test number 15, F2 exceeded the upper limit of formula (2). Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 16, F3 was less than the lower limit of formula (3). Therefore, in the core portion of the carburized steel part, part of the prior austenite grains became coarse grains.

In test number 17, F3 exceeded the upper limit of formula (3). Therefore, in the core portion of the carburized steel part, part of the prior austenite grains became coarse grains.

In test number 18, F4 was less than the lower limit of formula (4). Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 19, F4 exceeded the upper limit of formula (4). Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 20, the Al content was too high. Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 21, the Al content was too low. Furthermore, the core hardness was less than 250HV. Also, in the core portion of the carburized steel part, part of the prior austenite crystal grains became coarse grains.

In test number 22, the Ca content was too high. Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 23, the Ca content was too low. Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In test number 24, the N content was too high and the O content was too high. Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In Test No. 25, although the chemical composition was appropriate, the finish temperature T2 in the finish rolling step was too low. Therefore, the total area ratio of pearlite grains having an area of 200 μm ² or more was less than 20.0%. As a result, the chip weight exceeded 15 g, indicating poor chip disposability.

In Test No. 26, although the chemical composition was appropriate, the finish temperature T2 in the finish rolling step was too high. Therefore, the total area ratio of pearlite grains having an area of 200 μm ² or more exceeded 35.0%. Therefore, the limit processing rate of steel materials for carburized steel parts was low.

In Test No. 27, although the chemical composition was appropriate, the average cooling rate CR in the finish rolling process was too fast. Therefore, the total area ratio of bainite and martensite increased, and as a result, the total area ratio of ferrite and pearlite was less than 85.0%. As a result, the critical workability of steel materials for carburized steel parts was low.

In Test No. 28, although the chemical composition was appropriate, the average cooling rate CR in the finish rolling process was too fast. Therefore, although the total area ratio of ferrite and pearlite was 85.0% or more, the total area ratio of pearlite grains having an area of 200 μm ² or more was less than 20.0%. As a result, the chip weight exceeded 15 g, indicating poor chip disposability.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (2)

The chemical composition, in mass %,
C: 0.11 to 0.15%,
Si: 0.17 to 0.35%,
Mn: 0.45-0.80%,
S: 0.005 to 0.050%,
Cr: less than 1.50 to 1.90%,
B: 0.0005 to 0.0100%,
Al: 0.100 to 0.200%,
Ca: 0.0002 to 0.0030%,
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
Ti: 0 to less than 0.020%,
Nb: 0 to 0.100%,
Mo: 0-0.500%,
Ni: 0 to 0.500%,
Cu: 0 to 0.500%, and
The remainder consists of Fe and impurities, satisfying formulas (1) to (4),
In the microstructure, the total area ratio of ferrite and pearlite is 85.0% or more, and the total area ratio of pearlite grains having an area of 200 μm ² or more is 20.0 to less than 35.0%.
Steel material for carburized steel parts.
0.100<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.0003<Al×(N-Ti×(14/48))<0.0011 (3)
0.03≦Ca/S≦0.15 (4)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (4). The steel material for carburizing steel parts according to claim 1,
The chemical composition is
Ti: less than 0.001 to 0.020%,
Nb: 0.002 to 0.100%,
Mo: 0.005 to 0.500%,
Ni: 0.005 to 0.500%, and
Cu: 0.005 to 0.500%,
containing one or more elements selected from the group consisting of
Steel material for carburized steel parts.

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