JP6683073B2 - Steel for carburizing, carburized steel parts and method for manufacturing carburized steel parts - Google Patents

Description

  The present invention relates to a carburizing steel, a carburized steel part and a method of manufacturing a carburized steel part, and in particular, has a small deformation resistance during cold forging, a large limit working rate, and, after carburizing heat treatment, has a hardness equivalent to that of a conventional steel. TECHNICAL FIELD The present invention relates to a carburizing steel, a carburized steel part, and a method for manufacturing a carburized steel part having a layer and a steel part hardness.

  Generally, Mn, Cr, Mo, Ni, and the like are added in combination to steel used for machine structural parts. The carburizing steel having such chemical components and manufactured through the steps of casting, forging, rolling, etc. is further formed by mechanical processing such as forging and cutting, and then subjected to heat treatment such as carburizing. A carburized steel part including a carburized layer that is a hardened layer in the surface layer and a steel part that is a base material that is not affected by the carburizing treatment.

  Of the costs of manufacturing this carburized steel part, the costs related to cutting work are very large. The cutting process is not only expensive in terms of cutting tools but also disadvantageous in terms of yield because it produces a large amount of chips. Therefore, it has been attempted to replace the cutting process with forging. Forging methods can be roughly classified into hot forging, warm forging, and cold forging. Warm forging is characterized by less generation of scale and improved dimensional accuracy over hot forging. In addition, cold forging has the feature that scale does not occur and the dimensional accuracy is close to that of cutting. Therefore, performing rough machining by hot forging and then finishing by cold forging, performing mild cutting as finishing after performing warm forging, or forming by cold forging only Etc. have been considered. However, when the cutting resistance is replaced by warm or cold forging, if the deformation resistance of the carburizing steel is large, the surface pressure applied to the mold increases and the mold life decreases, so the cost advantage for cutting decreases. . Alternatively, in the case of molding into a complicated shape, a problem such as cracking at a portion to which large processing is applied occurs. Therefore, various techniques have been studied in order to soften the carburizing steel and improve the limit working rate.

  For example, Patent Documents 1 and 2 describe carburizing steels in which the contents of C, Si and Mn are reduced to soften the carburizing steel and improve the cold forgeability. In Patent Document 3, by reducing the C content, the density of fine Ti-based precipitates is controlled, and by suppressing an increase in hardness of the material, cold forgeability and crystal coarsening prevention characteristics are excellent. Carburizing steel is described. In both cases, it is said that the cold forgeability is improved by reducing the C content. In addition, in this specification, cold forgeability is evaluated by the deformation resistance at cold forging and the limit workability.

  Cold forging has a feature that the dimensional accuracy is close to that of cutting, but depending on the parts to be cold forged, a cutting process is included to some extent. In other words, cold forged steel is required to have not only cold forgeability but also machinability.

  Patent Documents 1 to 3 do not mention the machinability after cold forging, and the machinability improving effect is unclear. Further, Patent Documents 1 and 2 describe that by containing Ca or Mg, a protective coating is formed on the surface of the cutting tool at the time of cutting to improve machinability. However, this technique does not improve chip disposability. Therefore, in Patent Document 1 and Patent Document 2, there is a possibility that the chips may wind around the workpiece or the tool and the machining device may stop due to the lengthening of the chips.

  In order to suppress the amount of tool wear and improve chip disposability, it is necessary to increase the S content. However, if the S content is increased, a large amount of coarse sulfides are generated, which deteriorates the cold forgeability. That is, if the S content is increased in order to improve the machinability, the effect of improving the limit working rate due to the softening of the carburizing steel is canceled.

Japanese Patent No. 5135562 Japanese Patent No. 5135563 Japanese Patent No. 5458048

  In view of the above situation, the present invention is for carburizing steel in the stage of carburizing steel, which has smaller deformation resistance during cold forging than conventional steel, a large limit working ratio, and excellent machinability after cold forging. It is an object to provide a steel, a carburized steel part, and a method for manufacturing a carburized steel part.

  The present inventors have obtained the following findings as a result of detailed studies to solve such problems.

  (A) In order to improve machinability without increasing the S content, it is important to control the size and distribution of sulfides.

(B) As a result of various experiments on the relationship between the equivalent circle diameter of sulfide and the tool wear amount and chip disposability, it was found that the existence density of the equivalent circle diameter of less than 2 μm was 300 pieces / mm ² or more. If the wear is suppressed and the average distance between the sulfides in the steel is less than 30.0 μm, the chip disposability is improved. The interparticle distance between sulfides can be obtained by image analysis.

  (C) Sulfides in steel materials often crystallize before solidification (during molten steel) or at the time of solidification, and the size of sulfide is greatly affected by the cooling rate during solidification. Further, the solidification structure of the slab produced by continuous casting is usually in the form of dendrite, this dendrite is formed due to the diffusion of the solute element in the solidification process, the solute element is the interdendritic part of the dendrite. Thickens at. Mn is concentrated in the inter-tree portion, and sulfide crystallizes between the trees.

(D) In order to finely disperse the sulfide, it is necessary to shorten the interval between dendrite trees. Research on the primary arm spacing of dendrites has been performed conventionally (for example, the following non-patent document) and can be expressed by the following formula (A).
λ∝ (D × σ × ΔT) ^0.25 (A)
Where λ: primary dendrite arm spacing (μm), D: diffusion coefficient (m ² / s), σ
: Solid-liquid interface energy (J / m ² ), ΔT: solidification temperature range (° C).

  Non-Patent Document: W. Kurz and D. J. Fisher, "Fundamentals of Solidification", Trans Tech Publications Ltd. , (Switzerland), 1998, p. 256

From this equation (A), it is understood that the primary arm spacing λ of dendrite depends on the solid-liquid interface energy σ, and if this σ can be reduced, λ decreases. If λ can be reduced, the size of sulfide crystallized between dendrite trees can be reduced. The present inventors have added a trace amount of one kind or two or more kinds of Sb, Sn, and Te to the steel to reduce the sulfide content in the steel while keeping the deformation resistance during cold forging small. It has been found that the machinability of steel after cold forging is improved by dispersing it.

  The gist of the present invention is as follows.

[1] In mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.500%,
Mn: 0.0001% to 0.80%,
S: 0.0050% to 0.1000%,
Cr: more than 1.30% to 5.00%,
B: 0.0005% to 0.0100%,
Containing Ti: 0.020% to less than 0.100% and Al: 0.010% to 0.100%,
One or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050% are contained,
Furthermore, N, P and O are respectively
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content of each element in the chemical composition shown in mass% satisfies the formula (1) as a hardenability index,
In the rolling direction and parallel to the cross section of the steel material state, and are the density is 300 pieces / mm ² or more sulfide equivalent circle diameter is less than 2 [mu] m, the average distance between the sulphide Ru der less than 30.0μm Carburizing steel characterized by the following.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (1)
[2] Further, in mass%,
Nb: 0.002% to 0.100%,
V: 0.002% to 0.200%,
Mo: 0.005% to 0.500%,
Ni: 0.005% to 1.000%,
Cu: 0.005% to 0.500%,
Ca: 0.0002% to 0.0030%,
Mg: 0.0002% to 0.0030%,
Zr: 0.0002% to 0.0050%,
Rare Earth Metal: 0.0002% to 0.0050%,
Containing at least one or more of these elements,
The carburizing steel according to [1], wherein the content of each element in the chemical composition expressed in mass% satisfies formula (2) as a hardenability index instead of formula (1).
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (0.3633 × Ni + 1) <44.0 ... (2 )
[3] A carburized steel part comprising a steel part and a carburized layer having a thickness of more than 0.4 mm and less than 2 mm formed on the outer surface of the steel part,
The Vickers hardness of the carburized layer at a depth of 50 μm from the surface of the component is HV650 or more and HV1000 or less,
The Vickers hardness of the steel portion at a position of a depth of 2 mm from the surface of the component is HV250 or more and HV500 or less,
The steel portion, in mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.500%,
Mn: 0.0001% to 0.80%,
S: 0.0050% to 0.1000%,
Cr: more than 1.30% to 5.00%,
B: 0.0005% to 0.0100%
Containing Ti: 0.020% to less than 0.100% and Al: 0.010% to 0.100%,
One or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050% are contained,
Furthermore, N, P and O are respectively
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content of each element in the chemical composition shown in mass% satisfies the formula (3) as a hardenability index,
In the rolling direction and parallel to the cross section of the steel material state, and are the density is 300 pieces / mm ² or more sulfide equivalent circle diameter is less than 2 [mu] m, the average distance between the sulphide Ru der less than 30.0μm Carburized steel parts characterized by
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (3)
[4] The steel portion is further mass%,
Nb: 0.002% to 0.100%,
V: 0.002% to 0.20%,
Mo: 0.005% to 0.50%,
Ni: 0.005% to 1.00%,
Cu: 0.005% to 0.50%,
Ca: 0.0002% to 0.0030%,
Mg: 0.0002% to 0.0030%,
Zr: 0.0002% to 0.0050%,
Rare Earth Metal: 0.0002% to 0.0050%,
Containing at least one or more of these elements,
The carburized steel part according to [3], characterized in that the content of each element in the chemical composition expressed in mass% satisfies formula (4) as a hardenability index instead of formula (3).
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (0.3633 × Ni + 1) <44.0 ... (4 )
[5] A cold working step in which the carburizing steel described in [1] or [2] is subjected to cold plastic working to give a shape,
In the carburizing steel after the cold working step, a carburizing step of performing carburizing treatment or carbonitriding treatment,
The method for manufacturing a carburized steel part according to [3] or [4], characterized in that
[6] The method for manufacturing a carburized steel part according to [5], wherein after the carburizing step, a finishing heat treatment step of performing quenching treatment or quenching / tempering treatment is performed.
[7] The carburized steel part according to [5] or [6], further including a cutting step of performing a cutting process to give a shape after the cold working step and before the carburizing step. Production method.

  According to the present invention, at the carburizing steel stage, the carburizing steel has a smaller deformation resistance during cold forging than the conventional steel, a large limit working ratio, and excellent machinability after cold forging. A method for manufacturing a steel part and a carburized steel part can be provided.

  The present invention will be described in detail below.

  The content of each component element will be described. Here, "%" about a component is the mass%.

C: 0.07% to 0.13%
Carbon (C) is added in order to secure the hardness of the steel part in the carburized steel part including the carburized layer and the steel part. The C content of the conventional carburizing steel is about 0.2%, but in the carburizing steel according to the present embodiment and the steel portion of the carburized steel part, the C content is less than 0%. It is limited to less than 13%. The reason for this is that if the C content exceeds 0.13%, the hardness of the carburizing steel before forging remarkably increases and the critical working rate also decreases. However, if the C content is less than 0.07%, the hardness of the steel part of the carburized steel part is increased even if the hardness of the carburized steel part is increased as much as possible by adding a large amount of alloying elements described later that enhance hardenability. It is impossible to reach the level of conventional carburizing steel. Therefore, it is necessary to control the C content within the range of 0.07% to 0.13%. The preferred range is 0.08% to 0.12%. A more desirable range is 0.08% to 0.11%.

Si: 0.0001% to 0.500%
Silicon (Si) is an element that improves the tooth surface fatigue strength by significantly increasing the temper softening resistance of low temperature tempered martensitic steel such as carburized steel parts. In order to obtain this effect, the Si content needs to be 0.0001% or more. However, if the Si content exceeds 0.500%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical working rate decreases. Therefore, it is necessary to control the Si content in the range of 0.0001% to 0.500%. Within this range, Si is positively added when importance is placed on the tooth surface fatigue strength of carburized steel parts, and Si is actively added when emphasis is placed on reducing deformation resistance and improving the limit workability of carburizing steel. To reduce. The preferable range in the former case is 0.100% to 0.500%, and the preferable range in the latter case is 0.0001% to 0.200%.

Mn: 0.0001% to 0.80%
Manganese (Mn) is an element that enhances the hardenability of steel. In order to increase the martensite fraction after the carburizing heat treatment by this effect, the Mn content needs to be 0.0001% or more. However, if the Mn content exceeds 0.80%, the hardness of the carburizing steel before forging rises, the deformation resistance rises, and the critical working rate falls. Therefore, it is necessary to control the Mn content in the range of 0.0001% to 0.80%. The preferred range is 0.25% to 0.60%.

S: 0.0050% to 0.1000%
Sulfur (S) is an element that combines with Mn in steel to form MnS and improves machinability. If the S content exceeds 0.1000%, MnS may act as a starting point during forging to cause cracking, which may lower the critical working rate. Therefore, it is necessary to control the S content in the range of 0.0050% to 0.1000%. A suitable range is 0.0080% to 0.0200%.

Cr: over 1.30% to 5.00%
Chromium (Cr) is an element that enhances the hardenability of steel. In order to increase the martensite fraction after the carburizing heat treatment by this effect, the Cr content needs to be more than 1.30%. However, if the Cr content exceeds 5.00%, the hardness of the carburizing steel before forging rises, the deformation resistance rises, and the critical working rate falls. Therefore, it is necessary to control the Cr content in the range of more than 1.30% to 5.00%. Further, Cr is less effective in increasing the hardness of the carburizing steel and relatively effective in improving the hardenability, as compared with other elements such as Mn, Mo, and Ni, which have similar effects. Therefore, in the carburizing steel according to the present embodiment and the steel portion of the carburized steel component, Cr is added in a larger amount than in the conventional carburizing steel. The preferred range is 1.35% to 2.50%. A more desirable range is from more than 1.50% to 2.20%.

B: 0.0005% to 0.0100%
Boron (B) is an element that greatly enhances the hardenability of steel even if it is a small amount when it forms a solid solution in austenite. This effect can increase the martensite fraction after the carburizing heat treatment. Further, B does not need to be added in a large amount in order to obtain the above effect, so that it hardly increases the hardness of ferrite. That is, since the hardness of the carburizing steel before forging is hardly increased, B is positively used in the carburizing steel according to the present embodiment and the steel portion of the carburized steel part. If the B content is less than 0.0005%, the effect of improving the hardenability cannot be obtained. On the other hand, when the B content exceeds 0.0100%, the above effect is saturated. Therefore, it is necessary to control the B content in the range of 0.0005% to 0.0100%. A suitable range is 0.0010% to 0.0025%. When N is present in the steel in a certain amount or more, B combines with N to form BN, and the amount of solid solution B decreases. As a result, the effect of enhancing hardenability may not be obtained. Therefore, when B is added, it is necessary to simultaneously add an appropriate amount of Ti that fixes N.

Al: 0.010% to 0.100%
Aluminum (Al) has an effect of deoxidizing, and at the same time, easily combines with N to form AlN, and is an element effective in preventing austenite grain coarsening during carburizing and heating. However, if the Al content is less than 0.010%, coarsening of austenite grains cannot be stably prevented, and if coarsening occurs, bending fatigue strength decreases. On the other hand, if the Al content exceeds 0.100%, coarse oxides are likely to be formed, and bending fatigue strength is reduced. Therefore, the content of Al is set to 0.010 to 0.100%. The preferred range is 0.030% to 0.060%.

Ti: 0.020% to less than 0.100% Titanium (Ti) is an element having an effect of fixing N in steel as TiN. The addition of Ti prevents the formation of BN and secures the solid solution B that contributes to the hardenability. Further, a stoichiometric excess of Ti with respect to N forms TiC. This TiC has a pinning effect to prevent the coarsening of crystal grains during carburization. When the Ti content is less than 0.020%, the effect of improving the hardenability due to the addition of B cannot be obtained, and the coarsening of crystal grains during carburization cannot be prevented. On the other hand, when the Ti content is 0.100% or more, the precipitation amount of TiC becomes too large, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical working rate decreases. . Therefore, it is necessary to control the Ti content within the range of 0.020% to less than 0.100%. The preferred range is 0.025% to 0.050%.

Sb: 0.0001% to 0.0050%
Sn: 0.0001% to 0.0050%
Te: 0.0001% to 0.0050%
Antimony (Sb), tin (Sn) and tellurium (Te) are important elements in the present invention. By containing a trace amount of one kind or two or more kinds of Sb, Sn, and Te, the solidification structure of steel becomes fine and the sulfides are finely dispersed. In order to obtain the sulfide refining effect, the lower limits of the contents of Sb, Sn and Te must be 0.0001% or more. However, if the contents of Sb, Sn and Te each exceed 0.0050%, the hot workability of the steel deteriorates and continuous casting becomes difficult. Therefore, in the present invention, the contents of Sb, Sn and Te are each set to 0.0001% to 0.0050%. More preferably, each is 0.0015 to 0.0030%. In order to improve the hot workability, the total content of Sb, Sn and Te is more preferably 0.0015 to 0.0050%.

  In addition to the basic components described above, the carburizing steel according to the present embodiment and the steel portion of the carburized steel component contain impurities. Here, an impurity means an auxiliary material such as scrap, or an element such as N, P, O, Pb, Cd, Co, or Zn that is inevitably mixed in during the manufacturing process. Among these, N, P, and O need to be limited as follows in order to sufficiently exert the effects of one embodiment of the present invention. Here, the indicated% is% by mass. Further, although the limit range of the impurity content includes 0%, it is difficult to industrially stabilize the content to 0%.

N: 0.0080% or less Nitrogen (N) is an impurity that is inevitably contained and is an element that forms BN and reduces the amount of solid solution B. If the N content exceeds 0.0080%, it becomes impossible to fix N in the steel as TiN even if Ti is added, and it becomes impossible to secure the solid solution B that contributes to the hardenability. Further, if the N content exceeds 0.0080%, coarse TiN is formed, which becomes a starting point of cracking during forging, and the limit working rate of the carburizing steel before forging decreases. Therefore, it is necessary to limit the N content to 0.0080% or less. Preferably, the smaller the N content is, the more preferable it is. Therefore, 0% is included in the above limit range. However, it is not technically easy to set the N content to 0%, and even if the N content is stably set to less than 0.0030%, the steelmaking cost becomes high. Therefore, the limit range of the N content is preferably 0.0030% to 0.0080%. More preferably, it is 0.0030% to 0.0055%. Under normal operating conditions, N is unavoidably contained in an amount of about 0.0060%.

P: 0.050% or less Phosphorus (P) is an impurity. P reduces the fatigue strength and hot workability of steel. Therefore, it is preferable that the P content is small. The upper limit of the P content is 0.050% or less. The P content is preferably 0.035% or less, more preferably 0.020% or less. The smaller the P content, the more desirable, but it is technically not easy to set the P content to 0%, and even if the P content is stably set to less than 0.003%, the manufacturing cost becomes high. Therefore, the limit range of the P content is preferably 0.003% to 0.0050%. It is more preferably 0.003% to 0.035%, and even more preferably 0.003% to 0.020%.

O: 0.0030% or less Oxygen (O) is an unavoidable impurity and is an element that forms an oxide inclusion. If the O content exceeds 0.0030%, large inclusions that become the starting point of fatigue fracture increase, which causes deterioration of fatigue properties. Therefore, it is necessary to limit the O content to 0.0030% or less. Preferably, it is 0.0015% or less. The lower the O content is, the more preferable it is. However, it is not technically easy to set the O content to 0%, and even if the O content is stably set to less than 0.0007%, the steelmaking cost becomes high. Therefore, the limiting range of the O content is preferably 0.0007% to 0.0030%. More preferably, the limiting range of the O content is 0.0007% to 0.0015%. Under normal operating conditions, O is unavoidably contained in an amount of about 0.0020%.

  In addition to the basic components and impurity elements described above, the steel portion of the carburizing steel and the carburized steel component according to the present embodiment further has Nb, V, Mo, Ni, Cu, Ca, Mg, Te, and You may contain at least 1 sort (s) or 2 or more sorts of Zr and REM. Below, the numerical range of selection components and the reason for the limitation will be described. Here, the indicated% is% by mass.

  Of the above-mentioned selective components, Nb and V have the effect of preventing the coarsening of the tissue.

Nb: 0.002% to 0.100%
Niobium (Nb) is an element that combines with C and N in steel to form Nb (C, N). This Nb (C, N) suppresses grain growth by pinning the austenite crystal grain boundaries, and prevents coarsening of the structure. If the Nb content is less than 0.002%, the above effect cannot be obtained. If the Nb content exceeds 0.100%, the above effect is saturated. Therefore, the Nb content is preferably 0.002% to 0.100%. More preferably, it is 0.010% to 0.050%.

V: 0.002% to 0.200%
Vanadium (V) is an element that combines with C and N in steel to form V (C, N). This V (C, N) suppresses grain growth by pinning the austenite crystal grain boundaries and prevents coarsening of the structure. If the V content is less than 0.002%, the above effect cannot be obtained. When the V content exceeds 0.200%, the above effect is saturated. Therefore, the V content is preferably 0.002% to 0.200%. More preferably, it is 0.05% to 0.100%.

  Among the above-mentioned selected components, Mo, Ni, and Cu have the effect of increasing the martensite fraction during the carburizing heat treatment.

Mo: 0.005% to 0.500%
Molybdenum (Mo) is an element that enhances the hardenability of steel. In order to increase the martensite fraction after the carburizing heat treatment by this effect, the Mo content is preferably 0.005% or more. Further, Mo is an element that does not form an oxide and hardly forms a nitride in a gas carburizing gas atmosphere. The addition of Mo makes it difficult to form an oxide layer or a nitride layer on the surface of the carburized layer, or an abnormal carburized layer due to these. However, in addition to the high cost of Mo addition, when the Mo content exceeds 0.500%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical working rate increases. Is reduced. Therefore, the Mo content is preferably 0.005% to 0.500%. More preferably, it is 0.050% to 0.200%.

Ni: 0.005% to 1.000%
Nickel (Ni) is an element that enhances the hardenability of steel. In order to increase the martensite fraction after the carburizing heat treatment by this effect, the Ni content is preferably 0.005% or more. Further, Ni is an element that does not form an oxide or a nitride in a gas carburizing gas atmosphere. By adding Ni, it becomes difficult to form an oxide layer or a nitride layer on the surface of the carburized layer, or an abnormal carburized layer due to these. However, in addition to the high addition cost of Ni, when the Ni content exceeds 1.000%, the hardness of the carburizing steel before forging rises, the deformation resistance rises, and the critical working rate increases. Is reduced. Therefore, the Ni content is preferably 0.005% to 1.000%. More preferably, it is 0.050% to 0.500%.

Cu: 0.005% to 0.500%
Copper (Cu) is an element that enhances the hardenability of steel. In order to increase the martensite fraction after the carburizing heat treatment by this effect, the Cu content is preferably 0.005% or more. Further, Cu is an element that does not form an oxide or a nitride in a gas carburizing gas atmosphere. By adding Cu, it becomes difficult to form an oxide layer or a nitride layer on the surface of the carburized layer, or an abnormal carburized layer due to these. However, if the Cu content exceeds 0.500%, the ductility in the high temperature range of 1000 ° C. or higher decreases, which causes a decrease in yield during continuous casting and rolling. Further, if the Cu content exceeds 0.500%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the marginal workability decreases. Therefore, the Cu content is preferably 0.005% to 0.500%. More preferably, it is 0.050% to 0.300%. When Cu is added, it is preferable that the Ni content is at least ½ of the Cu content in mass% in order to improve the ductility in the high temperature range.

  Among the above-mentioned selected components, Ca, Mg, Zr and REM have an effect of improving machinability.

Ca: 0.0002% to 0.0030%
Calcium (Ca) is an element having a morphological control effect of making the shape of MnS generated due to S added for improving machinability into a spherical shape without elongating. Addition of Ca improves the anisotropy of the sulfide shape and does not impair the mechanical properties. Further, Ca is an element that forms a protective film on the surface of the cutting tool during cutting and improves machinability. In order to obtain these effects, the Ca content is preferably 0.0002% or more. When the Ca content exceeds 0.0030%, coarse oxides and sulfides are formed, which may adversely affect the fatigue strength of carburized steel parts. Therefore, the Ca content is preferably 0.0002% to 0.0030%. More preferably, it is 0.0008% to 0.0020%.

Mg: 0.0002% to 0.0030%
Magnesium (Mg) is an element that controls the morphology of the above-mentioned sulfide and forms a protective coating on the surface of the cutting tool during cutting to improve machinability. In order to obtain these effects, the Mg content is preferably 0.0002% or more. If the Mg content exceeds 0.0030%, coarse oxides are formed, which may adversely affect the fatigue strength of carburized steel parts. Therefore, the Mg content is preferably 0.0002% to 0.0030%. More preferably, it is 0.0008% to 0.0020%.

Zr: 0.0002% to 0.0050%
Zirconium (Zr) is an element that controls the morphology of sulfides. In order to obtain this effect, the Zr content is preferably 0.0002% or more. If the Zr content exceeds 0.0050%, coarse oxides are formed, which may adversely affect the fatigue strength of carburized steel parts. Therefore, the Zr content is preferably 0.0002% to 0.0050%. More preferably, it is 0.0008% to 0.0030%.

REM: 0.0002% to 0.0050%
REM (Rare Earth Metal) is an element that controls the morphology of sulfides. To obtain this effect, the REM content is preferably 0.0002% or more. If the REM content exceeds 0.0050%, coarse oxides may be formed, which may adversely affect the fatigue strength of carburized steel parts. Therefore, the REM content is preferably 0.0002% to 0.0050%. More preferably, it is 0.0008% to 0.0030%.
Note that REM is a general term for a total of 17 elements in which 15 elements from lanthanum having an atomic number of 57 to lutetium having an atomic number of 71 are added to scandium having an atomic number of 21 and yttrium having an atomic number of 39. Usually, it is supplied in the form of a misch metal which is a mixture of these elements and added to steel.

  As described above, the carburizing steel of the present embodiment contains the above-mentioned basic element, and the balance is a chemical composition consisting of Fe and impurities, or at least one selected from the above-mentioned basic element and the above-mentioned selected element. And has a chemical composition with the balance being Fe and impurities.

[Dendrite organization]
The solidified structure of the continuously cast slab used for producing the carburizing steel of the present embodiment usually has a dendrite form. Sulfides in carburizing steel often crystallize before solidification (during molten steel) or during solidification, and are greatly affected by the dendrite primary arm spacing. That is, if the primary dendrite arm spacing is small, the sulfides crystallized between the trees will be small. In the carburizing steel of the present embodiment, it is desirable that the primary dendrite arm interval at the stage of cast is less than 600 μm. In this specification, the sulfide is, for example, MnS or the like.

  In order to stably and effectively finely disperse sulfides, one or more kinds of trace amounts of Sb, Sn and Te are added to reduce the solid-liquid interface energy in molten steel. By reducing the solid-liquid interface energy, the dendrite structure becomes fine. When the dendrite structure is miniaturized, the sulfide crystallized from the dendrite primary arm is miniaturized.

[Sulfide]
Sulfide (MnS) contained in carburizing steel is useful for improving machinability, so it is necessary to secure its existing density. When the S content is increased, machinability is improved, but coarse sulfides are increased. Coarse sulfide stretched by hot rolling or the like impairs cold forgeability, so that it is necessary to control the size and shape. Further, in order to improve the chip controllability during machining, it is necessary to finely disperse the sulfide. That is, it is important to reduce the distance between sulfides. When MnS having an equivalent circle diameter of less than 2 μm exists in the steel at a density of 300 pieces / mm ² or more in a section parallel to the rolling direction of the steel material, wear of the tool is suppressed. The inclusion of sulfide may be confirmed by an energy dispersive X-ray analyzer attached to the scanning electron microscope. The equivalent circle diameter of sulfide is the diameter of a circle having an area equal to the area of sulfide, and can be obtained by image analysis. Similarly, the existing density of sulfide is obtained by image analysis.

  In addition, as a result of various experiments on the relationship between the average distance between sulfides (distance between particles between sulfides) and chip controllability, if the distance between particles between sulfides is less than 30.0 μm, It has been confirmed that good chip controllability can be obtained. The interparticle distance between sulfides can be obtained by image analysis.

[Hardenability index]
It is necessary that the content of each element in the above chemical components, expressed as mass%, satisfies the following formula (B), which is a hardenability index. In addition, when Mo and Ni which are selective components are contained, the hardenability index is redefined to the following formula (C) instead of the following formula (B).
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 ... (B)
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (0.3633 × Ni + 1) <44.0 ... (C )

  Carburizing and quenching of carburizing steels having various chemical components on the basis of the above-mentioned formulas (B) and (C) which are hardenability indexes, and under the same carburizing heat treatment conditions, the above conventional carburizing steels ( C content is about 0.2%), and a threshold value with which the hardness of the carburized layer and the effective hardened layer depth (the depth at which the Vickers hardness becomes HV550 or more) are equal to or higher than the threshold value. Obtained. That is, when the hardenability index is 7.5 or less, it is not possible to obtain the same properties as those of the conventional steel (C content is about 0.2%). Further, when the hardenability index is 44.0 or more, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the limit workability decreases. Therefore, the hardenability index needs to be more than 7.5 and less than 44.0. It is desirable that the hardenability index be as large as possible within the range that satisfies the hardness index described above. Preferably, it is 12.1 or more and less than 44.0. More preferably, it is 20.1 or more and less than 44.0.

  Further, the carburizing steel of this embodiment has a Vickers hardness of 125 Hv or less and a critical compressibility of 68% or more, and exhibits excellent cold forgeability.

Next, the carburized steel part of the present embodiment will be described.
The carburized steel component of the present embodiment is manufactured by subjecting the above carburizing steel to a cold working step and a carburizing step. After the carburizing step, a finishing heat treatment step may be performed if necessary. By the carburizing step, a carburized layer having a thickness of more than 0.4 mm and less than 2 mm is formed on the surface. The carburized layer in the present embodiment is a region where the Vickers hardness is HV550 or higher.

  More specifically, the carburized steel component of the present embodiment includes a steel part and a carburized layer formed on the outer surface of the steel part and having a thickness of more than 0.4 mm and less than 2 mm. The Vickers hardness of the carburized layer at a depth of 50 μm from the surface of the component is preferably HV650 or more and HV1000 or less. Further, the Vickers hardness of the steel portion at a position of a depth of 2 mm from the surface of the component is preferably HV250 or more and HV500 or less. The Vickers hardness of the carburized layer becomes harder than the carburizing steel, which is the material, by performing the carburizing treatment. Further, when the Vickers hardness of the steel portion after the carburizing step is insufficient, quenching or quenching / tempering may be performed as the finishing heat treatment step so that the Vickers hardness of the steel portion is HV250 or more.

The chemical composition of the steel part is almost the same as the chemical composition of the carburizing steel and satisfies the above formula (B) or the above formula (C), and the average distance between sulfides in the steel part. Is less than 30.0 μm, and the existence density of sulfides having a circle equivalent diameter of less than 2 μm is 300 / mm ² or more in a cross section parallel to the rolling direction of the steel material.

[Production method]
Next, a method for manufacturing the carburizing steel of the present embodiment and a method for manufacturing the carburized steel part will be described. In the method for manufacturing a carburized steel part, a process for manufacturing a cold forged product made of carburizing steel will be described as an example. The cold forged product is, for example, a machine part used in automobiles and construction machines, and is, for example, a steel part such as a gear, a shaft and a pulley.

  The method for producing the carburizing steel according to the present embodiment continuously casts a slab having the above chemical composition and having a dendrite primary arm interval of less than 600 μm within a range of 15 mm from the surface layer, and heats the slab. It is manufactured by performing inter-working and further annealing. Hot working may include hot rolling.

[Continuous casting process]
A steel slab satisfying the above chemical composition is manufactured by a continuous casting method. It may be made into an ingot (steel ingot) by the ingot making method. As the casting conditions, for example, using a 220 × 220 mm square mold, the superheat of the molten steel in the tundish is 10 to 50 ° C., and the casting speed is 1.0 to 1.5 m / min.

  Furthermore, in order to make the above-mentioned dendrite primary arm interval less than 600 μm, when casting molten steel having the above chemical composition, within the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the surface of the slab. It is desirable that the average cooling rate is 100 ° C./min or more and 500 ° C./min or less. If the average cooling rate is less than 100 ° C./min, it becomes difficult to set the dendrite primary arm interval at a depth position of 15 mm from the slab surface to less than 600 μm, and there is a possibility that sulfides cannot be finely dispersed. On the other hand, if it exceeds 500 ° C./min, the sulfide crystallized from the dendrite trees becomes too fine, and the machinability may deteriorate.

  The temperature range from the liquidus temperature to the solidus temperature is the temperature range from the start of solidification to the end of solidification. Therefore, the average cooling rate in this temperature range means the average solidification rate of the slab. The average cooling rate can be achieved, for example, by controlling the size of the mold cross section, the casting rate, etc. to appropriate values, or by increasing the amount of cooling water used for water cooling immediately after casting. This is applicable to both the continuous casting method and the ingot making method.

The cooling rate at the depth of 15 mm was obtained by etching the cross section of the obtained slab with picric acid, and at each of the positions at a depth of 15 mm from the surface of the slab, the dendrite secondary arm interval λ at a pitch of 5 mm in the casting direction. ₂ (μm) was measured at 100 points, and the cooling rate A (° C / sec) in the temperature range from the liquidus temperature to the solidus temperature of the slab was calculated from the value based on the following formula, and the arithmetic mean was calculated. Average.

λ ₂ = 710 × A ^−0.39

  For example, a plurality of slabs with different casting conditions are manufactured, the cooling rate of each slab is obtained by the above formula, and the optimum casting condition may be determined from the obtained cooling rate.

  A cast or ingot is hot-worked to produce a billet (steel slab), and the billet is further hot-worked into a steel bar or a wire rod.

  As a hot working step, the slab after the casting step is subjected to hot rolling, hot forging and the like to obtain a hot worked steel material. The plastic working conditions such as working temperature, working rate and strain rate in this hot working step are not particularly limited, and suitable conditions may be appropriately selected.

  Immediately after this hot working step, a temperature range in which the surface temperature of the hot worked steel material is 800 ° C. to 500 ° C. is 0 ° C. / The carburizing steel according to the present embodiment is obtained by performing gradual cooling at a cooling rate of more than 1 second / sec and not more than 1 second.

  If the cooling rate at 800 ° C to 500 ° C, which is the temperature at which austenite is transformed into ferrite and pearlite, exceeds 1 ° C / sec, the structural fraction of bainite and martensite becomes large. As a result, the hardness of the carburizing steel increases, the deformation resistance increases, and the critical working rate decreases. Therefore, it is preferable to limit the cooling rate in the above temperature range to more than 0 ° C / sec and 1 ° C / sec or less. More preferably, it is more than 0 ° C./sec and 0.7 ° C./sec or less. As a slow cooling process, in order to reduce the cooling rate of the hot-worked steel after the hot-working process, install a heat-retaining cover, a heat-retaining cover with a heat source, or a holding furnace after the rolling line or hot-forging line. do it. Further, as described later, spheroidizing annealing may be further performed to obtain the carburizing steel of this embodiment.

  Next, a method for manufacturing a carburized steel part according to this embodiment will be described.

  As a cold working step, the steel for carburizing, which is composed of the above basic components, selected components, and impurities, and finally manufactured through the slow cooling step is subjected to cold plastic working to give a shape. The plastic working conditions such as working ratio and strain rate in this cold working process are not particularly limited, and suitable conditions may be appropriately selected.

  Next, the carburizing steel having the shape after the cold working step is subjected to carburizing treatment or carbonitriding treatment as a carburizing step. In order to obtain a carburized steel part having the above-mentioned metal structure and hardness, the conditions of carburizing treatment or carbonitriding treatment are as follows: temperature: 830 ° C. to 1100 ° C., carbon potential: 0.5% to 1.2%, carburizing The time is preferably 1 hour or more. The carburized steel part according to the present embodiment is obtained through the above steps.

  After the carburizing step, the carburized steel part may be subjected to a quenching treatment or a quenching / tempering treatment as a finishing heat treatment step, if necessary. Particularly, the finish heat treatment step is preferably performed when the Vickers hardness of the steel portion after the carburizing step is insufficient. The conditions of the quenching treatment or the quenching / tempering treatment for obtaining the carburized steel part having the above-mentioned metal structure and hardness are preferably carried out at a temperature of the quenching medium in the range of room temperature to 250 ° C. Further, if necessary, sub-zero treatment may be performed after quenching.

  If necessary, the carburizing steel before the cold working step may be subjected to spheroidizing annealing. By performing the spheroidizing annealing treatment, the hardness of the carburizing steel is lowered, the deformation resistance is lowered, and the critical working rate is improved. The spheroidizing annealing conditions are not particularly limited, and suitable conditions may be appropriately selected.

  After the cold working step, the carburizing steel before the carburizing step is further subjected to a cutting step as a cutting step to give a shape. By performing the cutting process, it is possible to give the carburizing steel a precise shape, which is difficult only by cold plastic working. Since the carburizing steel of the present embodiment is excellent in machinability, the chip disposability is improved in this cutting process as compared with the conventional steel.

  If necessary, the carburized steel part after the finish heat treatment step may be further subjected to a grinding process or a shot peening process as a shot peening process. By performing the grinding process, it is possible to impart a precision shape to the carburizing steel, which is difficult only by cold plastic working. By performing the shot peening treatment, compressive residual stress is introduced into the surface layer of the carburized steel part. Since the compressive residual stress suppresses the occurrence and development of fatigue cracks, it is possible to further improve the tooth root and tooth surface fatigue strength of carburized steel parts. The shot peening treatment is preferably performed under the condition that the shot height is 0.4 mm or less and the arc height is 0.4 mm or more.

ADVANTAGE OF THE INVENTION According to this invention, the steel for carburizing excellent in cold forgeability and machinability and its manufacturing method can be provided. In the present invention, by casting a slab having a predetermined chemical composition, the dendrite structure that becomes the crystallization nuclei of sulfide can be refined, and the sulfide in steel can be finely dispersed. As a result, the machinability after cold forging, that is, the machinability before carburization, which is a raw material for steel parts such as gears, shafts and pulleys, can be improved.
As described above, the carburizing steel of the present invention is a machinability at the time of performing a cutting process directly or after roughly normalizing a rough formed product by cold forging after annealing. Is excellent. Therefore, the ratio of the cutting cost to the manufacturing cost of steel parts such as gears, shafts and pulleys for automobiles and industrial machines can be reduced, and the quality of parts can be improved.

According to the carburizing steel of the present embodiment, the carbon content is made relatively small, and a trace amount of one or more of Sb, Sn, and Te is added, and the above formula (B) or the above is used as a hardenability index. By having a component composition satisfying the formula (C) and having an average distance between sulfides of less than 30.0 μm and an existing density of sulfides of less than 2 μm of 300 / mm ² or more, cold The deformation resistance during forging is small, and the machinability after cold forging can be improved. In particular, the carburizing steel of the present embodiment has a Vickers hardness of HV125 or less and thus has a low deformation resistance during cold forging, and has a critical compressibility of 68% or more, and thus has good cold forgeability. . Then, the Vickers hardness of the steel portion becomes HV250 or more and the Vickers hardness of the carburized layer becomes HV650 or more by passing through the manufacturing process of the carburized steel component, which is suitable as a material of the carburized steel component.

The carburizing steel of the present embodiment is excellent in machinability when performing a cutting process directly or after normalizing a rough formed product by cold forging after annealing. . Therefore, the ratio of the cutting cost to the manufacturing cost of steel parts such as gears, shafts and pulleys for automobiles and industrial machines can be reduced, and the quality of parts can be improved.
Further, in the present embodiment, when producing carburizing steel, by casting a slab having a predetermined chemical composition, the dendrite structure that becomes the crystallization nuclei of sulfide is refined, and the sulfide in the steel Can be finely dispersed. As a result, the machinability after cold forging, that is, the machinability before carburization, which is a raw material for steel parts such as gears, shafts and pulleys, can be improved.

  Further, according to the carburized steel component of the present embodiment, the carburized layer is provided with the steel part and the carburized layer formed on the outer surface of the steel part, the carburized layer has a Vickers hardness of HV650 or more and HV1000 or less, and a Vickers hardness of the steel part. Since it is HV250 or more and HV500 or less, it can be suitably used as mechanical parts such as gears, shafts, and pulleys.

  As described above, according to the present embodiment, it is possible to provide a carburizing steel, a carburized steel part, and a manufacturing method thereof which are excellent in cold forgeability and machinability.

  Steels A to AC having the chemical compositions shown in Table 1 were melted in a 270 ton converter and continuously cast using a continuous casting machine. The casting conditions were such that a superheat of molten steel in the tundish was set to 10 to 50 ° C., a casting speed was set to 1.0 to 1.5 m / min, and a length of 15 mm from the surface of the slab was set using a 220 × 220 mm square mold. The average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at the depth position was 100 to 500 ° C / min. The reduction was applied during the solidification stage of continuous casting. The slab thus obtained was once cooled to room temperature, and the presence or absence of surface cracks in the slab was visually determined. Table 2 shows the results. Further, a cast piece once cooled to room temperature was taken as a test piece for observing the dendrite structure.

  In continuous casting of a slab, the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position 15 mm deep from the surface of the slab is changed by changing the cooling water amount of the mold. It was In this way, a slab having the chemical composition shown in Table 1 and having a primary dendrite arm interval of less than 600 μm within the range of 15 mm from the surface layer was continuously cast.

  Steels A to O shown in Table 1 are steels having the chemical composition specified in the present invention. Steels P to AC are comparative steels whose chemical compositions are out of the conditions specified in the present invention. The underline of the numerical value in Table 1 indicates that it is outside the range of the chemical composition of the carburizing steel according to the present embodiment.

  After that, hot forging was performed using the slab obtained by continuous casting as a raw material. Each slab was heated at 1250 ° C. for 2 hours, and the slab after heating was hot forged to manufacture a plurality of round bars having a circular cross section perpendicular to the longitudinal direction and a cross section having a diameter of 30 mm. After hot forging, the round bar was allowed to cool in the atmosphere. The cooling was carried out by keeping the round bar covered with a heat insulating cover so that the cooling rate at 800 ° C. to 500 ° C. was 1 ° C./second or less. Then, a spheroidizing heat treatment was performed as a spheroidizing annealing step (SA step: Spherodizing Annealing) on some of the round bars after cooling. In this way, steel materials made of carburizing steels of test numbers 1 to 29 were manufactured.

[Method for observing coagulated tissue]
The solidification structure was obtained by etching the cross section of the above slab with picric acid, and measuring 100 points of the dendrite primary arm interval and secondary arm interval at a depth of 15 mm from the surface of the slab at a pitch of 5 mm in the casting direction. , And the average value was obtained.

[Microstructure test]
The microstructure of the round bar of each steel number was observed. The L / 4 position where the axial length of the round bar was L was cut perpendicularly to the axial direction, and a test piece for microstructure observation was taken. The cut surface of the test piece was polished, the metal structure of steel was observed by an optical microscope, and the precipitate was discriminated from the contrast in the structure. The precipitate was identified using a scanning electron microscope and an energy dispersive X-ray spectroscopic analyzer (EDS). Ten polishing test pieces having a length of 10 mm and a width of 10 mm were prepared from a cross section including a longitudinal direction of a test piece described below, and predetermined positions of these polishing test pieces were photographed with an optical microscope at a magnification of 100. An image of an inspection reference area (region) of 9 mm ² was prepared for 10 fields of view. The equivalent circle diameter of each MnS in the observation visual field (image) was calculated. These dimensions (diameter) were converted into equivalent circle diameters indicating the diameter of a circle having the same area as the area of the precipitate. From the particle size distribution of the detected sulfides, the existing density of sulfides having an equivalent circle diameter of less than 2 μm and the average distance between the sulfides were calculated. The results are shown in Table 2. If the density of sulfides having an equivalent circle diameter of less than 2 μ is 300 / mm ² or more, and if the average distance between sulfides is less than 30.0 μm, the condition of the present invention is satisfied. And

[Hardness measurement test]
The hardness was measured 10 times in total using a Vickers hardness meter, and the average value was calculated. The results are shown in Table 2. When the hardness of the carburizing steel after the slow cooling step was HV125 or less, or when the hardness of the carburizing steel after the spheroidizing annealing was HV110 or less, the softening was sufficient and it was determined to be acceptable.

[Cold forgeability test]
A round bar test piece was prepared from a position of half the depth of the cut surface from the peripheral surface of the round bar having a diameter of 30 mm. The round bar test piece is a test piece with a diameter of 10 mm and a length of 15 mm centered on the position of the diameter ½ depth of the cut surface of the round bar with a diameter of 30 mm. The direction was parallel to the forging axis of a round bar having a diameter of 30 mm.

  For each steel, 10 round bar test pieces were prepared. A 500 ton hydraulic press was used for the cold compression test. Cold compression was performed at a speed of 10 mm / min using a restraining die, and when a minute crack of 0.5 mm or more occurred in the vicinity of the notch, the compression was stopped, and the compression rate at that time was calculated. This measurement was performed 10 times in total, the compression rate at which the cumulative damage probability was 50% was determined, and the compression rate was defined as the limit compression rate. Table 2 shows the measurement results. Since the conventional carburizing steel after spheroidizing has a critical compressibility of about 65%, a critical working ratio of 68% or more, which can be regarded as a value clearly higher than this value, is sufficient and the critical working ratio is acceptable. It was judged.

[Machinability test]
For each steel, the remainder of the steel bar having a diameter of 30 mm was used to apply strain by cold drawing instead of cold forging, and the machinability after the drawing was evaluated for the machinability after cold forging.

  Specifically, the remaining round steel bar having a diameter of 30 mm was cold drawn at a surface reduction rate of 30.6% to obtain a steel bar having a diameter of 25 mm. This cold drawn steel bar was cut into a length of 500 mm to obtain a test material for turning.

  The outer peripheral portion of the thus-obtained test material having a diameter of 25 mm and a length of 500 mm was turned under the following conditions using an NC lathe, and the machinability was investigated. Ten minutes after starting the turning, the wear amount (mm) of the flank of the cemented carbide tool was measured. Table 2 shows the measurement results. If the measured amount of wear of the flank was 0.2 mm or less, the machinability was sufficient and it was determined to be acceptable.

<Used chip>
Base material: Carbide P20 class grade Coating: None <Turning conditions>
Peripheral speed: 150m / min
Feed: 0.2 mm / rev.
Notch: 0.4 mm
Lubrication: Water-soluble cutting oil is used

  The chip controllability was evaluated by the following method. Chips ejected in 10 seconds during the machinability test were collected. The length of the collected chips was examined, and 10 chips were selected in order from the longest one. The total weight of the 10 selected chips was defined as "chip weight". When the total number of chips was less than 10 as a result of long chip connection, the total weight of the collected chips was measured, and the value converted into 10 chips was defined as "chip weight". 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 chips. Table 2 shows the measurement results. If the weight of the chips was 15 g or less, the chip disposability was sufficient and it was determined to be acceptable.

[Carburizing property evaluation test]
From the peripheral surface of the carburizing steel produced by the above method, a test piece for carburization (20 mmφ × 30 mm) was sampled from a position at a depth of 1/4 the diameter of the cut surface such that the longitudinal direction was the compression direction. . As the carburizing process, gas carburizing was carried out by the metamorphic furnace gas system. This gas carburizing was carried out at 950 ° C. for 5 hours with a carbon potential of 0.8%, and then at 850 ° C. for 0.5 hour. After the carburizing step, as a finishing heat treatment step, oil quenching to 130 ° C. was performed, and then tempering was performed at 150 ° C. for 90 minutes to obtain a carburized steel part.

The characteristics of the carburized layer and the steel portion of the manufactured carburized steel part were evaluated. Table 2 shows the measurement results.

  With respect to the carburized layer of the carburized steel component, the hardness at a position of a depth of 50 μm from the surface and the hardness at a position of a depth of 2 mm from the surface were measured 10 times in total using a Vickers hardness meter, and averaged. The value was calculated. Table 2 shows the measurement results. When the hardness at a depth of 50 μm from the surface was HV650 to HV1000, and when the hardness at a depth of 2 mm from the surface was HV250 to HV500, the hardness was sufficient and it was determined to be acceptable.

  Further, with respect to the carburized layer of the carburized steel component, the hardness distribution from the surface to the 5 mm position was measured three times in total using a Vickers hardness meter, and the average value was calculated. If the depth at the position of HV550 or more was more than 0.4 mm and less than 2 mm from the surface, it was judged as pass.

  With reference to Tables 1 and 2, the chemical compositions of the steels A to O are within the range of the chemical composition of the carburizing steel according to the present embodiment, and the hardenability index and the equivalent circle diameter are less than 2 μm. Both the density of existing substances and the average distance between sulfides satisfied the target. As a result, the performance required for carburizing steel and carburized steel parts is satisfied.

  Test No. 16 has the same composition as steel that satisfies the JIS standard SCr420H standard, which is a general-purpose steel type. In this steel, the content of chemical components C, Cr, Ti, B, Sb, Sn, Te, N does not satisfy the range of the present invention, so that the existing density of sulfides and the average distance between sulfides are This is an example in which the carburizing steel is not softened, the critical compressibility, the machinability and the chip disposability are not satisfied, without satisfying the scope of the invention.

  Test No. 17 did not contain any of Sb, Sn or Te. Therefore, the existing density of sulfides and the average distance between sulfides do not satisfy the scope of the present invention, which is an example in which machinability and chip disposability are insufficient. This is because no Sb, Sn, or Te was contained, so that the sulfide fine dispersion effect was not obtained.

  Test Nos. 18 to 21 are examples in which the Sb, Sn, or Te content does not satisfy the range of the present invention, and thus the slab after continuous casting is cracked. This is because the content of Sb, Sn or Te exceeds the upper limit of the present invention and the hot workability is lowered.

  Test No. 22 is an example in which the hardness of the steel part of the carburized steel part and the thickness of the carburized layer were insufficient because B was not contained. This is because the effect of improving hardenability was not obtained because B was not contained.

  Test No. 23, in which the N content of the chemical component does not satisfy the range of the present invention, the compressibility of the carburizing steel, the hardness of the steel portion of the carburized steel part, and the thickness of the carburized layer are insufficient. Is an example. The reason why the limit compressibility of the carburizing steel became insufficient is that coarse NIN was generated due to the large N content, and this became the starting point of fracture during cold working. Further, the hardness of the steel part of the carburized steel part was insufficient because the N content in the steel was too large to fix N in the steel as TiN, and the excess N and the solid solution B were separated. This is because BN was formed and the effect of improving the hardenability by solid solution B was not obtained.

  Test number 24 is an example in which the critical compressibility of the carburizing steel was insufficient because the S content of the chemical component did not satisfy the range of the present invention. The critical compressibility of the carburizing steel became insufficient because coarse MnS was generated due to the large S content, and this became the starting point of fracture during cold working.

  Test No. 25, the S content of the chemical component does not satisfy the range of the present invention, the existing density of sulfide and the average distance between sulfides do not satisfy the range of the present invention, and the carburizing steel This is an example of insufficient machinability and chip disposability.

  Test No. 26 is an example in which the hardness of the steel portion of the carburized steel component exceeds HV500 because the hardenability index does not satisfy the range of the present invention.

  Test number 27 is an example in which the hardness of the steel part of the carburized steel part and the thickness of the carburized layer were insufficient because the hardenability index did not satisfy the range of the present invention.

  Test No. 28 is an example in which the C content of the chemical component does not satisfy the range of the present invention, and therefore the hardness of the steel part of the carburized steel part and the thickness of the carburized layer are insufficient.

  Test No. 29 is an example in which the C content of the chemical component does not satisfy the range of the present invention, so that the carburizing steel has an insufficient softening and insufficient critical compressibility.

  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 thereof.

Claims (7)

In mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.500%,
Mn: 0.0001% to 0.80%,
S: 0.0050% to 0.1000%,
Cr: more than 1.30% to 5.00%,
B: 0.0005% to 0.0100%,
Containing Ti: 0.020% to less than 0.100% and Al: 0.010% to 0.100%,
One or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050% are contained,
Furthermore, N, P and O are respectively
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content of each element in the chemical composition shown in mass% satisfies the formula (1) as a hardenability index,
In the rolling direction and parallel to the cross section of the steel material state, and are the density is 300 pieces / mm ² or more sulfide equivalent circle diameter is less than 2 [mu] m, the average distance between the sulphide Ru der less than 30.0μm Carburizing steel characterized by the following.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (1) Furthermore, in mass%,
Nb: 0.002% to 0.100%,
V: 0.002% to 0.200%,
Mo: 0.005% to 0.500%,
Ni: 0.005% to 1.000%,
Cu: 0.005% to 0.500%,
Ca: 0.0002% to 0.0030%,
Mg: 0.0002% to 0.0030%,
Zr: 0.0002% to 0.0050%,
Rare Earth Metal: 0.0002% to 0.0050%,
Containing at least one or more of these elements,
The carburizing steel according to claim 1, wherein the content of each element in the chemical composition expressed in mass% satisfies the formula (2) instead of the formula (1) as a hardenability index.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (0.3633 × Ni + 1) <44.0 ... (2 ) A carburized steel part comprising a steel part and a carburized layer formed on the outer surface of the steel part and having a thickness of more than 0.4 mm and less than 2 mm,
The Vickers hardness of the carburized layer at a depth of 50 μm from the surface of the component is HV650 or more and HV1000 or less,
The Vickers hardness of the steel portion at a position of a depth of 2 mm from the surface of the component is HV250 or more and HV500 or less,
The steel portion, in mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.500%,
Mn: 0.0001% to 0.80%,
S: 0.0050% to 0.1000%,
Cr: more than 1.30% to 5.00%,
B: 0.0005% to 0.0100%
Containing Ti: 0.020% to less than 0.100% and Al: 0.010% to 0.100%,
One or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050% are contained,
Furthermore, N, P and O are respectively
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content of each element in the chemical composition shown in mass% satisfies the formula (3) as a hardenability index,
In the rolling direction and parallel to the cross section of the steel material state, and are the density is 300 pieces / mm ² or more sulfide equivalent circle diameter is less than 2 [mu] m, the average distance between the sulphide Ru der less than 30.0μm Carburized steel parts characterized by
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (3) The steel portion is further mass%,
Nb: 0.002% to 0.100%,
V: 0.002% to 0.200%,
Mo: 0.005% to 0.500%,
Ni: 0.005% to 1.000%,
Cu: 0.005% to 0.500%,
Ca: 0.0002% to 0.0030%,
Mg: 0.0002% to 0.0030%,
Zr: 0.0002% to 0.0050%,
Rare Earth Metal: 0.0002% to 0.0050%,
Containing at least one or more of these elements,
The carburized steel part according to claim 3, wherein the content expressed by mass% of each element in the chemical composition satisfies the formula (4) as the hardenability index instead of the formula (3).
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (0.3633 × Ni + 1) <44.0 ... (4 ) A cold working step in which the carburizing steel according to claim 1 or 2 is subjected to cold plastic working to give a shape;
In the carburizing steel after the cold working step, a carburizing step of performing carburizing treatment or carbonitriding treatment,
The method for manufacturing a carburized steel part according to claim 3 or 4, further comprising:   The method for manufacturing a carburized steel component according to claim 5, further comprising a finishing heat treatment step of performing quenching treatment or quenching / tempering treatment after the carburizing step.   The method for manufacturing a carburized steel part according to claim 5 or 6, further comprising a cutting step of performing a cutting process to give a shape after the cold working step and before the carburizing step.