JP2018035418A - Steel for carburization, carburization steel member and manufacturing method of carburization steel member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for carburization small in deformation resistance during cold forging and excellent in machinability after cold forging.SOLUTION: There is adopted a steel for carburization containing C:0.07 to 0.13%, Si:0.0001 to 0.50%, Mn:0.0001 to 0.80%, S:0.0050 to 0.1000%, Cr:over 1.30 to 5.00%, B:0.0005 to 0.0100%, Ti:0.020 to less than 0.100%, Al:0.010 to 0.100% and Bi:over 0.0001 to 0.0050%, one or more kind of Sb, Sn and Te and the balance Fe with impurities and further N:0.0080% or less, P:0.050% or less and O:0.0030% or less, satisfying the following formula (1) as a hardening property index and having presence density of sulfides with equivalent circle diameter of less than 2 μm of 300/mmor more. 7.5<(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)<44.0 (1).SELECTED DRAWING: None

Description

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

  In general, Mn, Cr, Mo, Ni, and the like are added in combination to steel used for machine structural parts. Carburizing steel that has such chemical components and is manufactured through processes such as casting, forging, and rolling is further molded by machining such as forging and cutting, and is subjected to heat treatment such as carburizing. The carburized steel part includes a carburized layer that is a hardened layer of the surface layer portion and a steel portion that is a base material that is not affected by the carburizing treatment.

  Of the cost of manufacturing the carburized steel part, the cost related to cutting is very large. Cutting is not only expensive in terms of cutting tools, but also produces a large amount of chips, which is disadvantageous from the viewpoint of yield. For this reason, attempts have been made to replace the cutting process with forging. Forging methods can be roughly divided into hot forging, warm forging, and cold forging. Warm forging is characterized by less scale generation and improved dimensional accuracy than hot forging. In addition, cold forging is characterized in that no scale is generated and dimensional accuracy is close to that of cutting. Therefore, after rough processing by hot forging, finish processing by cold forging, after light warm forging, perform light cutting as finishing, or perform molding only by cold forging Etc. have been studied. However, when replacing the cutting process with warm or cold forging, if the deformation resistance of the carburizing steel is large, the surface pressure applied to the mold is increased and the mold life is shortened. . Or when shape | molding in a complicated shape, problems, such as a crack arising in the site | part to which a big process is added generate | occur | produce. For this reason, various techniques have been studied in order to soften the carburizing steel and improve the limit working rate.

  For example, Patent Document 1 and Patent Document 2 describe carburizing steel in which the carburizing steel is softened by reducing the C, Si, and Mn contents to improve the cold forgeability. Patent Document 3 is excellent in cold forgeability and crystal coarsening prevention characteristics by controlling the density of fine Ti-based precipitates by reducing the C content and suppressing the increase in the hardness of the material. Carburizing steel is described. In any case, the cold forgeability is improved by reducing the C content. In the present specification, the cold forgeability is evaluated by the deformation resistance and the limit working rate during cold forging.

  Cold forging is characterized in that the dimensional accuracy is close to that of cutting, but depending on the parts to be cold forged, a cutting process is included. That is, steel for cold forging is required not only for cold forgeability but also for improved machinability.

  In Patent Document 3, machinability after cold forging is not mentioned, and the machinability improvement effect is unclear. Patent Document 1 and Patent Document 2 describe that by containing Ca or Mg, a protective film is formed on the surface of the cutting tool at the time of cutting to improve machinability. However, this technique does not improve chip disposal. Therefore, in patent document 1 and patent document 2, when a chip becomes long, there exists a possibility that a cutting | wrapping apparatus may stop when a chip wraps around a processed product or a tool.

  In order to improve the tool wear amount suppression and the chip disposal, it is necessary to increase the S content. However, when the S content is increased, a large amount of coarse sulfide is generated, and the cold forgeability is lowered. That is, if the S content is excessively increased in order to improve the machinability, the improvement effect of the limit working rate due to softening of the carburizing steel is canceled.

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

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

  As a result of detailed studies to solve such problems, the present inventors have obtained the following knowledge.

  (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 regarding the relationship between the equivalent circle diameter of sulfide, the amount of tool wear, and the chip disposability, the existence density of those having an equivalent circle diameter of less than 2 μm is 300 pieces / mm ² or more. When the wear is suppressed and the average distance between sulfides in the steel is less than 30.0 μm, the chip disposal is improved. The interparticle distance between sulfides can be determined by image analysis.

  (C) Sulfides in steel materials often crystallize before solidification (in molten steel) or at the time of solidification, and the size of sulfides is greatly influenced by the cooling rate during solidification. In addition, the solidification structure of the slab produced by continuous casting usually has a dendrite form, and this dendrite is formed due to diffusion of solute elements in the solidification process, and the solute elements are the dendritic part of the dendrites. Concentrate in Mn is concentrated in the part between trees, and sulfide is crystallized between trees.

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

  Non-patent literature: W. Kurz and D.C. J. et al. Fisher, “Fundamentals of Solidification”, Trans Tech Publications Ltd. (Switzerland), 1998, p. 256

From this equation (A), it can be seen that the primary arm interval λ of the 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 cold trace amounts of Bi and Sb, Sn, or Te to the steel, so that cold forging is achieved by finely dispersing sulfides in the steel while keeping the deformation resistance during cold forging small. It has been found that the machinability of the later steel is improved.

  The gist of the present invention is as follows.

[1] By mass%
C: 0.07% to 0.13%,
Si: 0.0001% to 0.50%
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%
Ti: 0.020% to less than 0.100%,
Al: 0.010% to 0.100% and Bi: more than 0.0001% to 0.0050%,
Containing one or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050%,
Furthermore, N, P and O are each
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content expressed by mass% of each element in the chemical component satisfies the formula (1) as a hardenability index,
The average distance between sulfides in steel is less than 30.0 μm, and the presence density of sulfides having an equivalent circle diameter of less than 2 μm in a cross section parallel to the rolling direction of the steel material is 300 pieces / mm ² or more. Features carburizing steel.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (1)
[2] Further, by 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 element or two or more elements,
The steel for carburizing according to [1], wherein a content expressed by mass% of each element in the chemical component satisfies Formula (2) instead of 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 )
[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 generated on the outer surface of the steel part,
The carburized layer has a Vickers hardness of HV650 or more and HV1000 or less at a depth of 50 μm from the surface of the component,
The Vickers hardness of the steel part at a position 2 mm deep from the part surface is HV250 or more and HV500 or less,
The steel part is mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.50%
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%
Ti: 0.020% to less than 0.100% Al: 0.010% to 0.100% and Bi: more than 0.0001% to 0.0050%,
Containing one or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050%,
Furthermore, N, P and O are each
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content expressed by mass% of each element in the chemical component satisfies the formula (3) as a hardenability index,
The average distance between sulfides in the steel part is less than 30.0 μm, and the abundance of sulfides having an equivalent circle diameter of less than 2 μm in a cross section parallel to the rolling direction of the steel is 300 pieces / mm ² or more. Carburized steel parts characterized by that.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (3)
[4] The steel part 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 element or two or more elements,
The carburized steel part according to [3], wherein a content expressed by mass% of each element in the chemical component satisfies Formula (4) instead of Formula (3) 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 (4) )
[5] A cold working step of applying a cold plastic working to the carburizing steel according to [1] or [2] to give a shape;
A carburizing step of subjecting the carburizing steel after the cold working step to carburizing or carbonitriding;
The method for producing a carburized steel part according to [3] or [4], comprising:
[6] The method for producing a carburized steel part according to [5], wherein after the carburizing step, a finishing heat treatment step for performing a quenching treatment or a quenching / tempering treatment is performed.
[7] The carburized steel part according to [5] or [6], further including a cutting step of giving a shape by performing a cutting process after the cold working step and before the carburizing step. Production method.

  According to the present invention, at the stage of carburizing steel, the carburizing steel, carburizing steel, which has a lower deformation resistance during cold forging than the conventional steel, a higher limit working rate, and excellent machinability after cold forging. A method for producing steel parts and carburized steel parts can be provided.

  The present invention will be described in detail below.

  The content of each component element will be described. Here, “%” for the component is mass%.

C: 0.07% to 0.13%
Carbon (C) is added to ensure 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%. However, in the carburizing steel according to the present embodiment and the steel part in the carburized steel part, the C content is 0 less than this amount. Limited to 13% or less. The reason for this is that when the C content exceeds 0.13%, the hardness of the carburizing steel before forging increases remarkably and the critical working rate decreases. However, if the C content is less than 0.07%, a large amount of an alloy element described later that enhances hardenability is added to increase the hardness as much as possible, but the hardness of the steel part of the carburized steel part is reduced. It is impossible to achieve the level of conventional carburizing steel. Therefore, it is necessary to control the C content in 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 0.50%
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 0.50%, 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 0.50%. Within this range, Si is actively added when emphasizing the tooth surface fatigue strength of carburized steel parts, and Si is actively added when reducing deformation resistance and improving the limit workability of carburizing steel. Reduction. The preferred range in the former case is 0.10% to 0.50%, and the preferred range in the latter case is 0.0001% to 0.20%.

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 increases, the deformation resistance increases, and the critical working rate decreases. 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 improve machinability. If the S content exceeds 0.1000%, MnS is the starting point at the time of forging, cracking may occur, and the critical processing rate may be reduced. Therefore, it is necessary to control the S content in the range of 0.0050% to 0.1000%. The preferred range is 0.0080% to 0.0200%.

Cr: more than 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 increases, the deformation resistance increases, and the critical working rate decreases. Therefore, it is necessary to control the Cr content within the range of more than 1.30% to 5.00%. In addition, Cr has a smaller degree of increasing the hardness of the carburizing steel than other elements such as Mn, Mo, and Ni having similar effects, and has a relatively large effect of improving the hardenability. Therefore, in the carburizing steel and the steel part in the carburized steel part according to the present embodiment, a larger amount of Cr is added 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 in a small amount when dissolved in austenite. This effect can increase the martensite fraction after the carburizing heat treatment. Further, since B does not need to be added in a large amount in order to obtain the above effect, the hardness of the ferrite is hardly increased. That is, since there is a feature that 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 part in 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%. The preferred range is 0.0010% to 0.0025%. When a certain amount or more of N is present in the steel, B combines with N to form BN, and the amount of solute B decreases. As a result, the effect of improving hardenability may not be obtained. Therefore, when adding B, it is necessary to add an appropriate amount of Ti for fixing N at the same time.

Al: 0.010% to 0.100%
Aluminum (Al) has a deoxidizing action and is easily combined with N to form AlN, and is an element effective in preventing austenite grain coarsening during carburizing heating. However, when the Al content is less than 0.010%, the austenite grains cannot be stably coarsened, and when the Al content is coarsened, the bending fatigue strength decreases. On the other hand, if the Al content exceeds 0.100%, it becomes easy to form a coarse oxide and the bending fatigue strength decreases. Therefore, the content of Al is set to 0.010 to 0.100%. The minimum with preferable Al content is 0.030% or more, and a preferable upper limit is 0.060% or less.

Ti: 0.020% to less than 0.100% Titanium (Ti) is an element having an effect of fixing N in steel as TiN. By adding Ti, formation of BN is prevented and solid solution B contributing to hardenability is secured. Further, Ti stoichiometrically excessive with respect to N forms TiC. This TiC has a pinning effect that prevents coarsening of crystal grains during carburizing. If the Ti content is less than 0.020%, the effect of improving hardenability by adding 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 increases too much, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical processing rate decreases. . Therefore, it is necessary to control the Ti content in the range of 0.020% to less than 0.100%. The preferred range is 0.025% to 0.050%.

Bi: more than 0.0001% to 0.0050%
Bismuth (Bi) is an important element in the present invention. By containing a small amount of Bi, the sulfide is finely dispersed as the solidified structure of the steel becomes finer.
In order to obtain a sulfide refinement effect, the Bi content needs to exceed 0.0001%. However, if the Bi content exceeds 0.0050%, the hot workability of steel deteriorates and continuous casting becomes difficult. For these reasons, in the present invention, the Bi content is more than 0.0001% and 0.0050% or less. Furthermore, in order to obtain machinability improvement and sulfide fine dispersion effect, the Bi content is preferably 0.001% or more.

Sb: 0.0001% to 0.0050%
Sn: 0.0001% to 0.0050%
Te: 0.0001% to 0.0050%
In the present invention, in addition to the above components, one or more of antimony (Sb), tin (Sn), and tellurium (Te) are added in the range of 0.0001% to 0.0050%, respectively. It is a feature. These elements are segregated at the grain boundaries or at the interface between the matrix and inclusions, and have the effect of reducing the deformation resistance during cutting even by adding a small amount by reducing the bonding force at the interface. In the present invention, the machinability can be improved by the addition of Bi, but the machinability improvement effect can be further exhibited by the addition of one or more of Sb, Sn, or Te. . Each lower limit is set to 0.0001% or more, but a preferable lower limit for sufficiently exhibiting the effect is set to 0.0015% or more. On the other hand, if the upper limit exceeds 0.0050%, the hot workability of the steel deteriorates and continuous casting becomes difficult. Preferably, the total concentration of Bi, Sb, Sn, and Te is 0.0015 to 0.0050%.

  In addition to the basic components described above, the steel for carburizing steel and carburized steel parts according to the present embodiment contains impurities. Here, the 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 from the manufacturing process. Among these, N, P, and O need to be limited as follows in order to sufficiently exhibit the effect of one embodiment of the present invention. Here, the described% is mass%. Moreover, although 0% is contained in the restriction | limiting range of impurity content, it is difficult to make it 0% stably industrially.

N: 0.0080% or less Nitrogen (N) is an inevitably contained impurity, and is an element that forms BN and reduces the amount of dissolved B. If the N content exceeds 0.0080%, even if Ti is added, N in the steel cannot be fixed as TiN, and solid solution B that contributes to hardenability cannot be secured. On the other hand, 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 is lowered. Therefore, it is necessary to limit the N content to 0.0080% or less. Preferably, the lower the N content, the better. Therefore, the above limit range includes 0%. However, it is not technically easy to reduce the N content to 0%, and even if the N content is stably set to less than 0.0030%, the steelmaking cost increases. Therefore, it is preferable that the limitation range of the N content is 0.0030% to 0.0080%. More preferably, the N content is 0.0030% to 0.0050%. A more preferable upper limit is 0.0055% or less. 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 P content is 0.050% or less. P content is preferably 0.035% or less, more preferably 0.020% or less. The lower the P content, the better. However, if the P content is stably less than 0.003%, the production cost is increased, which is economically disadvantageous. Therefore, the lower limit of the P content is 0.003. % Or more is preferable. Therefore, the P content limit range is preferably 0.003% to 0.050%. More preferably, it is 0.003% to 0.035%, and still more preferably 0.003% to 0.020%.

O: 0.0030% or less Oxygen (O) is an inevitably contained impurity and an element that forms oxide inclusions. When the O content exceeds 0.0030%, large inclusions that become the starting point of fatigue fracture increase, which causes a decrease in fatigue characteristics. Therefore, it is necessary to limit the O content to 0.0030% or less. Preferably, it is 0.0015% or less. The smaller the O content, the better. Therefore, 0% is included in the above limit range. However, it is not technically easy to reduce the O content to 0%, and even if the O content is stably less than 0.0007%, the steelmaking cost increases. Therefore, the O content limit range is preferably 0.0007% to 0.0030%. More preferably, the limit 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 part in the carburizing steel and carburized steel part according to the present embodiment further includes Nb, V, Mo, Ni, Cu, Ca, Mg, Zr, You may contain at least 1 sort (s) or 2 or more types of REM. Hereinafter, the numerical limitation range of the selected component and the reason for limitation will be described. Here, the described% is mass%.

  Among the above-described selective components, Nb and V have an effect of preventing the coarsening of the structure.

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

V: 0.002% to 0.200%
Vanadium (V) is an element that combines with N and C in steel to form V (C, N). This V (C, N) suppresses the grain growth by pinning the austenite grain boundary, and prevents the 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%.

  Of the above-described selective components, Mo, Ni, and Cu have the effect of increasing the martensite fraction during 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 due to this effect, the Mo content is preferably 0.005% or more. Mo is an element that does not form an oxide and hardly forms a nitride in a gas carburizing gas atmosphere. By adding Mo, it becomes difficult to form an oxide layer or a nitride layer on the surface of the carburized layer or a carburized abnormal layer due to them. However, in addition to the high addition cost of Mo, if the Mo content exceeds 0.500%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical processing rate Decreases. Therefore, the Mo content is preferably 0.005% to 0.500%. More preferably, it is 0.05 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. Ni is an element that does not form oxides or nitrides in a gas carburizing gas atmosphere. By adding Ni, it becomes difficult to form an oxide layer or nitride layer on the surface of the carburized layer or a carburized abnormal layer due to them. However, when the Ni addition cost is high and the Ni content exceeds 1.000%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical processing rate Decreases. Therefore, the Ni content is preferably 0.005% to 1.000%. More preferably, it is 0.050%-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 due to this effect, the Cu content is preferably 0.005% or more. Cu is an element that does not form oxides or nitrides 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 a carburized abnormal layer due to them. However, if the Cu content exceeds 0.500%, the ductility in a high temperature region of 1000 ° C. or higher is lowered, which causes a decrease in yield during continuous casting and rolling. On the other hand, if the Cu content exceeds 0.500%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical working rate decreases. Therefore, the Cu content is preferably 0.005% to 0.500%. More preferably, it is 0.050% to 0.300%. In addition, when adding Cu, in order to improve the ductility of above-mentioned high temperature range, it is desirable to make Ni content into the mass% and to be 1/2 or more of Cu content.

  Of the above-described selective components, Ca, Mg, Zr, and REM have an effect of improving machinability.

Ca: 0.0002% to 0.0030%
Calcium (Ca) is an element having an effect of form control in which the shape of MnS generated due to S added for improving machinability is made spherical without being elongated. By adding Ca, the anisotropy of the sulfide shape is improved and the mechanical properties are not impaired. Further, Ca is an element that improves the machinability by forming a protective film on the surface of the cutting tool during cutting. In order to obtain these effects, the Ca content is preferably 0.0002% or more. If the Ca content exceeds 0.0030%, coarse oxides and sulfides are formed, which may adversely affect the fatigue strength of the 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 improves the machinability by controlling the form of the sulfide and forming a protective film on the surface of the cutting tool during cutting. 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 form of sulfide. In order to obtain this effect, the Zr content is preferably 0.0002% or more. When 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 form of sulfide. In order to obtain this effect, the REM content is preferably 0.0002% or more. When the REM content exceeds 0.0050%, coarse oxides are 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%.
REM is a generic name for a total of 17 elements including 15 elements from lanthanum having an atomic number of 57 to lutesium having an atomic number of 57 plus scandium having an atomic number of 21 and yttrium having an atomic number of 39. Usually, it is supplied in the form of misch metal, which is a mixture of these elements, and added to the steel.

  As described above, the carburizing steel of the present embodiment includes at least one selected from the above-described basic elements and the chemical composition of which the balance is Fe and impurities, or the above-described basic elements and the above-described selective elements. And the balance has a chemical composition consisting of Fe and impurities.

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

  In order to finely disperse sulfides stably and effectively, a small amount of Bi and one or more of Sb, Sn, and Te are added to reduce the solid-liquid interface energy in the molten steel. The dendrite structure becomes fine because the solid-liquid interface energy is reduced. As the dendrite structure is refined, the sulfide crystallized from the dendrite primary arm is refined.

[Sulphides]
Since sulfide (MnS) contained in carburizing steel is useful for improving machinability, it is necessary to ensure its density. Increasing the S content improves machinability, but increases coarse sulfides. The coarse sulfide stretched by hot rolling or the like impairs the cold forgeability, and therefore it is necessary to control the size and shape. Furthermore, in order to improve the chip disposal at the time of cutting, it is necessary to finely disperse sulfides. That is, it is important to reduce the interval between sulfides. When the cross section parallel to the rolling direction of the steel material contains MnS with an equivalent circle diameter of less than 2 μm in the steel at a density of 300 / mm ² or more, tool wear is suppressed. In addition, what is necessary is just to confirm that an inclusion is a sulfide by the energy dispersive X-ray-analysis apparatus attached to a 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 abundance of sulfide is determined by image analysis.

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

[Hardenability index]
It is necessary that the content expressed by mass% of each element in the chemical component satisfies the following formula (B) which is a hardenability index. In addition, when Mo and Ni which are selective components are included, the hardenability index is redefined as the following formula (C) instead of the 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 )

  Based on the above formulas (B) and (C) that are hardenability indicators, carburizing and quenching of carburizing steel having various chemical components is performed, and the above-described conventional carburizing steel (C Compared with a content of about 0.2%), a threshold value can be obtained that can provide a carburized layer hardness equal to or higher than that and an effective hardened layer depth (depth at which the Vickers hardness is HV550 or higher). It was. That is, when the hardenability index is 7.5 or less, it is not possible to obtain the same characteristics as the above-described conventional steel (C content is about 0.2%). On the other hand, if the hardenability index is 44.0 or more, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical working rate decreases. Therefore, the hardenability index needs to be more than 7.5 and less than 44.0. This hardenability index is desirably as large as possible within a range that satisfies the above-described hardness index. 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 limit compression ratio of 68% or more, and exhibits excellent cold forgeability.

Next, the carburized steel part of this embodiment will be described.
The carburized steel part of the present embodiment is manufactured by subjecting the above-mentioned carburizing steel to a cold working process and a carburizing process. After the carburizing step, a finish heat treatment step may be performed as 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 this embodiment is a region where the Vickers hardness is HV550 or more.

  More specifically, the carburized steel part of the present embodiment includes a steel part and a carburized layer having a thickness of more than 0.4 mm and less than 2 mm generated on the outer surface of the steel part. The Vickers hardness of the carburized layer at a depth of 50 μm from the part surface is preferably HV650 or more and HV1000 or less. Moreover, it is preferable that the Vickers hardness of the steel part in the position of depth 2mm from the component surface is HV250 or more and HV500 or less. The Vickers hardness of the carburized layer becomes harder than that of the carburizing steel as a material by performing the carburizing treatment. Moreover, when the Vickers hardness of the steel part after a carburizing process is inadequate, hardening or quenching and tempering should be performed as a finishing heat treatment process, and the Vickers hardness of a steel part should just be set to HV250 or more.

The chemical composition of the steel part is substantially 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 abundance of sulfide having an equivalent circle diameter of less than 2 μm in a cross section parallel to the rolling direction of the steel material is 300 pieces / mm ² or more.

[Production method]
Next, the manufacturing method of the carburizing steel of this embodiment and the manufacturing method of carburized steel parts 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. A cold forged product is, for example, a machine part used in automobiles and construction machines, and is a steel part such as a gear, a shaft, or a pulley.

  The carburizing steel manufacturing method of this embodiment continuously casts a slab having the above chemical components and having a dendrite primary arm interval of less than 600 μm within a range of 15 mm from the surface layer. Manufactured by hot working and further annealing. Hot working may include hot rolling.

[Continuous casting process]
A steel slab satisfying the above chemical composition is produced by a continuous casting method. An ingot (steel ingot) may be formed by an ingot-making method. The casting conditions can be exemplified by a condition in which a 220 × 220 mm square mold is used, 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 the 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 slab surface. The average cooling rate is preferably 100 ° C./min to 500 ° C./min. If the average cooling rate is less than 100 ° C./min, it becomes difficult to make the dendrite primary arm interval less than 600 μm at a depth of 15 mm from the slab surface, and there is a possibility that the sulfide cannot be finely dispersed. On the other hand, if it exceeds 500 ° C./min, the sulfide crystallized from the dendritic trees becomes too fine, and the machinability may be reduced.

  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 by, for example, controlling the mold cross-sectional size, casting speed, etc. to appropriate values, or increasing the amount of cooling water used for water cooling immediately after casting. This is applicable to both continuous casting and ingot casting methods.

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

λ ₂ = 710 × A ^−0.39

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

  A billet (steel piece) is manufactured by hot-working a slab or an ingot, and further, the billet is hot-worked to obtain a bar steel or a wire rod.

  As the hot working process, the slab after the casting process is subjected to hot rolling, hot forging, etc. to obtain a hot worked steel material. Plastic processing conditions such as processing temperature, processing rate, and strain rate in the hot processing step are not particularly limited, and appropriate conditions may be selected as appropriate.

  Immediately after the hot working step, the temperature range in which the surface temperature of the hot worked steel material is 800 ° C. to 500 ° C. is set as 0 ° C. / Carburizing steel of the present embodiment is obtained by performing slow cooling at a cooling rate of more than 1 second / second or less.

  When the cooling rate at 800 ° C. to 500 ° C., which is a temperature at which austenite is transformed into ferrite and pearlite, exceeds 1 ° C./second, the structure fraction of bainite and martensite increases. 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./second and 1 ° C./second or less. More preferably, it is more than 0 ° C./second and 0.7 ° C./second or less. In order to reduce the cooling rate of hot-worked steel after the hot working process as a slow cooling process, a heat insulating cover, a heat insulating cover with a heat source, or a holding furnace is installed after the rolling line or hot forging line. do it. Further, as will be described later, spheroidizing annealing may be further performed to provide the carburizing steel of the present embodiment.

  Next, the manufacturing method of the carburized steel part which concerns on this embodiment is demonstrated.

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

  Next, carburizing treatment or carbonitriding treatment is performed as a carburizing step on the carburizing steel provided with the shape after the cold working step. In order to obtain a carburized steel part having the above-described metal structure and hardness, the conditions of carburizing or carbonitriding are as follows: temperature is 830 ° C. to 1100 ° C., carbon potential is 0.5% to 1.2%, and carburizing is performed. The time is preferably set to 1 hour or more. Through the above process, the carburized steel part according to the present invention is obtained.

  After the carburizing process, the carburized steel part may be subjected to a quenching process or a quenching / tempering process as a finishing heat treatment process, if necessary. In particular, the finish heat treatment step is preferably performed when the Vickers hardness of the steel portion after the carburizing step is insufficient. The quenching treatment or quenching / tempering treatment conditions for obtaining a carburized steel part having the above-described metal structure and hardness are preferably carried out when the temperature of the quenching medium is in the range of room temperature to 250 ° C. Moreover, you may perform a subzero process after hardening as needed.

  Moreover, you may perform a spheroidizing annealing process to the carburizing steel before the said cold work process as needed. 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 appropriate conditions may be selected as appropriate.

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

  Moreover, you may perform a shot peening process as a grinding process or a shot peening process further to the carburized steel components after the said finishing heat treatment process as needed. By performing the grinding process, it is possible to impart a precise 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, the tooth base and tooth surface fatigue strength of the carburized steel part can be further improved. The shot peening treatment is desirably performed using shot grains having a diameter of 0.7 mm or less and an arc height of 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 component, the dendrite structure that becomes the crystallization nucleus of the sulfide can be refined, and the sulfide in the steel can be finely dispersed. Thereby, the machinability after cold forging, that is, the machinability before carburizing, which is a material for steel parts such as gears, shafts, and pulleys, can be improved.
As described above, the carburizing steel according to the present invention is a machinability when performing a cutting process after directly or roughly performing a rough formed product by cold forging after annealing. Is excellent. For this reason, 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 the parts can be improved.

According to the carburizing steel of the present embodiment, the component composition satisfying the above formula (B) or (C) as a hardenability index by adding a small amount of Bi and Sb, Sn or Te with a relatively small amount of carbon. And having an average distance between sulfides of less than 30.0 μm and a density of sulfides of less than 2 μm of 300 pieces / mm ² or more, deformation resistance during cold forging is small, The machinability after cold forging can be improved. In particular, the carburizing steel of the present embodiment has a low Vickers hardness of HV125 or less and thus has a low deformation resistance during cold forging, and a critical compressibility of 68% or more, so that the cold forgeability is good. . And since the Vickers hardness of a steel part becomes HV250 or more by passing through the manufacturing process of a carburized steel part, and also the Vickers hardness of a carburized layer becomes HV650 or more, it is suitable as a raw material of a carburized steel part.

The carburizing steel of the present embodiment is excellent in machinability when performing a cutting process after directly or roughly performing a rough formed product by cold forging after annealing. . For this reason, 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 the parts can be improved.
Further, in this embodiment, when producing carburizing steel, by casting a slab having a predetermined chemical component, the dendrite structure that becomes the crystallization nucleus of sulfide is refined, and sulfide in steel Can be finely dispersed. Thereby, the machinability after cold forging, that is, the machinability before carburizing, which is a material for steel parts such as gears, shafts, and pulleys, can be improved.

  Further, according to the carburized steel part of the present embodiment, the steel part and the carburized layer generated on the outer surface of the steel part are provided, the carburized layer has a Vickers hardness of HV650 or more and HV1000 or less, and the steel part has a Vickers hardness. 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 that are excellent in cold forgeability and machinability.

  Steels A to AC having chemical compositions shown in Table 1 were melted in a 270 ton converter, and continuous casting was performed using a continuous casting machine. The casting conditions were as follows: using a 220 × 220 mm square mold, the superheat of the molten steel in the tundish was 10 to 50 ° C., the casting speed was 1.0 to 1.5 m / min, and 15 mm from the surface of the slab. 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. Note that reduction was applied during the solidification of continuous casting.

In continuous casting of a slab, the average cooling rate within 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 amount of cooling water in the mold. It was. In this way, a slab having the chemical components shown in Table 1 and having a dendrite primary arm interval of less than 600 μm within a range of 15 mm from the surface layer was continuously cast.

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

The slab thus obtained was once cooled to room temperature, and the presence or absence of surface cracks in the slab was visually determined. The results are shown in Table 2. Further, the slab once cooled to room temperature was collected as a test piece for observing a dendrite structure.

  Thereafter, each slab is heated at 1250 ° C. for 2 hours, and the heated slab is hot forged to produce a plurality of round bars having a circular cut surface perpendicular to the longitudinal direction and a cross-sectional diameter of 30 mm. did. After hot forging, the round bar was allowed to cool in the atmosphere. The natural cooling was performed by covering the round bar with a heat insulating cover so that the cooling rate at 800 ° C. to 500 ° C. was 1 ° C./second or less. Then, spheroidizing heat treatment was performed as a spheroidizing annealing process (SA process: Spheroidizing Annealing) on some round bars after being allowed to cool. Thus, the steel materials which consist of the steel for carburization of the test numbers 1-29 were manufactured.

[Coagulation structure observation method]
For the solidified structure, the cross section of the slab was etched with picric acid, and the dendrite primary arm interval and secondary arm interval were measured at 100 points at a 15 mm position in the depth direction from the slab surface at a pitch of 5 mm in the casting direction. The average value was obtained.

[Microstructure test]
The microstructure of each steel number round bar was observed. When the axial length of the round bar is L, the L / 4 position was cut perpendicular to the axial direction, and a specimen for microstructural observation was collected. The cut surface of the test piece was polished, the steel metal structure was observed with an optical microscope, and precipitates were discriminated from the contrast in the structure. In addition, the deposit was identified using the scanning electron microscope and the energy dispersive X-ray-spectral-analysis apparatus (EDS). Ten polishing test pieces each having a length of 10 mm and a width of 10 mm were prepared from a cross section including the longitudinal direction of the test piece described later, and a predetermined position of these polishing test pieces was photographed 100 times with an optical microscope. Images of 9 mm ² inspection reference area (region) were prepared for 10 fields of view. Ten MnS in the observation field of view (image) were selected in descending order, and the equivalent circle diameter of each MnS was calculated. These dimensions (diameters) were converted to equivalent circle diameters indicating the diameters of circles having the same area as the precipitates. From the detected particle size distribution of sulfides, the density of sulfides having an equivalent circle diameter of less than 2 μm and the average distance between sulfides were calculated. Table 2 shows the results. In order to satisfy the conditions of the present invention, the case where the abundance of sulfides was 300 pieces / mm ² or more and the case where the average distance between sulfides was less than 30.0 μm were regarded as acceptable.

[Hardness measurement test]
The hardness was measured 10 times in total using a Vickers hardness meter, and the average value was calculated. Table 2 shows the measurement results. 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 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 a half depth of the cut surface from the circumferential surface of a round bar having a diameter of 30 mm. The round bar test piece is a test piece having a diameter of 10 mm and a length of 15 mm centering on a position of a diameter 1/2 depth of the cut surface from the peripheral surface of a round bar having a diameter of 30 mm. The direction was parallel to the forging axis of a round bar with a diameter of 30 mm.

  Ten pieces of the above round bar test pieces were prepared for each steel. 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 constraining die. When a micro 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 a total of 10 times to obtain a compression rate at which the cumulative failure probability was 50%, and the compression rate was defined as the limit compression rate. Table 2 shows the measurement results. Since the limit compressibility of carburizing steel after conventional spheroidizing annealing is about 65%, when the limit compressibility is 68% or more which can be regarded as a value clearly higher than this value, the limit processing rate is sufficient, Judged.

[Machinability test]
Each steel was strained by cold drawing instead of cold forging using the remainder of the steel bar with a diameter of 30 mm, and the machinability after cold forging was evaluated by the machinability after drawing.

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

  The outer periphery of the test material having a diameter of 25 mm and a length of 500 mm thus obtained was turned using an NC lathe under the following conditions, and the machinability was investigated. After 10 minutes from the start of turning, the wear amount (mm) of the flank face of the carbide tool was measured. Table 2 shows the measurement results. If the measured wear amount of the flank was 0.20 mm or less, the machinability was sufficient and it was determined to be acceptable.

<Chip used>
Base material: Carbide P20 grade Coating: None <Turning conditions>
Peripheral speed: 150m / min
Feed: 0.2 mm / rev.
Cutting depth: 0.4mm
Lubrication: Uses water-soluble cutting oil

  Chip disposal was evaluated by the following method. Chips discharged 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. The total weight of the ten 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 to the number of 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 was calculated to be 12 g × 10 pieces / 7 pieces. Table 2 shows the measurement results. If the chip weight was 15 g or less, the chip processability was sufficient and it was determined to be acceptable.

[Carburization characteristics evaluation test]
A carburized test piece (20 mmφ × 30 mm) was sampled from the circumferential surface of the carburized steel produced by the above method so that the longitudinal direction was the compression direction from the position of the diameter ¼ depth of the cut surface. . As the carburizing process, gas carburizing was performed by the gas-shift furnace gas method. This gas carburization was carried out at 950 ° C. for 5 hours with a carbon potential of 0.8%, and subsequently at 850 ° C. for 0.5 hour. After the carburizing step, as a finish heat treatment step, oil quenching to 130 ° C. was performed, and tempering was performed at 150 ° C. for 90 minutes to obtain carburized steel parts.

  The characteristics of the carburized steel part and the steel part of the manufactured carburized steel part were evaluated. For the carburized layer of the carburized steel part, the hardness at a position 50 μm deep from the surface and the hardness at a position 2 mm deep from the surface are measured 10 times in total using a Vickers hardness meter, and the average The value was calculated. Table 2 shows the measurement results. The case where the hardness at the position of 50 μm depth from the surface is HV650 to HV1000, and the case where the hardness at the position of 2 mm depth from the surface is HV250 to HV500 is determined to be acceptable because the hardness is sufficient. .

  Furthermore, about the carburized layer of the said carburized steel part, the hardness distribution from the surface to a 5-mm position was measured 3 times in total using the Vickers hardness meter, and the average value was computed. If the depth of the position which is HV550 or more is more than 0.4 mm and less than 2 mm from the surface, it was determined to be acceptable.

  With reference to Table 1 and Table 2, the chemical composition of the steels A to O is within the range of the chemical composition of the carburizing steel according to the present embodiment, and the hardenability index, the abundance of sulfides, and between sulfides All of the average distances met the target. As a result, the performance required for the carburizing steel and carburized steel part of the present invention is satisfied.

  Test number 16 is the same component as steel that satisfies the standard of JIS standard SCr420H, which is common as a general-purpose steel type. For this reason, since the contents of chemical components C, Cr, Ti, B, Bi, Sb, Sn, Te and N do not satisfy the scope of the present invention, the abundance of sulfides and the average distance between sulfides are This is an example that does not satisfy the scope of the present invention, and that the carburizing steel is not sufficiently softened, critical compressibility, machinability, and chip disposal.

  Since test number 17 did not contain any of Bi, Sb, Sn, and Te, the abundance of sulfides and the average distance between sulfides did not satisfy the scope of the present invention. Therefore, this is an example in which machinability and chip disposal are insufficient. This is due to the fact that none of Bi, Sb, Sn and Te was contained, so that the sulfide refinement effect could not be obtained.

  Test Nos. 18 to 21 are examples in which the Bi, Sn, or Te content does not satisfy the scope of the present invention, the hot workability is deteriorated, and the slab after continuous casting is cracked.

  Since test number 22 did not contain B, the hardness of the steel part of the carburized steel part and the thickness of the carburized layer were insufficient. This is because the hardenability improvement effect was not obtained because B was not contained.

  Test No. 23 is an example in which the critical compression rate of the carburizing steel is insufficient because the N content of the chemical component does not satisfy the scope of the present invention. The reason why the critical compressibility of the carburizing steel became insufficient is that because of the high N content, coarse TiN was generated, which became the starting point of fracture during cold working.

  Test number 24 is an example in which the critical compressibility of the carburizing steel is insufficient because the S content of the chemical component does not satisfy the scope of the present invention. The reason why the critical compressibility of the carburizing steel became insufficient is that because of the large S content, coarse MnS was generated, which became the starting point of fracture during cold working.

  In Test No. 25, since the S content of the chemical component does not satisfy the scope of the present invention, the density of sulfides and the average distance between the sulfides do not satisfy the scope of the present invention, and the machinability of the carburizing steel. This is an example in which the chip disposal is insufficient.

  Test No. 26 is an example in which the hardness of the steel part of the carburized steel part and the thickness of the carburized layer are insufficient because the hardenability index does not satisfy the scope of the present invention.

  Test No. 27 is an example in which the hardenability index does not satisfy the scope of the present invention, so the hardness of the steel part of the carburized steel part exceeds HV500 and the carburized layer becomes too thick.

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

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

  As mentioned above, although embodiment of this invention was described, embodiment mentioned above is only the illustration for implementing this 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)

% By mass
C: 0.07% to 0.13%,
Si: 0.0001% to 0.50%
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%
Ti: 0.020% to less than 0.100%,
Al: 0.010% to 0.100% and Bi: more than 0.0001% to 0.0050%,
Containing one or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050%,
Furthermore, N, P and O are each
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content expressed by mass% of each element in the chemical component satisfies the formula (1) as a hardenability index,
The average distance between sulfides in steel is less than 30.0 μm, and the presence density of sulfides having an equivalent circle diameter of less than 2 μm in a cross section parallel to the rolling direction of the steel material is 300 pieces / mm ² or more. Features carburizing steel.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (1) In addition,
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 element or two or more elements,
The carburizing steel according to claim 1, wherein a content expressed by mass% of each element in the chemical component 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 having a thickness of more than 0.4 mm and less than 2 mm generated on the outer surface of the steel part,
The carburized layer has a Vickers hardness of HV650 or more and HV1000 or less at a depth of 50 μm from the surface of the component,
The Vickers hardness of the steel part at a position 2 mm deep from the part surface is HV250 or more and HV500 or less,
The steel part is mass%,
C: 0.07% to 0.13%,
Si: 0.0001% to 0.50%
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%
Ti: 0.020% to less than 0.100% Al: 0.010% to 0.100% and Bi: more than 0.0001% to 0.0050%,
Containing one or more of Sb: 0.0001% to 0.0050%, Sn: 0.0001% to 0.0050% and Te: 0.0001% to 0.0050%,
Furthermore, N, P and O are each
N: 0.0080% or less,
P: 0.050% or less,
O: 0.0030% or less,
The balance consists of Fe and impurities,
The content expressed by mass% of each element in the chemical component satisfies the formula (3) as a hardenability index,
The average distance between sulfides in the steel part is less than 30.0 μm, and the abundance of sulfides having an equivalent circle diameter of less than 2 μm in a cross section parallel to the rolling direction of the steel is 300 pieces / mm ² or more. Carburized steel parts characterized by that.
7.5 <(0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44.0 (3) The steel part 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 element or two or more elements,
4. The carburized steel part according to claim 3, wherein a content expressed by mass% of each element in the chemical component satisfies Formula (4) instead of Formula (3) 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 (4) ) A cold working step of applying a cold plastic working to the carburizing steel according to claim 1 or 2 to give a shape;
A carburizing step of subjecting the carburizing steel after the cold working step to carburizing or carbonitriding;
5. The method for manufacturing a carburized steel part according to claim 3, wherein:   6. The method for manufacturing a carburized steel part according to claim 5, wherein a finishing heat treatment step for performing a quenching treatment or a quenching / tempering treatment is performed after the carburizing step.   The method for manufacturing a carburized steel part according to claim 5 or 6, further comprising a cutting step of giving a shape by performing a cutting process after the cold working step and before the carburizing step.