JP6642236B2 - Cold forging steel - Google Patents

Description

  The present invention relates to a steel for cold forging.

  Steel parts such as gears, pulleys, and shafts used in automobiles and industrial machines are often roughly formed by hot forging or cold forging. Since cold forging has higher dimensional accuracy than hot forging, the amount of cutting after forging can be reduced. For this reason, in recent years, the number of steel parts roughly formed by cold forging has increased.

  When a steel material is roughly formed by cold forging, a steel material before cold forging is often subjected to spheroidizing annealing in order to reduce deformation resistance in forging and improve deformability. Further, the steel material after cold forging is often subjected to a cutting process and a surface hardening treatment such as carburizing quenching or carbonitriding quenching.

  However, when the austenite grains contained in the steel material are coarsened during the surface hardening treatment, problems such as insufficient fatigue strength of the steel part obtained after the surface hardening treatment and large deformation due to the surface hardening treatment are likely to occur. For this reason, there is a demand for a steel material capable of stably suppressing the austenite grain coarsening during the surface hardening treatment.

  Further, when spheroidizing annealing is performed on a steel material to form a spheroidizing annealed structure, there is a problem that machinability during cutting after cold forging is reduced. To solve this problem, it is known that machinability is improved by including sulfur (S) in steel. S combines with manganese (Mn) in steel to form a Mn sulfide-based inclusion mainly composed of MnS, and improves machinability.

  However, when the S content in the steel material is increased in order to enhance the machinability, a large amount of coarse sulfide is generated, and the cold forgeability is reduced. For this reason, in the conventional steels for cold forging, a reduction in the S content has suppressed a decrease in cold forgeability. As a result, the conventional cold forging steel had low machinability. Therefore, there is a demand for a steel material for cold forging in which the S content is increased and the machinability is improved without impairing the cold forgeability.

Conventionally, as a steel material formed into a component shape by cold forging after spheroidizing annealing, for example, those described in Patent Documents 1 to 3 are known.
Patent Document 1 discloses that the content of C is 0.2 to 0.6%, the fraction of proeutectoid ferrite is 5 to 30% by area, the remainder is composed of a structure mainly composed of bainite, and the cementite in the bainite is described. A steel wire and a steel bar having excellent cold forgeability after spheroidization having an average value of the lath interval of 0.3 μm or more is disclosed.

  Patent Literature 2 discloses a case-hardening steel wire / bar having a mixed structure containing ferrite, bainite, and pearlite, having an area fraction of bainite of 30% or more, and having excellent cold forgeability after spheroidization. It has been disclosed.

Patent Literature 3 discloses that the amount of Al precipitated as AlN and AlN-Nb (CN) in a region up to 半径 of the radius from the surface of the steel bar or the wire and in a region up to １ ／ of the radius from the center portion. 0.010% or less, AlN, Nb (CN) and AlN-Nb in which the amount of Nb precipitated as Nb (CN) and AlN-Nb (CN) is 0.020% or less and the diameter is 100 nm or more. A metal having a total number density of (CN) of 50/100 μm ² or less, an area ratio of a ferrite bainite structure of 80% or more, an area ratio of bainite of 30 to 70%, and an average ferrite grain size of 15 to 40 μm. A hot rolled steel bar or wire having a structure is disclosed.

Japanese Patent No. 3737323 Japanese Patent No. 4411096 Japanese Patent No. 5397247

However, the steel wire rods and bars described in Patent Literature 1 have a structure in which the fraction of proeutectoid ferrite is as low as 5 to 30% by area, so that the reduction of deformation resistance in cold forging is insufficient. Further, in the technique described in Patent Document 1, no measure is taken against coarsening of austenite grains during surface hardening treatment after cold forging.
Also, the technology described in Patent Document 2 does not consider the coarsening of austenite grains during surface hardening treatment after cold forging.
Furthermore, none of the techniques disclosed in Patent Documents 1 to 3 considers improvement in machinability after cold forging, and the machinability after cold forging is unclear.

  The present invention has been made in view of such circumstances, and has excellent cold forgeability and machinability after cold forging, and prevents coarsening of austenite grains during surface hardening treatment after cold forging. It is an object to provide a steel for cold forging that can be performed.

[1] In mass%,
C: 0.10 to 0.30%,
Si: 0.01 to 1.00%,
Mn: 0.40 to 1.80%,
S: 0.005 to 0.030%,
Cr: 0.01 to less than 1.60%,
Al: 0.010-0.045%,
N: 0.010-0.03%,
Bi: 0.0001 to 0.0050%,
Nb: 0.02 to 0.08%
Containing
P: 0.05% or less,
O: limited to 0.0025% or less, the balance being Fe and impurities,
A steel bar or wire rod having a chemical composition satisfying the following formula (1), having a microstructure composed of ferrite and pearlite,
The amount of Al precipitated as AlN and AlN-Nb (CN) is 0.010% or less in a region from the surface to １ ／ of the radius and in a region from the center to １ ／ of the radius, and Nb (CN) And the total number density of AlN, Nb (CN) and AlN-Nb (CN) having an amount of Nb precipitated as AlN-Nb (CN) of not more than 0.020% and a diameter of not less than 100 nm is 50. Pieces / 100 μm ² or less,
A steel for cold forging, characterized in that, in a cross section parallel to the rolling direction, the number density of sulfides having an equivalent circle diameter of less than 2 μm is 300 / mm ² or more.

0.8 ≦ (Nb / 93 + Al / 27) / (N / 14) ≦ 1.8 (1)
(Nb, Al, and N in the formula (1) are the contents of each element in mass%.)

[2] Instead of part of Fe, in mass%,
Mo: 1.5% or less,
Ni: 1.0% or less,
Cu: 0.4% or less,
V: 0.35% or less,
B: 0.020% or less,
Mg: The steel for cold forging according to [1], containing one or more kinds selected from the group consisting of 0.0035% or less.

  The steel for cold forging of the present invention is excellent in cold forgeability and machinability after cold forging, and can prevent coarsening of austenite grains during surface hardening treatment after cold forging. Therefore, the steel for cold forging of the present invention can be suitably used as a material for steel parts such as gears, pulleys, and shafts that are roughly formed by cold forging.

The present inventors have conducted research and studies on steels for cold forging in order to solve the above problems. As a result, the following findings (a) to (f) were obtained.
(A) By using cold forging steel (bar or wire) having a predetermined chemical composition containing a trace amount of Bi, solid solution of pinned particles (such as AlN) during surface hardening treatment after cold forging. It was clarified that the coarsening was suppressed and the generation of coarse austenite grains could be suppressed.

(B) Cutting is a breaking phenomenon that separates chips. To promote cutting, one of the points is to make the matrix brittle. When the sulfide is finely dispersed in the steel material to facilitate the destruction, the chip controllability (machinability) is improved. However, if a small number of coarse sulfides are dispersed in the steel material, the distance between the sulfides that is a starting point of chip separation becomes long. As a result, the chips tend to be longer.

The “sulfide” in the present embodiment means a general term for the following Mn sulfide-based inclusions.
That is, inclusions mainly containing MnS, sulfides such as Fe, Ca, Ti, Zr, Mg, and REM are dissolved or combined with MnS and coexist, and elements other than S such as MnTe are combined with Mn. Includes inclusions that form a compound and coexist with MnS by solid solution / bonding, and the above-mentioned inclusions precipitated with oxides as nuclei, and are represented by (Mn, X) (S, Y) ( Here, X is a general term for Mn sulfide-based inclusions that can be expressed as sulfide-forming elements other than Mn and Y: elements other than S that bind to Mn. In addition, it can be confirmed that the inclusion is a sulfide by an energy dispersive X-ray analysis attached to a scanning electron microscope.

(C) The present inventors conducted various experiments on the relationship between the equivalent circle diameter of sulfide and chip controllability. As a result, it has been found that when the number density of sulfides having an average equivalent circle diameter of less than 2 μm is 300 / mm ² or more, the chip controllability is improved.

(D) Sulfides in steel materials often crystallize before solidification (in molten steel) or during solidification. Therefore, the size of the sulfide in the steel material is greatly affected by the cooling rate during solidification. The solidified structure of the continuous cast slab usually has a dendrite form (dendrites). Dendrites are formed due to the diffusion of solute elements during the solidification process. The solute elements are concentrated in the dendritic tree. Specifically, Mn is concentrated in the dendritic tree portion, and Mn sulfide is crystallized.

(E) In order to finely disperse the sulfide in the steel material, it is necessary to shorten the interval between dendrite trees. The primary arm interval of the dendrite can be expressed by the following equation (A).
λ∝ (D × σ × ΔT) ^0.25 (A)
In the formula (A), λ is the primary arm interval of the dendrite (μm), D is the diffusion coefficient (m ² / s), σ is the solid-liquid interface energy (J / m ² ), and ΔT is the solidification temperature range (° C.). It is.

  From equation (A), it can be seen that the primary arm interval λ of the dendrite depends on the solid-liquid interface energy σ, and λ decreases as σ decreases. If the primary arm spacing λ can be reduced, the size of Mn sulfide crystallized in the dendrite tree can be reduced. The present inventors have found that by adding a small amount of Bi to steel, the primary arm interval λ can be reduced and the size of sulfide can be reduced.

(F) Further, in order to suppress the coarsening of austenite grains at the time of surface hardening treatment after cold forging, the present inventors have found that the entire area of the cross section of the steel for cold forging (bar steel or wire rod) must have a radius from the surface. In the region up to 1/5 of the area and in the region up to 1/5 of the radius from the center, the amount of Al precipitated as AlN and AlN-Nb (CN) is set to 0.010% or less, and Nb (CN) and AlN The amount of Nb precipitated as -Nb (CN) needs to be 0.020% or less. Further, in the region from the surface to 1/5 of the radius and the region from the center to 1/5 of the radius, the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more is It has been found that it is necessary to reduce the number to 50/100 μm ² or less.
The present invention has been completed based on the above findings (a) to (f).

Hereinafter, the steel for cold forging of the present invention will be described in detail.
BACKGROUND ART Cold forging steel used as a material for steel parts such as gears is manufactured by, for example, performing hot working such as hot rolling or hot forging on a continuously cast slab. The obtained cold forging steel is cold forged, then cut into a predetermined component shape, and then subjected to a surface hardening treatment such as carburizing and quenching to become a component.

  Sulfides in the steel for cold forging lower the cold forgeability, but are extremely effective for improving the machinability. That is, the sulfide in the steel for cold forging, which is a work material, suppresses a tool change due to wear of the cutting tool and has an effect of extending the tool life. Therefore, in order to enhance the machinability, it is desirable to generate sulfide in the steel.

  On the other hand, when hot working such as hot rolling or hot forging is performed in the process of manufacturing the steel for cold forging, coarse sulfides are stretched and machinability often decreases. In order to suppress the coarsening of the sulfide, it is desirable to reduce the solid-liquid interfacial energy in the molten steel and make the dendrite of the cast slab finer. Dendrite greatly affects the sulfide particle size. The finer the dendrite, the smaller the sulfide particle size.

  In order to stably and effectively finely disperse the sulfide in the steel for cold forging, it is preferable to reduce the solid-liquid interface energy in the molten steel by using a chemical composition containing a trace amount of Bi. When the solid-liquid interfacial energy is reduced, the dendrite of the slab becomes fine, and the sulfide crystallized from it becomes fine.

Next, the chemical composition of the steel for cold forging of the present embodiment will be described. In addition, “%” of the content of each element means “% by mass”.
The steel for cold forging of this embodiment is as follows: C: 0.10 to 0.30%, Si: 0.01 to 1.00%, Mn: 0.40 to 1.80%, S: 0.005 to 0.030%, Cr: 0.01 to less than 1.60%, Al: 0.010 to 0.045%, N: 0.010 to 0.03%, Bi: 0.0001 to 0.0050%, Nb: 0.02 to 0.08%, P: 0.05% or less, O: 0.0025% or less, the balance being Fe and impurities, chemical composition satisfying the following formula (1) Having.

0.8 ≦ (Nb / 93 + Al / 27) / (N / 14) ≦ 1.8 (1)
(Nb, Al, and N in the formula (1) are the contents of each element in mass%.)

(C: 0.10 to 0.30%)
Carbon (C) increases the tensile strength and fatigue strength of steel. On the other hand, if the C content is too large, the cold forgeability of the steel decreases, and the machinability decreases. Therefore, the C content is 0.10 to 0.30%. The preferred C content is 0.14 to 0.28%, more preferably 0.15 to 0.25%.

(Si: 0.01-1.00%)
Silicon (Si) forms a solid solution in ferrite in steel and increases the tensile strength of steel. On the other hand, if the Si content is too large, the cold forgeability of the steel decreases. Therefore, the Si content is 0.01 to 1.00%. The preferred Si content is 0.15 to 0.70%, more preferably 0.20 to 0.35%.

(Mn: 0.40 to 1.80%)
Manganese (Mn) forms a solid solution in steel to increase the tensile strength and fatigue strength of the steel, and to enhance the hardenability of the steel. Mn further combines with sulfur (S) in the steel to form MnS and enhances the machinability of the steel. On the other hand, if the Mn content is too high, coarse MnS is generated, and the fatigue strength decreases. Therefore, the Mn content is 0.40 to 1.80%. The preferred Mn content is 0.60 to 1.30%, more preferably 0.70 to 1.20%.

(S: 0.005 to 0.030%)
Sulfur (S) combines with Mn in steel to form Mn sulfide and enhances machinability of the steel. On the other hand, if S is excessively contained, the fatigue strength of steel decreases. Therefore, the S content is 0.005 to 0.030%. The preferred S content is 0.008 to 0.018%, more preferably 0.010 to 0.016%.

(Cr: 0.01 to less than 1.60%)
Chromium (Cr) enhances the hardenability and tensile strength of steel. When a steel part is manufactured using the steel for cold forging of the present embodiment, a surface hardening treatment such as carburizing or induction hardening may be performed on the steel for cold forging having a part shape. Cr enhances the hardenability of the steel and increases the surface hardness of the steel after carburizing or induction hardening. On the other hand, when the Cr content is too large, the cold forgeability and the fatigue strength of the steel decrease. Therefore, the Cr content is less than 0.01 to less than 1.60%. When increasing the hardenability and tensile strength of steel, the preferred Cr content is 0.03% to 1.50%, and more preferably 0.10% to 1.20%.

(Al: 0.010-0.045%)
Aluminum (Al) has a deoxidizing effect. Al is easily combined with N to form AlN, and is an element effective in preventing austenite grain coarsening during carburizing heating. However, if the Al content is less than 0.010%, the effect of preventing austenite grains from coarsening during the surface hardening treatment after cold forging and the effect of reducing the deformation resistance in cold forging cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.045%, AlN becomes coarse, and does not contribute to suppression of coarsening of crystal grains. Therefore, the Al content is set to 0.010% to 0.045%. A preferred lower limit of the Al content is 0.020%, and a preferred upper limit is 0.035%.

(N: 0.010-0.03%)
Nitrogen (N) is added for the purpose of refining the crystal grains during the surface hardening treatment after cold forging by precipitation of AlN and Nb (CN) and suppressing the coarsening of the crystal grains. If the N content is less than 0.010%, the above-described effects due to N cannot be sufficiently obtained. On the other hand, when the N content exceeds 0.030%, the above-described effect due to the N content is saturated. Further, excessive addition of N makes the steel brittle and causes cracks and scratches during casting and rolling. For the above reasons, the N content needs to be in the range of 0.010% to 0.03%. A preferable range of the N content is 0.013 to 0.02%.

(Bi: 0.0001-0.0050%)
Bismuth (Bi) is an important element in the present invention. By containing a small amount of Bi, the solidification structure of steel is refined, and sulfides are finely dispersed. In order to obtain the effect of reducing sulfide, the Bi content needs to be 0.0001% or more. However, if the Bi content exceeds 0.0050%, the effect of dendrite refining is saturated. If the Bi content exceeds 0.0050%, surface cracks occur during hot rolling. For this reason, the Bi content is set to 0.0001 to 0.0050%. In order to further improve the machinability, the Bi content is preferably set to 0.0010% or more. Further, the Bi content may be 0.0048% or less.

(Nb: 0.02-0.08%)
Niobium (Nb) is an element that combines with C and / or N to form NbC, NbN, and Nb (CN), and has an effect of preventing austenite grains from coarsening during surface hardening treatment after cold forging. However, if the Nb content is less than 0.02%, the above effects cannot be sufficiently obtained. On the other hand, if the Nb content exceeds 0.08%, the effect of preventing the austenite grains from coarsening is rather reduced. Therefore, the Nb content is set to 0.02 to 0.08%. Note that the Nb content is preferably 0.03 to 0.05%.

(P: 0.05% or less)
Phosphorus (P) is an impurity. P reduces the cold forgeability and hot workability of steel. Therefore, it is preferable that the P content is small. The P content is 0.05% or less. The preferred P content is 0.035% or less, more preferably 0.020% or less.

(O: 0.0025% or less)
Oxygen (O) combines with Al to form hard oxide-based inclusions. If oxide-based inclusions are present in a large amount in steel, they become precipitation sites for AlN and Nb (CN), and coarse AlN and / or Nb (CN) precipitate during hot rolling. As a result, it becomes impossible to suppress the coarsening of crystal grains during the surface hardening treatment after cold forging. Therefore, it is desirable to reduce the O content as much as possible. For the above reasons, it is necessary to limit the O content to 0.0025% or less. The preferable range of the O content is 0.0020% or less.

  The balance of the chemical composition of the steel for cold forging in the present embodiment consists of Fe and impurities. The impurities in the present embodiment are elements that are mixed in from the ore, scrap, the environment in the manufacturing process, or the like used as a raw material of steel when a steel material is manufactured industrially.

[About Equation (1)]
The chemical composition of the steel affects the total number density (dispersion state) of AlN, Nb (CN) and AlN-Nb (CN) in the steel for cold forging. If the content of Al and / or Nb with respect to the N content is too high, coarse AlN, Nb (CN) and AlN-Nb (CN) are precipitated, and the coarsening suppression characteristics of crystal grains are deteriorated. By setting the Al content and the Nb content relative to the N content in the steel in appropriate ranges, coarsening of AlN, Nb (CN) and AlN-Nb (CN) can be suppressed.

Since the steel for cold forging of the present embodiment has a chemical composition satisfying the following formula (1), coarsening of austenite grains during surface hardening treatment after cold forging is suppressed.
0.8 ≦ (Nb / 93 + Al / 27) / (N / 14) ≦ 1.8 (1)
(Nb, Al, and N in the formula (1) are the contents of each element in mass%. When the corresponding element is at the impurity level, “0” is assigned to the corresponding element symbol in the formula (1). Is assigned.)

  Equation (1) is an index of the crystal grain coarsening suppression characteristics. If (Nb / 93 + Al / 27) / (N / 14) in the formula (1) is too low, the number of precipitates that suppress the coarsening of the crystal grains decreases, and the crystal grain coarsening suppressing properties are inferior. On the other hand, when (Nb / 93 + Al / 27) / (N / 14) is large and the Al content and / or Nb content with respect to the N content is too high, coarse AlN, Nb (CN) and AlN-Nb ( CN) precipitates, and the crystal grain coarsening suppressing properties are poor.

  The steel for cold forging of the present embodiment may contain at least one selected from the group consisting of Mo, Ni, Cu, V, B, and Mg. Mo, Ni, Cu, V, B and Mg all increase the fatigue strength of steel.

(Mo: 1.5% or less)
Molybdenum (Mo) enhances the hardenability of the steel and increases the fatigue strength of the steel. Mo suppresses formation of an incompletely quenched layer in the carburizing treatment. The above effect can be obtained if Mo is contained even a little. On the other hand, if the Mo content is too large, the machinability of the steel decreases. In addition, the cost of producing the steel also increases. Therefore, the Mo content is 1.5% or less. When the Mo content is 0.02% or more, the above-described effects are remarkably obtained. The preferred Mo content is 0.05 to 0.50%, more preferably 0.10 to 0.30%.

(Ni: 1.0% or less)
Nickel (Ni) is an element effective for enhancing hardenability and increasing fatigue strength, and may be contained as necessary. However, if the Ni content exceeds 1.0%, not only the effect of improving the fatigue strength due to the improvement of the hardenability is saturated, but also the deformation resistance is increased and the cold forgeability is significantly reduced. Therefore, the Ni content is set to 1.0% or less. The Ni content is preferably 0.8% or less. Further, in order to stably obtain the effect of increasing the fatigue strength by improving the hardenability of Ni, the Ni content is preferably 0.1% or more.

(Cu: 0.4% or less)
Copper (Cu) is an element that enhances the hardenability of steel. In order to stably obtain the effect of improving hardenability by containing Cu, the Cu content is preferably 0.10% or more. However, the addition of a large amount of Cu causes deterioration of the surface properties of the steel material and an increase in alloy cost. For this reason, the upper limit of the Cu content is set to 0.4% or less.

(V: 0.35% or less)
Vanadium (V) forms carbides in the steel and increases the fatigue strength of the steel. Vanadium carbide precipitates in ferrite and increases the strength of the steel core (parts other than the surface layer). The above effect can be obtained if V is contained even a little. On the other hand, if the V content is too large, the cold forgeability and the fatigue strength of the steel decrease. Therefore, the V content is set to 0.35% or less. When the V content is 0.03% or more, the above effect due to the inclusion of V is remarkably obtained. The preferred V content is 0.04 to 0.20%, and more preferably 0.05 to 0.10%.

(B: 0.020% or less)
Boron (B) enhances the hardenability of steel and increases the fatigue strength of steel. The above effect can be obtained if B is contained even a little. When the B content exceeds 0.020%, the above effect is saturated. Therefore, the B content is 0.020% or less. When the B content is 0.0005% or more, the above-described effects are remarkably obtained. The preferred B content is 0.001 to 0.012%, and more preferably 0.002 to 0.010%.

(Mg: 0.0035% or less)
Like Al, magnesium (Mg) deoxidizes steel and refines oxides in steel. When the oxides in the steel are refined, the probability of fracture starting from the coarse oxides is reduced, and the fatigue strength of the steel is increased. The above effect can be obtained if Mg is contained even a little. On the other hand, if the Mg content is too large, the above effect is saturated, and the machinability of the steel decreases. Therefore, the Mg content is 0.0035% or less. When the Mg content is 0.0001% or more, the above-mentioned effects are remarkably obtained. The preferred Mg content is 0.0003 to 0.0030%, and more preferably 0.0005 to 0.0025%.

[Sulfide]
Since sulfides are useful for improving machinability, it is necessary to ensure their number density. Increasing the S content in the steel improves machinability, but increases coarse sulfides. Coarse sulfide drawn by hot rolling or the like impairs cold forgeability. For this reason, it is necessary to control the size and shape of the sulfide in the steel. Further, in order to improve the chip controllability at the time of machining, it is necessary to disperse sulfide finely in steel.
In the steel for cold forging according to the present embodiment, the number density of sulfides having an equivalent circle diameter of less than 2 μm is 300 / mm ² or more in a cross section parallel to the rolling direction of the steel for cold forging. Therefore, excellent cold forgeability and chip controllability (machinability) can be obtained.

[AlN, Nb (CN) and AlN-Nb (CN)]
In the steel for cold forging according to the present embodiment, a region extending from the surface to １ ／ of the radius (hereinafter, may be referred to as “surface region”) and a region extending from the center to １ ／ of the radius (hereinafter, referred to as “surface region”). In some cases, the amount of Al precipitated as AlN and AlN—Nb (CN) is 0.010% or less, and Nb precipitated as Nb (CN) and AlN—Nb (CN). The amount is not more than 0.020%.
In the present embodiment, “AlN—Nb (CN)” refers to a composite precipitate of AlN and Nb (CN).

Further, in the steel for cold forging of the present embodiment, the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more in the surface region and the central region is 50/100 μm ² or less. It is.
In the present embodiment, the “diameter” of AlN, Nb (CN) and AlN—Nb (CN) refers to AlN when an extracted replica sample is prepared by a general method and observed using a transmission electron microscope. , Nb (CN) and AlN-Nb (CN) mean the arithmetic mean of the major axis and minor axis, respectively.

  The slab and the slab produced by hot working the slab have a large cross-sectional area. For this reason, when a slab and a steel slab are heated, it takes a long time until the central part and the surface part reach the same temperature. Therefore, when the slab and the steel slab are heat-treated, the time for which the slab and the slab are kept at a predetermined temperature is generally shorter at the central portion than at the surface portion.

  Therefore, in a steel for cold forging (a bar or a wire) manufactured by hot working a cast slab to form a slab and then hot rolling the slab, the AlN, Nb (CN) ) And AlN-Nb (CN), and the dispersed state of AlN, Nb (CN) and AlN-Nb (CN). Therefore, the central part and the surface part of the steel for cold forging also have a difference in the austenite grain coarsening during the surface hardening treatment after the cold forging.

In the steel for cold forging of the present embodiment, the amount of Al precipitated as AlN and AlN—Nb (CN) in the surface region and the central region is 0.010% or less, and Nb (CN) and AlN—Nb (CN). ) Is 0.020% or less and the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more is 50 pieces / 100 μm ² or less. is there. For this reason, in the steel for cold forging of this embodiment, coarsening of austenite grains at the time of surface hardening treatment after cold forging can be suppressed in the entire region from the surface layer to the center.

  In order to obtain the effect of suppressing the austenite grain coarsening, the amount of Al precipitated as AlN and AlN—Nb (CN) in the surface region and the central region is 0.010% or less, and 0.008% The following is preferred. If the Al content is less than 0.002%, it is difficult to obtain the target effect of suppressing austenite grain coarsening. Therefore, the Al content is preferably 0.002% or more.

  In order to obtain the above-described effect of suppressing austenite grain coarsening, the amount of Nb precipitated as Nb (CN) and AlN-Nb (CN) in the surface region and the central region is 0.020% or less, and 0% or less. It is preferably at most 0.015%. If the Nb content is less than 0.005%, it becomes difficult to obtain the target effect of preventing austenite grains from becoming coarse. Therefore, the Nb content is preferably 0.005% or more.

In order to obtain the effect of suppressing the coarsening of austenite grains, the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more in the surface region and the central region is 50 / It is 100 μm ² or less, and preferably 40/100 μm ² or less.

  The amount of Al precipitated as AlN and AlN-Nb (CN), the amount of Nb precipitated as Nb (CN) and AlN-Nb (CN), AlN, in the surface region and the central region of the steel for cold forging. The total number density (dispersion state) of Nb (CN) and AlN-Nb (CN) is determined by the chemical composition of steel, the production conditions of slabs and slabs, segregation of component elements in slabs and slabs, hot working It varies depending on conditions and the cooling rate after hot working.

"Production method"
Next, a method of manufacturing the steel for cold forging of the present embodiment will be described.
[Continuous casting process]
A slab having the above chemical composition is manufactured by a continuous casting method. The slab may be made into an ingot (steel ingot) by an 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 set to 10 to 50 ° C., and the casting speed is set to 1.0 to 1.5 m / min. .

  In the method for producing a steel for cold forging according to the present embodiment, in order to cast a slab having a predetermined chemical composition, dendrites which are crystallization nuclei of sulfides are refined, and sulfides are finely dispersed in the steel. Is done. Thereby, a steel for cold forging having excellent machinability after cold forging can be obtained.

[Dendrite]
The solidified structure of the slab has a dendrite form (dendrites). Sulfides in cold forging steel are often crystallized before solidification (in molten steel) or during solidification, and are greatly affected by the primary arm spacing of dendrite in slabs. That is, if the primary arm spacing of the dendrite is small, the sulfide crystallized in the dendrite tree becomes small. In the steel for cold forging of this embodiment, it is desirable that the primary arm interval of the dendrite at a depth of 15 mm from the surface of the slab at the stage of the slab is less than 600 µm.

  In this embodiment, in order to make the primary arm interval of the above-mentioned dendrite less than 600 μm, when casting molten steel having the above-mentioned chemical composition, the solidus temperature is determined from the liquidus temperature at a depth of 15 mm from the slab surface. It is desirable that the average cooling rate in the temperature range up to the temperature be 100 ° C./min or more and 500 ° C./min or less. When the average cooling rate is less than 100 ° C./min, it is difficult to make the primary arm interval of the dendrite at a depth of 15 mm from the slab surface less than 600 μm, and the sulfide may not be finely dispersed. On the other hand, if the average cooling rate exceeds 500 ° C./min, the sulfide crystallized from the dendrite tree becomes too fine, and there is a possibility that the chip disposability is 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 temperature in this temperature range means the average solidification rate of the slab. The above average cooling rate can be achieved, for example, by controlling the size of the mold section, the casting speed, and the like 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 average cooling rate in the above temperature range at a depth of 15 mm from the slab surface is a numerical value measured by the following method.
The cross section of the cast slab is etched with picric acid, and 100 points of the secondary arm spacing λ ₂ (μm) of the dendrite are measured at a depth of 15 mm from the slab surface at a pitch of 5 mm in the casting direction. Then, the cooling rate A in the temperature range from the liquidus temperature to the solidus temperature of the slab (slab) is calculated from the measured value of the secondary arm interval λ ₂ (μm) using the following equation (2). (° C./sec) is the average value obtained by arithmetic averaging.
λ ₂ = 710 × A− ^0.39 Equation (2)

  The casting conditions in the present embodiment may be determined by the following method. For example, a plurality of slabs with different casting conditions are manufactured, and the average cooling rate in the above-mentioned temperature range at a depth of 15 mm from the slab surface in each slab is determined by the above-described method using Expression (2). Then, the optimal casting conditions are determined using the casting conditions in which the average cooling rate was 100 to 500 ° C./min at which the primary arm spacing of the dendrite was less than 600 μm.

[Hot rolling]
Next, the obtained cast slab or ingot is subjected to hot working such as slab rolling to produce a billet (steel slab). Further, the billet is hot-rolled into a bar or a wire. Hot working may include hot rolling. The reduction ratio in hot working is not particularly limited.
The hot rolling of the billet is performed, for example, by heating the billet at a heating temperature of 1270 to 1300 ° C for 1.5 hours or more, and then performing a hot rolling at a finishing temperature of 900 to 1100 ° C, preferably 950 to 1050 ° C. be able to.

  After the finish rolling at the above-mentioned finishing temperature, cooling is performed in the atmosphere at a cooling rate in a temperature range of 800 to 500 ° C. in a range of 0.1 to 1.0 ° C./sec. The preferred range of the cooling rate is 0.7 ° C./sec or less. After the finish rolling, cooling may be performed to the room temperature at the above cooling rate. In order to increase productivity, it is preferable to cool by appropriate means such as air cooling, mist cooling and water cooling when the temperature after finish rolling reaches 500 ° C.

The above-mentioned heating temperature and heating time in the hot rolling of the present embodiment mean the average temperature in the furnace and the time in the furnace, respectively.
The finishing temperature of hot rolling means the surface temperature of a steel bar or a bar at the exit of the last stand of a rolling mill having a plurality of stands.
In addition, the cooling rate after performing the finish rolling refers to the cooling rate on the surface of the bar or the wire.

Next, the manufactured steel bars and wires are annealed. The annealing is preferably spheroidizing annealing. By performing spheroidizing annealing, the cold forgeability of a steel bar or a wire can be enhanced.
Through the above steps, the steel for cold forging of the present embodiment is obtained.

  The steel for cold forging of the present embodiment is, for example, preferably used as a material before carburizing, carbonitriding or nitriding for producing steel parts such as automobiles, gears for industrial machines, shafts, pulleys and the like. it can.

Next, a method of manufacturing a part using the steel for cold forging of the present embodiment will be described.
For example, cold forging steel (bar, wire) is cold forged to produce a crude intermediate product. The manufactured intermediate product may be cut by machining as necessary to obtain a predetermined shape.
Next, the intermediate product is subjected to a surface hardening treatment under known conditions. Examples of the surface hardening treatment include carburizing treatment, nitriding treatment, and induction hardening. Next, the intermediate product after the surface hardening treatment is ground or polished into a predetermined shape. In this way, a steel part using the steel for cold forging as a raw material is obtained.
In the above-described method for manufacturing a component, the intermediate product is subjected to the surface hardening process, but the surface hardening process may not be performed.

Steels A to AA having the chemical compositions shown in Table 1 were melted in a 270 ton converter, and were continuously cast using a continuous casting machine to produce slabs of 220 × 220 mm square. The reduction was applied during the solidification of the continuous casting.
The average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface was changed by changing the cooling water amount of the mold when casting each slab. .

  Steels A to O shown in Table 1 are steels having a chemical composition specified in the present invention. Steels P to AA are steels of comparative examples whose chemical compositions deviate from 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 steel material for cold forging according to the present embodiment.

Each cast piece obtained by continuous casting was once cooled to room temperature, a test piece was collected, and dendrites were observed by the following method.
[Measurement of primary arm interval and secondary arm interval of dendrite]
The cross section of the test piece taken from the cast piece was etched with picric acid. Then, the primary arm interval and the secondary arm interval of the dendrite were measured at 100 points each at a position at a depth of 15 mm from the slab surface and at a pitch of 5 mm in the casting direction. Table 2 shows the average value of the primary arm interval of the dendrite.

[Average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface]
From the value of the secondary arm interval λ ₂ (μm) measured by the above method, the cooling rate A (° C./second) in the temperature range from the liquidus temperature to the solidus temperature using the following equation (2). ) Was calculated and calculated by arithmetic mean. As a result, the average cooling rate in the above temperature range was 100 ° C./min or more and 500 ° C./min or less in any of the cast pieces.
λ ₂ = 710 × A− ^0.39 Equation (2)

After collecting a test piece for dendrite observation, each cast piece was hot-rolled to form a billet (steel piece). Thereafter, the billet was heated at 1270-1300 ° C. for 3 hours, and hot-rolled at a finishing temperature of 900-1100 ° C. After the finish rolling, the sample was cooled in a temperature range of 800 to 500 ° C. at a cooling rate of 0.7 ° C./second or less, and was cooled when the temperature reached 500 ° C. to produce a round bar (steel bar) having a diameter of 36 mm.
The surface of each round bar thus obtained was visually observed to determine the presence or absence of surface cracks. Table 2 shows the results.

Next, spheroidizing annealing was performed on each round bar having a diameter of 36 mm obtained after hot rolling. Specifically, each of the above-mentioned round bars was soaked in a heating furnace at 760 ° C. for 4 hours, cooled to 660 ° C. at a cooling rate of 15 ° C./h, and then allowed to cool.
Through the above steps, steels for cold forging of test numbers 1 to 27 were manufactured.

[Microstructure observation]
The microstructure of the steel (round bar) for cold forging of Test Nos. 1 to 27 was observed by the method described below. First, a round bar was cut in parallel to the axial direction, and a test piece for microstructure observation in which a position of 1/4 of the diameter (D / 4 position) was the center of the observation surface was collected. The cut surface (observation surface) of the test piece was polished and corroded with a nital etchant. After the corrosion, the microstructure at the center of the cut surface was observed at 400 times using an optical microscope.
Each of the microstructures of the steels for cold forging of Test Nos. 1 to 27 was a structure composed of ferrite and pearlite.

Further, a Vickers hardness test specified in JIS Z2244 was performed using the test piece for microstructure observation. The hardness of each test piece was measured at five points.
As a result, the Vickers hardnesses of the steels for cold forging of Test Nos. 1 to 27 were all within the range of Hv 130 to 200, and each steel for cold forging had the same hardness.

Next, the sulfide density of the steels for cold forging of Test Nos. 1 to 27 was measured by the following method.
[Sulphide density measurement method]
After embedding the test piece in a resin in the same manner as the microstructure observation, the test surface (cut surface) was mirror-polished. The test surface is parallel to the longitudinal direction of the steel for cold forging. The sulfide in the test surface was identified by a scanning electron microscope and an energy dispersive X-ray spectrometer (EDS).

Specifically, a test piece of polished vertical 10mm × horizontal 10mm were prepared 10 pieces, and photographed at 100 times by a scanning electron microscope predetermined positions of these test pieces, test reference area of 0.9 mm ² (Region) images were prepared for 10 fields of view. The observation field of the sulfide is 9 mm ² . In each observation region, the sulfide was specified based on the contrast of the reflected electron image observed by the scanning electron microscope. In the backscattered electron image, the observation area was displayed as a grayscale image. The contrast of the matrix (base material), sulfide, and oxide in the backscattered electron image was different from each other.

The particle diameter of the sulfide having an equivalent circle diameter of 1 μm or more in each observation visual field (image) was detected. The particle size (diameter) of the sulfide was converted to a circle equivalent diameter indicating the diameter of a circle having the same area as the area of the sulfide. Then, the number of sulfides in each observation visual field (image) was determined by image analysis. Then, among the observed sulfides of 1 μm or more, the number of sulfides with an equivalent circle diameter of less than 2 μm is divided by the number of sulfides with an equivalent circle diameter of 1 μm or more, and the sulfides with an equivalent circle diameter of less than 2 μm Of the number density.
The reason why the circle equivalent diameter of the sulfide to be observed was set to 1 μm or more is that it is practically possible to use a general-purpose device to statistically handle the particle size and components, and to use a smaller sulfide. This is because even if the material is controlled, the influence on the cold forgeability and the chip disposability is small.

[Residual analysis]
With respect to each round bar having a diameter of 36 mm (steel for cold forging of test numbers 1 to 27) obtained as described above, the area from the surface to 1/5 of the radius (hereinafter, referred to as “surface area”) may be used. ) And in a region from the center to １ ／ of the radius (hereinafter, sometimes referred to as “center region”), the amount of Al precipitated as AlN and AlN—Nb (CN), Nb (CN) and The amount of Nb precipitated as AlN-Nb (CN) was determined by the following method.

  A scale is present on the surface of each round bar having a diameter of 36 mm. Therefore, extraction residue analysis cannot be performed with sufficient accuracy as it is. Therefore, after turning each round bar having a diameter of 36 mm, a test piece for surface area analysis (35 mm in diameter and 10 mm in length) and a test piece for central area analysis (7.2 mm in diameter and 20 mm in length) And were collected from concentric positions.

Next, the cross section of each test piece was masked with a resin so as not to be electropolished. Then, the masked test piece was subjected to a current density of 250 to 350 A / m ² using a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution (10% AA-based electrolyte) which is a general condition. Extracted (electrolyzed). Then, the extracted solution was filtered through a filter having a mesh size of 0.2 μm, and the filtrate (residue) was subjected to general chemical analysis. From the results, the Al amount and the Nb amount in the surface region and the central region were calculated. Table 2 shows the results.

[Precipitate number density]
In the surface region and the central region, the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more was determined by the following method.
Extracted replica samples were prepared by a general method from each of the “surface area” and “center area” of each round bar having a diameter of 36 mm (steel for cold forging of test numbers 1 to 27). Next, each extracted replica sample was observed randomly for 10 fields using a transmission electron microscope at a magnification of 20,000 times and an area per field of 10 μm ² , and AlN, Nb (CN) and AlN having a diameter of 100 nm or more were observed. The number of -Nb (CN) was determined. Using the results, the number density of AlN, Nb (CN) and AlN-Nb (CN) per 100 μm ² of the area of each region was calculated. Table 2 shows the results.

Next, a cold forgeability test, a machinability test, and a coarse grain generation temperature test were performed on the cold forging steels of test numbers 1 to 27 by the following methods.
[Cold forgeability test]
From each steel for cold forging (round bar with a diameter of 36 mm), eight round bar test pieces with a diameter of 10 mm and a length of 15 mm with a notch centered on a half radius (R / 2 position). Was collected at a time. The longitudinal direction of the round bar test piece was parallel to the forging axis of a round bar having a diameter of 36 mm. A 500 ton hydraulic press was used for the cold compression test.

  In the cold compression test, cold compression was performed by gradually increasing the compression ratio using eight round bar test pieces. Specifically, eight round bar specimens were cold-compressed at an initial compression ratio. After the first cold compression, it was visually confirmed whether or not each round bar test piece had cracked. Then, the round bar test piece in which a crack was confirmed was removed, and the compression ratio was raised for the remaining round bar test piece (that is, the round bar test piece in which no crack was observed), and the second cold compression was performed. Was carried out. After the second cold compression, each round bar test piece was checked for cracks. In the same manner as after the first cold compression, the round bar test piece in which cracks were confirmed was removed, and the remaining round bar test piece was subjected to a third cold compression by increasing the compression ratio. The above-described process was repeated until four of the eight round bar specimens were found to have cracks.

The compression ratio when cracks were confirmed in four round bar specimens among the eight round bar specimens was defined as “critical compression ratio”. In addition, after performing cold compression at a compression rate of 80%, when the number of round bar specimens in which cracks were confirmed was four or less, the critical compression rate of the steel was set to “80%”.
The target value of the critical compression ratio was 75% or more, which is practically no problem. Table 2 shows the critical compressibility of each steel for cold forging.

[Machinability test]
Each steel for cold forging (a round bar having a diameter of 36 mm) was strained by cold drawing, and the machinability of the round bar after drawing was evaluated by the machinability of the round bar after drawing.
Specifically, each cold forging steel (round bar having a diameter of 36 mm) was cold-drawn at a reduction of area of 30.6% to form a round bar having a diameter of 25 mm. The cold-drawn round bar was cut into a length of 500 mm to obtain a test material for turning. The outer peripheral portion of the obtained test material having a diameter of 25 mm and a length of 500 mm was turned using a numerically controlled (NC) lathe under the following conditions, and the machinability (chip controllability) was investigated.

<Used chip>
Base material: Carbide P20 grade.
Coating: None.

<Turning conditions>
Circumferential speed: 150 m / min.
Feed: 0.2 mm / rev.
Cut: 0.4 mm.
Lubrication: Uses water-soluble cutting oil.

For each test material, chips discharged during 10 seconds during the turning were collected. The length of the collected chips was examined, and ten chips were selected in order from the longest one. Then, the total weight of the selected ten chips was defined as “chip weight”.
If the total number of chips is less than 10 as a result of long chips, the total weight of the collected chips is measured, and the value converted to the number of chips is defined as "chip weight". did. For example, when the total number of chips is 7 and the total weight is 12 g, the chip weight was calculated as 12 g × 10/7.

  When the chip weight of each test material was 15 g or less, it was evaluated that the chip disposability was high. When the chip weight exceeded 15 g, the chip disposition was evaluated as low. Table 2 shows the evaluation results of the chip disposability of each steel for cold forging.

[Coarse grain generation temperature test]
Each steel for cold forging (round bar with a diameter of 36 mm) was subjected to 60% compression in the height direction simulating cold forging, and then heat-treated to simulate carburization. As a heat treatment to simulate carburization, a treatment was performed in which each was held at 950 ° C., 970 ° C., 990 ° C., 1010 ° C., 1030 ° C. and 1050 ° C. for 300 minutes, and then cooled to room temperature by water cooling.

  A longitudinal section (test surface) including the central portion was cut out from each cold forging steel (round bar having a diameter of 36 mm) that had been subjected to the heat treatment simulating the carburization described above, and the test surface was mirror-polished. Next, the test surface was corroded with a saturated aqueous solution of picric acid to which a surfactant was added, and 10 visual fields were randomly observed at 100 times magnification using an optical microscope. The size of each visual field was 1.0 mm × 1.0 mm.

In the optical microscope observation of the visual field, it was defined that when a total of two or more austenite grains having a grain size number of 5 or less were present, the austenite grains were coarsened.
When the austenite grains were not coarsened even after the heat treatment at 300 ° C. for 300 minutes, it was evaluated that the austenite grain coarsening prevention effect was excellent (described as “> 1050” in Table 2). On the other hand, when the austenite grains were coarsened by performing heat treatment at 300 ° C. or less for 300 minutes, it was evaluated that there was no effect of preventing austenite grains from being coarsened (Table 2 shows the heat treatment temperature at which the coarsening occurred. Write.).

  As shown in Table 1 and Table 2, the steels for cold forging of Test Nos. 1 to 15 had the chemical composition (Steel A to O) and the sulfide number density, the residual amount of Al and Nb in the surface region and the central region. As a result of the difference analysis, the precipitate number density is within the range of the present invention. The steels for cold forging of test numbers 1 to 15 are excellent in cold forgeability and machinability after cold forging, and the coarsening of austenite grains during heat treatment simulating carburization after cold forging was suppressed. Was something.

  The steel for cold forging of test number 16 is a steel specified in JIS SCr420. Since the steel for cold forging of Test No. 16 does not contain Bi and Nb, the amount of Nb in the surface region and the central region and Formula (1) are not satisfied, and the sulfide number density is low. Therefore, the steel for cold forging of Test No. 16 had a critical compressibility of less than 75% and low cold forgeability. Further, the steel for cold forging of Test No. 16 had a chip weight exceeding 15 g and low machinability. Moreover, the steel for cold forging of the test number 16 was insufficient in the effect of preventing austenite grain coarsening.

Test No. 17 for cold forging is an example that does not contain Bi and has a low sulfide number density. The steel for cold forging of Test No. 17 had low cold forgeability and machinability, and the effect of preventing austenite grain coarsening was insufficient.
Test No. 18 for cold forging is an example in which the Bi content exceeds the range specified in the present invention. The steel for cold forging of Test No. 18 had surface cracks during hot rolling.

The steel for cold forging of Test No. 19 is an example that does not satisfy Expression (1) because the content of Al is below the range specified in the present invention. The steel for cold forging of Test No. 19 had an insufficient effect of preventing austenite grain coarsening.
The steel for cold forging of Test No. 20 is an example that does not satisfy the Al content, the precipitate number density, and the formula (1) in the surface region and the central region because the Al content exceeds the range specified in the present invention. The steel for cold forging of Test No. 20 had low cold forgeability because coarse precipitates were deposited at high density in the surface region and the central region. Moreover, the steel for cold forging of the test number 20 was insufficient in the effect of preventing austenite grain coarsening.

The steel for cold forging of Test No. 21 is an example that does not satisfy Expression (1) because the content of Nb is below the range specified in the present invention. The steel for cold forging of Test No. 21 had an insufficient effect of preventing austenite grain coarsening.
The steel for cold forging of Test No. 22 is an example that does not satisfy the Nb content, the precipitate number density, and the formula (1) in the surface region and the central region because the Nb content exceeds the range specified in the present invention. The cold forging steel of Test No. 22 had low cold forgeability because coarse precipitates were deposited at high density in the surface region and the central region. Moreover, the steel for cold forging of the test number 22 was insufficient in the effect of preventing austenite grain coarsening.

The steel for cold forging of Test No. 23 is an example in which the sulfide number density is low because the S content is below the range specified in the present invention. The steel for cold forging of test number 23 had a chip weight exceeding 15 g and low machinability.
The steel for cold forging of Test No. 24 had insufficient cold forgeability because the S content exceeded the range specified in the present invention.

The steel for cold forging of test number 25 is an example in which the chemical composition does not satisfy the formula (1). The cold forging steel of Test No. 25 had low cold forgeability because coarse precipitates were precipitated at high density in the surface region and the central region. Moreover, the steel for cold forging of the test number 25 was insufficient in the effect of preventing austenite grain coarsening.
The steel for cold forging of the test number 26 is an example whose chemical composition does not satisfy the formula (1). The steel for cold forging of Test No. 26 had an insufficient effect of preventing austenite grain coarsening.
The steel for cold forging of Test No. 27 is an example having a chemical composition defined in the present invention and a low sulfide number density. The steel for cold forging of Test No. 27 had a chip weight of more than 15 g and low machinability.

  As described above, the embodiments of the present invention have been described, but the above-described embodiments are merely examples for implementing 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 (2)

In mass%,
C: 0.10 to 0.30%,
Si: 0.01 to 1.00%,
Mn: 0.40 to 1.80%,
S: 0.005 to 0.030%,
Cr: 0.01 to less than 1.60%,
Al: 0.010-0.045%,
N: 0.010-0.03%,
Bi: 0.0001 to 0.0050%,
Nb: 0.02 to 0.08%
Containing
P: 0.05% or less,
O: limited to 0.0025% or less, the balance being Fe and impurities,
A steel bar or wire rod having a chemical composition satisfying the following formula (1), having a microstructure composed of ferrite and pearlite,
The amount of Al precipitated as AlN and AlN-Nb (CN) is 0.010% or less in a region from the surface to １ ／ of the radius and in a region from the center to １ ／ of the radius, and Nb (CN) And the total number density of AlN, Nb (CN) and AlN-Nb (CN) having an amount of Nb precipitated as AlN-Nb (CN) of not more than 0.020% and a diameter of not less than 100 nm is 50. Pieces / 100 μm ² or less,
A steel for cold forging, characterized in that, in a cross section parallel to the rolling direction, the number density of sulfides having an equivalent circle diameter of less than 2 μm is 300 / mm ² or more.
0.8 ≦ (Nb / 93 + Al / 27) / (N / 14) ≦ 1.8 (1)
(Nb, Al, and N in the formula (1) are the contents of each element in mass%.) Instead of part of Fe, in mass%,
Mo: 1.5% or less,
Ni: 1.0% or less,
Cu: 0.4% or less,
V: 0.35% or less,
B: 0.020% or less,
The steel for cold forging according to claim 1, wherein the steel contains one or more kinds selected from the group consisting of Mg: 0.0035% or less.