JP2001181791A - Bar stock and wire rod for cold forging, excellent in induction hardenability and cold forgeability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bar stock and wire rod for cold forging, capable of preventing cracks of a steel stock at cold forging and excellent in cold forgeability as well as in ductility after spheroidizing, and to provide a manufacturing method thereof. SOLUTION: A steel, having a composition containing, by mass, 0.45-0.6% C, 0.01-0.2% Si, 0.2-0.7% Mn, 0.015-0.1% Al, 0.0005-0.007% B, <=0.15% Cr, <=0.035% P, <=0.015% S, <=0.01% N and <=0.003% O, is used, and the total decarburized depth DM-T is <=0.20 mm. Moreover, in the region between the surface and a position at a depth of (radius of bar stock/wire rod)×0.15 from the surface, the structural area ratio of ferrite is <=10% and the balance is composed of one or more kinds among martensite, bainite and pearlite. Further, in the region between the position at a depth of (radius of bar stock/wire rod)×0.5 and the center, the sum of the structural area ratios of martensite and bainite is <=10% and the average hardness in this region is lower by >=40 HV than that in the surface layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rod material for cold forging which is excellent in induction hardening property and cold forging property and is used for manufacturing parts for machine structures such as parts for automobiles and parts for construction machines. The present invention relates to a rod wire for cold forging having excellent ductility after spheroidizing annealing and excellent surface hardenability by high frequency after cold forging.

[0002]

2. Description of the Related Art Conventionally, carbon steel for machine structure and low alloy steel for machine structure have been used as structural steel materials for manufacturing machine structural parts such as automobile parts and construction machine parts. In order to manufacture mechanical structural parts such as automobile gears, engine parts and drive train parts from these steel materials,
Although hot forging is performed, the tendency to switch to cold forging is increasing. This is because the cold forging has good surface texture and dimensional accuracy of the product, and has a lower production cost and better yield than hot forging.

[0003] In the cold forging process, usually, after hot rolling material is subjected to spheroidizing annealing (SA) to ensure cold workability,
Cold forged. However, cold forging has a problem in that work hardening occurs in the steel material, and the ductility is reduced to cause cracking and shorten the life of the mold. Particularly, in cold forging having a large working ratio, cracking during cold forging, that is, lack of ductility of a steel material, is often a major obstacle to switching from the hot forging process to the cold forging process.

On the other hand, in spheroidizing annealing (SA), since it is necessary to heat a steel material to a high temperature and hold it for a long time, not only heat treatment equipment such as a heating furnace is required, but also energy for heating is consumed. Occupies a large weight in manufacturing costs. For this reason, various techniques have been proposed from the viewpoints of improving productivity and energy saving.

For example, in Japanese Patent Application Laid-Open No. 57-63638, in order to shorten the spheroidizing annealing time, after hot rolling, the steel sheet is cooled to 600 ° C. at a rate of 4 ° C./sec or more to form a quenched structure, and the scale adheres. In the method in which spheroidizing annealing is performed in an inert gas in a state of being made into a wire having excellent cold forgeability, and JP-A-60-152627, in order to enable rapid spheroidizing, finish rolling conditions are set. Quenched after rolling, fine pearlite into finely dispersed proeutectoid ferrite,
In a method of forming a structure in which bainite or martensite is mixed, and in JP-A-61-264158, the steel composition is improved, that is, P is reduced to 0.005% or less, and Mn is reduced.
/S≧1.7 and Al / N ≧ 4.0 by reducing the hardness of the steel after spheroidizing annealing by using a low carbon steel;
Japanese Patent Application Laid-Open No. Sho 60-114517 proposes a method of performing controlled rolling in order to omit a soft annealing treatment before cold working.

[0006] These conventional techniques are all techniques for improving or omitting spheroidizing annealing before cold forging, and mainly for switching from a hot forging step to a cold forging step in a part having a large workability. It is not a technology that seeks to improve the lack of ductility of steel, which is an obstacle.

Further, mechanical structural parts such as various gears, engine parts and drive train parts need to be surface hardened by high frequency after cold forging to secure wear resistance and improve fatigue strength. However, generally, in order to secure induction hardening properties to a steel material, the hardness of the material is usually hardened, and the cold forging property and the induction hardening property of the steel material are opposite properties.

For this reason, various steel materials having both cold forgeability and induction hardening and a method for producing the same have been proposed.

For example, in Japanese Patent Publication No. Hei 5-26850, finish rolling is finished at 650 to 750 ° C. using high carbon steel, and then cooled at a rate of 0.2 to 1.5 ° C./sec. By increasing the ferrite fraction and increasing the pearlite lamellar spacing, it is softened to obtain a high carbon rod for cold forging.

In Japanese Patent Application Laid-Open No. 11-217649, high-carbon steel is hot-rolled at a finishing temperature of 800 to 1000 ° C., and gradually cooled at a temperature of 800 to 500 ° C. at 1 ° C./sec or less. It has been proposed to use a steel material for induction hardening which has both workability and high strength characteristics.

[0011] Even with these proposed techniques, steel can have both cold forgeability and induction hardening, but it is still not satisfactory.

[0012]

Accordingly, in view of the above situation, the present invention has been a problem in the prior art when manufacturing a machine structural component by cold forging after spheroidizing and annealing a hot-rolled rod or wire. It is an object of the present invention to provide a rod material for cold forging having excellent ductility after spheroidizing annealing and having excellent induction hardening properties, which is capable of preventing cracking of a steel material generated during cold forging.

[0013]

The present inventor has studied the cold workability of a rod material for cold forging and found that only the surface layer of the rod material having a specific steel component was hardened and the central portion was soft. The present inventors have found that by forming the structure, it is possible to obtain a rod for cold forging having excellent ductility after spheroidizing annealing and excellent induction hardening properties, and completed the present invention.

The gist of the present invention is as follows.

(1) As mass%, C: 0.45 to
0.6%, Si: 0.01 to 0.2%, Mn: 0.2 to
0.7%, Al: 0.015 to 0.1%, B: 0.00
0.05-0.007%, Cr: 0.15% or less,
P: 0.035% or less, S: 0.015% or less, N:
A steel having a composition of 0.01% or less and O: 0.003% or less, with the balance being Fe and unavoidable impurities.
Total decarburization depth specified by SG0558: 0 for DM-T.
20 mm or less, and the rod wire radius from the surface × 0.
The area ratio of ferrite in the region up to the depth of 15 is 10%.
% Or less, the balance substantially consists of one or more of martensite, bainite, and pearlite, and further, the structure area ratio of martensite and bainite in a region from the rod wire radius x 0.5 to the center. Is less than 10%, and the average hardness in this region is HV4 higher than the hardness of the surface layer (the region from the surface to the depth of 0.15 of the rod or wire rod).
Bar wire for cold forging excellent in induction hardening property and cold forgeability after spheroidizing annealing characterized by being softer than 0.

(2) Ti: 0.1% by mass%
The bar wire for cold forging excellent in induction hardening property and cold forgeability after spheroidizing annealing according to the above (1), comprising:

(3) Mo: 0.4% by mass%
(1) or (2) above, characterized by containing:
A rod material for cold forging having excellent induction hardening properties and cold forgeability after spheroidizing annealing described in 1.

(4) Nb: 0.00% by mass.
The spherical shape according to any one of the above (1) to (3), containing one or two kinds of 5 to 0.1% and V: 0.03 to 0.3%. Bar wire for cold forging with excellent induction hardenability and cold forgeability after chemical annealing.

(5) Any one of the above (1) to (4), wherein the austenitic crystal grain size in the region from the surface to the depth of the rod wire radius × 0.15 is No. 8 or more. A rod material for cold forging having excellent induction hardening properties and cold forgeability after spheroidizing annealing described in 1.

(6) A spheroidized annealed material of the rod material for cold forging according to any one of the above (1) to (5), wherein the depth of the rod wire radius × 0.15 from the surface. The degree of the spheroidized tissue defined by JIS G3539 in the region up to
o. 2 and the degree of the spheroidized structure in the region where the depth was from the rod wire radius × 0.5 to the center was No. A steel material for cold forging excellent in induction hardenability and cold forgeability, characterized by being within 3.

(7) The induction hardening property and the cold forgeability as described in (6) above, wherein the ferrite crystal grain size in the region from the surface to the depth of the rod wire radius × 0.15 is 8 or more. Excellent cold forging steel.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

First, the present invention aims to achieve mechanical properties such as microstructure, hardness, ductility, and hardenability of a cold forging rod or wire which is excellent in cold forgeability and excellent in induction hardenability, which is the object of the present invention. The reason for limiting the steel composition required for the steel will be described.

C: C is an element necessary for increasing the strength as a component for machine structure and ensuring induction hardenability, but if it is less than 0.45%, the strength of the final product is insufficient.
Induction hardening cannot be ensured. On the other hand, if it exceeds 0.6%, it becomes rather hard and causes deterioration in cold workability.
The content was 0.45 to 0.6%.

Si: Si is added as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening, giving quenchability, and improving the tempering softening resistance. If it is less than 0.2%, these effects are insufficient. On the other hand, if it exceeds 0.2%, these effects are saturated, and rather, the hardness is increased and the cold workability is deteriorated. ０．２0.2%. But,
The upper limit of Si is preferably set to 0.1% or less.

Mn: Mn is an element effective for ensuring induction hardening, but if it is less than 0.2%, this effect is insufficient, while if it exceeds 0.7%, this effect is saturated,
Rather, it causes an increase in hardness and deteriorates cold workability.
The Mn content was set to 0.2 to 0.7%.

Al: Al is useful not only as a deoxidizing agent, but also for fixing solid solution N present in steel as AlN and securing solid solution B. However, if the amount of Al is too large, Al ₂ O ₃ will be generated excessively, increasing internal defects and deteriorating cold workability. Therefore, in the present invention, Al is 0.015 to 0.1.
%. In addition, when Ti which has the effect of fixing solid solution N is not added, Al is preferably set to 0.04 to 0.1%.

B: B is α / γ in the cooling process after hot rolling.
Fe, a boron iron-carbon compound, at the interface _{twenty three}(CB)₆As
Precipitates and promotes the growth of ferrite.
And contributes to improving cold workability. In addition, solid solution B is a grain boundary.
To improve the induction hardenability,
Improves the grain boundary strength of the quenched material, resulting in fatigue as a machine part
It has the effect of improving strength and impact strength. others
Therefore, the B content was set to 0.0005 to 0.007%. This
If the ratio is out of the range, the above effect cannot be obtained.

Cr: Although Cr is sometimes contained as an impurity in steel, it is an element that stabilizes cementite and deteriorates induction hardenability, so that the allowable amount of Cr is 0.15% or less (0% ).

P: P is a component unavoidably contained in steel, but P causes grain boundary segregation and center segregation in steel,
Since it causes ductility deterioration, it is desirable to suppress the content to 0.035% or less (including 0%), preferably 0.02% or less.

S: S is an element inevitably contained in steel and is a harmful element that degrades ductility for cold workability. Therefore, S is 0.015% or less (including 0%). , Preferably less than 0.01%.

N: N is a component unavoidably contained in steel, which is a harmful element that reacts with B to form BN and reduces the effect of B. Since they combine to form TiN, increase the hardness and cause cold forging cracking, the content must be 0.01% or less (including 0%), preferably 0.007% or less.

O: O is a component unavoidably contained in steel and reacts with Al to form Al ₂ O ₃ and deteriorates cold workability. %including),
Preferably, the content is desirably suppressed to 0.002% or less.

The above are the basic components of the steel targeted by the present invention. In the present invention, the following elements can be further contained.

Ti: By adding Ti, Ti can fix N as TiN and Ti (CN) by Ti, detoxify N, and fix B by fixing solid solution N.
The precipitation of N can be prevented, and solid solution B can be secured. Ti is an element having a deoxidizing effect. For this reason, if necessary, the content of Ti is set to 0.1% or less.
If Ti: exceeds 0.1%, precipitation hardening of TiC becomes remarkable, and cold workability is deteriorated, which is not preferable.

Mo: Mo is an element effective for imparting strength and hardenability to steel and improving the grain boundary strength after induction hardening to increase the strength characteristics. However, Mo exceeds 0.4%. When added, the hardness is increased and the cold workability is deteriorated. For the above reasons, the content is set to 0.4% or less.

Nb and / or V: Nb and V combine with C and N in the steel to form NbN, Nb (CN) or VN, V
Since (CN) is formed and is an element effective for refining crystal grains, one or two of Nb and V are contained. However, if the Nb content is less than 0.005%, V
If the content is less than 0.03%, the effect is insufficient, while the Nb content is more than 0.1% and the V content is 0.3
%, The effect is saturated, rather deteriorating the cold workability.
0.1%, V: 0.03 to 0.3%.

Next, the structure of the rod or wire according to the present invention will be described. The present inventor has studied a method of improving the ductility of a rod material for cold forging provided with induction hardening by adjusting the composition of a steel material.In order to improve the ductility of the spheroidized annealed material, a spheroidized annealing structure was used. The point is that it is uniform and fine. For that purpose, the ferrite fraction of the structure after hot rolling is suppressed to a specific amount or less, and the remainder is one or two of fine martensite, bainite, and pearlite. It was clarified that it was effective to use the above mixed structure.
Therefore, when the steel material is rapidly cooled after hot finish rolling, and then subjected to spheroidizing annealing, the ductility of the rod or wire is improved. However, if the entire cross section of the rod and wire is rapidly cooled to have a hard structure, there is a concern about sintering cracks, and the hardness does not decrease even after spheroidizing annealing, the cold deformation resistance increases, and the life of the cold forging die increases. Deteriorate. In order to solve this problem, the surface layer of the rod and wire is quenched after hot finish rolling, and then reheated by the sensible heat of the steel, thereby tempering the martensite generated in the surface layer, and spheroidizing annealing. It is effective to soften the hardness beforehand, and to make the inside a soft structure because the cooling rate is slow, so that the ductility after spheroidizing annealing is excellent and the cold deformation resistance is low. It was found that it became a bar wire for cold forging.

FIG. 1 is a diagram showing the relationship between the distance (mm) from the surface of the steel bar for cold forging of 54 mmφ of the present invention and the hardness (HV).

As shown in FIG. 1, the average hardness of the surface is HV
At 247, the average hardness at the center is HV168, and the hardness decreases toward the center.

As for the structure, as shown in FIG. 2 (a), the surface layer, and (b) the micrograph (× 400) of the center, the surface layer is mainly composed of tempered martensite, and the center is mainly composed of ferrite and pearlite. It is an organization.

After holding the steel bar of FIG. 1 at 740 ° C. for 4 hours and then performing spheroidizing annealing in which it is gradually cooled at a cooling rate of about 6 ° C./hour, the structure shown in FIG. b) As shown in the center micrograph (× 400), the surface has a uniform structure with a good degree of spheroidization.

When a gear having a diameter of 15 mm was manufactured by cold forging using the spheroidized and annealed steel bar, a product could be obtained without intermediate annealing.

Further, the induction hardening properties of gears manufactured by cold forging were investigated.

FIG. 5 is a diagram showing the hardness distribution of the gear after induction hardening.

As shown in FIG. 4, the hardness of the surface layer is about 700 HV, and the hardness of the central part is about 200 HV. The surface layer is hardened by induction hardening and has excellent wear resistance. The part was a gear with excellent ductility.

Therefore, in the present invention, the relationship between the structure of the surface layer and the hardness between the surface layer and the center portion, which is a condition that does not cause cracking even when cold forging is performed, while ensuring induction hardening properties, is examined by experiments and tests. Research progressed.

As a result, even if the surface layer has a tempered martensite structure (structure in which ferrite is present in a phase substantially composed of one or more of martensite, bainite, and pearlite). In order to improve the induction hardening property, the total decarburization depth specified in JIS G0558 must be 0.20 mm or less in DM-T, and the total decarburization depth is 0 in DM-T. .20m
If it exceeds m, the induction hardening property of the surface layer deteriorates. In order to make the total decarburization depth 0.20 mm or less by DM-T, it has been found that it is effective to control the heating atmosphere during hot rolling and to rapidly cool the surface after finish rolling. In order to improve cold forgeability, the area ratio of ferrite in the region from the surface to the depth of 0.15 of the diameter of the rod or wire is 10% or less. % Or less, it is impossible to prevent the occurrence of cracks during cold forging. In addition, in order to secure the ductility during cold forging and prevent cracks,
In the region from 0.5 to the center, the total area ratio of martensite and bainite is 10% or less, and preferably 5% or less in the case of forging with a large workability.
In order to secure the ductility during cold forging and prevent cracks from occurring and prevent deformation resistance from increasing, the surface layer structure is tempered at the stage of the rod and wire after rolling, and the martensite structure fraction is higher and more uniform. It is necessary to make a difference in hardness between the surface layer and the inside at the stage of the rod and wire after rolling, and the average hardness of the area from the rod wire radius x 0.5 to the center is required The hardness (HV) is HV40 or more as compared with the average hardness (HV) in the region from the surface to the depth of the rod wire radius × 0.15, and in the case of forging having a large workability, preferably HV60.
It has been found that softening is a necessary condition.

When spheroidizing annealing (SA) is performed on the above-described rod or wire, the degree of spheroidizing structure defined by JIS G3539 in the region from the surface to the depth of the rod and wire radius × 0.15 is no. 2 and the depth of the spheroidized structure in the region from the rod wire radius × 0.5 to the center is N
o. A rod wire for cold forging having excellent ductility of 3 or less is obtained. This spheroidized and annealed rod or wire was confirmed to be free from cold forging cracks even when subjected to a large upsetting test in which the true strain exceeded one. As the spheroidizing annealing, a conventionally known spheroidizing annealing method can be applied.

Regarding the grain size of the surface layer contributing to the improvement of ductility, before the spheroidizing annealing, the austenitic grain size (JIS G0551) in the region from the surface to the depth of 0.15 of the rod or wire rod diameter is set to No. 8. That's fine,
When higher characteristics are required, the number is preferably 9 or more, and when higher characteristics are required, the number is preferably 10 or more. After the spheroidizing annealing, the ferrite crystal grain size (JIS G3545) in the region from the surface to the depth of the rod wire radius × 0.15 may be set to No. 8 or more.
When higher characteristics are required, the number is preferably 9 or more, and when higher characteristics are required, the number is preferably 10 or more.

If the crystal grain size is less than the above specified value, sufficient ductility cannot be obtained.

Next, a method of manufacturing a rod material for cold forging according to the present invention will be described.

FIG. 5 is a diagram illustrating a rolling line according to the present invention.

As shown in FIG. 5, steel having the components defined in claims 1 to 4 is heated in the heating furnace 1 so that the total decarburization depth: DM-T specified in JIS G0558 is 0.20 mm or less. The atmosphere is controlled and heated, and low-temperature finish rolling is performed by the hot rolling mill 2 so that the surface temperature of the rod or wire at the final finish rolling output side is 700 to 1000 ° C. The outlet temperature is measured by the thermometer 3. Next, the finish-rolled rod 4 is quenched by pouring water onto the surface thereof with a cooling trough 5 (for example, preferably at an average cooling rate of 30 ° C./sec or more) to reduce the surface temperature to 600 ° C. or less, preferably 500 ° C. Hereinafter, the temperature is more preferably set to 400 ° C. or lower, and the surface is made to have a structure mainly composed of martensite. After passing through the cooling trough, the rod is reheated by sensible heat at the center of the rod or wire so that the surface temperature becomes 200 to 700 ° C. (measured by the thermometer 6), and the surface is made into a structure mainly of tempered martensite. In the present invention, the step of quenching and reheating is performed at least once,
Thereby, ductility can be significantly improved.

The reason why the steel material surface temperature is set to 700 to 1000 ° C. is that crystal grains can be refined by low-temperature rolling and the structure after rapid cooling can be refined. That is, the austenite grain size of the surface layer is No. 8 at 1000 ° C. or less, and 950 ° C.
Below, it is No. 9 and below 860 ° C. is No. 10. However, if the temperature is lower than 700 ° C., it is difficult to form the surface layer into a structure with less ferrite.

Although the object to be manufactured is different from the present invention, such a direct surface quenching method (DSQ) and apparatus are disclosed in JP-A-62-152323 and JP-A-1-25959.
It is a known one as disclosed in JP-A-18.

FIG. 6 is a diagram showing a CCT curve for explaining the surface layer of the rod and the structure of the central part.

As shown in FIG. 6, when the rod wire that has been subjected to low-temperature finish rolling is rapidly cooled and then re-heated, the surface layer 7 has a structure mainly composed of tempered martensite because the cooling rate is high, but the central portion 8 has Since the cooling rate is lower than that of the surface layer, the structure becomes ferrite and pearlite.

The surface temperature is reduced to 600 ° C. or less by rapid cooling.
Thereafter, the surface temperature is restored to 200 to 700 ° C. by sensible heat so that the surface layer has a structure mainly composed of tempered martensite with reduced hardness.

[0060]

Embodiments of the present invention will be described below.

The steel materials shown in Table 1 were rolled under the rolling conditions shown in Table 2.
Rolled into steel bars and wires. Rolled material size is 40m in diameter
m to 54 mm. Then, after performing spheroidizing annealing,
Induction hardening and tempering were performed. The state of the rod and rod after rolling, the stage after spheroidizing annealing, and the stage after quenching and tempering treatment were examined. Table 3 shows the results. Table 3 shows the “region from the surface to the depth of the rod wire radius × 0.15” described in the claims of the present application.
Then, it was simply described as “surface layer” (example: surface layer hardness). Also,
In Table 3, the “depth range from the rod wire radius × 0.5 to the center” described in the claims of the present application is simply described as “inside” (eg, internal hardness). The deformation resistance was measured by performing an upsetting test on a cylindrical test piece having a diameter 1.5 times the diameter of a rolled material. The critical compressibility was determined by performing an upsetting test using a test piece having a notch with a depth of 0.8 mm and a radius of curvature of 0.15 mm at the tip of the cylindrical test piece. Also, a tensile test piece was cut out from a position corresponding to the surface layer, and a tensile test was performed.
A drawing, which is an index of the tensile strength and ductility of the surface layer, was determined. Induction hardening was performed under the condition of a frequency of 30 kHz. Tempering is performed at 170 ° C. for 1 hour.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

[Table 3]

As is evident from Table 3, the examples of the present invention are remarkably excellent in the limit compressibility and the draw ratio, which are the indexes of the ductility of the steel material, as compared with the comparative example having the same carbon content. In addition, there is little decarburization of the rod and wire, and the hardness after induction hardening is sufficiently higher than that of the comparative example.

[0066]

According to the present invention, there is provided a rod material for cold forging according to the present invention, which is capable of preventing cracking of a steel material during cold forging, which has conventionally been a problem in cold forging after spheroidizing annealing. It is a rod wire for cold forging with excellent ductility after annealing.
And it has excellent induction hardening properties. For this reason, when manufacturing an induction hardened part, a forged part having a high working ratio can be manufactured by the cold forging process, so that there is a remarkable effect that productivity can be greatly improved and energy saving can be achieved.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the distance (mm) from the surface of a steel bar for cold forging of 54 mmφ of the present invention and the hardness (HV).

2 (a) is a micrograph (× 400) of the surface of a steel bar, and FIG.

FIG. 3 (a) shows the steel bars after spheroidizing annealing of the steel bars of FIG.
Is a surface, and (b) is a micrograph of the center (× 400).

FIG. 4 is a diagram illustrating a hardness distribution of a gear after induction hardening.

FIG. 5 is a diagram illustrating a rolling line according to the present invention.

6A is a diagram illustrating a CCT curve for explaining the surface layer and the structure of the central portion of the rod, and FIG. 6B is a diagram illustrating the cross-sectional structure of the rod after cooling and reheating. .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Hot rolling mill 3 Thermometer 4 Bar and wire 5 Cooling trough 6 Thermometer 7 Surface layer 8 Central part

Claims (7)

[Claims] C: 0.45 to 0.6 as mass%
%, Si: 0.01 to 0.2%, Mn: 0.2 to 0.7
%, Al: 0.015 to 0.1%, B: 0.0005 to
0.007%, Cr: 0.15% or less, P:
0.035% or less, S: 0.015% or less, N: 0.0
1% or less, O: 0.003% or less, steel having a balance of Fe and unavoidable impurities.
Decarburization depth specified in 0558: DM-T is 0.20m
m or less, and the area ratio of ferrite in the region from the surface to the depth of the rod wire radius × 0.15 is 10% or less, and the balance is substantially one or more of martensite, bainite, and pearlite. Further, in the region where the depth is from the rod wire radius x 0.5 to the center, the total area ratio of martensite and bainite is 10% or less, and the average hardness in this region is the surface layer (from the surface to the rod). Wire radius x 0.
A bar wire for cold forging excellent in induction hardening property and cold forgeability after spheroidizing annealing, characterized in that it is softer than HV 40 or more in hardness (area up to a depth of 15). 2. A cold forging having excellent induction hardenability and cold forgeability after spheroidizing annealing according to claim 1, further comprising, by mass%, Ti: 0.1% or less. Rod wire. 3. The cold steel excellent in induction hardenability and cold forgeability after spheroidizing annealing according to claim 1 or 2, further comprising Mo: 0.4% or less by mass%. Rod wire for forging. 4. Nb: 0.005% by mass%
4. The method according to claim 1, wherein the composition contains one or two of 0.1% and V: 0.03 to 0.3%.
Bar wire for cold forging excellent in induction hardening property and cold forgeability after spheroidizing annealing described in (1). 5. The spherical shape according to claim 1, wherein the austenitic crystal grain size in the region from the surface to the depth of the rod wire radius × 0.15 is No. 8 or more. Bar wire for cold forging with excellent induction hardenability and cold forgeability after chemical annealing. 6. A spheroidized annealed material of the rod and wire for cold forging according to any one of claims 1 to 5, wherein the spheroidized material is a region from a surface to a depth of 0.15 of a rod or wire radius. JIS G3
No. 539, the degree of spheroidized structure was 2 and the degree of the spheroidized structure in the region where the depth was from the rod wire radius × 0.5 to the center was No. A steel material for cold forging excellent in induction hardenability and cold forgeability, characterized by being within 3. 7. The induction hardening property and the cold forging property according to claim 6, wherein the ferrite crystal grain size in the region from the surface to the depth of the rod wire radius × 0.15 is 8 or more. Steel for cold forging.