JPH09287056A - Wire rod and bar steel excellent on cold forgeability and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decide conditions in which cracking is not generated on the boundary between ferrite and partite even if the cold working rate is increased as-hot rolled, and furthermore, deformation resistance is small. SOLUTION: A slab having a compsn, contg., by weight, 0.05 to 0.40% C, 0.3 to 3.0% Si and 0.3% to 3.0% Mn, furthermore contg. at least one kind among <=2.0% Cr, <=0.5% Mo, <=1.0% Ni, <=0.3% To, <=0.3% Nb and <=0.3 V, and the balance Fe with inevitable impurities is subjected to hot rolling to form into a bar steel or a wore rod, and then, this bar steel or wire rod is cooled at a cooling rate of >=10 deg.C/sec, by which the metallic structure of the bar steel or wire rod is formed into the one composed of, by volume, 3 to 20% residual austenite and 3 to 70% ferrite, and the balance baiting and/or martensite. Since softening can be obviated at the time of producing automobile parts, machine parts or the like and the service lives of tools at the time of cold intensive working can be prolonged, it contributes to the conservation of energy and the reduction of cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod and a steel bar having excellent cold workability as they are in hot rolling without heat treatment when manufacturing mechanical parts by cold forging, and a manufacturing method thereof. It is a thing.

[0002]

2. Description of the Related Art Generally, among steel materials used as mechanical parts such as bolts, shafts, gears and the like, steel materials that are cold worked such as cold forging are deformed during cold working such as cold forging. Low resistance and high ductility are required.
This is because if the deformation resistance of the steel material is large, the tool life will be reduced during cold working, and if the ductility is low, cracks will easily occur during cold working. However, in order to ensure hardenability after cold forging, the C content is 0.35
It was necessary to use high steel of wt.% or more. However, such a steel having a high C content generally has high tensile strength and low ductility when hot-rolled. Therefore, in the steel materials for these applications, spheroidizing annealing is generally performed before cold working to reduce the tensile strength of the steel material and enhance the ductility.
However, since this spheroidizing annealing treatment is a long-time heat treatment of 10 to 20 hours and requires heat treatment at a temperature of 700 ° C or higher, this treatment can be omitted and an excellent cooling treatment can be performed. There has been a demand for the development of hot-rolled material having hot forgeability.

On the other hand, as conventional methods, for example, Japanese Examined Patent Publication No. 61-37333 and Japanese Examined Patent Publication No. 1-12815 control the reduction amount and rolling temperature during hot rolling to refine austenite grains and By introducing a deformation zone,
By reducing the hardenability of steel, ferrite
Prevents the formation of bainite and martensite structures that promote pearlite transformation and contribute to the increase of deformation resistance,
Disclosed is a steel material (hereinafter referred to as "prior art") having excellent cold workability as hot rolled.

[0004]

However, in the ferrite + pearlite steel disclosed in the prior art, when it is processed, deformation of ferrite occurs preferentially, and this local deformation causes ferrite and pearlite to be deformed. There was a problem that cracks were likely to occur at the boundary with. In addition, in ferrite + pearlite steel, the deformation resistance gradually increases as the working rate increases, and there is a limit to reducing the deformation resistance in severe working, and there is a problem that the tool life decreases during cold forging. there were.

Therefore, the problem to be solved by the present invention is to solve the problems in the steel having a chemical composition such as a steel bar and a wire having predetermined mechanical properties, even if the cold working ratio becomes high in the as-hot-rolled state. The purpose is to determine the condition that cracks do not occur at the boundary between ferrite and pearlite and that the deformation resistance is smaller than a predetermined value. Thus, the object of the present invention is to solve the above-mentioned problems, without subjecting a steel bar and a wire rod as a hot-worked steel material to heat treatment such as spheroidizing annealing, and cracking in the as-hot-worked state. Wire rods and steel bars that can be cold forged without causing cracks and not impairing tool life,
Another object is to provide a method for manufacturing them.

[0006]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that ferrite has a structure containing an appropriate amount of retained austenite in order to reduce deformation resistance. It has been found that forming is effective. That is, after hot rolling, accelerated cooling is performed to a temperature of 550 ° C. or less, and the volume ratio in the metallographic structure is 3 to 20%.
By causing the retained austenite of 1 to be produced, deformation occurs in both the retained austenite and the ferrite in the early stage of processing. In this deformation of retained austenite, the retained austenite undergoes work-induced transformation and becomes hard martensite, so that the rate of hardening for a given strain, that is, work hardening rate, becomes larger than that of ferrite + pearlite steel. However, since the bainite and / or martensite structure is more likely to undergo plastic deformation in the structure than the pearlite structure,
It has been found that when further processing is performed, deformation mainly occurs in these bainite and / or martensite, and the work hardening rate becomes small.

Based on the above findings, the present invention is based on the above-mentioned findings. Concentration of C in austenite by ferrite transformation during cooling after hot rolling, and C in austenite by bainite and martensite formed after ferrite transformation. Was completed by studying the chemical composition of the steel that causes the formation of retained austenite and the cooling rate after hot rolling.

The steel bar and wire rod having excellent cold forgeability according to claim 1 have a chemical composition of C: 0.05 to 0.40 wt.%, Si:
0.3 to 3.0 wt.% And Mn: 0.3 to 3.0 wt.%, Balance: Fe and unavoidable impurities, and the metallographic structure is 3 to 20% by volume of retained austenite and
It is characterized by having 3 to 70% of ferrite and the balance being bainite and / or martensite.

The steel bar and wire rod having excellent cold forgeability according to claim 2 have a chemical composition of C: 0.05 to 0.40 wt.%, Si:
0.3 to 3.0 wt.% And Mn: 0.3 to 3.0 wt.%, And further select at least one of two groups having the following chemical composition, and select at least 1 from each selected group.
Containing seeds, balance: Fe and chemical composition consisting of inevitable impurities, and 3-20% by volume of metal structure
Of retained austenite and 3 to 70% of ferrite with the balance being bainite and / or martensite. However, the above two groups mean Cr: 2.0 wt.% Or less, Mo: 0.5 wt.% Or less, and N
i: 1.0 wt.% or less, Ti: 0.3 wt.% or less, Nb: 0.3 wt.% or less, and V: 0.3 wt.% or less.

In the method for producing a steel bar and a wire rod having excellent cold forgeability according to claim 3, the chemical composition is C: 0.05 to 0.40.
wt.%, Si: 0.3-3.0 wt.%, and Mn: 0.3-3.0 w
The steel bar or wire rod containing t.% and the balance: Fe and inevitable impurities is hot-rolled to form a steel bar or wire rod, and then the hot-rolled steel bar or wire rod is heated to 10 ° C / By cooling at a cooling rate of sec or more, the metal structure of the steel bar or wire is characterized by 3-20% by volume of retained austenite and 3-70% of ferrite and the balance bainite and / or martensite. Is to have.

In the method for producing a steel bar and a wire rod having excellent cold forgeability according to claim 4, the chemical composition is C: 0.05 to 0.40.
wt.%, Si: 0.3-3.0 wt.%, and Mn: 0.3-3.0 w
t.%, and at least one group selected from the two groups having the following chemical composition, containing at least one selected from each selected group, and the balance: Fe and inevitable impurities The steel bar or wire rod is formed into a steel bar or wire rod by hot rolling the piece, and then the steel bar or wire rod is cooled at a cooling rate of 10 ° C./sec or more, and then the metal structure of the steel bar rod or wire rod is cooled. Volume ratio of 3 to 20
% Residual austenite and 3-70% ferrite with the balance being bainite and / or martensite. However, the above two groups mean Cr: 2.0 wt.% Or less, Mo: 0.5 wt.% Or less, and N
i: 1.0 wt.% or less, Ti: 0.3 wt.% or less, Nb: 0.3 wt.% or less, and V: 0.3 wt.% or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, in order to obtain a steel material having excellent cold forgeability as it is in hot working, the chemical composition of the steel material and the cooling rate of the steel material after hot rolling are as described above. Must be limited. The reason for the limitation will be described below.

(1) Carbon (C) C is an essential element for obtaining a desired amount of retained austenite and a desired tensile strength. However, if the content is less than 0.05 wt.%, Retained austenite cannot be obtained even if the cooling rate after hot rolling is cooled at 10 ° C./sec or more, and sufficient tensile strength can be obtained. Can not. On the other hand, if it exceeds 0.40 wt.%, The hardenability of steel becomes excessively high, and retained austenite cannot be obtained. Therefore, the limit upsetting rate becomes too low if hot rolling is performed as it is. Therefore, the C content is 0.05 to
It should be limited to the range of 0.40 wt.%.

(2) Silicon (Si) Si is effective in expanding the temperature range of ferrite transformation during cooling after hot rolling, assists C to agglomerate into untransformed austenite, and is of cementite. Since it has the effect of delaying precipitation, it has an extremely effective effect in effectively generating and retaining retained austenite. However, if the Si content is less than 0.3 wt.%, The above effect cannot be obtained. On the other hand, if the Si content exceeds 3.0 wt.%, The strength becomes too high and the deformation resistance becomes too large. Decarburization of the steel surface progresses during heating before rolling and during rolling, which deteriorates the surface quality of the product. Therefore, the Si content should be limited to the range of 0.3 to 3.0 wt.%.

(3) Manganese (Mn) Mn is an element necessary for obtaining a desired amount of retained austenite, since Mn is effective for lowering the M _S point of untransformed austenite, and It serves to refine the structure of bainite and / or martensite and increase their toughness and ductility. However, M
If the n content is less than 0.3 wt.%, it is difficult to obtain the above effect. On the other hand, if the content exceeds 3.0 wt.%, The strength becomes too high and the deformation resistance becomes too high.
The scale generated during hot rolling becomes difficult to peel off, and it becomes difficult to perform pickling before cold working such as cold forging and descaling by shot blasting, which may lead to an increase in manufacturing cost in product processing. Therefore, the Mn content is 0.3
It should be limited to the range of ~ 3.0 wt.%.

In addition to C, Si and Mn as the essential component elements described above, the addition of Cr, Mo and Ni and Ti, Nb and V as the selective component elements produces the action and effect described below, and More desirable bar steels and wire rods having excellent forgeability can be obtained.

(4) Chromium (Cr), molybdenum (Mo)
In addition, nickel (Ni) Cr, Mo and Ni have the effect of improving the toughness of martensite, and are elements that effectively enhance the hardenability in the product used after quenching and tempering after cold forging.

However, if the Cr content exceeds 2.0 wt.% Or the Mo content exceeds 0.5 wt.%,
By increasing the hardenability of steel after hot rolling,
The structure becomes bainite and / or martensite, the desired amount of retained austenite cannot be obtained, and the deformation resistance during cold forging becomes high.

If the Ni content exceeds 1.0 wt.%, The stability of the retained austenite in cold working increases, and the retained austenite remains in the metal structure until it is deformed to a high degree of workability. It does not contribute to the reduction of deformation resistance with time. Therefore, the Cr content should be limited to 2.0 wt.% Or less, Mo to 0.5 wt.% And Ni to 1.0 wt.% Or less.

(5) Titanium (Ti), niobium (Nb) and vanadium (V) Ti are precipitated as TiN during melting of steel. Also, N
b and V are Nb during hot rolling and subsequent cooling.
Precipitates as CN or VN. Precipitation of these carbonitrides or nitrides reduces the amount of solute N in steel and reduces strain hardening during cold working. by this,
It is possible to keep the deformation resistance during cold working low.
Therefore, addition of Ti, Nb and V is an effective means for improving the cold forgeability of steel.

However, if any one of the Ti, Nb and V contents exceeds 0.3 wt.%, Not only the amount of precipitation of nitrides excessively increases but also carbides precipitate,
Hardening of steel occurs and cold forgeability of steel deteriorates. Therefore, the Ti, Nb, and V contents should all be limited to 0.3 wt.% Or less.

In the present invention, wire rods and steel bars having the chemical composition defined as described above are manufactured by hot rolling.
The reason for limiting the cooling rate after hot rolling to 10 ° C./sec or more is as follows.

(6) Cooling rate and metal structure after hot rolling At room temperature, it has a structure in which the residual austenite content is 3 to 20% and the ferrite content is 3 to 70% and the balance is bainite and / or martensite at room temperature. In order to obtain a steel material, during cooling after hot rolling, C atoms are concentrated in the remaining austenite by transformation of ferrite, bainite, and martensite, respectively, and the martensite transformation of austenite with the enriched C concentration is completed. In order to lower the point (M _f point) to a temperature below room temperature, the cooling rate during hot rolling must be limited to 10 ° C / sec or more. That is, if the cooling rate is less than 10 ° C / sec,
Unable to get bainite and martensite,
The structure of the steel material is a ferrite + pearlite structure, and not only a sufficient amount of ferrite cannot be obtained, but also a sufficient amount of retained austenite cannot be finally obtained.

In order to obtain a structure having a volume ratio of retained austenite of 3 to 20% and ferrite of 3 to 70% and the balance of bainite and / or martensite, the cooling rate after hot rolling is set to Must be 10 ° C / sec or higher. Further, since the alloying element can be reduced as the cooling rate increases, there is no upper limit.

(7) Metal Structure In the process of manufacturing the steel bar and wire rod according to the present invention, the retained austenite structure of the steel material after hot rolling has a function of reducing the deformation resistance and extending the life of the cold forging tool. However, the ferrite structure has the functions of improving the ductility of steel and improving the deformability, and the bainite and martensite structures are necessary for ensuring the strength of steel. And, if the retained austenite is less than 3%, the effect of reducing the deformation resistance is small, so that it is not possible, and if it exceeds 20%, the effect is not only saturated, but also a large amount of alloying elements is required and the cost becomes high, which is not possible. , Ferrite structure is 3%
If it is less than 70%, it is not possible because the ductility is insufficient and the cold forming ability is poor, and if it exceeds 70%, it is difficult because it is difficult to secure the strength of the steel.

Therefore, in order to obtain a hot-rolled steel bar and wire rod having excellent cold forgeability, the volume fraction of the metal structure is 3 to 20% of retained austenite and 3 to 70% of ferrite. Within the range, the balance should be bainite and / or martensite.

[0027]

Next, the present invention will be described with reference to examples and comparative examples. [Test 1] Molten steel having a predetermined chemical composition was melted in a 150 kg vacuum melting furnace, and a 160 mm square or 116 mm square steel ingot was cast. By hot rolling using the obtained steel ingot,
160 mm square steel ingot is 30 mm diameter steel bar, and 116 m
After rolling the m square steel ingot to a wire rod with a diameter of 16 mm, 10 ° C / se
It cooled at the cooling rate of c. The conventional steel JIS
Comparative example No. 20 corresponding to the chemical composition of S45C
Regarding (see Table 1 below), the wire rod after hot rolling was subjected to spheroidizing annealing treatment.

The amount of retained austenite was measured by a predetermined X-ray diffraction method for the test pieces collected from the wire rod and the steel bar obtained above, and the amount of ferrite was determined after the metallographic structure was revealed by using Nital corrosion. The image was measured by an image processing device, the balance was bainite and / or martensite, and the structure was confirmed by an optical microscope.

Table 1 shows Examples Nos. 1 to 13 which are wire rods and steel bars within the scope of the present invention, and Comparative Examples Nos. 14 to 20 which are wire rods and steel bars outside the scope of the present invention. .

[0030]

[Table 1]

Tensile properties and cold forging properties of the specimens of the above Examples and Comparative Examples were evaluated. Each evaluation method is as follows. The tensile properties were measured for tensile strength, elongation and reduction using JIS No. 14A tensile test pieces (parallel part: 10 mm diameter, gauge length: 50 mm). In addition, the cold forging characteristics were obtained by using compression test pieces (with 14 mmφ × 21 mm groove and without groove), the critical compression ratio and 70%.
The deformation resistance during compression processing was measured.

FIG. 1 shows a plan view (a) and a side view (b) of a test piece for measuring the critical compression rate. The test piece 1 for measuring the limit compression rate has a V-shaped groove 2 formed in the height direction of the circumferential surface of a cylindrical body having a diameter a = 14 mm and a height b = 21 mm.

FIG. 2 shows the detailed shape of the V-shaped groove 2. The V-shaped groove 2 has a depth c = 8 ± 0.05 mm, the bottom shape of the groove is a radius R = 0.15 ± 0.05 mm, and the opening angle θ of the V-shaped groove 2 =
It is 30 ± 0.4 °. The measurement of the critical compressibility is performed by applying a compressive load in the height direction of the test piece 1 for measuring the critical compressibility and applying the V-shaped groove 2
Compression rate at which cracks occur at the bottom 3 of the blade (critical compression rate)
φ _c was determined. The critical compression ratio φ _c was calculated by the following equation (1).

[0034] FIG. 3 shows a plan view (a) and a side view (b) of the deformation resistance measuring test piece. The deformation resistance measuring test piece 4 has a diameter d =
It is a cylindrical body having a size of 8 mm and a height e = 12 mm. The top view and side view of the test piece for deformation resistance measurement are shown. The deformation resistance is measured by applying compressive deformation in the height direction of the deformation resistance measuring test piece 4 and applying load stress to the test piece at the time when the strain applied to the test piece is 0.5 and 1.2, that is, The average value of the deformation resistance was calculated and defined as the deformation resistance.

Table 2 shows the results of the above measurement test.

[0036]

[Table 2]

The following matters can be seen from Table 2. Examples Nos. 1 to 13 within the scope of the present invention are all conventional steel J
It is superior to Comparative Example No. 20, which was subjected to spheroidizing heat treatment after hot rolling using IS S45C, in the limit compressibility and the deformation resistance during 70% compression processing. That is, it is excellent in cold forgeability.

Comparative Examples Nos. 14-16, 18 and 19
Is inferior to any of Examples 1 to 13 in at least one of the compression limit rate and the 70% deformation resistance during processing. That is, Comparative Example No. 14 in which the C content is lower than the range of the present invention and Comparative Example No. 16 in which the Si content is lower than the range of the present invention, residual austenite does not remain and the compressibility is limited. Is excellent, but inferior in deformation resistance. Comparative Example N with C content higher than the range of the present invention
O.15 is inferior in both the deformation resistance and the limit compression rate. However, in Comparative Example No. 17 in which the Si content is slightly higher than the range of the present invention, the deformation resistance is small, but the deformability is inferior.

In Comparative Example No. 18 in which the Mn content is lower than the range of the present invention, no retained austenite remains,
The critical compression ratio is inferior. In Comparative Example No. 19 in which the Mn content is higher than the range of the present invention, residual austenite does not remain, the critical compressibility is poor, and the elongation and the drawing are also poor.

Both the examples and the comparative examples are stretched as a whole and the drawing is good. However, Comparative Examples No. 17 and 19 are inferior in elongation, and Comparative Examples No. 17, 19 and 20 are inferior in drawing.

[Test 2] Example No. 1 and Comparative Example No. 1
The deformation resistance for various compressibility was measured for 14 and the relationship between the compressibility and the deformation resistance was investigated.

FIG. 4 shows Example No. 1 and Comparative Example No. 1.
4 shows the relationship between the compressibility and the deformation resistance for No. 4. The main difference in the chemical composition between Example No. 1 and Comparative Example No. 14 is that Comparative Example No. 14 having a C content is more effective than Example No.
The residual austenite does not remain in Comparative Example No. 14. As can be seen from the figure, Comparative Example No. 14 having no residual austenite has a smaller deformation resistance than Example No. 1 having residual austenite in a region where the compression processing is small, but about 50%.
In the compression processed region of not less than%, the increase of the deformation resistance is suppressed by the presence of the retained austenite, and the deformation resistance of Example No. 1 is smaller.

Similarly, Example No. 4 and Comparative Example No. 1
The deformation resistance for various compressibility was measured for 6 and the relationship between the compressibility and the deformation resistance was investigated. FIG. 5 shows the relationship between the compressibility and the deformation resistance for Example No. 4 and Comparative Example No. 16. Similarly, as can be seen from the figure, Comparative Example N in which residual austenite remains in the region where the compression processing is small is smaller than that in Example No. 4 in which residual austenite remains.
O.16 has a smaller deformation resistance, but in the compression working region of about 30% or more, the increase of the deformation resistance is suppressed due to the presence of retained austenite, and Example No. 4 is smaller.

[Test 3] For a wire rod having a diameter of 16 mm and having the same chemical composition as in Examples Nos. 6 and 9, after hot rolling, the cooling rate was variously fixed within the range of 5 to 45 ° / sec. Cooled to value. The test specimen thus obtained was evaluated for tensile properties and cold forging properties in the same manner as in Test 1.

Table 3 shows the test results.

[0046]

[Table 3]

The following matters can be seen from Table 3. The residual austenite amount when the cooling rate after hot rolling of the wire having the chemical composition within the range of the present invention is 5 and 7 ° C./sec is less than 3%, which is less than the range of the present invention. On the other hand, when the cooling rates are 15 and 45 ° C./sec, the amount of retained austenite is 12 to 17%, which is within the range of the present invention. And, Example No. cooled at 15 and 45 ° C./sec which are cooling rates within the scope of the present invention.
Nos. 21 to 24 were subjected to 70% compression processing despite having a higher tensile strength than Comparative Examples Nos. 25 to 28 cooled at 5 and 7 ° C./sec, which are cooling rates outside the range of the present invention. The deformation resistance when doing is small. In addition, the elongation is Example Nos. 21 to 24 and Comparative Examples No. 25 to 28.
Both are good, but the throttle has a high cooling rate.
Nos. 21 to 24 are more preferable than Comparative Examples No. 25 to 28 because the structure is finer.

As described above, when the retained austenite and the ferrite are present in the proper proportions and the balance is bainite and / or martensite, the ductility can be ensured as hot rolled and the cold forgeability is excellent. Can be obtained.

[0049]

As described above, according to the present invention, it is possible to produce a wire rod for cold forging and a steel bar which have a high limit compression rate in the as-hot-rolled state and a small deformation resistance even when subjected to a heavy working. You can Therefore, softening annealing can be omitted when manufacturing automobile parts and other mechanical parts, and the tool life during extended cold working can be extended, which contributes to energy saving and cost reduction. It is possible to provide excellent wire rods and steel bars, and methods for producing them, and to bring about an extremely great industrial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a plan view (a) and a side view (b) of a test piece for measuring a critical compression rate.

FIG. 2 is a diagram showing a detailed shape of a V-shaped groove.

FIG. 3 is a diagram showing a plan view (a) and a side view (b) of a deformation resistance measuring test piece.

FIG. 4 shows Example No. 1 and Comparative Example No. 14.
It is a graph which shows the relation between compression rate and deformation resistance by measuring the deformation resistance with respect to various compression rates.

5 is a graph showing the relationship between the compressibility and the deformation resistance for Example No. 4 and Comparative example No. 16. FIG.

[Explanation of symbols]

 1 Test piece for measuring critical compressibility 2 V-shaped groove 3 Bottom 4 Test piece for measuring deformation resistance a Diameter b Height c Depth d Diameter e Height R Radius θ Opening angle

Claims (4)

[Claims] 1. C: 0.05 to 0.40 wt.%, Si: 0.3 to 3.0 wt.
% And Mn: 0.3-3.0 wt.%, The balance: Fe and chemical composition consisting of inevitable impurities, and the metallographic structure is 3-20% by volume of retained austenite and 3- A wire rod and steel bar excellent in cold forgeability, which is characterized by 70% of ferrite and the balance of bainite and / or martensite. 2. C: 0.05 to 0.40 wt.%, Si: 0.3 to 3.0 wt.
%, And Mn: 0.3 to 3.0 wt.%, And a group consisting of the following chemical composition: Cr: 2.0 wt.% Or less, Mo: 0.5 wt.% Or less, and Ni: 1.0 wt.%. At least one of the following and / or a group consisting of the following chemical composition: Ti: 0.3 wt.% Or less, Nb: 0.3 wt.% Or less, and V: 0.3 wt.% Or less Contains, balance: Fe and chemical composition consisting of unavoidable impurities, and metallic structure with 3-20% by volume of retained austenite and 3-70%
Wire rod and steel bar having excellent cold forgeability, characterized in that the remainder is bainite and / or martensite. 3. C: 0.05 to 0.40 wt.%, Si: 0.3 to 3.0 wt.
%, And Mn: 0.3 to 3.0 wt.%, The balance: a steel slab having a chemical composition consisting of Fe and unavoidable impurities is formed into a steel bar or wire rod by hot rolling, Following hot rolling, the steel bar or wire was
By cooling at a cooling rate of ℃ / sec or more, the metal structure of the steel bar or wire rod is retained austenite of 3 to 20% by volume and ferrite of 3 to 70% with the balance being bainite and / or martensite. And a method of manufacturing a wire rod and a steel bar having excellent cold forgeability. 4. C: 0.05 to 0.40 wt.%, Si: 0.3 to 3.0 wt.
%, And Mn: 0.3 to 3.0 wt.%, And a group consisting of the following chemical composition: Cr: 2.0 wt.% Or less, Mo: 0.5 wt.% Or less, and Ni: 1.0 wt.%. At least one of the following and / or a group consisting of the following chemical composition: Ti: 0.3 wt.% Or less, Nb: 0.3 wt.% Or less, and V: 0.3 wt.% Or less Containing: balance: forming into a steel bar or wire rod by hot rolling a steel slab having a chemical composition consisting of Fe and unavoidable impurities, and after the hot rolling, the steel bar or wire rod is heated to 10 ° C. By cooling at a cooling rate of / sec or more, the metal structure of the steel bar or wire rod is retained austenite at a volume ratio of 3 to 20% and ferrite at a volume ratio of 3 to 70%, and the balance bainite and / or
Alternatively, a method for producing a wire rod and a steel bar having excellent cold forgeability, which is characterized by using martensite.