JP2003105495A - Wire-shaped or bar-shaped steel excellent in deformability and machine part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wire-shaped or bar-shaped steel which is excellent in deformablity and has high strength as a steel material itself only by hot rolling even when spheroidizing annealing is omitted. SOLUTION: The wire-shaped or bar-shaped steel substantially has a ferrite structure, and ferrite grain size number (A) in the ferrite structure within a range between a center and a diameter/4 of a rolled material is 7.0 to 10.0, ferrite grain size number (B) in the ferrite structure at an uppermost layer of the rolled material is 7.0 to 10.0. Further (A) and (B) satisfy a relation: 0<=(B)-(A)<=0.5.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a linear or rod-shaped steel (hereinafter sometimes abbreviated as steel) which is excellent in deformability even when hot-rolled as it is without being subjected to softening heat treatment and has high strength of steel itself. And a high-strength mechanical part having excellent deformability obtained by using such a steel [tensile strength 350 to 550].
N / mm ² , aperture 80.0% or more, twist value 120 times or more (100D conversion)]. The steel of the present invention is
By processing such as cold forging, cold forging, wire drawing, and cold rolling, for example, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases , Washers, tappets, saddles, balgs, inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, fittings, connectors, pulleys, fittings, yokes, bases , It is very useful for manufacturing mechanical parts such as valve lifters, spark plugs, special screw parts, etc., electrical parts, etc. (hereinafter sometimes represented by mechanical parts).

[0002]

2. Description of the Related Art Cold working has a higher productivity than hot working and cutting and has a good yield of steel materials, so that mechanical parts such as bolts, screws, nuts and electrical components can be efficiently manufactured. It is widely used as a method.

Therefore, the steel used for such cold working is essentially required to have excellent cold workability. Specifically, it is necessary that the deformation resistance during cold working is low (processing specific gravity is low) and the deformability [ductility (elongation, reduction, twist value)] is high. If the deformation resistance of steel is high, the life of the tool used for cold working will be shortened, while if the deformability is low, cracks will easily occur during cold working, causing defective products.

[0004] Conventionally, a method of descaling a rolled wire rod or steel bar by pickling, coating treatment, and then wire drawing by cold drawing (working rate 10 to 40%) and then cold working Was common. This method is effective when the cold working rate is low and the working load is low. However, in order to increase the processing rate and increase the processing load in response to the demand for more complicated and precise forged parts,
The above method is difficult to adopt, and there is a problem that the tool life for cold working becomes short.

Therefore, when the working load during cold working is high or cracks occur during cold working, heat treatment such as low temperature annealing, annealing, spheroidizing annealing, etc. is carried out before cold working. Therefore, a method of softening a steel material and cold working in a state where the ductility is enhanced is widely used.

However, the above heat treatment has a problem that it requires a heat treatment for a long time of several hours to several tens of hours. Therefore, for the purpose of improving productivity, energy saving measures, and eventually cost reduction, it is desired to develop a linear or bar steel having excellent cold workability, which can omit heat treatment such as spheroidizing annealing. There is.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide cold workability (particularly for cold workability as hot rolling even if the spheroidizing annealing treatment is omitted). The present invention is to provide a linear or rod-shaped steel having a high deformability) and a strength of steel itself; and a mechanical component obtained by using such a linear or rod-shaped steel.

[0008]

The wire rod or rod-shaped steel excellent in deformability according to the present invention (hereinafter sometimes referred to as steel again), which can solve the above-mentioned problems, means that a ferrite structure is substantially present. The ferrite grain size number (A) in the ferrite structure in the range of the center of the rolled material to the diameter / 4 is 7.0.
No. 10.0, the ferrite grain size number (B) in the ferrite structure in the outermost layer of the rolled material is 7.0 to 10.0, and the above (A) and (B) are 0 ≦. (B)-
The main point is that (A) ≦ 0.5 is satisfied. Here, the deformability [ductility (diaphragm, twist value)] is
It is a property of a material that expresses how much it can be deformed without breaking, and "having a large deformability (ductility)" means
It means that "cracking does not occur up to a large working degree".

The composition of the steel of the present invention is 0.008% C.
The content is not less than 0.05% and the deformability is remarkably enhanced. Furthermore, Ti: 0.005-0.0
By containing 25% and / or Nb: 0.02 to 0.07%, the strength of the steel material itself is further enhanced. Moreover, as a basic component, Si: 0.0
5 to 0.4%, Mn: 0.2 to 0.9%, N: 0.0015 to 0.007%, Al: 0.01 to 0.06% , Cr: 0.0
1 to 0.3%, P: 0.001 to 0.02%, S: 0.
It is preferable to satisfy 02% or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors pay particular attention to deformability (drawing and twist value) among cold workability, and are excellent in deformability even in hot rolling. Has been earnestly studying to provide steel having high strength as a steel material itself.
Specifically, in the present invention, the drawing is 80.0% or more; the tensile strength is 350 to 550 N / mm ² , the twist value is 120 times or more (10
The target level is to provide steel products that satisfy 0D conversion.

As a result, the present invention has been completed based on the following findings (a) to (c).

(A) As a systematic approach for increasing the deformability (drawing and twist value), controlling the structure constituting the inside of the linear or rod-shaped steel to a structure mainly composed of ferrite; and In order to enhance the deformability and perform the rolling process with high accuracy, it is preferable that the ferrite crystal grain size is not made so fine and uniform, and specifically, the center of the rolled material to the diameter / 4 range (hereinafter , The ferrite grain size number (A) in the ferrite structure in the ferrite structure, and the ferrite in the ferrite structure in the outermost layer of the rolled material (hereinafter, sometimes simply referred to as the “surface layer”) It is effective to control the grain size number (B) to be in the range of 7.0 to 10.0 and to make the ferrite grain size numbers of the surface layer and the inside substantially the same; (B) On the other hand, deformability (diaphragm, It has been found that controlling the C content in an extremely low region (C <0.05%) is effective as an approach from the chemical component side in order to increase (and twist value).

(C) However, if the structure and the components in the steel are controlled as described above, the strength of the steel material itself decreases, so in order to increase the strength, Ti and / or Nb is positively added in a predetermined amount, Ti in the ferrite and at the ferrite grain boundaries
Fine crystal grains such as C, TiN and Nb (C, N) were deposited to increase the strength.

As in the present invention, various methods have been proposed so far for producing steel excellent in cold workability even if the spheroidizing treatment is omitted, but especially from the viewpoint of improving deformability. Therefore, the technical ideas unique to the present invention represented by the above (a) to (c) have not been disclosed yet.

For example, JP-A-57-63635 describes A
_c1 transformation point, dissipate sufficiently aggregated cementite by 5 hours or more to a temperature not less than 50 ° C. from the transformation point A _c1, by fixing dissolved N by controlling the Al content, the machining tool life A method of making an enhanced cold forging steel bar is disclosed. This publication was made on the basis of the finding that "if the temperature after hot rolling is kept within a predetermined range, cementite can be coagulated and precipitated, and the strength can be reduced." If it occurs, cracking is likely to occur during cold forging. "From the viewpoint of controlling the components in the steel, the ferrite grain size numbers of the surface layer and the inside do not become so fine as in the present invention. There is no idea to improve the cold workability by controlling almost the same.

Further, in Japanese Unexamined Patent Publication No. 8-260047, in order to reduce the solid solution N which causes strain aging in cold forging, N
And a step of hot-rolling by specifying Al / N; a thermo-mechanical treatment step of applying plastic working of 50% or more in a predetermined temperature range in the final stage of the hot-rolling; Disclosed is a method for manufacturing a steel rod for cold forging, which includes overaging treatment in which a temperature range of ° C is heated for 3 hours or more. This publication was made based on the finding that "after miniaturizing ferrite grains, further overaging treatment for a long time is effective in suppressing strain aging and improving ductility", Similar to the present invention, the problems are the same in that the aim is to provide a wire or the like having excellent deformability and increased strength. However, the above-mentioned publication does not disclose the technical idea of the present invention described above. Moreover, in the above publication, in order to obtain desired characteristics,
Since special overaging treatment is added after cooling following thermomechanical treatment, such special overaging treatment is unnecessary,
This is different from the present invention in which desired characteristics are secured from the viewpoint of controlling the microstructure in steel and further the composition of components. As a matter of fact, in the above publication, although the strength is excellent, the aperture is about 50 to 75% and the twist value is at most 100.
It is not possible to secure the target level (diaphragm: 80.0% or more, twist value of 120 or more) in the number of turns (100D conversion).

The requirements for specifying the present invention will be described below.

First, as to the structure, in the linear or rod-shaped steel of the present invention, as described above, the ferrite crystal grain diameters of the surface layer and the inside are made substantially uniform and the deformability is enhanced without much refining. However, it has a technical idea.

Specifically, the structure of the linear or rod-shaped steel is substantially ferrite, and the ferrite grain size number (A) in the ferrite structure within the range (inside) from the center to the diameter / 4 of the rolled material is 7. No. 0 to 10.0, the ferrite grain size number (B) in the ferrite structure in the outermost layer (outermost layer) of the rolled material is 7.0 to 10.0, and (A) and (B) are , 0 ≦ (B) − (A) ≦ 0.5 must be satisfied.

It is necessary to have the above-described structure in order to secure the deformability equivalent to that of the spheroidizing material, which is the object of the present invention, to have a substantially ferrite structure . here,
“Substantially having a ferrite structure” means that the ferrite structure exists in 99% or more (including 100%) in space factor (area ratio) with respect to the entire structure, and the rest is pearlite. . This is because the deformability decreases as the area ratio of pearlite in the tissue increases.

[0021] The ferrite structure inside the rolled material
Elite grain size number (A) is 7.0-10.0 and rolled
Of the ferrite grain size in the ferrite structure in the outermost layer of the material
No. (B) is 7.0 to 10.0, and (A) and above
And (B) satisfy 0 ≦ (B) − (A) ≦ 0.5.
In the present invention, the "outermost layer" means the layer of the surface portion excluding the center of the rolled material to the diameter / 4 range, and the "inside" is the portion of the rolled material from the center to the diameter / 4 range. Means
Here, the ferrite grain size number is measured based on the ferrite grain size test method described in JIS G 0552. According to this test method, the larger the ferrite grain size number, the smaller the ferrite grain size. For example, ferrite grain size number 7 means grain size 32 μm, and ferrite grain size number 10 means 11 μm.

In the present invention, both the ferrite grain size number (B) of the outermost layer and the ferrite grain size number (A) of the inside are 7.
Control within the range of 0 to 10.0 and making the relationship between (B) and (A) substantially the same [(B)-(A) ≦ 0.
5]. That is, in the present invention, unlike the conventional general recognition that "the ferrite grain size is refined as much as possible to enhance the ductility", while controlling the ferrite grain size to a predetermined average grain size without making the ferrite grain size too small, The above-mentioned ferrite-based structure,
Further, in combination with the extremely reduction of the C content, the extremely high deformability of the aperture: 80.0% or more is achieved.

Here, if the ferrite grain size number (B) of the outermost layer / the inner ferrite grain size number (A) is less than 7, rolling occurs in the ferrite transformation region, so cracking occurs during rolling and cold forging. Tend to occur, and the deformability decreases. On the other hand, if the above (B) / (A) exceeds number 10,
There are problems that the strength becomes too high, the deformation resistance becomes high, the ferrite grains are apt to be flattened, the dimensional accuracy after cold forging is deteriorated, and the ferrite crystal grain size is easily varied.

Further, it is preferable that the relationship between (B) and (A) is the same. This makes it possible to uniformly secure the deformability and the deformation resistance in all the cross-sections; this is advantageous because the dimensional accuracy after cold forging is improved. However, if it is within the range of (B)-(A) ≦ 0.5, it is included in the scope of the present invention. This is because if the difference is in this range, it is considered that the surface layer and the inside have substantially the same ferrite grain size number, and desired characteristics can be secured. Incidentally, in the present invention, the ferrite grain size number (B) of the outermost layer is the same as or larger than the ferrite grain size number (A) of the inside, but this is because water, air, etc. This is because the outermost surface layer naturally cools at a higher cooling rate than the central portion because it is cooled by being applied to the surface of, and the time for crystal grain growth becomes shorter. The difference between the ferrite grain size number of the outermost layer of the rolled material and the ferrite grain size number inside the rolled material depends on the size of the rolled material, cooling conditions, etc.
It can be adjusted appropriately.

Preferably, the ferrite grain size number (B) of the outermost layer is 8.0 or more and 9.5 or less; the internal ferrite grain size number (A) is 7.5 or more and 9.0 or less. .

Next, preferable components in steel for obtaining such a structure will be described.

C: 0.008% or more and less than 0.05% C is an essential element for imparting strength to the steel material, but in the present invention, particularly desired deformability [drawing 80% or more,
And a twist value of 120 times or more (100D conversion)] are extremely important elements.

If it is less than 0.008%, even if Ti or N
Even if the precipitation strengthening element of b is added or the rolling conditions are controlled, the desired strength (350 N / mm ² or more) cannot be obtained. It is preferably 0.011% or more, more preferably 0.
It is 013% or more. On the other hand, if it exceeds 0.05%, the desired deformability cannot be obtained. Preferably 0.03% or less,
It is more preferably 0.023% or less.

Ti: 0.005-0.025%, and /
Or Nb: 0.02 to 0.07% All of these elements are nitride / carbonitride forming elements and fix free C and N to precipitate TiN in austenite, or Nb. By precipitating (C, N), there is an action of suppressing hot rolling cracks. The remaining Ti, which does not contribute to the precipitation of TiN, precipitates in the ferrite or in the ferrite grain boundaries as TiC, Ti, Nb, Cr compound carbides, etc., and contributes to the strength improvement.

In order to effectively exert such an action,
Ti is 0.005% or more (preferably 0.008% or more, more preferably more than 0.010%), 0.025% or less (preferably 0.02% or less, more preferably 0.0).
15% or less); Nb is 0.02% or more (preferably 0.
023% or more, more preferably 0.025% or more),
It is recommended to control to 0.07% or less (preferably 0.04% or less, more preferably 0.035% or less). These elements may be added alone or in combination.

N: 0.0015 to 0.007% N bonds with Al or Ti to form a nitride of AlN or TiN, and stabilizes ferrite crystal grains (ferrite having a desired average grain size is It is an element that contributes to stable generation). In order to effectively exert such an action,
It is recommended to add 0.0015% or more (preferably 0.002% or more, more preferably 0.003% or more). However, if added excessively, N will form a solid solution in the ferrite and cause the occurrence of strain aging during cold working, so the upper limit was set to 0.007%. Preferably 0.
It is 005% or less, more preferably 0.004% or less.

Si: 0.05 to 0.4% Si is a deoxidizing agent and an element useful for ensuring a desired strength. In order to effectively exhibit such an action,
Add more than 05%. It is preferably 0.10% or more, more preferably 0.15% or more. However, if added excessively, the desired ferrite grain size cannot be obtained, and since ferrite is solid-solution strengthened, the desired deformability cannot be secured even if the rolling conditions and the like are controlled. It is preferably 0.30% or less, more preferably 0.25% or less.

Mn: 0.2 to 0.9% Mn, like C, is an element useful for increasing the strength of steel. In order to exert such an effect effectively, 0.2% or more is added. It is preferably 0.25% or more, more preferably 0.30% or more. However, if added in excess, the ferrite / pearlite growth rate after hot rolling decreases,
Since the bainitic ferrite, which is harmful for improving the deformability, is likely to be generated, its upper limit is set to 0.9%. It is preferably 0.6% or less, more preferably 0.5% or less.

The steel of the present invention contains the above components, and the balance is substantially iron. However, in addition to the above components, other allowable components may be added within the range not impairing the action of the present invention. , Including impurities.

Specifically, it is recommended to positively add or control the following components for the purpose of imparting further characteristics or further improving the action of the present invention.

Al: 0.01 to 0.06% Al is an element useful for deoxidation, and by precipitating AlN, ferrite crystal grains are stabilized (a ferrite having a desired average grain size , Can be stably generated). To effectively exhibit such an effect, 0.01% or more is added. It is preferably 0.015% or more, more preferably 0.02% or more. However, if added excessively, the above action will be saturated, so the upper limit was made 0.06%. More preferably 0.05% or less (even more preferably 0.04% or less)
Is.

Cr: 0.01 to 0.3% Cr is an element that contributes to the strength increase and the aging suppressing action of C during cold forging. To effectively exhibit such an effect, 0.01% or more is added. It is more preferably 0.03% or more. However, the effect is saturated even if added in excess of 0.3%. It is preferably 0.2% or less, more preferably 0.15% or less.

P: 0.001 to 0.02% P is an element that contributes to work hardening, and is added in an amount of 0.001% or more in order to effectively exhibit such an action. It is preferably 0.004% or more. However, if added in excess of 0.02%, the deformability decreases, so the upper limit is made 0.02%. It is preferably 0.010% or less.

S: 0.02% or less (including 0) S mainly forms sulfide-based inclusions of MnS and reduces the deformability, so the upper limit is made 0.020%. It is preferably 0.010% or less.

Next, a method of manufacturing the wire rod or the rod material according to the present invention will be described.

Specifically, a steel slab satisfying the above composition is heated to the range of 975 to 1150 ° C., and then 900 to
Rolled to a predetermined wire diameter in the range of 1150 ° C, 950-1
Finish rolling at 050 ° C. Then, by mainly adjusting the water flow, etc., and quenching at a cooling rate of 600 to 6000 ° C./min until the adjusted cooling start temperature reaches 900 to 975 ° C., then 2 to
It is cooled to the adjusted cooling end temperature of 250 to 450 ° C. at an average cooling rate of 10 ° C./s.

Each step will be described in detail below.

Heating temperature of steel slab: 975 to 1150 ° C. This heating temperature is TiC or Nb (C,
It is set so that carbonitrides such as N) are dissolved as much as possible to obtain strength improvement by precipitation strengthening. here,
The "heating temperature of the billet" is measured by a radiation thermometer, and strictly means "the surface temperature of the billet". When heated above 1150 ° C., the ferrite crystal grain size becomes coarse, and the desired deformability cannot be obtained. It is preferably 1100 ° C or lower, more preferably 1050 ° C or lower. On the other hand, if the heating temperature is lower than 975 ° C., the precipitates do not form a solid solution, and the desired strength cannot be obtained even if the subsequent heat treatment is controlled. Preferably 10
The temperature is 00 ° C or higher, more preferably 1025 ° C or higher.

Rolling temperature: 900 to 1150 ° C. This temperature is TiC / Nb in Ti and Nb in steel in a series of rolling steps from rough rolling to intermediate rolling to finish rolling.
It is set in order to obtain a desired strength by precipitating carbonitrides such as (C, N). Here, the "rolling temperature" is measured by a radiation thermometer,
Strictly speaking, it means “surface temperature of steel billet”. 1150 ° C
If the rolling is carried out beyond the range, the pinning effect due to Ti or Nb cannot be obtained, the grain size of ferrite crystal after rolling becomes coarse, and the deformability decreases. It is preferably 1100 ° C or lower, more preferably 1050 ° C or lower. On the other hand, if the rolling temperature is lower than 900 ° C., the rolling is performed in the ferrite transformation region, so that cracking occurs at the interface between ferrite and austenite during rolling. It is preferably 950 ° C. or higher.

More specifically, in a series of rolling steps,
Rough rolling is 900 to 1150 ° C (preferably 950 ° C or higher and 1050 ° C or lower), and intermediate rolling is 925 to 1150 ° C.
(Preferably 950 ° C. or higher and 1100 ° C. or lower), finish rolling 950 to 1050 ° C. (preferably 975 ° C. or higher, 1
By controlling the temperature to be 025 ° C. or lower), the action of the present invention can be more efficiently exhibited.

Here, in the present invention, "rough rolling" means
Using 7 to 10 rolling mills, a process of rolling a billet having a surface area of 115 to 200 mm square with a surface reduction ratio of 75 to 95% into a square shape is meant; "intermediate rolling" refers to the above rough rolling. Continue to 4
~ 12 rolling mills are used to roll at a reduction rate of 70 to 98% and roll into a round shape; "finish rolling" means water rolling after the above "intermediate rolling". After adjusting, the area reduction rate is 5-9 using 1-2 block mills.
Each step of rolling 5% is meant.

In consideration of the increase of the load of the rolling roll, the decrease of the dimensional accuracy, the prevention of the occurrence of surface flaws, etc., it is practically 975
It is recommended to set the rolling temperature to about 1025 ° C.

Adjusted cooling start temperature: 900 to 975 ° C. After the above finish rolling, 600 to 60 mainly using water as a medium.
Minimum surface temperature of 500 at an average cooling rate of 00 ° C / min
After being rapidly cooled to about 900 ° C., it is wound on a cooling zone (cooling conveyor). At that time, the temperature is recovered by the heat (recovery) held by the steel slab, but in the present invention, this recovery temperature is referred to as “adjusted cooling start temperature” (synonymous with the coiling temperature) and is set to 900 to 975 ° C. . If the temperature is higher than 975 ° C, the scale after cooling becomes thick, the scale peels off during cooling, and a secondary scale is further generated, which easily causes troubles in the subsequent descaling step, and the obtained wire rod, etc. Since there is no stiffness in it, it becomes difficult to wind it into a ring-shaped desired shape. Preferably 960 ° C. or lower, more preferably 9
It is 50 ° C or lower. On the other hand, when it becomes lower than 900 ° C,
Even if the surface temperature of the wire rod is recrystallized due to the recuperation due to the above-mentioned rapid cooling treatment, the ferrite crystal grain size becomes too fine, and a predetermined average grain size cannot be obtained. Further, when the adjusted cooling start temperature is 900 ° C. or lower, Nb and the like start to precipitate as carbonitrides, so that the desired strength cannot be secured. The temperature is preferably 915 ° C or higher, more preferably 925 ° C or higher.

Adjusted cooling end temperature (250 to 500 ° C.)
Average cooling rate at: 2 to 10 ° C / sec This is the above adjusted cooling start temperature (900 to 975 ° C)
250 to 500 ° C (adjusted cooling end temperature)
The average cooling rate when cooling to the temperature is defined. The average cooling rate is set to secure a desired strength, and by controlling the average cooling rate within the above range, Ti and Nb carbonitrides that contribute to the strength improvement can be efficiently deposited. Preferably 3 ° C / sec or more,
8 ° C / sec or less; more preferably 4 ° C / sec or more and 6 ° C / sec or less.

According to the present invention, excellent cold workability can be obtained even with as-hot-rolled wire rod or steel bar. However, this wire rod or steel bar is immersed in a bath of acid (hydrochloric acid, sulfuric acid, etc.), After removing the scale by mechanically applying strain, the zinc phosphate coating, calcium phosphate coating, lime, etc. are subjected to pre-drawing treatment, and metal soap etc. is used as a lubricant for wire drawing, cold rolling, etc. The same excellent cold workability can be obtained with the steel wire subjected to the above.

The present invention will be described in detail below based on examples.
However, the following examples do not limit the present invention,
All changes and modifications made without departing from the spirits of the preceding and the following are included in the technical scope of the present invention.

[0052]

Examples a to d, f to having the composition of components shown in Table 1
n, r to u sample steels (units in the table are% by mass) and various wire rods (No. 1 to No. 1) under various manufacturing conditions shown in Table 2 are used.
26) was obtained. Of these, the test steels s, t, and u are J
IS SWRCH25K, JIS SWRCH45K,
JIS SWRCH20A is a currently used process material, and both steels have a large amount of C and a small amount of Ti and Nb. Further, the sample steel n is an example having a small amount of Ti, and r is an example having a large amount of Mn.

Next, for each of the above wire rods, the tensile strength and the drawing (index of deformability) were measured using JIS No. 9 test pieces. The twist value was calculated by using the JIS 9 test piece and performing a twist test at a gauge length of 100 mm, and the number of twists at a gauge distance of [100 × diameter (D)] based on the following formula.

Twisted value (100D conversion) = twisted times × (diameter / gage length) × 100 The obtained results are shown in Table 2.

[0055]

[Table 1]

[0056]

[Table 2]

From the above results, the following can be considered.

First, in Table 2, No. 1, 5, 7, 9, 11
Nos. 18 to 18 have the ferrite grain size numbers of the surface layer and the inside thereof controlled within the range of the present invention, and therefore are excellent in deformability as they are in hot rolling without spheroidizing annealing, and as steel materials themselves. Is also extremely high in strength. In particular, the deformability of these steels is that of No. It is higher than 20 to 26, and all of them are the target level of the present invention: diaphragm 80.0% or more, twist value 1
We were able to secure more than 20 times.

On the other hand, No. Nos. 2 and 6 are examples in which the adjusted cooling start temperature is low; Nos. 3 and 8 are examples in which the cooling rate to the adjusted cooling end temperature is slow and the end temperature is high; No. 4 is an example in which the adjustment cooling start temperature is low, and the cooling rate up to the adjustment cooling end temperature is slow and the end temperature is high; No. 10 is an example in which the adjusted cooling start temperature is low and the adjusted cooling end temperature is high. In all cases, the desired ferrite grain size cannot be obtained, and the strength of the steel material itself decreases, and the deformability decreases. It was observed.

No. Nos. 19 and 20 are examples in which steels that do not satisfy the preferred range of the present invention are used and the adjusted cooling start temperature is low, and desired ferrite grain sizes cannot be obtained, and examples in which the strength of the steel material itself decreases are also seen. It was

Further, No. Nos. 21 to 26 are examples in which the spheroidizing annealing process is applied to the current steel, and although the tensile strength is high because of the large amount of C, the drawing is 80.0% or more, which is the target level of the present invention, It is far below the turnover value of 120 times or more.

For reference, No. 2 in Table 2 is used. 9 (symbol d1)
And No. TEM (transmission electron microscope) photographs of No. 2 (symbol a2) are shown in FIGS. 1 and 2, respectively. Of these, No.
No. 9 is an example of the present invention in which the steel type d satisfying the requirements of the present invention was used and forged by the method d1 satisfying the requirements of the present invention, and the desired ferrite crystal grain size was obtained. On the other hand, No. No. 2 used steel type a that satisfied the requirements of the present invention, but the desired ferrite crystal grain size could not be obtained because it was forged by method a2 that did not satisfy the requirements of the present invention.

[0063]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, even if the spheroidizing annealing treatment is omitted, it remains hot-rolled, has excellent deformability, and has a high strength as a steel material itself. It was possible to efficiently provide bar steel.

[Brief description of drawings]

1 is a No. 2 table. 9 is a TEM photograph of 9 (symbol d1).

FIG. 2 No. of Table 2 2 is a TEM photograph of 2 (symbol a2).

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] November 14, 2001 (2001.11.
14)

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0062

[Correction method] Change

[Correction content]

For reference, No. 2 in Table 2 is used. 9 (symbol d1)
And No. 1 and 2 are optical microscope micrographs (magnification 400 times) of No. 2 (symbol a2), respectively. Of these, N
o. No. 9 is an example of the present invention in which the steel type d satisfying the requirements of the present invention was used and forged by the method d1 satisfying the requirements of the present invention, and the desired ferrite crystal grain size was obtained. On the other hand, No. No. 2 used steel type a that satisfied the requirements of the present invention, but the desired ferrite crystal grain size could not be obtained because it was forged by method a2 that did not satisfy the requirements of the present invention.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0063

[Correction method] Change

[Correction content]

[0063]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, even if the spheroidizing annealing treatment is omitted, it remains hot-rolled, has excellent deformability, and has a high strength as a steel material itself. It was possible to efficiently provide bar steel.

[Brief description of drawings]

1 is a No. 2 table. 9 is an optical micrograph (magnification 400 times) of 9 (symbol d1).

FIG. 2 No. of Table 2 2 is an optical micrograph (magnification 400 times) of No. 2 (symbol a2).

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Masataka Shimotsusa             2 Nadahama Higashi-cho, Nada-ku, Kobe             Steel Works Kobe Steel Works

Claims (7)

[Claims] 1. A ferrite grain size number (A) in the ferrite structure having a substantially ferrite structure in the range of the center of the rolled material to the diameter / 4 is 7.0 to 10.0, and the rolled material The ferrite grain size number (B) in the ferrite structure in the outermost layer is 7.0 to 10.0, and (A) and (B) are 0 ≦ (B) − (A) ≦ 0. A wire or rod-shaped steel excellent in deformability, which is characterized by satisfying 0.5. 2. As a steel component, C: 0.008% or more and less than 0.05% (meaning mass%,
The same shall apply hereinafter), The linear or bar steel according to claim 1. 3. The linear or bar steel according to claim 2, which further contains Si: 0.05 to 0.4% and Mn: 0.2 to 0.9%. 4. Further, Ti: 0.005 to 0.025%, and / or Nb:
The linear or rod-shaped steel according to claim 2 or 3, wherein the strength is increased by containing 0.02 to 0.07%. 5. The linear or rod-shaped steel according to claim 2, which further contains N: 0.0015 to 0.007%. 6. Further, Al: 0.01 to 0.06%, Cr: 0.01 to 0.3
%, P: 0.001 to 0.02%, S: 0.02%
The linear or rod-shaped steel according to any one of claims 2 to 5, containing the following. 7. A mechanical component obtained by using the linear or rod-shaped steel according to any one of claims 1 to 6.