JP3515923B2 - Wire or bar steel with excellent cold workability, and cold forged, cold forged or cold rolled products - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear or rod-shaped steel (hereinafter sometimes abbreviated as steel) in which deformation resistance is suppressed in a temperature rising region caused by heat generation during processing during cold working, and It relates to mechanical parts obtained using steel. According to the present invention, when manufacturing mechanical parts such as bolts and nuts by cold working such as cold forging, cold forging, and cold rolling, even if hot rolling is performed without heat treatment, heat generated by processing This is extremely useful because it can provide a linear or rod-shaped steel with suppressed deformation resistance in the temperature rise region.

[0002]

2. Description of the Related Art Cold working has a higher productivity than hot working and machine cutting, and also has a good yield of steel, so that bolts, nuts, and other mechanical parts can be manufactured efficiently. 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 and the ductility (elongation, drawing) is high. If the deformation resistance of the steel is high, the life of the tool used for cold working will be shortened, while if the ductility is low, cracks will easily occur during cold working, causing defective products.

Therefore, in order to reduce the deformation resistance of the steel and enhance the ductility, a spheroidizing annealing treatment is usually performed before the cold working, whereby the steel material is softened and the ductility is increased in the cold state. The method of processing has been conventionally adopted.

However, a long-time treatment (1
Since it takes 0 to 20 hours), for the purpose of improving productivity, energy saving measures, and eventually cost reduction,
It has been earnestly desired to develop a linear or rod-shaped steel in which the spheroidizing annealing treatment can be omitted and the deformation resistance in the temperature rising region caused by the heat generation during processing is suppressed during cold working.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to keep hot rolling even when the spheroidizing annealing treatment is omitted, and during cold working, It is an object of the present invention to provide a linear or rod-shaped steel in which deformation resistance is suppressed in a temperature rising region caused by heat generation during processing, and mechanical parts such as bolts and nuts obtained by using the linear or rod-shaped steel.

The wire rod or rod-shaped steel excellent in cold workability of the present invention which can solve the above problems is C: 0.001 to 0.
5% (mass%, the same below), Al: 0.005-0.1
%, N: 0.001 to 0.015%, Mn: 0.035
~ 2%, Si: 0.5% or less (not including 0%), S:
0.02% or less (not including 0%), balance: Satisfying iron and unavoidable impurities, 1/8 of the diameter when viewed from the surface of the rolled material
In the ferrite structure in the region closer to the center than the depth position, the carbide having a nucleus of nitride is 25 or more / 25 μm ^{2 on} average (preferably 35 or more / 25 μm on average). ² ) By precipitating, the deformation resistance in the temperature range (generally 100 to 350 ° C.) when the temperature rises due to heat generation during processing during cold working is suppressed to be lower than the deformation resistance at room temperature. Has the point.

The above wire or rod-shaped steel further contains Cr: 1.2% or less (not including 0%), Ti: 0.2% or less (not including 0%), B: 0.01% or less (0. %), Nb: 0.15% or less (0% is not included), V: 0.2% or less (0% is not included), Zr: 0.1% or less (0% is not included) It is preferable to contain at least one of the above. In addition, the case where a trace component or an unavoidable impurity is contained is also included in the technical scope of the present invention.

Mechanical parts obtained by using the above-mentioned linear or rod-shaped steel are also included in the scope of the present invention.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In order to provide a steel having excellent cold workability as hot rolled, the inventors of the present invention have solved the solid solution N and the cold-workability which control the deformation resistance. The solid solution C has been focused on and studied in detail. As a result, in the ferrite-pearlite structure that constitutes the internal structure of the linear or rod-shaped steel, a predetermined number of nitrides are finely precipitated in the ferrite grains, and with this as a nucleus, carbides such as cementite are well precipitated over a predetermined number. By doing so, solid solution N and solid solution C can be fixed as nitrides and carbides, and as a result of suppressing dynamic strain aging, it is possible to suppress an increase in deformation resistance; The present invention has been completed by finding that the deformation resistance at around 100 to 350 ° C. in the latter stage of cold working can be suppressed low without lowering the strength.

As in the present invention, attention has been paid to solid solution C and solid solution N, and a method for producing steel excellent in cold workability even if the spheroidizing annealing treatment is omitted has been proposed so far. ing.

For example, Japanese Examined Patent Publication No. 61-35249 discloses a method of controlling the rolling conditions and cooling conditions to reduce the solid solution C and the solid solution N to suppress the work hardening due to strain aging and to reduce the deformation resistance. Is disclosed.

In addition, JP-A-56-158841 and JP-A-57-39002 are based on the finding that fixing the solid solution N can reduce the hardness and the work hardening rate. In the latter, a method for producing a hot-rolled wire having an excellent die life by using Ti or B as the material and controlling Al / N is disclosed.

Further, Japanese Patent Laid-Open No. 57-63635 discloses 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.

However, in any of the above-mentioned methods (1) to (4), the chemical components in the steel are controlled and the rolling conditions and the cooling conditions are controlled in order to fix the solid solution C and the solid solution N which adversely affect the reduction of the deformation resistance. Even after scrutinizing the above publication, it is extremely effective to reduce the amount of solute N and solute C by precipitating a predetermined number or more of carbides having nitride as a nucleus in ferrite grains; As a result, the strength of the steel material itself does not decrease, and
There is no disclosure or suggestion that the deformation resistance can be kept low even in the vicinity of 0 to 350 ° C. Incidentally, the above items (1) to (4) do not pay attention to the reduction of the deformation resistance in the latter stage of processing. As described above, there has been no study that has been conducted by paying attention to the relationship between the number of carbides having nitride as a nucleus in the ferrite structure and the deformation resistance, which is a finding first found by the present inventors, In this respect, the technical significance of the present invention exists.

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

As described above, the linear or rod-shaped steel of the present invention includes a depth of ⅛ of the diameter when viewed from the surface of the rolled material, and is nitrided in the ferrite structure in the region closer to the center than the depth position. The average number of carbides whose core is 25 or more / 2
It is characterized by the presence of 5 μm ² . Here, for the observation position of “the ferrite structure including the depth of ⅛ of the diameter as viewed from the surface of the rolled material and located in the region closer to the center than the depth position”, refer to, for example, the structure observation position shown in FIG. can do. By thus precipitating a predetermined number or more of carbides having nuclei as nuclei in the ferrite structure, solid solution N and solid solution C that adversely affect the deformation resistance can be fixed, and not only in the initial stage of processing, Even when it reaches the latter stage of processing (around 100 to 350 ° C.), the excellent effect that the deformation resistance can be suppressed lower than the deformation resistance at room temperature is obtained.

Here, the above-mentioned nitride means Al, Cr, T.
i, B, Nb, V, Zr, etc. means a state in which one or more kinds of nitride-forming elements are combined with solid solution N and precipitated as nitrides.

Examples of the above-mentioned carbide include iron carbide such as cementite (Fe ₃ C), Cr, Ti, Nb,
Carbide in which one or more carbide-forming elements such as V and Zr are combined with C in steel; Mn, P, S, etc. are precipitated as a solid solution in these iron carbides and carbides Etc.

In the present invention, these nitrides and carbides are present as carbides having nitrides as nuclei. Specifically, for example, refer to FIG. 5 described later. In the following description, for convenience, a carbide containing a nitride having such an existing form as a nucleus may be abbreviated as “nitride and carbide” or “precipitate”.

Next, the reason for setting the number of precipitated nitrides and carbides will be described with reference to FIG.

This FIG. 1 shows No. 1 described in the embodiment described later.
Using test pieces 1 and 3, 25 ℃ (normal temperature), 78 ℃, 1
It is a graph of the deformation resistance when the temperature is raised to 50 ° C, 220 ° C, 330 ° C, 350 ° C, and 424 ° C. In the figure, ● (No. 1) is an example of the present invention (78 pieces) having a predetermined number of nitrides and carbides specified in the present invention, ◆
(No. 3) is a comparative example (21 pieces) which does not have a predetermined number.

From the figure, it can be seen that No.
3, the deformation resistance increases as the temperature rises, and 3
It can be seen that the deformation resistance reaches a maximum near 00 ° C. This is because dynamic strain aging due to solid solution C and solid solution N remarkably occurred. On the other hand, No. which satisfies the requirements of the invention.
In No. 1, since a predetermined number of nitrides and carbides were formed in the ferrite, the dynamic strain aging was suppressed,
Even if the processing temperature reaches around 300 ° C., the increase in deformation resistance can be effectively suppressed.

The reason why the deformation resistance in the vicinity of 300 ° C. could be remarkably reduced by forming a predetermined number of nitrides and carbides in the ferrite structure in this manner is considered as follows. Generally, solid solution N in ferrite
When the amount and the amount of solute C increase, the work hardening due to strain aging becomes large and the deformation resistance becomes high. However, in the present invention, the solute N which adversely affects the deformation resistance is a nitride forming element such as Al. It is considered that the deformation resistance could be suppressed to a low level by finely precipitating a predetermined number or more of nitrides by combining with the above, and successfully precipitating a predetermined number or more of carbides such as cementite using these as nuclei.

In order to effectively exert the deformation resistance reducing action due to the formation of such nitrides and carbides, nitrides and carbides are mixed or mixed in the ferrite structure in the range from the center of the rolled material to the diameter / 8. It is necessary that at least 25 or more / 25 μm ² are present on average in the combined state. This number is closely related to the average diameter of nitrides and carbides, and for example, the cooling rate becomes small, and the number of these precipitates becomes small as the average particle size of these precipitates becomes large. Therefore, the number of the above-mentioned nitrides and carbides should be determined strictly in relation to their average diameters, but generally, the average diameter of the nitrides is 1 to 1.
When the average diameter of the carbides is 0 to 50 nm at 0 nm,
With nitride and carbide mixed or compounded,
On average 35 or more / 25 μm ² (more preferably 40 or more / 25 μm ² , even more preferably 45 or more / 2
5 μm ² ) is recommended. The average diameter of the nitride is 10 to 50 nm and the average diameter of the carbide is 50 to 50 nm.
In the case of 500 nm, the average number of the precipitates is 25 or more / 25 μm ² (more preferably 30 or more / 25 μm
m ² , and even more preferably 35 or more / 25 μm ² ) is recommended.

The metallographic structure of the steel according to the present invention after hot rolling is mainly composed of the above-mentioned nitride and carbide. Specifically, the area ratio of ferrite in the metallographic structure is 20. % Or more is preferable. In the present invention, it is intended to suppress the deformation resistance to be low even if the ferrite fraction is the same, and in order to effectively exhibit the action of the above-described precipitate, the ferrite area ratio in the metal structure is set to 20%. % Or more (more preferably 25% or more) is recommended.

Next, the chemical components in the steel of the present invention will be described.

As described above, the most important point of the present invention is
This is where a predetermined number of nitrides and carbides are present in the ferrite structure. Therefore, it is recommended to add C, N, and Al as basic components and to positively add various carbide-forming elements and nitride-forming elements so that predetermined nitrides and carbides can be obtained.

C: 0.001 to 0.5% C is an essential element for imparting the required strength of the steel material. If it is less than 0.001%, the desired strength cannot be obtained, and if it is attempted to control such a low concentration, the cost is industrially high and it is not economical. It is preferably 0.003% or more, more preferably 0.005% or more. on the other hand,
If it exceeds 0.5%, the ferrite fraction becomes low and the desired effect cannot be obtained. It is preferably 0.48% or less.

Al: 0.1% or less (not including 0%) Al is useful for deoxidation, and is added to fix solid solution N and generate a nitride (AlN). For that, 0.
It is preferable to add 005% or more. However, 0.1%
Even if added in excess of, the above action will be saturated,
It is economically wasteful. It is more preferably 0.08% or less.

N: 0.015% or less (not including 0%) N is an unnecessary element in view of the fact that solid solution N adversely affects the reduction of deformation resistance, but in the present invention, AlN
A certain amount of addition is necessary in order to successfully precipitate carbides such as cementite by using such nitrides as nuclei. It is preferably 0.001% or more. On the other hand, if it exceeds 0.015%, the amount of alloying elements added for precipitating a predetermined nitride increases, so that the cost increases. Preferably 0.
It is 01% or less.

The steel of the present invention basically contains the above components, and the balance is iron and unavoidable impurities, but the following elements can be positively added.

Cr: 1.2% or less, Ti: 0.2% or less
Lower, B: 0.01% or less, Nb: 0.15% or less, V:
From the group consisting of 0.2% or less and Zr: 0.1% or less
At least one selected (all elements contain 0%
None) Of these, elements other than B (Cr, Ti, Nb, V, Zr)
Is a carbide and / or nitride forming element, B is a nitride forming element like Al, and addition of these elements can reduce solid solution C and solid solution N which adversely affect the deformation resistance. become. In order to effectively exhibit such an action, Cr: 0.02% or more, Ti: 0.01% or more, B: 0.0003% or more, Nb: 0.005% or more, V: 0.01 % Or more, and Zr: 0.005% or more is recommended to be added. However, even if it is added over the above range, the effect is saturated and it is economically useless. Preferably Cr: 1% or less, Ti: 0.15% or less, B:
0.008% or less, Nb: 0.1% or less, V: 0.15
%, Zr: 0.08% or less. These elements may be used alone or in combination of two or more.

Further, the following elements can be added.

Mn: 0.035 to 2% If the amount of Mn is less than 0.035%, S cannot be completely converted to MnS and the workability deteriorates. It is more preferably at least 0.05%. On the other hand, if it exceeds 2%, the rolling load is too high and the tool life is reduced. It is more preferably 1.8% or less.

Si: 0.5% or less (not including 0%) Si is one of deoxidizing agents, and it is preferable to add 0.005% or more in order to effectively exhibit such an action. . More preferably, it is 0.008% or more. on the other hand,
The effect is saturated even if added over 0.5%,
The deformation resistance also increases. More preferably 0.45%
It is the following.

S: 0.02% or less (not including 0%) When the content of S exceeds 0.02%, cracks are likely to occur during cold working. It is more preferably 0.018% or less.

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

In order to obtain a predetermined number of carbides of interest in the present invention, a steel slab is heated to a temperature in the range of 850 to 1050 ° C., rolled to a predetermined wire diameter in the range of 725 to 1000 ° C., and then subjected to water flow. Cool to 725 to 950 ° C at a cooling rate of 600 to 6000 ° C / min, and then 3 to 600
It is necessary to cool to 400 ° C at a cooling rate of ° C / min. Each requirement will be described below.

Heating temperature of steel slab: 850 to 1050 ° C. This heating temperature is set so that a nitride such as AlN is partially solid-dissolved and finely precipitated after rolling.
When heated above 1050 ° C., the precipitated nitride is completely solid-dissolved to form solid solution N, and the nitride can be precipitated no matter how the subsequent manufacturing process is controlled. It will be difficult. The temperature is preferably 1025 ° C or lower, more preferably 1000 ° C or lower. On the other hand, the heating temperature is 8
When the temperature is lower than 50 ° C., the nitride such as AlN does not form a solid solution at all and does not act as a precipitation nucleus of carbide. The temperature is preferably 870 ° C or higher, more preferably 890 ° C or higher.

Rolling temperature: 725 to 1000 ° C. This temperature is the same in rolling as in heating the billet.
It is set to prevent the solid solution of nitride, and at the same time, dislocation is imparted to the structure in the steel to finely reprecipitate the solid solution N as nitride in ferrite. Therefore, a synergistic effect of further precipitating a carbide such as cementite is set. However,
From the viewpoint of preventing the load on the rolling roll from increasing, the dimensional accuracy from decreasing, and the occurrence of surface defects, 750 to 750 are practically used.
It is recommended to set the rolling temperature to about 1000 ° C. It is preferably 775 ° C or higher and 975 ° C or lower.

Cooling rate: 3 to 600 ° C./min (up to 400 ° C.
In order to precipitate the solid solution C and the solid solution N as carbides having nitrides as nuclei, a slower cooling rate is preferable, but
If it is too slow, the lamellar spacing in pearlite (a lamellar structure of ferrite and cementite) becomes large, and the structure may have poor ductility. In practice, it is preferably 500 ° C / min or less.

Cooling rate: 3 to 600 ° C./min (up to 400 ° C.
In order to precipitate solid solution C and solid solution N as carbides and nitrides, it is preferable to slow the cooling rate, but if it is too slow, the lamellar spacing in pearlite (layer structure of ferrite and cementite) becomes wide. , There is a risk that the ductility will be poor. Practically 6 ℃ / min or more, 500 ℃
/ Min or less is preferable.

According to the present invention, an excellent cold workability can be obtained even with a wire rod or a steel bar that has been hot-rolled. However, an acid (hydrochloric acid, sulfuric acid, etc.) may be added to this wire rod or a steel bar, or mechanical treatment may be performed. The same applies to steel wire that has been subjected to wire drawing, cold rolling, etc., using a zinc phosphate film, calcium phosphate film, lime, metal soap, etc. as a lubricant after removing scale by applying strain to the Excellent cold workability can be obtained.

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.

[0046]

Example Using test steels (units in the table are% by mass) having the composition shown in Table 1, the wire rods of φ12 mm were rolled under various manufacturing conditions shown in Table 2. Using this wire, the following items were measured.

[Measurement of Average Number of Nitride and Carbide Precipitated in Wire Rod] In order to avoid the effect of decarburization by hot rolling, as shown in FIG. 2, ferrite in the range from the center of the rolled material to the diameter / 8 The number of precipitates at 5 points in the structure was counted. Specifically, the precipitate was photographed with a scanning electron microscope (SEM) (8000 times), and then subjected to image analysis (FRM Twool Kit) to count the average value. Figure 3
4 and FIG. 4 are electron micrographs showing the appearance of precipitates in the ferrite structure of the No. 1 test piece (invention example) and the No. 3 test piece (comparative example) shown in Table 2. Are shown respectively.

[Confirmation of composition of precipitate] Whether or not the precipitate is cementite having AlN as a nucleus is confirmed by a transmission electron microscope (FE-TEM).
The image was magnified to 1,000,000 times and analyzed by EELS (Energy Loss Spectroscopy). The result was imaged by GIF (imaging filter manufactured by GATAN), and then the composition was analyzed. 5 to 7 are micrographs showing the analysis results of the precipitates in the No. 2 test piece (Example of the present invention) shown in Table 2, of which, as described above, FIG. 6 is a micrograph showing a nitrogen composition image, and FIG. 7 is a micrograph showing a carbon composition image.

[Measurement of Deformation Resistance] This deformation resistance is an index of cold workability and was measured by performing an upsetting test by a press in the following manner. First, the shape recommended by the Japan Society for Plastic Processing (forging, plastic processing technology series 4, p55, Corona)
In order to achieve this, the wire rod was cut into a size of φ10 × 15 mm, and this was used as a cylindrical test piece for upsetting, and then upsetting was performed by using a constrained thick plate with concentric circular grooves as upsetting rolling. It was The test condition was a compressibility of 60%, the maximum load applied at this time was measured, and the deformation resistance was calculated by the following calculation formula.

Deformation resistance (kgf / mm ² ) = load (kg
f) / A / f in the formula, A: cross-sectional area of test piece (mm ² ) f: constraint coefficient In the above formula, for example, when φ = 10 mm, A = 78.
It is 5 mm ² , and f = 2.77 in the case of 60% compression.

In the actual operation, cold multi-step processing (strain rate 2 to
100 / sec), assuming that the material to be processed reaches several hundreds of degrees Celsius due to heat generated by processing, other than room temperature (25 degrees Celsius) 78
℃, 150 ℃, 220 ℃, 320 ℃, 350 ℃, 424
The deformation resistance when the temperature was raised to 0 ° C. was measured.
Further, in order to investigate the effect of dynamic strain aging on the deformation resistance, the amount of increase in the deformation resistance due to dynamic strain aging (kgf / mm ² ) was calculated based on the following formula.

Increase in deformation resistance = [deformation resistance at 320 ° C. (σ320)] − [at room temperature (25 ° C.)
Deformation resistance (σ25)] of these results are also shown in Table 2.

[0053]

[Table 1]

[0054]

[Table 2]

The following can be considered from the table.

First, Nos. 1, 2, 6 to 17 shown in Table 2
Is an example of the present invention in which a predetermined number or more of carbides having nitrides as nuclei were formed in the ferrite structure, and the amount of increase in deformation resistance due to dynamic strain aging could be suppressed to a low level. From the result of FIG. 5, it was confirmed that the composition of the nitride in the above-mentioned precipitate in the ferrite structure was AlN.

On the other hand, Nos. 3, 4, and 5 are comparative examples in which the carbides having the predetermined nitride as the nucleus are not formed because the manufacturing conditions do not satisfy the requirements of the present invention, and all of them have the deformation resistance. The amount of increase is high.

[0058]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, even if the spheroidizing annealing treatment is omitted, the temperature rise region (100 to 100) caused by the heat generation during the cold working is kept in the hot rolling. It was possible to efficiently provide the linear or bar steel in which the deformation resistance at around 350 ° C.) was suppressed.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between test temperature and deformation resistance.

FIG. 2 is a schematic explanatory diagram showing a method for measuring the number of deposits.

FIG. 3 is an electron micrograph showing the appearance of precipitates in the ferrite structure of the example of the present invention.

FIG. 4 is an electron micrograph showing a state of precipitates precipitated in a ferrite structure in Comparative Example.

FIG. 5 is a micrograph of a precipitate in an example of the present invention.

FIG. 6 is a micrograph showing a nitrogen composition image in FIG.

FIG. 7 is a micrograph showing a carbon composition image in FIG.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toyofumi Hasegawa 2 Nadahamahigashi-cho, Nada-ku, Kobe City Kamido Steel Works, Ltd. Inside the Kobe Steel Works (56) Reference JP-A-5-339676 (JP, A) JP HEI 8-199316 (JP, A) JP 9-78184 (JP, A) JP 4-228519 (JP, A) JP 60-121220 (JP, A) JP 2-213415 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00

Claims (4)

(57) [Claims] 1. C: 0.001-0.5% (mass%, the same applies hereinafter), Al: 0.005-0.1%, N: 0.001-0.015%, Mn: 0.035 ~ 2%, Si: 0.5% or less (0% is not included), S: 0.02% or less (0% is not included), balance: Satisfies iron and unavoidable impurities, and viewed from the surface of the rolled material. Including the depth of 1/8 of the diameter ,
In the ferrite structure in the region closer to the center than the center position , carbides having nitride as a nucleus are averaged 25 or more / 25 μm.
^{2 By} precipitating, during cold working, the deformation resistance in the temperature rise region generated by the heat generation of working is at room temperature.
Linear or bar steel, characterized in that suppress et a lower than deformation resistance when. 2. Further, Cr: 1.2% or less (0% is not included), Ti: 0.2% or less (0% is not included), B: 0.01% or less (0% is not included). ), Nb: 0.15% or less (0% is not included), V: 0.2% or less (0% is not included), Zr: 0.1% or less (0% is not included) The linear or rod-shaped steel according to claim 1, which comprises: 3. Deformation resistance in a temperature rise region caused by working heat during cold working by precipitating 35 or more / 25 μm ^{2 on} average of carbides having nitride as a nucleus in the ferrite structure. But deformation at room temperature
Those suppressed et the lower than the resistance according to claim 1 or 2
The linear or bar steel described in. 4. A cold forged product, a cold forged product, obtained by using the linear or rod-shaped steel according to claim 1 .
Or cold rolled product.