JP2002097546A - Hot rolled wire rod and heat treated steel product therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled wire rod which does not crack even when strong cold working, and has a superior cold workability, and to provide a heat treated steel product therefor. SOLUTION: This hot rolled wire rod includes 0.1-0.5 C, 0.1-0.5 Si, 0.2-2.0 Mn, 0.001-0.015 S, 0.01-0.10 Al, 0-0.03 N, 0.05 or less P, furthermore at least one kind among 0.1-2.0 Cr, 0.1-1.0 Mo, 0.1-1.5 Ni, 0.001-0.005 B, by mass%, and Fe and inevitable impurities as the balance. It further comprises Vickers hardness Hv of 160 or more and 220 or less before the cold working, and satisfying εθ<-0.004259Hv+0.677057×(-εz)0.296+0.724 in cold working.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold working process, particularly in the case of strong cold working such as cold forging bolt forming, by optimizing the chemical composition and material hardness of a hot-rolled wire or its heat-treated steel. The present invention relates to a hot-rolled wire or a heat-treated steel that can ensure the workability of the steel.

[0002]

[Prior Art] JIS G 3502 piano wire produced by hot rolling, JIS G 3507 carbon steel wire for cold heading, and
JIS G 4051, JIS G 4104 and JIS G 4105 etc.Carbon steel and alloy steel for machine structures described in JIS G 4805, bearing steel described in JIS G 4805, spring steel described in JIS G 4801 and
Wires based on tool steel etc. described in JIS G 4401 are processed to the required shape through cold working processes such as secondary processing, wire drawing, drawing, cutting, forging and cutting, which are tertiary processing. And finished products.

[0003] Many of the wires as hot rolled are:
Usually, the structure has hard phases such as pearlite, bainite and martensite. Therefore, in order to perform the secondary working and the tertiary working in the cold, the hot-rolled wire is previously subjected to a softening heat treatment such as annealing and spheroidization to reduce the strength and secure the ductility. Cracking has been suppressed.

[0004]

However, the above-mentioned softening heat treatment requires a long time of 10 to 20 hours, and from the viewpoint of improving productivity and energy saving, a hot work having excellent cold workability even as rolled. There has been a demand for a rolled wire and a heat-treated steel having excellent cold workability without being subjected to a long-time heat treatment. Therefore, the rolling technique and the cooling rate of the wire are adjusted by variously adjusting the rolling conditions and cooling conditions in the continuous hot rolling of the wire, and a rolling technique for obtaining a softened structure, and even a short-time heat treatment, the same cold treatment as the conventional heat-treated material. Techniques have been developed that can produce wires having processability. For example, JP
JP-A-114517, JP-B-2-6809 and JP-B-56-52991, etc., produce wires having excellent cold workability even if heat treatments such as softening annealing and spheroidization are omitted or shortened. Techniques that can be proposed.

[0005] The production method disclosed in Japanese Patent Application Laid-Open No. Sho 60-114517 discloses a method in which C: 0.1 to 0.9, S:
i: 0.05 to 0.8, Mn: 0.1 to 1.5, the balance being Fe and unavoidable impurities, or Cr: 0.1 to 1.5, Mo: 0.1 ~
0.8, Ni: 0.1-1.5, B: 0.0002-
For steel containing one or more of 0.005 and the balance consisting of Fe and unavoidable impurities,
850-Ms after partial finish rolling at 850 ° C
Cooling to the temperature range of
% Or less, and after finishing to the product size and shape, winding and passing the wound coil through a heat treatment apparatus to produce a steel wire rod capable of omitting soft annealing. However, in the hot-rolled wire manufactured by this method, although the softening heat treatment can be omitted and cold working can be performed, the amount of S is not specified, and strong cold working such as cold forging is performed. When applied, there is a problem that cracks are easily generated.

[0006] The manufacturing method disclosed in Japanese Patent Publication No. 2-6809 discloses a method in which the finish rolling is performed after the first finish rolling.
Rapidly cooled to a temperature higher than 850 ° C. and not higher than 850 ° C., and then subjected to a second finish rolling to give a strain of 20 to 80%,
Cool at a cooling rate of 0.15 to 10 ° C / sec until 50
Rapid cooling of 0 ° C or less at a cooling rate of 10 ° C / sec or more to obtain a structure in which fine pearlite, bainite or martensite is mixed with finely dispersed proeutectoid ferrite, thereby shortening the subsequent spheroidizing treatment time Is what you do.
However, the wire manufactured by this method has a hard phase of bainite and martensite structure, so that it is difficult to perform cold working as it is rolled, and when cold working is performed, the workability is low. There was a problem of lowering. Furthermore, when the wire obtained by the above-mentioned manufacturing method is subjected to a heat treatment, depending on the composition of the target steel, the heat treatment time can be reduced, but the cold workability cannot be ensured in some cases.

In Japanese Patent Publication No. 56-52991, C: 0.28 to 0.33, Si: less than 0.10.
Mn: 0.6 to 0.9, S: less than 0.015%, Cr:
0.5-0.7, SolAl: 0.04-0.09,
B: 0.0005 to less than 0.003, with the balance being F
e and unavoidable impurities. Annealing before cold forging is not required. Even with oil quenching, the center hardness at the position where only the shaft diameter enters from the end of the bolt shaft is HRC ≧ 47, and the tensile strength after tempering ≧ 106 kg / mm ² , yield strength ≧ 92 kg / mm ² , elongation ≧ 12
%, Drawing ≧ 35%, and hardness HRC ≧ 32, a low alloy steel for high tension bolts having excellent cold forgeability are disclosed. However, steel with defined chemical components as described above can be cold-worked without performing heat treatment before cold forging, but depending on the rolling conditions, the bainite and martensite structures, which are hard phases, may be formed. Cracks may occur when subjected to strong cold working typified by actual bolt forming, mixed in the rolled material structure, and if heat treatment before cold forging is omitted, excellent workability may not be secured. there were.

The present inventor has proposed two typical cases of cold working of a hot-rolled wire, specifically (1) cold working without heat treatment of a hot-rolled wire, 2) Paying attention to the case where the hot-rolled wire is subjected to at least one heat treatment during the cold working step, each of which is necessary to ensure excellent cold workability at the time of final cold working. The chemical composition and properties of the hot rolled wire were studied.

The present invention has been made in view of such circumstances, and specifies the chemical composition and material hardness of a hot-rolled wire that is subjected to cold working without performing heat treatment, and reduces circumferential distortion of the material. By making the circumferential strain smaller than the upper limit of the circumferential strain calculated from the crack generation limit formula, a hot-rolled wire rod that does not crack even when subjected to strong cold working and has excellent cold workability is provided. The purpose is to: In addition, the present invention defines the chemical component and the material hardness, and makes the circumferential strain of the material smaller than the upper limit of the circumferential strain obtained from the crack generation limit formula, so that at least one time during the cold working process. It is an object of the present invention to provide a heat-treated steel material of a hot-rolled wire having excellent cold workability, in which no crack is generated when cold work is finally performed even when the heat treatment is performed.

[0010]

Means for Solving the Problems The hot-rolled wire of the first invention is a hot-rolled wire which is subjected to cold working without heat treatment, and has a C: 0.1 to 0.5 in mass%. , Si:
0.1-0.5, Mn: 0.2-2.0, S: 0.00
1 to 0.015, Al: 0.01 to 0.10, N: 0
0.03, P: 0.05 or less, and Cr:
0.1 to 2.0, Mo: 0.1 to 1.0, Ni: 0.1
~ 1.5, B: contains at least one of 0.001 to 0.005, the balance consists of Fe and inevitable impurities, and before cold working, Vickers hardness Hv is 160 or more,
220 or less, and at the time of cold working, εθ <−0.004259Hv + 0.677057 × (−εz) ^0.296+
It satisfies 0.724. Here, εθ: circumferential strain, εz: axial strain, and the strain is positive in the tensile direction and negative in the compression direction.

Here, the hardness is a total of 9 at a central portion in the cross section of the material, 4 portions of the R / 2 portion at 90 ° pitch, and 4 portions of 1 mm inside from the material surface at 90 ° pitch. The average value obtained by measuring the Vickers hardness of a portion is referred to. The distortion means a true distortion, and is a value calculated by the finite element method. The chemical composition of the hot-rolled wire is defined as above, the chemical composition and the rolling conditions are adjusted so that the Vickers hardness falls within the above range, and the circumferential strain of the material is determined in the circumferential direction obtained from the crack generation limit equation. It has been confirmed by experiments that when the strain is smaller than the upper limit value, cracks do not occur even in strong cold working such as cold forging bolt forming, and the steel has excellent cold workability. It has been confirmed that, among the above-mentioned conditions, in the chemical components, particularly, the content of S greatly affects the cold workability.

The heat-treated steel material of the hot-rolled wire according to the second invention comprises:
In a heat-treated steel material of a hot-rolled wire that is subjected to at least one heat treatment during the cold working step, C:
0.1-0.5, Si: 0.1-0.5, Mn: 0.2
２．０2.0, S: 0.001 to 0.015, Al: 0.0
1 to 0.10, N: 0 to 0.03, P: 0.05 or less, Cr: 0.1 to 2.0, Mo: 0.1
-1.0, Ni: 0.1-1.5, B: 0.001-
0.005, the balance being F
e and inevitable impurities, and has a Vickers hardness Hv of 140 or more and 170 or less after final heat treatment, and satisfies εθ <−0.0049Hv−0.34εz + 1.2 during cold working. Here, εθ: circumferential strain, εz: axial strain, and the strain is positive in the tensile direction and negative in the compression direction.

The chemical composition of the hot-rolled wire rod to be subjected to at least one heat treatment during the cold working step is defined as described above, and the Vickers hardness after the final heat treatment is adjusted by adjusting the chemical composition and the heat treatment conditions. Is within the above range, and by setting the circumferential strain of the material to be smaller than the upper limit of the circumferential strain obtained from the crack generation limit formula, even in the final cold working, no crack is generated, and excellent. It has been confirmed by experiments that it has a cold workability.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. The chemical composition of the base steel was specified as follows. In addition,% of component content means mass%. (1) C C is an element having high hardenability and has an effect of increasing the product strength, but if it is less than 0.1%, the strength of the final product is insufficient, and if it exceeds 0.5%. , The hardenability is too high, the material hardness after rolling or heat treatment increases,
As the machinability of the final product deteriorates and the toughness value also decreases,
The content was defined as 0.1 to 0.5%.

(2) Si Si is used for stabilizing the deoxidation of steel. However, 0.
If it is less than 1%, a sufficient effect cannot be obtained. When the content exceeds 0.5%, since it is a ferrite phase strengthening element, toughness is reduced, and it is also a ferrite phase stabilizing element that raises the A3 transformation point, so that ferrite decarburization is promoted in the hot rolling process. There are cases. Therefore, the content is 0.1-0.5%
Stipulated.

(3) Mn Mn is an element effective for improving hardenability and strength of a final product. However, if it is less than 0.2%, a sufficient effect cannot be obtained, and if it exceeds 2.0%, the internal hardness increases,
The ductility and toughness decrease, and the cold workability deteriorates. Therefore, the content was specified to be 0.2 to 2.0%.

(4) S S is present as MnS in steel, improves machinability, is also effective as a ferrite precipitation nucleus, and can reduce the material hardness after rolling. However, the effect is poor at less than 0.001%, and 0.015%
If the ratio exceeds, the effect is saturated and the cold workability and the toughness of the product are significantly reduced. Therefore, the content is defined as 0.001 to 0.015%.

(5) Al Al is a deoxidizing agent and combines with N in steel to form a nitride. The formed nitride is effective in refining austenite crystal grains during hot rolling, but when the content is less than 0.01%, the effect is poor, and when it exceeds 0.1%, the effect is saturated. , Lowering the toughness value. Therefore, the content is defined as 0.01 to 0.1%.

(6) NN combines with Al in steel to form a nitride. The formed nitride is effective in refining austenite crystal grains during hot rolling, but if it exceeds 0.03%, the effect is saturated and the toughness value is reduced. 0.
03%.

(7) PP segregates at the grain boundaries to significantly lower the cold workability and lower the delayed fracture resistance at low temperatures. Therefore, the upper limit of the P content is set to 0.05%. On the other hand, the extreme reduction of the P content increases the cost of steelmaking and increases the cost of the raw steel, so the lower limit of the P content is preferably set to about 0.005%.

It contains at least one of Cr, Mo, Ni and B in addition to the above elements. These elements improve hardenability and improve final product strength. However,
When a large amount of these elements is added, a hard structure (bainite, martensite) is generated as it is rolled, and the addition cost is increased. When the content of Cr, Mo and Ni is less than 0.1% and the content of B is less than 0.001%, the effect of improving the strength is poor.
When Mo exceeds 1.0%, Ni exceeds 1.5%, and B exceeds 0.005%, the hardenability becomes too high, a hard structure is generated as it is rolled, and the addition cost increases.
0.1-2.0%, Mo: 0.1-1.0%, Ni:
0.1 to 1.5%, B: 0.001 to 0.005%.

Next, the limitation of the properties of the hot-rolled wire and its heat-treated steel will be described. First, when cold-working a hot-rolled wire without performing heat treatment, the Vickers hardness Hv of the hot-rolled wire before cold working is required to ensure excellent cold workability at the time of final cold working. When it is lower than 160,
Although the cold workability is good, it is necessary to reduce the addition amount of elements that improve hardenability such as C, Mn, Cr, Ni, and Mo in order to achieve Hv 160 or less as rolled. Cold-worked products (eg cold forged bolts)
Mechanical properties (hardening hardness, tensile strength during tempering, etc.) cannot be satisfied. In addition, it is necessary to extremely slow down the cooling rate in the cooling stage of the hot rolling process. For example, large-scale heat treatment facilities and other auxiliary heating facilities will be installed in the rolling line, resulting in a significant increase in facility costs. Leads to.

Further, Hv 220 or more can be realized by adjusting the chemical composition of the steel and the rolling conditions. In this case, bainite and martensite structures, which are hard phases, are formed in the wire obtained by hot rolling. Although it is present, the cold workability is extremely reduced, and it can be applied to light cold work in some cases. However, in strong working such as ordinary cold forging bolt forming, cracking occurs, and molding becomes impossible. Therefore, the Vickers hardness Hv is set to 160 or more and 220 or less.

Furthermore, in order to evaluate the cold workability, the critical crack initiation behavior was examined by a compression test using various test pieces. Εθ <−0.004259Hv + 0.677057 × (−εz) ^0.296+
It has been found that when 0.724 is satisfied (provided that εθ: circumferential strain and εz: axial strain), generation of cracks can be suppressed and excellent cold workability can be ensured.

When the hot-rolled wire is subjected to at least one heat treatment during the cold working step, the hardness of the material after the final heat treatment decreases the cold workability during the final cold working. Affect. Vickers hardness Hv after final heat treatment
Is smaller than 140, the cold workability is good,
Heat treatment needs to be performed for a long time of 15 hours or more, and Hv is set to 140 or more from the viewpoint of improvement in productivity and energy saving. When Hv exceeds 170, the heat treatment can be performed in a short time, and a certain degree of cold workability can be secured, but the cold workability is lower than that of a material that has been subjected to a normal long-time heat treatment. descend. Therefore, the material hardness is specified as Vickers hardness of Hv140 or more and 170 or less.

Further, in order to evaluate the cold workability, the behavior of critical crack initiation was examined by a compression test using various test pieces. As a result, the hot-rolled wire satisfies the chemical composition and material hardness described above, When εθ <−0.0049Hv−0.34εz + 1.2 (where εθ: circumferential strain, εz: axial strain), the generation of cracks can be suppressed, and excellent cold workability can be achieved. I found that I could secure it.

[0027]

The present invention will be described below in more detail with reference to examples. Example 1 As a test material, a material equivalent to SCM435 having the chemical components shown in Table 1 below was produced into a 1-ton billet by continuous casting and melting. Hot rolling was performed on this billet to obtain a wire having a diameter of 14 mm. The material hardness was adjusted by adjusting the cooling rate to 0.1 to 5 ° C./sec using a heat retaining cover at the cooling stage during hot rolling.

[0028]

[Table 1]

FIGS. 1, 2 and 3 are perspective views showing test pieces for a cold compression test. The obtained wire is formed into a test piece 1 having the shape shown in FIGS. 1, 2 and 3 by machining, and the cold workability is evaluated by a cold compression test, that is, the critical crack generation behavior is investigated. The Vickers hardness of the material used in the cold compression test was measured. Here, in the cold workability evaluation, the test piece 1 is cold-compressed at various compression ratios, and a compression ratio at which cracking is 50% or more (the compression ratio at this time is referred to as a critical compression ratio) is obtained. The axial strain εz and the circumferential strain εθ at the crack occurrence site at that time were calculated by numerical analysis (finite element method). Also,
Substituting Vickers hardness Hv and the axial strain εz to the next crack occurrence limit formula (1), the upper limit of εθ = -0.004259Hv + 0.677057 × (-εz ) 0.296 +0.724 ... (1) circumferential strain Ipushironshita I asked. The results are shown in Table 2 below.

[0030]

[Table 2]

From Table 2, it can be seen that the chemical composition and Vickers hardness satisfy the conditions of the present invention. For the steel materials of Nos. 1 to 4, the actual circumferential strain ε obtained by the cold compression test was
The limit value of θ coincides with the upper limit value of the circumferential strain εθ obtained by the crack generation limit formula (1). If the circumferential strain εθ is smaller than this value, 50% cracking may not occur. I understand. On the other hand, the chemical components satisfy the conditions of the present invention, but the Vickers hardness is out of the range of the present invention. The steel materials of Comparative Examples 5 to 8 and the Vickers hardness fall within the range of the present invention, but the S content of the chemical components is out of the conditions of the present invention. In the steel materials of Comparative Examples 9 to 12, the limit value of the actual circumferential strain εθ obtained by the cold compression test was smaller than the upper limit value of the circumferential strain εθ obtained by the crack generation limit expression (1). It can be seen that 50% cracking occurs at a value smaller than the upper limit. Therefore, it is understood that the steel material of the present invention has an improved limit value of the circumferential strain εθ.

[Example 2] With respect to the test materials shown in Table 1,
After hot rolling, perform cold drawing and spheroidizing heat treatment,
In the same manner as in Example 1, a test piece 1 having the shape shown in FIGS. 1, 2 and 3 was produced by machining, and the cold workability was evaluated by a cold compression test, that is, the critical crack generation behavior was investigated. At the same time, the Vickers hardness of the material used in the cold compression test was measured. The adjustment of the material hardness after the spheroidizing heat treatment was performed by changing the spheroidizing heat treatment conditions. Here, in the cold workability evaluation, the test piece 1 is cold-compressed at various compression ratios, and a compression ratio at which cracking is 50% or more (the compression ratio at this time is referred to as a critical compression ratio) is obtained. The axial strain εz and the circumferential strain εθ at the crack occurrence site at that time were calculated by numerical analysis. The Vickers hardness Hv and the axial strain εz were substituted into the following crack generation limit equation (2), and εθ = −0.0049 Hv−0.34εz + 1.2 (2) The upper limit of the circumferential strain εθ was determined. The results are shown in Table 3 below.

[0033]

[Table 3]

From Table 3, it can be seen that the chemical composition and Vickers hardness satisfy the conditions of the present invention. In steel materials 1 to 8, the actual circumferential strain ε obtained by the cold compression test was
The limit value of θ coincides with the upper limit of the circumferential strain εθ obtained by the crack initiation limit equation (2). If the circumferential strain εθ is smaller than this value, 50% cracking may not occur. I understand. On the other hand, the chemical components satisfy the conditions of the present invention, but the Vickers hardness is out of the range of the present invention. The steel materials of Comparative Examples 9 to 12 and the Vickers hardness are within the range of the present invention, but the S content of the chemical components is out of the conditions of the present invention. In the steel materials of Comparative Examples 13 to 16, the limit value of the actual circumferential strain εθ obtained by the cold compression test was smaller than the upper limit value of the circumferential strain εθ obtained by the crack initiation limit formula (2). It can be seen that 50% cracking occurs at a value smaller than the value. Therefore, it is understood that the steel material of the present invention has an improved limit value of the circumferential strain εθ.

As described above, in the hot-rolled wire of the present invention which is not subjected to the heat treatment, the chemical components are defined as described above, and the chemical components and rolling conditions are adjusted so that the Vickers hardness falls within the above range. Since the circumferential strain of the material is smaller than the upper limit of the circumferential strain obtained from the crack generation limit equation (1), no cracks are generated even in strong cold working such as cold forging bolt forming. Cold workability.

Further, for the heat-treated steel material of the hot-rolled wire to be subjected to at least one heat treatment during the cold working step, the chemical composition is specified as described above, and the chemical composition and the heat treatment conditions are adjusted to obtain the final product. Vickers hardness after heat treatment is set to fall within the above range, and the circumferential strain of the material is made smaller than the upper limit of the circumferential strain obtained from the crack occurrence limit equation, so that the heat treatment time can be reduced. In the final cold working, no cracks are generated, and excellent cold workability is obtained.

[0037]

As described above in detail, in the case of the first invention, the chemical composition of the hot-rolled wire to be subjected to cold working without heat treatment is defined, and the chemical composition and the rolling conditions are adjusted. The Vickers hardness before cold working is kept within a predetermined range, and the circumferential strain of the material is made smaller than the upper limit of the circumferential strain obtained from the crack generation limit equation. Cracking does not occur even during cold working,
Has excellent cold workability.

In the case of the second invention, the chemical composition of the hot-rolled wire to be subjected to at least one heat treatment during the cold working step is defined, and the chemical composition and the heat treatment conditions are adjusted so that the final heat treatment is performed. The Vickers hardness of the material falls within a predetermined range, and the circumferential strain of the material is made smaller than the upper limit of the circumferential strain obtained from the crack occurrence limit equation,
The heat treatment time can be shortened, and cracks do not occur in the final cold working, and excellent cold workability is obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a shape of a test piece used in a cold compression test of a hot-rolled wire according to the present invention.

FIG. 2 is a perspective view showing a shape of a test piece used for a cold compression test of the hot-rolled wire according to the present invention.

FIG. 3 is a perspective view showing a shape of a test piece used for a cold compression test of the hot-rolled wire according to the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takashi Kubogi 4-5-33 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Koichi Kuroda 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture 5-33 No. Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] 1. A hot-rolled wire rod that is subjected to cold working without heat treatment, wherein, by mass%, C: 0.1 to 0.5, Si: 0.1 to 0.
5, Mn: 0.2 to 2.0, S: 0.001 to 0.01
5, Al: 0.01 to 0.10, N: 0 to 0.03,
P: 0.05 or less, and Cr: 0.1 to
2.0, Mo: 0.1-1.0, Ni: 0.1-1.
5, B: at least one of 0.001 to 0.005
And the balance consists of Fe and unavoidable impurities. Before cold working, the Vickers hardness Hv is 160 or more, 22
0 or less, and at the time of cold working, εθ <−0.004259Hv + 0.677057 × (−εz) ^0.296+
A hot-rolled wire characterized by satisfying 0.724. Where ε
θ: circumferential strain, εz: axial strain, and the strain is positive in the tensile direction and negative in the compressive direction. 2. A heat-treated steel material of a hot-rolled wire which is subjected to at least one heat treatment during a cold working step, wherein, by mass%, C: 0.1 to 0.5, Si: 0.1 to 0.
5, Mn: 0.2 to 2.0, S: 0.001 to 0.01
5, Al: 0.01 to 0.10, N: 0 to 0.03,
P: 0.05 or less, and Cr: 0.1 to
2.0, Mo: 0.1-1.0, Ni: 0.1-1.
5, B: at least one of 0.001 to 0.005
And the balance consists of Fe and unavoidable impurities. After the final heat treatment, the Vickers hardness Hv is 140 or more and 170 or less, and satisfies εθ <-0.0049Hv-0.34εz + 1.2 during cold working. A heat-treated steel product of a hot-rolled wire rod, characterized in that: Here, εθ: circumferential strain, εz: axial strain,
The strain is positive in the tensile direction and negative in the compression direction.