JP2002194495A - Hot rolled steel wire rod for cold heating and method for producing cold heading article using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel wire rod for cold heading which exhibits excellent deformability even without performing softening annealing such as spheroidizing annealing in forming of a bolt, a nut or the other parts by cold heading, and a method for producing a cold headed article using the steel wire rod. SOLUTION: The hot rolled steel wire rod having excellent deformability in cold heading is consisting of steel having a C content of 0.25 to 0.5% (by mass% in the case of a chemical component, and so forth), and has tensile strength of <=650 MPa. The wire rod has a metallic structure of a dual-phase structure of ferrite + pearlite, the ferrite fraction in the dual-phase structure is >=55%, and the pearlite grain size is <=30 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel wire having excellent cold-forging properties and a method for producing a cold-formed product using the steel wire. When forming a part, a hot-rolled steel wire for cold heading that exhibits excellent deformability without softening annealing such as spheroidizing annealing, and a method of manufacturing a cold headed product using the steel wire It is about.

[0002]

2. Description of the Related Art As a method of manufacturing bolts, nuts and other cold headed products, carbon steel and various alloy steels are formed into a linear shape by hot working, and this is subjected to pickling and mechanical descaling. Thereafter, generally, a method of performing spheroidizing annealing or the like for improving workability, performing wire drawing, and then forging into a predetermined shape is generally adopted. Here, the spheroidizing annealing performed before the forging process is an important process for enhancing cold forging, but the spheroidizing annealing requires a large amount of time and heat energy, and the surface oxidized in the annealing process. Pickling treatment for cleaning is also indispensable,
Significant cost increases are inevitable.

[0003] In order to avoid such a problem of cost increase, depending on the shape and structure of parts, the hot-rolled steel wire after pickling is subjected to skin pass processing without spheroidizing annealing to improve the lubricity of the surface and then to forging. Some processing methods have been tried. However, if the softening heat treatment such as spheroidizing annealing is omitted, hot-rolled steel wire rods often have excessively high tensile strength or an inappropriate metallographic structure, often causing cracking during heading. In particular, in a component having a flange portion such as a bolt, it is a serious problem that a work crack occurs in the flange portion.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has as its object to heat-rolled steel wires and to perform softening heat treatment such as spheroidizing annealing. In addition to providing a hot-rolled steel wire that has excellent deformability at the time of cold heading and can perform cold heading smoothly without causing work cracks, etc., and a cold headed product using the steel wire To provide a manufacturing method.

[0005]

The hot-rolled steel wire rod for cold heading according to the present invention, which can solve the above-mentioned problems, comprises:
Content is 0.25 to 0.5% (in the case of a chemical component, mass%
A hot-rolled steel wire having a tensile strength of 650 MPa or less, wherein the metal structure is a ferrite + pearlite two-phase structure, and the ferrite fraction in the two-phase structure is It is a hot-rolled steel wire rod for cold heading which has a metal structure requirement of 55% or more and an average particle size of pearlite of 30 μm or less and which has excellent deformability during cold heading.

[0006] The method of manufacturing a cold-formed product according to the present invention means that a hot-rolled steel wire material satisfying the above requirements is used, and wire drawing is performed at a surface reduction rate of 20% or less without heat treatment of the steel wire material. Next, it has a gist in cold forging.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Under the above-mentioned circumstances, the present inventors have avoided the cost increase associated with softening heat annealing such as spheroidizing annealing performed before cold heading, and performed these heat treatments. In order to develop a hot-rolled steel wire that is stable and exhibits excellent cold-forging properties, we have been conducting intensive research.

[0008] In order to enhance the cold-forging property of a hot-rolled steel wire, it is desirable to lower the tensile strength of the raw material in order to reduce the deformation resistance during cold-forging and increase the deformability. Therefore, the present inventors first conducted research to clarify the strength level necessary for performing cold heading smoothly,
It was confirmed that it was necessary to suppress the tensile strength as a hot-rolled steel wire rod to 650 MPa or less. However, simply keeping the tensile strength at this level does not guarantee stable cold workability.However, as described above, it is necessary to surely suppress cracks that tend to occur when manufacturing flanged forged products etc. The metal structure is a ferrite + pearlite two-phase structure, and the ferrite fraction occupying 55% or more, more preferably 65% or more in the two-phase structure is ensured. It has been confirmed that controlling the particle size to 30 μm or less, more preferably 20 μm or less, is extremely important for increasing the crack generation limit during forging.

By the way, when the hot-rolled steel wire has a tensile strength exceeding 650 MPa, the strength is too high and the deformability at the time of cold heading is reduced, so that the phenomenon of cracking at a flange processed portion or the like cannot be avoided. . When applied to a higher-level forging process, the tensile strength is desirably reduced to about 620 MPa or less. In the case where the strength of the cold forged product is insufficient, it is possible to compensate for the insufficient strength by quenching after cold forging.

In the present invention, the metal structure is defined as a two-phase structure of ferrite + pearlite because other metal structures such as bainite and martensite and a structure of three or more phases containing them are intended by the present invention. This is because a sufficient level of deformability cannot be secured, and the occurrence of cracks during cold heading cannot be reliably suppressed.

In the present invention, the ferrite fraction in the two-phase structure is less than 55%, or the average particle size of pearlite is 3%.
Even when the thickness exceeds 0 μm, cracks during cold heading cannot be reliably eliminated. The reason is that sufficient deformability cannot be ensured for cold heading. To more reliably prevent cracking during cold heading, the ferrite fraction should be 65% or more,
More preferably, the average particle diameter of pearlite is 70% or more.
It is desirable that the thickness be suppressed to not more than 10 μm, more preferably not more than 10 μm.

As described above, the present invention has the greatest features in that the tensile strength and the metal structure as a hot-rolled steel wire are specified, and the premise is that the C content in steel is 0.1%.
It is essential to be in the range of 25 to 0.50%. The reason for this is that if the C content is less than 0.25%, the ferrite phase increases and cracks easily occur, while if it exceeds 0.50%, the tensile strength becomes too high. From such a viewpoint, a more preferable C content is 0.25 to 0.50%.
Range.

Incidentally, as described above, the tensile strength of the hot-rolled steel wire rod is 650 MPa or less, and the metal structure is ferrite +
A method for obtaining a hot-rolled steel wire satisfying the requirements of having a pearlite two-phase structure, a ferrite fraction in the structure of 55% or more, and an average particle diameter of pearlite surrounded by ferrite of 30 μm or less is not particularly limited. As preferred hot rolling conditions, the final finish rolling temperature during hot rolling is 700 to 900 ° C,
More preferably, it is in the range of 700 to 850 ° C, and further, the winding temperature is 700 to 850 ° C, more preferably 700 to 850 ° C.
It is desirable to be in the range of 00 ° C. That is, under these conditions, the austenite crystal grain size before the start of cooling after hot rolling is refined, the final metal structure is refined to minimize the average grain size of pearlite, and the supercooled structure during subsequent cooling is reduced. This is an effective condition for suppressing the generation of the ferrite, making the metal structure a ferrite + pearlite two-phase structure, and increasing the ferrite fraction. And in the cooling process,
The average cooling rate from the cooling start temperature to 500 ° C is 2.0
C./sec or less, more preferably 1.5.degree. C./sec or less, and even more preferably 0.7.degree. C./sec or less, promotes ferrite transformation while more reliably preventing the formation of a supercooled structure. desirable.

Although the type and content of elements other than C in the steel constituting the hot-rolled steel wire according to the present invention are not particularly limited, the above effects of the present invention are more effectively exhibited by Si
Content of 0.50% or less, Mn content of 2.0% or less,
r: 1.5% or less and / or Mo: 0.3% or less, and further, Al: 0.05% or less and / or B: 0.
005% or less, and the balance is substantially Fe, which may contain unavoidable impurities. The reason why the preferable content of the above elements is determined is as follows.

Si: 0.50% or less Si effectively acts as a deacidifying element, but if it is too much, it deteriorates cold forging, so that it is reduced to 0.50% or less, more preferably 0.40% or less. Should be suppressed.

Mn: 2.0% or less Mn works effectively to improve the hardenability, but if it is too much, it causes deterioration of cold forging, so that Mn is 2.0% or less.
It is desirable to keep it below. The more preferable content of Mn is 0.3% or more, 1.5% or less, and more preferably 1.2% or less.

Cr: 1.5% or less, and / or Mo:
0.3% Cr and Mo effectively act to improve hardenability,
If the Cr content exceeds 1.5% and the Mo content exceeds 0.3%, the cold forging property is remarkably deteriorated. Therefore, each of the contents of Cr and Mo has the above value as a preferable upper limit. These Cr,
A more preferable Cr content for suppressing Mo damage is 1.2.
% Or less, and the Mo content is 0.25% or less.

Al: 0.050% or less Al is usually mixed as a deacidifying element. However, since it becomes a source of nonmetallic inclusions and deteriorates cold forging properties and toughness of forged products, 0 It is desirable to keep the content at 0.050% or less, more preferably 0.045% or less, and even more preferably 0.040% or less.

B: 0.005% or less B is a trace element that is effective in improving the hardenability of the hot-rolled steel wire for cold heading, but its effect is saturated at about 0.005%. The above addition is meaningless. The more preferable amount of B is 0.0045% or less, further preferably 0.0040% or less.

Typical steel materials satisfying the above preferable contents of elements include SCR430, SC specified in JIS.
M435, SWRCH40K and the like, and their representative chemical compositions are as follows. [SCR430] C: 0.28-0.33%, Si:
0.15 to 0.35%, Mn: 0.60 to 0.85%,
Cr: 0.90 to 1.20% [SCM435] C: 0.33 to 0.38%, Si:
0.15 to 0.35%, Mn: 0.60 to 0.85%,
Cr: 0.90 to 1.20%, Mo: 0.15 to 0.3
0% [SWRCH40K] C: 0.37 to 0.43%, S
i: 0.10 to 0.35%, Mn: 0.60 to 0.90
%

However, the steel used in the present invention may contain an element other than the above, for example, Ti, Nb, Cu, Ni or the like in an appropriate amount depending on the required characteristics of the target forged product. It is possible, that is, all steel materials are applicable as long as they satisfy the above-mentioned C amount and satisfy the requirements of the tensile strength and the metal structure as a hot-rolled steel material for cold heading.

In forming a forged product using the hot-rolled steel wire for cold heading of the present invention, a softening heat treatment such as spheroidizing annealing is performed by utilizing the excellent deformability of the hot-rolled wire. In both cases, a forged product free from cracks and other defects can be obtained under normal forging conditions. However, in order to perform cold forging more smoothly, the hot-rolled steel wire is drawn with a surface reduction rate of 20% or less. It is desirable to cold forge after processing.

However, when wire drawing exceeding 20% is performed in wire drawing performed before cold heading, the deformability due to work hardening is remarkably reduced, and processing such as a flange portion during cold heading is performed. It may cause cracks in high parts,
This is because the object of the present invention cannot be achieved. Therefore,
When the hot-rolled steel wire of the present invention is used as a material for cold heading, the hot-rolled steel or the subsequent drawing is performed at a reduction of 2%.
It is desirable to keep it at 0% or less, more preferably at 10% or less.

Among the forged products using the hot-rolled steel wire for cold forging according to the present invention, typical are the bolts and nuts as described above, and their shapes and sizes are not limited at all. In addition to nuts, it can be widely and effectively used as a material for forging various machine parts.

[0025]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to experimental examples. However, the present invention is not limited to the following examples, and may be appropriately changed within a range that can conform to the purpose of the preceding and the following. And these are all included in the technical scope of the present invention.

Experimental Example 1 JIS standard steel types "SCR430" and "SCM43"
5 "and" SWRCH40K ", steel types having the chemical components shown in Table 1 were selected, and each steel material was 12 m in diameter by hot rolling.
The tensile strength and the metal structure of the hot-rolled steel wire were changed by variously changing the hot rolling conditions at that time as shown in Table 2. For each of the obtained hot-rolled steel wires, a tensile test was conducted, and a metal structure of a cross section was examined by microscopic observation. Further, a cold heading test was conducted to check for cracks. The results shown in Table 2 were obtained. .

The metal structure of the hot-rolled steel wire is as follows.
Observe the section of 1 / 8D to 1 / 4D in cross section of each steel wire rod at 400 times with an optical microscope, and obtain an area ratio of ferrite by image analysis of a micrograph. Was determined.
In the cold heading test, the hot-rolled steel wire was machined into a rod shape having a diameter of 10 mm and a length of 30 mm without heat treatment. This was performed and the presence or absence of processing cracks was visually evaluated.

[0028]

[Table 1]

[0029]

[Table 2]

From Table 2, the following can be considered.

No. Examples 1, 2, 3, 6, 9, 10, and 11 satisfy all of the requirements of the present invention, each having a tensile strength of 650 MPa or less, a metal structure of ferrite + pearlite two-phase structure, and a ferrite fraction of The average particle size of pearlite surrounded by ferrite is 60% or more, and the average particle size is 30 μm or less.

On the other hand, no. In Nos. 4, 5, and 11, the tensile strength was too high due to the low ferrite fraction probably because the cooling rate during hot rolling was too high. In No. 8, the finish rolling temperature was too high, the metal structure became a supercooled structure, and the ferrite fraction was remarkably reduced. No pearlite structure itself was observed, the strength was too high, and the performance intended by the present invention was too high. Has not been secured at all. No. No. 7 shows that the individual conditions of the finish rolling temperature, winding temperature and cooling rate satisfy the suitable requirements for the time being, but the tensile strength of the hot rolled steel wire obtained by combining them is too high and the ferrite fraction is insufficient. Therefore, cracks are generated during cold heading.

FIG. 1 shows No. 1 in the above experimental example. 9
FIG. 2 is a cross-sectional micrograph showing the metallographic structure of the hot-rolled steel wire of the present invention (material of the present invention). 12 is a cross-sectional micrograph of Comparative Example No. 12 (comparative material). As is apparent from comparison of these figures, the hot-rolled steel wire of the present invention has a ferrite + pearlite 2 in which the ferrite structure is uniformly distributed at a high area ratio. While the comparative material has a phase structure, the comparative material has a low area ratio of the ferrite structure, is sparsely distributed throughout the cross-sectional structure, and has a high area ratio of the pearlite structure.

EXPERIMENTAL EXAMPLE 2 The hot-rolled steel wire No. 9 obtained in Experimental Example 1 was subjected to wire drawing at a reduction rate of 10% or 25% without heat treatment. In the same manner, a cold heading test was performed to check for the occurrence of cracks.

The results are as shown in Table 3. The hot-rolled steel wire of the present invention cracks during cold forging when subjected to wire drawing with a reduction in area of more than 20%. If the rate is low, cold heading can be performed without cracking without heat treatment.

[0036]

[Table 3]

[0037]

The present invention is configured as described above, and specifies the C content of steel and specifies its metal structure, particularly the ferrite fraction and the average grain size of pearlite surrounded by ferrite. By providing a hot-rolled wire having excellent deformability in the cold state without performing a softening heat treatment such as spheroidizing annealing as it is as a hot-rolled wire material, and providing an inexpensive hot-rolled wire having excellent cold forging properties, As a result, the cost of cold forging products can be reduced.

[Brief description of the drawings]

FIG. 1 is a drawing-substituting micrograph showing a cross-sectional metal structure of a hot-rolled steel wire rod according to the present invention.

FIG. 2 is a micrograph instead of a drawing showing a cross-sectional metal structure of a comparative material.

Claims (2)

[Claims] 1. A hot-rolled steel wire made of steel having a C content of 0.25 to 0.5% (mean% in the case of a chemical component, the same applies hereinafter) and having a tensile strength of 650 MPa or less. Wherein the metal structure is a ferrite + pearlite two-phase structure, and the ferrite fraction in the two-phase structure is 55% or more;
A hot-rolled steel wire rod for cold heading, wherein the average particle size of pearlite is 30 μm or less. 2. The hot-rolled steel wire rod for cold heading according to claim 1, which is subjected to wire drawing at a reduction ratio of 20% or less without heat treatment, and then cold heading. Manufacturing method of cold forged products.