JP4116767B2 - Hot-rolled steel wire for cold heading and method for manufacturing heading products using the same - Google Patents

Description

【００２９】
【表２】

【００３０】
表２より次の様に考えることができる。
【００３１】
Ｎｏ．１，２，３，６，９，１０，１１は本発明の規定要件を全て満たす実施例で、引張強さはいずれも６５０ＭＰａ以下で、金属組織はフェライト＋パーライト２相組織でフェライト分率は６０％以上、フェライトに囲まれたパーライトの平均粒径は３０μｍ以下であり、冷間圧造時の変形能が高くて割れが全く見られない。
【００３２】
これらに対し、Ｎｏ．４，５，１１は、熱延時の冷却速度が速すぎたためかフェライト分率が低くて引張強さも過大であり、またＮｏ．８は仕上げ圧延温度が高すぎたためか、金属組織が過冷却組織となってフェライト分率が著しく低下しており、パーライト組織自体の生成が見られず、強度も過大で本発明で意図する性能を全く確保できていない。またＮｏ．７は、仕上げ圧延温度、巻取り温度、冷却速度の個々の条件は一応好適要件を満たしているが、それらの総合により得られる熱延鋼線材の引張強さが高すぎると共にフェライト分率も不足するため、冷間圧造時に割れを生じている。
【００３３】
なお図１は、上記実験例におけるＮｏ．９（本発明材）の熱延鋼線材の金属組織を示す断面顕微鏡写真、図２は、同じくＮｏ．１２（比較材）の断面顕微鏡写真であり、これらの図を対比すれば明らかである様に、本発明材の熱延鋼線材は、フェライト組織が高い面積率で均一に分布したフェライト＋パーライト２相組織を有しているのに対し、比較材はフェライト組織の面積率が低くて断面組織全体にまばらに分布しており、パーライト組織の面積率が高い。
【００３４】
実験例２
前記実験例１で得た符号９の熱延鋼線材に、熱処理を施すことなく減面率で１０％または２５％の伸線加工を施し、それぞれについて、実験例１と同様にして冷間圧造試験を行なって割れ発生の有無を調べた。
【００３５】
結果は表３に示す通りであり、本発明の熱延鋼線材でも、２０％を超える減面率の伸線加工を施すと冷間圧造時に割れを生じるが、これ以下の減面率であれば、熱処理をせずとも割れを生じることなく冷間圧造加工を行なうことができる。
【００３６】
【表３】

【００３７】
【発明の効果】
本発明は以上の様に構成されており、鋼のＣ含有量を特定すると共に、その金属組織、特にフェライト分率やフェライトに囲まれたパーライトの平均粒径を特定することにより、熱延線材のまま、即ち球状化焼鈍の如き軟化熱処理をせずとも冷間で優れた変形能を有し、安価で冷間圧造性に優れた熱延線材を提供すると共に、軟化熱処理に伴う費用を削減することにより冷間圧造製品を廉価に提供し得ることになった。
【図面の簡単な説明】
【図１】図１は、本発明にかかる熱延鋼線材の断面金属組織を示す図面代用顕微鏡写真である。
【図２】図２は、比較材の断面金属組織を示す図面代用顕微鏡写真である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel wire excellent in cold forging and a method for producing a cold forged product using the steel wire, and more specifically, when forming a bolt, a nut or other parts by cold forging. Furthermore, the present invention relates to a hot rolled steel wire for cold heading that exhibits excellent deformability without softening annealing such as spheroidizing annealing, and a method for manufacturing a cold headed product using the steel wire.
[0002]
[Prior art]
Bolts, nuts, and other cold forged products are manufactured by linearizing carbon steel or various alloy steels by hot working, and then pickling or mechanical descaling, followed by normal processing. In order to improve properties, a method is generally employed in which spheroidizing annealing or the like is performed before wire drawing, followed by forging and forming into a predetermined shape. The spheroidizing annealing performed before the heading is an important process for improving the cold heading, but the spheroidizing annealing requires a lot of time and heat energy, and the surface oxidized in the annealing process. Since pickling treatment for purifying the water becomes indispensable, a significant increase in cost is inevitable.
[0003]
Therefore, in order to avoid such a problem of cost increase, depending on the shape and structure of the part, the hot-rolled steel wire after pickling is subjected to skin pass processing without spheroidizing annealing, and the forging process is performed after improving the lubricity of the surface Some have also tried. However, if softening heat treatment such as spheroidizing annealing is omitted, the hot rolled steel wire rod is often too high in tensile strength, or the metal structure is not appropriate, which often causes work cracks during forging. In particular, in a part having a flange portion such as a bolt, causing a processing crack in the flange portion is a big problem.
[0004]
[Problems to be solved by the invention]
The present invention has been made paying attention to the circumstances as described above, and its purpose is to hot-rolled steel wire, and it is an excellent deformation during cold heading without softening heat treatment such as spheroidizing annealing. An object of the present invention is to provide a hot-rolled steel wire that has the ability to perform cold forging smoothly without causing work cracks and the like, and to provide a method for producing a cold-forged product using the steel wire.
[0005]
[Means for Solving the Problems]
The hot-rolled steel wire rod for cold heading according to the present invention that has solved the above problems has a C content of 0.25 to 0.5% (in the case of a chemical component, it means mass%, the same applies hereinafter). A hot-rolled steel wire rod having a tensile strength of 650 MPa or less, the metal structure being a ferrite + pearlite two-phase structure, the ferrite fraction in the two-phase structure being 55% or more, and a pearlite It is a hot rolled steel wire rod for cold forging that has excellent deformability during cold forging and satisfies the requirements on the metal structure such that the average particle size of the steel is 30 μm or less.
[0006]
And the manufacturing method of the cold forging product concerning this invention uses the hot-rolled steel wire which satisfy | fills the said requirements, and performs a wire-drawing process by the area reduction rate of 20% or less without heat-processing this steel wire, It has a gist where it is forged.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Under the circumstances as described above, the present inventors avoided the cost increase associated with softening thermal annealing such as spheroidizing annealing performed before cold heading, and were stable and excellent without performing these heat treatments. We have been diligently working on the development of hot-rolled steel wires that exhibit cold heading.
[0008]
In order to improve the cold forging property of hot-rolled steel wire, it is desirable to reduce the tensile strength of the material in order to reduce the deformation resistance during cold forging and increase the deformability. Therefore, the present inventors first conducted a study to clarify the strength level necessary for smoothly performing cold heading, and as a result, it was necessary to suppress the tensile strength as a hot-rolled steel wire to 650 MPa or less. It was confirmed. However, simply suppressing the tensile strength to this level does not guarantee stable cold workability, and to reliably suppress cracks that tend to occur when manufacturing a flanged forged product as described above. In addition, the metal structure is a ferrite + pearlite two-phase structure, and the ferrite fraction in the two-phase structure is 55% or more, more preferably 65% or more. Further, the average of pearlite surrounded by ferrite It was confirmed that suppressing the particle size to 30 μm or less, more preferably 20 μm or less, is extremely important in increasing the crack generation limit during forging.
[0009]
Incidentally, when the tensile strength as a hot-rolled steel wire exceeds 650 MPa, the strength is too high, so that the deformability during cold heading is reduced, and the phenomenon of cracking at the flanged portion cannot be avoided. When applied to a higher degree of forging, it is desirable to reduce the tensile strength to about 620 MPa or less. If the cold forged product has insufficient strength, it is possible to compensate for the lack of strength by quenching after cold forging.
[0010]
In addition, the metal structure is defined as a ferrite + pearlite two-phase structure in the present invention because other metal structures such as bainite and martensite and structures of three or more phases containing them have a level intended by the present invention. This is because the deformability cannot be ensured and the occurrence of cracks during cold heading cannot be reliably suppressed.
[0011]
Further, in the present invention, even when the ferrite fraction in the two-phase structure is less than 55% or the average particle size of pearlite exceeds 30 μm, cracks during cold forging cannot be reliably eliminated. The reason is that sufficient deformability for cold heading cannot be secured. In order to prevent cracking during cold forging more reliably, it is desirable to suppress the ferrite fraction to 65% or more, more preferably 70% or more, and the pearlite average particle size to 20 μm or less, more preferably 10 μm or less.
[0012]
As described above, the present invention has the greatest characteristics in specifying the tensile strength and metal structure as a hot-rolled steel wire. As a premise, the C content in the steel is 0.25 to 0. It is essential to be in the range of 50%. The reason is that if the C content is less than 0.25%, the ferrite phase increases and cracking tends to occur, and conversely if it exceeds 0.50%, the tensile strength becomes too high. From such a viewpoint, the more preferable C content is in the range of 0.25 to 0.50%.
[0013]
By the way, as described above, the tensile strength as a hot-rolled steel wire is 650 MPa or less, the metal structure is a ferrite + pearlite two-phase structure, the ferrite fraction in the structure is 55% or more, and the average of pearlite surrounded by ferrite A method for obtaining a hot-rolled steel wire satisfying the requirement of 30 μm or less in particle size is not particularly limited, but as a preferable hot-rolling condition, the final finish rolling temperature during hot-rolling is 700 to 900 ° C., more preferably 700 to 850. It is desirable that the temperature be in the range of ° C, and further that the coiling temperature be 700 to 850 ° C, more preferably 700 to 800 ° C. That is, these conditions are that the austenite crystal grain size is refined before the start of cooling after hot rolling, the final metal structure is refined to reduce the average grain size of pearlite as much as possible, and the supercooled structure during the subsequent cooling. This is an effective condition for suppressing the occurrence of the above, making the metal structure a ferrite + pearlite two-phase structure and increasing the ferrite fraction. 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 ° C./sec or less, further preferably 0.7 ° C./sec or less. It is desirable to promote the ferrite transformation while more reliably preventing the formation of a supercooled structure.
[0014]
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, but the effect of the present invention is more effectively exhibited when the Si content is 0.50. %: Mn content 2.0% or less, Cr: 1.5% or less and / or Mo: 0.3% or less, Al: 0.05% or less and / or B: 0. The reason for determining the preferable content of the above elements is as follows. It is a steel material containing 005% or less and the balance being substantially made of Fe which may contain inevitable impurities.
[0015]
Si: 0.50% or less Si acts effectively as a deoxidizing element, but if it is too much, the cold heading property deteriorates, so it should be kept to 0.50% or less, more preferably 0.40% or less. is there.
[0016]
Mn: 2.0% or less Mn works effectively to improve hardenability, but if it is too much, it will cause the cold heading to deteriorate, so it is desirable to keep it to 2.0% or less. The more preferable content rate of Mn is 0.3% or more, 1.5% or less, More preferably, it is 1.2% or less.
[0017]
Cr: 1.5% or less and / or Mo: 0.3%
Cr and Mo are effective in improving hardenability. However, if the Cr content exceeds 1.5% and the Mo content exceeds 0.3%, the cold heading property is remarkably deteriorated. Each content has the above value as a preferred upper limit. The Cr content is preferably 1.2% or less and the Mo content is 0.25% or less in order to suppress the failure of these Cr and Mo.
[0018]
Al: 0.050% or less Al is usually mixed as a deoxidizing element, but it becomes a non-metallic inclusion source and causes deterioration in cold forging and toughness of the forging product. In the following, it is more desirable to keep it to 0.045% or less, and more preferably 0.040% or less.
[0019]
B: 0.005% or less B is an element contained in a trace amount that is effective in improving the hardenability of hot-rolled steel wire for cold heading, but the effect is saturated at about 0.005%. Is meaningless. A more preferable amount of B is 0.0045% or less, and further preferably 0.0040% or less.
[0020]
Typical steel materials satisfying the above-mentioned preferable element content include SCR430, SCM435, and SWRCH40K defined by JIS, and their typical chemical compositions are as follows.
[SCR430]
C: 0.28 to 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 0.30%
[SWRCH40K]
C: 0.37 to 0.43%, Si: 0.10 to 0.35%, Mn: 0.60 to 0.90%
[0022]
In forming a forged product using the hot-rolled steel wire for cold forging according to the present invention, by utilizing the excellent deformability of the hot-rolled wire, it is usually possible to perform softening heat treatment such as spheroidizing annealing. In the forging conditions, it is possible to obtain a forged product with no defects such as cracks. However, in order to perform cold forging more smoothly, the hot-rolled steel wire is subjected to a drawing process with a reduction in area of 20% or less. It is desirable to carry out cold heading.
[0023]
Therefore, if the wire drawing performed before the cold heading is performed at a wire drawing process exceeding 20%, the deformability decreases due to work hardening, and the degree of processing such as a flange portion is high during cold heading. This is because cracks may occur in the parts, and the object of the present invention cannot be achieved. Therefore, when using the hot-rolled steel wire of the present invention as a raw material for cold heading, it is possible to keep hot rolling or the subsequent wire drawing process at a surface area reduction rate of 20% or less, more preferably 10% or less. desirable.
[0024]
In addition, typical of the forged products using the hot rolled steel wire rod for cold forging according to the present invention are the bolts and nuts as described above, and their shapes and sizes are not limited at all, and other than bolts and nuts. In addition, it can be used effectively as a material for forging various machine parts.
[0025]
【Example】
Hereinafter, the present invention will be described in more detail with reference to experimental examples.However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the purpose described above and below. Any of these may be included in the technical scope of the present invention.
[0026]
Experimental example 1
JIS standard steel types “SCR430”, “SCM435”, “SWRCH40K” are selected from the steel types of chemical composition shown in Table 1, and each steel material is hot rolled into a wire with a diameter of 12 mm. By changing variously as shown in Table 2, the tensile strength and metal structure as a hot-rolled steel wire were changed. About each obtained hot-rolled steel wire, while performing a tensile test, the metal structure of a cross-section was investigated by microscopic observation, and also the cold forging test was investigated for the presence or absence of cracks, and the results shown in Table 2 were obtained. .
[0027]
In addition, about the metal structure of a hot-rolled steel wire, the area of 1 / 8D-1 / 4D of each steel wire is observed 400 times with an optical microscope, and the ferrite area ratio is obtained by image analysis of the micrograph. As for the pearlite particle size, the average particle size of pearlite was determined from the structure photograph. In the cold heading test, the hot-rolled steel wire is processed into a rod shape with a diameter of 10 mm and a length of 30 mm by machining without heat treatment, and each test piece is subjected to 10 presses at a compression ratio of 90% by pressing each test piece. This was visually evaluated for the presence of processing cracks.
[0028]
[Table 1]

[0029]
[Table 2]

[0030]
From Table 2, it can be considered as follows.
[0031]
No. Examples 1, 2, 3, 6, 9, 10, and 11 are examples that satisfy all the requirements of the present invention. The tensile strength is 650 MPa or less, the metal structure is ferrite + pearlite two-phase structure, and the ferrite fraction is The average particle size of pearlite surrounded by ferrite of 60% or more is 30 μm or less, and the deformability during cold heading is high and no cracks are observed.
[0032]
In contrast, no. Nos. 4, 5, and 11 have a low ferrite fraction and an excessive tensile strength because the cooling rate during hot rolling is too high. No. 8 is because the finish rolling temperature is too high, the metal structure becomes an overcooled structure, the ferrite fraction is remarkably reduced, the formation of the pearlite structure itself is not observed, the strength is excessive, and the performance intended by the present invention. Is not secured at all. No. No. 7, the individual conditions of finish rolling temperature, coiling temperature, and cooling rate satisfy the preferred requirements, but the tensile strength of the hot-rolled steel wire obtained by combining them is too high and the ferrite fraction is insufficient. For this reason, cracks occur during cold heading.
[0033]
1 shows No. 1 in the above experimental example. No. 9 (material of the present invention) is a cross-sectional micrograph showing the metal structure of the hot-rolled steel wire, FIG. 12 (comparative material) is a cross-sectional photomicrograph, and as can be seen by comparing these figures, the hot rolled steel wire material of the present invention has a ferrite + pearlite 2 in which the ferrite structure is uniformly distributed at a high area ratio. In contrast to the phase structure, the comparative material has a low area ratio of the ferrite structure and is sparsely distributed throughout the cross-sectional structure, and the area ratio of the pearlite structure is high.
[0034]
Experimental example 2
The hot-rolled steel wire 9 of Example 9 obtained in Experimental Example 1 was subjected to wire drawing at a reduction rate of 10% or 25% without being subjected to heat treatment. A test was conducted to check for the occurrence of cracks.
[0035]
The results are as shown in Table 3. Even in the hot-rolled steel wire of the present invention, when wire drawing with a reduction in area exceeding 20% is performed, cracking occurs during cold heading, but the reduction in area is less than this. For example, cold forging can be performed without cracking without heat treatment.
[0036]
[Table 3]

[0037]
【The invention's effect】
The present invention is configured as described above, and by specifying the C content of steel and by specifying the metal structure, particularly the ferrite fraction and the average particle size of pearlite surrounded by ferrite, In other words, it provides hot-rolled wire that has excellent deformability in the cold without having to undergo softening heat treatment such as spheroidizing annealing, is inexpensive and has excellent cold heading properties, and reduces the costs associated with softening heat treatment. By doing so, cold forged products could be provided at low cost.
[Brief description of the drawings]
FIG. 1 is a drawing-substituting micrograph showing a cross-sectional metal structure of a hot-rolled steel wire according to the present invention.
FIG. 2 is a drawing-substituting micrograph showing a cross-sectional metallographic structure of a comparative material.

Claims (5)

  The C content is 0.25 to 0.5% (in the case of chemical components, it means mass%, the same shall apply hereinafter), Si: 0.50% or less, Mn: 2.0% or less, Al: 0.00. It is a steel made of Fe that contains no more than 05% and the balance can contain inevitable impurities, and is a hot-rolled steel wire with a tensile strength of 650 MPa or less, and the metal structure is a ferrite + pearlite two-phase structure, A hot rolled steel wire for cold heading, wherein a ferrite fraction in the two-phase structure is 55% or more and an average particle size of pearlite is 30 µm or less. The hot rolled steel wire rod for cold heading according to claim 1, wherein the average particle size of the pearlite is 20 μm or less. The hot rolled steel wire for cold heading according to claim 1 or 2 , further comprising Cr: 1.5% or less and / or Mo: 0.3% or less. Furthermore, B: The hot-rolled steel wire for cold heading in any one of Claims 1-3 containing 0.005% or less. The hot-rolled steel wire for cold heading according to any one of claims 1 to 4 is subjected to wire-drawing with a surface reduction ratio of 20% or less without heat treatment, and then cold heading. A method for manufacturing cold forged products.

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