WO2012005373A1 - 高強度ばね用伸線熱処理鋼線および高強度ばね用伸線前鋼線 - Google Patents
高強度ばね用伸線熱処理鋼線および高強度ばね用伸線前鋼線 Download PDFInfo
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- WO2012005373A1 WO2012005373A1 PCT/JP2011/065749 JP2011065749W WO2012005373A1 WO 2012005373 A1 WO2012005373 A1 WO 2012005373A1 JP 2011065749 W JP2011065749 W JP 2011065749W WO 2012005373 A1 WO2012005373 A1 WO 2012005373A1
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a heat-treated steel wire for a high-strength spring and a pre-drawn steel wire used as a material for a high-strength spring manufactured by cold coiling.
- the high-strength wire drawing heat-treated steel wire for springs has not only high strength but also high workability that does not break with cold coiling, and suppression of softening by heat treatment such as annealing and nitriding performed after coiling, that is, temper softening resistance Is required.
- the heat-treated steel wire for high-strength springs is used as a raw material, and the hardness of the spring surface layer is increased by nitriding and shot peening.
- Spring durability has fatigue characteristics and sag characteristics.
- Surface hardness affects fatigue properties. Not only the surface layer hardness but also the hardness of the base material of the spring has a great influence on the sagging characteristics (the property of causing plastic deformation in the load direction during use of the spring). Therefore, the surface strength after nitriding and the tempering softening resistance inside which nitrogen is not introduced by nitriding are important for the high strength spring steel wire.
- the spring when the spring is manufactured by cold coiling, an oil tempering treatment or a high frequency treatment capable of rapid heating and rapid cooling can be used when producing a heat-treated steel wire for a high-strength spring as a material. . Therefore, it is possible to reduce the prior austenite grain size of the heat-treated steel wire for high-strength springs, and a spring having excellent fracture characteristics can be obtained.
- cold coiling may cause breakage and may not be formed into a spring shape.
- Patent Document 1 a high-strength wire-drawn heat-treated steel wire for springs that controls carbides, refines prior austenite, and achieves both strength and cold coiling properties. Proposed (Patent Document 1). Furthermore, a high-strength wire-drawn heat-treated steel wire for springs was proposed in which retained austenite and carbides were controlled to refine the prior austenite to achieve both strength and cold coiling (Patent Documents 2 to 4). In particular, the fatigue properties of high-strength wire-drawn heat-treated steel wires by suppressing the formation of coarse oxides and carbides that are the starting point of fracture and making the distribution of cementite-based fine carbides required for securing strength uniform. And deterioration of workability is suppressed.
- Patent Document 2 defines the region by paying attention to the fact that the spherical carbide dilute region having an equivalent circle diameter of 2 ⁇ m or more in the region where the distribution of fine spherical carbide (particularly cementite) is dilute affects the mechanical properties.
- Patent Documents 3 and 4 focus on the effect of precipitation of fine carbides due to the addition of V, which is an alloy element, and limit the nitrogen (N) content to suppress undissolved spherical carbides.
- V which is an alloy element
- N nitrogen
- it uses the precipitation effect of V carbide, nitride and carbonitride, and can be used for hardening the steel wire at the tempering temperature and for hardening the surface layer during nitriding.
- it is effective in suppressing the coarsening of the austenite grain size due to the formation of precipitates, and the effect of V addition is remarkable.
- undissolved carbides and nitrides are easily generated, even if nitrogen (N) is suppressed, it is necessary to accurately control the precipitation.
- Patent Document 4 in order to obtain as much precipitated V carbide as effective in the final spring performance as much as possible, undissolved spherical carbide and precipitated carbide are quantitatively compared and defined by the amount. Specifically, it has been proposed to measure the V carbide residue in the electrolytic solution at a constant potential and compare it with the V amount passing through the filter (precipitation V amount).
- the above-described conventional heat-treated steel wire for high-strength springs has a certain degree of uniform dispersion of fine carbides to improve fatigue characteristics and workability.
- the temper softening resistance it is necessary to further uniformly disperse.
- the V addition proposed in Patent Document 3 and Patent Document 4 certainly has the effect of hardening the steel wire at the tempering temperature, hardening the surface layer during nitriding, and refining the austenite.
- N nitrogen
- Patent Document 3 also aims at supplementing excess nitrogen (N) by adding Nb or Ti. However, even in this case, it is not easy to control an appropriate N content.
- Patent Document 4 the resulting residue of undissolved spherical carbide is collected and compared with dissolved carbide. Therefore, the uniform dispersion of fine carbide is not actively controlled. From the above, the present invention suppresses the addition of alloy elements such as V as much as possible, that is, has excellent tensile strength, hardness and workability without accurately controlling the N content, and is excellent after nitriding treatment.
- An object of the present invention is to develop a heat-treated steel wire for high-strength springs having surface hardness and internal hardness.
- an object of the present invention is to develop a high-strength spring steel wire that does not contain undissolved spherical carbide having an equivalent circle diameter of 0.2 ⁇ m or more.
- V adversely affects the mechanical properties and fatigue strength of spring steel wires. That is, the steel material is repeatedly heated until it is processed into a spring after casting.
- undissolved spherical carbide is mainly composed of cementite (Fe 3 C).
- undissolved spherical carbide often contains Cr, V, etc. due to repeated heating, and mechanical properties (surface hardness, internal hardness, etc.) after nitriding are merely consumed wastefully by alloy elements such as Cr and V. ) could also make it worse.
- the addition of V is not easy to control the nitrogen (N) content, and as a result, coarse carbides, nitrides, and carbonitrides are precipitated, which causes a decrease in fatigue strength.
- V is not added, and even if it is added, it is kept in a very small amount. Further, as described above, the amount of Cr is controlled by the balance with the amount of Si, thereby suppressing the coarsening of undissolved spherical carbide. I found out that I can.
- the undissolved spherical carbide refers to an undissolved carbide having a ratio (aspect ratio) between the maximum diameter (major axis) and the minimum diameter (minor axis) of 2 or less.
- “carbide” and “spherical carbide” are simply undissolved, and are also referred to as “undissolved carbide” and “undissolved spherical carbide”, respectively, although they are synonymous in the emphasis here. This invention is made
- a method for producing a pre-drawn steel wire for a high-strength spring characterized by (9) Furthermore, in mass%, V: 0.03-0.10%, Nb: 0.015% or less Mo: 0.05-0.30% W: 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, Zr: contains one or more of 0.003% or less, When V is contained, 1.4% ⁇ Cr + V ⁇ 2.6% and 0.70% ⁇ Mn + V ⁇ 1.3% are satisfied, When Mo and W are contained, 0.05% ⁇ Mo + W ⁇ 0.5%
- a high-strength wire-drawn heat-treated steel wire for a high-strength spring having high surface layer hardness and high internal hardness even after nitrocarburizing treatment at 500 ° C. for 1 hour, particularly by being excellent in cold coiling properties and temper softening resistance, Furthermore, it becomes possible to provide a high-strength spring excellent in durability, and the industrial contribution is extremely remarkable.
- FIG. 1 is a micrograph of a metal structure showing an example of a spherical carbide of a heat-treated steel wire for high-strength springs according to the present invention. Undissolved spherical carbide is observed at the tip of the arrow in the figure.
- FIG. 2 is a diagram showing the shape of a punch provided with a notch in a test piece.
- FIG. 3 is a diagram illustrating a process of providing a notch in a test piece.
- FIG. 4 is a diagram showing an outline of the notch bending test.
- FIG. 5 is a diagram illustrating a method of measuring a notch bending angle.
- a spring wire is manufactured as follows. Of course, the manufacture of the spring is not limited to this process, and is introduced as an example.
- a steel bloom containing a predetermined component is rolled into a billet.
- the billet is rolled to produce a steel wire having a predetermined diameter.
- the steel wire manufactured at this stage is referred to as “steel wire before drawing”.
- the steel wire produced after rolling is patented, and the steel wire is further reduced in diameter by wire drawing, and heat treatment (quenching and tempering) is performed in order to remove the processing strain of the surface layer and to obtain subsequent cold coiling workability.
- the steel wire manufactured at this stage is referred to as “drawn heat-treated steel wire”.
- C 0.67% or more and less than 0.9% C is an important element that greatly affects the strength of the steel material and contributes to the formation of retained austenite.
- the lower limit value of the C amount is set to 0.67% or more so that sufficient strength can be obtained.
- the C content is preferably 0.70% or more. More preferably, it is 0.75% or more.
- the upper limit of the C content is less than 0.9%. From the viewpoint of suppressing the formation of spherical carbides, the upper limit of the C content is preferably 0.85%, and more preferably 0.80%.
- Si 2.0 to 3.5% Si is an important element that improves the temper softening resistance of the steel and the sag characteristics of the spring. To obtain these effects, it is necessary to add 2.0% or more. Si is also effective for spheroidizing and refining cementite, and it is preferable to add 2.1% or more of Si in order to suppress the formation of coarse spherical carbides. In order to increase the internal hardness after performing a treatment for curing the surface layer such as nitriding treatment, it is more preferable to add 2.2% or more of Si. Furthermore, Si is more preferably 2.3% or more from the balance with Cr. In some cases, Si may be 3.0% or more.
- the upper limit of Si content is 3.5% or less.
- the upper limit is preferably 3.4%, and more preferably 3.3% or less.
- Mn 0.5 to 1.2% Mn is an important element for improving the hardenability and stably securing the amount of retained austenite.
- Mn in order to increase the tensile strength of the steel wire and secure retained austenite, it is necessary to add Mn in an amount of 0.5% or more, preferably 0.65% or more. It should be 70% or more.
- the upper limit of the Mn content is preferably 1.2% or less, preferably 1.1% or less, and more preferably 1.0% or less.
- Cr 1.3-2.5% Cr is an element effective for improving hardenability and temper softening resistance. In order to obtain these effects, it is necessary to add 1.3% or more of Cr. When nitriding is performed, the hardened layer by nitriding can be deepened by adding Cr. Therefore, in the case of imparting hardening by nitriding and softening resistance at the nitriding temperature, it is preferable to add more than 1.5% Cr, and more preferably 1.7% or more. On the other hand, if the amount of Cr is excessive, not only the production cost increases, but also the dissolution of carbides is inhibited, and the amount of undissolved spherical carbides increases to inhibit coiling. Therefore, the upper limit of Cr amount is 2.5% or less.
- the upper limit of the Cr content is preferably set to 1.8% or less.
- N 0.003 to 0.007%
- N is an element that forms a nitride with Al or the like contained as an impurity in the steel.
- the upper limit of the N amount is 0.007% or less.
- the N amount is preferably 0.005% or less.
- P 0.025% or less
- P is an impurity and limits the upper limit of the amount of P to 0.025% or less in order to harden steel, cause segregation, and cause embrittlement. Further, P segregated at the prior austenite grain boundaries lowers the toughness, delayed fracture resistance, and the like, so the upper limit of the P content is preferably limited to 0.015% or less. Furthermore, when the tensile strength of the steel wire exceeds 2150 MPa, the P content is preferably limited to less than 0.010%.
- S 0.025% or less S is also an impurity.
- the steel is embrittled, so the upper limit of the amount of S is limited to 0.025% or less.
- Mn is an inclusion, and particularly in high-strength steel, MnS may be the starting point of fracture. Therefore, in order to suppress the occurrence of destruction, it is preferable to limit the upper limit of the S amount to 0.015% or less.
- the S content is preferably limited to less than 0.01%.
- Al 0.0005 to 0.003%
- Al is a deoxidizing element, affects the generation of oxides, and fatigue resistance decreases when hard oxides are generated. Particularly in a high-strength spring, if Al is added excessively, the fatigue strength varies and the stability is impaired.
- the Al content exceeds 0.003%, the fracture occurrence rate due to inclusions increases, so the Al content is limited to 0.003% or less.
- the upper limit of the amount of Al is desirably 0.0028%, and more desirably 0.0025%.
- the Al content is 0.0005% or more.
- the lower limit of the amount of Al is desirably 0.0007%, more desirably 0.0008%, and further desirably 0.001% or more.
- Si is an element that destabilizes cementite.
- an element that stabilizes cementite such as Cr
- it has an effect of promoting solid solution of cementite during heating. Accordingly, when the amount of Si added is small in spite of the addition of a large amount of Cr, the amount of undissolved spherical carbide increases and the workability is remarkably reduced.
- the present inventors have found that the difference between the Si content (mass%) and the Cr content (mass%) in the steel, that is, the Si—Cr content can be used as a guide.
- the value of Si—Cr is preferably 0.3 or more and 1.2% or less.
- the lower limit is preferably 0.35 ⁇ Si—Cr, more preferably 0.4 ⁇ Si—Cr.
- V 0.03-0.10%
- V is an element that generates nitrides, carbides, and carbonitrides. Fine V nitrides, carbides, and carbonitrides having an equivalent circle diameter of less than 0.2 ⁇ m are effective for refinement of prior austenite. Moreover, it can utilize also for hardening of the surface layer by nitriding treatment. However, on the other hand, undissolved carbides and nitrides are likely to be generated. Therefore, even if nitrogen (N) is suppressed, it is necessary to accurately control the precipitation. Therefore, in the present invention, V is not actively added. In order to obtain the effect of V addition as described above, a small amount can be added. In order to acquire these effects, it is good to add V 0.03% or more. Preferably it is 0.035% or more, and it is further better if it is 0.04% or more.
- the V content is preferably 0.1% or less.
- the addition of V facilitates the formation of a supercooled structure that causes cracks and breaks during wire drawing before wire drawing.
- the upper limit of the V amount is preferably 0.09% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.
- the amount of V added is desirably 0.05% or less.
- V is an element that greatly affects the formation of retained austenite, like Mn, it is necessary to precisely control the amount of V together with the amount of Mn.
- Nb 0.015% or less Nb is an element that forms nitrides, carbides, and carbonitrides in steel, and may be used to control the austenite grain size by the precipitates. At the same time, however, excessive addition reduces the hot ductility and makes cracking more likely during rolling or hot forging. Therefore, excessive addition must be avoided.
- Nb is intended to control the amount of N, not direct material control by precipitates.
- a spring such as a valve spring is manufactured by cold coiling after quenching and tempering. At this time, solute nitrogen prevents cold deformation and lowers its limit strain. Therefore, the coiling property is impaired. Therefore, Nb is added to produce nitrides at a high temperature, thereby reducing the solute nitrogen in the steel matrix steel and improving the cold workability.
- V is an element effective for improving the temper softening resistance and the outermost layer hardness during nitriding.
- V nitride, V carbide, and V carbonitride are also used in heating to obtain an austenitic phase such as patenting and quenching for producing a heat-treated steel wire for high-strength springs. In many cases, the solid solution is not sufficiently dissolved.
- V was not made an essential element.
- Nb produces nitride at a temperature higher than V. Therefore, in the steelmaking process, the formation of V nitride is suppressed by adding Nb. In other words, Nb forms a nitride in a high temperature region where V is dissolved and no nitride is formed. Further, since Nb consumes nitrogen at a high temperature generated by V nitride, it becomes difficult to generate V nitride even when cooled. Therefore, the addition of a small amount of Nb is particularly effective for suppressing undissolved spherical carbide and ensuring coiling properties when a large amount of V is added.
- the addition amount is 0.015% or less. Preferably, it is 0.010% or less, more preferably 0.005% or less, and even more preferably less than 0.001%.
- the effect that Nb controls the N content of spring steel appears from 0.0005%. Therefore, when Nb is added, it is preferable to add 0.0005% or more.
- V is added, it is more effective to add a trace amount of Nb, and the range is preferably 0.003 to 0.012%. Furthermore, the range of 0.005 to 0.009% is preferable, and the effect can be obtained even with 0.005 to 0.001%.
- V is not actively added. However, as described above, the addition of a small amount of V affects the refinement of prior austenite and the generation of retained austenite.
- the surface hardness and internal hardness after nitriding can be increased so as to be suitable for a high-strength spring.
- Both Cr and V are elements that impart so-called temper softening resistance that does not soften even when heated by annealing or nitriding after spring coiling.
- the surface hardness is improved by precipitating nitride at the nitriding portion of the surface layer, and the nitriding effect is increased.
- the decomposition of carbides is suppressed even in the inside where nitriding does not diffuse.
- both are elements that are liable to generate undissolved spherical carbides.
- V also has a solid solution temperature higher than the A3 point of the steel, and therefore tends to remain as undissolved spherical carbide.
- Cr + V which is the total content of Cr and V
- the surface hardness as a high-strength spring is less than HV750, and the internal hardness is also less than HV570. Therefore, Cr + V is preferably 1.4% or more. Furthermore, 1.5% or more is preferable.
- Cr + V exceeds 2.6%, excessive undissolved spherical carbides remain, so that the coiling property is impaired, so 2.6% is made the upper limit. Further, Cr + V is preferably 2% or less, and more preferably 1.8% or less.
- Mn and V are elements that improve the hardenability and have a great influence on the formation of retained austenite.
- Mn is more than specified, a large amount of retained austenite remains. Therefore, the sum of both Mn and V mixed as an unavoidable impurity directly affects the austenite behavior. If they exceed the limit, the amount of retained austenite increases, which not only affects workability but also greatly increases the yield point. It is affected, and sufficient durability cannot be secured.
- the total Mn + V content of Mn and V is set to 0.7 to 1.3%.
- Mo 0.05-0.30%
- Mo is an element that enhances hardenability and is extremely effective in improving temper softening resistance.
- 0.05% or more of Mo can be added.
- Mo is also an element which produces
- the addition amount of Mo exceeds 0.30%, a supercooled structure is likely to be generated by hot rolling, patenting before wire drawing, or the like.
- the upper limit of the Mo amount is set to 0.30% or less, preferably 0.25% or less, in order to suppress generation of a supercooled structure that causes breakage during cracking or wire drawing.
- the Mo amount is 0.20% or less, the patenting time is further shortened, and the pearlite transformation is stably terminated. Is preferably 0.15% or less.
- W 0.05-0.30% W, like Mo, is an element effective for improving hardenability and temper softening resistance, and is an element that precipitates as carbide in steel. In the present invention, 0.05% or more of W is added to increase the temper softening resistance.
- the W amount is preferably 0.1 to 0.2%, more preferably 0.13 to 0.18%.
- Mo + W are effective elements for improving the temper softening resistance.
- Mo + W needs to be 0.05% or more, preferably 0.15% or more.
- Mo + W exceeds 0.5%, a so-called supercooled structure such as martensite or bainite is generated by hot rolling or patenting before wire drawing, which causes breakage or breakage during wire drawing.
- the upper limit of Mo + W is set to 0.5% or less. Preferably it is 0.35% or less.
- Mg, Ca, and Zr will be described.
- Mg: 0.002% or less Mg generates an oxide in molten steel that is higher than the MnS generation temperature, and already exists in the molten steel when MnS is generated. Therefore, it can be used as MnS precipitation nuclei, whereby the distribution of MnS can be controlled. Also, the number distribution of Mg-based oxides is more finely dispersed in molten steel than Si and Al-based oxides often found in conventional steels. Therefore, MnS with Mg-based oxides as the core must be finely dispersed in steel. It becomes.
- the MnS distribution differs depending on the presence or absence of Mg, and the addition of them makes the MnS particle size finer.
- Mg is preferably added in an amount of 0.0002% or more, more preferably 0.0005% or more.
- the upper limit of the amount of Mg added is set to 0.002%. Preferably, it is 0.0015% or less. Further, in the case of spring steel, the amount of S added is suppressed more than other structural steels. Therefore, considering yield and the like, 0.001% or less is preferable. In addition, since inclusions are highly sensitive when used in high strength valve springs, Mg is effective in improving corrosion resistance, delayed fracture and preventing rolling cracks due to the effects of MnS distribution, etc. It is preferable to control the amount of addition within a very narrow range of 0002 to 0.001%.
- Ca is an oxide and sulfide-forming element.
- MnS metal-oxide-semiconductor
- the length of MnS as a fracture starting point such as fatigue can be suppressed and rendered harmless.
- the effect is similar to that of Mg, and 0.0002% or more is preferable.
- This addition amount is preferably 0.0015% or less, and more preferably 0.001% or less, because inclusion sensitivity is high when used for a high-strength valve spring.
- Zr is an oxide, sulfide and nitride-forming element.
- oxides are finely dispersed, so that, like Mg, MnS precipitate nuclei and MnS can be finely dispersed.
- Mg Mg
- MnS precipitate nuclei
- MnS metal-oxide-semiconductor
- oxides, nitrides such as ZrN and ZrS, and sulfides are generated, and the trouble in manufacturing and the fatigue durability characteristics of the spring are lowered. 0.003% or less.
- This addition amount is preferably 0.0025% or less, and when used for a high-strength valve spring, it has the effect of improving coiling properties by sulfide control. In order to minimize the influence, it is preferable to suppress it to 0.0015% or less.
- Undissolved spherical carbide plays an important role in securing strength in a high-strength spring steel wire.
- the presence of undissolved spherical carbides deteriorates the coiling property, and coarse carbides also deteriorate the fatigue characteristics. Therefore, it is indispensable for solving the problems of the present invention to suppress undissolved spherical carbides during coiling and to uniformly disperse fine carbides after the final nitriding treatment.
- the steel wire for high-strength springs of the present invention is characterized in that the major axis of the undissolved spherical carbide is 0.2 ⁇ m or less, that is, coarsening is suppressed.
- This undissolved spherical carbide already exists after the wire rod rolling (that is, the steel wire before drawing).
- This undissolved spherical carbide is difficult to be dissolved in the subsequent heat treatment (patenting, processing heat generation during wire drawing, quenching and tempering step). Rather, it may grow and become coarse in those heat treatment steps. That is, the undissolved spherical carbide in the pre-drawn steel wire may become the core of its own coarsening.
- the definition of the undissolved spherical carbide is important not only in the prestretched steel wire for high-strength springs according to the present invention but also in the heat-treated steel wire for high-strength springs.
- spherical alloy-based carbides and spherical cementite-based carbides are collectively referred to as spherical carbides.
- acicular carbides corresponding to the acicular structure of tempered martensite, but these acicular carbides are not included in the spherical carbide of the present invention.
- This acicular carbide is a carbide that does not exist immediately after quenching and precipitates during the tempering process.
- This tempered martensite structure is a structure suitable for achieving both strength, toughness, and workability, and in some sense, it is an ideal form among carbides.
- Undissolved spherical carbide is obtained by mirror-polishing a sample taken from a pre-drawn steel wire or a heat-treated steel wire for high-strength springs, and performing etching with picral or electrolytic etching, using a scanning electron microscope (SEM). Observation becomes possible. Moreover, it can observe with the replica method of a transmission electron microscope (TEM).
- SEM scanning electron microscope
- FIG. 1 shows an example of a structure photograph obtained by observing a sample after electrolytic etching with an SEM.
- the structure photograph of FIG. 1 two types of matrix acicular structure and spherical structure are recognized in the steel.
- the acicular structure is tempered martensite generated by quenching and tempering.
- the spherical structure is a carbide (undissolved spherical carbide) 1 that is not solid-dissolved in steel by hot rolling and is spheroidized by quenching and tempering by oil tempering or high-frequency treatment. Spherical carbide can be observed at the tip of the arrow in FIG.
- the equivalent circle diameter of undissolved spherical carbide is less than 0.2 ⁇ m.
- the size is controlled as follows.
- finer spherical carbides are defined as compared with the prior art to achieve both higher performance and workability.
- Spherical carbide having an equivalent circle diameter of less than 0.2 ⁇ m is extremely effective for ensuring the strength of the steel and the resistance to temper softening.
- the present invention is characterized in that spherical carbide having an equivalent circle diameter of 0.2 ⁇ m or more is not generated.
- the pre-drawn steel wire and the heat-treated steel wire of the present invention are characterized in that the equivalent circle diameter of undissolved spherical carbide is less than 0.2 ⁇ m. Therefore, workability can be secured while securing strength.
- the steel wire before wire drawing needs to be subjected to heat treatment such as patenting, wire drawing heating, and quenching and tempering, and thus undissolved spherical carbide may grow and become coarse. Therefore, it is desirable that the equivalent circle diameter of the undissolved spherical carbide in the pre-drawn steel wire is smaller than 0.2 ⁇ m. From the experimental results of the inventors, it has been confirmed that the equivalent circle diameter of the undissolved carbide of the pre-drawn steel wire can be 0.18 ⁇ m or less. It has also been confirmed that if the billet heating temperature is 1250 ° C. or more, it can be 0.15 ⁇ m or less.
- a sample taken from a steel wire for high-strength springs is polished and electrolytically etched.
- part observes what is called a 1 / 2R part near the center of the radius of heat-treated wire (steel wire) so that special conditions, such as decarburization and center segregation, can be excluded.
- the measurement area is 300 ⁇ m 2 or more.
- electrolytic action is performed using a low-potential current generator with the sample as the anode and platinum as the cathode in the electrolyte (a mixture of acetylacetone 10% by mass, tetramethylammonium chloride 1% mass%, residual methyl alcohol). Corrodes the sample surface.
- the potential is constant at a potential suitable for the sample in the range of ⁇ 50 to ⁇ 200 mV vs SCE.
- the energization amount can be obtained by the total surface area of the sample ⁇ 0.133 [c / cm 2 ].
- the total surface area of the sample is calculated by adding not only the polished surface but also the area of the sample surface in the resin. After the energization is started and held for 10 seconds, the energization is stopped and washed.
- the sample is observed with an SEM and a structure photograph of spherical carbide is taken.
- a structure that is observed in SEM relatively white and has a ratio of the maximum diameter (major axis) to the minimum diameter (minor axis) (aspect ratio) of 2 or less is a spherical carbide.
- the photographing magnification with SEM is 1000 times or more, preferably 5000 to 20000 times.
- the measurement site avoids the center segregation part, and randomly selects 10 fields of view at a depth of about 0.5 to 1 mm from the surface of the wire.
- the SEM structure photograph taken in this way is subjected to image processing, the minimum diameter (short diameter) and maximum diameter (long diameter) of the spherical carbides found in the measurement visual field are measured, and the equivalent circle diameter is calculated.
- the equivalent circle diameter is calculated by calculating the area of the target undissolved carbide in the field of view by image processing and converting it to a circle having the same area as the equivalent circle diameter. It is also possible to measure the existence density of spherical carbides having a circle-equivalent diameter of 0.2 ⁇ m or more found in the measurement visual field.
- the metal structure of the drawn heat-treated steel wire for high-strength springs according to the present invention is a retained austenite with a volume ratio of more than 6% and 15% or less. It consists of the balance tempered martensite and allows a small amount of inclusions.
- the trace inclusions are oxides and sulfides, oxides are deoxidation products such as Al and Si, and sulfides are MnS and CaS.
- the remaining tempered martensite structure contains a small amount of undissolved spherical carbide.
- the prior austenite grain size number in the structure is 10 or more, and the equivalent circle diameter of the spherical carbide is less than 0.2 ⁇ m.
- the pearlite structure accounts for 90% or more of the metal structure of the prestretched steel wire for high strength springs according to the present invention. It is desirably 95% or more, more desirably 98% or more, and ideally a pearlite structure.
- Former austenite grain size number 10 or more
- the drawn heat-treated steel wire for high-strength springs of the present invention has tempered martensite as the main structure, and the former austenite grain size has a great influence on the properties. That is, when the grain size of the prior austenite is made fine, fatigue characteristics and coiling properties are improved due to the effect of fine graining.
- the prior austenite grain size number is set to 10 or more.
- the refinement of the prior austenite is particularly effective for improving the properties of the heat-treated steel wire for high-strength springs, and the prior austenite grain size number is preferably 11, and more preferably 12 or more.
- the prior austenite grain size number is measured according to JIS G 0551.
- Substantially the austenite grain size can be made finer by treating the heating temperature during quenching at a low temperature for a short time.
- excessively low temperature and short-time treatment not only increases undissolved spherical carbide, but also austenite. In some cases, it is insufficient, resulting in a two-phase quenching, and conversely, coiling properties and fatigue characteristics may be reduced. Therefore, the upper limit is usually 13.5.
- Residual austenite more than 6% to 15% (volume ratio)
- the microstructure of the heat-treated steel wire for high-strength springs after quenching and tempering includes tempered martensite, retained austenite, and slight volume fraction inclusions (here, precipitates are also included in the inclusions). It is made up of.
- Residual austenite is effective in improving cold coiling properties.
- the volume fraction of retained austenite is set to more than 6% in order to ensure cold coiling properties. Preferably it is 7% or more, and more preferably 8% or more.
- the volume ratio of retained austenite is set to 15% or less. It is preferably 14% or less, and more preferably 12% or less.
- the volume fraction of retained austenite can be determined by an X-ray diffraction method or a magnetic measurement method. Among these, the magnetic measurement method is a preferable measurement method because the volume fraction of retained austenite can be easily measured.
- the volume ratio is measured, but the obtained numerical value is the same value as the area ratio.
- retained austenite is softer than tempered martensite and thus lowers the yield point. Moreover, since ductility is improved by transformation-induced plasticity, it contributes significantly to the improvement of cold coiling properties. On the other hand, retained austenite often remains in the vicinity of segregated parts, former austenite grain boundaries, and regions sandwiched between subgrains, so martensite generated by processing-induced transformation (processing-induced martensite) Become. Moreover, when retained austenite increases, tempered martensite relatively decreases.
- a high-strength spring is manufactured by bending a high-strength spring drawn heat-treated steel wire, which is a raw material, into a desired shape, and performing a surface hardening process such as nitriding or shot peening. In the nitriding process, the spring is heated to about 500 ° C., so that the spring may be softer than the high-strength wire heat-treated steel wire for the spring.
- the tensile strength of the high-strength spring-drawn heat-treated steel wire is high, the fatigue characteristics and the sag characteristics of the spring subjected to the treatment for hardening the surface such as nitriding treatment can be enhanced.
- the tensile strength of the heat-treated steel wire for high-strength springs is set to 2100 MPa or more in order to enhance the fatigue characteristics and sag characteristics of the springs. Further, the higher the tensile strength of the heat-treated steel wire for high-strength springs, the better the fatigue characteristics of the spring. Therefore, the tensile strength of the heat-treated steel wire for high-strength springs is preferably 2200 MPa or more, more preferably 2250 MPa or more. And
- the tensile strength of the heat-treated steel wire for high-strength springs is set to 2400 MPa or less.
- the yield strength or the yield point of a heat-treated steel wire for high-strength springs is the upper yield point and the yield point when a yield point is found in the stress-strain curve in a uniaxial tensile test. If not, the proof stress is 0.2%.
- it is preferable to increase the yield point of the heat-treated steel wire for high-strength springs which is a material.
- the yield point of the high-strength spring-drawn heat-treated steel wire is increased, the cold coiling property may be impaired. Therefore, the yield point of the heat-treated steel wire for high-strength spring is preferably 1600 MPa or more in order to ensure the strength and sag resistance of the spring.
- the yield point In order to impart even higher durability, it is preferably 1700 MPa or more. On the other hand, if the yield point exceeds 1980 MPa, the cold coiling property may be impaired, so the yield point is preferably 1980 MPa or less. In order to increase the yield point of a heat-treated steel wire for high-strength springs having the same tensile strength immediately after quenching and tempering for a short time, it is preferable to reduce the volume fraction of retained austenite.
- the high-strength spring is improved in surface layer hardness during nitriding, but the inside is softened.
- the heat-treated steel wire for high-strength springs of the present invention has excellent temper softening resistance, and can ensure the fatigue characteristics and sagability of the spring after heating at 500 ° C.
- the surface hardness and internal hardness after gas soft nitriding are defined.
- the surface hardness is 750 or more in terms of micro Vickers hardness at a depth of 50 to 100 ⁇ m from the surface layer. If it is less than 750, the surface layer hardness is insufficient and the fatigue durability is also inferior, so that the residual stress after shot peening cannot be sufficiently applied.
- the surface hardness is preferably 780 or more.
- the measurement of Vickers hardness is preferably performed at a depth of 500 ⁇ m from the surface because the temperature of the surface layer of the steel wire may be higher than the inside during quenching.
- the Vickers hardness after the heat treatment held at 500 ° C. for 1 hour may be 570 or more, and more preferably 575 or more.
- maintained at 500 degreeC for 1 hour is not prescribed
- the surface layer is cured by shot peening or nitriding treatment.
- the Vickers hardness (internal hardness) at a depth of 500 ⁇ m from the surface of the high-strength spring is affected by heating during nitriding. Therefore, when the spring is actually manufactured, the internal hardness varies depending on the temperature of the nitriding treatment.
- the component composition of the high-strength spring made of the heat-treated steel wire for high-strength spring of the present invention the spherical carbide, the prior austenite grain size
- the component composition of the heat-treated steel wire for high-strength spring of the present invention Spherical carbide, the same size as the prior austenite grain size.
- a steel slab (bloom) adjusted to a predetermined component is rolled to produce a steel slab (billet) downsized. And after heating a billet, it hot-rolls and makes it the steel wire before wire drawing for high strength springs.
- high-strength spring steel wire is patented, it is shaved, further annealed to soften the hardened layer, wire-drawn, quenched and tempered, and high-strength spring.
- a heat-treated steel wire is manufactured.
- the patenting treatment is a heat treatment in which the structure of the steel wire after hot rolling is ferrite pearlite, and is performed to soften the steel wire before wire drawing. After wire drawing, quenching and tempering such as oil tempering and high frequency treatment are performed to adjust the structure and properties of the steel wire.
- the heating temperature of the cast steel piece is set to 1250 ° C. or higher.
- the undissolved spherical carbide can be sufficiently dissolved. Therefore, in the subsequent heating during rolling, patenting and quenching, although undissolved spherical carbide tends to remain due to insufficient heating temperature and heating time, it is sufficiently solid solution at the beginning, so the dimensions of undissolved spherical carbide Can be suppressed to less than 0.2 ⁇ m.
- the bloom heating temperature is preferably 1270 ° C. or higher.
- the billet produced by rolling the bloom is further hot rolled (wire rolling) to produce a steel wire before drawing for high strength springs.
- the heating temperature of a billet shall be 1200 degreeC or more.
- the heating temperature of the billet is also 1250 ° C. or higher.
- the temperature decreases and precipitates grow. Therefore, it is preferable to complete hot rolling within 5 minutes after extraction from the heating furnace.
- the heating temperature before rolling of the bloom is preferably 1250 ° C. or higher, more preferably 1270 ° C. or higher.
- the undissolved carbide existing before the wire drawing (that is, after the wire rolling) is reduced as much as possible. Even if it exists, it is necessary to make its diameter fine so that it does not easily become coarse. Therefore, in the rolling process, which is a heating process before wire drawing, it is important to keep the bloom heating temperature and billet heating temperature sufficiently high so that the carbide dissolves. Thereby, the diameter of the undissolved spherical carbide can be suppressed small.
- the rolling of the spring steel is completed in a few minutes from the billet being extracted from the heating furnace to the diameter before drawing of about ⁇ 10 mm.
- the influence of billet heating temperature is the largest, and it is important to heat to 1200 ° C. or higher. It is more preferable if it is 1250 ° C. or higher, and further preferable if it is 1270 ° C. or higher.
- the microstructure of the steel wire before wire drawing (steel wire after wire rod rolling) usually has a high C content, it is composed only of ferrite / pearlite or pearlite phase having a high pearlite structure fraction. Undissolved spherical carbide exists in such a base material.
- Undissolved spherical carbide can be observed by observing a polished and etched microscopic sample with an SEM. Since undissolved carbide is spherical, it can be clearly distinguished from lamellar cementite contained in the pearlite structure of the base material. Can be distinguished. Of course, the size can also be measured.
- patenting is performed on the steel wire before drawing for spring.
- the heating temperature for this patenting is preferably 900 ° C. or higher in order to promote solid solution of carbides.
- a high temperature of 930 ° C. or higher is more preferable, and a temperature of 950 ° C. or higher is more preferable.
- a patenting process may be performed at 600 ° C. or lower.
- the patenting and drawing methods are not limited.
- a general steel wire patenting treatment or wire drawing method can be applied in the same manner as usual.
- the patenting step prior to the wire drawing step may be omitted. In that case, the solid solution of the undissolved spherical carbide is promoted by increasing the heating temperature of quenching described below (for example, 970 ° C. or higher).
- Quenching after wire drawing is performed by heating to a temperature of 3 points or more of A.
- the heating rate is 10 ° C./second or more
- the holding time at a temperature of A 3 point or more is 1 minute or more and 5 minutes or less.
- quenching is preferably performed at a cooling rate of 50 ° C./second or higher and cooled to 100 ° C. or lower.
- the temperature of the quenching refrigerant is preferably 100 ° C. or lower, more preferably 80 ° C. or lower, but in the present invention, the refrigerant temperature is set to 40 ° C. or higher in order to precisely control the amount of retained austenite.
- the refrigerant is not particularly limited as long as it can be quenched, such as oil, a water-soluble quenching agent, and water.
- the cooling time may be as short as oil tempering or high-frequency heat treatment, and it is desirable to avoid extending the holding time at low temperatures and reducing the coolant temperature to 30 ° C. or lower in order to extremely reduce residual austenite. . That is, it is preferable to complete the quenching within 5 minutes.
- tempering in order to suppress the growth of carbides, it is preferable to set the heating rate to 10 ° C./second or more and the holding time to 15 minutes or less.
- the holding temperature varies depending on the components and the target strength, but is usually held at 400 to 500 ° C.
- Pre-drawn steel wire for high-strength springs is processed into a desired spring shape by cold coiling, subjected to strain relief annealing, and further subjected to nitriding and shot peening to produce a spring.
- the cold-coiled steel wire is reheated by strain relief annealing or nitriding.
- the performance as a spring falls.
- the micro-Vickers hardness of the high-strength spring at a depth of 500 ⁇ m from the surface layer can be set to HV575 or more. The reason why the micro Vickers hardness is measured at a depth of 500 ⁇ m from the surface layer of the spring is to evaluate the Vickers hardness of the base material that is not affected by hardening by nitriding treatment and shot peening.
- the pre-drawing steel wire (rolled wire) having a diameter of 8 mm was patented before drawing in order to make the structure easy to draw.
- the heating temperature in the patenting is desirably 900 ° C. or higher so that carbides and the like are sufficiently dissolved, and after heating at 930 ° C., it is put into a fluidized bed at 600 ° C. for patenting. After patenting, a wire drawing material having a diameter of 4 mm was obtained by wire drawing. In this way, after heating the bloom at a high temperature, the growth of undissolved spherical carbide can be suppressed by increasing the heating during rolling, patenting and quenching as much as possible, and the dimension is 0.2 ⁇ m. The following can be suppressed.
- quenching and tempering treatment was performed to produce a pre-drawing steel wire for a spring.
- the sample in which the wire breakage has occurred is not subjected to quenching and tempering.
- Quenching and tempering are performed by heating the drawn steel wire to 950 ° C or 1100 ° C (temperature of 3 points or more) at a heating rate of 10 ° C / second or more and holding at that heating temperature for 4 to 5 minutes. Then, it was poured into a water bath at room temperature so as to have a cooling rate of 50 ° C./second or more, and cooled to 100 ° C. or less.
- the disconnection status, prior austenite grain number, retained austenite amount (volume%), equivalent circle diameter and abundance density of carbide, tensile strength, 0.2% proof stress, notch bending angle, average fatigue strength and after gas soft nitriding Indicates Vickers hardness.
- the target values to be accepted were as follows with reference to the conventional steel wire for high strength springs.
- Old austenite grain size number 10 degrees or more Residual austenite amount (volume%): 20% or less Spherical carbide equivalent circle diameter: 0.2 ⁇ m or less
- Vickers hardness after gas soft nitriding Internal hardness: 590 Hv or higher Vickers hardness after gas soft nitriding: Nitrided layer hardness: 750 Hv or higher
- the steel wire according to the present invention has strength and workability (coiling properties). Since it is necessary to make it compatible, workability will deteriorate if the yield ratio is too high. Therefore, the upper limit of the yield ratio is preferably 90%, more preferably 88% or less.
- Specimens were collected from the obtained heat-treated steel wire for springs and subjected to old austenite grain size, volume fraction of retained austenite, carbide evaluation, tensile test, notch bending test, and micro Vickers hardness test.
- the fatigue characteristics are as follows: gas soft nitriding treatment (500 ° C., 60 minutes), shot peening (simulating nitriding treatment applied to the spring after processing) as a process simulating spring manufacture (hereinafter referred to as spring manufacturing process).
- the diameter of the cut wire was 0.6 mm, 20 minutes), and low-temperature distortion removal treatment (180 ° C., 20 minutes) was performed for evaluation.
- the former austenite grain size number was measured according to JIS G 0551.
- the equivalent circle diameter and abundance density of the carbides were measured by taking a SEM structure photograph using a sample subjected to electrolytic etching, image processing, and the like.
- the volume fraction of retained austenite was measured by a magnetic measurement method.
- the fatigue test is a Nakamura-type rotating bending fatigue test (fatigue test in which a sample is bent with a two-point weight and rotated by a motor to apply compressive and tensile stress to the surface of the wire).
- the maximum load stress showing a life of 107 cycles or more was defined as the average fatigue strength.
- the notch bending test is a test for evaluating cold coiling properties, and was performed as follows.
- a groove (notch) having a maximum depth of 30 ⁇ m was provided in the test piece using the punch 2 shown in FIG. 2 having a tip angle of 120 °.
- the notch 4 was provided in the center part of the longitudinal direction of the test piece 3 at right angle with the longitudinal direction.
- the load P of the maximum tensile stress was applied from the opposite side of the notch 4 through the load jig 6 via the pressing jig 5, and three-point bending deformation was applied.
- D is the diameter of the test piece.
- the micro Vickers hardness after nitriding was evaluated as the hardness of the nitrided layer, with the internal hardness at a depth of 500 ⁇ m or more from the surface layer and the micro Vickers hardness at a depth of 50 ⁇ m from the surface layer.
- the measurement load is 10 g.
- Tables 1-5 to 8 These test results are shown in Tables 1-5 to 8.
- the metal structure is tempered martensite and slight inclusions except for retained austenite ( ⁇ ).
- the balance of the components is iron and inevitable impurities.
- Evaluation of the steel wire before wire drawing performed only the equivalent circle diameter of the undissolved spherical carbide. This is because before the heat treatment, it is not meaningful to measure mechanical properties, austenite grain size, and the like.
- Each of Examples 1 to 47 of the present invention has a good notch bending angle of 28 ° or more, which is an index of cold coiling property, and Nakamura rotary bending fatigue strength (index of spring durability) (The nitride layer hardness, which is an index of sagability and temper softening resistance, is also excellent.
- Comparative Examples 48 and 49 are examples in which the amount of C added is outside the scope of the claims, and when C exceeds the specified value (Comparative Example 48), the amount of undissolved spherical carbide increases, and the notch bending angle, which is an index of cold coiling properties Is low. On the other hand, when C is less than the specified amount (Comparative Example 49), sufficient tensile strength cannot be ensured. In particular, the internal hardness after nitriding becomes low, and the fatigue durability as a spring (Nakamura rotary bending fatigue strength) and sag characteristics (internal hardness after nitriding) are inferior.
- Comparative Examples 50 and 51 are examples in which the amount of Si added is outside the scope of the claims.
- Si exceeds the specified range, the matrix becomes brittle and the workability is impaired, that is, the notch bending angle is low.
- Si is less than the specified amount, the temper softening characteristic deteriorates, so that sufficient strength cannot be ensured after heating by nitriding.
- the internal hardness after nitriding and the nitrided layer hardness were low.
- Comparative Examples 52 and 53 are examples in which the amount of Mn added is outside the scope of the claims.
- Mn exceeds the specified value, the retained austenite increases, the yield strength decreases, and the fatigue durability (Nakamura rotary bending fatigue strength) is Inferior.
- the amount of Mn is less than the specified amount, the retained austenite is excessively decreased and the processing is deteriorated, so that the notch bending angle is decreased.
- Comparative Examples 54 and 55 are examples in which the amount of Cr added is out of the scope of the claims, and when Cr exceeds the specified value, cementite is stabilized, and undissolved carbides are increased even when heating a steel slab or quenching and tempering at a high temperature. Spring workability is greatly reduced. Therefore, the notch bending angle was lowered. On the other hand, if the Cr content is less than the specified amount, the so-called temper softening resistance is insufficient, such as softening due to heat treatment such as nitriding, and the hardness of the nitrided layer is lowered.
- Comparative Examples 56, 57, and 58 are examples in which the addition amounts of Mo, W, and Mo + W are excessively added outside the scope of the claims.
- Mo and W exceed the specified range, heat treatment such as rolling cooling or patenting is performed. Later, supercooled structures such as martensite and bainite were generated, and breakage occurred during conveyance and wire drawing, making it impossible to carry out a measurement test.
- Comparative Example 59 is an example in which V is excessively added.
- V is an element that generates carbides in the steel, and excessive addition results in insoluble carbides centered on V, resulting in deteriorated workability and notch bending. The angle decreased.
- Comparative Examples 60 and 61 are cases where the N amount is excessively included in the claims. This excessive N raises the temperature of formation of nitrides such as V and Nb and carbonitrides, and makes precipitates such as carbides coarse by using these as nuclei. Further, when repeatedly used as in the present invention, the solid solution of the nitride, carbonitride, and carbide becomes incomplete, and a large amount of coarse undissolved spherical carbide remains. As a result, workability is impaired. This is an example in which the notch bending angle is lowered.
- Comparative Examples 62 and 63 are examples in which the amount of Nb added is outside the scope of the claims, and when Nb exceeds the specified range, hot ductility is significantly impaired, surface flaws of the rolled material occur frequently, breakage occurs during wire drawing, and measurement The test was not possible.
- Comparative Example 64 is a case where the sum of the added amounts of Mn and V exceeds the range described in the present invention, and the amount of retained austenite of the steel wire remains more than specified.
- the notch portion is hardened by stress-induced transformation. As a result, the workability is reduced. This is an example in which the notch bending angle is lowered.
- V is not added, but because V is unavoidable as an inevitable impurity, this is a limitation for detoxifying the V.
- Comparative Example 65 is a case where the sum of the addition amounts of Mn and V is lower than the range described in the present invention, and the amount of retained austenite is less than the optimum range, so that the workability, that is, the notch bending angle is lowered.
- Comparative Example 66 is a case where the sum of the added amounts of Cr and V exceeds the range described in the present invention, and the undissolved spherical carbide remains excessively, and the workability, that is, the notch bending angle is lowered.
- Comparative Example 67 is a case where the sum of the added amounts of Cr and V is less than the range described in the present invention, and the workability is good, but the internal hardness after nitriding and the nitrided layer hardness are insufficient, and the spring performance Is not enough.
- Comparative Examples 68 to 70 are cases in which the difference between the Si amount and the Cr amount ([Si%] ⁇ [Cr%]) deviates from the scope of the claims, and the Cr amount is larger than the Si amount. If Cr is excessive with respect to the amount of Si, undissolved spherical carbide remains and the workability deteriorates, that is, the notch bending angle decreases.
- Comparative Examples 71 and 72 are cases in which the difference between the Si amount and the Cr amount ([Si%] ⁇ [Cr%]) is larger than the upper limit of the claims, and Si is significantly larger than the Cr amount. Excessive. In these cases, the surface decarburized layer of the rolled material grows large and cannot be removed sufficiently by a small amount of surface shaving. Therefore, it was inferior in fatigue endurance (Nakamura rotary bending fatigue strength).
- Comparative Examples 73 and 74 are obtained by rolling the steels of Invention Examples 1 and 23 at a billet heating temperature of 1100 ° C., respectively. Undissolved spherical carbide remained at the beginning of rolling, and the effect finally remained, so the workability deteriorated, that is, the notch bending angle decreased.
- Invention Examples 101 to 109 are examples of the pre-drawn steel wires of Invention Examples 1 to 5 and 20 to 23.
- the billet heating temperature of Invention Examples 101 and 106 is 1100 ° C. Since the evaluation is based on the pre-drawn steel wire, only the maximum equivalent circle diameter of the undissolved spherical carbide is evaluated. It can be seen that when the billet heating temperature is high, the equivalent circle diameter of the undissolved spherical carbide decreases.
- the present invention can be used for the production of steel wires for high-strength springs.
- High-strength spring materials can be used in many industrial fields including the automobile industry.
Abstract
Description
つまり、従来の高強度ばね用伸線熱処理鋼線よりも、さらに冷間コイリング性に優れ、500℃で1時間保持された後であっても焼戻し軟化抵抗が優れ、内部の軟化を最低限に抑制するとともに、最表層の硬さを高めることが求められている。
以上のことから、本発明は、Vなどの合金元素添加を極力抑え、つまりN含有量を精度よく制御することなく、優れた引張強度及び硬さと加工性を有し、窒化処理後にも優れた表層硬さと内部硬さを有する高強度ばね用伸線熱処理鋼線の開発を課題とするものである。
(a)鋼線中のC、Si、Mn、Crの含有量を厳密に制御して球状炭化物の生成を抑制し、かつ残留オーステナイトを活用することにより、Vのような合金元素を添加せずとも、高強度ばね用伸線熱処理鋼線の強度及び冷間コイリング性が従来よりも向上することを見出した。
つまり、疲労特性の高強度化にはCrの添加が有効であるが、Crは冷間コイリング性に悪影響を及ぼす未溶解の球状炭化物を残りやすくする元素である。そのため、その添加量を制限せざるを得なかった。本発明者らは、未溶解の球状炭化物の成長とセメンタイトの生成を抑制するSiにも着目した。Siの添加と、あわせてCr添加量を増量すれば、伸線熱処理鋼線の高強度化が図れることを見出した。定量的にはSiとCrの両者を多量に添加し、相互の関係としてSiの添加量とCr添加量の差、(Si−Cr)%を制御すればよいことを見出した。
未溶解球状炭化物は、鋳造後の鋼から存在し、コイリング性だけでなく、圧延や伸線においても断線の原因となる。そのため、鋳造後の分塊、線材圧延、パテンティング、焼入れ、伸線などの各工程で悪影響がでないように、各工程の加熱温度を高温化し、未溶解球状炭化物を常に抑制することが有効であることも見出した。
即ち、鋳造後からバネに加工されるまで、鋼材は加熱の繰り返しを受けることになる。通常、未溶解球状炭化物は、セメンタイト(Fe3C)が主体である。しかし、加熱の繰り返しにより未溶解球状炭化物にCrやVなどが含まれることが多く、CrやVなどの合金元素を無駄に消費するだけで、窒化後の機械的特性(表面硬度、内部硬度など)を悪化させる可能性があることもわかった。
また、前記したようにVの添加は窒素(N)含有量の制御が容易ではなく、結果として粗大炭化物や窒化物、炭窒化物を析出し、疲労強度の低下の原因ともなっている。
本発明はこれら知見に基づいてなされたものであり、その発明の要旨は以下のとおりである。
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、
残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下であり、
さらに、
未溶解球状炭化物の円相当径が0.2μm未満であることを特徴とする高強度ばね用伸線前鋼線。
(2) さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は0.05%≦Mo+W≦0.5%
を満たすことを特徴とする(1)に記載の高強度ばね用伸線前鋼線。
(3) 質量%で、
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、
残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下であり、
さらに、
金属組織として、少なくとも残留オーステナイトが体積率で6%超15%以下存在し、
旧オーステナイト粒度番号が10番以上であり、
未溶解球状炭化物の円相当径が0.2μm未満であることを特徴とする高強度ばね用伸線熱処理鋼線。
(4)さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は0.05%≦Mo+W≦0.5%
を満たすことを特徴とする(3)に記載の高強度ばね用伸線熱処理鋼線。
(5)前記高強度ばね用伸線熱処理鋼線の引張強度が2100~2400MPaであることを特徴とする(3)または(4)に記載の高強度ばね用伸線熱処理鋼線。
(6)前記高強度ばね用伸線熱処理鋼線の降伏点が1600~1980MPaであることを特徴とする(3)~(5)のいずれか1項に記載の高強度ばね用伸線熱処理鋼線。
(7)前記高強度ばね用伸線熱処理鋼線を500℃で1時間保持する軟窒化処理することにより、表層ビッカース硬さがHV750以上であり、内部ビッカース硬さがHV570以上となることを特徴とする(3)~(6)のいずれか1項に記載の高強度ばね用伸線熱処理鋼線。
(8)質量%で、
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
P :0.025%以下、
S :0.025%以下、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、
残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下であるブルームを1250℃以上に加熱後熱間圧延を施してビレット製造し、当該ビレットを1200℃以上に加熱後熱間圧延により伸線前鋼線を製造することを特徴とする高強度ばね用伸線前鋼線の製造方法。
(9)さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は0.05%≦Mo+W≦0.5%
を満たすことを特徴とする(8)に記載の高強度ばね用伸線前鋼線の製造方法。
(10)(8)または(9)に記載の伸線前鋼線を、さらに900℃以上に加熱し、その後600℃以下のパテンティング処理することを特徴とする高強度ばね用伸線前鋼線の製造方法。
(11)(8)~(10)のいずれか1項に記載の伸線前鋼線の製造方法により製造した前記伸線前鋼線を伸線加工した後、10℃/秒以上の加熱速度でA3点以上の温度まで加熱し、A3点以上の温度で1分以上5分以下保持した後、50℃/秒以上の冷却速度で100℃以下まで冷却することを特徴とする高強度ばね用熱処理鋼線の製造方法。
(12)さらに400~500℃で15分以下保持し焼戻し処理することを特徴とする(11)に記載の高強度ばね用熱処理鋼線の製造方法。
図2は、試験片にノッチを設けるポンチの形状を示す図である。
図3は、試験片にノッチを設ける工程を示す図である。
図4は、ノッチ曲げ試験の概要を示す図である。
図5は、ノッチ曲げ角度の測定方法を示す図である。
所定の成分を含有した鋼製のブルームを圧延しビレットとする。次にビレットを圧延し、所定の径の鋼線を製造する。この段階で製造された鋼線を「伸線前鋼線」と称する。
圧延後製造された鋼線をパテンティングし、伸線によりさらに細径の鋼線とし、表層の加工歪の除去と後続の冷間コイリング加工性を得るために熱処理(焼入れ焼戻し)を行う。この段階で製造された鋼線を「伸線熱処理鋼線」と称する。
まず、本発明の高強度ばね用伸線熱処理鋼線およびその素材である高強度ばね用伸線前鋼線の成分について説明する。ここで、成分についての%は、特に記載がある場合を除き、質量%を意味する。
Cは、鋼材の強度に大きな影響を及ぼし、残留オーステナイトの生成にも寄与する重要な元素である。本発明では、十分な強度を得られるように、C量の下限値を0.67%以上とする。強度を高めるには、C量を0.70%以上にすることが好ましい。さらに好ましくは0.75%以上とするとよい。
一方、C量が0.9%以上になると、過共析となり、粗大なセメンタイトが多量に析出し、靱性が著しく低下する。また、C量が過剰であると、粗大な球状炭化物が生成し、コイリング性を損なう。したがって、C量の上限を0.9%未満とする。球状炭化物の生成を抑える観点から、C量の上限は0.85%が好ましく、0.80%であればさらに好ましい。
Siは、鋼の焼戻し軟化抵抗及びばねのへたり特性を向上させる重要な元素であり、これらの効果を得るためには2.0%以上添加することが必要である。また、Siは、セメンタイトの球状化及び微細化にも有効であり、粗大な球状炭化物の生成を抑制するために、2.1%以上のSiを添加することが好ましい。窒化処理など、表層を硬化させる処理を行った後、内部硬度を高めるためには、2.2%以上のSiを添加することがさらに好ましい。さらに、CrとのバランスからSiを2.3%以上とすることがより好ましい。Siを3.0%以上とする場合もある。
一方、Siを過剰に添加すると、鋼線が硬化し、脆化するため、Si量の上限を3.5%以下とする。脆化防止の観点から、上限は3.4%とすることが好ましく、さらに好ましくは3.3%以下とするとよい。
Mnは、焼入れ性を高め、残留オーステナイト量を安定的に確保するために重要な元素である。本発明では、鋼線の引張強度を高め、残留オーステナイトを確保するために、Mnを0.5%以上添加することが必要であり、0.65%以上とすることが好ましく、さらには0.70%以上とするとよい。
一方、Mnを過剰に添加すると、残留オーステナイトが増加し、加工時に、加工誘起マルテンサイトが生成し、冷間コイリング性を損なう。過剰なMnの添加による脆化を防止するために、Mn量の上限を1.2%以下とし、1.1%以下にすることが好ましく、さらには1.0%以下とするとよい。
Crは、焼入れ性及び焼戻し軟化抵抗を向上させるために有効な元素であり、これらの効果を得るためには、1.3%以上のCrを添加することが必要である。窒化処理を行う場合には、Crの添加によって窒化による硬化層を深くすることができる。したがって、窒化での硬化と窒化温度での軟化抵抗を付与する場合には、1.5%超のCrを添加することが好ましく、さらには、1.7%以上添加することよい。
一方、Cr量が過剰であると、製造コストが高くなるだけでなく、炭化物の溶解を阻害し、未溶解球状炭化物が多くなりコイリング性を阻害するため、Cr量の上限を2.5%以下とする。また、C量が多い場合は、粗大なセメンタイトの生成を抑制するために、Cr量を2%以下に抑制することが好ましい。更に、強度とコイリング性とを両立させるためには、Cr量の上限を1.8%以下にすることが好ましい。
Nは、本発明では、鋼中に不純物として含まれるAl等と窒化物を形成する元素である。微細な窒化物を利用し、旧オーステナイトを微細化するために、0.003%以上のNを含有させることが必要である。一方、N量が過剰であると、窒化物が粗大化し、冷間コイリング性や疲労特性が低下する。したがって、N量の上限を0.007%以下とする。また、熱処理などの容易性を考慮するとN量は0.005%以下が好ましい。
Pは不純物であり、鋼を硬化させ、偏析を生じ、脆化させるため、P量の上限を0.025%以下に制限する。また、旧オーステナイト粒界に偏析したPは、靭性や耐遅れ破壊特性などを低下させるため、P量の上限を0.015%以下に制限することが好ましい。さらに、鋼線の引張強度が2150MPaを超えるような場合には、P量を0.010%未満に制限することが好ましい。
Sも不純物であり、鋼中に存在すると鋼を脆化させるため、S量の上限を0.025%以下に制限する。Sの影響を抑制するには、Mnの添加が有効である。しかし、MnSは介在物であり、特に高強度鋼では、MnSが破壊の起点になることがある。したがって、破壊の発生を抑制するには、S量の上限を0.015%以下に制限することが好ましい。更に、高強度ばね用伸線熱処理鋼線の引張強度が2150MPaを超えるような場合には、S量を0.01%未満に制限することが好ましい。
Alは脱酸元素であり、酸化物の生成に影響し、硬質の酸化物を生成すると、疲労耐久性が低下する。特に、高強度ばねにおいては、Alを過剰に添加すると、疲労強度がばらついて、安定性を損なう。本発明の高強度ばね用伸線熱処理鋼線では、Al量が0.003%を超えると、介在物に起因する破断発生率が多くなるため、Al量を0.003%以下に制限する。Al量の上限値は、望ましくは0.0028%、より望ましくは0.0025%とするとよい。
Si量が規定量を超えると脆化が激しくなりコイリング時の加工性が損なわれるだけでなく、中間工程における脱炭が激しくなる。そのため、最終製品のばねにおいて表層硬さが低くなり耐久性が低下する。また、脱炭部分がランダムに生じるため、ばね製品の強度の安定性が損なわれる。Si量が規定量よりも少ない場合には強度が低くなり、さらにへたり特性が不十分である。このことは窒化後の硬さにも現れ、表層、内部とも十分な硬さを確保できない。
Vは、窒化物、炭化物、炭窒化物を生成する元素である。円相当径が0.2μm未満である微細なVの窒化物、炭化物、炭窒化物は、旧オーステナイトの微細化に有効である。また、窒化処理による表層の硬化にも利用することができる。しかし、一方で未溶解の炭化物や窒化物が生成し易いため、窒素(N)を抑制したとしても、その析出制御は精度よく行う必要がある。
そのため、本発明においては、Vを積極的には添加しない。
前記したようなV添加の効果を得るために微量添加することはできる。これらの効果を得るためには、Vを0.03%以上添加するとよい。好ましくは0.035%以上であり、0.04%以上であればさらによい。
Nbは鋼中で窒化物、炭化物、炭窒化物を生成する元素であり、それら析出物によってオーステナイト粒径の制御などに用いられる場合がある。しかし同時に過剰な添加は熱間での延性を低下させ、圧延や熱間鍛造で割れを生じやすくする。そのため過剰な添加は避けなければならない。
一方、Nbがばね鋼のN量制御する効果は、0.0005%から表れるので、Nbを添加する場合は、0.0005%以上添加することが好ましい。また、Vを添加する場合などは、微量Nbを添加することさらに効果的であり、その範囲は0.003~0.012%の範囲が好ましく。さらには0.005~0.009%の範囲%が好ましく、0.005~0.001%でもその効果は得られる。
本発明においては、Vを積極的に添加しない。しかし、前記したように微量のV添加は、旧オーステナイトの微細化や残留オーステナイトの生成に影響する。Vに対し、CrとVの添加量の和を精密に制御することで窒化後の表層硬さと内部硬さを高強度ばねに適するように高強度化することができる。
MnとVは焼入れ性を向上させる元素であり、残留オーステナイトの生成に対する影響も大きい。Mnが規定より多い場合には残留オーステナイトが多く残留させる。したがって、Mnと不可避的不純物として混入するVの両者の和がオーステナイト挙動に直接影響し、それらが規定を超えると残留オーステナイト量が多くなり、加工性に影響するだけでなく、降伏点にも大きく影響し、十分な耐久性を確保できない。
Moは、焼入れ性を高める元素であり、また、焼戻し軟化抵抗の向上にも極めて有効である。本発明では、特に、焼戻し軟化抵抗をさらに高めるため、0.05%以上のMoを添加することができる。また、Moは、鋼中でMo系炭化物を生成する元素でもあり、Mo系炭化物が析出する温度は、V等の炭化物に比べると低い。そのため、適量のMoの添加は炭化物の粗大化の抑制にも有効であり、0.10%以上のMoを添加することが好ましい。一方、Moの添加量が0.30%を超えると、熱間圧延や、伸線加工前のパテンティングなどで過冷組織を生じ易くなる。したがって、割れや伸線時の断線の原因となる過冷組織の生成を抑制するため、Mo量の上限を0.30%以下、好ましくは0.25%以下とする。また、Mo量が多いと、パテンティング処理で、パーライト変態終了までの時間が長くなるため、Mo量を0.20%以下、さらにパテンティング時間を短時間化し、パーライト変態を安定して終了させるには0.15%以下にすることが好ましい。
Wは、Moと同様、焼入れ性及び焼戻し軟化抵抗の向上に有効な元素であり、かつ、鋼中で炭化物として析出する元素である。本発明では、特に、焼戻し軟化抵抗を高めるため、0.05%以上のWを添加する。
Mo及びWは、焼戻し軟化抵抗の向上に有効な元素である。両者を複合して添加すると、Mo、Wを単独で添加するよりも、炭化物の成長が抑制され、焼戻し軟化抵抗を著しく高めることができる。特に、500℃に加熱した際の焼戻し軟化抵抗を高めるには、Mo+Wを0.05%以上、好ましくは0.15%以上にすることが必要である。
一方、Mo+Wが0.5%を超えると、熱間圧延や、伸線加工前のパテンティングなどでマルテンサイトやベイナイトなどのいわゆる過冷組織を生じ、割れや伸線時の断線の原因となる過冷組織の生成を抑制するため、Mo+Wの上限を0.5%以下とする。好ましくは0.35%以下である。
Mg:0.002%以下
MgはMnS生成温度よりも高い溶鋼中で酸化物を生成し、MnS生成時には既に溶鋼中に存在している。従ってMnSの析出核として用いることができ、これによりMnSの分布を制御できる。またその個数分布もMg系酸化物は従来鋼に多く見られるSi、Al系酸化物より微細に溶鋼中に分散するため、Mg系酸化物を核としたMnSは鋼中に微細に分散することとなる。従って、同じS含有量であってもMgの有無によってMnS分布が異なり、それらを添加する方がMnS粒径はより微細になる。MnSを微細分散させることで、MnSの疲労等の破壊起点としての作用を無害化することができる。その効果は微量でも十分得られ、好ましくは、Mg0.0002%以上、さらに好ましくは0.0005%以上の添加がよい。
Caは酸化物および硫化物生成元素である。ばね鋼においてはMnSを球状化させることで、疲労等の破壊起点としてのMnSの長さを抑制し、無害化することができる。その効果はMgと似ており、0.0002%以上の添加が好ましい。また0.002%を超えて添加しても歩留まりが悪いばかりか、酸化物やCaSなどの硫化物を生成し、製造上のトラブルやばねの疲労耐久特性を低下させるので0.002%以下とした。この添加量は高強度弁ばねに用いる場合には介在物感受性が高いため、好ましくは0.0015%以下、さらには0.001%以下であることが好ましい。
Zrは酸化物、硫化物および窒化物生成元素である。ばね鋼においては酸化物を微細に分散するため、Mgと同様、MnSの析出核となり、MnSを微細に分散させることができる。それにより疲労耐久性を向上させ、また、延性を増すことでコイリング性を向上させる。0.0002%以上添加することが好ましい。また0.003%を超えて添加しても歩留まりが悪いばかりか、酸化物やZrN、ZrSなどの窒化物、硫化物を生成し、製造上のトラブルやばねの疲労耐久特性を低下させるので0.003%以下とした。この添加量は好ましくは0.0025%以下、さらに高強度弁ばねに用いる場合には硫化物制御によりコイリング性を向上させる効果もあるため、添加することがこのましいが、介在物寸法への影響を最小限にするために0.0015%以下に抑制することが好ましい。
未溶解球状炭化物は、高強度ばね用鋼線においては、強度確保のために重要な役割を果たす。反面、未溶解球状炭化物の存在はコイリング性を悪化させ、また、粗大な炭化物が疲労特性も悪化させる。したがって、コイリング時に未溶解球状炭化物を抑制し、最終窒化処理後には、微細炭化物を均一分散させることが、本発明の課題解決に不可欠である。
この未溶解球状炭化物は、その後の熱処理(パテンティング、伸線時の加工発熱、焼入れ焼き戻し工程)では固溶されにくい。むしろ、それらの熱処理工程において成長し、粗大化する場合もある。すなわち伸線前鋼線中の未溶解球状炭化物は自身の粗大化の核となる場合がある。
そのため、熱処理後の鋼線(熱処理鋼線)の粗大化した未溶解球状炭化物を制限するためには、伸線前鋼線中に存在する未溶解球状炭化物を極力小さいくしておくことが重要である。以上のことから、未溶解球状炭化物の規定は、本発明に係る高強度ばね用伸線前鋼線だけでなく高強度ばね用伸線熱処理鋼線においても重要な意味を持つ。
本発明では、未溶解球状炭化物が、高強度ばね用伸線熱処理鋼線の特性に影響を及ぼすため、サイズを以下のように制御する。なお、本発明では、従来技術に比べて、更に微細な球状炭化物について規定し、より高い性能と加工性の両立を図っている。円相当径で0.2μm未満の球状炭化物は、鋼の強度、焼戻し軟化抵抗を確保するために極めて有効である。
本発明の伸線前鋼線および伸線熱処理鋼線は、未溶解球状炭化物の円相当径が0.2μm未満となることが特徴である。そのため、強度を確保しつつ、加工性も確保できるのである。
前記したように、伸線前鋼線は、その後パテンティングや伸線加熱、焼入れ焼戻し等の熱処理をする必要があるため、未溶解球状炭化物が成長し、粗大化する可能性がある。そのため伸線前鋼線における未溶解球状炭化物の円相当径は、0.2μmよりさらに小さいことが望ましい。
発明者らの実験結果から、伸線前鋼線の未溶解炭化物の円相当径は、0.18μm以下にできることが確認されている。また、ビレット加熱温度を1250℃以上とすれば、0.15μm以下とできることも確認されている。
本発明に係る高強度ばね用伸線熱処理鋼線の金属組織は、体積率で6%超、15%以下の残留オーステナイトと、残部焼戻しマルテンサイトとからなり、微量の介在物を許容する。微量介在物とは、酸化物、硫化物であり、酸化物はAlやSiなどの脱酸生成物、硫化物はMnSやCaSなどが該当する。また、残部焼戻しマルテンサイト組織には、未溶解球状炭化物も微量ながら含まれる。
また、本発明に係る高強度ばね用伸線前鋼線の金属組織は、パーライト組織が90%以上を占める。望ましくは、95%以上であり、さらに望ましくは98%以上、ほぼ100%パーライト組織となることが理想的である。
本発明の高強度ばね用伸線熱処理鋼線は、焼戻しマルテンサイトを主要な組織としており、旧オーステナイト粒度が特性に大きな影響を及ぼす。即ち、旧オーステナイトの粒径を微細にすると、細粒化の効果により、疲労特性やコイリング性が向上する。本発明では、十分な疲労特性やコイリング性を得るため、旧オーステナイト粒度番号を10番以上とする。
焼入れ焼戻し後の高強度ばね用伸線熱処理鋼線でのミクロ組織は、焼き戻しマルテンサイトと残留オーステナイトおよびわずかな体積分率の介在物(ここでは、析出物も介在物に含めて表現する)とからなっている。残留オーステナイトは、冷間コイリング性の向上に有効である。本発明では冷間コイリング性を確保するために、残留オーステナイトの体積率を6%超とする。好ましくは7%以上であり、さらに好ましくは8%以上であるとよい。
残留オーステナイトの体積率は、X線回折法や、磁気測定法によって求めることができる。このうち、磁気測定法は、簡便に残留オーステナイトの体積率を測定できるため、好ましい測定方法である。ここでは、体積率を測定しているが、得られた数値は面積率と同じ値となる。
ばねの小型化や軽量化を図るためには、高強度化が有効であり、また、ばねには優れた疲労強度が要求される。本発明では、高強度ばねは、素材である高強度ばね用伸線熱処理鋼線を曲げ加工して所望の形状とし、窒化処理、ショットピーニングなど、表面を硬化させる処理を施して製造される。窒化処理では、500℃程度に加熱されるため、ばねは、素材である高強度ばね用伸線熱処理鋼線よりも軟化することがある。
高強度ばね用伸線熱処理鋼線の引張強度が高ければ、窒化処理などの表面を硬化する処理を施したばねの疲労特性及びへたり特性を高めることができる。本発明では、ばねの疲労特性及びへたり特性を高めるために、高強度ばね用伸線熱処理鋼線の引張強度を2100MPa以上とする。
また、高強度ばね用伸線熱処理鋼線の引張強度が高いほど、ばねの疲労特性が向上するため、高強度ばね用伸線熱処理鋼線の引張強度を好ましくは2200MPa以上、更に好ましくは2250MPa以上とする。
本発明において、高強度ばね用伸線熱処理鋼線の降伏強度または降伏点とは、単軸引張試験において、応力−歪み曲線に、降伏点が見られる場合は上降伏点、降伏点が見られない場合は0.2%耐力とする。繰り返し応力によって弾性変形するばねの強度や耐へたり性を確保するためには、降伏点を高めることが好ましい。ばねの降伏点を高めるには、素材である高強度ばね用伸線熱処理鋼線の降伏点を高めることが好ましい。
一方、降伏点が1980MPaを超えると、冷間コイリング性を損なうことがあるため、降伏点を1980MPa以下とすることが好ましい。なお短時間の焼入れ焼戻し直後に同一の引張強度を有する素材の高強度ばね用伸線熱処理鋼線の降伏点を高めるには、残留オーステナイトの体積率を低下させることが好ましい。
高強度ばねは、窒化処理の際に、表層硬さは向上するが、内部は軟化する。例えば500℃でのガス軟窒化では、従来加熱温度が500℃になると、高強度ばね用伸線熱処理鋼線内部の軟化を抑制することが困難であった。本発明の高強度ばね用伸線熱処理鋼線は、焼戻し軟化抵抗に優れており、500℃で加熱した後のばねの疲労特性及びへたり性を確保することができる。本発明では、ガス軟窒化後の表層硬さと内部硬さを規定する。
なお、500℃で1時間保持する加熱処理後のビッカース硬さの上限は、特に規定しないが、加熱処理前のビッカース硬さを超えることはないため、通常、783以下である。
しかし、成分組成、球状炭化物、旧オーステナイト結晶粒度は、冷間コイリング及び窒化処理による影響は小さいと考えられる。したがって、本発明の高強度ばね用伸線熱処理鋼線を素材とする高強度ばねの成分組成、球状炭化物、旧オーステナイト結晶粒度は、本発明の高強度ばね用伸線熱処理鋼線の成分組成、球状炭化物、旧オーステナイト結晶粒度と同程度である。
所定の成分に調整された鋼片(ブルーム)を圧延し、サイズダウンした鋼片(ビレット)製造する。そしてビレットを加熱後、熱間圧延し、高強度ばね用伸線前鋼線とする。この高強度ばね用伸線前鋼線をパテンティング処理後、シェービングを施し、さらに、硬化層を軟質化するための焼鈍を行い、伸線加工して、焼入れ及び焼戻しを施して、高強度ばね用伸線熱処理鋼線を製造する。パテンティング処理は、熱間圧延後の鋼線の組織をフェライト・パーライトとする熱処理であり、伸線加工前に鋼線を軟化させるために行う。伸線加工後、オイルテンパー処理や高周波処理などの焼入れ及び焼戻しを施し、鋼線の組織及び特性を調整する。
加熱炉から抽出した後は、温度が低下して析出物が成長する。そのため、加熱炉から抽出した後、5分以内に熱間圧延を完了させることが好ましい。ブルーム、ビレットの上記加熱により、鋼中の粗大炭化物が、均一に拡散、固溶され、その後の析出に際し、均一に微細析出することができる。
なお、ブルームからビレットを経ずに鋼線に圧延する場合は、ブルーム(鋼片)の圧延前加熱温度は1250℃以上、このましくは1270℃以上とするとよい。
従って、伸線前の加熱工程である圧延工程において、ブルーム加熱温度やビレット加熱温度を、炭化物が固溶するために十分高くしておくことが重要である。それによって未溶解球状炭化物の径は小さく抑制することができる。ばね鋼の圧延はビレットが加熱炉から抽出されてからφ10mm程度の伸線前素材径まで数分で完了する。このため、ビレット加熱温度の影響が最も大きく、1200℃以上に加熱することが重要である。1250℃以上であればより好ましく、1270℃以上であればさらに好ましい。
以上の工程により、ばね用伸線前鋼線(圧延線材)が得られる。
要求される線材径や精度によって伸線工程が省略される場合、伸線工程に先立つパテンティング工程も省略されることがある。その場合は、以下に述べる焼入れの加熱温度を高温化し(例えば、970℃以上)によって、未溶解球状炭化物の固溶を促進する。
合格とする目標数値は、従来の高強度ばね用鋼線を参考として、次のとおりとした。
旧オーステナイト粒度番号:10度以上
残留オーステナイト量(体積%):20%以下
球状炭化物の円相当径:0.2μm以下
引張強度:2100MPa以上
0.2%耐力:1800MPa以上
降伏比:75%以上95%以下
ノッチ曲げ角度:28度以上
平均疲労強度(中村式回転曲げ強度):900MPa以上、
ガス軟窒化後のビッカース硬さで内部硬さ:590Hv以上
ガス軟窒化後のビッカース硬さで窒化層硬さ:750Hv以上
なお、本発明による鋼線においては、強度と加工性(コイリング性)を両立させる必要があるため、降伏比が高すぎると加工性が悪化する。従って、降伏比の上限は好ましくは90%、さらに好ましくは88%以下とするとよい。
伸線前鋼線(線材圧延後の鋼線)の評価は、未溶解球状炭化物の円相当径のみを行った。熱処理前であるので、機械特性やオーステナイト粒度等の測定をしてもあまり意味がないからである。
伸線前鋼線での評価であるので、未溶解球状炭化物の最大円相当径のみで評価している。ビレット加熱温度が高いと、未溶解球状炭化物の円相当径が小さくなることがわかる。
2 ポンチ
3 試験片
4 ノッチ
5 押金具
6 負荷用治具
P 荷重
L 支え間の距離
θ ノッチ曲げ角度
Claims (14)
- 質量%で、
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下であり、
さらに、未溶解球状炭化物の円相当径が0.2μm未満であることを特徴とする高強度ばね用伸線前鋼線。 - さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は
0.05%≦Mo+W≦0.5%
を満たすことを特徴とする請求項1に記載の高強度ばね用伸線前鋼線。 - 質量%で、
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下であり、
さらに、金属組織として、少なくとも残留オーステナイトが体積率で6%超15%以下存在し、
旧オーステナイト粒度番号が10番以上であり、
未溶解球状炭化物の円相当径が0.2μm未満であることを特徴とする高強度ばね用伸線熱処理鋼線。 - さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は
0.05%≦Mo+W≦0.5%
を満たすことを特徴とする請求項3に記載の高強度ばね用伸線熱処理鋼線。 - 前記高強度ばね用伸線熱処理鋼線の引張強度が2100~2400MPaであることを特徴とする請求項3または4に記載の高強度ばね用伸線熱処理鋼線。
- 前記高強度ばね用伸線熱処理鋼線の降伏点が1600~1980MPaであることを特徴とする請求項5に記載の高強度ばね用伸線熱処理鋼線。
- 前記高強度ばね用伸線熱処理鋼線を500℃で1時間保持する軟窒化処理することにより、表層ビッカース硬さがHV750以上であり、内部ビッカース硬さがHV570以上となることを特徴とする請求項3または4に記載の高強度ばね用伸線熱処理鋼線。
- 質量%で、
C :0.67%以上、0.9%未満、
Si:2.0~3.5%、
Mn:0.5~1.2%、
Cr:1.3~2.5%、
N :0.003~0.007%、
Al:0.0005%~0.003%
を含有し、かつSiとCrが次式
0.3%≦Si−Cr≦1.2%
を満たし、残部が鉄及び不可避的不純物からなり、
不純物としてのP、Sが
P :0.025%以下、
S :0.025%以下
であるブルームを1250℃以上に加熱後熱間圧延を施してビレット製造し、当該ビレットを1200℃以上に加熱後熱間圧延により伸線前鋼線を製造することを特徴とする高強度ばね用伸線前鋼線の製造方法。 - さらに、質量%で、
V :0.03~0.10%、
Nb:0.015%以下
Mo:0.05~0.30%、
W :0.05~0.30%
Mg:0.002%以下、
Ca:0.002%以下、
Zr:0.003%以下
のうち1種または2種以上を含有し、
Vを含有する場合は
1.4%≦Cr+V≦2.6%、および0.70%≦Mn+V≦1.3%を満たし、
MoとWを含有する場合は
0.05%≦Mo+W≦0.5%
を満たすことを特徴とする請求項8に記載の高強度ばね用伸線前鋼線の製造方法。 - 請求項8または9に記載の伸線前鋼線を、さらに900℃以上に加熱し、その後600℃以下のパテンティング処理することを特徴とする高強度ばね用伸線前鋼線の製造方法。
- 請求項8または9に記載の伸線前鋼線を、伸線加工した後、10℃/秒以上の加熱速度でA3点以上の温度まで加熱し、A3点以上の温度で1分以上5分以下保持した後、50℃/秒以上の冷却速度で100℃以下まで冷却することを特徴とする高強度ばね用熱処理鋼線の製造方法。
- 請求項10に記載の伸線前鋼線を、伸線加工した後、10℃/秒以上の加熱速度でA3点以上の温度まで加熱し、A3点以上の温度で1分以上5分以下保持した後、50℃/秒以上の冷却速度で100℃以下まで冷却することを特徴とする高強度ばね用熱処理鋼線の製造方法。
- さらに400~500℃で15分以下保持し焼戻し処理することを特徴とする請求項11に記載の高強度ばね用熱処理鋼線の製造方法。
- さらに400~500℃で15分以下保持し焼戻し処理することを特徴とする請求項12に記載の高強度ばね用熱処理鋼線の製造方法。
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JP2011551341A JP4980496B2 (ja) | 2010-07-06 | 2011-07-05 | 高強度ばね用伸線熱処理鋼線および高強度ばね用伸線前鋼線 |
SE1250810A SE537538C2 (sv) | 2010-07-06 | 2011-07-05 | Dragen värmebehandlad ståltråd för höghållfasthetsfjäderanvändning, fördragen ståltråd för höghållfasthetsfjäderanvändning samt förfaranden för framställning av dessa trådar |
CN201180003762.1A CN102482747B (zh) | 2010-07-06 | 2011-07-05 | 高强度弹簧用拉伸热处理钢线及高强度弹簧用拉伸前钢线 |
US13/574,175 US20120291927A1 (en) | 2010-07-06 | 2011-07-05 | Drawn heat treated steel wire for high strength spring use and pre-drawn steel wire for high strength spring use |
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Cited By (3)
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WO2013024876A1 (ja) * | 2011-08-18 | 2013-02-21 | 新日鐵住金株式会社 | ばね鋼およびばね |
JP2017530258A (ja) * | 2014-09-04 | 2017-10-12 | ティッセンクルップ フェダーン ウント スタビリサトーレン ゲゼルシャフト ミット ベシュレンクテル ハフツング | 冷間成形鋼ばねを製造するための方法 |
WO2018211779A1 (ja) * | 2017-05-19 | 2018-11-22 | 住友電気工業株式会社 | オイルテンパー線 |
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JP2016014169A (ja) * | 2014-07-01 | 2016-01-28 | 株式会社神戸製鋼所 | 鋼線用線材および鋼線 |
JP6453138B2 (ja) * | 2015-03-31 | 2019-01-16 | 株式会社神戸製鋼所 | 曲げ加工性に優れた熱処理鋼線 |
WO2019010661A1 (zh) * | 2017-07-13 | 2019-01-17 | 田圣林 | 一种高韧性高强度耐腐蚀弹簧 |
WO2019186928A1 (ja) * | 2018-03-29 | 2019-10-03 | 日本製鉄株式会社 | ホットスタンプ成形体 |
WO2020173647A1 (en) * | 2019-02-26 | 2020-09-03 | Nv Bekaert Sa | Helical compression spring for an actuator for opening and closing a door or a tailgate of a car |
WO2020233872A1 (en) * | 2019-05-20 | 2020-11-26 | Nv Bekaert Sa | Method of making a spring core for a mattress or for seating products |
JP7388360B2 (ja) * | 2019-07-01 | 2023-11-29 | 住友電気工業株式会社 | 鋼線およびばね |
JP7321353B2 (ja) * | 2020-02-21 | 2023-08-04 | 日本製鉄株式会社 | 鋼線 |
CN112427484B (zh) * | 2020-11-11 | 2022-07-26 | 南京工程学院 | 一种再结晶退火调控不锈弹簧钢线成形制造方法 |
KR102492641B1 (ko) * | 2020-12-17 | 2023-01-30 | 주식회사 포스코 | 내피로특성과 질화처리 특성이 향상된 스프링용 선재, 강선, 스프링 및 그 제조 방법 |
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KR20220163153A (ko) * | 2021-06-02 | 2022-12-09 | 주식회사 포스코 | 강도 및 피로한도가 향상된 스프링용 선재, 강선, 스프링 및 그 제조방법 |
SE545660C2 (en) * | 2021-10-28 | 2023-11-28 | Suzuki Garphyttan Ab | Flat wire and method for production thereof |
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SE1250810A1 (sv) | 2013-03-20 |
JPWO2012005373A1 (ja) | 2013-09-05 |
SE537538C2 (sv) | 2015-06-09 |
CN102482747B (zh) | 2014-03-05 |
US20120291927A1 (en) | 2012-11-22 |
JP4980496B2 (ja) | 2012-07-18 |
CN102482747A (zh) | 2012-05-30 |
KR20120040728A (ko) | 2012-04-27 |
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