WO2012132821A1 - 伸線加工性および伸線後の疲労特性に優れたばね用鋼線材、ならびに疲労特性およびばね加工性に優れたばね用鋼線 - Google Patents
伸線加工性および伸線後の疲労特性に優れたばね用鋼線材、ならびに疲労特性およびばね加工性に優れたばね用鋼線 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- 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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- 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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- 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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16F—SPRINGS; SHOCK-ABSORBERS; MEANS FOR DAMPING VIBRATION
- F16F1/00—Springs
- F16F1/02—Springs made of steel or other material having low internal friction; Wound, torsion, leaf, cup, ring or the like springs, the material of the spring not being relevant
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a steel wire for springs excellent in wire drawing workability and fatigue properties after wire drawing, and a spring steel wire excellent in fatigue properties and spring workability.
- the present invention relates to a spring (processed spring) used by performing cold drawing.
- valve springs With the reduction in weight and stress of automobiles and the like, higher strength is also being sought for valve springs, clutch springs and suspension springs used in engines, clutches and suspensions. As a result, the stress applied to the spring increases, so a spring having excellent fatigue strength is required.
- valve springs and suspension springs are generally obtained by drawing a hot-rolled wire (steel wire) to produce a steel wire and quenching and tempering the steel wire.
- a tempered wire (OT wire) is manufactured by spring winding at room temperature. Since the structure of the OT wire is mainly composed of a tempered martensite structure, there is an advantage that high strength is easily secured and fatigue strength and sag resistance are excellent. However, there is a drawback in that large-scale equipment and processing costs are required for heat treatment such as quenching and tempering.
- a steel wire that has been drawn and not subjected to heat treatment (hard drawn wire) is used, and cold winding is applied to a spring (hereinafter referred to as a steel wire that has been drawn and is not subjected to heat treatment thereafter (in some cases, a spring obtained by using a hard-drawn wire) is sometimes referred to as a “hard-drawn spring”.
- a piano wire type V is defined particularly for valve springs or similar springs.
- the above-mentioned hard spring has the advantage that the manufacturing cost is low because it does not require heat treatment.
- the hard-drawn spring and the hard-drawn wire used for manufacturing the spring have a problem that it is difficult to ensure the fatigue characteristics at the OT-line level.
- Patent Document 1 discloses an improvement in the fatigue characteristics of a hard tension spring.
- the pearlite fraction is defined in relation to the carbon content, and further, the pearlite nodule size is refined by making V an essential element, and the tensile strength is 1890 MPa or more at a wire diameter of 3.5 mm. Both high strength and sag resistance are achieved.
- V is added as an essential element, the hardenability is likely to increase, so that it is necessary to reduce the wire speed in order to obtain a pearlite structure by the patenting process before wire drawing. It seems to cause an increase in.
- Patent Document 2 the maximum thickness of the TiN-based inclusion is specified in order to improve the fatigue characteristics of the spring wire. However, if the fatigue properties after wire drawing are further increased, the fatigue strength may decrease due to fatigue breakage starting from TiN-based inclusions. Seem.
- the present invention has been made to solve the above-described problems in the prior art, and its purpose is to provide a steel wire for a spring excellent in wire drawing workability and fatigue properties after wire drawing, as well as fatigue properties and spring workability.
- the object is to provide an excellent spring steel wire.
- the steel wire for springs of the present invention that has solved the above problems is C: 0.50% or more and less than 0.70% (% means “mass%”, the same applies to the chemical composition) Si: 1.0-2.5%, Mn: 0.50 to 1.50%, Cr: 0.5% or less (including 0%), B: 0.0010 to 0.0050%, N: 0.0050% or less (excluding 0%), P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Al: 0.03% or less (not including 0%), and O: 0.0020% or less (not including 0%)
- the balance consists of iron and inevitable impurities, It is characterized in that the area ratio of the pearlite structure in the entire structure is 85% or more.
- the steel wire for spring there are further 100 or less (including 0) BN compounds having an equivalent circle diameter of 100 nm or more and less than 1000 nm present in a pearlite structure in an observation visual field area of 2000 ⁇ m 2 , and a circle. It is preferable that the BN compound having an equivalent diameter of 1000 nm or more satisfies 10 or less (including 0).
- the spring steel wire material further includes one or more elements selected from the group consisting of V: 0.05 to 0.40% and Ni: 0.05 to 0.80%, It is preferable that N satisfies 0.0020% or less (including 0%).
- the present invention provides a spring steel wire obtained using the spring steel wire material, wherein the difference between the Vickers hardness (Hvs) of the surface layer portion and the internal Vickers hardness (Hvh) is expressed by the following formula (1): Also included are spring steel wires having features where (Hvs ⁇ Hvh) ⁇ 100 (1) [However, the above Hvs refers to the maximum Vickers hardness from 16 parts to D / 4 parts, D (refers to the diameter of the steel wire), and the above Hvh ranges from D / 4 parts to D / 2. The minimum Vickers hardness. ]
- the “equivalent circle diameter” means a diameter when the area of the BN compound is converted into a circle having the same area.
- the “BN compound” targeted in the present invention includes not only those composed of BN (boron nitride) but also those containing BN (mainly BN) and complex inclusions such as BN compounds having MnS as a nucleus. It is acceptable.
- the chemical composition is appropriately adjusted and the area ratio of the pearlite structure is adjusted, so that the steel wire for springs having excellent wire drawing workability and fatigue properties after drawing, and the fatigue properties and spring processing Spring steel wire with excellent properties can be realized. Furthermore, the fatigue characteristics can be further enhanced by controlling the form of the BN compound.
- the steel wire for spring of the present invention is a steel wire for cold winding that is subjected to quenching and tempering after drawing, a steel wire for cold winding that is used as it is, and a quenching and tempering treatment after spring processing. It can be used as a steel wire for springs to be applied. Such a spring steel wire is useful as a material for valve springs, clutch springs and suspension springs used in engines, clutches and suspensions.
- FIG. 1 is a structure observation photograph in the example.
- FIG. 2 is a diagram showing the relationship between ⁇ HV and the defective product rate (the number of defective products per 100) during spring processing.
- FIG. 3 is a diagram showing the relationship between the tensile strength and fatigue strength of the OT wire.
- FIG. 4 is a graph showing the relationship between the tensile strength of the hard drawn wire and the fatigue life of the hard drawn spring.
- the present inventors have studied from various angles in order to obtain a spring steel wire excellent in wire drawing workability and fatigue properties after wire drawing, and a spring steel wire excellent in fatigue properties and spring workability.
- A In order to improve the wire drawing workability of the spring steel wire rod, (A-1) The area ratio of the pearlite structure in the entire structure is 85% or more, (A-2) It is preferable to satisfy the component composition described later; (A-3) In order to further improve the wire drawing workability of the spring steel wire rod, it is preferable to reduce the amount of solute N in the steel to 0.0020% or less;
- the component composition described later is particularly satisfied.
- the final purpose is to increase the fatigue characteristics of the spring as the fatigue characteristics, but in order to achieve this, the steel wire used for manufacturing the spring must also have excellent fatigue characteristics. From such a viewpoint, the fatigue characteristics of spring steel wires (hard drawn wires or OT wires obtained by applying heat treatment thereto) are improved.
- the fatigue characteristics of a spring and the fatigue characteristics of a steel wire may be simply referred to as “fatigue characteristics”.
- the area ratio of the pearlite structure in the entire structure of the steel wire rod for spring is set to 85% or more.
- proeutectoid ferrite and bainite are formed as the structure of the spring steel wire rod.
- disconnection may occur during wire drawing (that is, the wire drawing workability is inferior).
- a defect tends to occur inside, and the fatigue characteristics are deteriorated due to the internal defect.
- the area ratio of the pearlite structure is increased, ferrite and the like are relatively suppressed, and the structure is made uniform, thereby reducing disconnection and internal defects, and improving wire drawing workability and fatigue characteristics.
- the area ratio of the pearlite structure is preferably 87% or more, more preferably 90% or more, still more preferably 91% or more, and particularly 93% or more.
- Examples of the structure other than the pearlite structure include the above-described pro-eutectoid ferrite and bainite, but the smaller the number, the better.
- the total is preferably 10% or less, more preferably 5% or less, and particularly 0. %.
- a BN compound having an equivalent circle diameter of 100 nm or more it is preferable to suppress a BN compound having an equivalent circle diameter of 100 nm or more from the viewpoint of further improving fatigue characteristics.
- no BN compound having an equivalent circle diameter of 100 nm or more is present, as described below, the influence can be suppressed to a minimum by limiting it within the specified range of the present invention.
- the present invention targets the BN compound of the above size existing in the pearlite structure.
- the number of BN compounds having an equivalent circle diameter of 100 nm or more and less than 1000 nm present in a pearlite structure with an observation visual field area of 2000 ⁇ m 2 is 100 or less (preferably 70 or less, more preferably 50 or less). By controlling, fatigue characteristics can be improved.
- the number of the coarse BN compounds present in the pearlite structure with an observation visual field area of 2000 ⁇ m 2 is suppressed to 10 or less (preferably 8 or less, more preferably 5 or less).
- a BN compound having an equivalent circle diameter of less than 100 nm in the pearlite structure has almost no adverse effect on fatigue properties, and therefore is not controlled in the present invention.
- C 0.50% or more and less than 0.70%
- C is an element useful for increasing the tensile strength of the wire drawing material and ensuring fatigue characteristics and sag resistance. It is also useful for securing the necessary tensile strength as a high stress spring. Furthermore, it is necessary for securing a desired structure, and it is also necessary for suppressing variation in fatigue life by suppressing an increase in the amount of pro-eutectoid ferrite. Therefore, the C amount is 0.50% or more. Preferably it is 0.52% or more, More preferably, it is 0.55% or more, More preferably, it is 0.60% or more.
- the C content is less than 0.70%.
- the C content is 0.68% or less, More preferably, it is 0.65% or less.
- Si 1.0-2.5%
- Si is an element that contributes to improving strength as a solid solution strengthening element and contributes to improving fatigue characteristics and sag resistance.
- heat processing is performed as distortion removal annealing after coiling in a spring processing process, it is also an element effective in exhibiting the outstanding softening resistance in that case.
- the lower limit of the Si amount is set to 1.0%.
- the Si content exceeds 2.5%, the surface decarburization increases and the fatigue characteristics may deteriorate.
- the upper limit of Si content is set to 2.5%.
- it is 2.3% or less, More preferably, it is 2.0% or less.
- Mn is an element that improves the fatigue characteristics by densifying and orderly the pearlite structure. In order to exert such an effect, 0.50% or more of Mn is contained. Preferably it is 0.70% or more, More preferably, it is 0.80% or more. On the other hand, Mn is an element that is easily segregated. When the content is excessive, segregation occurs, and micromartensite is generated in the segregated portion, and the wire drawing workability is likely to be lowered. In addition, fatigue characteristics are likely to deteriorate. Therefore, the upper limit of the amount of Mn is made 1.50%. Preferably it is 1.30% or less, More preferably, it is 1.20% or less.
- Cr 0.5% or less (including 0%)
- Cr is an element effective for narrowing the pearlite lamella spacing, increasing the strength after hot rolling or after patenting, which is heat treatment before wire drawing, and improving sag resistance and fatigue strength. From such a viewpoint, it is preferable to contain 0.05% or more of Cr, more preferably 0.10% or more, still more preferably 0.15% or more, and particularly preferably 0.20% or more.
- the amount of Cr becomes excessive, cementite is strengthened too much and toughness and ductility deteriorate. In addition, wire drawing workability and fatigue characteristics are also deteriorated.
- the upper limit of Cr content is 0.5%.
- it is 0.40% or less, More preferably, it is 0.35% or less, More preferably, it is 0.30% or less.
- B is an element effective for improving the wire drawing workability and fatigue characteristics of the wire by finely depositing solid solution N as a BN compound. It is also effective in improving spring workability. Furthermore, since a part of B exists in the steel as a solid solution B, it is effective for suppressing the formation of pro-eutectoid ferrite. In order to sufficiently exhibit these effects, the B amount needs to be 0.0010% or more. Preferably it is 0.0020% or more, More preferably, it is 0.0030 or more. On the other hand, if the amount of B exceeds 0.0050%, a coarse boron compound such as Fe 23 (CB) 6 may be generated and the fatigue strength may be deteriorated. Therefore, in the present invention, the upper limit of the B amount is set to 0.0050%. More preferably, it is 0.0040% or less.
- Ti and the like as elements for fixing solute N, but in the case of Ti, TiN obtained by fixing N has an angular shape as compared with BN, so it is a starting point for fatigue characteristics. It seems likely.
- N 0.0050% or less (excluding 0%)
- Solid solution N causes aging embrittlement during wire drawing, and easily causes disconnection and vertical cracking during wire drawing. That is, the wire drawing workability after rolling and the wire drawing workability after patenting tend to be reduced. In addition, when aging embrittlement progresses, the difference in hardness between the surface layer and the inside of the spring steel wire increases, and the spring workability of the spring steel wire tends to decrease. Therefore, it is preferable to suppress the solid solution N content to 0.0020% or less by precipitating the solid solution N as BN so that the contents of B and N in the steel satisfy the following formula (2). .
- the amount of solute N is more preferably 0.0015% or less, still more preferably 0.0010% or less, and still more preferably 0.0005% or less.
- P 0.020% or less (excluding 0%)
- P an inevitable impurity
- the amount is an element that induces disconnection during wire drawing.
- it since it is also an element which segregates in the prior austenite grain boundary, embrittles the grain boundary, and lowers fatigue characteristics, it is preferable that the amount be as small as possible. Therefore, in the present invention, it was made 0.020% or less. Preferably it is 0.016% or less, More preferably, it is 0.013% or less.
- S 0.020% or less (excluding 0%)
- S which is an unavoidable impurity, is an element that induces disconnection during wire drawing as in the case of P described above. Moreover, since it is also an element which segregates in the prior austenite grain boundary, embrittles the grain boundary, and lowers fatigue characteristics, it is preferable that the amount be as small as possible. Therefore, in the present invention, it was made 0.020% or less. Preferably it is 0.015% or less, More preferably, it is 0.010% or less.
- Al 0.03% or less (not including 0%)
- Al is contained as a deoxidizer at the time of steel making, but if contained excessively, coarse non-metallic inclusions are generated and fatigue strength is deteriorated, so the content is suppressed to 0.03% or less.
- it is 0.010% or less, More preferably, it is 0.005% or less.
- O is an element that, when contained in excess, forms coarse non-metallic inclusions and degrades fatigue strength. Therefore, in this invention, O content shall be 0.0020% or less. Preferably it is 0.0015% or less, More preferably, it is 0.0010% or less.
- the basic components of the steel of the present invention are as described above, and the balance is iron and inevitable impurities (impurities other than the above P, S, N, and O), but the status of raw materials, materials, manufacturing equipment, etc. as the inevitable impurities Incorporation of elements introduced by can be allowed.
- the steel wire or steel wire of the present invention can be further enhanced in characteristics by adding an appropriate amount of V or Ni as shown below, if necessary.
- V or Ni as shown below
- V and Ni are elements that are particularly effective for improving the wire drawing workability.
- V is an element useful for improving the toughness and sag resistance of the spring as well as improving the wire drawing workability by reducing the pearlite nodule size.
- it is preferable to contain 0.05% or more of V. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more.
- V exceeds 0.40% and is excessively contained, the hardenability increases and martensite and bainite are generated after hot rolling, and the wire drawing workability is deteriorated. In addition, fatigue properties are also reduced.
- V amount it is preferable to make V amount into 0.40% or less, More preferably, it is 0.30% or less.
- Ni is an element effective for improving the ductility of cementite and improving the wire drawing workability. It is also an element that improves the ductility of the steel wire itself. Furthermore, it has the effect of suppressing decarburization of the surface layer during hot rolling. In order to exert these effects, it is preferable to contain 0.05% or more of Ni. More preferably, it is 0.20% or more. However, when the amount of Ni becomes excessive, hardenability increases, and many structures such as martensite and bainite precipitate after hot rolling, leading to deterioration of wire drawing workability. In addition, fatigue properties are also reduced. Therefore, the upper limit of Ni content is preferably 0.80%. More preferably, it is 0.60% or less, More preferably, it is 0.50% or less.
- Hvs ⁇ Hvh 100 (1)
- the above Hvs refers to the maximum Vickers hardness from D (steel wire diameter) / 16 part to D / 4 part
- the above Hvh refers to the minimum Vickers hardness from D / 4 part to D / 2.
- this formula (1) it is necessary to suppress heat generation during wire drawing.
- this can be achieved by setting the wire drawing speed during wire drawing to 300 m / min or less. it can. It is also effective to set the B amount and N amount within the specified ranges and reduce the solid solution N amount, and to set the Cr amount and V amount within the specified ranges.
- the production conditions for obtaining the spring steel wire or spring steel wire of the present invention are not particularly limited as long as the specified structure and the form of the BN compound can be controlled, but the slab having the above chemical composition composition
- the coiling temperature after hot rolling and the cooling conditions after hot rolling should be controlled as follows in the production process, and the recommended BN system
- the heating temperature before the block rolling is preferably 1290 ° C or higher.
- heating at 1280 ° C. or higher is set to 30 minutes or longer. From the viewpoint of operation cost, heating at 1280 ° C. or higher is 90 minutes or shorter.
- the cooling rate in the temperature range of 1280 to 1100 ° C. before the start of the block rolling is preferably 1.0 ° C./second or more, more preferably 1.3 ° C./second or more, further preferably 1.5 ° C. / Second or more.
- the coiling temperature after hot rolling is preferably 850 ° C. or higher so that the load on the rolling mill does not become excessive. Preferably it is 880 degreeC or more.
- the coiling temperature after hot rolling is preferably 950 ° C. or less.
- the coiling temperature after hot rolling should be 950 ° C. or lower.
- the coiling temperature after hot rolling is preferably 940 ° C. or lower, more preferably 930 ° C. or lower.
- the cooling rate from after coiling after hot rolling to 600 ° C. is 10 ° C./second or more (preferably 15 ° C./second or more, more preferably 20 ° C. It is preferable to suppress the formation of pro-eutectoid ferrite as not less than 35 seconds / second and to suppress the formation of martensite and bainite structure as 35 ° C./second or less (preferably 30 ° C./second or less).
- the steel wire for spring specified in the present invention is used, Since it is necessary to suppress heat generation during processing, it is recommended that the wire drawing speed be 300 m / min or less (preferably 250 m / min or less). The lower limit of the drawing speed is preferably about 100 m / min from the viewpoint of productivity and the like.
- conditions other than the above are not particularly limited, and general production conditions may be employed.
- the steel having the chemical composition shown in Table 1 below was steeled in the converter, then subjected to secondary refining treatment and melted to produce a slab casted by a continuous casting method. Then, after the heating of the obtained slab before splitting (the heating time is constant for all steel types, the temperature is raised to the set temperature and held for 30 minutes) under the conditions shown in Table 2, the batch rolling is performed. To 155 mm square, then hot under the conditions shown in Table 2 [winding temperature after hot rolling (rolling winding temperature) and cooling rate to 600 ° C. after winding (cooling rate after winding)] Rolling was performed to produce a hot-rolled wire (steel wire) having a diameter of 8.0 mm. In addition, the solid solution N amount shown in the following Table 1 is measured by the following method.
- the total N amount in the steel was measured using an inert gas melting method-thermal conductivity method. Specifically, a sample was cut out from the test steel material, put in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and obtained from a change in thermal conductivity.
- N compounds in the steel were measured by ammonia distillation separation indophenol blue absorptiometry. Specifically, a sample is cut out from the test steel material, a 10% AA electrolyte (a non-aqueous solvent electrolyte that does not generate a passive film on the steel surface, specifically 10% acetylacetone, 10% chloride). Constant current electrolysis was performed in tetramethylammonium, the balance: methanol). About 0.5 g of the sample was dissolved by this constant current electrolysis, and the insoluble residue (N compound) was filtered through a polycarbonate filter having a hole size of 0.1 ⁇ m.
- AA electrolyte a non-aqueous solvent electrolyte that does not generate a passive film on the steel surface, specifically 10% acetylacetone, 10% chloride.
- Constant current electrolysis was performed in tetramethylammonium, the balance: methanol). About 0.5 g of the sample was dissolved by this constant current electrolysis, and
- the insoluble residue was decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips, and combined with the filtrate. Next, this solution was made alkaline with sodium hydroxide, and then steam distilled to absorb the distilled ammonia in dilute sulfuric acid. Then, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) were added to form a blue complex, and the absorbance was measured using a photometer to determine the total N compound amount.
- the pearlite area ratio is determined by embedding and polishing the surface of the cross section of the hot rolled wire (cross section perpendicular to the rolling direction; the same applies hereinafter), D / 4, and D / 2 (D: diameter of the wire). After performing chemical corrosion using acid, the optical microscope was used to make four positions at 90 degrees with each other (D / 2 parts were close to the center point of the cross section so that they would not overlap each other, and four positions at 90 degrees with each other. The same was applied to each field of view (magnification: 400 ⁇ , 200 ⁇ m ⁇ 200 ⁇ m area).
- the white part (ferrite and bainite) is painted with black magic, then the transparent film is taken into a personal computer with a scanner, and image analysis software (Media Cybernetics, Inc.) After binarizing the image using “Image Pro Plus” (trade name), the unfilled portion was determined as the pearlite area ratio, and the average value of a total of 12 fields of view was calculated.
- image Pro Plus trade name
- a BN compound having a diameter less than 1000 nm and a BN compound having an equivalent circle diameter of 1000 nm or more are determined, and the composition of each precipitate is determined by EDX (Energy Dispersive X-ray Spectrometer) and WDS (Wavelength Dispersive X-ray Spectrometer). confirmed. Then, after measuring the number of BN compounds of the above size in each field of view, the average number of four fields of view was calculated.
- FIG. 1 is an example of an observation photograph before blackening the ferrite and the like.
- the twist test uses a steel wire (test piece) with a length of 350 mm, and uses a torsion tester manufactured by Maekawa Test Machine Manufacturing Co., Ltd. until it breaks under the conditions of distance between gauge points: 200 mm and twist rate: 50 rpm. I let you.
- formula rotation bending fatigue test was implemented using the said sample.
- the test stress was set to 16 conditions in 25 MPa increments from 850 to 1150 MPa, and five test pieces were tested at each stress. The maximum test stress that reached 5 ⁇ 2 ⁇ 10 7 times was determined as the fatigue strength ( ⁇ w) of the sample. did.
- the tensile strength ( ⁇ B) of the steel wire was determined (strain rate 10 mm / min) using a tensile tester (“Autograph” (trade name) manufactured by Shimadzu Corporation). Then, ⁇ w / ⁇ B was obtained, and when ⁇ w / ⁇ B was 0.49 or more, it was evaluated that the fatigue characteristics of the OT wire were excellent, and the spring obtained by using this was also excellent in fatigue characteristics.
- Table 5 shows the shortest fatigue life value among the five springs. And the thing with the shortest fatigue life of 10 ⁇ 10 6 times or more was evaluated as being excellent in the fatigue characteristics of the spring (and the fatigue characteristics of the hard-drawn wire used in this spring). Further, the tensile strength after baking a steel wire having a diameter of 3.0 mm at 400 ° C. for 20 minutes was also measured.
- Example No. 1 to 9, 12, 13, and 16 to 19 (hereinafter simply referred to as “No.”) satisfy the requirements specified in the present invention, so that the wire drawing workability of the steel wire (hot rolled wire) is improved. It is excellent in fatigue characteristics of steel wire (fatigue characteristics after wire drawing of steel wire (hot rolled wire)) and spring workability. No. From the results of 16 to 19, in order to further improve the fatigue characteristics after wire drawing, heating before the partial rolling in the production process is performed under the recommended conditions, and the form of the BN compound is within the preferable range specified in the present invention. It turns out that it is good to be inside.
- the wire drawing workability was ⁇ , but the wire drawing conditions with a larger strain than this were shown.
- the wire drawing workability was x. From this result, it is understood that the solid solution N content is preferably 0.0020% or less in order to further improve the wire drawing workability.
- the steel wire for springs satisfies the provisions of the present invention and is excellent in wire drawing workability and fatigue properties after wire drawing, but the wire drawing speed during wire drawing using this wire is too fast. Therefore, ⁇ HV of the obtained steel wire is increased, and the spring workability is inferior.
- No. Nos. 14 and 15 use steel types satisfying the prescribed component composition in the present invention.
- No. 14 is because the rolling coiling temperature in the manufacturing process is too high.
- No. 15 had a slow cooling rate after winding in the manufacturing process, so that it was impossible to secure a sufficient pearlite structure, and the wire drawing workability and fatigue characteristics were inferior.
- No. No. 20 was inferior in fatigue characteristics because the amount of C was excessive.
- No. No. 21 lacks the amount of C. Since No. 22 had an excessive amount of Si, it was not possible to secure a sufficient pearlite structure, resulting in poor wire drawing workability and fatigue characteristics.
- No. No. 23 was inferior in wire drawing workability and fatigue properties because the amount of Mn was excessive.
- No. No. 24 has an excessive amount of P.
- No. 25 was inferior in wire drawing workability and fatigue characteristics because the amount of S was excessive.
- No. No. 26 was inferior in wire drawing workability and fatigue characteristics due to an excessive amount of Cr. Moreover, ⁇ HV is increased and the spring workability is inferior.
- No. No. 27 was inferior in wire drawing workability and fatigue characteristics due to an excessive amount of Ni.
- No. No. 28 was inferior in wire drawing workability and fatigue characteristics because the amount of V was excessive. Moreover, ⁇ HV is increased and the spring workability is inferior.
- No. No. 29 has an excessive amount of Al.
- No. 33 is inferior in fatigue characteristics because the amount of O is excessive.
- No. No. 31 has an insufficient amount of B and a relatively large amount of solid solution N, a sufficient pearlite structure cannot be secured, and all of wire drawing workability, fatigue characteristics, and spring workability are inferior.
- No. No. 32 is inferior in all of wire drawing workability, fatigue characteristics, and spring workability because the N amount is excessive.
- FIG. 2 is a diagram in which the relationship between ⁇ HV and the defective product rate (the number of defective products per 100 pieces) is arranged using the results of the above embodiment. From FIG. 2, it can be seen that the defective rate is sufficiently suppressed by setting ⁇ HV to 100 or less.
- FIG. 3 is a diagram in which the relationship between the tensile strength and the fatigue strength is arranged for the OT wire using the results of the above-described embodiment. From FIG. 3, it can be seen that the steel wire that satisfies the requirements of the present invention ( ⁇ and ⁇ in FIG. 3) has a higher fatigue limit ratio than the comparative example ( ⁇ ). In particular, it can be seen that the fatigue limit ratio is further increased by controlling the form of the BN compound ( ⁇ in FIG. 3).
- FIG. 4 is a diagram in which the relationship between the tensile strength of the hard drawn wire and the fatigue life of the hard drawn spring is arranged using the results of the above-described embodiment.
- FIG. 4 shows that the steel wire satisfying the requirements of the present invention ( ⁇ and ⁇ in FIG. 4) has a higher fatigue limit ratio than the comparative example ( ⁇ ). Also in FIG. 4, it can be seen that the fatigue limit ratio is further increased by controlling the form of the BN compound ( ⁇ in FIG. 4).
- the spring steel wire of the present invention is used not only when the OT wire is obtained as the spring steel wire but also when the hard drawn wire is obtained as the spring steel wire. It can be seen that the spring obtained using the wire exhibits excellent fatigue properties.
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Abstract
Description
C:0.50%以上0.70%未満(%は「質量%」の意味、化学成分組成について以下同じ)、
Si:1.0~2.5%、
Mn:0.50~1.50%、
Cr:0.5%以下(0%を含む)、
B:0.0010~0.0050%、
N:0.0050%以下(0%を含まない)、
P:0.020%以下(0%を含まない)、
S:0.020%以下(0%を含まない)、
Al:0.03%以下(0%を含まない)、および
O:0.0020%以下(0%を含まない)
を満たし、残部が鉄および不可避不純物からなり、
全組織に占めるパーライト組織の面積率が85%以上であるところに特徴を有する。
(Hvs-Hvh)≦100 …(1)
[但し、上記Hvsは、D(鋼線の直径をいう。以下同じ)/16部からD/4部までの最高ビッカース硬さをいい、上記Hvhは、D/4部からD/2までの最低ビッカース硬さをいう。]
(a)ばね用鋼線材の伸線加工性を高めるには、
(a-1)全組織に占めるパーライト組織の面積率を85%以上にすると共に、
(a-2)後述する成分組成を満たすようにするのがよいこと;
(a-3)ばね用鋼線材の伸線加工性を更に高めるには、鋼中の固溶N量を0.0020%以下に低減させるのが好ましいこと;
(b)疲労特性(ばね用鋼線材の伸線後の疲労特性、即ち、ばね用鋼線の疲労特性や、ばねの疲労特性)を高めるには、特に、後述する成分組成を満たすようにするのがよいこと、また疲労特性を更に高めるには、上記BN系化合物の析出サイズおよび個数を所定の範囲内に制御するのがよいこと;
(c)ばね用鋼線のばね加工性を高めるには、表層部のビッカース硬さ(Hvs)と内部のビッカース硬さ(Hvh)の差(以下、「ΔHV」と示すことがある)が、下記式(1)を満たすよう小さくすればよいこと;がわかった。
(Hvs-Hvh)≦100 …(1)
本発明では、ばね用鋼線材の全組織に占めるパーライト組織の面積率を85%以上とする。
一般に、ばね用鋼線材の組織として、パーライト組織以外に、初析フェライトやベイナイトが形成される。しかしパーライト組織以外の組織が増加すると、伸線中に断線が生じるおそれがある(即ち、伸線加工性に劣る)。また、断線にまで至らなくとも内部に欠陥が生じやすく、この内部欠陥を起因として疲労特性が低下する。本発明では、パーライト組織の面積率を高めて、相対的にフェライト等を抑制させ、組織の均一化を図ることによって、断線や内部欠陥を減少させ、伸線加工性や疲労特性を向上させる。前記パーライト組織の面積率は、好ましくは87%以上、より好ましくは90%以上、更に好ましくは91%以上、特には93%以上である。
本発明では、観察視野面積2000μm2におけるパーライト組織中に存在する、円相当直径が100nm以上1000nm未満のBN系化合物の個数を100個以下(好ましくは70個以下、より好ましくは50個以下)に制御することで、疲労特性を改善することができる。
円相当直径が1000nm以上と比較的大きめのサイズのBN系化合物の析出を抑制することも重要である。このような粗大なBN系化合物の析出個数が多くなると、疲労特性の低下を招きやすいからである。本発明では、観察視野面積2000μm2におけるパーライト組織中に存在する上記粗大なBN系化合物の個数を、10個以下(好ましくは8個以下、より好ましくは5個以下)に抑制する。
Cは、伸線材の引張強度を高め、疲労特性や耐へたり性を確保するために有用な元素である。また高応力ばねとして必要な引張強さの確保にも有用である。更に所望の組織の確保に必要であり、初析フェライト量の増大を抑えて疲労寿命のばらつきを抑制するのにも必要である。よって、C量は、0.50%以上とする。好ましくは0.52%以上、より好ましくは0.55%以上、更に好ましくは0.60%以上である。しかし本発明においてC量が0.70%以上になると、欠陥感受性が増大し、表面疵や介在物からのき裂の進展が容易になり、疲労寿命が劣化する。よって本発明では、C量を0.70%未満とする。好ましくは0.68%以下、より好ましくは0.65%以下である。
Siは、固溶強化元素として強度向上に寄与し、疲労特性と耐へたり性の改善に貢献する元素である。また、ばね加工工程におけるコイリング後の歪み取り焼鈍として熱処理が施されるが、その際に優れた軟化抵抗性を発揮させるのに有効な元素でもある。これらの効果を十分発揮させるべくSi量の下限を1.0%とする。好ましくは1.3%以上、より好ましくは1.5%以上である。しかし、Si量が2.5%を超えると、表面の脱炭が増大して疲労特性が劣化する恐れがある。また所望の組織が得られない。よってSi量の上限を2.5%とした。好ましくは2.3%以下であり、より好ましくは2.0%以下である。
Mnは、パーライト組織を緻密かつ整然化させて、疲労特性を向上させる元素である。こうした効果を発揮させるため、Mnを0.50%以上含有させる。好ましくは0.70%以上であり、より好ましくは0.80%以上である。一方、Mnは偏析しやすい元素であり、含有量が過剰になると偏析し、その偏析部にミクロマルテンサイトが生成して伸線加工性が低下しやすくなる。また疲労特性も低下しやすい。よってMn量の上限を1.50%とする。好ましくは1.30%以下であり、より好ましくは1.20%以下である。
Crは、パーライトラメラ間隔を狭くし、熱間圧延後や伸線前熱処理であるパテンティング後の強度を上昇させ、耐へたり性や疲労強度を向上する為に有効な元素である。このような観点からは、Crを0.05%以上含有させることが好ましく、より好ましくは0.10%以上、更に好ましくは0.15%以上、特に好ましくは0.20%以上である。しかし、Cr量が過剰になると、セメンタイトが強化されすぎて靱性や延性が劣化する。また伸線加工性や疲労特性も劣化する。更にCr量が過剰になると、時効脆化が生じやすくなり、鋼線の表層と内部の硬さの差が大きくなるため、ばね加工性に劣る。よってCr量の上限を0.5%とする。好ましくは0.40%以下であり、より好ましくは0.35%以下であり、更に好ましくは0.30%以下である。
Bは、固溶NをBN系化合物として微細析出させることで、線材の伸線加工性および疲労特性を向上させるのに有効な元素である。またばね加工性の向上にも有効である。更に、Bの一部が固溶Bとして鋼中に存在することで、初析フェライトの生成抑制にも有効である。これらの効果を十分発揮させるには、B量を0.0010%以上とする必要がある。好ましくは0.0020%以上、より好ましくは0.0030以上である。一方、B量が0.0050%を超えると粗大なFe23(CB)6等のボロン化合物が生成し疲労強度を劣化させるおそれがある。よって本発明では、B量の上限を0.0050%とした。より好ましくは0.0040%以下である。
Nが過剰であると、固溶Nが多くなり伸線加工性、疲労特性、ばね加工性が劣化する。この固溶Nを低減させるには、全N量を低減させることが有効である。しかし過分に少なくすることは製鋼コスト高を招くので、N量の上限を0.0050%とする。N量は、好ましくは0.0045%以下、より好ましくは0.0040%以下である。
固溶Nは、伸線加工中の時効脆化を引き起こし、伸線中の断線、縦割れの発生を招きやすい。即ち、圧延後の伸線加工性やパテンティング後の伸線加工性の低下を招きやすい。また、時効脆化が進むとばね用鋼線の表層と内部の硬度差が大きくなり、ばね用鋼線のばね加工性が低下しやすくなる。従って、鋼中のBとNの含有量が下記式(2)を満たすようにして、固溶NをBNとして析出させることにより、固溶N量を0.0020%以下に抑制することが好ましい。固溶N量は、より好ましくは0.0015%以下、更に好ましくは0.0010%以下、より更に好ましくは0.0005%以下である。
B-(N-0.0020)×0.77≧0.0000 ・・・(2)
不可避不純物であるPは、伸線加工時の断線を誘発させる元素である。また旧オーステナイト粒界に偏析して粒界を脆化させ、疲労特性を低下させる元素でもあるため、極力少ないほうがよい。従って本発明では0.020%以下とした。好ましくは0.016%以下、より好ましくは0.013%以下である。
不可避不純物であるSは、上記Pと同様に伸線加工時の断線を誘発する元素である。また、旧オーステナイト粒界に偏析して粒界を脆化させ、疲労特性を低下させる元素でもあるため、極力少ないほうがよい。従って本発明では0.020%以下とした。好ましくは0.015%以下、より好ましくは0.010%以下である。
Alは製鋼時の脱酸剤として含有されるが、過剰に含まれると粗大な非金属介在物を生成し、疲労強度を劣化させるので、その含有量を0.03%以下に抑制する。好ましくは0.010%以下であり、より好ましくは0.005%以下である。
Oは、過剰に含まれると粗大な非金属介在物を形成して疲労強度を劣化させる元素である。よって本発明では、O含有量を0.0020%以下とする。好ましくは0.0015%以下であり、より好ましくは0.0010%以下である。
V、Niは、特に伸線加工性を高めるのに有効な元素である。詳細には、Vはパーライトノジュールサイズを微細にして上記伸線加工性を高めるとともに、ばねの靱性および耐へたり性を向上させるのにも有用な元素である。こうした効果を発揮させるには、Vを0.05%以上含有させることが好ましい。より好ましくは0.1%以上、更に好ましくは0.2%以上である。しかしながらVが0.40%を超えて過剰に含まれると、焼入れ性が増大して熱間圧延後にマルテンサイトやベイナイトを生じ、伸線加工性がかえって悪くなる。また疲労特性も低下する。更に、ラメラセメンタイトとして使用されるべきCが減少し、強度がかえって低下する他、初析フェライトを過剰に生成させたり、フェライト脱炭を誘発させる傾向がある。またV量が過剰になると、時効脆化が生じやすくなり、鋼線の表層と内部の硬さの差が大きくなるため、ばね加工性に劣る。よって本発明では、V量を0.40%以下とすることが好ましく、より好ましくは0.30%以下である。
上記(c)の通り、ばね用鋼線のばね加工性を高めるには、ばね用鋼線の表層部のビッカース硬さ(Hvs)と内部のビッカース硬さ(Hvh)の差(ΔHV)が、下記式(1)を満たすよう小さくすればよい。ばね加工後のばねにはスプリングバックが生じるため、自由長の変化が生じるが、線材表層と線材内部の硬さの差が大きいと、このスプリングバックが大きくなり自由長が大きく変化するためである。
(Hvs-Hvh)≦100 …(1)
[但し、上記Hvsは、D(鋼線の直径)/16部からD/4部までの最高ビッカース硬さをいい、上記Hvhは、D/4部からD/2までの最低ビッカース硬さをいう。]
[分塊圧延開始前の1280~1100℃の温度範囲の冷却速度:0.5℃/秒以上]
分塊圧延前の加熱温度を1280℃以上とすることにより、BN系化合物を十分に鋼中に固溶させ、その後、分塊圧延開始前の1280~1100℃の温度範囲の冷却速度を0.5℃/秒以上に制御することで、BN系化合物の形態を規定範囲内として疲労特性をさらに高めることができる。分塊圧延前の加熱温度は、好ましくは1290℃以上である。
熱間圧延後の巻取り温度は、圧延機の負荷が過大とならないように、850℃以上とするのがよい。好ましくは880℃以上である。一方、初析フェライトを抑制してパーライト組織を面積率で85%以上確保するには、熱間圧延後の巻取り温度を950℃以下にするのがよい。表層脱炭、再結晶、粒成長を抑制し、微細粒組織にする観点からも、熱間圧延後の巻取り温度を950℃以下にするのがよい。熱間圧延後の巻取り温度は、好ましくは940℃以下、より好ましくは930℃以下である。
パーライト組織を面積率で85%以上確保するには、熱間圧延後の巻取り後から600℃までの冷却速度を、10℃/秒以上(好ましくは15℃/秒以上、より好ましくは20℃/秒以上)として初析フェライトの形成を抑制し、かつ35℃/秒以下(好ましくは30℃/秒以下)として、マルテンサイトやベイナイト組織の生成を抑制するのがよい。
本発明における「固溶N量」の値は、JIS G 1228に準拠し、下記(a)~(c)に示す通り、鋼中の全N量から全N化合物量を差し引いて鋼中の固溶N量を求めた。
パーライト面積率は、熱間圧延線材の横断面(圧延方向に垂直な断面。以下同じ)の表層、D/4、D/2(D:線材の直径)の各位置において、埋め込み研磨し、ピクリン酸を用いた化学腐食を実施した後、光学顕微鏡により、互いに90度をなす4箇所(D/2部については、各々が重ならない様に断面の中心点に近接し互いに90度をなす4箇所。以下同じ)にて夫々1視野撮影した(倍率:400倍で200μm×200μmの領域)。光学顕微鏡写真の画像をプリントアウトして、透明フィルムを重ねた上から白い部分(フェライトおよびベイナイト)を黒マジックで塗りつぶした後、透明フィルムをスキャナーでパソコンに取り込み、画像解析ソフト(Media Cybernetics社製「Image Pro Plus」(商品名))を用いて、画像を2値化した後、上記塗りつぶしていない部分をパーライト面積率として求め、合計12視野の平均値を算出した。尚、表層に脱炭層が存在する場合には、JIS G 0058の4で規定される全脱炭部は測定部位から除外した。
熱間圧延線材の横断面のD/4(D:線材直径)の位置において、埋め込み研磨し、ナイタルを用いた化学腐食を実施した後、互いに90度をなす4箇所にて夫々1視野、倍率:2000倍でFE-SEM観察を行った。尚、1視野を2000μm2とした。観察した画像のフェライトおよびベイナイト部分を黒く塗りつぶした後、画像解析ソフト(Media Cybernetics社製「Image Pro Plus」(商品名))を用いて画像を2値化することにより、円相当直径が100nm以上1000nm未満のBN系化合物、および円相当直径が1000nm以上であるBN系化合物を判定し、それぞれの析出物の組成をEDX(Energy Dispersive X-ray Spectrometer)およびWDS(Wavelength Dispersive X-ray Spectrometer)で確認した。そして、各視野において上記サイズのBN系化合物の個数をそれぞれ測定した後、4視野の平均個数を算出した。
本実施例では、伸線加工性について厳しく評価するため、伸線条件を、一般的に行われている伸線条件(例えば、後述する疲労試験用サンプル作製工程における伸線条件:ε=1.81)よりも厳しい条件(ε=2.17)で行った。詳細は以下の通りである。
熱間圧延線材を、SV(皮削り、これにより直径8.0mm→直径7.4mm)、乾式伸線(ダイススケジュールは下記表4の通りである)して直径3.0mmの鋼線(サンプル)を得た(直径3.0mmでの伸線速度は表5に示すとおりである)。このサンプルを用い、試験荷重を300gfとし、鋼線横断面の直径線上を表層から0.1mm深さの位置から0.1mm間隔で反対側の表層から0.1mm深さの位置までHvを測定した。そして、D/16部からD/4部までの最高HvをHvsとし、D/4部からD/2部までの最低HvをHvhとして、ΔHV(=Hvs-Hvh)を求めた。
上記ΔHVの測定で得た鋼線を用い、コイル平均径25mm、総巻数6、有効巻数4、自由長80mmの条件でコイリングマシンにより冷間コイリングした。各鋼線につき100本ずつコイリングし、投影機にて自由長を測定した。そして、80mm±0.5mmを良品とし、それ以外を不良品とした。不良品が5本以下の鋼線をばね加工性が良い(○)と評価し、不良品が5本を超える鋼線をばね加工性が悪い(×)と評価した。
熱間圧延線材を用いて、SV(皮削り)、乾式伸線(ダイススケジュールは上記表4の通りである)して直径3.0mmの鋼線を得た(直径3.0mmでの伸線速度は200m/min)のち、この鋼線に対して、オイルテンパー処理(加熱920℃、焼き戻し温度は表5に示すとおりである)、焼鈍、ショットピーニング、低温歪取焼鈍を順次行って、試験サンプルを作製した。
ばね加工性の評価に用いたばね(即ち、熱処理を施していない硬引き線を用いて形成された硬引きばねであって、各鋼種の良品)に歪取り焼鈍(400℃×20分)、座面研磨、2段ショットピーニング、低温焼鈍(220℃×20分)、および冷間セッチングを行った。得られたばねを用いて、650±500MPaの負荷応力下で疲労試験を行い、疲労寿命(破断寿命)を測定した(油圧サーボ式ばね疲労試験機を使用)。この測定を、各鋼種につきばね5個について行った。表5には、ばね5個のうち最も短い疲労寿命の値を示す。そして、最も短い疲労寿命が10×106回以上のものを、ばねの疲労特性(およびこのばねに用いた硬引き線の疲労特性)に優れると評価した。また、直径3.0mmの鋼線を400℃×20分でベーキングした後の引張強さも測定した。
Claims (5)
- C:0.50%以上0.70%未満(%は「質量%」の意味、化学成分組成について以
下同じ)、
Si:1.0~2.5%、
Mn:0.50~1.50%、
Cr:0.5%以下(0%を含む)、
B:0.0010~0.0050%、
N:0.0050%以下(0%を含まない)、
P:0.020%以下(0%を含まない)、
S:0.020%以下(0%を含まない)、
Al:0.03%以下(0%を含まない)、および
O:0.0020%以下(0%を含まない)
を満たし、残部が鉄および不可避不純物からなり、
全組織に占めるパーライト組織の面積率が85%以上であることを特徴とするばね用鋼線材。 - 更に、
観察視野面積2000μm2におけるパーライト組織中に存在する、
円相当直径が100nm以上1000nm未満のBN系化合物が100個以下(0個を含む)であり、かつ、
円相当直径が1000nm以上であるBN系化合物が10個以下(0個を含む)である
請求項1に記載のばね用鋼線材。 - 更に、
V:0.05~0.40%、および
Ni:0.05~0.80%
よりなる群から選択される1種以上の元素を含む請求項1に記載のばね用鋼線材。 - 更に、固溶Nが0.0020%以下(0%を含む)である請求項1に記載のばね用鋼線材。
- 請求項1~4のいずれかに記載のばね用鋼線材を用いて得られるばね用鋼線であって、表層部のビッカース硬さ(Hvs)と内部のビッカース硬さ(Hvh)の差が、下記式(1)を満たすことを特徴とする疲労特性およびばね加工性に優れたばね用鋼線。
(Hvs-Hvh)≦100 …(1)
[但し、上記Hvsは、D(鋼線の直径をいう。以下同じ)/16部からD/4部までの最高ビッカース硬さをいい、上記Hvhは、D/4部からD/2までの最低ビッカース硬さをいう。]
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EP3128031A4 (en) * | 2014-03-31 | 2017-10-25 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High-strength steel material having excellent fatigue characteristics |
WO2018212327A1 (ja) * | 2017-05-18 | 2018-11-22 | 新日鐵住金株式会社 | 線材、鋼線、及び鋼線の製造方法 |
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