Classifications

• • 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
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • 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
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
• • 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
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present invention relates to a steel wire rod for a spring excellent in fatigue properties and a spring.
• Non-Patent Document 1 describes that, in a steel for a valve spring, deformation during hot working is accelerated by controlling a composition of inclusions to a CaO-Al 2 O 3 -SiO 2 -based or MnO-Al 2 O 3 -SiO 2 -based amorphous stabilized composition, thereby preventing start points of fatigue breakage from being formed to improve fatigue properties.
• Patent Document 1 proposes a technology in which the whole inclusions have a low melting point and are easily deformable, and hard SiO 2 is less likely to be formed, even when phase separation occurs at the time of heating before hot rolling or during hot rolling.
• Patent Document 2 proposes a technology in which fragmentation of inclusions during hot rolling is accelerated by forming many fine grains in the inclusions to enhance refinement thereof.
• Patent Document 3 proposes a technology in which at least one of LiO 2, Na 2 O and K 2 O is allowed to be positively contained, in order to decrease the melting point and viscosity of composite oxide-based inclusions to be formed, thereby finally refining the inclusions.
• Patent Document 4 describes that addition of ZrO 2 as an unconventional oxide component contributes to securing of an amorphous phase.
• Patent Document 5 describes that oxide-based inclusions are finely fragmented by allowing B 2 O 3 to be contained in a composite oxide (for example, such as a CaO-Al 2 O 3 -SiO 2 -based composite oxide or a CaO-Al 2 O 3 -SiO 2 -MgO-based composite oxide), thereby being able to remarkably improve wire drawability and fatigue strength.
• a composite oxide for example, such as a CaO-Al 2 O 3 -SiO 2 -based composite oxide or a CaO-Al 2 O 3 -SiO 2 -MgO-based composite oxide
• Non-Patent Document 1 Tsuyoshi Mimura, 182nd and 183rd Nishiyama Memorial Technical Lecture “Inclusion Control and High Cleanliness Degree Steel Production Technology", edited by The Iron and Steel Institute of Japan, Tokyo, 2004 , p. 125
• the present invention has been made in view of the above circumstances, and an object thereof is to provide a steel wire rod for a spring extremely excellent in fatigue properties, and a spring.
• the steel wire rod for a spring excellent in fatigue properties in the present invention which is capable of solving the problems includes:
• the average composition of the oxide-based inclusions satisfies: CaO: 10 to 35%, Al 2 O 3 : 10 to 40%, SiO 2 : 30 to 70%, MgO: 8% or less (inclusive of 0%), MnO: 5% or less (inclusive of 0%), TiO 2 : 3 to 10%, and CaO+Al 2 O 3 +SiO 2 +MgO+MnO+TiO 2 ⁇ 80%, and the number of the oxide-based inclusions having a minor axis of 2 ⁇ m or more which are present in the cross section is more than 0.002 inclusions/mm 2 .
• the average composition of the oxide-based inclusions may further satisfy, by mass%, ZrO 2 : less than 1% (not inclusive of 0%) and Na 2 O: less than 5% (inclusive of 0%).
• the steel wire rod for a spring may further include Ni: 0.5% or less (not inclusive of 0%), and Cu: 0.5% or less (not inclusive of 0%).
• the steel wire rod for a spring may further include V: 0.5% or less (not inclusive of 0%).
• the spring excellent in fatigue properties which is capable of solving the problems, is obtained using the steel wire rod for a spring.
• Patent Document 2 In order to provide a steel wire rod for a spring having extremely excellent fatigue properties, the present inventors have made studies also after disclosure of the above Patent Documents 1 to 3 and the like. Conventionally, for improvement of fatigue properties, it is effective to refine, oxide-based inclusions by fragmentation due to extension during hot rolling, and there has been proposed a method of controlling a composition of oxide-based inclusions obtained by Si deoxidization to a SiO 2 -containing composition in which amorphousness is relatively stable, for example, SiO 2 -CaO-Al 2O 3-MgO-MnO or the like. In that case, as a means for realizing refinement even when the inclusions are crystallized, the technology of Patent Document 2 is proposed as a method for controlling a crystallization state (a fine grain phase is precipitated without being completely crystallized).
• the oxide-based inclusions in which TiO 2 is allowed to be contained based on SiO 2 -CaO-Al 2 O 3 -MgO-MnO, an average composition thereof satisfies, by mass%, CaO+Al 2 O 3 +SiO 2 +MgO+MnO+TiO 2 ⁇ 80%.
• the effect due to TiO 2 is exerted by containing 80% or more of these in total.
• containing of TiO 2 in the above oxide-based inclusions obtained by Si deoxidization causes separation into two phases of a TiO 2 -concentrated phase (A phase) and a SiO 2 -concentrated phase (B phase).
• the reason for the separation into two phases is considered because TiO 2 has a property to be separated from SiO 2 as two liquid phases at a molten steel stage.
• the SiO 2 concentration in the SiO 2 -concentrated phase (B phase) is increased to suppress crystallization of gehlenite, spinel (MgO-Al 2 O 3 ) and the like, which is liable to occur in Si-killed steel.
• the liquidus temperature is decreased to suppress crystallization by containing TiO 2 in the oxide-based inclusions.
• Patent Documents 1 to 5 described above do not disclose the above characterizing portion in the present invention.
• TiO 2 is mentioned as an impurity of the inclusions.
• examples of containing impurities such as FeO and TiO 2 in an amount of 1.0% are only disclosed, and this does not provide the effect of improving fatigue properties due to addition of TiO 2 (see No. 14 in the table described later).
• the above Patent Document 4 is different from the present invention in the composition of the oxide-based inclusions in that ZrO 2 is contained in an amount of 1% or more.
• Ti is contained within a range of 0.0003 to 0.010% as a component in steel and TiO 2 is contained in the above oxide-based inclusions of SiO 2 -CaO-Al 2 O 3 -MgO-MnO within a range of 3 to 10%.
• the steel wire rod for a spring includes both a steel after rolling (rolled steel) and a drawn wire rod obtained by subjecting the rolled steel to wire drawing. In the present invention, these are called the "steel wire rod" as a whole.
• the oxide-based inclusions mean oxide-based inclusions in which oxide forming elements such as Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr bond to oxygen.
• oxide forming elements such as Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr bond to oxygen.
• EDX energy dispersive X-ray spectrometry
• WDX wavelength-dispersive X-ray spectrometry
• the composition of the oxide-based inclusions is described.
• the average composition of the oxide-based inclusions having a minor axis of 1 ⁇ m or more which are present in a cross section parallel to a longitudinal direction of the steel satisfies, by mass%, CaO: 35% or less (inclusive of 0%), Al 2 O 3 : 40% or less (inclusive of 0%), SiO 2 : 30 to 95%, MgO: 8% or less (inclusive of 0%), MnO: 5% or less (inclusive of 0%), TiO 2 : 3 to 10%, and CaO+Al 2 O 3 +SiO 2 +MgO+MnO+TiO 2 ⁇ 80%, and preferably satisfies CaO: 10 to 35%, Al 2 O 3 : 10 to 40%, SiO 2 : 30 to 70%, MgO: 8% or less (inclusive of 0%), MnO: 5% or less (inclusiv
• the composition of the oxide-based inclusions is described.
• the average composition of the oxide-based inclusions having a minor axis of 1 ⁇ m or more which are present in a cross section parallel to a longitudinal direction of the steel satisfies, by mass%, CaO: 35% or less (inclusive of 0%), Al 2 O 3 : 40% or less (inclusive of 0%), SiO 2 : 30 to 95%, MgO: 8% or less (inclusive of 0%), MnO: 5% or less (inclusive of 0%), TiO 2 : 3 to 10%, and CaO+Al 2 O 3 +SiO 2 +MgO+MnO+TiO 2 ⁇ 80%, and the number of the oxide-based inclusions having a minor axis of 2 ⁇ m or more which are present in the above cross section is more than 0.002 inclusions/mm 2 .
• CaO is a basic oxide, and when it is contained in SiO 2 of an acidic oxide, the liquidus temperature of the oxide is decreased to have an effect of suppressing crystallization of the oxide-based inclusions. It may therefore be contained in the inclusions.
• the content thereof is desirably 10% or more, and more preferably 15% or more.
• the upper limit thereof is 35% or less.
• the upper limit of the CaO content is preferably 30% or less.
• SiO 2 is an acidic oxide and a component which is essential for making the oxide-based inclusions amorphous.
• the lower limit of the SiO 2 content is 30% or more.
• the lower limit of the SiO 2 content is preferably 40% or more.
• the upper limit of the SiO 2 content is 95% or less, preferably 70% or less, and more preferably 50% or less.
• Al 2 O 3 is an amphoteric oxide, and when it is contained in SiO 2 of an acidic oxide, the liquidus temperature of the oxide is decreased to have an effect of suppressing crystallization of the oxide. It may therefore be contained in the inclusions.
• the content thereof is desirably 10% or more, and more preferably 20% or more.
• an Al 2 O 3 crystal phase such as corundum is crystallized in molten steel and during a solidification process, or a MgO ⁇ Al 2 O 3 crystal phase such as spinel is crystallized together with MgO. Further, these crystal phases are formed in a rolling temperature range. These solid phases are hard and remain as coarse inclusions to deteriorate fatigue properties. From such a viewpoint, the upper limit of the Al 2 O 3 content is required to be 40% or less, and is preferably 30% or less.
• MgO is not an essential component in the present invention, but has an effect of controlling a SiO 2 -based oxide to an optimum composition to decrease the melting point thereof.
• the lower limit of the MgO content is preferably 0.2% or more.
• the upper limit thereof is 8% or less. It is preferably 5% or less, and more preferably 3% or less.
• MnO is also not an essential component in the present invention.
• MnO has an effect of decreasing the melting point of the SiO 2 -based oxide.
• the lower limit of the MnO content is preferably 0.1% or more, and more preferably 0.5% or more.
• the upper limit of the MnO content is 5% or less.
• TiO 2 is an oxide component which is the characteristics feature in the present invention.
• containing of TiO 2 in SiO 2 of the acidic oxide causes separation into two phases of the TiO 2 -concentrated phase (A phase) and the SiO 2 -concentrated phase (B phase), and both phases have an action of suppressing crystallization.
• a phase TiO 2 -concentrated phase
• B phase SiO 2 -concentrated phase
• crystallization of SiO 2 -cntaining oxide-based inclusions obtained in Si-killed steel during hot working is suppressed, and that occurrence of voids at interfaces between the steel and the oxide-based inclusions is suppressed, thereby further improving fatigue properties.
• Such effects are obtained by controlling the lower limit of the TiO 2 content to 3% or more.
• the TiO 2 content is 3% or more. It is preferably 4% or more, and more preferably 5% or more.
• the upper limit of the TiO 2 content is 10% or less. It is preferably 8% or less, and more preferably 7% or less.
• the present invention it is necessary to control the contents of the respective oxides as described above and to control the total of these contents to 80% or more, thereby keeping amorphousness of the oxide-based inclusions to improve fatigue properties.
• the larger total amount of the above oxides is better, and preferably 90% or more. It is most preferably 100%.
• the oxide-based inclusions contained in the steel wire rod for a spring in the present invention are basically CaO, Al 2 O 3 , SiO 2 , MgO, MnO and TiO 2 , and the balance is impurities.
• the above impurities include, for example, impurities unavoidably contained in a production process and the like.
• the above impurities can be contained to such an extent that desired fatigue properties are obtained without adversely affecting a crystallization state or form of the oxide-based inclusions.
• the total amount of the impurities is required to be controlled to at most 20%.
• the above impurities include, for example, ZrO 2 , Na 2 O, Cr 2 O 3 and the like.
• ZrO 2 when the concentration of ZrO 2 in the oxide-inclusions is increased, crystallization of the above inclusions is accelerated to deteriorate fatigue properties. It is therefore preferably decreased as much as possible.
• the ZrO 2 content is preferably less than 1%, more preferably 0.5% or less, and most preferably not contained.
• Na 2 O has a wide allowance, compared to the above ZrO 2 so that it may be contained in an amount of about 5%.
• the number of the oxide-based inclusions having a minor axis of 2 ⁇ m or more is more than 0.002 inclusions/mm 2 .
• the number of the oxide-based inclusions having a minor axis of 2 ⁇ m or more is preferably 0.005 inclusions/mm 2 or more, more preferably 0.01 inclusions/mm 2 or more and still more preferably 0.05 inclusions/mm 2 or more.
• oxide-based inclusions mean oxide-based inclusions in which oxide forming elements such as Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr bond to oxygen, and should not be limited to the above-mentioned oxides (CaO, Al 2 O 3 , SiO 2 , MgO, MnO and TiO 2 ). Further, of the above oxide-based inclusions, the ones having "a minor axis of 2 ⁇ m or more" are particularly specified, because the oxide-based inclusions having a minor axis of less than 2 ⁇ m have relatively little adverse effects on fatigue properties.
• the C content is an element necessary for securing predetermined strength, and in order to effectively exert such properties, the C content is 0.2% or more. It is preferably 0.5% or more. However, when the C content is excessive, steel becomes brittle and therefore does not become practical. Therefore, the upper limit thereof is 1.2% or less. The upper limit of the C content is preferably 0.8% or less, and more preferably 0.7% or less
• Si is an important element contributing to high strengthening of the steel wire rod for a spring and improvement of fatigue properties. Further, it is also a useful element for enhancing softening resistance and improving setting resistance. Furthermore, Si is an essential element also for controlling to a composition of desired oxide-based inclusions. In order to effectively exert such actions, the Si content is 1.0% or more. The Si content is preferably 1.4% or more, and more preferably 1.8% or more. However, when the Si content is excessive, hard pure SiO 2 may possibly be formed during solidification, and surface decarburization and surface flaws are increased to deteriorate fatigue properties in some cases. For this reason, the upper limit of the Si amount is 3% or less. It is preferably 2.4% or less, and more preferably 2.2% or less.
• Mn is an element acting as a deoxidizing agent and additionally increasing hardenability, thereby contributing to the enhancement of strength.
• the lower limit of the Mn content is 0.1% or more. It is preferably 0.5% or more.
• the upper limit thereof it is 2% or less. It is preferably 1% or less.
• Cr is an element for improving matrix strength of the steel wire rod for a spring by solid solution strengthening. Further, similarly to the case of Mn, Cr also effectively acts on improvement of hardenability.
• the Cr amount is preferably 0.5% or more, and more preferably 0.9% or more. However, when Cr is excessive, the steel wire rod for a spring tends to become brittle to increase sensitivity of the oxide-based inclusions. Therefore, fatigue properties are deteriorated.
• the upper limit of the Cr amount is 3%. As the upper limit of the Cr amount, it is preferably 2% or less, and more preferably 1 % or less.
• the Al content is 0.005% or less, preferably 0.002% or less, and more preferably 0.0015% or less.
• the Al content is less than 0.0002%, the Al 2 O 3 content in the oxide-based inclusions is excessively decreased to form crystal phases containing a large amount of SiO 2 Accordingly, the lower limit of the Al content is 0.0002% or more, and preferably 0.0005% or more.
• Ca is a component contained in the steel wire rod by slag refining for controlling the composition of the oxide-based inclusions.
• it is an effective element for controlling the CaO content in the oxide-based inclusions to suppress crystallization of the oxide-based inclusions, thereby improving fatigue properties.
• the Ca content is 0.0002% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.
• the Ca content is 0.002% or less, preferably 0.001% or less, and more preferably 0.0008% or less.
• Ti is an element which is the characteristic feature in the present invention.
• a predetermined amount of Ti is added to appropriately control the TiO 2 content in the oxide-based inclusions, thereby more enhancing amorphous stability to further improving fatigue properties.
• the Ti content is required to be 0.0003% or more. It is preferably 0.0005% or more, and more preferably 0.0008% or more.
• the Ti content is 0.010% or less. It is preferably 0.0050% or less, and more preferably 0.0030% or less.
• the elements in steel used in the present invention are as described above, and the balance is iron and unavoidable impurities.
• the above unavoidable impurities include, for example, elements introduced depending on situations of raw materials, materials, production facilities and the like, such as S, P, H and N.
• Ni is an effective element for suppressing decarburization of ferrite formed in hot rolling at the time of producing the steel wire rod for a spring or in heat treatment at the time of producing the spring. Further, Ni has an action to increase toughness of the spring after hardening and tempering.
• the lower limit of the Ni amount it is preferably 0.05% or more, more preferably 0.15 % or more, and still more preferably 0.2% or more.
• the Ni amount is preferably 0.5% or less, and more preferably 0.3% or less.
• Cu is an effective element for suppressing decarburization of ferrite formed during hot rolling at the time of producing the steel wire rod for a spring or in heat treatment at the time of producing the spring. Therefore, it may be contained in an amount of 0.05% or more. As the upper limit thereof, it is preferably 0.5% or less, and more preferably 0.3% or less.
• As the lower limit of V amount it is preferably 0.05% or more, and more preferably 0.10% or more.
• the V amount is excessive, the amount of carbide that is not soluted in austenite during heating for hardening is increased, resulting in difficulty to obtain sufficient strength and hardness. Additionally, coarsening of nitride is brought about to cause easy occurrence of fatigue breakage. Further, when the V amount is excessive, the residual austenite amount is increased to decrease the hardness of the spring.
• As the upper limit of the V amount it is preferably 0.5% or less, and more preferably 0.4% or less.
• a method for producing the steel wire rod for a spring in the present invention is described below.
• it is important to produce it, particularly paying attention to respective steps of a smelting step and hot working so as to obtain the desired composition and number of oxide-based inclusions.
• the other steps are not particularly limited, and methods usually used for the production of steel wire rods for springs can be appropriately selected and used.
• the preferred smelting step and hot step used in the present invention are as follows.
• slag refining is performed using CaO-SiO 2 -based slag according to a conventional method, thereby controlling to a composition of CaO-Al 2 O 3 -SiO 2 -MgO-MnO-TiO 2 .
• the above slag is fully suspended in molten steel, thereby being able to adjust the number of oxide-based inclusions having a minor axis of 2 ⁇ m or more to a predetermined range.
• a predetermined amount of TiO 2 is contained as the oxide-based inclusions.
• a controlling method thereof is also not particularly limited, and it should be sufficient to add Ti during smelting so that the Ti amount in the steel is controlled within a range of 0.0003 to 0.010%, based on a method usually used in the technical field in the present invention.
• a method for adding Ti is not particularly limited, and for example, an iron-based alloy containing Ti may be added to perform adjustment, or the Ti concentration in molten steel may be controlled by controlling a slag composition.
• the steel rod for a spring in the present invention is thus obtained.
• wire drawing may be further performed to obtain the steel wire rod for a spring.
• Wire drawing conditions are not particularly limited, and a method usually used can be employed.
• the steel wire rod for a spring in the present invention is very useful as a material for a processed product requiring high fatigue properties.
• the above processed products include, for example, springs such as valve springs to be used in engines or suspensions of automobiles, clutch springs, brake springs and suspension springs; steel wires such as steel cords; and the like.
• a production method of the above spring is not particularly limited, and the spring can be produced according to a conventional method. Specifically, the above steel wire rod for a spring is annealed as needed, and thereafter subjected to stripping treatment, lead patenting treatment, wire drawing and oil tempering treatment to produce the spring.
• test steels having various chemical components shown in the following Table 1 were smelted, and cast slabs of 245 mm diameter x 480. mm were prepared.
• smelting using a crucible of a MgO-based refractory during smelting, at least one of Ni and V was added as needed, as well as C, Si, Mn and Cr, and adjustment to a predetermined concentration was performed. Thereafter, Ti and Ca were added in this order, and each concentration of Ti and Ca was adjusted.
• a Ni-Ca alloy was used as Ca to be added to molten steel, and a Fe-Ti alloy was used as a Ti source.
• Each cast slab obtained was heated in a heating furnace at a temperature of 1100 to 1300°C, and then, blooming was performed at 900 to 1200°C. Thereafter, hot rolling was performed at 830 to 1100°C, thereby obtaining a hot-rolled steel having a diameter of 8.0 mm.
• the observation area of the polished surface was from 100 to 1000 mm 2 , and the component composition in a center part of the oxide-based inclusions was quantitatively analyzed by characteristic X-ray wavelength dispersion spectrometry.
• Elements to be analyzed were Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr.
• the average composition of the oxide-based inclusions was determined by previously determining the relationship between the X-ray intensity of each element and the element concentration, as a calibration curve by using a known substance, and quantifying the element amount contained in each sample from the X-ray intensity obtained from the above oxide-based inclusions to be analyzed and the above calibration curve, followed by conversion to the oxide.
• Ti oxides a plurality of valences can be taken, but all are calculated as TiO 2 .
• the value obtained by dividing the number of oxide-based inclusions having a minor axis of 2 ⁇ m or more by the above observation area (100 to 1000 mm 2 ) was taken as the inclusion number of oxide-based inclusions (inclusions/mm 2 ).
• the above hot-rolled steel (diameter: 8.0 mm) was stripped and wire-drawn to a diameter of 7.4 mm, followed by patenting and cold wire drawing to a diameter of 4.0 mm. Then, oil tempering treatment was performed by continuously performing oil quenching and tempering in a lead bath at about 450°C, and thereafter, a wire of 4.0 mm diameter x 650 mm length was obtained. After the wire thus obtained was subjected to treatment equivalent to strain relieving annealing at 400°C, shot peening was performed, and low temperature annealing at 200°C was performed. Thus, a test piece for measuring fatigue strength was prepared.
• Test Nos. 1 to 11, 17 and 18 in Table 2 satisfy the chemical component composition and oxide composition specified in the present invention, and it is known that they are excellent in fatigue properties.
• Test Nos. 12 to 16 did not satisfy any one of the requirements in the present invention. Therefore, fatigue properties were decreased.
• the Si amount and the Al amount were within the ranges in the present invention, but the Si amount and the Al amount were relatively low. Therefore, the CaO amount in the oxide-based inclusions was increased to cause a decrease in fatigue properties
• the Ti amount and the Al amount were within the ranges in the present invention, but the amounts thereof were relatively larger than in the other examples. Therefore, the SiO 2 amount in the oxide-based inclusions was decreased to cause a decrease in fatigue properties.
• the steel wire for a spring in the present invention is more excellent in fatigue properties than the conventional one, and suitable for valve springs, suspension springs and the like.

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• 2015
  • 2015-01-26 JP JP2015012546A patent/JP2015163735A/ja not_active Ceased
  • 2015-01-29 CN CN201580006045.2A patent/CN105940132B/zh not_active Expired - Fee Related
  • 2015-01-29 WO PCT/JP2015/052595 patent/WO2015115574A1/fr not_active Ceased
  • 2015-01-29 US US15/115,185 patent/US20160348221A1/en not_active Abandoned
  • 2015-01-29 KR KR1020167020516A patent/KR101815410B1/ko not_active Expired - Fee Related
  • 2015-01-29 MX MX2016009761A patent/MX2016009761A/es unknown
  • 2015-01-29 EP EP15744088.4A patent/EP3101148A4/fr not_active Withdrawn
• 2018
  • 2018-07-16 US US16/036,655 patent/US20180320255A1/en not_active Abandoned
  • 2018-07-17 JP JP2018134358A patent/JP2018193615A/ja active Pending

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