Classifications

• • 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
  • F16F1/021—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 characterised by their composition, e.g. comprising materials providing for particular spring properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
  • B24C1/10—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for compacting surfaces, e.g. shot-peening
• • 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
  • C22C38/34—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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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

Definitions

• the present invention relates to a high strength spring steel, high strength springs and manufacturing method thereof.
• the reduction in weight of the spring can be achieved by increasing design stress of the spring.
• the design stress is determined by permanent setting resistance and durability, and these characteristics can be improved by the following means. Improvement in the permanent setting resistance can be realized, from a material viewpoint by use of a steel grade (SUP7) having a increased amount of Si as ferrite strengthening element, or a steel grade (SUP12V) having V additionally as grain refining element, or from a processing viewpoint by conducting setting. On the other hand, improvement in the durability can be realized by increasing carbon and alloying elements so as to increase hardness after tempering from the material viewpoint, or by shot peening so as to give compression residual stress from the processing viewpoint.
• SUP7 steel grade
• SUP12V steel grade having V additionally as grain refining element
• the increase in the strength can only be achieved up to HRC52 in spring hardness and 1900 MPa in tensile strength from the durability viewpoint. That is, if the strength of the spring is intensified, the fracture toughness decreases so that the notch sensitivity increases thereby resulting in dispersion in durability and decrease in reliability. Further, it is also a problem that inclusions acting as notches decrease the durability. Further, there are also other problems such that adding any special element costs high in order to prevent decrease in fracture toughness, or that adding a large amount of alloy elements increases hardness after hot rolling so that cracking or breaking of wire occurs when wire is drawn.
• the present invention has been achieved in views of the above-described circumferences and an object of the present invention is to provide a high strength spring steel which has an excellent durability and can be manufactured for practical purposes even when its spring hardness is increased to higher than HRC52 in order to enhance the permanent setting resistance, and high strength spring and manufacturing method thereof.
• Patent document 1 Japanese Patent Application Publication No. 3064672
• the invention provides a high strength spring steel containing in percent of mass, C: 0.36-0.48%, Si: 1.80-2.80%, Mn: 0.20-1.40%, P: 0.015% or less, S: 0.010% or less, Cu: 0.10-0.50%, Ni: 0.10-2.00%, Cr: 0.05-1.20%, s-Al: 0.005-0.040%, N: 0.002-0.012%, O: 0.002% or less while the remainder is constituted of Fe and inevitable impurities, wherein following equations (1), (2), (3) are satisfied and the quantity of inclusions of 10 ⁇ m or larger in diameter per field of vision of 100 mm 2 is 10 or less: 1.2% ⁇ C(%)+Mn(%)+Cr(%) ⁇ 2.0% equation (1) 1.4% ⁇ Si(%)/3+Cr(%)/2+Mn(%) ⁇ 2.4% equation (2) 0.4% ⁇ Cu(%)+Ni(%) equation (3).
• the high strength spring steel has following three features when classified largely. First, hardness thereof after quenching and tempering is adjusted to HRC52 or higher while suppressing the hardness after rolling, by addition and adjustment of components such as Si, Mn, Cr or the like.
• the quantity of inclusions (particularly inclusion of oxides) having a diameter of 10 ⁇ m or larger, which can become an origin of fracture, is reduced by deoxidizing sufficiently (more specifically deoxidizing by Al) thereby providing high fatigue strength.
• the corrosion fatigue strength is enhanced by adding and adjusting anticorrosive elements such as Cu and Ni, and by optimizing the quantity of corrosion accelerating elements.
• a high strength spring steel having an excellent durability is realized even if the hardness is as high as HRC52 or higher. Then, a high strength spring having a maximum shearing stress of 1176 MPa or more is achieved by performing warm peening on such a high strength spring steel as described later.
• C is effective for obtaining a predetermined strength by annealing.
• C of 0.36% or more needs to be added.
• C is added at 0.38% or more.
• the upper limit is set to 0.48%.
• it is 0.46%.
• Si is effective for improving corrosion fatigue strength and permanent setting resistance. For that purpose, it needs to be added at 1.8% or larger. On the other hand, because excessive addition lowers the toughness, deteriorates the fatigue characteristic and accelerates generation of decarburization thereby worsening workability, its upper limit is set to 2.80%. Preferably, it is set to 2.60%.
• the upper limit of phosphor is set to 0.015% because phosphor is segregated in austenite grain boundary at the time of heating for austenitizing so that the grain boundary is embrittled.
• the upper limit of sulfur is set to 0.010% because sulfur embrittles austenite grain boundary like phosphor and forms MnS to induce deterioration of fatigue strength of the spring.
• Cu is effective for intensifying resistance to corrosion, thereby enhancing corrosion fatigue strength. Further, it is effective for preventing decarburization of ferrite. In order to obtain these effects, it needs to be added at 0.10% or larger. On the other hand, because excessive addition damages hot workability, the upper limit thereof is set to 0.50%. Preferably, it is set to 0.40%.
• Cr is an element which contributes to improvement of hardenability. For that purpose, it needs to be added at 0.05% or larger. On the other hand, excessive addition intensifies hardness of wire material after rolling and deteriorates workability of the wire material. Further, carbide of Cr turns into a local electrode on the surface of steel to increase a corrosion pit so that the corrosion fatigue strength decreases. Therefore, the upper limit thereof is set to 1.20%. Preferably, it is set to 1.1%.
• Al(aluminum) is a deoxidizing element and the s-Al needs to be added at 0.005% or more to obtain its effect.
• the upper limit thereof is set to 0.040%.
• it is set to 0.030%.
• the s-Al means Al soluble in acid.
• N has an effect of forming carbonitride and nitride which contributes to refinement of crystal grain in steel. In order to obtain this effect, it needs to be contained at 0.002% or more. On the other hand, since excessive addition generates bulky pieces of Nb carbonitride so that preventing effect of grain coarsening can not be obtained and generates TiN type inclusions thereby inducing decrease of fatigue strength, the upper limit thereof is set to 0.012%.
• C(%)+Mn(%)+Cr(%) needs to be 1.2% or larger. Preferably, it is set to 1.3% or more. On the other hand, if C(%)+Mn(%)+Cr(%) exceeds 2.0%, the steel hardens too much after rolling so that breaking or surface flaw occurs at the time of drawing. Preferably, it is set to 1.9% or less. *1.4% ⁇ Si(%)/3+Cr(%)/2+Mn(%) ⁇ 2.4% equation (2) [in case where B is added as described later: 1.4% ⁇ Si(%)/3+Cr(%)/2+Mn(%)+170B(%) ⁇ 2.4% equation (2)′]
• Si(%)/3+Cr(%)/2+Mn(%) also needs to be 1.4% or more. Preferably, it is set to 1.5% or more. On the other hand, if Si(%)/3+Cr(%)/2+Mn(%) exceeds 2.4%, the hardenability is intensified excessively so that crack occurs at the time of quenching. Preferably, it is set to 2.1% or less. *0.4% ⁇ Cu(%)+Ni(%) equation (3)
• Cu(%)+Ni(%) needs to be 0.4% or larger in order to secure fatigue characteristic under corrosive environment.
• the quantity of inclusions (oxide type inclusions) having a diameter of 10 ⁇ m or more is 10 or less per field of vision of 100 mm 2 .
• the high strength spring steel of the present invention can contain one or more of Ti: 0.020-0.070%, Nb: 0.020-0.050% and B: 0.0005-0.0030% as well as the above-described steel components.
• B the equation (2) is replaced with a following equation (2)′. 1.4% ⁇ Si(%)/3+Cr(%)/2+Mn(%)+170B(%) ⁇ 2.4% equation (2)′
• the high strength spring steel of the present invention is characterized in containing, in percent of mass, of: C: 0.36-0.48%; Si: 1.80-2.80%; Mn: 0.20-1.40%; P: 0.015% or less; S: 0.010% or less; Cu: 0.10-0.50%; Ni: 0.10-2.00%; Cr: 0.05-1.20%; s-Al: 0.005-0.040%; N: 0.002-0.012%; O: 0.002% or less, while further one or more of Ti: 0.020-0.070%; Nb: 0.020-0.050%; B: 0.0005-0.0030% are contained and the remainder is constituted of Fe and inevitable impurities, wherein following equations (1), (2)′, (3) are satisfied and the quantity of inclusions of 10 ⁇ m or larger in diameter per field of vision of 100 mm 2 is 10 or less: 1.2% ⁇ C(%)+Mn(%)+Cr(%) ⁇ 2.0% equation (1) 1.4% ⁇ Si(%)/3+Cr(%)/2+Mn(%)+170B(
• Ti forms carbonitride in steel and refines austenite grain, and contributes to precipitation hardening. In order to obtain this effect, it is preferable to be added at 0.020% or more. On the other hand, because excessive addition allows it to be left as a relatively large non-dissolved compound when steel is heated for quenching and can become an origin of fracture thereby reducing fatigue strength, preferably, it is set to 0.070% or less.
• Nb contributes to refinement of crystal grain and precipitation hardening and has an effect of enhancing permanent setting resistance. In order to obtain this effect, it is preferable to be added at 0.020% or larger. On the other hand, because excessive addition saturates its effect and lowers hot and cold workability, it is preferable to be added at 0.050% or less.
• B precipitates with preference at crystal grain boundary thereby preventing segregation of P and S at the crystal grain boundary and enhancing fatigue strength and delayed fracture property. In order to obtain this effect, it is preferable to be added at 0.0005% or more. On the other hand, since excessive addition forms B nitride thereby damaging toughness of steel and deteriorating fatigue characteristics, it is preferable to be added at 0.0030% or less. If B is added as described above, the aforementioned equation (2)′ needs to be satisfied.
• the high strength spring steel of the present invention may further contain one or two of Mo: 0.01-0.50% and V: 0.05-0.30% as steel components.
• Mo is an element which contributes to improvement of hardenability. It is an element which intensifies corrosion resistance, thereby enhancing corrosion fatigue strength. In order to obtain these effects, it is preferable to be added at 0.01% or more. On the other hand, since excessive addition generates bainite in wire material after rolling and can induce deterioration of cold workability, it is preferable to be 0.50% or less. More preferably, it is set to 0.40%.
• the magnitude of residual stress distribution by shot peening is related to deformation characteristics of the material, and for example, for the same tensile strength, a material having a smaller yield ratio (yield stress/tensile strength) can obtain a larger compressive residual stress by shot peening.
• warm shot peening WSP is performed on a formed spring steel.
• the warm shot peening is a method of carrying out the shot peening in a warm temperature range of 150° C.-350° C.
• the yield point at the time of shot peening of spring steel decreases so that a sufficient compressive residual stress is obtained and finally the fatigue strength and corrosion fatigue strength of the spring are enhanced.
• the warm shot peening is preferred to be carried out in a temperature range of 200-350° C.
• the quantity of C in the high strength spring steel of the present invention is lower as described above, and thus its yield ratio is small so that a sufficient compressive residual stress can be obtained by shot peening. Further, since the quantity of C is low so that a larger deformation can be introduced, improvement in hardness in the vicinity of the surface by dynamic strain aging is achieved, thereby securing an excellent fatigue strength enhancing effect.
• the ordinary shot peening has such a problem that an expensive SP material with high hardness needs to be used because the shot peening (SP) material is easy to crack.
• a high strength spring having hardness tempered to HRC52 or higher while its maximum shearing stress is 1176 MPa or higher can be obtained, provided with an excellent durability due to a sufficient compressive residual stress.
• a high strength spring can be employed preferably for a coil spring, leaf spring, torsion bar, stabilizer and the like used in an automobile suspension unit or the like.
• Table 2 shows values of C(%)+Mn(%)+Cr(%), Si(%)/3+Cr(%)/2+Mn(%), Cu(%)+Ni(%) .
• an example which is lower than the lower limit is supplied with a downward arrow and an example which is higher than the higher limit is supplied with an upward arrow ( ).
• Table 2 shows a result of the evaluations.
• Hardness of the rolled wire material was measured in its cut section.
• Rockwell C scale hardness was measured at 30 points and an average hardness plus 6 times of standard deviation ⁇ (dispersion) was regarded as “hardness after rolling”. For determination on whether the hardness after rolling was acceptable, HRC35 was set to be an upper limit.
• the rolled wire material of 13 mm in diameter was phosphated and cold drawn to 12 mm in diameter so as to obtain a drawn material. Presence or absence of breaking upon drawing processing was evaluated.
• the drawn material was heated at 900° C. or higher, it was quenched by water cooling immediately so as to obtain a quenched material. Presence or absence of crack was evaluated on the quenched material.
• Hardness-of a core portion in the section of the quenched material was measured.
• Rockwell C scale hardness was measured at 20 points and whether or not an average hardness satisfies a predetermined hardness (more than 52HRC) was evaluated.
• the quantity of oxide type inclusions of 10 ⁇ m or larger in diameter in the drawn material per 100 mm was evaluated.
• test piece was cut out from the drawn material of 20 mm in diameter and quenched from 900° C. or higher and tempered to 54HRC to obtain a fatigue test piece. ONO type rotating bending fatigue test was performed using the test piece so as to evaluate the fatigue strength.
• a tempered material having hardness of HRC52 was obtained.
• reversed torsion fatigue test was performed with stress amplitude set to 700 MPa. Corrosion fatigue property was evaluated with the number of repetitions until fracture. In the meantime, whether or not the number of repetitions until fracture reached 100,000 was adopted as a criterion for determining whether or not the property was acceptable.
• Depth of corrosion pit was measured at 40 points in a section of a corroded portion after the fatigue test so as to measure a maximum value (maximum pit depth) of a pit depth. Whether or not the maximum pitch depth was 100 ⁇ m or larger was adopted as a criterion for determining whether or not the pit depth was acceptable. Table 3 shows a relation between the corrosion pit depth and the number of repetitions until fracture.
• the comparative examples 13-15 were short in the value of C(%)+Mn(%)+Cr(%) to the range specified by the invention, and the comparative examples 15-17 were short in the value of Si(%)/3+Cr(%)/2+Mn(%)+170B(%) (also, the comparative example 13 is short in the quantity of C, the comparative example 14 is short in the quantity of Cr, the comparative example 15 is short in the quantity of Mn and the comparative example 16 is short in the quantity of Si) and (4) hardness after quenching was lower than HRC52 and the strength and hardenability were insufficient.
• the comparative examples 18 and 19 were excessive in the value of Si(%)/3+Cr(%)/2+Mn(%)+170B(%) to the range specified by the present invention, so that (3) quench crack occurred at the time of quenching.
• the comparative example 20 exceeded the range specified by the present invention in the quantity of Al, the comparative example 21 was excessive in the quantity of O, the comparative example 22 was excessive in the quantity of P, S, Al and O and (5) the quantity of inclusions was 10 or more while (6) the fatigue strength was insufficient. Further, (7) the corrosion fatigue strength was insufficient also.
• the comparative example 23 exceeded the range specified by the present invention in the quantity of C
• the comparative example 24 was excessive in the quantity of Si
• the comparative example 25 was excessive in the quantity of Cr
• the comparative example 26 was short in the quantity of Cu
• the comparative example 27 was short in the quantity of Ni
• the comparative example 28 was short in the value of Cu(%)+Ni(%)
• the depth of corrosion pit is greater than 100 ⁇ m and (7) the corrosion fatigue strength was insufficient.
• a coil spring obtained by performing warm shot peening on a spring steel in the range specified by the invention described above, in which an excellent fatigue strength, corrosion fatigue strength, and permanent setting resistance are achieved will be shown as an example.
• a cold forming method and a hot forming method are available as a spring forming method, a coil spring having such characteristics was obtained in both of the methods.
• the example A is a cold formed spring and the example B is a hot formed spring.
• the temperature condition of the warm spot peening was set to 250° C.
• a spring obtained by cold forming a conventional steel (SUP7) was used as a comparative example.
• Table 4 shows compositions of high strength spring steel of the present invention used in the examples A, B and the conventional steel (SUP7) used in the comparative example.
• the spring hardness was set to 52HRC and 54HRC. This is due to a following reason.
• the hardness of the spring varies in a certain range due to components and inevitable changes in tempering temperature.
• experiments were performed in a range from 52HRC to 54HRC as upper and lower limits.
• the fatigue strength and permanent setting property not relating to corrosion are higher as the hardness of the spring increases.
• the corrosion fatigue strength is lower as the hardness of the spring increases. Therefore, in order to achieve sufficient fatigue strength, permanent setting property and corrosion fatigue strength under a design stress of 1176 MPa, the fatigue strength and permanent setting property need to be satisfied even if the hardness is lower and the corrosion fatigue strength needs to be improved even if the hardness is higher.
• the fatigue strength and permanent setting property were compared with that of the conventional steel SUP7 having hardness of 49HRC and 54HRC. Further, the corrosion fatigue strength was compared with the conventional steel SUP7 having hardness of 51HRC.
• the tightening test is carried out in a following procedure.
• a load P1 is applied to the coil spring with a load testing machine so as to generate a predetermined shearing stress, for example, 1176 MPa so that the spring is allowed to deflect up to a corresponding height H. With this condition, the height of the spring is constrained with a jig.
• the spring is heated at a predetermined temperature in a predetermined interval of time , at 80° C. for 96 hours in this test, so as to generate creep deformation in the spring.
• This residual shearing distortion y indicates permanent setting property of the spring.
• Table 6 shows a comparison of the permanent setting property in the tightening test between the examples and the comparative examples.
• the example A having hardness of 52HRC indicates an far more better permanent setting property than the comparative example (SUP7) having lower hardness of 49 HRC. Further, it indicates a substantially equal permanent setting property under all tightening stresses as compared with the SUP7 having hardness of 54HRC, that is higher than the example A. Further, the example B having hardness of 52HRC indicates an excellent permanent setting property as compared to the SUP7 of the 54HRC having higher hardness.
• the corrosion fatigue test was carried out in a following procedure.
• the comparative example SUP7 has hardness of 51HRC and the vibration condition is 735 ⁇ 395 MPa.
• the number of repetitions of endurance is 59,000 times.
• the number of repetitions of endurance is equal or higher than the comparative example, that is, 60,000 times although its hardness is 54HRC and test stress is 735 ⁇ 490 MPa.
• the example B indicated an excellent corrosion fatigue strength as the number of repetitions of endurance is 74,000 times under the condition in which the hardness is 54HRC and the test condition is 735 ⁇ 490 MPa.

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• 2004
  • 2004-08-26 WO PCT/JP2004/012277 patent/WO2006022009A1/ja active Application Filing
  • 2004-08-26 US US11/661,139 patent/US20070256765A1/en not_active Abandoned
  • 2004-08-26 EP EP04772233.5A patent/EP1801253B1/de active Active
  • 2004-08-26 CN CNA2004800438097A patent/CN101001969A/zh active Pending
  • 2004-08-26 JP JP2006531172A patent/JP4588030B2/ja active Active
  • 2004-08-26 ES ES04772233.5T patent/ES2524326T3/es active Active

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