JP4097151B2 - High strength spring steel wire and high strength spring with excellent workability - Google Patents

Description

  The present invention relates to a high-strength spring steel wire and a high-strength spring that are excellent not only in fatigue characteristics and sag resistance but also in cold workability (coiling property).

  Automotive engine valve springs, suspension suspension springs, clutch springs, brake springs, and the like have been required to be designed for high stress in accordance with the recent reduction in weight and output of automobiles.

  For example, if the spring sag resistance is low, the amount of spring sag increases during high stress loading, and the engine speed does not increase as designed, resulting in poor responsiveness. An excellent spring is required.

  In order to improve the sag resistance of the spring, it is known that the strength of the spring material may be increased. In addition, if the spring material is strengthened, the fatigue characteristics are expected to be improved from the viewpoint of the fatigue limit. For example, a method for improving fatigue strength and sag resistance by adjusting chemical components and increasing tensile strength after oil quenching / tempering (after oil temper treatment) is known. Also known is a method for improving sag resistance by adding a large amount of an alloy element such as Si (see Patent Documents 1 and 2).

  However, the method of increasing the fatigue strength and sag resistance by increasing the tensile strength causes a problem that breakage occurs during coiling of the spring. In addition, the method for improving sag resistance by adding a large amount of alloy components increases the sensitivity to surface defects and internal defects, and breakage is likely to occur starting from these defects when the spring is assembled or used.

Therefore, it is difficult to further improve the cold workability while improving both the sag resistance and fatigue characteristics of the spring.
Japanese Patent No. 2898472 (Claim 1, paragraph 0015) JP 2000-169937 A (Claim 1, paragraph 0018, paragraph 0028)

  The present invention has been made in view of the above circumstances, and provides a high-strength spring steel wire and a high-strength spring that are excellent in both sag resistance and fatigue characteristics, and also excellent in workability (cold workability). .

As a result of intensive studies to solve the above problems, the present inventors have added a large amount of alloy elements to improve fatigue strength and sag resistance, and then the proof stress ratio (σ _0.2 / σ _B ). It has been found that when coiling is reduced to 0.85 or less, excellent coiling properties (cold workability) can be obtained. In addition, it has been found that if the crystal grains are made smaller, further improvement in fatigue life and improvement in sag resistance can be achieved, and even if a large amount of Cr is added, sag resistance can be improved without reducing defect sensitivity. The present invention has been completed.

That is, the steel wire for a high-strength spring excellent in workability according to the present invention has C: 0.53-0.68% (meaning mass%, the same shall apply hereinafter), Si: 1.2-2.5%, Mn : 0.2 to 1.5% (for example, 0.5 to 1.5%), Cr: 1.4 to 2.5%, and Al: 0.05% or less (not including 0%) Ni: 0.4% or less (not including 0%), V: 0.4% or less (not including 0%), Mo: 0.05 to 0.5%, and Nb: 0. It contains at least one selected from 05 to 0.5%, and the balance is Fe and inevitable impurities. Moreover, the spring steel wire of the present invention has a tempered martensite structure, the prior austenite grain size number is 11.0 or more, 0.2% proof stress (σ _0.2 ) and tensile strength (σ _B ) ratio (σ _0.2 / σ _B ) is 0.85 or less.

The steel wire for spring preferably has a 0.2% proof stress (σ _0.2 ) of 300 MPa or more when annealed at a temperature of 400 ° C. for 20 minutes.

  The spring of the present invention is made of the steel wire for high-strength spring described above, the core has a hardness of about Hv 550 to 700, and the depth at which the compressive residual stress of the surface turns to tension is 0.05 mm or more and 0.00. It is desirable that it is about 5 mm or less. The spring of the present invention may be subjected to surface hardening treatment (nitriding treatment or the like), but when the surface hardening treatment is not performed, it is desirable that the compressive residual stress on the spring surface is −400 MPa or less. When the surface is hardened (that is, when a nitriding layer is formed on the spring surface), it is desirable that the compressive residual stress of the spring surface is −800 MPa or less, and the spring surface hardness is Hv750. It is preferably about ˜1150. The depth of the hardened layer (the layer that is Hv15 or more harder than the core hardness) is, for example, 0.02 mm or more.

  According to the present invention, since the alloy components are appropriately adjusted, the strength is high, Cr is effectively used, and further, the crystal grain size and the proof stress ratio are also appropriately adjusted. Thus, it is possible to obtain a spring steel wire and a spring excellent in both sag resistance and cold workability.

  The steel wire and spring of the present invention contain C, Si, Mn, Cr, and Al, and further contain at least one selected from Ni, V, Mo, and Nb, and the balance is Fe and inevitable impurities . Hereinafter, the amount of each component and the reason for limitation will be described.

C: 0.53-0.68% (meaning mass%, the same shall apply hereinafter)
C is an element indispensable for securing sufficient high strength as a spring steel loaded with high stress and improving fatigue life, sag resistance, etc., so the lower limit was made 0.53%. However, if the amount is too large, the toughness becomes extremely poor, and cracking during spring processing or in use tends to occur due to surface defects or internal defects, so the upper limit was made 0.68%. A preferable amount of C is 0.58% or more and 0.65% or less.

Si: 1.2-2.5%
Si is an element necessary as a deoxidizer during steelmaking, and is an element useful for enhancing softening resistance and improving sag resistance. Therefore, the lower limit was set to 1.2%. However, too much not only deteriorates toughness and ductility, but also increases flaws, facilitates decarburization of the surface during heat treatment, and deepens the grain boundary oxide layer, shortening the fatigue life. In order to facilitate, the upper limit is set to 2.5%. A preferable Si amount is 1.3% or more and 2.4% or less.

Mn: 0.2 to 1.5%
Mn is an element that is effective for deoxidation during steelmaking, and also contributes to improving strength by improving hardenability, and also contributing to improvement in fatigue life and sag resistance. .2%. A preferable amount of Mn is 0.3% or more, particularly 0.4% or more (for example, 0.5% or more). However, the steel wire (and spring) of the present invention is obtained by hot rolling the steel and then patenting as necessary, followed by wire drawing, oil tempering, coiling, etc. If the amount is too high, a supercooled structure such as bainite is likely to be generated during hot rolling or patenting, and the drawability is likely to be lowered. Therefore, the upper limit is set to 1.5%. A preferable amount of Mn is 1.0% or less.

Cr: 1.4-2.5%
Cr has an effect of improving sag resistance and an effect of reducing defect sensitivity, and is an extremely important element for the present invention. Although Cr has the effect of reducing the fatigue life by thickening the grain boundary oxide layer, this point is controlled by controlling the atmosphere during oil tempering (specifically, about 3 to Since the grain boundary oxide layer can be thinned (by mixing about 80% by volume and forming a dense oxide film on the surface), the present invention can eliminate such a problem. Accordingly, the larger the amount of Cr, the more desirable, 1.4% or more, preferably 1.45% or more, more preferably 1.5% or more. If Cr is excessive, the patenting time at the time of wire drawing becomes too long, and the toughness and ductility are also lowered. Therefore, the content is made 2.5% or less, preferably 2.0% or less.

  In the steel wire and spring of the present invention, the depth of the grain boundary oxide layer is usually about 10 μm or less.

Al: 0.05% or less (excluding 0%)
Al has the effect of refining crystal grains during austenitization, and has the effect of improving toughness and ductility. However, if added excessively, the Al ₂ O ₃ -based coarse non-metallic inclusions increase and the fatigue characteristics are deteriorated, so the upper limit was made 0.05%, preferably 0.04%.

Ni: 0.4% or less (excluding 0%)
Ni is an element useful for enhancing hardenability and preventing low temperature embrittlement. However, if it is too much, a bainite or martensite structure is formed during hot rolling, and the toughness and ductility are lowered. Therefore, the upper limit is set to 0.4%, preferably 0.3%. A preferable amount of Ni is 0.1% or more.

V: 0.4% or less (excluding 0%)
V has the effect of refining crystal grains during heat treatment such as oil tempering (quenching and tempering), and has the effect of improving toughness and ductility. In addition, secondary precipitation hardening occurs during quenching / tempering treatment and strain relief annealing after coiling, thereby contributing to high strength. However, if added excessively, martensite and bainite structures are formed during rolling and patenting, and the workability deteriorates, so the upper limit was made 0.4%, preferably 0.3%. A preferable amount of V is 0.1% or more.

Mo: 0.05-0.5%
Mo is an element useful for improving the softening resistance, exhibiting precipitation hardening, and increasing the yield strength after low-temperature annealing. For example, Mo is 0.05% or more, preferably 0.10% or more. However, if added excessively, martensite and bainite structure are formed at the stage until the oil temper treatment, and the workability deteriorates. Therefore, the upper limit is 0.5%, preferably 0.3%, more preferably 0.8. 2%.

Nb: 0.05 to 0.5%
Since Nb forms Nb carbonitride having a pinning effect, Nb has the effect of refining crystal grains during heat treatment such as oil tempering (quenching and tempering), and can improve toughness and ductility. In order to exhibit such an effect effectively, it is 0.05% or more, preferably 0.10% or more. However, if excessively added, Nb carbonitride aggregates and the crystal grains tend to become coarser, so the upper limit was made 0.5%, preferably 0.3%.

  The structure of the spring steel wire of the present invention is usually a composite structure composed of tempered martensite and retained austenite (remaining austenite after cooling to room temperature). The tempered martensite is, for example, 90 area% or more, and the retained austenite is, for example, about 5 to 10 area%.

  The steel wire and spring of the present invention usually have a prior-austenite grain size number of 11.0 or more (preferably 13 or more). The larger the grain size number (that is, the smaller the grain size) is, the more effective for improving the fatigue life and sag resistance. The crystal grain size number can be increased by adjusting the amount of crystal grain refining elements (Cr, Al, V, Nb) added and by increasing the heating rate during quenching in the oil temper treatment.

Furthermore, the steel wire (oil tempered wire) and spring of the present invention have a ratio of 0.2% proof stress (σ _0.2 ) to tensile strength (σ _B ) (proof strength ratio; σ _0.2 / σ _B ) of 0.85 or less ( Preferably it is 0.80 or less. The smaller the yield ratio after the oil temper, the more the breakage during coiling can be prevented, and the cold workability can be improved. The yield strength ratio can be reduced, for example, by increasing the cooling rate after tempering in the oil temper treatment (for example, water cooling).

  The steel wires and springs of the present invention as described above have high strength because the alloy components are appropriately adjusted, and furthermore, the crystal grain size and the proof stress ratio are also appropriately adjusted. It is excellent in both sagability and cold workability. The Vickers hardness of the steel wire and the core of the spring can be adjusted as appropriate by heat treatment or the like in addition to the adjustment of the alloy components, and is, for example, Hv550 or higher (preferably Hv570 or higher, more preferably Hv600 or higher). The Vickers hardness may be, for example, about Hv 700 or less, or about Hv 650 or less. The surface hardness can be further increased by using a surface hardening treatment technique (such as nitriding treatment). For example, the surface hardness of a spring subjected to nitriding treatment (therefore, a nitriding treatment layer is formed on the surface) is about Hv 750 or more (preferably Hv 800 or more) and Hv 1150 or less (for example, Hv 1100 or less).

The spring steel wire (oil tempered wire) has a 0.2% yield strength (σ _0.2 ) of 300 MPa or higher (preferably 350 MPa or higher) when annealed at a temperature of 400 ° C. for 20 minutes. desirable. The greater the 0.2% yield strength increase (Δσ _0.2 ), the more the sag resistance can be improved. Δσ _0.2 can be increased by increasing the cooling rate (for example, water cooling) after the oil temper treatment (quenching and tempering), similarly to the yield strength ratio.

  In the spring of the present invention, it is desirable that the compressive residual stress on the surface of the spring is increased. The fatigue life can be increased as the residual stress is on the compression side. Desirable compressive residual stress varies depending on whether or not the spring is nitrided, but when the spring is not nitrided, for example, it is −400 MPa or less (preferably −500 MPa or less, more preferably −600 MPa or less). When the residual stress is a negative value, it means compression (and when it is a positive value, it means tensile), and the larger the absolute value, the larger the residual stress. When nitriding is performed (that is, when a nitriding layer is formed on the spring surface), for example, it is about −800 MPa or less (preferably −1000 MPa or less, more preferably −1200 MPa or less). The compressive residual stress on the surface of the spring can be increased, for example, by increasing the number of shot peening (for example, by increasing the number of times twice or more).

  Further, in the spring of the present invention, it is desirable that the depth (crossing point) at which the compressive residual stress on the surface turns into tension is deeper. The deeper the crossing point, the more the residual stress portion on the compression side can be increased and the fatigue life can be improved. The crossing point (depth) is, for example, 0.05 mm or more (preferably 0.10 mm or more, more preferably 0.15 mm or more), 0.5 mm or less (preferably 0.4 mm or less, more preferably 0.35 mm or less). ) The crossing point is, for example, by increasing the number of shot peening (for example, two times or more), and increasing the average grain size of shot grains during shot peening (for example, shot grains during first stage shot peening) The average particle diameter of the glass can be increased by 0.7 to 1.2 mm.

  Further, when the spring of the present invention is subjected to surface hardening treatment (nitriding treatment or the like), it is desirable that the depth of the hardened layer (layer in which Hv is 15 or more harder than the core hardness) is as deep as possible. As the hardened layer is deeper, the occurrence of fatigue cracks can be suppressed and the fatigue characteristics can be improved. The depth of the hardened layer is, for example, 0.02 mm or more (preferably 0.03 mm or more, more preferably 0.04 mm or more), 0.15 mm or less (preferably 0.13 mm or less, more preferably 0.10 mm or less). is there. The hardened layer can be deepened by increasing the nitriding time or increasing the nitriding temperature.

  Since the steel wire and spring of the present invention are excellent in fatigue characteristics, sag resistance, and workability, applications that require these characteristics, such as valve springs for automobile engines, suspension springs for suspensions, clutch springs, brakes, etc. This is particularly useful for a spring used for a restoring mechanism of a machine such as a spring.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
Steels A to R having chemical components shown in Table 1 (the balance being Fe and inevitable impurities) were melted and hot-rolled to prepare a wire having a diameter of 8.0 mm. Next, after softening annealing, surface cutting, and lead patenting treatment (heating temperature: 950 ° C., lead furnace temperature: 620 ° C.), the wire was drawn to a diameter of 4.0 mm. Thereafter, oil tempering treatment (heating rate during quenching: 250 ° C./second, heating temperature: 960 ° C., quenching oil temperature: 70 ° C., tempering temperature: 450 ° C., cooling rate after tempering: 300 ° C./second, furnace atmosphere : 10 vol% H ₂ O + 90 vol% N ₂ ) to produce an oil tempered wire (steel wire).

  In steel type E2, cooling after tempering in the oil temper treatment was air cooling. For steel type H2, the heating rate during quenching in the oil temper treatment was 20 ° C./second.

  The characteristics of the obtained oil tempered wire (grain boundary oxide layer depth: 10 μm or less) were evaluated as follows.

(1) Tensile strength (σ _B ), 0.2% yield strength (σ _0.2 ), grain size number Tensile test was conducted on the above oil tempered wire, tensile strength (σ _B ) and 0.2% yield strength (σ _0.2 ) And the yield strength ratio (σ _0.2 / σ _B ) was calculated. The crystal grain size number of the prior austenite grains was measured according to J1S G0551.

(2) Change in 0.2% yield strength after strain relief annealing (Δσ _0.2 )
After low-temperature annealing (400 ° C. × 20 minutes) of the oil tempered wire, 0.2% proof stress (σ _0.2 ) after low-temperature annealing is measured, and 0.2% proof stress (σ _0.2 ) after low-temperature annealing is used to measure the low temperature. The amount of change (Δσ _0.2 ) was determined by subtracting 0.2% proof stress (σ _0.2 ) before annealing.

(3) Workability The winding test of the oil tempered wire was performed according to JIS G 3560 (number of windings: 10 times).

(4) Fatigue life, residual shear strain Cold coiling (coil average diameter: 24.0 mm, number of turns: 6.0, number of effective turns: 3.5) of the above oil tempered wire, and strain relief annealing (400 ° C x 20 minutes), seat polishing, nitriding treatment (nitriding conditions: 80% by volume NH ₃ + 20% by volume N ₂ , 430 ° C. × 3 hours), shot peening [number of times: 3 times, average grain size of shot grains (first stage) : 1.0 mm, average particle diameter of shot grains (average of first to third stages): 0.5 mm], low-temperature annealing (230 ° C. × 20 minutes), cold setting was performed to obtain a spring.

Each obtained spring was subjected to a fatigue test under a load stress of 760 ± 650 MPa at a warm temperature (120 ° C.), and the number of repetition until the spring broke was measured (fatigue life). When the spring did not break, the test was stopped at a repetition number of 1 × 10 ⁷ times.

  Also, after each spring was tightened for 48 hours under a stress of 1372 MPa (temperature: 120 ° C.), the stress was removed, the amount of sag before and after the test was measured, and the residual shear strain was calculated. did.

(5) Hardness and residual stress The oil tempered wire was used as a spring in the same manner as "(4) Fatigue life and residual shear strain". The Vickers hardness (Hv) of the surface of the spring was measured by a method (cord method) in which the Vickers hardness (300 gf) was measured on a sample whose surface was polished and converted to the vertical direction. Moreover, the said spring is cut | disconnected and the Vickers hardness (Hv) of a cross section is measured according to JISZ2244, Vickers hardness (Hv) of a hardened layer, and a hardened layer (core part of a core part). The depth of the layer having a higher Hv15 or more than the hardness was determined. Furthermore, the residual stress was measured by the X-ray diffraction method, and the point (depth; crossing point) at which the compressive residual stress on the surface of the spring and the compressive residual stress on the surface side turned into the tensile residual stress were obtained.

  The results are shown in Table 2.

  As apparent from Tables 1 and 2, No. In No. 18, since the amount of C is insufficient, the predetermined strength is not achieved, and the fatigue life and sag resistance are insufficient. No. In No. 20, since the Al content is excessive, the oxide inclusions become coarse and become the starting point of fracture, so the fatigue life is short. No. Also in 14-17 and 19, since the amount of Cr is insufficient, the fatigue life is insufficient.

  On the other hand, no. In 1-5, 7-9, and 11-13, various chemical components are appropriately adjusted, and a predetermined amount of Cr is added. Further, the crystal grain size and the proof stress ratio are also appropriately controlled. Therefore, it is excellent in all of fatigue life, sag resistance, and workability.

No. As can be seen from FIG. 6, if the conditions of the yield strength ratio (σ _0.2 / σ _B ) and the 0.2% yield strength change amount (Δσ _0.2 ) are inappropriate, the workability deteriorates. In addition, the No. Although improved compared to 14-17, the sag resistance becomes insufficient.

  No. As is clear from FIG. 10, when the crystal grain becomes larger (when the grain size number becomes smaller), Although improved compared to 14-17, fatigue life and sag resistance are insufficient.

Claims (6)

A spring steel wire having a tempered martensite structure, the spring steel wire comprising:
C: 0.53-0.68% (meaning mass%, the same shall apply hereinafter)
Si: 1.2 to 2.5%,
Mn: 0.2 to 1.5%
Cr: 1.4-2.5%, and Al: 0.05% or less (not including 0%),
Furthermore, Ni: 0.4% or less (not including 0%), V: 0.4% or less (not including 0%), Mo: 0.05 to 0.5%, and Nb: 0.05 to 0 Including at least one selected from 5%,
The balance is Fe and inevitable impurities,
The crystal grain size number of the prior austenite grains is 11.0 or more,
0.2% proof stress (sigma _0.2) and tensile strength (sigma _B) ratio (σ _0.2 / σ _B) is a high strength spring steel wire with excellent workability, characterized in that it is 0.85 or less.   The steel wire for high-strength springs according to claim 1, wherein Mn is 0.5 to 1.5%. 3. The high strength spring steel according to claim 1, wherein the spring steel wire has a 0.2% yield strength (σ _0.2 ) of 300 MPa or more when annealed at a temperature of 400 ° C. × 20 minutes. line.   A high-strength spring comprising the steel wire for a high-strength spring according to any one of claims 1 to 3. The spring is
The hardness of the core is Hv550-700,
The high compressive residual stress of the surface of the spring is -400 MPa or less, and the depth at which the compressive residual stress of the surface turns into tension is 0.05 mm or more and 0.5 mm or less. Strength spring. The spring has a nitriding layer formed on the surface,
The surface hardness is Hv750-1150,
The hardness of the core is Hv550-700,
The depth of the hardened layer that is Hv15 or higher than the core hardness is 0.02 mm or more and 0.15 mm or less,
The high compressive residual stress according to claim 4, wherein the compressive residual stress on the surface of the spring is −800 MPa or less, and the depth at which the compressive residual stress on the surface turns into tension is 0.05 mm or more and 0.5 mm or less. Strength spring.