US20180230566A1 - Spring steel for suspension and method for producing same - Google Patents

Spring steel for suspension and method for producing same Download PDF

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US20180230566A1
US20180230566A1 US15/744,394 US201615744394A US2018230566A1 US 20180230566 A1 US20180230566 A1 US 20180230566A1 US 201615744394 A US201615744394 A US 201615744394A US 2018230566 A1 US2018230566 A1 US 2018230566A1
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less
rolling
area
reduction
steel
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Inventor
Takahisa Suzuki
Keisuke Chiba
Hirotsugu MIYAMOTO
Kengo Iwanaga
Tsukasa Okamura
Fumiaki FUJIWARA
Yukio EBANA
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Neturen Co Ltd
Nippon Steel Corp
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Neturen Co Ltd
Nippon Steel and Sumitomo Metal Corp
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Assigned to NETUREN CO., LTD., NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NETUREN CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: EBANA, Yukio, IWANAGA, KENGO, OKAMURA, TSUKASA, CHIBA, KEISUKE, MIYAMOTO, Hirotsugu, SUZUKI, TAKAHISA, FUJIWARA, Fumiaki
Publication of US20180230566A1 publication Critical patent/US20180230566A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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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
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • 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
    • 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
    • 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
    • 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
    • C21D2211/001—Austenite
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to spring steel used for suspension springs for automobiles etc., preferably high strength spring steel for suspension springs having a 1800 MPa or more tensile strength, and a method for producing the same.
  • a spring is usually manufactured by the cold spring making method of which drawing a hot rolled wire rod to a predetermined wire size, quench hardening it, tempering it, then shaping it in cold, or the method of which shaping a drawn wire rod while hot, quench hardening it, and tempering it.
  • high strength springs with a tensile strength of 1800 MPa or more are being developed in order to deal with environmental problems, further lightening of weight and lowering of fuel consumption have been sought from automobiles.
  • the cold spring making method has a high degree of freedom of design of spring shape and is also high in precision, so is a work method particularly suitable for high strength springs.
  • the material that is, the spring steel
  • An indicator of the formability is “reduction of area in tensile test”, that is, the rate of reduction of a cross-section at a location of break at the time of a tensile test.
  • a wire rod for spring use adjusted in relationship of amount of C and center segregation to raise the cold formability has been proposed (PLT 1). Further, the method of quenching a hot worked wire rod as is and tempering it to refine the microstructure and improve the strength and fatigue characteristic has also been proposed (PLT 2).
  • the delay fracture resistance is evaluated by the critical hydrogen content where the test piece does not break even if a certain load is applied (critical hydrogen content).
  • alloy elements such as Ni or Cu for improving the corrosion resistance, V or Mo for forming precipitates serving as sites for trapping hydrogen, etc. (PLT 3).
  • the inventors discovered that the reduction of area in tensile test is improved at the same strengths by controlling the crystal orientations of the martensite structures so that ⁇ 011 22 orientations preferentially appear and ⁇ 111> orientations are suppressed with respect to the rolling direction of spring steel. Further, they obtained the finding that both delayed fracture resistance and cold formability can be realized by controlling the crystal orientations of the martensite structures in addition to stretching of the prior austenite grains. Further, they investigated the hot rolling conditions required for stably obtaining such crystal orientations and as a result succeeded in stably stretching the prior austenite grains and increasing the fraction of the ⁇ 011> orientations of the martensite structures and suppressing the ⁇ 111> orientations with respect to the rolling direction.
  • a ratio of a length in a long axis direction of prior austenite grains and a length in a direction perpendicular to the long axis direction of the prior austenite grains is 1.5 or more and a ratio of ⁇ 011> fraction/ ⁇ 111> fraction of martensite texture as observed from the rolling direction is 3.0 or more.
  • V 0.50% or less
  • Ni 1.00% or less
  • Al 0.50% or less
  • Nb 0.10% or less.
  • a time from an end of the hot rolling to a start of the quenching is within 3 seconds.
  • FIG. 1 is a view explaining one example of a test piece for a delayed fracture test.
  • Prior austenite metal microstructures are structures before martensite transformation due to quenching. If rapidly cooling the structures after hot working, in particular the prior austenite grains at the surface become shapes stretched in the rolling direction, the long axis direction of the prior austenite grains substantially matches the rolling direction of the spring steel, and, for example, the deviation between the long axis direction and rolling direction is 15° or less. In the case of delayed fracture, cracks form from the surface of the steel and propagate along the grain boundaries of prior austenite grains. So if the prior austenite grains at the surface layer are stretched in the rolling direction, propagation of cracks in the diametrical direction of the wire rod and thickness direction of a plate material is suppressed.
  • the long axis direction of the prior austenite grains after rolling substantially matches the rolling direction of spring steel, while the direction perpendicular to the long axis direction substantially matches the direction perpendicular to the rolling direction, so if the ratio of the length in the long axis direction of prior austenite grains and the length in a direction perpendicular to the long axis direction (aspect ratio) of the surface layer is large, the delayed fracture resistance is improved.
  • the fact that texture has an effect on plastic deformation of steel is known in particular in steel sheet. Therefore, the inventors studied the relationship of a drawing action and texture required for improving the cold formability in the cold spring making method. As a result, as the method of improving the drawing action, they obtained the finding that it is effective to increase the fraction of ⁇ 011> orientations of the martensite texture with respect to the rolling direction and suppress the ⁇ 111> orientations at the martensite texture of the surface layer part of the steel.
  • the hot rolling conditions in order to increase the aspect ratio of the prior austenite grains and simultaneously suitably control the martensite texture. That is, in the hot rolling process, if giving a reduction of area in rolling 15% to less than 30% in the 1150 to 950° C. temperature region, and after that giving a reduction of area in rolling 20% or more in the 950 to 750° C. temperature region and giving a cumulative reduction of area in rolling 40% or more with the rate of reduction of area in rolling in the 1150 to 950° C.
  • the material when designating the diameter or thickness of the spring steel “D”, in the range of 0.1 D from the surface layer (10% of diameter or thickness from surface), the ratio of the length in a long axis direction of prior austenite grains and the length in the direction perpendicular to the long axis direction, that is, the aspect ratio, becomes 1.5 or more.
  • the ⁇ 011> orientations of the martensite structures were increased in fraction and ⁇ 111> orientations were suppressed in the rolling direction.
  • the steel was tempered to obtain spring steel with a tensile strength of 1800 MPa or more, a reduction of area in tensile test of 40% or more, and a critical hydrogen content of 0.30 ppm or more.
  • the chemical components of the spring steel for suspension of the present disclosure will be explained.
  • the percent marks indicate the mass % based on the total mass of the spring steel for suspension.
  • C is an element required for raising the strength of spring steel for suspension by quench hardening and tempering.
  • the amount of C has to be made 0.40% or more.
  • the amount of C is preferably 0.45% or more, more preferably 0.48% or more.
  • the upper limit of the amount of C must be 0.70%.
  • the amount of C is preferably 0.65% or less, more preferably 0.60% or less.
  • Si is an extremely important element which raises the tensile strength of spring steel for suspension and in particular raises the elastic limit and improves the resistance to change of the shape of the suspension spring during use, that is, sag loss.
  • the amount of Si is made 0.80% or more.
  • the addition of Si raises the temper softening resistance, so in particular from the viewpoint of improving the tensile strength, the amount of Si is preferably 1.20% or more.
  • the upper limit of the amount of Si is made 2.20%. From the viewpoint of improving the manufacturability in the steelmaking and rolling processes, the amount of Si is preferably 2.00% or less.
  • Mn is an element required for desulfurization of the spring steel for suspension.
  • the amount of Mn is 0.05% or more.
  • Mn is an element effective for improvement of hardenability at quench hardening. From the viewpoint of obtaining this effect, the amount of Mn is preferably 0.20% or more.
  • the upper limit of the amount of Mn must be 1.50%.
  • the amount of Cr is an element effective for raising the tensile strength of the spring steel for suspension and improving the hardenability. To obtain these effects, the amount of Cr is made 0.05% or more. Further, from the viewpoint of raising the temper softening resistance, the amount of Cr is preferably 0.15% or more. On the other hand, from the viewpoint of keeping undissolved carbides at the time of heating from remaining in the austenite temperature region and preventing unevenness of the components and formation of coarse carbides which may work as initiation points of fracture, the upper limit of the amount of Cr must be 1.00%.
  • P is an impurity. To suppress embrittlement of the steel, the amount of P is limited to 0.020% or less.
  • S is an impurity. To suppress embrittlement of the steel, the amount of S is limited to 0.020% or less.
  • spring steel for suspension may if necessary have one or more of Mo, V, Ni, Cu, Al, Ti, Nb, and B added to it.
  • Mo is an element contributing to improvement of hardenability by addition in a small amount.
  • Spring steel for suspension preferably contains 0.05% or more of Mo.
  • Mo is an element effective for raising the strength of spring steel for suspension by precipitation strengthening of carbides. From the viewpoint of obtaining this effect, spring steel for suspension preferably contains 0.10% or more of Mo.
  • the amount of Mo is preferably 0.50% or less.
  • Mo is an expensive alloy element, so from the viewpoint of the cost, the amount of Mo is preferably 0.45% or less, more preferably 0.20% or less, still more preferably is not contained.
  • the spring steel for suspension of the present disclosure can be given a large tensile strength and excellent delayed fracture resistance and cold formability even when Mo is not contained or is contained in a very small amount. If further including Mo, the spring steel for suspension of the present disclosure can be given better characteristics.
  • V is an element contributing to improvement of hardenability by addition in a small amount.
  • Spring steel for suspension preferably contains 0.03% or more of V.
  • V is an element effective for raising the strength of spring steel for suspension by precipitation strengthening of carbides. From the viewpoint of obtaining this effect, spring steel for suspension preferably contains 0.10% or more of V.
  • the amount of V is preferably 0.50% or less.
  • V is an expensive alloy element, so from the viewpoint of the cost, the amount of V is preferably 0.25% or less, more preferably 0.10% or less, still more preferably is not contained.
  • the spring steel for suspension of the present disclosure can be given a large tensile strength and excellent delayed fracture resistance and cold formability even when V is not contained or is contained in a very small amount. If V is further contained, the spring steel for suspension of the present disclosure can be given better characteristics.
  • Ni is an element contributing to improvement of the toughness and hardenability of spring steel for suspension.
  • Spring steel for suspension preferably contains 0.10% or more of Ni.
  • Ni is also an element effective for improvement of corrosion resistance. From the viewpoint of obtaining this effect, spring steel for suspension more preferably contains 0.15% or more of Ni.
  • the above effect is obtained by addition of a small amount of Ni. Even if excessively added, no increase in the effect can be expected, so the amount of Ni is preferably 1.00% or less.
  • Ni is an expensive alloy element, so from the viewpoint of the cost, the amount of N is preferably 0.60% or less, more preferably 0.30% or less, still more preferably is not included.
  • the spring steel for suspension of the present disclosure can be given a large tensile strength and excellent delayed fracture resistance and cold formability even when Ni is not contained or is contained in a very small amount. If further including Ni, the spring steel for suspension of the present disclosure can be given better characteristics.
  • Cu is an element contributing the improvement of the hardenability of spring steel for suspension.
  • Spring steel for suspension preferably contains 0.05% or more of Cu.
  • Cu is an element effective also for improvement of the corrosion resistance. From the viewpoint of obtaining this effect, the spring steel for suspension more preferably contains 0.10% or more of Cu.
  • the amount of Cu is preferably 0.50% or less, more preferably 0.30% or less.
  • B is an element improving the hardenability and simultaneously cleaning the grain boundaries to improve the toughness.
  • Spring steel for suspension preferably contains 0.0010% or more of B. However, if excessively adding B, this lowers the hot ductility of the steel and degrades the manufacturability, so the amount of B is preferably 0.0050% or less.
  • Al is a deoxidizing element.
  • the spring steel for suspension preferably contains 0.003% or more of Al.
  • Al is an element effective for suppressing coarsening of the austenite grains during heat treatment due to precipitates of AlN etc. From the viewpoint of obtaining this effect, the amount of Al is more preferably 0.020% or more. On the other hand, if the amount of Al is too large, coarse inclusions forming the initiation points of fracture are easily formed, so the amount of Al is preferably 0.500% or less.
  • the spring steel for suspension of the present disclosure can be given a large tensile strength and excellent delayed fracture resistance and cold formability even if not containing Al or if containing an extremely small amount of the level of an unavoidable impurity of 0.001% or less. Even if the spring steel for suspension contains 0.001% or less of Al, at the level of an unavoidable impurity of 0.001% or less, the effect as a deoxidizing element cannot substantially be obtained.
  • the spring steel for suspension contains the same amount of oxygen as the case where no Al is contained.
  • Ti is an element effective for suppressing coarsening of the austenite grains during heat treatment due to precipitates of TiN etc.
  • Spring steel for suspension preferably contains 0.002% or more of Ti.
  • the amount of Ti is more preferably 0.010% or more.
  • the amount of Ti is preferably 0.200% or less.
  • Nb is an element effective for suppressing coarsening of the austenite grains during heat treatment due to precipitates of NbN etc.
  • Spring steel for suspension preferably contains 0.002% or more of Nb.
  • the amount of Nb is more preferably 0.010% or more.
  • the amount of Nb is preferably 0.100% or less.
  • a suspension spring receives a high load and is required to return to its original shape even after deforming.
  • the main microstructure has to be made tempered martensite with a high tensile strength. Therefore, when quenching and tempering spring steel for suspension, phase fraction of tempered martensite has to be the largest phase fraction in the spring steel for suspension.
  • the area fraction of tempered martensite is 90% or more.
  • the balance is preferably retained austenite.
  • the area fraction of tempered martensite is evaluated by polishing the L-cross-section of the steel to a mirror finish, then corroding it by 3% Nital (3% nitric acid-ethanol solution), observing the microstructure by an optical microscope in the three fields of the surface layer part, 1 ⁇ 2 position of the radius or 1 ⁇ 4 position of the thickness, and center part, and obtaining the average area fraction of the martensite.
  • the “L-cross-section” means the surface when cutting the steel parallel to the rolling direction to pass through the center axis.
  • the tensile strength of the spring steel for suspension is preferably 1900 MPa or more, more preferably 1950 MPa or more, still more preferably 2000 MPa or more.
  • “Delayed fracture” is the phenomenon of a steel fracturing by a crack formed at the surface layer of the steel propagating to the inside when hydrogen penetrates the steel in the state where stress is applied.
  • the path of propagation of cracks is the grain boundaries of prior austenite.
  • the aspect ratio By making the aspect ratio larger, it is possible to ensure the direction of propagation of cracks from the surface layer part to the inside part and the direction of stretching of the austenite grain boundaries do not match, so there is the effect that the propagation of the cracks is suppressed. On the other hand, if the aspect ratio is small, the effect of suppressing propagation of cracks becomes smaller.
  • the ratio of the length in the long axis direction of prior austenite grain and the length in the direction perpendicular to the long axis direction that is, the aspect ratio of the length to the width, has to be 1.5 or more.
  • the aspect ratio is preferably 1.9 or more, more preferably 2.0 or more, still more preferably 2.1 or more, further more preferably 2.2 or more. From the viewpoint of suppressing the propagation of cracks, the larger the aspect ratio the better, but from the relationship between the amount of working and recrystallization at the time of hot rolling, the upper limit of the aspect ratio of the prior austenite grains is preferably less than 5.0. In the present application, the “aspect ratio of the prior austenite grains” means the average value of the aspect ratio.
  • the region where the aspect ratio of the prior austenite grains is 1.5 or more has to continue until a position of 10% of the diameter or thickness from the surface. Therefore, the aspect ratio of the prior austenite grains of the spring steel for suspension is measured at a position of 10% of the diameter or thickness from the surface. The aspect ratio of the prior austenite grains at this position is made 1.5 or more.
  • a microstructure texture where ⁇ 011> orientations gather in the rolling direction is known as a “drawing texture”. If ⁇ 011> orientation fraction is large in the rolling direction, the slip planes of the martensite, the ⁇ 011 ⁇ planes, easily become arranged in the maximum shear force direction. Motion of a slip system advantageous for reducing the cross-sectional area by the load in the rolling direction can be easily selected. For this reason, if ⁇ 011> orientation fraction is large in the rolling direction, this is considered advantageous in drawing deformation accompanying the reduction in cross-section.
  • the ⁇ 111> orientation fraction is large in the rolling direction, it becomes harder for the slip directions of martensite, that is, the ⁇ 011 ⁇ planes, to be arranged in the maximum shear force direction and it becomes difficult for slip deformation to occur with respect to the load in the rolling direction.
  • strain localizes at specific grains and easily becomes initiation points of fracture. For this reason, it is conceivable that the ⁇ 111> orientations gathering in the rolling direction is disadvantageous to drawing deformation.
  • the ratio of ⁇ 011> fraction/ ⁇ 111> fraction of the martensite texture observed from the rolling direction has to become 3.0 or more.
  • the ratio of ⁇ 011> fraction/ ⁇ 111> fraction is preferably 3.8 or more, more preferably 4.4 or more, still more preferably 4.5 or more, even more preferably 4.8 or more.
  • the ratio of ⁇ 011> fraction/ ⁇ 111> fraction of the martensite texture observed from the rolling direction is the area fraction of the grains having ⁇ 011> orientations within 5° in all orientations around from the rolling direction as the center axis and grains having ⁇ 111> orientations within 5° in all orientations around from the rolling direction as the center axis.
  • analysis of the crystal orientations by an SEM-EBSD or analysis of a pole figure by XRD is used. Further, with drawing deformation, the amount of deformation becomes greater the further to the surface layer of the steel, so the ratio of ⁇ 011> fraction/ ⁇ 111> fraction becoming larger near the surface layer is effective for improvement of the reduction of area in tensile test.
  • the reduction of area in tensile test is the rate of change of the cross-sectional area due to tensile test and is preferably 45% or more, more preferably 50% or more, still more preferably 55% or more.
  • the reduction of area in tensile test can be calculated by
  • the “cross-sectional area” is the area in the direction perpendicular to the tensile direction.
  • the critical hydrogen content is used.
  • the upper limit of the amount of hydrogen where no fracture occurs is defined as the “critical hydrogen content” wherein the critical hydrogen content is evaluated using a test piece of a shape shown in FIG. 1 simulating concentrated stress at the part of corrosion pits, precharging hydrogen, then running a constant load test by a load of 0.3 TS (tensile strength).
  • the critical hydrogen content is the upper limit of the amount of diffusible hydrogen where no delayed fracture occurs within 100 hours when charging hydrogen into a test piece of a shape shown in FIG. 1 and running a constant load test by a load of 0.3 TS (tensile strength).
  • the amount of diffusible hydrogen can be measured by graph of temperature-hydrogen emission relation obtained when heating a steel by a speed of 100° C./hour, as a curve having a peak at a temperature of about 100° C.
  • the critical hydrogen content is measured to estimate the delayed fracture resistance in a general corrosive environment.
  • the amount of hydrogen in spring steel for suspension in a general corrosive environment is estimated as a maximum 0.20 ppm, so if the critical hydrogen content of the above shape is 0.30 ppm or more, it is judged that delayed fracture in a general corrosive environment can be suppressed.
  • the critical hydrogen content is preferably 0.40 ppm, more preferably 0.50 ppm, still more preferably 0.60 ppm.
  • the spring steel for suspension of the present disclosure is produced starting from a general hot rolled wire rod and subjecting it to hot rolling, quenching, and tempering processes.
  • the material is made a wire rod of predetermined components produced through a general steelmaking process and rolling process.
  • the process of manufacture from the material to the wire rod is not particularly limited.
  • the wire size of the material is selected considering the reduction of area in subsequent hot rolling so as to become a predetermined wire size after the subsequent hot rolling, quenching, and tempering.
  • the diameter of the wire rod used as a material is ⁇ 14.0 mm when producing a ⁇ 10.0 mm spring steel for suspension after hot rolling by a reduction of area in rolling of 50%.
  • the “reduction of area in rolling” is the rate of change of the cross-sectional area during the rolling and can be calculated by
  • the “cross-sectional area” is the area in the direction perpendicular to the rolling direction.
  • the wire size of the material and the wire size after hot rolling are not particularly limited, but from the viewpoint of achieving both precision of rolling and an amount of rolling reduction well, the wire rod size of the material is preferably 11.5 mm or more, while the wire size after hot rolling is preferably 8.0 mm or more. From the viewpoint of better securing the hardenability due to cooling after rolling, the wire rod size of the material is preferably 20.0 mm or less and the wire size after the hot rolling is preferably 14.0 mm or less.
  • the steel is rapidly cooled to quench it after hot working, and then is tempered.
  • a sufficient reduction of area in rolling in hot rolling is necessary.
  • the control of the temperature and reduction of area in rolling at the time of hot rolling is important.
  • the hot rolling temperature is controlled divided into a less than 1150° C. to 950° C. temperature region, less than 950° C. to 750° C. temperature region, and less than 750° C. temperature region, but these mean the temperature regions of an integrated multistage hot rolling process.
  • Hot rolling in the less than 1150° C. to 950° C. (below, also referred to as “950° C. or more”) temperature region is effective for making the austenite grains largely deform and enlarging the ratio of ⁇ 011> fraction/ ⁇ 111> fraction.
  • the reduction of area in rolling in the 950° C. or more temperature region is large, recrystallization easily occurs. For this reason, to obtain a large ratio of ⁇ 011> fraction/ ⁇ 111> fraction, the reduction of area in rolling at 950° C. or more is controlled to 15% to less than 30%, preferably 20% to less than 25%.
  • 950 to 750° C. In the less than 950° C. to 750° C. (below, also referred to as “950 to 750° C.”) temperature region, it is hard for recrystallization to occur, so it is possible to increase the cumulative reduction of area in rolling with the reduction of area in rolling at less than 1150° C. to 950° C. In particular, by increasing the reduction of area in rolling at 950 to 750° C., it is possible to increase the aspect ratio of the prior austenite grains. By giving a 20% or more reduction of area in rolling in the 950 to 750° C. temperature region and giving a 40% or more cumulative reduction of area in rolling from the 950° C. or more temperature region, it is possible to make the aspect ratio of the prior austenite grains 1.5 or more.
  • the reduction of area in rolling in the 950 to 750° C. temperature region is preferably 25% or more.
  • the cumulative reduction of area in rolling is preferably 45% or more, more preferably 50% or more.
  • the upper limit of the reduction of area in rolling at the 950 to 750° C. temperature region is preferably 75% or less.
  • the steel given a 40% or more cumulative reduction of area in the above hot rolling process is quenched quickly after hot rolling, preferably within 3 seconds after final rolling, more preferably within 2 seconds after final rolling, then was tempered.
  • the quenching conditions are not particularly limited so long as conditions enabling 90% or more of the metal microstructure to be made the martensite structure, but for example the surface temperature of the steel is cooled to preferably 5 to 60° C., more preferably 10 to 40° C., for example 30° C., by cooling rate of preferably 5° C./sec or more, more preferably 10° C./sec or more.
  • the temperature of the tempering should be suitably set, but preferably is 350 to 550° C., more preferably 460° C. to 510° C.
  • the inventors used round bar steel having the components shown in Table 1 and diameters of 11.5 to 20.0 mm, and processed by hot rolling, quenching, and tempering under the condition shown in Table 2, and conducted observation of their microstructures, tensile tests, and delayed fracture tests to evaluate the martensite structure fraction (also called the “martensite fraction”, aspect ratio of prior austenite grains, ⁇ 011>/ ⁇ 111> fraction ratio, tensile strength, reduction of area in tensile test, and critical hydrogen content.
  • the quenching conditions were cooling by a cooling speed of 50° C./sec or more until the surface temperatures become 30° C.
  • Examples 1 to 19 are working examples while Examples X1 to X11 are comparative examples.
  • the martensite fraction was calculated from observation of the microstructure by an optical microscope.
  • the L-cross-section of the round bar steel cut parallel to the rolling direction was polished to a mirror finish, then 3% Nital was used to bring out the microstructure and a 400 ⁇ optical microscope was used to observe the microstructure at three locations of the surface layer part, 1 ⁇ 2 position of the radius, and center part at three fields, for a total of nine fields, and the average martensite fraction was calculated.
  • the aspect ratio of the prior austenite grains was calculated from observation of the structure by an optical microscope. An L-cross-section similar to the above was polished to a mirror surface, then was etched by a picric acid aqueous solution-based corrosive solution to bring out the prior austenite grains. Micrographs of powers of 400 ⁇ were taken in five fields at positions of 10% of the diameter from the surface of the steel. From the length in the long axis direction of prior austenite grains and the length in the direction perpendicular to the long axis direction, measured from the photographs by the interception method, as the aspect ratio, the “grain size (length) in long-axis direction”/“grain size (width) in direction perpendicular to long-axis direction” was calculated.
  • the ratio of ⁇ 011> fraction/ ⁇ 111> fraction of the martensite texture seen from the rolling direction was calculated by analyzing the above L-cross-section microstructure by an SEM-EBSD (made by JEOL, JSM7001-F, EDAX's OIM).
  • SEM-EBSD made by JEOL, JSM7001-F, EDAX's OIM
  • the data measured by the SEM-EBSD was processed using the EBSD analysis software “OIM Analysis” of TLS Solution.
  • the analysis mesh was made 5° and the inversed pole figure was analyzed to calculate the ratio of the ⁇ 011> fraction/ ⁇ 111> fraction.
  • the tensile test was made a round bar tensile test based on JIS Z 2201 and 2241.
  • the critical hydrogen content was calculated by a delayed fracture test using the hydrogen precharge method.
  • the hydrogen precharge method is the method of charging hydrogen into a test piece by cathode hydrogen charging in an aqueous solution for example containing NH 4 SCN, sealing in the hydrogen by HCP metal plating by Cd, Zn, etc., then running a delayed fracture test in that state.
  • hydrogen was precharged and a constant load delayed fracture test was run by a load of 0.3 time the tensile strength (0.3 TS) to measure the critical hydrogen content of the upper limit of the amount of hydrogen where no delayed fracture occurs within 100 hours.
  • the aspect ratio of the prior austenite grains was 1.5 or more while the ratio of the ⁇ 011> fraction/ ⁇ 111> fraction of the martensite texture as seen from the rolling direction was 3.0 or more.
  • the tensile strengths, reduction of area in tensile tests, and critical hydrogen content were realized together at high levels.
  • Example X1 the reduction of area in rolling in the 950° C. or more temperature range was large and the reduction of area in rolling in the 950° C. to 750° C. temperature range was small, so the aspect ratio of the prior austenite grains became smaller and the critical hydrogen content also became low.
  • Example X4 the rates of reduction of area in rolling in the 950° C. or more and 950 to 750° C. temperature ranges were small, the aspect ratio of the prior austenite grains was small, and the reduction of area in tensile test and critical hydrogen content were low.
  • Example X5 and Examples X7 to X11 the reduction of area in tensile tests were insufficient.
  • Example X10 and Example X11 the critical hydrogen contents were insufficient.
  • Example X5 was high in Si content
  • Example X7 was high in C content
  • Example X8 was high in Mn content
  • Example X9 was high in Cr content
  • Example X10 was high in P content
  • Example X11 was high in S content.
  • Example X12 was long in time from final rolling to quenching, fell in martensite fraction, and was insufficient in aspect ratio of prior austenite grains. It is estimated that proeutectoid ferrite nucleated from the worked structure before quenching.
  • Example X6 the C content was low and a sufficient tensile strength could not be obtained.
  • the spring steel for suspension of the present disclosure had a high tensile strength and excellent delayed fracture resistance and reduction of area in tensile test.
  • the spring steel for suspension of the present disclosure does not contain any expensive alloy elements or even if containing them contains it in a trace amount, has a 1800 MPa or more tensile strength, and has an excellent cold formability and delayed fracture resistance, so a suspension spring used in an automobile etc. can be lightened. The effect in industry is extremely remarkable.

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CN111519114B (zh) * 2020-05-14 2022-06-21 大冶特殊钢有限公司 一种弹簧扁钢材料及其制备方法
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