US20080308195A1 - Steel For Springs, Process Of Manufacture For Spring Using This Steel, And Spring Made From Such Steel - Google Patents

Steel For Springs, Process Of Manufacture For Spring Using This Steel, And Spring Made From Such Steel Download PDF

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US20080308195A1
US20080308195A1 US12/097,313 US9731306A US2008308195A1 US 20080308195 A1 US20080308195 A1 US 20080308195A1 US 9731306 A US9731306 A US 9731306A US 2008308195 A1 US2008308195 A1 US 2008308195A1
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steel
traces
spring
hardness
springs
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Nao Yoshihara
Kazuhisa Kawata
Julie Mougin
Jacques Languillaume
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Ascometal SA
Kobe Steel Ltd
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Ascometal SA
Kobe Steel Ltd
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Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.), ASCOMETAL reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KAWATA, KAZUHISA, YOSHIHARA, NAO, LANGUILLAUME, JACQUES, MOUGIN, JULIE
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/44—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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the invention relates to steel making, and more specifically, the field of spring steel.
  • springs need continually increasing hardness and tensile strength. Consequently, susceptibility to fractures that begin on defects, such as inclusions or surface defects generated during spring manufacture, increases, and fatigue resistance tends to become limited.
  • springs used in highly corrosive environments such as suspension springs, must have at least equivalent and preferably better fatigue properties in corrosive conditions since they use steels having higher hardness and tensile strength. Accordingly, such springs tend to fracture at the defects, immediately during the fatigue cycles in air, and more late during fatigue cycles in a corrosive medium. In particular, for fatigue in corrosive conditions, defects can begin in corrosion pits.
  • documents FR-A-2740476 and JP-3474373B describe a spring steel grade with good resistance to hydrogen embrittlement and good fatigue resistance, in which inclusions of carbonitrosulfides containing at least one of the elements titanium, niobium, zirconium, tantalium or hafnium are controlled so as to have lower mean size, less than 5 ⁇ m in diameter, and to be very numerous (10,000 or more on a cutting section).
  • this type of steels leads, after quenching and tempering according to the industrial spring manufacturing process, to a hardness level of only 50 HRC or a little higher, corresponding to a tensile strength of 1700 MPa or a little higher, but not much over 1900 MPa, corresponding to a hardness of 53.5 HRC. Because of this moderate hardness level, this steel only has moderate sag resistance, steel with a higher tensile strength being needed to improve sag resistance. Accordingly, such steel does not ensure an excellent compromise between high resistance, which would be above 2100 MPa, a hardness that would be higher than 55 HRC, a high fatigue resistance in air and fatigue resistance in corrosive conditions that is at least equivalent, if not higher than that needed for springs.
  • the purpose of the invention is to propose means to simultaneously increase, as compared to known steels, spring hardness and tensile strength, fatigue properties in air, making fatigue resistance in corrosive conditions at least equivalent, if not higher, increase spring sag resistance and to reduce susceptibility to surface defects that can be generated during spring coiling.
  • the object of the invention is a spring steel with high fatigue resistance in air and in corrosive conditions and with high resistance to cyclic sag, having the composition in weight percent:
  • Ceq % [C %]+0.12 [Si %]+0.17 [Mn %] ⁇ 0.1 [Ni %]+0.13 [Cr %] ⁇ 0.24 [V %]
  • the maximum size of titanium nitrides or carbonitrides observed at 1.5 ⁇ 0.5 mm of the surface area of a bar, or a wire rod, a slug or a spring over 100 mm 2 of the surface area of the section is preferably less than or equal to 20 ⁇ m, which size being the square root of the surface area of the inclusions considered as squares.
  • the composition of the steel is:
  • the balance being iron and impurities resulting from the steel making process.
  • a further object of the invention is a manufacturing process for a spring steel with high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, according to which a liquid steel is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature; that are rolled into bars, wire rods or slugs and transformed into springs, characterized in that:
  • the steel is of the previous type
  • billets or ingots have a minimum mean cooling rate of 0.3° C./s between 1450-1300° C.;
  • the blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles;
  • bars, wire rods or slugs, or springs made from these are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with hardness greater than or equal to 55 HRC.
  • a further object of the invention is springs made from such steel, and springs made of steel obtained by the previous process.
  • the inventors realized that a steel with the characteristics of the previously cited inclusion composition and morphology ensured, after steelmaking, casting, rolling, quenching and tempering done in specific conditions, a hardness greater than 55 HRC, while assuring excellent compromise between high endurance level to fatigue in air and to fatigue in corrosive conditions, high resistance to cyclic sag and low sensitivity to surface defects arising during manufacture of the spring.
  • FIG. 1 which shows the results of hardness and cyclic sag tests for steels according to the invention and reference steels;
  • FIG. 2 which shows the results of fatigue tests in air as a function of steel hardness for steels according to the invention and reference steels;
  • FIG. 3 which shows the results of Charpy impact tests as a function of the steel hardness for steels according to the invention and reference steels;
  • FIG. 4 which shows the results of fatigue tests in corrosive conditions as a function of steel hardness for steels according to the invention and reference steels.
  • the steel composition according to the invention must meet the following conditions.
  • the carbon content must be between 0.45% and 0.7%. After quenching and tempering carbon increases the tensile strength and hardness of the steel. If the carbon content is less than 0.45%, in the temperature range usually used to manufacture springs, no quenching and tempering treatment leads to the high strength and hardness of the steel described in the invention. Secondly, if the carbon content exceeds 0.7% preferably 0.65%, coarse and very hard carbides, combined with chromium, molybdenum and vanadium, can remain undissolved during the austenitization conducted before the quench, and can significantly affect fatigue lifetime in air, fatigue resistance in corrosive conditions and also toughness. Consequently carbon contents above 0.7% must be avoided. Preferably, it should not exceed 0.65%.
  • the silicon content is between 1.65% and 2.5%. Silicon is an important element that ensures, through its presence in solid solution, high levels of strength and hardness, as well as high carbon equivalent values Ceq and sag resistance. To have the tensile strength and hardness values of the steel according to the invention, the silicon content must not be less than 1.65%. Furthermore, silicon contributes at least partially to steel deoxidation. If its content exceeds 2.5%, preferably 2.2%, the oxygen content of the steel can be, by thermodynamic reaction, greater than 0.0020%, preferably 0.0025%. This involves formation of oxides of various compositions which are harmful to fatigue resistance in air.
  • silicon contents greater than 2.5% the various combined elements such as manganese, chromium or others can segregate during solidification, after casting. This segregation is very harmful to fatigue behavior in air and to fatigue resistance in corrosive conditions.
  • silicon content greater than 2.5% decarburization at the surface of bars or wire rods for springs becomes too high for the in-service properties of the springs. This is why the silicon content must not exceed 2.5%, and preferably 2.2%.
  • the manganese content is between 0.20% and 0.75%. In combination with residual sulfur at level of traces to 0.015%, the manganese content must be at least ten times higher than the sulfur content so as to avoid formation of iron sulfides that are extremely harmful to steel rolling. Consequently, a minimum manganese content of 0.20% is required. Furthermore, manganese contributes to solid solution hardening during the quenching of the steel as well as nickel, chromium, molybdenum and vanadium, which delivers high tensile strength and hardness values and the carbon equivalent Ceq value of the steel described in the invention. Manganese contents greater than 0.75%, preferably 0.65%, in combination with silicon, can segregate during the solidification stage, after steel making and casting. These segregations are harmful to the in-service properties and to the homogeneity of the steel. This is why the manganese content must not exceed 0.75%, and preferably 0.65%.
  • the chromium content must be between 0.60% and 2%, and preferably between 0.80% and 1.70%. Chromium is added to obtain, in solid solution after austenitization, quenching and tempering, high values for tensile strength and hardness, and to contribute to obtaining the carbon equivalent Ceq value, but also to increase fatigue resistance in corrosive conditions. To ensure these properties the chromium content must be at least 0.60%, and preferably at least 0.80%. Above 2%, preferably 1.7%, specific coarse, very hard chromium carbides, in combination with vanadium and molybdenum, can remain after the austenitization treatment that precedes the quench. Such carbides greatly affect the fatigue resistance in air. This is why the chromium content must not exceed 2%.
  • the nickel content is between 0.15% and 1%.
  • Nickel is added to increase steel hardenability, as well as tensile strength and hardness after quenching and tempering. Since it does not form carbides, nickel contributes to steel hardening, just like chromium, molybdenum and vanadium, without forming specific coarse, hard carbides which would not be dissolved during the austenitization that precedes the quench, and could be harmful to fatigue resistance in air. It also means that the carbon equivalent can be adjusted between 0.8% and 1% in the steel according to the invention as needed. As a non-oxidizable element, nickel improves fatigue resistance in corrosive conditions. To ensure that these effects are significant, the nickel content must not be lower than 0.15%.
  • nickel can lead to overly high residual austenite content, whose presence is very harmful to fatigue resistance in corrosive conditions. Furthermore, high nickel level significantly increase the cost of the steel. For all these reasons the nickel content must not exceed 1%, preferably 0.80%.
  • the molybdenum content must be between traces and 1%.
  • molybdenum increases steel hardenability, as well as strength. Furthermore, it has low oxidation potential. For these two reasons, molybdenum is favorable to fatigue resistance in air and in corrosive conditions. But for contents above 1%, preferably 0.80%, coarse, very hard molybdenum carbides can remain, optionally combined with vanadium and chromium, after the austenitization that precedes the quench. These particular carbides are very harmful for fatigue resistance in air. Finally, adding more than 1% molybdenum increases the cost of the steel unnecessarily. This is why the molybdenum content must not exceed 1%, and preferably 0.80%.
  • the vanadium content must be between 0.003% and 0.8%.
  • Vanadium is an element that increases hardenability, tensile strength and hardness after quenching and tempering. Furthermore, in combination with nitrogen, vanadium forms a large number of fine submicroscopic vanadium or vanadium and titanium nitrides that refine the grain and increase tensile strength and hardness levels, through structural hardening. To obtain formation of submicroscopic vanadium or vanadium and titanium nitrides that refine the grain, vanadium must be present with a minimum content of 0.003%. But this element is expensive and it has to be kept at this lower limit if a compromise is sought between the cost of steel making and the grain refinement.
  • Vanadium must not exceed 0.8% and, preferably, 0.5%, because beyond this value a precipitate of coarse, very hard vanadium-containing carbides, in combination with chromium and molybdenum, can remain undissolved during the austenitization that precedes the quench. This can be very unfavorable for fatigue resistance in air, for high values of strength and hardness in the steel according to the invention. Further, adding more than 0.8% vanadium increases the cost of the steel unnecessarily.
  • the copper content must be between 0.10% and 1%. Copper is an element that hardens steel when it is in solid solution after the quenching and tempering treatment. Accordingly, it can be added along with other elements that contribute in increasing the strength and hardness of the steel. As it does not combine with carbon, it hardens the steel without forming coarse, hard carbides that harm fatigue resistance in air. Form the electrochemical point of view, its passivation potential is higher than that of iron and, consequently, it favors steel fatigue resistance in corrosive conditions. To ensure that these effects are significant, the copper content must not be lower than 0.10%. In contrast, at contents of more than 1%, preferably 0.90%, copper has a very harmful influence on the behavior during hot rolling. This is why the copper content must not exceed 1%, and preferably 0.90%.
  • Titanium is added to form, in combination with nitrogen, preferably also carbon and/or vanadium, fine, submicroscopic nitrides or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench. According, it increases the surface area of the grain boundaries in the steel, thereby reducing the quantity of unavoidable impurities that segregate in the grain boundaries, such as phosphorus. Such intergranular segregations would be very harmful to toughness and fatigue resistance in air if they are present at high concentrations per unit of surface area at the grain boundaries.
  • titanium leads to the formation of other fine nitrides or carbonitrides producing an irreversible trapping effect for some elements, such as hydrogen formed during corrosion reactions, and which can be extremely harmful to fatigue resistance in corrosive conditions.
  • the titanium content must not be lower than 0.020%.
  • titanium can lead to the formation of coarse, hard carbonitrides that are very harmful to fatigue resistance in air. The latter effect is yet more harmful for high levels of tensile strength and hardness in the steel according to the invention.
  • the titanium content must not exceed 0.2, preferably 0.15%.
  • niobium content must be between traces and 0.2%.
  • Niobium is added to form, in combination with carbon and nitrogen, extremely fine, submicroscopic precipitates of nitrides and/or carbides and/or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench, especially when the aluminum content is low (0.002% for ample). Accordingly, niobium increases the surface area of the grain boundaries in the steel, and contributes to the same favorable effect as titanium as regards embrittlement of grain boundaries by unavoidable impurities such as phosphorus, whose effect is very harmful to toughness and fatigue resistance in corrosive conditions.
  • niobium nitrides or carbonitrides contribute to steel hardening through structural hardening.
  • the niobium content must not exceed 0.2%, preferably 0.15%, so that the nitrides or carbonitrides remain very fine, to ensure austenitic grain refining and to avoid cracks or splits forming during hot rolling.
  • the niobium content must not exceed 0.2%, preferably 0.15%.
  • the aluminum content must be between 0.002% and 0.050%.
  • Aluminum can be added to complete steel deoxidation and to obtain the lowest possible oxygen contents, certainly less than 0.0020% in the steel according to the invention.
  • aluminum contributes to refining the grain by forming submicroscopic nitrides. To ensure these two functions, the aluminum content must not be lower than 0.002%.
  • an aluminum content exceeding 0.05% can lead to the presence of large, isolated inclusions or to aluminates that are finer but hard and angular, in the form of long stringers that are harmful to the fatigue lifetime in air and to the cleanliness of the steel. This is why the aluminum content must not exceed 0.05%.
  • the phosphorus content must be between traces and 0.015%.
  • Phosphorus is an unavoidable impurity in steel. During a quenching and tempering treatment, it co-segregates with elements such as chromium or manganese in the former austenitic grain boundaries. The result is reduced cohesion in the grain boundaries and intergranular embrittlement that is very harmful to fatigue resistance in air. These effects are even more harmful for the high tensile strengths and hardnesses required in steels according to the invention. With the aim of simultaneously obtaining high spring steel tensile strength and hardness and good fatigue resistance in air and in corrosive conditions, the phosphorus content must be as low as possible and must not exceed 0.015%, preferably 0.010%.
  • the sulfur content is between traces and 0.015%. Sulfur is an unavoidable impurity in steel. Its content must be as low as possible, between traces and 0.015%, and preferably 0.010% at most. Accordingly, we wish to avoid the presence of sulfides that are unfavorable to fatigue resistance in corrosive conditions and fatigue resistance in air, for high values of strength and hardness in the steel according to the invention.
  • the oxygen content must be between traces and 0.0020%.
  • Oxygen is also an unavoidable impurity in steel.
  • oxygen can lead to isolated, coarse, very hard, angular inclusions appearing, or to inclusions that are finer but in the form of long stringers which are very harmful to fatigue resistance in air. These effects are even more harmful at the high tensile strengths and hardnesses of the steels according to the invention.
  • the oxygen content must not exceed 0.0020%.
  • the nitrogen content must be between 0.0020% and 0.0110%.
  • the nitrogen must be controlled in this range so as to form, in combination with titanium, niobium, aluminum or vanadium, a sufficient number of very fine submicroscopic nitrides, carbides or carbonitrides that refine the grain. Accordingly, to do so the minimum nitrogen content must be 0.0020%. Its content must not exceed 0.0110% so as to avoid forming coarse, hard titanium nitrides or carbonitrides larger than 20 ⁇ m, observed at 1.5 mm ⁇ 0.5 mm from the surface of the bars or wire rods used to manufacture the springs. This position is the place that is most critical as regards the fatigue loading of the springs.
  • a non-limiting steel making process that conforms to the invention is as follows. Liquid steel is produced either in a converter, or in an electric furnace, then undergoes a ladle metallurgy treatment during which alloy elements are added and deoxidation is performed, and in general all secondary metallurgy operations delivering a steel having the composition according to the invention and avoiding formation of sulfide or “carbonitrosulfide” complexes of elements such as titanium and/or niobium and/or vanadium. To avoid formation of such coarse precipitates during steel making, the inventors have discovered, in an unexpected way, that the contents of the various elements, in particular those of titanium, nitrogen, vanadium and sulfur, must be carefully controlled in the previously cited limits.
  • the steel is cast in the form of blooms or billets, or into ingots. But to completely avoid forming, or to avoid forming as much as possible, coarse titanium nitrides or carbonitrides during and after the solidification of these products, we have found that the mean cooling rate of these products (blooms, billets or ingots) must be controlled so as to be 0.3° C./s or higher between 1450-1300° C. When we operate in these conditions during the solidification and cooling stages, we observe in an unexpected way that the size of the coarsest titanium nitrides or carbonitrides observed on the springs is always less than 20 ⁇ m. The location and size of these titanium precipitates will be discussed hereinafter.
  • products having the precise composition according to the invention are next reheated and rolled between 1200-800° C. into the form of wire rods or bars in a single or double heating and rolling process.
  • the bars, rods, slugs, or even springs produced from these bars or wire rods are next subjected to a water quench treatment, a polymer quench or an oil quench after austenitization in a temperature range from 850-1000 C, so as to obtain a fine austenitic grain where there are no grains coarser than 9 on the ASTM grain size scale.
  • This quenching treatment is then followed by a tempering treatment specifically performed between 300-550° C., that delivers the high levels of tensile strength and hardness required for the steel, and avoids firstly a microstructure that would lead to embrittlement during tempering, and secondly, overly high residual austenite.
  • embrittlement during tempering and an overly high level of residual austentite are extremely harmful to fatigue resistance in corrosive conditions of the steel according to the invention.
  • the abovementioned treatments quenching and tempering
  • these heat treatments can be done on the bars, wire rods or slugs made from these bars before manufacturing the spring.
  • nitrides and carbonitrides are very hard, their size as previously defined does not change at all during the steel transformation steps. Therefore it is not important whether it is measured on the intermediate product (bar, wire rod or slug) which will be used to manufacture the spring or on the spring itself.
  • the invention delivers spring steels that can combine high hardness and tensile strength that are an improvement over the prior art, as well as improved fatigue properties in air and sag resistance, fatigue properties in corrosive conditions at least equivalent to those of known steels for this use, or even better, and lesser susceptibility to concentrations of stresses produced by surface defects that can form during spring manufacture, through addition of microalloyed elements, a reduction in residual elements and control of the analysis and production route of the steel.
  • Table 1 shows steel compositions according to the invention and reference steels.
  • the carbon equivalent Ceq is given by the following formula:
  • Table 2 shows the hardness values obtained for steels according to the invention and reference steels as a function of the quenching temperature that was used.
  • Table 3 shows the maximum size of the inclusions of titanium nitride or carbonitrides observed at 1.5 mm from the surface of steels according to the invention and reference steels, as previously defined. We have also reported the titanium contents of the various steels.
  • the maximum size of such titanium nitride or carbonitride inclusions is determined as follows. On a section of bar or wire rod coming from a given steel cast, a surface area of 100 mm 2 is examined at a point located 1.5 mm ⁇ 0.5 mm below the surface of the bar or wire rod. After the observations, the size of the titanium nitride or carbonitride inclusion having the largest surface area is determined by considering that the inclusions are squares and that the size of each of these inclusions, including the inclusion having the largest surface area, is equal to the square root of the surface area. All the inclusions are observed on a section of bar or wire rod for springs, and the observations are performed on 100 mm 2 of each section.
  • the steel cast conforms to the invention when the maximum size of the abovementioned inclusions observed on 100 mm 2 at 1.5 mm ⁇ 0.5 mm under the surface is less than 20 ⁇ m.
  • the corresponding results obtained on steels according to the invention and reference steels are given in table 3.
  • Samples for fatigue testing were taken from bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing included rough machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were torsion-fatigue tested in air. The shear stress applied was 856 ⁇ 494 MPa and the number of cycles to fracture was counted. The tests were stopped after 2.10 6 cycles if the samples had not broken.
  • Samples for fatigue testing in corrosive conditions were taken from bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing included rough machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were tested for fatigue in corrosive conditions, i.e. corrosion was applied at the same time as a fatigue load.
  • the fatigue load was a shear stress of 856 ⁇ 300 MPa.
  • the corrosion applied was cyclic corrosion in two alternating stages:
  • one stage being a wet stage, with spraying of a 5% NaCl solution for 5 minutes at 35° C.;
  • one stage being a dry stage without spraying, for 30 minutes at a temperature of 35° C.
  • the number of cycles to fracture was considered to be the fatigue life in corrosive conditions.
  • Sag resistance was determined using a cyclic compression test on cylindrical samples. The sample diameter was 7 mm and their height was 12 mm. They were taken from steel bars.
  • Preparation of the samples for sag testing included rough machining, austenitizing, oil quenching, tempering and final fine grinding.
  • the height of the sample was measured precisely before starting the test by using a comparator having 1 ⁇ m precision.
  • a preload was applied so as to simulate spring presetting, this presetting being a compression stress of 2200 MPa.
  • Reference steel 1 in particular, has sulfur content that is too high for good compromise between fatigue resistance in air and the content for fatigue in corrosive conditions. Furthermore, its manganese content is too high, which leads to segregations that are harmful for the homogeneity of the steel and fatigue resistance in air.
  • Reference steel 2 has too low carbon content and carbon equivalent to ensure high hardness. Its tensile strength is too low for good fatigue resistance in air.
  • Reference steel 3 in particular, has silicon content that is too low for good sag resistance and also good fatigue resistance in air.
  • Sag resistance is higher for the steels of the invention than for reference steels, as FIG. 1 shows, where it is clear that according to the abovementioned sag measurements, the values for sag are at least 32% lower for the worst case of the steels of the invention (steel of the invention 1) as compared to the best ease of the reference steels (reference steel 1).
  • the fatigue lifetime in air is clearly higher for the steels of the invention as compared to the reference steels. This is due to the increased hardness, as FIG. 2 shows, but increased hardness is not enough.
  • steels with high hardness are more susceptible to defects, such as inclusions and surface defects as the hardness increases.
  • steels according to the invention are less susceptible to defects, in particular to coarse inclusions such as titanium nitrides or carbonitrides, given that the invention prevents such large inclusions appearing.
  • table 3 shows, the largest inclusions found in steels according to the invention do not exceed 14.1 ⁇ m, where inclusions larger than 20 ⁇ m are found in reference steel 2.
  • steels according to the invention have the advantage that their fatigue resistance in corrosive conditions is higher than that of reference steels according to the prior art, and in particular hardness greater than 55 HRC as FIG. 4 shows.
  • the invention delivers higher hardness with a good compromise between fatigue lifetime in air and sag resistance, which are greatly increased, and fatigue lifetime in corrosive conditions which is better than those of reference steels according to the prior art. Furthermore, lesser susceptibility to possible surface defects, in particular those generated during spring manufacture or other operations, is also obtained.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Steel (AREA)
  • Wire Processing (AREA)
  • Springs (AREA)
US12/097,313 2005-12-15 2006-12-11 Steel For Springs, Process Of Manufacture For Spring Using This Steel, And Spring Made From Such Steel Abandoned US20080308195A1 (en)

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FR0512775A FR2894987B1 (fr) 2005-12-15 2005-12-15 Acier a ressorts, et procede de fabrication d'un ressort utilisant cet acier, et ressort realise en un tel acier
FR0512775 2005-12-15
PCT/FR2006/002700 WO2007080256A1 (fr) 2005-12-15 2006-12-11 Acier a ressorts, et procede de fabrication d'un ressort utilisant cet acier, et ressort realise en un tel acier.

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US20110074076A1 (en) * 2009-09-29 2011-03-31 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US8382918B2 (en) 2007-07-20 2013-02-26 Kobe Steel, Ltd. Steel wire material for spring and its producing method
US9068615B2 (en) 2011-01-06 2015-06-30 Chuo Hatsujo Kabushiki Kaisha Spring having excellent corrosion fatigue strength
EP2803742A4 (en) * 2012-01-11 2016-06-15 Kobe Steel Ltd STEEL FOR BOLTS, BOLTS AND METHOD FOR PRODUCING THE BOLT
US20170159161A1 (en) * 2015-12-07 2017-06-08 Hyundai Motor Company Ultra-high-strength spring steel for valve spring
CN107148483A (zh) * 2014-10-31 2017-09-08 株式会社神户制钢所 高强度中空弹簧用钢的制造方法
US20170298486A1 (en) * 2016-04-15 2017-10-19 Hyundai Motor Company High strength spring steel having excellent corrosion resistance
US20170298487A1 (en) * 2016-04-15 2017-10-19 Hyundai Motor Company High strength spring steel having excellent corrosion resistance
US10060013B2 (en) 2013-08-08 2018-08-28 Sanyo Special Steel Co., Ltd. Steel having superior rolling fatigue life
US10494705B2 (en) 2015-12-04 2019-12-03 Hyundai Motor Company Ultra high-strength spring steel
CN113930680A (zh) * 2021-09-29 2022-01-14 武汉钢铁有限公司 一种耐低温高强度弹簧扁钢及其生产方法
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Cited By (22)

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US8382918B2 (en) 2007-07-20 2013-02-26 Kobe Steel, Ltd. Steel wire material for spring and its producing method
US20110074076A1 (en) * 2009-09-29 2011-03-31 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US20110074077A1 (en) * 2009-09-29 2011-03-31 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US20110074079A1 (en) * 2009-09-29 2011-03-31 Chuo Hatsujo Kabushiki Kaisha Coil spring for automobile suspension and method of manufacturing the same
US20110074078A1 (en) * 2009-09-29 2011-03-31 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US8328169B2 (en) * 2009-09-29 2012-12-11 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US8349095B2 (en) 2009-09-29 2013-01-08 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US8789817B2 (en) 2009-09-29 2014-07-29 Chuo Hatsujo Kabushiki Kaisha Spring steel and spring having superior corrosion fatigue strength
US8936236B2 (en) 2009-09-29 2015-01-20 Chuo Hatsujo Kabushiki Kaisha Coil spring for automobile suspension and method of manufacturing the same
US9068615B2 (en) 2011-01-06 2015-06-30 Chuo Hatsujo Kabushiki Kaisha Spring having excellent corrosion fatigue strength
EP2803742A4 (en) * 2012-01-11 2016-06-15 Kobe Steel Ltd STEEL FOR BOLTS, BOLTS AND METHOD FOR PRODUCING THE BOLT
US9695488B2 (en) 2012-01-11 2017-07-04 Kobe Steel, Ltd. Steel for bolt use, bolt, and method for manufacturing bolt
US10060013B2 (en) 2013-08-08 2018-08-28 Sanyo Special Steel Co., Ltd. Steel having superior rolling fatigue life
CN107148483A (zh) * 2014-10-31 2017-09-08 株式会社神户制钢所 高强度中空弹簧用钢的制造方法
US10494705B2 (en) 2015-12-04 2019-12-03 Hyundai Motor Company Ultra high-strength spring steel
US20170159161A1 (en) * 2015-12-07 2017-06-08 Hyundai Motor Company Ultra-high-strength spring steel for valve spring
US10689736B2 (en) 2015-12-07 2020-06-23 Hyundai Motor Company Ultra-high-strength spring steel for valve spring
US20170298486A1 (en) * 2016-04-15 2017-10-19 Hyundai Motor Company High strength spring steel having excellent corrosion resistance
US20170298487A1 (en) * 2016-04-15 2017-10-19 Hyundai Motor Company High strength spring steel having excellent corrosion resistance
US10718039B2 (en) * 2016-04-15 2020-07-21 Hyundai Motor Company High strength spring steel having excellent corrosion resistance
CN113930680A (zh) * 2021-09-29 2022-01-14 武汉钢铁有限公司 一种耐低温高强度弹簧扁钢及其生产方法
CN115961215A (zh) * 2022-12-09 2023-04-14 铜陵有色金神耐磨材料有限责任公司 一种轻质半自磨机用高耐磨Cr-Mo钢衬板及制备方法

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EP1966407B1 (fr) 2009-10-07
BRPI0619892A2 (pt) 2011-10-25
KR20080090424A (ko) 2008-10-08
RU2008128865A (ru) 2010-01-20
RU2397270C2 (ru) 2010-08-20
CN101400818A (zh) 2009-04-01
WO2007080256A1 (fr) 2007-07-19
ME01062B (me) 2012-10-20
JP2007224413A (ja) 2007-09-06
NO20082766L (no) 2008-07-14
EP1966407A1 (fr) 2008-09-10
FR2894987B1 (fr) 2008-03-14
KR101048946B1 (ko) 2011-07-12
CA2633153C (en) 2013-05-07
BRPI0619892B1 (pt) 2016-06-07
SI1966407T1 (sl) 2009-12-31
CA2633153A1 (en) 2007-07-19
DE602006009705D1 (de) 2009-11-19
ATE445026T1 (de) 2009-10-15
ES2331539T3 (es) 2010-01-07
RS51070B (sr) 2010-10-31
PL1966407T3 (pl) 2010-04-30
CN101400818B (zh) 2012-08-29
FR2894987A1 (fr) 2007-06-22
NO341748B1 (no) 2018-01-15
JP4869051B2 (ja) 2012-02-01

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