HIGH STRENGTH SPRING STEEL WIRE WITH EXCELLENT COILING PROPERTIES AND HYDROGEN EMBRITTLEMENT RESISTANCE

Abstract

Disclosed herein is a high strength spring steel wire with excellent coiling properties and hydrogen embrittlement resistance. The steel wire comprises, by mass, 0.4 to 0.60% of C, 1.7 to 2.5% of Si, 0.1 to 0.4% of Mn, 0.5 to 2.0% of Cr, 0.015% or less of P (exceeding 0%), 0.015% or less of S (exceeding 0%), 0.006% or less of N (exceeding 0%), 0.001 to 0.07% of Al, and the remainder being Fe and unavoidable impurities. The steel wire has a structure wherein prior austenite has an average grain size of 12 μm or less, and retained austenite exists in an amount of 1.0 to 8.0 vol. % with respect to a whole structure of the steel wire. The retained austenite has an average grain size of 300 nm or less and a maximum grain size of 800 nm or less. The steel has a tensile strength of 1,900 MPa or more.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high strength spring steel wire with excellent coiling properties and hydrogen embrittlement resistance, and more particularly, to a spring steel wire, which has enhanced coiling properties and hydrogen embrittlement resistance in a high strength range for a tensile strength of 1,900 MPa or more.

2. Description of the Related Art

Requirement for weight reduction of an automotive vehicle is accompanied with reduction in size and thickness of vehicle components. In this regard, it is necessary for underbody components of the vehicle, such as a suspension spring and the like, to have high strength. However, since increase in strength of steel generally enhances atmospheric fatigue properties while reducing corrosion fatigue properties, it is difficult to improve both atmospheric fatigue properties and corrosion fatigue properties together with the strength of the steel.

In order to improve the corrosion fatigue properties, several methods, such as a method of improving corrosion resistance, a method of trapping hydrogen, etc. have been suggested in the art. However, according to these methods, an increase in required level results in an increase of added amounts of alloy elements, thereby increasing material costs while deteriorating workability.

In view of this, there has been suggested a method of enhancing properties of the material through an enhanced manufacturing process without increasing the content of alloy elements. For example, Patent Document 1 discloses that the high strength of the suspension spring as a final product can be realized without deteriorating the other properties described above by improving toughness or sagging resistance of steel applied to the suspension spring through modification of quenching and tempering conditions in a manufacturing process of a cold-wound coil spring.

As such, the cold-wound spring has a merit in that the properties thereof can be easily enhanced through modification of the manufacturing process. The process of manufacturing the cold-wound coil spring and a process of manufacturing a hot-wound coil spring will be disclosed below. Unlike the process of manufacturing the hot-wound coil spring, the process of manufacturing the cold-wound coil spring is performed in such a way that spring winding is performed after quenching and tempering. Thus, for the process of manufacturing the hot-wound coil spring, restriction of conditions for the quenching and tempering is not so severe in comparison to the process of manufacturing the hot-wound coil spring.

<Process of Manufacturing Hot-wound Coil Spring>

Spring steel→pickling→drawing→heating→hot spring winding→quenching→tempering→setting→shot peening→painting→product

<Process of Manufacturing Cold-wound Coil Spring>

Spring steel→pickling→drawing→heating→quenching→tempering→cold spring winding→annealing for strain relief→setting→shot peening→painting→product

However, for the process of manufacturing the cold-wound coil spring, since the spring winding is performed after the quenching and tempering unlike the process of manufacturing the hot-wound coil spring in which the quenching and tempering are performed after the spring winding in order to adjust the strength, a steel wire with high strength and low workability is provided to the spring winding process, whereby the steel wire is likely to be broken during the spring winding process. This phenomenon is remarkable as the steel is increased in strength. Accordingly, excellent ductility (coiling properties) is required for the steel wire which will be subjected to the quenching and tempering performed in the manufacturing process of the cold-wound coil spring.

In order to ensure the excellent ductility, for example, Patent Document 2 discloses a method which can ensure suitable coiling properties and high strength through refinement of austenite structure and reduction in content of C in a matrix by addition of Nb. In addition, Patent Document 3 discloses a method which can ensure excellent ductility and high strength through refinement of the austenite structure with TiN by adjusting added amounts of Ti and N. However, both methods require addition of alloy elements, and are insufficient to ensure workability or low manufacturing costs, which is evaluated as one of the merits of the cold wound coil spring.

In Patent Document 4, a method is disclosed, which improves the coiling properties, delayed failure properties and fatigue properties of a high strength spring steel wire without increasing the amounts of alloy elements through refinement of the austenite structure and adjustment of density and size of carbide. However, in order to satisfy this requirement, since it is necessary to use an additional technique which can heat the steel wire to a high temperature in a short period of time, it is not a versatile method.

In the manufacturing process of the cold wound coil spring, after being subjected to the quenching and tempering, the steel wire is wound and retained in a coil shape under stress until a coiling process, during which delayed failure can occur sometimes. The delayed failure is a kind of hydrogen embrittlement phenomenon caused by hydrogen diffusing into the steel wire from surroundings during heat treatment, and is likely to occur since sensitivity of hydrogen embrittlement is increased as the strength of the steel wire is increased. Accordingly, the steel wire for the cold wound coil spring must have excellent hydrogen embrittlement resistance in comparison to the steel wire for the hot wound coil spring.

As a technique attempting to enhance the hydrogen embrittlement resistance of the steel spring wire, Patent Document 5 discloses a method by which a steel wire for the spring with a good hydrogen fatigue resistance and a tensile strength of 1,700 MPa or more is manufactured in such a way of adding V, Mo, Ti, Nb and Zr to the steel such that precipitates thereof act as hydrogen trapping sites. However, in this method, since it is necessary to add a great amount of alloy elements, and to perform the tempering at a temperature of 500° C. or more in order to form the precipitates, it is difficult to ensure the high strength and the sagging resistance.

As described above, in order to realize the high strength (tensile strength of 1,900 MPa or more) of the spring such as a suspension spring generally used under severe conditions using the cold wound coil spring advantageous in terms of low cost and high capability, it is necessary to have both good coiling properties and hydrogen embrittlement resistance. However, a method of enhancing both coiling properties and hydrogen embrittlement resistance of a high strength spring steel wire having a tensile strength of 1,900 MPa or more has not been yet suggested in the related art. In particular, there has not been yet suggested a technique which can enhance both coiling properties and hydrogen embrittlement resistance at the same time without sacrificing the merits of the cold wound coil spring such as low cost and wide applicability.

Patent Document 1: Japanese Patent Laid-open Publication No. S59-96246

Patent Document 2: Japanese Patent Laid-open Publication No. H07-26347

Patent Document 3: Japanese Patent Laid-open Publication No. H11-29839

Patent Document 4: Japanese Patent Laid-open Publication No. 2002-180198

Patent Document 5: Japanese Patent Laid-open Publication No. 2001-288539

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and it is an object of the present invention to provide a high strength spring steel wire, which has a tensile strength of 1,900 MPa or more, and has enhanced coiling properties and hydrogen embrittlement resistance such that the steel wire may be suitably applied to a process of manufacturing a cold wound coil spring. Here, it is needless to say that the steel wire of the present invention may be applied to a process of manufacturing a hot wound coil spring.

In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a high strength spring steel wire with excellent coiling properties and hydrogen embrittlement resistance, the steel wire comprising, by mass: 0.4 to 0.60% of C, 1.7 to 2.5% of Si, 0.1 to 0.4% of Mn, 0.5 to 2.0% of Cr, 0.015% or less of P (exceeding 0%), 0.015% or less of S (exceeding 0%), 0.006% or less of N (exceeding 0%), 0.001 to 0.07% of Al, and the remainder being Fe and unavoidable impurities, the steel wire having a tensile strength of 1,900 MPa or more, and a structure wherein prior austenite has an average grain size of 12 μm or less, and retained austenite exists in an amount of 1.0 to 8.0 vol. % with respect to a whole structure of the steel wire, the retained austenite having an average grain size of 300 nm or less and a maximum grain size of 800 nm or less.

The steel wire may further comprise 1.0% or less of Ni (exceeding 0%) and/or 1.0% or less of Cu (exceeding 0%). The steel wire may further comprise at least one selected from the group consisting of 0.1% or less of Ti (exceeding 0%), 0.2% or less of V (exceeding 0%), 0.1% or less of Nb (exceeding 0%) and 1.0% or less of Mo (exceeding 0%).

As apparent from the above description, according to the present invention, the high strength spring steel wire permits an effective coiling operation in a process of cold spring winding as well as a process of hot spring winding, and has enhanced hydrogen embrittlement resistance and a tensile strength of 1,900 MPa or more. As a result, a suspension spring and the like having high strength as automobile parts, hardly causing delayed failure and the like can be supplied at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a conventional heat treatment process;

FIG. 2 is SEM micrographs showing retained austenite according to an SEM/EBSP method;

FIG. 3 is a side sectional view of a sample used for a tensile test;

FIG. 4 is a side sectional view of a sample used for a hydrogen embrittlement resistance test;

FIG. 5 is a graph depicting the relationship between tensile strength and total elongation; and

FIG. 6 is a graph depicting the relationship between tensile strength and failure life in the hydrogen embrittlement resistance test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Inventors of the present invention have investigated a spring steel wire suitable for a process of manufacturing a cold wound coil spring which has a high strength and has enhanced hydrogen embrittlement resistance without adding a large amount of alloy elements. As a result, they found that such a steel wire can be obtained by regulating the composition of the steel wire and the structure thereof in terms of an average grain size of prior austenite along with amount and grain size of retained austenite, as described below, and invented the present invention.

The structure of the steel wire according to the present invention will be described hereinafter.

<Average Grain Size of Prior Austenite: 12 μm or Less>

First, according to the present invention, the prior austenite has an average grain size of 12 μm or less. Refinement of the average grain size of the prior austenite enables reduction in stress concentration occurring at a grain boundary of the prior austenite, and improvement in toughness-ductility as well as hydrogen embrittlement resistance of the steel at the same time. The prior austenite preferably has an average grain size of 10 μm or less, and, more preferably, of 8 μm or less.

<Amount of Retained Austenite: 1.0 to 8.0 vol. % with Respect to the Whole Structure>

Generally, when a carbon steel is quenched, a great amount of retained austenite is formed in the structure. In this state, if the carbon steel is subjected to tempering at a temperature of, for example, about 250° C., the retained austenite is decomposed as known in the related art. However, if the contents of C and alloy elements are increased in order to improve the strength of steel, the amount of retained austenite is increased by quenching, and thus, it is difficult to decompose by the tempering. As such, when the retained austenite is present in a great amount within the steel after the tempering, the retained austenite causes deformation-induced transformation during a coiling process, resulting in failure of a spring (see Japanese Patent Laid-open Publication No. 2003-3241).

However, the inventors of the present invention found that, when controlling the amount and shape (size) of the retained austenite, the retained austenite contributes to enhance the toughness-ductility after tempering while effectively enhancing the hydrogen embrittlement resistance of the steel. Specifically, since the retained austenite causes lowering of the strength of the steel to some degree, the steel is enhanced in ductility while being lowered in sensitivity to hydrogen embrittlement, thereby enhancing the hydrogen embrittlement resistance. In addition, since the retained austenite serves as effective hydrogen trapping sites, it is effective to enhance the hydrogen embrittlement resistance through the hydrogen trapping.

These effects are realized by securing a predetermined amount of retained austenite. According to the present invention, the structure has 1.0% or more of retained austenite by a volumetric ratio with respect to the whole structure. As the amount of retained austenite increases, not only the hydrogen trapping effect is enhanced, but also the sensitivity for the hydrogen embrittlement is lowered, thereby enhancing the hydrogen embrittlement resistance. Accordingly, the amount of retained austenite is preferably 1.2% or more, and more preferably 1.5% or more. However, if the amount of retained austenite is excessive, hydrogen trapped by the retained austenite is released in a great amount due to decomposition of the retained austenite during a coiling process, and it is likely to cause the hydrogen embrittlement. Thus, according to the present invention, the amount of retained austenite is 8.0% or less as an upper limit by the volumetric ratio with respect to the whole structure. Preferably, an upper limit of the retained austenite is 7.5% or less.

<Average Grain Size of Retained Austenite: 300 nm or Less, Maximum Grain Size of Retained Austenite: 800 nm or Less>

Even when securing the amount of retained austenite as described above, if the amount of retained austenite is decreased due to the deformation-induced transformation by the coiling and the like, it is difficult to maintain excellent toughness-ductility and the hydrogen embrittlement resistance. In this regard, the inventors of the present invention found that grain refinement of the retained austenite suppresses the deformation-induced transformation, and releases a local stress concentration after deformation induction, thereby preventing delayed cracking or coiling failure.

Specifically, according to the present invention, the retained austenite is regulated to have an average grain size of 300 nm or less and a maximum grain size of 800 nm. With the retained austenite of the average grain size of 300 nm or less, since a possible deformation-induced transformation during the coiling does not cause an ultimately severe stress concentration, it is possible to prevent the failure. The average grain size of the retained austenite is preferably 280 nm or less, and more preferably 260 nm or less. In addition, it is an important feature of the present invention to regulate the maximum grain size of the retained austenite. That is, in the present invention, the retained austenite is regulated to have the maximum grain size of 800 nm or less, whereby the deformation-induced transformation is suppressed during the coiling after the quenching and tempering, thereby avoiding the delayed cracking. The maximum grain size of the retained austenite is preferably 600 nm, and more preferably 500 nm or less.

The amount of the retained austenite can be measured by an X-ray diffraction method, a saturation magnetization method, an electron back scattering pattern (EBSP) method, and the like (see Kobe Steel Engineering Reports, Vol. 52 (2002), p. 43). Among these methods, since the saturation magnetization method provides accurate measurements, it is recommended.

In addition, the size (average grain size and maximum grain size) of retained austenite can be measured by a transmission electron microscope (TEM) or a scanning electron microscope (SEM)/EBSP method. Meanwhile, since the TEM requires a predetermined time for observation of a predetermined range due to a narrow observation range, the SEM/EBSP method is recommended in measurement of the size of retained austenite.

Specifically, locations corresponding to D(diameter)/4 (the total area of measurement is 10,000 μm² or more and a distance between measuring points is 0.03 μm) of a surface (cross-section) vertical to a rolling direction of a specimen (bar shape) were determined as a target surface, and when polishing the target surface, electrolysis polishing was performed in order to prevent transformation of the retained austenite. Then, electron beam was irradiated to the specimen set in a lens barrel of the SEM by using an FE-SEM having an EBSP detector attached thereto such that the region could be analyzed by the EBSP detector while being observed by the SEM. Next, an EBSP image projected on a screen was taken by an intensified camera (VE-1000-SIT of Dage-MTI Inc.), and stored in a computer, followed by color mapping of an FCC phase determined through comparison of the image with a pattern obtained by simulation using a crystal system (for the retained austenite, FCC (Face Centered Cubic)) of a matrix. Then, a diameter of the mapped region was obtained through circular approximation of the mapped region after measuring an area of the mapped region. Finally, the average grain size and the maximum grain size of the retained austenite were obtained on the basis of the measurement.

As described above, the present invention has characteristics in terms of, particularly, regulation of the shape of the structure. In order to obtain the spring steel wire which permits easy control of the shape of the structure and has the desired high strength, it is necessary to control composition (by mass percent) of the steel as follows.

<C: 0.4 to 0.60%>

C is an element to secure high strength of steel. According to the present invention, the steel wire comprises C in an amount of 0.4% or more. Preferably, the content of C is 0.42% or more. However, if the content of C is excessive, the amount of retained austenite is increased after the quenching and tempering, thereby causing deterioration of the hydrogen embrittlement resistance. In addition, since C also serves to deteriorate corrosion resistance, it is necessary to suppress the content of C in order to improve the corrosion fatigue properties of a spring product (for example, a suspension spring) as a final product. Thus, according to the present invention, the content of C is 0.60% or less, and preferably 0.59% or less.

<Si: 1.7 to 2.5%>

Si is an element to improve sagging resistance required for the spring. In order to ensure the sagging resistance required for the spring having a strength level according to the present invention, the content of Si must be 1.7% or more. Preferably, the content of Si is 1.8% or more. Meanwhile, since Si also serves to promote decarburization, an excessive content of Si promotes formation of a decarburized layer on the surface of the steel, which requires a peeling process to remove the decarburized layer, causing a disadvantage in terms of manufacturing costs. Thus, according to the present invention, the upper limit of Si is 2.5%, and preferably 2.4% or less.

<Mn: 0.1 to 0.4%>

Mn is an element which acts as a deoxidation element while forming MnS via reaction with S acting as an adverse element in the steel, thereby removing the adverse effect of S. In order to allow Mn to exhibit such effect sufficiently, the content of Mn must be 0.1% or more. Preferably, the content of Mn is 0.12% or more. However, an excessive content of Mn creates precipitation bands, causing non-uniform properties or quenching crack. In addition, the excessive content of Mn causes the retained austenite to be coarsened in the precipitation bands during the quenching. Since it is difficult to decompose the coarse retained austenite during the tempering, the coarse retained austenite adversely influences the properties of the material. In this regard, according to the present invention, the upper limit of Mn is 0.4% or less, and preferably 0.38% or less.

<Cr: 0.5 to 2.0%>

Cr is an effective element in view of improvement in strength and corrosion resistance after the tempering. In particular, Cr is an important element for the suspension spring which requires the corrosion resistance of a high level. In order to allow Cr to exhibit such effect sufficiently, the content of Cr must be 0.5% or more. Preferably, the content of Cr is 0.7% or more. However, an excessive content of Cr creates Cr-rich carbides with poor solubility, and is not sufficiently dissolved as a solid solution in the steel during the quenching, causing a failure of securing a desired strength. Thus, according to the present invention, the upper limit of Cr is 2.0% or less, and preferably 1.9% or less.

<P: 0.015% or Less (Exceeding 0%)>

Since P deteriorates the toughness-ductility, it is desirable to have a low content of P, and the upper limit of P is 0.015%. In the present invention, the upper limit of P is preferably 0.01% or less, and more preferably 0.008% or less.

<S: 0.015% or Less (Exceeding 0%)>

Since S deteriorates the toughness-ductility like P, it is desirable to have a low content of S, and the upper limit of P is 0.015%. According to the present invention, the upper limit of P is preferably 0.01% or less, and more preferably 0.008% or less.

<N: 0.006% or Less (Exceeding 0%)>

If N is present in a solid-solution state in the steel, it deteriorates the toughness-ductility and the hydrogen embrittlement resistance. Here, if Al, Ti and the like are present in the steel, N forms a nitride therewith, causing refinement of the structure. In the present invention, the content of N is 0.006% or less to reduce solid-solution N as much as possible. The content of N is preferably 0.005% or less, and more preferably 0.004% or less.

<Al: 0.001 to 0.07%>

Al is usually added as a decarburization element. In addition, Al forms AlN with N, thereby removing the effect of solid-solution N while contributing to the refinement of the structure. In order to allow Al to exhibit such effect sufficiently, the content of Al must be 0.001% or more. In particular, in order to fix the solid solution N, it is desirable that the content of Al be regulated to become twice or more of the content of N by weight percent. However, since Al is an element serving to promote decarburization like Si, it is necessary to suppress the content of Al in the spring steel wire which comprises Si in a large amount. Thus, in the present invention, the content of Al is 0.07% or less, and preferably 0.06% or less.

Essential elements of the steel wire according to the present invention are set forth in the above, and the other components of the steel wire are Fe and unavoidable impurities. As the unavoidable impurities, additional alloy elements can be added according to circumstances such as raw material, manufacturing equipment, and the like. In addition, it is effective to further enhance the properties of the steel wire by adding elements described as follows.

<Ni: 1.0% or Less (Exceeding 0%)>

Ni is an effective element to suppress decarburization on the surface of the steel while enhancing the corrosion resistance. In order to allow Ni to exhibit such effect sufficiently, it is desirable that the content of Ni be 0.2% or more. However, since an excessive content of Ni causes an ultimate increase in amount of the retained austenite after the quenching, and deteriorates the toughness-ductility of the steel, the upper limit of Ni is 1.0% in the present invention. In particular, the content of Ni is preferably 0.7% or less, and more preferably 0.5% or less in view of hot deformation cracking or cost reduction.

<Cu: 1.0% or Less (Exceeding 0%)>

Like Ni described above, Cu is an effective element to suppress decarburization on the surface of the steel while enhancing the corrosion resistance. In order to allow Cu to exhibit such effect sufficiently, it is desirable that the content of Cu be 0.2% or more. However, an excessive content of Cu causes cracking during hot working or an ultimate increase in an amount of the retained austenite after the quenching, thereby deteriorating the toughness-ductility of the steel. Accordingly, the content of Cu is 1.0% as an upper limit, preferably 0.7% or less, and more preferably 0.5% or less. In addition, if the content of Cu exceeds 0.5%, the content of Ni can be controlled to be more than or equal to the content of Cu (that is, Ni (mass %) ≧Cu (mass %)), thereby suppressing hot embrittlement by Cu.

<Ti: 0.1% or Less (Exceeding 0%)>

Ti forms nitride or sulfide with N or S, thereby removing effect of N or S. In addition, Ti forms carbon nitride, thereby enabling the refinement of the structure. In order to allow Ti to exhibit such effect sufficiently, it is desirable that the content of Ti be 0.02% or more while exceeding 3.5× the content of N (mass %). An excessive content of Ti causes formation of coarse TiN, deteriorating the toughness-ductility. Thus, in the present invention, the upper limit of Ti is 0.1%. In particular, the content of Ti is preferably 0.07% or less in view of cost reduction.

<V: 0.2% or Less (Exceeding 0%)>

V is an element serving to form carbon nitride or sulfide with N or C, thereby contributing to the refinement of the structure. In order to allow V to exhibit such effect sufficiently, the content of V is preferably 0.02% or more, and more preferably 0.05% or more. However, since an excessive content of V causes an unnecessary increase of quenching properties, and results in formation of a supercooled structure during rolling, it is necessary to perform a softening process such as annealing in a post process, thereby lowering workability. Thus, the upper limit of V is preferably 0.2%. More preferably, the content of V is suppressed to 0.18% or less in view of cost reduction.

<Nb: 0.1% or Less (Exceeding 0%)>

Nb is an element serving to form carbon nitride or sulfide with N or C, thereby contributing to the refinement of the structure. In order to allow Nb to exhibit such effect sufficiently, the content of Nb is preferably 0.003% or more, and more preferably 0.005% or more. However, since an excessive content of Nb causes formation of coarse carbon nitride, and thus deteriorates the toughness-ductility of the steel. Thus, the upper limit of Nb is preferably 0.1%. It is desirable that the content of Nb be suppressed to 0.07% or less in view of cost reduction.

<Mo: 1.0% or Less (Exceeding 0%)>

Mo is an element serving to form carbon nitride or sulfide with N or C, thereby contributing to the refinement of the structure. In addition, Mo is effective to secure strength after the tempering. In order to allow Mo to exhibit such effect sufficiently, the content of Mo is preferably 0.15% or more, and more preferably 0.3% or more. However, an excessive content of Mo causes formation of coarse carbon nitride, and thus deteriorates the toughness-ductility of the steel. Thus, the upper limit of Mo is preferably 1.0% (more preferably, 0.7%). It is desirable that the content of Mo be suppressed to 0.5% or less in view of cost reduction.

The present invention does not restrict manufacture conditions. The spring steel wire according to the present invention can be manufactured in such a way of, for example, forming a steel billet from molten metal, rolling the steel billet into a steel rod, drawing the steel rod into a steel wire, quenching and tempering (oil tempering) the steel wire. In order to allow easy formation of the structure which can enhance the hydrogen embrittlement resistance and the coiling properties at the same time along with the strength, it is recommended to perform the quenching and tempering according to a method described below after the drawing.

Description will be now made of preferable conditions for the quenching and the tempering with reference to FIG. 1. First, in order to regulate prior austenite of the structure to have an average grain size of 12 μm or less, it is preferable that a heating retention temperature (T1 of FIG. 1) is 1,100° C. or less, and that a heating retention time (t1 of FIG. 1) is 1,500 seconds or less at the quenching. If T1 exceeds 1,100° C., carbides or nitrides acting as fixing pins to suppress growth of crystal grains are removed, and causes the prior austenite to be coarsened, thereby making it difficult for the prior austenite to have an average grain size of 12 μm or less. In addition, if t1 exceeds 1,500 seconds, carbides or nitrides become coarsened, failing to suppress the growth of the prior austenite. For the purpose of sufficiently dissolving a cementite-based carbide as a solid solution during heating, T1 is preferably 900° C. or more. More preferably, T1 is in the range of 920 to 1,050° C. In addition, t1 is preferably 1 second or more, and more preferably, in the range of 2 seconds≦t1≦1,200 seconds.

After heating the steel wire, cooling is performed, and at this time, the cooling rate significantly influences the amount and size of the retained austenite. In order to provide the amount and size of the retained austenite satisfying the conditions of the present invention, it is important to control a cooling rate, particularly, in the transformation range. An average cooling rate (CR1 in FIG. 1) is preferably 10 to 50° C./sec at a temperature of 300 to 50° C. If CR1 is less than 10° C./sec, there occurs an increase in amount of retained austenite, and coarsening of the retained austenite at the same time. In addition, if the quenching is performed at a CR1 exceeding 50° C./sec, the transformation of the steel is accelerated, failing to secure a predetermined amount of retained austenite.

The size of the retained austenite is affected by the average grain size of the prior austenite as well as the cooling rate during the quenching. According to the present invention, uniform refinement of the retained austenite can be achieved through regulating the prior austenite to have the average grain size of 12 μm or less, followed by controlling the CR1 as described above.

Controlling a condition of the tempering is also important in view of the amount of retained austenite. Since the retained austenite is decomposed during the tempering, it is preferable that the tempering is performed for a short period of time at a low temperature. In this regard, since suitable retention time and temperature are determined depending on the level of strength, they can be determined according to desired strength for the steel wire.

In addition, as heating furnaces used for heat treatment described above, the heat treatment can be performed in the sequence of an electric furnace, a salt furnace, and an Induction Heating (IH) furnace for a short period of time. Thus, the IH furnace is most advantageous for refinement of the prior austenite.

Before the drawing, it is possible to perform softening annealing, machining, lead patenting and the like as is performed in the art. In addition, after spring winding, it is possible to perform annealing for strain relief, double shot peening, low temperature annealing, cold setting and the like as is performed in the art.

The spring steel wire according to the present invention manufactured as above has excellent coiling properties and hydrogen embrittlement resistance in a high strength range of 1,900 MPa or more of tensile strength. Thus, the steel wire according to the present invention is useful for manufacture of the spring used in the field of, for example, automotive vehicles, industrial machinery, and the like. In particular, the steel wire according to the present invention is most suitable for a spring applicable to recovery mechanism of a machine, such as a suspension spring for a suspension of the vehicle, a valve spring, a clutch spring, a brake spring, and the like for an engine of the vehicle. In addition, since an excessive strength of the steel makes it difficult to performing the coiling process, the upper limit of the yield strength of the spring steel wire is about 2,300 MPa.

EXAMPLES

The present invention will be described in detail with reference to inventive and comparative examples hereinafter. It should be noted that the present invention is not limited to these examples, and that modification and variation of the examples are allowed without departing from the scope of the present invention.

After forming Steel A1 to A33, with compositions listed in Table 1, from molten metal, steel rods of φ14 mm were obtained through hot rolling. Then, for evaluation of properties, each of the steel rod was cut to a length of 200 mm, followed by quenching and tempering under the conditions listed in Tables 2 and 3 (T1, t1, CR1, T2, t2, and CR2 in Tables 2 and 3 indicate marks of FIG. 1). Quenching and tempering were performed using the electric furnace, the salt furnace or the IH furnace.

In these examples, an average grain size of prior austenite was regulated by controlling a treatment condition for the quenching, and at the same time, the amount and size of retained austenite was regulated by controlling a cooling rate of the quenching. In addition, conditions for the tempering were controlled to satisfy requirements for the amount of retained austenite and the strength of the present invention. Since a slow cooling rate after tempering can cause decomposition of the retained austenite even though the tempering is performed for a short period of time, the cooling rate CR2 after tempering was 30° C./sec or more.

Then, observation of the structure, tensile test, and hydrogen embrittlement test were performed using specimens obtained as above.

First, for observation of the structure, an average grain size of prior austenite was measured after extracting each specimen such that locations corresponding to D/4 in a cross-section of the steel wire become a target surface. Specifically, after extracting the specimen, the specimen was polished in a state of being embedded in a resin, and etched by using a picric acid based etching solution until grain boundaries of the prior austenite were exhibited. The grain size number of the prior austenite was measured according to a method of JISG 0551, and converted to the grain size.

Next, the amount of retained austenite was measured by the saturation magnetization method (see R&D Kobe Steel Engineering Reports, Vol. 52, No. 3, page 43, December 2002). In addition, the size of retained austenite was measured by the SEM/EBSP method described above. In FIG. 2, one example of results obtained by detecting the retained austenite with the SEM/EBSP method is shown. As shown in FIG. 2, after detecting the retained austenite, an image analysis was performed using an image analysis software (ImagePro) as described above to measure the grain size of the retained austenite. Specifically, after measuring an area of the retained austenite detected as above, a diameter of the area was obtained by the circular approximation with respect to the measured area of the retained austenite. Then, the average grain size and the maximum grain size of the retained austenite were obtained using the diameter. The measurement by the SEM/EBSP method was performed such that the total area of measurement is 10,000 μm² or more. The matrix structure of the spring steel wire may comprise martensite as a main structure, and a very small quantity of bainite and ferrite.

Tensile test was performed using a universal tester at a cross-head speed of 10 mm/min with a test specimen formed by a wire cut as shown in FIG. 3. With this test, a tensile strength and a total elongation were measured as indices for the strength and the coiling properties (ductility). In the examples, when the specimen had a tensile strength of 1,900 MPa or more, and a total elongation of 10% or more, it was evaluated as having excellent coiling properties (ductility).

For test of hydrogen embrittlement, after obtaining a failure life through a cathode charge-4 points bending test using a hydrogen embrittlement test specimen formed by the wire cut as shown in FIG. 3, hydrogen embrittlement resistance was evaluated using the failure life. In the examples, when the specimen had a tensile strength of 1,900 MPa or more, and a failure life of 1,000 seconds or more, it was evaluated as having excellent hydrogen embrittlement resistance.

Results of these tests are shown in Tables 2 and 3.

	TABLE 1
	Composition* (mass %)
Steel	C	Si	Mn	Cr	P	S	N	Al	Ni	Cu	Ti	V	Nb	Mo
A1	0.40	1.91	0.21	1.89	0.006	0.007	0.0030	0.0320	0.21	0.02	—	0.123	—	—
A2	0.42	1.72	0.33	1.88	0.010	0.012	0.0042	0.0351	0.02	—	—	—	—	0.21
A3	0.45	2.21	0.21	0.81	0.009	0.010	0.0045	0.0288	0.54	0.31	0.054	0.072	—	—
A4	0.46	1.92	0.18	1.22	0.008	0.005	0.0051	0.0333	0.21	0.22	0.051	—	—	—
A5	0.46	2.41	0.32	1.89	0.007	0.008	0.0050	0.0499	0.73	0.57	0.055	—	—	—
A6	0.47	2.03	0.33	1.78	0.013	0.012	0.0045	0.0522	0.32	0.29	0.022	—	0.011	—
A7	0.47	1.71	0.37	1.80	0.008	0.008	0.0033	0.0314	—	—	—	—	—	—
A8	0.50	1.98	0.20	1.21	0.007	0.008	0.0052	0.0344	0.20	—	—	0.082	0.007	—
A9	0.52	2.01	0.22	0.62	0.005	0.003	0.0037	0.0210	0.22	0.22	—	—	0.072	—
A10	0.51	2.45	0.39	1.77	0.009	0.011	0.0048	0.0012	0.81	0.62	0.078	0.171	0.007	—
A11	0.51	2.30	0.37	0.80	0.014	0.013	0.0036	0.0327	0.22	0.24	—	—	0.022	0.22
A12	0.54	2.02	0.22	0.99	0.008	0.007	0.0033	0.0271	0.11	0.13	—	—	—	0.52
A13	0.55	2.31	0.21	1.02	0.009	0.010	0.0035	0.0319	0.32	0.21	—	—	0.021	0.55
A14	0.54	1.72	0.22	1.21	0.007	0.006	0.0041	0.0582	0.22	—	0.081	0.082	—	—
A15	0.56	1.71	0.19	1.22	0.008	0.005	0.0039	0.0652	0.21	0.20	0.024	—	—	—
A16	0.55	1.84	0.20	0.74	0.012	0.011	0.0032	0.0316	0.33	0.21	0.025	—	—	0.30
A17	0.57	2.40	0.34	1.05	0.010	0.008	0.0052	0.0358	0.32	0.24	0.012	0.047	0.022	—
A18	0.57	1.94	0.33	1.03	0.009	0.007	0.0055	0.0364	0.33	0.25	0.025	—	—	0.31
A19	0.60	2.48	0.22	0.72	0.006	0.007	0.0032	0.0341	0.20	0.27	0.052	—	—	—
A20	0.33	2.48	0.49	1.21	0.010	0.012	0.0049	0.0422	0.78	0.53	0.078	—	—	—
A21	0.34	2.79	0.51	1.88	0.011	0.014	0.0038	0.0385	0.22	0.21	0.077	0.078	—	—
A22	0.45	2.22	0.82	1.89	0.010	0.010	0.0043	0.0398	0.31	0.22	0.021	—	—	0.22
A23	0.47	1.77	0.20	1.78	0.018	0.022	0.0051	0.0352	—	—	—	—	—	—
A24	0.46	1.45	0.77	0.21	0.012	0.014	0.0048	0.0318	—	—	0.022	—	0.026	—
A25	0.45	1.44	0.21	0.72	0.011	0.012	0.0045	0.0311	0.21	0.20	0.025	0.140	—	—
A26	0.50	1.82	0.38	0.98	0.012	0.014	0.0088	0.0289	0.41	0.22	—	0.071	—	—
A27	0.52	2.57	0.39	1.02	0.011	0.013	0.0051	0.0327	1.10	0.30	0.051	0.055	—	—
A28	0.55	2.12	0.34	1.24	0.009	0.008	0.0058	0.1020	0.24	0.22	0.110	0.122	—	—
A29	0.57	2.01	0.38	1.88	0.017	0.014	0.0055	0.0348	0.21	0.18	0.079	—	—	1.22
A30	0.56	2.00	0.80	1.51	0.010	0.009	0.0034	0.0316	0.72	0.54	0.051	0.221	—	—
A31	0.61	1.79	0.55	1.04	0.009	0.007	0.0032	0.0302	0.57	0.21	0.048	—	—	—
A32	0.67	2.10	0.32	1.23	0.012	0.013	0.0033	0.0324	0.52	0.23	0.053	0.140	—	—
A33	0.68	2.51	0.29	1.87	0.008	0.009	0.0051	0.0352	0.50	0.79	0.050	0.127	—	—
*Balance of Fe and unavoidable impurities

	TABLE 2
	Quenching	Tempering
			T1	t1	CR1		T2	t2	CR2
No.	Steel	Heating furnace	(° C.)	(sec)	(° C./sec)	Heating furnace	(° C.)	(sec)	(° C./sec)
1	A1	Electric furnace	952	600	32	Salt furnace	402	240	54
2	A1	Electric furnace	928	900	31	Electric furnace	373	3600	52
3	A1	Electric furnace	955	2400	37	Electric furnace	371	3600	55
4	A2	Salt furnace	931	900	35	Salt furnace	391	240	53
5	A3	IH furnace	1012	5	31	IH furnace	452	2	54
6	A3	IH furnace	1008	5	29	Electric furnace	381	3600	52
7	A4	IH furnace	981	10	29	IH furnace	451	2	57
8	A5	Salt furnace	1002	600	32	IH furnace	461	2	51
9	A6	Salt furnace	1004	600	31	IH furnace	458	2	54
10	A7	IH furnace	952	10	27	IH furnace	447	2	55
11	A7	IH furnace	948	10	79	IH furnace	452	2	52
12	A8	Salt furnace	934	600	21	Salt furnace	422	240	56
13	A9	Salt furnace	924	600	22	Salt furnace	405	240	53
14	A10	Electric furnace	951	1200	24	Salt furnace	482	240	52
15	A10	Electric furnace	951	1200	25	IH furnace	522	2	51
16	A10	Salt furnace	1022	60	26	IH furnace	476	2	54
17	A10	Salt furnace	1075	60	24	IH furnace	474	2	56
18	A10	IH furnace	1120	5	24	IH furnace	480	2	56
19	A11	Salt furnace	952	600	22	IH furnace	482	2	58
20	A12	Salt furnace	924	600	14	Salt furnace	449	240	54
21	A13	Salt furnace	921	600	17	Salt furnace	450	240	54
22	A14	IH furnace	943	5	16	IH furnace	491	2	55
				Average	Maximum			Hydrogen
		Grain size	Amount of	grain size of	grain size of			embrittlement
		of Prior-γ	Retained-γ	Retained-γ	Retained-γ	TS	El	Life
	No.	(μm)	(Vol. %)	(nm)	(nm)	(MPa)	(%)	(second)
	1	7.9	1.8	135	178	1932	21	1345
	2	8.8	1.6	136	164	1915	20	1117
	3	15.7	1.9	178	201	1933	9	578
	4	8.6	1.4	118	154	1922	20	1089
	5	6.4	3.1	153	198	2054	18	1236
	6	6.5	2.9	148	184	2061	17	1355
	7	8.8	2.4	163	225	2013	17	1174
	8	10.2	6.5	217	470	2061	16	1035
	9	8.9	4.7	206	341	2035	19	1175
	10	10.8	4.2	197	278	1943	16	1109
	11	11.1	0.6	82	101	1921	14	875
	12	8.4	2.4	134	186	2078	15	1176
	13	6.2	4.5	146	195	2049	17	1237
	14	5.4	3.7	164	182	2011	17	1304
	15	5.8	6.6	195	417	2156	15	1124
	16	4.7	7.8	206	434	2207	15	1195
	17	8.5	7.5	241	579	2215	15	1084
	18	11.8	7.9	288	811	2137	7	1017
	19	8.4	2.7	138	172	2055	16	1255
	20	7.8	1.9	124	168	2044	16	1302
	21	5.7	3.9	145	207	2106	15	1254
	22	5.1	4.2	136	182	1987	18	1372

	TABLE 3
	Quenching	Tempering
			T1	t1	CR1	Heating	T2	t2	CR2
No.	Steel	Heating furnace	(° C.)	(sec)	(° C./sec)	furnace	(° C.)	(sec)	(° C./sec)
23	A14	IH furnace	940	5	72	IH furnace	497	2	56
24	A15	IH furnace	941	5	38	IH furnace	484	2	53
25	A15	IH furnace	952	5	8	IH furnace	482	2	51
26	A16	IH furnace	935	5	39	IH furnace	488	2	52
27	A17	Salt furnace	927	600	45	Salt furnace	432	240	52
28	A18	Salt furnace	954	600	48	Salt furnace	433	240	55
29	A19	Salt furnace	975	600	47	IH furnace	482	2	54
30	A19	Salt furnace	978	600	5	IH furnace	461	2	56
31	A19	IH furnace	1102	5	28	IH furnace	497	2	52
32	A20	Electric furnace	955	900	29	IH furnace	442	3600	51
33	A21	Electric furnace	953	900	27	IH furnace	451	3600	58
34	A22	Salt furnace	931	600	25	IH furnace	451	2	57
35	A23	IH furnace	950	10	24	IH furnace	449	2	55
36	A24	IH furnace	948	5	28	IH furnace	452	2	54
37	A25	Salt furnace	934	600	27	IH furnace	452	2	53
38	A26	Salt furnace	935	600	32	IH furnace	451	2	52
39	A27	Salt furnace	933	600	34	IH furnace	455	2	54
40	A28	Salt furnace	927	600	38	IH furnace	454	2	51
41	A29	Salt furnace	925	600	37	IH furnace	449	2	52
42	A30	Salt furnace	923	600	39	IH furnace	432	2	51
43	A31	Salt furnace	934	600	35	IH furnace	440	2	53
44	A32	Salt furnace	928	600	8	IH furnace	437	2	54
45	A33	—	—	—	—	—	—	—	—
				Average	Maximum			Hydrogen
		Grain size	Amount of	grain size of	grain size of			embrittlement
		of Prior-γ	Retained γ	Retained γ	Retained γ	TS	EL	Life
	No.	(μm)	(Vol. %)	(nm)	(nm)	(MPa)	(%)	(sec)
	23	5.4	0.8	63	99	2011	16	941
	24	8.1	3.7	147	172	1944	18	1169
	25	8.2	6.1	318	602	1918	8	1054
	26	7.8	4.8	182	218	1964	18	1278
	27	7.2	2.1	165	207	1998	18	1214
	28	9.1	4.1	188	243	2078	16	1105
	29	8.8	7.4	246	398	2101	14	1098
	30	9.2	9.2	313	804	2240	7	1023
	31	24.7	7.1	255	465	2096	8	312
	32	9.4	0.4	40	72	1823	25	904
	33	6.4	0.8	33	62	1854	27	964
	34	8.4	8.8	287	814	2085	8	713
	35	10.5	5.1	204	321	2041	7	502
	36	7.9	8.4	234	798	1834	13	1123
	37	8.4	4.5	164	195	1866	20	1324
	38	7.8	4.4	162	185	2031	8	1034
	39	8.2	14.1	356	895	2015	9	824
	40	7.4	5.2	188	236	2054	9	1033
	41	6.5	6.1	189	234	2096	8	632
	42	6.4	10.2	304	811	2130	8	774
	43	12.8	8.7	245	546	2034	8	514
	44	7.1	15.2	365	862	2264	6	422
	45	—	—	—	—	—	—	—

From Tables 1 to 3, it can be appreciated as follows (in addition, “No.” indicates “No.” of Tables 2 and 3).

Nos. 1, 2, 4 to 10, 12 to 17, 19 to 22, 24, and 26 to 29 satisfying the requirements of the present invention have a high tensile strength of 1,900 MPa, and an excellent total elongation, thereby exhibiting excellent hydrogen embrittlement resistance in severe environments while providing good coiling properties.

On the contrary, Nos. 3, 11, 18, 23, 25, and 30 to 45 does not satisfy the requirements of the present invention, and have drawbacks as follows.

Although Nos. 3, 11, 18, 23, 25, 30, and 31 were made of steel satisfying the composition of the present invention, they were not subjected to quenching in the preferable condition of the present invention, thereby suffering from coarsening of prior austenite and retained austenite, and an increase in the amount of the retained austenite. As a result, the ductility and the hydrogen embrittlement resistance thereof are deteriorated. Specifically, since the heating retention time of No. 3 was excessively long for the quenching, it has a coarsened prior austenite. For Nos. 11 and 23, since the cooling rate for the quenching was excessively rapid, the amount of retained austenite could not sufficiently be secured. For No. 18, since it has excessive contents of Ti, V and Nb effectively contributing to refinement of structure, the grain size of the prior austenite is small, but since the heating temperature for the quenching was excessively high, the maximum grain size of the retained austenite exceeds that of the present invention. For No. 25, since the cooling rate for the quenching was slow, the average grain size of the retained austenite exceeds the upper limit of the requirement according to present invention. For No. 30, since the cooling rate for the quenching was excessively slow, coarse retained austenite is excessively formed. In addition, for No. 31, since the heating temperature for the quenching was excessively high, grains of the prior austenite became coarsened.

Nos. 32 to 45 do not satisfy the composition of the present invention, and thus do not have good properties. For Nos. 32 and 33, since they were made of Steel A20 and A21 having a lower content of C than that of the present invention, they do not have a desired strength, and fail to have a sufficient amount of retained austenite. For No. 33, since it was made of Steel A21 having an excessive content of Si, it suffers from decarburization during rolling.

For Nos. 34, 36, 42 and 43, since they were made of Steel A22, A24, A30 and A31 having an excessive content of Mn, the amount and size of the retained austenite increase above the requirements of the present invention.

For Nos. 35 and 41, since they were made of Steel A23 and A29 having an excessive content of P and/or S, they satisfy the requirements of the present invention in view of the average grain size of the prior austenite and the amount and size of the retained austenite, but they have reduced ductility or hydrogen embrittlement resistance.

For No. 37, since it was made of Steel A25 having an insufficient content of Si, it does not have the desired strength.

For No. 38, since it was made of Steel A26 having an excessive content of N, it has the structure satisfying the requirements of the present invention, but is lowered in ductility.

For No. 39, since it was made of Steel A27 having a high content of Si and an excessive content of Ni, it does not suffer from decarburization, but has an amount and size of the retained austenite above the requirements of the invention.

For No. 40, since it has excessive amounts of Al and Ti, it suffers from decarburization and has lowered ductility.

For No. 44, since it was made of Steel A32 having an excessive content of C, and quenched at an undesirable cooling speed below the cooling speed of the present invention, the amount and size of the retained austenite are increased. Finally, for No. 45, since it was made of Steel A33 having an excessive content of Cu, it was cracked, and thus cannot be subjected to a subsequent treatment.

FIG. 5 is a graph depicting the relationship between tensile strength and total elongation obtained by organizing the examples. As shown in FIG. 5, it can be appreciated that the spring steel wires of the present invention exhibit excellent coiling properties in the high strength range. In addition, FIG. 6 is a graph depicting the relationship between tensile strength and failure life from the hydrogen embrittlement resistance test obtained by organizing the examples. As shown in FIG. 6, it can be appreciated that the spring steel wires of the present invention exhibit excellent hydrogen embrittlement resistance in the high strength range.

It should be understood that the embodiments and the accompanying drawings have been described for illustrative purposes and the present invention is limited by the following claims. Further, those skilled in the art will appreciate that various modifications, additions and substitutions are allowed without departing from the scope and spirit of the invention as set forth in the accompanying claims.

Claims

1. A high strength spring steel wire with excellent coiling properties and hydrogen embrittlement resistance, the steel wire comprising, by mass: 0.4 to 0.60% of C, 1.7 to 2.5% of Si, 0.1 to 0.4% of Mn, 0.5 to 2.0% of Cr, 0.015% or less of P (exceeding 0%), 0.015% or less of S (exceeding 0%), 0.006% or less of N (exceeding 0%), 0.001 to 0.07% of Al, and the remainder being Fe and unavoidable impurities, the steel wire having a tensile strength of 1,900 MPa or more, and a structure wherein prior austenite has an average grain size of 12 μm or less, and retained austenite exists in an amount of 1.0 to 8.0 vol. % with respect to a whole structure of the steel wire, the retained austenite having an average grain size of 300 nm or less and a maximum grain size of 800 nm or less.

2. The spring steel wire according to claim 1, further comprising, by mass: 1.0% or less of Ni (exceeding 0%) and/or 1.0% or less of Cu (exceeding 0%).

3. The spring steel wire according to claim 1, further comprising, by mass: at least one selected from the group consisting of 0.1% or less of Ti (exceeding 0%), 0.2% or less of V (exceeding 0%), 0.1% or less of Nb (exceeding 0%) and 1.0% or less of Mo (exceeding 0%).

4. The spring steel wire according to claim 2, further comprising, by mass: at least one selected from the group consisting of 0.1% or less of Ti (exceeding 0%), 0.2% or less of V (exceeding 0%), 0.1% or less of Nb (exceeding 0%) and 1.0% or less of Mo (exceeding 0%).