EP3112491A1

EP3112491A1 - Rolled material for high strength spring, and wire for high strength spring

Info

Publication number: EP3112491A1
Application number: EP15755898.2A
Authority: EP
Inventors: Atsuhiko TAKEDA; Tomokazu Masuda; Sho Takayama
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2014-02-28
Filing date: 2015-02-03
Publication date: 2017-01-04
Also published as: TWI535856B; KR20160119216A; MX2016011156A; TW201540845A; US20170058376A1; CN106062229B; EP3112491A4; CN106062229A; JP6452454B2; WO2015129403A1; JP2015178673A; KR20180112882A

Abstract

It is an object of the present invention to provide a rolled material, which is a material for high strength spring, and also can exhibit excellent corrosion fatigue properties after quenching and tempering even when suppressing the addition amount of an alloying element; and a wire for high strength spring obtained from such a rolled material. The rolled material for high strength spring of the present invention includes, in % by mass: C: 0.39 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%, P: exceeding 0% and 0.015% or less, S: exceeding 0% and 0.015% or less, Al: 0.001 to 0.1%, Cu: 0.10 to 0.80%, Ni: 0.10 to 0.80%, and O: exceeding 0% and 0.0010% or less, with the balance being iron and inevitable impurities, wherein the number of oxide inclusions having an average diameter of 25 µm or more is 30 or less per 100 g of a steel material, and an amount of nondiffusible hydrogen is 0.40 ppm by mass or less.

Description

Field of the Invention

The present invention relates to a rolled material for high strength spring, and a wire for high strength spring using the same. More particularly, the present invention relates to a rolled material and a wire for high strength spring, which are useful as raw materials of high strength springs that are used in a state of being subjected to refining, namely, quenching and tempering, particularly a rolled material having excellent in corrosion fatigue properties after quench and temper, which are excellent in corrosion fatigue properties even though a tensile strength is a high strength in a range of 1,900 MPa or more after wire drawing.

Background Art

Coil springs used in automobiles, for example, a valve spring and a suspension spring used in the engine,
suspension, and the like are required to reduce the weight and to increase the strength so as to achieve exhaust gas reduction an improvement in fuel economy. The spring imparted with high strength is likely to cause hydrogen brittleness because of its poor toughness and ductility, leading to degradation of corrosion fatigue properties. Therefore, the steel wire (hereinafter, the steel wire is sometimes referred to as a wire) for high strength spring used in the manufacture of a spring is required to have excellent corrosion fatigue properties. Hydrogen generated by corrosion enters into a steel and may lead to embrittlement of a steel material, thus causing corrosion fatigue fracture, so that there is a need to improve corrosion resistance and hydrogen embrittlement resistance of the steel material so as to improve corrosion fatigue properties.
There has been known, as a method for enhancing corrosion fatigue properties of a wire for high strength spring, a method for controlling the chemical composition. However, such a method is not necessarily desirable from a viewpoint of an increase in manufacturing costs and resource saving because of use of a large amount of an alloying element.
Meanwhile, there have been known, as a method for manufacturing a spring, a method in which a steel wire is heating to a quenching temperature and hot-formed into a spring shape, followed by oil cooling and further tempering, and a method in which a steel wire is subjected to quenching and tempering, and then cold-formed into a spring shape. In the cold forming method of the latter, it is also known that quenching and tempering before forming is performed by high frequency induction heating. For example, Patent Document 1 discloses technology in which a wire material is cold-drawn and then the structure is adjusted by quenching and tempering through high frequency incuction heating. According to this technology, a structural fraction of pearlite is set at 30% or less and a structural fraction composed of martensite and bainite is set at 70% or more and then cold drawing is performed at a predetermined area reduction rate, followed by quenching and tempering to thereby reduce the insoluble carbides, leading to an improvement in delayed fracture properties.
In Patent Document 2, a rolled wire material is subjected to wire drawing, followed by a quenching and tempering treatment through high frequency induction heating in Examples. This technology focuses primarily on achievement of the reconciliation of high strength and coiling properties, and gives no consideration to corrosion fatigue properties.
While paying attention to the amount of hydrogen in a steel that is evaluated by the total amount of hydrogen released when the temperature is raised from room temperature to 350°C, Patent Document 3 proposes a hot rolled wire material having excellent wire drawability under high degree wire drawing conditions. However, Patent Document 3 focuses only on wire drawability during special processing such as high degree wire drawing, and also gives no consideration to corrosion fatigue properties after quenching and tempering, which becomes most important in a suspension spring.

Prior Art Document

Patent Document

Patent Document 1: JP 2004-143482 A
Patent Document 2: JP 2006-183137 A
Patent Document 3: JP 2007-231347 A

Summary of Invention

Problems to be Solved by the Invention

In light of aforementioned circumstances, the present invention has been made, and it is an object thereof is. to provide a rolled material, which is a material for high strength spring of hot coiling and cold coiling, and also can exhibit excellent corrosion fatigue properties after quenching and tempering even when suppressing the addition amount of an alloying element; and a wire for high strength spring obtained from such a rolled material.

Means for Solving the Problems

The present invention that can solve the foregoing problems provides a rolled material for high strength spring, including, in % by mass:

C: 0.39 to 0.65%,
Si: 1.5 to 2.5%,
Mn: 0.15 to 1.2%,
P: exceeding 0% and 015% or less,
S: exceeding 0% and 0.015% or less,
Al: 0.001 to 0.1%,
Cu: 0.10 to 0.80%,
Ni: 0.10 to 0.80%, and
O: exceeding 0% and 0.0010% or less, with the balance being iron and inevitable impurities, wherein
the number of oxide inclusions having an average diameter of 25 µm or more is 30 or less per 100 g of a steel material, and an amount of nondiffusible hydrogen is 0.40 ppm by mass or less.

When an average diameter of oxide inclusions is determined, a major axis and a minor axis of an oxide inclusion are respectively measured by observing using an electron probe micro analyzer (EPMA), and an average of the major axis and the minor axis of the oxide inclusion, namely, a value obtained by dividing the sum of the major axis and the minor axis by 2 is regarded as an average diameter. Inclusions exhibiting this average of 25 µm or more are objects that are subjected to measurement of the number in the present invention.
It is preferable that the rolled material for high strength spring of the present invention further includes, in % by mass, at least one of following (a) to (d):

(a) Cr: exceeding 0% and 1.2% or less,
(b) Ti: exceeding 0% and 0.13% or less,
(c) B: exceeding 0% and 0.01% or less, and
(d) at least one of Nb: exceeding 0% and 0.1% or less, and Mo: exceeding 0% and 0.5% or less.

The present invention also include a wire for high strength spring, including any of chemical components of the steel mentioned above, wherein an area ratio of tempered martensite is 80% or more, and a tensile strength,is 1,900 MPa or more.

Effects of the Invention

According to the present invention, oxide inclusions in the rolled material are reduced and the amount of nondiffusible hydrogen is suppressed without adding a large amount of an alloying element, thus making it possible to exhibit excellent corrosion fatigue properties after quenching and tempering. In such a rolled material, it is possible to improve corrosion fatigue properties of the wire even when suppressing the cost of steel materials, thus making it possible to supply a high strength spring which is very unlikely to cause corrosion fatigue fracture, for example, a coil spring such as a suspension spring that is one of automobile components, at a cheap price.

Brief Description of the Drawings

Fig. 1 is a graph showing an influence of the number of inclusions and an amount of nondiffusible hydrogen in a rolled material on corrosion fatigue properties.

Mode for Carrying Out the Invention

With the progress of corrosion of a wire, pits are generated on a surface of a wire rod, and also a wire diameter of the wire rod decreases due to thinning caused by corrosion. Hydrogen generated by corrosion enters into a steel, leading to embrittlement of the steel material. Corrosion fatigue fracture occurs from these corrosion pits, thinning positions and embrittled regions of steel.materials as starting points. Therefore, corrosion fatigue fracture can be suppressed by improving hydrogen embrittlement resistance and corrosion resistance of the wire rod.
Inventors of the present invention have made a study of factors, which affect hydrogen embrittlement resistance and corrosion resistance from every angle. As a result, it became apparent that corrosion fatigue properties are significantly improved by subjecting a rolled material, in which both the number of oxide inclusions having a predetermined size in the steel and the amount of hydrogen in the steel, especially the amount of nondiffusible hydrogen, to quenching and tempering treatment. Consequently, the inventors have found that, if numerous large oxide inclusions exist in the steel, not only durability in the atmosphere is degraded, but also "strain field" is formed around the oxide inclusions, leading to formation of the hydrogen accumulation place, thus causing embrittlement of grain boundaries around them and degradation of corrosion fatigue properties.
Appropriate control of the amount of oxide inclusions and the amount of hydrogen enables an improvement in corrosion fatigue properties even if the addition amount of a corrosion resistance-improving element is reduced. Hereinafter, description will be made of requirements defined in the present invention, for example, the number of oxide inclusions, the amount of nondiffusible hydrogen in the steel, and chemical composition.

Number of Oxide Inclusions

If numerous large oxide inclusions exist in the steel, not only durability in the atmosphere is degraded, but also the strain field is formed around the oxide inclusions, leading to formation of the hydrogen accumulation place, thus causing embrittlement of grain boundaries around them and degradation of corrosion fatigue properties. To reduce an adverse influence on corrosion fatigue properties, there is a need that the number of oxide inclusions having an average diameter of 25 µm or more is set at 30 or less per 100 g of a steel material (hereinafter sometimes referred to as "30 or less per 100 g). The number of oxide inclusions is preferably 20 or less per 100 g, and more preferably 10 or less per 100 g. To improve corrosion fatigue properties, there is no need to set the lower limit of the number of oxide inclusions. However, there will be production costs for setting the lower limit at 0 per 100 g, so that the lower limit is preferably 2 or more per 100 g in view of industrial production. Oxide inclusions having an average diameter of 25 µm or more serve as fracture starting point that is a stress concentration source, thus degrading corrosion fatigue properties, whereas, oxide inclusions having an average diameter of less than 25 µm do not exert an adverse influence on corrosion fatigue properties.

Amount of Nondiffusible Hydrogen

In the rolled material of the present invention, there is a need that the amount of nondiffusible hydrogen is set at 0.40 ppm by mass or less. If a large amount of nondiffusible hydrogen exists in the rolled material, the amount of nondiffusible hydrogen also increases in the wire after quenching and tempering. If a large amount of nondiffusible hydrogen exists in the wire, a permissible amount of hydrogen, which further enters until the steel material embrittles, decreases. Therefore, even though a small amount of hydrogen entered during use as a spring, embrittlement of the steel material occurs and early fracture is likely to occur, resulting in degraded hydrogen embrittlement resistance. The amount of nondiffusible hydrogen is preferably 0.35 ppm by mass or less, and more preferably 0.30 ppm by mass or less- The less the amount of nondiffusible hydrogen, the better. However, it is difficult to set at 0 ppm by mass and the lower limit is about 0.01 ppm by mass.
The amount of nondiffusible hydrogen is an amount of hydrogen measured by the method mentioned in Examples below, and specifically means the total amount of hydrogen released at 300 to 600°C when the temperature of a steel material is raised at 100°C/hour.
The rolled material for high strength spring according to the present invention is a low alloy steel in which the content of an alloying element is suppressed, and the chemical composition is as follows. The present invention also includes a wire obtained by wire-drawing the above-mentioned rolled material, followed by quenching and tempering, and the chemical composition is the same as that of the rolled material. As used herein, chemical composition means % by mass.

C: 0.39 to 0.65%

C is an element that is required to ensure the strength of a wire for spring, and is also required to generate fine carbides that serve as hydrogen trapping sites. From such a viewpoint, the amount of C was determined in a range of 0.39% or more. The lower limit of the amount of C is preferably 0.45% or more, and more preferably 0.50% or more. Excessive C amount, however, might generate coarse residual austenite and non-solid soluted carbides after quenching and tempering, which further degrades hydrogen embrittlement resistance. C is an element that degrades corrosion resistance, so that there is a need to suppress the amount of C so as to enhance corrosion fatigue properties of a spring product such as a suspension spring which is a final product. From such a viewpoint, the amount of C was determined in a range of 0.65% or less. The upper limit of the amount of C is preferably 0.62% or less, and more preferably 0.60% or less.

Si: 1.5 to 2.5%

Si is an element that is required to ensure the strength of a wire for spring, and also exhibits the effect of refining carbides. To effectively exhibit these effects, the amount of Si was determined in a range of 1.5% or more. The lower limit of the amount of Si is preferably 1.7% or more, and more preferably 1.9% or more. Meanwhile, since Si is also an element that accelerates decarburization, excessive Si amount accelerates formation of a decarburized layer on a surface of a wire rod, thus requiring the peeling step for removal of the decarburized layer, resulting in increased manufacturing costs. Non-solid solution carbides also increase, thus degrading hydrogen embrittlement resistance. From such a viewpoint, the amount of Si was determined in a range of 2.5% or less. The upper limit of the amount of Si is preferably 2.3% or less, more preferably 2.2% or less, and still more preferably 2.1% or less.

Mn: 0.15 to 1.2%

Mn is an element that is employed as a deoxidizing element and reacts with S, which is a harmful element in a steel, to form MnS, and is useful for detoxication of S. Mn is also an element that contributes to an improvement in strength. To effectively exhibit these effects, the amount of Mn was determined in a range of 0.15% or more. The lower limit of the amount of Mn is preferably 0.2% or more, and more preferably 0.3% or more. Excessive Mn amount, however, degrades toughness, thus causing embrittlement of a steel material. From such a viewpoint, the amount of Mn was determined in a range of 1.2% or less. The upper-limit of the amount of Mn is preferably 1.0% or less, and more preferably 0.85% or less.

P: exceeding 0% and 0.015% or less

P is a harmful element that degrades ductility such as coiling properties of a rolled material such as a wire rod, and the amount thereof is preferably as small as possible. P is likely to segregate in grain boundaries to cause grain boundary embrittlement, and hydrogen is likely to cause fracture of grain boundaries, thus exerting an adverse influence on hydrogen embrittlement resistance. From such a viewpoint, the amount of P was determined in a range of 0.015% or less. The upper limit of the amount of P is preferably 0.010% or less, and more preferably 0.008% or less. The amount of P is preferably as small as possible, and is contained usually about 0.001%.

S: exceeding 0% and 0.015% or less

Like P mentioned above, S is a harmful element that degrades ductility such as coiling properties of a rolled material, and the amount thereof is preferably as small as possible. S is likely to segregate in grain boundaries to cause grain boundary embrittlement, and hydrogen is likely to cause fracture of grain boundaries, thus exerting an adverse influence on hydrogen embrittlement resistance. From such a viewpoint, the amount of S was determined in a range of 0.015% or less. The upper limit of the amount of S is preferably 0.010% or less, and more preferably 0.008% or less. The amount of S is preferably as small as possible', and is usually contained about 0.001%.

Al: 0.001 to 0.1%

Al is mainly added as a deoxidizing element. This element reacts with N to form AlN to thereby detoxicate solid-soluted N, and also contributes to refining of the structure. To adequately exhibit these effects, the amount of Al was determined in a range of 0.001% or more. The lower limit of the amount of Al is preferably 0.002% or more, and more preferably 0.005% or more. However, since Al is an element that accelerates decarburization, like Si, there is a need to suppress the amount of Al in a steel for spring, which includes a large amount of Si. Therefore, in the present invention, the amount of Al was determined in a range of 0.1% or less. The upper limit of the amount of Al is preferably 0.07% or less, more preferably 0.030% or less, and particularly preferably 0.020% or less.

Cu: 0.10 to 0.80%

Cu is an element that is effective in suppressing surface decarburization and improving corrosion resistance. Therefore, the amount of Cu was determined in a range of 0.10% or more. The lower limit of the amount of Cu is preferably 0.15% or more, and more preferably 0.20% or more. Excessive Cu amount, however, causes cracks during hot working and increases costs. Therefore, the amount of Cu was determined in a range of 0.80% or less. The upper limit of the amount of Cu is preferably 0.70% or less, and more preferably 0.60% or less. The amount of Cu is preferably 0.48% or less, 0.35% or less, and 0.30% or less.

Ni: 0.10 to 0.80%

Like Cu, Ni is an element that is effective in suppressing surface decarburization and improving corrosion resistance. Therefore, the amount of Ni was determined in a range of 0.10% or more. The lower limit of the amount of Ni is preferably 0.15% or more, and more preferably 0.20% or more. Excessive Ni amount, however, increases costs. Therefore, the amount of Ni was determined in a range of 0.80% or less. The upper limit of the amount of Ni is preferably 0.70% or less, and more preferably 0.60% or less. The amount of Ni is preferably 0.48% or less, 0.35% or less, and 0.30% or less.

O: exceeding 0% and 0.0010% or less

If oxygen exists in a steel material, oxide inclusions such as Al₂O₃, SiO₂, CaO, MgO and TiO₂ are formed. Oxide inclusions are hard, and thus strain is generated around oxide inclusions due to a difference in hardness with a material around oxide inclusions. Hydrogen accumulated on the strain causes embrittlement of grain boundaries around the oxide inclusions. Therefore, reducing the amount of oxygen is important to improve corrosion fatigue properties. Therefore, the upper limit of the amount of O was set.at 0.0010% or less. The upper limit is preferably 0.0008% or less, and more preferably 0.0006% or less. Whereas, the lower limit of the amount of O is generally 0.0002% or more on industrial production.
Basic components of the rolled material of the present invention are as mentioned above, the balance being substantially iron. As a matter of course, inclusion of inevitable impurities such as Ca, Mg and N introduced by the state of raw material, material, manufacturing facility, and the like is permitted. The rolled material for spring of the present invention has the chemical composition mentioned above and can achieve excellent coiling properties and hydrogen embrittlement resistance while having high strength. Elements mentioned below may be further included for the purpose of improving corrosion resistance according to application.

Cr: exceeding 0% and 1.2% or less

Cr is an element that is effective in improving corrosion resistance. To effectively exhibit these effects, the amount of Cr is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. However, Cr is an element that has a strong tendency to form carbides, and forms peculiar carbides in a steel material and is likely to be dissolved in cementite in a high concentration. It is effective to include a small amount of Cr, however, the heating time of the quenching step decreases in high frequency induction heating, leading to insufficient austenitizing of dissolving carbide, cementite, and the like into a matrix. Therefore, when including a large amount of Cr, dissolving residue of cementite, in which Cr-based carbide and metallic Cr are solid-soluted in in high concentration is generated as a stress concentration source, so that fracture likely to occur, thus degrading hydrogen embrittlement resistance. Therefore, the amount of Cr is preferably 1.2% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.

Ti: exceeding 0% and 0.13% or less

Ti is an element that is useful to react with S to form sulfide to thereby detoxicate S. Ti also has the effect of refining the structure by forming carbonitride. To effectively exhibit these effects, the amount of Ti is preferably 0.02% or more, more preferably 0.05% or more, and still more preferably 0.06% or more. Excessive Ti amount, however, may form coarse Ti sulfide, thus degrading ductility. Therefore, the amount of Ti is preferably 0.13% or less. From a viewpoint of cost reduction, the amount of Ti is preferably 0.10% or less, and more preferably 0.09% or less.

B: exceeding 0% and 0.01% or less

B is an element that improve hardenability and strengthens prior austenite crystal grain boundaries, and also contributes to suppression of fracture. To effectively exhibit these effects, the amount of B is preferably 0.0005% or more, and more preferably 0.0010% or more. Excessive B amount, however, causes saturation of the above effects, so that the amount of B is preferably 0.01% or less, more preferably 0.0050% or less, and still more preferably 0.0040% or less.

At least one of Nb: exceeding 0% and 0.1% or less, and Mo: exceeding 0% and 0.5% or less

Nb is an element that forms carbonitride together with C and N, and mainly contributes to refining of the structure. To effectively exhibit these effects, the amount of Nb is preferably 0.003% or more, more preferably 0.005% or more, and still more preferably 0.01% or more. Excessive Nb amount, however, form coarse carbonitride, thus degrading ductility of a steel material. Therefore, the amount of Nb is preferably 0.1% or less. From a viewpoint of cost reduction, the amount is preferably set at 0.07% or less.
Like Nb, Mo is also an element that forms carbonitride together with C and N, and contributes to refining of the structure. Mo is an element that is also effective in ensuring the strength after tempering. To effectively exhibit these effects, the amount of Mo is preferably 0.15% or more, more preferably 0.20% or more, and still more preferably 0.25% or more. Excessive Mo amount, however, form coarse carbonitride, thus degrading ductility, for example, coiling properties of a steel material. Therefore, the amount of Mo is preferably 0.5% or less, and more preferably 0.4% or less.
Nb and Mo may be included individually, or both of them may be included in combination. The rolled material of the present invention includes N as inevitable impurities, and the amount of it is preferably adjusted in a range mentioned below.

N: exceeding 0% and 0.007% or less

N is an element included in inevitable impurities. As the amount of N increases, it forms coarse nitride together with Ti and Al, thus exerting an adverse influence on fatigue properties. Therefore, the amount of N is preferably as small as possible. The amount of N is, for example, 0.007% or less, and more preferably 0.005% or less. Meanwhile, if the amount of N is too reduced, productivity is drastically degraded. N forms nitride together with Al to thereby contribute to refining of crystal grains. From such a viewpoint, the amount of N is preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.003% or more.
A method for producing a rolled material of the present invention will be described below. In a series of steps of melting a steel having the above chemical composition, followed by continuous casting, blooming and hot rolling, it is possible to control the amount of nondiffusible hydrogen of the rolled material by adjusting at least one of (A) the amount of hydrogen in a molten steel stage, (B) the homogenizing treatment temperature and time before blooming, and (C) the cooling rate in a range of 400 to 100°C after hot rolling.
There is a need to remove hydrogen in a steel by diffusion so as to reduce hydrogen in the steel after solidification, and heating at a high temperature for a long time is effective to increase a diffusion rate of hydrogen so as to release hydrogen from a surface of a steel material. Specific examples of the method of reducing the amount of hydrogen in the steel include a method of adjusting in a molten steel stage, a method of adjusting in a stage of a continuously cast material at 1,000°C or higher after solidification, a method of adjusting in a heating stage before hot rolling, a method of adjusting in a heated stage during hot rolling, and a method of adjusting in a cooling stage after rolling. It is particularly preferred to perform at least one of treatments for reducing nondiffusible hydrogen (A) to (C) mentioned below.

(A) A degassing treatment is performed in a melting steel proc ess to thereby adjust the amount of hydrogen in a molten steel at 2.5 ppm by mass or less.
For example, it is effective that a vacuum tank equipped with two immersion tubes mounted in a ladle in a secondary refining step and then an Ar gas is blown from the side of one immersion tube, followed by vacuum degassing that enables circulation of a molten steel to the vacuum tank utilizing the buoyancy. This method is excellent in hydrogen removing capability and reduction in inclusion. The amount of hydrogen in the molten steel is preferably 2.0 ppm by mass or less, more preferably 1.5 ppm by mass or less, and particularly preferably 1.0 ppm by mass or less.
(B) A homogenizing treatment (heating) before blooming is performed at 1,100°C or higher, and preferably 1,200°C or higher for 10 hours or more.
(C) An average cooling rate in a range of 400 to 100°C after hot rolling is set at 0.5°C/second or less, and preferably 0.3°C/second or less-.

When a steel material has a large cross-sectional area, particularly, it becomes necessary to perform heating for a long time. If the steel material is heated for a long time, decarburization is accelerated, so that the amount of hydrogen in the steel is preferably reduced by performing the treatment (A) mentioned above.
There is no particular limitation on the coiling temperature TL after hot rolling, and cooling conditions beyond a temperature range of 400 to 100°C after coiling.
The coiling temperature TL can be set, for example, in a range of 900°C or higher and 1,000°C or lower, and is preferably 910°C or higher, and more preferably 930°C or higher. An average cooling rate at the coiling temperature of TL to 650°C can be set in a range of 2°C/second or more and 5°C/second or less. The lower limit of the average cooling rate at the coiling temperature of TL to 650°C is preferably 2.3°C/second or more, and more preferably 2.5°C/second or more. The upper limit of the average cooling rate at the coiling temperature of TL to 650°C is preferably 4.5°C/second or less, and more preferably 4°C/second or less. Further, the average cooling rate at 650 to 400°C can be set at 2°C/second or less. The average cooling rate at 650 to 400°C is preferably 1.5°C/second or less, and more preferably 1°C/second or less. There is no particular limitation on the lower limit of the average cooling rate, and the lower limit is, for example, about 0.3°C/second.

.Reduction in Oxide Inclusions

To reduce oxide inclusions, there is a need to set the content of oxygen of the wire rod at a defined value or less. Sufficient oxidizing with aluminum and silicon as well as sufficient degassing enable reduction in inclusions, leading to achievement of higher cleanliness and reduction in oxide inclusions.
To manufacture a coil spring used in automobiles, there is a need that a wire is manufactured by wire processing, namely, wire drawing of the rolled material mentioned above. For example, in a cold coiled spring, quenching and tempering by high frequency induction heating are performed after wire drawing, and such a wire is also included in the present invention.
A high strength wire having a tensile strength of 1,900 MPa or more can be obtained by subjecting the rolled material to wire working, namely, wire drawing, followed by quenching and tempering by high frequency induction heating. Specifically, the rolled material is subjected to wire drawing at an area reduction rate of about 5 to 35%, followed by quenching at about 900 to 1,000°C and further tempering at about 300 to 520°C. The quenching temperature is preferably 900°C or higher so as to sufficiently perform austenitizing, and preferably 1,000°C or lower so as to prevent grain coarsening. The heating temperature for tempering may be set at an appropriate temperature in a range of 300 to 520°C according to a target value of a wire strength. When quenching and tempering are performed by high frequency induction heating, quenching and tempering times are respectively in a range of about 10 to 60 seconds.
Regarding the structure after quenching and tempering, there is a need that the tempered martensite structure has 80 area% or more. As a result of increasing the proportions of non-solid-soluted ferrite and residual austenite, the strength decreases. Regarding the structure after quenching and tempering, the tempered martensite structure preferably has 88 area% or more. To set the proportion of the tempered martensite structure at 80 area% or more, it is preferable that the material is heated to 900°C or higher when heating before quenching, followed by sufficient austenitizing and further cooling to 100°C or lower by water cooling or oil cooling.
The thus obtained wire of the present invention can realize a high tensile strength in a range of 1,900 MPa or more. The tensile strength is usually selected in a range of 1,900 MPa to 2,200 MPa. Although there is no particular limitation on the upper limit of the tensile strength, and the upper limit is about 2,500 MPa. The wire of the present invention can exhibit corrosion fatigue properties even at a high strength in a range of 1,900 MPa or more because of use of the rolled material of the present invention.
This application claims priority based on Japanese Patent Application No. 2014-039368 filed on February 28, 2014 , the disclosure of which is incorporated by reference herein.
The present invention will be described in more detail below by way of Examples. It should be noted that, however, these examples are never construed to limit the scope of the invention; various modifications and changes may be made without departing from the scope and spirit of the invention and should be considered to be within the scope of the invention.

Examples

Each of steel materials having chemical compositions shown in Tables 1 to 3 was melted by melting in a converter and then subjecting to continuous casting and a homogenizing treatment at 1,100°C or higher. After the homogenizing treatment, blooming was performed, followed by heating at 1,000 to 1,280°C and further hot rolling to obtain a rolled material having a diameter of 14.3 mm, namely, a wire rod. It is as shown in Tables 4 to 6 below whether or not a degassing treatment of a molten steel by the above-mentioned material is implemented, and whether or not cooling is implemented after coiling, namely, whether or not cooling at an average cooling rate of 0.5°C/second or less is implemented at 400 to 100°C after rolling. The amount of O in the molten steel shown in Tables 4 to 6 was adjusted by controlling the degree of deoxidizing with aluminum and silicon.
The coiling temperature TL after hot rolling was set at 950°C, and other cooling after coiling was performed at an average cooling rate of 4°C/second at a temperature in a range of TL to 650°C, and performed at an average cooling rate of 1°C/second at a temperature in a range of 650 to 400°C. In test examples in which "Implementation" is written in the column of the homogenizing treatment, the homogenizing treatment is performed at 1,100°C for 10 hours or more. In test examples in which the mark "-" is written, the time of the homogenizing treatment at 1,100°C is less than 10 hours.
Regarding the wire rod thus obtained, the amount of nondiffusible hydrogen and the number of oxide inclusions were measured by the following procedures. The results are shown in Table 4 to 6. In Tables 4 to 6, the number of oxide inclusions having an average diameter of 25 µm or more in the rolled material was written as "Number of inclusions of 25 p m or more of rolled material".

Amount of Nondiffusible Hydrogen

A specimen measuring 20 mm in width × 40 mm in length was cut out from the rolled material, namely, wire rod. After raising the temperature of the specimen at a temperature rise rate of 100°C/hour, a hydrogen release amount at 300 to 600°C was measured using a gas chromatogram, and the hydrogen release amount was regarded as the amount of nondiffusible hydrogen.

Number of Oxide Inclusions

The number of oxide inclusions was determined by the following procedure: an average of the results of examination of six rolled material samples each having 50 g in weight was determined, followed by conversion into the number per 100 g. The number-of inclusions was examined by an acid dissolution method. Each sample (50 g) was dissolved with an acid and inclusions, remaining without being dissolved, was allowed to remain.on a filter paper. Inclusions having an average diameter of 25 µm or more were sorted by EPMA, analyzed by energy dispersive X-ray spectrometry (EDX), and then oxide inclusions were sorted. Regarding the above-mentioned six samples, the number of oxide inclusions having an average diameter of 25 µm or more was measured and an average thereof was determined, followed by conversion into the number per 100 g of a steel material. In this case, nitric acid adjusted so as not to dissolve oxide inclusions was.used for dissolution with an acid. An average diameter of oxide inclusions means an average of a major axis and a minor axis, namely, the value obtained by dividing the sum of the major axis and the minor axis by 2. To reduce the number of oxide inclusions, oxygen was removed by sufficiently perform vacuum degassing when melting in a converter.
Then, the wire rod was subjected to wire drawing, namely, cold drawing to thereby reduce to a diameter of 12.5 mm, followed by quenching and tempering. An area reduction rate of wire drawing is about 23.6%, and the conditions of quenching and tempering are as follows.

Quenching and Tempering Conditions

High frequency induction heating
Heating rate: 200°C/second
Quenching: 950°C, 20 seconds, water cooling
Tempering: each temperature in a range of 300 to 520°C, 20 seconds, water cooling

It is possible to obtain a structure in which an area ratio of tempered martensite is 80% or more, by quenching and tempering mentioned above. In this test, it was confirmed that the area ratio of the entire tempered martensite is 80% or more.
Regarding the wire after wire drawing, and quenching and tempering, the tensile strength and corrosion fatigue properties were evaluated. The results are collectively shown in Tables 4 to 6 below.

Measurement of Tensile Strength

After quenching and tempering, a wire was cut into a' predetermined length and a tensile test was performed at a distance between chucks of 200 mm and a tensile speed of 5 mm/minute in accordance with JIS Z2241 (2011).

Evaluation of Corrosion Fatigue Properties

Corrosion fatigue properties were evaluated by fracture life after subjecting to a corrosion treatment and performing the Ono-type rotating-bending fatigue test. Each wire.subjected to quenching and tempering was cut to fabricate a No. 1 test specimen (JIS Z 2274(1978)). The parallel part of this test specimen was polished using a sand paper of No. 800. A test was carried out without shot peening of a surface. First, the test specimen thus processed was subjected to a corrosion treatment under the following conditions.

Corrosion treatment

Using an aqueous 5% NaCl solution at 35°C, salt spraying was performed for 8 hours, followed by drying and holding in a wet atmosphere at 35°C and relative humidity of 60% for 16 hours (1 cycle). The test specimen was subjected to a corrosion treatment by repeating 10 cycles in total. After the corrosion treatment, the test specimen was subjected to the rotating-bending test and then corrosion fatigue properties were evaluated. Using ten test specimens for each test, load stress was set at 500 MPa and the Ono-type rotating-bending fatigue test was carried out. Measurement was made of fatigue life until fracture of each test specimen occurs. An average of each fatigue life of ten test specimens was measured. The case where the average of fatigue life is 100,000 times or more was rated as excellent corrosion fatigue life.

[Table 1]

Steel No.	Chemical composition (% by mass) The balance being iron and inevitable impurities
Steel No.	C	Si	Mn	P	S	Al	Cu	Ni	Cr	Ti	B	Nb	Mo	N
1	0.42	2.1	1.05	0.008	0.006	0.021	0.22	0.23	0.35	0.090	-	-	-	0.0039
2	0.43	1.8	0.77	0.005	0.007	0.020	0.21	0.24	0.30	0.120	-	-	-	0.0049
3	0.40	2.0	0.65	0.007	0.012	0.035	0.35	0.40	0.27	0.080	-	-	-	0.0040
4	0.44	2.1	0.95	0.012	0.007	0.003	0.30	0.22	0.36	0.070	-	-	-	0.0055
5	0.60	1.7	0.80	0.004	0.008	0.022	0.35	0.30	-	-	0.0030	-	-	0.0043
6	0.59	2.0	0.90	0.006	0.007	0.021	0.35	0.32	-	-	0.0035	-	-	0.0034
7	0.62	2.0	0.65	0.010	0.011	0.020	0.29	0.30	0.15	-	0.0020	-	-	0.0054
8	0.57	2.1	0.66	0.008	0.007	0.022	0.33	0.36	0.23	-	0.0019	-	-	0.0047
9	0.60	2.0	0.80	0.005	0.006	0.033	0.35	0.30	0.08	0.090	-	-	-	0.0054
10	0.59	1.6	0.50	0.008	0.006	0.002	0.27	0.27	0.32	0.070	-	-	-	0.0043
11	0.55	2.0	0.94	0.004	0.004	0.040	0.27	0.27	0.32	0.040	0.0030	-	-	0.0030
12	0.48	2.0	0.45	0.007	0.007	0.002	0.32	0.32	0.35	0.080	-	-	-	0.0052
13	0.62	1.5	0.95	0.006	0.008	0.045	0.26	0.26	0.75	-	-	-	-	0.0037
14	0.63	2.2	0.53	0.006	0.006	0.021	0.21	0.45	0.26	-	-	0.08	-	0.0043
15	0.58	2.0	0.20	0.010	0.011	0.020	0.14	0.14	0.19	-	-	-	0.25	0.0038
16	0.54	2.1	0.70	0.007	0.007	0.022	0.45	0.33	0.38	-	-	-	-	0.0025

[Table 2]

Steel No.	Chemical composition (% by mass) The balance being iron and inevitable impurities
Steel No.	C	Si	Mn	P	S	Al	Cu	Ni	Cr	Ti	B	Nb	Mo	N
17	0.43	2.0	0.65	0:004	0.002	0.021	-	-	0.25	-	-	-	-	0.0057
18	0.50	1.7	0.67	0.004	0.002	0.022	0.08	0.07	0.35	-	-	-	-	0.0045
19	0.47	2.0	0.95	0.005	0.008	0.020	0.21	0.25	0.36	0.050	-	-	-	0.0031
20	0.48	2.0	0.89	0.004	0.002	0.020	0.45	0.30	0.36	-	-	-	-	0.0043
21	0.58	2.1	0.78	0.004	0.002	0.021	0.42	0.40	-	-	0.0044	-	-	0.0032
22	0.61	1.9	0.52	0.008	0.005	0.020	0.41	0.29	-	-	0.0049	-	-	0.0043
23	0.50	2.1	0.59	0.008	0.008	0.023	0.43	0.43	0.48	0.020	-	0.02	-	0.0041
24	0.58	1.9	0.78	0.004	0.004	0.020	0.18	0.18	0.23	0.080	-	-	-	0.0043
25	0.42	1.6	0.83	0.008	0.008	0.023	0.43	0.43	0.48	-	-	-	-	0.0036
26	0.48	2.0	0.50	0.008	0.009	0.022	0.33	0.33	0.38	-	-	-	-	0.0047
27	0.52	2.1	0.51	0.005	0.006	0.021	0.24	0.24	0.29	-	-	-	-	0.0049
28	0.55	2.0	0.86	0.008	0.008	0.022	0.35	0.35	0.40	-	-	-	-	0.0043
29	0.60	1.5	0.63	0.010	0.011	0.022	0.38	0.38	0.43	-	-	-	-	0.0028
30	0.43	1.9	0.90	0.005	0.005	0.020	0.19	0.19	0.24	-	-	-	-	0.0052
31	0.50	1.9	0.93	0.009	0.005	0.020	0.19	0.19	0.24	-	-	-	-	0.0048

[Table 3]

Steel No.	Chemical composition (% by mass) The balance being iron and inevitable impurities
Steel No.	C	Si	Mn	P	S	Al	Cu	Ni	Cr	Ti	B	Nb	Mo	N
32	0.57	2.2	0.34	0.006	0.008	0.028	0.29	0.48	0.30	0.10	-	-	-	0.0039
33	0.52	2.1	0.53	0.007	0.008	0.028	0.30	0.50	0.28	0.09	0.0030	-	-	0.0041
34	0.60	2.2	0.40	0.006	0.008	0.025	0.31	0.58	0.34	0.08	0.0030	-	-	0.0052
35	0.54	2.1	0.57	0.010	0.010	0.033	0.28	0.79	0.26	0.07	-	-	-	0.0039
36	0.57	2.1	0.40	0.008	0.009	0.028	0.12	0.54	0.27	0.11	-	-	-	0.0042
37	0.58	2.2	0.71	0.007	0.006	0.031	0.21	0.58	0.27	0.08	-	-	-	0.0041
38	0.57	2.2	0.61	0.008	0.010	0.027	0.57	0.61	0.27	0.09	-	-	-	0.0053
39	0.60	2.3	0.47	0.008	0.008	0.024	0.28	0.54	0.31	-	-	-	-	0.0058
40	0.60	2.2	0.58	0.010	0.007	0.030	0.32	0.46	0.22	-	-	-	-	0.0039
41	0.50	2.2	0.60	0.007	0.009	0.032	0.32	0.55	0.20	0.08	0.0025	-	-	0.0041
42	0.52	2.2	0.50	0.009	0.006	0.025	0.27	0.50	0.19	0.10	-	-	-	0.0039
43	0.62	2.2	0.64	0.010	0.007	0.028	0.30	0.55	-	0.10	-	-	-	0.0055
44	0.55	2.3	0.74	0.008	0.008	0.029	0.27	0.49	-	0.08	-	-	-	0.0054
45	0.56	2.3	0.59	0.007	0.007	0.031	0.35	0.56	0.34	0.07	0.0030	-	-	0.0051
46	0.50	2.4	0.52	0.008	0.009	0.029	0.27	0.64	0.33	0.08	-	-	-	0.0040
47	0.59	2.2	0.49	0.009	0.008	0.024	0.28	0.60	0.18	0.07	-	-	-	0.0053
48	0.65	2.1	0.41	0.008	0.006	0.029	0.44	0.53	0.26	0.07	-	-	-	0.0043

[Table 4]

Test No.	Steel No.	Amount of O (% by mass)	Number of inclusions of 25 µm or more of rolled material (inclusions/100 g)	Whether or not treatment for reduction of hydrogen in steel is implemented			Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
Test No.	Steel No.	Amount of O (% by mass)		Molten steel treatment	Homogenizing treatment	Cooling at 400 to 100°C after rolling	Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
1	1	0.0005	1	Implementation	-	-	0.28	1,970	27.8
2	2	0.0008	7	-	Implementation	-	0.33	1,965	20.2
3	3	0.0006	1	-	-	Implementation	0.34	1,966	21.4
4	4	0.0009	24	Implementation	Implementation	Implementation	0.05	1,950	20.2
5	5	0.0004	1	Implementation	-	-	0.25	2,152	29.8
6	6	0.0007	5	-	Implementation	-	0.35	2,140	21.9
7	7	0.0007	5	-	-	Implementation	0.36	2,165	22.4
8	8	0.0008	15	Implementation	Implementation	Implementation	0.04	2,155	16.8
9	9	0.0007	5	Implementation	-	-	0.22	2,120	22.4
10	10	0.0006	1	-	Implementation	-	0.35	2,132	22.6
11	11	0.0009	28	Implementation	Implementation	Implementation	0.06	2,104	17.3
12	12	0.0008	11	Implementation	-	-	0.19	2,097	14.6
13	13	0.0008	13	Implementation	-	-	0.18	2,095	19.1
14	14	0.0007	1	Implementation	-	-	0.21	2,110	34.2
15	15	0.0008	18	Implementation	-	-	0.22	2,111	20.5
16	16	0.0005	1	Implementation	-	-	0.22	2,105	27.5

[Table 5]

Test No.	Steel	Amount of O (% by mass)	Number of inclusions of 25 µm or more of rolled material (inclusions/100 g)	Whether or not treatment for reduction of hydrogen in steel is implemented			Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
Test No.	Steel	Amount of O (% by mass)		Molten steel treatment	Homogenizing treatment	Cooling at 400 to 100°C after rolling	Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
17	17	0.0005	1	Implementation	-	-	0.19	2,105	8.0
18	18	0.0006	2	Implementation	-	-	0.19	2,104	8.5
19	19	0.0015	143	Implementation	-	-	0.28	1,948	9.5
20	20	0.0021	332	Implementation	-	-	0.18	1,953	6.1
21	21	0.0012	75	Implementation	-	-	0.26	2,152	8.7
22	22	0.0018	220	Implementation	-	-	0.30	2,155	6.6
23	23	0.0013	95	Implementation	-	-	0.21	2,120	8.0
24	24	0.0025	488	Implementation	-	-	0.22	2,118	2.6
25	25	0.0006	2	-	-	-	0.52	2,120	6.4
26	26	0.0006	1	-	-	-	0.45	2,148	8.5
27	27	0.0004	1	-	-	-	0.48	2,148	9.7
28	28	0.0003	1	-	-	-	0.63	2,123	6.8
29	29	0.0008	14	-	-	-	0.45	2,198	8.2
30	30	0.0023	440	-	-	-	0.56	1,955	7.8
31	31	0.0015	137	-	-	-	0.53	2,043	6.5

[Table 6]

Test No.	Steel No.	Amount of O (% by mass)	Number of inclusions of 25 µm or more of rolled material (inclusions/100 g)	Whether or not treatment for reduction of hydrogen in steel is implemented			Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
Test No.	Steel No.	Amount of O (% by mass)		Molten steel treatment	Homogenizing treatment	Cooling at 400 to 100°C after rolling	Amount of nondiffusible hydrogen of rolled material (ppm)	Tensile strength of wire (MPa)	Corrosion fatigue properties (×10⁴ times)
32	32	0.0007	5	Implementation	-	-	0.23	2,032	39.4
33	33	0.0004	8	Implementation	-	-	0.15	2,050	39.0
34	34	0.0009	13	Implementation	-	-	0.18	2,109	46.9
35	35	0.0004	6	Implementation	-	-	0.23	2,042	63.2
36	36	0.0007	8	Implementation	-	-	0.22	2,099	43.2
37	37	0.0005	7	Implementation	-	-	0.19	2,014	46.6
38	38	0.0006	6	Implementation	-	-	0.18	2,085	48.8
39	39	0.0007	6	Implementation	-	-	0.22	2,055	44.0
40	40	0.0004	5	Implementation	-	-	0.23	2,087	37.8
41	41	0.0007	4	Implementation	-	-	0.18	2,099	45.0
42	42	0.0007	4	Implementation	-	-	0.23	2,010	40.0
43	43	0.0004	1	Implementation	-	-	0.22	2.095	47.0
44	44	0.0006	1	Implementation	-	-	0.21	2,056	39.2
45	45	0.0007	9	Implementation	-	-	0.18	2,010	48.8
46	46	0.0007	5	Implementation	-	-	0.23	2,098	51.2
47	47	0.0004	3	Implementation	-	-	0.22	2,099	43.0
48	48	0.0004	1	Implementation	-	-	0.18	2,099	46.4

From these results, the following observations can be made. In the samples of test Nos. 1 to 16 shown in Table 4 and the samples of test Nos. 32 to 48 shown in Table 6, since steels in which the chemical composition of a steel material is appropriately adjusted are produced under preferable production conditions mentioned above, the number of oxide inclusions and the amount of nondiffusible hydrogen satisfy the ranges defined in the present invention. All of wires obtained by subjecting such a wire rod to wire drawing, followed by quenching and tempering have excellent tensile strength of 1,900 MPa or more. Moreover, all of wires exhibit fatigue life of 100,000 times or more after quenching and tempering, and are excellent in corrosion fatigue properties.
Whereas, the samples of test Nos. 17 to 31 shown in Table 5 are inferior in corrosion fatigue properties because at least any one of requirements of chemical composition of a steel material, the number of oxide inclusions and the amount of nondiffusible hydrogen defined in the present invention is invalid.
The samples of test Nos. 17 and 18 are examples using steels Nos. 17 ad 18 that contains neither Cu nor Ni added therein, or do not meet the defined lower limit, and thus corrosion fatigue properties were degraded. In the samples of test Nos. 19 to 24, insufficient deoxidizing treatment leads to excess amount of O in the steel, so that the number of oxide inclusions in the rolled material.increased and corrosion fatigue properties were degraded.
In the samples of test Nos. 25 to 29, although the amount of O in the steel is controlled in an appropriate range, the amount of nondiffusible hydrogen in the rolled material increased since the above-mentioned nondiffusible hydrogen reducing treatment was not performed, so that fatigue life decreased to less than 100,000 times and corrosion fatigue properties were degraded.
In the samples of test Nos. 30 and 31, insufficient deoxidizing treatment leads to excess amount of O in the steel, and also the number of oxide inclusions in the rolled material since the above-mentioned nondiffusible hydrogen reducing treatment was not performed. Because of increasing the amount of nondiffusible hydrogen in the rolled material, fatigue life decreased to less than 100,000 times and corrosion fatigue properties were degraded.
Based on these results, an influence of the number of oxide inclusions and the amount of nondiffusible hydrogen in the rolled material on corrosion fatigue properties is shown in Fig. 1. In Fig. 1, invented examples (expressed by the symbol "o" (circle)) denote samples of test Nos. 1 to 16 in Table 4 and comparative examples (expressed by the symbol "x" (cross)) denote samples of test Nos. 19 to 31 in Table 5, and the number of oxide inclusions in the rolled material was mentioned as "Number of Inclusions". These results reveal that rigid definition of the number of oxide inclusions and amount of nondiffusible hydrogen is effective to improve corrosion fatigue properties.

Industrial Applicability

The rolled material and the wire of the present invention are industrially useful since they can be suitably used for coil springs that are used in automobiles, for example, a valve spring, a suspension spring, and the like that are used in the engine, suspension, and the like.

Claims

A rolled material for high strength spring, comprising, in % by mass:
C: 0.39 to 0.65%,

Si: 1.5 to 2.5%,

Mn: 0.15 to 1.2%,

P: exceeding 0% and 0.015% or less,

S: exceeding 0% and 0.015% or less,

Al: 0.001 to 0.1%,

Cu: 0.10 to 0.80%,

Ni: 0.10 to 0.80%, and

O: exceeding 0% and 0.0010% or less, with the balance being iron and inevitable impurities, wherein
the number of oxide inclusions having an average diameter of 25 µm or more is 30 or less per 100 g of a steel material, and an amount of nondiffusible hydrogen is 0.40 ppm by mass or less.
The rolled material for high strength spring according to claim 1, further comprising, in % by mass, at least one of the following (a) to (d):
(a) Cr: exceeding 0% and 1.2% or less,

(b) Ti: exceeding 0% and 0.13% or less,

(c) B: exceeding 0% and 0.01% or less, and

(d) at least one of Nb: exceeding 0% and 0.1% or less, and Mo: exceeding 0% and 0.5% or less.
A wire for high strength spring, comprising chemical components of the steel according to claim 1 or 2, wherein an area ratio of tempered martensite is 80% or more, and a tensile strength is 1,900 MPa or more.