WO2011078165A1

WO2011078165A1 - High-strength spring steel

Info

Publication number: WO2011078165A1
Application number: PCT/JP2010/073003
Authority: WO
Inventors: 清佳永松; 知忠丸尾; 吉原　直
Original assignee: 株式会社神戸製鋼所
Priority date: 2009-12-22
Filing date: 2010-12-21
Publication date: 2011-06-30
Also published as: CN102482743A; US20170022580A1; ES2709515T3; CN102482743B; US20120285585A1; EP2518175A4; JP6027302B2; JP2011149089A; BR112012014178A2; KR20120084810A; EP2518175B1; JP2015214754A; EP2518175A1

Abstract

Provided is a spring steel that contains 0.15-0.40% carbon, 1-3.5% silicon, 0.20-2.0% manganese, 0.05-1.20% chromium, at most 0.030% phosphorus, at most 0.02% sulfur, and at least one of the following: 0.005-0.10% titanium, 0.005-0.05% niobium, and at most 0.25% vanadium. The remainder of said spring steel comprises iron and unavoidable impurities. The carbon equivalent (Ceq₁) of the provided spring steel, as calculated by formula (1), is at most 0.55. (1) Ceq₁ = [C] + 0.108×[Si] − 0.067×[Mn] + 0.024×[Cr] − 0.05×[Ni] + 0.074×[V] (In formula (1), each symbol in brackets represents the content (mass%) of the corresponding element.)

Description

High strength spring steel

TECHNICAL FIELD The present invention relates to a spring steel (spring steel) useful as a material for a coil spring, and more particularly to a spring steel used when manufacturing a coil spring, which has a tensile strength as it is quenched. The present invention relates to a steel for springs of 1900 MPa class.

For springs (suspension springs, etc.) used in automobiles and the like, weight reduction is required in order to reduce exhaust gas and improve fuel efficiency. Higher strength springs are more susceptible to defects, for example, are more likely to break from corrosion pits caused by the adhesion of snow melting agent, and there is a problem of early breakage due to corrosion fatigue. Therefore, a spring having high strength and excellent corrosion fatigue characteristics is required. For example, "UHS1900", which was previously developed as spring steel by the applicant, achieved good corrosion fatigue properties by coiling into a spring shape and then quenching and tempering, while the tensile strength was as high as 1900 MPa class. can do. Therefore, the coil spring obtained from this spring steel has both high strength and good corrosion fatigue characteristics.

Such a coil spring is generally manufactured by drawing spring steel (wire), polishing, heating, coiling hot, quenching and tempering, and setting. The quenching and tempering treatment after hot coiling is performed to adjust the strength of the spring. During heat treatment such as quenching and tempering, a large amount of CO ₂ is exhausted. However, in recent years, CO ₂ reduction has been strongly demanded as one of the measures for preventing global warming for the purpose of reducing the burden on the global environment. Therefore, it is required to reduce the CO ₂ emission amount even in the manufacturing process of the coil spring.

In addition, Patent Document 1 proposes a steel for a stabilizer that is water-quenched immediately after hot forming and has improved normal temperature toughness and low temperature toughness while still being water-quenched without tempering. In this stabilizer steel, the component composition is adjusted to a low C-high Mn-Cr system or a low C-high Mn-B-Cr system with one or more of Ti, V, and Nb added. There is a feature. The stabilizer that is the subject of this Patent Document 1 is different from the coiling spring in the technical field. For example, the strength level is an 800 MPa class in which corrosion fatigue characteristics do not become a problem, and a high strength region where compatibility with the corrosion fatigue characteristics is required. (For example, a 1900 MPa class) spring is not related.

In general, the strength of steel materials increases with increasing hardness, and toughness decreases with increasing hardness. In other words, the toughness decreases when the strength of the steel material increases, but the spring material is required to have fracture characteristics that can withstand the severe use environment of the spring, and even in springs such as suspension springs with increased strength, In particular, it is necessary to ensure low temperature toughness, which is important for use in cold regions.

For example, Patent Document 2 discloses that ductility and toughness have been improved in high-strength spring steel by adjusting various components, and Patent Document 3 has improved hardness and toughness by adjusting various components. It is disclosed that a combined spring steel is obtained. However, both Patent Documents 2 and 3 focus only on toughness at room temperature, and do not consider low-temperature toughness. The toughness at low temperature is usually inferior to the toughness at normal temperature, and considering the normal temperature toughness disclosed in Patent Documents 2 and 3, the low-temperature toughness in the techniques of Patent Documents 2 and 3 is insufficient.

Japanese Patent No. 4406341 Japanese Patent No. 3577411 Japanese Patent No. 3246733

The present invention has been made paying attention to the above circumstances, and its purpose is to provide high strength and good corrosion fatigue characteristics even when tempering after quenching is omitted when processing into a coil spring. An object of the present invention is to provide a spring steel that can produce a coil spring that is compatible and has excellent low-temperature toughness. Another object of the present invention is to provide a spring obtained from this spring steel.

The high-strength spring steel according to the present invention that can solve the above-mentioned problems is C: 0.15 to 0.40% (meaning mass%, the same applies hereinafter), Si: 1 to 3.5%, Selected from the group consisting of Mn: 0.20 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.25% or less At least one selected from the group consisting of Cr: 0.05 to 1.20%, P: 0.030% or less, S: 0.02% or less, the balance being iron and inevitable impurities, ), The carbon equivalent Ceq ₁ is 0.55 or less.
Ceq ₁ = [C] + 0.108 × [Si] −0.067 × [Mn] + 0.024 × [Cr] −0.05 × [Ni] + 0.074 × [V] (1)
(In the above formula (1), [] represents the content (mass%) of each element.)

The spring steel according to the present invention includes (a) Ni: 0.05 to 2% and Cu: 0.05 to 0.50%, (b) Ni: 0.15 to 2% and Cu: 0 as necessary. 0.05 to 0.50%, (c) B: 0.005% or less and / or Mo: 0.60% or less may be contained.

The spring steel of the present invention is selected from the group consisting of Ti: 0.035 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.05 to 0.25%. It is also preferable that at least one kind is contained and the crystal grain size after quenching is 7.5 or more.

If the spring steel is hot-coiled, quenched and quenched, and then set with the temper omitted, a spring that can achieve both of the above characteristics can be manufactured.

Since the spring steel of the present invention appropriately controls the element amount and blending balance of a specific alloy element, the tempering process after quenching is omitted when a coil spring is manufactured using this spring steel. Thus, it is possible to manufacture a spring having both high strength and good corrosion fatigue properties while still being quenched, and further having good low temperature toughness.

FIG. 1 is a graph showing the relationship between the carbon equivalent (Ceq ₁ ) of the test piece obtained in Example 1 and the hydrogen embrittlement crack life. FIG. 2 is a graph showing the relationship between the tensile strength and the low temperature toughness (vE- ₅₀ ) of the test piece obtained in Example 2.

When producing springs by coiling spring steel, the present inventors omit the tempering after quenching performed after coiling to achieve both high strength and good corrosion fatigue properties, and are excellent in low temperature toughness. In order to provide spring steel that can be manufactured, we have made extensive studies. As a result, the types of basic alloy elements to be contained in the spring steel are limited to at least one of C, Si, Mn, Cr and Ti, Nb and V, or (i) Ni and Cu are further added to these element groups. Or (ii) those containing B and / or Mo, and among these elements, the amount of C, Ti, Nb, V, Cr, Ni, and Cu is reduced as much as possible On the other hand, if Si and Mn are positively contained, the tempering treatment after quenching can be omitted when the spring steel is coiled to produce a spring, and the tensile strength of 1900 MPa class can be obtained in the as-quenched state. The inventors have found that a spring excellent in low-temperature toughness can be provided by satisfying both good corrosion fatigue characteristics and more precisely adjusting the contents of Ti, Nb and V, thereby completing the present invention.

The spring steel of the present invention is particularly characterized in that the amount of C is lower than the amount of C used in ordinary spring steel. By reducing the amount of C, the amount of carbide precipitated in the steel can be reduced, so that tempering after quenching that is performed in a normal spring manufacturing process can be omitted. That is, as described above, the spring is usually manufactured by drawing and polishing spring steel (wire rod), heating, hot coiling, quenching and tempering, and setting. After the setting, if necessary, coating is performed after shot peening. However, since the spring steel of the present invention has a reduced amount of C, the strength of the spring can be secured even if setting is performed without the tempering after quenching.

On the other hand, in the spring steel of the present invention, Si and Mn are positively added. Si and Mn are easily available elements, and stable supply can be maintained even if the amount of Si and Mn is increased. Moreover, since Si and Mn have the effect | action which raises an intensity | strength, without reducing toughness, high intensity | strength and favorable corrosion fatigue characteristics can be made compatible by adding Si and Mn positively.

Based on the above knowledge, in order to ensure both high strength and good corrosion fatigue properties, it is necessary to strictly define the amount of each element and also define the relationship between them. That is, in the present invention, the component composition of the spring steel is designed as follows, and the carbon equivalent Ceq _{1 represented} by the following formula (1) is set to 0.55 or less. In the following formula (1), [] represents the content (% by mass) of each element in the steel.

<Composition composition of spring steel>
C: 0.15 to 0.40%, Si: 1 to 3.5%, Mn: 0.20 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.0. 005 to 0.05%, and V: at least one selected from the group consisting of 0.25% or less, Cr: 0.05 to 1.20%
Ceq ₁ = [C] + 0.108 × [Si] −0.067 × [Mn] + 0.024 × [Cr] −0.05 × [Ni] + 0.074 × [V] (1)
The reason for setting the addition amount of each element and the reason for defining the carbon equivalent Ceq ₁ are as follows.

The reason why C is 0.15% or more is to improve the hardenability and ensure the strength. The reason why C is set to 0.40% or less is to prevent deterioration of toughness and corrosion fatigue characteristics. The lower limit of the C amount is preferably 0.2% or more, more preferably 0.25% or more, and the upper limit of the C amount is preferably 0.35% or less, more preferably 0.34% or less, particularly preferably. It is 0.33% or less.

The reason why Si is 1% or more is to make Si act as a solid solution strengthening element to ensure strength. If Si is less than 1%, the matrix strength is insufficient. On the other hand, when the amount of Si becomes excessive, the carbides are not sufficiently dissolved during quenching and heating, and high temperature heating is required for uniform austenitization, so that surface decarburization proceeds and the fatigue characteristics of the spring deteriorate. Therefore, by setting Si to 3.5% or less, it is possible to suppress the above-described decarburization and the occurrence of grain boundary oxidation, etc., and it is possible to prevent the formation of an abnormal structure and a decrease in strength. Si is preferably 1.5% or more and 3.0% or less, more preferably 1.80% or more and 2.5% or less.

When Mn is 0.20% or more, the hardenability is improved and the strength can be secured. Further, the formation of sulfide inclusions can suppress grain boundary embrittlement due to S and improve toughness and corrosion fatigue characteristics. Further, when Mn is 2.0% or less, it is possible to prevent a supercooled structure from being generated and deteriorate toughness and corrosion fatigue characteristics. Moreover, generation | occurrence | production and coarsening of an excessive sulfide type inclusion can be suppressed, and it can prevent that toughness and a corrosion fatigue characteristic deteriorate. The lower limit of the Mn amount is preferably 0.5% or more, more preferably 0.80% or more, and the upper limit of the Mn amount is preferably 1.8% or less, particularly preferably 1.5% or less.

The reason why Ti is made 0.005% or more is to refine the prior austenite crystal grains after quenching, improve the strength and yield ratio, and improve the toughness and corrosion fatigue characteristics. By improving toughness, sag resistance can be improved. The reason why Ti is made 0.10% or less is to prevent coarse inclusions (for example, Ti nitride) from precipitating and to suppress deterioration of corrosion fatigue characteristics. The lower limit of the Ti amount is preferably 0.01% or more (particularly preferably 0.05% or more), and the upper limit of the Ti amount is preferably 0.080% or less, more preferably 0.07% or less. .

V is an element that effectively acts to further enhance the hardenability and increase the strength. In addition to enhancing toughness and improving sag resistance, it is an element that refines crystal grains to improve strength and proof stress ratio. In order to exert such an action, V is preferably contained in an amount of 0.05% or more, more preferably 0.08% or more, and further preferably 0.1% or more. However, when V is excessive, coarse carbonitrides are formed, and toughness and corrosion fatigue properties are deteriorated. Therefore, V is 0.25% or less, preferably 0.22% or less, more preferably 0.2% or less.

Nb is an element that contributes to improvement in sag resistance by increasing toughness, and is an element that refines crystal grains to improve strength and proof stress ratio. In order to exert such an effect, the Nb amount is set to 0.005% or more. The amount of Nb is preferably 0.008% or more, and more preferably 0.01% or more. On the other hand, if the amount of Nb is excessive, the toughness is adversely affected. Therefore, the Nb content is 0.05% or less. The Nb amount is preferably 0.04% or less, more preferably 0.03% or less.

Ti, V and Nb may be added alone or in combination of two or more. The contents of Ti, V, and Nb are Ti: 0.035 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.05 to 0.25%, respectively. It is preferable to contain. Further, by containing Ti, V and Nb in such a range, the effect of crystal grain refinement can be effectively exhibited, and the crystal grain size after quenching can be set to 7.5 or more, Good low temperature toughness can be exhibited. The grain size after quenching is more preferably 8.0 or more, and even more preferably 9.0 or more. The low temperature toughness of the spring steel of the present invention is, for example, that Charpy absorbed energy at −50 ° C. is 50 J / cm ² or more, preferably 70 J / cm ² or more, more preferably 80 J / cm ² or more.

When Cr is 0.05% or more, the steel matrix is strengthened by solid solution strengthening, and the hardenability is improved and the strength can be secured. Further, it is an element that contributes to the improvement of corrosion resistance by making the rust generated in the surface layer portion under the corrosive condition amorphous and dense. On the other hand, when Cr is 1.20% or less, the Ms point is lowered to prevent formation of a supercooled structure, and toughness and corrosion fatigue characteristics can be secured, and strength due to insufficient penetration of Cr carbide during quenching. And a decrease in hardness can be prevented. Cr is preferably 0.1% or more and 1.10% or less, more preferably 0.5% or more and 1.05% or less.

The balance of the spring steel of the present invention is substantially iron. However, it is naturally allowed that steel contains unavoidable impurities brought in depending on the situation of materials such as iron raw materials (including scrap) and auxiliary materials, manufacturing equipment, and the like. Among unavoidable impurities, P is defined as 0.030% or less and S is defined as 0.02% or less. The reason for setting such a range is as follows.

The reason why P is set to 0.030% or less is to prevent segregation at the prior austenite grain boundaries and embrittle the grain boundaries, and to prevent deterioration of toughness and corrosion fatigue characteristics. P is preferably 0.02% or less, more preferably 0.01% or less. The smaller the amount of P, the better. However, it is usually contained in an amount of about 0.001%.

The reason why S is set to 0.02% or less is to prevent sulfide inclusions from forming in the steel and coarsening the steel to deteriorate the corrosion fatigue characteristics. S is preferably 0.015% or less, particularly 0.01% or less. Like S, P is preferably as small as P, but is usually contained in an amount of about 0.001%.

The total amount of P and S is preferably 0.015% or less, more preferably 0.010% or less.

The reason why the carbon equivalent Ceq ₁ is 0.55 or less is that, when coiled spring steel is manufactured to produce a coil spring, the strength and corrosion fatigue characteristics of the spring are compatible even if the tempering treatment after quenching is omitted. Because. That is, the carbon equivalent Ceq ₁ indicates the contribution of the alloy element that affects the hardness after quenching. By reducing this numerical value and omitting the tempering treatment after quenching, the core portion of the spring is obtained. Hardness can be secured and high strength can be achieved. In addition, by suppressing the carbon equivalent Ceq ₁ to 0.55 or less, the dependence of the alloy elements can be reduced, and the stable supply ability can be improved. The carbon equivalent Ceq ₁ is preferably 0.53 or less, more preferably 0.50 or less. The carbon equivalent Ceq ₁ can be reduced in cost by designing the component so as to be as small as possible. However, in order to achieve both high strength and corrosion fatigue characteristics, it is necessary to add an alloy element to some extent. Therefore, the lower limit value of the carbon equivalent Ceq ₁ is 0.30. In addition, in calculating the following formula (1), when there is an element that is not contained, the content of the element is assumed to be 0% by mass.

The spring steel of the present invention satisfies the above chemical composition and the carbon equivalent Ceq ₁ , but contains Ni and Cu, or contains B and / or Mo with the aim of further improving the characteristics. It may be contained.

When Ni and Cu are contained (that is, when Ni and Cu are used in combination), the Ni content is 0.05 to 2% and the Cu content is 0.05 to 0.50%. The reason why Ni is set to 0.05% or more is to increase toughness, reduce defect sensitivity, and improve corrosion fatigue characteristics. Further, Ni has an effect of improving corrosion resistance by forming amorphous rust that is amorphous and dense, and also has an effect of improving the sag characteristic important as a spring characteristic. On the other hand, by setting Ni to 2% or less, it is possible to prevent the formation of a supercooled structure by lowering the Ms point, and toughness and corrosion fatigue characteristics can be ensured. Ni is preferably 0.15% or more and 2% or less, more preferably 0.18% or more and 1.5% or less, further preferably 0.20% or more and 1% or less, particularly 0.5%. It is as follows.

Cu is an element that is electrochemically noble than iron (noble) and therefore has the effect of densifying rust and improving corrosion resistance. Therefore, when Cu is contained, the amount of Cu is set to 0.05% or more. However, even if added excessively, the effect is saturated, and there is a possibility of causing embrittlement of the material due to hot rolling. Therefore, the upper limit of Cu content is 0.50% or less. Cu is preferably 0.1% or more and 0.4% or less, more preferably 0.15% or more (particularly 0.18% or more) and 0.3% or less.

B is an element that further enhances hardenability to increase grain boundary strength, enhance toughness and improve sag resistance, and further densify rust generated on the surface to improve corrosion resistance. In order to exert such an effect, B is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more, and further preferably 0.0015% or more. However, when B is excessive, the above effect is saturated and coarse carbonitride is formed, resulting in deterioration of toughness and corrosion fatigue characteristics. Therefore, B is 0.005% or less, preferably 0.004% or less, more preferably 0.003% or less.

Mo is an element that increases toughness and contributes to improvement in sag resistance, and also ensures hardenability and increases the strength and toughness of steel. In order to effectively exhibit these actions, the Mo amount is preferably 0.05% or more, more preferably 0.08% or more, and further preferably 0.10% or more. On the other hand, even if the amount of Mo becomes excessive, the above effect is saturated. Therefore, the Mo content is preferably 0.60% or less, more preferably 0.50% or less, and still more preferably 0.35% or less. B and Mo may be contained alone or in combination.

As described above, the spring steel according to the present invention is characterized in that the amount of each alloying element is strictly defined and the relationship between them is specified. If this spring steel is used, it is performed after coiling. A tempering treatment after quenching can be omitted, and a spring having both high strength with a tensile strength of 1900 MPa or more and good corrosion fatigue characteristics can be produced even when quenched. Moreover, low temperature toughness can be improved by controlling the content of elements (Ti, Nb and V) having a grain refinement action more strictly. Hereinafter, a method for manufacturing a spring from the spring steel will be described.

When manufacturing a spring from the spring steel of the present invention, it is necessary to omit tempering after quenching. That is, it is the same as before until the spring steel (wire) satisfying the above chemical composition is drawn, polished, heated, coiled hot, formed into a spring shape, and quenched. After quenching, it is necessary to perform setting while omitting tempering. Since the spring steel of the present invention has a C content lower than that of the conventional spring steel, when it is tempered after quenching, it is too soft and deteriorates toughness and corrosion fatigue characteristics. Therefore, it is necessary to omit tempering after quenching.

Here, “omission of tempering” means that the material is not heated to a temperature exceeding 350 ° C. after quenching.

The above setting may be performed cold or warm. The temperature at the time of cold setting may be normal temperature, and the temperature at the time of cold setting may be about 200 to 250 ° C.

After setting, if necessary, you may paint after shot peening. The conditions for shot peening and painting are not particularly limited, and conventional conditions can be adopted.

The spring thus obtained can achieve both high strength and good corrosion fatigue properties, and is also excellent in low temperature toughness.

The production conditions of the spring steel according to the present invention are not particularly limited, but in order to make the crystal grain size 7.5 or more which is a preferred embodiment of the present invention, for example, the heating temperature at the time of quenching is set to 925 ° C. or less, and the heating is performed. It is recommended that the time be 15 minutes or less. Although the lower limit of the heating temperature and the heating time at the time of quenching is not particularly limited, the lower limit of the heating temperature is usually about 850 ° C., and the lower limit of the heating time is about 10 minutes.

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

Example 1
The steel of the chemical composition shown in Table 1 below (the balance is iron and inevitable impurities) was melted in a 150 kg vacuum melting furnace and held at 1200 ° C., then hot forged into a 155 mm square billet, This billet was hot-rolled to produce a spring steel (spring wire) having a diameter of 13.5 mm. The spring wire was subjected to a polishing rod process so that the diameter was 12.5 mm, and was then quenched after being cut to a length of 70 mm. Quenching was performed by heating at a temperature of 925 ° C. for 10 minutes and then placing in an oil bath at a temperature of 50 ° C. After quenching, a test piece having a width of 10 mm, a thickness of 1.5 mm and a length of 65 mm was cut out by machining.

No. shown in Table 1. 29 and No. 30 is data simulating a spring wire “UHS1900” manufactured by Kobe Steel (KOBEKSTEEL, LTD). No. 30 was quenched and held at 400 ° C. for 1 hour for tempering and then machined under the same conditions as above to prepare a test piece. Table 2 shows the presence or absence of tempering.

Table 1 below shows the results of calculating the amount of chemical components in the steel and the carbon equivalent (Ceq ₁ ) calculated from the above formula (1).

The strength and corrosion fatigue properties of the obtained specimens were examined as follows.

The strength and corrosion fatigue properties of the test specimens were measured by simulating the setting performed cold or warm. That is, when simulating cold setting, the test piece was used for each test as it was, and when simulating warm setting, the test piece heated at 200 ° C. for 60 minutes was used for each test. Table 2 below shows which of the cold setting and the warm setting is simulated.

<Strength>
The strength of the test piece was evaluated by measuring the hardness of the test piece with a Rockwell hardness tester on a C scale. The measurement results of C hardness are shown in Table 2 below. In the present invention, an HRC of 51 or more is considered acceptable.

<Corrosion fatigue characteristics>
The corrosion fatigue characteristics were evaluated by conducting a hydrogen embrittlement cracking test. In the hydrogen embrittlement cracking test, a stress of 1400 MPa was applied to the above test piece by four-point bending, and this test piece was treated with sulfuric acid (0.5 mol / L) and potassium thiocyanate (KSCN; 0.01 mol / L). The sample was immersed in a mixed aqueous solution, and a voltage of −700 mV, which is lower than the SCE electrode, was applied using a potentiostat, and the time until cracking occurred (hereinafter referred to as hydrogen embrittlement crack life) was measured. did. The measurement results of the hydrogen embrittlement cracking test are shown in Table 2 below. In the present invention, a case where the time until occurrence of cracking is 600 seconds or more is regarded as acceptable.

In addition, the standard that HRC is 51 or more and the time until crack generation is 600 seconds or more has the same or better characteristics as a conventional suspension spring (No. 30 in Table 2 below) obtained by tempering after quenching. It means that

FIG. 1 shows the relationship between the carbon equivalent (Ceq ₁ ) and the hydrogen embrittlement crack life (seconds). In FIG. The results of 1 to 15, 31, and 33 are indicated by □ and No. The results of 16 to 29, 32 are indicated by ● and No. The result of 30 (with tempering) is indicated by ○.

As is apparent from FIG. 1, it can be seen that the hydrogen embrittlement crack life can be extended and the corrosion fatigue characteristics can be improved by reducing the carbon equivalent (Ceq ₁ ).

From Table 2, it can be considered as follows.

No. 30 is an example of tempering after quenching. In this example, the core hardness can be secured, the strength is high, the hydrogen embrittlement crack life is good, and the corrosion fatigue characteristics can be improved. However, since tempering is performed after quenching, CO ₂ emissions cannot be reduced.

No. The component composition of No. 29 is the above No. This is an example similar to 30, but without tempering after quenching. In this example, the tempering process is omitted, so that the CO ₂ emission can be reduced. However, since the carbon equivalent exceeds 0.55 and the alloy element amount is large, the tempering is omitted. The partial hardness becomes too hard, the toughness is lowered, the hydrogen embrittlement crack life is shortened, and the corrosion fatigue characteristics are deteriorated.

No. Nos. 16 to 28 and 32 are examples that do not satisfy the requirements defined in the present invention, and cannot achieve both high strength and good corrosion fatigue characteristics. That is, the carbon equivalent (Ceq ₁ ) of the spring steel exceeds the range specified in the present invention, and the tempering after quenching is omitted, so that the CO ₂ emission can be reduced, but the core hardness is hard. As a result, the toughness is lowered, the hydrogen embrittlement crack life is shortened, and the corrosion fatigue characteristics are deteriorated.

No. Nos. 1 to 15 and 33 are examples that satisfy the requirements defined in the present invention, and both high strength and good corrosion fatigue characteristics can be achieved. That is, after suppressing the carbon equivalent (Ceq ₁ ) to 0.55 or less, and tempering after quenching is omitted, CO ₂ emission can be reduced, and the core hardness can be appropriately secured. High strength can be achieved. In addition, the hydrogen embrittlement crack life is long, and the corrosion fatigue characteristics can be improved. Moreover, since the carbon equivalent (Ceq ₁ ) of the spring steel is suppressed to 0.55 or less, the dependence of the alloy elements can be reduced, and stable supply can be realized. Therefore, when the spring steel of the present invention is used, the above No. 1 simulating the “UHS1900”. It can be seen that a spring exhibiting characteristics comparable to or higher than 30 can be provided.

Example 2
Steel materials having the chemical composition shown in Table 3 (the balance being iron and inevitable impurities) were melted in a 150 kg vacuum melting furnace, then cast by the ingot-making method or continuous casting method, and then 155 mm square by split rolling. The billet was prepared, further hot-rolled, and processed into a wire having a diameter of 13.5 mm to obtain a test material. These specimens were heated at a temperature of 925 ° C. for 10 minutes and then quenched in an oil bath at 50 ° C. No. Only 2-24 was tempered at 400 ° C. for 1 hour after the quenching.

<Low temperature toughness>
An impact test piece with a 2 mm U notch was taken from the specimen after quenching, and Charpy absorbed energy (vE _-50 ) at -50 ° C was determined according to JIS Z2242. Two tests were performed for each steel type, and the average value was defined as the Charpy absorbed energy of each steel type.

<Grain size number>
At the D / 4 position (D is the diameter of the wire) of the specimen after quenching, an arbitrary 15 mm ² region was observed with an optical microscope (magnification: 400 times), and the crystal grain size number was measured according to JIS G0551. . The measurement was performed for two fields of view, and the average of these values was defined as the austenite grain size.

The results are shown in Table 4.

No. in Table 4 2-1. No. 2-14 satisfies the requirements of the present invention, and in particular, the Ti, Nb, and V contents are appropriately adjusted, so that a steel having high strength and excellent low-temperature toughness could be realized.

On the other hand, No. 2-15 ~ No. No. 2-24 did not satisfy at least one of the requirements of the present invention, and thus toughness was insufficient.

No. 2-15 ~ No. No. 2-19 was an example in which the amount of C was excessive, and the low temperature toughness decreased due to the excessive increase in strength.

No. 2-20 ~ No. Since 2-22 does not contain any of Ti, Nb and V, the effect of refining crystal grains was not exhibited, and the low temperature toughness was lowered.

No. 2-23 and no. No. 2-24 is a steel grade corresponding to standard steel 9254. No. 2-24 has been tempered after quenching. No. No. 2-23 had a large amount of C and an excessively high strength, and did not contain any of Ti, Nb and V, so that the low temperature toughness decreased. No. Since No. 2-24 was tempered, its strength was No. 2-24. Although it is lower than that of 2-23, since it does not contain any of Ti, Nb and V, the low temperature toughness is reduced.

FIG. FIG. 2 is a graph showing the relationship between strength and low temperature toughness (Charpy absorbed energy at −50 ° C.) for 2-1 to 2-24. In FIG. The results of 2-1 to 2-14 and 2-25 are indicated by □ and No. 2-15 ~ No. The results of Nos. 2-23 and 2-26 are indicated by ●, The result of 2-24 was indicated by ○. According to FIG. 2, all the steels satisfying the requirements of the present invention (indicated by □ in FIG. 2) have a Charpy absorbed energy of 50 J / cm ² or more and steels that do not satisfy any of the requirements of the present invention ( In FIG. 2, it can be seen that high toughness is achieved when compared at the same strength as indicated by ● and ○.

Claims

C: 0.15 to 0.40% (meaning mass%, hereinafter the same)
Si: 1 to 3.5%,
Containing Mn: 0.20 to 2.0%,
At least one selected from the group consisting of Ti: 0.005 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.25% or less,
Cr: 0.05 to 1.20%,
P: 0.030% or less,
S: 0.02% or less, the balance is iron and inevitable impurities,
A high strength spring steel without tempering, characterized in that the carbon equivalent Ceq 1 represented by the following formula (1) is 0.55 or less.
Ceq 1 = [C] + 0.108 × [Si] −0.067 × [Mn] + 0.024 × [Cr] −0.05 × [Ni] + 0.074 × [V] (1)
(In the above formula (1), [] represents the content (mass%) of each element.)
Furthermore,
The high-strength spring steel according to claim 1, containing Ni: 0.05-2% and Cu: 0.05-0.50%.
The steel for high strength springs according to claim 2, containing Ni: 0.15 to 2%.
Containing at least one selected from the group consisting of Ti: 0.035 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.05 to 0.25%,
The steel for high strength springs according to claim 1, wherein the grain size after quenching is 7.5 or more.
Furthermore,
The steel for high-strength springs according to claim 1, containing B: 0.005% or less and / or Mo: 0.60% or less.
A method for producing a high-strength spring excellent in corrosion fatigue characteristics, characterized in that the spring steel according to any one of claims 1 to 5 is hot-coiled, quenched, and then set while omitting tempering.