KR20120084810A - High-strength spring steel - Google Patents

Abstract

From C: 0.15 to 0.40%, Si: 1 to 3.5%, Mn: 0.20 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.25% or less 1 or more selected, Cr: 0.05 to 1.20%, P: 0.030% or less, S: 0.02% or less, the balance consists of iron and inevitable impurities, and the carbon equivalent Ceq ₁ represented by the following formula (1) is Spring steel less than 0.55.
[Equation 1]
Ceq ₁ = [C] + 0.108 x [Si]-0.067 x [Mn] + 0.024 x [Cr]-0.05 x [Ni] + 0.074 x [V]
(In said Formula (1), [] shows content (mass%) of each element.)

Description

High Strength Spring Steel {HIGH-STRENGTH SPRING STEEL}

The present invention relates to a spring steel (spring steel) useful as a material of a coil spring, and specifically, a spring steel used when manufacturing a coil spring, and a spring having a tensile strength of 1900 MPa in a quenched state. It is about the molten steel.

Springs (suspension springs, etc.) used in automobiles and the like are required to be lighter in order to reduce exhaust gas and improve fuel consumption, and as a part thereof, high strength is being oriented. The spring with increased strength has a high susceptibility to defects, for example, the breakage from corrosion pits caused by adhesion of a snow melting agent tends to occur, and premature failure due to corrosion fatigue becomes a problem. Therefore, there is a demand for a spring having high strength and excellent corrosion fatigue characteristics. For example, "UHS1900", which was first developed by the applicant as a spring steel, is quenched and tempered after coiling in a spring shape, and has a high tensile strength of 1900 MPa class and good corrosion fatigue characteristics. Can be achieved. Therefore, the coil spring obtained from this spring steel is compatible with high strength and favorable corrosion fatigue characteristics.

In general, such a coil spring is drawn, pulverized, and heated in spring steel, and then coiled in hot, then quenched and tempered, and then set. Is produced. Quenching and tempering after hot coiling are performed in order to adjust the strength of a spring. During heat treatment, such as quenching and tempering treatment, large amounts of CO ² are emitted. By the way, in recent years, for the purpose of reducing the load on the global environment to treat, as a preventive measure for global warming it has become a strongly reduced CO ₂ requirements. Therefore, even in the manufacturing process of the coil spring it is not required to reduce the emissions of CO _2.

On the other hand, Patent Literature 1 proposes a stabilizer steel in which water quenching is performed immediately after hot forming and water quenching is performed without tempering, thereby improving room temperature toughness and low temperature toughness. This stabilizer steel is characterized by adjusting the component composition by adding one or two or more of Ti, V, and Nb to a low C-high Mn-Cr system or a low C-high Mn-B-Cr system. There is this. The stabilizer targeted by this patent document 1 is different from the coiling spring in the technical field. For example, the strength level is 800 MPa class where the corrosion fatigue characteristics are not a problem, and the high strength region where compatibility with the corrosion fatigue characteristics is required ( For example, 1900 MPa class).

In general, the strength of the steel material increases with the increase in hardness, and the toughness decreases when the hardness increases. In other words, when the strength of steel materials increases, the toughness decreases. However, in the spring material, the springs are required to have a fracture characteristic that can withstand harsh use environments. It is necessary to secure low temperature toughness which becomes important in the use of.

For example, Patent Document 2 discloses that ductility and toughness is improved in high strength spring steel by adjusting various components, and Patent Document 3 discloses that spring steel having hardness and toughness is obtained by adjusting various components. It is starting. However, Patent Documents 2 and 3 only pay attention to toughness at normal temperature, and are not considered regarding low temperature toughness. Toughness at low temperature is usually inferior to toughness at normal temperature, and considering the normal temperature toughness disclosed in patent documents 2 and 3, the low temperature toughness in the technique of patent documents 2 and 3 is inadequate.

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

The present invention has been made in view of the above circumstances, and an object thereof is to produce a coil spring having both high strength and good corrosion fatigue characteristics and excellent low temperature toughness even when omitting tempering after quenching when working with a coil spring. It is to provide spring steel which can be done. Moreover, another object of this invention is to provide the spring obtained from this spring steel.

High strength spring steel according to the present invention that can solve the above problems, C: 0.15 ~ 0.40% (the meaning of mass%. The same.), Si: 1 ~ 3.5%, Mn: 0.20 ~ 2.0% Further, 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, and S: 0.02% or less And the remainder are iron and unavoidable impurities, and the carbon equivalent Ceq ₁ represented by the following formula (1) is 0.55 or less.

(In said Formula (1), [] shows content (mass%) of each element.)

The spring steel of this invention is (a) Ni: 0.05-2% and Cu: 0.05-0.50%, (b) Ni: 0.15-2% and Cu: 0.05-0.50%, (c) B: as needed It may contain 0.005% or less and / or Mo: 0.60% or less.

In addition, the steel for springs of this invention contains 1 or more types chosen from the group which consists of Ti: 0.035-0.10%, Nb: 0.005-0.05%, and V: 0.05-0.25%, The crystal grain size after quenching is 7.5 It is also preferable that it is more than times.

By using the spring steel, the spring steel is hot coiled, quenched, and set to omit tempering to produce a spring capable of achieving the above characteristics.

Since the spring steel of this invention controls the element amount and compounding balance of a specific alloying element suitably, when manufacturing a coil spring using this spring steel, the tempering process after hardening can be skipped and it hardens as it is. It is possible to produce a spring having better low temperature toughness by making high strength and good corrosion fatigue characteristics compatible in the state of.

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

The inventors of the present invention can produce a spring having high strength and good corrosion fatigue characteristics by further eliminating the tempering after quenching performed after coiling in coiling the spring steel to produce a spring. In order to provide a spring steel which has been intensively studied intensively. As a result, the types of basic alloy elements contained in the spring steel are narrowed down to at least one of C, Si, Mn, Cr, Ti, Nb, and V, or i) Ni and Cu are added to these element groups. In addition, the amount of C, Ti, Nb, V, Cr, Ni, and Cu in these elements is reduced as much as possible, while further narrowing down by adding (ii) B and / or Mo. When actively containing and Mn, when the spring steel is coiled to produce a spring, the tempering after quenching can be omitted, and both the tensile strength of 1900 MPa grade and the good corrosion fatigue property can be achieved while being quenched. The present invention was completed by finding that a spring excellent in low temperature toughness can be provided by more precisely adjusting the contents of Ti, Nb and V.

In the spring steel of this invention, it is especially characterized by reducing the amount of C than the amount of C used for normal spring steel. By reducing the amount of C, the amount of carbides deposited in the steel can be reduced, so that tempering after quenching performed in the usual spring manufacturing process can be omitted. That is, as described above, the spring is usually produced by drawing and finishing the spring steel (wire rod), heating, coiling in hot, and then quenching and tempering to set. After setting, as needed, after shot peening, painting is performed. However, in the spring steel of the present invention, since the amount of C is reduced, the strength of the spring can be ensured even if the tempering after the quenching is omitted and set as it is.

On the other hand, in the spring steel of the present invention, Si and Mn are actively added. Si and Mn are easy to obtain elements, and stable supplyability can be maintained even if Si and Mn are increased. In addition, since Si and Mn have a function of increasing the strength without lowering the toughness, by adding Si and Mn actively, both high strength and good corrosion fatigue characteristics can be achieved.

Based on the above findings, in order to ensure both high strength and good corrosion fatigue characteristics, the amount of each element must be strictly defined and their relationship must also be defined. That is, in the present invention, was to design, and the following composition Steel components for a spring for the carbon equivalent Ceq represented by equation (1) ₁ to less than 0.55. In following formula (1), [] shows content (mass%) of each element in steel.

<Component composition of steel for spring>

From C: 0.15 to 0.40%, Si: 1 to 3.5%, Mn: 0.20 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.05%, and V: 0.25% or less 1 or more types selected, Cr: 0.05-1.20%

[Equation 1]

Ceq ₁ = [C] + 0.108 x [Si]-0.067 x [Mn] + 0.024 x [Cr]-0.05 x [Ni] + 0.074 x [V]

The reasons for setting the amount of each element added and the reasons for the regulation of the carbon equivalent Ceq ₁ are as follows.

C was made 0.15% or more in order to improve hardenability and to ensure strength. In addition, C is made 0.40% or less in order to prevent deterioration of toughness and corrosion fatigue characteristics. The lower limit of the amount of C is preferably 0.2% or more, more preferably 0.25% or more, and the upper limit of the amount of C is preferably 0.35% or less, more preferably 0.34% or less, particularly preferably 0.33% or less. .

Si is made 1% or more in order to make Si act as a solid solution strengthening element, and 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, dissolution of carbides at the time of quenching heating becomes inadequate, and in order to make it austenite uniformly, higher temperature heating is needed, surface decarburization advances, and the fatigue characteristic of a spring worsens. Therefore, by making Si into 3.5% or less, generation | occurrence | production of said suppression of decarburization, grain boundary oxidation, etc. can be suppressed, and abnormal structure is produced | generated and it can prevent that strength falls. Si is preferably 1.5% or more, 3.0% or less, more preferably 1.80% or more and 2.5% or less.

By making Mn 0.20% or more, hardenability can be improved and strength can be ensured. Furthermore, the formation of sulfide inclusions can suppress grain boundary embrittlement by S, thereby improving the toughness and corrosion fatigue properties. In addition, by setting the Mn to 2.0% or less, it is possible to prevent the supercooled structure from occurring and deterioration of the toughness and corrosion fatigue properties. In addition, generation and coarsening of excess sulfide inclusions can be suppressed to prevent deterioration in toughness and corrosion fatigue properties. The lower limit of the amount of Mn is preferably 0.5% or more, more preferably 0.80% or more, and the upper limit of the amount of Mn is preferably 1.8% or less, particularly preferably 1.5% or less.

Ti is made 0.005% or more in order to refine the quenched austenite grains, to improve strength and yield strength, and to improve toughness and corrosion fatigue characteristics. By improving the toughness, sag resistance can be improved. In addition, Ti is made into 0.10% or less in order to prevent coarse inclusions (for example, Ti nitride) from precipitation, and to suppress deterioration of a corrosion fatigue characteristic. The lower limit of the amount of Ti is preferably 0.01% or more (particularly preferably 0.05% or more), and the upper limit of the amount of Ti is preferably 0.080% or less, and more preferably 0.07% or less.

V is an element which acts effectively to raise hardenability further, and to raise strength. In addition, it is an element that increases the toughness and contributes to the improvement of the deflection resistance, and further refines the crystal grains to improve the strength and the strength ratio. In order to exhibit such an effect, it is preferable to contain V 0.05% or more, More preferably, it is 0.08% or more, More preferably, it is 0.1% or more. However, when V becomes excessive, coarse carbonitrides are formed, which deteriorate toughness and corrosion fatigue properties. Therefore V is 0.25% or less, preferably 0.22% or less, more preferably 0.2% or less.

Nb is an element which increases toughness and contributes to improvement of deflection resistance, and is an element which refines crystal grains and improves strength and yield ratio. In order to exhibit such an effect, Nb amount is made into 0.005% or more. Nb amount becomes like this. Preferably it is 0.008% or more, More preferably, it is 0.01% or more. On the other hand, excessive Nb adversely affects toughness. Therefore, Nb amount is made into 0.05% or less. Nb amount is preferably 0.04% or less, and more preferably 0.03% or less.

Ti, V, and Nb may be added alone or in combination of two or more kinds. 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, and it is preferable to contain one or more of them. Moreover, by containing Ti, V, and Nb in such a range, the grain refinement effect can be exhibited effectively, and the crystal grain size after quenching can be made into 7.5 times or more, and as a result, favorable low-temperature toughness can be exhibited. The crystal grain size after quenching is more preferably 8.0 or more, and still more preferably 9.0 or more. Steel low temperature toughness for the spring of the present invention, for example, a Charpy absorbed energy at -50 ℃ more than 50J / cm ^2, preferably 70J / cm ^2, more preferably at least 80J / cm ^2.

Cr may be 0.05% or more to strengthen the steel matrix by solid solution strengthening, to improve the hardenability, and to secure the strength. Moreover, it is an element which contributes to the improvement of corrosion resistance, making rust produced | generated at surface part under corrosion conditions amorphous and dense. On the other hand, Cr is 1.20% or less, the Ms point is lowered to prevent the formation of supercooled structure, thereby securing toughness and corrosion fatigue properties, and also reducing the strength and hardness due to insufficient dissolution of Cr carbide during quenching. 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 remainder of the spring steel of the present invention is substantially iron. However, it is naturally acceptable that steel contains unavoidable impurities brought in according to the situation of raw materials such as iron raw materials (including scrap), subsidiary materials, and manufacturing facilities. Among the unavoidable impurities, in particular, P was set at 0.030% or less, and S was set at 0.02% or less. The reason for determining this range is as follows.

P is made 0.030% or less in order to prevent segregation at the old austenite grain boundary and embrittlement of grain boundaries and to deteriorate toughness and corrosion fatigue characteristics. P becomes like this. Preferably it is 0.02% or less, More preferably, it is 0.01% or less. Although there are few P, it is more preferable, but it is contained normally about 0.001%.

S is made 0.02% or less in order to prevent sulfide inclusions from forming in the steel and coarsening thereof to lower the corrosion fatigue properties. S is preferably 0.015% or less, particularly 0.01% or less. The smaller the amount of S, the smaller the P is, the more preferable it is.

It is preferable to make the total amount of P and S into 0.015% or less, More preferably, it is 0.010% or less.

The carbon equivalent Ceq _{1 is set} to 0.55 or less in order to make both the strength and the corrosion fatigue characteristics of the spring even when the tempering after quenching is omitted when coiling the spring steel to produce the coil spring. That is, the carbon equivalent Ceq ₁ is, it represents the contribution of alloy elements influencing the hardness after quenching, by omitting tempering treatment after quenching after smaller this value, it is possible to secure the core hardness of the spring, an increase in strength Can be achieved. Further, by suppressing the carbon equivalent Ceq of 0.55 to less than ₁ can reduce the dependence of the alloy elements, it is possible to increase the stable supply property. The carbon equivalent Ceq ₁ is preferably 0.53 or less, and more preferably 0.50 or less. On the other hand, the carbon equivalent Ceq ₁ can reduce the cost of the component design to be as small as possible, but in order to achieve both high strength and corrosion fatigue properties, an alloying element must be added to some extent. Therefore, the lower limit of the carbon equivalent Ceq ₁ is 0.30. In addition, in calculating following formula (1), when there is an element which is not contained, content of the element shall be calculated as 0 mass%.

The spring steel of the present invention satisfies the chemical composition and the carbon equivalent Ceq ₁ , but may contain Ni and Cu or may contain B and / or Mo for the purpose of further improving the characteristics.

In the case of containing Ni and Cu (that is, using Ni and Cu together), the amount of Ni is made 0.05 to 2% and the amount of Cu is made 0.05 to 0.50%. Ni is made 0.05% or more in order to improve toughness, reduce defect susceptibility, and improve corrosion fatigue characteristics. In addition, Ni has an effect of improving the corrosion resistance by making the rust produced amorphous and dense, and also improving the sag characteristic, which is important as a spring characteristic. On the other hand, when Ni is 2% or less, the Ms point is lowered to prevent the formation of the supercooled structure, and the toughness and corrosion fatigue properties can be secured. Ni is preferably 0.15% or more and 2% or less, more preferably 0.18% or more, 1.5% or less, still more preferably 0.20% or more, 1% or less, particularly 0.5% or less.

Cu is an element that has a function of improving corrosion resistance by densifying rust because it is electrochemically more precious than iron. Therefore, when it contains Cu, Cu amount is made into 0.05% or more. However, even if it is added excessively, the effect is saturated, and there exists a possibility of causing brittleness of a raw material by hot rolling. Therefore, the upper limit of the amount of Cu was made into 0.50% or less. Cu is preferably 0.1% or more, 0.4% or less, more preferably 0.15% or more (particularly 0.18% or more) and 0.3% or less.

B is an element which further improves hardenability, increases grain boundary strength, increases toughness, improves sag resistance, and further densifies rust formed on the surface to improve corrosion resistance. In order to exhibit such an effect, it is preferable to contain B 0.0005% or more, More preferably, it is 0.001% or more, More preferably, it is 0.0015% or more. However, when B becomes excessive, in addition to saturation of the above effects, coarse carbonitrides are formed and the toughness and corrosion fatigue properties deteriorate. Therefore, B is 0.005% or less, Preferably it is 0.004% or less, More preferably, it is 0.003% or less.

Mo is an element which raises toughness and contributes to the improvement of deflection resistance, and also secures hardenability and improves the strength and toughness of steel. In order to exhibit such an effect effectively, the Mo amount is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. On the other hand, even if Mo amount becomes excessive, the said effect is saturated. Therefore, the Mo amount 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 independently and may be used together.

As described above, the steel for springs of the present invention is characterized in that the amount of each alloy element is strictly defined and their relations are defined, and when the spring steel is used, after the quenching performed after coiling The tempering treatment can be omitted, and a spring can be produced in which both high strength and good corrosion fatigue characteristics of tensile strength of 1900 MPa or more are achieved even with quenching. In addition, low-temperature toughness can be improved by more precisely controlling the content of elements (Ti, Nb and V) having a grain refining action. Hereinafter, the method at the time of manufacturing a spring from the said spring steel is demonstrated.

In manufacturing a spring from the spring steel of this invention, it is necessary to omit tempering after quenching. That is, the spring steel (wire) which satisfies the above chemical composition is drawn, finished, heated, coiled in hot shape to be formed into a spring shape, and quenched. You need to set it to one. Since the spring steel of this invention reduces C quantity compared with the conventional spring steel, when tempering after quenching, it will soften too much and deteriorate toughness and corrosion fatigue characteristics. Therefore, tempering after quenching needs to be omitted.

Here, "omission of tempering" means not heating to temperature exceeding 350 degreeC after hardening.

The setting may be performed cold or warm. What is necessary is just to make the temperature at the time of cold setting into normal temperature, and what is necessary is just to be about 200-250 degreeC in the temperature at the time of warm setting.

After setting, you may coat after shot peening as needed. The conditions of shot peening and painting are not specifically limited, The conditions of a conventional method can be employ | adopted.

The spring obtained in this way can make high strength and favorable corrosion fatigue characteristics compatible, and also is excellent in low-temperature toughness.

Although the manufacturing conditions of the spring steel which concerns on this invention are not specifically limited, In order to make the crystal grain size of 7.5 times or more which is a preferable aspect of this invention, the heating temperature at the time of quenching shall be 925 degreeC or less, and heating time shall be 15 minutes or less. It is recommended. Although the minimum of heating temperature and heating time at the time of said quenching is not specifically limited, Usually, the minimum of heating temperature is about 850 degreeC, and the minimum of heating time is about 10 minutes.

Example

Although an Example is given to the following and this invention is demonstrated to it further more concretely, this invention is not restrict | limited to the following example, of course, It is a matter of course that it changes and implements suitably in the range which can be suitable for the meaning of the pre and later. Possible and they are all included in the technical scope of this invention.

Example 1

The steel (residues of iron and unavoidable impurities) of the chemical composition shown in the following Table 1 was dissolved in a 150 kg vacuum melting furnace and kept at 1200 ° C., followed by hot forging to form billets of 155 mm angle, and the billets were hot rolled. The spring steel (spring wire rod) of diameter 13.5mm was produced. The spring wire rod was subjected to a finish machining so as to have a diameter of 12.5 mm, and then quenched after cutting to a length of 70 mm. Quenching was performed after heating at the temperature of 925 degreeC for 10 minutes, putting it in the oil bath of temperature 50 degreeC. After quenching, it machined and cut out the test piece of width 10mm x thickness 1.5mm x length 65mm.

No. shown in Table 1 29 and No. 30 is data which simulated the spring wire rod "UHS1900" made by Kobe Steel Co., Ltd. (KOBE STEEL, LTD.). 30 was quenched, held at 400 ° C. for 1 hour, tempered, and machined under the same conditions as above to prepare a test piece. Table 2 shows the presence or absence of tempering.

In addition, the chemical component amount in steel and the result of having computed the carbon equivalent (Ceq1) computed from said Formula ( ₁ ) are shown in following Table 1.

The strength and corrosion fatigue characteristics of the obtained test piece were examined as follows.

The strength and corrosion fatigue characteristics of the test pieces were measured by simulating the setting being cold or warm. That is, when simulating a cold setting, the said test piece was used for each test as it is, and when simulating a warm setting, what heated the said test piece at 200 degreeC for 60 minutes was used for each test. Table 2 shows which of the cold setting and the warm setting were simulated.

<Strength>

The strength of the test piece measured and evaluated the hardness of the test piece on the C scale with the Rockwell hardness tester. The measurement results of C hardness are shown in Table 2 below. In this invention, HRC makes 51 or more pass.

Corrosion Fatigue Characteristics

Corrosion fatigue characteristics were evaluated by performing a hydrogen embrittlement crack test. In the hydrogen embrittlement crack test, the test piece was applied to a mixed aqueous solution of sulfuric acid (0.5 mol / L) and potassium thiocyanate (KSCN; 0.01 mol / L) while applying a stress of 1400 MPa by four-point bending. Immersion was carried out, and a potent-start was applied to a voltage of -700 mV lower than that of the SCE electrode, and the time until a crack was generated (hereinafter referred to as hydrogen embrittlement crack life) was measured. The measurement results of the hydrogen embrittlement crack test are shown in Table 2 below. In this invention, the case where time until a crack generation is 600 second or more is made into a pass.

On the other hand, the criterion that the HRC is 51 or more and the time to crack generation is 600 seconds or more means that it has characteristics equivalent to or more than the conventional suspension spring (No. 30 in Table 2) obtained by tempering after quenching. have.

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

As can be seen from FIG. 1, the smaller the carbon equivalent (Ceq ₁ ) is, the longer the hydrogen embrittlement crack life can be lengthened, and the trend of corrosion fatigue characteristics can be read.

From Table 2, it can consider as follows.

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

No. The component composition of 29 is described in No. Similar to 30, but omitting tempering after quenching. In this example, the CO ₂ emissions can be reduced because the tempering treatment is omitted, but since the carbon equivalent exceeds 0.55 and tempering is omitted despite the large amount of alloying elements, the core hardness becomes too hard and the toughness is reduced. This lowers the hydrogen embrittlement crack life and shortens the corrosion fatigue characteristics.

No. 16-28 and 32 are the examples which do not satisfy the requirements prescribed | regulated by this invention, and do not make both high strength and favorable corrosion fatigue characteristics compatible. That is, the carbon equivalent (Ceq ₁ ) of the spring steel exceeds the range prescribed by the present invention, and furthermore, since the tempering after quenching is omitted, CO ₂ emissions can be reduced, but the core hardness is too hard, resulting in lowered toughness. Hydrogen embrittlement crack life is shortened, and corrosion fatigue characteristics deteriorate.

No. 1-15 and 33 are the examples which satisfy | fill the requirements prescribed | regulated by this invention, and both high strength and favorable corrosion fatigue property are compatible. That is, since the carbon equivalent (Ceq ₁ ) is suppressed to 0.55 or less and the tempering after quenching is omitted, CO ₂ emissions can be reduced, and further, the core hardness can be adequately secured and high strength is achieved. In addition, the hydrogen embrittlement crack life is long, and the corrosion fatigue characteristics are also improved. In addition, since the carbon equivalent (Ceq ₁ ) of the spring steel is suppressed to 0.55 or less, the dependence of the alloying element can be reduced, and stable supply can be realized. Therefore, when using the spring steel of this invention, the said No. which simulated the said "UHS1900". It can be seen that it is possible to provide a spring that exhibits the same or more characteristics as 30.

Example 2

The steel materials (residues of iron and unavoidable impurities) of the chemical composition shown in Table 3 were dissolved in a 150 kg vacuum melting furnace, cast by the ingot method or the continuous casting method, and then 155 mm square billets were made by ingot rolling. The material was further hot rolled and processed into a wire rod having a diameter of 13.5 mm to obtain a specimen. These test materials were heated at a temperature of 925 ° C for 10 minutes, and then quenched by placing them in a 50 ° C oil bath. No. Only 2-24 was tempered for 1 hour at 400 ° C after the quenching.

<Low temperature toughness>

The 2 mm U notch impact test piece was extract | collected from the test material after the said quenching, and the Charpy absorbed energy (vE- ₅₀ ) in _-50 degreeC was calculated | required according to JISZ2242. Two tests were carried out for each steel grade, and the average value was taken as the Charpy absorbed energy of each steel grade.

<Determination granularity number>

At the D / 4 position (D is the diameter of the wire rod) of the specimen after the quenching, an arbitrary area of 15 mm ² was observed under an optical microscope (magnification: 400 times), and the crystal grain size number was measured according to JIS G 0551. . The measurement was performed about 2 fields, and these average values were made into the austenite crystal grain size.

The results are shown in Table 4.

No. of Table 4 2-1? No. Since 2-14 satisfies the requirements of the present invention and especially Ti, Nb and V amount are appropriately adjusted, steel having high strength and excellent low temperature toughness has been realized.

On the other hand, 2-15? Since 2-24 did not satisfy at least one of the requirements of this invention, toughness was inadequate.

No. 2-15? 2-19 is an example where the amount of C was excessive, and low-temperature toughness fell as intensity | strength rose too much.

No. 2-20? Since 2-22 does not contain any of Ti, Nb, and V, grain refinement effect was not exhibited and low-temperature toughness fell.

No. 2-23 and No. All of 2-24 are steel grades corresponding to 9254 of standard steel. 2-24 is a tempering treatment after quenching. No. Low temperature toughness fell because 2-23 had a large amount of C and the strength was excessively increased and did not contain any of Ti, Nb and V. In addition, Since 2-24 was tempered, the strength was no. Although it fell compared with 2-23, since it did not contain any of Ti, Nb, and V, low-temperature toughness fell.

2 is No. It is a graph which shows the relationship between intensity | strength and low-temperature toughness (Charpy absorption energy in -50 degreeC) with respect to 2-1 to 2-24. In Figure 2 No. Results of 2-1? 2-14 and 2-25 in □, No. 2-15? Results of 2-23 and 2-26 in ●, No. The result of 2-24 is shown by (circle). According to FIG. 2, the steel which satisfies the requirements of the present invention (indicated by □ in FIG. 2) has a Charpy absorbed energy of 50 J / cm ² or more, and does not satisfy any of the requirements of the present invention. It can be seen that high toughness is achieved when compared at the same strength as shown in Figs.

Claims (6)

C: 0.15-0.40% (mean of mass%. Hereinafter, same.),
Si: 1-3.5%,
Mn: while containing 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: 0.05-1.20%,
P: 0.030% or less,
S: contains 0.02% or less, the remainder is iron and inevitable impurities,
Carbon steel Ceq ₁ represented by the following formula (1) is 0.55 or less high-strength spring steel, characterized in that the tempering is omitted.
[Equation 1]
Ceq ₁ = [C] + 0.108 x [Si]-0.067 x [Mn] + 0.024 x [Cr]-0.05 x [Ni] + 0.074 x [V]
(In said Formula (1), [] shows content (mass%) of each element.) The method of claim 1,
Furthermore, high strength spring steel containing Ni: 0.05-2% and Cu: 0.05-0.50%. The method of claim 2,
Ni: High strength spring steel containing 0.15-2%. The method of claim 1,
At least one member selected from the group consisting of Ti: 0.035-0.10%, Nb: 0.005-0.05%, and V: 0.05-0.25%,
High strength spring steel with a grain size of 7.5 or more after quenching. The method of claim 1,
In addition, B: 0.005% or less and / or Mo: 0.60% or less. A method for producing a high strength spring having excellent corrosion fatigue characteristics, characterized in that the steel for springs according to any one of claims 1 to 5 is hot coiled, quenched, and set with the tempering omitted.

Images