WO2011074600A1 - 高疲労強度板ばね用鋼及び板ばね部品 - Google Patents
高疲労強度板ばね用鋼及び板ばね部品 Download PDFInfo
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D7/00—Modifying the physical properties of iron or steel by deformation
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- C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
- C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
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- C21D7/00—Modifying the physical properties of iron or steel by deformation
- C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
Definitions
- the present invention is a high-fatigue strength leaf spring steel that can exhibit stable and excellent fatigue strength in a leaf spring subjected to high-strength shot peening treatment, and is excellent in toughness and hydrogen embrittlement characteristics at high strength, and from this It relates to a leaf spring component.
- leaf springs As suspension springs for automobiles, springs (torsion bars, stabilizers, (large diameter) coil springs, etc.) torsional stress are applied by leaf springs or springs made of round bars. ) Is used. Coil springs are generally used in many passenger cars, and leaf springs are often used in trucks. These leaf springs and round bar springs are one of the heavy parts of automobile undercarriage parts, and the study of increasing strength has been continued for weight reduction. It is a part. In increasing the strength, it is particularly important to improve the fatigue strength, and one of the countermeasures is to increase the hardness of the material.
- the leaf spring has a considerably larger cross-sectional area of the final product compared to the material of the round bar spring, so that the cooling rate after rolling is smaller than that of a round bar spring made of steel bars or wire rods, and It is necessary to consider that decarburization is likely to remain in the final product because the reduction rate of the cross-sectional area due to the is low. Furthermore, in the leaf spring, it is necessary to solve the problems common to the round bar spring, including improvement of hydrogen embrittlement resistance and toughness in the high hardness region. Steel needs to be provided.
- the present invention has been made to solve such a problem, and it is possible to increase the hardness for increasing the strength and to secure excellent toughness even in the hardness region where hydrogen embrittlement is a problem. It is possible to provide a steel for a high fatigue strength leaf spring and a leaf spring component capable of reliably improving the life by high strength shot peening.
- the inventors of the present invention conducted extensive research on the cause of early breakage in some leaf springs when high-strength shot peening treatment was performed. During the test, the presence of a coarse bainite structure was confirmed at the internal starting point instead of the surface where the stress was highest, and it was found that this bainite structure was considered to be the cause of the life reduction. And, as will be described later, by actively adding Ti in the range of 0.07 to 0.15% so as to satisfy the condition of Ti / N ⁇ 10, the generation of bainite structure can be suppressed, and as a result It has been found that excellent fatigue life can be obtained stably even when the strength shot peening treatment is performed.
- the inventors of the present application have found a component system in which ferrite decarburization does not easily occur even when a leaf spring is manufactured, and excellent characteristics can be secured in a high hardness region.
- a leaf spring component capable of stably securing an excellent fatigue life in a high hardness region can be manufactured, and the present invention has been completed.
- the first aspect of the present invention is, in mass%, C: 0.40 to 0.54%, Si: 0.40 to 0.90%, Mn: 0.40 to 1.20%, Cr: 0.70 to 1.50%, Ti: 0.070 to 0.150%, B: 0.0005 to 0.0050%, N: 0.0100% or less, with the balance being Fe and impurity elements ,
- the steel for high fatigue strength leaf springs is characterized in that the contents of Ti and N satisfy Ti / N ⁇ 10.
- the second aspect is, by mass%, C: 0.40 to 0.54%, Si: 0.40 to 0.90%, Mn: 0.40 to 1.20%, Cr: 0.70 to 1 .50%, Ti: 0.070 to 0.150%, B: 0.0005 to 0.0050%, N: 0.0100% or less, Further, in terms of mass%, Cu: 0.20 to 0.50%, Ni: 0.20 to 1.00%, V: 0.05 to 0.30%, and Nb: 0.01 to 0.30% Containing one or more selected, The balance consists of Fe and impurity elements, The steel for high fatigue strength leaf springs is characterized in that the contents of Ti and N satisfy Ti / N ⁇ 10.
- the third side surface is a leaf spring component formed by using the steel for high fatigue strength leaf springs of the first or second side surface.
- the high fatigue strength leaf spring steels of the first and second side surfaces have the specific composition.
- the ranges of Ti and Ti / N are defined as described above, fine TiC can be precipitated and fine austenite crystal grains can be obtained during quenching heating. Therefore, in the said steel for leaf
- fine TiC can be a hydrogen trap site. Therefore, even if hydrogen penetrates into the steel, hydrogen embrittlement hardly occurs, and the leaf spring steel can exhibit excellent hydrogen embrittlement resistance.
- temper softening resistance is increased by containing Si in the above specific range that causes no problem in increasing the amount of decarburization while the C content is relatively low. , Allowing tempering at higher temperatures.
- Ti and B as essential components, the hydrogen embrittlement resistance is improved and the grain boundary strength is improved. As a result, excellent toughness can be exhibited in the high hardness region. In particular, the effect becomes remarkable in a high hardness region of HV510 or higher.
- the hardness can be increased to increase the strength, and excellent toughness can be ensured even in the hardness region where hydrogen embrittlement is a problem. It is possible to provide a steel for high fatigue strength leaf springs that can reliably improve the life by strength shot peening.
- the leaf spring component on the third side surface is formed using the steel for high fatigue strength leaf springs on the first or second side surface.
- the plate spring component can be manufactured by forming the plate spring steel into a spring shape and performing quenching and tempering. Since the leaf spring component uses the steel for high fatigue strength leaf springs of the first or second side face, in the hardness region where hydrogen embrittlement is a problem, the hardness is increased for increasing the strength. Excellent toughness can be ensured, and the life can be reliably improved by high-strength shot peening. In particular, in a high hardness range of HV510 or higher, the effect of improving toughness becomes remarkable.
- Explanatory drawing which shows the relationship between the amount of carbon (C) and an impact value concerning an Example.
- Explanatory drawing which shows the relationship between the amount of silicon (Si) and an impact value concerning an Example.
- Explanatory drawing which shows the relationship between the amount of silicon (Si) and the decarburization depth concerning an Example.
- Explanatory drawing which shows the relationship between the amount of titanium (Ti) and an old gamma crystal grain size concerning an Example.
- Explanatory drawing which shows the relationship between Ti / N ratio and the old gamma crystal grain size concerning an Example.
- Explanatory drawing which shows the relationship between the titanium (Ti) amount and hydrogen embrittlement strength ratio concerning an Example.
- Explanatory drawing which shows the relationship between Ti / N ratio and hydrogen embrittlement strength ratio concerning an Example.
- Explanatory drawing which shows the relationship between hardness and an impact value concerning an Example.
- the plate spring steel contains C, Si, Mn, Cr, Ti, B, and N in the specific composition range as described above. Hereinafter, the reason which limited the range of content rate for every component is demonstrated.
- C 0.40 to 0.54% C is an element indispensable for securing sufficiently excellent strength and hardness after quenching and tempering treatment. If the C content is less than 0.4%, the spring strength may be insufficient. Further, when the C content decreases, tempering at a low temperature has to be performed to obtain high hardness, particularly HV510 or higher. As a result, the hydrogen embrittlement strength ratio is lowered, and hydrogen embrittlement tends to occur. On the other hand, if it exceeds 0.54%, the toughness in the high hardness region tends to decrease even when Ti and B are added, and hydrogen embrittlement may occur easily. In order to particularly improve toughness, the upper limit is preferably less than 0.50%.
- the spring steel can have a higher level of hardness and toughness. That is, usually, in a low hardness region, the lower the C content, the greater the toughness.
- the spring component targeted by the present invention aims at high hardness (preferably HV510 or more), when the C content is in the range of 0.40%, the tempering temperature is lowered to obtain high hardness. The possibility of becoming a low temperature temper brittle region becomes high. As a result, a reverse phenomenon occurs in which the toughness is lowered compared to the case where the C content is in the range of 0.50%.
- the toughness in the high hardness region is improved even with a low C content as a spring steel of the order of 0.40%. Compared with the case where the content exceeds 0.54%, the toughness can be further improved. In particular, when the C content is less than 0.50%, the effect of improving toughness becomes remarkable.
- Si 0.40-0.90%
- Si has the effect of increasing the temper softening resistance, and enables setting to a higher tempering temperature even when aiming for high hardness. As a result, it is an element that ensures high strength and high toughness, prevents embrittlement by hydrogen, and improves corrosion fatigue strength.
- the Si content is less than 0.40%, the target hardness cannot be obtained unless the tempering temperature is lowered, and the toughness may not be sufficiently improved. In this case, hydrogen embrittlement may not be sufficiently suppressed.
- it exceeds 0.90% the steel for springs with a larger cross-sectional area and a lower cooling rate after rolling will promote ferrite decarburization and fatigue strength. Cause a drop in Moreover, it is preferable to contain Si content exceeding 0.50% from a viewpoint that toughness can be improved more.
- Mn 0.40 to 1.20% Mn is an indispensable element in order to ensure the hardenability required as a steel for leaf springs. If the Mn content is less than 0.40%, it may be difficult to ensure the hardenability necessary for the leaf spring steel. On the other hand, if it exceeds 1.20%, the hardenability becomes excessive, and there is a possibility that quench cracks are likely to occur.
- Cr 0.70 to 1.50% Cr is an indispensable element in order to ensure the hardenability required for steel for leaf springs. If the Cr content is less than 0.70%, it may be difficult to ensure the hardenability and temper softening resistance necessary for the leaf spring steel. On the other hand, if it exceeds 1.50%, the hardenability becomes excessive, and there is a possibility that quench cracks are likely to occur.
- Ti 0.070 to 0.150% Ti is present in steel as TiC which can be a hydrogen trap site, and has the effect of improving hydrogen embrittlement resistance. Moreover, fine TiC can be formed together with C in the steel, the quenching and tempering structure can be refined, and the formation of coarse bainite can be suppressed. Moreover, by combining with N to become TiN, there is an effect of suppressing the generation of BN and preventing the later-described effects due to the addition of B from being obtained. When the Ti content is less than 0.070%, the above-described effects due to the addition of Ti may not be sufficiently obtained. On the other hand, if it exceeds 0.15%, TiC tends to be coarsened.
- B 0.0005 to 0.0050%
- B is an element necessary for ensuring the hardenability required for the steel for leaf springs, and is also effective for improving the grain boundary strength. If the B content is less than 0.0005%, it may be difficult to ensure the hardenability necessary for the leaf spring steel and to improve the grain boundary strength. Moreover, B is an element which can obtain an effect even when contained in a very small amount, and the effect is saturated even if contained in a large amount. Therefore, the upper limit of the B content can be set to 0.0050% as described above.
- N 0.0100% or less
- B is an element that is very easy to bond with N, and when combined with N contained as an impurity and present as BN, the above-described effects of B are sufficiently obtained. There is a risk that it will not be obtained. Therefore, the N content is set to 0.0100% or less.
- the content ratio of Ti and N satisfies Ti / N ⁇ 10.
- generation of coarse TiN can be suppressed and fine TiC can be produced
- the crystal grains can be refined and the fatigue strength can be improved.
- hydrogen embrittlement resistance can be improved.
- the plate spring steel on the first side contains C, Si, Mn, Cr, Ti, B, and N in the specific composition range, with the balance being Fe and impurity elements.
- the leaf spring steel of the second side surface contains the specific amount of C, Si, Mn, Cr, Ti, B, and N, as in the first side surface, and further, by mass%, Cu: Contains one or more selected from 0.20 to 0.50%, Ni: 0.20 to 1.00%, V: 0.05 to 0.30%, and Nb: 0.01 to 0.30%
- the balance consists of Fe and impurity elements.
- Cu and Ni have the effect of suppressing the growth of corrosion pits generated in a corrosive environment and improving the corrosion resistance.
- the Cu and Ni content is less than 0.20%, the effect of improving the corrosion resistance by these additive elements may not be sufficiently obtained.
- the upper limit of the Cu content is preferably 0.50%.
- the upper limit of the Ni content is preferably 1.00%.
- V and Nb have the effect of making the quenched and tempered structure finer and improving the strength and toughness in a well-balanced manner.
- the upper limit of the content ratio of V and Nb is preferably 0.30%.
- plate springs may contain the quantity of Al (about 0.040% or less) required for the deoxidation process which is an essential process at the time of manufacture of steel as an impurity.
- the leaf spring component can be produced by forming the leaf spring steel and quenching and tempering. Thereby, a tempered martensite structure can be obtained.
- the leaf spring component is preferably subjected to a shot peening treatment performed in a temperature range of room temperature to 400 ° C. with a bending stress of 650 to 1900 MPa. That is, it is preferable that the leaf spring component is subjected to high-strength shot peening. In this case, excellent fatigue strength can be exhibited.
- the leaf spring component has a Vickers hardness of 510 or more.
- the leaf spring steel of the present invention can exhibit excellent toughness and fatigue strength when applied to a high hardness leaf spring component, and in the high hardness region where the Vickers hardness is 510 or more as described above. , Such an effect becomes remarkable.
- the Vickers hardness can be adjusted to 510 or more as described above by controlling the temperature of tempering performed after quenching to be low, for example.
- Example 1 In this example, an example and a comparative example according to the above-described steel for leaf springs will be described.
- a plurality of types of leaf spring steels (samples E1 to E13 and samples C1 to C10) having chemical components shown in Table 1 were prepared.
- the above samples E1 to E13 are the steels of the present invention
- the above samples C1 to C7 have the contents of some components such as C, Si, Ti, TiN, etc.
- sample C8 is SUP10 which is a conventional steel
- sample C9 is SUP11A which is a conventional steel
- sample C10 is SUP6 which is a conventional steel.
- ⁇ Decarburization test> a cylindrical test piece having a diameter of 8 mm and a height of 12 mm was prepared by cutting from a round bar having a diameter of 18 mm (the amount of decarburization before the test was 0).
- the cylindrical specimen was heated in vacuum at a heating rate of 900 ° C./min and held at a temperature of 900 ° C. for 5 minutes. Then, it cooled in the air
- the test piece was cut and polished, and then etched with nital. Thereafter, the decarburization depth (DM-F) of the surface layer was measured with an optical microscope. The results are shown in Table 2. The relationship between the silicon (Si) content and the decarburization depth was plotted on a graph. This is shown in FIG.
- ⁇ Old austenite grain size measurement> A round bar test piece of ⁇ 18 mm ⁇ 30 mm was heated at a temperature of 950 ° C. and oil-quenched to obtain a martensite structure. Next, the test piece was cut and polished, and then immersed in a picric acid aqueous solution to reveal the prior austenite grain boundaries, and the crystal grain size (old ⁇ crystal grain size) was measured with an optical microscope. The results are shown in Table 2. Further, the relationship between the titanium (Ti) content and the old ⁇ crystal grain size, and the relationship between the Ti / N ratio and the old ⁇ crystal grain size were plotted in a graph. FIG. 4 shows the relationship between the Ti content and the old ⁇ crystal grain size, and FIG. 5 shows the relationship between the Ti / N ratio and the old ⁇ crystal grain size.
- ⁇ Hydrogen embrittlement characteristics test> A round bar test piece with a circular notch with a depth of 1 mm is prepared on the parallel part of a cylindrical test piece ( ⁇ 8 mm x 75 mm), and it is quenched and tempered so that the target hardness is HV540 (Vickers hardness). Tempered martensite structure. Next, this test piece was immersed in a 5 wt% ammonium thiocyanate aqueous solution (temperature: 50 ° C.) for 30 minutes to perform hydrogen charging. Next, a tensile test was carried out 5 minutes after the test piece was pulled up from the aqueous solution. The tensile test was performed under the condition of a strain rate of 2 ⁇ 10 ⁇ 5 / sec and evaluated by the load at break. For comparison, a similar test was performed on a test piece not charged with hydrogen.
- ⁇ Rolling material decarburization test> A rolled material having a width of 70 mm and a thickness of 20 mm produced by rolling was cut in a cross section perpendicular to the longitudinal direction, and the decarburization depth (DM-F) was measured by an optical microscope. The results are shown in Table 2. In addition, in order to clarify the influence on the decarburization depth due to the difference in shape, cross-sectional area, etc. from the plate material, the same steel ingot as the plate material was rolled to produce a ⁇ 12 mm round bar. The cross section was cut and the decarburization depth (DM-F) was measured. The results are shown in Table 2.
- ⁇ Durability test> A rolled material having a width of 70 mm and a thickness of 20 mm produced by hot rolling was formed into a leaf spring shape. Next, quenching and tempering were performed so that the target hardness was HV540 (Vickers hardness) to obtain a tempered martensite structure, and then high-strength shot peening was performed. High-strength shot peening was performed under conditions of a temperature of 300 ° C. and a bending stress of 1400 MPa.
- ⁇ Corrosion resistance evaluation> A rolled material having a width of 70 mm and a thickness of 20 mm produced by rolling was quenched and tempered to obtain a martensite structure, and then a plate-shaped test piece having a width of 30 mm, a thickness of 8 mm, and a length of 100 mm was produced by cutting. Then, a sodium chloride aqueous solution (salt water) having a concentration of 5 wt% and a temperature of 35 ° C. is sprayed on the plate-shaped test piece for 2 hours (salt water spray treatment), and dried with hot air at a temperature of 60 ° C. for 4 hours (drying treatment). It was moistened for 2 hours under conditions of 50 ° C.
- the sample C1 with a too low C content and the sample C3 with a too low Si content need to have a low tempering temperature in order to secure HV540.
- Hydrogen embrittlement easily occurs due to the influence.
- the sample C2 in which the C content is too high not only deteriorates the hydrogen embrittlement characteristics but also deteriorates toughness.
- the ferrite decarburization amount increased and the fatigue life decreased.
- the decarburization depth of a steel bar having a diameter of 12 mm corresponding to the shape and dimensions of a coil spring of an automobile is also shown.
- the Si content is high, the ferrite decarburization is performed.
- high Si materials that do not have a problem with coil springs for automobiles, etc., which are used at ⁇ 10 to ⁇ 20 mm or thinner valve springs have a high possibility of reduction in fatigue strength due to decarburization when used for leaf springs. Recognize.
- the sample C5 whose Ti content rate is too low deteriorates the hydrogen embrittlement characteristic. Further, in the sample C5, the old ⁇ crystal grain size is increased, the internal coarse structure is easily broken, and the durability is deteriorated. On the other hand, the sample C6 having an excessively high Ti content has inclusions in the internal structure, and the inclusions tend to break, resulting in poor durability. Moreover, in the sample C7 in which the Ti / N ratio is too low, the old ⁇ crystal grain size becomes large, the internal coarse structure tends to break down, and the durability deteriorates.
- the conventional steel samples C8 and C9 have low impact values and poor toughness when the hardness is increased as in this example.
- the hydrogen embrittlement characteristics are low, the old ⁇ crystal grain size is large, the internal coarse structure tends to break down, and the durability is poor.
- the sample C10 which is conventional steel has a large amount of ferrite decarburization.
- the samples E1 to E12 of the present invention have internal stresses even when they are subjected to bending stress and shot peening is performed at a temperature higher than room temperature (that is, when high-strength shot peening is performed). It is difficult to cause breakage due to, is excellent in durability, and can exhibit excellent fatigue strength. Moreover, it is excellent in hydrogen embrittlement characteristics, and is not easily embrittled even if hydrogen penetrates into the steel. Furthermore, it has strength and toughness in a well-balanced manner and has excellent durability. Therefore, it can be suitably used for a leaf spring for automobiles such as trucks. Further, in the present invention, the lower limit of the Si content is 0.40%, but as is known from Table 2 and FIG. 2, in order to increase the toughness by increasing the impact value in the high hardness region, It is preferable to increase the content to an amount exceeding 0.50%.
- leaf spring steel (sample E1 to sample E13) is preferable, with the balance being Fe and impurity elements and satisfying Ti / N ⁇ 10.
- Example 2 In Example 1, the hardness was aimed at HV540, but in this example, an impact test was performed on a test piece whose aim hardness was changed, and the relationship between the hardness and the impact value was examined. That is, with respect to Sample E1, Sample E12, Sample C3, and Sample C8 of Example 1, test specimens were produced by changing the target hardness and quenching and tempering, and the impact test was performed in the same manner as in Example 1. . The results are shown in Table 3 and FIG. FIG. 8 shows the relationship between hardness and impact value, with the horizontal axis representing the Vickers hardness (HV) of each sample and the vertical axis representing the impact value of each sample.
- HV Vickers hardness
- a leaf spring in a truck is a part that is considerably heavier than other parts, and if a technology that can reduce the weight is developed, the effect is great.
- it is not enough to simply improve toughness and hydrogen embrittlement resistance in a high hardness range, but by shot peening performed at a temperature higher than room temperature while applying bending stress, that is, high strength shot peening. It was necessary to develop a material that would increase the effect.
- the present invention completely satisfies the needs, and a great effect can be expected.
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Abstract
Description
この高強度化においては疲労強度の向上が特に重要であり、そのための対策の一つとして、材料の高硬度化がある。
このような理由から、高硬度化しても腐食疲労強度が低下しにくい鋼材の開発が強く望まれていた。
したがって、丸棒ばねでは顕著に生じないが板ばねでは顕著に生じる板ばね特有の問題を解決できる最適な成分系とはなっていなかった。
さらに、板ばねにおいては、丸棒ばねと共通の課題である耐水素脆性の向上や高硬さ域での靱性向上も含めて解決する必要があり、その点について考慮した上で最適な板ばね用鋼を提供する必要がある。
また、本願発明者らは、後述の通り、板ばね製造時においてもフェライト脱炭が生じ難く、かつ高硬度域で優れた特性を確保できる成分系を見出した。上述したTi添加と組み合わせて対策を実施することにより、高硬度域で優れた疲労寿命を安定して確保できる板ばね部品を製造できることを見出し、本願発明を完成させた。
TiとNの含有率がTi/N≧10を満足することを特徴とする高疲労強度板ばね用鋼にある。
さらに質量%で、Cu:0.20~0.50%、Ni:0.20~1.00%、V:0.05~0.30%、及びNb:0.01~0.30%から選ばれる1種以上を含有し、
残部がFe及び不純物元素からなり、
TiとNの含有率がTi/N≧10を満足することを特徴とする高疲労強度板ばね用鋼にある。
特に、Ti及びTi/Nの範囲を上記のごとく規定しているため、微細なTiCを析出させ、焼入加熱時に微細なオーステナイト結晶粒を得ることができる。そのため、上記板ばね用鋼においては、焼入焼戻し時に発生しうる粗大なベイナイトの生成を抑制することができる。それ故、上記板ばね用鋼は、これを用いて高強度ショットピーニング処理を施した板ばね部品を作製しても、粗大なベイナイトを起点とする早期折損を防止することができ、優れた疲労強度を発揮することができる。
また、上記板ばね用鋼においては、上記のごとく、C含有率を比較的低くしながら脱炭量の増加に問題が生じない上記特定範囲でSiを含有させることにより、焼もどし軟化抵抗を高め、より高い温度での焼き戻しを可能にする。さらに、Ti及びBを必須成分として添加することにより、耐水素脆性を高めると共に粒界強度の向上を図っている。
その結果、高硬度域において、優れた靱性を示すことができる。特に、HV510以上の高硬度域においてはその効果が顕著になる。
上記板ばね部品は、上記第1又は第2の側面の高疲労強度板ばね用鋼を用いているため、高強度化のために硬さを高め、水素脆化が問題となる硬さ領域においても優れた靱性を確保することができ、高強度ショットピーニングにより確実に寿命向上を図ることができる。
特に、HV510以上の高硬度域においては、靱性の向上効果が顕著になる。
以下、各成分毎に含有率の範囲を限定した理由について説明する。
Cは、焼入焼戻し処理後に十分に優れた強度及び硬さを確保するために不可欠な元素である。
Cの含有率が0.4%未満の場合には、ばねとしての強度が不十分になるおそれがある。また、C含有率が低下すると、高硬度、特にHV510以上の硬さを得るのに低い温度での焼き戻しを行わなければならなくなる。その結果、水素脆化強度比が低くなり、水素脆化が起こりやすくなるおそれがある。
一方、0.54%を越える場合には、高硬度域における靱性が、Ti、Bを添加しても低下傾向になると共に、水素脆化が起こりやすくなるおそれがある。靱性を特に向上させるためには、上限を0.50%未満とすることがが好ましい。
即ち、通常、低硬度域においてはC含有率が低い方が靱性は大きくなる。しかし、本発明の対象としているばね部品は、高硬度(好ましくはHV510以上)を狙いとするため、C含有率が0.40%台の場合には高硬度を得るために焼き戻し温度を低くする必要が生じ、低温焼き戻し脆性域になる可能性が高くなる。その結果、C含有率が0.50%台の場合に比べてかえって靱性が低下するという逆転現象が生じてしまう。しかし、本発明のように、Ti、Bの両方を必須成分として添加することにより、0.40%台というばね用鋼としては低いC含有率にしても高硬度域における靱性が向上し、C含有率が0.54%を超える場合と比較してさらに靱性を向上させることができる。特に、C含有率を0.50%未満とした場合に靱性の向上効果が顕著になる。
Siは、焼き戻し軟化抵抗を高める効果を有し、高硬度を狙う場合でもより高い焼き戻し温度への設定を可能にする。その結果、高強度及び高靱性を確保すると共に、水素による脆化を防止して、腐食疲労強度の改善を可能にする元素である。
Siの含有率が0.40%未満の場合には、焼き戻し温度を低くしないと狙いの硬さが得られなくなり、靱性を十分に向上できなくなるおそれがある。また、この場合には、水素脆化を十分に抑制することができなくなるおそれがある。一方、0.90%を超える場合には、丸棒を素材とするばねに比べて断面積が大きく、圧延後の冷却速度が小さくなる板ばね用鋼においてはフェライト脱炭を助長させ、疲労強度の低下の原因となる。
また、靱性をより向上できるという観点から、Si含有率は0.50%を超えて含有させることが好ましい。
Mnは、板ばね用鋼として必要な焼入性を確保するために必要不可欠な元素である。
Mnの含有率が0.40%未満の場合には、板ばね用鋼として必要な焼入性を確保することが困難になるおそれがある。一方、1.20%を超える場合には、焼入性が過剰になり、焼割れが発生し易くなるおそれがある。
Crは、板ばね用鋼として必要な焼入性を確保するために必要不可欠な元素である。
Crの含有率が0.70%未満の場合には、板ばね用鋼として必要な焼入性及び焼戻し軟化抵抗性を確保することが困難になるおそれがある。一方、1.50%を超える場合には、焼入性が過剰になり、焼割れが発生し易くなるおそれがある。
Tiは、水素のトラップサイトとなりうるTiCとなって鋼中に存在し、耐水素脆性を向上させる効果がある。また、鋼中のCと共に微細なTiCを形成し、焼入焼戻し組織を微細化し、粗大ベイナイトの生成を抑制することができる。また、Nと結合してTiNとなることにより、BNの生成を抑制し、Bを添加することによる後述の効果が得られなくなることを防止する効果がある。
Tiの含有率が0.070%未満の場合には、Ti添加による上述の効果が十分に得られなくなるおそれがある。一方、0.15%を超える場合には、TiCが粗大化し易くなるおそれがある。
Bは、板ばね用鋼として必要な焼入性を確保するために必要な元素であるが、さらに粒界強度の向上にも効果がある。
Bの含有率が0.0005%未満の場合には、板ばね用鋼として必要な焼入性の確保及び粒界強度の向上が困難になるおそれがある。また、Bは、極めて少量の含有で効果を得られる元素であり、多量に含有させてもその効果が飽和する。よって、B含有率の上限は上述のごとく0.0050%とすることができる。
上述のBは、Nと非常に結合しやすい元素であり、不純物として含有するNと結合し、BNとなって存在した場合には、Bによる上述の効果が十分に得られなくなるおそれがある。よって、Nの含有率は、0.0100%以下とする。
Ti/N<10の場合には、TiCの生成が不十分なため、結晶粒が粗大化し、疲労強度が低下したり、耐水素脆化特性が劣化したりするおそれがある。
また、後述する実施例に示すように、Ti≧0.07、Ti/N≧10とした鋼は、水素チャージによる強度低下を大きく抑制することができる。
一方、上記第2の側面の板ばね用鋼は、上記第1の側面と同様にC、Si、Mn、Cr、Ti、B、及びNを上記特定量含有し、さらに質量%で、Cu:0.20~0.50%、Ni:0.20~1.00%、V:0.05~0.30%、及びNb:0.01~0.30%から選ばれる1種以上を含有し、残部がFe及び不純物元素からなる。
このようにCu、Ni、V、及びNbから選ばれる1種以上を上記特定の含有率で含有する場合には、硬度域における靱性、及び耐食性をより向上させることができる。
以下、Cu、Ni、V、及びNbの各成分毎に含有率の範囲を限定した理由について説明する。
Cu及びNiの含有率が0.20%未満の場合には、これらの添加元素による耐食性の向上効果が十分に得られなくなるおそれがある。また、Cuを多量に含有させると耐食性の向上効果が飽和すると共に、熱間加工性が悪くなるおそれがあるため、Cu含有率の上限は0.50%が好ましい。また、Niを多量に含有させても耐食性の効果が飽和し、高コストの原因となるため、Ni含有率の上限は1.00%が好ましい。
Vの含有率が0.05%未満の場合又はNbの含有率が0.01%未満の場合には、これらの添加元素による結晶粒の微細化効果が十分に得られなくなるおそれがある。また、V及びNbを多量に含有させても靱性の効果が飽和し、高コストの原因となるため、V及びNbの含有率の上限は0.30%が好ましい。
即ち、上記板ばね部品には、高強度ショットピーニングが施されていることが好ましい。この場合には、優れた疲労強度を発揮することができる。
本発明の板ばね用鋼は、高硬度の板ばね部品に適用したときに、優れた靱性及び疲労強度を発揮することができ、上述のようにビッカース硬さが510以上という高硬度域においては、かかる作用効果が顕著になる。
ビッカース硬さは、焼入後に行う焼戻しの温度を例えば低く制御することにより、上述のごとく510以上に調整することができる。
本例は、上記板ばね用鋼にかかる実施例及び比較例について説明する。
まず、表1に示す化学成分を有する板ばね用鋼(試料E1~試料E13、及び試料C1~試料C10)を複数種類用意した。なお、表1に記載の成分のうちCu、Niについては、これらの一部は不純物としての含有率を示してある。
表1に示す板ばね用鋼のうち、上記試料E1~試料E13は本発明鋼であり、上記試料C1~試料C7はC、Si、Ti、TiN等の一部成分含有率が本発明鋼とは異なる比較鋼であり、試料C8は従来鋼であるSUP10、試料C9は従来鋼であるSUP11A、試料C10は従来鋼であるSUP6である。
このようにして得られた丸棒及び板材を用いて、後述の各種評価試験に用いる試験片(丸棒試験片又は板材試験片)を作製し、各種評価を行った。具体的には、丸棒については、後述の衝撃試験、脱炭試験、旧オーステナイト結晶粒径測定、及び水素脆化特性試験を実施し、板材については、後述の圧延材脱炭試験、耐久試験、及び耐食性評価を実施した。
<衝撃試験>
上述の丸棒からUノッチ試験片を作製し、ねらい硬さHV540(ビッカース硬さ)になるように成分の違いによる焼き戻し軟化抵抗の違いを考慮し、焼き戻し温度を調整して焼入及び焼戻しを施し(以下に記載の「焼入焼戻し」も同様)、組織を焼戻しマルテンサイト組織とした。その後、室温にて衝撃試験を実施した。
また、炭素(C)含有率と衝撃値、及びケイ素(Si)含有率と衝撃値との関係をグラフにプロットした。C含有率と衝撃値との関係を図1に示し、Si含有率と衝撃値との関係を図2に示す。
まず、φ18mmの丸棒から切削により直径φ8mm、高さ12mmの円柱型試験片を作製(試験前の脱炭量は0)した。次いで、円柱型試験片を真空中で昇温速度900℃/分で加熱し、温度900℃で5分間保持した。その後、大気雰囲気にて、予め測定しておいた上述の板材作製時における熱間圧延後の冷却曲線と同等の冷却速度で冷却した。次いで、試験片を切断し、研磨した後、ナイタールによりエッチングした。その後、光学顕微鏡により表層の脱炭深さ(DM-F)を測定した。その結果を表2に示す。
また、ケイ素(Si)含有率と脱炭深さとの関係をグラフにプロットした。これを図3に示す。
φ18mm×30mmの丸棒試験片を、温度950℃で加熱し、油焼入してマルテンサイト組織とした。次いで、試験片を切断研磨した後、ピクリン酸水溶液中に浸漬して旧オーステナイト結晶粒界を現出させ、光学顕微鏡にて結晶粒径(旧γ結晶粒径)を測定した。その結果を表2に示す。
また、チタン(Ti)含有率と旧γ結晶粒径、及びTi/N率と旧γ結晶粒径との関係をグラフにプロットした。Ti含有率と旧γ結晶粒径との関係を図4に示し、Ti/N率と旧γ結晶粒径との関係を図5に示す。
円柱形状の試験片(φ8mm×75mm)の平行部に深さ1mmの環状ノッチを付与した丸棒試験片を作製し、ねらい硬さHV540(ビッカース硬さ)になるように焼入及び焼戻しを施し、焼戻しマルテンサイト組織とした。次いで、この試験片を5wt%チオシアン酸アンモニウム水溶液(温度50℃)に30分間浸漬させることにより、水素チャージを実施した。次いで、試験片を水溶液中から引上げて5分後に引張試験を実施した。
引張試験は、歪速度2×10-5/secの条件で行い、破断時の荷重により評価した。比較のため、水素チャージを行っていない試験片についても同様の試験を行った。
また、チタン(Ti)含有率と水素脆化強度比、及びTi/N率と水素脆化強度比との関係をグラフにプロットした。Ti含有率と水素脆化強度比との関係を図6に示し、Ti/N率と水素脆化強度比との関係を図7に示す。
圧延により作製した幅70mm×厚さ20mmの圧延材を長手方向に垂直な断面で切断し、光学顕微鏡により脱炭深さ(DM-F)を測定した。その結果を表2に示す。また、板材との形状・断面積等の違いによる脱炭深さへの影響を明確するため、板材製造に用いた鋼塊と同じ鋼塊を圧延してφ12mmの丸棒を作製し、同様に断面を切断して脱炭深さ(DM-F)を測定した。その結果を表2に示す。
熱間圧延により作製した幅70mm×厚さ20mmの圧延材を板ばね形状に成形加工した。次いで、ねらい硬さHV540(ビッカース硬さ)になるように焼入及び焼戻しを施し、焼戻しマルテンサイト組織とした後、高強度ショットピーニング処理を施した。高強度ショットピーニングは、温度300℃、曲げ応力1400MPaの条件で行った。このようにして得られたショットピーニング処理を施した板ばね部品について、760±600MPaの応力で破断するまで耐久試験を実施し、各試料から得られた板ばね部品の破断寿命及び破壊起点を測定した。
破断寿命は、破断が生じるまでの回数を測定し、40万回を超える場合を「○」として評価し、40万回以下の場合を「×」として評価した。その結果を表2に示す。また、破断面を観察し、破壊起点を調べた。破壊起点が表面にある場合には「表面」とし、内部にある場合には「内部」としてその結果を表2に示す。さらに破壊起点が内部にあった場合には、破壊起点が粗大組織にあるか介在物にあるかを顕微鏡で確認した。その結果を表2に示す。
圧延により作製した幅70mm×厚さ20mmの圧延材に焼入及び焼戻しを施してマルテンサイト組織とした後、切削により幅30mm×厚さ8mm×長さ100mmの板状試験片を作製した。次いで、板状試験片に、濃度5wt%、温度35℃の塩化ナトリウム水溶液(塩水)を2時間噴霧し(塩水噴霧処理)、温度60℃の熱風で4時間乾燥させ(乾燥処理)、さらに温度50℃、湿度95%以上の条件で2時間湿潤させた(湿潤処理)。これらの塩水噴霧処理、乾燥処理、及び湿潤処理を1サイクルとし、これを合計60サイクル繰り返し行った。その後、試験片表面に生成した腐食生成物を除去し、腐食部の断面に現れる最大の腐食ピット深さを光学顕微鏡を用いて測定した。その結果を表2に示す。
また、Ti/N率が低すぎる試料C7においては、旧γ結晶粒径が大きくなり、内部の粗大組織で破壊が起こりやすくなり、耐久性が悪くなる。
また、本発明ではSiの含有率の下限を0.40%としているが、表2及び図2より知られるごとく、高硬度域における衝撃値を高くして靱性をより向上させるためには、Si含有率を0.50%を超える量まで高めることが好ましい。
実施例1においては、HV540を狙い硬さとしたが、本例においては、狙い硬さを変更した試験片について衝撃試験を行い、硬さと衝撃値との関係を調べた。
即ち、実施例1の試料E1、試料E12、試料C3、及び試料C8について、狙い硬さを変えて焼入及び焼戻しを施して試験片を作製し、実施例1と同様に衝撃試験を行った。その結果を表3及び図8に示す。図8は、横軸に各試料のビッカース硬さ(HV)をとり、縦軸に各試料の衝撃値をとり、硬さと衝撃値との関係を示すものである。
これに対し、本願発明の組成範囲にある試料E1及び試料E12は、硬さを高くしても、高い衝撃値を維持しており、優れた強度と靱性を兼ね備えることがわかる。
Claims (5)
- 質量%で、C:0.40~0.54%、Si:0.40~0.90%、Mn:0.40~1.20%、Cr:0.70~1.50%、Ti:0.070~0.150%、B:0.0005~0.0050%、N:0.0100%以下を含有し、残部がFe及び不純物元素からなり、
TiとNの含有率がTi/N≧10を満足することを特徴とする高疲労強度板ばね用鋼。 - 質量%で、C:0.40~0.54%、Si:0.40~0.90%、Mn:0.40~1.20%、Cr:0.70~1.50%、Ti:0.070~0.150%、B:0.0005~0.0050%、N:0.0100%以下を含有し、
さらに質量%で、Cu:0.20~0.50%、Ni:0.20~1.00%、V:0.05~0.30%、及びNb:0.01~0.30%から選ばれる1種以上を含有し、
残部がFe及び不純物元素からなり、
TiとNの含有率がTi/N≧10を満足することを特徴とする高疲労強度板ばね用鋼。 - 請求項1又は2に記載の板ばね用鋼を用いて成形されたことを特徴とする高疲労強度板ばね部品。
- 請求項3に記載の板ばね部品には、曲げ応力650~1900MPaを負荷した状態で、かつ室温~400℃の温度域で行うショットピーニング処理が施されていることを特徴とする高疲労強度板ばね部品。
- ビッカース硬さが510以上であることを特徴とする請求項3又は4に記載の高疲労強度板ばね部品。
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WO2017017290A1 (es) | 2015-07-28 | 2017-02-02 | Gerdau Investigacion Y Desarrollo Europa, S.A. | Acero para ballestas de alta resistencia y templabilidad |
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