[0001]The present invention relates to a spring steel, high strength after quenching and tempering, has high toughness and high corrosion resistance, of the preferred spring steel suspension spring.
　The present application, to 01 May 26, 2016, claiming priority based on Japanese Patent Application No. 2016-012427, filed in Japan, the contents of which are incorporated here.
BACKGROUND
[0002]As the performance and weight of the motor vehicle, the spring used in automotive parts are also being increased strength. For high strength of the spring, already high strength steel that exceeds the tensile strength 1800MPa after heat treatment, are subjected to the production of springs. In recent years, steel exceeding tensile strength 2000MPa has also begun to be used as a spring material.
　On the other hand, the suspension spring of an automobile not only high strength, high toughness for preventing damage is obtained in the impact load due to the uneven or the like of the road surface.
　The spring steel of high strength, or cause corrosion pits due to corrosion, the hydrogen from the surrounding environment or invade, it is known that the fatigue characteristics are deteriorated markedly due to stress concentration and hydrogen embrittlement of the pit portion . Therefore, the spring steel corrosion in an environment exposed to rain water or the like, corrosion fatigue characteristics are required.
[0003]
　Recently, high strength and a method to achieve both these properties have been proposed.
　For example, Patent Document 1, the grain boundaries by refining the grain size of the prior austenite grain as a starting point of brittle fracture, to achieve both high strength and high toughness of the steel. Control of prior austenite grain size, a nitride of Ti obtained by the addition of Ti, carbide, have been made using a carbonitride.
　In Patent Document 2, by trapping the hydrogen Ti precipitates, thereby suppressing the reduction of embrittlement and fatigue properties by hydrogen penetration.
　In Patent Document 3, to improve the corrosion resistance of the steel by adding a large amount of Ni, which suppressed the embrittlement caused by hydrogen penetration.
[0004]
　However, in Patent Document 1, measures against MnS inclusions as a starting point of corrosion have been conducted. Therefore, the corrosion resistance is not sufficient. Further, even in Patent Document 2, although measures have been studied to hydrogen entering after corrosion countermeasure against MnS inclusions as a starting point of corrosion have been conducted. Moreover, since Ti is an element results in embrittlement of the steel, nitrides of Ti as in Patent Document 2, a carbide, even when forming a carbonitride, it suppresses the amount of addition of Ti, a certain amount or more of Ti in the case of adding the large amount of expensive alloy elements such as Ni for improving toughness: it was necessary or added together (e.g., Ni 0.5 wt%). For improving corrosion resistance, even Patent Reference 3, by but requires the addition of a large amount of Ni, adding a large amount of Ni is increased and the raw material prices of steel, such as increased hot cracking risks during steel production leading to the deterioration of the manufacturability.
CITATION
Patent Document
[0005]
Patent Document 1: Japanese Patent 3577411 Patent Publication
Patent Document 2: Japanese Patent 2001-49337 JP
Patent Document 3: Japanese Patent 2839900 JP
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　The present invention is more tensile strength 1800MPa after heat treatment such as quenching and tempering, has high toughness and high corrosion resistance, it is an object to provide a spring steel.
Means for Solving the Problems
[0007]
　The present invention is a steel following the gist.
[0008]
(1) spring steel according to one embodiment of the present invention, chemical components, by mass%, C: 0.40 ~ 0.60% , Si: 0.90 ~ 3.00%, Mn: 0.10 ~ 0.60%, Cr: 0.10 ~ 1.00 %, Al: less than 0.010 ~ 0.050%, Ti: 0.040 ~ 0.100%, B: 0.0010 ~ 0.0060%, N: 0.0010 ~ 0.0070%, V : 0 ~ 1.00%, Mo: 0 ~ 1.00%, Ni: 0 less than ~ 0.45%, Cu: 0 ~ 0.50%, Nb: 0 to 0.10%, containing, P: less than 0.020% or, S: less than 0.020% or, limited to, the balance being Fe and impurities, satisfies the following formula 1 and formula 2, the surface among 1/4 of the circle equivalent diameter 1μm or more inclusions observed at the position of the diameter, the frequency of occurrence of MnS is less than 20%.
([Ti mass%] - 3.43 × [N wt%]) / [S% by weight> 4.0 Formula
1 [Ni wt%] + [Cu (mass%) <0.75 Equation 2
Here, the formula 1, in formula 2 [Ni wt%], [Cu (mass%), [Ti mass%], [N (mass%) and [S% by weight] is, Ni content in the unit mass%, respectively, Cu content the amount, Ti content, N content and represents a S content.
[0009]
(2) In the spring steel according to the above (1), the chemical composition, in mass%, V: 0.05 ~ 1.00%, Mo: 0.10 ~ 1.00%, Ni: 0.05 less than ~ 0.45%, Cu: 0.05 ~ 0.50%, Nb: 0.01 ~ 0.10%, may contain one or more.
Effect of the invention
[0010]
　According to the present invention, without reducing the productivity, possible to provide a spring steel having a, high toughness and high corrosion resistance with a tensile strength of at least 1800MPa after heat treatment such as quenching and tempering.
　Spring steel of the present invention, high strength after quenching and tempering, high toughness, and because it has a high corrosion resistance is preferably used for the suspension spring or the like.
DESCRIPTION OF THE INVENTION
[0011]
　The inventors have also a high strength after quenching and tempering and studied a method for obtaining a spring steel having sufficient toughness and corrosion resistance.
　As a result, the present inventors in order to obtain a spring steel having sufficient toughness after quenching and tempering was found that it is effective to reduce the Mn content. However, in general, Mn is an alloy element contained in order to harmless by fixing S adversely affects steels in toughness such as MnS. Therefore, it is necessary element for fixing S in place of Mn in lowering the Mn content.
[0012]
　The present inventors have focused on Ti as S fixing element in place of Mn, Ti, by controlling so that in the steel content of N and S satisfying the predetermined relationship, even if reducing the content of Mn It was found that S is harmless to be fixed. In general, although Ti is believed to be an element which embrittle the steel, the present inventors have found that refining the old austenite grain size by controlling the content of Ti at the same time N, and the particle by including a B to strengthen the field at the same time, and knowledge to be able to overcome this problem.
[0013]
　As for corrosion resistance, MnS in the steel are dissolved upon contact with water, it is known to promote the formation of rust and corrosion pit to form a local cell. In contrast, Ti 4 C 2 S 2 Ti-based sulfides, such as so stable to water, it is possible to increase the corrosion resistance of the steel material of the present invention.
　Therefore, to reduce the Mn content as described above, and to control the Ti content to satisfy the relationship between the N content and the S content, as well as by incorporating the prescribed amount of B, the intensity, in addition to toughness, corrosion resistance was also found that it is possible to improve. Also, when controlling the content of each element as described above, it is possible to suppress the content of such as Cu or Ni, manufacturability and cost is found to be ensured characteristics without deterioration.
[0014]
　Utilization Thus, the present inventors found that a toughness improvement by reducing the Mn, Ti, N, and detoxification and corrosion resistance improvement of S by controlling the S content, in combination with a grain boundary strengthening due to B by, newly found that spring steel is obtained having a high toughness and high corrosion resistance while having a high tensile strength after quenching and tempering. Further, since the corrosion resistance is improved by containing a small amount of Ti, it was also finding that it is possible to suppress the content of expensive Ni to increase the corrosion resistance.
　The following describes a spring steel according to one embodiment of the present invention based on this finding (spring steel according to the present embodiment).
[0015]
　It explained reasons for limiting the chemical components of the spring steel according to the present embodiment (chemical composition).
[0016]
[C: 0.40 ~ 0.60%]
　C is a great influence element on the strength of the steel. In order to impart sufficient strength to the steel after quenching and tempering, 0.40% of the lower limit of the C content. C preferred lower limit of the content 0.42%, more preferred lower limit is 0.45%. On the other hand, the C content is excessive, increasing untransformed austenite (retained austenite) is in steel after quenching, the strength increasing effect of C is reduced. Moreover, the toughness is remarkably lowered. Thus, 0.60% of the upper limit of the C content. The preferable upper limit of C content is 0.58%.
[0017]
[Si: 0.90 ~
　3.00%] Si is an element that increases the strength of the spring is manufactured from spring steel. Moreover, Si is an element which improves the resistance (characteristics sag resistance) against permanent deformation in the shape change during use of the spring. In order to obtain such an effect, the spring steel according to the present embodiment will be 0.90% of the lower limit of the Si content. Si preferred lower limit of the content of 1.20%, more preferred lower limit is 1.40%. On the other hand, when the Si content is excessive, the steel is remarkably embrittled. Therefore, the upper limit of the Si content is set to 3.00%. The preferable upper limit of the Si content is 2.50%.
[0018]
[Mn: 0.10 ~
　0.60%] Mn is an element that improves the hardenability of steel to improve the strength after quenching the steel. In order to obtain such an effect, the spring steel according to the present embodiment, and 0.10% the lower limit of the Mn content. Mn The preferred lower limit of the content is 0.20%, and more preferable lower limit is 0.25%. Meanwhile, Mn is an element that generates MnS by reacting with S in the steel, Mn content produces coarse MnS to be excessive. Further, conventionally had much is contained Mn to fix as MnS and S. However, MnS becomes a starting point of corrosion, cause corrosion pits generated as a result of rust or rust. This corrosion pit becomes a starting point of fatigue fracture, the spring steel according to the present embodiment, in order to suppress the formation of MnS, 0.60% of the upper limit of the Mn content. The preferable upper limit of the Mn content is 0.50%.
[0019]
[Cr: 0.10 ~ 1.00%] Cr
　is improves the hardenability of steel, by controlling the precipitation state of carbides, an element necessary for securing the strength of the steel after quenching and tempering. In order to obtain such an effect, the spring steel according to the present embodiment, and 0.10% the lower limit of the Cr content. A preferable lower limit of Cr content is 0.25%. On the other hand, when the Cr content is excessive, the steel becomes brittle after tempering quenching. Thus, 1.00% of the upper limit of the Cr content. The preferable upper limit of the Cr content is 0.90%.
[0020]
[Ti: 0.040 ~
　0.100%] Ti, along with improving the strength of the steel, Ti-based sulfides S reacts with S in the steel (TiS and / or Ti 4 C 2 S 2 as) by fixing an element that has a function to detoxify the S. Further, Ti has the effect of fixing N in steel as TiN by combine with N. Fixed effects of N are the essential to obtain the effect of solid solution B to be described later, it is necessary to include a sufficient amount of Ti for N fixed. To obtain these effects, the spring steel according to the present embodiment is 0.040% of the lower limit of the Ti content. Ti preferable lower limit of the content of 0.045%, more preferable lower limit is 0.050%. On the other hand, excessive Ti is adapted to generate a prone coarse TiN as starting points of fracture, the steel itself is brittle. Therefore, 0.100% of the upper limit of the Ti content. The preferable upper limit of the Ti content is 0.080%.
[0021]
[Al: less than 0.010 ~
　0.050%] Al is an element used as a deoxidizing element, also because they have the effect of fixing the excess N as AlN, O content and N content of the steel product it is a useful element in the control of. Since Al has deoxidizing power stronger than Ti, To take advantage of the Ti as described above as a nitride and / or sulfide, steel making, adding Al before Ti addition, it must be sufficiently deoxidized is there.
　To obtain these effects, 0.010% of the lower limit of the Al content. If Al content is less than 0.010%, sufficient Ti-based sulfides are obtained, MnS is increased. Preferred Al lower limit of the content is 0.015%, more preferable lower limit of Al content is 0.020%. On the other hand, excessive Al may cause coarse inclusions are generated, thereby deteriorating the fracture characteristics. Therefore, to avoid its adverse effects significant, the spring steel according to the present embodiment and less than 0.050% Al content. The preferable upper limit of Al content is 0.040%.
　Si also deoxidizing element, but because deoxidizing strength is lower than Ti, the above-mentioned effects can not be obtained in Si. Therefore, it is necessary to control the Al content within the above range.
[0022]
[B: 0.0010 ~ 0.0060%]
　B is an element having an effect of improving the hardenability of steel. Further B suppresses segregation such as P and S to grain boundaries by segregating preferentially starting from a prone prior austenite grain boundary fracture as a result contributes to increased and improved toughness of the grain boundary strength element it is. Above Ti is an element that could cause the spring steel is embrittled, simultaneously by incorporating the B, it is possible to suppress embrittlement Ti by toughness improving effect of B. However, in order to obtain these effects, to suppress the formation of BN, it is necessary to increase the amount of B in the solid solution state. To obtain the effect of improving the enhancing effect and the grain boundary strength of hardenability, the spring steel according to the present embodiment, the lower limit of the B content is 0.0010%. B preferred lower limit of the content 0.0015%, more preferred lower limit is 0.0020%. On the other hand, excessively these effects be contained B is not only saturated, there is a possibility that the toughness of the steel is lowered. Therefore, the upper limit of B content is set to 0.0060%. B preferable upper limit of the content is 0.0050%, and more preferable upper limit is 0.0040%.
[0023]
[N: 0.0010 ~ 0.0070%]
　N is an element that generates various nitrides in the steel. Stable nitride particles at elevated temperature exerts a prior austenite grain refinement effect by the pinning effect of austenite grain growth. The spring steel according to the present embodiment, a very stable and TiN particles precipitated in the quenched and tempered before the steel to refine the austenite grains of the steel after quenching and tempering, the lower limit of the N content 0.0010 % to. The preferable lower limit of the N content is 0.0020%. On the other hand, when the N content is excessive, TiN particles becomes starting points of fracture coarsened, toughness and fatigue characteristics are lowered. Furthermore, if the N content is excessive, N generates a BN combined with B, to reduce the solid solution B content. When the amount of solute B decreases, there is a possibility that the effect of improving the enhancing effect and the grain boundary strength of hardenability by the above-mentioned B is impaired. Therefore, the upper limit of the N content to 0.0070%. The preferable upper limit of the N content is 0.0060%.
[0024]
[P: less than 0.020%]
　P is present in steel as an impurity element, an element which embrittle the steel. In particular, P segregated in the old austenite grain boundary, cause the lowering the grain boundary strength causes embrittlement of steel. Therefore, P content is better less. To prevent embrittlement of the steel, the spring steel according to the present embodiment limits the P content to less than 0.020% or. The preferable upper limit of the P content is 0.015%.
[0025]
[S: less than 0.020%]
　S is present in steel as an impurity element similar to the P, is an element which embrittle the steel. S is can be fixed as MnS by containing Mn, MnS serves as starting points of fracture and the coarse, degrading the breakdown characteristics of the steel. To suppress these adverse effects, the S content is preferably lesser, the spring steel according to the present embodiment limits the S content to less than 0.020% or. S preferable upper limit of the content is 0.015%, and more preferable upper limit is 0.010%.
[0026]
　Spring steel according to the present embodiment includes the above elements, and the balance basically in that it consists of Fe and impurities. However, instead of a part of Fe, further, Ni, Mo, V, may be contained in the range described below one or more of Cu and Nb. However, Ni, Mo, V, Cu and Nb are optional elements, the chemical components of the steel according to the present embodiment may not contain them. Accordingly, Ni, Mo, V, the lower limit of Cu and Nb each content is 0%.
　The impurities, in producing the steel industrially, from raw materials such as ores or scraps, or a component that mixes the various environments of the production process, is allowed in a range that does not adversely affect the steel It means a thing.
[0027]
[Ni: 0 ~ less than
　0.45%] Ni is an element for improving the hardenability of steel. Further, Ni is an element for improving the corrosion resistance of steel, an element which contributes to the embrittlement inhibiting of suppressing hydrogen penetration in corrosive environments steel. To obtain these effects, the Ni content may be 0.05% or more in the spring steel according to the present embodiment. On the other hand, Ni content is significantly decreased is the manufacturability hot ductility is lowered in the steel at 0.45% or more. Therefore, even if to be contained, the Ni content is less than 0.45%. The preferable upper limit of the Ni content is 0.40%.
[0028]
[Mo: 0 ~ 1.00%] Mo
　is improves the hardenability of steel, by inhibiting temper softening, an element having the effect of increasing the strength of the steel after quenching and tempering. In order to obtain such an effect, the Mo content may be 0.10% or more. On the other hand, if the Mo content exceeds 1.00%, its effect is saturated. Mo is an expensive element, so is not preferable to contain more than necessary, even if to be contained, it is preferable to 1.00% the upper limit of the Mo content. More preferable upper limit of the Mo content is 0.60%.
[0029]
[V: 0 ~ 1.00%]
　V, along with improving the hardenability by suppressing the temper softening is an element having an effect of enhancing the strength of steel after quenching and tempering. To obtain such an effect, the V content may be 0.05% or more. On the other hand, if the V content exceeds 1.00%, the steel becomes brittle and generate coarse undissolved precipitates. Therefore, even if to be contained, and 1.00% the upper limit of the V content. The preferable upper limit of the V content is 0.50%.
[0030]
[Cu: 0 ~
　0.50%] Cu, it is the effect of suppressing the decarburization during hot rolling and has the effect of improving the corrosion resistance as with Ni. To obtain these effects, the Cu content may be 0.05% or more. On the other hand, Cu, there is a possibility that lowers the hot ductility of the steel, causing cracking occurs during hot rolling. Therefore, even if to be contained, and 0.50% the upper limit of the Cu content. The preferable upper limit of Cu content is 0.30%.
[0031]
[Nb: 0 ~ 0.10%] Nb
　is precipitated nitrides and carbides particles, the pinning effect of austenite grain growth, an element which contributes to the refinement of the prior austenite grains after quenching and tempering. In order to obtain such an effect, the Nb content may be 0.01% or more. On the other hand, if the Nb content exceeds 0.10%, the steel becomes brittle and generate coarse undissolved precipitates. Therefore, even if to be contained, and 0.10% the upper limit of Nb content. The preferable upper limit of Nb content is 0.06%.
[0032]
　As described above, the spring steel according to the present embodiment includes the essential element, if the balance being Fe and impurities, or comprise one or more and the essential elements and optional elements, the balance being Fe and impurities both of which are acceptable in the case.
　The spring steel according to the present embodiment, in addition to the respective content of each element, it is necessary to satisfy Ti, N, S, Cu, the relationship Ni will be described later.
[0033]
([Ti mass%] - 3.43 × [N wt%]) / [S% by weight> 4.0
　In a spring steel according to the present embodiment, by utilizing the Ti to fix the S, as described above , characterized in that to reduce the Mn content. Therefore, the spring steel according to the present embodiment, in order to secure a necessary and sufficient amount of Ti to fix the S, chemical component is required to meet the following formula 1.
([Ti mass%] - 3.43 × [N wt%]) / [S% by weight> 4.0 (Equation 1)
　where, in the formula 1 [Ti mass%], [N mass %] and [S% by weight] are each, Ti content in the steel, N content and S content (wt%).
[0034]
　Bonding force between the Ti is more of N is stronger than S. Therefore, Ti in the steel is first attached to the N to form a TiN, remaining Ti is sulfide. In Formula 1, the numerical value of the "3.43" in the left side of the numerator is a value obtained by dividing the atomic weight of Ti in the atomic weight of N. "3.43 × [N wt%]" is the maximum amount of Ti that can be consumed by the formation of TiN. Thus the left-hand side of Equation 1 is the ratio of the "Ti content remaining unconsumed by N", "S content". Ti as Ti-based sulfide 4 C 2 S 2 assuming the weight ratio of Ti and S, the molecular formula and the respective atomic weights, Ti: S = 3: Since 1 become, remaining without being consumed by the "N and Ti has, Ti 4 C 2 S 2 in order to be sufficient "to fix S as the left side of equation 1 it must be 4.0 or more, preferably 4.5 greater. Is less than the left side of the equation 1 is 4.0, Ti can not be sufficiently secure the S, MnS number generated as a result.
　The spring steel according to the present embodiment, the generation of MnS is suppressed since the Ti is fixed by S. Since MnS becomes a starting point of corrosion, by suppressing MnS product, it is possible to suppress the occurrence of corrosion pits caused by rust or rust.
[0035]
[Ni wt%] + [Cu (mass%) <0.75
　conventional, Cu, improvement of corrosion resistance by the inclusion of Ni has been achieved. However, in a large amount is contained Ni and Cu, the risk of hot tearing during production is increased, productivity is lowered. The spring steel according to the present embodiment, since the corrosion resistance is improved by the above MnS generation suppression, it is possible to reduce the content of Ni and Cu is an element for improving the corrosion resistance. Hot tearing measures by reducing the content of Ni and Cu allows mitigation, lead to improved manufacturability and the manufacturing cost reduced.
　Spring steel according to this embodiment, corrosion resistance, manufacturability, in order to ensure sufficient any of manufacturing cost, satisfy the following equation.
[Ni wt%] + [Cu (mass%) <0.75 (Equation 2)
　where, [Ni wt%] in the formula 2, [Cu (mass%) is, Ni content of each in the steel, it is a Cu content (wt%).
　Preferably, a <0.60 [Ni wt%] + [Cu (mass%).
　Since Ni and Cu are optional elements, the left side of the lower limit of equation 2 need not be specified.
[0036]
　For high-strength spring steel, it is also secured important issue of hardenability. The spring steel according to the present embodiment, since the corrosion pit generation inhibition, limiting the Mn is an element increasing the hardenability to 0.60%. However, Cr and B, further Mo, V, Cu, hardenability by leveraging such a complexly Ni can be secured if necessary. In particular B is so great effect of enhancing the hardenability even in trace amounts, in the spring steel according the present embodiment Cu, can also achieve high strength as less 0.75% in total content and Ni.
[0037]
　The spring steel according to the present embodiment, by fixing the S at Ti, the generation of MnS is suppressed. Since MnS becomes a starting point of corrosion, by suppressing the formation of MnS, it is possible to suppress the occurrence of rust and corrosion pits. Sufficient to obtain a rust and the effect of restraining the generation of the corrosion pit, out of the circle equivalent diameter 1μm or more inclusions observed in any of the cut surface of the steel material, the appearance frequency (circle equivalent diameter 1μm or more inclusions of MnS ratio of the number of MnS to total number of) is required to have been reduced to less than 20%. More preferably the frequency of occurrence of MnS is less than 10%. The object of observation was a circle equivalent diameter 1μm or more inclusions, equivalent circle diameter of generally sulfide inclusions is because it is 1μm or more. Frequency of MnS in 1μm or more inclusions, the cut surface of the steel material was observed more than 20 inclusions metallographic microscope (optical microscope) after mirror polishing, calculated from the number of MnS to the number of these inclusions to. At this time, the observation field is set to 1/4 of the position of the diameter from the surface (distance apart position corresponding to 1/4 of the diameter of toward the center from the surface of the steel material steel), to observe 20 or more inclusions for the, to observe 10 or more field of view in the observation magnification of 1000 while moving, for example, in the rolling direction. Inclusions of determining whether the at metallographic microscope observation color is MnS (MnS gray, Ti-based white-pink-yellow) but can be estimated from, it is desirable to verify by EPMA or SEM-EDS .
[0038]
　Spring steel according to the present embodiment, casting a slab having the above chemical components obtained from molten steel by Al deoxidation, obtained by hot rolling a slab. For example, at a temperature below 1200 ° C. 950 ° C. or higher a steel ingot having the above components were heated for a time not exceeding 120min, obtained by hot rolling in a known manner.
　Spring steel according to the present embodiment further performs coiling after returning hardened baked, or by quenching and tempering after coiling in hot can be a spring.
Example
[0039]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention may employ various conditions.
[0040]
　Each component of the Examples and Comparative Examples, and ([Ti mass%] - 3.43 × [N wt%]) / [S% by weight] (in the table (Ti-3.43 × N) / S), [Cu wt%] + [Ni wt%] of (in the table Cu + Ni) shown in Table 1 and Table 2. In Table 1 and Table 2, the symbol "-" indicates that no addition of elements according to the symbol. Also, Table 1, the balance of Table 2 is Fe and impurities.
　Table 1, at a temperature below 1200 ° C. 950 ° C. or higher a steel ingot having the ingredients shown in Table 2, and heated for a time not exceeding 120min, by hot rolling, phi (diameter) 12 ~ 18 mm steel (spring It was steel).
[0041]
[Table 1]

[0042]
[Table 2]

[0043]
　Carried the obtained spring steel, in order to evaluate the characteristics after quenching and tempering, a process of quenching by heating to a temperature of 900 ° C. or higher 1050 ° C. or less, and a step of tempering as tensile strength becomes 1900 ~ 2000 MPa It was. Tempering conditions, for example, 300 ° C. As a preliminary test, 400 ° C., by measuring the intensity Te tempering at 500 ° C., was determined by estimating the tempering temperature at which a predetermined strength.
　The obtained test piece from the steel after quenching and tempering were harvested, tensile test, Charpy impact test, the observation of inclusions, and was subjected to constant temperature and humidity test.
[0044]
　
　tensile test in conformity with "JIS Z 2241", was carried out to prepare a No. 14 test piece parallel portion diameter 8 mm. If the tensile strength is 1800MPa or more, it is determined that sufficient strength is obtained.
[0045]
　
　Charpy impact test in conformity with "JIS Z 2242" was tested at room temperature (23 ° C.) was prepared U-notch test piece (notched under height 8 mm, 5mm wide subsize). Impact value (absorbed energy) is 70.0J / cm 2 if the above is determined that sufficient toughness can be obtained.
[0046]
　
　frequency of MnS in yen equivalent diameter 1μm or more inclusions, steel cut parallel to the rolling direction, the circle equivalent diameter 1μm or more inclusions in metallographic microscope cut surface after mirror polishing observing 20 or more was calculated from the number of MnS to the number of inclusions observed. At this time, the observation field is 1/4 position of diameter were observed over 10 visual field observation magnification 1000 times while moving, for example, in the rolling direction. The MnS determination color during metallographic microscopy (MnS gray, Ti-based white-pink-yellow) on estimated from, it was verified by EPMA or SEM-EDS. The frequency of occurrence of MnS was regarded as passed less than 20%.
[0047]
　
　exposing the test piece to a constant temperature and humidity for one week (temperature 35 ° C., 95% humidity), were examined visually the presence of rust. Rust was judged to be excellent in corrosion resistance when there is no.
[0048]
　Table 3 and Table 4, the mechanical properties of Examples and Comparative Examples (tensile strength, impact value), MnS frequency in inclusions, after constant temperature and humidity test 1 week (temperature 35 ° C., 95% humidity) indicating the presence or absence of rusting in.
[0049]
[table 3]

[0050]
[Table 4]

[0051]
　Example Both tensile strength and 70.0J / cm of 1900 ~ 2000 MPa 2 has a higher impact value, indicating that both the strength and toughness at a high level. Further, in all embodiments, the frequency of occurrence of MnS is less than 20%, rusting in the constant temperature and humidity test was observed.
[0052]
　On the other hand, Comparative Example 24,25,26,27,28,29,30,31,32,33,34,35,37,39 the C content, Si content, Mn content, P content, S content is / [S% by weight] - amount, Cr amount, Mo amount, V content, Al content, Ti content, B content, (3.43 × [N wt%] [Ti mass%]) excess or where is a shortage, as a result, steel is embrittled or tissue is coarse, and the impact value was decreased.
[0053]
　Moreover, since Comparative Examples 21, 22, 27 is of Ti lack 23,39 is ([Ti mass%] - 3.43 × [N wt%]) / for lack of [S% by weight], 30 S for over 36 are N excess, 38 for Al insufficient, rusting was observed corrosion resistance decreases.
Industrial Applicability
[0054]
　Spring steel according to the present invention, since the austenite grains are refined after quenching and tempering, has excellent mechanical properties after quenching and tempering. Therefore, according to the present invention, the impact value can be ensured while having a high strength of at least 1800 MPa, it is possible to further obtain even higher spring steel corrosion.

claims

Chemical composition, in
　mass%,
　C:
　0.40 ~ 0.60%, Si: 0.90 ~ 3.00%,
　Mn: 0.10 ~ 0.60%, Cr: 0.10 ~ 1.00
　%, Al: less than ~ 0.050%
　0.010,
　Ti: 0.040 ~
　0.100%, B: 0.0010 ~ 0.0060%, N: 0.0010 ~
　0.0070%, V: 0
　1.00% ~, Mo:
　0 ~ 1.00%, Ni: 0 ~ less than%
　0.45, Cu:
　0 ~ 0.50%, Nb: 0 ~ 0.10%,
containing,
　P: 0 less than
　.020% S: less than 0.020% or,
limited to, the balance being Fe and impurities,
　it satisfies the following formula 1 and formula 2,
　the equivalent circle diameter which is observed from the surface at a 1/4 position of the diameter among the above inclusions 1 [mu] m, the frequency of occurrence of MnS is less than 20%
spring steel, characterized in that.
([Ti mass%] - 3.43 × [N wt%]) / [S% by weight> 4.0 Formula 1
[Ni wt%] + [Cu (mass%) <0.75 Equation 2
, where the formula 1, in the formula 2 [Ni wt%], [Cu (mass%), [Ti mass%], [N wt% ] and [S% by weight] is, Ni content in the unit mass%, respectively, Cu content, Ti content, represents the N content and S content.
[Requested item 2]
　The chemical composition, in
　mass%,
　V: 0.05 ~ 1.00%,
　Mo: 0.10 ~ 1.00%, Ni: less than ~ 0.45%
　0.05, Cu: 0.05 ~ 0
　% .50, Nb: 0.01 ~ 0.10%,
containing one or more of
the spring steel according to claim 1, characterized in that.