JP6728817B2 - High strength spring steel and spring - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength spring steel and a spring, and more particularly to a high-strength spring steel that is hot- or cold-coiled and used for a suspension spring having high strength and high toughness after heat treatment.

As the performance of automobiles increases, the suspension springs are required to have higher strength. Recently, high-strength spring steel having a tensile strength of more than 1800 MPa after heat treatment has been used for suspension springs, and spring steel having a tensile strength of more than 2000 MPa has been used in recent years. Such high-strength spring steel generally has a tempered martensite structure, but in the tempered martensite structure, ductility and toughness have a trade-off relationship with respect to strength. It is gradually becoming difficult to improve strength while maintaining balance.

On the other hand, Patent Document 1 proposes a spring steel that pursues a balance of strength, ductility, and toughness different from that of a tempered martensite structure by using bainite as the spring steel structure after heat treatment. Generally, when trying to obtain a bainite structure in a spring steel containing a large amount of Si, a relatively large amount of retained austenite occurs after heat treatment. However, in Patent Document 1, by utilizing the TRIP effect of this retained austenite, conventional tempering is performed. We have achieved a spring steel with high strength and ductility that could not be achieved with martensitic spring steel.

Further, Patent Document 2 focuses on that retained austenite having an FCC structure has a higher hydrogen storage capacity than tempered martensite and bainite having a BCC structure, and a spring steel having improved hydrogen embrittlement resistance due to retained austenite is disclosed. Proposed.

On the other hand, spring steel having a bainite structure also has some problems. One of them is that the hardness is greatly reduced by heating. In order to increase the strength of bainite spring steel, it is necessary to lower the bainite transformation temperature. For example, in Patent Document 1, a tensile strength of 1800 to 2100 MPa is achieved in a spring steel wire state by performing bainite transformation at 250 to 350°C. However, when it is actually used as a spring, it is necessary to subject this wire to a spring working, and usually to carry out strain relief annealing at about 350° C. in order to remove the residual stress due to the working. In this case, since strain relief annealing is performed at a temperature higher than the transformation temperature of bainite, the bainite structure is tempered, and when used as a spring, the strength of the wire before spring processing is reduced.

Since the conventional tempered martensitic spring steel is subjected to heat treatment at 350°C or higher at the time of tempering, its strength hardly deteriorates even after strain relief annealing. Therefore, the reduction in strength after strain relief annealing is a problem peculiar to spring steel having a bainite structure.

Further, in martensitic transformation structure and bainite transformation structure, the interior of the former austenite grains is divided into lower structures called packets and blocks, but it is known that the larger the packets and blocks, the lower the resistance to brittle fracture. .. Packets and blocks tend to become larger as the transformation temperature becomes higher. In spring steel, martensite transforms at about 250°C, whereas bainite transforms at 300 to 400°C, resulting in larger packet and block sizes. Toughness tends to decrease.

Conventionally, there has been a tendency that the hardness of the bainite structure is lowered and the toughness is improved by increasing the transformation temperature. However, as the strength of the bainite structure increases, the decrease in toughness due to the coarsening of the bainite structure size is becoming a significant problem.

JP, 2010-222671, A JP-A-5-255733

The present invention has been made in view of the above circumstances, and has both strength and ductility, and at the same time, strength reduction does not occur even after strain relief annealing during spring production, and toughness reduction due to structure coarsening is suppressed, It is an object to provide a steel for high strength spring and a spring.

The transformation temperature of bainite must be equal to or higher than the strain relief annealing temperature in order not to cause the strength reduction after the strain relief annealing. Further, it is necessary to exert a strength/ductility balance equal to or higher than that of the tempered martensite structure by the bainite transformation.

Generally, as compared with a martensite structure that is tempered at a temperature of T ₁ after quenching, carbide coarsely precipitates in a structure that is transformed to bainite at a temperature of T ₁ , and thus there is a concern that the strength of steel may be reduced.

Therefore, as a result of researching a method for obtaining high strength even if the bainite transformation temperature is increased, in the present invention, carbon concentration at the transformation interface characteristic of bainite transformation and alloying elements that precipitate fine carbides are combined. It has been found that by utilizing the alloys, it is possible to precipitate carbides with the addition of less alloying elements than before, and it is possible to suppress the strength reduction regardless of bainite structure or strain relief annealing temperature.

Further, even if transformation is performed with the same heating temperature and the same austenite grain size, the packet block size of bainite having a high transformation temperature is larger and the packet block size of martensite having a lower transformation temperature is smaller. In order to reduce the bainite packet block size, it is effective to disperse fine precipitation particles in austenite to suppress austenite grain growth and to reduce the austenite grain size. Found.

(1) Steel component is mass %, C: 0.40 to 0.80%, Si: 0.80 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.10 to 2 0.00%, Mo: 0.10 to 1.00%, Ti: 0.030 to 0.100%, N: 0.0010 to 0.0070%, P: 0.030% or less, S:0. 0.030% or less , Al: less than 0.050% , the balance consisting of Fe and impurities,
In the microstructure in any cross section, the grain size number of old austenite is 8 or more, bainite is 70% or more in volume fraction, retained austenite is 5% or more and 15% or less in volume fraction, and the balance is 0 to 25% or less. high-strength spring steel which is a martensite.
(2) In the steel composition, when [Ti mass%], [N mass%] and [S mass%] respectively represent Ti content, N content and S content in unit mass %, the chemical composition Is ([Ti mass%]-3.43 [N mass%])/[S mass%]>3.0, The high strength spring steel according to (1).
(3) The high-strength spring steel according to (1) or (2), wherein the steel composition further contains V: 0.05 to 1.00%.
(4) Ni: 0.10 to 1.00%, Cu: 0.10 to 0.50%, W: 0.10 to 0.50%, Nb: 0.020 to 0. 100% , B: 0.0010 to 0.0060%, The high-strength spring steel according to any one of (1) to (3), containing one or more.
(5) A spring made of the high-strength spring steel according to any one of (1) to (4).

According to the present invention, it is possible to provide a spring steel and a spring that have both strength and ductility, do not undergo strength reduction even after strain relief annealing during spring manufacture, and have little toughness reduction.

The inventor intends to invent a spring steel excellent in temper softening resistance while maintaining high strength and high ductility of the conventional bainite structure spring steel by utilizing bainite transformation and addition of alloying elements in combination. I arrived.

It is also known that MnS in steel dissolves when it comes into contact with water to form a local battery and promote corrosion of steel. On the other hand, Ti-based sulfides such as Ti ₄ C ₂ S ₂ are stable to water. Therefore, in the present invention, Ti-based sulfides such as Ti ₄ C ₂ S ₂ are formed by controlling the contents of Ti, Mn, and S in the steel. This increases the corrosion resistance of the steel. Further, since formation of corrosion pits and entry of hydrogen into the steel material are suppressed, the corrosion fatigue characteristics required for suspension springs are also improved.

Hereinafter, the chemical composition of the spring steel of this embodiment will be described.

[C: 0.40 to 0.80%]
C is an element that greatly affects the basic strength of the steel material, and 0.40% or more of C must be added to obtain sufficient strength as spring steel. On the other hand, if C is added excessively, coarse carbides are formed after bainite transformation, and ductility and toughness are remarkably lowered. Therefore, the upper limit of the amount of C added is set to 0.80%.

[Si: 0.80 to 3.00%]
Si is an element necessary for ensuring the strength and sag resistance of the spring, and also has the function of suppressing coarsening of cementite and improving the bainite strength. In order to obtain these effects, it is necessary to add 0.80% or more of Si. On the other hand, if Si is added excessively, the ductility of the steel material is significantly reduced and the steel material becomes brittle. Therefore, the upper limit of the amount of Si added is set to 3.00%.

[Mn: 0.10 to 1.50%]
Mn is an element that improves the hardenability of steel and is an element necessary for fixing S existing in steel as MnS. To obtain these effects, it is necessary to add 0.10% or more of Mn. It is more preferably 0.20% or more. On the other hand, if Mn is excessively added, segregation of components in the steel material is significantly deteriorated and the amount of retained austenite is excessively added, which deteriorates the toughness and fatigue characteristics of the steel material. Therefore, the upper limit of the amount of Mn added is set to 1.50%.

[Cr: 0.10 to 2.00%]
Cr is an element effective for improving the hardenability of steel and refining cementite precipitation to improve the strength of bainite. To obtain these effects, it is necessary to add Cr in an amount of 0.10% or more. On the other hand, when Cr is excessively added, undissolved carbides increase during quenching to deteriorate the hardenability of the steel material, and at the same time, these carbides act as fracture starting points to deteriorate fatigue properties. Therefore, the strength of the steel material also decreases. Therefore, the upper limit of the Cr addition amount is set to 2.00%.

[Mo: 0.10 to 1.00%]
Mo is an element effective for improving the strength of bainite by adding cement with Cr to finely precipitate cementite and also fine CrMo carbide (carbide containing Cr and Mo). To obtain these effects, it is necessary to add 0.10% or more of Mo. On the other hand, when excessive Mo is added, coarse carbides are formed in the austenite temperature range, which does not contribute to the increase in strength during bainite transformation, and these coarse carbides serve as the starting points of fatigue fracture. Therefore, the upper limit of the amount of Mo added is set to 1.00%.

[Ti: 0.030 to 0.100%]
Ti is an element that precipitates fine TiN and Ti(C,N) that function as pinning particles in austenite, and has a function of suppressing coarsening of austenite grains during heat treatment. Ti also has a function of forming Ti-based sulfides (TiS, Ti ₄ C ₂ S ₂ ) and rendering S harmless. In order to obtain this effect, addition of 0.030% or more of Ti is necessary. On the other hand, if the Ti addition amount is excessive, coarse TiN is generated immediately after solidification, and the fatigue life is reduced as a fracture starting point. Therefore, the upper limit of the Ti addition amount is set to 0.100%.

[N: 0.0010 to 0.0070%]
N is an element that forms various kinds of nitrides in steel, and nitride particles that are stable even at high temperatures exert the effect of refining old austenite grains due to the pinning effect of austenite grain growth. In order to obtain these effects, it is necessary to add 0.001% or more of N. On the other hand, if the amount of N is excessive, coarse nitride particles such as CrN and TiN are generated, resulting in deterioration of toughness and fatigue characteristics. Therefore, the upper limit of the amount of N added is set to 0.007%.

[P: less than 0.030%]
P is present in steel as an impurity element and embrittles steel. In particular, P segregated at the former austenite grain boundaries causes a decrease in impact value and delayed fracture due to hydrogen intrusion. Therefore, the P content is preferably low. In order to prevent the embrittlement of steel, it is necessary to limit the P content to less than 0.030%.

[S: less than 0.030%]
S, like P, exists in steel as an impurity element and embrittles steel. S can be fixed as MnS by containing Mn, but MnS also acts as a fracture starting point when coarsened, thereby deteriorating the fracture characteristics of steel. Further, since MnS can also be a starting point of corrosion, excessive S also causes a decrease in corrosion resistance. In order to suppress these adverse effects, it is necessary to limit the S content to less than 0.030%.

Further, in the spring steel according to the present embodiment, the Mn content can be reduced by utilizing Ti for fixing S as described above for the purpose of reducing MnS which is a starting point of corrosion. Therefore, in the spring steel according to the present embodiment, the chemical composition may satisfy the following formula 1 in order to secure a Ti amount necessary and sufficient for fixing S.
([Ti mass%]-3.43 x [N mass%])/[S mass%]>3.0...(Equation 1)

In the numerical formula 1, the numerical value “3.43” of the molecular part on the left side is a value obtained by dividing the atomic weight of Ti by the atomic weight of N. “3.43×[N mass %]” is the maximum amount of Ti that can be consumed by the formation of TiN. Therefore, the left side of the mathematical expression is the ratio of the “Ti amount remaining without being consumed by N” and the “S amount”. When Ti4C2S2 is assumed as the Ti-based sulfide, the weight ratio of Ti and S is Ti:S=3:1 based on the molecular formula and the atomic weights of the respective elements. Therefore, “Ti remaining without being consumed by N is Ti _In order to fix S as ₄ C ₂ S ₂ , it is preferable that the lower limit value of the left side of Expression 1 is more than 3.0.

The chemical composition of the steel according to this embodiment may further contain V. Moreover, you may contain 1 type(s) or 2 or more types among Ni, Cu, W, Nb, Al, and B. However, V, Ni, Cu, W, Nb, Al, and B are arbitrary elements, and the chemical composition of the steel according to this embodiment does not need to contain these.

[V: 0.05 to 1.00%]
V improves bainite strength by refining cementite and precipitated carbides, and at the same time, VN and V(N,C) particles that precipitate in the austenite region suppress austenite grain growth as pinning particles, and It also contributes to high toughness. In order to obtain these effects, it is desirable to add V in an amount of 0.05% or more. On the other hand, if V is added excessively, coarse carbides will be formed and the fatigue characteristics will be deteriorated. Therefore, the amount of V added is preferably 1.00% or less.

[Ni: 0.10 to 1.00%]
Ni is an element that improves the corrosion resistance and toughness of the steel material, and in order to obtain these effects, addition of Ni of 0.10% or more is desirable. On the other hand, excessive addition of Ni causes an increase in retained austenite, resulting in deterioration of sag resistance and spring fatigue characteristics. Therefore, the amount of Ni added is preferably 1.00% or less.

[Cu: 0.10 to 0.50%]
Like Ni, i is an element that improves the corrosion resistance and toughness of steel materials, and in order to obtain these effects, addition of Cu of 0.10% or more is desirable. On the other hand, if Cu is excessively added, embrittlement is caused during hot rolling, and manufacturability is significantly reduced. Therefore, the Cu addition amount is preferably 0.50% or less.

[W: 0.10 to 0.50%]
W is an element that improves the bainite strength by refining precipitated carbides, and in order to obtain this effect, W addition of 0.10% or more is desirable. On the other hand, since excessive addition of W embrittles the steel material, the amount of W added is preferably 0.50% or less.

[Nb: 0.020 to 0.100%]
Nb is an element that precipitates fine NbN and Nb(C,N) that function as pinning particles in austenite, and has a function of suppressing coarsening of austenite grains during heat treatment. In order to obtain this effect, it is desirable to add 0.020% or more of Nb. On the other hand, if the amount of Nb added is excessive, coarse NbN is generated immediately after solidification, and the fatigue life is reduced as a fracture starting point. Therefore, the amount of Nb added is preferably 0.100% or less.

[Al: less than 0.050%]
Al is an element used as a deoxidizing element, and the Al content after a normal deoxidizing step is less than about 0.050%. Excessive Al causes the generation of coarse inclusions and deteriorates the fracture characteristics. In order to suppress these adverse effects, it is desirable to limit the Al content to less than 0.050%.

[B: 0.0010 to 0.0060%]
B has the effect of improving the hardenability of steel. Further, B is an element that suppresses segregation of P and S and the like at the grain boundaries by preferentially segregating to the old austenite grain boundaries, which tend to become the starting point of fracture, and as a result contributes to an increase in grain boundary strength and an improvement in toughness. Is. In order to obtain these effects, it is desirable to add 0.0010% or more of B. On the other hand, even if B is excessively contained, these effects are saturated, and further, the toughness of the steel may be impaired. Therefore, the B addition amount is preferably 0.006% or less.

The balance of the steel components is iron and impurities. The impurities include, in addition to P and S described above, trace elements that are inevitably mixed in from the raw materials and the manufacturing process.

Next, in the metallographic structure of the spring steel according to the present embodiment, the microstructure in an arbitrary cross section has bainite of 70% or more, and the balance is martensite and retained austenite.

By mainly using the bainite structure, a structure with high dislocation density and high strength due to shear transformation can be obtained, and the strength after heat treatment can be increased. Further, since transformation proceeds from the former austenite grain boundaries toward the inside, segregation of embrittlement elements and precipitation of grain boundary carbides at the grain boundaries can be suppressed, so that the ductility can also be improved. The volume fraction of bainite is preferably 70% or more.

The balance other than bainite is martensite and retained austenite. The volume fraction of martensite is in the range of 0 to 25%, and the volume fraction of retained austenite is in the range of 5 to 15%. In the steel for springs of the present embodiment, it is preferable that residual austenite is present in an amount of 5% or more from the viewpoint of improving ductility. However, since the steel softens when the volume fraction of retained austenite is high, it is necessary to limit the amount of retained austenite to 15% or less in order to maintain high strength.

Bainite is a structure formed by cooling steel heated to an Ac3 point or higher to a bainite transformation start temperature (Bs point) or lower. Note that the Ac3 point is the temperature at which the transformation from the ferrite phase to the austenite phase is completed by heating, and the Bs point is the upper limit temperature at which the bainite transformation starts upon cooling from the Ac3 point or higher. Bainite is a set of fine plate-like structures called lath, and carbon in austenite is concentrated between bainite laths as bainite transformation progresses. This thickened carbon combines with Mo that was in solid solution in the steel to form Mo carbide.

It is desirable that CrMo carbide is precipitated in the metal structure of the spring steel of the present embodiment. Since Cr and Mo are contained in the steel, a large amount of CrMo carbide is finely precipitated, whereby the bainite structure is precipitation strengthened and the strength is increased. Further, since Mo carbide is relatively stable as compared with Cr carbide when it is heated, it can continue to exist without being coarsened/disappeared even if strain relief annealing is performed at the time of manufacturing a spring. As a result, even if a spring is used, the reduction in strength is suppressed. In the case of only Cr carbide, when strain relief annealing is carried out at the time of manufacturing a spring, grain growth occurs and the number density of Cr oxide is reduced, so that precipitation strengthening ability is lowered and it is difficult to maintain strength. become.

In the spring steel of the present embodiment, the prior austenite grains are refined in order to reduce the size of the packets and blocks existing inside the former austenite grains and improve the toughness. This effect becomes particularly large by refining so that the former austenite grain size number becomes 8 or more. Therefore, the former austenite grain size number is defined as 8.0 or more. Preferably, the prior austenite grain size number is 9.0 or more.

Note that the old austenite grain size number, by corroding with an appropriate corrosive solution such as a picric acid saturated aqueous solution, the old austenite grain boundaries are exposed, and may be measured by observing with an optical microscope or the like, typically, It may be based on JIS G 0551. Further, here, the measurement of the prior austenite grain size may be performed on a cross section (C cross section) orthogonal to the length direction of the steel wire for spring after quenching and tempering, and in practice, for example, as shown in Examples described later. Further, it may be represented by a value measured at a position ¼ of the diameter D from the outer peripheral surface position of the steel wire in the C section of the spring steel wire after quenching and tempering.

The spring steel of the present embodiment is such that the steel material having the above composition is austenitized at a temperature of more than Ac3 point and (Ac3 point+200° C.) or less, and then cooled at a rate of 10° C./s or more (Bs point−250° C. ) Above Bs point for 300 to 3600 s, and then cooled to room temperature at a cooling rate of 10°C/s or more.

In order to transform the steel material having the above components into bainite and effectively utilize the alloying elements, it is necessary to control the temperature condition of the bainite transformation. The details are shown below. The steel material can be obtained as a steel wire material by continuously casting molten steel having the adjusted composition into a billet and hot rolling the billet. After that, pickling and, if necessary, wire drawing and then heat treatment for obtaining a bainite structure are performed. The conditions for hot rolling are not particularly limited.

In order to obtain the bainite structure, it is necessary to heat the steel material to transform it into an austenite phase and then to cool it to transform it into bainite. It is appropriate that the temperature at which the austenite phase is transformed is higher than the Ac3 point (Ac3 point+200° C.) or lower. At temperatures lower than the Ac3 point, an austenite single phase cannot be obtained, and untransformed proeutectoid ferrite remains in the final structure, so that the required strength cannot be obtained. On the other hand, if the temperature is higher than (Ac3 point+200° C.), Ti(C,N) that pinches and suppresses austenite grain growth is likely to be coarsened, and the pinning effect is lost, and austenite grains are likely to be coarsened. When the austenite grain size becomes coarse, the toughness after heat treatment decreases, so the upper limit of the heating temperature in the austenite region is (Ac3 point+200° C.).

After heating to the austenite phase, after holding for an appropriate time for controlling the austenite grain, it is cooled to obtain a bainite structure. In order to obtain the high-strength bainite of the present invention, when the upper limit temperature at which bainite transformation starts by cooling from a temperature of Ac3 point or higher is set to Bs point, cooling is performed from an Ac3 point or higher at a rate of 10°C/s or higher, and ( It is necessary to hold at a temperature above (Bs point −250° C.) and below Bs point for 300 to 3600 s and then cool to room temperature at a cooling rate of 10° C./s or more.

If the cooling rate from the Ac3 point is low, proeutectoid ferrite precipitates, and the required strength may not be obtained in some cases. If the holding temperature is higher than the Bs point, a bainite structure cannot be obtained. When the holding temperature is lower than (Bs point −250° C.), the final spring cannot maintain the strength during working because it is too close to the strain relief annealing temperature after spring working. If the holding time at a temperature higher than (Bs point −250° C.) and lower than Bs point is shorter than 300 s, the fraction of martensite becomes too high due to uncompleted bainite transformation, resulting in insufficient toughness after heat treatment. .. If the holding time is longer than 3600 s or the cooling rate after holding is lower than 10°C/s, the strength required for softening the bainite transformed structure cannot be maintained.

The spring steel of this embodiment is manufactured as described above.
Further, the obtained spring steel is processed into a spring shape, and further strain relief annealing at about 350° C. is performed in order to remove the residual stress due to the processing. In the present embodiment, since the temperature for bainating is higher than the temperature for strain relief annealing, the bainite structure is not tempered during strain relief annealing, and when used as a spring, it is equivalent to the wire before spring processing. The strength of can be demonstrated.

Next, an example of the present invention will be described. The condition in the example is one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one condition example. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

The components of Examples and Comparative Examples, Ac3 point, Bs point, and (Ti-3.43N)/S values are shown in Table 1, and bainite (B) after bainite transformation, martensite (M), and retained austenite (γR). ), former austenite grain size number (γ grain size), tensile strength (TS), elongation (EL), Charpy impact value, presence or absence of precipitation of CrMo carbide in the structure, 1 week constant temperature and humidity test (temperature 35 Table 2 shows the presence or absence of rust at ℃ and 95% humidity.

The Ac3 point and the Bs point are values calculated from the following mathematical expressions, respectively. The notation such as [C] means mass% of each alloy element.
In each of the examples and comparative examples, steel having a diameter of 14 to 16 mm was subjected to bainite transformation so as to have a pressure of 1900 to 2000 MPa, and test pieces were collected. The tensile test was carried out in accordance with "JIS Z 2201" by producing a No. 14 test piece having a parallel part diameter of 8 mm. The Charpy impact test was carried out by preparing a U-notch test piece (height under the notch: 8 mm, width: 5 mm subsize) in accordance with "JIS Z 2204".
Further, the presence or absence of precipitation of CrMo carbide in the structure was not found when the carbide in the steel was observed by a transmission electron microscope and a carbide containing Cr and Mo at the same time was found in a rectangular region of 100 μm square. The case was marked as x.
Further, the presence or absence of rust after the constant temperature and constant humidity test was visually determined.
The former austenite grain size number was measured according to JIS G 0551 by exposing it to an austenite grain boundary by corroding it with an appropriate corrosive solution such as a saturated picric acid aqueous solution and observing it with an optical microscope. The former austenite grain size was measured at a position ¼ of the diameter D from the outer peripheral surface position of the steel wire in the C cross section of the spring steel wire after quenching and tempering.

Ac3 (° C.)=910−203×√[C]−15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]
Bs (° C.)=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo]

In these Examples and Comparative Examples, a step of hot rolling the steel ingot at a temperature of 950° C. or higher and 1200° C. or lower before hot rolling for a time not exceeding 120 min and hot rolling after Ac3 (° C.) (Ac3+200) After heating at a temperature of (° C.) or lower, it is cooled at a rate of 10° C./s or higher, and is isothermally held at a temperature of (Bs-250) (° C.) or higher and Bs (° C.) or lower for 300 to 3600 seconds, and then 10° C. It was manufactured by a step of cooling to room temperature at a rate of /s or more and performing bainite transformation. Table 3 shows the details of the manufacturing conditions.

Each of the examples has a structure composed of 70% or more bainite, 5% or more and 15% or less retained austenite, and the balance martensite, and has a tensile strength (TS) of 1800 MPa or more and an elongation (EL) of 6% or more, The Charpy impact value was 50 J/cm ² or more. This indicates that all the inventions have a structure mainly composed of bainite and satisfy strength-ductility at a high level.

On the other hand, among the comparative examples, 20, 22, and 25 have insufficient strength after bainite transformation because they lack C, Si, and Mo, respectively.

Further, among the comparative examples, 21, 23, 24, 26, 33, 34 and 35 are hard-quenched because bainite transformation does not proceed because C, Mn, Cr, Mo, Ni, Cu and W are excessive. Ductility is reduced due to excessive generation of martensite.

In Comparative Example 32, the residual austenite fraction was insufficient because V was excessive, and the TRIP effect was not exhibited and the ductility was reduced.

In Comparative Examples 30, 31, 38, the ductility is lowered because P, S, and B are excessive. Further, 28, 29, 36, and 37 each have an excessive amount of Ti, N, Nb, and Al and have coarse precipitates such as nitrides, so that the ductility decreases. Further, in Comparative Example 35, the steel becomes brittle due to excessive W, and as a result, the ductility decreases.

In addition, since 27, 29, and 31 of the comparative examples are insufficient in Ti, excessive in N, and excessive in S, respectively, ([Ti mass %]-3.43 [N mass %])/[S mass %]>3. Without satisfying 0, MnS also precipitates in addition to the Ti-based sulfide. Therefore, it rusted in the constant temperature and humidity test and no improvement in corrosion resistance was observed.

Further, in Comparative Example 39, the heating temperature exceeded (Ac3+200)° C., so the prior austenite grain size was reduced.
Further, since the holding temperature of Comparative Example 40 is low and the holding time of Comparative Example 41 is short, the volume fraction of bainite decreases and the volume fraction of martensite becomes excessive, so that the ductility EL decreases.
Further, in Comparative Example 42, since the holding time is long, the strength TS is insufficient.

The spring steel according to the present invention can achieve both strength and ductility at a high level by being transformed into bainite at an appropriate temperature after being heated to austenite. Therefore, according to the present invention, it is possible to obtain a spring steel having a bainite structure, which has a high strength of 1800 MPa or more and sufficient ductility.

Claims (5)

Steel composition is% by mass, C: 0.40 to 0.80%, Si: 0.80 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.10 to 2.00% , Mo: 0.10 to 1.00%, Ti: 0.030 to 0.100%, N: 0.0010 to 0.0070%, P: 0.030% or less, S: 0.030% Hereinafter , Al: limited to less than 0.050% , the balance consisting of Fe and impurities,
In the microstructure in any cross section, the grain size number of old austenite is 8 or more, bainite is 70% or more in volume fraction, retained austenite is 5% or more and 15% or less in volume fraction, and the balance is 0 to 25% or less. high-strength spring steel which is a martensite. When the steel components are [Ti mass%], [N mass%], and [S mass%] respectively, the Ti content, the N content, and the S content are represented by unit mass%, the chemical component is ([Ti Mass%]-3.43 [N mass%])/[S mass%]>3.0, The high strength spring steel according to claim 1. The high-strength spring steel according to claim 1 or 2, further comprising V: 0.05 to 1.00% in the steel composition. In addition to the above steel components, Ni: 0.10 to 1.00%, Cu: 0.10 to 0.50%, W: 0.10 to 0.50%, Nb: 0.020 to 0.100% , B: 0.0010 to 0.0060%, The high-strength spring steel according to any one of claims 1 to 3, containing one or more of them. A spring made of the high-strength spring steel according to any one of claims 1 to 4.