JP4177403B2 - Si-killed steel wire rod and spring with excellent fatigue characteristics - Google Patents

Description

  The present invention relates to a Si-killed steel wire having excellent fatigue characteristics and a spring obtained from the steel wire. For example, when a high-strength spring (particularly a valve spring) is used, high fatigue characteristics can be exhibited. It is useful as a material for required automotive engine valve springs, clutch springs, brake springs, suspension springs, steel cords and the like.

  Recently, as demands for weight reduction and high output of automobiles increase, high stress design is directed to valve springs and suspension springs used for engines and suspensions. Therefore, in order to cope with an increase in load stress, those having excellent fatigue resistance and sag resistance are strongly desired for these springs. In particular, the demand for increasing the fatigue strength of valve springs is very strong, and SWOSC-V (JIS G 3566), which is considered to be excellent in fatigue strength among conventional steels, has become difficult to cope with.

  In a spring wire that requires high fatigue strength, it is necessary to reduce hard non-metallic inclusions present in the wire as much as possible. From such a point of view, as a steel material used for the above-described applications, it is common to use highly clean steel in which the presence of the non-metallic inclusions is reduced as much as possible. In addition, as the strength of materials increases, the risk of breakage and fatigue breakage due to non-metallic inclusions increases, so the demand for reduction and downsizing of non-metallic inclusions, which are the main factor, is more severe. It has become a thing.

Various techniques have been proposed so far from the viewpoint of reducing and miniaturizing hard non-metallic inclusions in steel materials. For example, Non-Patent Document 1 discloses that inclusions are kept in a glassy state so that the inclusions are refined during rolling, and CaO—Al ₂ O ₃ —SiO ₂ -based components are involved in a glassy and stable composition. It is described that the thing exists. It has also been proposed that it is effective to lower the melting point of inclusions in order to promote the deformation of the glass portion (for example, Patent Document 1).

In Patent Document 2, the chemical composition composition of the steel material is appropriately adjusted while controlling the amounts of Ca, Mg, and (La + Ce) within an appropriate range, and the composition ratio of the average composition of nonmetallic inclusions in the steel is disclosed. It has been shown that a spring steel having excellent fatigue characteristics can be obtained by adjusting (the composition ratio of SiO ₂ , MnO, Al ₂ O ₃ , MgO and CaO) to an appropriate range.

  On the other hand, Patent Document 3 controls basic components such as C, Si, Mn, and Cr, and contains at least one of Ca, Mg, Ba, and Sr in a range of 0.0005 to 0.005%. In addition, there has been proposed a high-strength spring wire material that exhibits excellent “sag characteristics” by making the size of the nonmetallic inclusions 20 μm or less.

Various conventional techniques that have been proposed so far center on the aim of miniaturization by controlling the inclusion composition in the low melting point region. For example, a CaO—Al ₂ O ₃ —SiO ₂ ternary inclusion is known to have a low melting point region in a certain composition range in a ternary phase diagram generally known. However, in a composition in which any of the components is high, the melting point is high and the fatigue strength of the wire is reduced. Such a tendency is the same even in the case of MgO—Al ₂ O ₃ —SiO ₂ ternary inclusions.

  In the various techniques described above, directions for improving characteristics such as fatigue characteristics are shown. However, in the heating time and temperature at the time of hot working, for example, it is not always possible to maintain a complete glass state by controlling the composition as shown in Non-Patent Document 1, and crystals may be generated. is there. Moreover, in order to meet the needs for further steel fatigue strength in recent years, it is necessary to further promote the deformation of the glass portion.

Furthermore, as the strength of steel increases, the components of the steel increase in Si, and the difficulty of pinpoint control to the target composition in the conventionally known CaO—Al ₂ O ₃ —SiO ₂ system increases. For example, as shown in Patent Document 4, advanced control such as controlling not only the total but also dissolved components is required.

  Moreover, in the said patent document 3, although mentioning utilization of Ba, Ca, Mg, Sr, etc., it pays attention only to these low melting-point effects, and can utilize the difference of each component, and the effect of compounding. As a result, it has become a technology that cannot realize the fatigue strength that can withstand the current high demands.

Of the non-metallic inclusions, those containing a large amount of Al ₂ O ₃ are difficult to obtain low melting point inclusions. Therefore, the steel material for obtaining such a wire is not Al killed steel but Si. It is common to use a wire made of so-called “Si killed steel” which is deoxidized by using it.
“182th and 183th Nishiyama Memorial Technology Course”, Japan Iron and Steel Institute, pages 131-134 JP-A-5-320827 Japanese Patent Laid-Open No. 63-140068 JP-A 63-227748 JP 9-310145 A

  The present invention has been made under such circumstances, the purpose of which is to reduce the melting point of the entire inclusion to facilitate deformation, Si killed steel wire for obtaining a spring and the like excellent in fatigue characteristics, And it is providing the spring excellent in the fatigue characteristic obtained from such a steel wire.

  Under such circumstances, the present inventors control the inclusions in the molten steel to an appropriate composition by controlling the concentrations of Ba, Si, Al, Mg, and Ca in a well-balanced manner, and are also harmful during casting. The present inventors have found that it is possible to prevent the formation of complex inclusions.

In general, it is conceivable to lower the melting point by combining oxides. However, due to the limited components that can be controlled as inclusions in steel, it is not easy to lower the melting point of inclusions in Si killed steel and keep the glass stable, and no concrete means has been realized so far. . In contrast, the present inventors have realized this by controlling Ba, Si, Al, Mg, and Ca with an optimal balance. In particular, it is important to control Ba and (Mg + Ca), and to contain them, among Ba, Ca, and Mg, which are conventionally considered similar. In addition, the fatigue strength can be remarkably improved by appropriately controlling Al that has a complicated influence on the stability of the SiO ₂ glass.

  That is, the Si-killed steel wire rod of the present invention that has achieved the above-mentioned object is Ba: 0.03 to 30 ppm (meaning “mass ppm”, the same applies hereinafter), Al: 1 to 30 ppm, and Si: 0.2 to In addition to containing 4% (meaning “mass%”, the same shall apply hereinafter), it also includes Mg and / or Ca in a total range of 0.5 to 30 ppm.

  In said various Si killed steel wire, what contains Li in the range of 0.03-20 ppm is also preferable embodiment.

  The chemical composition of the Si-killed steel wire rod of the present invention is not particularly limited as long as it is used as a “spring”. For example, C: 1.2% or less (0%) Steel materials containing Mn: 0.1 to 2.0%. Further, such a steel material may further include one or more selected from the group consisting of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Co, and rare earth elements (REM). . The preferable content when these are contained varies depending on each element, but Cr: 0.5 to 3%, Ni: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less Mo: 0.5% or less, W: 0.5% or less, Cu: 0.1% or less, Ti: 0.1% or less, Co: 0.5% or less. Moreover, you may add about 0.05% or less of REM as an element which lowers the viscosity of inclusions and exhibits more effect.

  Other than the above components (remainder) are basically Fe and inevitable impurities. In addition, even if the component (for example, B, Pb, Bi) which does not have a big influence on an inclusion is added for the purpose of improving steel properties, the effect of the present invention is exhibited.

  A spring having excellent fatigue strength can be realized by forming the spring using the Si-killed steel wire as described above.

The present invention is configured as described above, and by appropriately adjusting the chemical component composition while containing Ba, the entire inclusions are lowered in melting point to be easily deformed, and before hot rolling or during hot rolling. Even when the phase was separated during heating, SiO ₂ was hardly generated, and a Si-killed steel wire for obtaining a spring having excellent fatigue characteristics could be realized.

  It is known that in a wire rod having a large deformation ratio at the time of hot rolling, it is useful to make the inclusions fine by stretching and cutting at the time of hot rolling. Under these circumstances, the present inventors considered the composition and form of individual inclusions for improving the fatigue characteristics of the spring, taking into account changes in the form of inclusions due to heating after solidification and hot rolling. Considered from various angles. As a result, it has been found that by appropriately controlling the concentrations of Ba, Al, Si, Mg, and Ca, deformation during hot rolling of oxide inclusions is remarkably promoted and becomes finer.

Conventionally, it is known that addition of trace amounts of alkaline earth metal elements such as Ba, Sr, Mg, and Ca is effective in improving the spring characteristics (for example, Patent Document 3). However, it is not necessary to add a trace amount, and it has been found that the fatigue characteristics of the Si-killed steel wire can be remarkably improved by containing them in a balanced manner. For example, a CaO—Al ₂ O ₃ —SiO ₂ ternary inclusion is known to have a low melting point region in a certain composition range in a ternary phase diagram generally known. However, in a composition in which any of the components is high, the melting point of the inclusions is increased and the fatigue properties of the wire are lowered. On the other hand, by appropriately controlling the concentrations of Ba, Al, Si, Mg, and Ca, none of the components in the ternary inclusions become too high, and there is no component. It is considered that the inclusions are more easily deformed than those.

  The Si killed steel wire rod according to the present invention is characterized by containing components such as Ba, Al, Si, Mg, and Ca in a balanced manner as described above. The reasons for limiting the ranges of these components are as follows. It is.

[Ba: 0.03 to 30 ppm]
Ba is a component that is indispensable for complexing inclusions to lower the melting point. Inclusion of BaO in the inclusion has the effect of lowering the melting point without reducing the stability of the glass so much. Further, by containing Ba having a strong binding force with oxygen in steel having a high Si concentration, there is an effect of maintaining a certain melting point even when inclusions having a remarkably high SiO ₂ concentration are formed during solidification. In order to exhibit these effects, at least 0.03 ppm of Ba is required. Preferably it is 0.2 ppm or more. On the other hand, if the Ba concentration becomes too high, the concentration of the other components (Mg, Ca, Al, Si, Mn, etc.) of the inclusions is lowered, and the composition with the lowest melting point cannot be controlled. Therefore, the Ba concentration needs to be 30 ppm or less, preferably 10 ppm or less.

[Al: 1 to 30 ppm]
Al has the effect of lowering the melting point of the inclusion composition of Si killed steel. In addition, there is an effect of controlling vitrification when the concentration of CaO or the like in the inclusion is increased. Furthermore, Al is a component that is more easily dissolved in steel than Ca, Ba, and the like, and has a high effect of suppressing the formation of inclusions having a significantly high SiO ₂ concentration during solidification. In order to exert these effects, it is necessary to contain 1 ppm or more. However, if the Al content is high, there is a risk that pure Al ₂ O ₃ is generated during solidification, so it is necessary to set it to 30 ppm or less. In addition, in order to control to the optimal composition which lowers the melting | fusing point of an inclusion most, it is preferable to set it as 20 ppm or less.

[Si: 0.2-4%]
Si is a main deoxidizer during the production of Si killed steel, and is an essential element for obtaining the wire of the present invention. It is also an important element in that it contributes to higher strength and the remarkable effect of improving the fatigue characteristics of the present invention. Further, it is an element useful for increasing softening resistance and improving sag resistance. In order to exert such effects, the Si content is 0.2% or more (preferably 2% or more). However, if the Si content is excessive, pure SiO ₂ may be generated during solidification, and surface decarburization and surface flaws increase, so that the fatigue characteristics are deteriorated. For these reasons, Si is made 4% or less, preferably 3% or less.

[Mg and / or Ca: 0.5 to 30 ppm in total]
Mg and Ca are indispensable components for making inclusions into an optimal composite composition and lowering the melting point. If Ba alone, Mg alone, Ca alone, or Al alone is contained, inclusions with a high melting point are obtained. Therefore, any of them must be contained. In addition, Mg and Ca have a strong affinity for oxygen, and when pure SiO ₂ is rarely produced, there is an effect that they can be easily modified into a composite composition. In order to exert these effects, the content of Mg and Ca (Mg, Ca alone or in combination) (the total content when used in combination) needs to be 0.5 ppm or more. Preferably, each element is used in combination by containing at least 0.1 ppm or more (however, the total content is 0.5 ppm or more). However, when these elements become excessive, the concentration of other components in the inclusions becomes low, and an optimum low melting point composition cannot be maintained. Therefore, the upper limit is 30 ppm (preferably 20 ppm or less).

  In the Si killed steel wire rod of the present invention, the fatigue characteristics are improved by containing each of the above components in a balanced manner, but it is also useful to contain Li if necessary. Li has the effect of making the crystals in the inclusions finer, and in the steel of the present invention in which the glass is stable and controlled to have a low melting point, it has the effect of preventing the crystals from becoming coarse even if crystals are produced. Therefore, it is also useful to contain Li. In order to exert such an effect, it is preferable to contain Li in an amount of 0.2 to 20 ppm. However, it is considered that some effects are exhibited even by addition of about 0.03 ppm, and at least an adverse effect is exerted by addition of a low concentration. Presumed not.

  In the present invention, Si killed steel wire useful as a spring material is assumed, and the steel type is not particularly limited, but Mn is an element contributing to deoxidation of steel, Increases hardenability and contributes to strength improvement. From such a viewpoint, it is preferable that Mn is contained by 0.1% or more. However, if the Mn content is excessive, the toughness and ductility deteriorate, so it should be 2% or less.

  About C content which is a basic component as spring steel, it is preferable that it is 1.2% or less. If the C content exceeds 1.2%, the steel material becomes brittle and becomes impractical.

  In addition to the above basic components, Fe and inevitable impurities (for example, S of 0.02% or less, P of 0.02% or less, etc.), Cr, Ni, V, Nb, Mo, W, Cu, One or more selected from the group consisting of Ti, Co, and rare earth elements (REM) may be included. The preferable content when these are contained varies depending on each element, but Cr: 0.5 to 3%, Ni: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less Mo: 0.5% or less, W: 0.5% or less, Cu: 0.1% or less, Ti: 0.1% or less, Co: 0.5% or less, REM: 0.05% or less .

  As described above, a spring having excellent fatigue characteristics can be realized by forming a spring using a Si-killed steel wire whose chemical composition is appropriately adjusted.

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

  The experiment was performed on a real machine (or laboratory level). That is, in the actual machine, the molten steel melted in the converter is put into a ladle (in the laboratory, 500 kg of molten steel simulating the molten steel discharged from the converter is melted), and various fluxes are added to the components. Adjustment, electrode heating, and argon bubbling were performed, and molten steel treatment (slag refining) was performed. Moreover, after adjusting other components, Ca, Mg, Ce, Ba, Li, etc. were added during the molten steel process as needed, and were hold | maintained for 5 minutes or more. The obtained steel ingot was forged and hot-rolled to obtain a wire having a diameter of 8.0 mm.

  About each obtained wire, content of Ba and Li in steel was measured with the following method, and the evaluation test by the rotation bending fatigue test which simulated the valve spring was done.

[Contents of Ba and Li in Steel]
1) When the content is 0.2 ppm (mg / kg) or more (lower limit of determination is 0.2 ppm)
A 0.5 g sample was taken from the target wire, taken into a beaker, and heated and decomposed by adding pure water, hydrochloric acid and nitric acid. After standing to cool, it was transferred to a 100 mL (milliliter) volumetric flask to prepare a measurement solution. This measurement solution was diluted with pure water, and Ba and Li were quantitatively analyzed using an ICP mass spectrometer (model SPQ8000: manufactured by Seiko Instruments Inc.).

2) When the content is less than 0.2 ppm (mg / kg) (lower limit of determination is 0.03 ppm)
A 0.5 g sample was collected from the target wire and taken in a beaker, and hydrolyzed by adding pure water, hydrochloric acid and nitric acid. Thereafter, hydrochloric acid was added to adjust the acid concentration, methyl isobutyl ketone (MIBK) was added and shaken, and iron was extracted into the MIBK phase. After standing, only the aqueous phase was taken out and transferred to a 100 mL volumetric flask to obtain a measurement solution. This measurement solution was diluted with pure water, and Ba and Li were quantitatively analyzed under the above conditions using an ICP mass spectrometer (model SPQ8000: manufactured by Seiko Instruments Inc.).

[Fatigue strength test (breaking rate)]
For each hot-rolled wire (diameter: 8.0 mm), shaving (diameter: 7.4 mm) → patenting → cold drawing (diameter: 4 mm) → oil temper [oil quenching and lead bath (about 450 ° C.) A wire having a diameter of 4.0 mm × 650 mm was produced in the continuous tempering step]. The obtained wire was subjected to strain relief annealing treatment (400 ° C.) → shot peening → low temperature annealing 200 ° C., then using a Nakamura rotary bending tester, nominal stress of 908 MPa, rotation speed: 4000 to 5000 rpm, discontinued Number of times: The test was conducted at 2 × 10 ⁷ times. And the fracture | rupture rate was calculated | required by the following formula about what the inclusion broke among the fracture | ruptured things.
Breaking rate (%) = [number of inclusions broken / (number of inclusions broken + number of broken inclusions reached a predetermined number of times)] × 100

These results are shown in Table 1 below together with the chemical composition of each wire. In addition, about elements other than Ba and Li, it measured by the following method.
C: Combustion infrared absorption method Si, Mn, Ni, Cr, V and Ti: ICP emission spectrometry Al, Mg, Zr and REM: ICP mass spectrometry Ca: Flameless atomic absorption analysis O: Inert gas melting method

  From these results, it can be considered as follows. Test No. In the case of 1 to 3, 5, 6, 9, 10, 13, 18 to 27, the chemical component composition is appropriate, and the inclusion composition is also controlled within an appropriate range, and good fatigue strength is obtained. I understand that

  In contrast, test no. In the case of 4, 7, 8, 11, 12, 14-17, the chemical composition composition is outside the appropriate range, and the inclusion composition is not controlled within the appropriate range, so the fatigue test result is good. Absent.

  For details, see test no. In Nos. 4 and 7, the concentrations of Ba, Ca and Mg are appropriately controlled, but the Al concentration is high or low, and the fracture rate is high.

  Test No. In 8, 11 and 12, the concentrations of Al, Ca and Mg are appropriately controlled, but the Ba concentration is high or low, and the fracture rate is high.

  Test No. In 14 and 16, the concentrations of Ba and Al are appropriate, but the concentrations of Ca and Mg are low and the fracture rate is high.

  Test No. In 15 and 17, the concentrations of Ba and Al are appropriate, but the concentrations of Ca and Mg are too high and the fracture rate is high. Test No. No. 18 shows that the Li concentration is outside the preferred upper limit. The effect is saturated compared to 19 items.

  Thus, it can be seen that it is necessary to appropriately control all of Ba, Ca, Mg and Al.

Claims (4)

  C: 1.2% (meaning “mass%”, the same shall apply hereinafter) or less (excluding 0%), Mn: 0.1 to 2.0%, Ba: 0.03 to 30 ppm (of “mass ppm” Meaning, the same applies hereinafter), Al: 1 to 30 ppm and Si: 2 to 4%, respectively, and Mg and / or Ca in a total range of 0.5 to 30 ppm, with the balance being Fe and inevitable impurities Si-killed steel wire rod with excellent fatigue characteristics.   The Si-killed steel wire rod according to claim 1, which contains Li in a range of 0.03 to 20 ppm. Further, Cr : 0.5 to 3% , Ni : 0.5% or less (not including 0%) , V : 0.5% or less (not including 0%) , Nb : 0.1% or less (0 ) , Mo : 0.5% or less (not including 0%) , W : 0.5% or less (not including 0%) , Cu : 0.1% or less (not including 0%) , Ti : 0.1% or less (not including 0%) and Co : 0.5% or less (not including 0%) , containing one or more elements selected from the group consisting of: The Si killed steel wire according to 1 or 2.   A spring obtained from the Si-killed steel wire rod according to any one of claims 1 to 3.