Si-killed steel wire rod and spring

Abstract

A Si-killed steel wire rod for obtaining a spring excellent in fatigue properties and a spring excellent in fatigue properties obtained from such steel wire rod are provided. In the Si-killed steel wire rod of the present invention, oxide-based inclusions present in the wire rod contain SiO.sub.2: 30-90%, Al.sub.2O.sub.3: 2-35%, MgO: 35% or below (not inclusive of 0%), CaO: 50% or below (not inclusive of 0%), MnO: 20% or below (not inclusive of 0%) and BaO: 0.2-20% respectively, and total content of (CaO+MgO) is 3% or above.

Claims

1. A Si-killed steel wire rod comprising: steel and oxide-based inclusions, wherein the Si-killed steel wire rod comprises greater than 0 mass % to 1.2 mass % of C, 0.1-4.0 mass % of Si, 0.1-2.0 mass % of Mn, 1-30 ppm of Al, and 0.5-30 ppm in total of Mg and/or Ca, based on the total mass of the Si-killed steel wire rod, and the oxide-based inclusions in steel comprise 30-90 mass % of SiO.sub.2, 2-35 mass % of Al.sub.2O.sub.3, greater than 0 mass % to 35 mass % of MgO, greater than 0 mass % to 50 mass % of CaO, greater than 0 mass % to 20 mass % of MnO and 0.2-20 mass % of BaO, and the total content of MgO and CaO is 3% or greater, each based on the total mass of the oxide-based inclusions, and wherein the balance is Fe and inevitable impurities.

2. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel wire rod further comprises 0.1-20 mass % of Li.sub.2O as the oxide-based inclusion, based on the total mass of the oxide-based inclusions, and wherein a content of Li in the Si-killed steel wire rod is 0.03-20 ppm, based on the total mass of Si-killed steel wire rod.

3. The Si-killed steel wire rod of claim 1, further comprising at least one element selected from the group consisting of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Co, and a rare earth element, wherein a content of Cr is 0.5-3.0 mass %, of Ni is 0.5 mass % or less, of V is 0.5 mass % or less, of Nb is 0.1 mass % or less, of Mo is 0.5 mass % or less, of W is 0.5 mass % or less, of Cu is 0.1 mass % or less, of Ti is 0.1 mass % or less, of Co is 0.5 mass % or less, and of the rare earth element is 0.05% or less, based on the total mass of the Si-killed steel wire rod.

4. A spring obtained from the Si-killed steel wire rod of claim 1.

5. The Si-killed steel wire rod of claim 1, wherein a content of Ba in the Si-killed steel wire rod is 0.03-30 ppm, based on the total mass of the Si-killed steel wire rod.

6. The Si-killed steel wire rod of claim 2, wherein a content of Li in the Si-killed steel wire rod is 0.2-20 ppm, based on the total mass of the Si-killed steel wire rod.

7. The Si-killed steel wire rod of claim 1, wherein the oxide-based inclusions comprise 1-10 mass % of BaO, based on the total mass of the oxide-based inclusions.

8. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel wire rod further comprises 15 mass % or less of SrO, as the oxide-based inclusion, based on the total mass of the oxide-based inclusions.

9. The Si-killed steel wire rod of claim 2, wherein the Si-killed steel wire rod comprises 2.0-20 mass % of Li.sub.2O, as the oxide-based inclusion, based on the total mass of the oxide-based inclusions.

10. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel rod comprises 1.4-4.0 mass % of Si, based on the total mass of the Si-killed steel rod.

11. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel rod comprises 1.9-4.0 mass % of Si, based on the total mass of the Si-killed steel rod.

12. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel rod comprises 1.4-3.0 mass % of Si, based on the total mass of the Si-killed steel rod.

13. The Si-killed steel wire rod of claim 1, wherein a content of Ba in the Si-killed steel wire rod is 0.2-10 ppm, based on the total mass of the Si-killed steel rod.

14. The Si-killed steel wire rod of claim 1, wherein the Si-killed steel wire rod comprises 7-20 mass % of BaO, as the oxide-based inclusion, based on the total mass of the oxide-based inclusions.

Description

BEST MODE FOR CARRYING OUT THE INVENTION

(1) It is known that, in the wire rod with large deformation ratio in hot rolling, refinement of inclusions by extending tearing off in hot rolling is useful. Under such circumstance, the present inventors made investigations from various angles on the composition and forms of each inclusion for improving fatigue strength of springs with variation in form of inclusions by heating after solidification and heat rolling also taken into consideration. As a result, it was found out that, by properly controlling concentration of BaO, Al.sub.2O.sub.3, SiO.sub.2, MgO, CaO and MnO and making the ratio of each oxide component in oxide-based inclusions appropriate, deformation of oxide-based inclusions in hot rolling was remarkably promoted and became easy to be refined.

(2) It is known that, in the wire rod with large deformation ratio in hot rolling, refinement of inclusions by extending tearing off in hot rolling is useful. Under such circumstance, the present inventors made investigations from various angles on the composition and forms of each inclusion for improving fatigue properties of springs with variation in form of inclusions by heating after solidification and heat rolling also taken into consideration. As a result, it was found out that, by properly controlling concentration of BaO, SrO, Al.sub.2O.sub.3, SiO.sub.2, MgO, CaO and MnO and making the ratio of each oxide component in oxide-based inclusions appropriate, deformation of oxide-based inclusions in hot rolling was remarkably promoted and became easy to be refined.

(3) It was known conventionally that to make the ratio of each oxide in oxide-based inclusions appropriate was effective for improving properties of steel (the Patent Documents 1-6, for example), however fatigue strength did not necessarily become excellent, and it was revealed that, by containing these components with excellent balance, fatigue properties of Si-killed steel wire rod could be remarkably improved. In CaO—Al.sub.2O.sub.3—SiO.sub.2 three-component based inclusions for example, it is known that a low melting point region is present in a composition area of three components in the three component system phase diagram which is generally known, however, in a composition where any of the components becomes high, the melting point of inclusions becomes high on the contrary and the fatigue properties of the wire rod are lowered.

(4) The Si-killed steel wire rod of the present invention is characterized in that the composition of oxide-based inclusions present in the wire rod is properly adjusted, and the reasons content of each oxide consisting oxide-based inclusions is stipulated are as described below.

(5) [BaO: 0.2-20%]

(6) BaO is a component indispensable for compositing inclusions and lowering the melting point. If BaO is contained in inclusions, there is an effect that stabilization of glass is not deteriorated much and the melting point is lowered. In order to exert these effects, 0.2% BaO is necessary in the minimum, preferably 1% or above. On the other hand, if concentration of BaO becomes excessively high, the melting point of inclusions becomes high on the contrary. Therefore, BaO should be made 20% or below (preferably 10% or below).

(7) [BaO and SrO: 0.2-20% in Total (However, SrO≦15%)]

(8) BaO and SrO are components indispensable for compositing inclusions and lowering the melting point. If BaO and SrO are contained in inclusions, there is an effect that stabilization of glass is not deteriorated much and the melting point is lowered. In order to exert these effects, 0.2% BaO and/or SrO in total (solely or using both) is necessary in the minimum, preferably 1% or above. On the other hand, if BaO concentration becomes excessively high, the melting point of inclusions becomes high on the contrary. Therefore, the total should be made 20% or below (preferably 10% or below). However, even if SrO content in the total exceeds 15%, the melting point of inclusions becomes high, therefore Sr in the total content should be made 15% or below.

(9) [SiO.sub.2: 30-90%]

(10) SiO.sub.2 is a component indispensable for making glass stable inclusions, and it is necessary by 30% in the minimum. On the other hand, if SiO.sub.2 content becomes excessive, a hard SiO.sub.2 crystal phase is formed and extending tearing off in hot rolling is hindered, therefore it should be made 90% or below.

(11) [Al.sub.2O.sub.3: 2-35%]

(12) Al.sub.2O.sub.3 has an effect of lowering the melting point of the composition of inclusions of Si-killed steel. Further, it has also an effect of inhibiting crystallization when concentration of CaO or the like in inclusions becomes high. In order to exert these effects, it is necessary to be contained by 2% or above. However, if content of Al.sub.2O.sub.3 becomes excessively high, Al.sub.2O.sub.3 crystals are formed in inclusions and extending tearing off in hot rolling is hindered, therefore it should be made 35% or below.

(13) [MgO: 35% or Below (not Inclusive of 0%), CaO: 50% or Below (not Inclusive of 0%), MgO+CaO: 3% or Above in Total Content]

(14) MgO and CaO are indispensable components for making inclusions of optimal composite composition and lowering the melting point. Either of MgO and CaO is of high melting point singly, but has an effect of lowering the melting point of SiO.sub.2-based oxide. In order to exert such an effect, 3% or above should be contained for either one or for total. However, if the concentration of them becomes excessively high, the melting point of inclusions becomes high, crystals of MgO, CaO are formed, and extending tearing off during hot rolling is hindered. Therefore there is an upper limit. Because there is a difference in crystal formation performance between MgO and CaO, the upper limit is different which is to be 35% or below for MgO and 50% or below for CaO.

(15) [MnO: 20% or Below (not Inclusive of 0%)]

(16) Although MnO has an effect of lowering the melting point of SiO.sub.2-based oxide, it is not rather realistic to control to high concentration in high-Si steel, therefore it was made 20% or below.

(17) In the Si-killed steel wire rod of the present invention, fatigue strength is improved by containing respective components described above with excellent balance, but it is also useful to contain Li.sub.2O according to necessity. The reasons of setting the range when Li.sub.2O is contained are as follows.

(18) [Li.sub.2O: 0.1-20%]

(19) Li.sub.2O has an effect of refining crystals in inclusions, and, in the steel of the present invention wherein glass is controlled stable and of low melting point, even if crystals were very exceptionally formed, it has an effect of preventing the crystals from becoming coarse. Therefore, it is also useful to contain Li.sub.2O. In order to exert such effects, it is preferable to contain Li.sub.2O by approximately 2% or above, it is considered that the effects are exerted to some degree even by addition by approximately 0.1%, and it is presumed that addition of low concentration at least does not cause a harmful incident. However, even if Li.sub.2O content exceeds 20% to be contained excessively, its effect saturates.

(20) A spring excellent in fatigue properties can be realized by forming the spring using a Si-killed steel wire rod whose respective component ratios in inclusions have been properly adjusted as described above.

(21) The present invention was developed on the assumption of a Si-killed steel wire rod useful as material for a spring, and its steel kind is not particularly limited, however, in order to control the composition of inclusions, it is preferable to contain Si and Mn which are deoxidizing components by 0.1 mass % or above. Si: 1.4% or above is more preferable and 1.9% or above is further more preferable. However, if these components are contained excessively, steel becomes easy to be embrittled, therefore they should be made 4.0% or below for Si and 2.0% or below for Mn.

(22) Although Al can be positively contained in order to perform composition control of oxide-based inclusions, if it is excessive, concentration of Al.sub.2O.sub.3 in inclusions becomes high and coarse Al.sub.2O.sub.3 which becomes the cause of wire breakage is possibly formed, therefore 0.01% or below is preferable.

(23) With respect to content of C which is a fundamental component as steel for a spring, 1.2% or below is preferable. If C content exceeds 1.2%, steel is embrittled and becomes impractical.

(24) Those other than above fundamental components are Fe and inevitable impurities (0.02% or below S, 0.02% or below P, or the like, for example), however if necessary, it may contain one or more kinds selected from a group consisting of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Co, and a rare earth element (REM). The preferable content when these are contained differs according to each element, which is, Cr: 0.5-3%, Ni: 0.5% or below, V: 0.5% or below, Nb: 0.1% or below, Mo: 0.5% or below, W: 0.5% or below, Cu: 0.1% or below, Ti: 0.1% or below, Co: 0.5% or below. Also, as an element for lowering the viscosity of inclusions and exerting the effect more, REM may be added by approximately 0.05% or below.

(25) It is known that, in the wire rod with large deformation ratio in hot rolling, refinement of inclusions by extending tearing off in hot rolling is useful. Under such circumstance, the present inventors made investigations from various angles on the composition and forms of each inclusion for improving fatigue properties of springs with variation in form of inclusions by heating after solidification and heat rolling also taken into consideration. As a result, it was found out that, by properly controlling concentration of Ba, Al, Si, Mg and Ca, deformation of oxide-based inclusions in hot rolling was remarkably promoted and became easy to be refined.

(26) Also, it is known that, in the wire rod with large deformation ratio in hot rolling, refinement of inclusions by extending tearing off in hot rolling is useful. Under such circumstance, the present inventors made investigations from various angles on the composition and forms of each inclusion for improving fatigue properties of springs with variation in form of inclusions by heating after solidification and heat rolling also taken into consideration. As a result, it was found out that, by properly controlling concentration of Ba, Sr, Al, Si, Mg, Ca, deformation of oxide-based inclusions in hot rolling was remarkably promoted and became easy to be refined.

(27) It was known conventionally that addition of a fine amount of an alkaline-earth metal element such as Ba, Sr, Mg, Ca, or the like was useful for improvement of properties of a spring (the Patent Document 6, for example), however it was revealed that addition of a fine amount without consideration on the kind of component did not work, but fatigue strength of a Si-killed steel wire rod could be remarkably improved by containing them with excellent balance. In CaO—Al.sub.2O.sub.3—SiO.sub.2 three-component based inclusions for example, it is known that a low melting point region is present in a composition area of three components in the three component system phase diagram which is generally known, however, in a composition where any of the components becomes high, the melting point of inclusions becomes high on the contrary and the fatigue properties of the wire rod are lowered. On the other hand, it is considered that, by properly controlling concentration of Ba, Al, Si, Mg, Ca, any component in the three-component based inclusions described above does not become excessively high, and the inclusions become more easily deformed compared with the case where any of the components is lacking.

(28) Also, it was known conventionally that addition of a fine amount of an alkaline-earth metal element such as Ba, Sr, Mg, Ca, or the like was useful for improvement of properties of a spring (the Patent Document 6, for example), however it was revealed that addition of a fine amount without consideration on the kind of component did not work, but fatigue properties of a Si-killed steel wire rod could be remarkably improved by containing them with excellent balance. In CaO—Al.sub.2O.sub.3—SiO.sub.2 three-component based inclusions for example, it is known that a low melting point region is present in a composition area of three components in the three component system phase diagram which is generally known, however, in a composition where any of the components becomes high, the melting point of inclusions becomes high on the contrary and the fatigue properties of the wire rod are lowered. On the other hand, it is considered that, by properly controlling concentration of Ba, Sr, Al, Si, Mg, Ca, any component in the three-component based inclusions described above does not become excessively high, and the inclusions become more easily deformed compared with the case where any of the components is lacking.

(29) As described above, the Si-killed steel wire rod of the present invention is characterized by containing components such as Ba, Al, Si, Mg and Ca with excellent balance, and the reasons of limiting the range of these components will be described below.

(30) Also, as described above, the Si-killed steel wire rod of the present invention is characterized by containing components such as Ba, Sr, Al, Si, Mg, Ca with excellent balance, and the reasons of limiting the range of these components are as described below.

(31) [Ba: 0.03-30 ppm]

(32) Ba is a component indispensable for compositing inclusions and lowering the melting point. If BaO is contained in inclusions, there is an effect that stability of glass is not lowered much and the melting point is lowered. Also, if Ba, which has strong bonding force with oxygen, is contained in steel with high Si concentration, there is an effect that, even if inclusions with extremely high SiO.sub.2 concentration are formed in solidification, the melting point of a certain degree can be maintained. In order to exert these effects, 0.03 ppm Ba is necessary in the minimum. It is preferable to contain 0.2 ppm or above. On the other hand, if concentration of Ba becomes excessively high, concentration of other components of inclusions (Mg, Ca, Al, Si, Mn, or the like) is lowered, and controlling to the composition where the melting point becomes lowest becomes impossible. Therefore, concentration of Ba should be made 30 ppm or below, preferably 10 ppm or below.

(33) [Ba and Sr: 0.04-30 ppm in Total (However, Sr≦20 ppm)]

(34) Ba and Sr are components indispensable for compositing inclusions and lowering the melting point. If BaO and SrO are contained in inclusions, there is an effect that stabilization of glass is not deteriorated much and the melting point is lowered. Also, even if inclusions with extremely high SiO.sub.2 concentration are formed in solidification, by containing Ba and Sr, which have strong bonding force with oxygen, in steel with high Si concentration, there is an effect that, the melting point of a certain degree can be maintained. In order to exert these effects, 0.04 ppm Ba and Sr are necessary in the minimum (total). It is preferable to contain 0.2 ppm or above. On the other hand, if the concentration of Ba and Sr becomes excessively high, concentration of other components of inclusions (Mg, Ca, Al, Si, Mn, or the like) is lowered, and controlling to the composition where the melting point becomes lowest becomes impossible. Therefore, concentration of Ba and Sr should be made 30 ppm or below, preferably 10 ppm or below. However, if Sr content out of the total content exceeds 20 ppm, above inconvenience is liable to occur, therefore Sr content should be 20 ppm or below.

(35) [Al: 1-30 ppm]

(36) Al has an effect of lowering the melting point of the composition of inclusions of Si-killed steel. Further, there is also an effect of controlling vitrification when concentration of CaO or the like in inclusions becomes high. Furthermore, Al is a component easily dissolved in steel compared with Ca, Ba, or the like, and the effect of inhibiting formation of inclusions with extremely high SiO.sub.2 concentration in solidification is excellent. In order to exert these effects, it is necessary to be contained by 1 ppm or above. However, if Al content becomes high, there is a risk of forming pure Al.sub.2O.sub.3 in solidification, therefore it is necessary to make it 30 ppm or below. Also, in order to control to an optimal composition where the melting point of inclusions is lowered most, it is preferable to make it 20 ppm or below.

(37) Also, Al has an effect of lowering the melting point of the composition of inclusions of Si-killed steel. Further, there is also an effect of controlling vitrification when concentration of CaO or the like in inclusions becomes high. Furthermore, Al is a component easily dissolved in steel compared with Ca, Sr, Ba, or the like, and the effect of inhibiting formation of inclusions with extremely high SiO.sub.2 concentration in solidification is excellent. In order to exert these effects, it is necessary to be contained by 1 ppm or above. However, if Al content becomes high, there is a risk of forming pure Al.sub.2O.sub.3 in solidification, therefore it is necessary to make it 30 ppm or below. Also, in order to control to an optimal composition where the melting point of inclusions is lowered most, it is preferable to make it 20 ppm or below.

(38) [Si: 0.2-4%]

(39) Si is a main oxidizing agent in steel making of Si-killed steel and is an indispensable element for obtaining the wire rod of the present invention. Further, it contributes also to high strengthening and is an important element from the point that the effect of improving fatigue properties of the present invention is exerted remarkably. Furthermore, it is a useful element for enhancing softening resistance and improving setting resistance properties as well. In order to exert such effects, Si content is to be made 0.2% or above (preferably 2% or above). However, if Si content becomes excessive, pure SiO.sub.2 may possibly be formed during solidification, and surface decarburization and surface flaws increase, therefore fatigue properties lower on the contrary. Consequently, Si is to be made 4% or below, preferably 3% or below.

(40) [Mg and/or Ca: 0.5-30 ppm in Total]

(41) Mg and Ca are indispensable components for making inclusions of optimal composite composition and lowering the melting point. If containing Ba solely, Mg solely, Ca solely, Al solely, inclusions become of high melting point. Therefore, it is necessary to surely contain some of them. Further, Mg and Ca have strong affinity against oxygen, and have also an effect that, when pure SiO.sub.2 is formed exceptionally, it is easily reformed to a composite composition. In order to exert these effects, content (total content if both are used) of Mg and Ca (Mg, Ca solely or using both) necessarily is to be made 0.5 ppm of above. Also, it is preferable to contain both of them with each element by at least 0.1 ppm or above (total content however is 0.5 ppm or above). However, if these elements become excessive, concentration of other elements in inclusions becomes low, and optimal low melting point composition cannot be kept. Therefore, its upper limit is to be made 30 ppm (preferably 20 ppm or below).

(42) In the Si-killed steel wire rod of the present invention, fatigue properties are improved by containing respective components described above with excellent balance, but it is also useful to contain Li according to necessity. Li has an effect of refining crystals in inclusions, and, in the steel of the present invention wherein glass is controlled stable and of low melting point, even if crystals were very exceptionally formed, it has an effect of preventing the crystals from becoming coarse. Therefore, it is also useful to contain Li. In order to exert such effects, it is preferable to contain Li by 0.2-20 ppm, however, it is considered that some effects are exerted to some degree even by addition by approximately 0.03 ppm, and it is presumed that addition of low concentration at least does not exert a harmful influence.

(43) The present invention was developed on the assumption of a Si-killed steel wire rod useful as material for a spring, and its steel kind is not particularly limited, but Mn is an element contributing to deoxidation of steel, and improves quenchability and contributes to enhancing the strength. From such viewpoint, it is preferable to contain Mn by 0.1% or above. However, if Mn content becomes excessive, toughness and ductility are deteriorated, therefore it should be made 2% or below.

(44) With respect to content of C which is a fundamental component as steel for a spring, 1.2% or below is preferable. If C content exceeds 1.2%, steel is embrittled and becomes impractical.

(45) Those other than above fundamental components are Fe and inevitable impurities (0.02% or below S, 0.02% or below P, or the like, for example), however if necessary, it may contain one or more kinds selected from a group consisting of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Co, and a rare earth element (REM). The preferable content when these are contained differs according to each element, which is, Cr: 0.5-3%, Ni: 0.5% or below, V: 0.5% or below, Nb: 0.1% or below, Mo: 0.5% or below, W: 0.5% or below, Cu: 0.1% or below, Ti: 0.1% or below, Co: 0.5% or below, REM: 0.05% or below.

(46) A spring excellent in fatigue properties can be realized by forming the spring using a Si-killed steel wire rod whose chemical components are properly adjusted as described above.

(47) Although the present invention is described below further specifically by referring to the examples, the present invention is by no means limited by the examples below and can of course be implemented with modifications properly added within the scope adaptable to the purposes described above and below, and any of them is to be included within the technical range of the present invention.

EXAMPLE 1

(48) The experiment was performed with actual machines or on a laboratory level. That means, with the actual machines, molten steel smelted by a converter was discharged to a ladle (molten steel of 500 kg imitating the molten steel discharged from a converter was smelted, in a laboratory), various flux was added, component adjustment, electrode-heating (and argon bubbling) were performed, and a smelting treatment (slag refining) was performed. Also, alloy elements such as Ca, Mg, Ce, Ba, Li, or the like were added during the smelting treatment according to necessity. Then, the molten steel was casted and made a steel ingot (was casted by a mold which could obtain the cooling speed equivalent to the actual machines, on a laboratory level). A steel ingot obtained was forged and hot rolled, and a steel wire rod of a diameter: 8.0 mm was made.

(49) For each steel wire rod obtained, the composition of oxide-based inclusions in steel was measured and an evaluation test by a rotary bending fatigue test imitating a valve spring was performed. These measuring methods are as described below.

(50) [Composition of Inclusions (but Excluding Li.sub.2O)]

(51) An L-section (a section including the axis) of each hot rolled steel wire rod was ground, composition analysis was performed for 300 oxide-based inclusions present on the ground section by an EPMA (Electron Probe Micro Analyzer), and the average value was obtained after converted to oxide. Also, those with 5% or below concentration of S were regarded as oxide-based inclusions. The measuring condition of the EPMA then is as described below. EPMA apparatus: JXA-8621MX (made by JEOL Ltd.) Analyzer (EDS): TN-5500 (made by Tracor Northern) Acceleration voltage: 20 kV Scanning current: 5 nA Measuring method: Quantitative analysis by energy dispersion analysis (measuring the entire area of a particle)

(52) [Measurement of Li.sub.2O]

(53) Because concentration of Li.sub.2O in inclusions could not be measured by the EPMA, an analyzing method by SIMS (Secondary Ion Mass Spectroscopy) was originally developed and the measurement was performed in a procedure described below.

(54) (1) Primary Standard Sample 1) First, concentration of each CaO, MgO, Al.sub.2O.sub.3, MnO, SiO.sub.2, SrO or the like of inclusions in steel is analyzed by an EDX, EPMA or the like. 2) The synthesized oxide with the composition same to the composition of inclusions other than Li.sub.2O and the synthesized oxide added with various Li.sub.2O to them are prepared in a large number, concentration of Li.sub.2O of them are quantitatively analyzed by chemical analysis, and standard samples are prepared. 3) The relative secondary ion strength of Li against Si of each synthesized oxide prepared is measured. 4) A calibration curve of the relative secondary ion strength of Li against Si and concentration of Li.sub.2O chemically analyzed in 1) above is drawn.

(55) (2) Secondary Standard Sample (for Measuring Environment Correction) 5) For environment correction purpose in measuring, a standard sample wherein Li ions have been ion-implanted on a Si wafer is prepared separately, the relative secondary ion strength of Li against Si is measured, and correction is done when above 2) is performed.

(56) (3) Actual Measurement 6) The relative secondary ion strength of Li against Si of inclusions in steel is measured, and concentration of Li.sub.2O is obtained by the calibration curve obtained in 4) above.

(57) [Fatigue Strength Test (Rupture Ratio)]

(58) For each hot rolled wire rod (diameter: 8.0 mm), stripping (diameter: 7.4 mm).fwdarw.patenting.fwdarw.cold wire drawing (diameter: 4 mm).fwdarw.oil tempering [oil quenching and lead bathing (approximately 450° C.) tempering continuous process] were performed and a wire with 4.0 mm diameter×650 mm was manufactured. The wire obtained was subjected to treatment equivalent to strain relieving annealing (400° C.).fwdarw.shot peening.fwdarw.low temperature annealing, thereafter the test was performed using a Nakamura Method rotational bending tester with 908 MPa nominal stress, rotational speed: 4,000-5,000 rpm, number of times of stoppage: 2×10.sup.7 times. Then, for those the breakage was caused by inclusions out of those ruptured, the rupture ratio was obtained by the equation below.
Rupture ratio (%)=[number of samples broken by inclusions/(number of samples broken by inclusions+number of samples wherein the test was stopped after attaining prescribed number of times)]×100

(59) The chemical componential compositions of the steel wire rods are shown in Table 1 below along with the slag composition in smelting, and the composition of inclusions and fatigue properties (rupture ratio) of each steel wire rod are shown in Table 2 below respectively.

(60) TABLE-US-00001 TABLE 1 Test Chemical componential composition* (mass %) Slag composition (mass %) No. C Si Mn P S Others CaO Al.sub.2O.sub.3 SiO.sub.2 MnO MgO BaO LiO.sub.2 1 0.6 2.2 0.5 0.01 0.01 — 35 15 35 3 3 5 tr 2 0.8 1.5 0.7 0.01 0.01 — 20 15 48 6 3 5 tr 3 0.6 2.2 0.7 0.01 0.01 — 5 1 80 2 3 5 tr 4 0.6 2.2 0.5 0.01 0.01 — 10 3 46 2 30 5 tr 5 0.7 1.6 0.7 0.01 0.01 — 10 37 32 2 10 5 tr 6 0.7 1.6 0.7 0.01 0.01 — 20 29 37 2 3 5 tr 7 0.6 1.9 0.9 0.01 0.01 — 45 1 37 2 3 10 tr 8 0.6 1.9 0.9 0.01 0.01 — 45 1 35 2 3 10 tr 9 0.6 2.2 0.5 0.01 0.01 — 30 12 30 2 3 20 tr 10 0.6 2.2 0.5 0.01 0.01 — 30 10 32 2 3 20 tr 11 0.5 2.0 0.5 0.01 0.01 — 30 15 45 2 3 1 tr 12 0.8 2.0 0.7 0.01 0.01 — 29 15 44 4 3 1 tr 13 0.8 2.0 0.3 0.01 0.01 — 2 6 42 2 39 5 tr 14 0.6 2.2 0.6 0.01 0.01 — 51 5 33 2 3 5 tr 15 0.8 2.2 0.5 0.01 0.01 — 3 20 56 2 3 13 tr 16 0.6 2.1 0.5 0.01 0.01 — 33 10 33 2 3 10 5 17 0.6 2.0 0.4 0.01 0.01 — 35 10 35 2 3 10 1 18 0.6 2.2 0.7 0.01 0.01 — 2 1 78 2 3 5 5 19 0.6 2.2 0.5 0.01 0.01 — 19 6 42 2 3 5 20  20 0.6 2.0 0.9 0.01 0.01 Cr: 0.9, Ni: 0.25, 35 10 30 2 3 10 tr V: 0.1 21 0.6 1.5 0.7 0.01 0.01 Cr: 0.65, V: 0.1 20 15 45 5 3 5 tr 22 0.6 3.0 0.5 0.01 0.01 V: 0.5, Mo: 0.3 2 1 80 2 3 5 tr 23 1.0 2.2 2.0 0.01 0.01 Nb: 0.1, 10 5 45 2 25 10 tr Ce: 0.0005, Ti: 0.01 *Balance: Iron and inevitable impurities

(61) TABLE-US-00002 TABLE 2 Rupture Test Inclusions composition (mass %) ratio No. CaO Al.sub.2O.sub.3 SiO.sub.2 MnO MgO BaO LiO.sub.2 (%) 1 32 14 37 2 3 7 — 2 2 18 16 52 6 1 2 — 4 3 2 3 87 1 2 5 — 4 4 7 8 48 2 24 8 — 4 5 8 42 33 2 8 5 — 21 6 19 33 34 2 2 5 — 4 7 42 3 39 2 4 10 — 4 8 40 1 39 1 4 9 — 22 9 26 13 33 1 1 33 — 22 10 31 12 34 1 2 26 — 5 11 26 17 47 2 3 0.4 — 5 12 25 17 48 2 4 0.1 — 18 13 2 9 48 1 37 2 — 24 14 52 5 33 1 2 5 — 22 15 1 19 57 2 1 14 — 22 16 32 15 38 1 2 7 3 0 17 33 14 38 2 2 7 0.1 3 18 2 2 79 1 1 5 5 2 19 16 13 47 1 2 2 18 4 20 33 15 38 2 3 7 — 2 21 18 16 52 6 1 2 — 4 22 2 3 84 1 1 5 — 4 23 7 8 49 2 24 8 — 4

(62) From these results, following consideration is possible. In those in Test Nos. 1-4, 6, 7, 10, 11, 16-23 in Tables 1, 2, it is understood that the composition of inclusions is properly controlled and excellent fatigue strength is obtained.

(63) On the other hand, in those in Test Nos. 5, 8, 9, 12-15 in Tables 1, 2, the composition in inclusions deviates from the region stipulated in the present invention, therefore the result of fatigue test is not good.

(64) More specifically, in Test Nos. 5, 8 in Tables 1, 2, although concentration of SiO.sub.2, CaO and MgO is properly controlled, concentration of Al.sub.2O.sub.3 is high or low, and the rupture ratio becomes high.

(65) In Test Nos. 9, 12 in Tables 1, 2, although the SiO.sub.2, CaO, MgO and Al.sub.2O.sub.3 is properly controlled, concentration of BaO is high or low, and the rupture ratio becomes high.

(66) In Test No. 13 in Tables 1, 2, although concentration of SiO.sub.2, CaO and Al.sub.2O.sub.3 is properly controlled, concentration of MgO is too high, and the rupture ratio becomes high.

(67) In Test No. 14 in Tables 1, 2, although concentration of SiO.sub.2, MgO and Al.sub.2O.sub.3 is properly controlled, concentration of CaO is too high, and the rupture ratio becomes high.

(68) In Test No. 15 in Tables 1, 2, although concentration of SiO.sub.2, MgO, Al.sub.2O.sub.3 and BaO is properly controlled, concentration of CaO+MgO is low, and the rupture ratio becomes high.

EXAMPLE2

(69) The experiment was performed with actual machines or on a laboratory level. That means, with the actual machines, molten steel smelted by a converter was discharged to a ladle (molten steel of 500 kg imitating the molten steel discharged from a converter was smelted, in a laboratory), various flux was added, component was adjusted, electrode-heating (and argon bubbling) was appropriately performed, and a smelting treatment (slag refining) was performed. Also, alloy metal such as Ca, Mg, Ce, Ba, Sr, Li, or the like was added during the smelting treatment according to necessity. Then, the molten steel was casted and made a steel ingot (was casted by a mold which could obtain the cooling speed equivalent to the actual machines, on a laboratory level). A steel ingot obtained was forged and hot rolled, and a steel wire rod of a diameter: 8.0 mm was made.

(70) For each steel wire rod obtained, the composition of oxide-based inclusions in the wire rod was measured and an evaluation test by a rotary bending fatigue test imitating a valve spring was performed. These measuring methods are as described below.

(71) [Composition of Inclusions (but Excluding Li.sub.2O)]

(72) An L-section (a section including the axis) of each hot rolled steel wire rod was ground, composition analysis was performed for 300 oxide-based inclusions present on the ground section by an EPMA (Electron Probe Micro analyzer), and the average value was obtained after converted to oxide. Also, those with 5% or below S concentration were regarded as oxide-based inclusions. The measuring condition of the EPMA then is as described below. EPMA apparatus: JXA-8621MX (made by JEOL Ltd.) Analyzer (EDS): TN-5500 (made by Tracor Northern) Acceleration voltage: 20 kV Scanning current: 5 nA Measuring method: Quantitative analysis by energy dispersion analysis (measuring the entire area of a particle)

(73) [Measurement of Li.sub.2O]

(74) Because concentration of Li.sub.2O in inclusions could not be measured by the EPMA, an analyzing method by SIMS (Secondary Ion Mass Spectroscopy) was originally developed and the measurement was performed in a procedure described below.

(75) (1) Primary Standard Sample 1) First, concentration of each CaO, MgO, Al.sub.2O.sub.3, MnO, SiO.sub.2, BaO, SrO or the like of inclusions in steel is analyzed by an EDX, EPMA or the like. 2) The synthesized oxide with the composition same to the composition of inclusions other than Li.sub.2O and the synthesized oxide added with various Li.sub.2O to them are prepared in a large number, concentration of Li.sub.2O of them are quantitatively analyzed by chemical analysis, and standard samples are prepared. 3) The relative secondary ion strength of Li against Si of each synthesized oxide prepared is measured. 4) A calibration curve of the relative secondary ion strength of Li against Si and concentration of Li.sub.2O chemically analyzed in 1) above is drawn.

(76) (2) Secondary Standard Sample (for Measuring Environment Correction) 5) For environment correction purpose in measuring, a standard sample wherein Li ions have been ion-implanted on a Si wafer is prepared separately, the relative secondary ion strength of Li against Si is measured, and correction is done when above 2) is performed.

(77) (3) Actual Measurement 6) The relative secondary ion strength of Li against Si of inclusions in steel is measured, and concentration of Li.sub.2O is obtained by the calibration curve obtained in 4) above.

(78) [Fatigue Strength Test (Rupture Ratio)]

(79) For each hot rolled wire rod (diameter: 8.0 mm), stripping (diameter: 7.4 mm).fwdarw.patenting.fwdarw.cold wire drawing (diameter: 4 mm).fwdarw.oil tempering [oil quenching and lead bathing (approximately 450° C.) tempering continuous process] were performed and a wire with 4.0 mm diameter×650 mm was manufactured. The wire obtained was subjected to treatment equivalent to strain relieving annealing (400° C.).fwdarw.shot peening.fwdarw.low temperature annealing, thereafter the test was performed using a Nakamura Method rotational bending tester with 908 MPa nominal stress, rotational speed: 4,000-5,000 rpm, number of times of stoppage: 2×10.sup.7 times. Then, for those the breakage was caused by inclusions out of those ruptured, the rupture ratio was obtained by the equation below.
Rupture ratio (%)=[number of samples broken by inclusions/(number of samples broken by inclusions+number of samples wherein the test was stopped after attaining prescribed number of times)]×100

(80) The chemical componential compositions of the steel wire rods are shown in Table 3 below along with the slag composition in smelting, and the composition of inclusions and fatigue properties (rupture ratio) of each steel wire rod are shown in Table 4 below respectively.

(81) TABLE-US-00003 TABLE 3 Test Chemical componential composition* (mass %) Slag composition (mass %) No. C Si Mn P S Others CaO Al.sub.2O.sub.3 SiO.sub.2 MnO MgO BaO SrO LiO.sub.2 1 0.6 2.2 0.5 0.01 0.01 — 34 13 33 3 3 3 5 tr 2 0.8 1.5 0.7 0.01 0.01 — 21 13 49 6 3 3 2 tr 3 0.6 2.2 0.7 0.01 0.01 — 5 1 80 1 3 3 3 tr 4 0.6 2.2 0.5 0.01 0.01 — 9 6 44 2 27 7 1 tr 5 0.7 1.6 0.7 0.01 0.01 — 10 37 29 2 10 2 5 tr 6 0.7 1.6 0.7 0.01 0.01 — 20 30 34 2 3 5 2 tr 7 0.6 1.9 0.9 0.01 0.01 — 44 3 34 2 3 5 5 tr 8 0.6 1.9 0.9 0.01 0.01 — 42 1 37 2 3 5 5 tr 9 0.6 2.2 0.5 0.01 0.01 — 24 9 31 1 2 17 14 tr 10 0.6 2.2 0.5 0.01 0.01 — 29 10 31 2 3 15 5 tr 11 0.5 2.0 0.5 0.01 0.01 — 29 17 40 2 3 1 1 tr 12 0.8 2.0 0.7 0.01 0.01 — 29 15 44 4 3 0.4 0.5 tr 13 0.8 2.0 0.3 0.01 0.01 — 2 6 46 2 40 1 1 tr 14 0.6 2.2 0.6 0.01 0.01 — 50 3 33 1 3 3 2 tr 15 0.8 2.2 0.5 0.01 0.01 — 1 20 53 2 3 13 4 tr 16 0.6 2.1 0.5 0.01 0.01 — 33 12 35 2 3 3 4 4 17 0.6 2.0 0.4 0.01 0.01 — 34 11 36 2 3 2 7 2 18 0.6 2.2 0.7 0.01 0.01 — 2 1 79 2 1 2 3 7 19 0.6 2.2 0.5 0.01 0.01 — 19 4 33 2 3 2 3 21  20 0.6 2.0 0.9 0.01 0.01 Cr: 0.9, Ni: 0.25, 34 10 37 2 3 4 5 tr V: 0.1 21 0.6 1.5 0.7 0.01 0.01 Cr: 2, V: 0.1 20 15 47 6 3 1 1 tr 22 0.6 3.0 0.5 0.01 0.01 V: 0.5, Mo: 0.3 5 1 80 2 3 5 5 tr 23 1.0 2.2 2.0 0.01 0.01 Nb: 0.1, 10 7 46 2 26 3 5 tr Ce: 0.0005, Ti: 0.01 *Balance: Iron and inevitable impurities

(82) TABLE-US-00004 TABLE 4 Rupture Test Inclusions composition (mass %) ratio No. CaO Al.sub.2O.sub.3 SiO.sub.2 MnO MgO BaO SrO LiO.sub.2 (%) 1 32 14 37 2 3 3 4 — 3 2 18 16 52 6 1 1 1 — 4 3 2 2 88 1 1 3 3 — 4 4 7 8 48 1 24 8 1 — 5 5 8 42 33 1 8 1 5 — 25 6 19 33 34 2 2 4 1 — 4 7 42 3 39 2 4 5 5 — 5 8 40 1 39 1 4 5 5 — 25 9 22 12 32 1 1 14 12 — 22 10 30 12 33 1 2 13 5 — 5 11 26 17 47 1 3 0.4 0.4 — 5 12 25 17 48 2 4 0.1 0.1 — 18 13 2 9 48 1 39 1 1 — 20 14 53 5 33 1 2 3 2 — 26 15 1 19 57 1 1 11 4 — 28 16 33 15 38 2 2 2 3 3 1 17 33 14 37 2 2 2 5 1 3 18 2 3 79 1 1 2 2 5 3 19 16 13 46 1 2 1 2 18  4 20 33 15 38 2 3 3 4 — 3 21 18 16 52 6 1 1 1 — 4 22 2 3 84 1 1 1 4 — 4 23 7 8 49 2 24 2 6 — 4

(83) From these results, following consideration is possible. In those in Test Nos. 1-4, 6, 7, 10, 11, 16-23 in Tables 3, 4, it is understood that the composition of inclusions is properly controlled and excellent fatigue strength is obtained.

(84) On the other hand, in those in Test Nos. 5, 8, 9, 12-15 in Tables 3, 4, the composition of inclusions deviates from the region stipulated in the present invention, therefore the result of fatigue test is not good.

(85) More specifically, in Test Nos. 5, 8 in Tables 3, 4, although concentration of SiO.sub.2, CaO and MgO is properly controlled, concentration of Al.sub.2O.sub.3 is high or low, and the rupture ratio becomes high.

(86) In Test Nos. 9, 12 in Tables 3, 4, total content of (BaO+SrO) is high or low, and the rupture ratio becomes high.

(87) In Test No. 13 in Tables 3, 4, although concentration of SiO.sub.2, CaO and Al.sub.2O.sub.3 is properly controlled, concentration of MgO is too high, and the rupture ratio becomes high.

(88) In Test No. 14 in Tables 3, 4, although concentration of SiO.sub.2, CaO and Al.sub.2O.sub.3 is properly controlled, concentration of CaO is too high, and the rupture ratio becomes high.

(89) In Test No. 15 in Tables 3, 4, although concentration of MgO, Al.sub.2O.sub.3 and SrO is properly controlled, the total of CaO+MgO is low, and the rupture ratio becomes high.

EXAMPLE 3

(90) The experiment was performed with actual machines (or on a laboratory level). That means, with the actual machines, molten steel smelted by a converter was discharged to a ladle (molten steel of 500 kg imitating the molten steel discharged from a converter was smelted, in a laboratory), various flux was added, component adjustment, electrode-heating, and argon bubbling were performed, and a smelting treatment (slag refining) was performed. Also, after other components were adjusted, Ca, Mg, Ce, Ba, Li or the like were added during the smelting treatment according to necessity to be maintained for 5 minutes or more. A steel ingot obtained was forged and hot rolled, and a wire rod of a diameter: 8.0 mm was made.

(91) For each wire rod obtained, Ba and Li content in steel were measured by a method described below, and an evaluation test by a rotary bending fatigue test imitating a valve spring was performed.

(92) [Ba and Li Content in Steel] 1) When content is 0.2 ppm (mg/kg) or above (0.2 ppm quantitative lower limit value)

(93) A 0.5 g sample was taken from a wire rod of an object, was put in a beaker, demineralized water, hydrochloric acid and nitric acid were added, and was thermally decomposed. After it was natural-cooled, was transferred into a 100 mL (milliliter) measuring flask, and was made a measuring solution. This measuring solution was diluted with demineralized water and Ba and Li were quantitatively analyzed using an ICP mass spectrometer (model SPQ8000: made by Seiko Instruments Inc.). 2) When content is below 0.2 ppm (mg/kg) (0.03 ppm quantitative lower limit value)

(94) A 0.5 g sample was taken from a wire rod of an object, was put in a beaker, demineralized water, hydrochloric acid and nitric acid were added, and hydrolysis was performed. Thereafter acid concentration was adjusted by adding hydrochloric acid, added with methyl isobutyl keton (MIBK), shaked, and the iron content was extracted to the MIBK phase. After left to stand, only the water phase was taken out, was transferred into a 100 mL measuring flask, and was made a measuring solution. This measuring solution was diluted with demineralized water, and Ba and Li were quantitatively analyzed with the condition described above using an ICP mass spectrometer (model SPQ8000: made by Seiko Instruments Inc.).

(95) [Fatigue Strength Test (Rupture Ratio)]

(96) For each hot rolled wire rod (diameter: 8.0 mm), stripping (diameter: 7.4 mm).fwdarw.patenting.fwdarw.cold wire drawing (diameter: 4 mm).fwdarw.oil tempering [oil quenching and lead bathing (approximately 450° C.) tempering continuous process] were performed and a wire with 4.0 mm diameter×650 mm was manufactured. The wire obtained was subjected to treatment equivalent to strain relieving annealing (400° C.).fwdarw.shot peening.fwdarw.200° C. low temperature annealing, thereafter the test was performed using a Nakamura Method rotational bending tester with 908 MPa nominal stress, rotational speed: 4,000-5,000 rpm, number of times of stoppage: 2×10.sup.7 times. Then, for those the breakage was caused by inclusions out of those ruptured, the rupture ratio was obtained by the equation below.
Rupture ratio (%)=[number of samples broken by inclusions/(number of samples broken by inclusions+number of samples wherein the test was stopped after attaining prescribed number of times)]×100

(97) These results are shown in Table 5 below along with the chemical componential composition of each wire rod. Also, with respect to the elements other than Ba and Li, measurement was performed in accordance with the methods described below.

(98) C: Burning infrared absorption method

(99) Si, Mn, Ni, Cr, V and Ti: ICP emission spectrometry method

(100) Al, Mg, Zr and REM: ICP mass spectrometry method

(101) Ca: Frameless atomic absorption spectrometry method

(102) O: Inert gas fusion method

(103) TABLE-US-00005 TABLE 5 Rupture Test Chemical componential composition (mass %, Al, Ba, Ca, Mg and Li are in mass ppm) ratio No. C Si Mn P S Al Ba Ca Mg Li Others (%) 1 0.6 2.2 0.5 0.01 0.01 8 3 6 0.3 — — 5 2 0.8 1.5 0.7 0.01 0.01 10 1.4 3 0 — — 6 3 0.8 0.2 0.5 0.01 0.01 5 2 0.5 3 — — 5 4 0.7 1.6 0.7 0.01 0.01 32 3 6 0.2 — — 38 5 0.6 2.4 0.3 0.01 0.01 24 10 1 6 — — 10 6 0.6 1.9 0.9 0.01 0.01 2 5 7 0.3 — — 10 7 0.7 1.5 0.7 0.01 0.01 0.4 6 10 0.1 — — 36 8 0.5 1.5 0.7 0.01 0.01 11 34 6 1 — — 49 9 0.7 1.5 0.7 0.01 0.01 18 27 6 0.3 — — 11 10 1.0 2.0 1.6 0.01 0.01 10 0.05 0.3 6 — — 10 11 0.5 2.0 0.9 0.01 0.01 6 0 5 0 — — 23 12 0.5 2.0 0.9 0.01 0.01 14 0 6 0.3 — — 21 13 0.6 2.4 0.4 0.01 0.02 20 27 15 10 — — 9 14 0.6 2.4 0.5 0.01 0.01 6 20 0 0 — — 31 15 0.9 1.6 0.7 0.01 0.01 8 12 16 19 — — 35 16 0.6 1.6 0.7 0.01 0.01 5 12 0.3 0 — — 28 17 0.6 1.6 0.7 0.01 0.01 3 7 33 0.3 — — 35 18 0.6 2.0 0.9 0.01 0.01 2 6 7 0.5 25 — 6 19 0.7 2.0 0.9 0.01 0.01 1 4 5 1 17 — 5 20 0.6 2.4 0.5 0.01 0.01 9 7 4 2 0.5 — 5 21 0.5 2.0 0.7 0.01 0.01 3 0.1 0 5 0.03 — 7 22 0.6 2.0 0.9 0.02 0.01 8 3 6 0.4 — Cr: 0.9, Ni: 0.25, 5 V: 0.1 23 0.6 1.5 0.7 0.01 0.02 10 1.5 3 0 — Cr: 0.65, V: 0.1 6 24 0.6 1.9 0.9 0.01 0.01 2 5 0.4 7 — V: 0.5, Mo: 0.3 9 25 0.6 2.4 0.4 0.01 0.01 20 27 15 10 — V: 0.5, Ti: 0.01, 8 W: 0.003 26 0.6 2.4 0.5 0.001 0.01 9 7 4 2 0.5 Cr: 3, Nb: 0.1, 5 Co: 0.01 27 0.8 1.5 0.7 0.01 0.01 10 1.5 3 0 — Ni: 0.5, Ca: 0.0005 6

(104) From these results, following consideration is possible. In those in Test Nos. 1-3, 5, 6, 9, 10, 13, 18-27 in Table 5, it is understood that the chemical componential composition is appropriate, and the composition of inclusions is controlled to a proper region and excellent fatigue strength is obtained.

(105) On the other hand, in those in Test Nos. 4, 7, 8, 11, 12, 14-17 in Table 5, the chemical componential composition deviates from a proper region and the composition of inclusions is not controlled to a proper region, therefore the result of fatigue test is not good.

(106) More specifically, in Test Nos. 4, 7 in Table 5, although concentration of Ba, Ca and Mg is properly controlled, concentration of Al is high or low, and the rupture ratio becomes high.

(107) In Test Nos. 8, 11, 12 in Table 5, although concentration of Al, Ca and Mg is properly controlled, concentration of Ba is high or low, and the rupture ratio becomes high.

(108) In Test Nos. 14, 16 in Table 5, although concentration of Ba and Al is appropriate, concentration of Ca and Mg is low, and the rupture ratio becomes high.

(109) In Test Nos. 15, 17 in Table 5, although concentration of Ba and Al is appropriate, concentration of Ca and Mg is excessively high, and the breakage ratio becomes high. Also, in Test No. 18 in Table 5, concentration of Li deviates from a preferable upper limit, however the effect saturates compared with the one in Test No. 19 in Table 5.

(110) Thus, it is understood that proper controlling all of Ba, Ca, Mg and Al is necessary.

EXAMPLE 4

(111) The experiment was performed with actual machines (or on a laboratory level). That means, with the actual machines, molten steel smelted by a converter was discharged to a ladle (molten steel of 500 kg imitating the molten steel discharged from a converter was smelted, in a laboratory), various flux was added, component adjustment, electrode-heating, and argon bubbling were performed, and a smelting treatment (slag refining) was performed. Also, after other components were adjusted, Ca, Mg, Ce, Ba, Li, or the like were added during the smelting treatment according to necessity to be maintained for 5 minutes or more. A steel ingot obtained was forged and hot rolled, and a wire rod of a diameter: 8.0 mm was made.

(112) For each wire rod obtained, concentration of Ba, Sr and Li in steel were measured by a method described below, and an evaluation test by a rotary bending fatigue test imitating a valve spring was performed.

(113) [Ba, Sr, Li Content in Steel] 1) When content is 0.2 ppm (mg/kg) or above (0.2 ppm quantitative lower limit value)

(114) A 0.5 g sample was taken from a wire rod of an object, was put in a beaker, demineralized water, hydrochloric acid and nitric acid were added, and was thermally decomposed. After it was natural-cooled, was transferred into a 100 mL (milliliter) measuring flask, and was made a measuring solution. This measuring solution was diluted with demineralized water and Ba, Sr and Li were quantitatively analyzed using an ICP mass spectrometer (model SPQ8000: made by Seiko Instruments Inc.). 2) When content is below 0.2 ppm (mg/kg) (0.03 ppm quantitative lower limit value)

(115) A 0.5 g sample was taken from a wire rod of an object, was put in a beaker, demineralized water, hydrochloric acid and nitric acid were added, and hydrolysis was performed. Thereafter acid concentration was adjusted by adding hydrochloric acid, added with methyl isobutyl keton (MIBK), shaked, and the iron content was extracted to the MIBK phase. After left to stand, only the water phase was taken out, was transferred into a 100 mL measuring flask, and was made a measuring solution. This measuring solution was diluted with demineralized water, and Ba, Sr and Li were quantitatively analyzed with the condition described above using an ICP mass spectrometer (model SPQ8000: made by Seiko Instruments Inc.).

(116) [Fatigue Strength Test (Rupture Ratio)]

(117) For each hot rolled wire rod (diameter: 8.0 mm), stripping (diameter: 7.4 mm).fwdarw.patenting.fwdarw.cold wire drawing (diameter: 4 mm).fwdarw.oil tempering [oil quenching and lead bathing (approximately 450° C.) tempering continuous process] were performed and a wire with 4.0 mm diameter×650 mm was manufactured. The wire obtained was subjected to treatment equivalent to strain relieving annealing (400° C.).fwdarw.shot peening.fwdarw.200° C. low temperature annealing, thereafter the test was performed using a Nakamura Method rotational bending tester with 908 MPa nominal stress, rotational speed: 4,000-5,000 rpm, number of times of stoppage: 2×10.sup.7 times. Then, for those the breakage was caused by inclusions out of those ruptured, the rupture ratio was obtained by the equation below.
Rupture ratio (%)=[number of samples broken by inclusions/(number of samples broken by inclusions+number of samples wherein the test was stopped after attaining prescribed number of times)]×100

(118) These results are shown in Table 6 below along with the chemical componential composition of each wire rod. Also, with respect to the elements other than Ba, Sr and Li, measurement was performed in accordance with the methods described below.

(119) C: Burning infrared absorption method

(120) Si, Mn, Ni, Cr, V and Ti: ICP emission spectrometry method

(121) Al, Mg, Zr and REM: ICP mass spectrometry method

(122) Ca: Frameless atomic absorption spectrometry method

(123) O: Inert gas fusion method

(124) TABLE-US-00006 TABLE 6 Rupture Test Chemical componential composition (mass %, Al, Ba, Ca, Mg and Li are in mass ppm) ratio No. C Si Mn P S Al Ba Sr Ca Mg Li Others (%) 1 0.6 2.2 0.5 0.01 0.01 8 2 1 6 0.3 — — 5 2 0.8 1.5 0.7 0.01 0.01 10 1 0.4 3 0 — — 7 3 0.8 0.2 0.5 0.01 0.01 5 1 1 0.5 3 — — 6 4 0.7 1.6 0.7 0.01 0.01 32 2 1 6 0.2 — — 38 5 0.6 2.4 0.3 0.01 0.01 24 4 6 1 6 — — 9 6 0.6 1.9 0.9 0.01 0.01 2 4 2 0.3 7 — — 9 7 0.7 1.5 0.7 0.01 0.01 0.4 3 4 10 0.1 — — 37 8 0.5 1.5 0.7 0.01 0.01 11 16 17 6 1 — — 55 9 0.7 1.5 0.7 0.01 0.01 18 10 17 6 0.3 — — 13 10 1.0 2.0 1.6 0.01 0.01 10 0.03 0.05 0.3 6 — — 10 11 0.5 2.0 0.9 0.01 0.01 6 0 0 5 0 — — 22 12 0.6 2.4 0.4 0.01 0.02 20 15 12 15 10 — — 8 13 0.6 2.4 0.5 0.01 0.01 6 7 12 0 0 — — 33 14 0.9 1.6 0.7 0.01 0.01 8 4 8 16 20 — — 35 15 0.6 1.6 0.7 0.01 0.01 5 8 4 0.3 0 — — 28 16 0.6 1.6 0.7 0.01 0.01 3 4 3 35 0.3 — — 38 17 0.6 2.0 0.9 0.01 0.01 2 3 3 7 0.5 25 — 6 18 0.7 2.0 0.9 0.01 0.01 1 2 2 5 1 17 — 4 19 0.5 2.4 0.5 0.01 0.01 9 6 1 4 2 0.5 — 4 20 0.5 2.0 0.7 0.01 0.01 3 0.1 0.1 0 5 0.04 — 7 21 0.6 2.0 0.9 0.01 0.01 8 1 2 6 0.4 — Cr: 0.9, Ni: 0.25, 5 V: 0.1- 22 0.6 1.5 0.7 0.01 0.02 10 0.8 0.5 3 0 — Cr: 0.65, V: 0.1 5 23 0.6 1.9 0.9 0.02 0.01 2 3 3 0.4 7 — V: 0.5, Mo: 0.3 9 24 0.6 2.4 0.4 0.01 0.01 20 10 17 15 10 — V: 0.5, Ti: 0.01, 7 W: 0.003 25 0.6 2.4 0.5 0.001 0.01 9 4 2 4 2 0.5 Cr: 3, Nb: 0.1, 5 Co: 0.01 26 0.8 1.5 0.7 0.01 0.01 10 1 1 3 0 — Ni: 0.5, Ca: 0.0005 5

(125) From these results, following consideration is possible. In those in Test Nos. 1-3, 5, 6, 9, 10, 12, 17-26 in Table 6, it is understood that the chemical componential composition is appropriate, and the composition of inclusions is controlled to a proper region and excellent fatigue strength is obtained.

(126) On the other hand, in those in Test Nos. 4, 7, 8, 11, 13-6 in Table 6, the chemical componential composition deviates from a proper region and the composition of inclusions is not controlled to a proper region, therefore the result of fatigue test is not good.

(127) More specifically, in Test Nos. 4, 7 in Table 6, although Ba, Sr, Ca and Mg are properly controlled, concentration of Al is high or low, and the rupture ratio becomes high.

(128) In Test No. 8 in Table 6, concentration of Ba and Sr is excessive, therefore the rupture ratio becomes high.

(129) In Test No. 11 in Table 6, Ba and Sr are not contained, therefore the rupture ratio becomes high.

(130) In Test Nos. 13-16 in Table 6, although concentration of Ba, Sr and Al is appropriate, concentration of Ca and Mg is high or low, and the breakage ratio becomes high. Also, in Test No. 17 in Table 6, concentration of Li deviates from a preferable upper limit, however the effect saturates compared with the one in Test No. 18 in Table 6.

(131) Thus, it is understood that proper controlling all of Ba, Sr, Ca, Mg and Al is necessary.

(132) Although the present invention was described in detail referring to specific embodiments, it is apparent to those with the ordinary skill in the art that a variety of alterations and modifications can be added without deviating from the spirit and scope of the present invention. The present application is on the basis of four Japanes Patent Applications applied on Dec. 28, 2006 (Patent Application No. 2006-356308, Patent Application No. 2006-356309, Patent Application No. 2006-356311, Patent Application No. 2006-356313) whose contents are incorporated herein as references.

INDUSTRIAL APPLICABILITY

(133) By properly controlling the composition of oxide-based inclusions (compositing with optimum balance), low melting point and glass state in hot rolling are kept, thereby refinement of inclusions in hot rolling is promoted and a Si-killed steel wire rod excellent in fatigue properties can be provided.