JP2005029888A - Superclean steel having excellent fatigue strength and cold workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide superclean steel which is intended to be used for valve springs, suspension springs, fine steel wire or the like and exhibits performance exceeding that of the conventional material in cold workability and fatigue properties by controlling oxide inclusions in the steel. <P>SOLUTION: The superclean steel having excellent fatigue strength and cold workability contains by mass, 15 to 55% CaO, 20 to 70% SiO<SB>2</SB>, ≤35% Al<SB>2</SB>O<SB>3</SB>, ≤20% MgO, and 0.5 to 20% of one or more kinds selected from among Li<SB>2</SB>O, Na<SB>2</SB>O and K<SB>2</SB>O as oxide inclusions present in the steel. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high cleanliness steel excellent in cold workability and fatigue characteristics, and particularly relates to a high cleanliness steel exhibiting excellent performance when used as a high-strength steel wire, ultrafine steel wire, high-strength valve spring, etc. Is.

  In steel wires that are drawn to a diameter of about 0.1 to 0.5 mm by cold working and spring steels that require high fatigue properties, hard non-metallic inclusions contained in steel materials are drawn and fatigue properties. It is desirable to keep it as small as possible. From such a viewpoint, high cleanliness steel in which hard non-metallic inclusions are reduced as much as possible is used for the above-described applications.

  On the other hand, in recent years, as the demand for automobile weight reduction and higher output for the purpose of reducing exhaust gas and improving fuel consumption has increased, valve springs and suspension springs used in engines and suspensions are increasingly designed for high stress. Has been. For this reason, spring steel has a tendency to further increase in strength and diameter, load stress increases further, and spring steel having further improved fatigue resistance and sag resistance is demanded. In particular, higher fatigue properties are required for valve spring steel.

  On the other hand, ultra-fine steel wires represented by tire cords are also required to have higher strength for the purpose of weight reduction of tires, and recently steel cords having a strength of 4000 MPa class are also required. Along with this, disconnection is likely to occur during cold working, and it is also necessary to improve cold workability.

Generally, in steels used for these applications, wire breakage and fatigue breakage due to non-metallic inclusions are likely to occur as the strength is increased, and the amount of hard non-metallic inclusions that are the main cause is reduced as much as possible. Many improvements have been made in the direction of reducing the size. For example, in Non-Patent Document 1, it is effective to reduce the amount of non-ductile inclusions such as Al ₂ O ₃ and SiO ₂ contained in the steel material for tire cords as much as possible. Is controlled to a CaO—Al ₂ O ₃ —SiO ₂ low melting point composition having a melting point of about 1400 to 1500 ° C. or less, it has been clarified that it is difficult to become a starting point of fatigue fracture.

Patent Documents 1 and 2 disclose a non-metallic inclusion composition for making a non-metallic inclusion easy to be stretched or broken at the time of cold working and to be soft so as not to cause substantial breakage. .
The Japan Iron and Steel Institute edition, "126th and 127th Nishiyama Memorial Technology Course", Japan Iron and Steel Institute Publishing, November 14, 1988, pp. 145-167 Japanese Patent Publication No. 6-74484 Japanese Patent Publication No. 6-74485

  However, as mentioned above, in the fields of recent fine valve springs, suspension springs, and ultra-fine steel wires typified by high-performance tire cords, it is no longer possible to satisfy the demands of customers with the improvement methods on the conventional extension line. Therefore, there is an urgent need to establish performance improvement measures that surpass conventional methods.

  The present invention has been made by paying attention to such a situation, and its purpose is to focus on oxide inclusions that are included in steel and have a remarkable effect on fatigue properties, etc., and stretch in the hot rolling process. It is an object of the present invention to provide a high cleanliness steel that exhibits performance superior to that of conventional materials in cold workability and fatigue characteristics by reducing the size of inclusions as much as possible.

The high cleanliness steel excellent in fatigue strength and cold workability according to the present invention that has been able to achieve the above-mentioned problem is that oxide inclusions present in the steel are CaO: 15 to 55% (mass%) The same shall apply hereinafter), SiO ₂ : 20 to 70%, MgO: 20% or less, Al ₂ O ₃ : 35% or less, and one or more of Li ₂ O, Na ₂ O, K ₂ O : It is characterized by containing 0.5 to 20%. It is recommended that the oxide inclusions have a Li ₂ O / SiO ₂ (mass ratio) of about 0.01 to 0.5. More preferably the content of SiO ₂ contained in the oxide inclusions is 30% or more and less than 45%, more preferably 40% or less.

  Further, the present invention specifies the component composition of oxide inclusions present in the steel as described above, thereby making the inclusions soft and highly ductile as will be described in detail later, and dividing them in the hot rolling step. By making them finer, they have the greatest feature in that they do not become the starting point of fatigue breakage or breakage that occurs during wire drawing. The basic composition of the steel material itself is not particularly limited, but valve springs and suspension springs, In order to exhibit desired strength characteristics by applying to uses such as steel cords, C: 1.2% or less, Si: 0.1 to 4%, Mn: 0.1 to 2.0%, Al: 0 A steel material satisfying 0.005% or less is preferable. The steel material may contain one or more elements selected from the group consisting of Cr, Ni, V, Nb, Mo, W, Cu, and Ti as other elements, with the balance being Fe and Inevitable impurities may be used.

  When the oxide inclusions are as described above, the oxide inclusions can be controlled to a soft and low melting point composition that can be easily stretched and divided in the hot rolling step, and the oxide inclusions are sufficient. Can be miniaturized and miniaturized. Therefore, high cleanliness steel excellent in fatigue characteristics and cold workability can be provided by reducing as much as possible the coarse hard inclusions that are the starting points of fatigue failure and disconnection.

A major cause of individual oxides and complex oxides such as SiO ₂ , Al ₂ O ₃ , CaO, and MgO existing as oxide inclusions in various steels, causing fatigue fracture and breakage during wire drawing It is well known that there are many techniques for improving fatigue characteristics and the like by changing the component composition of these oxide inclusions, including the above-mentioned patent documents. However, it is also a fact that the improvement method on the extension line of the conventional reforming technology as described above cannot satisfy the demands of the customers in recent years. Therefore, the present inventors do not attempt modification in the category of oxide-based inclusion composition inevitably mixed in the steel, but actively add a third component into the steel so that the above-mentioned oxide-based inclusions are added. In order to improve the inclusions, various additives were studied.

As a result, SiO ₂ , Al ₂ O ₃ , CaO, and MgO that are almost inevitable in the steel are effectively used, and an appropriate amount of Li ₂ O, Na ₂ O, and K ₂ O is added to these. When the seeds or more are actively contained, the oxide inclusions generated in the steel become highly ductile that surpasses the conventional oxide inclusions, and the high ductility oxide inclusions generated Is easily stretched in the hot-rolling process and finely divided, and the oxide inclusions are finely and uniformly dispersed as hot-rolled steel, and the fatigue characteristics and wire drawing workability are dramatically improved. And the present invention has been conceived.

  Hereinafter, the present invention will be described in detail mainly on the reason for determining the content of each oxide constituting the oxide inclusions.

CaO: 15-55%
CaO is an essential component for making oxide inclusions soft and easy to refine in the hot rolling process of steel materials. If the CaO content is insufficient, high SiO ₂ type or SiO ₂ · Al ₂ O ₃ type It becomes a hard inclusion and is difficult to be miniaturized in the hot rolling process, which is a major cause of deterioration of fatigue characteristics and wire drawing workability. Therefore, CaO must be contained at least 15%, preferably 20% or more, more preferably 25% or more. However, if the CaO content in the oxide inclusions increases too much, the hot deformability of the inclusions decreases, and hard high CaO inclusions may be generated and become the starting point of destruction. Therefore, it is preferable to keep it at 50% or less, more preferably 45% or less.

SiO ₂ : 20 to 70%
SiO ₂ is an essential component for producing soft oxide inclusions having a low melting point together with CaO, Al ₂ O _{3 and the} like, and if it is less than 20%, the oxide inclusions are CaO or Al ₂ O _3. It becomes a large or hard inclusion mainly composed of and becomes the starting point of destruction. Therefore, it is essential to contain 20% or more, more preferably 30% or more. However, if the SiO ₂ content is too large, the oxide inclusions become high melting point and hard inclusions mainly composed of SiO ₂ , and the possibility of becoming a starting point of disconnection or destruction increases. Such a tendency appears remarkably when the SiO ₂ content exceeds 70%. Therefore, it is extremely important to keep the SiO ₂ content at 70% or less. More preferably, it is 65% or less, more preferably less than 45%, and particularly preferably 40% or less.

Al ₂ O ₃ : 35% or less Al ₂ O ₃ is not an essential component for the formation of soft inclusions, but CaO and SiO ₂ , and further Li ₂ O and Na ₂ that are essential in the present invention. Depending on the proper composition control of the oxide inclusions, including the O, K ₂ O content, etc., it may be substantially free of Al ₂ O ₃ . However, if an appropriate amount of Al ₂ O ₃ is contained, the oxide inclusions tend to be softer with a lower melting point, and therefore it is preferable to contain about 5% or more, more preferably 10% or more. . However, if there is too much Al ₂ O ₃ in the oxide inclusions, it becomes hard and difficult to miniaturize alumina inclusions, which also becomes difficult to refine in the hot rolling process, and it becomes the starting point of destruction and breakage, At most, it should be suppressed to 35% or less, preferably about 30% or less.

MgO: 20% or less MgO is a generation source of MgO.SiO ₂ hard inclusions and easily causes breakage or breakage. Such an obstacle appears remarkably when the MgO content exceeds 20%. Therefore, it is desirable to suppress it to 20% or less in order not to cause such a failure. More preferably, it is 15% or less.

Total of one or more of Li ₂ O, Na ₂ O, K ₂ O: 0.5 to 20%
Li ₂ O, Na ₂ O, and K ₂ O are the most specific and important components in the present invention, and exhibit extremely important actions for lowering the melting point and viscosity of the complex oxide inclusions to be produced. To do. In order to increase the refinement of inclusions by lowering the melting point and lowering the viscosity of oxide inclusions, and to ensure the fatigue property improvement effect at the level intended in the present invention, Li ₂ O, Na ₂ It is desirable to contain one or more of O and K ₂ O in total at least 0.5% or more, more preferably 1% or more, and even more preferably 2% or more. However, when the total of one or more of Li ₂ O, Na ₂ O, and K ₂ O exceeds 20%, the oxide inclusions are excessively lowered in melting point, and the resistance to refractories is significantly increased. The amount of hard inclusions derived from the elution of the lining refractories used increases, and the fatigue characteristics and cold workability are reduced. Therefore, the total of one or more of Li ₂ O, Na ₂ O, and K ₂ O in the oxide inclusions must be suppressed to 20% or less, preferably 15% or less.

As described above, Li ₂ O, Na ₂ O, and K ₂ O exhibit a very important effect in reducing the melting point and viscosity of the complex oxide inclusions that are all formed, and finally miniaturizing them. However, these are not equivalent, and the effect is enhanced when an appropriate amount of Li ₂ O is contained in the oxide inclusions by positively adding Li having a strong deoxidizing power as an origin of oxide inclusions. According to a separate confirmation by the present inventors, Li ₂ O also has an effect of facilitating crystallization of glassy oxide inclusions, which also promotes refinement of oxide inclusions. As a result, it was confirmed that the fatigue characteristics were significantly affected. That is, when an appropriate amount of Li ₂ O is included in the component oxide inclusions, the oxide inclusions are easily crystallized, and a large number of fine crystals are formed in the glassy oxide inclusions. Precipitate. As a result, the load applied to the oxide inclusions in the hot rolling process is concentrated at the boundary between the vitreous and the crystalline, and the separation of the inclusions is further promoted. The oxide inclusions to be produced are further miniaturized. Even if Li ₂ O, Na ₂ O, or K ₂ O alone is added, the effect is great. However, when Li ₂ O is further added in the presence of Na ₂ O, K ₂ O, the effect is further improved. This is also considered to be synergistically positive and contribute to improving fatigue properties.

In addition, Li has a strong deoxidizing power and contributes to the reduction of the amount of dissolved oxygen present in the steel, so it suppresses the formation and coarsening of high SiO ₂ inclusions that precipitate during solidification. Also demonstrates. Further, SiO ₂ -Li ₂ O during solidification by the action of dissolved to Li, Na, K, by generating a _{SiO 2 -Na 2 O, SiO 2} -K 2 O and mixtures thereof, high SiO ₂ based inclusions It also has the effect of suppressing the production of objects.

When Li is essential, it is recommended that the mass ratio of Li ₂ O to SiO ₂ (Li ₂ O / SiO ₂ ) in the oxide inclusions be in a predetermined range. Li ₂ O is important in reducing the melting point and viscosity of the complex oxide and promoting the refinement of complex oxide inclusions, especially in the ratio with SiO ₂ that forms a network and increases the viscosity. Because it is important to think. By sufficiently large as compared with li ₂ O to SiO _2, it is possible to further exhibit the effect of lowering the melting point and viscosity of the composite oxide inclusions, fine inclusions are promoted, SiO ₂ Breakage starting from large inclusions in the system can be prevented more reliably. Even if Li ₂ O is too much compared with SiO ₂ , the melting point and viscosity of the composite oxide inclusions are lowered, the refractory is melted, and hard inclusions derived from the refractory increase. Fatigue properties and cold workability are reduced. From the above viewpoint, when Li is essential, the mass ratio of Li ₂ O to SiO ₂ (Li ₂ O / SiO ₂ ) is, for example, about 0.01 (preferably about 0.02 or more, more preferably 0). 0.03 or more) and 0.5 or less (preferably about 0.4 or less).

  In the present invention, MnO may be mixed in the oxide inclusions as other oxides. However, MnO itself is less likely to cause fatigue fracture or breakage, and Ca, Al , Li is reduced by the addition of a strong deacidifying element such as Li, and the content in the oxide inclusions is reduced, so the content is not particularly limited.

By the way, JP 2002-167647 A discloses a technique for improving fatigue properties for Si deoxidized steel, and this publication discloses SiO in oxide inclusions contained in Si deoxidized steel. _{2 The} content is specified to be 45% or more, and 0.5 to 10% of an oxide (R ₂ O) of alkali metal R (R = Na, K, Li) is specified.

However, paragraph 0013 of the publication states that “the state in which R ₂ O (R: Na, K, Li) is contained in the SiO ₂ -based inclusions is that ... SiO ₂ contains CaO, Al ₂ O ₃ and MgO. Since the reactivity with molten steel is higher than in the contained state, the interfacial energy is lowered, and the effect enables the SiO ₂ inclusions containing R ₂ O to be refined. ” In addition, in claim 1, "addition of 0.5 to 10% of an oxide of an alkali metal R in 45% or more of SiO ₂ ", and in an example, "as high as about 50 to 80%" as is apparent from the fact that it is contained several% of Na ₂ O to SiO ₂ inclusions "is described, an oxide of an alkali metal R to" high SiO ₂ inclusions 0.5% to 10% It is the basic idea of “contain”. In this publication, on the premise that Li ₂ O, K ₂ O, and Na ₂ O are almost equivalent, an example using Na ₂ O that is the cheapest and easy to obtain in the experiment is given. Among these, the specific action of Li ₂ O having particularly strong deoxidizing power is not recognized at all.

On the other hand, in the present invention, as described above, high SiO ₂ inclusions are considered to be excluded as much as possible because they are the main cause of disconnection and breakage. Therefore, the basic composition of oxide inclusions is CaO: 15 to 15 55%, SiO ₂ : 20 to 70% (preferably less than 45%, more preferably 40% or less), Al ₂ O ₃ : 35% or less, MgO: 20% or less, and Li ₂ O, An appropriate amount of Na ₂ O and K ₂ O is contained to synergistically exert these effects as described above. Therefore, the technical idea is different between the present invention and the invention disclosed in Japanese Patent Laid-Open No. 2002-167647.

The present applicant has previously proposed Japanese Patent No. 2654099 and Japanese Patent Laid-Open No. 2-15111 as a composition control technique for inclusions in steel using Li. Among them, Japanese Patent No. 2654099 “controls a deoxidation product to an alkali metal-containing composition by using a mixture of a Si-based deoxidizing agent and an alkali metal compound”. The use of silicates (Na ₂ SiO ₃ , K ₂ SiO ₃ ) and fluorides (LiF, NaF), which have relatively high thermal stability, is recommended. Further, in the above-mentioned Japanese Patent Application Laid-Open No. 2-15111, “contains 10 to 1% by mass of one or more selected from the group consisting of Li, Na and K (provided that all of them are alloyed) ), A technology for improving fatigue characteristics by controlling the form of inclusions to be easily deformable by using a deoxidizing material for refining containing 60 to 99% of Si.

  However, according to laboratory experiments conducted in the course of the present invention, the addition of the silicate and fluoride + Fe · Si alloy disclosed in the above-mentioned Patent No. 2654099 makes it difficult for the desired amount of Li to be retained in the molten steel. As such, it is difficult to control the inclusion composition intended in the present invention.

Further, the 10-1% Li · Si alloy disclosed in Japanese Patent Laid-Open No. 2-15111 has a better yield to molten steel than the former, but an oxide inclusion containing a predetermined amount of Li ₂ O. Therefore, the composition control as intended in the present invention could not be performed. Moreover, the 10% to 1% Li · Si alloy has a high liquidus temperature when produced by the premelt method, so that Li is liable to evaporate, and the Li yield is poor, resulting in high costs.

  Therefore, in utilizing Li in the present invention, pre-melting is relatively easy in the composition range of the Li—Si phase diagram, and since Li is present as an intermetallic compound, the Li activity is low, so In order to prevent explosive evaporation loss even when it is added, the liquidus is relatively low, and a composition in which a large number of intermetallic compounds composed of Li and Si exist in its composition range is “Li: 11 to 50 %, Balance Si and inevitable impurities "is preferably used. If a premelt body having such a composition is manufactured in advance and added to molten steel, a predetermined amount of Li can be easily produced in the steel, and a predetermined oxide system intended by the present invention is used. The inclusion composition can be controlled. Further, the above-described Li—Si alloy “Li: 11 to 50%, remaining Si and inevitable impurities” may be blended or premelted with Ca, Mg, Na, K or the like as necessary.

As the Li, Na, and K sources, carbonates, that is, Li ₂ CO ₃ , Na ₂ CO ₃ , and K ₂ CO ₃ are used, and a predetermined yield can be obtained even if these are mixed with Ca or Mg alloy. Therefore, these may be used. If these oxides are added to the slag, the yield is further improved.

  As described above, in the present invention, by appropriately controlling the composition of oxide inclusions contained in the steel, the inclusions have a low melting point and low viscosity, and can be refined in a hot rolling process. This is characterized in that the fatigue characteristics and wire drawing workability are improved, and the component composition of the steel itself is not particularly limited, but in order to satisfy the composition of the oxide inclusions described above, It is desirable to control the content to 0.1% or more, and to control the Li, Na, K, Mg, Ca, Al content to a level of 1 to 100 ppm in total (dissolved concentration + content in inclusions).

  In addition, in order to apply to the use as the above-described high-tensile steel wire, ultrafine steel wire, high-strength valve spring and the like intended by the present invention, C: 1.2% or less, Si: 0.1-4%, Mn : 0.1 to 2.0%, Al: 0.005% or less steel is preferable, and if necessary, one or more of Cr, Ni, V, Nb, Mo, W, Cu, Ti, etc. as physical property improving elements The balance may be Fe and inevitable impurities. Preferable contents of these additional elements are Cr: 0.01 to 3.0%, Ni: 0.05 to 1.0%, V: 0.005 to 0.5%, Nb: 0.005 to 0 10%, Mo: 0.01 to 1%, W: 0.01 to 1.0%, Cu: 0.05 to 2%, Ti: 0.005 to 0.06%.

  The preferable C content is set to 1.2% or less for applications from high-strength steel wire (C content: about 1.1% level) to extra fine mild steel wire (C content: about 0.01% level). The intention is that when the carbon content exceeds 1.2%, the steel is excessively hardened and the workability is lowered, which makes it impractical. In consideration of the fact that the characteristics of the present invention are effectively exhibited particularly for high-strength steel wires, the preferable lower limit of the C content is 0.2%, more preferably 0.3%, and still more preferably 0.4. % Or more.

  Further, in order to secure the oxide inclusions having the above-described composition, the Si and Mn contents in the steel should be 0.1% or more, respectively, but if these contents are too large, the steel becomes brittle. Therefore, it is preferable to suppress Si to 4.0% or less and Mn to 2.0% or less. Al can be positively contained to control the composition of oxide inclusions as described above, but if too much, the amount of hard alumina inclusions increases, leading to a decrease in cleanliness. It should be kept below 0.005%. Since Ca hardly dissolves in molten steel, almost all of it moves to slag as oxides and sulfides at the time of melting, and the remainder only remains as a composite oxide. Can be ignored.

  The high cleanliness steel of the present invention thus obtained is a composition whose oxide inclusions present as impurities are controlled to have a low melting point and low viscosity, and are drawn and refined in the hot rolling process. Therefore, they do not become sources of fatigue failure or breakage during wire drawing, and therefore this clean steel has excellent wire drawing workability and fatigue characteristics. Therefore, this cleanliness steel can be used effectively in a wide range of applications such as high-tensile steel wire, ultrafine steel wire, and high-strength valve spring steel.

  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. It is also possible to implement, and they are all included in the technical scope of the present invention.

Experimental Examples Experiments were performed on 90 ton and 250 ton real machines (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. Component adjustment, electrode heating, and argon bubbling were performed, and slag refining was performed. In the slag refining (molten steel treatment), during the treatment, 30% Li-70% Si, Ca—Si wire, Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ and Ca wire, Mg wire, etc. Addition of a mixture and the like was also performed. After the refining, the molten steel was cast (in the laboratory, it was cast into a mold capable of obtaining a cooling rate equivalent to that of the actual machine). The obtained steel ingot was forged and hot-rolled to obtain a steel wire having a diameter of 5.5 mm. As a comparative material, a material equivalent to a conventional product was prototyped and evaluated in the same process. As steel components, spring steel components and steel cord components were used.

  In the evaluation, the L-section inclusions in each steel wire are observed with a microscope and the composition is investigated, and each steel wire is acid-dissolved to investigate the composition, number and size of hard inclusions, while the spring steel is rotated. The bending fatigue test and steel cord steel were evaluated by a wire drawing test.

(Inclusions in steel wire)
The L section of a steel wire rod (diameter: 5.5 mm) having a length of 80 mm was polished, and the thickness, length, number, and composition of inclusions were determined.

(Number of hard inclusions with a length of 20 μm or more)
The target hot-rolled steel wire 1500 g is cut at intervals of about 100 g, and after removing the surface scale, it is placed in a warm nitric acid solution at about 90 ° C. and dissolved in acid. This solution is filtered through a filter having a mesh size of 20 μm, and the inclusions extracted on the filter paper are analyzed by EPMA and the length is measured to obtain a hard inclusion (oxide) having a maximum length of 20 μm or more. The number of system inclusions (for example, alumina, zirconia, etc.) was measured, and the number per 50 g of steel was calculated.

(Fatigue strength)
For each hot-rolled steel wire (diameter 5.5 mm), skin cutting (SV) → low temperature annealing (LA) → cold drawing (diameter 4.0 mm) → oil temper [oil quenching and lead bath (about 450 ° C.) Continuous tempering process] → Simple strain relief annealing (Bluing: approx. 400 ° C.) → Shot peening → Strain relief annealing, then a 4.0mm x 650mm diameter wire is taken as a test material, and Nakamura rotary bending tester The test is performed at a nominal stress of 880 MPa, a rotational speed of 4000 to 5000 rpm, and a number of cancellations of 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. Further, the composition of inclusions appearing on the fracture surface was examined by EPMA, and the maximum inclusion size (width) was measured.
Fracture rate = [inclusion breakage number / (inclusion breakage + number of breaks after reaching a predetermined number of times)]
× 100 (%)

(Drawing workability)
A test wire drawing machine was used for evaluation of wire drawing workability. That is, the wire after hot rolling (diameter 5.5 mm) is first drawn to a diameter of 2.5 mm, heat-treated (air patenting), and then secondarily drawn to a diameter of 0.8 mm. Subsequently, heat treatment (lead patenting) and brass plating were performed, followed by wet drawing to a diameter of 0.15 mm, and evaluation was performed in terms of the number of breaks per 10 tons of steel wire. As for the broken one, the composition of the inclusions appearing in the cross section was examined by EMPA, and the maximum inclusion size (width) was measured.

(Inclusion composition analysis)
Since the concentration of Li ₂ O in the inclusions cannot be measured by conventional EPMA, an analysis method by SIMS was independently developed and measured by the following procedure.

(1) Primary standard sample
1) a synthetic oxides range covering the inclusions composition excluding Li ₂ O, to produce a large number of these synthetic oxides plus Li ₂ O, and their Li ₂ O concentration was quantitatively analyzed by chemical analysis, A standard sample is prepared.

  2) Measure the relative secondary ion intensity of Li with respect to Si of each prepared synthetic oxide.

3) Draw a calibration curve of the relative secondary ion intensity of Li with respect to Si and the Li ₂ O concentration chemically analyzed in 1) above.

(2) Secondary standard sample (for measurement environment correction)
4) For environmental correction at the time of measurement, separately prepare a standard sample in which Li is ion-implanted on a Si wafer, measure the relative secondary ion intensity of Li with respect to Si, and correct when performing the above 2).

(3) Actual measurement
5) First, each concentration of inclusions in the steel, such as CaO, MgO, Al ₂ O ₃ , MnO, SiO ₂ , Na ₂ O, K ₂ O, etc. is analyzed by EDX, EPMA or the like.

6) The relative secondary ion strength of Li against Si of inclusions in steel is measured, selects the closest calibration curve analysis of the 5) of the calibration curve obtained in the above 3), thereby Li ₂ O Determine the concentration.

  The results are collectively shown in Table 1 for steel cord steel wires and in Table 2 for spring steel wires.

  From Tables 1 and 2, the following can be considered.

  Table 1 is an example for steel cords for steel cords. Reference numerals 1 to 13 satisfy the requirements of the present invention, so the number of coarse hard inclusions of 20 μm or more is small and the maximum inclusion size is shown. Is relatively small, and the number of wire breaks during wire drawing is small. Judging from this table, when the number of coarse hard inclusions of 20 μm or more is 0.3 or less per 50 g of steel, the number of disconnections is clearly reduced.

  On the other hand, reference numerals 14 to 21 are comparative examples lacking any of the requirements defined in the present invention, and the number of breaks exceeds 20 times per 10 tons of steel wire, and the coarseness is 20 μm or more. The number of hard inclusions exceeds 0.4 per 50 g of steel.

  1 to 3 show the relationship between the number of disconnections and the number of hard inclusions of 20 μm or more (FIG. 1), the relationship between the number of disconnections and the maximum inclusion size (FIG. 2), and the maximum fracture surface. FIG. 3 is a graph showing the relationship between the size of inclusions and the number of hard inclusions of 20 μm or more (FIG. 3), and the following tendency can be confirmed from these figures.

  From FIG. 1, if the number of hard inclusions of 20 μm or more is suppressed to about 0.4 or less per 50 g of steel, the number of disconnections can be suppressed to less than 20 per 10 tons of steel wire, and more than 20 μm. If the number of hard inclusions is suppressed to about 0.25 or less per 50 g of steel, the number of disconnections can be suppressed to 10 or less per 10 tons of steel wire.

  From FIG. 2, if the fracture surface maximum inclusion size is suppressed to 30 μm level or less, the number of disconnections can be suppressed to 10 times or less per 10 tons of steel wire.

  From FIG. 3, if the number of hard inclusions is decreased by appropriately controlling the inclusion composition, the fracture surface maximum inclusion size tends to be reduced.

FIG. 4 is a graph showing the relationship between the Li ₂ O / SiO ₂ ratio and the number of hard inclusions of 20 μm or more based on the results shown in Table 1 above. From this graph, it can be seen that when the Li ₂ O / SiO ₂ ratio is in the optimum range, the number of hard inclusions can be easily suppressed.

  Table 2 is an example for a spring steel wire. Reference numerals 22 to 35 satisfy the prescribed requirements of the present invention, so the fracture rate is relatively small and the maximum fracture inclusion size is also small. On the other hand, reference numerals 36 to 45 are comparative examples that deviate from any of the prescribed requirements defined in the present invention, and have a relatively high fracture rate and a large maximum fracture inclusion size.

FIG. 5 is a graph in which the results of Table 2 above are arranged as a relationship between the fracture rate and the maximum fracture inclusion size. From this figure, the maximum inclusion size is reduced to a level of 30 μm or less by controlling the inclusion composition. If it can be suppressed, it can be seen that the fracture rate can be suppressed to 20 levels or less. FIG. 6 is a graph arranged as a relationship between the total content of Li ₂ O, Na ₂ O, and K ₂ O in the inclusion composition and the maximum rupture inclusion size. If the total content of Li ₂ O, Na ₂ O and K ₂ O is controlled within the range of 0.5 to 20%, it can be confirmed that the maximum fracture inclusion size is reduced.

FIG. 7 is a graph in which the relationship between the Li ₂ O / SiO ₂ ratio and the maximum inclusion size of the fracture surface is arranged based on the results of Table 2 above. It can be seen from this graph that inclusions can be easily miniaturized when the Li ₂ O / SiO ₂ ratio is within an appropriate range.

It is a graph which shows the relationship between the number of the hard inclusions 20 micrometers or more obtained by experiment of steel for steel cords, and the frequency | count of a disconnection. It is a graph which shows the relationship between the fracture surface maximum inclusion size obtained by experiment of steel for steel cords, and the frequency | count of a disconnection. It is a graph which shows the relationship between the fracture surface maximum inclusion size obtained by experiment of steel for steel cords, and the number of hard inclusions of 20 micrometers or more. Was obtained in a steel cord for steel experiments is a graph showing the relationship between the Li ₂ O / SiO ₂ ratio and 20μm more hard inclusions number. It is a graph which shows the relationship between the fracture surface maximum inclusion size and the fracture | rupture rate obtained by experiment of the steel for springs. Was obtained in a spring steel experiment is a graph showing an oxide Li ₂ O in inclusions, the fracture surface maximum inclusion size relationship between the total content of Na ₂ O and K ₂ O. Was obtained in a spring steel experiment is a graph showing the fracture surface maximum inclusion size relationship between Li ₂ O / SiO ₂ ratio.

Claims (5)

Oxide inclusions present in the steel are CaO: 15 to 55% (meaning mass%, the same applies hereinafter), SiO ₂ : 20 to 70%, Al ₂ O ₃ : 35% or less, MgO: 20% High cleanliness steel excellent in fatigue strength and cold workability, characterized by containing at least one of Li ₂ O, Na ₂ O, and K ₂ O: 0.5 to 20% . The oxide inclusions are Li ₂ O / SiO ₂ (weight ratio) is high cleanliness steel according to claim 1 in which the 0.01 to 0.5. The oxide-based high cleanliness steel according to claim 1 or 2 SiO ₂ content of inclusions is less than 45% to 30%.   4. The steel according to claim 1, wherein the steel satisfies C: 1.2% or less, Si: 0.1 to 4%, Mn: 0.1 to 2.0%, Al: 0.005% or less. High cleanliness steel according to any one of the above.   The high cleanliness steel according to claim 4, wherein the other clean element contains at least one element selected from the group consisting of Cr, Ni, V, Nb, Mo, W, Cu, and Ti.

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