Title:
PROCESS FOR PRODUCING STEEL FOR HIGH-CARBON STEEL WIRE MATERIAL WITH EXCELLENT DRAWABILITY AND FATIQUE CHARACTERISTICS
Kind Code:
A1


Abstract:
The present invention provides a method suitable for manufacturing a steel material for obtaining a steel wire rod with decreased amount of hard nonmetallic inclusions and improved drawability and fatigue property by adequately controlling the conditions of secondary refining and manufacturing conditions in a converter. Converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that the molten iron takes 96 to 100% (means wt. %, same hereinbelow), the cold iron takes 4% or less, and the steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and operations are carried out such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm3/min or more to 0.004 Nm3/min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm3/min or more to 0.10 Nm3/min or less per 1 t of molten steel within the tundish.



Inventors:
Kimura, Sei (Hyogo, JP)
Mimura, Tsuyoshi (Hyogo, JP)
Application Number:
12/162550
Publication Date:
02/12/2009
Filing Date:
03/26/2007
Assignee:
Kabushiki Kaisha Kobe Seiko Sho(Kobe Steel Ltd) (Kobe-shi Hyogo, JP)
Primary Class:
International Classes:
C21C5/30
View Patent Images:



Primary Examiner:
MCGUTHRY BANKS, TIMA MICHELE
Attorney, Agent or Firm:
OBLON, MCCLELLAND, MAIER & NEUSTADT, L.L.P. (ALEXANDRIA, VA, US)
Claims:
1. A method for manufacturing steel for high-carbon steel wire rod that excels in drawability and fatigue property, characterized in that converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that the molten iron takes 96 to 100% (means wt. %, same hereinbelow), the cold iron takes 4% or less, and the steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and operations are conducted such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm3/min or more to 0.004 Nm3/min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm3/min or more to 0.10 Nm3/min or less per 1 t of molten steel within the tundish.

2. The manufacturing method according to claim 1, wherein a chemical composition of the steel for steel wire rod comprises C: 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less with Fe and unavoidable impurities as the balance.

3. The manufacturing method according to claim 1, wherein the steel for steel wire rod further comprises as another element at least one element selected from the group consisting of Ni: 0.05 to 1%, Cu: 0.05 to 1%, and Cr: 0.05 to 1.5%.

4. The manufacturing method according to claim 1, wherein the steel for steel wire rod further comprises as another element at least one element selected from the group consisting of Li: 0.02 to ppm, Mg: 0.02 to 20 ppm, Ce: 3 to 100 ppm, and La: 3 to 100 ppm.

Description:

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a steel material for obtaining a high-carbon steel wire rod with excellent drawability and fatigue property, and more particularly to a method suitable for manufacturing a steel for a steel wire rod in which the amount of nonmetallic inclusions having high hardness and very low ductility is reduced and the drawability and fatigue property are improved.

BACKGROUND ART

Where nonmetallic inclusions (in particular oxide inclusions; referred to hereinbelow simply as “inclusions”) that are hard and have a very low ductility, such as alumina (Al2O3) or spinel (Al2O3.MgO) are contained in a steel for tire cords or a steel for springs, the inclusions cause a loss of drawability in the process of drawing into an ultrafine steel wire and become the starting points for fatigue fracture at the product stage. Therefore, it is important to reduce the content of inclusions to a minimum or increase the ductility thereof by softening, thereby rendering them harmless, in the process of manufacturing the steel wire rod.

A variety of methods have heretofore been suggested from a view point of reducing to a minimum the amount of impurities present in steel wire rods. For example, Patent Documents 1, 2 disclose a method for reducing the amount of impurities by using Si and Mn as deoxidizing agents for a steel melt and regulating the concentration of Al. Furthermore, Patent References 3, 4 suggest a technology relating to the reduction in the amount of impurities by regulating the concentration of Al2O3 present in a refractory container that accommodates the steel melt. A technology for reducing the amount of impurities in steels by refining the steel melt by using a CaO—SiO2 flux with a low concentration of Al2O3 has also been suggested (for example, Patent Documents 5, 6).

However, the technologies that have been heretofore suggested relate to a secondary refining process performed with respect to a molten steel tapped from a converter, and even if the conditions of such secondary refining process are adequately controlled, the amount of impurities cannot be sufficiently reduced. Accordingly, it is necessary to control adequately the conditions of both the secondary refining and the processes preceding the secondary refining.

[Patent Document 1] Japanese Patent Application Laid-open No. 50-081907, Claims, etc.

[Patent Document 2] Japanese Patent Application Laid-open No. 50-11618, Claims, etc.

[Patent Document 3] Japanese Patent Application Laid-open No. 2003-245758, Claims, etc.

[Patent Document 4] Japanese Patent Application Laid-open No. 2004-211148, Claims, etc.

[Patent Document 5] Japanese Patent Application Laid-open No. 4-110413, Claims, etc.

[Patent Document 6] Japanese Patent Application Laid-open No. 9-059744, Claims, etc.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present invention was made in view of above circumstances, it is an object of the present invention to provide a suitable method for manufacturing a steel material for obtaining a steel wire rod that has improved drawability and fatigue property and in which the amount of hard nonmetallic inclusions is reduced by adequately controlling the conditions in secondary refining and manufacturing conditions in a converter.

Means for Resolving the Problems

The essence of the manufacturing method in accordance with the present invention that successfully attains the above-described object is that converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that molten iron takes 96 to 100% (means wt. %, same hereinbelow), cold iron takes 4% or less, and steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and that operations are carried out such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm3/min (N means “normal” and represents a volume at 298 K and 105 Pa; same hereinbelow) or more to 0.004 Nm3/min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm3/min or more to 0.10 Nm3/min or less per 1 t of molten steel within the tundish.

The steel for a steel wire rod that is an object of the method in accordance with the present invention preferably has a chemical composition comprising C, 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less with Fe and unavoidable impurities as the balance.

If necessary, the steel for a steel wire rod that is an object of the method in accordance with the present invention also can contain as another element at least one element selected from the group consisting of (a) Ni: 0.05 to 1%, Cu: 0.05 to 1%, and Cr: 0.05 to 1.5% and at least one element selected from the group consisting of (b) Li: 0.02 to 20 ppm, Mg: 0.02 to 20 ppm, Ce: 3 to 100 ppm, and La: 3 to 100 ppm, and the properties of the steel wire rod can thus be further modified according to the type of the contained components.

EFFECT OF THE INVENTION

In accordance with the present invention, by adequately controlling the ratio of raw materials and an average P concentration in the main raw materials to be charged into a converter and also adequately controlling the flow rate of a stirring gas of molten steel in secondary refining and a flow rate of Ar used for purging the inside of the tundish during continuous casting, it is possible to reduce the amount of hard nonmetallic inclusions and obtain a steel for a steel wire rod that has excellent fatigue property and to provide with good efficiency a steel for a steel wire rod that is optimum for the manufacture of high-strength ultrafine wires for tire cords and springs that are required to have good fatigue property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between the molten iron ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 2 is a graph illustrating the relationship between the cold iron ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 3 is a graph illustrating the relationship between the steel scrap ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 4 is a graph illustrating the relationship between the flow rate of purge Ar in a tundish and the number of wire breakages per 10 t of steel.

BEST MODE FOR CARRYING OUT THE INVENTION

In converter operations, steel scrap from inside and outside the steelmaking plant or cold iron, which is a solidified molten iron, is charged in addition to molten iron manufactured in a blast furnace, the molten iron temperature is raised, while oxidizing and removing C, and a molten steel with a C concentration of about 0.03 to 1% is manufactured. The temperature of the molten iron that is charged into the converter in this process is about 1200 to 1400° C. The higher is this temperature, the larger amount of steel scrap or cold iron can be charged into the converter, thereby making it possible to manufacture a large amount of molten steel by using a small amount of molten iron.

However, when high-carbon steel such as steel for tire cord or steel for springs is manufactured, P is difficult to remove in a converter, without decreasing the production efficiency. Therefore, it is necessary to reduce as much as possible the concentration of P in the main raw materials such as molted iron, cold iron, and steel scrap that will be charged into the converter. Examples of other raw materials that are charged into the converter in addition to the molten iron, cold iron, and steel scrap include an iron ore for adjusting slag formation and dolomite for protecting the converter refractory, but in the present invention, raw materials (molten iron, cold iron, and steel scrap) that do not include these additional components are referred to as the main raw materials.

From amongst the main raw materials, molten iron is subjected to dephosphorization treatment prior to charging into a converter. Therefore, the temperature of the molten iron charged into the converter is low and a thermal margin necessary for charging the steel scrap is small. The low thermal margin can be compensated by excess oxidation of C contained in the molten iron, but in the manufacture of steel for tire cords or steel for springs that are adversely affected by hard inclusions such as alumina every effort has to be made to avoid the excess oxidation of C. Thus, the excess oxidation of C increases the concentration of dissolved oxygen in molten steel at the completion of converter blowing and increases the amount of FeSi alloy used in deoxidation of molten steel. Furthermore, a small amount of Al is unavoidably contained in the FeSi alloy and, as a result, the amount of alumina-based inclusions in the molten steel is increased.

Accordingly, in order to reduce the content of alumina-based inclusions in molten steel, it is necessary to maximize the thermal margin of the main raw materials that are charged into a converter. The investigation conducted by the inventors with this consideration in view demonstrated that adequately selecting the ratio of molten iron, cold iron, and steel scrap in the main raw materials is effective in terms of increasing the thermal margin of the main raw materials.

First, the inventors examines the relationship between the molten iron ratio, cold iron ratio, and steel scrap ratio in the main raw materials (the ratio of each raw material when the sum total of the molten iron, cold iron, and steel scrap is taken as 100%) that will be charged into a converter and the number of wire breakages during drawing of 10 t of steel wire rods. The relationship between the molten iron ratio in the main raw materials and the number of wire breakages is shown in FIG. 1, the relationship between the cold iron ratio in the main raw materials and the number of wire breakages is shown in FIG. 2, and the relationship between the steel scrap ratio in the main raw materials and the number of wire breakages is shown in FIG. 3 (the meaning of the number of wire breakages is explained in the below-described embodiments).

The results clearly indicate that the number of wire breakages increases when the molten iron ratio is less than 96%, cold iron ratio is more than 4%, or steel scrap ratio is more than 2%. Therefore, it is clear that using main raw materials for charging into a converter that comprise molten iron at a ratio of 96% or more, cold iron at a ratio of 4% or less, and steel scrap at a ratio of 2% or less is effective in terms of reducing the number of breakages. The molten iron ratio is preferably 98% or more, even more preferably 100%. The preferred range of the cold iron ratio is 2% or less.

In accordance with the present invention, the average P concentration in the raw materials charged into a converter has to be 0.02% or less. Dephosphorization during converter blowing proceeds simultaneously with decarburization, but because in high-carbon steels such as steels for tire cords and steels for springs, decarburization has to be suppressed, dephosphorization cannot be expected to proceed during converter blowing. Further, if the concentration of P in the steel wire rod exceeds 0.02%, the segregation of P causes more frequent occurrence of wire breakage and decrease in fatigue strength. Accordingly the concentration of P in the entire raw material has to be decreased to 0.02% or less before the converter blowing. It is preferred that this concentration be 0.015% or less.

In accordance with the present invention, the stirring gas flow rate (sometimes referred to hereinbelow simply as “gas flow rate”) during stirring of slag and molten steel also has to be adequately controlled during secondary refining (for example, ladle refining) after completion of converter blowing. Molten steel after completion of converter blowing is deoxidized with Si or Mn, but in this process Al2O3 is generated by a very small amount of Al contained in the alloy (deoxidizing agent) such as FeSi, FeMn, and SiMn, and harmful inclusions remain in the product. Accordingly, Al2O3 has to be removed during secondary refining.

From this standpoint, it is necessary to intensify the contact between slag and molten steel by increasing the gas flow rate to 0.0005 Nm3/min or more per 1 t (ton) of molten steel. The gas flow rate is preferably 0.0006 Nm3/min or more, more preferably 0.0007 Nm3/min or more. However, if the gas flow rate is too high, loss of ladle refractories on melting becomes significant and undesirable in terms of operation. Moreover, the refractories are admixed to the molten steel, thereby adversely affecting the product. Accordingly, the gas flow rate has to be 0.004 Nm3/min or less, preferably 0.0035 Nm3/min or less, more preferably 0.003 Nm3/min or less.

No specific limitation is placed on the gas to be used for the stirring, and argon, which does not react with molten steel and is available at a low cost, is a suitable gas. No specific limitation is also placed on the gas blowing method, and a method for blowing through a refractory nozzle from above the molten steel or a method for blowing from the bottom or side surface of the ladle can be employed.

The molten steel subjected to secondary refining is cast in a continuous casting machine. In continuous casting, the molten steel is transferred and temporarily accommodated in a container called “tundish”. If air remains in the space inside the tundish, then molten steel is oxidized by oxygen present in the air, thereby generating inclusions and increasing the breaking frequency of wire when a steel wire rod is produced. Accordingly, the inside of the tundish has to be purged during casting with Ar gas.

FIG. 4 shows the relationship between the purge Ar flow rate in the tundish (flow rate per 1 t of molten steel inside the tundish) and the number of wire breakages (per 1 t of molten steel). The results clearly indicate that when the purge Ar flow rate is less than 0.04 Nm3/min, the oxidation of molten steel by oxygen contained in the air becomes significant and the number of wire breakages increases. Accordingly, the purge Ar flow rate has to be 0.04 Nm3/min or more. However, if the purge Ar flow rate per 1 t of molten steel exceeds 0.10 Nm3/min, the aforementioned effect tends to be saturated.

In the steel material (high-carbon steel) that is the object of the present invention, only the amount of Al from amongst the steel components is limited as described hereinbelow, no special limitation is placed on other components and, as shown hereinabove, they are contained in the same amounts as in generally used steel for springs or steel materials for drawing such as steel cords. More specifically, the steel contains C, 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less (excluding 0%). Reasons for setting such preferred ranges of components are described below.

[C: 0.4 to 1.3%]

C is an element useful for increasing the strength and is preferably contained at 0.4% or more to demonstrate this effect. Even more preferred content is 0.5% or more. However, if the content of C becomes too high, the steel is embrittled and loses its drawability. Therefore, it is preferred that the content of carbon be suppressed to 1.3% or less (more preferably, 1.2% or less).

[Si: 0.1 to 2.5%]

Si is an element demonstrating a deoxidizing function. For this function to be demonstrated, it is preferred that the content of silicon be 0.1% or more, even more preferably 0.2% or more. However, if the content of Si is too high, a large amount of SiO2 is generated as a deoxidization product and the steel loses its drawability. Therefore, the content of Si is preferably suppressed to 2.5% or less (more preferably 2.3% or less).

[Mn: 0.2 to 1.0%]

Mn is an element that demonstrates a deoxidizing function, similarly to Si, and also has a function for controlling inclusion property. For this functions to be demonstrated effectively, it is preferred that the content of Mn be 0.2% or more (more preferably 0.3% or more). On the other hand, if the amount of Mn is too high, the steel is embrittled and loses its drawability. Therefore, it is preferred that the content of manganese be suppressed to 1.0% or less (more preferably, 0.9% or less).

[Al: 0.003% or Less]

If the content of Al increases, the concentration of Al2O3 in inclusions rises, and large-sized Al2O3 that causes breakage of wire can be formed. Therefore, the content of aluminum is preferably suppressed to a minimum. From this standpoint, it is preferred that the content of aluminum be suppressed to 0.003% or less (more preferably 0.002% or less).

The basic components of the steel material that is the object of the present invention are described above, the balance being iron and unavoidable impurities. As for impurities, the admixture of elements carried by raw materials, source materials, and production equipment can be allowed. Furthermore, properties can be further effectively improved by introducing the following elements.

[At Least One Element Selected from the Group Consisting of Ni: 0.01 to 1%, Cu: 0.01 to 1%, and Cr: 0.05 to 1.5%]

Ni is an element making no significant contribution to the increase in strength of steel wire, but demonstrating an effect of increasing the toughness of drawn wire material. For this effect to be demonstrated, it is preferred that the content of nickel be 0.01% or more, more preferably 0.02% or more. However, if the content of Ni is too high, the aforementioned effect reaches saturation. Therefore, it is preferred that the content of nickel be 1% or less (more preferably 0.9% or less).

Cu is an element making contribution to strengthening of steel wire due to precipitation hardening function thereof. To demonstrate such an effect, the contents of copper is preferably 0.01% or more, more preferably 0.02% or more. However, if the content of Cu is too high, it precipitates on crystal grain boundaries, and cracks or scratches easily occur in the process of hot rolling the steel wire. Therefore, the content of copper is preferably 1% or less (more preferably, 0.9% or less).

Cr increases a work hardening ratio during wire drawing and easily ensures a high strength even at a comparatively low processing ratio. Moreover, Cr also acts to increase the corrosion resistance of steel. For example, when the wire is to be used as a reinforcing material for rubber (ultrafine steel wire), e.g. in tires, chromium effectively acts to inhibit the corrosion of the ultrafine steel wire. For this effect to be demonstrated, it is preferred that the content of Cr be 0.05% or more, more preferably 0.1% or more. However, if the amount of Cr is too high, hardenability with respect to pearlitic transformation increases and the patenting treatment is made difficult. Furthermore, the amount of secondary scale and density thereof increase, and the mechanical descaling ability and pickling ability are degraded. Therefore, the content of Cr is preferably 1.5% or less, more preferably 1.4% or less.

[At Least One Element Selected from the Group Consisting of Li: 0.02 to 20 ppm, Mg: 0.02 to 20 ppm, Ce 3 to 100 ppm and La: 3 to 100 ppm]

These elements act to further soften the nonmetallic inclusions present in steep. For this effect to be demonstrated, it is preferred that in the case of Li the content be 0.02 ppm or more (more preferably 0.03 ppm or more), in the case of Mg the content be 0.02 ppm or more (more preferably 0.03 ppm or more), in the case of Ce the content be 3 ppm or more (more preferably 5 ppm or more), and in the case of La the content be 3 ppm or more (more preferably 5 ppm or more). However, because the effect reaches saturation even if these elements are introduced in excess, the amount of Li and Na may be restricted to 20 ppm or less (more preferably 10 ppm or less) each. Further, the amount of Ce and La may be restricted to 100 ppm or less (more preferably 80 ppm or less) each.

The steel material obtained by the manufacturing method in accordance with the present invention is thereafter hot rolled to obtain a steel wire rod. The cross section diameter of the wire rod is 3 to 10 mm. This steel wire rod is suitable as a workpiece for ultrafine high-strength steel wire such as a tire cord or a piano wire that is required to have high drawability, for example, in a cold drawing process. Further, the steel wire rod is also suitable as a workpiece for springs or wires that require good fatigue property.

The present invention will be described below in greater detail with reference to embodiments thereof, but these embodiments place no limitation on the present invention, and the invention can be implemented by introducing appropriate changes within a range that can conform to the above- and below-described essence of the invention.

EMBODIMENTS

Manufacture of Steel Material

Molten iron in which the content of P and S was decreased to 0.007 to 0.020% and 0.002 to 0.01%, respectively, in the preliminary treatment process of the molten iron, or such molten iron mixed at various ratios with cold iron and/or steel scrap was charged into a converter and decarburization blowing was performed to a predetermined C concentration. Then, the steel was tapped into a ladle, and composition adjustment (the composition is presented in Tables 2, 5 below) and slag refining (secondary refining) were preformed in a ladle furnace. The slag during ladle refining was of a CaO—SiO2—Al2O3 system with a CaO/SiO2=0.7 to 1.7 and Al2O3=4 to 25%. Further, Ar was used as a stirring gas for molten steel during ladle refining, and the flow rate of argon was changed within a range of 0.0002 to 0.0080 Nm3/min/t per 1 t of molten steel. The gas stirring time was 15 min or more in all the cases.

Continuous casting was conducted following the ladle refining and a bloom with a cross section of 600 mm×380 mm was obtained. The amount of molten steel in a tundish during casting was 20 t, and the purge Ar flow rate was changed within a range of 0.02 to 0.13 Nm3/min/t per 1 t of molten steel. The bloom was then heated to 1260° C. and subjected to billetting to obtain a square cross section with a size of 155 mm. Then, hot rolling was performed to obtain a steel wire rod with a diameter of 5.5 mm or 8.0 mm.

A piece of 1000 g was cut from the obtained steel wire rod and used for inclusion extraction by acid dissolution and composition analysis of inclusions. Methods for the inclusion extraction and composition analysis (quantities) of inclusions are described below.

[Method for Inclusion Extraction]

First, a beaker was prepared containing an acid solution in which pure water, nitric acid (concentration 60%), and sulfuric acid (concentration 96%) were mixed at a volume ratio of 5:25:1, respectively, and the steel wire rod (1000 g) was placed into the beaker. The beaker was heated, and the wire rod was completely dissolved, while maintaining the solution temperature at 90 to 95° C. Upon dissolution, filtration with a 10 μm filter was performed. The composition of inclusions with a long diameter of 20 μm or more, from amongst the inclusions remaining in the filter, was analyzed and the number of such inclusions was measured.

[Quantitative Determination of Inclusions]

An EPMA [Electron Probe Microanalyzer, manufactured by Japan Electron Optics Laboratory Co., Ltd. (JXA-8000 Series)] was used for quantitative determination of inclusions, and quantitative analysis was conducted by characteristic X-ray energy dispersion spectroscopy under the conditions of accelerating voltage 20 kV and probe current 0.01 μA. Elements that were the objects of the quantitative analysis included Al, Mn, Si, Mg, Ca, Ti, Zr, and O. The method for quantitative determination included the steps of measuring X-ray intensity of a substance of already known concentration of these elements, plotting the relationship between the X-ray intensity and element concentration in advance as a calibration curve, and finding the concentration at which each element is present from the X-ray intensity of the inclusions that are the observation objects by using the calibration curve. Each element was assumed to be present in the form of Al2O3, MnO, SiO2, MgO, CaO, TiO2, ZrO2, the concentration at which the Al2O3, MnO, SiO2, MgO, CaO, TiO2, ZrO2 were present in the inclusions was calculated based on the concentration of each element found by the above-described quantitative analysis, inclusions containing 80% or more of Al2O3 were taken as alumina-based inclusions, and the long diameter and number thereof were measured.

Embodiment 1

Evaluation of Drawability

Drawability in the case of applying the steel wire rod with a diameter of 5.5 mm that was obtained in the above-described manner to a tire cord was evaluated according to the following items.

(Evaluation Method)

Number of wire breakages during drawing from a diameter of 5.5 mm to a diameter of 0.2 mm.

(Drawing Method)

An oxidation surface film on the steel wire rod with a diameter of 5.5 mm was removed with hydrochloric acid and then dry drawing was conducted to a diameter of 1.2 mm with a continuous drawing machine (model CD-610−7+BD610 manufactured by Showa Machine Works, Ltd.). The diameter of drawing dies used in the drawing process was 4.8, 4.2, 3.7, 3.26, 2.85, 2.5, 2.2, 1.93, 1.69, 1.48, 1.3 (units: mm for all the dies). The drawing speed in the die with a diameter of 1.2 mm was 400 m/min. The wire rod surface was coated with zinc phosphate prior to drawing, and the drawing was preformed by using a lubricant based on sodium stearate.

The wire rod that was drawn to a diameter of 1.2 mm was heated to 1230 K, and subjected to patenting in a lead bath at 830 K to obtain a fine pearlitic structure, followed by plating with brass (film thickness: about 1.5 um) containing Cu and Zn at a 7:3 ratio (mass ratio). Finally, the wire rod was drawn to the diameter of 0.2 mm with a wet-type drawing machine (Type KPZIII/25-SPZ250, manufactured by Koch, Ernst & Co., Ltd.). In the dipping bath employed for wire drawing, a solution containing 75% water and prepared by mixing a natural fatty acid, an amino acid, and a surfactant was used. The diameter of the dies used in the drawing process was 1.176, 0.959, 0.880, 0.806, 0.741, 0.680, 0.625, 0.574, 0.527, 0.484, 0.444, 0.408, 0.374, 0.343, 0.313, 0.287, 0.260, 0.237, and 0.216 (units: mm for all the dies). The drawing speed at a diameter of 0.2 mm was 500 m/min.

The conditions relating to the main raw materials for a converter are shown in Table 1 below, the chemical compositions of the steel wire rods are shown in Table 2 below, and the results on drawability, together with the conditions of secondary refining, are shown in Table 3 below.

TABLE 1
Raw materials for converter
MoltenColdMoltenCold ironScrapP
TestironironScrapTotaliron ratioratioratioconcentration
No.(t)(t)(t)(t)(wt. %)(wt. %)(wt. %)(wt. %)
126400264100.00.00.00.007
226000260100.00.00.00.010
32600226299.20.00.80.015
42570526298.10.01.90.011
52525326096.91.91.20.013
62525526296.21.91.90.013
72525025798.11.90.00.014
825010026096.23.80.00.017
92529026196.63.40.00.022
102580426298.50.01.50.025
1126400264100.00.00.00.007
122600426498.50.01.50.010
132604226697.71.50.80.012
142549026396.63.40.00.009
1525500255100.00.00.00.010
1626400264100.00.00.00.012
1726000260100.00.00.00.013
1824219026192.77.30.00.013
1924816026493.96.10.00.015
2025111026295.84.20.00.016
2125012026295.44.60.00.012
222438926093.53.13.50.014
232530726097.30.02.70.014
2425001026096.20.03.80.011
2525101326495.10.04.90.012
2625014026494.75.30.00.015
2724801626493.90.06.10.016

TABLE 2
Chemical composition of steel material
TestCSiMnAlNiCuCrLiMgCeLa
No.(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(ppm)(ppm)(ppm)(ppm)
10.880.340.470.0030.000.620.000.000.00680
20.820.240.490.0020.550.000.650.050.0000
30.710.220.450.0010.520.841.371.870.061337
40.830.250.510.0010.000.000.510.000.0000
50.730.210.480.0020.000.000.000.028.4000
60.780.350.520.0010.000.000.000.000.00050
70.770.230.500.0020.000.350.350.000.0000
80.800.180.520.0020.000.000.000.000.0000
90.780.200.490.0020.000.000.000.000.0000
100.830.180.540.0020.450.000.000.0010.0000
110.880.340.470.0030.000.620.000.000.00680
120.780.350.520.0010.000.000.000.000.0000
130.770.230.500.0020.000.350.350.000.0000
140.820.240.490.0020.550.000.650.030.0000
150.820.210.450.0010.000.000.000.000.0000
160.820.240.490.0020.800.000.570.000.0000
170.850.240.550.0030.000.000.000.007.5000
180.810.190.550.0030.000.000.000.000.00270
190.830.220.610.0010.000.000.000.000.0000
201.050.250.580.0010.000.000.000.000.001524
210.970.230.560.0020.250.310.4315.450.0000
220.770.240.630.0010.000.500.000.000.003255
230.850.340.500.0020.000.001.210.000.00680
240.720.310.420.0010.000.000.000.000.0000
250.770.200.630.0020.000.651.410.000.00260
260.820.150.630.0010.000.000.000.000.0000
270.970.300.800.0010.510.000.570.000.0000

TABLE 3
Gas flow rate inFlow rate ofNumber of Al2O3-
stirring of moltenpurgingtype inclusions withNumber of
steel inAr gas toa long diameter of 20 μmbreakages of
Testsecondary refiningtundishor moresteel wire per 10 t
No.(Nm3/min/t)(Nm3/min/t)(number/1000 g)(number/10 t)
10.00100.0821.9
20.00250.0732.8
30.00370.1053.9
40.00300.0965.6
50.00050.0497.8
60.00380.0787.0
70.00090.0556.1
80.00130.0677.8
90.00090.10613.5
100.00110.081416.1
110.00030.081515.4
120.00450.081314.2
130.00020.081716.8
140.00750.081917.7
150.00110.1265.0
160.00230.032024.1
170.00250.022528.0
180.00100.071517.3
190.00250.071513.6
200.00130.091612.0
210.00300.081312.7
220.00050.121716.1
230.00100.081312.2
240.00380.111814.3
250.00090.081818.0
260.00030.082119.5
270.00700.082520.0

These results suggest the following (No. below denotes the Test No. in Tables 1 to 3).

In No. 1 to 8, the conditions stipulated by the present invention were satisfied. Therefore, it is clear that the number of Al2O3 inclusions decreased, the number of wire breakages during wire drawing was small, and excellent drawability was attained. By contrast, in No. 9 to 27, the conditions stipulated by the present invention were not satisfied. As a result, the number of wire breakages during wire drawing was large and drawability degraded.

More specifically, in No. 9, 10, the concentration of P in the raw materials for a converter exceeded 0.02%. Therefore, excellent drawability could not be ensured. In No. 11 to 14, the mixing ratio of raw materials, P concentration, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the flow rate of the stirring gas for the molten steel in secondary refining was outside the range stipulated by the present invention. As a result, drawability was degraded. In No. 15 to 17, the mixing ratio of main raw materials, P concentration, and flow rate of the stirring gas for the molten steel in secondary refining were within the ranges stipulated by the present invention, but the purge Ar flow rate in the tundish was outside the range stipulated by the present invention. As a result, drawability was degraded. In No. 18 to No. 25, P concentration of main raw materials, flow rate of the stirring gas for the molten steel in secondary refining and the purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the mixing ratio of the main raw materials was outside the range stipulated by the present invention, therefore drawability was degraded. In No. 26, 27, the purge Ar flow rate in the tundish was within the range stipulated by the present invention, but the mixing ratio of raw materials and the flow rate of the stirring gas for the molten steel in secondary refining were outside the ranges stipulated by the present invention. As a result, drawability was degraded.

Embodiment 2

Evaluation of Fatigue Property

Fatigue property in the case of applying the steel wire rod with a diameter of 8.0 mm that was obtained in the above-described manner to a spring was evaluated according to the following items.

(Evaluation method) Nakamura-type rotating-bending fatigue test of a steel wire rod with a diameter of 8.0 mm

(Sample Preparation Method and Test Method)

A steel wire rod with a diameter of 8.0 mm was subjected sequentially to oil tempering, stress-relief annealing, shot peening, and secondary stress-relief annealing. Then, a fatigue test was performed under the following conditions by using a Nakamura-type rotating-bending fatigue machine and the fatigue property was evaluated by finding a wire breakage ratio.

(Fatigue Test Conditions)

Test piece length: 650 mm,

Number of test pieces: 30,

Test load: 95.8 kgf/mm2 (940 MPa),

Rotating speed: 4,500 rpm,

Frequency of test stop: 2×107,


Calculation formula for wire breakage ratio: Wire breakage ratio=(number of broken test pieces)/(total number of test pieces)×100(%)

The conditions relating to the main raw materials for a converter are shown in Table 4 below, the chemical compositions of the steel wire rods are shown in Table 5 below, and the results on drawability, together with the conditions of secondary refining, are shown in Table 6 below.

TABLE 4
Raw materials for converter
MoltenColdMoltenCold ironScrapP
TestironironScrapTotaliron ratioratioratioconcentration
No.(t)(t)(t)(t)(wt. %)(wt. %)(wt. %)(wt. %)
2826300263100.00.00.00.010
2925800258100.00.00.00.013
302610226399.20.00.80.011
312520525798.10.01.90.011
322505526096.21.91.90.015
332603026398.91.10.00.014
342555026098.11.90.00.017
352567026397.32.70.00.016
3625110026196.23.80.00.015
372555026098.11.90.00.022
3826100261100.00.00.00.026
3926000260100.00.00.00.007
402600426498.50.01.50.010
412604226697.71.50.80.012
4226000260100.00.00.00.012
4326400264100.00.00.00.012
4424910025996.13.90.00.013
4525113026495.14.90.00.013
4624515026094.25.80.00.013
4723420025492.17.90.00.011
482439525794.63.51.90.015
4924741126294.31.54.20.014
5025301026396.20.03.80.013
5124501425994.60.05.40.011
5224501626193.90.06.10.011
5324501626193.90.06.10.011
5424519026492.87.20.00.014

TABLE 5
Chemical composition of steel material
TestCSiMnAlNiCuCrLiMgCeLa
No.(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(ppm)(ppm)(ppm)(ppm)
280.701.990.750.0030.000.001.150.000.0050
290.711.800.770.0020.000.000.000.000.00094
300.581.450.550.0020.000.000.000.006.5000
310.621.850.850.0020.000.000.000.000.0000
320.671.930.770.0010.000.000.000.190.0000
330.702.020.820.0010.330.001.210.000.0500
340.651.510.610.0010.000.210.000.000.0005
350.721.990.750.0030.920.841.370.320.205281
360.681.870.770.0020.000.000.000.000.007378
370.701.800.810.0010.000.000.000.000.0000
380.651.960.770.0020.000.000.560.0014.0000
390.661.500.800.0030.000.000.620.000.0000
400.681.850.910.0010.000.000.630.230.0000
410.672.060.840.0020.350.000.350.000.0000
420.691.450.810.0020.000.000.000.000.0000
430.621.490.690.0020.000.000.000.000.0000
440.591.670.780.0030.000.000.000.000.0000
450.681.910.880.0010.000.000.750.000.0000
460.681.910.880.0010.000.310.750.000.00055
470.591.460.810.0030.000.000.000.000.00098
480.611.470.680.0020.000.000.000.000.0000
490.681.990.760.0010.000.000.860.520.0600
500.681.910.880.0010.340.001.250.000.0000
510.591.460.810.0030.000.000.000.000.0000
520.621.740.740.0020.000.481.290.300.00440
530.621.740.740.0020.000.000.350.000.0000
540.702.010.870.0020.000.000.420.220.0000

TABLE 5
Chemical composition of steel material
TestCSiMnAlNiCuCrLiMgCeLa
No.(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(wt. %)(ppm)(ppm)(ppm)(ppm)
280.701.990.750.0030.000.001.150.000.0050
290.711.800.770.0020.000.000.000.000.00094
300.581.450.550.0020.000.000.000.006.5000
310.621.850.850.0020.000.000.000.000.0000
320.671.930.770.0010.000.000.000.190.0000
330.702.020.820.0010.330.001.210.000.0500
340.651.510.610.0010.000.210.000.000.0005
350.721.990.750.0030.920.841.370.320.205281
360.681.870.770.0020.000.000.000.000.007378
370.701.800.810.0010.000.000.000.000.0000
380.651.960.770.0020.000.000.560.0014.0000
390.661.500.800.0030.000.000.620.000.0000
400.681.850.910.0010.000.000.630.230.0000
410.672.060.840.0020.350.000.350.000.0000
420.691.450.810.0020.000.000.000.000.0000
430.621.490.690.0020.000.000.000.000.0000
440.591.670.780.0030.000.000.000.000.0000
450.681.910.880.0010.000.000.750.000.0000
460.681.910.880.0010.000.310.750.000.00055
470.591.460.810.0030.000.000.000.000.00098
480.611.470.680.0020.000.000.000.000.0000
490.681.990.760.0010.000.000.860.520.0600
500.681.910.880.0010.340.001.250.000.0000
510.591.460.810.0030.000.000.000.000.0000
520.621.740.740.0020.000.481.290.300.00440
530.621.740.740.0020.000.000.350.000.0000
540.702.010.870.0020.000.000.420.220.0000

TABLE 6
Gas flow rate in
stirring of moltenFlow rate ofNumber of Al2O3-type
steel inpurging Arinclusions with a long
Testsecondary refininggas to tundishdiameter of 20 μm orWire breakage ratio
No.(Nm3/min/t)(Nm3/min/t)more (number/1000 g)(%)
280.00100.08317
290.00250.07217
300.00370.10427
310.00300.09533
320.00050.04937
330.00380.10523
340.00090.05427
350.00130.06837
360.00120.10843
370.00090.09753
380.00070.101663
390.00030.081657
400.00450.081253
410.00550.081960
420.00100.12427
430.00130.021960
440.00180.012570
450.00130.121763
460.00130.131763
470.00300.101777
480.00100.092080
490.00250.091767
500.00130.091763
510.00300.101777
520.00050.092277
530.00020.092483
540.00800.092583

These results suggest the following (No. below denotes the Test No. in Tables 4 to 6).

In No. 28 to 36, the conditions stipulated by the present invention were satisfied. Therefore, it is clear that the breakage ratio during the fatigue test was low and excellent fatigue property was attained. By contrast, in No. 37 to 54, the conditions stipulated by the present invention were not satisfied. As a result, the number of wire breakages during fatigue test was high and the fatigue property degraded.

More specifically, in No. 37, 38, the concentration of P in the raw materials for a converter exceeded 0.02%. Therefore, excellent fatigue property could not be ensured. In No. 39 to 41, the mixing ratio of raw materials, P concentration, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the flow rate of the stirring gas for the molten steel in secondary refining was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 42 to 44, the mixing ratio of raw materials, P concentration, and flow rate of the stirring gas for the molten steel in secondary refining were within the ranges stipulated by the present invention, but the purge Ar flow rate in the tundish was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 45 to 52, the P concentration of the main raw materials, flow rate of the stirring gas for the molten steel in secondary refining, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the mixing ratio of raw materials was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 53, 54, the purge Ar flow rate in the tundish was within the range stipulated by the present invention, but the mixing ratio of raw materials and the flow rate of the stirring gas for the molten steel in secondary refining were outside the ranges stipulated by the present invention. As a result, drawability was degraded.