KR100891764B1 - Spring steel wire rod excellent in pickling performance - Google Patents

Abstract

The steel wire for spring of the present invention is C: 0.35 to 0.7% (meaning the mass%, hereinafter, the same), Si: 1.5 to 2.50%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.9%, Cu: 0.05 to 0.7%, Ni: 0.15 to 0.8%, P: 0.02% or less (not including 0%), and S: 0.02% or less (not including 0%), and the ratio of Si and Cu (Si / Cu) is in the range of 4 or more, the difference between the Cu concentration in the surface layer and the Cu concentration in the steel is 0.50% or less, and the difference between the Ni concentration in the surface layer and the Ni concentration in the steel is 1.00% or less. In the spring steel wire which contains much Si and contains Cr, Cu, and Ni further, pickling property improves.

Description

Steel wire for pickling excellent springs {SPRING STEEL WIRE ROD EXCELLENT IN PICKLING PERFORMANCE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire rod for springs having excellent pickling performance, and specifically, for springs containing a large amount of Si and further containing Cr, Cu, and Ni. It relates to a technology for increasing pickling of steel wire rods. The steel wire for spring of the present invention is suitably used for valve springs used in engines such as automobiles, suspension springs such as clutch springs, brake springs, stabilizers, torsion bars, and the like. .

The chemical composition of the spring steel used for valve springs, suspension springs, etc. is prescribed | regulated to JIS G 3565-JIS G 4801 etc., for example, and the appropriate steel grade is used according to the kind of spring design, etc. In recent years, as the weight reduction of the spring progresses with the reduction of the exhaust gas and the fuel consumption rate, the design stress of the spring also increases, for example, the high strength of the spring element wire (quenching tempering material) of about 1600 MPa or more. There is a desire to provide a steel wire for the spring that can realize the. In addition, in order to increase the durability in the atmosphere, which is one of the important characteristics of the spring, improvement of proof stress is also desired, and Si and Cr alloy elements which can improve the proof strength by solid solution strengthening are required. There is a tendency to use a steel wire containing a lot as. Moreover, in order to improve the corrosion fatigue characteristic which is one of the important characteristics of the spring of that excepting the above, Cu and Ni are added and the improvement of corrosion resistance is tried.

In general, the spring is drawn to a predetermined wire diameter after applying a lubricant to the surface of the wire rod (rolled wire rod) heated and hot rolled to a steel sheet, if necessary, and performing a coating treatment (surface coating treatment). It is manufactured by processing (hot forming or cold forming). Since heating is normally performed in an oxidizing atmosphere, an oxide layer of Fe oxide called "rolling scale" or "scale" is formed on the surface of the rolled wire rod. If the spring is manufactured using the rolled wire rod with the scale attached, surface defects or the like may occur, resulting in deterioration in quality. Therefore, pickling treatment for removing the scale is performed before the drawing process. .

In FIG. 1, the photograph which observed the cross section of the rolled wire rod with scale on the surface of the steel which contains a large amount of Si and further contains Cr, Cu, and Ni using the Fe-SEM apparatus is shown. This is No. of the Example mentioned later. It corresponds to E-1. As shown in FIG. 1, the scale is sequentially from the surface layer side, hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), wustite (FeO), pyalite (fayalite; 2FeOSiO ₂ ). Between the steel (base steel) and the scale, a subscale in which Si or Cr are concentrated is additionally generated.

Among these, pyalite is a low melting oxide seen in the case of using a steel containing a large amount of Si, and is a refractory material that is difficult to peel off in a normal pickling process. For example, when a steel containing Si and Cr is heated above the process temperature (about 1170 ° C.) of pyalite and wustite, a dense molten phase in which these oxides are intricately intertwined is formed, and also 1200 ° C. When heated above, Cr invades into the said molten phase and a pialite, it concentrates in the interface with a branch iron, and a thickening layer (details later) is formed. First, it is very difficult to remove the concentrated Cr by a subsequent step, and the pickling property is lowered. When pickling property falls, scale remains after pickling treatment, and adhesion with the lubricant (used for surface coating treatment) applied to the surface is lowered, and there is a risk of disconnection during drawing. Even if the wire does not reach a disconnection, cracks (cracking) may occur during the drawing process, and may break during spring forming (cold coiling). These problems can be reduced, for example, by increasing the pickling treatment time to completely remove the scale. However, since the immersion time in the acid solution becomes long, the attack by acid on the ground iron becomes severe and the surface roughness is increased. This results in a decrease in the temperature and finally also the durability in the atmosphere. In addition, part of the hydrogen generated during the attack by acid on the iron is rapidly diffused and absorbed in the steel, so that the amount of hydrogen occlusion increases, causing embrittlement of the steel (hydrogen embrittlement), leading to disconnection during drawing. .

In addition, although not shown in FIG. 1, a concentrated layer of Cu or Ni is also generated between the steel (ferrous iron) and the scale. This concentrated layer is seen when Cu and Ni are contained in steel. Metals that are more precious than iron such as Cu and Ni are not oxidized by heating, so these metals are concentrated in the interface between scale and base iron or pyolite, and surface cracks due to red shortness are prevented. It is known to occur. For example, although the melting point of Cu is about 1080 ° C., the heating of the steel before hot rolling is usually carried out at a temperature higher than the melting point of Cu (eg, about 1100 to 1250 ° C.), so that the molten Cu (liquid phase) is scaled and branched. It is thought that the surface cracks due to the red brittleness are generated because the Cu is formed at the interface of the Cu, and the Cu is concentrated and the Cu is thickened and it cannot withstand the shear stress or tensile stress during hot rolling. This phenomenon is referred to as liquid metal melt embrittlement, and consequently is considered to result in a drop in pickling.

Various proposals have been made to prevent the thickening of Cu to the base iron interface and to prevent surface cracking due to red brittleness.

For example, Japanese Patent Application Laid-Open No. 194-297026 discloses that Si is added to steel, and Si is present in the scale in the heating step before hot rolling to produce a SiO ₂ -FeO-based low melting oxide liquid scale, and the Cu melt is The method of preventing surface cracking by receiving in the scale is described.

Japanese Patent Laid-Open No. 2004-223523 is a technique relating to a heating method for Cu-containing steels proposed by the same applicant as Japanese Patent Laid-Open No. 194-297026. In this document, as in Japanese Patent Application Laid-Open No. 1994-97026, a method capable of preventing surface cracks caused by the thickening of Cu without changing the steel component, the heating atmosphere temperature and the heating conditions before hot rolling. A method of controlling the oxygen concentration in the atmosphere is described.

As mentioned above, although the spring steel wire material containing a large amount of Si and containing Cr, Cu, and Ni is mainly used for the improvement of a load-bearing force and corrosion fatigue property, such a spring steel wire material is a scale (especially a pialite). Since Cu is concentrated at the interface between the iron and the iron, the pickling property is deteriorated.

However, a method of sufficiently increasing the pickling property of such a spring steel wire is not disclosed. Japanese Laid-Open Patent Publication No. 2004-223523 described above is a technique for preventing surface cracking due to the concentration of Cu without adding a large amount of Si, and improvement of the yield strength by addition of Si is not obtained. On the other hand, as described in Japanese Patent Laid-Open Publication No. 1994-297026, the prior art column of the Japanese Laid-Open Patent Publication No. 2004-223523 proposed later, "Steel-added steel is poor in peelability of scale. Descaling and descaling by high-pressure water before rolling also makes it difficult to peel and remove scales, leaving the scales red, resulting in redness of the surface of the steel, and impairing surface properties. If there is, the scale is hard to be dissolved by pickling, so there is a problem of an increase in the cost of the pickling process and a decrease in productivity. And have problems.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a spring steel wire material having improved pickling property in a spring steel wire material containing a large amount of Si and further containing Cr, Cu, and Ni. .

The spring steel wire rod of the present invention is excellent in pickling property because the thickening of the scale is very thin in addition to the significant concentration of Cr and Ni in the surface layer portion being significantly suppressed. When a spring is manufactured using the spring steel wire material of this invention, since a scale peels easily by a pickling process, the spring excellent in the surface property can be provided.

Steel wire for the spring according to the present invention that can solve the above problems, C: 0.35 to 0.7% (meaning of the mass%, hereinafter, the same unless otherwise specified), Si: 1.5 to 2.50%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.9%, Cu: 0.05 to 0.7%, Ni: 0.15 to 0.8%, P: 0.02% or less (not including 0%), and S: 0.02% or less (not including 0%) And the ratio of Si and Cu (Si / Cu) is in the range of 4.0 or more, the difference between the Cu concentration in the surface layer and the Cu concentration in the steel is 0.50% or less, and the difference between the Ni concentration in the surface layer and the Ni concentration in the steel is 1.00. % Or less

In a preferred embodiment, the steel wire for spring according to the present invention further contains at least one member selected from the group consisting of V: 0.07 to 0.4%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1%.

The spring of this invention which can solve the said subject is obtained using the steel wire material for any one of the above.

The spring steel wire rod of the present invention is excellent in pickling property because not only the concentration of Cr and Ni in the surface layer portion is significantly suppressed but also the thickness of the scale is very thin. When the spring is manufactured using the spring steel wire of the present invention, the scale can be easily peeled off by the pickling process, so that a spring having excellent surface properties can be provided.

MEANS TO SOLVE THE PROBLEM This inventor earnestly examined in order to improve the pickling property of the steel wire material for spring which contains a large amount of Si and contains Cr, Cu, and Ni further. As a result, as will be described in detail later, in particular, if (i) the descale process before hot rolling and (ii) the hot rolling process are appropriately controlled, the concentration of Cu and Ni on the surface of the wire rod (particularly, Cu, Ni concentration) is suppressed, and the difference between the Cu concentration in the surface layer and the Cu concentration in the steel (hereinafter abbreviated as ΔCu) and the difference between the Ni concentration in the surface layer and the Ni concentration in the steel (hereinafter abbreviated as ΔNi) In some cases, it has been found that the pickling properties are remarkably improved, thereby completing the present invention.

Specifically, (i) In the descale step before hot rolling, a predetermined high water pressure shower is performed. Thereby, Cu taken in in a pialite peels and can reduce the Cu concentration of a surface layer part. In addition, in the (ii) hot rolling process, a predetermined high water pressure shower is performed. Thereby, the thickening of Cu to the pialite produced during hot rolling can be suppressed remarkably.

Hereinafter, the present invention will be described in detail.

As described above, the steel wire for spring of the present invention is C: 0.35 to 0.7%, Si: 1.5 to 2.50%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.9%, Cu: 0.05 to 0.7%, Ni: 0.15 to 0.8%, P: 0.02% or less (does not contain 0%), S: 0.02% or less (does not contain 0%), remainder: Fe and a spring steel wire satisfying inevitable impurities, Si and The ratio of Cu (Si / Cu) is in the range of 4.0 or more, the difference between the Cu concentration in the surface layer and the Cu concentration in the steel is 0.50% or less, and the difference between the Ni concentration in the surface layer and the Ni concentration in the steel is 1.00% or less. Doing.

In this specification, a "steel wire material" is a steel material (rolled material) hot-rolled linearly after heating a steel piece, and means the thing before a pickling process is performed.

First, the component in steel is demonstrated.

C: 0.35 to 0.7%

C is an element that contributes to the improvement of the strength (hardness) after quenching and tempering, thereby enhancing atmospheric durability. If the amount of C is less than 0.35%, the above effect cannot be exerted effectively. On the other hand, if the amount of C is more than 0.7%, toughness and ductility deteriorates, cracks are easily propagated, and durability is lowered. It has an adverse effect. It is preferable that amount of C is 0.39% or more and 0.54% or less.

Si 1.5 to 2.50%

Si is an element which can contribute to strength improvement and also improve the strength as a solid solution strengthening element. If Si is less than 1.5%, the matrix strength is insufficient. However, when Si exceeds 2.50% and becomes excessive, the ratio of the pialite which occupies in a scale will increase, and pickling property will fall. It is preferable that Si is 1.70% or more and 2.1% or less.

Mn 0.05 to 1.0%

Mn is an element which raises hardenability in steel. In order to exert such an effect effectively, the amount of Mn added is made 0.05% or more. However, when the amount of Mn is excessively added beyond 1.0%, hardenability increases and supercooled structure tends to be produced, and drawing workability deteriorates. In addition, when carrying out an annealing process for the purpose of softening a wire rod before the pickling process after hot rolling like the "spring process (c)" mentioned later, high cost cannot be avoided. It is preferable that Mn is 0.12% or more and 0.8% or less. In the present invention, in order to prevent the formation of MnS, which is a starting point of destruction, MnS is not generated as much as possible by reducing the content of S or adding other sulfide forming elements such as Cu as described later. .

Cr 0.05 to 1.9%

Cr is an element that not only contributes to the improvement of corrosion resistance by making amorphous and dense rust formed in the surface portion under corrosion conditions, but also effectively improves the hardenability like Mn. In order to exert this effect effectively, Cr is added at 0.05% or more. However, when Cr exceeds 1.9%, the supercooled structure is likely to be formed during cooling after rolling, and the drawing workability is lowered. When the annealing process is performed for the purpose of softening the wire rod before the pickling treatment after hot rolling, as in the "spring process (c)" described later, high cost cannot be avoided. It is preferable that Cr is 0.15% or more and 1.75% or less.

Cu 0.05 to 0.7%

Cu is an element that is electrochemically more precious than iron and has an effect of improving corrosion resistance. In order to exert such an effect effectively, Cu is added at 0.05% or more. However, if Cu exceeds 0.7%, the amount of Cu concentration in the surface of the ground iron layer, the interface between the ground iron and the rolling scale, and the pialite increases, and the pickling property is impaired. It is preferable that Cu is 0.20% or more and 0.5% or less.

Ni 0.15 to 0.8%

In addition to increasing toughness after quenching and tempering, Ni has an effect of raising the corrosion resistance by making the rust generated in the surface layer portion amorphous and dense under corrosion conditions. It also has the effect of suppressing ferrite decarburization occurring before and during rolling. In order to exert such an effect effectively, Ni is added at least 0.15%. However, when Ni exceeds 0.8%, hardenability increases and it becomes easy to produce a supercooled structure after rolling. In addition, the amount of retained austenite also increases, and the spring hardness decreases. It is preferable that Ni is 0.25% or more and 0.55% or less.

P: 0.02% or less (does not include 0%)

P segregates at the old austenite grain boundary, embrittles the grain boundary, and degrades the delayed fracture characteristics. Therefore, P is as small as possible. In this invention, an upper limit shall be 0.02% on industrial production.

S: 0.02% or less (not including 0%)

Since S segregates to the old austenite grain boundary, embrittles the grain boundary, and degrades the delayed fracture property, it is better to use S as little as possible. In this invention, an upper limit shall be 0.02% on industrial production.

Si Wow Cu Of ( Si Of Cu ): 4.0 or higher

In the present invention, not only the amounts of Si and Cu are determined, but also the lower limit of (Si / Cu) is determined. As a result, the amount of Cu to be concentrated at the interface between the iron and the scale by controlling the addition of the pyolite produced by the Si addition and the Cu is controlled appropriately, so that the concentration of Cu in the pyolite is suppressed and the ΔCu is small. do. As a result, pickling becomes high (refer to the Example mentioned later). The upper limit of (Si / Cu) is set to 50 by content of Si and Cu mentioned above. (Si / Cu) is preferably 4.5 or more and 35 or less.

The steel wire for spring of this invention contains the said component, and remainder is iron and an unavoidable impurity.

In the present invention, the composition further contains at least one selected from the group consisting of V: 0.07 to 0.4%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1% for the purpose of further improving the hydrogen embrittlement resistance. It is desirable to. Hereinafter, each element is explained concretely.

V: 0.07 to 0.4%

V is an element which forms fine carbides or nitrides and contributes to the improvement of hydrogen embrittlement resistance. In addition, fatigue characteristics are also increased. In addition, the grain refining effect improves toughness and strength, and contributes to improvement of corrosion resistance and resistance to permanent set in fatigue. In order to exhibit such an effect effectively, it is preferable to add V 0.07% or more. However, when V is added in excess of 0.4%, the amount of carbide which is not dissolved in austenite during quenching heating increases, not only sufficient strength and hardness can not be obtained, but also the amount of retained austenite increases and the spring hardness decreases. do. As for V, it is more preferable that they are 0.1% or more and 0.2% or less.

Ti 0.01 to 0.1%

Ti is an element effective in refining the former austenite grains after quenching and tempering, and improving the hydrogen embrittlement resistance. It also has atmospheric durability. In order to exhibit such an effect effectively, it is preferable to add Ti or more. However, when Ti is excessively added, coarse nitride tends to precipitate and atmospheric durability decreases. Therefore, the upper limit is preferably made 0.1%. As for Ti, it is more preferable that they are 0.04% or more and 0.09% or less.

Nb 0.01 to 0.1%

Nb is an element that forms fine precipitates composed of carbides, nitrides, sulfides, and complex compounds thereof to contribute to the improvement of hydrogen embrittlement resistance. In addition, the toughness and the yield strength are also improved by the grain refinement effect. In order to exhibit such an effect effectively, it is preferable to add Nb or more. However, if Nb is added in excess of 0.1%, the amount of carbide which is not dissolved in austenite during quenching heating increases, and a predetermined tensile strength is not obtained. As for Nb, it is more preferable that they are 0.02% or more and 0.05% or less.

In the above, the component in steel which concerns on this invention was demonstrated.

Superficial Cu  Concentration and river Cu  Difference in concentration (Δ Cu ): 0.50% or less

In the present invention, ΔCu is suppressed as low as 0.50% or less. As mentioned above, although the scale (primary scale) produced | generated by heating can be removed most by the normal descale process performed before hot rolling, the scale produced | generated during hot rolling and cooling after hot rolling (Secondary scale), especially in the case of steel containing high Si and Cu, as in the present invention, is not easily removed by a normal descale treatment. Cu precipitates in the interface or in the pilite. As a result, pickling property is reduced. In the present invention, as described in detail later, not only the descaling step before hot rolling but also the finishing rolling step are appropriately controlled, so that ΔCu can be kept low. It is so preferable that there are few (DELTA) Cu, it is preferable that it is 0.45% or less, and it is more preferable that it is 0.40% or less.

Here, the measuring method of "Cu concentration of surface layer" is demonstrated using FIG. Fig. 2 is a drawing measured by EPMA line quantitative analysis of the following conditions directed from the surface layer portion to the center of the interior using a specimen prepared as follows, showing the relationship between the Cu concentration (%) and the distance from the surface. have. Here, regarding the Fe, when the EPMA line quantitative analysis is performed in the above manner, the point when the X-ray intensity (cps) of Fe reaches the maximum value is the ferrous interface (the boundary between scale and ferrous). The area | region between a surface and a base steel interface is defined as "surface layer part", and the maximum value of Cu amount in the said surface layer part is defined as "Cu concentration of a surface layer." Although the state of said "surface layer part" changes also with manufacturing conditions, etc. of components in steel, etc., it contains at least pialite.

EPMA measuring device: X-ray microanalyzer `` JXA-8800 RL '' made by JEOL Ltd. is used

Specimen: The steel material with the scale attached to it was embedded in the resin, and mirror-finished the cross section perpendicular to the rolling direction, followed by vapor deposition using osmium to maintain conductivity.

Acceleration Voltage: 15kV

Irradiation Current: 0.3μA

Quantitative line analysis: 1 µm distribution interval, measuring 300 points in total

Surface layer Ni  Concentration and river Ni  Difference in concentration (Δ Ni ): 1.00% or less

In the present invention, ΔNi is also suppressed as low as 1.00%. As described above, Ni is an element that contributes to the improvement of corrosion resistance similarly to Cu, and is similar to Cu in that Ni precipitates in the interface between scale (pyalite) and branch iron or pyalite in the heating step and the hot rolling step. . As a result, pickling property is reduced. Therefore, in the present invention, similarly to Cu, it is also necessary to suppress the concentration of Ni. As there are few (DELTA) Ni, it is so good that it is preferable, for example, it is preferable that it is 0.90% or less, and it is more preferable that it is 0.85% or less.

Here, the "Ni concentration of the surface layer" is the same as above in the range of 0.3 mm from the surface layer part toward the center inside using the test material produced by the same method used for the above-mentioned measurement of "Cu concentration of the surface layer". It means the maximum amount of Ni when Ni amount is measured by EPMA line quantitative analysis of conditions. 3 shows the relationship between the Ni concentration (%) and the distance from the surface layer portion.

In the above, (DELTA) Cu and (DELTA) Ni which characterize this invention were demonstrated.

As shown below, it is preferable that the thickness and composition of a scale are suitably controlled, and, as a result, the pickling property is further improved.

(Thickness of scale)

It is preferable that the thickness of a scale is 40 micrometers or less. As described in detail below, in consideration of scale peeling due to cracks (cracks) generated in the scale, the thickness of the scale is more preferably 5 µm or more and 35 µm or less.

In the scale, for example, microlevel cracks may be generated during the cooling process after rolling or the handling of the rolled wire rod. It is thought that pickling property improves because there are many cracks, since peeling of the scale from a surface of an iron is easy. In general, as the thickness of the scale becomes thinner, the crack tends to occur due to the decrease in scale strength, but when the thickness of the scale is too thin, the ductility of the scale itself increases, so that the internal stress decreases, so the crack becomes less. . Therefore, the thickness of the scale is preferably in the above range.

(The composition of the scale)

The composition of the scale is generally in volume proportions: pialite: 2 to 10% (more preferably 3 to 7%), wustite: 2 to 20% (more preferably 10 to 18%), magnetite: 35 to It is preferred to satisfy 70% (more preferably 37-50%), and hematite: 20-60% (more preferably 30-55%). As described above, according to the present invention, since the composition of the scale is controlled so that the ratio of refractory pialite is low and the ratio of ustite and magnetite excellent in scale peelability is increased, pickling performance is further improved.

In the above, the spring steel wire material of this invention was demonstrated.

Next, a method of manufacturing the spring steel wire material will be described.

The production method of the spring steel wire rod includes (a) a heating step, (b) a soaking step, (c) a descaling step before the hot rolling step, and (d) a hot rolling step. In the present invention, in particular, (c) in a descale process before hot rolling, for example, a predetermined high-pressure shower is performed to quickly remove scales containing pyalite, and (d) produced during hot rolling in a hot rolling process. In order to prevent the concentration of Cu in the used pialite, a predetermined water-cooled shower is performed, whereby it is possible to significantly suppress ΔCu and ΔNi. As shown in the examples described later, according to the present invention, despite the use of steel containing a large amount of Si and containing Cr, Cu, Ni, the concentration of Cu and Ni to the surface layer portion is remarkably suppressed. Since the thickness is also thin, it is possible to provide a spring having a tensile strength of about 1600 MPa or more and excellent surface properties.

Hereinafter, each process is explained in full detail.

(A) heating process

Although heating conditions are not specifically limited, Usually, it heats at the temperature of 700 degreeC-1000 degreeC (750 degreeC-900 degreeC) at the temperature increase rate of 10 degreeC / min or more. When the temperature increase rate is less than 10 ° C / min, the concentration of Cr to the surface layer portion cannot be effectively prevented. The temperature increase rate should be as fast as possible, and it is preferable that it is 15 degreeC / min or more. Moreover, when heating temperature exceeds the said range, concentration of Cu and Ni advances and Cu amount and Ni amount of a surface layer increase. On the other hand, when heating temperature is less than the said range, since heating is not fully performed, rough rolling is impossible.

(B) cracking process

It is preferable to crack here for 20 minutes-60 minutes (more preferably, 30 minutes-50 minutes) at the temperature of 1050 degreeC-1250 degreeC (preferably 1100 degreeC-1200 degreeC) generally. This cracking condition is determined in order to prevent the concentration of Cu or Ni in the surface layer and to suppress the progression of grain boundary oxidation. For example, when the cracking temperature and the cracking time exceed the above ranges, the concentration of Cu and Ni tends to proceed easily. On the other hand, when the cracking temperature or the cracking time is lower than the above range, the concentration of Cu to the austenite grain boundary proceeds.

In the present invention, the heating temperature of the heating process and the cracking temperature of the cracking process need not necessarily coincide. For example, in a later example, although the crack temperature is about 200 to 300 ° C higher than the heating temperature, this is because the temperature at the time of cracking increases due to the residence time before cracking after heating.

(C) before hot rolling Descale  fair

Here, in order to quickly remove the pie Al-containing light scale, about 80kgf / mm ² (≒ 785MPa) to ^{160kgf / mm 2 (≒ 1569MPa)} , more preferably from about 100kgf / mm ² (≒ 981MPa) to 120kgf / mm ² Under a water pressure of (# 1176 MPa), it is preferable to take a high water pressure shower for about 1 second to 10 seconds (more preferably, 3 seconds to 7 seconds). Thereby, a subsequent hot rolling process can be performed quickly. When the water pressure of the shower is less than 80 kgf / mm ² , the scale becomes thick, which may cause the occurrence of surface defects due to biting during hot rolling and an increase in the surface layer Cu concentration. On the other hand, when the water pressure of a shower exceeds 160 kgf / mm < ² >, the temperature of the billet before hot rolling will fall, and rolling will become difficult.

On the other hand, the descale process before hot rolling is not limited to said high-pressure shower, For example, mechanical descaling, such as shot blasting, may be performed.

(D) hot rolling process

Here, a predetermined water-cooled shower is performed in order to prevent the concentration of Cu in the pialite generated during hot rolling and to control the composition of the scale appropriately.

Specifically, cooling by shower is performed in the finish rolling process after rough rolling. It is preferable that it is 100t / hr or more and 200t / hr or less generally, and, as for the quantity of a shower, it is more preferable that they are 120t / hr or more and 180t / hr or less. If the quantity of the shower is less than 100 t / hr, the desired scale (phialite) removing action and the reducing action of ΔCu or ΔNi are not effectively exhibited. On the other hand, when the quantity of shower exceeds 200 t / hr, steel materials will be cooled too much and a supercooled structure will precipitate.

In order to control suitably the thickness and composition of a scale mainly, finishing rolling temperature is generally made into the range of 800 degreeC-1000 degreeC (more preferably, 900 degreeC-950 degreeC).

In addition, from the above point of view, for example, after completion of the finish rolling, the cooling rate to a temperature range of about 700 ° C. is 4 ° C./sec to 20 ° C./sec (more preferably 6 ° C./sec to 15 ° C./sec). It is preferable to control in the range of. When the cooling rate in the said temperature range is less than 4 degree-C / sec, scale thickness etc. increase and pickling property falls. On the other hand, when said cooling rate exceeds 20 degree-C / sec, since the holding time of the said temperature range becomes short and the ratio of the wustite produced | generated in the said temperature range falls, pickling property falls.

In addition to said spring steel wire material, this invention also includes the spring obtained using the said steel wire material. According to the present invention, a scale flaw called red scale does not occur at all, the surface properties are excellent, and a spring with improved fatigue characteristics can be produced.

The method for producing the spring is not particularly limited, and a method commonly used may be appropriately employed. Typically, it can manufacture by either of the following spring process (a)-(c).

(a) Pickling → surface coating treatment → drawing → quenching tempering (oil tempering)

(b) Lead parting (LP) → pickling → surface coating → drawing → oil tempering

(c) loosening → pickling → surface coating → skin shaving (SV) → LP → pickling → surface coating → drawing → oil tempering

As shown in the examples which will be described later, according to the present invention, since a rolled wire rod having low ΔCu and ΔNi is obtained, even if any of the above spring steps (a) to (c) is used, the surface properties are very high. Excellent springs are obtained.

Each processing method described in said spring process (a)-(c) is not specifically limited, Usually, the method performed normally can be selected suitably. For example, the pickling treatment is typically carried out by dipping in 5 to 25% H ₂ SO ₄ at a temperature of 60 ° C to 90 ° C or in 5 to 15% of HCl at a temperature of 20 ° C to 50 ° C. .

<Example>

Hereinafter, the present invention will be described in detail by way of examples. However, the following Examples do not limit the present invention, and modifications appropriately within the scope not departing from the gist of the preceding and the latter are included within the technical scope of the present invention.

150 kg of various steels (steel grades A to K, the remainder being iron and unavoidable impurities) shown in Table 1 were melted in a small vacuum melting furnace and hot forged with billets of 155 cm angle, followed by wire rod steps 1 to 6 shown below. A steel wire with a diameter of 13.5 mm was produced under the conditions of heating, cracking, and hot rolling. Among the wire rod processes, wire rod steps 1 to 2 are examples of the present invention that satisfy all of the manufacturing conditions specified in the present invention, and wire rod steps 3 to 6 are comparative examples which do not satisfy any of the manufacturing conditions specified in the present invention. All.

(Wire Rod Process 1)

After heating to about 900 degreeC at the heating rate of 15 degree-C / min, it cracked for about 50 minutes at about 1150 degreeC, and performed the high-pressure shower for about 5 seconds under the water pressure of 100 kgf / mm <^2>(# 981 MPa), and descaled. Next, after rough rolling, finishing rolling was performed while shower cooling of 150 t / hr (finishing rolling temperature 920 degreeC), and after completion | finish rolling finish, the range to about 700 degreeC cooling rate of 7 degrees C / sec. Cooled to. Winding temperature was 875 degreeC.

(Wire Rod Process 2)

After heating to about 800 degreeC by the heating rate of 20 degree-C / min, it cracked for 30 minutes at about 1100 degreeC, descaling was performed by carrying out high pressure shower for about 7 seconds under the water pressure of 150 kgf / mm <^2>(# 1471 Mpa). Next, after rough rolling, finishing rolling was performed while shower cooling of 130t / hr (finishing rolling temperature 950 degreeC), and after completion | finish rolling, the cooling rate of 6 degreeC / sec was made into the range up to about 700 degreeC. Cooled to. Winding temperature was 925 degreeC.

(Wire Rod Process 3)

After heating up to about 1050 degreeC by the heating rate of 15 degree-C / min, it cracked for 60 minutes at about 1280 degreeC, descaling was performed by carrying out high pressure shower for about 10 second under the water pressure of 50 kgf / mm <^2> (\ 490Mpa). Next, after rough rolling, finishing rolling was performed while shower cooling of 50 t / hr (finishing rolling temperature 1000 degreeC), and after completion | finish rolling, the range to about 700 degreeC was cooled down to 2 degreeC / sec. Cooled to. Winding temperature was 980 degreeC.

Wire rod process 3 is a comparative example which does not satisfy the manufacturing conditions of the present invention, and is a heating process (high heating temperature), a cracking process (high cracking temperature), a descale process (low water pressure) before hot rolling, and hot rolling It is out of the condition of this invention in all the processes (a finishing rolling temperature is high, the quantity of shower is few, and the cooling rate from finishing rolling temperature to 700 degreeC is slow).

(Wire Rod Process 4)

As in the wire rod step 3 described above, descaling was performed before heating, cracking, and rolling. Next, finish rolling, cooling, and winding were performed similarly to the wire rod process 2 mentioned above.

The wire rod process 4 and the wire rod process 6 described later are comparative examples which do not satisfy the manufacturing conditions of the present invention, and are a heating step (high heating temperature), a cracking step (high cracking temperature), and a descaling step before hot rolling. (Low water pressure) is out of the condition of the present invention.

(Wire Rod Process 5)

As in the wire rod step 1 described above, descaling was performed before heating, cracking, and rolling. Next, finish rolling, cooling, and winding were performed similarly to the wire rod process 3 mentioned above.

Wire rod process 5 is a comparative example which does not satisfy the manufacturing conditions of the present invention, and the hot rolling process (high finish rolling temperature, low quantity of showers, slow cooling rate from finishing rolling temperature to 700 ° C) is seen. It is out of the conditions of the invention.

(Wire Rod Process 6)

As in the wire rod step 3 described above, descaling was performed before heating, cracking, and rolling. Next, finish rolling, cooling, and winding were performed similarly to the wire rod process 2 mentioned above.

About each steel wire obtained in this way, while measuring (DELTA) Cu and (DELTA) Ni by the method mentioned above, the pickling property and the thickness of the scale were measured and evaluated as follows.

(Evaluation of pickling)

The steel wire was cut into a length of 100 mm, the sample number n was 3, and the following pickling test (beaker test) was performed. Here, the experiment was carried out under the same conditions as the pickling treatment in actual operation.

Acid solution: 15% sulfuric acid, 0.5% of inhibitor (cationic amine derivative) to prevent dissolution of ferrous iron, 20g / L of ferric iron as iron powder

Immersion conditions: 10 minutes at 60 ℃

Next, the scale peeling rate after pickling was measured as follows. In the present Example, it is "the scale peeling rate after pickling" by the percentage (B / A * 100 (%)) of the scale peeling rate (B) when pickling with respect to the original scale adhesion rate (A to mention later). Defined.

Where

A is the original scale adhesion rate (scale adhesion rate of steel wire),

W ₀ is the weight in grams of the steel wire (as rolled, with scale) before immersion,

W ₁ means the weight (g) of the steel wire after immersing in the acid solution (60 ° C.) until the scale is completely peeled off.

B is a scale peeling rate after pickling in the said conditions,

W ₀₁ is the weight (g) of the steel wire (as rolled) before immersion,

W ₂ denotes the weight (g) of the steel wire rod after dipped in the acid solution, the immersion conditions of the.

In the above Equations 1 and 2, W ₀ and W ₀₁ both mean the weight of the steel wire as it is rolled, but is "the weight of a separate sample (rolled steel rod as it is) manufactured under the same conditions". I used different symbols to clarify. This is because A and B cannot be measured using the same sample.

In this invention, it was determined that pickling property which was 100% measured as mentioned above is excellent in pickling property (passing (circle)).

(Thickness of scale)

For the steel wire, the thickness (maximum thickness) of the scale was measured based on a photograph (magnification: 3000 times) observed using a Fe-SEM apparatus (S-4500 field emission scanning electron microscope manufactured by Hitachi, Ltd.). did.

In this invention, it was determined that the thickness of the scale measured as mentioned above is 40 micrometers or less as a pass.

(Manufacture of Steel Wire for Spring)

Next, using each said steel wire material, the spring steel wire (oil temper wire) of diameter 4.0mm was manufactured on condition of the following (corresponding to spring process (a) mentioned above).

Surface coating treatment → drawing (dry drawing) → oil tempering (heating temperature: 930 ° C, quenching oil temperature: 70 ° C, tempering temperature: 450 ° C, cooling after tempering: water cooling)

(Surface roughness)

Next, in order to evaluate the surface properties of the spring thus obtained, surface roughness (Ry, maximum roughness) was measured according to JIS B 0601 1994.

In this invention, it was determined that the surface roughness Ry measured as mentioned above is 18.0 micrometers or less, "surface property is excellent (passing (circle))."

These results are shown in Table 2 and Table 3.

Among steel grades A to K shown in Table 1, steel grades A to H are examples satisfying the steel component of the present invention, and steel grades I and J are comparative examples in which the ratio of (Si / Cu) is less than the scope of the present invention, K is a comparative example in which the amount of Si exceeds the range of this invention.

In Table 2 and Table 3, for example, "A-1" means the example manufactured by the method of the wire rod process 1 using the steel grade A shown in Table 1, and "A-2" is shown in Table 1 By using steel grade A, the example manufactured by the method of the wire rod process 2 is meant. The same is true of other examples.

The following can be considered from Table 2 and Table 3.

First of all, No. 1, 4, 7 to 8, 11 to 12, 15, 18 to 19, 22 to 23, and 26 are examples of the present invention in which ΔCu and ΔNi defined in the present invention satisfy the scope of the present invention. Is 100% and has very good pickling. Moreover, when the scale composition of the said wire rod was investigated by the X-ray diffraction method, it confirmed that all were controlled in the above-mentioned preferable range (not shown in the table). Moreover, the surface property of the oil temper wire obtained using said wire rod was favorable. In addition, when the tensile strength of the oil temper wire was measured according to JIS Z 2241, it was confirmed that all had a high strength of about 1900 to 2100 MPa or more (not shown in the table).

In contrast, no. 2 to 3, 5 to 6, 9 to 10, 13 to 14, 16 to 17, 20 to 21, 24 to 25, 27 to 28, any of the wire rod process 3 to 6 outside the manufacturing conditions of the present invention It is the comparative example which employ | adopted and produced the spring, and since (DELTA) Cu or (DELTA) Ni is out of the range of this invention, scale peeling rate fell and the desired pickling property was not obtained. Moreover, the surface properties of the oil temper wire obtained using the said wire rod also fell.

In addition, No. As for 29-32, all are the comparative examples in which a component in steel deviates from the requirements of this invention, and since (DELTA) Cu or (DELTA) Ni is out of the range of this invention, scale peeling rate fell and the desired pickling property was not obtained. Moreover, the surface properties of the oil temper wire obtained using the said wire rod also fell.

On the other hand, in this embodiment, although the steel wire for spring was manufactured by the above-mentioned spring process (a), it is not limited to this, For example, even if it uses the above-mentioned spring process (b) or (c), surface property will be It is confirmed by experiment that a very good spring steel wire is obtained.

1 is a Fe-SEM photograph of a cross section of a scaled rolled wire rod.

2 is a diagram illustrating a relationship between Cu concentration (%) and the distance from the surface layer portion.

3 is a diagram showing the relationship between the Ni concentration (%) and the distance from the surface layer portion.

Claims (5)

As a steel wire for spring, C: 0.35 to 0.7% (mean of mass%, hereinafter same unless otherwise specified), Si: 1.5-2.50%, Mn: 0.05-1.0%, Cr: 0.05 to 1.9%, Cu: 0.05-0.7%, Ni: 0.15 to 0.8%, P: 0.02% or less (not including 0%), and S: 0.02% or less (not including 0%) Including, The balance consists of Fe and inevitable impurities, The ratio of Si and Cu (Si / Cu) is in the range of 4.0 or more, The difference between the Cu concentration in the surface layer and the Cu concentration in the steel is 0.50% or less, and the difference between the Ni concentration in the surface layer and the Ni concentration in the steel is 1.00% or less, The steel wire for spring manufactured by the manufacturing method containing the hot-rolling process including the descale process by the shower before a hot rolling process, and the finishing rolling process performed while shower cooling. The method of claim 1, A steel wire for spring further containing at least one selected from the group consisting of V: 0.07 to 0.4%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1%. As a method of manufacturing a steel wire for the spring according to claim 1, Heating process; Cracking process; Descaling process before hot rolling; And Hot rolling process Including, The hot rolling step is a method for producing a steel wire for spring comprising a finish rolling step performed while shower cooling. The method of claim 3, wherein The manufacturing method of the spring steel wire material which performs the high pressure shower of water pressure 785MPa-1569MPa in the descale process before the said hot rolling. The method of claim 4 The manufacturing method of the steel wire material for spring which cools by sour of 100t / hr or more and 200t / hr or less in the finishing rolling process performed while performing the said shower cooling.

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