KR20070053095A - Steel wire rod for steel spring excellent in pickling and descaling property - Google Patents

Abstract

The steel wire for spring of this invention is C: 0.35-0.7% (meaning the mass%. The same below), Si: 1.4-2.5%, Mn: 0.05-1.0%, Cr: 0.5-1.9%, P: 0.02% or less (Not including 0%), and S: 0.02% or less (not including 0%), and the difference between the Cr concentration in the surface layer and the Cr concentration in the steel is 2.50% or less. In the spring steel wire material containing a large amount of Si and Cr, it is excellent in pickling property even without adding an alloying component such as Mo as an essential component.

Description

Steel wire rod for steel spring excellent in pickling and descaling property}

(Technology)

The present invention relates to a spring steel wire material having excellent pickling properties, and more particularly, to a spring steel wire material containing a large amount of Si and Cr as alloy elements. The spring steel wire rod of the present invention is suitably used for valve springs, clutch springs, brake springs, stubby risers, torsion bars, etc. used in engines such as automobiles.

(Background)

The chemical composition of the spring steel used for valve springs, suspension springs, etc. is prescribed | regulated, for example by JIS G 3565-JIS G 4801, etc., and steel grade suitable for the kind of spring design, etc. are used. In recent years, as the size and weight of the spring are reduced with the reduction of the exhaust gas and fuel economy, the design stress of the spring is also increased. It can be expected to provide a spring steel wire rod that can achieve high strength with a tensile strength of about 1600 MPa or more. In addition, in order to increase the durability in the atmosphere, which is one of the important characteristics of the spring, it is also required to improve the yield strength, and steel wire materials containing a large amount of Si and Cr as alloy elements that can improve the strength by solid solution strengthening are used. It is a trend.

In general, a spring is heated by heating a steel strip and applying a lubricant to the surface as needed to perform a coating treatment (surface coating treatment), and then drawing, processing (hot forming or Cold forming). Since heating is usually performed in an oxidizing atmosphere, an oxide layer of Fe oxide called "rolling scale" or "scale" is formed on the surface of the hot wire. If the spring is manufactured by using the rolled wire with the scale attached, scratches or the like may occur on the surface, resulting in deterioration of the quality. Thus, a pickling treatment for removing the scale is performed before the drawing process.

1 shows a photograph of a cross section of a rolled wire rod having a scale attached to a surface of a steel (high Si high Cr containing steel) containing a large amount of Si and Cr, using a Fe-SEM apparatus. This is the No. of the Example mentioned later. Corresponds to E-1. As shown in Fig. 1, the scale is in the order from the surface layer side, and hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), wustite (FeO), and white light (2FeOSiO ₂ ). Consists of. A subscale in which Si and Cr are concentrated is formed between the steel (iron) and the scale, and a grain boundary oxide layer mainly composed of Cr oxide is also generated.

Among these, fiyalite is a low melting point oxide that can be seen when using a steel containing a large amount of Si, and is a refractory material that is difficult to peel off the scale layer by a normal pickling treatment. For example, when a high Si and high Cr-containing steel is heated above the process temperature (about 1170 ° C) between fayalite and wustite, a dense molten layer in which these oxides are intricately bonded When formed and heated to 1200 ° C or more, Cr infiltrates into the molten phase and the fiber, and concentrates at the interface with the iron, thereby forming a grain boundary oxide layer (described in detail later). It is extremely difficult to remove Cr once concentrated by a subsequent process.

In addition, it is known that the subscale lowers the activation of the iron in the pickling process and causes the pickling to decrease. If the pickling property is deteriorated, the rolling scale mainly based on the subscale remains after pickling, so that the adhesion to the lubricant applied to the surface (used for the surface coating treatment) is reduced, and the disconnection during the drawing process is performed. There is concern. Even if the wire is not disconnected, cracks may occur during the drawing process and break during spring forming (cold coiling). This problem can be reduced, for example, by lengthening the pickling treatment time and completely removing the rolling scale. However, the immersion time in the acid solution becomes long. It causes deterioration of the table, and finally durability in the atmosphere is also impaired. In addition, some of the hydrogen generated when an 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), which may cause disconnection during drawing processing. .

The grain boundary oxide layer can be seen in the case of using Cr-rich steel. This is because Cr has a strong affinity with oxygen and, when oxygen invades during hot rolling, the oxidation rate of the grain boundary becomes larger than in the grains. If the grain boundary oxidation progresses to a certain degree, the spring is broken by the notch effect in the grain boundary oxidation part during spring forming. In addition, there is a fear of fatigue loss due to the notch effect in the grain boundary oxidation portion during the use of the spring. Generally, the thicker the subscaled layer is, the thicker the subscale is.

Therefore, when using the steel wire for spring containing much Si and Cr in steel, there exists a problem that pickling property falls and fatigue property of a spring falls for the reason mentioned above.

In order to solve the above problem, for example, Japanese Patent Laid-Open No. Hei 6-299295 discloses that an alloy element such as Mo is added to control the heating temperature and the rolling end temperature during hot rolling, thereby controlling the film thickness of the scale to 10 m or less. A method is disclosed.

`(A problem to be solved by the invention)

However, in the above method, expensive alloying elements such as Mo and the like must be added as essential components in addition to Cr and Si.

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 excellent pickling properties without adding an alloying component such as Mo as an essential component to a spring steel wire material containing much Si and Cr. There is.

(Means to solve the task)

Spring steel wire rod of the present invention that can solve the above problems, C: 0.35 ~ 0.7% (meaning of mass%. The same below), Si: 1.4 ~ 2.5%, Mn: 0.05 ~ 1.0%, Cr: 0.5 ~ 1.9% , P: 0.02% or less (not including 0%), and S: 0.02% or less (not including 0%), and the difference between the Cr concentration in the surface layer and the Cr concentration in the steel is 2.50% or less.

As a preferable embodiment, 1 or more types are further contained by the at least one selected from the group which consists of V: 0.07 to 0.4%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1%.

Moreover, as preferable embodiment, Ni: 0.15 to 0.8% is further contained.

The spring of this invention which can solve the said subject can be obtained even using any said spring steel wire material.

(The best form to carry out invention)

MEANS TO SOLVE THE PROBLEM This inventor earnestly examined in order to improve the pickling property of the spring steel wire material containing a large amount of Si and Cr. As a result, as described in detail later, in particular, if the heating process and the cracking process before hot rolling are properly controlled, the concentration of Cr on the surface of the wire rod (particularly, the concentration of Cr in fiyalite) is suppressed, and the Cr concentration of the surface layer and the Cr concentration in the steel are suppressed. The difference (hereinafter, it may be briefly described as ΔCr) was found to be significantly lowered, and pickling was noticeably improved, thereby completing the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

As described above, the steel wire for spring of the present invention is C: 0.35 to 0.7%, Si: 1.40 to 2.5%, Mn: 0.05 to 1.0%, Cr: 0.50 to 1.9%, P: 0.02% or less (0% ), And S: 0.02% or less (does not contain 0%), remainder: Fe and a spring steel wire satisfying inevitable impurities, the difference between the Cr concentration of the surface layer and the Cr concentration in the steel (ΔCr) Is 2.50% or less.

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

First, the steel component will be described.

C: 0.35 ~ 0.7%

C is It is an element which contributes to the improvement of the strength (hardness) after quenching and tempering and increases the atmospheric durability. If the amount of C is less than 0.35%, the above effect cannot be effectively exhibited. On the other hand, if the amount of C is more than 0.7%, the toughness and ductility deteriorate, cracks tend to propagate, and durability decreases. Moreover, in the spring to which cold coiling was performed like a valve spring etc., the break from the surface flaw may generate | occur | produce as a result of deterioration of a ductility and ductility. Therefore, it is preferable that amount of C is 0.51% or more and 0.61% or less.

Si  1.4-2.5%

Si is an element that can contribute to the improvement of strength as a solid solution strengthening element and can also improve the strength. If Si is less than 1.4%, the matrix strength is insufficient. However, if Si is excessively added in excess of 2.5%, ferrite decarburization is likely to occur on the surface by heat treatment beyond the A ₃ transformation point, so that the solid solution strengthening effect is not effectively exerted. Si is preferably 1.7% or more and 2.1% or less.

Mn  0.05 to 1.0%

Mn of the river It is an element to increase the title. In order to exhibit such an effect effectively, the amount of Mn added is made 0.05% or more. However, if the amount of Mn is added in excess of 1.0%, Because of its increased tendency and easy generation of supercooled tissue, the drawability deteriorates. In addition, when the annealing step is performed for the purpose of softening the wire rod before the pickling treatment after hot rolling, as in the "spring step (c)" described later, high cost cannot be avoided. Therefore, it is preferable that Mn is 0.4% or more and 0.9% or less.

Cr  : 0.5 ~ 1.9%

Cr is an element that strengthens the matrix of wire rod by solid solution strengthening. Also, like Mn, It also works effectively to improve the title. In order to exert this effect effectively, 0.5% or more of Cr is added. However, if Cr exceeds 1.9%, the supercooled structure is liable to be generated during cold after dark smoke, and the drawing processability is deteriorated. Therefore, Cr is preferably added at 0.6% or more and 1.75% or less.

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

P segregates in the old austenite grain boundary, embrittles the grain boundary, and lowers the fatigue characteristics. Therefore, P is as small as possible. In the present invention, the industrial upper limit is made 0.02%.

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

S is segregated to the former austenite grain boundary, embrittles the grain boundary, and lowers the fatigue characteristics. Therefore, it is better to use S as little as possible. In the present invention, the industrial upper limit is made 0.02%.

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

In the present invention, 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 enhancing hydrogen embrittlement resistance. Moreover, it is preferable to contain. Also, It is preferable to contain Ni for the purpose of improving the toughness after quenching and tempering. Hereinafter, each element is explained in full detail.

V: 0.07 ~ 0.4%

V is an element which forms fine carbides or nitrides and contributes to the improvement of hydrogen embrittlement resistance. In addition, the fatigue characteristics can be improved. In addition, the toughness and strength are improved by the grain refining effect, which contributes to the improvement of the setting resistance. In order to exhibit such an effect effectively, it is preferable to add V 0.07% or more. However, if V is added in excess of 0.4%, When the heating is carried out, the amount of carbide not dissolved in austenite increases, and not only sufficient strength and firmness cannot be obtained, but also the amount of retained austenite increases and the spring strength decreases. It is preferable to add V to 0.1% or more and 0.35% or less.

Ti  : 0.01 ~ 0.1%

Ti is It is an element effective in miniaturizing old austenite crystal grains after quenching and tempering and improving hydrogen embrittlement resistance. It also has the effect of improving atmospheric durability. In order to exhibit such an effect effectively, it is preferable to add Ti 0.01% or more. However, when Ti is excessively added, coarse nitride tends to precipitate and atmospheric durability decreases. Therefore, the upper limit is preferably 0.1%. As for Ti, it is more preferable that it is 0.04% or more and 0.085% or less.

Nb  : 0.01 ~ 0.1%

Nb is an element that forms fine precipitates composed of carbides, nitrides, emulsions, and complex compounds thereof to contribute to the improvement of hydrogen embrittlement resistance. In addition, toughness and yield strength are also improved by the grain refining effect. In order to exhibit such an effect effectively, it is preferable to add Nb 0.01% or more. However, when Nb is added in excess of 0.1%, At the time of quenching heating, the amount of carbide which is not dissolved in austenite increases, so that a predetermined tensile strength cannot be obtained. In addition, fatigue loss due to coarse nitride is likely to occur. It is preferable that Nb is 0.02% or more and 0.05% or less.

Ni  0.15 to 0.8%

Ni is It is an element which raises toughness after quenching and tempering. Moreover, it also has an effect of suppressing the ferrite decarburization occurring before and during rolling. In order to exhibit this effect effectively, 0.15% or more of Ni is added. However, if Ni exceeds 0.8% Due to the increase in tendency, a supercooled structure tends to occur after rolling. In addition, the amount of retained austenite also increases, and the spring strength decreases. It is preferable that Ni is 0.25% or more and 0.55% or less.

In the above, the steel component of this invention was demonstrated.

Superficial Cr  Concentration and In the river Cr  Difference from concentration (Δ Cr ): 2.50% or less

As for the spring steel wire material of this invention, (DELTA) Cr is suppressed low as 2.50% or less. As shown in Examples below, when ΔCr is high, pickling property is lowered, because CrO (OH) is generated in the pickling solution, and a passive film of Cr is formed on the surface of the ferrous iron. In particular, the present invention can appropriately control the heating step and the cracking step before hot rolling to suppress ΔCr low. The smaller the ΔCr is, the better it is. For example, it is preferably 2.0% or less, and more preferably 1.5% or less. The lower limit thereof is not particularly limited, but is preferably 0.4% or more in consideration of the actual industrial level.

Here, the measuring method of "Cr concentration of surface layer" is demonstrated using FIG. FIG. 2 is a view measured by EPMA line quantitative analysis of the following conditions in a range of 0.3 mm from the surface layer portion toward the center of the interior using a test material manufactured as follows. The relationship between the X-ray intensity (cps) and the distance from the surface layer portion is shown in FIG. 2, and the relationship between the Cr concentration (%) and the distance from the surface layer portion is shown in FIG. As shown in Fig. 2 (a), the point when the X-ray intensity of Fe reaches the maximum value is defined as the “iron surface” (the boundary between the scale and the iron), and this region is defined as the “surface layer”, and the Cr at the surface layer portion is defined. The maximum value of the amount is defined as "Cr concentration in the surface layer" (see Fig. 2 (b)). Although the said "surface layer" part differs also depending on a steel component, the manufacturing conditions of a wire rod, etc., it contains at least a white light.

EPMA measuring device: Japan-made X-ray microanalyzer `` JXA-8800 RL ''

Specimen: Dip the steel as it is attached to the resin, finish the cross section (measurement surface) perpendicular to the rolling direction with an abrasive, and then deposit it using osmium to maintain conductivity. Was performed.

Acceleration Voltage: 15 kV

Irradiation current: 0.3 ㎂

Quantitative line analysis: Measure the interval of 1㎛, 300 points in total

In the above, (DELTA) Cr which characterizes this invention most was demonstrated.

As for the steel wire for spring of this invention, it is preferable that the thickness of a scale, a composition, and the thickness of a subscale are controlled suitably as follows, and pickling property can be improved further by this.

(Thickness of scale)

It is preferable that the thickness of a scale is 40 micrometers or less. Although it demonstrates in detail below, when the scale peeling by the crack (cracks) which generate | occur | produced in the scale is considered, it is more preferable that the thickness of a scale is generally 5 micrometers or more and 25 micrometers or less.

In the scale, for example, a micro level crack may be generated during the cooling process after rolling or the handling of the rolled wire. The more cracks, the easier the peeling of scale from the base iron surface, and therefore, the pickling property is considered to be improved. In general, the smaller the thickness of the scale, the lower the strength of the scale, which tends to occur. However, if the thickness of the scale is too thin, the ductility of the scale itself is increased and the internal stress decreases, so the crack is less. Therefore, the thickness of the scale is preferably in the above range.

(The composition of the scale)

Composition of scale is approximately white volume: 2-10% (more preferably 3-7%), ustatite: 2-20% (more preferably 10-18%), magnetite: 35- by volume ratio It is preferable to satisfy the range of 70% (more preferably 37-50%) and hematite: 20-60% (more preferably 30-55%). According to the present invention, since the composition of the scale is controlled so that the ratio of refractory filament is low and the ratio of ustite and magnetite excellent in scale peeling property is increased, pickling becomes even higher.

(Subscale thickness)

It is preferable that the thickness of a subscale is 2 micrometers or less. One of a steel material containing Cr as in the present invention, there is scale and the interface a subscale of Cr as the main component in the metal part (mainly Cr ₂ O ₃₎ is generated as described above, Cr ₂ O ₃ is CrO during pickling ( OH) and poorly soluble antifreeze film is generated for the pickling solution, so the pickling properties are lowered. In addition, when the passivation film is formed, activation of the ground iron is reduced, and generation of hydrogen is suppressed, which requires a long time for scale peeling. The thinner the sub-scale, the better, and more preferably 8 µm or less.

In the steel wire for spring of the present invention, the thickness of the grain boundary oxide layer is preferably controlled as follows, whereby the fatigue characteristics are increased mainly when molded into a spring.

(Depth of grain boundary layer)

It is preferable that the depth (thickness) of a grain boundary oxide layer is 10 micrometers or less. The thickness of a grain boundary oxide layer is so good that it is thin, and it is more preferable that it is 8 micrometers or less.

In addition, grain boundary oxidation is not only a steel wire for the spring, but also processed into a spring (for example, in the austenite region). May occur). It is preferable that the depth of the grain boundary oxide layer at the time of working with a spring is 15 micrometers or less.

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

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

The manufacturing method of the steel wire for spring includes (a) heating step, (b) cracking step, (c) descaling step before hot rolling step, and (d) hot rolling step. In the present invention, in order to prevent Cr thickening to the surface layer portion and to suppress grain boundary oxidation, in particular, (a) the heating rate and heating temperature in the heating step, and (b) the cracking time and the cracking temperature in the cracking step are closely controlled. As a result, it is possible to remarkably reduce the Cr concentration of the surface layer. As shown in the examples below, according to the present invention, even when the steel containing a large amount of Si and Cr is not used, Cr concentration to the surface layer is remarkably suppressed, so that the scale and subscale thickness are thin and no grain boundary oxide layer is formed. Therefore, it is possible to provide a spring having a tensile strength of about 1600 MPa or more and excellent fatigue characteristics.

Hereinafter, each process is explained in full detail.

(Iii) heating process

Here, it heats to the temperature of 700 degreeC-1000 degreeC at the temperature increase rate of about 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 Cr advances and Cr amount of surface layer increases. On the other hand, when heating temperature is less than the said range, steel materials are not heated enough and crude rolling cannot be carried out. It is preferable that heating temperature is 750 degreeC or more and 900 degrees C or less.

(Ii) cracking process

Here, it cracks for 20 minutes-60 minutes (more preferably, 30 minutes-50 minutes) at the temperature of about 1050 degreeC-1250 degreeC (preferably 1100 degreeC-1200 degreeC). This cracking condition is determined to prevent the thickening of Cr to the surface layer and to suppress the progression of grain boundary oxidation. For example, when the cracking temperature or the cracking time exceeds the above range, the thickening of Cr is likely to proceed, and the cracking is performed. If the temperature or the cracking time is less than the above range, grain boundary oxidation proceeds.

In the present invention, the heating temperature of the heating step and the cracking temperature of the cracking step do not necessarily have to coincide. For example, in Examples described later, the cracking temperature is higher by about 50 to 150 ° C than the heating temperature, because the temperature at the time of cracking is increased by the residence time before cracking after heating.

(Iii) before hot rolling Descale  fair

The steel wire material for spring of this invention needs to be manufactured especially paying attention to the process of said (i) and (ii), and is another process, for example, (i) the descaling process before hot rolling, and the following (i) hot rolling process. Although is not particularly limited, the conditions usually used can be appropriately selected. For example, the following control is recommended.

In this case, in order to quickly remove the fiber containing scale, about 80 kgf / mm 2 (≒ 785 MPa) to 160 kgf / mm 2 (≒ 1569 MPa), more preferably about 100 kgf / mm 2 (≒ 981 MPa) to 120 kgf / mm 2 (≒ 1176 MPa) It is preferable to perform a high water pressure shower for about 1 second to 10 seconds (more preferably, 3 seconds to 7 seconds) under a water pressure of). 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 the bite during hot rolling and an increase in the surface layer Cr concentration. On the other hand, when the water pressure of the shower exceeds 160 kgf / mm 2, the billet temperature before hot rolling decreases and rolling becomes difficult.

In addition, the descaling process before hot rolling is not limited to the said high water pressure shower, For example, you may perform mechanical scale peeling, such as a shot blast.

(Iii) hot rolling process

Here, a predetermined water-cooled shower is performed in order to prevent Cr thickening of the white light generated during hot rolling and to appropriately control the composition of the scale.

Specifically, in the finishing rolling process after rough rolling, cooling is performed by a shower. The amount of water in the shower is preferably about 100 t / hr or more and 200 t / hr or less, and more preferably 120 t / hr or more and 180 t / hr or less. If the amount of water in the shower is less than 100 t / hr, the desired scale (fiyalite) removal action and the reduction action of ΔCu are not effectively exhibited. On the other hand, when the amount of water in the shower exceeds 200 t / hr, the steel is excessively cooled to precipitate the supercooled structure.

It is preferable to perform finish rolling temperature mainly in the range of about 800 degreeC-1000 degreeC (more preferably, 950 degreeC-980 degreeC), in order to control suitably the thickness and composition of a scale.

From the above point of view, for example, after finishing rolling, the cooling rate is controlled within a range of 4 ° C / sec to 20 ° C / sec (more preferably 6 ° C / sec to 15 ° C / sec) to a temperature range of about 700 ° C. It is desirable to. When the cooling rate in the temperature range is less than 4 ° C / sec, the scale thickness and the like increase, and the pickling property is lowered. On the other hand, when the said cooling rate exceeds 20 degreeC / sec, the holding time of the said temperature range will become short, and since the ratio of the wustite produced | generated in this temperature range will fall, pickling property will fall.

The present invention includes, in addition to the spring steel wire material, a spring obtained by using the steel wire material. According to the present invention, scale defects called red scales do not occur at all, and it is possible to manufacture a spring having excellent surface properties and high fatigue characteristics.

Since the method of manufacturing a spring is not specifically limited, What is necessary is just to employ | adopt the method normally used suitably. Representatively, there are the following spring steps (a) to (c), for example, and any of them can be used to manufacture the spring of the present invention.

(a) Pickling → surface coating → drawing → Ching and Tempering (Oil Tempering)

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

(c) annealing → pickling → surface coating → machining → SV → pickling → surface coating → drawing → oil temper

As can be seen from the examples described later, according to the present invention, a rolled wire rod having a low ΔCr can be obtained. Therefore, even if any of the spring processes (a) to (c) is used, a spring having excellent surface properties can be obtained. have. In addition, since the thickness of the grain boundary oxide layer of the rolled wire is also controlled to be thin, a spring excellent in fatigue characteristics can be obtained using any of the spring processes described above. In addition, even if the thickness of the grained oxide layer of the rolled wire is outside the preferred range of the present invention, if the machining process is performed as in the spring step (c), the grained oxide layer after the spring processing will be thin, so that good fatigue characteristics can be obtained. Check through.

Since each treatment method described in said spring process (a)-(c) is not specifically limited, The method normally performed can be selected suitably. For example, pickling is typically performed 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 ]

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail based on an Example. 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.

(Manufacture of Steel Wire for Spring)

150 kg of each steel (steel grades A to G, all of which satisfy the steel composition defined in the present invention, the balance consisting of iron and inevitable impurities) is dissolved in a small vacuum melting furnace, and billets of 155 cm angle are shown in Table 1. After hot forging, a steel wire with a diameter of 7.4 mm was produced under the heating and hot rolling conditions of the wire rods 1 to 3 shown below. Among the wire rod steps, wire rod steps 1 and 2 are examples of the present invention that satisfy all of the manufacturing conditions specified in the present invention, and wire rod step 3 is a comparative example in which both heating temperature and crack temperature are higher than the scope of the present invention.

(Wire Rod Process 1)

After heating up to about 1000 degreeC by the heating rate of 15 degree-C / min, it cracked for 20 minutes at about 1060 degreeC, showered with high water pressure for about 5 second under the water pressure of 100 kgf / mm <2> (# 981 MPa), and descaled. Subsequently, after rough rolling, finishing rolling was performed while shower cooling at 150 t / hr (finishing rolling temperature 950 ° C.), and after finishing rolling, the range up to about 700 ° C. was cooled at a cooling rate of 6 ° C./sec. It was. Winding temperature was 875 degreeC.

(Wire Rod Process 2)

After heating to about 980 degreeC at the heating rate of 20 degreeC / min, it cracked at about 1100 degreeC for 30 minutes, showered by high water pressure for about 5 second under the water pressure of 100 kgf / mm <2> (# 981 MPa), and descaled. Subsequently, after rough rolling, finishing rolling was performed while shower cooling at 150 t / hr (finishing rolling temperature 950 ° C.), and after finishing rolling, the range up to about 700 ° C. was cooled at a cooling rate of 6 ° C./sec. It was. Winding temperature was 875 degreeC.

(Wire Rod Process 3)

After heating to about 1100 degreeC by the heating rate of 18 degree-C / min, it cracked for 40 minutes at about 1300 degreeC, showered with high water pressure for about 5 seconds under the water pressure of 100 kgf / mm <2> (# 981 MPa), and descaled. Subsequently, after rough rolling, finish rolling was performed while shower cooling at 150 t / hr (finishing rolling temperature 1050 ° C.), and after finishing rolling, the range up to about 700 ° C. was cooled at a cooling rate of 4 ° C./sec. It was. Winding temperature was 875 degreeC.

About each steel wire obtained in this way, while measuring (DELTA) Cr by the method mentioned above, pickling property was evaluated.

(Evaluation of pickling)

The steel wire was cut into a length of 100 mm, and the following pickling test (beaker test) was performed with the number of samples n as three. Here, experiments were conducted under the same conditions as pickling treatment in the actual industry.

Acid solution: 15% sulfuric acid

         0.5% inhibitor (cationic amine derivative) to prevent dissolution of iron

         20g / L biferrous iron with iron

Immersion condition: 10 minutes at 60 ℃

Next, the scale peeling rate after pickling was measured as follows. In the present embodiment, "scale peeling rate after pickling" is expressed as a percentage (B / A x 100 (%)) of the scale peeling rate B when pickling with respect to the original scale adhesion rate (A described later). Defined.

(1) A (%) = [(W ₀ -W ₁ ) / W ₀ ] × 100

In the formula,

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

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

W ₁ means the weight (g) of the steel wire after immersion under the immersion conditions.

(2) B (%) = [(W ₀₁ -W ₂ ) / (W ₀₁ )]

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

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

W ₂ means the weight (g) after pickling experiment.

In the above formulas (1) and (2), W ₀ and W ₀₁ both mean the weight of the steel wire as it is rolled, but the "weight of each sample (steel wire as it is rolled) manufactured under the same conditions". Other symbols are used to clarify. This is because it is impossible to measure A and B using the same sample.

 In this invention, it was judged that the thing of 100% of scale peeling rates measured as mentioned above is excellent in pickling property (passing (circle)).

In addition, the scale, the thickness of the subscale and the grain boundary oxide layer, and the composition of the scale were measured as follows. In measuring them, the steel wire is immersed in the resin, and the end surface (measurement surface) perpendicular to the rolling direction is mirror-finished with an abrasive, followed by deposition using osmium to maintain conductivity. Was used.

(Thickness of scale)

The 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 Seisakusho Co., Ltd.). Was taken as "thickness of scale".

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

(The composition of the scale)

The cross section of the specimen was subjected to X-ray diffraction analysis under the following conditions to measure the composition (volume ratio) of the scale.

Equipment: Rigakudenki `` RAD-RU300 ''

Target: Cr

Target output: 40kV-200mA

Monochrome Light Slit: 0.6mm

Slit: 1 ° divergence, 1 ° divergence, 0.15 mm light receiving

Scanning Speed: 2 ° / min

Measuring range (2θ): 15 ° or 110 °

Sampling Width: 0.02 ° / step

(Subscale thickness)

By the same method as the "thickness of scale" mentioned above, the thickness of the subscale was measured based on the Fe-SEM photograph, and the maximum thickness was made into the "subscale thickness". In the Fe-SEM photograph, since the subscale is observed black compared to the scale (see FIG. 1), both can be distinguished by light and shade of color.

In this invention, it was judged as the pass that the thickness of the subscale measured as mentioned above is 2 micrometers or less.

(Thickness of grain boundary layer)

About the cross section of the said specimen, the grain boundary oxide layer was measured using the optical microscope (400x magnification), and the maximum depth was made into the "thickness of the grain boundary oxide layer."

In this invention, it was judged as pass that the depth of the grain boundary oxide layer measured as mentioned above is 10 micrometers or less.

(Manufacture of Steel Wire for Spring)

Subsequently, any of the spring steps (a) to (c) shown below was selected and performed using the respective steel wires, thereby producing a steel wire for use (oil temper wire) having a diameter of 4.0 mm. In the following process, drawing, lead parting (LP), and workpiece (SV) were all performed under the same conditions.

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

(b) LP, heated to 930 ° C. → maintained at 600 ° C. → pickling → surface coating treatment → drawing → oil temper

(c) annealing (holding at 660 ° C for 2 hours) → pickling → SV → LP → pickling → drawing → oil temper

(Depth of grain boundary layer)

Using the oil temper wire thus obtained, the depth of the grain boundary oxide layer was measured in the same manner as above. In this invention, it was determined by pass that the depth of a grain boundary oxide layer is 10 micrometers or less.

(Fatigue Life (Crack Resistance))

Fifty samples (about 650 mm in length) cut | disconnected the said oil temper line were prepared, and the Nakamura-type rotary bending fatigue test was done. A fatigue test (10 million times) under a load stress of 45% of the tensile strength of each sample was performed, and the number of breakages was measured. In 50 samples, the ratio (loss rate) which a loss generate | occur | produced was computed.

In the present invention, it was determined that the fatigue rate measured as described above was 5% or less, which was excellent in fatigue characteristics (passed, ○).

These results are shown in Table 2 and Table 3.

TABLE 1

TABLE 2

TABLE 3

In Table 2 and Table 3, for example, "A-1" means the example which manufactured the steel wire material by the wire rod process 1 using the steel grade A shown in Table 1, and "A-2" is shown in Table 1 It refers to an example of manufacturing a steel wire in the wire rod process 2 using the steel grade A. Other examples can be understood in the same way. In addition, in Table 3, the type of spring process at the time of manufacturing an oil temper line was written together. For example, in Table 2 and Table 3, No. 1 is an example which manufactured the oil-tempered wire by the spring process (a) after manufacturing a steel wire by the wire rod process 1 using the steel grade A. FIG. Other examples can be understood in the same way.

Table 2 and Table 3 can be considered as follows.

First, Nos. 1 to 2, 4 to 5, 7 to 8, 10, 12, and 14 to 15 are ΔCr satisfying the scope of the present invention, and the scale and subscale thickness satisfy the preferred range of the present invention. As an honor, they show extremely excellent pickling properties with a 100% peel rate as shown in Table 2. Moreover, when the scale composition of the said wire rod was investigated by the X-ray diffraction method, it can confirm that all are controlled in the above-mentioned preferable range (not shown in the table).

In addition, both of the wire rod and the oil tempered wire obtained by using the wire rod showed no generation of grain boundary oxide at all, and it was found that the fatigue characteristics were excellent. In addition, when the tensile strength of the oil-tempered wire was measured based on JIS Z 2241, it was confirmed that all had a high strength of about 1900-2100 MPa or more (not shown in the table).

In the examples of the present invention, Nos. 1, 2, 4, 5, 7 and 8 were produced by the spring step (a), and Nos. 10, 12, 14 and 15 were produced by the spring step (b). The springs with excellent fatigue characteristics could be obtained by any method. In addition, although not shown in the table, it was confirmed through experiments that a spring having excellent fatigue characteristics can be obtained even when the oil tempered wire was produced by the spring process (c).

In contrast, No. 3, 6, 9, 11, and 13 are all comparative examples in which the springs were manufactured by adopting wire rod process 3 in which the heating temperature and the cracking temperature were out of the range of the present invention. This was reduced (see Table 2). In addition, the depth of the grain boundary oxide layer of the steel wire and the spring obtained using the comparative example were all outside the preferred range of the present invention, and the fatigue life of the spring also decreased.

In addition, as in the comparative example, even if the grain boundary layer depth of the steel wire is out of the scope of the present invention and subjected to the machining process as described above in the spring process (c), the grain boundary oxide layer after the spring processing becomes thin to obtain excellent fatigue characteristics It was confirmed through the experiment.

The spring steel wire of the present invention is excellent in pickling property because the thickness of the scale, the subscale, and the grain boundary oxide layer are all very thin, in addition to the significant concentration of Cr on the surface layer. When the spring is manufactured using the spring steel wire of the present invention, since the scale and the subscale are easily peeled off by the pickling process, it is possible to provide a spring having excellent surface properties and high fatigue characteristics.

1 is a Fe-SEM photograph observing the cross section of the rolled wire rod with a scale.

FIG. 2 is a diagram in which predetermined specimens are measured by EPMA line quantitative analysis in order to measure the Cr concentration of the surface layer. FIG. 2 (a) shows the X-ray intensity (cps) and the It is a figure which shows the relationship with distance, and FIG.2 (b) is a figure which shows the relationship between Cr concentration (%) and the distance from a surface layer part.

Claims (4)

C: 0.35-0.7% (mean of mass%. The same below), Si: 1.4-2.5%, Mn: 0.05-1.0%, Cr: 0.5 to 1.9%, P: 0.02% or less (not including 0%), and S: 0.02% or less (not including 0%) Including, A steel wire for excellent pickling, characterized in that the difference between the Cr concentration in the surface layer and the Cr concentration in the steel is 2.50% or less. The method of claim 1, V: 0.07 to 0.4%, Ti: 0.01-0.1%, and Nb: 0.01 ~ 0.1% A spring steel wire having excellent pickling properties, further comprising at least one selected from the group consisting of: The steel wire for spring having excellent pickling property according to Claim 1, further comprising Ni: 0.15 to 0.8%. A spring obtainable by using the steel wire for spring according to claim 1.