JP5796782B2 - High strength spring steel wire rod and high strength spring with excellent skin machinability - Google Patents

Description

  The present invention relates to a high-strength spring steel wire useful as a material for a high-strength spring (particularly a valve spring) used in automobile clutches, engines, fuel injection devices, suspension mechanisms, and the like, and a high-strength spring. In particular, the present invention relates to a steel wire for high-strength springs that can exhibit good shaving properties in a skin-cutting process, and a high-strength spring obtained from the steel wires for high-strength springs.

  Since the spring used in the above environment is used with a high stress for a long period of time, a high level of fatigue resistance is required. In order to improve fatigue resistance, excellent surface properties and excellent inclusion control are required. Of these, the surface texture is flattened and cured by shot peening or nitriding after the spring is formed.If wrinkles of only a few tens of microns remain or occur, Breakage occurs as a starting point.

  Therefore, a decarburized portion of the wire surface layer after rolling and a skin removal process (hereinafter, also referred to as “SV process”) for removing fine wrinkles on the wire surface layer are performed. This SV process is a process that uses a chipper die to scrape the entire surface of the wire layer by several hundred microns in the depth direction. However, when the SV processability (skinning processability) is poor, breakage occurs during the SV process. In addition to such problems, there are problems such as chipper die chipping, wire surface disturbance, and tool life shortening. In addition, in a wire with poor SV processability, the swarf is finely divided, the swarf is entangled with the breaker to improve the swarf discharge performance, the motor load for driving the breaker becomes excessive, and the device stops. There is also a problem that the yield decreases.

  By improving the SV processability, it is possible to greatly improve the yield and quality. As techniques for improving SV processability, tissue control, inclusion composition control, and the like are mainstream, but various techniques have been proposed so far.

  For example, Patent Document 1 proposes improving machinability by increasing the grain size of austenite. However, in order to achieve high fatigue strength with spring steel, fine crystal grains are required, and in consideration of manufacturability such as SV treatment and wire drawing, a fine crystal grain size is preferable. .

  In Patent Document 2, the SV processability is improved by defining the composition of oxide inclusions and the size and distribution density of oxide inclusions existing on the surface layer. However, the factor that lowers the SV processability is that the influence of alloy-based carbides and nitrides that influence the ductility and toughness of the structure is large.

  On the other hand, in Patent Document 3, the SV processability is improved by defining the mechanical characteristics. However, in this technique, the amount of alloy addition is increased, and in spring steel where precipitation of alloy carbides and nitrides is large. At present, the SV processability cannot be improved only by satisfying the mechanical characteristics.

JP 2000-256785 A JP 2010-222604 A JP 2000-239797 A

  In particular, a valve spring needs to have high fatigue strength and high fatigue life. In order to satisfy these characteristics, the surface property of the spring is required to be good, and SV treatment is performed to remove the decarburized layer and surface flaws of the rolled material. This SV treatment consists of a skin pass process for improving the roundness of the rolled material and preventing one-side cutting and a skinning process using a chipper die, and in order to prevent disconnection in the skin pass process. It is required to properly control the cooling conditions on the conveyor during rolling so that the rolled material structure does not contain a supercooled structure (bainite or martensite).

  Further, in a chipping process with a chipper die, a stable line surface quality that can cut the entire length of a 2-ton coil and that does not have a die mark or the like is required. Therefore, in addition to the supercooled structure that is likely to cause disconnection in the rolled material structure, the chipper die is less likely to chip and the load on the tool is small. It is necessary. Furthermore, the swarf generated when chipping with a chipper die is discharged after being finely divided by a breaker, but it is also a swarf that can be easily divided by a breaker, that is, the swarf is well discharged. Needed.

  The present invention was made in order to solve the problems in the prior art, and its purpose is not only to have good cutting properties and shavings discharge properties, but also to prevent disconnection during SV processing. An object of the present invention is to provide a steel wire for a high-strength spring capable of exhibiting good SV processability, and a high-strength spring obtained using such a steel wire for a high-strength spring as a raw material.

The steel wire for high-strength springs of the present invention that has solved the above problems is a steel wire after hot rolling, and C: 0.4% or more and less than 1.2% (meaning “mass%”) The same applies to the chemical component composition), Si: 1.5 to 3.0%, Mn: 0.5 to 1.5%, Cr: 0.5% or less (not including 0%), and Al: 0. Each structure contains 01% or less (excluding 0%), the balance is composed of iron and inevitable impurities, and is mainly composed of pearlite. The average value Pave of the pearlite nodule particle size number satisfies the following formula (1) In addition, the main decarburized layer depth of the surface layer is 0.20 mm or less, and the grit is that the Cr-based alloy carbide amount is 7.5% or less.
6.0 ≦ Pave ≦ 12.0 (1)

  The steel wire for a high-strength spring of the present invention may further include (a) V: 0.5% or less (not including 0%) and / or Nb: 0.5% or less (not including 0%) if necessary. (B) Mo: 0.5% or less (not including 0%), (c) Ni: 1.0% or less (not including 0%), (d) Cu: 0.5% or less (0% (E) B: 0.010% or less (excluding 0%) is also effective, and the properties of the steel wire for high-strength springs are further improved according to the components contained. The

  The present invention also includes a high-strength spring obtained from the steel wire for a high-strength spring as described above.

  In the present invention, by appropriately adjusting the chemical component composition and making the manufacturing conditions appropriate, the structure is mainly composed of pearlite, the average value Pave of the pearlite nodule particle size number is within a predetermined range, and the surface layer Since the total decarburized layer depth and Cr alloy carbide content are appropriately controlled, good SV treatment that prevents cutting and swarf discharge and prevents disconnection during SV treatment A steel wire for a high-strength spring capable of exhibiting the properties can be realized, and such a steel wire for a high-strength spring is extremely useful as a material for producing a high-strength spring.

It is explanatory drawing which shows the sampling method (ring division position) of the sample for evaluation. It is a cross-sectional view which shows typically the structure | tissue observation position of a wire. It is a cross-sectional view which shows typically the surface decarburization observation position of a wire. Test No. It is the graph which showed the displacement of the breaker electric current value in 2 (invention example). Test No. It is the graph which showed the displacement of the breaker electric current value in 27 (comparative example).

  The present inventors have studied from various angles in order to realize a high-strength spring steel wire suitable for the above-described purpose. As a result, by appropriately controlling the chemical composition, structure, pearlite nodule particle size number, surface decarburization depth, and amount of Cr-based alloy carbide on the surface of the rolled material, it is possible to control the cutting and cutting properties. It was clarified that in addition to good waste discharging performance, the SV processing performance is greatly improved so that disconnection does not occur during the SV processing. In addition, below, it may be called "SV processability" including the shaving property and shavings discharge property. Next, each requirement prescribed | regulated by this invention is demonstrated.

[Perlite-based organization]
The steel wire rod of the present invention (steel wire rod after hot rolling: rolled wire rod) has a structure mainly composed of pearlite. The rolled wire rod having a structure mainly composed of pearlite means a rolled wire rod having a bainite, a martensite supercooled structure, and a ferrite having a small area ratio in the cross section of the rolled wire rod. The pearlite-based rolled wire can be subjected to the SV treatment without disconnection during the SV treatment. However, the rolled wire with a certain amount or more of supercooled structures such as bainite and martensite has poor ductility and toughness, and the SV treatment. SV processability deteriorates, such as disconnection sometimes occurring.

  Ferrite does not decrease the SV processability as much as the supercooled structure such as bainite and martensite, and may be partially contained. However, when the amount is excessive, the structure becomes non-uniform, so the SV processability is improved. Is not preferred. From such a viewpoint, in the steel wire of the present invention, the area ratio of pearlite is preferably 90 area% or more. The area ratio of pearlite is more preferably 92 area% or more (more preferably 95 area% or more).

[Average value of particle numbers of pearlite nodules Pave: 6.0 ≦ Pave ≦ 12.0]
The average value Pave of the grain number of pearlite nodules (hereinafter sometimes referred to as “pearlite nodule size”) greatly affects the ductility of the rolled wire rod. A rolled wire rod having a small pearlite nodule size is poor in ductility and causes disconnection during the SV treatment. In addition, the larger the pearlite nodule size, the better the ductility, but in order to make the pearlite nodule extremely fine, the mounting temperature in the hot rolling is extremely lowered and excessive cooling equipment is used for rapid cooling. Is actually difficult to manufacture. From such a viewpoint, the average value Pave of the pearlite nodule size was set to 6.0 ≦ Pave ≦ 12.0. Preferably, 7.0 ≦ Pave ≦ 11.0.

[Total decarburized layer depth of surface layer: 0.20 mm or less]
Decarburization of the surface layer is usually removed by the SV treatment, but if the surface decarburization layer is deep, the ductility of the shavings generated during the SV processing becomes high, so that the severability of the shavings by the chip breaker deteriorates, and the shavings The discharge performance is lowered, and the SV processability is lowered. When the surface decarburized layer is deep, the surface fully decarburized layer remains even after the SV treatment, and the fatigue strength of the spring is significantly reduced. Therefore, the surface layer total decarburization layer depth was set to 0.20 mm or less. Preferably it is 0.15 mm or less (more preferably 0.10 mm or less).

[Cr-based alloy carbide content ≦ 7.5 mass%]
Since the Cr-based alloy carbide is remarkably harder than the iron-based carbide, a small amount of precipitation causes chipping of the tip of the chipper, a decrease in the life of the chipper die, a deterioration in chip dischargeability, and the like, thereby reducing the SV processability. Therefore, the upper limit of the amount of Cr-based alloy carbide is set to 7.5%. The amount of Cr-based alloy carbide is preferably 5.0% or less (more preferably 4.0% or less). The Cr-based alloy carbide targeted in the present invention is basically a carbide containing Cr as a main component, but when it contains carbide-forming elements such as V, Nb, and Mo, a composite alloy carbide with these It is the main point including. Moreover, a trace amount nitride and carbonitride may be contained in Cr system alloy carbide amount.

  In producing the steel wire for a high-strength spring as described above, it is necessary to appropriately control the production conditions. The procedure for producing a steel wire for high strength springs is as follows. First, a steel billet having a predetermined chemical composition is hot-rolled and processed into a desired wire diameter. When the heating temperature at the time of rolling is too high, it causes the structure embrittlement associated with the coarsening of the prior austenite grain size, and decreases the SV processability. If the heating temperature is too low, the deformation resistance of the steel material increases, so the load on the rolling mill increases and the productivity decreases. Therefore, the heating temperature before rolling is preferably 900 ° C. or higher and 1100 ° C. or lower. More preferably, it is 950 degreeC or more and 1050 degreeC or less.

  Subsequently, the steel wire after hot rolling is coiled and placed on a cooling conveyor. When the temperature (mounting temperature) at this time exceeds 1100 ° C., the prior austenite grain size becomes coarse, and the pearlite nodule size When the temperature becomes less than 860 ° C., deep surface decarburization tends to occur, deformation resistance becomes high, and winding shape defects tend to occur. For these reasons, the mounting temperature is preferably 860 to 1100 ° C., and by controlling the temperature within such a range, coarsening of the pearlite nodule size and surface decarburization can be suppressed. The mounting temperature is more preferably 900 ° C. or higher and 1050 ° C. or lower.

  After placing on the conveyor, the average cooling rate up to 600 ° C. which is the end temperature range of pearlite transformation is 1.0 ° C./second or more (preferably 3.5 ° C./second or more), 10 ° C./second or less (preferably 8 ° C. / Sec), a rolled material structure that is a pearlite-based structure and suppresses coarsening of the pearlite nodule size is obtained. The average cooling rate from less than 600 ° C. to 400 ° C. is 3 ° C./second or more (preferably 3.5 ° C./second or more), 10 ° C./second or less (preferably 8 ° C./second or less), and 400 ° C. or less ( By cooling to preferably 375 ° C. or lower, a rolled wire rod excellent in SV processability is obtained in which precipitation of Cr-based alloy carbide is suppressed in the pearlite-based structure.

  The steel wire rod for high-strength springs of the present invention needs to appropriately adjust the chemical component composition in order to exhibit the characteristics as a final product (high-strength spring). The reason for the range limitation by each component (element) in the chemical component composition is as follows.

[C: 0.4% or more and less than 1.2%]
C is an element that secures the basic strength of the steel material and is effective in increasing the strength and sag resistance of the spring. For that purpose, it is necessary to contain 0.4% or more. As the C content increases, the strength and sag resistance of the spring will improve, but if it is excessive, a large amount of coarse cementite will precipitate, reducing ductility and toughness, which will adversely affect spring workability and spring characteristics. become. From such a viewpoint, the C content needs to be less than 1.2%. The preferable lower limit of the C content is 0.5% or more, and the preferable upper limit is 1.0% or less.

[Si: 1.5 to 3.0%]
Si is an element necessary for deoxidation of steel, and is also an element necessary for ensuring the strength, hardness and sag resistance of the spring. In order to exhibit these effects, it is necessary to contain 1.5% or more of Si. However, when the Si content is excessive, not only the material is hardened, but also ductility and toughness are reduced, and surface decarburization is increased to reduce SV processability and fatigue characteristics. It is necessary to do the following. The preferable lower limit of the Si content is 1.6% or more (more preferably 1.7% or more), and the preferable upper limit is 2.8% or less (more preferably 2.5% or less).

[Mn: 0.5 to 1.5%]
Mn, like Si, is an element necessary for deoxidation of steel. In addition to fixing S in steel as MnS, it enhances hardenability and contributes to improvement of spring strength. In order to exhibit these effects, it is necessary to contain 0.5% or more. However, when the Mn content is excessive, the hardenability becomes excessively high, and a supercooled structure such as martensite and bainite is easily generated. Therefore, the Mn content needs to be 1.5% or less. A preferable lower limit of the Mn content is 0.6% or more (more preferably 0.7% or more), and a preferable upper limit is 1.4% or less (more preferably 1.3% or less).

[Cr: 0.5% or less (excluding 0%)]
In addition to improving hardenability and temper softening resistance and improving spring strength, Cr has the effect of reducing the activity of C and preventing decarburization during rolling and heat treatment. However, when the Cr content is excessive, precipitation of Cr-based alloy carbide, nitride, and carbonitride becomes excessive, and the SV processability is lowered. Therefore, the Cr content needs to be 0.5% or less. In order to exhibit the said effect, Cr content is 0.05% or more. A more preferable lower limit of the Cr content is 0.10% or more, and a preferable upper limit is 0.45% or less (more preferably 0.40% or less).

[Al: 0.01% or less (excluding 0%)]
Al is a deoxidizing element, but forms inclusions of Al ₂ O ₃ and AlN in the steel. Since these inclusions significantly reduce the fatigue life of the spring, Al should be reduced as much as possible. From such a viewpoint, the Al content needs to be 0.01% or less, preferably 0.008% or less. More preferably, it is 0.005% or less.

  The basic components in the steel wire for high-strength springs according to the present invention are as described above, and the balance is iron and inevitable impurities (for example, P, S, etc.). In the steel wire for high-strength springs according to the present invention, if necessary, (a) V: 0.5% or less (not including 0%) and / or Nb: 0.5% or less (not including 0%), (B) Mo: 0.5% or less (not including 0%), (c) Ni: 1.0% or less (not including 0%), (d) Cu: 0.5% or less (0% (E) B: 0.010% or less (not including 0%) or the like may be included, and the properties of the steel wire are further improved depending on the type of element to be included. The reason for setting a preferable range of these elements is as follows.

[V: 0.5% or less (not including 0%) and / or Nb: 0.5% or less (not including 0%)]
V and Nb both have the effect of refining crystal grains in hot rolling and quenching and tempering treatments, and improve ductility and toughness. Among these, V also has an effect of causing secondary precipitation hardening at the time of stress relief annealing after spring forming and contributing to improvement of spring strength. However, if excessively contained, precipitation of composite alloy carbides of Cr and V or Nb becomes excessive, and the SV processability is lowered. For this reason, the content is preferably 0.5% or less. The preferable lower limit for exhibiting the above effects is 0.05% or more (more preferably 0.10% or more), and the more preferable upper limit is 0.45% or less (more preferably 0.40%). The following).

[Mo: 0.5% or less (excluding 0%)]
Mo contributes to improving the strength of the spring by causing secondary precipitation hardening at the time of stress relief annealing after spring forming. However, when the Mo content is excessive, precipitation of the composite alloy carbide of Cr and Mo becomes excessive, and the SV processability is lowered. Therefore, the Mo content is preferably 0.5% or less. A preferable content for exhibiting the above effect is 0.05% or more. A more preferable lower limit of the Mo content is 0.10% or more, and a more preferable upper limit is 0.45% or less (more preferably 0.40% or less).

[Ni: 1.0% or less (excluding 0%)]
Ni contributes to the improvement of ductility, toughness, and corrosion resistance after quenching and tempering as well as suppressing decarburization during hot rolling. However, when the content is excessive, the hardenability is excessively improved, so that a supercooled structure such as martensite and bainite is easily generated. In addition, since retained austenite is excessively generated by quenching and tempering, which is a manufacturing process of the OT wire (oil tempered wire), the sag resistance of the spring is significantly reduced. Therefore, the Ni content is preferably 1.0% or less. A preferable lower limit of the Ni content is 0.05% or more (more preferably 0.10% or more), and a more preferable upper limit is 0.9% or less (more preferably 0.8% or less).

[Cu: 0.5% or less (excluding 0%)]
Cu not only suppresses decarburization during hot rolling, but also contributes to improvement of corrosion resistance. However, if it is contained excessively, the hot ductility is lowered and there is a risk of cracking during hot rolling. Therefore, the Cu addition amount is preferably 0.5% or less. The minimum with preferable Cu content is 0.05% or more (more preferably 0.1% or more), and a more preferable upper limit is 0.45% or less (more preferably 0.40% or less).

[B: 0.010% or less (excluding 0%)]
B has an effect of improving ductility and toughness by improving hardenability and cleaning austenite grain boundaries. However, if it is contained excessively, a composite compound of Fe and B precipitates and there is a risk of causing cracks during hot rolling. Moreover, since hardenability improves excessively, it becomes easy to produce | generate supercooled structures, such as a martensite and a bainite. Therefore, the B content is preferably 0.010% or less. A preferable lower limit of the B content is 0.0010% or more (more preferably 0.0015% or more, still more preferably 0.0020% or more), and a more preferable upper limit is 0.0080% or less (more preferably 0.0060). % Or less).

  The high-strength steel wire rod according to the present invention assumes that after hot rolling, but this high-strength steel wire rod is subsequently subjected to skinning treatment, annealing, wire drawing pretreatment (pickling treatment), wire drawing, A high-strength spring is formed by performing coiling, quenching and tempering treatment, surface treatment, and the like. The high-strength spring thus obtained exhibits good characteristics.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  After melting steel ingots having the chemical composition shown in Tables 1 and 2 below in a converter, this steel ingot is subjected to ingot rolling to produce a steel billet having a cross section of 155 mm × 155 mm and heated to 1000 ° C. Conveyor placement temperature (mounting temperature after rolling) and average cooling rate shown in Tables 3 and 4 below, average cooling rate (average cooling rate from placement temperature to 600 ° C. and from less than 600 ° C. to 400 ° C.) Then, a coil having a wire diameter of 8.0 mmφ and a single weight of 2 ton was manufactured (Test Nos. 1 to 31).

  The pearlite area ratio, pearlite nodule size, total decarburized layer depth, Cr-based alloy carbide content, and SV processability of each obtained coil were investigated. In the investigation of the SV processability, the total amount of the 2-ton coil was used. In addition to the investigation of SV processability, one ring is cut out from the end of each 2-ton coil for workability investigation, and it is sampled by dividing into 8 parts in the circumferential direction (corresponding to 8 parts in the longitudinal direction of the wire) as shown in FIG. The representative value of each coil was calculated | required by using each sample and averaging each measured value.

  As shown in FIG. 2 (cross-sectional view schematically showing the structure observation position), the pearlite area ratio is a 1 / 4D position (D is the D from the surface layer (2 fields of view), as shown in FIG. Wire rod diameter: 2 visual fields), 1 / 2D (D / 4 to D / 4 central region: 1 visual field) (total 5 visual fields) were measured using an optical microscope. After embedding and polishing the cross section of the hot-rolled wire, and performing chemical corrosion using picric acid, an optical microscope was used to take a structure photograph of an area of 200 μm × 200 μm at a magnification of 400 times, and image analysis software (“Image” The image was binarized using “Pro Plus” (Media Cybermetrics), and then the perlite area ratio was determined, and the average value was calculated. The pearlite area ratio of 5 fields of view was obtained at 8 sites, and the pearlite area ratio for each coil was calculated by averaging them. In addition, when the decarburized layer existed in the surface layer, the entire decarburized part specified by 4 of JIS G 0558 was excluded from the measurement site. A structure having a pearlite area ratio of 90% or more was expressed as P, and when pearlite was less than 90% and bainite or martensite was formed, it was expressed as “P + B + M” or “B + M”.

  The pearlite nodule size is measured at each of the eight layers of the hot-rolled wire, as shown in FIG. 2, with each surface layer (2 fields of view), 1 / 4D position from the surface (D is the wire diameter: 2 fields of view), 1 / In the 2D (D / 4 to D / 4 central region: 1 field of view) portion (total of 5 fields of view), measurement was performed using an optical microscope. Here, the pearlite nodule indicates a region in which the ferrite crystal grains in the pearlite structure have the same orientation, and the measurement method is as follows. First, the cross section of the hot rolled wire is embedded and polished, and then corroded using a solution of concentrated nitric acid (62%): alcohol = 1: 100 (volume ratio). The nodule grains are observed to emerge), and the particle number number of the pearlite nodules was measured. The average number Pave of the pearlite nodule size for each coil was calculated by measuring the particle size numbers of pearlite nodules with 5 fields of view at 8 sites and averaging them. The particle number of pearlite nodules was measured according to “Measurement of austenite crystal particle size” described in JIS G 0551.

  The total decarburized layer depth was measured using an optical microscope at 8 locations on each surface layer as shown in FIG. 3 (cross sectional view schematically showing the decarburization observation position) at 8 locations of the hot-rolled wire rod. Measured. The cross section of hot-rolled wire is embedded and polished, and after chemical corrosion using picric acid is observed, the maximum depth at 8 locations is measured at each location, and the deepest total decarburized layer depth at 8 locations. Was the total decarburization depth of the coil. The measurement calculated | required the total decarburization layer depth according to "the decarburization layer depth measuring method of steel" as described in JISG0558.

  The amount of Cr-based alloy carbide was determined by the electrolytic extraction method. First, after removing the scale of the rolled wire with sandpaper and washing with acetone, this sample was immersed in an electrolytic solution (ethanol solution containing 10% by mass of acetylacetone). After about 0.5 g, the sample was taken out), the metal phase metal Fe was electrolyzed, and alloy precipitates in the steel existing in the electrolyte (including carbides, trace amounts of nitrides, carbonitrides) Was collected as a residue, and the amount of Cr-based alloy carbide (mass%) was determined by dividing the residue mass by the amount of electrolysis. The alloy precipitate to be measured is mainly a Cr-based alloy carbide, but when a selective element is added, it includes a composite alloy carbide of Cr and V, Nb, Mo or the like. A filter having a mesh diameter of 0.1 μm [a membrane filter manufactured by Advantech Toyo Co., Ltd.] was used as a filter for collecting the residue.

  SV treatment is performed without applying heat treatment to the coil, the presence or absence of disconnection in this SV treatment, the load of the breaker that breaks up the chip set on the chipper die entry side, the chipper die missing The presence or absence, etc. were evaluated.

[SV Evaluation Criteria]
(1) The presence or absence of disconnection: When the entire 2-ton coil is subjected to the SV process, the coil in which the disconnection does not occur has good SV processability: ○, the coil in which the disconnection has occurred once or more is evaluated as poor in SV processability: x did.

  (2) Breaker load: The displacement (0 to 10 A) of the current value of the breaker was measured with a data logger at a sampling interval of 1 second, and data excluding each 10 kg of TOP and BOT terminals during SV processing was used. A coil whose measurement data has a part of the 60-point average moving line that does not exceed 9A has good SV processability: A coil that has a part of the 60-point average moving line that has a value that exceeds 9A has a poor SV processing ability: X was evaluated (see FIGS. 4 and 5 below).

  (3) Chipper die chipping: After the entire amount of the 2-ton coil was subjected to SV treatment, the chipper die was removed, and chipping of the wire contact portion of the chipper die was confirmed with a stereomicroscope. A coil having no chipping (chipper chipping) in the wire contact portion of the chipper die was evaluated as having good SV processability: ○, and a coil having chipping in the wire contact portion was evaluated as having poor SV processing performance: x.

  These evaluation results are shown in Tables 5 and 6 below together with the rolled wire structure (perlite area ratio, average value Pave of pearlite nodule size) and Cr alloy carbide amount.

  Test No. Nos. 1 to 15 (Table 5) are examples satisfying the requirements defined in the present invention, Test No. Examples 16 to 23 (Table 6) satisfy the chemical composition (steel types B1, B2, C1, C2, E1, G1, G2, L1), but the manufacturing conditions do not satisfy the requirements defined in the present invention. , Test No. 24 to 31 (Table 6) are those in which the chemical composition is outside the range defined in the present invention (steel types P to W).

  From these results, it can be considered as follows. First, test no. Nos. 1 to 15 satisfy the requirements defined in the present invention, and these steel wires have good results in all items of SV processability (presence of disconnection, breaker load, chipper chipping). .

  In contrast, test no. In No. 16, since the mounting temperature after rolling is high, the pearlite nodule size of the rolled material is coarse, and disconnection occurs in the SV treatment. Test No. In No. 17, since the mounting temperature after rolling is low, the entire decarburized layer of the rolled wire rod surface layer is deep, and the load of the breaker is increased.

  Test No. Nos. 18 and 21 have a slow average cooling rate up to 600 ° C. after placing the conveyor, so that the pearlite nodule size of the rolled material is coarse, and disconnection occurs in the SV process. Test No. In Nos. 19 and 22, since the average cooling rate from less than 600 ° C. to 400 ° C. is slow, the amount of Cr-based alloy carbide increases, the load on the breaker increases, and chipping in the chipper occurs.

  Test No. No. 20 has a high average cooling rate up to 600 ° C. after placing on the conveyor, so it does not have a pearlite single phase structure, but martensite and bainite are generated, and disconnection occurs during SV processing. Test No. Since No. 23 has a high average cooling rate from less than 600 ° C. to 400 ° C., it does not become a pearlite single phase structure, but martensite and bainite are generated, and disconnection occurs during the SV treatment.

  Test No. No. 24 is an example using a steel type with excessive Si content (steel type P in Table 2), where the entire decarburized layer of the rolled wire rod surface layer is deep, and the breaker load is increased.

  Test No. 25, 26, 31 are examples using steel types (steel types Q, R, W in Table 2) in which the content of each component (Mn, Ni, B) is excessive, and the hardenability increased excessively. It does not become a pearlite single phase, but bainite and martensite are generated, and disconnection occurs during the SV treatment.

  Test No. 27 to 30 are examples using steel types (steel types S, T, U, V in Table 2) in which the content of each component (Cr, V, Mo, Nb) is excessive, and the amount of Cr-based alloy carbide increases. In addition, the load on the breaker increases and chipping on the chipper occurs.

  FIG. 2 (invention example) shows the displacement of the breaker current value, and it can be seen that the current value is stable. On the other hand, FIG. 27 (comparative example) shows the displacement of the breaker current value, and it can be seen that the load of the breaker is partially increased (the breaker load in the portion surrounded by the broken line is high and the current value is large).

Claims (7)

It is a steel wire after hot rolling, C: 0.4% or more, less than 1.2% (meaning “mass%”, the same applies to the chemical composition), Si: 1.5-3.0%, Mn: 0.5 to 1.5%, Cr: 0.5% or less (not including 0%) and Al: 0.01% or less (not including 0%), respectively, the balance being iron and inevitable It consists of impurities and is a structure mainly composed of pearlite, the average value Pave of the pearlite nodule particle size number satisfies the following formula (1), the total decarburized layer depth of the surface layer is 0.20 mm or less, and Cr A steel wire for high-strength springs with excellent carving characteristics, characterized in that the amount of carbide of the alloy is 7.5% or less.
6.0 ≦ Pave ≦ 12.0 (1)   The steel wire for high-strength springs according to claim 1, containing V: 0.5% or less (not including 0%) and / or Nb: 0.5% or less (not including 0%).   The steel wire for high-strength springs according to claim 1 or 2, containing Mo: 0.5% or less (not including 0%).   Furthermore, Ni: 1.0% or less (0% is not included) The steel wire for high-strength springs in any one of Claims 1-3 containing.   Furthermore, Cu: 0.5% or less (0% is not included) The steel wire for high strength springs in any one of Claims 1-4 containing.   Furthermore, the steel wire for high strength springs according to any one of claims 1 to 5, further comprising B: 0.010% or less (not including 0%).   A high-strength spring obtained from the steel wire for a high-strength spring according to any one of claims 1 to 6.