KR101601582B1 - Steel wire rod with excellent shavability for high-strength spring, and high-strength spring - Google Patents

Abstract

The steel wire rod for high strength steel of the present invention is a steel wire rod after hot rolling and has a predetermined chemical composition and a pearlite area ratio of 90% or more, and the average value Pave of the particle size number of the pearlite nodule satisfies the following expression (1) , The total decarburized layer depth of the surface layer is 0.20 mm or less, and the amount of the Cr-based alloy carbide is 7.5% or less with respect to the total mass of the steel wire rod.
6.0? Pave? 12.0 ... (One)
Strength springs for high-strength springs and high-strength springs obtained by using such high-strength spring steel rods as materials are excellent in machinability and dischargeability of cutting debris and have excellent SV treatmentability .

Description

STEEL WIRE ROD WITH EXCELLENT SHAVABILITY FOR HIGH-STRENGTH SPRING, AND HIGH-STRENGTH SPRING

 The present invention relates to a steel wire rod for a high strength spring useful as a material of a high strength spring (particularly a valve spring) used for a clutch, an engine, a fuel injecting device, a suspension mechanism of an automobile, Strength steel wire for a high-strength spring which can exert a satisfactory machinability in a cutting process, and a high-strength spring obtained from the steel wire for high strength steel.

Since the spring used under the circumstances described above is used with high stress over a long period of time, a high level of endothelial property is required. In order to improve the endothelial property, excellent surface properties and excellent inclusion control are required. Among them, the surface property is subjected to planarization and curing treatment by shot peening, nitriding treatment or the like after the spring forming, but when scratches of only about several tens of microns remain or occur, surface scratches One break occurs with the origin.

Thus, a machining process (hereinafter also referred to as " SV treatment ") is carried out to remove fine decarburization of the decarburized portion and the wire rod surface layer of the wire rod surface layer after rolling. This SV treatment is a process of cutting the entire surface layer of the wire by several hundred microns in depth by using a chipper die, but in the case of a wire having poor SV treatmentability (machinability) There are disadvantages such as breakage of the chipping die, scattering of the surface of the wire rod, and shortening of the tool life. In addition, in the case of a wire rod having poor SV treatmentability, cutting chips are finely divided and cutter debris is entangled in the breaker for increasing the dischargeability of cutting debris, resulting in an excessive motor load for driving the breaker, Problems also arise.

By improving the SV processability, it is possible to greatly improve the yield and improve the quality. As techniques for improving the SV processing property, there are main control methods such as the tissue control and the inclusion composition control, but such techniques have already been proposed.

For example, Patent Document 1 proposes to improve machinability by roughening the austenite grain size. However, in order to realize the high fatigue strength with the spring steel, fine crystal grains are required, and it is preferable that the crystal grains have a fine grain size in consideration of the composition such as the SV treatment and the drawing process.

In Patent Document 2, the SV treatmentability is improved by defining the composition of the oxide inclusion, the size of the oxide inclusion present in the surface layer, and the distribution density. However, the reason for lowering the SV treatmentability is the fact that the effects of alloy-based carbides, nitrides, etc., which determine the ductility and toughness of the structure, are large.

On the other hand, in Patent Document 3, the SV treatmentability is improved by defining the mechanical properties. However, in this technique, the amount of alloy added is increased, and in spring steels such as alloyed carbide or nitride, Can not improve the SV processability.

Japanese Patent Application Laid-Open No. 2000-256785 Japanese Patent Application Laid-Open No. 2010-222604 Japanese Patent Application Laid-Open No. 2000-239797

Especially in valve springs, high fatigue strength and high fatigue life are required. In order to satisfy these characteristics, it is required that the surface property of the spring is good, and SV treatment is performed in order to remove the decarburized layer and surface scratches of the rolled material. This SV treatment consists of a skin pass process to increase the toughness of the rolled material to prevent chipping and a machining process using a chipping dice. In order to prevent disconnection in the skin pass process It is required to control the cooling condition on the conveyor properly during rolling so that the rolled material does not contain supercooled structure (bainite or martensite).

In addition, in the machining process in the chipping dies, the entire length of the 2-ton (tone) coil can be machined, and stable, linear skin quality without die marks is required. For this reason, it is necessary that the rolled material is a rolled material having excellent machinability such that cracked structure is not included in the rolled material, which is likely to cause breakage, and cracks are unlikely to occur in the chippedper die and the load on the tool is small. In addition, it is also necessary that the cutting debris generated during the machining in the chipping die is discharged after being finely divided by the breaker, but it should be a cutting debris which is likely to be broken by the breaker, that is, good dischargeability of the cutting debris.

SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems in the prior art, and an object of the present invention is to provide a cutting tool which is excellent in machinability and dischargeability of cutting debris, A high strength spring wire for a high strength spring which can be used for a high strength spring, and a high strength spring obtained by using such a high strength steel wire for a spring as a material.

The steel wire rod for high strength steel of the present invention which can solve the above problems is a steel wire rod after hot rolling and contains C: not less than 0.4% and not more than 1.2% (meaning "mass%", And the balance of iron and inevitable impurities and having a pearlite area ratio of 90% or more, and the pearlite The average value Pave of the particle size of the nodule satisfies the following expression (1), the depth of the entire decarburized layer in the surface layer is 0.20 mm or less, and the amount of the Cr-based alloy carbide is 7.5% or less with respect to the total mass of the steel wire rod .

6.0? Pave? 12.0 ... (One)

(A) at least one of V: 0.5% or less (not including 0%) and Nb: 0.5% or less (excluding 0%), ( b) Mo: not more than 0.5% (not including 0%), (c) Ni: not more than 1.0% (not including 0%), (d) Cu: not more than 0.5% (e) B: 0.010% or less (not including 0%) is also effective, and the characteristics of the high strength spring steel wire rod are further improved depending on the contained components.

The present invention also includes a high-strength spring obtained from the above-mentioned high-strength spring steel wire rod.

In the present invention, by appropriately adjusting the chemical composition and appropriately preparing conditions, the pearlite area ratio is set to 90% or more, and the average value Pave of the pearlite nodule particle number is set to a predetermined range, It is possible to control the depth and the amount of the Cr-based alloy carbide appropriately so that the machinability and the dischargeability of the cutting debris are good and the steel wire rod for a high strength spring And the steel wire rod for a high strength spring is very useful as a material for manufacturing a high strength spring.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view showing a sampling method (ring splitting position) of a sample for evaluation. FIG.
Fig. 2 is a cross-sectional view schematically showing a position where a wire is observed.
3 is a cross-sectional view schematically showing a surface decarburization observation position of the wire rod.
Fig. 2 (in the case of the present invention).
Fig. 27 is a graph showing the displacement of the breaker current value in Comparative Example.

The present inventors have studied at various angles in order to realize a steel wire for high strength springs suitable for the above purpose. As a result, by appropriately controlling the composition of the chemical components constituting the rolled material, the texture, the particle size number of the pearlite nodules, the depth of the surface layer decarburization, the amount of the Cr-based alloy carbide in the surface layer of the rolled material, and the like, In addition, it has been found that the SV processability is remarkably improved such that disconnection does not occur in the SV process. On the other hand, hereinafter referred to as " SV processability ", including machinability and cutting debris dischargeability. Next, each of the requirements defined in the present invention will be described.

[Tissue having pearlite area ratio of 90% or more]

The steel wire rod (hot rolled steel wire: rolled wire) of the present invention is a structure having a pearlite area ratio of 90% or more. A rolled wire having a structure having a pearlite area ratio of 90% or more refers to a rolled wire having an area ratio of bainite, martensite, and ferrite occupying less than 10% in the cross section of the rolled wire. The rolled wire having a pearlite area ratio of 90% or more can be processed without being broken at the time of the SV treatment, but in the case of the rolled wire having a supercooled structure such as bainite or martensite having an area ratio of 10% or more, The SV processability is deteriorated such as disconnection occurs in the SV process.

In addition, the ferrite does not lower the SV processability by a supercooled structure such as bainite or martensite but may contain a part thereof. If the amount is excessive, the structure becomes uneven and is not preferable for the SV processability. From this viewpoint, in the steel wire rod of the present invention, the area ratio of pearlite is preferably 90 percent by area or more. The area ratio of the pearlite is more preferably 92% by area or more (more preferably 95% by area or more).

[Average value of particle size numbers of pearlite nodule Pave: 6.0? Pave? 12.0]

The average value Pave of the particle number of the pearlite nodule (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 deficient in ductility, resulting in disconnection in the SV treatment. The larger the pearlite nodule size is, the more ductility is improved. However, in order to extremely reduce the pearlitic nodule, an extremely large cooling capacity is required for the hot rolling since the temperature of the hot rolling is extremely lowered. it's difficult. From this viewpoint, the average value Pave of the pearlite nodule size was 6.0? Pave? 12.0. Preferably, 7.0? Pave? 11.0.

[Total decarbonization layer depth in the surface layer: 0.20 mm or less]

The decarburization of the surface layer is usually removed by the SV treatment. However, if the whole surface decarburization layer is deeper, the ductility of the cutting debris generated during the SV treatment becomes higher, so that the segmentation of the cutting debris by the chip breaker is deteriorated, So that the SV processability is lowered. Further, when the entire surface decarbonization layer is deep, the entire surface decarbonization layer remains even after the SV treatment, and the fatigue strength of the spring is remarkably lowered. Therefore, the depth of the entire decarbonization layer in the surface layer was set to 0.20 mm or less. Preferably 0.15 mm or less (more preferably 0.10 mm or less).

[Amount of Cr-based alloy carbonized material to total weight of steel wire rod < = 7.5% by mass]

The Cr-based alloy carbide is remarkably harder than the iron-based carbide, so that a small amount of precipitation causes breakage of the end of the chipping blade, deterioration of the life of the chipping dies and deterioration of dischargeability of cutting chips. Therefore, the upper limit of the amount of the Cr-based alloy carbonaceous material with respect to the total mass of the steel wire rod was 7.5%. The amount of Cr-based alloy carbide is preferably 5.0% or less (more preferably 4.0% or less). On the other hand, the Cr-based alloy carbide to be used 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, it also includes a composite alloy carbide It is the main point. Further, the amount of the Cr-based alloy carbonized material may contain a small amount of nitride or carbonitride.

In manufacturing such a steel wire rod for high strength springs, it is necessary to appropriately control the production conditions thereof. The procedure for manufacturing a high strength steel wire for a spring is as follows. First, a steel billet having a predetermined chemical composition is hot-rolled to a desired wire diameter. Regarding the heating temperature at the time of rolling, if too high, it becomes a cause of brittleness accompanying the coarsening of the old austenite grain size, and the SV processability is lowered. If the heating temperature is too low, the deformation resistance of the steel becomes high, so that the load of the rolling mill is increased and the productivity is lowered. Therefore, it is preferable that the heating temperature before rolling is 900 DEG C or more and 1100 DEG C or less. More preferably 950 DEG C or more and 1050 DEG C or less.

Subsequently, the hot-rolled steel wire rod is placed on a cooling conveyor in the form of a coil. When the temperature (setting temperature) at this time exceeds 1100 ° C, the grain size of the old austenite crystal is increased, It becomes a cause of accompanying texture embrittlement. When it is less than 860 DEG C, deep surface layer decarburization tends to easily occur, and the deformation resistance is increased, and the wound shape defect tends to occur easily. For this reason, it is preferable that the setting temperature is 860 to 1100 占 폚. By controlling the temperature within this range, coarsening of the pearlite nodule size and surface layer decarburization can be suppressed. The deposition temperature is more preferably 900 DEG C or more and 1050 DEG C or less.

The average cooling rate up to 600 占 폚, which is the end temperature of the pearlite transformation, is set to 1.0 占 폚 / sec or more (preferably 3.5 占 폚 / sec or more) and 10 占 폚 / sec or less (preferably 8 占 폚 / sec) A rolled material structure in which pearlite-based structure and pearlite nodule-size coarsening are suppressed is obtained. And the average cooling rate from 600 deg. C to 400 deg. C is set at 3 deg. C / sec or more (preferably 3.5 deg. C / sec or more) at 10 deg. C / Preferably 375 DEG C or lower), precipitation of the Cr-based alloy carbide in the structure of the pearlite main body is suppressed, and a rolled wire rod excellent in SV treatment property is obtained.

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

[C: 0.4% or more and less than 1.2%]

C is an element effective in securing the basic strength in the steel material and raising the strength and fatigue resistance of the spring. For this, it is necessary to contain 0.4% or more of the element. As the C content increases, the strength and fatigue resistance of the springs are improved. However, if the C content is excessive, a large amount of coarse cementite precipitates and the ductility and toughness deteriorate, adversely affecting spring workability and spring characteristics. From this viewpoint, the C content needs to be less than 1.2%. A preferable lower limit of the C content is 0.5% or more, and a 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 strength, hardness and fatigue resistance of springs. In order to exhibit these effects, it is necessary to contain not less than 1.5% of Si. However, if the Si content is excessive, not only the material is hardened but also the ductility and toughness are lowered, besides the decarbonization of the surface is increased and the SV treatment property and the fatigue property are lowered. . The lower limit of the Si content is preferably 1.6% or more (more preferably 1.7% or more), and the upper limit is preferably 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, and in addition to fixing S in the steel as MnS, it contributes to the improvement of the hardenability and the improvement of the spring strength. In order to exhibit these effects, it is necessary to contain not less than 0.5%. However, if the Mn content is excessive, the quenching property becomes excessively high, and undercoating structures such as martensite and bainite are easily produced. Therefore, the Mn content should be 1.5% or less. The lower limit of the Mn content is preferably 0.6% or more (more preferably 0.7% or more), and the upper limit is preferably 1.4% or less (more preferably 1.3% or less).

[Cr: 0.02-0.5%]

Cr improves the hardenability and softening resistance and improves the spring strength. In addition, it reduces the amount of C to prevent decarburization at the time of rolling or heat treatment. However, if the Cr content is excessive, precipitation of Cr-based alloy carbides, nitrides and carbonitrides becomes excessive, thereby lowering the SV processability. Therefore, the Cr content should be 0.5% or less (the preferred upper limit is 0.45% or less (more preferably 0.40% or less)). In order to exhibit the above effect, the Cr content is 0.02% or more. A more preferable lower limit of the Cr content is at least 0.05% (more preferably at least 0.10%).

[Al: 0.010% or less]

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 must be reduced as much as possible. From this viewpoint, the Al content should be 0.010% or less, preferably 0.008% or less. More preferably 0.005% or less.

The basic components of the steel wire rod for high-strength spring according to the present invention are as described above, and the remainder are iron and unavoidable impurities (for example, P, S and the like). (A) at least one of V: not more than 0.5% (excluding 0%) and Nb: not more than 0.5% (not including 0%), (b) a steel wire rod for high strength steel according to the present invention, ) Mo: not more than 0.5% (not including 0%), (c) Ni: not more than 1.0% (not including 0%), (d) Cu: not more than 0.5% e) B: not more than 0.010% (not including 0%), and the properties of the steel wire rod are further improved depending on the kind of element to be contained. The reasons for setting the preferable ranges of these elements are as follows.

[At least one of V: not more than 0.5% (not including 0%) and Nb: not more than 0.5% (not including 0%)]

Both V and Nb have the function of refining the crystal grains in hot rolling and quenching tempering treatment, thereby improving ductility and toughness. Among these, V has an effect of contributing to the improvement of the strength of the spring by causing secondary precipitation hardening at the time of deformation correcting annealing after the spring forming. However, if it is contained in excess, precipitation of Cr and composite alloy carbide of V or Nb becomes excessive, and the SV processability is lowered. Therefore, it is preferable that the content is 0.5% or less. The lower limit for achieving the above effect is preferably not less than 0.05% (more preferably not less than 0.10%), and more preferably not more than 0.45% (more preferably not more than 0.40%).

[Mo: 0.5% or less (not including 0%)]

Mo causes secondary precipitation hardening at the time of deformation correcting annealing after the spring forming, thereby contributing to improvement of the strength of the spring. However, if the Mo content is excessive, precipitation of the composite alloy carbide of Cr and Mo becomes excessive and the SV treatment property is deteriorated. 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: not more than 1.0% (not including 0%)]

Ni not only inhibits decarburization during hot rolling but also contributes to improvement in ductility, toughness and corrosion resistance after quenching and tempering. However, when the content is excessive, the quenchability is excessively improved, and therefore supercooled structure such as martensitic or bainite is easily produced. Further, since the retained austenite is excessively produced in the quenching tempering process, which is a manufacturing process of the OT line (oil tempering line), the fatigue resistance of the spring is remarkably lowered. Therefore, the Ni content is preferably 1.0% or less. The lower limit of the Ni content is preferably not less than 0.05% (more preferably not less than 0.10%), and more preferably not more than 0.9% (more preferably not more than 0.8%).

[Cu: not more than 0.5% (not including 0%)]

Cu not only inhibits decarburization in hot rolling but also contributes to improvement of corrosion resistance. However, if it is contained in an excessive amount, there is a risk that the hot ductility is lowered and cracks are generated during hot rolling. Therefore, the amount of Cu added is preferably 0.5% or less. The lower limit of the Cu content is preferably 0.05% or more (more preferably 0.1% or more), and the more preferable upper limit is 0.45% or less (more preferably 0.40% or less).

[B: not more than 0.010% (not including 0%)]

B has the effect of improving the hardenability and improving the ductility and toughness by the purification of the austenite grain boundary. However, if it is contained in excess, there is a risk that a composite compound of Fe and B is precipitated, causing cracking during hot rolling. Further, since the quenching property is excessively improved, supercooled textures such as martensite and bainite are easily produced. Therefore, the B content is preferably 0.010% or less. The lower limit of the B content is preferably 0.0010% or more (more preferably 0.0015% or more, and still more preferably 0.0020% or more), and still more preferably 0.0080% or less (more preferably 0.0060% or less).

The high-strength steel wire rod according to the present invention is one obtained after hot-rolling. The high-strength steel wire rod is then subjected to machining, annealing, fresh pre-treatment (pickling treatment), drawing, coiling, Treatment or the like is carried out to form a high strength spring. The high-strength spring thus obtained exhibits good characteristics.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is of course not limited by the following Examples, and it is needless to say that the present invention can be carried out by modifying it appropriately within a range suitable for the purposes And are all included in the technical scope of the present invention.

The steel billets having the chemical compositions shown in Tables 1 and 2 were melted in a converter, and the steel billets were crushed and rolled to prepare steel billets having a section of 155 mm x 155 mm. After heating to 1000 deg. C, (An average cooling rate from 600 deg. C to less than 400 deg. C), the diameter of the conveyor was 8.0 mm, the diameter of the shaft was 8.0 mm, (Test No. 1 to 31).

The pearlite area ratio, pearlite nodule size, total decarburized layer depth, Cr-based alloy carbonization amount and SV treatment property of each of the obtained coils were examined. Investigation of the SV processability was carried out using the total amount of two-tone coils. Except for the investigation of the SV treatmentability, one ring was cut out from the terminal of the two-tone coil for each working condition, and as shown in Fig. 1, a sample taken in eight parts in the circumferential direction (corresponding to eight parts in the longitudinal direction of the wire) , And a representative value of each coil was obtained by averaging each measured value.

As shown in Fig. 2 (a cross-sectional view schematically showing the structure observation position), the pearlite area ratio is set to 1 / 4D position (D: wire diameter (Total area of 5 o'clock) of 1 / 2D (a central area of D / 4 to D / The cross-section of the hot-rolled wire was buried and polished, and chemical etching was performed using pyric acid. Then, a tissue photograph of an area of 200 mu m x 200 mu m at a magnification of 400x was taken by an optical microscope, Quot; Pro Plus ", manufactured by Media Cybemetics), the pearlite area ratio was obtained, and the average value was calculated. The pearlite area ratios of the five fields of view at each of the eight sites were obtained and averaged, thereby calculating the perlite area ratio for each coil. On the other hand, when a decarburized layer is present in the surface layer, the entire decarburized portion specified in JIS G 0558-4 is excluded from the measurement site. P + B + M " or " B + M " when the structure having pearlite area ratio of 90% or more is defined as P and the pearlite is less than 90% and bainite or martensite is produced.

As shown in Fig. 2, the pearlite nodule size was measured at each of the eight surface portions of the hot-rolled wire rods in the order of 1 / 4D (D: wire rod diameter: 2) (A total area of 5 o'clock) in a region of the center (a central area of D / 4 to D / 4: 1 field of view) using an optical microscope. Here, the pearlite nodule refers to a region in which the ferrite grains in the pearlite structure exhibit the same orientation, and the measuring method thereof is as follows. First, the cross-sectional surface of the hot-rolled wire was embedded and polished, and the steel was corroded using a solution of concentrated nitric acid (62%): alcohol = 1: 100 (volume ratio) (pearlite nodule particles And the particle size of the pearlite nodule was measured. The particle size numbers of the pearlite nodules at the five fields of view at each of the eight sites were measured and averaged to calculate the average value Pave of the pearlite nodule size for each coil. The particle size number of the pearlite nodule was measured in accordance with "Measurement of the austenite crystal grain size" described in JIS G 0551.

The total decarburized layer depth was measured at eight locations on each surface layer using an optical microscope as shown in Fig. 3 (a cross-sectional view schematically showing decarburized observation positions) at eight sites of hot-rolled wire rods. The cross-sectional surface of the hot-rolled wire was buried and polished, and chemical etching was carried out using pyric acid. Observation was performed, and the maximum depth of eight locations was measured at each site. Of the total decarburized depth. The depth of the entire decarburized layer was determined in accordance with "Method for measuring the depth of decarburized layer of steel" described in JIS G 0558.

The amount of Cr-based alloy carbonized material was determined by an electrolytic extraction method. First, the scale of the rolled wire rod was removed with a sandpaper, and the sample was washed with acetone. The sample was immersed in an electrolytic solution (ethanol solution containing 10% by mass of acetylacetone) (the electrolytic amount was 0.4 to 0.5 g (The sample is taken out), the metal Fe of the mother phase is electrolyzed to collect alloy precipitates (including carbide and trace amounts of nitride and carbonitride) in the steel present in the electrolytic solution as a residue, By dividing the residue mass by the electrolytic amount, the amount of Cr-based alloy carbide (mass%) was obtained. The alloy precipitates to be measured are mainly Cr-based alloy carbides, but when the selected elements are added, they include Cr and composite alloy carbides such as V, Nb and Mo. On the other hand, a filter having a mesh diameter of 0.1 mu m (membrane filter made by Advantec Toyo Kaisha, Ltd.) was used as a filter for collecting the residue.

The SV processability is determined by performing SV treatment without applying a heat treatment to the coil, determining whether there is a break in the SV process, a load of a breaker for dividing the cutting debris set on the entrance side of the chipping die, .

[Evaluation Criteria of SV Processability]

(1) The presence or absence of disconnection: When the total amount of two-tone coils was subjected to the SV treatment, the coils in which the disconnection did not occur were evaluated as having excellent SV treatmentability: .

(2) Load of breaker: The displacement (0 to 10A) of the current value of the breaker was measured with a data logger at a sampling interval of 1 second, and the data excluding the terminal 10 kg of the TOP and BOT at the time of the SV processing were used. 60 points of measurement data Average of the moving average of the moving line is not more than 9A. SV: Good: SV: Average of 60 points Average moving line: (See later Figs. 4 and 5).

(3) Breaking of Chipper Dice: The entire amount of two-tone coils was subjected to SV treatment, the chipping dies were removed, and the breakage of the wire contact portions of the chipping dies was confirmed by a stereoscopic microscope. The coil having no breakage (chipping crack) in the wire contact portion of the chippers dice was evaluated as having good SV treatmentability: o, and the coil in which the breakage occurred in the wire contact portion was poor in SV treatmentability.

The results of these evaluations are shown in Tables 5 and 6 together with the rolled wire structure (pearlite area ratio, average value of pearlite nodule size Pave) and Cr-based alloy carbide amount.

Test No. Examples 1 to 15 (Table 5) show examples satisfying the requirements specified in the present invention, Test No. 1, (Steel materials B1, B2, C1, C2, E1, G1, G2, and L1) satisfying the chemical composition, but the production conditions necessary for obtaining the steel material of the present invention are not satisfied For example, 24 to 31 (Table 6) are those in which the chemical composition is out of the range specified by the present invention (steel types P to W).

From these results, it can be considered as follows. First, 1 to 15 satisfied the requirements specified in the present invention, and these steel wire rods obtained satisfactory results in all the items of the SV treatmentability (presence of disconnection, breaker load, and chip breakage).

On the contrary, 16, the pearlite nodule size of the rolled material was coarse because of the high placement temperature after rolling, and breakage occurred in the SV treatment. Test No. 17, since the placement temperature after rolling is low, the entire decarburized layer of the rolled wire rod surface layer is deepened, and the load of the breaker is increased.

Test No. 18, and 21, since the average cooling rate to 600 ° C was slow after the conveyor was mounted, the pearlite nodule size of the rolled material was rough, resulting in breakage in the SV treatment. Test No. 19, and 22, the average cooling rate from less than 600 ° C to 400 ° C was slow, so that the amount of Cr-based alloy carbide increased, so that the load on the breaker was increased and cracking occurred in the chippers.

Test No. 20 had an average cooling rate of up to 600 ° C after the conveyor was installed. Therefore, martensite or bainite was produced, resulting in breakage in the SV treatment. Test No. 23 had an average cooling rate of less than 600 DEG C to 400 DEG C, so that the structure of the pearlite single phase was not formed, and martensite or bainite was formed, resulting in breakage during the SV treatment.

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

Test No. 25, 26, and 31 are examples using a steel grade (steel grade Q, R, W in Table 2) in which the content of each component (Mn, Ni, B) is excessive and the hardenability is excessively increased, But bainite and martensite were generated, and breakage occurred in the SV treatment.

Test No. 27 to 30 are examples using a steel grade (steel grade S, T, U, V in Table 2) having an excessive content of each component (Cr, V, Mo and Nb), and the amount of Cr-based alloy carbide increases, As the load increased, there was a break in the chippers.

Fig. 2 (state of the art), and it can be seen that the current value is stable. On the other hand, Fig. 27 (comparative example), it can be seen that the load of the breaker is partially increased (the breaker load at the portion surrounded by the broken line is high and the current value is large).

Claims (7)

C: not less than 0.4% and not more than 1.2% (meaning "mass%", the same applies hereinafter for chemical composition), Si: 1.5 to 3.0%, Mn: 0.6 to 1.5%, Cr: 0.5% and Al: 0.008% or less, the balance being iron and inevitable impurities, the pearlite area ratio being 90% or more, and the average value Pave of the particle size numbers of the pearlite nodules satisfying the following expression (1) The steel wire rod for high strength steel wire excellent in machinability, wherein the total decarburized layer depth of the surface layer is not more than 0.20 mm and the amount of Cr-based alloy carbide is not more than 7.5% with respect to the total mass of the steel wire rod.
6.0? Pave? 12.0 ... (One) The method according to claim 1,
V: not more than 0.5% (not including 0%),
Nb: not more than 0.5% (not including 0%),
Mo: not more than 0.5% (not including 0%),
Ni: not more than 1.0% (not including 0%),
Cu: not more than 0.5% (not including 0%), and
And B: not more than 0.010% (excluding 0%). delete delete delete delete A high-strength spring obtained from the steel wire rod for high-strength springs according to claim 1 or 2.

Images