SE1250810A1 - Pulled heat treated steel wire for high strength spring use and preferred steel wire for high strength spring use - Google Patents

Abstract

lO _65_ ABSTRACT Drawn heat treated steel wire for high strengthspring use is provided containing, by mass%, C: 0.67% toless than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to l.2%, Cr: l.3to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%,having Si and Cr satisfying the following formula: 0.3%SSi-Cršl.2%,and having a balance of iron and unavoidable impurities,having as impurities, P: 0.025% or less and S: 0.025% orless, furthermore having a circle equivalent diameter ofundissolved spherical carbides of less than 0.2 um,further having, as a metal structure, at least residualaustenite in a volume rate of over 6% to l5%, having aprior austenite grain size number of #10 or more, and having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 um.

Description

1 DESCRIPTION Technical area

The present invention relates to drawn heat-treated steel wire, for high-strength spring application, which can be used as a material for high-strength springs, made by cold winding, and to the preferred steel wire.

The state of the art

Springs used for motor vehicle engines, clutches, etc. tend to offer more advanced performance and higher durability to cope with the trend towards lighter weight and higher performance of motor vehicles. For this reason, the material of the springs, that is to say, drawn heat-treated steel row for pasture strength spring use, also has a high material half-strength. Usually in the manufacture of such small, high-strength springs, the material of the drawn heat-treated steel wire for high-strength spring application is toughened to give higher material strength, in the drawn heat-treated steel wire for high-strength spring use, it is then called a spiral spring. In addition, stress-relieving glazing or other heat treatment and nitriding is performed to obtain a finished coil spring. For this reason, the features of heat-treated steel line, for high-strength fiber application, need not only high high-strength, but also sufficiently high working capacity so that it does not break during cold winding and to vaporize softening due to glazing, nitriding and other heat treatment performed. , to resist heat-related softening.

[0003] A spring needs to have good fatigue properties, therefore the drawn heat-treated steel row is used intended for high-strength spring use, t material, which is further nitrided or ball-bombed to increase the height of the surface layer of the spring. The durability of a spring includes fatigue properties and a resilience property. The fatigue properties are affected by the surface hardness. The resilience property (the property of the spring resulting in plastic deformation in the load direction during use) is greatly affected, not only by the surface hardness, but also the hardness of the base material in the spring. For this reason, the surface strength after nitriding and resistance to heat-related softening on the inside, where nitrogen is not introduced by nitriding, are important in the stable line, for high-strength spring application.

In addition, in the case of spring production by cold winding, tempering in oil, induction hardening treatment, etc., where rapid heating and rapid cooling are possible, can be used in the manufacture of the material of the drawn heat-treated steel row, for high-strength spring application. For this reason, the drawn heat-treated steel row for high-strength spring application can be reduced to previous austenite grain size, so that a spring with excellent fracture properties can be obtained. 10 However, if the drawn heat-treated stable line for high-strength spring use has a higher half-strength, during cold winding, breakage can occur and the spring shape may not be formed.

In order to deal with the problem, some of the inventors suggested that the carbides 15 should be controlled, which would make the previous austenite finer, in this way both the strength and cold winding capacity (PLT 1) of the drawn heat-treated steel row, for high-strength spring use. Furthermore, the inventors proposed control of residual austenite and residual carbides, invention of prior austenite, to achieve both strength and cold winding shape (PLT 2 to PLT 4), of the drawn heat treated steel row, for high strength spring application. In particular, the starting points are suppressed for fractures caused by the formation of coarse oxides and carbides, and the distribution of fine carbides of cementite which are required to ensure that the strength becomes uniform in order to vaporize the fatigue properties and machinability of the drawn heat treated steel fiber.

PLT 2 focuses on the fact that the areas of sparsely distributed spherical carbides, with a circle equivalent diameter of 2 μm or more, in the areas with sparse distribution of fine spherical carbides (in particular, cementite), affect the dynamic properties and define it the area.

PLT 3 and PLT 4 note the effect of precipitation of fine carbides due to the addition of the alloying substance V, and limit the nitrogen (N) content to suppress undissolved spherical carbides. That is to say, they use the effect of precipitation of carbides, nitrides, and carbon nitrides of V Mr to allow the assimilation of hardening of the steel row, at the annealing temperature or hardening of the surface layer during nitriding. In addition, they also have the effect of vapor coarsening of the austenite grain size, due to the formation of precipitate. The effect of adding V is remark value. Despite this, undissolved spherical carbides or nitrides are formed at night, so even if nitrogen (N) is suppressed, control of the precipitation needs to be done precisely.

Daft '. compares PLT 4 quantitatively undissolved spherical carbides and precipitated carbides and defines the amount to obtain as much precipitated V carbides as possible, which affect the final spring performance. In particular, the & PLT 4 aft vague fall of V carbides in the electrolytic solution, at constant potential, and compare the amount with the amount V passing through the filter (the amount of excreted V).

Reference list 15 Patent literature

PLT 1: Japanese Patent Publication (A) No. 2002-180198 PLT 2: Japanese Patent Publication (A) No. 2006-1 831 37 PLT 3: Japanese Patent Publication (A) No. 2006-342400 PLT 4: International Publication WO2007 / 114491 Summary of the Invention Technical Problem

In recent years, surface hardening by nitriding has become a conventional method for increasing the durability of high strength springs. Furthermore, increasing the nitriding depth and shortening the nitriding time, by increasing the treatment temperature, have been studied. For this reason, the drawn heat-treated steel row, for high-strength springs, needs further improved resistance to heat-related softening.

That is to say, when further improved cold winding ability is sought than even conventionally drawn heat treated steel, for 30 high strength spring application, excellent resistance to heat related softening 3 4 even after a residence time of 1 hour at 500 ° C, minimal internal softening, and greater surface softness.

The above conventional drawn heat-treated steel row, for high-strength spring use, ensures a certain degree of uniform dispersion of fine carbides for improved fatigue properties and workability. But for all the improved resistance to heat-related softening, further spreading is necessary. In particular, the addition of V, proposed in PLT 3 and PLT 4, has the effect of hardening the steel row at the annealing temperature, hardening the surface layer during nitriding, and refining the austenite. However, the control of the nitrogen content (N) is not simple. As a result, coarse carbides, nitrides, and carbon nitrides fall out, causing deterioration in fatigue durability.

PLT 3 adds Nb and Ti, with the effect of capturing Excess Nitrogen (N), as mai. Even with added Ors, controlling the appropriate amount of N is still not easy.

PLT 4 takes samples of remaining undissolved spherical carbides, obtained as a result, and compares with the dissolved carbides. Therefore, PLT4 does not proactively control the uniform distribution of fine carbides.

In view of the above, the present invention aims to keep the addition of V and other alloying substances to a minimum, that is to say, without all precisely controlling the N content, to develop drawn heat-treated stable line for high-strength spring application, which has excellent tensile strength and hardness and excellent machining ability and which has superior surface hardness and inner hardness even after nitriding.

Furthermore, as described in PLT 3 and PLT 4, in order to obtain excellent tensile strength and hardness and excellent machining ability, the size of the undissolved spherical carbides in the steel should be small. Effective size is preferably 0.1 μm or less. If the size is Over 1 pm, the contribution to the hall strength and machining capacity is lost and a deterioration of the deformation properties is obtained only. For this reason, the grain density of undissolved spherical carbides with a circle equivalent diameter of 0.2 μm or more becomes an important indicator.

Therefore, the present invention has for its object not to allow the occurrence of undissolved 4 spherical carbides with a circle equivalent diameter of 0.2 .mu.m or more, when improving the shape of the half-strength spring spring.

Solution to the problem

The inventors conducted intensive research to solve the above problems and as a result obtained the following discoveries: (a) They discovered aft by aft carefully controlling the content of C, Si, Mn, and Cr in the steel row, to vaporize the formation of spherical carbides and by all means assimilating residual austenite, even without the addition of alloying substances such as V, the drawn heat-treated stable spring high-strength spring use has improved half-strength and cold-winding ability compared with conventional stable row.

(B) It was also found that by adding both Cr and Si in appropriate amounts to the steel row, the formation of undissolved spherical carbides and the softening on annealing or nitriding after winding is vaporized, and in addition, higher hardness of the nitrated layer can be achieved.

That is to say, for all the increasing strength in the fatigue properties, the addition of Cr is effective, but Cr is a substance which still leaves undissolved spherical carbides, which have a negative effect on the cold winding capacity. For this reason, the amount of Cr added should be limited. The inventions also noted that Si vaporizes the growth of undissolved spherical carbides and the formation of cementite. The inventors discovered that if Si was added together with Okad addition of Cr, then the half-strength of the drawn heat-treated stable Oka. Quantitatively, it is sufficient to add a large amount of both Si and Cr, since the ratio between them controls the difference in the amount of added Si and the amount of added Cr, that is, (Si-Cr)%.

(C) Furthermore, it was again discovered by heating Wen to 1250 ° C or more, it is possible that all Ora Cr and other alloying substances in the steel material are uniformly dispersed and vaporize the formation of coarse undissolved spherical carbides and also Ora fine carbides uniformly dispersed.

Undissolved spherical carbides are present in the steel material immediately after casting and become the cause, not only of poor winding capacity, but also of breakage during rolling and drawing. For this reason, it is effective to increase the heating temperature at normal steps and constantly vaporize undissolved spherical carbides, to prevent a negative effect in the cast after casting, wire rod rolling, patenting, slackening, and drawing.

(D) Furthermore, it was discovered that the addition of V has a detrimental effect on the mechanical properties and fatigue strength of the stable row, for spring use.

It viii saga from immediately after casting until it aft it is processed into a spring, hot steel material repeatedly. Usually, the undissolved spherical carbides are mainly cementite (Fe3C). However, by repeated heating, the undissolved spherical carbides often include Cr, V, etc. It has been understood that not only is Cr, V and other alloying substances consumed in a waste manner, but it is also possible to reduce the mechanical properties after nitriding (surface hardness, internal hardness, etc.) Furthermore, as explained above, when adding V, control of nitrogen content (N) is not possible. As a result, coarse carbides, nitrides and carbon nitrides form precipitates and cause impaired fatigue strength.

From these facts, the inventors discovered that it is possible to vaporize coarsening of undissolved spherical carbides, by not adding V, or by adding an extremely small amount, and further, as explained above, controlling the amount Cr in 20 equilibrium with the amount Si.

Has means "undissolved spherical carbides" undissolved carbides with a ratio between the maximum size (longest size) and the minimum size (shortest size) (side ratio) of 2 or less. In fact, "carbides" and "spherical carbides" are also undissolved. To emphasize, these are also called "undissolved carbides" and "undissolved spherical carbides, even though they are synonymous. Despite being synonymous, they are also called" undissolved carbides "and" undissolved spherical carbides ", respectively.

The present invention was made on the basis of these discoveries. The content of the invention is as follows:

(1) Preferred stable line for high semi-solid state use, can be characterized by content in mass% of, C: 0.67% to 0.9%, 6 Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, Si and Cr having the following expression: 0.3% sSi-Crs1, 2%, with a balance of iron and unavoidable impurities, wherein P and S as impurities comprising P: 0.025% or less and S: 0.025 cl% o or less, and further, wherein a circle equivalent diameter for undissolved spherical carbides is cm less than 0.2 pm. (2) Preferred design for high-strength spring use, as set forth in (1), therefor can be drawn from content in mass%, of one or more of, V: 0.03 to 0.10%, Nb: 0.015% or less, Mo: 0 .05 to 0.30%, W: 0.05 to 0.30%, Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when content of V is satisfied 1.4 % sCr + V52.6% and 0.70% sMn + Vs1.3%, and, when containing Mo and W, 0.05% sMo + Ws0.5% is satisfied. (3) Drawn heat-treated stable row for high-strength spring use, can be drawn content in mass% of, C: 0.67% to 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2% , Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, Si and Cr satisfying the following expression: 0.3 ° A) Si-Cr1, 2%, and 5 with balance of jam and unavoidable impurities, with P and S as impurities comprising P: 0.025% or less and S: 0.025 ° A) or less, and further, with a metal structure comprising at least residual austenite of a volume mat of Over 6% to 15%, with a previous austenitic grain size number # 10 or more, and further wherein a circle equivalent diameter for undissolved spherical carbides is less than 0.2 μm. (4) Drawn heat-treated stable row for high-strength spring application, according to (3), can be characterized by content in mass ° / 0, of one or more of, 0.03 to 0.10%, Nb: 0.015% or less Mo: 0.05 to 0.30%, 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when content of V is satisfied 1.4 ° / 0Cri-V52, 613/0 and 0.70% 5Mn + V51.3%, and, when containing Mo and W, 0.05% Oilo + W5_0.5% is met. (5) Drawn heat-treated steel line for high-strength spring use, according to (3) or (4), characterized in that the drawn heat-treated steel line, for high-strength spring use, has a tensile strength of 2100 to 2400 MPa. 8 9 (6) Drawn heat-treated steel line for high-strength spring use, according to any of (3) to (5), may be characterized by the fact that the drawn heat-treated steel line, called high-strength spring use, has a yield strength of 1600 to 1980 MPa. (7) Drawn heat-treated steel line for high-strength spring application, according to any one of (3) to (6), characterized in that the drawn heat-treated steel line, for high-strength spring use, has surface hardness HV750 or more according to the Vickers scale, and internal hardness according to VV nitration at 500 ° C for 1 hour.

Production Procedure for Prefabricated Stable High Strength Spring Application Can be characterized by the fact that a good containing in mass%, C: 0.67% to 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, with Si and Cr satisfying the following terms: 0.3% 5Si-Cr51.2 (Yo, with balance of jams and unavoidable impurities, with P and S as impurities comprising P: 0.025% or less and S: 0.025% or less, the ingot being heated to 1250 ° C or more, then the ingot is hot rolled to produce a roll and heat the roll. to 1200 ° C or more, with subsequent hot rolling to produce a preferred steel line.

The production process for the preferred stage of high-strength spring application, according to (8), may be characterized by the carcass also containing, in mass%, one or more of 0.03 to 0.10%, Nb: 0.015% or less Mo: 0.05 to 0.30%, 0.05 to 0.30% 9 Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when containing V 5, 1.4% 5Cr + Vs2.6% and 0 are met. 70% sMn + V51.3%, and, when containing Mo and W, 0.05% 5Mo + W50.5% is satisfied.

Production Procedure for Prefabricated Stable Row 10 High-strength spring application, further characterized by heating the preferred stall row according to (8) or (9), to 900 ° C or more, after which it is patented at 600 ° C or less.

Production process Preheated treated wire for 15 high strength spring application, characterized by drawing the preferred steel wire which is manufactured by the production method for the preferred steel wire, according to (8) to (10), heating the wire at a rate of 10 ° C / A or at a temperature of 10 ° C / sec. A3 or more for 1 minute to 5 minutes, then cooling at a rate of 50 ° C / sec or more down to 100 ° C or less.

Production process for the production of heat-treated stable line for high-strength spring use, according to (11), characterized by further maintenance and annealing at 400 to 500 ° C for 15 minutes or less.

Beneficial effects of the invention

According to the present invention, in particular, the drawn heat-treated steel row, for high-strength spring application, with a high surface layer hardness and an inner hardness, and also high-strength spring with excellent durability, can be obtained, due to the excellent cold winding resistance and resistance to resistance. 500 ° C for 1 hour. The contribution to industry is extremely large.

Brief description of the drawings 11

FIG. 1 is a photomicrograph of the metal structure showing an example of spherical carbides, in the drawn heat treated steel row, for high strength spring application, in the present invention. Undissolved spherical carbides are seen at the tips of the arrows in the figure.

FIG. 2 shows a view of the shape of a punch for creating a groove in a test piece.

FIG. 3 shows a view of the steps for all create eft savings in one test piece.

FIG. 4 shows a view of an overview of a buoy test of the spar.

FIG. 5 is a view showing a method of feeding the beet angle of the spare.

Description of embodiments

In general, a coiled spring is manufactured as follows: Of course, the manufacture of springs is not limited to the method described. Della describes only one example.

Goods made of steel containing predetermined chemical composition, rolled to obtain a rolling stock. Thereafter, the roll is rolled for all manufactures formed of predetermined diameter. The stalt line produced in this step is called the "preferred stalt line". The steel row which is manufactured by rolling is patented and drawn to obtain an even finer steel row, after which the working stress in the surface layer is removed and the subsequent cold winding capacity is obtained by heat treatment (toughening), which is produced after rolling, patented and drawn for all the other working coils. at the surface layer and cold winding processing ability is obtained by heat treatment (toughening). The stalt line manufactured at this stage is called the "drawn heat treated stalt line".

Thereafter, the spring is machined by cold winding for improved strength and nitriding for improved surface hardness. In this way, a "spring" is manufactured as a final product.

First, the chemical composition of the drawn heat-treated steel row, for high-strength spring use, in the present invention and its material, that is, the preferred steel row, for high-strength spring use, will all be explained. Has means "%" in the chemical composition mass%, unless otherwise stated. 11 12

C: 0.67% to less than 0.9% C is an important substance which has a strong effect on the strength of the steel material and C also contributes to the formation of residual austenite. In the present invention, the lower limit of the amount C has been set to 0.67% or more to obtain sufficient strength. To increase the strength, the amount C is 0.70% or more, preferably 0.75% or more.

On the other hand, if the amount C becomes 0.9% or more, it results in Excessive coincidence, that a large amount of coarse cementite falls off and that the toughness decreases remarkably black. Furthermore, the amount C is Excessive, coarse spherical carbides are formed and the curvature is reduced. Therefore, the upper limit for the amount C is set to less than 0.9%. In order to vaporize the formation of spherical carbides, the upper limit of the amount C is preferably 0.85 ° A), more preferably 0.80%.

Si: 2, In addition to 3.5% Si is an important substance for improving the resistance to heat-related softening of the steel and the resilience properties of the spring. To obtain these effects, 2.0% or more Si needs to be added. Furthermore, Si is effective for spheroidization and refining of the cementite. To vaporize the formation of coarse spherical carbides, 2.1% or more of Si is preferably added. To increase the inner hardness, after nitriding and other treatments, to increase the surface layer harder, 2.2% or more of Si is preferably added. Furthermore, from the balance sheet with Cr, Si is more advantageously set at 2.3% or more. Si is sometimes set to 3.0% or more.

On the other hand, with excessive addition of Si, the steel line hardens and becomes scattered, thus making the upper limit of Si 3.5% or less. In order to prevent proliferation, the Owe border is preferably reduced to 3.4 ° A, more preferably 3.3% or less.

Mn: 0.5 to 1.2% Mn is a substance that is important for increasing the slackening capacity and for stably securing the amount of residual austenite. In the present invention, Mn has been added in 0.5% or more, more preferably 0.65% or more, even more preferably 0.70% or more, to increase the yield strength of the steel and to secure residual austenite. 12 13 On the other hand, in case of excessive addition of Mn Okar the amount of residual austenite. During machining, machining-induced martensite is formed and the cold winding forearm is assembled. To prevent proliferation due to Excessive addition of Mn, the Upper Limit for Mn is set at 1.2% or less, preferably 1.1% or less, more preferably 1.0% or less.

Cr: 1.3 to 2.5% Cr is a substance which is effective in improving the slackening shape and the resistance to heat-related softening. To obtain these effects, 1.3% or more Cr needs to be added. Upon nitration, it is possible to thicken the hardened layer obtained by nitration by adding Cr. Therefore, preferably more than 1.5% of Cr is added, in order to achieve hardening by nitration and softening resistance at the nitration temperature. More advantageously 1.7% or more of Cr.

On the other hand, if the quantity Cr is Exaggerated, the manufacturing cost will be higher. Not only that without the dissolution of carbides is dissolved, undissolved spherical carbides become more numerous, and the winding capacity is increased, so the Upper boundary kir Cr is set at 2.5% or less. Furthermore, if the amount of Cr is large, the amount of Cr is preferably suppressed to 2% or less, in order to vaporize the formation of coarse cementites. Furthermore, in order to obtain both hall strength and formability, the upper limit for the amount of Cr is 1.8% or less.

N: 0.003 to 0.007% N is the substance, which forms nitrides with Al, etc., included as impurities in the steel, in the present invention. To use fine nitrides and fine residual austenite, 0.003% or more of N must be included. On the other hand, if the amount N is 25 Excessive, the nitrides are overgrown and the cold winding shape and fatigue properties decrease. If the upper limit for the amount N is 0.007% or less. Furthermore, with regard to the capacity for heat treatment etc., the amount N is preferably 0.005% or less.

P: 0.025% or less P is a contaminant. Causes the steel to harden, form segregations, and cause proliferation, so the Upper limit of P is set at 0.025% or less. Furthermore, P, which has segregated at previous austenitic grain boundaries, causes the toughness and resistance to delayed fractures, etc. to decrease, so the upper limit for the amount of P is preferably set at 0.015% or less. In addition, the amount of P is preferably limited to less than 0.010% when the yield strength of the steel row exceeds 2150 MPa.

S: 0.025% or less S is also an impurity. If S is present in steel, it causes the steel to crack, so the Upper limit of the amount S is set to 0.025% or less. To dampen the effect of S, the addition of Mn is effective. Nevertheless, MnS is an inclusion. Especially in the case of high-strength strength, MnS sometimes becomes the beginning of crime. Therefore, in order to reduce the incidence of fractures, the upper limit for the amount of fractures has been set at 0.015% or less. Furthermore, the amount S is preferably limited to less than 0.01%, when the yield strength of the drawn heat-treated steel row, for high-strength spring application, will exceed 2150 MPa.

Al: 0.0005 to 0.003% Al is a deoxidizing agent. It affects the formation of oxides. If hard oxides are formed, the fatigue durability decreases. In particular, if Al is added in Nefflode, the fatigue strength becomes variable and the stability is impaired, in high-strength springs. If the amount of Al exceeds 0.003%, the incidence of fractures to WO of 20 inclusions becomes large, so that the amount of Al is limited to 0.003% or less in the drawn heat-treated steel row for high-strength spring use, in the present invention. The Ore spansvardet of the amount Al is preferably 0.0028%, more the preference 0.0025%.

[0034] On the other hand, if the amount of Al becomes less than 0.0005%, silicon-based hard oxides are formed. For this reason, the amount of Al is set at 0.0005% or more. The lower limit for the quantity Al is preferably 0.0007%, more preferred 0.0008%, even more preferred 0.001% or more.

Most recently, the standpoint of the present invention, that is, the relationship between Si and Cr, will be explained. It is already clear that both Si and Cr are important for increased strength in spring numbers.

Despite this, Excessive additive causes problems. 14

0.3% 5Si-Cr1, 2% If the amount of Si exceeds the prescribed amount, the dispersion becomes external and the processing capacity during winding is reduced. Not only that without 5 decarburization in the Transition process will be startling. For this reason, the surface layer hardness becomes lower and the durability decreases, in the final spring product. Furthermore, charred parts are formed randomly, which causes the duration of the half-strength, of the manufactured spring, to be reduced. When the quantity Si is less than the prescribed quantity, the strength decreases. Furthermore, the forgiveness property is insufficient. This is also evident in the hardness after nitriding. Sufficient hardness can not be ensured either at the surface layer or inside.

Nevertheless, the ratio of Si to Cr, in the cementite, in the steel is important. The viii saga, Si is a substance which destabilizes cementite. If a large amount of Cr is added or other substances which stabilize cementite on heating, the effect of the formation of a solid solution of cementite is promoted. Therefore, the amount of undissolved spherical carbides becomes larger and the processing power decreases remarkably, if the amount Si is small, regardless of whether a large amount of Cr is added. The inventors discovered that it is possible to use the difference between the Si content (mass%) and the Cr content (mass%) in the steel, that is to say, the Si-Cr amount, as a carpet. The viii saga, when the value of Si-Cr is less than 0.3%, the amount of Cr becomes relatively large and undissolved spherical carbides remain lath However, if Si-Cr is Over 1.2%, Si becomes relatively redundant, which causes proliferation, decarburization or other problems. Therefore, the value of Si-Cr should be set to 0.3 to 1.2%. [0038] In order to vaporize the formation of carbides, a large amount of Si-Cr of undissolved carbides is suppressed, but industrially, if Si is present in an excessive amount, the thickness of the hardened layer by nitration becomes thin. For this reason, Si-Cr is preferably 50.9%, more preferably Si-Cr is 50.75%, in view of the behavior of undissolved spherical carbides and the hardened layer formed by nitration. Furthermore, with the intention of reducing the amount of Cr and reducing the residual occurrence of undissolved spherical carbides, respectively, the lower limit is advantageously 0.35% 5Si-Cr, more advantageously 0.4% Si-Cr. 16

Most recently, the selectively added chemical composition will be explained.

V: 0.03 to 0.10% V is a substance which forms nitrides, carbides, and carbon nitrides. Fine V nitrides, carbides, and carbon nitrides with a circle equivalent diameter of less than 0.2 microns are effective for the invention of residual austenite. Furthermore, these can also be used for hardening the surface layer by nitration. On the other hand, it is necessary to precisely control the precipitation, as undissolved carbides and nitrides are formed, even if nitrogen (N) is suppressed.

For that reason, according to the present invention, V is not intentionally added.

To obtain such an effect, a fine amount of V can be added. For all the effect to be obtained, V should be added in 0.03% or more, preferably 0.035% or more, more advantageously 0.04% or more.

On the other hand, if V is added to more than 0.10%, coarse spherical carbides are formed and the cold winding and spring fatigue properties are compromised. Therefore, the V content should be set to 0.1% or less. Furthermore, by adding V, pre-drawing, all of the subcooled structure is formed which causes cracks and fractures during drawing. For this reason, the upper limit for the amount V is preferably set at 0.09% or less, more preferably 0.08% or less, most preferably 0.05% or less. In particular, in the case of the addition of a small amount of Nb, the amount of addition of V is preferably set to 0.05% or less. Furthermore, V is a substance which strongly affects the formation of residual austenite in the same way as Mn, so the amount of V must be carefully controlled together with the amount of Mn.

Nb: 0.015% or less Nb is a substance which forms nitrides, carbides and carbon nitrides in steel. These precipitates are sometimes used to control the austenite grain size, etc. But at the same time, excessive addition gives reduced ductility and results in crack cracking forming more easily during rolling and thermoforming. For this reason, Excessive addition of Nb 30 should be avoided.

Nb is allowed for the purpose of controlling the quantity N. The precipitates are not used directly to control the quality. Valve springs and other springs are manufactured by slackening, tempering, then cold winding, but at that time, dissolved nitrogen prevents the color deformation and reduces the stress limit. For this reason, the cold winding stock is mixed. Therefore, the effect is through the addition of Nb and the formation of nitrides at high temperature, that dissolved nitrogen in the steel matrix is reduced and that the cold winding stomach is improved.

Furthermore, the addition of a small amount of Nb is also effective in vaporizing V and other undissolved spherical carbides involved as unavoidable impurities. Is a substance which is effective in improving the resistance to heat-related softening during nitriding and the hardness of the outermost layer. However, if the amount of added V becomes larger, V-nitrides, V-carbides, and V-carbon nitrides are often not sufficiently loose, even in patenting, slackening, and other heatings, such as Ors to obtain an austeniphase, for the manufacture of heat-treated drawn steel for high-strength spring use. Undissolved spherical carbides of V wax frail the nuclei of the V-based nitrides, formed at the time of normal high temperature. As a result, undissolved spherical carbides remain and the winding belly accumulates. For this reason, it is necessary to vaporize the amount of added V when undissolved spherical carbides are suppressed. In the present invention, V was not a necessary substance.

In contrast, Nb nitrides form at a higher temperature than V. For this reason, Nb suppresses the formation of V nitrides in the steelmaking process. That is to say, Nb forms nitrides in the high temperature region, where V dissolves and does not form nitrides. Furthermore, at high temperatures where V nitrides are formed, Nb consumes nitrogen, so all the formation of V nitrides is justified, even when cooling. For this reason, the addition of a small amount of Nb is particularly effective in vaporizing undissolved spherical carbides and securing the winding capacity when adding a large amount of V.

If the amount of Nb added is greater than 0.015%, the hot ductility is impaired and defects and other problems during rolling appear more easily. For this reason, the amount of Nb added is set to 0.015% or less, preferably 0.0% or less, more preferably 0.005% or less, most preferably less than 0.001%.

On the other hand, the effect of Nb in controlling the amount of N in the spring number from 0.0005% is seen, so when Nb is added, 0.0005% or more is preferable. Furthermore, when V etc. 17 18 is added, the addition of a small amount of Nb is more efficient. A range of 0.003 to 0.012% is preferable. Furthermore, a range of 0.005 to 0.009% is more advantageous. The effect is also achieved at 0.005 to 0.001%.

1.4% 5Cr + V 2. 2.6% In the present invention, V is not intentionally added. However, as explained above, the addition of a small amount of V has an effect on the refinement of the previous austenite and the formation of residual austenite. By carefully controlling the sum of the amount of Cr and V added with respect to V, it is possible to increase the hall strength to increase the surface layer hardness after nitriding and the inner hardness suitable for high strength springs.

Cr and V are both substances which prevent softening when heated by annealing or nitriding etc. performed after spring winding, the viii saga partly so-called heat-related resistance to softening. In particular, nitriding causes nitrides to precipitate at the nitrided part of the surface layer, thereby improving the surface hardness and increasing the nitriding effect. Furthermore, even at the inside, where nitriding does not spread, carbide probes are steamed. On the other hand, Cr and V are both substances which facilitate the formation of unread spherical carbides. Cr is dissolved in the cementite to increase the resistance, so that the solution of the cementite is vaporized in the heating step kir dissolution of the cementite (heating at the time of patenting and heating at the time of slackening), often remaining as undissolved spherical carbides. Furthermore, V also has a dissolution temperature for the precipitates, which is higher than the A3 point for steel, so that V remains as undissolved spherical carbides.

If the total content of Cr and V, that is, Cr + V, is less than 1.4 6) / 0, the surface hardness of the high-strength spring decreases below HV750 and the inner-half-strength decreases below HV570. For that reason, Cr + V is preferably 1.4% or more. Furthermore, 1.5% or more are all preferred. On the other hand, Excessive additive Cr + V, of more than 2.6%, leaves large amounts of undissolved spherical carbides, which leads to the winding capacity being condensed. Therefore, 2.6% is set to the Upper border. Furthermore, Cr + V is preferably 2% or less, more preferably 1.8% or less.

0.7% 5Mn + V% 18 19 Mn and V are the substances which improve the quenching capacity and which in addition have a great effect on the formation of residual austenite. If the amount Mn is greater than the prescribed amount, the amount of residual austenite increases. Therefore, the sum of both Mn and V, which occur as unavoidable pollutants, has a direct effect on the behavior of austenite. It is not only the working capacity that is affected, but also the stretch limit is strongly affected. Sufficient durability cannot be ensured.

For this reason, the total content of Mn and V, the so-called Mn + V, is set at 0.7 to 1.3% in the present invention. In order to secure a volume mat of more than 6% of residual austenite, the lower limit for Mn + V must be set to 0.7% or more.

As a result, transformation-induced plasticity causes the toughness to improve and the possibilities of cold winding to sag are sacrificed. On the other hand, the Upper limit of Mn + V must be set to 1.3% or less, for aft Ora residual austenite volume mat 15% or less. Due to this, the formation of processing-induced martensite, as an RV of the impact field during cold winding, suppressed and local propagation can be prevented.

Mo: 0.05 to 0.30% Mo is a substance which improves the slackening ability. Furthermore, Mo is also extremely effective in improving resistance to heat-related softening. In particular in the present invention, 0.05% or more of Mo may be added, to further improve the resistance to warning-related softening. Furthermore, Mo is a substance which forms Mo-based carbides in the steel. The temperature at which the Mobasated carbides precipitate is lower than the temperature at which V-carbides etc. precipitate. For this reason, the addition of an appropriate amount of Mo is also effective in vapor deposition of carbides. Addition of 0.10% or more of Mo ar aft fOredra. On the other hand, if the amount added Mo is more than 0.30%, a supercooled structure is formed, during hot rolling, and patenting drawing, etc. Therefore, to vaporize the formation of a subcooled structure, causing cracking or wire breakage during drawing, the upper limit is of the amount Mo was set at 0.30% or less, preferably 0.25% or less. Furthermore, if the amount of Mo is large, in the case of patenting, the time until the end of the perlite transition becomes longer, so the amount of Mo is preferably set to 0.20% or less. 19 In addition, to shorten the patenting period and stably stop the perlite transition is 0.15% or less.

W: 0.05 to 0.30% W, similar to Mo, is a substance which is effective in improving the sagging capacity and the resistance to heat-related softening and is a substance which precipitates in the steel as carbides. In particular, in the present invention, 0.05% or more W is added to improve the resistance to heat-related softening.

On the other hand, if W is added (WeltOd forms a supercooled structure which causes cracking or wire breakage during drawing, then the amount W must be set to 0.30 ° A) or less.

In addition, in view of the simplicity of heat treatment, etc., the amount W is preferably 0.1 to 0.2%, more preferably 0.13 to 0.18%.

0.05% sMo + M 0.5% Mo and W are the substances which are effective in improving the resistance to heat-related softening. If! Dada tilsants in combination with each other vaporizes the growth of carbides and the resistance to heat-related softening can be remarkably black compared with the addition of only Mo or W. In particular to improve the resistance to heat-related softening when heated to 500 ° C must Mo + W be set to 0.05% or more, preferably 0.15% or more.

On the other hand, if Mo + W is more than 0.5%, in hot rolling and patenting pre-drawing, etc., a so-called subcooled structure of martensite, bainite, etc. is formed to evaporate the formation of a subcooled structure which causes cracks and steepness during drawing, the Upper limit of MO + W is set to 0.5% or less, preferably 0.35 ° A or less.

Most recently, Mg, Ca, and Zr will all be explained.

Mg: 0.002% or less Mg forms oxides in narrow steel at a higher temperature than the temperature for the formation of MnS. At the time of the formation of MnS, it is already present in 21 the narrow steel. Therefore, Mg can be used as the nuclei for precipitation of MnS. This has resulted in the distribution of MnS being controlled. Furthermore, Mg-based oxides are nevertheless finely distributed in the number of distributions in the narrow state, in the sense that MnS formed around nuclei of the Mg-based oxides are finely distributed in the steel, compared with Si- and Al-based oxides which are often appears in conventional steel. Therefore, the MnS distribution varies depending on the presence of M, even if the S content is the same. Addition of these makes the MnS cross size finer. Due to the fact that Ora MnS is nicely divided, it is possible to consider MnS as harmed by what applies to MnS as a starting point for fatigue. Sufficient effect is achieved with small amounts. Preferably Mg 0.0002% or more, more preferably 0.0005% or more, should be added.

With the addition of more than 0.001%, however, it is black for Mg to remain in the molten steel, it is an effect of the oxide composition and the amount of emerging oxides that form the starting point for fatigue becomes larger, so 0.002% Mg is the Upper the border. Therefore, the upper limit of the amount of Mg added was set to 0.002%, preferably 0.0015% or less. Furthermore, when it comes to spring numbers compared to other steels for construction use, the amount of added S is suppressed, considering the yield, etc., is 0.001% or less. Furthermore, when used for high-strength valve springs, the containment probability is high, so Mg has the effect of improving the corrosion resistance and the resistance to postpone fracture-preventing rolling cracks due to the effect of the distribution of MnS, etc.. the range 0.0002 to 0.001 is still preferred.

Ca: 0.002% or less Ca is an oxide and sulfide-forming substance. In spring figures Or Ca, so often MnS becomes spherical and thereby the extent of the MnS is steamed, which serves as the initiating place for fatigue and other fractures, which makes MnS harmless. The effect is similar to the effect of Mg. Addition of 0.0002% or more is preferred. Furthermore, not only the yield is poor, even if more than 0.002% is added, but oxides and CaS and other sulfides are also formed and problems in manufacturing and preserving the spring fatigue durability properties fall, so the amount was set to 0.002% or less. In view of the amount added, if the inclusion probability is high for use with a high-strength valve, the advantageous amount is preferably 0.0015% or less, more preferably 0.001%.

Zr: 0.003% or less Zr is an oxide, sulfide, and nitride-forming substance. In spring numbers, the oxides are finely distributed, and in the same way as with Mg, they form nuclei for precipitation of MnS, and in this way Ora MnS can be finely distributed. As a result, it is possible to all improve the fatigue durability and further increase the ductility and thereby improve the winding capacity. Preferably 0.0002% or more is added. Furthermore, even if more than 0.003% is added, not only the yield is added, but oxides and ZrN, ZrS, and other nitrides and sulfides are formed and problems in the production or degradation of the spring durability properties are burned, so the amount is set to 0.003% or less. The amount added is approximately 0.0025% or less. In addition, Zr has the effect of improving the winding capacity by controlling the sulphides, so the addition is advantageous to maintain high-strength valve springs, but to minimize the effect of inclusions, suppression to 0.0015 ° A or less is preferred.

Notes that the above voluntarily added chemical compositions, if they contain small amounts, do not detract from the effect of the stable radicals comprising the basic chemical composition of the present invention.

The resin structure metal structure of the steel wire, for high strength spring use in the present invention, is explained.

[0064] Undissolved spherical carbides Undissolved spherical carbides play an important role when it comes to all the safe half-strength of the stable line for high-strength spring use. On the other hand, the presence of undissolved spherical carbides causes the winding force to contract. Furthermore, coarse carbides also cause the fatigue properties to deteriorate. Therefore, it is necessary to vaporize undissolved spherical carbides on winding and, after final nitriding, to create a uniform distribution of fine carbides in order to solve the problem of the present invention. 22 23

The stall row for haghall strength spring use in the present invention has a long size of unplastic spherical carbides of 0.2 μm or less which are suppressed during color roughening. The unresolved spherical carbides are already present after the rolling of the wire rod (the viii saga fardragen staltrad).

Uncharged spherical carbides are responsible for being read in the latter heat treatment (patenting, formation of processing heat during drawing, and toughening, for example). sometimes they rather wax during these heat treatment steps and are dyed. The viii saga sometimes acts as unloaded spherical carbides in the lined sternrad karnor father color excavation by themselves.

For this reason, it is important to reduce as much as possible undissolved spherical carbides which are present in the drawn steel row in order to limit the color roughening of unsplastic spherical carbides in the steel row after heat treatment (heat-treated steel row). In view of the above, the definition of "unloaded spherical carbides" has an important meaning not only for coated steel springboard spring use according to the present invention, but also for heat-treated steel springboard steel.

The strength in the stable line for high-strength spring use in the present invention increases by the addition of C, the addition of Mn and Cr, further the addition of Mo, W, 20 and other so-called alloying elements. When large amounts of C and in particular Cr and other alloying substances, which form nitrides, carbides, and carbon nitrides, are added, spherical cementite carbides and alloy-based carbides remain in the steel. Spherical cementite carbide and alloy-based carbides are unplastic spherical carbides that do not dissolve in the steel during heating during hot rolling.

It is noted that in the present invention, both spherical alloy-based carbides and spherical cementite carbides will be referred to as spherical carbides. The steel contains nal-shaped carbides corresponding to the nal-shaped structure of tempered martensite, but these nal-shaped carbides are not included in the spherical carbides of the present invention. Squeegee-shaped carbides are not present immediately after slackening but fall out during the rolling process. The tempered martensite structure is a structure suitable for achieving both strength and toughness and machining ability. Nalform is in a sense the ideal form of carbides. 23 24

Strictly speaking, the machining capacity can also be compromised if carbides with a proportion of 2 or more (nal-shaped carbides) are coarse. But in fact, needle-shaped carbides become coarse when the annealing temperature is high or when the annealing time at annealing is extremely long. The effect of the performance is that the strength and hardness become insufficient. Problems arise in other areas with undissolved spherical carbides. Coarse-grained nal-shaped carbides are not formed in the 2100 MPa or so strong steel row included in the present invention. Therefore, nal-shaped carbides are not included in the present invention. As explained above, the precipitated carbides are normally undissolved, but in the present invention the term "undissolved" has been added. This only emphasizes their unresolved character. In the present invention, "undissolved spherical carbides" and "spherical carbides" are synonymous.

Undissolved spherical carbides can be observed with a tracer electron microscope (SEM) by polishing a sample obtained from a preferred steel line or a heat-treated steel line, for high-strength spring application, to mirror gloss and etching the sample by picral or electrolytic etching. Furthermore, spherical carbides can be observed by model procedure by transmission electron microscopy (TEM).

Figure 1 shows an example of a structural SEM image of a sample after electrolytic sketching. In the structural picture in Figure 1, it is observed that the matrix of the steel has two types of structures, that is to say, squeegee-shaped structures and spherical structures. Among these, the needle-shaped structures are called martensite formed by toughening. On the other hand, the spherical structures are carbides 1 which have taken their spherical shape in that they have not been dissolved in the steel by also being made spherical by toughening by annealing in oil or induction hardening treatment (undissolved spherical carbides) due to heating by hot rolling. Spherical carbides can be observed at the front of the arrow in Figure 1.

Undissolved spherical carbides with a circle equivalent diameter of less than 0.2 μm 24 In the present invention, undissolved spherical carbides affect the properties of the drawn heat-treated steel row for high-strength spring use and are therefore controlled in size according to the following: Note that the prior art is as follows. Spherical carbides further defined to achieve higher performance and machining ability, in the present invention. Spherical carbides with a circle equivalent diameter of less than 0.2 μm are extremely effective in maintaining the strength and resistance to heat-related softening of the steel.

In contrast, spherical carbides with a circle equivalent diameter of 0.2 μm or more do not contribute to improving the strength and resistance to heat-related softening and the degree of cold winding capacity. For this reason, the present invention is characterized by not allowing the formation of spherical carbides with a circle equivalent diameter of 0.2 μm or more.

Preferred stable row and drawn heat treated stable row, in the present invention, are characterized by undissolved spherical carbides having a circle equivalent diameter of less than 0.2 microns. As a result, it is possible to secure strength and at the same time secure processing ability.

As explained above, the compositions must be patented, drawn and heated, toughened, or otherwise heat treated so that undissolved spherical carbides can wax and coarse. For this reason, the circle equivalent diameter of undissolved spherical carbides in the preferred steel row is preferably made less than 0.2 microns. From the results of the inventors' experiments, the circle equivalent diameter of undissolved spherical carbides, in the preferred embodiment, has been confirmed to be able to be reduced to 0.18 μm or less. Furthermore, it is also confirmed that if the rolling stock manufacturing temperature is set to 1250 ° C or more, the diameter can be made 0.15 μm or less.

The method for feeding the circle equivalent diameter and the density of the present spherical carbides will be explained. After samples taken from the stable row, for high-strength spring application, are polished and electrolytically etched.

Note that the observed area is randomly selected near the center of the radius of the heat treated roll line (steel line), that is, the so-called "1 / 2R part", to eliminate special conditions such as decarburization and segregation in the middle. Furthermore, the feed area is 300 pm2 or more. In electrolytic sketching, the surface of the 26 sample is corroded by electrolysis in an electrolytic solution (a mixture of acetylacetone 10% by mass, tetamethylammonium chloride 1% by mass, and an equilibrium of methyl alcohol) where the sample is used as anode and platinum as cathode and has a current generator used. The potential becomes constant at a potential that is suitable for the test in the range -50 to -200 mV vs SCE. It is preferred that the potential be constant at -100 mV vs SCE, for the steel line of the present invention.

The amount of current used can be determined by the total surface area of the sample x 0.133 [c / cm 2]. Note that not only the polished surface but also the surface of the sample embedded in resin is added to the total surface area of the sample.

The power is turned on and the sample is maintained for 10 seconds then the power is turned off and the sample is cleaned.

After that, the sample is observed with SEM and a structural image of the spherical carbides is tagged. In the SEM, the structure of the spherical carbides appears relatively white, and has a proportion (partial proportion) of largest size (rank size) and a minimum size (short size) of 2 or smaller, spherical carbides. The magnification of the image taken with SEM is X1000 or more, with X5000 to X20000 to be preferred. As feeding positions, the solids were selected at a thickness of about 0.5 to 1 mm from the surface of the rolling bar, the segregated middle portions being avoided. The SEM structure image was processed with image processing to feed the smallest size (short size) and the largest size (long size) of the spherical carbides observed in the feed area and the circle equivalent diameter is calculated. The circle equivalent diameter is the diameter near the area, calculated by image processing, of an undissolved carbide in an area further converted to a circle of the same area. Furthermore, it is also possible to feed the density of present spherical carbides with a circle equivalent diameter of 0.2 μm or more observed in the feed area.

The metal structure of the drawn heat treated spring use and drawn heat treated steel line The metal structure of the drawn heat treated steel line for high strength spring use, according to the present invention, comprises in volume mat over 6% to 15% of residual austenite and a balance of residual anlite. Fine 26 27 inclusions are allowed. The "fine inclusions" are oxides and sulfides. The oxides are deoxidation products of Al and Si, etc., the sulphides corresponding to MnS, CaS, etc. Furthermore, the balance of the molten martensite structure also contains undissolved spherical carbides in small amounts.

The previous austenite grain size in the structure is # 10 or more, with the circle equivalent diameter of the spherical carbides being less than 0.2 μm. Furthermore, the perlite structure constitutes 90% or more, preferably 95% or more, more preferably 98% or more, of the metal structure of the coated steel wire, for high strength spring application, according to the present invention. A significant 100% perlite structure is ideal.

Previous austenite grain size number: # 10 or more The structure has been heat treated for high spring strength application in the present invention is mainly comprised of tempered martensite. Previous austenite grain size has a large effect on the properties. That is, the fatigue properties and formability are improved to WO by the invention of the previous austenite grain size as an effect of grain size reduction. In the present invention, the previous austenite grain size is made # 10, in order to achieve sufficient fatigue properties and formability.

The invention of prior austenite is particularly effective for all the improved properties of the heat treated steel for high strength spring application. The previous austenite grain size number is preferably made # 11, more preferably # 12. For all the refined grain size of previous austenite, it is effective to reduce the heating temperature at slackening. Note that "previous austenite grain size number" is based on JIS G 0551. If slackening is performed by reducing the heating temperature and shortening the time, the previous austenite grain size can be refined, but unreasonably low temperature and short treatment time increases not only the amount of undissolved spherical carbides, also in insufficient austenite transformation per se and slackening in two phases. Conversely, the formability and fatigue properties are sometimes reduced. For that reason, # 13.5 is usually the upper limit. 27 28

Residual austenite: Over 6% to 15% (unit by volume) The microstructure which is heat-treated is designed for high-strength spring application after toughening, tempered martensite, residual austenite, and a small volume fraction of inclusions (the precipitates described are also included). Residual austenite is effective in improving cold winding ability. The volume unit of residual austenite is made 6%, preferably 7% or more, more preferably 8% or more, to ensure cold winding capacity, in the present invention.

On the other hand, if residual austenite exceeds the volume mat 15%, martensite, which is formed as a result of work-induced conversion, causes the cold winding properties to decrease. Therefore, the volume content of residual austenite is set at 15% or less, preferably 14% or less, more preferably 12% or less.

The volume mat of the most austenitic can be determined by X-ray diffraction procedures and magnetic food procedures. The magnetic mapping procedures allow easy feeding of the volume mat residual austenite, so this is the preferred mapping procedure. If the volume mat is math, but the information obtained is the same as the math on the area.

Note that residual austenite is softer than annealed martensite, which reduces the yield strength. Furthermore, the transformation-induced plasticity is used to improve the formability, which contributes remarkably to improving the cold formability. On the other hand, residual austenite often remains in the segregated parts, 25 former austenite basket boundaries, and areas clamped by the grains, so that martensite which is formed by work-induced phase transformation (work-induced martensite) becomes the starting point for crime. Furthermore, if residual austenite Mar, anlOpt martensite decreases proportionally. For this reason, previous decreases in strength and cold winding ability, due to residual austenite, have been considered a problem. But in haghall strength steel radar of Over 2000 MPa, the amount added C, Si, Mn, Cr, etc. becomes larger, which makes the use of residual austenite extremely efficient to improve cold winding capacity. Furthermore, new high performance spring machining technology has made it possible to vaporize the deterioration of the formability properties even if high hardness parts are formed locally due to the formation of machining induced martensite in forming the spring.

Most recently, the mechanical properties of the drawn heat-treated steel row for high-strength spring use will be explained.

In order to reduce the size and to make the spring lighter, it is effective to make the ear more durable. Furthermore, it is required that the spring has Overlagsen fatigue resistance. In the present invention, high-strength spring is manufactured by bending the material of the drawn heat-treated steel row, for high-strength spring application, to the desired shape, then nitriding, ball-bombing, or on another sail hardened surface. During nitration, the spring is heated to 500 ° C or so, which means that all the spring is sometimes softer than the material of the drawn heat-treated steel row, for high-strength spring use.

Therefore, it is necessary to ensure the straight limits for material of the drawn heat-treated wire, for high-strength spring application, to increase the half-strength of the spring and to improve the fatigue properties. Furthermore, the requirement for cold-wrapping capacity. If the drawn heat-treated steel row, for high-strength spring use, must be able to be processed to the desired shape, the upper limit for the stretch limit must be limited.

Strain limit: 2100 to 2400 MPa If the preferred heat-treated stable line, such as high-strength spring application, hardened at the surface by nitriding, etc., has a large stretch limit, it is possible to improve the fatigue properties and the resilience property of the spring. In the present invention, the yield strength of the drawn heat-treated steel row for high-strength spring use is made 2100 MPa or more, in order to improve the fatigue properties and resilience of the spring.

Furthermore, the larger the stretch limit drawn heat-treated steel row for high-strength spring use has, the better the fatigue properties of the spring, so the drawn heat-treated steel row for high-strength spring use has a stretch limit of preferably 2200 MPa or more, more preferred. 29

On the other hand, the heat treatment provided for high-strength spring application has too large a stretch limit, the cold winding shape decreases so that the stretch limit sails to 2400 MPa or less.

In the present invention, the yield strength or yield point of drawn heat-treated steel row, for high-strength spring application, means the highest yield strength when a yield point is seen on stress / strength. - If yield strength cannot be observed, 0.2% test stress indicates: 1600 to 1980 MPa. The elongation curve in a one-axis tensile strength test and 0.2% shows tension when no tensile point is visible. To ensure the strength and resilience of the spring, which is elastically deformed by repeated tensioning, increased tensile strength is preferable. In order to increase the yield strength of the spring, it is preferable to increase the yield strength of the material which has been heat-treated steel-treated steel, for high-strength spring use.

[0089] On the other hand, if heat-treated steel is used for high-strength spring application, it will sometimes be cold-rolled. For this purpose, the heat-treated steel line has been drawn for 20 high-strength spring applications, preferably a yield strength of 1600 MPa or more, for all the strength and resilience of the spring.

For all give further higher durability, 1700 MPa or more is preferable.

On the other hand, if the yield strength exceeds 1980 MPa, the cold winding capacity is sometimes reduced so that the yield strength is preferably set at 1980 MPa or less.

Note that it is preferable to reduce the volume mat of residual austenite in order to increase the yield strength of the material with the same yield strength immediately after rapid toughening of the heat-treated plant for high-strength spring application.

Vickers hardness after nitriding by maintaining at 500 ° C for 1 hour: Surface hardness HW750, inner hardness HW.570 A high-strength spring is improved in surface hardness during nitriding, the inside is softened. For example, when tau gas nitriding at 500 ° C, it was black to vaporize softening of the inside of the drawn heat-treated steel row, for high-strength spring use, if the conventional heating temperature becomes 500 ° C. The drawn heat-treated stable line for high-strength spring use, in the present invention, has excellent resistance to heat-related softening and allows for substantial proofing of the spring fatigue properties and the resilience property after heating at 500 ° C.

In the present invention, the surface hardness and the inner hardness after soft gas nitriding are defined.

The surface layer hardness is set to the micro Vickers hardness at a depth of 50 to 100 μm from the surface layer with the Vickers hardness 750 or more. If the Vickers hardness is 750 or more, the surface layer hardness becomes insufficient and fatigue durability is also compromised, which means that the remaining stress after the ball bombing is not sufficiently reduced. Preferably, the surface layer hardness is 780 or more.

On the other hand, the inner hardness is sometimes measured. The Vickers hardness is sometimes measured during slackening, when the temperature at the surface layer of the steel row is higher than inside, which means that feeding of the inner hardness is preferably done at a position of 500 m deep in the surface. In order to ensure the yield strength and yield properties, the Vickers hardness after heat treatment maintaining the range at 500 ° C for 1 hour should be 570 or more. Furthermore, 575 or more are the Lecture.

Note that the upper limit of Vickers hardness after maintaining at 500 ° C for 1 hour for heat treatment is not specifically defined, but to cause the Vickers hardness before heat treatment not to be exceeded, it is usually set to 783 or less.

Furthermore, the surface layer of the drawn heat-treated steel row for high-strength spring application is used as a material for high-strength springs, hardened by ball bombardment, nitriding, etc. Therefore, the inner hardness will vary depending on the nitriding temperature during actual production of a spring. 31 32

Note that when drawn heat-treated steel line for high-strength spring use, in the three-pronged invention, is used as a material for the production of high-strength spring, it is cold-wound and nitrided. For this reason, residual austenite decreases somewhat, at a depth of 500 μm from the surface of the high-strength spring, compared with the material for the drawn heat-treated steel line for high-strength spring use.

However, the chemical composition of spherical carbides and former austenite crystal grain size has been found to be slightly affected by cold winding and nitriding. There, the chemical composition is spherical carbides and former austenite crystal grain size of high strength steel, made using drawn heat treated steel used for high strength spring use, in the present invention, as a material is the same chemical composition, spherical thermocarbons of carbon d for high-strength spring use, in the three-pronged invention.

Next, the process for producing the drawn heat-treated steel row for high-strength spring use, in the present invention, will be explained. A steel roll adapted to a trout-determined chemical composition is rolled to produce a steel roll blank of reduced size. Furthermore, the roll assembly is heated, after which it is hot-rolled to obtain the preferred steel line for high-strength spring use. The wire-drawn wire line for high-strength spring use is patented, shaped and, in addition, the hard surface is softened. Thereafter, the steel row is drawn, slacks and antps the steel line to produce the drawn heat-treated steel line for high-strength spring use. The "patenting" involved heat treatment in order to fine-tune the structure of the steel after hot rolling and perlite and to soften the steel row before drawing. After drawing, annealing in oil, induction hardening treatment, and toughening are carried out to adjust the structure and properties of the stable line.

The method of preventing wood digging of spherical carbides is important in the production of wire rods designed for high strength spring use, in the three present invention. 32 33

In particular, when the content of C and Cr is high, in the present invention, it is extremely important to heat the grout or roll sufficiently pre-rolling at that stage and to simplify precipitation inside the steel and to dissolve the internal coarse carbides (alloy carbides and cementites) and make the material homogeneous.

In order to prevent the formation of coarse carbides, coarse carbides which are formed in the ingot or rolling stock must be dissolved in the steel. Furthermore, it is necessary to create uniform spreading steel. For this reason, increased heating temperature is preferable.

Therefore, the casting or rolling stock is made only after casting at a heating temperature of 1250 ° C or more. To NA of this it is possible to Ora the undissolved spherical carbides sufficiently dissolved. For this reason, the heating temperature and the heating time are insufficient during the heating after subsequent rolling, patenting and slackening, so undissolved spherical carbides remain intact, but in order to allow sufficient dissolution from the brine, the dimensions of the undissolved spherical minor carbides can be controlled. The warming temperature of the litter must be 1270 ° C or more.

Thereafter, the roll assembly, which is manufactured by rolling the gesture, is further hot-rolled (rolled-rolled is rolled) to manufacture the contracts designed for high-strength spring application. At this time, the heating temperature of the roll is set to 1200 ° C or more. Preferably, the heating temperature of the roll should be set to 1250 ° C or more.

After the steel has been extracted from the heating furnace, the temperature drops and precipitation increases. For this reason, it is preferable that hot rolling be completed within 5 minutes, after the steel has been extracted from the heating furnace. Due to the above heating of the grout and rolling stock, the coarse carbides in the steel are homogeneously dispersed and dissolved and the carbides can uniformly precipitate on the later precipitation.

Note that when rolling the ingot into a stable row, without first making a rolling stock, the heating temperature before rolling the ingot should be set to 1250 ° C or more, preferably 1270 ° C or more.

In the above process, it is necessary to reduce the size to avoid coarsening in order to vaporize the coarseness of undissolved spherical carbides in the steel row after heat treatment of the undissolved spherical carbides, 33 34 which are presently preferred (as shown). rolling), is greatly reduced or if, for example, undissolved carbides are present.

Therefore, it is important that the heating temperature and the rolling stock heating temperature be sufficiently high in the rolling step for heating before dissolving carbides. As a result, the size of the spherical carbides can be kept small. The rolling of spring figures to a size of the material preference of about (1) 10 mm, is completed several minutes after extraction of the roll stock from the heating furnace. For this reason, it is important to heat to 1200 ° C or more when the effect of the rolling stock's heating temperature is large. Heating to 1250 ° C or more is preferable. Heating to 1270 ° C or more is more preferred.

After rolling, the frac is taken up in a coil and air cooled according to a conventional method. For this reason, the microstructure of the preferred staltra (staltra after rolling of roll) usually comprises ferrite and perlite or perlite with a high perlite structure fraction, since the amount C is high. Undissolved spherical carbides are present in the base material.

Undissolved spherical carbides can be observed by any polished and etched detection sample observed by SEM. Undissolved carbides can be clearly distinguished from laminar cementite enclosed in the perlite structure of the base material because they are spherical. Of course, the size can also be fed.

Due to the above-mentioned steps, the Treaties stable for spring use (rolled rolled row) are observed.

After hot rolling, the preferred steel row is patented for spring use.

The heating temperature during patenting can be set to 900 ° C or more to facilitate dissolution of carbides. A higher temperature of 930 ° C or more is preferred. Furthermore, 950 ° C or more is preferred. The trade can then be patented at 600 ° C or less. In the Treaties Designed for Spring Use, in accordance with the present invention, the procedures for patenting and drawing are not limited. If conventional patenting and drawing procedure for staltrad, the same treatment as usual can be performed.

When drawing the wire diameter and required precision are excluded, the patenting step of drawing can be excluded. In this case, dissolution of the 34 undissolved spherical carbides can be promoted by gars Nig (for example, 970 ° C or more) in the warming temperature, in the later explained slackening.

The slackening after drawing is carried out by heating to a temperature of A3 point or more. To promote the dissolution of carbides, it is advisable to increase the heating temperature during slackening. At slackening, the heating rate is preferably set to 10 ° C / sec or more and the retention time at the temperature of the A3 point or more is preferably to 1 minute to 5 minutes, to vaporize the growth of carbides. In order to dampen the growth of grain from austenite, it is preferable to shorten the retention time. To promote slackening and martensite transformation, the cooling rate is preferably 50 ° C / sec to 100 ° C.

The refrigerant in the quenching process is preferably salted to 100 ° C or less, more preferably a low temperature of 80 ° C or less, for all precisely controlling the amount of residual austenite, or the coolant temperature is 40 ° C or more in the present invention. The choice of coolant is not particularly limited as long as it is an oil, a water-soluble quencher or other coolant that promotes quenching. Furthermore, the cooling time can be shortened to the same salt as when tempering in oil and induction hardening treatment. Parts are preferred to avoid prolonged retention time at low temperature to greatly reduce residual austenite and lower the refrigerant temperature to 30 ° C or less. That is to say, it is preferable that the slacking is completed within 5 minutes.

[0107] After slackening, annealing is performed. Tempering suppresses the growth of 25 carbides, so it is preferable to all the heating rate is 10 ° C / sec or more and that the retention time is 15 minutes or less. The retention temperature varies due to the chemical composition and the target for the half strength, but the material is usually maintained at 400 to 500 ° C.

The preferred shapes for high strength spring use are cold wound to be processed into the desired spring shape, relieved of tension and nitrided and ball bombing to make the spring. 36

The cold-wound steel row is reheated by stress-relieving annealing, nitriding, etc. At this time, the inside is softened, which means that the performance of the spring decreases. In particular, in the present invention, sufficient hardness is preserved even when nitriding is carried out at a high temperature of about 500 ° C. As a result, it is possible to make the micro Vickers hardness at a depth of 500 .mu.m from the surface layer of high-strength springs HV575 or more. for high-strength spring use. Note that the micro Vickers hardness is fed at a depth of 500 μm from the surface layer of the spring in order to evaluate Vickers hardness has the basic material which is not affected by nitriding and ball bombing during curing.

Example

Steels with the chemical composition shown in Tables 1-1 to 1-4 are melted in a 10 kg vacuum malting furnace and cast to obtain a cast or roll blank. These vacuum narrow materials were hot pressed up to (1) 8 mm. Then the materials which are hot pressing up to 8 mm were heated at 1270 ° Cx4 hours. Furthermore, a part of the sample is refined in a 250 ton converter, continuously cast to color the goth, then heated at 1270 ° Cx4 hours or more, then made into rolls with a cross section of 160 mm x 160 mm. Furthermore, these are rolled to obtain c1) 8 mm rolled rolled wire. The heating temperature of the rolling stock before rolling was set to 1200 ° C or more.

A diameter of 8 mm of the preferred steel row (rolled rolled bar) is preferably formed into a single drawn structure by patenting the drawing. The heating temperature during patenting is preferably 900 ° C or more so that sufficient dissolution of carbides etc. is achieved. Patenting is performed by heating at 9 25 ° C then the sample is fed into a flowing bath at 600 ° C. After patenting, the steel wire is drawn to obtain a 4 mm diameter wire rolled wire. In this way, by heating the cast iron at a high temperature, after which the temperature in rolling process, patenting, and slackening is as high as possible, it is possible to vaporize the growth of undissolved spherical carbides and keep the dimensions down to 0.2 microns or less.

In order to adapt the stretch boundary of the patented and drawn steel wire, the steel wire is hardened to manufacture the preferred steel wire for spring use. Note that the sample that broke during drawing was not toughened. 36 37 Toughening was done by heating the drawn stalks at a heating rate of 10 ° C / sec or more at 950 ° C or 1100 ° C (temperature at A3 point or more), maintaining at the peak of the heating temperature for 4 minutes to 5 minutes, then placing the steel in a room temperature water tank said that the cooling rate will be 50 ° C / sec or more and the cooling Ors to 100 ° C or less.

The results of the evaluation show the state of the fracture, previous austenite grain size number, residual austenite quantity (vol%), circle equivalent diameter and density of carbide presence, yield strength, sample stress 0.2%, saving bend angle, average discharge strength after Vickers hardness strength.

The templates to be obtained were set as follows with male reference to conventional stable line for high strength spring use.

Previous austenite grain size number: 10 degrees or more Amount of residual austenite (vol%): 20% or less Circle equivalent diameter of spherical carbides: 0.2 μm or less Strain strength: 2100 MPa or more 0.2% test stress 1800 MPa or more Yield ratio: 75 % to 95% Spare bending angle: 28 degrees or more Average fatigue strength (Nakamura type of rotating bending strength): 900 MPa or more Internal hardness via Vickers hardness after gas nitriding: 590 HV or more Nitrated layer hardness via Vickers hardness after Vickers hardness After gas nitriding: 750 HV or more and working capacity (winding capacity) must be obtained in the stable line, according to the present invention, because if the exchange ratio is too high, the processing capacity is reduced. Therefore, the upper limit for the exchange ratio is dangerously 90%, more that the preference is 88% or less.

A sample taken from the obtained drawn heat-treated steel row for spring use, evaluated for previous austenite grain size, volume fraction of residual austenite, and carbides, then subjected to the tensile test, bending test of the 37 38 spar, and micro Vickers hardness test. Note all fatigue properties were evaluated by simulating treatment and production of the spring (hereinafter, referred to as "spring production and treatment") including gas nitriding, simulating nitriding challenge on a spring after processing (500 ° C, 60 minutes), ball bombing (diameter of the cut wire 0.6 mm, 20 minutes), and de-stressing treatment at range temperature (180 ° C, 20 minutes).

Previous austenite grain size numbers were measured based on JIS G 0551. The circle equivalent density and density of the carbides were measured by using an electronically etched sample to obtain a SEM structural image, and by analyzing the image. Furthermore, the volume mat of residual austenite is fed by the magnetic feeding method.

The fatigue test is a Nakamura type of rotary bending fatigue test (fatigue test bending through two-point skid weight and turning the motor all apply compressive and tensile stress to the surface of the wire). The highest loading force was set at the average fatigue strength for 10 samples, having a life of 7 cycles or more with a probability of 50% or more. The bending test at the save is a test for all evaluating the cold winding capacity and is performed as follows.

A punch 2 with an angle of the tip shown in Figure 2 of 120 ° was used to provide a notch (notch) of a maximum depth of 30 μm in a specimen. Note all that is shown in Figure 3, the groove 4 is provided at a steering angle in relation to the longitudinal direction in the middle of the test piece 3 in the longitudinal direction. Then, as shown in Figure 4 from the opposite side of the notch 4, a punch 5 was used to apply a load P of a maximum tensile strength stress through a load applying device 6 and the specimen was deformed by three-point bending. Note that the radius of curvature of the tip of load loading device 6 was made 4.0 mm, the difference L between reinforcements being made L = 2r + 3D. Where D is the diameter of the specimen.

The bending deformation was applied until the part with the spare was broken.

The bending angle at the time of the fracture (saving bending angle) was measured as shown in Figure 38 39 5. Note that after the specimen was broken, the broken parts were placed together to feed the bending angle of the spar e. In the present invention, a sample having a saving bending angle of 28 ° or more beclOrnts have excellent cold-winding Ability.

Micro Vickers hardness after nitriding was evaluated at a depth of 500 μm or more from the surface layer because the inner hardness was defined as "nitrated layer hardness of micro Vickers hardness at a depth of 50 μm from the surface layer. The feed weight was 10 grams.

The results of these tests are shown in Tables 1-5 to 1-8. Note all in Tables 1-5 to 1-8, the metal structure includes residual austenite (y) and anlOpt martensite as well as certain inclusions. Furthermore, jam and unavoidable impurities were the weight of the chemical compositions.

Preferred stable row (stable row after rolling rolled) was evaluated only on the basis of the circle equivalent diameter of undissolved spherical carbides. Part of this is due to everything that has happened before the heat treatment, and that it then does not make much sense with the images, despite all the mechanical properties or austenite grain size etc. are fed.

Examples 1 to 47, in the present invention, show all the cold winding ability, the viii saga, the bending angle of the spade, of a good 28 ° or more and are an excellent indication of spring half strength, the viii saga, Nakamura type of rotary bending fatigue strength and hairline. simply referred to as the "fatigue strength") and an excellent indication of the resilience and resistance to heat-related softening, that is, the nitride layer hardness.

Comparative Examples 48 and 49 are examples where the amount of added C is outside the range of the claims. If the amount C is added above the prescribed amount (comparative example 48), the undissolved spherical carbides become larger and the convergence of the cold winding forceps and the bending angle of the spar are indicated. On the other hand, if C is present in a smaller amount than the prescribed amount (comparative example 49), the toughening property deteriorates, which means that sufficient half-strength cannot be ensured. In particular, the inner hardness after nitriding and spring spring strength 39 (Nakamura type rotating bending fatigue strength) and the resilience property (internal hardness after nitriding) are lower.

Comparative Examples 50 and 51 are examples where the amount of Si added is outside the range of the claims. If the quantity Si Exceeds the prescribed quantity, the matrix is propagated and the working capacity is assumed, that is to say, the turning angle of the saving is law. On the other hand, if Si occurs to a lesser extent than prescribed, the toughening properties deteriorate, which means that sufficient half-strength cannot be sacrificed, after heating by nitration. In particular, the inner hardness after nitriding and the hardness of the nitrided layer become law.

Comparative Examples 52 and 53 are examples where the amount of added Mn is outside the prescribed range in the claims. If Mn is present in a larger amount than in the prescribed interval, the residual austenite becomes coarser, the yield strength decreases, and the fatigue strength (Nakamura type of rotary pickling fatigue strength) is subordinate. On the other hand, when Mn is present in a smaller amount than the prescribed amount, residual austenite decreases a lot and the working capacity is reduced, which has the consequence that the bending angle of the saving decreases.

Comparative Examples 54 and 55 are examples where the amount of Cr added is outside the range of the claims. If the amount Cr exceeds the prescribed interval, the cementite and even when heating the Viten or the roll is stabilized to high temperature, toughening, etc., unresolved carbides and the working capacity of the spring are greatly reduced. For this reason, the bending angle of the spar decreases. On the other hand, if Cr is present in a smaller amount than the prescribed amount, the steel is softened during heat treatment during nitration, etc., and the so-called resistance to heat-related softening becomes insufficient, which causes the hardness of the nitrated layer to decrease.

Comparative Examples 56, 57, and 58 are examples where the amount of Mo, W, and Mo + W added is greater than the range of the claims. If the quantity Mo and W exceeds the prescribed quantities, a supercooled structure of martensite, bainite etc. is formed, during rolling and cooling and after patenting and other heat treatment 41 which has the consequence that all the wire is broken during transport or during the drawing process, and all the feed test can not utfOras.

Comparative Example 59 is an example of Excessive addition of V. It is a substance which forms carbides in the steel. Excessive addition causes undissolved carbides to form around the V, the machining force is reduced and the bending angle of the groove decreases.

Comparative Examples 60 and 61 are cases where the amount N is Exaggerated compared to the range of claims. The excessive amount of N raises the temperature RN- the formation of nitrides and carbon nitrides of V, Nb, etc. and causes the coarsening of carbides and other precipitates which use these as nuclei. Furthermore, Ulysses nitrides, carbon nitrides and carbides are incomplete and a large amount of coarse undissolved spherical carbides remains when repeated heating, as used in the present invention. As a result, the processing capacity is compromised. Della is an example where the saving angle of bending decreases.

Comparative Examples 62 and 63 are examples where the amount of Nb added is outside the range of the claims. If Nb exceeds the prescribed quantity, the hot formability is noticeably blackened, a number of surface cracks occurred in the rolled material, wire steeply occurred during drawing, and a feed test could not be performed.

Comparative Example 64 is the case where the sum of the amount added Mn and V is more than the range according to the present invention. The amount of residual austenite in the stable row then becomes greater than the prescribed value. In the bending test of the groove, the hardness of the part decreases with the groove due to stress-induced phase transformation and the machining capacity decreases. Delia is an example where the savings' living angle decreases. Repeatedly, V itself is not added in the present invention, but sometimes V occurs as an unavoidable contaminant, which means that all V cannot be declared irrelevant.

Comparative Example 65 is the case where the sum of the amounts added Mn and V is lower than the range of the present invention. The amount of residual austenite is less than the optimal range, which means that the working capacity, ie the bending angle of the saving, decreases. 41 42

Comparative Example 66 is the case where the sum of the amounts added Cr and V is greater than the content explained in the present invention. Undissolved spherical carbides remain in Excessive quantity and machining capacity, the viii saga, the bending angle of the spar decreases.

Comparative Example 67 is the case where the sum of the amount of Cr added and V is less than the range explained in the present invention. The working capacity is excellent, but the inner hardness after nitriding and the hardness of the nitrided layer is insufficient and the performance of the spring is insufficient.

Comparative Examples 68 to 70 are cases where the difference between the amount of Si and the amount of Cr ([Si%] - [Cr%]) differs from the content of the claims and the amount of Cr is greater than the amount of Si. If the amount of Cr is Excessive in relation to the amount of Si, undissolved spherical carbides remain and the working capacity is reduced, the bending angle of the sai sag decreases.

In the same way, Comparative Examples 71 and 72 are cases where the difference between the amount Si and the amount Cr ([Si `) / 0] - [Cr%]) is a storm at the other limit of the range of the claims. The quantity Si is very Exaggerated in relation to the quantity Cr. In these cases, the charred layer of the rolled material of the rolled material grows strongly and cannot be removed to a sufficient extent by shaving the surface layer in a small amount. For this reason, the fatigue strength (Nakamura type of rotary bending fatigue strength) was irrelevant.

Comparative Examples 73 and 74 are Example 1 and Example 23, respectively, of the invention, in which the steel is rolled at the heating temperature of the rolling stock 1100 ° C. At the beginning of the rolling, undissolved spherical carbides remain. The effects remain so that the working capacity is finally collected, the bending angle of the saw blade decreases.

Examples 101 to 109 are examples of the invention of preferred steel radars of Examples 1 to 5 and 20 to 23 of the invention. Comparative Examples 110 and 111 are Examples 101 and 106 of the invention, where the heating temperature of the roll is Ors 1100 C. 42 43 The preferred shapes are evaluated in such a way that only the highest circle equivalent diameter of undissolved spherical carbides is examined. If the heating temperature of the roll is high, it has been found that the circle equivalent diameter of dissolved spherical carbides becomes smaller. 43

Table 1-1Chemical compositions (mass%) Ex.

C Si Mn P S Cr Al N V Nb Mo W Mg Zr Ca Mn + V Cr + V Si-Cr Mo + W 1 Inv.ex. 0.78 2.48 0.68 0.0076 0.001.57 0.0022 0.0031 - - - 0.90 2 Inv.ex. 0.77 2.41 0.68 0.0034 0.0047 2.00.0011 0.0042 - - - 0. 3 Inv.ex. 0.68 2.38 0.87 0.000.0061 1.53 0.0013 0.0033 - - - 0.8 4 Inv.ex. 0.88 2.0.87 0.0063 0.0071 1.71 0.0018 0.00- - - 0.79 Inv.ex. 0.78 2.11 0.84 0.0057 0.0039 1.0.000.0031 - - 0.61 6 Inv.ex. 0.72 2.62 0.62 0.0054 0.0031 1.96 0.0022 0.0038 - - 0.66 7 Inv.ex. 0.72 2.67 0.58 0.0037 0.001.51 0.000.0032 - - 1.16 8 Inv.ex. 0.76 2.28 1.02 0.0067 0.0069 1.82 0.002 0.0028 0.0031 - - 0.46 - 9 Inv.ex. 0.73 2.23 0.73 0.0054 0.0077 1.53 0.000.0058 - 0.70 Inv.ex. 0.77 2.0.80 0.0041 0.0047 1.52 0.0019 0.0032 - 0.83 11 Inv.ex. 0.77 2.59 0.83 0.0060 0.0074 1.53 0.0012 0.0063 - 0.008 - 1.07 12 Inv.ex. 0.72.52 0.68 0.0054 0.0056 1.67 0.0013 0.0039 0.06 0.74 1.74 0.84 13 Inv.ex. 0.72.33 0.83 0.0066 0.0060 2.00 0.000.0037 0.09 0.001 - 0.93 2.0.32 14 Inv.ex. 0.72 2.41 0.86 0.000.0068 1.77 0.0013 0.0033 - 0.12 0.64 0.12 Inv.ex. 0.77 2.57 0.70.0078 0.0071.91 0.0018 0.0043 - - 0.0.66 0. 16 Inv.ex. 0.72 2.42 0.84 0.0044 0.0053 1.90 0.0019 0.0056 - 0.12 0.16 - 0.52 0.27 17 Inv.ex. 0.76 2.53 0.67 0.0061 0.0076 1.60.0028 0.0039 - - 0.000- 0.87 18 Inv.ex. 0.73 2.46 0.66 0.0051 0.0060 1.60 0.0023 0.00- - 0.0002 - 0.86 19 Inv.ex. 0.73 2.34 0.70.0039 0.0071 1.97 0.0028 0.0032 - 0.0011 0.37 Inv.ex. 0.73 2.0.70 0.0046 0.0033 1.91 0.0016 0.0034 0.0.008 0.12 0.17 0.0002 0.0003 0.0011 0.79 2.00 0.44 0.28 21 Inv.ex. 0.77 2.46 0.71 0.0031 0.0054 1.90.0013 0.0038 0.03 0.003 0.0.16 0.0003 0.0002 0.0003 0.74 1.98 0.51 0.31 22 Inv.ex. 0.76 2.0.64 0.0051 0.0072 1.72 0.0016 0.0044 0.07 0.007 0.11 0.17 0.0000.0003 0.0006 0.71 1.79 0.64 0.28 23 Inv.ex. 0.73 2.36 0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 0.11 0.0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27 24 Inv.ex. 0.76 3.0.72 0.0076 0.0038 2.0.000.0037 0.06 - 0.0.77 2.16 1.0.

Table 1-2 Chemical compositions (mass%) Ex.

C Si Mn P S Cr Al N V Nb Mc W Mg Zr Ca Mn + V Cr + V Si-Cr Mo + W Inv.ex. 0.73 2.00.80 0.0062 0.0052 1.71 0.0011 0.0033 0.08 0.009 0.0.16 0.88 1.79 0.34 0.31 26 Inv.ex. 0.76 2.52 0.71 0.0069 0.0080 1.69 0.0024 0.0039 0.00.28 - 0.0002 0.0003 0.000.76 1.74 0.84 0.28 27 Inv.ex. 0.72.0.82 0.0074 0.0051.73 0.0018 0.0038 0.08 0.00.13 0.0.0004 0.0001 0.0000.91 1.82 0.56 0.28 28 Inv.ex. 0.73 2.24 1.0.0052 0.0074 1.52 0.0027 0.0033 0.09 0.11 - 0.0003 0.0001 0.0011 1.29 1.62 0.72 0.11 29 Inv.ex. 0.73 2.44 0.74 0.0076 0.0046 1.73 0.0024 0.0032 0.00.00.12 0.0.0001 0.0003 0.0013 0.79 1.78 0.71 0.27 Inv.ex. 0.74 2.24 0.76 0.0052 0.0076 1.41 0.0016 0.000.00.000.11 0.16 0.0003 0.0001 0.0011 0.81 1.46 0.83 0.27 31 Inv.ex. 0.72.28 0.89 0.0060.0043 1.70 0.0011 0.0043 0.03 0.00.14 0.16 0.0004 0.0002 0.0012 0.93 1.73 0.58 0.31 32 Inv.ex. 0.74 2.23 0.89 0.0043 0.0049 1.0.0022 0.0032 0.04 0.11 - 0.0001 0.0001 0.0002 0.93 1.54 0.73 0.11 33 Inv.ex. 0.77 2.86 0.77 0.0057 0.0057 2.48 0.000.000.09 0.000.11 0.16 0.0002 0.0002 0.0000.86 2.57 0.38 0.27 34 Inv.ex. 0.77 2.29 0.79 0.0050.0072 1.80.0016 0.0032 0.09 0.004 0.23 0.0.0004 0.0003 0.0006 0.88 1.93 0.0.38 Inv.ex. 0.73 2.52 0.76 0.0078 0.0067 2.00 0.0022 0.0036 0.09 0.009 0.18 0.28 0.0000.0001 0.0000.84 2.09 0.52 0.46 36 Inv.ex. 0.77 2.31 0.89 0.0046 0.0080 1.91 0.0021 0.0032 0.06 0.000 0.13 0.0.0001 0.0002 0.0006 0.91.97 0.0.28 37 Inv.ex. 0.73 2.53 0.69 0.0046 0.0060 2.01 0.0022 0.0042 - 0.000.14 0.17 0.0000.0002 0.0006 0.52 0.31 38 Inv.ex. 0.76 2.0.81 0.0034 0.0036 1.80 0.0016 0.000.04 0.006 0.12 0.09 0.0004 0.0001 0.0009 0.81.84 0.50.21 39 Inv.ex. 0.74 2.38 0.72 0.0031 0.0064 1.82 0.000.0043 0.07 0.009 0.11 0.28 0.0002 0.0002 0.0007 0.80 1.89 0.50.39 Inv.ex. 0.72.32 0.72 0.0034 0.0038 1.56 0.0017 0.0033 0.09 0.002 0.13 0.0.0004 0.0002 0.0012 0.81 1.60.76 0.28 41 Inv.ex. 0.76 2.37 0.69 0.0056 0.0067 1.63 0.0028 0.0041 0.04 0.14 0.17 0.0002 0.0002 0.0008 0.73 1.67 0.74 0. 42 Inv.ex. 0.76 2.48 0.73 0.000.001.97 0.0016 0.0034 0.09 0.0.16 0.82 2.06 0.51 0.26 43 Inv.ex. 0.72 2.26 0.68 0.0053 0.0043 1.51 0.0027 0.0032 0.00.14 0.14 0.0002 0.0003 0.0002 0.73 1.56 0.70.29 44 Inv.ex. 0.76 2.38 0.86 0.0077 0.0039 1.61 0.000.0052 - 0.14 0.0.77 0.28 Inv.ex. 0.77 2.23 0.76 0.0060 0.0061 1.69 0.0029 0.0053 0.00.006 0.11 0.0.81 1.74 0.54 0.26 46 Inv.ex. 0.76 2.0.86 0.0060 0.0067 1.87 0.000.000.00.00.0.0.0000.0003 0.000.90 1.92 0.48 0. 47 Inv.ex. 0.71 2.28 0.77 0.000.0058 1.88 0.000.0033 0.07 0.003 0.0.17 0.0000.0003 0.0006 0.84 1.90.41 0.27

Table 1-3Chemical compositions (mass%) Ex.

C Si Mn P S Cr Al N V Nb Mo W Mg Zr Ca Mn + V Cr + V Si-Cr Mo + W 48 Comp.ex. 0.92.47 0.61 0.0033 0.0078 1.54 0.0022 0.0043 0.09 0.006 0.12 0.14 0.0004 0.0001 0.0013 0.70 1.63 0.93 0.26 49 Comp.ex. 0.58 2.59 0.62 0.0079 0.0057 1.81 0.0021 0.0066 0.09 0.000.0.0.0003 0.0001 0.0014 0.71 1.90 0.77 0.29 Comp.ex. 0.71 3.80 0.80.0049 0.002.03 0.0017 0.0032 0.00.002 0.14 0.16 0.0002 0.0003 0.0008 0.90 2.09 1.77 0.31 51 Comp.ex. 0.73 1.86 0.83 0.0034 0.0060 1.32 0.0013 0.0031 0.09 0.004 0.14 0.14 0.0004 0.0003 0.0000.92 1.41 0.54 0.29 52 Comp.ex. 0.72 2.46 1.54 0.0076 0.0066 2.02 0.0024 0.0043 0.06 0.008 0.14 0.0.0003 0.0003 0.0014 1.60 2.08 0.44 0.29 53 Comp.ex. 0.72 2.22 0.21 0.0057 0.0064 1.58 0.0024 0.0036 0.09 0.000.0.17 0.0001 0.0002 0.0011 0.1.67 0.64 0.31 54 Comp.ex. 0.73 3.13 0.71 0.0048 0.002.72 0.0014 0.000.04 0.007 0.14 0.0.0003 0.0002 0.0011 0.72.76 0.41 0.29 5Comp.ex. 0.78 2.21 0.68 0.0051 0.0041 1.02 0.0013 0.0031 0.09 0.011 0.14 0.17 0.0002 0.0001 0.0012 0.77 1.11 1.19 0.31 56 Comp.ex. 0.72.42 0.74 0.0034 0.0067 1.78 0.0017 0.0033 0.09 0.000.42 0.07 0.0004 0.0003 0.0000.83 1.87 0.63 0.49 57 Comp.ex. 0.73 2.0.81 0.0046 0.0061 1.70.0027 0.0046 0.11 0.00.12 0.0.0003 0.0001 0.0008 0.92 1.86 0.70 0.62 58 Comp.ex. 0.73 2.0.64 0.0044 0.0034 1.80 0.0011 0.0037 - 0.00_ 0.26 0.2 / 0.0000.0002 0.0007 0.0.53 59 Comp.ex. 0.74 2.23 0.80 0.0060 0.0046 1.72 0.0026 0.0032 0.46 0.007 0.11 0.16 0.0004 0.0001 0.001.26 2.18 0.51 0.27 60 Comp.ex. 0.77 2.43 0.83 0.0062 0.0072 1.96 0.000.0076 0.08 0.0.17 0.0002 0.0001 0.0004 0.91 2.04 0.47 0.32 61 Comp.ex. 0.72.26 0.78 0.0069 0.0058 1.99 0.0013 0.0080.07 0.00.0.16 0.0003 0.0002 0.0000.82.06 0.27 0.27 62 Comp.ex. 0.73 2.0.62 0.0049 0.0062 1.73 0.000.0053 0.00.00.13 0.17 0.67 1.78 0.77 0.29 63 Comp.ex. 0.72 2.21 0.72 0.0074 0.0051 1.86 0.0026 0.0036 - 0.024 0.12 0.17 0.0002 0.0001 0.0012 0.34 0.29 64 Comp.ex. 0.74 2.1.18 0.0053 0.0056 2.02 0.0022 0.0042 0.09 0.006 0.0.16 - - 1.27 2.11 0.48 0.26 6Comp.ex. 0.74 2.52 0.51 0.0064 0.0046 1.73 0.0019 0.0039 0.06 0.003 0.12 0.16 0.0004 0.0002 0.0007 0.57 1.79 0.79 0.28 66 Comp.ex. 0.72.76 0.72 0.0048 0.0058 2.0.0014 0.0032 0.09 0.004 0.11 0.0.0002 0.0002 0.000.81 2.54 0.31 0.26 67 Comp.ex. 0.77 2.0.79 0.0059 0.0059 1.31 0.0016 0.000.03 0.002 0.14 0.16 0.0003 0.0001 0.0011 0.82 1.34 0.89 0. 68 Comp.ex. 0.76 2.12 0.66 0.0068 0.0072.31 0.0026 0.0033 0.09 0.0.16 0.0003 0.0001 0.0013 0.72.-0.19 0.31 69 Comp.ex. 0.74 2.0.88 0.0056 0.0056 2.23 0.0023 0.0043 0.06 0.000.14 0.16 0.94 2.29 -0.13 0. 70 Corp.ex. 0.78 2.23 0.73 0.0067 0.0072.41 0.0023 0.0048 0.11 0.000 0.12 0.17 0.0004 0.0003 0.0013 0.84 2.52 -0.18 0.28 71 Comp.ex. 0.71 3.12 0.76 0.0068 0.0044 1.54 0.0021 0.000.07 0.000.12 0.0.84 1.61 1.58 0.26 72 Comp.ex. 0.76 3.0.66 0.0071 0.0052 1.66 0.0017 0.0036 0.04 0.000.11 0.16 0.0004 0.0002 0.0002 0.70 1.70 1.79 0.26 73 Comp.ex. 0.78 2.48 0.68 0.0076 0.001.57 0.0022 0.0031 - 0.90 74 Comp.ex. 0.73 2.36 0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 0.11 0.0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27

Table 1-4 Chemical compositions (mass%) Ex.

C Si Mn P S Cr Al N V Nb Mo W Mg Zr Ca Mn + V Cr + V Si-Cr Mo + W 101 Inv.ex. 0.78 2.48 0.68 0.0076 0.001.57 0.0022 0.0031 - - 0.90 102 Inv.ex. 0.77 2.41 0.68 0.0034 0.0047 2.00.0011 0.0042 - 0. 103 Inv.ex. 0.68 2.38 0.87 0.000.0061 1.53 0.0013 0.0033 - - 0.8 104 Inv.ex. 0.88 2.0.87 0.0063 0.0071 1.71 0.0018 0.00- - 0.79 10Inv.ex. 0.78 2.11 0.84 0.0057 0.0039 1.0.000.0031 - - - 0.61 106 Inv.ex. 0.73 2.0.70 0.0046 0.0033 1.91 0.0016 0.0034 0.0.008 0.12 0.17 0.0002 0.0003 0.0011 0.79 2.00 0.44 0.28 107 Inv.ex. 0.77 2.46 0.71 0.0031 0.0054 1.90.0013 0.0038 0.03 0.003 0.0.16 0.0003 0.0002 0.0003 0.74 1.98 0.51 0.31 108 Inv.ex. 0.76 2.0.64 0.0051 0.0072 1.72 0.0016 0.0044 0.07 0.007 0.11 0.17 0.0000.0003 0.0006 0.71 1.79 0.64 0.28 109 Inv.ex. 0.73 2.36 0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 0.11 0.0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27 1Comp.ex. 0.78 2.48 0.68 0.0076 0.001.57 0.0022 0.0031 - 0.90, 111 Comp.ex. 0.73 2.36 0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 0.11 0.0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27

Table 1- Ex. Roller Item temperature (° C) Patenting temperature (° C) Extinguishing temperature (° C) Wood breakage etc.

[Good no abnormality] Maximum spherical carbide diameter Previous austenite grain size (V) Residual austenite (vol%) Tensile strength (MPa) 0.2% Sample voltage Dtbytesfor relating (%) Sprout bedding degrees (degrees) Macamura type of rotary bedding (MPa) Internal hardening ( HY) Hardness nitrided layer (HIT) 1 Inv.ex. 12990.06 9 2214 1887 836 927 597 788 2 Inv.ex. 12990.13 12 9 2288 1939 839 918 638 787 3 Inv.ex. 12990.06 11 8 2383 1982 37 924 599 819 4 Inv.ex. 12990.09 12 13 2217 1811 82 39 9621 799 Inv.ex. 12990.13 13 11 2301844 80 36 918 627 817 6 Inv.ex. 12990.12 2241 1899 839 918 623 816 7 Inv.ex. 1200 990.08 2243 1978 88 39 911 618 793 8 Inv.ex. 12990.02 6 2301 1811 79 38 914 600 789 9 Inv.ex. 12990.07 13 11 2293 1880 82 928 607 806 Inv.ex. 1200 9100.11 13 9 2263 1801 80 37 912 612 79 11 Inv.ex. 129100.13 2283 1939 837 929 608 808 12 Inv.ex. 129100.12 7 231844 79 38 913 616 780 13 Inv.ex. 1299.a I 0.13 13 2169 1904 88 39 9627 819 14 Inv.ex. 12990.07 13 2346 1854 79 919 612 808 Inv.ex. 12990.11 12 2189 1917 88 36 924 599 804 16 Inv.ex. 12990.01 12 9 2281822 80 38 916 680 17 Inv.ex. 12990.12 11 13 2248 1987 929 59787 18 Inv.ex. 12990.09 12 9 2193 1904 87 38 9616 804 19 Inv.ex. 12990.11 7 2180 1949 89 36 928 598 786 Inv.ex. 12990.09 8 2374 1891 80 39 924 612 817 21 Inv.ex. 12990.04 12 12 2368 1846 78 36 911 638 806 22 Inv.ex. 12970 0.04 12 2254 1853 82 914 629 79 23 Inv.ex. 12990.06 12 11 2311 1852 80 39 923 6783 24 Inv.ex. 1200 990.11 11 221956 87 911 616 793

Table 1-6 Ex. rolling stock temperature (° C) Patenting temperature (° C) ^ 02 o 0 a. - n N. 0 I-- P (0 0 Am 0 M 0 m 0 m Tradbrott etc.

[Good = no abnormality] Maximum spherical carbide diameter AM Previous austenite grain size (y #) Residual austenite (vol%) Tensile strength (MPa) 0.2% Test spinning Yield ratio (%) Saving bending degrees (degrees) Nakamura type of rotating bending (MPa) Internal hardness after rivets (HV) Hardness hoe nitrated layer (HV) Inv.ex. 12990.8 2357 1953 83 39 916 619 792 26 Inv.ex. 12990.12 11 13 21918689628 794 27 Inv.ex. 12990.11 11 12 2226 1872 84 38 922 592 79 28 Inv.ex. 1200 990.011 13 2181 1854 838 9631 784 29 Inv.ex. 12990.09 13 9 2277 1890 83 37 916 6786 Inv.ex. 12990.04 11 7 231938 83 37 922 593 798 31 Inv.ex. 12990.07 12 2301867 81 38 921 6791 32 Inv.ex. 12990.01 11 12 2216 1880 8919 592 813 33 Inv.ex. 1200 990.02 2281929 84 39 911 597 797 34 Inv.ex. 12990.03 11 11 2329 1944 83 39 916 601 794 Inv.ex. 12990.01 12 9 231824 79 911 609 793 36 Inv.ex. 12990.012 13 2153 1951 91 37 927 68 37 Inv.ex. 12970 0.01 9 221904 86 919 606 816 38 Inv.ex. 12990.07 11 6 2178 1971 91 38 921 60803 39 Inv.ex. 12990.12 6 2268 1880 39 9590 784 Inv.ex. 12990.06 13 8 2302 1860 81 39 919 604 814 41 Inv.ex. 12990.01 8 2190 1896 87 37 924 611 806 42 Inv.ex. 12990.06 13 6 2218 1893 837 927 639 787 43 Inv.ex. 12990.08 12 12 2382 1949 82 929 624 819 44 Inv.ex. 12990.06 12 8 2269 1369 82 918 618 806 Inv.ex. 12990.09 12 6 2151880 87 37 919 627 782 46 Inv.ex. 12990.13 12 12 2314 1964 838 912 639 813 47 Inv.ex. 12990.03 11 7 221892 837 919 6798

Table 1-7 Ex. '0 <2 1 ..; . . 0 4 m. 0 "m H Patent temperature. (° C) Slackening temperature (° C) Wood breakage etc.

(Good = no abnormality] Maximum spherical carbide diameter (gm) Previous austenite grain size (y1) Residual austenite (vol%) Tensile strength (MPa) 0.2% Test stress Yield ratio (%) Spire bending degrees (degrees) Nakamura type of rotating bending (MPa) hardness after nitriding (HV) Hardness nitrated layer (MN) 48 Comp.ex. 12990.31 12 9 231857 80 24 9617 807 49 Comp.ex. 1299 0.02 13 7 2002 1806 90 39 812 554 813 Comp.ex. 1299 0.04 13 8 2338 1841 79 23 926 612 812 51 Comp.ex. 1299Bra 0.12 11 7 2236 1954 87 36 9568 728 52 Comp.ex. 1299 0.08 12 21 2318 1680 737 8612 816 53 Comp.ex. 1299 0.11 2 2391971 82 923 637 801 54 Comp .ex 1299 0.26 13 2363 1909 81 23 9629 79 5Comp.ex 1299 0.03 7 2382 1877 39 919 608 731 56 Corrp.ex 1299. 57 Comp.

Cop.ex. 1299Tra I I - 58 Comp.ex. 1299brott 59 Comp.ex. 12990.42 12 7 2227 188821 929 68 60 Comp.ex. 1299Bra I1 0.12 8 221881 84 17 914 6816 61 Comp.ex. 1299 0.13 12 6 2384 1877 17 916 606 817 62 Comp.ex. 12990 I I - - - - - 63 Comp.ex. 1299Tradbr. - - - - - 64 Comp.ex. 12990.03 11 17 2233 1560 70 21 786 561 783 6Comp.ex. 1299 0.11 11 3 2232 1860 83 24 911 606 8 66 Comp.ex. 1299 0.33 11 2311 1838 80 21 916 613 783 67 Comp.ex. 1299Bra 0.03 9 2276 1977 87 43 789 561 732 68 Comp.ex. 1299 0.28 12 11 2297 1934 84 24 923 612 809 69 Comp.ex. 1299 0.22 13 6 2243 1838 82 22 914 629 813 70 Comp.ex. 1299 0.26 12 8 2329 1813 78 21 922 596 787 71 Comp.ex. 1299 * I 12 9 2359 1898 80 43 774 609 780 72 Comp.ex. 1299 12 2371 1877 43 781 631 797 73 Comp.ex. 1100 990.26 9 2114 1827 86 911 591 793 74 Comp.ex. 1100 99 * * 0.28 6 2251 1822 81 22 9602 751 * decarburised ** Good Table 1-8 51 Hardness nitrided layer (HV) 11111111111 Internal hardness after nitriding (MV) 11 111 1 111 1 Nakamura type of rotating bending Ili! It ti i II SpArbojning grader (grader) IIIIIIII (It Utbytesfdrhallande (%) IIIIIIII pp 0,2% provspinning 11111111111 Draghillfastthet (MPa) 11111111 1 1 Restaustennit (%) IIIIIIIIIII Tidigare austenitkornstor- lek (14) IIIIIIIIIII 08iam siddisk 03I 02 03 22 0 0 CO Tridbrott etc.

[Good. no abnormality] 1 11111 III Extinguishing temperature (° C) IIIIIIIIIII Patenting temperature (° C) IIIIIIII1 It Roll sample temperature (o C) 0 1.0 Ln 117 00000 1.7 L, 1, no 00 o N rH ZT, -I, -, -i - ■ NNNNN. [Th 12, -I e, X X X X X X X X 1Comp. ex .1 CD (t) (I) (1) (1.7 (1) 1) 0.)>>>>>>>>> CGC HHH NN HHH NINM, C LID 1.0 f - COMO N • X r-1 c - 1 = 1 r-1 000000000.-1, -1 I-1 W 51 52 IndustrieII applicability

The present invention can be used for the production of staltra for high-strength spring use. The fluff resistance spring material can be used in many industrial areas based on the automotive industry.

List of male designation designations

1 spherical carbides 2 punch 10 3 test piece 4 groove 5 feeder 6 stand used for load P load L distance between supports 0 spar bending angle 52 53

Claims (14)

1. Claim lPre-drawn steel wire for high strength spring use characterized by containing, by mass%, C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to l.2%, Cr: l.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the following formula:0.3%SSi-CrSl.2%, having a balance of iron and unavoidable impurities,having P and S as impurities comprising P: 0.025% or less and S: 0.025% or less, and, furthermore, having a circle equivalent diameter of undissolvedspherical carbides of less than 0.2 um.

2. Claim 2 Pre-drawn steel wire for high strength spring use as set forth in claim l characterized by, further,containing, by mass%, one or more of V: 0.03 to 0.l0%, Nb: 0.0l5% or less Mo: 0.05 to 0.30%, W: 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when containing V satisfying l.4%SCr+VS2.6% and 0.70%SMn+Všl.3%, and,when containing Mo and W, satisfying 0.05%SMo+Wš0.5%.

3. Claim 3 Drawn heat treated steel wire for high strength lO _6]__ spring use characterized by containing, by mass%, C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to l.2%, Cr: l.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the following formula:0.3%šSi-Cršl.2%, and having a balance of iron and unavoidable impurities,having P and S as impurities comprising P: 0.025% or less and S: 0.025% or less, furthermore, having a metal structure comprised of at least residualaustenite in a volume rate of over 6% to l5%, having prior austenite grain size number of #10 or more,and having a circle equivalent diameter of undissolvedspherical carbides of less than 0.2 um.

4. Claim 4 Drawn heat treated steel wire for high strength spring use as set forth in claim 3 characterized by,further, containing, by mass%, one or more of V: 0.03 to 0.l0%, Nb: 0.0l5% or less Mo: 0.05 to 0.30%, W: 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when containing V satisfying l.4%SCr+VS2.6% and 0.70%šMn+Všl.3%, and,when containing Mo and W, satisfying 0.05%SMo+Wš0.5%.

5. Claim 5 _62_ Drawn heat treated steel wire for high strengthspring use as set forth in claim 3 or 4 characterized inthat said drawn heat treated steel wire for high strengthspring use has a tensile strength of 2100 to 2400 MPa.

6. Claim 6 Drawn heat treated steel wire for high strengthspring use as set forth in any one of claims 3) to 5characterized in that said drawn heat treated steel wirefor high strength spring use has a yield strength of 1600to 1980 MPa.

7. Claim 7 Drawn heat treated steel wire for high strengthspring use as set forth in any one of claims 3 to 6characterized said drawn heat treated steel wire for highstrength spring use has a a surface Vicker's hardness of HV750 or more and an internal Vicker's hardness of HV570 or more aftersoft nitriding of keeping at 500°C for 1hour.

8. Claim 8 A method of production of pre-drawn steel wire for high strength spring use characterized by taking a bloomcontaining, by mass%, C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the following formula:0.3%SSi-CrS1.2%, having a balance of iron and unavoidable impurities,having P and S as impurities comprising P: 0.025% or less and S: 0.025% or less, heating the bloom to 1250°C or more, then hot rolling the bloom to produce a billet and heating the billet to 1200°C lO _63_ or more, then hot rolling to produce pre-drawn steelwire.

9. Claim 9 A method of production of pre-drawn steel wire for high strength spring use as set forth in claim 8characterized by the bloom further, containing, by mass%,one or more of V: 0.03 to 0.l0%, Nb: 0.0l5% or less Mo: 0.05 to 0.30%, W: 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or less, when containing V satisfying l.4%SCr+VS2.6% and 0.70%šMn+Všl.3%, and,when containing Mo and W, satisfying 0.05%SMo+Wš0.5%.

10. Claim l0 A method of production of pre-drawn steel wire for high strength spring use characterized by further heating pre-drawn steel wire as set forth in claim 8 or 9 to 900°C or more, then patenting it at 600°C or less.

11. Claim llA method of production of heat treated steel wire for high strength spring use characterized by drawing said pre-drawn steel wire which was produced bythe method of production of pre-drawn steel wire as setforth in any one of claim 8 or 9, heating it by a heating rate of l0°C/sec or more up to anA3 point, holding it at a temperature of the A3 point or more for lminute to 5 minutes, then cooling it by a cooling rate of 50°C/sec or more down to l00°C or less.

12. Claim l2 lO _64_ A method of production of heat treated steel wire for high strength spring use characterized by drawing said pre-drawn steel wire which was produced bythe method of production of pre-drawn steel wire as setforth in any one of claim l0, heating it by a heating rate of l0°C/sec or more up to anA3 point, holding it at a temperature of the A3 point or more for lminute to 5 minutes, then cooling it by a cooling rate of 50°C/sec or more down to l00°C or less.

13. Claim l3 A method of production of heat treated steel wirefor high strength spring use as set forth in claim llcharacterized by further holding and tempering it at 400to 500°C for l5 minutes or less.

14. Claim l4 A method of production of heat treated steel wirefor high strength spring use as set forth in claim l2 characterized by further holding and tempering it at 400 to 500°C for l5 minutes or less.