DE112014001616T5 - A steel wire rod for a high strength spring having excellent drawability into a wire and excellent post bending ability, process for its production, high strength spring and method of making the same - Google Patents

Abstract

There is provided a steel wire rod having an excellent drawability into a wire and an excellent post bending ability. The steel wire rod for a high strength spring of the present invention contains C: 0.5 to 0.8%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.5%, Ni: 0.05 to 0 , 5%, Cr: 0.05 to 2.5% and V: 0.05 to 0.5%, with the remainder being iron and unavoidable impurities, with the perlite microstructure area ratio being 95% of the total area or more, the tensile strength (TS) and the necking (RA) satisfy the expression (1) [TS ≦ 1250 MPa, RA ≥ 35%], the standard deviation of tensile strength (TSσ) and the standard deviation of necking (RAσ) of the term (2) [TSσ ≦ 55.0, RAσ ≦ 6.0], and the area ratio of the pearlite microstructure having an interlamellum pitch of 0.9 μm or more is found at the position of d × 1/4 where d is the diameter of the steel wire rod is 20% or less.

Description

[Technical area]

The present invention relates to a steel wire rod for a high-strength spring used for a valve spring of a motor vehicle and the like and which has excellent workability, particularly excellent wire drawing property, excellent post-draw bending ability and excellent peeling properties, as described below is a method for its preparation, and a high-strength spring, such. A hard-drawn spring and an oil-starting spring obtained from the steel wire rod for a high-strength spring.

[State of the art]

Since a spring used for an engine, a clutch, a fuel injection device and the like of a motor vehicle is used at a high load for a long period of time, a high level of fatigue resistance is required. Further, in recent years, fuel economy limitation of a motor vehicle due to environmental problems has become more stringent, and achievement of low fuel consumption of a motor vehicle is urgently required. Therefore, there is a strong demand for high strength for the purpose of weight reduction of the spring components used for a motor vehicle. On the other hand, market competition is increasing due to the growth of developing countries, and the development of high-strength and low-cost steel is required.

In view of the fact that the valve spring, which is used in the fuel injection device of a motor vehicle, is subject to a high load, many alloys are added to the properties such. B. to improve the fatigue strength and the resistance to permanent deflection. As the addition amount of an alloy increases, the ductility of the material tends to decrease and the drawability to a wire, the bending workability after drawing such. The helical forming properties and the like tend to deteriorate.

An example of a method of manufacturing a valve spring that is primarily used for an engine is shown below. First, a steel ingot having a predetermined composition, which has been melted and rough-rolled, is processed by hot rolling into a round wire having a diameter of about 5.0 to 8.0 mm, the wire is wound into a coil shape, and cooled. Thereafter, a peeling treatment (hereinafter referred to as "SV treatment") of removing the decarburized portion of the surface layer is performed without subjecting the steel wire rod (hereinafter referred to as "hot-rolled wire rod") to heat treatment and the like. Further, thereafter, by means of high frequency and the like, a soft annealing treatment is carried out and a processing into a wire having a desired wire diameter, in which it is z. B. is about a diameter of about 3 to 4 mm in the case of the valve spring is performed. Thereafter, the wire is subjected to a quenching-tempering treatment to improve the spring properties and then processed into a spring shape.

Although the manufacturing method described above is an example, the drawability to a wire related to the microstructure of the hot-rolled wire rod is influenced by the pre-process of the heat-treatment step comprising austenitization. However, internal defects and the like which are formed into a wire during processing remain as a defect in the microstructure even after the heat treatment comprising austenitization and also affect the properties of the wire after the heat treatment. Therefore, as the production steps are progressively reduced, securing the drawability to a wire and the post-drawing bending ability, which are ensured by improving the ductility of the hot-rolled wire rod, are a very important goal.

Therefore, there has hitherto been proposed a technology in which the workability of the hot rolled wire rod is improved by adjusting the production conditions of the hot rolling to improve the ductility of the hot rolled wire rod.

For example, in Patent Document 1, there is proposed a steel wire rod for a high-strength spring capable of excellent drawability into a wire by adjusting the chemical composition of the steel, setting the assembly temperature of the rolled wire rod to 900 ° C or more, and the starting temperature of the pearlite conversion to 650 to 750 ° C, whereby a microstructure is obtained mainly from perlite and the distribution of the pearlite beads is reduced.

[Document List]

[Patent Document]

• Patent Document 1: Japanese Unexamined Patent Application Publication No. 2012-072492 ,

Summary of the Invention

[Technical problem]

However, as a result of the investigation by the present inventors, it has been found that the internal defects in the microstructure according to Patent Document 1 have remained and that the bending ability after drawing has deteriorated.

As described above, in the past, there has not been a steel for a spring having an excellent drawability into a wire and an excellent post bending ability.

The present invention has been made in view of the circumstances as described above, and the object of the present invention is to provide a steel wire rod having an excellent drawability into a wire and an excellent post-drawing bending ability. In particular, the object of the present invention is to provide a steel wire rod for a high-strength spring, in which a pulling to a wire end diameter of about 85% of the cross-sectional change in the drawing z. Example, for a spring with a diameter of 2.0 to 4.0 mm only by applying a Zwischenwärmebehandlung a strong plasticizing annealing or high frequency heating is possible, in which no austenitization occurs, and which has an excellent bending capacity after drawing, and a A method suitable for producing such a steel wire rod for a high strength spring.

[The solution of the problem]

The steel wire rod for a high-strength spring of the present invention, which can solve the problems described above, is characterized by being C: 0.5 to 0.8% (% stands for mass%, hereinafter, the same applies to the chemical composition Si: 1.5 to 2.5%, Mn: 0.5 to 1.5%, Ni: 0.05 to 0.5%, Cr: 0.05 to 2.5% and V: 0, 05 to 0.5%, the remainder being iron and unavoidable impurities, the perlite microstructure area ratio being 95% or more in total area, the tensile strength (TS) and the necking (RA) in the following expression (1) satisfy the standard deviation of tensile strength (TSσ) and the standard deviation of necking (RAσ) in the following expression (2), and the area ratio of the pearlite microstructure having an inter-blade distance of 0.9 μm or more the position of d × 1/4 is determined, where d is the diameter r of the steel wire rod is 20% or less. TS ≤ 1250 MPa, RA ≥ 35% (1) TSσ ≤ 55.0, RAσ ≤ 6.0 (2)

Further, according to the present invention, in the chemical composition, at least one member selected from the group consisting of Nb: more than 0% to 0.10%, Mo: more than 0% to 0.50%, Cu: more than 0 % to 0.50% and B: 0.0010 to 0.0100%.

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

The present invention also includes a method for producing the steel wire rod for a high-strength spring described above. In particular, the method is characterized in that it comprises the steps of winding a steel wire rod after hot rolling, which is the above to a helical shape at an assembly temperature of 750 to 890 ° C, then cooling the dense portion of the helix and the loose portion of the helix on a cooling conveyor to a starting temperature of slow cooling with an average cooling rate of 2.0 to 10.0 ° C / second and then the slow cooling of the steel wire rod for 120 seconds or more at an average cooling rate of 1.0 ° C / second or less, wherein the starting temperature of the slow cooling is set so that the temperature of the dense Part of the helix and the loose part of the helix is within the range of 600 to 650 ° C and the temperature difference of the dense part of the helix and the loose part of the helix is 30 ° C or less.

Further, the present invention also includes the steps of subjecting the steel wire rod for a high strength spring described above, a peeling treatment without performing a heat treatment, thereafter performing soft annealing or high frequency heating, thereafter performing drawing into a wire, and thereafter Perform quenching and tempering treatment and forming into a spring.

[Advantageous Effects of Invention]

In the present invention, the chemical composition and the metal microstructure were appropriately set, the tensile strength and the necking of the steel wire rod were set within a predetermined range, and the area ratio of the predetermined pearlite microstructure within the steel wire rod was appropriately adjusted. As a result, a steel wire rod for a high-strength spring having an excellent drawability into a wire and an excellent bending ability after drawing can be provided. Furthermore, such a steel wire rod for a high-strength spring as a starting material for producing a high-strength spring is very well suited.

[Brief Description of the Drawings]

1 is a schematic explanatory drawing of the sampling position of a coil.

2 Fig. 12 is a schematic explanatory drawing showing a state of a coil on a cooling conveyor.

3 Fig. 12 is a schematic explanatory drawing of a method for sampling a coil which is a test piece for evaluation.

4 Fig. 15 is a cross-sectional view of a wire rod which is a test piece for evaluation, and is a schematic explanatory drawing of the measuring position of the pearlite microstructure.

5 is a drawing substitute photograph showing a pearlite colony with an interlamellar distance of 0.9 μm or more.

[Description of Embodiments]

In general, in the production of a steel wire rod for a spring, the steel wire rod after hot rolling is wound into a helical shape, placed on a cooling conveyor, and cooled by air cooling and the like. A state of the coil on the cooling conveyor is in the 2 shown. When cooling is performed in such a state, a difference in the cooling rate between a comparatively close-overlapping portion (hereinafter referred to as "dense portion") of the steel wire rod and a comparatively loose portion (hereinafter referred to as "loose portion") ), and after cooling, a difference in the microstructure is produced.

The present inventors have studied the relationship between the roll microstructure of the steel for a high-strength spring and the workability or, in particular, the drawability to wire and the bending ability after drawing. As a result, it has been found that the drawability to a wire and the bending ability after drawing the steel wire rod by adjusting the distribution of mechanical properties (the tensile strength and the necking can be collectively referred to as "mechanical properties" hereinafter) in the longitudinal direction, e.g. B. the circumferential direction, in the 3 and the area ratio of the pearlite microstructure having a coarse interlamellar spacing (hereinafter may be referred to as "coarse pearlitic microstructure") to adjust the roll microstructure to a uniform microstructure consisting primarily of perlite.

Here, with respect to the distribution of the mechanical properties in the longitudinal direction, the reduction of the distribution of the microstructure in the longitudinal direction or the dense part and the loose part of the filament due to the cooling speed becomes important.

Further, regarding the suppression of the pearlite microstructure having a large inter-blade distance, it has been found that an appropriate adjustment of the temperature range in which the pearlite transformation starts on the roller conveyor was also important by adjusting the cooling speed.

The present inventors further have the conditions for ensuring a sufficient drawability to a wire and bending ability after drawing in the steel wire rod for a high strength spring by performing the SV treatment with the heat treatment omitted after rolling, and even only by soft annealing by a high frequency heat treatment and the like. As a result, it was found that the steel wire rod for a high-strength spring which solves the above-mentioned problem could be obtained when 95% or more of the area ratio of the microstructure of the steel wire rod was formed of the pearlite microstructure, the cooling speed of the dense portion, and The helical portion of the helix was adjusted to provide tensile strength (TS) and necking (RA), which are the mechanical properties, and the standard deviation of tensile strength (TSσ) and the standard deviation of necking (RAσ) at which these are indicators of the distribution of mechanical properties in the longitudinal direction which satisfied expressions (1) and (2) below, and the starting temperature of pearlite conversion was set so that the area ratio of the pearlite microstructure was 0.9 μm in interleaf pitch or more, which would possibly lead to an internal defect and the drawing operation is 20% or less, and the present invention has been made. TS ≤ 1250 MPa, RA ≥ 35% (1) TSσ ≤ 55.0, RAσ ≤ 6.0 (2)

The reasons for setting the above requirements are as follows.

[Perlite microstructure area ratio: 95% or more]

The steel wire rod for a spring of the present invention has a main phase of the perlite microstructure. As a microstructure, which is different from the perlite microstructure, optionally a supercooled microstructure, such as. As bainite and martensite, and ferrite may be included. If the supercooled microstructure, such. As bainite and martensite increases, the ductility and toughness deteriorate, and it is likely that wire break occurs in the peeling treatment and the drawing treatment. As ferrite increases, the microstructure becomes nonuniform, and therefore, if necessary, the drawability deteriorates into a wire, and internal defects such as the like are deteriorated. As a cavity, are generated after the drawing process and therefore, if necessary, the bending ability deteriorates after drawing. In the present invention, by increasing the area fraction of the perlite microstructure, the undesirable microstructure such. B. bainite is relatively suppressed and a uniform microstructure is achieved mainly from pearlite, a wire break, internal defects and the like reduced and the drawability to a wire and the bending capacity after drawing are improved. To ensure such effects, the areal ratio of the perlite microstructure relative to the entire microstructure should be 95% or more, preferably 97% or more, and more preferably 98% or more.

Further, the reasons for setting tensile strength (TS) and standard deviation of tensile strength (TSσ) and necking (RA) and standard deviation of necking (RAσ) of the steel wire rod for a spring in the present invention are as follows.

[Tensile strength (TS): 1250 MPa or less, necking (RA): 35% or more (1)]

Both tensile strength (TS) and necking (RA) are important indicators for suppressing wire breakage of the steel wire rod during drawing. In the present invention, the steel wire rod, which has been obtained by hot rolling, subjected to a peeling treatment (SV treatment) without performing a heat treatment, is then subjected to a soft annealing treatment or a Subjected to high frequency heating and is then subjected to a drawing process. In these manufacturing steps, if the ductility is poor, it is likely that a wire break occurs in the SV treatment and the drawing process. In particular, when the tensile strength (TS) is excessively high and the necking (RA) is excessively low, it is likely that a wire break occurs in the SV treatment and the drawing process. With a view to suppressing such wire breakage, the mechanical properties were appropriately adjusted in the present invention. Specifically, the tensile strength (TS) of the steel wire rod is set to 1250 MPa or less, preferably 1200 MPa or less. Further, the necking (RA) of the steel wire rod is set to 35% or more, preferably 40% or more.

[TSσ: 55.0 or less, RAσ: 6.0 or less (2)]

Further, in the present invention, the distribution of mechanical properties has been suitably adjusted. Since the rolled wire rod has different cooling rates between the dense part and the loose part of the coil when cooled, there is a case where a large difference is generated in the mechanical properties. Therefore, in view of suppressing wire breakage in the SV treatment and the drawing treatment, and producing defects in the microstructure after the drawing process and improving the bending ability after drawing, it is preferable to improve the distribution of tensile strength (TS) and necking (FIG. RA). Accordingly, in the present invention, the standard deviation (TSσ) of the tensile strength is set to 55.0 or less, preferably 50.0 or less, and more preferably 48 or less. Further, the standard deviation (RAσ) of the necking is set to 6.0 or less, preferably 5.0 or less, and more preferably 4.8 or less.

[Total area ratio of the pearlite microstructure having an interlamellar distance of 0.9 μm or more at a position d × 1/4, where d is the diameter of the steel wire: 20% or less]

The adjustment of the area ratio of a specific pearlite microstructure at a predetermined position of the steel wire rod is effective for suppressing wire breakage in the drawing process, the bending process after the drawing process, and the like. In the present invention, although the wire rod has a metal microstructure whose main phase is pearlite as described above, when a pearlite microstructure having a coarse interlamellar spacing exists inside the wire rod, since the pearlite microstructure has poor ductility, likely to become a defect in the microstructure which becomes a cause of wire breakage. Further, since the defect tends to remain in the microstructure even after the heat treatment, it is preferable that it is suppressed. In view of suppressing such a defect, in the present invention, the total area ratio of the pearlite microstructure having an interlamellar pitch of 0.9 μm or more at the 1/4 position of the diameter of the steel wire rod becomes 20% or less, preferably 15%. or less, and more preferably 10% or less. Further, in view of the suppression of a defect, it is preferable to set the total area ratio of the pearlite microstructure to the above-described range with an inter-blade distance of preferably 0.8 μm or more, and more preferably 0.7 μm or more. Further, the wire rod satisfying the above-described requirements is also excellent in SV characteristics.

In the production of such a steel wire rod for a high-strength spring as described above, it is necessary to suitably set the production conditions as described below. First, a steel block having a predetermined chemical composition is hot rolled and processed to a desired wire diameter. Although the heating temperature is not particularly limited in this rolling, processing at as low a temperature as possible in view of miniaturization of the microstructure is preferable. However, since the deformation resistance of the steel and the plant load increase as the temperature is lowered, the temperature must be properly adjusted according to the existing equipment. Normally, the heating temperature during hot rolling, which is z. B. is the heating temperature of the steel block, about 900 to 1000 ° C.

Subsequently, although the steel wire rod is placed in a helical shape after being hot-rolled on the cooling conveyor, if the assembly temperature exceeds 890 ° C, the microstructure becomes coarser, the necking (RA) decreases, the ductility decreases, and the drawability diminishes into a wire and the bending capacity after drawing may be deteriorated. If the arrangement temperature is below 750 ° C, the deformation resistance increases and the Plant load increases, and therefore, the placement temperature is set to 750 ° C or higher and 890 ° C or lower. It is preferable that the arrangement temperature is 770 ° C or higher and 830 ° C or lower.

Although the steel wire rod is cooled to the temperature range at which the pearlite transformation starts, which is the slow cooling starting temperature of 600 to 650 ° C after being placed on the cooling conveyor, it is set to a predetermined one for the post-rolling microstructure Area required to cool the wire rod, which has been formed into a helical shape, quickly and uniformly. Specifically, the dense portion and the loose portion of the filament are respectively cooled at an average cooling rate of 2.0 ° C / second or more and 10.0 ° C / second or less, then for 120 seconds or more at an average cooling rate of 1 , 0 ° C / second or less, and the rolling wire temperature at the beginning of the slow cooling is set within a range of 600 ° C or higher and 650 ° C or lower for both the loose part and the dense part. Since it is normal to start the slow cooling of a region by arranging a cover for slow cooling, the slow cooling region may be hereinafter referred to as "within the slow cooling cover", and the slow cooling start position may be hereinafter be referred to as "inlet of the cover for a slow cooling".

When the average cooling rate up to the slow cooling start temperature described above is less than 2.0 ° C / second, the microstructure can not be sufficiently adjusted, the area ratio of the coarse pearlite microstructure increases, and the drawability becomes one Wire and the bending ability after drawing deteriorate. On the other hand, if the average cooling rate exceeds 10.0 ° C / second, a local supercooled microstructure, such as e.g. B. bainite formed, the area ratio of the pearlite microstructure decreases, the ductility decreases and therefore the drawability deteriorate into a wire and the bending ability after drawing. Preferably, the average cooling rate is 3.0 ° C / second or more and 7.0 ° C / second or less.

In order to suppress the distribution of the mechanical properties in the longitudinal direction of the steel wire rod and to set the distribution in a desired range, it is important to set the cooling conditions of the dense portion and the loose portion of the rolled coil to the temperature range at which the pearlite transformation occurs after the placement of the Steel wire rod on the cooling conveyor begins.

Specifically, the cooling conditions are set so that the dense portion and the loose portion of the filament are respectively cooled at the cooling speed of the above-described temperature range, the temperature of the steel wire rod at the beginning of the slow cooling being within the range of 600 ° C or more and 650 ° C or less is set for both the dense part and the loose part of the coil, and the temperature difference between the dense part and the loose part of the coil is 30 ° C or less.

When the start temperature of the slow cooling of the dense portion and the loose portion of the filament exceeds 650 ° C, a coarse pearlite microstructure is formed, the area ratio of which increases, and the internal defects decrease. As a result, the bending ability after pulling deteriorates. On the other hand, if the starting temperature of the slow cooling is lower than 600 ° C, the pearlite conversion within the cover for slow cooling becomes insufficient. As a result, a supercooled microstructure, such. B. bainite formed, the perlite area ratio decreases, the ductility decreases and the drawability to a wire and the bending ability after drawing deteriorate. Preferably, the temperature of the inlet of the slow cooling cover is 620 ° C or more and 640 ° C or less.

Further, the temperature difference of the inlet of the cover for slow cooling between the dense portion and the loosening portion of the coil is set to 30 ° C or less, preferably 20 ° C or less. By making the temperature difference between the dense part and the loose part of the filament at the inlet of the slow cooling cover 30 ° C or less, the distribution of the mechanical properties in the longitudinal direction of the rolled wire rod (TSσ, RAσ) on the predetermined range, which has been described above, be reduced.

With respect to the cooling speed of the dense portion and the loosening portion of the filament at the inlet of the slow cooling cover after the rolled wire rod is placed on the cooling conveyor, the temperature difference between the dense portion and the loosened portion of the filament may be at the entrance side of the coil Slow cooling range by adjusting the air volume of the Cooling fan can be reduced to the dense part or the loose part of the coil. Since the cooling speed of the dense portion and the loosening portion of the filament change in accordance with the wire rod speed, the speed of the conveyor, and the like, it is necessary to adjust the air volume to suit each rolling condition.

If the cooling rate within the slow cooling cover is too high and the dwell time, particularly the dwell time in the slow cooling range, or the slow cooling time is too short, the slow cooling is completed before the conversion is complete, and the supercooled microstructure such. B. bainite and martensite, can be cooled by, for. B. normally formed by water cooling. Therefore, the cooling speed at the slow cooling range as described above is set to 1.0 ° C / second or less, preferably 0.4 ° C / second or less, and it is preferable that the above-described residence time to 120 seconds or more.

In the steel wire rod for a high-strength spring of the present invention, it is for the presence of the properties as a final product or z. B. as a high-strength spring required to adjust its chemical composition in a suitable manner. The reasons for the restriction of the range according to each constituent (element) in the chemical composition are as follows.

[C: 0.5 to 0.8%]

C is an element effective in ensuring the basic strength of steel and improving the strength and resistance to permanent deflection of the spring. To ensure such effects, C should be contained in an amount of 0.5% or more. Along with the increase in C content, the strength and resistance to permanent deflection of the spring improve. However, when the addition amount of C becomes excessive, coarse cementite is precipitated in a large amount, the ductility deteriorates and the drawability becomes a wire, the post-draw bending ability and spring properties may be adversely affected. Therefore, the C content should be 0.8% or less. A preferable lower limit of the C content is 0.55% or more, more preferably 0.6% or more, and a preferable upper limit is 0.75% or less, more preferably 0.7% or less.

[Si: 1.5 to 2.5%]

Si is an element required for deoxidizing steel and ensuring strength, hardness and resistance to permanent deflection of the spring. To ensure such effects, Si should be contained in an amount of 1.5% or more. However, when the Si content becomes excessive, not only the material is hardened, but also the drawability becomes a wire, and the bending ability after drawing is deteriorated due to the reduction in ductility, the surface decarburization increases, and the SV treatment ability the fatigue properties of the spring may be worsened if necessary. Therefore, the Si content should be 2.5% or less. The preferable lower limit of the Si content is 1.7% or more, more preferably 1.8% or more, and the preferable upper limit is 2.3% or less, more preferably 2.2% or less.

[Mn: 0.5 to 1.5%]

Like Si, Mn is an element required for the deoxidation of steel. Further, in addition to binding S in steel as MnS, Mn helps to improve the quenchability and to increase the strength of the spring. To ensure such effects, Mn should be contained in an amount of 0.5% or more. However, when the Mn content becomes excessive, the quenching ability is excessively increased, and there is a tendency that a supercooled microstructure such as a microstructure may be generated. As martensite and bainite are formed. As a result, the drawability deteriorates into a wire and the bending ability after drawing. Therefore, the Mn content should be 1.5% or less. The preferable lower limit of the Mn content is 0.7% or more, more preferably 0.8% or more, and the preferable upper limit is 1.3% or less, more preferably 1.2% or less.

[Ni: 0.05 to 0.5%]

In addition to suppressing decarburization in hot rolling, Ni contributes to the improvement of ductility, toughness, and corrosion resistance after quenching and tempering. To the To ensure such effects, Ni should be contained in an amount of 0.05% or more. However, when the Ni content becomes excessive, the quenching ability is excessively increased, and therefore, there is a tendency that a supercooled microstructure such as a microstructure may be generated. As martensite and bainite are formed. As a result, the drawability deteriorates into a wire and the bending ability after drawing. Further, since quenching and tempering, which are the manufacturing steps for an oil tempering wire (OT wire), residual austenite is excessively formed, the resistance to permanent deflection of the spring is extremely deteriorated. Therefore, the Ni content should be 0.5% or less. The preferable lower limit of the Ni content is 0.10% or more, more preferably 0.15% or more, and the preferable upper limit is 0.30% or less, more preferably 0.25% or less.

[Cr: 0.05 to 2.5%]

In addition to improving the quenching property and increasing the strength of the spring, Cr has an effect of reducing the activity of C and preventing decarburization in rolling and heat treatment. To ensure such effects, Cr should be contained in an amount of 0.05% or more. However, when the Cr content becomes excessive, Cr-based alloy carbide, nitride and carbonitride are excessively precipitated, the ductility decreases and the drawability becomes a wire, and the bending ability after drawing deteriorates. Therefore, the Cr content should be 2.5% or less. The preferable lower limit of the Cr content is 0.10% or more, more preferably 0.20% or more, and the preferable upper limit is 2.2% or less, more preferably 2.0% or less.

[V: 0.05 to 0.5%]

V has the effect of reducing grain size during hot rolling and quenching and tempering, and improves ductility and toughness. Further, V causes secondary precipitation hardening in stress relief annealing after forming the spring and contributes to the improvement of the strength of the spring. To ensure such effects, V should be contained in an amount of 0.05% or more. However, when the V content becomes excessive, a complex alloy carbide of Cr and V is excessively precipitated, the ductility decreases and the drawability becomes a wire, and the bending ability after drawing deteriorates. Therefore, the V content should be 0.5% or less. The preferable lower limit of the V content is 0.10% or more, more preferably 0.15% or more, and the preferable upper limit is 0.3% or less, more preferably 0.2% or less.

The basic composition of the steel wire rod for a high-strength spring according to the present invention is as described above, and the remainder are iron and unavoidable impurities (for example, P, S, and the like). In the steel wire rod for a high-strength spring according to the present invention, optionally, at least one element selected from the group consisting of Nb: more than 0% and 0.10% or less, Mo: more than 0% and 0.50% or less, Cu: more than 0% and 0.50% or less, and B: 0.0010 to 0.0100%, and these elements have the same effect and the same effect, and the properties of the steel wire rod according to the kind of contained Elements are further improved. The reasons for setting the preferable range of these elements are as described below.

[Nb: more than 0% and 0.10% or less]

Nb has the effect of reducing grains in hot rolling, quenching and tempering, and has an effect of improving ductility. However, if Nb is excessively contained, a complex alloy carbide of Cr and Nb is excessively precipitated, the ductility decreases, and the drawability becomes a wire, and the bending ability after drawing deteriorates. Therefore, the Nb content is set to 0.10% or less. The preferable lower limit of the Nb content is 0.01% or more, more preferably 0.02% or more, and the preferable upper limit is 0.07% or less, more preferably 0.05% or less.

[Mo: more than 0% and 0.50% or less]

Mo causes secondary precipitation hardening in stress relief annealing after forming the spring and contributes to the improvement of the strength of the spring. However, when the Mo content becomes excessive, a complex alloy carbide of Cr and Mo is excessively precipitated, the ductility decreases and the drawability becomes a wire, and the bending ability after drawing deteriorates. Therefore the Mo content is set to 0.50% or less. Preferably, the lower limit of the Mo content is 0.05% or more, more preferably 0.10% or more, and preferably the upper limit is 0.40% or less, more preferably 0.30% or less.

[Cu: more than 0% and 0.50% or less]

Cu has the effect of suppressing decarburization during hot rolling. Further, Cu also contributes to an improvement in corrosion resistance. However, if Cu is excessively contained, hot ductility is deteriorated and there is a risk of cracking during hot rolling. Therefore, the Cu content is set to 0.50% or less. Preferably, the lower limit of the Cu content is 0.05% or more, more preferably 0.10% or more, and preferably the upper limit is 0.40% or less, more preferably 0.30% or less.

[B: 0.0010 to 0.0100%]

B has effects to improve the quenching ability and to increase ductility and toughness by cleaning the austenitic grain boundary. For ensuring such effects, it is preferable that B is contained in an amount of 0.0010% or more. However, if B is excessively contained, a complex compound of Fe and B is precipitated and there is a risk of cracking during hot rolling. Further, since the quenching capability is excessively improved, a supercooled microstructure such as e.g. B. bainite, formed, and the ductility of the rolled wire rod may optionally be reduced. Therefore, the B content is set to 0.0100% or less. Preferably, the lower limit of the B content is 0.0015% or more, more preferably 0.0020% or more, and preferably the upper limit is 0.0050% or less, more preferably 0.0040% or less.

Although the steel wire rod for a high strength spring of the present invention is a steel wire rod obtained after hot rolling, this steel wire rod for a high strength spring is then formed into a high strength spring by processing into a spring, and a spring having excellent properties can be obtained.

The present application claims the priority right on the basis of Japanese Patent Application No. 2013-063012 , which was registered on March 25, 2013. The entire content of the description of the Japanese Patent Application No. 2013-063012 , filed on Mar. 25, 2013, is incorporated by reference into the present application.

[Examples]

Although the present invention will be described more specifically below with reference to Examples, the present invention is not limited to the following examples, it being understood that the present invention can be practiced also with modifications that can be conveniently added within the range which can be adapted for the purposes described above and below, and any of these modifications should be included in the technical scope of the present invention.

A steel ingot having the chemical composition shown in Table 1 below was melted by a converter, the steel ingot was then subjected to rough rolling, and a steel block having a cross section of 155 mm x 155 mm was prepared. The steel ingot was heated to 950 ° C, then hot rolled, placed at the conveyor arrangement temperature shown in Table 2 below ("arrangement temperature" in the table), and thereafter a coil having a wire diameter of 5.5 to 8.0 mm and a unit weight of 2 tons at the temperature conditions shown in Table 2 (Test Nos. 1 to 29). With respect to each helix obtained, the following tests were carried out.

[Tensile strength (TS), necking (RA) and standard deviation thereof (TSσ, RAσ)]

As it is in the 1 from each position of a 2-ton helix, ie, from above ("TOP"), which is 1/3 of the total helix, from below ("BOT"), which is 1/3 the entire helix, and taken from the center ("MID"), which is 1/3 as the remainder of TOP and BOT described above, taken in each case 3 turns or a total of 9 turns as a sample. With the samples by parts each ring has been obtained in 8 parts in the circumferential direction or by dividing each ring into 8 parts in the longitudinal direction of the helix as shown in FIG 3 or with a sample number (n) = 72, a tensile test (TS) and a cross-sectional degradation test (RA) were performed. Furthermore, the standard deviation of the tensile strength (TSσ) and the standard deviation of the cross-sectional reduction (RAσ) of all samples were obtained (n = 72).

Perlite area ratio and coarse pearlite area ratio of the steel wire rod or coil were measured in the manner described below.

[Pearlite area fraction]

At each position of TOP, MID and BOT of the 2-ton helix as described above, 2 pieces or a total of 6 pieces whose tensile strength (T s) was maximum and minimum were sampled. First, the cross-section was embedded and polished in the transverse direction of each sample and subjected to chemical corrosion by picric acid. Next, as it was in the 4 4, the d × 1/4 position of the cross section in the transverse direction, where d is the diameter, is photographed at 400 magnification in the range of 200 μm × 200 μm using an optical microscope. As shown, the photographing was performed at 4 places forming an angle of 90 degrees to each other. The image of the photo from the optical microscope was printed, white portions were blackened by means of a black felt pen from the top of a superimposed transparent film, the transparent film was then scanned by a scanner in a PC, the image was analyzed by image analysis software ("Image Pro Plus "manufactured by Media Cybernetics, Inc.) and the perlite area fraction was then obtained. The portion where the image of the photo from the optical microscope was white was assigned to ferrite and lower bainite. The average value was obtained from the perlite area ratio of 4 fields of view per sample. In the present invention, the average value was calculated from the perlite area of a total of 6 samples (4 digits each). When a decarburized layer existed in the surface layer, the completely decarburized section determined in accordance with JIS G 0058 was excluded from the measuring sections. The area ratio was given in Table 3 according to the following criteria (the column "Rolled wire microstructure" in the table).
P: The area ratio of the pearlite microstructure is 95% or more
P + B: The area ratio of the pearlite microstructure is less than 95% and a bainite microstructure is formed
P + B + M: The perlite microstructure area fraction is less than 95%, and a bainite microstructure and a martensite microstructure are formed

[Area fraction of pearlite having an interlamellar spacing of 0.9 μm or more]

At each position (TOP, MID, BOT) of the 2-ton helix as described above, 1 part or a total of 3 parts whose tensile strength (TS) was minimum was sampled. As with the perlite area ratio described above, polishing and chemical corrosion were performed. Next, as it was in the 4 is shown, the d × 1/4 position of the cross section in the transverse direction, where d is the diameter, with a scanning electron microscope (SEM, magnification: 2000 times, range of 50 microns × 40 microns) photographed. As shown, the photographing was performed at 4 places.

Next, the area ratio of a pearlite colony having an interlamellar spacing of 0.9 μm or more at at least one location in the perlite microstructure of each photograph (FIG. 5 ) receive. The SEM image was printed, the perlite colonies sections with an inter-lamellar spacing of 0.9 μm or more at at least one location were blackened from the top of a superimposed transparent film by means of a black felt-tip pen, and the transparent film was then scanned by a scanner PC was scanned, the image was binarized by the image analysis software, and the area ratio of pearlite having an inter-lamella distance of 0.9 μm or more was obtained thereafter. In the present invention, the average value was calculated from the area ratio of perlite having an interlamellar spacing of 0.9 μm or more out of a total of 3 samples (4 digits each). Although locally having a pro-eutectoid ferritic microstructure, the area ratio of coarse pearlite having an interlamellar spacing of 0.9 μm or more was obtained after the case was excluded in advance. With respect to the inter-blade pitch, a line segment was drawn to be perpendicular to the blade, and the center distance of cementite sandwiching ferrite was measured.

The SV processability, the drawability into a wire, and the bendability after the drawing of each rolled wire rod were evaluated in the manner described below.

(Peel treatment capability)

The peelability was evaluated by the presence / absence of wire breakage in the peeling treatment when a total amount of 1 ton on the TOP side of the filament (total filament × 1/2) was subjected to a peeling treatment without performing a heat treatment ("SV characteristics " in the table).
A (passed, excellent peelability): helix where wire break did not occur
F (failed, poor peelability): helix at which a wire break occurred

(Pulling power to a wire)

The drawability to a wire was evaluated by the presence / absence of wire breakage in the drawing, when the filament after the peeling treatment was subjected to an annealing treatment at 600 ° C for 3 hours, then subjected to a pickling treatment and a phosphatizing treatment and by a continuous drawing machine having a Cross-section reduction of 15 to 20% per 1 pass was taken to 85% of the total cross-section reduction ("drawability to a wire" in the table). Further, "-" in the table means that a drawing operation was not performed because wire break occurred due to the above-described peeling treatment ("SV characteristics" are "F" in the table).
A (passed, excellent drawability to a wire): helix, in which a wire break did not take place
F (failed, poor drawability to a wire): Wendel, in which a wire break took place

(Bending capacity after drawing)

The bending ability after drawing was evaluated by the number of breakages when self-winding was performed for 1000 turns after drawing (number of breaks in the self-diameter winding) ("spiraling properties" in the table). Further, "-" in the table indicates that a bending operation after drawing was not performed because of wire breakage due to the above-described peeling treatment ("F" in the table) or wire breakage due to the drawing operation ("F" in FIG Table).
Number of fractions of 0 (excellent coil forming properties): coil in which no breakage occurred in the self-wound coil
Fraction number of 1 or more (poor helix forming properties): Helix in which breakage occurred at the self-diameter winding

The test no. 1 to 10 and 25 to 29 are test specimens which satisfy the requirement of the present invention, and they were excellent in peelability (SV properties), wire drawability, and post-draw bendability (helical properties).

In the test no. 11 grew the austenitic grains, since the arrangement temperature was high. As a result, the necking (RA) decreased, the ductility deteriorated, causing wire breakage to occur at about 80% of the neck drawdown, and the drawability to a wire was poor.

In the test no. 12, the area ratio of coarse pearlite became high because the temperature of the inlet of the cover was slow to cool the dense portion of the coil slowly. In this test piece, the peeling treatment ability (SV properties) and the drawability to a wire were excellent. Since the internal defects in the drawn microstructure were large in number, the bending ability after drawing (coil forming properties) was poor.

The test no. Fig. 13 is an example in which the temperature of the inlet of the cover for slowly cooling the dense portion of the coil was low. Therefore, the perlite area fraction of the loose part of the helix and a supercooled microstructure, such as, for example, decreased. B. bainite was formed. As a result, as the ductility deteriorated, wire breakage occurred at about 75% of necking on drawing and the drawability to a wire was poor.

The test no. 14 is an example in which the difference in temperature of the inlet of the cover for slow cooling of the dense portion of the coil and the loose portion of the coil was large. In this example, the standard deviation of tensile strength (TSσ) and the standard deviation of necking (RAσ) became large. In this test piece, the peeling treatment ability (SV properties) and the drawability to a wire were excellent. However, since the internal defects in the drawn microstructure were large in number, the bending ability after drawing (coil forming properties) was poor.

The test no. Fig. 15 is an example in which the average cooling speed from the placing to the inlet of the slow cooling cover was small. Therefore, the growth of the austenitic grains continued and the areal proportion of coarse pearlite became high. As a result, the necking (RA) decreased, the ductility deteriorated, wire break occurred at about 70% of the neck drawdown, and the drawability to a wire was poor.

The test no. Fig. 16 is an example in which the average cooling speed from arranging the loose part of the coil to the inlet of the slow cooling cover was high. Therefore, the perlite area ratio of the loose part of the helix was reduced and a supercooled microstructure such. B. bainite was formed. As a result, the ductility deteriorated, wire breakage occurred at about 75% of the neck drawdown, and the drawability to a wire was poor.

The test no. 17 is an example in which the cooling speed of the dense portion of the coil and the loose portion of the coil within the slow cooling cover was too high. Therefore, the cooling continued before the pearlite conversion was completed, the perlite area ratio of the loose portion of the coil decreased, and a supercooled microstructure such B. bainite was formed. As a result, the ductility deteriorated, and wire-breakage occurred in the peeling treatment.

The test no. Fig. 18 is an example where the hold time within the slow cooling cover was short. Therefore, the slow cooling was completed before the pearlite conversion was completed, the cooling rate became high, the perlite area ratio of the loose portion of the coil decreased, and a supercooled microstructure such. B. bainite was formed. As a result, the ductility deteriorated, and wire-breakage occurred in the peeling treatment.

The test no. 19 is an example in which the C content is high. Therefore, the tensile strength (TS) became high and the necking (RA) decreased. As a result, the ductility deteriorated, a wire break occurred at about 70% of the neck drawdown, and the drawability to a wire was poor.

The test no. 20 is an example in which the Si content is high. Therefore, the tensile strength (TS) became high, the necking (RA) decreased, and the standard deviation of necking (RAσ) was wide. As a result, the ductility deteriorated and wire breakage occurred in the peeling treatment.

The test no. 21 is an example in which the content of Si and Mn is high. Furthermore, the test no. 22 shows an example in which the Ni content is high. In these examples, the quenching ability became excessively high, the perlite conversion within the slow cooling cover was not complete, a supercooled microstructure such B. a bainite microstructure, was formed, and the area ratio of perlite was reduced. As a result, the tensile strength (TS) became high, the necking (RA) decreased and its distribution (the standard deviation of tensile strength (TSσ) and the standard deviation of necking (RAσ)) also became large, and wire breakage occurred in the peeling treatment.

The test no. 23 is an example in which the Cr content is high. Therefore, the precipitation of a Cr-based carbide became excessive and the ductility deteriorated. As a result, the ductility deteriorated, wire breakage occurred at about 75% of the neck drawdown, and the drawability to a wire was poor.

The test no. 24 is an example where the V content is high. Therefore, the precipitation of a V-based carbide became excessive and the ductility deteriorated. As a result, the ductility deteriorated, a wire break occurred at about 80% of the neck drawdown, and the drawability to a wire was poor.

Claims (5)

A steel wire rod for a high-strength spring, which has an excellent drawability to a wire and bending ability after drawing, comprising: C: 0.5 to 0.8% (% is% by mass, hereinafter, the same applies to the chemical composition), Si: 1.5 to 2.5%, Mn: 0.5 to 1.5%, Ni: 0.05 to 0.5%, Cr: 0.05 to 2.5%, and V: 0.05 to 0.5%, with the remainder being iron and unavoidable impurities, the perlite microstructure area ratio being 95% or more in total area, tensile strength (TS) and necking (RA) in the following expression (1) satisfy the standard deviation of tensile strength (TSσ) and the standard deviation of the cross-sectional reduction (RAσ) the following expression (2), and the area ratio of the pearlite microstructure having an interlamellar distance of 0.9 microns or more, which at the position of d × 1/4, where d is the diameter of the steel wire rod i st, 20% or less. TS ≤ 1250 MPa, RA ≥ 35% (1) TSσ ≤ 55.0, RAσ ≤ 6.0 (2) The steel wire rod for a high strength spring according to claim 1, further comprising at least one member selected from the group consisting of Nb: more than 0% to 0.10%, Mo: more than 0% to 0.50%, Cu: more than 0% to 0.50% and B: 0.0010 to 0.0100%, is selected. High strength spring obtained from the steel wire rod for a high strength spring according to claim 1 or 2. A method for producing a steel wire rod for a high-strength spring according to claim 1 or 2, comprising the steps of: winding a steel rod wire after hot rolling having the chemical composition according to claim 1 or 2 into a helical form at an assembly temperature of 750 to 890 ° C, thereafter cooling the dense portion of the coil and the loose portion of the coil on a cooling conveyor to a starting temperature of slow cooling with an average cooling rate of 2.0 to 10.0 ° C / second and then slowly cooling the steel wire rod for 120 seconds or more at an average cooling rate of 1.0 ° C / second or less, wherein the starting temperature of the slow cooling is set so that the temperature of the dense portion of the coil and the loose portion of the coil within of the range of 600 to 650 ° C, and the temperature difference of the dense portion of the coil and the loose portion of the coil is 30 ° C or less. Method for producing a high-strength spring, comprising the steps: Subjecting the steel wire rod for a high-strength spring according to claim 1 or 2 to a peeling treatment without performing a heat treatment, thereafter performing soft annealing or high frequency heating, then performing a drag to a wire and then performing a quench and temper treatment and forming into a spring.

Images