CN117751206A

CN117751206A - Wire rod for spring, steel wire for spring, spring with improved strength and fatigue limit, and method for manufacturing same

Info

Publication number: CN117751206A
Application number: CN202280049907.XA
Authority: CN
Inventors: 李埈模; 崔锡欢; 崔榠洙
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-06-02
Filing date: 2022-05-26
Publication date: 2024-03-22
Also published as: KR20220163153A; WO2022255727A1; EP4332265A1

Abstract

Disclosed are a wire rod for a spring and a steel wire for a spring, which have improved strength and fatigue limit, a spring, and a method of manufacturing the same. The disclosed wire for springs with improved strength and fatigue limit according to one embodiment comprises, in weight percent (wt.%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr.ltoreq.1.8% and 0.05 at.. Ltoreq.Mo+W.ltoreq.0.15 at% can be satisfied.

Description

Wire rod for spring, steel wire for spring, spring with improved strength and fatigue limit, and method for manufacturing same

Technical Field

The present disclosure relates to a wire and a wire for a spring, a spring having improved strength and fatigue limit, and a method of manufacturing the same, and more particularly, to a wire and a wire for a spring, and a spring having ultra-high strength and excellent workability of 2,200mpa level, such that nitriding is easy at high temperature, and having improved nitriding characteristics and fatigue limit, and a method of manufacturing the same.

Background

With the development of light vehicles, springs used in the transmission and engine valves of the vehicle are also required to have high strength in order to meet the continuing demand for light vehicle components. However, as the strength of the spring material increases, the wire diameter decreases to increase sensitivity to inclusions, thereby reducing the fatigue limit. That is, there is a limit to increase the fatigue limit by increasing the strength. To overcome this, spring manufacturers have attempted to increase the fatigue limit of materials for springs by increasing the surface hardness while maintaining strength by nitriding.

Although nitriding of other parts is generally performed at a temperature higher than 500 ℃, nitriding of steel for springs is performed at a temperature of 420 ℃ to 460 ℃ to prevent strength degradation, and is continued for a long time exceeding 10 hours to obtain a sufficient nitrogen penetration depth.

Since the tempering heat treatment temperature of the steel for general springs is 450 ℃ or less, the heat treatment at a temperature of 420 ℃ to 450 ℃ for a long time may decrease the strength of most springs, and thus a high alloy material containing an element capable of improving softening resistance by forming carbide should be used. However, in the case where a large amount of carbide-forming elements such as Mo and V is used, a decrease in strength can be suppressed during nitriding, but a low-temperature structure may be formed by a center segregation region, and a problem of a decrease in area reduction may be caused.

In addition, since the high-temperature heat treatment process is repeated when processing the spring material, problems may occur in controlling the prior austenite grain size (prior austenite grain size, PAGS), and a technique of controlling carbide is required during the heat treatment.

Meanwhile, spring manufacturers need to shorten the nitriding time by nitriding at a temperature as high as possible to shorten the nitriding time, and also need high-strength wires that do not cause productivity problems in the field.

Therefore, there is a need to develop wires and steel wires having excellent qualities such as strength and workability and improved nitriding characteristics and fatigue limit.

(patent document 0001) Korean patent laid-open No. 10-2000-0043776 (published 7/15/2000)

Disclosure of Invention

Technical problem

In order to solve the problems as described above, wires, steel wires and springs each having excellent strength and workability so as to be easily nitrided at high temperature and having improved nitriding characteristics and fatigue limit, and a method of manufacturing the same are provided.

Technical proposal

According to one aspect of the present disclosure, a wire for a spring having improved strength and fatigue limit comprises, in weight percent (wt.%): 0.6 to 0.7% C, 2.0 to 2.5% Si, 0.2 to 0.7% Mn, 0.9 to 1.5% Cr, 0.015% or less P, 0.01% or less S, 0.01% or less Al, 0.01% or less N, 0.25% or less Mo, 0.2 5% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr is satisfied by 1.8% or less, mo+W is satisfied by 0.05 at% or less and 0.15 at% or less, 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Within the area, satisfy C>0.85％、Si>3.0％、Mn>0.8% and Cr>The proportion (wt%) of the area of one or more of 2.0% is 10% or less.

In this regard, the wire rod may include a pearlite structure of 80% or more and a rest of a bainite structure or a martensite structure in terms of an area fraction.

In this regard, the prior austenite average grain size may be 20 μm or less.

In this respect, the number of carbonitrides distributed in a cross section parallel to the longitudinal direction within a surface depth of 1mm and having a maximum diameter of 15 μm or more may be less than 2/cm ² 。

In this regard, the tensile strength may be 1,400mpa or less, and the reduction of area may be 35% or more.

According to another aspect of the present disclosure, a method for manufacturing a wire for a spring having improved strength and fatigue limit includes: a bloom is prepared by continuous casting of molten steel comprising, in weight percent (wt%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities; heating the bloom at a temperature above 1,200 ℃ and rolling the bloom to prepare a billet (billet); heating the billet at a temperature of 1,030 ℃ or higher and rolling the billet at a temperature of 1,000 ℃ or lower to prepare a wire rod; coiling the rolled wire rod at a temperature of 800-900 ℃; and cooling the coiled wire at a rate of 0.5 ℃/sec to 2 ℃/sec.

In this regard, the continuous casting process may include soft reduction at a total reduction of 20mm or more.

In this regard, light reduction may be performed so that each roller is rolled by reducing 4mm or less and may have a cumulative reduction of 60% or more at a solidification fraction of 0.6 or more.

According to another aspect of the present disclosure, a steel wire for a spring having improved strength and fatigue limit comprises, in weight percent (wt.%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr is satisfied by 1.8% or less, 0.05 at% or less of Mo+W is satisfied by 0.15 at% or less, and the steel wire contains a tempered martensitic structure of 85% or more and a balance of austenitic structure in terms of area fraction.

In this regard, the prior austenite average grain size may be 15 μm or less.

In this respect, at 100. Mu.m ² The number of carbides in the region of (a) may be 10 to 50, the maximum diameter of the carbides may be 5nm to 50nm, and the content of V or Nb may be 10 at% or more.

In this regard, the tensile strength may be 2,100mpa or more, and the reduction of area may be 45% or more.

According to another aspect of the present disclosure, a method for manufacturing a steel wire for a spring having improved strength and fatigue limit includes: performing LP heat treatment on the wire rod; drawing the LP heat-treated wire rod to prepare a steel wire; and QT heat treating the steel wire, wherein the LP heat treatment comprises: a primary austenitizing process heated to a temperature of 950 ℃ to 1100 ℃ in 3 minutes and maintained for 3 minutes or less; and a process of passing the wire rod primarily austenitized through a Pb bath at a temperature of 650 to 700 ℃ in 3 minutes.

In this regard, in the LP heat treatment, the pearlite transformation completion time may be less than 130 seconds.

In this aspect, the method may further comprise subjecting the wire to LA heat treatment prior to the LP heat treatment, wherein the LA heat treatment may further comprise heat treatment at a temperature of 650 ℃ to 750 ℃; and (5) carrying out acid washing.

In this regard, the QT heat treatment may include a secondary austenitizing process that is heated to a temperature of 900 ℃ to 1000 ℃ within 3 minutes and maintained for 3 minutes or less; and a primary oil quenching process performed at 70 ℃ or less; a tempering process of heating to a temperature of 450 ℃ to 550 ℃ within 3 minutes and maintaining for 3 minutes or less; and a secondary oil quenching process performed at 70 ℃ or lower.

According to another aspect of the present disclosure, a spring having improved strength and fatigue limit may comprise, in weight percent (wt.%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr.ltoreq.1.8% and 0.05 at.ltoreq.Mo+W.ltoreq.0.15 at%, and a fatigue limit subjected to repeated stress of more than 1000 ten thousand times is 700MPa or more.

According to another aspect of the present disclosure, a method for manufacturing a spring with improved strength and fatigue limit includes: cold forming a steel wire according to one embodiment of the present disclosure in the form of a spring; performing stress relief heat treatment on the formed spring; and nitriding at a temperature of 420 ℃ to 450 ℃ for 10 hours or more.

Further, according to the method for manufacturing a spring having improved strength and fatigue limit, the fatigue limit can be increased by 10% or more after nitriding.

Advantageous effects

According to one aspect of the present disclosure, there are provided a wire rod, a steel wire, and a spring capable of suppressing formation of a low-temperature structure in a central region by reducing center segregation and obtaining excellent reduction of area and tensile strength of 2,200mpa or more, and a manufacturing method thereof.

According to another aspect of the present disclosure, there are provided a wire rod, a steel wire, and a spring having improved nitriding characteristics and fatigue limit by controlling grain size and the number of precipitates, and a method of manufacturing the same.

Detailed Description

The wire for springs with improved strength and fatigue limit according to the present disclosure comprises in weight percent (wt.%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr.ltoreq.1.8% and 0.05 at.ltoreq.Mo+W.ltoreq.0.15 at%, and 1mm in a central region of a cross section perpendicular to the longitudinal direction are satisfied ² Within the area, satisfy C>0.85％、Si>3.0％、Mn>0.8% and Cr>The proportion (wt%) of the area of one or more of 2.0% is 10% or less.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present disclosure will now be described. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

The terminology used herein is for the purpose of describing particular embodiments only. Thus, unless the expression used in the singular has a distinct meaning in the context, it encompasses plural expressions. Furthermore, it should be understood that terms such as "comprises" or "comprising" are intended to mean that there is a feature, process, function, component, or combination thereof disclosed in the specification, and are not intended to exclude the possibility that one or more other features, processes, functions, components, or combinations thereof may be present or added.

Meanwhile, unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure pertains. Thus, unless expressly so defined herein, these terms should not be construed in an idealized or overly formal sense. As used herein, the singular is intended to include the plural unless the context clearly indicates otherwise.

Furthermore, the terms "about," "substantially," and the like, as used throughout the specification, mean that when an allowable error of natural manufacturing and substance is introduced, such allowable error corresponds to or is similar to the value, and such value is intended for the purpose of clearly understanding the present invention or preventing an unintended infringer from illegally using the disclosure of the present invention.

A wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure comprises, in weight percent (wt.%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities.

Hereinafter, the reason for numerical limitation concerning the content of the alloy element will be described. Hereinafter, unless otherwise indicated, units are% by weight.

The content of C is 0.6% to 0.7%.

C is an element that increases the strength of the material and may be added in an amount of 0.6% or more to obtain sufficient material strength. However, excessive C may cause significant deterioration of impact characteristics after Quenching and Tempering (QT) heat treatment and an increased possibility of forming a low-temperature structure during the manufacturing process of the wire rod, thereby deteriorating the quality of the wire rod. Further, if the C content is excessive, the heat treatment time of the LP heat treatment (one of the wire manufacturing processes) is significantly increased, thereby decreasing productivity. In view of this, the upper limit of the C content can be controlled to 0.7%.

The Si content is 2.0% to 2.5%.

Si used for deoxidation of steel is also effective for obtaining strength by solid solution strengthening, and may be added in an amount of 2.0% or more to suppress a decrease in strength during nitriding and improve the deformation resistance of the spring. However, excessive Si may cause surface decarburization of the material and deterioration of workability. In view of this, the upper limit of the Si content can be controlled to 2.5%.

The Mn content is 0.2% to 0.7%.

Mn as a hardenability enhancing element may be added in an amount of 0.2% or more to obtain hardenability of the material, form a high-strength tempered martensitic structure, and make S harmless by fixing S. However, excessive Mn may cause degradation of quality due to segregation. In view of this, the upper limit of the Mn content can be controlled to 0.7%.

The Cr content is 0.9% to 1.5%.

Cr is a hardenability enhancing element together with Mn, and may be added in an amount of 0.9% or more to enhance the softening resistance of the steel. However, excessive Cr may cause a significant decrease in toughness of the steel wire and promote formation of low-temperature structure when the wire is cooled. In view of this, the upper limit of the Cr content may be controlled to 1.5%.

The content of P is 0.015% or less.

P is an element segregated in grain boundaries to cause deterioration of toughness and resistance to hydrogen-induced delayed fracture of the material, and thus it is desirable to remove P from the steel material. In view of this, the upper limit of the P content can be controlled to 0.015%.

The content of S is 0.01% or less.

Like P, S can segregate in grain boundaries to cause deterioration of toughness and deterioration of hydrogen-induced delayed fracture resistance of the material by formation of MnS. In view of this, the upper limit of the S content can be controlled to 0.01%.

The content of Al is 0.01% or less.

Although Al increases purity as a powerful deoxidizing element by removing oxygen from the steel, al may be formed thereby ₂ O ₃ Inclusions, thereby causing a decrease in fatigue resistance. In view of this, the upper limit of the Al content can be controlled to 0.01%.

The content of N is 0.01% or less.

Although N is an impurity, N combines with Al or V to form coarse AlN or VN precipitates that do not melt during heat treatment. In view of this, the upper limit of the N content may be controlled to 0.01%.

The content of Mo is 0.25% or less.

In the nitriding material, mo is an element that improves softening resistance and forms carbide with V to improve strength during tempering. Furthermore, mo forms MC carbides and maintains the strength of the material even after long heat treatment. However, excessive Mo suppresses the formation of pearlite structure, and thus the quality of the wire rod may be deteriorated due to the formation of low-temperature structure after rolling the wire rod. In addition, excessive Mo suppresses pearlite transformation during LP heat treatment before drawing to increase pearlite transformation time, resulting in significant reduction of productivity. In view of this, the upper limit of the Mo content may be controlled to 0.25%.

The content of W is 0.25% or less.

As with Mo, W, as an element that improves softening resistance together with Mo in the nitriding material, forms MC carbide so as to maintain the strength of the material even after long-time heat treatment. However, an excessive amount of W may inhibit the formation of pearlite in the wire rod and promote the formation of low temperature tissues. In view of this, the upper limit of the W content may be controlled to 0.25%.

The content of V is 0.05% to 0.2%.

V, which is an element that improves softening resistance together with Mo in the nitriding material, forms carbide to increase strength during tempering, and can maintain strength even after nitriding for a long time. Further, unlike Mo and W, V has a high solid solution temperature for carbide holding the prior austenite grain size. Further, since V accelerates pearlite transformation, formation of low temperature tissues can be suppressed at the time of producing the wire rod, constant temperature transformation time can be reduced during LP heat treatment, and productivity can be improved during the wire manufacturing process, so V can be added in an amount of 0.05% or more. However, if the V content is excessive, coarse carbonitrides may be formed during the wire production process, and the temperature should be raised by heating when rolling the wire. In view of this, the upper limit of the V content may be controlled to 0.2%.

The content of Nb is 0.05% or less.

Nb, as a carbonitride forming element, has a higher solid solution temperature than V and thus has an effect of controlling the prior austenite grain size better than V. However, if the Nb content is excessive, a problem of increasing the prior austenite grain size may occur. In view of this, the upper limit of the Nb content may be controlled to 0.05%, and the addition of Nb may be omitted in the case where the prior austenite grain size is controlled during the manufacturing process.

The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may contain unintended impurities that are inevitably incorporated from the raw materials or the surrounding environment, and thus the addition of other alloy components is not precluded. No specific mention of impurities is made in this disclosure, as they are known to any person skilled in the art of manufacture.

Meanwhile, the wire rod having improved strength and fatigue limit according to one embodiment of the present disclosure may satisfy mn+cr < 1.8% in weight percent (wt%).

If the sum of Mn and Cr exceeds 1.8%, a low-temperature structure such as bainite or martensite may be formed during the process of cooling the wire rod, and the pearlite transformation completion time may increase during the LP heat treatment. Further, if the sum of Mn and Cr exceeds 1.8%, the carbon equivalent (Ceq) increases significantly to limit the addition amount of W and Mo, and thus the decrease in the material strength cannot be prevented during nitriding. Further, if the carbon equivalent (Ceq) is increased, the pearlite transformation time is increased, so that a complete pearlite structure cannot be obtained during the process of cooling the wire rod, and the LP heat treatment time is increased, thereby causing a decrease in productivity.

Further, the wire rod having improved strength and fatigue limit according to one embodiment of the present disclosure may satisfy 0.05 at% mo+w% 0.15 at%. In this regard, atomic% refers to atomic weight percent.

If the sum of the atomic% of Mo and W is less than 0.05 atomic%, the decrease in strength cannot be suppressed during nitriding, and thus the steel material cannot be used as nitrided steel. In contrast, if the sum of the atomic% of Mo and W exceeds 0.15 atomic%, the carbon equivalent increases to increase the pearlite transformation time, thereby causing a problem of decreasing productivity.

Meanwhile, the reason why the control is performed by atomic% is to control the ratio of Mo and W to carbide to 1:1, because Mo and W contribute to the increase in strength by forming carbide in the form of MC (where m=mo or W and c=carbon).

Further, in the wire rod according to one embodiment of the present disclosure, the pearlite transformation completion time during the patenting (LP) heat treatment may be less than 130 seconds. In this regard, the LP heat treatment process may include a process of heating at a temperature of 950 ℃ to 1100 ℃ and rapidly cooling to a temperature of 650 ℃ to 750 ℃. If the pearlite transformation completion time exceeds 130 seconds during the LP heat treatment, a problem of lowering productivity may occur.

Further, the wire rod having improved strength and fatigue limit according to one embodiment of the present disclosure may include a pearlite structure at an area fraction of 80% or more.

Further, the prior austenite average grain size of the wire rod having improved strength and fatigue limit according to one embodiment of the present disclosure may be 20 μm or less. When the prior austenite average grain size exceeds 20 μm, the time of the LP heat treatment process increases and a problem of deteriorating the workability of the wire rod may occur.

Furthermore, in accordance with the present disclosureIn the wire rod with improved strength and fatigue limit of one embodiment of the disclosure, 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Within the area, satisfy C>0.85％、Si>3.0％、Mn>0.8% and Cr>The proportion (wt%) of the area of one or more of 2.0% may be 10% or less.

When the above area ratio exceeds 10%, deterioration of material quality may be caused, for example, formation of a low-temperature structure due to center segregation, and deterioration of workability due to reduction of area shrinkage (reduction of area, RA) after manufacturing the steel wire, thereby increasing the frequency of breakage at the time of working the spring. In addition, when the above area exceeds 10%, carbide action may be reduced due to concentration of carbide forming elements at the center.

Further, in the wire rod with improved strength and fatigue limit according to an embodiment of the present disclosure, the number of carbonitrides having a maximum diameter of 15 μm or more distributed in a cross section parallel to the longitudinal direction within a surface depth of 1mm may be less than 2/cm ² 。

In the case where carbonitrides having a diameter of 15 μm or more are present on the surface of the wire rod, fatigue fracture may occur in the material. Therefore, it may be preferable that the number of carbonitrides having a maximum diameter of 15 μm or more present in a cross section parallel to the longitudinal direction within a surface depth of 1mm may be less than 2/cm ² 。

Further, a wire rod having improved strength and fatigue limit according to one embodiment of the present disclosure may have a tensile strength of 1,400mpa or less and a Reduction of Area (RA) of 35% or more.

Hereinafter, a method for manufacturing a wire rod for a spring having improved strength and fatigue limit according to one embodiment of the present disclosure will be described.

A method for manufacturing a wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure includes: a bloom is prepared by continuous casting of molten steel comprising, in weight percent (wt%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities; heating the blooming square billet at a temperature of more than 1,200 ℃ and rolling the blooming square billet to prepare a small square billet; heating the billet at a temperature of 1,030 ℃ or higher and rolling the billet at a temperature of 1,000 ℃ or lower to prepare a wire rod; coiling the rolled wire rod at a temperature of 800-900 ℃; and cooling the coiled wire at a rate of 0.5 ℃/sec to 2 ℃/sec.

The reason for the numerical limitation regarding the content of the alloy element is as described above, and hereinafter, the process of the manufacturing method thereof will be described in more detail.

According to one embodiment of the present disclosure, the continuous casting process may include soft reduction at a total reduction of 20mm or more.

A method in which a slab having an uncured layer is cast in a final solidification stage in a continuous casting machine while gradually compressing the slab at a total reduction amount and a compression rate approximately corresponding to the sum of a solidification shrinkage amount and a thermal shrinkage amount by passing the slab through a set of rolls is called soft reduction. In this regard, the total reduction refers to the reduction from the start to the end of compression. When the total reduction is less than 20mm, it is difficult to obtain a segregation removing effect by the soft reduction, and thus the total reduction under the soft reduction can be controlled to 20mm or more to minimize segregation of the wire rod.

Further, according to one embodiment of the present disclosure, the soft reduction may be performed such that each roller is reduced by 4mm or less, and the cumulative reduction is 60% or more at a coagulation fraction of 0.6 or more. The solidification fraction refers to the ratio of the weight of solid phase molten steel to the total weight of the entire molten steel.

Meanwhile, if the casting speed is too low, solidification is completed before the soft reduction, so that the ratio of the liquid phase to the solid phase is too low to obtain the segregation removal effect by the soft reduction. Conversely, if the casting speed is too high, the ratio of the liquid phase to the solid phase becomes too high, resulting in segregation caused by solidification shrinkage. Therefore, it is necessary to control the casting speed so that the reduction is 60% or more at a solidification fraction of 0.6 or more.

The amount of coolant is appropriately adjusted so that solidification can be completed until the light depression is completed. The mold electromagnetic stirrer (Mold Electro Magnetic Stirrer, mold-EMS) and strand-EMS may be set according to conditions for a conventional spring or may be arbitrarily set according to equipment.

Meanwhile, unlike a conventional spring wire rod, spring steel for nitriding contains a large amount of high alloy elements, and carbonitrides therein need to be controlled. Thus, according to one embodiment of the present disclosure, internal carbonitrides may be minimized by heating the prepared blooms at a temperature above 1,200 ℃ and rolling the heated blooms into billets.

Subsequently, the billet may be heat-treated at a temperature of 1,030 ℃ or more and rolled at a temperature of 1,000 ℃ or less to prepare a wire rod.

If the heat treatment temperature of the billet is lower than 1030 ℃, the component V in the material cannot be sufficiently melted, and thus a solid solution of carbide cannot be formed, thereby causing a problem of deterioration of softening resistance in the final product. The rolling of the billets into wire rods can be performed at temperatures below 1000 ℃ to perform coiling at temperatures below 900 ℃.

Subsequently, the rolled wire rod may be coiled at a temperature of 800 ℃ to 900 ℃.

As the difference between the rolling temperature and the coiling temperature of the prepared wire rod increases, severe F decarburization may be caused by partial supercooling. In view of this, the process of coiling the rolled wire rod may be performed at a temperature of 800 to 900 ℃.

The coiled wire may then be cooled at a rate of 0.5 ℃/sec to 2 ℃/sec.

Unlike the conventional wire rod for springs, the steel for springs used for nitriding contains a large amount of high alloy elements, and thus it is necessary to suppress the formation of low-temperature tissues. Decarburization may occur if the coiled wire is cooled at a rate of less than 0.5 ℃/sec. Conversely, if the cooling rate exceeds 2 ℃/sec, the material may fracture due to the low temperature tissue.

Hereinafter, a steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure will be described.

A steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure may include, in weight percent (wt%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities.

Furthermore, the steel wire for springs having improved strength and fatigue limit according to one embodiment of the present disclosure may satisfy mn+cr < 1.8%.

Further, the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure may satisfy 0.05 at% or less mo+w or less than 0.15 at%.

The reasons for numerical limitation regarding the content of the alloy element are as described above.

Further, the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure may include a tempered martensite structure of 85% or more and an austenite structure of the balance in terms of area fraction.

Further, the prior austenite average grain size of the steel wire for a spring having improved strength and fatigue limit according to one embodiment of the present disclosure may be 15 μm or less.

Further, in the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure, 1mm in the central area of the cross section perpendicular to the longitudinal direction ² Within the area, satisfy C>0.85％、Si>3.0％、Mn>0.8% and Cr>The proportion (wt%) of the area of one or more of 2.0% may be 10% or less.

If the proportion (wt%) of the above area exceeds 10%, deterioration in material quality, such as formation of a low-temperature structure due to center segregation, may be caused, and deterioration in workability may be caused, thereby increasing the frequency of breakage at the time of working the spring. In addition, if the proportion (wt%) of the above area exceeds 10%, carbide action may be reduced due to concentration of carbide-forming elements at the center.

Further, in the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure, the number of carbonitrides having a maximum diameter of 15 μm or more distributed in a cross section parallel to the longitudinal direction within a surface depth of 1mm may be less than 2/100 mm in length.

In the case where carbonitrides having a diameter of 15 μm or more are present on the surface of the steel wire, fatigue fracture may occur in the material. Therefore, it may be preferable to control the number of carbonitrides having a maximum diameter of 15 μm or more to less than 2/100 mm length in a section parallel to the longitudinal direction within a surface depth of 1 mm.

Furthermore, in the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure, the strength and fatigue limit are improved at 100 μm ² The number of carbides in the region of (a) may be 10 to 50, the maximum diameter of the carbides may be 5nm to 50nm, and the content of V or Nb may be 10 at% or more.

Once carbide containing V or Nb grows beyond 10nm, other carbide forming elements such as Cr and Mo are contained therein in addition to V, and thus it is necessary to appropriately distribute carbide forming elements for suppressing the prior austenite grain growth and carbide forming elements for precipitation hardening.

If the number of carbides having a maximum diameter of 5nm to 50nm is less than 10, it is difficult to control the prior austenite grain size. In contrast, if the number of carbides having a maximum diameter of 5nm to 50nm is more than 50, carbides of 5nm or less for precipitation hardening are reduced, thereby decreasing the tensile strength of the steel wire.

Further, the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure may have a tensile strength of 2,100mpa or more and a Reduction of Area (RA) of 45% or more.

Hereinafter, a method for manufacturing a steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present disclosure will be described.

A method for manufacturing a steel wire for a spring according to an embodiment of the present disclosure includes: subjecting a wire rod according to one embodiment of the present disclosure to LA heat treatment; performing LP heat treatment; drawing the wire rod to prepare a steel wire; and carrying out QT heat treatment on the steel wire.

First, a wire rod according to one embodiment of the present disclosure may be subjected to low temperature annealing (LA) at a temperature of 650 ℃ to 750 ℃.

Although not limited thereto, as the treatment time of the LA heat treatment increases, carbides coarsen, making it difficult to control the carbides during the subsequent process, so the LA heat treatment may be performed within 2 hours. By the LA heat treatment, the strength of the wire rod can be reduced to 1,200mpa or less. The LA heat treatment process may be omitted if desired.

Then, the LA heat-treated wire rod is pickled and may be subjected to patenting (LP) heat treatment.

The LP heat treatment may include a primary austenitizing process heated to a temperature of 950 ℃ to 1100 ℃ in 3 minutes and maintained for 3 minutes or less; and a process of passing the wire rod primarily austenitized through a Pb bath at a temperature of 650 to 700 ℃ in 3 minutes.

By performing an austenitizing process heated to a temperature of 950 to 1100 ℃ in 3 minutes and maintained for 3 minutes or less, an austenite structure can be obtained and carbides coarsened in the LA process can be reformed into a solid solution.

Subsequently, the wire rod which is primarily austenitized may be isothermally transformed via rapid cooling by passing through a Pb bath at a temperature of 650 ℃ to 750 ℃ within 3 minutes, and a pearlite structure may be obtained. If the Pb bath temperature is lower than 650 ℃, a low-temperature tissue may be formed. In contrast, if the Pb bath temperature is higher than 750 ℃, carbide coarsens and the strength may be lowered.

Subsequently, the LP heat treated wire rod may be drawn to prepare a steel wire. In this respect, the wire diameter of the prepared steel wire may be 5mm. The LP heat treatment may be further performed to control the wire diameter of the steel wire to 2mm or less.

Subsequently, the prepared steel wire may be subjected to QT heat treatment process to obtain tempered martensitic structure.

According to one embodiment of the present disclosure, QT heat treatment may include a secondary austenitizing process that heats to a temperature of 900 ℃ to 1000 ℃ within 3 minutes and holds for 3 minutes or less; and a primary oil quenching process performed at 70 ℃ or less; a tempering process of heating to a temperature of 450 ℃ to 550 ℃ within 3 minutes and maintaining for 3 minutes or less; and a secondary oil quenching process performed at 70 ℃ or lower.

In QT heat treatment, the austenitizing temperature may be 900 ℃ to 1000 ℃ so that fine carbides precipitated during LP heat treatment are maintained. Although not limited thereto, in QT heat treatment, the austenitizing process may be performed for 6 minutes or less.

If the tempering temperature is lower than 450 ℃ in QT heat treatment, the nitriding temperature is lowered, formation of additional carbide cannot be induced, and toughness may deteriorate. In contrast, if the tempering temperature exceeds 550 ℃ in QT heat treatment, sufficient strength cannot be obtained.

Hereinafter, a spring having improved strength and fatigue limit according to an embodiment of the present disclosure will be described.

A spring with improved strength and fatigue limit according to one embodiment of the present disclosure comprises in weight percent (wt.%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, satisfies Mn+Cr.ltoreq.1.8%, and satisfies 0.05 at.ltoreq.Mo+W.ltoreq.0.15 at%.

Further, in the spring of one embodiment of the present disclosure, the fatigue limit increases by 10% or more after nitriding. In this respect, the fatigue limit refers to a limit to withstand repeated loads more than 1000 ten thousand times during a fatigue test after designing a spring.

Further, a spring according to one embodiment of the present disclosure may have a fatigue limit of 700MPa or more, which is subjected to repeated stress more than 1000 ten thousand times.

Further, in the spring of one embodiment of the present disclosure, the strength before and after nitriding is changed to 15% or less, and the nitriding temperature may be 430 ℃ or higher.

Hereinafter, a method for manufacturing a spring having improved strength and fatigue limit according to one embodiment of the present disclosure will be described.

A method for manufacturing a spring having improved strength and fatigue limit according to one embodiment of the present disclosure includes: cold forming a steel wire according to one embodiment of the present disclosure in the form of a spring; performing stress relief heat treatment on the formed spring; nitriding the resultant.

The fatigue limit of the steel wire according to one embodiment of the present disclosure may be improved by nitriding before shot peening in the spring manufacturing process. In this regard, if the nitriding temperature is too low, nitrogen cannot penetrate into the surface properly. If the nitriding temperature is too high, the hardness of the central region of the material decreases and the desired material strength cannot be obtained. In view of this, the nitriding process may be performed at a temperature of 420 ℃ to 450 ℃ for 10 hours or more.

Hereinafter, the present disclosure will be described in more detail by way of examples. It should be noted, however, that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. As the scope of the present disclosure is determined by what is described in the claims and can be reasonably inferred therefrom.

Examples

Steel materials containing various alloy element compositions shown in table 1 below were continuously cast at a total light reduction of 10mm to 25mm to prepare blooms. The prepared blooming billets were subjected to heat treatment at 1,200 ℃ and 1050 ℃ for homogenization, and then hot rolled to a final wire diameter of 6.5mm while cooling to 850 ℃ to prepare wires having a final wire diameter of 6.5 mm. The hot rolled wire was then coiled at a temperature of 800 ℃ to 900 ℃ and cooled at a rate of 1 ℃/sec.

TABLE 1

The following table 2 shows the atomic% content and total light reduction of w+mo of examples and comparative examples. Segregation area of the wire rod prepared by analysis of 1mm in cross section perpendicular to the longitudinal direction of the wire rod in Table 2 below ² A central region. The `C segregation area` of Table 2 means 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Satisfy C in area>0.85% by weight of the total weight of the composition. The `Si segregation area` means 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Satisfy Si in area>3.0% by weight of the area. The `Mn segregation area` means 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Satisfy Mn in area>0.8% by weight of the total weight of the composition. 'Cr segregation area' means 1mm in the central region of the cross section perpendicular to the longitudinal direction ² Satisfy Cr in area>2.0% by weight of the area. The segregation area was measured by using an electron probe X-ray microanalyzer EPMA (model EMPA-1600).

TABLE 2

Referring to table 2, since examples 1 and 2 satisfy the alloy element composition and total light reduction proposed in the present disclosure, the sum of the segregation areas of C, si, mn, and Cr is not more than 10%. In contrast, since the total soft reduction of comparative example 1 is 10mm, which is less than 20mm, the sum of C, si, mn, cr segregation areas is 30%.

Table 3 below shows the tensile strength, reduction of Area (RA), central low temperature structure, prior austenite average grain size, pearlite structure, and number of carbonitrides of the prepared wire rods. The prior austenite average grain size, pearlite structure, and the number of carbonitrides were measured by using a Scanning Electron Microscope (SEM) (model JEOL, JSM-6610 LV).

The 'O' of table 3 represents the case where the area fraction of the low-temperature tissue exceeds 20%, and the 'X' represents the case where the area fraction of the low-temperature tissue is not more than 20%.

8 samples were prepared by cutting 3m long wire into 8 sections. The pearlite structure of table 3 below refers to the number of samples in which the area fraction of the pearlite structure in the microstructure of the cross section perpendicular to the longitudinal direction of each sample is 80% or more.

10 samples were prepared by cutting 10cm long wires into 10 sections each 1cm long. The number of carbonitrides in table 3 below refers to the number of carbonitrides having a maximum diameter of 15 μm or more measured in the microstructure of a section parallel to the longitudinal direction in a surface depth of 1mm of the sample.

TABLE 3 Table 3

Referring to table 3, in examples 1 and 2, a low temperature structure was not formed in the central region, and the prior austenite average grain size was not more than 20 μm. Further, according to examples 1 and 2, 6 or more samples showed a pearlite structure of 80% or more and a tensile strength of not more than 1400MPa among 8 samples, thereby indicating excellent workability. In addition, in examples 1 and 2, carbonitrides were not formed on the surface.

In contrast, according to comparative example 1, the tensile strength exceeding 1400MPa, the reduction of area of less than 35% exhibited poor workability, and a low-temperature structure was formed in the central region. In addition, according to comparative example 1, only 5 samples among 8 samples contained 80% or more of pearlite structure, and 80% or more of pearlite structure was unevenly formed.

In comparative example 2, the alloy element shown in Table 1 has a V content of less than 0.15%, and thus the prior austenite average grain size is 24 μm or more and 20 μm or more, indicating coarsening of the grains.

In comparative example 3, since the tensile strength was 1510MPa and the area reduction was only 10%, the workability was poor and a low temperature structure was formed in the central region. Further, in comparative example 3, only 2 samples among 8 samples contained 80% or more of the pearlite structure, thereby indicating that the pearlite structure was not sufficiently formed.

Subsequently, the samples of examples and comparative examples were subjected to LA heat treatment at 720 ℃ for 2 hours and acid washing, and then subjected to LP heat treatment. The LP heat treatment was performed by heating to the primary austenitizing temperature in 3 minutes and then under the conditions shown in table 4 below. In addition, table 4 shows pearlite transformation times according to LP heat treatments of examples and comparative examples. Pearlite transformation time is measured by obtaining a time-temperature-transformation (TTT) curve through an expansion assay experiment.

TABLE 4

The pearlite transformation times of examples 1 and 2 were 110 seconds and 105 seconds, respectively, less than 130 seconds, indicating excellent productivity. In contrast, the pearlite transformation time of comparative example 3 was 130 seconds, indicating that the productivity was poor to the extent that it was difficult to produce on site.

Subsequently, the LP heat treated materials of examples and comparative examples were drawn to prepare steel wires having a wire diameter of 3 mm. The prepared steel wire is subjected to a secondary austenitizing process and a primary quenching process, and then tempered and subjected to a secondary quenching process to obtain QT steel wire. The steel wire is heated to the secondary austenitizing temperature in 3 minutes, and the primary and secondary quenching processes are performed in 60 ℃ oil. The remaining process was performed under the conditions of table 5 below.

TABLE 5

Table 6 below shows the tensile strength, reduction of Area (RA) and the number of carbides of the QT steel wire prepared. In this connection, the number of carbides means that the carbide is in the range of 100. Mu.m ² Has a maximum diameter of 5nm to 50nm and contains the amount of V or Nb carbide in an amount of 10 at% or more. The number of carbides means 100 μm from the wire surface by using a Transmission Electron Microscope (TEM) of FEI Tecnai OSIRIS ² An average of 8 values measured at 8 locations in the area of (c).

TABLE 6

Referring to table 6, examples 1 and 2 exhibited excellent tensile strength of 2200MPa or more and reduction of area of 45% or more. Further, the number of carbides of examples 1 and 2 is 10 to 50.

In contrast, comparative example 1 has a reduction of area of only 32% and the number of carbides exceeds 50. According to comparative example 2, poor tensile strength of not more than 2200MPa was obtained, and the number of carbides was less than 10, thereby causing a problem that it was difficult to control the prior austenite average grain size. Comparative example 4 shows poor tensile strength of 2200MPa or less and the number of carbides exceeds 50.

Subsequently, the QT wire is cold formed in the shape of a spring and the formed spring is heat treated and nitrided at a temperature of 420 ℃ to 450 ℃.

Table 7 below shows whether the spring breaks when it is formed, the fatigue limit value and the fatigue limit after nitriding.

The fatigue limit before and after nitriding is measured under conditions of a stress ratio R (tensile/compressive) of-1 and a test speed of 30Hz to 60 Hz.

In the following table 7, 'X' indicates that no breakage occurs when the spring is formed, and 'O' indicates that breakage occurs when the spring is formed.

TABLE 7

The samples of examples 1 and 2 did not break due to excellent workability and had excellent fatigue limit of more than 650MPa before nitriding. Furthermore, the samples of examples 1 and 2 had a fatigue limit after nitriding of more than 750 MPa. Since the fatigue limit after nitriding is 10% or more higher than the fatigue limit before nitriding, excellent nitriding characteristics are obtained.

In contrast, comparative examples 1 and 2 exhibited fracture due to poor workability, and the fatigue limit after nitriding increased by less than 10% compared to the fatigue limit before nitriding.

Although the spring of comparative example 4 was not broken during processing, the fatigue limit after nitriding could not be increased by 10% or more as compared with the fatigue limit before nitriding, thereby indicating poor nitriding characteristics.

According to the disclosed embodiments, by optimizing the composition of the alloy element and the conditions of the manufacturing process, excellent tensile strength and area reduction can be obtained, and nitriding characteristics and fatigue limit can also be improved, and thus the spring can be suitably used as a material for a transmission and an engine valve of a vehicle.

[ Industrial applicability ]

According to one embodiment of the present disclosure, a wire and a wire for a spring and a spring having improved strength and fatigue limit, and a method of manufacturing the same may be provided.

Claims

1. A wire for springs having improved strength and fatigue limit, the wire comprising in weight percent (wt.%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities,

Wherein Mn+Cr is less than or equal to 1.8 percent,

satisfies that Mo+W is more than or equal to 0.05 atomic percent and less than or equal to 0.15 atomic percent,

1mm in the central region of the cross section perpendicular to the longitudinal direction ² Within the area, satisfy in weight percent C>0.85％、Si>3.0％、Mn>0.8% and Cr>The proportion of the area of one or more of 2.0% is 10% or less.

2. The wire rod according to claim 1, wherein the wire rod comprises 80% or more of a pearlite structure and the balance of a bainite structure or a martensite structure in an area fraction.

3. The wire rod according to claim 1, wherein the prior austenite average grain size is 20 μm or less.

4. The wire rod according to claim 1, wherein the number of carbonitrides distributed in a cross section parallel to the longitudinal direction within a surface depth of 1mm having a maximum diameter of 15 μm or more is less than 2/cm ² 。

5. The wire rod of claim 1, wherein the tensile strength is 1,400mpa or less and the reduction of area is 35% or more.

6. A method for manufacturing a wire for a spring with improved strength and fatigue limit, the method comprising:

a bloom is prepared by continuous casting of molten steel comprising, in weight percent (wt%): 0.6% to 0.7% of C, 2.0% to 2.5% of Si, 0.2% to 0.7% of Mn, 0.9% to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities;

Heating the blooming square billet at a temperature of 1,200 ℃ or higher and rolling the blooming square billet to prepare a small square billet;

heating the billet at a temperature of 1,030 ℃ or higher and rolling the billet at a temperature of 1,000 ℃ or lower to produce a wire rod;

coiling the rolled wire rod at a temperature of 800-900 ℃; and

the coiled wire is cooled at a rate of 0.5 to 2 deg.c/sec.

7. The method of claim 6, wherein the continuous casting process comprises soft reduction at a total reduction of 20mm or greater.

8. The method according to claim 7, wherein the light reduction is performed such that each roller is rolled by reducing 4mm or less and has a cumulative reduction of 60% or more at a solidification fraction of 0.6 or more.

9. A steel wire for springs having improved strength and fatigue limit, the steel wire comprising in weight percent (wt%): 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05 to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities,

Wherein Mn+Cr is less than or equal to 1.8 percent,

satisfies 0.05 atom% or less Mo+W or less than 0.15 atom%

The steel wire comprises a tempered martensitic structure of 85% or more and an austenitic structure of the balance in area fraction.

10. The steel wire according to claim 9, wherein the prior austenite has an average grain size of 15 μm or less.

11. The steel wire according to claim 9, wherein the number of carbonitrides distributed in a cross section parallel to the longitudinal direction within a surface depth of 1mm having a maximum diameter of 15 μm or more is less than 2/cm ² 。

12. A steel wire according to claim 9, wherein the thickness is 100 μm ² The number of carbides in the region of (a) is 10 to 50, the maximum diameter of the carbide is 5nm to 50nm, and the content of V or Nb is 10 at% or more.

13. The steel wire according to claim 9, wherein the tensile strength is 2,100mpa or more and the reduction of area is 45% or more.

14. A method for manufacturing a steel wire for a spring with improved strength and fatigue limit, the method comprising:

subjecting the wire rod according to any one of claims 1 to 5 to LP heat treatment;

drawing the LP heat-treated wire rod to prepare a steel wire; and

QT heat treatment is performed to the steel wire,

wherein the LP heat treatment comprises:

a primary austenitizing process heated to a temperature of 950 ℃ to 1100 ℃ in 3 minutes and maintained for 3 minutes or less; and

a process of passing the wire rod primarily austenitized through a Pb bath at a temperature of 650 ℃ to 700 ℃ in 3 minutes.

15. The method of claim 14, wherein the pearlite transformation completion time is less than 130 seconds in the LP heat treatment.