Title of Invention: Wire rod for high-strength spring, steel wire and manufacturing method thereof
technical field
[One]
The present invention relates to a wire and a steel wire for an ultra-high strength spring of 1,800 MPa class, and more particularly, to a wire for a high-stress suspension spring for a motorcycle that is easy to decarburize and suppress low-temperature structure during cooling, a steel wire, and a method for manufacturing the same.
background
[2]
Similar to the automotive material market, the motorcycle market is continuously reducing weight or restructuring, and the demand for high-strength spring steel is increasing as the dual-type suspension used in existing motorcycles is used as a mono-type recently.
[3]
Existing spring materials used in motorcycle suspensions have problems with insufficient strength and fatigue resistance to be used in mono-type suspensions as wire rods. Therefore, although the use of wire rods made of tempered martensite for automobiles has been reviewed, automobile suspension springs are difficult to apply to motorcycle suspension springs because they have difficult management standards, are difficult to manufacture, and are expensive. In particular, suspension springs for automobiles have a relatively thick diameter compared to those for motorcycles, making it difficult to manage low-temperature structures. Therefore, there is a need for a new high-strength suspension spring that can be used for motorcycles.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[4]
An object of the present invention is to provide a high-strength wire rod for a motorcycle spring, a steel wire, and a method for manufacturing the same, in which decarburization and low-temperature structure are easily suppressed through the use of low C eq and a minimum of alloying elements, and optimization of heat treatment in manufacturing the steel wire.
means of solving the problem
[5]
Wire rod for high strength spring according to an embodiment of the present invention, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, contains the remaining Fe and unavoidable impurities, and satisfies the following formula (1), and the microstructure contains 80% or more of pearlite and the remaining ferrite do.
[6]
(1) 0.77 ≤ C + (1/6)Mn + (1/5)Cr + (1/24)Si ≤ 0.83
[7]
Here, C, Mn, Cr, and Si mean the content (wt%) of each element.
[8]
In addition, according to an embodiment of the present invention, the thickness of the ferrite decarburized layer on the surface of the wire rod may be 1㎛ or less.
[9]
In addition, according to an embodiment of the present invention, a low-temperature structure having a hardness of 430 Hv or more in the C section of the wire rod may exist in an area fraction of 5% or less.
[10]
In addition, according to an embodiment of the present invention, the tensile strength of the wire rod may be 1,200 MPa or less.
[11]
High strength spring steel wire according to an embodiment of the present invention, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remaining Fe and unavoidable impurities, and satisfies the following formula (1), and includes a tempered martensite structure of 90% or more.
[12]
(1) 0.77 ≤ C + (1/6)Mn + (1/5)Cr + (1/24)Si ≤ 0.83
[13]
In addition, according to an embodiment of the present invention, the prior austenite average grain size of the steel wire may be 25 ㎛ or less.
[14]
In addition, according to an embodiment of the present invention, the steel wire may have a tensile strength of 1,700 MPa or more and a reduction in area (RA) of 35% or more.
[15]
The method of manufacturing a high-strength spring steel wire according to an embodiment of the present invention, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P : 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, remaining Fe and unavoidable impurities, including the following formula (1) by drawing a wire that satisfies the following formula (1) to produce a steel wire with a wire diameter of 15 mm or less preparing; heating the steel wire to a temperature range of 900 to 1,000° C. within 10 seconds and maintaining it for 5 to 30 seconds; High-pressure water cooling the heated steel wire; Tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500° C. within 10 seconds and maintaining it within 30 seconds; and water cooling the tempered steel wire.
[16]
(1) 0.77 ≤ C + (1/6)Mn + (1/5)Cr + (1/24)Si ≤ 0.83
[17]
In addition, according to an embodiment of the present invention, the microstructure of the wire rod may include 80% or more of pearlite and the remaining ferrite.
[18]
In addition, according to an embodiment of the present invention, the thickness of the ferrite decarburized layer on the surface of the wire rod may be 1㎛ or less.
[19]
In addition, according to an embodiment of the present invention, in the wire rod, a low-temperature structure having a hardness of 430 Hv or more in the C section may be present in an area fraction of 5% or less.
[20]
In addition, according to an embodiment of the present invention, the steel wire cooled by water after tempering may contain 90% or more of the tempered martensite structure.
[21]
In addition, according to an embodiment of the present invention, the average austenite grain size of the heated steel wire may be 25 ㎛ or less.
Effects of the Invention
[22]
According to the present invention, it is easy to control the low-temperature structure through optimization (C eq) even in a low-cost component system with a small amount of alloying elements while excluding expensive alloying elements such as Mo and V as much as possible, and the surface of the wire rod through a low Si content decarburization can be minimized.
[23]
In addition, it is possible to provide a steel wire for spring of high strength and high ductility that is easy to control decarburization and low temperature structure by optimizing heat treatment during steel wire manufacturing along with optimizing alloy elements.
Best mode for carrying out the invention
[24]
The wire rod for a high-strength spring according to an embodiment of the present invention is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less , S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, contains the remaining Fe and unavoidable impurities, satisfies the following formula (1), and the microstructure contains 80% or more of pearlite and the remaining ferrite .
[25]
(1) 0.77 ≤ C + (1/6)Mn + (1/5)Cr + (1/24)Si ≤ 0.83
[26]
Here, C, Mn, Cr, and Si mean the content (wt%) of each element.
Modes for carrying out the invention
[27]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein, and may be embodied in other forms. The drawings may omit the illustration of parts not related to the description in order to clarify the present invention, and slightly exaggerate the size of the components to help understanding.
[28]
Also, when a part "includes" a component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.
[29]
The singular expression includes the plural expression unless the context clearly dictates otherwise.
[30]
As used herein, the term "low temperature structure" refers to bainite and martensite, and collectively refers to a hard structure of steel formed by quenching steel from the point of view of a person skilled in the art.
[31]
In order to realize high strength, applying tempered martensitic steel for automobiles to mono-type suspension for motorcycles has problems of cost and low temperature structure control. In the past, when forming a tempered martensite structure, oil quenching was used to sufficiently secure hardenability after heating in a heat treatment furnace, so Mn and Cr had to be included in a certain amount or more. However, due to the development of induction heating treatment technology, sufficient cooling capacity can be secured by using water cooling, and since the diameter of the spring steel material for motorcycles is relatively smaller than that for automobiles, the possibility of utilizing low C eq alloy components has increased. Therefore, it was possible to achieve the target strength while lowering the alloying element compared to the spring steel material for automobiles.
[32]
In the case of low C eq, since decarburization and low-temperature structure management can be advantageous in relatively narrow materials, the price of the material can be stabilized, and the price can be lowered by reducing alloying elements.
[33]
Accordingly, the high-strength spring wire of the present invention, the steel wire is C: 0.55 to 0.65% by weight, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S : 0.01% or less, Al: 0.01% or less, N: 0.005% or less, remaining Fe and unavoidable impurities, and must satisfy the following formula (1).
[34]
Hereinafter, the reason for numerical limitation of the alloying element content in the embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.
[35]
The content of C is 0.55 to 0.65%.
[36]
C is an element added to secure product strength. When the C content is less than 0.55%, since C eq cannot be secured, the martensitic structure is not completely formed during cooling, so it is difficult to secure strength, and even if an intact martensitic structure is formed, it may be difficult to secure the target strength. If the C content exceeds 0.65%, the impact properties are lowered and quenching cracks may occur during water cooling, so the content is limited.
[37]
The content of Si is 0.5 to 0.9%.
[38]
Si is not only used for deoxidation of steel, but also is an element advantageous for securing strength through solid solution strengthening. However, excessive addition may cause surface decarburization, and since it is difficult to process the material, the amount of addition is controlled to 0.5 to 0.9% depending on the target strength and the degree of material processing.
[39]
The content of Mn is 0.3 to 0.8%.
[40]
Mn is a hardenability improving element and is one of the essential elements for making a high-strength tempered martensitic structure. That However, since toughness is lowered when the Mn content is increased in tempered martensitic steel, the Mn addition is controlled to 0.3 to 0.8%.
[41]
The content of Cr is 0.3 to 0.6%.
[42]
Cr, together with Mn, is effective in improving hardenability and is an element that improves the corrosion resistance of steel, so it can be added at a certain level. limited to %.
[43]
The content of P is 0.015% or less.
[44]
Since P is an element that segregates at grain boundaries to reduce toughness and reduces resistance to delayed hydrogen fracture, it is preferable to exclude it from steel materials as much as possible, so an upper limit of 0.015% is placed.
[45]
The content of S is 0.01% or less.
[46]
Like P, S is segregated at grain boundaries to reduce toughness, as well as to form MnS to reduce the hydrogen delayed fracture resistance, so the amount added is limited to 0.01% or less.
[47]
The content of Al is 0.01% or less.
[48]
Al is a powerful deoxidizing element that can remove oxygen in steel to improve cleanliness, but since Al 2O 3 inclusions can be formed, fatigue resistance can be reduced when added in a certain amount or more. Therefore, the amount added is limited to 0.01% or less.
[49]
The content of N is 0.005% or less.
[50]
N is an impurity, but it combines with Al or V to form coarse AlN or VN precipitates that are not dissolved during heat treatment. Therefore, it should be suppressed to 0.005% or less.
[51]
In addition to the above composition, the remainder is Fe, and other impurities are included inevitably in the manufacturing process. The present invention does not exclude the addition of other alloying elements other than the above-mentioned alloy composition.
[52]
In the present invention, the content of C, Si, Mn, and Cr in the alloy component system is determined by the following formula (1) to suppress low-temperature structures such as martensite or bainite while applying a fast cooling rate to prevent surface decarburization when cooling the wire rod. should be satisfied with
[53]
(1) 0.77 ≤ C + (1/6)*Mn + (1/5)*Cr + (1/24)*Si ≤ 0.83
[54]
When the value of Equation (1) is less than 0.77, it may be difficult to secure the amount of martensitic structure or strength of the martensitic structure at a fast cooling rate. On the other hand, when it exceeds 0.83, a low-temperature structure is generated during cooling, which leads to a decrease in wire-drawing workability and additional heat treatment, and it may be difficult to secure an appropriate surface hardness.
[55]
When the content of C, Si, Mn, and Cr is controlled to satisfy Equation (1) with a low Si content in the range of 0.5 to 0.9%, the surface decarburization is suppressed during cooling during the manufacturing process to reduce the thickness of the ferrite decarburized layer on the surface to 1 μm It can be controlled below, and a low-temperature tissue having a hardness of 430 Hv or higher in the C section may not be generated. In the present invention, a cross-sectional area means a cross-section of the wire rod perpendicular to the longitudinal direction.
[56]
The wire rod for a high strength spring according to the present invention may be manufactured through a conventional process for manufacturing the wire rod for a spring. For example, a billet satisfying the above-described alloy composition and formula (1) may be manufactured by heating, hot rolling a wire rod, winding, and cooling.
[57]
The manufactured wire rod contains 80% or more of pearlite and the remaining ferrite as a microstructure, and the ferrite decarburized layer on the surface may be formed to have a thickness of 1 μm or less. In addition, the tensile strength of the wire rod may be 1,200 MPa or less.
[58]
Next, a method for manufacturing a steel wire for a spring using the above-described high-strength wire will be described.
[59]
The method for manufacturing a high-strength spring steel wire according to an embodiment of the present invention, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P : 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, remaining Fe and unavoidable impurities. preparing; heating the steel wire to a temperature range of 900 to 1,000° C. within 10 seconds and maintaining it for 5 to 30 seconds; High-pressure water cooling the heated steel wire; Tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500° C. within 10 seconds and maintaining it within 30 seconds; and water cooling the tempered steel wire.
[60]
In general, when manufacturing a steel wire for a spring, it includes the step of processing and heat-treating the steel wire after manufacturing the steel wire by drawing the wire rod. The working heat treatment consists of heating the steel wire to austenitize it, cooling it with water, and then tempering it.
[61]
In the steel wire for high-strength spring according to the present invention, a wire that satisfies the above-described alloy composition and Equation (1) is drawn to a wire diameter of 15 mm or less used in a suspension spring for a motorcycle to manufacture a steel wire.
[62]
Then, in order to QT heat-treat the drawn steel wire, it is heated within 10 seconds to a temperature range of 900 to 1,000° C. and maintained for 5 to 30 seconds to austenitize. When austenitization takes more than 10 seconds to reach the target temperature range, crystal grains grow and it is difficult to secure desired properties. In addition, if the holding time is less than 5 seconds, the pearlite structure may not be transformed into austenite, and if the holding time is longer than 30 seconds, the crystal grains may become coarse, so it is preferable to control the holding time to 5 to 30 seconds.
[63]
The austenitized steel wire is cooled with water at high pressure enough to remove the boiling film. When cooling is performed by oil cooling instead of water cooling, desired strength cannot be obtained due to low C eq. In addition, if water pressure high enough to remove the boiling film is not used for water cooling, the probability of cracking cracks increases.
[64]
The water-cooled steel wire is heated within 10 seconds to a temperature range of 400 to 500° C. and tempered by maintaining it within 30 seconds. If the tempering temperature is less than 400 ℃, toughness is not secured, so processing is difficult and the risk of product damage increases. In addition, if the tempering is not heated to the target temperature range within 10 seconds, coarse carbides are formed and toughness is lowered. Therefore, it must be heated within 10 seconds.
[65]
Afterwards, the tempered steel wire is water-cooled to room temperature.
[66]
The steel wire for spring manufactured under the manufacturing conditions according to the present invention contains 90% or more of tempered martensite structure after heat treatment. In addition, the average grain size of prior austenite of the steel wire is 25 μm or less, and the tensile strength is 1,700 MPa or more, so that high-strength properties required for a suspension spring for motorcycles can be secured. In addition, it is possible to ensure high ductility by exhibiting an excellent reduction in area (RA) of 35% or more.
[67]
Hereinafter, it will be described in more detail through preferred embodiments of the present invention.
[68]
Example
[69]
After casting the material having the alloy composition in Table 1 below into an ingot, homogenization heat treatment at 1,200 ° C., and hot rolling to a final thickness of 14 mm while lowering the temperature from 980 ° C to 820 ° C. did Table 2 is a table showing the measurement results for the C-section low-temperature tissue area fraction, hardness, tensile strength, and thickness of the decarburized layer of the wire rod manufactured according to the above-described manufacturing process. In Table 2, the area fraction (%) of the low-temperature tissue means the area fraction of the low-temperature tissue on the C section of the wire rod.
[70]
[Table 1]
alloy element (wt%)
C Si Mn Cr P S Al N
Comparative Example 1 0.6 1.5 0.6 0.4 0.011 0.004 <0.003 <0.005
Comparative Example 2 0.45 0.8 0.8 0.6 0.01 0.005 <0.003 <0.005
Comparative Example 3 0.6 0.5 1.0 0.0 0.01 0.004 <0.003 <0.005
Inventive Example 1 0.6 0.8 0.6 0.4 0.009 0.005 <0.003 <0.005
Inventive Example 2 0.6 0.6 0.3 0.6 0.011 0.005 <0.003 <0.005
[71]
[Table 2]
Formula (1) Low temperature structure area fraction (%) Wire rod hardness (Hv) Wire rod tensile strength (MPa) Ferrite decarburized layer thickness (㎛) Total decarburized layer thickness (㎛)
Comparative Example 1 0.803 0 318 1,030 22 60.2
Comparative Example 2 0.737 0 235 762 - 22.8
Comparative Example 3 0.788 0 266 881 - 38.1
Invention Example 1 0.813 0 291 950 - 34.5
Invention Example 2 0.795 0 288 930 - 22.4
[72]
The wire rods of Comparative Examples 1 to 3 and Inventive Examples 1 and 2 were drawn to prepare a steel wire having a wire diameter of 12 mm, and heat treatment was performed under the conditions shown in Table 3 below. After austenitization, high-pressure water cooling was performed, and after tempering, it was cooled with general water cooling.
[73]
[Table 3]
Formula (1) Austenitization temperature (℃) Tempering temperature (℃) Average hardness (Hv) RA (%) Tensile strength (MPa)
Comparative Example 1 0.803 950 430 573 48 1,920
Comparative Example 2 0.737 950 430 498 43 1,670
Comparative Example 3 0.771 950 430 502 34 1,690
Invention Example 1 0.813 950 430 550 42 1,820
Invention Example 2 0.795 950 430 540 48 1,790
[74]
Referring to Tables 1 to 3, Inventive Examples 1 and 2 satisfying all of the alloy composition, formula (1) and manufacturing conditions of the present invention all had a strength of 1,700 MPa or more, and it was confirmed that a low-temperature structure was not generated upon cooling. .
[75]
On the other hand, in Comparative Example 1, the Si content was excessive, and a ferrite decarburized layer was formed upon cooling. Comparative Example 2 did not secure the target strength of 1,700 MPa or more because the value of Equation (1) was less than 0.77. In Comparative Example 3, the value of Equation (1) was 0.77 or more and 0.83 or less, but the Cr content did not secure the target strength of 1,700 MPa or more outside the range limited by the present invention, and the reduction in area (RA) was less than 35%.
[76]
In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will It will be understood that various changes and modifications can be made without departing from the spirit and scope of the appended claims.
Industrial Applicability
[77]
The wire rod for a high-strength spring according to the present invention may be applied as a suspension spring for automobiles, motorcycles, various moving means, or a spring used in various industrial fields.
Claims
[Claim 1]
By weight%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, including the remaining Fe and unavoidable impurities, satisfying the following formula (1), the microstructure is a high-strength spring wire containing 80% or more of pearlite and the remaining ferrite. (1) 0.77 ≤ C + (1/6)*Mn + (1/5)*Cr + (1/24)*Si ≤ 0.83 (where C, Mn, Cr, and Si are the content of each element (wt%) ) means)
[Claim 2]
The wire rod for a high-strength spring according to claim 1, wherein the thickness of the ferrite decarburized layer on the surface is 1 μm or less.
[Claim 3]
The wire rod for a high-strength spring according to claim 1, wherein a low-temperature structure having a hardness of 430 Hv or more in the C section is present in an area fraction of 5% or less.
[Claim 4]
The wire rod for a high-strength spring according to claim 1, wherein the tensile strength is 1,200 MPa or less.
[Claim 5]
By weight%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, including the remaining Fe and unavoidable impurities, satisfying the following formula (1), high-strength spring steel wire containing 90% or more of tempered martensite structure. (1) 0.77 ≤ C + (1/6)*Mn + (1/5)*Cr + (1/24)*Si ≤ 0.83 (where C, Mn, Cr, and Si are the content of each element (wt%) ) means)
[Claim 6]
The steel wire for high-strength springs according to claim 5, wherein the prior austenite average grain size is 25 µm or less.
[Claim 7]
The steel wire for a high-strength spring according to claim 5, wherein the tensile strength is 1,700 MPa or more and the reduction in area (RA) is 35% or more.
[Claim 8]
By weight%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: preparing a steel wire with a wire diameter of 15 mm or less by drawing a wire containing N: 0.005% or less, the remaining Fe and unavoidable impurities, and satisfying the following formula (1); heating the steel wire to a temperature range of 900 to 1,000° C. within 10 seconds and maintaining it for 5 to 30 seconds; High-pressure water cooling the heated steel wire; Tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500° C. within 10 seconds and maintaining it within 30 seconds; and water-cooling the tempered steel wire. (1) 0.77 ≤ C + (1/6)*Mn + (1/5)*Cr + (1/24)*Si ≤ 0.83 (where C, Mn, Cr, and Si are the content of each element (wt%) ) means)
[Claim 9]
[Claim 9] The method of claim 8, wherein the microstructure of the wire rod contains 80% or more of pearlite and the remaining ferrite.
[Claim 10]
The method of claim 8, wherein the thickness of the ferrite decarburized layer on the surface of the wire rod is 1 μm or less.
[Claim 11]
The method of claim 8, wherein, in the wire rod, a low-temperature structure having a hardness of 430 Hv or more in the C section exists in an area fraction of 5% or less.
[Claim 12]
The method of claim 8, wherein the water-cooled steel wire after tempering contains 90% or more of a tempered martensitic structure.
[Claim 13]
The method of claim 8, wherein the austenite average grain size of the heated steel wire is 25 μm or less.