Patent Description:
In recent years, a demand for lightness of materials for vehicle manufacture has greatly increased in order to improve fuel efficiency of vehicles. In particular, a suspension spring has been designed to be manufactured using a high strength material having strength of <NUM> MPa or higher after quenching and tempering in order to respond to the demand for lightness.

A predetermined wire rod is produced through hot rolling by using steel for a spring, and then, in a case of a hot-rolled spring, a heating process, a forming process, and a quenching and tempering process are sequentially performed, and in a case of a cold-rolled spring, a drawing process and a quenching and tempering process are sequentially performed, thereby forming a spring.

In general, when high strength of a material is achieved, toughness is degraded and crack sensitivity is increased due to grain boundary embrittlement or the like. Thus, although the high strength of the material is achieved, when corrosion resistance of the material is degraded, a component that is exposed to the outside, such as a suspension spring of a vehicle, has a corrosion pit formed at a portion from which paint is peeled off. Therefore, the component may be damaged at an early stage due to fatigue cracks spreading from the corrosion pit.

In particular, a corrosive environment of a suspension spring becomes more severe due to an increase in spraying of a snow-melting agent used to prevent a road surface from freezing in winter. Therefore, a demand for steel for a spring having high strength and excellent corrosion fatigue resistance properties has increased.

Corrosion fatigue of a suspension spring is a phenomenon in which paint of a surface of the spring is peeled off by pebbles on a road surface or foreign matter, a material of a portion from which the paint is peeled off is exposed to outside to cause a pitting corrosion reaction, a generated corrosion pit is gradually grown, cracks are generated and spread from the corrosion pit, hydrogen flowing from the outside is concentrated on the cracks at some point to cause hydrogen embrittlement, and thus the spring becomes broken.

In order to improve corrosion fatigue resistance of a spring, a method of increasing types and contents of alloy elements has been used in the related art. In Patent Document <NUM>, a content of Ni is increased to <NUM> wt % to improve corrosion resistance, thereby improving a corrosion fatigue life of a spring. In Patent Document <NUM>, a content of Si is increased to obtain fine carbide to be precipitated during tempering, thereby increasing corrosion fatigue strength.

In addition, in Patent Document <NUM>, a Ti precipitate, which is a strong hydrogen trapping site, and V, Nb, Zr and Hf precipitates, which are weak hydrogen trapping sites, are adequately combined to improve hydrogen-delayed fracture resistance, thereby improving a corrosion fatigue life of a spring.

However, Ni is a very expensive element, and when a large amount of Ni is added, material costs increase. Since Si is a representative element that causes decarburization, an increase in content of Si may be significantly dangerous. Elements constituting a precipitate, such as Ti, V, and Nb, may degrade a corrosion fatigue life of a spring because the elements crystallize coarse carbonitrides from a liquid when the material is solidified.

Meanwhile, in order to increase strength of a spring, a method of adding an alloy element and a method of lowering a tempering temperature have been used in the related art. As the method of increasing strength of a spring by adding an alloy element, a method of increasing a quenching hardness by using C, Si, Mn, Cr, and the like is basically used. Strength of a steel material is increased through a rapid cooling and a tempering heat treatment by using Mo, Ni, V, Ti, Nb, and the like that are expensive alloy elements. However, in the case of these techniques, a cost price may increase.

In addition, a method of increasing strength of a steel material by changing heat treatment conditions in a general component system without changing of an alloy composition is used. That is, in a case where tempering is performed at a low temperature, the strength of the material is increased. However, when a tempering temperature is lowered, an area reduction rate of the material is decreased, which may cause degradation in toughness. As a result, a breakage during formation and use of the spring may occur at an early stage. Therefore, there is a need to develop a steel for springs with improved toughness and corrosion fatigue properties and capable of securing strength.

Patent Document <NUM> discloses a wire for a high-strength spring containing, in % by mass, C: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, P: more than <NUM>% but no more than <NUM>%, S: more than <NUM>% but no more than <NUM>%, Al: <NUM>-<NUM>%, Cu: <NUM>-<NUM>%, Ni: <NUM>-<NUM>%, Cr: <NUM>-<NUM>%, and Ti: <NUM>-<NUM>%, with the remainder being made up of iron and unavoidable impurities, and an ideal critical diameter DCI represented by formula (<NUM>) not exceeding <NUM>, the hydrogen content of steel not exceeding <NUM>. 30ppm, the prior austenite crystal grain size being at least <NUM>, and the percentage of the entire structure accounted for by pearlite not exceeding <NUM>% by area; and a method for producing the wire for a high-strength spring. (<NUM>) DCI = <NUM> × (<NUM> + <NUM> × [C] + <NUM> × [C]<NUM>) × (<NUM> × [Mn] + <NUM>) × (<NUM> + <NUM> × [Si]) × (<NUM> + <NUM> × [Ni]) × (<NUM> + <NUM> × [Cr]) × (<NUM> + <NUM> × [Cu]) × (<NUM> + <NUM> × [V]) × (<NUM> + <NUM> × [Mo]).

Patent Document <NUM> discloses a high strength spring steel wire with excellent coiling properties and hydrogen embrittlement resistance. The steel wire comprises, by mass, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Si, <NUM> to <NUM>% of Mn, <NUM> to <NUM>% of Cr, <NUM>% or less of P (exceeding <NUM>%), <NUM>% or less of S (exceeding <NUM>%), <NUM>% or less of N (exceeding <NUM>%), <NUM> to <NUM>% of Al, and the remainder being Fe and unavoidable impurities. The steel wire has a structure wherein prior austenite has an average grain size of <NUM> or less, and retained austenite exists in an amount of <NUM> to <NUM> vol. % with respect to a whole structure of the steel wire. The retained austenite has an average grain size of <NUM> or less and a maximum grain size of <NUM> or less. The steel has a tensile strength of <NUM>,<NUM> MPa or more.

According to the invention there is provided a wire rod, a steel wire for springs and their manufacturing method, with improved corrosion fatigue properties capable of securing toughness.

In accordance with an aspect of the invention, a wire rod for springs with improved toughness and corrosion fatigue properties includes: by weight percent, carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities, and wherein a grain size is <NUM> or less, a Charpy impact energy value is <NUM> J/cm<NUM> or more and a colony size of pearlite is <NUM> - <NUM>, wherein.

In accordance with another aspect of the invention, a method of manufacturing a wire rod for springs with improved toughness and corrosion fatigue properties includes: manufacturing a billet including carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities; heating the billet at <NUM> to <NUM>° C; and finishing rolling the heated billet at <NUM> to <NUM>° C; winding to manufacture a wire rod; and
cooling the wire rod at a cooling rate of <NUM>/s or less, wherein a cooling start temperature of the wire rod is <NUM>° C or less.

In accordance with still another aspect of the invention, a steel wire for springs with improved toughness and corrosion fatigue properties includes: by weight percent, carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities and wherein a grain size is <NUM> or less, and a Charpy impact energy value is <NUM> J/cm<NUM> or more.

In accordance with still another aspect of the invention, a method of manufacturing a steel wire for springs with improved toughness and corrosion fatigue properties includes: drawing a wire rod including carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities to manufacture a steel wire; austenitizing the steel wire by heating the steel wire in a range of <NUM> to <NUM>,<NUM>° C and then maintaining it for at least <NUM> second; and quenching the austenitized steel wire in a range of <NUM> to <NUM>° C and tempering in a range of <NUM> to <NUM>° C.

As is apparent from the above, according to the invention, by reducing a grain size toughness is improved, and at the same time a depth of a corrosion pit becomes shallow, as the path through which the crack generated from the corrosion pit is spread and the path through which hydrogen introduced from the outside diffuses to the crack area are increased, it is possible to provide a wire rod and a steel wire for springs with excellent corrosion fatigue properties.

Hereinafter, the embodiments of the invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to transfer the technical concepts of the present disclosure to one of ordinary skill in the art. However, the present disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, parts that are irrelevant to the descriptions may be not shown in order to clarify the present disclosure, and also, for easy understanding, the sizes of components are more or less exaggeratedly shown.

Also, when a part "includes" or "comprises" an element, unless there is a particular description contrary thereto, the part may further include other elements, not excluding the other elements.

Hereinafter, embodiments according to the invention will be described in detail with reference to the accompanying drawings.

The inventor was able to obtain the following findings in providing a wire rod for springs while examining various factors affecting a corrosion resistance of spring steel, and focusing on a fact that corrosion fatigue of springs is a phenomenon in which paint of a surface of a spring is peeled off, and a corrosion pit occurs, and the corrosion pit from which cracks are generated and spread from the corrosion pit, and hydrogen flowing from the outside is concentrated in the crack area and thus the spring may become broken.

The inventor has found that if a grain size of wire rod and steel wire for springs is reduced by optimizing component compositions and manufacturing conditions of steel, toughness may be improved and a depth of the corrosion pit may become shallow. In addition, a path through which cracks generated from the corrosion pit are spread and a path through which hydrogen flowing from the outside travels to diffuse to the crack area is increased, and thus a time to fracture is delayed. This can improve corrosion fatigue properties, as is an aim of the invention.

In accordance with an aspect of the invention, a wire rod for springs with improved toughness and corrosion fatigue properties includes: by weight percent, carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities and wherein the sum of the content of V + Nb is <NUM>% or less.

Hereinafter, the reason for numerical limitation of alloy element contents in the embodiments of the invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.

Carbon (C) is an essential element added to secure the strength of a spring, and is added in an amount of at least <NUM>%. However, if the content is excessive, a twin martensite structure is formed during quenching and heat treatment, cracks of a material are generated, and a fatigue life is degraded, defect sensitivity is increased. When a corrosion pit occurs, the fatigue life or a fracture stress is degraded, there is a problem that the fatigue life or the fracture stress decreases, so the upper limit of the content of C is <NUM>%.

Silicon (Si) is an element that is dissolved in ferrite to enhance strength and improve deformation resistance, and the lower limit of its content is <NUM>%. More preferably, Si may be added in an amount of at least <NUM>%. However if the content is excessive, not only the effect of improving deformation resistance is saturated, but also surface decarburization occurs during heat treatment. Hence the upper limit of the content of Si is <NUM>%.

Manganese (Mn) is an element that serves to secure strength by improving the hardenability of steel, and is added in an amount of <NUM>% or more. However, if the content is excessive hardenability is excessively increased, the hard structure is likely to occur during cooling after hot rolling, and the formation of MnS inclusions increases, and corrosion fatigue properties may be reduced. Therefore the upper limit of the content of Mn is <NUM>%.

Chromium (Cr) is an element useful for preventing oxidation resistance, temper softening properties, and surface decarburization and securing hardenability. In embodiments it is added in an amount of <NUM>% or more. However, if the content is excessive, there is a problem that the strength is rather inferior due to a decrease in deformation resistance. Hence the upper limit of the content of Cr is <NUM>%.

In addition, a wire rod for springs with improved toughness and corrosion fatigue properties according to an embodiment of the present disclosure as noted may include one or more of V, Nb, Ti, and Mo among elements forming carbonitrides.

The content of V may be <NUM> to <NUM>%.

Vanadium (V) is an element that contributes to strength improvement and crystal grain refinement. In addition, it combines with carbon (C) or nitrogen (N) to form carbonitrides, which act as a trap site for hydrogen, and it inhibits hydrogen infiltration and reduces corrosion. It may be added in an amount of <NUM>% or more. However if the content is excessive, the manufacturing cost increases so the upper limit of the content of V when added is <NUM>%.

The content of Nb may be <NUM> to <NUM>%.

Niobium (Nb) is an element that forms carbon or nitrogen and carbonitrides, contributing to microstructure, and acts as a trap site for hydrogen, and may be added in an amount of <NUM>% or more. However, if the content is excessive, coarse carbonitride is formed and the ductility of the steel material is reduced. Therefore the upper limit of the content of Nb when added is <NUM>%.

The content of Ti may be <NUM> to <NUM>%.

Titanium (Ti) combines with carbon (C) or nitrogen (N) to form carbonitrides, and the generated carbonitrides act as a trap site for hydrogen, suppressing hydrogen infiltration in the inside of the steel, reducing the occurrence of corrosion and improving spring properties by causing precipitation hardening action. In addition, since Ti improves strength and toughness through particle refinement and precipitation reinforcement, it may be added in an amount of <NUM>% or more.

However, if the content is excessive, the manufacturing cost increases rapidly and the effect of improving spring properties due to precipitates is saturated. In addition, the amount of coarse alloy carbides not dissolved in the base material increases during austenite heat treatment, which lowers the fatigue properties and precipitation reinforcement effect. Therefore the upper limit of the content of Ti when added is <NUM>%.

The content of Mo may be <NUM> to <NUM>%.

Molybdenum (Mo) combines with carbon (C) or nitrogen (N) to form carbonitrides. The generated carbonitrides contribute to the microstructure and act as a trap site for hydrogen.

In order to effectively exhibit the above effect, it is preferable to add <NUM>% or more of Mo. However, if the content is excessive, a hard structure is likely to occur during cooling after hot rolling, and coarse carbonitrides are formed, reducing the ductility of the steel. Therefore the upper limit of the content of Mo when added is <NUM>%.

In addition as also stated, the wire rod for springs with improved toughness and corrosion fatigue properties according to the invention, optionally may further include at least one of Cu and Ni.

The content of Cu may be <NUM> to <NUM>%.

Copper (Cu) is an element that improves corrosion resistance, and may be added in an amount of <NUM>% or more. However, if the content is excessive, the brittle properties are degraded during hot rolling, causing problems such as cracking, so the upper limit of the content of Cu when added is <NUM>%.

The content of Ni may be <NUM> to <NUM>%.

Nickel (Ni) is an element added to improve hardenability and toughness, and may be added in an amount of at least <NUM>%. However, if the content is excessive, the residual austenite content increases to reduce the fatigue life and a rapid increase in manufacturing cost may be caused by the high-priced Ni properties. Hence the upper limit of the content of Ni when added is <NUM>%.

The remaining component of the present disclosure is iron (Fe). However, in the normal manufacturing process, impurities that are not intended from the raw material or the surrounding environment can inevitably become mixed, and therefore cannot be excluded. Since these impurities are known to those of skill in the ordinary manufacturing process, they are not specifically mentioned in this specification.

The microstructure of the wire rod of the present disclosure that satisfies the above-described alloy composition is made of a mixed structure of ferrite and pearlite, and the pearlite structure is again divided into colonies in which cementite has one direction. At this time, when the grain size of a wire rod is measured with an Electron Backscatter Diffraction (EBSD) apparatus, the size is measured on average without distinction between ferrite and colony.

In the wire rod of the present disclosure, which satisfies the alloy composition described above, the microstructure phase fraction is given as an area ratio, ferrite is <NUM> to <NUM>%, the remainder is pearlite and bainite or martensite does not exist. In addition, the colony size of pearlite is <NUM> ~ <NUM>.

The average grain size of the wire rod of the invention that satisfies the above-described alloy composition is <NUM> or less.

A wire rod having the grain size as described above may be obtained by controlling the aforementioned alloy composition and optimizing the wire rod rolling and cooling processes to be described later.

In order to reduce the grain size of wire rod for springs, the grain size of the billet, which is a material before rolling, is reduced, and at the same time, the finishing rolling temperature, which is the point immediately before the start of cooling after rolling, is important. Specifically, by controlling the heating temperature of the billet the grain size of the billet, which is the material before rolling, is refined; and simultaneously the austenite grain size may be effectively controlled by controlling the finishing rolling temperature.

Hereinafter, a method of manufacturing a wire rod for springs with improved toughness and corrosion fatigue properties, which is another aspect of the invention, will be described in detail.

The wire rod for springs of the invention may be manufactured, after a billet having the above-described alloy composition is manufactured, through reheating - wire rod rolling - cooling processes.

Particularly, a method of manufacturing a wire rod for springs in accordance with another aspect of the invention includes manufacturing a billet satisfying the alloy composition described above; heating the billet at <NUM> to <NUM>; finishing rolling the heated billet at <NUM> to <NUM> and winding to manufacture a wire rod; and cooling the wire rod at cooling rate of <NUM>/s or less.

After manufacturing the billet, it is preferable for it to undergo a heating step of homogenizing the billet. Through the heating process, coarsening of the grain size of the billet may be prevented.

For this purpose, heating of the billet takes place in a temperature range of <NUM> to <NUM>. If the heating temperature is less than <NUM>. , the load of the rolling roll increases; and all the coarse carbides generated during casting are not dissolved, so that the alloy elements cannot be uniformly distributed in the austenite. On the other hand when the temperature exceeds <NUM>, the grains of the billet form coarsely, so even if hot rolling of the wire rod under the same rolling conditions it is difficult to secure the grain size of the targeted level in the final wire rod.

Subsequently, the heated billet is subjected to finishing rolling at <NUM> to <NUM>° C to manufacture a wire rod. The finishing rolling temperature is an important factor that finally determines the grain size of the wire rod because cooling starts immediately afterwards. When the finishing rolling temperature is less than <NUM>. , the load of the rolling roll is increased. On the other hand when the temperature exceeds <NUM>, the austenite grain size before the start of cooling increases; and the grain size becomes coarse after the final cooling, which may reduce ductility.

Thereafter, the wire rod is subjected to winding and cooling at a cooling rate of <NUM>/s or less. It is preferable to manufacture the wire rod having a pearlite structure through the cooling.

The cooling rate after winding is an important factor because depending on the range a hard structure such as bainite or martensite may be formed, or decarburization may occur severely without completing the transforming pearlite after ferrite is created.

If the hard structure is created during cooling, the material is disconnected or the drawing becomes impossible in the process of drawing a wire rod to obtain a steel wire for springs of an appropriate diameter. In addition when decarburization occurs severely the hardness of the surface portion is lowered, thereby reducing the corrosion fatigue properties of the spring.

If the cooling rate exceeds <NUM>/s, there is a problem in that a hard structure is generated during cooling and a sufficient time for the transformation of pearlite to be completed may not be secured. Therefore, the cooling rate after winding is limited to <NUM>/s or less.

At this time cooling is started in a temperature range of <NUM> or less. The cooling start temperature means the temperature after finishing hot rolling, the lower the temperature, the more preferable. If the cooling start temperature exceeds <NUM>° C, since sufficient deformation energy may not be supplied it is difficult to refine crystal grains.

Hereinafter, a method of manufacturing a steel wire for springs with improved toughness and corrosion fatigue properties will be described in detail.

Such a method of manufacturing a steel wire for springs includes drawing the wire rod to manufacture a steel wire; austenitizing the steel wire by heating the steel wire in a range of <NUM> to <NUM>,<NUM>° C; maintaining the temperature for at least <NUM> second; and quenching the austenitized steel wire in a range of <NUM> to <NUM>° C and tempering in a range of <NUM> to <NUM>° C.

The wire rod obtained via the method described above is drawn to obtain a steel wire.

Thereafter, an austenitizing step is performed. The steel wire is heat treated at a temperature in a range of <NUM> to <NUM>,<NUM> ° C. At this time the heat treatment maintaining time is <NUM> second or more.

In recent years, induction heat treatment facilities are increasingly used to manufacture steel wire for springs. In this case if the heat treatment maintaining time is less than <NUM> second, the ferrite and pearlite structures may not be sufficiently heated and thus may not be transformed into austenite.

Subsequently, the austenitized steel wire is subjected to quenching in a range of <NUM> to <NUM>, and heat treatment (tempering) in a range of <NUM> to <NUM>. The heat treatment is a step for securing desired mechanical properties, and is required to secure toughness and strength.

If the tempering temperature is less than <NUM>, toughness is not secured and there is a risk of damage in forming and product conditions. If it exceeds <NUM>, it may be difficult to secure high strength due to a sharp decrease in strength.

Hereinafter, the invention will be described in more detail through embodiments. However, it should be noted that the following examples are only intended to illustrate the disclosure in more detail and not to limit the scope of the invention or the disclosure hereof.

After preparing a cast piece having an alloy composition shown in Table <NUM> below, through a series of casting processes each wire rod was manufactured through a reheating - hot rolling - cooling process under the conditions shown in Table <NUM> below.

Thereafter, the wire rod was subjected to an austenitizing step of heating at <NUM> for <NUM> minutes, and then quenched by immersing in oil at <NUM>. Subsequently, tempering treatment was performed at <NUM> for <NUM> minutes to manufacture a steel wire.

The grain size of the wire rod, the presence or absence of the hard structure, a Charpy U-notch energy and the grain size of the steel wire, the Charpy impact energy, the tensile strength and the relative corrosion fatigue life after quenching and tempering heat treatment are shown in Table <NUM> below.

The grain size was measured using an Electron Backscatter Diffraction (EBSD) apparatus.

Charpy impact energy was measured by processing an impact specimen according to ASTM E23 standard.

Tensile strength is measured by performing a tensile test after following the above-described steel wire manufacturing method, after processing the hot-rolled wire rod into a tensile specimen according to ASTM E8 standard.

For the relative corrosion fatigue life, a cycle in which the tempered steel wire specimen was put into a salt water spray tester, <NUM>% salt water was sprayed at an atmosphere of <NUM>. for <NUM> hours, the steel wire was dried in an atmosphere of a temperature of <NUM>. and a humidity of <NUM>% for <NUM> hours, and the steel wire was wetted at an atmosphere of <NUM>. for <NUM> hours until the humidity became <NUM>% was repeated <NUM> times, and then a rotary bending fatigue test was performed, thereby measuring the corrosion fatigue life. At this time, a speed of the fatigue test was <NUM>,<NUM> rpm, a weight applied to the specimen was <NUM>% of the tensile strength, each of <NUM> specimens were tested, an average value of fatigue lives of <NUM> specimens excluding the specimen having the highest fatigue life and the specimen having the lowest fatigue life was calculated, and the obtained average value was defined as a corrosion fatigue life of the specimen. In Table <NUM> below, when the corrosion fatigue life of Comparative Example <NUM> is <NUM>, the relative corrosion fatigue life of the remaining specimens is shown.

Comparative Examples <NUM> to <NUM> are described as comparative examples because the alloy composition satisfies the proposal in the present disclosure but the manufacturing process conditions deviate from the claimed conditions. Particularly, in Comparative Example <NUM> the heating temperature of the billet was <NUM>,<NUM>° C which is out of the range of <NUM> to <NUM>° C; Comparative Example <NUM> has a finishing rolling temperature of <NUM>° C which is out of the range of <NUM> to <NUM>° C; and in Comparative Example <NUM>, the cooling rate after rolling is <NUM>° C/s, exceeding <NUM>° C/s.

<FIG> and <FIG> are microstructure photographs taken with an Electron Backscatter Diffraction apparatus to measure the grain size of the wire rod in Comparative Example <NUM> and Inventive Example <NUM>, respectively.

Referring to <FIG> and <FIG>, while the grain size was coarse in the case of Comparative Example <NUM>, it could be confirmed that the average grain size is fine in Inventive Example <NUM>.

In addition, referring to Table <NUM>, the grain size of the hot-rolled wire rod was in the range of <NUM> to <NUM> in the case of the comparative examples; but it was <NUM> ~ <NUM> in the case of the inventive examples, which was finer than that of the Comparative examples. Also the Charpy impact energy value is <NUM> to <NUM> J/cm<NUM> in the case of the comparative examples, but the high value of <NUM> to <NUM> J/cm<NUM> was shown in the case of the inventive examples, and it could be confirmed that toughness was improved.

Therefore, the wire rod obtained according to the alloy composition and manufacturing conditions proposed herein has excellent toughness and may be suitably used for a spring.

On the other hand in the case of Comparative Example <NUM>, as a result of analyzing the microstructure of the wire rod it was found that the transforming to pearlite was not completed after ferrite was created, resulting in the creation of a hard structure such as bainite or martensite. This is because the cooling rate exceeded <NUM>° C/s and it was not possible to secure sufficient time to complete the transforming to pearlite.

<FIG> and <FIG> are microstructure photographs taken with an Electron Backscatter Diffraction apparatus to measure the grain size of the steel wire in Comparative Example <NUM> and Inventive Example <NUM>, respectively.

Referring to <FIG> and <FIG>, it can be confirmed that the crystal grains in the steel wire subjected to heat treatment were formed finely in Example <NUM> compared to Comparative Example <NUM>.

In addition, referring to Table <NUM>, the heat-treated steel wire exhibited tensile strength in the vicinity of <NUM>,<NUM> MPa in both a comparative example and an inventive example. The grain size was in the range of <NUM> ~ <NUM> in the case of the comparative example whereas in the case of the inventive example the grain size is <NUM> to <NUM>, which is significantly finer compared to the Comparative example.

<FIG> is a graph showing a correlation between grain size, toughness, and relative corrosion fatigue life of a wire rod for springs according to an embodiment hereof.

<FIG> is a graph showing a correlation between grain size, toughness and relative corrosion fatigue life of a steel wire for springs according to an embodiment hereof.

Referring to <FIG> and <FIG>, it can be confirmed that the smaller the average grain size of the hot-rolled wire rod and the steel wire for springs, the lower the Charpy impact energy value increases, and the relative corrosion fatigue life is improved.

Particularly, according to Table <NUM>, the Charpy impact energy value was at the level of <NUM> to <NUM> J/cm<NUM> in the case of the comparative examples, but it was a high value of <NUM> to <NUM> J/cm<NUM> in the case of the inventive examples. It therefore may be confirmed that toughness is improved. In addition, the relative corrosion fatigue life was <NUM> to <NUM> in the case of the comparative examples, whereas it was <NUM> to <NUM> in the case of the inventive examples. It could be confirmed that the corrosion fatigue properties are significantly improved compared to the comparative examples.

Claim 1:
A wire rod for springs with improved toughness and corrosion fatigue properties, the wire rod comprising: by weight percent, carbon (C): <NUM> to <NUM>%, silicon (Si): <NUM> to <NUM>%, manganese (Mn): <NUM> to <NUM>%, chromium (Cr): <NUM> to <NUM>%, optionally one or more elements selected from vanadium (V): <NUM> to <NUM>%, niobium (Nb): <NUM> to <NUM>%, titanium (Ti): <NUM> to <NUM>%, and molybdenum (Mo): <NUM> to <NUM>%, optionally one or more elements selected from copper (Cu): <NUM> to <NUM>% and nickel (Ni): <NUM> to <NUM>% and a balance of iron (Fe) and inevitable impurities, wherein
a grain size is <NUM> or less;
a Charpy impact energy is <NUM> J/cm<NUM> or more;
a colony size of pearlite is <NUM> - <NUM>; and
a microstructure of the wire rod comprises, by area fraction, <NUM> to <NUM>% of ferrite; and
the remainder is pearlite, with bainite or martensite not being present in the microstructure.