Title of invention: spring wire, steel wire with improved toughness and corrosion fatigue characteristics, and their manufacturing method
Technical field
[One]
The present invention relates to a wire rod for a spring, a steel wire, and a manufacturing method thereof, and in particular, to a wire rod for a spring, a steel wire, and a method of manufacturing the same, while securing strength and improving toughness and corrosion fatigue characteristics.
Background
[2]
In recent years, for the purpose of improving vehicle fuel efficiency, weight reduction of automotive materials is required, and in particular, in the case of suspension springs, in order to respond to the demand for weight reduction, a spring design using a high-strength material having a strength of 1800 MPa or more after quenching and tempering is applied.
[3]
For spring steel, a predetermined wire is manufactured by hot rolling, and in the case of a hot-formed spring, it is heated and then molded and then quenched and tempered. It is molded with.
[4]
In general, when the material is high-strength, the toughness decreases due to grain boundary embrittlement, and the crack susceptibility increases. Therefore, if high strength is achieved but the toughness and corrosion fatigue characteristics of the material are deteriorated, corrosion pits are formed in the parts exposed to the outside such as automobile suspension springs, and the fatigue crack propagation from these corrosion pits as the starting point is caused by the formation of corrosion pits. There is a risk of premature damage to parts.
[5]
In particular, in recent years, the corrosion environment of suspension springs is becoming more and more severe due to a lot of spraying of snow removal agent to prevent freezing of the road surface in winter. Therefore, development of steel for springs having excellent corrosion resistance while securing strength is required.
[6]
Corrosion fatigue of the suspension spring is when the paint on the spring surface is peeled off by gravel or other foreign substances on the road surface, the material in this part is exposed to the outside, causing a pitting corrosion reaction, and the resulting corrosion pit grows gradually, causing the pit to grow. This is a phenomenon in which cracks are generated and propagated as a starting point, and at some point, hydrogen introduced from the outside is concentrated in the cracks and the spring is broken due to hydrogen embrittlement.
[7]
As a conventional technique for improving the resistance to corrosion fatigue of the spring, there may be mentioned a method of increasing the type and amount of alloying elements. In Japanese Patent Laid-Open No. JP 2008-190042, the Ni content was increased to 0.55% by weight to increase the corrosion resistance, thereby increasing the corrosion fatigue life. The corrosion fatigue strength was improved by miniaturization of
[8]
In addition, Japanese Patent Application Publication No. JP 2005-023404 attempts to improve the life of spring corrosion fatigue by improving the hydrogen delayed fracture resistance by appropriately matching the Ti precipitate, which is a strong hydrogen trapping site, and the precipitate (V, Nb, Zr, Hf), which is a weak site. .
[9]
However, Ni is a very expensive element, and if it is added in a large amount, it causes a problem of an increase in material cost.Si is a representative element that promotes decarburization, there may be a risk due to an increase in the amount added, and precipitates such as Ti, V, and Nb are formed. There is a risk that the elements may crystallize coarse carbonitrides from the liquid phase when the material is solidified, thereby reducing the life of corrosion fatigue.
[10]
On the other hand, conventional techniques for increasing the strength of the spring include a method of adding an alloying element and a method of lowering the tempering temperature. In the method of increasing strength by adding alloying elements, there is basically a method of increasing the small particle hardness by using C, Si, Mn, Cr, etc., and rapid cooling and quenching using expensive alloying elements such as Mo, Ni, V, Ti, Nb, etc. The strength of steel is increased by tempering heat treatment. However, this technology has a problem that the cost cost increases.
[11]
In addition, there is a method of increasing the strength of the steel material by changing the heat treatment conditions in the existing component system without changing the alloy composition. That is, when the tempering temperature is performed at a low temperature, the strength of the material increases. However, when the tempering temperature is lowered, the reduction rate of the sectional area of ​​the material is lowered, resulting in a problem of lowering toughness, and problems such as premature fracture during spring forming and use. Therefore, there is a need to develop a spring steel with improved toughness and corrosion fatigue properties while securing strength.
Detailed description of the invention
Technical challenge
[12]
Embodiments of the present invention are intended to provide a wire rod for a spring, a steel wire, and a method of manufacturing the same while securing toughness and improved corrosion fatigue characteristics.
Means of solving the task
[13]
The wire rod for a spring having improved toughness and corrosion fatigue properties according to an embodiment of the present invention is, by weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8 %, the remainder contains Fe and inevitable impurities, the crystal grain size is 13.2 µm or less, and the Charpy impact energy is 38 J/cm 2 or more.
[14]
In addition, according to an embodiment of the present invention, the microstructure of the wire rod may be an area fraction, 5 to 37% of ferrite, and the rest may be a mixed structure including pearlite.
[15]
Further, according to an embodiment of the present invention, at least one of V: ​​0.01 to 0.2%, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.15%, and Mo: 0.01 to 0.4% may be further included.
[16]
In addition, according to an embodiment of the present invention, it may further include one or more of Cu: 0.01 to 0.4% and Ni: 0.01 to 0.6%.
[17]
A method of manufacturing a spring wire rod having improved toughness and corrosion fatigue properties according to another embodiment of the present invention is, in weight%, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr : Preparing a billet containing 0.2 to 0.8%, the balance Fe and inevitable impurities; Heating the billet at 800 to 950°C; Finishing rolling the heated billet at 700 to 1,100° C. and then winding to manufacture a wire rod; Includes.
[18]
Further, according to an embodiment of the present invention, at least one of V: ​​0.01 to 0.2%, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.15%, and Mo: 0.01 to 0.4% may be further included.
[19]
In addition, according to an embodiment of the present invention, it may further include one or more of Cu: 0.01 to 0.4% and Ni: 0.01 to 0.6%.
[20]
In addition, according to an embodiment of the present invention, the cooling start temperature of the wire rod may be less than or equal to 820°C.
[21]
The steel wire for a spring having improved toughness and corrosion fatigue properties according to another embodiment of the present invention is by weight %, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the remainder contains Fe and inevitable impurities, the grain size is 10.3 µm or less, and the Charpy impact energy is 45 J/cm 2 or more.
[22]
A method of manufacturing a spring steel wire having improved toughness and corrosion fatigue properties according to another embodiment of the present invention, in weight%, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the rest of the step of producing a steel wire by drawing a wire rod containing Fe and inevitable impurities; An austenitization step of heating the steel wire in the range of 850 to 1,000° C. and maintaining it for at least 1 second; And quenching the steel wire that has undergone the austenitization step in the range of 25 to 80°C and tempering in the range of 350 to 500°C.
Effects of the Invention
[23]
According to an embodiment of the present invention, the toughness is improved by reducing the grain size, and the depth of the corrosion pit is reduced, and the path through which the crack generated from the corrosion pit is propagated and the hydrogen introduced from the outside diffuses to the crack portion By increasing the moving path, it is possible to provide a wire rod and a steel wire for a spring having excellent corrosion and fatigue properties.
Brief description of the drawing
[24]
1 and 2 are microstructure photographs taken by an electron back scattering diffraction device to measure the grain size of the wire rods in Comparative Examples 1 and 3, respectively.
[25]
3 and 4 are microstructure photographs taken by an electron back scattering diffraction apparatus to measure the grain size of the steel wires of Comparative Examples 1 and 3, respectively.
[26]
5 is a graph showing the relationship between the grain size and toughness and relative corrosion fatigue life of a spring wire according to an embodiment of the present invention.
[27]
6 is a graph showing the relationship between the grain size and toughness and relative corrosion fatigue life of a steel wire for a spring according to an embodiment of the present invention.
Best mode for carrying out the invention
[28]
The wire rod for a spring having improved toughness and corrosion fatigue properties according to an embodiment of the present invention is, by weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8 %, the remainder contains Fe and inevitable impurities, the crystal grain size is 13.2 µm or less, and the Charpy impact energy is 38 J/cm 2 or more.
Mode for carrying out the invention
[29]
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 exemplary embodiments presented here, but may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be somewhat exaggerated to aid understanding.
[30]
Throughout the specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.
[31]
Expressions in the singular number include expressions in the plural unless the context clearly has exceptions.
[32]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[33]
In providing a wire for a spring, the present inventors reviewed various factors affecting the corrosion resistance of the spring steel, and at the same time, the corrosion fatigue of the spring causes corrosion pit as the paint on the spring surface is peeled off, and this corrosion pit is the starting point. The following knowledge could be obtained by focusing on the fact that the spring breaks due to the occurrence and propagation of cracks and the concentration of hydrogen introduced from the outside into the crack.
[34]
By optimizing the alloy composition and manufacturing conditions and reducing the grain size of the spring wire and steel wire, the toughness can be improved and the depth of the corrosion pit can be reduced. It was confirmed that the time taken until fracture was delayed by increasing the path through which the generated hydrogen travels to diffuse to the crack portion, thereby improving corrosion fatigue characteristics, and to complete the present invention.
[35]
A wire rod for a spring having improved toughness and corrosion fatigue properties according to an aspect of the present invention is, by weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8% , The rest contains Fe and unavoidable impurities.
[36]
Hereinafter, the reason for limiting the numerical value of the content of the alloying component in the examples of the present invention will be described. Hereinafter, unless otherwise specified, the unit is% by weight.
[37]
The content of C is 0.4 to 0.7%.
[38]
Carbon (C) is an essential element added to secure the strength of the spring, and can be added 0.4% or more. However, if the content is excessive, a twin martensite structure is formed during quenching and tempering, resulting in material cracking, which not only results in inferior fatigue life, but also increases the susceptibility to defects. There is a problem that the stress decreases, so the upper limit can be limited to 0.7%.
[39]
The content of Si is 1.2 to 2.3%.
[40]
Silicon (Si) is an element that is dissolved in ferrite to enhance strength and improve deformation resistance, and its lower limit may be limited to 1.2%. More preferably, it may be added at least 1.4%. However, if the content is excessive, not only the effect of improving deformation resistance is saturated, but also surface decarburization occurs during heat treatment, so that the upper limit may be limited to 2.3%.
[41]
The content of Mn is 0.2 to 0.8%.
[42]
Manganese (Mn) is an element that serves to secure strength by improving the hardenability of steel, and can be added by 0.2% or more. However, if the content is excessive, the hardening property is excessively increased and hard texture is easily generated during cooling after hot rolling, and the formation of MnS inclusions may increase, thereby reducing the corrosion fatigue property, so the upper limit is limited to 0.8%. can do.
[43]
The content of Cr is 0.2 to 0.8%.
[44]
Chromium (Cr) is an element useful for securing oxidation resistance, temper softening, surface decarburization prevention and quenching properties, and can be added 0.2% or more. However, when the content is excessive, there is a problem that the strength is rather inferior due to a decrease in deformation resistance, and the upper limit thereof may be limited to 0.8%.
[45]
In addition, the wire rod for a spring having improved toughness and corrosion fatigue properties according to an embodiment of the present invention may further include at least one of V, Nb, Ti, and Mo among the elements forming carbon-nitride in addition to the above-described alloy composition. I can.
[46]
The content of V is 0.01 to 0.2%.
[47]
Vanadium (V) is an element that contributes to strength improvement and grain refinement. In addition, it combines with carbon (C) or nitrogen (N) to form carbon/nitride, which acts as a trap site for hydrogen, suppressing hydrogen intrusion and reducing the occurrence of corrosion. Can be added. However, if the content is excessive, the manufacturing cost increases, so the upper limit may be limited to 0.2%.
[48]
The content of Nb is 0.01 to 0.1%.
[49]
Niobium (Nb) is an element that contributes to microstructure by forming carbon or nitrogen and carbonitrides, and acts as a trap site for hydrogen, and can be added by 0.01% or more. However, if the content is excessive, coarse carbonitrides are formed and the ductility of the steel is lowered, so the upper limit may be limited to 0.1%.
[50]
The content of Ti is 0.01 to 0.15%.
[51]
Titanium (Ti) combines with carbon (C) or nitrogen (N) to form carbon/nitride, and the generated carbon/nitride acts as a trap site for hydrogen, suppressing hydrogen intrusion and preventing corrosion. It not only reduces, but also improves spring properties by causing precipitation hardening action. In addition, since Ti improves strength and toughness through particle refinement and precipitation strengthening, it may be added by 0.01% or more.
[52]
However, if the content is excessive, the manufacturing cost increases rapidly and the effect of improving the spring characteristics due to the precipitate is saturated. In addition, since the amount of coarse alloy carbides not dissolved in the base material increases during the austenite heat treatment, the fatigue properties and the precipitation strengthening effect are deteriorated, so the upper limit can be limited to 0.15%.
[53]
The content of Mo is 0.01 to 0.4%.
[54]
Molybdenum (Mo) combines with carbon (C) or nitrogen (N) to form carbon/nitride, and the generated carbon/nitride contributes to the microstructure and acts as a trap site for hydrogen. In order to effectively exhibit the above effect, it is preferable to add 0.01% or more of Mo. However, if the content is excessive, the possibility of hard structure formation during cooling after hot rolling increases, and coarse carbonitrides are formed to reduce the ductility of the steel, so the upper limit can be limited to 0.4%.
[55]
In addition, the wire rod for a spring having improved toughness and corrosion fatigue properties according to an embodiment of the present invention may further include at least one of Cu and Ni.
[56]
The content of Cu is 0.01 to 0.4%.
[57]
Copper (Cu) is an element that improves corrosion resistance, and can be added by 0.01% or more. However, if the content is excessive, the brittle property is lowered during hot rolling, causing problems such as cracking, so the upper limit may be limited to 0.4%.
[58]
The content of Ni is 0.01 to 0.6%.
[59]
Nickel (Ni) is an element added to improve hardenability and toughness, and may be added in an amount of 0.01% or more. However, if the content is excessive, the residual austenite content increases to reduce the fatigue life, and causes a rapid increase in manufacturing cost due to the expensive Ni property, and the upper limit may be limited to 0.6%.
[60]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal steel manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.
[61]
The microstructure of the wire rod of the present invention that satisfies the above-described alloy composition is composed of a mixed structure of ferrite and pearlite, and the pearlite structure is divided into colonies in which cementite has one direction. Lose. At this time, when the grain size of the wire is measured with an Electron BackScatter Diffraction (EBSD) device, the size is averaged without distinction between ferrite and colony.
[62]
In the wire rod of the present invention that satisfies the above-described alloy composition, the microstructure phase fraction is an area ratio, ferrite is 5 to 37%, and the balance is pearlite, and bainite or martensite does not exist. In addition, the colony size of pearlite is 1.7 ~ 5.6 ㎛.
[63]
It is preferable that the average grain size of the wire rod of the present invention that satisfies the above-described alloy composition is 13.2 μm or less.
[64]
A wire rod having the crystal grain size as described above can be obtained by controlling the aforementioned alloy composition and optimizing a wire rod rolling process and a cooling process to be described later.
[65]
In order to reduce the grain size of the spring wire rod, it is important to reduce the grain size of the billet, which is a material before rolling, and the finish rolling temperature, which is the point just before cooling begins after rolling. Specifically, by controlling the heating temperature of the billet, the grain size of the billet, which is a raw material before rolling, can be refined, and at the same time, the austenite grain size can be effectively controlled by controlling the finish rolling temperature.
[66]
Hereinafter, a method of manufacturing a wire rod for a spring having improved toughness and corrosion fatigue properties, which is another aspect of the present invention, will be described in detail.
[67]
The wire rod for a spring of the present invention can be manufactured through a reheating-wire rolling-cooling process after manufacturing a billet having the above-described alloy composition.
[68]
Specifically, a method of manufacturing a wire rod for a spring according to another aspect of the present invention includes the steps of manufacturing a billet that satisfies the alloy composition described above; Heating the billet at 800 to 950°C; Finishing rolling the heated billet at 700 to 1,100° C. and then winding to manufacture a wire rod; And cooling the wire rod at a cooling rate of 5° C./s or less.
[69]
After preparing the billet, it is preferable to go through a heating step of homogenizing the billet. Through the heating process, coarsening of the grain size of the billet can be prevented.
[70]
For this purpose, it is preferable to heat the billet in a temperature range of 800 to 950°C. If the heating temperature is less than 800°C, the load on the rolling roll increases and all the coarse carbides generated during casting are not dissolved, so that the alloying elements cannot be uniformly distributed in the austenite, whereas the temperature exceeds 950°C. If this is done, the crystal grains of the billet are coarse, so even if the wire rod is hot-rolled under the same rolling conditions, it is difficult to secure the grain size of the target level in the final wire rod.
[71]
Subsequently, the heated billet is finished wire rolled at 700 to 850° C. to manufacture a wire. The finish rolling temperature is an important factor that can finally determine the grain size of the wire rod because cooling starts immediately afterwards. If the finish rolling temperature is less than 700℃, the load on the rolling roll increases, whereas if the temperature exceeds 850℃, the austenite grain size before the start of cooling increases, and the grain size after the final cooling becomes coarse, resulting in a decrease in ductility. There is a risk of becoming.
[72]
Thereafter, the wire is wound and then cooled at a cooling rate of 5° C./s or less. It is preferable to manufacture a wire rod having a pearlite structure through the cooling.
[73]
The cooling rate after winding is an important factor because, depending on the range, a hard structure such as bainite or martensite may be generated without completion of the pearlite transformation after ferrite generation, and decarburization may occur severely.
[74]
This is because if a hard structure is generated during cooling, the material is disconnected or the drawing or drawing becomes impossible in the process of drawing or drawing the wire rod in order to obtain a spring steel wire with an appropriate wire diameter. In addition, when decarburization occurs severely, the hardness of the surface portion is lowered, thereby reducing the corrosion fatigue characteristics of the spring.
[75]
If the cooling rate exceeds 5°C/s, there is a problem in that a hard structure is generated during cooling and a sufficient time for the pearlite transformation to be completed cannot be secured. In the present invention, the cooling rate after winding is limited to 5°C/s or less. I did.
[76]
At this time, cooling can be started in a temperature range of 820°C or less. The cooling start temperature means the temperature after finish hot rolling, and the lower the temperature is, the more preferable. When the cooling start temperature exceeds 820°C, it is difficult to refine crystal grains because sufficient strain energy cannot be supplied.
[77]
Hereinafter, a method of manufacturing a spring steel wire having improved toughness and corrosion fatigue properties, which is another aspect of the present invention, will be described in detail.
[78]
A method of manufacturing a steel wire for a spring according to another aspect of the present invention includes the steps of manufacturing a steel wire by drawing a wire rod; An austenitization step of heating the steel wire in the range of 850 to 1,000° C. and maintaining it for at least 1 second; And quenching the steel wire that has undergone the austenitization step in the range of 25 to 80°C and tempering in the range of 350 to 500°C.
[79]
The wire rod thus obtained is drawn to obtain a steel wire.
[80]
Thereafter, it undergoes an austenitization step. The steel wire is heat-treated at a temperature in the range of 850 to 1,000°C. At this time, the heat treatment holding time is preferably 1 second or more.
[81]
Recently, induction heat treatment facilities are used to manufacture spring steel wires. In this case, if the heat treatment holding time is less than 1 second, the ferrite and pearlite structures are not sufficiently heated and thus do not transform into austenite. May not.
[82]
Subsequently, the steel wire that has undergone the austenitization step is quenched in the range of 25 to 80°C, and heat treated (tempered) in the range of 350 to 500°C. The heat treatment is a step for securing desired mechanical properties of the present invention, and is required to secure toughness and strength.
[83]
When the tempering temperature is less than 350°C, toughness is not secured and there is a risk of damage in molding and product conditions, and when it exceeds 500°C, it may be difficult to secure high strength due to a sharp decrease in strength.
[84]
[85]
Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.
[86]
After preparing a cast steel having the alloy composition shown in Table 1 below, through a series of casting processes, each wire was manufactured through a reheating-wire hot rolling-cooling process under the conditions shown in Table 2 below.
[87]
Thereafter, the wire rod was subjected to an austenitization step of heating at 975°C for 15 minutes, and then quenched (quenched) by immersing in oil at 70°C. Subsequently, a tempering treatment held at 390° C. for 30 minutes was performed to prepare a steel wire.
[88]
Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.
[89]
After preparing a cast steel having the alloy composition shown in Table 1 below, through a series of casting processes, each wire was manufactured through a reheating-wire hot rolling-cooling process under the conditions shown in Table 2 below.
[90]
Thereafter, the wire rod was subjected to an austenitization step of heating at 975°C for 15 minutes, and then quenched (quenched) by immersing in oil at 70°C. Subsequently, a tempering treatment held at 390° C. for 30 minutes was performed to prepare a steel wire.
[91]
[Table 1]
division C Si Mn Cr V Ti Nb Mo Cu Ni
Example 1 0.55 1.51 0.67 0.69 - - - - - -
Example 2 0.52 1.49 0.68 0.62 0.10 - - - - 0.27
Example 3 0.61 1.65 0.56 0.58 - 0.03 0.02 0.12 0.17 0.21
Example 4 0.53 1.50 0.69 0.63 - - - - - -
Example 5 0.51 1.52 0.40 0.31 0.09 - 0.02 0.11 0.21 0.22
Comparative Example 1 0.61 1.48 0.43 0.33 0.11 - 0.03 - 0.18 0.43
Comparative Example 2 0.52 1.64 0.55 0.51 - 0.02 0.02 - 0.20 0.26
Comparative Example 3 0.53 2.25 0.52 0.29 0.12 - 0.02 0.12 - 0.28
[92]
[Table 2]
division Reheating temperature (℃) Finishing hot rolling temperature (℃) Cooling start temperature (℃) Cooling rate (℃/s)
Example 1 941 774 751 0.8
Example 2 918 840 819 2.0
Example 3 885 829 804 4.8
Example 4 832 712 706 0.7
Example 5 859 803 778 3.3
Comparative Example 1 1025 841 835 1.5
Comparative Example 2 940 874 860 3.2
Comparative Example 3 924 836 828 5.8
[93]
The grain size of the wire rod, the presence or absence of hard structure formation, the Charpy U-notch energy and the grain size of the steel wire, the Charpy impact energy, the tensile strength after quenching and tempering heat treatment, and the relative corrosion fatigue life are shown in Table 3 below.
[94]
The grain size was measured using an Electron Backscatter Diffraction (EBSD) device.
[95]
Charpy impact energy was measured by processing an impact specimen in accordance with ASTM E23 standards.
[96]
Tensile strength was measured by processing a tensile specimen of the hot-rolled wire rod in accordance with ASTM E8 standards, and then performing a tensile test according to the above-described steel wire manufacturing method.
[97]
For the relative corrosion fatigue life, the tempered steel wire specimen was put into a salt spray tester, sprayed with 5% brine for 4 hours in an atmosphere of 35°C, dried for 4 hours in an atmosphere of 25°C/humidity 50%, and dried for 4 hours in an atmosphere of 40°C. The process of wetting to 100% for 16 hours was repeated 14 times, and then a rotational bending fatigue test was conducted and measured. At this time, the fatigue test speed was 3,000 rpm, and the load applied to the specimen was 40% of the tensile strength, and each ten tests were performed, and the remaining eight fatigue lives were averaged after excluding the largest and the smallest ones. It was defined as life span. In Table 3 below, when the corrosion fatigue life of Comparative Example 1 is 1, the relative corrosion fatigue life of the remaining specimens is shown.
[98]
[Table 3]
division Wire rod Steel wire
Grain size (㎛) Charpy U-notch energy(J/㎠) Phase fraction (%) Pearlite colony size (㎛) Grain size (㎛) Charpy U-notch energy(J/㎠) Tensile strength after quenching and tempering heat treatment (MPa) Relative corrosion fatigue life
ferrite Pearlite Bainite + Martensite
Example 1 13.2 38 10 90 0 5.6 10.3 45 1,979 2.38
Example 2 12.0 39 5 95 0 3.8 9.4 58 1,997 2.75
Example 3 8.2 44 18 82 0 2.8 6.8 61 2,008 5.18
Example 4 5.1 56 7 93 0 1.7 3.1 68 1,983 11.4
Example 5 6.4 51 37 63 0 2.2 4.5 67 2,051 7.30
Comparative Example 1 20.2 18 4 96 0 9.7 15.0 23 1,988 1.00
Comparative Example 2 19.1 29 3 97 0 9.4 13.4 28 1,994 0.96
Comparative Example 3 18.4 14 20 67 13 8.9 12.8 14 1,987 0.99
[99]
In Comparative Examples 1 to 3, the alloy composition satisfies the suggestions in the present invention, but the manufacturing process conditions deviate from the present invention, so they are indicated as comparative examples. Specifically, in Comparative Example 1, the heating temperature of the billet is out of the range of 800 to 950 °C at 1,025 °C, and in Comparative Example 2, the finish rolling temperature is out of the range of 700 to 850 °C at 874 °C, and in Comparative Example 3 after rolling The cooling rate exceeds 5°C/s at 5.8°C/s.
[100]
1 and 2 are microstructure photographs taken by an electron back scattering diffraction device to measure the grain size of the wire rods in Comparative Examples 1 and 3, respectively.
[101]
1 and 2, it can be seen that in Comparative Example 1, the grain size was coarse, whereas in Example 3, the average grain size was fine.
[102]
In addition, referring to Table 3, the grain size of the hot-rolled wire rod was in the range of 18.4 to 20.2 µm in the case of the comparative example, but in the case of the example, it was 5.1 to 13.2 µm, which was finer than that of the comparative example, and the Charpy impact energy value was 14 in the case of the comparative example. The level was ~ 29 J/cm2, but in the case of the example, the high value of 38 ~ 56 J/cm2 was shown, indicating that the toughness was improved.
[103]
Therefore, the wire rod obtained according to the alloy composition and manufacturing conditions proposed in the present invention has excellent toughness and can be suitably used for a spring.
[104]
On the other hand, in the case of Comparative Example 3, as a result of the microstructure analysis of the wire rod, it can be confirmed that the pearlite transformation was not completed after ferrite generation, and thus a hard structure such as bainite or martensite was generated. This is because the cooling rate exceeded 5°C/s, so that sufficient time to complete the pearlite transformation could not be secured.
[105]
3 and 4 are microstructure photographs taken by an electron back scattering diffraction apparatus to measure the grain size of the steel wires of Comparative Examples 1 and 3, respectively.
[106]
3 and 4, it can be seen that crystal grains in the heat-treated steel wire are finely formed in Example 3 compared to Comparative Example 1.
[107]
In addition, referring to Table 3, the heat-treated steel wire exhibited tensile strength in the vicinity of 2,000 MPa in both Comparative Examples and Examples. In the case of the comparative example, the crystal grain size was in the range of 12.8 to 15.0 μm, but in the case of the example, it can be seen that it is significantly finer than that of the comparative example, as 3.1 to 10.3 μm.
[108]
5 is a graph showing the relationship between the grain size and toughness and relative corrosion fatigue life of a spring wire according to an embodiment of the present invention.
[109]
6 is a graph showing the relationship between the grain size and toughness and relative corrosion fatigue life of a steel wire for a spring according to an embodiment of the present invention.
[110]
5 and 6, it can be seen that the smaller the average grain size of the hot-rolled wire rod and the spring steel wire, the greater the Charpy impact energy value and the relative corrosion fatigue life improved.
[111]
Specifically, according to Table 3, the Charpy impact energy value was at the level of 14 to 28 J/cm 2 in the case of the comparative example, but exhibited a high value of 45 to 68 J/cm 2 in the case of the Example, it can be confirmed that toughness was improved. In addition, it can be seen that the relative corrosion fatigue life was 0.96 to 1.00 in the case of the comparative example, while 2.38 to 11.4 in the case of the example, which significantly improved corrosion fatigue characteristics compared to the comparative example.
[112]
As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those of ordinary skill in the art are within the scope not departing from the concept and scope of the following claims. It will be appreciated that various changes and modifications are possible in
Industrial applicability
[113]
The wire rod and the steel wire according to the present invention can be used as a material such as a suspension spring, a torsion bar, a stabilizer and the like because corrosion fatigue characteristics and toughness are improved.
Claims
[Claim 1]
In% by weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the remainder contains Fe and inevitable impurities, and the grain size is 13.2 μm or less, Spring wire with improved toughness and corrosion fatigue properties with Charpy impact energy of 38 J/㎠ or more.
[Claim 2]
The wire rod for a spring according to claim 1, wherein the microstructure of the wire rod is an area fraction, 5 to 37% of ferrite, and the balance of pearlite.
[Claim 3]
According to claim 1, V: 0.01 to 0.2%, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.15% and Mo: for a spring having improved toughness and corrosion fatigue properties further comprising at least one of 0.01 to 0.4% Wire rod.
[Claim 4]
According to claim 1, Cu: 0.01 to 0.4% and Ni: 0.01 to 0.6% of the wire rod for a spring with improved toughness and corrosion fatigue properties further comprising at least one.
[Claim 5]
In weight %, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the balance is Fe and preparing a billet containing unavoidable impurities; Heating the billet at 800 to 950°C; Finishing rolling the heated billet at 700 to 1,100° C. and then winding to manufacture a wire rod; And cooling the wire rod at a cooling rate of 5° C./s or less.
[Claim 6]
According to claim 5, wherein the billet has toughness and corrosion fatigue properties further comprising at least one of V: ​​0.01 to 0.2%, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.15%, and Mo: 0.01 to 0.4% An improved method of manufacturing a spring wire.
[Claim 7]
[6] The method of claim 5, wherein the billet further comprises at least one of Cu: 0.01 to 0.4% and Ni: 0.01 to 0.6%.
[Claim 8]
[6] The method of claim 5, wherein the cooling start temperature of the wire rod is 820°C or less, with improved toughness and corrosion fatigue characteristics.
[Claim 9]
In% by weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the remainder contains Fe and inevitable impurities, and the grain size is 10.3 μm or less, Steel wire for spring with improved toughness and corrosion fatigue properties with Charpy impact energy of 45 J/㎠ or more.
[Claim 10]
By weight, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, the rest is a step of drawing a wire rod containing Fe and inevitable impurities to prepare a steel wire ; An austenitization step of heating the steel wire in the range of 850 to 1,000° C. and maintaining it for at least 1 second; And quenching the steel wire that has undergone the austenitization step in the range of 25 to 80°C and tempering in the range of 350 to 500°C.