Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon

Definitions

• the present invention relates to a spring steel that is cold-coated and has high strength and high toughness, and a spring heat-treated steel wire.
• nitriding and shot peening are known to increase the surface hardness and dramatically improve durability against spring fatigue, but the sag characteristics of springs are not determined by the surface hardness. The strength or hardness inside the spring material is greatly affected. Therefore, it is important to make it a component that can maintain an extremely high internal hardness.
• elements such as V, Nb, and Mo are added to form fine carbides that are solid-dissolved by quenching and precipitate by tempering, thereby restricting the movement of dislocations and providing sag resistance characteristics.
• elements such as V, Nb, and Mo are added to form fine carbides that are solid-dissolved by quenching and precipitate by tempering, thereby restricting the movement of dislocations and providing sag resistance characteristics.
• the inventors have found that by controlling N, which has not been attracting attention until now, it is possible to suppress the formation of undissolved carbides even when alloying elements are added, and toughness can be secured.
• Te 0.0002 to 0.01%
• Sb 0.0002 to 0.01%
• Mg 0.0001 to 0.0005%
• Zr 0.0001 to 0.0005%
• Ca 0.0002 to 0.01%
• Hf High strength spring steel according to any one of (1) to (3), characterized by containing one or more of 0.0002 to 0.01%
• Occupied area ratio of equivalent circle diameter of 0.2 ⁇ m or more is 7% or less, Equivalent density of circle equivalent diameter of 0.2 / m or more satisfies 1 // m 2 or less, the old austenite grain size number is 10 or more, and residual austenite ⁇ is 15% by mass or less.
• Heat-treated steel wire for high-strength springs Brief Description of Drawings
• Figure 1 illustrates the effect of Nb addition when N is reduced (relationship between tempering temperature and Charpy impact value).
• Fig. 2 (a) is a photograph showing an example of observation of undissolved carbide using a scanning electron microscope.
• (B) is an example of elemental analysis by X-ray of alloy-based undissolved carbide X
• (c) is an example of elemental analysis by X-ray of cementitious undissolved carbide Y.
• the present inventor has invented a steel wire that has sufficient coiling characteristics to produce a spring by controlling the shape of carbide in the steel by heat treatment while defining the chemical composition to obtain high strength. It came.
• C is an element that has a great influence on the basic strength of the steel, and is set to 0.5 to 0.9% so that a sufficient strength can be obtained. If it is less than 5%, sufficient strength cannot be obtained. In particular, even when nitriding is omitted to improve spring performance, 0.5% or more of C is required to ensure sufficient spring strength. If it exceeds 0.9%, substantial hypereutectoid precipitation occurs, and a large amount of coarse cementite precipitates, resulting in a significant reduction in toughness. This simultaneously degrades the coiling characteristics.
• carbide dilute area The area ratio of the area smaller than the part (hereinafter referred to as carbide dilute area) is likely to increase, and it is difficult to obtain sufficient strength and toughness or coiling (ductility). Therefore, it is preferably 0.55% or more, and more preferably 0.6% or more from the viewpoint of balance of strength and coiling.
• S i is a necessary element to ensure the strength, hardness and sag resistance of the spring, and if it is small, the required strength and sag resistance are insufficient. .
• Si also has the effect of spheroidizing and refining the carbide-based precipitates at the grain boundaries, and by adding it positively, it has the effect of reducing the area occupied by the grain boundaries. However, adding too much will not only cure the material, but will also embrittle. So after quenching and tempering In order to prevent embrittlement, 3.0% was made the upper limit.
• Si is an element that also contributes to temper softening resistance, and is preferably added in a large amount to prepare a high-strength wire. Specifically, it is preferable to add 2% or more. On the other hand, in order to obtain stable coating properties, it is preferably 2.6% or less.
• Mn is frequently used for deoxidation and fixing S in steel as MnS, as well as enhancing the hardenability and obtaining sufficient hardness after heat treatment.
• the lower limit is 0.1%.
• the upper limit was set to 2.0%. Further, in order to achieve both strength and coiling properties, it is preferably 0.3 to 1%. If priority is given to coiling, it is effective to make it 1.0% or less.
• Cr is an effective element for improving hardenability and temper softening resistance. Furthermore, it is an effective element not only for ensuring the tempering hardness but also for increasing the surface hardness after nitriding and the depth of the hardened layer in the nitriding treatment found in recent high-strength valve springs.
• the amount added is large, not only will the cost be increased, but the cementite seen after quenching and tempering will be coarsened. It also has the effect of stabilizing and coarsening the alloy carbide. As a result, since the wire becomes brittle, there is also a detrimental effect that breakage is likely to occur during coiling. Therefore, when Cr is added, the effect is not clear unless it is 0.1% or more.
• the upper limit was set to 2.5%, at which embrittlement becomes significant.
• carbides are finely controlled by defining N, a large amount of Cr can be added.
• the addition of Cr can deepen the hardened layer by nitriding. Therefore, addition of 1.1% or more is preferable, and addition of 1.2% or more is preferable in order to make it suitable for nitriding for an unprecedented high strength spring.
• Cr inhibits dissolution of cementite by heating, especially when C> 0.55% and the amount of C increases, suppressing the amount of Cr can suppress the formation of coarse carbides, making it easier to achieve both strength and coiling properties. . Therefore, it is preferable that the amount of addition be 2.0% or less. More preferably, it is about 1.7% or less.
• V is a secondary precipitation hardening that precipitates and hardens carbides during tempering. Therefore, V can be used for hardening the steel layer at the tempering temperature and for hardening the surface layer during nitriding. Furthermore, it is effective in suppressing the coarsening of the austenite particle size due to the formation of nitrides, carbides and carbonitrides, and it is preferable to add them. Until now, however, V nitrides, carbides, and carbonitrides have been generated even at austenization temperature A of 3 or higher in steel, so if the solid solution is insufficient, undissolved carbides (nitriding) It was easy to remain as a product.
• the added amount is 0.15% or less, the effect of adding V, such as increasing the hardness of the nitrided layer if the hardness of the nitrided layer is increased, is small, and sufficient fatigue durability over conventional steel cannot be ensured. If the added amount exceeds 1.0%, coarse undissolved inclusions are formed, and the toughness is reduced. Like Mo, an undercooled structure is likely to occur, causing cracks and wire breakage during wire drawing. Cheap. For this reason, the upper limit was set to 1.0%, which is industrially stable and easy to handle.
• V nitride, carbide, carbonitride is the austenization temperature of steel A 3 Since it is formed even above the point, it is likely to remain as undissolved carbide (nitride) if the solid solution is insufficient. Therefore, considering the current industrial nitrogen amount control capability, it is industrially preferably 0.5% or less, and more preferably 0.4% or less.
• the surface hardening treatment by nitriding is most heated to a temperature of 300 or more, it is necessary to add more than 0.15% in order to suppress hardening of the outermost layer and softening of internal hardness due to nitriding. Is preferably added at 0.2% or more.
• a 1 is a deoxidizing element and affects oxide formation. Especially in high-strength valve springs, hard oxides centering on A 1 2 0 3 tend to be the starting point of fracture, so this must be avoided. To that end, it is important to strictly control the amount of A1. In particular, when the tensile strength of a heat-treated steel wire exceeds 2 100 MPa, strict control of oxide-forming elements is essential to reduce the variation in fatigue strength. In the present invention, it is defined that A 1: 0.005% or less. This is because if it exceeds 0.005%, an oxide mainly composed of A 1 2 0 3 is produced and breakage due to the oxide occurs, so that sufficient fatigue strength and quality stability cannot be secured. Further, when high fatigue strength is required, it is preferably 0.003% or less.
• N is a big point, and in the present invention, a strict limit value is defined as N ⁇ 0.007%. This is because of the new focus on the role of N in spring steel.
• the effect of N control and the reason for the definition in the present invention are described below.
• the effects of N in steel are as follows: 1) Solid solution N exists in the ferrite and hardens the ferrite by suppressing the movement of dislocations in the ferrite. 2) Ti, Nb, V, A l , B and other alloying elements and nitrides are produced, affecting steel performance. The mechanism will be described later. 3) It affects the precipitation behavior of ferrous carbides such as cementite and affects steel performance.
• V produces precipitates in steel at high temperatures. Its chemical composition is mainly nitride at high temperatures, and changes its form to carbonitride and carbide with cooling. Therefore, nitrides formed at high temperatures tend to become V carbide precipitation nuclei. This is easy to produce undissolved carbides during heating in the patenting quenching process, and since it becomes a nucleus, it is easy to grow its size. Furthermore, from the viewpoint of cement tying, high strength springs like this one are tempered at a tempering temperature of 300-500 because of their required strength.
• one or two of Ti and Nb are added in a trace amount.
• the N content can be suppressed to 0.003% or less, good performance can be obtained without adding any one or two of Ti and Nb, but industrially stable 0.003% or less. This is disadvantageous in terms of manufacturing costs. Therefore, add a trace amount of either 1 or 2 of Ti and Nb.
• Ti or Nb When Ti or Nb is added, these elements produce nitrides at high temperatures, so the amount of dissolved nitrogen is substantially reduced, so that the same effect as that obtained by reducing the amount of N added can be obtained. Giru. Therefore, the amount of N added The upper limit may be increased.
• the N content exceeds 0.007%, the amount of nitride of V, Nb or Ti increases, resulting in an increase in undissolved carbides and an increase in hard inclusions such as TiN. For this reason, the upper limit of the N content was limited to 0.007% because the toughness was reduced and the fatigue durability was degraded.
• the upper limit of the soot amount is preferably 0.005% or less, and more preferably 0.004% or less.
• Such precise soot control suppresses the embrittlement of the ferrite and suppresses the formation and growth of undissolved carbides by suppressing the formation of V-based nitriding and products.
• toughness can be improved by controlling the form of the iron-based carbide. In other words, if the soot exceeds 0.007%, V-based nitrides are likely to be formed, a large amount of undissolved carbides are formed, and the steel becomes brittle due to the form of ferrite and carbides.
• the content is preferably 0.005% or less in consideration of the capacity for heat treatment and the like.
• the lower limit of the N amount is preferable, it is preferable that the production cost is less than 0.0015% considering the ease in the denitrification process due to the contamination from the atmosphere such as the steelmaking process. Is preferred.
• Nb produces nitrides, carbides, and carbonitrides, and nitrides are produced at a higher temperature than V. For this reason, Nb in the steel is consumed by generating Nb nitride during cooling, and the formation of V-based nitride can be suppressed. As a result, the formation of V-based undissolved carbide can be suppressed, so that temper softening resistance and workability can be ensured.
• austenite with Nb-based carbonitrides reduces the coarsening of the soot grain size.
• Hardening of steel wire at tempering temperature can be used for hardening of surface layer during nitriding
• FIG. 1 shows the results of measuring the impact values of the materials having the chemical components shown in Table 1, and shows the results of measuring the impact values of samples A and B heat-treated by the methods of the examples described later. As shown in Fig. 1, it can be seen that the steel with Nb slightly added and N controlled has a higher impact value overall.
• the amount added when Ti is added, the amount added is 0.001% or more and less than 0.005%. Since Ti is a deoxidizing element and a nitride and sulfide-forming element, it affects the generation of oxides, nitrides, and sulfides. Therefore, if a large amount is added, hard oxides and nitrides are likely to be formed. If carelessly added, hard carbides are formed and fatigue resistance is reduced. As with A1, especially for high-strength springs, the fatigue stability of the spring is reduced more than the fatigue limit of the spring itself, and if the amount of Ti is large, the incidence of fracture due to inclusions increases. It is necessary to control less than 0.005%.
• T i generates T i N at a high temperature in the molten steel, and thus has a function of reducing sol. N in the molten steel.
• the point of technology is to suppress the formation of V-based nitrides and further suppress the growth of V-based undissolved carbides. Therefore, if N is consumed at a temperature equal to or higher than the V-based nitride formation temperature, the growth of V-based nitrides and V-based carbonitrides that grow during cooling can be suppressed.
• the amount of N that binds to V substantially decreases, so that the temperature of formation of V-based nitrides can be lowered, and V-based undissolved carbides can be suppressed.
• addition of a large amount of Ti should be avoided from the viewpoint of Ti-based undissolved carbonitride and oxide formation, but addition of a small amount can lower the V-based nitride formation temperature. Can be reduced.
• the amount added is 0.001% or more, and if it is less than 0.001%, there is no effect of N consumption, no effect of suppressing V-based undissolved carbides, and no improvement in workability.
• the amount of Ti added is preferably 0.003% or less.
• the above-mentioned components are used as basic components, and further components for improving the properties of the steel can be added.
• one or two of W and Mo are applied to strengthen the temper softening resistance.
• W not only improves hardenability but also produces carbides in the steel and increases strength, and is effective in imparting temper softening resistance. Therefore, it is preferable to add as much as possible. Since W generates carbides at low temperatures, including Ti and Nb, it is difficult to generate undissolved carbides.
• temper softening resistance can be imparted by precipitation hardening. In other words, the internal hardness is not significantly reduced even during nitriding or strain relief annealing.
• the added amount is 0.05% or less, no effect is observed. If the added amount is 0.5% or more, coarse carbides are formed, and mechanical properties such as ductility may be impaired. 05 to 0.5%. Furthermore, if considering the ease of heat treatment, 0.1 to 0.4% is preferable. Especially bad effects such as supercooled structure immediately after rolling. In order to obtain the maximum temper softening resistance while avoiding harm, addition of 0.15% or more is further preferable.
• Mo improves hardenability and precipitates as carbides at a temperature of about the tempering nitriding temperature, so it can provide temper softening resistance. Therefore, even after heat treatment such as tempering at a high temperature or strain relief annealing or nitriding that is performed in the process, high strength can be exhibited without softening. This suppresses a decrease in the internal hardness of the spring after nitriding, and facilitates hot settling and strain relief annealing, thus improving the fatigue characteristics of the final spring. That is, the tempering temperature when controlling the strength can be increased. This increase in the tempering temperature is advantageous for reducing the grain boundary area ratio of the grain boundary carbide.
• the grain boundary carbides precipitated in a film form are tempered by tempering at a high temperature, and the interfacial area ratio is reduced.
• Mo also produces Mo-based carbides separately from cementite in steel.
• its precipitation temperature is lower than that of V and so on, so it has the effect of suppressing carbide coarsening. The effect is not observed when the amount added is 0.05% or less.
• the addition amount is large, an overcooled structure is likely to be generated by softening heat treatment before rolling or wire drawing, which is likely to cause wire breakage during wire drawing. That is, at the time of wire drawing, it is preferable that the steel material is drawn before forming a ferrite toprite structure by patenting.
• Mo is an element that greatly imparts hardenability, so if the amount of addition increases, the time until the end of the perlite transformation becomes longer, and an overcooled structure is likely to occur in the patenting process during cooling after rolling. If it causes breakage or does not break and exists as an internal crack, the characteristics of the final product will be greatly degraded. If Mo exceeds 0.5%, the hardenability increases, making it difficult to industrially make a ferrite toprite structure, so this is the upper limit. Rolling and wire drawing Suppresses the formation of martensite structures that reduce manufacturability in the manufacturing process.
• the content is preferably 0.4% or less, and more preferably about 0.2%.
• V, Nb and Ti produce nitrides as described above, and further use them as nuclei for carbides.
• W and Mo produce almost no nitride, so they can be added to strengthen the softening resistance without being affected by the amount of N.
• the softening resistance can be strengthened even with V, Nb, and Ti, the addition amount is naturally limited to strengthen the softening resistance while avoiding undissolved carbides. Therefore, when no undissolved carbides are generated, and when higher softening resistance is required, nitrides are not generated, and carbides are precipitated at a relatively low temperature, and the addition of W or Mo functions as a precipitation strengthening element.
• Ni improves the hardenability and can be stably strengthened by heat treatment.
• the ductility of the matrix is improved to improve the coilability.
• quenching and tempering increases residual austenite soot, which is inferior in terms of sag and material uniformity after spring forming. If the added amount is 0.05% or less, no effect is observed in increasing strength and improving ductility.
• decarburization can be prevented by adding Cu.
• the decarburized layer reduces the fatigue life after spring processing, so minimize it as much as possible. Efforts are being made.
• the surface layer is removed by a peeling process called peeling.
• it has the effect of improving corrosion resistance.
• the effect of suppressing decarburization and improving corrosion resistance of Cu can be exerted at 0.05% or more. Even if Ni is added as described later, if it exceeds 0.5%, it tends to cause rolling flaws due to embrittlement.
• the lower limit to 0.05% and the upper limit to 0.5%.
• Almost no mechanical properties at room temperature are impaired by the addition of Cu.
• the hot ductility may be deteriorated and cracking may occur on the surface of the billet during rolling.
• the amount of Ni added to prevent cracking during rolling is [Cu%] ⁇ [Ni%] according to the amount of Cu added. In the range of Cu 0.3% or less, rolling flaws do not occur, so there is no need to regulate the amount of Ni added to prevent rolling flaws.
• Co can reduce the hardenability, but can improve the high-temperature strength. Further, since it inhibits the formation of carbides, it functions to suppress the formation of coarse carbides that are a problem in the present invention. Therefore, coarsening of carbides including cementite can be suppressed. Therefore, it is preferable to add. When added, the effect is small at 0.05% or less. However, adding a large amount increases the hardness of the ferrite phase and lowers the ductility, so the upper limit was made 3.0%. Industrially, stable performance can be obtained at 0.5% or less.
• B is effective in cleaning hardenability improving elements and austenite grain boundaries.
• elements such as P and S that reduce the toughness by praying to the grain boundaries.
• the lower limit is 0.0005% at which the effect becomes clear, and the upper limit is 0.0060% where the effect is saturated. .
• it is preferably 0.003 or less, and more preferably it is effective to fix free N by a nitride-forming element such as Ti or Nb to make B: 0.0010 to 0.0010% It is.
• Ni, Cu, Co, and B are mainly effective in strengthening the matrix phase.
• this element is effective in securing strength by strengthening the matrix if the optimum balance between softening resistance and workability by carbide control cannot be obtained.
• Te, Sb, Mg, Zr, Ca, and Hf is required to be further improved in performance and stability, the form of oxide and sulfide It is added as an element for controlling.
• Te has the effect of spheroidizing MnS.
• the effect is not clear if the content is less than 0.02%, and if the content exceeds 0.01%, the toughness of the matrix decreases, hot cracking occurs, and fatigue resistance decreases. Therefore, the upper limit is 0.0 1%.
• Sb has the effect of spheroidizing MnS, and if it is less than 0.00002%, the effect is not clear. If it exceeds 0.01%, the toughness of the matrix is reduced, causing hot cracking or fatigue. The harmful effect of lowering durability becomes prominent, so 0.0 1% is made the upper limit.
• Mg forms oxides in the molten steel at a temperature higher than the MnS formation temperature, and already exists in the molten steel when MnS is formed. Therefore, it can be used as Mn S precipitation nuclei, which can control the distribution of MnS.
• the number distribution of Mg-based oxides is more finely dispersed in the molten steel than the Si and A1-based oxides often found in conventional steels. Will be distributed. Therefore, even if the S content is the same, the MnS distribution varies depending on the presence or absence of Mg, and the force to add them ⁇ MnS particle size becomes finer. The effect can be obtained even in a small amount, and if Mg is added, MnS is fine Turn into.
• the amount of Mg added was set to 0.0001 to 0.0005%.
• the content is preferably 0.0003% or less.
• Zr is an oxide and sulfide-forming element.
• oxides are finely dispersed, and like Mg, they become precipitation nuclei for MnS. As a result, fatigue durability is improved, and ductility is increased to improve coiling. If less than 0.001%, the effect is not seen, and even if added over 0.0005%, the formation of hard oxide is promoted, so even if the sulfide is finely dispersed, troubles due to oxide occur. It becomes easy.
• nitrides and sulfides such as ZrN and ZrS are generated in addition to oxides, which reduces manufacturing troubles and fatigue durability of springs.
• the amount added is preferably 0.0003% or less. Although these elements are in trace amounts, they can be controlled by carefully selecting by-products and controlling the refractory etc. precisely.
• Zr refractories can be used in places such as ladle, tundish, and nozzle where they are in contact with molten steel for a long time, so about 1 ppm can be added to about 200 t of molten steel.
• auxiliary materials so that the specified range is not exceeded while taking this into consideration.
• the analysis method of Zr in steel is to collect 2 g from the part not affected by the surface scale of the steel to be measured, treat the sample in the same way as in JISG 1237-1997 Annex 3, It can be measured. At this time, the calibration curve for ICP is set to be suitable for trace Zr.
• Ca is an oxide and sulfide-forming element.
• MnS for spring steel By spheroidizing, the length of MnS as a starting point of fracture such as fatigue can be suppressed and rendered harmless. The effect is not clear if it is less than 0.0002%, and even if added over 0.01%, not only the yield is bad, but oxides and sulfides such as CaS are generated, and manufacturing trouble and fatigue resistance characteristics of the spring Since it decreases, it was made 0.01% or less.
• the amount added is preferably 0.001% or less.
• Zr is an oxide and sulfide-forming element when finely dispersed.
• oxides are finely dispersed, and like Mg, they become MnS precipitation nuclei.
• fatigue durability is improved, and the ductility is increased to improve the coilability.
• the effect is not clear if it is less than 0.0002%, and even if added over 0.01%, not only the yield is bad, but also oxides, nitrides such as ZrN, ZrS, and sulfides are generated, and manufacturing troubles and To reduce the fatigue endurance characteristics of the spring, it was set to 0.01% or less. This addition amount is preferably 0.003% or less.
• P and S are not included in the claims, but restrictions are necessary. 'P hardens the steel, but further segregates and embrittles the material.
• P that has broken to the austenite grain boundary causes a delayed fracture due to a drop in impact value or hydrogen penetration. Therefore, it is better to have less. Therefore, it is preferable to set P: 0.015% or less where embrittlement tendency becomes remarkable.
• the tensile strength of the heat-treated steel wire is high such that it exceeds 2150 MPa, it is preferably less than 0.01%.
• MnS MnS also takes the form of inclusions, so the fracture characteristics deteriorate.
• high-strength steel may cause fracture from a small amount of MnS, and it is desirable to reduce S as much as possible.
• the adverse effect is obvious It is preferable that the content is 0.015% or less.
• the tensile strength of the heat-treated steel wire is high such that it exceeds 2 150 MPa, it is preferable to make it less than 0.01%.
• t — ⁇ is 0.002 to 0.01%.
• this total oxygen content (t 1 O) is large, it means that there are many oxide inclusions.
• the size of the oxide inclusions is small, the spring performance will not be affected, but if a large amount of large oxides are present, the spring performance will be greatly affected.
• the oxygen content exceeds 0.01%, the spring performance is remarkably deteriorated, so the upper limit is preferably made 0.01%.
• the amount of oxygen is small, but even if it is less than 0.0002%, the effect is saturated, so this is preferably set as the lower limit. Considering the ease of practical deoxidation process, it is desirable to adjust to 0.0005% to 0.005%.
• the tensile strength is preferably 2000 MPa or more. If the tensile strength is high, the fatigue characteristics of the spring tend to improve. Even when a surface hardening treatment such as nitriding is performed, if the basic strength of the steel wire is high, even higher fatigue characteristics and sag characteristics can be obtained. On the other hand, if the strength is high, the coiling property is lowered, and the spring manufacturing becomes difficult. Therefore, it is important not only to improve the strength, but also to provide ductility that can be simultaneously coated.
• Steel wire strength is required from the viewpoint of fatigue and sag, but the tensile strength TS ⁇ 2000 MPa is the lower limit. Furthermore, when applying to a high-strength spring, higher strength is desirable, preferably 2200 MPa or more, and for applying to a high-strength spring, 2250, 2300 MPa or more, the range that does not impair the coiling property. It is preferable to increase the strength.
• Undissolved carbides For undissolved carbides, C and other so-called alloy elements such as Mn, Ti, V, Nb are added to obtain high strength. When a large amount of elements that form fluoride, carbide, or carbonitride are added, undissolved carbide tends to remain. Undissolved carbide is generally spherical, and is mainly composed of alloying elements and cementite.
• Figure 2 shows a typical observation example.
• Fig. 2 (a) is an example of observation of undissolved carbide using a scanning electron microscope
• (b) is an elemental analysis example of X-ray of alloy undissolved carbide X
• (c) is cementite undissolved carbide.
• An example of elemental analysis by X-ray of Y is shown.
• two types of matrix acicular and spherical structures are found in steel.
• steel is known to form a martensitic needle-like structure by quenching and to generate carbides by tempering to achieve both strength and toughness.
• FIGS. 2 (a) X and Y, not only the needle-like structure but also many spherical structures may remain.
• the undissolved carbide mentioned here is mainly composed of Fe carbide (cementite) as well as so-called alloy-based spherical carbide (X) in which the above alloy forms nitride, carbide, and carbonitride.
• X cementite spherical carbide
• Figures 2 (b) 'and (c) show examples of analysis by EDX attached to SEM.
• the conventional invention focuses only on carbides of alloy elements such as V and Nb, an example of which is shown in Fig. 2 (b).
• the Fe peak in the carbide is relatively small, and the alloy peak (in this example) V) is large.
• alloy-based carbides become composite carbides with nitrides (so-called carbonitrides). Therefore, here, these alloy-based carbides, nitrides and their composite alloy-based spherical precipitates are used. Collectively referred to as alloy-based spherical carbide.
• the following regulations are important for the alloy spherical carbide and cementite spherical carbide occupying the mirror surface, and the following regulations are important in order to eliminate the harmful effects caused by these.
• Occupied area ratio of equivalent circle diameter 0.2 / m or more is 7% or less
• These steel alloys and cementite-based carbides can be subjected to etching such as picral or electrolytic etching on mirror-polished samples. However, it is necessary to observe at a high magnification of 3000 times or more with a scanning electron microscope for detailed observation and evaluation of the dimensions and the like.
• the target alloy-based spherical carbide and cementite-based spherical Carbide has an equivalent circle diameter of 0.2 / m or more.
• carbides in steel are indispensable for securing the strength of steel and resistance to tempering and softening, but the effective particle size is 0.1 / zm or less. Austenite does not contribute to particle size refinement, it simply degrades deformation characteristics.
• the old austenite grain size number is 10 or more.
• the old austenite grain size has a great influence on the basic properties of steel wires along with carbides.
• the smaller the old austenite grain size the better the fatigue characteristics.
• no matter how small the austenite grain size If it is contained in a large amount, the effect is small.
• it is effective to lower the heating temperature during quenching to reduce the austenite particle size but this increases the undissolved spherical carbide. Therefore, it is important to finish the steel wire with a balance between the carbide content and the old austenite grain size.
• the carbides satisfy the above requirements, if the old austenite particle size number is less than 10, sufficient fatigue properties cannot be obtained, so that the old austenite particle size number is 10 or more. Stipulated.
• finer grains are preferred for application to high-strength springs ⁇ , 11 and even more than 12 to achieve both high strength and coiling ( ⁇ Is possible.
• the reason why the residual austenite is 15% by mass or less is that the residual austenite often remains in the segregated portion or in the vicinity of the area between the old austenite grain boundaries and subgrains. Residual austenite becomes martensitic due to processing-induced transformation, and when it is transformed during spring forming, a local high-hardness portion is generated in the material, and rather the coiling characteristics as a spring are degraded. Also, recent springs have the ability to reinforce the surface by plastic deformation, such as shot peening and settling, etc. If there is a manufacturing process that includes multiple processes that apply plastic deformation in this way, the induced martensite that occurs at an early stage breaks down. Reduces strain and reduces workability and fracture characteristics of the spring in use.
• the residual austenite is reduced as much as possible, and the processability is improved by suppressing the generation of process-induced martensite. Specifically, the residual austenite amount is 15% (mass%) Beyond the limit, the sensitivity to crushing and the like becomes high, and it easily breaks during coiling and other handling, so it was limited to 15% or less.
• the amount of residual austenite varies depending on the amount of alloying elements such as C and Mn and the heat treatment conditions. Therefore, it is important to enhance not only the component design but also the heat treatment conditions.
• the martensite generation temperature (start temperature M s point, end temperature M f point) is low, martensite will not be generated unless the temperature is sufficiently lowered during quenching, and residual austenite tends to remain. Water or oil is used in industrial quenching, but high heat treatment control is required to control residual austenite defects. Specifically, it is necessary to maintain the cooling refrigerant at a low temperature, maintain a low temperature as much as possible after cooling, and ensure a long transformation time to martensite. Since it is processed on a continuous line industrially, the temperature of the cooling refrigerant easily rises to near 100, but is preferably maintained below at 60, and more preferably below 40 at 40.
• the structure should be avoided where the distribution of carbides is less than in other parts. Specifically, the distribution of carbides in the lens martensite and its tempered structure is smaller than in other parts and the microstructure is inhomogeneous, which adversely affects fatigue strength and workability.
• the tensile strength, the hardness after annealing, the impact value, and the drawing measured in the tensile test are shown as evaluation items.
• the tensile strength is directly related to the durability of the spring, and the higher the strength, the higher the durability.
• the drawing simultaneously measured during tensile strength measurement shows the plastic deformation behavior of the material and is an evaluation index of workability (coiling characteristics) to the spring. This shows that the larger the aperture, the easier it can be processed. Generally, the higher the strength, the smaller the aperture. From the example of conventional steel, this wire diameter evaluation shows that when the restriction exceeds 30%, it is difficult to cause obstacles in industrial mass production even at other wire diameters.
• the prepared specimen was quenched and tempered to a material with a diameter of 13mm exceeding 2200MPa, then created with the HSZ 220 No. 19 specimen, tested in accordance with JISZ 2241, and pulled from its breaking load. Intensity was calculated.
• the surface layer is often hardened by nitriding to increase the strength of the spring.
• nitriding the spring is heated to 400 to 500 in a nitriding atmosphere gas, and the surface layer is hardened by holding for about several minutes to 1 hour. At that time, the inside where nitrogen does not enter is heated and thus annealed and softened. Since it is important to suppress this softening, the hardness after annealing in which nitriding was simulated was used as an evaluation item for the softening resistance.
• Charpy impact value was used as an evaluation item in order to evaluate the workability and fracture resistance of the material.
• materials with good impact values are considered to have good fracture resistance including fatigue properties.
• materials with high toughness are considered to be excellent in workability.
• the Charpy impact value of a material subjected to the same heat treatment as that for measuring the tensile strength after quenching and tempering was measured. Since the Charpy impact value is also affected by the austenite particle size, the austenite particle size of the same material was also measured.
• Charpy impact test specimens are 2 mm from 13 thigh heat-treated materials to so-called half-size (5 X 10 mm cross-section) materials. The u-notch was processed.
• the spring has a smaller diameter of about 4 mm and finishes the heat treatment in a relatively short time. For this reason, it is known that undissolved carbide tends to remain and the workability is lowered. Therefore, in the examples of the present invention, patenting was performed to obtain ⁇ 4 ⁇ , and the wire was heat treated to measure the distribution of carbides and the austenite particle size. In general, when the heating temperature is low and the time is short, the austenite particle size decreases, but undissolved carbides tend to increase, and should be evaluated comprehensively based on a balance between the two. Since the results appear in the tensile strength and elongation, both of them were evaluated. Small diameter materials with ⁇ 5 mm or less have a small cross-sectional area, so the plastic deformation behavior clearly shows a difference in elongation rather than squeezing.
• test pieces with a parallel part of ⁇ 6 M were prepared according to JIS, and the tensile strength and elongation were measured.
• the amount of residual austenite was measured by X-ray after mirror polishing after quenching and tempering.
• the hardness after annealing was mirror-polished after the heat treatment, and three points of Vickers hardness at a radius of 12 from the surface were measured, and the average value was taken as the hardness after annealing. .
• Invention Example 16 of the present invention produced a billet by rolling after melting in a 2 t-vacuum melting furnace. At that time, in the invention example, it was held at a high temperature of 1200 or more for a certain time. After that, in all cases, it was rolled from billet ⁇ to ⁇ 13 dragons.
• the tempering temperature varies depending on the chemical component, but in the present invention, heat treatment was performed according to the chemical component so that the tensile strength was 2200 MPa or more.
• the comparative example was simply heat-treated so as to match the tensile strength.
• the softening resistance was evaluated by annealing 400 X 20m imin simulating nitriding and measuring its hardness.
• Tables 2 to 9 show the chemical composition of the present invention and comparative steel when treated at ⁇ 4 mm, cementite carbide dilute area ratio, alloy cementite ⁇ system spherical carbide occupied area ratio, equivalent circle diameter 0.2 to 3 m cementite spherical system Carbide density, cementite spherical carbide density over 3 im equivalent circle diameter, maximum oxide diameter, former austenite grain size number, residual austenite amount (mass%), and resulting tensile strength
• the drawing shows the hardness measured after annealing, impact value, and tensile test. That is, Tables 2 and 3 show the chemical components of Invention Examples Nos. 1 to 25, and Tables 4 and 5 show the chemical components of Invention Examples Nos. 26 to 51.
• Table 6 shows the chemical compositions of Comparative Examples Nos. 52-77.
• Table 7 shows the characteristics of Invention Examples Nos. 1 to 25 and Table 8 of Invention Examples 26 to 51 with and without wire drawing.
• Table 9 shows the characteristics of Comparative Examples No. 52 to 77 with and without wire drawing.
• T i was added and N was fixed as T i N, and the T i addition amount was excessive, and the adverse effect due to T iN became healthy.
• the inclusion distribution increases, and as a result, the elongation after drawing or wire drawing in the tensile test is low, and the workability is reduced.
• Example 57 by reducing the heating temperature during quenching, This is a case where undissolved carbide is produced.
• Examples 60 to 62 Nb was added, but since the amount added was excessive, a large amount of undissolved carbide was observed, and the elongation after drawing or drawing in the tensile test was low, resulting in reduced workability. . In Examples 63 and 64, since A 1 was excessive, the oxide became larger and the fatigue characteristics deteriorated.
• Examples 65 and 66 the amount of V added is too small. In this case, the hardness after annealing in which nitriding has occurred is low, and the grain size of old austenite tends to become coarse, resulting in fatigue. Characteristics are degraded. Furthermore, in actual nitridation, compared with the invention example in which a prescribed amount of V was added, the surface layer hardness was low, and the nitridation depth was shallow even in the same nitridation time. .
• Examples 74 to 77 are cases where C or Si is less than specified, and the fatigue strength cannot be ensured because the tensile strength after annealing is reduced.
• the steel of the present invention is a cementite system and steel in cold coil spring steel wires.
• the strength is increased to 2000 MPa or more, and the coiling property is secured and high strength is achieved.

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