JP3971569B2 - Hot rolled wire rod for high strength springs - Google Patents

Description

【００６５】
【発明の効果】
本発明ばね用熱間圧延線材は、鋼中セメンタイトを含む炭化物の析出を制御可能な成分とすることで高強度化可能な成分系を有しているにもかかわらず工業的に製造可能にした。また熱処理加工後には高強度のばね製造を可能にした。特に冷間コイリングするばねにおいても強度を１９００ＭＰａ以上に高強度化するとともに、コイリング性を確保し高強度かつ破壊特性に優れたばねを製造可能になる。
【図面の簡単な説明】
【図１】焼入れ焼戻し組織を示す鋼の顕微鏡写真である。
【図２】セメンタイト系球状炭化物分析例を示す図である。
【図３】ノッチ曲げ試験方法を示す図である。
【符号の説明】
１ 球状炭化物
２ 溝（ノッチ）
３ 荷重
θ 測定角度[0001]
BACKGROUND OF THE INVENTION
The present invention has high strength and high toughness after heat treatment and is used for a spring for automobiles and general machines. Hot rolled wire rod It is about.
[0002]
[Prior art]
With the reduction in weight and performance of automobiles, springs have also been strengthened, and high-strength steel having a tensile strength exceeding 1600 MPa after heat treatment is used for the springs. In recent years, steel having a tensile strength exceeding 1900 MPa has also been used.
[0003]
In the coil spring manufacturing method using steel, the steel is heated to the austenite region and coiled, and then hot coiling in which quenching and tempering is performed and high-strength steel wire that has been previously quenched and tempered are cold-coiled. There is cold coiling. In any case, the basic strength of the spring is determined by quenching and tempering. Therefore, for spring steel, component design considering the characteristics after quenching and tempering is important.
[0004]
Specifically, in Japanese Patent Application Laid-Open No. 57-32353, the addition of elements such as V, Nb, and Mo improves the hardenability and generates fine carbides that precipitate by tempering. It limits movement and improves sag resistance.
[0005]
However, in the steel manufacturing process, heating is repeated many times, such as converter-casting-billet rolling-wire rolling, and at the same time, cooling to room temperature is performed many times. In such a case, the added carbide-generating elements such as Cr, V, Nb, and Mo tend to remain in the steel as coarse carbides at the same time as hardening the steel. In particular, when high strength exceeding a tensile strength of 1900 MPa is directed, the amount of addition of these alloy elements increases, so that a large amount of carbide remains. Until now, JP-A-11-6033 and the like have invented inventions that define the size of carbides such as Cr, V, Nb and Mo (hereinafter referred to as alloy carbides). However, it is not these fine carbides that actually dominate the strength of steel, but the behavior of iron carbides, that is, carbides mainly composed of cementite (hereinafter referred to as cementite-based carbides). Important for steel.
[0006]
Regarding the particle size of the alloy-based carbide, for example, as disclosed in Japanese Patent Application Laid-Open No. 10-251804, there has been an invention focusing on the average particle size of Nb and V-based carbides. There is a description concerned about the occurrence of an abnormal structure (paragraph 0015), and practically recommends dry rolling. This is an industrially unsteady operation, and it is presumed that it is clearly different from normal rolling, suggesting that even if the average grain size is controlled, unevenness in the surrounding matrix structure will cause rolling trouble. is doing. Therefore, it is shown that it is industrially insufficient only to control the average particle size of alloy carbides such as V and Nb carbides.
[0007]
[Problems to be solved by the invention]
The present invention is a spring that can be manufactured industrially and can provide strength and coiling for a spring after quenching and tempering. Hot rolled wire rod It is an issue to provide.
[0008]
[Means for Solving the Problems]
The inventors have used conventional springs Hot rolled wire rod By reducing the size of carbides in steel, especially cementite, which were not noticed in Japan, both high strength and coiling properties can be achieved after quenching and tempering. Hot rolled wire rod for spring Led to the development.
[0009]
That is, the present invention provides the following spring Hot rolled wire rod Is the gist.
[0010]
(1) In mass%,
C: 0.4-0.8%
Si: 0.9-3.0%
Mn: 0.1 to 2.0%,
P: 0.015% or less,
S: 0.015% or less,
Cr: 0.16 to 0.30%,
N: 0.001 to 0.007%,
The remaining density of iron and inevitable impurities, and the density of cementite-based spherical carbide having an equivalent circle diameter of 0.2 to 3 μm in the microstructure after hot rolling is 0.5 / μm ² Hereinafter, the existence density of cementite-based spherical carbide having an equivalent circle diameter of more than 3 μm is 0.005 / μm. ² Spring characterized by Hot rolled wire rod .
[0011]
(2) Furthermore, in mass%,
W: 0.05-1.0%
Co: 0.05-5.0%
1 type or 2 types of these are contained, The spring of said (1) description characterized by the above-mentioned Hot rolled wire rod .
[0012]
(3) Furthermore, in mass%,
Ti: 0.005 to 0.1%,
Mo: 0.05-1.0%,
V: 0.05-0.7%
Nb: 0.01 to 0.05%
The spring according to (1) or (2) above, comprising one or more of Hot rolled wire rod .
[0013]
(4) Furthermore, in mass%,
B: 0.0005 to 0.006%
The spring according to any one of the above (1) to (3), characterized in that Hot rolled wire rod .
[0014]
(5) Furthermore, in mass%,
Ni: 0.05-5.0%,
Cu: 0.05 to 0.5%
The spring according to any one of (1) to (4) above, which contains one or two of Hot rolled wire rod .
[0015]
(6) Furthermore, in mass%,
Mg: 0.0002 to 0.01%
The spring according to any one of (1) to (5) above, characterized in that Hot rolled wire rod .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The inventor obtains high strength by defining an appropriate chemical composition, and controls the shape of carbide in steel by heat treatment to ensure sufficient coiling characteristics when manufacturing the spring. Hot rolled wire rod It came to invent.
[0017]
Details are shown below. First, the reason for defining the chemical composition of steel will be described.
[0018]
C is an element that greatly affects the basic strength of the steel material, and is set to 0.4 to 0.8% in order to obtain sufficient strength. If the C content is less than 0.4%, sufficient strength cannot be obtained, and a larger amount of other alloy elements must be added. If it exceeds 0.8%, a large amount of coarse cementite precipitates after normal rolling. , Significantly reduce toughness. This reduction in toughness simultaneously reduces the coiling characteristics. In addition, there are industrial disadvantages such as the necessity of setting a high heat treatment temperature in the spring steel manufacturing process and difficulty in high-frequency treatment.
[0019]
Si is an element necessary for ensuring the strength, hardness and sag resistance of the spring, and if it is small, the necessary strength and sag resistance are insufficient, so 0.9% was made the lower limit. Si also has the effect of spheroidizing and refining the carbide-based precipitates at the grain boundaries, and positively adding it has the effect of reducing the grain boundary occupation area ratio of the grain boundary precipitates. However, adding too much will not only cure the material, but will also embrittle. Therefore, in order to prevent embrittlement after quenching and tempering, the upper limit was made 3.0%.
[0020]
Mn improves hardenability and hardens the matrix. Moreover, S which exists in steel can be fixed as MnS, and S can be made harmless. Further, it is an element that can ensure strength without making carbide against the behavior of carbide that is particularly focused on in the present invention. Therefore, 0.1% is made the lower limit in order to fix S as MnS. In order to ensure strength, Mn is preferably 0.5% or more. In order to prevent embrittlement due to Mn, the upper limit was made 2.0%.
[0021]
Cr is an effective element for improving hardenability and temper softening resistance. When hardening the spring surface by nitriding to improve the spring fatigue strength, the hardened layer has a shorter nitriding treatment when the Cr content is higher. Deepens and the maximum hardness tends to increase. Therefore, when nitriding is assumed, Add 0.16% or more of Cr. However, if the amount added is large, not only the cost is increased, but cementite that is found after quenching and tempering is coarsened. As a result, since the wire becomes brittle, it is easy to cause breakage during coiling. In particular, since Cr dissolves in the cementite precipitated after rolling, it stabilizes the cementite and tends to be undissolved during quenching heating. This point has a great influence on oil tempered wires and high-frequency heat treatment materials. Therefore, it is difficult to solidify cementite during quenching and heating during spring manufacturing, and heat treatment during spring or spring steel wire manufacturing becomes extremely difficult. 0.30% Was the upper limit.
[0022]
N easily forms a nitride when an element that generates a nitride such as V or Nb is added. They facilitate the formation of carbonitrides. Since these carbonitrides are pinned particles that suppress austenite grain growth during quenching, they are effective in reducing the austenite grain size. For this purpose, 0.001% or more of N is added. On the other hand, excessive N causes coarsening of nitrides and carbonitrides and carbides produced using nitrides as nuclei, so the upper limit was made 0.007%.
[0023]
P hardens the steel but further segregates and embrittles the material. In particular, P segregated at the austenite grain boundaries causes a delayed fracture or the like due to a drop in impact value or hydrogen penetration. Therefore, it is better to have less. Therefore, it was limited to 0.015% or less where the embrittlement tendency becomes remarkable.
[0024]
If S is present in the steel as in the case of P, the steel is embrittled. Although the effect is reduced as much as possible by Mn, since MnS also takes the form of inclusions, the fracture characteristics are lowered. Particularly in the case of high-strength steel, it may break down from a small amount of MnS, and it is desirable to reduce S as much as possible. The upper limit was set to 0.015% at which the adverse effect becomes significant.
[0025]
W improves the hardenability and generates carbides in the steel to increase the strength. This is particularly important because it can suppress the coarsening of cementite and other alloy carbides. If the addition amount is less than 0.05%, no effect is observed, and if it exceeds 1.0%, coarse carbides are formed, and mechanical properties such as ductility may be impaired. 1.0%.
[0026]
Co reduces the hardenability, but can ensure strength at high temperatures. Moreover, in order to inhibit the production | generation of a carbide | carbonized_material, it has the effect | action which suppresses the production | generation of the coarse carbide | carbonized_material which becomes a problem by this invention. If it is less than 0.05%, the effect is small, and if it exceeds 5.0%, the effect is saturated.
[0027]
Although W and Co have different mechanisms in steel, both have the effect of reducing cementite-based carbides. Therefore, when steel has a high C content as in the present invention, cementite is refined and easily solidified. It is effective for solubilization. Therefore, it was decided to add either one or two of W and Co.
[0028]
Ti, Mo, V and Nb precipitate in the steel as nitrides, carbides and carbonitrides. Therefore, if one or more of these elements are added, these precipitates can be produced, temper softening resistance can be obtained, and heat treatment such as tempering at high temperature and strain relief annealing and nitriding that can be performed in the process can be performed. Even if it passes, high strength can be exhibited, without softening. This suppresses a decrease in the internal hardness of the spring after nitriding, and facilitates hot setting and strain relief annealing, so that the fatigue characteristics of the final spring are improved. However, when Ti, Mo, V, and Nb are added in too large amounts, their precipitates become too large and combine with carbon in the steel to produce coarse carbides. This reduces the amount of C that should contribute to increasing the strength of the steel wire, and the strength corresponding to the added amount of C cannot be obtained. Furthermore, since coarse carbide becomes a stress concentration source, it is easy to break due to deformation during coiling.
[0029]
As for Ti, the precipitation temperature of nitride is high, and it is already precipitated in the molten steel. Moreover, since the binding force is strong, it is used also when N in steel is fixed. When B is added, it is necessary to add N as much as possible in order to prevent B from becoming BN. Therefore, it is preferable to fix N with Ti. The minimum amount of Ti added is 0.005%, which is the minimum required amount that can reduce the austenite grain size, and the maximum amount is 0.1% where the precipitate size does not adversely affect the fracture characteristics.
[0030]
Mo improves the hardenability by adding 0.05 to 1.0% and can provide temper softening resistance. That is, the tempering temperature when controlling the strength can be increased. This is advantageous for reducing the grain boundary area ratio of the grain boundary carbide. That is, the grain boundary carbide precipitated in a film shape is tempered by tempering at a high temperature, and it is effective in reducing the grain boundary area ratio. Mo produces Mo-based carbides separately from cementite in steel. In particular, since the precipitation temperature is lower than V or the like, there is an effect of suppressing the coarsening of the carbide. If the added amount is less than 0.05%, no effect is observed, and if it exceeds 1.0%, the effect is saturated.
[0031]
V can also be used for hardening of the steel wire at the tempering temperature and hardening of the surface layer during nitriding, in addition to suppressing the coarsening of the austenite grain size due to the formation of nitrides, carbides, and carbonitrides. If the addition amount is less than 0.05%, the effect of addition is hardly recognized, and if it exceeds 0.7%, coarse undissolved inclusions are generated and toughness is lowered.
[0032]
Nb can also be used for hardening of the steel wire at the tempering temperature and hardening of the surface layer during nitriding, as well as suppressing the coarsening of the austenite grain size due to the formation of nitrides, carbides and carbonitrides. Since Nb produces fine carbides even at higher temperatures than V, Mo, etc., Nb is a very effective element that has a large effect on refining the austenite grain size during the manufacture of heat-treated steel wire, even if the addition amount is very small. If it is less than 0.01%, almost no effect is observed. If it exceeds 0.05%, coarse undissolved inclusions are generated and the toughness is lowered, so the content was made 0.01 to 0.05%.
[0033]
B is known as a hardenability improving element. Furthermore, it is effective in cleaning the austenite grain boundary. In other words, elements such as P and S that segregate at the grain boundaries and reduce toughness are made harmless by adding B, and the fracture characteristics are improved. At that time, if B is combined with N to generate BN, the effect is lost. The lower limit of the amount added is 0.0005% at which the effect becomes clear, and the upper limit is 0.006% at which the effect is saturated.
[0034]
Ni improves the hardenability and can increase the strength stably by heat treatment. In addition, the ductility of the matrix is improved to improve the coilability. Furthermore, the corrosion resistance of the spring is improved, which is effective for a spring used in a corrosive environment. If the added amount is less than 0.05%, no effect is observed in increasing strength and improving ductility, and if it exceeds 5.0%, the effect is saturated, which is disadvantageous in terms of cost.
[0035]
About Cu, decarburization can be prevented by adding Cu. In order to reduce the fatigue life of the decarburized layer after spring processing, efforts have been made to reduce it as much as possible. When the decarburized layer becomes deep, the surface layer is removed by a peeling process called peeling. Moreover, it has the effect of improving corrosion resistance like Ni.
[0036]
Therefore, by suppressing the decarburized layer, the fatigue life of the spring and the peeling process can be omitted. The decarburization suppressing effect and corrosion resistance improving effect of Cu can be exhibited at 0.05% or more, and even if Ni is added as described later, if it exceeds 0.5%, it tends to cause rolling defects due to embrittlement. Therefore, the lower limit is set to 0.05% and the upper limit is set to 0.5%. Although the mechanical properties at room temperature are hardly impaired by addition of Cu, when Cu is added in excess of 0.3%, cracks may occur on the billet surface during rolling in order to deteriorate hot ductility. Therefore, it is preferable that the amount of Ni added to prevent cracking during rolling is [Cu%] <[Ni%] according to the amount of Cu added.
[0037]
Mg is an oxide generating element and generates an oxide in molten steel. The temperature range is higher than the generation temperature of MnS, and already exists in the molten steel when MnS is generated. Therefore, it has been found that it can be used as a precipitation nucleus of MnS, and thereby the distribution of MnS can be controlled. That is, since Mg-based oxides are dispersed in molten steel more finely than Si and Al-based oxides often found in conventional steels, MnS having Mg-based oxides as a nucleus is finely dispersed in steel. Therefore, even if the S content is the same, the MnS distribution differs depending on the presence or absence of Mg, and the addition of them makes the MnS particle size finer. The effect is sufficiently obtained even in a minute amount, and if Mg is 0.0002% or more, MnS is refined. However, since 0.01% or more hardly remains in the molten steel, 0.01% is considered to be the upper limit industrially. Therefore, the amount of Mg added is set to 0.0002 to 0.01%. This Mg is effective in improving corrosion resistance, delayed fracture and preventing rolling cracking due to the effects of MnS distribution and the like, and it is desirable to add Mg as much as possible, and the preferable addition amount is 0.0005 to 0.01%.
[0038]
The problems in manufacturing the spring steel directed to higher strength than the conventional one targeted by the present invention will be described. In springs, the strength is increased by quenching and tempering, but in the conventional component system, the tempering temperature has to be lowered, and generally it becomes brittle and cannot be put into practical use. Further, in the manufacture by cold coiling, coiling is performed after quenching and tempering, and therefore breakage occurs during coiling. Therefore, it is generally performed to slightly increase the amount of C or add an alloy element. However, when alloy elements such as Cr and V are increased, segregation occurs, and the melting point locally lowers at the concentrated portion, so that cracking is likely to occur. This is considered to be a cause of wrinkles during rolling.
[0039]
Further, the carbide to be noted in the present invention will be described. When considering the performance of steel, the form of carbides in the steel becomes important. The term “carbide in steel” as used herein refers to cementite found in the steel after heat treatment in the steel and carbide in which the alloy elements are dissolved (hereinafter, both are collectively referred to as cementite) and alloy elements such as Nb, V, and Ti. Carbides and carbonitrides (hereinafter referred to as alloy carbides). These carbides can be observed by mirror-polishing and etching a steel wire.
[0040]
FIG. 1 shows a photomicrograph of a typical example of a quenched and tempered structure. According to this, two types of carbides, pearlite-like or plate-like precipitates and spherical carbides 1 are observed in the steel. Spring steel is cast, rolled into a billet shape, once cooled to room temperature, and then rolled into a wire size according to the order. Furthermore, in the manufacture of spring steel, quenching and tempering are performed, but pearlite-like or plate-like cementite is easily dissolved, but spheroidized and stabilized carbides are not easily dissolved in the quenching and tempering process in the next process. The strength corresponding to the added amount of C cannot be secured, or the ductility during coiling is reduced. Moreover, it also becomes a cause of rolling defects during wire rod rolling.
[0041]
This residual carbide does not contribute at all to the strength and toughness by quenching and tempering. Therefore, it does not only waste the added C by fixing C in the steel, but it also becomes a source of stress concentration, so the mechanical properties of the steel wire. It becomes a factor to reduce. Since this spherical carbide did not dissolve at the time of reheating after cooling (wire rolling, quenching at the time of spring production, etc.), the carbide grew into a spherical shape. Therefore, it is preferable that the amount is as small as possible immediately after rolling the wire. In particular, the spherical carbide further grows and becomes coarse by heat treatment after rolling such as oil tempering. From this point of view, even if the carbide equivalent to a circle-equivalent diameter of 3 μm or less is not normally a problem, there is a high possibility of a problem. In the present invention, it has been found that cementite mainly composed of Fe and C, which has not been noticed so far, is no exception. This coarse undissolved carbide not only affects the spring production but also causes defects during rolling.
[0042]
This cementite-based carbide includes those in which alloy elements such as Cr and Mo are solid-dissolved in cementite, and in general, cementite in which these solid-solution is stabilized is stabilized and difficult to dissolve. As a feature of the detection, when the carbide appearing by etching is analyzed by EDX, Fe and C are detected as main components, and a solid solution alloy element may also be detected. Hereinafter, such a carbide containing Fe and C as main components is referred to as cementite-based carbide, and a case where the shape is spherical is particularly referred to as cementite-based spherical carbide.
[0043]
FIGS. 2A and 2B show examples of analysis of carbides by EDX attached to the SEM. Similar analysis results can be obtained from the replica method using a transmission electron microscope. The conventional invention focuses only on carbides of alloy elements such as V and Nb added to obtain high strength, and an example thereof is characterized in that the Fe peak in the carbide is very small in FIG. 2 (a). It is. However, in the present invention, not only conventional alloy element carbides but also Fe having an equivalent circle diameter of 3 μm or less as shown in FIG. _Three Attention was paid to precipitation of cementite-based spherical carbides in which C and its alloying elements were slightly dissolved. When achieving both high strength and workability higher than those of conventional steel wires as in the present invention, if there are many cementite-based spherical carbides of 3 μm or less, the workability is greatly impaired.
[0044]
In the case of coiling after quenching and tempering the steel, cementite-based spherical carbide affects the coiling characteristics, that is, the bending characteristics up to fracture. When a large amount of not only C but also alloy elements such as Cr and V are added in order to obtain high strength, a large amount of coarse spherical carbides are generated, which causes deterioration of coiling characteristics and generation of rolling defects. Although it has been attracting attention, as shown in FIG. _Three A so-called cementite-type spherical carbide in which C and its alloying elements are slightly dissolved, and when it is present in large quantities or grows coarsely, it promotes cracking in rolling and mechanical properties of the steel wire after heat treatment Especially, reducing the coiling characteristics.
[0045]
These spherical carbides can be observed by subjecting a mirror-polished sample to etching such as picral, but the target carbides here are observations and dimensions of cementite-based spherical carbides having an equivalent circle diameter of 0.2 to 3 μm. Therefore, it is necessary to observe at a high magnification of 3000 times or more with a scanning electron microscope. Conventionally, fine carbides in steel are indispensable to ensure the strength and resistance to temper softening of steel, but the present inventor has effective particle diameter of 0.1 μm or less, and conversely, equivalent circle diameter. It has been found that when the thickness exceeds 1 μm, there is no contribution to strength and austenite grain size refinement, and the deformation characteristics are merely deteriorated.
[0046]
In the present invention, when the cementite-based spherical carbide size (equivalent circle diameter) is 3 μm or less, not only the size but also the number is a major factor. Therefore, the scope of the present invention is defined in consideration of both. That is, even if the equivalent circle diameter is as small as 0.2 to 3 μm, the number is very large, and the existence density on the microscopic surface is 0.5 / μm. ² If it exceeds, the degradation of the coiling characteristics becomes remarkable.
[0047]
Further, when the size of the carbide exceeds 3 μm, the influence of the size becomes larger, so the existence density on the microscopic surface is 0.005 / μm. ² If it exceeds, the degradation of the coiling characteristics becomes remarkable.
[0048]
Even if these remain immediately after hot rolling, they are not easily dissolved by various heat treatments in the subsequent wire drawing-spring manufacturing process. Accordingly, the density of cementite-based spherical carbide having an equivalent circle diameter of 0.2 to 3 μm in the microstructure after rolling is 0.5 / μm. ² Hereinafter, the existence density of cementite-based spherical carbide having an equivalent circle diameter of more than 3 μm is 0.005 / μm. ² It was as follows.
[0049]
For wire rod rolling, continuous casting → billet rolling → wire rod rolling or continuous casting → wire rod rolling is performed. ₁ Since the temperature is lower than the transformation point, carbide has already precipitated after continuous casting. Therefore, in order to reduce the cementite-based spherical carbide remaining after wire rod rolling, heating for billet rolling and heating for wire rod rolling must be performed at a sufficiently high temperature and for a long time so that coarse carbides dissolve. There is.
[0050]
【Example】
Table 1 shows examples of the present invention and comparative examples.
[0051]
In Example 1 of the present invention, billets were prepared by continuous casting of what was refined by a 250 t converter. In other examples, billets were prepared by rolling after melting in a 2t-vacuum melting furnace. At that time, in the invention example, it was held at a high temperature of 1200 ° C. or higher for a certain time. Thereafter, in each case, the billet was rolled to φ8 mm and drawn to φ4 mm. On the other hand, the comparative example was rolled under normal rolling conditions and used for wire drawing.
[0052]
Since the present invention has excellent characteristics different from those of the prior art in the characteristics after rolling and quenching and tempering after rolling, the evaluation was performed by characteristics immediately after rolling and after quenching and tempering. The soot immediately after rolling was visually observed for the presence of the soot.
[0053]
After drawing to φ4 mm by drawing, so-called oil temper treatment was performed by quenching by passing through the radiation furnace and immediately quenching in oil, and further tempering by passing through molten Pb, followed by quenching and tempering.
[0054]
In the oil temper treatment, the wire passing material was continuously passed through the heating furnace, and the heating furnace passing time was set so that the steel internal temperature was sufficiently heated. If this heating is insufficient, quenching will be insufficient and sufficient strength cannot be achieved. In this example, the heating temperature was 950 ° C., the heating time was 150 sec, and the quenching temperature was 50 ° C. (oil tank). Further, the strength was adjusted by tempering at a tempering temperature of 400 to 550 ° C. and a tempering time of 1 min. The heating temperature during quenching and tempering and the resulting tensile strength in the air atmosphere were as specified in Table 1, and the tensile strength was adjusted to about 2100 MPa.
[0055]
In the examples, spherical carbides in steel containing cementite considered to be important in the present invention are also described. The size and number of the carbides were evaluated by polishing them to the mirror surface in the longitudinal section of the hot-rolled wire and the heat-treated steel wire, and then slightly etching with picric acid to raise the carbides. Since it is difficult to measure the size of carbides at the optical microscope level, photographs of 10 fields of view were randomly taken with a scanning electron microscope at a magnification of 5000 times in a 1 / 2R portion of a hot-rolled wire and a steel wire. Furthermore, the X-ray microanalyzer attached to the scanning electron microscope for the spherical carbide (cementite-based spherical carbide) from the photograph is confirmed to be cementite-based, and the size and number of the image processing apparatus are determined. And measured. Using the data, the equivalent circular diameter and density of each spherical carbide were calculated. Total measurement area is 3088.8 μm ² It is.
[0056]
Tensile properties were measured according to JIS Z 2241 using a JIS Z 2201 No. 9 test piece, and the tensile strength was calculated from the breaking load.
[0057]
The ductility was evaluated by a notch bending test. An outline of the notch bending test is shown in FIG. The following procedure was followed. As shown in FIG. 3 (a), a groove (notch) 2 having a maximum depth of 30 μm is attached perpendicularly to the longitudinal direction of the steel wire by a punch having a tip radius of 50 μm, and both sides are loaded so as to apply a maximum tensile stress to the groove. A three-point bending deformation was applied, which was deformed by applying a load 3 at the center. Bending deformation was continuously applied until breaking from the notch, and the bending angle at the time of breaking was measured. The measurement angle is as shown in FIG. 3B, and the larger the measurement angle (θ), the better the coiling characteristics. Empirically, coiling is difficult for a φ4 mm steel wire at a notch bending angle of 25 ° or less.
[0058]
In the invention example, the size of the spherical carbide after rolling was small, preventing rolling flaws, and exhibiting high strength and good notch bending characteristics after quenching and tempering. However, it was suggested that the comparative example was inferior in notch bending properties and inferior in coiling properties. In addition, rolling wrinkles were observed, which revealed that rolling was difficult.
[0062]
It can also be seen that the steel grades that deviate from the scope of the claims of the present invention basically tend to leave the rolling iron. This is also due to the large amount of cementite-based spherical carbides. Involved It is considered that the cementite-based spherical carbide exceeding the regulation of the present invention is detected in the observation result from the sound part after rolling. This means that even when austenite is formed by heating the steel a plurality of times, if carbide remains in the previous structure, it can be easily used in subsequent processes such as wire rolling, coiling after oil temper treatment, hot coiling, etc. Suggests that this may occur.
[0063]
[Table 1]

[0065]
【The invention's effect】
The present invention Hot rolled wire rod for spring Made it possible to manufacture industrially despite having a component system capable of increasing the strength by controlling the precipitation of carbide containing cementite in steel. In addition, high-strength springs can be manufactured after heat treatment. In particular, even in the case of a cold coiling spring, the strength can be increased to 1900 MPa or more, and the coiling property can be ensured and a spring having high strength and excellent fracture characteristics can be manufactured.
[Brief description of the drawings]
FIG. 1 is a micrograph of steel showing a quenched and tempered structure.
FIG. 2 is a diagram showing an analysis example of cementite-based spherical carbide.
FIG. 3 is a diagram showing a notch bending test method.
[Explanation of symbols]
1 Spherical carbide
2 groove (notch)
3 Load
θ Measurement angle

Claims (6)

In mass%
C: 0.4-0.8%
Si: 0.9-3.0%
Mn: 0.1 to 2.0%,
P: 0.015% or less,
S: 0.015% or less,
Cr: 0.16 to 0.30%,
N: 0.001 to 0.007%,
A cementite system containing the balance iron and unavoidable impurities and having a cementite-based spherical carbide density of 0.5-3 μm or less equivalent circle diameter in the microstructure after hot rolling and 0.5 or less / μm ² equivalent circle diameter. A hot-rolled wire rod for springs, wherein the existence density of spherical carbides is 0.005 pieces / μm ² or less. Furthermore, in mass%,
W: 0.05-1.0%
Co: 0.05-5.0%
The hot-rolled wire rod for springs according to claim 1, comprising one or two of the following. Furthermore, in mass%,
Ti: 0.005 to 0.1%,
Mo: 0.05-1.0%,
V: 0.05-0.7%
Nb: 0.01 to 0.05%
The hot-rolled wire rod for springs according to claim 1 or 2, comprising one or more of the following. Furthermore, in mass%,
B: 0.0005 to 0.006%
The hot-rolled wire rod for a spring according to any one of claims 1 to 3, characterized by comprising: Furthermore, in mass%,
Ni: 0.05-5.0%,
Cu: 0.05 to 0.5%
1 type or 2 types of these are contained , The hot-rolled wire rod for springs in any one of Claims 1-4 characterized by the above-mentioned. Furthermore, in mass%,
Mg: 0.0002 to 0.01%
The hot-rolled wire rod for springs according to any one of claims 1 to 5, characterized in that

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