JP2018062690A - Steel wire and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire excellent in heat conduction characteristics and high temperature strength, and a manufacturing method therefor.SOLUTION: There is provided a steel wire containing C:0.3 to 1.2 mass%, Si:0.01 to 2.5 mass%, Mn:0.01 to 1.0 mass%, P:over 0 mass% and 0.05 mass% or less, S:over 0 mass% and 0.05 mass% or less and Cr:0.7 to 2.0 mass% and the balance iron with inevitable impurities. In the steel wire, maximum diameter of Cr-based carbide contained in the steel wire is 0.3 μm or less and the number thereof is 1.5/μmor less, and tempered martensite is 90% or more in an area ratio.SELECTED DRAWING: None

Description

  The present invention relates to a steel wire and a manufacturing method thereof, and more particularly to a steel wire that can be used as a material for a mechanical structural member such as a piston ring that requires excellent heat conduction characteristics and high-temperature strength, and a manufacturing method thereof.

  With the reduction in weight of automobiles and the increase in output of automobile engines, various parts used in engines are required to have improved various mechanical properties such as strength and toughness. In particular, in parts exposed to high heat, such as pistons and piston rings, in addition to strength and toughness, heat conduction characteristics are also required.

  For recent piston rings, special steels and stainless steels that have been provided with various functions by addition of various alloy elements have been used instead of conventionally used cast irons.

  Special steel can ensure high strength by using martensite as the main phase, and can also ensure excellent fatigue strength. However, when used in a high-temperature environment such as an engine, the strength of the high-strength member is significantly reduced, and the wear is accelerated and the performance of the engine is easily lowered due to the strength reduction. In order to cope with such a problem that occurs in a high temperature environment, the following techniques have been proposed so far.

  For example, in Patent Document 1, by appropriately controlling the short diameter (width) of sulfide inclusions present in steel, a steel material for piston ring and a deformed wire for piston ring having high seizure resistance and high wear resistance. As well as a piston ring.

  Patent Document 2 discloses a piston ring wire that can obtain appropriate hardness by optimizing the composition of the wire and the heat treatment conditions.

Japanese Patent No. 4616209 International Publication No. 2014/054130

  However, the shape control of sulfide inclusions described in Patent Document 1 can improve the wear resistance, but cannot improve the heat resistance and heat conduction characteristics, and cannot fully exhibit the performance as a piston ring. There is a case.

  In Patent Document 2, the Cr content is as high as 11.0 to 14.0% in order to re-optimize the heat treatment conditions, and a large amount of Cr-based carbides are precipitated, so that the thermal conductivity tends to be lowered, and the durability of the engine. May not be enough.

  The present invention has been made in view of the above circumstances, and a main object thereof is to provide a steel wire excellent in heat conduction characteristics and high-temperature strength and a method for producing the same.

The steel wire according to the present invention has C: 0.3-1.2% by mass, Si: 0.01-2.5% by mass, Mn: 0.01-1.0% by mass, P: more than 0% by mass. 0.05% by mass or less, S: more than 0% by mass, 0.05% by mass or less, and Cr: 0.7 to 2.0% by mass with the balance being iron and inevitable impurities. The maximum diameter of the Cr-based carbide contained in the steel wire is 0.3 μm or less, the number thereof is 1.5 pieces / μm ² or less, and the tempered martensite is 90% or more in area ratio. is there.

  The steel wire according to the present invention includes Cu: more than 0% by mass, 0.5% by mass or less, Ni: more than 0% by mass, 1.0% by mass or less, and Mo: more than 0% by mass, 1.5% by mass or less. It may further contain one or more selected from the group consisting of:

  Steel wire according to the present invention is Ti: more than 0% by mass, 0.1% by mass or less, Nb: more than 0% by mass, 0.5% by mass or less, and V: more than 0% by mass, 1.0% by mass or less. It may further contain one or more selected from the group consisting of:

  The method for manufacturing a steel wire according to the present invention includes (a) a step of manufacturing a steel material having the chemical component composition according to any one of claims 1 to 3, and (b) after heating the steel material to 1150 to 1300 ° C. , A step of hot working at a hot working temperature of 750 to 1000 ° C., and (c) after the step (b), cooling from a cooling start temperature of 750 ° C. or higher to 400 ° C. at an average cooling rate of 1 ° C./second or lower. And (d) a step of drawing the wire, and (e) a step of quenching and tempering the drawn wire after the step (d).

  In the method of manufacturing a steel wire according to the present invention, in the step (d), the surface of the wire may be cut before drawing.

  According to the present invention, a steel wire excellent in heat conduction characteristics and high-temperature strength and a method for producing the same are provided.

  Embodiments of the present invention will be described below. However, it should be noted that the embodiments described below are intended to embody the technical idea of the present invention and are not intended to limit the technical scope of the present invention.

  In order to increase the strength of steel, it is effective to make the structure martensite. However, when heat is applied to martensite, C in solid solution aggregates and precipitates as carbide. As the temperature rises, the surrounding carbon absorbs C in the process of growing, so the strength decreases as the temperature rises. In order to increase the high-temperature strength, it is known that suppression of C aggregation is effective, and addition of Si or Cr is effective.

On the other hand, regarding the heat conduction characteristics, the smaller the alloy elements, the better the heat conduction characteristics can be exhibited. However, when Si or Cr is added to improve the high temperature strength, the thermal conductivity is lowered, so it is difficult to provide a steel material that achieves both high temperature strength and high thermal conductivity.
From such a viewpoint, the present inventors have conducted various studies to obtain a steel wire that can achieve both high-temperature strength and high thermal conductivity. As a result of investigating the thermal conductivity when steel is used at high temperature, the dispersion of solid solution elements accompanying the coarsening of carbides is involved in the inhibition of thermal conductivity. It was found that the rate could be demonstrated. For that purpose, if the amount of precipitates generated when adding a large amount of Si or Cr in order to obtain high-temperature strength is sufficiently reduced, it is determined that carbides are not easily generated even when the temperature rises, and high-temperature strength and excellent heat It came to obtain the means with which a conduction characteristic is compatible.

As will be described in detail later, the present inventors set the components in a predetermined range and appropriately control the hot working conditions such as rolling, so that the maximum diameter of the Cr-based carbide is reduced to 0 even after quenching and tempering. It has been found that the thickness can be made 3 μm or less, and the number thereof can be 1.5 pieces / μm ² or less. And it discovered that the steel wire obtained using the wire which has such a characteristic has the outstanding high temperature strength and high heat conductive characteristic.

  Below, the detail of the steel wire which concerns on embodiment of this invention, and its manufacturing method is demonstrated.

  In this specification, “wire” means a steel material that has been hot-formed, such as rolled, and processed into a predetermined size, and “steel wire” is drawn (drawing). It means a heat treatment line obtained by quenching and tempering. In addition, when processing a wire and obtaining a steel wire, you may cut (shaving) as needed before a drawing process.

  In the present specification, the “maximum diameter” means the maximum value of the diameter of the target Cr-based carbide. The maximum diameter can be measured from, for example, an SEM photograph.

<1. Steel wire>
[1-1. Metallographic structure]
In the steel wire according to the present invention, the maximum diameter of the Cr-based carbide contained in the steel wire is 0.3 μm or less, the number thereof is 1.5 pieces / μm ² or less, and tempered martensite has an area ratio. 90% or more.
Hereinafter, each configuration will be described in detail.

(Cr-based carbide contained in steel wire: maximum diameter is 0.3 μm or less, number is 1.5 / μm ² or less)
Cr-based carbide aggregates Cr and forms a Cr-depleted region around it, thereby causing local strength reduction. Further, Cr-based carbide itself also causes a decrease in thermal conductivity. Therefore, Cr-based carbides must be sufficiently small and small. In steel wires, the maximum diameter of Cr-based carbides contained in steel wires is 0.3 μm or less, and if the number is 1.5 pieces / μm ² or less, Si and Cr necessary for securing high-temperature strength are added. Even if the required amount is added, a decrease in strength and a decrease in thermal conductivity can be suppressed, and both high thermal conductivity and high temperature strength can be achieved.
When the maximum diameter of the Cr-based carbide exceeds 0.3 μm, the thermal conductivity-inhibiting action by the Cr-based carbide becomes remarkable, so that the thermal conductivity tends to decrease. From the viewpoint of obtaining more excellent thermal conductivity and high temperature strength, it is preferably 0.25 μm or less, more preferably 0.20 μm or less.
On the other hand, if the number of Cr-based carbides exceeds 1.5 / μm ² , the distance between Cr-based carbides becomes too close, and the thermal conductivity inhibiting action becomes remarkable, so that the thermal conductivity tends to decrease. Furthermore, aggregation of Cr-based carbides tends to occur, and high-temperature strength tends to decrease. From the viewpoint of obtaining more excellent thermal conductivity and high temperature strength, it is preferably 1.2 pieces / μm ² or less, more preferably 1 piece / μm ² or less.

(Tempered martensite is over 90% in area ratio)
In order to ensure the strength of the steel wire structure, the tempered martensite has an area ratio of 90% or more. If it is less than 90%, not only the strength cannot be secured, but also the carbide tends to aggregate and the thermal conductivity tends to decrease. From the viewpoint of securing a superior strength, the area ratio is preferably 92% or more, more preferably 95% or more. In addition, pearlite, bainite, residual austenite, etc. are mentioned as main structures other than tempered martensite.

The maximum diameter and number of Cr-based carbides are measured by cutting a steel sample from D / 4 in a cross section (cross section) perpendicular to the hot working direction (for example, the rolling direction when hot working is rolling), A sample for microscopic observation is prepared and performed. The obtained sample for microscopic observation was observed with a scanning electron microscope equipped with a composition analyzer such as EDX. The total mass of elements other than Fe was 100%, and the total ratio of C and Cr was 10% or more. It is determined whether or not the maximum diameter (the size of the portion where the size of one carbide is the maximum on the SEM photograph) is 0.3 μm or less. Thereafter, the number of Cr carbides per unit area is counted. Observation with a scanning electron microscope is performed by observing three or more fields of view having an area of 100 μm ² or more.
Note that image analysis software may be used as necessary.

[1-2. Chemical composition]
The steel wire and the wire according to the embodiment of the present invention are: C: 0.3 to 1.2 mass%, Si: 0.01 to 2.5 mass%, Mn: 0.01 to 1 mass, P: 0 mass. More than%, 0.05 mass% or less, S: more than 0 mass%, 0.05 mass% or less, Cr: 0.7-2 mass%, and the balance consists of iron and inevitable impurities.

  Steel wires and wires according to embodiments of the present invention include Cu: more than 0 mass%, 0.5 mass% or less, Ni: more than 0 mass%, 1.0 mass% or less, and Mo: more than 0 mass%, 1 or more selected from the group consisting of .5% by mass or less).

  Steel wires and wires according to embodiments of the present invention are Ti: more than 0% by mass, 0.1% by mass or less, Nb: more than 0% by mass, 0.5% by mass or less, and V: more than 0% by mass, One or more selected from the group consisting of 0.0 mass% or less may be further contained.

  Each element is described in detail below.

(1) C: 0.3 to 1.2% by mass
C (carbon) is an element effective for improving the strength of a steel wire. In order to effectively exhibit such an effect, the C content is 0.3% by mass or more, preferably 0.4% by mass or more, and more preferably 0.5% by mass or more. If the C content is excessive, a structure other than tempered martensite is likely to be formed, so that the required toughness cannot be ensured and the thermal conductivity becomes insufficient. Therefore, the C content is 1.2% by mass or less, preferably 1.1% by mass or less, more preferably 1% by mass or less.

(2) Si: 0.01 to 2.5% by mass
Si (silicon) is an element effective for improving the strength of steel wires and ensuring high temperature strength. In order to effectively exhibit such an effect, the Si content is 0.01% by mass or more, preferably 0.05% by mass or more, more preferably 0.1% by mass or more. On the other hand, if the Si content is excessive, a structure other than tempered martensite is likely to be formed, so that necessary toughness cannot be ensured and thermal conductivity becomes insufficient. Therefore, Si content is 2.5 mass% or less, Preferably it is 2.4 mass% or less, More preferably, it is 2.3 mass% or less.

(3) Mn: 0.01 to 1.0% by mass
Mn (manganese) has the effect of improving the hardenability and improving the strength of the steel wire. In order to effectively exhibit such an effect, the Mn content is 0.01% by mass or more, preferably 0.05% by mass or more, more preferably 0.1% by mass or more. On the other hand, when the Mn content is excessive, a structure other than tempered martensite is likely to be formed, so that necessary toughness cannot be ensured and thermal conductivity becomes insufficient. Therefore, the Mn content is 1.0% by mass or less, preferably 0.9% by mass or less, more preferably 0.8% by mass or less.

(4) P: more than 0% by mass, 0.05% by mass or less P (phosphorus) is an inevitable impurity, and its content should be as small as possible. P is an element that easily segregates at grain boundaries, and may reduce toughness and workability of the wire. Therefore, the P content is 0.05% by mass or less. The content of P is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. The smaller the content of P, the better. However, it is difficult to industrially make it less than 0.001% by mass, and considering the mass productivity, the content is generally about 0.001% by mass or more.

(5) S: more than 0% by mass and 0.05% by mass or less S (sulfur) is an unavoidable impurity, and it is preferable that it is as small as possible. In particular, since S forms sulfide inclusion MnS and segregates during hot working, the wire may be embrittled, so the S content is 0.05% by mass or less. The S content is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. The smaller the S content, the better. However, since it is difficult to industrially make it less than 0.001% by mass, it is generally contained in an amount of about 0.001% by mass or more in consideration of mass productivity.

(6) Cr: 0.7 to 2.0% by mass
Cr (chromium) is an element useful for improving the high-temperature strength of a steel wire. In order to effectively exhibit such an effect, the Cr content is set to 0.7% by mass or more. The Cr content is preferably 0.8% by mass or more, more preferably 0.9% by mass or more. However, when the Cr content is excessive, it becomes difficult to dissolve the Cr-based carbide, and the thermal conductivity tends to decrease. Therefore, the Cr content is set to 2.0% by mass or less. The Cr content is preferably 1.9% by mass or less, and more preferably 1.8% by mass or less.

  The steel wire and wire according to one embodiment of the present invention may further selectively contain the following components.

One selected from the group consisting of Cu: more than 0 mass%, 0.5 mass% or less, Ni: more than 0 mass%, 1.0 mass% or less, and Mo: more than 0 mass%, 1.5 mass% or less These selective components are useful elements for increasing the strength of steel wires. These may be contained alone or in combination of two or more. Details of the content of each element when added are shown below.

(7) Cu: More than 0 mass%, 0.5 mass% or less Cu (copper) is an element useful for increasing the strength of a steel wire. In order to exert such effects, the Cu content is preferably more than 0% by mass. The Cu content is more preferably 0.05% by mass or more, further preferably 0.1% by mass or more, and still more preferably 0.2% by mass or more. On the other hand, when the Cu content is excessive, it becomes a liquid phase at a constant temperature (1356 K) and segregates at the austenite grain boundaries during deformation during hot rolling to generate surface cracks. It is preferable that it is 5 mass% or less. The Cu content is more preferably 0.4% by mass or less, and still more preferably 0.3% by mass or less.

(8) Ni: more than 0 mass%, 1.0 mass% or less Ni (nickel) is an element useful for increasing the strength and toughness of a steel wire. In order to exert such effects, the Ni content is preferably more than 0% by mass. The Ni content is more preferably 0.05% by mass or more, further preferably 0.1% by mass or more, and still more preferably 0.2% by mass or more. On the other hand, if the Ni content is excessive, the steel wire surface is unevenly concentrated, the irregularities on the steel wire surface become large, the surface properties are deteriorated, and the fatigue properties are adversely affected. The amount is preferably 1% by mass or less. The Ni content is more preferably 0.9% by mass or less, and still more preferably 0.8% by mass or less.

(9) Mo: more than 0 mass%, 1.5 mass% or less Mo (molybdenum) is an element useful for increasing the strength and toughness of a steel wire. In order to exhibit such effects, the Mo content is preferably more than 0% by mass. The content of Mo is more preferably 0.05% by mass or more, further preferably 0.08% by mass or more, and still more preferably 0.1% by mass or more. On the other hand, if the Mo content is excessive, the toughness and fatigue properties are adversely affected, so the Mo content is preferably 1.5% by mass or less. The content of Mo is more preferably 1.3% by mass or less, and further preferably 1.0% by mass or less.

  The wire and steel wire according to one embodiment of the present invention may further selectively contain the following components.

One selected from the group consisting of Ti: more than 0% by weight, 0.1% by weight or less, Nb: more than 0% by weight, 0.5% by weight or less, and V: more than 0% by weight, 1.0% by weight or less. These selected components are useful elements for improving the toughness of steel wires. These may be contained alone or in combination of two or more. Details of the content of each element when added are shown below.

(10) Ti: more than 0% by mass and 0.1% by mass or less Ti (titanium) is an element that contributes to improving the toughness of the steel wire by forming carbonitrides and making crystal grains finer. In order to effectively exhibit such an effect, the Ti content is preferably more than 0% by mass. The content of Ti is more preferably 0.02% by mass or more, and further preferably 0.03% by mass or more. On the other hand, if the Ti content is excessive, the toughness of the steel wire is lowered, so the Ti content is preferably 0.1% by mass or less. The Ti content is more preferably 0.08% by mass or less, and still more preferably 0.05% by mass or less.

(11) Nb: more than 0% by mass and 0.5% by mass or less Nb (niobium) is an element that contributes to improving the toughness of the steel wire by forming carbonitrides and refining crystal grains. In order to exhibit such an effect effectively, the Nb content is preferably more than 0% by mass. The Nb content is more preferably 0.02% by mass or more, further preferably 0.04% by mass or more, and still more preferably 0.06% by mass or more. On the other hand, when the content of Nb is excessive, not only the cost increases, but also the yield point (yield ratio) is raised to deteriorate the workability for skin cutting and drawing, so the content of Nb is 0.5. It is preferable that it is below mass%. The Nb content is more preferably 0.4% by mass or less, and still more preferably 0.3% by mass or less.

(12) V: more than 0% by mass and 1.0% by mass or less V (vanadium) is an element that contributes to improving the toughness of the steel wire by forming carbonitrides and refining crystal grains. In order to effectively exhibit such an effect, the V content is preferably more than 0% by mass. The content of V is more preferably 0.01% by mass or more, further preferably 0.03% by mass or more, and still more preferably 0.05% by mass or more. On the other hand, when the V content is excessive, not only the cost is increased, but also the yield point (yield ratio) is increased to deteriorate the workability for skin cutting and drawing, so the Nb content is 0.5. It is preferable that it is below mass%. The Nb content is more preferably 0.4% by mass or less, and still more preferably 0.3% by mass or less.

  The basic components of the steel wire and wire according to the embodiment of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that steel contains inevitable impurities such as Ca and Na which are inevitably mixed depending on the situation of materials such as iron raw materials (including scrap), auxiliary materials, and manufacturing equipment.

  In addition, about P and S, it is so preferable that there is little content, Therefore It is an inevitable impurity, However, The composition range is prescribed | regulated separately as mentioned above. For this reason, in the present specification, the “unavoidable impurities” constituting the balance is a concept excluding elements whose composition range is separately defined.

<2. Manufacturing method of steel wire>
Next, a method for producing the steel wire of the present invention will be described. The method for manufacturing a steel wire according to an embodiment of the present invention includes (a) a step of manufacturing a steel material having the above-described chemical composition, and (b) after heating the steel material to 1150 to 1300 ° C, A step of hot working at a hot working temperature, and a step (c) of obtaining a wire after cooling at a mean cooling rate of 1 ° C./second or less from a cooling start temperature of 750 ° C. or higher to 400 ° C. after step (b). As a result, the maximum diameter of the Cr-based carbide contained in the steel can be 0.3 μm or less, and the number can be 1.5 pieces / μm ² or less. The manufacturing method further includes (d) a step of drawing the wire, and (e) a step of quenching and tempering the drawn wire after the step (d), thereby obtaining a steel wire. be able to.

  Hereafter, each structure is explained in full detail. In the following description, hot working will be described by taking rolling as an example. However, the hot working is not limited to rolling, and other hot working methods such as forging may be used.

[(A) Step of producing a steel material having the above-described chemical composition]
A steel material satisfying the predetermined chemical composition for hot working is obtained by a steelmaking process, a casting process, and the like. For example, steel having a predetermined chemical composition may be melted in accordance with a normal steel making method, and an ingot (bloom) obtained by a continuous casting process may be subjected to ingot rolling to produce a billet of a predetermined size.

  In a hot working process such as rolling, in order to dissolve Cr-based carbides, it is necessary to control heating, hot working and cooling conditions as shown below.

[(B) Step of hot-working the steel material to 1150 to 1300 ° C. and then hot working at a temperature of 750 to 1000 ° C.]
(B-1) Heating the steel material to 1150 to 1300 ° C. In heating before rolling, it is necessary to dissolve coarse Cr-based carbides present in the billet (steel material for hot working). The heating temperature is 1150 to 1300 ° C. When the heating temperature is lower than 1150 ° C., Cr-based carbides remain, and Cr-based carbides continue to remain after the subsequent cooling, making it impossible to obtain a predetermined structure.
The heating temperature is preferably 1175 ° C or higher, more preferably 1200 ° C or higher. As the billet is heated to a higher temperature, the dissolution of Cr-based carbides becomes easier, but the austenite grains tend to become coarser, so that Cr tends to aggregate and the size of Cr-based carbides after quenching and tempering becomes excessive. , High temperature strength and thermal conductivity tend to decrease. Therefore, the upper limit of the heating temperature is 1300 ° C. or lower, preferably 1275 ° C. or lower, more preferably 1250 ° C. or lower. In addition, what is necessary is just to select the holding time in heating temperature suitably according to operation conditions, for example, is 30 minutes or more. The heating temperature may be confirmed, for example, by measuring the temperature of the steel material at the outlet from the heating furnace with a radiation thermometer or the like. If the holding time is sufficiently long, the heating furnace temperature may be used as the heating temperature. Good.

(B-2) Hot working at a hot working temperature of 750 to 1000 ° C. The hot working conditions greatly affect the size and the number per unit area of the Cr-based carbide after quenching and tempering. When Cr-based carbide is not subjected to hot working such as rolling, or when hot working is performed at a high temperature exceeding 1000 ° C., Cr is concentrated locally, such as austenite grain boundaries, and then after pearlite transformation, Cr Highly concentrated pearlite is partially formed. Since the lamellar cementite constituting these pearlites is suppressed from diffusion of C by Cr, it is difficult to dissolve during austenite heating for quenching and remains as an insoluble carbide. As a result, a Cr-deficient region is formed around the undissolved carbide, which causes deterioration in high temperature strength and thermal conductivity. In order to suppress the formation of insoluble carbides, it is effective to perform hot working on the low temperature side of the austenitizing temperature after hot heating. By processing at as low a temperature range as possible, many strains can be introduced into the steel material, and these strains can make Cr uniformly present in the steel material, and the entire amount is dissolved during austenite heating for subsequent quenching. Thus, a steel structure that can achieve both high-temperature strength and thermal conductivity can be obtained. The hot working temperature for uniformly presenting Cr is 1000 ° C. or lower, preferably 975 ° C. or lower, more preferably 950 ° C. or lower. When the hot working temperature is lower than 750 ° C., the concentration of Cr proceeds before hot working, and a desired structure cannot be obtained by subsequent quenching and tempering. Therefore, the hot working temperature is 750 ° C. or higher, preferably 775 ° C. or higher, more preferably 800 ° C. or higher.
In addition, you may confirm hot processing temperature by measuring the temperature of the steel materials just before hot processing with a radiation thermometer etc., for example.

[(C) After step (b), a step of obtaining a wire by cooling from a cooling start temperature of 750 ° C. or higher to 400 ° C. at an average cooling rate of 1 ° C./second or less]
The steel material after the hot working is cooled from the cooling start temperature set to 750 ° C. or higher (1000 ° C. or lower) to 400 ° C. at an average cooling rate of 1 ° C./second or lower. When the cooling start temperature is lower than 750 ° C., the concentration of Cr proceeds before the start of cooling, and a desired structure cannot be obtained by subsequent quenching and tempering. The cooling start temperature is preferably 775 ° C. or higher, more preferably 800 ° C. or higher.
As long as an appropriate cooling temperature is selected, there is no influence on the structure control from the completion of hot working to the start of controlled cooling, so that it is possible to select cooling conditions suitable for the operation such as natural cooling and controlled cooling.
The strain during hot working is important to disperse Cr-based carbides, but if a cross-section reduction rate of approximately 50% or more is ensured, the size and unit of the predetermined Cr-based carbides after quenching and tempering. The number per area can be obtained more reliably.

By performing the cooling from the controlled cooling temperature to 400 ° C. at an average cooling rate of 1 ° C./second or less, it is possible to obtain pearlite in which Cr is uniformly dispersed, and by subsequent quenching and tempering, a predetermined Cr carbide And the number per unit area can be obtained.
When the average cooling rate is faster than 1 ° C./second, a supercooled structure such as bainite and martensite is generated, and it is difficult to perform subsequent processes such as skinning and drawing. The average cooling rate is preferably 0.9 ° C./second or less, more preferably 0.8 ° C./second or less. The lower limit of the average cooling rate is not particularly limited, but is usually 0.2 ° C./second.

  By setting the controlled cooling stop temperature to 400 ° C. or lower, uniform pearlite can be obtained. When the controlled cooling stop temperature exceeds 400 ° C., a supercooled structure such as bainite or martensite is generated, and it is difficult to perform subsequent processes such as skinning and drawing. The controlled cooling stop temperature is preferably 375 ° C. or lower, more preferably 350 ° C. or lower. Although the minimum of control cooling stop temperature is not specifically limited, Usually, it is 200 degreeC.

The average cooling rate may be adjusted, for example, by controlling the conditions for cooling the slermore (air volume, wind speed, and presence / absence of cover).
The average cooling rate may be obtained by measuring the temperature of the rolled material with a radiation thermometer or the like.

  By producing a steel wire using the wire thus obtained, a steel wire excellent in high-temperature strength and excellent in heat conduction characteristics can be obtained.

The method for manufacturing a steel wire according to an embodiment of the present invention further includes (d) a step of drawing the wire, and (e) a step of performing quenching and tempering on the drawn wire after step (d). including.
The process for obtaining a steel wire from a wire including the said process (d) and (e) may use known general manufacturing conditions. Before drawing, the surface of the wire is shaved for the purpose of removing decarburized layers, wrinkles, etc. near the surface of the wire (rolled wire), and then drawn (drawn) to the desired wire diameter. A steel wire is obtained by quenching and tempering. In addition, you may abbreviate | omit the shaving according to the use or use conditions, etc. of a steel wire.
Below, the manufacturing conditions for obtaining a steel wire from a wire are illustrated.

In the quenching and tempering treatment, the heating temperature at the time of quenching is set so as to redissolve the Cr-based carbide, but at the same time, it is necessary to suppress the regeneration and growth of the Cr-based carbide. Therefore, it is 850 ° C. or higher, preferably 870 ° C. or higher, more preferably 890 ° C. or higher. On the other hand, when the heating temperature is excessively high, the austenite grains become coarse and Cr tends to aggregate at the austenite grain boundaries. Therefore, the heating temperature is 1000 ° C. or lower, preferably 980 ° C. or lower, more preferably 960 ° C. or lower. is there. Tempering is performed after quenching in oil that has been heated to a predetermined temperature and then heated, for example, approximately 50 to 80 ° C. Tempering may be performed by appropriately adjusting the temperature and time so that a desired tensile strength such as 2000 MPa or more is obtained. For example, tempering is generally preferably performed at a heating temperature of 350 ° C. or higher and 450 ° C. or lower. By performing such treatment, the steel wire obtained in the present invention exhibits excellent high-temperature strength and excellent heat conduction characteristics. By processing the steel wire of the present invention into a desired shape, a structural member such as a piston ring that requires heat resistance can be manufactured. You may perform well-known various surface hardening processes, such as shot peening and nitriding, as needed after component processing.
Further, for example, depending on the usage of the steel wire such as the shape of the piston or piston ring, the cross section of the steel wire (the cross section perpendicular to the longitudinal direction) has various shapes such as a square, a quadrangle, a circle, or an ellipse. May be processed as follows. The said process may be performed with respect to the steel wire obtained by the said process (e) by rolling or a drawing process etc., for example, is performed at the time of a wire drawing process in the said process (d), and quenching tempering treatment is carried out after that. You can go.

  As described above, the method of manufacturing the steel wire according to the present invention has been described. However, a person skilled in the art who understands the desired characteristics of the steel wire according to the present invention performs trial and error, and has the desired characteristics according to the present invention. There is a possibility of finding a method other than the above manufacturing method.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a scope that can meet the gist of the present invention. Of course, any of these is also included in the technical scope of the present invention.

(1) Sample preparation Melting and casting was performed in a small vacuum melting furnace, and 150 kg of steel ingots having the chemical composition shown in Table 1 were prepared. In addition, the underline attached | subjected under the numerical value of each table | surface has shown that the said numerical value has remove | deviated from embodiment of this invention mentioned above. The ingot was heated at a heating temperature simulating the lump temperature (the same temperature as the hot working heating temperature shown in Tables 2 and 3), and then forged to produce a 155 mm square steel slab. A dummy billet was welded to this steel piece, and a wire rod having a diameter of 7 mm was prepared by hot rolling under the hot working conditions (rolling conditions) and cooling conditions shown in Tables 2 and 3. The “control cooling rate” in Tables 2 and 3 means an average cooling rate from the control cooling start temperature to the control cooling stop temperature.
Next, the wire rod was subjected to skinning and drawing, and then subjected to quenching and tempering treatment to obtain a steel wire having a wire diameter of φ3.0 mm. The tempering conditions of the quenching and tempering treatment were adjusted so that the tensile strength of the steel wire was 1950 to 2050 MPa.

  The obtained Cr-based carbide existing in the steel wire was evaluated for the maximum diameter and the number per unit area by the following methods. Moreover, the hardness test after heating to 450 degreeC about the obtained steel wire, and the heat conductive characteristic evaluation were done with the following method.

(2) Maximum diameter of Cr-based carbide and number per unit area A test sample was cut out from a cross section (cross section) perpendicular to the rolling direction of the wire. Using this sample, an SEM structure observation sample was prepared.
Specifically, after cutting out the sample, the cut surface was mirror-polished by emery paper, diamond buffing, or electrolytic polishing. The mirror-polished surface of the test piece was observed and photographed with a field emission scanning electron microscope (FE-SEM: observation magnification 10,000 times, acceleration voltage 20 kV). Observation was performed at five arbitrary locations, and photographs of each observation location were taken (total of 5 images). Identification of the Cr-based carbide was performed using an EDX analyzer attached to the FE-SEM. The total mass of elements other than Fe was 100%, and a carbide containing 10% or more of C and Cr by mass ratio was defined as a Cr-based carbide. The evaluation was performed for 3 visual fields with an area of μm ² per visual field. Thereby, the maximum diameter of the Cr-based carbide and the number of Cr-based carbides were obtained. Regarding the number, the number of Cr-based carbides having a major axis of 0.05 μm or more was obtained using image analysis software (Image Pro Plus) manufactured by Media Cybernet, and the number density per 1 μm ² was obtained.
The measurement results are shown in Tables 4 and 5.

(4) Vickers hardness test Using the FE-SEM observation sample as a hardness test piece, the cross section was polished into a mirror surface and finished, and then measured using a Vickers hardness tester.
Specifically, in order to evaluate the hardness of the entire steel material, the surface layer part (0.05 mm inside from the outermost surface of the test piece), D / 4 part, and D / 2 part (D is the thickness of the test piece) ) Was measured at three locations. At the time of measurement, the measurement load was set to 300 g, and measurement was performed three times to obtain an average value. In the present invention, a Vickers hardness of 500 Hv or more was evaluated as acceptable (high strength), and less than 500 Hv was evaluated as unacceptable (insufficient strength). The measurement results are shown in Tables 4 and 5.

(5) Evaluation of thermal conductivity characteristics Thermal conductivity characteristics were measured by measuring the thermal conductivity by laser flash method at room temperature and in the atmosphere. The measurement conditions are as follows.
Measuring method: Laser flash method Measuring temperature: Room temperature Measuring device: Thermal constant measuring device TC-7000 manufactured by ULVAC-RIKO
Atmosphere: Air Test piece size: φ7 × 2tmm
A thermal conductivity of 20 W / m · K or higher was evaluated as acceptable. The evaluation results are shown in Tables 4 and 5.

  In Tables 1 to 5, the underlined items mean that they are not within the scope of the present invention.

  Test No. in Table 4 Nos. 1 to 15 are samples in which the heat treatment conditions were variously changed as shown in Table 2 using the steel type E shown in Table 1. Of these, test no. 2 to 3, 6 to 7, 10 to 11, and 13 to 14 are all samples manufactured under the conditions according to the embodiment of the present invention described above, and the maximum diameter and number density of Cr-based carbides and tempering. Since all of the martensite area ratio is controlled within the range of the embodiment of the present invention, it exhibits excellent high-temperature strength and heat conduction characteristics.

  On the other hand, test No. in which the heating temperature of the ingot is lower than the lower limit. No. 1 was insufficiently heated and the Cr-based carbide could not be sufficiently dissolved, so the maximum diameter of the Cr-based carbide was excessive and the thermal conductivity of the steel wire was insufficient.

  On the other hand, test No. in which the heating temperature of the ingot is higher than the upper limit. In No. 4, heating was excessive and the austenite grain size became coarse, and it was impossible to form sufficient Cr-based carbide precipitation sites by subsequent hot working, resulting in an excessively large Cr-based carbide size. As a result, the high-temperature strength and thermal conductivity of the steel wire were insufficient.

  Test No. whose hot working temperature is lower than the lower limit. No. 5, coarse pearlite is generated, and sufficient Cr-based carbide precipitation sites cannot be formed by hot working, resulting in an excessive number density of Cr-based carbides. As a result, the thermal conductivity of the steel wire Was insufficient.

  Test No. whose hot working temperature is higher than the upper limit. No. 8 is excessively heated and the austenite grain size becomes coarse even after hot working and the precipitation site of Cr carbide is insufficient, so the maximum diameter of Cr carbide is excessive. As a result, the high temperature strength and heat The conductivity was insufficient.

  Test No. with controlled cooling start temperature lower than lower limit. No. 9, coarse pearlite is generated, and sufficient Cr-based carbide precipitation sites cannot be formed by hot working, resulting in an excessive number density of Cr-based carbides. As a result, the thermal conductivity of the steel wire is reduced. It was insufficient.

  Test No. in which the average cooling rate from the control cooling start temperature to the control cooling stop temperature is faster than the upper limit. No. 12 could not complete the pearlite transformation during cooling, and bainite was partially generated, resulting in an excessive number density of Cr-based carbides. As a result, the high-temperature strength of the steel wire was insufficient.

  Test No. with controlled cooling stop temperature higher than the upper limit. No. 15 could not complete the pearlite transformation during cooling, and bainite was generated in the subsequent cooling, resulting in an excessive number density of Cr-based carbides. As a result, the high-temperature strength of the steel wire was insufficient. .

  Test No. in Table 4 16-20, test No. in Table 5. 21 to 39 are samples in which the hot working conditions and the cooling conditions are constant as shown in Tables 2 and 3, and the steel types are changed to A to Y (except E). Of these, test no. Nos. 16 to 33 all satisfy the above-described component range according to the embodiment of the present invention, and the maximum diameter and number density of Cr-based carbides and the area fraction of tempered martensite are all within the scope of the present invention. Therefore, excellent high-temperature strength and thermal conductivity were exhibited.

  On the other hand, test No. using the steel type T whose C amount is lower than the lower limit. No. 34 is liable to decrease in hardness at high temperatures, and as a result, the high-temperature strength of the steel wire is insufficient.

  Test No. using a steel type U having a C amount higher than the upper limit. No. 35 could not complete the pearlite transformation by controlled cooling, and retained austenite was generated after cooling, and some of the transformation was insufficient after tempering. As a result, the thermal conductivity of the steel wire Was insufficient.

  Test No. using steel type V with Si amount higher than the upper limit. No. 36 could not complete the pearlite transformation by controlled cooling, and retained austenite was produced after cooling, and some of the transformation was insufficient after tempering. Was insufficient.

  Test No. using steel type W with Mn amount higher than the upper limit. No. 37 could not complete the pearlite transformation by controlled cooling, and retained austenite was generated even after cooling, and some of the transformation was insufficient after tempering. As a result, the thermal conductivity of the steel wire Was insufficient.

  Test No. using steel type X having a Cr content lower than the lower limit. No. 38 was easily reduced in hardness at high temperatures, and as a result, the high-temperature strength of the steel wire was insufficient.

  Test No. using steel type Y with a Cr content higher than the upper limit. No. 39 could not sufficiently decompose the Cr-based carbide in the heating or hot working process, and the size and area ratio of the Cr-based carbide after quenching and tempering were excessive. As a result, the thermal conductivity of the steel wire was insufficient.

Claims (5)

C: 0.3 to 1.2% by mass,
Si: 0.01 to 2.5% by mass,
Mn: 0.01 to 1.0% by mass,
P: more than 0% by mass, 0.05% by mass or less,
S: more than 0 mass%, 0.05 mass% or less, and Cr: 0.7-2.0 mass%
And the balance is a steel wire composed of iron and inevitable impurities,
The maximum diameter of the Cr-based carbide contained in the steel wire is 0.3 μm or less, and the number thereof is 1.5 pieces / μm ² or less,
Steel wire with tempered martensite of 90% or more in area ratio. Cu: more than 0% by mass, 0.5% by mass or less,
The steel wire according to claim 1, further comprising at least one selected from the group consisting of Ni: more than 0 mass%, 1.0 mass% or less, and Mo: more than 0 mass%, 1.5 mass% or less. Ti: more than 0% by mass, 0.1% by mass or less,
The steel according to claim 1 or 2, further comprising at least one selected from the group consisting of Nb: more than 0% by mass, 0.5% by mass or less, and V: more than 0% by mass, 1.0% by mass or less. line. (A) a step of producing a steel material having the chemical component composition according to any one of claims 1 to 3;
(B) after the steel material is heated to 1150 to 1300 ° C., hot working at a hot working temperature of 750 to 1000 ° C .;
(C) after the step (b), a step of cooling from a cooling start temperature of 750 ° C. or higher to 400 ° C. at an average cooling rate of 1 ° C./second or less to obtain a wire;
(D) a step of drawing the wire;
(E) After the said process (d), the manufacturing method of the steel wire including the process of performing the quenching tempering process to the said drawn wire.   The method of manufacturing a steel wire according to claim 4, wherein in the step (d), the surface of the wire is cut before drawing.