KR102107507B1 - Development of heat treatment method for shot-peening ball - Google Patents

Abstract

The present invention relates to a heat treatment method of a shot-peening ball for a spring. A heat treatment step includes a heating step, a quenching step, and a tempering step to allow a shot-peening ball to form a tempered martensite structure, have wear resistance of 5,500-5,700 cycles, and improve strength and toughness to improve the durability of the shot-peening ball. In comparison to the use of a shot-peening ball manufactured by simple mechanical machining, a raw material is heat-treated to improve hardness and toughness, improve wear resistance by impact during projection, allow reuse, respond to high hardness of a spring material, and improve the service life by 1.6 times. The heat treatment is realized in stages to improve the durability of a shot-peening ball for a spring in comparison to a conventional manufacturing method of a shot-peening ball for a spring produced in a small quantity without heat treatment.

Description

{Development of heat treatment method for shot-peening ball}

The present invention relates to a method for heat treatment of spring short balls, in particular, heat treatment of raw materials for spring short balls to improve durability by heat treatment of shot-peening balls for spring (hereinafter referred to as short balls) It's about how.

In general, springs are used to achieve the purpose of cushioning by absorbing and accumulating energy using the elastic force of a steel wire, or to secure the movement function of various mechanical elements by using resilient elasticity to rebound after compression.

In addition, each component used in transportation such as automobiles, ships, and railway vehicles, such as springs, requires light weight, and the high strength of metal, which is considered in the design of metals, reduces the weight of materials and fuels by realizing weight reduction of parts. Therefore, efforts are being made to realize high strength of materials due to weight reduction.

However, due to defects in the surface of the metal or the spring, cracks are initiated even at stresses below the yield stress, and the stability is gradually increased, resulting in a rapid breakdown.

A manufacturing method known to have certain characteristics in such springs,

A preparation step of preparing a coiling device and a heating furnace for forming a spring, and a material having a predetermined length after the core is wound to be changed in the coiling device so as to correspond to the shape and size of the spring being manufactured. A material transfer step is transferred to the heating furnace, a heating step in which a part of the material is instantaneously heated in the heating furnace, and the material passing through the heating step is wound by the coiling device to form a spring, and the The material transfer step is partially progressed, and the material is transferred to a heating furnace and, at the same time, a coiling step wound in a coiling device, and after the coiling step, spring processing is performed to process the end of the spring to the required size of the molded product. Step, the heat treatment step of the spring is quenched and tempered heat treatment, and the short pinning step of the heat-treated spring is short pinned (the A manufacturing method consisting of a short peening (shot peening process) in which the short ball is processed at a throwing speed of 70 kPa to form a compressive residual stress on the surface of the fring steel, and a setting step of reducing the permanent deformation of the spring after the short peening is completed. This is a generally known method.

Among them, in particular, materials for manufacturing shot peening balls (hereinafter referred to as shot balls) used for shot peening are used in mild steel, hard steel, tool steel, structural steel, spring steel, duralumin, aluminum alloy, stainless steel, zinc, gold steel, etc. do.

There are two types of short ball manufacturing methods, such as cast steel short ball manufacturing and short ball manufacturing using grit. Both of these processes are made of iron grains made of iron. Used to boost. Both shots and grits are a kind of projection material mainly used in industries that use metal materials such as automobiles, construction, shipbuilding, forging, and steel as raw materials, and various manufacturing methods satisfying various shapes and performances are known.

That is, it has been disclosed in Patent Registration No. 10-0124392, Patent Registration No. 10-0524533, Patent Publication No. 10-2006-0110361, Patent Registration No. 10-1606126, Registration Patent No. 10-1497892, etc. Most of these prior arts are continuously forming a short ball by cutting a steel wire continuously into a high-speed rotary roller having a cutting blade and cutting it to a predetermined length to form a large amount of short balls in a batch. In addition, as a short ball manufacturing method, iron scrap is melted in an induction furnace or the like to perform water spraying such as atomizing, followed by heat treatment and sieving to produce a short ball or crushing of the material after the water death, followed by heat treatment again to adjust the particle size. There is also a way to do it. As another method of manufacturing a short ball, a short ball (short grit) may be manufactured by shortly cutting an iron wire. In any case, the above methods consume high energy because they not only use high-temperature energy to melt expensive iron, but also require an annealing process.

In addition, although a method of using spherical iron generated in the blast furnace slag crushing process is also known, the shortball manufacturing method uses a spherical iron generated as a by-product, and thus has a disadvantage in that its amount is small and its composition is not uniform.

The above-described "short ball manufacturing methods" have a problem of sorting misalignment and short loss according to the diameter of the short ball during the forming process of the short ball, and the short ball is destroyed or cracked during the short peening process. In addition, the short ball itself has not been used as a solution to the compressive residual stress of the surface of the spring formed by the short ball processing. Therefore, measures to improve the durability of the short ball have also been required.

Republic of Korea Registered Patent No. 10-0524533 Republic of Korea Registered Patent No. 10-1676709 Republic of Korea Patent Publication No. 10-2006-0110361

In an embodiment of the present invention, an object of the present invention is to improve durability by heat-treating the raw material of a spring short ball.

In an embodiment of the present invention, it is an object of the present invention to improve the residual stress of a spring that is a target for shot peening by heat treating the shot ball.

In an embodiment of the present invention, in order to improve the hardness (hardness) and toughness (toughness) of the short ball, it is an object to improve durability through a three-step heat treatment process of heating, quenching and tempering.

It is an object of the present invention to provide a short ball with improved abrasion resistance due to impact during projection as the heat treatment process.

In an embodiment of the present invention, as a short ball having improved hardness and toughness and improved abrasion resistance, it is an object of the present invention to reduce material costs by allowing reuse and repeated use.

In an embodiment of the present invention, it is an object of the present invention to be able to cope with the high strength of the spring material because a short ball with improved strength can be applied through heat treatment.

In an embodiment of the present invention, it is an object of the present invention to provide a short ball raw material in a large amount during the spring manufacturing process and to heat and heat it together with the spring.

In an embodiment of the present invention, it is an object of the present invention to improve the short ball that improves the residual stress on the surface of the spring by the heat treatment method.

According to an embodiment of the present invention, a method of heat-treating a short ball for a spring includes cutting a short ball (process, process, hereinafter referred to as step) and heat-treating the cut short ball (hereinafter referred to as heat treatment step). However, the step of heat treatment includes a heating step and a quenching step; And tempering (tempering); by including, durability is increased, hardness, strength, abrasion resistance and toughness is improved heat treatment method of spring short balls.

According to an embodiment of the present invention, in the step of heat treatment, the short ball forms a tempered martensitic structure, and is a heat treatment method for a spring short ball, characterized in that it has an abrasion resistance of 5,500 to 5,700 cycles.

According to an embodiment of the present invention, the heating step of the heat treatment step is a method of heat treatment of a spring short ball, characterized in that heating is performed at 900 ° C or higher for 10 minutes.

According to an embodiment of the present invention, during the heat treatment step, the quenching step is a short for spring having a characteristic that a predetermined hardness and strength are secured by cooling using oil having a predetermined temperature and viscosity. It is the heat treatment method of the ball.

According to an embodiment of the present invention, the oil in the quenching step is mixed with a base oil having a kinematic viscosity of a predetermined value or higher, and a pressure-reducing oil pressure is adjusted by reducing the pressure of the oil during oil quenching. It is a method of heat treatment of spring short balls that can adjust performance.

According to an embodiment of the present invention, the oil in the quenching step is mixed with a base oil having a kinematic viscosity of 40 kPa / sec or higher at 40 ° C., a vapor film rupture agent is added, and the pressure is reduced, and the heat treatment time of the base oil is 2.5 seconds. It is the heat treatment method of the short ball for springs characterized by below (sec) and having a kinematic viscosity of 40 to 300 kPa / sec of the base oil at 40 ° C.

According to an embodiment of the present invention, the oil in the quenching step is a method for heat treatment of a spring short ball, characterized in that the blending amount of the steam film breaking agent is 5% by mass or more based on the oil-quenched oil.

According to an embodiment of the present invention, during the heat treatment step, the tempering step is a heat treatment method for a spring short ball, characterized in that the hardness is in the range of Hv 600 to 630 by tempering at 300 to 400 ° C for 30 minutes or more. to be.

In the embodiment of the present invention, compared to the conventional shot peening ball using a short ball manufactured only by simple mechanical cutting of thin wires, the raw material is heat-treated to improve hardness and toughness.

In an embodiment of the present invention, the heat-treated short ball has an effect of improving abrasion resistance due to impact during projection.

In the exemplary embodiment of the present invention, the number of repetitive uses increases, thereby reducing the material cost.

In the embodiment of the present invention, since a short ball having high strength and high strength can be applied through heat treatment, there is an effect corresponding to high strength of the spring material.

In the embodiment of the present invention, a short ball is heated during the spring processing process without separately performing the short process of the short ball by performing a tempering process after the heating process with the spring by containing a large amount of the short ball raw material during the spring manufacturing process. There is an effect that the process equipment for performing separate tempering is omitted.

According to an embodiment of the present invention, the homogenization of the tissue is improved, and the hardness and strength against impact are improved, resulting in the short ball of a high-strength spring, compared to the short ball in which the conventional parrite and pearlite are mixed on the homogeneous screen of the tissue. .

According to an embodiment of the present invention, compared to 3,400 to 3,500 cycles of the existing short ball in abrasion resistance, the service life is increased by about 1.6 times to 5,500 to 5,700 cycles.

As a result, according to an embodiment of the present invention, unlike the method for manufacturing a short shot for spring, which was produced in a small amount without an existing heat treatment, it has an effect of improving the durability of the short shot for spring by implementing heat treatment step by step.

Hereinafter, a preferred embodiment of the present invention will be described.

In describing the embodiments of the present invention, descriptions of technical contents well known in the technical field to which the present invention pertains and which are not directly related to the present invention will be omitted.

In addition, technical terms used in this specification should be interpreted as meanings generally understood by those of ordinary skill in the art to which the present invention pertains, unless defined otherwise. It should not be interpreted as a meaning, or an excessively reduced meaning.

In addition, the singular expression used in this specification includes the plural expression, unless the context means otherwise.

In the present invention, terms such as “consisting of” or “comprising” should not be construed as including all of the various components, or various steps described in the specification, among which some components or some steps are It may not be included, or it should be construed to further include additional components or steps.

Hereinafter, "shot peening ball" will be referred to as "short ball", "Quenching" as "quenching", and "Tempering" as "tempering".

Example

The present invention is not limited to the following examples.

In general, quenching is one of heat treatment processes in which steel is heated to a suitable temperature to reach an austenite structure, and then rapidly cooled to change to a martensitic structure.

The quenching is intended to increase the hardness and strength of the steel. If the quenching temperature of the steel is too high, the austenite crystal grains of the steel will grow, resulting in poor mechanical properties even after quenching and easy to crack or deform.

In addition, the tempered steel is brittle while the hardened steel has increased hardness, and if the residual stress remains on the surface, it is unstable and easy to break, so it must be tempered after quenching to impart moderate toughness to the material.

That is, the purpose of tempering is to change the quenched tissue into a stable tissue and to reduce the residual stress to obtain the required properties and conditions.

The quenched steel is heated to a suitable temperature and cooled again.

The short ball for spring of the present invention is subjected to heat treatment to have a predetermined structure after manufacturing the short ball, and through this heat treatment, the structure of the primary manufactured short ball is changed to tempered martensite to improve toughness when blasting. To have.

Example 1

According to an embodiment of the present invention, the method for heat-treating a short ball for a spring comprises a step (a) of cutting the short ball and a heat-treating step (b) of heat-treating the cut short ball, improving tissue homogenization and impact resistance. Performances such as abrasion resistance (irving test) are in accordance with Table 1 and Table 2 below.

First, the step of cutting (a) is a process of cutting a steel material to cut a chip into an appropriate size, and since this step is conventionally known, a detailed description is omitted.

One of the conventional short ball manufacturing methods is dissolving iron scrap in an induction furnace or the like to perform water spraying such as atomizing, then heat-treating it and sieving it to produce short balls, or after crushing the material after water spraying, re-heating and particle size. There is a method of adjusting, and in another method, a short ball (short grit) may be manufactured by shortly cutting an iron wire.

In the above method, the former consumes a lot of energy because it uses high-temperature energy to melt expensive iron in any case and requires an annealing process.

The latter is a method of using spheroidal iron generated in the blast furnace slag crushing process, and since it uses spheroidal iron generated as a by-product, its production is small and its composition is not uniform. Therefore, there is a need for a method to enable mass production input.

Second, the heat treatment step (B) is divided into a heating step, a quenching step, and a tempering step, and will be described separately for each step.

Although divided into stages, in an embodiment, the heating stage, the quenching stage, and the tempering stage may be selectively selected, or may include all stages.

The heating step of the heat treatment step (B) according to the embodiment of the present invention is a step of heating for 900 minutes or more, for 10 minutes.

The quenching (oil quenching) step of the heat treatment step (B) according to an embodiment of the present invention is a step of cooling using oil having a predetermined temperature and viscosity.

In general, quenching is one of the heat treatment processes to increase the hardness and strength of the steel by heating the steel to a suitable temperature to reach the austenite structure and then rapidly cooling it to change to the martensitic structure.

The quenching oil is a step of quenching and hardening a metal material such as steel, and if the quenching temperature of the steel is too high, the austenite crystal grains of the steel grow to deteriorate mechanical properties even after quenching and cracks or deformation occur. It is easy, so be careful with the quenching temperature.

Therefore, quenching oil is required to have high cooling properties, but in general, quenching distortion may occur when quenching oil is too high in quenching step, and quenching distortion is insufficient to suppress quenching hardness, resulting in insufficient quenching hardness, so quenching hardness and quenching It is difficult to satisfy distortions and the like at the same time.

Therefore, two types of so-called low-temperature oil and high-temperature oil (Martempering Oil) exist in the quenching oil.

Low-temperature oil usually uses a low-viscosity base oil, so the cooling rate is fast and has high cooling properties, but because the vapor blanket stage is long, it is easy to cause quench staining and to cause quench distortion. In most cases, the steam film step is shortened by adding a steam film breaker.

On the other hand, high-temperature oil usually uses a high-viscosity base oil, so the vapor film stage is short and the quenching distortion is small, but the boiling point is high and the convection stage initiation temperature is high, so the coolability is low.

Therefore, when emphasizing the hardness of the quenched product, a low temperature oil is used, and when emphasizing the suppression of distortion of the quenched product, a high temperature oil is used.

That is, it is necessary to select and use quenched oil in accordance with the required quality, and oil change becomes inevitable for each quenched.

On the other hand, by using a high-viscosity quenching oil and quenching treatment under reduced pressure, the vapor film step is long and stable to suppress quenching distortion and to change the cooling performance.

The present invention is to include a quenching step in the heat treatment step that can express a wide range of coolability ranging from low temperature oil to high temperature oil in one quenching oil.

The quenching oil of the quenching step according to an embodiment of the present invention is mixed with a base oil having a kinematic viscosity of a predetermined value or higher, and a vapor film rupture agent is blended, and at the same time, the pressure of the oil is adjusted under reduced pressure to cool the oil. Performance can be adjusted.

In particular, the quenched oil is preferably mixed with a base oil having a kinematic viscosity of 40 kPa / sec or higher at 40 ° C. and a pressure-reducing agent while simultaneously reducing the pressure, and the heat treatment time of the base oil is 2.5 seconds (sec) or less. .

The kinematic viscosity of the base oil at 40 ° C is preferably 40 to 300 kPa / sec, and most preferably, the blending amount of the vapor film breaking agent is 5% by mass or more based on the quenching oil.

That is, in the quenching oil of the quenching step according to the embodiment of the present invention, a base oil having a kinematic viscosity at 40 ° C. of 40 kPa / sec or more, preferably 40 to 300 kPa / sec, is used as the base oil.

When the kinematic viscosity at 40 ° C. is less than 40 kPa / sec, the vapor film stage becomes longer, so the cooling performance is lowered, and there is a possibility that quenching distortion occurs and quenching distortion occurs.

In addition, the upper limit of the kinematic viscosity at 40 ° C of the base oil used in the quenching step according to the embodiment of the present invention is not particularly limited, but is preferably 300 Pa / sec or less. When this kinematic viscosity is 300 kPa / sec or less, the cooling property can be adjusted widely while maintaining the length of the vapor film step appropriately.

In addition, the base oil used in the present invention preferably has a characteristic time (seconds) of 2.5 or less in the heat treatment oil test of the Korean Industrial Standard, and this characteristic time (seconds) is the cooling performance prescribed by the Japanese Industrial Standard (JISK2242). In the test, it refers to the time from reaching the temperature at which the vapor film collapses, and the length of the vapor film step is quantified.

When the characteristic time (second) is 2.5 or less, the cooling property is good, and it is possible to suppress the occurrence of quenching distortion due to quenching unevenness.

In addition, the base oil used in the quenching step according to the embodiment of the present invention preferably has a flash point of 230 ° C or higher, particularly 250 ° C or higher.

When the flash point is 230 ° C or higher, it is possible to suppress the change in cooling properties over time due to evaporation of light components contained in the base oil, which is satisfactory in safety.

The base oil used in the quenching step preferably has a light oil content of less than 400 ° C. at a boiling point of 5% by mass or less in the same manner as to increase the flash point described above.

Specifically, as the base oil used in the quenching step of the heat treatment step of the spring short ball according to the embodiment of the present invention, mineral oil is generally used.

As a specific mineral oil, paraffin machine crude oil, intermediate machine crude oil, naphthenic machine crude oil, aromatic crude oil, etc., are distilled under normal pressure or distilled under reduced pressure by distilling the residual oil under atmospheric pressure, or refined oil obtained by purifying them according to a conventional method, e.g. For example, a solvent refined oil, a hydrogenated refined oil, a hydrolysis refined refined oil, a solvent dewaxing or hydrogenated lead-free refined oil, and a clay processing oil etc. are mentioned.

Further, alkylbenzene, alkylnaphthalene, α-olefin oligomer (PAO), α-olefin copolymer, polybutene, dibasic acid ester, hindered ester, polyoxyalkylene glycol, polyoxyalkylene glycol ester, polyoxyalkylene Synthetic oils such as glycol ether and silicone oil can also be used.

These base oils may be used alone or in combination of two or more.

However, when mixing a low-viscosity base oil and a high-viscosity base oil, the flash point may not be able to maintain the flash point sufficiently high due to the presence of the low-viscosity base oil, or there may be a large amount of light cut. It is preferable to be careful not to lower this or to increase the number of light oils below 400 ° C.

The quenching oil in the quenching step of the heat treatment step for the short ball for spring according to the embodiment of the present invention is mixed with a vapor film breaker in the base oil. By mixing the base oil with a vapor film breaker, there is an effect of shortening the vapor film step of the base oil under reduced pressure or the like to expand the range of coolability that can be adjusted.

As the steam film breaking agent, a known steam film breaking agent compounded in a low temperature oil can be used. Specifically, a high molecular weight organic compound such as a high molecular weight polymer such as ethylene-α-olefin copolymer, polyolefin, polymethacrylate, or asphalt, or an inorganic dispersion type of oil dispersion may be mentioned.

These vapor film breakers may be used alone or in combination of two or more.

The blending amount of the steam film rupture agent is not particularly limited, and may be 1% by mass or more, but is preferably 5% by mass or more, particularly preferably 6% by mass or more, and the effect is remarkable.

Although there is no restriction | limiting in particular about the upper limit of the compounding quantity of a steam film breaking agent, 30 mass% or less, especially 20 mass% or less are preferable.

When the blending amount of the steam film breaking agent is 30% by mass or less, a change in properties such as viscosity of the quenched oil can be suppressed.

The quenching step of the short-ball heat treatment step of the present invention is a quenching process in which quenching is performed by adjusting the pressure on the oil surface of the heat treatment furnace using quenching oil.

That is, by using a sealing furnace heat treatment furnace such as a vacuum furnace or a vacuum carburizing furnace, the pressure on the oil side of the quenching oil is adjusted from normal pressure to reduced pressure, so that quenching is adjusted according to the purpose of quenching treatment. Is how to do it.

In this case, the adjustment range of the pressure on the oil surface is preferably performed between normal pressure (about 0.1 MPa) and 13 kPa.

When the pressure on the oil surface is within the above range, the effect of blending the vapor film breaking agent is exhibited satisfactorily.

By adjusting the pressure on the oil surface in the above range, the quenching stiffness (H value) exhibiting cooling while maintaining the number of seconds of the characteristic can be adjusted in the range of at least 0.10 to 0.14 / cm.

The quenching intensity is generally abbreviated as H value, and is a numerical value indicating cooling property. It is obtained from the time required for cooling from 800 ° C to 300 ° C as a cooling curve in the heat treatment oil test of the industrial standard. Becomes

In addition, since the range of the H value of the conventional low temperature oil is 0.12 to 0.14, and the range of the H value of the high temperature oil is 0.10 to 0.12, depending on the quenching process, the range of the H value of the low temperature oil and the high temperature oil may be included. .

Specifically, when quenching oil is used as a low temperature oil, in order to lower the pressure on the oil surface, for example, it is adjusted to about 15 to 70 kPa, and when used as a high temperature oil, to increase the pressure on the oil surface, for example For example, quenching can be performed by adjusting to about 80 to 101 kPa.

Thus, without using one quenching oil to replace the oil, it can be used as a low-temperature oil or a high-temperature oil.

In addition, in the case of quenching one quenching component as another embodiment of the quenching process of the short ball heat treatment step of the present invention, it is a method of quenching by changing the pressure on the oil surface during the quenching process.

For example, there is a quenching process in which the vapor film step is performed under reduced pressure, and then continuously under normal pressure or under reduced pressure near atmospheric pressure. According to this process, it is possible to quickly enter the boiling stage and increase the cooling performance while suppressing quenching distortion.

In addition, a quenching method in which quenching is started in a reduced pressure state at or near normal pressure and the pressure is rapidly reduced at the same time as the vapor membrane is broken is also exemplified. According to this method, the effect of expanding the boiling step can be increased without lengthening the vapor film step.

In addition, although the pressure on the oil surface is adjusted and quenched, a method such as changing the oil temperature or changing the stirring flow rate can be introduced at the same time. Thereby, the adjustment range of the coolability (H value) may be further extended in some cases.

Third, the tempering step of the heat treatment step according to the embodiment of the present invention may include a step of tempering at 300 to 400 ° C for 30 minutes or more.

Here, tempering is a kind of heat treatment, and the hardened steel refers to heat treatment for mainly lowering the hardness and increasing the viscosity by reheating to a temperature below the A1 transformation point because the hardness increases but the material becomes softer. The tempering methods include dry, wet, and direct tempering.

That is, when the product heated in the furnace (S45C: BOLT, NUT) is cooled (Oil-Quenching), the highest hardness value (HRC 542) is obtained, and the resulting tissue is called martensite. The characteristic of martensite structure is strong and hard to be fragile at all, and if left at room temperature for a long time, natural cracks may occur.

Therefore, in order to overcome this disadvantage, tempering is performed according to the required hardness.

This tempering is for controlling the hardness according to the product, and the temperature of the tempering furnace must be variable according to the required hardness. At this time, when the temperature is raised, there is no problem because the proper temperature can be maintained by heating through the heater, but when the temperature is lowered, the cooling time is long and loss occurs in the working time. The problem of dropping occurs.

Therefore, in order to solve this problem, an apparatus or method for rapidly decreasing the temperature of the furnace is required.

Looking at the prior art related to the tempering treatment, the registered utility model No. 20-0220107 relates to a tempering heat treatment furnace of a small mechanical element, and a plurality of heating heater rods are arranged above and below the heat treatment chamber formed in the housing of the tempering heat treatment furnace. Then, the net conveyor is arranged to circulate through the wide conveyor roller between the upper and lower heat transfer heater rods, and the net side walls are vertically formed on both sides of the net conveyor, and the inclined plates are extended from the side walls disposed on both sides of the net conveyor. Disclosed is that the end portion can cover the edge portion between the net side wall and the net conveyor.

On the other hand, quenching-Tempering (QT) is a transformation temperature (A3 ~ A1) + (30 ~ 50 ℃) to a suitable temperature (about 840 ~ 880 ℃) to maintain a stable Austenite zone and then a suitable refrigerant (OIL or Refers to heat treatment after quenching in water and then tempering again according to mechanical properties.

In general, automotive parts require high strength and high toughness due to the characteristics of parts, and for this reason, low carbon alloy steel is hardened through carburizing heat treatment to produce parts having high wear resistance and toughness inside. In particular, due to the small degree of surface hardening during the carburizing heat treatment and the limitation of the material used, the interior of the surface other than the carburizing part shows the original ferrite + pearlite structure, and thus the fatigue strength is not excellent. Therefore, it was necessary to improve the hardness.

In other words, the martensitic structure can be molded, austenite, heated, solvated, mold-cooled or quenched after extraction, and finally martensite of the desired strength can be obtained, but martensite-based tissue steel is brittle and durable. Separate tempering must be performed to improve the life or to increase toughness.

The tempering heat treatment is usually performed in the temperature range of 180 ~ 220 ℃, and the yield strength is increased compared to the quenching state, but the tensile strength is lowered to obtain a yield ratio in the range of 0.7 to 0.85, and the uniform elongation and total elongation are slightly increased compared to quenching. Is done. Therefore, in the case of a high temperature tempering heat treatment, the tensile and yield strength is lowered than the quenching state, and the yield ratio can be increased in the range of 0.9 to 0.98.

In addition, there is an increasing demand to further improve strength in these heat-treated parts. As a way to increase the strength, the composition of the bar regulated in the existing boron-added heat-treated steel, that is, Mn is fixed in the range of 0.5 to 1.5%, Cr in the range of 0.1 to 0.3%, and when the C content is increased considering the strength after heat treatment, quenching strength Increases in proportion to the content of C, Mn, etc., but when heat treatment is performed as before to impart toughness and ductility, yield strength and tensile strength are significantly reduced, and the effect of adding C, Mn, etc. is halved to increase strength. There is also a problem that the toughness increases proportionally.

The quenching and tempering step according to an embodiment of the present invention is a step of resolving the residual stress and processing to have a required strength, so that quenching and tampering can be sequentially performed to minimize deformation occurring in the heat treatment process, In this step, after the short ball is heated to a temperature of at least 1000 ° C. or higher, it is precipitated in a salt bath for 30 to 60 minutes in a salt bath to prevent deformation occurring during cooling, and then air-cooled to perform martensite transformation.

At this time, the preferred heating temperature is 1000 ° C or higher, and when heated to a temperature too low, it is preferable to set it to 1000 ° C or higher since it causes unnecessary energy consumption.

  division   Conventional short ball   Short ball according to the present invention
Organization

 Ferrite + pearlite  Tempered martensitic   Hardness  Hv 550 ~ 580   Hv 600 ~ 630

As can be seen from Table 1, the tempering step of the heat treatment step shown in the embodiment of the present invention is a step of tempering at 300 to 400 ° C for 30 minutes or more. This tempering step usually refers to a treatment in which steel is heated to austenitization temperature and then quenched to form martensite and harden. The heating temperature should be 30 ~ 50 ℃ above Ac3 point in the perforated steel. In order to obtain a martensite structure, it is necessary to cool at a rate higher than the critical cooling rate unique to each steel, and for this reason, oil or water is often used. Although the cooling capacity of the temperature zone that requires oil quenching is large and is an excellent cooling liquid, water-soluble quenching liquids have been developed and put into practical use in the event of a fire or pollution. Special methods of avoiding this risk can be used, such as martenpa, ostenpa, and time quenching. The hardness of the martensite structure increases with increasing carbon content of the steel, but the cooling rate in quenching is not sufficient, and when other structures are incorporated, not only the quenching hardness decreases as shown in the figure, but also the toughness and fatigue when released. The strength and the like are lowered.

In general, it may also refer to an operation of rapidly cooling a metal at a high temperature. In Patent No. 10-0587761, the viscosity of the quench oil circulated in the pyrolysis fractional distillation apparatus is brought into contact with the quench oil effluent stream of 0.1-0.5 kg / kg, and the resulting vapor-liquid mixture is separated. In order to remove tar liquid, a technique was introduced to control residual steam by supplying it to a fractional distiller.

By removing the tar liquid from the fractional distiller feed in this method, it is possible to operate the fractional distiller to reduce reflux, raise the bottom temperature, and recover more heat at higher temperatures.

In Patent Registration No. 10-1254782, when high strength and high toughness are required due to the characteristics of a component, a technique for manufacturing a component having high wear resistance while having high wear resistance by hardening the surface of a low carbon alloy steel through carburizing heat treatment is known.

As shown in Table 2, the existing shot ball has a small degree of surface hardening during heat treatment, and due to the limitations of the material used, the interior of the surface other than the carburizing portion has the original ferrite + pearlite structure, so the fatigue strength is not excellent.

As a method of omitting oil quenching after heat treatment, in the case of European Patent No. 1098011 (An air hardenable low to medium carbon steel composion), carbon (C) 0.10 to 0.55 wt% or 0.39 wt% was used to reduce the carburization time. . This reduces the carburization time in the range of 0.10 to 0.38% by weight of the comparative steel, which is impossible when considering a conventional carburizing process.

In addition, the quenching and tempering process was omitted in order to secure the strength during curing of the comparative steel, but a large amount of expensive molybdenum (Mo) is used, which has a disadvantage of high production cost. Therefore, it is possible to achieve the object of the present invention as a short ball having a tempered martensite structure and having a hardness of Hv 600 to 630.

     division   Conventional short ball Short ball according to the present invention   Development effect Tissue homogenization and
Improving impact resistance Raw material texture
(Ferrite + Pearlite) Homogenized tissue
(Tempered martensitic) Into a homogenized tissue
Improving hardness and strength against impact → Can be used for high strength springs Abrasion resistance
(Irving test) 3,400 ~ 3,500 cycles 5,500 ~ 5,700 cycles Improved service life
(1.6 times higher improvement) Shortball application diversity        - Applicable regardless of the size of the short ball. Expandable

Table 2 shows the result of comparing the short ball made by the heat treatment method of the spring short ball according to the present invention and the conventional short ball. As described in Table 3, the raw material of the existing short ball forms a structure with ferrite + pearlite. Short ball cutting with abrasion resistance of 3,400 to 3,500 cycles, tempered martensitic structure, and abrasive resistance of 5,500 to 5,700. It has an effect that is improved 1.6 times compared to that produced only by bay.

In addition, in the embodiment of the present invention, by carrying out a tempering process after the heating process with a spring containing a large amount of the short ball raw material during the spring manufacturing process, the short ball is not added to the spring processing process without separately performing the tempering process of the short ball. There is also an effect that the process equipment for performing separate tempering by performing the tempering process is omitted. As the volume of the short ball increases, it is preferable to use it as a high alloy steel gradually because of the mass effect.

Particularly, since the tempered martensite is a structure obtained by tempering a structure, it is composed of a mixed structure of ferrite and microcarbide. In contrast to the layered structure of pearlite, tempered martensite has a uniform distribution of fine granular cementite and hardness Since it is Hv 600 ~ 630, it has higher elastic limit, improved toughness and higher strength than pearlite compared to martensite.

Although embodiments of the present invention have been described with reference to the above, those skilled in the art to which the present invention pertains will appreciate that the present invention may be implemented in other specific forms without changing its technical spirit or essential features.

Therefore, the above-described embodiments are to be understood as illustrative and not restrictive in all respects, and the scope of the present invention described in the detailed description is indicated by the claims below, and the meaning of the claims and It should be construed that all modifications or variations derived from the scope and equivalent concepts are included in the scope of the present invention.

Claims (9)

In the heat treatment method of the spring short ball,
Cutting the short ball; And
Heat-treating the cut short ball;
Including, but, the heat treatment step, including the heating (Heating), quenching (quenching) and tempering (tempering), the durability of the short ball is increased, hardness, strength, abrasion resistance and toughness is improved, short The ball forms a tempered martensitic tissue, has a wear resistance of 5,500 to 5,700 cycles,
The quenching step,
Cooling using oil having a predetermined temperature and viscosity ensures a predetermined hardness and strength,
The oil,
Of the base oil having a kinematic viscosity of 40 kPa / sec or higher at 40 ° C., a vapor film rupture agent is added and the pressure is reduced, and the heat treatment time of the base oil is 2.5 seconds (sec) or less. Heat treatment method. According to claim 1,
In the quenching step and tempering step, the heat treatment method of the spring short ball, characterized in that it is precipitated for 30 to 60 minutes in a salt bath. According to claim 1,
The heating step,
Method for heat treatment of a spring short ball, characterized in that heating is performed at 900 ° C or higher for 10 minutes. According to claim 1,
In the quenching step and the tempering step, the heat treatment method of the spring short ball, characterized in that heating the short ball to a temperature of at least 1000 ℃. The method of claim 1, wherein the tempering step,
Heat treatment method for spring short balls, characterized in that the hardness is in the range of Hv 600 ~ 630 by tempering at 300 ~ 400 ℃ for more than 30 minutes. delete delete The method of claim 1, wherein the oil has a kinematic viscosity at 40 ° C. of 40 to 300 mm 2 / sec. The method of claim 1, wherein the amount of the steam film breaking agent is 5% by mass or more based on the oil quenched oil.