KR100940724B1 - Manufacturing method of high carbon steel wire rod for bearing steel, heat treatment method of high carbon steel bearing and high carbon steel bearing - Google Patents

Abstract

The present invention relates to a method for manufacturing a wire rod for high carbon bearing steel, a heat treatment method for high carbon bearing steel, and a high carbon bearing steel, and more particularly, a heat treatment method for manufacturing high strength high carbon bearing steel having excellent wear resistance, fatigue resistance, and the like. The present invention relates to a method for producing a wire rod for high carbon bearing steel provided in the heat treatment and to a high carbon bearing steel produced from the heat treatment.

Bearing heat treatment method of the present invention, the step of quenching the bearing-shaped steel parts comprising carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight; Maintaining the quenched component for at least 1 minute; And dispensing at least 10 minutes at a temperature of 100 to 150 ° C for the retained parts.

Bearing, toughness, fatigue characteristics, heat treatment, distribution treatment

Description

MANUFACTURING METHOD OF HIGH CARBON STEEL WIRE ROD FOR BEARING STEEL, HEAT TREATMENT METHOD OF HIGH CARBON STEEL BEARING AND HIGH CARBON STEEL BEARING}

The present invention relates to a method for manufacturing a wire rod for high carbon bearing steel, a heat treatment method for high carbon bearing steel, and a high carbon bearing steel, and more particularly, a heat treatment method for manufacturing high strength high carbon bearing steel having excellent wear resistance, fatigue resistance, and the like. The present invention relates to a method for producing a wire rod for high carbon bearing steel provided in the heat treatment and to a high carbon bearing steel produced from the heat treatment.

Bearing means a mechanical element that serves to rotate the shaft while fixing the shaft of the rotating machine to a certain position and supporting the weight of the shaft and the load on the shaft.

In a rotating machine, a large number of rotations are generated per unit time, so that a cyclic load is transmitted to the bearing supporting the rotating shaft in proportion to the rotational speed. Since the cyclic load is transmitted to the bearing, the bearing must have high resistance to fatigue failure due to the cyclic load, and the wear resistance needs to be excellent. In order to manufacture the bearing as described above, a process of appropriately controlling the composition of steel as a material and manufacturing the steel wire through a process leading to steelmaking-casting-rolling, and then processing the manufactured steel wire into a bearing is required. .

As a bearing material, high carbon chromium steel containing about 1.0% by weight of carbon and about 1.5% by weight of chromium has been most used.

In general, high carbon chromium bearing steel is often made of BLUM through a process of converter-vacuum degassing (RH) -continuous casting. In this case, in the converter process, the work of converting the molten iron into molten steel proceeds, and the operation of lowering the total oxygen (T. [O]) content, which is one measure of the inclusion content in the vacuum degassing process, is performed. The obtained bloom is rolled into a bile after cracking soking and natural air cooling to remove segregation and large carbides present in the center of the cast steel. The manufactured billet is made of wire rod through wire rod rolling.

Bearing processing is performed by processing the shape of the ball, roller or inner and outer rings, which are the rolling elements of the bearing, through the drawing and heat treatment of the wire obtained, and then hardening heat treatment to have sufficient fatigue strength and wear resistance.

At this time, the conventional hardening heat treatment used for the bearing is made by a so-called QT process of tempering after quenching. The quenching treatment is a step of quenching the bearing after heating the bearing to a temperature higher than Ac3, which is the temperature at which the steel is austenitized. The internal structure of the steel is hardened from austenite to hard martensite by constant diffusionless transformation. The martensite produced by the quenching is very hard but cannot be used directly as a bearing because it has very poor toughness. Thus, the tempering treatment is followed by the quenching treatment to improve the toughness of the bearing. The tempering treatment is a process of decomposition precipitation of martensite formed by diffusionless transformation. By maintaining the bearing at a constant temperature, the supersaturated carbon present in martensite is precipitated as a carbide to reduce or eliminate residual stress caused by quenching. Refers to the treatment of stiff microstructure. That is, when tempering, fine carbide precipitates in martensite, and martensite is changed to tempered martensite.

The bearing steel produced by the above process is made of tempered martensite and carbides therein, and has a surface hardness of about 60 HRC or more. However, even in the case of bearings heat-treated by the QT process, when used in harsh conditions such as rolling contact, the wear resistance and fatigue resistance are often insufficient.

This is because, as mentioned above, the martensite structure has a very high hardness but a very poor toughness or ductility, so that even if tempered martensite is formed by tempering treatment, the toughness and ductility improvement effect is insufficient.

In addition, since the QT process requires at least two hours or more, there is a problem of cost increase and productivity decrease.

The present invention is to solve the above problems of the prior art, according to one aspect of the present invention, by a method and a heat treatment method of a new bearing steel that toughness is significantly improved compared to the conventional QT treatment even with a relatively short heat treatment time Bearings are provided.

In addition, according to another aspect of the present invention there is provided a steel wire for a bearing that can be usefully used in manufacturing a bearing according to the aspect of the present invention.

Bearing heat treatment method of the present invention for solving the above problems of the present invention, the step of quenching the bearing-shaped steel parts comprising carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight; Maintaining the quenched component for at least 1 minute; And dispensing at least 10 minutes at a temperature of 100 to 150 ° C for the retained parts.

In this case, the bearing-shaped steel parts are selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less, copper: 1.0% by weight or less It is more preferable to further include 2 or more types of elements.

In addition, the bearing-shaped steel parts preferably contain P and S of 0.025% by weight or less as impurities.

In addition, it is effective that the treatment for holding the quenched component is performed for 10 minutes or less.

In addition, the dispensing treatment is advantageously carried out for 30 minutes or less.

Another aspect of the present invention, the bearing comprises carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight, characterized in that the internal structure consists of martensite and residual austenite.

At this time, one or two or more elements selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less and copper: 1.0% by weight or less It is good to include.

In addition, the residual austenite is preferably contained 5 to 15% by area fraction.

In addition, it is effective that carbides are not substantially included in the internal structure.

In another aspect of the present invention, a method of manufacturing a wire rod provided in the manufacture of the bearing includes cracking and diffusing a cast steel including carbon: 0.95 to 1.05 weight% and silicon: 1.0 to 2.0 weight%, followed by cooling by rolling a steel piece. In the method of manufacturing the wire rod for bearing, the cooling rate is characterized in that less than 1 ℃ / sec.

At this time, one or two or more elements selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less and copper: 1.0% by weight or less It is preferable to include.

According to the present invention can provide a bearing significantly improved toughness than the conventional QT heat treatment, the wear resistance and fatigue resistance of the manufactured bearing can be greatly improved.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention, while trying to solve the above problems of the prior art, in order to improve the toughness of the bearing, it is necessary not only to form martensite into the internal structure but to retain austenite and to retain residual austenite The facts were discovered and the present invention was reached.

However, the component system of commonly used high carbon chromium bearings is not only able to achieve a sufficient ratio of residual austenite formed therein by austenitizing and quenching the bearings to form martensite. Even though the residual austenite is produced, the bearing steel is composed of only tempered martensite and martensite by the so-called plastic organic transformation, which is transformed into martensite by the load applied to the bearing during the use of the bearing. Is obtained.

In addition, the transformation process of the retained austenite to martensite is accompanied by a dimensional change, the dimensional accuracy of the bearing is significantly impaired by the transformation process, resulting in noise or fatigue failure and wear of the bearing. Problems such as significant acceleration may occur.

Therefore, in bearing steel, it is important to stabilize the retained austenite so as not to transform into martensite even when stress is applied to the retained austenite.

In order to solve the problems of the present invention, the inventors of the present invention provide a method for stabilizing the retained austenite as well as forming the retained austenite at an appropriate fraction. Controlling and applying a new process called the QP (Quenching and Partitioning) method was effective.

That is, the QP method is a method first proposed in International Patent Publication No. WO2004 / 022794 and refers to a method of partitioning after quenching. The dispensing process is quenched to keep the steel containing martensite and retained austenite at low temperatures, so that the carbon present in the martensite diffuses into the retained austenite without forming carbides so that the retained austenite is further reduced at low temperatures. How to make it stable. The QT process (a) and the QP process (b) were compared in FIG. In the figure, Ms and Mf mean martensite transformation start temperature and transformation end temperature, respectively. As can be seen in the figure, since the residual austenite is essential in the QP treatment, the quenching temperature is more preferably between Ms and Mf.

Therefore, when the dispensing treatment is carried out, martensite free of substantially no carbide therein and stable residual austenite are formed.

However, such known QP treatment cannot be directly applied to bearing steel. That is, in the case of bearing steel, carbon is contained in a large amount, and even if it is treated at a distribution treatment temperature, carbon does not diffuse into residual austenite and is likely to precipitate inside martensite, thus inherently stabilizing residual austenite. There is a problem that the purpose cannot be achieved.

Therefore, it is necessary to change the composition of the steel so that carbon can be easily diffused into the retained austenite during the distribution treatment to form a stable retained austenite. In order to solve this problem, the inventors set the content of carbon contained in steel to 0.95 to 1.05% by weight, and at the same time, the content of silicon is significantly higher than that contained in the SUJ2 bearing, which has been widely used in the past, 1.0 to 2.0. It was found that changing the weight percent by weight was effective for diffusion of residual austenite in the dispensing process.

That is, silicon is an element belonging to the fourth group of the periodic table like carbon, and is an element which competes with carbon. Therefore, when the silicon content is high, carbon becomes very thermodynamically unstable, so it is diffused from martensite to residual austenite having higher solubility for carbon. As a result, the retained austenite is further stabilized to enjoy the effects intended in the present invention. Hereinafter, the reason for limiting the content of carbon and silicon as described above will be described in more detail.

 C: 0.95 to 1.05 wt%

Carbon is not only a very important element for securing the strength of the bearing, but also an element essential for stabilizing residual austenite in the present invention. If the content of carbon is low, the strength and fatigue strength of the bearing is low, so it is not suitable as a bearing part, the content of carbon is preferably at least 0.95% by weight. On the other hand, when the carbon content is too high, the undissolved huge carbides remain to lower the fatigue strength as well as the workability before quenching, so the upper limit of the carbon content is set to 1.05% by weight.

Si: 1.0-2.0 wt%

Silicon is preferably added in an amount of 1.0% by weight or more to enable dispensing treatment, which has been difficult to implement in conventional bearings as described above. However, if the silicon content is too high, decarburization may occur according to the competition reaction with carbon, and like C, the workability before quenching is inferior, so the silicon content is 2.0% by weight.

That is, the bearing of the present invention and the wire for producing the same is made of a component containing carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight. The term 'comprises' herein means not only the above elements but also an open meaning that other elements other than the above elements may be included together, and a person having ordinary knowledge in the art to which the present invention belongs will be known in the past. From the composition and specification of many provided bearing steels, and common knowledge that has been provided in the art, it may be easy to select elements that may be included in addition to the above elements to be included in the steel composition.

However, as one example thereof, the bearing or steel wire may more preferably include one or two or more of the following components. Hereinafter, the component which can be added is demonstrated.

Mn: 0.25-1.20 wt%

Manganese is an important element in securing strength by improving the hardenability of steel. Therefore, it is preferable that Mn is contained by 0.25 weight% or more. However, when the content of manganese is too high, the workability before quenching is inferior, so the content of manganese is 1.20% by weight or less.

Cr: 1.30 ~ 1.60wt%

Since chromium improves the hardenability of steel and gives hardening ability, it is preferable to add 1.30 weight% or more since it is an effective element to refine | miniaturize steel structure. However, since the effect is saturated when the content of Cr is excessive, the content of chromium is made 1.60 wt% or less.

Al: 0.1 wt% or less

Aluminum is an element that acts as a strong deoxidizer at the time of solvent of steel, has an effect of cleaning steel, and forms a compound with nitrogen in steel to refine crystal grains. However, when 0.1 wt% or more is added, the Al content is made 0.1 wt% or less because not only the cleansing action of the steel is lowered but also the fatigue life is reduced.

Ni: 1.0 wt% or less

Nickel is an element which improves the hardenability of steel and improves the toughness of the quenched portion, and adds 1.0 wt% as an upper limit. In particular, when adding copper, it is preferable to add Ni 1/2 of Cu addition amount in order to suppress hot brittleness.

Cu: 1.0 wt% or less

Copper is added because it has the effect of improving the hardenability and increasing the hardness of the quenched portion, and when added in an amount of 1.0% by weight or more, it is added in an amount of 1.0% by weight or less.

In addition, in order to manufacture the bearing and it may inevitably include impurities. The amount of the impurity may be included to the extent that does not significantly affect the amount of the bearing, and those skilled in the art to which the present invention belongs to easily limit the amount and refine to meet the range of impurity content Wire rods and bearings for controlled bearings may be manufactured. However, in particular, since phosphorus (P), sulfur (S) and titanium (Ti) have a great influence on the physical properties of the bearing, a more preferable content will be described.

P: 0.025% by weight or less

Phosphorus is an element that segregates at grain boundaries and degrades the toughness of steel materials. Therefore, it is more preferable to actively limit the content. However, when considering the load of the steelmaking process, it is preferable to limit the content to 0.025% by weight or less.

S: 0.025% by weight or less

Although sulfur acts to increase the machinability of steel, it is preferable to limit its content since it adversely affects not only segregation at grain boundaries but also toughness as in phosphorus, but also adversely affects fatigue life by forming an emulsion in combination with Mn. When considering the load of the steelmaking process, the content is preferably 0.025% by weight or less.

Ti: 0.01 wt% or less

Titanium combines with nitrogen to form coarse nitrides, which reduces fatigue life, so it is best to limit the content to 0.01% by weight or less.

The present invention is carried out a process of quenching and then quenching the austenitic bearing-shaped parts having the above-described composition, and then performing a distribution heat treatment. Quenching processes are similar to those commonly known in the art. The quenching is required to quench (water-cool or oil-cool) the material to a temperature below the martensite transformation start temperature (Ms). As the martensite transformation start temperature, the following equation may be used.

Ms (° C) = 512-453C-16.9Ni + 15Cr-9.5Mo + 217C ² -71.5C × Mn-67.6C × Cr

Here, C, Ni, Cr, Mo, Mn and the like means the weight percent of each corresponding element.

The subsequent distribution heat treatment process is different from that described in the above-mentioned International Patent Publication No. WO2004 / 022794. In the present invention, a more unique process is proposed to secure hardness and toughness after heat treatment of the bearing steel.

That is, in the present invention, it is not necessary to perform distribution heat treatment immediately after the quenching treatment, but a process of maintaining the quenched bearing for a predetermined time or longer is required. In other words, the invention features another process feature that is quenching-maintenance-dispensing of the material. In the present invention, 'maintain' means to maintain the specimen at the end of the quenching temperature without heating.

According to the results of the inventors of the present invention, it is an essential process for stabilizing residual austenite retaining quenched specimens. That is, the specimen immediately after quenching, the temperature difference between the surface and the center occurs, there is a fear that the tissue is uneven if the temperature is raised immediately. Therefore, in order to prevent this it is necessary to maintain the process for some time after quenching. Therefore, the holding time after the quenching is preferably 1 minute or more. In addition, since there is no problem even if the specimen is maintained after the quenching, there is no need to specifically limit the upper limit of the holding time after the quenching. However, if the maintenance for an excessive time may cause a problem in productivity, when considering this, it is more preferable that the holding time is 30 minutes or less.

After holding the bearing after quenching, dispensing treatment is preferred. In the present invention, the distribution treatment temperature is limited to 100 ~ 150 ℃. If the dispensing temperature is too low, it is difficult to obtain the effect of the dispensing treatment intended in the present invention. On the other hand, if the temperature of the dispensing treatment is excessively high, temper embrittlement may occur, and only carbon should be diffused into the austenite region by the dispensing treatment. not. Therefore, it is preferable to limit the said distribution process temperature to 150 degrees C or less.

In order to obtain sufficient dispensing effect, the dispensing treatment is preferably performed for 10 minutes or more. The distribution treatment is a one-sided process in which carbon dissolved in martensite is diffused toward the retained austenite, so even if the treatment time is long, there is no particular problem. Therefore, there is no need to specifically set an upper limit of the processing time. However, when the dispensing treatment time is increased, the productivity may be deteriorated, and when the dispensing treatment is performed for about 30 minutes, no further significant effect increase occurs, and the time is more preferably 30 minutes or less. This time required for the dispensing process is significantly shorter than the time required for the tempering treatment in the conventional QT method, and can greatly contribute to productivity improvement.

Therefore, the bearing heat treatment method of the present invention comprises the steps of: quenching the parts machined into the bearing shape with the advantageous composition of the present invention described above; Maintaining the quenched bearing part for at least 1 minute; And it consists of a step of dispensing the retained components at a temperature of 100 ~ 150 ℃ for 10 minutes or more.

The bearing manufactured by the conventional QT treatment method has a structure in which tempered martensite and carbide are formed therein, but the bearing manufactured by the present invention has a structure in which martensite and residual austenite are present.

In addition, the proportion of the retained austenite present in the bearing steel produced by the present invention is preferably 5 to 15% by area fraction. Since the residual austenite is an essential structure for improving the toughness of the bearing as described above, the ratio is preferably 5% or more. However, when the ratio of residual austenite is excessively high, carbon may be dispersed and diffused, so that stabilization of residual austenite may not be sufficient. Accordingly, some of the residual austenite may be transformed into martensite. In this case, the dimensional accuracy of the bearing may be seriously impaired, and the bearing may be damaged during use due to poor dimensional accuracy. Therefore, the upper limit of the ratio of the retained austenite is preferably set to 15%.

In addition, the bearing of the present invention has a structure in which carbides are not substantially formed inside martensite, unlike bearings manufactured by the conventional QT method.

Bearings of the present invention having the advantageous features as described above is preferably provided to the bearing heat treatment after the bearing steel wire is processed into a bearing shape to be manufactured by drawing and spheroidizing heat treatment in a conventional manner.

At this time, the wire rod for bearing steel is subjected to a crack-diffusion process for the cast steel and then subjected to a conventional rolling process of rolling the steel sheet, followed by ultra-cooling at a cooling rate of 1 ° C / sec or less so as to have properties suitable for subsequent drawing and spheroidizing heat treatment. It is more preferable to manufacture by. This is because the ductility of the material is improved by the ultra-low cooling process, and thus the drawing is facilitated.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

Inventive Example

Fe-0.972% C-1.71% Si-0.349% Mn-0.019% P-0.009% S-0.032% Al-1.46Cr-0.051Ni-0.009Cu-0.005Ti after wire drawing and heat treatment Machined into a bearing shape. As a result of performing the Ms measurement experiment to confirm the Ms temperature according to Equation 1 of the processed specimen, as shown in FIG. 2, the Ms temperature was found to be 173 ° C, which was in good agreement with the calculated value in Equation 1.

It was heated at 840 ° C. for 30 minutes and then quenched in oil at 100 ° C. lower than the Ms temperature to give a quenching treatment to obtain martensite microstructure. Then, after maintaining for 5 minutes and then heated to a temperature of 150 ℃ to maintain for 10 minutes to prepare a bearing of the invention example.

Conventional example

After quenching in the same manner as in the invention example was subjected to a tempering treatment in a manner that is maintained for 1 hour at a temperature of 180 ℃ to prepare a bearing of the prior art.

Organization comparison

The internal structure of the bearing obtained by the said invention example and the prior art example is shown in FIG. As can be seen in the photomicrograph of Figure 3, the bearing obtained by the conventional example is composed of the main structure of the tempered martensite inside, it can be seen that the carbide is distributed in the tempered martensite. However, the invention according to the present invention is composed of martensite and residual austenite, it can be seen that almost no carbide. In addition, as a result of comparing the residual austenite fraction of the conventional example and the inventive example according to the present invention, the residual austenite ratio of the conventional example was only about 0.64%, but the residual austenite ratio of the inventive example was 8.0%, which is defined in the present invention. It was found that the night ratio was met.

Hardness

The hardness in the conventional example and invention was compared. In the case of the conventional example, the hardness was 60.2 HRC, whereas in the case of the invention example, the hardness was 62HRC level, and it was confirmed that the invention had an excellent hardness equal to or higher than that of the conventional example.

Therefore, when the bearing is manufactured by another method according to the present invention, it is determined that the rolling fatigue life of the bearing steel is also improved according to the high hardness value.

1 is a schematic diagram comparing the QT method and the QP method,

Figure 2 is a graph showing the result of measuring the martensite transformation start temperature of the bearing component used in one embodiment of the present invention, and

3 is a microscopic picture of the structure comparing the bearing (a) manufactured by the conventional example and the bearing (b) manufactured by the invention example.

Claims (11)

Quenching the bearing-shaped steel part comprising carbon: 0.95 to 1.05% by weight and silicon: 1.0 to 2.0% by weight; Maintaining the quenched component for at least 1 minute; And Dispensing at least 10 minutes at a temperature of 100 to 150 ° C for the retained parts; Heat treatment method of a high carbon steel bearing comprising a. The method of claim 1, wherein the bearing-shaped steel parts are selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less, copper: 1.0% by weight or less Heat treatment method of a high carbon steel bearing, characterized in that it further comprises one or two or more elements. 3. The method of heat-treating a high carbon steel bearing according to claim 1 or 2, wherein the bearing-shaped steel parts contain P and S as impurities at 0.025% by weight or less, respectively. The method of claim 1 or 2, wherein the treatment for holding the quenched component is performed for 10 minutes or less. 3. The heat treatment method of a high carbon steel bearing according to claim 1 or 2, wherein the dispensing treatment is performed for 30 minutes or less. A high carbon steel bearing comprising carbon: 0.95 to 1.05% by weight and silicon: 1.0 to 2.0% by weight, the internal structure consisting of martensite and residual austenite. The method according to claim 6, wherein one or two selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less and copper: 1.0% by weight or less A high carbon steel bearing, further comprising the above elements. 8. The high carbon steel bearing according to claim 6 or 7, wherein the residual austenite is contained in an area fraction of 5 to 15%. 9. The high carbon steel bearing of claim 8, wherein substantially no carbide is included in the internal tissue. In the method of producing a wire for bearings by crack-diffusion, steel-rolling and cooling a cast steel containing 0.95 to 1.05% by weight of silicon and 1.0 to 2.0% by weight of silicon, The cooling rate is a manufacturing method of a high carbon steel wire for bearings, characterized in that less than 1 ℃ / second. The method according to claim 10, wherein one or two selected from manganese: 0.25-1.20% by weight, chromium: 1.30-1.60% by weight, aluminum: 0.1% by weight or less, nickel: 1.0% by weight or less and copper: 1.0% by weight or less The manufacturing method of the high carbon steel wire for bearings further containing the above element.

Images