KR100940723B1 - Manufacturing method of high carbon bearing - Google Patents

Abstract

The present invention relates to a method for manufacturing a high carbon bearing, and more particularly, to remove segregation that tends to exist in a cast steel cast steel due to a high amount of carbon, and to perform an effective crack diffusion treatment that can contribute to improving the fatigue life of the bearing. After the bearing is produced and a special heat treatment for the bearing.

Bearing production method of the present invention is prepared by the bearing cast piece comprising carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight crack diffusion treatment step of removing segregation present in the interior; Manufacturing a bearing from the crack diffusion cast slab; And quenching and distributing heat treatment of the manufactured bearing, wherein the crack diffusion treatment is performed at a temperature of 1190 to 1250 ° C. for at least 1 hour.

Crack Diffusion, Segregation, Cast, Distribution, Bearing Steel

Description

MANUFACTURING METHOD OF HIGH CARBON BEARING

The present invention relates to a method for manufacturing a high carbon bearing, and more particularly, to remove segregation that tends to exist in a cast steel cast steel due to a high amount of carbon, and to perform an effective crack diffusion treatment that can contribute to improving the fatigue life of the bearing. After the bearing is produced and a special heat treatment for the bearing.

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.

Hardening heat treatment is carried out to improve fatigue resistance and wear resistance of such a bearing, and a normal hardening heat treatment used for a bearing is performed 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.

In addition, the bearing steel is often made of a high carbon steel containing a large amount of carbon therein to be advantageous for the above-mentioned hardening heat treatment. The high-carbon bearing steel as described above converts the molten iron into molten steel through a converter process, and then finely adjusts the components by vacuum degassing (RH) refining or ladle furnace refining, and at the same time, includes non-metallic inclusions or Through the process of removing other unnecessary impurities, molten steel having the final desired component system is manufactured.

Previously, molten steel manufactured by the above process was manufactured into an ingot by an ingot casting method, and then the manufactured cast steel was processed and put into wire rod rolling to manufacture bearing steel. However, since the continuous casting method has been introduced worldwide, most steel materials are manufactured by continuous casting as part of the productivity improvement. If such continuous casting is performed, the possibility of carbon segregation with a very high carbon content will increase. .

This phenomenon causes the liquid line to move toward the process line during solidification, so that the final solidified portion contains carbon as a process point composition.In the nature of the solidification process, at the center of the cast steel farthest from the surface of the cast steel from which the heat flux escapes Carbon segregation zone of the process point composition as described above is generated. Of course, this phenomenon can occur in all carbon steels having carbon above the ferrite solid solution limit, but as the carbon content increases, the size of the segregation zone increases. In the case of the bearing steel of the present invention, since the carbon content is close to 1%, this segregation phenomenon makes it possible to easily generate huge carbides, namely, segregation and cementite cementite. It can occur frequently and cause serious problems with the quality of the product.

Various alternatives have been proposed in the past to solve the problem of carbon segregation. One of them is a technique for reducing segregation by increasing the ratio of equiaxed crystals by controlling molten steel superheat below a certain level and adjusting casting speed.

In addition, the method of improving the material quality by reducing the amount of cementite cementite produced in the center of the wire rod by reducing the amount of cementite cementite produced at the center of the wire rod by adjusting the cooling rate in the forced air cooling zone during wire rod manufacturing. there was.

In addition, Japanese Patent Application Laid-Open No. 1996-132205 proposes a method of reducing segregation of bearing steel slabs by performing a light pressure of 10 to 100 mm when operating in a vertical type instrument, and in Japanese Patent Laid-Open Publication No. 199-248302. Several techniques have been proposed to control segregation under light pressure, such as attempting to reduce the pressure by installing a roll on the solidified part for control.

However, when using this conventional technique, segregation cannot be reduced to a certain level, but there is a limit in the removal thereof, and thus problems such as remaining of a large level of oxides have not been solved.

As a method of solving this technical limitation, a method of crack diffusion has emerged. The crack diffusion treatment is used to heat the slab to a temperature where carbon diffusion can occur easily so that the concentration of carbon is uniform throughout the slab. Refers to heat treatment.

Although some preferred methods for such crack diffusion heat treatment have been proposed in the related art, the application of crack diffusion heat treatment conditions as described above will have to be changed depending on the alloy component system present in steel, and appropriate research is required.

The present invention is to solve the problems of the prior art described above, according to the present invention, of course, to propose a new method and a bearing steel component system suitable for applying the method to solve the problems caused by the conventional QT process. A crack cracking treatment and a heat treatment method of a bearing steel slab of the component system are proposed.

The bearing manufacturing method of the present invention for solving the problems of the present invention is prepared by the bearing cast steel comprising carbon: 0.95 ~ 1.05% by weight and silicon: 1.0 ~ 2.0% by weight cracks to remove segregation present inside Processing step; Manufacturing a bearing from the crack diffusion cast slab; And quenching and distributing heat treatment of the manufactured bearing, wherein the crack diffusion treatment is performed at a temperature of 1190 to 1250 ° C. for at least 1 hour.

At this time, the bearing cast steel is one or two selected from manganese: 0.25 to 1.20% by weight, chromium: 1.30 to 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 further contain an element of species or more.

In addition, it is effective that the bearing cast piece contains P and S in an amount of 0.025% by weight or less, respectively.

The crack diffusion treatment may be performed for 6 hours or less.

According to the present invention, the fatigue life of the final bearing can be remarkably improved by deriving a bearing steel component system suitable for quenching-distributing (QP) heat treatment and providing a crack diffusion treatment method and a heat treatment method suitable for slabs having the component system.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention, while trying to solve the problems of the prior art described above, in order to improve the toughness of the bearing, it is necessary not only to form martensite as an internal structure but to retain austenite to retain residual austenite And found the present invention.

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 from the figure, since the residual austenite remains essential in the QP treatment, the distribution temperature is more preferably between Ms and Mf.

Accordingly, the dispensing treatment results in the formation of tempered martensite and stable residual austenite in which carbides are not substantially present therein.

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 to be included in the composition of the steel other than the above elements.

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 Cu, 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 it does not have a significant adverse effect on 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.

When the component system of a bearing steel (casting) is made into the component system mentioned above, it becomes suitable for QP process of high carbon steel. However, such a component system needs to be subjected to crack diffusion heat treatment as the core segregation is very easily developed when the continuous casting is performed as described above. By the way, the crack diffusion treatment method for the new component system as described above has not been proposed. Therefore, a detailed crack diffusion treatment method for the component system will be proposed below. The crack diffusion treatment is described here because the treatment temperature and treatment time are important conditions.

Treatment temperature: 1190 ~ 1250 ℃

In order to facilitate the diffusion of carbon into the component system proposed in the present invention, the treatment temperature should be 1190 ° C or higher. If the treatment temperature is low, it is not preferable because the large carbide located in the center of the cast steel is difficult to completely dissolve and diffuse. On the contrary, if the treatment temperature is too high, energy is not consumed unnecessarily, and decarburization occurs on the surface of the cast steel, and the decarburization part is insufficient to be used as a bearing due to lack of hardness. This may cause problems such as load on the process and loss of material. Therefore, the upper limit of processing temperature is set to 1205 degreeC.

Treatment time: over 1 hour

Treatment time is also a significant factor in the diffusion of segregated components. In order to induce sufficient diffusion, it is necessary to maintain the treatment time for at least 1 hour. More preferably, the treatment time needs to be maintained for 1.5 hours or longer. However, even if the treatment time is extended, the crack diffusion treatment efficiency does not decrease, so the upper limit of the treatment time does not need to be specifically determined. However, in consideration of decarburization and treatment costs, the upper limit of the treatment time is preferably 6 hours.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings and 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)

300mm cast by continuous casting with a tundish component of 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 A specimen of 20 mm × 20 mm × 10 mm size was taken from the center of the specimen with a cross section of 400 mm size. Figure 2 shows a micrograph photographing the formation of a large carbide inside the cast steel.

The sample was then subjected to crack diffusion treatment at various temperatures and holding times as shown in Table 1, and the cross section of the specimen was observed to observe whether the macrocarbide was completely removed and the results are also shown in Table 1. . In the table, the symbol x indicates the case where the macrocarbide is not completely removed and the symbol ○ indicates that the carbide is completely removed.

1190 ℃ 1200 ℃ 1210 ℃ 1225 ℃ 1240 ℃ 1250 ℃ 0.5 hours × × × × × × 1 hours × × ○ ○ ○ ○ 1.5 hours ○ ○ ○ ○ ○ ○ 2 hours ○ ○ ○ ○ ○ ○ 3 hours ○ ○ ○ ○ ○ ○ 4 hours ○ ○ ○ ○ ○ ○ 5 hours ○ ○ ○ ○ ○ ○ 6 hours ○ ○ ○ ○ ○ ○

As can be seen from the table, it was found that the carbides could not be completely removed regardless of the temperature after 0.5 hour crack diffusion treatment. However, in the case of 1-hour crack diffusion heat treatment, when the heat treatment temperature was 1190 ° C and 1200 ° C, large carbides remained slightly to obtain a full crack diffusion effect, but at the rest of the temperature, only a slight amount of fine carbide remained Carbide was hardly present and it was confirmed that sufficient treatment effect could be obtained.

In addition, since the treatment time passed 1.5 hours, it was confirmed that all of the large carbides were removed at a temperature of 1190 ° C or higher specified in the present invention.

As a result supporting this, micrographs of the specimens according to the treatment temperature and the treatment time of some of the conditions described in Table 1 are shown in FIG. 3. 3 (a) and 3 (b) show the results of 0.5 hours treatment at 1200 ° C. and 1225 ° C., respectively. However, FIG. 3 (c) shows that as a result of 1.5 hours of treatment at 1200 ° C., most of the large carbides were removed and only some of the fine carbides remained. These fine carbides have a very small adverse impact on the fatigue life of the bearings compared to the large carbides. FIG. 3 (d) shows the result of the treatment at 1225 ° C. for 1 hour. Here, it can be confirmed that the macrocarbide is removed well.

Thus, the advantageous effects of the present invention could be confirmed.

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

Figure 2 is a micrograph observing the presence of segregation in the cast steel used in the embodiment of the present invention, and

3 is an internal tissue micrograph according to the conditions of each crack diffusion treatment.

Claims (4)

A crack diffusion treatment step of removing a segregation present therein after preparing a bearing cast piece including carbon: 0.95 to 1.05% by weight and silicon: 1.0 to 2.0% by weight; Manufacturing a bearing from the crack diffusion cast slab; And Quenching and distributing heat treatment of the manufactured bearing; The crack diffusion treatment is a bearing manufacturing method, characterized in that carried out for 1 hour or more at a temperature of 1190 ~ 1250 ℃. The method of claim 1, wherein the bearing cast steel is selected from manganese: 0.25 to 1.20% by weight, chromium: 1.30 to 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 The manufacturing method of the bearing further containing 1 type, or 2 or more types of elements. 3. The bearing manufacturing method according to claim 1 or 2, wherein the bearing slab contains P and S as impurities at 0.025 wt% or less, respectively. The bearing manufacturing method according to claim 1 or 2, wherein the crack diffusion treatment is performed for 6 hours or less.

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