KR101262458B1 - Steel for bearing having excellent thermal deformation resistance and method for manufacturing thereof - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire for bearings widely used in the automobile and machinery industries, such as engines, transmissions and wheels of automobiles, rolling rolls of steel machines, construction machinery, and the like, and more particularly, to steel wire for bearings. By adding silicon, which is a solid solution to the substrate, to minimize the density change in the material during heating and cooling, the steel wire for bearings with excellent heat deformation resistance and minimization of microstructure degradation due to temperature change and its manufacturing method It is about.
To this end, the steel wire manufacturing method of the present invention, in weight%, C: 0.9 ~ 1.4%, Si: 1.5 ~ 3.0%, Mn: 0.3 ~ 0.7%, Cr: 0.8 ~ 1.5%, the remaining Fe and other unavoidable Performing a spheroidizing heat treatment of the steel wire including the element at a two-phase temperature of 850 to 900 ° C. for 1 to 3 hours; Cooling the heat treated steel wire at a rate of 0.3 ° C./s or less; Performing a homogenization heat treatment of the cooled steel wire at 830 ° C. for at least 1 hour and quenching at a cooling rate of 5˜10 ° C./s; It includes the step of soaking the quenched steel wire at less than 300 ℃ 1 hour.

Description

Steel wire for bearing with excellent heat deformation resistance and manufacturing method thereof {STEEL FOR BEARING HAVING EXCELLENT THERMAL DEFORMATION RESISTANCE AND METHOD FOR MANUFACTURING THEREOF}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire for bearings widely used in the automobile and machinery industries, such as engines, transmissions and wheels of automobiles, rolling rolls of steel machines, construction machinery, and the like, and more particularly, to steel wire for bearings. By adding silicon, which is a solid solution to the substrate, to minimize the density change in the material during heating and cooling, the steel wire for bearings with excellent heat deformation resistance and minimization of microstructure degradation due to temperature change and its manufacturing method It is about.

Recently, solving the environmental problems caused by the use of energy along with the efficiency of fossil energy has emerged as an urgent obstacle. In addition, the green business has been in the spotlight as a new growth engine for 21C, and the automobile and machinery industries are strongly required to reduce the weight and efficiency of materials.

A bearing is a mechanical element that fixes the shaft of a rotating machine to a fixed position and supports the shaft's own weight and the load applied to the shaft while helping to rotate the shaft. Widely used in rolling rolls, etc.

Therefore, the bearings require high strength and toughness to support the load of the machine, and also require high dimensional accuracy and resistance to thermal deformation during rotation to enable fast rotational motion.

Most steel wires, except stainless steel, which do not cause phase transformation due to temperature change, exist as ferrite phase, which is a body-centered cubic (BCC) structure at low temperature, and when heated above the A1 transformation point, transforms to austenitic phase, which is a face-centered cubic (FCC) structure. Will start. At this time, before and after the phase transformation temperature, the crystal structure in the material is changed due to the phase transformation and thus accompanied by the density change, so that the material expands or contracts rapidly at this temperature region. Therefore, in a use environment in which heating and cooling occur repeatedly, surface cracks continuously grow due to heat deformation, which may lead to premature loss. In particular, it is urgent to solve these problems in products requiring high dimensional accuracy such as bearings and frequent temperature changes due to friction during use, and in recent years, such thermal deformation due to high engine power and miniaturization of peripheral components. Is becoming more important.

In general, when strength and heat resistance are both required, expensive austenitic stainless steels containing a large amount of Ni, Cr, etc. have been mainly used. However, these austenitic alloys have a lattice constant larger than that of ferritic, and thus are repeatedly used in bearings. The thermal fatigue of expansion / contraction (heating / cooling) which is indicated by

In addition, the existing ferritic bearing steel has a problem that the strength is reduced due to the coarsening of carbide when used in a high temperature environment of 200 ~ 400 ℃.

Therefore, the development of bearing steel, which solves both the problems of strength degradation due to thermal deformation and coarsening of carbides in a thermal fatigue environment, has emerged as an important task.

The present invention is to solve the above problems, by minimizing the density change in the material during heating and cooling by adding silicon to minimize thermal deformation and microstructure degradation, as well as the effect of solid solution strengthening and precipitation strengthening at high temperature It is an object of the present invention to provide a ferritic bearing steel wire having excellent thermal deformation resistance and a method of manufacturing the same, which can ensure the stability of carbide.

The present invention comprises, by weight%, C: 0.9-1.4%, Si: 1.5-3.0%, Mn: 0.3-0.7%, Cr: 0.8-1.5%, the remaining Fe and other unavoidable elements, the structure of which is tempered. To provide a steel wire for bearings with excellent heat deformation resistance composed of two phases of de-martensite or tempered martensite and spheroidized cementite.

The steel wire for bearing is preferably a maximum shrinkage amount of less than 0.0015mm due to the phase transformation when the phase transformation from ferrite to austenite during heating of the 10mm test piece.

The steel wire for bearing is preferably a maximum expansion amount of less than 0.0002mm due to phase transformation during a phase transformation from austenite to ferrite during cooling of the 10mm test piece.

In addition, the method for producing a steel wire according to the present invention as a means for realizing this,

By weight%, C: 0.9-1.4%, Si: 1.5-3.0%, Mn: 0.3-0.7%, Cr: 0.8-1.5%, and the steel wire containing the remaining Fe and other unavoidable elements is 850-900 ℃ 2 Performing a nodular heat treatment for 1 to 3 hours at a normal temperature; Cooling the heat treated steel wire at a rate of 0.3 ° C./s or less; Performing a homogenization heat treatment of the cooled steel wire at 830 ° C. for at least 1 hour and quenching at a cooling rate of 5˜10 ° C./s; It includes the step of soaking the quenched steel wire at less than 300 ℃ 1 hour.

According to the manufacturing method of the steel wire for bearings according to the present invention, by minimizing the amount of heat deformation due to phase transformation during heating / cooling, it is possible to significantly reduce the cracks and fractures due to heat deformation, used in parts requiring high dimensional accuracy There is a possible effect. In addition, there is a more economical effect by producing a steel wire for bearings excellent in strength and heat deformation resistance without using expensive alloying elements.

Figure 1 (a) is a graph showing the change in thermal deformation amount according to the amount of Si added during heating, Figure 1 (b) is a graph showing the change in thermal strain according to the amount of Si added during cooling.
Figure 2 is for confirming the difference between the microstructure according to the soaking temperature, (a) shows a microstructure photograph when the comparative example 2 and Example 2 are considered at 300 ℃, respectively, (b) is Comparative Example 2 And a microstructure photograph in case of considering Example 2 at 450 ° C., respectively.

The present invention relates to a steel wire for bearing, in order to minimize the thermal deformation and microstructure degradation of the material according to the temperature change, by adding silicon to reduce the density change in the material during heating and cooling, the effect of solid solution strengthening and precipitation strengthening To ensure the stability of the carbide.

Hereinafter, the present invention will be described in detail by dividing the components of the steel wire and its manufacturing method.

First, the steel wire component will be described. The content of each element below represents weight percent.

Carbon: 0.9 ~ 1.4%

Carbon is an essential element added to secure the strength of the material. If the carbon content is less than 0.9%, it is impossible to secure the minimum strength required for the steel wire, and if the content is more than 1.4%, it is impossible to suppress the cementite cementite and the ductility of the material is significantly reduced. Therefore, the content of C is preferably limited to 0.9 to 1.4%.

Silicon: 1.5 ~ 3.0%

Silicon is a representative ferrite former, and its atomic size is smaller than that of iron atoms, thereby minimizing thermal strain during phase transformation. In addition, silicon has little solubility in cementite and is an element that greatly enhances the activity (chemical or activity) of carbon atoms in the material, and is selectively concentrated only on ferrite to improve the high temperature stability of cementite during use. In other words, the growth rate is greatly delayed.

In terms of strength contribution, silicon is expected to have a higher solid solution strengthening effect than other elements, thereby minimizing tissue degradation and strength deterioration due to high temperature. In general, when the martensitic structure is considered, supersaturated carbides in the base material are precipitated and grown. At this time, silicon is also known to lower the growth rate of carbides, and thus, a great effect can be expected in terms of precipitation strengthening. When the content of Si is less than 1.5%, the effect is weak, and when the content is more than 3.0%, the effect disappears due to the graphitization of carbides. Therefore, the content of Si is preferably limited to 1.5 to 3.0%.

Manganese: 0.3 ~ 0.7%

Manganese is an element that is beneficial in securing strength by improving the hardenability of steel wire when present in steel wire. When the content of Mn is less than 0.3%, it is difficult to obtain required strength and quenchability, and when it exceeds 0.7%, toughness is lowered. Therefore, the content of Mn is preferably limited to 0.3 ~ 0.7%.

Chromium: 0.8 ~ 1.5%

Chromium is a useful element to secure graphitization, temper softening and quenchability. When the Cr content is less than 0.8%, it is difficult to secure sufficient temper softening and quenching properties, and it is difficult to suppress graphitization due to the addition of silicon. In addition, when the content exceeds 1.5%, the deformation resistance may be lowered, which may lead to a decrease in strength. Therefore, the content of Cr is preferably limited to 0.8 ~ 1.5%.

Hereinafter, the manufacturing conditions of the steel wire of the present invention will be described.

In the present invention, after the spheroidization heat treatment is performed at a two-phase temperature of 850 ~ 900 ℃ to the steel wire composition as described above, the spheroidized heat treatment steel wire is cooled.

Next, after the homogenization heat treatment is performed on the cooled steel wire material, the steel wire material is hardened, and the steel wire material for bearing having excellent heat deformation resistance is manufactured by considering the hardened steel wire material.

The steel wire used in the manufacturing method of this invention is manufactured by a conventional method.

The spheroidization heat treatment time is preferably limited to 1 hour or more as a softening treatment for ensuring sufficient formability of the material. However, when the spheroidization heat treatment time exceeds 3 hours, it is difficult to suppress the growth coarsening of carbides. It is preferable to limit to 1 to 3 hours.

When cooling after the spheroidizing heat treatment, the cooling rate is preferably limited to 0.30 ° C / s. More preferably, it is preferable to limit to 0.05-0.3 ° C / s. If the cooling rate is less than 0.05 ℃ / s may be connected to a low productivity due to the cooling rate is slow, and if the cooling rate exceeds 0.3 ℃ / s to reduce the hardness through the inhibition of the production of pearlite structure and the growth of spheroidized carbide to form a desired level It is difficult to expect the effect to get sex.

It is preferable to perform a homogenization heat treatment of the steel wire cooled as described above at 830 ° C. for 1 hour or more.

When the quenching, the cooling rate is preferably limited to 5 ~ 10 ℃ / s. If it is less than 5 ° C./s, the slow cooling rate may result in a failure to obtain complete quenched tissue.

It is preferable that the steel wire quenched as described above is treated at least 300 ° C. for at least 1 hour. In case of consideration at 300 ° C. or higher, the recovery may proceed rapidly, leading to a large deterioration of the microstructure, which may cause a problem of failing to secure sufficient hardness in the final material.

The microstructure of the steel wire of the present invention manufactured by the above manufacturing method is composed of a form in which fine spheroidized cementite (carbide) is distributed in a tempered martensite or a tempered martensite matrix. The fine spheroidized carbides serve to minimize wear during driving of the bearing and to stabilize the microstructure at high temperatures.

Steel wire manufactured by the above manufacturing method is preferably a maximum shrinkage amount of less than 0.0015mm by phase transformation when the phase transformation from ferrite to austenite on the basis of 10mm test piece during heating, when the phase transformation from austenite to ferrite based on 10mm test piece during cooling It is preferable that the maximum expansion amount by phase transformation is 0.0002 mm or less.

When the shrinkage of the test piece is 0.0015 mm or less or the expansion amount is 0.0002 mm or less, the thermal deformation during heating / cooling has the advantage of excellent fatigue life, while the shrinkage exceeds 0.0015 mm or the expansion amount exceeds 0.0002 mm. Due to the high shrinkage / expansion amount during use, problems such as premature loss in the fatigue environment may occur, so the amount of shrinkage and expansion is preferably limited as described above.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example 1)

For the steel wire having the chemical composition and structure as shown in Table 1, the heat deformation during the heating and cooling process was measured using a dilator, and then the L10 fatigue life evaluation was performed for each of them through a 4 ball fatigue tester. It is shown in Table 1 below. (Shrinkage and swelling tests for phase transformation of materials shown in Table 1 were based on 10 mm test pieces.)

In order to investigate the thermal strain change graphs for Comparative Material 2, Inventive Materials 1 and 2 of Table 1, and the results are shown in FIG. 1.

In addition, for the comparative material 1 and the invention material 2 was investigated the microstructure change according to the soaking temperature and the results are shown in Figure 2, Figure 2 (a) shows the microstructure of the steel wire material at 300 ℃, 2 (b) shows the microstructure of the steel wire considered at 400 ° C.

Chemical composition / wt% Heat Strain / mm L10 fatigue life
C Si Mn Cr Heating (shrinkage) Cooling (expansion amount) Comparison 1 0.97 - 0.42 1.20 0.0032 0.0005 130,000 times Comparative material 2 1.00 0.25 0.45 1.21 0.0030 0.0005 250,000 times Comparative material 3 1.05 0.30 0.40 1.18 0.0031 0.0005 720,000 times Inventory 1 1.00 1.52 0.43 1.23 0.0013 0.0001 1.14 million times Inventory 2 1.10 2.00 0.42 1.22 0.0008 0.0001 2.12 million times Inventory 3 1.03 1.73 0.45 1.19 0.0002 0.0001 1.8 million times

1 is a graph showing changes in thermal strain of Comparative Material 2 and Examples 1 and 2 during heating and cooling. Under heating conditions, the specimen expands continuously with increasing temperature. However, in the section in which the phase transformation occurs, shrinkage of the test piece occurs as the temperature increases. Referring to FIG. 1 (a), it can be seen that the shrinkage tends to decrease rapidly as the Si addition amount increases. On the contrary, in the cooling conditions, the test piece is continuously contracted as the temperature decreases, but the expansion of the test piece occurs in a section in which the phase transformation occurs. Referring to FIG. 1 (b), it is confirmed that the amount of expansion decreases as the Si addition amount is increased. Can be. The change according to the various amounts of Si addition is summarized in Table 1.

Referring to the results of Table 1, it can be seen that when the addition of Si or more than 1.5%, the amount of heat deformation during heating and cooling is reduced by 50% or more. This is due to the effect of the substitutional solid solution strengthening element Si on the lattice constants of austenite and ferrite during phase transformation. The addition of Si has little effect on the austenite lattice constant change, while the lattice constant of ferrite is greatly reduced. This causes the above result. In cooling, this effect of Si is more clearly observed. As can be seen in FIG. 1, the transformation from austenite to ferrite and cementite occurs at almost the same temperature range, and the deformation amount of the specimen is austenite in which C is completely dissolved. It can be seen that the change occurs when the phase transformation is generated, and when Si is added more than 1.5%, the length reduction of the specimen becomes more pronounced when the transformation occurs during cooling. As shown in Table 1, the thermal strain shows a fatigue life of about 700,000 times in the comparative example, while in the examples, it has a value of 1.4 million times or more, indicating that the fatigue life is twice or more. In the case of having the amount of shrinkage and expansion of the present invention, fatigue life was significantly improved compared to the comparative example.

In addition, in order to examine the change in microstructure according to the soaking temperature, the microstructure of Comparative Example 2 (0.25% Si) and Example 2 (2.0% Si) after soaking at 300 ° C and 450 ° C, respectively, is shown in FIG. 2. .

Compared with (a) and (b) of FIG. 2, carbides are homogeneously distributed at 300 ° C. compared with (b) at 450 ° C., and their size is significantly smaller than that at 450 ° C., even at high temperatures. It can be confirmed that a stable steel wire for bearings having excellent thermal deformation resistance can be obtained. In addition, as can be seen in (a) and (b) of Figure 2, when containing 2.0% than when containing Si as 0.25% carbide is homogeneous and small distribution can be obtained high temperature stability of carbide Could confirm.

Claims (5)

In weight%, C: 0.9-1.4%, Si: 1.5-3.0%, Mn: 0.3-0.7%, Cr: 0.8-1.5%, the rest of Fe and other unavoidable elements, the structure being tempered martensite or Steel wire for bearings with excellent heat deformation resistance composed of two phases of tempered martensite and spheroidized cementite.
The method according to claim 1,
Steel wire rod bearing excellent heat deformation resistance of the maximum amount of shrinkage due to phase transformation when the phase transformation from ferrite to austenite during heating of the 10mm test piece of the steel wire less than 0.0015mm.
The method according to claim 1 or 2,
Steel wire rod bearing excellent heat deformation resistance of the maximum expansion amount due to the phase transformation when the phase transformation from austenite to ferrite during cooling of the 10mm test piece of the steel wire less than 0.0002mm.
By weight%, a steel wire containing C: 0.9-1.4%, Si: 1.5-3.0%, Mn: 0.3-0.7%, Cr: 0.8-1.5%, remaining Fe and other unavoidable elements
Performing a spherical heat treatment for 1 to 3 hours at a 2-phase temperature of 850 to 900 ° C;
Cooling the heat treated steel wire at a rate of 0.3 ° C./s or less;
Performing a homogenization heat treatment of the cooled steel wire for at least 1 hour and quenching at a cooling rate of 5 to 10 ° C./s;
Method for producing a steel wire for bearings having excellent heat deformation resistance comprising the step of treating the hardened steel wire material.
The method of claim 4,
The said method is a manufacturing method of the steel wire bearing excellent in heat deformation resistance including the step of performing at least 1 hour at the temperature below 300 degreeC.

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