JP2005187888A - Method for quenching hyper-eutectoid steel excellent in static strength used for rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for quenching hyper-eutectoid steel with which the unevenness of static strength in a bearing is eliminated and the bearing having high strength can be obtained. <P>SOLUTION: In the method for quenching the hyper-eutectoid steel excellent in the static strength used for a rolling bearing, as to the hyper-eutectoid steel containing by mass% of 0.8-1.2% C, 0.01-1.5% Si, 0.1-2.0% Mn, 0.1-3.0% Cr, ≤0.03% P, ≤0.03% S and the balance Fe with inevitable impurities, at the quenching time, the temperature rising speed is made to be 1°C/min-25°C/min from 720°C to 780-850°C quenching holding temperature and the holding time at the quenching holding temperature is made to be 15-60 min. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for quenching hypereutectoid steel having excellent static strength used for rolling bearings.

  For rolling bearings, high-carbon chromium bearing steels defined in JIS G4805 and hypereutectoid steels containing 0.8% or more by mass in accordance with C are used in many cases. These hypereutectoid steels are subjected to spheroidizing annealing after processing in the hot region, and after forming into the required shape, quenching and tempering treatment are performed to adjust to the desired hardness, and then the final polishing step is performed. After that, it becomes a bearing.

  By the way, hypereutectoid steel has a structure in which pro-eutectoid cementite and pearlite are mixed in the air-cooled state after hot working and is hard. Spheroidizing annealing is performed to reduce the hardness by spheroidizing to a size, and then processing into a final shape.

  In the subsequent quenching and tempering treatment, in order to ensure rolling fatigue resistance and wear resistance, it is necessary to change the structure to a structure in which appropriate amounts of austenite and carbide remain in martensite. Furthermore, the quenching temperature is about 800 to 850 ° C. for SUJ1, SUJ2 and SUJ4, about 790 to 830 ° C. for SUJ3 and SUJ5, and the holding time is about 30 to 60 min, respectively. In order to remove the internal stress caused by quenching and stabilize the structure, 120 min is standard at 140 to 180 ° C. (see, for example, Non-Patent Document 1). However, even when quenched at such a temperature, the variation in static strength was large.

  Further, as the characteristics required for the bearing, in addition to the rolling fatigue resistance and wear resistance, the crushing strength (static strength) is evaluated. The crushing strength is evaluated by preparing a test piece on the ring and applying a load when the ring is crushed (for example, see Non-Patent Document 2). According to this, it is shown that the crushing value is lower in strength as the austenitizing temperature is higher and the holding time is longer, and the optimum temperature is determined based on this fact.

  However, the fact that the crushing strength of the bearings processed in an actual mass production furnace varies greatly and is low in strength despite the processing under the quenching conditions described above is the reality.

“Special Steel”, Vol. 46, No. 4, p. 46-48 “Iron and Steel”, 49th No. 1, p. 47-54

  The problem to be solved by the present invention is a method of quenching the hypereutectoid steel to eliminate the static strength variation of the hypereutectoid steel which is the cause of the crushing strength of the bearing and to obtain a high strength bearing. Is to provide.

  As a result of studying the cause of variation in static strength in hypereutectoid steel, the inventor found that the amount of workpieces to be placed in the heat treatment furnace, the size of the workpiece, We have earnestly studied that there is a difference in the heating rate depending on the placement position, etc., and that there is a difference in static strength despite the same quenching temperature and holding time due to the difference in heating rate. I found it.

  Therefore, the means of the present invention for solving the above-mentioned problem is that, in the invention of claim 1, the average rate of temperature increase from 720 ° C. to the quenching holding temperature is limited to 1 ° C./min to 25 ° C./min during quenching. This is a method for quenching hypereutectoid steel that suppresses variations in static strength and increases strength.

  In the invention of claim 2, the quenching holding temperature is 780 to 850 ° C., and the holding time at the quenching holding temperature is 15 to 60 min. This is a method of quenching hypereutectoid steel with excellent mechanical strength.

  In the invention of claim 3, the hypereutectoid steel is, in mass%, C: 0.8 to 1.2%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, 3. A steel comprising Cr: 0.1 to 3.0%, P: 0.03% or less, and S: 0.03% or less, the balance being Fe and inevitable impurities. This is a method for quenching hypereutectoid steel with excellent static strength used in rolling bearings.

  In the invention of claim 4, the hypereutectoid steel is, in addition to the above-mentioned components, Ni: 0.1-2.0%, Mo: 0.01- It is a hardening method of hypereutectoid steel excellent in the static strength used for the rolling bearing of the means of the means of Claim 3 characterized by containing 1 type or 2 types out of 1.5%.

  In the invention of claim 5, in addition to the above components, the hypereutectoid steel is Nb: 0.001 to 0.15% as an element that further refines the crystal grains and further improves the static strength by mass%. The method of quenching hypereutectoid steel excellent in static strength used for a rolling bearing according to the means of claim 3 or 4 characterized by comprising:

  By the way, the above-mentioned from 720 ° C. to the quenching holding temperature is a temperature range in which the ferrite is transformed into austenite and the carbide precipitated in a spherical shape is dissolved in the austenite. The reason for limiting the average heating rate during this period to 1 ° C./min or more is that when the average heating rate is less than 1 ° C./min, the transformation from ferrite to austenite is slow, so that impurities segregate at the grain boundaries. At the same time, carbides are excessively dissolved in austenite, and martensite becomes lens martensite, resulting in a decrease in strength. More desirably, a higher quenching effect can be obtained by setting the heating rate to 10 ° C./min or more. Furthermore, the reason for limiting the upper limit of the average temperature rising rate to 25 ° C./min is that if the average temperature rising rate exceeds 25 ° C./min, the solid solution of the carbide becomes non-uniform, and the crystal grains are coarse at the portion where the carbide does not remain. This is because the strength decreases. Therefore, it is desirable to obtain a higher effect by setting the upper limit of the average heating rate to 20 ° C./min or less.

  Note that the temperature increase rate up to 720 ° C. is not particularly defined in terms of characteristics, and may be any temperature increase rate. Further, from the industrial viewpoint, it is desirable that the heating rate is as fast as possible. For example, 200 ° C./hr or more is suitable. In that case, it is also possible to keep the temperature between 500 to 720 ° C. for about 30 min to 60 min for the purpose of suppressing variation in the temperature of the processed product.

  The quenching holding temperature is preferably 780 to 850 ° C., and the holding time at that temperature is preferably 15 to 60 min. The tempering temperature after quenching is 140 to 200 ° C., and the holding time at that temperature is preferably 60 to 180 min.

  As an example of a preferable component of the hypereutectoid steel in the present invention, in mass%, C is 0.8 to 1.2%, Si is 0.01 to 1.5%, and Mn is 0.1 to 2.0%. , Cr is a steel containing 0.1 to 3.0%, and 0.1 to 2.0% of Ni and 0.01 to 1.5% of Mo as elements for improving hardenability. 1 type (s) or 2 or more types can be contained. Moreover, in order to refine | miniaturize a crystal grain and to improve a static strength further, 0.001 to 0.15% of Nb can also be contained. Further, S is suppressed to 0.03% or less as an impurity.

The reasons for limiting the above steel component examples will be described. In addition, each% shows the mass%.
C: 0.8 to 1.2%
When C is less than 0.8%, when hardening and tempering are performed while leaving residual carbides, sufficient hardness cannot be obtained, and as a result, static strength decreases. On the other hand, if it exceeds 1.2%, eutectic carbides cannot be avoided during solidification, and enormous carbides remain after quenching and tempering, and the static strength is lowered. Therefore, C is set to 0.8 to 1.2%.

Si: 0.01 to 1.5%
If Si is less than 0.01%, the effect of improving hardenability cannot be expected, and the strength after quenching cannot be ensured. If it exceeds 1.5%, the effect of improving hardenability is saturated, and the workability is lowered by strengthening ferrite.

Mn: 0.1 to 2.0%
If Mn is less than 0.1%, the effect of improving hardenability cannot be expected, and the strength after quenching cannot be ensured. If it exceeds 2.0%, the effect of improving hardenability is saturated, and the workability is lowered by strengthening ferrite.

Cr: 0.1-3.0%
If Cr is less than 0.1, good spheroidized carbide cannot be obtained by spheroidizing annealing which is a pre-heat treatment, and the carbide cannot be uniformly dissolved during quenching, resulting in a decrease in static strength. If it exceeds 3.0%, eutectic carbides cannot be avoided at the time of solidification, and huge carbides remain after quenching and tempering, thereby reducing the static strength.

Ni: 0.1 to 2.0%
If Ni is less than 0.1%, there is no effect of improving hardenability and toughness. If it exceeds 2.0%, the effect is saturated.

Mo: 0.01 to 1.5%
If Mo is less than 0.01%, there is no effect of improving hardenability and toughness. If it exceeds 1.5%, the effect is saturated.

Nb: 0.001 to 0.15%
If Nb is less than 0.001%, there is no effect of refining. If it exceeds 0.15%, the effect is saturated.

In addition, about the element contained as another unavoidable impurity, it is desirable to limit to the following range.
P: 0.03% or less P is preferably lower because P is unevenly distributed at the grain boundaries and lowers the strength. If it is 0.03% or less, it is acceptable.

S: 0.03% or less S is desirable because it generates MnS and serves as a starting point for fracture and lowers the static strength. However, when machinability is required, it is 0.010% to 0.03%. Acceptable in range.

O: 0.002% or less O is contained as an inevitable impurity, because a large oxide serves as a starting point of destruction and lowers the static strength. Therefore, O is made 0.002% or less.

Ti: 0.01% or less Ti contained as an unavoidable impurity combines with N to generate large TiN, which becomes a starting point of fracture and lowers the static strength. Therefore, Ti is made 0.01% or less.

  The present invention has excellent effects such as obtaining a bearing having high tensile strength without variation in static strength by devising a quenching method in hypereutectoid steel such as bearing steel.

  The best mode for carrying out the present invention will be described below. An ingot of the components shown in Table 1 melted in a 100 kg vacuum melting furnace was heated to 1100 ° C., hot forged to φ32 mm, held at 870 ° C. for 1 hour, then air-cooled, and then heated to an austenitizing temperature of 800 ° C. The specimen was slowly cooled and subjected to spheroidizing annealing for a total of 15 hours to prepare a test material.

  A tensile test piece was produced from the steel material prepared above. In order to perform heat treatment in the atmosphere, a sample having a parallel portion of φ12 mm was prepared as rough processing of a tensile test piece, and the average rate of temperature increase from 720 ° C. to the quenching holding temperature was 0.5 ° C./min to 30 ° C. / After raising the temperature at the rate of temperature rise shown in Table 2, the temperature was kept at the quenching holding temperature shown in Table 2 between 780 ° C. and 850 ° C. for 30 minutes, and quenching was performed in 50 ° C. oil. Then, after tempering at 180 ° C. for 90 minutes, the parallel part φ10 mm was finished and processed, and a tensile test was performed. In the tensile test, the number of n was set to 5 and the average value was evaluated, and 2000 MPa or more was judged good. The rate of temperature increase was confirmed by inserting a thermocouple in the center of a φ12 mm steel bar and measuring the time from 720 ° C. until reaching the quenching holding temperature ± 5 ° C.

  As seen in Table 2, steel materials A, B, C, and D each have an average rate of temperature increase from 720 ° C. to their respective quenching holding temperatures of 1 ° C./min to 25 ° C./min. In all cases, the tensile strength was 2000 MPa or more. On the other hand, those having an average temperature rising rate of less than 1 ° C./min or exceeding 25 ° C./min all had a tensile strength of less than 2000 MPa.

Claims (5)

  In hypereutectoid steel, the static heating rate used for rolling bearings is characterized in that the average temperature increase rate from 720 ° C. to the quenching holding temperature is 1 ° C./min to 25 ° C./min during quenching. Hardening method for hypereutectoid steel with excellent strength.   2. The heat retention excellent in static strength used for the rolling bearing according to claim 1, wherein the quenching holding temperature is 780 to 850 ° C., and the holding time at the quenching holding temperature is 15 to 60 min. Hardening method for deposited steel.   Hypereutectoid steel is mass%, C: 0.8-1.2%, Si: 0.01-1.5%, Mn: 0.1-2.0%, Cr: 0.1-3 0.0%, P: 0.03% or less, S: 0.03% or less, and the steel comprising the balance Fe and inevitable impurities, used for a rolling bearing according to claim 1 or 2 Hardening method for hypereutectoid steel with excellent static strength.   Hypereutectoid steel, in addition to the above-mentioned components, is 1% of Ni: 0.1 to 2.0%, Mo: 0.01 to 1.5% as an element that improves hardenability by mass%. A method for quenching hypereutectoid steel excellent in static strength used for a rolling bearing according to claim 3, comprising seeds or two kinds.   The hypereutectoid steel is characterized by containing Nb: 0.001 to 0.15% as an element that further refines the crystal grains and further improves the static strength by mass% in addition to the above components. A method for quenching hypereutectoid steel having excellent static strength used in the rolling bearing according to claim 3.