JP2013241674A - Method for manufacturing bearing steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for softening-annealing a bearing steel, by which an enough saw property can be made to appear by a simple softening-annealing process for a short time.SOLUTION: A rolled material of a bearing steel is composed, by mass, of 0.7-1.3% C, 0.2-1.0% Si, 0.1-1.5% Mn, 0.01-0.2% Al, 0.5-2.5% Cr and the balance Fe with inevitable impurities. The rolled material is heated to 720-850°C temperature zone with heating speed in the temperature zone of ≥700°C as 10-100°C/h, and successively, is cooled at a 0.02-0.2°C/s speed to the temperature of a pearlite transformation point or lower.

Description

The present invention relates to a method for producing a high carbon chromium bearing steel among bearing steels, and in particular, aims to improve the sawing ability at the same time as shortening the processing time in the soft annealing process.
Here, sawability means the low man-hour required for the sawing operation, and the man-hour means the time required for saw cutting and the time for replacing the saw blade due to the deterioration of the saw blade. To do. This saw cutting property has a strong correlation with the hardness of the steel material. If the Vickers hardness Hv of the steel material is 270 or less, it can be said that the saw cutting property is sufficient.

Conventionally, in spheroidizing annealing for softening of bearing steel, for example, as described in Patent Document 1, it has been necessary to maintain a high temperature for nearly 12 hours. Therefore, it has been desired to shorten the processing time.
In Patent Document 1, the processing time is shortened by controlling the rolling temperature.

  Patent Document 2 discloses that a high carbon chromium bearing steel having a Mo content of 0.08% by mass or less is subjected to a secondary spheroidizing treatment three or more times following the primary spheroidizing treatment at a relatively high heating rate. A method for reducing the total processing time by implementing the cooling rate is proposed.

  Furthermore, in Patent Document 3, when a bearing steel wire coil is spheroidized and annealed, a furnace capable of forcibly convection of the atmosphere in the furnace is used, and variation in processing time is caused by uniformizing coarsening of carbides. A reduction method has been proposed.

Japanese Patent Laid-Open No. 11-286724 Japanese Patent Publication No.6-2898 JP-A-6-73437

However, the above-described conventional technique has the following problems.
That is, in Patent Document 1, since the rolling temperature is low, a heavy load is applied to the rolling mill, and when the capacity of the existing rolling mill is insufficient, a large amount of equipment costs are required. It was difficult to realize. In addition, from the viewpoint of thermal followability, only relatively small steel bars having a diameter of up to 60 mm are applicable, and further expansion of the application range has been desired.

On the other hand, in Patent Document 2, it is necessary to hold the heat four times or more in total, and in the case of a relatively large diameter steel bar having a diameter exceeding 90 mm, the temperature difference between the material surface layer part and the inside is remarkable, and the heat follow-up The need to wait results in an inefficient manufacturing method.
Furthermore, for example, in the case of SUJ4 or SUJ5 defined in JIS G 4805, since 0.1 to 0.25 mass% of Mo is contained, the relatively high heating rate and cooling rate described in Patent Document 2 are used. There is a problem that a normal spheroidized structure cannot be obtained, and as a result, the hardness does not soften.

  In addition, since Patent Document 3 is held at 790 ° C. for 4 hours and then cooled to 660 ° C. at 20 ° C./h (0.006 ° C./s), it takes a long time for the softening annealing process. There was a problem that the possible amount was limited and it was not suitable for mass production.

  In recent years, the demand for large-sized bearing parts for wind power generation has increased, and since such parts are large, they are generally manufactured by hot forging instead of cold forging. Previously, a saw cut is made to adjust the length of the round bar. When processing such as sawing is performed, spheroidizing annealing performed under strict conditions is not necessarily required to ensure cold forgeability as described in Patent Documents 1 to 3 above. Rather, mass productivity is required in order to meet the increasing demand for steel materials, and it is more important to balance such requirements with sawability.

  The present invention was developed in view of the above-described present situation, and an object thereof is to propose a method for producing a high carbon chromium bearing steel capable of expressing sufficient sawing properties by a simple and short-time softening annealing treatment. And

Now, in order to achieve the above object, the inventors have conducted intensive research on soft annealing treatment of high carbon chromium bearing steel. As a result, the intended purpose is achieved by carrying out two-stage heat treatment and controlled cooling treatment. Has been found to be advantageously achieved.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.7 to 1.3% by mass,
Si: 0.2 to 1.0 mass%,
Mn: 0.1 to 1.5% by mass,
Al: 0.01 to 0.2% by mass and
Cr: 0.5-2.5% by mass
The balance is heated to a temperature range of 720 to 850 ° C. at a heating rate of 10 to 100 ° C./h with respect to the rolling material of the bearing steel consisting of Fe and inevitable impurities. Then, softening annealing is performed to cool to a temperature below the pearlite transformation point at a rate of 0.02 to 0.2 ° C./s.

(2) The method for manufacturing a bearing steel according to (1), wherein the rolled material is a steel bar having a diameter of 90 to 450 mm.

(3) The bearing steel further
Cu: 0.5% by mass or less,
Ni: 0.5 mass% or less and
Mo: 1 or 2 types or more selected from 0.5 mass% or less are contained, The manufacturing method of the bearing steel as described in said (1) or (2) characterized by the above-mentioned.

(4) The bearing steel further
Sb: 0.005 mass% or less is contained, The manufacturing method of the bearing steel in any one of said (1)-(3) characterized by the above-mentioned.

  According to the present invention, the soft annealing treatment of the high carbon chromium bearing steel is made into a two-stage heating treatment and a controlled cooling treatment under appropriate conditions, so that a good sawing property can be obtained in a short softening annealing time. As a result, the production efficiency of bearing steel can be remarkably improved.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the high carbon chromium bearing steel is limited to the above range in the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.7 to 1.3%
In order to ensure sufficient strength necessary for bearing steel, C of 0.7% or more is necessary. On the other hand, when C is added exceeding 1.3%, the amount of retained austenite after quenching increases, leading to a decrease in strength. Therefore, the C amount is set to a range of 0.7 to 1.3%.

Si: 0.2-1.0%
Si is an element added as a deoxidizer and to enhance the strength of the steel by solid solution strengthening and to improve the rolling fatigue resistance of the steel. In the present invention, Si is contained in an amount of 0.2% or more. However, addition exceeding 1.0% deteriorates the machinability and forgeability of steel. Moreover, Si combines with oxygen in the steel and causes deterioration of the rolling fatigue life characteristics by being present in the steel as an oxide. Further, when Si is concentrated in the segregation part, eutectic carbide is easily generated. From the above, in the present invention, the upper limit of Si is 1.0%. Preferably it is 0.3 to 0.9% of range, More preferably, it is 0.4 to 0.8% of range.

Mn: 0.1-1.5%
Mn is an element added to improve hardenability, increase the toughness of the steel, and improve the rolling fatigue resistance of the steel, and is contained in an amount of 0.1% or more in the present invention. However, addition over 1.5% not only lowers the machinability, but also hardenability becomes too high, and a hard martensite structure may be formed in air cooling after cold rolling. However, the altitude may not be lowered. From the above, in the present invention, the upper limit of Mn is 1.5%. Preferably it is 0.15 to 1.4% of range, More preferably, it is 0.2 to 1.3% of range.

Al: 0.01-0.2%
Al is an element effective for deoxidation, and is contained by 0.01% or more in the present invention. However, if added over 0.2%, coarse oxide inclusions are present in the steel, leading to a reduction in the rolling fatigue life of the steel. From the above, in the present invention, the upper limit of Al is 0.2%. Preferably it is 0.012 to 0.1% of range, more preferably 0.015 to 0.05% of range.

Cr: 0.5-2.5%
Cr enhances hardenability and promotes spheroidization of carbides during soft annealing, so at least 0.5% is included in the present invention. However, if over 2.5% is added in excess, the hardenability becomes too high, a hard martensite structure is formed in air cooling after rolling, and the hardness does not decrease even when soft annealing is performed. From this viewpoint, the Cr content is in the range of 0.5 to 2.5%. Preferably it is 0.6 to 2.4% of range.

The basic components have been described above. In the present invention, the following elements can be appropriately contained as necessary.
One or more selected from Cu: 0.5% or less, Ni: 0.5% or less, and Mo: 0.5% or less
Cu, Ni and Mo are all elements that increase the hardenability and strength after tempering and improve the rolling fatigue life of steel, and can be appropriately selected and added according to the required strength. In order to obtain such an effect, it is preferable to contain Cu and Ni in an amount of 0.005% or more and Mo in an amount of 0.01% or more. However, if Cu, Ni, and Mo are added in excess of 0.5%, the forgeability of the steel deteriorates, so the upper limit of the content is 0.5%. A more preferred upper limit is 0.4%.

Sb: 0.005% or less
Sb can be added as necessary to suppress surface decarburization during heat treatment. In order to acquire this effect, it is preferable to make it contain 0.0001% or more. However, even if added over 0.005%, the effect of suppressing surface decarburization is saturated, so Sb is preferably contained at 0.005% or less. More preferably, it is in the range of 0.0004 to 0.004%, and further preferably in the range of 0.001 to 0.0035%.

  The balance composition other than the above is Fe and inevitable impurities.

Next, the reason for limiting the heat treatment conditions of the present invention will be described.
Heating rate up to 700 ° C When heating high-carbon chromium bearing steel, if the temperature difference between the material surface layer and the inside is large, thermal stress is generated in the circumferential direction, which may cause cracking. In particular, since the temperature at which the thermal stress increases is near the transformation temperature, the difference between the surface temperature and the internal temperature must be reduced by gradual heating near the temperature. In this respect, a large thermal stress does not occur in the temperature range up to 700 ° C. and there is no risk of cracking. Accordingly, the heating rate up to 700 ° C. is not particularly limited, and rapid heating can be performed. Here, it is preferable to heat up to 700 ° C. at a rate of 20 to 150 ° C./h. More preferably, the speed range is 50 to 150 ° C./h. The heating rate is an average heating rate.

Slowly heat a temperature range of 700 ° C or higher at a rate of 10-100 ° C / h and heat it to a temperature range of 720-850 ° C. To soften high-carbon chromium bearing steel, the layered pearlite lamellar is broken to promote spheroidization. The preferred temperature range for this is 720-850 ° C. Further, since the temperature range from 700 ° C. is also a transformation temperature range, the thermal stress becomes high. Therefore, the heating rate in the temperature range of 700 ° C. or higher is set to 10 to 100 ° C./h, and the heating is performed gradually to the temperature range of 720 to 850 ° C. From the viewpoint of promoting spheroidization and thermal stress, it is more preferably a treatment of gradually heating from 720 to 830 ° C. at a heating rate of 10 to 80 ° C./h. The heating rate is an average heating rate.

Holding in a temperature range of 720 to 850 ° C. As described above, in order to break down the layered pearlite lamellar and promote spheroidization, gradually heat to 10 to 100 ° C./h to the temperature range of 720 to 850 ° C. The desired spheroidization is achieved by slow heating. In order to further promote the spheroidization, the holding treatment may be performed in this temperature range.

Slow cooling at a rate of 0.02 to 0.2 ° C./s to a temperature below the pearlite transformation point. By the above gradual heat treatment or holding treatment, the pearlite lamellar collapses and spheroidization is promoted, but if the subsequent cooling rate becomes too fast, A new pearlite lamella is generated again, increasing the hardness. On the other hand, if the cooling rate is excessively slow, a huge amount of time is required for the cooling process, resulting in inefficient operation. Therefore, the slow cooling rate is limited to a range of 0.02 to 0.2 ° C./s. More preferably, the speed range is 0.05 to 0.15 ° C./s. The cooling rate is an average cooling rate.
Further, since the structure does not change after the transformation is completed, the hardness does not change. Therefore, the slow cooling stop temperature is set to the pearlite transformation point or lower. Preferably it is 700 degrees C or less. More preferably, it is 680 degrees C or less.

  There is no particular limitation on the cooling process after the cooling to the pearlite transformation point or less by the controlled cooling described above, and the cooling process may be performed at the same speed. preferable. However, if the speed is too high, the steel material may be warped or bent, so the upper limit of the cooling rate is preferably 100 ° C./s. The cooling rate is an average cooling rate.

  Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. It should be noted that the present invention is not limited by the examples described below, and can be appropriately changed within a range that can be adapted to the gist of the present invention, all of which fall within the technical scope of the present invention. included.

A steel slab comprising the composition shown in Table 1 (the balance is Fe and inevitable impurities) is heated (steel slab heating) and hot-rolled into a steel bar having a 90 to 450 mm round cross section to obtain a sample for soft annealing. . Subsequently, after softening annealing was performed on the obtained sample under the conditions shown in Table 2, Vickers hardness was measured. The softening annealing is performed by performing a first heat treatment for heating a normal temperature sample to the first heating temperature at the first heating rate, and then performing the second heating from the first heating temperature to the second heating temperature. A second heat treatment for heating at a heating rate was performed. The hardness was measured at 20 points with a load of 9.8 N (10 kgf), and the average value was defined as Vickers hardness Hv.
In all the experimental examples in Table 2, the pearlite transformation temperature is in the range of 700 to 650 ° C.
The obtained results are also shown in Table 2.

  As shown in Table 2, it can be seen that all of the inventive examples according to the present invention are excellent in sawing ability because the Vickers hardness Hv after softening annealing is reduced to 270 or less by a simple method.

Claims (4)

C: 0.7 to 1.3% by mass,
Si: 0.2 to 1.0 mass%,
Mn: 0.1 to 1.5% by mass,
Al: 0.01 to 0.2% by mass and
Cr: 0.5-2.5% by mass
The balance is heated to a temperature range of 720 to 850 ° C. at a heating rate of 10 to 100 ° C./h with respect to the rolling material of the bearing steel composed of Fe and inevitable impurities. Then, softening annealing is performed to cool to a temperature below the pearlite transformation point at a rate of 0.02 to 0.2 ° C./s.   The method for producing bearing steel according to claim 1, wherein the rolled material is a steel bar having a diameter of 90 to 450 mm. The bearing steel further
Cu: 0.5% by mass or less,
Ni: 0.5 mass% or less and
Mo: 1 or 2 types or more selected from 0.5 mass% or less are contained, The manufacturing method of the bearing steel of Claim 1 or 2 characterized by the above-mentioned. The bearing steel further
Sb: 0.005 mass% or less is contained, The manufacturing method of the bearing steel in any one of Claims 1-3 characterized by the above-mentioned.