JP3291068B2 - Manufacturing method of bearing steel with excellent spheroidizing annealing characteristics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material for bearings having excellent spheroidizing annealing properties, and more particularly, to reducing net pro-eutectoid cementite in the production of steel for bearings, and then performing spheroidizing annealing. It is possible to obtain a uniform spherical carbide structure that does not contain coarse plate-like carbides, and is excellent in cold workability such as cutting, cold forging, and cutting after annealing, and in quenching and tempering bearing parts. The present invention relates to a method for producing a bearing steel material capable of obtaining excellent rolling fatigue characteristics.

[0002]

2. Description of the Related Art Bearing parts are usually manufactured by cutting a bar steel wire rod and performing cold working such as cold forging and cutting. In cold working, since cold working is too hard as it is rolled, spheroidizing annealing is performed before cold working for the purpose of improving cold workability. However, the normal spheroidized annealed structure is composed of fine spherical carbides and coarse plate-like carbides, and it is extremely difficult at present to obtain a uniform spherical carbide structure. Such non-uniformity of the carbide structure deteriorates cold workability and causes deterioration of rolling fatigue life in the final part.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 64-55330 discloses that a steel composed of a specific component is made of _three points of Ac or Accm.
Heating above the point, processing rate 1 in austenite unrecrystallized area
A method for producing a steel material capable of spheroidization in a short time, characterized in that hot rolling of 0 to 80% is performed and cooling is performed at a cooling rate of 1.5 to 100 ° C / sec to a temperature range just above the Ms point. Have been. However, in this method, since the rolled material has a fine pearlite or bainite structure, spheroidization can be performed in a short time. However, the spherical carbide obtained from the fine pearlite or bainite structure portion is remarkably fine, and The difference from the size of the spherical carbide obtained from the boundary pro-eutectoid cementite portion is large, and it cannot be said that a sufficiently uniform spherical carbide structure can be obtained.

[0004] Also, Japanese Patent Application Laid-Open No. 3-53021 discloses that
A steel composed of a specific component is heated to a temperature of Ar ₁ + 200 ° C. or higher, hot-rolled at a reduction rate of 20 to 90%, and further hot-rolled at Ar ₁ to Ar ₁ + 200 ° C. at a temperature of 20 to 90%. 5 to a temperature of Ar ₁ ~Ar ₁ + 200 ℃ after performing
A method for producing a high carbon steel material for spheroidizing annealing characterized by holding for 900 seconds is shown. It is said that the use of this method can provide a prestructure desirable for shortening the spheroidizing annealing time, but the reduction of reticulated primary cementite by this method is not always sufficient.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a uniform spheroidal carbide structure which does not contain coarse plate-like carbides by ordinary spheroidizing annealing. An object of the present invention is to provide a method for producing a bearing steel material which is excellent in cold workability such as forging and cutting, and which can obtain excellent rolling fatigue characteristics in a quenched and tempered bearing component.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies and obtained the following knowledge in order to obtain a uniform spheroidized structure that does not contain coarse plate-like carbides by ordinary spheroidizing annealing. . (1) In order to obtain a uniform spheroidized structure that does not contain coarse plate-like carbides by ordinary spheroidizing annealing, it is necessary to reduce the reticulated pro-eutectoid cementite in the rolled material, and intragranular pro-eutectoid cementite in austenite grains. The point is to increase the generation of pearls and increase the lamella spacing of pearlite.

(2) To realize the above, the following five points are essential. A steel material containing a specific amount of a carbonitride forming element such as Al or N is used, and the heating temperature before rolling is 900 to 115.
Limited to 0 ° C., while preventing the coarsening of austenite grains during rolling heating, leaving some fine carbides,
To reduce the net amount of solute carbon to make it difficult to form reticulated primary cementite after rolling. Rolling with a total area reduction of 50% or more in a temperature range of a heating temperature to 880 ° C. or more, and reducing the austenite grains to about 6 to 7 by recrystallization.

In a temperature range of less than 880 ° C. to 400 ° C.,
"After rolling with a reduction of area of 10% or more, the steel temperature
Cooling to s point to 700 ° C., and subsequently rolling at a reduction of area of 10% or more ”is performed in a step having at least one step, and the steel material temperature at the final rolling exit side is set to 700 to
By setting the temperature to 880 ° C., a deformation zone is introduced into the grains of the austenite grains to generate intragranular pro-eutectoid cementite, thereby reducing reticulated pro-eutectoid cementite and suppressing the growth and coarsening of austenite grains.

[0009] Immediately after the final rolling, the steel is quenched to a final rolling quenching temperature of less than 550 to 700 ° C or quenched in a molten salt maintained at a molten salt temperature of less than 550 to 700 ° C. And to suppress the formation and growth of reticulated cementite at grain boundaries. Thereafter, the temperature is reduced to 450 ° C. at a cooling rate of 0.05 to 1.0 ° C./sec to coarsen the lamella spacing of pearlite generated in the austenite grains.

(3) Further, at the time of heating before hot rolling, by setting the heating rate at 650 to 750 ° C. to 10 to 100 ° C./hour, the residual amount of fine carbides at the time of heating increases, and The amount of dissolved carbon is further reduced, and the amount of reticulated cementite after rolling is also reduced. The present invention has been made based on the above-described novel findings, and the gist thereof is as follows.

According to the first aspect of the present invention, as a weight ratio, C: 0.80 to 1.20%, Si: 0.15 to 1.50%, Mn: 0.15 to 1.50%, Cr : 0.50 to 1.60%, S: 0.003 to 0.02%, Al: 0.015 to 0.05%, N: 0.004 to 0.015%, P: 0 0.020% or less, Ti: 0.0020% or less, O:
When hot-rolling a steel consisting of iron and unavoidable impurities with the balance being not more than 0.0015%, A) heating to a heating temperature of 900 to 1150 ° C, and B) heating temperature to 880 ° C or more. Total area reduction rate 5 in the temperature range
0% or more rolling; and C) after that, in a temperature range of less than 880 ° C to 400 ° C,
"After rolling with a reduction of area of 10% or more, the steel temperature
s point to 700 ° C., and then rolling to a rolling rate of 10% or more ”.
And D) quenching immediately after the final rolling to raise the temperature of the steel material to 550 ° C.
E) elongation of the microstructure substantially, characterized by a step of setting the final rolling quenching temperature to a temperature of less than ~ 700 ° C, and a step of thereafter cooling to 450 ° C at a cooling rate of 0.05 to 1.0 ° C / sec. This is a method for producing a bearing steel material comprising cementite and pearlite and having excellent spheroidizing annealing characteristics.

The invention of claim 2 of the present invention is characterized in that D) and E) are: D) immediately after the final rolling, quenching in a molten salt maintained at a molten salt temperature of less than 550 to 700 ° C; 2. The structure according to claim 1, characterized in that it comprises: a step of holding for 0.5 to 30 seconds in a molten salt; and E) a step of cooling to 450 ° C. at a cooling rate of 0.05 to 1.0 ° C./second thereafter. Is a method for producing a bearing steel material comprising substantially proeutectoid cementite and pearlite and having excellent spheroidizing annealing characteristics.

[0013] The invention of claim 3 of the present invention is directed to claim 1 of the present invention .
The method for producing a bearing steel material having excellent spheroidized annealing characteristics according to claim 1 or 2, wherein a heating rate at 650 to 750 ° C is 10 to 100 ° C / hour upon heating in the step A) .

According to a fourth aspect of the present invention, the composition further comprises Ni: 0.50 to 2.00%, Mo: 0.05 to 0.5.
The method for producing a bearing steel material having excellent spheroidizing annealing characteristics according to claim 1 or 2 or 3, which contains 0% or 1 or 2 types.

According to a fifth aspect of the present invention, the composition further comprises Nb: 0.01 to 0.3%, V: 0.03 to 0.3.
5. The method for producing a bearing steel material having excellent spheroidized annealing characteristics according to claim 1, wherein the steel material comprises one or two of the following.

[0016]

Hereinafter, the present invention will be described in detail. First, the reason for limiting the component content range of the steel of the present invention as described above will be described. C: 0.80 to 1.20%, C is an effective element for obtaining the rolling fatigue strength and wear resistance required for bearing components of the final product. If it exceeds 1.20%, the amount of reticulated grain boundary pro-eutectoid cementite becomes remarkable, leading to deterioration in the workability after spheroidizing annealing and the strength of the final product. .20%.

Si: 0.15 to 1.50%, Si is added as a deoxidizing element and for the purpose of increasing the strength of the final product by suppressing the structural change in the rolling fatigue process, but 0.15% If it is less than 1.5%, the effect is not sufficient. On the other hand, if it exceeds 1.50%, these effects are saturated and rather deteriorate the toughness of the final product.
It was set to 5 to 1.50%.

Mn: 0.15-1.50%, Mn is an effective element for increasing the strength of the final product through improvement of hardenability, but if less than 0.15%, this effect is insufficient. On the other hand, if it exceeds 1.5%, this effect is not saturated but rather deteriorates the toughness of the final product, so its content was made 0.15-1.5%.

Cr: 0.50 to 1.60%, Cr is effective in improving hardenability, increasing toughness, and improving wear resistance by promoting the formation of carbides. This effect is not sufficient at less than 0.5%, while 1.
If the content exceeds 6%, the effect is not saturated but rather deteriorates the toughness of the final product. Therefore, the content is set to 0.50 to 1.60%.

S: 0.003-0.02%, S is M in steel
Although present as nS, it contributes to improvement of machinability and miniaturization of the structure, but if it is less than 0.003%, its effect is insufficient. On the other hand, if it exceeds 0.02%, the effect is saturated,
Rather, the toughness is degraded and the anisotropy is increased. For the above reasons, the content of S is set to 0.003 to 0.02%.

Al: 0.015 to 0.05%, Al is added as a deoxidizing element and a crystal grain refining element.
If it is less than 15%, the effect is insufficient, while 0.0
If the content exceeds 5%, the effect is saturated and the toughness is rather deteriorated. Therefore, the content is set to 0.015 to 0.05%.

N: 0.004 to 0.015%, N is AlN
Contributes to the refinement of austenite grains through the precipitation behavior of, but if its content is less than 0.004%, its effect is insufficient, while if it exceeds 0.015%, its effect saturates and rather leads to deterioration of toughness. , The content of which is N: 0.004
-0.015%.

P: 0.020% or less, P causes grain boundary segregation and central segregation in steel, and causes deterioration in the strength of the final product. In particular, if P exceeds 0.020%, the deterioration of strength becomes remarkable, so 0.020% was made the upper limit.

Ti: 0.0020% or less Ti forms hard precipitate TiN, which becomes a starting point of rolling fatigue fracture in the final product, and causes deterioration of strength. In particular, Ti is 0.
If it exceeds 0020%, the deterioration of strength becomes remarkable,
The upper limit is 0.0020%.

O: 0.0015% or less O generates hard inclusions Al ₂ O ₃ , which become the starting point of rolling fatigue fracture in the final product and cause deterioration of strength. In particular, when O exceeds 0.0015%, the strength is significantly deteriorated, so the upper limit is 0.0015%.

Next, in the steel according to the fourth aspect of the invention, one or two of Ni and Mo can be contained for the purpose of improving hardenability. Ni: 0.50 to 2.00%, Mo:
0.05 to 0.50%, Ni and Mo are effective elements for increasing the strength of the final product through improvement of hardenability, but Ni: less than 0.50%, Mo: less than 0.05% In this case, the effect is insufficient. On the other hand, Ni: 2.00%, M
If o: more than 0.5%, this effect saturates and rather causes deterioration of the toughness of the final product.
0 to 2.00%, Mo: 0.05 to 0.50%.

Next, in the steel according to the fifth aspect of the present invention, one or two types of Nb and V can be contained for the purpose of refining the structure. Nb: 0.01 to 0.3%, V: 0.0
3 to 0.3%, Nb and V are added as crystal grain refining elements, but if the Nb is less than 0.01% and the V is less than 0.03%, the effect is insufficient. 0.3%, V:
When the content exceeds 0.3%, the effect is saturated, and the toughness is rather deteriorated. Therefore, the content is set to Nb: 0.01 to 0.3%,
V: 0.03 to 0.3%.

Next, the reason for limiting the hot rolling conditions in the present invention will be described. First, set the heating temperature to 900-
The reason why the heating temperature is set to 1150 ° C. is that if the heating temperature is lower than 900 ° C., the rough rolling-intermediate rolling temperature becomes low and the austenite grains are insufficiently refined by recrystallization zone rolling. In this case, austenite crystal grains are remarkably coarsened, and the effect of reducing the net amount of solute carbon due to the remaining fine carbides cannot be expected.

Next, rolling at a total area reduction of 50% or more in the temperature range of the heating temperature to 880 ° C. or more is for reducing the austenite grains to about 6 to 7 by recrystallization. The reason why the total area reduction rate is 50% or more is that if the total area reduction rate is less than 50%, the effect of recrystallization refinement is small.

Further, in a temperature range of less than 880 ° C. to 400 ° C., immediately after the rolling at a reduction rate of 10% or more, the steel material is cooled once to the Ms point to 700 ° C., and then the reduction rate is reduced. Rolling in a step having at least one step of “rolling at least 10%” is performed by introducing a deformation zone in the austenite grains, nucleating intra-granular pro-eutectoid cementite, and reducing reticulated pro-eutectoid cementite. To do that. 88
Rolling in a temperature range of less than 0 ° C to 400 ° C causes processing heat
Temperature rises, the processing strain is released by recovery.
This makes it difficult to introduce deformation zones. That is, the reason why cooling is interposed during rolling is to suppress the release of processing strain due to recovery, accumulate strain between passes, and form a deformation zone in which nucleation of intragranular proeutectoid cementite can be generated.

Here, the reason why the area reduction ratio before and after cooling is set to 10% or more is that if the area reduction ratio is less than 10%, a sufficient deformation zone is not formed in the austenite grains because the accumulated strain is small. is there. Further, the temperature after cooling is set to the Ms point to 700
The reason why the temperature was set to ° C. is that when the temperature after cooling exceeds 700 ° C., it is not enough to prevent deformation band disappearance due to release of processing strain, and when it is lower than the Ms point, a martensite structure is generated. Since the effect of preventing deformation band disappearance becomes particularly significant when the temperature is cooled to less than 600 ° C., it is desirable to set the temperature after cooling to the Ms point or more and less than 600 ° C. if possible.

Further, in the present invention, in a temperature range of less than 880 ° C., the steel is cooled immediately after the rolling at a reduction of area of 10% or more so that the temperature of the steel material once becomes Ms point to 700 ° C. Any rolling can be performed before and after the “rolling of 10% or more” step. When this step is repeated two or more times, it can be performed continuously or with any rolling performed. good.

Further, the temperature of the steel material on the final rolling exit side is set to 700.
The reason for setting the range to ８880 ° C. is that at a final rolling exit temperature of less than 700 ° C., the rolling load is significantly increased,
Also, if the final rolling exit side temperature exceeds 880 ° C., there is a risk that austenite grains immediately after rolling may grow and coarsen.

Next, in the first aspect of the present invention, the steel is quenched immediately after the final rolling to set the temperature of the steel material to the final rolling quenching temperature of less than 550 to 700 ° C. Quenching in a molten salt maintained at a molten salt temperature of less than ７００700 ° C., and holding in the molten salt for 0.5 to 30 seconds, causes formation and growth of reticulated primary cementite at grain boundaries. This is for suppressing. The reason why the quenching temperature range is set to less than 550 to 700 ° C.
If the temperature is 0 ° C. or higher , the formation and growth of reticulated cementite are not sufficiently suppressed, and if the temperature is lower than 550 ° C., there is a risk that a bainite structure may be generated. After the final rolling, 5
The rapid cooling rate to 50 to less than 700 ° C. is desirably 10 ° C./s or more.

Next, the temperature of the molten salt to be quenched according to the second aspect of the invention is set to be less than 550 to 700 ° C.
If the temperature is higher than 700 ° C., the formation-growth of network-precipitated cementite is not sufficiently suppressed, and if the molten salt temperature is lower than 550 ° C., there is a risk that a bainite structure may be formed. The reason for setting the holding time in the molten salt to 0.5 to 30 seconds is that if the holding time is less than 0.5 seconds, there is a risk that the center of the steel material cross section cannot be sufficiently cooled. This is because productivity decreases.

Next, "From 450 to 0.05 ° C.
"Cooling at a cooling rate of 1.0 ° C./sec" is for increasing the lamella spacing of pearlite generated in austenite grains. The cooling rate in this temperature range is 1.0 ° C /
If it exceeds 2 seconds, the lamella spacing of pearlite becomes remarkably fine, while if it is less than 0.05 ° C./second, the effect of slow cooling is saturated,
In order to waste time unnecessarily, the cooling rate is 0.05
1.0 ° C./sec. Here, in order to obtain a more uniform spherical carbide structure after the spheroidizing annealing, the cooling rate in this temperature range is desirably 0.4 ° C./sec or less. An arbitrary cooling rate can be selected for cooling at 500 ° C. or lower. As a method of the adjustment cooling, a method of attaching a slow cooling cover or the like can be considered.

The reason why the structure obtained after rolling is substantially composed of proeutectoid cementite and pearlite is as follows.
If the structure contains bainite or martensite, these parts become fine spherical carbide structures after spheroidizing annealing, and a uniform spherical carbide structure cannot be obtained.

Next, a third aspect of the present invention relates to a method for producing a bearing steel material for obtaining a more uniform spheroidized annealed structure after spheroidizing annealing. In the invention before the hot rolling in the invention of claim 3, the heating rate of 650 to 750 ° C. is set to 10 to 100 ° C./hour by gradually heating in a temperature range just below the A ₁ point in the temperature increasing process. The purpose is to further stabilize the carbide in austenite, increase the amount of fine carbide remaining during rolling heating, and further reduce the net amount of solute carbon. However, if the heating rate at 650 to 750 ° C. is 10
When the temperature exceeds 0 ° C./hour, the effect is small.
If the time is less than the time, the effect of the slow heating is saturated and the time is unnecessarily consumed, so that the heating rate is set to 10 to 100 ° C./hour.

In order to obtain an even more uniform spherical carbide structure after the spheroidizing annealing, the steel material for bearings manufactured by the manufacturing method of the present invention must be prepared by rolling the steel structure after rolling with a prior austenite grain size of 9 or more. It is desirable to use a certain bearing steel material. This is because by setting the prior austenite grain size of the structure after rolling to No. 9 or more, the austenite grain size during spheroidizing annealing heating retention is refined, and the uniformity of carbide after spheroidizing annealing is further increased. Because we can
This is because the effect is particularly remarkable at the ninth or higher. less than,
The effects of the present invention will be more specifically shown by examples.

[0040]

EXAMPLES Table 1 shows the chemical components of the test materials. These were all cast by continuous casting after converter melting. After slab rolling to a 162 mm square steel slab, it was rolled into a round bar under the rolling conditions shown in Table 2.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

In the method of the present invention, after rolling, the cooling was adjusted by applying a slow cooling cover to the cooling floor. Also,
For some steel types, 50 m from the above 162 mm square slab
A square piece of m-square was cut out and had a thickness of 15 mm under the rolling conditions shown in Table 3.
mm. After rolling, these rolled materials were subjected to spheroidizing annealing.

With respect to the spheroidized annealed material, the spheroidization rate, average and maximum values of carbide diameter (equivalent circle diameter), and hardness (index of cold workability) were evaluated. The spheroidization ratio was represented by the ratio of the number of cementite having a major axis / minor axis of 5.0 or less to the total number of cementite. Table 4 shows the evaluation results of each steel material in comparison with the present invention and comparative examples. As is clear from the above, according to the method of the present invention,
In each case, the spheroidization rate was 100%. When the average carbide diameter is 1 μm and the maximum carbide diameter is less than 2 μm, a uniform spherical carbide structure without coarse plate-like carbide is obtained. Also, the hardness is 90 or less HRB, which indicates a favorable softening level as compared with the comparative example.

On the other hand, in Comparative Examples 5 and 35, the heating temperature during rolling exceeded the upper limit of the range of the present invention, and in Comparative Examples 6 and 36, the rolling temperature in the temperature range of less than 880 ° C.
"After rolling with a reduction of area of 10% or more, the steel temperature
Comparative Example 7 is a case where the step of “cooling to s point to 700 ° C. and subsequently performing rolling with a reduction of area of 10% or more” was not performed. Comparative Examples 8 and 37 show cases where the cooling rate exceeded the upper limit of the range of the present invention up to 450 ° C. after quenching immediately after rolling. The carbonization ratio is 70% or less, the size of the spheroidized carbide is not uniform, and the degree of softening is not sufficient.

Next, the steel materials manufactured in Examples 3, 4, and 34 of the present invention and Comparative Example 5 shown in Table 4 were 12 mm in diameter and 2 mm in length.
A 2mm cylindrical rolling fatigue test piece was prepared, and after tempering treatment,
Hertz maximum contact stress 60 by point contact type rolling fatigue tester
A rolling fatigue test was performed at 0 kgf / mm ² . As a measure of the fatigue life, usually, "stress repetition of the test results to fatigue failure in a cumulative failure probability of 10% obtained by plotting the Weibull probability paper" is used as the L ₁₀ life. L ₁₀ life of the present invention example, compared to the L ₁₀ life of Comparative Example 5, 1.5-fold in the present invention example 3, 1.8 times in Invention Example 4, Inventive Example 34
, The life is improved 1.5 times.

[0049]

As described above, when the method of the present invention is used, a uniform spheroidized structure containing no coarse plate-like carbide can be obtained by ordinary spheroidizing annealing. It is possible to manufacture bearing steel materials that are excellent in cold workability such as cold forging and cutting, and that can obtain excellent rolling fatigue characteristics in quenched and tempered bearing parts. Some are remarkable.

Claims (5)

(57) [Claims] 1. Weight ratios of C: 0.80 to 1.20%, Si: 0.15 to 1.50%, Mn: 0.15 to 1.50%, Cr: 0.50 to 1.50%. 60%, S: 0.003 to 0.02%. Al: 0.015 to 0.05%, N: 0.004 to 0.015%, P: 0.020% or less, Ti: 0.0020% or less, O:
When hot-rolling a steel consisting of iron and unavoidable impurities with the balance being not more than 0.0015%, A) heating to a heating temperature of 900 to 1150 ° C, and B) heating temperature to 880 ° C or more. Total area reduction rate 5 in the temperature range
0% or more rolling; and C) after that, in a temperature range of less than 880 ° C to 400 ° C,
"After rolling with a reduction of area of 10% or more, the steel temperature
is cooled to s point to 700 ° C., and subsequently rolled at a reduction of 10% or more ”.
And D) quenching immediately after the final rolling to raise the temperature of the steel material to 550 ° C.
E) elongation of the microstructure substantially, characterized by a step of setting the final rolling quenching temperature to a temperature of less than ~ 700 ° C, and a step of thereafter cooling to 450 ° C at a cooling rate of 0.05 to 1.0 ° C / sec. A method for producing a bearing steel material having excellent spheroidizing annealing properties comprising cementite and pearlite. 2. The method according to claim 1, wherein D) and E) are quenched immediately after the final rolling in a molten salt maintained at a molten salt temperature of less than 550 to 700 ° C. The method according to claim 1, further comprising: a step of holding for about 30 seconds; and E) a step of cooling at a cooling rate of 0.05 to 1.0 ° C./sec to 450 ° C. thereafter.
A method for producing a bearing steel material having excellent spheroidizing annealing properties, the structure of which is substantially composed of proeutectoid cementite and pearlite. 3. The method according to claim 1, wherein the heating in the step A) comprises the step of:
The method according to claim 1 or 2, wherein the heating rate at 0 to 750 ° C is 10 to 100 ° C / hour. 4. The composition further comprises: Ni: 0.50-2.00%, Mo: 0.05-0.50
The method for producing a bearing steel material having excellent spheroidizing annealing characteristics according to claim 1 or 2 or 3, which comprises one or two of the following. 5. The composition according to claim 1, wherein the component further comprises one or two of Nb: 0.01 to 0.3% and V: 0.03 to 0.3%. Method for producing steel for bearings having excellent spheroidizing annealing characteristics.