JP5018305B2 - Manufacturing method of rough bearing product - Google Patents

Description

  The present invention relates to a method for producing a bearing rough molded product by a plurality of forging processes, and more specifically, omits spheroidizing annealing after rough forming into a predetermined shape by hot forging or shortens the time for spheroidizing annealing. The present invention relates to a method for manufacturing a roughly molded bearing product.

  2. Description of the Related Art Conventionally, among bearing parts used in automobiles and industrial machines, parts such as bearing rings are generally made of high-carbon chromium bearing steel materials represented by SUJ1-5 specified in JIS G 4805 (1999). After rough forming into a predetermined shape by hot forging, it is subjected to cold processing such as rolling and cutting to make the final shape, and then finished by further so-called tempering treatment of quenching and tempering It was. And as the hot forging at that time, as shown in Non-Patent Document 1, a method is adopted in which the inner and outer rings are simultaneously roughly formed by performing forging processes a plurality of times.

  However, since the microstructure of the rough bearing product after hot forging is usually a single-phase structure of pearlite or a mixed structure containing a hard phase such as bainite in addition to the pearlite structure, the rough bearing product as hot forged. Is inferior in cold workability and cutting workability.

  For this reason, in order to improve the cold workability and cutting workability, the rough bearing product after hot forging is subjected to a long-time heat treatment over 20 hours called spheroidizing annealing, and the microstructure is ferrite. It has been generally performed to change to a mixed structure of spherical cementite.

  However, the spheroidizing annealing for a long time not only consumes a large amount of energy but also reduces productivity and increases costs.

  Therefore, from the industry, the spheroidizing annealing of bearings after hot forging is omitted, or even if it cannot be omitted, the time is greatly reduced, energy consumption is reduced, and the process is simplified. There is a growing demand to increase productivity.

  In order to improve the productivity in the manufacturing process of bearing parts, a manufacturing method that improves the thermal history of the manufacturing process of the bearing steel and omits spheroidizing annealing or shortens the spheroidizing annealing time is disclosed in Patent Document 1, for example. Proposed in ~ 4.

That is, Patent Document 1 discloses a stage in which a carbon steel adjusted to a specific component range is heated to a temperature of 850 ° C. or higher and hot rolling is performed at a reduction rate of 30 to 60%, and the Ac ₁ transformation after the hot rolling. A stage in which hot rolling at a rolling reduction of 30 to 60% is performed in the same temperature range after holding for 60 to 900 seconds in the temperature range of point to A _cm transformation point, and 1 ° C up to a temperature of 600 ° C after the hot rolling. And a step of gradual cooling at a cooling rate of / s or less, and “a direct softening heat treatment method for high carbon steel” is disclosed.

In addition, the technique proposed in this Patent Document 1 is a method in which pro-eutectoid cementite is retained from austenite grains by holding for 60 to 900 seconds in the temperature range of Ac ₁ transformation point to A _cm transformation point in the course of hot rolling. Precipitating and hot rolling at a rolling reduction of 30 to 60% and then gradually cooling to a temperature of 600 ° C. at a cooling rate of 1 ° C./s or less to obtain a soft pearlite structure or a pseudo pearlite structure It is a feature.

  In Patent Document 2, a steel material containing C: 0.8 to 1.3% by mass is controlled by controlling the finish rolling temperature in hot rolling to 850 ° C. or lower and the cooling start temperature to 850 ° C. or lower, and Cooling at an average cooling rate in the range of 600 ° C. from the cooling start temperature at 0.1 to 5 ° C./s, “a method for producing a linear or rod-like steel excellent in wire drawing workability that can omit heat treatment before wire drawing” Is disclosed.

  Note that the technique proposed in Patent Document 2 described above is such that, in a series of manufacturing processes of “heating” → “rolling” → “cooling” to a predetermined wire diameter, proeutectoid cementite is precipitated before finish rolling, and finish rolling. By controlling the temperature to 850 ° C. or less, the pro-eutectoid cementite is broken in the finish rolling process so as to have an aspect ratio of 10 or less (“major axis / minor axis”), and further within the range of 600 ° C. from the cooling start temperature. By cooling at an average cooling rate of 0.1 to 5 ° C./s, pro-eutectoid cementite having a minor axis of 2 μm or less is obtained.

Patent Document 3 discloses that a steel material having a specific chemical composition is subjected to hot rolling finish rolling in a temperature range of (Ar ₁ −50 ° C.) to (Ar ₁ + 50 ° C.) of the steel material. Disclosed is "Method for producing wire rod / bearing steel for bearings having a spheroidized carbide structure as it is hot-rolled", which is performed at a cooling rate of 0.5 ° C / s or less and immediately cooled to 500 ° C or less. Has been.

In the technique proposed in Patent Document 3, the plate-like cementite constituting the layered pearlite is finely divided by the accumulation of processing strain caused by rolling at a temperature near the Ar ₁ point, and at the same time pearlite and ferrite The entire structure is also processed, and the dislocation density increases and the interfacial energy between the phases increases, and the cementite is spheroidized by the subsequent slow cooling at a cooling rate of 0.5 ° C./s or less.

  Furthermore, Patent Document 4 describes a high-carbon chromium bearing steel containing C: 0.8 to 1.2% and Cr: 0.9 to 1.8% by weight, from extraction to finish rolling. , To obtain a steel bar or wire rod by obtaining a spheroidized structure by rolling while controlling so that the temperature is between the points A1 to Acm in the entire cross section, and omitting or shortening the subsequent spheroidizing annealing step A method for producing a rolled steel material is disclosed.

  The technique proposed in Patent Document 4 described above is intended to be rolled by performing temperature rolling so that the entire cross section is in the same two-phase region and rolling in a temperature range lower than that of the conventional method. A large strain is generated in the material, and the strain is used as a driving force for rapid spheroidization in the subsequent annealing process.

JP-A-1-255623 JP 2003-129176 A JP 2004-190127 A JP-A-11-286724 “Ring material” (Kazuhiko Hiraoka: Special Steel, Vol. 47 (1998) No. 2, p. 42)

  All the techniques proposed in Patent Documents 1 to 4 are techniques in the process of manufacturing steel bars and wire rods by “hot rolling”. For this reason, among bearing parts used for automobiles, industrial machines, etc., it is not necessarily applicable to parts that are roughly formed into a predetermined shape by “hot forging”, such as races.

  This is the same as “hot rolling” in which “hot forging” applies multiple times of processing to the material, but the following points (a) to (c). This is because the process is very different from “hot rolling”.

(B) The time from the start to the end of processing is short.
(B) The processing interval that is the time from processing to the next processing (hereinafter also referred to as “interval time between processing”) is short.
(C) Unlike a rolling facility, in the case of a forging facility, a water cooling facility, a reheating facility, or the like cannot be continuously installed between the forging machines.

  Therefore, the thermal history of the hot rolling disclosed in Patent Documents 1 to 4 is not directly applicable to hot forging, and even if applied, the effect of omitting the intended spheroidizing annealing and the spheroidizing annealing time The shortening effect was not obtained.

That is, the technique proposed in Patent Document 1 requires a holding process for 60 to 900 seconds in the temperature range from the Ac ₁ transformation point to the A _cm transformation point in the middle stage of hot working. In order to introduce such a long holding process, it becomes necessary to divide the forging process and provide a new facility such as a tunnel heating furnace.

  In addition, even if the technique proposed in Patent Document 2 and Patent Document 3 in which the finishing processing temperature is lowered and gradually cooled is applied to hot forging, it is sufficient to shorten the spheroidizing annealing time. It was not obtained.

  Furthermore, sufficient spheroidizing annealing time can be obtained even when the hot working technique proposed in Patent Document 4 in which the temperature of the entire cross section is a two-phase region is applied to hot forging during the period from extraction to finish rolling. The shortening effect was not obtained.

  Therefore, an object of the present invention is to spheroidize annealing for a long time exceeding 20 hours, which has been applied to a bearing rough molded part after hot forging to improve cold workability and cutting workability. It is to provide a method for producing a coarse bearing molded product capable of omitting or shortening the spheroidizing annealing time.

  More specifically, the present invention is to provide a method for producing a rough bearing molded product that can shorten the spheroidizing annealing time to about half of the conventional time. Furthermore, it is providing the manufacturing method of the rough bearing molded product which can obtain the spherical cementite obtained by the conventional spheroidizing annealing, without performing spheroidizing annealing.

  In order to solve the above-described problems, the present inventors first examined an optimal microstructure for obtaining a spheroidizing annealing time shortening effect. As a result, the following findings (1) to (4) were obtained.

  (1) In order to shorten the spheroidizing annealing time, it is necessary to keep the cementite in a hot-forged state so that the aspect ratio is as small as possible, that is, as close to spherical as possible.

  (2) However, in normal hot forging, proeutectoid cementite precipitates in a network form along the prior austenite grain boundaries. Therefore, nucleation and growth of pro-eutectoid cementite at a large number of precipitation sites other than the prior austenite grain boundary is effective in suppressing the precipitation of network-like pro-eutectoid cementite along the prior austenite grain boundary.

  (3) Although it is effective to shorten the spheroidizing annealing time if there is a lot of fine cementite with a small aspect ratio, if the cementite is too fine, the hardness of the coarse bearing molded product will become too high, so that the final shape will be obtained. There is a possibility of hindering workability in cold working and cutting.

  (4) Accordingly, cementite needs to be grown to an appropriate size.

  Therefore, the present inventors examined means for obtaining the above microstructure, and the “interval between machining” in (b), which was considered to be a disadvantage of “hot forging” with respect to “hot rolling”. I came up with the following (5), which actively uses the “short time”.

  (5) Since the processing strain can be accumulated if the processing temperature of the hot forging is lowered and the processing interval, which is the time from processing to the next processing, is limited, the initial analysis is performed using this processing strain. If the cementite is uniformly processed and induced in the prior austenite grains, the formation of network-like proeutectoid cementite that precipitates along the prior austenite grain boundaries may be suppressed.

  Therefore, when a confirmation test was performed based on the idea of (5) above, the following items (6) were clarified.

  (6) If the processing temperature of the hot forging is lowered and the processing interval is shortened, the pro-eutectoid cementite precipitated along the prior austenite grain boundaries can be changed from a network shape to a shape with a small aspect ratio. . However, the precipitation sites are still limited to the prior austenite grain boundaries, and plate-like cementite due to pearlite transformation precipitates in the prior austenite grains. That is, by simply lowering the hot forging processing temperature and shortening the processing interval, pro-eutectoid cementite having a small aspect ratio cannot be induced in the prior austenite grains.

  Therefore, the present inventors have made various studies on means for obtaining cementite having a small aspect ratio even in the prior austenite grains, and obtained the following findings (7) to (10).

  (7) If cementite is allowed to remain in the prior austenite grains in advance, it becomes possible to accumulate processing strain in the vicinity of the remaining cementite, so that the proeutectoid cementite is uniformly induced by processing in the prior austenite grains. Could be possible.

  (8) That is, the material is not completely austenitic during the hot forging heating, but the cementite in the pearlite that was present in the material before heating remains in a certain granular or spherical state, that is, heated rapidly. If hot forging is started immediately, the fine granular or spherical residual cementite in the prior austenite grains becomes the precipitation site for the pro-eutectoid cementite-induced precipitation.

  (9) Then, the forging temperature is lowered and the machining interval is shortened, the forging process is performed a plurality of times with a deformation amount greater than a specific amount to form a predetermined rough molded product shape, and then cooled at a specific cooling rate. For example, pro-eutectoid cementite is finely induced in the prior austenite grains as well as in the prior austenite grain boundaries, and carbon diffusion is also promoted by the accumulated work strain. Residual cementite can be grown to an appropriate size.

  (10) As a result, network-like pro-eutectoid cementite along the former austenite grain boundary that was precipitated in the case of the conventional hot forging method, and further, the conventional hot forging method and the processing temperature are lowered and processed. In the case of the hot forging method with only a short interval, the formation of pearlite formed in the prior austenite grains is suppressed, so that the formation of cementite with a small aspect ratio is reduced. By doing so, cementite having a nearly spherical shape can be obtained.

  Therefore, the present inventors further used various high-carbon chromium bearing steel materials containing 0.7 to 1.2% C and 0.8 to 1.8% Cr in terms of mass%. In particular, the test was repeated under various hot forging conditions. As a result, the following findings (11) to (15) were obtained.

(11) Heating is performed at a heating rate of 600 ° C. or higher to 10 ° C./s or higher to a temperature T ° C. of Ae ₁ point to (Aem point + 50 ° C.), and then within 10 min (including 10 min) after reaching this temperature. When the forging is started, the fine granular or spherical cementite in the pearlite that was present in the material being forged, which is the material for hot forging, will remain in the prior austenite grains at the start of hot forging. Can do.

(12) In the temperature range of (Ar ₁ point + 150 ° C.) to Ar ₁ point, fine fore-positioning is performed by performing the machining multiple times with a deformation amount of 5% or more per machining at a forging machining interval of 2 s or less. Cementite can be uniformly processed and induced to precipitate in the prior austenite grain boundaries and prior austenite grains.In addition, during the cooling process after processing, this proeutectoid cementite and residual cementite are grown to an appropriate size, and the aspect ratio is increased. It is possible to make cementite in a form close to a very small sphere.

(13) However, since the forging in (12) is performed at Ar ₁ or more, not all C (carbon) dissolved in the austenite is processed and precipitated as proeutectoid cementite during forging. Cementite precipitation still occurs in the subsequent cooling process. However, since the amount of solid solution of carbon in austenite is reduced due to processing-induced precipitation of pro-eutectoid cementite, if the temperature range up to 400 ° C. after forging ends is cooled at a cooling rate of 5 ° C./s or less, The pearlite transformation is suppressed, in other words, the precipitation of plate-like cementite is suppressed, so that a pro-eutectoid cementite having a relatively small aspect ratio is precipitated although it is not spherical.

(14) Then, by hot forging as described above, a rough bearing product having a microstructure composed of nearly spherical cementite and ferrite satisfying the following (a) to (c) is obtained.
(A) Of the cementite, the proportion of those having an aspect ratio of 2.0 or less is 50% or more,
(B) Of the cementite, the proportion of those having an aspect ratio of 5.0 or less is 75% or more,
(C) The average particle size of cementite having the aspect ratio of (a) of 2.0 or less is 0.16 μm or more.

  The “aspect ratio” means “major axis / minor axis”. In the following description, the major axis may be referred to as “L”, the minor axis may be referred to as “W”, and the aspect ratio may be referred to as “L / W”.

  (15) By having a microstructure composed of nearly spherical cementite and ferrite satisfying the above (a) to (c), it becomes possible to shorten the spheroidizing annealing time, and further more desirable heating conditions and forging conditions. If selected, the spheroidizing annealing can be omitted.

  The present invention has been completed on the basis of the above findings, and the gist of the present invention resides in a method for manufacturing a rough bearing molded product shown below.

“A high carbon chrome bearing comprising a plurality of forging processes for producing a rough bearing product, and containing, by mass, C: 0.7 to 1.2% and Cr: 0.8 to 1.8% The steel material is heated to a temperature T ° C. in the temperature range of Ae ₁ to (Aem point + 50 ° C.) at a heating rate of 600 ° C. or higher at 10 ° C./s or higher, and then within 10 min after reaching the temperature T ° C. (10 min In a temperature range from (Ar ₁ point + 150 ° C.) to Ar ₁ point, a forging process is performed a plurality of times with a deformation amount of 5% or more per process at a processing interval of 2 s or less. A method for producing a coarse bearing molded product, characterized in that the temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or less after the shape of the coarse molded product is formed.
In the present invention, “Ae ₁ point” and “Aem point” refer to the eutectoid temperature in an equilibrium state and the temperature at which cementite completely dissolves in austenite in the equilibrium state, respectively.

  The above “deformation amount” refers to the average value of equivalent plastic strain applied to a rough molded product. The equivalent plastic strain is calculated by Masuda et al., “Revised Industrial Plastic Mechanics” (February 20, 1995, 15th edition). , Issued by Yokendo Co., Ltd.) on page 113, the strain in the cross section deformed by forging can be converted into a plastic strain of uniaxial tension.

  Hereinafter, the invention relating to the above-described method for producing a coarse bearing molded product is referred to as “the present invention”.

  According to the present invention, the spheroidizing annealing for a long time exceeding 20 hours, which has been conventionally performed to improve the cold workability and the cutting workability, is performed on the roughly formed bearing after hot forging. Since it can be omitted or the time for spheroidizing annealing can be shortened, bearing parts, especially parts such as bearing rings, can be manufactured at low cost and high productivity with low energy consumption.

  Hereinafter, each requirement of the present invention will be described in detail. Note that. In the following description, “%” notation of the content of each element means “mass%”.

(A) Chemical composition of high carbon chromium bearing steel C: 0.7-1.2%
C is an essential element for ensuring the necessary strength for bearing parts used in automobiles and industrial machines as final products. In particular, since it is necessary to increase the amount of cementite for the purpose of improving the fatigue life, it is necessary to contain an amount of 0.7% or more. However, if its content exceeds 1.2%, the hardness of the coarse bearing molded product formed into a predetermined shape by hot forging becomes too high, so cold workability and cutting work to make the final shape Will be reduced. Moreover, it becomes easy to produce a crack at the time of the hardening process performed after making it into a final shape. Therefore, the content of C is set to 0.7 to 1.2%. In addition, in order to acquire a desired effect stably, it is preferable that content of C shall be 0.8 to 1.1%.

Cr: 0.8 to 1.8%
Cr has the effect of enhancing the hardenability of the steel and thermally stabilizing the cementite and inhibiting solid solution of the cementite in the matrix at high temperatures. This effect is exhibited when the Cr content is 0.8% or more. However, if the content of Cr exceeds 1.8%, not only the above effects are saturated, but also during the quenching process after making the final shape, it is easy to cause cracking, and fatigue resistance characteristics, etc. The mechanical properties are degraded. Therefore, the Cr content is set to 0.8 to 1.8%. The Cr content is preferably 0.9 to 1.6%.

  For the above reasons, in the method of manufacturing a coarse bearing molded product according to the present invention, a high carbon chromium bearing steel material containing C: 0.7 to 1.2% and Cr: 0.8 to 1.8% is used. I decided to use it.

  As a preferable chemical composition of the high carbon chromium bearing steel, for example, C: 0.7 to 1.2%, Cr: 0.8 to 1.8%, Si: 1.2% or less, Mn: 1.5 % Or less, P: 0.03% or less, and S: 0.025% or less, with the balance being Fe and impurities.

  Among the above high carbon chromium bearing steel materials, more preferable chemical compositions are C: 0.7 to 1.2%, Cr: 0.8 to 1.8%, Si: 0.05 to 1.0%, Examples include Mn: 0.2 to 1.2%, P: 0.02% or less, S: 0.02% or less, and the balance consisting of impurities.

  As impurities of each of the high carbon chromium bearing steels described above, the content of elements such as Cu, Ni, Al, N, and O that do not form carbides is Cu: 0.2% or less, Ni: 0.25% Hereinafter, if Al: 0.05% or less, N: 0.015% or less, and O: 0.002% or less, spheroidization is not affected at all. On the other hand, in the case of an element that forms a carbide among impurities, it is preferable that the content of Mo in particular be 0.08% or less.

  Moreover, as a preferable chemical composition of the high carbon chromium bearing steel, C: 0.7 to 1.2%, Cr: 0.8 to 1.8%, Si: 1.2% or less, Mn: 1.5 %, P: 0.03% or less, S: 0.025% or less, Mo: 0.5% or less, with the balance being Fe and impurities.

  Among the above high carbon chromium bearing steels, more preferable chemical compositions are C: 0.7 to 1.2%, Cr: 0.8 to 1.8%, Si: 0.05 to 1.0%, Mn: 0.2 to 1.2%, P: 0.02% or less, S: 0.02% or less, Mo: 0.05 to 0.4%, with the balance being Fe and impurities Can be mentioned.

  The amount of elements that do not form carbides such as Cu, Ni, Al, N, and O as impurities in each of the high-carbon chromium bearing steels described above is Cu: 0.2% or less, Ni: 0.25% or less Al: 0.05% or less, N: 0.015% or less, and O: 0.002% or less have no effect on spheroidization.

(B) Heating condition of high carbon chromium bearing steel In the present invention, the heating rate of 600 ° C. or higher is set to 10 ° C./s or higher for the high carbon chromium bearing steel having the chemical composition described in the above section (A). It is necessary to heat up to a temperature T ° C. in a temperature range from Ae ₁ point to (Aem point + 50 ° C.), and then start processing within 10 min (including 10 min) after reaching the temperature T ° C.

  This means that all cementite in pearlite that was present in the material to be forged, that is, the steel material before being roughly formed into a predetermined shape by hot forging, is dissolved in the matrix in the heating stage for forging. This is because it is important to leave as much as possible in a fine granular or spherical state so that it can be used as a precipitation site of pro-eutectoid cementite in the hot forging process.

  Therefore, it is necessary to heat the forged material rapidly and start hot forging promptly.

  By setting the heating rate of 600 ° C. or higher to 10 ° C./s or higher, a non-equilibrium state is maintained up to a relatively high temperature range, and cementite can remain for a short time even at a temperature slightly exceeding the Aem point. When the heating rate of 600 ° C. or higher is less than 10 ° C./s, the temperature range in which cementite can remain during heating and holding becomes narrow. Therefore, the heating rate of 600 ° C. or higher is set to 10 ° C./s or higher. The heating rate of 600 ° C. or higher is preferably 15 ° C./s or higher.

  If the heating rate is increased too much, excessive heating equipment is required and the cost is increased, so that the heating rate of 600 ° C. or higher is preferably 500 ° C./s or lower.

  Even when the heating rate of 600 ° C. or higher is 10 ° C./s or higher, if the heating temperature T ° C. exceeds (Aem point + 50 ° C.), the material is completely austenitic at the start of hot forging, Since carbon is completely dissolved, cementite cannot remain, and austenite undergoes pearlite transformation in the cooling process after hot forging, and plate-like cementite is precipitated in the prior austenite grains.

On the other hand, when the heating temperature T ° C. is lower than the Ae ₁ point, the pearlite itself of the material remains, the microstructure after hot forging becomes a pearlite structure, and a large number of plate-like cementite remains. In such a case, the effect of shortening the spheroidizing annealing time cannot be obtained.

Therefore, the heating temperature T ° C. was set to Ae ₁ point to (Aem point + 50 ° C.). In order to obtain the effect of omitting the spheroidizing annealing, the heating temperature T ° C. is preferably (Ae ₁ point + 15 ° C.) to (Aem point−45 ° C.).

  As the time from reaching the heating temperature T ° C. to the start of processing becomes longer, the number of residual cementite decreases with time, and when it exceeds 10 min, carbon completely dissolves, so cementite cannot remain, In the cooling process after hot forging, austenite undergoes pearlite transformation, and plate-like cementite is precipitated in the prior austenite grains. Therefore, the time from reaching the heating temperature T ° C. to the start of processing is within 10 min (including 10 min). It is desirable that the time from reaching the heating temperature T ° C. to the start of processing is within 5 min (including 5 min).

  If the heating temperature reaches T ° C., there is no problem even if the processing is started immediately after that.

  Further, the heating method is not particularly required, and it is only necessary that the heating rate of 600 ° C. or higher is 10 ° C./s or higher. For example, it is desirable to use high-frequency induction heating or current heating method.

(C) Forging conditions after heating of high carbon chromium bearing steel material In the present invention, the high carbon chromium bearing steel material having the chemical composition described in the item (A) is subjected to the conditions described in the item (B). Then, it is necessary to perform multiple forgings with a deformation amount of 5% or more per process at a processing interval of 2 s or less in a temperature range of (Ar ₁ point + 150 ° C.) to Ar ₁ point.

  In order to reduce the aspect ratio of pro-eutectoid cementite, it is necessary to suppress network-like pro-eutectoid cementite that precipitates along the prior austenite grain boundaries. This is because it is important to cause the pro-eutectoid cementite to be induced by processing-induced precipitation in the prior austenite grains and further grow.

  Therefore, it is necessary to lower the processing temperature of hot forging and shorten the processing interval.

First, processing strain can be accumulated by lowering the forging temperature. That is, many dislocations are introduced by forging, but when the forging temperature is low, the introduced dislocations do not disappear easily and accumulate in the vicinity of old austenite grain boundaries and residual cementite in the old austenite grains. As a result, the dislocation density is increased, and proeutectoid cementite is preferentially precipitated in the vicinity thereof, that is, processing-induced precipitation occurs. Further, since the diffusion of C (carbon) is promoted by the processing strain, the pro-eutectoid cementite or residual cementite grows to an appropriate size. Then, such an effect can be obtained by carrying out the forging in a temperature range of (Ar ₁ point + 150 ℃) ~Ar ₁ point.

When the forging temperature is higher than (Ar ₁ point + 150 ° C.), the introduced dislocations are consumed as the driving force for recrystallization of austenite grains, so that processing-induced precipitation does not occur and proeutectoid cementite is not in the austenite grain boundaries. Along the network. On the other hand, when the forging temperature is lower than the Ar ₁ point, many dislocations can be introduced, but austenite starts pearlite transformation, which is a decomposition reaction to ferrite and cementite, before forging. As a result of processing, some plate-like cementite in pearlite is slightly divided, but the aspect ratio of cementite is not so small. In such a case, the effect of shortening the spheroidizing annealing time cannot be obtained. Therefore, it is necessary to forge in the temperature range of (Ar ₁ point + 150 ° C.) to Ar ₁ point. In order to obtain the effect of omitting the spheroidizing annealing, forging is preferably performed in a temperature range of (Ar ₁ point + 120 ° C.) to (Ar ₁ point + 20 ° C.).

However, even when forging in the temperature range of (Ar ₁ point + 150 ° C.) to Ar ₁ point, if the deformation amount per processing is less than 5%, the processing strain is not accumulated. Since fine pro-eutectoid cementite cannot be induced by processing-induced precipitation in the austenite grain boundaries and the prior austenite grains, the pro-eutectoid cementite precipitates in a network form along the prior austenite grain boundaries. Therefore, the deformation amount per process at the time of hot forging needs to be 5% or more.

  The above "deformation amount" refers to the average value of equivalent plastic strain applied to a rough molded product. The equivalent plastic strain is calculated by Masuda et al. "Revised Industrial Plastic Mechanics" (February 20, 1995, 15th edition, stock As described above, the strain in the cross section deformed by forging can be obtained by converting it into a plastic strain of uniaxial tension by the method shown on page 113 of the company Yokendo.

  In addition, as already stated, for parts such as races, a method is adopted in which the inner and outer rings are simultaneously roughly formed by forging a plurality of times. When the amount of deformation is less than 5%, the working strain is not accumulated, and fine pro-eutectoid cementite cannot be induced to work in the prior austenite grain boundaries and the prior austenite grains. Along the network.

  Accordingly, if the forging process of 5% or more in the deformation amount per process described above is 2 times or more, the upper limit number does not need to be specifically defined, and may be appropriately determined from the dimensions and shape of the raw material and the final product. .

  Even when forging is performed multiple times with a deformation amount of 5% or more per processing, if the processing interval exceeds 2 s, the effect of accumulating processing strain becomes poor, and the former austenite grain boundaries and the old austenite grains are not sufficient. Fine proeutectoid cementite cannot be induced by processing, and proeutectoid cementite precipitates in a network form along the prior austenite grain boundaries. Therefore, it is necessary to forge by setting the processing interval to 2 s or less. In order to exhibit the effect, it is preferable to perform forging with a processing interval of 1 s or less.

  Note that the shorter the processing interval is, the more effective the processing strain can be accumulated. However, the lower limit is about 0.1 s due to the limitations of the forging equipment.

(D) Cooling conditions after forging of high carbon chrome bearing steel In the present invention, the high carbon chrome bearing steel with the chemical composition described in (A) is used under the conditions described in (B). After heating and then forging as described in the above section (C) to obtain a predetermined rough molded product shape, it is necessary to cool the temperature range up to 400 ° C. at a cooling rate of 5 ° C./s or less.

  When the cooling rate in the temperature range up to 400 ° C. after the forging is over 5 ° C./s, growth of pro-eutectoid cementite and residual cementite during cooling is inhibited and pearlite transformation occurs, so in the prior austenite grains Plate-like cementite constituting pearlite is precipitated, and the amount of cementite having an extremely large aspect ratio increases as a whole. In addition, when the cooling rate becomes extremely high, bainite transformation and martensite transformation occur instead of pearlite transformation, so that precipitation of plate-like cementite constituting pearlite can be suppressed, but the hardness of the bearing rough molded product is low. Since it becomes too high, the cold workability for making a final shape and the cutting work will be reduced. Therefore, it is necessary to cool the temperature region up to 400 ° C. at a cooling rate of 5 ° C./s or less after making the shape of a predetermined rough molded product.

  It should be noted that the temperature range for cooling at the cooling rate of 5 ° C./s or less is sufficient if the temperature range is up to 400 ° C. after forging, and the temperature range below 400 ° C. need not be specified. For this reason, it may be determined as appropriate from, for example, air cooling (cooling), forced air cooling, mist cooling, etc. in consideration of manufacturing equipment and productivity.

  In addition, the lower limit of the cooling rate in the temperature range up to 400 ° C. is that if the cooling rate is slowed, the effect of suppressing pearlite increases, but a temperature control facility for slowing the cooling rate is required, resulting in production. Since it causes an increase in cost, it is preferably 5 ° C./h.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Steels A to C shown in Table 1 were melted in a 150 kg vacuum melting furnace and then cast into an ingot. Table 1 also shows the Ae ₁ point and Aem point of each steel obtained by the phase diagram calculation software “Pandat ver. 6.0” developed and sold by Materials Design Technology Laboratory, Inc. .

  Ingots of Steel A to Steel C were heated at 1250 ° C. for 60 minutes and hot forged at a finishing temperature of 1000 ° C. or higher to obtain a round bar having a diameter of 30 mm.

  From the round bar having a diameter of 30 mm obtained in this way, a test piece for measuring the transformation point having a diameter of 3 mm and a length of 10 mm and a cylindrical test piece for processing having a diameter of 8 mm and a height of 12 mm are produced by machining. did.

Next, using a test piece having a diameter of 3 mm and a length of 10 mm, the Ar ₁ point in the cooling process of each steel was measured with a Formaster tester. Table 1 shows the Ar ₁ point of each steel.

  In addition, using a cylindrical test piece having a diameter of 8 mm and a height of 12 mm, a hot working test machine (working for master test machine) is used to perform hot two-stage compression under various conditions shown in Table 2. Went.

  After hot working, the cooling range is controlled by changing the flow rate of the cooling gas (He gas) in the temperature range up to 400 ° C., and cooling is not performed in the temperature range below 400 ° C. .

  Next, the microstructure of the test piece after each hot working was investigated by the following method.

  First, it passes through the central axis of each hot-worked test piece and is embedded in the resin so that a cross section cut out in parallel with the compression axis that is the processing direction (hereinafter referred to as “longitudinal cross section”) becomes the test surface. After mirror polishing, the microstructure was corroded with picric alcohol (picral liquid), and a microstructure image was taken for 10 fields of view using a scanning electron microscope (SEM) at a magnification of 5000 times. The area of each visual field is 25 μm × 20 μm.

  Next, the long diameter L and the short diameter W of each cementite are individually measured by the image processing software using the photographed image, and the ratio of the cementite whose L / W is 5.0 or less and the L / W is The ratio of the cementite which is 2.0 or less was calculated, respectively.

  Further, an equivalent circle diameter of each cementite having an L / W of 2.0 or less was derived by image processing software, and the average particle diameter of the cementite having an L / W of 2.0 or less was obtained by arithmetic averaging. .

  Furthermore, HV hardness of each test piece was measured at five locations with a test force of 2.94 N using the test specimens observing the microstructure, and the values were arithmetically averaged to calculate the HV hardness of each test specimen. Investigate.

  In the following description, the ratio of cementite whose L / W determined as described above is 2.0 or less is referred to as “spheroidization rate”. Further, as will be described later, since the spheroidization rate is about 85% when it is obtained by a long-time treatment exceeding 20 hours generally used as the spheroidization treatment conditions, the spheroidization rate is 85%. Was used as a criterion for determining whether or not

  Table 2 also shows the results of the above tests. In Table 2, “◎” in the “Evaluation” column indicates that the spheroidizing rate is 85% or more, that is, the effect of omitting the spheroidizing annealing is obtained, and the spheroidizing rate of 85% or more is not satisfied. , "-".

From Table 2, among the test numbers 1 to 12 that satisfy the conditions defined in the present invention, the heating temperature T ° C is (Ae ₁ point + 15 ° C) to (Aem point -45 ° C), and (Ar ₁ point + 120 ° C) Test numbers 2 to 4, test number 8, test number 10 and test number 11 forged in the temperature range of (Ar ₁ point + 20 ° C.) are “◎” and the spheroidization rate is 85% or more. And spheroidizing annealing can be omitted. In this case, it can be seen that the HV hardness is as low as 270 or less.

(Example 2)
Using the cylindrical test piece having a diameter of 8 mm and a height of 12 mm produced in Example 1 above, hot two-stage compression processing was performed under the conditions shown in Table 3.

  In addition, the two-stage compression processing of test numbers 21 to 34 shown in Table 3 has heating conditions, processing conditions and cooling conditions, respectively, test number 1, test numbers 5 to 7, test number 9 and test number in Table 2. These are the same as 12 to 20, and these are evaluations in Table 2 that are "-", that is, the effect of omitting the spheroidizing annealing was not obtained.

  Next, among the test pieces processed as described above, the test pieces of Test Nos. 21 to 33 have a total furnace time of 12 hours shown in FIG. The spheroidization treatment was performed using a box-type electric heating furnace apparatus in an air atmosphere under the condition that the total furnace time shown in FIG.

  The heat treatment pattern shown in FIG. 2 is an example of a long-time treatment that is generally used as a spheroidizing treatment condition.

  The microstructure of each test piece obtained as described above was examined.

  That is, it passes through the central axis of each hot-worked test piece, is embedded in a resin so that the longitudinal section becomes the test surface, mirror-polished, and then corroded with picric acid alcohol (picral solution) to give a magnification of 5000. Microscopic images were taken for 10 fields using a scanning electron microscope (SEM) as a magnification. The area of each visual field is 25 μm × 20 μm.

  Next, the long diameter L and the short diameter W of each cementite are individually measured by the image processing software using the above-mentioned photographed image, and the ratio of the cementite whose L / W is 2.0 or less, that is, the spheroidization ratio is determined. Calculated.

  For the spheroidization rate, 85% of test number 34 simulating long-time treatment, which is generally used as spheroidization treatment conditions, was used as a criterion.

  Table 3 shows the test results. In addition, “◯” and “×” in the “Evaluation” column of Table 3 respectively indicate that “the spheroidization rate has reached 85% of the target” and “the spheroidization rate has not reached 85% of the target. Means "."

  From Table 3, in the case of test numbers 21 to 26 that satisfy the conditions specified in the present invention, even if the spheroidizing treatment is performed with the heat treatment pattern of the total furnace time of 12 hours in FIG. The spheroidization rate higher than that in the case of test number 34 in which the spheroidizing treatment was performed with a long heat treatment pattern of 24 hours in all furnaces, that is, a spheroidizing rate exceeding 85% was obtained, and the effect of shortening the spheroidizing annealing time Is clearly obtained.

  On the other hand, in the case of test numbers 27 to 33 that do not satisfy the conditions defined in the present invention, the test number 34 in which the spheroidizing treatment is performed with the long-time heat treatment pattern of FIG. It is clear that the spheroidizing rate is lower than that in the case of, and the time for spheroidizing annealing cannot be reduced.

  FIG. 3 shows the test number 2 as an example of the microstructure when the effect of omitting the spheroidizing annealing was obtained, that is, the evaluation in Table 2 was “◎”. 4 and 5, when the effect of omitting the spheroidizing annealing and the effect of shortening the spheroidizing annealing time were not obtained, that is, the evaluation of Table 2 was “−” and the evaluation of Table 3 was “×”. As an example of the microstructure before spheroidizing annealing, those in Test No. 15 and Test No. 17 are shown.

  As shown in FIG. 3, it is clear that the microstructure when the evaluation in Table 2 is “◎” has very little plate-like cementite, and the proportion of spherical cementite is large, and is spheroidized. On the other hand, as shown in FIG. 4 and FIG. 5, in the microstructure in which the evaluation in Table 2 is “−” and the evaluation in Table 3 is “x”, almost no spherical cementite is generated, and plate-like cementite, It is clear that a pearlite structure is formed and a preferable microstructure suitable for shortening the spheroidizing time is not obtained.

  According to the present invention, the spheroidizing annealing for a long time exceeding 20 hours, which has been conventionally performed to improve the cold workability and the cutting workability, is performed on the roughly formed bearing after hot forging. Since it can be omitted or the time for spheroidizing annealing can be shortened, bearing parts, especially parts such as bearing rings, can be manufactured at low cost and high productivity with low energy consumption.

It is a figure explaining the spheroidization process conditions performed about the test piece of the test numbers 21-33 of Example 2 using the box-type electric heating furnace apparatus of an atmospheric condition. It is a figure explaining the spheroidization process conditions performed about the test piece of the test number 34 of Example 2 using the box-type electric heating furnace apparatus of an atmospheric atmosphere, and is the long-time process generally used as the spheroidization process conditions It is an example. It is a figure which shows the microstructure observed with the scanning electron microscope about the test piece of the test number 2 of Example 1. FIG. It is a figure which shows the microstructure observed with the scanning electron microscope about the test piece of the test number 15 of Example 1. FIG. It is a figure which shows the microstructure observed with the scanning electron microscope about the test piece of the test number 17 of Example 1. FIG.

Claims (1)

A high carbon chromium bearing steel which is a method for producing a roughly formed bearing product by forging a plurality of times and contains C: 0.7 to 1.2% and Cr: 0.8 to 1.8% by mass. The steel material is heated at a heating rate of 600 ° C. or higher to 10 ° C./s or higher to a temperature T ° C. in the temperature range of Ae ₁ point to (Aem point + 50 ° C.), and then within 10 min after reaching the temperature T ° C. (10 min. In the temperature range of (Ar ₁ point + 150 ° C.) to Ar ₁ point, a forging process is performed a plurality of times with a deformation amount of 5% or more per process with a process interval of 2 s or less. A method for producing a coarse bearing molded product, characterized by cooling the temperature range up to 400 ° C. at a cooling rate of 5 ° C./s or less after forming the rough molded product shape.

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