JP5124847B2 - Rolling method of high carbon chromium bearing steel - Google Patents

Description

  The present invention relates to a method for rolling a high carbon chromium bearing steel, and more specifically, a high carbon chromium bearing steel that has a spheroidized structure as it is hot-rolled and can shorten the time for spheroidizing heat treatment. It relates to a rolling method.

  Bearing parts used for automobiles and industrial machines are generally hot forged using high carbon chromium bearing steels represented by SUJ1-5 specified in JIS G 4805 (1999) as a raw material, and then cold forging, It is finished in a desired shape through a cold working such as wire drawing and a secondary working process such as cutting.

  In addition, the high carbon chromium bearing steel as hot rolled usually has a microstructure that is a single phase structure of pearlite or a mixed structure of a hard phase such as bainite and pearlite, and has a high hardness. Therefore, if secondary processing such as cutting or cold forging is performed on hot-rolled high carbon chrome bearing steel, the tool life will be reduced, and cracks may occur during cold forging. Inevitable.

  For this reason, in order to improve cold workability and cutting workability as a pretreatment of the secondary processing step, conventionally, a long spheroidizing heat treatment exceeding 20 hours called “spheroidizing annealing” is performed, The structure has been changed to a mixed structure of ferrite and spherical cementite.

  However, the long spheroidizing heat treatment requires expensive heat treatment equipment, consumes a lot of energy, and lowers productivity, leading to an increase in cost.

  Therefore, from the industry, even if the spheroidizing heat treatment as the pretreatment of the secondary processing process cannot be omitted, the time is greatly shortened, the energy consumption is reduced and the equipment cost is reduced, and the process is further simplified. There is a growing demand to increase productivity.

  Therefore, various techniques are proposed in Patent Documents 1 to 5, for example, in order to meet the above-described demand.

  That is, in Patent Document 1, a high-carbon chromium bearing steel containing C: 0.8-1.2% and Cr: 0.9-1.8% by weight is extracted 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.

Patent Document 2 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 Patent Document 3, after finishing the hot rolling, Ar ₁ −200 ° C. or higher and Ar ₁ + 100 ° C. or lower in a temperature range of 15% or higher after finishing rolling, Once the finish-rolled steel material is cooled to a temperature range of Ar ₁ or less, it is subsequently reheated to a temperature range of Ac ₁ or more, Ac ₃ or Acm or less, and then 0.05 ° C./s or less. “A method for producing a wire / bar excellent in cold forgeability” is disclosed in which the cooling rate is 600 ° C. or lower and the cooling stop temperature is 600 ° C. or less.

In Patent Document 4, hot rolling is performed on a low-alloy steel material containing a specific amount of C and Cr at a final finishing temperature of 900 to 1200 ° C. to obtain a wire, and then (Ar ₁ transformation point −30 ° C.). After cooling to the following temperature, the wire is heated, cooled, and subjected to spheroidizing annealing. When heating the wire, the maximum heating temperature is (Ac ₁ transformation point + 30 ° C.) to (Ac ₁ transformation). And a cooling rate from the temperature of (Ar ₁ transformation point −30 ° C.) to the maximum heating temperature is 1.0 ° C./second or less, and from the maximum heating temperature to (Ar ₁ transformation) Point) A "direct spheroidizing annealing method for low alloy wire" is disclosed in which the following temperature is continuously cooled at a cooling rate of 0.2 to 5 ° C / second.

  In Patent Document 5, 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, the cooling start temperature to 850 ° C. or lower, and the cooling. “A method for producing a linear or bar-shaped steel excellent in wire drawing workability that can omit the heat treatment before wire drawing” in which the average cooling rate in the range of 600 ° C. from the start temperature is 0.1 to 5 ° C./s. It is disclosed.

JP-A-11-286724 JP 2004-190127 A JP 2005-120432 A JP 2000-192147 A JP 2003-129176 A

  All the techniques proposed in Patent Documents 1 to 3 require finish rolling at a low temperature. For this reason, a normal finish rolling mill cannot withstand a large mill load, and it is necessary to install a rolling mill with a large power, which requires a large equipment investment, resulting in an increase in manufacturing cost.

  In the case of the technique proposed in Patent Document 4, since the austenite grain size after finish rolling becomes coarse, the austenite grain interface area becomes small. For this reason, since the precipitation site | part of the pro-eutectoid cementite in the cooling process after finish rolling decreases, it cannot be avoided that a pro-eutectoid cementite with a large thickness is produced in the prior austenite grain boundary.

  The structure obtained by the technique proposed in Patent Document 5 is mainly a pearlite structure. For this reason, as a secondary processing, for example, a spherical shape as a pre-processing of the secondary processing step can be applied in the case of special wire drawing processing or cold forging such as a reduction in cross-section and a small reduction amount. This is not necessarily sufficient to omit the heat treatment or to greatly shorten the time.

  Therefore, the object of the present invention is that, as it is hot rolled, its microstructure is a single phase structure of pearlite or a mixed structure of a hard phase and pearlite such as bainite, so that cold workability and machinability are low, and 20 hours. Spheroidizing heat treatment after hot rolling for high carbon chrome bearing steel, especially high carbon chrome bearing steel rod or high carbon chrome bearing steel wire, which has been subjected to long spheroidizing heat treatment exceeding Even if it is a case, it is providing the manufacturing method which can ensure the microstructure which is inferior to the case where the conventional spheroidization heat processing is carried out.

  In order to solve the above-described problems, the present inventors first examined a microstructure for shortening the spheroidizing heat treatment time. As a result, the following findings (1) to (3) were obtained.

  (1) In order to shorten at least the spheroidizing heat treatment time, it is necessary to suppress pro-eutectoid cementite as much as possible or to control the form of pro-eutectoid cementite in the microstructure as hot-rolled.

  (2) In order to grow cementite into a spherical shape during spheroidizing heat treatment, fine cementite that forms the nuclei of spherical cementite in the microstructure before spheroidizing heat treatment, especially with a small aspect ratio, that is, close to spherical It is necessary to have as much cementite as possible.

  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”.

  (3) If the area ratio in the microstructure of pearlite which is a layered structure is 60% or less, the effect of shortening the spheroidizing heat treatment time can be obtained.

  Next, the present inventors suppressed the generation of pro-eutectoid cementite while maintaining the hot rolling without applying a large load to the rolling mill as in the case of normal hot rolling, or the formation of pro-eutectoid cementite. Various studies were made on the method capable of controlling the morphology to a small aspect ratio and limiting the ratio of pearlite, which is a layered structure, to the microstructure.

  As a result, the following important findings (4) to (7) were obtained.

  (4) It is necessary to increase as much as possible the interfacial area of austenite, which is the precipitation site of pro-eutectoid cementite after finish rolling, in other words, it is necessary to make the prior austenite grain size as fine as possible.

(5) Formation of film-like pro-eutectoid cementite along the prior austenite grain boundary by cooling from finisher rolling to the Ar ₁ point after finish rolling at a specific cooling rate. And the pearlite transformation can be completed.

  (6) After the pearlite transformation is carried out once by the treatment of (5) above, it is reheated to a specific temperature range and held to disperse a very large amount of fine cementite having a small aspect ratio that becomes a nucleus of spherical cementite. be able to.

  (7) The fine cementite dispersion effect of (6) can be ensured by cooling at a specific cooling rate after the reheating treatment.

  Therefore, the present inventors further examined specific means for increasing the austenite grain interface area, that is, for refining the prior austenite grain size.

  That is, the relationship with the Zener-Holomon parameter Z expressed by the following equation (1) was examined.

Z = {εdot} × {exp (Q / RT)} (1).
In the above equation (1),
ε dot: average strain rate (1 / s) in the final pass of finish rolling,
Q: activation energy (kJ / mol),
R: gas constant, 8.31 × 10 ⁻³ kJ / (mol · K),
T: surface temperature (K) of the material to be rolled on the exit side of the rolling mill in the final pass of finish rolling,
It is.

  The “average strain rate in the final pass of finish rolling” refers to a value obtained by dividing “true strain” in the final pass of finish rolling by “time (s)” required for rolling in the final pass.

  The “final rolling final pass” in the case of a continuous rolling method using a so-called “tandem mill” refers to a pass at the final rolling mill in the finishing rolling mill row. Therefore, in the following description, the pass in the final rolling mill in the finish rolling mill row in the case of the continuous rolling method may be referred to as “final rolling final pass”.

In addition, Namba “Influence of various conditions on microstructure refinement in hot rolling” (recrystallization / texture and its application to microstructure control, March 1999, p.235, Japan Iron and Steel Institute) As discussed in the Material Structure and Properties Subcommittee, Recrystallization / Gene Texture Study Group), the prior austenite grain size d is hot-rolled in a high temperature region to cause “dynamic recrystallization”. , And can be expressed as an exponential function of the Zener-Holomon parameter Z, ie, “d = AZ ⁻ⁿ ”.

  That is, when “dynamic recrystallization” occurs, the prior austenite grain size d is affected by the processing temperature and strain rate.

  And when it is processed continuously like hot rolling, "quasi-dynamic recrystallization" occurs, and the prior austenite grain size d in that case is also the case of the above "dynamic recrystallization" Similarly, it is shown that it can be organized with the Zener-Holomon parameter Z.

  Moreover, as described in Umemoto et al. In “Measurement method of crystal grain size and related formulas” (heat treatment, 24 (1984), p. 334), the grain interfacial area per unit volume is the crystal grain size. It is proportional to the reciprocal of the diameter. For this reason, the austenite grain interface area per unit volume after finish rolling is proportional to the reciprocal of the prior austenite grain size d.

From the above, the present inventors first come to the conclusion that “1 / d = A′Z ⁿ ” can be expressed using the reciprocal “1 / d” of the prior austenite grain size d as an evaluation index. It was.

  Then, as a specific high carbon chromium bearing steel, steels J1 to J7 having the chemical composition shown in Table 1 were melted in a 150 kg vacuum melting furnace and examined.

  That is, the above 7 steel types were melted in a vacuum melting furnace and cast into a mold. The ingot was heated at 1250 ° C. for 60 minutes and hot forged at a finishing temperature of 1000 ° C. or more to obtain a round bar with a diameter of 30 mm. It was.

  Regarding steel J5 in Table 1, particularly Ti was added to make its content 0.020% by mass. In addition, since the high carbon chromium bearing steel produced by actual furnace melting may contain a small amount of Ti, the Ti content of other steels excluding steel J5 is 0.001% by mass or 0%. It adjusted so that it might become 0.008 mass%.

  From the round bar having a diameter of 30 mm obtained as described above, a test piece for measuring the transformation point having a diameter of 3 mm and a length of 10 mm and a cylindrical test for hot working having a diameter of 8 mm and a height of 12 mm by machining. A piece was made.

Subsequently, the Ac ₁ point, the Acm point, and the Ar ₁ point of each steel were measured by a Formaster tester using the test piece having a diameter of 3 mm and a length of 10 mm. Table 1 shows the Ac ₁ point, Acm point, and Ar ₁ point of each steel.

  Further, using the cylindrical test piece having a diameter of 8 mm and a height of 12 mm, hot compression processing was performed under various conditions shown in Table 2 using a hot compression processing tester (processing for master tester). . The hot compression process was performed in one pass in order to simulate the “final rolling final pass” in hot rolling.

  “Processing temperature” in Table 2 refers to the surface temperature of the test piece after completion of hot compression processing in ° C., and “strain rate” refers to the average strain rate in hot compression processing. Here, the “strain amount” in Table 2 is also shown in the “%” unit multiplied by 100.

The value of the Zener-Holomon parameter Z shown in Table 2 uses “276 kJ / mol” as the value of the activation energy Q, and 8.31 × 10 ⁻³ kJ / (mol · K).

  The above-mentioned value of 276 kJ / mol in the activation energy Q is that the steels J1 to J7, which are target steels, are high-carbon chromium steels. “Carbon steel low alloy steel” at Crystal Steel and Texture and its Application to Structure Control, March 1999, p.235, Japan Iron and Steel Institute, Material Structure and Properties Subcommittee, Recrystallization and Texture The value close to the upper limit of 230 to 280 kJ / mol arranged as the Q value of “

  Immediately after the above hot compression processing, the structure was frozen by quenching with He gas so that the cooling rate became 50 ° C./s, and the prior austenite particle size under each compression processing condition was investigated.

  In other words, after embedding in a resin so that a so-called “longitudinal section” cut through the central axis of the compressed test specimen and parallel to the compression processing direction becomes the test surface and mirror-polished, picrin with a surfactant added Corrosion with an acid saturated aqueous solution reveals the prior austenite grain boundaries, and using an optical micrograph of magnification 400 times taken arbitrarily in four fields of view, an “average section length” L is determined by a cutting method, and “1.128 × The so-called ASTM nominal particle size of “L” was used as the prior austenite particle size.

  Table 2 also shows the prior austenite grain size determined as described above.

  Further, in FIG. 1, “lnZ” that is a natural logarithmic notation of the Zener-Holomon parameter Z and “ln (1 / d)” that is a natural logarithmic notation of (1 / d) that is the reciprocal of the prior austenite particle diameter d. Organize and show the relationship.

  The notation in () attached to each symbol in FIG. 1, for example, “0.7C” means that the C content of steel J1 is “0.7 mass%”. In addition, in FIG. 1, it describes with "Ti addition steel" only regarding steel J5 which added Ti especially and made the content 0.020 mass%, and Ti content is 0.01% or less About steel J1-J4, steel J6, and steel J7 which are these, description of Ti content was abbreviate | omitted.

From FIG. 1, except for the case of steel J5 in which Ti is added to make the content 0.020% by mass, 0.7 to 1.2% by mass of C and 0.8 to 1.8% by mass. In the steel containing% Cr, the coefficient A ′ and the index n in “1 / d = A′Z ⁿ ” are almost the same value, and it was found that the steel can be arranged as follows.

1 / d = 1.01 × 10 ⁻⁴ × Z ^0.28
= 1.01 × 10 ⁻⁴ × [{ε dot} × {exp (Q / RT)] ^0.28 .

On the other hand, in the case of steel J5 containing 0.020% Ti having a pinning action as a precipitate, there is a correlation between the prior austenite grain size d after hot compression processing and the processing temperature and strain rate. However, the coefficient A ′ and the index n in “1 / d = A′Z ⁿ ” are different from those of steels J1 to J4, steel J6 and steel J7 having a Ti content of 0.01% or less. I understood.

Then, the present inventors then heated the above-mentioned steel J1 to J4, steel J6, and steel J7 by using a processing for master tester, using the cylindrical test piece having a diameter of 8 mm and a height of 12 mm. Heating is performed at a temperature of 880 to 1200 ° C. and a holding time of 10 minutes, and the processing condition is a test piece surface temperature (hereinafter, also referred to as “processing temperature”) after the hot compression processing is 880 to 1200 ° C. in% units. The amount of strain at 30 to 60% and the strain rate to 0.5 to 25 (1 / s) are variously changed to perform hot compression processing in one pass, and from the processing temperature to Ar ₁ point. The cooling rate V (° C./s) is changed in the range of 0.05 to 5 ° C./s and cooled to 500 ° C., which is a temperature below the Ar ₁ point, to produce proeutectoid cementite precipitated at the prior austenite grain boundaries. The aforementioned cooling rate V and Zener-H The influence of the olomon parameter Z was investigated in detail. In addition, after cooling to the Ar ₁ point at the cooling rate of 0.05 to 5 ° C./s, the cooling to 500 ° C. is performed by adjusting the He gas flow rate using He gas as a cooling medium, and thereafter It was allowed to cool naturally and cooled to room temperature (25 ° C.).

  FIG. 2 shows the effect of “lnZ”, which is the natural logarithmic expression of the cooling rate V and the Zener-Holomon parameter Z, on the formation of proeutectoid cementite precipitated at the prior austenite grain boundaries.

  In FIG. 2, “◯” indicates that the formation of pro-eutectoid cementite at the grain boundaries of fine prior austenite is suppressed, and “△” indicates that pro-eutectoid cementite is generated at the grain boundaries of fine prior austenite. “L / W” is small and less than 5.0, and “×” indicates film-like pro-eutectoid cementite with “L / W” exceeding 5.0 at the grain boundary of coarse old austenite. Indicates that

From FIG. 2, when lnZ ≧ 25 and the cooling rate from the processing temperature to the Ar ₁ point is 0.5 ° C./s or more, even if processing is performed in the austenite single phase region, the prior austenite grain boundary is reached. The precipitation of pro-eutectoid cementite can be suppressed, or even if pro-eutectoid cementite is formed at the grain boundaries of the prior austenite, the pro-eutectoid cementite in a finely dispersed state with an "L / W" of 5.0 or less Obviously you can.

  3 to 5 show specific examples of “◯”, “Δ”, and “×” in FIG. Each of FIGS. 3 to 5 is a so-called “longitudinal cross section” in which a hot-compressed test piece of steel J2 passes through the central axis of the compressed test piece and is cut out in parallel with the compression processing direction. After being mirror-polished by embedding in a resin so as to become a test surface, it was corroded with 3% nitric alcohol (nitral liquid) and photographed using a scanning electron microscope (SEM).

Therefore, the present inventors further used the processing for master tester in FIG. 2 to test the cylindrical test pieces having a diameter of 8 mm and a height of 12 mm for the steels J1 to J4, the steel J6, and the steel J7. After performing hot compression processing and cooling under conditions where the evaluation of pro-eutectoid cementite precipitated at the austenite grain boundaries was “◯” and “Δ”, and cooling to 500 ° C., which is a temperature of Ar ₁ point or less, Furthermore, it is reheated to a temperature range of (Ac ₁ point−50 ° C.) to (Ac ₁ point + 150 ° C.) and held in this temperature range for 0 to 90 minutes, and then the temperature range up to 400 ° C. is a cooling gas (He gas). ), The cooling rate was controlled, and the temperature range below 400 ° C. was not controlled, but it was allowed to cool naturally and cooled, and the formation state of proeutectoid cementite precipitated at the prior austenite grain boundaries was investigated. .

As a result, once pearlite transformation was performed, if reheated to a temperature range of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) and held for 5 min or longer in this temperature range, It is possible to disperse a very large amount of fine cementite with a small aspect ratio, and the effect is ensured by cooling the temperature range up to 400 ° C. at a cooling rate of 5 ° C./s or less after reheating treatment. It turns out that you can.

  It was also clarified that the cooling rate in the temperature range below 400 ° C. does not affect the dispersion of fine cementite having a small aspect ratio, which is a nucleus for forming spherical cementite.

  In addition, the present inventors also conducted a similar investigation on steel J5, in which Ti was added to make the content 0.020% by mass.

  As a result, compared with the steels J1 to J4, steel J6 and steel J7 having a low Ti content, the austenite grain size can be refined when processed in a high temperature region, so that the pro-eutectoid cementite precipitated at the prior austenite grain boundaries. It has been found that it is possible to more reliably suppress or prevent proeutectoid cementite in a finely dispersed state with an “L / W” of 5.0 or less.

  However, in the steel J5, as compared with other steels, generation of hard TiN combined with N in the steel was recognized. This TiN is known to greatly reduce the “rolling fatigue life” of the bearing steel product, and therefore it was confirmed that the Ti content needs to be limited.

  This invention is completed based on said knowledge, The summary exists in the rolling method of the high carbon chromium bearing steel materials shown to following (1)-(3).

(1) A hot rolling method for high carbon chromium bearing steel, in mass%, C: 0.7-1.2%, Cr: 0.8-1.8%, Si: 1. 2% or less, Mn: 1.5% or less, the balance being Fe and impurities, Ti in impurities: 0.01% or less, P: 0.03% or less, S: 0.025% Hereinafter, Mo: 0.08% or less, 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 A high carbon chromium bearing steel material having a certain chemical composition is heated to a temperature range of the Acm point or higher to start rolling, and the final pass of finish rolling in the rolling is expressed by Z as represented by the following formula (1). -As a Hollomon parameter, the material to be rolled on the exit side of the rolling mill is used while satisfying the following formula (2) After finishing under the condition that the surface temperature T is from Acm point to 1150 ° C., the temperature T to the Ar ₁ point is cooled at a cooling rate of 0.5 ° C./s or more and cooled to a temperature T ₁ that is not more than the Ar ₁ point. The temperature T1 is reheated to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.), held in the temperature range for 5 to 90 minutes, and then cooled at a cooling rate of 5 ° C./s or less. A method for rolling high-carbon chromium bearing steel.
Z = {εdot} × {exp (Q / RT)} (1),
lnZ ≧ 25 (2).
In the equation (1),
ε dot: average strain rate (1 / s) in the final pass of finish rolling,
Q: activation energy (kJ / mol),
R: gas constant, 8.31 × 10 ⁻³ kJ / (mol · K),
T: surface temperature (K) of the material to be rolled on the exit side of the rolling mill in the final pass of finish rolling,
It is.

(2) Hot rolling method of high carbon chromium bearing steel by continuous rolling method, in mass%, C: 0.7-1.2%, Cr: 0.8-1.8%, Si: 1.2% or less, Mn: 1.5% or less, with the balance being Fe and impurities, and Ti in impurities: 0.01% or less, P: 0.03% or less, S: 0.025% or less, Mo: 0.08% or less, Cu: 0.2% or less, Ni: 0.25% or less, Al: 0.05% or less, N: 0.015% or less, and O: 0.0. A high carbon chrome bearing steel material having a chemical composition of 002% or less is heated to a temperature range of Acm point or higher to start continuous rolling, and the pass at the final rolling mill of finish rolling in the continuous rolling is set to Z. Conditions that satisfy the following equation (2) as the Zener-Holomon parameter represented by the following equation (1) And after finishing under the condition that the surface temperature T of the material to be rolled on the exit side of the rolling mill is Acm point to 1150 ° C., the temperature T to the Ar ₁ point are cooled at a cooling rate of 0.5 ° C./s or more. Then, it is cooled to a temperature T1 that is not more than Ar ₁ point, reheated from the temperature T1 to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) and held in the temperature range for 5 to 90 minutes, Then, the rolling method of the high carbon chromium bearing steel material which cools with the cooling rate of 5 degrees C / s or less.
Z = {εdot} × {exp (Q / RT)} (1),
lnZ ≧ 25 (2).
In the equation (1),
ε dot: average strain rate (1 / s) in the pass in the final rolling mill of finish rolling,
Q: activation energy (kJ / mol),
R: gas constant, 8.31 × 10 ⁻³ kJ / (mol · K),
T: surface temperature (K) of the material to be rolled on the exit side of the rolling mill in the pass in the final rolling mill for finish rolling,
It is.

  (3) The method for rolling high carbon chrome bearing steel according to (2) above, wherein the high carbon chrome bearing steel is a bar or wire.

  The “average strain rate in the final pass of finish rolling” refers to a value obtained by dividing “true strain” in the final pass of finish rolling by “time (s)” required for rolling in the final pass.

  The “continuous rolling method” is, for example, “rough rolling mill (row) —finish rolling mill (row)” or “rough rolling mill (row) —intermediate rolling mill (row) —finish rolling mill (row)”. Such a rolling method using a tandem mill. “Finishing rolling” refers to rolling in the “finishing rolling mill (row)”, and “final finishing rolling mill” refers to the final rolling mill in the finishing mill row in the tandem mill.

  Note that the “final finish rolling mill” in the case where the “finish rolling” in the continuous rolling method is performed by one “finish rolling mill” refers to the finish rolling mill.

Furthermore, reheating to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) and holding for 5 to 90 minutes in this temperature range means that (Ac ₁ point + 20 ° C.) to T 2 in the temperature range. It means that the total holding time is 5 to 90 min.

  The “rod” refers to a steel material that is hot rolled into a rod shape and cut to a predetermined length, and includes a so-called “burn-in coil” wound in a coil shape. The “wire” refers to steel that is hot rolled into a rod shape and wound in a coil shape.

  Hereinafter, the inventions relating to the rolling methods of the high carbon chromium bearing steel materials of (1) to (3) are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

  According to the present invention, as hot-rolled, the microstructure is a single-phase structure of pearlite or a mixed structure of a hard phase and pearlite such as bainite, so that cold workability and cutting workability are low, and it exceeds 20 hours. For high carbon chrome bearing steels that have been subjected to such long spheroidizing heat treatment, especially high carbon chrome bearing steel rods or wires, even if the time for spheroidizing heat treatment after hot rolling is shortened It is possible to secure a microstructure that is inferior to that of the conventional spheroidizing heat treatment.

  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 has the effect | action which ensures the intensity | strength of steel. In the case of bearing steel, in particular, from the viewpoint of improving the rolling fatigue life and wear resistance at the final product stage, the quenching conditions are optimized, and the microstructure after quenching / tempering treatment is changed between martensite and cementite. It is an essential element for a mixed structure. In order to obtain the strengthening effect by precipitation and dispersion of cementite in the final product stage described above, it is necessary to contain 0.7% or more of C. However, if the content of C exceeds 1.2%, the above-mentioned effect at the product stage can be obtained, but the form of pro-eutectoid cementite in the cooling process after finish rolling cannot be controlled. It is not possible to reduce the spheroidizing heat treatment time. Therefore, the content of C is set to 0.7 to 1.2%. In addition, 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 making cementite uniform and fine, improving the hardenability of the steel, and improving the rolling fatigue life. This effect is exhibited when the Cr content is 0.8% or more. However, if the Cr content exceeds 1.8%, not only the above-described uniform refinement and hardenability improvement effect of cementite is saturated, but also the rolling fatigue life is reduced, and further, cold workability is reduced. It also causes a decline. Therefore, the Cr content is set to 0.8 to 1.8%. The Cr content is preferably 0.9 to 1.6%.

  In the present invention, the Ti content in the impurities is limited as follows.

Ti: 0.01% or less Ti combines with N present in the steel to form hard TiN, and this TiN greatly reduces the “rolling fatigue life” of the bearing steel product. In particular, if the Ti content exceeds 0.01%, the formation of TiN increases and the rolling fatigue life of the bearing steel product is significantly reduced. Therefore, the Ti content in the impurities is set to 0.01% or less. In addition, since the lower the content of Ti in the impurities, the better. Therefore, the content is preferably 0.008% or less.

  For the above reason, the rolling method of the high carbon chromium bearing steel according to the present invention contains C: 0.7 to 1.2% and Cr: 0.8 to 1.8%, and Ti in impurities It was decided to use a high carbon chrome bearing steel with a 0.01% or less.

  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 %, With the balance being Fe and impurities, Ti in the impurities being 0.01% or less, P being 0.03% or less, S being 0.025% or less and Mo being 0.08% or less Examples include high carbon chromium bearing steels.

  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%, Mn: 0.2 to 1.2%, the balance is Fe and impurities, Ti in impurities is 0.01% or less, P is 0.02% or less, S is 0.02% or less, and Mo Is a high carbon chromium bearing steel with 0.08% or less.

  In each of the high carbon chromium bearing steels described above, among the impurities, the amount of elements such as Cu, Ni, Al, N, and O that do not form carbides is Cu: 0.2% or less, Ni: 0.0. If it is about 25% or less, Al: 0.05% or less, N: 0.015% or less, and O: 0.002% or less, the spheroidization is not affected at all.

(B) Heating condition of high carbon chrome bearing steel In the present invention (1), the high carbon chrome bearing steel having the chemical composition described in the above section (A) is heated to a temperature range above the Acm point. It is necessary to start rolling.

  Further, in the present invention (2) and the present invention (3), the high carbon chromium bearing steel having the chemical composition described in the above section (A) is heated to a temperature range equal to or higher than the Acm point to start continuous rolling. There is a need to.

  This is because at the heating temperature below the Acm point, the pro-eutectoid cementite remains, and the shortening effect of the spherical heat treatment, which is a subsequent process, is significantly inhibited.

  Accordingly, in the present invention (1), rolling is started by heating to a temperature range equal to or higher than the Acm point, and in the present invention (2) and present invention (3), the temperature range is equal to or higher than the Acm point. It was decided to start continuous rolling by heating.

  From the viewpoint of heating cost, the heating temperature is preferably 1250 ° C. or lower.

(C) Hot rolling conditions and cooling conditions after rolling of the high carbon chrome bearing steel material In the present invention (1), a high carbon chrome bearing steel material having the chemical composition described in the above section (A) is used. For the hot rolling that starts by heating under the conditions described in the item (B), the final pass of the finish rolling is defined as (2), where Z is the Zener-Holomon parameter represented by the formula (1). ) And the condition that the surface temperature T of the material to be rolled on the rolling mill exit side is Acm point to 1150 ° C., and thereafter the temperature T to Ar ₁ point is 0.5 ° C./s. It is necessary to cool to the temperature T1 below the Ar ₁ point by cooling at the above cooling rate.

Further, in the present invention (2) and the present invention (3), the high carbon chromium bearing steel having the chemical composition described in the item (A) is heated under the conditions described in the item (B). In the continuous hot rolling, the pass in the final rolling mill of the finish rolling (final pass of the finish rolling) is expressed by the formula (2), where Z is the Zener-Hollonmon parameter represented by the formula (1). And the condition that the surface temperature T of the material to be rolled on the exit side of the rolling mill is Acm point to 1150 ° C., and thereafter the temperature T to Ar ₁ point is 0.5 ° C./s or more. It is necessary to cool to a temperature T1 that is not higher than Ar ₁ by cooling at a cooling rate.

  The reason will be described below.

  First, in the final pass of finish rolling, when the surface temperature T of the material to be rolled on the exit side of the rolling mill is below the Acm point, the deformation resistance during hot rolling increases, resulting in an increase in the mill load of the rolling mill, On the other hand, when the surface temperature T of the material to be rolled on the exit side of the rolling mill exceeds 1150 ° C., the surface property is rapidly deteriorated, for example, the scale of the surface layer portion becomes too thick.

Next, the final pass of the finish rolling is set to a condition satisfying the expression (2) with Z being a Zener-Holomon parameter represented by the expression (1), that is, “lnZ ≧ 25”, and the temperature T From ₁ to Ar _{1 at} a cooling rate of 0.5 ° C./s or more, as already shown in FIG. 2, if this condition is not satisfied, in the cooling process after the final pass of finish rolling, This is because film-like pro-eutectoid cementite is formed along the prior austenite grain boundaries, which significantly impairs the shortening effect of the spheroidizing heat treatment, which is a subsequent process.

  In addition, “ε dot” and “T” in the Zener-Holomon parameter of the equation (1) are respectively the average strain rate (1 / s) in the final pass of finish rolling and the surface temperature of the material to be rolled on the exit side of the rolling mill ( K).

  The “average strain rate in the final pass of finish rolling” refers to a value obtained by dividing “true strain” in the final pass of finish rolling by “time (s)” required for the rolling process in the final pass.

Also, the cool from the temperature T up to _a point Ar to 0.5 ° C. / s or more temperature cooled to less than ₁ point Ar at a cooling rate T1, when stopping the cooling at a temperature higher than _a point Ar Because it will be reheated and held in the specific temperature range that follows from the austenite state without pearlite transformation, it will not be possible to disperse a very large amount of fine cementite with a small aspect ratio that will be the nucleus of spherical cementite. It is. Therefore, the ratio of pearlite, which is a layered structure, to the microstructure is limited to 60% or less, and a microstructure in which a very small amount of fine cementite having a small aspect ratio is dispersed cannot be obtained.

By cooling to the Ar ₁ point at a cooling rate of 0.5 ° C./s or more, the formation of film-like pro-eutectoid cementite along the prior austenite grain boundaries can be suppressed, and the pearlite transformation can be completed. Therefore, the temperature T1 may be the Ar ₁ point itself. Further, the temperature T1 may be room temperature as long as the temperature is equal to or lower than the Ar ₁ point. However, if the temperature T1 is too low, in the reheating step (D) described later, the heat treatment time until the steel material reaches the predetermined temperature T2 becomes long, so that the productivity is lowered and the manufacturing cost is increased. I'll be stuck. Therefore, the temperature T1 in order to as short a time as possible treatment time in the reheating step is preferably in the range of (Ar ₁ point -200 ° C.) to Ar ₁ point.

When the temperature T1 is lower than the Ar ₁ point, the cooling rate from the Ar ₁ point to the temperature T1 does not need to be specified. Therefore, the cooling method is appropriately determined in consideration of productivity and equipment. Can be determined.

(D) Conditions for reheating and cooling from temperature T1 In the present invention, after pearlite transformation is performed once by the treatment of (C) above, from temperature T1 (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) It is necessary to reheat to a temperature T2 of (5) and hold in that temperature range for 5 to 90 minutes, and then cool at a cooling rate of 5 ° C./s or less.

First, when the reheating temperature T2 from the temperature T1 is lower than (Ac ₁ point + 20 ° C.), the pearlite cannot be sufficiently dissolved in the substrate. On the other hand, T2 is (Ac ₁ point + 80 ° C.). In this case, since the number of fine cementite with a small aspect ratio that forms nuclei of spherical cementite is drastically reduced because pearlite is dissolved in the substrate too much, in each case, the desired microstructure, i.e., the initial The formation of precipitated cementite can be suppressed, or the morphology of pro-eutectoid cementite can be controlled to a low aspect ratio, and the proportion of pearlite, a lamellar structure, in the microstructure is limited to 60% or less, and the fine aspect ratio is small. A microstructure in which a large amount of cementite is dispersed cannot be obtained.

In addition, even if the reheating temperature T2 from the temperature T1 is a temperature of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.), if the holding time in the temperature range is less than 5 min, the layered structure As pearlite cannot be dissolved in the substrate, layered cementite remains, and even if held for more than 90 min, there is no change in the quantity of fine cementite with a small aspect ratio that forms the nuclei of spherical cementite. As a result, it only increases costs and decreases productivity.

As already stated, reheating to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) and holding for 5 to 90 minutes in this temperature range means that (Ac ₁ point + 20 ° C.) to T 2 It means that the total holding time in the temperature range is 5 to 90 min.

Next, it is reheated to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.), held in the temperature range for 5 to 90 minutes, and then cooled at a cooling rate of 5 ° C./s or less. This is because when the cooling rate exceeds 5 ° C./s, the cementite is prevented from growing in a spherical shape during cooling, and the austenite is transformed into pearlite, which is a layered structure, so that the layered cementite remains.

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

Example 1
Steel A having the chemical composition shown in Table 3 was melted in a converter, vacuum degassed, continuously cast into bloom, and then subjected to ingot rolling to produce a 160 mm square billet.

  Steel A in Table 3 is SUJ2 steel described in JIS G 4805 (1999).

Next, a part of the 160 mm square billet was used and heated at 1250 ° C. for 60 minutes to perform hot forging at a finishing temperature of 1000 ° C. or higher to obtain a round bar having a diameter of 30 mm. A test piece for measuring the transformation point having a diameter of 3 mm and a length of 10 mm is produced by machining with reference to a portion of the diameter D 1/4 from the center of the cross section of the round bar. The Acm point and Ac ₁ point in the heating process and the Ar ₁ point in the cooling process were measured.

Table 4 shows the Acm point, Ac ₁ point, and Ar ₁ point determined as described above.

  Further, a test piece for hot plate rolling having a cross-sectional shape of 40 mm × 40 mm and a length of 100 mm was cut out from the 160 mm square billet and rolled as shown in Table 5 using a two-roll type reverse rolling mill. A plate material having a thickness of 10 mm was finished depending on the conditions.

  After the final pass of finish rolling, the cooling rate was controlled by changing the air cooling or the cooling medium, and the temperature was cooled to the temperature T1 shown in Table 5.

Of test numbers 1 to 7, test number 9 and test number 10 where temperature T1 is lower than Ar ₁ point, test numbers 1 to 5, test number 9 and test number 10 are from Ar ₁ point to temperature T1. Each of the cooling was performed under air cooling conditions. On the other hand, for Test No. 6 and Test No. 7, the rolling material was wrapped with a heat insulating material to control the cooling.

  Next, after reheating from the temperature T1 to the temperature T2 shown in Table 5 using an electric heating furnace, the temperature was cooled to 400 ° C. at a cooling rate of 1.0 ° C./s.

The holding time described in Table 5 indicates the total holding time in the temperature range of (Ac ₁ point + 20 ° C.) to T2, that is, 740 ° C. to T2, as already described. In addition, when T2 is 740 degreeC, the holding time in 740 degreeC is pointed out.

  The plate material having a thickness of 10 mm subjected to reheating and cooling as described above was subjected to spheroidizing heat treatment using a box-type electric heating furnace in an air atmosphere under the conditions shown in FIG.

  Note that the heat treatment pattern shown in FIG. 6 has a total furnace time halved compared to the heat treatment pattern of FIG. 7 shown as an example of the long-time treatment used as a general spheroidizing heat treatment. This is for investigating the effect of shortening the heat treatment time.

  Next, the microstructure of each plate material having a thickness of 10 mm subjected to the spheroidizing heat treatment was examined by the following method.

  First, after passing through the center of the width of each plate having a thickness of 10 mm and embedding in a resin so that a so-called “longitudinal section” cut out parallel to the rolling direction becomes a test surface, and mirror-polished, picric alcohol (picral The microstructure was corroded at a magnification of 5000 times and a microstructure image was taken with respect to 10 visual fields in the center using a scanning electron microscope (SEM). The area of each visual field is 25 μm × 20 μm.

  Next, using the microstructure image, the major axis L and the minor axis W of each cementite were individually measured by image processing software, and the ratio of cementite having L / W of 2.0 or less was calculated. 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”.

  In addition, since the spheroidization rate in the case of being obtained by a long-time treatment exceeding 20 hours used as a general spheroidizing heat treatment as shown in FIG. It was determined that the spheroidizing heat treatment time could be shortened when the spheroidizing ratio was 85% or more.

  Table 5 also shows the results of the above tests. Note that “◯” in the “Evaluation” column of Table 5 indicates that “the spheroidization rate has reached the target 85%”, that is, the effect of shortening the spheroidizing heat treatment time can be obtained, and “×” Means “the spheroidization rate does not reach 85% of the target”.

  From Table 5, it is clear that in the case of test numbers 1 to 4 that satisfy the conditions defined in the present invention, the evaluation is “◯” and the spheroidizing heat treatment time can be shortened.

  On the other hand, in the case of test numbers 5 to 10 where the manufacturing conditions deviate from the conditions specified in the present invention, the evaluation is “x”, and the effect of shortening the spheroidizing heat treatment time is not recognized.

(Example 2)
A 160 mm square billet of steel A having the chemical composition shown in Table 3 used in Example 1 was 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.

  A test piece having a diameter of 20 mm and a length of 1000 mm was cut out from the above round bar with a diameter of 30 mm produced by hot forging, and the diameter was changed according to the rolling conditions shown in Table 6 using a continuous model mill. Finished into a 12 mm round bar.

  After the final pass of finish rolling, that is, after the pass in the final rolling mill in the finish rolling mill row, the cooling rate was controlled by changing the air cooling or the cooling medium, and the temperature was cooled to the temperature T1 shown in Table 6.

Of test numbers 11 to 17, test number 19 and test number 20 where temperature T1 is lower than Ar ₁ point, test numbers 11 to 15, test number 19 and test number 20 are from Ar ₁ point to temperature T1. Each of the cooling of was performed under the condition of air cooling. On the other hand, for test number 16 and test number 17, the rolling material was wrapped with a heat insulating material to control the cooling.

  Next, after reheating from the temperature T1 to the temperature T2 shown in Table 6 using an electric heating furnace, the temperature was cooled to 400 ° C. at a cooling rate of 1.0 ° C./s.

In addition, the holding time described in Table 6 refers to the total holding time in the temperature range of (Ac ₁ point + 20 ° C.) to T2, that is, 740 ° C. to T2. In addition, when T2 is 740 degreeC, the holding time in 740 degreeC is pointed out.

  A round bar having a diameter of 12 mm subjected to reheating and cooling as described above was subjected to spheroidizing heat treatment using a box-type electric heating furnace in an air atmosphere under the conditions shown in FIG.

  As already described, the heat treatment pattern shown in FIG. 6 is longer than the heat treatment pattern shown in FIG. 7 as an example of a long-time treatment used as a general spheroidizing heat treatment. This is a halved measure for investigating the effect of shortening the spheroidizing heat treatment time.

  Next, the microstructure of each round bar having a diameter of 12 mm subjected to the spheroidizing heat treatment was examined by the following method.

  First, a so-called “longitudinal section” cut through the central axis of each round bar having a diameter of 12 mm and parallel to the rolling direction is embedded in a resin so as to be a test surface, mirror-polished, and then picric alcohol (picral solution). ) And a microstructure image was taken with respect to 10 visual fields in the central part using a scanning electron microscope (SEM) at a magnification of 5000 times. The area of each visual field is 25 μm × 20 μm.

  Next, using the microstructure image, the major axis L and the minor axis W of each cementite are individually measured by image processing software, and the ratio of cementite having L / W of 2.0 or less, that is, “spherical” The conversion rate was calculated.

  As already described, the spheroidization rate when obtained by a long-time treatment exceeding 20 hours used as a general spheroidizing heat treatment as shown in FIG. 7 is about 85%. For this reason, the aim was to obtain a spheroidization rate of 85% or more, and it was determined that the spheroidization heat treatment time could be shortened when the spheroidization rate was 85% or more.

  Table 6 shows the results of the above tests together. Note that “◯” in the “Evaluation” column of Table 6 indicates that “the spheroidization rate has reached the target 85%”, that is, the effect of shortening the spheroidization heat treatment time can be obtained, and “×” Means “the spheroidization rate does not reach 85% of the target”.

  From Table 6, in the case of test numbers 11 to 14 that satisfy the conditions defined in the present invention, the evaluation is “◯”, and it is clear that the spheroidizing heat treatment time can be shortened.

  On the other hand, in the case of test numbers 15 to 20 where the manufacturing conditions deviate from the conditions specified in the present invention, the evaluation is “x”, and the effect of shortening the spheroidizing heat treatment time is not recognized.

  According to the present invention, as hot-rolled, the microstructure is a single-phase structure of pearlite or a mixed structure of a hard phase and pearlite such as bainite, so that cold workability and cutting workability are low, and it exceeds 20 hours. For high carbon chrome bearing steels that have been subjected to such long spheroidizing heat treatment, especially high carbon chrome bearing steel rods or wires, even if the time for spheroidizing heat treatment after hot rolling is shortened It is possible to secure a microstructure that is inferior to that of the conventional spheroidizing heat treatment.

For steels J1 to J7, “lnZ”, which is the natural logarithm of the Zener-Holomon parameter Z, and “ln (1 / d)”, which is the natural logarithm of (1 / d), which is the reciprocal of the prior austenite grain size d. FIG. For steels J1 to J4, steel J6 and steel J7, the cooling rate V from the processing temperature to the Ar ₁ point and the natural logarithmic notation “lnZ” of the Zener-Holomon parameter Z are precipitated in the prior austenite grain boundaries. FIG. FIG. 3 is a specific example of “◯” in FIG. 2 and shows that generation of pro-eutectoid cementite at grain boundaries of fine prior austenite is suppressed for steel J2. FIG. 2 is a specific example of “Δ” in FIG. 2, and shows that although the proeutectoid cementite was formed at the grain boundaries of fine prior austenite for steel J2, the aspect ratio was small and 5.0 or less. . FIG. 3 is a specific example of “x” in FIG. 2 and shows that film-like pro-eutectoid cementite having an aspect ratio of more than 5.0 was generated at grain boundaries of coarse prior austenite for steel J2. In Example 1 and Example 2, it is a figure explaining the spheroidization heat processing pattern performed using the box-type electric heating furnace apparatus of an atmospheric atmosphere on the board | plate material of thickness 10mm, and the round bar of diameter 12mm, respectively. It is a figure which shows an example of the long-time heat processing pattern used as general spheroidization heat processing.

Claims (3)

A hot rolling method for high carbon chromium bearing steel, in mass%, C: 0.7-1.2%, Cr: 0.8-1.8%, Si: 1.2% or less , Mn: 1.5% or less, the balance being Fe and impurities, and Ti in impurities: 0.01% or less, P: 0.03% or less, S: 0.025% or less, Mo : Chemical composition of 0.08% or less, 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 A high carbon chrome bearing steel material having a temperature of Acm is heated to a temperature range equal to or higher than the Acm point and rolling is started. The final pass of finish rolling in the rolling is expressed by a Zener-Holomon parameter represented by the following formula (1). As well as being performed under the conditions satisfying the following formula (2), the surface temperature of the material to be rolled on the rolling mill exit side After finishing under the condition that the temperature T is from Acm point to 1150 ° C., the temperature T to the Ar ₁ point is cooled at a cooling rate of 0.5 ° C./s or more and cooled to the temperature T1 of the Ar ₁ point or less, Reheat from the temperature T1 to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.), hold in this temperature range for 5 to 90 minutes, and then cool at a cooling rate of 5 ° C./s or less. A method of rolling a high carbon chromium bearing steel.
Z = {εdot} × {exp (Q / RT)} (1),
lnZ ≧ 25 (2).
In the equation (1),
ε dot: average strain rate (1 / s) in the final pass of finish rolling,
Q: activation energy (kJ / mol),
R: gas constant, 8.31 × 10 ⁻³ kJ / (mol · K),
T: surface temperature (K) of the material to be rolled on the exit side of the rolling mill in the final pass of finish rolling,
It is. A hot rolling method for high carbon chromium bearing steel by continuous rolling method, in mass%, C: 0.7-1.2%, Cr: 0.8-1.8%, Si: 1 .2% or less, Mn: 1.5% or less, the balance being Fe and impurities, Ti in impurities: 0.01% or less, P: 0.03% or less, S: 0.025 %: Mo: 0.08% or less, 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 A high carbon chromium bearing steel material having a chemical composition is heated to a temperature range of the Acm point or higher to start continuous rolling, and Z is defined as (1) ) As a Zener-Holomon parameter represented by the following formula, it is performed under the condition satisfying the following formula (2). At the same time, after finishing on the condition that the surface temperature T of the material to be rolled on the exit side of the rolling mill is Acm point to 1150 ° C., the temperature T to Ar ₁ point is cooled at a cooling rate of 0.5 ° C./s or more. It is cooled to a temperature T1 of Ar ₁ point or less, reheated from the temperature T1 to a temperature T2 of (Ac ₁ point + 20 ° C.) to (Ac ₁ point + 80 ° C.) and held in the temperature range for 5 to 90 minutes, A method for rolling a high-carbon chromium bearing steel, characterized by cooling at a cooling rate of 5 ° C./s or less.
Z = {εdot} × {exp (Q / RT)} (1),
lnZ ≧ 25 (2).
In the equation (1),
ε dot: average strain rate (1 / s) in the pass in the final rolling mill of finish rolling,
Q: activation energy (kJ / mol),
R: gas constant, 8.31 × 10 ⁻³ kJ / (mol · K),
T: surface temperature (K) of the material to be rolled on the exit side of the rolling mill in the pass in the final rolling mill for finish rolling,
It is. The high carbon chromium bearing steel material is a rod or wire rod, The rolling method of the high carbon chromium bearing steel material according to claim 2.

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