CN110993040B - Method for determining critical value of 30Cr2Ni4MoV steel converted from cast state to forged state - Google Patents
Method for determining critical value of 30Cr2Ni4MoV steel converted from cast state to forged state Download PDFInfo
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- CN110993040B CN110993040B CN201911195690.XA CN201911195690A CN110993040B CN 110993040 B CN110993040 B CN 110993040B CN 201911195690 A CN201911195690 A CN 201911195690A CN 110993040 B CN110993040 B CN 110993040B
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- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- B21B2265/00—Forming parameters
- B21B2265/10—Compression, e.g. longitudinal compression
Abstract
The invention provides a method for determining a critical value of a 30Cr2Ni4MoV steel from a cast state to a forged state, and relates to the technical field of free forging forming. The method comprises the steps of firstly adopting numerical simulation software DEFORM to carry out numerical simulation on the forging process of the as-cast 30Cr2Ni4MoV steel, then actually forging the as-cast steel on a hydraulic press, and then determining the critical value of the conversion of the as-cast state of the material into the forging state. The method simulates the forging process of a similar cavity test piece, sets different forging ratios, observes the closing condition of the cavity through compression deformation, and simultaneously forges a non-cavity test piece for observing the change of coarse grains; when the hollow in the test piece with the hollow can be closed, the coarse grains are refined, and the mechanical property is recovered to be more than 95% of that of the test piece without the hollow, the corresponding forging ratio is the critical value of the transition from the cast state to the forged state.
Description
Technical Field
The invention relates to the technical field of free forging forming, in particular to a method for determining a critical value of a 30Cr2Ni4MoV steel converted from a cast state to a forged state.
Background
The 30Cr2Ni4MoV steel is a medium-alloy low-pressure rotor steel, is mainly used as a steam turbine rotor material, and has the characteristics of high hardenability, good comprehensive mechanical property and hot workability and the like. As-cast 30Cr2Ni4MoV steel has interstitial defects such as coarse grains, cracks, cavities and the like, if the defects existing in as-cast materials in the forging process cannot be completely eliminated, the mechanical property of the forge piece is seriously deteriorated, and the service life of the forge piece in the service process is shortened.
In actual forging, whether the as-cast 30Cr2Ni4MoV steel can be completely converted into a forged state, namely whether a coarse as-cast structure can be converted into a fine and uniform as-forged structure, whether defects such as cavities, inclusions, looseness and the like in the center can be eliminated, whether cracks in a forging piece can be repaired or not and the method has important significance for the quality of a final product. At present, the judgment of the critical value from the casting state to the forging state in the material forging mainly depends on experience, and an accurate method is not available.
Disclosure of Invention
In view of the above, the present invention is directed to a method for determining the as-cast transformation threshold of 30Cr2Ni4MoV steel into as-forged steel. The method provided by the invention is accurate and reliable, changes the method for judging the critical value from the cast state to the forging state in the material forging process by depending on experience, and has important significance for actual production.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides a method for determining a critical value of a 30Cr2Ni4MoV steel from a casting state to a forging state, which comprises the following steps:
(1) Manufacturing a plurality of 30Cr2Ni4MoV castings with cavities inside to simulate cast 30Cr2Ni4MoV steel, and manufacturing a plurality of 30Cr2Ni4MoV castings without cavities inside; the 30Cr2Ni4MoV castings are the same in size;
(2) Numerical simulation is carried out on the forging process of the as-cast 30Cr2Ni4MoV steel by adopting numerical simulation software DEFORM:
setting different forging ratios by taking the forging temperature in the actual production of the 30Cr2Ni4MoV steel as the deformation temperature, and respectively carrying out simulated forging on a 30Cr2Ni4MoV casting with a cavity inside and a 30Cr2Ni4MoV casting without a cavity inside at the deformation temperature and the different forging ratios; under the same forging ratio, respectively forging 1 piece of 30Cr2Ni4MoV casting with a cavity inside and 1 piece of 30Cr2Ni4MoV casting without a cavity inside;
observing the closing condition of the cavity of the 30Cr2Ni4MoV casting with the cavity in the interior under different forging ratios through a post-processing window of numerical simulation software DEFORM; observing the conditions that the 30Cr2Ni4MoV casting without a cavity inside is subjected to dynamic recrystallization and coarse grain refinement under different forging ratios; determining the forging ratio of closed cavity and coarse grain refinement:
(3) The as-cast 30Cr2Ni4MoV steel is actually forged on a hydraulic press:
setting the deformation temperature same as that in the step (2), actually forging the 30Cr2Ni4MoV casting with the cavity inside and the 30Cr2Ni4MoV casting without the cavity inside respectively by using a hydraulic machine under the forging ratio determined in the step (2), and putting the obtained test piece into water for quenching after the forging is finished;
respectively cutting the obtained test pieces, observing the closing condition of the cavity, the grain distribution around the cavity and whether cracks formed after the cavity is closed completely disappear in the grains of the 30Cr2Ni4MoV casting with the cavity in the inner part under different forging ratios, and taking the complete disappearance of the cracks as a mark for completely eliminating the cavity; meanwhile, the conditions of dynamic recrystallization and coarse grain refinement of the 30Cr2Ni4MoV casting without a cavity inside under different forging ratios are observed, and a test piece with stable grain size is used as a reference for grain refinement and uniformity;
and respectively carrying out mechanical property test on the obtained test pieces;
(4) Determining the critical value of the cast-state transformation of the 30Cr2Ni4MoV steel into the forged state:
when the 30Cr2Ni4MoV casting with the cavity inside is actually forged at a certain forging ratio, the cavity is thoroughly eliminated, the crystal grains are uniform and refined, and the mechanical property is recovered to more than 95% of the mechanical property of the 30Cr2Ni4MoV casting without the cavity inside after actual forging, the 30Cr2Ni4MoV steel is converted from an as-cast state to a as-forged state, and the corresponding forging ratio is the critical value of the 30Cr2Ni4MoV steel converted from the as-cast state to the as-forged state.
Preferably, the forging in the step (2) and the step (3) adopts four-pass overturning in the range of 90-90 degrees.
Preferably, when the mechanical property test is performed in the step (3), a sample obtained after the 30Cr2Ni4MoV casting with the cavity therein is forged is sampled, and the sample is taken so that the cavity is located at the middle position.
Preferably, when the mechanical property test is performed in the step (3), a sample obtained after the 30Cr2Ni4MoV casting without a cavity inside is forged is sampled, and the samples are respectively taken from the center and the outermost part of the sample.
Preferably, the mechanical property test in the step (3) comprises a tensile property test and an impact property test.
Preferably, the tensile property tests include elongation, reduction of area, yield strength and tensile strength tests.
Preferably, the size of the crystal grains in the step (4) is less than 60 μm.
The invention provides a method for determining a critical value of a 30Cr2Ni4MoV steel converted from an as-cast state to a forged state, which comprises the steps of firstly adopting numerical simulation software DEFORM to carry out numerical simulation on the forging process of the as-cast 30Cr2Ni4MoV steel, then actually forging the as-cast 30Cr2Ni4MoV steel on a hydraulic press, and then determining the critical value of the 30Cr2Ni4MoV steel converted from the as-cast state to the forged state. The method has the advantages that the internal of the as-cast 30Cr2Ni4MoV steel has void defects such as voids and looseness, the forging process of a similar void test piece is simulated, different forging ratios are set, the closing condition of the voids is observed through compression deformation, and meanwhile, a void-free test piece is forged and used for observing the change condition of coarse grains; when the hollow in the test piece with the hollow can be closed, the coarse grains are refined, and the mechanical property is recovered to more than 95% of that of the test piece without the hollow, the material is converted from the cast state to the forged state, and the corresponding forging ratio is the critical value of converting the cast state of the 30Cr2Ni4MoV steel into the forged state. The method provided by the invention is accurate and reliable, changes the method of judging the critical value from the cast state to the forging state in the material forging by depending on experience, and has important significance for actual production.
Drawings
Fig. 1 is a schematic view of four-pass upset forging of the present invention, wherein 1 represents an upper die, 2 represents a workpiece, 3 represents a lower die, a represents a first pass compression, b represents a second pass compression after the first pass compression specimen is turned by 90 °, c represents a third pass compression after the second pass compression specimen is turned by 90 °, and d represents a fourth pass compression after the third pass compression specimen is turned by 90 °.
Detailed Description
The invention provides a method for determining a critical value of a 30Cr2Ni4MoV steel from a casting state to a forging state, which comprises the following steps:
(1) Manufacturing a plurality of 30Cr2Ni4MoV castings with cavities inside to simulate cast 30Cr2Ni4MoV steel, and manufacturing a plurality of 30Cr2Ni4MoV castings without cavities inside; the 30Cr2Ni4MoV castings are the same in size;
(2) Numerical simulation software DEFORM is adopted to carry out numerical simulation on the forging process of the as-cast 30Cr2Ni4MoV steel:
setting different forging ratios by taking the forging temperature in the actual production of the 30Cr2Ni4MoV steel as the deformation temperature, and respectively carrying out simulated forging on a 30Cr2Ni4MoV casting with a cavity inside and a 30Cr2Ni4MoV casting without a cavity inside at the deformation temperature and the different forging ratios; under the same forging ratio, respectively forging 1 piece of 30Cr2Ni4MoV casting with a cavity inside and 1 piece of 30Cr2Ni4MoV casting without a cavity inside;
observing the closing condition of the cavity of the 30Cr2Ni4MoV casting with the cavity in the interior under different forging ratios through a post-processing window of numerical simulation software DEFORM; and observing the conditions that the 30Cr2Ni4MoV casting without the inner cavity is dynamically recrystallized and coarse grains are refined under different forging ratios; determining the forging ratio of closed cavities and coarse grain refinement;
(3) The cast 30Cr2Ni4MoV steel is actually forged on a hydraulic press:
setting the deformation temperature same as that in the step (2), actually forging the 30Cr2Ni4MoV casting with the cavity inside and the 30Cr2Ni4MoV casting without the cavity inside respectively by using a hydraulic machine under the forging ratio determined in the step (2), and putting the obtained test piece into water for quenching after the forging is finished;
respectively cutting the obtained test pieces, observing the closing condition of the cavity, the grain distribution around the cavity and whether cracks formed after the cavity is closed completely disappear in the grains of the 30Cr2Ni4MoV casting with the cavity in the inner part under different forging ratios, and taking the complete disappearance of the cracks as a mark for completely eliminating the cavity; meanwhile, the conditions of dynamic recrystallization and coarse grain refinement of the 30Cr2Ni4MoV casting without a cavity inside under different forging ratios are observed, and a test piece with stable grain size is used as a reference for grain refinement and uniformity;
and respectively carrying out mechanical property test on the obtained test pieces;
(4) Determining the critical value of the cast-state transformation of the 30Cr2Ni4MoV steel into the forged state:
when the 30Cr2Ni4MoV casting with the cavity inside is actually forged at a certain forging ratio, the cavity is thoroughly eliminated, the crystal grains are uniform and refined, and the mechanical property is recovered to more than 95% of the mechanical property of the 30Cr2Ni4MoV casting without the cavity inside after actual forging, the 30Cr2Ni4MoV steel is converted from an as-cast state to a as-forged state, and the corresponding forging ratio is the critical value of the 30Cr2Ni4MoV steel converted from the as-cast state to the as-forged state.
The invention manufactures a plurality of 30Cr2Ni4MoV castings with cavities inside and manufactures a plurality of 30Cr2Ni4MoV castings without cavities inside. In the invention, the sizes and the forms of the holes of the 30Cr2Ni4MoV castings with the holes inside are the same or similar. The invention simulates the cast 30Cr2Ni4MoV steel by manufacturing the 30Cr2Ni4MoV casting with the internal cavity, and observes the change condition of coarse grains by manufacturing the 30Cr2Ni4MoV casting without the internal cavity.
The invention adopts numerical simulation software DEFORM to carry out numerical simulation on the forging process of the as-cast 30Cr2Ni4MoV steel. The present invention has no particular requirements for the numerical simulation software DEFORM, and DEFORM software known in the art may be used. In the numerical simulation, the forging temperature in the actual production of the 30Cr2Ni4MoV steel is used as the deformation temperature, in the specific embodiment of the invention, the deformation temperature of the 30Cr2Ni4MoV steel is set to 1200 ℃, in the numerical simulation, the invention sets different forging ratios, and the critical value of the transformation from the as-cast state to the forging state is screened from the forging ratios; the number of the forging ratios is not particularly required, and the forging ratios can be set according to actual conditions; in the embodiment of the present invention, the forging ratios are preferably set to 1.1,1.6,1.8,2.0, and 2.2, respectively. Respectively carrying out simulated forging on a 30Cr2Ni4MoV casting with a cavity inside and a 30Cr2Ni4MoV casting without the cavity inside at the deformation temperature and different forging ratios; under the same forging ratio, 1 piece of 30Cr2Ni4MoV casting with a cavity inside and 1 piece of 30Cr2Ni4MoV casting without a cavity inside are respectively forged. In the present invention, the forging is preferably carried out by four-pass turning in the range of 90-90 degrees, the turning process being shown in fig. 1. The casting state 30Cr2Ni4MoV steel has void defects such as voids and looseness, similar voids are simulated to forge 30Cr2Ni4MoV castings with the voids inside, and the closing conditions of the voids of the 30Cr2Ni4MoV castings with the voids inside under different forging ratios are observed; and observing the conditions that the 30Cr2Ni4MoV casting without the inner cavity is dynamically recrystallized and coarse grains are refined under different forging ratios; and determining the forging ratio of closed cavities and coarse grain refinement.
The invention actually forges the as-cast 30Cr2Ni4MoV steel on a hydraulic press. In the present invention, the hydraulic machine is preferably a 500t hydraulic machine. The method adopts the deformation temperature which is the same as that in the numerical simulation process, and utilizes a hydraulic press to actually forge the 30Cr2Ni4MoV casting with the cavity inside and the 30Cr2Ni4MoV casting without the cavity inside respectively at the forging ratio determined in the numerical simulation process, and the obtained test piece is put into water for quenching after the forging. In the present invention, it is preferable to observe the distribution of crystal grains around the voids, whether or not cracks formed after the closure of the voids completely disappear inside the crystal grains, and the occurrence of dynamic recrystallization and coarse grain refinement under an optical microscope after polishing and corroding the obtained test piece. In the invention, the corrosion solution for corrosion is preferably a mixed solution of saturated picric acid and a detergent; the corrosion is preferably carried out under the condition of water bath at 40 ℃, and the corrosion time is preferably 3-5 min; after corrosion, the invention preferably wipes the corroded test piece clean with alcohol. When mechanical property testing is carried out, a test piece obtained after a 30Cr2Ni4MoV casting with a cavity inside is forged is sampled, and the obtained test piece preferably enables the cavity to be located at the middle position; sampling a test piece obtained after the 30Cr2Ni4MoV casting without a cavity inside is forged, wherein the taken samples are preferably respectively arranged at the core part and the outermost part of the test piece, and in the specific operation, one sample is preferably taken at the core part and one sample is preferably taken at the outer part of the test piece respectively so as to judge whether the mechanical properties of the core part and the outer part are consistent. In the present invention, the mechanical property test preferably includes a tensile property test and an impact property test; the tensile property test preferably comprises elongation, reduction of area, yield strength and tensile strength tests; the impact performance test is used to test the impact absorption work of a material. The invention respectively tests the mechanical properties of the obtained test pieces, and is used for comparing the recovery condition of the mechanical properties of the 30Cr2Ni4MoV casting with a cavity inside with the test pieces of the 30Cr2Ni4MoV casting without the cavity inside after forging under different forging ratios.
After numerical simulation and actual forging are completed, the method determines the critical value of the transition of the cast state of the 30Cr2Ni4MoV steel to the forged state. When the 30Cr2Ni4MoV casting with the cavity inside is actually forged at a certain forging ratio, the cavity is thoroughly eliminated, the crystal grains are uniform and refined, and the mechanical property is recovered to more than 95% of the mechanical property of the 30Cr2Ni4MoV casting without the cavity inside after actual forging, the 30Cr2Ni4MoV steel is converted from an as-cast state to a forged state, and the corresponding forging ratio is the critical value of the 30Cr2Ni4MoV steel converted from the as-cast state to the forged state. In the present invention, the size of the crystal grains is preferably 60 μm or less.
The method for determining the transition of the casting state of the 30Cr2Ni4MoV steel into the forging state critical value is accurate and reliable, changes the method of judging the critical value from the casting state to the forging state in material forging by depending on experience, and has important significance for actual production.
The method for determining the as-cast transformation critical value of the 30Cr2Ni4MoV steel provided by the present invention is described in detail with reference to the following examples, but they should not be construed as limiting the scope of the present invention.
Example 1
A method for determining the as-cast to as-forged critical value of 30Cr2Ni4MoV steel comprises the following steps:
(1) Manufacturing a 30Cr2Ni4MoV casting with a cavity inside to simulate cast 30Cr2Ni4MoV steel, and manufacturing a 30Cr2Ni4MoV casting without a cavity inside; the sizes of the 30Cr2Ni4MoV castings are the same;
(2) Numerical simulation is carried out on the forging process of the as-cast 30Cr2Ni4MoV steel by adopting numerical simulation software DEFORM: the forging is carried out in four passes, the schematic drawing of the forging process is shown in figure 1, the forging temperature is set to 1200 ℃, the forging ratios are respectively set to 1.1,1.6,1.8,2.0 and 2.2, and the 30Cr2Ni4MoV casting with a cavity inside and the 30Cr2Ni4MoV casting without a cavity inside are respectively subjected to simulated forging under different forging ratios; under the same forging ratio, 1 piece of each of a 30Cr2Ni4MoV casting with a cavity inside and a 30Cr2Ni4MoV casting without a cavity inside is forged. Observing the closing condition of the cavity of the 30Cr2Ni4MoV casting with the cavity in the interior under different forging ratios through a post-processing window of numerical simulation software DEFORM; observing the conditions that the 30Cr2Ni4MoV casting without a cavity inside is subjected to dynamic recrystallization and coarse grain refinement under different forging ratios; determining the forging ratio of closed cavities and coarse grain refinement;
(3) The actual forging was carried out on a 500t hydraulic press as in the simulated forging process: and (3) the forging temperature and the overturning process are consistent with the simulation, the 30Cr2Ni4MoV casting with the cavity inside and the 30Cr2Ni4MoV casting without the cavity inside are actually forged by utilizing a hydraulic machine under the forging ratio determined in the step (2), and the obtained test piece is placed into water for quenching after the forging is finished. A test piece obtained after a 30Cr2Ni4MoV casting with a cavity inside is forged is cut, the closing condition of the cavity under different deformation process parameters is observed, a small sample is taken out and polished, then the small sample is placed under an optical microscope to observe the evolution law of the cavity, whether cracks formed after the cavity is closed are completely repaired or not is judged, and after the sample is corroded, the distribution of crystal grains around the cavity and whether the formed cracks completely disappear in the crystal grains or not are observed, and the complete disappearance of the cracks is used as a mark for completely eliminating the cavity. And simultaneously cutting a test piece obtained after the forging of the 30Cr2Ni4MoV casting without a cavity inside, polishing and corroding, and observing the dynamic recrystallization condition of the as-cast coarse crystal under different deformation and the grain refining condition by using an optical microscope to find out the critical process condition when the grain size tends to be stable.
And taking out a tensile sample and an impact sample from the actually forged test piece, and performing a mechanical property test, wherein when the test piece obtained after the 30Cr2Ni4MoV casting with a cavity inside is forged, the cavity position is ensured to be in the middle of the mechanical property test piece, and when the test piece obtained after the 30Cr2Ni4MoV casting without the cavity inside is forged, the sampling positions are respectively in the center and the outermost part. After sampling, the elongation, the reduction of area, the yield strength and the tensile strength of the material are tested by a tensile test, and the impact absorption work of the material is tested by an impact test.
(4) Compared with the situation that the mechanical property of the 30Cr2Ni4MoV casting with the cavity inside is recovered after being compressed by different deformation amounts, the mechanical property of the 30Cr2Ni4MoV casting without the cavity inside is recovered to be more than 95% of the mechanical property after being actually forged, the cavity of the test piece is thoroughly eliminated, coarse grains on the whole section are refined, the grains are uniform, the size of the grains reaches below 60 mu m, and the as-cast state of the material can be considered to be converted into the forged state. In the embodiment, the critical process parameters of the 30Cr2Ni4MoV steel from the cast state to the forged state are the forging temperature of 1200 ℃ and the forging ratio of 2.2.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (7)
1. A method for determining a critical value of a 30Cr2Ni4MoV steel from an as-cast state to a as-forged state, comprising the steps of:
(1) Manufacturing a plurality of 30Cr2Ni4MoV castings with cavities inside to simulate as-cast 30Cr2Ni4MoV steel, and manufacturing a plurality of 30Cr2Ni4MoV castings without cavities inside; the 30Cr2Ni4MoV castings are the same in size;
(2) Numerical simulation software DEFORM is adopted to carry out numerical simulation on the forging process of the as-cast 30Cr2Ni4MoV steel:
setting different forging ratios by taking the forging temperature in the actual production of the 30Cr2Ni4MoV steel as the deformation temperature, and respectively carrying out simulated forging on a 30Cr2Ni4MoV casting with a cavity inside and a 30Cr2Ni4MoV casting without a cavity inside at the deformation temperature and the different forging ratios; under the same forging ratio, respectively forging 1 piece of 30Cr2Ni4MoV casting with a cavity inside and 1 piece of 30Cr2Ni4MoV casting without a cavity inside;
observing the closing condition of the cavity of the 30Cr2Ni4MoV casting with the cavity in the interior under different forging ratios through a post-processing window of numerical simulation software DEFORM; and observing the conditions that the 30Cr2Ni4MoV casting without the inner cavity is dynamically recrystallized and coarse grains are refined under different forging ratios; determining the forging ratio of closed cavities and coarse grain refinement;
(3) The as-cast 30Cr2Ni4MoV steel is actually forged on a hydraulic press:
setting the same deformation temperature as that in the step (2), actually forging the 30Cr2Ni4MoV casting with the cavity inside and the 30Cr2Ni4MoV casting without the cavity inside respectively by using a hydraulic machine at the forging ratio determined in the step (2), and quenching the obtained test piece in water after forging;
respectively cutting the obtained test pieces, observing the closing condition of the cavity, the grain distribution around the cavity and whether cracks formed after the cavity is closed completely disappear in the grains of the 30Cr2Ni4MoV casting with the cavity in the inner part under different forging ratios, and taking the complete disappearance of the cracks as a mark for completely eliminating the cavity; meanwhile, observing the conditions of dynamic recrystallization and coarse grain refinement of the 30Cr2Ni4MoV casting without a cavity inside under different forging ratios, and taking a test piece with stable grain size as a reference for grain refinement and uniformity;
and respectively carrying out mechanical property test on the obtained test pieces;
(4) Determining the critical value of the cast-state transformation of the 30Cr2Ni4MoV steel into the forged state:
when the 30Cr2Ni4MoV casting with the cavity inside is actually forged at a certain forging ratio, the cavity is thoroughly eliminated, the crystal grains are uniform and refined, and the mechanical property is recovered to more than 95% of the mechanical property of the 30Cr2Ni4MoV casting without the cavity inside after actual forging, the 30Cr2Ni4MoV steel is converted from an as-cast state to a forged state, and the corresponding forging ratio is the critical value of the 30Cr2Ni4MoV steel converted from the as-cast state to the forged state.
2. The method according to claim 1, wherein the forging in step (2) and step (3) are turned in four passes of 90 ° -90 ° -90 ° -90 °.
3. The method according to claim 1, wherein the mechanical property test in step (3) is carried out by sampling a sample obtained after forging of a 30Cr2Ni4MoV casting having a hollow inside, the sample being taken such that the hollow is located at a middle position.
4. The method according to claim 1, wherein in the step (3), when the mechanical property test is performed, a sample obtained after the 30Cr2Ni4MoV casting without a cavity inside is forged is sampled, and the samples are respectively taken at the center part and the outermost part of the sample.
5. The method of claim 1, wherein the mechanical property test in step (3) comprises a tensile property test and an impact property test.
6. The method of claim 5, wherein the tensile property tests include elongation, reduction of area, yield strength, and tensile strength tests.
7. The method according to claim 1, wherein the size of the crystal grains in the step (4) is 60 μm or less.
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