JP5093118B2 - Tool steel manufacturing method - Google Patents

Description

  The present invention relates to a method for producing tool steel.

Many proposals have been made on heat treatment methods for annealing, quenching, and tempering tool steel. In general, the crystal grain size is reduced by repeating transformation. An example is a heat treatment method in which the steel is cooled to a martensite and bainite transformation region after hot working, and then completely austenite transformed at the Ac3 point or higher, annealed, quenched, and tempered.
In addition, the metal structure in the annealed state is preferably a metal structure in which carbides are dispersed as uniformly as possible. For example, tool steel containing C: 0.10 to 2.0% is steel that can be used for many tools including molds. Therefore, it is necessary to adjust the metal structure and mechanical properties by performing heat treatment on the tool steel under optimum heat treatment conditions.
In order to obtain a desired metal structure and mechanical properties more economically and efficiently, a proposal has been made to shift to the next heat treatment during the cooling of hot working represented by hot forging.
For example, as a method for producing medium carbon steel containing 0.25 to 0.55% of C, there is JP 2000-204414 (see Patent Document 1). In this proposal, pearlite transformation is performed in the pearlite transformation temperature range during cooling from forging, and further subjected to normalizing treatment at least once at a temperature of Ac3 point or higher, and then heated to Ac3 point or higher and quenched. Thereafter, it is disclosed that medium carbon steel having excellent strength and toughness is produced by performing tempering once or more.

JP 2000-204414 A

In order to refine crystal grains using the heat treatment method disclosed in Patent Document 1, the effect of making the crystal grains fine can be obtained by performing the normalization process.
In the method of Patent Document 1, since a pearlite treatment is performed in the course of cooling from forging, net-like carbides precipitate at the crystal grain boundaries of the heat-treated material. The net-like carbide precipitated at the grain boundaries causes a problem in that it remains in the subsequent quenching and tempering and inhibits toughness. Therefore, in the method of Patent Document 1, the normalizing process is essential for the purpose of eliminating the net-like carbides and refining the crystal grains by transformation.
An object of the present invention is to produce a tool steel intermediate material capable of making crystal grains fine without requiring a normalizing process, and a method for producing tool steel using the tool steel intermediate material obtained thereby. I will provide a.

The present invention has been made in view of the above problems.
As a result of intensive studies on the cooling conditions after hot working, the present inventors have found that there are the following two important points in the cooling process.
Important point 1: forced cooling around 900 ° C,
Important point 2: Keep isothermal at a lower temperature range than pearlite nose.
As an effect of the above important points 1 and 2,
The forcible cooling near 900 ° C., which is an important point 1, can suppress the precipitation of carbon dissolved in the hot working as a net-like carbide at the grain boundaries during the cooling process. By forced cooling around 900 ° C., there is an effect that a normalizing process for eliminating the net-like carbide is not necessary.
In the temperature range lower than the important point 2 pearlite nose, the carbide nucleation density is high due to the supercooling effect and the diffusion of carbon is slow, so that the carbide can be finely precipitated. In addition, austenite can be completely diffused and transformed into a ferrite and carbide precipitate structure by holding for a long time. There is an effect of making the crystal grains fine by quenching and tempering with the ferrite and carbide precipitate structure.
By combining the knowledge of the important point 1 and the important point 2 described above, it is possible to obtain a tool steel intermediate material before quenching and tempering that is optimal for making the crystal grains fine. And it discovered that the crystal grain diameter of tool steel could be made fine by performing quenching and tempering using the above-mentioned tool steel intermediate material, and reached the present invention.

That is, the present invention provides a hot working step in which hot working is performed by heating a tool steel material containing C: 0.10 to 2.0% to 1050 to 1250 ° C., and after the hot working step, A hot work cooling process for cooling at a cooling rate equal to or higher than air cooling until the surface temperature of the steel material reaches 500 to 700 ° C., and after the hot work cooling process, the tool steel material is inserted into a heating furnace to 400 to After the first heating / holding step for heating / holding to a temperature of 700 ° C. and the first heating / holding step, the tool steel material is heated, and the tool steel material temperature is set to be higher than that of pearlite nose and pearlite nose. A second heating / holding step for heating and holding in this temperature range, and cooling after the second heating / holding step to cool the ferrite structure Tool steel intermediate material with metallized carbides A step, which is followed, method for producing tool steel and a step of performing quenching and tempering for performing quenching and tempering on the tool steel intermediate material.
The chemical composition of a preferable tool steel material contains Si: 2.0% or less, Mn: 2.0% or less, Cr: 1.0 to 15.0%, Mo: 10.0% or less in mass%, Furthermore, it contains any one or more of Ni: 4.0% or less, V: 4.0% or less, W: 20.0% or less, Co: 10.0% or less, with the balance being substantially Fe and The composition is an inevitable impurity.
Further, according to the present invention, when the metallographic structure of the above-described tool steel intermediate material is observed at a magnification of 10000 times, a carbide in which 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² is dense. Tool having a metal structure in which a region having a small number of carbides having a circle equivalent diameter of 0.1 to 0.5 μm and less than 100 carbides is mixed in 100 μm ^{2 with} respect to a region in which the carbide is dense It is a manufacturing method of steel.
Further, the present invention uses the above-mentioned tool steel intermediate material, heats it to a temperature of Ac3 point or higher, quenches it, and thereafter performs tempering once or more to make the average grain size number finer than No. 6. It is a manufacturing method.
Furthermore, according to the other viewpoint of this invention, the hot-work process which heats the tool steel raw material containing C: 0.10-2.0% to 1050-1250 degreeC, and performs hot working, The said heat | fever After the hot working process is finished, the tool steel raw material is charged into the heating furnace after the cooling process in which the surface temperature of the tool steel raw material is 500 to 700 ° C. Then, after the first heating / holding step for heating / holding to a temperature of 400 to 700 ° C. and the first heating / holding step, the tool steel material is heated, and the tool steel material temperature is set to a pearlite nose. A second heating / holding step in which the temperature is raised to a temperature range between 100 ° C. lower than the pearlite nose, and heating / holding is performed in a temperature range between the pearlite nose and a temperature lower than the pearlite nose by 100 ° C. And after the second heating / holding step Performing cooling, and a step to tool steel intermediate material having a metal structure of ferrite structure precipitated carbide, the manufacturing method of a tool steel intermediate material is provided.

  According to the present invention, tool steel having excellent strength and toughness can be obtained by making the average grain size number finer than No. 6.

The reason specified in the present invention will be described in detail below using the heat pattern shown in FIG.
First, in the present invention, a tool steel material containing C: 0.10 to 2.00% is an object of the present invention.
The reason why the C content is 0.10% to 2.00% is that when the C content is less than 0.10%, the C content is too small and C does not diffuse into the crystal grains, and carbides are not present in the crystal grains. It does not precipitate. On the other hand, if it is 2.00% or more, the carbide becomes excessive and the toughness is lowered. Preferably, C: 0.20 to 0.60%.
And the hot-working process which heats the tool steel raw material containing C: 0.10-2.00% to 1050-1250 degreeC and performs hot working (not shown in FIG. 3) is performed. The heating temperature was set to 1050 ° C. or higher in order to obtain a complete austenite structure in consideration of the plastic workability of the tool steel material. Moreover, since there is a possibility that the tool steel material partially melts at 1250 ° C. or higher, the temperature range is set to 1050 to 1250 ° C. Preferably it exists in the range of 1070-1170 degreeC. When heating and holding the tool steel material during the hot working process, the austenite crystal grains grow coarsely as the heating and holding time becomes longer. Therefore, the heating / holding time may be appropriately determined in consideration of the growth of austenite crystal grains, and it is sufficient if it is about 3 to 10 hours.
In the hot working of the tool steel material, forging such as free forging and die forging may be applied. As conditions for hot forging, the hot working end temperature may be in the range where the surface temperature of the tool steel material is 950 to 1050 ° C., and the forging ratio should be larger than 5 in order to accumulate more strain in hot working. preferable.

And after completion | finish of said hot working process, it cools with the cooling rate more than air cooling until the surface temperature of a tool steel raw material becomes 500-700 degreeC ((1) in FIG. 3). The carbide precipitation temperature is shown by line A in FIG.
The tool steel material temperature after hot working is at a temperature at which carbides can be precipitated at the grain boundaries. When carbides are excessively precipitated at the crystal grain boundaries after the hot working is finished, there is a problem that the carbides remain at the crystal grain boundaries even after quenching and tempering, thereby inhibiting toughness. For this reason, it is necessary to urge cooling to a temperature range of 700 ° C. or lower where carbides are unlikely to precipitate at the grain boundaries.
The cooling at this time is performed at a speed that does not affect the region where carbides precipitate at the grain boundaries when continuous cooling is performed. The cooling rate may be about 25 ° C./min. For example, if the cross-sectional dimension of the tool steel material is smaller than about 300 mm (t) × 300 mm (w), air cooling is sufficient. What has a cross-sectional dimension larger than approximately 300 mm (t) × 300 mm (w) may be forcibly cooled using, for example, a large electric fan.
And although it cools to the temperature range of 700 degrees C or less where a carbide | carbonized_material does not precipitate easily at a crystal grain boundary by said cooling, when it cools to an excessively low temperature, austenite may transform | transform into a bainite. If the metal structure is transformed into bainite, there are problems that the precipitation of carbides cannot be controlled by subsequent isothermal holding, and that crystal grains are likely to be coarsened during quenching heating. In order to suppress this, the lower limit of the surface temperature of the tool steel material is set to 500 ° C.

After the above-described cooling step, a tool steel material is put into a heating furnace, and a first heating / holding step is performed in which heating / holding is performed at a temperature of 400 to 700 ° C. ((2) in FIG. 3). This treatment is performed for the purpose of soaking the temperature to a temperature below the pearlite nose up to the center of the tool steel material.
The reason why the first heating / holding temperature is limited to 400 to 700 ° C. is that when the temperature is higher than 700 ° C., carbides are precipitated at the crystal grain boundaries as described above. Further, if the temperature is lower than 400 ° C., the metal structure may be transformed into bainite.
If the heating / holding time is excessively long, transformation to bainite may occur during heating / holding. Therefore, what is necessary is just to determine suitably the time which does not transform a metal structure into a bainite, equalizing to the temperature below a pearlite nose to the center part of the raw material of tool steel. For example, a heating / holding time of about 0.5 to 5 hours is sufficient.

Following the first heating / holding step, the tool steel material is heated ((3) in FIG. 3), and the tool steel material temperature is set to a temperature range between the pearlite nose and a temperature 100 ° C. lower than the pearlite nose. A second heating / holding step for heating and holding in this temperature range is performed ((4) in FIG. 3), and the tool steel material is cooled after the second heating / holding step (in FIG. 3). (5)) to obtain a tool steel intermediate material.
The reason for setting the temperature in the second heating / holding step to a temperature range between pearlite nose and a temperature lower by 100 ° C. than pearlite nose is within the vicinity of the prior austenite grain boundary as shown in FIGS. 1A and 1B within this temperature range. This is because a metal structure in which carbides are dense and carbides are sparsely precipitated in the prior austenite grains is obtained. By making the metal structure in which the carbide is dense in the vicinity of the prior austenite grain boundary and the carbide in the former austenite grain is sparsely precipitated, it is possible to achieve grain refinement by quenching and tempering performed after annealing.
When the temperature of the second heating / holding step is higher than that of the pearlite nose, a pearlite structure in which carbides are dispersed almost uniformly is obtained. On the other hand, at a temperature lower than the temperature lower by 100 ° C. than the pearlite nose, the time until the pearlite transformation is completed becomes longer. There is a problem that it is difficult to obtain an organization. Therefore, in this invention, it was set as the temperature range between 100 degreeC lower temperature than pearlite nose and pearlite nose.
The heating / holding time may be appropriately determined in consideration of holding from the start of pearlite transformation to the end of pearlite transformation, and it is sufficient if it is about 10 to 50 hours. In the present invention, the process from forced cooling after forging to cooling after heating and holding in a temperature range between pearlite nose and a temperature lower by 100 ° C. than pearlite nose is called annealing.

When this method of the present invention is applied, a tool steel intermediate material having a metal structure in which carbides are densely deposited in the vicinity of the prior austenite grain boundaries and carbides are sparsely precipitated in the former austenite grains is obtained. And when quenching with the metal structure in which the carbides are dense in the vicinity of the prior austenite grain boundaries and the carbides are precipitated sparsely in the former austenite grains, new carbides are used regardless of the grain boundaries and within the grains. Austenite is produced. Thereby, it can suppress that a crystal grain grows coarsely at the time of quenching heating, and refinement | miniaturization of a crystal grain can be achieved. Fine grains can be maintained after tempering after quenching.
There are two factors involved in the refinement of crystal grains: the formation of new austenite grains with carbides as the core and the suppression of coarse growth of austenite grains due to the different crystal orientations of adjacent ferrite grains. thinking.
Specifically, in the present invention, the metal structure is a ferrite structure. In the ferrite structure, since the orientation of crystal grains is random, it is considered that during quenching heating, each crystal grain suppresses the growth of each other and the austenite crystal grains are refined. Another mechanism for crystal grain refinement is the formation of new austenite grains with carbides as nuclei. The crystal grains can be refined by suppressing the growth of the grains. The mechanism of crystal grain refinement with carbide as a nucleus will be described in detail later.

The hardness of the tool steel intermediate material obtained by applying the method of the present invention can be 300 HBW or less. Therefore, since the hardness of the material to be heat-treated is low, the workability of the material to be heat-treated is also good.
The metal structure in the tool steel intermediate material is adjusted to a metal structure in which carbides are precipitated in the ferrite structure. As shown in FIGS. 1A and B, the metal structure in which carbide is precipitated in the ferrite structure is a metal structure in which carbides are densely deposited in the vicinity of the prior austenite grain boundary, and carbides are sparsely precipitated in the former austenite grain. Such a state is called a metal structure in which carbides are precipitated in a ferrite structure. In order to confirm this, a test piece for observing a metallographic structure is cut out, and the observation surface is mirror-polished, then corroded and directly observed with a microscope.

In the vicinity of the prior austenite grain boundary, the carbide is dense, and in the former austenite grain, the carbide is sparsely precipitated. For example, when the metal structure is observed with an electron microscope at a magnification of 10,000 times, the equivalent circle diameter in the 100 μm ² is 0.00. With respect to a region where the number of carbides of 1 to 0.5 μm is 300 or more is dense (near the prior austenite grain boundary) and a region where the carbide is dense, an equivalent circle diameter of 0.1 in 100 μm ² A region (within the former austenite grains) in which the number of carbides of ˜0.5 μm is less than 100 or less is mixed.
As for the distribution situation of the carbide described above, the observation of the dense carbide area and the sparse carbide area may be performed in the field of view of 10 or more fields selected at random. Are essential to observe the two areas of the prior austenite grain boundary or the vicinity thereof and the two fields of view of the prior austenite grain.
In addition, as the region where the carbides are dense, the more the carbide having an equivalent circle diameter of 0.1 to 0.5 μm is contained in the range of 300 or more in 100 μm ² , the more the quenching heating becomes the core of austenite generation. The crystal grain refinement by tempering can be expected. Since the upper limit of the number of carbides in the dense carbide region varies depending on, for example, the amount of C contained and the amount of carbide forming elements, it is difficult to set the upper limit, but empirically, it is substantially equivalent to a circle in 100 μm ² The limit is that about 600 carbides having a diameter of 0.1 to 0.5 μm are formed.
Regarding the lower limit of the number of carbides in the region where the carbides are sparse, if the number is less than 200, the effect of crystal grain refinement becomes difficult to obtain. Therefore, the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm in 100 μm ^{2 of} the region B The lower limit is about 200.

The mechanism of crystal grain refinement with carbide as the core is that the carbide-rich region has many fine carbides of 0.1 to 0.5 μm, and the carbide-sparse region is carbide compared to the carbide-dense region. Therefore, by quenching and heating, carbides are dissolved in the matrix (matrix), and a difference in carbon concentration occurs in the matrix. Since the carbide concentration region is higher in the carbide dense region than the carbide sparse region, the transformation point of the carbide dense region is lower than the transformation point of the carbide sparse region. Therefore, the carbide is preferentially transformed from a dense region to austenite.
Austenite in a dense carbide region tends to grow, but since many fine carbides are precipitated, there are many austenite nucleation sites generated by quenching heating, and each crystal grain suppresses grain growth. meet. Next, it is considered that the region where carbides are sparse also transforms to austenite and suppresses the growth of austenite.
In order to more reliably realize the difference between the carbon concentration necessary to obtain a carbide refinement mechanism and the partial carbon concentration, both the dense carbide region and the sparse carbide region are easily dissolved. It is preferable that 80% or more (preferably 90% or more, more preferably 95% or more) of the total number of carbides that can be confirmed when observed at 10000 times is 0.1 to 0.5 μm. It is good that it is the size.
As described above, it is desirable that the number of carbides having a size that is easily dissolved with a difference between the carbon concentration necessary for obtaining a carbide refinement mechanism and a partial carbon concentration should be greater than a specific number. Thus, in the present invention, the carbide structure is dense in the vicinity of the prior austenite grain boundary, and the inside of the prior austenite grain has a metal structure in which the carbide is sparsely precipitated.

Next, a preferred composition of the tool steel material of the present invention will be described. In addition, content is represented by the mass%.
Si: 2.0% or less Si is added as a deoxidizer during melting in tool steel. However, when added in a large amount, the toughness decreases. Therefore, in the present invention, it was made 2.0% or less. Preferably it is 0.15 to 1.20%.
Mn: 2.0% or less Mn is added as a deoxidizing and desulfurizing agent during melting in tool steel. However, when added in a large amount, the toughness decreases. Therefore, in the present invention, it was made 2.0% or less. Preferably it is 0.30 to 1.00%.

Cr: 1.0-15.0%
Cr is added for the purpose of improving hardenability in tool steel and improving tensile strength and toughness. However, if added in a large amount, the toughness is reduced. Therefore, in this invention, it was set as 1.0 to 15.0%. Preferably it is 1.0 to 13.0%.
Mo: 10.0% or less Mo improves hardenability in tool steel. Further, it is added for the purpose of forming fine carbides by tempering and increasing the high-temperature tensile strength. However, if added in a large amount, the toughness is reduced. Therefore, in the present invention, it was made 10.00% or less. Preferably it is 0.20 to 5.00%.

Ni: 4.00% or less Ni is added for the purpose of improving hardenability and improving toughness in tool steel. However, if added in a large amount, the transformation point is lowered and the high-temperature strength is lowered. Therefore, in the present invention, it was made 4.00% or less. Preferably it is 2.0% or less.
V: 4.00% or less V improves the toughness by making crystal grains finer in tool steel. Further, it is added for the purpose of forming a high hardness carbonitride by tempering and increasing the tensile strength. However, if added in a large amount, the toughness is reduced. Therefore, in the present invention, it was made 4.00% or less. Preferably it is 0.10 to 1.10%.

W: 20.00% or less W improves hardenability in tool steel. Further, it is added for the purpose of forming fine carbides by tempering and increasing the high-temperature tensile strength. However, if added in a large amount, the toughness is reduced. Therefore, in the present invention, it was made 4.00% or less. Preferably it is 0.10 to 1.10%.
Co: 10.00% or less Co is added for the purpose of increasing red hot hardness and increasing high-temperature tensile strength in tool steel. In the present invention, it was made 10.00% or less.
The balance is substantially Fe
In the present invention, the elements other than those specified are substantially Fe, but impurities inevitably contained are naturally included. Further, for example, Nb and Ti are effective elements for refining crystal grains, and therefore may be contained in a range of 0.20% or less to the extent that toughness does not deteriorate. Al is an element that accelerates the diffusion of carbon, and has the effect of promoting precipitation of carbides by pearlite transformation. Therefore, Al may be contained in a range of 0.20% or less.

Next, using the tool steel intermediate material obtained by the above-described method of the present invention, it is quenched by heating to a temperature of Ac3 point or higher, and then performing tempering once or more, as shown in FIGS. 2A and 2B. Finer tool steel than No. 6 in grain size number can be obtained.
The reason for quenching by heating to a temperature not lower than the Ac3 point is that if the heating is not performed at a temperature higher than the Ac3 point, it is not completely transformed into austenite and a normal quenched structure cannot be obtained. In addition, the holding time at the time of quenching may be appropriately determined in consideration of the fact that it reaches a predetermined temperature up to the inside of the tool steel intermediate material, completely transforms into austenite, and austenite grains do not grow coarsely, About 0.5 to 3 hours is sufficient.
The quenched intermediate material is tempered one or more times. The number of times of tempering is preferably performed once or more in consideration of obtaining a tempered martensite structure without austenite remaining. Further, the heating and holding time may be appropriately determined in consideration of obtaining the required hardness and strength, and it is sufficient if it is about 1 to 10 hours within the temperature range of 540 to 650 ° C. It is.
By performing this quenching and tempering heat treatment, the average grain size number can be made finer than No. 6. When fine grains having an average grain size number of 6 or more are obtained, there is an effect that toughness is improved. A preferred average grain size is finer than No. 8.

The following examples further illustrate the present invention.
Example 1
First, the tool steel was melted to obtain a 10 Ton steel ingot. The composition is shown in Table 1.
And this steel ingot was divided into 3 and it was set as the tool steel raw material to which this invention method is applied, and the tool steel raw material for a comparative example.

This tool steel material was heated to 1100 ° C. and held for 8 hours. And hot working was performed by hot forging (hot pressing). The processing rate at this time was 18% (forging ratio 5.5), and finished to 350 mm (t) × 350 mm (w) × 1500 mm (l). The hot working finish temperature was 950 ° C. at the surface temperature.
And as a cooling process, forced cooling is performed using a large electric fan until the surface temperature of the tool steel material reaches 550 ° C. ((1) in FIG. 3), and the carbon dissolved in the hot working is cooled. In order to suppress precipitation as net-like carbides at the grain boundaries in the process, forced cooling using a large fan was also performed in the vicinity of 900 ° C. The surface temperature was measured using a radiation thermometer.
After that, as the first heating / holding step, the tool steel material is put into a 450 ° C. heating furnace, held at a temperature of 450 ° C. for 3 hours ((2) in FIG. 3), and then the second heating. In the holding step, the material temperature of the material heated and held at the temperature of 450 ° C. is 700 ° C. (method 1 of the present invention) and 725 ° C. in the temperature range between the pearlite nose and the temperature lower than the pearlite nose by 100 ° C. Inventive method 2), heating to raise the material temperature to a temperature of 800 ° C. (comparative method) higher than the pearlite nose ((3) in FIG. 3), and holding for 20 hours (FIG. 3) (4)) and furnace cooling ((5) in FIG. 3) to obtain a tool steel intermediate material. The temperature of the pearlite nose of the tool steel shown in Table 1 was 775 ° C.

Test pieces for hardness measurement were cut out from the tool steel intermediate material of the present invention and the tool steel intermediate material of the comparative example, and the hardness was measured by a Brinell hardness test. The hardness test results are shown in Table 2.
In addition, when the metal structure observation specimen was cut out and the metal structure was observed, the metal structure to which the method of the present invention was applied had a dense carbide in the vicinity of the prior austenite grain boundary as shown in FIGS. 1A and B. It was confirmed that the inside of the prior austenite grains was a metal structure in which carbides were sparsely precipitated and carbides were precipitated in a ferrite structure. The metal structure of the comparative tool steel intermediate material was a pearlite structure in which carbides were dispersed almost uniformly.

Next, using the tool steel intermediate material and the comparative example tool steel intermediate material obtained by the production method of the present invention, the steel is heated to 1030 ° C. at a temperature of Ac3 or higher, and then tempered at 600 ° C. Was used as a tool steel. The heating and holding time was 2 hours during quenching and 7 hours during tempering.
A specimen for metallographic observation and a specimen for hardness measurement were cut out from the tool steel of the present invention and the tool steel of the comparative example, and the average crystal grain size and hardness were measured. Moreover, the test for the Charpy impact test was cut out and the Charpy impact was measured. These test results are shown in Table 3, and metal structure photographs are shown in FIGS.

  As described above, according to the heat treatment method of the present invention, in the cooling process from hot working, forced cooling is performed with a fan, and then pearlite transformation is performed in a temperature range of minus 100 ° C. from the pearlite nose, thereby forming a ferrite structure. In addition, it is hardened by heating to a temperature of 3 or more points of Ac, and then tempering once or more, so the average grain size number becomes finer than No. 8, and the toughness is greatly improved. There is an effect to.

(Example 2)
The following examples further illustrate the present invention.
First, tool steel having the composition shown in Table 1 was melted to obtain a 10-ton steel ingot. The steel ingot was divided into two and the production method of the present invention was applied.

This tool steel material was heated to 1100 ° C. and held for 8 hours. And hot working was performed by hot forging (hot pressing). The processing rate at this time was 18% (forging ratio 5.5), and finished to 350 mm (t) × 350 mm (w) × 1500 mm (l). The hot working finish temperature was 950 ° C. at the surface temperature.
And as a cooling process, forced cooling is performed using a large electric fan until the surface temperature of the tool steel material reaches 550 ° C. ((1) in FIG. 3), and the carbon dissolved in the hot working is cooled. In order to prevent precipitation as a net-like carbide at the crystal grain boundaries during the process, forced cooling using a large electric fan was also performed near 900 ° C. The surface temperature was measured using a radiation thermometer.
After that, as the first heating / holding step, the tool steel material is put into a 450 ° C. heating furnace, held at a temperature of 450 ° C. for 3 hours ((2) in FIG. 3), and then the second heating. In the holding step, the material temperature of the material heated and held at the temperature of 450 ° C. is 700 ° C. (method 1 of the present invention) and 725 ° C. in the temperature range between the pearlite nose and the temperature lower than the pearlite nose by 100 ° C. Inventive method 2), heating to raise the material temperature to a temperature of 800 ° C. (comparative method) higher than the pearlite nose ((3) in FIG. 3), and holding for 20 hours (FIG. 3) (4)) and furnace cooling ((5) in FIG. 3) to obtain a tool steel intermediate material. The temperature of the pearlite nose of the tool steel shown in Table 1 was 775 ° C.

A test specimen for observing the metal structure was cut out from the tool steel intermediate material, and the metal structure was observed. After 4% nital corrosion, 10 views of the metal structure were observed at 10,000 times using a scanning electron microscope (SEM), and the image was analyzed with image analysis software “WinROOF®” manufactured by Mitani Corporation. ^The number of carbides present in ² and the carbide size were confirmed.
The metal structure obtained in the present invention is shown in FIGS. 4A and 4B. 4A is a 100 × electron micrograph, and FIG. 4B is an enlarged photograph (× 400) of a portion surrounded by a white frame in FIG. 4A. In FIGS. 4 and B, white portions are regions where the carbides are dense, and black portions are regions where the carbides are sparse.
It can be seen that the region where the carbides are dense is seen at and near the prior austenite grain boundary. In addition, the area | region where a carbide | carbonized_material is dense is shown in FIG. 5, and the area | region where a carbide | carbonized_material is sparse is shown in FIG. 5 and 6 are 10000 × electron micrographs.
Table 5 shows the results of the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm in 100 μm ² when the tool steel intermediate material of the present invention is observed at a magnification of 10,000 using a scanning electron microscope (SEM). Show.
In addition, although the test piece for X-ray diffraction was extract | collected from the tool steel intermediate material of this invention, and the presence or absence of residual austenite was confirmed using the wide angle X-ray diffractometer (target Co, voltage 40KV, current 200mA), residual austenite can be confirmed. None (the amount of retained austenite was 0%).

As shown in Table 5, when the metallographic structure is observed at a magnification of 10,000 times, a region in which 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² is a dense region (old austenite grains). Near the boundary) and a region where the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm is less than 100 in 100 μm ² (the inside of the prior austenite grains). It was confirmed that they were mixed.

Next, using the tool steel intermediate material obtained by the production method of the present invention, it was quenched by heating to 1030 ° C. having a temperature of Ac3 or higher, and then tempering was performed once at 600 ° C. The heating and holding time was 2 hours during quenching and 7 hours during tempering.
A specimen for metallographic observation was cut out from the tool steel of the present invention and the tool steel of the comparative example, and the average crystal grain size was measured. As a result, the ASTM crystal grain size number was 9.0.

  According to the heat treatment method of the present invention, forced cooling is performed using a large electric fan in the cooling process from hot working, and then the pearlite transformation is performed in a temperature range between the pearlite nose and a temperature lower than the pearlite nose by 100 ° C. In order to obtain a metal structure in which carbide is precipitated in the ferrite structure, and further quenching by heating to a temperature of Ac3 point or higher, and then tempering once or more, the average grain size number becomes finer than No. 8, resulting in toughness. Has the effect of significantly improving.

  According to the heat treatment method of the present invention, since the crystal grains after quenching and tempering become fine, the present invention can be used for applications requiring toughness of tool steel.

It is the microscope picture after the annealing obtained by this invention method 1. It is the microscope picture after annealing obtained by this invention method 2. FIG. 2 is a photomicrograph after annealing obtained by Comparative Method 1. It is the microscope picture after hardening and tempering obtained by the method 1 of this invention. It is the microscope picture after hardening and tempering obtained by this invention method 2. FIG. 2 is a photomicrograph after quenching and tempering obtained by Comparative Method 1. It is a schematic diagram which shows the heat pattern of this invention. It is an electron micrograph of the metal structure of a tool steel intermediate material. 4B is an enlarged photograph of a portion surrounded by a white frame in FIG. 4A. It is a 10,000 times as many electron micrographs of the metal structure | tissue of a carbide | carbonized_material dense area. It is a 10,000 times as many electron micrographs of the metal structure of a sparse carbide region.

Claims (3)

By mass%, C: 0.10~2.0%, Si : 2.0% or less, Mn: 2.0% or less, Cr: 1.0~15.0%, Mo: 10.0% below Ni: 4.0% or less, V: 4.0% or less, W: 20.0% or less, Co: 10.0% or less, and the balance is Fe and A hot working step of performing hot working by heating the tool steel material, which is an inevitable impurity, to 1050 to 1250 ° C;
After the hot working step is completed, a cooling step of cooling at a cooling rate of air cooling or higher until the surface temperature of the tool steel material becomes 500 to 700 ° C;
After the hot working cooling step, a first heating / holding step in which the tool steel material is inserted into a heating furnace and heated / held at a temperature of 400 to 700 ° C .;
Subsequent to the first heating / holding step, the tool steel material is heated to raise the tool steel material temperature to a temperature range between a pearlite nose and a temperature lower by 100 ° C. than the pearlite nose, A second heating / holding step for heating / holding in a temperature range between 100 ° C. lower than the pearlite nose;
When cooling is performed after the second heating / holding step and the metal structure is observed at a magnification of 10,000, 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² . A ferrite in which a carbide-dense region and a region in which the number of carbides having a circle-equivalent diameter of 0.1 to 0.5 μm is less than 100 is small in 100 μm ² are mixed with the carbide-dense region. A step of forming a tool steel intermediate material having a metal structure in which carbide is precipitated in the structure;
A method for producing tool steel, comprising a step of quenching and tempering the tool steel intermediate material. The tool steel intermediate material, and hardening by heating to a temperature above Ac3 point, then, the production of tool steel according to claim 1, the fine from the sixth with an average grain size number of tempered once or more Method. By mass%, C: 0.10~2.0%, Si : 2.0% or less, Mn: 2.0% or less, Cr: 1.0~15.0%, Mo: 10.0% below Ni: 4.0% or less, V: 4.0% or less, W: 20.0% or less, Co: 10.0% or less, and the balance is Fe and A hot working step of performing hot working by heating the tool steel material, which is an inevitable impurity, to 1050 to 1250 ° C;
After the hot working step is completed, a cooling step of cooling at a cooling rate of air cooling or higher until the surface temperature of the tool steel material becomes 500 to 700 ° C;
After the hot working cooling step, a first heating / holding step in which the tool steel material is inserted into a heating furnace and heated / held at a temperature of 400 to 700 ° C .;
Subsequent to the first heating / holding step, the tool steel material is heated to raise the tool steel material temperature to a temperature range between a pearlite nose and a temperature lower by 100 ° C. than the pearlite nose, A second heating / holding step for heating / holding in a temperature range between 100 ° C. lower than the pearlite nose;
When cooling is performed after the second heating / holding step and the metal structure is observed at a magnification of 10,000, 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² . A ferrite in which a carbide-dense region and a region in which the number of carbides having a circle-equivalent diameter of 0.1 to 0.5 μm is less than 100 is small in 100 μm ² are mixed with the carbide-dense region. A method for producing a tool steel intermediate material, comprising a step of forming a tool steel intermediate material having a metal structure in which carbide is precipitated in the structure.

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