JP4423608B2 - Hardened tool steel material - Google Patents

Description

  The present invention relates to a tool steel material for quenching having a metal structure capable of obtaining uniform fine crystal grains by quenching and tempering.

As a technique for obtaining uniform fine crystal grains by quenching and tempering, it is possible to repeat transformation in the process of manufacturing a tool steel material for quenching. An example of this method is a heat treatment method in which the steel is cooled to a martensite and bainite transformation region after hot working, then completely austenite transformed at the Ac3 point or higher, annealed, and then quenched and tempered.
In addition, the metal structure in the annealed state of the tool steel material for quenching is preferably a metal structure in which carbides are uniformly dispersed.
However, unexplained abnormal grain growth has been reported, including (1) reverse martensitic transformation theory (Non-patent document 1), (2) crystal orientation regulation theory (Non-patent document 2), (3 ) The retained austenite coalescence theory (Non-Patent Document 3) is known.

Reverse martensitic transformation theory: Matsuda et al .; Iron and steel 60, 2 (1974) p. 226. Crystal orientation regulation theory: Watanabe et al .; Iron and steel 61, 1 (1975) p. 96. Residual austenite coalescence theory: T.A. Kimmins and D. J. et al. Gooch; Metal science 17, (1983) p. 519.

In the conventional crystal grain refinement method, even if the metal structure in the annealed state is a uniform structure, a phenomenon that uniform fine crystal grains cannot be obtained after quenching and tempering may occur. This is likely to occur when conditions such as the holding temperature, heating rate, and cooling rate of the quenching process are inappropriate, and as a solution to these problems, measures have been taken by reviewing the holding temperature, rapid heating, and rapid cooling. .
However, with the recent increase in size of products, there have been cases where these measures cannot be physically addressed. For example, if the heating rate is slow, the crystal grains grow coarsely, so measures are taken to increase the heating rate as much as possible, but the heat transfer coefficient of the product is constant, and the heating rate becomes slower as the product becomes larger. Also, the cooling rate tended to be slow. Further, abnormal growth of crystal grains of unknown cause has been reported, and there has been a problem that it is impossible to obtain completely uniform fine crystal grains.
The objective of this invention is providing the tool steel raw material for hardening which has a metal structure which can obtain a uniform fine crystal grain more reliably by quenching and tempering.

The present invention has been made in view of the above problems.
That is, in the present invention, C: 0.1 to 0.8% , Si: 2.0% or less, Mn: 2.0% or less, Cr: 1.0 to 15.0%, Mo: 10. 0% or less, Ni: 4.0% or less, V: 4.0% or less, the balance is a tool steel material for quenching made of Fe and inevitable impurities , and the metallographic structure of the tool steel material is observed at a magnification of 10,000 times the time, and carbides dense areas a carbides number of equivalent circle diameter 0.1~0.5μm in 100 [mu] m ² is formed at least 300, with respect to the region a, circle equivalent diameter in the 100 [mu] m ² It is a tool steel material for quenching made of a metal structure in which the number of carbides of 0.1 to 0.5 μm is mixed with a region B having a small number of carbides less than 100.
Preferably, the above-described tool steel material is a tool steel material for quenching having a retained austenite of 1% or less (including 0) as measured by X-ray diffraction .

  According to the present invention, by controlling the metallographic structure of the tool steel material for quenching with a completely different concept from the knowledge of uniformly dispersing carbide in conventional crystal grain refinement, it is possible to more reliably obtain uniform crystal grains. Can be obtained.

As described above, the greatest feature of the present invention resides in the metal structure of a tool steel material for quenching that can obtain uniform fine crystal grains by quenching and tempering. The present invention is described in detail below.
First, in this invention, the tool steel raw material for hardening containing C: 0.1-0.8% by mass% is made into the object of this invention. The reason why the C content is 0.1 to 0.8% is that when the C content is less than 0.1%, the C content is too small, particularly in the crystal grains, C is insufficient and carbides are not precipitated. If it is .8% or more, C becomes excessive, there is no difference between the carbide dense region and the carbide sparse region, and it becomes impossible to obtain a metal structure for obtaining uniform fine crystal grains by quenching and tempering. is there. Preferably it is C: 0.2-0.6%, More preferably, it is 0.25-0.55%.

Next, the metal structure will be described. In the metal structure, the tool steel material for quenching according to the present invention is greatly different from the tool steel material for quenching which has been said to be preferable in the past.
A specimen for metal structure observation was cut out from a large steel material of 350 mm (t) × 350 mm (w) × 1500 mm (l) as a tool steel material for quenching adjusted to a metal structure that has been considered preferable in the past and observed. The metal structure of the tool steel material for quenching obtained by the conventional method (annealed state before quenching) is shown in FIG.
Observed after 4% nital corrosion test pieces cut out from the tool steel material for hardening, which was observed at 10,000-fold using a scanning electron microscope (SEM), circle equivalent diameter from 0.1 to 0 in 100 [mu] m ^2. A metal structure in which 100 to 150 carbides of 5 μm are uniformly dispersed. The chemical composition of the conventional material is C: 0.39%, Si: 0.96%, Mn: 0.43%, Cr: 5.11%, Mo: 1.21%, Ni: 0.23%, V: 0.81% contained with the balance being Fe, and the metal structure adjustment method will be shown in the examples described later.

Then, the metal structure prescribed | regulated by this invention is demonstrated. The chemical composition is the same as the above-mentioned conventional material.
The metal structure of the present invention has a dense region A in which 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² when observed at a magnification of 10,000 times. The region A exhibits a metal structure in which a region B in which the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm is less than 100 is sparse is mixed in 100 μm ² .
That is, the metal structure defined in the present invention has a carbide dense region in which a larger number of carbides than the number of carbides found in the metal structure of the conventional material are formed, and the carbides are dense. It exhibits a special metal structure called dispersion.
When the metallographic structure is observed at a magnification of 10,000, the region A is defined as a region having a dense 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. A region where carbides are sparse compared to (the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm in 100 μm ² is 100 or more less than region A) is referred to as region B.

As a tool steel material for quenching according to the present invention, a specimen for metallographic observation was cut out from a large-sized steel material of 350 mm (t) × 350 mm (w) × 1500 mm (l) and observed.
The observation was made by observing a specimen cut out from a tool steel material for quenching at a magnification of 10,000 using a scanning electron microscope (SEM) after 4% nital corrosion. Region A in FIG. 1 and region B in FIG. Show.
In the observation of the above-described region A and region B, the distribution state of carbide is measured in a field of view of 10 fields or more with the observation surface selected at random, but the area A is found at or near the prior austenite grain boundary. The region B is often found in a portion corresponding to the inside of the prior austenite grains.
Therefore, for the visual field to be observed at random, the prior austenite grain boundary or its vicinity and the two visual fields in the prior austenite grain are essential.

In the present invention, it is important to have a metal structure in which the above-described carbide-rich region A and carbide-sparse region B are mixed. By using this mixed structure, uniform fine crystal grains can be obtained after quenching and tempering. Obtainable.
In this grain refinement mechanism, region A has many fine carbides of 0.1 to 0.5 μm, and region B has less than 100 carbides compared to region A. The carbon concentration of the region A is higher than that of the region B, the transformation point of the region A is lower than the transformation point of the region B, and transformation from the region A to austenite is performed preferentially.
This is because the austenite transformed prior to the region A is grain-grown, but the region B is also transformed to austenite and has the effect of suppressing the growth of austenite transformed prior to the region A. In addition, since many fine carbides are precipitated, the number of nucleation sites of austenite generated by quenching heating increases, and each crystal grain suppresses grain growth, so that uniform fine crystal grains after quenching and tempering. I believe you can get

Therefore, a difference between the carbon concentration necessary for obtaining the carbide refinement mechanism and a partial carbon concentration is required between the region A and the region B. In addition, if the number of carbides having a size that is easy to dissolve is not greater than a specific number, the above-described crystal grain refining effect cannot be obtained.
Therefore, it is easy to dissolve in the base at the time of quenching heating, and as a size capable of realizing the preferential transformation necessary to obtain crystal grain refinement, a carbide size of 0.1 to 0.5 μm and The number was defined as 300 or more. Even if the number of carbides defined in the present invention is small or / and the size of the carbide is other than 0.1 to 0.5 μm, the carbon concentration necessary for obtaining a carbide refinement mechanism is not sufficient. It becomes sufficient, or the difference in partial carbon concentration becomes insufficient, so that the effect of crystal grain refinement cannot be obtained.
Further, in order to obtain a carbon concentration difference necessary for crystal grain refinement, the region B in which the carbides are sparse is defined as having 100 or less carbides having an equivalent circle diameter of 0.1 to 0.5 μm in the region 100 μm ² than the region A. The reason for this is that if the difference in the number of carbides is less than 100 between region A and region B, the difference in carbon concentration is small and the effect of crystal grain refinement cannot be obtained.

By the way, in order to more surely realize the difference between the carbon concentration necessary for obtaining the carbide refinement mechanism and the partial carbon concentration, both the region A and the region B are mainly composed of carbides having a size that is easily dissolved. 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. Is good.
Further, even if the number of carbides having a size of 0.1 to 0.5 μm in the region A is varied depending on the field of view to be observed, the number of carbides in the region A for specifying the region B is at least 10,000. The average value obtained by observing and measuring the double field of view is preferably used as a reference for the number of carbides in the region A.
The lower limit of 200 fewer than the the carbides number of equivalent circle diameter 0.1~0.5μm to 100μm ² in area B since the grain refining effect is difficult to obtain a lower limit of the carbide number of region B 200 is good.
Further, as the region A, 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 core of austenite generation at the time of quenching heating, and more Fine grain can be expected. C amount and the upper limit of the carbide number of region A containing for example, to vary with a carbide forming element content, but it is difficult to set an upper limit, Empirically, the circle equivalent diameter 0 in substantially 100 [mu] m ^2. The limit is that about 600 1-0.5 μm carbides are formed.

A more preferable embodiment of the metal structure of the present invention will be described.
The metal structure of the present invention desirably has substantially no retained austenite. If the retained austenite remains excessively, it may cause aging cracking in the annealed state, or coarse austenite may be generated starting from the retained austenite during quenching heating. Therefore, in the present invention, it is defined that it is desirable that there is substantially no retained austenite.
The fact that there is substantially no retained austenite is measured quantitatively by X-ray diffraction, and the retained austenite is substantially absent if it is within 1% including 0.

Next will be described the set configuration of tool steel to be applied to the heat treatment method of the present invention. 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%, and more preferably 1.0 to 6.0%.
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%, More preferably, it is 0.20 to 2.5%.

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% .

Except elements these provisions in the present invention are impurities contained in F e and unavoidable.

The manufacturing method of the tool steel raw material for hardening which has the metal structure of this invention below is demonstrated using the heat pattern shown in FIG.
A tool steel material having a C content of 0.1 to 0.8% necessary for precipitation of carbides and preferably having a chemical composition within the above-mentioned preferred range is heated to 1050 to 1250 ° C. Perform inter-processing. (Not shown in FIG. 5). Considering the plastic workability of the tool steel material, the heating temperature was set to 1050 ° C. or higher in order to obtain a complete austenite structure. Moreover, since the tool steel raw material may partially melt 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. Further, the heating and holding time may be appropriately determined in consideration of the fact that the austenite crystal grains grow coarsely as it is held for a long time, and about 3 to 10 hours is sufficient.
In the hot working of the tool steel material, it is preferable to apply hot forging such as free forging and die forging. As other conditions for the hot working, the hot working finish temperature is 950 to 950. It may be in the range of 1050 ° C., and the forging ratio is preferably larger than 5 in order to accumulate more strain in hot working.

And after completion | finish of said hot working, 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. 5).
The tool steel material temperature after hot working is at a temperature at which carbides can be precipitated at the grain boundaries. In the case where carbide is excessively precipitated at the crystal grain boundary after the hot working is finished, there is a problem that if quenching and tempering is performed, the carbide remains in the crystal grain boundary and the toughness is hindered. 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.
Cooling at this time is performed at a speed that does not affect the nose of the grain boundary carbide, and when the cross-sectional dimension of the tool steel material is smaller than about 300 mm (t) × 300 mm (w), air cooling is performed. Larger ones may be forcedly cooled by applying a fan with a fan, and may be about 25 ° C./min.
Then, cooling is performed to a temperature range of 700 ° C. or less where carbides are not easily precipitated at the grain boundaries by the above cooling, but if cooled to an excessively low temperature, austenite may be transformed into bainite, resulting in bainite transformation. And there is a problem that the precipitation of carbide cannot be controlled by the subsequent isothermal holding. In order to suppress this, the lower limit of cooling above air cooling was set to 500 ° C.

Next, after cooling at a cooling rate of air cooling or higher until the surface temperature of the tool steel material reaches 500 to 700 ° C., the tool steel material is placed in a heating furnace and heated and held at a temperature of 400 to 700 ° C. Perform ((2) in FIG. 5).
The reason why the temperature is limited to 400 to 700 ° C. is that if the temperature is higher than 700 ° C., carbides precipitate at the grain boundaries as described above, and if the temperature is lower than 400 ° C., there is a possibility of transformation into bainite. Note that the heating / holding time may be appropriately determined in consideration of the fact that, when held for a long time, there is a possibility of transformation to bainite at that time, and it is sufficient if it is about 0.5 to 5 hours. is there. By this treatment, the temperature is equalized to a temperature equal to or lower than the pearlite nose up to the center of the heat-treated material.

Next, heating is performed to increase the material temperature of the tool steel material heated and held at the temperature of 400 to 700 ° C. ((3) in FIG. 5). The tool steel material temperature is changed from pearlite nose to a temperature range of −100 ° C. Then, heating and holding from the pearlite nose in a temperature range of minus 100 ° C. ((4) in FIG. 5) and cooling ((5) in FIG. 5) to obtain a tool steel intermediate material.
The temperature range of −100 ° C. from the pearlite nose is that within this range, as shown in FIG. 1 , a metal structure in which carbides are densely deposited in the vicinity of the prior austenite grain boundaries and carbides are loosely precipitated in the former austenite grains is obtained. This is to adjust the metal structure necessary to achieve grain refinement by quenching and tempering after annealing.
On the higher temperature side than the pearlite nose, the carbide has a pearlite structure in which the carbides are almost uniformly dispersed, and at a temperature lower than minus 100 ° C. from the pearlite nose, the time until the pearlite transformation is completed becomes long. In the former austenite grains, there is a problem that it is difficult to obtain a metal structure in which carbides are sparsely precipitated, and the crystal grain size is not refined by quenching and tempering after annealing. Therefore, in this invention, it was set as the temperature range of minus 100 degreeC from 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.

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 three parts, and a tool steel material according to the present invention was used as a one-part material.

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.
Then, forced cooling is performed by blowing a case with a fan until the surface temperature of the tool steel material reaches 550 ° C. ((1) in FIG. 5), and the carbon dissolved in the hot working is cooled. In order to prevent precipitation as a net-like carbide in the crystal grain boundary during the process, forced cooling was performed by spraying caseo at around 900 ° C. The surface temperature was measured using a radiation thermometer.
After that, the tool steel material was put into a 450 ° C. heating furnace, held at 450 ° C. for 3 hours ((2) in FIG. 5), and then the material temperature of the material heated and held at the above 450 ° C. temperature. The pearlite nose is heated to raise the material temperature to a temperature of 725 ° C. within the temperature range of −100 ° C. ((3) in FIG. 5), and after holding for 20 hours ((4) in FIG. 5), It was furnace-cooled ((5) in FIG. 5) to obtain a tool steel material for quenching. The temperature of the pearlite nose of the tool steel shown in Table 1 was 775 ° C.
Moreover, as a conventional example, the steel was cooled to the martensite and bainite transformation region, and then completely austenite transformed at the Ac3 point or higher, thereby obtaining a conventional tool steel material for quenching that was annealed. The hot forging conditions were the same as the conditions for producing the tool steel material for quenching according to the present invention.

A test specimen for observing the metal structure was cut out from the tool steel of the present invention and the quenched tool steel material of the comparative example, 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 by the present invention is shown in FIG. FIG. 3A is a 100 × magnification electron micrograph, and FIG. 3B is an enlarged photograph (× 400) of a portion surrounded by a white frame in FIG. The portion that appears white in FIG. 3 is the region A, and the black portion is the region B. It can be seen that region A is seen at the prior austenite grain boundary and its vicinity. A region A in which carbides are dense is shown in FIG. 1, and a region B in which carbides are sparse is shown in FIG. 1 and 2 are 10000 × electron micrographs.
The test piece cut out from the tool steel material for quenching was also observed at 10,000 times using a scanning electron microscope (SEM) after 4% nital corrosion. The metal structure of the conventional material is shown in FIG.

Then, specimens for X-ray diffraction were sampled from the quenching tool steel material of the present invention and the conventional quenching tool steel material, and remained at a target Co, a voltage of 40 KV, and a current of 200 mA using a wide-angle X-ray diffraction apparatus. The presence or absence of austenite was confirmed, and the retained austenite of the present invention material and the conventional material could not be confirmed (the amount of retained austenite was 0%).
Moreover, when observed at 10,000 times using a scanning electron microscope (SEM) of the quenching tool steel material of the present invention and the conventional quenching tool steel material, an equivalent circle diameter of 0.1 to 0.5 μm in 100 μm ² Tables 2 and 3 show the results of the number of carbides.

First, from the results of image analysis, the metal structure of the tool steel material for quenching according to the present invention is not precipitated continuously in a certain direction as a carbide precipitation form, but each carbide is dispersed without orientation. However, it had a characteristic metal structure with an acicular ratio of 1-2.
Further, when the metal structure is observed at a magnification of 10,000, the carbide structure in which 300 or more carbides having an equivalent circle diameter of 0.1 to 0.5 μm are formed in 100 μm ² has a dense region A, and the metal structure It was confirmed that in 100 μm ² , the number of carbides having an equivalent circle diameter of 0.1 to 0.5 μm in the region A was a metal structure in which a region B in which 100 or more carbides were sparse was mixed.
In the conventional material, a specimen cut from a tool steel material for quenching was observed at 10000 times using a scanning electron microscope (SEM) after 4% nital corrosion, and an equivalent circle diameter of 0.1 in 100 μm ² was obtained. It was a metal structure in which approximately 100 to 150 carbides of ˜0.5 μm were uniformly dispersed.

Next, the tool steel of the present invention and the tool steel of the comparative example were heated to 1030 ° C. at a temperature of Ac3 point or higher and quenched, and then tempered once at 600 ° C. The heating and holding time was 2 hours during quenching and 7 hours during tempering.
Table 4 shows the results of cutting out test pieces for observing the metal structure from the tool steel of the present invention and the tool steel of the comparative example and measuring the average crystal grain size.

  As explained above, by using the tool steel material for quenching having the metal structure of the present invention and quenching and tempering, uniform fine crystal grains can be obtained, and the average grain size number becomes finer than No. 8. The toughness is greatly improved.

  If the tool steel material for quenching having the metal structure of the present invention is used, uniform fine crystals can be obtained by quenching and tempering, so that it can be used for applications requiring toughness of tool steel.

It is a 10,000-fold electron micrograph of the metal structure (region A) of the present invention. It is a 10,000-fold electron micrograph of the metal structure (region B) of the present invention. It is an electron micrograph of the metal structure (area | region A, B) of this invention. It is a 10,000 times as many electron micrographs of the metal structure obtained by the prior art. It is a schematic diagram which shows the heat pattern of the manufacturing method of this invention tool steel raw material.

Claims (2)

C: 0.1 to 0.8% by mass , Si: 2.0% or less, Mn: 2.0% or less, Cr: 1.0 to 15.0%, Mo: 10.0% or less, Ni : 4.0% or less, V: 4.0% or less, and the balance is a tool steel material for quenching made of Fe and inevitable impurities , and when the metal structure of the tool steel material is observed at a magnification of 10000 times 100 μm ² A region A in which 300 or more carbides having a circle equivalent diameter of 0.1 to 0.5 μm are formed is dense, and the region A has a circle equivalent diameter of 0.1 to 0 in 100 μm ^2. A tool steel material for quenching characterized in that it has a metal structure in which the number of carbides of 5 μm is less than 100 and a region B in which carbides are sparse is mixed. The tool steel material for quenching according to claim 1 , wherein the retained austenite is 1% or less (including 0) when measured by X-ray diffraction .