JP2009132990A - Alloy tool steel and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an alloy tool steel which exhibits an isotropic size change in quenching and tempering. <P>SOLUTION: The alloy tool steel comprises, by mass, 0.55-0.85% C, 0.20-2.50% Si, 0.30-1.20% Mn, ≤0.50% Cu, 0.01-0.50% Ni, 6.00-9.00% Cr, 0.1-2.00% of Mo+0.5W, 0.01-4.00% V and the balance being Fe and inevitable impurities. Here, the value L (%), which is defined as an area ratio of a coarse carbide having a circle equivalent diameter of ≥2 μm in a cross section parallel to a forging direction, and a value T (%), which is defined as an area ratio of the coarse carbide in a cross section perpendicular to the forging direction, are both ≥0.001%, and the ratio of L/T is within a range of from 0.90 to 3.00. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an alloy tool steel, and more particularly to an alloy tool steel having a property of isotropic expansion during quenching and tempering and a method for producing the same.

  Conventional molds for cold forging, precision forging, progressive press, plastic molding, warm forging, powder molding, magnet molding (trim, die, draw etc.) and molds Alloy tool steel is widely used as mold parts.

  This alloy tool steel is a material that is required to be hard, and is subjected to quenching and tempering to make the structure martensite and used as the above-mentioned mold or the like with a desired hardness.

  This alloy tool steel undergoes volume expansion by its quenching and tempering treatment. If the expansion is isotropic expansion, there is no problem, but the conventional alloy tool steel is an anisotropic non-uniform expansion, which is a major obstacle in manufacturing molds and the like It was.

Such anisotropic non-uniform expansion in alloy tool steels tends to be particularly noticeable when the alloy tool steel contains a large amount of carbides, but the reason has not been elucidated so far. .
This anisotropic non-uniform expansion causes the following problems in manufacturing a mold, for example.

  When manufacturing a mold, the desired mold dimensions are roughly processed in advance with shapes and dimensions that allow for dimensional changes due to heat treatment, followed by quenching and tempering, and then finishing. The mold is finally sized.

  When the mold material (alloy tool steel) isotropically expands by quenching and tempering, the mold may be roughly machined with dimensions and shapes that allow for the same amount of expansion in any direction.

  However, when the mold material is greatly expanded (expanded) in one direction by quenching and tempering, and the elongation is small or contracted in the other direction, the dimension before quenching and tempering is determined based on the dimensional change in the other direction. I have to keep it.

  However, the direction of elongation due to quenching and tempering changes depending on the direction of the material from which the mold is taken, so there is no reproducibility of dimensions after quenching and tempering, and the dimensions of the mold are desired. It could not be controlled with accuracy, and this was a major obstacle to manufacturing the mold.

  Therefore, for example, the mold dimensional accuracy required by general users is ± 0.03% (when the length is 100 mm, the dimensional accuracy is ± 30 μm), but conventionally, the size before heat treatment is uniformly large (approximately +0.06). %), Even if the dimensions are out of order due to quenching and tempering (+ 0.06 ± 0.03% = 0.03% to 0.09%), sufficient machining allowance (+ 0.03% or more. 0.03% If it is less than 1 to 30 μm, 1 to 30 μm is removed by cutting, which is difficult from the viewpoint of mechanical rigidity and the like.

However, in this case, the machining allowance for the finishing process is 0.09% at the maximum, and since tool steel is originally a hard material with high hardness, it takes a lot of time to process after heat treatment (cutting process by 0.03% each) If so, 3 cutting operations are required).
Alternatively, the load on the cutting tool becomes too large (when 0.09% is processed at a time), which leads to a serious problem that the tool is damaged.

  For this reason, there has been a strong demand for reducing the machining allowance. However, since the factors governing the non-uniform expansion due to the heat treatment are unknown, the actual situation is that no countermeasure has been found.

In addition, what aimed at the solution of the problem of the anisotropy of the thermal expansion coefficient in hot tool steel is disclosed by the following patent document 1. The coefficient of thermal expansion in this case is a rate at which the material expands (when no phase transformation is involved) in the material after performing the heat treatment for quenching and tempering.
The present invention relates to heat treatment of quenching and tempering, that is, isotropic of dimensional change when accompanied by phase transformation, and is essentially different from that disclosed in the cited document 1 in terms of the presence or absence of phase transformation. ing. Therefore, from Patent Document 1, the isotropy of dimensional change when accompanied by the phase transformation shown in the present invention is not inferred.

Patent Document 2 below discloses that machinability can be improved by controlling the particle size and amount of carbides and non-metallic inclusions in hot tool steel.
However, this Patent Document 2 does not disclose the problem of the present invention, and the technique disclosed in this document for solving the problem is different from the present invention.

JP-A-2005-113161 JP 2003-226939 A

  In view of the above circumstances, the present invention provides an alloy tool steel that is isotropic in dimensional change accompanied by phase transformation by quenching and tempering while satisfying a working hardness of 55 HRC as a tool steel, and a method for producing the same. It was made for the purpose of doing.

  Thus, claim 1 relates to an alloy tool steel, and in mass%, C: 0.55 to 0.85%, Si; 0.20 to 2.50%, Mn: 0.30 to 1.20%, Cu: ≦ 0.50%, Ni: 0.01 to 0.50% , Cr: 6.00 to 9.00%, Mo + 0.5W: 0.1 to 2.00%, V: 0.01 to 0.40%, composition of balance Fe and inevitable components, and 2 μm in equivalent circle diameter in the cross section parallel to the forging direction When the area ratio of the coarse carbide is L (%) and the area ratio of the coarse carbide in the cross section perpendicular to the forging direction is T (%), both L and T are 0.001% or more. / T is in the range of 0.90 to 3.00.

  According to a second aspect of the present invention, in the first aspect, the L and T are each 0.5% or less.

  Claim 3 further contains at least one of S: 0.040 to 0.100%, Se: 0.040 to 0.100%, Te: 0.040 to 0.100% by mass% in any one of claims 1 and 2. It is characterized by.

  A fourth aspect of the present invention is characterized in that, in the third aspect, Ca: 0.0001 to 0.0150% is further contained by mass%.

  A fifth aspect of the present invention is characterized in that, in any one of the first to fourth aspects, Al: ≦ 0.50%, O: ≦ 0.0050%, and N: ≦ 0.0200 in mass%.

  A sixth aspect of the present invention is the method according to any one of the first to fifth aspects, wherein at least 1% by mass of Nb: 0.01 to 0.15%, Ta: 0.01 to 0.15%, Ti: 0.01 to 0.15%, Zr: 0.01 to 0.15%. It further contains more than seeds.

  Claim 7 relates to a method for producing alloy tool steel, wherein hot forging is performed with a forging ratio in the range of 0.85 to 30, and a coarse carbide having a circle equivalent diameter of 2 μm or more in a cross section parallel to the forging direction. Is L (%), and the area ratio of the coarse carbide in the cross section perpendicular to the forging direction is T (%), both L and T are 0.001% or more and L / T is 0. It is characterized by being in the range of .90 to 3.00.

Effects and effects of the invention

As described above, according to the present invention, the alloy tool steel has the above composition, and the area ratio of coarse carbides having a circle equivalent diameter of 2 μm or more in a cross section parallel to the forging direction is expressed as L (%). When the area ratio of coarse carbides in the cross section perpendicular to L is T (%), both L and T are 0.001% or more and L / T is in the range of 0.90 to 3.00. By doing in this way, the expansion when the alloy tool steel is quenched and tempered can be made isotropic.
In the present invention, forging is a concept including roll forging (generally rolling).

  The present inventor, while conducting research to elucidate the phenomenon that alloy tool steel expands anisotropically and unevenly by quenching and tempering, pays attention to the distribution of carbides, and the distribution and expansion of carbides. As a result, I found the fact that there is a close relationship between them.

Specifically, under the fact that the expansion after quenching and tempering in alloy tool steel is large in the forging direction and small in the direction perpendicular to the forging direction, the distribution of carbides in the cross section parallel to the forging direction and the direction perpendicular thereto When the distribution of carbides in the cross section was examined, in a cross section parallel to the forging direction, coarse carbides having a diameter equivalent to a circle of 2 μm or more formed aggregates, and the aggregates were elongated in the forging direction. Unlike the shape of the carbide in the cross section perpendicular to the forging direction, the carbide is non-aggregate and relatively uniformly distributed, while the area ratio of the carbide is large. And the area ratio of the carbide was found to be small.
Moreover, when the distribution of this carbide and the relationship between expansion by quenching and tempering were investigated, the magnitude of expansion was correlated with the area ratio of carbide, and the greater the area ratio, the greater the degree of expansion. It was also found.

  The reason why the expansion due to quenching and tempering is increased by increasing the area ratio of coarse carbides, and the expansion is decreased by decreasing the area ratio of carbides is not yet elucidated, This is probably due to the following reasons.

When the strength of the carbide and the surrounding base metal, that is, the metal matrix is compared, the strength of the carbide is much higher in all ranges from room temperature to the quenching temperature. For this reason, the base metal, that is, the metal matrix, is distorted and relaxed by the thermal stress due to heat treatment, particularly the austenite transformation during heating and the transformation stress due to martensite transformation during cooling.
If the carbide area ratio is different between the forging direction and the direction perpendicular to the alloy tool steel, the distortion of the base metal, that is, the metal matrix, may differ depending on the direction, which causes anisotropy of expansion. It is thought that.

  Therefore, in order to equalize the expansion due to quenching and tempering in the forging direction and the direction perpendicular thereto, that is, in order to make the expansion isotropic, the distribution of coarse carbides in the forging direction and the direction perpendicular thereto. It is sufficient to equalize.

When the inventor actually conducted the confirmation test, the ratio of the area ratio L of the coarse carbide in the cross section parallel to the forging direction and the area ratio T of the coarse carbide in the cross section perpendicular to the forging direction is as follows. It has been found that the expansion due to quenching and tempering becomes isotropic as it becomes smaller.
The area ratio L / T is ideally set to 1, but a sufficiently uniform dimensional change (by quenching and tempering) is within the range of 0.9 to 3.00 in manufacturing a mold or the like. ) Can be obtained.
The present invention has been made based on these findings.

Here, the sufficiently uniform dimensional change means that the difference between the dimensional change rate (%) in the forging direction and the dimensional change rate (%) in the direction perpendicular thereto is within a range of -0.03 to 0.03. Say there is.
If it is not within this range, generally required mold dimensional accuracy of ± 0.03% cannot be satisfied (even if the dimensional accuracy is satisfied in the forging direction, the dimensional accuracy in the direction perpendicular to the forging direction cannot be satisfied. For).

As a method for realizing the carbide distribution as described above, the following manufacturing method can be suitably applied.
(1) Casting under conditions where the cooling rate from the start of casting to the completion of solidification (1200 ° C) is 0.1 to 5.0 ° C / min. Alternatively, the casting material is remelted (secondary melting) and re-solidified (generally, secondary melting / casting technology using VAR (vacuum arc remelting method) or ESR (electroslag remelting method)). . Furthermore, the method of manufacturing by HIP (high temperature isostatic pressing) using a powder raw material.
(2) A soaking process of at least 1 hour at 1100 to 1250 ° C. is performed at least once, and hot forging (including rolling) is started at a temperature range of 900 to 1250 ° C. The manufacturing method performed as follows.

  The method (1) is a method for refining coarse carbides generated by casting. The faster the cooling rate from the start of casting to the completion of solidification, the smaller the size of the coarse carbide formed. In order to control the size, amount, and distribution state of this coarse carbide to an appropriate range by the soaking process of (2), the cooling rate during casting must be 0.1 ° C./min or more. However, in reality, it is difficult to rapidly cool at a rate exceeding 5.0 ° C./min from the casting amount and the like.

The application of secondary dissolution corresponds to an increase in cooling rate because dissolution and solidification are performed in a short time. When a powder material is used, the carbide particle size is finer than that of a normal cast material. However, since the manufacturing cost is high, there is a practical problem in this respect.
The method (2) is an optimum method for controlling coarse carbides within an appropriate range. The soaking process needs to be performed at a temperature higher than the quenching temperature and lower than the melting point. If the soaking process is properly performed, the coarse carbide formed can be reduced in a small amount and uniformly dispersed in the cast material produced in (1). The soaking temperature and time have different values depending on the components.

The appropriate value of temperature is to heat the cast material produced in (1) to a range of −50 to −10 ° C. from the melting point (including the meaning of local melting of the component segregation part). If the cast material is partially melted by soaking, the material will break. On the contrary, when the temperature is lower than the appropriate value, the solid carbide is not sufficiently dissolved, and the carbide distribution cannot be controlled within an appropriate range.
The appropriate time for soaking varies depending on the soaking temperature, but it may be 10 hours or more in consideration of manufacturing in a factory.

The forging temperature is equal to or lower than the soaking temperature, and any temperature may be selected as long as it is 900 ° C. or higher at which hot forging can be practically performed.
However, if the carbide solid-dissolved by soaking is reprecipitated at a low forging temperature, the carbide distribution within the scope of the present invention cannot be obtained. Therefore, it is desirable to start forging at a temperature as close as possible to the soaking temperature (within 50 ° C. with respect to the soaking temperature).

The forging ratio is a value defined by (cross-sectional area before forging) / (cross-sectional area after forging). In general, the larger the forging ratio, the longer the carbide extends in the forging direction.
If the production methods (1) and (2) are applied, coarse carbides can be dissolved and controlled in the first place, so the magnitude of the forging ratio and the area ratio of the carbide distribution state (L / T) are not necessarily correlated. do not have.
However, when the forging ratio becomes extremely large, the base metal, that is, the structure of the metal matrix is in a strong orientation state (the crystal orientation is not random, but is aligned in a specific direction). Anisotropy of change occurs.

Since isotropic property is essential in the present invention, the forging ratio needs to be suppressed to less than 30. On the other hand, a forging ratio of less than 1 means that the cross-sectional area after forging becomes larger than the cross-sectional area of the cast material, and is generally realized by forging called upsetting. In general, a large amount of coarse carbides during casting remains in the upset state, so it cannot be used in this state.However, when the production methods (1) and (2) are applied, Even if it exists, the isotropy of the dimensional change by heat processing is ensured.
Applying the production methods (1) and (2) is particularly useful in obtaining the effects of the present invention.

  According to claim 2, when the area ratios in the L and T directions are each 0.5% or less, the change rate difference (difference in dimensional change rate) is very high mold dimensional accuracy ± 0.01%. Can be satisfied.

  As described above, it is ideal that the carbide area ratio L in the cross section in the forging direction and the carbide area ratio T in the cross section perpendicular to the forging direction have a ratio of 1: 1.

When the area ratios L and T are 0.5% or less according to claim 2, the area ratio of carbide in each of the cross section parallel to the forging direction and the cross section perpendicular to the forging direction is small. In other words, the carbide distribution itself is extremely small, and it is difficult to form a carbide aggregate in the first place. Therefore, the carbide distribution non-uniformity caused by the carbide aggregate being stretched in the forging direction is difficult to occur. The distribution of carbides in and the distribution of carbides in the direction perpendicular to the forging direction are almost equalized.
That is, claim 2 is a useful means for making the ratio of L and T close to 1: 1.

  Furthermore, according to claim 3, at least one of S, Se, and Te can be added as a selective element. In this case, Ca can be added together with S, Se, and Te according to claim 4. Further, according to claim 5, Al, O, and N can be regulated. Further, according to claim 6, at least one of Nb, Ta, Ti and Zr can be further added.

  Next, claim 7 performs hot forging with a forging ratio in a range of 0.85 to 30, and an area ratio L of coarse carbides in a cross section parallel to the forging direction and a coarseness in a cross section perpendicular to the forging direction. Alloy tool steel is manufactured so that the ratio L / T to the area ratio T of the carbide is in the range of 0.90 to 3.00. By doing so, expansion by quenching and tempering is forged. It is possible to satisfactorily produce an alloy tool steel that equalizes in the direction and the direction perpendicular thereto.

Next, the reasons for limiting the chemical components and the like in the present invention will be described in detail below.
“The area ratio L of coarse carbides of 2 μm or more in the cross section parallel to the forging direction, and the area ratio T of coarse carbides in the cross section perpendicular to the forging direction are both 0.001% or more and L / T is 0.90. Within 3.00 "
In order to satisfy the required dimensional tolerance in both directions, expansion in the forging direction and in a direction perpendicular to the forging direction is approximately isotropic, and the difference in dimensional change (difference in dimensional change) is -0.03 to 0.03%. It is desirable that
In order to satisfy this, (L / T) needs to be in the range of 0.90 to 3.00.

Fine carbides cause solid solution and precipitation due to quenching and tempering, and the influence on the change rate is substantially not recognized.
Therefore, it is necessary to treat a coarse thing having a diameter equivalent to a circle of 2 μm or more which is difficult to be dissolved or precipitated by heat treatment as a target (carbide).
Here, the equivalent circle diameter means an equivalent diameter when the area of the observed carbide is obtained and converted into a circle.

C: 0.55-0.85%
C is an element necessary for obtaining a working hardness of 55 HRC or more as tool steel. The amount of C is appropriately adjusted according to the required hardness. If 0.55% or more is not contained, 55 HRC or more cannot be obtained, and conversely, even if added over 0.85%, the contribution to the increase in carbides and hardness is saturated.
A preferable range of C is 0.60 to 0.70%.

Si; 0.20-2.50%
Si is an element added as a deoxidizing element. In actual production, it is costly to make it less than 0.20%, and if added over 2.50%, the form of carbide changes from granular to rod-like, and coarse carbide tends to remain, so it is necessary to keep it below the upper limit It is.
The preferred range of Si is 0.90-2.20%.

Mn: 0.30 to 1.20%
In order to be applied to large molds and parts as tool steel, it is necessary to have high hardenability. From the viewpoint of hardenability, quenching cannot be performed by air cooling unless 0.30% or more is added, and if it exceeds 1.20%, hardenability is sufficient, but the amount of retained austenite increases and hardness increases. Since it falls, it is necessary to keep below an upper limit.
A preferable range of Mn is 0.70 to 1.20%.

Cu: ≤0.50%
Inevitable element contained in steel. If Cu exceeds 0.50%, red heat embrittlement occurs during forging, making it impossible to manufacture. Therefore, it is necessary to keep it below 0.50%.
However, since it takes a great deal of cost to make it less than 0.01% in actual production, 0.01% or more is acceptable.

Ni: 0.01-0.50%
As tool steel, in order to apply it to large molds and parts, it is necessary to have high hardenability. From the viewpoint of hardenability, it is impossible to quench by air cooling unless 0.01% or more is added. If added over 0.50%, the hardenability is sufficient, but the amount of retained austenite increases and the hardness increases. Since it falls, it is necessary to keep below an upper limit.

Cr: 6.00 to 9.00%
Since it forms a carbide by combining with carbon, it is an essential element for obtaining a hardened and tempered hardness. Addition of 6.00% or more is necessary to form a sufficient carbide that contributes to hardness. However, even if added over 9.00%, a large amount of carbides that do not contribute to hardness are formed, so it is necessary to keep it below the upper limit. A preferable range of Cr is 6.50 to 8.00%.

Mo + 0.5W: 0.1-2.00%
Since it combines with carbon to form a carbide, it is an essential element for obtaining a hardened and tempered hardness. In order to form sufficient carbides that contribute to hardness, addition of 0.1% or more is necessary. However, even if added over 2.00%, the amount of carbide becomes too large and the toughness is greatly deteriorated, so it is necessary to keep it below the upper limit.

V: 0.01 to 0.40%
Since it forms a carbide by combining with carbon, it is an essential element for obtaining a hardened and tempered hardness. In order to form sufficient carbides that contribute to hardness, addition of 0.01% or more is necessary. However, if added over 0.40%, very coarse carbides are formed and the toughness is very deteriorated, so it is necessary to keep it below the upper limit.
A preferable range of V is 0.03 to 0.20%.

S, Se, Te: 0.040 to 0.100%
S, Se, and Te all have the same effect, so any element may be selected (at least one or more). All of them combine with Mn in the material to form MnS, MnSe, MnTe and the like.
The presence of MnS, MnSe, and MnTe has the effect of reducing the amount of tool wear due to cutting, such as drill machinability, and improving the cutting speed as compared with the prior art. Addition of S or the like uses Mn in the material, so if it is added in a large amount exceeding 0.100%, the amount of Mn in the matrix is lowered. On the other hand, in order to obtain the effect of free cutting, addition of 0.040% or more is necessary. S and the like do not contribute at all to the amount, size, and distribution of carbides, and can be added freely.

Ca: 0.0001 to 0.0150%
When Ca is added simultaneously with S, it exists in MnS as Ca oxide or solute Ca. In this case, it is known that the free cutting effect is larger than that of MnS alone. In order to obtain the effect, positive addition of 0.0001% or more is essential. However, even if added over 0.0150%, the effect of free cutting is saturated, so it is limited to the upper limit or less. Like S, it does not contribute at all to the amount, size and distribution of carbides, so it can be added freely.

Al: ≤0.50%
O: ≦ 0.0050%
N: ≤ 0.0200
These elements are contained in steel as inevitable impurities. However, if each of these elements exceeds the upper limit, a large amount of Al oxide or Al nitride is formed. If a large amount of oxide or nitride is formed, this corresponds to a large amount of coarse carbide remaining, and therefore it is desirable to reduce it as much as possible from the viewpoint of isotropic dimensional change. However, reducing these elements causes a cost increase such as a long refining time.

Nb, Ta, Ti, Zr: 0.01-0.15%
These elements form oxides, nitrides, and carbides. By positively adding, these non-metallic inclusions are formed, crystal grain coarsening is suppressed during quenching, and toughness is improved. In the steel of the present invention, coarse carbides are uniformly dispersed, but added when the amount of carbides decreases and the crystal grains at the time of quenching become coarse.

  The alloy tool steel according to the present invention is mainly used as a mold. In particular, among alloy tool steels, cold die steel and high-speed tool steel are preferably used because they contain a large amount of coarse amorphous carbide. Further, among these, cold die steel is preferably used because anisotropic deformation behavior is easily recognized.

Next, embodiments of the present invention will be described in detail below.
A 30 kg steel material having the composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and then ingot-formed. The cooling rate during casting was 1.2 ° C./min. Moreover, about the comparative steel 2, the heating control by a heater was implemented and the cooling rate at the time of casting was manufactured at 0.01 degrees C / min. The steel ingot was held at the plastic working temperature (forging heating temperature) shown in Table 2 for 10 hours or more, and then hot forging was performed using a 500-ton hammer type forging machine to produce cold die steel.

Here, forging was performed so that the forging ratio shown in Table 1 was obtained. The forging ratio is the cross-sectional area before forging / the cross-sectional area after forging.
Slow cooling was performed after forging, followed by spheroidizing annealing.
The obtained test steel and comparative steel were subjected to the following tests and evaluations.

<Area ratio of carbide>
After cutting so that a plane parallel to the forging direction (L direction) was obtained as a 15 mm square, this surface was subjected to final diamond polishing and then corroded with nital or billera. The surface perpendicular to the forging direction (T direction) was similarly cut, polished and corroded. After corrosion, 10 fields of view were taken at a magnification of 100 times that of an optical microscope, and the area ratio of carbides in these 10 fields was measured. The area ratio was an average value of 10 fields of view by measuring the area ratio of carbides for each field of view, in which the circle equivalent diameter of carbides was 2 μm or more. This average value was defined as the carbide area ratio.

<Heat treatment conditions>
Quenching and tempering were performed at the temperatures shown in Table 2.
<Quantification of the amount of retained austenite>
Test pieces were cut out from the manufactured inventive steel and comparative steel.
Quenching was carried out by holding at the temperature shown in Table 2 for 30 minutes and then cooling at an average cooling rate of 50 ° C./min. Thereafter, the surface was ground and polished, and the final finish was removed by 0.05 μm by electrolytic polishing. The average ratio was obtained from the peak intensity ratio of the martensite structure and the austenite structure with an X-ray diffractometer.
The amount of residual γ in Table 2 indicates the volume ratio (%) of the amount of retained austenite after quenching and tempering in steel.

<Change rate difference>
A test piece of φ10 × 50 mm was cut out from the manufactured inventive steel and comparative steel and processed. At this time, the length of the test piece was measured in units of 1 μm using a micrometer for the sample taken so that the length direction of the test piece was parallel to the forging direction and the sample taken from the right angle direction. Was used as a reference value. The test pieces were quenched and tempered at the temperatures shown in Table 2. These heat treatments were performed in a vacuum heat treatment furnace so that the test pieces were not oxidized.
The length was measured after quenching and after tempering, and the rate of change in length relative to the reference value was determined. Then, the difference in the length change ratios of the test pieces in the forging direction (L direction) and the direction perpendicular to the forging direction (T direction) (the dimensional change ratio in the L direction−the dimensional change ratio in the T direction) is defined as the difference in the change ratio. evaluated.

The results are shown in Table 2 and FIG.
FIG. 1 shows the result of the same test in addition to the result of Table 2 (the ● and ▲ marks in the figure are a part of the results of Table 2).

Here, FIG. 1A shows the relationship between the area ratio (L / T) on the horizontal axis and the change in size ratio on the vertical axis.
In FIG. 1B, the horizontal axis represents the area ratio L of the carbide in the cross section in the direction parallel to the forging direction, and the vertical axis represents the difference in size change ratio to express the relationship.
Claim 2 is such that each of L and T is 0.5% or less, but here, only the relationship between L and the change in size ratio is shown. The relationship between T and the difference in size change rate is exactly the same.

First, from the result shown in FIG. 1 (A), when the area ratio (L / T) is in the range of 0.9 to 3.00, the required change in size ratio -0.03 to 0.03 is satisfied. You can see what you get.
Further, from the results shown in (B), it can be seen that the more desirable change rate difference -0.01 to 0.01 can be satisfied by setting the area ratio L of the carbide in the forging direction to 0.5% or less.

In the results of Table 2, Comparative Steel 1 is the same component as Invention Steel 15, but is heated (soaked) at a temperature lower than the temperature considered appropriate from the melting temperature and added with a large forging ratio, so that it is coarse. A large amount of carbide remains, and (L / T) deviates from the appropriate range. For this reason, the difference in change rate is increased.
The comparative steel 2 is the same component as the inventive steel 15 but is manufactured by slowing the cooling rate at the time of casting. Therefore, even if an appropriate heating temperature and forging ratio are added, the carbide distribution cannot be controlled, and (L / T ) Is out of the proper range, and the difference in size change becomes large.
In Comparative Steel 3, C and Cr are out of the proper range, and a large forging ratio is added, so that (L / T) is out of the proper range and the difference in change rate becomes large.
Since the comparative steels 4, 5, and 6 deviate from the appropriate components, the hardness is less than 40 HRC, and the use hardness as the tool steel cannot be satisfied. However, since the area ratio is in an appropriate range, the difference in size change is equivalent to that of the invention steel.
On the other hand, all the inventive steels have good results.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

(A) It is the figure showing the relationship between area ratio ratio (L / T) and a size change rate difference. (B) It is the figure showing the relationship between the area ratio L of the carbide | carbonized_material of the cross section in a parallel direction with a forge direction, and a size change rate difference.

Claims (7)

In mass%
C: 0.55-0.85%
Si; 0.20-2.50%
Mn: 0.30 to 1.20%
Cu: ≤0.50%
Ni: 0.01-0.50%
Cr: 6.00 to 9.00%
Mo + 0.5W: 0.1-2.00%
V: 0.01 to 0.40%
L (%) represents the area ratio of coarse carbides having a composition of the balance Fe and inevitable components, and having a circle equivalent diameter of 2 μm or more in a cross section parallel to the forging direction, and the cross section in a direction perpendicular to the forging direction. An alloy tool steel characterized in that when the area ratio of coarse carbide is T (%), both L and T are 0.001% or more and L / T is in the range of 0.90 to 3.00.   The alloy tool steel according to claim 1, wherein L and T are each 0.5% or less. In mass%
S: 0.040 to 0.100%
Se: 0.040 to 0.100%
Te: 0.040 to 0.100%
The alloy tool steel according to any one of claims 1 and 2, further comprising at least one of the following. In mass%
Ca: 0.0001 to 0.0150%
The alloy tool steel according to claim 3, further comprising: In mass%
Al: ≤0.50%
O: ≦ 0.0050%
N: ≤ 0.0200
The alloy tool steel according to any one of claims 1 to 4, wherein the alloy tool steel is restricted. In mass%
Nb: 0.01-0.15%
Ta: 0.01-0.15%
Ti: 0.01-0.15%
Zr: 0.01-0.15%
The alloy tool steel according to any one of claims 1 to 5, further comprising at least one of the following.   Hot forging is performed with a forging ratio in the range of 0.85 to 30, and the area ratio of coarse carbides having a circle equivalent diameter of 2 μm or more in a cross section parallel to the forging direction is L (%), perpendicular to the forging direction. When the area ratio of the coarse carbide in the cross section in the direction is T (%), both L and T are 0.001% or more and L / T is in the range of 0.90 to 3.00. A method for producing alloy tool steel.

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