EP3050986A1

EP3050986A1 - High-speed-tool steel and method for producing same

Info

Publication number: EP3050986A1
Application number: EP14847363.0A
Authority: EP
Inventors: Shiho Fukumoto
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2013-09-27
Filing date: 2014-06-24
Publication date: 2016-08-03
Anticipated expiration: 2034-06-24
Also published as: TW201527549A; JP2018048407A; TWI654318B; EP3050986B1; CN111411293A; WO2015045528A1; CN105579604A; JPWO2015045528A1; JP6474348B2; EP3050986A4

Abstract

A high speed tool steel including by mass%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo at a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb at a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%, in which the content of N is 0.0200% or less, the balance includes Fe and impurities, and the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less.

Description

Technical Field

The present invention relates to a high speed tool steel to be used for a tool, such as a die or a punch, as well as a method for producing the same.

Background Art

As a material for various tools, such as a die or a punch, to be used for warm-to-hot working, hitherto, a low-alloy high speed tool steel has been used, having a toughness that is improved by reducing the content of C (carbon), and of Mo, W, and V, which form a carbide together with C, relative to the ingredient composition of SKH51, which is a typical steel type of high speed tool steels. With respect to this kind of low-alloy high speed tool steel, a high speed tool steel has been proposed (Patent Document 1 below) whose toughness is enhanced by the following: a steel ingot composed of the ingredient composition of the high speed tool steel is subjected to a soaking treatment at a high temperature of from 1,200 to 1,300°C, and then cooled at a cooling rate of 3°C/min or more until the surface temperature of the steel ingot reaches 900°C or lower, so that aggregation of a carbide in the structure after quenching and tempering can be suppressed, and as a result, the average particle diameter of a carbide is limited to 0.5 µm or less, which makes the distribution density of the same 80 x 10³ mm² or higher.
Patent Document 1: Japanese Patent Application Laid-Open ( JP-A) No. 2004-307963

SUMMARY OF INVENTION

Technical Problem

The technique of Patent Document 1 is effective in improving the toughness of a low alloy high speed tool steel.
However, even in the case of a high speed tool steel produced according to the technique of Patent Document 1, there may occasionally be more than a few carbide particles with an individual particle diameter higher than 0.5 µm in the structure after quenching and tempering. Therefore, using the technique of Patent Document 1, the effect of improvement on the toughness of a high speed tool steel cannot always be sufficiently obtained.
An object of the present invention is to provide a high speed tool steel whose toughness is further improved, and a method for producing the same.

Solution to Problem

Specific means for attaining the object are as follows.

<1> A high speed tool steel including by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo at a content determined by a relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb at a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%; wherein a content of N is 0.0200% or less by mass-%, the balance includes Fe and impurities, and a maximum value of an equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less.
<2> The high speed tool steel according to <1> above further including: Ni at 1.00% or less by mass-%.
<3> The high speed tool steel according to <1> or <2> above further including: Co at 5.00% or less by mass-%.
<4> The high speed tool steel according to any one of <1> to <3> above, wherein the content of Si is 0.20% or less by mass-%.
<5> The high speed tool steel according to any one of <1> to <4> above, having hardness of 45 HRC or higher.
<6> A method for producing a high speed tool steel, the method including:
- a preparation step of preparing a steel ingot including by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo at a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb at a content determined by a relational expression (V+Nb) of from 0.50 to 3.00%, wherein a content of N is 0.0200% or less by mass-%, and the balance includes Fe and impurities;
- a soaking treatment step of performing a soaking treatment by heating the steel ingot at from 1,200 to 1,300°C;
- a cooling step for cooling the steel ingot after the soaking treatment step until a surface temperature of the steel ingot reaches 900°C or less, wherein at least after the surface temperature has decreased to a temperature T1 within a range of not higher than 1,000°C but higher than 900°C, cooling is performed at a cooling rate of the surface temperature of 3°C/min or more until the surface temperature reaches 900°C or less;
- a hot working step of reheating the steel ingot after the cooling step to a hot working temperature higher than 900°C, and hot-working the reheated steel ingot into a steel product; and
- a quenching and tempering step for quenching and tempering the steel product.
<7> The method for producing a high speed tool steel according to <6> above, wherein, in the cooling step, cooling of the steel ingot is performed at a cooling rate of the surface temperature of the steel ingot of less than 3 °C/min until the surface temperature has decreased to the temperature T1.
<8> The method for producing a high speed tool steel according to <6> or <7> above, wherein the steel ingot prepared in the preparation step is a steel ingot yielded by casting molten steel that has been refined by a deoxidizing refining method.
<9> The method for producing a high speed tool steel according to <8> above, wherein the steel ingot prepared in the preparation step is a steel ingot yielded by casting molten steel that has been refined by a deoxidizing refining method to yield an electrode for remelting, and by applying a remelting method to the electrode for remelting.
<10> The method for producing a high speed tool steel according to any one of <6> to <9> above, wherein the steel ingot prepared in the preparation step further includes Ni at 1.00% or less by mass-%.
<11> The method for producing a high speed tool steel according to any one of <6> to <10> above, wherein the steel ingot prepared in the preparation step further includes Co at 5.00% or less by mass-%.
<12> The method for producing a high speed tool steel according to any one of<6> to <11> above, wherein the content of Si in the steel ingot prepared in the preparation step is 0.20% or less by mass-%.
<13> The method for producing a high speed tool steel according to any one of <6> to <12> above, wherein, in the quenching and tempering step, the hardness of the steel product is adjusted to 45 HRC or more by the quenching and tempering.
<14> The method for producing a high speed tool steel according to any one of <6> to <13> above, further including a machining step of machining the steel product into a tool shape after the hot working step but before the quenching and tempering step,
wherein, in the quenching and tempering step, the steel product that has been machined into a tool shape is subjected to quenching and tempering.

Advantageous Effects of Invention

According to the invention, the toughness of a high speed tool steel can be further improved.

BRIEF DESCRIPTION OF DRAWINGS

[Figure 1] Figure 1 is a schematic diagram illustrating a soaking treatment and a cooling process (cooling conditions 1 to 4) performed on a steel ingot in Example 1.
[Figure 2] Figure 2 is binarized images showing carbide distributions in sectional structures, obtained by analyses with an EPMA (electron probe micro analyzer) on respective sectional structures of a steel ingot A and a steel ingot B cooled by cooling conditions 1 to 4 respectively in Example 1.
[Figure 3] Figure 3 is a scanning electron micrograph showing an example of a carbide distributed in a sectional structure of a high speed tool steel of Inventive Example in Example 2.
[Figure 4] Figure 4 is a scanning electron micrograph showing an example of a carbide distributed in a sectional structure of a high speed tool steel of Comparative Example in Example 2.
[Figure 5] Figure 5 is a graph showing the relationship between an equivalent circle diameter and a number density (mm^-2) of a carbide in sectional structures of high speed tool steels of Inventive Example and Comparative Example in Example 2.
[Figure 6] Figure 6 is a scanning electron micrograph showing an example of a fracture surface after a Charpy impact test performed on a high speed tool steel of Inventive Example in Example 2.
[Figure 7] Figure 7 is a scanning electron micrograph showing an example of a fracture surface after a Charpy impact test performed on a high speed tool steel of Comparative Example in Example 2.

DESCRIPTION OF EMBODIMENTS

The expression of "%" with respect to an ingredient (an element) means herein "mass-%".
A numerical range expressed by "x to y" herein includes the values of x and y in the range as the minimum and maximum values, respectively.
A "hardness" expressed in unit "HRC" means herein a Rockwell hardness on C scale according to JIS G 0202(2013).
A high speed tool steel and a method for producing the same according to the invention will be described in detail below.

A high speed tool steel according to the invention includes by mass-%: C (carbon) at from 0.40 to 0.90%; Si (silicon) at 1.00% or less; Mn (manganese) at 1.00% or less; Cr (chromium) at from 4.00 to 6.00%; one or both of W (tungsten) and Mo (molybdenum) in a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V (vanadium) and Nb (niobium) in a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%, wherein the content of N (nitrogen) is 0.0200% or less by mass-%, and the balance includes Fe (iron) and impurities; and wherein the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less.
The concept of "carbide" according to the invention in a sectional structure of a high speed tool steel includes not only a carbide free from nitrogen but also a carbide containing nitrogen (namely, carbonitride).
The N content in a high speed tool steel according to the invention is 0.0200% or less as described above.
N is an impurity element inevitably contained in a steel ingot after casting.
In a case in which the ingredient composition of a molten steel before casting has been adjusted solely in the air environment, generally a steel ingot after casting may contain N ordinarily at approx. 0.0300% or more.
N is an element having strong affinity for V and Nb, which are elements forming a carbide.
Therefore, in a high speed tool steel with a high content of N, before V or Nb bonds with C to crystallize out as a carbide (eutectic carbide) in the course of solidification during casting, the same bonds with N to crystallize out as a nitride. Then, a carbide crystallizes out in the surroundings of the nitride to form a carbonitride.
The carbonitride is a thermally stable compound.
Therefore, when a large amount of carbonitride is formed in a steel ingot, it is difficult to dissolve the carbonitride in a matrix in the following steps of a soaking treatment step or a hot working step. As the result, there remains a large amount of carbonitride in the structure of a high speed tool steel (including a tool product; the same shall apply hereinbelow) produced through a soaking treatment step and a hot working step, and the toughness of a high speed tool steel decreases. Further, the carbonitride becomes an origin of destruction, which promotes early crack of a high speed tool steel and therefore decreases the lifetime of a high speed tool steel.
In view of the above, the N content in a high speed tool steel according to the invention is 0.0200% or less so as to suppress the formation amount of of the carbonitride.
By this measures, the form of the carbonitride crystallized in a steel ingot can be changed to a form of a carbide not containing nitrogen.
A carbide not containing nitrogen can be easily dissolved in a matrix by a soaking treatment, etc. Therefore, by reducing the N content to 0.0200% or less, a carbide distributed in a high speed tool steel can be micronized, so that the toughness of a high speed tool steel can be improved.
Further, as described above, with respect to a high speed tool steel according to the invention, the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm less.
With respect to the particle diameter of a carbide, the average particle diameter of a carbide in the high speed tool steel described in Patent Document 1 is 0.5 µm or less.
However, it became clear through investigations by the inventors that even in a case where the average particle diameter of a carbide in a high speed tool steel is 0.5 µm or less, a coarse carbide with a particle diameter exceeding 1.00 µm by far may be present in the high speed tool steel. Further, it became clear that the toughness of a high speed tool steel may not be improved adequately due to presence of such a coarse carbide.
With respect to such problems, in the case of a high speed tool steel according to the invention, since the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less, the toughness of a high speed tool steel is further improved.
When the maximum value of the equivalent circle diameter of a carbide in a sectional structure of a high speed tool steel exceeds 1.00 µm, a carbide with a large particle diameter (especially, a carbide with an equivalent circle diameter higher than 1.00 µm) is apt to become a destruction origin, so as to decrease the toughness of a high speed tool steel. A carbide having an equivalent circle diameter higher than 1.00 µm is mainly a carbide that does not dissolve into a matrix (insoluble carbide) in a quenching step at the quenching temperature (an austenitizing temperature about 900°C or higher).
In this regard, it is only required for a high speed tool steel according to the invention that the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less, and it is needless to say that the average particle diameter of a carbide may be 0.5 µm or less, insofar as the above condition is satisfied.
As described above, in a case in which the N content in a high speed tool steel according to the invention is 0.0200% or less, and the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less, the toughness can be further improved compared to a conventional high speed tool steel (for example, a high speed tool steel described in Patent Document 1).
The N content in a high speed tool steel according to the invention is preferably 0.0180% or less, and more preferably 0.0150% or less.
There is no particular restriction on the lower limit of the N content in a high speed tool steel according to the invention, and the lower limit of the N content may be, for example, 0.0005%, or 0.0010%.
The maximum value of the equivalent circle diameter of a carbide in a sectional structure of a high speed tool steel according to the invention is 1.00 µm or less as described above, but the maximum value of the equivalent circle diameter is preferably 0.90 µm or less, and more preferably 0.80 µm or less.
Further, the distribution density of a carbide in a high speed tool steel according to the invention can be 80 × 10³ mm^-2 or higher. When the distribution density of a carbide is high, the prior austenite particle diameter after quenching and tempering can be reduced, so that the toughness of a high speed tool steel can be further enhanced.
For the purpose of the invention, it is enough for determination of the maximum value of an equivalent circle diameter of a carbide, or a distribution density of a carbide to observe and examine a sectional structure of a high speed tool steel with a scanning electron microscope, for example, at a magnification of 4000x over a total visual field area of 5000 µm² or more.
In this regard, when the carbide is examined, a high speed tool steel has generally a shape of one of various tool products. A region apparently vulnerable to cracking due to the carbide in the shape of a tool product is, for example, a working surface of the tool product, especially a corner part in contact with another component (external corner, and internal corner) in the working surface. Therefore, as a region of a high speed tool steel for examining the carbide, a sectional structure including the corner part may be selected.
A carbide having a strong influence on the toughness of a tool is mainly a carbide that does not dissolve into a matrix (insoluble carbide) in a quenching step at the quenching temperature (an austenitizing temperature about 900°C or higher).
A high speed tool steel according to the invention has preferably a hardness of 45 HRC or higher.
In applying a high speed tool steel according to the invention to various tools, when the in-use hardness is 45 HRC or more, a superior tensile strength can be imparted to tools. Especially, for application to various hot work tools, when the in-use hardness (hardness at room temperature) is 45 HRC or more, superior tensile strength at a high temperature can be imparted.
The hardness of a high speed tool steel according to the invention is more preferably from 45 HRC to 60 HRC.
The ingredient composition of a high speed tool steel according to the invention is common to the ingredient composition of the high speed tool steel in Patent Document 1 in terms of a basic composition except the N content.
Each ingredient except N of a high speed tool steel according to the invention will be described below

C at from 0.40 to 0.90%

C is an element which forms a hard double carbide by bonding with a carbide forming element, such as Cr, Mo, W, V, and Nb, and thereby imparting abrasion resistance to a high speed tool steel. Further, a part of C dissolves in a matrix, and thereby strengthening the matrix. Further, due to the above, a part of C imparts hardness to a martensite structure after quenching and tempering. However, an excessive amount of C encourages segregation of a carbide. Therefore, the C content is set between 0.40 and 0.90%.

Si at 1.00% or less

Si is ordinarily used in a melting step as a deoxidizing agent, and is an element that a cast steel ingot inevitably contains. However, when the Si content is too high, the toughness of a high speed tool steel decreases. Therefore, the Si content is set at 1.00% or less. Meanwhile, Si has an action to micronize a rod-like M₂C-type primary carbide to spheres. Therefore, the Si content is preferably 0.10% or more.
Meanwhile, from the following viewpoint, the Si content is preferably 0.20% or less.
Namely, when the Si content is 0.20% or less, the action to micronize a primary carbide to spheres tends to weaken. Therefore, when Si is 0.20% or less, the effect of decreasing the N content to 0.0200% or less, and the effect of limiting the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less, can be obtained more remarkably compared to a case where Si is higher than 0.20%.

Mn at 1.00% or less

Mn is, similarly to Si, used in a melting step as a deoxidizing agent, and is an element that a cast steel ingot inevitably contains. However, when the Mn content is too high, the A₁ transformation temperature decreases excessively, and the hardness after anneal becomes high, so that the machinability (cutting performance) of a high speed tool steel is impaired. Therefore, the Mn content is set at 1.00% or less. Meanwhile, Mn has an action to improve hardenability. Therefore, the Mn content is preferably 0.10% or more.

Cr at from 4.00 to 6.00%

Cr is an element that bonds with C to form a carbide and improves the abrasion resistance of a high speed tool steel. Further, Cr is an element that contributes also to improvement of the hardenability of a high speed tool steel. However, when the Cr content is too high, stripe segregation is encouraged, and the toughness of a high speed tool steel decreases. Therefore, the Cr content is set between 4.00 and 6.00%.
One or both of W and Mo at content determined by relational expression (Mo+0.5W) from 1.50 to 6.00%
W and Mo are elements that bond with C to form a carbide, and dissolve into a matrix during quenching to increase hardness, so as to improve the abrasion resistance of a high speed tool steel. However, when the content of W and Mo is too high, stripe segregation is encouraged, and the toughness of a high speed tool steel decreases.
With respect to effects of the actions, the content of W and Mo means a content determined by the relational expression (Mo+0.5W). In the relational expression (Mo+0.5W), "Mo" represents the content (%) of Mo (molybdenum), and "W" represents the content (%) of W (tungsten).
In a high speed tool steel according to the invention, the content of one or both of W and Mo is set between 1.50 and 6.00% in terms of a content determined by the relational expression (Mo+0.5W).
A high speed tool steel according to the invention may contain one (either) of W and Mo, or contain two (both) of W and Mo. Namely, either of "Mo" and "W" in the relational expression (Mo+0.5W) may be 0%.
In this regard, W has a stronger encouraging power for stripe segregation than Mo, and is apt to impair the toughness of a high speed tool steel. Therefore, the W content in a high speed tool steel is preferably 3.00% or less (1.50% or less in terms of 0.5W in the relational expression (Mo+0.5W)).
One or both of V and Nb at content determined by relational expression (V+Nb) from 0.50 to 3.00%
V and Nb bond with C to form a carbide to improve the abrasion resistance and seizure resistance of a high speed tool steel. Further, V and Nb dissolve into a matrix during quenching and precipitate carbides that are minute and resistant to aggregation during tempering, so as to improve the softening resistance of a high speed tool steel in a high temperature environment, and impart superior high temperature load-carrying capacity thereto. Further, V and Nb micronize crystal grains and increase the A₁ transformation temperature, so as to improve the toughness and the heat crack resistance of a high speed tool steel. However, when the content of V and Nb is too high, a large carbide is formed, and cracking in use as a tool is encouraged.
With respect to effects of the actions, the content of V and Nb means a content determined by the relational expression (V+Nb).
In a high speed tool steel according to the invention, the content of one or both of V and Nb is set between 0.50 and 3.00% in terms of a content determined by the relational expression (V+Nb).
In the relational expression (V+Nb), "V" represents the content (%) of V (vanadium), and "Nb" represents the content (%) of Nb (niobium).
A high speed tool steel according to the invention may contain one (either) of V and Nb, or contain two (both) of V and Nb. Namely, either of "V" and "Nb" in the relational expression (V+Nb) may be 0%.
The content determined by the relational expression (V+Nb) is preferably 1.50% or less.
In this regard, Nb is superior to V in terms of softening resistance, improvement effect on high temperature strength, and inhibitory effect on coarsening of crystal grains. Therefore, a high speed tool steel according to the invention should preferably contain Nb (namely, the Nb content is higher than 0%).

Ni at preferably 1.00% or less

Ni imparts superior hardenability to a high speed tool steel. In this way, a martensite rich quenched structure can be formed, and the toughness intrinsic to a matrix itself can be improved. However, when the Ni content is too high, the A₁ transformation temperature decreases excessively, and the hardness after anneal of a high speed tool steel becomes high, so that the machinability of the high speed tool steel deteriorates. Consequently, even when a high speed tool steel contains Ni, the Ni content is preferably 1.00% or less. When a high speed tool steel contains Ni, the Ni content is preferably 0.05% or more.

Co at preferably 5.00% or less

Co has an effect of forming a protective oxide film, which is extremely compact and has excellent adhesiveness, on a tool surface, when the temperature of the tool in use is elevated. In this way, a metal contact between the tool surface and a mating material is reduced, so that a temperature increase of the tool surface is mitigated thereby imparting superior abrasion resistance to the tool. Further, by formation of the protective oxide film, the heat insulating effect is strengthened and the heat crack resistance is also improved. However, when the Co content is too high, the toughness of a high speed tool steel decreases. Consequently, even when a high speed tool steel contains Co, the Co content is preferably 5.00% or less. And, when a high speed tool steel contains Co, the Co content is preferably 0.30% or more.
Additionally, a high speed tool steel according to the invention possibly contains, for example, S (sulfur), or P (phosphorus) as an unavoidable impurity element.
When the S content is too high, the hot workability of a high speed tool steel may be impaired, and therefore the S content is preferably adjusted to 0.0100% or less. The S content is more preferably 0.0050% or less.
When the P content is too high, the toughness of a high speed tool steel may be deteriorated, and therefore the P content is preferably adjusted to 0.050% or less. The P content is more preferably 0.025% or less.
There is no particular restriction on a method for producing a high speed tool steel according to the invention. There is, for example, a producing method by which on a steel ingot having an ingredient composition of a high speed tool steel according to the invention, a soaking treatment (preferably, a soaking treatment in which the steel ingot is heated to between 1200 and 1300°C), cooling (preferably, cooling in which the steel ingot after the soaking treatment is cooled until the surface temperature of the steel ingot reaches 900°C or less), hot working (preferably, hot working in which the cooled steel ingot is reheated higher than 900°C), and quenching and tempering (preferably, quenching and tempering in which the quenching temperature is 900°C or more, and the tempering temperature is from 500 to 650°C) are performed successively. In this regard, a steel material may be machined to a tool shape between the hot working and the quenching and tempering.
With respect to the producing methods referred to above, by the following method for producing a high speed tool steel according to the invention, a high speed tool steel according to the invention can be especially easily produced.

A method for producing a high speed tool steel according to the invention (hereinafter also referred to as the "present producing method") includes:

a preparation step of preparing a steel ingot including by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo in a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb in a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%, wherein the content of N is 0.0200% or less, and the balance includes Fe and impurities;
a soaking treatment step of performing a soaking treatment by heating the steel ingot at from 1,200 to 1,300°C;
a cooling step of cooling the steel ingot after the soaking treatment step until a surface temperature of the steel ingot reaches 900°C or less, at least after the surface temperature has decreased to a temperature T1 within a range of not higher than 1,000°C but higher than 900°C, cooling is performed at a cooling rate of the surface temperature of 3°C/min or more until the surface temperature reaches 900°C or less;
a hot working step of reheating the steel ingot after the cooling step to a hot working temperature higher than 900°C, and hot-working the reheated steel ingot to a steel product; and
a quenching and tempering step of quenching and tempering the steel product.

A cooling rate of the surface temperature of a steel ingot may be herein simply referred to as a "cooling rate".
The present inventors investigated deeply the method for producing a high speed tool steel including a soaking treatment proposed in Patent Document 1. As the result, it has been confirmed that a high temperature soaking treatment between 1,200 and 1,300°C is effective indeed for dissolving a carbide in a steel ingot of a high speed tool steel having a low alloy ingredient composition as in Patent Document 1.
However, the inventors have also observed that in a case in which the control of a cooling process after the soaking treatment is inappropriate, a carbide which is insoluble or newly precipitated may occasionally grow coarser. Now, the inventors have finally found out that by an appropriate control of the cooling conditions the growth of a carbide in a cooling process can be suppressed, and as the result a carbide in the structure of a high speed tool steel can be micronized. Further, the inventors have found out that for keeping the effect of micronization of a carbide by appropriate cooling conditions, there is an especially suitable ingredient composition of an steel ingot itself which is an object of the soaking treatment, thereby completing a method for producing a high speed tool steel according to the invention.
Namely, in a method for producing a high speed tool steel according to the invention, a steel ingot with a N content of 0.0200% or less by mass-% is used. In this way, a carbide distributed in a produced high speed tool steel can be micronized as described in a section concerning "high speed tool steel", and therefore a high speed tool steel with improved toughness can be produced.
Further, in the present producing method, regulation of the N content in a steel ingot which is an object of the soaking treatment at 0.0200% or less plays an important role together with a cooling step in the present producing method in micronizing a carbide (including carbonitride) in a structure. The details will be described below.
According to the technique of Patent Document 1, the carbide crystallized in a steel ingot can be dissolved into a matrix in the next soaking treatment step at from 1,200 to 1,300°C. Then in a cooling process after the soaking treatment, precipitation and growth of a carbide of V or Nb can be suppressed by cooling the surface temperature of a steel ingot to 900°C or less at a cooling rate of 3°C/min or more.
However, in an actual operation it is difficult to cool a steel ingot immediately after the completion of a soaking treatment from the soaking treatment temperature to the temperature of 900°C or less at a cooling rate of 3°C/min or more. In other words, in an actual operation, during a time period in which a steel ingot is taken out from a soaking furnace slow cooling at a cooling rate less than 3°C/min advances (for example, furnace cooling advances in the soaking treatment furnace) and when cooling at the designated cooling rate is initiated, the surface temperature of a steel ingot is realistically already below the soaking treatment temperature.
According to the study of the inventors, it has been found that when the surface temperature of a steel ingot decreases to approx. 1,000°C, there are already a large amount of precipitated carbides of V and Nb, and their growth have also started.
Meanwhile, by regulating the N content in a steel ingot which is an object of a soaking treatment to 0.0200% or less, the temperature at which the carbide precipitates and grows in the cooling process after the soaking treatment can be lowered. Specifically, the temperature at which the carbide precipitates and grows can be lowered to 1,000°C or less in terms of the surface temperature of a steel ingot. And owing to the decrease in the temperature at which the carbide precipitates and grows, even in a case where the surface temperature of a steel ingot taken out from a soaking treatment furnace has been lowered around 1,000C, precipitation and growth of the carbide can be suppressed by conducting cooling thereafter at a cooling rate of 3°C/min or more, so that micronization of a carbide can be achieved with higher certainty.
Consequently, by the present producing method, by using a steel ingot with a N content of 0.0200% or less as an object of the soaking treatment, and by providing a cooling step in which the steel ingot is cooled such that at least after the surface temperature of a steel ingot has decreased to a temperature T1 within a range of not higher than 1,000°C but higher than 900°C, the cooling rate of the surface temperature is 3°C/min or more until the surface temperature reaches 900°C or less, micronization of a carbide can be achieved more surely. Therefore, by the present producing method, a high speed tool steel having improved toughness compared to a conventional high speed tool steel (for example, a high speed tool steel described in Patent Document 1) can be produced.
By the present producing method, for example, a high speed tool steel in which the maximum value of the equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less (for example, the high speed tool steel according to the invention) can be produced.
Further, by the present producing method, owing to the presence of the cooling step, an effect that enough time becomes available for handling a steel ingot after the soaking treatment can be obtained.
Each step of the present producing method will be described below.

- Preparation Step -

A preparation step is a step for preparing a steel ingot including by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo in a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb in a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%, wherein the content of N is 0.0200% or less by mass-%, and the balance includes Fe and impurities.
The preparation step is a step for the sake of convenience.
The preparation step may be a step for producing a steel ingot, or may be a step for preparing a steel ingot produced in advance prior to the production of a high speed tool steel.
The ingredient composition of a steel ingot to be prepared in a preparation step is the same as the ingredient composition of a high speed tool steel according to the invention, and the preferable range is also the same.
In an actual operation the amount of a molten steel to be melted at one time is large. Therefore, it is not easy to lower the N content in a steel ingot to 0.0200% or less by a simple melting in the air.
In a case in which the N content in a steel ingot is lowered to 0.0200% or less solely by melting in the air, a high quality source material with a reduced N content must be used as a source material before melting, and therefore it is costwise disadvantageous.
Therefore, a steel ingot to be prepared in a preparation step in the present producing method is preferably a steel ingot yielded by casting a molten steel refined by a deoxidizing refining method.
Examples of a deoxidizing refining method include various ladle refining methods, such as a LF method, an ASEA-SKF method, a VAD method, and a VOD method; and various vacuum degassing methods, such as a RH method, and a DH method.
Further, since in the case of an actual operation, each single steel ingot is massive, and segregation in a steel ingot may become severe.
Therefore, a steel ingot to be prepared in a preparation step is more preferably a steel ingot, which is yielded by casting a molten steel refined by a deoxidizing refining method to an electrode for remelting, and by subjecting the yielded electrode for remelting to a remelting method. By conducting a remelting method, segregation in a steel ingot can be mitigated.
Examples of a remelting method include an electro-slag remelting method, a vacuum arc remelting method, a plasma arc remelting method, and an electron beam remelting method. Especially, an electro-slag remelting method is advantageous for reducing an impurity element such as S, because slag is used.

- Soaking treatment step -

A soaking treatment step is a step for conducting a soaking treatment by heating a steel ingot prepared in the preparation step at from 1,200 to 1,300°C.
In a soaking treatment step by subjecting a steel ingot with the ingredient composition to a soaking treatment at a high temperature of from 1,200 to 1,300°C similarly as the technique according to Patent Document 1, so that a very large carbide present in casting is dissolved, and composition ingredients are dissolved and dispersed, and that the distribution of a carbide can be improved.
Although the temperature for a soaking treatment is from 1,200 to 1,300°C, it is preferably from 1,260 to 1,300°C.
The duration of a soaking treatment is preferably from 10 to 20 hours.
In this regard, an ordinary temperature of a soaking treatment for a high speed tool steel is around 1,150°C, and the temperature of a soaking treatment according to the present producing method is higher than the ordinary temperature of a soaking treatment.

- Cooling Step -

A cooling step is a step for cooling a steel ingot after the soaking treatment step until the surface temperature of the steel ingot reaches 900°C or less, such that in the course of cooling down af the surface temperature of the steel ingot to 900°C or less at least after the surface temperature of the steel ingot has decreased to a temperature T1 within a range of not higher than 1,000°C but higher than 900°C, cooling is performed at a cooling rate of the surface temperature of the steel ingot of 3°C/min or more.
In a cooling step, cooling at a cooling rate of 3°C/min or more is carried out until the surface temperature of the steel ingot reaches 900°C or less. The cooling step is a step by which a temperature range down to 900°C, where carbides of V and Nb are apt to precipitate and grow, is passed through quickly so as to suppress formation of a coarse particle of a carbide, and preferably to form solely small particles of a carbide finely dispersed in a matrix.
However, as described above, it is difficult to cool a steel ingot having finished a soaking treatment at a cooling rate of 3°C/min or more from a time point still keeping the soaking treatment temperature to a temperature of 900°C or less.
Responding thereto, by the present producing method, the N content in a steel ingot which is an object of a soaking treatment is limited to 0.0200% or less, and as the result the temperature of precipitation and growth of a carbide in the course of cooling can be successfully lowered to approx. 1,000°C.
By performing a soaking treatment on a steel ingot, in which the N content is reduced to 0.0200% or less according to the present producing method, even when in a cooling step after the soaking treatment, cooling from a temperature of the soaking treatment to approx. 1,000°C is performed at a cooling rate as slow as less than 3°C/min, insofar as cooling thereafter down to 900°C or less is performed at a cooling rate as fast as 3°C/min or more, carbide micronization can be effectively achieved.
In other words, a cooling step according to the present producing method cools a steel ingot after the soaking treatment step until the surface temperature of the steel ingot reaches 900°C or less, such that in the course of cooling down of the surface temperature to 900°C or less at least after the surface temperature has decreased to a temperature T1 within a range of not higher than 1000°C but higher than 900°C, cooling is performed at a cooling rate of the surface temperature of 3°C/min or more.
In the cooling step, cooling until the surface temperature of a steel ingot declines to the temperature T1 may be performed at a cooling rate of the surface temperature of below 3°C/min, however it may also be performed at a cooling rate of the surface temperature of 3°C/min or more.
The cooling rate of 3°C/min or more can be achieved, for example, by air cooling (radiational cooling) or fan cooling on a steel ingot taken out from a soaking treatment furnace.
Since a mode that cooling until the surface temperature of a steel ingot declines to the temperature T1 is performed at a cooling rate of the surface temperature of below 3°C/min, allows spare time in handling a steel ingot after a soaking treatment, and therefore there is an advantage that production of a high speed tool steel becomes easier.
The temperature T1 is a temperature that falls within a range of not higher than 1,000°C but higher than 900°C, preferably a temperature that falls within a range of from 1,000°C to 950°C, more preferably a temperature that falls within a range of from 1,000°C to 970°C, and especially preferably 1,000°C.
A cooling step is preferably a step in which cooling is performed, at least after the surface temperature of a steel ingot is cooled down to 950°C, at a cooling rate of the surface temperature of a steel ingot of 3°C/min or more until the surface temperature of the steel ingot reaches 900°C or less.
Further, a cooling step is more preferably a step in which cooling is performed, at least after the surface temperature of a steel ingot is cooled down to 1,000°C, at a cooling rate of the surface temperature of a steel ingot of 3°C/min or more until the surface temperature of the steel ingot reaches 900°C or less.
In a cooling step, a cooling rate after cooled down to the temperature T1 is 3°C/min or more, and the cooling rate is preferably 10°C/min or more, more preferably 20°C/min or more, further preferably 30°C/min or more, and especially preferably 40°C/min or more.
Meanwhile, in a cooling step, there is no particular restriction on the upper limit of a cooling rate after cooled down to the temperature T1, and the upper limit is preferably 100°C/min, and more preferably 80°C/min.

- Hot Working Step -

A hot working step is a step for reheating a steel ingot after the cooling step to a hot working temperature higher than 900°C, and hot-working the reheated steel ingot into a steel product. The hot working temperature means a temperature for initiating the hot working.
Reheating and hot working to be performed in a hot working step may be carried out by the same methods as in Patent Document 1. For example, hot working is performed for purposes of improvement of a cast structure of a steel ingot, adjustment to a predetermined size of a steel material, etc. Hot working may be carried out following prevailing cogging conditions of forging, rolling, etc.
A hot working temperature of a steel ingot after the cooling step is higher than 900°C, preferably 950°C or more, more preferably 1,000°C or more, and especially preferably 1,050°C or more.
There is no particular restriction on the upper limit of a hot working temperature of a steel ingot after the cooling step, and the upper limit is preferably 1,250°C, more preferably 1,200°C, and especially preferably 1,150°C.

- Quenching and Tempering Step -

A quenching and tempering step is a step for quenching and tempering a steel material yielded by the hot working. A steel material after quenching and tempering is superior in toughness, since a carbide contained in a structure is adjusted to minute particles.
Quenching and tempering in a quenching and tempering step may be carried out by the same methods as in Patent Document 1, and carried out according to prevailing conditions, etc.
With respect to quenching and tempering in a quenching and tempering step, a quenching temperature may be selected appropriately in a range of 900°C or higher. A quenching temperature is more preferably 950°C or more, and further preferably 1,000°C or more. There is no particular restriction on the upper limit of a quenching temperature, and it is preferably 1,250°C, and more preferably 1,200°C
With respect to quenching and tempering in a quenching and tempering step, a tempering temperature may be appropriately selected in a range of from 500 to 650°C.
A quenching and tempering step is preferably a step for adjusting the hardness of a steel material (steel product) by quenching and tempering to 45 HRC or more (more preferably from 45 to 60 HRC).
In other words, the hardness of a steel product after quenching and tempering in the step is preferably 45 HRC or more (more preferably from 45 to 60 HRC).

- Machining Step -

The present producing method may further include a machining step for machining the steel material into a tool shape after the hot working step and before the quenching and tempering step, and the quenching and tempering step may be a step for quenching and tempering the steel material machined into a tool shape.
Such a mode of the present producing method can produce a steel material in a tool shape (namely, tool product) efficiently. In other words, in view of production of a tool product, such as a die or a punch, using a steel material, the state of a steel material after hot working is preferably an annealed state with a low hardness. For producing a tool product, it is efficient to machine a steel material in such an annealed state, and thereafter to conduct quenching and tempering.

[Example]

The invention will be described more specifically below by way of Examples, provided that he invention be not limited in any way by the Example.

[Example 1]

A molten steel adjusted to a predetermined ingredient composition was prepared by an atmospheric dissolving method.
For a molten steel to be used for the the present Inventive Example (steel ingot A), the molten steel was subjected further to refining by a ladle refining method to lower the N content.

Next, the molten steel (with respect to a molten steel to be used for Inventive Example is a molten steel adjusted to a low N content) was cast to prepare an electrode (electrode for remelting) for electro-slag remelting. Next, electro-slag remelting was conducted on the electrode to produce a steel ingot A or a steel ingot B of a high speed tool steel having an ingredient composition listed in Table 1 in which the balance was Fe and impurities.

[Table 1]

Steel ingot	Ingredient composition (mass%) *[N]: ppm													Remarks
Steel ingot	C	Si	Mn	P	S	Ni	Cr	W	Mo	V	Co	Nb	[N]	Remarks
A	0.52	0.16	0.48	0.019	0.0006	0.28	4.13	1.56	1.96	1.15	0.77	0.03	128	Inventive Example
B	0.53	0.13	0.43	0.018	0.0001	0.19	4.13	1.55	1.97	1.15	0.77	0.03	296	Comparative Example

A soaking treatment was conducted on each of the steel ingot A and the steel ingot B, by which the ingot was kept at 1,280°C for 10 hours (soaking treatment step), then cooled under any one of the cooling conditions 1 to 4 presented in Figure 1 (cooling step).
The cooling condition 1 is a cooling condition, under which a steel ingot after a soaking treatment is cooled slowly (cooling rate: 0.5°C/min) until the surface temperature of the steel ingot decreases from a soaking treatment temperature (1,280°C) to 1,200°C, and after the surface temperature of the steel ingot is lowered to 1,200°C air cooling by fan cooling (cooling rate: approx. 50°C/min) is conducted until the surface temperature of the steel ingot reaches 900°C or less.
The cooling condition 2 is a condition, under which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1,200°C of the cooling condition 1 to 1,100°C.
The cooling condition 3 is a condition, under which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1,200°C of the cooling condition 1 to 1,000°C.
The cooling condition 4 is a condition, under which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1,200°C of the cooling condition 1 to 900°C.
With respect to each steel ingot after the cooling step, the distribution situation of a carbide in a structure (dissolved situation in a matrix) was examined.
The sectional structure of each sample taken from a steel ingot was observed with a scanning electron microscope (magnification 50x) and the observed visual field was analyzed by an EPMA. Then, based on the contents of V and Nb forming carbides, a binarization processing was conducted with respect to the analysis result putting 10 cps of detected intensities for V and Nb as a threshold value. From this, a binarized image showing carbides of V and Nb distributed in a sectional structure was obtained.
Figure 2 shows a binarized image for each steel ingot. In Figure 2 carbides appear as dispersed black spots.
As shown in Figure 2 for the steel ingot A cooled under the cooling condition 1, the steel ingot A cooled under the cooling condition 2, the steel ingot A cooled under the cooling condition 3, the steel ingot B cooled under the cooling condition 1, and the steel ingot B cooled under the cooling condition 2, dispersed black spots (clear existence of a carbide) were not recognized.
From Figure 2, in the case of the steel ingot A with a N content of 0.0200% or less, even when the surface temperature of the steel ingot was cooled slowly until it decreased to 1,000°C before the same was cooled at a cooling rate of 3°C/min or more in a cooling process after a soaking treatment (cooling condition 3), a large carbide was not recognized in a steel ingot structure after cooling. The same results were obtained for the steel ingot A whose N content was adjusted to a level of 150 ppm or 180 ppm (not illustrated).
In contrast thereto, in the case of the steel ingot B with a N content higher than 0.0200%, when the surface temperature of the steel ingot was cooled slowly until it decreased to 1,000°C (cooling condition 3), even if the same was cooled at a cooling rate of 3°C/min or more after the surface temperature of the steel ingot decreased to 1,000°C, a carbide was clearly recognized.
The results were obtained because by regulating the N content in a high speed tool steel to 0.0200% or less (steel ingot A), the precipitation and growth temperature of a carbide during cooling was lowered to approx. 1,000°C.

[Example 2]

The steel ingot A (N at 0.0128%) cooled under the cooling condition 1 (after a soaking treatment, cooled slowly to 1,200°C) in Example 1, and the steel ingot B (N at 0.0296%) cooled under the cooling condition 1 (after a soaking treatment, cooled slowly to 1,200°C) in Example 1 were respectively reheated to a hot working temperature of 1,100°C, and the reheated steel ingots were hot-pressed and hot-rolled for cogging. The respective cogged steel ingots (billets) were subjected to hot rolling to complete round bar steel materials with a cross-section diameter of 100 mem (hot working step).
A portion was sampled from each round bar steel material and each of the obtained sample was subjected to quenching from 1,080°C and tempering at 560°C to obtain an evaluation sample (high speed tool steel) adjusted to a hardness of 56 HRC (quenching and tempering step).
As above, evaluation samples for Inventive Example (high speed tool steel produced using the steel ingot A), and evaluation samples for Comparative Example (high speed tool steel produced using the steel ingot B) were obtained respectively.
Next, a carbide distribution in a sectional structure of each of the the evaluation samples was examined as follows.
Firstly, a sectional structure of the evaluation sample was observed with a scanning electron microscope (magnification 4,000x).
Figure 3 is a scanning electron micrograph of a sectional structure of an evaluation sample of Inventive Example (high speed tool steel produced using the steel ingot A), and Figure 4 is a scanning electron micrograph of a sectional structure of an evaluation sample of Comparative Example (high speed tool steel produced using the steel ingot B).
In Figure 3 and Figure 4, carbides that did not dissolve and remained in a matrix (insoluble carbides) are recognizable.
Next, a visual field observed as above was analyzed by an EPMA to obtain a structure image having per each visual field 1,200 x 1,000 pixels (area 29.19 µm x 23.92 µm). For each evaluation sample structure images of 10 visual fields (total area of 6,982.2 µm² per each evaluation sample) were obtained.
Then the structure images were subjected to a image processing for contrasting a carbide with a matrix using an image analysis software (SCANDIUM software, produced by Olympus Corporation). A matrix and a carbide were distinguished by this means, and the particle size distribution of a carbide was measured.
The particle size distribution of a carbide was measured by examining a relationship between an equivalent circle diameter of a carbide and a number density (mm^-2).
Figure 5 is a graph showing the relationship between an equivalent circle diameter and a number density (mm^-2) of a carbide.
The note of "total 176 × 10³ mm^-2" or "total 180 × 10³ mm^-2" in Figure 5 refers to a number density of all carbides (mm^-2) obtained by summing up each number density of an equivalent circle diameter.
As shown in Figure 3 to Figure 5, in the case of an evaluation sample of Inventive Example (high speed tool steel), the maximum value of an equivalent circle diameter of a carbide in a sectional structure was 1.00 µm or less.
On the other hand in the case of an evaluation sample of Comparative Example (high speed tool steel), not a few carbides have an equivalent circle diameter higher than 1.00 µm.
As above, it has become clear that a carbide in a high speed tool steel of Inventive Example is finer than a carbide in a high speed tool steel of Comparative Example. Further, in a high speed tool steel of Inventive Example, a number density of all carbides was 80 x 10³ mm^-2 or more indicating that a large number of fine carbides were formed.
Next, the toughness was evaluated on an evaluation sample of Inventive Example and an evaluation sample of Comparative Example respectively by conducting a Charpy impact test.
The notch shape of a specimen for a Charpy impact test was 10R.
As a specimen for a Charpy impact test, 2 types, namely a specimen cut out such that the length of the specimen corresponded to the longitudinal direction (hot-working direction) of the round bar steel material, and a specimen cut out such that the length of the specimen corresponded to the radial direction of the round bar steel material, were used.
With respect to each of the 2 types, 3 specimens cut out from different positions (TP1, TP2, and TP3) were prepared respectively and subjected to a Charpy impact test.

The test results of Charpy impact tests are shown in Table 2.

[Table 2]

	Charpy impact value (J/cm²)
	Longitudinal direction of round bar steel material				Radial direction of round bar steel material
	TP1	TP2	TP3	Mean	TP1	TP2	TP3	Mean
Inventive Example (Steel ingot A)	401	378	401	393	252	288	273	271
Comparative Example (Steel ingot B)	275	285	322	294	201	150	147	166

As seen in Table 2, a high speed tool steel material of Inventive Example had a larger Charpy impact value than a high speed tool steel of Comparative Example, and was superior in toughness.
Figure 6 and Figure 7 are individually a scanning electron micrograph showing a fracture surface near a notch after a Charpy impact test on a specimen TP2 cut out in a radial direction of a round bar steel material with respect to a high speed tool steel of Inventive Example or Comparative Example.
As shown in Figure 6, in the case of a high speed tool steel of Inventive Example, a large factor which may impair an impact value was not recognized at an origin of the fracture surface.
On the other hand, as shown in Figure 7, in the case of a high speed tool steel of Comparative Example, a large carbide with an equivalent circle diameter exceeding 1.00 µm was observed at an origin of the fracture surface (circled region). In other words, it was confirmed that the large carbide constituted an origin of destruction to impair the toughness of a high speed tool steel of Comparative Example.
The entire disclosures of Japanese Patent Application No. 2013-201392 filed on 27 September 2013 are hereby incorporated by reference.
All the document, patent application, and technical standards cited herein are also herein incorporated to the same extent as provided for specifically and severally with respect to an individual document, patent application, and technical standard to the effect that the same should be so incorporated by reference.

Claims

A high speed tool steel, comprising by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo at a content determined by a relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb at a content determined by the relational expression (V+Nb) of from 0.50 to 3.00%, wherein a content of N is 0.0200% or less by mass%, the balance comprises Fe and impurities, and a maximum value of an equivalent circle diameter of a carbide in a sectional structure is 1.00 µm or less.
The high speed tool steel according to claim 1, further comprising: Ni at 1.00% or less by mass%.
The high speed tool steel according to claim 1 or claim 2, further comprising: Co at 5.00% or less by mass%.
The high speed tool steel according to any one of claim 1 to claim 3, wherein the content of Si is 0.20% or less by mass%.
The high speed tool steel according to any one of claim 1 to claim 4, having a hardness of 45 HRC or higher.
A method for producing a high speed tool steel, the method comprising:
a preparation step of preparing a steel ingot comprising by mass-%: C at from 0.40 to 0.90%; Si at 1.00% or less; Mn at 1.00% or less; Cr at from 4.00 to 6.00%; one or both of W and Mo at a content determined by the relational expression (Mo+0.5W) of from 1.50 to 6.00%; and one or both of V and Nb at a content determined by a relational expression (V+Nb) of from 0.50 to 3.00%, wherein a content of N is 0.0200% or less by mass%, and the balance comprises Fe and impurities;

a soaking treatment step of performing a soaking treatment by heating the steel ingot at from 1,200 to 1,300°C;

a cooling step of cooling the steel ingot after the soaking treatment step until a surface temperature of the steel ingot reaches 900°C or less, wherein at least after the surface temperature has decreased to a temperature T1 within a range of not higher than 1,000°C but higher than 900°C, cooling is performed at a cooling rate of the surface temperature of 3°C/min or more until the surface temperature reaches 900°C or less,;

a hot working step of reheating the steel ingot after the cooling step to a hot working temperature higher than 900°C, and hot-working the reheated steel ingot into a steel product; and

a quenching and tempering step of quenching and tempering the steel product.
The method for producing a high speed tool steel according to claim 6, wherein, in the cooling step, cooling of the steel ingot is performed at a cooling rate of the surface temperature of the steel ingot of less than 3°C/min until the surface temperature has decreased to the temperature T1.
The method for producing a high speed tool steel according to claim 6 or claim 7, wherein the steel ingot prepared in the preparation step is a steel ingot yielded by casting molten steel that has been refined by a deoxidizing refining method.
The method for producing a high speed tool steel according to claim 8, wherein the steel ingot prepared in the preparation step is a steel ingot yielded by casting molten steel that has been refined by a deoxidizing refining method to yield an electrode for remelting, and by applying a remelting method to the electrode for remelting.
The method for producing a high speed tool steel according to any one of claim 6 to claim 9, wherein the steel ingot prepared in the preparation step further comprises Ni at 1.00% or less by mass%.
The method for producing a high speed tool steel according to any one of claim 6 to claim 10, wherein the steel ingot prepared in the preparation step further comprises Co at 5.00% or less by mass%.
The method for producing a high speed tool steel according to any one of claim 6 to claim 11, wherein the content of Si in the steel ingot prepared in the preparation step is 0.20% or less by mass%.
The method for producing a high speed tool steel according to any one of claim 6 to claim 12, wherein, in the quenching and tempering step, the hardness of the steel product is adjusted to 45 HRC or more by the quenching and tempering.
The method for producing a high speed tool steel according to any one of claim 6 to claim 13, further comprising a machining step of machining the steel product into a tool shape after the hot working step but before the quenching and tempering step, wherein, in the quenching and tempering step, the steel product that has been machined into a tool shape is subjected to quenching and tempering.