WO2015045528A1 - High-speed-tool steel and method for producing same - Google Patents

Abstract

This high-speed-tool steel contains, in mass%: C in the amount of 0.40-0.90%; Si in the amount of 1.00% or less; Mn in the amount of 1.00% of less; Cr in the amount of 4.00-6.00% or less; W and/or Mo in the amount of 1.50-6.00% calculated using the relational expression (Mo+0.5W); V and/or Nb in the amount of 0.50-3.00% calculated using the relational expression (V+Nb); and N in the amount of 0.0200 mass% or less; with Fe and impurities constituting the remainder. Therein, the maximum value of the equivalent circle diameter of a carbide in the sectional structure is 1.00μm or less.

Description

High speed tool steel and manufacturing method thereof

The present invention relates to a high-speed tool steel used for tools such as dies and punches and a method for producing the same.

Conventionally, as materials for various tools such as dies and punches used for hot working, C (carbon) and carbide together with C are formed with respect to the component composition of SKH51, which is a typical steel type of high-speed tool steel. Low-alloy high-speed tool steel that has improved toughness by reducing the contents of Mo, W, and V is used. With regard to this low alloy high speed tool steel, a steel ingot having the composition of the high speed tool steel is subjected to high temperature soaking (soaking) at 1200 to 1300 ° C. and then cooled at 3 ° C./min or more. By cooling at a speed until the surface temperature of the steel ingot becomes 900 ° C. or less, the agglomeration of carbides in the structure after quenching and tempering is suppressed, the average particle size of the carbides is 0.5 μm or less, and the distribution thereof A high-speed tool steel with increased toughness with a density of 80 × 10 ³ pieces / mm ² or more has been proposed (Patent Document 1 below).

Patent Document 1: Japanese Patent Application Laid-Open No. 2004-307963

The technique of Patent Document 1 is effective in improving the toughness of a low-alloy high-speed tool steel.
However, even in the high-speed tool steel manufactured by the technique of Patent Document 1, there are not a few carbides whose individual particle sizes exceed 0.5 μm in the structure after quenching and tempering. For this reason, in the method of Patent Document 1, there is a case where the effect of improving the toughness of the high-speed tool steel cannot be obtained sufficiently.

An object of the present invention is to provide a high-speed tool steel having further improved toughness and a method for producing the same.

Specific means for solving the above problems are as follows.
<1> By mass%, C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 4.00 to 6.00%, W and Mo One or two of: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and one or two of V and Nb: determined by the relational expression (V + Nb) The content is 0.50 to 3.00%, the N content is 0.0200% or less by mass, the balance is Fe and impurities, and the equivalent circle diameter of the carbide in the cross-sectional structure is High-speed tool steel with a maximum value of 1.00 μm or less.
<2> The high-speed tool steel according to <1>, further containing Ni: 1.00% or less by mass%.
<3> The high-speed tool steel according to <1> or <2>, further containing, by mass%, Co: 5.00% or less.
<4> The high-speed tool steel according to any one of <1> to <3>, wherein the Si content is 0.20% or less by mass%.
<5> The high-speed tool steel according to any one of <1> to <4>, wherein the hardness is 45 HRC or more.

<6> By mass%, C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 4.00 to 6.00%, W and Mo One or two of: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and one or two of V and Nb: determined by the relational expression (V + Nb) A preparatory step of preparing a steel ingot containing 0.50 to 3.00% as content, N content is 0.0200% or less by mass, and the balance is Fe and impurities;
A soaking process in which the steel ingot is soaked by heating to 1200 to 1300 ° C .;
In the process of cooling the steel ingot after the soaking process until the surface temperature of the steel ingot becomes 900 ° C. or lower, at least the surface temperature falls to a temperature T1 included in the range of 1000 ° C. or lower and over 900 ° C. After that, a cooling step of cooling until the surface temperature becomes 900 ° C. or less under the condition that the cooling rate of the surface temperature is 3 ° C./min or more,
Reheating the steel ingot after the cooling step to a hot working temperature of more than 900 ° C., hot working the reheated steel ingot to obtain a steel material; and
A quenching and tempering step for quenching and tempering the steel material;
The manufacturing method of the high-speed tool steel which has.

<7> In the cooling step, the steel ingot is cooled under the condition that the cooling rate of the surface temperature is less than 3 ° C./min until the surface temperature of the steel ingot decreases to the temperature T1. The manufacturing method of the described high-speed tool steel.
<8> The high speed tool steel according to <6> or <7>, wherein the steel ingot prepared in the preparation step is a steel ingot obtained by casting molten steel refined by a deoxidation refining method. Production method.
<9> The steel ingot to be prepared in the preparation step is obtained by remelting using the obtained remelting electrode by casting molten steel refined by deoxidation refining to obtain a remelting electrode. The manufacturing method of the high-speed tool steel as described in <8> which is a steel ingot.
<10> The high speed tool steel according to any one of <6> to <9>, wherein the steel ingot prepared in the preparation step further contains, by mass%, Ni: 1.00% or less. Method.
<11> The high-speed tool steel according to any one of <6> to <10>, wherein the steel ingot to be prepared in the preparation step further contains, by mass%, Co: 5.00% or less. Method.
<12> The steel ingot to be prepared in the preparation step has a Si content of 0.20% or less by mass%, and the high-speed tool steel according to any one of <6> to <11>. Production method.
<13> The method for producing high-speed tool steel according to any one of <6> to <12>, wherein the quenching and tempering step adjusts the hardness of the steel material to 45 HRC or more by the quenching and tempering.
<14> After the hot working step and before the quenching and tempering step, further comprising a machining step of machining the steel material into a tool shape,
The method for producing high-speed tool steel according to any one of <6> to <13>, wherein the quenching and tempering step includes quenching and tempering a steel material machined into a tool shape.

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

FIG. 3 is a conceptual diagram for explaining soaking and cooling processes (cooling conditions 1 to 4) performed on a steel ingot in Example 1. FIG. In Example 1, the binarized images obtained by analyzing the cross-sectional structures of the steel ingot A and the steel ingot B cooled under each of the cooling conditions 1 to 4 with an EPMA (electron beam microanalyzer) It is a figure which shows carbide distribution in a cross-sectional structure | tissue. In Example 2, it is a scanning electron micrograph which shows an example of the carbide | carbonized_material distributed in the cross-sectional structure of the high speed tool steel of the example of this invention. In Example 2, it is a scanning electron micrograph which shows an example of the carbide | carbonized_material distributed in the cross-sectional structure of the high speed tool steel of a comparative example. In Example 2, it is a graph which shows the relationship between the circle equivalent diameter and number density (pieces / mm < ² >) of the carbide | carbonized_material in the cross-sectional structure | tissue of the high-speed tool steel of the example of this invention and a comparative example. In Example 2, it is a scanning electron micrograph which shows an example of the fracture surface after a test when the Charpy impact test is implemented to the high speed tool steel of the example of this invention. In Example 2, it is a scanning electron micrograph which shows an example of the fracture surface after a test when the Charpy impact test is implemented to the high-speed tool steel of a comparative example.

In the present specification, “%” indicating the content of each component (each element) indicates “mass%”.
In the present specification, numerical ranges indicated by using “to” indicate ranges including the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the present specification, “hardness” represented by the unit “HRC” represents C-scale Rockwell hardness defined in JIS G 0202 (2013).

Hereinafter, the high-speed tool steel of the present invention and the manufacturing method thereof will be described in detail.

<High speed tool steel>
The high-speed tool steel of the present invention is, in mass%, C (carbon): 0.40 to 0.90%, Si (silicon): 1.00% or less, Mn (manganese): 1.00% or less, Cr (Chromium): 4.00 to 6.00%, one or two of W (tungsten) and Mo (molybdenum): 1.50 to 6.5 as the content determined by the relational expression (Mo + 0.5W) 00%, and one or two of V (vanadium) and Nb (niobium): 0.50 to 3.00% as a content determined by the relational expression (V + Nb), N (nitrogen) The content of is not more than 0.0200% by mass, the balance is made of Fe (iron) and impurities, and the maximum equivalent circle diameter of the carbide in the cross-sectional structure is 1.00 μm or less.

In the present invention, the concept of “carbide” in the cross-sectional structure of the high-speed tool steel includes not only carbides not containing nitrogen but also carbides containing nitrogen (ie, carbonitrides).

As described above, the high-speed tool steel of the present invention has an N content of 0.0200% or less.
N is an impurity element inevitably contained in the steel ingot after casting.

Generally, when the composition of the molten steel before casting is adjusted exclusively in the air environment, the steel ingot after casting usually has an N content of about 0.0300% or more. May be included.
N is an element having strong affinity with V and Nb which are carbide forming elements.
Therefore, in high-speed tool steel containing a large amount of N, in the solidification process at the time of casting, V and Nb are combined with N before being crystallized as carbides (eutectic carbides). Crystallize. Next, carbides crystallize around the nitrides to form carbonitrides.
The carbonitride is a thermally stable compound.
Therefore, if a large amount of the carbonitride is formed in the steel ingot, it is difficult to dissolve the carbonitride in the base in the soaking process or the hot working process as the next process. As a result, a large amount of the carbonitride remains in the structure of the high-speed tool steel (including tool products; the same applies hereinafter) manufactured through the soaking process and the hot working process. The toughness of the steel decreases. And since the said carbonitride becomes the starting point of destruction, the early crack of high-speed tool steel becomes easy to occur, and the lifetime of high-speed tool steel falls by extension.

In view of the above points, in the high-speed tool steel of the present invention, the N content is 0.0200% or less in order to suppress the formation amount of the carbonitride.
Thereby, the form of the carbonitride crystallized in the steel ingot can be changed to a form of carbide not containing nitrogen.
Carbide containing no nitrogen can be easily dissolved in the matrix by soaking. For this reason, since the carbide | carbonized_material distributed in high speed tool steel can be made finer by making content of N into 0.0200% or less, the toughness of high speed tool steel can be improved more. .

Further, as described above, in the high-speed tool steel of the present invention, the maximum value of the equivalent circle diameter of the carbide in the cross-sectional structure is 1.00 μm or less.

Regarding the carbide particle size, in the high-speed tool steel described in Patent Document 1 described above, the average particle size of the carbide is 0.5 μm or less.
However, according to the inventor's study, even when the average particle size of carbides in the high-speed tool steel is 0.5 μm or less, the particle size greatly exceeds 1.00 μm in the high-speed tool steel. It has been found that coarse carbides may be present. Furthermore, it has been found that the presence of such coarse carbides may not sufficiently improve the toughness of high-speed tool steel.
With respect to these points, in the high-speed tool steel of the present invention, the toughness of the high-speed tool steel is further improved because the maximum value of the equivalent circle diameter of the carbide in the cross-sectional structure is 1.00 μm or less.
When the maximum equivalent circle diameter of carbide in the cross-sectional structure of high-speed tool steel exceeds 1.00 μm, carbide with a large particle size (especially carbide with an equivalent circle diameter exceeding 1.00 μm) becomes the starting point of fracture. It becomes easy and the toughness of high-speed tool steel decreases. Carbides having an equivalent circle diameter of more than 1.00 μm are carbides (undissolved carbides) that do not form a solid solution in the base at the quenching temperature (approximately austenitizing temperature of approximately 900 ° C. or higher) in the quenching step.

In the high-speed tool steel of the present invention, the maximum value of the equivalent circle diameter of the carbide in the cross-sectional structure may be 1.00 μm or less, and as long as this condition is satisfied, the average particle diameter of the carbide is 0.5 μm or less. Needless to say, it may be.

As described above, in the high-speed tool steel of the present invention, the N content is 0.0200% or less, and the maximum circle equivalent diameter of carbide in the cross-sectional structure is 1.00 μm or less. As a result, the toughness is further improved as compared with conventional high-speed tool steel (for example, the high-speed tool steel described in Patent Document 1).

In the high-speed tool steel of the present invention, the N content is preferably 0.0180% or less, more preferably 0.0150% or less.
In the high-speed tool steel of the present invention, the lower limit of the N content is not particularly limited, but the lower limit of the N content can be 0.0005%, for example, or can be 0.0010%. it can.

In the high-speed tool steel of the present invention, as described above, the maximum value of the equivalent circle diameter of the carbide in the cross-sectional structure is 1.00 μm or less, and the maximum value of the equivalent circle diameter is preferably 0.90 μm. Below, more preferably 0.80 μm or less.

In the high-speed tool steel of the present invention, the distribution density of carbides can be 80 × 10 ³ pieces / mm ² or more. By increasing the distribution density of the carbide, the prior austenite grain size after quenching and tempering can be made finer, and the toughness of the high-speed tool steel can be further increased.

In the present invention, in order to specify the maximum value of the equivalent circle diameter of carbide and the distribution density of carbide, the cross-sectional structure of the high-speed tool steel is, for example, a total field of view of 5000 μm ² or more using a scanning electron microscope with a magnification of 4000 times. It is sufficient to observe and specify the area.
And the high speed tool steel when the said carbide | carbonized_material is specified generally has the shape of various tool products. The site where the occurrence of cracks due to the carbide in the shape of the tool product is a concern, for example, is the work surface of the tool product, and in particular, the corner portion (in contact with other members of the work surface ( Outer corner, inner corner). Therefore, the site | part of the high speed tool steel which specifies the said carbide | carbonized_material can be made into the cross-sectional structure | tissue containing the said corner part, for example.
Carbides that have a great influence on the toughness of the tool are carbides (undissolved carbides) that do not dissolve in the matrix at the quenching temperature (approximately 900 ° C. or higher austenitizing temperature).

The high-speed tool steel of the present invention preferably has a hardness of 45 HRC or more.
When the high-speed tool steel of the present invention is used for various tools, the tensile strength excellent in the tool can be imparted by setting the use hardness to 45 HRC or more. In particular, when used in various hot tools, excellent tensile strength at high temperatures can be imparted by setting the used hardness (hardness at room temperature) to 45 HRC or higher.
More preferably, the high-speed tool steel of the present invention has a hardness of 45 HRC to 60 HRC.

The component composition of the high-speed tool steel of the present invention is common to the component composition of the high-speed tool steel of Patent Document 1 in the basic configuration other than the N content.
Hereinafter, each component other than N of the high-speed tool steel of the present invention will be described.

・ C: 0.40-0.90%
C is an element that imparts wear resistance to high-speed tool steel by combining with carbide-forming elements such as Cr, Mo, W, V, and Nb to form hard double carbide. A part of C is dissolved in the base to strengthen the base. Thereby, a part of C gives hardness to the martensitic structure after quenching and tempering. However, an excessive amount of C promotes segregation of carbides. Therefore, the C content is set to 0.40 to 0.90%.

-Si: 1.00% or less Si is an element that is normally used as a deoxidizer in the melting step and is unavoidably contained in the steel ingot after casting. However, when there is too much content of Si, the toughness of high speed tool steel will fall. Therefore, the Si content is set to 1.00% or less. Note that the Si, an effect of refining the primary carbides in the spherical rod-shaped M 2 _C type. Therefore, the Si content is preferably 0.10% or more.
Moreover, it is preferable that content of Si is 0.20% or less from the following viewpoints.
That is, when the Si content is 0.20% or less, the effect of refining the primary carbide into a spherical shape tends to be weakened. Therefore, when Si is 0.20% or less, compared with the case where Si exceeds 0.20%, the effect of making the N content 0.0200% or less, and in the cross-sectional structure The effect by making the maximum value of the equivalent circle diameter of the carbide of 1.00 μm or less is more remarkable.

-Mn: 1.00% or less Mn is an element that is used as a deoxidizer in the melting step, as in Si, and is unavoidably contained in the steel ingot after casting. However, when the Mn content is too large, the A ₁ transformation point is excessively reduced, annealed hardness increases, machinability of high-speed tool steel (machinability) is lowered. Therefore, the Mn content is 1.00% or less. In addition, Mn has the effect | action which improves hardenability. Accordingly, the content is preferably 0.10% or more.

・ Cr: 4.00 ~ 6.00%
Cr is an element that combines with C to form carbides and improves the wear resistance of the high-speed tool steel. Cr is an element that also contributes to improving the hardenability of the high-speed tool steel. However, when there is too much content of Cr, striped segregation is promoted and the toughness of high speed tool steel will fall. Therefore, the Cr content is 4.00 to 6.00%.

One or two of W and Mo: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W)
W and Mo are elements that combine with C to form carbides, and dissolve in the matrix during quenching to increase the hardness and improve the wear resistance of high-speed tool steel. However, when there is too much content of W and Mo, striped segregation is promoted and the toughness of high-speed tool steel will fall.

Regarding the above-described effects, the contents of W and Mo refer to the contents obtained 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 the high-speed tool steel of the present invention, the content of one or two of W and Mo is set to 1.50 to 6.00% as the content obtained by the relational expression (Mo + 0.5W).
The high speed tool steel of the present invention may contain only one (one) of W and Mo, or may contain two (both) of W and Mo. That is, either “Mo” or “W” in the relational expression (Mo + 0.5W) may be 0%.

Note that W has the ability to promote striped segregation compared to Mo and tends to impair the toughness of high-speed tool steel. Therefore, the W content in the high-speed tool steel is preferably 3.00% or less (0.50 or less as 0.5 W in the relational expression (Mo + 0.5W)).

One or two of V and Nb: 0.50 to 3.00% as the content determined by the relational expression (V + Nb)
V and Nb combine with C to form a carbide, and improve the wear resistance and seizure resistance of the high-speed tool steel. V and Nb are dissolved in the matrix during quenching and precipitate fine and hard-to-aggregate carbides during tempering, thereby improving softening resistance in high-temperature environments and high temperature Gives strength. Then, V and Nb, as well to the fine grain, A ₁ transformation point be increased, improving the toughness and heat crack resistance of high-speed tool steel. However, when there is too much content of V and Nb, a big carbide | carbonized_material will be produced | generated and generation | occurrence | production of the crack at the time of use as a tool will be promoted.

Regarding the above-described effects, the contents of V and Nb refer to the contents obtained by the relational expression (V + Nb).
In the high-speed tool steel of the present invention, the content of one or two of V and Nb is 0.50 to 3.00% as the 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).
The high speed tool steel of the present invention may contain only one (one) of V and Nb, or may contain two (both) of V and Nb. That is, either “V” or “Nb” in the relational expression (V + Nb) may be 0%.
The content obtained by the relational expression (V + Nb) is preferably 1.50% or less.

Note that Nb is superior to V in softening resistance, high temperature strength improvement effect, and crystal grain coarsening suppression effect. Therefore, it is preferable that the high-speed tool steel of the present invention contains Nb (that is, the Nb content is more than 0%).

-Preferably, Ni: 1.00% or less Ni imparts excellent hardenability to the high-speed tool steel. As a result, a hardened structure mainly composed of martensite can be formed, and the essential toughness of the base itself can be improved. However, when the content of Ni is too large A ₁ transformation point is excessively lowered, the higher the annealing hardness of high speed tool steel, machining of high-speed tool steel is reduced. Therefore, even when the high-speed tool steel contains Ni, the Ni content is preferably 1.00% or less. And when high-speed tool steel contains Ni, 0.05% or more of content of Ni is preferable.

-Preferably, Co: 5.00% or less Co has an effect of forming a protective oxide film having a very dense and good adhesion on the surface of the tool when the temperature of the tool in use is raised. This reduces the metal contact between the surface of the tool and the counterpart material, reduces the temperature rise on the surface of the tool, and provides the tool with excellent wear resistance. And formation of this protective oxide film increases the heat insulation effect and also improves the heat crack resistance. However, when there is too much content of Co, the toughness of high speed tool steel will fall. Therefore, even when the high-speed tool steel contains Co, the Co content is preferably 5.00% or less. And when high-speed tool steel contains Co, content of Co is preferably 0.30% or more.

In addition, the high-speed tool steel of the present invention may contain, for example, S (sulfur) and P (phosphorus) as inevitable impurity elements.
If the S content is too large, the hot workability of the high-speed tool steel is hindered, so the S content is preferably regulated to 0.0100% or less. The S content is more preferably 0.0050% or less.
If the P content is too high, the toughness of the high-speed tool steel deteriorates, so the P content is preferably regulated to 0.050% or less. The content of P is more preferably 0.025% or less.

The method for producing the high-speed tool steel of the present invention is not particularly limited. For example, the steel ingot having the composition of the high-speed tool steel of the present invention is subjected to soaking (preferably, the steel ingot is 1200 to 1300. Soaking by heating to ℃), cooling (preferably cooling the steel ingot after soaking) until the surface temperature of the steel ingot becomes 900 ° C. or less, hot working (preferably after cooling) A hot working performed by reheating the steel ingot to over 900 ° C.) and quenching and tempering (preferably quenching and tempering at a quenching temperature of 900 ° C. or higher and a tempering temperature of 500 to 650 ° C.). Here, the steel material may be machined into a tool shape between hot working and quenching and tempering.
Among the manufacturing methods described above, the high-speed tool steel of the present invention is particularly easy to manufacture according to the method of manufacturing the high-speed tool steel of the present invention described later.

<Method for producing high-speed tool steel>
The production method of the high-speed tool steel of the present invention (hereinafter also referred to as “the present production method”) is mass%, C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1. 00% or less, Cr: 4.00 to 6.00%, one or two of W and Mo: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and 1 or 2 of V and Nb: 0.50 to 3.00% as a content determined by the relational expression (V + Nb), and N content is 0.0200% or less by mass% There is a preparation step for preparing a steel ingot with the balance being Fe and impurities,
A soaking process in which the steel ingot is soaked by heating to 1200 to 1300 ° C .;
In the process of cooling the steel ingot after the soaking process until the surface temperature of the steel ingot becomes 900 ° C. or lower, at least the surface temperature falls to a temperature T1 included in the range of 1000 ° C. or lower and over 900 ° C. After that, a cooling step of cooling until the surface temperature becomes 900 ° C. or less under the condition that the cooling rate of the surface temperature is 3 ° C./min or more,
Reheating the steel ingot after the cooling step to a hot working temperature of more than 900 ° C., hot working the reheated steel ingot to obtain a steel material; and
A quenching and tempering step for quenching and tempering the steel material;
Have

In this specification, the cooling rate of the surface temperature of the steel ingot is sometimes simply referred to as “cooling rate”.

The inventor examined details of a method of manufacturing high-speed tool steel including soaking, which is proposed in Patent Document 1. As a result, it was confirmed that high-temperature soaking at 1200 to 1300 ° C is really effective for dissolving high-speed carbides in steel ingots for high-alloy tool steels with low alloy composition as in Patent Document 1. did.
However, the present inventor has found that if the cooling process after the soaking is inappropriate, the undissolved or newly precipitated carbide may be coarsened. Therefore, the present inventor has found that by appropriately managing this cooling condition, it is possible to suppress the coarsening of the carbide in the cooling process, and as a result, it is possible to further refine the carbide in the structure of the high-speed tool steel. . Furthermore, the present inventor has determined that the steel ingot itself to be subjected to soaking has a more optimal component composition in order to maintain the effect of carbide refinement under the appropriate cooling conditions. A high-speed tool steel manufacturing method has been reached.

That is, in the method for producing a high-speed tool steel of the present invention, a steel ingot having an N content of 0.0200% or less by mass% is used. For this reason, as explained in the section of “High-speed tool steel”, carbides distributed in the manufactured high-speed tool steel can be made finer, so that high-speed tool steel with further improved toughness can be manufactured. can do.

Furthermore, in this manufacturing method, adjusting the N content in the steel ingot to be subjected to soaking is 0.0200% or less, together with the cooling step in this manufacturing method, the carbide (carbonitride in the structure) Plays an important role in making fine. Details will be described below.

First, according to the method of Patent Document 1, the carbide crystallized in the steel ingot can be dissolved in the matrix by soaking at 1200 to 1300 ° C., which is the next step. And, in the cooling process after the soaking process, the precipitation and growth of carbides of V and Nb can be suppressed by cooling until the surface temperature of the steel ingot becomes 900 ° C. or less at a cooling rate of 3 ° C./min or more. Is possible.
However, in actual operation, it is difficult to cool the steel ingot immediately after the soaking process from the soaking temperature to the temperature of 900 ° C. or less at a cooling rate of 3 ° C./min or more. That is, in actual operation, slow cooling at a cooling rate of less than 3 ° C./min proceeds until the steel ingot is removed from the soaking furnace (that is, furnace cooling progresses in the soaking furnace). When starting cooling at the cooling rate, it is realistic that the surface temperature of the steel ingot is lowered from the soaking temperature.
According to the research of the present inventor, when the surface temperature of the steel ingot has decreased to around 1000 ° C., a large amount of carbides of V and Nb have already precipitated, and the growth has started. all right.

Therefore, by adjusting the N content in the steel ingot to be subjected to soaking, the temperature at which the carbides precipitate and grow can be lowered in the cooling process after the soaking. it can. Specifically, the temperature at which the carbide precipitates and grows can be lowered to 1000 ° C. or less at the surface temperature of the steel ingot. Even if the surface temperature of the steel ingot taken out from the soaking furnace is lowered to around 1000 ° C. due to the decrease in the carbide precipitation and growth temperature, the subsequent cooling is 3 ° C./min or more. If performed at a speed, precipitation and growth of the carbide can be suppressed, so that the refinement of the carbide can be achieved more reliably.

Therefore, in this production method, a steel ingot having a N content of 0.0200% or less is used as a steel ingot to be subjected to soaking, and at least the surface temperature of the steel ingot is 1000 ° C. or less and over 900 ° C. After the temperature T1 falls within the range, a cooling step of cooling until the surface temperature becomes 900 ° C. or less under the condition that the cooling rate of the surface temperature becomes 3 ° C./min or more is obtained. Miniaturization can be achieved more reliably. For this reason, according to this manufacturing method, compared with the conventional high speed tool steel (for example, the high speed tool steel of patent document 1), the high speed tool steel in which the toughness was further improved can be manufactured. .
According to this manufacturing method, for example, a high-speed tool steel (for example, the above-described high-speed tool steel of the present invention) having a maximum equivalent circle diameter of carbide in a cross-sectional structure of 1.00 μm or less can be manufactured. it can.

In addition, according to the present manufacturing method, there is an effect that the time for handling the steel ingot after the soaking process can be afforded by having the cooling step.

Hereinafter, each step of the manufacturing method will be described.

-Preparation process-
The preparatory process is, by mass%, C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 4.00 to 6.00%, W and Mo. One or two of them: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and one or two of V and Nb: by the relational expression (V + Nb) This is a step of preparing a steel ingot containing 0.50 to 3.00% as the required content, the N content being 0.0200% or less by mass, and the balance being Fe and impurities.
The preparation process is a process for convenience.
The preparation step may be a step of manufacturing a steel ingot, or a step of preparing a steel ingot that has been manufactured in advance prior to the manufacture of high-speed tool steel.

About the component composition of the steel ingot prepared by a preparatory process, it is the same as that of the high speed tool steel of this invention mentioned above, and its preferable range is also the same.

By the way, in actual operation, there is a large amount of molten steel to be melted at a time. For this reason, it is not easy to reduce the N content in the steel ingot to 0.0200% or less only by air dissolution.
When trying to reduce the N content in the steel ingot to 0.0200% or less only by air melting, a high-grade raw material with a reduced N content must be used as the raw material before melting. It is disadvantageous in terms of cost.
Therefore, in this production method, the steel ingot prepared in the preparation step is preferably a steel ingot obtained by casting molten steel refined by a deoxidation refining method.
Examples of the deoxidation refining method include various ladle smelting methods such as LF method, ASEA-SKF method, VAD method and VOD method; various vacuum degassing methods such as RH method and DH method.

In actual operation, since each steel ingot is large, segregation in the steel ingot may be increased.
Therefore, the steel ingot prepared in the preparation step is a steel ingot obtained by remelting using the obtained remelting electrode by casting molten steel refined by the deoxidation refining method to obtain a remelting electrode. It is more preferable that By performing the remelting method, segregation in the steel ingot can be improved.
Examples of the remelting method include an electroslag remelting method, a vacuum arc remelting method, a plasma arc remelting method, and an electron beam remelting method. In particular, since the electroslag remelting method uses slag, it is advantageous in reducing impurity elements such as S.

-Soaking process-
The soaking process is a process of soaking by heating the steel ingot prepared in the preparation process to 1200 to 1300 ° C.
In the soaking process, as in the method of Patent Document 1, the steel ingot having the above-described composition is soaked at a high temperature of 1200 to 1300 ° C., so that the huge carbides at the time of casting are dissolved, and the composition components are solidified. It can be dissolved and diffused to improve the distribution of carbides.
The temperature for soaking is 1200 to 1300 ° C., preferably 1260 to 1300 ° C.
The soaking time is preferably 10 to 20 hours.
In addition, while the temperature of general soaking of high-speed tool steel is around 1150 ° C., the temperature of soaking in the soaking process of this production method is higher than the temperature of general soaking.

-Cooling process-
The cooling step is a process in which the steel ingot after the soaking process is cooled until the surface temperature of the steel ingot becomes 900 ° C. or lower, and at least the surface temperature of the steel ingot is included in the range of 1000 ° C. or lower and over 900 ° C. After the temperature T1 is lowered, the steel ingot is cooled until the surface temperature of the steel ingot is 900 ° C. or less under the condition that the cooling rate of the surface temperature of the steel ingot is 3 ° C./min or more.
In the cooling step, the steel ingot is cooled at a cooling rate of 3 ° C./min or more until the surface temperature of the steel ingot becomes 900 ° C. or less. This cooling process reduces the formation of carbides with a large particle size by passing quickly through a temperature range up to 900 ° C. where V and Nb carbides are likely to precipitate and grow, and is preferably finely dispersed in the matrix. This is a step of forming only a small particle size carbide.
However, as described above, it is difficult to cool the steel ingot after soaking at a cooling rate of 3 ° C./min or more from the time when the soaking temperature is maintained to a temperature of 900 ° C. or less.
Therefore, in this production method, the precipitation and growth temperature of carbide during cooling is lowered to around 1000 ° C. by setting the N content in the steel ingot to be subjected to soaking to 0.0200% or less. Succeeded.

In this production method, the steel ingot in which the N content is reduced to 0.0200% or less is soaked, so that in the cooling step after soaking, cooling from the soaking temperature to around 1000 ° C. is performed. Even if the cooling is performed at a slow cooling rate of less than 3 ° C./minute, if the subsequent cooling to 900 ° C. or less is performed at a fast cooling rate of 3 ° C./minute or more, the refinement of carbide can be effectively performed. Can be achieved.
That is, the cooling step in this production method is a process in which the steel ingot after the soaking process is cooled until the surface temperature of the steel ingot becomes 900 ° C. or lower, and at least the surface temperature is 1000 ° C. or lower and over 900 ° C. After the temperature T1 is lowered, cooling is performed until the surface temperature becomes 900 ° C. or lower under the condition that the cooling rate of the surface temperature is 3 ° C./min or higher.

In the cooling step, the cooling until the surface temperature of the steel ingot is lowered to the temperature T1 may be performed under the condition that the cooling rate of the surface temperature is less than 3 ° C / min. The cooling rate of the surface temperature is 3 ° C / min. You may carry out on the conditions which become the above.
The cooling rate of 3 ° C./min or more can be achieved by removing the steel ingot from the soaking furnace and cooling it with air (cooling) or cooling with a fan, for example.
In the embodiment in which the cooling until the surface temperature of the steel ingot is lowered to the temperature T1 is performed under the condition that the cooling rate of the surface temperature is less than 3 ° C./min, the handling time of the steel ingot after soaking can be afforded. It has the advantage that the production of high-speed tool steel is easier.

The temperature T1 is a temperature included in a range of 1000 ° C. or lower and over 900 ° C., but is preferably a temperature included in a range of 1000 ° C. or lower and 950 ° C. or higher, and in a range of 1000 ° C. or lower and 970 ° C. or higher. It is more preferable that the temperature is within the range of 1000 ° C.

In the cooling step, at least after the surface temperature of the steel ingot has decreased to 950 ° C., until the surface temperature of the steel ingot becomes 900 ° C. or less under the condition that the cooling rate of the surface temperature of the steel ingot is 3 ° C./min or more. A cooling step is preferred.
In the cooling step, at least after the surface temperature of the steel ingot has decreased to 1000 ° C., the surface temperature of the steel ingot is set to 900 ° C. or less under the condition that the cooling rate of the surface temperature of the steel ingot is 3 ° C./min or more. It is more preferable that it is the process of cooling until it becomes.

Further, in the cooling step, the cooling rate after the temperature T1 is lowered is 3 ° C./min or more, but this cooling rate is preferably 10 ° C./min or more, more preferably 20 ° C./min or more, and 30 ° C./min. Min. Or more is more preferable, and 40 ° C./min or more is particularly preferable.
In the cooling step, the upper limit of the cooling rate after the temperature T1 is lowered is not particularly limited, but the upper limit is preferably 100 ° C./min, and more preferably 80 ° C./min.

-Hot working process-
The hot working step is a step in which the steel ingot after the cooling step is reheated to a hot working temperature exceeding 900 ° C., and the reheated steel ingot is hot worked to obtain a steel material. The hot working temperature is a temperature at which the hot working is started.
The reheating and hot working performed in the hot working process may be performed in the same manner as in Patent Document 1. For example, the hot working is performed for the purpose of improving the cast structure of the steel ingot, adjusting to a predetermined steel material size, and the like. What is necessary is just to perform hot processing according to the lump conditions, such as forging and rolling currently implemented normally.
The hot working temperature of the steel ingot after the cooling step is higher than 900 ° C, preferably 950 ° C or higher, more preferably 1000 ° C or higher, and particularly preferably 1050 ° C or higher.
The upper limit of the hot working temperature of the steel ingot after the cooling step is not particularly limited, but the upper limit is preferably 1250 ° C, more preferably 1200 ° C, and particularly preferably 1150 ° C.

-Quenching and tempering process-
The quenching and tempering step is a step of quenching and tempering the steel material obtained by the hot working. The steel material after quenching and tempering has fine toughness in carbides contained in the structure and has excellent toughness.
Quenching and tempering in the quenching and tempering step may be performed in the same manner as in Patent Document 1, and may be performed in accordance with conditions or the like that are normally performed.
In the quenching and tempering in the quenching and tempering step, the quenching temperature can be appropriately selected from a range of 900 ° C. or higher. The quenching temperature is more preferably 950 ° C. or higher, further preferably 1000 ° C. or higher. Although there is no restriction | limiting in particular in the upper limit of quenching temperature, 1250 degreeC is preferable and 1200 degreeC is more preferable.
In the quenching and tempering in the quenching and tempering step, the tempering temperature can be appropriately selected from the range of 500 to 650 ° C.

The quenching and tempering step is preferably a step of adjusting the hardness of the steel material to 45 HRC or more (more preferably 45 to 60 HRC) by quenching and tempering.
That is, the hardness of the steel material after quenching and tempering in this step is preferably 45 HRC or more (more preferably 45 to 60 HRC).

-Machining process-
The manufacturing method further includes a machining step of 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 is machined into a tool shape. It may be a step of quenching and tempering the steel material.
When this manufacturing method is such an embodiment, a tool-shaped steel material (that is, a tool product) can be efficiently manufactured. That is, considering the production of tool products such as dies and punches using steel, it is preferable that the state of the steel after hot working is an annealed state with low hardness. Performing quenching and tempering after machining this annealed steel material is efficient for the manufacture of tool products.

Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to these examples.

[Example 1]
The molten steel adjusted to the predetermined component composition was prepared by the atmospheric melting method.
About the molten steel provided to the example of this invention (steel ingot A), refinement | purification by the ladle smelting method was further implemented with respect to the said molten steel, and N content was adjusted low.
Next, molten steel (molten steel after the N content was adjusted low for molten steel to be used in the present invention) was cast to finish an electrode for remelting electroslag (remelting electrode). Next, the electrode was subjected to electroslag remelting to produce steel ingot A and steel ingot B of high-speed tool steel having the composition shown in Table 1 and the balance being Fe and impurities.

Each of the steel ingot A and the steel ingot B is subjected to a soaking treatment that is held at 1280 ° C. for 10 hours (soaking treatment step), and then cooling under any one of the cooling conditions 1 to 4 shown in FIG. Performed (cooling step).
Cooling condition 1 is that a steel ingot that has undergone soaking is gradually cooled (cooling rate: 0.5 ° C./min) until the surface temperature of this ingot decreases from the soaking temperature (1280 ° C.) to 1200 ° C. After the surface temperature of the ingot has decreased to 1200 ° C., cooling is performed until the surface temperature of the ingot is 900 ° C. or less by air cooling (cooling rate: about 50 ° C./min) by fan cooling.
The cooling condition 2 is a condition in which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1200 ° C. in the cooling condition 1 to 1100 ° C.
The cooling condition 3 is a condition in which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1200 ° C. in the cooling condition 1 to 1000 ° C.
The cooling condition 4 is a condition in which the temperature for switching from slow cooling to air cooling in the cooling condition 1 is changed from 1200 ° C. in the cooling condition 1 to 900 ° C.

About each steel ingot after the said cooling process, the distribution condition (solid solution state in a base | substrate) in a structure | tissue was investigated as follows.
First, the cross-sectional structure of each sample collected from the steel ingot was observed with a scanning electron microscope (magnification 50 times), and the observed visual field was analyzed with EPMA. Then, based on the contents of V and Nb that form carbides, a binarization process is performed on the analysis result with the detected intensity of V and Nb being 10 counts (cps) or more as a threshold value. It was. Thereby, the binarized image which showed the carbide | carbonized_material of V and Nb distributed in a cross-sectional structure | tissue was obtained.
In FIG. 2, the binarized image about each steel ingot is shown. In FIG. 2, the carbides are shown with a black distribution.
As shown in FIG. 2, the steel ingot A cooled in the cooling condition 1, the steel ingot A cooled in the cooling condition 2, the steel ingot A cooled in the cooling condition 3, and the steel ingot B cooled in the cooling condition 1 In the steel ingot B cooled under the cooling condition 2, no black distribution (a clear presence of carbide) was confirmed.

From FIG. 2, in the case of a steel ingot A having a N content of 0.0200% or less, the surface temperature of the steel ingot is reduced before cooling at a cooling rate of 3 ° C./min or more in the cooling process after soaking. Even if it was gradually cooled to 1000 ° C. (cooling condition 3), large carbides could not be confirmed in the steel ingot structure after cooling. And this result was the same also about what adjusted N content of the steel ingot A to the level of 150 ppm and 180 ppm (not shown).
On the other hand, in the case of the steel ingot B in which the N content exceeds 0.0200%, if the steel ingot is gradually cooled until the surface temperature of the steel ingot decreases to 1000 ° C. (cooling condition 3), the surface temperature of the steel ingot After cooling down to 1000 ° C., carbides were clearly confirmed even when cooled at a cooling rate of 3 ° C./min or more.
These results are attributed to the fact that the precipitation and growth temperature of carbide during cooling was lowered to around 1000 ° C. by setting the N content in the high-speed tool steel to 0.0200% or less (steel ingot A).

[Example 2]
In Example 1, the steel ingot A (N: 0.0128%) cooled under cooling condition 1 (after soaking and gradually cooled to 1200 ° C.) and cooling condition 1 (after soaking and gradually cooled to 1200 ° C.) Each of the cooled steel ingot B (N: 0.0296%) was reheated to a hot working temperature of 1100 ° C., and the reheated steel ingot was subjected to hot pressing and hot rolling to perform the ingot processing. . Each steel ingot (steel slab) after the batch processing was hot-rolled to finish a round bar steel material having a cross-sectional diameter of 100 mm (hot processing step).

Next, a part is sampled from each round bar steel material, and the sampled part is subjected to quenching from 1080 ° C. and tempering at 560 ° C., and an evaluation sample (high speed tool steel) adjusted to a hardness of 56 HRC. Was obtained (quenching and tempering step).
By the above, the sample for evaluation of the present invention example (high-speed tool steel produced using the steel ingot A) and the sample for evaluation of the comparative example (high-speed tool steel produced using the steel ingot B) are obtained, respectively. It was.

Next, the carbide distribution in the cross-sectional structure of the evaluation sample was examined as follows.
First, the cross-sectional structure of the sample for evaluation was observed with a scanning electron microscope (magnification 4000 times).
FIG. 3 is a scanning electron micrograph of the cross-sectional structure of the sample for evaluation of the present invention example (high-speed tool steel produced using the steel ingot A), and FIG. 4 is the sample for evaluation of the comparative example (steel) It is the scanning electron micrograph of the cross-sectional structure | tissue of the high speed tool steel produced using the lump B. FIG.
In FIG. 3 and FIG. 4, the carbide | carbonized_material (non-solid solution carbide | carbonized_material) which remained in the base | substrate, without being dissolved can be confirmed.

Next, the visual field observed above was analyzed by EPMA, and a tissue image having a number of pixels per visual field of 1200 × 1000 [area 29.19 μm × 23.92 μm]) was obtained. Tissue images were obtained for 10 visual fields per evaluation sample (total area 6982.2 μm ² per evaluation sample).
These tissue images were subjected to image processing using image analysis software (Software SCANDIUM manufactured by Olympus Corporation) to highlight the contrast between the base and the carbide. Thereby, the base and the carbide were identified, and the particle size distribution of the carbide was measured.
The particle size distribution of the carbide was measured by examining the relationship between the equivalent circle diameter of the carbide and the number density (pieces / mm ² ).
FIG. 5 is a graph showing the relationship between the equivalent circle diameter of carbide and the number density (pieces / mm ² ).
In FIG. 5, the notations “total 176 × 10 ³ pieces / mm ² ” and “total 180 × 10 ³ pieces / mm ² ” are the total carbides obtained by adding the number density for each equivalent circle diameter. The number density (pieces / mm ² ) is shown.

As shown in FIGS. 3 to 5, in the evaluation sample (high speed tool steel) of the example of the present invention, the maximum value of the equivalent circle diameter of the carbide in the cross-sectional structure was 1.00 μm or less.
On the other hand, in the sample for evaluation of the comparative example (high speed tool steel), there were many carbides having an equivalent circle diameter exceeding 1.00 μm.
As mentioned above, it turned out that the carbide | carbonized_material in the high speed tool steel of the example of this invention is fine compared with the carbide | carbonized_material of the high speed tool steel of a comparative example. In the high-speed tool steel of the example of the present invention, the number density of the entire carbide was 80 × 10 ³ pieces / mm ² or more, and a large amount of fine carbide was formed.

Next, the toughness was evaluated by conducting a Charpy impact test for each of the evaluation sample of the present invention and the evaluation sample of the comparative example.
The notch shape of the specimen for the Charpy impact test was 10R.
As a test piece for the Charpy impact test, a test piece taken so that the length direction (hot working direction) of the round bar steel material matches the length of the test piece, and the test piece in the cross-sectional radial direction of the round bar steel product Two types of test pieces collected so as to have the same length were used.
Then, three test pieces (TP1, TP2, TP3) having different sampling positions were prepared for the two types of test pieces, and a Charpy impact test was performed.
Table 2 shows the results of the Charpy impact test.

As shown in Table 2, the high-speed tool steel of the example of the present invention had a larger Charpy impact value and excellent toughness than the high-speed tool steel of the comparative example.

6 and 7 are scanning types showing fracture surfaces near the notch after the Charpy impact test of the test piece TP2 taken in the cross-sectional radial direction of the round bar steel material for the high-speed tool steels of the present invention and the comparative example, respectively. It is an electron micrograph.
As shown in FIG. 6, in the case of the high-speed tool steel of the example of the present invention, no major factor for lowering the impact value was confirmed at the starting point of the fracture surface.
On the other hand, as shown in FIG. 7, in the case of the high-speed tool steel of the comparative example, large carbides having an equivalent circle diameter exceeding 1.00 μm were confirmed at the fracture surface starting point (rounded portion). That is, it was confirmed that this large carbide was the starting point of fracture, and the toughness of the high-speed tool steel of the comparative example was lowered.

The disclosure of Japanese application 2013-201392 filed on September 27, 2013 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Claims (14)

1. % By mass: C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 4.00 to 6.00%, one of W and Mo Or 2 types: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and 1 or 2 types of V and Nb: the content determined by the relational expression (V + Nb) 0.50 to 3.00% as the content, N content is 0.0200% or less by mass, the balance is Fe and impurities, and the maximum equivalent circle diameter of the carbide in the cross-sectional structure is High speed tool steel of 1.00 μm or less.
2. The high-speed tool steel according to claim 1, further comprising Ni: 1.00% or less in mass%.
3. The high-speed tool steel according to claim 1 or 2, further comprising, by mass%, Co: 5.00% or less.
4. The high-speed tool steel according to any one of claims 1 to 3, wherein the Si content is 0.20% or less by mass.
5. The high-speed tool steel according to any one of claims 1 to 4, wherein the hardness is 45 HRC or more.
6. % By mass: C: 0.40 to 0.90%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 4.00 to 6.00%, one of W and Mo Or 2 types: 1.50 to 6.00% as the content determined by the relational expression (Mo + 0.5W), and 1 or 2 types of V and Nb: the content determined by the relational expression (V + Nb) As a preparatory step for preparing a steel ingot containing 0.50 to 3.00%, N content is 0.0200% by mass or less, and the balance is Fe and impurities;
   A soaking process in which the steel ingot is soaked by heating to 1200 to 1300 ° C .;
   In the process of cooling the steel ingot after the soaking process until the surface temperature of the steel ingot becomes 900 ° C. or lower, at least the surface temperature falls to a temperature T1 included in the range of 1000 ° C. or lower and over 900 ° C. After that, a cooling step of cooling until the surface temperature becomes 900 ° C. or less under the condition that the cooling rate of the surface temperature is 3 ° C./min or more,
   Reheating the steel ingot after the cooling step to a hot working temperature of more than 900 ° C., hot working the reheated steel ingot to obtain a steel material; and
   A quenching and tempering step for quenching and tempering the steel material;
   The manufacturing method of the high-speed tool steel which has.
7. The high cooling according to claim 6, wherein the cooling step cools the steel ingot under a condition that the cooling rate of the surface temperature is less than 3 ° C./min until the surface temperature of the steel ingot decreases to the temperature T1. Manufacturing method of speed tool steel.
8. The method for producing high-speed tool steel according to claim 6 or 7, wherein the steel ingot prepared in the preparation step is a steel ingot obtained by casting molten steel refined by a deoxidation refining method.
9. The steel ingot prepared in the preparation step is obtained by casting a molten steel refined by a deoxidation refining method to obtain a remelting electrode, and using the obtained remelting electrode, a steel ingot obtained by a remelting method The method for producing a high-speed tool steel according to claim 8.
10. The high-speed tool steel manufacturing method according to any one of claims 6 to 9, wherein the steel ingot prepared in the preparation step further contains Ni: 1.00% or less in mass%.
11. The method for producing high-speed tool steel according to any one of claims 6 to 10, wherein the steel ingot prepared in the preparation step further contains Co: 5.00% or less in mass%.
12. The method for producing high-speed tool steel according to any one of claims 6 to 11, wherein the steel ingot prepared in the preparation step has a Si content of 0.20% or less by mass%.
13. The method for producing high-speed tool steel according to any one of claims 6 to 12, wherein the quenching and tempering step adjusts the hardness of the steel material to 45 HRC or more by the quenching and tempering.
14. After the hot working step and before the quenching and tempering step, further comprising a machining step for machining the steel material into a tool shape,
    The method for producing high-speed tool steel according to any one of claims 6 to 13, wherein the quenching and tempering step performs quenching and tempering on a steel material machined into a tool shape.

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