TWI654318B - High speed tool steel and manufacturing method thereof - Google Patents

Abstract

The invention relates to a high-speed tool steel, which contains 0.40 to 0.90% of C, Si of 1.00% or less, Mn of 1.00% or less, and 4.00 to 6.00% of Cr according to mass%. One or two of W and Mo with a content of 1.50 to 6.00%, and one of V and Nb with a content of 0.50 to 3.00% obtained by the relationship (V + Nb) Or two kinds; the content of N is 0.0200 mass% or less; the remainder is composed of Fe and impurities; the maximum value of the equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less.

Description

High-speed tool steel and manufacturing method thereof

The invention relates to a high-speed tool steel used for tools such as a wire drawing die and a die, and a manufacturing method thereof.

Conventionally, as materials for various tools such as wire drawing dies and punches used for hot working, low-alloy high-speed tool steels, which are representative of high-speed tool steels, are used. Their component composition compared to SKH51 reduces C (carbon ) And the contents of Mo, W, and V that form carbides with C, thereby improving toughness. Next, regarding this low-alloy high-speed tool steel, a high-speed tool steel has been proposed. The high-temperature tool steel is subjected to a high temperature soaking at a temperature of 1200 to 1300 ° C. A cooling rate of ℃ / minute or more enables the surface temperature of the steel block to be 900 ° C. or lower, thereby suppressing the aggregation of carbides in the structure after quenching and tempering, so that the average particle diameter of the carbides is 0.5 μm or less. Further, the distribution density is made 80 × 10 ³ pieces / mm ² or more to further improve the toughness (Patent Document 1 described below).

[Patent Document 1] Japanese Patent Laid-Open No. 2004-307963

[Questions to be Solved by the Invention]

The method of Patent Document 1 effectively improves the toughness of a low-alloy high-speed tool steel. However, even in a high-speed tool steel manufactured by the method of Patent Document 1, there may be many carbides each having a particle size exceeding 0.5 μm in the microstructure after quenching and tempering. Therefore, the method of Patent Document 1 may not be able to sufficiently obtain the effect of improving the toughness of high-speed tool steel.

The object of the present invention is to provide a high-speed tool steel with improved toughness and a manufacturing method thereof. [Means for solving problems]

Specific means to solve this problem are as follows. <1> A high-speed tool steel containing 0.40 to 0.90% of C, Si of 1.00% or less, Mn of 1.00% or less, and Cr of 4.00 to 6.00% in terms of mass%. One or two of W and Mo with a content of 1.50 to 6.00%, and one or two of V and Nb with a content of 0.50 to 3.00% obtained by the relationship (V + Nb) The content of N is 0.0200 mass% or less, and the remainder is composed of Fe and impurities; the maximum value of the equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less. <2> The high-speed tool steel according to <1>, further including Ni in an amount of 1.00% by mass or less. <3> The high-speed tool steel according to <1> or <2>, further including Co in an amount of 5.00 mass% or less. <4> The high-speed tool steel according to any one of <1> to <3>, wherein the Si content is 0.20% by mass or less. <5> The high speed tool steel according to any one of <1> to <4>, wherein the hardness is 45 HRC or more.

<6> A method for manufacturing high-speed tool steel, which includes the steps of preparing the following steel blocks: in terms of mass%, containing 0.40 to 0.90% of C, Si of 1.00% or less, Mn of 1.00% or less, 4.00 ~ 6.00% of Cr, one or two of W and Mo with a content of 1.50 ~ 6.00% obtained from the relationship (Mo + 0.5W), and one obtained from the relationship (V + Nb) One or two of V and Nb with a content of 0.50 to 3.00%, the content of N is 0.0200 mass% or less, and the remainder is a steel block composed of Fe and impurities; a soaking step, by using the steel The block is heated to 1200 to 1300 ° C for soaking; the cooling step, in the process of cooling the steel block after the soaking step to a surface temperature of the steel block below 900 ° C, at least the surface temperature drops to 1000 After the temperature T1 in the range of below 900 ° C and exceeding 900 ° C, the surface temperature is cooled to below 900 ° C under the condition that the cooling rate of the surface temperature is 3 ° C / min or more; The steel block is reheated to a hot working temperature exceeding 900 ° C, and the reheated steel block is hot processed to It becomes steel; and the step of quenching and tempering, quenching and tempering the steel.

<7> The method for manufacturing a high-speed tool steel according to <6>, wherein the cooling step is performed before the surface temperature of the steel block reaches the temperature T1, and the cooling rate at the surface temperature is less than 3 ° C / min. Condition to cool the steel block. <8> The method for manufacturing a high-speed tool steel according to <6> or <7>, wherein the steel block prepared in the preparation step is a steel obtained by casting a molten steel refined by a deoxidizing refining method. Piece. <9> The method for manufacturing a high-speed tool steel according to <8>, wherein the steel block prepared in the preparation step is obtained by casting a molten steel refined by a deoxidizing refining method to obtain an electrode for redissolution, and using the obtained electrode The electrode for redissolving the steel block obtained by the redissolving method. <10> The method for producing a high-speed tool steel according to any one of <6> to <9>, wherein the steel block prepared in the preparation step further contains 1.00% by mass or less of Ni. <11> The method for producing a high-speed tool steel according to any one of <6> to <10>, wherein the steel block prepared in the preparation step further contains Co in an amount of 5.00% by mass or less. <12> The method for producing a high-speed tool steel according to any one of <6> to <11>, wherein the content of Si in the steel block prepared in the preparation step is 0.20% by mass or less. <13> The method for producing a high-speed tool steel according to any one of <6> to <12>, wherein in the quenching and tempering step, the hardness of the steel is adjusted to 45 HRC or more by the quenching and tempering. <14> The method for manufacturing a high-speed tool steel according to any one of <6> to <13>, wherein after the hot working step and before the quenching and tempering step, a machining step is further included, and the steel is subjected to Machining to make it into the shape of a tool; This quenching and tempering step is to quench and temper the steel material that has been formed into a tool shape by machining. [Effect of the invention]

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

In this specification, "%" showing the content of each component (each element) means "mass%". In this specification, the numerical range indicated by "~" means a range in which the numerical values described before and after "~" are included as the minimum value and the maximum value, respectively. It should be noted that the "hardness" indicated by the unit "HRC" in this specification is the Rockwell hardness on the C scale specified in JIS G 0202 (2013).

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

<High-speed tool steel> The high-speed tool steel of the present invention includes, by mass%, C (carbon) of 0.40 to 0.90%, Si (silicon) of 1.00% or less, Mn (manganese) of 1.00% or less, and 4.00 to 6.00%. Cr (chromium), one or two of W (tungsten) and Mo (molybdenum) with a content of 1.50 to 6.00% obtained by the relationship (Mo + 0.5W), and the relationship (V + Nb) One or two of V (vanadium) and Nb (niobium) whose content is 0.50 to 3.00%; The content of N (nitrogen) is 0.0200 mass% or less; the remainder is composed of Fe (iron ) And impurities; the maximum value of the equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less.

In the present invention, the concept of "carbides" in the cross-section structure of high-speed tool steel is not only nitrogen-free carbides, but also nitrogen-containing carbides (that is, carbonitrides).

As described above, the high-speed tool steel of the present invention has an N content of 0.0200% by mass or less. The N-type cast steel block cannot avoid the inclusion of impurity elements.

In general, when the composition of the molten steel before casting is completely adjusted in the atmospheric environment, the steel block after casting usually contains about 0.0300% or more of N. N is a carbide-forming element, that is, an element having a strong affinity for V or Nb. Therefore, in a high-speed tool steel containing a large amount of N, in the solidification process during casting, before V or Nb is bonded to C to precipitate carbides (eutectic carbides), nitrides are bonded to N to precipitate. Next, a carbide is precipitated around the nitride to form a carbonitride. This carbonitride is a thermally stable compound. Therefore, if the carbonitride is formed in a large amount in the steel block, it is difficult to dissolve the carbonitride in the base material in the soaking step or the hot working step of the next step. As a result, a large amount of the carbonitride remains in the structure of the high-speed tool steel (including tool products, hereinafter the same) produced through the soaking step and the hot working step, so that the toughness of the high-speed tool steel is reduced. Then, because this carbonitride becomes the starting point of destruction, early cracking of high-speed tool steel easily occurs, and even the life of high-speed tool steel is reduced.

In view of the above-mentioned problems, in order to suppress the amount of carbonitride formation, the high-speed tool steel of the present invention has an N content of 0.0200% or less. As a result, the form of the carbonitrides precipitated in the steel block can be changed to a form of carbides not containing nitrogen. Nitrogen-free carbides easily dissolve in the substrate during soaking and the like. Therefore, by setting the N content to 0.0200% or less, the carbides distributed in the high-speed tool steel can be made finer, so that the toughness of the high-speed tool steel can be further improved.

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-section structure is 1.00 μm or less.

Regarding the particle size of carbides, in the high-speed tool steel described in Patent Document 1, the average particle size of carbides is 0.5 μm or less. However, according to the study by the inventor of the present case, it was found that even when the average particle diameter of carbides in high-speed tool steel is 0.5 μm or less, the high-speed tool steel has carbides with particle diameters far exceeding 1.00 μm. Happening. Furthermore, it is also known that the toughness of high-speed tool steel cannot be sufficiently improved due to the presence of this coarse carbide. Regarding these problems, the high-speed tool steel of the present invention can increase the toughness of the high-speed tool steel by making the maximum value of the equivalent circle diameter of carbides in the cross-section structure 1.00 μm or less. If the maximum equivalent circle diameter of carbides in the cross-section structure of high-speed tool steel exceeds 1.00 μm, carbides with large particle sizes (especially carbides with equivalent circular diameters exceeding 1.00 μm) tend to become the starting point of failure. As a result, the toughness of high-speed tool steel is reduced. Carbides with an equivalent circle diameter of more than 1.00 μm are carbides that are not solid-soluble in the substrate at the quenching temperature (about 900 ° C or above Wastfield ironing temperature) (non-solid solution carbonization)物).

In addition, the high-speed tool steel of the present invention need only have a maximum equivalent circle diameter of carbides in a cross-section structure of 1.00 μm or less. In the case where this condition is satisfied, the average particle diameter of the carbides may of course be 0.5 μm or less.

As described above, the high-speed tool steel of the present invention is similar to the conventional high-speed tool steel in that the content of N is 0.0200% or less and the maximum value of the equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less. (For example, the high-speed tool steel of Patent Document 1), the toughness is further improved.

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

In the high-speed tool steel of the present invention, as described above, although the maximum equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less, the maximum equivalent circle diameter is preferably 0.90 μm or less. It is preferably 0.80 μm or less.

Furthermore, in the high-speed tool steel of the present invention, the distribution density of carbides may be set to 80 × 10 ³ pieces / mm ² or more. By making the distribution density of carbides larger, the particle size of the old Vosted iron after quenching and tempering can be reduced, and the toughness of high-speed tool steel can be further improved.

In the present invention, in order to specify the maximum equivalent circle diameter of carbides and the distribution density of carbides, for example, a scanning electron microscope with a magnification of 4,000 times and a total screen area of 5000 μm ² or more can sufficiently observe high-speed tool steels. The profile is organized for specific. Next, the high-speed tool steel used when specifying this carbide generally has the shape of various tool products. The shape of the tool product has a part that may be broken due to the carbide. For example, the working surface of the tool product, especially the corner surface (outer corner, inner corner) of the tool surface that is in contact with other members. unit). Therefore, the portion of the high-speed tool steel that specifies the carbide can be, for example, a cross-sectional structure including the corner portion. The carbides that have a great impact on the toughness of the tool are carbides that are completely insoluble in the substrate (not solid solution carbides) at the quenching temperature (about 900 ° C and above). ).

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 in various tools, the use of a hardness of 45 HRC or more can impart excellent tensile strength to the tool. In particular, when it is used in various hot-pressing tools, its use hardness (hardness at room temperature) is 45 HRC or more, and it can provide excellent tensile strength at high temperatures. The high-speed tool steel of the present invention preferably has a hardness of 45HRC ~ 60HRC.

The component composition of the high-speed tool steel of the present invention is the same as 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 this invention is demonstrated.

‧0.40 ~ 0.90% C C is an element that forms hard complex carbides by forming elemental bonds with carbides such as Cr, Mo, W, V, Nb, etc., and further imparts abrasion resistance to high-speed tool steel. In addition, a part of C is solid-dissolved in the base material to strengthen the base material. Thereby, a part of C imparts hardness to the Asada loose iron structure after quenching and tempering. However, excess C promotes segregation of carbides. Therefore, the C content is set to 0.40 to 0.90%.

‧Si of 1.00% or less Si is usually used as a deoxidizing agent in the dissolution step. It is an element that cannot be avoided in the steel block after casting. However, when the content of Si is too large, the toughness of the high-speed tool steel is lowered. Therefore, the Si content is set to 1.00% or less. In addition, Si has a function of spheroidizing the M ₂ C-type rod-shaped primary carbide. Therefore, the Si content should preferably be 0.10% or more. The Si content is preferably 0.20% or less from the following viewpoints. That is, when the content of Si is 0.20% or less, there is a tendency that the effect of making the primary carbide spherical in size becomes weak. Therefore, in the case where Si is 0.20% or less, compared with the case where Si exceeds 0.20%, the effect of setting the N content to 0.0200% or less and the maximum value of the equivalent circle diameter of the carbide in the cross section structure are 1.00 The effect caused by μm or less is more remarkable.

‧1.00% or less Mn Mn is the same as Si and is used as a deoxidizing agent in the dissolution step. It is an element that cannot be avoided in the steel block after casting. However, if the content of Mn is too large, the A ₁ transformation point is excessively lowered, resulting in an increase in annealing hardness, resulting in a decrease in the machinability (machinability) of the high-speed tool steel. Therefore, the Mn content is set to 1.00% or less. In addition, Mn has an effect of improving hardenability. Therefore, it is preferably 0.10% or more.

‧ 4.00 ~ 6.00% Cr Cr is bonded with C to form carbides, and it is an element that improves the wear resistance of high-speed tool steel. In addition, Cr is an element that contributes to improving the hardenability of high-speed tool steels. However, if the content of Cr is too large, banded segregation is promoted, and the toughness of high-speed tool steel is lowered. Therefore, the Cr content is set to 4.00 to 6.00%.

‧ One or two types of W and Mo among W and Mo with a content of 1.50 to 6.00% obtained by the relational formula (Mo + 0.5W) are bonded with C to form carbides. In addition, it is at the time of quenching An element that solidly dissolves in a substrate to increase its hardness, thereby improving the wear resistance of high-speed tool steel. However, if the W and Mo contents are too large, band-like segregation is promoted, and the toughness of the high-speed tool steel is lowered.

Regarding the above-mentioned 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). The high-speed tool steel of the present invention uses the content obtained by the relational formula (Mo + 0.5W) as one or two content of W and Mo, which is 1.50 to 6.00%. The high-speed tool steel of the present invention may contain only one (one) of W and Mo, or two (two) of W and Mo. That is, any one of "Mo" and "W" in the relational expression (Mo + 0.5W) may be 0%.

In addition, compared with Mo, W has the ability to promote band-like segregation, and it is easy to damage the toughness of high-speed tool steel. Therefore, the W content in the high-speed tool steel is preferably 3.00% or less (in the case of 0.5W in the relational expression (Mo + 0.5W), it is 1.50% or less).

‧ One or two V and Nb of V and Nb with a content of 0.50 ~ 3.00% obtained by the relationship (V + Nb), bond with C to form carbides, which improves the resistance of high-speed tool steel Wear resistance and baking resistance. In addition, V and Nb are solid-dissolved in the substrate during quenching, and precipitate fine and difficult to agglomerate carbides during tempering, thereby improving the softening resistance of high-speed tool steels in high-temperature environments and imparting excellent high-temperature resistance. . Next, while V and Nb make the crystal grains fine, the A ₁ abnormal point also rises, and the toughness and heat crack resistance of high-speed tool steel are also improved. However, when the content of V and Nb is too large, large carbides are generated, which promotes the occurrence of cracks when used as a tool.

Regarding the aforementioned 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 determined as a content obtained by a relational expression (V + Nb), and it is 0.50 to 3.00%. In the relationship (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 kind (one) of V and Nb, and may contain two kinds (both) of V and Nb. That is, any one of "V" and "Nb" in the relationship (V + Nb) may be 0%. The content obtained by the relational expression (V + Nb) should preferably be 1.50% or less.

In addition, Nb is superior to V in the effects of softening resistance, improving high-temperature strength, and suppressing coarsening of crystal grains. Therefore, the high-speed tool steel of the present invention preferably contains Nb (that is, the Nb content exceeds 0%).

‧Ni should be 1.00% or less. Ni imparts excellent hardenability to high-speed tool steel. This makes it possible to form a quenched structure mainly composed of Asada scattered iron, and to improve the inherent toughness of the substrate itself. However, if the content of Ni is too large, the A ₁ transformation point is excessively reduced, which causes the annealing hardness of the high-speed tool steel to be increased, and the machinability of the high-speed tool steel to be reduced. Therefore, even if the high-speed tool steel contains Ni, the Ni content should be 1.00% or less. Next, when the high-speed tool steel contains Ni, the Ni content is preferably 0.05% or more.

‧Co should be 5.00% or less Co has the effect of forming an extremely dense and good adhesion protective oxide film on the surface of the tool when the tool in use is heated. Thereby, the metal contact between the surface of the tool and the target material is reduced, the temperature rise of the surface of the tool is reduced, and the abrasion resistance of the tool is excellent. Next, by forming the protective oxide film, the heat insulation effect is increased, and the heat crack resistance is also improved. However, if the content of Co is too large, the toughness of the high-speed tool steel decreases. Therefore, even when high-speed tool steel contains Co, it is desirable to set the Co content to 5.00% or less. Next, when high-speed tool steel contains Co, the Co content is preferably 0.30% or more.

In addition, the high-speed tool steel of the present invention may include, for example, S (sulfur) and P (phosphorus) as unavoidable impurity elements. If the content of S is too large, the hot workability of high-speed tool steel is impeded, so the content of S should be limited to 0.0100% or less. The content of S is more preferably 0.0050% or less. If the content of P is too large, the toughness of high-speed tool steel will be deteriorated. Therefore, the content of P should be set to 0.050% or less. The content of P is more preferably 0.025% or less.

Although the method for manufacturing the high-speed tool steel of the present invention is not particularly limited, for example, the following manufacturing method may be used: For ingots having the component composition of the high-speed tool steel of the present invention, a soaking treatment is sequentially performed (preferably by Soaking by heating to 1200 to 1300 ° C), cooling (should reduce the surface temperature of the ingot after soaking to 900 ° C or lower for cooling), hot working (should make the cooled steel block again Hot working by heating to more than 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 may be machined between hot working and quenching and tempering to make it into a tool shape. Even among the above-mentioned manufacturing methods, it is particularly easy to manufacture the high-speed tool steel of the present invention as long as the method of manufacturing the high-speed tool steel of the present invention is described later.

<Manufacturing method of high-speed tool steel> The manufacturing method of the high-speed tool steel of the present invention (hereinafter also referred to as "this manufacturing method") includes a preparation step of preparing a steel block composed of the following steel blocks: 0.40% by mass% C of 0.90%, Si of 1.00% or less, Mn of 1.00% or less, Cr of 4.00 to 6.00%, W and Mo in the content of 1.50 to 6.00% obtained by the relationship (Mo + 0.5W) One or two kinds, and one or two kinds of V and Nb with a content of 0.50 to 3.00% obtained by the relationship (V + Nb); the content of N is less than 0.0200% by mass, and the remaining part is From Fe and impurities; soaking step, soaking the steel block to 1200 ~ 1300 ° C to perform soaking treatment; cooling step, cooling the steel block after the soaking step to the surface temperature of the steel block is In the process of 900 ° C or lower, at least the surface temperature is lowered to a temperature T1 in the range of less than 1000 ° C and higher than 900 ° C, and the surface temperature is set to a cooling rate of 3 ° C / min or more. Cooling below 900 ° C; hot working step, reheating the steel block after the cooling step to a heat exceeding 900 ° C Process temperature, the reheated steel ingot hot working to make it of steel; and the step of quenching and tempering, quenching and tempering the steel.

In this specification, only the cooling rate of the surface temperature of a steel block is called "cooling rate".

The inventor of the present case discusses the details of a method for manufacturing a high-speed tool steel including soaking treatment proposed in Patent Document 1. As a result, it was confirmed that the high-temperature soaking treatment at 1200 to 1300 ° C. can indeed effectively dissolve carbides in a steel block for a high-speed tool steel having a low alloy composition such as that of Patent Document 1. However, the inventors of the present invention have found that if the cooling process after the soaking treatment is not properly managed, the carbides that have not been dissolved or newly precipitated may be coarsened. Then, the present inventors have further found that by appropriately managing the cooling conditions, coarsening of the carbides during cooling can be suppressed, and as a result, the carbides in the microstructure of the high-speed tool steel can be further refined. Furthermore, the inventors of the present invention have found that, in order to maintain the effect of miniaturizing the carbides caused by the proper cooling conditions, the steel block itself, which is the object of the soaking treatment, has the most appropriate composition and composition, thereby achieving the present invention. Manufacturing method of high-speed tool steel.

That is, the method for manufacturing a high-speed tool steel according to the present invention uses a steel block having a N content of 0.0200% or less by mass%. Therefore, as explained in the item of "High-speed tool steel", the carbides distributed in the manufactured high-speed tool steel can be made finer, so high-speed tool steel with improved toughness can be manufactured.

Furthermore, in this manufacturing method, the N content in the steel block to be subjected to the soaking treatment is adjusted to 0.0200% or less, which is the same as the cooling step in this manufacturing method, and the carbides (including carbonitrides) in the structure are adjusted The subtle aspect plays an important role. This is explained in detail below.

First, according to the method of Patent Document 1, the carbides precipitated by crystallization in the steel block can be solid-dissolved in the substrate by the next step, namely, a soaking treatment at 1200 to 1300 ° C. Next, in the cooling process after the soaking treatment, the surface temperature of the steel block is cooled to 900 ° C or lower at a cooling rate of 3 ° C / min or more, thereby suppressing the precipitation and growth of V or Nb carbides. However, in actual operation, it is difficult to cool the steel block immediately after the soaking treatment at a cooling rate of 3 ° C./min or more from the soaking treatment temperature to the temperature of 900 ° C. or less. That is, during the actual operation, until the steel block is taken out from the soaking furnace, the cooling is performed at a cooling rate of less than 3 ° C / minute (that is, the furnace is cooled in a soaking furnace, etc.). When cooling at this cooling rate started, the surface temperature of the steel block actually dropped from the soaking temperature. Next, according to the study by the inventor of the present case, it was found that when the surface temperature of the steel block dropped to around 1000 ° C., carbides of V and Nb had precipitated in large amounts, and also began to grow.

Therefore, by adjusting the N content in the steel block to be subjected to the soaking treatment to 0.0200% or less, the temperature at which the carbides precipitate and grow can be reduced during the cooling process after the soaking treatment. Specifically, the temperature at which the carbides are precipitated and grown can be lowered to a surface temperature of the steel block of 1,000 ° C or lower. Next, because the temperature of precipitation and growth of the carbides is reduced, even if the surface temperature of the steel block taken out from the soaking furnace is reduced to around 1000 ° C, the subsequent cooling can be suppressed at a cooling rate of 3 ° C / min or more, and the temperature can be suppressed. The precipitation and growth of the carbides can surely achieve the miniaturization of the carbides.

Therefore, in the present manufacturing method, a steel ingot having an N content of 0.0200% or less is used as the steel ingot to be subjected to the soaking treatment, and has a temperature at least in which the surface temperature of the steel ingot falls to a range between 1000 ° C. and 900 ° After the temperature T1, the cooling step of cooling the surface temperature to 900 ° C or lower under the condition that the cooling rate of the surface temperature is 3 ° C / min or more can achieve more reliable refinement of carbides. Therefore, according to this manufacturing method, a high-speed tool steel with improved toughness can be manufactured compared with a conventional high-speed tool steel (for example, the high-speed tool steel described in Patent Document 1). According to this manufacturing method, for example, a high-speed tool steel (for example, the high-speed tool steel of the present invention described above) having a maximum equivalent circle diameter of carbides in a cross-section structure can be manufactured.

Moreover, according to this manufacturing method, since this cooling step is provided, it is possible to achieve the effect of making the processing time of a steel block after soaking heat more sufficient.

Hereinafter, each step of this manufacturing method is demonstrated.

-Preparation step- The preparation step is a step of preparing the following steel blocks: mass%, containing 0.40 to 0.90% C, 1.00% or less Si, 1.00% or less Mn, 4.00 to 6.00% Cr, based on the relationship (Mo + 0.5W) One or two of W and Mo whose content is 1.50 to 6.00%, and V whose content is 0.50 to 3.00% obtained by the relation (V + Nb) And one or two of Nb; the content of N is 0.0200 mass% or less, and the remainder is composed of Fe and impurities. The preparation steps are convenient steps. The preparation step may also be a step of manufacturing a steel block, or a step of preparing a pre-manufactured steel block before manufacturing a high-speed tool steel.

The composition of the steel block prepared in the preparation step is the same as the composition of the high-speed tool steel of the present invention, and the preferred range is also the same.

In addition, in actual operation, the amount of molten steel in one dissolution treatment is large. Therefore, it is not easy to reduce the N content in the steel block to 0.0200% or less only by atmospheric dissolution. When it is desired to reduce the N content in the steel block to 0.0200% or less by atmospheric dissolution, it is necessary to use a high-grade raw material having a reduced N content as a raw material before dissolution, which is disadvantageous in terms of cost. Therefore, in this manufacturing method, the steel block prepared in the preparation step is preferably a steel block obtained by casting a molten steel refined by a deoxidizing refining method. Examples of the deoxidizing refining method include various barrel refining methods such as the LF method, the ASEA-SKF method, the VAD method, and the VOD method; and various vacuum degassing methods such as the RH method and the DH method.

In addition, since each steel block is large in actual operation, segregation in the steel block may increase. Therefore, the steel block prepared in the preparation step is preferably a steel block obtained by casting the molten steel refined by a deoxidizing refining method to obtain an electrode for redissolving, and then using the electrode for redissolving and using the redox method. . By implementing the redissolving method, segregation in a steel block can be improved. Examples of the redissolving method include an electroslag redissolving method, a vacuum arc redissolving method, a plasma arc redissolving method, and an electron beam redissolving method. In particular, the electroslag redissolution method is advantageous for reducing impurity elements such as S because slag is used.

-Heat Treatment Step-The heat treatment step is a step of heating the steel block prepared in the preparation step to 1200 to 1300 ° C and performing a heat treatment. In the soaking treatment step, the steel block having the composition composition is subjected to the soaking treatment at a high temperature of 1200 to 1300 ° C in the same method as in Patent Document 1, thereby solid-dissolving the giant carbides at the time of casting and making the composition thereof. Solid solution diffusion, which can improve the distribution of carbides. The temperature of the soaking treatment is 1200 ~ 1300 ° C, preferably 1260 ~ 1300 ° C. In addition, the soaking time is preferably 10 to 20 hours. In addition, the general soaking temperature of the high-speed tool steel is about 1150 ° C. In contrast, the soaking temperature in the soaking step of this manufacturing method is higher than the temperature of the usual soaking treatment.

-Cooling step- The cooling step is to cool the steel block after the soaking step to a surface temperature of the steel block below 900 ° C, at least when the surface temperature of the steel block falls below 1000 ° C and exceeds 900 ° C. After the temperature T1 in the range of ℃, the step of cooling the surface temperature of the steel block to 900 ° C or lower under the condition that the cooling rate of the surface temperature of the steel block is 3 ° C / min or more. In the cooling step, the steel block is cooled at a cooling rate of 3 ° C./min or more before the surface temperature of the steel block becomes 900 ° C. or lower. This cooling step is to reduce the formation of carbides with a large particle size by quickly passing through a temperature range up to 900 ° C where carbides of V and Nb are easily precipitated and grown, and it is preferable to form finely dispersed particles only in the substrate. Diameter of the carbide step. However, as described above, it is difficult to cool the steel block whose soaking treatment is completed to a temperature of 900 ° C. or less at a cooling rate of 3 ° C./min or more from a time point at which the soaking treatment temperature is maintained. Therefore, in the present manufacturing method, the precipitation and growth temperature of carbides during cooling was successfully reduced to around 1000 ° C. by setting the N content in the steel block to be subjected to the soaking treatment to 0.0200% or less.

Next, in this manufacturing method, since the steel block having the N content reduced to 0.0200% or less is subjected to soaking treatment, even in the cooling step after the soaking treatment, the homogenization is performed at a slower cooling rate of less than 3 ° C / min. Cooling when the temperature of the heat treatment is reduced to around 1000 ° C, if the cooling is performed at a rapid cooling rate of 3 ° C / min or more to 900 ° C or less, the carbides can be effectively refined. That is, in the cooling step in this manufacturing method, in the process of cooling the steel block after the soaking step until the surface temperature of the steel block becomes 900 ° C or lower, at least the surface temperature drops below 1000 ° C and exceeds 900 ° C. After the temperature T1 within the range, the surface temperature is cooled to 900 ° C or lower under the condition that the cooling rate of the surface temperature is 3 ° C / min or more.

In the cooling step, the cooling of the surface temperature of the steel block to the temperature T1 may be performed under the condition that the cooling rate of the surface temperature is less than 3 ° C / min, but the cooling rate of the surface temperature may be 3 ° C / min. The above conditions are performed. The cooling rate of 3 ° C / min or more can be achieved by taking out the steel block from the soaking furnace, such as air cooling (cooling) or fan cooling. When the cooling rate of the surface temperature is less than 3 ° C / min, cooling the surface temperature of the steel block to the temperature T1 is performed, so that the processing time of the steel block after soaking can be sufficient, so it has high-speed tools The advantage of making steel easier.

The temperature T1 is a temperature included in a range of 1000 ° C or lower and more than 900 ° C, preferably a temperature in a range of 1000 ° C or lower and 950 ° C or higher, and more preferably in a range of 1000 ° C or lower and 970 ° C or higher. The temperature is particularly preferably 1000 ° C.

In the cooling step, at least after the surface temperature of the steel block is reduced to 950 ° C, the cooling rate of the surface temperature of the steel block is 3 ° C / min or more, and the surface temperature of the steel block is cooled to 900 ° C or less. step. In the cooling step, it is preferable that the surface temperature of the steel block is cooled to below 900 ° C. under the condition that the cooling rate of the surface temperature of the steel block is at least 3 ° C./min after the surface temperature of the steel block is reduced to 1000 ° C. or more. A step of.

In the cooling step, the cooling rate after falling to the temperature T1 is 3 ° C / min or more. The cooling rate is preferably 10 ° C / min or more, more preferably 20 ° C / min or more, and more preferably 30 ° C / min or more. It is particularly preferably at least 40 ° C / minute. In the cooling step, although the upper limit of the cooling rate after the temperature is lowered to the temperature T1 is not particularly limited, the upper limit is preferably 100 ° C / minute, and more preferably 80 ° C / minute.

-Hot working step- In the hot working step, the steel block after the cooling step is reheated to a hot working temperature exceeding 900 ° C, and the reheated steel block is hot processed to become a steel material. The hot working temperature is a temperature at which the hot working is started. The reheating and thermal processing performed in the thermal processing step may be performed in the same manner as in Patent Document 1. For example, the purpose of hot working is to improve the cast structure of the steel block and adjust it to a predetermined steel size. The hot working may be performed under conditions such as forging, rolling, and the like which are generally performed. The hot working temperature of the steel block after this cooling step exceeds 900 ° C, preferably 950 ° C or higher, more preferably 1000 ° C or higher, and particularly preferably 1050 ° C or higher. Although the upper limit of the hot working temperature of the steel block after the cooling step is not particularly limited, the upper limit is preferably 1250 ° C, more preferably 1200 ° C, and particularly preferably 1150 ° C.

-Quenching and tempering step-The quenching and tempering step is a step of quenching and tempering the steel obtained by the hot working. After quenching and tempering, the steel contained in the structure is finely adjusted to have excellent toughness. The 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 under conditions and the like for general implementation. In the quenching and tempering in the quenching and tempering step, the quenching temperature may be appropriately selected from the range of 900 ° C or higher. The quenching temperature is preferably above 950 ° C, and more preferably above 1000 ° C. Although the upper limit of the quenching temperature is not particularly limited, it is preferably 1250 ° C, and more preferably 1200 ° C. In the quenching and tempering in the quenching and tempering step, the tempering temperature may 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 to more than 45HRC (preferably 45 ~ 60HRC) 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 step- In this manufacturing method, after the hot working step and before the quenching and tempering step, there is further a machining step of machining the steel material to make it into a tool shape; the quenching and tempering step is also This may be a step of quenching and tempering a steel material formed into a tool shape by machining. According to the aspect of the manufacturing method, a tool-shaped steel (that is, a tool product) can be efficiently manufactured. That is, if it is considered that a tool product such as a wire drawing die or a punch is made of a steel material, the state of the steel material after hot working is preferably an annealed state with low hardness. This annealed steel is machined and then quenched and tempered, which is effective for the manufacture of tool products. [Example]

Hereinafter, although the present invention will be specifically described with examples, the present invention is not limited to those examples.

[Example 1] A molten steel adjusted to a predetermined composition was prepared by an atmospheric dissolution method. Regarding the molten steel supplied to the example of the present invention (steel block A), the molten steel was further subjected to refining by the ladle refining method to adjust the N content to be low. Next, the molten steel (the molten steel supplied to the example of the present invention is a molten steel whose N content is adjusted to be low) is cast to complete an electrode for redissolving electroslag (electrode for redissolving). Next, the electrode was re-dissolved with electroslag to produce a steel block A and a steel block B of a high-speed tool steel having the composition and composition of Table 1 and the remainder consisting of Fe and impurities.

The steel block A and the steel block B were respectively subjected to a soaking treatment (a soaking treatment step) held at 1280 ° C for 10 hours, and then cooled under any of the cooling conditions 1 to 4 shown in FIG. 1 (cooling step). . Cooling condition 1 is a method of reducing the surface temperature of the steel block from the soaking temperature (1280 ° C) to 1200 ° C (cooling rate: 0.5 ° C / min), and slowly cooling the steel block after the soaking treatment. After the surface temperature of the steel was reduced to 1200 ° C, air cooling (cooling rate: about 50 ° C / minute) by fan cooling was used to cool the surface temperature of the steel block to below 900 ° C. The cooling condition 2 is a temperature in the cooling condition 1 which is switched from the slow cooling to the air cooling, and the cooling condition 1 is changed from 1200 ° C to 1100 ° C. The cooling condition 3 is a temperature in the cooling condition 1 which is switched from the slow cooling to the air cooling, and is changed from 1200 ° C to 1000 ° C in the cooling condition 1. Cooling condition 4 refers to the cooling condition 1, and the temperature is changed from the simmer cooling to the air cooling, and the cooling condition 1 is changed from 1200 ° C to 900 ° C.

About each steel block after this cooling step, the distribution state of the carbide | carbonized_material in a structure (solid solution state in a base material) was investigated as follows. First, the cross-sectional structure of each sample taken from a steel block was observed with a scanning electron microscope (50 times magnification), and the observed image was analyzed with EPMA. Next, based on the contents of V and Nb that form carbides, a binarization process is performed with the detection intensity of V and Nb 10 times (cps; count per second) or more as a critical value based on the analysis results. Thereby, a binary image showing the carbides of V and Nb distributed in the cross-section structure is obtained. A binary image of each steel block is shown in FIG. 2. In FIG. 2, the carbide system is represented by a black distribution. As shown in FIG. 2, among the steel block A cooled under the cooling condition 1, among the steel block A cooled under the cooling condition 2, among the steel block A cooled under the cooling condition 3, and among the steel block B cooled under the cooling condition 1, In the steel block B cooled under the cooling condition 2, no black distribution (clear presence of carbides) was confirmed.

According to FIG. 2, in the case of steel block A with an N content of 0.0200% or less, during the cooling process after soaking, the surface of the steel block is slowly cooled even before cooling at a cooling rate of 3 ° C./min or more. Even when the temperature dropped to 1000 ° C (cooling condition 3), large carbides could not be confirmed in the structure of the steel block after cooling. Moreover, the results were the same for those who adjusted the N content of the steel block A to 150 ppm and 180 ppm (not shown in the figure). In contrast, in the case of steel block B with an N content exceeding 0.0200%, if the surface temperature of the steel block is reduced to 1000 ° C (cooling condition 3) in a slow cooling manner, even if the surface temperature of the steel block is reduced to 1000 ° C Thereafter, it was cooled at a cooling rate of 3 ° C./min or more, and the carbides were clearly confirmed. The above-mentioned result is achieved by reducing the N content in the high-speed tool steel to 0.0200% or less (steel block A) and reducing the precipitation and growth temperature of the carbides during cooling to around 1000 ° C.

[Example 2] In Example 1, steel block A (N: 0.0128%) cooled under cooling condition 1 (slow cooling to 1200 ° C after soaking) and cooling condition 1 (slow cooling to 1200 ° C after soaking) The cooled steel block B (N: 0.0296%) is respectively reheated to a hot working temperature of 1100 ° C., and the reheated steel block is subjected to hot stamping and hot rolling, and is subjected to block processing. Each steel block (steel sheet) after the block processing is hot-rolled to complete a rod-shaped steel having a cross-section diameter of 100 mm (the above-mentioned thermal processing steps).

Next, a portion was sampled from each of the rod-shaped steel materials, and the sampled portion was quenched from 1080 ° C and tempered at 560 ° C, and the test pieces (high-speed tool steel) for hardness adjustment to 56HRC (quenching) Tempering step). As described above, the test pieces for evaluation according to the present invention (high-speed tool steel produced using steel block A) and the test samples for evaluation (compared to high-speed tool steel produced using steel block B) were obtained.

Next, as described below, the carbide distribution in the cross-sectional structure of the test piece for evaluation was investigated. First, the cross-sectional structure of the test piece for evaluation was observed with a scanning electron microscope (magnification: 4000 times). Fig. 3 is a scanning electron microscope image of a cross-sectional structure of an evaluation test piece (high-speed tool steel produced using steel block A) according to an example of the present invention, and Fig. 4 is an evaluation test piece (made using steel block B) of a comparative example. Scanning electron microscope image of the cross-section structure of high-speed tool steel). In FIG. 3 and FIG. 4, it was confirmed that carbides (non-solid solution carbides) remained in the substrate without solid solution.

Then, the above-mentioned observed images were analyzed by EPMA, and an image of each tissue having an image number of 1200 × 1000 [area 29.19 μm × 23.92 μm] was obtained. For each sample for evaluation, a microstructure image of 10 screens was obtained (the total area of each sample for evaluation was 6982.2 μm ² ). Next, image processing software (OLYMPAS Co., Ltd. SCANDIUM) was used to perform such image processing on the structure images, so that the contrast between the substrate and the carbides was obvious. Thereby, the substrate and the carbide are discriminated to measure the particle size distribution of the carbide. The particle size distribution of the carbide is measured by examining the relationship between the equivalent circle diameter of the carbide and the number density (number / mm ² ). FIG. 5 is a graph showing the relationship between the equivalent circle diameter of a carbide and the number density (number / mm ² ). In FIG. 5, the expressions of “counting 176 × 10 ³ pieces / mm ² ” and “counting 180 × 10 ³ pieces / mm ² ” are obtained by adding the number density of each equivalent circle diameter. This represents the number density (number / mm ² ) of the entire carbide.

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-section structure is 1.00 μm or less. On the other hand, in the evaluation sample (high-speed tool steel) of the comparative example, carbides having an equivalent circle diameter exceeding 1.00 μm were reduced. As described above, it can be seen that the carbides in the high-speed tool steel of the present example are finer than the carbides of the high-speed tool steel of the comparative example. Next, in the high-speed tool steel according to the example of the present invention, the number density of the entire carbide is 80 × 10 ³ pieces / mm ² or more, and a large amount of fine carbides are formed.

Next, the test piece for evaluation of the example of the present invention and the test piece for evaluation of the comparative example were subjected to a sand test to evaluate the toughness. The cut shape of the test piece of the sand blast impact test was 10R. The following two types of test pieces were used as the test pieces for the sand test: a test piece sampled so that the longitudinal direction (hot working direction) of the rod-shaped steel material matches the length of the test piece, and a cross-section of the rod-shaped steel material. The specimen is sampled in such a way that the radial direction matches the length of the specimen. Next, for the above two types of test pieces, three test pieces (TP1, TP2, TP3) with different sampling positions were prepared to perform a sand blast impact test. Table 2 shows the test results of the sand blast impact test.

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

FIGS. 6 and 7 show the scanning formulas of the fracture surface near the notch of the test piece TP2 sampled in the cross-sectional diameter direction of the bar-shaped steel for the high-speed tool steel of the present invention and the comparative example, respectively. Electron microscope image. As shown in FIG. 6, in the case of the high-speed tool steel according to the example of the present invention, no significant cause of the reduction in the impact value was found at the starting point of the cross section. On the other hand, as shown in FIG. 7, in the case of the high-speed tool steel of the comparative example, a large carbide having an equivalent circle diameter exceeding 1.00 μm was confirmed at the starting point (circle portion) of the cross section. That is, it was confirmed that this large carbide becomes a starting point of failure, and the toughness of the high-speed tool steel of the comparative example is reduced.

The present invention refers to the disclosure of Japanese Application 2013-201392 filed on September 27, 2013, and the entirety thereof is incorporated herein by reference. All documents, patent applications, and technical specifications described in this specification are cited with reference to each document, patent application, and technical specification, and are specifically the same as those in each description, and are further incorporated into this specification by reference.

[Fig. 1] A schematic diagram for explaining the soaking treatment and cooling process (cooling conditions 1 to 4) performed on the steel block in Example 1. [Fig. [Fig. 2] In Example 1, an electron microanalyzer (EPMA; electron probe microanalyzer) was used to analyze the cross-section structure of the steel block A and the steel block B cooled under each of the cooling conditions 1-4. A binary image is a diagram showing the distribution of carbides in a cross-section structure. 3 is a scanning electron microscope image showing an example of carbides distributed in a cross-sectional structure of a high-speed tool steel according to the present invention in Example 2. FIG. FIG. 4 is a scanning electron microscope image showing an example of carbides distributed in a cross-sectional structure of a high-speed tool steel of a comparative example in Example 2. FIG. FIG. 5 is a graph showing the relationship between the equivalent circle diameter of carbides and the number density (number / mm ² ) in the cross-sectional structure of the high-speed tool steel of the present invention and the comparative example in Example 2. FIG. FIG. 6 is a scanning electron microscope image showing an example of a fracture surface after the test when a charpy impact test is performed on the high-speed tool steel of the present invention in Example 2. FIG. FIG. 7 is a scanning electron microscope image showing an example of a fracture surface after a test when a sand impact test is performed on a high-speed tool steel of a comparative example in Example 2. FIG.

Claims (13)

A high-speed tool steel, characterized by mass%, containing 0.40 to 0.90% C, Si less than 1.00%, Mn less than 1.00%, Cr 4.00 to 6.00%, and expressed by the relationship (Mo + 0.5W) One or two of W and Mo with a content of 1.50 to 6.00%, and one of V and Nb with a content of 0.50 to 1.50% based on the relationship (V + Nb) Or two kinds; N content is 0.0200% by mass or less; the remainder is composed of Fe and impurities; the maximum equivalent circle diameter of the carbide in the cross-section structure is 1.00 μm or less; the hardness is 45 to 60 HRC. For example, the high-speed tool steel of the scope of patent application No. 1 further includes Ni of 1.00% by mass or less. For example, the high-speed tool steel of the scope of application for patent No. 1 or 2 further includes Co of 5.00 mass% or less. For example, the high-speed tool steel of the patent application scope item 1 or 2, wherein the content of Si is 0.20% by mass or less. A method for manufacturing a high-speed tool steel, which comprises the following steps: preparing a steel block having the following structure: by mass%, containing 0.40 to 0.90% of C, Si of 1.00% or less, Mn of 1.00% or less, 4.00 ~ 6.00% Cr, one or two of W and Mo with a content of 1.50 ~ 6.00% obtained from the relational formula (Mo + 0.5W), and obtained from the relational formula (V + Nb) The content is 0.5 or 1.50% of one or two of V and Nb; the content of N is 0.0200 mass% or less, and the remainder is Fe and impurities; the heat treatment step is to heat the steel block to 1200 to 1300 ° C A soaking process; a cooling step, in the process of cooling the steel block after the soaking process to a surface temperature of the steel block of 900 ° C or lower, at least when the surface temperature falls below 1000 ° C and above 950 ° C After the temperature T1 in the range, the surface temperature is cooled to 900 ° C or lower under the condition that the cooling rate of the surface temperature is 3 ° C / min or more, and the surface temperature is reduced to at least a part of the temperature T1, and the surface temperature is If the cooling rate is less than 3 ° C / min, cooling is performed; After the step, the steel block is reheated to a hot working temperature exceeding 900 ° C, and the reheated steel block is hot-worked to become a steel material; in the quenching and tempering step, the steel material is quenched and tempered to hardness Adjust to 45 ~ 60HRC. For example, the method for manufacturing a high-speed tool steel according to item 5 of the application, wherein in the cooling step, the steel block is cooled under the condition that the cooling rate of the surface temperature of the steel block is less than 3 ° C./minute until the steel block The surface temperature of T1 drops to this temperature T1. For example, the method for manufacturing a high-speed tool steel according to claim 5 or 6, wherein the steel block prepared in the preparation step is a steel block obtained by casting a molten steel refined by a deoxidizing refining method. For example, the method for manufacturing a high-speed tool steel according to item 7 of the application, wherein the steel block prepared in the preparation step is obtained by casting a molten steel refined by a deoxidizing refining method to obtain an electrode for redissolution, and then using the obtained electrode. An electrode for re-dissolution, and a steel block obtained by the re-dissolution method. For example, the method for manufacturing a high-speed tool steel according to item 5 or 6 of the patent application scope, wherein the steel block prepared in the preparation step further contains 1.00 mass% or less of Ni. For example, the method for manufacturing a high-speed tool steel according to item 5 or 6 of the patent application scope, wherein the steel block prepared in the preparation step further includes Co in an amount of 5.00% by mass or less. For example, the method for manufacturing a high-speed tool steel according to item 5 or 6 of the patent application scope, wherein the steel block prepared in the preparation step further includes Ni of 1.00 mass% or less and Co of 5.00 mass% or less. For example, the method for manufacturing a high-speed tool steel according to claim 5 or 6, wherein the content of Si in the steel block prepared in the preparation step is 0.20% by mass or less. For example, the method for manufacturing a high-speed tool steel according to the scope of patent application No. 5 or 6, wherein after the hot working step and before the quenching and tempering step, a machining step is further included, and the steel is mechanically processed into a tool shape. This quenching and tempering step is quenching and tempering the steel material that is machined into a tool shape.