JP2016060961A - High-speed tool steel having high toughness and softening resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-steep tool steel having softening resistance by optimization of added amount of alloy elements in the steel and having, in particular, high toughness by considering added amount of Mo to added amounts of Cr, V, W, Nb.SOLUTION: There is provided the high-speed tool steel which contains, by mass%, C:0.60 to 0.75%, Si:0.10 to 2.50%, Mn:0.10 to 1.00%, Cr:3.00% to less than 5.00%, Mo:2.00 to 3.60%, W:3.00% or less with 2Mo+W:5.00 to 9.00%, V:0.50 to 2.00%, P:0.050% or less, S:0.030% or less, O:0.0100% or less, N:0.0400% or less and the balance Fe with inevitable impurities and in which ΔC is -0.1000 to 0.0600 and A value is 0.255 to 0.350.SELECTED DRAWING: None

Description

  The present invention relates to a high-speed tool steel having high toughness and softening resistance suitable for dies such as cold forging, warm forging, hot forging, hot extrusion, and die casting.

  As a high-speed tool steel capable of obtaining hardness and wear resistance, for example, there is JIS standard SKH51. However, since SKH51 contains many alloying elements such as Mo, V, and W, the primary carbide tends to increase and the toughness tends to decrease. Matrix high-speed high-speed tool steel is known as a high-speed tool steel having high toughness while obtaining the same hardness as this high-speed tool steel. This matrix high speed high-speed tool steel is composed of a component system in which the content of alloy elements such as Mo, V, and W is reduced with reference to the composition of the matrix (matrix) of high-speed tool steel such as JIS-SKH51. Tool steel having both high strength and toughness is known as the prior art.

  As one of these conventional technologies, high-speed tool steel, especially high-speed tools capable of low-temperature quenching, are used for cutting tools and various tools such as hot, warm or cold forging, forging, upsetting, and shearing. An invention of steel has been proposed (see, for example, Patent Document 1). However, since the upper limit of the component amount of (2Mo + W) in the component system shown as an example of the invention of Patent Document 1 is as high as 20%, stable toughness cannot be obtained.

  Further, as another prior art, inventions of low alloy high speed tool steels having excellent impact resistance and friction resistance used for forging tools for plastic working have been proposed (for example, Patent Document 2 and Patent Document 3). reference.). In addition, by using the relational expression of alloy elements such as C content, optimization of the addition amount of alloy elements is proposed, and the invention of a mold steel capable of maintaining softening resistance and wear resistance in a high temperature environment is proposed. (For example, refer to Patent Document 4). However, these documents do not consider the impurity elements such as P, S, O, and N that cause the toughness to decrease, and stable toughness cannot be obtained.

  Furthermore, in addition to the above Patent Documents 1 to 4, inventions that take into account the toughness of matrix high-speed high-speed tool steel have been proposed (see, for example, Patent Document 5 and Patent Document 6). However, since the invention of Patent Document 5 has a high Cr content, the softening resistance may be insufficient, and the value considering the ratio of Mo-based carbide to the carbide of each alloy element is not considered, and the toughness is not considered. May decrease. In addition, the invention of Patent Document 6 does not consider the value considering the ratio of the Mo-based carbide to the carbide of each alloy element. Further, in the component system shown as an example of the invention of this Patent Document 6, In addition, there is a case where components such as C, Mo, and W considering the softening resistance at the same time have not been studied, and the toughness and the softening resistance may be insufficient.

JP 2003-268499 A Japanese Patent Laid-Open No. 1-108348 JP-A-1-159349 JP 2013-213256 A JP 2004-285444 A JP-A-2005-206913

  The problem to be solved by the present invention is to combine softening resistance by optimizing the addition amount of alloying elements in steel, and in particular, by considering the addition amount of Mo with respect to the addition amounts of Cr, V, W, and Nb. An object of the present invention is to provide tool steel for a mold that can stabilize toughness at a high level.

In order to solve the above-described problems as the conventional technique, the inventor has intensively developed, and as a result, the alloy composition shown in the claims, the formula consisting of C amount-C _eq = ΔC, and the A value = 0.063. It has been found that a tool steel excellent in the balance between toughness and softening resistance can be obtained by satisfying the formula consisting of ×% Mo / C _eq .

That is, the means of the present invention for solving the above-mentioned problem is that in the means of claim 1, C: 0.60 to 0.75%, Si: 0.10 to 2.50%, Mn : 0.10 to 1.00%, Cr: 3.00% to less than 5.00%, Mo: 2.00 to 3.60%, W: 3.00% or less, provided that 2Mo + W: 5.00 9.00%, V: 0.50-2.00%, P: 0.050% or less, S: 0.030% or less, O: 0.0100% or less, N: 0.0400% or less Further, it is a high-speed tool steel having high toughness, characterized by consisting of the balance Fe and inevitable impurities, ΔC: −0.1000 to 0.0600, and A value: 0.255 to 0.350.
Here, ΔC = C−C _eq and C _eq = 0.06 ×% Cr + 0.063 ×% Mo + 0.033 ×% W + 0.2% V + 0.1 ×% Nb. C _eq is mainly used as a measure of the amount of C required when all of the added alloy elements are all carbides.
That is, ΔC is a value that takes into account the amount of dissolved C from the relationship between C in steel and the amount of each alloying element.
Further, A value = 0.063 ×% Mo / C _eq , and the A value was considered with respect to the ratio of the Mo-based carbide to the carbide of each alloy element in order to combine both toughness and softening resistance characteristics. Value.

The means of claim 2 contains, in addition to the chemical components of the means of claim 1, one or two kinds of Nb: 1.00% or less and Co: 5.00% or less in mass%, with the balance being Fe. And high-speed tool steel having high toughness, characterized by comprising ΔC: −0.1000 to 0.0600 and A value: 0.255 to 0.350.
Here, ΔC is ΔC = C−C _eq , and C _eq is C _eq = 0.06 ×% Cr + 0.063 ×% Mo + 0.033 ×% W + 0.2% V + 0.1 ×% Nb. C _eq is mainly used as a measure of the amount of C required when all of the added alloy elements are all carbides.
That is, ΔC is a value that takes into account the amount of dissolved C from the relationship between C in steel and the amount of each alloying element.
Further, the A value is A value = 0.063 ×% Mo / C _eq , and the A value is a ratio of the Mo-based carbide to the carbide of each alloy element in order to combine both characteristics of toughness and softening resistance. Is a value that is taken into consideration.

  This invention is a high-speed tool steel having high toughness suitable for dies such as cold forging, warm forging, hot forging, hot extrusion, die casting and the like, and by optimizing the addition amount of alloying elements in the steel It is a high-speed tool steel that has high toughness after quenching, excellent softening resistance with a small softening amount when held at high temperature for a long time, and excellent balance between toughness and softening resistance.

Prior to the embodiments for carrying out the present invention, the reasons for limiting the chemical components of the claimed invention and ΔC (= C−C _eq ) and A value (= 0.063% Mo / C _eq ) will be described. To do. In addition,% in the following each chemical component is mass%.

C: 0.60 to 0.75%
C is an element necessary for securing sufficient hardenability and quenching and tempering hardness and for obtaining wear resistance and high-temperature strength by forming carbides. When C is less than 0.60%, sufficient hardness, high temperature strength and wear resistance cannot be obtained. On the other hand, when C is more than 0.75%, solidification segregation and carbide segregation in steel are promoted, and toughness is inhibited. Therefore, C is set to 0.60 to 0.75%.

Si: 0.10 to 2.50%
Si is an element necessary for ensuring the deoxidation effect and hardenability in steelmaking. However, if Si is less than 0.10%, the effects of deoxidation and hardenability in steelmaking cannot be obtained. On the other hand, if the Si content is more than 2.50%, the toughness is lowered. Therefore, Si is set to 0.10 to 2.50%.

Mn: 0.10 to 1.00%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.10%, the effect of hardenability cannot be obtained. On the other hand, if Mn is contained in an amount of more than 1.00%, workability is lowered. Therefore, Mn is set to 0.10 to 1.00%.

Cr: 3.00 to 5.00%
Cr is an element that improves hardenability. However, if Cr is less than 3.00%, the hardenability is not sufficiently improved. On the other hand, when Cr is more than 5.00%, segregation of Cr-based carbides is promoted during quenching and tempering, and high temperature strength and softening resistance are lowered. Therefore, Cr is set to 3.00 to 5.00%.

Mo: 2.00 to 3.60%, W: 3.00% or less, and 2Mo + W: 5.00 to 9.00%
Mo and W are elements that contribute to hardenability, secondary hardening during tempering, high-temperature strength, and softening resistance. In addition, fine carbides that are insoluble during quenching suppress the coarsening of crystal grains and suppress the decrease in toughness. However, if Mo is less than 2.00% and 2Mo + W is less than 5.00%, hardenability, secondary hardening at the time of tempering, high temperature strength, softening resistance cannot be obtained, and it is not dissolved at the time of quenching. Fine carbides cannot suppress the coarsening of crystal grains and cannot suppress the decrease in toughness. On the other hand, when Mo is contained in excess of 3.60%, W is contained in excess of 3.00%, and 2Mo + W is contained in excess of 9.00%, solidified segregation is promoted to crystallize coarse carbides, and toughness Reduce. In particular, Mo needs to contain twice the amount of W in order to obtain the same effect as W, but if Mo is contained excessively, the cost increases. Therefore, Mo is set to 2.00 to 3.60%, W is set to 3.00% or less, and 2Mo + W is set to 5.00 to 9.00%.

V: 0.50 to 2.00%
V is an element that precipitates fine and hard MC-type carbides, nitrides, and carbonitrides during tempering and contributes to high-temperature strength and wear resistance. In addition, fine carbides and carbonitrides that are not dissolved at the time of quenching suppress coarsening of crystal grains and suppress a decrease in toughness. However, if V is less than 0.50%, high-temperature strength and wear resistance cannot be obtained, and coarsening of crystal grains and deterioration of toughness cannot be suppressed. On the other hand, if V is more than 2.00%, solidification segregation is promoted, coarse carbides, nitrides, carbonitrides are crystallized, and toughness is impaired. Therefore, V is set to 0.50 to 2.00%.

P: 0.050% or less P is an element inevitably contained as an impurity element. By the way, when P is more than 0.050%, it segregates to a grain boundary and lowers toughness. Therefore, P is set to 0.050% or less.

S: 0.030% or less S is an element inevitably contained as an impurity element. By the way, when S is more than 0.030%, sulfide is formed, and toughness and hot workability are lowered. Therefore, S is set to 0.030% or less.

O: 0.0100% or less O is an element inevitably contained as an impurity element. By the way, when O is more than 0.0100%, an oxide is formed and becomes a starting point of fracture, and toughness and fatigue strength are reduced. Therefore, O is set to 0.0100% or less.

N: 0.0400% or less N combines with V, Nb, and Ti to form MC-type nitrides and carbonitrides, and contributes to hardness and wear resistance. These MC type nitrides and carbonitrides suppress the coarsening of crystal grains during quenching and improve toughness. By the way, when N is more than 0.0400%, in order to promote the bonding with V, Nb, Ti at a higher temperature in the solidification process, the crystallized MC type nitride or carbonitride becomes coarse, Conversely, toughness is impaired. Therefore, N is set to 0.0400% or less.

Nb: 1.00% or less Nb, like V, is an element that precipitates fine and hard MC-type carbides, nitrides, and carbonitrides during tempering and contributes to high-temperature strength and wear resistance. Moreover, the fine carbide | carbonized_material and carbonitride which became insoluble at the time of quenching suppress the coarsening of a crystal grain, and suppress the fall of toughness. By the way, when Nb is more than 1.00%, solidification segregation is promoted, coarse carbides, nitrides, carbonitrides are crystallized, and toughness is inhibited. Therefore, Nb is made 1.00% or less.

Co: 5.00% or less Co is an element that strengthens the matrix and improves high-temperature strength. It is an element that suppresses the coarsening of carbides at high temperatures and increases high-temperature softening resistance and hardness. By the way, when Co is contained more than 5.00%, toughness is deteriorated and cost is increased. Therefore, Co is set to 5.00% or less.

ΔC: −0.1000 to 0.0600
ΔC is a value defined by ΔC = C−C _eq . When this ΔC value is less than −0.1000, not only is it difficult to obtain sufficient quenching and tempering hardness, but also the amount of carbide is reduced and softening resistance is lowered. On the other hand, when ΔC exceeds 0.0600, component segregation and carbide segregation are promoted, and toughness deteriorates. Therefore, ΔC is set to −0.1000 to 0.0600.

A value: 0.255-0.350
The A value is a value defined by A value = 0.063% Mo / C _eq . When this A value is smaller than 0.255, in order to increase the toughness and softening resistance, the balance of components of Mo and other elements is important. In the steel after quenching and tempering, The ratio of the Mo type carbide | carbonized_material which falls exists and softening resistance falls. On the other hand, if the A value exceeds 0.350, the proportion of Mo-based carbide becomes excessive, promotes carbide segregation, and decreases toughness. Therefore, the A value is set to 0.255 to 0.350.

Embodiments of the present invention will be described below.
First, a cast material composed of the chemical components of the steel of the present invention is secondarily melted and resolidified by a vacuum arc remelting method (VAR). In this method, solidification after remelting is performed in a short time by secondary melting, so that solidification segregation hardly occurs and local agglomeration and segregation of carbides can be suppressed. In addition, it can replace with said vacuum arc remelting method (VAR), and can also perform secondary melting by an electroslag furnace remelting method (ESR).

  Next, the soaked secondary remelted steel is subjected to a soaking process at 1000 to 1200 ° C. for 10 hours or more. This production method is an optimum production method for controlling coarse carbides precipitated in the steel within a suitable range. This soaking process needs to be performed at a temperature higher than the quenching temperature and lower than the melting point. If the soaking process is appropriately performed, it is possible to reduce the amount of coarse carbide formed and further reduce the amount of carbide to uniformly disperse. It should be noted that suitability values of the soaking temperature and time vary depending on the components.

  That is, in the embodiment of the present invention, the ingot comprising the composition of each chemical component and the balance Fe and unavoidable impurities of the inventive steel shown in symbols A to Q in Table 1 and the comparative steel shown in symbols 1 to 17, After melting using a 1-ton vacuum melting furnace, ingots are formed into ingots, and the obtained ingots are subjected to a soaking treatment at 1200 ° C. for 10 hours or more, followed by hot forging to produce a forging ratio. Was manufactured as a steel bar having a diameter of 160 mm, which is approximately 6S.

  In Table 1, “-” in Nb and Co in the component composition represents no addition. Further, numerical values with an underline indicate that they are outside the scope of the claims of the present invention.

  A sample consisting of a block of 25 mm in length, 25 mm in width and 25 mm in height, calculated from the central part of a steel bar of 160 mm in diameter and made of the composition of the inventive steel consisting of symbols A to Q in Table 1 is used. As shown in Table 2, as an example, after holding for 10 minutes at the quenching temperature of 1180 ° C. of (1) and as an example, after holding for 10 minutes at the quenching temperature of 1100 ° C. of (2), The sample was put into 50 ° C. oil which was being stirred and quenched. Next, the operation of air-cooling was performed three times by holding the samples composed of these quenched blocks at the tempering temperature in (2) of Table 2 and the tempering temperature in (2) for 60 minutes, respectively. Cut each obtained sample, use the cut surface as the measurement surface, remove the heat-affected layer on the measurement surface and the scale layer on the opposite surface with a flat grinder to improve parallel accuracy, then lock The hardness was measured with a well hardness meter, and the hardness is shown in Table 2 as HRC.

The toughness in Table 2 was evaluated by a Charpy impact test. The test piece used was taken from the rolling direction of the central part of the hot forged material having a diameter of 160 mm, and the quenching temperature and tempering temperature of 1180 ° C. in (1) described in Table 2 and (2) After quenching and tempering at a quenching temperature and a tempering temperature of 1100 ° C., respectively, each was processed into a Charpy test piece having a 10RC notch. The evaluation was performed by selecting two conditions of HRC63 or higher which is the maximum hardness of the example of (1) and HRC58 of the example of (2). However, the steel material in which 63 HRC or more was not obtained with the comparative steel was tempered to its highest quenching and tempering hardness. In Table 2, in the case of the hardness of 63HRC or more in the example of (1), when the Charpy impact value is 25.0 J / cm ² or more, ○, and when it is less than 25.0 J / cm ² , × did. In the case of the hardness of HRC58 in the example of (2), the Charpy impact value was evaluated as ◯ when it was 120.0 J / cm ² or more, and when it was less than 120.0 J / cm ² , it was rated as x.

  The method of the softening resistance test is shown below. From the central part of each steel material of the present invention steel and comparative steel, the test material consisting of blocks of 25 mm length, 25 mm width, 25 mm height was indexed, and after quenching at 1180 ° C. in the example of (1), By tempering, HRC 63-65 was tempered. In order to check the softening resistance, these test materials were held at 600 ° C. for 50 hours, and then these steel materials were air-cooled. Similarly to the above, after removing the scale layer on the surface of the steel material, the hardness of each test material was measured with a Rockwell hardness meter, and the difference between this hardness value and the initial hardness of 1180 ° C. quenching. That is, the softening resistance was evaluated by the softening amount (ΔHRC). However, the steel material in which HRC63 or more was not obtained with comparative steel was tempered to its highest quenching and tempering hardness. The evaluation is shown in Table 2 as ◯ when the softening amount ΔHRC is 20.0 or less, and when the softening amount ΔHRC exceeds 20.0, the numerical value of the softening amount ΔHRC is underlined and indicated as x. .

  The comparative steels in Tables 1 and 2 will be described below.

In Comparative Steel 1, C is 0.86%, which is higher than 0.75% of the upper limit value of C of Claim 1, and thus the hardness is (1) HRC65.2 and (2) HRC58.4. although, since promotes segregation, 14.2J / cm ² in toughness (1), and lower than the value specified in the 35.2J / cm ^2, both in the present application (2), any toughness Is also x.

  In Comparative Steel 2, C is 0.53%, which is lower than 0.60% of the lower limit value of C of Claim 1, so that the hardness is sufficient as (1) HRC61.2 and (2) HRC56.8. Hardness cannot be obtained. Moreover, the amount of carbide effective for high-temperature strength decreases, and ΔHRC of the softening resistance softening amount is 23.6.

In Comparative Steel 3, Mo is 4.30%, which is higher than the upper limit of 3.60% of Mo of Claim 1, and therefore the A value is 0.365, which exceeds the upper limit of 0.350 of the A value of the present application. since promotes segregation, 23.1J / cm ² in toughness (1), and 110.3J / cm ² (2), both are lower than the value specified in this application, both toughness X.

In Comparative Steel 4, since 2Mo + W is 9.10 and exceeds the upper limit of 9.00 of 2Mo + W of claim 1, and ΔC is 0.1575 and exceeds the upper limit of 0.0600 of ΔC of claim 1, segregation occurs. since promoting, 18.4J / cm ² in toughness (1), and 70.2J / cm ² (2), both are lower than the value specified in this application, both toughness is a × .

  In Comparative Steel 5, Mo is 1.85%, which is lower than 2.00% of the lower limit value of Mo of Claim 1, and W is 0.51%, so 2Mo + W is 4.21, and the lower limit of 2Mo + W of Claim 1 Since the value is lower than 5.00, the amount of carbide effective for high-temperature strength is reduced, softening resistance is reduced, and ΔHRC of softening amount is 22.7, which exceeds the evaluation standard of 20.0. is there.

  In Comparative Steel 6, Cr is 5.04%, which is higher than 5.00% of the upper limit value of Cr of claim 1, so the A value is 0.242, which is lower than the lower limit value of 0.256 of claim 1, and the high temperature strength. Since the effective amount of carbide is reduced, the softening resistance is lowered, and the ΔHRC of the softening amount is 23.2, which exceeds 20.0 of the evaluation standard, and therefore, x.

  Since the comparative steel 7 has a poor component balance and ΔC is −0.1068, which is lower than the lower limit value of ΔC of −0.1000 of claim 1, the softening resistance softening amount ΔHRC 20.6 is 20. Since it exceeds 0, it is x.

  In Comparative Steel 8, V is 2.67%, which is higher than 2.00% of the upper limit value of V in Claim 1, so that the solid solution C effective for hardness is reduced and the amount of carbides precipitated during tempering is insufficient. Therefore, it is difficult to obtain hardness, and the amount of carbide effective for high-temperature strength is reduced, so the softening resistance is reduced, and ΔHRC of softening amount is 20.5, which exceeds 20.0 of the evaluation standard. is there.

In comparative steel 9, V is 0.44%, which is lower than 0.50% of the lower limit value of V in claim 1, so the A value is 0.390, which exceeds the upper limit of 0.350 of the A value of the present application. , 23.9J / cm ² in toughness (1), is lower than the value specified in the 114.1J / cm ^2, both in the present application (2). That is, a decrease in the amount of insoluble carbides during quenching and a decrease in the coarsening suppression ability of crystal grains is one factor, and the toughness is (1) 23.4 J / cm ² and (2) 114.1 J / cm. ² and both are inferior to the toughness value specified in the present application, and thus the toughness is x.

  In Comparative Steel 10, Nb is 1.37%, which is higher than 1.00% of the upper limit value of Nb of Claim 2, so that the solid solution C effective for hardness is reduced, and it is difficult to obtain the target hardness at high temperature strength. Since the effective amount of carbide is also reduced, the softening resistance is lowered, and the ΔHRC of the softening amount is 23.3, which exceeds 20.0 of the evaluation standard, and is x.

Since the comparative steel 11 has 7.0% Co, which exceeds 5.00% of the upper limit value of Co in claim 2, the ductility of the base structure is lowered, and the toughness is 22.5 J / cm at (1). ² and (2) are 97.8 J / cm ² , both of which are inferior to the toughness values specified in the present application, and thus the toughness is x.

  Comparative Steel 12 has a poor balance of Cr, Mo, W, and V components, and the A value is 0.226, which is lower than the lower limit value of 0.256 in Claim 1, so that the amount of carbide effective for high-temperature strength is also reduced. The softening resistance is lowered, and the ΔHRC of the softening amount is 21.0, which exceeds 20.0 of the evaluation standard, and is x.

Comparative Steel 13 has a poor balance of Cr, Mo, W, and V components, and has an A value of 0.364, which is higher than the upper limit of 0.350 of Claim 1, and therefore promotes segregation and has a toughness of (1). 23.7J / cm ^2, and 100.6J / cm ² (2), since both are inferior toughness value defined by the present, both toughness is ×.

In the comparative steel 14, S is 0.041%, which is higher than 0.030% of the upper limit of S in claim 1, and a large amount of sulfide is formed, and the toughness is (1) 21.4 J / cm ² ( In 2), 105.7 J / cm ² , both of which are inferior to the toughness value specified in the present application, the toughness is x.

In Comparative Steel 15, O is 0.0230%, which is higher than 0.0100% of the upper limit value of O of Claim 1, so that a large amount of oxide is formed, which becomes a starting point of fracture, and the toughness is 20. 0 J / cm ^2, and 103.4J / cm ² (2), since both are inferior toughness value defined by the present, both toughness is ×.

In Comparative Steel 16, P is 0.061%, which is higher than 0.050% of the upper limit value of P in claim 1, so that P is segregated at the grain boundary, and N is 0.049%. is higher than 0.0400% of the upper limit of N, promotes segregation of carbonitrides, 19.4J / cm ² in toughness (1), and 87.6J / cm ² (2), both present Therefore, the toughness is x in all cases.

Since the comparative steel 17 has 2.78% Si and more than 2.50% of the upper limit value of Si of Claim 1, the ductility of the base structure is lowered, and the toughness is (1) 24.1 J / cm ² , In (2), 111.4 J / cm ² , both of which are inferior to the toughness value specified in the present application, the toughness is x.

Compared to the above comparative steels, A to Q of the steels of the present invention are all excellent in softening resistance, ΔHRC of softening resistance is 20.0 or less, and all are ○, and the toughness is (1) 25 It is 0.0 J / cm ² or more, and (2) is 20.0 J / cm ² or more. Both have high toughness, and the toughness is all good. Therefore, A to Q of the steel of the present invention can provide a high-speed tool steel having high toughness and softening resistance.

Claims (2)

In mass%, C: 0.60 to 0.75%, Si: 0.10 to 2.50%, Mn: 0.10 to 1.00%, Cr: 3.00% to less than 5.00%, Mo: 2.00 to 3.60%, W: 3.00% or less, provided that 2Mo + W: 5.00 to 9.00%, V: 0.50 to 2.00%, P: 0.050% or less , S: 0.030% or less, O: 0.0100% or less, N: 0.0400% or less, consisting of the balance Fe and inevitable impurities, ΔC: −0.1000 to 0.0600, A Value: High-speed tool steel having high toughness and softening resistance, characterized by being from 0.255 to 0.350.
Here, ΔC = C−C _eq , C _eq = 0.06 ×% Cr + 0.063 ×% Mo + 0.033 ×% W + 0.2% V + 0.1 ×% Nb, A value = 0.063 ×% Mo / C _eq . In addition to the chemical component according to claim 1, it contains one or two of Nb: 1.00% or less and Co: 5.00% or less in mass%, and consists of the balance Fe and inevitable impurities, and ΔC : High speed tool steel having high toughness and softening resistance, characterized in that -0.1000 to 0.0600 and A value: 0.255 to 0.350.
Here, ΔC = C−C _eq , C _eq = 0.06 ×% Cr + 0.063 ×% Mo + 0.033 ×% W + 0.2% V + 0.1 ×% Nb, A value = 0.063 ×% Mo / C _eq .