WO2012118053A1

WO2012118053A1 - Hot work tool steel having excellent toughness, and process of producing same

Info

Publication number: WO2012118053A1
Application number: PCT/JP2012/054868
Authority: WO
Inventors: 公太片岡; 洋佑中野
Original assignee: 日立金属株式会社
Priority date: 2011-03-03
Filing date: 2012-02-28
Publication date: 2012-09-07
Also published as: CN103403209A; EP2682491B1; EP2682491A1; EP2682491A4; TWI447237B; CN103403209B; TW201250011A

Abstract

Provided are a hot work tool steel having improved toughness and a process of producing the hot work tool steel. A hot work tool steel comprising, in mass%, 0.3% or more and less than 0.6% of C, 1.5% or less of Si, 1.5% or less of Mn and 3.0% or more and less than 6.0% of Cr, wherein more than 0.0025% and 0.025% or less of Zn and 0.005% or more of P are contained and the Zn/P ratio is more than 0.5; and a process of producing a hot work tool steel, comprising a first step of preparing a molten steel having a chemical composition of a hot work tool steel containing 0.005 mass% or more of P, a second step of adding Zn to the molten steel having the chemical composition of the hot work tool steel, and a third step of casting the Zn-added molten steel to produce a steel ingot, wherein the second step is a step of adding Zn in such a manner that more than 0.0025 mass% and 0.025 mass% or less of Zn and 0.005 mass% or more of P can be contained in the chemical composition of the steel ingot after the casting in the third step and the hot work tool steel can have a Zn/P ratio of more than 0.5.

Description

Hot tool steel with excellent toughness and method for producing the same

The present invention relates to hot tool steel with improved toughness that is optimal for various hot tools such as press dies, forging dies, die casting dies, and extrusion tools, and a method for producing the same.

Since hot tools are used in contact with high-temperature workpieces and hard workpieces, they must have both strength and toughness that can withstand thermal fatigue and impact. Therefore, conventionally, for example, SKD61-based alloy tool steel, which is a JIS steel type, has been used as a steel type used in the field of hot tools (hereinafter referred to as hot tool steel). And the method of improving the toughness of the hot tool steel by reviewing the addition amount of the main elements constituting the hot tool steel and further regulating and controlling various impurities such as As, Bi, Sn, Zn, Sb Has been proposed (see Patent Document 1). However, adjusting each of the various impurity elements within the specified range can lead to an increase in manufacturing cost.

On the other hand, the present inventor has conducted intensive investigations on elements that have not been actively added as alloys in the field of steel materials, regardless of the addition of expensive and special elements. It has been found that the toughness can be greatly improved by adding Zn in a predetermined content range (see Patent Document 2). That is, hot work tool steel containing, by mass, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 3.00 to 5.65% Further, it is a hot tool steel of Zn: 0.001 to 0.015%.

JP 2003-155540 A JP 2007-224418 A

The Zn addition technique proposed in Patent Document 2 is effective as a new technique for improving the toughness of hot tool steel. And by utilizing the method of patent document 2, the scrap of Zn-plated steel can be utilized as a recycling raw material, and it is suitable also for reduction of an environmental load. The inventor examined the possibility of compensating for the deterioration of toughness due to other impurity elements, paying attention to the effect of improving toughness by this positive Zn addition. If the permissible amount of these impurity elements can be increased appropriately, the amount of energy used for removing impurities can be reduced while increasing the usage rate of low scrap with a large impurity content, which is expected to increase in the future. It is possible to further reduce the environmental load in the manufacturing process of hot tool steel.

An object of the present invention is to provide a hot work tool steel excellent in toughness capable of reducing an environmental load and a method for producing the same.

The inventor investigated the influence of impurity elements contained in hot tool steel on toughness and the environment. As a result, in particular, P (phosphorus) is an element that greatly reduces the toughness of hot tool steel, and the energy required for removal is large, and for these reasons, the use of low-grade scrap is also stagnant. The knowledge that is a large element was obtained. Therefore, in order to reduce this environmental load, a method that can maintain sufficient toughness even if the allowable amount of P is increased was examined. As a result, it has been found that the deterioration of toughness due to the increase in the P content can be compensated for by adding an appropriate amount of Zn to the P content. In addition to the specific relationship between P and Zn that can sufficiently utilize this toughness complementation effect, a component adjustment method suitable for achieving the relationship is also clarified, thereby achieving the present invention. did.

That is, the present invention includes, by mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 3.0 to less than 6.0% Hot tool steel with excellent toughness characterized by Zn: more than 0.0025 to 0.025%, P: 0.005% or more, and Zn / P: more than 0.5 Hot tool steel. Preferably, P: 0.01% or more. Further, if necessary, Mo and W may be contained alone or in combination (Mo + 1 / 2W): 3.5% or less, or V: 1.5% or less.

Specifically, by mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 1.5% or less (including 0%) Cr: 3.0 to less than 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 3.5% or less, V: 1.5% or less, Nb: 0.3% or less (0 Co: 5.0% or less (including 0%), Zn: more than 0.0025 to 0.025%, P: 0.005% or more, and Zn / P: more than 0.5 And it is the hot work tool steel excellent in toughness which consists of remainder Fe and an unavoidable impurity. Preferably, P: 0.01% or more.

Further, the present invention is a method for producing a hot tool steel excellent in toughness, the first step of obtaining a molten steel having a component composition of hot tool steel containing P of 0.005 mass% or more, It consists of a second step of adding Zn to the molten steel having the component composition of the hot tool steel and a third step of obtaining the steel ingot by casting the molten steel to which the Zn is added. The composition of the steel ingot after casting in the third step includes Zn: more than 0.0025 to 0.025 mass%, P: 0.005 mass% or more, and Zn / P: more than 0.5 hot It is a manufacturing method of hot tool steel excellent in toughness characterized by adding Zn so as to become tool steel. Preferably, the component composition of the molten steel obtained in the first step is mass% and includes P: 0.01% or more, and the component composition of the steel ingot after casting in the third step is mass%. , P: contains 0.01% or more. The composition of the steel ingot is, in mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 3.0 to 6 It is preferable that the hot tool steel contains less than 0.0%. If necessary, Mo and W may be contained alone or in combination (Mo + 1 / 2W): 3.5% or less, or V: 1.5% or less.

The most typical component composition of the steel ingot after casting in the third step is mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5 % Or less, Ni: 1.5% or less (including 0%), Cr: 3.0 to less than 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 3.5% or less, V : 1.5% or less, Nb: 0.3% or less (including 0%), Co: 5.0% or less (including 0%), Zn: more than 0.0025 to 0.025%, P: 0 0.005% or more and Zn / P: more than 0.5, which is a hot tool steel composed of the remaining Fe and inevitable impurities. Preferably, P: 0.01% or more.

According to the present invention, since sufficient toughness can be maintained without managing P contained in hot tool steel as an impurity to an extremely low value, energy consumption for lowering P can be saved and the environment can be saved. Can reduce the load. And the amount of Zn for maintaining sufficient toughness can be accurately adjusted by the addition method of the present invention. As described above, the present invention can drastically improve the toughness of hot tool steel, and is an effective technique for the practical application of hot tool steel that can be applied to a wide variety of uses and environments.

In Example 1, it is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel tempered to various hardness with respect to the hardness of a test piece. In Example 1, it is the figure which plotted the 2 mmV notch Charpy impact value between room temperature and 400 degreeC of this invention steel and comparative steel which were tempered to the hardness of 45HRC with respect to test temperature. In Example 2, it is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel tempered to various hardness with respect to the hardness of a test piece. In Example 2, it is the figure which plotted the 2 mmV notch Charpy impact value between room temperature to 400 degreeC of this invention steel and comparative steel tempered to the hardness of 45HRC with respect to test temperature.

A major feature of the present invention resides in that Zn, which has been conventionally treated as an impurity, is positively added in order to improve the toughness of hot work tool steel. And P which is an element which greatly reduces toughness is in a place where its inclusion is allowed in a specific range by clarifying the relationship with the amount of added Zn of the present invention. In other words, for hot tool steel, it has been found that if Zn is used as an alloy element, the effect of improving toughness can be exhibited even if the P content increases. In the present invention, a large amount of P, which has conventionally been required to be extremely reduced, can be tolerated. Therefore, in the selection of raw materials, the amount of high-grade low P scrap used can be reduced, which is suitable for scrap recycling. . Furthermore, the energy and time required for removing P in the refining process can be reduced. Hereinafter, the reason for limiting the components of the hot work tool steel manufactured according to the present invention will be described (mass% is simply expressed as%).

Zn: more than 0.0025 to 0.025%
Zn is the most important additive element for the present invention, and the toughness is remarkably improved by adding Zn. And this effect can fully be acquired by adding exceeding 0.0025%. Preferably it is 0.003% or more. On the other hand, even if a large amount is added, the effect reaches a peak. Furthermore, if excessive segregation occurs at the grain boundary or the like due to excessive addition, this may return and cause deterioration in toughness. Further, the addition technique becomes complicated, so the upper limit was made 0.025%. Preferably it is 0.020% or less, More preferably, it is 0.015% or less.

P: 0.005% or more P is an element that segregates at the prior austenite grain boundaries during the heat treatment such as tempering and embrittles the grain boundaries. Therefore, in order to improve the toughness of hot tool steel, it is an impurity element that has been usually managed as low as possible. However, in the present invention, the toughness degradation effect due to P can be compensated for by making maximum use of the above-described toughness improving effect by the addition of Zn. And the remarkable effect of Zn addition required for this can be acquired by adjustment of Zn addition amount with respect to P content mentioned later. Thereby, the hot tool steel of the present invention can tolerate a P content of 0.005% or more. Even if it is preferably 0.01% or more, more preferably 0.02% or more, sufficient toughness can be maintained.

-Zn / P: More than 0.5 Even when the hot tool steel of the present invention contains 0.005% or more of P, it is necessary to ensure a Zn addition amount sufficient to maintain sufficient toughness. Therefore, it is necessary to adjust the Zn addition amount with respect to the P content. Specifically, sufficient toughness can be ensured by setting the value of Zn / P to more than 0.5. Preferably it is more than Zn / P: 0.55. A Zn / P value exceeding 0.55 is a preferable condition even when 0.01% or more of P, further 0.02% or more of P is contained.

And, in the case of obtaining hot tool steel satisfying the above-mentioned relationship between P and Zn by the melt casting method, there is a component adjustment method suitable for this. That is, if the component composition of the molten steel at the time of casting is adjusted to the one containing Zn by various methods, the ingot of the hot tool steel according to the present invention can be obtained by casting this. However, metal Zn is a volatile element having a low melting point and escapes from the molten steel with time. Therefore, for the Zn amount to be maintained in the molten steel at the time of casting, it is effective to set a larger amount of Zn at the time of addition to the molten steel in consideration of the above allowance. However, if the amount of Zn in the molten steel is controlled to adjust it to the exact value of the present invention at the time of casting, it is difficult to achieve by the addition method only assuming the above-mentioned allowance. . Therefore, in the present invention, Zn is not adjusted at the same timing as other additive elements, but is adjusted in advance to the component composition of the hot tool steel and added to the molten steel with a fixed P content. Thus, the elapsed time until the subsequent casting can be shortened, and the fluctuation of the Zn / P ratio due to the volatilization of Zn or the like can be suppressed.

Specifically, Zn is added to the first step of obtaining molten steel having a component composition of hot tool steel containing 0.005 mass% or more of P, and to the molten steel having the component composition of hot tool steel. It consists of a 2nd process and the 3rd process which casts the molten steel which added said Zn, and obtains a steel ingot. In the second step, the composition of the steel ingot after casting in the third step includes Zn: more than 0.0025 to 0.025 mass%, P: 0.005 mass% or more, and Zn / P: This is a method for producing hot tool steel in which Zn is added so that the hot tool steel exceeds 0.5. P contained in the steel ingot after casting in the third step is preferably 0.01% by mass or more, more preferably 0.02% by mass or more. Hereinafter, each step will be described.

-1st process: It is the process of obtaining the molten steel of the component composition of the hot tool steel containing 0.005 mass% or more of P.
By adjusting the component composition of the molten steel to that of the hot work tool steel, the second step described later can be specialized in adjusting the Zn content exclusively. And since it can transfer to casting of the 3rd process mentioned later immediately after a predetermined amount of Zn addition, it is easy to adjust Zn content of the steel ingot after casting exactly. In the first step of the present invention, “to obtain molten steel having a component composition of hot tool steel” means that before the second step described later, the molten steel of hot tool steel adjusted to various component compositions is used. It means to make a state. And it is preferable to adjust the component composition at this time to the intended component composition of the steel ingot. Further, at this time, depending on the type (component composition) of the Zn source used in the next Zn addition, if the component composition fluctuates before and after the addition of Zn, it is preferable to take this fluctuation into account. Therefore, the process is not limited to alloy addition applied to molten steel, and may be an operation of preparing raw materials before melting, for example. And after melt | dissolution, you may give the various refining process which removes an inclusion and an impurity. In addition, content of P which the molten steel obtained at the 1st process contains becomes like this. Preferably it is 0.01 mass% or more, More preferably, it is 0.02 mass% or more.

-2nd process: It is the process of adding Zn to the molten steel of the hot tool steel by a 1st process.
In the present invention, as described above, it is important to manage the second step of adding Zn separately from the first step. If Zn is added to the molten steel that has been previously adjusted to the component composition of the hot tool steel, it is easy to adjust the Zn content with respect to P, and it is possible to quickly shift to casting in the third step described later. In addition, the 2nd process of this invention does not exclude the process for the purpose other than adding Zn. Therefore, at this time, if there is a slight difference between the elemental composition other than Zn and the component composition of the target steel ingot, fine adjustment is performed again unless a new refining treatment is performed. Also good.

-3rd process: It is the process of casting the molten steel by a 2nd process, and obtaining a steel ingot.
By casting the molten steel that has undergone the first and second steps to obtain a steel ingot, hot tool steel that satisfies the component composition of the present invention is melted. The casting in the third step of the present invention is not limited to the ordinary ingot casting method, and may be a continuous casting method or other special ingot casting methods.

The effect of improving toughness by adding Zn of the present invention is greatly inhibited and weakened when a large amount of carbide is distributed in the structure. That is, the above-described toughness improving effect is sufficiently exhibited by hot tool steel with less carbides than cold tool steel in which many carbides are distributed in the structure. Therefore, the object of the present invention is limited to hot tool steel. Hot tool steel refers to steel having a component composition defined in JIS-G-4404, for example. In addition to standard steel types such as JIS, conventionally proposed hot work tool steel can also be applied. Element types other than those defined in the above hot work tool steel can be added as necessary. The following are preferred component compositions of the hot work tool steel produced by the present invention. The reason for the numerical limitation will be described.

-C: 0.3 to less than 0.6% C is an element that increases the wear resistance and seizure resistance by partly forming a solid solution in the matrix to give strength and partly forming carbides. is there. Further, when C, which is a solid interstitial atom, is co-added with a substitution atom having a high affinity with C such as Cr, the I (interstitial atom) -S (substitution atom) effect; It acts as a resistance and is also expected to increase the strength of steel. However, excessive addition causes a decrease in toughness and hot strength. Therefore, it is preferable to set it to 0.3 to less than 0.6%. More preferably, it is less than 0.55%.

Si: 1.5% or less Si is a deoxidizing agent during steel making and an element that enhances the machinability of the material. In order to obtain these effects, addition of less than 0.2% may be performed, but addition of 0.2% or more is preferable. However, since it will lead to the formation of ferrite if too much, it is preferably 1.5% or less.

-Mn: 1.5% or less Mn has the effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, since it exists as MnS of a nonmetallic inclusion, there is a great effect in improving machinability. In order to obtain these effects, addition of less than 0.1% may be performed, but addition of 0.1% or more is preferable. However, if it is too much, the viscosity of the base is increased and the machinability is lowered, so the content is preferably 1.5% or less.

Cr: 3.0 to less than 6.0% Cr is an element that enhances hardenability and forms carbides, and has an effect on strengthening the base and improving wear resistance. And it is an element which contributes also to the improvement of temper softening resistance and high temperature strength. However, excessive addition causes a decrease in hardenability and high temperature strength. Therefore, 3.0 to less than 6.0% is preferable. More preferably, it is 5.65% or less.

Mo and W are single or composite (Mo + 1 / 2W): 3.5% or less Mo and W are single or composite for precipitating or agglomerating fine carbides by tempering to give strength and improving softening resistance. Can be added. The amount of addition at this time can be defined together with the Mo equivalent of (Mo + 1 / 2W) since W is about twice the atomic weight of Mo (of course, either one or both may be added) Can be added together). And in order to acquire an above-described effect, although addition of less than 1.0% may be sufficient as the value of (Mo + 1 / 2W), addition of 1.0% or more is preferable. However, since it will lead to a decrease in machinability and toughness if it is too much, the value of (Mo + 1 / 2W) is preferably 3.5% or less.

V: 1.5% or less V has the effect of forming carbides and improving base strengthening and wear resistance. In addition, the temper softening resistance is increased and the coarsening of crystal grains is suppressed, thereby contributing to the improvement of toughness. In order to obtain these effects, addition of less than 0.5% may be performed, but addition of 0.5% or more is preferable. However, since too much will lead to a decrease in machinability and toughness, it is preferably made 1.5% or less.

Ni: 1.5% or less Ni is an element that suppresses the formation of ferrite. Also, together with C, Cr, Mn, Mo, W, etc., it imparts excellent hardenability to hot tool steel and forms a martensite-based structure even when the cooling rate during quenching is slow, resulting in toughness It is an effective element for preventing a decrease in the amount. Furthermore, since the essential toughness of the base is also improved, it is added as necessary in the present invention. However, if it is too much, the base becomes thicker and the machinability decreases. Therefore, even when it is added, the content is preferably 1.5% or less. In addition, when adding, 0.1% or more is preferable.

-Nb: 0.3% or less Nb has the effect of forming a carbide | carbonized_material and improving the reinforcement | strengthening of a base and abrasion resistance. In addition, the temper softening resistance is increased and the coarsening of the crystal grains is suppressed to contribute to the improvement of toughness. Therefore, in the present invention, it is added as necessary. However, if too much, machinability and toughness are reduced. Therefore, even when it is added, the content is preferably 0.3% or less. When adding, 0.05% or more is preferable.

Co: 5.0% or less Co forms a protective oxide film that is extremely dense and has good adhesion on the surface when the hot tool steel according to the present invention is used as a tool. This oxide film prevents metal contact with the counterpart material, suppresses temperature rise on the tool surface, and provides excellent wear resistance. Therefore, in this invention, it adds as needed. However, if it is too much, the toughness is lowered, so even if it is added, it is preferably 5.0% or less. When adding, 0.3% or more is preferable.

Main elements that may remain in the steel as inevitable impurities are S, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), and the like. In order to effectively obtain the effects of the addition of Zn of the present invention, these elements are preferably as low as possible. However, on the other hand, in order to obtain additional functions and effects such as control of the shape of inclusions, other mechanical properties, and improvement in production efficiency, some inclusions and / or additions can be made. In this case, S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, N ≦ 0.03 % Range is sufficiently acceptable and is the preferred upper limit of regulation of the present invention.

The hot tool steel according to the present invention may be subjected to a homogenization heat treatment while processing a steel ingot after casting into a steel material, for example, as a form for sufficiently exerting the toughness improving effect by addition of Zn. preferable. Alternatively, the quenching and tempering hardness is preferably 50 HRC or less. More preferably, it is 48 HRC or less.

First, the molten steel adjusted to the component composition of hot tool steel containing P of 0.005 mass% or more was maintained by a vacuum induction melting furnace. The component composition at this time was adjusted so that it might become the target component composition of the steel ingot after the next Zn addition (that is, injection of Zn source). Then, using a Zn-plated steel sheet as a Zn source, Zn was added to the molten steel and cast to produce a steel ingot of 7 to 10 kg. Table 1 shows the composition of the steel ingot after casting. Zn content was measured by fluorescent X-ray analysis. The present invention steel is obtained by adding Zn to the component composition of the commonly used hot work tool steel JIS-SKD61 (P standard: 0.030% or less) so as to satisfy the Zn / P ratio of the present invention. A large amount of P is allowed. In all the steel ingots, S, Cu, Al, Ca, Mg, O, and N are not added (including the case where Al is added as a deoxidizer in the melting step), and S ≦ 0. The results were 01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, and N ≦ 0.03%. And the content of P and Zn in the steel of the present invention to which Zn was added before casting could be accurately adjusted to a predetermined relationship. On the other hand, when Zn was added at the initial stage of the above-described vacuum induction melting, Zn evaporated, and a predetermined amount of Zn could not be maintained in the molten steel before casting.

On the other hand, comparative steels 1 to 6 in Table 1 were also prepared in order to explain the effect of the addition of Zn of the present invention. These comparative steels are obtained by increasing only the P content of SKD61 without adding Zn (except comparative steel 6). And S, Cu, Al, Ca, Mg, O, and N are not added (however, including the case where Al is added as a deoxidizing agent in the dissolving step), S ≦ 0.01%, Cu ≦ 0 .25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, and N ≦ 0.03%.

These steel ingots were subjected to a homogenization heat treatment at 1250 ° C. for 5 hours and then hot forged at 1150 ° C. to produce a steel material 20 mm thick × 60 mm wide × about 500 to 800 mm long. Then, after annealing at 860 ° C., it is processed into the size of a Charpy impact test piece used for the following evaluation, oil quenching is performed from 1030 ° C., and tempering is performed at various temperatures. The toughness evaluation sample was used.

[Test 1]
FIG. 1 shows the results of a 2 mm U notch Charpy impact test at room temperature for each of the inventive steels 1, 3, 5 to 7 and the comparative steels 1 and 3 to 6 at room temperature. The Charpy test piece was in the TL direction according to ASTM E399-90. In the combination of the inventive steels 1, 3 and comparative steel 1 containing the same level of P, the inventive steel 5 and comparative steel 3, and the inventive steels 6, 7 and comparative steel 4, the Zn / P of the invention The inventive steels 1, 3, 5 to 7 to which Zn is added so as to satisfy the ratio are superior in Charpy impact value to the comparative steels 1, 3, and 4 to which Zn is not added. Moreover, although the steel 6 of the present invention contains Zn at a high concentration exceeding 0.02%, the addition of Zn results in the same level as the comparative steel 1 having a P of less than 0.01%. Maintains toughness. The comparative steel 6 is obtained by adding Zn to the comparative steel 5 containing the same level of P, but does not satisfy the Zn / P of the present invention, and no improvement in toughness is observed.

[Test 2]
FIG. 2 shows the results of a 2 mm V notch Charpy impact test between room temperature and 400 ° C. when the steels of the present invention 2, 4 to 7 and comparative steels 2 to 6 were tempered to a hardness of 45 HRC. The Charpy test piece was in the TL direction according to ASTM E399-90. In the combination of the present invention steels 2, 4 and comparative steel 2 containing the same level of P, the present invention steel 5 and comparative steel 3, and the present invention steels 6, 7 and comparative steel 4, the Zn / P of the present invention Inventive steels 2, 4 to 7 to which Zn is added so as to satisfy the ratio are superior in Charpy impact value at any test temperature compared to comparative steels 2 to 4 containing the same level of P. And this invention steel 6 which satisfied Zn / P of this invention by adding Zn while containing P with a high density | concentration exceeding 0.02% is comparative steel 2 with P less than 0.01%. Maintains the same level of toughness.

In the same manner as described in Example 1, a steel ingot having the component composition shown in Table 2 was produced. Invention steel A is obtained by adding Zn to the component composition of hot work tool steel so as to satisfy the Zn / P ratio of the present invention. And the comparison steel B is set as the component composition equivalent to this invention steel A except not adding Zn. In both steel ingots, S, Cu, Al, Ca, Mg, O, and N were not added (however, Al was added as a deoxidizer in the melting step), S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, and N ≦ 0.03%.

Next, these steel ingots were subjected to the same hot forging and various heat treatments as in Example 1 to prepare Charpy impact test pieces tempered to the respective hardness. And the Charpy impact tests 1 and 2 implemented in Example 1 were implemented, and the toughness of each sample was evaluated.

[Test 1]
FIG. 3 shows the results of the 2 mm U-notch Charpy impact test at room temperature for the steel A of the present invention and the comparative steel B. The Charpy test piece was in the LS direction according to ASTM E399-90. These steels have high toughness due to the addition of Ni. And this invention steel A which added Zn so that the Zn / P ratio of this invention may be satisfy | filled on this is excellent in the Charpy impact value compared with the comparative steel B which did not add Zn.

[Test 2]
FIG. 4 shows the result of a 2 mmV notch Charpy impact test between room temperature and 400 ° C. when the steel A of the present invention and the comparative steel B were tempered to a hardness of 45 HRC. The Charpy test piece was in the LS direction according to ASTM E399-90. The steel A of the present invention in which Zn was added so as to satisfy the Zn / P ratio of the present invention in both steels having higher toughness as compared with the comparative steel B in which Zn was not added at any test temperature, Excellent Charpy impact value.

Claims

A hot work tool steel containing, by mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 3.0 to less than 6.0% A hot work tool steel excellent in toughness, characterized by Zn: more than 0.0025 to 0.025%, P: 0.005% or more, and Zn / P: more than 0.5.
The hot work tool steel excellent in toughness according to claim 1, characterized in that Mo and W are contained alone or in combination (Mo + 1 / 2W): 3.5% or less.
The hot work tool steel excellent in toughness according to claim 1 or 2, characterized by containing, by mass%, V: 1.5% or less.
% By mass
C: 0.3 to less than 0.6%,
Si: 1.5% or less,
Mn: 1.5% or less,
Ni: 1.5% or less (including 0%),
Cr: 3.0 to less than 6.0%,
Mo and W are single or composite (Mo + 1 / 2W): 3.5% or less,
V: 1.5% or less,
Nb: 0.3% or less (including 0%),
Co: 5.0% or less (including 0%),
Zn: more than 0.0025 to 0.025%,
P: 0.005% or more,
And Zn / P: more than 0.5,
The hot work tool steel excellent in toughness according to claim 1, comprising the balance Fe and inevitable impurities.
The hot work tool steel excellent in toughness according to claim 1 or 4, wherein P: 0.01% or more in mass%.
A method for producing hot work tool steel with excellent toughness,
A first step of obtaining a molten steel having a component composition of hot tool steel containing 0.005 mass% or more of P;
A second step of adding Zn to the molten steel having the component composition of the hot tool steel;
It consists of a third step of casting the molten steel added with Zn to obtain a steel ingot,
In the second step, the composition of the steel ingot after casting in the third step includes Zn: more than 0.0025 to 0.025 mass%, P: 0.005 mass% or more, and Zn / P: A method for producing hot tool steel with excellent toughness, characterized by adding Zn so as to be a hot tool steel exceeding 0.5.
The component composition of the steel ingot after casting in the third step is mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Cr: The method for producing hot work tool steel with excellent toughness according to claim 6, wherein the hot work tool steel contains 3.0 to less than 6.0%.
The component composition of the steel ingot after casting in the third step is mass%, and Mo and W are hot tool steels containing a single or composite (Mo + 1 / 2W): 3.5% or less. The manufacturing method of the hot tool steel excellent in toughness of Claim 7.
The toughness according to claim 7 or 8, wherein the composition of the steel ingot after casting in the third step is hot tool steel containing V: 1.5% or less in mass%. Excellent hot tool steel manufacturing method.
The component composition of the steel ingot after casting in the third step is mass%,
C: 0.3 to less than 0.6%,
Si: 1.5% or less,
Mn: 1.5% or less,
Ni: 1.5% or less (including 0%),
Cr: 3.0 to less than 6.0%,
Mo and W are single or composite (Mo + 1 / 2W): 3.5% or less,
V: 1.5% or less,
Nb: 0.3% or less (including 0%),
Co: 5.0% or less (including 0%),
Zn: more than 0.0025 to 0.025%,
P: 0.005% or more,
And Zn / P: more than 0.5,
The method for producing hot tool steel with excellent toughness according to claim 6, wherein the hot tool steel is made of the remaining Fe and inevitable impurities.
The component composition of the molten steel obtained in the first step is mass%, including P: 0.01% or more, and the component composition of the steel ingot after the casting in the third step is mass%, P: It contains 0.01% or more, The manufacturing method of the hot tool steel excellent in toughness of Claim 6 or 10 characterized by the above-mentioned.