WO2008032816A1

WO2008032816A1 - Hot-working tool steel having excellent stiffness and high-temperature strength and method for production thereof

Info

Publication number: WO2008032816A1
Application number: PCT/JP2007/067915
Authority: WO
Inventors: Kouta Kataoka; Hideshi Nakatsu; Yasushi Tamura; Masayuki Nagasawa
Original assignee: Hitachi Metals, Ltd.
Priority date: 2006-09-15
Filing date: 2007-09-14
Publication date: 2008-03-20
Also published as: KR20090043556A; CN102994902A; EP2065483A4; KR20130036076A; KR20120006091A; EP2065483A1; CN101517114A; US20100193089A1

Abstract

Disclosed is a hot-working tool steel having improved stiffness and high-temperature strength. Also disclosed is a method for producing the hot-working tool steel. The hot-working tool steel comprises the following components (by mass): C: 0.34-0.40%, Si: 0.3-0.5%, Mn: 0.45-0.75%, Ni: 0-0.5% (exclusive), Cr: 4.9-5.5%, (Mo+1/2W): 2.5-2.9% (provided that Mo and W are contained singly or in combination), and V: 0.5-0.7%, with the remainder being Fe and unavoidable impurities. Preferably, the cross-sectional structure of the hot-working tool steel upon quenching contains a bulk structure and a needle-like structure, wherein the bulk structure (A%) accounts for 45 area% or less, the needle-like structure (B%) accounts for 40 area% or less, and the remaining austenite (C%) accounts for 5 to 20 volume%. Also disclosed is a method for producing a hot-working tool steel, which comprises tempering the above-mentioned hot-working tool steel so that a value X determined by the following relational expression between a tempered hardness (HRC) and the percentages of the tissues becomes 40 or greater. X = [-0.36×(HRC)-1.47×(A%)-1.67×(B%)+6.55×(C%)+72.91]

Description

Specification

Hot work tool steel with excellent toughness and high temperature strength and its manufacturing method

[0001] The present invention relates to a hot tool steel with improved toughness and high-temperature strength 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. It is about the law.

Background art

[0002] Since a hot tool is used while being in contact with a high-temperature work material or a hard work material, it must have both strength and toughness that can withstand thermal fatigue and impact. For this reason, for example, SKD61-based alloy tool steel, which is a JIS steel type, has been used in the field of hot tools. More recently, if the manufacturing time of products manufactured using hot tools has been shortened, the temperature of workpieces has increased due to the formation of complex shapes, and the number of products has increased due to simultaneous simultaneous production of products. As hot tools such as molds have become larger! /, Etc., hot tool materials are required to have high high-temperature strength and high toughness even at large sizes.

[0003] For the purpose of improving the toughness and high-temperature strength of alloy tool steel, a method of improving high-temperature strength while maintaining toughness by determining the chemical composition range (see Patent Document 1), residual A technique for improving toughness and high-temperature strength by regulating the amount of carbide has been proposed (see Patent Document 2).

Patent Document 1: Japanese Patent Laid-Open No. 2-179848

Patent Document 2: JP 2000-328196 A

Disclosure of the invention

Problems to be solved by the invention

[0004] However, the above-mentioned Patent Document 1 cannot evaluate the level of toughness because there is no specific measurement value of toughness. In order to combine high-temperature strength at a sufficiently high level, the range of chemical composition is insufficiently limited. Also in the method of Patent Document 2 described above, toughness and high temperature strength are hardened. Since it is greatly influenced by structures such as the martensite structure and bainite structure after this, it is not sufficient to specify the amount of residual carbide to control toughness and high-temperature strength at a high level.

[0005] An object of the present invention is to provide a hot work tool steel having more reliably superior toughness and high temperature strength, and a method for producing the same.

Means for solving the problem

[0006] As a result of intensive studies by the present inventors, it has been found that the structure after quenching has a great influence on toughness and high-temperature strength, and a structure after quenching suitable for combining excellent toughness and high-temperature strength has been found. Revealed. In order to obtain a suitable quenched structure, it is found that there is an extremely narrow and favorable composition range that can be obtained by controlling the content of each element within the optimum range. Reached.

[0007] That is, the present invention is, by mass%, C: 0.34-0.40%, Si: 0.3-0. 5%, Mn: 0.4 5-0. 75%, Ni: 0 to less than 0.5%, Cr: 4.9 to 5.5%, Mo and W are single or composite (Mo + l / 2W): 2.5 ~ 2.9%, V: 0.5 ~ 0.7%, balance Fe and inevitable impurities It is a hot work tool steel with excellent toughness and high temperature strength. For example, the hot work tool steel of the present invention may be used after its hardness is adjusted to 40 HRC or higher, but it has excellent toughness and high temperature strength in a high hardness range of 43 HRC or higher, particularly 45 HRC or higher. Demonstrate the effect. 49HRC or less is desired!

[0008] Here, the hot work tool steel of the present invention is preferably C, Si, Mn, Ni, Cr,

One or more of the elements of Mo, W, and V, force S, and the following narrow composition range. In this, of course, it is desirable to satisfy all of them.

C: 0.35-0.39%,

Si: 0.35-0.45%,

Μη: 0.5 to 7%,

Ni: 0.01—0.3%,

Cr: 5.0—5.4%,

Mo and W alone or in combination (Mo + l / 2W): 2 · 6 to 2 · 8%

V: 0.55-0.65% [0009] Further, the present invention is a hot tool steel having the above component composition, the cross-sectional structure at the time of quenching includes a massive structure and a needle-like structure,

Bulk texture (A%): 45 area% or less,

Needle-like tissue (B%): 40 area% or less,

It retained austenite (C%): 5~20 volume ^_0/0

It is a hot work tool steel excellent in toughness and high temperature strength.

[0010] And, toughness and high temperature strength characterized by tempering the above hot tool steel so that the relationship value X between the tempering hardness (HRC) and the structure ratio represented by the following formula is 40 or more This is an excellent method for producing hot work tool steel. The tempering hardness may be set to 40 to 49 HRC, but the preferable tempering hardness is 43 to 49 HRC, more preferably 45 to 49 HRC.

X = [-0. 36 X (HRC)-l. 47 X (A%) — 1 · 67 Χ (Β%) + 6. 55 Χ (C%) + 72. 91]

The invention's effect

[0011] According to the present invention, it is possible to combine the toughness and high-temperature strength of hot tool steel at a very high level. This effect is maximized not only in the hardness range of 40HRC or higher, but also when tempered in a high hardness range of 43HRC or higher, or even 45HRC or higher and 46HRC or higher. Therefore, it is an effective technology for the practical application of hot tool steel that can be applied to various environments.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] As described above, one of the important features of the present invention is that the content of each element is controlled within an optimum range. In other words, the content of each element is controlled to a limited range, and more preferably, by recognizing the quenching structure described later, the manufacturing method remains the same as before, for example, a wide range of quenching cooling rates. In addition, there is a narrow composition range in which a toughness and high-temperature strength can be obtained by any quenching method, and a structure that can be combined at a level can be obtained. That is, in the basic elements, while the relationship of C Cr content follows the conventional balance, the optimum adjustment of Mo, W, V of other carbide forming elements that are related to this, and the basic elements It is important to adjust Si and Ni, which have a great influence on the resulting characteristics of the adjustment. Hereinafter, it is composed of a narrow composition range of the steel of the present invention. The reasons for limiting the ingredients are described!

[0013] Part of C is an essential element important for hot work tool steel, partly forming a solid solution in the base to give strength, and partly forming carbides to increase wear resistance and seizure resistance. It is. In addition, C, which is a solid interstitial atom, co-added with a substitution atom having a high affinity for C, such as Cr, has an I (interstitial atom) S (substitution atom) effect; Bow I Acts as a scratch resistance and is expected to increase strength. However, if the content is less than 0.34 mass%, sufficient hardness and wear resistance as a tool member cannot be secured. On the other hand, excessive addition causes toughness and a decrease in hot strength, so the upper limit is made 0.40% by mass. It is preferably 0.35-0.39%, more preferably 0.36-0.38%.

[0014] Si is an element for improving machinability as well as a deoxidizer during steelmaking. In order to obtain these effects, it is necessary to add 0.3% by mass or more. However, if it is too much, a needle-like structure described later is developed and toughness is lowered. In addition, by suppressing the precipitation of cementite carbides in the bainite structure during quenching and cooling, the precipitation of carbides during alloying during tempering-aggregation and coarsening can be indirectly promoted to reduce high-temperature strength. Therefore, it should be 0.5% by mass or less. Preferably, it is 0.35–0.45%.

[0015] Mn has an effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, if it exists in the structure as non-metallic inclusions MnS, it has a great effect on improving machinability. In order to obtain these effects, a force S that requires addition of 0.45% by mass or more is required S, and if it is too much, the viscosity of the base is increased and machinability is lowered, so the content is made 0.75% by mass or less. Preferably it is 0.5 to 0.7%.

[0016] Ni is an element that suppresses the formation of ferrite. Also, together with C, Cr, Mn, Mo, W, etc., it imparts excellent hardenability to the steel of the present invention, and suppresses the formation of needle-like structures described later even at moderate quenching and cooling rates. It is an effective additive element in order to form a martensite-based structure and prevent toughness deterioration. Furthermore, since it provides the essential toughness improvement effect of the base, it is a preferable element to be added, for example, 0.01% or more. The most important thing for the present invention is that the upper limit is strictly controlled even when Ni is added. In other words, if the amount is too large, the viscosity of the base is increased to reduce machinability, the high temperature strength is decreased, or the massive structure described later is developed to reduce toughness. It is necessary to make it less than 0.5% by mass. Preferably, the amount is restricted to 0.3% by mass or less.

[0017] Cr is an element that has the effect of enhancing hardenability and forming carbides to improve the base and strengthening wear resistance, and contributes to the improvement of temper softening resistance and high-temperature strength. It is an essential element for the hot work tool steel of the present invention. In order to obtain these effects, it is necessary to add 4.9% by mass or more. However, excessive addition causes a decrease in hardenability and high-temperature strength, so the upper limit is made 5.5% by mass. Preferably it is 5.0 to 5 · 4%, more preferably 5 · ;! to 5 · 3%.

[0018] Mo and W can be added singly or in combination in order to enhance hardenability and to give strength by precipitating fine carbides by tempering and to improve softening resistance. Since W is about twice the atomic weight of Mo, it can be defined as Mo + 1 / 2W (Of course,! /, Either one of them can be added, or both can be added together) . In order to obtain the above effect, it is necessary to add 2.5% by mass or more in terms of (Mo + 1 / 2W). Since lowering the toughness due to the development of too large a decrease in machinability described later acicular structure, a 2-9 mass ^_0/0 or less (Mo + 1 / 2W). Preferably, (Mo + 1 / 2W) is 2 · 6 to 2 · 8%.

[0019] V forms carbides and has the effect of enhancing the wear resistance of the base. It also increases resistance to temper softening and suppresses coarsening of crystal grains, contributing to improved toughness. In order to obtain this effect, it is necessary to add 0.5% by mass or more, but if it is too much, the machinability is reduced toughness. Preferably it is 0.55-0.65%.

[0020] The main elements that may remain as unavoidable impurities are P, S, Co, Cu, Al, Ca, Mg, 0, N, and the like. In order to achieve the maximum effect of the present invention, these should be as low as possible, but on the other hand, such as the form control of inclusions, other mechanical properties, or the improvement of production efficiency, Some inclusions and / or additions may be made for the purpose of obtaining additional effects. In this case, in mass%, P≤0.03%, S≤0.01%, Co≤0.05%, Cu≤0.25%, A1≤0.025%, Ca≤0.01%, Mg If it is ≤0.01%, O≤0.01%, N≤0.03%, it is considered that the basic characteristics of the hot work tool steel of the present invention are not particularly affected! Is acceptable and is the preferred upper limit of regulation. [0021] In addition to the importance of the above-described component composition, the present invention preferably has a great feature also when a solution approach from the organization is attempted. In other words, by studying the “structural factors” that affect the mechanical properties of alloy tool steels, we have identified the optimum structure in addition to the optimum component range in the extremely narrow range of the present invention. Is. That is, the hot tool steel of the present invention satisfying the above component composition includes a massive structure and a needle-like structure in a cross-sectional structure at the time of quenching,

Bulk texture (A%): 45 area% or less,

Needle-like tissue (B%): 40 area% or less,

It retained austenite (C%): 5~20 volume ^_0/0

It is.

[0022] First, the quenched structure is a structure mainly composed of martensite and / or bainite obtained by cooling from the austenite temperature range, as is usually used. The hardened structure of the present invention is substantially composed of the above-mentioned martensite and / or bainite and the remaining austenite in an appropriate amount, and the above-mentioned massive structure and acicular structure are the martensite. And / or part of bainite. Here, the massive structure and the acicular structure defined in the hardened structure of the present invention are feathered bainite (upper bainite) and acicular bainite (lower part) used for ordinary bainite fractionation. It is different from what follows the definition of (Bainite).

That is, the massive structure of the present invention is a structure in which a large number of fine carbides having several kinds of orientations have grown inside the structure. And in the cross-sectional structure of steel, the block structure of the present invention is pushed and arranged in a “bulk shape” as shown. Since this massive structure is generated even at a cooling rate that is fast enough to air-cool a small sample of about 10 mm square, it is still difficult to reduce the massive structure when quenching a practical steel ingot, but it accounts for most of the entire structure. And toughness reduction. Therefore, in the present invention, it is preferable that the massive structure occupying in the quenched structure be 45% by area or less. More preferably, it is 40 area% or less, and further preferably 30 area% or less.

[0024] Next, the needle-like structure of the present invention is a structure in which a large number of carbides have grown in the structure in comparison with the carbides in the massive structure having one direction. And then In the surface structure, the needle-like structure of the present invention has a “needle-like” shape. This acicular structure is a force generated at a cooling rate slower than the cooling rate at which the massive structure begins to form. Again, it is difficult to reduce this acicular structure when quenching a practical steel ingot. However, if it accounts for most of the entire structure, the toughness is greatly degraded. Therefore, in the present invention, it is preferable that the acicular structure occupying the hardened structure be 40 area% or less. More preferably, it is 25 area% or less.

[0025] Here, the massive tissue and the acicular tissue of the present invention can be visually sorted and quantified by cross-sectional observation by utilizing the difference in shape. In other words, in any cross section of the structure, for example, by performing corrosion by the controlled potential electrolytic etching method (SPEED method), both structures having inferior corrosion resistance as compared with the martensite base with no precipitation of carbide are preferentially corroded. The Fig. 1 shows a structure photograph of the corroded surface observed with a scanning electron microscope (X5000). As shown in Figs. 2 and 3 as supplemental schematic diagrams, It is possible to quantitatively determine the acicular tissue. In this case, in the present invention, observation of three arbitrary fields of view with both tissues having a maximum length of about 0.5 in or more is sufficient to identify the action and effect. FIG. 1 shows one field of view corresponding to the steel 6 of the present invention of Example 3 to be described later, in which the block structure is 27 area% and the acicular structure force is 0 area%. FIG. 4 shows one field of view corresponding to the conventional steel 31 of Example 3 to be described later, in which the massive structure is 44 area% and the acicular structure is 16 area%.

[0026] Further, in the quenched structure structure of the present invention, another important force is retained austenite. Although this structure is a preferable structure for reduction as a deterioration factor of strength characteristics, an appropriate residual amount contributes to improvement of toughness in the present invention. Therefore, in the present invention, the retained austenite in the quenched structure is preferably 5 to 20% by volume. More preferably, it is 10% by volume or more. For determination of retained austenite, volume ratio measurement using the diffraction intensity by X-ray diffractometry may be performed according to a conventional method, for example, using an electropolished sample.

[0027] Then, in the method for producing a hot work tool steel of the present invention, after satisfying the above component composition and quenching structure configuration, the target tempering hardness is determined by tempering in the next step. Therefore, a hot tool with excellent toughness can be obtained by performing tempering so that X in the following relational expression is 40 or more. Steel is formed.

X = [— 0. 36X (HRC) -l.47X (A%)-1. 67X (B%) + 6.55X (C%) + 72. 91]

A%: block structure area%, B%: needle structure area%, C%: residual austenite volume%

[0028] That is, the above formula clarifies the specific influence parameters by studying the influence of the structure at the time of quenching and the tempering hardness on the toughness after tempering. In order to secure toughness after tempering, it is effective to reduce the massive structure and the needle-like structure. Among them, it is particularly effective to reduce the needle-like structure that has a large coefficient on the negative side. It is. On the other hand, since the formula has a large coefficient on the positive side, it can be seen that an appropriate amount of retained austenite works to secure toughness. And as the target hardness, for example, 40HRC or more established as hot tool steel may be set, but it is even higher if it is hot tool steel satisfying the composition of composition and the quenching structure of the present invention. Sufficient toughness can be secured even when aiming for hardness, for example, 43HRC or more, and hardness of 45HRC or more. However, in order to maintain a remarkable toughness improving effect, it is preferable to keep the tempering hardness below 49 HRC.

Example 1

[0029] Table 1 shows chemical compositions of the steels of the present invention, comparative steels and conventional steels. The comparative steel is a steel having a chemical composition that is outside the limited narrow component range of the present invention, and the conventional steel is a hot-work tool steel that is, of course, currently in general use, outside the component range of the present invention. .

[0030] [Table 1]

(Mass ° /.)

* Contains impurities

These steels of the present invention, comparative steels, and conventional steels were subjected to homogenization heat treatment at 1250 ° C for 5 hours on steel ingots that were melted by 10kg each in a vacuum induction melting furnace, and then hot forged at 1150 ° C. As a result, a steel material 30 mm thick x 60 mm wide was produced. Then, after annealing at 860 ° C, it was quenched at 1030 ° C. Quenching is performed by pressurized gas cooling, and the time required for cooling from the quenching temperature (1030 ° C) to the intermediate temperature (525 ° C) between the quenching temperature and room temperature (20 ° C) is semi-cooled. If it is defined as time (for example, it takes 10 minutes to cool from 1030 ° C to 525 ° C, it is expressed as “semi-cooled 10 minutes”). It was cooled in about 40 minutes semi-cooling to correspond to the part where the cooling rate was slow, such as the central part of steel. Then tempered at different temperatures, and adjusted to the hardness of 46HRC Bruno ₀

Replacement paper (Regulation 26) [0032] From the steel of the present invention, the comparative steel and the conventional steel produced as described above, the longitudinal direction of the test piece is in the width direction of the steel material after forging, and the notch direction of the test piece is in the longitudinal direction of the steel material. Table 2 shows the results of a Charpy impact test at room temperature using a 2 mm U-notch Charpy impact test piece prepared in such a way (ie, T direction force). When a Charpy impact test is performed on a specimen that has been sampled from this T-direction force and tempered to 46HRC, which is a relatively high hardness, the impact value tends to be low due to the influence of the forged structure! If an impact value exceeding 34 a / cm ² ) is obtained, it can be said to have excellent toughness. In particular, if an impact value exceeding 40 a / cm ² ) is obtained, its toughness is extremely excellent!

[0033] [Table 2]

2 mm U-notch Charpy impact value (J / cm ² ) i5 formula fee

Semi-cold 3 minutes quench Semi-cold 4 0 minutes quench

Invention steel 1 49. 8 37.5

Invention steel 2 52. 5 36. 2

Invention steel 3 57. 2 43. 3

Invention steel 4 53. 9 34.4

Invention steel 5 53. 0 40. 6

Invention steel 6 52. 5 41.1

Invention steel 7 52. 5 47. 9

Invention steel 8 54. 9 41.1

Invention Steel 9 48. 4 41.5

Invention steel 1 0 42. 4 36. 6

Invention steel 1 1 47. 0 34.4

Invention steel 1 2 49. 3 39. 7

Invention steel 1 3 43. 8 34. 4

Comparative steel 2 1 38. 0 29. 8

Comparative steel 2 2 52. 1 32. 3

Comparative steel 2 3 54. 9 43. 8

Comparative steel 2 4 44. 3-31.4

Comparative steel 2 5 37. 5 33. 6

Comparative steel 2 6 5 1. 6 30. 1

Comparative steel 2 7 47. 9 33. 6

Conventional steel 3 1 42. 4 21.6

Conventional steel 3 2 41.1 43.4

[0034] From the results shown in Table 2, if quenching by quenching is performed, a comparatively high impact value can be obtained even for a test piece taken from the T direction, whether it is a comparative steel or a conventional steel outside the composition of the present invention. However, when quenching at a slow cooling rate of about 40 minutes semi-cooled, the amount of Mo in the comparative steel 21 is low as well as the comparative steel 21, and in addition, Ni is also an additive-free metal. Each of them has a low hardenability and a low impact value. Also, comparative steels 24-27 with too much Si content have low impact values. '

[0035] Conventional steel 31 not only has a low Mo content, but also has a low C content and Ni, which has no added iron. The impact value tends to be low with a large amount of Mo

Replacement paper (2 One conventional steel 32 has poor machinability due to its very low Si.

[0036] On the other hand, the inventive steels 1 to 13 having the optimum chemical composition maintain excellent toughness even when quenched at a slow cooling rate. The comparative steel 23 is a composition in which only Ni, which is an element that enhances toughness as compared with the optimum composition according to the present invention, deviates from a high level. Therefore, the toughness is good even when the cooling rate is slow.

Example 2

[0037] Next, the high-temperature strength was compared using the steel of the present invention in Table 1 and comparative steel 23, which had a good impact value among the comparative steels. Tensile test specimens were sampled so that the longitudinal direction of the test specimen was aligned with the longitudinal direction of the steel material after forging (ie, L direction force was sampled), and evaluated by the tensile strength when a high temperature tensile test was performed at 650 ° C. . The tensile test was started after the specimen had been held for 10 minutes after reaching 650 ° C. The results are shown in Table 3.

[0038] [Table 3]

Difference rule 26 6 Tensile strength at 50 ° C (MP a)

Sample

Semi-cooled 3 minutes quenching Semi-cold 4 0 minutes quenching Invention steel 1 645 603

Invention steel 2 656 606

Invention Steel 3 675 621 Invention Steel 4 638 632 Invention Steel 5 650 635

Invention steel 6 678 641

Invention Steel 7 664 628 Invention Steel 8 657 638 Invention Steel 9 661 621 Invention Steel 1 ό 648 622 Invention Steel 1 1 667 614 Invention Steel 1 2 638 596 Invention Steel 1 3 675 636 Comparison Steel 2 3 603 589

It can be seen that Comparative Steel 23, which is out of the narrow range of the optimum composition of the present invention, was excellent in toughness but inferior in high-temperature strength due to its excessive Ni content. On the other hand, the steel of the present invention Yes It can be seen that the deviation is high and has high temperature strength.

Example 3

[0040] The steel of the present invention 6, the comparative steels 21 to 23 and 26, and the conventional steels 31 and 32 produced in Example 1 are targeted for steel materials quenched at a slow cooling rate of about 40 minutes. Before the tempering, the following preliminary tissue observation was performed. First, 10 mm square tissue observation samples were collected from these steel materials, and the samples corroded by the S, PEED method were used and observed 5000 times with a scanning electron microscope. As an example, images obtained from steel 6 of the present invention and conventional steel 31 are shown in FIGS. Using these images, the area ratio of the massive tissue and needle-like tissue was measured by image analysis. As an example, Fig. 2 and Fig. 5 show the schematic diagrams of the massive structure of Invention Steel 6 and Conventional Steel 31 and Fig. 3 shows the schematic diagram of the measurement of the acicular structure of Invention Steel 6 and Conventional Steel 31 (mark X). And is shown in FIG. By performing such measurements for each tissue of each sample in three fields, the average was taken as the area ratio. Further, the amount of retained austenite was measured by the X-ray diffraction method for the sample finished by electropolishing after repolishing the sample. Table 4 summarizes the above results.

[0041] [Table 4]

[0042] Although disclosed in Example 1, the result of the Charpy impact test is obtained together with the tempering hardness of the test piece and the value X derived from the relationship between the hardness and the tissue ratio of the present invention. Table 5 shows.

[0043] [Table 5]

Difference 2 2 mm U notch

Charpy impact test piece

Sample Charpy impact value X

Hardness (H R C)

(J / cm ² )

Invention steel 6 41. 1 45. 2 41.3 Comparative steel 2 1 29. 8 45. 1 30.7 Comparative steel 2 2 32. 3 46. 0 37. 8 Comparative steel 2 3 43. 8 45. 3 49 0 Comparative steel 2 6 30. 1 45. 6 26. 2 Conventional steel 3 1 '21. 6 46. 2 24. 5 Conventional steel 3 2 34. 4 46. 5' 31. 3

* X = [— 0.36 * (HRC)-1.47 * (A%) — 1.67 * (B%) + 6.55 * (C%) +72.91]

[0044] From these results, if the quenching structure of the comparative steel 21 and the conventional steel 31 is evaluated with a low impact value when quenching at a slow rate of about 40 minutes and a cooling rate, the comparative steel 21 and the conventional steel 31 are relatively When there are many massive structures and low retained austenite, acicular structures with a high negative impact on toughness develop—and the X value is also low. Conventional copper 32, which has a low impact value, has a hardened structure similar to that of comparative steel 21 and conventional steel 31, but tends to improve toughness due to improved balance of each structural structure (that is, an increase in X value). .

[0045] Comparative Steel 22 and Comparative Steel 26 with a low Mo content have a low impact value because a massive structure develops because the Ni content is too large. In both samples, the comparative steel 26 also has a high Si content, so a tendency to form a needle-like structure is also observed.

[0046] On the other hand, if the quenched structure of the steel 6 of the present invention with the optimum chemical composition adjusted is evaluated, the needle-like braid develops, but the bulk structure is small, and above all, the toughness is improved. A lot of effective retained austenite remains. It is also excellent in the balance (ie, X value) of the above organization. Note that the hardened structure of comparative steel 23, which has good toughness because it contains a large amount of Ni, satisfies the X value force O or more even though it has a lot of massive structure. As described above, the high-temperature strength is inferior, though the strength is low.

Industrial applicability ' By applying the present invention to improve the toughness and high temperature strength of hot work tool steel, it can be applied to various hot tools such as press dies, forging dies, die casting dies, extrusion tools, Furthermore, it is applicable also to hot tool members, such as a metal mold | die with a heavy use load.

Replacement paper (Rule 2® Brief Description of Drawings

FIG. 1 is a cross-sectional microphotograph showing an example of a quenched structure of the hot tool steel of the present invention.

FIG. 2 is a schematic diagram in which a massive structure is selected in the quenched structure of FIG.

FIG. 3 is a schematic diagram in which a needle-like structure is selected in the quenched structure of FIG.

FIG. 4 is a cross-sectional micrograph showing an example of a quenched structure of a hot work tool steel of a comparative example.

FIG. 5 is a schematic diagram in which a massive structure is selected for the quenched structure of FIG.

[FIG. 6] A schematic diagram in which a needle-like structure is selected in the quenched structure of FIG.

Claims

The scope of the claims

Mass: ⁰ / o, C: 0.34—0.40%, Si: 0.3—0.5%, Mn: 0.45—0.75%, Ni: 0 to less than 0.5%, Cr: 4.9 to 5.5%, Mo and W alone or Composite (Mo + 1 / 2W): 2.5 to 2.9%, V: 0.5 to 0.7%, balance Fe and inevitable impurities, hot tool steel with excellent toughness and high temperature strength.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that C: 0.35-0.39% in mass%.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that, in mass%, Si: 0.35-0.45%.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that, in mass%, Mn: 0.5 to 0.7%.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that Ni: 0.01-0.3% in mass%.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, wherein Cr: 5.0 to 5.4% in mass%.

The hot work tool steel excellent in toughness and high-temperature strength according to claim 1, characterized in that Mo and W are each alone or in combination of (Mo + l / 2W): 2.6 to 2.8%.

The hot work tool steel excellent in toughness and high-temperature strength according to claim 1, characterized in that, in mass%, V: 0.55-0.65%.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that the hardness force is S, 40HRC or more.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that the hardness force is S, 43HRC or more.

The hot work tool steel excellent in toughness and high temperature strength according to claim 1, characterized in that the hardness force is S, 45HRC or more.

The hot work tool steel with excellent toughness and high temperature strength according to claim 9, wherein the hardness is 49 HRC or less.

The cross-sectional structure at the time of quenching includes massive structure and acicular structure, Bulk tissue (A%): 45 area% or less,

Needle-like tissue (B%): 40 area% or less,

It retained austenite (C%): 5~20 volume ^_0/0

2. The hot work tool steel having excellent toughness and high temperature strength according to claim 1.

[14] A toughness characterized by tempering the hot work tool steel according to claim 13 so that the relationship value X between the tempering hardness (HRC) and the structural ratio represented by the following formula is 40 or more. And a method for producing hot work tool steel with excellent high temperature strength.

X = [-0.36X (HRC) -l.47X (A%) — 1 · 67Χ (Β%) + 6.55Χ (C%) + 72. 91]

[15] The method for producing hot work tool steel excellent in toughness and high temperature strength according to claim 14, characterized by tempering to 40 to 49HRC.

[16] The method for producing hot work tool steel excellent in toughness and high-temperature strength according to claim 14, characterized by tempering to 43 to 49HRC.

[17] The method for producing hot work tool steel excellent in toughness and high-temperature strength according to claim 14, characterized by tempering to 45 to 49HRC.