KR20130036076A - Hot-working tool steel having excellent toughness and high-temperature strength and method for production thereof - Google Patents

Abstract

The present invention provides a hot tool steel with improved toughness and high temperature strength and a method of manufacturing the same. In mass%, C: 0.34 to 0.40%, Si: 0.3 to 0.5%, Mn: 0.45 to 0.75%, Ni: less than 0 to 0.5%, Cr: 4.9 to 5.5%, Mo and W alone or in combination (Mo +1/2 W): 2.5-2.9%, V: 0.5-0.7%, hot tool steel consisting of the remaining amount of Fe and unavoidable impurities. Preferably, the cross-sectional structure at the time of its quenching comprises mass and acicular tissue, mass structure (A%): 45 area% or less, needle structure (B%): 40 area% or less, residual austenite (C %): 5-20% by volume. A method for producing hot tool steel, wherein the hot tool steel is tempered so that the relationship X between the tempering hardness (HRC) and the tissue ratio represented by the following formula is equal to or greater than 40:
X = [-0.36 × (HRC) -1.47 × (A%)-1.67 × (B%) + 6.55 × (C%) + 72.91]

Description

HOT-WORKING TOOL STEEL HAVING EXCELLENT TOUGHNESS AND HIGH-TEMPERATURE STRENGTH AND METHOD FOR PRODUCTION THEREOF}

FIELD OF THE INVENTION The present invention relates to hot tool steels having improved toughness and high temperature strength so as to be optimal when used as various hot tools such as press dies, forging dies, die cast dies, and extrusion tools, and methods for producing the same.

Since the hot tool is used while being in contact with a hot work material or a hard work material, it is necessary to have strength and toughness that can withstand thermal fatigue and impact. Thus, for example, SKD61 alloy tool steel, which is a type of JIS steel, has been used in the field of hot tools. More recently, in order to shorten the manufacturing time of a product manufactured by using a hot tool and to form a complicated shape, the material to be processed is heated to high temperatures, and hot tools such as molds are being enlarged according to simultaneous processing of a plurality of products. Due to the point and the like, hot tool materials are required to be able to secure higher internal strength even at a higher temperature strength and larger sizes.

In order to improve the toughness and high temperature strength of the alloy tool steel, a method of improving the high temperature strength while maintaining toughness by defining a range of chemical composition (see Patent Document 1), toughness and high temperature strength by specifying the amount of residual carbide A method of improving the quality of the resin is proposed (see Patent Document 2).

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

Patent Document 2: Japanese Patent Application Laid-Open No. 2000-328196

The above-mentioned Patent Document 1 cannot evaluate the level of toughness because there is no specific measurement of toughness. However, judging from the results of the study conducted by the present inventors, in order to combine the toughness and high temperature strength to a sufficiently high level, the range of the chemical composition is limited. It is not enough to limit. Also in the method of Patent Document 2 described above, toughness and high temperature strength are largely affected by structures such as martensite structure and bainite structure after quenching, so that the toughness and high temperature strength can be controlled at a high level. It is not enough to define the amount of residual carbide.

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

As a result of careful study by the present inventors, it has been found that the structure after quenching has a great influence on toughness and high temperature strength, and the preferred after quenching structure is found in order to combine excellent toughness and high temperature strength. In order to obtain a structure after a preferable quenching, it discovered that there exists a very narrow favorable composition area | region obtained only by controlling content of each element to the optimum range, and arrived at this invention.

That is, in the present invention, in mass%, C: 0.34 to 0.40%, Si: 0.3 to 0.5%, Mn: 0.45 to 0.75%, Ni: less than 0 to 0.5%, Cr: 4.9 to 5.5%, Mo and W are (Mo + 1 / 2W) alone or in combination, 2.5 to 2.9%, V: 0.5 to 0.7%, remaining amount of Fe and inevitable impurities, characterized in that the hot tool steel with excellent toughness and high temperature strength. The hot tool steel of the present invention may be used, for example, by adjusting its hardness to 40 HRC or higher, but exhibits a combined effect of excellent toughness and high temperature strength in a high hardness region of 43 HRC or higher, particularly 45 HRC or higher. 49 HRC or less is preferable.

Here, in the hot tool steel of the present invention, preferably one or two or more of the elements of C, Si, Mn, Ni, Cr, Mo, W, and V constituting the same further have the following narrow composition To meet the realm. Of course, it is desirable to satisfy all of them.

C: 0.35 to 0.39%,

Si: O. 35-0.45%,

Mn: 0.5-0.7%,

Ni: 0.01-0.3%,

Cr: 5.0-5.4%,

Mo and W alone or in combination (Mo + 1 / 2W): 2.6-2.8%,

V: 0.55 to 0.65%.

Moreover, this invention is a hot tool steel which has the said component composition, Comprising: The cross-sectional structure at the time of quenching contains a mass structure and a needle-like structure,

Massive tissue (A%): 45 area% or less,

Couch tissue (B%): 40 area% or less,

Residual Austenite (C%): 5-20% by volume

It is a hot tool steel excellent in toughness and high temperature strength, characterized by the above-mentioned.

The hot tool steel is tempered so that the relationship X between the tempering hardness (HRC) represented by the following formula and the tissue ratio is 40 or more, characterized in that it is a method for producing hot tool steel with excellent toughness and high temperature strength. Although tempering hardness should just set 40-49HRC, preferable tempering hardness is 43-49HRC, More preferably, it is 45-49HRC.

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

According to the present invention, the toughness and high temperature strength of hot tool steel can be combined at a very high level. This effect is maximized not only in the hardness region of 40 HRC or more, but also in the high hardness region of 43 HRC or more, moreover 45 HRC or more, and 46 HRC or more, for example. Therefore, it becomes an effective technique in practical use of hot tool steel which can be applied to various uses and environments of heat.

1 is a cross-sectional microphotograph showing an example of a quenched structure of hot tool steel of the present invention.
FIG. 2 is a schematic diagram in which mass structures are selected in the quenched tissue of FIG. 1. FIG.
3 is a schematic view of selecting a needle-like tissue in the quenched tissue of FIG.
4 is a cross-sectional microphotograph showing an example of a quenched structure of hot tool steel of a comparative example.
FIG. 5 is a schematic diagram illustrating a mass structure in the quenched tissue of FIG. 4.
FIG. 6 is a schematic view of selecting a needle-like tissue in the quenched tissue of FIG. 4.

As mentioned above, one of the important characteristics of this invention is to control content of each element to an optimal range. That is, one aspect of the present invention controls the content of each element in a limited range, and more preferably, by recognizing the quenching structure described later, for example, at a wide range of quenching cooling rates in the conventional manufacturing method, The quenching method also found that there is a narrow compositional area that can provide high levels of toughness and high temperature strength. That is, in the basic element, the relationship between the C-Cr content is a result of the optimum adjustment of Mo, W, V, which are other carbide-forming elements which are mutually related to this, while following the conventional balance, and the characteristics as a result of the adjustment of these basic elements. It is important to adjust the Si or Ni, which has a big impact on it. Hereinafter, the reason for component limitation comprised by the narrow composition area of the steel of this invention is demonstrated.

C is an essential element that is important for hot tool steels in which a part is dissolved in the base to impart strength, and a part forms carbide to increase wear resistance and sticking resistance. Moreover, C which is a solid solution invading atom has the effect of I (invasive atom) -S (substituted atom) when co-added with a substituted atom having a high affinity with carbon such as Cr; The effect of increasing the strength by acting as the dragging resistance of the solute atom is also expected. However, when content is less than 0.34 mass%, hardness and abrasion resistance sufficient as a tool member cannot be ensured. On the other hand, since excessive addition causes a fall of toughness and hot strength, an upper limit is made into 0.40 mass%. Preferably it is 0.35 to 0.39%, More preferably, it is 0.36 to 0.38%.

Si is an deoxidizer at the time of steelmaking and an element which raises machinability. In order to acquire such an effect, 0.3 mass% or more must be added, but when too much, the needle structure mentioned later will be developed and toughness will fall. In addition, by suppressing the precipitation of cementite-based carbides in the bainite structure during quenching and cooling, it is possible to indirectly promote the precipitation, coagulation, and coarsening of alloy carbides during tempering, thereby lowering the high-temperature strength. do. Preferably it is 0.35 to 0.45%.

Mn has the effect of increasing hardenability, suppressing the formation of ferrite, and obtaining an appropriate hardening tempering hardness. Moreover, when it exists in a structure as nonmetallic inclusion MnS, it has a big effect in improving machinability. In order to acquire such an effect, 0.45 mass% or more must be added, but when too much, since the viscosity of a base will become high and machinability will fall, it shall be 0.75 mass% or less. Preferably it is 0.5 to 0.7%.

Ni is an element that suppresses the formation of ferrite. In addition, C, Cr, Mn, Mo, W and the like to provide excellent hardenability to the steel of the present invention, also in the case of a slow quenching cooling rate, there is an effect of suppressing the formation of the needle-like structure described later, martensite It is an important additional element because it forms a main body structure and prevents the fall of toughness. In addition, since the base imparts an intrinsic toughness improving effect, for example, addition of 0.01% or more is a preferred element. In the present invention, the most important thing is to strictly control the upper limit even when Ni is added. That is, when too much, the viscosity of a base will become high and machinability will fall, high temperature strength will fall, or the bulk structure mentioned later will be developed and toughness will fall, and it is necessary to be less than 0.5 mass%. Preferably, it is regulated to 0.3 mass% or less.

Cr is an element having an effect of improving hardenability and forming carbides to enhance the strength and wear resistance of the base, and is an essential element for the hot tool steel of the present invention, which also contributes to the improvement of tempering softening resistance and high temperature strength. . In order to acquire such an effect, it is necessary to add 4.9 mass% or more. However, excessive addition causes a decrease in hardenability and high temperature strength, so the upper limit is set to 5.5% by mass. Preferably it is 5.0 to 5.4%, More preferably, it is 5.1 to 5.3%.

Mo and W can be added alone or in combination in order to increase hardenability, precipitate fine carbides by tempering, impart strength, and improve softening resistance. Since W has an atomic weight of about twice that of Mo, it can be defined as Mo + 1 / 2W (of course, only one may be added or both may be co-added). And in order to acquire the said effect, addition of 2.5 mass% or more in (Mo + 1 / 2W) is required. Too much will cause a decrease in machinability and a decrease in toughness due to the development of a needle-like structure to be described later. Therefore, the Mo content should be 2.9 mass% or less. Preferably it is (Mo + 1 / 2W) 2.6-2.8%.

V forms carbide and has the effect of strengthening a matrix and improving abrasion resistance. In addition, the temper softening resistance is increased, the coarsening of crystal grains is suppressed and the toughness is improved. In order to acquire this effect, it is necessary to add 0.5 mass% or more, but when it is too large, it will cause the fall of machinability and toughness, and shall be 0.7 mass% or less. Preferably it is 0.55 to 0.65%.

On the other hand, the main elements which may remain as unavoidable impurities are P, S, Co, Cu, A1, Ca, Mg, O, N and the like. In order to achieve the effect of the present invention to the maximum, it is preferable that such impurities are as low as possible, but on the other hand, additional effect such as control of the shape of the inclusions, other mechanical properties, or improvement in manufacturing efficiency In order to obtain, some inclusion and / or addition may be possible. In this case, P≤0.03%, S≤0.01%, Co≤0.05%, Cu≤0.25%, Al≤0.025%, Ca≤0.01%, Mg≤0.01%, O≤0.01%, N≤ If it is 0.03%, since it is considered that it does not have a big influence in particular on the basic characteristic of the hot tool steel of this invention, if it is this range, it is acceptable and it is a preferable upper limit of regulation.

In addition to the importance of the above-described component composition, preferably, the present invention also has great features even when the solution approach from the structure is tested. That is, by studying the "structure factor" that affects the mechanical properties of the alloy tool steel, the optimum structure is specified in addition to the optimum component range made in the extremely narrow region of the present invention. That is, the hot tool steel of this invention which satisfy | fills said component composition contains mass structure and acicular structure in the cross-sectional structure at the time of quenching,

Massive tissue (A%): 45 area% or less,

Couch tissue (B%): 40 area% or less,

Residual Austenite (C%): 5-20% by volume

to be.

First, the quenched structure is a structure mainly composed of martensite and / or bainite, obtained by cooling in an austenite temperature region as is usually used. The quenched tissue of the present invention is substantially composed of the martensite and / or bainite, and then a suitable small amount of retained austenite. It is composed of a part of bainite. Here, the mass and acicular structure defined in the quenched tissue of the present invention are defined in the definition of feather-shaped bainite (upper bainite) or acicular bainite (lower bainite) that can be used for ordinary bainite fractionation. It is different from following.

That is, the mass structure of this invention is a structure | tissue in which many fine carbides with various kinds of directionality were grown inside the structure. And the cross-sectional structure of steel WHEREIN: The mass structure of this invention all shows "bulk shape" like the notation. This mass structure is produced even at a high cooling rate as long as air cooling a small sample of about 100 mm in all directions. Therefore, when quenching of practical steel ingots, it is difficult to further reduce the mass structure. Results in. Therefore, in this invention, it is preferable to make the blocky structure occupied in a hardening structure into 45 area% or less. More preferably, it is 40 area% or less, More preferably, it is 30 area% or less.

Next, the acicular tissue of the present invention is a tissue in which a large number of long carbides are grown in the tissue as compared with the carbide in the bulky structure having one orientation. In the cross-sectional structure, the needle-like structure of the present invention exhibits a "needle shape". This needle-like structure is produced at a cooling rate slower than the cooling rate at which the bulky tissue starts to form, but it is also difficult to reduce the needle-like structure at the time of quenching the practical steel ingot. However, when they occupy most of the entire organization, toughness deteriorates greatly. Therefore, in this invention, it is preferable to make the acicular structure occupied in a hardening structure into 40 area% or less. More preferably, it is 25 area% or less.

Here, the mass structure and the needle structure of this invention can visually discriminate and quantify by cross-sectional observation by using the difference of the shape. That is, in any structure cross section, by performing corrosion by, for example, the electropotential electrolytic etching method (SPEED method), both structures which are inferior in corrosion resistance are preferentially corroded compared with the martensitic base on which carbides are not deposited. And although the photograph of the structure which observed the corroded surface with the scanning electron microscope (x 5000 times) is FIG. 1, as shown in FIG. 2 and FIG. 3 as a schematic diagram of supplementation, the classification | quantitative quantification of the mass structure and the acicular tissue of this invention is It is possible. In this case, on the other hand, in the present invention, when both tissues are observed as the maximum length of about 0.5 µm or more, and any of the three fields of view is observed, it is sufficient to specify the effect. FIG. 1: is 1 visual field in the correspondence of this invention steel 6 of Example 3 mentioned later whose mass structure is 27 area% and needle shape structure is 30 area%. 4 is a visual field of 1 corresponded to the conventional steel 31 of Example 3 mentioned later whose mass structure is 44 area% and needle shape structure is 16 area%.

In addition, in the quenched tissue structure of the present invention, one more important thing is retained austenite. This structure is a preferable structure for reduction as a deterioration factor of the strength characteristics, and therefore, in the present invention, a suitable residual amount contributes to the improvement of toughness. Therefore, in this invention, it is preferable to set residual austenite to 5-20 volume% in a hardening structure. More preferably, it is 10 volume% or more. In addition, what is necessary is just to perform volume fraction measurement using the diffraction intensity | strength by X-ray-diffraction method using the sample electrolytically polished according to a conventional method, for example, to quantify residual austenite.

In the manufacturing method of the hot tool steel of this invention, after satisfy | filling said component composition and hardening structure constitution, after tempering the tempered hardness target by tempering of the next process, after tempering that X becomes 40 or more in the following relational formulas, By performing this, hot tool steel with excellent toughness is produced.

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

A%:% of bulk tissue, B%:% of needle tissue, C%:% of retained austenite

That is, the above formula is a study of the influence of the tempering hardness and the structure at the time of quenching on the toughness after tempering, and the specific influence parameter is clarified. In order to secure the toughness after tempering, reduction of mass structure and acicular structure is effective, and reduction of acicular structure which has a large coefficient on the negative side in a formula is especially effective in both of them. 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 acts advantageously to secure toughness. The target hardness may be, for example, 40 HRC or more, which is established as a hot tool steel. However, in the case of hot tool steel that satisfies the component composition and the quenched structure configuration of the present invention, a higher hardness, for example, 43 HRC or more, And even if it aims at the hardness of 45HRC or more, sufficient toughness can be ensured. However, after maintaining a remarkable toughness improvement effect, it is preferable to stop at tempering hardness of 49 HRC or less.

Example  One

Table 1 shows the chemical components of the inventive steel, comparative steel and conventional steel. Comparative steels are steels of chemical composition that deviate from the limited narrow component range of the present invention, and conventional steels are hot tool steels, both natural and out of the component range of the present invention, which are currently commonly used.

Such steels of the present invention, comparative steels and conventional steels were subjected to homogenization heat treatment at 1250 ° C. for 5 hours to a steel ingot 100 kg in a vacuum induction furnace, followed by hot forging at 1150 ° C. to produce a 30 mm thick × 60 mm wide steel material. did. Thereafter, annealing was performed at 860 ° C, and then 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 defined as semi-cooling time (for example, If it takes 10 minutes to cool from 1030 ° C to 525 ° C, it is referred to as "semi-cooling 10 minutes"). As a counterpart, it cooled by about 40 minutes of semi-cooling. Then, it tempered at the various temperature and adjusted to the hardness of 46HRC.

From the inventive steel, comparative steel, and conventional steel of Table 1 manufactured as described above, the notch direction of the test piece is in the longitudinal direction of the test piece in the width direction of the steel material after forging, and the notch direction of the test piece in the longitudinal direction of the steel material (that is, T Table 2 shows the results of the Charpy impact test at room temperature using a 2 mm U-notched Charpy impact test specimen prepared by collecting from the direction). When the Charpy impact test was taken from the T-direction and a comparatively high hardness 46HRC tempered specimen, the impact value tends to be lowered due to the influence of the forging structure, so that an impact value exceeding 34 (J / cm 2) is obtained. It can be said to have toughness. Especially when the impact value exceeding 40 (J / cm <2>) is obtained, the toughness is very excellent.

sample 2mm U-notch Charpy impact value (J / ㎠) Semi-cooled 3 minutes quenching Semi-cooled 40 minutes quenching 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 Inventive Steel 7 52.5 47.9 Inventive Steel 8 54.9 41.1 Inventive Steel 9 48.4 41.5 Inventive Steel 10 42.4 36.6 Inventive Steel 11 47.0 34.4 Inventive Steel 12 49.3 39.7 Invention steel 13 43.8 34.4 Comparison river 21 38.0 29.8 Comparison river 22 52.1 32.3 Comparative River 23 54.9 43.8 Comparative River 24 44.3 31.4 Comparative River 25 37.5 33.6 Comparative River 26 51.6 30.1 Comparative River 27 47.9 33.6 Conventional Steel 31 42.4 21.6 Conventional Steel 32 41.1 34.4

From the results of Table 2, when quenching by quenching is performed, even in the case of comparative steels or conventional steels other than the composition of the present invention, a relatively high impact value can be obtained even from test pieces taken from the T direction. However, when quenched at a slow cooling rate of about 40 minutes of semi-cooling, comparative steel 21 has a low Mo content in addition to the low Mo content from the beginning, and since comparative steel 22 has a low Mo content, the quenchability is inferior, respectively. The impact value is also lowered. In addition, the comparative steels 24 to 27 with too much Si also have a low impact value.

In the conventional steel 31, in addition to low Mo content from the beginning, slightly lower C content and Ni are also added, so hardenability is considerably inferior, and the impact value is the lowest. The conventional steel 32, which has a large amount of Mo and tends to have a low impact value, is also very low in Si, and therefore has insufficient machinability.

On the contrary, the steels 1 to 13 of the present invention, in which the chemical composition is optimally adjusted, maintain excellent toughness even when quenched at a slow cooling rate. On the other hand, Comparative Steel 23 is a composition in which only Ni, which is an element that improves toughness, is higher than the optimum composition according to the present invention, so that the toughness is good even if the cooling rate is slow.

Example  2

Next, high temperature strength was compared using the comparative steel 23 in which the impact value was favorable in the steel of this invention of Table 1, and a comparative steel. The tensile test piece was sampled so that the longitudinal direction of the test piece might come in the longitudinal direction of the steel material after forging (that is, taken from the L direction), and evaluated by the tensile strength at the time of high temperature tensile test at 650 degreeC. The tensile test was started after the test piece reached 650 degreeC for 10 minutes. The results are shown in Table 3.

sample Tensile Strength at 650 ° C (MPa) Semi-cooled 3 minutes quenching Semi-cooled 40 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 Inventive Steel 7 664 628 Inventive Steel 8 657 638 Inventive Steel 9 661 621 Inventive Steel 10 648 622 Inventive Steel 11 667 614 Inventive Steel 12 638 596 Invention steel 13 675 636 Comparative River 23 603 589

Since comparative steel 23 deviating from the narrow range of the optimal composition of this invention has too much Ni content, it is excellent in toughness but it turns out that high temperature strength falls. On the other hand, it can be seen that the steels of the present invention all have high high temperature strength.

Example  3

For inventive steels 6, comparative steels 2i to 23, 26 and conventional steels 31 and 32 prepared in Example 1, the steels quenched at a slow cooling rate of about 40 minutes for the semi-cooling thereof were used, and before tempering, Structure observation was performed previously as follows. First, samples for tissue observation of 100 mm square were collected from such steel materials, and 5000 times observation by the scanning electron microscope was performed using the sample corroded by the SPEED method. As an example, images obtained from inventive steel 6 and conventional steel 31 are shown in FIGS. 1 and 4. Using such an image, the area ratio was measured by image analysis of the mass structure and the acicular structure. As an example, a schematic diagram of the measurement of the bulk structure of the inventive steel 6 and the conventional steel 31 is shown in FIGS. 2 and 5, and a schematic diagram of the measurement of the needle-like structure of the inventive steel 6 and the conventional steel 31 is shown in FIG. ) And FIG. 6. The average was made into area ratio by performing these measurements by 3 visual fields about each structure of each sample. Moreover, after regrinding the sample, the amount of retained austenite by X-ray diffraction was measured for the sample which was finished by electropolishing. Table 4 summarizes the above results.

sample Lumpy tissue
Area% (A%) Couch tissue
Area% (B%) Residual austenite
Volume% (C%) Invention steel 6 23 36 12 Comparison river 21 34 17 8 Comparison river 22 46 2 8 Comparative River 23 43 2 9 Comparative River 26 55 5 9 Conventional Steel 31 39 20 9 Conventional Steel 32 32 22 9

Although already disclosed in Example 1, the result of a Charpy impact test is shown in Table 5 with the value X derived from the tempering hardness of a test piece, and the relationship of the hardness and structure ratio of this invention.

sample 2mm U-notch Charpy
Impact value (J / ㎠) Charpy impact test piece
Hardness (HRC) X Invention steel 6 41.1 45.2 41.3 Comparison river 21 29.8 45.1 30.7 Comparison river 22 32.3 46.0 37.8 Comparative River 23 43.8 45.3 49.0 Comparative River 26 30.1 45.6 26.2 Conventional Steel 31 21.6 46.2 24.5 Conventional Steel 32 34.4 46.5 31.3

From these results, when the quenched structures of comparative steel 21 and conventional steel 31 having a low impact value were evaluated when quenched at a slow cooling rate of about 40 minutes of semi-cooling, in a state in which not only relatively bulky structures but also retained austenite are low, Needle tissues with a high degree of adverse effect on toughness develop, and the X value is also considerably low. The conventional steel 32 degree with low impact value and its hardening structure are close to the comparative steel 21 and the conventional steel 31, but there exists a tendency for toughness to improve by balance improvement (that is, increase of X value) of each structural structure.

Comparative steel 22 and comparative steel 26 having a low Mo amount have a low impact value because the amount of Ni is excessively large and a bulk structure develops. On the other hand, in both samples, comparative steel 26 also has a large amount of Si, and therefore, a tendency for needle-like structure to be produced is also seen.

On the contrary, when the quenched structure of the steel 6 of the present invention with the optimum chemical composition adjusted was evaluated, even when the needle-like structure developed, there was less mass structure, and above all, much residual austenite effective for improving toughness remained. In addition, the balance of the structural structure (that is, X value) is also excellent. On the other hand, the quenched structure of Comparative Steel 23 having good toughness because it contains a large amount of Ni, while having a large number of bulky structures, the X value satisfies 40 or more. However, the drop in high temperature strength is as described above.

By applying the present invention to improve the toughness and high temperature strength of the hot tool steel, it can be applied to various hot tools such as press dies, forging dies, die cast molds, extrusion tools, as well as hot dies for heavy loads. It can also be applied to tool members.

Claims (18)

In mass%, C: 0.34 to 0.40%, Si: 0.3 to 0.5%, Mn: 0.45 to 0.75%, S: 0.01% or less, Cr: 4.9 to 5.5%, Mo and W are used alone or in combination (Mo + 1 / 2W): 2.5 to 2.9%, V: 0.5 to 0.7%, containing no Ni or in an amount less than 0.3%, characterized in that consisting of the remaining amount of Fe and unavoidable impurities, Hot tool steel with good toughness and high temperature strength. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized by a mass% of 0.35% to 0.39%. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized by a mass% of Si: 0.35 to 0.45%. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized by a mass percentage of Mn of 0.5 to 0.7%. The method of claim 1,
Ni: 0.01% to 0.3% by mass, hot tool steel excellent in toughness and high temperature strength. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized by a mass% of Cr: 5.0 to 5.4%. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized in that, by mass%, Mo and W alone or in combination (Mo + 1 / 2W): 2.6 to 2.8%. The method of claim 1,
Hot tool steel with excellent toughness and high temperature strength, characterized by a mass% of V: 0.55 to 0.65%. The method of claim 1,
Hot tool steel excellent in toughness and high temperature strength, wherein hardness is 40 HRC or more. The method of claim 1,
Hot tool steel excellent in toughness and high temperature strength characterized by the hardness being 43 HRC or more. The method of claim 1,
Hot tool steel excellent in toughness and high temperature strength characterized by the hardness of 45 HRC or more. 12. The method according to any one of claims 9 to 11,
Hot tool steel excellent in toughness and high temperature strength characterized by the hardness of 49 HRC or less. The method of claim 1,
The cross-sectional structure at the time of quenching includes block structure and needle structure,
Massive tissue (A%): 45 area% or less,
Couch tissue (B%): 40 area% or less,
Residual Austenite (C%): 5-20% by volume
Hot tool steel excellent in toughness and high temperature strength, characterized by the above-mentioned. The hot tool steel of Claim 13 is tempered so that relationship X between tempering hardness (HRC) and a structure ratio which are represented by a following formula may be 40 or more, The hot tool steel excellent in toughness and high temperature strength is characterized by the above-mentioned. Manufacturing method:
X = [-0.36 × (HRC) -1.47 × (A%)-1.67 × (B%) + 6.55 × (C%) + 72.91] 15. The method of claim 14,
A method for producing hot tool steel having excellent toughness and high temperature strength, characterized by tempering at 40 to 49 HRC. 15. The method of claim 14,
A method for producing hot tool steel having excellent toughness and high temperature strength, characterized by tempering at 43 to 49 HRC. 15. The method of claim 14,
A method for producing hot tool steel having excellent toughness and high temperature strength, characterized by tempering at 45 to 49 HRC. The method of claim 1,
Hot tool steel with good toughness and high temperature strength, characterized by containing 0.01 to 0.11% Ni.

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