JPH0688163A - Hot tool steel - Google Patents

Abstract

PURPOSE:To obtain hot tool steel improved in toughness and high temp. softening resistance by specifying the compsn. constituted of C, Si, Mn, Ni, Cr, V, Nb, Mo, W and Fe. CONSTITUTION:The hot tool steel having high strength and high toughness is one contg., by weight, 0.20 to 0.45% C, <=0.30% Si, 0.5 to 2.0% Mn, 0.5 to 2.0% Ni, 1.0 to 4.0% Cr, 0.05 to <0.20% V, 0.01 to 0.15% Nb and one or two kinds of 0.2 to 2.0% Mo and 0.4 to 4.0% W (0.2 to 2.0% 1/2W+Mo in the case of two kinds), furthermore contg., at need, 0.2 to 3.0% Co and contg., at need, <=0.005% S and <=0.020% P, and the balance Fe with inevitable impurities, and its warmability is also excellent. Thus, a die using this steel is remarkably improved in heat cracks and large cracks or wear and settling caused by softening in the process of using.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a mold used for forging large products, in particular, heat cracks during use.
The present invention relates to a high-strength and high-toughness hot work tool steel capable of remarkably improving wear and fatigue due to large cracks or softening.

[0002]

2. Description of the Related Art In a die for a large forged part typified by an automobile, JIS-SK having a high toughness and a low cracking susceptibility is used for applications in which a large crack from a corner portion of a die is a problem.
JIS T4 is used for applications where bending due to insufficient strength and wear due to softening due to heat effect of the die are severe.
-SKD 61, 62 types and the like are used. Further, for some applications, the above-mentioned materials have been improved into Japanese Patent Publication No.
The materials proposed in Japanese Patent No. 75 and Japanese Patent Publication No. 54-38570 are also used. The following characteristics are required for the mold material used for such applications. 1) There is little softening of the mold surface due to the influence of heat during use. 2) Low susceptibility to large cracks. 3) High strength and little settling. 4) Excellent heat check resistance. 5) Good hardenability and high toughness even inside the mold.

[0003]

Among the conventionally used materials, JIS-SKT4 type has a very high sensitivity to large cracks, but has a low resistance to high temperature softening, and is either scrapped or reworked due to its wear life. ing. Further, when the mold engraved surface is lowered by reworking several times, the hardenability is low, so that the hardness of the central part of the mold material is lowered, and fatigue, cracking, etc. occur due to insufficient strength. Therefore, in order to further improve the mold life, it is necessary to greatly improve the high temperature softening resistance and the hardenability. On the other hand, JIS-S
In KD61 and 62 types, since the strength of the mold material is high and the resistance to softening at high temperature is relatively excellent, wear and fatigue are improved, but in the case of a large material, the toughness of the center portion of the mold material is low, Large cracks may occur from the corners of the mold bottom,
Improvement of toughness is indispensable for improving the life. Furthermore, the materials proposed in JP-B-54-26975 and JP-B-54-38570, which are improvements thereof, are also
It is not always sufficient in terms of toughness and softening resistance.

In view of such conventional problems, an object of the present invention is to provide a hot work tool steel having improved toughness and high temperature softening resistance.

[0005]

The present inventor has conducted a lot of researches on conventional components to solve the above-mentioned problems, and as a result, in order to simultaneously improve the toughness and the high temperature softening resistance, C was added. In order to compensate for the effect of V, it is effective to keep it as low as possible, and V is effective for wear resistance, but on the other hand, it is intentionally restricted to a low value to promote striped segregation. We have found that adding a small amount of Nb is effective for improving toughness without impairing wear resistance, and completed the present invention.

That is, the hot work tool steel according to the first invention of the present application is, by weight%, C: 0.20 to 0.45%, Si:
0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.
5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.0
5 to less than 0.20%, Nb: 0.01 to 0.15%, and Mo: 0.2 to 2.0%, W: 0.4 to 4.0.
%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo:
0.2 to 2.0%), and the balance Fe and unavoidable impurities.

Further, the hot work tool steel according to the second invention of the present application is, by weight%, C: 0.20 to 0.45%, Si:
0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.
5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.0
5 to less than 0.20%, Nb: 0.01 to 0.15%, C
o: 0.2 to 3.0% and Mo: 0.2 to 2.0
%, W: 0.4 to 4.0%, one or two kinds (1 / 2W + Mo: 0.2 to 2.0% in the case of two kinds), and the balance Fe and unavoidable impurities. It is characterized by

Further, the hot work tool steel according to the third invention of the present application is, by weight%, C: 0.20 to 0.45%, Si:
0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.
5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.0
5 to less than 0.20%, Nb: 0.01 to 0.15%, and Mo: 0.2 to 2.0%, W: 0.4 to 4.0.
%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo:
0.2 to 2.0%), and further S: 0.005
% Or less, P: 0.020% or less, and the balance is Fe and inevitable impurities.

The hot work tool steel according to the fourth invention of the present application is, by weight%, C: 0.20 to 0.45%, Si:
0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.
5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.0
5 to less than 0.20%, Nb: 0.01 to 0.15%, C
o: 0.2 to 3.0% and Mo: 0.2 to 2.0
%, W: 0.4 to 4.0%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo: 0.2 to 2.0%), and S: 0.005%. Hereinafter, P: 0.020
% Or less, and the balance is Fe and inevitable impurities.

[0010]

Next, the reasons for limiting the chemical components of the present invention will be described. C: C is an element that gives sufficient matrix hardness by quenching and tempering, forms a carbide by combining with Cr, Mo, W, V, Nb and the like, and gives high temperature strength and wear resistance. However, if it exceeds 0.45% and is too much, microsegregation is promoted to form a large amount of huge eutectic carbides, and toughness and high temperature strength are lowered, so the upper limit is set to 0.
It was set to 45%. On the other hand, if less than 0.2%, sufficient hardness cannot be obtained, so the lower limit was made 0.20%. Si: If Si is added in a large amount exceeding 0.3%, the thermal conductivity deteriorates, the heat check resistance is impaired, and the toughness of the matrix decreases, so the content was made 0.30% or less. Mn: Mn is an element that is added as a deoxidizer to improve the cleanliness of steel and hardenability, and is at least 0.5.
Need to be added. However, if added over 2.0%, the toughness decreases and the machinability deteriorates, so the upper limit was made 2.0%. Ni: Ni is a very effective component in the present invention for enhancing the hardenability, and at least 0.5% is necessary for obtaining the effect. However, if added over 2.0%, the A ₁ transformation point is lowered, heat resistance is deteriorated, and machinability is also deteriorated, so the upper limit was made 2.0%. Cr: Cr is an element that combines with C to form hard carbides to improve wear resistance and enhance hardenability. For that purpose, 1.0% or more is required. However, if added in a large amount in excess of 4.0%, agglomeration and coarsening of Cr carbides are caused, and the softening resistance, which is important for the steel of the present invention, is significantly impaired, so the Cr content is set to 1.0 to 4.0%. did. V: V is an element that forms a carbide that is difficult to form a solid solution and enhances wear resistance and softening resistance, but is also an element that promotes microsegregation (striped segregation), and is excessive in terms of toughness in the steel of the present invention. Is not preferable, the upper limit is 0.20.
It was less than%. On the other hand, if it is less than 0.05%, the effect of improving wear resistance cannot be obtained, so the lower limit was made 0.05%. Nb: Nb is an element that forms carbides that are difficult to form a solid solution like V and improves wear resistance and softening resistance. Further, coarsening of austenite crystal grains is suppressed during quenching and heating, and toughness is improved. This effect is more remarkable than V, and a larger effect can be obtained by adding a small amount. Therefore, in the present invention, Nb is positively added for the purpose of compensating for the effect of V, and the amount thereof is set to 0.01 to 0.15%. Mo, W: Mo and W are elements that both form fine carbides and improve wear resistance and softening resistance. However, the effect of Mo is twice as strong as that of W, and W needs twice as much as Mo to obtain the same effect. If too much of both elements are added, they will form huge carbides, promote striped segregation, and reduce toughness and heat check resistance.
The upper limit was set to 2.0% in terms of Mo equivalent (1/2 W + Mo).
If the amount is too small, the above addition effect cannot be obtained.
The lower limit was 0.2% in terms of Mo equivalent.

Co: Co is added to the second and fourth invention steels of the present application. Co is an element that suppresses the coagulation and coarsening of carbides at high temperatures, and is particularly effective in resistance to softening,
In particular, it is effective when used for a die for forging such as a high alloy steel having high high temperature deformation resistance. In order to obtain these effects, at least 0.2% is required, and if it exceeds 3.0%, the heat check property and toughness are deteriorated, so the upper limit was made 3.0%.

S, P: In the third and fourth inventions of the present application, S and P are limited to extremely low values in order to prevent large cracks especially in the case of a large die having a complicated shape in which stress concentration is severe. And toughness was especially improved. S forms MnS and the like, which extends in the hot working direction, thereby lowering the toughness particularly in the lateral direction. If it exceeds 0.005%, this effect appears. Therefore, when particularly high toughness is required, the amount of S is limited to 0.005% or less. P promotes segregation of alloy elements during solidification and increases striped segregation after hot working. In addition, during toughening treatment before use and when the mold surface temperature rises during use, it segregates at the matrix grain boundaries and reduces toughness. This effect becomes remarkable when the content exceeds 0.02%, and therefore, especially for applications requiring high toughness,
It was limited to 0.020% or less.

[0013]

EXAMPLES The present invention will be described in detail below with reference to examples. Example 1 100 heats of 13 heats A to M shown in Table 1
The steel was tapped in a kg vacuum induction melting furnace, cast into an ingot having an average diameter of 190 mm, and forged to a diameter of 30 mm to obtain a test material,
The properties of each test material were tested.

[0014]

[Table 1]

Each characteristic test method is shown below. 1) In the softening resistance test, each test piece having a diameter of 30 mm and a height of 30 mm was quenched and tempered to HRC44, held at 600 ° C. for 30 hours in a canthal furnace, and then air-cooled to give D of the test piece.
The hardness of / 4 part (center part of the radius) was measured to evaluate the degree of softening. 2) For the Charpy impact test, a test piece of JIS No. 4 angle 10 mm x length 55 mm and V notch was quenched and tempered to HRC44 and tested at 600 ° C. 3) High temperature strength test, parallel part diameter 5mm, gauge length 25
A test piece of mm was quenched and tempered to HRC44 and tested at 600 ° C. 4) Heat check test is 20mm in diameter x 15mm in height
Quenching and tempering the test piece of
This is heated to 600 ° C. with a gas burner, put in water at 30 ° C. and cooled. Heating-cooling is repeated 500 times,
The average crack length per 10 mm of the test piece end face and the maximum crack length were measured and evaluated.

The test results are shown in Table 2.

[0017]

[Table 2]

As shown in Table 2, the steels of the present invention (A to H) are superior to the comparative steels (I to M) in softening resistance, toughness, high temperature strength and heat checkability. Became clear. In particular, compared with SKT4 (Comparative Steel I), which is often used for large-scale molds, the steel of the present invention has the same or higher toughness, but resistance to softening, high temperature strength, heat checkability, etc. Has extremely high high-temperature strength characteristics, and due to the influence of heat during use, mold dripping, wear,
It shows that sagging is extremely suppressed.

(Example 2) Steel C of the present invention shown in Table 1 was tapped in an electric furnace to produce a block material having a width of 400 mm, a height of 300 mm and a length of 550 mm, which was quenched and tempered in HRC44. The cross-sectional hardness distribution of the central part of this material is shown in FIG. 1 in comparison with SKT4 (Comparative Steel I) manufactured in the same manner. From FIG. 1, it is expected that the steel of the present invention has good hardenability and sufficient hardness up to the center portion, and that the life will not be shortened even if the die engraved surface is dug down by reworking and used. .

[0020]

As described above, the steel of the present invention is a high-strength and high-toughness hot tool in which heat cracks, large cracks during use, wear due to softening, and fatigue are remarkably improved as compared with conventional steels. Since it is steel and is excellent in hardenability, it can be used for a large-sized die used for forging a large-sized part, and the die cost can be reduced. Further, it is a die material having a wide utility value as a general hot hammer and a press die, and has great industrial contributions.

[Brief description of drawings]

FIG. 1 is a diagram showing the cross-sectional hardness distributions of the case-hardened materials of the present invention steel C and comparative steel I.

[Explanation of symbols]

 None

Claims (4)

[Claims] 1. By weight%, C: 0.20 to 0.45%, Si: 0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.05 to less than 0.20%, Nb: 0.01 to 0.15%, and Mo: 0.2 to 2.0%, W: 0 4 to 4.0%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo: 0.
2 to 2.0%) and the balance Fe and unavoidable impurities. 2. By weight%, C: 0.20 to 0.45%, Si: 0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.05 to less than 0.20%, Nb: 0.01 to 0.15%, Co: 0.2 to 3.0%, and Mo: 0 2 to 2.0%, W: 0.4 to 4.0%, 1 or 2 types (in the case of 2 types, 1/2 W + Mo: 0.
2 to 2.0%) and the balance Fe and unavoidable impurities. 3. By weight%, C: 0.20 to 0.45%, Si: 0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.05 to less than 0.20%, Nb: 0.01 to 0.15%, and Mo: 0.2 to 2.0%, W: 0 4 to 4.0%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo: 0.
2 to 2.0%), S: 0.005% or less, P: 0.020% or less, and the balance Fe and inevitable impurities. 4. In% by weight, C: 0.20 to 0.45%, Si: 0.30% or less, Mn: 0.5 to 2.0%, Ni: 0.5 to 2.0%, Cr: 1.0 to 4.0%, V: 0.05 to less than 0.20%, Nb: 0.01 to 0.15%, Co: 0.2 to 3.0%, and Mo: 0 .2 to 2.0%, W: 0.4 to 4.0%, 1 type or 2 types (in the case of 2 types, 1/2 W + Mo:
0.2 to 2.0%), and further S: 0.005% or less, P: 0.020% or less, and the balance Fe and unavoidable impurities.