JPS62161942A - Tool steel for hot working - Google Patents

Abstract

PURPOSE:To improve the strength at high temp. and the toughness by specifying the amounts of C, Si, Mn, Ni, Cr, W, Mo, V and N. CONSTITUTION:This tool steel for hot working contains by weight, 0.36-0.5% C, <=1% Si, 1.5% Mn, 0.5-2.3% Ni, 4.05-6.5% Cr, 1-1.85% (1/2W+Mo), 0.4-1.3% V and 0.025-0.15% N. Even when the steel is tempered by slow cooling at a low cooling rate, the toughness is not reduced. Even in case of a large-sized tool, high strength at high temp. and high toughness are maintained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention has particularly high hardenability, suppresses the formation of upper bainite even in slow quenching, maintains high toughness, and has a suitable hardness. It relates to high-temperature strength, easily hardenable, high-toughness hot working tool steels, and more specifically, to tools with large dimensions that slow the quenching cooling rate, or with sharp corners and large stress concentrations. This relates to mold materials that can be applied to hot tool applications and provide long life.In recent years, aluminum die-casting molds have become larger due to the use of F/F in automobiles, and sharp corners and mold surfaces of forging molds are used to improve dimensional accuracy. It can withstand harsher usage conditions, such as enhanced cooling, and has a long life without heat cracks or large cracks.

[Conventional technology]

Conventionally, in the heat treatment of aluminum die-casting molds, rapid cooling treatment such as oil cooling was not used to prevent heat treatment distortion, and SCr-based 5KD61 and 5KD6, which are so-called air-hardened steels, were used, and blast hardening was used. ing.

[Problem that the invention seeks to solve]

However, the cooling rate decreases as the mold size increases, leading to the formation of a large amount of upper bainite, resulting in a significant decrease in toughness after tempering, leading to early heat cracks and large cracks.

In addition, even with 5KD61 and 5K06, which have high hardenability, the amount of upper bainite gradually increases due to a decrease in cooling rate, and decreases accordingly. Even with small to medium sized molds, air cooling did not provide a satisfactory lifespan, and the molds could not be used unless oil cooled. However, oil cooling caused problems with distortion and cracking.

As mentioned above, it is necessary to develop a mold material that has much better hardenability than 5KD6L and 5KD6, which have the highest toughness among conventional tool steels for hot working, and has equivalent or higher high-temperature strength. Ta.

The present invention has toughness and high-temperature strength equivalent to or higher than 5KD61 and 5KD6, and has much greater hardenability than 5KD61 and 5KD6, and is suitable for use with large molds (for example, 700 mm x 700 mm).
The purpose of the present invention is to develop a hot work tool steel that suppresses the formation of upper bainite even in slow cooling quenching such as blast quenching of mm x 10100O and maintains high toughness to solve the above problems.

[Means for solving problems]

The chemical composition of the steel of the present invention has medium C-medium Cr-Ni-low to medium (W-MO)-medium V-N as its basic components, and suppresses the formation of coarse V-based and W, Mo-based carbides. , and C1Cr,
Hardenability is improved by appropriately combining alloy compositions mainly composed of NiMn, and toughness is improved by refining the solidified structure and austenite crystal grain size.
Even in the case of slow cooling quenching over 5 m1n, the formation of upper bainite that is harmful to toughness is suppressed, and excellent toughness is maintained.

In addition, 5KD can be achieved by appropriate combination of C, W, Mo, and V amounts.
In addition to providing excellent high-temperature strength and high-temperature wear resistance equivalent to or higher than 61 and 5KD6, the addition of Cr, Go, and Ni and adjustment of the amount of Si make it dense and highly adhesive to the mold surface as the temperature rises during use. An oxide film with an appropriate thickness is formed, which also provides high high-temperature wear resistance and seizure resistance.

That is, the claimed invention.

Weight%: C 0.36-0.50%, Si 1.00% or less, Mn 1.50% or less, Ni 0.50-2.30%
, Cr4.05-6.50%.

(1/2W + Mo) L, 0O~1.85%, V
0.40-1.30%.

A hot working tool steel characterized by comprising 0.025 to 0.15% N, the balance Fe and normal impurities.

C0.36-0.50% by weight, Si 1.0 part or less, Mn 1.50% or less, Ni 0.50-2.30
%, Cr 4.05-6.502, (1/2W +
Mo) 1.00-1.85%, V 0.40-1.3
0%, Go 0.50-4.00%, N 00025-
Tool steel for hot working, characterized in that it consists of 0.15%, balance Fe and normal impurities, C 0.36 to 0.50% by weight, Si 1.00% or less, Mn 1.50% or less, Ni 0.50-2.30%
, Cr 4.05-6.50%, (L/2W+Mo)1
．． 00-1.85%, V 0.40-1.30%.

Nb 0.02-0.15%, N 0.025-0.
15%, balance Fe and normal impurities; Ni 0.50-2.30%
, Cr 4,05-6.50%, (1/2W +
Mo) 1,004.85%, V 0.40-1.3
0%.

Co 0.50-4.00%, Nb 0.02-0.1
5%, N 0.025-0.15%, the balance Fe and normal impurities.

[Effect]

Next, the reason for limiting the composition of the steel of the present invention will be described.

C maintains the excellent hardenability, tempering hardness, and high temperature hardness of the steel of the present invention, and also contains W, Mo, V, Nb and C.
It combines with carbide-forming elements such as r to form carbides, and is added to provide grain refinement, wear resistance, temper softening resistance, and high-temperature hardness.

If the content is too high, the toughness will decrease, so the content should be 0.50% or less, and if it is too low, the effect of the above addition cannot be obtained.
The content shall be 0.36% or more.

Si is added to increase the A-transformation point and to provide oxidation resistance depending on the application.

Addition of a large amount is disadvantageous in terms of toughness, makes it difficult to form a protective oxide film during use, and causes a decrease in thermal conductivity, so the content should be 1.00% or less.

Mn is added to impart particularly excellent hardenability, which is a characteristic of the steel of the present invention.

Addition of a large amount increases annealing hardness, reduces machinability, and lowers the A transformation point, so the content should be 1.50% or less.

Nj, together with C, CrlMn, Mo, W, etc., imparts excellent hardenability to the steel of the present invention and forms a martensite-based structure even at a slow quenching cooling rate to prevent a decrease in toughness. is an important additive element. It also gives an essential toughness improvement to the base.

Ni is added to obtain the above effects, but if it is too large, it will excessively lower the A1 transformation point, leading to a decrease in wear resistance life, and will also excessively increase annealing hardness, reducing machinability. The content should be 2.30% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 0.50% or more.

Cr is added for the purpose of improving the hardenability, which is a characteristic of the steel of the present invention, and is one of the most important elements in the steel of the present invention, along with Ni, and the setting of Cr is extremely important. Cr is added to improve temper softening resistance and high-temperature strength, provide appropriate oxide film properties, improve wear resistance by combining with C to form carbides, and improve A□ transformation point. be done.

If the temperature is too low, it will be difficult to maintain the particularly excellent hardenability that is a characteristic of the steel of the present invention, and the oxidation resistance will be insufficient, which will easily cause roughness during use. 4.05% because it becomes difficult to achieve both good toughness and wear resistance due to deterioration of softening resistance and reduction of residual carbides.
The content should be 6.50% or less, since if it is too high, carbides are likely to aggregate when the temperature rises, reducing high-temperature strength and softening resistance.

In order to obtain sufficient hardenability even by slow cooling quenching, 4.
It is more desirable to add 50 to 6.50%.

W forms a large amount of carbides that are difficult to dissolve in the solid state during quenching and heating, which has a unique effect on improving wear resistance, and during tempering, it precipitates fine carbides that are difficult to aggregate, increasing high-temperature yield strength. It is added to increase the softening resistance when the temperature rises, and to improve the density of the surface oxide film formed when the temperature rises during use. If too much W is present, there is a strong tendency to form coarse carbides similar to conventional steels, leading to a decrease in toughness.

MO forms carbides to improve wear resistance, dissolves in the matrix to improve hardenability, and forms fine carbides during tempering to improve high-temperature strength and resistance to softening at elevated temperatures. It also has the effect of improving the ease of forming a protective oxide film during use.

W and Mo are added to obtain the above effects, but if they are too large, the toughness decreases due to the formation of coarse carbides and an increase in the amount of carbides, making it difficult to maintain the excellent toughness that is the characteristic of the steel of the present invention. Therefore, (1/2W+Mo) is 1
．． The content should be 8% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 1.00% or more.

Note that the effects of adding W and MO are similar.

W is more advantageous in terms of high-temperature strength and wear resistance, and Mo is more advantageous in terms of toughness, and these may be added alone or in combination depending on the purpose and use.

(2) Forms a large amount of carbides that are difficult to dissolve in solid solution, which improves wear resistance and seizure resistance. Precipitates and improves softening resistance in high temperature ranges.

It also brings about the effect of grain refinement during quenching, improving toughness, and increases the A1 transformation point, which together with the effect of improving high-temperature strength, improves heat crack resistance. If it is too high, coarse carbides will be formed and it will be difficult to maintain the excellent toughness that is a characteristic of the steel of the present invention, so it should be set at 1.30% or less, and if it is too low, the effect of the above addition cannot be obtained, so 0. 40
% or more.

Together with Nb, N refines the solidification structure and also refines the crystal grains during quenching, prevents the formation of coarse upper bainite even during slow quenching, maintains high toughness, and enhances the effects of the steel of the present invention. It is an essential and important additive element to achieve this goal.

N is added to obtain the above effect, but it is not necessary to add a large amount, and there are restrictions from the solid solubility limit, so it is limited to 0.15% or less.
If it is too low, the effect of the above addition cannot be obtained, so 0.025
% or more.

Co is an additive element for imparting extremely high high-temperature wear resistance to the steel of the present invention. The addition of CO forms a protective oxide film that is extremely dense and has good adhesion when the temperature rises during use, thereby preventing metal contact with the other material and preventing the temperature rise of the steel of the present invention. It also provides excellent wear resistance.

This effect of co + Cr, Ni, W, M.

It differs depending on the relationship with the amount of other added elements, and in the case of the steel of the present invention, it is not necessary to add a large amount. If it is too large, the toughness will decrease, so the content should be 4.00% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 0.50% or more.

Nb forms fine carbides that are particularly difficult to agglomerate during tempering, particularly increases softening resistance during temperature rise, and is added to refine crystal grains during quenching.

In addition, it combines with N and C and acts as a nucleus during the formation of primary crystals during solidification, thereby refining the solidified structure and improving toughness.

Nb is added to obtain the above effect, but if it is too large, it will form coarse carbides and reduce toughness, so it should be added to 0.15
% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 0.02% or more.

〔Example〕

The present invention will be described in detail below based on examples.

Table 1 shows the chemical compositions of the steel of the present invention and the conventional tool steel for hot working.

Table 2 shows the heat treatment conditions (target hardness HRC45) and high-temperature strength of the inventive steel and conventional steel shown in Table 1. The inventive steel has high-temperature strength equivalent to or similar to the conventional steel. It can be seen that

Tables 2 and 3 show the inventive steel and conventional steel samples shown in Table 1 at a cooling rate of 45 m1n (cooling rate such that the time required to lower the temperature to an intermediate temperature between the quenching temperature and room temperature is 45 m1n).
The fracture toughness value (KIC) when tempered to I-IRC45 after quenching and cooling is shown.

Table 3 In the case of conventional w4L, when a small-sized specimen is quenched by oil cooling and then tempered, a good fracture toughness value exceeding 200 kg/+am3h can be obtained, but when half-cooled to 45 m1n, the structure is mainly composed of upper bainite. Therefore, the fracture toughness value has decreased to 200 kg/B, Vz or less.

On the other hand, in the case of the steel of the present invention, the hardenability is large and
Even in the case of quenching and cooling, a martensite-based structure is generated and shows high toughness after tempering.

In this way, the steel of the present invention has a major feature in that it can maintain sufficient toughness even when air-cooled and quenched, even in large-sized molds, since it can obtain sufficient toughness even when it is slowly quenched. .

Table 4 shows the heat check resistance of the steel of the present invention.

The test pieces were partially quenched at 45nine at each austenitizing temperature shown in Table 2, and then tempered to I (RC45).

Test piece 2011111φX 30mm Q Terror 60
The operation of rapidly heating to ℃ and rapidly cooling to 20℃ in water for 4,00
This is the result of repeating 0 times.

Table 4 It can be seen that although the number of cracks in the steel of the present invention is slightly higher than that of the conventional steel, the average and maximum depths are small and the steel has significantly excellent heat check resistance.

This is because the steel of the present invention has excellent high-temperature strength equivalent to or similar to conventional steel, and has high hardenability, and the formation of upper baini 1 is suppressed even when quenched to a depth of 45 ml. This is due to the maintenance of high toughness, and the effect of improving toughness due to the small tendency to form coarse carbides during solidification due to the alloy composition and the nucleation effect of the initial product during solidification.

〔Effect of the invention〕

As explained above, the steel of the present invention does not cause a decrease in toughness even when subjected to slow cooling quenching for a half-time of about 45 m1n or at a slower cooling rate, and also has an appropriate combination of alloy structures.
This is a new tool steel for hot working, which is characterized by its fine solidification structure, which allows it to maintain excellent high-temperature strength and high-level toughness even in the case of large-sized tools.

Claims (1)

[Claims] 1% by weight: C 0.36-0.50%, Si 1.00%
Below, Mn 1.50% or less, Ni 0.50-2.30%
, Cr4.05-6.50%, (1/2W+Mo)1.
00~1.85%, V0.40~1.30%, N0.0
A tool steel for hot working, characterized in that it consists of 25 to 0.15%, the balance being Fe and normal impurities. 2% by weight: C0.36-0.50%, Si1.00%
Below, Mn 1.50% or less, Ni 0.50-2.30%
, Cr4.05-6.50%, (1/2W+Mo)1.
00-1.85%, V0.40-1.30%, Co0.
50-4.00%, N0.025-0.15%, balance F
A tool steel for hot working, characterized in that it consists of e and normal impurities. 3 Weight%: C0.36-0.50%, Si1.00%
Below, Mn 1.50% or less, Ni 0.50-2.30%
, Cr4.05-6.50%, (1/2W+Mo)1.
00-1.85%, V0.40-1.30%, Nb0.
02-0.15%, N0.025-0.15%, balance F
A tool steel for hot working, characterized in that it consists of e and normal impurities. 4 C0.36-0.50%, Si1.00% by weight%
Below, Mn 1.50% or less, Ni 0.50-2.30%
, Cr4.05-6.50%, (1/2W+Mo)1.
00-1.85%, V0.30-1.30%, Co0.
50-4.00%, Nb0.02-0.15%, N0.
A tool steel for hot working, characterized in that it consists of 0.025% to 0.15%, the balance being Fe and normal impurities.