KR20200075383A - Hot forging die steel enhancing durability - Google Patents

Abstract

The present invention relates to hot forging die steel, which contains: 0.40 to 0.50 wt% of carbon (C); 0.8 to 1.2 wt% of silicon (Si); 0.3 to 0.6 wt% of manganese (Mn); 4.8 to 5.5 wt% of chromium (Cr); 1.5 to 2.5 wt% of molybdenum (Mo); 0.8 to 1.2 wt% of vanadium (V); 0.01 to 0.3 wt% of tungsten (W); and the balance Fe and unavoidable impurities. According to the present invention, durability can be improved and thus the life of a die can be further extended.

Description

Hot forging die steel with improved durability{HOT FORGING DIE STEEL ENHANCING DURABILITY}

The present invention relates to a mold steel for hot forging, and more particularly to a mold steel with improved durability for hot forging.

Die steel for hot forging processing has high hardness and toughness, high wear resistance, high heat resistance, little dimensional change, good hardening, and easy heat treatment Cr, W, Cr-W-Mn steel, etc. are generally used because properties such as a thing, a small heat treatment strain, a good workability, and the like are required.

Such a conventional hot forging die steel is discarded after taking a specific target stroke.

That is, if the mold life can be increased, the production cost can be reduced by that amount, and in order to do so, it is required to improve the durability of the mold steel.

The above description of the background art is for helping understanding of the background of the invention, and may include matters other than the prior art already known to those skilled in the art to which this technology belongs.

Korean Registered Patent Publication No. 10-1312822

The present invention has been devised to solve the above-mentioned problems, and the present invention has an object to provide a hot-forged mold steel capable of improving durability and extending mold life.

Hot forging mold steel according to one aspect of the present invention, by weight, carbon (C) 0.40 ~ 0.50%, silicon (Si) 0.8 ~ 1.2%, manganese (Mn) 0.3 ~ 0.6%, chromium (Cr) 4.8 ~ Contains 5.5%, molybdenum (Mo) 1.5-2.5%, vanadium (V) 0.8-1.2%, tungsten (W) 0.01-0.3 wt%, balance Fe and other unavoidable impurities.

And, the microstructure is characterized in that it contains bainite and martensite.

More specifically, the mold steel may have a composition in which the [Mo] + [W] value satisfies the range of 1.5 to 2.8.

In addition, the mold steel may have a composition in which the [V]/[C] value satisfies the range of 1.6 to 3.0.

Furthermore, the mold steel may have a composition in which [V]/[C] values satisfy 2.7.

According to the hot forging mold steel of the present invention, the mold life can be improved by about 20% by securing an impact value of 20 J/cm 2 or more at a hardness of HRC 50 or higher in order to satisfy the load condition for hot forging.

In particular, if the hardness HRC 50 is satisfied by adjusting the alloying components, the mold can reduce the wear of the mold surface, which acts as a high load at a high temperature, and thus is effective in realizing the shape and dimensions of the forged product.

And, if the toughness is maintained at 20 J/cm 2 or more, the life of the crack can be further extended by delaying the occurrence of cracks on the impact on the mold during forging.

In addition, by adding chromium and tungsten, the oxidation resistance and corrosion resistance at high temperatures can be improved to maintain the cleanliness of the surface even when the mold is used at high temperatures or not.

As described above, when the mold steel material of the present invention is applied, the cost reduction effect is superior to that of the prior art.

1 shows a C-Cr state diagram curve.
Figure 2 shows the formation of alloy carbides by Mo, Cr, V.
3 shows alloy carbide formation according to the V content.
4 and 5 are evaluation results for strength according to the Mo content.
6 is a strength evaluation result according to the W content.
7 to 9 are results of hardness evaluation according to the Mo content.
10 and 11 are results of hardness evaluation according to the W content.
12 is a result of toughness evaluation according to the Mo content.
13 and 14 are results of toughness evaluation according to the W content.
15 shows the change in the amount of wear according to the Mo content.
16 and 17 show the change in the amount of wear according to the W content.
Figure 18 shows the effect of tensile strength and toughness by the addition of Mo and W.
19 shows an example of heat treatment for manufacturing the mold steel of the present invention.

In order to fully understand the present invention, the operational advantages of the present invention, and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings and the contents described in the accompanying drawings, which illustrate preferred embodiments of the present invention.

In describing preferred embodiments of the present invention, well-known techniques or repetitive descriptions that may unnecessarily obscure the subject matter of the present invention will be reduced or omitted.

Depending on the durability of the die steel for hot forging, the life of the mold produced thereby depends.

As an example of the existing prior art mold steel, the composition of STD61 is 0.40% by weight of carbon (C), 1.0% by weight of silicon (Si), 0.4% by weight of manganese (Mn), 5.1% by weight of chromium (Cr), and molybdenum (Mo) 1.1% by weight, vanadium (V) 0.8% by weight and balance iron (Fe).

The present invention overcomes the limitation of durability due to the composition of the mold steel of the related art, and the composition of the hot forged mold steel according to the present invention is as follows.

Carbon (C) 0.40 to 0.50 wt%, silicon (Si) 0.8 to 1.2 wt%, manganese (Mn) 0.3 to 0.6 wt%, chromium (Cr) 4.8 to 5.5 wt%, molybdenum (Mo) 1.5 relative to the total weight% It consists of ~2.5 wt%, vanadium (V) 0.8-1.2 wt%, and tungsten (W) 0.01-0.3 wt% and the balance iron (Fe).

Hereinafter, the effect of each composition constituting the hot forged mold steel of the present invention will be described.

1 shows a state diagram curve of C-Cr.

The content of C is 0.40 to 0.50% by weight, and when the carbon content is low, thermal stability is lowered to form coarse resin-like crystals, and when the content of C is large, metal carbides are formed to generate brittleness.

Carbon increases quenching property by rapid cooling after heating, increases strength and hardness, and is necessary for forming carbides with Cr, Mo, V, and W in the present invention.

Therefore, when the carbon content is lower than 0.40%, it is difficult to increase the strength or hardness to HRC 50 or higher, and when it is higher than 0.50%, it is difficult to simultaneously satisfy toughness of 20J or higher at the hardness level of the hardness HRC 50.

Next, the content of chromium (Cr) is 4.8 to 5.5% by weight, improving thermal stability, thermal fatigue strength and wear resistance while forming carbon and carbide.

When 4.8% or more of Cr is added, the softening resistance and oxidation resistance are improved, and when it is less than 4.8%, the softening resistance and oxidation resistance of the steel are insufficient. Conversely, if it exceeds 5.5%, crack stability may be deteriorated at high temperatures.

In terms of wear resistance, it is best when the Cr content is 4.8%, but it does not show a significant difference from that when it is 5.5%.

Therefore, the content of chromium in the present invention is preferably 4.8% to 5.5%.

In addition, silicon is an element that is added as a deoxidizer in the steelmaking process, and is partially dissolved in steel to improve hardenability and hardness.

If silicon is less than 0.8%, proper hardness control is difficult, and if it exceeds 1.2%, there is a problem in that toughness is greatly reduced by promoting graphitization of carbides.

Therefore, the content of silicon (Si) is preferably 0.8% to 1.2%.

And, manganese (Mn) is an element that enhances hardenability and hardness and wear resistance together with carbon, inhibits ferrite formation and promotes formation of martensite.

In order to match the hardness in the present invention, 0.3% or more should be added, and if it exceeds 0.6%, the moldability is difficult due to the reduction in machinability and workability due to the miniaturization of martensite and bainite, so it is preferably 0.3% to 0.6%. .

Next, Mo and W are elements that improve high temperature hardness and wear resistance. Mo and W interact with each other at high temperatures.

Mo induces an increase in the atomic binding force in the crystal lattice and reduces the flow permeability, and thus serves to stabilize the steel structure.

In addition, as the content of Mo increases, the development and solidification of carbides at high temperatures are reduced.

If Mo is less than 1.5%, thermal stability or texture stability is weakened, which is not sufficient as an alloy steel for molds, and if it exceeds 2.5%, carbides develop at the boundary, which eventually leads to a decrease in metal viscosity and embrittlement. .

In terms of abrasion resistance, when Mo is 1.5%, it has the best abrasion resistance, but when it is less than 1.5%, the amount of abrasion increases, resulting in poor wear resistance.

In the present invention, an additional alloy element (Cr, W, etc.) is added to limit the content of Mo to a maximum of 2.5% to prevent a decrease in toughness, and to a minimum of 1.5% to show the effect on thermal stability.

Next, W serves to strengthen the ferrite matrix and improve physical properties such as high temperature strength.

W forms special carbides such as W ₆ C, and secondary hardening occurs at high temperatures to maintain high temperature hardness. Then, corrosion resistance is improved, such as Cr.

If W is less than 0.01%, there is no effect of improving corrosion resistance and high temperature strength, and when it exceeds 0.3%, W is 0.01 to 0.3 wt. % Is preferred.

2 shows the formation of alloy carbides by Mo, Cr, and V, and FIG. 3 shows the formation of alloy carbides according to the V content.

The addition of V leads to improved thermal stability, high temperature fatigue strength and wear resistance.

If V is less than 0.8%, there is little effect due to the dispersion strengthening of V, and if it exceeds 1.2%, a reticular V nitride is formed at the interface, causing microscopic deformation of the metal, and residual stress is generated, and this part acts as a weak part.

In terms of abrasion resistance, V/C=1.6 to 3.0 is preferable, and more preferably, when V/C=2.7, abrasion resistance is the best, and in the present invention, to maximize wear resistance, V is 1.2 when C content is 0.45%. %.

However, there is a disadvantage in that the price is high when V is added, and thus the V content is minimized to minimize the V content in order to satisfy economic efficiency and the secondary curing effect.

When the V content is less than 0.8%, the abrasion resistance is weak, and when the V content exceeds 1.2%, alloy carbides are formed to have a secondary hardening effect, but the effect is insufficient, so an appropriate amount should be added in consideration of economic efficiency.

The present invention was able to implement a hot forged mold steel with improved durability compared to the mold steel according to the prior art by the composition as described above, and the evaluation of the effect of the additive content based on the examples and comparative examples in Table 1 below Explain the results.

division C(%) Si(%) Mn(%) Cr(%) V(%) Mo(%) W(%) result Example 1 0.40 1.00 0.40 5.10 1.0 1.1 - Similar to conventional STD61 (target standard) Example 2 0.40 1.00 0.40 5.10 1.0 1.3 - Mo lower than the lower limit Example 3 0.40 1.00 0.40 5.10 1.0 1.5 - Check the effect on the content of W at 1.5% Mo Mo content section (1.5~2.5%) Example 4 0.40 1.00 0.40 5.10 1.0 1.5 0.3 Example 5 0.40 1.00 0.40 5.10 1.0 1.5 1.0 Example 6 0.40 1.00 0.40 5.10 1.0 1.5 2.0 Example 7 0.40 1.00 0.40 5.10 1.0 2.0 - Check the effect on the content of W at Mo 2.0% Example 8 0.40 1.00 0.40 5.10 1.0 2.0 0.3 Example 9 0.40 1.00 0.40 5.10 1.0 2.0 1.0 Example 10 0.40 1.00 0.40 5.10 1.0 2.0 2.0 Example 11 0.40 1.00 0.40 5.10 1.0 2.5 - Comparative example 0.40 1.00 0.40 5.10 1.0 3.0 - Mo upper limit

The evaluation results according to the examples and comparative examples in Table 1 are summarized as in Table 2.

division High temperature strength (MPa) Hardness (HRC) Impact toughness (J) Abrasion resistance (mg) result Cr(%) Mo(%) W(%) High temperature tensile strength (at 650℃ High temperature yield strength (at 650℃ 575℃ 600℃ 625℃ 575℃ 600℃ 625℃ 575℃ 600℃ 625℃ 5.1 1.1 - 570 350 50 45 33 20 22 31 1.2 1.32 1.6 A 5.1 1.3 - 615 402 50 45 35 20 21 30 1.17 1.30 1.56 B 1.5 0.3 682 421 51 46 37 18 19 26 0.91 1.05 1.57 C 1.0 688 428 52 47 38 9 11 12 0.96 1.07 1.59 D 2.0 695 435 52 47 38 4 9 9 0.97 1.10 1.62 2 0.3 692 438 53 48 39 11 14 17 0.95 1.05 1.32 E 1.0 705 455 54 48 40 8 10 11 0.95 1.11 1.38 F 2.0 711 460 54 49 40 3 8 9 0.94 1.12 1.40 2.5 - 685 430 53 48 40 10 11 14 0.96 1.18 1.30 G 3.0 - 682 428 54 50 44 8 9 10 0.95 1.15 1.28 H

In the results of the table, A is similar to the conventional STD61 (target basis), B has insufficient strength below the lower limit, and C, D, E, F, and G are Mo content target intervals (1.5 to 2.5%), and C deteriorates toughness. Selects tungsten content that is not large, D excludes the toughness decrease compared to the strength increase, E selects the tungsten content that does not have the toughness decrease, F excludes the toughness decrease compared to the increase in strength, G is the upper limit of strength increase and toughness It corresponds.

And, H has a toughness deterioration problem above the upper limit.

4 and 5 are evaluation results for strength according to the Mo content.

As shown in the figure, as a result of evaluating the high temperature strength according to the Mo content, the strength increases as the content of Mo increases, but it can be seen that there is no effect of increasing the strength at 2.5% or more.

And, referring to the results of the strength according to the W content in FIG. 6, the high temperature strength increases according to the content of W, but the effect of the increase in strength is insignificant even when 0.3% or more is added.

Next, FIGS. 7 to 9 are hardness evaluation results according to the Mo content.

The change in hardness according to the Mo content is effective at 1.5% or more, and at 2.5% or more, the effect is insignificant.

The higher the hardness at each tempering temperature, the better the quenchability, so it is advantageous to secure a hardness of HRC 50 or higher during heat treatment of the forged mold steel.

In addition, FIGS. 10 and 11 are results of hardness evaluation according to the W content.

The change in hardness according to the W content shows its effect at 0.01% or more, and when it exceeds 0.3%, the effect is insignificant.

12 is a result of toughness evaluation according to the Mo content, and FIGS. 13 and 14 are results of toughness evaluation according to the W content.

Toughness decreases as the Mo content increases, and toughness increases as the tempering temperature increases. The maximum value of the Mo content is limited to properties that can satisfy both strength and toughness.

In addition, it can be seen that as the W content increases, the toughness decreases, and when the W content is 1.0% or more, the effect of improving the toughness decreases even when the tempering temperature is increased. Therefore, W is limited to a maximum of 0.3%, and beyond that, it is difficult to secure toughness.

Next, FIG. 15 shows the change in the amount of wear according to the Mo content, and FIGS. 16 and 17 show the change in the amount of wear according to the W content.

As a result of evaluation of the amount of wear according to the Mo content, the effect of abrasion resistance appears when the Mo content is added more than 1.5%, the wear amount is the smallest at 1.5% under the tempering condition of 600°C or less, and the effect is good when added at least 2.0% at 625°C. Able to know.

In addition, as a result of the evaluation of the amount of wear according to the W content, as the content of W increases, the amount of wear increases, thereby exhibiting poor wear resistance.

Therefore, it is desirable to reduce the W content to a minimum.

The following Figure 18 shows the effect of tensile strength and toughness by the addition of Mo and W.

The high temperature strength increases with the addition of Mo, but the toughness tends to drop rapidly.

Since the mold life is increased only by increasing the high temperature tensile strength, the high temperature tensile strength should be higher than 650 MPa, and the toughness should be secured at least 10J so as not to cause brittle fracture in the overload received from the mold.

Therefore, it can be seen that the content of Mo is appropriately 1.5% to 2.5%, and W must be added at 0.3% or less to ensure the maximum toughness.

The mold steel according to the present invention having a composition and content proved through the above experiments can be produced through a tempering heat treatment by quenching and tertiary as shown in FIG. 19 as an example, and according to the present invention manufactured as described above. The mold steel has improved durability compared to the conventional mold steel, so that the life of the mold can be further extended.

Although the present invention as described above has been described with reference to the exemplified drawings, it is not limited to the described embodiments, and it can be modified and modified in various ways without departing from the spirit and scope of the present invention. It is obvious to those who have it. Therefore, such modifications or variations will have to belong to the claims of the present invention, the scope of the present invention should be interpreted based on the appended claims.

Claims (5)

In weight percent, carbon (C) 0.40 to 0.50%, silicon (Si) 0.8 to 1.2%, manganese (Mn) 0.3 to 0.6%, chromium (Cr) 4.8 to 5.5%, molybdenum (Mo) 1.5 to 2.5%, vanadium (V) 0.8-1.2%, tungsten (W) 0.01-0.3%, containing the balance Fe and other inevitable impurities,
Hot forged mold steel. The method according to claim 1,
The microstructure comprises bainite and martensite,
Hot forged mold steel. The method according to claim 1,
The mold steel is [Mo] + [W] value satisfies the range of 1.5 to 2.8,
Hot forged mold steel. The method according to claim 1,
The mold steel is [V] / [C] value satisfies the range of 1.6 to 3.0,
Hot forged mold steel. The method according to claim 1,
The mold steel is characterized in that the [V] / [C] value is 2.7,
Hot forged mold steel.

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