JP2952245B2 - Tool steel for hot working - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot forging die, an aluminum die casting die, an aluminum extrusion die and the like, which is used for various kinds of hot die applications. , It does not cause cracks due to the action of mechanical stress, it can obtain a long life, and it is hard to crack, so it can be used with increased hardness,
The present invention relates to a tool steel material for hot working having a high level of toughness and ductility, which is capable of obtaining an excellent wear life, and has isotropy with little orientation. 2. Description of the Related Art In recent years, the shape of a mold has become complicated and large, the cooling from the mold surface has been severe in order to increase molding efficiency, and the sharp corner of the mold has been increased early in order to increase the forging accuracy. The problem of cracking has been raised, and the improvement of forging accuracy has led to an increase in the number of cases in which the mold reaches the end of its service life at an early stage because the product dimensions and shape become poor at the stage of slight sagging and wear of the mold surface. in this case,
Consideration was given to increasing hardness to prevent early wear and abrasion, but this resulted in early large cracks. [0003] In the case of a conventional tool steel for hot working, a toughness value in the case where cracks are generated, propagate, or break along a fiber during hot working of the material, that is, a toughness value in a direction perpendicular to the forging direction ( (T-direction toughness value) is lower than the toughness value in the case where a crack propagates and breaks in a direction perpendicular to the fiber, that is, the toughness value in the forging direction (L-direction toughness value).
(Toughness value in direction / Ltoughness value in L direction = 0.6, etc.) Therefore, fracture is likely to progress along the fiber direction, and improvement of toughness and ductility in the T direction of the material has been the most important issue for improving the life. [0004] In the case of a conventional steel material, the toughness value in the direction parallel to the forging direction (L direction toughness value) is:
As described above, the level of toughness and ductility (T-direction toughness value) of the sample in the perpendicular direction is usually clearly lower, for example, 60% of that of the sample in the parallel direction, as described above. In many cases, the ductility was affected by the level of ductility and the ductility in the right angle direction. The cause of the difference is that non-metallic inclusions elongated in the forging direction and dense inclusions are liable to cause delamination failure, and therefore cracks are generated and propagated along the fiber direction, In addition, when the segregation concentration of the stripe-like segregation extending in the forging direction is high, the stripe width is wide, and the fibers are arranged with strong directionality in the fiber direction, the cracks proceed linearly along the stripe-like segregation. This was the main reason that the toughness in the perpendicular direction was reduced. In the present invention, the amount and size of sulfide-based inclusions, which are particularly easy to extend in the forging direction, are reduced to the utmost, and both silicate-based and oxide-based inclusions are extremely small. Efficiently obtain ultra-clean steel with reduced diameter, further reduce micro-segregation by appropriate diffusion soaking, control the shape of non-metallic inclusions by hot working with appropriate management of elongation coefficient, etc. To increase the toughness level in both the forging direction and the perpendicular direction, and to increase the toughness value in the perpendicular direction to a level equivalent to or similar to that in the parallel direction (isotropic). Regarding the melting and agglomeration methods, large quantities such as electric furnace refining-out-of-furnace refining etc. are used regardless of the special methods that increase the cost and reduce the efficiency such as vacuum remelting and consumable electrode type remelting. The solution was made in the production system. [0006] That is, the first invention of the present invention is a C:
0.10 to 0.70%, Si: 0.10 to 2.00%,
Mn ≦ 2.00%, Cr ≦ 7.00%, W or Mo alone or in combination (1 / 2W + Mo): 0.20 to 1
2.00%, V ≦ 3.00%, S: 0.005%
, O is less than 30 ppm, and the balance is substantially Fe
And the cleanliness of nonmetallic inclusions present in the steel is JIS dA60 × 400 ≦ 0.010%, d
(B + C) 60 × 400 ≦ 0.020% is a tool steel for hot working. The steel of the first invention has a T-direction toughness value / L, which is a ratio of a toughness value in the forging direction (L-direction toughness value) to a toughness value in a direction perpendicular thereto (T-direction toughness value).
It is preferable that the directional toughness is more than 0.70 and isotropic.
Further, in the range of the steel of the first invention, S is less than 0.003%, and cleanliness of nonmetallic inclusions is JIS dA60 × 400.
≦ 0.005%, T direction toughness value / L direction toughness value is 0.8
5 or more isotropic. Further, in the second invention, C: 0.10 by weight ratio.
0.70%, Si: 0.10-2.00%, Mn ≦
2.00%, Cr ≦ 7.00%, alone or in combination of W and Mo (１ ／ W + Mo): 0.20 to 12.00
%, V ≦ 3.00%, Ni ≦ 4.00%,
Co ≦ 6.50%, N ≦ 0.20% or more, furthermore, S: less than 0.005%, O is less than 30 ppm, the balance is substantially composed of Fe, and present in steel Cleanliness of non-metallic inclusions is JIS dA60 × 400 ≦
0.010%, d (B + C) 60 × 400 ≦ 0.020
% Of tool steel for hot working. The steel of the second invention has a toughness value in the forging direction (L-direction toughness value).
Which is the ratio of the toughness value in the direction perpendicular to the direction (T direction toughness value)
It is preferable that the directional toughness value / L-direction toughness value is more than 0.70 and isotropic. Further, in the scope of the second invention, S is equal to 0.
Less than 003%, cleanliness of non-metallic inclusions is JIS dA6
0 × 400 ≦ 0.005%, and it is preferable that the T-direction toughness value / L-direction toughness value be 0.85 or more isotropic. The compositions of the steel of the first invention and the steel of the second invention contain 0.50% or less of a special carbide-forming element Nb, Ti or the like alone or in combination, and precipitation-enhancing elements Cu, B by intermetallic compound formation. , Al, Be, etc., alone or in combination, may contain various additive elements such as 3.00% or less. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The role of various elements necessary for the hot working tool steel of the present invention will be described below. C forms a solid solution in the matrix at the time of quenching and heating to provide a necessary quenching hardness, and at the time of tempering, forms and precipitates a special carbide with a special carbide-forming element, giving softening resistance and high-temperature strength in tempering, In addition, it has an effect of forming residual carbides to impart wear resistance at high temperatures and preventing crystal grains from being coarsened during quenching and heating, and is an indispensable important element. If it is too large, the amount of carbides excessively increases, the required toughness as a hot tool cannot be maintained, and the high-temperature strength is reduced, so that the content is set to 0.70% or less. If it is too low, the effect of the above addition cannot be obtained. Therefore, it is set to 0.10% or more. [0010] Si is generally required to be used as a deoxidizing element in production, and has an oxidation resistance or 500 to 60 depending on the use.
0 ℃ enhanced temper softening resistance below and amount object, the application for increasing the A ₁ transformation point is adjusted. If the content is too large, the toughness is reduced and the thermal conductivity is excessively reduced. Mn
Has a great effect of improving the hardenability by forming a solid solution in the matrix. Mn
In order to obtain the above effect, the amount of addition is adjusted depending on the purpose and application. Too much and too high annealed hardness, it lowers the machinability, also to 2.00% or less because excessively lower the A ₁ transformation point. [0011] Cr is the most important element for providing the hardenability required as a tool. Also, increase in oxidation resistance and the A ₁ transformation point, also to form a residual carbides suppressing grain coarsening during heating for quenching, also enhance the abrasion resistance,
It is added for the purpose of precipitating a special carbide during tempering to improve the softening resistance at the time of raising the temperature and giving an effect such as increasing the high-temperature strength. If the content is too large, Cr carbides are excessively formed and the high-temperature strength is reduced, so the content is set to 7.00% or less.
In some cases, it is not added, but in order to obtain the effect of the above addition, it is generally preferable to contain 0.70% or more. W and Mo form special carbides. The formation of residual carbides prevents the coarsening of the structure during quenching and heating. Precipitates fine special carbides during tempering to reduce tempering softening resistance and high-temperature strength. It is the most important additive element to increase. Also it has the effect of increasing the A ₁ transformation point.
W is particularly effective in increasing high temperature strength and wear resistance.
On the other hand, Mo is more advantageous than W in terms of toughness. If the content is too large, coarse carbides are formed and the toughness is excessively reduced, so that the content of W and Mo alone or in combination (1 / 2W + Mo) is set to 12.00% or less. If the content is too low, the effect of the above addition is insufficient. 20% or more. V is a strong carbide-forming element, forms a residual carbide, has a large effect of refining the crystal grains, and gives an improvement in wear resistance at high temperatures. At the time of tempering, fine carbides are precipitated in the matrix, and co-added with W and Mo.
Effect of increasing the strength at from 00 to 650 ° C. or higher high-temperature range is large and gives an effect of enhancing A ₁ transformation point. V is added to obtain the above effect, but if it is too much, coarse carbides are formed and the toughness is reduced. In some cases, it is not added, but in order to obtain the effect of the above addition, it is generally preferable to contain 0.05% or more. Ni is added depending on the purpose and application in order to increase the toughness by solid solution in the matrix and to enhance the hardenability. Too much and too high annealed hardness, lowers the machinability, also since lead to excessive decrease of A ₁ transformation point 4.00
% Or less. Co has a function of increasing the high-temperature strength by forming a solid solution in the matrix. In addition, the solid solubility limit of carbides in austenite during quenching and heating is increased, the amount of special carbides precipitated during tempering is increased, and the agglomeration resistance of precipitated carbides during heating is increased. It has the effect of improving the characteristics. In addition, by raising the temperature during use of the tool, a dense adhesive oxide film is formed on the surface, which has the effect of increasing wear resistance and seizure resistance at high temperatures. Co is added depending on the purpose and application for the above purpose, but if too much, the toughness is reduced, so Co is set to 6.50% or less. N forms a solid solution in the matrix and carbides to refine the crystal grains and enhance toughness. Also, even in the case of low C as an austenite former, ferrite remains during quenching heating and is excellent in toughness. This enables a combination of alloy compositions. N is added depending on the purpose and application in order to obtain the above effects, but is limited to 0.20% or less because there is a limit amount that can be added within the range of the alloy composition of the hot work tool steel such as Cr. Nb and Ti are strong carbide-forming elements, and have the effect of increasing softening resistance and high-temperature strength in a high-temperature region of 650 ° C. or more by precipitation of fine carbides, which have particularly large agglomeration resistance during grain refinement and tempering. is there. It is added depending on the purpose and application to obtain the above effects. If the content is too large, coarse carbides which hardly form a solid solution are formed, and the toughness is reduced. Cu, B,
Al and Be form an intermetallic compound and have a precipitation effect, and have an effect of improving softening resistance at the time of temperature rise and high-temperature strength. If the content is too large, the toughness is reduced. EXAMPLES Table 1 shows the compositions of the steels of the present invention, comparative steels and conventional steels having a composition equivalent to SKD61 of JIS and the cleanliness of nonmetallic inclusions. FIG. 1 shows the relationship between the S content and the cleanliness of nonmetallic inclusions according to the JIS method, the plane strain fracture toughness value KIC in the forging direction (L direction) and the direction perpendicular to the direction (T direction) in the SKD61 composition steel material for molds. An experimental example will be described. The forging ratio in this case is 15 (the elongation coefficient is 6.5). While the S content decreases from 0.014 to 0.006%, the amount and size of the sulfide-based inclusions gradually decrease, and the KIC gradually increases with the decrease. However, particularly when the S content is less than 0.005%, When the S amount is less than 0.003%, the KIC value in the T direction sharply increases, and L, T
It can be seen that the difference between the directions decreases sharply. It has been conventionally pointed out that the toughness value in the T direction TP increases due to the decrease in the S content, and that the toughness value approaches the direction in the L direction. However, as a result of detailed research and investigation by the present inventors, the hot work In machining tool steel, there is a special point where the effect increases remarkably when the S content is less than 0.005%, especially around 0.003%, and the toughness value in the T direction sharply increases when the S content is less than this. And obtained excellent properties far beyond expectations as various types of hot dies. [Table 1] FIG. 2 shows the heat treatment (quenching, tempering) hardness H
The relationship between S: 0.002% of the steel of the present invention and S: 0.014% of the conventional steel of SKD61 equivalent steel of RC45 of SKD61 and the KIC values in the L and T directions are shown.
In this case, upsetting is performed before moving to forging, and the total forging ratio is 0 to 50. In this result, S:
In the conventional steel of 0.014%, the toughness value of the specimen in the T direction increases when the elongation coefficient is 2 or more, and the toughness value becomes maximum around the elongation coefficient of 4 to 6, but the KIC value in the L direction is larger. It is only about 60% (the ratio of T-direction toughness value / L-direction toughness value is about 0.6), and tends to decrease when the elongation coefficient is around 10 or more. On the other hand, the toughness value of the sample in the T direction of the steel of the present invention having S: 0.002% increases greatly near the elongation coefficient of 2 as compared with the case of the conventional material, and reaches a maximum around 4 to 10;
The value is clearly higher than the L direction as well as the T direction of the conventional steel of S: 0.014%.
The KIC value is 90% or more of the C value (the ratio of T direction toughness value / L direction toughness value is 0.85 or more). In addition, the decrease in the KIC value of the sample in the T direction due to the increase in the elongation coefficient is unlikely to occur even when compared with the conventional material, and the decrease in the KIC value in the T direction TP is small even at around the elongation coefficient of 20. That is, the forging ratio is 1.5 or more (however, the elongation coefficient is 1 to 20), preferably 4 or more (however, the elongation coefficient is 4 to 10). FIG. 3 shows the heat treatment (quenching, tempering) hardness H
Ingot of steel of the present invention having an SKD61 composition of S: 0.002% with RC45, a stage of soaking and a forging forming ratio of 2.3 (forging factor 1) when the forging ratio is 5.0 and the forging ratio is 12.0. 5 shows the effect of improving the Charpy impact value in the T direction after the forging finish when the billet soaking treatment is performed. The soaking temperature in this case is 1200 ° C. or higher. By reducing micro-segregation during solidification by soaking treatment, the ratio of the Charpy impact value in the T direction / Charpy impact value in the L direction is 0.88 without soaking, whereas the ratio of the steel ingot soaked is 0. It was recognized that the Charpy impact value was improved by applying soaking at 0.92 to that obtained by soaking with a slab of 0.92. In order to obtain the steel of the present invention, the oxidation refining and the reduction refining are advanced in advance in an electric furnace to reduce the amount of [O] in the molten steel to 100 ppm or less. It is effective to proceed. At this time, slag
In order to efficiently advance desulfurization by molten steel reaction, desulfurization is advanced to an extremely low level of S: less than 0.005% in a short period of time by electromagnetic refining external furnace refining, and at the same time, molten steel is injected by injecting Ar from below. It is more effective to further reduce the amount of [O] therein to less than 30 ppm and further accelerate the desulfurization effect. As shown in Table 1 above, in the steel of the present invention, S is less than 0.005% and O is less than 30 ppm, desirably, S is less than 0.003% and O is 30 ppm.
Less than the conventional steel. The cleanliness of non-metallic inclusions in steel is measured in accordance with JIS dA
60 × 400 ≦ 0.010%, d (B + C) 60 × 40
0 ≦ 0.020%, desirably dA60 × 400
At ≦ 0.005%, the amount and size of sulfide-based inclusions and oxide-based inclusions are extremely reduced as compared with conventional steel. FIG. 4 shows a heat treatment (quenching, tempering) hardness H
The RC44 shows the impact transition characteristics of the steel material of the present invention having a composition of SKD61 of S: 0.002% and the conventional steel material of a composition of SKD61 of 0.014% by S in a T-direction test piece. The test piece was a JIS V notch Charpy test piece, which was tested at 20 to 300 ° C., and the change in absorbed energy at break was examined.
The forging ratio of the material is 12.5, and the elongation coefficient is 5.0. In the case of the conventional steel material of S: 0.014%, the 50% brittle fracture transition temperature is 50 to 100 ° C, and the absorbed energy is increased with respect to the test temperature. However, the degree of the increase is small in a temperature range exceeding 100 ° C. . On the other hand, in the case of the steel material of the present invention, the 50% brittle fracture transition temperature is similarly 50 to 100.
° C, but the increase in absorbed energy with increasing test temperature is clearly large. For this reason, in the case of the mold using the steel material of the present invention, the impact absorption energy due to the preheating of the mold can be increased, and it is recognized that the effect of reducing cracking is remarkably greater than that of the conventional steel material. Table 2 shows that 0.52% C-0.21% Si-0.85% Mn-1.
SK of 65% Ni-1.03% Cr-0.40% Mo-0.16% V-balFe
T4, 0.40% C-0.22% Si-0.34% Mn-4.36% Cr-4.35%
SKD8 of W-0.35% Mo-1.98% V-4.30% Co-balFe,
0.19% C-0.25% Si-0.60% Mn-3.32% Ni-3.42% Mo-ba
1Fe 3Ni-3Mo, 0.31% C-0.33% Si-0.65% M
n-10.25% Cr-1.58% Mo-0.97% V-balFe 10Cr
With respect to several types of -Mo-V-N type hot work tool steels, the plane strain fracture toughness values in the L direction and the T direction and the ratio of the T direction toughness value to the L direction toughness value of the steel material of the present invention and the conventional steel material are shown. In the case of the conventional steel material, the toughness in the T direction is low and the ratio of the toughness value in the T direction / the toughness value in the L direction is less than 0.70, whereas the toughness in the T direction of the steel material of the present invention is remarkably excellent. It can be seen that the material has excellent isotropy of 0.85 or more in which the ratio of the direction toughness value / L direction toughness value far exceeds 0.70. [Table 2] Table 3 shows a comparative example of die life when the steel material of the present invention and the conventional material are used for a hot press forging die. [Table 3] The use of the steel material of the present invention slows the occurrence of cracks, makes it difficult to progress, and does not cause large cracks, so that the mold life is twice as long as that of the conventional material, stabilization is achieved, and practical performance is greatly improved. It became clear that it would be. Also SKD
Also in the aluminum die casting mold using the steel material of the present invention having 61 composition and the hot hammer mold using the steel material of the present invention having the SKT4 composition, the life is 2-3 times longer than that of the conventional material. . As described above, the tool steel for hot working of the present invention has a high level of toughness and ductility, and has a low
Since it has the isotropy with little difference in directional characteristics, it does not cause early large cracks, slow crack generation, and does not easily progress in various applied hot dies, so the long life and stabilization of the mold can be achieved. Can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS [FIG. 1] S content, sulfide inclusion area ratio, plane strain fracture toughness K in the forging direction (L direction) and its perpendicular direction (T direction)
FIG. 3 is a diagram illustrating a relationship with an IC. FIG. 2 is a diagram showing the relationship between the elongation coefficient and the Charpy impact value. FIG. 3 is a diagram showing the influence of soaking on the Charpy impact value. FIG. 4 is a diagram showing an impact transition characteristic in a T direction.

Continuation of the front page (56) References JP-A-60-59053 (JP, A) JP-A-58-117863 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38 / 00-38/60 C21D 6/00

Claims (1)

(57) [Claims] C: 0.10 to 0.70% by weight, Si: 0.1
0-2.00%, Mn ≦ 2.00%, Cr ≦ 7.00
%, W and Mo alone or in combination (1 / 2W + M
o): 0.20 to 12.00%, V ≦ 3.00%, S: less than 0.005%, O is less than 30 ppm,
The balance has a composition substantially consisting of Fe, and the cleanliness of nonmetallic inclusions present in the steel is JIS dA60 × 400 ≦
0.010%, d (B + C) 60 × 400 ≦ 0.020
% Tool steel for hot working. 2. C: 0.10 to 0.70% by weight, Si: 0.1
0-2.00%, Mn ≦ 2.00%, Cr ≦ 7.00
%, W and Mo alone or in combination (1 / 2W + M
o): contains 0.20 to 12.00%, V ≦ 3.00%, further Ni ≦ 4.00%, Co ≦ 6.50%, N ≦
0.20% or more, S: less than 0.005%,
O is less than 30 ppm, the balance is substantially composed of Fe, and the cleanliness of the nonmetallic inclusions present in the steel is JIS dA60 × 400 ≦ 0.010%, d (B +
C) Tool steel for hot working, wherein 60 × 400 ≦ 0.020%. 3. The tool steel for hot working according to claim 1, wherein the tool steel is subjected to a soaking treatment.