JP2960496B2 - Cold tool steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a cold tool steel suitable for a forging die material mainly used in a cold state.

[Conventional technology]

Conventionally, high C-high Cr steels such as JIS SKD11 have been mainly used for mold materials for cold forging, but for applications requiring more impact resistance, high-speed tool steel The use of mold materials has improved the mold life and expanded the application of cold forging. JIS SKH51 is generally used as a high-speed tool steel-based mold material, but for more severe applications, Japanese Patent Publication No. 42-20609 and Japanese Patent Publication No. 50-108
08, JP-B 55-49148, JP-B 57-24063, JP-B
Low alloy high speed tool steels as disclosed in JP-A-62-28503 and JP-A-1-159349 have been developed and used.

[Problems to be solved by the invention]

However, in recent years, as a result of difficult processing of a work material and complicated and refined forged shapes, a sufficient mold life cannot be obtained with a conventional high-speed tool steel-based material in many cases. The main cause is the toughness of the mold material, especially the direction perpendicular to the direction (T
Direction) due to insufficient toughness, cracking, chipping, and the like. To improve this, a material with higher toughness is desired. On the other hand, to improve wear resistance and fatigue strength, a material having high hardness is required.

An object of the present invention is to provide a cold work tool steel having a high hardness of HRC 61 to 64 and having higher toughness in the L direction and the T direction than conventional steel, which is optimal for a cold forging die. .

[Means for solving the problem]

The present inventors have conducted the following studies in order to obtain a material having high hardness and high toughness.

First, in order to improve toughness, reduction of huge primary carbides,
The reduction of the stripe structure of the carbide, the refinement of the structure, and the like were studied. Furthermore, in order to obtain high toughness and high hardness, not only the individual and the amount of each element but also the interaction between each element were examined in detail.As a result, by limiting the amount of the added element to a predetermined narrow range, The inventors have found that it is possible to obtain a steel having high toughness in a high hardness region (HRC 61 to 64), which cannot be obtained with steel, and have completed the present invention.

That is, the first invention of the present invention is C.O.
5% or more and less than 0.7%, Si 0.5-1.5%, Mn 1.5% or less, Cr
3.5-6.5%, one or two of W and Mo at 1 / 2W + Mo
2.0-3.5%, V 0.8-1.5%, Nb 0.05-0.20%, balance Fe
A cold tool steel comprising N and 300 ppm or less of P and 0.02% or less of the inevitable impurities.
With 0.5% or more and less than 0.7% of C, more than 0.6% of Si and less than 1.0%, M
n 1.5% or less, Cr 3.5 to 6.5%, one or two of W and Mo at 1 / 2W + Mo 2.0 to 3.5%, V over 1.0% and 1.5% or less, Nb 0.05% or more and less than 0.15%, balance Fe and It has a composition consisting of unavoidable impurities, and among the unavoidable impurities, N
Is 300 ppm or less, and P is 0.02% or less.
The fourth invention is the cold tool steel according to any one of the first to third inventions, which contains 1.5% or less of Ni, and the fifth invention Is less than 5%
The cold tool steel according to any one of the first to fourth inventions containing Co, wherein the sixth invention is characterized in that S in the inevitable impurities has a content of 0.1%.
The cold tool steel according to any one of the first to fifth inventions, wherein 005% or less and O is 30 ppm or less.

[Action]

 The reasons for limiting the components of the present invention will be described.

C combines with carbide forming elements such as Cr, W, Mo, V, and Nb to form a hard primary carbide, and improves wear resistance. Further, in high-temperature tempering, the height is increased by combining with secondary hardening elements such as Mo, W, V, and Nb and precipitating as secondary carbide. In addition, a part is dissolved in the base to strengthen the base. To obtain hardness of HRC61 or more
0.5% or more is necessary, but if added excessively, the amount of carbides increases and the toughness decreases. In the steel of the present invention, the upper limit is set to less than 0.7% from the viewpoint of emphasizing toughness. Therefore C
The amount was limited to a range of 0.5% or more and less than 0.7%.

Si has an effect of increasing the hardness of the matrix by forming a solid solution in the matrix, but if added excessively, it lowers the toughness. In a hot working steel used at a hardness of 60 or less in HRC, the amount of Si is limited to 0.6% or less in consideration of toughness as in the steel described in JP-A-2-8347. On the other hand, in the steels disclosed in JP-B-55-49148 and JP-B-57-24063, in order to improve the hardness by Si, Si generally exceeds 1.0% and is added mainly in the range of 1.4 to 1.5%. ing.

In order to obtain a steel with high hardness and toughness as an object of the present invention, it is necessary to optimize the amount of Si in relation to the amounts of other elements. That is, as will be described later, the hardness can be improved by the secondary effect of increasing W, Mo, etc., but it is desirable to minimize the addition of W, Mo to secure toughness. It is effective to add a predetermined amount to improve the height of the base itself.

In the component range of the present invention, 0.5% or more of Si is required in order to obtain a hardness of HRC 61 or more, but if it exceeds 1.5%, the toughness is greatly reduced, so the amount of Si is limited to 0.5 to 1.5%.

Further, according to the result of the detailed examination, the contribution of the Si content to the hardness tends to saturate when it exceeds 1.0%. Therefore, when the toughness is particularly considered, the Si content exceeds 0.6% and is 1.0% or less. It is desirable that

Mn is usually added as a deoxidizing agent, but is also an effective element for improving hardenability. However, excessive addition impairs hot workability, so the content was limited to 1.5% or less.

Cr improves hardenability and combines with C to form a carbide, thereby improving wear resistance. To obtain this effect, 3.5% or more is required. However, if it exceeds 6.5%, giant carbides and stripe-like segregation of carbides are generated, and the toughness is reduced. Therefore, the Cr content was limited to 3.5-6.5%.

W and Mo can be added alone or in combination to form a primary carbide in combination with C to improve wear resistance, and to precipitate a fine secondary carbide during tempering to have a strong secondary effect. Element. Since the atomic weight of W is about twice as large as that of Mo, one or more of W and Mo are converted to Mo equivalent (1 / 2W + M
o). If the Mo equivalent is less than 2.0%, sufficient hardness cannot be obtained, while if it exceeds 3.5%, the amount of carbides becomes excessive and the toughness is reduced due to the distribution in the form of stripes. Therefore, 1 / 2W + Mo is limited to the range of 2.0 to 3.5%.

V forms a primary carbide at the time of solidification in combination with C to improve wear resistance and to improve toughness by making crystal grains fine. Also, because it is a secondary effect element,
It is effective for increasing the hardness by high temperature tempering. V amount is 0.8
If it is less than 1.5%, the above effect cannot be obtained, and if it exceeds 1.5%, the amount of carbides becomes excessive, and the toughness is reduced due to the distribution in the form of stripes. Therefore, the amount of V was limited to the range of 0.8 to 1.5%.

Further, as described later, in order to prevent excessive formation of NbC due to the addition of Nb, the amount of V is desirably more than 1.0% and 1.5% or less.

Nb is an important additive element in the present invention, and is an element that greatly contributes to improvement in toughness. That is, Nb affects the crystallization morphology of carbides during solidification, and forms primary carbides that are fine and hardly form a solid solution.

In order to obtain the high hardness intended in the present invention, it is necessary to increase the quenching heating temperature. At that time, since the carbide prevents the crystal grains from becoming coarse, high toughness can be obtained. .

However, if the primary carbide (NbC) of Nb alone becomes too large, the carbide is distributed in a striped manner, so that the toughness rapidly decreases. Since the crystallization form of the primary carbides is affected not only by the amount of Nb but also by the amounts of C and other carbide-forming elements, the optimum amount of Nb needs to be determined in relation to the amounts of other elements.

That is, C is a relatively high and carbide forming element W,
When Mo and V are relatively low, C and Nb combine to form Nb
A single primary carbide is likely to be striped, which is a factor in reducing toughness. For example, in the steel described in JP-B-57-24063, since the V content is low and the Nb content is high, the steel described in Japanese Patent Application Laid-Open No. 1-159349 has a low C and Nb content. Is high, it is considered that both increase NbC and are distributed in a striped form, thereby reducing toughness.

In the component range of the present invention, in order to obtain the above effect of Nb addition, the amount of Nb must be at least 0.05%, but if it exceeds 0.20%, the toughness is reduced. Therefore, the Nb amount is 0.
Limited to 05-0.20%.

Also, in order to optimize the carbide form and obtain the effect of improving toughness, the Nb content is desirably in the range of 0.05% or more and less than 015%, and more desirably 0.05% or more and less than 0.10%.

N is added in the steel described in JP-B-54-24063 in order to reduce the amount of C added and to prevent austenite crystal grains from becoming coarse. However, according to the study of the present inventors, the crystallization temperature of MC-type carbide crystallized at the time of solidification increases due to an increase in the amount of N, so that the MC-type carbide coarsens, which lowers the toughness particularly in the T direction. Tend. Therefore, in the present invention for improving the toughness in the T direction, it is necessary to regulate N to a low level of 300 ppm or less.

Ni is an element that forms a solid solution in the matrix, has an effect of improving toughness, and imparts hardenability. However, if added excessively, the annealing hardness becomes excessively high and the workability is deteriorated.

Co is a solid solution in the matrix and is an element effective for improving heat resistance and seizure resistance. However, if added excessively, the toughness is reduced. Therefore, when added, the content is limited to 5% or less.

P, S, and O are usually contained in trace amounts as impurity elements. P
Segregates at the crystal grain boundaries (micro-segregation), not only lowering the grain boundary strength, but also promotes matrix segregation (macro-segregation) during solidification and causes the directionality of the material.

S and O mainly exist in steel as nonmetallic inclusions, and adversely affect fatigue strength and the like. Therefore, the toughness is improved by reducing the amounts of these impurity elements. In the case of the steel of the present invention used for high hardness, P 0.02% or less, S 0.
Since an improvement effect was obtained when 005% or less and O 30 ppm or less were satisfied, it is desirable to reduce P, S and O below this level.

〔Example〕

 Hereinafter, the present invention will be described based on examples.

As the test steel, the steel of the present invention (No.
1, 3 to 7), comparative steels (No. 2, 8 to 11) and conventional steels (No. 12 to 19) were melted and forged, predetermined quenching and tempering treatments (1160 for all samples) Quenching at 560 ° C. and tempering twice at 560 ° C. for 1 hour). However, as for the samples No. 7 and No. 11, the quenching temperature was changed as shown in FIG. Samples for the Charpy impact test were collected from a direction equilibrium with the forging direction (L direction) and a direction perpendicular to the forging direction (T direction) to prepare a 10 mmR test piece.

Fig. 1 shows the effect of Nb addition on No. 7 and No. 11 samples, in which other alloy components were almost the same and only the Nb content was different, by examining the crystal grain size and hardness by changing the quenching temperature. It is.
In both samples, the hardness increases as the quenching temperature increases.
However, in the case of No. 11 sample without Nb, the quenching temperature was substantially reduced because the crystal grains became coarse and the toughness decreased as the quenching temperature increased.
The temperature is below 1140 ° C, so the hardness has to be used at maximum HRC61. On the other hand, the No. 7 sample to which Nb was added can be used in a state of high hardness because the crystal grains hardly become coarse even when the quenching temperature is increased.

Table 2 shows the hardness and the Charpy impact value of the steel of the present invention, the comparative steel, and the conventional steel when subjected to the standard heat treatment. The steels of the present invention all have a high hardness of HRC 61 to 64, and have a high Charpy impact value as compared with comparative steels and conventional steels. In particular, the characteristic is that the level of the Charpy impact value in the T direction is high. This is because the composition balance of the steel of the present invention, in particular, the optimization of the amount of carbides and the reduction of striped segregation by optimizing the amounts of C, W, Mo and V, the optimization of the balance between hardness and toughness by optimizing the amount of Si, This is due to the effect of refinement of crystal grains by adding Nb and improvement of toughness by reducing the amount of impurities.

A detailed comparison between the steels of the present invention (No. 1, 3 to 7) shows that
No. 1 has a slightly lower Charpy impact value because of a higher Si content than Nos. 3 to 6. Therefore, the upper limit of Si is 1.0%
It is desirable that In addition, No. 6 compared with other samples,
Since the amount of V is low, NbC is slightly excessive and the Charpy impact value is slightly low as described above, and the lower limit of V is preferably higher than 1.0%.

On the other hand, the comparative steels 8 and 9 have the same components as the steel of the present invention except for N, or have a large amount of N, and have a high level of impurity elements. I have.

Comparative steel 10 was intended to obtain a hardness equivalent to that of the steel of the present invention with a lower Si content and a higher Mo content instead, but the Charpy impact value was reduced, like
This indicates that the decrease in toughness due to the increase in Mo is large. Comparative Example 11 is an Nb-free material and has a low Charpy impact value due to coarsening of crystal grains.

Conventional steels 12 (SKH51) and 13, 14 have a hardness higher than HRC62, but have low toughness in both the L and T directions. This is C
Since the amount and 1 / 2W + Mo amount are too high, excessive carbides are formed and stripe-like segregation is caused, resulting in a decrease in toughness. Conventional steel 15 has a composition close to that of the steel of the present invention, but has a slightly lower hardness and a lower toughness level than the steel of the present invention. This is firstly because Si is relatively high. As described above, Si is necessary to secure hardness, but if it is excessive, it adversely affects toughness. Second, V is too high and does not contain Nb.

Conventional steel 16 also has a composition close to that of the steel of the present invention, but has a slightly lower hardness due to a slightly lower C content, and has a lower V
Since the amount is relatively high, NbC becomes excessive and N is contained as an essential element, so that the toughness value in the T direction is particularly low.

The conventional steels 17 and 18 have a higher C content than the steels of the present invention, and thus have a low level of toughness.

The conventional steel 19 is for a hot-working die and has a slightly different application from the steel of the present invention, but is described for comparison. This steel has a low Si content and has a high toughness level but a much lower hardness than the steel of the present invention.

Table 3 compares the seizure properties. The seizure resistance is determined by pressing the end face against the counterpart material (SCM415) while rotating a cylindrical sample at high speed, and calculating the maximum load (seizure critical load) at which seizure does not occur. Was. It can be seen that the steel of the present invention exhibits seizure resistance equal to or higher than that of SKH51, and particularly the samples to which Co is added (Nos. 4 and 6) have high seizure resistance.

[Effects of the Invention] As described above, the steel of the present invention has a higher hardness than the conventional cold tool steel, and also has a high toughness centering on a remarkable improvement in the toughness particularly in the T direction. Die for cold forging or complex shape, precision shape cold forging of processed material,
Alternatively, it can be used for other cold tools to obtain a long life.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the grain size and the hardness when the quenching temperature is changed for the steel of the present invention to which Nb is added and the comparative steel to which Nb is not added.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-117863 (JP, A) JP-A-61-213349 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (6)

(57) [Claims] C. 0.5% to less than 0.7% by weight of Si, 0.5% to less than 0.5% by weight.
1.5%, Mn 1.5% or less, Cr 3.5-6.5%, 1 of W and Mo
Species or 2 types at 1 / 2W + Mo 2.0-3.5%, V 0.8-1.5%,
A cold tool steel having a composition consisting of 0.05 to 0.20% Nb, the balance being Fe and unavoidable impurities, wherein, among the unavoidable impurities, N is 300 ppm or less and P is 0.02% or less. 2. A composition in which, by weight%, C is 0.5% or more and less than 0.7%, and Si is 0.6%.
1.0% or less, Mn 1.5% or less, Cr 3.5-6.5%, one or two of W and Mo at 1 / 2W + Mo 2.0-3.5%, V1.
0% to 1.5% or less, Nb 0.05% to less than 0.15%, balance F
A cold tool steel having a composition consisting of e and unavoidable impurities, wherein, among the unavoidable impurities, N is 300 ppm or less and P is 0.02% or less. 3. The method according to claim 2, wherein Nb is at least 0.05% and less than 0.1%.
A cold tool steel according to claim 1. 4. The cold work tool steel according to claim 1, which contains 1.5% or less of Ni. 5. The cold work tool steel according to claim 1, comprising 5% or less of Co. 6. S is 0.005% or less of unavoidable impurities,
The cold tool steel according to any one of claims 1 to 5, wherein O is 30 ppm or less.