JP3417922B2 - Cast iron for sizing press die - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast iron material used in a sizing press die of a sizing press device for sizing hot slabs and the like.

[0002]

2. Description of the Related Art Sizing press dies such as hot slabs are prone to heat cracks under the conditions of repeated rapid heating and rapid cooling, and as they are further used, cracks develop and chip off on the surface to be used. Occurs, and flaws may be printed on the product, limiting the period of use. Conventionally, as a material for the sizing press die, for example, a cast iron-based material such as alloy ductile cast iron, a cast steel-based material such as heat-resistant cast steel or high chromium cast steel, and a forged steel-based material have been used.

[0003]

However, although the materials of the cast steel and the forged steel are extremely excellent in toughness, there is a problem that they are inferior in crack resistance. On the other hand, the alloy ductile cast iron is a material that is also used for hot rolling rolls, has some crack resistance, toughness, and wear resistance, and is a promising material for sizing press dies. Although it is considered that there are different conditions of use between hot rolling and sizing, since sizing is used in more severe situations involving repeated rapid heating and rapid cooling, the occurrence of cracks also poses a problem, Usage is limited depending on progress. That is, as the characteristics required for the sizing press die, it is sufficient that the toughness has such strength that it does not break or break, and the wear resistance is not so necessary. It was a place where development was desired.

[0004] Therefore, an object of the present invention is to provide a cast iron material for a sizing press die, which has the characteristics of excellent crack resistance as a characteristic overcoming the drawbacks of the conventional materials for a sizing press die.

[0005]

In order to achieve the above object, the inventors of the present invention have conducted extensive studies, and as a result, in a use environment of rapid heating and rapid cooling in which a sizing press die is placed, cracks such as heat cracks and In order to sufficiently improve the resistance to crack propagation, that is, the crack resistance, it is necessary to: exist as much graphite as possible; at least 15% or more, preferably 20% or more in area ratio of graphite in the structure.・ In addition, use graphite with a particle size as large as possible ・ 50% of graphite with a particle size of 100 microns or more
As mentioned above, it has been found that the content is preferably 70% or more, and the above points are important. Graphite improves the thermal conductivity of the material and suppresses the occurrence of heat cracks. The larger the particle size of graphite, the more effective it is as a stop hole for cracks, which is more effective in preventing the progress of cracks. On the other hand, since cracks propagate along cementite, cementite deteriorates crack resistance.

Based on the above concept, the present invention having the following component composition and structure was completed. That is, the cast iron material for a sizing press die of the present invention has a composition of wt%.
And C: 3.2 to 4.2%, Si: 1.0 to 2.5%,
Mn: 0.1 to 0.6%, P: 0.10% or less, S:
0.02% or less, Cr: 0.15% or less, Mg: 0.0
4 to 0.10% and the balance is substantially Fe
Is a ductile cast iron material consisting of 15% or more of graphite crystallized in the structure in an area ratio, and the grain size of which is 1
The first feature is that graphite having a size of 00 microns or more occupies 50% or more. Further, the cast iron material for a sizing press die of the present invention has, in addition to the above-mentioned first feature, a further composition in weight%, Ni: 1.0 to 2.5%, Mo:
The second feature of the ductile cast iron material containing 0.2 to 0.8% is that a compressive residual stress is applied to the use layer by bainizing the matrix structure of the use layer.

According to the first feature, the crack resistance is greatly improved as compared with the conventional materials for sizing press molds. Therefore, it is possible to extend the interval period for cutting the sizing press mold, and also reduce the amount of cutting when cutting the mold. Therefore, the number of man-hours for changing the size of the sizing press die can be significantly reduced, and the life of the sizing press die can be expected to be improved. Regarding the component composition and the structure constitution shown in the above-mentioned first characteristic, the heat treatment required in the case of the second characteristic described later is basically unnecessary. The crystallized area ratio of graphite is preferably 20% or more. Similarly, the particle size of graphite is preferably 100% or more and occupies 70% or more. According to a second aspect of the present invention, the above first
In addition to the action and effect due to the feature of N, N having the same graphite structure as the first feature, and N which is an element that does not generate carbides
When i and Mo are contained and heat treatment is performed so that the use surface (use layer) is hardened, the matrix structure of the use layer becomes bainite, and the transformation expansion thereof imparts compressive residual stress. This compressive residual stress acts as resistance to crack propagation, and crack resistance is further improved.

With respect to the cast iron material for a sizing press die of the present invention, the reasons for limiting the content range of each component element in the component composition will be described below. In addition, all component compositions are shown by weight%.

The content of C is 3.2 to 4.2%. A large amount of graphite is contained, and more than the usual ductile component is contained in order to crystallize coarse graphite. The C content is more preferably set to 3.6 to 4.0%.

The Si content is 1.0 to 2.5%.
Si is a graphitization promoting element. If it is too small, the amount of crystallized graphite will decrease, so the lower limit is made 1.0%. If too much, it becomes brittle against heat. Therefore, in the present invention, it is set slightly lower than the normal ductile component. However, in order to crystallize a large amount of graphite and coarse graphite, a hypereutectic component (C + Si / 3
= 4.3 or more) is desirable. The Si content is more preferably 1.4 to 2.0%.

The Mn content is 0.1 to 0.6%.
Mn is a carbide-forming element. If it is contained in a large amount, the material may become hard and brittle and cementite may be formed. Therefore, it is preferable that the content is as small as possible. However, since MnS is formed to prevent the damage of S and also has a deoxidizing action, it is added in a small amount. The Mn content is more preferably 0.20 to 0.
40% is preferable.

The P content is 0.10 or less. P is an element that can be regarded as an impurity, and if it exceeds 0.1%, the material may become brittle and steadite (phosphorus eutectic carbide) may be generated.

The S content is 0.02% or less. S is a graphite spheroidization inhibiting element. Therefore, it is preferable that the amount is as small as possible.

The Cr content is 0.15% or less. C
r is a strong carbide-forming element. Since it inhibits graphitization, it is not actively added in this material. It is desirable that the amount of unavoidable impurities is as small as possible. C
The content of r is more preferably 0.10% or less, and further preferably 0.05% or less.

The Mg content is 0.04 to 0.10%. Mg is an element for spheroidizing general graphite.

The Ni content is 1.0 to 2.5%.
Ni is a graphitization promoting element. It also promotes base strengthening,
Improves hardenability. In the present invention, it is added in order to facilitate the bainite formation of the matrix in the quenching treatment. The Ni content is preferably 1.5 to 2.0%.

The Mo content is 0.2 to 0.8%.
When Mo is used together with Ni, it strengthens the matrix and improves hardenability. It is added for the same purpose as Ni. The Mo content is preferably 0.3 to 0.5%.

With regard to the material having the above-mentioned first characteristic, basically it may be as-cast, and heat treatment is unnecessary. On the other hand, regarding the material having the second characteristic, the material is heated to 800 ° C. or higher, preferably 900 ° C. or higher for a predetermined time (determined by the product. Considering the case where cementite crystallizes, the wall thickness is 50 mm. After that, the surface to be used is quenched at a cooling rate of 10 ° C./minute or more, and when the quenched surface reaches 400 to 500 ° C., the temperature is immediately increased to 500 to 580 ° C. Tempering is performed at temperature. The holding time for tempering is set to about 1 hour for a wall thickness of 50 mm, and cooling is performed in the furnace as slowly as possible.
The heat treatment as described above bainites the used layer and the surrounding structure, and 10 kg /
A compressive residual stress of about mm is applied.

[0019]

EXAMPLE A sizing press mold was cast using the materials having the component composition according to the first feature of the present invention as Examples 1 and 2 and the material having the component composition according to the second feature as Example 3. . As a comparative example, a sizing press mold was similarly cast using a material whose Mn and Cr components were out of the range of the present invention. The composition of the components is shown in Table 1.

[0020]

[Table 1]

In Examples 1 and 2, products were processed without heat treatment. For Example 3, 92
After holding at 0 ° C for 10 hours, only the used surface is quenched at a cooling rate of 15 ° C / min or more, and the used surface is 450 to 500 ° C.
Immediately at 5 (other than that, 600 ℃ or higher)
A tempering treatment was applied at a temperature of 60 ° C. For the comparative example, after holding at 830 ° C. for 10 hours, the whole is 450 to 500.
Quenching was carried out at a cooling rate of 15 ° C./min or more until the temperature reached ℃, and immediately the same tempering treatment as described above was performed.

With respect to Examples 1 to 3 and Comparative Example, the obtained sizing press molds were assembled in a sizing press apparatus and used for the same period, respectively, and then the crack generation state and crack depth were investigated. The results are shown in Table 2. Table 2 also shows the characteristics of the tissues of each example.

[0023]

[Table 2]

From Table 2, it can be seen from Examples 1 to 3 that cementite does not crystallize, while the amount of graphite is very large at 20% or more, and the particle size of graphite is 70% at 100 μm or more.
It turns out that it is very large. This is due to the fact that the component system of the present invention has a high C content and a low Si, Mn, and Cr content, and that the inoculation treatment generally performed for finely stabilizing graphite is not intentionally performed. It is a thing. On the other hand, in the comparative example, crystallization of cementite was observed, and the graphite was also fine and the crystallization amount was small.

A clear difference was observed between the crack generation state and the crack depth investigation result after a certain period of use. Many heat cracks have occurred in the comparative example,
The cracks were opened and the steel material was inserted, and the crack intersections were also found to be missing. On the other hand, in Examples 1 and 2, the occurrence of heat cracks was extremely small. In Example 3, although the number of heat cracks was larger than that in Examples 1 and 2, the size and depth of the heat cracks were small and the heat cracks were minute. Of course, in Examples 1 to 3, neither the insertion of the steel material nor the chipping of the crack intersection was observed, and it is clear that the crack resistance is significantly improved. As a result, it is possible to fully expect the extension of the service life (life) of the mold.

[0026]

According to the cast iron material for a sizing press die according to claim 1, the present invention has the above-mentioned constitutions and functions, and the composition of the components is wt%, and C: 3.2 to 4.2%. , S
i: 1.0 to 2.5%, Mn: 0.1 to 0.6%, P:
0.10% or less, S: 0.02% or less, Cr: 0.15
% Or less, Mg: 0.04 to 0.10%, and the remainder is a ductile cast iron material substantially composed of Fe, and graphite is crystallized in the structure in an area ratio of 15% or more, and 5 of which are graphite with a particle size of 100 microns or more
Since it occupies 0% or more, the crack resistance of the sizing press die can be significantly improved as compared with the past. Therefore, it is possible to extend the interval period of the cutting of the sizing press die, and it is possible to reduce the amount of cutting when the die is cut. As a result, it can be expected that the number of man-hours for changing the size of the sizing press die is significantly reduced and the life of the sizing press die is improved. Further, according to the cast iron material for a sizing press die set forth in claim 2, in addition to the effect of the configuration of claim 1,
As the component composition, further by weight%, Ni: 1.0 to 2.5
%, Mo: 0.2 to 0.8%, and a compressive residual stress is applied to the use layer by bainizing the matrix structure of the use layer. By imparting a compressive residual stress due to this, the crack propagation resistance can be further increased, and the crack resistance of the sizing press die can be further improved.

Continuation of the front page (56) Reference JP-A-6-304612 (JP, A) JP-A-2-243736 (JP, A) JP-A-64-83641 (JP, A) JP-A-54-136533 (JP , A) JP 3-79739 (JP, A) JP 61-133360 (JP, A) JP 60-110842 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) C22C 37/04 C22C 37/08

Claims (2)

(57) [Claims] 1. The composition of the composition is% by weight, C: 3.2 to 4.2% Si: 1.0 to 2.5% Mn: 0.1 to 0.6% P: 0.10% or less S It is a ductile cast iron material containing 0.02% or less Cr: 0.15% or less Mg: 0.04 to 0.10% and the balance substantially consisting of Fe, and graphite has an area ratio in the structure. The cast iron material for a sizing press die is characterized by being crystallized in an amount of 15% or more, and graphite having a particle size of 100 microns or more accounts for 50% or more. 2. A ductile cast iron material further containing, by weight, Ni: 1.0 to 2.5% and Mo: 0.2 to 0.8% as a component composition, wherein the base structure of the used layer is bainite. The cast iron material for a sizing press die according to claim 1, characterized in that a compressive residual stress is applied to the used layer by making it into a material.