JP3217661B2 - High strength ductile cast iron - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ductile cast iron material having improved toughness and surface roughness, and having a very fine and uniform graphite structure.

[0002]

2. Description of the Related Art Ductile cast iron is obtained by performing spheroidizing treatment such as addition of Mg to spheroidize graphite, thereby improving mechanical properties compared to flake graphite cast iron. JP-A-6-116677 discloses a ductile cast iron material in which spheroidal graphite is finely dispersed by adding Bi to the ductile cast iron to improve the toughness and the surface roughness resistance. However, in the case of JP-A-6-116677, although the graphite structure can be improved by finely dispersing spheroidal graphite,
No improvement in matrix structure is obtained.

[0003]

For example, when a ductile cast iron material is applied to a member having a large load such as a rolling roll, the material strength is particularly important. Further, since the rolling roll comes into contact with the rolled material at the time of use, irregularities on the roll surface are likely to cause a problem. Therefore, it is necessary to further improve the toughness and skin roughness resistance of the ductile cast iron material. For that purpose, it is considered effective to uniformly and finely disperse the graphite and to make the matrix structure pearlite or bainite. . The easiest way to do this is to increase the casting solidification speed and quench, but in the case of large castings such as rolling rolls, a sufficient solidification speed cannot be obtained, especially inside, so that the spheroidal graphite becomes coarse. In addition, the matrix tends to ferrite, and it is difficult to improve toughness and skin roughness. An object of the present invention is to provide a ductile cast iron material in which graphite is uniformly finely dispersed even when a sufficient cooling rate is not obtained, and the matrix is made pearlite or bainite to improve toughness and skin roughness resistance. Things.

[0004]

The present invention SUMMARY OF THE INVENTION The aim of the graphite grains fine by addition of Bi, in which the matrix was pearlite or bainite by Cu addition, chemical composition by wt%, C: from 3.0 to 4 0.0% , Si: 1.0 to
4.0%, Mn: 0.3~1.3%, P: 0.1% or less <br/> under, S: 0.05% or less, Ni: 1.0~4.0%, C
r: 2.0% or less , Mo: 0.1 to 2.0% , Cu:
0.1~4.0%, Mg: 0.1~1.0%, Bi:
0.0005 to 0.05% , with the balance being impurities and substantially Fe , the matrix inside the roll being pearlite
Alternatively, it is a ductile cast iron material for rolling rolls having excellent toughness and skin resistance, which is characterized by being formed as bainite .

[0005]

According to the above composition, graphite is uniformly and finely dispersed to improve toughness and skin roughness resistance.
The reason for limiting the alloy components to the above ranges will be described below. C:
Regarding the limitation of 3.0 to 4.0%, if it is less than 3.0%, the crystallization of graphite is insufficient, and if it exceeds 4.0%, the amount of graphite becomes excessive and the toughness and skin resistance deteriorate. . Si:
Regarding the limitation of 1.0 to 4.0%, Si is an element indispensable for crystallization of graphite, and if it is less than 1.0%, the eutectic carbide crystallization amount is excessive and the toughness is deteriorated. On the other hand, if it exceeds 4.0%, the toughness and skin resistance deteriorate due to embrittlement of the matrix.

Mn: With respect to the limitation of 0.3 to 1.3%,
Mn is a pearlitization promoting element of the matrix,
If less than 0.3%, no pearlitizing effect is observed, and
If it exceeds 3%, the toughness and the skin resistance deteriorate due to the embrittlement of the matrix. Regarding the limitation of P: 0.1% or less, P is set to 0.1% or less from the viewpoint of making the material brittle. S: For the limitation of 0.05% or less, S is set to 0.05% or less from the viewpoint of making the material brittle.

Ni: 1.0 to 4.0%
Ni is an element that improves the matrix structure and is a graphitization promoting element, as is evident in nihard cast iron. 1.0%
If it is less than 4.0%, no improvement effect is observed, and if it exceeds 4.0%, spheroidization of graphite is inhibited. For the limitation of Cr: 2.0% or less, Cr is an element for promoting white iron and also an element for promoting pearlite. In the present invention, since Cu, which is a strong pearlitization promoting element, is added, the eutectic carbide content is excessive (white iron), and the toughness is not deteriorated by 2.0%.
It was as follows.

Mo: About the limitation of 0.1 to 2.0%,
Mo has an effect of maintaining quenching and tempering resistance.
If it is less than 0.1%, no improvement effect is observed, and if it exceeds 2.0%, the eutectic carbide content becomes excessive and the toughness deteriorates. C
u: 0.1 to 4.0%, Cu has the effect of strongly promoting the pearlitization of the matrix without turning into white iron. Even when a sufficient solidification rate cannot be obtained as in a large cast product such as a rolling roll, precipitation of ferrite in a matrix is suppressed, and precipitation of pearlite or bainite is promoted. Moreover, it has an effect of suppressing eutectic carbides (white iron). If it is less than 0.1%, no effect is obtained due to promotion of pearlite or bainite, and 4.0%
If it exceeds 300, Cu segregates at the grain boundary and the toughness is deteriorated.

Bi: Regarding the limitation of 0.0005 to 0.05%, Bi is an element which is generally preferred in ductile cast iron, since addition of Bi to cast iron inhibits graphite crystallization and spheroidization. But not conventionally added. However, when a trace amount of Bi is added, there is no adverse effect such as inhibition of graphite crystallization, and the graphite can be uniformly and finely dispersed even in the inside where the cooling rate is low. Where Bi
Is excessively added, the amount of graphite is reduced more than the effect of uniform fine dispersion, and the tendency to cause embrittlement of cast iron appears remarkably.
Therefore, the range was 0.0005 to 0.05%. The component according to the present invention is composed of the above components, the remaining impurities and substantially Fe.

Next, an example of basic data that led to the present invention will be described. This basic data is obtained by casting a mold as shown in FIG. 1 with a molten metal having the chemical composition shown in Table 1 and shaping the Cu content to prepare a test piece, and then forming a chill surface (surface in contact with the mold surface plate). ), The amount of ferrite and the tensile strength were measured on a test piece taken from a position 100 mm from the above. The results of the basic test will be described below. FIG. 2 shows the amount of ferrite (area ratio) and C
This shows the relationship with the amount of u added. The amount of ferrite was measured by a generally known image analysis method. Cu is 0.1
% Has an effect of suppressing ferrite precipitation.
When it exceeded 3.0%, the amount of ferrite (area ratio) was found to be 0%, that is, the matrix was all pearlite. FIG. 3 shows the relationship between the tensile strength and the amount of Cu added. C
By adding u (0.1 to 4.0%), a remarkable increase in tensile strength was observed. If it exceeds 4.0%, Cu segregates at the grain boundaries, and the strength is degraded.

[0011]

[Table 1]

[0012]

Example: Product body diameter 800 mmφ, body length 2500 mm,
Ductile cast iron integrated rolls having a total length of 5120 mm were manufactured as described below and subjected to various investigations. As a ductile cast iron material, a molten metal of the chemical composition which is the material of the present invention shown in Table 2 was used to apply a mold to the inside of the mold.
The casting was performed at a casting temperature of 50 ° C. Table 2 shows the chemical composition of the molten metal of the ductile cast iron integrated roll manufactured in the present invention. After being completely cooled, the roll was extracted from the mold, and the same shape as that of the roll manufacturing method described above was used as a comparative example with the molten metal chemical components (with and without Bi, without Bi and without Cu) shown in Table 3. Were manufactured, and the production quality of both rolls was compared and investigated.

[0013]

[Table 2]

[0014]

[Table 3]

FIG. 4 is a photograph of the microstructure of the three persons (1 from the surface).
00 mm). 4A shows Bi and Cu, FIG. 4B shows Bi and Cu, and FIG. 4C shows B and Cu.
This is the case without i and Cu. In the case where Bi and Cu were added, it was confirmed that the target graphite was uniformly finely dispersed and crystallized, and that the matrix was entirely pearlitized. Table 4 shows the production quality of both rolls. The Bi and Cu-added roll of the present invention simultaneously achieves an increase in material strength and a finer graphite structure.

[0016]

[Table 4]

As described above, in the roll of the present invention, B
i (0.0005 to 0.05%) and Cu (0.1 to
By adding 4.0%), graphite can be uniformly finely dispersed and crystallized, and the matrix can be entirely pearlitized. This is extremely effective in improving toughness and improving skin roughness resistance. It is also effective. In this embodiment, the application of the rolling roll has been described, but the material of the present invention is not limited to this, and can be widely applied to, for example, large machine parts requiring high strength.

[0018]

As described above, the ductile cast iron of the present invention has a significantly higher toughness by uniformly dispersing and crystallizing graphite than conventional ductile cast iron and crystallizing the entire matrix. In addition to the improvement, the skin resistance was also improved.

[Brief description of the drawings]

FIG. 1 is a view showing a mold for producing a test piece,

FIG. 2 is a graph showing the relationship between the amount of ferrite and the amount of Cu added;

FIG. 3 is a diagram showing the relationship between tensile strength and the amount of Cu added;

FIG. 4 is a metallographic photograph (× 50) of a roll of the present invention and a comparative roll.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Gen Nakamura, Inventor 46-59 Ohara Nakahara, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Machinery & Plant Business Department (56) References JP-A-6-116677 (JP) JP-A-5-339672 (JP, A) JP-A-7-145444 (JP, A) JP-A-60-36644 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) C22C 37/00-38/60

Claims (1)

(57) [Claims] [Claim 1] In chemical composition by weight%, C: 3.0~4.0%, Si : 1.0~4.0%, Mn: 0.3~1.3%, P: 0.1 % or less, S: 0.05% or less, Ni: 1.0~4.0%, Cr: 2.0% or less, Mo: 0.1~2.0%, Cu: 0.1~4.0 % , Mg: 0.1 to 1.0% , Bi: 0.0005 to 0.05% , the balance of impurities and substantially Fe .
Matrix as perlite or bainite
Excellent rolling toughness and耐肌Arasei, characterized in that that
Ductile cast iron for rolls .