JPH0723520B2 - Cast iron with excellent weather resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to cast iron for construction and structural / design members, and more particularly to cast iron having excellent weather resistance in the atmosphere.

<Prior art> High Si cast iron containing 4 to 15% Si, Ni-resist cast iron containing 12 to 25% Ni, high Cr cast iron containing 10 to 35% Cr (for example, Foundry Handbook, Japan Foundry Association) Cast iron with excellent corrosion resistance already exists, but high Si cast iron has poor mechanical properties and is hard, so it is unsuitable for building members and difficult to machine. Niresist cast iron contains a large amount of Ni and has excellent mechanical properties, but it is expensive. High Cr cast iron has corrosion resistance close to that of stainless steel, but has extremely hard and low toughness, etc., which are difficult to handle, and they are generally used only as industrial members for special purposes. .

As described above, the conventional corrosion-resistant cast iron is unsuitable for use as a building member in terms of cost, workability, material, and the like.

On the other hand, in the category of steel materials, weather resistant steel is known as a structural member that is inexpensive and has excellent corrosion resistance (weather resistance), but this is generally a rolled steel material, and its designability as a design member is low. There are also cases in which this is cast into a prescribed shape as cast steel (for example, Hashizome and Foundation, 81-3, P40-). These weather-resistant cast steels have poor castability and the reproducibility of complicated design patterns and shapes However, it is lacking in applicability as an architecturally-prepared design member, such as poor production cost and higher manufacturing cost than cast iron castings.

Further, when the conventional corrosion-resistant cast iron is cast into a thin product, particularly a product having a thickness of 10 mm or less, it is difficult to prevent chill. If it is chilled, the casting tends to be cracked or cracked, and there is a problem in application to building members such as deterioration of weather resistance.

<Problems to be Solved by the Invention> An object of the present invention is to construct,
As a design member, we are going to provide cast iron with excellent weather resistance and workability that is inexpensive and has good castability. Especially, the weather resistance is highly applicable as a design member with a thin wall and a complicated shape and pattern. It provides cast iron.

<Means for Solving the Problem> The cast iron of the present invention is based on a cast iron composition having excellent castability and workability in its material, and by blending a trace amount of alloying elements, chill formation is prevented as much as possible, In addition, the weather resistance of the rust layer formed in the atmosphere is improved.

That is, according to the present invention, the carbon saturation {Sc = C% / (4.23−
Si% / 3.2)} is in the range of 0.9 to 1.2, C: 2.8 to 3.8%, Si: 2.
6 to 3.8%, Mn: 0.01 to 0.6%, S: 0.015% or less, P: 0.05 to 0.3
%, Cr: 0.1 to 0.6%, Cu: 0.2 to 2.0%, Ni: 0.1 to 2.0%, Mg: 0.
Cast iron with excellent weather resistance, characterized by containing 003 to 0.060% by weight, the balance being Fe and unavoidable impurities, and Bi: 0.0003 to 0.005%, RE: if necessary.
It further contains one or two selected from 0.0003 to 0.005% by weight respectively, and preferably has an area ratio of carbides contained in the metal structure of 5% or less and has excellent weather resistance. It is cast iron.

<Operation> Next, the reason for limiting the range of constituent components of the weather-resistant cast iron of the present invention will be described.

C, Si; C, Si is a basic element in cast iron and affects the material and castability of cast iron. In the case of the cast iron of the present invention, good properties are obtained when the carbon saturation {Sc = C% / (4.23-Si% / 3.2)} is in the range of 0.9 to 1.2. That is, carbon saturation is 0.8
If it is less than 1, the material tends to be chilled and the material becomes hard and brittle. Further, if the carbon saturation exceeds 1.2, the amount of graphite becomes excessive and the mechanical properties deteriorate. Here, if the Si content is less than 2.6%, the castability and graphitization tendency are deteriorated, and if it exceeds 3.8%, the precipitated ferrite becomes brittle, so the content is limited to the range of 2.6 to 3.8%. Further, the amount of C is limited to the range of 2.8 to 3.8% in relation to the amount of Si.

S; S is detrimental to the weather resistance and adversely affects the shape of graphite, and it is desirable to reduce it as much as possible in order to promote the chilling of cast iron. However, the content of 0.015% is acceptable.

Mn; Mn stabilizes pearlite and is effective for increasing strength, so 0.01% or more is included, but if it exceeds 0.60%, the chilling tendency increases and the material becomes hard and brittle, so 0.01 to 0.60
It was limited to the range of%.

P; P is an element useful for improving the weather resistance of cast iron and is effective from 0.05%. However, if the content exceeds 0.3%, a large amount of iron phosphide crystallizes and the material becomes hard and brittle, so 0.05 to 0.
Limited to 3% range.

Cu; Cu is an extremely important element for improving the weather resistance of cast iron,
Weather resistance is improved by containing 0.2% or more.
On the other hand, even if added in excess of 2%, no further effect is obtained and the tendency of uneven bending increases, which is not preferable. Therefore, the Cu content is limited to the range of 0.2 to 2.0%.

Ni; Ni has the effect of reducing the bias of Cr and stabilizing the rust layer. Addition of a large amount will not adversely affect, but the upper limit was set to 2.0% to prevent cost increase. On the other hand, the effect of Ni cannot be obtained unless 0.1% or more is added, so the lower limit was set to 0.1%.

Cr; Cr has little effect on the weather resistance when added alone, but when it is added in combination with P, Cu, and Ni, it becomes effective from 0.1% or more. On the other hand, addition of more than 0.6% does not improve the effect, but also significantly increases the tendency of white pig iron to form a large amount of carbides, so 0.1-0.6%
Limited to.

Mg; Mg has the effect of changing the graphite shape of cast iron from flaky to caterpillar or spherical, thereby improving strength and toughness. A thin-walled product having a wall thickness of 10 mm or less has an effect of improving the graphite shape at a content of 0.003% or more, but a thick-walled material preferably has a content of about 0.05%. However, when Mg exceeds 0.060%, the chilling tendency of cast iron increases and it becomes impossible to prevent crystallization of carbides, so the content was limited to 0.003 to 0.060%.

RE (Rare earth element), Bi; Rare earth element and Bi are both graphite refining elements, and have the effect of increasing the number of eutectic cells, and evenly disperse alloy elements such as P, Cu, Ni, Cr. . Therefore, the addition of rare earth elements and Bi also has an advantageous effect on the improvement of weather resistance, so it is desirable to add them individually or in combination as required. In particular, addition of Bi and a rare earth element alone is effective for making graphite finer, but the effect is remarkably increased by adding them in combination. By adding Bi and rare earth elements, chill is prevented as the graphite becomes finer, that is, the number of graphite particles increases.

Therefore, the addition of the rare earth element, Bi, has a remarkable effect in preventing chilling of thin products with a thickness of 10 mm or less. In the cast iron of the present invention, the above effect cannot be sufficiently obtained unless it is added in an amount of 0.0003% or more. When the content exceeds 0.005%, the number of graphite particles decreases, so the content is limited to the range of 0.0003% to 0.005%. Any rare earth element has an effect, but in the present invention, the combined addition of La and Ce is particularly preferable.

Next, the preferable range of the carbide area ratio in the weather resistant cast iron of the present invention will be described below.

The reason why the cast iron of the present invention has excellent weather resistance is that the alloy elements dissolved in the iron matrix are concentrated on the iron surface as rust occurs and form a dense protective film. Therefore, unless the alloying element is unevenly distributed or is dissolved in the base metal in a predetermined amount, the weather resistance is deteriorated. On the other hand, carbides in cast iron absorb a large amount of these alloy elements (which are Cr and P in the cast iron of the present invention), resulting in a decrease in the amount of alloy solid solution in the base iron and deterioration in weather resistance. . Further, when the amount of carbides becomes large, the material becomes hard and brittle, so that the excellent workability characteristic of the present weather-resistant cast iron is also deteriorated. As shown in FIG. 3, when the area ratio of carbides in cast iron is high, the hardness is remarkably increased, and when the area ratio is close to 10%, it becomes difficult to machine with a general machine tool. For these reasons, it is better that the cast iron of the present invention has a smaller amount of carbide formation, and the carbide area ratio is preferably 5% or less.

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

<Examples> Plate-shaped samples having a chemical composition shown in Table 1 and having a wall thickness of 3 mm and 10 mm were melted in a vacuum molding mold.

Table 2 shows the results of examining the area ratio (%) of carbides by micro-observing each sample. A typical metallographic photograph of a 3 mm thick sample is shown in FIG. D to H of the present invention show a healthy bull's eye structure with no crystallization of carbide even in a thin-walled sample having a wall thickness of 3 mm, which proves that it is advantageous for casting a thin-walled casting. Inventive Examples B and C produced only a small amount of carbide in a thin sample having a wall thickness of 3 mm, and Inventive Example A had a wall thickness of 10 mm.
Since it does not crystallize carbides in mm, it is recommended to use it for casting thick products. In Comparative Example I in which the amount of Cr deviates from the scope of claims of the present invention, carbide crystallizes even with a wall thickness of 10 mm, which is not preferable in terms of hard embrittlement.

Figure 2 shows the results of two years of atmospheric exposure tests conducted on the samples A to K with a wall thickness of 10 mm. Inventive Examples A to H
Has a significantly smaller amount of corrosion than Comparative Example J, and the effectiveness of the present invention is clear.

<Effects of the Invention> As described above, according to the present invention, the thickness is 10 mm, as well as the thickness.
Even if the following thin-walled castings are cast, chill is hardly generated, and cast iron having excellent weather resistance and machinability can be obtained. Therefore, for construction, structure / design members such as inner / outer wall materials, balustrades,
It is an extremely useful material as a thin-walled design member such as a bridge member, a gate pillar, a roof tile, and a gate.

In addition, this weather-resistant cast iron, when painted such as paint,
The rust layer formed at the defective part and the locally deteriorated part of the coating due to long-term use forms a protective film with excellent weather resistance, so it also has the effect of suppressing the diffusion of rust and extending the life of the coating. It is a useful material in this respect as well.

[Brief description of drawings]

Fig. 1 is a photograph of a metal etching structure of 100 times the thickness of a 3 mm thick cast sample, Fig. 2 is a graph showing the amount of corrosion in the atmospheric exposure test for 2 years, and Fig. 3 is a diagram showing the relationship between the carbide area ratio and hardness. Is.

Claims (3)

[Claims] 1. Carbon saturation {Sc = C% / (4.23-Si% / 3.
2)} is in the range of 0.9 to 1.2, C: 2.8 to 3.8%, Si: 2.6 to 3.8
%, Mn: 0.01 to 0.6%, S: 0.015% or less, P: 0.05 to 0.3%, Cr:
0.1 to 0.6%, Cu: 0.2 to 2.0%, Ni: 0.1 to 2.0%, Mg: 0.003 to
Cast iron with excellent weather resistance, characterized by containing 0.060% by weight and the balance Fe and unavoidable impurities. [Claim 2] Bi: 0.0003 to 0.005%, RE: 0.0003 to 0.005%
The cast iron having excellent weather resistance according to claim 1, further comprising one or two selected from the above, each in a weight percentage. 3. The area ratio of carbide contained in the metal structure is 5%.
The cast iron having excellent weather resistance according to claim 1 or 2, wherein: