JP4636395B1 - Method for producing flake graphite cast iron - Google Patents

Abstract

An object of the present invention is to easily form a dent due to sink marks on the outer surface of a flake graphite casting and a sink cavity inside. The structure of the thick part is coarsened, the strength is lowered, and the processed surface roughness is lowered. Moreover, the structure of the thin part tends to harden and the machinability tends to decrease.
To produce flake graphite cast iron castings with low sink tendency and low material thickness sensitivity.
(I) In order to reduce the tendency to sink, the amount of graphite that crystallizes when the casting solidifies is increased. As a means for this, the silicon content is lowered. The silicon content is in the range of 0.9 to 1.4%, and the carbon content is in the range of 3.0 to 3.4%. The relationship between the silicon content and the carbon content is in the range of the following formula.
C% + 0.23 × Si% ≦ 4.23%
(Ii) In order to reduce the thickness sensitivity, the silicon content is lowered.
(Iii) Various inoculations are performed in order to promote the purpose of the preceding item and the preceding item, and at the same time prevent tissue hardening of the thin portion.
[Selection figure] None

Description

  The present invention relates to a method for fundamentally improving the material of flake graphite cast iron used in various parts of machines and rough parts of molds.

The flake graphite cast iron has an appropriate tensile strength (200 to 350 N / mm ² ), pressure resistance, a low melting temperature (1200 ° C. or less), and good fluidity based on it, and it is easy to make castings of various shapes. For this reason, it is often used for general machine structural parts and rough mold materials. In general, what defines the basic properties of flake graphite cast iron is its chemical composition and dissolution method. Among these, chemical components are composed of carbon, silicon, manganese, phosphorus, sulfur and unavoidable trace elements called five components. In the usual case, the elements that have the most dominant influence on the properties of cast iron are carbon and silicon, which are determined by the strength and product shape that require these two element contents. In the prior art, the carbon content is in the range of 3.0 to 3.5% and the silicon content is in the range of 1.5 to 2.2%. As a melting method, it is a general method to melt the materials of these components at a temperature of about 1400 ° C. or more, and to inject the inoculant when the hot water is discharged from the melting furnace to make the metal structure uniform.
When the material strength of flake graphite cast iron manufactured by these methods is made higher (for example, 300 N / mm ² or more), the most reliable method for achieving the purpose is to adjust the carbon content to a low level. This operation increases the amount of shrinkage when the casting is solidified and tends to cause sink defects.
In addition, flake graphite cast iron generally has a large thickness sensitivity, and the thick part of the product, that is, the part where the cooling rate is slow, tends to make the metal structure rough, and the desired strength and dense finished surface cannot be obtained. On the other hand, the material is hard at the thin part, that is, the part where the cooling rate is fast, and in the extreme case, there is a problem that a chill structure containing a compound of iron and carbon is generated, the material becomes brittle, and machining is difficult. .

Hideo Nakae New Edition Casting Engineering p. 177. R. W. Heine The Carbon Equivalent Fe-C-Si Diagram and it Applications to Cast Irons. AFS Cast Metals Journal June 1971 p. 49. E. PiWarsky GuBeisen p. 88.

The object of the present invention is to eliminate or reduce the occurrence of sink marks on the outer surface and cast marks generated inside the casting, which is a drawback when increasing the strength of flake graphite cast iron. .
It is another object of the present invention to further reduce the difference in the metal structure between the thick part and the thin part of the casting, in particular, to obtain a dense finished surface by preventing the thick part structure from coarsening and precipitation of the ferrite phase. .
In addition to this, an object of the present invention is to prevent the occurrence of a chill structure that makes the metal structure of a cast thin part brittle and makes machining difficult.

(1) In order to reduce the sink tendency of flake graphite cast iron, the present inventor has paid attention to the fact that the amount of graphite crystallized inside when the casting solidifies must be increased. For this purpose, it has been found that the silicon content must be reduced as much as possible, the carbon content must be increased to some extent, and the inoculation operation of the molten metal must be performed strongly.
(2) Next, in order to make the metal structure of the cast wall thicker more dense and to reduce the precipitation of the ferrite phase, it has been found that the silicon content must be reduced and inoculation must be sufficiently performed. It was.
(3) Further, the inoculation operation is sufficiently performed to prevent the thin portion of the casting from becoming hard and generating a chill structure in the worst case. This operation is common to these three items and has a role of strengthening the means (1) and (2).

  According to the present invention, there is no sink phenomenon on both the outside and inside of the casting, and at the same time, the thick portion, for example, a portion having a thickness of 300 mm or more is dense in structure and the ferrite phase is less precipitated, while the thin portion, for example, The chill structure does not appear even at the corner of the 10 mm thick portion, and good machinability can be obtained.

  The silicon content of the flake graphite cast iron of the present invention is preferably in the range of 0.9 to 1.4%, more preferably 1.0 to 1.3%. Although it can be adjusted by the minimum thickness of the casting, it is desirable to perform a sufficient inoculation operation to select a lower silicon content.

  The carbon content of the flake graphite cast iron of the present invention is preferably 3.0 to 3.4%, and more preferably 3.2 to 3.25%. A lower carbon content is selected when higher strength is desired, and a higher carbon content range is selected when the sink tendency is reduced and when machinability is desired to be increased.

The relationship between the carbon content and the silicon content of the flake graphite cast iron of the present invention preferably follows the following formula.
C% + 0.23 × Si% ≦ 4.23%

The inoculation operation to the flake graphite cast iron melt of the present invention is preferably performed three or more times in the following order.
(1) Add the inoculant to the molten metal in the furnace immediately before leaving the melting furnace.
(2) Add the inoculum when pouring from the melting furnace to the hot water ladle.
(3) When pouring from the casting ladle or pouring furnace into the mold, add the inoculant to the pouring stream, or set the inoculant in the pouring basin, pouring bowl or mold in advance Pour hot water.

  The inoculation operation to the flake graphite cast iron melt of the present invention can be omitted depending on the situation. However, the inoculation operation (3) cannot be omitted.

The invention's effect

According to the present invention, it is possible to produce a flake graphite cast iron casting in which no sink phenomenon is observed both inside and outside.
According to the present invention, a flake graphite cast iron casting having a dense metal structure and a good machined surface can be produced even when the thickness portion, for example, the thickness is 200 mm or more.
According to the present invention, it is possible to produce flake graphite cast iron having a thin part, for example, a hard chill structure or the like and having good machinability even when the thickness is 10 mm.

  The outline of the experiment is as follows. After inoculating the molten metal whose silicon content and carbon content were adjusted to specified amounts, the molten metal was poured into a mold. A test material with a main body was attached to the mold according to JIS 5501, and a JIS No. 8 test piece was manufactured from the test material by machining, and a mechanical property test, a microscopic examination, and a chemical component analysis were performed. As a result, this material was found to be FC350.

  The components of the flake graphite cast iron and the molten metal treatment were as follows. The target chemical components of the experimental molten metal were 1.3% silicon content and 3.2% carbon content. This molten metal was heated to 1540 ° C. by a low-frequency electric induction furnace, and 0.1% of an inoculum in the furnace mainly composed of silicon carbide was added immediately before pouring. Next, 0.1% calcium silicon inoculant was added to the hot water stream when the hot water was taken out into the ladle. After the molten metal is cooled to 1380 ° C., which is the casting temperature, 0.13% of a ferrosilicon-based bulk inoculant containing a small amount of calcium and barium is continuously introduced into the dam during the period of pouring into the mold. Added and evenly distributed in the molten casting.

  The mold in which the molten metal of the previous item was cast was a block having a width of 300 mm, a height of 275 mm, and a mass of 1,490 kg. A specimen with a main body was attached to the side surface of the mold in accordance with JIS G5501. Since the distance between the product and the test material was 55 mm and the thickness of the product was 80 mm or more, the diameter of the test material was φ80 mm.

The specimen with the main body was machined into a JIS No. 8 test piece, and the mechanical properties and the microstructure were examined. As a result, the tensile strength was 234 N / mm ² and the Brinell hardness was HB 156. As a result of the microscopic examination, the base was basically pearlite, and a small amount of ferrite phase was observed accompanying graphite. The situation is shown in FIG.

Microstructure of a tensile specimen taken from a specimen with a body. Magnification is x100.

  As a result of the wet chemical component analysis of the residual material of the tensile test piece, the silicon content was 1.33%, the carbon content was 3.19%, the manganese content was 0.70%, the phosphorus content was 0.040%, and sulfur. The content was 0.088%.

The tensile strength of the test piece with flake graphite cast iron body produced by this experiment was 234 N / mm ² . As a result of collating this value with the standard of the specimen with the body of JIS 5501, it corresponds to the item of cast iron thickness 150 to 300 mm and satisfies the standard value 210 N / mm ² or more and is equivalent to FC350. found.

  As a result of arbitrarily cutting each part of the block material into which the molten metal was cast and cutting deeply as a rough mold material, no sink defect was found.

Claims (1)

(B) A piece containing 0.9 to 1.4% of silicon content, 3.0 to 3.4% of carbon content, 0.05 to 0.2% of sulfur, and other elements that cannot be avoided Molten graphite cast iron melt.
(B) The relationship between the silicon content and the carbon content is according to the following equation.
C% + 0.23 × Si% ≦ 4.23%
(C) Various inoculation operations are performed on this molten metal.
(1) Immediately before pouring from the melting furnace, 0.1% of an inoculum mainly composed of SiC is added to the molten metal in the furnace.
(2) When pouring hot water from the melting furnace to the ladle, add 0.1% of calcium silicon inoculum on the hot water flow.
(3) During the period of pouring into the mold, 0.13% of a ferrosilicon-based bulk inoculant containing a small amount of calcium and valeum is continuously added into the dam and uniformly distributed in the cast molten metal.
A method for producing flake graphite cast iron constructed as described above.