RU2586730C1 - Method of producing high-strength cast iron - Google Patents

Abstract

FIELD: metallurgy; technological processes.

SUBSTANCE: invention relates to methods of producing high-strength iron with spherical shape of graphite, and can be used in production of medium and large-sized castings with wall thickness of 50 mm or higher. Method involves melting charge in melting unit, considerable overheating of melt to 1,480-1,520 °C for implementation of modification by treatment of its alloy containing rare-earth metals, and containing a modifier, modifier and ligature is placed on ladle bottom, heated to 750-800 °C, layer by layer in form of a layer of a modifier, powder in form of magnetoactive slag, next layer alloy and final layer in form of cast iron crushed with slag-forming additives, while filling ladle with melt is performed in time interval from 1 to 1.50 minutes, after which cast iron melt is held for 8-9 minutes.

EFFECT: invention provides stable production of spherical shape of graphite, reduces cost owing to use of cheap ligatures and reduces consumption owing to use of waste from blast-furnace process and cast iron crushed with slag-forming additives fraction to 10 mm.

1 cl, 1 ex, 3 tbl, 1 dwg

Description

The invention relates to metallurgy and foundry, in particular to methods for producing ductile iron with spherical shape of graphite, and can be used in the production of medium and large castings with a wall thickness of 50 mm and above.

One of the methods for producing cast iron with spherical and vermicular form of graphite is the use of ligatures containing 30% rare earth metals (REM), 43% silicon, 8% aluminum. Depending on the sulfur, the amount of ligature introduced varies from 0.8 to 2.5% (see the book. "High-strength cast iron for castings." / Ed. By Doctor of Technical Sciences Aleksandrova NN - M .: Mechanical Engineering, 1982, p. 193).

When the residual content of rare-earth metals (REM) in the alloy significantly exceeds its optimal concentration of 0.5-0.6%, the overmodification effect occurs, namely, the metal matrix of the castings consists of ledeburite or structurally free cementite, and part graphite has a star shape. Both of these factors do not allow cast iron in the cast state to have mechanical properties corresponding to the brands of ductile iron. To achieve the required properties in such cast irons, additional heat treatment is required. To improve the properties in the molten state, secondary modification is carried out with 75% ferrosilicon in an amount of 0.5-0.8% by weight of the liquid metal. The disadvantage of this process is the saturation of the matrix ferrite with silicon and an increase in the amount of star-shaped graphite in the structure, which does not allow the production of cast iron with high mechanical properties. The appearance of vermicular graphite in the metal matrix is a structural deviation for high-strength cast iron with spherical shape of graphite, which noticeably affects the mechanical characteristics of cast iron.

Closest to the proposed method is a method of processing an REM-containing ligature and secondary treatment with a magnesium-containing modifier (see the description of the invention of RF patent No. 2341562, C21C 1/00, C22C 37/04, 12/20/2008).

The method is hampered by the fact that the method is initially aimed at achieving vermicular graphite, and then at changing to a spherical shape. When conducting secondary modifications aimed at correcting the structure of graphite, often either incomplete or partial correction of the structure. The REM-containing ligature is introduced twice, respectively, the matrix ferrite is oversaturated with silicon and the amount of star-shaped graphite in the structure increases, as a result of which the mechanical characteristics of the products significantly decrease. The number of introduced ligatures, due to the instability of the composition of the ligature, is an approximate value, which leads to instability of the method. The primary modification process is carried out by loading the primary REM-containing ligature into a special dispenser and feeding it when filling the dispensing bucket onto a stream of metal to be drained. Then it was fed to the stand to fill the casting ladles. Accordingly, the implementation of this method requires the presence of at least two buckets, which undoubtedly complicates the process of modifying the melt. Subsequently, a modifier was added to the bottom of the casting ladle with an additional input of ligature in an amount of up to 0.5%, which in some cases leads to the formation of ledeburite and cementite, and, accordingly, leads to a decrease in the mechanical properties of cast iron. As you know, the modifier pops up when the metal is drained into the bucket, since the density is much lower than cast iron. The specified method is carried out for castings weighing from 5 to 30 kg with a wall thickness of 5 to 40 mm, which significantly limits the range of products.

The technical result obtained by using the invention is to increase the stability of the formation of spherical graphite in high-strength cast iron by simultaneously entering the ligature and modifier.

To achieve this technical result, a method for producing high-strength cast iron with spherical graphite form from the initial one, including melting the mixture to the melting temperature, analyzing the sulfur content in the melt, modifying the rare-earth metal-containing alloy and magnesium-containing modifier, the modification being carried out at a sulfur content of 0.01-0, 50% with significant overheating of the melt 1480-1520 ° C, in addition, the modifier and ligature are placed on the bottom of the bucket, heated to 750-800 ° C, layer by layer in the form of a modifier layer, dust in the form of slag m gnito-active, subsequent layer of the ligature and the final layer of cast iron crushed with slag-forming additives, filled in a period of 1-1.50 minutes, after which the modified iron is kept for 8-9 minutes, while magnetically active slag is used as powders according to TU 14-128 -Sh-7-06 and crushed cast iron with slag-forming additives according to TU 14-128-D-5-06 with a fraction of up to 10 mm.

The invention is illustrated in the drawing, where in FIG. 1 shows a diagram of laying and pouring components; and tables, where table 1 shows the composition of the ligatures, selected from two different batches, the composition of the modifier, the composition of magnetically slag and crushed cast iron with slag-forming additives with a fraction of up to 10 mm; table 2 shows the dependence of the input components on the sulfur content in the source iron; table 3 shows the dependence of the formation of 100% spherical graphite on the time of discharge of cast iron.

The production of ligatures and modifiers has some variation in chemical composition: 30-40% rare-earth metals, 30-40% silicon, 2.0-5.0% aluminum class A, 5.0-15.0 aluminum class B, the rest is iron according to TU 14-5-136-81. Modifier: 0.5-1.0% REM, 5.5-6.5% magnesium, up to 1.2% aluminum, 45.0-50.0% silicon, 0.3-0.5 calcium, the rest is iron according to TU 0826-002-72684889-09. In view of the constant growth of the economic component, the technical result achieved using the invention is: an increase in the stability of the formation of spherical graphite, independent of some deviations in the composition of the ligature; reduction in the amount of ligature due to blast furnace wastes, simultaneously leading to their disposal.

The specified technical result is achieved by the fact that in the method of producing high-strength cast iron from the source, the main indicator in calculating the amount of modifier and ligature used in the chemical composition is the sulfur content.

Table 1 shows: ligature formulations selected from two different lots; modifier composition; magneto-slag composition; crushed cast iron with slag-forming additives with a fraction of up to 10 mm.

The method is implemented as follows. Cast iron was smelted in an IChT-6 induction furnace. Due to the heterogeneity of the charge materials of the chemical composition has a large run-up, in this regard, the composition of the source cast iron must be kept within: C 2.7-3.5%; Si 1.7-2.0%; Mn 0.5-1.0; S 0.01-0.5%; Cr not more than 0.2%. Subsequently, before draining the metal into the ladle, it is overheated to a temperature of 1480-1520 ° C to carry out the modification process.

Depending on the sulfur content in the source cast iron, the amount of ligature, modifier, magnetically active slag and crushed cast iron with slag-forming additives used is shown in table 2.

To carry out the modification process, having determined the optimal percentage of components, they are laid in a preheated bucket 1 to a temperature of 750-800 ° C, with lower heating, incomplete dissolution of the components sintered at its bottom with even layers according to the principle of FIG. one:

1. Modifier 2 is put in the first even layer to reduce the ascent rate due to enveloping with slag inclusions.

2. The next layer is magnetically active slag 3. It is a coating layer of a modifier that prevents its rapid dissolution, and as a result, the extension of the "aging effect" - the transition from spherical graphite to vermicular. It is evenly distributed over all previously laid components.

3. The third layer is the ligature 4. It is also laid in an even layer on top of previously laid components.

4. The fourth layer is crushed cast iron with slag-forming additives 5. According to TU 14-128-D-5-06 its permissible fraction is 0-20 mm, but when using a fraction of 10-20 mm a temperature drop was observed to 1350 ° C and the modification process proceeded not completely, due to which the formation of spherical graphite reached a maximum of 75%. Therefore, crushed cast iron with slag-forming additives 5 is preliminarily selected to a fraction of 1-10 mm. This layer, like the second one, is a cover layer, and its main function is to reduce the rate of modification by enveloping all components with slag inclusions.

Example

The metal was drained into a 6-ton bucket 1. Since the bulk of the castings produced on average had a weight of 3000 kg, respectively, the sample was taken from this calculation. To accurately determine the amount of metal to be poured during lining, a partition 6 was placed located in the middle of the bucket 1. Before draining, the bucket 1 is heated to a temperature of 750-800 ° C to avoid large temperature losses, otherwise the modification process will not be carried out, since all components will be melted will not happen, which will lead to segregation and mismatch of the grade of cast iron.

Draining was carried out strictly at a temperature of 1480-1520 ° C, since when the melt was fed, the temperature dropped significantly to 1400-1430 ° C. The filling of the bucket 1 was carried out in the direction 7 strictly to the center of the bucket 1 for better divergence of all components. The time spent on filling the bucket 1 should be as small as possible - 1-1.50 min. An increase in the drain time from 2–2,50 min led to incomplete assimilation of modifier 1, as a result of which the appearance of a vermicular form of graphite was observed. The results are shown in table 3. At the time of discharge of the metal, the modifier is enveloped with slag-active components, as a result of which the pyroeffect practically did not appear. After filling the bucket, the slag was not removed from the mirror, since a modification process was still taking place on the surface due to the activity of all components. Due to enveloping the modifier with slag inclusions, activity was observed for 7-10 minutes, which significantly prolongs the "modifying effect". The pouring process was carried out after 8–9 min, since an increase in time of more than 9 min led to a decrease in the number of graphite crystallization centers, which contributed to the formation of vermicular graphite.

After pouring the metal into the ladle without removing slag from the mirror, a specially prepared spoon took a sample and poured samples of size 15 × 15 × 40. After collecting the sample, the bucket was sent to fill the mold. This whole procedure is carried out in the range of 1-9 minutes, then the process of removing slag and filling the mold should be carried out. At this point, the sample cooled. Then it broke to confirm the spherical shape of graphite. Typically, small inclusions of cementite were observed at the edges of the sample. This is because the sample cross section is too small. Accordingly, with a wall thickness of more than 40 mm, ledeburite and cementite were practically not observed.

Castings were made of the type “blast furnace chill plates” with wall thicknesses of 40 mm and higher. Pouring was carried out in dry sandy-clay forms. The pouring temperature was 1400-1430 ° C.

Metallographic studies were carried out on the details, mechanical tests on the samples. Control of the structure was carried out before filling the mold and after.

The operational properties of the products manufactured by the described method correspond to the characteristics of products from ductile iron.

Using the proposed method for producing high-strength cast iron provides an increase in the stability of the process of formation of spherical shape of graphite in castings with a wall thickness of 50 mm and higher, cost reduction due to the simultaneous introduction of a modifier and ligature using dusting components of blast furnace waste, which, firstly, favorably affects the economic component of the products, secondly, allows you to get a spherical shape of graphite in thick-walled castings through the use of cheaper REM-containing ligatures and also helps to dispose of the magnetically active slag formed during the production of blast furnace iron.

Claims (2)

1. A method of producing high-strength cast iron with spherical graphite form from the original, including melting the mixture to a melting point, analyzing the sulfur content in the melt, subsequent modification in the ladle with a ligature containing rare-earth metals, and a magnesium-containing modifier, characterized in that the modification is carried out at a sulfur content of 0 , 01-0.50% with overheating of the melt to 1480-1520 ° C, while the above-mentioned modifier and ligature are placed on the bottom of the bucket, heated to 750-800 ° C, layer by layer in the form of a modifier layer, dusting powder in the form of slurry Single magnetically, subsequent ligation and a final layer of the layer of powder in the form of iron from the crushed slag-forming additives, wherein the content of the melt in the ladle is carried out a period of from 1 to 1.50 min after which promodifitsirovanny iron incubated for 8-9 min. 2. The method according to p. 1, characterized in that crushed cast iron with slag-forming additives with a fraction of up to 10 mm is used as powder.

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