RU2841804C1 - Modifying mixture for cast iron - Google Patents

Abstract

FIELD: metallurgy; foundry production.

SUBSTANCE: invention relates to metallurgy and can be used for out-of-furnace graphitizing treatment of melts of grey and high-strength cast irons. Mixture contains ferrosilicon aspiration dust obtained during crushing thereof and graphite aspiration dust obtained during mechanical treatment of graphitised electrodes, with the following ratio of components, wt. %: ferrosilicon aspiration dust 35-65, graphite aspiration dust 35-65.

EFFECT: improving graphitizing ability of modifying agent and hardness of cast iron in various sections of casting, as well as stability and homogeneity of microstructure.

1 cl, 3 tbl, 11 dwg

Description

The most common in terms of the achieved result with the proposed modifier is an alloy of iron with silicon - ferrosilicon. [Girshovich N.G. Crystallization and properties of cast iron in castings. - L.: Mechanical Engineering, 1996. - 562 p.]. High-grade ferrosilicon is used for modification: FS65, FS 75. The disadvantages of this modifier are the need for preliminary crushing of the modifier, dusting during modification, large errors in dosing, as well as increased consumption of ferrosilicon and its low viability in the melt.

A promising direction for modifying cast irons is the use of mixed modifiers. Mixed reagents are environmentally friendly, provide advantages in the accuracy of material dosing, do not require a melting operation during production and have a lower cost than molten materials. [Kobelev N.I. Mixed modifiers for cast irons / N.I. Kobelev, A.V. Kozlov, S.S. Zuikov [et al.] // Foundry production. - 1989. - No. 12. - Pp. 6 - 8]. Abroad, they are following the path of introducing free C or SiC into mixed modifiers for cast irons, which create graphite nuclei upon contact with liquid cast iron, which remain stable for a long time and significantly increase the time of action of the modifier [Lekakh S.N. Out-of-furnace processing of high-quality cast irons in mechanical engineering / S.N. Lekakh, N.I. Bestuzhev. - Mn.: Navuka and technology, 1992. - 269 p.].

The closest in technical essence, general features and achieved result is the Modifying mixture [RU 2373290] for modifying gray cast iron, containing highly dispersed silicon powder formed during the production of organohalogenosilanes, and graphite, characterized in that the graphite is used in the form of artificial graphite subjected to temperature treatment above 5000°C and fine grinding in the following ratio of components, wt.%: ferrosilicon 35-65; graphite 35 - 65.

The disadvantage of this technical solution is that the technology for producing organohalogenosilanes has recently changed, which does not provide for the formation of highly dispersed silicon powder, and this material is becoming increasingly scarce. In addition, artificial graphite subjected to special treatment is expensive. And finally, the modifying mixture does not stimulate the involvement of production waste in the process of obtaining the modifier.

At the same time, there is currently an urgent need in the machine-building industry to process metallurgical waste, including waste from the production of ferrosilicon and graphite products. To solve environmental problems, new modern aspiration systems are being built, in which more than 90% of small fractions are captured by filters. The main part of the aspiration dust is subject to burial, which is accompanied by significant financial costs. Therefore, its involvement in production is very important.

The task, which the proposed invention is aimed at solving, is to create a modifying mixture that is easy to manufacture. When implementing the claimed invention, the technical result achieved consists of: a) involving high-grade ferrosilicon (FS) aspiration waste in the technological process of manufacturing mixtures, b) involving aspiration waste generated during mechanical processing of graphite products in the production process. A mixture containing 35% graphite and 65% FS is used to modify cast iron, which is close in composition to eutectic. With such a ratio of components, the most favorable synergetics of the modification process is formed. As the carbon equivalent decreases, the ratio of components changes. The graphite content increases, and FS decreases.

When producing mixed modifiers to eliminate cementite in the cast iron structure, it is important to use a component whose crystallographic parameters will be as close as possible to the graphite released in liquid cast iron during crystallization. Such a component is carbon, the main component of graphite. Thus, graphite is universal and suitable for producing additives in the production of cast iron. There are many graphites in the Russian Federation, both natural and artificial. However, they all have high ash and sulfur content, which significantly reduces the efficiency of cast iron modification. Therefore, special processing of graphite is required, sharply reducing the amount of S and ash in it, and also ensuring dispersion of graphite particles. Graphitized electrodes (GOST R 56973), which are an important element of electric arc furnaces in the production of steel and alloys, meet these requirements. They maintain the arc burning between the ends of the electrodes and the metal, and to a certain extent limit the productivity of electric furnace units. Modern electrodes are made only from low-sulfur and low-ash graphites. All electrodes and connecting nipples are subjected to mechanical processing. For this purpose, CNC machines are increasingly used. During processing, a large amount of dust is generated, which can have negative consequences for the equipment and operators. Therefore, a highly efficient aspiration system is installed on electrode processing machines. The average size of aspiration dust of graphite is 13-21 µm.

Ferroalloy plants install modern aspiration systems. At the same time, dust emissions into the atmosphere, which occur during the crushing of ferrosilicon, are captured almost one hundred percent. For example, the operation of gas cleaning aspiration systems, including 21 filters at OJSC Kuznetsk Ferroalloys, allows air to be cleaned by 99%. The average size of FS aspiration dust is 17.12 microns.

The efficiency of dispersion of components for mixed modifiers was confirmed by modeling the behavior of particles in the cast iron melt when it fills the cavity of the casting mold, for which the FLOW-3D program was used.

For modeling, the technology of the mold of a responsible railway casting "friction wedge" made of cast iron SCh30 was used. Four castings were symmetrically placed in the mold (Fig. 1), which made it possible to model the filling of 1/4 of the casting mold with cast iron and modifier, i.e. one friction wedge.

For mathematical modeling of the process of filling the mold with melt, the Navier-Stokes system of hydrodynamic equations was used, supplemented by the continuity equation.

We also added the Fourier equation, which takes into account the movement of heat flows in the form, and the Fourier-Kirchhoff equation for heat transfer in metal.

We simulated the pouring of cast iron grade SCh30 with the following thermal and physical properties:= 39 W/(m⋅K);= 841 J/(kg⋅K);= 7360 kg/m³;t _hall = 1415 °C;t _face = 1230 °C;t _Sol = 1140 °C;Tfilling the form 22 s. The form was made from a sand-clay mixture:t _f = 25 °C,= 0.712 W/(m⋅K);= 1133 J/(kg K);= 1595 kg/m³;= 0.19 W/(m⋅K);= 0.29 mm. The basis of the mixed modifier is dispersed particles of graphite and FS. Individual calculations were performed for each component. Calculations were also made for large pieces of FS75 to compare their behavior with dispersed particles. The size and density of the granules during the process of filling the mold with cast iron were assumed to be constant. Graphite, FS and FS75 granules were modeled as ideal spheres with parameters equal to their average values in the modifier. The average diameters of graphite and FS granules are 13.21 and 17.12 μm, respectively, FS75 - 5.5 mm. The density of the materials is as follows: graphite - 2.19 g / cm³; FS and FS75 - 3.46 g/cm³. Form filling time is 22 seconds.

The distribution of modifier granules was analyzed in the inclined and vertical walls of the "friction wedge" in its lower, middle and upper fragments during the rise of the melt and modifier in the casting during the mold filling. The order of the graphite particles in the casting during the process of filling the casting with liquid cast iron is shown in Figure 2.

As can be seen in Figure 2, the graphite particles were uniformly distributed throughout the entire volume of the casting at all observed heights of the controlled walls. The simulation revealed that the graphite and FS particles in the casting behave similarly during the mold filling process, since the materials are close in density and size. As a result, it can be confidently stated that the identified patterns of granule arrangement will also be observed when using a mixture of FS and graphite. The FS and graphite particles serve as crystallization centers throughout the entire time of casting solidification, which ensures the absence of chill in the controlled walls of the casting. The casting will be qualitatively modified and the modification effect will be preserved over time, since the dispersed particles of the additive are practically not controlled by gravity.

According to the simulation results shown in Figure 3, it is evident that large granules of FS65, unlike dispersed USM and KSM, immediately float to the surface of the liquid cast iron and at the end of the mold filling end up mainly in the upper wall of the casting. This wall turns out to be the most modified.

Thus, the selected arrangement of castings in the mold determines a high probability of the occurrence of chilling in the inclined and vertical walls, which was subsequently confirmed by production experience.

Modeling has shown that for effective modification the presence of dispersed Si and C particles in the modifier is necessary.

The mixture for modifying gray cast iron is prepared by weighing the calculated doses of components in accordance with the proposed quantitative ratio, thoroughly mixed in a mixer, and then packed in paper or polyethylene bags. The weight of the bags depends on the mass of the melt being processed and is determined by the customer. The amount of the mixture introduced into the melt is 0.1 - 0.2%, depends on the composition of the cast iron being processed, the required level of mechanical properties, and is optimized experimentally. The bags are placed on the bottom of the ladle before pouring it with metal or are given to the stream. This ensures accurate dosing, no dusting, and improves the ecology of the foundry. In addition, the modifier can be introduced by blowing into the melt or rolled into a powder wire and other known methods.

Example 1.

At AvtoVAZ OJSC, a casting was modified - 12101-3103015 "Front wheel hub" made of high-strength cast iron grade VCh50. The chemical composition of the cast iron in the channel furnace was as follows: C = 3.60%, Si = 1.97%, Mn = 0.40%, P = 0.019%, S = 0.010%, Cr = 0.072%, Ni = 0.15%, Cu = 0.24%, Sn = 0.018%. The temperature of the cast iron melt in the furnace is 1530°C. The ladle capacity is 1200 kg.

Ladle No. 1. For spheroidizing modification, 4.8 kg (0.4%) of the "heavy" Ni-Mg-Ce ligature were used. Graphitizing modification was carried out with a mixture consisting of 35% aspiration graphite dust and 65% FS dust. This ratio was selected because the cast iron had a high carbon equivalent (3.6% C). The amount of mixed modifier was 1.2 kg (0.1%). The modifiers were introduced "under the stream in a single portion". After that, the ladle was transferred to AFL SPO-4, a ladle sample of the metal was taken for chemical analysis: C = 3.66%, Si = 2.00%, Ni = 0.40%, Mg = 0.035%, the content of other elements - see the analysis of metal from the furnace. The temperature of the beginning of ladle pouring is 1490 ° C.

Ladle No. 2. Everything was done similarly, only the graphitizing modification was carried out with a mixture in the amount of 2.4 kg (0.2%)

The analysis of the microstructure and mechanical properties of the cast iron of the experimental castings are given in Table 1. The microstructure of the castings is shown in Fig. 4-6.

Table 1 - Controlled parameters.

Casting Microstructure HB _5/750/10 σ _in , kgf/mm ² Shape, distribution and size of graphite inclusions Metal base 1 Nodular graphite, SSG 85% Lamellar pearlite - 60%, ferrite 40% 207 73 2 Nodular graphite, SSG 80% Lamellar pearlite - 60%, ferrite 40% 197…207 70 Requirements for
And 12011.37.101.66 Nodular graphite, SSG not less than 80% Metal base - ferrite-pearlite, cementite up to 5% 170…210

As can be seen from the data in the table and figures 4 and 5, the introduction of the mixture material in the amount of 1.2 kg (0.1%) together with 4.8 kg (0.4%) of the "heavy" Ni-Mg-Ce ligature into the ladle using the "under the stream in a single portion" method made it possible to obtain the VCh70 grade (σ _в = 73 kgf/mm ² ) with the required Brinell hardness (85%), pearlite-ferrite metal matrix and the required Brinell hardness (HB 207). Modification with the mixture in the amount of 2.4 kg (0.2%) made it possible to slightly reduce the hardness of the cast iron (to 197 HB) while maintaining the strength and microstructure indicators (Brinell hardness and pearlite/ferrite ratio, figure 6).

Thus, it is possible to obtain castings from high-strength cast iron of high grades due to ladle modification with a mixed modifier in the amount of 1.2 kg (0.1%). It should also be noted that the modifying mixture is quite well absorbed in the ladle when it is introduced using the "under the stream in a single portion" method.

Example 2

At AvtoVAZ OJSC, they modified the casting - 2101-1601093 "Clutch pressure plate" made of pearlitic grey cast iron grade SCh26.

The chemical composition of the cast iron in the channel furnace was as follows: C = 3.31%, Si = 1.96%, Mn = 0.57%, P = 0.021%, S = 0.020%, Cr = 0.22%, Ni = 0.18%, Cu = 0.15%, Sn = 0.032%. The temperature of the cast iron melt in the furnace was 1460°C. A mixed modifier consisting of 50% aspiration graphite dust and 50% FS dust was used for modification, which corresponds to a chemical composition of C = 3.31%.

1 ladle. A mixed modifier in the amount of 1.2 kg (0.1%) was fed into a 1200 kg ladle for graphitizing modification under the melt stream in a single portion. After that, the ladle was transferred to AFL SPO-4, a ladle sample of metal was taken for chemical analysis: C = 3.39%, Si = 1.92%, the content of other elements - see the analysis of metal from the furnace.

The temperature of the cast iron start pouring is 1410°C. The chill value by wedge sample is 2.0 mm with the standard being 1.6…2.0 mm. The wedge sample of cast iron from the furnace is 4.3 mm (the standard range is 4…6 mm). Consequently, the efficiency of ladle modification of gray cast iron with a modifier in the amount of 0.1% should be satisfactory.

The molds were poured from the test ladle immediately after the temperature was measured.

The analysis of the microstructure and the results of determining the mechanical properties of cast iron in castings are given in Table 2 and in Figures 7, 8.

Table 2 - Tested parameters.

Casting Microstructure HB _5/750/10 σ _in , kgf/mm ² Shape, distribution and size of graphite inclusions Metal base 1 PGr8, PGd4, PGd5 Lamellar pearlite, ferrite up to 10%,
cementite in the form of a mesh and in the corner of the section to a depth of 5 mm (20%) 224…229 24.5 Requirements for the standard FIAT-VAZ 52205 Type PGr1 - predominates, Perlite is lamellar, traces of cementite are allowed (<1%) 190…240 ≥ 26

Ladle modification of cast iron melt with a mixture in the amount of 0.1% (1.2 kg), despite the satisfactory value of the wedge sample - 2.0 mm - with a regulated range of 1.6 ... 2.0 mm, did not ensure the required quality of cast iron in the casting due to its design features. The microstructure of the cast iron contains free cementite in the form of a mesh and in the corner of the section to a depth of 5 mm in the amount of 10%, ferrite in the amount of ~ 10% (Figure 8) and interdendritic graphite of the PGr8 type (Figure 7). The low value of the tensile strength of cast iron (24.5 kgf/ ^mm2 , the actual grade is SCh24 instead of SCh26) is associated with the release of free cementite in the form of an embrittlement network along the boundaries of the pearlite grain of interdendritic graphite of the PGr8 type, which occurs due to the design features of the casting, cooling conditions, etc. The simultaneous presence of ferrite, pearlite and cementite in the structure of cast iron also indicates its insufficient modification, since the required structure should be represented only by pearlite, in which small traces of ferrite are allowed.

2 ladle. In a ladle with a capacity of 1200 kg, a mixed modifier in the amount of 1.8 kg (0.15%) was fed under the melt stream in a single portion to carry out graphitizing modification. After that, the ladle was transferred to AFL SPO-4, a ladle sample of metal was taken for chemical analysis: C = 3.37%, Si = 1.90%, the content of other elements - see the analysis of metal from the furnace.

The temperature of the cast iron start pouring is 1410°C. The chill value by wedge sample is 1.7 mm with the standard of 1.6…2.0 mm. The wedge sample of cast iron from the furnace is 4.1 mm (the standard range is 4…6 mm). Therefore, the efficiency of ladle modification of gray cast iron with a modifier in the amount of 0.15% should also be satisfactory.

When modified with a mixture in the amount of 1.8 kg (0.15%), the quality of the cast iron structure improved: there is no cementite in the form of a mesh, the chill depth in the corner of the section decreased from 5 to 1 mm, with a decrease in its distribution over the area from 20 to 10% (Figure 10); the content of free ferrite decreased from 10 to 5%. The amount of interdendritic graphite of the PGr8 type was also reduced, the predominant form of graphite became the PGr1 type (Figure 9). But the presence of a small amount of cementite and PGr8 graphite indicates undermodification of the cast iron.

Ladle No. 3. A mixed modifier in the amount of 2.4 kg (0.2%) was fed into a 1200 kg ladle for graphitizing modification under the melt stream in a single portion. After that, the ladle was transferred to AFL SPO-4, a ladle sample of metal was taken for chemical analysis: C = 3.38%, Si = 1.89%, the content of other elements - see the analysis of metal from the furnace.

The temperature of the cast iron start pouring is 1410°C. The chill value by wedge sample is 1.6 mm with the standard being 1.6…2.0 mm. The wedge sample of cast iron from the furnace is 4.0 mm (the standard range is 4…6 mm). Consequently, the efficiency of ladle modification of gray cast iron with a modifier in the amount of 0.2% should also be satisfactory.

When introducing 2.4 kg (0.2%) of the mixture into the ladle, the obtained values of the tensile strength of cast iron in the castings correspond to the SCh26 grade. There is no cementite in the casting (Figure 11). The Brinell hardness of the cast iron has noticeably decreased (from 224…229 to 197…207 HB). Thus, introducing 0.2% of the mixed modifier into the ladle allows obtaining the required quality of cast iron in the casting.

Example 3

The casting of the "friction wedge" from cast iron grade SCh30 was modified at the Cheboksary Aggregate Plant (ChAZ). The technical conditions for cast wedges regulate: σ _B ≥ 300 MPa; HB = 197 - 260. The required chemical composition of the cast iron is given in Table 3.

Table 3. - Elemental composition of cast iron SCh30 for "friction wedge" castings

Product name Elemental composition of cast iron, % by weight C Si Mn P S Cr Ni Cu No more Friction wedge 3.0-3.2 1.3-1.9 0.7-1.0 0.2 0.12 0.4 0.4 0.4

Three experimental melts were carried out. The chemical composition of the melts is given in Table 4.

Table 4 - Chemical composition of cast iron

Experimental melt no. Chemical composition, % by weight C Mn Si Cr Ni Cu S 1 3.11 0.97 1.79 0.16 0.12 0.13 0.059 2 3.13 0.95 1.76 0.14 0.13 0.15 0.056 3 3.15 0.86 1.81 0.13 0.12 0.12 0.057

The chemical composition of the cast iron complied with the requirements of the NTD. Graphitizing modification of the melt was carried out with a mixture consisting of 65% aspiration graphite dust and 35% FS dust. This ratio was chosen due to the fact that the cast iron had a low carbon equivalent (3.11 - 3.15% C). Increasing additives of mixtures from 0.1 to 0.2% of the mass of liquid cast iron and, for comparison, FS75 in an amount of 0.5% were added to the ladle (Table 5).

Table 5 - Quantity of additives introduced

Type
additives Additive value, % (kg) FS75 (0.5) (2.5) Mixture 0.1 (0.5) Mixture 0.15 (1) Mixture 0.2 (1.5)

Three cast iron melts were modified for friction wedge castings. The results of modification with FS75 additives and mixtures are presented in Table 6.

Table 6 - Results of modification of experimental melts

Experimental melt no. Additive Additive mass, %, kg Sampling location Chill on wedge, mm t, °С Mechanical properties σ _B , MPa NV 1 - - Bake 20 1465 - - FS75 0.5(2.5) Ladle 10 1405 301 225 Mixture 0.1(0.5) Ladle 10 1390 289 369 Mixture 0.15(0.75) Ladle 9 1375 330 250 Mixture 0.2(1) Ladle 7 1340 345 239 2 - - Bake 15 1458 - - mixture 0.2(1) Ladle 8 1395 301 231 mixture 0.1(0.5) Ladle 10 1369 285 383 mixture 0.15(0.75) Ladle 8 1350 297 225 FS75 0.5(2.5) Ladle 10 1330 319 237 3 - - Bake 14 1460 - - mixture 0.2(1) Ladle 6 1400 333 219 mixture 0.15(0.75) Ladle 8 1379 305 225 FS75 0.5(2.5) Ladle 8 1370 315 231 mixture 0.1(0.5) Ladle 7 1343 319 239

As can be seen from the data in the table, the best results were obtained with the addition of a modifying mixture in the ladle in an amount of 0.2%. In this case, the mechanical properties of the cast iron met the requirements of the NTD.

Thus, the claimed mixed modifier allows solving the problem of eliminating chill in castings made of gray and high-strength cast iron, reducing modifier consumption and recycling foundry waste.

Claims (2)

A mixture for modifying grey and high-strength cast iron, characterized in that it contains aspiration dust of ferrosilicon obtained by crushing it, and aspiration dust of graphite obtained by mechanical processing of graphitized electrodes, in the following ratio of components, wt.%: ferrosilicon aspiration dust 35-65 aspiration graphite dust 35-65