RU2489511C2 - Grey cast iron obtaining method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves obtaining of molten metal containing carbon, silicon, manganese, chrome, titanium, zirconium and copper as the main alloying components at the following ratio, wt %: carbon 2.9-3.6, silicon 1.2-1.7, manganese 0.2, 0.2-0.5, chrome 0.03-0.08, titanium 0.03-0.08, zirconium 0.02-0.10, copper 0.2-1.5, and iron is the rest, and introduction to molten metal to the bottom of a ladle of 0.03-0.10 wt % of a complex additive based on stannum, which contains the following, wt %: bismuth 14, selenium 14, boron 9, antimony 7, and stannum 7.

EFFECT: improvement of processibility and provision of high strength owing to stabilising pearlite structure in primary dendrites and reducing tendency of cast iron to formation of triple phosphide-cementite eutectics.

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Description

The invention relates to the field of metallurgy and is intended to produce thin-walled case castings of critical use from gray cast iron in the agricultural, oil and gas and other engineering industries.

Known perlite gray cast iron for the manufacture of castings according to the Lanz method, which, with moderate hardness HB 160 ... 180, and, therefore, good machinability by cutting, has a tensile strength σ _in = 250 ... 330 MPa. Such high strength characteristics of this cast iron are ensured by a decrease in its composition of silicon content to 0.5 ... 1.5%. Under normal conditions, cast iron with such a low silicon content solidifies in white. To achieve complete graphitization of the eutectic and to obtain castings from gray cast iron without bleaching, the metal is poured into preheated forms. The mold heating temperature depends on the composition of cast iron and the wall thickness of the casting. By slowing down the cooling rate, it is possible to obtain a sufficiently soft and easily machined cast iron containing, for example, at 3.3% C and 0.8% Si, while the same cast iron under normal cooling conditions should be so hard (bleached) that it can be processed it would be impossible [1].

The disadvantages of this cast iron include the fact that the preheating of the whole form, necessary for its use, does not provide optimal cooling conditions and the formation of a pearlite structure in the walls of the castings of different thicknesses. The casting receives a slowed down cooling rate in all structurally sensitive temperature ranges, including in the range of dendritic crystallization and pearlite transformation, where such a slowdown is undesirable from the point of view of obtaining the optimal structure of the metal base. The complexity of the technology for casting thus limits the range of possible products.

Known high quality gray cast iron for castings, in which the tensile strength σ _to reach 280 ... 335 MPa by introducing into the charge of from 50 to 80% steel. The main feature of the production of these cast irons is that the carbon content in them does not exceed 3.0%, and to prevent bleaching of castings, the silicon content in the metal should be from 2.0 to 2.5% [1, see also (CN 101671754, C21C 1/10. Date of publication: 2010.03.17)].

, %С, %Si, %Р - содержание углерода, кремния и фосфора в чугуне в % масс., широко применялось в практике литейного производства. При затвердевании низкоэвтектического чугуна образуется большое количество дендритных кристаллов первичного аустенита, которые способны эффективно упрочнять металл, армируя малопрочную эвтектическую матрицу, если в результате последующих превращений они приобретают чисто перлитную структуру [2, 3, 4, 5, 6].Improving the strength characteristics of gray cast iron with lamellar graphite by adjusting the composition in the direction of decreasing the content of Si or C, which reduces the degree of eutectic of cast iron $$S_c(S_c=\frac{\%c}{4.3-\raisebox{1ex}{$\mathrm{one}$}\!\left/ \!\raisebox{-1ex}{$3$}\right.(\%Si+\%P)})$$

,% C,% Si,% P - the content of carbon, silicon and phosphorus in cast iron in wt.%, Has been widely used in foundry practice. Upon hardening of low-eutectic iron, a large number of dendritic crystals of primary austenite are formed, which are able to efficiently harden the metal, reinforcing a low-strength eutectic matrix, if as a result of subsequent transformations they acquire a pure pearlite structure [2, 3, 4, 5, 6].

A common drawback of these technical solutions is the tendency of low eutectic cast irons to perlite instability and the formation of ferrite in the primary dendrite region, which reduces their reinforcing ability and the strength of cast iron as a whole. This feature of structure formation is associated with patterns of microliquation of cast iron components, which oppositely affect the thermodynamic activity of carbon in austenite and ferrite.

All elements that increase the activity of carbon a _c segregate into primary dendritic crystals, and elements that reduce a _c enrich the eutectic component. Due to this “segregation polarization” of the primary structure, the activity of carbon in dendrites is always (for any composition of cast iron), and in the eutectic it is reduced, which inevitably causes a difference in the activity of carbon Δ a _s between these micro-segregation zones. A decrease in carbon activity in the eutectic increases the tendency to crystallization of cast iron in the metastable system and the likelihood of bleaching in castings.

The segregation formed during the crystallization process due to the low diffusion mobility of almost all components of cast iron (except carbon) has a very high resistance and persists not only until the eutectoid transformation interval, but also to room temperature. Moreover, they are preserved and even aggravated by repeated heating [7]. Cast iron, like any system, seeks to equalize its thermodynamic characteristics, in particular, carbon activity. Due to the low diffusion mobility of other components, equalization of a _with in real conditions is possible only due to the mass transfer of carbon from dendrites to eutectic. Thus, the inevitable enrichment of primary dendrites with elements that increase the activity of carbon determines the instability of the pearlite structure in these microliquations.

Graphization of the eutectoid with the formation of a pearlite-ferritic and even purely ferritic structure in the region of primary dendrites can also develop during cooling of castings in the mold or during subsequent technological or operational subcritical heating. Full or partial ferritization of primary dendrites reduces their reinforcing ability, and the strength of cast iron as a whole drops sharply. Moreover, it was found in [8] that the softening effect of ferrite located in primary dendrites is 15–20 times stronger than that of localized in a eutectic matrix.

The intensity of decarburization of dendrites is proportional to the difference in activity Δ a _c in microliquation zones, the value of which depends on the chemical nature of the components of cast iron and, due to the above-mentioned “liquation polarization”, always grows with an increase in the content of components of any complex. In other words, it is impossible to select the components that compensate for the increase in ac in dendrites, since the elements that reduce the activity of carbon segregate into the liquid phase and are collected mainly in the eutectic. An increase in the volume fraction of dendrites is also accompanied by an increase in microliquation heterogeneity and an increase in the activity difference Δ a _s .

In [9], it was shown that the stability of the pearlite structure in the region of primary dendrites increases in low manganese cast irons with balancing with a lower silicon content.

Based on this study, we selected the closest to the proposed cast iron in technical essence and the achieved effect of gray perlite cast iron with the following ratio of ingredients, wt.%:

carbon 2.9-3.6;

silicon 1.2-1.7;

manganese 0.03-0.30;

chrome 0.03-0.08:

titanium 0.03-0.08;

zirconium 0.02-0.10;

copper 0.20-1.50;

iron the rest.

(see SU 1294862 A1, IPC C22c 37/06).

The disadvantage of this method is a sharp deterioration in machinability by cutting with a high sulfur content in cast iron with a manganese content close to the lower value of the declared interval. In this case, a triple phosphide-cementite eutectic is formed - steadite with a very high hardness [10], which sharply affects the machinability of cast iron by cutting.

The problem to which the invention is directed, is to eliminate the harmful effects of sulfur, expressed in the formation of a triple phosphide-cementite eutectic, which reduces machinability, and to prevent the formation of ferrite in dendritic branches, causing a decrease in the strength of cast iron.

The technical result achieved by the invention is to increase the strength by stabilizing the pearlite structure in the primary dendrites and improving the machinability of cast iron by preventing the formation of triple phosphide-cementite eutectic.

The specified technical result in the implementation of the invention is achieved by the fact that in the well-known cast iron, a complex additive consisting of 14% Bi is additionally introduced; 14% Se; 14% Zn; 9% B; 7% Sb; 7% Sn; the rest of Pb, in an amount of 0.03 ... 0.10% of the mass, with the following content of the main components,% of the mass:

carbon 2.9-3.6;

silicon 1.2-1.7;

manganese 0.20-0.50;

chromium 0.03-0.08;

titanium 0.03-0.08;

zirconium 0.02-0.10;

copper 0.20-1.50;

iron the rest.

.The increase in the lower and upper limits of the manganese content is due to the need to bind sulfur to manganese sulfide. Being in solution, sulfur very liquates in the liquid phase, causing severe overcooling and the formation of a developed triple phosphide-cementite eutectic. The manganese content cannot be lower than determined by the formula $$Mn,\%=1.72\times S,\%+0.1\text{[eleven]}$$

.

The introduction of a complex additive of surface-active elements changes the crystallization mechanism of dendrites of primary austenite, and, consequently, the segregation characteristics of the components also change. The dendritic growth of primary austenite crystals under the action of the introduced additive ceases, and excess austenite with a higher silicon content crystallizes in layers on its surface. The resulting high-silicon shell is subsequently a barrier to the mass transfer of carbon from dendritic crystals to eutectic. The shell itself will have a ferritic structure, but decarburization and ferritization of the central regions of the dendrites will be difficult. As a result, dendrites with a stable perlite structure will have a high reinforcing ability, which will provide increased strength of cast iron as a whole.

The invention is illustrated by examples in which a complex additive of surface-active elements was introduced in an amount corresponding to the recommended interval (options 2-4, see table 1), as well as in an amount outside it (options 1, 5)), for comparison, cast iron was also smelted by a known method (SU 1294862 A1, IPC C22c 37/06).

Table 1. Compositions of a complex additive in experimental cast iron melts (per 10 kg of molten metal) The component of the additive, the content of the active element The content of the component in experimental additives, g one 2 3 four 5 Alloy 3.6% Sb - 13.6% Sn - 72.8% Pb (cast fraction) 1.25 2.0 3.0 4.0 5,0 Bismuth metal, 99.98% Bi (granular) 0.2 0.5 1,0 1,5 2.0 Technical selenium, 99.8% Se (powder) 0.25 0.5 1,0 1,5 2.0 Zinc, 99.94% Zn (granular) 0.25 0.5 1,0 1,5 2.0 Boric acid, 18% V (powder) 1,0 2.0 3.0 4.0 5,0 The total amount of surface-active elements introduced with the addition,% of the mass. 0,021 0,031 0,065 0.0923 0.119

Cast iron was smelted in an induction furnace IS 1-004 on a charge consisting of metallized pellets and ferroalloys. The liquid metal was modified with silico-zirconium with a grain size of 3-5 mm, introducing it into the casting bucket under a stream of cast iron. The copper content was regulated by introducing cathode copper into the charge. The complex additive was introduced to the bottom of the casting bucket. From each version of cast iron at a metal temperature of 1420-1400 ° C, technological samples were cast using an immersion thermocouple for bleaching (wedge) and sample blanks to determine the mechanical properties of 030 mm and a length of 340 mm. The compositions of cast irons are shown in table 2.

Table 2. Chemical composition * of test cast irons Chemical elements of the composition The content of elements,% of the mass. Complex Additive Options By a known method one 2 3 four 5 6 basic FROM 3,55 3.20 3.3 3.05 2.9 3.57 Si 1.7 1.45 1.68 1.4 1,62 1.7 Mn 0.29 0.25 0.45 0.20 0.30 0.05 Cr 0,062 0,030 0,037 0,030 0.08 0,045 Ti 0,035 0.04 0,042 0,050 0,055 0,040 Zr 0,03 0.05 0.04 0,025 0.06 0,025 Cu 0.7 0.5 0.3 1,0 0.8 0.72 S 0,07 0.06 0.04 0.05 0.08 0.05 Introduced with a comprehensive supplement Bi 0,0008 0.0012 0.002 0.0025 0.0045 - Se 0.001 0.002 0.004 0.004 0.006 - Zn 0.003 0.006 0.008 0.009 0.010 - AT 0,0007 0.0014 0.002 0.003 0.007 - Sb 0.004 0.006 0.009 0.004 0.005 - Sn 0.005 0.0065 0.009 0.007 0.006 - Pb 0.004 0.0045 0.004 0.009 0.009 - * The rest is iron.

The tendency of cast iron to bleach was studied using wedge samples.

Comparative tests of machinability by cutting were carried out according to the Kesner method when drilling square bars with a side of 30 mm. The method consists in drilling the test material with a drill at a constant spindle speed and constant force on it.

The workability was characterized by the depth of penetration of the drill (average of 10 measurements) into the material for a fixed time (15 s).

The tests were carried out on a vertical drilling machine at a spindle speed of 350 min ^-1 and a force on the spindle of 490 N, which was created by the load. In this case, a ⌀10 mm drill made of P6M5 steel, a normal sharpening form (2φ = 118 °), was used.

The microstructure of the metal base of cast iron was studied after etching with 4% alcohol solution of nitric acid microsections cut from samples for mechanical testing.

The data obtained as a result of research are shown in table 3.

Table 3. The results of comparative studies of cast iron obtained by the proposed (option 1-5) and known methods Test cast iron Metal base structure Bleached by wedge test, mm Machinability, drilling depth,
mm Hardness, HB Tensile strength, σ _in , MPa with complex additive options one Perlite + 5% Ferrite 1,5 6.0 140 150 2 Perlite + ferrite border around the branches of dendrites 2.0 5.2 208 230 3 2,5 4.85 216 278 four 3.0 4.75 221 318 5 Perlite 7.0 3.0 254 300 by a known method 6 Perlite + 8% ferrite + steadite 4.0 1.8 265 175

When the manganese content is less than the lower limit, and the complex additive is greater than the upper limit (options 5, 6, see table 1, 2), the machinability of cast iron deteriorates, and the tendency to bleach increases.

With an insufficient amount of complex additives (option 1 of table 1, 2), the strength of cast iron is sharply reduced.

The limitation of the lower limit of the manganese content of at least 0.20% reliably protects the cast iron from the formation of steadite, and a complex additive stabilizes the pearlite structure in the region of primary dendritic crystals. The increase in the upper limit of the manganese content to 0.5 wt.% Is dictated by the technological need to have a sufficient predetermined concentration range. At the same time, such an increase in the manganese content cannot cause softening of cast iron due to ferritization of dendritic branches, since the pearlite structure in them is stabilized by the action of a complex additive.

Analysis of the data in the table shows that, in comparison with the known analogue, the proposed method for producing gray iron castings has an optimal complex of mechanical and technological properties. The use of this cast iron for the manufacture of complex case castings will increase the quality of casting and will allow to obtain a significant economic effect.

Used sources of literature

1. K. Geiger. Foundry. Per. from German. T.1. ONTI. Leningrad 1934, 320 p.

2. Patterson V., Microstructure of cast iron and its properties // In the book. 29th International Foundry Congress / In Patterson / - M.: Mechanical Engineering, - 1967, - p. 55-63.

3. Girshovich NG, Primary structure as a criterion for assessing the mechanical properties of gray cast iron / N.G. Girshovich, I.A. Ioffe, G.A. Kosnikov // - L .: LDNTP, 1967 .-- 30 p.

4. Ilyinsky VA, On the composite nature of the crystallization structure of cast irons with varying degrees of eutectic. / B.A. Ilyinsky, L.V. Kostyleva // Proceedings of the USSR Academy of Sciences. Metals -1986. - No. 5. - S.116-118.

5. Arzamasov BN, Construction materials. Directory. // B.N. Arzamasov / - M.: Mechanical Engineering - 1990, - 687 p.

6. Eliot R., Management of eutectic solidification // R. Eliot / - M. Metallurgy, - 1987, - 350 p.

7. Il'insky V.A., Mechanism of Microsegregation in Iron-Carbon Alloys and New Possibilities in Foundry Technology // V.A. l'insky, A.A. Zhukov, L.V. Kostyleva / Cast Metals, - 1990, - v.3, - No. 1, - P. 42-48.

8. Ilyinsky V.A., Effect of dendritic segregation on the pearlite-ferrite structure of gray cast iron // V.A. Ilyinsky, L.V. Kostyleva / Metallurgy and heat treatment of metals. - 1987, - No. 5, - p. 47.

9. Ilyinsky VA, Features of crystallization of cast iron, limiting the effectiveness of its alloying // V.A. Ilyinsky, L.V. Kostyleva / Foundry, - 1990, - No. 4, - S.5-6.

10. Ilyinsky V.A., Foundry defects in the structure of thin-walled cast iron castings // V.A. Ilyinsky, L.V. Kostyleva, N.P. Rubtsova, V.G. Maikov / Foundry, - 1988, - No. 12, - P.5-6.

11. Girshovich N.G., Iron casting // N.G. Girshovich / - L. - M .: Metallurgizdat, - 1949, - 708 p.

Claims (1)

A method of producing gray cast iron, including the production of a melt containing carbon, silicon, manganese, chromium, titanium, zirconium, copper as the main alloying elements, characterized in that the melt containing the main alloying elements is melted in the following ratio of components, wt.%:
carbon 2.9-3.6 silicon 1.4-1.8 manganese 0.20-0.50 chromium 0.03-0.08 titanium 0.03-0.08 zirconium 0.02-0.10 copper 0.20-1.50 iron rest,
and injected into the melt at the bottom of the pouring bucket from 0.03 to 0.10 wt.% complex additives containing, wt.%:
bismuth fourteen selenium fourteen zinc fourteen boron 9 antimony 7 tin 7 lead rest