RU2318900C2 - Complex modifier for steel - Google Patents

Abstract

FIELD: metallurgy, namely production of heavy steel products with high level of strength, ductile and viscous characteristics and improved wear resistance in given temperature range, widely used in machine engineering, ship making, at manufacture of welded constructions and articles for North regions.

SUBSTANCE: modifier includes in addition nitrogen at next relation of components, mass %: silicon, 40 - 50; calcium, 21 - 42; aluminum, 1 - 2; nitrogen, 1 - 20, iron, the balance. Its phase composition includes at least 2.5 - 25 vol.% of nitrides. Modifier also includes barium in such quantity that total content of calcium and barium is no less than 21 mass%.

EFFECT: possibility for optimizing structural state of steel due to its improved uniformity and fraction size formed at diffusion decomposition of austenite during article cooling.

2 cl, 2 tbl

Description

The invention relates to metallurgy, in particular to the production of massive steel products having a high level of strength, plastic and viscosity characteristics, wear resistance, durability in the temperature range from -80 ° C to + 200 ° C, which are widely used in mechanical engineering, shipbuilding, railway transport, in the manufacture of welded structures, products in the "northern version", etc.

The operational reliability and durability of such products is determined by resistance to brittle fracture, ductility and toughness of steel. This is achieved:

a) a decrease in the number of stress concentrators in steel - inclusions (oxides, sulfides, oxysulfides, spinels, etc.), pores and other defects that are sources of cracking;

b) the dispersion of the matrix structure in finished products, providing an increase in the work of crack propagation.

The implementation of these tasks, in addition to the well-known special methods of steel smelting - ESR, VDP, inert gas treatment, etc., is carried out by modifying the metal during smelting with special compounds - modifiers.

So it is known to use silicocalcium for deoxidation and steel modification. Silicocalcium contains 10-30% Ca, 45-55% Si, 1-2% Al, 0.2-1.0% C, 0.02-0.04% P (see GOST 4762-71 "Silicocalcium. Technical conditions.").

The disadvantages of silicocalcium are its weak ability to disperse austenitic grain, which leads to coarsening of the structure of finished products even in the event of their subsequent heat treatment. As a result, there is an unsatisfactory level of viscous and plastic properties.

The closest in technical essence, the achieved result and selected as a prototype is an inoculant modifier for steel of the INSTEEL series. This modifier is an iron-silicon based alloy with the addition of active elements such as calcium, barium, RMZ, aluminum and others in various combinations. So the INSTEEL-1 modifier contains (in wt.%): 45-50% Si, 8-10% Ca, 8-10% Ba, 8-10% Al, the rest is Fe; the INSTEEL-4 modifier contains (in wt.%): 40-45% Si, 10-12% Ca, 7-8% RMZ, 7-8% Al, 1-1.5% Mg, 4-5% Ti, the rest is Fe (see the catalog of OOO Industrial Company NPP "Modifiers for out-of-furnace treatment of cast iron and steel", 2004, p.10).

This composition, contributing to the deoxidation and desulfurization of steel, as well as the formation of globular non-metallic inclusions, does not contain a sufficient number of elements that contribute to the grinding of the grain structure and, therefore, does not provide a high level of strength, ductility and toughness of the metal.

The present invention is to increase the strength, ductility and toughness of steel.

The technical result obtained by the implementation of the present invention is the optimization of the structural state of steel, namely, increasing the uniformity and dispersion of the structure formed during the diffuse decomposition of austenite in the process of cooling products.

This problem is solved due to the fact that the complex modifier for steel, containing silicon, calcium, aluminum and iron, according to the invention additionally contains nitrogen in the following ratios of components, wt.%:

Silicon 40-50 Calcium 21-42 Aluminum 1-2 Nitrogen 1-20 Iron rest

and its phase composition includes at least 2.5-25 vol. % nitrides.

The modifier may further include barium in such an amount that the total content of calcium and barium is at least 21 wt.%.

Studies conducted on the sources of patent and scientific and technical information have shown that the claimed modifier is unknown and does not follow explicitly from the studied prior art, i.e. meets the criteria of novelty and inventive step.

The inventive modifier can be manufactured at any enterprise specializing in this industry, because this requires well-known materials and standard equipment, and is widely used in the manufacture of steel products, i.e. is industrially applicable.

The introduction of a modifier in steel to improve the structural characteristics of the metal should solve four main problems:

a) additional deoxidation and desulfurization;

b) a decrease in the number of oxides, oxysulfides, sulfides and globularization of the remaining non-metallic inclusions;

c) purification of grain boundaries and near-boundary sections of particles and segregation, leading to embrittlement of the metal;

d) the formation of a fine-grained structure both after crystallization, and in subsequent technological operations to obtain finished products.

The effectiveness of each of the tasks depends on the composition of the modifier used and the conditions for its entry into the metal. Since at present the introduction of the modifier is usually carried out with cored wire, in which the modifier is a filler and the sheath is a steel tape rolled into a tube, the correct selection of the modifier composition is of particular importance. A decrease in the oxygen and sulfur content, a decrease in the number of nonmetallic inclusions, and the formation of a globular particle shape in the metal are usually ensured by the presence of 10-15% Ca or Ca + Ba in the modifier, since Behavior in liquid steel Ba, as one of the alkaline earth elements, is largely similar to Ca, as well as 1-2% Al.

The use of scanning electron microscopy, X-ray microspectral and X-ray diffraction analysis showed that the Ca and Ba in the modifier, as a rule, are in the form of alloys and compounds with silicon, forming various phases: silicocalcium - CaSi ₂ , silicobarium - Ba (AlSi) ₄ , complex alloy - Ca-Si-Ba. The amount and composition of these phases in the structure of the modifier depends on the content of Ca, Ba, Si, Al, and other elements. It is well known that when the content of Ca (or the total amount of Ca + Ba) in the modifier is not more than 20%, along with the above phases, various low-temperature eutectics are present in the structure: Si-Ca-Fe-Al, Ca ₂ MgSi ₃ , Fe-Si -Ca-Ti and others, concentrated mainly along the grain boundaries.

Purification of grain boundaries and boundary volumes from particles, segregations and eutectics, embrittlement of metal and additionally containing phosphorus, sulfur, antimony, etc., is a more difficult task. To solve it, both the presence itself and the quantitative content of Ca are important, which, in addition to deoxidizing properties, is a horophilic element and, having limited solubility in the solid state in steels (up to 0.003%), it is concentrated mainly in the boundary volumes of grains, displacing compounds and phases from there containing phosphorus, sulfur, etc. The presence of Ca (at least 0.002%) in solid solution and its presence in border volumes only in the case of high concentrations of Ca in the modifier was found to be at least 21%. At these concentrations, the maximum modifying effect is ensured, and in the structure of such a modifier the amount of Ca and Ba containing phases is at least 50 vol.%. At a lower Ca content, a significant amount of low-temperature eutectics is present in the modifier, which concentrate along grain boundaries in steel, while Ca is only enough for deoxidation, desulfurization, and globularization of the remaining inclusions. In practice, a partial replacement in the Ca modifier with Ba is possible. Their joint presence in steel leads to a decrease in the activity of each element, reduces the partial vapor pressure of Ca, and thereby increases its “survivability,” leading to an increase in the residual Ca content in steel. Metallurgical practice shows that the use of large amounts of Ca, or Ca + Ba in the modifier (> 42-45 wt.%) Is undesirable, because In this case, when processing steel, a significant pyroelectric effect is observed, accompanied by the ejection of metal from the ladle.

Previously, with all known modification options, the task of grinding the grain structure of steel was solved, as a rule, by using dispersed high-temperature particles (inoculants) introduced into the molten metal as part of the modifier. In this case, the particles must have certain crystallographic parameters and disperse the structure due to an increase in the number of nucleus centers. Another option to refine the structure is the introduction of surface-active additives, which precipitate during crystallization at the boundaries of growing dendrites (grains), reduce the surface energy of the boundaries and slow down their speed.

The present invention provides a different solution. The suppression of grain growth, and first of all, the limitation of the size of austenitic crystallites, is carried out by dispersed nitride particles released from the solid solution both directly during cooling of the cast metal and during subsequent heat treatment of the products. Their required number and degree of dispersion are provided by a certain ratio of nitride-forming elements and a change in their equilibrium content in solution at various temperatures. We emphasize that we are not talking about primary non-metallic inclusions (1-3 microns in size) formed during the crystallization of steel, but about particles of 100-700 Å released from a solid solution, which inhibit the growth of ferrite and austenite grains during heating, aging and cooling of the metal thereby providing a high level of strength, plastic and viscosity characteristics of the finished product. It was found that controlling the dispersion and uniformity of the grain structure is most effective if the dispersed phases are nitrides (carbonitrides) of aluminum, silicon, and vanadium, i.e. such inclusions that can dissolve and stand out in the temperature range of the heat treatment. It was shown that the formation of nitrides in an amount sufficient to inhibit grain growth occurs when the nitrogen content in the modifier is 1-20 wt.%, Which corresponds to the total amount of chemical compounds of silicon nitrides, aluminum, etc. in the structure of the modifier within 2.5 -25 vol. %

When the nitrogen content in the modifier is less than 1 wt.%, A sufficient density of secondary nitrides does not form and a coarse-grained structure of the finished products is formed. When the nitrogen content in the modifier is more than 20 wt.% And, accordingly, in the presence of more than 25 vol.% Phases containing nitrogen in the structure, large nitrides are formed in the steel and the brittleness of the metal increases.

The inventive modifier was tested in the production of side frames of freight wagon trolleys.

A comprehensive modifier of the proposed composition was obtained as follows. The starting materials, including silicocalcium (20 and 30 wt.% Ca), nitrided ferrosilicon (34 wt.% N), silicobarium (22 wt.% Ba), calcium metal, were mechanically mixed in various proportions, obtaining complex modifiers that differ in chemical and phase composition.

Smelting of steel containing (in wt.%): 0.20% C; 0.26% Si; 1.2% Mn; 0.14% Cr; 0.13% Ni; 0.13% Ca; 0.05% Al; 0.008 N was carried out in a 30-ton electric furnace. Next, the metal was portioned into the pouring ladles, in which the final deoxidation of Al was carried out at the rate of 0.5 kg / t of liquid steel, and then the modification with flux-cored wire with different chemical and phase composition of the modifiers filling it in the amount of 1.1 kg of wire per ton of liquid steel . The modified metal was poured into molds and, after crystallization, the products were subjected to heat treatment at 900 ° C for one hour, followed by cooling in air. In the heat-treated metal, the structure, tensile strength, elongation, and impact strength were evaluated on samples with a V-shaped notch at a temperature of -60 ° C according to GOST 9454-78. The phase composition of the modifiers used was studied using scanning electron microscopy, X-ray spectral and X-ray diffraction analysis.

The results of determining the chemical and phase composition of the modifiers are given in table 1. The composition of the modifiers in options 1 and 2 correspond to the inoculant modifiers INSTEEL-1 and INSTEEL-4 (the composition of the modifiers is given in the description of the prototype), selected as a prototype.

Table 2 presents the results of assessing the grain structure and mechanical properties of heat-treated products obtained from castings modified with the modifier compositions given in the table.

From the analysis of the data shown in tables 1 and 2, it is seen that:

1. The use of a modifier, the composition of which corresponds to the prototype (options 1 and 2) leads to the formation of calcium and barium-containing phases in the modifier in the amount of 28-37 vol.%, And in the finished products a coarse-grained (d> 27 μm) structure, low strength values σ _c , ductility δ and impact toughness a _KCV- 60 ° C

2. The use of a modifier having the stated chemical composition leads to the formation in the modifier of 2.5-25 vol.% Nitrides of at least 50 vol.% Calcium and barium-containing phases (options 4-7, 10-13), and in the finished products fine-grained structure (d≤23 μm) and high strength values (σ _in > 58 kg / cm ² ), ductility (δ> 26.8%) and impact strength (and _KCV- 60 ° C≥2 kgf · m / mm ² ) .

3. The use of a modifier containing a sufficient total amount of Ca and Ba, but not containing nitrogen (options 3, 9) or having more than 25 wt.% Nitrogen (options 8, 14), leads to a coarsening of the finished product structure and a fall level of mechanical properties.

Thus, the analysis of the data in the tables shows that the inventive modifier provides an increase in strength, ductility and toughness by optimizing the structural state of steel, namely, increasing the uniformity and dispersion of the structure formed during diffuse decomposition of austenite in the process of cooling products.

Table 1 The chemical and phase composition of the modifiers used The chemical composition of the modifiers, wt.% The phase composition of modifiers, vol.% No. p / p Si Ca Ba Al RMZ Ti Mg N Fe Silicocalcium CaSi ₂ Silicobarium Ba (SiAl) ₄ Silicon-calcium barium Ca-Si-Ba Nitrides Si ₃ N ₄ , AlN ₄ , etc. one fifty 10 10 8 - - - - rest twenty 5 12 - 2 45 12 - 8 8 four one - rest 28 - - - 3 45 21 - 1,0 - - - - rest fifty - - - four 45 21 - 1,2 - - - one rest 51 - - 2,5 5 45 thirty - 1,0 10 2 - 5 rest 70 - - 13 6 45 thirty - 1,0 - - - 10 rest 72 - - eighteen 7 fifty 25 - 1,2 - - - twenty rest 58 - - 25 8 45 25 - 1,2 - - - 25 rest 60 - - thirty 9 45 fifteen 6 1,0 - - - - rest 28 3 22 - 10 fifty fifteen 6 1,0 - - - one rest 28 2 21 2,5 eleven 45 fifteen 6 1,0 8 2 - 5 rest thirty 2 21 fourteen 12 45 25 5 1,5 - - - 10 rest 52 one fifteen eighteen 13 45 25 5 1,0 - - - twenty rest fifty one 16 25 fourteen 45 twenty 5 1,0 - - - 25 rest 42 one fifteen 29th

table 2 The effect of the composition of modifiers on the grain structure and mechanical properties of heat-treated products No. p / p The content of elements, wt.% The total content of phases, vol.% d, μm σ _in , kg / cm ² δ,% and _{KCV-60 ° C} , kgf · m / mm ² Si Ca Ba Al Calcium-barium-containing phases (table 1) Nitrides (table 1) one fifty 10 10 8 37 - 29th 55,2 21,2 1,0 2 45 12 - 8 28 - thirty 54.0 23.0 1,1 3 45 21 - 1,0 fifty - 28 55.3 22.3 1,2 four 45 21 - 1,2 51 2,5 23 58.3 27.6 2.0 5 45 thirty - 1,0 70 13 19 59.0 28.3 2.6 6 45 thirty - 1,0 72 eighteen eighteen 59.1 28,2 3.0 7 fifty 25 - 1,2 58 25 17 58.9 27.8 2.9 8 45 25 - 1,2 60 thirty 27 56.1 21.0 1,0 9 45 fifteen 6 1,0 53 - thirty 55,2 21.0 1,0 10 fifty fifteen 6 1,0 51 2,5 23 58.2 26.8 2.2 eleven 45 fifteen 6 1,0 53 fourteen twenty 58.0 27,2 2,8 12 45 25 5 1,5 68 eighteen eighteen 59.3 28.1 3.0 13 45 25 5 1,0 67 25 16 60.5 28.0 2.95 fourteen 45 twenty 5 1,0 58 29th 26 56.2 22.3 1.15

Claims (2)

1. A complex modifier for steel containing silicon, calcium, aluminum and iron, characterized in that it additionally contains nitrogen in the following ratios of components, wt.%: Silicon 40-50 Calcium 21-42 Aluminum 1-2 Nitrogen 1-20 Iron Rest, and its phase composition includes at least 2.5-25 vol.% nitrides. 2. The modifier according to claim 1, characterized in that it additionally contains barium in such an amount that the total content of calcium and barium is at least 21 wt.%.