RU2175985C1 - Method of making electrical-sheet anisotropic steel - Google Patents

Abstract

FIELD: ferrous metallurgy; making grain-oriented electrical- sheet steels at high permeability of magnetic flux. SUBSTANCE: method consists in maximum realization of principle of grain- oriented heredity in production line making electrical-sheet anisotropic steel. Method includes heating of steel before hot rolling in temperature range ensuring single-phase (ferrite) structure; time and temperature conditions of hot deformation of metal at which 3 to 8% of austenite appears in temperature range from 1100 to 1150 C; method includes also heating of steel for primary recrystallization after second cold rolling procedure at rather slow rate. EFFECT: high magnetic properties of steel made according to this method. 3 cl, 1 dwg, 1 tbl

Description

 The invention relates to the field of ferrous metallurgy, in particular to a technology for the production of cold rolled anisotropic steel with high magnetic permeability.

 Sheet electrical steel is the most important soft magnetic material used for the manufacture of magnetic cores and magnetically active parts of various electrical devices. The properties of electrical steel largely determine the characteristics, efficiency, dimensions of devices and the possibility of improving them; therefore, much attention is paid to improving the production technology and improving the characteristics of steels, especially magnetic properties.

 High magnetic properties of the finished electrical anisotropic steel (EAS) are ensured by the presence of a perfect crystallographic texture (110) [001] in the steel (rib texture, Goss texture), which is formed during secondary recrystallization (HR) during high-temperature annealing. For BP to occur, it is necessary, firstly, to create a certain structural and textural heterogeneity already during hot rolling of steel and, secondly, the presence of dispersed particles of the inhibitor phase in the metal.

 Obtaining the necessary crystallographic texture in EAS is achieved through the implementation of the structural heredity mechanism. The inhibitory phase delays the normal growth of grains, allowing the BP process to be realized.

 The texture state of iron-silicon materials after high-temperature deformation is one of the most important structural elements that determine the development of the texture formation process during subsequent cold rolling and recrystallization. In the process of hot rolling (GP), the basic structural parameters are laid that affect the texture formation processes and, as a result, the magnetic properties of the finished electrical steel. The effect on structure and texturing is manifested in the inheritance of the features of the initial structure of hot-rolled steel according to the technological redistribution of the end-to-end anisotropic steel production cycle.

 Currently, there are three main options for the production of electrical transformer steel: sulfide, nitride and sulfonitride. These options differ in chemical compositions and processing modes.

 The sulfide variant has been known since the late 40s and is currently the most common. The inhibitory phase in this steel is manganese sulfide -MnS. The main technological operations in the production of steel according to the sulfide variant are to limit the concentration of manganese, high-temperature heating in front of the GP, hot rolling, two cold rolling separated by recrystallization annealing, decarburization annealing and high temperature annealing (WTO). Finished steel has a magnetic induction in the field of 800 A / m - 1.81-1.84 T. Of fundamental importance during hot rolling of steel is the formation in the subsurface layer of the region of elongated polygonized crystals with a pronounced deformation texture - (110) [001]. The presence of this layer is ensured by two cold rolling with deformations of 40-60%, separated by recrystallization annealing, in order to obtain a sufficiently large number of grains with perfect orientation (110) [001] in the steel structure before the WTO, some of which will be secondary nuclei recrystallization.

 Steel nitride variant has a high content of carbon, nitrogen and copper. The inhibitory phase is aluminum nitride (AlN). The main operations after hot rolling are the first cold rolling, decarburization annealing, the second cold rolling and high temperature annealing. Magnetic induction in the field of 800 A / m - 1.85-1.89 T.

A significant difference between the steel of the nitride variant and the sulfide one is the lower heating of the metal before hot rolling (~ 1250 ^o C, versus 1400 ^o C). The consequence of this, as well as a higher carbon content in steel, is the formation of (110) [uvw] recrystallization texture in hot rolling in the subsurface layer, in which the perfect component (110) [001] is very weakly expressed. For this reason, it is fundamentally important to heat up the primary recrystallization after the second cold rolling at a slower rate. The low-temperature, time-stretched primary recrystallization in the presence of segregation of impurities and / or dispersed particles is a kind of “filter” for nucleation and growth of grains with a (110) [uvw] orientation in the deformed metal, which allows crystallites with a (110) [001] texture to form mainly ]. It should also be noted that such treatment is possible only for steel with a more stable inhibitory phase, which is AlN.

Steel sulfonitride variant has a high content (compared with sulfide) of carbon and aluminum. The main operations after hot rolling are normalization, single cold rolling, decarburization annealing and high temperature annealing. Magnetic inductions in the field of 800 A / m -1.89-1.94 T are the highest for finished steel, which is ensured by the formation of a superdense dispersed inhibitory phase during heat treatment (and not during hot rolling) and a powerful force effect on the texture steel, which is a single rolling. Fundamentally important in this technology is the presence of high-temperature normalizing annealing (1120-1150 ^o C) with a strictly regulated cooling law. It should be noted that the production of this type of steel in Russia and a number of other countries is currently impossible due to the lack of the necessary equipment.

 The present invention will provide a highly permeable state in electrical anisotropic steel in the process of double rolling (without the use of normalization) due to the maximum implementation of the texture heredity mechanism in combination with a decrease in the heating rate of the metal in the range of polygonization and primary recrystallization temperatures. In other words, it is about taking advantage of the sulfide and nitride variants.

 Studies on the structure of the hot-rolled strip emphasize the presence of significant heterogeneity in the morphology of the grains and texture along the thickness of the tack. According to the data presented, the formation of the structural features of the EAS tackle occurs at the stage of finish rolling, where the main role is played by the temperature-deformation processing regimes. First, the presence of distinct zones of subsurface equiaxial recrystallized grains with a predominant texture of (110) [uvw] and the central region of elongated polygonized crystallites with a predominant orientation of (110) [001], and secondly, the presence of regions decomposition products of austenite.

 The texture inhomogeneity formed during the HH process is due to differences in the path of metal flow in the surface and central layers as it passes through the deformation zone. Obviously, the most perfect deformation texture (in the surface layer is (110) [001], (100) [011] in the central region), which must be obtained to produce the highest quality EAS, deformed but not recrystallized grains will possess. The recrystallization processes occurring during HP should contribute to the scattering of the deformation texture. The higher the degree of recrystallization of the structure, the weaker the deformation orientations will be expressed in it. In the sulfide variant, this is achieved by limiting the carbon concentration to 0.028 wt.%.

For steel nitride variant containing 0.03-0.05 wt.% Carbon, up to 0.2 wt.% Manganese and up to 0.6 wt.% Copper, the lines of the decay products of the gamma phase are observed after rough rolling of steel. This allows us to talk about the presence of austenite in steel when it is heated before HF in the temperature range 1250-1300 ^o C. According to metallographic analysis (length of lines and their number per unit area in the metal at different stages of HF), the average amount of gamma phase in nitride steels the inhibition variant with a typical chemical composition is 10-20%. For this reason, the severity and perfection of the texture (110) [001] in this steel is inferior to the steel of the sulfide variant.

 In addition, in the EAS of the nitride variant of inhibition, secondary aluminum nitrides, which are released during the decarburization operation due to nitrogen released from areas with austenite decomposition products, are important for the formation of the final structure and properties of steel.

Studies conducted at the Verkh-Isetsk Metallurgical Plant on steels of various chemical compositions (0.01-0.05 wt.% C, 2.90-3.50 wt.% Si) after heating the slabs to temperatures of 1250-1400 ^o C and hot rolling mode on a continuous mill, characterized by temperature and time parameters, allowed to establish a number of patterns of texture and structure of hot rolled steel, as well as finished metal, which formed the basis of the invention.

A known method of producing cold-rolled EAS, including steel smelting, casting, heating a slab to 1320 ^o C, hot rolling, double cold rolling with intermediate annealing, high temperature annealing, final annealing (see SU 1664854 A, C 21 D 8/12, 1991) .

 A known method of producing cold-rolled EAS, including the smelting and casting of metal, hot rolling of ingots into slabs, heating the slab, hot rolling, etching, cold rolling with intermediate and final annealing (see SU 996474, C 21 D 8/12, 1983).

 The closest analogue to the claimed invention is a known method for the production of EAS, including after-furnace treatment, continuous casting of steel, heating the slab to the temperature of formation of the ferrite structure, hot rolling, etching, double cold rolling and heat treatment (see RU 2142020 C1, C 21 D 8 / 12, 1999).

 The technical result of the invention is the production of steel with high magnetic properties.

To achieve a technical result in a known method for the production of electrical anisotropic steel, including steel smelting, after-furnace treatment, continuous casting or casting into ingots, heating the slab to the temperature of formation of a single-phase ferritic structure, rough and finish hot rolling, double cold rolling and heat treatment, new is that the ratio between ferrite-forming and austenite-forming elements is chosen from the condition of achieving an austenite concentration in the structure of 3-8% with hot rolling in the temperature range 1150-1100 ^o C. In addition, the amount of austenite in steel is determined based on the following mathematical dependence:
V _γ = 694 • [C] - 23 • [Si] + 64.8
where V _γ is the fraction of austenite,%;
C is the concentration of carbon, wt.%;
Si is the concentration of silicon, wt.%,
and the carbon concentration in the range of 0.018-0.028 wt.% is adjusted depending on the silicon concentration, while with an increase in silicon content for every 0.05 wt.% in excess of 3 wt.%, the carbon content is increased by 0.005 wt.%, starting from 0.020 wt.%.

 Finished anisotropic electrical steel has the highest magnetic properties, which in the hot-rolled state was characterized by the presence of elongated polygonized crystallites in the subsurface region with a perfect orientation of (110) [001], and then processed according to the technological scheme, including two cold rolling, separation by recrystallization-decarburizing annealing , and slow heating for primary recrystallization after the second cold deformation.

 The presence of a limited amount of austenite in steel (3-8%) during hot rolling is necessary for the formation of a dispersed nitride inhibitor phase during further processing of the metal during its heat treatment, which ensures the complete passage of secondary recrystallization in steel that has previously undergone primary recrystallization.

 With the formation of a large amount of austenite and its decomposition, before or during the GP process, the processes of recrystallization, intensified by phase recrystallization (phase hardening), develop significantly. The process of recrystallization of a deformed metal leads to the replacement of the perfect texture (110) [001] in the subsurface layers with a texture with a predominance of orientations (110) [112] - [113].

With the formation of a small amount of austenite (~ 3-8%), and only with GP, and not before it, the recrystallization process does not receive such a strong development. This allows you to save the structure of the deformation with a perfect rib texture. Moreover, the most perfect rib texture of the subsurface layer has a metal heated before hot rolling to temperatures that provide a single-phase state, and the rolling mode provides the occurrence of austenite in the range of 1100-1150 ^o C
The single-phase (ferritic) state when heated before the start of hot rolling should be ensured not only by temperature (1350-1400 ^o C), but also by the optimal combination of austenitic and ferrite-forming elements (mainly, respectively, carbon and silicon). The optimum chemical composition corresponds to 0.020-0.028 wt. % carbon and 3.05-3.15 wt.% silicon, and can be adjusted: with an increase in silicon by 0.05%, it is necessary to increase carbon by 0.005%.

 In the absence of austenite in steel during HP, the recrystallization process is characterized by a small number of nuclei, but at the same time, their boundaries are highly mobile. The result is to obtain in the subsurface layer a recrystallized structure with a relatively large grain, characterized by perfect rib texture. In addition, aluminum nitride particles formed during hot rolling are characterized by relatively large sizes and, accordingly, low density, i.e. have a low ability to inhibit normal grain growth (essentially not an inhibitory phase).

 These patterns are illustrated by confirmed examples of the implementation of the invention in an industrial environment.

 Example 1

 In electric arc furnaces with a capacity of 170 tons, a low-carbon semi-product is smelted, which is then subjected to evacuation in an unsweetened state, additional heating, alloying, and modification on an AISA-SKF unit.

 In total, two melts were melted, the composition of which is given in table 1.

Casting was performed on vertical continuous casting machines. Before hot rolling, the slabs were heated to a temperature of 1400 ^° C. and then rolled to a thickness of 2.2 mm. During the rolling process, the temperature of completion of the rough deformation was maintained within the range of 1230-1250 ^o C; the temperature of the beginning of the final deformation - 1140-1160 ^o C; the temperature of the end of the final deformation - 1150-1100 ^o C; winding of strips - 580-610 ^o C. Part of the metal (individual slabs) was heated before hot rolling in low temperature conditions (T - 1250 ^o C). Subsequently, the metal was processed using the following technology: etching, the first cold rolling to a thickness of 0.6 mm, recrystallization-decarburizing annealing, the second cold rolling to a thickness of 0.30 mm, application to a magnesia coating strip, high-temperature annealing, with a metal heating rate in the range temperatures of 400-700 ^o C in the range of 15-20 ^o C / hour, rectifying annealing, metal testing.

It is important to emphasize that when the melts were docked during the mixing of the metal in the intermediate ladle, a metal averaged to various degrees was obtained (mainly by carbon concentration). His subsequent test showed the extreme sensitivity of the magnetic properties on the carbon concentration and, consequently, on the amount of austenite forming and then decomposing during hot rolling in the temperature range 1150-1000 ^o C (see drawing). It is important to emphasize: this temperature range initiates the formation of a large number of centers both at -α → γ and the reverse γ → -α transformation. In the volume of carbon concentrations of 0.018-0.028%, which is equivalent to 3-8% of austenite at 1100-1150 ^o C.

If the concentration of the main ferrite-forming element-silicon is deviated, an adjustment of the carbon concentration will be required in accordance with the empirical equation:
V _γ = 694 • [C] - 23 • [Si] + 64.8, (1)
where V _γ is the fraction of austenite,%;
C is the concentration of carbon, wt.%;
Si is the concentration of silicon, wt.%.

 Example 2

Steel was smelted in oxygen converters, the chemical composition was adjusted in accordance with equation (1) after alloying the metal. The hot rolled tackle contained 0.021 wt.% C, 3.10 wt.% Si, 0.19 wt. % Mn, 0.015 wt.% S, 0.020 wt.% Al, 0.009 wt.% N ₂ and 0.45 wt.% Cu. The temperature parameters of the heating of slabs and hot rolling, as well as the redistribution of hot rolled coils, were similar to those shown in example 1.

The finished metal was characterized by the following level of magnetic properties: P _{1.7 / 50} - 1.03-1.10 W / kg, B ₈₀₀ - 1.90-1.92 T, B ₂₅₀₀ - 1.98 T.

Thus, subject to the obtained laws (heating slabs into a ferrite region, rolling in a two-phase region with a limitation of the amount of austenite in the range of 3-8% at temperatures of 1100-1150 ^o C), the metal is characterized by the level of properties typical of a highly permeable state.

Claims (3)

1. A method of manufacturing electrical anisotropic steel, including steel smelting, after-furnace treatment, continuous casting or casting into ingots, heating the slab to the temperature of formation of a single-phase ferrite structure, rough and finish hot rolling, double cold rolling and heat treatment, characterized in that the ratio between the ferritic and austenite-forming elements are selected from the condition of achieving an austenite concentration in the structure of 3-8% during hot rolling in the temperature range 1150-1100 ^o C. 2. The method according to p. 1, characterized in that the amount of austenite in steel is determined from the following mathematical dependence:
V _γ = 694 • [C] -23 • [Si] +64.8;
where V _γ is the fraction of austenite,%;
C is the carbon concentration, wt.%;
Si is the concentration of silicon, wt.%.  3. The method according to claim 1, characterized in that the carbon concentration in the range of 0.018-0.028 wt. % correct depending on the concentration of silicon, while with an increase in silicon content for every 0.05 wt.% in excess of 3 wt.%, the carbon content is increased by 0.005 wt.%, starting from 0.020 wt.%.

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