RU2142020C1 - Method of production of antisotropic electrical steel - Google Patents

Abstract

FIELD: ferrous metallurgy; applicable in production of grain-oriented electrical steels. SUBSTANCE: the essence of invention consists in optimization of chemical composition of anisotropic electrical steel, and in adjustment of conversion technology depending on concentration of carbon in metal in heat. Invention provides for changing temperature of hot rolling end and degree of deformation in the second cold rolling with change of carbon concentration in steel. EFFECT: improved magnetic properties of steel with nitride inhibiting up to level of high permeability steel. 4 cl, 1 tbl

Description

 The invention relates to the field of ferrous metallurgy, specifically to the production of transformer steel with an oriented structure.

 The following basic requirements are imposed on this steel: high magnetic permeability, minimal losses during magnetization reversal, high magnetic induction. These requirements are met only if a perfect texture {110} <001> (rib texture) is formed in the steel, which is realized during secondary recrystallization at the final stages of heat treatment.

The main conditions for the development of the process of texture formation and secondary recrystallization are:
- stabilization of the structure by dispersed inclusions of the second phase (manganese sulfides or aluminum nitrides);
- the presence in the texture of the matrix of sufficiently perfect grains {110} <001>, which are centers of secondary recrystallization and a pronounced octahedral component {111} <112>, which is easily absorbed by rib grains.

 Note that as the texture situation changes, the requirements for the second phase also change. In particular, with a decrease in the ratio {110} <001> / {111} <112>, the stabilizing function of the second phase should be strengthened.

 The fulfillment of the first condition (stabilization of the structure) is achieved either due to the formation of inclusions during hot rolling (for example, the sulfide version), or during heat treatment (nitride version), or during hot rolling and during heat treatment (sulfonitride version).

 The second condition (the formation of the optimal matrix texture) is realized either as a result of the inheritance of the sharp rib component from the subsurface layers of hot-rolled strips [1] (a scheme with double deformation and recrystallization annealing between the stages of cold rolling), or by rolling with a high degree of deformation (scheme with a single rolling) [2].

 An exception is the redistribution scheme, which provides for a sharp strengthening of the {111} <112> texture during slow heating in the range of return temperatures and primary recrystallization [3]. This technology applies exclusively to the nitride method of inhibiting structure.

 Each of the redistribution schemes has its inherent advantages and disadvantages, which is taken into account in the structure of world steel production.

 So the technology based on sulfonitride inhibition, providing steel with a sharp rib texture and maximum permeability in the direction of deformation, is used to manufacture power transformers. For the manufacture of distribution and domestic transformers, steel is used with sulfide inhibition of the structure and, processed according to the double-rolling scheme, as the most stable and low-cost. The technology, which provides nitride inhibition of the structure and slow heating at the softening stage, as well as steel with sulfide inhibition, is used in distribution and less in power transformer construction.

 The main objective of the invention is to improve the magnetic properties of steel with nitride inhibition to the level of highly permeable steel, which makes it possible to use it in both distribution and power transformer construction.

 This goal is achieved by a combination of elements of the technology of sulfide and nride methods of inhibiting structure. A technology element is borrowed from the sulfide variant, which contributes to the formation of a layer of rib grains in the subsurface layer of hot-rolled strips and, therefore, to the improvement of the texture of grains {110} <001> - nuclei of secondary recrystallization. From the nitride variant, the amplification of the octahedral component of the matrix texture is taken and, therefore, conditions are created for the facilitated growth of the most perfect rib grains during secondary recrystallization.

 A patent [4] was selected as the closest analogue (prototype) of the present invention, which governs the parameters of the final (finish) hot rolling (GP) of electrical anisotropic steel of the nitride inhibition variant in order to obtain finished products with a high level of magnetic properties. A significant disadvantage of the prototype should include the absence of any connection between the parameters of the GP with possible variations in the chemical composition of the EAS. The latter does not allow the implementation of the manufacture of sheet steel with the highest possible magnetic properties, potentially incorporated by the GP process.

 The technical result of the present invention is the selection of the parameters of technological operations depending on the chemical composition of the smelted steel, in which stable production of EAS with a sufficiently high level of magnetic properties is guaranteed.

The invention is based on the following laws:
1. In the process of hot rolling, 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 influence on the structure and texture formation is manifested in the inheritance of the initial structure of the hot rolled steel according to the technological redistribution of the end-to-end anisotropic steel production cycle. In order for a sharp rib texture to provide high magnetic properties to form during secondary recrystallization in the EAS, a necessary condition is the presence in the structure of hot-rolled steel in the subsurface layer (1 / 10-1 / 4 thickness) of crystallites with an orientation of {110} <001>. Moreover, the sharper this orientation after GP, the more perfect the texture in the finished steel.

 2. The formation of the structural features of the EAS tackle occurs at the stage of finish rolling, where the temperature-deformation processing conditions play the main role, the texture heterogeneity formed in the process of GP is caused by differences in the path of the metal in the surface and central layers when it passes through the deformation zone. The most perfect deformation texture ({110} <001> in the surface layer, {100} <001> in the central region) is possessed by deformed, but not recrystallized grains. The processes of recrystallization that occur during HPE contribute to the scattering of the deformation texture. The higher the degree of recrystallization of the structure, the weaker the deformation orientations are expressed in it, and, accordingly, the texture of the subsurface layer becomes {more} {110} <001>.

 3. Depending on the ratio of ferrite and austenite-forming elements in steel (mainly silicon and carbon), the degree of perfection of the texture of the subsurface layer can vary over a wide range. An increase in the carbon content in the EAS leads to the formation of a large amount of austenite (with its subsequent decay) during the GP process, which results in the development of a recrystallization process intensified by phase recrystallization (phase hardening). The process of recrystallization leads to the replacement of the deformation texture (in the subsurface layers the perfect texture {110} <001>) with orientations {110} <112> ... <113>.

 4. In the event that the EAS contains, after smelting, a comparatively small amount of carbon (<0.025 wt.%, For Si> 3.0 wt.%), Austenite is practically absent in the steel structure during HF. This also leads during GP to the development of a recrystallization process, which is characterized by a small number of new grain nuclei, but with a high mobility of their boundaries. The consequence of this is the obtaining in the subsurface layer of a recrystallized structure with a relatively large grain, characterized by low perfection of the rib texture.

 In addition, the complete absence of the austenitic phase in the process of EAS GP negatively affects the formation of a finely dispersed inhibitor phase, which is associated with the early release of aluminum nitrides from solid solution (ferrite) and, accordingly, their coarsening already at the last stages of high-temperature deformation. Obtaining stable secondary recrystallization (and the corresponding level of magnetic properties) in such a metal becomes problematic.

Of all these regularities, the existence of an optimal chemical composition of steel follows, which ensures a stably maximum level of magnetic properties after completion of EAS processing. This optimum chemical composition of the EAS of the nitride inhibition variant was determined on the basis of a statistical analysis of the magnetic properties of several hundred steel melts smelted at the Magnitogorsk and Novo-Lipetsk metallurgical plants and processed at the Verkh-Isetsk Metallurgical Plant. It was shown that steel containing 3.0 ... 3.4 wt.% Silicon, 0.025..0.035 wt.% Carbon, 0.1. . . 0.3 wt.% Manganese, 0.4 ... 0.6 wt.% Copper, 0.0011 ... 0.017 wt.% Aluminum, 0.007 ... 0.012 wt.% Nitrogen. Slabs are produced in a continuous casting unit, followed by heating of the slab, the first rolling in several passes in the roughing stand to an intermediate thickness, the second hot rolling in three passes in the finishing reverse stand, etching, decarburizing annealing, the second cold rolling, high-temperature annealing and straightening annealing. After final processing, the steel stably has an increased value of magnetic properties corresponding to the characteristics of highly permeable steel of class N1-B, if the temperature of the end of hot rolling was not less than 970 ^o C, and the strain during the second cold rolling of steel was 45 ... 50% .

 However, obtaining a stable carbon concentration during steelmaking within such narrow limits is a rather complicated problem. For this reason, the present invention provides for the selection of alternative means for stably obtaining EAS with sufficiently high magnetic properties with a significantly greater variation of carbon in steel.

 In order to obtain a more perfect texture of the subsurface layer during high-temperature steel deformation, it is necessary to finish the GP at the lowest possible temperatures. In this case, rolling will occur in the temperature range when the steel is mainly in a single-phase (ferritic) state, i.e., phase recrystallization will not have a significant effect on the process of texture formation. As a result of this, the deformation texture will remain in the subsurface layer - perfect orientation {110} <001>. Moreover, the higher the carbon content in steel, the wider the temperature range of austenite existence and, correspondingly, the lower the temperature, it is necessary to finish the GP (fine rolling) to form a texture with a sufficient degree of perfection in the subsurface layer. It should be noted that a decrease in the temperature of the end of a GP for a metal with a chemical composition close to optimal is not only impractical, but harmful, since it leads to early precipitation and coarsening of A1N from the solid solution and, accordingly, to a decrease in the efficiency of the inhibitory phase. Moreover, the higher the carbon concentration in steel (i.e., the more austenite is in it), the lower the temperature range for the release of the finely dispersed phase and, accordingly, the weaker the effect of weakening the inhibitory ability of aluminum nitrides.

The described qualitative patterns form the basis of one of the methods of the present invention. Quantitative patterns were established experimentally on a representative batch. An analysis of the correlation between the magnetic properties of the finished EAS, the carbon content in the steel after smelting, and the temperature of the end of rolling (T _kgp ) allowed us to obtain an empirical formula for choosing the optimal T _kgp depending on the carbon concentration (% C):
T _kgr = 970 - (% C - 0.035) • 3000, (1)
where T _kgr is the temperature of the end of hot rolling in degrees Celsius;
% C is the carbon concentration in steel in wt.%.

It is also known that a significant improvement in the magnetic properties of the finished EAS can be achieved by increasing the size of the reductions in the last (most often second) cold rolling in steel production. In this case, in the metal structure during primary recrystallization, preceding the abnormal grain growth, crystals form (nuclei of secondary recrystallization) with minimal deviations from the rib orientation. However, this technique for improving magnetic properties should be used carefully, since an increase in the strain inevitably leads to dispersion of the primary recrystallization structure and, accordingly, requires an increased density and dispersion of the inhibitory phase for the implementation of the process of abnormal grain growth. Steel nitride variant of inhibition with a relatively high optimum carbon content has a phase with a high inhibitory ability, since the latter is formed at sufficiently low temperatures. This allows you to successfully use the increase in the strain during the second cold rolling to improve the magnetic properties of the EAS. The described qualitative regularities formed the basis of the experimentally obtained relationship between the strain in the second cold rolling and the carbon concentration in steel, taking into account which ensures stable production of EAS with high magnetic properties from rolled stock with an initially increased (relatively optimal) carbon concentration:
E = 50 + (% C - 0.035) • 400, (2)
where E is the magnitude of the deformation during the second cold rolling in%;
% C - carbon concentration in steel in wt.%
To illustrate the essence of the invention, the table shows the magnetic properties (minimum, maximum and average) of steel with a thickness of 0.30 mm, made in accordance with the methods set forth in the present invention, as well as steel made by the usual production technology of EAS nitride inhibition variant.

Literature
1. Tsyrlina M. B. Abstract of a doctoral dissertation "Principles and methods of modifying and controlling the structure of electrical steel." Moscow 1987

 2. Taguchi S. Nihov Kinzoku Kaiho, 1974, v. 3, p. 49.

 3. Tsyrlin M. B., Sukhanov G. P. Copyright certificate N 835151 "Method for the manufacture of textured electrical steel." Priority from 08.24.81 g.

 4. Patent of the Russian Federation 2017837. Published on 08/15/94 (Application 5013424/02 of 11/29/91). Zaveryukha A.A., Sharshakov I.M., Kalinin V.N. et al. Method for the production of anisotropic electrical steel.

Claims (4)

1. Method for the production of cold-rolled, electrical anisotropic steel, including the smelting of steel containing carbon, silicon, manganese, copper, aluminum, nitrogen, casting, hot rolling, pickling, two cold rolling with decarburization annealing between them, high temperature and straightening annealing, characterized the fact that smelted steel containing, wt.%:
Carbon - 0.025-0.060
Silicon - 3.0-3.4
Manganese - 0.1-0.3
Copper - 0.4-0.6
Aluminum - 0.011-0.017
Nitrogen - 0.007-0.012
Iron and Inevitable Impurities - Else
when the carbon content in the steel is 0.025-0.035 wt.% the temperature of the end of the hot rolling is not less than 970 ^o C. 2. The method according to claim 1, characterized in that the strain value during the second cold rolling for steel is selected depending on its carbon content according to the following empirical formula:
E = 50 + (% C - 0.035) • 400,
where E is the magnitude of the deformation during the second cold rolling,%;
% C is the carbon concentration in steel in wt.%. 3. A method for the production of cold rolled electrical anisotropic steel, including the smelting of steel containing carbon, silicon, manganese, copper, aluminum, nitrogen, casting, hot rolling, pickling, two cold rolling with decarburization annealing between them, high-temperature and straightening annealing, characterized in that that melt steel containing, wt.%:
Carbon - 0.025-0.060
Silicon - 3.0-3.4
Manganese - 0.1-0.3
Copper - 0.4-0.6
Aluminum - 0.011-0.017
Nitrogen - 0.007-0.012
Iron and Inevitable Impurities - Else
when the carbon content is more than 0.035 wt.% the temperature of the end of the hot rolling is selected according to the following empirical formula:
T _kgr = 970 - (% C - 0.035) • 3000,
where T _kgr is the temperature of the end of hot rolling, ^o C;
% C is the carbon concentration in steel, wt.%. 4. The method according to claim 3, characterized in that the strain during the second cold rolling for steel is selected depending on the carbon content in it according to the following empirical formula:
E = 50 + (% C - 0.035) • 400,
where E is the magnitude of the deformation during the second cold rolling,%;
% C is the carbon concentration in steel, wt.%.

Images