RU2199595C1 - Process for making cold rolled electrical anisotropic steel - Google Patents

Abstract

FIELD: ferrous metallurgy, possibly manufacture of textured electrical steels. SUBSTANCE: process is realized due to optimization of chemical composition of electrical anisotropic steel and changing technological mode of its conversion depending upon content of austenite- and ferrite-forming elements in metal after melting out. Process comprises step of changing temperature value during final stage of hot rolling at varying relation of concentration values of austenite- and ferrite-forming elements in steel. EFFECT: stable process of making steel with rather high level of magnetic properties. 2 tbl, 2 ex

Description

 The invention relates to the field of ferrous metallurgy, or rather to the production of electrical steel (EAS) with oriented grain structure.

 According to the conditions of its operation, the following basic requirements are imposed on this steel: ease of magnetization and magnetization reversal (ie high values of magnetic permeability); high values of magnetic induction; minimal losses during magnetization reversal. The fulfillment of the first two requirements determines the size and weight of the electrical windings and magnetic cores of the transformers. Minimum losses on magnetization reversal determine the efficiency of transformers and their operating temperature.

 The above 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 matrix texture of a small number of perfect grains with orientation {110} <001>, which are centers of secondary recrystallization and a pronounced octahedral component {111} <112>, which is easily absorbed by rib grains.

 The fulfillment of the first condition (stabilization of the structure) is achieved either by forming inclusions during hot rolling (GP) (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 (a steelmaking process including two cold rolling with recrystallization annealing between them), or by rolling with a large degree of deformation (single-shot scheme by rolling). The first technological scheme is used for the production of steel according to the sulfide variant, and the second according to the sulfonitride variant.

 An exception is the technology of steel production, which provides for the formation of the necessary texture contrast during slow heating in the range of return temperatures and primary recrystallization. This technology applies exclusively to the nitride method of inhibiting structure.

 The sulfide variant has been known since the late 40s and is currently the most common. The main technological operations in the production of steel according to the sulfide variant are high temperature heating, hot rolling (GP), two cold rolling separated by recrystallization annealing, decarburization annealing and high temperature annealing (WTO) (see Molotilov B.V. et al. "Sulfur in electrical steel ", 1976, S. 176) [1]. 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 elongated polygonized crystallites with a pronounced deformation texture - {110} <001>. The presence of this layer makes it possible to obtain a sufficiently large number of grains with perfect orientation {110} <001> in the steel structure before high-temperature annealing, some of which will be secondary recrystallization nuclei.

 Steel nitride variant has a high content of carbon, nitrogen and copper. 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 -1.85-1.90 T (see the journal Steel 10, 1994, S. 35-37) [2].

A significant difference between the steel of the nitride variant and the sulfide one is a lower heating of the metal before hot rolling (~ 1250 ^o С, against 1400 ^o С). A 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-prolonged 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 only possible for steel with a more stable inhibitory phase, which is AIN.

To become a sulfonitride variant (see Fiedler HC A New High Jnduetion Grain Oriented 3% Silicon Jron // JEEE Trans. On Magnetics, 1977, v. 13. 5 P. 1433-1436) [3] has an increased content (compared with sulfide) carbon and aluminum. The main operations after hot rolling are normalization, single cold rolling, decarburization annealing and high temperature annealing. Magnetic induction in a field of 800 A / m - 1.89 ... 1.94 T - is 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 powerful force on the texture of steel, which is a single rolling. Fundamentally important in this technology is the presence of high-temperature normalizing annealing (1120 ... 1150 ^o С) after GP with a strictly regulated cooling law. It should be noted that steel production under this technological regulation is not always possible due to the lack of the necessary equipment.

 Significant disadvantages of sulfide and sulfonitride technologies for the production of electrical anisotropic steel include, firstly, the presence of high-temperature heating of slabs before hot rolling, which is a super-energy-intensive operation that requires special equipment and, secondly, for hot-rolled steel, conducting an expensive normalizing operation after hot rolling annealing, also requiring an additional unit. Thus, the least expensive, not requiring additional equipment is nitride technology for the production of electrical anisotropic steel. However, the magnetic properties of the metal produced by nitride technology are somewhat inferior to steel with the initial double inhibition.

 The closest analogue to the proposed one is the known method for the production of cold rolled electrical anisotropic steel, including steel smelting, casting, hot rolling, descaling, two cold rolling with intermediate decarburization annealing, high temperature and straightening annealing (see RU 2017837 C1, IPC 7 C 21 D 8/12, 08/15/1994) [4].

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

To achieve the specified technical result in a known method for the production of cold rolled electrical anisotropic steel, including steel smelting, casting, hot rolling, descaling, two cold rolling with intermediate decarburization annealing, high temperature and straightening annealing, steel containing, wt.%: Is melted:
Carbon - 0.021-0.055
Silicon - 2.8-3.6
Manganese - 0.1-0.3
Copper - 0.4-0.6
Acid-soluble aluminum - 0.011-0.018
Nitrogen - 0.007-0.012
Iron and the Inevitable
impurities - the rest
and the temperature of the end of hot rolling (TKGP) is selected based on the following expression:
T _kgn = {970 - ([% C] -0.018) • 3000 + ([% Si] -2.8) (60} ± 20, ^o C,
where [% C] and [% Si] are the concentrations of carbon and silicon in steel, wt.%.

 The invention is based on the following laws.

 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 of crystallites with the orientation {110} <001> in the structure of hot rolled steel in the subsurface layer (1 / 10-1 / 4 of the thickness). Moreover, the sharper this orientation after GP, the more perfect the texture in the finished steel.

 The formation of the structural features of the EAS tackle occurs at the stage of finishing rolling, where the temperature-deformation processing regimes play the main role. 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. The most perfect deformation texture {110} <001> - in the surface layer, {100} <011> - in the central region have deformed, but not recrystallized grains [5]. 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> [6, 7].

 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 within wide enough limits. 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>.

 If the EAS contains, after smelting, a comparatively small amount of carbon (C <0.025 wt.%, With Si> 3.0 wt.%), Austenite is practically absent in the steel structure during HP. This also leads during GP to the development of a recrystallization process, which is characterized by a small number of nuclei of new grains, but at the same time 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 [6]. In addition, the complete absence of the austenitic phase during the 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 and GP parameters follows, i.e. the optimal ratio of the concentrations of austenite and ferrite-forming elements in the metal (mainly carbon and silicon) in combination with the temperature range of the GP, providing a stable maximum level of magnetic properties after completion of the EAS treatment. 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 coils whose chemical composition was in the following limits: 2.8 ... 3.4 wt.% Silicon, 0.01 ... 0.055 wt.% Carbon, 0.1 ... 0.3 wt.% manganese, 0.4 ... 0.6 wt.% copper, 0.011. . . 0.018 wt. % acid-soluble aluminum, 0.007 ... 0.012 wt.% nitrogen, stably had after finishing a higher value of magnetic properties corresponding to the characteristics of high-permeability steel of class HI-B, if the ratio of the concentrations of carbon and silicon in the steel satisfied the following expression:
0.025≤ ([% C] -0.018) / ([% Si] -2.8) <0.035, (1)
where [% C] and [% Si] are the concentrations of carbon and silicon in steel, respectively, wt. %

 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 and the lower the silicon content, the wider the temperature range of austenite existence, and, accordingly, at a lower temperature, it is necessary to finish the GP (fine rolling) to form a texture of 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 just impractical, but harmful, since it leads to early precipitation and coarsening of AIN 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 laws form the basis of the main 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 and silicon 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 (in degrees Celsius) depending on the concentrations of carbon [% C] and silicon [ % Si] (in wt.%):
T _kgn = {970 - ([% C] -0.018) (3000 + ([% Si] -2.8) • 60) ± 20. (2)
The invention is illustrated by the following examples.

Example 1. A series of melts of electrical steel (chemical composition, wt.%: Si - 2.92-3.5; Mn - 0.15-0.22; C - 0.021-0.055; acid-soluble Al - 0.013-0.015; N - 0.009-0.011; Cu - 0.45-0.51; the remaining iron and unavoidable impurities) were smelted for research in 350-ton converters, cast on continuous casting machines in slabs with a cross section of 250x1080 mm, most of the slabs from this series were rolled on a broadband hot rolling mill . The temperature of the end of hot rolling was 930 ... 970 ^o C. Hot rolled coils were processed according to the scheme: descaling, the first cold rolling to a thickness of 0.65 mm, decarburizing annealing, the second cold rolling to a thickness of 0.30 mm, coating of magnesium oxide , high temperature and straightening annealing. After the final treatment, the magnetic properties of the obtained steel were measured. The initial concentration of carbon and silicon in steel, their ratio in accordance with the formula, as well as the measurement results of the magnetic properties of the finished steel are shown in table 1.

 As the results show, the best magnetic properties are 9 ... 11 steel with an optimal ratio of austenitic and ferrite-forming elements. Moreover, it should be noted that the closer the value ([% C] - 0.018) / ([% Si] - 2.8) to the optimum, the better the magnetic properties of the finished EAS.

 Example 2. Part of the slabs of heats 3, 5, 7 of example 1 was subjected to hot rolling with different temperatures of its end. Further metal processing occurred similarly to that described in example 1. The temperatures of the end of hot rolling (actual and calculated by formula (2)), as well as the results of measurements of the magnetic properties of the finished steel are shown in table 2.

 As the results show, the best magnetic properties are the bands of the finished EAS, hot rolling which was carried out in accordance with the invention.

Sources of information
1. Sulfur in electrical steel / Molotilov B.V., Petrov A.K., Borevsky V.M. et al., Moscow: Metallurgy, 1973.176 p.

 2. Frantsenyuk I.V., Kazadzhan VB, Baryatinsky V.P. Achievements in improving the quality of electrical steel at NLMK // Steel. 1994.10. P. 35 ... 37.

 3. Fiedler H.C. A New High Induction Grain Oriented 3% Silicon Iron // IEEE Trans. on Magnetics. 1977. V.13. 5. P. 1433 ... 1436.

 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.

 5. Pashchenko S.V., Goldstein V.Ya., Sery A.V., Grazhdankin S.N. Texture formation during hot rolling of a silicon alloy // FMM. 1984.T.58. Vol. 1. S. 63 ... 68.

 6. The effect of phase recrystallization on the structure of the roll of electrical anisotropic steel / Lobanov M. L., Shabanov V. A., Tsyrlin M. B., Mineev F. V. // Steel. 2000 2.P. 59 ... 63.

 7. The effect of hot rolling temperature on the structure and properties of electrical anisotropic steel / Lobanov M. L., Shabanov V. A., Tsyrlin M. B., Pervushina O. V. // Steel. 2001. 7. P.65 ... 67.

Claims (1)

A method for the production of cold rolled electrical anisotropic steel, including steel smelting, casting, hot rolling, descaling, two cold rolling with intermediate decarburization annealing, high temperature and straightening annealing, characterized in that steel containing, wt.%: Is smelted:
Carbon - 0.021-0.055
Silicon - 2.8-3.6
Manganese - 0.1-0.3
Copper - 0.4-0.6
Acid-soluble aluminum - 0.011-0.018
Nitrogen - 0.007-0.012
Iron and Inevitable Impurities - Else
and the temperature of the end of the hot rolling T _kgp choose, based on the following expression:
T _kgn = {970 - ([% C] - 0.018) • 3000 + ([% Si] -2.8) • 60} ± 20, ^o C,
where [% C] and [% Si] are the concentrations of carbon and silicon in steel, wt.%, respectively.

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