RU2180357C1 - Method for making cold rolled strip of electrical anisotropic steel - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: method for making cold rolled strip of electrical steels with texture (110)[001] comprises steps of setting temperature of final stage of decarburizing annealing of cold rolled steel in dependence upon temperature of heating slabs before hot rolling and upon temperature of coiling hot rolled strip; nitriding strip during process of decarburizing annealing or after such process. EFFECT: optimal production process for making electrical anisotropic steels with use of nitriding process. 3 tbl

Description

 The invention relates to ferrous metallurgy and can be used in the production of cold rolled strips from electrical anisotropic steel.

 The following basic requirements are imposed on electrical anisotropic steel (EAS) related to the conditions of its operation - high magnetic permeability, minimal losses during magnetization reversal, high magnetic induction. These requirements are met only if the steel has a perfect texture (110) {001} (Goss texture, rib texture). Goss texture formation occurs during secondary recrystallization (VR) during high-temperature annealing of steel at the final stages of its processing. Secondary recrystallization with the formation of texture proceeds due to the presence of a dispersed inhibitory phase in the steel structure, which inhibits normal grain growth. The technology of industrial production of EAS, used in world practice to date (sulfide, sulfonitride and nitride), is based on the method of congenital inhibitor. Elements that are part of the inhibitor phase (Mn, S, Se, Al, N) are introduced into the steel at the smelting stage in strictly regulated quantities. The primary coarse precipitates present in the cast metal are completely dissolved by prolonged high-temperature aging of the slabs before hot rolling, so that in the future (during hot deformation or during annealing of the cold-rolled metal) the phase precipitates out and can play the role of an inhibitor. Thus, the chemical composition and treatment regimes of steel should provide, among other things, the necessary parameters of the inhibitory phase: high bulk density, small particle size and their resistance to dissolution to certain temperatures.

All the mentioned technological schemes (sulfide, sulfonitride, nitride variants) have common very significant drawbacks:
1) during hot rolling and cooling after it, to obtain a highly dispersed phase, continuous exposure of slabs at high temperatures is required, which is not possible on any equipment and is very uneconomical;
2) it is necessary in the process of subsequent redistribution to monitor the particles of the inhibitory phase, preventing their enlargement.

 In this regard, the creation of an inhibitory phase, not at the initial stage of production, but at the intermediate, was of great practical interest, which allows us to significantly simplify and make more economical the production of EAS. New technological schemes for the production of textured steel have been actively developed since the late 80s of the 20th century [1, 2, 3, 4]. The principal differences between the new method and the classical ones are the introduction of one of the phase forming elements (nitrogen) into the steel after decarburization annealing and the formation of an inhibitory phase in the metal structure directly during high-temperature annealing.

 Despite the fact that the new method is implemented in the industrial production of EAS, it is also not without drawbacks. An arbitrary choice of the parameters of technological operations (steel heating temperature before hot rolling, winding temperature, temperature-time regimes of normalizing, decarburizing and nitriding annealing) with their wide enough spectrum does not allow EAS with guaranteed high magnetic properties.

Studies carried out at the Verkh-Isetsk Metallurgical Plant showed the following regularities in the formation of the EAS structure during its production using the nitriding process:
1. The larger the average crystallite size after decarburization annealing, which grew during primary recrystallization and further normal growth, the better the texture after secondary recrystallization and, accordingly, the higher the magnetic properties of steel. Steel has the best magnetic properties, the average crystallite size of which after decarburization annealing was 20–25 μm. However, if separate crystallites are observed in the steel structure after decarburization annealing, the size of which is 2.5-3.0 times larger than the average grain size (the beginning of abnormal growth), then the properties of EAS after high-temperature annealing will be quite low.

 2. The average crystal size after decarburization annealing is the higher, the lower the temperature of steel heating before hot rolling, the higher the temperature of winding of a hot-rolled strip, the higher the temperature of the end of decarburization annealing.

 3. It is impossible to achieve the complete absence of the inhibitory phase in the steel structure during decarburization annealing due to lowering the heating temperature before hot rolling and / or increasing the temperature of winding of the hot-rolled strip. As a result of this, a certain threshold temperature arises of the final stage of decarburization annealing, at which collective recrystallization is replaced in part by anomalous growth, which leads to a deterioration in the magnetic properties of the finished EAS.

 Establishment of the above-mentioned qualitative regularities made it possible to assume that only a rational combination of certain technological parameters can guarantee obtaining EAS with stably high magnetic characteristics.

 The closest analogue to the claimed invention is a known method for the production of cold rolled electrical anisotropic steel, with the A1N phase as an inhibitor of normal grain growth, including steel smelting, casting, hot rolling, one or two cold rolling, decarburization annealing, nitriding, high temperature and straightening annealing (see SU 1520116) (2).

 An object of the invention is to optimize the production of EAS using nitriding.

To achieve the specified technical result in a known method for producing a cold rolled strip from electrical anisotropic steel, with the A1N phase as an inhibitor of normal grain growth, including steel smelting, casting, hot rolling, winding a hot rolled strip into a roll, one or two cold rolling strips, decarburizing annealing, nitriding, high-temperature and straightening annealing, the temperature of the final stage of decarburization annealing T _{1 of the} cold-rolled strip is set depending on t of the heating temperature of the steel before hot rolling and from the temperature of the winding of the hot rolled strip into a roll according to the formula:
T ₁ = 0.6 • (T ₂ -T ₃ ) +540,
where T ₂ is the heating temperature of the steel before hot rolling, ^o C;
T ₃ - temperature of the coil of the hot rolled coil, ^o C,
and nitriding of the strip is carried out in the process of decarburization annealing or immediately after it.

 Using this formula, it is possible, on the basis of the known temperatures for heating slabs before hot rolling and winding the hot rolled strip, which are determined by the capabilities of the hot rolling mill, to select the optimal temperature for the end of decarburization annealing and to obtain an EAS with the sharpest BP texture and, accordingly, the best magnetic properties.

 The invention is illustrated by the following examples.

 Steel for research is smelted using the converter method, cast on continuous casting machines in slabs. Then, the slabs are rolled in a hot rolling mill into strips 2.2 mm thick.

 The chemical composition of steel melts is given in table. 1.

Example 1. During hot rolling of the first heat, the temperatures of heating the slabs (1150 and 1250 ^o C) and the temperature of the winding of hot-rolled strips (580 and 720 ^o C) were varied.

After hot rolling, the melting rolls 1 are subjected to etching, single cold rolling to a thickness of 0.30 mm, two-stage decarburization and high-temperature annealing. In the first stage of decarburization annealing, hydrogen is removed due to the interaction of steel with a moist nitrogen-hydrogen atmosphere at 800 ^° C for 150 s, and in the second stage, normal grain growth occurred within 120 s. Steel nitriding is carried out during high-temperature annealing due to the addition of silicon nitride (Si ₃ N ₄ ) to the anti-welding coating (MgO). After the final processing, the magnetic properties of the obtained EAS are measured. The measurement results and processing parameters are given in table. 2.

Example 2. The heating temperature of the slabs of melting 2 (table. 1) before hot rolling was 1220 ^o C, while the temperature of the winding coils (560 ^o C, 620 ^o C, 700 ^o C) was varied. After hot rolling, the melting rolls 2 were processed similarly to the melting rolls 1, except for the nitriding process. Nitriding was carried out after decarburization annealing in a continuous furnace at a temperature of 800 ^o C for 30 s in a nitrogen-hydrogen atmosphere containing ammonia. After the final processing, the magnetic properties of the obtained EAS were measured. The measurement results and processing parameters are given in table. 3.

 The results shown in examples 1 and 2 of the experiments show the high efficiency of the methods of the present invention.

Literature
1. N. Takahashi, J. Harase. Recent Development of Technology of Grain Oriented Silicon Steel / Materials Science Forum Vols. 204-206 // Switzerland, Transtec Publication, 1996. P. 143-154.

 2. Dneprenko K.V. et al. Method for heat treatment of anisotropic electrical steel. USSR copyright certificate 1520116, claimed 02.19.1988, published 11.11.1989

 3. Process for Preparation of Grain-Oriented electrical steel sheet containing a nitriding treatment. H. Kobayashi, K. Kuroki, M. Minakuchi et. al. United States Patent 4979996. Dec. 25, 1990.

 4. Process for production of grain oriented electrical steel sheet having high flux density. Takahashi; Nobuyuki Suga et.al. United States Patent 4994120. February 19, 1991.

Claims (1)

A method of manufacturing a cold rolled strip of electrical anisotropic steel, with an AlN phase as an inhibitor of normal grain growth, including steel smelting, casting, hot rolling, winding a hot rolled strip into a roll, one or two cold rolling strips, decarburizing annealing, nitriding, high temperature and straightening annealing, characterized in that the temperature of the final stage of decarburization annealing the cold-rolled strip T ₁ is set depending on the heating temperature of steel before hot rokatkoy and coiling temperature of the hot strip into a roll, according to the formula
T ₁ = 0.6x (T ₂ -T ₃ ) +540,
where T ₂ is the heating temperature of the steel before hot rolling, ^o C;
T ₃ - temperature of the winding of the hot-rolled strip into a roll, ^o С,
and nitriding of the strip is carried out in the process of decarburization annealing or immediately after it.

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