EP1795617A1 - Process for heat treating electrical steel sheet - Google Patents

Abstract

Heat treatment method for steel strip comprises continuous annealing followed by cooling in three stages. In the first stage the strip is cooled to a temperature 50[deg]C above the Curie temperature. In the second stage it is cooled to 80[deg]C below the Curie temperature and in the final stage with a cooling gradient up to 85[deg]C/s. Heat treatment method is used for steel strip with an Si and Al content (wt%) given by the relationship: 0.9=Si + 2 x Al =1.8, an Mn content up to 2.5 wt%, up to 1 wt% of each of P, Cr and Ni and up to 0.1 wt% of each of C, N and S. The method comprises continuous annealing followed by cooling in three stages. In the first stage the strip is cooled to a temperature 50[deg]C above the Curie temperature with a cooling gradient of -38[deg] to -1[deg]C/s. In the second stage it is cooled to 80[deg]C below the Curie temperature. The cooling gradient has an upper limit (CR 2 M A X) given by: CR 2 M A X = pi (cos(2(T B A N D - T C)pi /2 6>((T B A N D - T C)(pi /2 6>) 2> + 1)) - 1) - pi . Its lower limit (CR 2 M I N) is given by: (CR 2 M I N) = 3pi (cos(2(T B A N D - T C)pi /2 6>((T B A N D - T C)(pi /2 6>) 2> + 1)) - 1) - pi . In the final stage the cooling gradient is up to 85[deg]C/s.

Description

The invention relates to a method for heat treating steel strips consisting of a steel, the Si and / or Al, with the proviso that for the Si content% Si and the Al content% Al% Si + 2 x% Al ≤ 6 , 6 wt .-%, up to 2.5 wt .-% Mn, optionally one or more elements from the group "P, Cr, Ni" in each case up to 1 wt .-%, optionally one or more elements from the group "C, N, S" with contents of up to 0.1 wt .-% and the remainder containing iron and unavoidable impurities are produced, in which the steel strips are first pass annealed and then cooled controlled in at least three stages , Such a method is for example from the EP 0 357 797 A1 known.

Steel strips made in long lengths and large widths are used in a very large area as construction materials. In a thickness range of about 0.1 to 2 mm uniform mechanical properties are required for such steel strips. These are achieved in part only after a heat treatment with subsequent forming. The heat treatment usually includes annealing and controlled cooling. The annealing and cooling lead in practice to uneven planes and internal stresses of the band.

In such manufacturing processes in which a deformation of the strip material takes place after annealing, the unevennesses and internal material stresses generated in the annealing process are tolerable to a certain extent. However, they are problematic in those steel materials in which an annealing process has a decisive influence on the expression of their properties. An example of such uses of steel strip are so-called "electrical sheets" which are used in the field of electrical engineering because of their particular electromagnetic properties.

In the case of the sheets, where the annealing has a decisive influence on the properties, a subsequent transformation leads to a deterioration of the property in question. Therefore, in such materials, subsequent reforming usually eliminates to improve planarity. With increasing automation, increasing quality requirements and the use of new steels, therefore, there is the need to improve the flatness level during heat treatment and to minimize the generation of internal stresses.

This is of particular importance for steels for electrotechnical applications. These steels are particularly demanding in terms of flatness level, because they must not be subjected to post-deformation in the finally annealed state in order not to lose their good soft magnetic properties set during annealing. In addition, low residual stress levels in these materials are particularly favorable in terms of their magnetic properties. Therefore, at the To further develop these materials pay particular attention to the annealing process.

The use of electrotechnical steels is in the field of motors for driving machines and vehicles as well as in the field of the transformation of energy between different electrical voltage levels. They are used here as an iron core for guiding the magnetic flux in the form of stacked electric sheet metal lamellae. For the application of energy distribution special low-loss varieties have been developed. These are essentially grain oriented materials that are characterized by banded Goss oriented textures. They are produced in complex processes with silicon contents of about 3% by weight. These materials are only mentioned here, but not further treated.

The application in the field of actuators, so the motors, is far more common application and is usually covered with non-oriented steels. Here, silicon with proportions of up to 3.5 wt .-% is the main alloying element.

The soft and ferromagnetic materials are of great importance in electrical engineering and are used here eg for energy transformation and in the field of actuators (motors). Here, highly permeable materials in their amorphous, nanocrystalline or crystalline state are mentioned, all of which are generated in a coordinated manner, good conducting properties in the soft magnetic core for the provide magnetic flux. It is important to minimize the coercivity and to maximize the polarization even with low field requirements. This results in low core losses for small core sizes with low copper requirements for the windings. A special economic importance in the case of soft magnetic materials is occupied by the silicon-alloyed steels. They are manufactured with up to 3.5 wt .-% Si in standard sheet thicknesses of 0.1 to 1.0 mm as a band and used for example as a stack of sheets in the flux guide of electric machines or transformers. Since the respective processed electrical sheets directly influence the energy requirements of the finished pumps, fans, compressors, chokes and motors, steel grades with specially tailored magnetization behavior have been developed for the respective application.

A group of these steels is partially deformed at the steel strip producer in the finishing stand and / or in the stretch leveler and delivered to the customer in the so-called semi-finished state. At the customer, sheet metal parts are punched or cut from the semi-finished material, which are subsequently annealed. Only after this annealing, the required good soft magnetic properties set.

The other group of these electrotechnical steels, which is called "fully-finished", is already heat treated at the steel strip producer prior to delivery to the customer so that it has the required magnetic Retains properties even after punching or cutting at the customer, without the need for a final annealing must be carried out at the customer. Especially with such a "fully finished material", increasing the efficiency of further processing requires a good flatness before and after the punching process.

In order to achieve this, steel strips are required in which only minimal flatness errors are present in the delivery state. Of particular importance in this context is the edge waviness of the steel strip, since optimal material utilization is only possible with minimized edge ripple. Due to the sometimes filigree stampings with only millimeter-sized structures in addition, the internal stresses in the sheet must be kept low so that these structures do not discard, which would hamper the stacking of the stampings.

In the already mentioned from the beginning EP 0 357 797 A1 In order to avoid the formation of internal thermal stresses in a non-grain-oriented electrical steel sheet as a result of the final annealing, the cooling of the sheet after annealing is carried out in three stages. In the first, reaching from 620 - 550 ° C stage while a cooling rate of at most 8 ° C / s is set. The average cooling rate in the second, from <550 ° C - 300 ° C reaching cooling stage should be greater than the cooling rate of the first stage, on the other hand, but also less than four times the cooling rate of the first stage. In the third, subsequent to the second cooling stage stage, finally, an average cooling rate of at least 5 ° C / s should be maintained. The practice of this process has resulted in electrical sheets having improved magnetic properties. The problem of the often poor for the further processing flatness in the from the EP 0 357 797 A1 However, known manner heat-treated electrical sheets persisted.

The present invention has therefore dealt in particular with the heat treatment carried out in the course of the production of fully-finished electrical tapes and has answered the question of how annealing with subsequent cooling is to be carried out in order to ensure good flatness states in the case of electrical sheet material at low internal stresses.

• dass die erste Stufe der Abkühlung von der maximalen Bandtemperatur T_max bis zu einer um 50 °C über der Curie-Temperatur T_C liegenden Temperatur T_C+50°C reicht und das Stahlband während dieser ersten Stufe der Abkühlung mit einem Abkühlgradienten CR₁ von -38 °C/s bis -1 °C/s gekühlt wird,
• dass die zweite Stufe der Abkühlung von der Temperatur T_C+50°C bis zu einer um 80 °C unterhalb der Curie-Temperatur T_C liegenden Temperatur T_C-80°c reicht und das Stahlband während der zweiten Stufe der Abkühlung mit einem Abkühlgradienten CR₂ gekühlt wird, die in einem Bereich liegt, dessen obere Grenze CR_2MAX und untere Grenze CR_2MIN wie folgt bestimmt werden: $${\mathrm{CR}}_{\mathrm{2}\mathrm{MAX}}\mathrm{=}\mathrm{\pi }\mathrm{\cdot }\left(\cos \left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{Band}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{Band}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)\mathrm{+}\mathrm{1}\right)\mathrm{-}\mathrm{<figure-callout\; id="2"\; label="\pi \; CR"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}$$
  
  $${\mathrm{CR}}_{}$$$${}_{\mathrm{2}\mathrm{MIN}}\mathrm{=}3\cdot \mathrm{\pi }\mathrm{\cdot }\left(\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{Band}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{Band}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)-\mathrm{1}\right)\mathrm{-}\mathrm{\pi }$$
  
  
  und
• dass in der unterhalb der Temperatur T_C-80°C beginnenden dritten Stufe der Abkühlung mit einem Abkühlgradienten CR₃ von bis zu -85 °C/s beschleunigt abgekühlt wird.

Based on a method of the type described above, this object has been achieved according to the invention,

• that the first stage of the cooling from the maximum belt temperature T _max to a lying by 50 ° C above the Curie temperature T _C temperature T _{C + 50 ° C} and the steel strip during this first stage of cooling with a cooling gradient CR ₁ of From -38 ° C / s to -1 ° C / s,
• that the second stage of the cooling from the temperature T _{C + 50 ° C} up to a lying by 80 ° C below the Curie temperature T _C temperature T _{C-80 ° c} and the steel strip is cooled during the second stage of the cooling with a cooling gradient CR ₂ , which lies in a range whose upper limit CR _2MAX and lower limit CR _{2MIN are determined} as follows: $${\mathrm{<figure-callout\; id="2"\; label="CR"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">CR</figure-callout>}}_{\mathrm{2}\mathrm{MAX}}\mathrm{=}\mathrm{\pi }\mathrm{\cdot }\left(\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)\mathrm{+}\mathrm{1}\right)\mathrm{-}\mathrm{<figure-callout\; id="2"\; label="\pi \; CR"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi \; </figure-callout>}$$
  
  $${\mathrm{CR}}_{}$$$${}_{\mathrm{2}\mathrm{MIN}}\mathrm{=}3\cdot \mathrm{\pi }\mathrm{\cdot }\left(\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)-\mathrm{1}\right)\mathrm{-}\mathrm{\pi }$$
  
  
  and
• that in the third stage of the cooling, beginning below the temperature T _{C-80 ° C} , cooling is accelerated up to -85 ° C / s with a cooling gradient CR ₃ .

The "cooling gradients" CR1, CR2 and CR3 denote the temperature decreases per unit time [° C / s] which are achieved in the respective cooling stage (CR1, CR2, CR3 in each case <0 ° C / s).

The invention is based on the finding that, in the conventional manufacture of electrical steel, inhomogeneous temperature conditions over the bandwidth during continuous annealing are the primary cause for the development of deviations in planarity and internal stresses. These are not only to a limited extent due to inhomogeneities in the temperature distribution in the furnace cross-section, but result essentially from the surface quality and the applied pass schedule during cold rolling. Here are playing when heating due to high Heat transfer coefficients remaining partial oxide layers from the pre-process (hot strip, cold strip) a strong role.

During cooling, the surface roughness gains importance in addition to the oxide layers formed during the annealing process. The roughness index RPc (roughness peak number) controls, in addition to the alloy composition of the surface, the energy absorption of the material surface primarily at relatively constant mean roughness depths. The resulting small changes in the emission number are sufficient to trigger a further change in the emission number via the temperature dependence. These effects cumulate roughness-dependent, so that over longer periods (about 30 - 60 s) with correspondingly steep cooling gradients without interruptions in the cooling and without adjustment of the cooling curve significant temperature differences of up to 70 ° C over the bandwidth (depending on the starting temperature, gradient and Time) occur.

This results in a dependence on the surface quality for the uniformity of the material elongation in the continuous furnace, wherein a response of the plastic strain could be detected on different cooling curves with identical starting material with inhomogeneous surfaces.

In all cases, the cold rolling process provides the recrystallization nuclei and thus the starting point for a uniform or uneven recrystallization. Inhomogeneous microstructural conditions over the bandwidth are thus due to high shear during cold rolling due to high Frictions on the surface caused. Good homogeneously pickled surfaces without residual oxide layers are thus important prerequisites for a favorable homogeneous cold rolling and a low stress level of the material after annealing.

Already from the homogeneity of the hot rolling process in the form of a uniform temperature distribution over the bandwidth and thus possibly a Umformregimes in other crystallographic phase areas results in a change in the starting conditions during cold rolling. Also, the oxidation after hot rolling is particularly important here in the edge region of the strip.

The cold rolling step can not eliminate these structural states or residual scale surfaces. On the contrary, in the case of severely shearing cold rolling conditions and high rolling forces in the sense of dislocation inhomogeneities, it only creates worse starting conditions for the recrystallizing continuous annealing. All this can cause z. B. in the middle and edge region during continuous annealing fluctuations of grain size distributions but also of coercive force values arise, which are then a sign of high internal stresses.

As a result, the respective surface quality and the grain size distribution of the cold rolling process could be identified as a significant influence on the homogeneity of a strip with regard to low internal stresses and good flatness.

For performance and plant technical reasons, it is often possible only with great effort and high costs to change the cold rolling process so that an optimized flat product of the type in question can be produced.

The invention has now shown a way in which the generation of internal stresses and flatness problems in the manufacture of steel strips of the type in question can be minimized without the need to change the cold rolling process or the other process steps preceding the heat treatment step mentioned above.

By following the annealing, still starting in the outlet of the continuous furnace cooling is carried out according to the invention at least three stages with a precisely predetermined temperature control around the Curie temperature, can surprisingly compensate for such influences that the steel strip from the annealing step itself and the annealing upstream Brings processes.

In the course of the method according to the invention, a part of the heat energy is required for the dissolution of the ferromagnetic order during heating. In this area, the temperature of the strip temperature converges over the strip width, so that volume cross sections across the bandwidth in the uncritical surface quality case all reach their desired maximum temperature and at most different times for the recrystallization at this temperature are available. In the critical case with an inhomogeneous surface and only a short or no holding time, reaching the maximum strip temperature is not guaranteed. However, in both cases grain size differences over the bandwidth can occur, which can only be homogenized to a limited extent by the cooling process. The deformation in the heating section due to steadily rising temperatures with easier forming capability is reflected in band constrictions and maximum lengthened edge regions due to short or homogeneous length ratios of the band edge to the band center. Tape running difficulties can also contribute to this.

The critical areas for elongation in continuous annealing lie in two areas of cooling. On the one hand in the range of the Curie temperature, which leads by the anomalies of the specific heat capacity and the emission behavior to the temperature-dependent utilization of the irradiated heat energy. The construction of the ferromagnetic coupling of the atoms during the cooling process costs energy that is not available for lowering the temperature. As a result, a previously existing small temperature difference in band areas across the bandwidth can be excited to a significant increase. The two main influencing factors in the temperature transfer, emission number and specific heat capacity, are temperature-dependent here and contribute in an unfavorable case to a mutual influence and to create significantly different values across the bandwidth. Due to the resulting temperature difference results in evaluation of the resulting due to the local temperature difference across the bandwidth different expansions, which are also temperature-dependent, and the band trains, which add up in "colder", shorter band areas, a transformation of the material.

Otherwise, a homogeneous elongation pattern of the material can be assumed. When cooling, however, reduces the formability of the material with decreasing temperature. In this case, local deformation effects can occur in the region of the Curie temperature because of rather high m-values (m = d (ln σ _F ) / d (ln) & = reforming speed; σ _F = yield stress (temperature-dependent)) Forming speeds change from 0.1 s ^-1 to 1.0 s ^-1 (shear effects). This results in deformations in the respective crystal (grain) and thus stress in the structure, which take place not only in the grain boundary area (lattice construction error). Between 550 ° C and 700 ° C, annealing effects are only limitedly possible at low forming speeds (<0.01 s ^-1 ).

For an electrical steel alloy containing 1.3% by weight of Si and 0.15% by weight of Al, the stresses arising in the course of the heat treatment according to the invention could be detected by means of magnetic measurements, in particular the coercive force. An adaptation of the annealing curve in this temperature range of the cooling in the form of a lying at 730 - 750 ° C (740 ° C, the Curie temperature of this alloy) temperature maintenance level showed a significant improvement in the magnetic properties compared to the driving without the inventive cooling stop. This is especially true if inhomogeneous roughness profiles across the bandwidth were measured. Here, the roughness profiles of the cold-rolled behave Material with respect to average surface roughness and roughness peak number almost identical, so that after the non-oxidizing annealing, the panel samples could be used after the annealing for comparison.

Corresponding marked improvements of the magnetic properties are achieved when for a 3.0 wt .-% Si and 0.4 wt .-% Al containing sheet metal alloy whose Curie temperature is 746 ° C, the glow curve in the form of a 730 ° C - 765 ° C temperature curve is adjusted.

In the case of an electroplated metal alloy with 0.15% by weight of Si and 0.1% by weight of Al and a Curie temperature of 767 ° C., corresponding improvements in the magnetic properties can be achieved with a temperature maintenance step in the annealing curve of around 750-775 ° C. become.

mit t_schnell [sec]: Jeweiliger Zeitpunkt der Abkühlung, für den der einzustellende Abkühlgradient CR₃ bestimmt werden soll; CR3 ≤ 0 °C/s.
Typischerweise beträgt dabei die die Dauer der dritten Stufe der Abkühlung 14 - 30 Sekunden.After leaving the critical zone of the second stage of the cooling, the cooling can be accelerated during the third stage. In this case, at the respective time of the third stage of the cooling, the respectively optimum cooling gradient CR ₃ with a tolerance of ± 20 ° C. can be determined as follows: $${\mathrm{CR}}_{\mathrm{3}}\mathrm{=}\mathrm{3}\mathrm{.}\mathrm{4}\mathrm{\cdot }{\mathrm{t}}_{\mathrm{fast}}\mathrm{-}\mathrm{70}$$

with t _fast [sec]: respective time of cooling, for which the cooling gradient CR ₃ to be set is to be determined; CR3 ≤ 0 ° C / s.
Typically, the duration of the third stage of cooling is 14-30 seconds.

A further advantageous embodiment of the invention is characterized in that the cooling from a 5 - 10 seconds before reaching the Curie temperature T _C lying time and up to a 5 - 10 seconds after reaching the Curie temperature T _C lying time with a Cooling gradient CR _{2 is set} near the minimum cooling gradient CR _2MIN . In this way, the construction of thermal stresses in the material is reliably prevented, which may otherwise occur due to the non-linear in the critical temperature range of T _C specific heat capacity of the processed sheet metal material. The structure of the ferromagnetic order taking place in the region of the Curie temperature requires energy which is then not available for changing the temperature. By reducing the cooling gradient in this temperature range according to the invention, the potential for the generation of internal stresses is thus minimized.

Likewise, it contributes to the further optimization of the properties of the steel strip processed according to the invention if the cooling is carried out from a point in time 5 to 15 seconds before reaching the temperature T _{C-80 ° C} with a cooling gradient CR ₂ which is close to the minimum cooling gradient CR _2MIN is set. This measure creates optimal starting conditions over the bandwidth for the subsequently completed third stage of cooling in terms of temperature homogeneity. As a result, this measure leads to a optimum flatness of the heat-treated hot strip according to the invention.

An oxidation of the steel strip during annealing can also be avoided in the procedure according to the invention in that the continuous annealing is carried out under a protective gas atmosphere. This protective gas atmosphere can also decarburizing in a manner known per se in order to achieve minimized C contents in the finished steel strip.

According to a first variant of the annealing step, the steel strip is heated to the annealing temperature during continuous annealing in a single heating stage. In this case, a rapid heating device can be used to heat the steel strip, which heats the steel strip in the range of 20 - 450 ° C in an open-heated combustion gas atmosphere at a heating rate of more than 100 ° C / s. The rapid heating causes a surface oxidation that makes the energy absorption uniform throughout the annealing process, thus minimizing temperature differences across the bandwidth.

According to an alternative variant of the annealing step, the heating of the steel strip to the annealing temperature is carried out in at least two stages. This ensures uniform heating of the steel strip to maximum temperature.

It has proven to be particularly favorable in terms of optimizing the properties of the resulting steel strip, when temperature maintenance levels by T _c and before T _{c-80 ° C} by lowering the furnace chamber temperature for 3 to 10 s be set before the respective temperature maintenance level by 30 - 200 ° C below the oven room temperature of the subsequent oven room temperatures. In this way, higher energy transitions are generated which expand the possibilities of the subsequent reduction of the cooling gradients according to the invention in the material.

The method according to the invention is particularly suitable for steel strips made of steels with Si contents% Si and Al contents% A1, for which% Si + 2 ×% A1 ≦ 6.6% by weight, and their mean specific Heat capacity C _P at T _c shows a non-linear increase.

The inventive method has a higher tolerance width over the bandwidth on the top and bottom surface fluctuating surface roughness. Thus, with the inventive method also for such steel strips low internal stress and a good flatness ensure fluctuate in which the width of the bands Rauheitsspitzenzahlen (RPc) from 100 to 300, and R _z values from 2 to 7 microns.

It is also found that in the case of the procedure according to the invention, low magnetic scattering (standard deviation) of less than 1.2% for the coercive field strength change can be ensured even in the case of residual-stress inhomogeneous structures after the annealing process by a subsequent stress relief anneal at 650 ° C.

Even with residual stress-dependent homogeneous starting structures after the annealing process, the invention enables low magnetic scattering (standard deviation) below 1.0% for the coercive field strength change by subsequent stress relief annealing at 650 ° C versus annealing without holding steps. In addition, there is an improvement in the mean values in the coercitive force change due to the stress relief annealing of <5% to <2.5%.

It has also been demonstrated that 5 to 10% lower remagnetization losses P _1.5 (1.5 T polarization) for the annealed material can be achieved in the inventive procedure compared to a conventionally performed linear standard cooling (without cooling stop according to the invention). Also, up to 1% higher polarization values at 2500 and 5000 A / m can be achieved for the heat-treated material according to the invention.

Furthermore, the invention gave higher permeability values at 1.0 and 1.5 T polarization over conventionally processed steel strip.

Particularly advantageous permeability values at 1.0 and 1.5 T polarization with an improvement of up to 20% can be achieved with processing according to the invention for electro-sheet alloys for whose Si content% Si and Al content% Al is 0.9% by weight. % ≤% Si + 2 x% Al ≤ 1.8 wt%.

In addition, heat-treated steel strip according to the invention regularly has up to 40% lower coercive field strength values at 2500 and 5000 A / m for the annealed material compared to the linear standard cooling (without cooling stop according to the invention).

It could also be demonstrated that in the case of processing according to the invention, at most symmetrical constrictions and associated elongations in the annealing direction (band longitudinal direction) are set across the band width with reduced edge undulations for the annealed material.

Finally, according to the present invention, material cooled after annealing has a greater tolerance to material that has been conventionally cooled linearly with respect to the influence of carbon residue levels on magnetic aging. Thus, in a four-year aging at room temperature with material produced according to the invention, only a max. 5% higher re-magnetization loss compared to 10-20% increase in the re-magnetization loss occurring in conventional cooling processes. This is especially true when the carbon content is limited to less than 30 ppm, and it has been found that in the case of electrical steel sheets for whose S content% Si and Al content% Al% Si + 2 x% Al ≥ 1.8 Wt .-%, the improvement of the invention also sets at carbon contents of up to 50 ppm.

Fig. 1

eine Anlage zur Wärmebehandlung von kaltgewalztem Stahlband;

Fig. 2

einen in der Anlage gemäß Fig. 1 eingesetzten Durchlaufofen.

The invention will be explained in more detail with reference to exemplary embodiments illustrative drawings. Each show schematically:

Fig. 1

a plant for the heat treatment of cold-rolled steel strip;

Fig. 2

a continuous furnace used in the system according to FIG.

The conventionally constructed plant A for heat treatment of a cold-rolled steel strip S comprises in the direction of belt B successively a coiler C, an inlet storage D, a continuous furnace E, connected to the continuous furnace E cooling device F, a discharge storage G and a coiler H.

The continuous furnace E is divided into 26 successively traversed in the strip running direction B, in each case immediately merging into one another furnace zones 1 to 26, of which the furnace zone 1 to the kiln inlet E _and the furnace zone 26 is assigned to the kiln outlet E _from. The furnace zones 1 to 12 form a jet tube heating section E _A , while the furnace zones 13 - 26 together form a radiation cooling section E _K. Each individual furnace zone 1-26 is controllable with respect to the temperatures T _o prevailing in it.

The cooling device F which is connected directly to the outlet of the continuous furnace E is designed in a manner also known per se as a protective gas jet rapid cooling device. In order to demonstrate the effect of the invention, in Appendix A 0.5 mm thick and 1265 mm wide cold-rolled steel sheet steel strips were produced, which were produced from a steel alloy which in a first example (Example 1) 1.3 wt. % Si and 0.15 wt .-% Al, in a second example (Example 2) 3.0 wt .-% Si and 0.4 wt .-% Al and in a third example (Example 3) 0.15 wt In addition, each of the steel alloys further contained in total up to 1% by weight contents of Mn, P, Cr, Ni and in total up to 0.1% by weight of Al. % Contents of C, N, S, balance iron and unavoidable impurities had. The correspondingly assembled strips were first hot rolled in a conventional manner to hot strips and then cold rolled in the same conventional manner to final thickness. The supplied as coils I, cold-rolled steel sheet steel steel strips S are unwound in the Abhaspeleinrichtung C and passed over the tape storage D in the continuous furnace E. There, they were first heated in the first 12 furnace zones 1 to 12 until they had a uniform temperature of at least 800 ° C. when leaving the furnace zone 12 over their strip cross-section.

The control of the respective furnace temperature T _o in the furnace zones 1 to 12 was carried out so that the temperature at the entrance of the steel strip S in the furnace zone 13 above the critical Curie temperature T _C was, in Example 1 740 ° C, in Example 2 746 ° C and 3 767 ° C in Example.

In the furnace zone 14, a significantly lower furnace temperature T _{o has been} set than in the adjacent furnace zones 13 and 15 in order to ensure the lowest possible temperature change of the strip as it passes through this furnace zone 14. The consequence of this was that the belt temperature T _{B has} been kept essentially constant up to about the furnace zone 19. From oven zone 20, the furnace temperature T _o local then been up to the furnace zone 22 significantly lowered again so that approximately in the oven zone 22, the belt temperature T _B has been cooled to a more than 80 ° C lying below the Curie temperature Temperature , The oven temperature T _o in the oven zones 23 to 26 was compared to the furnace temperature T _o in the region of the furnace zone 22 at a much higher level, so that on the furnace zones 23 - 26, the strip temperature T _B only slightly decreased. After exiting the continuous furnace E and entering the cooling device F, the strip was then accelerated accelerated by exposure to a gas stream.

In Diag. For a heat-treated steel strip S _E according to the invention, the respective strip temperature T _{B is} plotted over the furnace zones 1 to 26. The entries relating to the heat-treated steel strip S _E according to the invention are characterized by unfilled squares. In addition, in Diag. 2 for a linearly cooled in a conventional manner steel strip S _{K applied} the respective strip temperature T _B over the furnace zones 1 to 26. The temperature values determined for the conventionally heated and cooled steel strip S _K are indicated by circles. The temperature gradients CR ₁ , CR ₂ , CR ₃ set in inventive and conventional heat treatment are given in the following table. gradient According to the invention Conventional CR ₁ -2 ° C / s - 4.8 ° C / s CR ₂ From T _C -80 to T _C -66: -0.9 ° C / s From T _C -80 to T _C +10: -1.1 ° C / s - 4.8 ° C / s CR ₃ -9.14 ° C / s 12.21 ° C / s In Diag. 1 for the inventively heat-treated and conventionally processed steel strip S _E, the respective furnace temperatures T _{O are} plotted over the respectively assigned furnace zone 1 - 26. The temperatures T _O determined for the steel strip S _E processed according to the invention are characterized by unfilled squares and the temperatures T _o set for the conventionally heat-treated steel strip S _K are indicated by circles.

The properties of the steel strips according to the invention and conventionally heat-treated are compared in the following table: Heat treated according to the invention Conventionally heat treated example 1 2 3 1 2 3 Edge ripple [%] <1 <1 <1 <1.5 <1.5 <1.5 P _1.5 [W / kg] 4.8 4.2 7.3 5.2 4.8 7.8 J ₂₅₀₀ [T] 1.68 1,623 1,646 1.67 1,606 1,630 μ1,0 5800 6000 3850 4800 3700 3400 μ1,5 3200 2000 1800 2800 1400 1600 It shows a clear superiority of the flatness (edge waviness) and the magnetic properties of the heat-treated steel strip S _E according to the invention.

Claims (12)

Process for heat treating steel strips made of a steel, the Si and / or Al, with the proviso that for the Si content% Si and the Al content% Al% Si + 2 x% Al ≤ 6.6 wt. up to 2.5% by weight of Mn, optionally one or more elements from the group "P, Cr, Ni" in each case up to 1% by weight, optionally one or more elements from the group " C, N, S "containing in each case up to 0.1 wt .-% and the remainder containing iron and unavoidable impurities are produced, in which the steel strips are first continuously annealed and then cooled in controlled at least three stages, characterized: - That the first stage of the cooling from the maximum belt temperature T _max up to a lying by 50 ° C above the Curie temperature T _C temperature T _{C + 50 ° C} and the steel strip during this first stage of cooling with a Abkühlgradienten CR ₁ from -38 ° C / s to -1 ° C / s, - That the second stage of cooling from the temperature T _{C + 50 ° C} up to 80 ° C below the Curie temperature T _C lying temperature T _{C-80 ° C} ranges and the steel strip during the second stage of cooling with a Cooling gradient CR _{2 is} cooled, which is in a range whose upper limit CR _2MAX and lower limit CR _{2MIN are determined} as follows: $${\mathrm{CR}}_{\mathrm{2}\mathrm{MIN}}\mathrm{=}3\cdot \mathrm{\pi }\mathrm{\cdot }\left(\cos \left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)-\mathrm{1}\right)\mathrm{-}\mathrm{\pi }$$

$${\mathrm{CR}}_{\mathrm{2}\mathrm{MAX}}\mathrm{=}\mathrm{\pi }\mathrm{\cdot }\left(\cos \left(\mathrm{2}\mathrm{\cdot }\left({\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\right)\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\mathrm{\cdot }\left(\mathrm{(}{\mathrm{T}}_{\mathrm{tape}}\mathrm{-}{\mathrm{T}}_{\mathrm{C}}\mathrm{)}{\left(\frac{\mathrm{\pi }}{{\mathrm{2}}^{\mathrm{6}}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{1}\right)\right)\mathrm{+}\mathrm{1}\right)\mathrm{-}\mathrm{\pi }$$

- And that accelerated in the starting below the temperature T _{C-80 ° C} third stage of cooling with a cooling gradient CR ₃ of up to -85 ° C / s accelerated. A method according to claim 1, characterized in that during the third stage of the cooling at the respective time instant t _quickly set cooling gradient CR _{3 is determined} with a tolerance of ± 20 ° C as follows: $${\mathrm{CR}}_{\mathrm{3}}\mathrm{=}\mathrm{3}\mathrm{.}\mathrm{4}\mathrm{\cdot }{\mathrm{t}}_{\mathrm{fast}}\mathrm{-}\mathrm{70}$$

with t _fast [sec]: respective time of cooling, for which the cooling gradient CR ₃ to be set is to be determined; CR ₃ ≤ 0 ° C / s. A method according to claim 2, characterized in that the duration of the third stage of the cooling is 14 - 30 seconds. Method according to one of the preceding claims, characterized in that the cooling from a 5-10 seconds before reaching the Curie temperature T _C lying time and up to a 5-10 seconds after reaching the Curie temperature T _C point is set with a cooling gradient CR ₂ near the minimum cooling gradient CR _2MIN . Method according to one of the preceding claims, characterized in that the cooling is carried out from a 5 - 15 seconds before reaching the temperature T _C -80 _° C lying time with a Abkühlgradientenienten CR ₂ , which is set close to the minimum cooling gradient CR _2MIN . Method according to one of the preceding claims, characterized in that the continuous annealing is carried out under a protective gas atmosphere. A method according to claim 6, characterized in that the protective gas atmosphere acts decarburizing. Method according to one of the preceding claims, characterized in that the steel strip is heated in the continuous annealing in a heating stage to the annealing temperature. A method according to claim 8, characterized in that for heating the steel strip, a rapid heating device is used, the steel strip in the range of 20 - 450 ° C in a heated heated combustion gas atmosphere at a heating rate of more than 100 ° C / s. Method according to one of claims 1 to 7, characterized in that the heating of the steel strip to the annealing temperature is carried out in at least two stages. A method according to claim 10, characterized in that temperature maintenance levels are adjusted by T _c and T _c -80 ° C by lowering the furnace chamber temperature for 3 to 10 s before the respective temperature maintenance stage by 30 - 200 ° C below the furnace chamber temperature of the subsequent furnace chamber temperatures. Method according to one of the preceding claims, characterized in that for the Si content% Si and the Al content% Al is 0.9 wt .-% ≤% Si + 2 x% Al ≤ 1.8 wt .-%.

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