Patent Application
20160108493
1. Field of the Invention
The invention is directed to a process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, said process comprising the process steps of a) smelting a steel, b) continuously casting the smelt by thin slab continuous casting, c) heating up the thin slabs, d) heating the thin slabs, e) subjecting the thin slabs to continuous hot rolling, f) cooling and reeling the hot-rolled strip to form a coil, g) annealing the hot-rolled strip after reeling and prior to subsequent cold rolling, h) cold rolling the hot-rolled strip in a first cold rolling stage to an (intermediate) thickness, i) subjecting the resulting cold-rolled strip to recrystallization and decarburization annealing, j) in a second cold rolling stage, cold rolling the cold strip that has been subjected to recrystallization and decarburization annealing to its final thickness, k) applying an annealing separator (non-stick layer) to the strip surface of the cold-rolled strip that has been rolled to its final thickness, l) subjecting the cold-rolled strip that has been coated with the annealing separator to secondary recrystallization annealing by high-temperature annealing in a bell-type furnace, forming a finished steel strip having a pronounced Goss texture, and m) coating the finished steel strip that has undergone secondary recrystallization annealing with an electrically insulating layer, and then stress-free annealing or stress-relief annealing the coated finished steel strip.
The invention is further directed to a grain-oriented electrical steel strip that is obtained by said process.
2. Description of the Prior Art
The grain-oriented electrical steel strip produced by said process is intended for use in transformers. The material of the grain-oriented electrical steel strip is characterized by a particularly sharp {110}<001> texture (Miller indices), which has an easymagnetization direction parallel to the rolling direction. A method for forming such a texture was first described by N. P. Goss, and therefore, such textures are generally referred to as “Goss texture”. The Goss texture is formed by selective, anomalous grain growth, also referred to as secondary recrystallization. In this process, the normal, natural tendency of a metallic matrix toward grain growth is suppressed by the presence of grain growth inhibitors, also referred to as inhibitors or an inhibitor phase. An inhibitor phase consists of very fine and optimally homogeneously distributed particles of one or more second phases. These particles have a natural interfacial energy on their boundary with the matrix, which inhibits the grain boundary movement because the further savings of interfacial energy is minimized throughout the system. Such an inhibitor phase is of central significance to the development of the Goss texture and therefore to the magnetic properties that can be achieved in such a material. It is critical in this process to achieve a homogeneous distribution of a very large number of very small particles, which is more advantageous than a small number of coarser particles. Since the number of precipitated particles cannot be determined through experimentation, their size is used as an indication of their efficacy. It is thus assumed that the particles of the inhibitor phase should not be substantially larger than 100 nm, on average.
In U.S. Pat. No. 1,965,559 A, N. P. Goss describes a process in which a grain-oriented electrical steel strip (silicon steel) is produced by heating up a steel strip, subjecting said strip to a first cold rolling step, and then subjecting the strip to further heat treatment followed by a second cold rolling step.
Also known in practice are processes in which manganese(II) sulfite (MnS) is used as the inhibitor. The slabs produced by block casting or continuous casting are heated to very high temperatures, close to 1,400° C., in order to bring primary, coarse MnS precipitates back into solution. This diluted MnS is then precipitated finely dispersed during the hot working process. Since the hot-rolled strip thus produced already has the necessary grain growth inhibition this is referred to as inherent inhibition.
However, the grain growth inhibiting effect of the MnS phase is limited, so that, assuming customary hot-rolled strip thicknesses of, e.g., 2.30 mm, at least two cold rolling stages are required to bring the steel strip to its nominal usable thickness, with an intermediate recrystallization annealing being performed between the individual cold rolling stages. Moreover, material that is inhibited by manganese(II) sulfite can achieve only a limited texture sharpness, in which the Goss orientation is scattered on average by 7° around the ideal position. This texture sharpness is reflected, i.a., in the magnetic polarization at a field strength of 800 A/m, which is only rarely able to exceed values of 1.86 T. Such material is traditionally referred to as Conventional Grain Oriented or CGO.
The traditional production process, proceeding from the hot-rolled strip, further comprises a two-stage cold working process in which an intermediate, continuous recrystallization annealing step is performed between the two stages. Prior to the first cold rolling stage, a continuous hot strip annealing step is optionally performed, and is frequently combined with the essential hot strip pickling. The last cold working step is traditionally followed by a continuous recrystallization annealing step. This annealing step also removes the carbon from the steel strip below the magnetic aging limit, which is determined by the maximum carbon content that is soluble in ferrite, or approximately 30 ppm C in a composition of Fe with 3 wt % Si. (Carbon is essential because it establishes the correct microstructure during hot rolling.) The recrystallized microstructure of the steel strip that has been reduced to its nominal usable thickness represents the starting basis for the subsequent step of secondary recrystallization. This secondary recrystallization is accomplished by high temperature annealing in a bell-type furnace. Before the coiled rings (coils) are placed in the bell-type annealing furnace, the surface of the steel strip must be provided with a non-stick layer. An aqueous slurry of magnesium oxide (MgO) is usually used for this purpose. Once the magnetically desirable Goss texture has formed during the high temperature annealing in a bell-type furnace, the outer shape of the steel strip is further improved, and an electrically insulating layer is applied to the two opposing, large-area, wide surfaces of the strip. This is carried out in a continuous annealing furnace.
In this traditional production method, MnS is particularly used as the inhibitor phase, which must be precipitated very finely dispersed throughout the entire steel matrix. This is accomplished by preheating and through-heating the cast slabs at temperatures of approximately 1,400° C., followed immediately by hot working in a hot rolling train. These high temperatures are necessary because the solubility of [MnS] [Mn]+[S] in both ferrite and austenite requires such high temperatures. In the subsequent hot rolling process, the material cools down again, and the dislocations formed during plastic working serve as nucleation sites for the re-precipitation of MnS, this time very finely dispersed.
This traditional “high heating method” has a number of economic disadvantages: For one, it requires a particularly large amount of energy. For another, this method of production necessitates special logistical features in the overall operations of a smelting plant, which in quantitative terms uses typical slab preheating temperatures of 1,200° C. to 1,250° C. for the overwhelming majority of standard types of flat-rolled steel. But the greatest disadvantage is the formation of liquid slag at temperatures above 1,350° C. due to a low-melting Fe—Si eutectic mixture. This liquid slag results in material losses of >1%, and necessitates considerable expenditure on equipment needed to protect the annealing device. As a result, so-called “low-heating” methods with substantially reduced slab reheating temperatures have been developed.
SU 688527 A1 discloses a production process which likewise involves a two-stage cold rolling process with a continuous recrystallization strip annealing step between the two stages. However, during this intermediate annealing stage, the strip is also simultaneously decarburized to a maximum residual carbon content of 30 ppm. This has the advantage that, after the final cold rolling to the nominal usable thickness, no further continuous strip annealing step is required. The strip is simply coated with the non-stick layer (usually MgO) and then fed directly to a high-temperature bell-type annealing furnace. However, the microstructure of the resulting strip is not recrystallized, but is instead as-rolled. As a result, during the very gradual heating of the steel strip during annealing in a bell-type furnace, a microstructural recovery is first achieved, followed by a primary recrystallization, and then secondary grain growth, which causes the formation of the Goss texture. This process offers the advantages of relatively cost-effective and reliable production.
In this process/production method, aluminum nitride (AlN) is used as the inhibitor phase, which has the advantage that the solubility thereof [AlN] [Al]+[N] even in ferrite is below that of MnS, and is much lower in austenite. In a chemical composition that is typically used for the production of electrical steel strip, the microstructure at the temperatures used for hot rolling is predominantly ferritic, but with austenitic proportions of up to 50%.
The above-described processes relate to conventional slab technology with slab thicknesses significantly greater than 150 mm, typically 210 mm-260 mm. Another important development in the history of grain-oriented electrical steel strip is the use of so-called thin slab technology, as described in EP 1 025 268 B1. The main economic advantage of this technology is that thin slabs, which are understood to be (cast) slabs having a thickness of 30 to 100 mm, typically 60 mm-90 mm, are no longer cooled to the ambient temperature and later reheated to high temperatures, but are instead fed at a controlled temperature to an inline homogenization furnace, in which they need only to be reheated somewhat in order to compensate for temperature losses, and to homogenize their temperature over the length and width of the slab. Immediately thereafter, these thin slabs are then hot rolled. In practical use, this results in substantial cost advantages due to the savings of energy, and an improved hot-rolled strip edge condition, with the resulting yield improvement (improvement of physical yield).
Due to the limited high temperature strength of the thin slabs and the necessity to transport them through a roller hearth furnace, the temperature that can be reached by heating is limited by the thickness of the slab. For example, with a slab thickness of 65 mm for a typical grain-oriented Si steel, 1,200° C. is the critical upper limit for ensuring sufficient practical production reliability. For this reason, process routes that are based on thin slab technology, i.e., thin slab continuous casting, are all essentially low-heating methods, in which the slab through-heating temperature is substantially lower than 1,300° C. However, at such temperatures an inherent inhibitor phase can no longer be formed during hot rolling because the precipitations that form these inhibitor phases cannot be dissolved prior to hot rolling. In processes of this type, the inhibitor phase is formed by a nitridation process (nitridation annealing) applied to the steel strip that has been cold rolled to the finished strip thickness. Such processes, in which only the use of inhibitors acquired by nitridation treatment is considered, are described in U.S. Pat. No. 8,038,806 B2 and in U.S. Pat. No. 8,088,229 B2.
However, at the low temperatures of the low-heating method, an inhibitor phase cannot be precipitated in the hot strip during the hot working process, i.e., during hot rolling.
One prior art, which discloses the fundamental and essential process steps of producing grain-oriented electrical steel strip by means of thin slab continuous casting with a two-stage cold rolling process and a recrystallization and decarburization annealing stage that is performed between the two cold rolling stages, along with homogenization annealing of the thin slabs at a maximum temperature of 1,250° C. prior to hot rolling, is described in U.S. Pat. No. 7,736,444 B1 and US 2014/0076464 A1.
In general, the problem with such/these processes is that, although the method of two-stage cold rolling with intermediate decarburization annealing is effective in preventing the formation of liquid slag during slab heating, the decrease in the slab heating temperature to levels that will enable the use of thin slab technology results in problems with respect to the formation of an inhibitor phase.
The object of the invention is therefore to devise a process that will enable the cost-efficient production of high-grade grain-oriented electrical steel using thin slab continuous casting systems, and in particular, to devise a solution that will provide a further improved process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, in which an inhibitor can be introduced into the steel strip that is capable of controlling secondary grain growth during secondary recrystallization annealing in a high temperature bell-type annealing furnace.
In a process according to the invention for producing grain-oriented electrical steel strip by means of thin slab continuous casting, the aforementioned object is attained in that it comprises the following process steps:
For a grain-oriented electrical steel strip according to the invention, the object of the invention is attained in that the grain-oriented electrical steel strip is obtained by such a process.
The invention is based on the following fundamental principle and involves sequential process steps:
The cast thin slabs are homogenization annealed in an equalization furnace at temperatures above 1,050° C., but below 1,200° C., typically 1,150° C. Immediately before the thin slabs are fed to the hot working process, they are passed through a continuous induction heating device, which is capable of heating the thin slabs to a temperature above a respective homogenization temperature of at least 1,250° C. and to temperatures above 1,300° C., maximum 1,350° C. This brief heating step is sufficient to dissolve the AlN inhibitor phase and to precipitate it immediately thereafter in a fine dispersion during the hot working process (hot rolling). Such heating by means of an induction heating device is described in DE 10 2012 224 531 A1.
In addition, the invention proposes relatively very high chemical proportions of copper in the smelted alloy, wherein copper is elementally precipitated, thereby favorably affecting the strength of the inhibitor. As a result, slab reheating can be carried out at temperatures below 1,350° C., which at least prevents the problematic formation of liquid slag.
It is particularly advantageous in the embodiment of the invention for the hot rolling in process step e) to be carried out at an initial rolling temperature during the first working pass of greater than 1,150° C., preferably greater than 1,200° C., a final rolling temperature ranging from 850° C.-980° C., and a final rolling speed of less than 12 m/s, preferably less than 10 m/s.
In a further development, the invention also provides that in process step e), during hot rolling, a hot-rolled strip having a maximum relative thickness crown of less than 2%, preferably less than 1%, in particular, less than 0.7% is rolled.
In this case it is then further expedient for a hot-rolled strip having a maximum relative thickness taper of less than 2% to be rolled during hot rolling in process step e), which is likewise proposed by the invention.
To achieve good recrystallization, it is also advantageous according to a further embodiment of the invention for the cold-rolled strip to be heated at a heating rate of more than 100 K/s after the first cold rolling stage at the start of recrystallization and decarburization annealing in process step i).
For thin slab continuous casting, it is advantageous for the casting in process step b) to be carried out at a superheating temperature of the smelt during casting of less than 40K, preferably less than 20K, in particular, less than 12K, which is likewise proposed by the invention.
One expedient procedure for producing a thin slab is the “liquid core reduction” method. The invention is therefore further characterized in that during casting in process step b), the thickness of the strand that is produced by the “liquid core reduction” method is reduced just below the metal mold while the core of the strand is liquid. Here, the strand is cast without being exposed to inert gases. Under casting without exposure of the strand to inert gas, is understood, wherein a conventional and usual protection of the strand can be provided and become available in the tundish and the mold.
To form the desired microstructure in the steel strip, it is further advantageous according to the embodiment of the invention that, during annealing of the hot-rolled strip in process step g), the annealed hot-rolled strip is quenched after annealing at a cooling rate of more than 25 K/s, preferably more than 30 K/s, in particular more than 40 K/s, preferably at a cooling rate ranging from 25 K/s-52 K/s.
To intensify the AlN inhibitor phase, the invention further provides that, as part of the recrystallization and decarburization annealing in process step i), the cold-rolled strip is also nitridation annealed, wherein in this nitridation annealing phase, the total nitrogen content of the annealing atmosphere is increased by at least 50% of its initial value in process step i) by adding ammonia (NH3) to the annealing gas, wherein the ammonia (NH3) is preferably added separately and is particularly blown onto the two opposing large-area strip surfaces of the cold-rolled strip.
Finally, the invention also provides that, following process step m), in particular optionally, a process step is performed which effects a magnetic domain refinement of the coated finished steel strip.
Overall, the invention is based on a basic alloy system that is customarily used for grain-oriented electrical steel strip and comprises iron at a proportion of 2 to 4 wt % and Si, typically a Si content of 3.2 wt %. Other suitable alloy elements are carbon, manganese, copper and aluminum, along with sulfur and nitrogen. The smelt is cast to form a strand without its exposure to an inert gas, using a thin slab casting machine. This strand is then divided into thin slabs, and these thin slabs are subjected to homogenization annealing in an equalization furnace. The thin slabs are then rapidly heated up continuously to a temperature above 1,050° C. using a linear induction heating apparatus, and immediately thereafter, the thin slabs are hot worked to a hot strip thickness ranging from 1.8 mm-3.0 mm, preferably from 1.80-2.30 mm. Once the hot-rolled strip produced in this manner has been annealed and pickled, the hot strip is cold rolled in a first cold rolling stage to an (intermediate) thickness ranging from 0.50 mm-0.80 mm, preferably approximately 0.65 mm. The cold-rolled strip thus produced is then recrystallized and decarburized in a continuous annealing line. At the primary recrystalization, decarburization, and optional nitridation annealing, neither texture ratios nor oxygen content are controlled. Rather, the related values are determined automatically by control of respective system and process. There is not any control and/or regulating system for controlling the texture ratios or the oxygen content. The cold strip which has been cold rolled to an intermediate thickness and then has been recrystallization and decarburization annealed is then rolled in a second cold rolling stage to the final thickness or nominal usable thickness of 0.15 mm-0.40 mm. After a non-stick coating (annealing separator), particularly consisting of MgO, is applied, the material is subjected to high-temperature annealing in a bell-type annealing furnace, at a temperature above 1,150° C. and up to 1,210° C., for the purpose of adjusting and forming the magnetically required Goss texture. An insulating coating is then applied, which is followed immediately by continuous stress-relief annealing. Following inspection, certification and adjustment, the result is a grain-oriented electrical steel strip in the form of a finished strip ready for use. ° C.
The chemical composition of the smelt for casting is based as follows:
After a non-stick coating (annealing separator), particularly consisting of MgO, is applied, the material is subjected to high-temperature annealing in a bell-type annealing furnace, at a temperature above 1,150° C. and up to 1,200° C., for the purpose of adjusting and forming the magnetically required Goss texture. An insulating coating is then applied, which is followed immediately by continuous stress-relief annealing. Following inspection, certification and adjustment, the result is a grain-oriented electrical steel strip in the form of a finished strip ready for use. During carrying out the process, during the secondary recrystallization the texture ratios and the oxygen content are again not controlled. Rather, these values are adjusted automatically by control of the respective system and process. There is not any control or regulating system for controlling the texture ratios or oxygen content.
The chemical composition of the smelt for casting is stated for the following reasons:
Silicon causes an increase in specific electric resistivity and therefore a decrease in the classic magnetization losses. Below an alloying degree of 2 wt %, its use as grain-oriented electrical steel does not make sense. An alloying percentage above 4 wt % impedes processing tremendously due to the massive brittleness that results. In practical applications, Si alloying percentages of 3.15 to 3.30 wt % have proven advantageous. Beyond even 3.45 wt %, the aforementioned problems with brittleness are observed.
During the heating process and during annealing treatments, carbon causes structural homogenization as a result of ferrite-austenite transformation processes. Carbon contents of between 0.025 and 0.100 wt % are generally standard. This effect is intensified with high C contents; however, the decarburization step that is necessary during this process then requires more time, thereby reducing productivity. In the process proposed here, decarburization is necessary with a relatively large strip thickness. Therefore, relatively low smelt carbon contents are sought, although they must be sufficient to enable the structure homogenizing effect. Carbon contents ranging from 0.025 to 0.045 wt % have proven advantageous.
The alloy element manganese generally has a favorable effect on casting and hot working properties. Moreover, a certain Mn content is helpful in reducing wear and tear on refractory material during the liquid metallurgical treatment steps. Mn contents ranging from 0.060-0.500 wt % have proven advantageous in practice.
In the process considered here, sulfur is more of a detractive element and is decreased to contents of less than 0.010 wt %, in particular of less than 100 ppm. The sulfur content should preferably be less than 40 ppm. During solidification of the smelt, MnS particles form, which are retained in the very coarse state in which they are precipitated during solidification of the smelt throughout the entire process, and are magnetically deleterious in the finished product. However, reducing the S content will result in the formation of only a small number of coarse MnS particles, which do not have a deleterious effect. It is further known that the ratio of manganese to sulfur is correlative to the quality of the hot-rolled strip edges in terms of the occurrence of edge crack. This ratio should therefore be at least Mn/S>6, more preferably >20. In the process considered here, sulfur is more of a detractive element and is decreased to contents of less than 0.010 wt %, in particular of less than 100 ppm. The sulfur content should preferably be less than 40 ppm.
Aluminum is the (main) carrier of grain growth inhibition, and is based on the acid soluble proportion of the aluminum. (The remainder is alumina Al2O3). To adjust the effect of the inhibitor phase correctly, the Al content should be between 0.010 and 0.030 wt %, ideally between 0.010 and 0.020 wt %.
Nitrogen, together with the acid soluble aluminum, acts as an inhibitor by way of the finely dispersed precipitation of AlN particles. For the sufficiently effective and reliable formation of the AlN inhibitor phase, the N content of the smelt should range from 80 to 120 ppm.
Copper is an element that in most cases becomes a steel admixture through the addition of scrap metal. Copper forms elemental Cu precipitates, which also have a favorable effect on secondary grain growth in the microstructure, with even quite high Cu contents being tolerable. Therefore, Cu contents ranging from 0.200 to 0.550 wt % are provided according to the invention.
In addition to iron and unavoidable impurities, additional alloy elements, such as chromium, molybdenum, vanadium, nickel and others, may also be contained, as long as they do not exceed a maximum quantity of 0.100 wt %. The oxygen and boron contents must definitely be adjusted to values of less than 5 ppm. (Oxygen forms oxides, which as particles diminish the magnetic properties. Boron produces extreme brittleness problems and must be avoided wherever possible.)
The method and manner of smelt production, e.g., the type and frequency of secondary metallurgical treatments, is not important as long as the desired alloy constituents can be prepared with reproducible precision. In particular, the secondary metallurgical treatment of the smelt should be such that the addition of calcium to improve pourability is highly limited. This is because calcium causes precipitations which must be avoided in principle for magnetic reasons.
The smelt is cast to form a strand at a maximum superheating temperature of 40 K, ideally less than 20 K, and optimally less than 12 K, in each case referred to the liquidus temperature, which for the steel alloy considered here is very close to 1,493° C. Casting at a temperature just above the liquidus point will result in an advantageously homogeneous solidification structure with a high globulitic primary microstructure ratio. However, with all of the above, production reliability must take priority, in which too great a decrease in the superheating temperature is associated with the risk of premature solidification. The strand is cast without being exposed to an inert gas, with the conventional protection being provided during casting of the strand in a tundish or a mold.
The Liquid Core Reduction (LCR) casting method is also used, i.e., casting is carried out into a metal mold having a thickness of 80-120 mm, for example, after which the strand, which has not yet fully solidified and still has a liquid core, is reduced by adjusting the segments, preferably the first two segments, to a lower thickness range of between 50 and 120 mm, preferably 50-90 mm, in particular, 65-85 mm. In this manner, the more critical conditions which can occur during thin slab continuous casting as compared with the previously customary thick slab continuous casting are mitigated. Furthermore, this method facilitates casting at a lower superheating temperature. The vertical, rectilinear arrangement used during continuous casting over the entire metallurgical length is advantageous for ensuring a high degree of metallurgical cleanliness, since the particles of alumina contained in the smelt raise upward and can reach the slag level. The fully solidified strand is bent to the horizontal position at temperatures above 1,100° C., which has a favorable impact on the homogeneity of the inner microstructure.
The resulting strand is separated into individual thin slabs by cross-cutting, and is through-heated homogeneously in a homogenization furnace to a maximum temperature of 1,250° C., but at least to a temperature that will allow the softened thin slab to be further worked in a reliable process. The time required for through-heating can be between 15 and 60 minutes.
Before the thin slab, which has undergone homogenization annealing in the continuous furnace, is hot rolled, it passes through a high-frequency induction heating device, as in a first step, which is situated immediately upstream of the hot rolling line, as described in U.S. Pat. No. 8,408,035 B2 and in DE 10 2012 224 531 A1. This inductor is ideally designed to be capable of raising the temperature of a thin slab measuring 60-90 mm in thickness, for example, and typically 1,000-1,300 mm in width by 150-300 K as it is being continuously advanced lengthwise into the hot rolling line at a typical infeed rate of less than 1 m/s. The structure of the induction device is designed with respect to its electrical specifications (particularly frequency) such that uniform through-heating (skin depth) up to the core can be achieved.
An induction heating device of this type offers several technical advantages:
For one, this technical option gives the hot working process substantial thermomechanical degrees of freedom and therefore enormous flexibility in designing the hot working/temperature/time process.
For another, it offers the technical option of selecting the homogenization temperature for the thin slabs as advantageously low, for example, around 1,150° C., so that the thin slabs can thereafter be heated individually to any desired initial hot rolling temperature, up to approximately 1,350° C. In addition to the tremendous gain in logistical flexibility in production, this allows a substantial savings of energy in the large homogenization furnace. It is also possible to optimize the technology for the roller hearth of this furnace. For example, at an appropriate, constant equalization temperature that is not overly high, water-cooled furnace rollers can be dispensed with, and in their place, simpler, uncooled rollers can be used. Substantial amounts of energy are saved as a result, since no thermal energy must be discharged to the exterior unused due to the water cooling of the rollers.
Most importantly, however, for grain-oriented electrical steel strip produced by the production process proposed here according to the invention, the very important possibility is created of producing grain-oriented electrical steel strip by means of thin slab continuous casting technology/thin slab casting, without requiring the technically costly nitridation process. This is achieved in that heating of the thin slab to a temperature in the range from 1,250° C. to 1,350° C. in process step d) while the thin slab was inductively heated for several seconds after process step c), immediately prior to the first rolling pass of the hot rolling process, brings the AlN inhibitor phase into chemical solution, so that said phase is precipitated finely dispersed in the subsequent hot rolling process, in the same manner used in the traditional thick slab high temperature heating process. Because this reheating takes place immediately prior to the infeed part of the hot rolling stage, the otherwise known problems in terms of low high temperature strength of the thin slabs do not occur, since the thin slab is already drawn into the roller nip of the first hot rolling stand, and therefore has a firm mechanical coupling thereto.
Immediately after the inductive reheating of the thin slab, the slab is hot rolled in the linear hot rolling stage to a hot strip thickness ranging from 1.80 to 3.0 mm, preferably from 1.8 mm to 2.5 mm. Based on the overall temperature schedule, the initial rolling temperature is generally substantially higher than 1,200° C. This ensures that full recrystallization of the hot worked cast structure will take place after the first, and at the latest after the second hot working pass. The high initial rolling temperature likewise ensures maintenance of a safe final rolling speed at the required high final rolling temperatures of generally >950° C. In the present case, the maximum speed at which the steel strip can be safely transported to reeling is 12 m/s. By selecting the proper final temperature of the thin slab after induction heating, the actual speeds can be reduced to 7.5 m/s, thereby decreasing the risk of roller breaks and increasing yield as a result.
Specifically, care must be taken to ensure the proper steel strip profile, particularly the thickness crown and the thickness taper. This is very important for grain-oriented electrical steel strip, because the finished steel strip will be stacked in the form of a plurality of laminates to form magnetic yokes and machine parts, and therefore, the requirements for precise dimensional accuracy are extremely high. The geometric characteristics with respect to relative thickness crown and relative thickness taper which are established or which result during hot rolling can no longer be corrected during cold rolling and must therefore be properly adjusted in the hot strip during the hot rolling process.
The thickness crown is defined as C40=2·dm−d40l−d40r
and the thickness taper is defined as T40=|d40l−d40r|,
wherein d40l, d40r denotes the strip thicknesses at a distance of 40 mm from the hot strip edges to the left and the right, and dm denotes the strip thickness in the hot strip center position. The definitions refer to the absolute values, divided by the hot strip thickness dm, each resulting in the relative values.
Although a certain thickness crown is frequently desired for the purpose of stable strip guidance over the deflecting rollers of the roller system and also in the roller nip, it should not exceed 2%. For a typical hot-rolled strip thickness of 2.30 mm, this corresponds to an absolute thickness crown of 46 μm. The threshold value for the thickness taper in the same order of magnitude is 1.5%, although a value of zero is most desirable here.
These characteristics can be adjusted by correct strand guidance, especially at the end of the metallurgical length, but particularly also by using hot rolling stands having CVC-ground working rollers and asymmetrical cooling/lubricating devices.
One preferred embodiment of the production process therefore comprises adjusting a hot strip thickness crown of <2%, particularly preferably <1%, and a hot strip thickness taper of <1.5%, particularly preferably <0.7%.
The hot rolled steel strip is subjected to hot rolled strip annealing, which is carried out over a period of 180-300 s, typically 240 s, at temperatures of 950-1,150° C. Particularly important in hot rolled strip annealing is the rapid quenching of the steel strip that has just been annealed, at a cooling rate of >30 K/s, preferably >40 K/s, and particularly >45 K/s, ordinarily by means of water-spray nozzles at high water pressure. With the method described in SU 688 527 A involving two-stage cold rolling and intermediate decarburization annealing, followed immediately by high-temperature annealing in a bell-type furnace, no hot rolled strip annealing is performed. However, it has been found that hot rolled strip annealing in the overall combination presented here, consisting of thin slab casting and additional inductive heating of the thin slabs, is highly advantageous. For one thing, hot rolled strip annealing fulfills the function of microstructure homogenization. However, the regions of the hot rolled strip close to the surface, in which the Goss texture is already present due to the shear deformation during hot working, are made somewhat more coarse, which promotes the process of Goss texture formation as part of the subsequent process steps of cold rolling and annealing. Moreover, the rapid cooling causes a finely dispersed carbide precipitation. In the subsequent cold rolling process, this leads to increased strain-hardening and therefore to energy being introduced into the matrix. Immediately after water quenching, the surface of the hot-rolled strip is freed of the annealing scale by customary descaling and pickling techniques.
Once the hot-rolled strip has been annealed, cold rolling is performed, in which, in a first cold rolling stage, the hot strip is rolled in a first step to an intermediate thickness ranging from 0.50 to 0.80 mm, preferably around 0.65 mm. In this step it is irrelevant whether the cold rolling is performed in a multiple stand tandem train or in a reversive cold rolling mill.
Before the cold-rolled strip is then cold rolled in a second cold rolling stage to the final usable thickness, it is subjected to intermediate recrystallization annealing. This intermediate annealing step offers the last possibility for decarburizing the strip before the subsequent high-temperature annealing in a bell-type furnace, in which secondary recrystallization will take place. During the intermediate recrystallization annealing, the cold rolled steel strip having an intermediate thickness of 0.50 mm to 0.80 mm, preferably around 0.65 mm, is fully decarburized, or is at least decarburized to a residual carbon content of less than 30 ppm. Decarburization is carried out at the same time as recrystallization. The temperature during this annealing step ranges from 820 to 890° C., ideally from 840 to 850° C., at which temperature the strip surface gas reaction is most effective. The annealing time in this case is 300 to 600 seconds, usually 400 seconds. A moist annealing atmosphere containing hydrogen, nitrogen and water vapor is used for the decarburizing strip surface gas reaction. The constituents thereof can be varied within broad limits, as long as the oxidation potential remains properly adjusted. This is the case if the water vapor/hydrogen partial pressure ratio pH2O/pH2 ranges from 0.30 to 0.60, preferably from 0.35 to 0.46.
Optionally, during the intermediate decarburization annealing, a nitriding effect can be achieved by adding ammonia gas (NH3) to the annealing atmosphere, which intensifies the AlN inhibitor phase. In this case, various methods may be used, in which the ammonia gas is either admixed into the furnace atmosphere or is blown directly via nozzle pen stocks onto the strip surface. In such a nitriding annealing phase or annealing treatment, a quantity of ammonia (NH3) corresponding to approximately 5% of the gas flow volume, which otherwise consists entirely of N2+H2, is blown directly onto the two strip surfaces. It is important in this connection that the total nitrogen content of the steel strip is thereby (substantially) detectably increased as compared with the nitrogen content of the cast smelt. Increases in the total nitrogen content beyond 20% generate process-stabilizing effects in terms of the homogeneity of the magnetic properties of the finished steel strip.
At the primary recrystalization, decarburization, and nitridation annealing, neither texture ratios nor oxygen content are controlled. Rather, the related values are determined automatically by control of respective system and process. There is not any control and/or regulating system for controlling the texture ratios or the oxygen content.
The steel strip that has been recrystallized and decarburized as described above is then cold rolled to its final thickness or its nominal usable thickness by means of a further cold rolling process in a second cold rolling stage. The finished steel strip nominal thicknesses that are customary for grain-oriented electrical steel strip are 0.35 mm, 0.30 mm, 0.27 mm, 0.23 mm and 0.18 mm. In this further cold rolling process, it also is irrelevant whether this second cold rolling stage is carried out on a multiple-stand tandem train or on a reversive cold rolling mill.
The steel strip that has therefore been cold rolled to its usable thickness is then coated with a non-stick layer (annealing separator), before it can preferably be further processed, wound to form a coil, in high-temperature annealing in a bell-type furnace. The non-stick coating is applied to the steel strip as an aqueous slurry of MgO powder in demineralized water. Here, it is important to minimize the crystal water pick-up in the MgO, for which purpose measures such as minimizing the contact time between the MgO and the water, cooling the entire MgO slurry and coating system and the coated cold-rolled strip itself to 4° C., and rapidly drying the coating offer possible options.
The formation of the Goss texture by the process of secondary grain growth is carried out by means of traditional high-temperature annealing in a bell-type furnace. The coils, which have been coated with a non-stick layer, are placed on highly heat-resistant steel plates through which the annealing gas is directed, and are encompassed by protective hoods. The heating hoods are then placed over these, and are either fired with gas or electrically heated. Once the entire annealing assembly has been flushed with dry nitrogen gas at the start of each annealing pass, a rapid heating to 400° C. is performed, followed by slow heating at approximately 15-20 K/h, up to a holding temperature of 1,190-1,210° C. In this process, an intermediate holding stage lasting 5 to 10 hours can advantageously be introduced, at a temperature ranging from 600-700° C., particularly at 650° C., which serves to compensate for temperature gradients of heavy and thermally sluggish coils. This is recommended especially for particularly high coil weights. During this slow heating phase, the protective hoods are supplied with a mixture of dry nitrogen and hydrogen. Dry annealing gas is particularly important in this case, because any water vapor fractions will disrupt the sensitive process of texture formation. However, a certain increase in humidity is unavoidable beyond a temperature of 400° C., as a result of evaporating of hydration water from the MgO non-stick layer. That is why it is so important to minimize the crystal water pick-up by the above-described measures.
With respect to the composition of the annealing gas during the heating phase, an annealing atmosphere having a strongly predominant nitrogen proportion of up to 90 wt % N2 is used. Such an excess of nitrogen allows the period of action of the AlN inhibitor phase to be extended somewhat, because the decomposition of AlN and the removal of the released nitrogen are delayed somewhat.
Once the holding temperature is reached, the gas infeed is switched to 100% hydrogen, and is maintained for at least 20 hours at 1,190-1,210° C. To optimize the holding time and the holding temperature, the total purification of sulfur and nitrogen must be ensured, and the formation of edge defects on the stand edge of the coils (bottom buckles) must be minimized.
When this high-temperature holding time has expired, the resulting finished steel strip is cooled to the ambient temperature. During this process, gassing with 100% hydrogen is initially maintained, in order to avoid any nitrogen pick-up. However, as soon as the temperature in the coils drops below approximately 600° C., the annealing atmosphere is switched to 100% dry nitrogen. As soon as the temperature drops below 400° C., the heating hoods can be lifted, and when it drops below approximately 100° C. the protective hoods can also be taken off.
Following the high-temperature annealing in a bell-type furnace, the secondary recrystallized finished steel strip is mechanically cleaned of excess residual MgO (using water and rotating brushes), then advantageously pickled in a bath with phosphoric acid, and immediately thereafter and directly downstream, said strip is fed to a continuous annealing line, where it is stress-relief annealed. As is known in practice, the moist, coated steel strip is usually suspended in a long loop in the intake region of a continuous annealing line. In this furnace region, the steel strip is heated with high heating power, in which process the insulating coating is also fully set and dried. Only then is the steel strip permitted to touch the first furnace transport roller. The annealing atmosphere that is used is non-critical, as is the heating speed, however, the maximum temperature that is reached must be between 840 and 880° C., and is ideally 860° C., in order to remove any mechanical stresses and to produce a steel strip that is evenly flattened. If the temperature drops too far below this level, the desired effect will not be sufficiently produced. If it is too far above this level, the insulating coating can sustain damage. However, it is particularly important for the cooling in which the steel strip is brought back to the ambient temperature to be as homogeneous as possible. This is usually achieved by using ventilators over a relatively long cooling pass.
In the outlet region of this last annealing line in the overall process of producing grain-oriented electrical steel strip, the finished product is ready, and can be evaluated and certified on the basis of quality-relevant criteria.
The grain-oriented electrical steel strip that has been processed to form the final product can also be optionally subjected to a subsequent magnetic domain refinement, which can reduce the magnetization losses by an additional 12-20%. Such a device for domain refinement can be installed in the outlet part of the final insulation/stress-relief annealing system or can optionally be performed offline.
The procedure for one embodiment example is as follows:
A steel smelt having a chemical composition of
3.230 wt % Si
0.029 wt % C
0.215 wt % Mn
0.485 wt % Cu
0.011-0.025 wt % S
0.018 wt % Al (acid soluble)
0.011 wt % N
0.087 wt % Cr
0.001 wt % Ti
0.085 wt % Ni
and small, unavoidable quantities of impurities is cast in a metal mold to a casting thickness of 110 mm using the thin slab continuous casting technique, and is formed by the “liquid core reduction” method (with thickness reduction according to the “liquid core reduction” method, the strand thickness is reduced just below the metal mold while the interior of the strand has a liquid core. Also possible is the so-called “soft reduction” method, in which a selective thickness reduction of the cast strand is (first) performed at the solidification point close to final solidification), without exposure of the strand to inert gas to a strand, having a thickness of 65 mm to 85 mm (the latter thickness being achieved without liquid core reduction) and a width of 1,100 mm to 1,250 mm.
Following a subsequent controlled cooling, the strand that is produced reaches a temperature behind the end of its metallurgical length of 1,190° C., at which the strand is bent from vertical to horizontal and is then divided crosswise into individual slabs. Thus the slabs are produced by means of the so-called thin slab technology. The slabs are then subjected to 20 minutes of homogenization annealing at 1,150° C.
This is followed by hot rolling treatment in the form of a hot rolling process in a rolling train, wherein the slabs are guided through an electrically powered continuous induction heating device, immediately prior to the first hot rolling pass, and are brought by said device to a temperature of 1,350° C. at least for a short time of several seconds, preferably under continuous heating. The (thin) slabs then pass through a high-pressure descaling device.
The first hot working pass is carried out approximately 10 seconds after leaving the induction heating device, at a temperature of approximately 1,280° C.
In the embodiment example, the thin slab is hot rolled to a hot strip in a hot-rolling train comprising 6 stands, wherein after leaving the last stand, each slab has a thickness of 2.30 mm.
Upon completion of hot-rolling treatment, the hot-rolled strip that is produced from the thin slab has a final rolling temperature of 930° C. It then passes, after approximately 5 s, through a laminar cooling path, before being wound at a reeling temperature of approximately 580° C. to a reel to form a coil.
The hot-rolled strip produced/generated in this manner will later be fed to a cold rolling process.
The cold rolling process begins with trimming the rough hot strip edges of the hot-rolled strip, after which it is fed to a continuous annealing process, by means of which it is annealed over a period of 220 seconds under non-oxidizing conditions (gas atmosphere with 95 wt % dry N2 and 5 wt % H2) at a maximum temperature of between 920° C. and 1,150° C., in particular at a temperature of 950° C. or 1,050° C. or 1,120° C.
Immediately after the annealing process, within 5 seconds of leaving the furnace that was used for the annealing process, the annealed hot-rolled strip is subjected to a high-pressure water quenching and a cooling rate ranging from 28 K/s to 52 K/s, in particular, defined cooling rates of 52 K/s or 45 K/s or 38 K/s or 28 K/s.
The hot-rolled strip that has been annealed and quenched in this manner is then pickled, wherein the surface scales are broken up and are dissolved by pickling chemicals.
The hot-rolled strip which is now prepared for cold rolling is then fed to a cold rolling process, in which it is brought in a first cold rolling stage to a first thickness reduction by cold rolling to an intermediate thickness of 0.65 mm.
The cold-rolled strip that has been rolled to this intermediate thickness is then subjected to recrystallization and simultaneously decarburization annealing treatment in the form of a continuous strip annealing process. In this process, the following process conditions are established: Heating is carried out at an average heating rate of 25 K/s to a holding temperature of 850° C., wherein the annealing atmosphere consists of a moist mixture of 60% N2 and 40% H2 at a saturation temperature of 54° C., so that a water vapor/hydrogen partial pressure ratio of pH2O/pH2=0.44 is established. The annealing treatment lasts 400 seconds, after which the cold-rolled strip is cooled to the ambient temperature.
At the primary recrystalization, decarburization, and nitridation annealing, neither texture ratios nor oxygen content are controlled. Rather, the related values are determined automatically by control of respective system and process. There is not any control and/or regulating system for controlling the texture ratios or the oxygen content.
After this, in a second cold rolling stage of the cold rolling process, cold rolling involving multiple passes is performed, to the desired nominal usable thickness, which preferably ranges from 0.23 mm-0.30 mm.
A coating of an annealing separator (non-stick layer) consisting of MgO with additives of 5 wt % TiO2, 0.5 wt % Na2B4O7 and 0.05 wt % MgCl2 (quantities referred to the quantity of MgO) is then applied to the annealed cold-rolled strip which has been treated and prepared in this manner. Both the steel strip and the annealing separator in the form of an aqueous anti-adhesion slurry are cooled to 4° C. prior to coating. Immediately after the coating with the annealing separator (non-stick layer), the two opposing large-area steel strip surfaces (surface areas) are dried using intensive infrared radiation. The cold-rolled strip is then wound onto a reel to form a coil, tilted to a position in which the coil axis is vertical, and transferred to the next step, i.e. high temperature annealing in a bell-type furnace.
To obtain a grain-oriented electrical steel strip having a Goss texture for use in transformers, which is characterized by a particularly sharp {110}<001> texture (Miller indices) and which has a easy magnetization direction parallel to the rolling direction, a Goss texture is then formed in a secondary recrystallization process, for which purpose the coils are annealed in a high-temperature, bell-type annealing furnace, in which a heating rate of 20 K/h is established. The heating phase is interrupted by a holding stage at 650° C., during which the temperature is maintained for a period of 5 hours for the purpose of temperature equalization. Heating is then continued as before, until a temperature of 1,200° C. is reached. Throughout this time, a dry gas consisting of 75 wt % N2 and 25 wt % H2 flows through the annealing hood. The temperature of 1,200° C. represents the holding temperature at which, when reached, the gas atmosphere prevailing in the annealing hood is switched to 100% dry hydrogen. The coils are annealed for 24 hours at this high-temperature holding stage of 1,200° C. This is followed by gradual cooling to the ambient temperature, in which, when the temperature drops below 600° C., the gas atmosphere in the annealing hood is switched to 100% N2. Following this secondary recrystallization annealing, the grain-oriented electrical steel strip with Goss texture is finished.
Once the finished steel strip obtained in this manner has cooled to the ambient temperature, it is washed, pickled in phosphoric acid, coated with a liquid phosphating agent and finally continuously stress-relief annealed at a maximum temperature of 860° C. and then uniformly cooled.
The grain-oriented electrical steel strip produced in this manner has very good magnetic characteristics. The magnetization losses at 50 Hz and 1.7 T modulation for such a steel strip having a finished steel strip nominal thickness/usable nominal thickness of 0.30 mm is 1.08 W/kg with a polarization of 1.88 T at a field strength of 800 A/m.
