WO1998048062A1

WO1998048062A1 - New process for the production of high-permeability electrical steel from thin slabs

Info

Publication number: WO1998048062A1
Application number: PCT/EP1997/002090
Authority: WO
Inventors: Stefano Fortunati; Giuseppe Abbruzzese; Stefano Cicale'; Romeo Capotosti; Giancarlo Di Schino
Original assignee: Acciai Speciali Terni S.P.A.
Priority date: 1997-04-24
Filing date: 1997-04-24
Publication date: 1998-10-29
Also published as: AU2701197A

Abstract

According to present invention, the classic composition of high-characteristics silicon steels is improved, utilizing contents of carbon and aluminum shifted towards the lower limits thereof up to now accepted and maintaining anyway low the total inhibitors content, thus allowing to lower the slabs heating temperature, to attenuate the effect of lumping the inhibitors in coarse precipitates, to obtain a good glass-film and to utilize the thin-slab continuous casting technique.

Description

NEW PROCESS FOR THE PRODUCTION OF HIGH-PERMEABILITY ELECTRICAL STEEL FROM THIN SLABS Field of the invention

The present invention refers to a new process for the production of high-permeability electrical steel from thin strips and, more precisely, refers to a process utilizing specific steel composition in combination relationship with specific thin slab continuous casting parameters, to eliminate some critical aspects of the processes known in the state of the art, simplifying the production cycle and some plants.

Background of the invention

Before describing the state of the art referring to this kind of products, it seems appropriate to remind relevant scientific and technical basis. Silicon steel consists of a plurality of separate contiguous grains (or crystals), each having a body-centered cubic lattice, in which the axes corresponding to the cube corners, crystallographically designed with <100>, are directions of easest magnetization. The magnetic characteristics of such steels are defined by their maximum value of magnetic permeability and by the energy to be utilized to reach a given permeability; in order to obtain good values for those quantities it is necessary to maintain within giving limits number and dimensions of grains which moreover, must be all oriented in a similar way, with a minimum of disorientation degree between relevant <001> axis.

Only by keeping said general conditions it is possible to obtain a material having good magnetization characteristics, i.e magnetic permeability, expressed as magnetic flux density induced in the nucleus by a magnetic field of given value, and energy loss in operation, usually referred to as core losses at given frequency and permeability and expressed in W/kg

When the steel is heated over specific temperatures , a gram-growth process starts in which the larger crystals and/or those energetically or kmetically more "charged" grow at the expenses of the other ones. However, the production process of silicon steel strips comprises a number of high-temperature treatments, during some of which a gra growth could start which, should it occur with wrong modalities and timing, will prevent to reach the wanted final results. The secondary recrystallization is controlled by some compounds, such as manganese sulfide, manganese selenide, aluminum nitride and the like, which, duly precipitated within the steel, inhibit the gram growth up to a temperature at which are solubilized, thus permitting the secondary recrystallization to start.

The extremely complex process conditions above mentioned become more critic in the production of high-magnetic-characteπstics strips, i.e. of a product whose permeability and core losses are respectively higher than 1.9 T and lower than 1.11 W/kg. Such characteristics are connected to a good orientation of grains which must have their <001> axis oriented parallel to the surface and rolling direction of the strip, with a disorientation of less than 4 degrees, while conventional gram oriented strips have a disorientation level of 6-7 degrees . Only with high inhibitors contents it is possible to utilize very high coil-dolling rates (of more than 80 % ) which in turn allow a grain growth with the orientation necessary to obtain the desired superior magnetic characteristics, for such results, however, it is necessary to maintain an active inhibition for a duration and temperatures higher than those required for lower qualitative levels.

The above temperatures, however, are higher than the solubilization temperature of the common gram growth inhibitors (such as manganese or copper sulfides, manganese selenides and the like); it was hence necessary to found new inhibitors having a higher solubilization temperature and the best one was found to be the aluminum nitride.

According to other researchers, it is necessary to have, during the cold rolling and in addition to aluminum nitride, also a specific steel microstructure comprising specific contents of martensite, obtainable with a two-step annealing cycle of the hot rolled strip. However, also the aluminum nitride forms coarse precipitates during the steel solidification and subsequent cooling; the slabs must be then annealed at high temperature before hot rolling, to solubilize aluminum nitride and allow it to reprecipitate , along with the sulfides, in the desired dimensions. As the technological aspect is concerned, modern production of gram oriented silicon steel strips requires preparing a molten steel of controlled composition, with particular reference to the content of silicon, carbon, oxygen, manganese, sulfur, aluminum, nitrogen, and continuously casting it in slabs having a thickness usually comprised between 15 and 25 cm, a width of around a metre and a length of some metres . Such slabs are translated at a temperature of no less than 300 °C, and then reheated at high temperature, and hot rolled; the strip, if necessary annealed, is cold rolled to the final thickness, usually comprised between O.lδ and 0.35 mm. and then subjected to a number of high-temperature final treatments, intended to drastically reduce the content of carbon (decarburization annealing) , as well as of other elements harmful for the final product, to obtain the desired magnetic properties (secondary recrystallization annealing), to form on the strip surface insulating inorganic coatings, for instance magnesium phosphate and silica based. Each of the above steps is fundamental for the reaching of final characteristics of the product, and thus it must be carefully planned and controlled.

For instance, the continuous casting requires a quick initial cooling of the molten steel in the mould, to allow a quick extraction of the slab comprising a solid skin still containing a quantity of liquid steel, which will more slowly solidify later. Already from such initial conditions some consequencies ensue requiring opportune careful control. In fact, the metal undergoes two radically different cooling rates, a first very quick at the surface and then a second more slow at the core, thus solidifying in two different structures depending on solidification times, at the surface in small equiaxic crystals and at the core in elongated much larger crystals, called columnar. This starting difference of grain dimensions, if not amended brings, during the subsequent process steps, to a non omogeneous structure in the final product, and to a lesser quality.

Moreover, the relatively slow cooling rate of the slabs brings to the segregation of some elements as well as to the coagulation of some compounds, in particular of the grain growth inhibitors and more specifically of aluminum nitride, in large lumps not uniformly dispersed in the steel bulk and therefore unable to inhibit the gram growth. Thus, since the very beginning of the production process, it is necessary to accurately control a number of variables in order to avoid an excessive dimension and orientation difference of grains , and to obtain a sufficiently fine and homogeneous distribution of inhibitors. To obtain the above, the slabs are heated at a temperature higher than 1330 °C, typically above 1380 °C, to dissolve the compounds precipitated during the slab cooling as large lumps, and to allow them to be more homogeneously diffused within the metal. Such a high heating temperature have a number of inconveniences , among which very important are temperature differences found between surface and core of the slabs and the high overheating of the slab surface, necessary to let the core assume the desired temperature within an acceptable period of time, which factors induce an unwanted grain growth, as well as the formation of liquid slag on the slab surface, which calls for special extremely costly furnaces. During the hot rolling process, the metal undergoes a thickness reduction at such temperature and reduction rates to obtain acceptable gram dimensions and to precipitate in fine particles the aluminum nitride. To avoid an abnormal gram growth during the heating a pre-roll g is usually utilized, consisting m a first hot rolling pass carried out before the maximum heating temperature is reached; this obviously calls for higher costs, mainly due to the fact that slabs have to be extracted from the furnace, rolled and then put again in the furnace. In addition to, or instead of, such treatment, it is possible to utilize high initial carbon contents, which however are more crytical in terms of rolling defectosity and require longer and more costly decarbuπzation treatments.

It also interesting to remind that the higher carbon and aluminum contents on one hand are useful to obtain high quality products but on the other hand entail some inconveniences; for instance, to eliminate the carbon it s necessary to utilize high dew-point atmospheres, favouring the iron oxidation instead of the silicon oxidation; moreover, during the last process steps, aluminum can reduce silica to form alumina .

This is an unfavourable fact, in that during the last annealing there is no sufficient silica to form through a reaction with magnesium oxide, coated on the strip as annealing separator, the glass-film, necessary both as electric insulator between the stacked bands in a transformer and a bonding layer for other coatings utilized to improve some magnetic characteristics. It is easy to understand now how complex and costly is the production of a good gram oriented silicon steel strip having superior magnetic characteristics, and hence how important is to utilize in the more efficient way any possible technique to reduce production costs. Thus it is highly desirable to simplify the production process of this k nd of steel through the elimination, or at least the simplification, of some critical steps.

As already mentioned, in the common casting techniques the steel solidification conditions have a great, and generally unfavourable, importance in defining the crystalline structure of the steel and the precipitation of the annealing separators.

For different reasons and for different steels, in particular for carbon steels, new methods of low-thickness continuous casting have been devised, in particular the strip casting, in which the cast product has a thickness of less than 15 mm, and the thin slab casting, n which the cast product has a thickness of some tens of millimetres, tipically 40-100 mm. The experience gained with common steels, points out that the thin slab continuous casting cannot solve the problem of different grain dimensions at the surface and in the core; hence, the common behaviour s that such technology is not advantageous in the production of electrical steels with high magnetic characteristics, in that it keeps the complexities of the classic production processes of said steels and adds the need of a new kind of casting machine, whose management moreover is not simple.

The interest of the majority of the steel producers is hence mainly directed to the strip continuous casting, m which practically no problems due to different solidification structures are encountered. However, also this technology did not have a full industrial utilization; one of the reasons for this could be the unsatisfactory surface quality of the strip, the initial roughness being not elimmable w th the low reduction rates necessary to pass from some mm to some tens of mm. Coming back to the traditional casting technique of slabs 200-250 mm thick, a concept was introduced to shift the mam crytical points as far as possible towards the last phases of the production process; thus it was proposed, for instance, to considerably lower the slabs heating temperature before hot rolling, to avoid the dissolution of significant quantities of aluminum nitride; the finely precipitated and uniformly diffused aluminum nitride precipitates, necessary to control the grain growth, are obtained just before the secondary recrystallization annealing by an addition if nitrogen to the steel through a solid phase nitπdmg process.

Other tentatives to simplify the process consist in eliminating the prerollmg step and in reducing the slab heating temperature before hot rolling, this last step being particularly costly, essentially due to the high temperature to be reached, the long treating time, the large dimensions of the slabs to be treated and the necessity to utilize specific furnaces, as already mentioned. Concerning this last aspect, since the dissolution temperature of manganese sulfide in the steel is a function of a number of factors, among which the content of oxygen (and then of the internal oxidation level of the steel) , manganese and sulfur, by careful controlling such elements it s possible to reduce by many tens of degrees the slab heating temperature. Another crytical aspect is the final annealings, in order to eliminate some elements such as carbon (continuous annealing) and sulfur (box annealing) , said elements having an adverse influence on the quality. The efforts up to now put into practice did permit to obtain good results, not so good however to eliminate some important process complexities. In the specific case of high magnetic characteristics silicon steel strips, some problems still remain unsolved, for instance linked to the casting of high thickness slabs. Summing up, in spite of important industrial and research efforts, the production of high characteristics electrical silicon steel strip still remains a complex and costly process, comprising many crytical points to be carefully controlled to avoid quality losses an ensuing heavy downgrading. State of the art lip to now, we have no information referring to the thin-slab continuous casting of high characteristics gram oriented electrical steel strips. with reference to the above outlined complexity of the process, also due to the fact that a number of questions are not yet completely understood, many solutions have been proposed, referring to different problems . The classic production process of high characteristics silicon steels requires, as described in the Italian patents 860.631, 860.946 and 982.690, to continuously cast the steel in slabs to be heated at high temperature, typically above 1300 °C, in some cases in two steps with an interposed rolling by 30-70 % before reaching the maximum heating temperature, to hot roll the slabs, to anneal the strip thus obtained at a temperature of between 750 and 1200 °C and to controlledly cool it within 2-200 s down to 400 °C and th cold roll the strip with a reduction rate between 50 and 95 % •

Italian patent 808.108 refers to the production of an adherent glass- film by mixing, in the annealing separator, to the magnesium oxide also other oxides, such as manganese, zinc, chromium oxides.

The Italian patent 8 0.926 refers to the same problem, but mixes titanium and manganese oxides to magnesium oxide. The European patent application EP-33 47 -A discloses the casting of a slab containing, in wt % , 0.025-0.075 C, 2.5-4.5 Si, <0.012 S, 0.01- 0.06 Al_s, < 0.01 N. 0.08-0.45 Mn, 0.015-0.045 P, remaining being essentially iron, said slab being heated at a temperature of no more than 1200 °C and hot rolled; the strip is then cold rolled in one or more passes with intermediate annealing. Follow a decarbuπzation annealing and a continuous nitrid g process, the coating with an annealing separator and the final high temperature annealing. The nitridmg time s between 15 and 60 s at a temperature of 500 to 900 °C, m a nitrogen-hydrogen atmosphere containing at least 50 % in volume of hydrogen and from 0.01 to 10 % of NHo , to introduce into the steel a nitrogen amount of at least 100 ppm.

Similar concepts are contained in the Japanese patent applications J63 093824-A, J06 145803 and J06 184638-A. Other patents and patent applications refer to the nitriding conditions .

Though doubtely interesting, this nitridmg technology has some problems, which complicate an already complicated process, for instance the necessity to utilize very low heating temperatures before the hot rolling, to ensure the inhibitors absence up to the nitrid g operations; however, it can be possible for such low temperature to be not compatible with a good hot rolling procedure. Description of the invention Present invention is directed to the simplification of the high magnetic characteristics silicon steel production process, by identification of the composition and process conditions permitting to utilize the thin slab continuous casting process and to attenuate some important disadvantages of the known processes

In particular, it was found that it is possible to lower the annealing temperature of the slabs, well below the liquid slag formation temperature, to attenuate the coarsening effect of grain growth inhibitors precipitates, to eliminate quality problems due to faulty formation of glass film, and to maintain a high quality of the final product by utilizing a carbon and aluminum content shifted towards the lower limits of the known classic compositions and by keeping low the total inhibitors content. According to present invention, a steel is prepared comprising, n wt % . C 0.023-0.080, Si 2.5-4.5, Mn 0.040-0.100, S 0.008-0.023, P < 0.20, Sn < 0.20, Al_s 0.010-0.025, N 0.0040-0.0070, Cu 0.100-0.250, remaining being iron and minor impurities. This steel, having an overheating, or temperature above its liquidus temperature, of between 20 and 40 °C, is continuously cast m thin slabs having a thickness of 4θ-6θ mm, with a casting speed of 3 to 5 m/min and a solidification time of 30 to 100 s. The casting mould is an oscillating one, having a maximum oscillation excursion (also non- smusoidal) comprised between 1 and 100 mm and an oscillation frequency comprised between 200 and 400 cycles per minute.

The slabs are then brought to a temperature comprised between ll8θ and 1320 °C, preferably between 1220 and 1290 °C, hot rolled keeping, within 30 °C, a temperature of between 1100 and 1200 °C at the entrance in the finishing stand, while at the exit from the finishing stand the strip temperature is comprised between 900 and 1000 °C. The thickness of the hot rolled strip is comprised between 2 and 3 mm; the strip undergoes a forced cooling starting 4-12 s after the strip leaves the finishing stand and is then coiled at a temperature of 680 °C or less, preferably comprised between 600 and 65O °C. The strip s then heat-treated according the following cycle: heating at 1000-1150 °C, cooling to 85O-95O °C, keeping this temperature for 30-90 s, cooling in non-oxidmg atmosphere to 600-800 °C, and quenching in boiling water.

All the remaining of production process is conventional; however, it is preferable to have a single-pass cold rolling, i.e. without intermediate annealings, down to the final thickness, utilizing m the last pass a reduction rate of between 60 and 90 % ■ Moreover, m the decarbuπzation annealing the effective treatment can be kept within 1 0 s for a strip thickness of 0.30 mm.

The present invention will be now more precisely described with reference to some embodiements , to be considered exclusively as examples not limiting the scope of the invention itself. EXAMPLE 1

A steel was prepared having the following wt % composition: S 3-10; C 0.040; Mn 0.050; Cu 0.170; S 0.0140; P 0.070; Sn O.O98O; Al_s 0.0140; N 0.0050; remaining being iron and minor impurities. Part of this steel was cast in a continuous casting mould having a thickness of 50 mm, with an overheating of 25 °C, a casting speed of 4.8 m/mm and a solidification time of around 50 s. The mould was an oscillating one, with a frequency of 250 cycles per mmute and an oscillation excursion of 5 mm. The remaining part of the steel was cast in a traditional continuous casting machine with a thickness of 240 mm, at a speed of 0.5 m/mm, solidification time 1200 s, overheating 30 °C, oscillation excursion and frequency respectively of 5 ^m and 8θ cycles per minute. The slabs were heated at 1230 °C and hot rolled to 2,1 mm. The maximum coiling temperature was 640 °C. The strip was annealed at 1135 °C, cooled at 900 °C, this temperature being maintained for 60 s, and then quenched in boiling water.

The strip, after being sanded and pickled, was cold rolled to 0.30, 0.27 and 0.23 mm.

The cold rolled strips were decarbuπzed for 130, 115 and 100 s, respectively, in wet hydrogen-nitrogen atmosphere, coated with magnesium oxide, box-annealed with a heating rate of 1 °C/s in a nitrogen-hydrogen 25~75 atmosphere, held at 1200 °C, cooled; the strip then received a tensioning coating and was thermically flattened. The magnetic characteristics obtained (permeability, BδOO, in T; core losses at 50 Hz and 1,7 T , PI.7, m W/kg), are reported in the following Table 1:

TABLE 1 Thin slab Traditional slab

Thickness , mm PI.7 BδOO PI.7 B800

0.23 0.90 1-935 1.85 1.601

0.27 1.01 1-930 2.00 1.598

0.30 1.09 1.928 2.50 1.570

EXAMPLE 2

A steel was prepared, having the following wt% composition:

Si 3.30; C 0,045; Mn 0.040; Cu 0.150; S 0.016; P 0.075; Sn 0.090; Al_g

0.015; N 0.045; remaining being iron and minor impurities. The steel was continuously cast in thin slabs, according to Example 1, and then hot rolled at 3 mm with a slab heating temperature of 1290 °C. The hot rolled strip was heat-treated according to Example 1, then cold rolled according to the following Table 2 and then treated according to Example 1, obtaining the following magnetic characteristics (without tensioning coating) :

TABLE 2

1st rolling pass 2nd rolling pass PI.7 BδOO thickness reduction thickness reduction W/kg T mm % mm %

1 .0 66.7 0. 30 70.0 1 .45 1 .820

1 . 5 46.7 0. 30 δ2.0 1 . 30 1 .900

2 .0 33. 3 0. 30 δ5.0 1 . 10 1 .918

2.5 16.7 0.30 8δ.O 1.01 1.930

2.7 10.7 0.30 88.9 1.12 1.914

3.0 0 0.30 90.0 2.50 1.600

EXAMPLE 3

Some of the thin slabs of Example 1 were heated at 1150 °C and then treated according to Example 1, to obtain a finished product. The obtained results are shown in Table 3:

TABLE 3

Thickness , mm PI.7 /W/kg) B800 (T)

0.23 1.95 1.570

0.27 2.02 1.600

0.30 2.45 1.589 EXAMPLE 4

Two steels were prepared having the following wt % composition:

Si C Mn Cu S Sn Al_g N

Steel #1 3.20 0.043 0.045 0.17 0.015 0.10 0.015 0.0043 Steel #2 3.18 0.079 0.075 0.07 0.020 0.07 0.029 0.00δ3

remaining being iron and minor ompurities.

Steel 1 was cast in thin slabd 0 mm thick, and then transformed in cold rolled strip 0.30 mm thick, according to the present invention, Steel 2 was conrinuously cast in slabs 24θ mm thick, and then transformed in cold rolled strip 0,30 mm thick, according to the traditional process for this kind of steel.

Cold rolled strips of both steels were continuously annealed according the following cycles : CYCLE A: Treatment at δ30 °C for 210 s in N₂-H₂ wet atmosphere with PH2O/PH2 = O.56 and subsequent treatment at 860 °C for 30 s in N2-H2 wet atmosphere with PH2/H2 < 0.01;

CYCLE B: Treatment at 83O °C for 240 s in N₂-H₂ wet atmosphere with pH₂0/pH₂ = O.56; CYCLE C: Treatment at δ 0 °C for 150 s in N₂-H₂ wet atmosphere with pH₂0/pH₂ = 0.50.

The strips were then coated with MgO based annealing separator, box- annealed at 1210 °C and then thermally flattened, and coated with a tensioning/insulating layer according to the common practice for this kind of products. The final strips were then evaluated and confronted as per C content, surface quality and magnetic characteristics; the results are shown in Table 4: TABLE 4

1+A 1+B 1+C 2+A 2+B 2+C

C cont . , PPm 12 10 21 20 18 150

Glass film look good good good good some some discontinuities

Coating adherence good good good good scarce scarce

Coating tension, Kg/mm' 0.82 0.85 0.82 0.50 0.54

BδOO, T Tesla 1.93 1.92 1.93 1.93 1.89 1.78

PI.7 (50 Hz) W/Kg 0.93 O.96 0.95 0.97 1.06 ι.4θ

As it is apparent , only by following the teachings of present invention very good results can be obtained.

Claims

1 1 . A process for the production of high-permeability electrical

2 s teel from continuously cas t thin s l abs , charac terized by the

3 combination in cooperation relationship of the following steps :

4 1 ) preparing a molten steel having the following weight % composition :

5 C 0.023-0.080 , Si 2.5-4 .5 , Mn 0.040-0. 100 , S O . OOδ-0.023 , P < 0.20 , Sn

6 < 0.20, Al_s 0.010-0.025, N 0.0040-0.0070, Cu 0.100-0.250, remaining

7 being iron and minor impurities. δ 11) continuously casting this steel in a mould, having a thickness of

9 40-60 mm, with an overheating of between 20 and 40 °C;

10 111) bringing the thus obtained slabs to a temperature comprised

11 between ll8θ and 1320 °C and then hot rolling the same keeping, within

12 30 °C, a temperature of between 1100 and 1200 °C at the entrance in

13 the finishing stand, while at the exit from the finishing stand the

1 strip temperature is comprised between 900 and 1000 °C;

15 iv) forcibly cooling the strip starting 4-12 s after it leaves the

16 finishing stand and is then coiling the same at a temperature of 680

17 °C or less; lδ v) heat-treating the strip according the following cycle: heating at

19 1000-1150 °C, cooling to δ50-950 °C, keeping this temperature for 30-

20 90 s , cooling in non-oxidmg atmosphere to 6θ0-δ00 °C, and quenching

21 in boiling water;

22 vi ) cold rolling the strip without intermediate annealings, down to

23 the final thickness, utilizing in the last pass a reduction rate of

24 between 60 and 90 % .

1 2. A process according to claim 1, characterized in that the casting 2 speed is comprised between 3 and 5 m/mm and the solidification time is comprised between 30 and 100 s. 3- A process according to any one of the preceding claims, characterized in that the casting mould is an oscillating one, having a maximum oscillation excursion (also non-sinusoidal) comprised between 1 and 100 mm and an oscillation frequency comprised between 200 and 400 cycles per minute. 4. A process according to any one of the preceding claims, characterized in that the slabs are heated at a temperature of between 1220 and 1290 °C. 5 - A process according to any one of the preceding claims , characterized in that the strip temperature at the exi t from the finishing slab is comprised between 600 and 650 °C .