A process for the production of a grain-oriented electrical steel

Abstract

 The invention relates to a process for the production of grain oriented electrical steel by providing a hot rolled strip having a carbon content of less than 0.006 wt.%, subjecting said strip to cold rolling, intermediate annealing, cold rolling to final thickness and thereafter subjecting the cold rolled strip to a batch annealing and a final continuous annealing in order to produce a final product having a maximum core loss at 1.7 T and 50 Hz (P_17/50) of < 2 W/kg and/or a minimum magnetic flux density at 800 A/m (J₈₀₀) of > 1.70 T.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for the production of grain oriented electrical steel, particularly grain oriented steel sheets to be used for cores in transformers and other electrical machines.

BACKGROUND OF THE INVENTION

[0002] Ever since the very beginning of the commercial use of alternating-current transformers and other alternating-current machinery, hysteresis and eddy current losses in the iron cores have been a problem. Very early it was found, however, that a low content of carbon and other impurities in the steels that were employed as a core material promoted a high permeability and a low coercive field intensity, i.e. reduced hysteresis losses. Further, it was recognized that a high content of silicon and possibly also aluminum in the steel could reduce the eddy current losses.

[0003] A major technical achievement in this field was the invention of the cold rolled grain-oriented electrical steel by Norman P. Goss in the early thirties. The cold rolled grain-oriented electrical steel, often abbreviated CRGO, or called Goss- or GO-steel, is processed through a series of hot rolling, heat treatment, and cold rolling operations in a way aiming at achieving optimum properties in the rolling direction through control of the orientation of the crystals relative to the steel sheet. During the many years that have passed after the disclosure of Goss's invention, numerous modifications of the process have been suggested. Apart from the use of different combinations of inhibitors, it is also known to perform a decarburization of the cold rolled strip at final gauge through an annealing in wet atmosphere containing H₂ (introduced by Carpenter with US 2287467; see example of applications in EP 1 577 405 and EP 869 190), and to perform a low heating rate final batch annealing at high soaking temperature to generate Goss ({110}<001>) grains through secondary recrystallization (EP 0789093).

[0004] During decarburization annealing the carbon is oxidized at strip surface through the reaction with water contained in the annealing atmosphere and the strip is progressively decarburized by diffusion of carbon through the sheet thickness. Such decarburization requires several minutes to be completed and constitutes a relevant cost in the production process for the production of GO electrical steel.

[0005] The "low heating rate" "high soaking temperature" batch annealing during which secondary recrystallization is performed, constitutes a high time-consumption and high cost in the known CRGO production process.

[0006] Even if some improvements may have been achieved in terms of magnetic properties of the steel sheets, they have as a rule been gained at the cost of more complicated and hence less economic manufacturing facilities.

BRIEF DISCLOSURE OF THE INVENTION

[0007] It is the purpose of the present invention to provide a process for the manufacturing of grain oriented electrical steel sheets, i.e. a steel sheet with crystal grains predominantly in the so called Goss-orientation, which highly corresponds to the rolling direction of the steel sheet, but without provision of complicated manufacturing facilities or modes of operation which significantly raise the total manufacturing costs. Sheets in this context are defined as any flattened steel products that are thinner than a plate, including also strips regardless if the products are slit from wider sheets or not.

[0008] Further, it is an objective of the invention that the final product in which the steel sheets manufactured according to the process of the invention are intended to be employed as core material has a maximum core loss at 1.7 T and 50 Hz (P_17/50) of < 2 W/kg and/or a magnetic polarization at 800 A/m (J₈₀₀) of > 1.7 T. The thickness of such sheets is typically in the range of 0.23 - 0.35 mm.

[0009] These and other objectives can be achieved by a process for the manufacturing of a grain oriented electrical steel sheet comprising the steps of:
a) providing a hot rolled strip comprising in weight %:

C	< 0.006	preferably	< 0.003
Si	3.0 - 3.5	preferably	3.1-3.3
Mn	0.4 - 2.0	preferably	0.45 - 0.65
Als	0.005-0.03	preferably	0.01-0.02
N	0.004 - 0.009	preferably	0.005 -0.008
S	< 0.008	preferably	< 0.005
Ti	<0.006	preferably	< 0.004

optionally one or more of

Cu	0.05 - 0.3
Sn	0.04 - 0.15
Ni	< 0.1	preferably	< 0.05
Cr	< 0.2	preferably	< 0.05
P	< 0.02	preferably	< 0.01
B	< 0.01	preferably	0.001 - 0.005
Te	< 0.01	preferably	< 0.005
Cd	< 0.01	preferably	< 0.005
Zn	< 0.01	preferably	< 0.005
As	< 0.01	preferably	< 0.005
Pb	< 0.01	preferably	< 0.005
Bi	< 0.002	preferably	0.0005 - 0.002

balance Fe apart from impurities, wherein Als is acid soluble aluminium,
b) optionally annealing and/or pickling the hot rolled strip,
c) cold rolling the hot rolled strip to an intermediate thickness,
d) annealing the cold rolled strip in order to recrystallize the cold rolled strip,
e) cold rolling the recrystallized strip to a final thickness,
f) batch annealing the cold rolled strip having said final thickness, preferably by heating the strip at a rate of ≤ 200°C/h to a holding temperature within the range of 860 - 950 °C, and holding the strip at such a temperature for 2 - 20 h,
g) continuously annealing said batch annealed strip at a temperature of 800 - 1200 °C, preferably 1050 - 1150 °C, for a time of 30 - 600 s.

[0010] The new and efficient process results in a final product typically having a maximum core loss at 1.7 T at a sheet thickness of 0.30 mm and 50 Hz (P_17/50) of < 2 W/kg and a magnetic polarization at 800 A/m (J₈₀₀) of > 1.7 T.

[0011] Key features of the inventive process includes the provision of a hot rolled strip having a carefully balance composition, in particular a very low carbon content. A decarburization annealing need not be performed during the transformation process of the hot rolled strip down to finished product.

[0012] A further key feature is that the final annealing during which the secondary recrystallization happens is divided in two steps:

• the first step, performed in batch annealing, at a relatively low temperature (860°C-950°C) if compared to the high temperature at which the batch annealing for secondary recrystallization is performed in the state of art.
• the second step performed in continuous annealing at a temperature included in the range 800°C-1200°C, preferably 1050-1150 °C.

[0013] This two step annealing allows the possibility of finalizing the recrystallization annealing in the continuous annealing line if not already completed during the batch annealing.

[0014] The reason why it is advantageous to divide the final annealing in two steps is not yet completely clarified, but the inventors have hypothesized that, differently from what happens in the state of art cycle, the secondary recrystallization is onset during the batch annealing and may be completed in the continuous annealing line, if not already completed during batch annealing.

[0015] The invention is defined in the claims.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The drawing schematically illustrates a process line for the manufacturing of a grain oriented electrical steel sheet according to the invention.

DISCLOSURE OF THE INVENTION

[0017] The steel composition of the hot rolled band is defined in claim 1. The carbon level is closely reflected in the carbon level of the secondary recrystallized strip since the process dispenses with any deliberate intermediate decarburization annealing. For this reason it is of the utmost importance to control the carbon content during steelmaking such that the carbon content is less than 30 ppm (0.003 wt. %) in the melt to be cast. The inhibition to control the secondary recrystallization is mainly based on the precipitation of AlN. Accordingly, the content of Al and N should be controlled such that AlN is dissolved during slab reheating, precipitation is minimized or avoided during hot rolling but occur during the annealing of the strip performed at intermediate thickness. For best result the contents of Als (acid soluble Al) and N are controlled such that the ratio Als/N is stoichiometric. However, from a practical point of view ± 35 % of the stoichiometric ratio or preferably within ± 15 % of the stoichiometric ratio can be tolerated.

[0018] The sulfur content should be maintained at a low level in order to avoid undue precipitation of MnS. The content should be less than 30 ppm, preferably less than 20 ppm and most preferred less than 10 ppm. The manganese content is maintained at 0.4 - 2 % in order to increase the resistivity of the alloy and thereby decreasing the core loss.

[0019] The content of titanium should also be closely controlled since Ti is a strong nitride former. Ti enters the steel melt from the raw materials used in iron- and steelmaking. Accordingly, the ladle slag is normally to be skimmed after tapping and FeSi having a low Ti-content should be used. The content of Ti should be less than 0.006 % , preferably less than 0.004% or even less than 0.0020% (20 ppm).

[0020] In addition to AlN other inhibitors may be used to assist the control. Other possible elements that may be present are defined in claim 1. It may be noted that in some cases these elements are present as impurities. Cu and Sn may be added, typical in an amount of 0.1 %. As, Pb, P and Zn may be added as defined in claim 1. However, preferably, the total amount of these elements is less than 0.2%, in particular less than 0.05%. B, Ni, Cr, Te and Cd may be present as defined in claim 1. However, in most cases it is preferred that the total amount of these elements is restricted to 0.30%. Bismuth, when used, need to be present in an amount of at least 5 ppm in order to provide an effect. However, if the content is higher than 20 ppm brittleness problems may occur.

[0021] In the drawing, reference numeral I represents a section for the provision of a steel alloy having the adequate chemical composition prepared for cold rolling, and of a hot rolled strip of the steel alloy according to a) and b) in the foregoing while reference numeral II represents a section for cold rolling of the hot rolled steel strip and for heat treatment of the strip in connection therewith according to c), d) and e) in the foregoing.

[0022] In section I, molten steel is manufactured in a mode, which principles may be conventional per se, by means a complex of iron and steel manufacturing facilities which also may be conventional such as a number of the following ones: blast furnace, LD converter, electric arc furnace, VOD, RH degasser and others. In the drawing, any chosen combination of apparatuses for making molten steel is symbolically represented by complex 1. For the achievement of a steel having the following composition (in weight - %) adapted for electrical steel sheet production according to an aspect of the invention:

C	0.002
Si	3.2
Mn	0.5
S	< 0.008
Als	0.01 - 0.02
N	0.006
Cu	0.1
Sn	0.1

wherein the contents of Als (acid soluble aluminium) and N are such that the ratio Als/N is within ± 15 % of the stoichiometric ratio, balance iron and impurities,

the following route may be followed

• Iron production in blast furnace and hot metal desulphurization. The sulphur content may be 40 - 60 ppm in the raw iron before the LD converter treatment.
• Treatment in LD converter to get aimed carbon content.
• Tapping into a ladle and performing ladle skimming.
• Ladle Furnace (LF) treatment to reheat steel before vacuum decarburization.
• Recirculation Degassing (RH Process), if necessary, in order to reduce the carbon content to the desired level and adjustment of the steel analysis.
• The molten steel is then sent to a ladle furnace 2, where the steel composition is controlled and if necessary adjusted to comply with what is stated above and/or with the composition defined in the appending patent claims. For the manufacturing of a strip of steel from the molten steel with said proper composition, a number of methods can be contemplated, including any of the steps of ingot casting, continuous slab casting, thin slab casting or strip casting. Conveniently, however, molten steel with the proper composition is transferred to a conventional slab caster 3 from the ladle furnace 2 for continuous slab casting. In this connection, the steel has to be adequately protected from contact with air to avoid N₂ pick-up and oxidation of aluminum.

[0023] The continuously cast strand 4 is successively cut to slabs 5 which are reheated in a walking beam furnace 6 to a temperature of between 1220 and 1300 °C, suitably to a temperature of about 1260 °C. On the transfer table, at furnace exit, a series of induction heating devices locally reheat the slabs eliminating any skid mark effect. Successively, each reheated slab 5 is subjected to roughing at > 1200 °C. in a roughing facility 7 in order to produce a plate or bar having a thickness of 20 - 80 mm. Next the plate or bar is hot rolled in a hot rolling mill 8 to form a strip 9 with a thickness of 1.5 - 4 mm, preferably 2 - 3 mm or 2 - 2.5 mm. The starting temperature of the hot rolling in the hot rolling mill 8 is > 920 °C, preferably 950 - 1200 °C, and most preferable 970 - 1150 °C, while the finish rolling temperature is > 850 °C. Before coiling, the hot rolled steel strip is cooled at a rate of 5 - 100 C/s down to a coiling temperature < 600 °C. and coiled at that temperature.

[0024] Optionally, before cold rolling, the hot rolled strip is passed through a scale breaker (not shown), pickled in the pickling unit 10 and optionally annealed (not shown). When employed, pickling may be performed in sulphuric acid at a concentration of 235-245 g/l, which is regenerated by crystallization and centrifugation. The pickling time can be varied in the range 20 - 60 seconds. After pickling, the strip is trimmed and recoiled.

[0025] Now, in section II, the hot rolled strip is cold rolled in a cold rolling mill 12 to an intermediate thickness of 0.38 - 1.2 mm, preferably to 0.5 - 1.0 mm. Then, the cold rolled strip having said intermediate thickness is subjected to continuous intermediate annealing in a continuous type annealing furnace 13 at a temperature of 850 - 1000 °C, preferably at 880 - 930 °C, such that the cold rolled strip material is recrystallized. The atmosphere in the annealing furnace may be 100 % hydrogen or a mixture of hydrogen and nitrogen. Next, the recrystallized strip is cooled and then cold rolled a second time in cold rolling mill 12, now with a reduction rate of 40 - 70 % to a final thickness of 0.23 - 0.35 mm. During one or both of the cold rolling operations, the temperature of the strip is maintained in the range of 80 - 400 °C, preferably in the range of 100 - 200 °C.

[0026] The cold rolled strip is now prepared to be annealed to provoke secondary recrystallization. The annealing is performed in two steps according to the invention. The final microstructure is obtained by secondary recrystallization annealing, which is incubated at low temperature in the batch annealing furnace and may subsequently be completed by the continuous annealing at high temperature, if not already completely recrystallized after batch annealing. In a first step, the cold rolled and coiled strip 15 having said final thickness is batch annealed in a batch annealing furnace 16 by heating the strip at a rate of less than 200 C/h to a holding temperature of 860 - 950 °C. and holding the strip at that temperature for a period of time of 2 - 20 hours. The batch annealing is preferably performed in an atmosphere of dry hydrogen. In another preferred embodiment an annealing separator, in particular a MgO powder layer is built on the strip surface before entering the batch annealing furnace in order to prevent sticking. In the framework of this preferred embodiment it is useful if the atmosphere of the continuous intermediate annealing performed in a continuous type annealing furnace 13 contains water so that the water partial pressure and hydrogen partial pressure is in the range 0.1-0.7. Due to this presence of water an oxide layer, which is composed of Silica, Fayalite and Iron Oxide, adherent to the strip surface is built on the strip surface. During cold rolling such iron oxide remains adherent to the strip surface, and during box annealing react with MgO powder to form and adherent layer of forsterite, which act as a coating of the strip surface.

[0027] If MgO powder is not used during box annealing, oxidation during the continuous intermediate annealing, performed in a continuous type annealing furnace 13, is detrimental and the presence of water during the annealing has to be avoided; in such a case the water partial pressure and hydrogen partial pressure has to be lower than 0.01.

[0028] Subsequent to batch annealing in furnace 16, the cold rolled and batch annealed strip 17 is continuously annealed at a temperature of 800-1200 °C for 30 - 600 seconds in a continuous annealing furnace 18 in an atmosphere preferably consisting of dry hydrogen or dry hydrogen/nitrogen mixture. If the secondary recrystallization is completed after the batch annealing, then the continuous annealing mainly effects thermo-flattening and the temperature may be in the range of 800-950 °C. On the other hand, if the secondary recrystallization is to be completed during this step, then it is better to perform the continuous annealing in the range of 950-1200 °C, preferably 1050-1150°C.

[0029] Finally, the process, according to the preferred embodiment, further includes coating the continuously annealed sheet 19 with an organic or inorganic coating as an annealing separator, optionally with tensioning properties and curing the sheet for more than 20 seconds at a temperature exceeding 150 °C. in a continuous curing furnace 20 before coiling. Due to the invention as described in this patent specification and claims, the final product will have a maximum core loss at 1.7 T and 50 Hz (P_17/50) of < 2 W/kg and/or a magnetic polarization at 800 A/m (J₈₀₀) of > 1.7 T.

[0030] In a second embodiment of the present invention it has been found that the final annealing temperature may be optimized according to the starting rolling temperature. In particular, it has been found that if the starting finishing rolling temperature is ≤ 1050°C then the final annealing as specified in point g) of claim 1 can be performed with a soaking temperature laying in the lower part of said range. The temperature could be in the range of 800-950 °C, preferably 860-950° because an annealing temperature above 950°C does not improve the final characteristics of the material but it increases the consumption of energy necessary to perform the annealing.

[0031] In a further embodiment of the present invention it has instead found that if the starting finishing rolling temperature is higher than 1050°C then the final annealing temperature specified in point g) of claim 1 should be the upper part of said. The temperature should in this case be in the range of 950°C-1200°C, preferably in the range of 1050°C-1150°C.

[0032] The reason why the final annealing optimal temperature range varies depending on the starting rolling temperature is not completely clear but it would appear that when the starting rolling temperature is below 1050°C then the secondary recrystallization is virtually completed during the low temperature batch annealing. In this case, the final annealing is necessary only for performing thermo-flattening of the steel strip when the batch annealing is performed with the strip wound in a coil.

[0033] In case instead the starting rolling temperature falls above 1050 °C the secondary recrystallization after low temperature batch annealing is not complete and an high temperature final annealing is necessary to complete the secondary recrystallization.

EXAMPLES

Example 1:

[0034] A steel with a chemical composition as reported in Table 1 has been cast to form a slab, slab has been reheated at 1260 °C temperature, and hot rolled.

• Samples after hot rolling have been cold rolled down to 0,70 mm intermediate thickness.
• Samples at intermediate thickness have been annealed at 900 °C for 100 sec.
• Annealed samples have been cold rolled down to final thickness of 0,30 mm.
• Cold rolled samples at final thickness have been subjected to batch annealing with the following cycle:

  Heating from 25°C to 900 °C in 12 h; holding at 900 °C for 10 h; cooling from 900 °C to 25°C in 18 h.

• Batch annealed samples have been annealed at 1100 °C for 180 sec.

Magnetic characteristic measured after final annealing have been reported in Table 2

Table 1: Chemical composition of steel used in the example

EX.	Chemical Analysis
	Fe	Sn	Al sol	C	N	Cu	Si	Mn	S
A	Bal	0.110	0.0150	0. 0033	0.0075	0.12	3.15	0.49	0.0036

Table 2: Final magnetic characteristics

	P17	J800
	[W/kg]	[mT]
A	1,79	1740

Example 2:

[0035] A steel with a chemical composition as reported in Table 3 has been cast, slab have been treated at 1250 °C slab reheating temperature and hot rolled.

• Samples after hot rolling have been cold rolled down to 0,70 mm intermediate thickness.
• Samples at intermediate thickness have been annealed at 900°C for 100 sec.
• Annealed samples have been cold rolled down to final thickness of 0,30 mm.
• Cold rolled samples at final thickness have been subjected to batch annealing with the following cycle:

  Heating from 25°C to 900 °C in12 h; holding at 900 °C for 10 h; cooling from 900 °C to 25 °C in 18 h.

• Batch annealed samples have been annealed at 1100 °C for 60 sec and 180 sec.
  Magnetic characteristic measured after final annealing have been reported in Table 4.

Table 3: Chemical composition of steel used in example 2

EX.	Chemical Analysis
	Fe	Sn	Al sol	C	N	Cu	Si	Mn	S
B	Bal	0.100	0.0149	0.004	0. 0061	0.12	3.14	0.50	0.002

Table 4: Final magnetic characteristics of Example 2 samples

Final annealing time	P17	J800
	[W/kg]	[mT]
60 sec	1,82	1720
180 sec	1,81	1710

Example 3

[0036] A steel with a chemical composition as reported in Table 5 has been cast in 3 different slabs: a, b, c. Cast slabs have been reheated at A:1210 °C, B:1240 °C and C:1260 °C, and hot rolled.

• Samples after hot rolling have been cold rolled down to 0,70 mm intermediate thickness.
• Samples at intermediate thickness have been annealed at 900°C for 100 sec.
• Annealed samples have been cold rolled down to final thickness of 0,30 mm.
• Cold rolled samples at final thickness have been separated in two groups of samples subjected to batch annealing with two different cycles:

  Cycle I: Heating from 25°C to 900°C in 6 h ; holding at 900°C for 10 h;

  cooling from 900°C to 25°C in 18 h

  Cycle II: Heating from 25°C to 880°C in 6 h ; holding at 880°C for 10 h;

  cooling from 880°C to 25°C in 18 h.

• Batch annealed samples have been annealed at 1100°C for 180 sec.
  Magnetic characteristic measured after final annealing have been reported in Table 6.

Table 5: Chemical composition of steel used in example 3

EX.	Chemical Analysis
	Fe	Sn	Alsol	C	N	Cu	Si	Mn	S
C	Bal	0.100	0.0098	0.003	0.0042	0.12	3.14	0.50	0. 0023

Table 6: Final magnetic characteristics

		Batch annealing cycle I		Batch annealing cycle II
	Slab	P17	J800	P17	J800
	RH T	[W/kg]	[mT]	[W/kg]	[mT]
Slab a	1210°C	1,89	1695	1,78	1695
Slab b	1240°C	1,80	1710	1,68	1745
Slab c	1260°C	1,65	1747	1,71	1730

Example 4:

[0037] Steel with chemical composition as reported in Table 7 has been cast, cast slab have been treated at 1270 °C slab reheating temperature, and hot rolled.

• Samples after hot rolling have been cold rolled down to 0,70 mm intermediate thickness.
• Samples at intermediate thickness have been annealed at 900 °C for 100 sec.
• Annealed samples have been cold rolled down to final thickness of 0,30 mm.
• Cold rolled samples at final thickness have been separated in two groups of samples subjected to batch annealing with two different following cycles:

  Cycle III: Heating from 25 °C to 850 °C in 6 h ; holding at 850 °C for 10 h; cooling from 850 °C to 25 °C in 18 h (condition out of the invention) Cycle IV: Heating from 25 °C to 900 °C in 6 h ; holding at 900 °C for 10 h; cooling from 900 °C to 25 °C in 18 h.

• Batch annealed samples have been annealed at 1100 °C for 180 sec.
  Magnetic characteristic measured after final annealing have been reported in Table 8

Table 7: Chemical composition of steel used in example 4

EX.	Chemical Analysis
	Fe	Sn	Al sol	C	N	Cu	Si	Mn	S
D	Bal	0.100	0.012	0.003	0.0048	0.12	3.10	0.50	0.0023

Table 8: Final magnetic characteristics

Batch annealing cycle IV		Batch annealing cycle III (out of the invention limit)
P17	J800	P17	J800
[W/kg]	[mT]	[W/kg]	[mT]
1,82	1720	2,18	1608

Example 5:

[0038] A steel with a chemical composition as reported in Table 9 has been cast in 2 different slabs: a, b. Cast slabs have been reheated at 1240 °C and hot rolled, after roughing, with two different starting finishing rolling temperature:

• slabs a, b, had a starting finishing rolling temperature of 1140°C,
• slabs c, d, had a starting finishing rolling temperature of 980°C.

[0039] Samples after hot rolling have been cold rolled down to 0,70 mm intermediate thickness.

[0040] Samples at intermediate thickness have been annealed at 900°C for 100 sec.

[0041] Annealed samples have been cold rolled down to final thickness of 0,30 mm and have been undergone to batch annealing with following cycle: heating from 25°C to 900°C in 6 h ; holding at 900°C for 10 h; cooling from 900°C to 25°C in 18 h.

[0042] Batch annealed samples have been separated in two groups of samples subjected to final annealing with two different following cycles:

Cycle (i): 1100°C for 180 sec.

Cycle (ii): 850°C for 180 sec.

Magnetic characteristic measured after final annealing have been reported in Table 10.

Table 9: Chemical composition of steel used in example 5

EX.	Chemical Analysis
	Fe	Sn	Al sol	C	N	Cu	Si	Mn	S
E	Bal	0.100	0.0149	0.004	0. 0061	0.12	3.14	0.50	0.002

Table 10: Final magnetic characteristics

			Final annealing cycle (i)		Final annealing cycle (ii)
			1100°C 180 s		850°C 180 s
	Slab RH T [°C]	Start. Fin. Roll. Temp. [°C]	P17 [W/kg]	J800 [mT]	P17 [W/kg]	J800 [mT]
Slab a	1240	1140	1,75	1740	1,80	1720
Slab b	1240	980	1,85	1720	1,60	1720

Claims

1. A process for the production of a grain oriented electrical steel sheet comprising the steps of:

a) providing a hot rolled strip comprising in weight %:

C	< 0.006	preferably	< 0.003
Si	3.0 - 3.5	preferably	3.1-3.3
Mn	0.4 - 2.0	preferably	0.45 - 0.65
Als	0.005-0.03	preferably	0.01-0.02
N	0.004 - 0.009	preferably	0.005 -0.008
S	< 0.008	preferably	< 0.005
Ti	<0.006	preferably	< 0.004

optionally one or more of

Cu	0.05 - 0.3
Sn	0.04 - 0.15
Ni	< 0.1	preferably	< 0.05
Cr	< 0.2	preferably	< 0.05
P	< 0.02	preferably	< 0.01
B	< 0.01	preferably	0.001 - 0.005
Te	< 0.01	preferably	< 0.005
Cd	< 0.01	preferably	< 0.005
Zn	< 0.01	preferably	< 0.005
As	< 0.01	preferably	< 0.005
Pb	< 0.01	preferably	< 0.005
Bi	< 0.002	preferably	0.0005 - 0.002

balance Fe apart from impurities, wherein Als is acid soluble aluminium,

b) optionally annealing and/or pickling the hot rolled strip,

c) cold rolling the hot rolled strip to an intermediate thickness,

d) annealing the cold rolled strip in order to recrystallize the cold rolled strip,

e) cold rolling the recrystallized strip to a final thickness,

f) batch annealing the cold rolled strip having said final thickness, preferably by heating the strip at a rate of ≤ 200°C/h to a holding temperature within the range 860 - 950 °C, and holding the strip at such temperature for 2 - 20 h,

g) continuously annealing said batch annealed strip at a temperature of 800 - 1200 °C, for a time of 30 - 600 s.

2. A process according to claim 1, wherein the intermediate thickness of the strip in step c) is 0.38 - 1.2 mm, preferably 0.5 - 1.0 mm.

3. A process according to claim 1 or 2, wherein the strip in step e) is cold rolled with a reduction rate of 40 - 70% to a final thickness of 0.23 - 0.35 mm.

4. A process according to any of the preceding claims, wherein the annealing temperature in step d) is 850 - 1000 °C, preferably 880 - 930 °C.

5. A process according to any of the preceding claims, wherein the composition of the hot rolled strip fulfils at least one of the following requirements:

C	< 0.003	preferably	< 0.002
Si	3.0 - 3.5	preferably	3.1-3.3
Mn	0.4 - 1.0	preferably	0.45 - 0.65
ls	0.012-0.017	preferably	0.014 - 0.016
N	0.005 - 0.008	preferably	0.006 - 0.007
S	< 0.003	preferably	< 0.002
Ti	< 0.004	preferably	< 0.002

optionally one or more of

Cu	0.08 - 0.12
Sn	0.08 - 0.12
Ni	< 0.04	preferably	< 0.01
Cr	< 0.04	preferably	< 0.01
P	< 0.015	preferably	< 0.01
B	< 0.005	preferably	0.001 - 0.002
Te	< 0.01	preferably	< 0.005
Cd	< 0.01	preferably	< 0.005
Zn	< 0.01	preferably	< 0.005
As	< 0.01	preferably	< 0.005
Pb	< 0.01	preferably	< 0.005
Bi	< 0.002	preferably	0.0005 - 0.002

6. A process according to any of the preceding claims, wherein the temperature of the strip during one or both of the cold rolling steps c) and e) respectively, is maintained in the range of 80 - 400 °C, preferably 100 - 200 °C.

7. A process according to any of the preceding claims, wherein the step of providing the hot rolled strip includes any of the steps of ingot casting, continuous slab casting, thin slab casting or strip casting.

8. A process according to any of the preceding claims, wherein the step a) of providing a hot rolled strip includes one or more of the steps of continuously casting a slab, reheating the slab to a T>1220°C, roughing the slab at > 1100 °C in order to produce a bar having a thickness 20 - 80 mm and hot rolling the bar to produce the hot rolled strip.

9. A process according to claim 8, wherein the starting temperature of the hot rolling is > 920 °C, preferably 950 -1200 °C most preferable 970°C- 1150 °C and wherein the finish rolling temperature is > 850 °C.

10. A process according to claim 9 wherein the starting temperature of the hot rolling is lower than 1050°C and/or the continuously annealing specified in point g) of claim 1 is performed at a temperature in the range 800-950 °C.

11. A process according to claim 9 wherein the starting temperature of the hot rolling is greater than 1050°C and/or the continuously annealing specified in point g) of claim 1 is in the range 950-1200°C and preferably in the range 1050-1150 °C.

12. A process according to any of the claims 8, 9, 10 or 11 wherein the hot rolled steel sheet is cooled at a rate of 5 -100 °C/s down to coiling temperature of < 600 °C and coiled at said temperature.

13. A process according to any of the preceding claims, wherein the hot rolled strip has a thickness of 1.5 - 4 mm, preferably 2 - 3 mm.

14. A process according to any of the preceding claims, wherein the contents of Als and N are controlled such that the ratio Als/N is within ± 35 % of the stoichiometric ratio, preferably within ± 15 % of the stoichiometric ratio.

15. A process according to any of the preceding claims, wherein the process further includes at least one of the steps of: coating the continuously annealed strip obtained in step g) with an organic or inorganic coating as an annealing separator, optionally with tensioning properties and curing at >150 °C for > 20s.

Drawing