Patent Description:
Imparting magnetic properties to Fe-Si steel is the most economical source of magnetic induction. From a chemical composition standpoint, adding silicon to iron is a very common way to increase electrical resistivity, hence improving magnetic properties, and reducing at the same time the total power losses. Two families presently co-exist for the construction of steels for electrical equipment: grain-oriented and non grain-oriented steels.

Non grain-oriented steels have the advantage of possessing magnetic properties that are nearly equivalent in all the magnetizing directions. As a consequence, such material is more adapted for applications that require rotative movements such as motors or generators for instance.

The following properties are used to evaluate the efficiency of electrical steels when it comes to magnetic properties:.

Many metallurgical parameters may influence the above mentioned properties, the most common ones being: the alloying content, material texture, the ferritic grain size, precipitates size and distribution, and the material thickness. Henceforth, the thermomechanical processing from the cast to the final cold rolled steel annealing is essential to reach the targeted specifications.

<CIT> discloses a method for producing an electromagnetic steel sheet which comprises <NUM>% or less of C, <NUM>-<NUM>% of Si, <NUM>-<NUM>% of Al, <NUM>-<NUM>% of Mn, and <NUM>-<NUM>% of Sn, wherein Si+Al+Sn ≤ <NUM>%. Such steel is subjected to hot rolling, and then primary cold rolling with a rolling rate of <NUM>-<NUM>% to produce a steel sheet with a middle thickness. Then, the steel sheet is subjected to process annealing, then secondary cold rolling with a rolling rate of <NUM>-<NUM>%, and further final annealing at <NUM> or more for <NUM>-<NUM> seconds. Such method is rather energy consuming and involves a long production route.

<CIT> relates to a non grain-oriented electromagnetic steel sheet having a chemical composition comprising, by mass%, <NUM>% or less C, <NUM> to <NUM>% Si, <NUM>% or less Mn, <NUM> to <NUM>% Al, <NUM> to <NUM>% Sn, and the balance Fe with unavoidable impurities. The non grain-oriented electromagnetic steel sheet with a thickness of <NUM> to <NUM> is manufactured by the steps of: cold-rolling the hot-rolled plate before and after an intermediate annealing step and subsequently recrystallization-annealing the sheet. Such processing route is as for the first application detrimental to productivity since it involves a long production route.

<CIT> discloses an example of a method of production of an annealed cold-rolled non grain-oriented Fe-Si steel sheet.

It appears that a need remains for a production method of such FeSi steels that would be simplified and more robust while not comprising on power loss and induction properties.

The steel according to the invention follows a simplified production route to reach good compromises of power loss and induction. Furthermore, tool wear is limited with the steel according to the invention.

The present invention aims at providing a method of production of annealed cold-rolled non grain-oriented Fe-Si steel sheet consisting of the successive following steps:.

In a preferred embodiment, the method of production of non grain-oriented Fe-Si steel sheet according to the invention has a silicon content such that: <NUM> ≤ Si ≤ <NUM>, even more preferably, <NUM> ≤ Si ≤ <NUM>.

In a preferred embodiment, the method of production of non grain-oriented Fe-Si steel sheet according to the invention has an aluminum content such that: <NUM> ≤ Al ≤ <NUM>, even more preferably, <NUM> ≤ Al ≤ <NUM>.

In a preferred embodiment, the method of production of non grain-oriented Fe-Si steel sheet according to the invention has a manganese content such that: <NUM> ≤ Mn ≤ <NUM>.

Preferably, the method of production of non grain-oriented Fe-Si steel sheet according to the invention has a tin content such that: <NUM> ≤ Sn ≤ <NUM>, even more preferably, <NUM> ≤ Sn ≤ <NUM>.

In another preferred embodiment, the method of production of non grain-oriented Fe-Si steel sheet according to the invention involves an optional hot band annealing done using a continuous annealing line.

In another preferred embodiment, the method of production of non grain-oriented Fe-Si steel sheet according to the invention involves an optional hot band annealing done using a batch annealing.

In a preferred embodiment, the soaking temperature is between <NUM> and <NUM>.

In another embodiment, the non grain-oriented cold rolled annealed steel sheet according to the invention is coated.

Another object is the non grain-oriented steel obtained using the method of the invention.

High efficiency industry motors, generators for electricity production, motors for electrical vehicles using the non grain-oriented steel produced according to the invention are also an object as well as motors for hybrid vehicle using the nor grain-oriented steel produced according to the invention.

In order to reach the desired properties, the steel according to the invention includes the following chemical composition elements in weight percent:
Carbon in an amount limited to <NUM> included. This element can be harmful because it can provoke steel ageing and/or precipitation which would deteriorate the magnetic properties. The concentration should therefore be limited to below <NUM> ppm (<NUM> wt%).

Si minimum content is <NUM>% while its maximum is limited to <NUM>%, both limits included. Si plays a major role in increasing the resistivity of the steel and thus reducing the Eddy current losses. Below <NUM> wt% of Si, loss levels for low loss grades are hard to achieve. Above <NUM> wt% Si, the steel becomes fragile and subsequent industrial processing becomes difficult. Consequently, Si content is such that: <NUM> wt% ≤ Si ≤ <NUM> wt%, in a preferred embodiment, <NUM> wt% ≤ Si ≤ <NUM> wt%, even more preferably, <NUM> wt% ≤ Si ≤ <NUM> wt%.

Aluminium content shall be between <NUM> and <NUM> %, both included. This element acts in a similar way to that of silicon in terms of resistivity effect. Below <NUM> wt% of Al, there is no real effect on resistivity or losses. Above <NUM> wt% Al, the steel becomes fragile and subsequent industrial processing becomes difficult. Consequently, Al is such that: <NUM> wt% ≤ Al ≤ <NUM> wt%, in a preferred embodiment, <NUM> wt% ≤ Al ≤ <NUM> wt%, even more preferably, <NUM> wt% ≤ Al ≤ <NUM> wt%.

Manganese content shall be between <NUM> and <NUM> %, both included. This element acts in a similar way to that of Si or Al for resistivity: it increases resistivity and thus lowers Eddy current losses. Also, Mn helps harden the steel and can be useful for grades that require higher mechanical properties. Below <NUM> wt% Mn, there is not a real effect on resistivity, losses or on mechanical properties. Above <NUM> wt% Mn, sulphides such as MnS will form and can be detrimental to core losses. Consequently, Mn is such that <NUM> wt% ≤ Mn ≤ <NUM> wt%, in a preferred embodiment, <NUM> wt% ≤ Mn ≤ <NUM> wt%,.

Just as carbon, nitrogen can be harmful because it can result in AIN or TiN precipitation which can deteriorate the magnetic properties. Free nitrogen can also cause ageing which would deteriorate the magnetic properties. The concentration of nitrogen should therefore be limited to <NUM> ppm (<NUM> wt%).

Tin is an essential element of the steel of this invention. Its content must be between <NUM> and <NUM>%, both limits included. It plays a beneficial role on magnetic properties, especially through texture improvement. It helps reduce the (<NUM>) component in the final texture and by doing so it helps improve magnetic properties in general and polarization/induction in particular. Below <NUM> wt% of tin, the effect is negligible and above <NUM> wt%, steel brittleness will become an issue. Consequently, tin is such that: <NUM> wt% ≤ Sn ≤ <NUM> wt%, in a preferred embodiment, <NUM> wt% ≤ Sn ≤ <NUM> wt%.

Sulphur concentration needs to be limited to <NUM> wt% because S might form precipitates such as MnS or TiS that would deteriorate magnetic properties.

Phosphorous content must be below <NUM> wt%. P increases resistivity which reduces losses and also might improve texture and magnetic properties due to the fact that is a segregating element that might play a role on recrystallization and texture. It can also increase mechanical properties. If the concentration is above <NUM> wt%, industrial processing will be difficult due to increasing fragility of the steel. Consequently, P is such that P ≤ <NUM> wt% but in a preferred embodiment, to limit segregation issues, P ≤ <NUM> wt%.

Titanium is a precipitate forming element that may form precipitates such as: TiN, TiS, Ti<NUM>C<NUM>S<NUM>, Ti(C,N), and TiC that are harmful to the magnetic properties. Its concentration should be below <NUM> wt%.

The balance is iron and unavoidable impurities such as the ones listed here below with their maximum contents allowed in the steel according to the invention: <MAT> <MAT> <MAT> <MAT> <MAT> <MAT>.

Other possible impurities are: As, Pb, Se, Zr, Ca, O, Co, Sb, and Zn, that may be present at traces level.

The cast with the chemical composition according to the invention is afterwards reheated, the Slab Reheating Temperature (SRT) lying between <NUM> and <NUM> until the temperature is homogeneous through the whole slab. Below <NUM>, rolling becomes difficult and forces on the mill will be too high. Above <NUM>, high silicon grades become very soft and might show some sagging and thus become difficult to handle.

Hot rolling finishing temperature plays a role on the final hot rolled microstructure and takes place between <NUM> and <NUM>. When the Finishing Rolling Temperature (FRT) is below <NUM>, recrystallization is limited and the microstructure is highly deformed. Above <NUM> would mean more impurities in solid solution and possible consequent precipitation and deterioration of magnetic properties as well.

The Coiling Temperature (CT) of the hot rolled band also plays a role on the final hot rolled product; it takes place between <NUM> and <NUM>. Coiling at temperatures below <NUM> would not allow sufficient recovery to take place while this metallurgical step is necessary for magnetic properties. Above <NUM>, a thick oxide layer would appear and it will cause difficulties for subsequent processing steps such as cold rolling and/or pickling.

The hot rolled steel band presents a surface layer with Goss texture having orientation component as {<NUM>}<<NUM>>, the said Goss texture being measured at <NUM>% thickness of the hot rolled steel band. Goss texture provides the band with enhanced magnetic flux density thereby decreasing the core loss which is well evident from Table <NUM>, <NUM> and <NUM> provided hereinafter. The nucleation of Goss texture is promoted during hot rolling by keeping the finishing rolling temperature above <NUM> degree Celsius.

The thickness of the hot strip band varies from <NUM> to <NUM>. It is difficult to get a thickness below <NUM> by the usual hot rolling mills. Cold rolling from more than <NUM> thick band down to the targeted cold rolled thickness would strongly reduce productivity after the coiling step and that would also deteriorate the final magnetic properties.

The optional Hot Band Annealing (HBA) can be performed at temperatures between <NUM> and <NUM>, this step is optional. It can be a continuous annealing or a batch annealing. Below a soaking temperature of <NUM>, recrystallization will not be complete and the improvement of final magnetic properties will be limited. Above a soaking temperature <NUM>, recrystallized grains will become too large and the metal will become brittle and difficult to handle during the subsequent industrial steps. The duration of the soaking will depend on whether it is continuous annealing (between <NUM> and <NUM>) or batch annealing (between <NUM> and <NUM>). Afterwards, the band (annealed or not) is cold rolled. In this invention, cold rolling is done in one step i. e without intermediate annealing.

Pickling can be done before or after the annealing step.

Finally, the cold rolled steel undergoes a final annealing at a temperature (FAT) lying between between <NUM> and <NUM>, preferably between <NUM> and <NUM>, for a time between <NUM> and <NUM> depending on the temperature used and on the targeted grain size. Below <NUM>, recrystallization will not be complete and losses will not reach their full potential. Above <NUM>, grain size will be too high and induction will deteriorate. As for the soaking time, below <NUM> seconds, not enough time is given for recrystallization whereas above <NUM> the grain size will be too big and will negatively affect the final magnetic properties such as the induction level.

The Final Sheet Thickness (FST) is between <NUM> and <NUM>.

The microstructure of the final sheet produced according to this invention contains ferrite with grain size between <NUM> and <NUM>. Below <NUM>, the losses will be too high while above <NUM>, the induction level will be too low.

As for mechanical properties, the yield strength will be between <NUM> MPa and <NUM> MPa, while ultimate tensile strength shall be between <NUM> MPa and <NUM> MPa.

The following examples are for the purposes of illustration and are not meant to be construed to limit the scope of the disclosure herein:.

Two laboratory heats were produced with the compositions given in the table <NUM> below. The underlined values are not according to the invention. Then, successively: hot rolling was done after reheating the slabs at <NUM>. The finished rolling temperature was <NUM> and the steels were coiled at <NUM>. The hot bands were batch annealed at <NUM> during <NUM>. The steels were cold rolled down to <NUM>. No intermediate annealing took place. The final annealing was done at a soaking temperature of <NUM> and the soaking time was <NUM>.

Magnetic measurements were done on both of these heats. Total magnetic losses at <NUM>. 5T and <NUM> as well as the induction B5000 were measured and the results are shown in the table below. It can be seen that Sn addition results in a significant improvement of magnetic properties using this processing route.

Two heats were produced with the compositions given in the table <NUM> below. The underlined values are not according to the invention. Hot rolling was done after reheating the slabs at <NUM>. The finishing rolling temperature was <NUM>, coiling temperature was <NUM>. The hot bands were batch annealed at <NUM> during <NUM>. Then cold rolling took place down to <NUM>. no intermediate annealing took place. The final annealing was done at a soaking temperature of <NUM> and the soaking time was <NUM>.

Two heats were produced with the compositions given in the table <NUM> below. The underlined values are not according to the invention. Then, successively: hot rolling was done after reheating the slabs at <NUM>. The finished rolling temperature was <NUM> and the steels were coiled at <NUM>. The hot bands were batch annealed at <NUM> during <NUM>. The steels were cold rolled down to <NUM>. No intermediate annealing took place. The final annealing was done at a soaking temperature of <NUM> and the soaking time was <NUM>.

Magnetic measurements were done on both of these heats. Total magnetic losses at <NUM>. 5T and <NUM>, at 1T and <NUM> as well as the induction B5000 were measured and the results are shown in the table below. It can be seen that <NUM> wt% Sn addition results in an improvement of magnetic properties using this processing route.

As can be seen, from both of these examples, Sn improves magnetic properties using the metallurgical route according to the invention with different chemical compositions.

Claim 1:
Method of production of an annealed cold-rolled non grain-oriented Fe-Si steel sheet consisting of the successive following steps:
- melting a steel composition that contains in weight percentage: <MAT> <MAT> <MAT><MAT> <MAT> <MAT> <MAT> <MAT> <MAT> the balance being Fe and inevitable impurities
- casting said melt into a slab
- reheating said slab at a temperature between <NUM> and <NUM>
- hot rolling said slab with a hot rolling finishing temperature between <NUM> and <NUM> to obtain a hot rolled steel band,
- coiling said hot rolled steel band at a temperature between <NUM> and <NUM>,
- said hot rolled steel band being annealed at a temperature between <NUM> and <NUM> for a time between <NUM> and <NUM> hours
- cold rolling the hot rolled steel band to obtain a cold rolled steel sheet
- heating the cold rolled steel sheet up to a soaking temperature between <NUM> and <NUM>
- holding the cold rolled steel at the soaking temperature for a time between <NUM> and <NUM>
- cooling the cold rolled steel down to room.