EP0905263A1 - Process for manufacturing iron-nickel alloy strip from a continuously cast thin strip - Google Patents

Abstract

Production of iron-nickel alloy strip from a continuously cast slab or strip includes homogenization treatment to obtain a standard segregation ratio for nickel of less than 0.4%. In the production of a strip of iron-nickel alloy, of composition (by wt.) 25-50% Ni, 50-75% Fe, optionally less than 8% one or more alloying elements (especially Co, Cr, Mo, Mn, Si, V, Ta, Ti and/or Al) and balance impurities, from a continuously cast slab or strip, the above homogenization treatment is carried out. Independent claims are also included for: (i) hot rolled strip of less than 10 mm thickness, produced as described above; (ii) cold rolled strip obtained by cold rolling a hot rolled strip produced as described above; (iii) cold rolled strip produced from a continuously cast iron-nickel alloy having the above composition, the Ni content being 35-37% and the coercive field strength (Hc) of the strip being less than 45 A/m after annealing at 750[deg]C for 15 mins.; and (iv) cold rolled strip produced from a continuously cast iron-nickel alloy having the above composition, the Ni content being 32-34%, the Co content being 3.5-6.5% and the coercive field strength (Hc) of the strip being less than 55 A/m after annealing at 750[deg]C for 15 mins.

Description

The present invention relates to the manufacture of an alloy strip of the type iron-nickel obtained from a thin strip obtained by direct casting of a thin strip.

Iron-nickel alloys are well known and used for their magnetic properties or for their expansion properties. Their composition chemical mainly comprises from 25% to 50% by weight of nickel, from 50% to 75% by weight of iron, possibly at least one alloying element taken from the cobalt, chromium and molybdenum, in contents of less than 5%. We know, by for example, alloys with very low expansion coefficients containing about 36% of nickel or about 33% nickel and about 4% cobalt, the rest being essentially iron, possibly some additional elements in small amounts, and impurities. These alloys with low coefficient of expansion are also used for their good magnetic properties, especially under form of cold rolled strip of thickness generally between one tenth and a few tenths of a millimeter. The magnetic properties obtained are characterized in particular by a coercive field less than 55 A / m.

To manufacture these cold strips, the alloy is poured in the form of ingots or slabs thicker than 100 mm, then hot rolling is carried out to obtain a hot strip of thickness less than 5 mm. This hot strip is cold rolled to obtain a cold strip which is annealed recrystallization at a temperature in the region of 750 ° C. This technique presents the disadvantage of requiring significant hot rolling operations.

To limit or even avoid hot rolling, it is possible to cast the alloy continuously directly in the form of a thin strip with a thickness of less than 10 mm. For this, a continuous thin strip casting machine can be used between two rotary cylinders with horizontal axes. However, the inventors have found surprisingly that the cold rolled strips obtained from strips thin continuous castings have a significantly higher coercive field than that cold rolled strips from ingots or slabs.

The object of the present invention is to remedy this drawback by proposing a means for manufacturing cold-rolled strips of alloy of the type iron-nickel obtained from a thin strip obtained by direct strip casting thin, having magnetic properties as good as the properties magnetic strips of the same alloy, made from ingots or slabs.

To this end, the subject of the invention is a method of manufacturing an iron-nickel type alloy strip containing, mainly, from 25% to 50% by weight of nickel and from 50% to 75% by weight of iron. , and, optionally, one or more alloying elements such as, in particular, cobalt, chromium, molybdenum, manganese, silicon, vanadium, tantalum, titanium, aluminum, in contents lower than 8% by weight, the remainder being impurities resulting from the preparation, according to which a thin strip of thickness less than 10 mm is continuously cast, the thin strip is laminated, and a treatment is carried out before or after rolling. '' homogenization consisting in maintaining at a temperature T (in ° C) for a time t (in hours) such that: t ≥ 0.5 x 10 -12 exp (38000 / (T + 273)) so as to obtain a "standard segregation rate" of nickel of less than 0.4%.

Preferably, the sum of the contents of cobalt, chromium, molybdenum, manganese, silicon, vanadium, tantalum, titanium and aluminum is less than or equal to 8%.

Preferably, the standard nickel segregation rate is less than 0.35%.

Homogenization treatment can be carried out on the thin strip directly from continuous casting or after hot rolling of the strip thin, or even after a cold rolling operation.

Preferably, after the homogenization treatment, a cold rolling to the final thickness of the strip, so as to give the band a controlled texture.

When the nickel content is between 35% and 37%, the field coercive Hc is less than 45 A / m after annealing at 750 ° C for 15 minutes.

When the nickel content is between 32% and 34% and the content of cobalt is between 3.5% and 6.5%, the coercive field Hc is less than 55 A / m after annealing at 750 ° C for 15 minutes.

The invention will now be described in more detail but not limitative, and be illustrated by examples.

The inventors have found, in a new and completely surprising manner, that the magnetic properties of iron-nickel alloys were affected by the micro segregation of nickel in interdendritic spaces resulting from the solidification.

To highlight this effect, they defined a "segregation rate standard ”which characterizes the micro segregation of nickel as well on a cold rolled strip, than on a hot strip intended to be rolled cold. The hot strip intended to be cold rolled can be a thin strip obtained directly by continuous thin strip casting and possibly having undergone hot rolling or additional heat treatment. To fix the ideas, and as an indication, the thin strip obtained directly by continuous casting of thin strip has a thickness between 1 mm and 10 mm.

The "standard segregation rate" defined by the inventors is the estimator unbiased standard deviation of the nickel content distribution in thickness a cold rolled strip of thickness less than 0.2 mm, which has been annealed recrystallization at 850 ° C for 15 minutes.

Dans cette formule, (Ni)_moy est la moyenne arithmétique des (Ni)_j, et n le nombre de mesures. To determine this standard segregation rate, take a strip of thickness less than 0.2 mm and measure, by scanning electron microscopy coupled with energy dispersive spectrometry (EDS), the nickel content in one plurality of points distributed in the thickness, every 1 μm, along a line perpendicular to the surface. We thus obtain n values of nickel (Ni) _{j content} , j varying from 1 to n , and we calculate the standard segregation rate, called σ _Ni , by the formula:

In this formula, (Ni) _avg is the average of the (Ni) _j, and n the number of measurements.

With this definition, in the absence of special treatment, in accordance with the prior art, the "standard segregation rate" of nickel is greater than 0.5% for a cold strip from a thin hot strip obtained directly by direct continuous thin strip casting, when it is less than 0.35% for a cold strip from an ingot.

To manufacture a cold rolled strip from continuous strip casting thin, an alloy of the electric arc is produced and by refining in a pocket iron-nickel type as defined above, and for which we aim for example a content 36% nickel and a manganese content preferably between 0.02% and 0.5%, the remainder being iron and impurities resulting from processing. The liquid alloy thus obtained is cast in the form of a thin strip using a continuous thin strip casting machine which has two cylinders arranged horizontally, parallel to each other, so as to form a width slot less than 10 mm, and generally between 1 mm and 5 mm. Both cylinders rotate in opposite directions from each other, about their respective axes, by so as to drive the alloy down by passing it through the slot. Both rollers are cooled by an internal circulation of water, so that the alloy is cooled in contact with the cylinders and leaves their grip in the form of a strip solidified thickness substantially equal to the width of the air gap of the cylinders. The thin strip is then wound up using a winder to obtain a reel which is allowed to cool, in general, naturally.

After casting, the thin strip can optionally undergo rolling with warm, preferably after reheating between 1050 ° C and 1300 ° C.

The hot strip thus obtained is then cold rolled after pickling. to obtain a cold strip having the desired final thickness which can be between 0.1 mm and 0.25 mm. In general, this cold rolling is done in several stages separated by recrystallization annealing at temperatures of around 1000 ° C. For example, a first step achieves a thickness between 0.5 mm and 2 mm, a second step allows to reach a thickness of 0.15 mm to 0.3 mm, and a final step leads to the thickness final. Cold rolling is not only used to obtain the final thickness, but also to give the strip a texture which, preferably, must be of the type "Cubic", and to control the size of the grain which should preferably have an index AFNOR between 8 and 9, approximately.

In addition to the hot and cold rolling operations, the production of the strip is completed by a homogenization treatment consisting in at least one maintenance of a duration t (in hours) at a temperature T (in ° C), such than : t ≥ A exp (38000 / (T + 273)) The coefficient A is greater than or equal to 0.5 x 10 ^-12 , and preferably greater than 1 x 10 ^-12 .

(avec A supérieur à 0,5 x 10^-12, et de préférence supérieur à 1 x 10^-12).It is also possible to carry out several successive homogenization treatments, at temperatures T ₁ , T ₂ ... T _n , for periods of time t ₁ , t ₂ , .... t _n . In this case, the times and temperatures must be such that:

(with A greater than 0.5 x 10 ^-12 , and preferably greater than 1 x 10 ^-12 ).

These conditions correspond, for example, to maintaining 0.5 hours at 1150 ° C or 10 minutes at 1200 ° C for a 3 mm thick strip (obtained directly by continuous thin strip casting).

Although the homogenization treatment is all the more effective and easy to realize that the product on which it is made is thin, when cold rolling end of the cold strip is intended, in particular, to impart a texture and a size of grain, the homogenization treatment must be carried out before this final cold rolling. Otherwise, it would destroy the structure and make the grain.

When the homogenization treatment is finished on the thin strip, the latter or the resulting cold-rolled strip is characterized by a "rate of standard "segregation" of nickel less than 0.4% or even 0.35%. When the homogenization treatment is only completed on the cold rolled strip, only this is characterized by a “standard segregation rate” of nickel less than 0.4% or 0.35%.

When the cold rolled strip according to the invention is made of a low-alloy coefficient of expansion mainly containing 35% to 37% nickel, its Hc coercive field is less than 45 A / m after annealing at 750 ° C for 15 minutes. When the cold rolled strip is made of a low coefficient alloy expansion mainly containing from 32% to 34% of nickel and from 3.5% to 6.5% of cobalt, its coercive field Hc is less than 55 A / m after annealing at 750 ° C for 15 minutes.

By way of example and comparison, there are two direct tapes of bands thin CCBM3 and CCBM2, and two castings in ingots CL1 and CL2 of an alloy containing 36% nickel, 0.3% manganese, the rest being iron and impurities resulting from processing.

With these castings, 0.12 mm cold rolled strips were made thick which was annealed at 750 ° C and 850 ° C.

The ingots from CL1 and CL2 were first hot rolled to obtain hot strips 3 mm thick, which were then rolled to cold to the final thickness. BL1 and BL2 cold bands were thus obtained, data for comparison.

• à titre de comparaison, BM3C et BM2C, qui ont été obtenues directement par laminage à froid,
• conformément à l'invention, BM3I1, BM3I2, BM3I3, BM2I1 et BM2I2 qui, outre le laminage à froid, ont été soumises à des traitements d'homogénéisation.

The thin strips obtained by direct casting of thin strips were 3 mm thick and were used to manufacture several cold-rolled strips:

• for comparison, BM3C and BM2C, which were obtained directly by cold rolling,
• in accordance with the invention, BM3I1, BM3I2, BM3I3, BM2I1 and BM2I2 which, in addition to cold rolling, have been subjected to homogenization treatments.

• BM3I1 : maintien à 1150°C pendant 30 mn,
• BM3I2 : maintien à 1250°C pendant 30 mn,
• BM3I3 : maintien à 1200°C pendant 20 mn précédé d'un laminage à froid avec réduction de 50%,
• BM2I2 : maintien à 1250°C pendant 30 minutes, suivi d'un laminage à chaud avec réduction de 35 %.

The homogenization treatments were:

• BM3I1: holding at 1150 ° C for 30 min,
• BM3I2: holding at 1250 ° C for 30 min,
• BM3I3: holding at 1200 ° C for 20 min preceded by cold rolling with reduction of 50%,
• BM2I2: maintained at 1250 ° C for 30 minutes, followed by hot rolling with reduction of 35%.

On all the cold-rolled strips thus obtained, the "standard segregation rate" of nickel σ _Ni , the coercive field Hc and the magnetic permeability µ _{maxcc were measured} .

The results were as follows:

These results show on the one hand that by directly manufacturing the strip cold from a thin strip continuously poured, without processing homogenization, it is not possible to obtain a coercive field less than 50 A / m after a heat treatment of 15 min at 750 ° C, as obtained without difficulty with strips from ingots. However, when we complete the manufacture of the cold rolled strip from a thin strip continuously cast by a homogenization treatment allowing the segregation of the nickel below 0.4%, a coercive field less than 45 A / m is obtained.

In addition, the strips according to the invention had a coefficient of expansion between 20 ° C and 100 ° C of 0.99x10 ^-6 , a grain size G _AFNOR of 9, a cubic texture {100} <001>, a pole density I / I ₀ = 5, and the texture was isotropic after a heat treatment of recrystallization for 15 min at 750 ° C.

Claims (8)

Method for manufacturing an iron-nickel alloy strip containing, mainly, from 25% to 50% by weight of nickel and from 50% to 75% by weight of iron, and, optionally, one or more elements of alloys such as, in particular, cobalt, chromium, molybdenum, manganese, silicon, vanadium, tantalum, titanium, aluminum, in contents of less than 8% by weight, the rest being impurities resulting production, characterized in that a thin strip of thickness less than 10 mm is continuously cast, the thin strip is laminated, and a homogenization treatment consisting of maintaining at a temperature is carried out before or after rolling T (in ° C) for a time t (in hours) such that: t ≥ 0.5 x 10 -12 exp (38000 / (T + 273)) so as to obtain a "standard segregation rate" of nickel of less than 0.4%. A method according to claim 1 characterized in that the rate of standard nickel segregation is less than 0.35%. Method according to any one of Claims 1 to 2, characterized in that that the homogenization treatment is carried out on the thin strip directly from continuous casting. Method according to any one of Claims 1 to 2, characterized in that that the homogenization treatment is carried out after hot rolling of the thin strip from continuous casting. Method according to any one of Claims 1 to 2, characterized in that that the homogenization treatment is carried out after a rolling operation at cold. Method according to any one of Claims 1 to 5, characterized in that that after the homogenization treatment cold rolling is carried out until the final thickness of the strip, so as to give the strip a controlled texture. Process according to any one of Claims 1 to 6, characterized in that that the chemical composition of the alloy is such that the nickel content is understood between 35% and 37%, and the coercive field Hc is less than 45 A / m after annealing at 750 ° C for 15 minutes. Process according to any one of Claims 1 to 6, characterized in that that the chemical composition of the alloy is such that the nickel content is understood between 32% and 34%, the cobalt content is between 3.5% and 6.5%, and the Hc coercive field is less than 55 A / m after annealing at 750 ° C for 15 minutes.