EP0681031B1 - Process for manufacturing mild steel - Google Patents

Abstract

Mild steel contains (in %) max 1 C; max 1 Si; less than 1 Mn; less than 0.1 P; less than 0.1 S; less than 0.1 N; max 0.25 Al, and is cast into slabs and hot rolled at least partially in the ferritic phase to a thickness of 1-12 mm. The strip is then auto-annealed and/or control annealed to produce the ferritic phase.

Description

The present invention relates to a process for manufacturing mild steel, in particular a mild coercive field mild steel for manufacturing of magnets used in particle accelerators or the like, such as a ring storage, etc.

In general, the accelerators of particles of a certain dimension require magnets of a steel presenting a set of very specific characteristics. So it takes everything first that this steel has mechanical strength sufficient and special magnetic properties, such as a relatively low coercive field with a slightly dispersed mean value, an absence of aging magnetic properties, magnetic permeability ensuring high levels induction as well as a very high level of magnetic loss bottom allowing to work with sheet thicknesses equal to or greater than 1.5 mm.

In addition, the flatness of the parts of the magnet must be such that it ensures a high filling coefficient, preferably greater than 98%. These parts must also have very thick thickness tolerances tight, have no mechanical tension, be practically carbon-free, resist corrosion, can be easily machined and cut and finally be able to come in thicknesses too as high as possible.

We know a process for producing air plates comprising a plate rolling which corresponds in the preamble of claim 1 (see EP-A-0 388 776).

We also know a method of manufacturing a strip thin mild steel, hot rolled and having stamping qualities (see EP-A-0 524 162).

The invention aims to present a method economically justified for the treatment of steel in view of obtaining a mild steel responding as much as possible with the criteria defined above.

To solve this problem, we have developed the following the invention, a method as indicated in the claim 1 below.

C ≤ 0,010 %

Si ≤ 0,30 %

Mn < 1 %

P < 0,030 %

S < 0,020 %

N < 0,010 %

Al < 0,100 %,

les brames formées à partir d'un tel acier étant soumises à un laminage dans la phase ferritique à une température de 200 à 900°C avant le recuit précité.Advantageously, we start from a steel corresponding to the following composition:

C ≤ 0.010%

If ≤ 0.30%

Mn <1%

P <0.030%

S <0.020%

N <0.010%

Al <0.100%,

the slabs formed from such a steel being subjected to rolling in the ferritic phase at a temperature of 200 to 900 ° C before the aforementioned annealing.

Other embodiments of the process according to the invention are indicated in the secondary claims.

Other details and peculiarities of the invention will emerge from the description given below, non-limiting, of some forms of carrying out the process of the invention illustrated by concrete examples of application.

C ≤ 1 %

Si ≤ 1 %

Mn < 1 %

P < 0,100 %

S < 0,100 %

N < 0,100 %

Al ≤ 0,250 %.

A partir de cet acier, on coule des brames, par exemple d'une épaisseur de 200 à 300 mm. A noter qu'une décarburation de l'acier liquide peut avantageusement être réalisée. Cette décarburation peut être favorisée par l'addition à l'acier à l'état fondu d'éléments carburigènes notamment Nb, Ti, V, de préférence à des concentrations qui sont telles que le carbone libre dans les brames coulées soit inférieur à 0,006 %.In general, depending on the parameters of the successive stages of the process of the invention, one starts from a steel corresponding to the following general composition:

C ≤ 1%

If ≤ 1%

Mn <1%

P <0.100%

S <0.100%

N <0.100%

Al ≤ 0.250%.

From this steel, slabs are poured, for example with a thickness of 200 to 300 mm. Note that decarburization of liquid steel can advantageously be carried out. This decarburization can be favored by the addition to the molten steel of carburogenic elements, in particular Nb, Ti, V, preferably at concentrations which are such that the free carbon in the cast slabs is less than 0.006%. .

If necessary, the slabs thus obtained are heated in order to increase the malleability steel and thus facilitate rolling ulterior. This reheating is carried out for example at a temperature from 900 to 1250 ° C.

Then, after roughing these slabs on a conventional train, we then proceed to a rolling in the ferritic phase.

So, to ensure that at least the end of this rolling will actually take place in this phase ferritic, this will advantageously take place at a temperature below 900 ° C.

This allows to obtain a non-crystallized product work hardened, somewhat in the form very hard fibers.

This rolling is carried out until a steel strip 2.5 to 12 mm thick, depending on the intended application.

This steel strip thus obtained undergoes a self-annealing and / or is subjected to a controlled annealing of so as to obtain a crystallization identical to that rolled steel in the ferritic phase.

Self-annealing which occurs automatically, due to the temperature the tape takes place during and after the winding of these rolled steel strips.

On the other hand, in case a controlled annealing is desired, the steel coil is introduced into an oven for a specified time and is brought to a temperature which is notably a function of this time of stay in the oven.

So, we anneal controlled at a temperature between 750 and 910 ° C for a period of 30 minutes to 48 hours, ensuring that the shortest durations in the oven correspond to the most annealing temperatures and vice versa than the longest durations correspond to the lowest temperatures.

In addition, according to the invention, it is possible also provide for a strip stripping step thus rolled steel.

This pickling preferably takes place before annealing. However, pickling and annealing can if necessary, be carried out simultaneously, for example in a reducing atmosphere rich in hydrogen.

Some examples are given below examples of carrying out the process according to the invention from a steel of a well-defined composition.

Example 1

C = 0,002 %

Mn = 0,110 %

S = 0,004 %

Si = 0,005 %

P = 0,012 %

Al = 0,012 %

N = 0,0015 %

The steel used for slab formation had the following composition:

C = 0.002%

Mn = 0.110%

S = 0.004%

If = 0.005%

P = 0.012%

Al = 0.012%

N = 0.0015%

The slabs obtained were subjected to a reheating for 1 hour 30 minutes at a temperature of 1250 ° C.

Then the rolling took place at a temperature which was 900 ° C at the inlet and 780 ° C at the finisher outlet.

The thickness of the steel strip obtained was 2.5 mm. Then took place the winding of the tape steel at an average temperature of around 680 ° C and therefore it undergoes self-annealing. Finally, a classic chemical pickling was applied to the tape of steel after having laid it flat and cut into leaves.

Grain size in the finished strip was 5 ASTM and the coercive field was equal to 95 A / m.

Example 2

C = 0,003 %

Mn = 0,300 %

S = 0,001 % globulaire

Si = 0,250 %

P = 0,015 %

Al = 0,250 %

V = 0,150 %.

The steel used had the following composition:

C = 0.003%

Mn = 0.300%

S = 0.001% globular

If = 0.250%

P = 0.015%

Al = 0.250%

V = 0.150%.

The reheat temperature was 1050 ° C for 2 hours.

The average temperature at the paver was 840 ° C.

The winding temperature was 730 ° C.

Self-annealing took place automatically on reel.

Then, an unwinding followed by a leveling of the "black sheet", that is to say without pickling, were performed.

This sheet then presented a field coercive of 50 A / m and an induction of 1.95 Ts at a magnetic field of 100 A / cm.

Example 3

C = 0,017 %

Ti = 0,150 %

Al = 0,025 %

Si = 0,030 %

P = 0,017 %

S = 0,007 %

N = 0,0065 %.

The steel used had the following composition:

C = 0.017%

Ti = 0.150%

Al = 0.025%

If = 0.030%

P = 0.017%

S = 0.007%

N = 0.0065%.

The reheat temperature was 1250 ° C for 1 hour 30 minutes.

Average temperature in the finisher was 760 ° C.

The winding temperature was 300 ° C and the latter was followed by a chemical pickling.

Annealing was carried out in a medium of hydrogen at a temperature of 750 ° C.

The steel obtained had an ASTM grain of 5-6.

The coercive field was 110 A / m.

The invention is of course not limited to the different embodiments described above and, within the framework of the invention, several variants can be considered, particularly with regard to the type of annealing and pickling applied. So a calamine removal treatment can, if necessary, intervene at different stages of the process. This treatment can be a chemical pickling or a mechanical descaling such as, respectively, a reduction of the metal oxide by a reducing agent, such as hydrogen, or shot blasting.

Furthermore, the rolled steel strip can be kept at a substantially uniform temperature relatively low at most 150 ° C before winding.

Claims (10)

1. Method of producing a low carbon steel with a low coercivity less than or equal to 110 A/m and improved magnetic permeability, having the following composition:

   C = 1%

   Si = 1%

   Mn < 1%

   P < 0.100%

   S < 0.100%

   N < 0.100%

   Al = 0.250%

   which consists in forming slabs from a steel of said composition, subjecting these slabs to a rolling process conducted at least partly during the ferritic phase until a product w ith a thickness of 2.5 to 12 mm is obtained and annealing this product to bring about crystallisation of the steel in the ferritic phase, the rolling followed by annealing being such that the finished steel product has a grain size of 1 to 6 ASTM, characterised in that it involves a single process of rolling a strip of steel, performed at least during the ferritic phase, between non-lubricated laminating rolls, reeling the strip once rolled to said thickness and a final step of annealing the strip by automatic annealing as it is reeled at a temperature of at least 680 °C and/or by annealing the strip in the reeled state by a controlled process at a temperature of from 750 °C to 910°C for a period of at least 30 minutes.
2. Method as claimed in claim 1, characLerised in that a steel of the following composition is used:

   C = 0.010%

   Si = 0.30%

   Mn < 1%

   P < 0.030%

   S < 0.020%

   N < 0.010%

   Al < 0.100%
3. Method as claimed in claim 1 or 2, characterised in that the steel is subjec ted to a decarburization treatment, preferably prior to rolling.
4. Method as claimed in claim 3, characterised in that the slabs are brought to a temperature of from 900 to 1200°C prior to rolling.
5. Method as claimed in any one of claims 1 to 4, characterised in that a steel containing decarburizing additives is used, in particular Nb, Ti, V, at concentrations such that the free carbon is less than 0.006%.
6. Method as claimed in any one of claims 1 to 5, characterised in that the strip of steel ob tained by said rolling is subjected to a pickling process.
7. Method as claimed in claim 6, characterised in that the strip of steel is subjected to a pickling process prior to and/or simultaneously with said controlled annealing process.
8. Method as claimed in claim 1, characterised in that said annealing lasts for a period of from 30 minutes to 48 hours, the shortest periods corresponding to the highest temperatures and, conversely, the longest periods corresponding to the lowest temperatures.
9. Method as claimed in any one of claims 1 to 8, characterised in that the rolled strip of steel is cooled to a substantially uniform temperature of at most 150°C before it is reeled.
10. Method as claimed in any one of claims 1 to 9, characterised in that rolling is effected at an input temperature of 900 °C maximum and in that reeling is performed at a temperature at which the strip undergoes said automatic annealing in the ferritic phase.