EP1299569B1 - Ferritic stainless steel for ferromagnetic parts - Google Patents

Abstract

Ferritic stainless steel, comprising the following composition by weight: 0%<C<=0.030% 1%<=Si<=3% 0%<Mn<=0.5% 10%<=Cr<=13% 0%<Ni<=0.5% 0%<Mo<=3% N<=0.030% Cu<=0.5% Ti<=0.5% Nb<=1% Ca>=1x10<SUP>-4</SUP>% 0>=10x10<SUP>-4</SUP>% S<=0.030% P<=0.030% the remainder being iron and the impurities which are inevitable from the production of the steel.

Description

The present invention relates to a usable ferritic stainless steel for ferromagnetic parts.

Ferritic stainless steels are characterized by their composition determined, the ferritic structure being ensured in particular, after rolling and cooling the composition by annealing heat treatment conferring on them said structure.

• les aciers inoxydables ferritiques pouvant contenir jusqu'à 0,17% de carbone. Ces aciers, après le refroidissement qui suit leur élaboration, ont une structure biphasée austéno-ferritique. Ils peuvent cependant être transformés en aciers inoxydables ferritiques après recuit malgré une teneur en carbone relativement élevée.
• les aciers inoxydables ferritiques dont la teneur en chrome est de l'ordre de 11 ou 12 %. Ils sont assez proches des aciers martensitiques contenant 12 % de chrome, mais différents par leur teneur en carbone qui est relativement faible.

Among the major families of ferritic stainless steels, defined in particular by their chromium and carbon content, we quote:

• ferritic stainless steels up to 0.17% carbon. These steels, after cooling after their development, have a two-phase austenitic-ferritic structure. However, they can be converted into ferritic stainless steels after annealing despite a relatively high carbon content.
• ferritic stainless steels whose chromium content is of the order of 11 or 12%. They are quite close to martensitic steels containing 12% of chromium, but different in their carbon content which is relatively low.

When rolling hot steels, the structure of the steel can be biphasic, ferritic and austenitic. If the cooling is, for example energetic, the final structure is ferritic and martensitic. If he is slower, the austenite is broken down partly into ferrite and carbides, but with a carbide content richer than the surrounding matrix, the austenite having hot solubilized more carbon than ferrite. In both cases, a income or annealing must therefore be practiced on hot-rolled steels and cooled to generate a completely ferritic structure. Income can be at a temperature of about 820 ° C lower than the Ac1 temperature of alpha → gamma transition, which causes a precipitation of carbides.

In the field of ferritic steels for one application using magnetic properties, the ferritic structure is obtained by limiting the amount of carbides, this is why stainless steels ferritics, developed in this area, have a carbon content less than 0.02%.

Steels that can be used for their magnetic properties are known as for example in US 5,769,974 which describes a process ferritic steel that is resistant to corrosion and can reduce value of the coercive field of said steel. The steel used in the process is a steel of the sulfide type. Sulfur reduces cold deformation properties. The steel obtained by the process is therefore difficult to use for realization of cold-struck pieces.

It is also known from US Pat. No. 5,091,024 in which it is presented corrosion-resistant magnetic articles formed of a alloy consisting of basically in a low carbon composition and low silicon content, that is to say respectively less than 0.03% and 0.5%. However, in the magnetic field, it is important for steel to contain a high silicon content to increase the resistivity of the material and reduce eddy currents.

The present invention aims to present a stainless steel of ferritic structure usable for magnetic parts having high magnetic properties and having good properties of implementation in terms of cold stamping and good machinability properties.

0% < C ≤ 0,030%

1%≤ Si ≤3%

0% < Mn ≤ 0,5%

10%≤ Cr ≤13%

0% < Ni ≤ 0,5%

0% < Mo ≤ 3%

N ≤ 0,030%

Cu ≤ 0,5%

Ti ≤ 0,5%

Nb ≤ 1%

Ca ≥ 30 10^-4%,

O ≥ 70 10^-4%,

S ≤ 0,030%,

P ≤ 0,030%

   le reste étant du fer et les impuretés inévitables à l'élaboration de l'acier.The subject of the invention is a ferritic stainless steel that can be used for ferromagnetic parts, which is characterized in that it comprises, in its weight composition:

0% <C ≤ 0.030%

1% ≤ If ≤3%

0% <Mn ≤ 0.5%

10% ≤ Cr ≤13%

0% <Ni ≤ 0.5%

0% <Mo ≤ 3%

N ≤ 0.030%

Cu ≤ 0.5%

Ti ≤ 0.5%

Nb ≤ 1%

Ca ≥ 30 10 ^-4 %,

O ≥ 70 10 ^-4 %,

S ≤ 0.030%,

P ≤ 0.030%

the rest being iron and the inevitable impurities in the making of steel.

• le rapport entre la teneur en calcium et en oxygène Ca/O étant
     0,2 ≤ Ca/O ≤ 0,6.
• l'acier contient des inclusions de silico-aluminale de chaux de type anorthite et/ou pseudo-wollastonite et/ou gehlénite.
• de préférence l'acier-comprend dans sa composition pondérale : 1
• 0% < C 0.015%
• 1% ≤ Si ≤ 3%
• 0 ≤ Mn ≤ 0,4%
• 10% ≤ Cr ≤ 13%
• 0% < Ni ≤ 0,2%
• 0,2% ≤ Mo ≤ 2%
• N ≤ 0,015%
• Cu ≤ 0,2%
• Ti ≤ 0,2%
• Nb ≤ 1%
• Ca ≥ 30 10^-4%
• O ≥ 70 10^-4%
• S ≤ 0,003%
• P ≤ 0,030%
   le reste étant du fer et les impuretés inévitables à l'élaboration de l'acier.
• de préférence l'acier comprend dans sa composition pondérale :
• 0% < C ≤ 0.015%
• 1% ≤ Si ≤ 3%
• 0 ≤ Mn ≤ 0,4%
• 10% ≤ Cr ≤ 13%
• 0% < Ni ≤ 0,2%
• 0,2% ≤ Mo ≤ 2%
• N ≤ 0,015%
• Cu ≤ 0,2%
• Ti ≤ 0,2%
• Nb ≤ 1%
• Ca ≥ 30 10^-4%
• O ≥ 70 10^-4%
• 0,015 ≤ S ≤ 0,03%
• P ≤ 0,030%
   le reste étant du fer et les impuretés inévitables à l'élaboration de l'acier.

The other features of the invention are:

• the ratio between the calcium and oxygen content Ca / O being
  0.2 ≤ Ca / O ≤ 0.6.
• the steel contains silicoaluminium inclusions of lime type anorthite and / or pseudo-wollastonite and / or gehlenite.
• preferably the steel-comprises in its weight composition: 1
• 0% <C 0.015%
• 1% ≤ If ≤ 3%
• 0 ≤ Mn ≤ 0.4%
• 10% ≤ Cr ≤ 13%
• 0% <Ni ≤ 0.2%
• 0.2% ≤ Mo ≤ 2%
• N ≤ 0.015%
• Cu ≤ 0.2%
• Ti ≤ 0.2%
• Nb ≤ 1%
• Ca ≥ 30 10 ^-4 %
• O ≥ 70 10 ^-4 %
• S ≤ 0.003%
• P ≤ 0.030%
the rest being iron and the inevitable impurities in the making of steel.
• preferably the steel comprises in its weight composition:
• 0% <C ≤ 0.015%
• 1% ≤ If ≤ 3%
• 0 ≤ Mn ≤ 0.4%
• 10% ≤ Cr ≤ 13%
• 0% <Ni ≤ 0.2%
• 0.2% ≤ Mo ≤ 2%
• N ≤ 0.015%
• Cu ≤ 0.2%
• Ti ≤ 0.2%
• Nb ≤ 1%
• Ca ≥ 30 10 ^-4 %
• O ≥ 70 10 ^-4 %
• 0.015 ≤ S ≤ 0.03%
• P ≤ 0.030%
the rest being iron and the inevitable impurities in the making of steel.

The invention also relates to a method for producing a steel characterized in that the weight composition is subjected, after hot rolling and cooling, annealing heat treatment then to a section modification of the drawing or drawing type.

Drawn or drawn steel may be subsequently annealed complementary recrystallization to perfect the magnetic properties of the room.

The following description by way of example non-limiting, will make clear the invention.

0% < C ≤ 0,030%

1% ≤ Si ≤ 3%

0% < Mn ≤ 0,5%

10% ≤ Cr ≤ 13%

0% <Ni ≤ 0,5%

0% < Mo ≤ 3%

N ≤ 0,030%

Cu ≤ 0,5%

Ti ≤ 0,5%

Nb ≤ 1%

Ca ≥ 30 10^-4%

O ≥ 70 10^-4%

S ≤ 0,030%

P ≤ 0,030%

   le reste étant du fer et les impuretés inévitables à l'élaboration de l'acier.The invention relates to a steel of the following general composition:

0% <C ≤ 0.030%

1% ≤ If ≤ 3%

0% <Mn ≤ 0.5%

10% ≤ Cr ≤ 13%

0% <Ni ≤ 0.5%

0% <Mo ≤ 3%

N ≤ 0.030%

Cu ≤ 0.5%

Ti ≤ 0.5%

Nb ≤ 1%

Ca ≥ 30 10 ^-4 %

O ≥ 70 10 ^-4 %

S ≤ 0.030%

P ≤ 0.030%

the rest being iron and the inevitable impurities in the making of steel.

From a metallurgical point of view, certain elements contained in the composition of a steel favor the appearance of the ferritic phase of centered cubic structure. These elements are called alpha-genes. Among these include chromium and molybdenum. Other elements said gamma-genes promote the appearance of the gamma-austenitic phase of cubic structure with centered faces. These elements include nickel as well than carbon and nitrogen. It is therefore necessary to reduce the content of these elements and it is for these reasons that the steel according to the invention comprises in its composition less than 0.030% of carbon, less than 0.5% of nickel, less than 0.030% nitrogen.

Carbon is bad for striking, corrosion and machinability. Of Generally speaking, in the field of magnetic properties, precipitates must be reduced because they constitute obstacles to movements of the walls of Blocks.

As regards the other elements of the composition, nickel, manganese, copper in the composition, due to the industrial development of steel are only residual elements that we seek to reduce and even to eliminate.

Titanium and / or niobium form compounds whose carbide titanium and / or niobium, which avoids the formation of carbides and nitrides chromium. They favor, therefore, the resistance to corrosion and in particular the holding corrosion of welds.

Sulfur is limited to optimize the behavior of steel in the field of cold stamping and to optimize magnetic properties.

Silicon is needed to increase the resistivity of steel in order to reduce eddy currents, and is favorable for corrosion resistance.

The steels according to the invention may also preferably contain from 0.2% to 3% molybdenum, an element that improves the resistance to corrosion and promotes formation of ferrite.

In the field of their use, ferritic stainless steels pose problems of machinability.

Indeed, a big drawback of ferritic steels is the bad conformation of the chip. They produce long, entangled chips that are very difficult to fragment. This disadvantage can become very penalizing in machining modes where the chip is confined, for example in deep drilling, bucking.

A solution to overcome the problems of machining steels ferritics is to introduce sulfur into their composition or elements type lead, tellurium, selenium that adversely affect the mechanical properties of cold deformation of corrosion resistance, or the properties magnetic. Said ferritic steels usually contain Hard inclusions of chromite type (Cr Mn, AI Ti) O, alumina (AIMg) O, silicate (SiMn) O, abrasive for cutting tools.

According to the invention, the ferritic stainless steel further contains in its composition by weight more than 10 ^-4 % calcium and more than 70 10 ^-4 % oxygen.

The controlled and voluntary introduction of calcium and oxygen, verifying the 0.2 ≤ Ca / O 0.6 ratio, favors in ferritic steel the formation of malleable oxides of the lime silicoaluminates type, the malleable oxides being preferably selected from the region of the triple anorthite point, gehlenite, pseudo-wollastonite, of the ternary Al ₂ O ₃ diagram; SiO ₂ ; CaO.

The presence of calcium and oxygen significantly reduces the formation of hard and abrasive inclusions of the chromite, alumina type, silicate. On the other hand, the formation of inclusions of silicoaluminates of lime promotes chip splitting and improves tool life cutting.

It has been found that the introduction of calcium-based oxides into a Ferritic structure steel, replacing the existing hard oxides, does not alters the other characteristics of ferritic steel in the field of hot deformation, cold stamping, resistance to corrosion and magnetic properties.

It has been found that a steel of ferritic structure according to the invention does not containing no or very little sulfur, has a machining ensuring its use in industrial turning, while presenting increased resistance to corrosion.

The presence of so-called malleable oxides in a ferritic steel, leads to advantages in the field of drawing and drawing.

Indeed, malleable oxides are likely to deform in the direction of rolling, while the hard oxides they replace remain in grain shape.

In the field of drawing ferritic steel wire of small diameter, the inclusions chosen according to the invention substantially reduce the breakage rate of drawn wire.

In another field of application, for example in operations hard inclusions become encrusted in ferritic steel and cause furrows on the surface.

Ferritic steel, according to the invention comprising malleable inclusions, can be polished with much greater ease for obtaining a state of polished surface improved.

The steel is made by electrical fusion and then poured continuously for to form blooms.

The blooms are then subjected to hot rolling for forming, for example wire rod or bars.

An annealing is necessary to ensure the transformation operations to cold product eg wire drawing and drawing.

The steel is subjected to a supplementary annealing of recrystallization for restore and perfect the magnetic properties.

Then follows a surface treatment.

In one application example, two steels according to the invention referenced steel 1 and steel 2 have been produced, as well as two reference steels A and B whose compositions are represented in the following Table 1: % VS Cr Yes MB mn P NOT S Or Cu Ti Nb It O Steel 1 0,010 12.2 1.58 0.48 0.25 0,011 0,009 0,001 0,135 0.04 0,002 0,002 0.0048 0,009 Steel 2 0,011 11.9 1.47 0.49 0.22 0,015 0,007 0,029 0.126 0.06 0,003 0,002 0.0062 0.012 Ref A 0015 17.4 1.25 0.35 0.5 0.02 0.02 0.28 0.3 0.1 0003 0002 0002 0006 Ref B 0016 17.5 1.37 1.53 0.38 0018 0017 0277 0.29 0.06 0003 0003 0.0017 0007

• un laminage à chaud de rond de 11 mm,
• un recuit,
• un étirage en diamètre de 10 mm,
• un recuit final,
• un dressage et une rectification,

   puis ils ont été caractérisés en propriétés magnétiques en usinabilité, en frappe à froid et en corrosion.These steels were transformed into 10 mm diameter bars according to the following method:

• hot rolling of 11 mm round,
• an annealing,
• drawing in diameter of 10 mm,
• a final annealing,
• a training and a correction,

then they were characterized in magnetic properties in machinability, cold stamping and corrosion.

The steels according to the invention have better magnetic characteristics than the reference steels, as shown in Table 2 below. Steel Hc (A / m) Coercive field Steel 1 109 Steel 2 115 Ref A 184 Ref B 177

These characteristics are due to a low content of elements in particular a chromium content of about 12%.

Steel 2 behaves very well in the field of machining bar turning, despite a limited sulfur content. This is explained by the presence of calcium and oxygen.

Steel 1 has a very good ability to cold stamp, because of of its low sulfur content. On previously struck coins, the finishing machining by turning is carried out correctly, without particular problem.

Steels 1 and 2 behave very well in the field of corrosion, despite their low chromium content, as can be seen in Table 3 below. This is due to the low sulfur steel 1 and the low sulfur 2 steel associated with a low manganese content. Potential for puncture in 0.02M NaCl at 23 ° C I corrosion in H2SO4 2M at 23 ° C Steel 1 220 mV / DHW 10 mA / cm ² Steel 2 215 mV / ECS 11 mA / cm ² Ref A 205 mV / ECS 24 mA / cm ² Ref B 330 mV / DHW 6 mA / cm ²

The steel according to the invention can be used particularly for the manufacture ferromagnetic parts such as, for example, parts of solenoid valves, injector for gasoline direct injection system, closures centralized door in the automotive field or any application requiring parts of the magnetic core type or inductor. Under the sheet form, it can be used in current transformers or magnetic shielding.

Claims (7)

1. Ferritic stainless steel which can be used for ferromagnetic parts, characterised in that its composition by weight is as follows:

   0% < C ≤ 0.030%

   1% ≤ Si ≤ 3%

   0% < Mn ≤ 0.5%

   10% ≤ Cr ≤ 13%

   0% < Ni ≤ 0.5%

   0% < Mo ≤ 3%

   N ≤ 0.030%

   Cu ≤ 0.5%

   Ti ≤ 0.5%

   Nb ≤ 1%

   Ca > 30 10^-4%

   O > 70 10^-4%

   S ≤ 0.030%

   P ≤ 0.030%

   the remainder being iron and the impurities which are inevitable in the production of the steel.
2. Steel according to claim 1, characterised in that the ratio between the calcium content and the oxygen content Ca/O is:

   0.2 ≤ Ca/O ≤ 0.6
3. Steel according to claim 1, characterised in that it contains silico-aluminate of lime inclusions of the anorthite and/or pseudo-wollastonite and/or gehlenite type.
4. Steel according to claim 1, characterised in that its composition by weight is as follows:

   C ≤ 0. 012%

   1% ≤ Si ≤ 3%

   0% ≤ Mn ≤ 0.4%

   10% ≤ Cr ≤ 13%

   0% ≤ Ni ≤ 0.2%

   0.2% ≤ Mo ≤ 2%

   N ≤ 0.015%

   Cu ≤ 0.2%

   Ti ≤ 0.2%

   Nb ≤ 1%

   Ca ≤ 30 10^-4%

   O ≥ 70 10^-4%

   S ≤ 0.003%

   P ≤ 0.030%

   the remainder being iron and the impurities which are inevitable in the production of the steel.
5. Steel according to claim 1, characterised in that its composition by weight is as follows:

   0% < C ≤ 0.012%

   1% ≤ Si ≤ 3%

   0% ≤ Mn ≤ 0.4%

   10% ≤ Cr ≤ 13%

   0% < Ni ≤ 0.2%

   0.2% ≤ Mo ≤ 2%

   N ≤ 0.015%

   Cu ≤ 0.2%

   Ti ≤ 0.2%

   Nb ≤ 1%

   Ca ≥ 30 10^-4%

   O ≥ 70 10^-4 %

   0.015 ≥ S ≤ 0.03%

   P ≤ 0.030%

   the remainder being iron and the impurities which are inevitable in the production of the steel.
6. Method of producing a ferritic steel according to one of claims 1 to 5, characterised in that, after hot rolling and cooling, the steel is subjected to an annealing heat treatment then to a modification of cross-section by means of drawing or stretch-forming.
7. Method according to claim 6, characterised in that the drawn or stretch-formed steel can later be subjected to additional recrystallisation annealing to perfect the magnetic properties of the part.