EP0685567B1 - Ferritic stainless steel with improved machinability - Google Patents

Abstract

Stainless steel contains in %:- less than 0.17C; max. 2Si; max. 2Mn; 11-20Cr; max O.1Ni; max 0.55S; min 30x10-4Ca; min 70x10-40; with ratio of Ca/O being 0.2-0.6.

Description

The present invention relates to a structural stainless steel ferritic and improved machinability, usable in particular in the field bar turning.

By stainless steels, we mean iron alloys containing at least 10.5% chromium. Document FR-A-2 456 785 discloses a free-cutting stainless steel having excellent machinability and which can contain up to 40% Ni and Te, so that the% Te /% S ratio is greater than 0.04 .

• les aciers inoxydables de structure martensitiques,
• les aciers inoxydables de structure austénitiques,
• les aciers inoxydables de structure austéno-ferritiques,
• les aciers inoxydables de structure ferritiques.

Other elements enter into the composition of steels in order to modify their structure and their properties. There are known four typical families of stainless steels differentiated by their structure. Those are :

• stainless steels with martensitic structure,
• stainless steels of austenitic structure,
• stainless steels with an austenitic-ferritic structure,
• stainless steels with ferritic structure.

Ferritic stainless steels are characterized by a determined composition, the ferritic structure being notably ensured, after lamination and cooling of the composition, by treatment thermal annealing giving 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 sont transformés en aciers inoxydables ferritiques après recuit malgré une teneur en carbone relativement élevée.
• les aciers inoxydables ferritiques dont la teneur en chrome varie de 11 à 12 %. Ils sont assez proches des aciers martensitiques contenant 12 % de chrome, mais différent par leur teneur en carbone qui est nettement plus faible.

Among the four main families of ferritic stainless steels defined in particular according to their chromium and carbon content, we cite:

• ferritic stainless steels which may contain up to 0.17% carbon. These steels, after the cooling which follows their development, have a two-phase austeno-ferritic structure. They are transformed into ferritic stainless steels after annealing despite a relatively high carbon content.
• ferritic stainless steels whose chromium content varies from 11 to 12%. They are quite close to martensitic steels containing 12% chromium, but different in their carbon content which is significantly lower.

• les aciers inoxydables ferritiques à 17 % de chrome. Ce sont les plus courants. Il en existe de nombreuses variantes, en particulier au niveau de la teneur en carbone. L'addition de molybdène permet d'améliorer leur résistance à la corrosion.

For example, the following table presents a series of ferritic and martensitic steels with the carbon content imposed by the standard. Shade Content imposed by the standard FERRITICS AISI 430 (Z 8 C 17) C <0.12% AISI 434 = (Z8CD17-01) C <0.12% AISI430 F = (Z10CF) C <0.12% MARTENSITICS AISI 420 A (Z 20 C 13) 0.15% <C <0.24% AISI 416 (Z 12 CF 13) 0.08% <C <0.15%

• ferritic stainless steels with 17% chromium. These are the most common. There are many variations, especially in terms of carbon content. The addition of molybdenum improves their resistance to corrosion.

• les aciers inoxydables ferritiques à 17 % de chrome stabilisés par addition d'éléments ayant une forte affinité pour le carbone ou l'azote, tels que le titane, le niobium, le zirconium.
• les aciers inoxydables ferritiques à haute teneur en chrome, généralement supérieure à 24 %.

In general, the ferritic structure of steels is preferably obtained by limiting the quantity of chromium carbide, this is why most ferritic stainless steels have a carbon content of less than 0.12% or even 0.08%. .

• ferritic stainless steels with 17% chromium stabilized by the addition of elements having a strong affinity for carbon or nitrogen, such as titanium, niobium, zirconium.
• ferritic stainless steels with a high chromium content, generally greater than 24%.

From a metallurgical point of view, it is known that certain elements contained in the composition of the steel favor the appearance of the ferritic phase with a centered cubic structure. These elements are said alpha-genes. Among these are chromium and molybdenum. Others so-called gamma-gene elements favor the appearance of the gamma-austenitic phase of cubic structure with centered faces. Among these elements include nickel as well as carbon and nitrogen.

When hot rolling, the structure of the steel can be two-phase, ferritic and austenitic. If the cooling is, by energetic example, the final structure is ferritic and martensitic. If he is slower, the austenite partially decomposes into ferrite and carbides, but with a richer carbide content than the surrounding matrix, the austenite having solubilized hot more carbon than the ferrite. In the two cases, an income or annealing must therefore be practiced on steels hot rolled and cooled to generate a fully structured ferritic. Tempering can be done at a temperature of around 820 ° C lower than the transition temperature alpha → gamma A1, which generates a precipitation of carbides.

It is also possible to anneal at a higher temperature. high such as 870 ° C which leads to more softening marked with martensite but causes a partial transformation into austenite. Slow cooling is then necessary to decompose austenite formed of ferrite and carbides, thus avoiding the formation of new martensite.

In the development of so-called stabilized ferritic steels, the carbon combines with stabilizing elements such as titanium and / or niobium and no longer participates in the formation of the gamma-gene phase, no longer present in the matrix. In this case, it is possible to obtain after hot rolling a steel whose structure is completely ferritic.

From the point of view of physical properties, the most significant difference apparent between ferritic steels and austenitic steels is the ferromagnetic behavior of the former.

The thermal conductivity of ferritic steels is very low. It is between that of martensitic steels and that of steels austenitics at room temperature. It is equivalent to the thermal conductivity of austenitic steels at temperatures between 800 ° C and 1000 ° C, temperatures which correspond to steel temperatures during machining.

From a machining point of view, the expansion coefficient thermal of ferritic steels is about 60% higher than that austenitic steels.

In addition, ferritic steels have characteristics significantly lower than those of martensitic steels and austenitics.

In an example, the following table presents a series of stainless, ferritic, martensitic, austenitic steels and the corresponding mechanical characteristics (Rm). Stainless steel Standardized RM (MPa) Ferritic AISI 430 (Z8 C17) 440 - 640 AISI 430F (Z20 CF17) 440 - 640 Martensitic AISI 420A (Z20 C13) 700 - 850 AISI 420B (Z33 C13) 850 - 1000 F16 H (Z7CNU16-04) (hardened) 930 - 1100 Austenitic AISI 304 (Z6 CNT18 10) 510 - 710

In the development of steels of ferritic structures, the flow stresses at rolling temperatures are markedly weaker than those of austenitic steels or steels martensitics. Therefore, lamination is carried out at temperatures relatively lower.

As an example, the flow stress at a rolling temperature of 1100 ° C and for a deformation speed of 1 s ^-1 is 110 MPa for martensitic steel of type AISI 420 A, of 130 MPa for an austenitic steel of type AISI 304 whereas it is 30 MPa for a ferritic steel of type AISI 430.

Steels of ferritic structure are not subjected to a rapid quenching or quenching type cooling like steels martensitic or austenitic. However, they are generally subject very specific delayed heat treatments which give them their structure. Deferred heat treatments also aim to homogenize the chromium element and avoid the creation of carbide chromium and the appearance of chromium-depleted areas.

For example, steels of ferritic structure with 17% chromium non-stabilized have, after rolling, a ferritic and martensitic structure. Heat treatment on the one hand ensures the transformation of the martensite made of ferrite and carbides and on the other hand, a distribution uniform chrome.

In the field of their use, stainless steels ferritics pose very different machinability problems than those encountered with stainless steels of austenitic structure or martensitic.

Indeed, a big drawback of ferritic steels is the poor chip conformation. They produce long chips and tangled, which are very difficult to fragment. It is then necessary operators to stay close to the machine to clear tools. This disadvantage can become very disadvantageous in machining modes where the chip is confined, such as in deep drilling, the cutting ...

One solution to solve this problem is speed machining high cutting to cause chip fragmentation, but hand, increasing the cutting speed critically decreases the lifetime of tools, on the other hand machines do not allow always reach sufficiently high speeds, especially when the production of small diameters, especially in bar turning.

Another solution to overcome machining problems ferritic steels is to introduce sulfur into their composition. The sulfur forms with manganese manganese sulfides which have a favorable effect on chip fragmentation and incidentally on tool life. However, sulfur degrades the properties of ferritic steel, in particular hot and cold deformability, and corrosion resistance.

Said ferritic steels usually contain hard inclusions of chromite (Cr Mn, Al Ti) O, alumina (AlMg) O, silicate (SiMn) O type, abrasives for cutting tools.
It has been found that resulphurized ferritic steels have good machinability, however, in addition to corrosion resistance, the mechanical properties in the cross direction are greatly degraded.

The object of the invention is to propose a ferritic steel with machinability improved with characteristics far superior to those, for example, resulphurized ferritic steels and, in another form, to present a machinable ferritic steel containing little or no sulfur.

• carbone ≤ 0,17 %
• silicium ≤ 2 %
• manganèse ≤ 2 %
• chrome : 11 - 20 %
• nickel < 1 %
• soufre ≤ 0,55 %
• calcium ≥ 30 x 10 - 4 %
• oxygène ≥ 70 x 10^-4 %

Optionnellement : Mo < 3 %,
reste Fe et les impuretés inévitables,
le rapport de la teneur en calcium et en oxygène Ca/O étant 0,2 ≤ Ca/O ≤ 0,6.
De préférence, l'acier inoxydable de structure ferritique comprend dans sa composition :

• carbone ≤ 0,12 %
• silicium ≤ 2 %
• manganèse ≤ 2 %
• chrome 15 - 19 %
• nickel < 1 %
• soufre ≤ 0,55 %
• calcium ≥ 35 x 10^-4 %
• oxygène ≥ 70 x 10^-4 %

un rapport de la teneur en calcium et oxygène Ca/O compris dans l'intervalle 0,35 < Ca/O <0,6.The subject of the invention is a stainless steel of ferritic structure and with improved machinability usable in particular in the field of bar turning and which comprises in its weight composition:

• carbon ≤ 0.17%
• silicon ≤ 2%
• manganese ≤ 2%
• chromium: 11 - 20%
• nickel <1%
• sulfur ≤ 0.55%
• calcium ≥ 30 x 10 - 4%
• oxygen ≥ 70 x 10 ^-4 %

Optionally: Mo <3%,
remains Fe and the inevitable impurities,
the ratio of calcium and oxygen content Ca / O being 0.2 ≤ Ca / O ≤ 0.6.
Preferably, the ferritic structure stainless steel comprises in its composition:

• carbon ≤ 0.12%
• silicon ≤ 2%
• manganese ≤ 2%
• chromium 15 - 19%
• nickel <1%
• sulfur ≤ 0.55%
• calcium ≥ 35 x 10 ^-4 %
• oxygen ≥ 70 x 10 ^-4 %

a Ca / O calcium and oxygen content ratio in the range 0.35 <Ca / O <0.6.

• l'acier inoxydable de struture ferritique comprend dans sa composition :
  • C ≤ 0,08 %
  • Si ≤ 2,0 %
  • Mn ≤ 2,0 %
  • Cr 15 - 19 %
  • Ni < 1 %
  • S ≤ 0,55 %
  • Ca ≥ 35 x 10^-4 %
  • O ≥ 70 x 10^-4 %
  le rapport entre le teneur en calcium et en oxygène Ca/O satisfaisant à la relation 0,35 ≤ Ca/O ≤ 0,6.

In one form of the invention:

• stainless steel with ferritic structure includes in its composition:
  • C ≤ 0.08%
  • If ≤ 2.0%
  • Mn ≤ 2.0%
  • Cr 15 - 19%
  • Ni <1%
  • S ≤ 0.55%
  • Ca ≥ 35 x 10 ^-4 %
  • O ≥ 70 x 10 ^-4 %
  the ratio between the calcium and oxygen Ca / O content satisfying the relationship 0.35 ≤ Ca / O ≤ 0.6.

• l'acier ferritique comprend de 0,15 % à 0,45 % de soufre,

The other characteristics of the invention are:

• ferritic steel comprises from 0.15% to 0.45% sulfur,

• l'acier ferritique comprend moins de 0,035 % de soufre,
• l'acier ferritique comprend de 0,05 à 0,15 % de soufre,
• l'acier ferritique peut contenir dans sa composition moins de 3 % de mobybdène.

In another form of the invention:

• ferritic steel contains less than 0.035% sulfur,
• ferritic steel comprises from 0.05 to 0.15% sulfur,
• ferritic steel may contain less than 3% mobybdenum in its composition.

The following description and the attached drawings, all given to by way of nonlimiting example, will make the invention well understood.

Figures 1 and 2 show a conformation diagram of the chips depending on the machining conditions respectively for a known non-resulfurized AISI 430 ferritic steel, designated by the reference A and for an AISI 304 austenitic steel.

Figure 3 shows different chip conformations machining results when turning different metals.

Figure 4 is a ternary diagram defining the compositions of the malleable oxides introduced into the composition of ferritic steel according to the invention.

Figures 5 and 6 show a conformation diagram of chips depending on the machining conditions respectively for a known ferritic steel C AISI 430F resulfurized and for a ferritic steel resulfurized S according to the invention.

Figure 7 is a diagram showing three curves machinability test characteristics, one of which corresponds to steel of reference A, the other two corresponding to two steels in the the invention C1 and C2 and containing little sulfur.

Figure 8 presents a diagram schematically the conformation of chips depending on the tool feed and the depth of the machining pass for a C2 steel according to the invention.

In the field of machinability of stainless steels in general and according to the different structures of the steels used, the problems encountered turn out to be different, but also particularly specific. Problems encountered during machining ferritic steels are unrelated to the problems encountered during the machining of austenitic or martensitic steels.

For example, austenitic stainless steels have the disadvantage of being hardenable and of wearing out the cutting tools very quickly, the conformation of the chips is bad, but without comparison with that of ferritic steels.

Figures 1 and 2 show a conformation diagram of shavings depending on the feed and the depth of the machining pass determined respectively for non-resulfurized AISI 430 ferritic steel corresponding to the reference A and an AISI 304 austenitic steel.

In order to be able to compare chip conformations, the Figure 3 is a table which associates with different chip conformations a coefficient comprising several successive digits, the first digit defining different general images of the chip, forming the columns of the table such as 1: ribbon chip; 2: tubular chip; 3: chip spiral ..., 4: helical chip in washer; 5: conical helical chip; 6: arched chip; 7: elementary chip; 8: needle chip, the second figure defining a dimension and shape characteristic classified in each of the columns such as: 1: long; 2: short .; 3: tangled ; 4: flat; 5: conical; 6: attached; 7: detached.

Martensitic stainless steels have characteristics high mechanical properties, which generates high cutting temperatures and rapid tool wear.

Due to the poor mechanical properties of steels of ferritic structure, said steels do not have the same modes of machining and degradation of cutting tools than those of martensitic steels.

• des aciers de décolletage qui ont une teneur en soufre comprise entre 0,15 % et 0,55 %. Ce type d'acier utilisé en décolletage présente une bonne usinabilité, au détriment de la résistance à la corrosion,
• des aciers standard qui ont une teneur en soufre inférieure à 0,035 %. Ce type d'acier présente une bonne résistance à la corrosion, mais est peu ou pas usiné, justement à cause des difficultés rencontrées lors du décolletage.
• les aciers ayant des niveaux intermédiaires de soufre correspondant à une teneur comprise entre 0,05 % et 0,15 % ne sont pas commercialisés. En effet, leur usinabilité n'est que très modérément améliorée pour ces teneurs en soufre, en comparaison avec les aciers dits resulfurés. Ils ne présentent pas un avantage réel à côté de l'inconvévient qui reste la dégradation de la résistance à la corrosion.

There are two types of ferritic stainless steels, depending on their sulfur content:

• free-cutting steels which have a sulfur content of between 0.15% and 0.55%. This type of steel used in bar turning has good machinability, to the detriment of corrosion resistance,
• standard steels which have a sulfur content of less than 0.035%. This type of steel has good corrosion resistance, but is hardly or not machined, precisely because of the difficulties encountered during bar turning.
• steels with intermediate sulfur levels corresponding to a content between 0.05% and 0.15% are not sold. In fact, their machinability is only very moderately improved for these sulfur contents, in comparison with so-called resulfurized steels. They do not present a real advantage beside the disadvantage which remains the degradation of the resistance to corrosion.

According to the invention, ferritic stainless steel with improved machinability, usable in particular in the field of bar turning, comprises in its composition by weight, less than 0.17% of carbon, less than 2% of silicon, less than 2% of manganese, 11 to 20% chromium, less than 1% nickel, less than 0.55% sulfur, more than 30 10 ^-4 % calcium and more than 70 10 ^-4 % oxygen, steel being subjected, after preparation, to an annealing treatment to give it a ferritic structure.

The presence of nickel in the composition due to the preparation steel industry is only a residual element that we seek to reduce and even to eliminate.

The controlled and voluntary introduction of calcium and oxygen at high levels and verifying the relationship 0.2 ≤ Ca / O ≤ 0.6 promotes in ferric steel, the formation of malleable oxides, chosen in a Al ₂ O ₃ ternary diagram; SiO ₂ ; CaO, in the area of the triple point anorthite, gehlenite, pseudowollastonite as shown in Figure 4.

The presence of calcium and oxygen significantly reduces the formation of hard and abrasive inclusions such as chromite, alumina, silicate.

It has been found that the introduction of calcium-based oxides and of oxygen in a steel of ferritic structure, replacing the existing hard oxides, does not affect other characteristics in any way of ferritic steel in the field of hot or cold deformation or in the field of corrosion resistance.

While resulphurized ferritic steels have good machinability, chip fragmentation being ensured by the presence of sulfur in the composition of said steel, surprisingly, the introduction of malleable oxides in the steel structure improves yet dramatically machinability.

The so-called malleable inclusions also contained in the steel malleable, cannot behave like inclusions malleable in non-malleable steel of an austenitic structure or martensitic.

In fact, the rolling temperatures of ferritic steels are lower than the rolling temperatures of steels of another structure, and the flow stress of ferritic steels remains very low at these rolling temperatures.

It is indeed unexpected, due to the weakness of flow constraints that so-called malleable oxides can be distorted to influence conformation and chip behavior during machining.

FIGS. 5 and 6 represent a diagram of conformation of chips as a function of a feed of the tool and a depth of machining pass determined, respectively for a steel referenced C of the type AISI 430F resulfurized and for a steel S , resulfurized according to the invention. The composition of the reference steel C is presented in Table 1. steel VS Yes Mn Or Cr Ref. C 0.062 0.505 0.680 0.273 16.1 steel Mo Cu S P N2 Ref. C 0.214 0.091 0.298 0.022 0.037 The composition of the steel S according to the invention is given in table 2.

For a steel according to the invention, the phenomenon of removal of the chip is very special. Without being clearly marked on the chip, fragmentation is significantly increased.

The controlled introduction of calcium and oxygen has also carried out in a ferritic steel having, in its composition, a sulfur content of less than 0.035%

The steels according to the invention may also contain less than 3% molybdenum, an element improving resistance to corrosion. It has been found that a steel of ferritic structure according to the invention does not containing little or no sulfur, has greatly improved machining of in such a way that this steel can be used industrially in bar turning, while having good resistance to corrosion.

In an example of application, a machinability comparison is presented between the steel referenced A, non-resulfurized ferritic and not containing an oxide of the anorthite, gehlenite and pseudowollastonite type and two steels C1 and C2, C2 being in the field of the invention.
Table 3 shows the composition of the reference steel A.
Table 4 shows the composition of steels C1 and C2, C2 being in the field of the invention. steel VS Yes Mn Or Cr Ref.A 0.058 0.356 0.514 0.212 16.35 steel Mo Cu S P N2 Ref.A 0.226 0.021 0.0114 0.019 0.046

In a machinability test presented in Figure 7, we during the machining of reference steel A, steel C1 and C2 steel, the different wear rates of a coated carbide tool. The essay is carried out without lubrication in order to be more severe. We see a less wear and tear on the tool when we compare steel reference A (curve A), steel C1 (curve C1) and steel C2 (curve C2) according to the invention.

Indeed, comparative steel C1 because of its composition does not contain enough so-called malleable oxides of the anothite, gehlenite type, pseudowollostonite, due to the lack of calcium in the metal. In addition, we observe on the diagrams of figure 8, that the steel C2 according to the invention has a clearly higher fragmentation zone than steel reference A, and even close to the reference steel C which is a steel resulphurized ferritic.

Concerning steels with intermediate contents in sulfur, between 0.05% and 0.15%, we find that steels according to the invention have a machinability comparable to that of steels resulfurized while having better resistance to corrosion.

In another application, it turned out that the presence of so-called malleable oxides in ferritic steel, had advantages individuals.

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

In the field of low-ferritic steel wire drawing diameter, the inclusions chosen according to the invention reduce so consequent the rate of breakage of the drawn wire.

In the field of steel wool shaving ferritic stainless steel wire, hard inclusions that wear out shaving tools also quickly provoke due to their shape in grain important breaks which affect the quality of the wool of steel.

According to the invention, ferritic stainless steels in the form of wires comprising malleable inclusions, subjected to shaving, have characteristics which ensure the formation of steel wool strands of longer average length and allow shaving with wires much lower residuals, which saves on material.

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

Ferritic steel, according to the invention comprising inclusions malleable can be polished much more easily to obtain an improved polished surface finish.

Claims (7)

1. Stainless steel of ferritic structure and with improved machinability which can be used, especially, in the field of screw-machining, characterized in that it comprises, in its composition by weight:

   C < 0.17%

   Si ≤ 2.0%

   Mn ≤ 2.0%

   Cr 11 - 20%

   Ni ≤ 1%

   S ≤ 0.55%

   Ca ≥ 30 x 10^-4%

   O ≥ 70 x 10^-4%

   Optionally: Mo < 3%, the balance Fe and the inevitable impurities,
   the Ca/O ratio, between the calcium content and the oxygen content, being given by 0.2 ≤ Ca/O ≤ 0.6.
2. Steel of ferritic structure according to Claim 1, characterized in that it comprises, in its composition by weight:

   C < 0.12%

   Si < 2.0%

   Mn < 2.0%

   Cr 15 - 19%

   Ni < 1%

   S ≤ 0.55%

   Ca ≥ 35 x 10^-4%

   O ≥ 70 x 10^-4%

   Optionally: Mo < 3%, the balance Fe and the inevitable impurities,
   the Ca/O ratio, between the calcium content and the oxygen content, satisfying the relationship: 0.35 ≤ Ca/O ≤ 0.6.
3. Stainless steel of ferritic structure according to Claims 1 and 2, characterized in that it comprises, in its composition by weight:

   C ≤ 0.08%

   Si ≤ 2.0%

   Mn ≤ 2.0%

   Cr 15 - 19%

   Ni < 1%

   S ≤ 0.55%

   Ca ≥ 35x10^-4%

   O ≥ 70 x 10^-4%

   Optionally: Mo < 3%, the balance Fe and the inevitable impurities,
   the Ca/O ratio, between the calcium content and the oxygen content, satisfying the relationship 0.35 ≤ Ca/O ≤ 0.6.
4. Steel of ferritic structure according to Claims 1 and 3, characterized in that it includes less than 0.035% of sulphur.
5. Steel of ferritic structure according to Claims 1 and 3, characterized in that it includes between 0.15% and 0.45% of sulphur.
6. Steel of ferritic structure according to Claims 1 to 3, characterized in that it includes between 0.05% and 0.15% of sulphur.
7. Steel of ferritic structure according to Claims 1 to 6, characterized in that it contains silica/alumina/calcium-oxide inclusions of the anorthite and/or pseudo-wollastonite and/or gehlenite type.

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