EP0685567A1 - 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 stainless steel of ferritic structure and improved machinability, usable in particular in the field of bar turning.

The term “stainless steels” means iron alloys containing at least 10.5% chromium.

• 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 in particular ensured, after rolling and cooling of the composition, by an annealing heat treatment 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. lls 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 %. lls 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.
  De manière générale, la structure ferritique des aciers est de préférence obtenue en limitant la quantité de carbure de chrome, c'est pour cela que la plupart des aciers inoxydables ferritiques ont une teneur en carbone inférieure à 0,12 % voire 0,08 %.
• les aciers inoxydables ferritiques à 17 % de chrome stabilités 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 %.

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

When hot rolling, the steel structure can be two-phase, ferritic and austenitic. If the cooling is, for example energetic, the final structure is ferritic and martensitic. If it is slower, the austenite partially decomposes into ferrite and carbides, but with a content of carbide richer than the surrounding matrix, the austenite having hot dissolved more carbon than ferrite. In both cases, tempering or annealing must therefore be carried out on hot rolled and cooled steels to generate a totally ferritic structure. Tempering can take place at a temperature of approximately 820 ° C lower than the alpha transition alpha → gamma temperature, which generates a precipitation of carbides.

One can also carry out an annealing at a higher temperature such as for example 870 ° C. which leads to a more marked softening of the martensite but causes a partial transformation into austenite. Slow cooling is then necessary to decompose the 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, carbon combines with stabilizing elements such as titanium and / or nobium and no longer participates in the formation of the gamma-gene phase, being 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 apparent difference 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 austenitic steels 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 the temperatures of steels during machining.

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

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

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 production of steels with ferritic structures, the flow constraints at rolling temperatures are clearly weaker than those of austenitic steels or martensitic steels. As a result, rolling is carried out at relatively lower temperatures.

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 rapid cooling of the quenching or hyper quenching type like martensitic or austenitic steels. On the other hand, they are generally subjected to very specific deferred heat treatments which give them their structure. Deferred heat treatments also have the aim of homogenizing the chromium element and avoiding the creation of chromium carbide and the appearance of zones depleted in chromium.

For example, steels of ferritic structure with 17% chromium not stabilized have, after rolling, a ferritic and martensitic structure. A heat treatment ensures on the one hand the transformation of martensite into ferrite and carbides and on the other hand, a uniform distribution of the chromium.

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

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

One solution to solve this problem is machining at high cutting speed to cause chip fragmentation, but on the one hand, increasing the cutting speed critically decreases the life of the tools, on the other hand apart from the machines, it is not always possible to reach sufficiently high speeds, in particular when producing small diameters, in particular in bar turning.

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

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 improved machinability having 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 < 10 %
• soufre ≦ 0,55 %
• calcium ≧ 30 10⁻⁴ %
• oxygène ≧ 70 10⁻⁴ %

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 10⁻⁴ %
• oxygène ≧ 70 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 composition:

• carbon ≦ 0.17%
• silicon ≦ 2%
• manganese ≦ 2%
• chromium: [11 - 20]%
• nickel <10%
• sulfur ≦ 0.55%
• calcium ≧ 30 10⁻⁴%
• oxygen ≧ 70 10⁻⁴%

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 10⁻⁴%
• oxygen ≧ 70 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 10⁻⁴ %
  • O ≧ 70 10⁻⁴ %
    le rapport entre le teneur en calcium et en oxygène Ca/O satisfaisant à la relation 0,35 / Ca/O / 0,6.
  Les autres caractéristiques de l'invention sont :
• l'acier ferritique comprend de 0,15 % à 0,45 % de soufre,
     Dans une autre forme de l'invention :
• 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 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 10⁻⁴%
  • O ≧ 70 10⁻⁴%
    the ratio between the calcium and oxygen Ca / O content satisfying the relationship 0.35 / Ca / O / 0.6.
  The other characteristics of the invention are:
• ferritic steel comprises from 0.15% to 0.45% sulfur,
  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 accompanying drawings, all given by way of non-limiting example, will clearly understand the invention.

FIGS. 1 and 2 present a diagram of conformation of the chips as a function of the machining conditions respectively for a known ferritic steel AISI 430 non-resulfurized, designated by the reference A and for an austenitic steel AISI 304.

Figure 3 shows different conformations of shavings from machining during the turning of different metals.

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

Figures 5 and 6 show a chip shaping diagram as a function of the machining conditions respectively for a known ferritic steel C AISI 430F resulfurized and for a resulfurized ferritic steel S according to the invention.

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

FIG. 8 presents a diagram diagramming the conformation of chips as a function of the advance of the tool and the depth of the machining pass for a steel C2 according to the invention.

In the field of machinability of stainless steels in general and depending on the different structures of the steels used, the problems encountered turn out to be, on the one hand, different, but also particularly specific. The problems encountered during the machining of 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 poor, but without comparison with that of ferritic steels.

FIGS. 1 and 2 show a diagram of chip shaping as a function of the advance and the depth of the machining pass determined respectively for a non-resulfurized AISI 430 ferritic steel corresponding to the reference A and an AISI 304 austenitic steel.

In order to be able to compare the chip conformations, FIG. 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: spiral chip ..., 4: helical chip in a washer; 5: conical helical chip; 6: arched chip; 7: elementary chip; 8: needle chip, the second digit defining a characteristic of size and shape 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 high mechanical properties, which generate high cutting temperatures and rapid tool wear.

Due to the poor mechanical characteristics of stainless steels with ferritic structure, said steels do not have the same machining and degradation methods for cutting tools as 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 slightly 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 industrial production of steel is only a residual element that we seek to reduce and even eliminate.

The controlled and voluntary introduction of calcium and oxygen at high contents and verifying the relation 0.2 ≦ Ca / O ≦ 0.6 promotes in ferric steel, the formation of malleable oxides, chosen in a Al₂O₃ ternary diagram; SiO₂; CaO, in the point area triple anorthite, gehlenite, pseudowollastonite as shown in figure 4.

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

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

While the resulphurized ferritic steels have good machinability, the 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 further improves spectacular machinability.

The so-called malleable inclusions contained in the equally malleable steel cannot have the same behavior as malleable inclusions in non-malleable steel with an austenitic or martensitic structure.

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 the flow constraints that the so-called malleable oxides can be deformed to influence the conformation and the behavior of the chip 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. 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 chip removal phenomenon is very specific. Without being clearly marked on the chip, the fragmentation is increased significantly.

The controlled introduction of calcium and oxygen was 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% of molybdenum, an element improving the resistance to corrosion. It has been found that a steel of ferritic structure according to the invention, containing no or very little sulfur, has a greatly improved machining such that this steel can be used industrially in bar turning, while having good resistance corrosion.

In an example of application, a machinability comparison is presented between the steel referenced A, non-resulfurized ferritic and containing no oxide of anorthite, gehlenite and pseudowollastonite type and two steels C1 and C2 from the field of invention.
Table 3 shows the composition of the reference steel A.
Table 4 shows the composition of steels C1 and C2 in the field of the invention. table 3 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 note during the machining of reference steel A, steel C1 and steel C2, the different wear rates of a tool in coated carbide. The test is carried out without lubrication in order to be more severe. We observe a reduction in the wear in the tool clearance when we compare the reference steel A (curve A), steel C1 (curve C1) and steel C2 (curve C2) according to the invention .

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

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

In another application, it has been found that the presence of so-called malleable oxides in ferritic steel has particular advantages.

In fact, the malleable oxides are liable to deform in the rolling direction, while the hard oxides which they replace have the form of grains.

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

In the field of steel wool manufacturing by shaving ferritic stainless steel wire, the hard inclusions which quickly wear down the shaving tools also cause, due to their grain form, significant ruptures which adversely affect the quality of the steel wool.

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 strands of steel wool of greater average length and allow shaving with much residual wires. more reduced, which saves material.

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

The ferritic steel according to the invention comprising malleable inclusions can be polished with much more ease in order to obtain an improved polished surface state.

Claims (8)

1.- Stainless steel of ferritic structure and with improved machinability which can be used, in particular in the field of bar turning, characterized in that it comprises in its composition:
C <0.17%
If ≦ 2.0%
Mn ≦ 2.0%
Cr [11 - 20]%
Ni ≦ 0.1%
S ≦ 0.55%
Ca ≧ 30 10⁻⁴%
O ≧ 70 10⁻⁴%
The ratio between the calcium and oxygen content Ca / O being 0.2 ≦ Ca / O ≦ 0.6. 2.- Steel of ferritic structure according to claim 1, characterized in that it comprises in its composition:
c <0.1%
If <
Mn <
Cr [15 - 19]%
Ni <1%
S ≦ 0.55%
Ca ≧ 35 10⁻⁴%
0 ≧ 70 10⁻⁴%
the ratio between the calcium and oxygen Ca / O content satisfying the relationship $$\text{0.35 ≦ Ca / O ≦ 0.6}$$

3.- Stainless steel of ferritic structure according to claims 1 and 2 characterized in qui'l comprises in its composition: - C ≦ 0.08% - If ≦ 2.0% - Mn ≦ 2.0% - Cr [15 - 19]% - Ni <1% - S ≦ 0.55% - Ca ≧ 35 10⁻⁴% - O ≧ 70 10⁻⁴% the relationship between calcium and oxygen Ca / O satisfying the relation 0.35 / Ca / O / 0.6. 4.- Ferritic structure steel according to claims 1 and 3, characterized in that it comprises less than 0.035% sulfur. 5.- Steel of ferritic structure according to claims 1 and 3, characterized in that it comprises between 0.15% and 0.45% of sulfur. 6.- Steel of ferritic structure according to claims 1 to 3 characterized in that it comprises between 0.05% and 0.15% of sulfur. 7.- Steel of ferritic structure according to claims 1 to 6, characterized in that it further comprises less than 3% molybdenum. 8.- Ferritic structure steel according to claims 1 to 7, characterized in that it contains inclusions of silico-aluminale of lime of the anorthite and / or pseudo-wollastonite and / or gehlenite type.

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