EP0629714B1 - Martensitic stainless steel with improved machinability - Google Patents

Abstract

Martensitic stainless steel with improved machinability, characterised in that its composition by weight is the following: - carbon lower than 1.2 % - silicon lower than or equal to 2 % - manganese lower than or equal to 2 % - chromium: 10.5 < Cr < 19 % - sulphur lower than or equal to 0.55 % - calcium higher than 32 x 10<-4> % - oxygen higher than 70 x 10<-4> %, - the ratio of the calcium and oxygen contents Ca/O being 0.2 < Ca/O < 0.6, the said steel being subjected to at least one quenching heat treatment in order to give it a martensitic structure.

Description

The present invention relates to a steel stainless steel of the martensitic type with improved machinability.

We designate by stainless steels the iron alloys containing at least 10.5% chromium.

• les aciers martensitiques,
• les aciers ferritiques,
• les aciers austénitiques,
• les aciers austéno-ferritiques.

Other elements enter into the composition of the steel in order to modify the structure and the properties of the alloy. The main four are:

• martensitic steels,
• ferritic steels,
• austenitic steels,
• austenitic-ferritic steels.

Martensite steels generally include 12 to 18% chromium and carbon contents which can go up to about 1%. Many alloying elements, such as Ni, Mo, Si, Ti, V, Nb ... allow a wide range properties and lead to applications too varied as: mechanical engineering, tools, cutlery, hot oxides ...

Their originality is to combine good resistance to corrosion mainly due to chromium, high mechanical characteristics which can be explained by the martensitic structure.

• les nuances au chrome-carbone sans nickel. Les caractéristiques recherchées sont la dureté, la résistance à la corrosion, la polissabilité.
• les nuances à 16% de chrome plus nickel. La présence de chrome leur confère une bonne résistance à la corrosion, le nickel (2 à 4%) permet d'obtenir une structure martensitique après trempe.
• les nuances à durcissement structural. Elles ont une excellente résistance à la corrosion avec de hautes caractéristiques mécaniques.
• les 12% de chrome améliorées (addition d'éléments comme le vanadium, le molybdène, le tungstène, le silicium, le niobium, le titane...). Le but est d'optimiser une ou plusieurs propriétés d'emploi du matériau, comme résistance à chaud, le fluage, le résilience, la résistance à la corrosion...

There is a wide range of martensitic stainless steels, with very varied compositions and properties of use. Among the most common nuances, we can cite:

• nickel-free chrome-carbon grades. The characteristics sought are hardness, corrosion resistance, polishability.
• shades with 16% chromium plus nickel. The presence of chromium gives them good resistance to corrosion, nickel (2 to 4%) makes it possible to obtain a martensitic structure after quenching.
• structural hardening grades. They have excellent resistance to corrosion with high mechanical characteristics.
• 12% improved chromium (addition of elements such as vanadium, molybdenum, tungsten, silicon, niobium, titanium ...). The aim is to optimize one or more properties of use of the material, such as hot resistance, creep, resilience, corrosion resistance ...

For all these nuances, the structure of the final product and its mechanical characteristics depend largely heat treatments. The three common treatments are quenching, tempering and softening annealing.

The purpose of hardening is to give the steel a martensitic structure and very high hardness.

The income increases the ductility which is very weak after quenching and softening annealing allows to obtain a metal which can undergo sophisticated implementation operations, such as certain modes of machining or forming.

All treatments are defined according to shade composition (temperature adjustment income, its duration, the type of cooling ...).

Martensitic stainless steels are difficult to machine. Several reasons explain this state of affairs.

Indeed, their great hardness causes a mechanical fatigue of tools under stress very important cutting and can see their limit of rupture exceeded.

In addition, the friction forces added to poor thermal conductivity will induce high temperatures at the interface tool / material, resulting in thermal fatigue and degradation by broadcast.

On the other hand, the splitting domains chips are often reduced.

Finally, the presence of hard oxides such as alumina or chromite is an aggravating factor cutting tools.

Tool wear therefore has origins different for martensitic steels (hardness high, high friction) than for steels austenitics (hardenability, poor conductivity thermal bad chip fractionation).

Many channels are used to improve machinability, but all have drawbacks.

The addition of sulfur, which will form with the manganese sulfides, sometimes substituted for chromium, deteriorates corrosion resistance, deformability hot and cold, weldability, as well as mechanical characteristics in cross direction.

The addition of selenium serves to complement the sulfur, it tends to globulate sulphides and improves this makes the mechanical characteristics in the cross direction. In addition to the cost, this element is highly toxic.

The addition of tellurium also makes it possible to globulate the sulphides, and therefore tends to reduce the anisotropy of the steel, in particular the anisotropy of its mechanical properties. It also improves, in itself, the machinability but has the drawback of reducing the ability to be processed hot. For this reason, its use is limited. FR-A-2456785 describes a free-cutting steel containing inclusions of type A and B, inclusions A consist of an element from the group: Pb, Bi, MnS-TeS, SiO ₂ -K ₂ O, SiO ₂ -K ₂ O, SiO ₂ -Na ₂ O, SiO ₂ -K ₂ O-Al ₂ O ₃ , SiO ₂ -Na ₂ O and SiO ₂ -Na ₂ O- CaO-MnO and inclusions B consist of an element of the group: MnS , MnSe and Mn (S, Se).

The addition of lead which is insoluble in steel appears as spherical nodules, but this element has the disadvantage of being toxic and degrade forgeability.

We know by FR-A-2 648 477 (EP-A-40332), a steel resulfurized austenitic with improved machinability containing in its weight composition a proportion of calcium and oxygen which improves the machinability. Document JP-A-55-122858 describes the use of gehenite in high Mn steel with improved machinability.

However, it is well known that stainless steels austenitics are difficult to machine, in large part because of their low thermal conductivity, hence a poor flow of the heat produced at the tip of a cutting tool and rapid deterioration of the tool and their high inducing hardenability locally areas of high hardness.

During the machining of steel, due to high cutting temperatures, these inclusions play a lubricant role at the interface - steel to be machined - tool cutting, thus leading to reduced tool wear cutting and better surface appearance of the parts machined.

In addition, in the field of machining, the austenitic steels do not require treatment significant thermal likely to change the state physicochemical of steel and inclusions.

Martensitic steels are hardenable and have the characteristics of being of high hardness. Of this fact, the problem of the difficulty of the machining is not not fully resolved.

The invention aims to reduce the difficulties encountered in machining steels martensitic, while retaining their properties of hot and cold deformability or forgeability, their mechanical characteristics and their peculiarities heat treatments.

• carbone inférieur à 1,2%
• silicium inférieur ou égal à 2%
• manganèse inférieur ou égal à 2%
• chrome : 10,5<Cr<19%
• soufre inférieur ou égal à 0,55%
• calcium supérieur à 32.10^-4%
• oxygène supérieur à 70.10^-4%
• le rapport de la teneur en calcium et en oxygène Ca/O étant 0,2<Ca/O<0,6, ledit acier étant soumis à au moins un traitement thermique de trempe pour lui conférer une structure martensitique.

The subject of the invention is a martensitic steel with high machinability, which is characterized by the following weight composition:

• carbon less than 1.2%
• silicon less than or equal to 2%
• manganese less than or equal to 2%
• chromium: 10.5 <Cr <19%
• sulfur less than or equal to 0.55%
• calcium greater than 32.10 ^-4 %
• oxygen greater than 70.10 ^-4 %
• the ratio of the calcium and oxygen content Ca / O being 0.2 <Ca / O <0.6, said steel being subjected to at least one quenching heat treatment to give it a martensitic structure.

• l'acier comprend de façon préferentielle du soufre dans une proportion inférieure ou égale à 0,035%,
• l'acier comprend de façon préferentielle du soufre dans une proportion 0,15%<S<0,45%, ledit acier étant resulfuré,
• l'acier comprend, en outre, du nickel dans une proportion inférieure ou égale à 6%,
• l'acier comprend, en outre, du molybdène dans une proportion inférieure ou égale à 3%,
• l'acier comprend, en outre, dans sa composition pondérale des éléments choisis parmi le tungstène, le cobalt, le niobium, le titane, le tantale, le zirconium, le vanadium, le molybdène dans les proportions pondérales suivantes :
  • tungstène inférieur ou égal à 4%
  • cobalt inférieur ou égal à 4,5%
  • niobium inférieur ou égal à 1%
  • titane inférieur ou égal à 1%
  • tantale inférieur ou égal à 1%
  • zirconium inférieur ou égal à 1%
  • vanadium inférieur ou égal à 1%
  • molybdène inférieur ou égale à 3%,
• outre l'acier comprend du nickel dans une proportion 2%<Ni<6% et du cuivre dans une proportion de 1%<Cu<5%
• l'acier contient des inclusions de silicoaluminate de chaux du type anorthite et/ou pseudo-wollastonite et/ou géhlénite.

According to the invention:

• the steel preferably comprises sulfur in a proportion less than or equal to 0.035%,
• the steel preferably comprises sulfur in a proportion 0.15% <S <0.45%, said steel being resulfurized,
• the steel also comprises nickel in a proportion less than or equal to 6%,
• the steel also comprises molybdenum in a proportion less than or equal to 3%,
• the steel furthermore comprises, in its weight composition, elements chosen from tungsten, cobalt, niobium, titanium, tantalum, zirconium, vanadium, molybdenum in the following weight proportions:
  • tungsten less than or equal to 4%
  • cobalt less than or equal to 4.5%
  • niobium less than or equal to 1%
  • titanium less than or equal to 1%
  • tantalum less than or equal to 1%
  • zirconium less than or equal to 1%
  • vanadium less than or equal to 1%
  • molybdenum less than or equal to 3%,
• besides the steel includes nickel in a proportion 2% <Ni <6% and copper in a proportion of 1% <Cu <5%
• the steel contains inclusions of lime silicoaluminate of the anorthite and / or pseudo-wollastonite and / or gehenite type.

• la figure 1 représente sur diagramme ternaire SiO₂ - CaO - Al₂O₃ donnant les compositions des oxydes introduit dans l'acier selon l'invention,
• la figure 2 montre des courbes représentant l'évolution de l'usure d'un outil pour différents exemples donnés.

The tests described below and the appended figures will make the invention better understood.

• FIG. 1 represents on a ternary diagram SiO ₂ - CaO - Al ₂ O ₃ giving the compositions of the oxides introduced into the steel according to the invention,
• FIG. 2 shows curves representing the evolution of the wear of a tool for various examples given.

Martensitic steels have compositions and above all a completely different structure, compared to steels, for example austenitic. The behavior of martensitic steels during machining are related to specific issues.

A change in the composition of steels martensitics does not safely keep their properties or even more to improve them.

Martensitic steels are hardenable and have the characteristics of being of high hardness.

These steels are metallurgically very different from austenitic steels. On the one hand, they can undergo quenching and crystal structure obtained cold on these steels is not comparable to the austenitic structure.

On the other hand, the development of steels martensitics differs in many respects from that of austenitic steels.

In particular, heat treatments are numerous on the former and give metal its job characteristics. Quenching (cooling fast from a high temperature below a Martensitic transformation start temperature Ms which depends on the composition of the steel) makes it possible to obtain, from a hot austenitic structure, a martensitic structure. It is generally followed an income (maintaining at an intermediate temperature dependent on steel) which increases ductility which is very weak after quenching.

Certain grades of martensitic steels undergo softening treatments. This last one is used when the metal has to undergo operations of sophisticated implementation, such as certain modes machining or forming. The metal structure is not then no longer a martensitic structure, but a structure ferritic with chromium carbides at the level of grain boundaries.

However, it regains its martensitic structure and its mechanical characteristics after appropriate heat treatments.

Finally, the chemical composition of steels martensitics is very different from that of steels austenitic which is also partly explained by the need to have the temperature at the start of transformation Martensitic Ms high enough. They don't contain little nickel (minus 6%), low chromium for stainless steels (from 11 to 19% chrome).

According to the invention, martensitic steel is characterized by its weight composition as defined in claim 1.

Unexpectedly, by introducing malleable oxides in a martensitic composition it It turned out that the chosen oxides, i.e. lime silicoaluminates of the anorthite type and / or pseudowollastonite and / or gehenite represented on the ternary diagram of figure 1, keep the main properties of martensitic steel after heat treatments that said steel undergoes, without degradation of mechanical properties and improving notably the machinability properties,

However, the inclusions of malleable oxides have no favorable action for machinability only because let the matrix lend itself to it.

The Applicant was surprised to note that in a matrix of structure as different as the structure of martensitic steels, these oxides have also a beneficial effect on machinability.

Furthermore, it was not clear that, because differences in development, the Applicant arrives at get the same type of inclusions in steel.

The Applicant in particular had the surprised to find that the heat treatments changed nothing about the nature of the inclusions.

It does not happen, or at least does not significant change in composition analysis of inclusions, among other things by diffusion to the solid state and this during heat treatments that martensitic steels undergo.

The problems of machining steels martensitics are also very different from the problems posed by austenitic steels.

Unlike these, they are not workable and their thermal conductivity is not also bad.

On the other hand, the main problem of steels Martensitics for machining is hardness.

There was nothing to suggest that inclusions identical can have a beneficial effect then that the machining problems had causes if different.

It turned out that when machining the martensitic steels, malleable oxides, machining temperatures of these steels are sufficiently permanently heated to form a lubricating film regenerated by the inclusions of oxides present in the metal. This lubricating film reduces friction material on the tool. So the effect of the heavy load due to the high hardness of the material finds it reduced.

Two families of martensitic steels have been tested, one including in its weight composition sulfur in a proportion between 0.15 and 0.45%, the other including in its weight composition sulfur in a proportion less than 0.035%.

It has been noted that the presence of oxides malleable in steel does not affect resistance to corrosion, either by pitting or cavernous, too good for the low sulfur composition as in the resulfurized composition.

Generally speaking, the gain in machinability is not done, in any case, to the detriment of characteristics such as forgeability or hot or cold deformability.

It has also been noted, that oxides introduced keep their properties whatever the heat treatment performed.

According to the invention, the introduction of oxides malleable is done without taking into account the carbon content added with nitrogen whose decrease, tends as it is proven, to reduce mechanical characteristics.

The invention also relates to a steel martensitic to which it was added, in its weight composition of 2 to 6% nickel and 1 to 5% copper or even less than 3% molybdenum.

Nickel is needed in steels containing more than 16% chromium to obtain, after quenching, a martensitic structure.

In the grade known as structural hardening, nickel, in addition to its role previously mentioned (reduction in the quantity of delta ferrite), will form with copper the "NI ₃ Cu" phase which will harden the metal. Curing is not obtained here only by carbon, which, moreover, is relatively low.

Copper allows in combination with metal to obtain a structural hardening and therefore to increase mechanical characteristics.

Molybdenum improves resistance to corrosion and has a beneficial effect on hardness after income and it also improves resilience.

• tungstène inférieur ou égal à 4%
• cobalt inférieur ou égal à 4,5%
• niobium inférieur ou égal à 1%
• titane inférieur ou égal à 1%
• tantale inférieur ou égal à 1%
• zirconium inférieur ou égal à 1%.

The martensitic steel according to the invention may also contain stabilizing elements chosen from tungsten, cobalt, niobium, titanium, tantalum, zirconium in the following weight proportions:

• tungsten less than or equal to 4%
• cobalt less than or equal to 4.5%
• niobium less than or equal to 1%
• titanium less than or equal to 1%
• tantalum less than or equal to 1%
• zirconium less than or equal to 1%.

Ca = 30.10^-4%

0 = 129.10^-4%

le rapport de la teneur en calcium et un oxygène Ca/O étant égal à 0,22.In an example of application, a martensitic steel A according to the invention, the composition of which is as follows: VS Yes Mn STEEL A 0.205 0.462 0.52 Cr Mo S P NOT 12.34 0.041 0.024 0.022 0.046 in which it is introduced:

Ca = 30.10 ^-4 %

0 = 129.10 ^-4 %

the ratio of calcium content and Ca / O oxygen being 0.22.

In this example, steel A contains, as residual, less than 0.5% nickel and less than 0.2% copper.

This steel was compared to two reference steels, the compositions of which are as follows: VS Yes Mn Or Ref. 1 0.184 0.359 0.530 0.180 Ref. 2 0.194 0.364 0.731 0.313 Cr Mo Cu S P NOT 12.63 0.135 0.084 0.022 0.018 0.056 12.77 0.093 0.088 0.002 0.017 0.049

All three steels have undergone machinability tests shooting.

Filming is carried out with inserts solid carbide, test designated by Vb 30 / 0.3, which is to determine the speed at which wear clearance is 0.3mm after 30 minutes of machining and also, with coated carbide inserts, test designated by Vb 15 / 0.15 which consists in determining the speed for which the wear and tear is 0.15 mm after 15 minutes of machining.

It can be seen in table 1 below that the mechanical properties are in no way altered by the introduction of inclusions of malleable oxides for two thermal softening treatments, that is to say comprising an oil quenching at 950 ° C, holding for four hours at 820 ° C, slow cooling to 650 ° C, then air cooling and "treated" that is to say having undergone quenching at 950 ° C, an income at 640 ° C and air cooling. SHADE HEAT TREATMENT Rm MPa INV AT SOFTENED 535 REF 2 SOFTENED 544 REF 1 SOFTENED 544 INV AT TREATY 858 REF 2 TREATY 967 REF 1 TREATY 899 Rp0.2 MPa AT% Z% HARDNESS HRB / HRC 282 29 82 296 29.2 64.1 82.3 HRB 280 28.6 60.6 80.6 HRB 737 14 51 837 12 52.6 29.1 HRC 754 15.5 55.8 27.3 HRC

Tests have shown that so-called steels "treated" can be machined better than softened steels.

In another example of application, a martensitic steel according to the invention and the only weight composition of which is as follows: VS Yes Mn Cr Mo STEEL B 0.196 0.444 0.555 12.10 0.073 S P NOT It O Ca / O SteelB 0.0263 0.019 0.053 41.10 ^-4 99.10 ^-4 0.41

In this example, steel B contains, as residual, less than 0.5% nickel and less than 0.2% copper.

This steel was compared to a standard reference steel which does not contain malleable oxides in its composition and whose composition is as follows: VS Yes Mn Or Cr Mo REF 3 0.214 0.344 0.564 0.354 12.32 0.097 Cu S P NOT It O Ca / O _REF 3 0.106 0.261 0.017 0.054 45.10 ^-4

It is noted in table 2 below that the mechanical characteristics compared between the reference steel 3 and the steel B according to the invention do not show significant differences both in the case of a softened state than treated. REF. 3 STEEL B SOFTENED TREATY SOFTENED TREATY Rm (MPa) 559 803 566 787 Rp0.2 (MPa) 418 636 408 600 AT% 29 18.7 29 19 Z% 67.5 60.5 67 63

Table 3 below presents characteristic values of the machining tests and shows that the steels treated according to the invention give a machinability gain of 25 to 30%. METALLURGICAL CONDITION TREATY SOFTENED test: Vb 30 / 0.3 (m / min) Vb 15 / 0.15 (m / min) Vb 30 / 0.3 (m / min) Vb 15 / 0.15 (m / min) Steel ref 1 195 250 - - Steel ref 2 150 205 - - Steel ref 3 230 250 200 220 Steel A 250 - - - Steel B 250 290 - -

In a third example of application, two martensitic steels C and D according to the invention, the compositions of which are as follows: VS Yes Mn Or Cr Mo Steel C 0.018 0.443 0.825 4.517 15.2 0.005 Steel D 0.012 0.448 0.818 3,739 15.37 0.005 Cu P NOT Nb S.10 ^-4 Ca.10 ^-4 0.10 ^-4 3.189 0.01 0.018 0.202 110 65 132 3,236 0.01 0.021 0.192 233 70 157

Steels C and D were compared with reference steels, not containing malleable oxides and whose weight compositions are as follows: VS Yes Mn Or Cr Mo Ref. 4 0.011 0.45 0.815 4.548 15.26 0.006 Ref. 5 0.013 0.405 0.878 4.509 15.26 0.006 Cu P NOT Nb S.10 ^-4 Ca.10 ^-4 0.10 ^-4 3.245 0.011 0.017 0.182 270 <5 138 3,228 0.011 0.016 0.202 110 <5 48

These reference steels contain in their composition of copper and nickel and are part of structural hardening grades.

• l'état trempé : trempe huile à 1050°C, puis revenu à 250°C. Rm 1000 MPa,
• l'état vieilli, dans lequel le métal a sa dureté maximale : trempe 1050°C, puis revenu vers 450°C. Rm 1400 MPa
• l'état adouci : trempe 1050°C, revenu à 760°C pendant 4 heures, deuxième revenu vers 620°C. Rm 900 MPa

Three metallurgical states corresponding to different heat treatments are commonly encountered:

• quenched state: quenched oil at 1050 ° C, then returned to 250 ° C. Rm 1000 MPa,
• the aged state, in which the metal has its maximum hardness: quenching 1050 ° C, then returned to 450 ° C. Rm 1400 MPa
• the softened state: quenching 1050 ° C, tempering at 760 ° C for 4 hours, second tempering around 620 ° C. Rm 900 MPa

The peculiarity of this type of shade is that it does not undergo dimensional variations following heat treatments. It can therefore be machined, then aged.

The steel D according to the invention was treated by machining in the quenched state. That is to say that he underwent a soak at 1050 ° C in oil. He appeared, as represented on the curves of figure 2, that the presence of malleable oxides increased machinability well which can be seen on the curves by the decrease tool wear. This wear does indeed pass 0.15 mm after 15 minutes of machining at a speed of 190 m / min, an advance of 0.15 mm / revolution, a depth of cut 1.5 mm for steel reference 4, with wear of 0.125 mm for steel D.

Steel D according to the invention made it possible to obtain in the softened state a cutting speed of 240 m / min while reference steel 5 allowed a speed of 210 m / min cut. The gain recorded is 20%.

It is well highlighted with these different application examples, that martensitic steels containing oxides in their composition malleable have improved machinability, oxides do not not deteriorating the other characteristics of said steels.

Claims (4)

1. Martensitic stainless steel with improved machinability, characterized in that its composition (% weight) is as follows:

   carbon less than 1.2%

   silicon not more than 2%

   manganese not more than 2%

   chromium: 10.5<Cr<19%

   sulphur not more than 0.55%

   calcium more than 32.10^-4%

   oxygen more than 70.10^-4%

   nickel not more than 6%

   molybdenum not more than 3%

   elements chosen from tungsten, cobalt, niobium, titanium, tantalum, zirconium, vanadium in the following proportions:

   tungsten not more than 4%

   cobalt not more than 4.5%

   niobium not more than 1%

   titanium not more than 1%

   tantalum not more than 1%

   zirconium not more than 1%

   vanadium not more than 1%

   furthermore, copper in a proportion 1%<Cu<5% with the condition 2%<Ni<6%>
   and inclusions of calcium aluminium silicate of gehlenite and/or pseudowollastonite and/or anorthite type,
   the rest being constituted by iron and unavoidable impurities,
   the ratio between calcium content and oxygen content Ca/O being 0.2<Ca/O<0.6, the said steel being subjected to at least one hardering heat treatment to give it a martensitic structure.
2. Steel according to claim 1, characterized in that it includes sulphur in a proportion not more than 0.035%.
3. Steel according to claim 1, characterized in that it includes sulphur in a proportion 0,15%<S<0,45%, the said steel being re-sulphurated.
4. Steel according to claim 1, characterized in that it includes nickel in a proportion 2%<Ni<6% and copper in a proportion 1%<Cu<5%.

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