EP1896624B1 - Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part - Google Patents
Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part Download PDFInfo
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- EP1896624B1 EP1896624B1 EP06778669A EP06778669A EP1896624B1 EP 1896624 B1 EP1896624 B1 EP 1896624B1 EP 06778669 A EP06778669 A EP 06778669A EP 06778669 A EP06778669 A EP 06778669A EP 1896624 B1 EP1896624 B1 EP 1896624B1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 124
- 239000010959 steel Substances 0.000 title claims abstract description 124
- 238000000034 method Methods 0.000 title claims abstract description 19
- 239000000203 mixture Substances 0.000 title claims abstract description 18
- 229910001105 martensitic stainless steel Inorganic materials 0.000 title claims abstract description 6
- 230000009466 transformation Effects 0.000 claims description 30
- 230000007797 corrosion Effects 0.000 claims description 26
- 238000005260 corrosion Methods 0.000 claims description 26
- 238000011282 treatment Methods 0.000 claims description 22
- 238000001816 cooling Methods 0.000 claims description 13
- 238000010438 heat treatment Methods 0.000 claims description 12
- 238000010791 quenching Methods 0.000 claims description 12
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 10
- 230000000171 quenching effect Effects 0.000 claims description 10
- 238000004519 manufacturing process Methods 0.000 claims description 8
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims description 5
- 235000011089 carbon dioxide Nutrition 0.000 claims description 5
- 239000012535 impurity Substances 0.000 claims description 5
- 238000000265 homogenisation Methods 0.000 claims description 4
- 239000013067 intermediate product Substances 0.000 claims description 4
- 238000005496 tempering Methods 0.000 claims description 3
- 229910052742 iron Inorganic materials 0.000 claims description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 59
- 239000011651 chromium Substances 0.000 description 40
- 239000010936 titanium Substances 0.000 description 34
- 229910000734 martensite Inorganic materials 0.000 description 27
- 239000012071 phase Substances 0.000 description 25
- 229910052750 molybdenum Inorganic materials 0.000 description 23
- 229910052759 nickel Inorganic materials 0.000 description 22
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 20
- 229910001566 austenite Inorganic materials 0.000 description 20
- 239000011572 manganese Substances 0.000 description 20
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 19
- 229910052804 chromium Inorganic materials 0.000 description 19
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 16
- 239000011733 molybdenum Substances 0.000 description 16
- 229910052719 titanium Inorganic materials 0.000 description 15
- 238000001556 precipitation Methods 0.000 description 14
- 239000000243 solution Substances 0.000 description 14
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 13
- 230000032683 aging Effects 0.000 description 13
- 229910052782 aluminium Inorganic materials 0.000 description 13
- 229910052739 hydrogen Inorganic materials 0.000 description 13
- 239000001257 hydrogen Substances 0.000 description 13
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 12
- 230000000694 effects Effects 0.000 description 12
- 229910052748 manganese Inorganic materials 0.000 description 12
- 229910000859 α-Fe Inorganic materials 0.000 description 12
- 239000010949 copper Substances 0.000 description 11
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 10
- 238000007792 addition Methods 0.000 description 10
- 238000007711 solidification Methods 0.000 description 10
- 230000008023 solidification Effects 0.000 description 10
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 9
- 230000009931 harmful effect Effects 0.000 description 9
- 239000010941 cobalt Substances 0.000 description 8
- 229910017052 cobalt Inorganic materials 0.000 description 8
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 description 8
- 229910052757 nitrogen Inorganic materials 0.000 description 8
- 238000004090 dissolution Methods 0.000 description 7
- 230000000930 thermomechanical effect Effects 0.000 description 7
- 229910052721 tungsten Inorganic materials 0.000 description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 6
- 229910052799 carbon Inorganic materials 0.000 description 6
- 230000008569 process Effects 0.000 description 6
- 239000000047 product Substances 0.000 description 6
- 239000002994 raw material Substances 0.000 description 6
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 5
- 238000005204 segregation Methods 0.000 description 5
- 229910052717 sulfur Inorganic materials 0.000 description 5
- 239000011593 sulfur Substances 0.000 description 5
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 4
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 4
- 239000000956 alloy Substances 0.000 description 4
- 238000005275 alloying Methods 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000000576 coating method Methods 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 238000005336 cracking Methods 0.000 description 4
- 229910052751 metal Inorganic materials 0.000 description 4
- 239000002184 metal Substances 0.000 description 4
- 229910052710 silicon Inorganic materials 0.000 description 4
- 239000010703 silicon Substances 0.000 description 4
- 230000035882 stress Effects 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 238000006467 substitution reaction Methods 0.000 description 4
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 4
- 229910000943 NiAl Inorganic materials 0.000 description 3
- 240000008042 Zea mays Species 0.000 description 3
- 239000012736 aqueous medium Substances 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000005242 forging Methods 0.000 description 3
- 150000002431 hydrogen Chemical class 0.000 description 3
- 239000007788 liquid Substances 0.000 description 3
- 239000011159 matrix material Substances 0.000 description 3
- 230000007246 mechanism Effects 0.000 description 3
- 238000002360 preparation method Methods 0.000 description 3
- 239000011265 semifinished product Substances 0.000 description 3
- 239000007787 solid Substances 0.000 description 3
- 238000000844 transformation Methods 0.000 description 3
- 239000010937 tungsten Substances 0.000 description 3
- 229910000851 Alloy steel Inorganic materials 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 2
- 229910000604 Ferrochrome Inorganic materials 0.000 description 2
- 241001080024 Telles Species 0.000 description 2
- 239000000654 additive Substances 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 229910052793 cadmium Inorganic materials 0.000 description 2
- BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 230000018109 developmental process Effects 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 230000002349 favourable effect Effects 0.000 description 2
- 230000006698 induction Effects 0.000 description 2
- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 2
- 239000002609 medium Substances 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 230000002829 reductive effect Effects 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- 230000003313 weakening effect Effects 0.000 description 2
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- 208000035126 Facies Diseases 0.000 description 1
- 229910000760 Hardened steel Inorganic materials 0.000 description 1
- 229910000922 High-strength low-alloy steel Inorganic materials 0.000 description 1
- 229910001240 Maraging steel Inorganic materials 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- -1 TiN nitrides Chemical class 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 235000015115 caffè latte Nutrition 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 239000010962 carbon steel Substances 0.000 description 1
- 150000001805 chlorine compounds Chemical class 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 239000002826 coolant Substances 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 230000003292 diminished effect Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000005496 eutectics Effects 0.000 description 1
- 230000002431 foraging effect Effects 0.000 description 1
- 239000011229 interlayer Substances 0.000 description 1
- 230000000670 limiting effect Effects 0.000 description 1
- 229910001338 liquidmetal Inorganic materials 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- VCTOKJRTAUILIH-UHFFFAOYSA-N manganese(2+);sulfide Chemical class [S-2].[Mn+2] VCTOKJRTAUILIH-UHFFFAOYSA-N 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 230000007935 neutral effect Effects 0.000 description 1
- QJGQUHMNIGDVPM-UHFFFAOYSA-N nitrogen group Chemical group [N] QJGQUHMNIGDVPM-UHFFFAOYSA-N 0.000 description 1
- 239000003921 oil Substances 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 230000000704 physical effect Effects 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
- 230000000135 prohibitive effect Effects 0.000 description 1
- 230000001681 protective effect Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 230000002787 reinforcement Effects 0.000 description 1
- 230000003252 repetitive effect Effects 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 238000007493 shaping process Methods 0.000 description 1
- 239000002893 slag Substances 0.000 description 1
- 239000007790 solid phase Substances 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 239000003381 stabilizer Substances 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 238000007619 statistical method Methods 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 150000003467 sulfuric acid derivatives Chemical class 0.000 description 1
- 229910021653 sulphate ion Inorganic materials 0.000 description 1
- 230000004083 survival effect Effects 0.000 description 1
- 230000009897 systematic effect Effects 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 238000003878 thermal aging Methods 0.000 description 1
- 238000007666 vacuum forming Methods 0.000 description 1
- 238000009489 vacuum treatment Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/02—Hardening by precipitation
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/04—Hardening by cooling below 0 degrees Celsius
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a martensitic stainless steel, and in particular to an alloy steel containing mainly chromium, nickel, molybdenum and / or tungsten, titanium, aluminum and optionally manganese elements, and offering a unique combination of corrosion resistance and mechanical strength. high.
- Low-alloyed carbon martensitic steels that is to say none of the alloying elements exceeds 5% by weight
- quenched and tempered are most suitable when operating temperatures remain below their temperature. of income.
- those alloyed with silicon can withstand slightly higher operating temperatures because their tempering temperature to obtain the best compromise between the breaking strength (R m ) and the toughness (K 1C ) is typically located around 250 ° C. / 300 ° C.
- R m / K 1C compromises of the order of 1900 ⁇ MPa / 70 ⁇ MPa ⁇ m and 2000 ⁇ MPa / 60 ⁇ MPa ⁇ m where m is expressed in meters, are commonly obtained with these categories of steels, through an appropriate development that is now controlled with known industrial means.
- cadmium is a highly harmful element to the environment, and its use is severely controlled by certain regulations.
- the solid substrate remains intrinsically very sensitive to the fragile cracking favored by external hydrogen of any source.
- the aim of the steel composition of the invention is to solve these technical problems by proposing a martensitic stainless steel having an intrinsic resistance to corrosion in an atmospheric medium (marine or urban environment) for which the external source of hydrogen is eradicated , and simultaneously having a high tensile strength (of the order of 1800 MPa and more) and toughness equivalent to that of low alloyed carbon steel and very high strength.
- Said cryogenic treatment may be a quenching in dry ice.
- Said cryogenic treatment can be carried out at a temperature of -80 ° C for at least 4 hours.
- At least one homogenizing heat treatment may be carried out between 1200 and 1300 ° C. for at least 24 hours on the ingot or during its hot transformations into semi-finished product, but before the last of these hot transformations.
- the invention also relates to a mechanical part made of steel with high resistance to corrosion and mechanical strength, characterized in that it was obtained by the above method.
- the invention is based primarily on a composition of the steel as defined above. It has particular characteristics as Ni, Al, Ti, Mo, Cr and Mn that are or can be quite high.
- Thermomechanical treatments are also proposed, whereby the desired properties for the final metal are obtained.
- the steel of the invention allows a structural hardening by simultaneous precipitation of the secondary phases of ⁇ -NiAl type, ⁇ -Ni 3 Ti and optionally ⁇ -Fe 7 (Mo, W) 6 according to the effect called “maraging", this which gives, after thermal aging, ensuring precipitation, a very high level of mechanical strength of at least 1800 MPa, combined with good resistance to corrosion, in particular to stress corrosion in atmospheric corrosive environments.
- the steel of the invention has good resistance to heating and can therefore withstand temperatures up to 300 ° C for short durations and of the order of 250 ° C for long periods. Its sensitivity to hydrogen is lower than that of low alloyed steels.
- Very high strength steels are very sensitive to stress corrosion.
- the steel composition of the invention is such that the very origin of the stress corrosion fracture, which is the production of hydrogen by the corrosion mechanisms and then the embrittlement of the metal by internal diffusion of this hydrogen, is circumvented in atmospheric environments thanks to an outfit reinforced with corrosion in general.
- a minimum chromium content of 9 to 11% is necessary to give a steel a protection capacity against corrosion in a humid atmosphere, thanks to the formation on its surface of an oxide film. rich in chromium. But this protective film is insufficient in the case where the atmospheric medium is polluted by sulphate or chloride ions that can develop pitting corrosion and then crevice, both likely to provide hydrogen embrittlement.
- the molybdenum element has a very favorable effect on the reinforcement of the passive film with respect to corrosion in aqueous media polluted by chlorides or sulphates.
- the curing effect which gives a very high mechanical strength to the steel is obtained by precipitation of several hardening secondary phases during a thermal heat treatment of a completely martensitic structure.
- This martensitic structure prior to the income results from a preliminary solution treatment in the austenitic domain, then a cooling (or quenching) until a sufficiently low temperature so that all the austenite is transformed into martensite.
- the steel of the invention undergoes this hardening thanks to the precipitation of intermetallic prototype phases ⁇ -NiAl, ⁇ -Ni 3 Ti and possibly ⁇ -Fe 7 (Mo, W) 6 .
- the strongest hardening is achieved with the highest additions of aluminum, titanium and molybdenum.
- the nickel content must be very precisely adjusted so that the maximum hardening is obtained from a purely martensitic structure, without any residual ferrite or quench austenite.
- the steel of the invention has maximum ductility and toughness, which are obtained in particular by limiting at best the effects of anisotropy related to the solidification of ingots.
- the steel must be free of the ⁇ ferrite phase and the residual austenite phase after dissolution and cooling.
- the steel of the invention does not contain ferrite because its composition meets the conditions described below.
- the ferrite ⁇ formed transiently during the solidification of the steel of the invention can be completely resorbed during a heat treatment at high temperature and in the solid phase, for example between 1200 and 1300 ° C. , when : Cr eq / Neither eq ⁇ 1 , 05
- the structural homogeneity of the steel of the invention which is therefore dictated by the solidification conditions, is preferably optimized by means of homogenization heat treatments at very high temperatures, between 1200 and 1300.degree. longer than 24 hours, applied on the ingots and / or the intermediate products, that is to say on the half-products being processed hot. Such treatment should not, however, occur after the last hot transformation, otherwise we would end up with too large grain size before further processing.
- the best properties of the steel of the invention are obtained after being dissolved between 850 and 950 ° C., in the austenitic field, followed by cooling sufficiently energetic to allow the total transformation of the austenite. in martensite. This transformation must be total for two reasons.
- the width of the domain of the martensitic transformation of a high-alloy steel a range between the transformation start temperature Ms and the end-of-transformation temperature M f, is approximately 150 ° C., and that This area is all the larger as the structure of the steel is less homogeneous.
- the temperature Ms of a steel that is cooled to room temperature (about 25 ° C) from its austenitic dissolution range must be at least 175 ° C.
- the steel of the invention has a balanced composition such that the transformation temperature Ms is ⁇ 50 ° C, and preferably close to or greater than 70 ° C.
- its cooling to -80 ° C, or lower, in a cooling medium allows the transformation of austenite into martensite. This is made possible by searching for a temperature range Ms-Mf of at least 140 ° C., preferably at least 160 ° C., by the application, after the treatment. dissolving between 850 and 950 ° C, a cooling completed for example in dry ice at -80 ° C or lower, for a time sufficient to ensure complete cooling of the products and a complete transformation of the austenite in martensite.
- the steel of the invention must have a repetitive and reliable value of Ms which must satisfy the following relationship, a function of all the additive elements included in the steel and which have a significant influence on Ms, y. including the elements present in residual contents but whose effect is strong on the value of Ms.
- Chromium and molybdenum are the elements that give steel its good resistance to corrosion: molybdenum is also likely to participate, in addition, in hardening during the precipitation of the intermetallic phase Fe 7 Mo 6 .
- the molybdenum content is at least 1.5% to obtain the desired anticorrosion effect.
- the maximum content is 3%.
- the solvus temperature of a ét type molybdenum-rich intermetallic phase, stable at high temperature becomes greater than 950 ° C; in addition, in some cases, the solidification is completed by a eutectic system which produces massive intermetallic phases, rich in molybdenum, and whose subsequent solution requires solution temperatures above 950 ° C.
- the steel also contains tungsten, it will partially replace the molybdenum at the rate of one tungsten atom for two molybdenum atoms. In this case, the maximum limit of 3% applies to the sum Mo + (W / 2).
- the chromium and molybdenum contents must make it possible to obtain a pitting index of at least 16.5.
- the austenite content dispersed in the steel must be limited to a maximum of 10% to maintain very high mechanical strength: the nickel content is, in this perspective, a maximum of 14%; its preferred content between 10.5 and 12.5% is finally adjusted precisely using the two previously described relationships: Cr eq / Ni eq ⁇ 1.05; M s ⁇ 50 ° C;
- Aluminum is a necessary element for the hardening of steel; the desired maximum resistance levels (Rm ⁇ 1800 MPa) are only achieved with an addition of at least 1% aluminum, and preferably at least 1.2%. Aluminum strongly stabilizes ferrite ⁇ and the steel of the invention can not contain more than 2% of aluminum without appearance of this phase. Thus, the aluminum content is preferably limited to 1.6%, as a precaution, so as to take into account the analytical variations of the other elements which promote ferrite, and which are mainly chromium, molybdenum and titanium.
- Titanium just like aluminum, is a necessary element for the hardening of steel. It allows its hardening by precipitation of the phase ⁇ - Ni 3 Ti.
- the increase in titanium Rm strength is approximately 400MPa per percent titanium.
- the very high strength values referred to are obtained only when the sum Al + Ti is at least equal to 2.25% by weight.
- titanium very effectively binds the carbon contained in the steel in the form of TiC carbide, which makes it possible to avoid the harmful effects of free carbon as indicated below.
- solubility of the TiC carbide being quite low, it is possible to precipitate this carbide in a homogeneous manner in the steel during the final stages of the thermomechanical transformation at low temperatures in the austenitic domain of the steel: this avoids the intergranular weakening of the carbide.
- the titanium content must be between 0.5 and 1.5%, preferably between 0.75 and 1.25%.
- Cobalt in substitution for nickel in a proportion of 2% by weight of cobalt per 1% of nickel, is advantageous because it makes it possible to stabilize the austenite at the dissolution temperatures, while allowing the solidification of the steel to be maintained.
- of the invention according to the desired ferritic mode (it very weakly stabilizes the austenite at solidification temperatures): in this, cobalt widens the range of the compositions according to the invention as they are delimited by the Cr eq binding relationships and Neither eq.
- the substitution of 1% of nickel with 2% of cobalt makes it possible to record the starting point of the martensitic transformation of the steel as clearly as possible. be deduced from Ms.'s calculation formula
- cobalt gives the martensitic structure a stronger ability to respond to hardening; however, cobalt does not participate directly in precipitation hardening of the ⁇ - NiAl phase and does not have the ductilizing effect of nickel. On the contrary, it favors the precipitation of the ⁇ - FeCr weakening phase at the expense of the ⁇ - Fe 7 Mo 6 phase, which can have a hardening effect.
- cobalt is limited to 2%, preferably to 0.5% in the restricted range where all the properties of the steel of the invention can be acquired without resorting to the effects of cobalt.
- Tungsten can be added in substitution for molybdenum because it participates more actively in the hardening of the solid solution of martensite, and it is also likely to participate in the precipitation of the intermetallic phase type ⁇ -Fe 7 (Mo, W). ) 6 .
- Phosphorus tends to segregate at the grain boundaries, which reduces the adhesion of these joints and decreases the tenacity and ductility of the steels by intergranular embrittlement.
- a maximum content of 0.02%, preferably 0.01%, is not to be exceeded in the steel of the invention.
- Sulfur is known to induce strong embrittlement of high strength steels according to various modes such as intergranular segregation and precipitation of sulphide inclusions: the objective is therefore to minimize its content in the steel, according to the means. available. Very low sulfur contents are easily accessible in the raw materials with conventional refining means. It is therefore easy to meet the requirement of the steel of the invention which specifies that the required mechanical properties require a sulfur content of less than 0.0050%, preferably less than 0.0010% and ideally less than 0, 0005%, subject to an appropriate choice of raw materials.
- the nitrogen content must also be kept at the lowest possible value with the available means of elaboration, firstly to obtain the best ductility of the steel, and secondly to obtain the fatigue endurance limit. the highest possible, especially since the steel contains the titanium element. Indeed, in the presence of titanium, nitrogen forms insoluble cubic TiN nitrides which are extremely harmful by their shape and their physical properties. They constitute systematic primers of fatigue cracking.
- the industrial vacuum production method makes it possible to obtain residual nitrogen contents of between 0.0030 and 0.0100%, typically centered on 0.0050 to 0.0060% in the case of the steel of the invention. 'invention.
- the best solution for the steel of the invention is therefore to seek a residual nitrogen content as low as possible, less than 0.0060%.
- nitrogen contents of less than 0.0030% may be sought by the choice of raw materials and methods of preparation. specific.
- the maximum carbon content of the steel of the invention is limited to 0.025% at most, preferably 0.0120% at most.
- Copper which is a residual element found in commercial raw materials, must not be present at more than 0.5%, and preferably a final copper content of 0.25 or less is recommended. % in the steel of the invention. The presence of copper in larger quantities would unbalance the overall behavior of the steel: the copper easily tends to move the mode of solidification out of the desired range, and unnecessarily lowers the point of transformation Ms.
- Manganese and silicon are commonly present in steels, in particular because they are used as deoxidants of the liquid metal during conventional furnace processes where the liquid steel is in contact with the atmosphere.
- Manganese is also used in steels to fix free sulfur, extremely harmful, in the form of less harmful manganese sulphides. Since the steel of the invention has very low sulfur contents and that it is developed under vacuum, the elements manganese and silicon are from this point of view of any utility, and their contents can be limited to those of the raw materials.
- the silicon content must therefore be maintained at most 0.25%, preferably at most 0.10%.
- the manganese content can also be maintained within these same limits.
- Manganese widens the austenitic loop, and in particular it lowers the Ac1 temperature almost as much as nickel. Since, moreover, it has a lower effect of lowering Ms than nickel, it may be advantageous to replace part of the nickel with manganese to avoid the presence of ⁇ ferrite and help form reversion austenite when aging curing. This substitution must, of course, be done in compliance with the conditions on Cr eq / Ni eq and Ms as seen above. The maximum Mn content can thus be increased to 3%.
- the method of production of the steel must be adapted so that this content is well controlled.
- the oxygen present in the steel of the invention forms oxides that are detrimental to ductility and fatigue strength. For this reason, it is necessary to contain its concentration at the lowest possible value, that is to say at most 0.0050%, preferably below 0.0020%, which is permitted by the industrial means of preparation. under vacuum.
- the steel of the invention is evacuated according to conventional industrial practices by means of, for example, a vacuum induction furnace or a double vacuum forming phase, for example by forming and molding in a vacuum.
- a vacuum furnace of a first electrode then by at least one vacuum remelting operation of this electrode to obtain a final ingot.
- the development of an ingot may comprise a vacuum elaboration phase of an electrode in an induction furnace followed by a remelting phase according to the slag remelting process (ESR ); different ESR or VAR (vacuum arc reflow remelting) methods can be combined.
- Thermomechanical processes at high temperature allow easy shaping of molded ingots under usual conditions. These processes make it possible to obtain all kinds of semi-finished products with the steel of the invention (plates, bars, blocks, forged or stamped parts, etc.).
- a good structural homogeneity in the semi-finished products is preferably ensured by means of a homogenization heat treatment between 1200 and 1300 ° C., practiced before and / or during the range of thermomechanical hot transformations, but not after the last hot transformation to avoid that subsequent treatments take place on semi-products too large grain size.
- the products are then dissolved at a temperature of between 850 and 950 ° C., and the parts are then rapidly cooled to a final temperature of less than or equal to -75 ° C. uninterrupted below the transformation point Ms, possibly by placing an isothermal quenching stage above Ms.
- Ms point is low, it can easily be hot oil quenched at T ⁇ Ms. This allows to equalize the temperature in massive pieces and, above all, to avoid quenching taps due to the differential martensitic transformation between the surface of the massive pieces and the warm heart of the pieces.
- the martensitic transformation during the cryogenic passage occurs continuously.
- the temperature is of the order of -80 ° C. when this quenching is carried out in dry ice.
- the maintenance at low temperature is of sufficient duration to ensure complete cooling throughout the thickness of the parts. It typically lasts at least 4 hours at -80 ° C.
- the metal consisting of a ductile martensite and of low hardness, can be optionally cold-formed and, again, dissolved in order to achieve homogeneous properties.
- Table 1 groups together the compositions of the steels tested.
- Table 1 Composition of the steels tested References Invention AT B CD E F BOY WUT H I J VS % 0.0080 0.0040 0,013 ⁇ 0.0020 0.0091 0.0028 0.0120 0.0120 0.0044 0.0024 Yes % 0.073 ⁇ 0.030 ⁇ 0.030 ⁇ 0.030 0,021 0,038 0,036 0,038 ⁇ 0.03 0.033 Mn% ⁇ 0.030 ⁇ 0.030 ⁇ 0.030 ⁇ 0.050 0.016 0,019 0,023 ⁇ 0.03 ⁇ 0.030 Neither% 10.71 10.96 10.46 11.83 11.16 10.58 10.85 11,84 10.95 12.47 Cr% 11.53 11,44 10.75 11.63 11.36 11.40 10.89 9.00 10.35 10.00 Mo% 2.01 2.00 3.48 2.34 1.94 1.98 2.45 2.96 2.85 2.00 Al% 1.60 1.43 1.21 1.55 1.35 1.38 1.41 1.33 1.41 Ti% 0.322 0.605 0,321 1.00 1.03
- the reference samples have compositions which differ from the invention mainly on their too low titanium content (A and C) and / or on their sum Ti + Al too low (A, B, C) or on their point Ms too much low because less than 50 ° C (D).
- Sample C also has a molybdenum content that is too high.
- Table 2 Structural and mechanical characteristics of the steels tested. References Invention AT B VS D E F BOY WUT H I J Rm (MPa) 1778 1815 1690 1671 1888 1896 1920 1908 1947 1842 Rp0.2 (MPa) 1667 1710 1595 1439 1763 1800 9822 1795 1895 1661 Z (%) 59 61 61 61 53 56 53 55 50 51 KV (J) 15 14 35 20 9/13 6/7 8/9 8/8 6 - AT (%) 10.9 10.7 10.7 11.5 9.5 9.1 9.2 9.4 9.1 11.7 K 1c (TL) (MPa ) 85 70 101 - - - 46 - - 76
- the reference steel D of which only the value of Ms does not correspond to the invention, does not reach the desired level of hardening, whereas its sum Al + Ti satisfies the condition Al + Ti ⁇ 2.25. Indeed, it contains 16% residual austenite after the cryogenic treatment.
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Abstract
Description
La présente invention concerne un acier inoxydable martensitique, et en particulier un acier allié contenant principalement les éléments chrome, nickel, molybdène et/ou tungstène, titane, aluminium et éventuellement manganèse, et proposant une combinaison unique de résistance à la corrosion et de résistance mécanique élevées.The present invention relates to a martensitic stainless steel, and in particular to an alloy steel containing mainly chromium, nickel, molybdenum and / or tungsten, titanium, aluminum and optionally manganese elements, and offering a unique combination of corrosion resistance and mechanical strength. high.
Pour certaines applications critiques où les pièces mécaniques en acier sont soumises à des efforts très importants et pour lesquelles la masse de ces pièces est un facteur majeur, par exemple dans les domaines de l'aéronautique (caissons de trains d'atterrissage) ou de l'espace, on doit recourir à des aciers martensitiques à très haute résistance mécanique et, en outre, offrant encore une bonne ténacité telle que mesurée par l'essai de rupture brutale K1C.For certain critical applications where steel mechanical parts are subjected to very high forces and for which the mass of these parts is a major factor, for example in the fields of aeronautics (landing gear housings) or the space, it is necessary to use martensitic steels with very high mechanical strength and, in addition, still offering good toughness as measured by the brutal failure test K 1C .
Les aciers martensitiques au carbone faiblement alliés (c'est-à-dire dont aucun des éléments d'alliage ne dépasse 5% en masse), trempés et revenus, conviennent la plupart du temps lorsque les températures en service restent en dessous de leur température de revenu.Low-alloyed carbon martensitic steels (that is to say none of the alloying elements exceeds 5% by weight), quenched and tempered, are most suitable when operating temperatures remain below their temperature. of income.
Parmi ces aciers, ceux alliés au silicium peuvent supporter des températures en service un peu plus hautes car leur température de revenu pour obtenir le meilleur compromis entre la résistance à la rupture (Rm) et la ténacité (K1C) est typiquement située vers 250/300°C.Among these steels, those alloyed with silicon can withstand slightly higher operating temperatures because their tempering temperature to obtain the best compromise between the breaking strength (R m ) and the toughness (K 1C ) is typically located around 250 ° C. / 300 ° C.
Lorsque les températures en service dépassent ponctuellement ou de façon permanente ces valeurs, il faut recourir aux aciers « maraging » (martensitiques à bas carbone durcis par précipitation d'éléments intermétalliques), dont le revenu est effectué à 450°C ou plus en fonction du compromis Rm/K1C recherché.When operating temperatures temporarily or permanently exceed these values, it is necessary to use "maraging" steels (low carbon martensitic hardened by precipitation of intermetallic elements), whose income is made at 450 ° C or more depending on the compromise R m / K 1C sought.
Des compromis Rm/K1C de l'ordre de
Ces classes d'aciers sont extrêmement sensibles à ce qui est couramment dénommé « corrosion sous contrainte », mais qui est en fait l'une des formes de fragilisation par l'hydrogène externe produit par des réactions de corrosion superficielle (piqûres, corrosion intergranulaire en particulier). Le seuil de propagation de fissures dans ces aciers en présence de réactions de corrosion (K1CSC) est très inférieur à leur valeur de K1C; pour les aciers faiblement alliés traités au-delà de 1600MPa de Rm, la valeur de K1CSC présente une valeur minimale entre la température ambiante et 80°C qui est de l'ordre de
La sensibilité des aciers maraging non inoxydables, quoique moins marquée que dans les aciers peu alliés car la diffusion de l'hydrogène dans leur matrice très alliée est plus faible et les modes de piégeage de l'hydrogène sont apparemment moins nocifs, reste aussi très forte à des températures de l'ordre de 20 à 100°C qui correspondent à des phases d'utilisation en service.The sensitivity of non-stainless maraging steels, although less marked than in low alloyed steels because the diffusion of hydrogen in their highly alloyed matrix is lower and the hydrogen trapping modes are apparently less harmful, also remains very strong at temperatures of the order of 20 to 100 ° C which correspond to use phases in service.
Jusqu'à aujourd'hui, le seul moyen de protection contre ces phénomènes très dommageables était la protection des surfaces par des revêtements anticorrosion comme le cadmiage, qui est très utilisé en aéronautique. Ces revêtements posent cependant des problèmes importants.Until today, the only means of protection against these very damaging phenomena was the protection of surfaces by anticorrosion coatings such as cadmium, which is widely used in aeronautics. These coatings, however, pose significant problems.
En effet, ces revêtements sont sujets à l'écaillage et à la fissuration, ce qui impose une surveillance régulière et attentive de l'état de surface.Indeed, these coatings are prone to chipping and cracking, which requires regular and careful monitoring of the surface condition.
En outre, le cadmium est un élément fortement nocif vis-à-vis de l'environnement, et son usage est sévèrement contrôlé par certaines réglementations.In addition, cadmium is a highly harmful element to the environment, and its use is severely controlled by certain regulations.
Par ailleurs, les différentes opérations de revêtement de type chimique ou électrolytique dégagent de l'hydrogène qui est susceptible d'endommager irrémédiablement les pièces à protéger par le phénomène bien connu de « rupture retardée » ou de « fatigue statique » avant leur mise en service, et les méthodes de prévention sont très lourdes et coûteuses.In addition, the various chemical or electrolytic type coating operations release hydrogen, which is liable to irreparably damage the parts to be protected by the well-known phenomenon of "delayed failure" or "static fatigue" before being put into service. , and prevention methods are very cumbersome and expensive.
Dans tous les cas, le substrat massif reste intrinsèquement très sensible à la fissuration fragile favorisée par l'hydrogène externe de provenance quelconque.In all cases, the solid substrate remains intrinsically very sensitive to the fragile cracking favored by external hydrogen of any source.
Actuellement, aucun acier faiblement allié et à très haute résistance (Rm > 1900MPa) ne présente une valeur de K1CSC dans les milieux aqueux atmosphériques ou urbains qui approcherait la valeur de K1C mesurée en atmosphère neutre, et l'étude fine des mécanismes de propagation de fissures en présence d'hydrogène interne ou externe tendrait à prouver que les rapports K1CSC/K1C des aciers actuels à très haute résistance sont toujours très nettement inférieurs à l'unité, sauf en cas d'introduction dans ces aciers, d'éléments de la classe des platinoïdes. Ceux-ci agissent comme « repoussoir » de l'hydrogène, mais leur coût prohibitif interdit aujourd'hui leur utilisation comme éléments d'addition.At present, no low-alloy, very high-strength steel (R m > 1900MPa) has a K 1CSC value in atmospheric or urban aqueous media approaching the value of K 1C measured in a neutral atmosphere, and the detailed study of the mechanisms propagation of cracks in the presence of internal or external hydrogen would tend to prove that the K 1CSC / K 1C ratios of the current high-strength steels are still very much less than unity, except if they are introduced into these steels, of elements of the class of platinoids. These act as a "booster" for hydrogen, but their prohibitive cost now prohibits their use as additives.
Par ailleurs, il existe aussi des aciers maraging, à teneurs élevées en chrome (> 10% Cr) et qui sont considérés inoxydables en atmosphères « urbaines » ; un exemple d'acier représentatif de cette catégorie est décrit dans le document
Aucun de ces aciers maraging inoxydables actuellement connus ne permet cependant d'atteindre les niveaux de résistance mécanique qu'offrent les aciers maraging sans chrome et les aciers faiblement alliés, à savoir une résistance à la traction Rm de 1900MPa et plus.None of these currently known stainless maraging steels, however, allows to achieve the levels of mechanical resistance that offer chromium-free maraging steels and low-alloy steels, namely a tensile strength Rm of 1900MPa and more.
La composition d'acier de l'invention a pour but de résoudre ces problèmes techniques en proposant un acier inoxydable martensitique, ayant une résistance intrinsèque à la corrosion en milieu atmosphérique (environnement marin ou urbain) pour lequel la source externe d'hydrogène est éradiquée, et présentant simultanément une résistance à la traction élevée (de l'ordre de 1800MPa et davantage) et une ténacité équivalente à celle des aciers au carbone faiblement alliés et à très haute résistance.The aim of the steel composition of the invention is to solve these technical problems by proposing a martensitic stainless steel having an intrinsic resistance to corrosion in an atmospheric medium (marine or urban environment) for which the external source of hydrogen is eradicated , and simultaneously having a high tensile strength (of the order of 1800 MPa and more) and toughness equivalent to that of low alloyed carbon steel and very high strength.
A cet effet, l'invention a pour objet un acier inoxydable martensitique, caractérisé en ce que sa composition est, en pourcentages pondéraux :
- 9% ≤ Cr ≤ 13%
- 1,5% ≤ Mo ≤ 3%
- 8% ≤ Ni ≤14%
- 1% ≤ Al ≤ 2%
- 0,5% ≤ Ti ≤ 1,5% avec Al + Ti ≥ 2,25%
- traces ≤ Co ≤ 2%
- traces ≤ W ≤ 1% avec Mo + (W/2) ≤ 3%
- traces ≤ P ≤ 0,02%
- traces ≤ S ≤ 0,0050%
- traces ≤ N ≤ 0,0060%
- traces ≤ C ≤ 0,025%
- traces ≤ Cu ≤ 0,5%
- traces ≤ Mn ≤ 3%
- traces ≤ Si ≤ 0,25%
- traces ≤ O ≤ 0,0050%
- Ms (°C) = 1302 - 42Cr - 63Ni - 30Mo + 20Al - 15W - 33Mn - 28Si - 30Cu - 13Co + 10Ti ≥ 50
- Creq/Nieq ≤1,05
Ni eq (%) = 2Ni + 0,5Mn + 30C + 25N + Co + 0,3Cu, le reste étant du fer et des impuretés inévitable,
De préférence 10% ≤ Cr ≤ 11,75%.
De préférence 2% ≤ Mo ≤ 3%.
De préférence 10,5% ≤ Ni ≤ 12,5%.
De préférence 1,2% ≤ Al ≤ 1,6%.
De préférence 0,75% ≤ Ti ≤ 1,25%
De préférence traces ≤ Co ≤ 0,5%
De préférence traces ≤ P ≤ 0,01 %
De préférence traces ≤ S ≤ 0,0010%
De préférence traces ≤ S ≤ 0,0005%
De préférence traces ≤ N ≤ 0,0030%
De préférence traces ≤ C ≤ 0,0120%
De préférence traces ≤ Cu ≤ 0,25%
De préférence traces ≤ Si ≤ 0,25%
De préférence traces ≤ Si ≤ 0,10%
De préférence traces ≤ Mn ≤ 0,25%
De préférence traces ≤ Mn ≤ 0,10%
De préférence traces ≤ O ≤ 0,0020%.For this purpose, the subject of the invention is a martensitic stainless steel, characterized in that its composition is, in weight percentages:
- 9% ≤ Cr ≤ 13%
- 1.5% ≤ Mo ≤ 3%
- 8% ≤ Ni ≤14%
- 1% ≤ Al ≤ 2%
- 0.5% ≤ Ti ≤ 1.5% with Al + Ti ≥ 2.25%
- traces ≤ Co ≤ 2%
- traces ≤ W ≤ 1% with Mo + (W / 2) ≤ 3%
- traces ≤ P ≤ 0.02%
- traces ≤ S ≤ 0.0050%
- traces ≤ N ≤ 0.0060%
- traces ≤ C ≤ 0.025%
- traces ≤ Cu ≤ 0.5%
- traces ≤ Mn ≤ 3%
- traces ≤ If ≤ 0.25%
- traces ≤ O ≤ 0.0050%
- M s (° C) = 1302 - 42Cr - 63Ni - 30Mo + 20Al - 15W - 33Mn - 28Si - 30Cu - 13Co + 10Ti ≥ 50
- Creq / Nieq ≤1.05
Ni eq (%) = 2Ni + 0.5Mn + 30C + 25N + Co + 0.3Cu, the remainder being iron and unavoidable impurities,
Preferably 10% ≤ Cr ≤ 11.75%.
Preferably 2% ≤ Mo ≤ 3%.
Preferably 10.5% ≤ Ni ≤ 12.5%.
Preferably 1.2% ≤ Al ≤ 1.6%.
Preferably 0.75% ≤ Ti ≤ 1.25%
Preferably traces ≤ Co ≤ 0.5%
Preferably traces ≤ P ≤ 0.01%
Preferably traces ≤ S ≤ 0.0010%
Preferably traces ≤ S ≤ 0.0005%
Preferably traces ≤ N ≤ 0.0030%
Preferably traces ≤ C ≤ 0.0120%
Preferably traces ≤ Cu ≤ 0.25%
Preferably traces ≤ Si ≤ 0.25%
Preferably traces ≤ Si ≤ 0.10%
Preferably traces ≤ Mn ≤ 0.25%
Preferably traces ≤ Mn ≤ 0.10%
Preferably traces ≤ 0 ≤ 0.0020%.
L'invention a également pour objet un procédé de fabrication d'une pièce mécanique en acier à hautes résistance mécanique et résistance à la corrosion, caractérisé en ce que :
- on élabore un demi-produit par préparation puis transformation à chaud d'un lingot de composition telle que précédemment décrite ;
- on exécute un traitement thermique de mise en solution sur ledit demi-produit entre 850 et 950°C, immédiatement suivi par un traitement cryogénique de refroidissement rapide jusqu'à une température inférieure ou égale à -75°C sans interruption en dessous du point de transformation Ms et pendant une durée suffisante pour assurer un refroidissement complet dans toute l'épaisseur de la pièce ;
- on exécute un revenu de vieillissement entre 450 et 600°C pour une durée de maintien isotherme de 4 à 32 h.
- a half-product is produced by preparation and then hot transformation of an ingot of composition as previously described;
- a solution heat treatment is carried out on said half-product between 850 and 950 ° C, immediately followed by cryogenic rapid cooling treatment to a temperature of -75 ° C or lower without interruption below the transformation Ms and for a time sufficient to ensure complete cooling throughout the thickness of the room;
- an aging income of between 450 and 600 ° C is performed for an isothermal holding time of 4 to 32 hours.
Ledit traitement cryogénique peut être une trempe dans de la neige carbonique.Said cryogenic treatment may be a quenching in dry ice.
Ledit traitement cryogénique peut être effectué à une température de -80°C pendant au moins 4 h.Said cryogenic treatment can be carried out at a temperature of -80 ° C for at least 4 hours.
Entre ledit traitement de mise en solution et ledit traitement cryogénique, on peut procéder à une trempe isotherme à une température supérieure au point de transformation Ms.Between said solution treatment treatment and said cryogenic treatment, it is possible to proceed with isothermal quenching at a temperature above the point of transformation Ms.
Après le traitement cryogénique et avant le revenu de vieillissement, on peut procéder à une mise en forme à froid et à un traitement thermique de mise en solution.After the cryogenic treatment and before the aging income, it is possible to carry out a cold forming and a solution heat treatment.
On peut exécuter au moins un traitement thermique d'homogénéisation entre 1200 et 1300°C pendant au moins 24 h sur le lingot ou lors de ses transformations à chaud en demi-produit, mais avant la dernière de ces transformations à chaud.At least one homogenizing heat treatment may be carried out between 1200 and 1300 ° C. for at least 24 hours on the ingot or during its hot transformations into semi-finished product, but before the last of these hot transformations.
L'invention a également pour objet une pièce mécanique en acier à hautes résistance à la corrosion et résistance mécanique, caractérisée en ce qu'elle a été obtenue par le procédé précédent.The invention also relates to a mechanical part made of steel with high resistance to corrosion and mechanical strength, characterized in that it was obtained by the above method.
Il s'agit par exemple d'un caisson de train d'atterrissage d'aéronef.This is for example an aircraft landing gear box.
Comme on l'aura compris, l'invention repose en premier lieu sur une composition de l'acier telle que définie ci-dessus. Elle présente notamment comme particularités des teneurs en Ni, Al, Ti, Mo, Cr et Mn qui sont ou peuvent être assez élevées.As will be understood, the invention is based primarily on a composition of the steel as defined above. It has particular characteristics as Ni, Al, Ti, Mo, Cr and Mn that are or can be quite high.
Des traitements thermomécaniques sont également proposés, grâce auxquels les propriétés désirées pour le métal final sont obtenues.Thermomechanical treatments are also proposed, whereby the desired properties for the final metal are obtained.
L'acier de l'invention permet un durcissement structural par précipitation simultanée des phases secondaires de type β-NiAl, η-Ni3Ti et éventuellement µ-Fe7(Mo, W)6 selon l'effet dit « maraging », ce qui lui confère après un vieillissement thermique, assurant la précipitation, un très haut niveau de résistance mécanique d'au moins 1800MPa, combiné à une bonne tenue à la corrosion, en particulier à la corrosion sous contrainte en milieux corrosifs atmosphériques.The steel of the invention allows a structural hardening by simultaneous precipitation of the secondary phases of β-NiAl type, η-Ni 3 Ti and optionally μ-Fe 7 (Mo, W) 6 according to the effect called "maraging", this which gives, after thermal aging, ensuring precipitation, a very high level of mechanical strength of at least 1800 MPa, combined with good resistance to corrosion, in particular to stress corrosion in atmospheric corrosive environments.
Sa tenue en fatigue est également améliorée moyennant le strict contrôle des impuretés réputées nocives (azote, oxygène).Its resistance in fatigue is also improved by means of the strict control of impurities deemed harmful (nitrogen, oxygen).
En outre, l'acier de l'invention possède une bonne résistance à l'échauffement et peut donc supporter des températures atteignant 300°C pour de courtes durées et de l'ordre de 250°C pour de longues durées. Sa sensibilité à l'hydrogène est plus faible que celles des aciers faiblement alliés.In addition, the steel of the invention has good resistance to heating and can therefore withstand temperatures up to 300 ° C for short durations and of the order of 250 ° C for long periods. Its sensitivity to hydrogen is lower than that of low alloyed steels.
L'invention sera mieux comprise à la lecture de la description qui va suivre.The invention will be better understood on reading the description which follows.
Les aciers à très haute résistance sont très sensibles à la corrosion sous tension. La composition d'acier de l'invention est telle que l'origine même de la rupture par corrosion sous tension, qui est la production d'hydrogène par les mécanismes de corrosion puis la fragilisation du métal par diffusion interne de cet hydrogène, est circonvenue en milieux atmosphériques grâce à une tenue renforcée à la corrosion en général. Dans ce but, les teneurs en chrome et molybdène sont d'au moins respectivement 9% et 1,5%, préférentiellement d'au moins 10% et 2% de façon dans ce dernier cas à atteindre un indice de piqûration I.P., défini par I.P. = Cr + 3,3 Mo, d'au moins 16,5, comme celui des aciers inoxydables austénitiques du type AlSI 304 à 16-18% Cr. En effet, une teneur en chrome minimale de 9 à 11 % est nécessaire pour conférer à un acier une capacité de protection vis-à-vis de la corrosion en atmosphère humide, grâce à la formation à sa surface d'un film d'oxyde riche en chrome. Mais ce film protecteur est insuffisant dans le cas où le milieu atmosphérique est pollué par des ions sulfates ou chlorures qui peuvent développer la corrosion par piqûre puis par crevasse, toutes deux susceptibles de fournir de l'hydrogène fragilisant.Very high strength steels are very sensitive to stress corrosion. The steel composition of the invention is such that the very origin of the stress corrosion fracture, which is the production of hydrogen by the corrosion mechanisms and then the embrittlement of the metal by internal diffusion of this hydrogen, is circumvented in atmospheric environments thanks to an outfit reinforced with corrosion in general. For this purpose, the chromium and molybdenum contents are at least 9% and 1.5%, preferably at least 10% and 2% respectively, in the latter case to reach an IP pitting index, defined by IP = Cr + 3.3 Mo, at least 16.5, like that of the austenitic stainless steels of the type AlSI 304 at 16-18% Cr. Indeed, a minimum chromium content of 9 to 11% is necessary to give a steel a protection capacity against corrosion in a humid atmosphere, thanks to the formation on its surface of an oxide film. rich in chromium. But this protective film is insufficient in the case where the atmospheric medium is polluted by sulphate or chloride ions that can develop pitting corrosion and then crevice, both likely to provide hydrogen embrittlement.
L'élément molybdène a, lui, un effet très favorable sur le renforcement du film passif vis-à-vis de la corrosion en milieux aqueux pollués par des chlorures ou des sulfates.The molybdenum element has a very favorable effect on the reinforcement of the passive film with respect to corrosion in aqueous media polluted by chlorides or sulphates.
En deuxième lieu, l'effet de durcissement qui procure une très haute résistance mécanique à l'acier est obtenu par précipitation de plusieurs phases secondaires durcissantes lors d'un traitement thermique de revenu d'une structure entièrement martensitique. Cette structure martensitique préalable au revenu résulte d'un traitement de mise en solution préalable dans le domaine austénitique, puis d'un refroidissement (ou trempe) jusqu'à une température suffisamment basse pour que toute l'austénite se transforme en martensite.Secondly, the curing effect which gives a very high mechanical strength to the steel is obtained by precipitation of several hardening secondary phases during a thermal heat treatment of a completely martensitic structure. This martensitic structure prior to the income results from a preliminary solution treatment in the austenitic domain, then a cooling (or quenching) until a sufficiently low temperature so that all the austenite is transformed into martensite.
L'acier de l'invention subit ce durcissement grâce à la précipitation de phases intermétalliques de prototype β-NiAl, η-Ni3Ti et éventuellement µ-Fe7 (Mo, W)6. Le plus fort durcissement est obtenu avec les additions les plus élevées en aluminium, titane et molybdène. La teneur en nickel doit être très précisément ajustée de façon à ce que le durcissement maximal soit obtenu à partir d'une structure purement martensitique, sans ferrite ni austénite résiduelle de trempe.The steel of the invention undergoes this hardening thanks to the precipitation of intermetallic prototype phases β-NiAl, η-Ni 3 Ti and possibly μ-Fe 7 (Mo, W) 6 . The strongest hardening is achieved with the highest additions of aluminum, titanium and molybdenum. The nickel content must be very precisely adjusted so that the maximum hardening is obtained from a purely martensitic structure, without any residual ferrite or quench austenite.
En troisième lieu, l'acier de l'invention possède une ductilité et une ténacité maximales, qui sont obtenues notamment en limitant au mieux les effets d'anisotropie liés à la solidification des lingots.Third, the steel of the invention has maximum ductility and toughness, which are obtained in particular by limiting at best the effects of anisotropy related to the solidification of ingots.
Dans ce but, l'acier doit être exempt de la phase ferrite δ et de la phase austénite résiduelle après mise en solution et refroidissement.For this purpose, the steel must be free of the δ ferrite phase and the residual austenite phase after dissolution and cooling.
C'est la raison pour laquelle l'acier de l'invention se caractérise par un équilibrage spécifique de ses éléments d'addition comme cela est décrit ci-après.This is the reason why the steel of the invention is characterized by a specific balancing of its addition elements as described below.
Cette phase est néfaste pour deux raisons majeures :
- i) - elle provoque une fragilisation du métal,
- ii) - elle modifie la réponse au durcissement de l'acier et ne lui permet plus d'atteindre ses propriétés mécaniques optimales.
- i) - it causes embrittlement of the metal,
- ii) - it modifies the response to hardening of the steel and no longer allows it to reach its optimum mechanical properties.
L'acier de l'invention ne contient pas de ferrite du fait que sa composition répond aux conditions décrites ci-après.The steel of the invention does not contain ferrite because its composition meets the conditions described below.
Les formules qui vont être citées s'appuient sur deux relations entre les éléments d'alliage, l'une étant une somme pondérée des teneurs en % massique des éléments qui stabilisent la ferrite, et exprimée par une variable Cr équivalent (Cr eq), l'autre étant une somme pondérée des teneurs en % massique des éléments qui stabilisent l'austénite, et exprimée par la variable Ni équivalent (Ni eq)
Il s'avère que la ferrite δ formée de façon transitoire lors de la solidification de l'acier de l'invention peut être totalement résorbée lors d'un traitement thermique à haute température et en phase solide, par exemple entre 1200 et 1300°C, lorsque :
La ségrégation chimique d'un acier lors de sa solidification est un phénomène inévitable qui résulte du partage des éléments entre la fraction solide et la fraction liquide autour du solide. En fin de solidification, le liquide résiduel se fige dans des zones qui sont classiquement soit intergranulaires, soit interdendritiques, et on retrouve dans ces zones un enrichissement en certains éléments d'alliage, et/ou un appauvrissement en d'autres éléments d'alliage. Les cellules de ségrégation ainsi formées sont ensuite déformées et partiellement réhomogénéisées lors des opérations de transformation thermomécanique. Après ces opérations de déformation, il subsiste une structure dite en « bandes » selon le sens de la déformation, qui est nettement anisotropique. La réponse aux traitements thermiques de ces bandes ségrégées est très différenciée, ce qui aboutit à des propriétés mécaniques inégales en fonction de la direction des efforts exercés : d'une façon quasi-généralisée, les propriétés de ductilité et de ténacité (K1C) sont amoindries dans tous les cas où les efforts sont exercés plus ou moins perpendiculairement à la structure en bandes.The chemical segregation of a steel during its solidification is an inevitable phenomenon that results from the sharing of elements between the solid fraction and the liquid fraction around the solid. At the end of solidification, the residual liquid congeals in areas that are conventionally intergranular or interdendritic, and in these zones there is an enrichment in certain areas. alloying elements, and / or depletion of other alloying elements. The segregation cells thus formed are then deformed and partially rehomogenized during the thermomechanical transformation operations. After these deformation operations, there remains a structure called "bands" according to the direction of the deformation, which is clearly anisotropic. The response to the heat treatments of these segregated strips is highly differentiated, which results in unequal mechanical properties as a function of the direction of the forces exerted: in a quasi-generalized manner, the properties of ductility and toughness (K 1C ) are diminished in all cases where the forces are exerted more or less perpendicular to the band structure.
L'homogénéité structurale de l'acier de l'invention, qui est donc dictée par les conditions de solidification, est de préférence optimisée à l'aide de traitements thermiques d'homogénéisation à très hautes températures, entre 1200 et 1300°C, de durée supérieure à 24 h, pratiqués sur les lingots et/ou les produits intermédiaires, c'est-à-dire sur les demi-produits en cours de transformation à chaud. Un tel traitement ne doit, cependant, pas intervenir après la dernière transformation à chaud, sinon on se retrouverait avec une taille de grains trop importante avant la suite des traitements.The structural homogeneity of the steel of the invention, which is therefore dictated by the solidification conditions, is preferably optimized by means of homogenization heat treatments at very high temperatures, between 1200 and 1300.degree. longer than 24 hours, applied on the ingots and / or the intermediate products, that is to say on the half-products being processed hot. Such treatment should not, however, occur after the last hot transformation, otherwise we would end up with too large grain size before further processing.
Les meilleures propriétés de l'acier de l'invention sont obtenues à la suite d'une mise en solution entre 850 et 950°C, dans le domaine austénitique, suivie d'un refroidissement suffisamment énergique pour permettre la transformation totale de l'austénite en martensite. Cette transformation doit être totale pour deux raisons.The best properties of the steel of the invention are obtained after being dissolved between 850 and 950 ° C., in the austenitic field, followed by cooling sufficiently energetic to allow the total transformation of the austenite. in martensite. This transformation must be total for two reasons.
En premier lieu, le durcissement par précipitation des phases intermétalliques lors du vieillissement ultérieur n'opère qu'à partir de la structure martensitique. Ainsi, toutes les plages d'austénite résiduelle non transformées après la fin du refroidissement ne répondent pas au durcissement. Cela nuit fortement aux propriétés globales de l'acier de l'invention, d'autant plus que ces plages sont très souvent celles issues de la ségrégation résiduelle des lingots et sont donc fortement anisotropes.In the first place, the hardening by precipitation of the intermetallic phases during the subsequent aging only operates from the martensitic structure. Thus, all residual austenite ranges not transformed after the end of cooling do not respond to hardening. This strongly affects the overall properties of the steel of the invention, especially since these ranges are very often those resulting from the residual segregation of the ingots and are therefore strongly anisotropic.
En second lieu, les meilleurs compromis entre résistance, ductilité et ténacité de l'acier sont obtenus lorsque le revenu de vieillissement permet la formation simultanée des précipités durcissants et d'une faible fraction d'austénite de réversion disposée en films dans les défauts de la structure tels que les joints interlattes de la martensite. La structure sandwich constituée des latte de martensite séparées par des films d'austénite de réversion procure une grande ductilité à l'acier durci. Pour que cette austénite de réversion en faible quantité puisse se former à partir de la structure martensitique, il faut impérativement que celle-ci soit martensitique, c'est-à-dire exempte le plus possible d'austénite résiduelle non transformée à la fin du refroidissement depuis le cycle de mise en solution. En effet, à une température de vieillissement donnée, il n'existe qu'une seule teneur d'austénite à l'équilibre, qu'elle soit de type résiduel ou de réversion, cette dernière étant recherchée.Secondly, the best trade-offs between strength, ductility and toughness of the steel are obtained when the aging income allows the simultaneous formation of the hardening precipitates and a small fraction of reversion austenite arranged in films in the defects of the structure such as the interlayer joints of martensite. The sandwich structure consisting of martensite slats separated by reversion austenite films provides high ductility to the hardened steel. In order for this low-level reversion austenite to form from the martensitic structure, it is imperative that it be martensitic, that is to say as free as possible of residual non-transformed austenite at the end of the process. cooling since the dissolution cycle. Indeed, at a given aging temperature, there is only one equilibrium austenite content, whether residual type or reversion, the latter being sought.
Il est communément admis que la largeur du domaine de la transformation martensitique d'un acier très allié, domaine compris entre la température de début de transformation Ms et la température de fin de transformation Mf, est d'environ 150°C, et que ce domaine est d'autant plus large que la structure de l'acier est moins homogène. Cela signifie que la température Ms d'un acier que l'on refroidit à température ambiante (environ 25°C) depuis son domaine de mise en solution austénitique, doit être d'au moins 175°C.It is generally accepted that the width of the domain of the martensitic transformation of a high-alloy steel, a range between the transformation start temperature Ms and the end-of-transformation temperature M f, is approximately 150 ° C., and that This area is all the larger as the structure of the steel is less homogeneous. This means that the temperature Ms of a steel that is cooled to room temperature (about 25 ° C) from its austenitic dissolution range must be at least 175 ° C.
Les technologies modernes permettent aisément de refroidir les aciers à des températures inférieures à la température ambiante (traitements dits « cryogéniques ») ce qui permet d'achever la transformation martensitique d'aciers dont la température Ms est inférieure à 175°C ; toutefois, il y a une limite à cela dans le sens où cette transformation de phase, thermiquement activée, est fortement contrariée à de très basses températures.Modern technologies easily make it possible to cool the steels to temperatures below room temperature (so-called "cryogenic" treatments), which makes it possible to complete the martensitic transformation of steels whose Ms temperature is below 175 ° C .; however, there is a limit to this in the sense that this phase transformation, thermally activated, is strongly thwarted at very low temperatures.
L'acier de l'invention a une composition équilibrée de telle façon que la température de transformation Ms soit ≥ 50°C, et préférentiellement voisine de ou supérieure à 70°C. Ainsi, son refroidissement à -80°C, ou plus bas, dans un milieu réfrigérant; permet la transformation de l'austénite en martensite. Cela est rendu possible en recherchant un intervalle de température Ms - Mf d'au moins 140°C, préférentiellement d'au moins 160°C, par l'application, après le traitement de mise en solution entre 850 et 950°C, d'un refroidissement achevé par exemple dans de la neige carbonique à -80°C ou plus bas, pendant une durée suffisante pour assurer le refroidissement complet des produits et une transformation complète de l'austénite en martensite.The steel of the invention has a balanced composition such that the transformation temperature Ms is ≥ 50 ° C, and preferably close to or greater than 70 ° C. Thus, its cooling to -80 ° C, or lower, in a cooling medium; allows the transformation of austenite into martensite. This is made possible by searching for a temperature range Ms-Mf of at least 140 ° C., preferably at least 160 ° C., by the application, after the treatment. dissolving between 850 and 950 ° C, a cooling completed for example in dry ice at -80 ° C or lower, for a time sufficient to ensure complete cooling of the products and a complete transformation of the austenite in martensite.
Pour obtenir cet effet, l'acier de l'invention doit présenter une valeur répétitive et fiable de Ms qui doit répondre à la relation suivante, fonction de tous les éléments d'additions inclus dans l'acier et qui influent notablement sur Ms, y compris les éléments présents en teneurs résiduelles mais dont l'effet est fort sur la valeur de Ms. Cette valeur est calculée par la formule (les teneurs des différents éléments sont en % pondéraux) :
L'analyse statistique de coulées expérimentales a permis de valider cette relation pour des valeurs de Ms de 0 à 225°C, et de déduire la valeur minimale que doit avoir le point Ms pour l'acier de l'invention. Cette valeur est de +50°C et préférentiellement +70°C.Statistical analysis of experimental flows made it possible to validate this relationship for Ms values of 0 to 225 ° C., and to deduce the minimum value that the Ms point must have for the steel of the invention. This value is + 50 ° C. and preferably + 70 ° C.
Les rôles des éléments d'addition principaux sont détaillés ci-après :The roles of the main addition elements are detailed below:
Le chrome et le molybdène sont les éléments qui confèrent à l'acier sa bonne résistance à la corrosion : le molybdène est également susceptible de participer, en outre, au durcissement lors de la précipitation au revenu de la phase intermétallique de type Fe7Mo6.Chromium and molybdenum are the elements that give steel its good resistance to corrosion: molybdenum is also likely to participate, in addition, in hardening during the precipitation of the intermetallic phase Fe 7 Mo 6 .
La teneur en chrome des aciers de l'invention est comprise entre 9 et 13%, de préférence entre 10 et 11,75%. Au-delà de 13% de chrome, l'équilibrage global de l'acier n'est plus possible. En effet, en minorant les éléments qui favorisent la ferrite delta résiduelle (Mo = 1,5%, Al = 1,5% et Ti = 0,75%, Ti + Al = 2,25%), la relation liant Cr eq et Ni eq implique que la teneur en nickel soit d'au moins 11 %. Or, une telle composition, qui se trouve donc en limite des domaines de l'invention ne répond plus à la relation Ms ≥ 50°C.The chromium content of the steels of the invention is between 9 and 13%, preferably between 10 and 11.75%. Beyond 13% of chromium, global balancing of steel is no longer possible. Indeed, by reducing the elements which favor the residual delta ferrite (Mo = 1.5%, Al = 1.5% and Ti = 0.75%, Ti + Al = 2.25%), the relation binding Cr eq and Ni eq implies that the nickel content is at least 11%. However, such a composition, which is therefore at the limit of the fields of the invention no longer responds to the Ms ≥ 50 ° C relationship.
Et cela est d'autant plus vrai que les teneurs en éléments durcissants Al, Ti et Mo sont plus élevées, d'où la limite supérieure préférentielle en chrome de 11,75%.And this is all the more true that the contents of hardening elements Al, Ti and Mo are higher, hence the preferred upper limit in chromium of 11.75%.
La teneur en molybdène est d'au moins 1,5% pour qu'on puisse obtenir l'effet anticorrosion recherché. La teneur maximale est de 3%. Au-delà de 3% de molybdène, la température de solvus d'une phase intermétallique riche en molybdène de type χ, stable à haute température, devient supérieure à 950°C ; en outre, dans certains cas, la solidification s'achève par un système eutectique qui produit des phases intermétalliques massives, riches en molybdène, et dont la mise en solution ultérieure réclame des températures de mise en solution supérieures à 950°C.The molybdenum content is at least 1.5% to obtain the desired anticorrosion effect. The maximum content is 3%. Above 3% of molybdenum, the solvus temperature of a ét type molybdenum-rich intermetallic phase, stable at high temperature, becomes greater than 950 ° C; in addition, in some cases, the solidification is completed by a eutectic system which produces massive intermetallic phases, rich in molybdenum, and whose subsequent solution requires solution temperatures above 950 ° C.
Dans ces deux cas, des températures d'austénisation supérieures à 950°C conduisent à un grossissement exagéré de la structure granulaire, incompatible avec les propriétés mécaniques requises.In both cases, austenization temperatures above 950 ° C lead to exaggerated magnification of the granular structure, incompatible with the required mechanical properties.
Toutefois, si l'acier contient également du tungstène, celui-ci va se substituer partiellement au molybdène à raison d'un atome de tungstène pour deux atomes de molybdène. Dans ce cas, la limite maximale de 3% s'applique à la somme Mo + (W/2).However, if the steel also contains tungsten, it will partially replace the molybdenum at the rate of one tungsten atom for two molybdenum atoms. In this case, the maximum limit of 3% applies to the sum Mo + (W / 2).
Comme on l'a dit, de préférence, les teneurs en chrome et molybdène doivent permettre d'obtenir un indice de piqûration d'au moins 16,5.As has been said, preferably, the chromium and molybdenum contents must make it possible to obtain a pitting index of at least 16.5.
Le nickel est indispensable à l'acier pour exercer trois fonctions essentielles :
- stabiliser la phase austénitique aux températures de mise en solution et éliminer toute trace de ferrite δ ; dans ce but, l'acier de l'invention doit comporter au moins 10% de nickel et de préférence au moins 10,5%, à moins qu'un autre élément gammagène ne soit ajouté à l'acier, par exemple du manganèse ; pour une addition de manganèse allant jusqu'à 3%, on peut descendre la teneur en nickel jusqu'à 8% ;
- favoriser la ductilité de l'acier, en particulier pour les vieillissements à températures supérieures ou égales à 500°C, car il provoque dans ce cas la formation d'une petite fraction d'austénite dite de réversion, très ductile, finement dispersée dans tout l'acier, entre les lattes de la martensite dure et fragile ; toutefois, cet effet ductile est obtenu au détriment de la valeur de la résistance mécanique ;
- participer directement au durcissement de l'acier lors du vieillissement par précipitation des phases : β-Ni Al et η-Ni3Ti.
- stabilize the austenitic phase at solution temperatures and remove any trace of δ ferrite; for this purpose, the steel of the invention must comprise at least 10% nickel and preferably at least 10.5%, unless another gamma element is added to the steel, for example manganese; for a manganese addition of up to 3%, the nickel content can be reduced to 8%;
- promote the ductility of steel, especially for aging at temperatures of 500 ° C or higher, because it causes in this case the formation of a small fraction of austenite called reversion, very ductile, finely dispersed in any steel, between the laths of hard and fragile martensite; however, this ductile effect is obtained to the detriment of the value of the mechanical strength;
- participate directly in the hardening of the steel during aging by precipitation of the phases: β-Ni Al and η-Ni 3 Ti.
La teneur en austénite dispersée dans l'acier doit être limitée à 10% maximum pour conserver de très hautes résistances mécaniques : la teneur en nickel est, dans cette perspective, au maximum de 14% ; sa teneur préférée entre 10,5 et 12,5% est finalement ajustée précisément à l'aide des deux relations décrites précédemment : Cr eq / Ni eq ≤ 1,05 ; Ms ≥ 50°C ;The austenite content dispersed in the steel must be limited to a maximum of 10% to maintain very high mechanical strength: the nickel content is, in this perspective, a maximum of 14%; its preferred content between 10.5 and 12.5% is finally adjusted precisely using the two previously described relationships: Cr eq / Ni eq ≤ 1.05; M s ≥ 50 ° C;
L'aluminium est un élément nécessaire au durcissement de l'acier ; les niveaux de résistance maximale recherchés (Rm ≥ 1800MPa) ne sont atteints qu'avec une addition d'au moins 1% d'aluminium, et préférentiellement d'au moins 1,2%. L'aluminium stabilise fortement la ferrite δ et l'acier de l'invention ne peut pas comporter plus de 2% d'aluminium sans apparition de cette phase. Ainsi, la teneur en aluminium est elle de préférence limitée à 1,6%, par précaution, de façon à tenir compte des variations analytiques des autres éléments qui favorisent la ferrite, et qui sont principalement le chrome, le molybdène et le titane.Aluminum is a necessary element for the hardening of steel; the desired maximum resistance levels (Rm ≥ 1800 MPa) are only achieved with an addition of at least 1% aluminum, and preferably at least 1.2%. Aluminum strongly stabilizes ferrite δ and the steel of the invention can not contain more than 2% of aluminum without appearance of this phase. Thus, the aluminum content is preferably limited to 1.6%, as a precaution, so as to take into account the analytical variations of the other elements which promote ferrite, and which are mainly chromium, molybdenum and titanium.
Le titane, au même titre que l'aluminium, est un élément nécessaire au durcissement de l'acier. Il permet son durcissement par précipitation de la phase η - Ni3Ti.Titanium, just like aluminum, is a necessary element for the hardening of steel. It allows its hardening by precipitation of the phase η - Ni 3 Ti.
Dans l'acier maraging du type PM 13-8Mo et contenant plus de 1 % Al, l'accroissement de la valeur de résistance mécanique Rm procuré par le titane est approximativement de 400MPa par pourcent de titane.In PM 13-8Mo type maraging steel and containing more than 1% Al, the increase in titanium Rm strength is approximately 400MPa per percent titanium.
Dans l'acier de l'invention, contenant au moins 1% d'aluminium, les très hautes valeurs de résistance mécanique visées ne sont obtenues que lorsque la somme Al + Ti est au moins égale à 2,25% en poids.In the steel of the invention, containing at least 1% aluminum, the very high strength values referred to are obtained only when the sum Al + Ti is at least equal to 2.25% by weight.
D'autre part, le titane fixe très efficacement le carbone contenu dans l'acier sous forme du carbure TiC, ce qui permet d'éviter les effets nocifs du carbone libre comme indiqué ci-après. En outre, la solubilité du carbure TiC étant assez faible, il est possible de précipiter ce carbure d'une façon homogène dans l'acier lors des phases finales de la transformation thermomécanique à de basses températures dans le domaine austénitique de l'acier : ceci permet d'éviter la précipitation intergranulaire fragilisante du carbure.On the other hand, titanium very effectively binds the carbon contained in the steel in the form of TiC carbide, which makes it possible to avoid the harmful effects of free carbon as indicated below. In addition, the solubility of the TiC carbide being quite low, it is possible to precipitate this carbide in a homogeneous manner in the steel during the final stages of the thermomechanical transformation at low temperatures in the austenitic domain of the steel: this avoids the intergranular weakening of the carbide.
Pour l'obtention optimale de ces effets, la teneur en titane doit être comprise entre 0,5 et 1,5%, de préférence entre 0,75 et 1,25%To obtain these effects optimally, the titanium content must be between 0.5 and 1.5%, preferably between 0.75 and 1.25%.
Le cobalt, en substitution au nickel en proportion de 2% en poids de cobalt pour 1 % de nickel, est avantageux car il permet de stabiliser l'austénite aux températures de mise en solution, tout en permettant de conserver une solidification de l'acier de l'invention selon le mode ferritique recherché (il stabilise très faiblement l'austénite aux températures de solidification) : en cela, le cobalt élargit le domaine des compositions selon l'invention telles qu'elles sont délimitées par les relations liant Cr eq et Ni eq. En outre, tout en stabilisant la structure austénitique aux températures de mise en solution, la substitution de 1 % de nickel par 2% de cobalt permet de relever assez nettement le point Ms de début de la transformation martensitique de l'acier, comme cela peut être déduit de la formule de calcul de Ms.Cobalt, in substitution for nickel in a proportion of 2% by weight of cobalt per 1% of nickel, is advantageous because it makes it possible to stabilize the austenite at the dissolution temperatures, while allowing the solidification of the steel to be maintained. of the invention according to the desired ferritic mode (it very weakly stabilizes the austenite at solidification temperatures): in this, cobalt widens the range of the compositions according to the invention as they are delimited by the Cr eq binding relationships and Neither eq. In addition, while stabilizing the austenitic structure at the dissolution temperatures, the substitution of 1% of nickel with 2% of cobalt makes it possible to record the starting point of the martensitic transformation of the steel as clearly as possible. be deduced from Ms.'s calculation formula
Enfin, le cobalt confère à la structure martensitique une plus forte capacité de réponse au durcissement ; toutefois, le cobalt ne participe pas directement au durcissement par précipitation de la phase β - NiAl et n'a pas l'effet ductilisant du nickel. Au contraire, il favorise la précipitation de la phase fragilisante σ - FeCr au détriment de la phase µ - Fe7Mo6 qui peut avoir un effet durcissant.Finally, cobalt gives the martensitic structure a stronger ability to respond to hardening; however, cobalt does not participate directly in precipitation hardening of the β - NiAl phase and does not have the ductilizing effect of nickel. On the contrary, it favors the precipitation of the σ - FeCr weakening phase at the expense of the μ - Fe 7 Mo 6 phase, which can have a hardening effect.
Pour ces deux dernières raisons, l'addition de cobalt est limitée à 2%, préférentiellement à 0,5% dans le domaine restreint où toutes les propriétés de l'acier de l'invention peuvent être acquises sans avoir recours aux effets du cobalt.For the latter two reasons, the addition of cobalt is limited to 2%, preferably to 0.5% in the restricted range where all the properties of the steel of the invention can be acquired without resorting to the effects of cobalt.
Le tungstène peut être ajouté en substitution au molybdène car il participe plus activement au durcissement de la solution solide de la martensite, et il est aussi susceptible de participer à la précipitation au revenu de la phase intermétallique de type µ - Fe7 (Mo, W)6. On peut en ajouter jusqu'à 1%, si la somme Mo +(W/2) ne dépasse pas 3%.Tungsten can be added in substitution for molybdenum because it participates more actively in the hardening of the solid solution of martensite, and it is also likely to participate in the precipitation of the intermetallic phase type μ-Fe 7 (Mo, W). ) 6 . We can add up to 1% if the sum Mo + (W / 2) does not exceed 3%.
En général, de petites quantités de certains éléments ou d'impuretés, métalliques, métalloïdes ou non métalliques, peuvent modifier considérablement les propriétés de tous les alliages.In general, small amounts of certain elements or impurities, metallic, metalloidal or nonmetallic, can significantly alter the properties of all alloys.
Le phosphore tend à ségréger aux joints des grains, ce qui réduit l'adhésion de ces joints et diminue la ténacité et la ductilité des aciers par fragilisation intergranulaire. Une teneur maximale de 0,02%, préférentiellement de 0,01 %, est à ne pas dépasser dans l'acier de l'invention.Phosphorus tends to segregate at the grain boundaries, which reduces the adhesion of these joints and decreases the tenacity and ductility of the steels by intergranular embrittlement. A maximum content of 0.02%, preferably 0.01%, is not to be exceeded in the steel of the invention.
Le soufre est connu pour induire une forte fragilisation des aciers à haute résistance selon divers modes comme la ségrégation intergranulaire et la précipitation d'inclusions de sulfures : l'objectif est donc de minimiser au mieux sa teneur dans l'acier, en fonction des moyens d'élaboration disponibles. De très basses teneurs en soufre sont accessibles assez facilement dans les matières premières avec les moyens d'affinage classique. Il est donc aisé de répondre à l'exigence de l'acier de l'invention qui spécifie que les propriétés mécaniques requises demandent une teneur en soufre inférieure à 0,0050%, préférentiellement inférieure à 0,0010% et idéalement inférieure à 0,0005%, moyennant un choix approprié des matières premières.Sulfur is known to induce strong embrittlement of high strength steels according to various modes such as intergranular segregation and precipitation of sulphide inclusions: the objective is therefore to minimize its content in the steel, according to the means. available. Very low sulfur contents are easily accessible in the raw materials with conventional refining means. It is therefore easy to meet the requirement of the steel of the invention which specifies that the required mechanical properties require a sulfur content of less than 0.0050%, preferably less than 0.0010% and ideally less than 0, 0005%, subject to an appropriate choice of raw materials.
La teneur en azote doit aussi être maintenue à la plus basse valeur possible avec les moyens d'élaboration disponibles, d'une part pour obtenir la meilleure ductilité de l'acier, et d'autre part pour obtenir la limite d'endurance en fatigue la plus élevée possible, en particulier puisque l'acier contient l'élément titane. En effet, en présence de titane, l'azote forme des nitrures cubiques TiN insolubles qui sont extrêmement nocifs par leur forme et leurs propriétés physiques. Ils constituent des amorces systématiques de fissuration en fatigue.The nitrogen content must also be kept at the lowest possible value with the available means of elaboration, firstly to obtain the best ductility of the steel, and secondly to obtain the fatigue endurance limit. the highest possible, especially since the steel contains the titanium element. Indeed, in the presence of titanium, nitrogen forms insoluble cubic TiN nitrides which are extremely harmful by their shape and their physical properties. They constitute systematic primers of fatigue cracking.
Toutefois, les concentrations en azote que l'on obtient couramment avec les méthodes industrielles d'élaboration sous vide restent relativement élevées, en particulier en présence d'additions de titane.However, the nitrogen concentrations commonly obtained with industrial vacuum production methods remain relatively high, especially in the presence of titanium additions.
De très basses teneurs en azote ne peuvent être obtenues qu'avec une sélection soignée de matières premières, en particulier de ferro-chrome à très basses teneurs en azote, ce qui est très onéreux.Very low nitrogen contents can only be obtained with careful selection of raw materials, in particular ferro-chromium with very low nitrogen contents, which is very expensive.
Généralement, la méthode industrielle d'élaboration sous vide permet d'obtenir des teneurs en azote résiduel comprises entre 0,0030 et 0,0100%, typiquement centrées sur 0,0050 - 0,0060% dans le cas de l'acier de l'invention. La meilleure solution pour l'acier de l'invention est donc de rechercher une teneur résiduelle en azote aussi basse que possible, soit inférieure à 0,0060%.Generally, the industrial vacuum production method makes it possible to obtain residual nitrogen contents of between 0.0030 and 0.0100%, typically centered on 0.0050 to 0.0060% in the case of the steel of the invention. 'invention. The best solution for the steel of the invention is therefore to seek a residual nitrogen content as low as possible, less than 0.0060%.
Si nécessaire, et lorsque l'application requiert des caractéristiques exceptionnelles de tenue en fatigue, de ténacité et/ou de ductilité, on peut rechercher des teneurs en azote inférieures à 0,0030% par le choix de matières premières et de méthodes d'élaboration spécifiques.If necessary, and where the application requires exceptional fatigue strength, toughness and / or ductility, nitrogen contents of less than 0.0030% may be sought by the choice of raw materials and methods of preparation. specific.
Le carbone, communément présent dans les aciers, est un élément indésirable dans l'acier de l'invention à plusieurs titres :
- il provoque la précipitation de carbures qui réduisent la ductilité et la ténacité,
- il fixe du chrome sous forme du carbure M23C6, facilement soluble et dont la précipitation lors des divers cycles thermiques de la fabrication se produit en partie dans les joints des grains dont la matrice environnante est ainsi appauvrie en chrome : ce mécanisme est à l'origine du phénomène très nocif et bien connu de la corrosion intergranulaire,
- il durcit la matrice martensitique à l'état de mise en solution et trempe, ce qui la rend plus fragile et notamment plus sensible aux « tapures » (fissurations superficielles apparaissant lors de la trempe).
- it causes the precipitation of carbides that reduce ductility and toughness,
- it fixes chromium in the form of carbide M 23 C 6 , easily soluble and whose precipitation during the various thermal cycles of manufacture occurs partly in the grain boundaries whose surrounding matrix is thus depleted in chromium: this mechanism is to the origin of the very harmful and well-known phenomenon of intergranular corrosion,
- it hardens the martensitic matrix in the state of dissolution and quenching, which makes it more fragile and in particular more sensitive to "taps" (superficial cracking occurring during quenching).
Pour toutes ces raisons, la teneur maximale en carbone de l'acier de l'invention est limitée à 0,025% au plus, préférentiellement 0,0120% au plus.For all these reasons, the maximum carbon content of the steel of the invention is limited to 0.025% at most, preferably 0.0120% at most.
Le cuivre, qui est un élément que l'on trouve de façon résiduelle dans les matières premières commerciales, ne doit pas être présent à plus de 0,5%, et préférentiellement on recommande une teneur finale en cuivre inférieure ou égale à 0,25% dans l'acier de l'invention. La présence de cuivre en plus forte quantité déséquilibrerait le comportement global de l'acier : le cuivre tend facilement à déplacer le mode de solidification en dehors du domaine recherché, et abaisse inutilement le point de transformation Ms.Copper, which is a residual element found in commercial raw materials, must not be present at more than 0.5%, and preferably a final copper content of 0.25 or less is recommended. % in the steel of the invention. The presence of copper in larger quantities would unbalance the overall behavior of the steel: the copper easily tends to move the mode of solidification out of the desired range, and unnecessarily lowers the point of transformation Ms.
Le manganèse et le silicium sont communément présents dans les aciers, en particulier parce qu'ils sont utilisés comme désoxydants du métal liquide lors d'élaborations classiques en four où l'acier liquide est en contact avec l'atmosphère.Manganese and silicon are commonly present in steels, in particular because they are used as deoxidants of the liquid metal during conventional furnace processes where the liquid steel is in contact with the atmosphere.
Le manganèse est aussi utilisé dans les aciers pour fixer le soufre libre, extrêmement nocif, sous forme de sulfures de manganèse, moins nocifs. Etant donné que l'acier de l'invention comprend de très faibles teneurs en soufre et qu'il est élaboré sous vide, les éléments manganèse et silicium ne sont de ce point de vue d'aucune utilité, et leurs teneurs peuvent être limitées à celles des matières premières.Manganese is also used in steels to fix free sulfur, extremely harmful, in the form of less harmful manganese sulphides. Since the steel of the invention has very low sulfur contents and that it is developed under vacuum, the elements manganese and silicon are from this point of view of any utility, and their contents can be limited to those of the raw materials.
D'autre part, ces deux éléments abaissent le point de transformation Ms, ce qui réduit d'autant les concentrations tolérables des éléments favorables aux propriétés mécaniques et anticorrosion (Ni, Mo, Cr) pour maintenir Ms à un niveau suffisamment élevé, comme il est possible de le déduire de la relation entre Ms et la composition chimique.On the other hand, these two elements lower the transformation point Ms, which reduces by the same tolerable concentrations of elements favorable to the mechanical properties and anticorrosion (Ni, Mo, Cr) to maintain Ms at a sufficiently high level, as it is possible to deduce from the relationship between Ms and the chemical composition.
La teneur en silicium doit donc être maintenue à au plus 0,25%, de préférence à au plus 0,10%. La teneur en manganèse peut aussi être maintenue dans ces mêmes limites.The silicon content must therefore be maintained at most 0.25%, preferably at most 0.10%. The manganese content can also be maintained within these same limits.
Toutefois, il est aussi envisageable de jouer sur la teneur en manganèse de l'acier de l'invention pour ajuster le compromis entre une résistance à la traction élevée et une ténacité élevée qu'il est souhaitable d'obtenir pour les applications envisagées. Le manganèse élargit la boucle austénitique, et en particulier il abaisse la température Ac1 presque autant que le nickel. Comme, de plus, il a un moindre effet d'abaissement de Ms que le nickel, il peut être avantageux de remplacer une partie du nickel par du manganèse pour éviter la présence de ferrite δ et aider à former de l'austénite de réversion lors du vieillissement de durcissement. Cette substitution doit, bien entendu, se faire dans le respect des conditions sur Cr eq / Ni eq et Ms telles que vues plus haut. La teneur maximale en Mn peut ainsi être portée jusqu'à 3%. Dans le cas d'une haute teneur en manganèse, le mode d'élaboration de l'acier doit être adapté pour que cette teneur soit bien contrôlée. En particulier, il pourra être préférable de ne pas effectuer de traitement sous vide postérieurement à l'addition principale de manganèse, cet élément tendant à s'évaporer sous pression réduite.However, it is also conceivable to play on the manganese content of the steel of the invention to adjust the compromise between a high tensile strength and a high toughness that it is desirable to obtain for the intended applications. Manganese widens the austenitic loop, and in particular it lowers the Ac1 temperature almost as much as nickel. Since, moreover, it has a lower effect of lowering Ms than nickel, it may be advantageous to replace part of the nickel with manganese to avoid the presence of δ ferrite and help form reversion austenite when aging curing. This substitution must, of course, be done in compliance with the conditions on Cr eq / Ni eq and Ms as seen above. The maximum Mn content can thus be increased to 3%. In the case of a high manganese content, the method of production of the steel must be adapted so that this content is well controlled. In particular, it may be preferable not to perform vacuum treatment subsequent to the main addition of manganese, this element tending to evaporate under reduced pressure.
L'oxygène présent dans l'acier de l'invention forme des oxydes néfastes à la ductilité et à la tenue en fatigue. Pour cette raison, il est nécessaire de contenir sa concentration à la plus basse valeur possible, c'est-à-dire au maximum 0,0050%, préférentiellement en dessous de 0,0020%, ce que permettent les moyens industriels d'élaboration sous vide.The oxygen present in the steel of the invention forms oxides that are detrimental to ductility and fatigue strength. For this reason, it is necessary to contain its concentration at the lowest possible value, that is to say at most 0.0050%, preferably below 0.0020%, which is permitted by the industrial means of preparation. under vacuum.
Les éléments que l'on n'a pas cités ne sont éventuellement présentes qu'en tant qu'impuretés résultant de l'élaboration.The elements that have not been mentioned are only present as impurities resulting from the elaboration.
Les teneurs données comme préférentielles pour les divers éléments sont indépendantes les unes des autres.The contents given as preferential for the various elements are independent of each other.
Typiquement, l'acier de l'invention est élaboré sous vide selon des pratiques industrielles traditionnelles au moyen, par exemple, d'un four à induction sous vide ou selon une double phase d'élaboration sous vide, par exemple par élaboration et moulage dans un four sous vide d'une première électrode, puis par au moins une opération de refusion sous vide de cette électrode pour obtenir un lingot final. En cas d'addition volontaire de manganèse, l'élaboration d'un lingot peut comprendre une phase d'élaboration sous vide d'une électrode dans un four à induction suivi d'une phase de refusion selon le procédé de refusion sous laitier (ESR) ; les différentes méthodes de refusion ESR ou VAR (refusion à l'arc sous vide) peuvent être combinées.Typically, the steel of the invention is evacuated according to conventional industrial practices by means of, for example, a vacuum induction furnace or a double vacuum forming phase, for example by forming and molding in a vacuum. a vacuum furnace of a first electrode, then by at least one vacuum remelting operation of this electrode to obtain a final ingot. In the case of deliberate addition of manganese, the development of an ingot may comprise a vacuum elaboration phase of an electrode in an induction furnace followed by a remelting phase according to the slag remelting process (ESR ); different ESR or VAR (vacuum arc reflow remelting) methods can be combined.
Les procédés de transformation thermomécanique à haute température, par exemple le forgeage ou le laminage, permettent une mise en forme aisée des lingots moulés, dans des conditions habituelles. Ces procédés permettent l'obtention de toutes sortes de demi-produits avec l'acier de l'invention (plats, barres, blocs, pièces forgées ou matricées...).Thermomechanical processes at high temperature, for example forging or rolling, allow easy shaping of molded ingots under usual conditions. These processes make it possible to obtain all kinds of semi-finished products with the steel of the invention (plates, bars, blocks, forged or stamped parts, etc.).
Une bonne homogénéité structurale dans les demi-produits est, de préférence, assurée à l'aide d'un traitement thermique d'homogénéisation entre 1200 et 1300°C, pratiqué avant et/ou pendant la gamme de transformations thermomécaniques à chaud, mais pas après la dernière transformation à chaud afin d'éviter que les traitements ultérieurs n'aient lieu sur des demi-produits à trop forte taille de grains.A good structural homogeneity in the semi-finished products is preferably ensured by means of a homogenization heat treatment between 1200 and 1300 ° C., practiced before and / or during the range of thermomechanical hot transformations, but not after the last hot transformation to avoid that subsequent treatments take place on semi-products too large grain size.
Lorsque les opérations de transformation thermomécanique à chaud sont achevées, les produits sont alors mis en solution à une température comprise entre 850 et 950°C, puis les pièces sont refroidies rapidement jusqu'à une température finale inférieure ou égale à -75°C, sans interruption en dessous du point de transformation Ms, éventuellement en plaçant un palier de trempe isotherme au-dessus de Ms. Comme le point Ms est peu élevé, on peut facilement faire des trempes à l'huile chaude à T ≥ Ms. Cela permet d'égaliser la température dans des pièces massives et, surtout, d'éviter les tapures de trempe dues à la transformation martensitique différentielle entre la surface des pièces massives et le coeur chaud des pièces. En outre, en partant d'une pièce égalisée à une température supérieure à Ms, la transformation martensitique lors du passage cryogénique se produit de façon continue. Typiquement la température est de l'ordre de -80°C lorsque cette trempe est effectuée dans de la neige carbonique. Le maintien à basse température est d'une durée suffisante pour assurer un refroidissement complet dans toute l'épaisseur des pièces. Il dure typiquement au moins 4h à -80°C.When the hot thermomechanical processing operations are completed, the products are then dissolved at a temperature of between 850 and 950 ° C., and the parts are then rapidly cooled to a final temperature of less than or equal to -75 ° C. uninterrupted below the transformation point Ms, possibly by placing an isothermal quenching stage above Ms. As the Ms point is low, it can easily be hot oil quenched at T ≥ Ms. This allows to equalize the temperature in massive pieces and, above all, to avoid quenching taps due to the differential martensitic transformation between the surface of the massive pieces and the warm heart of the pieces. In addition, starting from a piece equalized at a temperature greater than Ms, the martensitic transformation during the cryogenic passage occurs continuously. Typically the temperature is of the order of -80 ° C. when this quenching is carried out in dry ice. The maintenance at low temperature is of sufficient duration to ensure complete cooling throughout the thickness of the parts. It typically lasts at least 4 hours at -80 ° C.
Après retour à la température ambiante, le métal, constitué d'une martensite ductile et de faible dureté, peut être éventuellement mis en forme à froid puis, de nouveau, mis en solution pour atteindre des propriétés homogènes.After returning to ambient temperature, the metal, consisting of a ductile martensite and of low hardness, can be optionally cold-formed and, again, dissolved in order to achieve homogeneous properties.
Les propriétés finales de l'acier sont finalement obtenues par un revenu de vieillissement à des températures comprises entre 450 et 600°C pour des durée de maintien isothermes comprises entre 4 et 32h, en fonction des caractéristiques recherchées. En effet, le couple des variables temps et température de vieillissement est choisi en considérant les critères suivants dans le domaine 450-600°C :
- la résistance maximale atteinte diminue lorsque la température de vieillissement croît mais, réciproquement, les valeurs de ductilité et de ténacité croissent,
- la durée de vieillissement nécessaire pour provoquer le durcissement croît lorsque la température diminue,
- à chaque niveau de température, la résistance passe par un maximum pour une durée déterminée, qui est appelé « pic de durcissement »,
- pour chaque niveau de résistance visé, qui peut être atteint par plusieurs couples de variables temps et température de vieillissement, il existe un seul couple temps/température qui confère le meilleur compromis résistance/ductilité à l'acier de l'invention. Ces conditions optimales correspondant à un début de survieillissement de la structure, obtenues lorsqu'on va au-delà du « pic de durcissement » défini ci-dessus.
- the maximum resistance reached decreases as the aging temperature increases but conversely, the values of ductility and toughness increase,
- the aging time necessary to cause the hardening increases when the temperature decreases,
- at each temperature level, the resistance goes through a maximum for a determined duration, which is called "curing peak",
- for each desired level of resistance, which can be achieved by several pairs of time and aging temperature variables, there is a single time / temperature pair which gives the best resistance / ductility compromise to the steel of the invention. These optimum conditions correspond to an early survival of the structure, obtained when going beyond the "hardening peak" defined above.
On va à présent décrire des exemples d'aciers selon l'invention et de procédés selon l'invention qui leur sont appliqués, ainsi que des exemples de référence pour comparaison des résultats obtenus.Examples of steels according to the invention and processes according to the invention which are applied to them, as well as reference examples for comparison of the results obtained, will now be described.
Le tableau 1 regroupe les compositions des aciers testés.
Les échantillons de référence ont des compositions qui diffèrent de l'invention essentiellement sur leur teneur en titane trop faible (A et C) et/ou sur leur somme Ti + Al trop faible (A, B, C) ou sur leur point Ms trop bas car inférieur à 50°C (D). L'échantillon C présente également une teneur en molybdène trop élevée.The reference samples have compositions which differ from the invention mainly on their too low titanium content (A and C) and / or on their sum Ti + Al too low (A, B, C) or on their point Ms too much low because less than 50 ° C (D). Sample C also has a molybdenum content that is too high.
Ces échantillons ont été obtenus par élaboration d'une électrode de 1t (échantillons A, D, I et J) ou 200kg (les autres) dans un four sous vide, électrode ensuite refondue dans un four à électrode consommable, et ont subi les traitements thermomécaniques suivants :
- homogénéisation pendant 24 heures à 1250°C ;
- forgeage à leur sortie de four avec une réduction d'épaisseur supérieure ou égale à 4 ;
- forgeage de finition avec un taux de corroyage d'au moins 2 après réchauffage à 950°C
- mise en solution à des températures de 900°C environ pendant 2h, suivie d'une trempe à l'eau et d'un traitement cryogénique à -80°C dans de la neige carbonique pendant 8h (sauf pour l'échantillon I où la mise en solution a été effectuée à 950°C pendant 1h30),
- revenu de vieillissement à 510°C pendant 8h.
- homogenization for 24 hours at 1250 ° C;
- forging at their furnace exit with a reduction in thickness greater than or equal to 4;
- finishing forging with a cure ratio of at least 2 after reheating to 950 ° C
- solution at temperatures of about 900 ° C for 2h, followed by quenching with water and cryogenic treatment at -80 ° C in dry ice for 8h (except for sample I where the dissolution was carried out at 950 ° C for 1:30),
- aging income at 510 ° C for 8h.
Les principales caractéristiques structurelles et mécaniques des échantillons sont regroupés dans le tableau 2.
Les aciers selon l'invention permettent donc :
- d'obtenir les niveaux visés de résistance à la rupture Rm de plus de 1800 MPa, ainsi qu'une limite élastique Rp 0,2 élevée ;
- de maintenir une ductilité qui n'est pas trop dégradée par rapport aux aciers de référence.
- to obtain the desired levels of resistance to rupture Rm of more than 1800 MPa, as well as a high elastic limit Rp 0.2;
- maintain a ductility that is not too degraded compared to reference steels.
L'acier de référence D, dont seule la valeur de Ms ne répond pas à l'invention, n'atteint pas le niveau de durcissement désiré, alors que sa somme Al + Ti répond bien à la condition Al + Ti ≥ 2,25. En effet, il contient 16% d'austénite résiduelle après le traitement cryogénique.The reference steel D, of which only the value of Ms does not correspond to the invention, does not reach the desired level of hardening, whereas its sum Al + Ti satisfies the condition Al + Ti ≥ 2.25. Indeed, it contains 16% residual austenite after the cryogenic treatment.
Parmi les aciers de l'invention, on peut distinguer deux catégories :
- ceux qui ont une tenue à la corrosion supérieure (chrome et molybdène élevés), mais qui ont une plus grande fragilité car leur teneur en nickel est nécessairement plus basse si on veut respecter la condition sur Ms : E, F, G, H, I relèvent de cette catégorie ;
- ceux qui offrent une meilleure ductilité que les précédents car leur teneur en nickel est élevée, mais dont la tenue à la corrosion est moindre car leurs teneurs en chrome et molybdène sont nécessairement limitées pour que la condition sur Ms soit respectée : J relève de cette catégorie.
- those with higher corrosion resistance (high chromium and molybdenum), but which are more fragile because their nickel content is necessarily lower if we want to respect the condition on Ms: E, F, G, H, I fall into this category;
- those which offer a better ductility than the previous ones because their nickel content is high, but whose resistance to corrosion is lower because their chromium and molybdenum contents are necessarily limited so that the condition on Ms is respected: J falls within this category .
Claims (26)
- Martensitic stainless steel, characterised in that its composition in percent by weight is:- 9% ≤ Cr ≤ 13%- 1.5% ≤ Mo ≤ 3%- 8% ≤ Ni ≤ 14%- 1% ≤ Al ≤ 2%- 0.5% ≤ Ti ≤ 1.5% with Al + Ti ≥ 2.25%- traces ≤ Co ≤ 2%- traces ≤ W ≤ 1 % with Mo + (W/2) ≤ 3%- traces ≤ P ≤ 0.02%- traces ≤ S ≤ 0.0050%- traces ≤ N ≤ 0.0060%- traces ≤ C ≤ 0.025%- traces ≤ Cu ≤ 0.5%- traces ≤ Mn ≤ 3%- traces ≤ Si ≤ 0.25%- traces ≤ O ≤ 0.0050%and is such that:• Ms (°C) = 1302 - 42Cr - 63Ni - 30Mo + 20Al - 15W - 33Mn - 28Si - 30Cu - 13Co + 10Ti ≥ 50• Creq/Ni eq ≤1.05with Cr eq (%) = Cr + 2Si + Mo + 1.5Ti + 5.5Al + 0.6W
Ni eq (%) = 2Ni + 0.5Mn + 30C + 25N + Co + 0.3Cu,
the remainder being iron and unavoidable impurities. - Steel according to claim 1, characterised in that
10% ≤ Cr ≤ 11.75%. - Steel according to claim 1 or 2, characterised in that
2% ≤ Mo ≤ 3%. - Steel according to one of claims 1 to 3, characterised in that
10.5% ≤ Ni ≤ 12.5% - Steel according to one of claims 1 to 4, characterised in that
1.2% ≤ Al ≤ 1.6% - Steel according to one of claims 1 to 5, characterised in that
0.75% ≤ Ti ≤ 1.25% - Steel according to one of claims 1 to 6, characterised in that
traces ≤ Co ≤ 0.5% - Steel according to one of claims 1 to 7, characterised in that
traces ≤ P ≤ 0.01 % - Steel according to one of claims 1 to 8, characterised in that
traces ≤ S ≤ 0.0010% - Steel according to one of claims 1 to 9, characterised in that
traces ≤ S ≤ 0.0005% - Steel according to one of claims 1 to 10, characterised in that
traces ≤ N ≤ 0.0030% - Steel according to one of claims 1 to 11, characterised in that
traces ≤ C ≤ 0.0120% - Steel according to one of claims 1 to 12, characterised in that
traces ≤ Cu ≤ 0.25% - Steel according to one of claims 1 to 13, characterised in that
traces ≤ Si ≤ 0.25% - Steel according to one of claims 1 to 14, characterised in that
traces ≤ Si ≤ 0.10% - Steel according to one of claims 1 to 15, characterised in that
traces ≤ Mn ≤ 0.25% - Steel according to claim 16, characterised in that
traces ≤ Mn ≤ 0.10% - Steel according to one of claims 1 to 17, characterised in that
traces ≤ O ≤ 0.0020%. - Method of manufacturing a mechanical part in steel having high mechanical strength and high corrosion resistance, characterised in that:- an intermediate product is made by preparing then hot-forming an ingot having the composition according to any one of claims 1 to 18;- a solution heat treatment is performed on said intermediate product between 850 and 950°C, immediately followed by a rapid cooling cryogenic treatment down to a temperature below or equal to -75°C without interruption below the transformation point Ms and for a sufficient period of time to ensure complete cooling through the full thickness of the part;- age tempering is performed between 450 and 600°C for an isothermal soak time of 4 to 32 hrs.
- Method according to claim 19, characterised in that said cryogenic treatment is quenching in dry ice.
- Method according to claim 19 or 20, characterised in that said cryogenic treatment is performed at a temperature of -80°C for at least 4 hrs.
- Method according to any of claims 19 to 21, characterised in that, between said solution treatment and said cryogenic treatment, an isothermal quench is performed at a temperature above the transformation point Ms.
- Method according to any one of claims 19 to 22, characterised in that after the cryogenic treatment and before the age tempering, a cold-forming and a solution heat treatment are performed.
- Method according to any one of claims 20 to 23, characterised in that at least one homogenisation heat treatment is performed between 1200 and 1300°C for at least 24 hrs on the ingot or during hot-forming operations to produce the intermediate product, but before the last of these hot-forming operations.
- Mechanical part in steel having high corrosion resistance and high mechanical strength, characterised in that it was obtained by the method according to any one of claims 19 to 24.
- Mechanical part according to claim 25, characterised in that it is an aircraft landing gear strut assembly.
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
SI200630767T SI1896624T1 (en) | 2005-06-28 | 2006-06-26 | Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part |
PL06778669T PL1896624T3 (en) | 2005-06-28 | 2006-06-26 | Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR0506591A FR2887558B1 (en) | 2005-06-28 | 2005-06-28 | MARTENSITIC STAINLESS STEEL COMPOSITION, PROCESS FOR MANUFACTURING A MECHANICAL PART THEREFROM, AND PIECE THUS OBTAINED |
PCT/FR2006/001472 WO2007003748A1 (en) | 2005-06-28 | 2006-06-26 | Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part |
Publications (2)
Publication Number | Publication Date |
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EP1896624A1 EP1896624A1 (en) | 2008-03-12 |
EP1896624B1 true EP1896624B1 (en) | 2010-08-18 |
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EP06778669A Active EP1896624B1 (en) | 2005-06-28 | 2006-06-26 | Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part |
Country Status (15)
Country | Link |
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US (1) | US8097098B2 (en) |
EP (1) | EP1896624B1 (en) |
JP (1) | JP5243243B2 (en) |
CN (1) | CN101248205B (en) |
AT (1) | ATE478165T1 (en) |
BR (1) | BRPI0613291B1 (en) |
CA (1) | CA2612718C (en) |
DE (1) | DE602006016281D1 (en) |
DK (1) | DK1896624T3 (en) |
ES (1) | ES2349785T3 (en) |
FR (1) | FR2887558B1 (en) |
PL (1) | PL1896624T3 (en) |
RU (1) | RU2415196C2 (en) |
SI (1) | SI1896624T1 (en) |
WO (1) | WO2007003748A1 (en) |
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2005
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2006
- 2006-06-26 WO PCT/FR2006/001472 patent/WO2007003748A1/en active Application Filing
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- 2006-06-26 BR BRPI0613291-0A patent/BRPI0613291B1/en active IP Right Grant
- 2006-06-26 DK DK06778669.9T patent/DK1896624T3/en active
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US20100139817A1 (en) | 2010-06-10 |
JP5243243B2 (en) | 2013-07-24 |
BRPI0613291A2 (en) | 2010-12-28 |
RU2415196C2 (en) | 2011-03-27 |
CN101248205B (en) | 2014-05-07 |
ATE478165T1 (en) | 2010-09-15 |
CN101248205A (en) | 2008-08-20 |
FR2887558A1 (en) | 2006-12-29 |
PL1896624T3 (en) | 2010-12-31 |
US8097098B2 (en) | 2012-01-17 |
DK1896624T3 (en) | 2010-09-20 |
CA2612718C (en) | 2015-01-06 |
ES2349785T3 (en) | 2011-01-11 |
WO2007003748A1 (en) | 2007-01-11 |
FR2887558B1 (en) | 2007-08-17 |
BRPI0613291B1 (en) | 2014-08-26 |
EP1896624A1 (en) | 2008-03-12 |
SI1896624T1 (en) | 2010-10-29 |
DE602006016281D1 (en) | 2010-09-30 |
RU2008102988A (en) | 2009-08-10 |
JP2008546912A (en) | 2008-12-25 |
CA2612718A1 (en) | 2007-01-11 |
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