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
  • C22C38/02—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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the improvement relates to a high-strength austenitic-martensitic lightweight steel alloyed with chromium, silicon, manganese, and aluminum and having a tensile strength greater than 800 to 1,200 MPa and an elongation at break greater than 25%, and relates to its use.
• Steels with tensile strengths above 600 MPa are referred to as lightweight steels because the tensile strength per weight unit is higher than that of aluminum.
• austenitic-martensitic steels there are various possibilities for increasing the strength of multi-phase steels such as austenitic-martensitic steels. For example, the increase of the phase proportion of martensite and/or coldforming and/or precipitation hardening. In austenitic-martensitic steels, the 0.2% technical elastic limit, the tensile strength, and the hardness are increased in comparison to austenitic steels as a result of the martensite content. Rustproof austenitic-martensitic CrNi steels combine the advantages of the austenitic steels and of the preferably soft-martensitic steels.
• the disadvantage of the aforementioned methods for increasing strength resides in that generally they entail a deterioration of the toughness properties and thus in general of the transformation properties.
• Austenitic steels with TRIP/TWIP effect transformation-induced plasticity and twinning-induced plasticity compensate this disadvantage in that one or several transformation-induced martensites or twinning are induced during coldforming. These effects cause a simultaneous increase of the tensile strength and of the elongation at break so that coldforming properties are improved and energy absorption capacity increases.
• austenitic-martensitic steels there are no solutions disclosed yet for eliminating this disadvantage and the loss of toughness as strength is increased.
• High-alloy austenitic-martensitic steels are rustproof steels [1] or high-manganese steels and obviously also LIP steels (light induced plasticity) [2, 3, 4]. There is no information available yet in the literature in regard to LIP steels. Comprehensive test results in regard to the TRIP/TWIP effect and its effects on the mechanical properties and the energy absorption capacity are available only for high-manganese steels [2, 3]. These high-manganese steels contain no chromium and are thus not corrosion-resistant and weather-resistant or slow to corrode.
• the high-manganese steels have 0.2% technical elastic limits of 200 to 450 MPA and tensile strengths of 780 MPa to 1,100 MPa and elongation at break between 39 and 47%.
• a steel with 15% manganese and silicon content of 4 to 2% and aluminum content of 2 to 4% exhibits these properties [1, 2].
• the alloying range in which the austenitic-martensitic steels with TRIP effect exist has been specified partially for high-manganese steels but not for rustproof steels [3].
• aluminum and silicon have a detectable positive effect on the passivation behavior of rustproof steels and on the rust layer formation in weather-resistant steels and corrosion-resistant steels.
• these elements however can cause deterioration of the coldforming properties and the surface quality of the products. This is a disadvantage when relatively large aluminum-containing and silicon-containing oxide inclusions are preferably formed in steels.
• EP 1 0901 006 B1 [8], EP 1 006 706 B1 [9], and EP 0 031 800 B1 [10] disclose ultra high strength steels whose tensile strengths are above 2,200 MPa. These steels are originally austenitic steels that have been subjected to coldforming and subsequently have been subjected to an aging or precipitation hardening process. The high tensile strengths are then achieved in the thus treated material. This coldformed material is very brittle and can hardly be elongated. It is no longer designed for a further coldforming process.
• the product of tensile strength and maximum elongation can be utilized as a characteristic value.
• the product of maximum elongation and tensile strength for austenitic-martensitic steels is in the range above 20,000 MPa % [3-5].
• the steels still be coldformed relatively well.
• the steels still have residual energy absorption capacity. This means that in case of crash loading the austenitic martensitic steels still have a satisfactorily high elongation buffer [3-5].
• transformation-induced ⁇ ′ martensite is that the micro-structure is comprised at least partially of austenite. Moreover, the austenite must be metastable in order to have a correspondingly high tendency for forming transformation-induced martensite. For these reasons, for the chemical composition of the steels an appropriate chromium equivalent and nickel equivalent are required. This means that in the chemical composition of the steels the ferrite-stabilizing and austenite-stabilizing elements must be adjusted relative to one another. For this reason, a modified chromium equivalent and a known nickel equivalent have been used in order to specify, as formulated in the claim, the range of existence of transformation-induced ⁇ ′ phase formation. Under these conditions, the required chemical composition of the steel according to the invention can be determined.
• the invention as defined in the independent claims therefore concerns the problem of providing austenitic-martensitic lightweight steels with excellent coldforming properties and with tensile strengths between 800 to 1,200 MPa and elongation at break greater than 25%.
• the lightweight steels according to the invention can be divided into two different steel types.
• the first steel type comprises rustproof lightweight steels with TRIP effect and with chromium content in the limits of greater than 12.0 to 18%.
• the second steel type comprises lightweight steels with TRIP/TWIP effect and with chromium content of more than 0.5% and smaller than 12.0% that generally are weather-resistant and corrosion-resistant.
• the inventive high-strength lightweight steel with TRIP effect has a carbon content of 0.03%, a chromium content of 14.1%, a silicon content of 1.23%, a nickel content of 6.3%, a manganese content of 7.94%, an aluminum content of 0.051%, and a niobium content of 0.5%, the remainder being essentially iron.
• the micro-structure of the steel is comprised primarily of metastable austenite and martensite.
• the steel exhibits a TRIP effect at room temperature. A high hardening capacity is observed.
• the 0.2% technical elastic limit is approximately at 300 MPa and the tensile strength is at 890.
• the steel exhibits a maximum elongation of 45%.
• the inventive high-strength lightweight steel with TWIG/TRIP effect has a carbon content of 0.04%, a chromium content of 0.52%, a silicon content of 1.5%, a nickel content of 2.1%, a manganese content of 11.5%, and an aluminum content of 0.051%, the remainder being essentially iron.
• the micro-structure of the steel is comprised of metastable austenite and martensite.
• the steel exhibits a TRIP/TWIG effect.
• a relatively high hardening capacity is observed.
• the 0.2% technical elastic limit is at 310 MPa and the tensile strength is at 1170 MPa and maximum elongation is at 31%.
• these steels are alloyed with chromium, silicon, and aluminum and partially with nickel, they have increased resistance with regard to material loss through rust. A variety of these steels can therefore be viewed as weather-resistant or corrosion-resistant. In particular steels with chromium content of 10 to 12% have a distinct corrosion resistance.
• the mechanical properties of the stainless steels according to the invention with chromium content greater 12 and less than 18% are comparable to the mechanical properties of rustproof soft-martensitic steels inasmuch as there is still residual austenite in the micro-structure.
• the rustproof steels according to the invention have in general in comparison to soft-martensitic steels low martensite and no ferrite proportions in the un-transformed initial micro-structure. Only as a result of a TRIP effect in the process of coldforming, the martensite proportion in the steels according to the invention will increase and reach values that are existing in soft-martensitic steels in general already in the un-transformed initial state.
• the steels according to the invention generally have lower 0.2% technical elongation limits.
• the steels will harden strongly in the process of mechanical loading and will reach almost the same or higher tensile strengths and high elongation at break. For this reason, these steels can still be coldformed well.
• the nickel content can be lowered in comparison to the commercially available soft-martensitic CrNi steels. This provides a cost-effective production of these steels.
• the steel according to the invention can be differentiated from steels as they are disclosed in [7] by a lower nickel equivalent.
• the micro-structure of the un-transformed initial state is comprised of martensite and austenite.
• the advantage of the austenitic lightweight steels according to the invention with chromium content between 0.5 and 12% relative to high-strength chromium-free lightweight steels resides in their weather resistance and corrosion resistance. These properties are achieved in the case of a tightly adhering rust layer.
• the strength and toughness properties of this group of steels according to the invention in individual situations approach the excellent mechanical properties of high-manganese TRIP/TWIP steels.
• These steels according to the invention with rust layer formation can also still be coldformed and still have a relatively high energy absorption capacity.
• the austenite in the steels according to the invention is metastable.
• a mechanical treatment it is possible to affect the micro-structure of the austenite with regard to generating stacking faults, twinning, and transformation-induced martensite, preferably transformation-induced ⁇ ′ martensite.
• the formation of preferably transformation-induced ⁇ ′ martensite in an austenitic-martensitic micro-structure is activated in the steel according to the invention.
• the nickel equivalent relative to the coldformable austenitic lightweight steels [7] is lowered.
• the steels according to the invention differ in this respect from the austenitic lightweight steels that can be coldformed well.
• the indicated property potential is however achieved in the process of mechanical loading as a result of transformation-induced martensite formation and without after treatment.
• the steels according to the present invention differ in principle from the ultra high strength steels as they are disclosed in [8, 9, 10].
• the steel according to the invention can possibly have a chemical composition as observed in aluminum-containing CrNi steels [8, 10] as well as in those that contain Ti, Si, Nb, and V [9].
• Manganese is alloyed in the steels according to the invention as an austenite former and as a substitution element for nickel.
• Titanium and niobium improve moreover the formation of austenitic fine grain and cause a fine martensite structure. Accordingly, these elements have a positive effect on the mechanical properties. Moreover, niobium and titanium effect binding of carbon and cause thus an improvement of the corrosion properties.
• the steels according to the invention differ from the known austenitic TRIP/TWIP steels in that the TRIP effect is induced not in an austenitic initial micro-structure but in an austenitic-martensitic micro-structure.
• the tensile strengths of more than 800 MPa are thus mainly the result of the already existing annealed martensite and of the transformation martensite. Elongation at break of more than 25% is caused primarily by the TRIP effect and thus the formation of transformation martensite. Precipitation hardening or aging is not required in order to obtain the indicated mechanical properties.
• Aluminum is special with regard to its alloying effect. As a ferrite-stabilizing element it has an effect on the chromium equivalent as expressed in the relationship 1 of claim 1 .
• the effective factor of aluminum on the nickel equivalent in the relationship 2 indicated in claim 1 has been set to ⁇ 0.2.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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• 2005
  • 2005-06-28 DE DE102005030413A patent/DE102005030413C5/de not_active Expired - Fee Related
• 2006
  • 2006-06-28 WO PCT/DE2006/001124 patent/WO2007000156A1/de not_active Ceased
  • 2006-06-28 US US11/994,119 patent/US20080247902A1/en not_active Abandoned
  • 2006-06-28 EP EP06761728A patent/EP1896623A1/de not_active Withdrawn
  • 2006-06-28 KR KR1020087002316A patent/KR20080034903A/ko not_active Withdrawn

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