EP1430161B1 - High-strength duplex/triplex steel for lightweight construction and use thereof - Google Patents

Abstract

The invention concerns a steel for lightweight construction, consisting of a multiphase structure. In the case of a duplex steel, it consists of mixed ferrite (alpha) and austenite (gamma) crystals. In the case of a triplex steel, it comprises, additioally to said two phases, martensitic (epsilon) and/or (kappa) phases. The volumetric weight of the inventive steel is low as a result of the high proportion of light alloys A1, Si Mn, Mg, Ga and Be. The inventive alloys have a volumetric weight of less than 7b/cm<SUP>3</SUP>.

Description

The invention relates to a high-strength and very deep-drawable Duplex or Triplex lightweight steel and its Use.

High strength steel is used for the automotive industry, construction industry as well as in aerospace applications with different Properties developed and already in production used. This is especially in use in the Automobile industry increasingly the desire in the foreground, through new materials a weight reduction of the vehicle make. The aim is to produce lighter ones Steel alloys, which otherwise the previous favorable Maintain or improve properties.

From DE 43 03 316 are steels with 13-16 wt .-% Al and the Higher levels of other alloying elements, such as Cr, Nb, Ta, Si, B, Ti, for oxidation and corrosion resistant Components known. Such alloys are distinguished Temperatures above 700 ° C by a high oxidation and Corrosion resistance and are used in building components although they can be used at high temperatures, however preferably oxidizing only at low mechanical stress and corrosive conditions.

Furthermore, for example, from DE 199 00 199 high strength Lightweight steels known which a higher proportion Aluminum, chromium and nickel as well as manganese and as a result of which have a lower density than iron. As expected, the steel alloy is characterized by good corrosion resistance and stress corrosion cracking resistance as well as high strength out. Such steels have aluminum contents of up to 10% by weight. on.

For example, the austenitic or austenitic / ferritic Lightweight steel a composition on with 7-27 Wt% Mn, 1-10 wt% Al, less than 10 wt% Cr, less than 10% by weight of Ni, more than 0.7-4% by weight of Si, less than 3 Wt .-% Cu, less than 0.5 wt .-% C and a remainder Composition of iron and melting Impurities. The other alloy components can from small amounts of nitrogen, niobium, titanium, vanadium and Phosphorus exist.

From DE 12 62 613 B1 also the use of Lightweight steels for aircraft components and engines as well Projectiles and the like known which with 4 to 20% Al, 18 to 40% Mn and 0.15 to 2% C high levels of light Have alloy constituents. If necessary, 0 to 3% Si and 0 to 4% Nb come.

Also known from DE 197 27 759 is the use of a cold-formable, in particular readily deep-drawable, austenitic Lightweight structural steel, which has a tensile strength up to 1100 MPa and TRIP and / or TWIP properties. In the Composition with 1 to 6% Si, 1 to 8% Al, where (Al + Si) < 12%, 10 to 30% Mn, balance essentially iron inclusive usual steel accompanying elements he serves as a material for stiffening bodywork parts.

The known lightweight structural steels listed above have Although important advantageous properties for use in the these technology areas, but they are with significant disadvantages. Further weight savings, For example, in the auto industry, can be with the known steels only by further reducing the Sheet thickness or by additional design measures to reach. Good formability, d. H. deep and stretchable, cold-rolled and recrystallizing annealed deep drawing steels higher aluminum content, as they are especially for the application are needed in the automotive industry, are due to a still to high specific gravity from the state of the Technique not known in this form.

EP-A-414 949 describes an α-γ duplex lightweight steel having a composition similar that of the invention, but with lower Si content and without Mg and / or Ga and / or Be contents.

The invention is based on the object, a good kaltumformbaren, especially good deep drawing and stretchable high strength Lightweight steel to create its density under the specific density of previously known steels.

The invention is achieved by the subject matter of claim 1 played. The other claims contain advantageous Training and developments of the invention.

18 bis 35% Mn

8 bis 12% Al

3 bis zu 6% Si, wobei Al + Si > 12%

0,5 bis 2% C

höchstens 0,05% B

0 bis 3 % Ti

zumindest eines der Elemente Mg, Ga, Be mit einem Gehalt von jeweils 0,3 bis zu 3%

mit Rest, im wesentlichen Eisen, einschließlich üblicher Stahlbegleitelemente.

The solution according to the invention relates to a high-strength α / γ-duplex or α / γ / ε (κ) -triplex lightweight steel having the following composition (contents in% by weight):

18 to 35% Mn

8 to 12% Al

3 up to 6% Si, where Al + Si> 12%

0.5 to 2% C

at most 0.05% B

0 to 3% Ti

at least one of the elements Mg, Ga, Be with a content of 0.3 to 3% each

with remainder, essentially iron, including common steel components.

0,03 bis 2% Ti

N kleiner als 0,3%

Nb kleiner als 0,5%

V kleiner als 0,5%.

According to further developments of the alloy according to the invention, the elements N, Nb, V and optionally Ti having the following proportions are preferably used as further alloying elements:

0.03 to 2% Ti

N less than 0.3%

Nb less than 0.5%

V less than 0.5%.

The lightweight structural steel according to the invention is formed from a multi-phase structure in the case of duplex steel made of α-ferrite and γ-austenite mixed crystals. In the case of the triplex steel, a martensitic ε-phase and / or κ-phase is added to the first two phases. The specific weight of the steel according to the invention is lowered to low values by the high proportions of the light alloying elements Al, Si, C and Mn and at least one of the elements Mg, Ga, Be and optionally Ti. With both alloys, a density of less than 7 g / cm ^{3 is} achieved, which is significantly reduced by up to 15% compared to conventional steels with values of between 7.3 and 7.5 g / cm ³ . Also compared to the lightweight steels known from the literature with up to 8% aluminum content, the solution according to the invention has a further reduction in density.

The lightweight steel according to the invention achieved by the Element Mg, as far as it is present in the alloy, a further lowering its density, due to the very low specific gravity of Mg. The same applies to Be, as far as it is present in the alloy, here additionally still an increase in the strength is achieved, which under Maintaining the Dukltilität is achievable. Through the element Ti is, if it is present in the alloy, another Increase in strength through grain refining and mixed critical hardening reached. Also, the element Ga serves, provided it is in the alloy exists, increasing strength and hardness. In addition, it increases the castability of the alloy, since these by the proportion of Ga at comparable Temperature conditions becomes more fluid.

The inventive duplex or triplex lightweight steel is characterized in the cold-rolled and recrystallized state by a fine-grained two- or three-phase structure with equiaxial i. isotropic morphology of α-ferrite, γ-austenite or ε-martensite grains.

The solidification exponent of duplex / triplex steel is included n = 0.23 to 0.24 and the mean r-value (planar anisotropy) is: r = (r0 ° + r90 ° + 2r45 °) / 4≅1. this lightweight steel behaves with respect to a planar change in shape during Deep and ironing quasi-isotropic (Δr ≅ 0).

The respective fine-grained two- or three-phase structure increases the energy absorption - the dissipative energy - of this steel when subjected to high strain rate, as they are for example, by impact load or in the event of a crash occurs.

The lightweight steel is characterized by yield stresses of over 400 MPa. Due to a high solidification due to strong Interaction of the displacements of the coexisting α / γ or α / γ / ε (κ) phases become tensile strengths on the hot strip of up to Achieved 1000 MPa and uniform expansions up to 40% as well maximum strains up to 50% achieved. The recrystallizing annealed cold-rolled strip has strengths in the range of 900 MPa maximum strains of 70%.

Particularly advantageous is compared to conventional steels significantly reduced density. The lightweight construction steel according to the invention also indicates that known from the prior art Lightweight steels with aluminum components one not yet Known density reduction on.

Another advantage of the solution according to the invention is that, despite its high strength, the material is a very has good formability. These properties have been so far only achieved with high-alloyed stainless steels. Especially also worth mentioning is its pourability during processing, which, as already mentioned, in the presence of Ga is improved again.

The described property spectrum of a higher component strength, the higher freedom of design in geometry As well as the reduced material density leads to the goal, a reduced component weight due to fabric and lightweight design too receive.

Thus, the α / γ-duplex or α / γ / ε (κ) -triplex Lightweight steel is another improvement in the Combination of previously unknown advantageous properties result.

Due to the high proportion of alloy components with one specific gravity below the specific weight of Iron and previously known lightweight steels will be one for the Auto industry advantageous weight reduction while maintaining achieved the previous design construction. in the Further, the lightweight steel has an excellent Ductility, high strength and extremely high Solidification rate on. To emphasize is the property of a high load speed in crash behavior at a Accident, making this steel alloy especially for the Motor vehicle construction is suitable. Furthermore, there is an increased Corrosion and in particular stress cracking corrosion resistance, so that this steel alloy is also suitable for use in other technological fields such as construction suitable.

For concrete structures, d. H. especially as prestressed concrete steels and Monierisen (- steels) or crash barriers and Sheet piling can be the lightweight steels of the invention excellent use. The corrosion resistance can in others by chemical, electrochemical, organic, non-metallic or metallic coatings are improved.

There is also the possibility of chemical, electrochemical or thermal treatment is a reimbursement of Steel alloy bring about.

The formation of a protective cover layer can thereby be achieved be that the surface enriched with aluminum and / or is coated.

The deep and stretchable aluminum-containing steel is used in the Production process melted, in the continuous casting process potted, rolled in the temperature range above the Recrystallization temperature or by casting, thin strip casting preferably, poured off as near-net-near band.

The steel can either be processed directly as a hot strip or cold-rolled after hot rolling.

In addition to the aforementioned uses in vehicle construction the lightweight structural steel according to the invention is suitable in particular for the production of components for body shell components / body, Integral beams, suspension structures and space Frames. Other lightweight components in motor vehicles are steering, Axles and axle components, attachments, seat rails, Fasteners and systems for passive safety, Wheel suspensions, driveline and fuel tank.

Similarly, the scope extends to rail and Watercraft and aerospace, there preferably in thin-walled stiffness-relevant components.

In addition to the already described use in the construction industry, especially in building construction, the material is also suitable for Conveying systems, conveyor belts and metallurgy.

Claims (8)

1. High-strength α/γ duplex or α/γ/ε triplex lightweight structural steel having the following composition (contents in % by weight):

   18 to 35% of Mn,

   8 to 12% of Al,

   3 to 6% of Si, with Al + Si > 12%,

   0.5 to 2% of C,

   at most 0.05% of B,

   at most 3% of Ti,

   at least one of the elements Mg, Ga, Be in an amount of in each case 0.3% to 3%,

   remainder mainly iron plus standard steel accompanying elements.
2. High-strength lightweight structural steel according to Claim 1, characterized by a Ti content of from 0.03 to 2%.
3. High-strength lightweight structural steel according to either of Claims 1 and 2, characterized by the following contents of further alloying elements

   up to 0.3% of N

   up to 0.5% of Nb

   up to 0.5% of V.
4. High-strength lightweight structural steel according to one of Claims 1 to 3, characterized by a fine-grained two-phase α/γ duplex microstructure or three-phase α/γ/ε triplex microstructure with an equiaxial morphology in the cold-rolled and recrystallized state.
5. High-strength lightweight structural steel according to one of Claims 1 to 4, characterized by a planar isotropy with r = 1, i.e. a steel which in the plane of the metal sheet has virtually isotropic mechanical properties in terms of strength and elongation.
6. Production of a component from high-strength lightweight structural steel according to one of Claims 1 to 5 by means of a casting process.
7. Use of the lightweight structural steel according to one of Claims 1 to 5 as a material for bodyshell or bodywork components, integral beams, chassis structures or space frames in vehicles.
8. Use of the lightweight structural steel according to one of Claims 1 to 5 as a material for building, conveying installations, metallurgy applications and guard rails and sheet pile walls.