Austenitic Lightweight Steel and Use Thereof

[0001] The innovation relates to an austenitic lightweight steel, and the use thereof Steels exhibiting tensile strengths of more than 600 MPa are referred to as lightweight steels due to the tensile strength per unit of weight being higher compared to aluminum.

[0002] Stainless austenitic steels are distinguished by a high corrosion resistance and, as a rule, good cold formability as well. The cold formability and the energy absorptivity of stainless austenitic steels can be increased by a TRIP effect (transformation induced plasticity). Both tensile strengths and fracture strains obtained are relatively high. The alloying range in which a TRIP effect occurs in stainless, cold-formable CrNi steels and CrNiMn steels has not been specified yet. Previously, stainless, cold-formable austenitic steels exhibiting the TRIP effect have only been characterized by some special properties. Said steels exhibit tensile strengths of about 520 to 850 MPa while exhibiting fracture strains of about 60 to 45% [1, 2]. A typical steel showing the TRIP effect is a stainless steel with 17 to 18% chromium and 8 to 10% nickel such as the steel X5 CrNi 18 10 (1.4301).

[0003] In addition to the stainless austenitic steels, high-manganese TRIP/TWIP-steels (twinning induced plasticity) and the LIP-steels (light induced plasticity) are cold-formable steels. Due to the increased tensile strength of the TRIP/TWIP- and the LIP-steels they are also referred to as lightweight steels. Austenitic TRIP/TWIP-steels exhibit tensile strengths of higher than about 650 to 1100 MPa. The associated fracture strains range from about 80 to 40% [1, 3, 4].

[0004] The chemical composition of the steels is disclosed in DE 197 27 759 A [3]. According to [3], these steels contain 10 to 30% manganese, typically with silicon and aluminum added. They are not alloyed with chromium. A steel with 20% manganese, 3% silicon and 3% aluminum is typical [3, 4, 5].

[0005] Austenitic LIP-steels have only been tested on a laboratory scale. They are reported to reach tensile strengths of about 1000 to 1100 MPa and fracture strains ranging from about 60 to 50%. According to [6], the chemical composition has not yet been published.

[0006] Cold formability and energy absorptivity, tensile strength and fracture strain of said steels are increased by a TRIP, TWIP, or superimposed TRIP and TWIP effects.

[0007] When during mechanical straining the austenite transforms, induced by deformation, to ε- and/or α′-martensite, a TRIP effect is observed. As a result, the plastic deformability and the tensile strength will rise. Twinning will further intensify these property changes. High strain hardening capability will then be observed. Relatively high tensile strengths are then reached with relatively low 0.2% yield strengths so that, typically, low yield strength ratios are recorded.

[0008] For assessing the cold formability of said steels, the product of the tensile strength multiplied by the maximum elongation can be used as characteristic value. The product of tensile strength times maximum elongation is in the range of from about 25,000 to 38,000 MPa % for the austenitic TRIP steels, over 38,000 to 57,000 MPa % for the TRIP/TWIP steels, and over 57,000 MPa % for the LIP steels [3-7]. The energy absorptivity of the TRIP steels and TRIP/TWIP steels reaches values of 0.45 to 0.5 J/mm³. This means that on crash loading these steels exhibit a large strain reserve [3, 4, 5]. For the LIP steels, corresponding values have not been published.

[0009] In the austenitic TRIP and TRIP/TWIP steels, cold formability and energy absorptivity are achieved by affecting the austenitic structure as a result of mechanical straining during a cold deformation process. The microstructure of the austenite, particularly with respect to the formation of stacking faults and twins, as well as the formation of deformation-induced ε-martensite and α′-martensite are affected. The different mechanisms can principally be influenced via the stacking fault energy of the austenite, which depends on the chemical composition of the austenite [5, 8]. Furthermore, niobium aids in developing fine grain and thus has a good effect on the mechanical properties. In addition, niobium causes setting of the carbon and hence leads to improved corrosion properties.

[0010] Until now, silicon contents of more than 1% are alloyed to austenitic steels in order to achieve heat resistance, or improved scaling resistance, respectively, in connection with high chromium contents. Silicon and aluminum show a high oxygen activity, which can have an effect on the castability and the level of purity. For this reason, as a rule, the contents of said elements are chosen to be at a minimum unless they are alloyed with the aim of improving special properties.

[0011] Silicon and aluminum are ferrite stabilizing elements. Therefore, the contents of these elements are limited in austenitic steels in order to prevent ferrite formation. With the exception of the high manganese TWIP steels, aluminum has not been used as alloying constituent in austenitic steels. Unlike other accompanying and alloying elements, the influence of aluminum on the chromium and/or nickel equivalent has not yet been reported.

[0012] A chromium content of more than about 12% causes a passive layer to form, making the stainless steels corrosion-resistant. Austenitic steels with 12% chromium are, as a rule, weather-resistant and slow-corroding. Resistance against rusting is increased in these steels. High-manganese austenitic steels, however, are not chromium-alloyed. Thus, they do not rank among the stainless, slow-corroding weather-resistant steels.

In the usual austenitic steels, manganese is used as an austenite-forming and nickel-substituting element. Therefore manganese is added to austenitic steels mainly for cost reasons.

[0013] It is a prerequisite of the creation of deformation-induced ε-martensite that the structure consists of austenite. In order to reach the TRIP effect, corresponding chromium and nickel equivalents are required in the chemical composition. This means that the chemical composition of the steels must be adjusted with respect to the ferrite and austenite stabilizing elements.

[0014] The stainless manganese and nitrogen alloyed austenitic steels 1.4371 (X2 CrMnNiN 17 7 5), 1.4372 (X12 CrMnNiN 17 7 5) and 1.4373 (X12 CrMnNiN 17 9 5), and the steels AISI 201 and 202, which may be nitrogen-alloyed or do not contain nitrogen, are, as far as their Cr, Ni and Mn contents is concerned, covered by the patent in some partial ranges. Said steels are listed in the Stahlschlüssel [7]. However, they do not contain aluminum.

• [0015] [1] Schröder, T.: Technische Rundschau 1/2 (2005), pp. 48-52
• [0016] [2] DIN 17 440 and DIN 17 441
• [0017] [3] Frommeyer, G.: Unexamined laid-open patent application, DE 197 27 759 A1.
• [0018] [4] Frommeyer, G.: Patent specification, DE 197 27 759 C2
• [0019] [5] Grässel, O., L. Krüger, G. Frommeyer and L. W. Meyer: Int. J. Plasticity 16(2000), pp. 1391-1409
• [0020] [6] Bode, R. et al.: stahl und eisen 8(2004), pp. 19 to 26
• [0021] [7] Stahischlüssel (Steel Index), Verlag Stahlschlüssel Wegst GmbH
• [0022] [8] Martinez, L. G. et al.: Steel research 63(1992)5, pp. 221-223

[0023] Therefore, the invention, which is disclosed in the independent claims, aims at providing further austenitic lightweight steels with good cold formability, a characteristic value of the cold formability of higher than 30,000 MPa %, as well as tensile strengths between 600 and 800 MPa and fracture strains over 50%.

[0024] The invention solves the problem in that the austenitic steel according to the invention is alloyed with silicon, aluminum and chromium while containing manganese. An improvement of the formability of said steel is achieved with the aid of alloying measures, especially by adding silicon within the limits of higher than 1.0% up to 4.0%, aluminum within the limits of 0.05% up to 4.0% while lowering the chromium content to less than 18%.

It has been found that aluminum affects both the chromium and the nickel equivalents. The efficiency factor for aluminum takes this into account when calculating the chromium equivalent according to claim 1. Also in claim 1, an effect on the nickel equivalent is taken into account by a coefficient. As a consequence, aluminum lowers the nickel equivalent.

[0025] Preferably, aluminum leads to improved mechanical properties and increased cold formability and energy absorption at temperatures above room temperature, i.e. at temperatures where most technological cold forming processes take place.

[0026] Based on these conditions, the required chemical composition of the steel according to the invention can be determined as mentioned above. As far as the Cr-, Ni- and Mn-contents are concerned, the known stainless manganese- and nitrogen-alloyed austenitic steels 1.4371 (X2 CrMnNiN 17 7 5), 1.4372 (X12 CrMnNiN 17 7 5) and 1.4373 (X12 CrMnNiN 17 9 5), and the steels AISI 201 and 202, which may be nitrogen-alloyed or do not contain nitrogen, are covered by the claim in partial ranges. Said steels are listed in the Stahischlüssel [7]. However, they do not contain aluminum. In addition, the steel according to the invention differs from these steels in containing more silicon, and also partly in regard to its use. Particularly, the solid-solution strengthening effect of the nitrogen in said steels is used in order to obtain, unlike with steels exhibiting good cold formability, relatively high 0.2% yield strengths. The nitrogen-alloyed steels are then preferably used as spring steels. The steels of the 201 and 202 grades that are not nitrogen-alloyed are characterized by lower 0.2% yield strengths compared with the nitrogen-alloyed steels of the same grade. Therefore the cold formability of these steels is a little better so that components made of these steels are used in household articles, in apparatus construction, building industry, etc.

[0027] The advantages achieved by the invention particularly resides in that with the lightweight steels of the invention improved mechanical properties and increased cold formability as well as energy absorption are reached. It is thus possible to manufacture cost-effective steels such as austenitic CrNiMn steels with lowered Ni contents. Said steels exhibit better properties or similar properties in comparison to the properties of, for example, commercial stainless CrNi steels of the 18/8 or 18/10 grades. Furthermore, weather-resistant or slow-corroding lightweight steels with high levels of strength and toughness can successfully be produced. The steels according to the invention cold-form very well, similar to the chromium-free, high-manganese TWIP-steels.

[0028] The invention will be explained with the aid of the following preferred embodiments.

[0029] The austenitic steels according to the invention comprise two different steel grades. The first steel grade comprises stainless austenitic steels containing about 12.0% to 18.0% chromium. The second steel grade comprises austenitic steels containing more than 2.0% and less than 12.0% chromium. Steels of the second grade are not stainless, but exhibit a higher resistance against rusting as a result of the chromium-, nickel- and silicon content thereof, in this respect, they are thus different from the previous austenitic TRIP/TWIP steels in spite of a similar potential of properties. A multitude of said steels can therefore be considered as weather-resistant or slow-corroding. Particularly steels containing 10% to 12% chromium exhibit distinct slow corrosion rates.

[0030] According to claim 2, a preferred composition is that the nickel content is lower than 10% but also 0%, the niobium content is lower than 1.2% but also 0%, the carbon content is between 0.01% and 0.15%, the nitrogen content is lower than 0.1% but also 0%, the copper content is lower than 4% but also 0%, the cobalt content is lower than 1% but also 0%, the molybdenum content is lower than 4% but also 0%, the tungsten content is lower than 3% but also 0%, the titanium content is lower than 1% but also 0%, and the vanadium content is lower than 0.15% but also 0%.

[0031] Preferably, such an austenitic steel with ε-TRIP effect, good cold formability and increased rusting resistance according to claim 3 has a carbon content of 0.04%, a chromium content of 13%, a silicon content of 1.5%, a niobium content of 0.15%, a nickel content of 7.9%, a manganese content of 8.1%, a nitrogen content of 0.02% and an aluminum content of 0.11%, balance largely iron. The structure of the steel consists of metastable austenite. The steel shows a marked ε-TRIP effect. A relatively high hardening capability is achieved. The 0.2% yield strength is 210 MPa, the tensile strength is 645 MPa. The steel reaches a maximum elongation of 65%. This means that the characteristic value calculated as the product of the fracture strain times the tensile strength is determined to be 38,055 MPa %. The energy absorption is about 0.5 J/mm³. The steel forms an oxidation layer containing iron, chromium and silicon, said layer under atmospheric conditions causing weather resistance, or slow corrosion, respectively.

[0032] Particularly preferred is a stainless austenitic steel with ε-TRIP effect and good cold formability according to claim 4 that has a carbon content of 0.03%, a chromium content of 15,82%, a silicon content of 1.22%, a nickel content of 7.50%, a manganese content of 5.80% and an aluminum content of 0.11%, balance largely iron. The structure of the steel consists of metastable austenite. The steel shows an austenitic basic structure with a marked ε-TRIP effect at room temperature. A relatively low yield strength ratio is observed as a result of high hardening capability. The 0.2% yield strength is about 197 MPa, the tensile strength is 620 MPa. The steel reaches a maximum elongation of 64%. That means that the value that characterizes cold formability, calculated as the product of the fracture strain times the tensile strength, is determined to be 39,820 MPa %. The energy absorption is about 0.5 J/mm³.