JP5755153B2 - High corrosion resistance austenitic steel - Google Patents

Description

  The present invention relates to a high corrosion resistant austenitic steel, its production method and the use of this steel.

  The strength of austenitic steels is enhanced by atoms dissolved in particular between the carbon and nitrogen elemental lattices. Chromium and manganese are typically alloyed to dissolve the volatile element nitrogen in the melt. Chromium alone promotes ferrite formation, while so-called solution annealing with manganese results in an austenitic structure, which is stabilized by quenching to room temperature.

  One type of austenitic steel is the so-called TWIP steel (Twinning Induced Plasticity, Zwillingsbildung induzierte Platistizitaet in German) in which intensive twin formation occurs during plastic deformation. This process is usually already carried out with a slight load and the steel is hardened, with the elongation at break exceeding 60%. Due to these properties, the steel is outstandingly suitable for the production of thin plates in the automotive industry, especially in the areas involved in car body accidents. TWIP steels typically have a carbon content of about 0.02 to 0.5% by weight, as alloying elements in quantities of 20 to 30% by weight of manganese, and in particular TWIP steels 3% of aluminum and silicon each. Used up to%.

  European Patent (EP) 0 889 144 discloses so-called TWIP steels, lightweight structural steels, which exhibit a tensile strength of up to 1,100 MPa and have a Si 1-6 mass%, Al 1-8 mass% (here the total content of Al and Si is 12 mass% or less) and Mn 10-30 mass% are contained. The disclosed steel is characterized by a higher flow stress of 400 MPa and a uniform elongation value of up to 70% and a breaking elongation of up to 90%. Disadvantageous to the steel disclosed in this print is the slight corrosion resistance.

  In DE 101, high-strength austenitic stainless steel is melted under normal atmospheric pressure of about 1 bar, and in addition to iron, 12-15% by mass of chromium, 17-21% by mass of manganese, silicon <0.7 % By weight, carbon and nitrogen in total 0.4-0.7% by weight and other elements related to production <1.0% by weight in total, the ratio of carbon content and nitrogen content being 0. It is characterized by being 6-1.0. The disclosed steels do not exhibit TWIP action and can form martensite upon strong deformation, which manifests itself in particularly less nominal strain.

  International Publication (WO) No. 2006/025412 discloses a high corrosion resistant TWIP steel containing Fe, Al, Si, Mn, Cr and Ni as main elements. The resulting steel exhibits a uniform elongation value of over 50% and a tensile strength of 600-800 MPa. The mechanical properties are comparable to those of steels based on Fe, Al, Si and Mn disclosed in EP 0 889 144, however, the addition of nickel is costly to produce. And the lack of interstitial atoms results in a slight strength. Another austenitic steel containing C and N as alloying elements is disclosed in WO 2006/027091, in which the steel described therein contains chromium and chromium in amounts of 16 to 21% by weight, respectively. In addition to the manganese alloy metal, it also contains 0.5-2.0% by weight of molybdenum and a total of 0.8-1.1% by weight of carbon and nitrogen with a carbon / nitrogen ratio of 0.5-1.1. To do. The disclosed steel exhibits mechanical strength, ductility, wear resistance and corrosion resistance and does not exhibit ferromagnetism. However, the disadvantage is that during the production of these alloys, primary ferrite formation occurs during solidification, which can lead to nitrogen leakage during melting and / or welding.

  The present invention was based on the problem of providing a weldable, highly corrosion-resistant austenitic steel that has high yield strength and also high tensile strength and elongation at break exceeding 90% and at the same time is corrosion resistant.

The subject of the present invention is based on 100% by mass, in addition to iron,
Manganese 20-32%,
10-15% chromium,
Carbon and nitrogen in total 0.5 to 1.3%, with the ratio of carbon to nitrogen being 0.5 to 1.5,
As well as high corrosion resistant austenitic steels which contain impurities related to melting (erschmelzungsbedingt).

  The austenitic steel according to the invention exhibits TWIP properties (TWIP = Twinning Induced Plasticity) as well as good corrosion resistance. The essential property of this TWIP steel is that it is plastic due to the formation of twin grain boundaries with good corrosion resistance, i.e. a large number of twin grain boundaries are formed in the microstructure during deformation, thereby making it powerful and It is to obtain a steel that hardens uniformly, has a high nominal strain in tensile tests, and remains completely austenite without forming martensite.

  The steel according to the invention has a stabilized austenitic structure formed by a combination of the main alloying elements Fe, Mn and Cr and the interstitial elements C and N. The steel according to the invention exhibits a breaking elongation of more than 90%, a yield strength of more than 400 MPa and a tensile strength of more than 900 MPa in the tensile test. Based on the combination of high elongation at break and yield strength, the steel according to the invention is extremely deformable. Furthermore, the alloys according to the invention show no α-martensite or ε-martensite formation detectable using X-ray diffraction after intentional deformation.

  It has been confirmed that the alloys according to the invention enable primary austenite solidification at the stated proportions of Cr, Mn, C and N, thereby obtaining a melt from which nitrogen does not leak during solidification and / or welding. . Said alloys can therefore be produced and processed under normal pressure. The alloy according to the invention exhibits a stable austenite structure that prevents the formation of ferrite. The alloy metal Cr and the existing N give rise to a higher corrosion resistance compared to TWIP steel from the state of the art.

  The individual quantitative ratios of the alloy metals Cr and Mn and additives N and C are such that the amount of Cr not only improves the solubility of N in the melt, but also does not form primary ferrites during the solidification of the melt. The ratio is adjusted to have an advantageous effect on the corrosion resistance of the alloy. The formation of ferrite is disadvantageous because it can result in less solubility in nitrogen and thus pore formation. Furthermore, in the alloys according to the invention, the formation of precipitates, such as carbides and nitrides, has been shifted to lower temperatures. This allows for slower cooling from the austenitizing temperature as well as trouble-free production of larger component cross sections.

  Also, the weldability of the alloy according to the invention is determined by avoiding nitrogen gas leakage during solidification after fusion welding and by precipitation during subsequent cooling of the solid material in the zone affected by the weld seam and heat to room temperature. It is advantageously influenced by avoiding the formation of objects. This is particularly technically important, since the material cools relatively slowly after welding, and the formation of precipitates at the weld seam and in the heat affected zone is undesirable. It is.

  The amount of Mn improves the deformability (plasticity, deformability). The other components C and N improve the mechanical properties and corrosion resistance without the formation of nitrides and carbides. The ratio of C and N according to the present invention allows complete austenite solidification without leakage of gas during melting or formation of carbides or nitrides during accelerated cooling. The solubility of the desired amount of nitrogen in the melt is preferably given at 1500 ° C. and a pressure of 1 bar.

  In a preferred embodiment, there is an alloy metal of Mn in an amount of 22.0-30.0% by weight and chromium in an amount of 11.0-13.0% by weight, in particular 12.0-13.0% by weight. . A total content of carbon and nitrogen of 0.5 to 0.8% by weight with a ratio of carbon to nitrogen of 0.5 to 0.8 has been found to be particularly advantageous. Because the alloys of this embodiment exhibit advantageous material properties, they are suitable for use in lightweight construction.

  In a further embodiment, the alloy according to the invention contains a secondary alloy metal, which can be used to further alter the mechanical properties. The secondary alloy element is preferably selected from Mo, Si, Nb, Hf, V, Zr, Ti and Nd. Among these alloy metals, Mo is preferably contained in an amount of 1.0 to 2.0% by mass, and Si is contained in an amount of 0.1 to 2% by mass. The metals Nb, Hf, V, Zr, Ti and Nd may be included in smaller amounts and are also referred to as microalloy elements. Among the microalloy elements, Nb is present in an amount of 0.02 to 0.1% by mass, and metals Hf, V, Zr, Ti and Nd are present independently of each other in an amount of 0 to 0.5% by mass. It may be.

  A further object of the present invention is a method for producing a highly corrosion-resistant austenitic steel having TWIP properties, in which the individual alloy metals are melted under normal pressure and the diffusion annealing is in the temperature range from 1,000 to 1,250 ° C. Over a period of 1 to 72 hours with subsequent quenching and hot / cold working.

  The melting process can be carried out in pure nitrogen at a pressure of 800-1000 mbar or in an open furnace at an ambient pressure corresponding to a nitrogen partial pressure of about 800 mbar.

  A further subject of the invention relates to the use of the austenitic steel according to the invention for the manufacture of structural components in buildings, in particular in the automotive industry.

Stress strain diagram of TWIP steel 25Mn12CrCN Notch impact bend value as a function of temperature Phase diagram of high corrosion resistant TWIP steel with primary austenite solidification

In Table 1 below, examples of alloys according to the invention are shown:

  The mechanical properties are shown in Table 2.

  In the following figure, a calculated phase diagram (FIG. 3) is shown which can read the elongation curve under load at room temperature (FIG. 1), impact strength (FIG. 2) and primary austenite formation. Yes.

Claims (9)

Based on 100% by mass, in addition to iron,
Manganese 22-30%,
12-13% chromium,
Carbon and nitrogen in total 0.5-0.8%, in which case the ratio of carbon to nitrogen is 0.5-0.8,
And Ru impurity <br/> or Rana related to melt, TWIP properties, yield strength> 400 MPa and a breaking elongation> High corrosion resistant austenitic steel having 90%.   The high corrosion-resistant austenitic steel according to claim 1, wherein Mo is contained in an amount of 1.0 to 2.0 mass%.   The high corrosion-resistant austenitic steel according to claim 1, wherein Si is contained in an amount of 0.1 to 2.0% by mass.   The high corrosion-resistant austenitic steel according to any one of claims 1 to 3, wherein Nb is contained in an amount of 0.02 to 0.1 mass%.   The high corrosion-resistant austenitic steel according to any one of claims 1 to 4, wherein Hf, V, Zr, Ti, and Nd are each contained in an amount of up to 0.5% by mass. The high corrosion-resistant austenitic steel according to any one of claims 1 to 5 , which has a tensile strength exceeding 900 MPa. A method for producing a highly corrosion-resistant austenitic steel according to any one of claims 1 to 6 ,
a) melting an austenitic steel having the chemical composition according to any one of claims 1 to 5 under normal pressure;
7. The high of any one of claims 1 to 6 , characterized in that b) annealing in the temperature range of 1,000 to 1,250 [deg.] C. for a period of 1 to 72 hours, and c) subsequent quenching. A method for producing corrosion-resistant austenitic steel. The method according to claim 7 , wherein hot working and / or cold working is carried out subsequent to processing step c). The method of using the high corrosion-resistant austenitic steel according to any one of claims 1 to 6 for manufacturing a structural member in the automobile industry.

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