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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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 invention deals with a low nickel, high manganese austenitic stainless, featuring, with regard to the well-known AISI-304 type, lower nickel (about 2wt%) and higher manganese (about 10wt%) contents, remarkably lower production cost due to its low nickel content, equivalent or somewhat superior mechanical and forming properties, improved resistance to cold deformation-induced martensite transformation and good weldability.
• the stainless steel term is related to both chromium and chromium/nickel steels group, having a noteworthy corrosion resistance in atmospheric and chemical environments. To get it, more than 12wt% chromium is needed.
• stainless group may be classified as follows: ferritic, martensitic and austenitic stainless steels.
• Austenitic stainless steels have a 18wt% chromium (Cr) / 8wt% nickel (Ni) base alloy; the former provides corrosion resistance and the latter undertakes structural stability of the austenite (together with less than 0.1%wt carbon (C)).
• These steels may be alloyed with different contents of several elements, such as silicon (Si), manganese (Mn), copper (Cu), molibdenum (Mo), nitrogen (N), with the aim of obtaining steels with special properties.
• Guiraldenq [P. Guiraldenq, "Action alpha somehow and gamma relies des Serx élements d'addition dans les aciers inoxydables nickel-chrome dérive du type 18-10", Memoires Scientifiques Rev . Metallurg . LXIV , N° 11 (1967)] worked with Mn contents up to 8% but not having low Ni levels; individual variations on a base alloy led to determine equivalent coeffiecients for each element.
• Austenitic stainless steel type 304 is used for general purposes comprising architecture, transportation systems, furniture, power generation, and petrochemical plants among others. However, specific cases within these application examples represent extreme conditions for type 304 steel (and any other of the austenitic grade), that can not be overcome with guarantee. Among them, those in which non-magnetic condition would be lost by required increasing deformation. Moreover, type 304 is a high cost steel due to its Ni content, what contributes to increase dependence on the strategic value of a scanty element in european mining resources.
• an austenitic stainless steel suitable both for the above mentioned specific purposes, and for general applications, those in which type 304 is commonly used. Taking this into account, mechanical properties similar to those of AISI-304 steel, higher resistance to cold-induced martensite transformation, good weldability and low production cost should be the characteristics of a high-performance new austenitic stainless steel.
• the invention has been carried out by the applicant through the framework of a ECSC R&D program (7210.MA/934), and deals with a new austenitic stainless steel developed with the aim of providing lower cost and similar mechanical, corrosion and welding properties, in relation to those of AISI-304 austenitic stainless steel. These objectives can be achieved by lowering the Ni content down to 1.5-3.5wt% and balancing the Ni effect on the new austenitic structure by those of Mn, C, N, and Cu.
• spectrometric button technique By using spectrometric button technique, base compositions having known ferrite contents were used to vary elements increasingly and determine chromium and nickel equivalents. A population of 450 spectrometric buttons were manufactured following the above mentioned ranges. One hundred buttons were also cast having individual variations for element contents lying out of the proposed limits.
• Experimental ingots were 300 mm height, truncated-pyramid shaped, having square bases, 140 mm side up and 120 mm side down.
• Ingot structure consisted of randomly distributed ferrite in austenite matrix. The structure was studied by metallographic techniques on lengthwise and crosswise sections of the ingots, comprising electrochemical etching with oxalic acid or 10% NaOH for observation by light microscopy, and magnetic measurement (ferritoscope) of ferrite.
• Hot deformability of low-Ni austenitic stainless steel was studied by tensile tests and plane compression tests, and compared with that of type-304. As it is shown in the example no. 2, hot tensile tests to fracture were performed at 900 through 1250 °C, and strain rates of 1 and 5 per second. Deformability results, as reduction of area, show lower average values for low-Ni alloys with regard to those of 304 steel at any strain rate (25% vs 45% at 1000 °C and 5 /s), although the difference decreases as temperature rises (50% vs 65% at 1200 °C and 5 /s). Results of 3-stage plane compression tests at 1180 °C, having 15% reduction and 39 mm/s each pass, exhibit higher stress values for low-Ni austenitic steel, but also a greater restoration rate between passes.
• the following feasible low-Ni composition range is selected (I'): C: 0.075 - 0.085% Si: 0.20 - 0.35% Mn: 10.2 - 10.4% Ni: 1.65 - 2.6% Cu: 2.0% Cr: 16.4 - 17.0% Mo: ⁇ 3.0% N: 0.135 - 0.175% Fe: balance, which has to be balanced by adjusting the elements and ferrite content from the above mentioned experimental formula [1].
• FLD's Forming-limit diagrams
• Hardness and strain-induced martensite were also measured on samples from low-Ni and 304 cold rolled sheets. Hardness data showed that structural hardening causes on low-Ni steel the same effect than martensite-precipitation hardening on 304 steel, that is, hardness increase (example no. 3.3.2). On the other hand, no ⁇ martensite has been detected by X-ray diffraction in the cold strained Low-Ni steel (example no. 3.3.1).
• Hot rolling of low-Ni slabs can be performed by using the program of common austenitic stainless steels, featuring a stress level at half way between AISI-316 and AISI-304 steels, for each pass. Black coil is subsequently annealed in the same way than AISI-304. For pickling, the strip goes through two acid bath (HNO3 and HF). Low-Ni strips can be cold rolled, reducing thickness from 3.0 to 0.7 mm, without problems. Final annealing of low-Ni steel was performed under the usual conditions of temperature and speed, with final air cooling.
• the application field of the new low nickel austenitic stainless steel from this invention comprises cold deformation requiring suitable drawability and strength, and use under atmospheric conditions (i.e. cold-storage plants, bar counters, cookware and cutlery).
• Industrial low-Ni heats were produced to get cold sheets in several thicknesses which were supplied to different manufacturers. Finished products as spoons, knives, beer barrels, pans, cooker among others have been manufactured with good results, only requiring setting machine parameters for the somewhat higher strength of the new alloy.
• the steels provided by this invention can be used in non-magnetic applications, such as nuclear industry, computer hardware and electronics, as well as in some applications of ferritic stainless steels (those involving excessive high manufacture costs, since they can not be welded or deep-drawn).
• Example no. 1 Manufacturing low-Ni steels.
• Low-Ni alloys have been manufactured as 40 kg vacuum melted ingots (300 mm height, truncated-pyramids shaped, having square bases, 140 mm side up and 120 mm side down), and 100 Tn melts subsequently continuous cast (1000 mm width). From the former, samples and specimens were obtained to perform the tests appearing described from example no. 2 through 5. Industrial heats were produced to assess the characteristics of the new alloy along the production process. Moreover, cold sheets of those heats were to be manufactured as final products (cutlery, beer barrels, etc.) and check their fabricability. Below are two examples of chemical compositions of low-Ni ingots (the variations of element contents correspond to the usual tolerances in steelmaking process). Ni Mn C N Si Cu Cr 1.69 10.45 0.079 0.141 0.36 1.97 16.4 1.61 10.29 0.079 0.138 0.37 1.98 16.3
• the chemical compositions of industrial heats differ from those of ingots in Ni ( ⁇ 2.5%), Si ( ⁇ 0.20%), N ( ⁇ 0.170%) and Cr ( ⁇ 17.0%).
• Hot rolling low-Ni slabs could be performed by using the program of common austenitic stainless steels, featuring a stress level at half way between AISI-316 and AISI-304 steels, for each pass. Black coil was subsequently annealed in the same way than AISI-304, that is, at 1130 °C and 25 m/min. For pickling, the strip went through two acid bath (15-20% HNO3 and 4-5% HF). Low-Ni strips were cold rolled, reducing thickness from 3.0 to 0.7 mm, without problems. Final annealing of low-Ni steel was performed under the usual conditions of temperature (1160 °C) and speed (45 m/min), with final air cooling.
• Example no. 2 Hot deformability.
• Hot deformability of low-Ni austenitic stainless steel was studied by tensile tests and plane compression tests, and compared with that of type-304.
• Hot tensile test procedure met the so-called "Gleeble test”. Test conditions were: temperature from 900 to 1250 °C (100 °C intervals); and strain rates of 1 and 5 s ⁇ 1.
• Deformability results assessed by means of reduction of area percentage, show lower mean values for low-Ni steel, in relation to AISI-304 at any strain rate (25% vs 45% at 1000 °C and 5 s ⁇ 1), although difference decreases as temperature rises (50% vs 65% at 1200 °C and 5 s ⁇ 1).
• Results exhibit higher stress values for low-Ni austenitic steel, but also a greater restoration rate between one pass and the following.
• Example no. 3 Mechanical and forming properties.
• Forming-limit diagrams were determined by four kind of tests:
• Samples from hot and cold rolled sheets (both low-Ni -example no. 1- and AISI-304 steels) were subsequently cold rolled in a laboratory mill with the aim of providing successive reductions of thickness and assessing the acquired hardness and the amount of cold-induced martensite.
• the initial dimensions of samples were: 50 mm x 350 mm x 2 mm (width, length, thickness).
• Example no. 4 Welding tests.
• Example no. 5 Corrosion resistance tests.
• Corrosion resistance of low-Ni steel was evaluated, in comparison with that of 304 grade, in relation to its response to pitting and crevice and SCC. Intergranular corrosion tests were also performed.
• TTT curves Intergranular corrosion tests (TTT curves) and microstructural analysis were carried out according to practice ASTM-A.262-A, on samples previously treated at 1100 °C during 30 minutes for carbide disolution, that gave rise to 6-sized grains.
• Samples (30x20x2mm) underwent heat treatments at 700-900 °C, with holding time in the 10 min to 6 h range and water-quenching.
• Low-Ni steel shows sensitization to intergranular corrosion at temperatures within 700-800 °C interval. This phenomenon, due to chromium carbide precipitation, can be observed also in AISI-304 in the same temperature range, although corrosion rate is slightly lower in this latter steel.

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Cited By (20)

Citations (6)

• 1994
  • 1994-07-26 EP EP94500134A patent/EP0694626A1/fr not_active Withdrawn

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