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
  • C22C38/18—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
• • 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
  • C22C38/02—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper

Definitions

• the present invention refers to the field of low- nickel nickel-chromium-manganese-copper-nitrogen steels.
• 3XX series austenitic stainless steels like, e.g., AISI 304 having a substantially austenitic microstructure at room temperature, are highly appreciated for their high ductility and moldability, their corrosion resistance and workability.
• these steels exhibit a mechanical strength not sufficient for structural uses finalized to the making of light-weight structures, like e.g. those of the automotive field (such as chassis, shock absorbers, suspensions) .
• Another relevant drawback of these alloys is their high cost, owing to the remarkable amounts of nickel required to stabilize the austenitic phase at room temperature.
• the alloy has the following composition: up to about 0.12% carbon, from about 5 to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3.5 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, the remainder being, occasional impurities apart, iron.
• This alloy exhibits a lower limit of 5%, since a content lower than this value might require a higher content of expensive nickel, or excessive amounts of copper, making the alloy hot-short. Moreover, for manganese an upper limit of 8.5% is recommended, taking into account that between manganese and copper there should be maintained a proportion suitable to prevent hot-shortness induced by too high Cu contents.
• nickel reduction below the lower limit of the range indicated therein should be compensated for, by increasing the amount of manganese and/or copper to prevent formation of martensite and of an excess of delta-ferrite .
• Mn and Cu contents should be balanced to the upper limit of the ranges indicated for these alloy elements, i.e., to about 8% Mn and 2-2.5% Cu, in order to assure sufficient stability of the austenitic phase and a good combination of mechanical and corrosion resistance properties.
• Mn percents may range from 10.5 to 11.5%, therefore being higher than those envisaged in the above-mentioned prior art, without prejudice of mechanical and corrosion resistance properties and with the advantage of increasing nitrogen solubility, thereby contributing to prevent the appearance of porosities, even in the presence of higher contents of this element.
• Austenitic phase stabilization is also achieved in the alloy, thanks to the high N content (0.25-0.40%).
• This high nitrogen content entails an improvement of the corrosion resistance (in particular by pitting) and of the mechanical properties, by effect of the strengthening induced by its interstitial solid solution.
• the high concentration of nitrogen thanks to its enhanced solubility, by effect of the increase of the Mg percent, prevents formation of porosities, in particular superficial ones.
• Subject-matter of the present invention is a low- nickel austenitic stainless steel, containing the following components expressed in percent by weight: C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S ⁇ 0.01; P ⁇ 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron.
• the lower limit of the nickel range is 3.8%.
• Carbon though being a strong stabilizer for the austenitic phase, cannot be used in high (>0.1%) contents in order not to cause an excessive decrease of intergranular corrosion resistance and not to create weldability problems.
• Chromium fosters corrosion resistance and enhances nitrogen solubility, preventing the appearance of porosities. Chromium content cannot be higher than 19% in order to avoid effects of its alphagenic character and tendency to form undesired intermetallic phases.
• Nickel is the primary austenitizing element. A reduction of its content in the alloy, in order to meet economic and strategic needs, should foresee the introduction of replacing elements that may compensate for the consequent reduction in the gammagenic character of the alloy.
• Manganese has a marked stabilizing effect on the austenitic phase and considerably enhances nitrogen solubility, thereby contributing to prevent appearance of porosities. Balancing of Mn and other alloy elements of the steel of this invention, such as S and N, even in the presence of relatively high copper values, prevents problems related to hot-shortness and allows to obtain a corrosion resistance comparable to the more expensive AISI 304.
• Copper besides fostering formation of austenitic phase and contributing to its stability, improves the resistance of stainless steels towards generalized corrosion.
• copper content should not be higher than 3% in order to prevent hot workability problems .
• Silicon is an important element, both for the fluidifying effect it exerts on the metal bath and for oxidation resistance. Due to its alphagenic character, it should be limited to 0.60% and, at higher contents, may create problems during pickling. However, for a good steel castability it should be present in contents higher than 0.15%.
• Nitrogen is a strong stabilizer of the austenitic phase, moreover determining an improvement of pitting corrosion resistance.
• nitrogen due to its reduced solubility in the liquid phase, nitrogen cannot be introduced in the alloy by conventional casting methods in contents sufficient to completely replace nickel.
• An important effect of nitrogen is the strengthening, induced by its interstitial solid solution, bringing about strength characteristics tendentially higher than the AISI 3XX class.
• Molybdenum in the percent range indicated, besides acting as alphagenic element and increasing nitrogen solubility in the alloy, is essential for the improvement of corrosion resistance and, in particular, of pitting corrosion resistance. However, a high percent of this element would not allow nickel to be decreased to desired levels. In this regard, it has to be pointed out that, in the context of the invention, molybdenum percents lower than the lower limit of the indicated range should be considered as impurities.
• Boron in the percent range indicated and suitably balanced with nitrogen, is effective at improving cold formability and mechanical strength (yield) .
• yield mechanical strength
• boron percents lower than the lower limit of the indicated range should be considered as impurities .
• Sulphur in the composition range according to the invention and suitably balanced with manganese, also contributes to improve hot workability.
• Phosphor in the composition range according to the invention, has no negative effect on mechanical properties and corrosion resistance.
• the composition of the steel according to the present invention is as follows: C 0.02-0.06; Cr 17.8-18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15- 0.40; N 0.25-0.33; S ⁇ 0.01; P ⁇ 0.03, and optionally Mo 0,2-1,0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron.
• subject-matter of the present invention is also a process for producing rolled sections of low- nickel austenitic stainless steel, characterized by subjecting a steel containing the following components expressed as percent by weight: C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S ⁇ 0.01; P ⁇ 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron, to the following operations: continuous casting in an ingot mold with a casting rate ranging from 0.5 to 5 m/min and a steel overheating at the casting ranging from 10 to 60 0 C; solidification of said steel cast in the form
• the above-described process is applied to a steel having the following composition expressed as percent by weight: C 0.02-0.06; Cr 17.8-18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15-0.40; N 0.25-0.37; S ⁇ 0.01; P ⁇ 0.03, and optionally Mo 0.2-1.0 and B 0.001- 0.003, the remainder being substantially, unavoidable impurities apart, iron.
• the greater amount of nitrogen in solution with respect to the traditional casting cycle allows to reduce Ni content and concomitantly increase the mechanical characteristics of the steel.
• these rolled sections exhibit mechanical properties higher than those of conventional AISI 3XX steel products, and, production costs being substantially equal, analogous formability and corrosion resistance higher than that of conventional AISI 2XX steel products.
• Example 1 In the following Table 1 there are reported the chemical compositions of steels according to the present invention and of conventional comparison steels.
• steels according to the invention exhibit mechanical strength higher than that of conventional steels taken into account, high corrosion resistance and good formability properties.
• a steel was made, complying with the chemical composition denoted by A in Table 1.
• This steel was cast by means of continuous casting technology, making slabs having a 220mm-thickness .
• the resulting steel has, as shown in Table 2, a yield strength as RpO.2 improved with respect to that of the conventional steels shown in Table 1 and denoted by F and G.
• This steel was cast by means of continuous casting technology, making slabs having a 220mm-thickness .
• Continuous casting occurs in an ingot mold with a casting rate of Im/min and a steel overheating at the casting of 40°C.
• Heat equalization treatment of the slabs occurs at a temperature of 128O 0 C.
• Hot rolling of the slabs is performed with a start- of-rolling temperature of HOO 0 C and an end-of-rolling temperature of 950 0 C, so as to obtain said rolled sections .
• the resulting steel has, as shown in Table 2, case A-I, a yield strength as RpO.2 lower than that of steel A obtained according to the thermo-mechanical treatment complying with what is subject-matter of the present invention.
• This steel was cast by means of a traditional casting technology, making slabs having a 220mm- thickness .
• the resulting steel has, as shown in Table 2, an improved formability/ability to undergo drawing (Erichsen Index) with respect to steel A of Table 1.
• This steel there were made, by means of forming and hydroforming techniques, members intended for the automotive field, in particular suspension arms.
• This steel was cast by means of a continuous casting technology, making slabs having a 180mm ⁇ thickness .
• Continuous casting occurs in an ingot mold with a casting rate of 0.8m/min and a steel overheating at the casting of 50 0 C. Solidification of this steel, cast in the form of slabs, occurs with a cooling rate such as to complete solidification in 750 s.
• Heat equalization treatment of the slabs occurs at a temperature of 1310 0 C.
• Hot rolling of the slabs is performed with a start- of-rolling temperature of 116O 0 C and an end-of-rolling temperature of 98O 0 C, so as to obtain said rolled sections .
• a steel was made complying, in terms of composition, with the chemical composition denoted by C in Table 1, and this steel was cast by a thermo- mechanical treatment differing from the thermo-mechanical cycle proposed by the present invention.
• the resulting steel has, as shown in Table 2, case C-I, a yield strength as RpO.2 lower than that of steel C obtained according to the thermo-mechanical treatment complying with what is subject matter of the present invention.
• This steel has a nitrogen content higher than that of the steels having compositions A and C. Moreover, it possesses a mechanical strength and a corrosion resistance higher than those of the same steels having compositions A and C.

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• Chemical & Material Sciences (AREA)
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• Heat Treatment Of Steel (AREA)
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• 2008
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