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
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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 present invention relates to duplex stainless steel containing Mn of high content and Cr, Mo, N and Ni of low contents as compared with S32205 duplex stainless steel, and more particularly, to duplex stainless steel that includes low contents of Cr, Mo, N and Ni to thereby decrease a production cost thereof, has excellent corrosion resistance better than STS 304 steel and 316L steel, and has a low incidence of an edge crack when it is hot-rolled.
• austenite stainless steel excellent in formability and corrosion resistance uses Fe as base metal and mainly contains Cr and Ni. Further, the austenite stainless steel has been variously developed by adding other elements such as Mo, Cu, etc. for various purposes.
• 316L steel is excellent in corrosion resistance, pitting resistance and high temperature strength.
• the 316L steel is low carbon steel and contains Ni more than 10wt% and Mo more than 2wt%, so that a cost price thereof heavily fluctuates according to the price of Ni and Mo, thereby decreasing competitive power.
• S32205 duplex stainless steel (hereinafter, referred to as "2205 steel” contains high percentage of Cr, Mo and N to secure excellent corrosion resistance, and contains Ni more than 5wt% to secure a volume fraction.
• duplex stainless steel contains a relatively low percentage of Ni as compared with STS 316L steel containing 10% Ni, so that its production cost is low and thus its price is competitive, thereby increasing added value.
• the 2205 steel has poor hot-formability and thus has a very low production yield of 80%.
• the 2205 steel has high contents of Cr and Mo, so that a sigma-phase deposition rate is high, thereby deteriorating the property of steel and having a high load on winding and cooling processes. Thus, it is hard to replace the 316L steel by the 2205 steel.
• duplex stainless steel that has low contents of Cr, Mo, N and Ni as compared with 2205 steel to thereby decrease a production cost thereof, increases production yield by lowering an incidence of an edge crack when it is hot-rolled, and has excellent corrosion resistance better than STS 304 steel and 316L steel.
• the present inventor develops duplex stainless steel that contains relatively low content of Ni, and limits constituents of Cr-Mo-Mn-N to make volume fraction of ⁇ and ⁇ have about 50:50, so that a production cost is reduced; a CPT is secured to be higher than 20°C of that of the STS 304 steel and the 316L steel; and the incidence of a edge crack is minimized to enhance a production yield and decrease a processing load.
• duplex stainless steel includes Cr of 19.5-22.5%, Mo of 0.5-2.5%, Ni of 1.0- 3.0%, Mn of 1.5-4.5%, N of 0.15% ⁇ 0.25%, C of 0.03% and less, P of 0.03% and less, Si of 2% and less, Fe and unavoidable elements.
• FIG. 1 are photographs showing an edge crack of a hot-rolled sample according to alloy constituents
• FIG. 2 is a table showing alloy constituents and a volume fraction of steel samples according to the present invention and comparative steel samples;
• FIG. 3 is a table showing critical pitting temperatures (CPT) of the samples of FIG.
• FIG. 4 is a table showing total test results of the samples of FIG. 2 with regard to corrosion resistance and hot formability.
• FIG. 5 is a table showing oxidation increment of the steel according to the present invention and the conventional steel.
• Carbon (C) is effective for strengthening a material by solid solution strengthening.
• C is easily combined with a carbide-forming element such as Cr, which is effective for corrosion resistance in a boundary between ferrite-austenite phases.
• Cr carbide-forming element
• C lowers the content of Cr around a grain boundary, so that the corrosion resistance is deteriorated.
• the content of C is lowered into 0.03% and below.
• N Nitrogen (N): N, together with Ni, is one of elements that contribute stabilization of an austenite phase. As the content of N increases, the corrosion resistance and high strengthening are achieved. However, when the content of N is too high, the hot formability of steel is deteriorated, thereby lowering the production yield thereof. On the other hand, when the content of N is too low, the contents of Cr and Mo should be lowered to secure the volume fraction of steel, and it is difficult to secure the strength of a welding part and phase stability. Therefore, the content of N preferably ranges between 0.15% and 0.25%.
• Mn generally has a content of about 1.5% to adjust a metal flow rate.
• Mn can be additionally contained instead of Ni.
• the hot formability can be secondarily improved.
• the upper limit content of Mn is limited to 4.5%.
• the content of Mn ranges between 1.5% and 4.5%.
• Cr is essential to not only primarily securing the ferrite phase of duplex stainless steel but also securing excellent corrosion resistance.
• the content of Cr increases, the corrosion resistance increases, but the content of expensive Ni should be also increased to maintain the volume fraction.
• the content of Cr is preferably limited between 19.5% and 22.5% so as to maintain the volume fraction of t he duplex stainless steel and the corrosion resistance better than that of STS 304 and 316L steel.
• Molybdenum (Mo) like Cr, Mo is used for not only stabilizing the ferrite phase but also largely enhancing the corrosion resistance. However, if the content of Mo is excessive, Mo is likely to form the sigma phase when it is annealed, thereby deteriorating the corrosion resistance and impact resistance. In the present embodiment, Mo just assists Cr in securing the ferrite volume fraction and secures the proper corrosion resistance, so that the content of Mo is preferably limited between 0.5% and 2.5%.
• Nickel (Ni) Ni, together with Mn and N, is an element to stabilize the austenite phase, and mainly used in securing the austenite phase of the duplex stainless steel. To reduce a production cost, if the content of expensive Ni is lowered, the decrement of Ni can be replaced by increasing the content of Mn and N that form the austenite phase. However, if the content of Ni is excessively lowered, Mn and N should be ex- cessively needed so that the corrosion resistance and the hot formability are deteriorated, or the contents of Cr and Mo are lowered so that it is difficult to secure the corrosion resistance better than the 316L steel. Thus, the content of Ni preferably ranges between 1.0% and 3.0%.
• Phosphorous (P) P is seeded in the grain boundary or an interface, and is likely to deteriorate the corrosion resistance and toughness. Therefore, the content of P is lowered as low as possible.
• the upper limit content of P is limited to 0.03% in consideration of the efficiency of a refining process.
• S S deteriorates the hot formability, or forms MnS together with Mn, thereby deteriorating the corrosion resistance. Therefore, the content of S is lowered as low as possible. Preferably, the content of S is lower than 0.03%.
• Si is added for deoxidization, but it can act as an element for stabilizing the ferrite phase. If the content of Si is excessive, Si deteriorates the mechanical property such as impact toughness of steel. Therefore, the content of Si is preferably limited to 2% and below.
• samples of duplex stainless steel having constituents according to an embodiment of the present invention are prepared and they are tested about the volume fraction, the corrosive resistance and the hot formability.
• FlG. 2 shows alloy constituents of the samples and ⁇ -volume fractions after they are annealed at a temperature of 1050°C. In these alloys, the ⁇ -volume fractions thereof range from about 40 to about 60%.
• the duplex stainless steel has an ⁇ -volume fraction ranging from 44 to 51% is excellent (O); the duplex stainless steel has an ⁇ -volume fraction lower than 44% or higher than 54% is defective (X); and the duplex stainless steel has an ⁇ -volume fraction ranging from 51 to 54% is good ( ⁇ ).
• FlG. 3 is a table showing critical pitting temperatures (CPT) of the samples of FlG.
• the CPT means the corrosion resistance.
• X the steel having a CPT of 20°C and below is defective (X); the steel having a CPT ranging from 20°C to 25°C and below is good (?); and the steel having a CPT of 20°C and higher is excellent (O).
• the ASTM G48 method suggests calculating the CPTs and selecting the closest value at intervals of 5°C.
• the CPT measuring starting temperature estimated by the ASTM G48 has a large deviation. Why the deviation is large is because the corrosion resistance deteriorated by Mn is not considered. That is, because the duplex stainless steel having lowered contents of Ni has relatively high content of Mn, the deviation arises in the estimated CPT obtained by the foregoing ASTM G48 method.
• the present inventor calculates the CPT by considering Mn as follows.
• the estimated CPT is approximately equal to the measured CPT.
• FlG. 4 shows total test results of the samples of FlG. 2 with regard to the volume fraction, the corrosion resistance, and the hot formability.
• Four steels (steel Nos. 3, 4, 14, 15) satisfy the formation properties of the duplex stainless steel, has the corrosion resistance better than that of the 316L steel, and is excellent in the hot formability.
• other steels (steel Nos. 1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 20, 21, and 22) can be selected as preferred steel, but they are inferior to the foregoing steels.
• the hot formability and the corrosion resistance are deteriorated as the content of Mn becomes higher.
• the hot formability is deteriorated as the content of N becomes higher.
• the steels having the high content of Mn needs relatively higher content of Mo.
• FlG. 5 shows difference in high temperature oxidation between the excellent and good steels according to the embodiment of the present invention and comparative steels such as STS304, STS316L and 2205 when they are reheated as slabs for hot- rolling.
• the high temperature oxidation measurement is performed by measuring oxidation increment under the condition that the hot-rolled sample having a size of 10mm(L)xl0mm(W)x3mm(T) is heated at a temperature of 1250°C and remained in a heating furnace for 180 minutes.
• the oxidation increment of the steel according to the present invention is 4 through 6 times lower than the convention 2205 steel, and about 1/3 through 1/2 times higher than the 316L steel.
• the surface quality of the steel according to the present invention is enhanced as a surface defective is decreased by the surface lubrication effect due to an oxidation layer formed on a surface of a reheating slab while being hot-rolled.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=36615116

Family Applications (1)

Country Status (6)

Families Citing this family (14)

Citations (6)

Family Cites Families (8)

• 2004
  • 2004-12-27 KR KR1020040113129A patent/KR20060074400A/ko not_active Application Discontinuation
• 2005
  • 2005-12-22 JP JP2007548086A patent/JP2008525636A/ja active Pending
  • 2005-12-22 US US11/722,341 patent/US20080112840A1/en not_active Abandoned
  • 2005-12-22 EP EP05822027A patent/EP1838890A4/de not_active Withdrawn
  • 2005-12-22 WO PCT/KR2005/004472 patent/WO2006071027A1/en active Application Filing
  • 2005-12-22 CN CNA200580045052XA patent/CN101090988A/zh active Pending

Patent Citations (6)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events