Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/0006—Adding metallic additives
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/005—Manufacture of stainless steel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/06—Deoxidising, e.g. killing
• • 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
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C2300/00—Process aspects
  • C21C2300/08—Particular sequence of the process steps

Definitions

• the present invention relates to a ferritic stainless steel, with high and stable grain refining potency and its production method.
• a ferritic stainless steel comprising C : max 24 mass%, Ti- ' 200 to 1000 mass ppm, Zr ⁇ at least 200 mass ppm, whereby the Zr/Ti ratio is greater than 0.3, O: 10 to 150 mass ppm, N: at least 70 mass ppm, whereby the N/O ratio is greater than 1.5, C: max. 0.03 mass%, and balance Fe and residual elements.
• the ferritic stainless steel comprises Ti and/or Zr oxides/nitrides (i.e. oxides and/or nitrides containing Titanium, oxides and/or nitrides containing Zirconium, and oxides and/or nitrides containing both Titanium and Zirconium) with a maximum diameter of 5 ⁇ .
• the expression "diameter” is intended to mean the diameter of an equivalent spherical volume corresponding to the volume of the oxide/nitride.
• a metal sample was dissolved by using the potentiostatic electrolytic extraction method with an electrolyte of 10% acetylacceton - 1% tetramethylammonium chloride-methanol (150 mV, 40-50 mA, 300 Coulombs). After extraction, the electrolyte was filtered using a polycarbonate membrane (PC) filter with an open-pore size of 50nm.
• PC polycarbonate membrane
• the ferritic stainless steel comprises a total number of more than lxl0 6 Ti and/or Zr oxides/nitrides per mm 3 of the steel.
• the present invention also concerns a method for producing of a ferritic stainless steel, which may be used to produce a ferritic stainless steel according to the prevent invention.
• the method compromises comprises the step of adding deoxidizers Si, Mn, Ti and Zr to the melt in the order of weak deoxidizer to strong deoxidizer, i.e. firstly Si and Mn are added to the melt either at the same time or separately, then Ti is added to the melt, and finally Zr is added to the melt, i,e, the deoxidizers are preferably added to the melt in the following order: Si-Mn, Ti, Zr.
• the method comprises the step of deoxidizing a melt with the deoxidizers Ti and Zr in said order with an interval of 1 to 5 minutes between additions, holding the melt for at least one minute and casting the melt.
• the method according to the present invention may be used to produce ferritic stainless steel suitable, such as AISI 430, 434, 436 or 444 for many applications with weldability and workability.
• TABLE 1 is the content of main elements and characteristics of particles and microstructure in Fe-20mass% Cr experiments.
• FIGURE 1 shows the size distributions of oxide/nitride particles with different compositions in Ti'Zr addition experiments at different N contents
• FIGURE 2 shows the average grain size in specimens of Ti-Zr addition experiments with different N content as a function of holding time at 1200 and 1400°C.
• the inventors performed experiments concerning the addition of Ti, Zr, Ce, and examined the distribution characteristics of oxides/nitrides produced, and the consequent microstructure, for stably enhancing the grain refining potency in ferritic stainless steel with higher N content and lower C content. Especially, the inventors focused on the selection of oxides/nitrides and the fine particle dispersion technique (with high amount and uniform space-distribution) fit to the above objective. It was found that Ti-Zr oxides/nitrides, i.e. oxides/nitrides containing both Titanium and Zirconium and Zirconium nitrides were the most promising particles with following grain refining potencies:
• Ti"Zr oxides/nitrides have a high potency as ferrite solidification nucleants, and enhance equiaxed grain zone ratio and equiaxed grain refining in as-cast structure.
• Zr nitrides have a thermal-stable potency as pinning particles on ferrite grain growth in reheated structure.
• the present invention which was based on the ⁇ outcome from the above examinations, provides a new "Oxide Metallurgy” technique with effective application of nitrides, in other words, an "Oxide/Nitride Metallurgy” technique for the production of ferritic stainless steel with higher N content specification and lower C content specification.
• the oxides/nitrides according to the present invention consist of Ti-Zr oxides/nitrides and Zr nitrides. The above selection of the oxide/nitride was performed considering the following points ' -
• (l) ZrN has a high potency as a ferrite solidification nucleant like that of TiN and VN, from the point of lattice disregistry, and enhances the equiaxed grain zone ratio and equiaxed grain refining in the as-cast structure.
• both Ti-Zr oxides/nitrides and Zr nitrides have a partial or complete ZrN surface.
• ZrN has a thermal- stable potency as pinning particles on ferrite grain growth, in comparison to TIN, in reheated structure.
• Zr nitrides include a little TiN, because ZrN is mainly precipitated in the melt and TiN might be precipitated on ZrN surface in the solidification/cooling.
• TiZr oxides/nitrides means a mainly Ti(0,N) core covered partially or completely by Zr oxide, Zr nitrides or their complexes (for example, ZrCVZrN with a low content of Ti(0,N)).
• Zr nitride means mainly ZrN including TiN (for example, ZrN-TiN). This naming due to the content of Ti, Zr, 0 and N was done for convenience.
• the Zr/Ti ratio should be controlled to be over 0.3, and the N/O ratio should be controlled to be over 1.5.
• the ferritic stainless steel according to the present invention has a chemical composition recited in claim 1.
• the O content is set 10 to 150 mass ppm.
• N is an essential element for producing Ti-Zr oxides/nitrides and Zr nitrides by Ti and Zr addition.
• the N content is set over 70 mass ppm.
• the N/O ratio is over 1.5.
• Ti contributes the primary formation of Ti(0,N) core and then the consequent formation of fine Ti-Zr oxides/nitrides which act as nucleants for ferrite solidification in the melt, when the Al content is very low.
• Ti contributes the formation of fine TiN in the solidification/cooling which acts as pinning particles for ferrite grain growth in the reheating.
• the Ti content 200 mass ppm is necessary to achieve the above effect.
• excessive Ti content causes the deterioration of toughness characteristics by carbide formation. Therefore, the Ti content is set below 1000 mass ppm to avoid/reduce the deterioration of toughness characteristics
• the Zr content is set over 200 mass ppm under the Oless condition.
• the Zr/Ti ratio is over 0.3.
• Residual elements are essential elements as first deoxidizers for producing ferritic stainless steel.
• one or more ferrite stabilizing elements such as Mo, Nb and V, which then constitute residual elements in the ferritic stainless steel, can also be added.
• one or more corrosion resistance elements such as Ni and Cu, which then constitute residual elements in the ferritic stainless steel, can also be added.
• a method recited in claim 4 for controlling the fine particle dispersion (with a high number of particles and uniform space -distribution) of Ti-Zr oxides/nitrides and Zr nitrides has a deoxidation order characterized by the following:
• the deoxidation operation of the steel is based on Ti and Zr addition in the melt after SrMn deoxidation.
• the melt is deoxidized with Ti and then Zr, using 1 to 5 minutes intervals between additions. Thereafter, the melt is cast after at least 1 minute holding. That is, all deoxidizers should be added in the order of weak deoxidizer to strong deoxidizer, i.e. Si and Mn, then Ti and finally Zr.
• the AHess condition should also be carefully kept for preventing particle agglomeration.
• the ferritic stainless steel might be contaminated unavoidably with Al in its production process, although it is essentially an AHess material. Therefore, the Al content might be set below 0.01 mass%, and preferably 0.005 mass%.
• Fe-20mass%Cr ferritic stainless steels having chemical compositions shown in Table 1 were melted at 1600°C in a MgO crucible under an Ar gas atmosphere in a high frequency induction furnace.
• the process of sample making was based on melting a Fe-20mass%Cr alloy with constant free 0 content ' - 150 mass ppm, and with different N contents: 65 mass ppm, 250 mass ppm and 500 mass ppm. Thereafter, the melt was deoxidized with Ti (0.1 mass ) and Zr or Ce (0.1 mass%) using 60s interval between additions and then after 30s cooled to 1400°C. After then, the ingot sample was quenched in water.
• the final chemical compositions, the as-cast structure in ing ot (the ratio of equiaxed grain zone to columnar grain zone, REC, and the mean equiaxed grain size, DA), and the particle characteristics (total number of particles per unit volume, Nv, and median diameter of particles, d) are shown in Table 1.
• TiZr l to TiZr-3 samples correspond to Ti-Zr addition experiments
• TiCe- 1 and TiCe-2 samples correspond to Ti-Ce addition experiments.
• grain refining in as-cast structure can be generally promising.
• TiZr2 and TiZr-3 samples produced according to the present invention have an as-cast structure with the ratio of equiaxed grain zone to columnar grain zone, REC: over 50% and the mean equiaxed grain size, Dp,- ' below 300 micron.
• grain refining in as-cast structure was not confirmed in TiZr- 1 sample with the lack of N and Zr content.
• New specimens were cut from the equiaxed grain zones of TiZ l to TiZr-3 samples with Ti-Zr addition, and then were reheated at 1200°C and 1400°C during 60min under an Ar atmosphere by using a Confocal Scanning Laser Microscope (CSLM). By in-situ observation the grain growth behavior in each reheated specimen was investigated. The change of the average equiaxed grain size of each specimen during heat treatment is shown in Figure 2.
• CSLM Confocal Scanning Laser Microscope
• TiZr-2 and TiZr-3 samples produced according to the present invention could keep the perfect pinning effect on ferrite grain growth during 60min reheating at 1200°C.
• the TiZr3 sample could keep the perfect pinning effect on ferrite grain growth for a few minutes of reheating at 1400°C.
• a clear pinning effect was not confirmed in TiZr 1 sample with the lack of N and Zr contents.

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• 2011
  • 2011-04-26 US US13/643,703 patent/US20130121870A1/en not_active Abandoned
  • 2011-04-26 KR KR1020127030746A patent/KR20130108071A/ko not_active Application Discontinuation
  • 2011-04-26 JP JP2013507917A patent/JP2013531130A/ja active Pending
  • 2011-04-26 WO PCT/SE2011/050495 patent/WO2011136724A1/en active Application Filing
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