US20130129560A1 - Ferritic stainless steel - Google Patents

Ferritic stainless steel Download PDF

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US20130129560A1
US20130129560A1 US13/813,511 US201113813511A US2013129560A1 US 20130129560 A1 US20130129560 A1 US 20130129560A1 US 201113813511 A US201113813511 A US 201113813511A US 2013129560 A1 US2013129560 A1 US 2013129560A1
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black spots
stainless steel
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Tooru matsuhashi
Michio Nakata
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Nippon Steel Stainless Steel Corp
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    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
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    • 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
    • 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
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
    • F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
    • F28F21/08—Constructions of heat-exchange apparatus characterised by the selection of particular materials of metal
    • F28F21/081—Heat exchange elements made from metals or metal alloys
    • F28F21/082—Heat exchange elements made from metals or metal alloys from steel or ferrous alloys
    • F28F21/083—Heat exchange elements made from metals or metal alloys from steel or ferrous alloys from stainless steel

Definitions

  • the present invention relates to a ferritic stainless steel that generates only a small amount of black spots in the TIG weld zone.
  • Ferritic stainless steel generally has properties such as a small expansion coefficient relative to the austenitic stainless steel and excellent resistance to stress corrosion cracking in addition to excellent corrosion resistance. Therefore, the ferritic stainless steel is widely used in tableware, kitchen equipment, building exterior materials such as roof materials, water or hot water container materials or the like. Further, there is increasing demand for replacement of austenitic stainless steel due to recent increasing price of nickel raw material, and the uses of ferritic stainless steel have expanded.
  • Patent Reference 3 discloses a technique of improving crevice corrosion resistance of the weld zone by adding a certain amount or more of Si in addition to combined addition of Al and Ti.
  • Patent Reference 4 discloses a technique to improve the corrosion resistance of a weld zone by satisfying 4Al+Ti ⁇ 0.32 (wherein each of Ti and Al denotes amount of each component in the steel), thereby reducing heat input during the welding and suppressing generation of scale in the weld zone, and improving corrosion resistance of the weld zone.
  • Patent Reference 5 there is a technique to improve the weather resistance and crevice corrosion resistance of not the weld zone but the material itself by intentionally adding P and appropriate amounts of Ca and Al (for example, see Patent Reference 5).
  • Ca and Al are added to control the shapes and distribution of non-metallic inclusions in the steel.
  • the most important feature of Patent Reference 5 is the addition of P in excess of 0.04%, and Patent Reference 5 describes nothing about an effect during welding.
  • Patent Reference 1 Japanese Examined Patent Application, Second Publication No. S55-21102.
  • Patent Reference 2 Japanese Unexamined Patent Application, First Publication No. 1105-70899.
  • Patent Reference 3 Japanese Unexamined Patent Application, First Publication No. 2006-241564.
  • Patent Reference 4 Japanese Unexamined Patent Application, First Publication No. 2007-270290.
  • Patent Reference 5 Japanese Unexamined Patent Application, First Publication No. H07-34205.
  • black spots In the conventional ferritic stainless steel, dark spots (so called black spots or slag spots) occasionally occurred on the surfaces of back beads after the welding even though shielding conditions in the weld zone were controlled appropriately.
  • Elements such as Al, Ti, Si, Ca have a strong affinity to oxygen.
  • Black spots are oxides of these elements that are solidified on the surface of the weld metal during the solidification process of the weld metal formed by Tungsten Inert Gas (TIG) welding.
  • Tungsten Inert Gas (TIG) welding Tungsten Inert Gas
  • the black spot itself is an oxide
  • the black spots are generated in a large amount or generated in a continuously linked manner, the appearance of the weld material used without polishing the weld zone is worsened.
  • exfoliation of the black spots portion occasionally occurs during working of the weld zone.
  • problems such as deterioration of workability and/or occurrence of crevice corrosion in the interstitial area between the weld metal and the exfoliated black spots occasionally occur.
  • exfoliation of the black spots and the resultant deterioration of corrosion resistance occasionally occur in the member having a structure in which stress is loaded on the weld zone.
  • the corrosion resistance of the TIG weld zone it is important to not only improve the corrosion resistance of the weld bead portion itself and weld scale portion itself, but also to control the occurrence of black spots in the weld zone.
  • the scale that is generated during the welding and accompanies the change of color can be mostly suppressed by the method of enhancing shielding conditions during welding.
  • the black spots generated in the TIG weld zone could not be suppressed sufficiently in accordance with the prior arts even when the shielding conditions were enhanced.
  • an object of the present invention is to provide a ferritic stainless steel that is resistant to the generation of black spots in the TIG weld zone and exhibits excellent corrosion resistance and workability in the weld zone.
  • the inventors performed extensive research to suppress the amount of generation of black spots. As a result, the inventors found that the generation of black spots in the TIG weld zone could be suppressed by optimizing the amounts of Al, Ti , Si, and Ca, and reached the invention of the ferritic stainless steel in which black spots are generated only in a small amount.
  • a first aspect of the present invention is a ferritic stainless steel containing, in mass %, 0.020% or less of C, 0.025% or less of N, 1.0% or less of Si, 1.0% or less of Mn, 0.035% or less of P, 0.01% or less of S, 16.0 to 25.0% of Cr, 0.12% or less of Al, 0.05 to 0.35% of Ti, and 0.0015% or less of Ca, and the balance consisting of Fe and unavoidable impurities, wherein the following formula 1 is satisfied.
  • Al, Ti, Si, and Ca in the formula 1 each denotes an amount of each element in mass % of the steel.
  • a second aspect of the present invention is a ferritic stainless steel according to the above-described first aspect, and further contains, in mass %, 0.6% or less of Nb.
  • a third aspect of the present invention is a ferritic stainless steel according to the above-described first or second aspect, and further contains, in mass %, 3.0% or less of Mo.
  • a fourth aspect of the present invention is a ferritic stainless steel according to any one of the above-described first to third aspects, and further contains, in mass %, one or two selected from 2.0% or less of Cu and 2.0% or less of Ni.
  • a fifth aspect of the present invention is a ferritic stainless steel according to any one of the above-described first to fourth aspects, and further contains, in mass %, one or two selected from 0.2% or less of V and 0.2% or less of Zr.
  • a sixth aspect of the present invention is a ferritic stainless steel according to any one of the above-described first to fifth aspects, and further contains, in mass %, 0.005% or less of B.
  • FIG. 1A is a photograph that shows appearance of black spots that were generated in the back side during TIG welding.
  • FIG. 1B is a schematic drawing that corresponds to the photograph of FIG. 1A and shows an appearance of black spots that were generated in the back side during TIG welding.
  • FIG. 2A is a graph that shows a result of AES measurement of depth profile of elements (concentration distributions of elements in the depth direction) of the weld bead portion in the back side of a test piece.
  • FIG. 2B is a graph that shows a result of AES measurement of depth profile of elements (concentration distributions of elements in the depth direction) of a black spot in the back side of a test piece.
  • FIG. 3 is a graph that shows a relationship between the BI value and the formation length ratio of black spots.
  • FIG. 4 is a graph showing the relationship between the BI value and corrosion, where concentric double circles ( ⁇ ) show excellent results, circles ( ⁇ ) show good results, and crosses (x) show bad results.
  • a ferritic stainless steel according to the present invention that generates only a small amount of black spot satisfies the below described formula 1.
  • Al, Ti, Si, and Ca in the formula 1 each denotes an amount of each element in the steel in mass %.
  • Al, Ti, Si, and Ca are elements that have a specifically strong affinity with oxygen and that generate black spots at the time of TIG welding. As the amount of Al, Ti, Si, and Ca contained in the steel increases, black spots are easily generated.
  • the coefficients of the amounts of Al, Ti, Si, and Ca in the above-described formula 1 are determined based on the degree (strength) of the effect of enhancing the generation of black spots and the amounts in the steel. Specifically, as it is shown in the following experimental example, Al is an element that is included in the black spot at the highest concentration and has a specifically large effect of enhancing the generation of black spots. Therefore, coefficient of the Al content in the above-described formula 1 is determined to be 3.
  • the coefficient of the Ca content is determined to be 200.
  • the generation of black spots is prominent.
  • the BI value is 0.8 or less
  • the generation of black spots in the TIG weld zone is sufficiently reduced, and an excellent corrosion resistance is obtained.
  • the BI value is 0.6 or less
  • the BI value is 0.4 or less
  • the generation of black spots is mostly suppressed, and it is possible to further improve the corrosion resistance of the TIG weld zone.
  • the black spot portion Under conditions in which a large amount of black spots tend to occur, the black spot portion has a large thickness. It is considered that such a portion is easily exfoliated at the time of working, and that severe working such as bulging causes the exfoliation and provides a starting point for corrosion. On the contrary, the black spots have thin thickness under conditions in which black spots are generated in a small amount. Therefore, it is considered that even when black spots are generated, the black spots are not exfoliated easily. Therefore, it is considered that corrosion resistance of the weld zone can be suppressed by suppressing the generation of black spots.
  • Al is important as an deoxidizing element and has an effect in grain-refinement of the microstructure of the steel by controlling the composition of non-metallic inclusion.
  • Al is an element that has the largest contribution to the generation of black spots.
  • excessive addition of Al causes coarsening of the non-metallic inclusions that may act as starting points of scratching in the product. Therefore, the upper limit of the amount of Al was determined to be 0.12%. It is preferable for the steel to contain 0.01% or more of Al with the purpose of deoxidization. More preferably, the amount of Al is 0.03% to 0.10%.
  • Ti is an very important element for fixing C and N and suppressing grain boundary corrosion in the weld zone, thereby improving workability.
  • excessive addition of Ti results in not only generation of black spots but also occurrence of surface scratches during the production process. Therefore, the range of the amount of Ti was determined to be 0.05% to 0.35%. More preferable range is 0.07% to 0.20%.
  • Si is an important element as an deoxidizing element and is effective in improvement of corrosion resistance and oxidation resistance.
  • excessive addition of Si results in not only enhancement of generation of black spots but also deterioration of workability and productivity. Therefore, the upper limit of the amount of Si was determined to be 1.0%. It is preferable to make the steel contain 0.01% or more of Si for the purpose of deoxidization. Preferably, the amount of Si is 0.05% to 0.55%.
  • Ca is very important as an deoxidizing element and is contained in the steel in small amount in the form of non-metallic inclusion.
  • Ca since Ca is oxidized very easily, Ca behaves as strong cause of generation of black spots in the time of welding.
  • the amount of Ca it is preferable to control the amount of Ca to be as small as possible, and the upper limit of the amount of Ca was determined to be 0.0015%. More preferably, the amount of Ca is 0.0012% or less.
  • the amount of C must be suppressed since C deteriorates grain boundary corrosion resistance and workability. Therefore the upper limit of C was determined to be 0.020%. On the other hand, excessive reduction of C causes an increase in the refining cost. Therefore, the amount of C is preferably 0.002% to 0.015%.
  • the amount of N is preferably 0.002% to 0.015%.
  • the amount of Mn was controlled to be 1.0% or less.
  • the steel it is preferable for the steel to contain 0.01% or more of Mn.
  • the amount of Mn is 0.05% to 0.5%, and more preferably, 0.05% to 0.3%.
  • the amount of P must be suppressed to low value since P deteriorates weldability and workability and makes the grain boundary corrosion easily occur. Therefore, the amount of P was controlled to be 0.035% or less. A more preferable amount of P is 0.001% to 0.02%.
  • the amount of S must be reduced since S generates water-soluble inclusions such as CaS and MnS that act as starting points of corrosion. Therefore, the amount of S is controlled to be 0.01% or less. On the other hand, excessive reduction of S results in deterioration of the cost. Therefore, the amount of S is preferably 0.0001% to 0.005%.
  • Cr is the most important element for ensuring corrosion resistance of the stainless steel. In order to stabilize the ferrite microstructure, it is necessary for the steel to contain 16.0% or more of Cr. On the other hand Cr deteriorates the workability and productivity of the steel. Therefore, the upper limit of Cr content was determined to be 25.0%. Preferably, the amount of Cr is 16.5% to 23.0%, and more preferably, 18.0% to 22.5%.
  • Nb has properties such that Nb may be added alone, or in combination with Ti. Where Nb and Ti are added in combination, it is preferable that the formula: (Ti+Nb)/(C+N) ⁇ 6 is satisfied, where Ti, Nb, C, and N in the formula each denote the amount in mass % of each element in the steel.
  • Nb fixes C and N, suppress grain boundary corrosion of weld-zone, and improves workability.
  • excessive addition of Nb deteriorates workability of the steel. Therefore, it is preferable to control the upper limit of the amount of Nb to be 0.6%.
  • the steel In order to improve the above-described properties by the content of Nb, it is preferable for the steel to contain 0.05% or more of Nb.
  • the amount of Nb is 0.15% to 0.55%.
  • Mo is an element that has an effect of repairing the passive film and is very effective in improvement of corrosion resistance.
  • Mo has an effect of improving pitting corrosion resistance of the steel.
  • Mo has an effect of improving the resistance to outflow rust.
  • increased Mo reduces workability and increases the cost. Therefore, it is preferable to control the upper limit of the amount of Mo to be 3.0%.
  • the steel it is preferable for the steel to contain 0.30% or more of Mo.
  • the amount of Mo is 0.60% to 2.5%, more preferably, 0.9% to 2.0%.
  • Ni has an effect of suppressing active dissolution rate, and is excellent in repassivation properties because of the small hydrogen overvoltage. On the other hand, excessive addition of Ni deteriorates workability and destabilizes the ferrite microstructure. Therefore, it is preferable to control the upper limit of the amount of Ni to be 2.0%. In order to improve the above-described properties by the content of Ni, it is preferable for the steel to contain 0.05% or more of Ni. Preferably, the amount of Ni is 0.1% to 1.2%, more preferably, 0.2% to 1.1%.
  • the steel In the same manner as Ni, Cu reduces the active dissolution rate, and has an effect of enhancing repassivation. However, an excessive addition of Cu deteriorates the workability. Therefore, where Cu is added, it is preferable to control the upper limit of the amount to be 2.0%. In order to improve the above-described property by the content of Cu, it is preferable for the steel to contain 0.05% or more of Cu. Preferably, the amount of Cu is 0.2% to 1.5%, more preferably, 0.25% to 1.1%.
  • V Vanadium
  • Zr Zirconium
  • V and Zr improve weather resistance and crevice corrosion resistance.
  • V is added while suppressing the use of Cr and Mo, it is possible to ensure an excellent workability.
  • excessive addition of V and/or Zr deteriorates workability, and saturates the effect of improving corrosion resistance. Therefore, where V and/or Zr is contained, it is preferable to control the upper limit of the amount to be 0.2%.
  • the steel In order to improve the above-described properties by the content of V and/or Zr, it is preferable for the steel to contain 0.03% or more of V and/or Zr. More preferably, the amount of V and/or Zr is 0.05% to 0.1%.
  • B is an element that strengthens grain boundary and is effective in improvement of secondary working embrittlement.
  • excessive addition of B causes reduction of ductility by solution-strengthening of ferrite. Therefore, where B is added, it is preferable to control the lower limit to be 0.0001%, and the upper limit to be 0.005%. More preferably, the amount of B is controlled to be 0.0002% to 0.0020%.
  • Test pieces composed of ferritic stainless steel each having a chemical component (composition) shown in Table 1 were produced in accordance with the below described method. Firstly, a cast steel having a chemical component (composition) shown in Table 1 was molten by vacuum melting, and an ingot having a thickness of 40 mm was produced from the melt. The ingot was rolled to a thickness of 5 mm by hot rolling. After that, in accordance with the recrystallization behavior of each steel, the rolled steel was heat treated for one minute at a temperature of 800 to 1000° C. After removing a scale by grinding, steel plates having a thickness of 0.8 mm were produced by cold rolling After removing the surface oxide scale by pickling (acid cleaning), test materials were obtained.
  • test pieces of Nos. 1 to 28 were produced.
  • the amount of each element is shown in mass %, and the balance is iron and unavoidable impurities.
  • the under-line shows that the value is outside the range of the present invention.
  • test pieces of Nos. 1 to 28 were subjected to TIG welding under the following welding conditions, and formation length ratio of black spot was measured in accordance with the following manner. In addition, corrosion experiment was performed for the test piece of Nos. 1 to 28.
  • Pieces of the same steel were butted and subjected to TIG welding with feed rate of 50 cm/min, and heat input of 550 to 650 J/cm 2 .
  • Argon was used both in shielding of torch side and shielding of back side.
  • the formation length ratio of black spots was determined as a standard for showing the amount of generated black spots after the TIG welding.
  • the formation length ratio of black spots was obtained by integrating length of each black spot along the welding direction, and dividing the integrated value by the total welding length. Practically, photograph of a 10 cm length weld zone was taken by a digital camera, length of each black spot was measured, and ratio of the integrated length of black spots to the welding length was calculated using image processing.
  • TIG weld zones of test pieces were subjected to bulging and were used as test pieces for corrosion test. Erichsen test conditions in accordance with JIS2247 were used as the bulging conditions. The penetration side was used as the surface, and a punch of 20 mm ⁇ was used in the bulging. The working of each test piece was paused so as to adjust the working conditions with respect to the height of the bulge. The paused height (height of bulge) was standardized to 6 mm and 7 mm Each test piece was subjected to continuous spray test of 5% NaCl in accordance with JIS Z 2371, and corrosion was evaluated by presence or absence of outflow rust in the test piece at 48 hours after the spray test.
  • formation length ratios of black spots were small, that is, amounts of generated black spots after the TIG welding were small in the test pieces Nos. 1 to 21 that had chemical component (composition) within the range of the present invention and BI value of 0.8 or less.
  • test pieces Nos. 22, 24, 26 to 28 having BI value exceeding 0.8 showed large formation length ratio of black spots after the TIG welding and rust from the weld zone was observed in each specimen in the corrosion test.
  • Magnified images of rust portions of test pieces Nos. 22, 24, 26 to 28 were observed using a magnifying glass. As a result, exfoliation was observed in the boundary between the black spots and the weld bead portion.
  • Rust was generated in corrosion test of Nos. 22, 26, 27, and 28 in which concentrations of Al, Ti, Si, and Ca exceeded the regulated values.
  • Test materials were produced in accordance with the same manner as test piece No.1 except for that steel plates of 1 mm thick were produced by cold-rolling of ferritic stainless steels having the below described chemical components (compositions). Test piece A and test piece B were obtained using the test materials.
  • test piece A and test piece B were subjected to TIG welding under the similar welding conditions as the test piece No. 1, and appearance of black spots generated in the back side during TIG welding was observed.
  • FIG. 1A and FIG. 1B The results are shown in FIG. 1A and FIG. 1B .
  • FIG. 1A is a photograph that shows an appearance of black spots generated in back side during TIG welding.
  • FIG. 1B is a schematic drawing that shows an appearance of black spots generated in back side during TIG welding and that corresponds to FIG. 1A .
  • FIG. 1A and FIG. 1B the left-side figures show a photograph and a drawing of test piece A in which BI value is 0.49, and the right-side figures show a photograph and a drawing of test piece B in which BI value is 1.07.
  • a FE (Field Emission) Auger Electron Spectroscopy was used in the AES analysis, and the analysis was performed until intensity of oxygen spectrum was almost not detected under conditions of an acceleration voltage of 10 keV, a spot diameter of 40 nm, and a sputtering rate of 15 nm/min. Although a position dependent error may occur due to small spot of AES measurement, the results were applied as values showing approximate thickness.
  • FIG. 2A and FIG. 2B are graphs that show depth profile of elements (concentration distribution of elements in depth direction) in the black spot and the weld bead portion on the back side of the test piece as a result of AES analysis.
  • FIG. 2A is a result of weld bead portion
  • FIG. 2B is a result of a black spot.
  • the weld bead portion was an oxide having a thickness of several hundred A that mainly contained Ti, and also contained Al and Si.
  • the black spot was a thick oxide having a thickness of several thousand A that mainly contained Al, and also contained Ti, Si, and Ca. From the graph of black spot shown in FIG. 2B , it was confirmed that Al was contained in the black spot at the highest concentration and Ca was contained in the black spot at high concentration irrespective of its small amount in the steel.
  • Test materials of ferritic stainless steel having various component ratios (compositions) were produced in accordance with the production method similar to that of test piece A, where each steel had a basic composition of C: 0.002 to 0.015%, N: 0.02 to 0.015%, Cr: 16.5 to 23%, Ni: 0 to 1.5%, Mo: 0 to 2.5%, and also contained main components of black spots such as Al, Ti, Si, and Ca in different amount. Using the test materials, a plurality of test pieces was obtained.
  • test piece No. 1 The thus obtained plurality of test pieces were subjected to TIG welding under similar welding conditions as the test piece No. 1, and ratios of formation length of black spots were calculated in accordance with the same manner as the test piece No. 1.
  • the formation length ratio of black spots tended to increase in accordance with the increasing amounts of Al, Ti, Si, and Ca. Those elements are elements having specifically strong affinity with oxygen. It was found that Al had the highest effect, and Ca had a strong influence on black spots irrespective of its small amount in the steel. It was also confirmed that Ti and Si also contributed to the generation of black spots.
  • BI value shown by the below described formula 1 was calculated for the plurality of test pieces, and the relationship between the BI value and the ratio of formation length of black spots was examined.
  • Al, Ti, Si, and Ca in the formula 1 respectively denotes an amount of each element in mass % of the steel.
  • FIG. 3 is a graph showing the relationship between the BI value and the ratio of formation length of black spots. As shown in FIG. 3 , it is recognized that the ratio of formation length of black spots increases with an increasing BI value.
  • FIG. 4 is a graph that shows a relationship between the BI values and the results of corrosion resistance evaluation after the spray test after working.
  • double circles ( ⁇ ) show excellent results
  • circles ( ⁇ ) show good results
  • crosses (x) show bad results.
  • corrosion did not occur in the specimen with bulge height of 6 mm where BI value was 0.8 or less.
  • BI value was 0.4 or less
  • specifically good results were achieved such that corrosion was not observed in the specimen with bulge height of 7 mm.
  • the ferritic stainless steel of the present invention can be appropriately used in members requiring corrosion resistance in a structure of, for example, exterior materials, building materials, water or hot water containers, consumer electronics, kitchen equipment, drain water collectors and heat exchanger of gas condensing boiler, various welded pipes or the like that are formed by TIG welding and used in general indoor or outdoor environment.
  • the ferritic stainless steel of the present invention is appropriately used in a member that is subjected to working after the welding. Since the ferritic stainless steel of the present invention is excellent not only in corrosion resistance but also in workability of TIG weld zone, the steel can be widely used in the usage suffering severe working conditions.

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US10385429B2 (en) 2013-03-27 2019-08-20 Nippon Steel & Sumikin Stainless Steel Corporation Hot-rolled ferritic stainless-steel plate, process for producing same, and steel strip
US11220732B2 (en) 2016-06-27 2022-01-11 Jfe Steel Corporation Ferritic stainless steel sheet
US11230756B2 (en) 2016-09-02 2022-01-25 Jfe Steel Corporation Ferritic stainless steel
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US11230756B2 (en) 2016-09-02 2022-01-25 Jfe Steel Corporation Ferritic stainless steel
US12084406B2 (en) 2019-07-05 2024-09-10 Stamicarbon B.V. Ferritic steel parts in urea plants
CN114746569A (zh) * 2019-12-20 2022-07-12 株式会社Posco 具有改善的磁化强度的铁素体不锈钢及其制造方法

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